S D J Inglethorpe, D J D E Highley and A J Bloodworth TT · S D J Inglethorpe, D J Morgan, D E Highley and A J Bloodworth TT TT Mineralogy and Petrology Group British Geological Survey

British Geological Survey

TECHNICAL REPORT WG/93/20 Mineralogy and Petrology Series

Industrial Minerals Laboratory Manual BENTONITE S D J Inglethorpe, D J Morgan, D E Highley and A J Bloodworth

T T T T

Mineralogy and Petrology Group British Geological Survey

Keyworth Nottingham

United Kingdom NG12 5GG

British Geological Survey

TECHNICAL REPORT WG/93/20 Mineralogy and Petrology Series

Industrial Mberals Laboratory Manual

BENTONITE S D J Inglethorpe, D J Morgan, D E Highley and A J Bloodworth

A Report prepared for the Overseas Development Administration under the ODABGS Technology Development and Research Programme, Project 91/1

ODA Classzjkation: Subsector: Geoscience Theme: G 1 - Promote environmentally sensitive development of non-renewable resources Project title: Minerals for Development Reference number: E541

Bibliographic reference: Inglethorpe, S D J, Morgan, D J, Highley, D E & Bloodworth, A J Industrial Minerals Laboratory Manual: Bentonite BGS Technical Report WG/93/20

Subject index: Industrial minerals, bentonite, laboratory techniques

Cover illustration: Schematic representation of the crystal structure of smectite group minerals

0 NERC 1993

Keyworth, Nottingham, British Geological Survey, 1993

CONTENTS

1. INTRODUCTION

2. GEOLOGICAL OCCURRENCE

3. MINING AND PROCESSING

4. INDUSTRIAL APPLICATIONS

5. LABORATORY ASSESSMENT OF BENTONITE

6. USE-SPECIFIC TESTS

REFERENCES

APPENDICES:

1. Release of extractable cations from bentonite by ammonium acetate leach

2. Cation-exchange capacity (BaClrngS04 method)

3. Methylene blue cation-exchange capacity

4.2-ethoxyethanol surface area method

5. Bentonite swelling test

6. Bentonite liquid limits

7. Plastic limit and plasticity index

8. Drilling fluid properties

9. Mechanism of acid activation and assessment of bleaching performance of bentonite for use in edible oil clarification

Page

1

5

9

13

25

48

61

65

68

72

78

82

84

90

92

98

115 10. Water absorbency of pet litters

Preface

Industrial mineral raw materials are essential for economic development. Infrastructure improvement and growth of the manufacturing sector requires a reliable supply of good quality construction minerals and a wide range of other industrial mineral raw materials.

Although many less developed countries have significant potential industrial mineral resources, some continue to import these materials supply their industries. Indigenous resources may not be exploited (or are exploited ineffectively) because they do not meet industrial specifications, and facilities and expertise to carry out the necessary evaluation and testwork are unavailable. Unlike metallic and energy minerals, the suitability of industrial minerals generally depends on physical behaviour, as well as on chemical and mineralogical properties. Laboratory evaluation often involves determination of a wide range of inter-related properties and must be carried out with knowledge of the requirements of consuming industries. Evaluation may also include investigation of likely processing required to enable the commodity to meet industry specifications.

Over the last 10 years, funding from the Overseas Development Administration has enabled the British Geological Survey to provide assistance to less developed countries in the evaluation of their industrial mineral resources. This series of laboratory manuals sets out experience gained during this period. The manuals are intended to be practical bench-top guides for use by organisations such as Geological Surveys and Mines Departments and are not exhaustive in their coverage of every test and specification. The following manuals have been published to date:

Limestone Flake Graphite Diatomite Kaolin Bentonite Construction Materials

A complementary series of Exploration Guides is also being produced. These are intended to pr6vide ideas and advice for geoscientists involved in the identification and field evaluation of industrial minerals in the developing world. The following guide has been published to date:

Biogenic Sedimentary Rocks

A J Bloodworth Series Editor

D J Morgan Project Manager

1

Industrial Minerals Laboratory Manual

Bentonite

1. INTRODUCTION

Bentonites, which consist essentially of clay minerals of the smectite group, have a wide range of industrial uses. A particular feature of this group of minerals is the substitution of Si4+ and Al3+ in the crystal structure by lower valency cations. This leaves unsatisfied negative charges which are balanced by loosely-held ‘exchangeable’ cations such as Na+, Ca2+, Mg2+ and H+ located mainly on the interlayer crystal surfaces. The structure, chemical composition, exchangeable ion type and small crystal size of smectite are responsible for several unique properties, including a large chemically active surface area, a high cation- exchange capacity, interlayer surfaces having unusual hydration characteristics, and sometimes the ability to modify strongly the flow behaviour of liquids (Odom, 1984). The crystal structure and chemistry of smectites are summarized in Table 1 and Figure 1.

Depending on the dominant exchangeable cations present the clay may be referred to as either calcium bentonite or sodium bentonite, the two varieties exhibiting markedly different properties and thus uses. The terms nonswelling bentonite and swelling bentonite are synonymous with calcium bentonite and sodium bentonite respectively. When mixed with water, swelling bentonites exhibit a greater degree of dispersion and better plastic and rheological properties than nonswelling bentonites. Natural sodium bentonites, such as those in Wyoming, USA (Wyoming or Western bentonite), are comparatively rare although the cation-exchange properties of smectite enable the more widespread calcium form to be easily converted to high-swelling sodium bentonite by a simple sodium-exchange process. In some countries, notably Britain, calcium bentonite is referred to as fuller’s earth, although elsewhere, and particularly in the USA, this term is applied to any clay that has the capacity to decolourise oil and may consist of smectite or attapulgite. The latter is mineralogically distinct but has similar properties of adsorption to calcium bentonite and can substitute in certain applications. Sepiolite (the magnesium analogue of attapulgite) is also used as an alternative to calcium bentonite, particularly in Europe (Singer & Galan, 1984).

Mineralogy and Petrology Group, British Geological Survey 0 NERC 1993

2

Bentonite

The physical and chemical properties of bentonites typically vary both within and between deposits due to differences in the degree of chemical substitution within the smectite structure and the nature of the exchangeable cations present, and also due to the type and amount of impurities present. No bentonite is universally acceptable for every application. In this context a distinction can be made between the grade and quality of bentonites. The grade is defined as the smectite content of the bentonite, whilst its quality is related to the inherent physico- chemical properties of the clay, either in its natural or modified form, and is a measure of likely industrial performance. However, there is no recognised minimum grade or smectite content below which a clay is no longer considered to be a bentonite. Commercial bentonites rarely contain less than 60% smectite and usually more than 70%, associated minerals typically being quartz, cristobalite, feldspars, zeolites, calcite, volcanic glass, and other clay minerals such as kaolinite. Amounts and type of associated minerals are related to the origin of the bentonite.

The aim of this manual is to show how data gathered during initial mineralogical and chemical examination of a bentonite may be used to indicate the suitability of the clay for different applications. Following on from this, some physico-chemical tests are described which can be used to screen large numbers of exploration samples for bentonite grade and quality. These physico-chemical tests are all relatively simple and can be carried out in a reasonably well-equipped mineralogical or chemical laboratory. Finally, some use-specific test procedures are described which could be carried out in the same setting.

This manual is one of a series produced as part of the BGS/ODA R&D Project ‘Minerals for Development’.


3

Bentonite

Table 1. Crystal structure and chemistry of smectite-group clays.

STRUCTURE

* Classed as 2:l Layer phyllosilicates Composed of alternatmg octahedral and tetrahedral sheets

* Ratio of tetrahedral : octahedral sheets is 2 : 1

CATION COORDINATION

* X positions - exchangeable cations Y positions - structural octahedral cations

* 2 positions - structural tetrahedral cations

* Structural cations - fixed * Exchangeable cations (surface cations) available for chemical exchange

CHEMICAL COMPOSITION

General formula of smecfite group:

Chemical composition of the smectite group

Pyrophyllite Dioctahedral smectites: Montmorillonite Beidellite Nontronite

Talc Trioctahedral smectites: Saponite Hectorite

Octahedral Exchange Chargei Y X

Si8

Si8 Mg6

(ca. Na, Mg) 0.2-0.6 (ca, Na, Mg) 0.2-0.6 (cap Na, Mg) 0.2-0.6

Negative charge per formula unit layer [Ole (OH)2] Trioctahedral - all 3 octahedral sites (per unit cell) occupied by divalent cations

Dioaahedral - 2 of 3 octahedral sites (per unit cell) occupied by trivalent cations

* Isomorphous substitution of of lower valency cations for Si4+ and Al3+ results in a negative crystal charge which is balanced by exchangeable cations


4

Bentonite n

Q z 6 6 2 c.

P f 6

tif 0,

f n

X-SIT€

INTERLAYER

Z-SITE Y-SITE

TETRAHEDRAL OCTAHEDW

Figure 1. Crystal structure and chemistry of smectite-group clay minerals.


5

Bentonite

2. GEOLOGICAL OCCURRENCE

Smectite is the essential and active component of bentonites on which their economic importance is based. The majority of commercial bentonite deposits contain Ca2+ and Mg2+ as the main exchangeable cations. However, smectite clay minerals become unstable with increasing age, depth of burial, and diagenesis, and alter to mixed-layer, illite-smectite clays, sometimes referred to as K- or meta-bentonites, in which their valuable properties have been largely destroyed. Consequently, pure smectite clay minerals are essentially absent in rocks of pre-Mesozoic age (Moorlock & Highley, 1991). Whilst Jurassic bentonites are known, most economic deposits are Cretaceous or younger in age and this fact is an important exploration criterion.

Most bentonites have formed by alteration of igneous material. Such deposits are of two markedly different types: (i) those resulting fi-om sub-aqueous alteration of fine-grained volcanic ash and (ii) those

. c resulting from in situ hydrothermal alteration of acid volcanic rocks (Grim & Guven, 1978).

2.1 Sedimentary bentonites

These bentonites are formed by the alteration of volcanic ash deposited in the sedimentary environment. This material may have been subsequently reworked and concentrated by sedimentary processes. This type of deposit may be found associated with a wide range of lithologies, mainly of shallow, marine origin, although occurrences in non-marine environments have been reported. Most bentonites in this category have formed from ash of mainly rhyolitic to trachytic composition. Beds vary in thickness from a few millimetres up to 10 m and whilst they are often lenticular, in some cases markedly so, some deposits may extend over hundreds of square kilometres, a feature consistent with their formation from airborne volcanic ash. Typically bentonites exhibit sharp contacts with the underlying sediments but their upper surfaces may be both erosive, and therefore sharp, or gradational into the overlying sediment. The most important deposits of this type, accounting for a major proportion of world production of bentonite, are those in the western USA High-swelling or sodium bentonites occur in the Northern Black Hills district straddling the Montana-Wyoming-


6

Bentonite

South Dakota borders and in the Hardin district of south-central Montana and north-central Wyoming. Numerous bentonite beds ranging from a few millimetres up to several metres in thickness occur in Cretaceous strata over a stratigraphical interval of over 1000 m. However, only seven beds are sufficiently thick and extensive to be of commercial interest. The best known is the Clay Spur Bentonite within the Mowry Shale. The bed, which is between 1- 1.5 m thick, is the source of most of the sodium bentonite with very high colloidal properties for which the area is famous (Harben & Bates, 1990). Commercially exploited bentonites generally contain less than 30% non- clay minerals but exhibit varying properties, which influences their ultimate use. The source of the volcanic ash is considered to be the volcanic centres of the Rocky Mountains and the beds thus thin towards the east. Since the exchangeable cations present in bentonites are easily replaceable, Na+ is readily replaced by Ca2+ and Mg2+ under leaching conditions. Locally this has happened in Wyoming, where groundwater leaching of the overlying overburden has provided the source of Ca2+ and Mg2+. Only rarely are Ca2+ and Mg2+ replaced by Na+ (Odom, 1984).

As sedimentary bentonites are not usually associated with volcanic rocks, the ultimate source of the volcanic ash may be several hundred kilometres away and no longer exposed. This is the case in Britain where Cretaceous bentonites occur as a number of isolated deposits of varying size (Figure 2) but where there is no evidence of contemporaneous volcanic activity onshore. A volcanic source in the North Sea is most likely. However, primary ash falls from volcanic centres some hundreds of kilometres distant are unlikely to be more than a few centimetres thick. The processes which lead to the formation of this type of commercial bentonite deposits are complex. Moorlock & Highley (1 99 1) recognised five genetic stages in their formation:

e Eruption of ash and its airborne transport. e Water sorting and concentration of volcanic ash in shallow-

e Protection of the bentonite beds from subsequent erosion. 0 No subsequent thermal alteration of the smectite to other clay

marine environments to give thick accumulations. e Conversion of the volcanic ash to smectite.

mineral phases.


Bentonite

2.2 Hydrothermal bentonites

Some bentonite deposits have formed by the in situ hydrothermal alteration of volcanic rocks, normally of acidic composition. Such deposits may be irregular in shape and controlled by fault and joint systems and they may also be highly variable in quality due to partial alteration of the parent rock. Often they may be associated with recent volcanism, which should also be regarded as an important exploration criterion. Many of the important bentonite deposits in the Mediterranean region are related to the recent volcanism and associated hydrothermal activity produced by the subduction of the African plate under the Eurasian plate. The important deposits on the island of Milos, Greece, are of this type and have formed in the recent geological past (since Pliocene times) by the alteration of acid volcanic rocks and associated tuffs in a marine environment. The degree of alteration is variable with the main impurities being quartz, feldspar, unaltered volcanic glass and kaolinite.

2.3 Field characteristics

Bentonites range in colour from black through to white but most frequently are bluish-green when fresh, weathering to a yellowish- brown colour at or near outcrop due to the oxidation of ferrous iron. Material from near outcrop often exhibits enhanced swelling properties. Despite their often characteristic appearance at outcrop, where they tend to exhibit a 'frothy' or 'popcorn' texture due to successive wetting and drying, deposits may be easily overlooked during field mapping, particularly in tropical areas where this feature may be obscured. In addition, because bentonite deposits sometimes exhibit a highly lenticular form, some may have no surface expression with no outcrops. Detection then can prove extremely difficult.


8

Bentonite

AGE BIO- WSUSSEX / SURREY WEST EAST BEDFORD- ZONE HANTS KENT KENT SHIRE

LC- fuller's earth

I .-.- unconformity

Limestone F l d Sandy clay

F j Cloy

[ml No sediment present

Figure 2. Geological section showing occurrences of fuller's earth (Ca-bentonite) in Lower Cretaceous sediments in south east England (from Moorlock & Highley, 1991).


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Bentonite

3. MINING AND PROCESSING

3.1 Mining

Bentonite is almost always extracted in open pit workings, usually by opencast methods in which the overburden is dumped into the worked out sections of the pit and rapidly restored. This method can be applied to stratified deposits which are either flat-lying or dip at an angle less than the safe angle of repose of waste rock. The relative proportion of overburden to mineral (the overburden ratio). Overburden ratios may be as high as 20: 1 for high quality bentonites and in Britain up to 45 m of sand overburden is removed at one operation to extract 2-3 m of bentonite. In stripping overburden, care must be taken to prevent contamination and in some operations the top of the bentonite bed may be separately recovered for lower quality uses. Variations in the properties of bentonite within the same bed, as well as different beds, means that selective mining is often necessary to match different qualities with specific applications. Underground mining is rarely used but was practised in Britain until 1979 to recover Jurassic bentonite from beneath a thick cover of limestone.

3.2 Processing

Bentonite is rarely used in the raw form but undergoes processing essentially to modify its properties for specific industrial applications rather than to increase its smectite content. The major product groups are listed below.

e Ca-bentonite (fine powders and granules) e Na-bentonite and Na-exchanged bentonite e Acid-activated clays 0 Speciality clays (white bentonite and organoclays)

A simplified flow diagram for the processing of Ca-bentonite is given in Figure 3. Initial processing consists of drying and grinding, the dried clay being either screened and marketed in the granular fonn or milled to a fine powder, size grading being carried out by air-classification. Fines powders may be pelletized and crushed to produce a higher yield of granules. Granules may be lightly calcined to make them water-stable.


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Bentonite

I Raw Ca-bentonite I

Grinding to fine powder

I Granules I /I

I \ pi-zzGGl

/ 1 produce 1 1 Calcination to Sodium carbkate addition i\

I I

Figure 3. Typical processing routes for Ca-bentonite.


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Bentonite

It is common practice, particularly in Europe, to attempt to convert Ca-bentonite to the Na-variety, and therefore extend greatly the number of potential outlets for the clay. The ion exchange is usually effected by mixing sodium carbonate (soda ash) with the crude, moist clay using various mechanical methods. After the soda ash is added, the clay is generally stockpiled for several days or weeks to allow maximum exchange to occur. In Europe, clays are often extruded as part of the sodium-exchange process. In addition to increasing the intimacy of mixing, extrusion shears the clay aggregates and particles making more interlamellar surfaces accessible for exchange. A number of mechanical processes used to help sodium exchange in bentonites have been described by Alther (1982). This process is sometimes referred to as activation, but should not be confused with acid-activation.

