The kinetics of desilication of synthetic spent Bayer liquor seeded with cancrinite and cancrinite/sodalite mixed-phase crystals

Isothermal, batch desilication kinetics of synthetic, sodium aluminate solution (spent Bayer liquor) via cancrinite and cancrinite/sodalite mixed-phase crystal growth, have been studied under conditions at which sodium aluminosilicate scale forms at the surfaces of steel heat exchangers of alumina plant. Seeding with the pure cancrinite and mixed-phase crystals results in the suppression of scale formation and a faster rate of liquor desilication in comparison with its sodalite dimorph. Cancrinite seed crystals prepared from NO3--rich solutions exhibited crystal growth mechanism and kinetic behaviour different from dimorphic mixed-phase crystals prepared from CO32--rich solutions, when both were used to desilicate CO32--rich spent Bayer liquor. The rate of desilication due to crystal growth on CO32- -cancrinite/sodalite mixed phase crystals followed a second-order dependence on the relative supersaturation of SiO2. An activation energy of 52 kJ mol(-1) was estimated for the crystal growth process. For desilication kinetics involving NO3--cancrinite seed crystal growth, a third-order dependence on relative supersaturation of SiO2 and an activation energy of 63 kJ mol(-1) were obtained. (C) 1999 Elsevier Science B.V. All rights reserved.