QUANTIFICATION AND CHARACTERISATION OF HYDROGARNET AND CANCRlNITE PRESENT IN DESILICATION PRODUCT (DSP) BY POWDER X-RAY DIFFRACTION

Kaolin present in bauxite reacts with Bayer liquor to a sodium silicon aluminium hydrate of the general chemical composition 3(Na₂0.Al₂0₃.2Si0₂).2NaOH.(0-2)H₂0. Since this is discarded with the red mud, its formation results in a loss of soda from the refinery. Addition of lime during desilication can reduce the soda losses in the red mud by forming hydrogarnet (HG; "Ca₃Al(SiO₄)_n(OH)_(12-4n)") or, at high temperatures, a calcium-containing cancrinite (Ca-CAN). However, the observed Na₂O/SiO₂ ratio, lime efficiency and alumina efficiency depends upon both the quantity and the composition of the HG and CAN present in the desilication product (DSP). This paper discusses methods for the characterisation and quantification of HG and CAN present in DSP samples. In particular, we examine the non-Rietveld analysis of powder X-ray diffraction data for the determination of the HG silica incorporation "n" or the degree of substitution of calcium ions for sodium ions in CAN. The application of this method to DSP prepared from bauxite or synthetic reaction mixtures illustrates how we can gain an understanding of the reactions of tricalcium aluminate (TCA) or HG occurring during pre-desilication and digestion.