Abstract
Organic compounds are known to interfere with the precipitation of trihydroxy aluminium (gibbsite) in the Bayer process. Studies have been carried out of the effect of 3,4-dihydroxybenzoic acid on precipitation and of the beneficial effect that can be achieved by oxidising the acid. Measurements of precipitate yields both in the presence of the acid and of products obtained by oxidising the acid were made. Oxidation in the presence of various metal ions was also examined. The yield of precipitate was found to decrease in the order Cu2+ (1·9 mM) > Fe3+ (2·2 mM) > blank (no added organics) > Mn4+ (44·3 and 2·3 mM) > Mn2+ (2·3 mM) > Mn2+ (44·3 mM) ≈ uncatalyset oxidation > Fe3+ (43·5 mM) > Cu2+ (38·2 mM) > untreated sample. This relative ordering was generally unaffected by reaction temperature or oxygen partial pressure. Soluble manganese salts were formed by partial dissolution of MnO2 in the alkaline solutions. These salts were required for oxidation of the 3,4-dihydroxybenzoic acid to products which, collectively, did not poison the trihydroxy aluminium precipitation process.
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