European Ben to Nites as Alternatives to MX-80

8/2/2019 European Ben to Nites as Alternatives to MX-80

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Science & Technology Series n 334 (2008) Andra 23

European Bentonites as alternatives to MX-80

Dr. Dietrich Koch

S&B Industrial Minerals GmbH, Ruhrorter Strae 72, D-68219 Mannheim

(Phone: 0049/621/80427-0, fax: 0621/80427-50, e-mail: d.koch@ikomineral s.com)

Abstract

For the management and disposal of radioactive waste, most of the European countries are working on solutions with bentonite as

a key component. While over many years almost exclusively North American Wyoming bentonite, a natural sodium bentonite (MX-80

type), was tested as reference buffer material, since some years also different bentonites from European sources are examined and

commercially available for this application. Bentonite has special characteristics, which make it very suitable for different environmental

protection techniques. The possible future use of bentonite as sealing component in final repositories for radioactive waste is under

investigation for two applications: (1) High grade bentonite as buffer material for embedment of the containers w ith radioactive waste, (2)

Medium grade bentonite, possibly mixed with soil (e.g. crushed rock) or non swelling clays as backfill material for sealing the deposition

tunnels. Differences in characteristics of Ca bentonite and Na bentonite are explained and recommendations are given for the use as

buffer bentonite and as backfill material. The substantial characteristics of European bentonites are compared with those of MX-80

for the application as buffer bentonite. Information is given about high quality European bentonite mines and processing facilities. It is

concluded by these recent research results that high grade Ca bentonite is equivalent with the so far preferred natural Na bentoniteMX-80 for buffer application. The Milos Ca bentonite IBECO RWC is mentioned now as SKB reference bentonite.

Keywords: bentonite, Ca bentonite, Na bentonite, buffer material, backfill material, disposal of radioactive waste

Introduction1.

Because of its unusual characteristics (2:1-layer nano-

sized structure of the montmorillonite mineral, cation

exchange capacity, intracrystalline swelling) bentonite is

used as a sealing component for different environmental

protection techniques, (Koch, 2002). For the disposal of

radioactive waste, most of the European countries are

working on solutions with bentonite as a key component

for buffer and backfill material. While over many years

almost exclusively North American Wyoming bentonite, a

natural sodium bentonite (MX-80 type), was analyzed as

reference buffer material, since some years also different

bentonites from European sources are examined for this

application. Results so far confirm that there are alterna-

tives to MX-80 for application as buffer (Pusch, 2001a,

b) but also for backfilling. The main quality determiningparameters of high-level radioactive waste (HLRW) ben-

tonites are described in chapter 3.2.

Materials & methods2.

The bentonites described in this paper originate from

Greece (island of Milos), Georgia (CIS), and the USA

(Wyoming). The bentonite characteristics, as mentioned

in this paper, were determined using methods listed in

table 1.

Bentonite characteristics3.

Bentonite is the name for clay (weathered volcanic

rock) which contains swellable 2:1 layer clay minerals

of the smectite group, in particular the clay mineral

montmorillonite as main component. The term smectite

is used to describe a family of expansible 2:1 phyllosili-

cate minerals having permanent layer charge (between

0.2 and 0.6 charges per half unit cell) (Bailey, 1980) andthe ability to embed and exchange inorganic and organic

cations as well as liquids. The properties of bentonites


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24 D. Koch

are based on the structure of smectites, mainly montmo-

rillonite (Grim & Gven, 1978). Basic units of smectites

are constructed of a single octahedral sheet sandwiched

between two tetrahedral sheets sharing the apical oxy-

gens of the tetrahedral sheets (Figure 1).

Layer charge arises from substitutions in either the

octahedral sheet (typically from the substitution of low

charge species such as Mg2+, Fe2+, or Mn2+ for Al3+) or the

tetrahedral sheet (where Al3+ or occasionally Fe3+ substi-

tutes for Si4+), producing one negative charge for each

substitution. This negative charge is distributed over the

surface of the 2:1 layer platelets, while the edges and cor-

ners can bear some positive charges depending on the

pH of the surrounding solution (Odom, 1984). The nega-

tive layer charges are neutralized by positively charged

cations in the interlayer without entering the structural

Characteristic Unit Method

Cation exchange capacity (CEC) mmol(eq)/kg Cu-trien (Meier & Kahr, 1999)

