Precipitation of desilication products in CaO-Na2O-Al2O3-SiO2-H2O system based on the Bayer process

The formation and evolution of desilication products (DSPs) in synthetic sodium aluminate solution with different reaction temperatures, holding time, CaO additions and carbonate concentrations were investigated based on the Bayer process. The DSPs precipitated at 145 degrees C and 195 degrees C are mainly composed of hydrogarnet, tricalcium aluminate hexahydrate and zeolite, and the precipitation sequence is: tricalcium aluminate hexahydrate -> hydrogarnet -> zeolite. As the desilication temperature increases to 245 degrees C for 1 h, the ZEO disappears and the HG content reaches 97.8% when the mass ratio of added CaO to SiO2 in solution is 3:1. The SiO2 saturation coefficient of hydrogarnet increases from 0.79 to 1.23 when the desilication temperature rises from 145 degrees C to 245 degrees C for 1 h. The crystallinity of various DSPs is enhanced by the reaction temperature, time and CaO addition. The presence of carbonate in sodium aluminate solution promotes the formation of calcium-cancrinite, inhibits the precipitation of hydrogarnet, and decreases the crystallinity and SiO2 saturation coefficients of DSPs. The lattice constants of DSPs increase with the increasing desilication temperature and carbonate concentration.