The production of valued-added acid-activated clays involves more complex processing but the starting raw material is calcium bentonite, as sodium bentonites do not respond to the same extent to acid treatment. The clay is weighed into mixing vessels with water and mineral acid (either sulphuric or hydrochloric acid). The clay-acid slurry is pumped to agitated reaction vessels which can be heated to near boiling by steam or other means until the desired degree of activation or leaching has been achieved. The clay is then washed and dewatered to yield a filter cake which is subsequently dried, ground and sized. A.wide range of grades are produced for different applications. Product quality and perfomance can be modified by control of the following parameters:

e acid-clay ratio e degree of washing e degree of drying e particle size

The mechanism of activation is complex and is considered in more detail in Section 6.4. Other proerty modifications of smectite, usually involving acid pretreatment and often followed by precipitation of AlOH ‘pillars’ in the interlayer space, is canied out to prepare catalysts for small- or large-scale organic syntheses (Adams, 1987).

Although most bentonites are produced by dry processing methods, modest quantities of high-value white bentonite, in both the calcium and sodium form, are wet-refined using centrifuges to remove coarser impurities such as quartz, cristobalite and feldspar, and to improve their


12

Bentonite

rheological properties. These speciality and highly-priced bentonites are used in water-based paints, cosmetics, a range of pharmaceutical products, detergents and certain ceramics, but only clays with an initial high brightness are suitable for these applications.

Through the exchange of inorganic cations, usually sodium, with positively charged long-chain ammonium compounds, a range of organophilic (and thus hydrophobic) clays can be produced which are known generically as organoclays. They are effective gellants in a wide range of organic fluids, such as paint, grease, ink and oil-based drilling muds (Jones, 1983). They are based on natural and sodium- exchanged bentonites, as well as hectorites (a lithium-bearing smectite) which provide greater gel strength.


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Bentonite

4. INDUSTRIAL APPLICATIONS

Bentonites have a wide range of industrial applications. Based on production data for the USA, by far the largest producer, the major uses of bentonite have always been considered to be in bonding foundry sands, drilling fluids, and iron ore pelletising (Vista, 1990). These uses accounted for 75% of total US production (3.5 Mt) in 1988, although total usage is at much lower levels than in the late 1970s and early 1980s due to a downturn in oil drilling activity, as well as in the steel and foundry industries. Demand for bentonite varies significantly from country to country. In western Europe, the largest market for bentonite is currently pet litter. It is also used as a clarifying agent for oils and fats, in agriculture (as a carrier for pesticides and fertilizers and as a coating for seeds), in civil engineering, in papexmaking, and in paints, pharmaceuticals and cosmetics. Descriptions of these and many other applications have been given by Ross (1963, Grim & Guven (1978) and Odom (1984).

Relationships between the physico-chemical properties of bentonite and commercial applications are summarised in Table 2. Major application areas of the main bentonite product groups are summarised in Figure 4.


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Bentonite

Table 2. Commercial applications of bentonite in relation to physico-chemical properties (after Highley, 1972).

Sorptive properties (may be increased by acid treatment) A: In suspension Refining and bleaching of glyceride oils

Clarification and purification of sugar solutions, syrups and wines Water purification, sewage and effluent treatment Pharmaceutical and therapeutic preparations Absorbent (pet and animal litter oil spillage on factory floors)

B: In dry state (or paste)

Sur$ace area (may be increased by acid-activation)

Rheological properties A: Viscosity and suspending powers

B: Thixotropy

Impermeability, coating properties

Bonding properties

Plasticity

Catalytic action, carrier for catalysts Carrier for insecticides and fungicides Mineral filler and extender

Drilling fluids Paints (oil and water-based) Fertilizer sprays Bitumen emulsions Formulation of ceramic glazes Wall support for boreholes Civil engineering (diaphragm wall construction) Non-drip paints

Civil engineering (grouting, impermeable membrane) Drilling in permeable strata

Bonding foundry moulding sands Pelletising iron ore concentrates Pelletising animal feedstuffs

Formulation of mortars, putties, adhesives, some ceramic bodies


Bentonite

I I

I I

Crude bentonite I Activated with acid Naturally active Alkalino mctivatod Organically activated lact lvatd b luching omrthl (N@-oxchongod bentonitel (orgonophilic bentonite)

Foodstuffs Refining, decdorising, purifying and stablising of vegetaMe and industry animal oils and fats

Forest and water conservation I Powder fire-extinguishing agenlslbindmg agents for oil on water I

I I

Refining, decolorising and purifying of mineral oils, fats. waxes, mraffin/cstalysts for oil cracking Grease thickening 1

I I I I I I I

Beverages and suaar industry Fining of wine, must and juiceslbser stabibtion/purifying of saccharine juice and syrup 1 I

industry waste-water purification/adsorbenn for radioactive materials Catabts/cetalvst carriers. insecticides and fungicideslfillers, dehydrating agentslwater and

of impurities in white water system Pigment and colour d e w l m r for carbon- copying paper/adsorptbn Paper industry

I I I Cleaning and Regeneration or organic Pobhm mtd dressings/additives for wash i i and cleaning agents detergents fluids for drv cleanina and for moo nroduetion

Pharmaceutical industry I Starting m a t e r U for heding earths and medicaments/bases

for creams and cosmetics I Ore production Binding agents for ore pelletising

Supporting suspensions for cut-off diaphragm wall constructions and shield tunndling/subsoil ses l i (eg dumps)/anti-friction agents for pipejacking and shaft sinking/additive for soil concrete, concrete and mortar

Building industry

Ceramics industry Plesticidng of ceramic compoundslimprovement of strength/fluxing agents

Horticulture, agriculture. I Soil improvement/compostinglar,imal.leed peIkt&ing/~~d-ma&~e I-- ~

animal husbandry treatment/cat litter I I 1 I I I I

Drilling industry Borehole scavenging for saltwater

Thixotropic suspensions for borehole scavenging

Tar exploitation 1 - I Emukifiition and thixotroping of ter-&ter e&ioGpter and asphalt coatinos . I Thickening, thixotroping, stabilising and anti-setting agents for

adhesives Paint and varnish industry paints, varnishes. coating materials. sealing cements, waxes,

I I

Foundries

Figure 4. Major application areas of bentonite (from Sud Chemie technical literature).


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Bentonite

4.1 Foundry moulding sands

One of the largest applications for bentonite worldwide is the bonding of silica sand for the production of greensand moulds in foundries. Both calcium and sodium bentonite are used in the foundry industry and typically between 5-10% bentonite is mixed with sand and water to prepare a plastic and cohesive material that can be compressed around a pattern to produce a mould. The bentonite must give the sand sufficient strength to maintain the shape of the pattern before, during and after pouring of the molten metal. After casting the moulding sand is recycled with small additions of new silica sand and bentonite to replace losses and restore the bond. Properties of bentonite/sand mixtures that are crucial for foundry usage are green compressive strength, dry compressive strength, shatter index, and compactibility; properties such as wet tensile strength, hot compressive strength, flowability and durability may also be quoted in specifications. The nature of these specialised tests is discussed in Section 6.2.

4.1.1 Behaviour sf different types of bentonite in foundry sand mixes

Sodium bentonite, both natural and sodium-exchanged, and calcium bentonite are used by the foundry industry, but they exhibit quite different properties. Natural sodium bentonites have a medium green strength and a high dry strength which increases their resistance to molten metal. They also have good high-temperature durability which means that bonding properties are not destroyed by moderate heating. When molten metal is poured into a mould a proportion of the bentonite is heated above its dehydroxylation temperature with the loss of structural water. This loss is associated with a loss in bonding power (green and dry compressive strength) and new clay additions are required to restore the strength of the moulding sand. Most bentonites lose structural water in the range 50O0-75O0C and as a general rule the higher the temperature of OH loss the greater the durability of the clay. An assessment of the durability of bentonites can be obtained from differential thermal analysis (see Section 5.1.2).

Calcium bentonites have a lower durability than sodium bentonites, although this property can be improved by sodium-exchange. Because


17

Bentonite

of the higher casting temperature of steel, natural sodium bentonites are the preferred material for steel castings. The hot and dry strengths and thermal stability of sodium bentonites-are preferred for the production of steel and heavy iron castings. However, high dry strength is not usually wanted in the majority of iron foundries. Mould breakdown becomes more difficult where a high dry strength bentonite is used with resultant high sand losses.

Sodium-exchanged bentonites have high green strengths and moderate dry strengths, whilst calcium bentonites produce moulds with a medium green strength and a low dry strength. However, greensands mixed with calcium bentonites tend to have a low resistance to erosion by the molten metal and are prone to giving scabbing and expansion defects.

For most foundry purposes it is necessary to produce a sand mixture with a high green strength and low dry strength with the minimum amount of clay. For this reason calcium bentonites are rarely used alone and most foundries use a blend of calcium and sodium bentonites to

- - achieve the desired combination of properties.

In summary:

0 Sand mixtures produced from natural Na-bentonites have medium to low green strengths but high dry strengths, which increase the resistance to erosion by molten metal. Wet tensile strengths are high. Na-bentonites usually show good high-temperature durability and the bonding properties are not destroyed by moderate heat. These are used in casting steel and high-duty iron.

0 Sand mixtures made from Ca-bentonites have medium green strengths and low dry strengths. They have low resistance to erosion by molten metal and are likely to cause scabbing and expansion defects in the moulded metal.

0 Sand mixtures made from sodium-exchanged Ca-bentonite have high green strengths, low dry strengths and improved resistance to scabbing and expansion defects. These are used in casting iron, non-fen-ous metals and light-section steel.


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Bentonite

Natural Na-bentonite may be blended with a Ca-bentonite or an activated bentonite to give a specific set of properties in the sand. Such blends are used where high clay levels are necessary to produce strong sands at low moisture contents, for instance in high-throughput mechanized casting of iron and steel products.

When a greensand mould is poured the steam flashed off from the molten metaVsand surface recondenses on cool sand just behind the interface. If the bentonite is unable to hold this additional water, the mould will loose its strength and fail. Liquid limit, which is a measure of the ability of a clay to hold water without flowing, is a useful indicator of the suitability of a clay for bonding purposes. Sodium bentonites have a greater capacity for adsorbing water and thus have much higher liquid limits than calcium bentonites. The Cast Metal Technology Centre of the UK has recommended a minimum liquid limit of 525 for bentonites for use in steel casting (see Section 5.2.2). Typically, bentonites from the north-west USA have liquid limits of between 600-700. Table 3 shows typical foundry moulding sand test values for a range of commercial bentonites.

Table 3. Typical test values for commercial bentonites used in foundry moulding sands.

Green Dry Shatter Compactibility Liquid strength strength index ( W limit (kN/ma) (kN/m2) N o . (%)

Natural Na-bentonite Wyoming, USA 41-49 (med) 655-897 (high) 80-83 NA Fully-activated Ca-bentonite

Na- treated Ca-bentonite

Natural Ca-bentonite mssissippi, USA 41 (med) 448 (low) 78 NA

IdY 55 (high) 482 (low) 79 NA

m 80 (high) 240 (low) 78 62.5

600-620

530

NA

118

All figures based on a 5% addition of bentonite to a standard silica sand at 3.5% moisture content


19

Bentonite

4.2 Drilling fluids

8

The most important functions of bentonite in a drilling fluid are:

e To increase the carrying capacity of the drilling fluid through increased viscosity at low solids concentrations.

e To suspend weighting agents and cuttings when drilling fluid circulation ceases for any reason.

e To impede loss of fluid into permeable, low-pressure strata by the formation of an impermeable filter cake on the sides of the borehole. The filter cake not only prevents fluid loss but also acts as a wall-stabilising membrane.

e To lubricate the drill string and bit.

The ease of dispersion of sodium bentonite in water to produce a fluid with a high viscosity and thixotropic properties (the development of a rigid structure when shear stress is removed) is widely employed in preparing water-based drilling fluids particularly for the petroleum industry. Bentonites used for drilling fluids must meet specifications set by the American Petroleum Institute (MI), which is based on Wyoming or natural sodium bentonite, and the Oil Companies Materials Association (OCMA), which is used for ‘lower quality’ clays, including Na-exchanged bentonites. Specifications relate to the viscosities of suspensions of fixed solids content, requiring the measurement of apparent viscosity, plastic viscosity and yield value, and the filtrate loss of a suspension at the same solids content. Specifications are shown in Table 4.


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Bentonite

Table 4. Specifications for bentonite for drilling fluids.

Measurement API OCMA

Viscometer dial reading at 600 rpm* 30, min Yield pointjplastic viscosity ratio* 3, max Filtrate volume* 15.0 ml Residue > 75 pm 4%, max Moisture 10% max

30, min

6, Max 16.0 ml 2.5%, max 13% max

* Measurements made on suspensions of 22.5 g bentonite in 350 ml water.

Consumption of bentonite in drilling fluids is related to drilling activity (number of holes and depth) and ultimately this is related to the price of 0-il. There appears to be no direct correlation with meterage drilled, but in the USA consumption is between 3.3 to 4.6 tonnes for each 300 m drilled.

In areas where salt horizons are persistently encountered during drilling, then the drilling fluid must be salt-saturated. In the presence of an electrolyte, such as salt solution, smectite clays flocculate and it is impossible to maintain viscosity. Attapulgite and sepiolite are not flocculated by these conditions and are used as alternatives.

4.3 Civil engineering and waste management

The properties of sodium bentonite that are desirable in its use in drilling fluids, that is ease of dispersion in water, viscosity, thixotropy and impermeability, also find application in a wide range of civil engineering applications. Slurries containing 4-8% bentonite are used in grouting fissures in rocks. It is also is used in diaphragm wall construction to provide nonmechanical support for the walls of trenches, during piling and in other excavations, and to lubricate caissons and piles. Bentonite improves the pumpability of concrete and has been used in soft ground tunnelling by the formation of a gel ahead of the tunnel face which can


21

Bentonite

then be excavated and the bentonite suspension separated and recycled for further use. Only sodium bentonite is suitable for these applications.

Natural sodium-bentonites and sodium-exchanged calcium bentonites are used to improve the performance of clay liners in engineered landfill sites for solid and liquid waste (Bath, 1993). Current practise for landfill is to ‘contain and treat’ leachates to protect nearby aquifers and/or discharges to surface water courses, rather than ‘dilute and disperse’ as in the past. Clay liners must have maximum hydraulic conductivities of 10-9 m.s-1 (UK) or 10-6 m.s-1 (USA) and are generally used in conjunction with a plastic ‘geomembrane’. Some natural clays may be acceptable for this purpose, although the use of bentonite is increasing since the consistency and high-swelling capacity of this clay allows for a much thinner lining. Natural clay liner performance is improved by rotovating-in bentonite granules to produce a compacted bentonite-enriched soil (Harries-Rees, 1993). Alternatively, bentonite may be incorporated in a geocomposite fabric. This material is easy to lay, flexible and has low permeability. It also has the ability to self-seal if punctured. Sodium bentonites used for landfill liners are generally treated with a polymer (anionic polyacrylamide) to prevent flocculation where solute concentrations in leachates are likely to be high (>lo00 PPm).

Much research is being conducted at present into the possible use of compacted sodium bentonite as backfill in radioactive waste repositories (Pusch, 1992). The main properties relevant to this application are the low hydraulic conductivity of the compacted bentonite (which serves to isolate the radioactive waste canisters from circulating groundwater) and the cation-exchange capacity (to capture any radionuclides escaping from the canisters). Further information is given by Bucher & Muller- Vonmoos (1989).

4.4 Iron ore pelletizing

Some iron ore concentrates consist of finely divided particles and must be pelletized with a binder before they can be used as a blast furnace feed. The conventional binder (0.5% addition) is high-swelling Wyoming bentonite, although sodium-exchanged bentonite may also be used. The mixture of iron ore and bentonite is rolled to form pellets


22

Bentonite

which are sintered at about 1250°C to develop enough strength to avoid breakage during handling. A proportion of olivine is added to some pellets to improve their high-temperature properties. Bentonite in pellets:

e Absorbs excess water from the iron ore fines. e Provides sufficient drop and compressive strength to the green.

pellet to withstand handling. e Provides adequate dry strength. e Improves the mechanical properties of the fired pellets, reducing

the quantity of fine particles produced.

Other than testing the strength of the pellet, swelling volume and Enslin value (water absorption) are the best clay characterisation techniques for predicting the efficiency of clay samples to make pellets with high compressive strengths. Although bentonite is a relatively inexpensive binder it does add to costs by increasing handling requirements and contamination by an aluminium silicate gangue. Bentonite usually decreases the iron content of the pellet by 0.8%. As a result pellets have been produced using lime, and some investigations have been carried out on the use of organic binders. However, other binders have not received widespread acceptance.

Bentonites are also used to pelletize animal feeds, to which they also impart some nutritional benefits.

4.5 Acid-activated clays

Acid-activated clays find application in three main market areas;

e Refining (bleaching, or decolourising, and purifying) edible

e Catalysis e Colour developers for carbonless copy paper

vegetable and animal oils, fats and solvents

The fxst of these is the most important, accounting for 70-80% of the total market. Acid-activated clays are used in the bleaching of colouring pigments and the adsorption of impurities such as phosphatides, gums,


23

Bentonite

trace metals and free fatty acids. A review of applications in this market area is given by Griffiths (1990).

Acid-activation (or bleaching) enhances the adsorptive and catalytic activity of the clay by increasing its surface area and particularly its surface acidity. During refining the coloured pigments in the oil are 'captured by the donation of a proton (H+) from the acid cations, yielding a positively charged organic cation which is attracted and effectively bound to the surface of the clay in the same way as exchangeable cations. The large surface area of the clay can easily accommodate the large organic cations. Similarly catalytic activity is promoted by proton donation and in carbonless copy papers the colourless dye encapsulated in the paper is converted to a coloured dye on protonation.