Chemical composition wt.% XRF

Density g/cm3 Pusch (2002b), p. 144

Mineralogical composition wt.% XRD

Montmorillonite content (Methylene-blue adsorption) wt.% VDG P 69

pH value pH meter

Swelling index ml/2g ASTM D 5890

Swelling pressure kPa Karnland et al. (2006), p. 35

Water adsorption (Enslin/Neff-method) wt.% DIN 18132

Water content wt.% DIN 18121 (12 hrs, 105 C)

Hydraulic conductivity m/s DIN 18130

Sulphur content wt.% Thermogravimetric Analyzer

Total organic content TOC wt.% Thermogravimetric Analyzer

Bentonite characteristics and testing methodsTable 1:

O HO Si, Al, Fe, Mg

exchangeable cationsneutral molecules (H2O)

Tetrahedral sheet

Octahedral sheet

Tetrahedral sheet

Interlayer (Gallery)

Tetrahedral sheet

Basic units of smectite structure (Grim & Gven, 1978)Figure 1:


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Ca bentonite can adsorb between 150% and 200%

water related to its own weight. Contrary is the behaviour

of Na bentonite. As the electrical interaction between

the monovalent Na+-ions and the negatively charged

platelets is much weaker compared to the Ca2+-ions with

two positive charges, excess water molecules can enter

completely the interlayer area, surrounding the Na+-ions

with a larger hydration shell. The distances between the

superimposed platelets are increased so much that the

Na montmorillonite crystal bundles are disbanded into

15-20 single platelets each (see figures 3 and 5). One

single 2:1 platelet has a thickness of ca. 1 n m (dehy-

drated). Its dimensions in length and width are in the

range of 500-800 nm.

The differences in swelling behaviour between

Na- and C a bentonite can be demonstrated by the

swelling index (ASTM D 5890, figure 4) which is much

larger for Na bentonite. Because of these differencesin structure and propert ies, Na bentonite is preferred

for most of the industrial and environmental

applications.

For most technical applications the European Ca ben-

tonites are converted to Na bentonites by a technical

ion-exchange process, called activation (Jasmund &

Lagaly, 1993). Based on the patent of Prof. Endell (British

Patent 4770, 1935), the soda-activation by an exchange

of the Ca2+-ions by Na+-ions improves most of the prop-

erties of the basic Ca bentonites (Hofmann et al., 1933,

1934, see figure 5).

2:1 layers (Figure 2). In natural bentonites these can be

predominantly Na+-ions (Wyoming bentonite, e.g. MX-80)

or Ca2+-, Mg2+-ions (European bentonites).

The interlayer (the space between the 2:1 layers)

is hydrated and expansible. Therefore smectites are

referred to as swelling clays (Lagaly, 1992). Bringing

bentonite in contact with a surplus of water, e.g. by pre-

paring an aqueous slurry, the dipole molecules of water

are entering the interlayer area, accumulate to the posi-

tively charged cations and increase their hydration-shell.

The effect is that the distance of the superimposed smec-

tite particles is increasing (intracrystalline swelling). The

characteristic properties of montmorillonite can be sum-

marized in these three points:

Structure of very small, thin, and flexible platelets

with large surface

Cation exchange capacity because of the negative

charge of the elementary cellCapability of reversible adsorption of water in the

interlayer space (intracrystalline swelling).

Differences in characteristics of Ca-bentonite3.1.

and Na-bentonite

In case of Ca bentonite, the electrical bonding forces

between the Ca2+-ions und the negative charged parti-

cle surfaces are strong enough to preserve bundles of

15-20 2:1-layer platelets, building Ca montmorillonite

quasi crystals. Therefore spatial conditions are limited

for taking up additional water molecules.

Model of Montmorillonite structure (Mller-Vonmoos, 1988)Figure 2:

soda-activation

Ca2+ - Montmorillonite + Na2CO

3


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26 D. Koch

On industrial scale, the natural Ca bentonite (with

28 % water content) is intensively mixed with an ade-

quate amount of Na2CO

3that corresponds to the CEC.

