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EFFECT OF BAUXITE AND DIGESTION CONDITIONS ON IRON IN SGA

Roach, G. and Jamieson, E.

There are many bauxites which, when processed, result in very high iron in solution and consequently product alumina. It is well known that low iron bauxites are more likely to produce high iron in liquor, but that is not always the case. The solubility of iron in liquor from bauxites was studied at varying temperatures and free caustic concentrations. The solubility of iron for iron oxides increases with temperature and free caustic concentration.

Bauxites can contain high levels of a soluble iron species that appears to be present as iron substituted in the gibbsite. On dissolution of that gibbsite the iron is released and, if there is insufficient iron oxide seed present, a high iron in liquor results. The high level of iron within such gibbsites is postulated to occur if there is insufficient iron in the laterisation process for it to precipitate as a separate phase. A soluble iron test has been developed to assist in predicting and controlling iron in product for such bauxites.