4.6 Absorbents

Natural Ca-bentonite is used as a granular absorbent and particularly as the basic ingredient of many of the popular types of pet litter. This use has emerged as one the largest applications for bentonite in Europe and America, although the mineral is in direct competition with other granular absorbent minerals, particularly sepiolite, attapulgite and diatomite. However, Ca-bentonite behaves in quite a different way to the other minerals when wetted, forming clumps which can easily be separated and removed for disposal. There are no rigid specifications, the requirements being for a high liquid and odour absorbency, freedom from dust and uniformity in size. Granules are generally in the size range 2-5 mm and the bentonite used must have some inherent strength to prevent dusting. A review of market developments in this area is given by Santarh (1993).

As Ca-bentonite tends to slake in oil and grease, producing a slippery surface, attapulgite and sepiolite are the preferred granules for use as oil absorbents on factory floors where spillage is likely.

In the calcined form Ca-bentonites are water stable and can be used as carriers for pesticides and herbicides. They are abrasion resistant, free fiom dust, uniform in size and have good mechanical and flow


24

Bentonite

properties providing a safe carrier from which highly toxic pesticides can be formulated.

4.7 Other Uses

Bentonite has numerous other applications (Figure 4), for example in cosmetics and pharmaceuticals (Gamiz et al., 1992). However, a relatively new application which has appeared in recent years and which is showing high growth rates, is in the paper industry. Na-bentonite is used in combination with water-soluble polymers in the papermaking process where it acts as a filler and fibre retention aid. More importantly, however, it improves the wet permeability of the paper web, allowing improved dewatering and thus savings on drying costs. The process appears to work by microparticle flocculation which improves the porosity of the paper web. Detailed specifications are not available but coarse impurities such as quartz must be kept to an absolute minimum because of abrasion to the papermaking machinery. Consequently bentonites are subjected to air classification to remove oversize particles.


25

Bentonite

5. LABORATORY ASSESSMENT OF BENTONITE

The laboratory assessment of bentonites involves a variety of mineralogical, chemical and physico-chemical techniques. These are used to characterize the clay, and to assess grade and quality. A more detailed investigation programme of testing related to specific applications on materials which have met certain minimum grade and quality criteria might may be carried out following this initial resource assessment phase. Figure 5 sets out an idealised laboratory investigation scheme which includes all these elements. Examples of simplified resource assessment schemes for the laboratory screening of relatively large numbers of suspected bentonites are set out in Figures 6 and 7. Figure 6 illustrates a scheme which might be used by a laboratory with fairly basic facilities whilst a scheme which utilises more sophisticated equipment is shown in Figure 7.

5.1 Mineralogical and chemical examination

A number of mineralogical and chemical techniques may be used to characterize bentonitic clays, including X-ray diffraction (XRD), differential thermal analysis (DTA), infrared (IR) spectroscopy, scanning and transmission electron microscopy, optical microscopy (mainly to identify textures relevant to origin), and major- and minor-element determinations, including those for exchangeable cations. Some of these techniques are discussed in more detail below.


26

Bentonite

GRADE I

I 1

MB dye absorption (appmx CEC) 2-ethoxyethanol surfam area (%smectile)

Bulk XRD analysis (wholerock mineralogy) Q XRD analysis (clay mineralogy) XRF analysis (major and nace element chemistry) 'True' CEC Exchangeable cations (clay surface chemistry) Thermal anatysis (dehydroxyhtin behaviour. quantitatiie mineralogy) Elearon microprobe (smeaite cyaal chemistry) Scanning electron microscope (petrography 8 smectite texture) Transmission electron microscopy (smectite crystal size, shape 8

aggregation)

I NATURAL CLAY

I I

b U SODIUM

CARBONATE Liquid limit (LL) 8 plastic limit (PL) (plastic properties)

I _s

-ty Plastic vlscwity Yield point Gel strength Grit content (>75 pm) Filtrate volume

7

9 Green 8 dry strength - Wet tensile strength Shatter index Conpactability Green permeabilty nun

AClD-ACllVATlO~ Acid treatment Surface area

= Crystal structure

Bleaching performance OIL-BLEACHING

Figure 5. Ideal scheme for laboratory assessment of bentonites.

Mineralogy and Petrology Group, British Geologicd Survey 0 NERC 1993

27

Bentonite

.................................................................................................................

LOW CEC

as bentonites

I ................................................... ~~ .~ .~ .~ .~ .~ . .~ .~ . . .~ . . . . .~ .~ . . .....................

Plot resuls on a $Aasticity chart to 'confm' : the presence of smectite.

Identify whether -----.--e-

; Ca-montmorillonite I or Na-montmorillonite

is present.

Assess quality of I bentonite by comparison I of maximum liquid limit : and swelling index values : with those of commercial

bcntonites

..................................

"r' I POOR

QUALITY swelling index SAMPLES

_.--.......-_... . and liquid limit values after

additions of 1.5% sodium carbonalc

................... .......................

Add optimum addition of

? Reject samples i as bentonites or i use in low-value applications

........................... i

FAILS OCMA

Compare tcsl data with hcological and ? Bentonite for OCMA specs. for ..e-...... I .....

iluate loss ESIS non-drilling fluid drilling fluids. applications

MEETS W M A SPECS I

? Bentonite for drilling fluid use

..................................................................................................................

Figure 6. Example of a scheme used in Costa Rica for screening large numbers of suspected bentonites (Inglethorpe, 1990)


28

Bentonite

....................................................................................................................................................................................................................................

STAGE 1 i

; 'Screening test'

: All samples are tested using this : procedure. A characteristic of : smectite-group clay minerals (the i principal constituents of fuller's j earths) is their very high surface ! area compared to other clay and ....... i non-clay minerals. Moderate and : high surface area values infer the

reference to a standard - enable a ' pre~en~e of sme~tite and - by

i maximum possible smectite ; contenttobecalculated. ; Although the test is sensitive to ' the presence of smectite, it does

not definitely indicae its presence

......

Inferred smedite content ~30%

Calculate % smectite Reject samples

smedite content >30%

.......................................................................................................................... ~ ......................................................................................................

I STAGE 2

bw-grade of smectite with high levels of

Mlneraiogkal anaiysls impurities

the relative proporlions of non- clay mineral constituents.

High-grade of smectite with low levels of

impurities

?Possible further testing (e.g. examinatlon of response to

Na-exchange, userelated tests etc.)

.......

Figure 7. Example of a scheme used by BGS for initial screening of large numbers of suspected bentonites (Moorlock & Highley, 1991).


29

Bentonite

5.1 .I X-ray diffraction

X-ray diffraction (XRD) provides definitive information on the mineralogical composition of a bentonite, both in terms of clay and non-clay constituents. In the first instance, the sample is examined as a random powder mount. From this, using standard JCPDS search procedures, the bulk mineralogy of the bentonite can be determined (Figure 8). By comparing peak heights of particular minerals with those on diffraction charts of artificial mixtures, semi-quantitative data can be obtained. At this stage, impurities which may be a problem in use can be identified: e.g. calcite, which is an acid user during acid-activation, gypsum, the presence of which can lead to poor rheological properties, and cristobalite, which is a possible health hazard in certain applications, and which may also act as a cementing agent, thus leading to poor physico-chemical performance.

XRD examination of an 'oriented mount of the smectite constituent will show whether this contains mainly divalent (Ca2+,Mg2+) or monovalent (Na+) exchangeable cations. Under normal relative humidity, the basal spacing, dWl, of a divalent cation-saturated smectite is -15-4,8,, whereas for a monovalent cation-saturated smectite it is 12-6 A. Subscquent basal spacings are exact sub-multiples of either 15.4 or 12.6 A. With smectites having a mixed monovalent/divalent exchangeable cation assemblage, the dool spacing is between 15-4 and 12.6 A and subsequent orders are not exact sub-multiples of dool. All cation-saturated varieties expand to 17.2 A with ethylene glycol or to 17.8 A with glycerol. Identification of mixed monovalent / divalent cation-saturated smectites may be significant if an application involving viscosity is likely, as it has been demonstrated that 'optimum' rheological properties are given by a bentonite having an exchangeable Na+ to Ca2+@Ig2+ratio of 3:2 (Alther, 1986). A d oolspacing of between 10 and 15-4 A in the air-dried state, together with an irrational series of peaks from both the air-dried and glycerolated mounts, would indicate a mixed-layer illite-smectite clay. 'Oriented' XRD curves of bentonites from Thailand are illustrated in Figures 9a and 9b.

XRD examination of powder mounts of a bentonite may also provide limited information on the nature of the smectite species. The d&, spacing will show wheteer the smectite is dioctahedral(1-49-1-50 A) or trioctahedral(l.52- 1 3 3 A). Dioctahedral smectites include


30

Bentonite

montmorillonite, the most common species, and beidellite, and contain predominantly A1 in octahedral structural sites. Trioctahedral smectites are less common in general and large deposits are consequently relatively rare. Hectorite, a smectite containing Mg and Li in octahedral sites, occurs in the western USA and is marketed mainly for its gel-forming and thixotropic properties. Saponite contains almost exclusively Mg in octahedral sites, and a deposit in central Spain has recently been described (Galan et al., 1986) which shows certain physico-chemical properties equal to those of the more common montmorillonitic bentonites. The b-dimension of the smectite unit-cell, which can be determined by XRD from powder mounts, has been linked to the degree of swelling exhibited in suspension. An investigation into this relationship is described by Low (1980).


31

Bentonite

Peak intensity

Sm

il

Anicr An

Figure 8. Whole-rock (random) XRD curves of two bentonites from Thailand. Sm: smectite; An: calcic plagioclase; Qz: quartz; Cr: ctistobalite; Hm: hematite; Ca: calcite; At: anatase.


32

Bentonite

Peak intensity

a

glycol

air-dried

OOI ( I 6.70 A)

I \ glycol

air-dried

Figure 9. XRD curves of oriented e2 pm fraction of two Thailand bentonites in air-dried and glycol-saturated states. Smectite basal spacings are labelled. Note low-angle shoulder on 001 peak of air-dry trace of bentonite (b) which may be related to exchangeable Na content,

Mineralogy and Petrology Group, British Geological Swrvey 0 NERC 1993

33

Bentonite

5.1.2 Thermal analysis

In the absence of XRD, differential thermal analysis (DTA) can be used to a limited extent to characterize the smectite constituent of a bentonite and to identify impurites such as calcite and quartz.

DTA curves of smectites are usually divided into three regions:

e the low-temperature region (<300"C) within which adsorbed and exchangeable-cation coordinated water are released;

e the dehydroxylation region (400"-75OoC);

e the high-temperature region (>800"C), where new phases crystallize from the dehydroxylated clay.

In the low-temperature region, smectites containing monovalent exchangeable cations show only a single endothermic peak, whereas those containing divalent exchangeable cations show a well-defined shoulder to this peak at -200°C representing expulsion of water loosely bound (the 'hydration shell') to these cations (Figure lo).

Smectites are often described as showing 'normal' or 'abnormal' dehydroxylation behaviour depending on the temperature range in which dehydroxylation occurs (Mackenzie, 1970). Normal smectites show a single dehydroxylation endotherm between 650"-72O"C, abnormal varieties show either a single endotherm at -550°C or a dual endothermic system with peaks at -550°C and -65OOC. The dehydroxylation behaviour of a smectite is important in foundry sand applications. When metal is poured into a bentonite-bonded sand, the heat drives adsorbed and hydroxyl water from the clay nearest the sand-metal interface. As moulding sands are recycled many times, the structure of the smectite is gradually destroyed, with consequent deterioration of bonding performance. Smectites showing high dehydroxylation temperatures exhibit greater durability although, as Odom (1984) has pointed out, this also depends on the nature of the exchangeable cations.


34

Bentonite

Figure 10. DTA curves of bentonites showing different dehydroxylation behaviour. Note the well-defined shoulder at -200 "C on the low temperature endotherm of sample 5-2-70 representing the expulsion of water loosely bound to divalent exchangeable cations.

Other thermal methods such as thermogravimetry (TG) or evolved gas analysis (EGA) can give accurate quantitative information on impurities such as carbonates and sulphides (Morgan, 1978).

5.1.3 Chemical analysis

Before determining the chemical composition of a smectite it is fxst necessary to remove impurities by separating a <2 pm (ideally a e1 pm or smaller) fraction by either sedimentation/decantation or centrifugation. Prior to separation, the clay must be saturated with Na ions to provide a stable (i.e. non-flocculating) suspension, and after separation excess salt must be removed from the clay product by sequential water and alcohol washing. A good description of the methodology for clay separation is given by Bain & Smith (1987); these authors also describe methods appropriate for major-element analysis of


35

Bentonite

the clay product. Distributions of atoms within the different structural sites of the smectite can then be detennined by calculating its structural formula from the chemical analysis; this procedure is described by Newman & Brown (1987).

As indicated in Table 1 , smectite-group minerals show considerable variation in chemical composition through substitutions within tetrahedral and octahedral structural sites. Members of the solid-solution series montmorillonite-beidellite are the most common species found in commercial bentonites. In this solid-solution series, up to one of the eight Si atoms in tetrahedral sites can be replaced by A1 atoms, and up to one of the four A1 atoms in octahedral sites can be replaced by Mg and Fe atoms. Variations in structural chemistry within members of the montmorillonite-beidellite series can cause differences in physical properties such as viscosity in suspension, but in commercial practice other factors such as the nature and amount of impurities, variations in exchangeable cation assemblage, and state of aggregation of the clay particles have far more influence on behaviour.

Nearly all chemical analyses of bentonites quoted in the trade literature are of unseparated, production-run materials and, as such, are of limited value for relating to behaviour in use. Likewise, major-element analyses of impure bentonite exploration samples are unlikely to provide information of value in relation to use. Only in the latter stages of a laboratory exploration programme when promising material has been identified by other more appropriate methods, would chemical analysis of a purified smectite product be justified.

However, some chemical determinations are essential in assessment programmes. The most important are determination of the cation-exchange capacity and the composition of the exchangeable cation assemblage (see next section). The presence of trace elements such as lead and arsenic could mean that a bentonite was unsuitable for pharmaceutical use or as a carrier for agricultural chemicals. Gamiz et al. (1992) discuss the suitability of Spanish bentonites in relation to existing pharmaceutical specifications. Because certain trace elements are usually retained when an igneous rock alters to, clay, plots of ratios of these trace-elements on a geochemical grid (Figure 11) can often indicate the composition of the parent igneous material, and thus can be of use in bentonite exploration.


36

Bentonite

""I 1 o a 1 0:10 i 10

NblY

Figure 11. Plots of bentonites from Thailand (A)9 Malaysia k) and Indonesia (a) on a geochemical grid.

5.1.4 Cation-exchange capacity and exchangeable cations

The cation-exchange capacity (CEC) of a smectite is of fundamental significance to applied properties and is often quoted in use-specifications. The composition of the exchangeable cation assemblage is also of practical significance as this is a prime criterion of suitability for most major applications. Discussions on the relationship between exchangeable cations and hydration/swelling of bentonites are given by Odom (1984), Alther (1986) and Elzea & Murray (1990). Effects of differences in bentonite exchangeable cations on khaviour in foundry moulding sands are described by Grim & Guven (1978) and Stephens & Waterworth (1968).

ExchangeabZe cations Exchangeable cations are usually released from the bentonite by ammonium acetate (Appendix l), and the leach solutions analysed for Ca, Mg, Na and K by, e.g., flame photometry / atomic absorption spectrometry. Table 5 gives values for exchangeable cations of two bentonites from Thailand However, totals for 'exchangeable cations' released in this way may be greater than 'true' cation exchange capacity of the clay, because fine-grained calcite and gypsum are partially dissolved by this treatment and release calcium ions into solution. Erroneously high values for individual cations may result from the presence of these minerals (sample 'B' in Table 5 shows anomalously high Ca because of the presence of calcite).


37

Bentonite

Table 5. Exchangeable cations of two bentonites from Thailand.

A 5 1.5 9.5 12.2 1.9 C0.1 B 99.9 11.5 20.6 3.2 CO. 1

‘True CEC’ Common methods for determination of cation-exchange capacity are discussed in Van Olphen & Fripiat (1979) and Bain & Smith (1987). A method for measurement of CEC using barium as the index ion is described in Appendix 2. It is useful to distinguish between ‘true’ CEC methods described above and the methylene blue method commonly used to determine bentonite grade. Table 6 gives values for ‘true’ CEC, and methylene blue CEC for some bentonites from Thailand.

Table 6 . ‘True’ CEC and methylene blue CEC of two bentonites from Thailand.

A B

73.7 74.4

50.2 44.5

~

T: ‘True’ CEC * CEC by methylene blue method.

5.2 Physico-chemical tests

A number of use-related physico-chemical test procedures can be applied to a bentonite as part of a laboratory assessment programme. These can be divided into tests which measure either the grade or quality of the clay (see Figure 5).