The Ca2+-ions react with the CO3

2--anions forming cal-

cium carbonate of low solubility. According to this proc-

ess, the Na+-ions can completely replace the Ca2+-ionsas interlayer cations (Lagaly et al., 1981). The resulting

activated Na bentonite can easily be dispersed in water,

building up a colloidal dispersion. Because of their elec-

trical charges on the surface and at the edges and cor-

ners, the dispersed platelets are building up voluminous

structures like cardhouses or ribbons, depending on the

pH value of the dispersion (see figure 5). These volumi-

nous structures are responsible for the special rheologi-

cal properties (e.g. thixotropy) of bentonite suspensions.

Reaction (1) can run in reverse if a Na bentonite comes

in contact with a surplus of free Ca2+-ions, e.g. by hard

water with high content of Ca2+-ions.

The polyvalent cations can (again) partly or totally

replace the Na+-ions (de-activation) with the effect, that

the montmorillonite platelets agglomerate to semi-

crystallites.

During the technical use of bentonite suspensions,

e.g. as drilling slurries, this de-activation of Na ben-

tonite can be identified by (unwanted) flocculation effects

and solid/liquid separations. In case of buffer applica-

tion, the reaction of Na bentonite with saline rock water

could result in structural changes like shrinking or crack-

ing because of ion-exchange reactions.If the water adsorption can take place without spatial

restrictions, Na bentonite has a clearly higher swelling

Swelling index. Ca-bentoniteFigure 4:(2 g) in water: 5 ml (left); Na-bentonite (2 g)

in water: 32 ml (right).

Cation-exchange and swelling behaviour of Ca- and Na bentonitesFigure 3:

2 2

DRY IN SUSPENSION

hydration shell of 6 water molecules

hydration shell of 4 water molecules

d001: 2.0 nm

Calcium bentonite

limited swelling capacity

high swelling capacity

hydrated cations

water molecules

Sodium bentonite

d 001: 1.2 nm

hydrated cations+ Na2CO3(soda activation)

Ca2+ -ions

=Na+ -ions

d 001:

d001: 1.5 nm


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Na bentonite or Ca bentonite as buffer?3.2.

The Wyoming type bentonite MX-80 was analyzed

for more than 20 years by all institutes that were con-

cerned with this topic, as reference bentonite for the

buffer application. In 2001 in a meeting of the author with

Roland Push and the Swedish Nuclear Fuel and Waste

Management Co (SKB), the long-term stability of HLRW

bentonite in a saline environment was discussed. The

possibilities of ion-exchange of Na bentonite by ground-

water containing Ca2+/Mg2+-ions and as consequence

possible structural changes like shrinking or cracking

were discussed. Therefore the author made the proposal

for examination of Ca bentonite as less sensitive alterna-

tive for host rock formations with saline rock water. After

several years of analyzing, the principle suitability of high

grade Ca bentonite (montmorillonite content > 75 wt.%) as

volume than Ca bentonite (Figure 4). Upon loading, the

swelling pressure depends on the density of the ben-

tonite, the salinity of the pore water, and the major type

of interlayer cation (Lagaly, 2006). Therefore Na ben-

tonite like MX-80 was preferred as a standard buffer

material for a long time. Under spatially limited condi-

tions without free swelling, e.g. after compaction, ben-

tonite can develop rather high swelling pressures. The

space between the montmorillonite platelets is smaller

and the repulsion between the negatively charged sur-

faces is greater (Pusch, 2006). The difference in swell-

ing pressure between Na bentonite and C a bentonite

becomes negligible at densities lager than about 1900

kg/m , when the stacks of lamellae have been forced

together so much that the microstructural patterns are

similar (Pusch, 2002a, p. 130).

Bentonite typeGrain-size

% < 2 m

Water adsorption

(E.N. Value, %)Swelling volume (ml) pH-Value

Ca bentonite ca. 20 150 - 200 5 - 8 7.5 - 8

Na bentonite ca. 80 500 - 700 25 - 35 9.5 - 10.5

Comparison of properties of typical Na and Ca bentonitesTable 2:

Soda-activation and de-activation by ion-exchangeFigure 5:


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28 D. Koch

buffer material was confirmed by SKB (Sellin et al., 2005).

The substantial influence of electrolyte concentration in

pore water on Na and Ca bentonite has been described by

Pusch (2006). In the meantime S&Bs calcium dominated

Milos bentonite IBECO RWC is listed as SKB reference

buffer material, as well as the Wyoming bentonite MX-80.

