The composition of Desilication Product (DSP) has been determined as a function of liquor composition from which it was made, under the conditions of predesilication and high temperature digestion characteristic of the Queensland Alumina (QAL) refinery. DSPs were formed in synthetic liquor by reacting kaolin with and gibbsite. Langmuir type models have been developed to predict the inclusion of both sulphate and carbonate into DSP, whereas chloride and alumina contents are described well by regression models. The soda content of DSP is related to lime charge, but is unrelated to liquor composition. DSP formed under predesilication conditions was predominantly hydroxysodalite with low silica TCAS (hydrogarnet). After subsequent digestion, the TCAS contents were reduced, and a particular form of carbonate containing sodium calcium cancrinite was identified. Semi-quantitative estimates of the phase composition indicated that under conditions of medium to high lime charge and high carbonate in solution, DSP is dominated by this cancrinite. When this occurs, carbonate removal is boosted at the expense of sulphate and chloride. An estimate has also been made of the soda savings associated with lime addition. A soda saving of 3.8 percent for each 1 percent CaO on bauxite is predicted which is in good agreement with laboratory tests with refinery liquor and bauxite. Measurements of soda savings from plant trials were substantially below this predicted value. The reasons for these differences are unknown but are suspected to originate from differences in the formation conditions in the refinery.
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