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 exp ressed as beta = C/C0, where C0 is the solubility at given temperature and caustic soda concentration, then the growth rates can be describe d by the model G(mum/h) = k0(beta - beta(c))2 exp(-DELTAG/RT), where k 0 = 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 nor mally is 1 in pure systems, arises from the fact that there are dead z ones 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 describe s pretty well the results of previous works where the kinetic coeffici ents of the growth rate equations are dependent not only on temperatur e but also on caustic soda concentration.