Desilication of sodium aluminate solution by Friedel's salt (FS: 3CaO centre dot Al_2O_3 centre dot CaCl_2 centre dot 10H_2O)

摘要

Controlling silica level in Bayer liquor is critical in order to prevent scaling or alumina quality issues. The conventional method of removing silicates from aluminate solutions requires the introduction of calcium oxide or calcium hydroxide. In this work, Friedel's salt (FS: 3CaO centre dot Al_2O_3 centre dot CaCl_2 centre dot 10H_2O) is proved for the first time to remove up to 95 percent silica from sodium aluminate solution. FS is a mineral anion exchanger belonging to the layered double hydroxides (LDHs) which was prepared by adding calcium chloride to sodium aluminate over the temperature range of 50-90 deg C. It was characterized by XRD, SEM, Particle Size Analyzer and TG-DSC. FS prepared at 50 deg C has a relatively high desilication capacity, better than calcium oxide. Experimental parameters affecting the desilication process, such as temperature, sodium aluminate liquor composition, initial silica concentration (4-10 g/L) and FS dosage were investigated in detail and a comparison of desilication between FS and CaO was carried out. The desilication products (DSP) were mainly calcium aluminium silicates, identified by XRD to be Chabazite and Wadalite and the final chloride concentration in the sodium aluminate solution after anion exchange with FS was approx 0.015 g/L. The rate of desilication by FS was first order with a rate constant 2.582 X 10~6 min~(-1). The apparent activation energy was estimated to be 57.7 kJ mol~(-1) over the temperature range of 80-110 deg C.