MINERAL PROCESSING TECHNOLOGIES IN THE BAUXITE …€¦ · Proceedings of the 8th International Alumina Quality Workshop • 2008 149 MINERAL PROCESSING TECHNOLOGIES IN THE BAUXITE

Proceedings of the 8th International Alumina Quality Workshop • 2008 149

MINERAL PROCESSING TECHNOLOGIES IN THE BAUXITE AND ALUMINA INDUSTRY

Buntenbach S

AKW Apparate + Verfahren, Germany

Abstract

The Bauxite and Alumina Industry is facing some important challenges to improve the process chain for the production of aluminium. The process chain has to be improved from the bauxite mining and processing up to the refinery and smelter process.

Up-to-date technologies of mineral processing, like

- Washing and Elutriation- Separation of Fines- Gravity Separation- Magnetic Separation

are able to contribute to the beneficiation of ROM (Run of Mine) Bauxite to increase the Al2O3 content of the ore, the Al203/SiO2 ratio, or even the content of Fe+.

1. Introduction

The global production of bauxite has reached a peak of nearly 190M.t/a. With regard to the use of bauxite, metallurgical bauxite for aluminium production accounts for the main share, making up around 85–90 % of this total. A typical range of the metallurgical bauxite composition is given in Table 1.

Table 1. Bauxite: Typical Composition

ComponentsWt. %

(as metallic oxide if not indicated otherwise)

Al2O3 30 - 60

Fe2O3 1 - 30

SiO2 < 0.5 - 10

TiO2 < 0.5 - 10

Organic Carbon (as C) 0.02 - 0.40

P2O5 0.02 - 1.0

CaO 0.1 - 2

Ga2O3 0.004 - 0.013

The wide range in the composition originates from the differences in the origin and geologic history of the different deposits. It is well accepted that the quality influences the ability to be refined.

1.1 Industrial production of aluminium

The industrial production of aluminium is still based on a two-stage process that was introduced in principle more than 100 years ago:

I. In the alumina factory (i.e. the refinery), the Bayer process is used to produce alumina as an intermediate product from the bauxite raw material

II. In the smelter, the oxide dissolved in a cryolite melt is reduced electrochemically to metal (fusion electrolysis according to the Hall-Héroult method).

The Bayer process can be described as an extremely selective extraction process in which the aluminium hydroxides of the bauxite are dissolved in an aqueous soda caustic liquor at elevated temperature and pressure, while the alkaline constituents remain in the residue.

The temperature chosen for the liberation is crucial for which of the aluminium hydroxides contained in the bauxite are dissolved.

Gibbsite is dissolved at a liberation temperature of 110 to 140° C, whereas a temperature of 180 to 280° C is necessary for boehmite and 250 to 280° C is needed for diaspore. The iron- and silicate-containing components, referred to as red mud, remaining in the residue, are separated from the aluminate-saturated lye in clarifying thickeners by means of gravity separation into a liquor overflow and thickened slurry underflow.

1.2 Material streams in primary aluminium production

The multistage process of primary aluminium production partly described above involves a series of direct and induced material streams (Figure 1).

3,5 - 6,5 tBauxite

Bayer Process

11,9 - 31,2 GJEnergy (Fuel)

2 - 4 tRed Mud

2,89 t Al(OH)3

Calcination6,6 - 9,6 GJEnergy (Fuel)

1,89 tAl2O3

Aluminium Smelter M

ediu

m to

Low

Var

iatio

n

1 tAluminium

1.500 - 1.600 kg Gases0,25 - 1,7 kg Dust

10 - 20 kg Spent Potlining

12,5 - 18,6 MWhelectrical Energy

15 -35 kgCryolit, Al F3425 - 460 kgElectrodes

High

Var

iatio

n

1t H2O1 - 2 kg Dust

300 - 600 kg NaOH50 - 100kg CaCO3

Figure 1. Direct and induced material streams involved in the production of 1 t of primary aluminium

The most important input in terms of volume is the bauxite raw material. The wide variation in the required quantity can be explained by the varying grades of the ores used. In the following process steps, the bauxite quality has a lesser influence on the necessary material and energy quantities. The differences here result mainly from the application of various methods for calcination and electrolysis.

