GROWTH-KINETICS OF HYDRARGILLITE AL(OH)3 FROM CAUSTIC SODA SOLUTIONS

The growth rates of hydrargillite (alumina trihydroxide) from caustic alumina solutions were studied in a semi-continuous crystallizer as a function of temperature and supersaturation. If supersaturation is expressed as beta = C/C0, where C0 is the solubility at given temperature and caustic soda concentration, then the growth rates can be described by the model G(mum/h) = k0(beta - beta(c))2 exp(-DELTAG/RT), where k0 = 1.92 x 10(19) mum/h is independent of temperature and caustic soda concentration, and DELTAG = 120.7 kJ/mol. The term beta(c), which normally is 1 in pure systems, arises from the fact that there are dead zones at low supersaturation where growth is inhibited, probably by Fe ions. Accordingly, beta(c) is a critical supersaturation which must be exceeded for growing the crystals. The model we propose also describes pretty well the results of previous works where the kinetic coefficients of the growth rate equations are dependent not only on temperature but also on caustic soda concentration.