PLANT IMPURITY BALANCES AND IMPURITY INCLUSION IN DSP

The effect of liquor composition on the incorporation of sulphate, carbonate and chloride in desilication product (DSP) has been determined in a comprehensive series of laboratory tests. The DSPs were produced by reacting kaolinite and gibbsite in a wide range of synthetic liquors at temperatures and holding times corresponding to both Bayer predesilication and low temperature digestion conditions. The methodology used and the principal outcomes are described. The impurity incorporation depended on the desilication conditions; the DSPs produced under predesilication conditions contained less sulphate and aluminate but more chloride than those produced from the same liquors under digestion conditions. However the Na₂O:SiO₂ ratios for the digestion DSPs corresponded to full occupancy of the sodalite cages; the ratios for the predesilication DSPs were slightly lower. Sulphate incorporation was best described by a Langmuir type relationship with respect to sulphate concentration and with negative contributions from carbonate and chloride concentrations. Carbonate and chloride inclusions were better fitted by regression models involving linear dependence on the predicted sulphate content. Aluminate incorporation was insensitive to variations in alumina content of the liquors within either the set of predesilication or the set of digestion DSPs, but the average incorporation for the predesilication DSPs was slightly below that for the digestion DSPs. Fluoride and hydroxide levels in the DSPs were below their quantitation limits. The relationships obtained have a sound scientific basis and are now being used in predictive models for plant impurity concentrations.