38

Bentonite

5.2.1 Grade of bentonite

Methylene blue cation-exchange capacity For a number of reasons mainly connected with the large size of the methylene blue ion, this test does not measure the 'true' CEC of a bentonite. CEC values obtained by the methylene blue method are usually lower than 'true' CEC values (Section 5.1.4). However, this test is sensitive to the smectite content and has the advantage of being relatively rapid. As a consequence, it is useful in estimating the grade of large batches of samples such as might be encountered in an exploration context (Figure 6). The method involves dispersing a known weight of clay in water and adding a methylene blue solution of known concentration in 1- or 2-ml increments to the clay suspension. Uptake of dye by the clay is monitored by 'spotting' a small portion of the clay-dye complex onto a filter paper (Figure 12a). While the dye is still being adsorbed by the clay, the 'spot' is blue and is surrounded by clear water (Figure 12b). At and beyond the end-point, the blue spot is surrounded by a halo of free dye (Figure 12c). The methylene blue CEC is calculated from the

- amount of dye adsorbed at this end-point and is normally about 80% of the true CEC. To ensure efficient adsorption of dye, a waiting period of 5 min or so is necessary after each addition and the method thus lends itself to batch testing; 20 samples can easily be dealt with in a working day. Step-by-step instruction for this test are given in Appendix 3. Figure 13 illustrates the range of methylene blue CEC values expected from bentonites and other clays. Background to this test is given by Nevins & Weintritt (1967) and Taylor (1985).


39

Bentonite

a

b

Figure 12a. Filter paper showing methylene blue dye spots following testing of a poor- quality @a-bentonite.

Figure 12b. Appearance of methylene blue dye spot before end-point is reached.

Figure l2c. Appearance of methylene blue dye spot at end-point.

Mheralogy and Petrology Group, British Geological Survey El NERC 1993

40

~~ ~

Bentonite

0. 425 a

Figure 13. Range of methylene blue CEC values (after Taylor, 1985).

0

10

20

30

40

50

120

shown by bentonites and other clays


41

Bentonite

2-ethoxyethunol [ethylene glycol monoethyl ether (EGME)] surface area This test measures the total available surface area of a sample, in contrast to the conventional BET nitrogen adsorption procedure which measures only 'accessible' surface area. The procedure followed is that of Carter et al. (1965) and involves monolayer formation of EGME on the clay surface under vacuum. Details of the method are given in Appendix 4; some typical values are given in Table 5. Weighing the clay-EGME complex, and knowing the cross-sectional area of the EGME molecule, allows the surface area of the clay to be calculated. The standard material used is one for which the surface area has originally been determined, e.g. a pure smectite which has been analysed chemically, the structural foxmula calculated, and the surface area determined from the unit-cell parameters. As pure smectites have surface areas of the order of 800 m2/g, other clay minerals such as kaolin <40 m2/g, and non-clay minerals <5 m2/g by this method, the surface area of an unknown bentonite can easily be converted to a reasonably accurate % smectite figure. The method was originally developed using Ca-bentonite and some difficulties may be experienced when assessing Na-bentonites as surface area values appear to be influenced by exchangeable cation-EGME reaction as well as by the real extent of the clay surface. A discussion of this aspect is given in Kellomaki et al. (1987).

Table 5. EGME surface area values for two bentonites from Thailand; three separate determinations (figures in parentheses represent estimated smectite content).

A 582 547 540 556 (90) B 505 507 53 1 514 (64)

Mineralogy and Petrology Group, British Geological Suwey 0 NERC 1993

42

Bentonite

5.2.2 Quality of bentonite

Tests which are covered by this heading are those which can be used to ~ assess the likely performance of the bentonite in use. They measure

indirectly the extent to which the bentonite disperses into 'individual' smectite particles, and thus the efficiency with which the large active surface area is utilized. Factors that can lead to poor dispersion in bentonites are compaction due to overburden and cementation by secondary silica.

Sodium carbonate exchange The production of sodium-exchanged bentonites involves the addition of small amounts of sodium carbonate (usually 2 to 4%) prior to drying and grinding. On subsequent addition of water, the calcium bentonite converts to sodium bentonite. The amount of sodium carbonate required for optimum property development is usually determined by preliminary laboratory trials. It may vary from 2% for a mixed Ca,Mg,Na-bentonite to 5% for a completely Ca,Mg-saturated clay. The tests described below can also be used to assess the response of a bentonite to sodium exchange.

Swelling test The generally accepted test for measuring the extent of swelling of a Na-bentonite consists of sprinkling a small amount of clay into a graduated cylinder filled with distilled water and measuring the volume of the swollen bentonite after 24 h. A detailed procedure is given in Appendix 5. Typical swelling values for bentonites are given in Figure 14. A moderately swelling bentonite will produce 15-20 ml gel, a good variety about 25 ml and an excellent grade will produce 30 ml or more. By progressively mixing amounts of 1 to 6 wt% sodium carbonate to a Ca-bentonite and drying gently, the swelling test can be used to give an indication of the ease with which the bentonite can be converted to the Na-form, as shown in Figure 14.


43

Bentonite

"Or FULLERS EARTH FULLERS EARTH BAULKING BH.1 FERNHAM F.11

140 - I30 - 120 -

cg * 2 110- - 0

100 - E

CI, 90- 0 t 80- a

5 7 0 -

x 60-

z I I 1 BENEF!CIATED COMMERCIAL

from FULLERS EARTH FULLER'S EARTH

F 1 REDHILL SURREY c.r.c 65

0 1 2 3 4 5 6

ADDITION OF SODIUM CARBONATE (96 by welght ol a1r-dt-y clay)

Figure 14. Typical swelling values for sodium-exchanged bentonites.

Atterberg limits The Atterberg liquid limit (LL) is an indicator of the bonding property of a bentonite. It is the minimum % water that will cause a bentonite-water mixture to flow when tested in a prescribed manner using the Casagrande liquid limit apparatus, or the % water relating to a penetration depth of 20 mm using a cone penetrometer. Of the two tests, the cone penetrometer method is considered to be fundamentally more satisfactory since it is essentially a static test depending on the shear strength of the clay. It is also easier to perform and more reproducible than the Casagrande technique (British Standard 1990).

The LL of a bentonite can form part of the specification for foundry use and is commonly mentioned on specification sheets of bentonites offered for many other applications. Generally, Ca-bentonites give LEs in the range 100-200 and Na-bentonites values between 550 and 750. In the higher range, LL values are markedly affected by the method of preparation used. The LL specification developed by the UK Cast Metals Technology Centre (minimum LL 525) is based on values obtained with minimum mixing between clay and water. In contrast, the Steel Founders Society of America's LL test is based on a bentonite which has been dispersed'in water using a high-speed mixer and then allowed to dry from a slurry. Consequently, LLs are higher: values up to 900 are quoted by Alther (1986).

The LL test can be a sensitive indicator of the response of a Ca,Mg-bentonite to sodium exchange. The procedure developed in the laboratories of the British Geological Survey involves determining LLs after addition of 1-6% sodium carbonate in a similar manner to that


44

Bentonite

described for the swelling test above. Detailed methods for LL determination using both the Casagrande apparatus and the cone penetrometer apparatus are given in Appendix 6. Figure 15 shows LL results for four UK Ca,Mg-bentonites (all containing >90% smectite) subjected to Na-exchange. Maximum LLs are obtained after 4-5% sodium carbonate addition, but these vary from 340 to just over 600. This difference is caused by variations in the extent of disaggregation of the clays, this being proved by subjecting sample 4 to high-shear dispersion which increased the maximum LL to 750.

Although primarily used for the assessment of bentonite quality, LL can be used in conjunction with Atterberg Plastic Limit (PL) as a simple indicator of bentonite grade (Bain, 197 1). PL is defined as the moisture content of a clay at the point where it becomes plastic. By plotting PL against the ‘Plasticity Index (PI)’ (LL value minus PL value), an indication of bentonite grade and response to Na2CO3 addition is obtained (Figures 16 and 17). In the absence of more sophisticated techniques, this method may be useful in providing additional complementary data for (say), a screening exercise on suspected bentonites using methylene blue CEC. A method for plastic limit determination is given in Appendix 7.

Mineralogy and Petrology Group, British Geological Survey 0 NER C 1993

45

Bentonite

1

1 -800

-700

-600

-500

E* -

-400 E -. - g.

-300

-200

-1 00

0 1 2 3 4 5 6 7 percentage Na2C03 added

Figure 15. Liquid limit response of four Ca-bentonites (all more than 90% smectite) to Na2C03 addition. High shear treatment of sample 4 induced aggregate breakdown and resulted in improved response to Na2C03 addition.


46

Bentonite

60

40

30 ----

- *

X X

x *

*

>

0 Holloysile e Kaolinite 0 Plastic Kaolins

c Illite A Ca-Montmorillonite

A ‘No-hlontmorillonite * Sepiolire x Atrapulgile

L A

t ,A C /

0,’

0

1 0

A I I I

I A A I

i I

/

,/ Trace of A Casagrande’s ,’ Kline

A

A A

Plasticity index

Figure 16. Clay identification chart using plastic limit and plasticity index as parameters. Note the clear distinction between natural Na-bentonites (montmorillonites) and Ca-bentonites (from Bain, 1971),


47

Bentonite

Colcium Montmorillonite

A (C)

Sodium Montmorillonite

*!dl

(e) A I r

!t 1

Plasticity index

Figure 17. Location of bentonites and .smectite-bearing clays on the PL/PI chart (from Bain, 1971). (a),(b) Wyoming Na-bentonite; (c) saponite (Tanzania); (d),(e) North African Na-bentonite; (9 Wyoming bentonite; (g),(h) UK Ca-bentonites; (i) smectite-bearing soil, Nevis Island; u),(k) UK Ca-bentonites; (1) Ca-bentonite, Hungary; (m) smectite-bearing soil, Mauritius; (n) Ca-bentonite, Botswana; (0) London Clay (smectite-bearing mixed-assemblage clay). Arrows indicate the effect on Na-saturation on natural Ca- bentonites.


48

Bentonite

6. USE-SPECIFIC TESTS

6.1 Foundry moulding sands

Properties of bentonite/sand mixtures that are crucial for €oundry usage are green compressive strength, dry compressive strength, shatter index, and compactibility; properties such as wet tensile strength, hot compressive strength, flowability and durability may also be quoted in specifications.

Factors such as sand:clay mix milling time, moisture content and sand:clay ratio have considerable influence on some of these properties (Figures 18a, b, c). For a preliminary evaluation of the suitability of a bentonite for foundry use, it is sufficient to determine the green and dry compressive strengths of a 5% mixture of the bentonite with a standard sand at a fixed moisture content, typically 3.5%. Green and dry compressive strength values indicate the ability of a mould to retain its shape during forming, handling and casting. Shatter index indicates the cohesiveness of the greensand during emplacement and subsequent removal of the mould from its pattern. Compactibility is the volume occupied by a fixed amount of greensand during moulding and indicates both mould density and greensand requirement. Permeability indicates the susceptibility of a greensand mould to gas and metal penetration during casting; slow/moderate gas permeability is needed to prevent the formation of porosity within the metal cast, but if permeability is too high then metal will penetrate into the mould. Some typical foundry moulding sand test results for bentonites from Thailand are given in Table 6.


. . 49

BOND DEVELOPMENT

MILLING TIME (MINUTES)

Figure 18a. Rate of development of green strength with 'milling t ime.

a Activated bentonite

Colcium kntonite Wyommq sodium

201 I I I I I I I 1 2 3 ~ 5 6 7

Water *I.

Figure 18b. Relationship between

1 2 0 0 V m i n g sodium

Activated bentonite Calcium bentonite

Water '1.

green strength (left) and dry strength (right) and water content for mixtures of standard sand and 5% clay.

lCOt

Figure 18c. Effect of increased clay content on green strength (left) and dry strength (right) of mixtures of standard sand and clay.


5 0

Bentonite

Table 6. Foundry moulding test results for two bentonites from Thailand.

Green permeability Green strength Dry strength Shatter index Compactability Sample No W h 2 ) W/m2) No. (%)

Thailand bentonites SDI 144 130 (145) 66 (63) 669 (648) 73 (68) 63 (59) SDI 145 130 (140) 58 (57) 572 (662) 67 (69) 62 (64) Commercial bentonites (A) 155 80 240 713 62.5

NA 4 1-48 655-896 80-83 NA

A) Na-treated natural Ca-bentonite (B) Natural Na-bentonite, Wyoming, USA

Values in parentheses are after N a 2 Q addition.

The tests were conducted using a mixture of 92.5% Chelford 60 (silica) sand, 5% clay and 2.5% moisture (figures in parentheses indicate results after addition of 2% sodium carbonate).

Detailed test methods for the determination of these physical properties in clay sand mixes are not given in this manual, as they require equipment which is not generally found outside specialized laboratories. Preparation of test pieces of the clay/sand mix and procedures for these specialized tests are described by Parks (1971) and in a series of method sheets published by the British Casting Industries Research Association (1985).

6.2 Drilling fluids

The main properties of bentonite that detennine its suitability for drilling fluids are its ability to form viscous suspensions in water or other fluids at low solids concentrations, the thixotropic behaviour of these suspensions, and the impermeability of thin films of the dispersed particles. Determination of these properties forms the basis of the


5 1

Bentonite

American Petroleum Institute (API) testing procedures for drilling fluid bentonites. These tests are also used to determine suitability of bentonite for civil engineering applications.

6.2.4 API and OCMA specifications for bentonites

Two specifications exist - the main API specification which is based on the performance of Wyoming bentonite, and one that is less rigid, based on an earlier Oil Companies Materials Association (OCMA) specification aimed at bentonites that, although providing satisfactory oil-well drilling mud performance, could not meet that of Wyoming bentonite. Viscosities of suspensions of fixed solids content are determined on a direct-reading Fann viscometer (Figure 19) and require the measurement of apparent viscosity, plastic viscosity and yield value (although this is not immediately apparent from the specifications); 10-second and lo-minute gel strengths are also usually measured in a comprehensive evaluation of a bentonite for drilling mud applications. Filtrate loss of a suspension of the same solids concentration also forms part of the specifications, as does the amount of material retained on a 75 pm screen and the moisture content of the clay.

Step-by-step test procedures are described in Appendix 9 (OCMA tests) and in API (1990). A useful discussion of the rheological properties of bentonite suspensions is given by Brandenburg & Lagaly (1988). Some typical APYOCMA test results are given in Table 7. Table 8 details typical gel strength values for a number of commercial-grade bentonites and other clays.


52

Bentonite

Table 7. OCMA/API test results of bentonites from Thailand compared to commercial bentonites used in drilling fluids.

(3600 (3300 Apparent Plastic Yield point Filtrate >75 /un Moisture (Cps) (Cps) viscosity viscosity lbs/100sq ft volume (mass %) (mass 9%)

(CPS) KPS) (cm3) OCMA specs >30 <(6*’p.v . ) c l 6 c2.5 ~ 1 3 . 0

Thuiland bentonites A (0% Na2C03) 3 1.5 1.5 1.5 0 A (5% Na2C03) 60 51 30 9 42 B (0% Na2C03) 3 1.5 1.5 1.5 0 B (5% Na2C03) 38 30 19 8 22

~ ~~ ~ ~~~

Commercial bentonites C D E

45 34 22.5 31 20 15.5

11 11 27

23 9

22 6.8 19.6 8.4

13 11.5 13.7 3 -2

7.8

5.3

(C) Na-activated bentonite, Mexico (Alther, 1986). @) Cedars type bentonite, Wyoming (Alther, 1986).

. (E) Wyoming & Montana (Elzea & Murray, 1990)

Table 8. Typical gel strength values for commercial-grade bentonites and other clays. Values in parentheses after Na2C03 addition.

€I3 10 sec gel (lbs/100 ft2)

€I3 10 min gel (lbs/lOO ft2)

Wyoming bentonite UK Ca-bentonite Ca-bentonite (Morocco)

London Clay (UK smectite-rich mixed-assemblage clay) 1.5 3


5 3

Figure 190 Fann viscometer.

54

Bentonite

6.2.2 Measurement of viscosity of bentonite suspensions using direct reading Fann viscometer

Preparation of suspensions The bentonite suspension is prepared in a high-shear mixer (the API specification requires that this has a sine-wave impeller of approximately 25 mm in diameter which rotates at 11 000 rpm). Total mixing time should be 20 minutes. The bentonite suspension is then stored overnight (sealed to prevent evaporation) to allow it to 'age'. This is very important as the viscosity of a bentonite suspension does not develop fully without a sufficient ageing period After ageing, the suspension should be mixed under the same conditions for 5 minutes immediately before viscosity determination.

Viscosity measurement Starting with 600 rpm, the dial deflection on the Fann viscometer should be recorded at 5 min intervals until a constant reading is reached. This may take up to 30 minutes whilst the suspension adjusts to the new shear conditions of the viscometer rotor. Once this constant reading is achieved, the speed is changed to 300 rpm and dial deflections again noted at 5 min intervals. A constant reading will be obtained sooner than for 600 rpm. These two readings give sufficient information for the MI specification, but if a 'consistency curve' is required then the same procedure should be followed for 200, 100 and 6 rpm. Dial deflections for a typical bentonite are given in Table 9.

Table 9. Typical readings on Fann viscometer for bentonite suspension during API test.

Viscometer dial readings (" deflection) Rotor speed 0 min 5 min 10 min 15 min 30 min

600 rpm 21 300 rpm 31 200 rpm 30 100 rpm 26 6 rpm 23

30 33 31 28 26

34 36 40" 34 34* 31 29 29 26

* readings used to calculate apparent and plastic viscosities and yield value.


