THE MECHANISM OF SODIUM ALUMINOSILICATE SCALE FORMATION IN ALUMINA REFINERIES

The formation of sodium aluminosilicate scale in heat exchangers is a costly by-product of the Bayer process and can result in reduced heat-exchanger efficiency, reduction of flow and excess caustic consumption. Three aspects of the crystallisation of sodium alununosilicates have been examined:

(I) Mechanism of sodium alminosilicate formation. X-ray diffraction and Fourier transform infra-red spectroscopy have shown that sodalite is always the first phase to crystallise in the temperature range studied, 90^oC to 160^oC. The initial sodalite phase (sodalite) contains a high concentration of carbonate. As carbonate diffuses out of the sodalite lattice, it shrinks to give a distinct second sodalite phase (sodalite2) which then transforms to cancrinite.

(II) Sodium aluminosilicate phases found in plant scale. Plant sodium aluminosilicate scale has been analysed as a function of conditions of formation and age. The mechanism of formation has been shown to be the same as that observed in the laboratory. Depth profiling of plant scale samples has shown that the conversion of sodalite2 to cancrinite is the rate determining step in cancrinite formation.

(III) Solution nucleation versus substrate nucleation. In batch precipitation conditions the rate of desilication of was found to increase with increasing Na₂CO₃, increasing temperature and increasing initial SiO₂ content. A high substrate surface area appears to promote scale nucleation/growth.