In table 4 the chemical and mineralogical composition and

some physical properties of bentonites are summarized,

which had been tested as alternatives to MX-80 as buffer

bentonite. Besides the Ca bentonite IBECO RWC also as

another natural Na bentonite IBECO RWN was analyzed.

From our experience as bentonite producer, the ques-

tion for the most suitable bentonite type as buffer can

be answered in such a way that for hard rock formations

with saline rock water environment Ca bentonite should

be preferred. For repositories in a clay stone as host rock

formation, where no saline water is to be expected, a Na

bentonite can be used. In order to achieve a long-termstability under final repository conditions we recommend

to specify the following main quality determining param-

eters of HLRW bentonites for buffer application:

High montmorillonite content

(e.g. 75-90 wt.%)

High cation exchange capacity

(e.g. 0.80-0.95 mmol(eq)/g)

Medium water adsorption

(e.g. 150-200% Enslin/Neff value)

Low hydraulic conductivity (e.g. Kf

= 10-11-10-14 m/s)

Low sulphur content (still to specify)

Low organic matter content (still to specify)

and additionally for repositories in hard rock formation

with saline rock water environment:

Mechanical stability (low dispersibility) against flow-

ing rock water (Kaufhold & Dohrmann, 2008)

Chemical stability against electrolyte rich rock water.

For backfill there are different options for the fill-

ing material as well as for the construction technique of

backfill operation (Pusch, 2002b). As backfilling mate-

rial either a high grade bentonite can be blended with

inert fillers (crushed rock, sand, or non swelling clay) or

a medium grade bentonite (with a montmorillonite con-

tent between 50 and 65 wt.%) could be used, if available

in constant quality and sufficient quantity. For backfill we

give the same recommendation for a suitable bentonite

type as for buffer. Depending on the host rock forma-

tion, non-activated Ca bentonite should be used, when

electrolyte-rich rock water can come into contact with

the backfill, while Na bentonite can be used in environ-

ment without any saline load.

For a medium grade bentonite as backfill material,we recommend to specify as the main quality determin-

ing parameters:

Medium montmorillonite content (e.g. 50-65 wt.%)

Medium cation exchange capacity

(e.g. 0.55-0.70 mmol(eq)/g)

Medium water adsorption

(e.g. 150-200% Enslin/Neff value)

Low hydraulic conductivity (e.g. Kf

= 10-10-10-13 m/s)

and additionally for repositories in hard rock formation

with saline rock water environment:

Mechanical stability (low dispersibility) against flow-

ing rock water and upward movement of the buffer

bentonite (Kaufhold & Dohrmann, 2008)

Chemical stability against electrolyte rich rock water.

Density &

saturation1300 1500 1700 1800 1900 2000 2100

MX-80 0.06 0.2 0.4 0.8-0.9 1.4 4-5 10-12

IBECO, Na - - - 0.6-1 - 4-5 -

IBECO, Ca - - - 0.2 - 5 -

RMN - - 0.45 1.9* 2.5** 3.9** -

Beidellite - - - 1.5 - 4.2 -

Saponite - - - 2.5 - 8.8 -

Kunigel - - 0.2 - 0.9 -

Friedland - - 0.05 0.1 0.3 0.8-1 2.2.5

* Density at saturation: 1850 kg/m3

** Mixture with 5% graphite and 10% quartz powder- Density at saturation: 1950 kg/m3

Comparison of swelling pressures of Na and Ca bentonites (Pusch, 2002a, p. 130)Table 3:


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Summary and conclusions4.

Recent research results (Karnland et al., 2006)

confirmed the equivalence of high grade Ca bentonite

with so far preferred natural Na bentonite (MX-80 type)

for buffer application. The Milos Ca bentonite IBECO

RWC is mentioned now also as SKB reference bentonite.

Depending on the type of host rock, the presence of

(saline) water, and the application as buffer or backfill

bentonite, S&B Industrial Minerals recommends different

types of bentonites (Table 5).

S&B Industrial Minerals as the leading bentonite

producer in Europe owns several bentonite mines and

production plants in Europe and outside of Europe, and

can offer all grades of bentonite that can be used in the

above mentioned buffer and backfill applications.

Natural Na bentonite

Technical activated Na bentonite

Non-activated Ca bentonite.