5 5

Bentonite

Gel strength may also be measured using the Fann instrument. This property is not required by the API specification, but is nonetheless an important property of a drilling mud. Gel strength is measured by recording the dial deflection at the moment of gel break. The suspension is allowed to stand for a set period before restarting the motor at very low revolutions. 10 second and 10 minute gel strength values are generally determined using this method. .

Calculations The design of the Fann viscometer and experimental conditions (the geometry of the rotor and bob, rpms used, and torsion of the bob spring) are such that direct readings can be made of apparent and plastic viscosity (in cp) and gel strengths (in lbsjft2). Thus:

R600/2 = apparent viscosity R600 - R300 = plastic viscosity R300 - plastic viscosity = yield value (lbs/lOOft2)

0 10 2 0 30 40 50

Figure 20. Curve showing the effect of clay concentration on flow properties of a bentonite suspension determined on a direct-reading Fann viscometer.


5 6

Bentonite

Consistency curves obtained from Fann viscometer If the rotor speeds (= shear rate) are plotted against maximum dial deflection at the particular speed (= shear stress) then a consistency curve can be drawn for a particular bentonite concentration. Figure 20 shows consistency (or flow) curves for suspensions of 5 ,6 and 7% bentonite concentration. These curves well illustrate the Bingham flow nature of bentonite suspensions, i.e. that the suspensions only begin to yield to an applied stress above a finite value (the Bingham yield stress). This is conventionally measured by extrapolating the linear portion of the flow curve to the horizontal axis.

Figure 21 shows the effect of adding (a) calgon (a dispersing agent) and (b) sodium carbonate to a 6% bentonite suspension. Calgon breaks down the electrostatic (+ve edge / -ve face) interaction of the bentonite particles (‘cardhouse effect’) and because of this the flow behaviour of the suspension approaches that of a Newtonian liquid (in other words the suspension has been ‘thinned’). Sodium carbonate addition, however, ‘thickens’ the suspension and apparent viscosity (R600/2) increases. However, the slope of the upper part of the flow curve does not increase, i.e. the plastic viscosity remains the same. Bentonites which are below drilling mud quality can be treated with sodium carbonate to increase the apparent viscosity to meet the OCMA specification, but because of the yield point constraint they are unlikely to meet the main API specification.


57

Bentonite

77------

0 10 20 30

Vip#nnaepdial&g 40

I

50

a

0 10 20 30 40 50 60

ViQiatrcat i ing

Figure 21. Curves showing the effect of sodium carbonate (a) and calgon dispersing agent (b) on flow properties of 6% bentonite suspension.


5 8

Bentonite

6.2.3 Measurement of filtrate volume

This is measured on the suspension used for viscosity determinations, which is re-mixed for 1 min and then immediately poured into a fiter press cell. The model most commonly used for laboratory tests is the Baroid @area cell which filters over a 21/2-in diameter area at 100 lb/in2. Filtration takes place over a 30-min period but only the volume of filtrate emerging after the last 22.5 min is collected, multiplied by 2, and matched to specification.

6.3 Iron ore pelletizing

As in foundry sand applications, testing a bentonite for iron ore pelletizing requires specialized equipment, therefore detailed methods are not given in this manual. Tests are conducted by the industry on iron ore pellets containing between 0.5 and 2% bentonite, these pellets having been formed by a process that simulates the actual process as closely as possible (Auer et al., 1978; Banks et al., 1962). The green drop test involves dropping each of 10 freshly prepared pellets a distance of 18 in on to a steel plate until it breaks; the drop number is the average number of drops before breaking. Green, dry and fired compressive strengths of the pellets may be measured with a standard foundry compression apparatus equipped for pellets, or with a specially designed pellet tester. Pellets are dried at 105°C for dry testing, and heated to 1300°C and cooled €or fired strength determinations. An important test of the dried and fired pellets is the 'tumble index', where they are rotated in a drum for 200 revolutions at 24 rpm; the percentage of material retained on a designated screen is the tumble index.

As well as acting as a binder, the bentonite is required to absorb excess water that is not removed from the ore during processing. For this reason, Na-bentonites having a high moisture adsorption capacity are preferred by iron ore pelletizing companies and a minimum moisture adsorption capacity is often specified. This can be determined using the Enslin apparatus (White & Pichler, 1959) and a simplified test procedure is given by Auer & Thayer (1978).


59

Bentonite

6.4 Bleaching/decolourizing of oils

The use of acid-activated bentonite in the physical and chemical refining of fats and oils such as palm, coconut, soya bean and rapeseed is a . .

widely accepted process (Morgan et al., 1985). Evaluation of the suitability of a bentonite for this application involves two. operations:

. ..

0 response of the clay to acid treatment;

e measurement of the bleaching efficiency of the products towards the raw oils.

Methods for laboratory acid-activation of bentonites and assessment of bleaching perfomance are given in Appendix 9.

6.4.1 Acid activation a

Although the laboratory procedure for acid activation is relatively simple, the optimization of activation conditions may be a time- consuming procedure in which a number of experimental parameters must be adjusted. A small sample of clay is heated under reflux with sulphuric or hydrochloric acids of known but varying strengths for varying periods of time. Variation in acid type, strength, and reflux time will affect the degree of activation of the clay (Appendix 9). Considerable difficulties may be encountered in trying to optimise these conditions. Relatively high concentrations of acid might be necessary if the clay was known to contain calcite.

6.4.2 Measurement of bleaching efficiency

Bleaching efficiency of a bentonite can be measured in the laboratory b mixing heated palm oil with a small amount of activated earth for a fme time period. The change in red colour of the filtered oil is then measured using a ‘Tintometer’. Using the decrease in red colour as a guide, the most appropriate acid strength for activation can be identified. Usually there is a very rapid initial development of good bleached colour as acid strength increases, followed by a slower improvement at higher levels of treatment; bleaching properties can deteriorate above a certain acid level due to breakdown of the pore structure of the clay. A summary of


60

Bentonite

the relationship between acid activation and bleaching performance of bentonites is given in Appendix. 9 and also in Taylor & Jenkins (1986).

Bleaching earths perform many functions besides the removal of coloured impurities from the oils and fats and thus the above test is only a preliminary indication of suitability. Comparison of bleaching performance of the activated clay with that of a commercial product is an essential part of the testing procedure. A variation of the test applied to mineral oils involves heating a mixture of oil and 4% clay at 23OOC for 15 min, filtering, and measuring the transmittance of the oil.

6.5 Absorbents

A standard test for water absorbency of bentonites for use in pet litter applications has been developed by MNOR (the National Standards Organisation of France). Details of a water absorbency test based on this standard are given in Appendix 10. Typical values for bentonite are 100-140%. However, the suitability of materials for materials used for pet litter is not a function of moisture absorption alone, but also on how moisture absorption takes place, and also on odour absorption. Bentonite-based pet litters tend to ‘clump’ when moisture absorption takes place. This allows the clogged lumps to be removed, giving a lower overall consumption rate compared to other clay granules used for this purpose such as sepiolite or attapulgite.


6 1

Bentonite

REFERENCES

Adams J M (1987) Synthetic organic chemistry using pillared cation-exchanged and acid-treated montmorillonite catalysts - a review. Appl. Clay Sei. 2, 309-342.

Alther G R (1982) Improvement of drilling mud properties of low grade bentonites by simultaneous chemical activation and compaction. Interceram 5,501-503.

Alther G R (1986) The effect of the exchangeable cations on the physico-chemical properties of Wyoming bentonites. Appl. Clay Sei. 1, 273-284.

American Petroleum Institute (1990) Specifications for Drilling Fluid. (Spec 13A, 1 July 1990).

Auer D L & Thayer R L (1978) Bentonite update: production, reserves, quality control and testing. SOC. Min. Eng. AIME preprint 78-B-59 ( I 978 meeting) 8 pp.

Bain D C & Smith B F L (1987) Chemical analysis. 248-274 in: A Handbook of Determinative Methods in Clay Mineralogy (M J Wilson, editor). Blackie, Glasgow & London.

Bain J A (197 1) A plasticity chart as an aid to identification and assessment of industrial clays. Clay Miner. 9, 1-17.

Banks G N, Campbell R A & Viens G E (1962) Iron ore pelletizing - a literature survey. Trans. Can. Min. Metall. SOC. 65,428-433.

Bath A H (1993) Clays as chemical and hydraulic barriers in waste disposal: evidence from pore waters. 3 16-330 in: Geochemistry of Clay-Pore Fluid Interactions. (D A C Manning, P L Hall & C R Hughes, editors). Chapaman & Hall, London.

Brandenburg U & Lagaly G (1988) Rheological properties of sodium montmorillonite dispersions. Appl. Clay Sei. 3, 263-279.


62

Bentonite

British Casting Industries Research Association (1985) Broadsheets on Test Procedures for Moulding Sand. BCIRA, Alvechurch, Birmingham B48 7QB, UK.

British Standards Institution (1990) Soils for civil engineering purposes, BSS 1377 Part 2.

Bucher F & Muller-Vonmoos M (1989) Bentonite as a containment barrier for the disposal of highly-radioactive wastes. App. Clay Sei. 4, 157-177.

Carter D L, Heilman M D & Gonzalez F L (1965) Ethylene glycol monoethyl ether for determining surface area of silicate minerals. Soil Sci. 100, 356-360.

Elzea J M & Murray H H (1990) Variation in mineralogical, chemical and physical properties of the Cretaceous Clay Spur bentonite in Wyoming and Montana (USA). Appl. Clay Sci. 5,229-248.

Galan E, Alvarez A & Esteban M A (1986) Characterization and technical properties of a Mg-rich bentonite. Appl. Clay Sci. 1, 295-309.

Gamiz E, Linares J & Delgado, R (1992) Assessment of two Spanish bentonites for pharmaceutical uses. App. Clay Sci. 6, 359-368.

Griffiths J (1990) Acid-activated bleaching clays. Ind. Miner. 276. 55- 67.

Grim R E & Guven N (1978) Bentonites: Geology, Mineralogy, Properties and Uses. Developments in Sedimentology. 24, Elsevier.

Harben P W & Bates P L (1984) Clays. Pp 62-89 in: Industrial Minerals: Geology and World Deposits. Metal Bulletin, London.

Harries-Rees K (1993) Minerals in waste and effluent treatment. Ind. Miner. 308, 29-39.

Highley D E (1972) Fuller’s Earth. Miner. Resour. Consult. Comm. Mineral Dossier 3. HMSO, London.


63

Bentonite

Inglethorpe S D J (1990) A visit to San Jose, Costa Rica, to demonstrate laboratory methods for assessing bentonites. Tech. Rep. Brit. Geol. Surv. WG/90/22R.

Jones T R (1983) The properties and uses of clays which swell in organic solvents. Clay Miner. 18,399-410.

Kellomaki A, Nieminen P & Ritmaki L (1987) Sorption of ethylene glycol monoethyl ether on homoionic montmorillonites. Clay Miner. 22, 297-303.

Low P F (1980) The swelling of clay: II. Montmorillonites. Soil Sci. SOC. Am. 44, 667-676.

Mackenzie R C (1970) Simple phyllosilicates based on gibbsite- and brucite-like sheets. Pp 498-534 in: Differential Thermal Analysis I (R Mackenzie, editor). Academic Press.

Moorlock B S P & Highley D E (1991) An appraisal of fuller’s earth resources in England and Wales. Tech. Rep. Brit. Geol. Sum. WG/9 1/75

Morgan D J (1978) Evolved gas analysis of minerals and natural mineral mixtures. J. Thermal Anal. 12,245-263.

Morgan D A, Shaw D By Sidebottom M J, Soon T C & Taylor R S (1985) The function of bleaching earths in the processing of palm, palm kernel and coconut oils. JAOCS 62,292-299.

Nevins M J & Weintritt D J (196’7) Determination of cation exchange capacity by methylene blue adsorption. Am. Ceram. SOC. Bull. 46, 587-592.

Newman A C D & Brown G (1987) The chemical constitution of clays. Pp 1-128 in: Chemistry of Clays and Clay Minerals (A C D Newman, editor). Mineralogical Society, London.

Odom I E (1984) Smectite clay minerals: properties and uses. Phil. Trans. R. SOC. Lond. A 3 l l , 391-409.


64

Bentonite

Parkes W B (197 1) Clay-Bonded Foundry Sand. Applied Science Publishers, 367 pp.

Pusch R (1992) The use of bentonite in the isolation of radioactive waste products. Clay Miner. 27, 353-361.

Ross J S (1964) Bentonite in Canada. Dept. Mines & Techn. Surv, Ottawa, Monograph 873.

Santarkn J (1993) European market developments for absorbent clays. Ind. Miner. 304. 35-47.

Singer A & Galan E (1984) Palygorskite and Sepiolite: Occurence, Genesis and Uses. Developments in Sedimentology. 37. Elsevier.

Stephens H A & Waterworth A N (1968) Significance of exchangeable cation in foundry bentonite. Brit. Foundryman May, 202-219.

Taylor R K (1985) Cation exchange in clays and mudrocks. J . Chem. Biotechrzol. 35A, 195-207.

Taylor D B & Jenkins D B (1986) Acid activated clay. Am. SOC. Min. Eng. Mtng. Preprint 86-365.

Van Olphen H & Fripiat J J (1979) Data Handbook for Clay Materials and other Non-Metallic Minerals. Pergamon Press.

Vista R L (1992) Clays. Annual Report 1990, US Bureau of Mines.

White W A & Pichler E (1959) Water sorption characteristics of clay minerals. Illinois State Geol. Sum. Circular 266.


65

Bentonite

Appendix 1: Release of exchangeable cations from bentonite by ammon.ium acetate leach

Apparatus & reagents

50 ml graduated centrifuge tube 100 ml volumetric flask 1 litre volumetric flask Magnetic stirring plate/magnetic followers Centrifuge Balance 0.01 g readability, 1 kg approx. capacity

Ammonium acetate, analytical grade Distilled/deionised water

Method

1. Dissolve 75 g of ammonium acetate in 1 litre of water. Check that pH of solution is neutral (7) and adjust with a dilute acid or alkali if necessary.

2. Weigh out 1 g of clay into a centrifuge bottle. Add a magnetic follower.

3. Add 30 ml of 1M ammonium acetate solution and stir the suspension on a magnetic stirring plate for 30 minutes. Centrifuge for 15 minutes at 2000 rpm approx. Decant the supernatent liquid into a 100 ml volumetric flask (leach solution).

4. Carry out the operation described in step 3 three times in total.

5. Make the leach solution in the volumetric flask up to the 100 ml mark with distilled water.


6 6

Bentonite

Appendix 1 (continued)

6. Determine exchangeable cation concentration (Ca, Mg, Na, K) of an aliquot of the leach solution using standard chemical methods (e.g. inductively-coupled plasma optical emission spectroscopy [ICP-OES] or emission flame photometry/atomic absorption spectroscopy).

NOTE Gypsum, calcite and dolomite are all to some extent soluble in ammonium acetate solution and therefore can produce erroneous exchangeable Ca and Mg results. If these minerals are suspected to be present then a barium chloride/triethanolaine solution (see Appendix 2 for preparation) should be substituted for ammonium acetate as the leach solution. However, exchangeable Na and K can not be determined using the latter solution as barium chloride contains significant amounts of both these elements. Therefore, if either carbonates or gypsum are present, two determinations may need to be carried out using different leach solutions: an ammonium acetate leach for Na, K and a barium chloride/triethanolamine leach for Ca and Mg.

Calculation

~2 = (V * C)/(10000 * M1) = C/lOO

CEC [meq/lOOg] = (M2/A)* 1000

M 2 : Exchangeable cation content of clay [%] (M2 is sometimes quoted as mass of exchangeable cation (g) per 100 g of clay)

V: Volume of solution [ml] = 100 M1: Mass of clay [g] = 1 C: Cation concentration in solution [ppm] A: Atomic mass of cation [ m u ]


67

Bentonite


Example

C = 230 ppm Na M 2 = C/lOO

= 2.3% exchangeble Na content [2.3 g exchangeable Na per 100 g of clay]

A = 23 CEC = (M%/A)*l000

= 100 meq/100 g


68

Bentonite

Appendix 2: Cation exchange capacity (BaCIz/MgS04 method)

Equipment & reagents

250 ml wide-necked polythene bottles 250 ml measuring cylinder 5 ml pipette 100 ml pipette Magnetic stirring plate + magnetic foliowers Centrifuge pH meter (+ pH calibration solutions) Analytical balance, 200 g capacity, 0.1 mg readability 100 ml conical flask 50 ml burette 100 ml stoppered flaskbottle PTFE/glass rod

Distilled or deionised water Triethanolamine, general purpose reagent (GPR) quality Barium chloride dihydrate (BaC12.2H20), analytical (Analar) quality Magnesium sulphate heptahydrate (MgS04.7H20), Analar quality Ammonium chloride, Analar quality Ammonia ‘880’ solution (specific gravity 0.88) Solochrome black 6B Ethanol, GPR grade Concentrated hydrochloric acid (HCl) ‘solution, Analar quality Di-sodium EDTA (ethylene-diamine-tetra-acetic-acid), Analar quality

Preparation of solutions

1. Dilute (2M) HC1 solution: slowly add 18 ml of concentrated HCl to 75 ml of distilled water (Caution: DO NOT dilute by adding water to concentrated acid, always dilute by adding concentrated acid to water). Dilute to 100 ml with more distilled water.

