ABOUT SUPERSATURATION AND GROWTH-RATES OF HYDRARGILLITE AL(OH)3 IN ALUMINA CAUSTIC SOLUTIONS

Growth rates of hydrargillite crystals, Al(OH)3, growing from concentr ated caustic solutions, are traditionally plotted and discussed as a f unction of the difference between actual concentration and solubility of alumina. This way to express supersaturation is probably due to pra ctical or technical reasons, as hydrargillite is mainly grown in indus trial plants. However, as the solubility of hydrargillite is greatly a ffected by the presence of caustic soda there are as many growth rate curves as there are solutions at different soda concentrations, if sup ersaturation is expressed as a concentration difference. In the presen t paper we show that all growth rates, measured in different caustic s olutions, lie on a single curve if supersaturation is normalized with respect to solubility, i.e. expressed as a ratio of actual concentrati on over solubility. Accordingly, growth rates become independent of th e caustic concentrations when growth takes place at the same supersatu ration.