THE MECHANISM AND KINETICS OF SODIUM ALUMINOSILICATE SCALE FORMATION IN BAYER PLANT HEAT EXCHANGERS

Abstract

The mechanisms and kinetics of precipitation of heat exchanger scale-forming sodium aluminosilicates (sodalite and cancrinite crystals) investigated under a range of conditions reflecting the Bayer alumina-refining process are reported. A systematic, 6-year study shows that, over the temperature range 90 - 240 °C, steel substrate fouling by heated, optically-clear, synthetic, spent Bayer liquors occurs by the formation of the more soluble sodalite scale, via substrate-mediated heterogeneous nucleation. Sodalite subsequently undergoes a dimorphic transformation to the less soluble cancrinite phase at a rate which is first order (with respect to the sodalite concentration). The activation energy for heterogeneous nucleation of sodalite is 95 kJ mol^−1. The transformation mechanism is solution-mediated, involving sodalite dissolution and subsequent nucleation and growth of cancrinite, the activation energy of which is 133 kJ mol^−1. Sodalite and cancrinite seeding significantly enhanced liquor desilication, suppressing the scale formation process. The activation energies for sodalite and cancrinite crystal growth are 30 kJ mol^−1 and 80 kJ mol^−1 respectively. The kinetics of growth depended upon Si0₂ relative supersaturation to the power of 2 and 3 for sodalite and cancrinite, respectively. Agitation rate, solution flow velocity and the nature of steel substrate surface roughness had no noticeable effect on scale deposition rate. Data and correlations for predicting dimorphic phase-dependent, equilibrium Si0₂ solubility in spent Bayer liquor and scale-related desilication/crystal growth rates are presented with a discussion on scale mitigation strategy.