2. Triethanolamine solution: dilute 90 ml of triethanolamine to 1 litre with distilled water. Add dilute (2M) HCl (approx. 180 ml) until a


69

Bentonite


3.

4.

5.

6.

7.

pH value of 8.1 is obtained and dilute to 2 litres with distilled water.

2M barium chloride solution: dissolve 244 g of BaC12.2H20 in 1 litre of distilled water.

Buffered barium chloride solution: mix equal volumes of barium chloride and triethanolamine solutions.

0.02 M EDTA solution: dissolve 3.723 g of disodium EDTA in 1 litre of distilled water, or use ampoules of concentrated EDTA solution available commercially (e.g. BDH Convol) and dilute according to the manufacturer’s instructions.

Ammonia buffer: dissolve 7 g of ammonium chloride in 57 ml of ammonia ‘880’ solution, and make up to 100 ml with distilled water (caution: carry out in a fume cupboard and DO NOT inhale fumes from ammonia ‘880’ solution).

Indicator: dissolve 0.25 g of Solochrome Black 6B in 50 ml of ethanol.

Method

1.

2.

3.

4.

Weigh 5 g of sample into a 250 ml polythene bottle and add a magnetic follower.

Note weight of bottle and contents (MI).

Add 100 ml of buffered barium chloride solution and agitate mixture for 1 hour on a magnetic stirring plate. Ensure that all the sample is dispersed - it may be necessary to dislodge some of the sample using a PTFE/glass rod. This stage can be omitted if samples are non-calcareous and non-saline.

Centrifuge at 1500 rpm for 15 minutes and discard the supernatant (if it is difficult to remove the majority of the solution by decantation then remove by a suction method instead).


70

Bentonite


5. Add a further 200 ml buffered barium chloride solution, agitate the mixture for 1 hour on magnetic stining plate and then leave overnight.

6. Centrifuge at 1500 rpm for 15 minutes and discard the supernatant.

7. Add 200 ml of distilled water and agitate for a few minutes on a magnetic stirring plate. Centrifuge at 1500 rpm for 15 minutes and discard the supernatant.

8. Note weight of bottle and contents (M2).

9. Pipette 100 ml of MgS04 solution into the bottle, mix well and leave for approx. 2 hours, occasionally agitating on magnetic stirring plate.

10. Centrifuge at 1500 rpm for 15 minutes and decant the supernatant into a stoppered flask or bottle.

11. Pipette a 5 ml aliquot of this solution into a 100 ml conical beaker and add 5 ml of ammonia buffer, plus 6 drops of indicator.

12. Titrate with standard EDTA (titre A1 ml).

13. Titrate a 5 ml aliquot of 0.05 M MgS04 solution (titre B ml). The end-point is indicated by a blue to pink colour change.


Bentonite


Sample code

Calculation(s)

The titration end-point (Al) must be corrected for the volume of residual water (retained by the sample after washing with distilled water). The corrected value (A2) can then be used to calculate the CEC value.

CEC = 8(B-A2) - - meq/lOO g

M1: weight of bottle plus dry contents (g) M 2 : weight of bottle plus wet contents (g) A2: corrected end-point = [A1 *( lOO+M2-M1)]/100 ml Al: titration end-point of sample (ml) B: titration end-point of MgS04 solution (ml)

M1 Wt of bottle + contents DRY (8)

M2 Wt. of bottle + contents WET

~ (8)

M2 - M1

Wt. of residual water (8)

A 1

Titration end-point (ml)

A 2

Mineralogy and Petrology Group, British Geological Survey 0 NERC I993

72

-~

Bentonite

Appendix 3: Determination of methylene blue cation exchange capacity

Equipment, reagents & materials

500 ml plastic screw-top bottle Analytical balance, lmg readability, 200 g capacity Drylng oven set at 105”-110°C 50 ml burette 100 ml flask Glass stirring rod 541 filter paper 1 litre volumetric flask Crystalline methylene blue chloride dye 5 M sulphuric acid solution Air-dried clay ground to approximately 4 2 5 pm

Determination of dry weight of clay samples and methylene blue chloride

1. Label a small aluminium dish with sample code and weigh. [Record as “A.... weight of dish”]

2. Weigh out 1 g of sample (clay or methylene blue) into the dish. [Record as “B.... weight of dish + sample before drying”]

3. Place in a 105°C oven and leave overnight. Weigh again the following morning. [Record as “C.... weight of dish +sample after drymg” J

Preparation of 1 litre of nominally 0.01 M methylene blue chloride solution:

1. Weigh out 3 g of methylene blue. [Record as “D .... actual weight of methylene blue”]


73

Bentonite


2. Place in a 1 litre volumetric flask and add approximately 250 ml of warm distilled water (alternatively add 250 ml of cold distilled water and warm using a water bath).

3. Gently agitate flask until all the dye has dissolved and no solids remain on floor of flask. Allow to cool and add distilled water up to the 1 litre mark.

Determination of CEC value

1.

2.

3.

4.

5.

6.

Weigh out 1.5 g of sample [record as ‘E .... weight of sample as received’] and place in a wide-necked plastic screw-topped bottle. Add 20 ml distilled water and disperse the sample by agitating for 1-2 hours in a reciprocal/end-over-endorbital shaker. Leave overnight.

Using as little water as possible, wash contents of bottle into 100 ml flask, ensuring that all clay is transferred.

Add 1 m15 M sulphuric acid: this stage is optional but increasing acidity is believed to give a ‘sharper’ titration end-point.

Add 2 ml of the nominally 0.01 M methylene blue chloride solution from a 50 ml burette and gently rotate the flask to mix the contents. After each 2 ml addition, place a drop of the suspension onto a filter paper using a glass stirring rod (“spotting”). Repeat until the end-point is reached.

Spotting initially produces a dstinct dark blue spot of clay- absorbed dye surrounded by a clear halo of water. However, near the end-point spotting produces a dark blue spot of clay- absorbed dye surrounded by a pale blue halo of excess dye (see Figure 12 in Section 5.2.1).

When this pale-blue halo is obtained leave to stand for 5 minutes and then repeat spotting. If the pale-blue halo disappears add a further 2 ml increment of dye.


74

Bentonite


7. If after 5 minutes the pale blue halo persists, allow to stand for a further 20 minutes and repeat spotting. If the pale blue halo then disappears cautiously add further dye.

8. If the pale blue halo is still present after a period of 25 minutes then the end-point has been reached.

9. Record volume of dye added at the end-point [“F.. .. volume of methylene blue”]. Calculate the methylene blue cation exchange capacity as shown, quoting the CEC value to the nearest whole number.

NOTES (i) Typical CEC values for pure clay minerals obtained using the methylene

(ii) The resolution of the method can be improved by changing to 1 ml blue method are shown in Figure 13.

additions of dye near the end-point. Repeatability of the test is typically of the order of & 1 meq/lOOg.

(iii) The overall rate of dye addition should be approximately 30 ml of dye in 2 hours. An excessive rate of dye addition may produce a premature end-point.

(iv) Moisture content can be determined by a standard method if preferred, such as Test 1(A) of British Standard BS1377: 1990.

(v) Maximum efficiency can be achieved in this test by dealing with samples in batches of 10-12.


75

Bentonite


Worksheets and calculations

Calculation of percentage dry weight of sample

% dry weight (Ps) = [(C-A)/(B-A)]*lOO/l

A: weight of dish B: weight of dish + sample before drying C: weight of dish +sample after drying

Calculation of concentration of methylene blue chloride solution

Concentration of methylene blue chloride solution (MBCONC) = (D*PMB)/~ 19.9 - - M

D actual weight of methylene blue (g) PMB: % dry weight of methylene blue

Molecular weight of anhydrous methylene blue = 319.9

Calculation of cation exchange capacity (CEC) values

Ps: percentage dry weight of sample MBCONC: Concentration of methylene blue chloride solution E: weight of sample as received F: volume of methylene blue


76

Bentonite

Sample

7

a

LO

Methylene Blue


A Wt. of dish

(8)

B Wt. of dish + sample BEFORE DRYING (g)

C Wt. of dish + sample AFER DRYING (8)

P Percentage dry weight

(wt.%)


77

Bentonite

Sample

I

5

7


E Wt. of sample as received (g)

PS Percentage dry wt, of sample (%I

F Volume of Methylene Blue (ml)

~ ~~~


7 8

Bentonite

Appendix 4: 2-ethoxyethanol (ethylene glycol monoethyl ether [EGME]) surface area method

I


Rotary vacuum pump (e.g. Edwards model 5 single-stage rotary pump) Analytical balance, 200 g capacity, 0.1 mg readability 2 glass or polycarbonate vacuum desiccators, small, approx. 15 cm &meter, including desiccant tray with a perforated cover 10 Aluminium weighing dishes, 6 cm diameter, labelled 1,2,3,4,5 & A, B, c, D, E Dropper pipette

8-16 mesh, fused, granular anhydrous calcium chloride (CaCl2), desiccant, GPR grade (ground to e100 mesh and stored in an airtight container; can be recycled after use by placing in 105°C oven) Anhydrous phosphorus pentoxide (P2O5), GPR grade 2-ethoxyethanol (ethylene glycol monoethyl ether, EGME), Analar quality Control sample - ‘pure’ Ca-montmorillonite standard.

Method

1. Weigh aluminium dish to four figures. Record weight (Ml). Add approximately 1.1 g of clay. Record weight (M2). Repeat this operation for a further four clay samples, including the control sample (in dish No 3 or C).

2. Place approx. 70 g of anhydrous phosphorus pentoxide in the tray of the first desiccator and replace desiccant tray cover. Arrange the five aluminium dishes in order (1,2,3,4,5 or A, B, C, D, E) around the circumference of the tray cover. Replace desiccator lid ensuring that no dishes are placed directly below air inlet valve.

3. Evacuate for approx. 45 minutes and allow to stand for 3-4 hours.


Bentonite


4. Gradually release vacuum by SLOWLY opening air-inlet. Weigh dishes as quickly as possible (M3, 1st weighing). Replace immediately in desiccator and arrange the dishes in order 3,4,5, 1 ,2 (or C, D, E, A, B) so that the control sample is in the first position. Evacuate for approx. 45 minutes and stand overnight under vacuum.

5. Next morning, gradually release vacuum and rapidly weigh dishes again (M3,2nd weighing). Immediately after weighing, add approx. 3 ml of EGME reagent to the dry clay in each dish and transfer samples to second desiccator containing dry calcium chloride desiccant. Arrange &shes in original order around circumference of the desiccant tray cover. Replace desiccator lid and allow to stand for 1- 11/2 hours. Evacuate for approx 15 minutes with air-ballast setting of vacuum pump turned on, then evacuate for a further 45 minutes with the air ballast turned off. Allow to stand overnight under vacuum.

6. Next morning, gradually release vacuum and weigh dishes rapidly (M4).


80

Bentonite


Calculation

M1 Weight of dish (g) M 2 Weight of dish + sample (8) M3 Weight of dish + dry sample (1 st weighing) (g)? M3 Weight of dish + dry sample (2nd weighing) (g)? M4 Weight of dish + dry sample + 2-ethoxyethanol (g) K Weight of EGME required to form a monolayer over 1 m2 of

surface = 2.86*10-4 g/m2

3. t Select lowest value of M3

NOTES (i) Before attempting routine surface area analysis of samples, the surface area

of the control sample should be determined repeatedly (approx. 10-20 analyses) until a relatively stable surface area value is obtained. This enables calculation of an average surface area value for the control sample.

(ii) If, during routine analysis of a batch of samples, the surface area of the control sample exceeds +lo m2/g of the average value then the surface area values of samples should be multiplied by a correction factor ‘X’ [‘X’ = (average surface area of control sample/measured surface area of the control sample)].

(iii) If, during routine analysis of a batch of samples, the surface area of the control sample exceeds +50 m2/g of the average value then the analysis should be repeated.

(iv) The moisture content of samples can be calculated using the following equation:


Wor

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et fo

r the 2

-eth

oxye

lhan

ol su

rfac

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a met

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~~~~

~

Wei

ghin

g

dish

num

ber

I I

1 I

I

Sam

ple

de

scrip

tion

I I

I I

I

M1.

.. W

t. of

con

tain

er (9

) I

I I

I I

M2 ..

. Wt.

of c

onta

iner

t s

ampl

e (9)

I I

I I

I

M3,

1st

w

eigh

ing

... W

t. of

con

tain

er +

dry

sa

mpl

e (9

) I

'

12

1

3

l4

I

s

M3,

2nd

w

eigh

lng

... Wt.

of c

onta

iner

+ d

ry

sa

mpl

e (9)

l4

Is

I

t I*

1

3

~~~

~~

~~~~

~ ~

M4.

..Wt .

of c

onta

iner

+ dr

y sa

mpl

e +

2-et

hoxy

etha

nol (9)

I I

I I

I

Wt.

2-

etho

xyet

hano

l

divi

ded

by

wt

. of d

ry

sa

mpl

e I

I I

I I

Surf

ace

area

(m2/

g) (

abov

e v

alue

div

ided

by 0

.000

286)

Cor

rect

ed

su

rface

area

(m2/

g)

(m

ultip

ly

by

fact

or'>

(')

I I

I I

I

b

P

82

Bentonite

Appendix 5: Laboratory procedure for bentonite swelling test


Analytical balance, 0.01g readability, 200 g capacity 6 watch glasses 6 10 ml measuring cylinders Filter funnel Drymg oven (60°C) Spatula Analytical balance, 0.01g readability, 200 g capacity

Anhydrous sodium carbonate (Na2CO& analytical grade Distilled/deionised water

Method

1.

2.

3.

4.

5.

6.

Take seven 4 g portions of 4 2 5 pm dry clay, and to six of these add 1,2,3,4,5 and 6% by weight of sodium carbonate. Gently grind the clays to mix in the sodium carbonate, add 1.5 ml distilled water and mix to a paste.

Dry at 60°C in oven overnight.

Crush the clay gently and screen through 500 and 250 pn. Weigh out 1 g of 500-250 pm material onto a watch glass.

Fill a 10 ml measuring cylinder to the mark with distilled water and place a filter funnel in the neck so that the tip is about 1 cm from the surface of the water.

Divide the clay sample roughly into 8 portions with a spatula and slowly add one portion every 5 minutes.

After each addition, lightly tap the bottom of the cylinder to settle the clay. The final surface of the clay should be level.


83

Bentonite


7. Leave the sample for 24 hours and read the volume of the clay to the nearest 0.1 ml. Multiply the result by 10. Samples of good bentonite should swell in excess of 10 ml. In such cases, addition of clay should be stopped when the swollen clay approaches the 9 ml mark. The amount of clay left is then ,weighed and the swelling volume for the total 1 g calculated.

NOTES (i) The relative quantities of clay and Na2CO3 required to obtain 1,2,3,4,5,

& 6% concentrations in 4 g of material are as follows:

% Na2C03 0 1 2 3 4 5 6

Wt. ofNa2C03 (g) 0

0.04 0.08 0.12 0.16 0.20 0.24

Wt. of clay (8) 4

3.96 3 -92 3.88 3.84 3.80 3.76

(ii) Typical swelling volumes (see also Figure 13, Section 5.2.2):

Non-swelling material Kaolinitic clays UK Ca-bentonite: + 1% Na2C03 + 2% Na2CO3 + 3% Na2CO3 + 4% Na2C03 + 5% Na2C03 + 6% Na2CO3 Natural Na-bentonite

10-12 ml 15-20 ml 25-35 ml 40-60 ml 50-90 ml 70-105 ml 70-130 ml 70-100 ml 70-90 ml 100-150 ml

(iii) Maximum swelling volumes are obtained with high-grade Ca-bentonites after artificial Na-exchange with 4-5% Na2C03 additions.


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Bentonite

Appendix 6: Laboratory procedure for the determination of bentonite liquid limits


2 large palette knives 1 small palette knife Wash bottle 500 ml beakers Large watch glass Casagrande apparatus/Cone penetrometer Glass weighing bottles Analytical balance, 0.01 g readability, 1 kg capacity Drying oven (1 10°C) Large square glass plate (400 by 400 mm approx.)

Distilled/deionised water Anhydrous sodium carbonate (Na2C03), analytical grade

Preparation of clay pastes

1. Take six 100 g portions of d 2 5 m dry clay, and to five of these add 1,2,3,4 and 5% by weight of sodium carbonate. Mix thoroughly with a small mount of distilled water on the glass plate using the palette knives for at least 15 minutes to obtain a stiff paste. DO NOT add an excessive amount of distilled water as the resulting paste must be not be too fluid.

2. Place each clay paste in a beaker, cover with a watch glass to prevent evaporation and allow to 'age' overnight.

Determination of liquid limit: Cone penetrometer method

1. Remove the clay paste from the beaker and work the clay on the glass plate with the palette knives for about 10 minutes.

2. Place a portion of the clay into the cup with a palette knife taking care not to trap any air. Strike-off excess clay with the straight edge of palette knife to obtain a smooth level surface.


8 5


3.

4.

5.

6.

7.

8.

9.

Position the cup in the centre of penetrometer base plate. Lower the penetration cone until the tip of the cone just touches the surface of the clay: if the cone is correctly positioned a slight movement of the cup will just mark the surface of the clay. Gently lower the dial gauge stem until it contacts the top of the cone shaft by using the INNER knob at the centre of the gauge. Set the dial gauge reading to zero by adjusting the OUTER knob at the centre of the gauge.

