THE KINETICS OF DESILICATION OF SYNTHETIC BAYER SPENT LIQUOR SEEDED WITH SYNTHETIC SODALITE AND CANCRINITE

Sodium aluminosilicate scaling occurs in Bayer refineries, pulp and paper industries and high level nuclear waste plants due to the presence of silica-rich caustic aluminate liquors. Numerous studies have been undertaken to investigate the mechanisms of scale formation and the development of scale inhibition strategies. However, given the paucity of relevant studies on single stream Bayer plants, the objective of this study was to understand the mechanisms and kinetics of sodium aluminosilicate crystallisation from spent liquors under single stream pressure and temperature conditions relating to digester heat exchangers.

Solution desilication tests were performed using synthetic spent Bayer liquor at 140 °C, with and without addition of sodalite or cancrinite seeding (seed loading 1.125 g L^-1), in the presence of steel substrates. The solution desilication rate showed significant increase on addition of seeds and was found to be greater in the presence of cancrinite seeds (0.52 mM Si m^-2) than sodalite (0.21 mM Si m^-2). The desilication kinetics of the synthetic spent Bayer liquor suggest only growth in the presence of sodalite seed and both secondary-nucleation and growth in the presence of cancrinite seed. A first amorphous layer on the coupon surface, containing high Al and low Si concentrations, was observed. In addition zeolite was found on the coupons from both unseeded and seeded desilication experiments.