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ADVANCED TECHNIQUES FOR REMOVING ULTRAFINE SOLIDS FROM ALUMINATE PROCESS LIQUOR

Rogers, M.G. Jauhiainen, T.

Dissolution of alumina from bauxite ore generates a liquor stream containing soluble aluminium salts and an insoluble residue of suspended solids having a broad particle size distribution. Removal of the suspended solids, typically oxides of iron, silica and titanium, to less than 10 ppm in the process liquor is essential for production of high-grade alumina and subsequently high-grade aluminium metal.

Current alumina refineries try to minimize capital and operating costs by maximizing liquor throughput using existing clarification and filtration equipment. The use of very active flocculants, with faster solids settling rates has led to smaller diameter high rate thickeners. The effect has been a shorter residence time in the thickener allowing unconsumed flocculant to pass to security filtration. This surplus flocculant binds a thin layer of filter cake to the filter media. After cake discharge, this layer stays attached to the media, restricting the filtration rate on the next and subsequent cycles.

Reclaiming and reusing fouled filter media can minimize the frequency of media replacement. The result is lower maintenance cost and increased online productive time. Keeping the filter media clean, effective removal of deposited filter cake at the end of each cycle, alternative filter aids, improved process operation and modified process equipment have made it possible to produce liquor containing less than 2 ppm of residue.