Release the cone for a period of 5 seconds using either the automatic or manual release (if using manual release, avoid jarring the instrument during this action). After locking the cone in position, gently lower the dial gauge stem until it contacts the top of cone shaft by using the INNER knob at the centre of the gauge. Record the gauge dial reading (penetration depth) to the nearest 0.1 mm.

If penetration depth is less than 15 mm, add more water to the clay and repeat steps 2-4. If the penetration depth is greater than 15 mm proceed to step 6.

Repeat steps 2-4 until two subsequent dial readings are within - +1 mm. This ensures consistent results.

If the conhtions in step 6 are satisfied remove a small amount of clay paste (10 g approx.) from the area penetrated by the cone and place in a weighing bottle (mass M1) and replace airtight lid. Weigh the bottle containing the wet clay (mass M2). Open the lid and place overnight in an 105OC oven. The bottle plus dried clay is then weighed the following morning (mass M3). Moisture content is then calculated as percentage of the dry mass of sample (see worksheet).

Repeat steps 1-7 at least three more times adding additional increments of distilled water. Mix very thoroughly each time more water is added. Amounts of water added should be such that penetration values in the range 15 mm to 25 mm are obtained. Moisture content/penetration depth co-ordinates are plotted on the graph paper on the worksheet and are then connected by a straight line ‘best fit’ (the flow curve).

The liquid limit is reported as the moisture content, expressed to the nearest whole-number, corresponding to a penetration depth of 20 rnrn on the f l ~ w curve.

Mineralogy and Petrology Group, British Geological Survey 0 NERC 7 993

86

Bentonite


Determination of liquid limit: Casagrande method.

1. Remove the clay paste from the beaker and work the clay on the glass plate with the palette knives for about 10 minutes.

2. Place a portion of the remixed clay in the cup of the Casagrande apparatus and level-off parallel to the base of the apparatus, ensuring that there are no entrapped air bubbles. Carefully draw the grooving tool supplied through the paste, keeping it noma1 to the surface of the cup with the chamfered edge facing the direction of movement.

3. Having first ensured that the counter is set to zero, steadily turn the crank at the rate of two revolutions per second until the paste at the bottom of the groove closes up over a length of 13 mm. This distance may be measured with a ruler or the end of the grooving tool. Record the number of bumps at which this occurs.

4. If the number of bumps is greater than 50, add more water to the clay and repeat steps 2 and 3. If the number is less than 50 proceed to step 6.

5. If the conditions in step 4 are satisfied remove a small amount of clay paste (10 g approx.) from the area of the groove and place in a weighing bottle (mass M1) and replace airtight lid. Weigh the bottle containing the wet clay (mass M2). Open the lid and place overnight in an 105°C oven. The bottle plus dried clay is then weighed the following morning (mass M3). Moisture content is then calculated as percentage of the dry mass of sample (see worksheet).

6. Repeat steps 1-5 at least three more times adding additional increments of distilled water. Mix very thoroughly each time more water is added. Amounts of water added should be such that bump values are evenly scattered above and below 25. Bump values of less than 10 are discounted. Moisture content/bump value co-ordinates are plotted on the semi- logarithmic graph paper on the worksheet and are then connected by a straight-line ‘best fit’ (the flow curve).


87

Bentonite


7. The liquid limit is reported as the moisture content, expressed to the nearest whole-number, corresponding to 25 bumps on the flow curve.

NOTES (i) The results obtained by using the two methods do differ (see Section 5.2.2).

Where the liquid limit is less than 100 differences are not considered significant as they are within the normal variation typically obtained using the Casagrande method. However, for liquid limit values above 100 the cone penetrometer method is known to give LOWER values in comparison to the Casagrande test. Because high-grade bentonites have liquid limit values >>lo0 it is therefore important to record which of the methods was used when quoting results.

(ii) Typical liquid limit values (determined using the Casagrande method):

UK plastic kaolin (‘ball clay’) 61 UK mixed-assemblage clay (‘common clay’-26% smectite) 84 UK commercial Ca-bentonite, S teetley ‘Berkbond 1 ’ 150-200 Commercial Ca-bentonites, artificially Na-exchanged (‘alkaline treated’):

Steetley ‘Berkbond 2’ (Wobum, UK, partially Na-exchanged) 250-300 Steetley ‘Berkbent D.M. (drilling mud grade)’ 300400 Steetley ‘Berkbent C.E. (civil engineering grade)’ 400-450 Steetley ‘Berkonite’ (Ponza, Italy) 500-600

Steetley ‘Wyoming bentonite’yAquage1’ 600-700 Natural Na-bentonite:


88

Bentonite


LIQUID LfMIT: CONE PENETROMETER METHOD

Sample ........................... Operator ......................... Remarks .......................... Date ................

Penetration depth (mm)

\ J

Liquid limit worksheet (Cone penetrometer method)


89


LIQUID LIMIT: CASAGRANDE METHOD

Sample Operator ......................... ........................... Remarks .......................... Date ................

Determination No. 1 2 3 4 5

Bottle No.

M1 ... Mass.ernpty bottle (9)

Mass water (9) =M2-M3

Mass dry clay (9) =M3-M1

Moisture content (VO) I (M2-M3)I(M3-M1)'100

Number of blows I I

10 1s 20 LO so Number of bumps

Liquid limit worksheet (Casagrande method)


90

Bentonite

Appendix 7: Determination of plastic limit and plasticity index


Wash bottle Glass weighing bottles Analytical balance, 0.01 g readability, 1 kg capacity Drying oven (1 10°C) Large square glass mixing plate (400 by 400 mm approx.) Polished glass test plate Metal or glass rod, 3 mm diameter, approx. 100 rmn length

Distilled/deionised water

Method

1. Take a sub-sample of approx 20 g of clay and place on mixing plate. Using your hands mix water with the sample until a plastic ball can be formed.

2. Mould the ball between the fingers and roll it between the palm of the hands until the heat of the hands has dried the clay sufficiently for slight cracks to appear on the surface.

3. Divide the ball into two sub-samples and set one of these aside.

4. Divide the remaining sub-sample into four equal parts and carry out steps 5-8 separately on each part

5. Mould the clay using the fingers to equalize the moisture distribution. Fom the clay into a thread of about 6 mm diameter between the first finger and thumb of each hand.

6. Roll the thread between the fingers of one hand (from the fingertips to the second joint) and the surface of the glass test plate. Use enough pressure to reduce the diameter of the thread from 6 mm to 3 mm in five to 10 complete forward-and-back hand movements. Use the rod to gauge the thickness of the thread. All clays tend to harden near the plastic limit; some “heavy” clays may require 10- 15 movements. It is important to


91

Bentonite


maintain uniform rolling pressure: do not change hand pressure as the thread diameter approaches 3 mm.

7. On completing the operation in step 6, pick up the clay thread and mould using fingers to dry the clay further. Gradual drying is carried out by alternately rolling on the test plate and moulding with the fingers. Continual rolling should not be attempted as this only dries the surface of the clay. Reform the clay into a ball as described in step 5.

8. Repeat steps 6 and 7 until the thread shears (cracks) both longitudinally and transversely when it has been rolled to about 3 mrn diameter. Gather the cracked portions of clay and transfer to an airtight bottle (mass M1) and replace the lid immediately.

9. Carry out steps 5-8 on each of the four portions of the clay, placing all the cracked pieces of clay obtained into the same airtight bottle. The bottle and its moist clay contents is then weighed (mass M2). After opening lid of bottle, place overnight in an 105°C oven. The bottle and Bry clay is then weighed the following morning (mass M3). Results can be recorded on the liquid limit worksheets and the moisture content then calculated.

10. Carry out steps 4-9 on the remaining material.

1 1. Calculate the average of the two moisture contents obtained and express to the nearest whole number. This is the plastic limit.

12. The plastic index (PI) can be calculated by subtracting the plastic limit (PL) from the liquid limit (LL).

NOTES

(i) Test should be carried out on a polished glass surface. If possible use separate plates for mixing and testing as discrepancies will result from use of a textured or scratched glass surface.

test. (ii) If moisture content of each sub-sample differs by more than +I% repeat the


92

Bentonite

Appendix 8: Determination of drilling fluid properties


Drylng oven set at 1O5"-11O0C Balance, 0.01 g readability, 1 kg load High-speed mixer: e.g. Hamilton-Beach type, 11000 rpm, single corrugated impeller of approx. 25 mm diameter Direct-reading couvette type viscometer: e.g. Model 35s Fann V-G viscometer Filter-press: e.g. NL Industries/Baroid Division 1/2 area filter press (includes 'sparklet' type C02 bulbs, filter paper and 10 ml measuring cylinder) Wire cloth sieve, approx. 80 mm diameter, 75 pm aperture Water spray nozzle attached to water line, water pressure ideally regulated at 10 psi Evaporating basins Spatula Stirring rod 500 ml measuring cylinder 500 ml beaker and watch glass

Distilled/deionised water Anhydrous sodium carbonate (Na2CO3), analytical grade Sodium hexametaphosphate 'Calgon' GPR grade

IMPORTANT NOTE: prior to carrying out this test the % dry weight of sample and optimum %Na2C03 at maximum swelling volume need to be determined (see methylene blue cation exchange capacity method and swelling volume test method respectively).

Mineralogy and Petrology Group, British Geological Survey 0 NERC f 993

93

Rheological properties

1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

11,

Calculate the weight of ground clay and weight of Na2CQ required for test (see calculations and worked example).

Add 400 ml of water from measuring cylinder to cup of high- speed mixer. Turn mixer on and add the ground clay and sodium carbonate (in total 25.7 g).

Turn off mixer after 5 minutes and dislodge any bentonite adhering to sides of cup using a stimng rod.

Turn on mixer and continue stirring for an additional 15 minutes (total mixing time of 20 minutes).

Transfer the suspension to a 500 ml beaker, cover with a watch glass and allow to age overnight (16 hours).

The following day, place the suspension in mixer and stir for a further 5 minutes.

Pour the agitated suspension into viscometer test cup up to the 350 ml line inscribed inside the cup. Raise cup, immersing the viscometer rotor sleeve and bob until the line inscribed on rotor sleeve is level with the top of the suspension.

Set viscometer rotor sleeve running at 600 rpm and record dial reading at consecutive 5 minute intervals. A stable reading is indicated by two consecutive readings within 0.5 centipoise. A good quality bentonite may take up to 30 minutes to stabilize at 600 rpm [record value as 86001.

Repeat step 8 with rotor sleeve running at 300 rpm [record value as 83001. The suspension should stabilize more quickly at lower speeds. Calculate apparent viscosity (a.v.), plastic viscosity (P.v.), yield value (Y.v.) and API specification values.

(OPTIONAL) If, in addition to API specifications, a complete consistency curve is required, repeat step 8 with rotor sleeve running at consecutive settings of 200, 100,6 and 3 rpm and record dial readings [record as 8200,e 100,06 and 831.

(OPTIONAL) Determine gel strength as follows. Stir sample at 600 rpm until dial reading stabilizes. Turn viscometer off for 10


94

Bentonite


seconds and then switch to 3 rpm. Record maximum dial reading which occurs at the instant of gel break [record value as e310 set]. Repeat but this time turn off the viscometer for 10 minutes instead of 10 seconds [record value as 8310muJ.

Filtrate volume of suspension

1.

2.

3.

4.

5.

6.

Using the suspension prepared in method (A), stir suspension for one minute using high-speed mixer.

Pour the suspension into the rubber ‘boot’ of the 1/2 area filter press to within approx. 13 mm of the top. Place filter paper on top of ‘boot’ and screw-in bottom lidscreen assembly tightly (ensure that gaskets are not distorted or worn and that screen is clean and dry). Mount 1/2 area press in upright position and place 10 ml measuring cylinder under drain hole.

Start timer: within 30 seconds close the air-vent valve and adjust the gas regulator to 100 25 psi.

Wait for 7.5 minutes and then empty the measuring cylinder of the water collected. It is not necessary to record the volume of water collected during the first 7.5 minutes.

Immediately replace empty measuring cylinder and restart timer. Continue the test for a further 22.5 minutes and record the volume of water collected [record as ‘volume of filtrate’].

Calculate the filtrate-relative 30 minutes value as specified in ‘Calculations’ section below.


95

Bentonite


Wet screen analysis

1. Measure out 350 ml of water using a measuring cylinder. Add 350 ml of water to cup of high-speed mixer. Turn mixer on and add log of bentonite and 0.2g of ‘calgon’. Stir on mixer for 30 minutes

2. ‘Age’ suspension for approx. 2 hours.

3. Re-stir for 5 minutes and transfer suspension to the 75 pm sieve. Wash the material on the screen for 2 minutes using water from the spray nozzle set at approx. 10 psi.

4. Transfer the >75 pm sieve residue into an evaporating dish and dry in oven.

5. Remove residue from oven and weigh (record as ‘weight of >75 pm sieve residue’.

6.. Calculate residue greater than 75 pm value as specified below.

Calculation of weight of ground clay and weight of sodium carbonate required for test (including a worked example)

Weight of gound clay (‘as received’) required for test = (100/ps) * (25.7 - [ 25.7 * %Na/100J)

Weight of Na2C03 required for test = (%Na/100) * 25.7

Ps percentage dry weight of sample %Na optimum %Na2CO3 to obtain maximum swelling volume

(see methylene blue CEC and swelling volume methods respectively)


96

Bentonite


Example:

PS = 90% %Na = 5 %

Weight of ground clay (‘as received’) required for test = (100/90) * (25.7 - [ 25.7 * 5/100]) = 27.128 g

Weight of Na2C03 required for test = (5/100) * 25.7 = 1.285 g

Is this correct ? - check calculation as follows:

Weight of ground clay ( d r y weight) + weight of Na2C03 should = 25.7 g total

Equivalent weight of dry clay = 27.128 * 90% = 24.415 g

Weight of Na2C03 = 1.285g

Total = 24.415 + 1.285 = 25.7 g Total - correct!


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Bentonite


Calculation of rheological properties

Apparent viscosity (a.v.) [centipoise] = e600/2

Plastic viscosity (P.v.) [centipoise] = e600 - e300

Yield value (Y.v.) [lbs/100 sq ft] = e300 - P.V.

83 10 see= 10 second gel strength (as recorded)

8310 mh= 10 minute gel strength (as recorded)

Filtrate-relative 30 minutes [cm3] = 2 * volume of filtrate

Residue greater than 75 pm [ % J = (100 * weight of >75 pm sieve residue)/lO

Moisture [ %] = 100-Ps

8600: 600 rpm dial reading 86300: 300 rpm dial reading 8200: 200 rpm dial reading 8100: 100 rpm dial reading 86: 6 rprn dial reading 83: 3 rprn dial reading

Ps: percentage dry weight of sample


98

Bentonite

Appendix 9: Mechanism of acid activation and assessment of bleaching performance of bentonite for use in edible oil clarification

9.1 Background

Acid activation of bentonite is carried out commercially to provide material for for three main applications:

e Refining oils, fats and solvents 0 Catalysis 0 Colour developers for dyes impregnated in carbonless copy

paper.

Further details are given in Table 9.1. This account will deal only with acid-activation of smectite - the clay constituent of bentonite - for edible oil clarification; descriptions of smectite-catalysed reactions are given by Atkins et al. (1983) and Gregory et al. (1983), and the use of smectite in carbonless copy paper is excellently described by Fahn & Fender (1983).

The refining of vegetable and animal oils for edible purposes involves the removal of a variety of impurities which include phosphatides, fatty acids, gums and trace metals, followed by decolourization and deodourization. The various stages in the edible oil refining process are shown in Figure 9.1. It is the purification and decolourization stage that employs an acid-activated bleaching earth which is later removed from the oil by filtration. A good description of the commercial process is given by Richardson (1978).


99

Bentonite


Table 9.1. Major applications for acid-activated clays (from Taylor & Jenkins, 1986).

Application Function Method of use ~~

Refining agent for Edible and inedible oils: decolourization and 1/4-4% clay dosage; heated oils and solvents stabilization by removal of coloured pigments and under vacuum for 15-30 min

~~

impurities at 90-130°C then filtered

Used lubricant oils: reclamation by removal 3 4 % clay dosage; heated of degradation products and contaminants with steam to 390-575°F for

10-30 min then filtered

BTX refining: olefin removal and decolourization Fixed bed contact at 170-200°C

Catalysis Acid-catalysed reactions: polymerization, Reagents either passed over alkylations, esterifications, isomerizations and bed of granulated particles or fatty acid dimerizations added to reagents in powder

form at moderate to high temperatures

Carbonless Colour developer: developing colour of micro- 20-30% clay dosage in COPY paper encpsulated dye beads on copy paper coating mixture; coating

spread on front of bottom (copy) sheet


100

Bentonite


DEGUMMING

PHYSICAL REFINING

POLISHING

EDIBLE OIL STORAGE

Figure 9.1 Edible oil refining process (from Taylor & Jenkins, 1986) .

