Desilication of low alumina to caustic liquor seeded with sodalite or cancrinite

Abstract

The aim of this study was to improve understanding of scale formation mechanisms and to provide theoretical support for development of scale inhibition strategies. Sodium aluminosilicate scaling on plant surfaces is a critical problem in Bayer refineries, particularly within digesters and heat exchangers. Although numerous studies have been carried out to understand the mechanisms of scale formation and to develop scale inhibition strategies in double-stream circuit Bayer plants, there is still insufficient relevant research on single-stream Bayer plants. We have studied the desilication behavior of a liquor of similar composition to spent cleaning caustic liquor, for which the aluminate concentration and A/C are both less than heat exchanger liquor. These concentration conditions were chosen in order to investigate the early stage of scale formation in single-stream Bayer plants by increasing the component of aluminosilicate bulk precipitate upon seeding and decreasing the relative degree of scaling.Solution desilication experiments were performed using spent cleaning caustic liquor at 140°C either without seeding or with sodalite (seed loading 1.125g L−1) or cancrinite seeding (seed loadings 1.125gL−1 and 2.850gL−1) in the presence of steel substrates. The solution desilication rate showed significant increase in the presence of seeds and was found to be greater in the presence of cancrinite seeds than sodalite of the same surface area. The desilication kinetics of the synthetic spent cleaning caustic liquor suggests predominant heterogeneous nucleation for unseeded desilication, a mixed secondary-nucleation and growth process for desilication on addition of sodalite seeding, and cancrinite seeding with both smaller and greater loading.A first layer, possibly amorphous, with high Al and low Si concentrations was observed by SEM and FIB/SEM on the coupon surface after desilication with or without seeds. We note that this layers forms despite the low A/C and high Si supersaturation suggesting that under single-stream heat exchanger conditions this may also be the first layer to form. XRD confirmed that sodalite formed on the coupons in unseeded and sodalite seeded desilication systems while cancrinite formed on the coupons upon cancrinite seeded desilication with both smaller and greater seed loading. The coupon from the cancrinite seeded liquor with high seed loading is least covered by scale amongst the coupons, suggesting that cancrinite seeding may be a potential strategy for reducing scale formation in single-stream Bayer circuits.