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EFFECT OF HUMATE ON THE BATCH AND CONTINUOUS CRYSTALLIZATION OF ALUMINIUM HYDROXIDE

Chan, V.A. and Ang, H.M.

Due to the requirement of high purity crystalline products, an understanding of the effects of impurities on the crystallization process is necessary. Trace amounts of impurities present in a system have long been recognised for their ability to induce striking changes in all kinds of crystallization characteristics such as crystal morphology and growth rates, nucleation rates and tendencies to agglomerate. The effect of impurities on crystallization kinetics is very unpredictable and cannot be explained by any general rule. In common with the majority of industrial crystallization processes, the green liquor which aluminium hydroxide crystallizes in the Bayer process is by no means pure, but contains many impurities. The presence of impurities in the alumina industry (inorganic and organic substances from the bauxite and Na2CO3 through atmospheric uptake) is known to adversely affect the Bayer precipitation process. From batch experiments investigating the effect of various common impurities on the kinetics of crystallization, sodium humate was identified to have the most significant effect on the aluminium hydroxide crystallization. A continuous laboratory crystallizer was developed, enabling crystallization studies under steady state conditions. Subsequent crystallization runs using a continuous laboratory crystallization setup were conducted to study the effect of humate on crystallization kinetics: nucleation rate, mass and linear growth rates and yield. The results of both batch and continuous experiments showed that humate impurity adversely affected the kinetics of aluminium hydroxide crystallization.