The efficiency of the process chain is determined essentially by the mineralogical characteristics of the bauxite used. Key factors are:

AQW2008_Proceedings_FINAL-121-37149 149 4/08/2008 2:57:16 AM

150 Proceedings of the 8th International Alumina Quality Workshop • 2008

- the content of recoverable Al2O3

- the quantity of reactive silica R.SiO2

- the TiO2 content- the bauxite/Al2O3 ratio- the residual moisture.

For economically efficient utilization, the bauxite should have a minimum content of > 40 wt. % recoverable Al2O3.

If we look at the long- and mid-term development of these mineralogical characteristics, different trends become apparent. This is shown based on the example of the Al2O3 content. Up until the mid-1980s, the quality of the bauxite extracted and used in the refineries rose, but then registered a steady decline (Figure 2).

Figure 2. Long- and mid-term development of the worldwide bauxite quality, based on the example of the Al2O3 content

This development can also be observed for the other above-mentioned quality criteria. One of the main reasons for this is the limited availability of high grade bauxite, a situation that will not improve in the next few years. To obtain the necessary qualities, various possibilities are available to plant operators for the beneficiation of the ROM bauxite.

These generally low-cost processes can extend the life of existing bauxite mines considerably, and substantially improve the technical and economic basis of the Bayer process.

2. Typical process concepts developed by AKW A+V

In this paper processes with different objectives are presented. Common to all processes is that the feed material consists of bauxite that was previously not used owing to its low quality.

One process was developed to lower the silica content as silica increases the required energy, wear and the quantity of red mud in the Bayer process. Another process was designed to lower the content of reactive silica in the beneficiated bauxite.

2.1 Reducing the silica content

The bauxite district of Los Pijiguaos lies in the western part of the State of Bolivar in the Bolivarian Republic of Venezuela. It is currently the only functional bauxite district in this South American country.

Geographically, they form part of the South American platform province, which is the world‘s third most important bauxite province in terms of reserves and the second most important bauxite province with regard to production. All deposits in this district have a high recoverable content of Al2O3, at around 50 wt. %. The assured reserves total some 170 mill. t bauxite with an average of 49 wt. % Al2O3 and 10.2 wt. % SiO2. Figure 3 shows a typical deposit profile of this district.

Figure 3. Typical deposit profile of the bauxite district Los Pijiguaos [4]

At present the bottommost high-Al2O3 zone is not being mixed as the silica content at > 20 wt. % is too high.

In the year 2003, the geological department of the CVG-Bauxilum-Operadora de Bauxita took a representative sample of the high-silica bauxite (HS-bauxite) and brought four large bags, each containing around 750 kg, to the test centre of AKW A+V. Working together with Rio Tinto Alcan Bauxite and Alumina in Gardanne, AKW A+V started with an extensive characterization of the ROM Bauxite. (Table 2).

Table 2. Analysis of ROM HS-Bauxite, Los Pijiguaos

Bauxite sample Global content

Al2O3 44.5 %

Fe2O3 8.5 %

SiO2 Total 21.62 %

SiO2 Quartz 21.56 %

TiO2 1.1 %

CaO < 0.05 %

Na2O < 0.03 %

LOI 23.6 %

Ctotal 0.072 %

Total FX 99.5 %

Al2O3 100 % Gibbsite

In the raw material characterization of the starting material it was established that this type of bauxite has a significant concentration of silica in the particle size range 0.2–2 mm (Figure 4).

