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UNDERSTANDING GROWTH OF DSP IN THE PRESENCE OF INORGANIC IONS

Smith, P., Lowe, J.L., Rohl, A.L., Pennifold, R. and Parkinson, G.M.

The effect of inorganic salt addition on the precipitation of desilication product (DSP) from kaolin-containing synthetic spent Bayer liquors at 95˚C has been examined. Experiments were conducted to examine DSP formation in the presence of added sodium sulfate (Na2SO4), sodium carbonate (Na2CO3), sodium hydroxide (NaOH), sodium chloride (NaCl), potassium hydroxide (KOH) and potassium sulfate (K2SO4). The anions examined were found to influence the onset of precipitation, but did not influence the rate of precipitation, with the exception of hydroxide ions. Substitution of the sodium cation with potassium was found to delay onset of precipitation. The observed onset effects are thought likely to be related to the ability of individual ions to template the DSP, and/or to differences in the solubility of sodalites that contain different ions. It is proposed that the DSP precipitation rate is limited by the dissolution of kaolin. Dissolution was unaffected by the presence of anions except hydroxide, which elevated the dissolution rate, and also the DSP formation rate. Substitution of added NaOH in the liquor with KOH, resulted in a slowed rate of kaolin dissolution and DSP formation, suggesting that the sodium ions may be important for the elevation in the rates of these processes observed from the NaOH liquor. Sulfate was found to be the dominant inclusion in the DSP in competitive uptake experiments, and computer modelling results suggest that sulfatic DSP is thermodynamically more stable than DSP containing carbonate anions.