Desilication of hematite, goethite and iron powder seeded low alumina to caustic liquors

Abstract

Even though many studies have been carried out aimed at the development of methodologies for the inhibition of scale formation during the Bayer process there is still limited understanding of the underlying desilication processes and kinetics, in particular for the high solids density environment of single-stream heat exchangers. Our objective was to gain insight into the relative desilication due to nucleation and/or growth, and the kinetics of scale formation in the presence of typical solid phases. We have chosen a caustic aluminosilicate liquor composition in which both aluminate concentration and A/C are less than for heat exchanger feed liquor and hence aluminosilicate supersaturation is increased. These concentration conditions were chosen to investigate the early stage of scale formation in single-stream Bayer plant heat exchangers by increasing the component of aluminosilicate bulk precipitate upon seeding and decreasing the relative degree of scaling. The precipitation of aluminosilicate phases from this liquor at 140 and 180°C seeded with hematite, goethite and iron powder crystals was investigated. Upon seeding rapid desilication took place with significant suppression of scale formation on steel coupon surfaces as compared to unseeded experiments. Seeding with hematite resulted in the most rapid reduction in aluminosilicate supersaturation. Additionally, the presence of hematite, in contrast to the other seed types, appears to promote the transformation of sodalite to cancrinite. Precipitation was found to occur predominantly via heterogeneous nucleation during the 240min reaction time. The results of this study suggest that a seeding strategy, especially using hematite, may be effective in reducing scale formation on mild steel process equipment in a single-stream Bayer heat exchanger process environment.