Figure 4. Content of Al2O

3, Fe

2O

3 and SiO

2 in different size ranges [5]



The objective of cost-efficient beneficiation is therefore the removal of the high-silica particle size range. In this context, it should be noted that thanks to its relatively low carbon content (Ctotal) of 0.07 wt. %, the HS-bauxite does have some processing advantages compared to the standard bauxite with a carbon content of 0.20 to 0.22 wt. %.

The earthy-clay-like matrix of the raw material can be broken down by intensive washing and elutriation, and the washed bauxite can then be sized into different ranges. As gibbsitic Al minerals are also found in the fine particle size ranges < 100–150 μm, the plant design requires the careful balance of the recoverable Al2O3 against the investment and operating costs.

As the refineries cannot tolerate any unnecessary entrainment of water in the process, the washed bauxite must be dewatered to a maximum residual moisture of < 12 wt. %. The necessary process steps are associated with considerable investment costs. A flowsheet of a process for the maximized recovery of Al2O3 is shown in Figure 5.

The influence of the washing and elutriation process on process efficiency is considerable. The design of the washing and elutriation drum (WLT) and the process flow was adapted to the raw material.

Table 3 shows the various results with consideration of the separated particle size range. These results are compared with those for the extraction of coarse bauxite by the simple separation of the fraction < 2 mm, of medium bauxite by the removal of the fraction 2–0.2 mm and of fine bauxite by the removal of the fraction 2 – 0.1 mm.

Table 3. Composition of beneficiated bauxite R O M

B auxite B E N E FIC IAT E D B A U XIT E

global Coarse Bauxite

Medium Bauxite

Fine Bauxite

M.-Recovery (%) 61 .0 70 .0 66 .5 LOI (%) 23 ,6 27 ,5 27 ,2 27 ,5

Al2O3 (%) 43 ,4 51 ,2 50 ,6 51 ,1 TiO2 (%) 1 ,0 0 ,9 1 ,1 1 ,0

Fe2O3 (%) 7 ,9 8 ,4 9 ,3 9 ,0 SiO2 total (%) 23 ,8 11 ,8 11 ,5 11 ,2

Quartz (%) 22 ,8 10 ,8 10 ,6 10 ,3 Al2O3/SiO2 total 1 .8 4 .3 4 .4 4 .6

Separated Fraction: - 2mm 2 – 0,2 mm 2 – 0,1 mm

2.2 Reduction of the reactive silica

Another South American plant operator approached AKW A+V with another target. To adapt the quantities extracted to the increased demand without reducing the remaining life of the established mine areas, the company wanted to mine parts of the deposit that had been ignored in the past owing to an excessive content of reactive silica. For this purpose, a semi-mobile plant was to be designed that can be set up and used comparatively easily in the vicinity of the different mining sites.

The high content of reactive silica of up to 15 wt. % is caused mainly by the kaolinite contained in the rock. Thanks to its small particle size, this can be fully removed by classification after reasonable elutriation. Following a successful test programme conducted at the AKW A+V test centre, this plant was put into operation in 2007. The first results of this plant are satisfactory and in accordance to the previous test work.

The material with a particle size of up to 1,000 mm is fed to a roller grizzly with a variable cut-point. Depending on the composition of the raw material, material in the size range from < 100 to 150 mm is fed to the downstream washing and elutriation drum (WLT). The separated material > 100–150 mm is considered product.

Figure 5. Simplified flowsheet of the process for the beneficiation of HS bauxite with recovery of the fine particles


152 Proceedings of the 8th International Alumina Quality Workshop • 2008

The WLT completely dissolves any kaolinite contained. At the end of the WLT, a strainer basket removes the fraction > 16 mm. The underflow from the strainer basket is sized at around 2 mm on a vibrating screen. The screen overflow is discharged together with the material >16 mm on to a belt conveyor at the side. The screen underflow is collected in a pump sump and fed to a hydrocyclone with a centrifugal pump. Its overflow has a high content of kaolinite, the underflow is dewatered on a dewatering screen and fed to the product belt conveyor. In the plant – installed next to the independent feed unit in three [3] containers – a TAK hydrosizer can be added in the future if the bauxite quality requires this. Some images of the mounted plant on site are shown in Figure 6.

