Bauxite is a mixture of the minerals gibbsite (Al2O3 - 3H2O) boehmite (alpha monohydrate, Al2O3 H2O) and Diaspora (beta monohydrate, Al2O3 H2O). It is a unique mineral in the sense that, by and large, this is the only mineral from which aluminum is extracted economically. Beside aluminum its other important uses are in the manufacture of refractories, abrasive and chemicals. Low-grade bauxite finds use in cement industry. Among minor uses mention may be made of its use as a fluxing material in steel melting shop and in forro-alloy industries in place of fluorspar and as absorbent in the refining of kerosene. Metallurgical use: Mineralogical character of bauxite has a bearing on its digestion in caustic soda. If monohydrate, high temperature and higher concentration of soda is necessary while the trihydrate variety is digested at low temperature and lower concentrates. The important chemical impurities are reactive silica, titania and phosphorus penta-oxide. The reactive silica causes the loss of alumina and caustic soda, while titania and phosphorus penta-oxide bring about loss of soda. The reactive silica is the most harmful impurity. It forms insoluble double aluminum, sodium silicate which goes in the red mud resulting in the loss of alumina and caustic soda, as one molecule of silica will ties up one molecule of soda and one molecule alumina to form the double silicate. The Bureau of Indian Standards (BIS) has prescribed the following specification of bauxite for metallurgical uses.
Note: For extraction of A12O3 by lime sinter process or by a combination of Bayers and lime-sinter process, low-grade ore containing up to 15% SiO2 can be used. Future Trends: The mineralogical composition is no longer a handicap due to the adoption of high-pressure system in most of the plants in the country. There is also a trend to use even low alumina bauxite but with low content of reactive silica, For example, NALCO has reported that they would be using in their plant bauxite containing 42% A12O3 and less than 4% reactive silica. A. In the manufacture of refractory, the diaspora variety is preferred although gibbsite or a mixture of monohydrates can also be used. Fe2O3 and TiO2 are the most deleterious constituents. Besides A12O3 should be as high as possible. Excess of iron causes deformation and warping in the refractory products. Similarly, alkalies and lime cause fusion at low temperature. The Sub-Committee on refractory raw materials of the DGTD in their report dated August 1985 has stipulated the following five region-wise specifications of bauxite for the refractory industry depending upon the desired quality of dead-brunt product.
Future Trends: At present the user industries generally prefer bauxite having 55% (min.) A12O3 and 3% (max.) TiO2 each. The above-mentioned specifications of the DGTD indicate relaxation in the tolerance limit of Fe2O3 and TiO2. The trend seems to be to use lower grade materials than what has so far been used. B. Abrasive: When bauxite is fused in an electric furnace, synthetic corundum is formed which is used in making of grinding wheels, rubbing bricks, polishing powers, etc. In the manufacture of abrasives, silica in bauxite is the most undesirable constituent. Excess of silica causes high consumption of power and carbon and also causes erratic furnace operations and power control of the chemical reactions involved. Silica also forms ferro-silicon alloy with iron, which is non-magnetic and cannot be separated magnetically from the product. There is no BIS specification for abrasive industry. The user industries prefer A12O3 50% (min.), SiO2 4% (max.), Fe2O3 10% (max.) and TiO2 3% (max.). C. Chemical and Petroleum Industries: For use in chemical industry bauxite should have high alumina and low iron content. Iron causes setting difficulties and imparts coloring effects. The BIS (IS: 305 1984) has prescribed specifications for chemical and petroleum industries according to which bauxite should have A12O3 58% (min.), Fe2O3 2% (max.), SiO2 3% (max.), TiO2 4% (max.), P2O5 2.5% (max.), MnO 0.1% (max.), CaO 2.0% (max.), LOI 32% (max.). D. Should have A12O3 40% SiO2 5% and Fe2O3 30 40% bauxite for use in cement industry. In the steel industry metallurgical grade bauxite is used as a flux as partial replacement of fluorspar. Consumption has varied from a high of about 27,500 tonnes in 1985. The reserves of bauxite have been categorized as metallurgical, chemicals, refractory and abrasive and low-grade. The metallurgical grade has been further divided into the following: (a) Metallurgical - I A12O3 + 50% SiO2 up to 5% (b) Metallurgical II A12O3 48 - 50% SiO2 up to 5% (c) Metallurgical - III A12O3 44 - 48% SiO2 up to 5% (d) Metallurgical mixed A12O3 44 - 50% SiO2 up to 5% The categorization of reserves into metallurgical I, II, & III is based on the BIS specifications in so far as alumina and silica percentages are concerned. The BIS has also prescribed the limit of other chemical constituents such as P2O5, V2O5, Fe2O3 and TiO2, but these have not been considered due to the lack of information in this regard. The classification into chemical grade containing A12O3 53% (min.) and refractory abrasive grade containing A12O3 55% (min.) is based on a single chemical constituent, i.e., alumina. The other chemical constituents in these cases have also not been taken in to account. There is a low-grade category analyzing A12O3 (-) 44% and SiO2 (+) 5%.
The BSI has prescribed specification for bauxite for chemical and petroleum industries ,(IS: 3605 1984):
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