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Proceedings of an International Conference on Mitigation of Heat Exchanger Fouling and Its Economic and Environmental Implications

ISSN:
1-56700-172-6 (Print)

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

J. Addai-Mensah
Ian Wark Research Institute, University of South Australia, Mawson Lakes, Adelaide. 5095 Australia

M.C. Barnes
Ian Wark Research Institute, University of South Australia, Mawson Lakes, Adelaide. 5095 Australia

R. Jones
Ian Wark Research Institute, University of South Australia, Mawson Lakes, Adelaide. 5095 Australia

A. R. Gerson
Ian Wark Research Institute, University of South Australia, Mawson Lakes, Adelaide. 5095 Australia

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.