Figure 6. Pictures of a semi-mobile Bauxite Washing Plant build by AKW A+V

3. Bauxite for the Production of Abrasives, Refractories and Ceramics

The bauxite extracted for non-metallurgical applications must meet much higher specifications with regard to its Al2O3 content and remaining accessory minerals than those for metallurgical bauxite.

Commercially available grades for the different applications contain 85–90 % Al2O3 and 1.5–2.5 % Fe2O3 and exhibit a high content of diaspore. For these applications, the lion‘s share of the traded bauxite comes from China, where around 300 bauxite mines are in operation, particularly in the provinces of Shanxi and Guizhou.

In response to the rising prices for Chinese bauxite in recent years, the South American plant operators have become active again and have even re-started some already shut-down operations.

For the bauxite used in metallurgical applications, a study of the beneficiation process for this ”speciality division“ is like looking into the future. For this reason a South American beneficiation plant with gravity separation is described below where typically, the washing of the extracted and comminuted ROM bauxite takes place. After being washed in a washing and elutriation drum

(WLT), the material is sized into various ranges to supply the downstream processes with feed of appropriate size.

Gravity separation processes are suitable for the separation of the iron-rich components from the ROM bauxite. Figure 7 shows the dense media separator in a Brazilian beneficiation plant. The products can be differentiated based on their colour. The darker, reddish fraction contains the iron-rich components removed in the high-gravity product.

Bauxites Parnasse Mining Co. S.A. is the largest bauxite mining enterprise in the European Union. The company operates a beneficiation plant to meet quality demands of their different customers.

The market segments that are supplied, can be classified as follows:

- Alumina Refineries- Aluminous Cement producers- Iron and Steel producers- Portland Cement producers- Abrasive producers.

Among the practical technologies is gravity separation by heavy media separators.

4. Future Trends

It can be stated that among the common mineral processing technologies used for several types of metallic ores, coal ore minerals:

- Crushing and Milling- Screening- Elutriation (Scrubbing)- Cycloning (in combination with Dewatering screens)- Dewatering of fine ground ore (for pipeline transportation).

are partly used for Beneficiation of Bauxite ores and state of the art. Among the Bauxite deposits which are mined today, the Trombetas site in Brazil, the Awaso site of the Ghana Bauxite Company in Ghana, Rawmin Mining and Industries in India and Bauxite Parnasse Mining Co. do have a beneficiatim plant for the ROM Bauxite to improve the quality of the Bauxite ore.

China is also reporting Bauxite Beneficiation Plants, which may use Flotation Technology.

Figure 7. Light product (Bauxite) Heavy Media Separator Heavy product (Fe cont. minerals)



Future trends will probably be:

- Gravity Separation (Jig, Spiral, Hydroseparator)- Magnetic Separation (WHIMS, LIMS).

To get an impression of the potential application of the different processes it is advantageous to consider the applicable particle size range of these processes (Figure 7).

0,1 1,0 10,0

(mm)

0,2 0,5

Separation Process

Jigs

Spirals

Flotation

0,01 0,02 2 50,05

Magnetic Separator(Drum)

WHIMS

Particle Size

0,1 1,0 10,0

(mm)

0,2 0,5

Separation Process

Jigs

Spirals

Flotation

0,01 0,02 2 50,05

Magnetic Separator(Drum)

WHIMS

Particle Size

Figure 7. Granulometric size ranges for different mineral processing technologies

Gravity Separation technologies can be used to lower the Iron content of the ROM Bauxite. Depending on the grain size which has to be processed, the Jig or the Spiral is an appropriate type of equipment (Figure 8).