Figure 6 shows a part of the worldwide largest

bentonite mine Angeria on the Greek island Milos. This

mine is 2.5 km long, 600 m wide, 130 m deep, and

has 27 mio tons current proven reserves in different

qualities. The bentonite excavation is done by open pit

mining.

IBECO RWC IBECO RWN MX-80

Origin Greece Ca-bentoniteGeorgia (CI S)

Nat. Na-bentonite

Wyoming

Nat. Na-bentonite

Chemical Composition [XRF]

SiO2

[wt.%] 55.2 59.5 55.8

Al2O

3[wt.%] 16.5 18.6 18.1

Fe2O

3[wt.%] 5.6 3.5 5.5

TiO2

[wt.%] 0.7 0.4 0.7

CaO [wt.%] 7.4 2.3 5.1

MgO [wt.%] 3.1 4.5 3.8

Na2O [wt.%] 0.5 2.4 2.8

K2O [wt.%] 0.5 1.3 0.9

LOI [wt.%] 9.7 7.2 7.3

Mineralogy [XRD]

Smectite [wt.%] 75 85 85 95 65 75

Illite/Mica [wt.%] < 4 2 6 2 4

Quartz [wt.%] < 4 < 4 5 7

Calcite [wt.%] 8 12 3 5 6 9

Others [wt.%] < 4 < 4 5 16

[Physical Properties]

CEC mmol(eq)/kg 0.80 - 0.90 0.95 - 1.05 0.70 - 0.80

Swelling vol. ml/2g 6 - 10 22 - 26 28 - 32

Enslin-Neff % 110 - 150 480 - 520 650 - 800

Characteristics of different buffer bentonitesTable 4:

Host rock Application Bentonite type

Hard rock with saline water Buffer High grade Ca bentonite

Hard rock with saline water Backfill Medium grade Ca bentonite

Clay rock Buffer High grade Na bentonite

Clay rock Backfill Medium grade Na bentonite

Recommendations of bentonite types for different host rocks and different HLRW applicationsTable 5:


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30 D. Koch

Milos, largest bentonite mine worldwideFigure 6:

Bailey S.W. (1980). Summary of recommendations of AIPEA

nomenclature committeeon clay minerals. American Mineralogist,

65, 1-7

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Properties and Uses. Elsevier, Amsterdam

Hofmann U., Endell K., Wilm D. (1933). Kristallstruktur und

Quellung von Montmorillonit. (Das Tonmineral der Bentonittone).

Z Kristallographie 86, 340-348

Hofmann U., Endell K., Wilm D. (1934). Rntgenographische

und kolloidchemische Untersuchungen ber Ton. Angewandte

Chemie, 47, 539-547

Jasmund K. & Lagaly G. (1993). Tonminerale und Tone. Steinkopff

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Kaufhold S. & Dohrmann R. (2008). Detachment of colloids from

bentonites in water. Applied Clay Science, 39, 50-59.Karnland O., Olsson S. & Nilsson U. (2006). Mineralogy and seal-

ing properties of various bentonites and smectite-rich clay mate-

rials. SKB Technical Report TR-06-30.

Koch D. (2002). Bentonites as a basic material for technical base

liners and site encapsulation cut-off walls. Applied Clay Science

21, 1-11.

Lagaly G. (1992). From clay mineral crystals to clay mineral dis-

persions. In: Dobis B (ed) Coagulation and flocculation: theory

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Lagaly G. (2006). Colloid Clay Science. In: Bergaya F., Theng

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186-188.

Lagaly G., Mller-Vonmoos M., Kahr G. & Fahn R. (1981).

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Meier L.P, & Kahr G. (1999). Determination of the cation exchange

capacity (CEC) of clay minerals using the complexes of copper

(II) ion with triethylenetetraamine and tetraethylenepentamine.

Clays and Clay Minerals 47, 386-388.

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Eidgenssische Hochschule Zrich, No. 133.

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Phil Trans R Soc London A, 311-391.

Pusch R. (2001a). Origin and exploitable amounts of Greek ben-

tonites and comparison of one representative (IBECO-Seal M

90), with MX-80. Unpublished report.

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blocks of IBECO-SEAL M 90 Clay material. Unpublished report.

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Definitions, basic relationships and laboratory methods. SKB

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VDG Merkblatt P 69, (1988) Prfung von Bindetonen p. 5

References

Documents

European Ben to Nites as Alternatives to MX-80