It is important to note that bleaching earths perform many functions beside the removal of coloured impurities from oils and fats. Traditionally, a simple oil bleaching evaluation is regarded as a goad initial sorting test to establish whether a bentonite has the required properties for commercial use, but before this can be carried out the clay has to be acid-activated and this requires much experimentation to identify the best combination of acid strength, contact time and temperature. Although the bleached oil colour may be optimized for a particular acid treatment, other oil parameters such as oxidative stability, trace metal content and free fatty acid content may require different levels of acid treatment to produce an optimum product, so even more careful experimentation is required. This level of investigation would normally be beyond the resources and brief of a Geological Survey.

The following sections are intended to provide sufficient background information to enable a Geological Survey laboratory to set up a simple evaluation scheme for bentonite acid activation and evaluation of bleaching performance of the products. Enough key references are provided to enable further detailed reading on the subject.


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Bentonite


9.2. Changes in properties of smectite with acid activation

Structure and X-ray diffraction patterns If any carbonate impurity is present in the bentonite, this is dissolved by the acid. The first effect of acid attack on the smectite itself is that the exchangeable cations are replaced by H+. Subsequently, the main structure is attacked, and Al, Mg and Fe are removed from the octahedral sheet exposed at the edges of the smectite platelets. Some of these ions may then migrate to exchange positions. As a result of acid attack the surface area of the smectite is increased and the pore-size distribution of the clay is modified. The smectite layers take on the appearance of bundles of used bank notes, relatively unaffected in the centre but separating at the edges. The mechanism is illustrated in Figure 9.2.; Figure 9.3. shows the changes in XRD patterns resulting from structural deterioration.

2. STRUCTURE DISRUPTED- 1. ACID ATYACKS OCTAHEDRAL LAYER SURFACE AREA INCREASES

P

0-

P

8'

k

d&

3. AClDCCATlONS RELEASED FROM STRUCTURE EXCHANQED FOR CALCIUM CATIONS

4. ACID ACTIVATED CLAY WITH ACID CATIONS IN EXCHANQE SITES a

Figure 9.2. Mechanism of acid attack on smectite (from Taylor lk Jenkins, 1986).


102

Bentonite


Surface area and pore structure In general, surface area increases with increase in seventy of acid treatment, but it does pass through a maximum beyond which additional acid treatment actually reduces the surface area. Figure 9.4. illustrates the important point that smectites from different bentonite deposits respond individually with respect both to the maximum surface area they achieve and the acid requirement to reach that maximum. Thus when assessing a bentonite for oil-clarification applications a considerable amount of experimentation is usually necessary to determine optimum acid-activation conditions. Maximum surface area is usually in the range 200-400 m2/g, and in general the optimum bleaching properties are found below the maximum surface area obtainable.


103

Bentonite


I

o f I I I 1 I I I I I 1 1

0 20 40 60 60 100 120 I

lncrearing Add Treatment __c

Figure 9.4. Effect of acid dosage on the surface area of various smectites (from Taylor & Jenkins, 1986).

The physical changes in the structure of acid-activated smectite may be examined quantitatively from nitrogen adsorption/desorption isotherms, from which surface area and pore volumes are calculated (a good introduction to this topic is given by Greenland & Mott (1978), and Fijal et al. (1 975) deal comprehensively with the calculation and significance of pore volume distribution in smectites). An adsorption/desorption nitrogen isotherm for Ca-smectite is shown in Figure 9.5a. This isotherm corresponds to a surface area of 70 m2/g, typical of Ca-smectite (range 50-120 m2/g). The hysterisis occurring during the


104

Bentonite


desorption cycle is given by a capillary system consisting of a combination of slit-shaped and wedge-shaped capillaries with a closed edge. The same clay when acid-activated gives the nitrogen adsorption/desorption isothenn shown in Figure 9.5b. The total volume of nitrogen adsorbed is three times greater and corresponds to a surface area of 227 m2/g. The isothenn is also changed in shape; in particular, the degree of hysteresis is reduced, corresponding to a greater contribution to the pore structure by wedge-shaped capillaries.

The change in pore type between original clay and acid-activated product are emphasized by computer treatment of the nitrogen isotherm to give incremental pore-size distribution plots (Figure 9.6.).

0 0.5 1 -0

200-

100-

01 0

0.5 I 1 -0 1

RELATNE PRESSURE

Figure 9.5a-b. Nitrogen adsorption/desorption isotherms for Ca-smectite and acid-activated product (from Morgan et al., 1985).


105

Bentonite


0 50 AVERAGE PORE DlAM€IER(A)

Figure 9.6. Pore-size distribution plots for Ca-smestite and acid-activated product (from Morgan et al., 1985).

These plots show that in natural Ca-smectite the surface of the clay is composed of relatively small pores <20 A in diameter. These pores correspond to the smectite interlayer spaces. Following acid activation the average diameter of the pores has increased and over 80% of the available surface is in pores having diameters between 20 and 60 A. These are pores that begin to take on a three-dimensional appearance with less restricted openings.

Su@ace acidity Surface acidity, as shown in Figure 9.7., measures both the number of acidic sites on the clay surface and their relative strength. In this case surface acidity has been measured by base titration using Hammett indicators of known pKa. The pKa value quantifies the strength of the acid sites, the lower the number the higher the acid strength. Acid sites possessing a pKa value of -3 are approximately as strong as sulphuric acid; those possessing a pKa value of 5 are considered weak sites with the approximate strength of acetic acid. As with surface area, surface acidity also passes through a maximum with increasing acid treatment. The trend also shows that it is possible to over-activate a clay and lose surface acidity entirely.


106

Bentonite


Figure 9.7. Effect of acid dosage on surface acidity (from Taylor & Jenkins, 1986).

110

^o 100 0

0, 90 \

a 0

E eo

- 5 70

a u *n 60 0 a

0

Y

a 50

5 4 0 s 0

30

,o 20

0 10

C

c 6

0

Cation exchange capaciry Figure 9.8. shows that the CEC decreases with extent of acid treatment.

I

20 4.0 60 80 Increeaing A d d Treatment - 100 120

Figure 9.8. Effect of acid dosage on CEC of various smectites (from Taylor & Jenkins, 1986).


107

Bentonite


9.3. Laboratory activation trials and assessment of bleaching efficiency of products

Measurement of surface area will often give a good indication of the bleaching potential of an acid-activated bentonite but there is no absolute correlation between bleaching performance and surface area. This is illustrated in Figure 9.9. Although a rough correspondence is obtained between bleaching perfomance and surface area for clay A, for clay C the bleaching efficiency maximum is achieved well after surface area for that clay has begun to drop off. Additional evidence that surface area alone is inadequate as the sole indicator of adsorptive capacity can be deduced from the fact that A, which has a much higher surface area than C, is only about two-thirds as active as C.

I CLAY A 1

0 m .Q a0 0' l o o 120

Incroaaing Acid froatmont -

.OD

m

400

34,

1100

100

0 0 20 .o a0 .o l o o l t o

Incroaslng A d d Troatmont-

Figure 9.9. Correlation of surface area with bleaching efficiency (from Taylor & Jenkins, 1986) .

For a bentonite from a new deposit there is, therefore, no alternative to carrying out acid-treatments with different strengths of acid and for different contact times, and then measuring the oil- decolourizing ability of the various products. Because there is no generally accepted procedure for acid activation, these procedures vary in detail between different workers; some.of these are described briefly below.


108

Bentonite

15

10


X Oxfordshire

Q Surrey

I j

'.

% Fe,O,

0 2 4 6 S io 12 % H,SO, on slurry

Surface area

5% SiO,

70

66

62

58

54

2

X Oxfordshire

0 Surrey

m:

400

300

200

10

4 6 8 10 % HSO, on slurry

12

-1

x Oxfordshire

0 Surrey

% HSO. on slurry

X Oxfordshire

o Surrey

!% H,SO, on slurry Figure 9.10a-d. Changes in chemical composition and surface area for two UK fuller's earths following sulphuric acid treatment (from Yates, 1986).


109

Bentonite


Procedure of Yates ( I 986) Raw materials were two fuller's earths (Ca-smectite) from Oxfordshire and Surrey, UK. Slurries of the clays at constant (but not given) solids content were reacted in glass vessels with sulphuric acid in amounts up to 1 1.2% of the total mix. The reaction slumy was heated to boiling point under reflux for a standard time (again not given) with continuous stining. After reaction, the treated clays were filtered and washed with deionized water to a low residual acidity level and dried slowly to 10% moisture levels. Changes in Al, Mg, Fe content and surface area of the two clays with acid treatment are shown in Figure 9.10a-d.

Oil-bleaching ability of the activated products was measured on neutralized soya bean oil. This was carried out at a contact temperature of 95OC in a stirred open vessel, the contact time being 30 min. After treatment, the oil/clay mixture was filtered and the colour of the bleached oils determined using a Lovibond Tintometer with a 51/4-in cell, the red colours obtained being recorded. Results are plotted in Figure 9.1 1.

o Surrey

2 4 6 8 lo i2 % H,SO. on slurry

Figure 9.1 1. Decoloutization of soya bean oil by fuller's earths as function of acid treatment (from Yates, 1986).

There is a very rapid development of bleached colour as the acid strength increases, followed by a slower improvement at the higher levels of treatment.

Procedure of Kolta et al. (1976) Materials used were three medium- to poor-grade bentonites from the Quasr-el-Sagha deposit, Egypt. 100 g of bentonite was added to 100 ml water in a three-necked flask, with a thermometer, a condenser and a stirrer.. The flask was placed in a water


110

Bentonite


bath, thermostatically controlled at 95°C. 60 ml of 37% HCl was added and the slurry subjected to leaching for various times ranging up to 10 hours. At the end of the treatment the sluny was filtered, thoroughly washed until an acid-free filtrate was obtained, and then dried at 110°C.

For determination of bleaching capacity, a fixed amount of the acid-activated product was added to 20 ml of crude cotton oil in a wide glass test tube immersed in an oil bath to maintain the bleaching temperature (100°C). The bleaching process was conducted for 30 min with stimng. The slurry was filtered and the absorbance of the bleached oil was measured at 460 nm using a Beckmann spectrophotometer. The relative bleaching abilities of the activated products were measured by comparing the absorbances of the oil bleached with each product and that bleached with the same amount of commercial bleaching clay Tonsil AC, applying the following equation:

BA = (A1 - Az) / (AI - A3) X 100

where BA is the bleaching ability, AI is the absorbance of the crude oil, A2 is the absorbance of the oil after bleaching with the activated product, and A3 is the absorbance of the oil after bleaching with a similar amount of Tonsil AC. The relationships between bleaching abilities and time of acid activation for the three clays are shown in Figure 9.12a and corresponding development of surface area in Figure 9.12b.

Only sample C gives a conventional response to acid activation, after 360 min contact with HC1 giving a product with 90% of the bleaching ability of Tonsil AC. Sample B cannot be used for activation because of its low smectite content. Sample B still had not developed its full bleaching potential even after 10 hours' acid contact, which suggests that an appreciable amount of kaolin is also present in this sample.

Procedure of Zaki et al. (1986) Materials were bentonites from three further Egyptian deposits. Acid treatment involved adding 166 g clay to 1 litre of 1N HC1 in a two-necked flask equipped with a thermometer and a condenser. The flask was placed in a water bath at 95°C. Contact times, with continuous stirring, were 60,90 and 120 min. Recovery of products followed Novak et al., as did the assessment of bleaching ability, except that a Lovibond Tintometer was used.


111

Bentonite


l f t l # I , l t l

0 !20 240 360 480 600 0 I20 240 360 480 60C Time of acid oetivolim (mid Time of oeid oetiwtlonh)

Figure 9.12a-b. Relationships between bleaching ability (a) and surface area (b) with increasing acid treatment of Egyptian bentonites. Smectite contents of samples: 33%; A 40%; 0 60% (from Novak el al., 1976).

Procedure of Srasra et al. (1989) This paper deals with a bentonite from Tunisia, the main component (80%) of which is an irregularly interstratified smectite-illite. Acid activation was accomplished by stirring 10 g of bentonite with 100 ml of hydrochloric, sulphuric and nitric acids of different normalities (0.1-10 N ). After centrifugation and filtration, the products were washed with hot water to pH 6 and dried at 105OC. For assessment of decolourization ability, 2 g of the activated product was thoroughly mixed with 100 ml rapeseed oil (previously neutralized with NaOH) for 20 min at 90°C. Bleaching ability was assessed by recording the absorbance at 430 nm of 2.5 ml of the treated oil in 10 mi carbon tetrachloride on a LMB spectrophotometer and applying the equation:

BE = [ A430 (neurral oil) - 4 3 0 (treated oil) ] I A430 (neutral oil)

HC1 was found to be the best activator. Total absorption spectra for the untreated oil, that bleached by the crude bentonite and that bleached by the product after a 15 hour treatment with 7 N HCl are shown in Figure 9.13.


112

Bentonite


Figure 9.13. Total absorption spectra for rapeseed oil (a) and for products bleached with crude (b) and HCI-activated (c) bentonite (from Srasra et af.,1989).

Procedures of Stoch et al. (1979a,b) These two papers describe the effects of acid attack on surface area, pore structure, surface acidity and other properties of impure bentonites from Poland and the bleaching performance of the products to rapeseed oil. It would not be practical - or even necessary - to replicate all the various combinations of experimental conditions in a routine assessment of a possible bleaching earth, but these investigations provide an excellent example of the use of the various techniques and the type of interpretation possible.


113

Bentonite


References

Atkins M P, Smith D J H & Westlake D J (1983) Montmorillonite catalysts for ethylene hydration. Clay Miner. 18,423-429.

Fahn R & Fender1 K (1983) Reaction products of organic dye molecules with acid-treated montmorillonite. Clay Miner. 18,447-458.

Greenland D J & Mott C J B (1978) Surfaces of clay particles. Pp. 321-353 in: The Chemistry of Soil Constituents (D J Greenland and M H B Hayes, editors). John Wiley & Sons, Chichester, UK

Gregory R, Smith D J H & Westlake D J (1983) The production of ethyl acetate from ethylene and acetic acid using clay catalysts. Clay Miner. 18, 431-435.

Fijal J, Klapyta 2, KwiecinskA B, Zietkiewicz J & Zyla M (1975) On the mechanism of acid activation of montmorillonite: 11. Changes in the morphology and porosity in the light of electron microscopic and adsorption investigations. Mineralogica Polonica 6,49-57.

Kolta G A, Novak I, Samir 2 EL-T & Kamilia A EL-B (1976) Evaluation of bleaching capacity of acid-leached Egyptian bentonites. J . Appl. Chern. Biotechnol. 26, 355-360.

Morgan D A, Shaw D B, Sidebottom T C, Soon T C & Taylor R S (1985) The function of bleaching earths in the processing of palm, palm kernel and coconut oils. J . Am. Oil Chem. SOC. 62, 292-299.

Richardson L L (1978) Use of bleaching clays in processing edible oils. J. Am. Oil Chem SOC. 55, 777-780.

Srasra E, Bergaya F, Van Damme H & Ariguib N K (1989) Surface properties of an activated bentonite - decolorisation of rape-seed oils. Appl. Clay Sci. 4,411-421.

Stoch L, Bahranowski K & Gatarz 2 (1979a) Bleaching properties of non-bentonitic clay materials and their modification: 11. Bleaching ability of natural and activated Krakowiec clays from Machow. Mineralogica Polonica 10,Zl-38.

Stoch L, Bahranowski K, Eilmes J & Fijal J (1979b) Bleaching properties of non-bentonitic clay materials and their modification: III. Modification of bleaching properties of Kracowiec clays from Machow with some organic compounds. Mineralogica Polonica 10,3947.


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Bentonite


Taylor D B & Jenkins D B (1986) Acid activated clay. Am. SOc. Min. Eng. Mtng. Preprint 86-365.

Yate M H (1986) Acid treatment of English montmorillonite and its effect on clay structure and properties. Unpublished paper (Luporte Inorganics, Widnes, UK).

Zaki M I, Abdel-Khalik M & Habashi G M (1986) Acid-leaching and consequent pore structure and bleaching capacity modifications of Egyptian clays. Colloids & Surfaces 17,24 1-249.


Bentonite


Appendix I O : Water absorbency sf pet litters

Equipment and reagents

Stainless steel mesh (355 pm aperture) cone, 75 mm high by 70 mm diameter; with holes cut in rim to allow suspension from a stand. 1000 ml beaker Analytical balance, 0.1 g readability Stopwatch or timer Clamp and stand

Distillfldeionised water

Sample preparation

1. Oven-dry lump material at 60°C overnight.

2. Jaw-crush and screen on 10 mm and 5 mrn, repass >10 rnm through jaw-crusher.

3. Retain 5-10 mm material for testing (store in a sealed container).

Method

1 . Weigh the mesh cone (Wl).

2. Add approximately 20 g representative sub-sample of 5- 10 mm bentonite to the cone and weigh cone + sample (W2).

3. Suspending the cone + sample from the clamp and stand, immerse the cone in the beaker filled with distilled water for 20 minutes.

4.

5 .

6 .

Remove the cone from the water and allow to drain for 30 minutes.

Weigh the cone + saturated. sample (W3).

Repeat the test twice further to give a total of three determinations.


116


Calculation

Absorbency as a percentage of mass

= W3-W2 * 100 w2-w1

Quote the mean of the three results.

Mineralogy and Petrology Group, British Geologlcal Survey 0 NERC 1993

Documents

S D J Inglethorpe, D J D E Highley and A J Bloodworth TT · S D J Inglethorpe, D J Morgan, D E Highley and A J Bloodworth TT TT Mineralogy and Petrology Group British Geological Survey