Jig

Coarse Separation

Spiral

Fine Separation

Jig

Coarse Separation

JigJig

Coarse Separation

Spiral

Fine Separation

SpiralSpiral

Fine Separation

Figure 8. Gravity Separation Equipment

As an alternative technology, Magnetic Separation can also be used to lower the Iron content of the ROM Bauxite. Depending on the grain size and the iron containing mineral which has to be processed, the Drum Separator or the Wet High Intensity Magnetic Separator (WHIMS) is an appropriate type of equipment.

5. Conclusions and/or Recommendations

The processes that have long been used for the beneficiation of non-metallurgical bauxite, e.g. gravity separation, will no doubt be applied in aluminium production in the mid-term. Besides gravity separation, magnetic separation (low- and high intensity magnetic separation) shows promise for potential application in bauxite beneficiation.

Even more technically complex and more expensive processes such as flotation have already been successfully tested for the reduction of silica content. A more extensive beneficiation of the ROM bauxite will lower the costs of the Bayer process and reduce the environmental impact.

Presumably, in response to market developments, more bauxite mines will introduce beneficiation of their ROM material in the mid-term. This applies particularly to deposits in India, Western Australia and China.

References

Two approaches for reducing wasted ‘Red Mud’: Possibility of upgrading Bauxite and the ‘Red Mud’; Owada, S.; Okajima, D., Nakamura, Y., Ito, M., Proceedings of the 7th International Alumina Quality Workshop, Perth 2005.

MINERAL COMMODITY SUMARIES 2007; U.S.G.S. http://minerals.usgs.gov/minerals/pubs/commodity/bauxite/index.html#myb.

METHODEN DER EDV-GESTÜTZTEN, DREIDIMENSIONALEN LAGER-STÄTTENMODELLIERUNG AM BEISPIEL AUSGEWÄHLTER BAUXITVORKOMMEN; Von der Fakultät für Bergbau, Hüttenwesen und Geowissenschaften der Rheinisch-Westfälischen Technischen Hochschule Aachen zur Erlangung des akademischen Grades eines Doktors der Naturwissenschaften genehmigte Dissertation vorgelegt von Diplom-Geologe Uwe Happel, 2001.

CONSTRAINTS ON THE GLOBAL AVAILABILITY OF BAUXITE RESERVES* J. Hausberg, F. M. Meyer, U. Happel Institute of Mineralogy and Economic Geology University of Technology Aachen, Germany M. Mistry Institute of Mining Engineering I University of Technology Aachen, Germany; Applied Mineralogy. Rammlmair et al., (eds), 2000 Balkema, Rotterdam, pp. 341 - 344.

BENEFICIATION OF HIGH QUARTZ CONTENT BAUXITE FROM LOS PIJIGUAOS - Jean-Marc Rousseaux, Hans Verschuur, Pedro Flores, Stephan Buntenbach, Fred Donhauser ; Light Metals 2006 Edited by A.T. Tabereaux TMS (The Minerals, Metals & Materials Society), 2006.

MINERAL PROCESSING TECHNOLOGIES IN THE BAUXITE & ALUMINA INDUSTRY – Dr. S. Buntenbach, F. Donhauser, G. Basu, S. Chatterjee; MBD 2006, Beneficiation And Value Added Mineral Products, November 27-28, 2006, Nagpur, pp. 43 – 60.

STUDY ON IMPROVED SILICATES REMOVAL TECHNOLOGY IN DIRECT BAUXITE FLOTATION – X. Cheng, A. Ren, G. Zheng, XXIII International Mineral Processing Congress 2006; Istanbul, Turkey, Volume 1, pp 719 - 722.

1.

2.

3.

4.

5.

6.

7.


Documents

MINERAL PROCESSING TECHNOLOGIES IN THE BAUXITE …€¦ · Proceedings of the 8th International Alumina Quality Workshop • 2008 149 MINERAL PROCESSING TECHNOLOGIES IN THE BAUXITE