The influence of sodium carbonate on sodium aluminosilicate crystallisation and solubility in sodium aluminate solutions

Isothermal batch precipitation experiments have been carried out in synthetic Bayer liquors to investigate the effects of sodium carbonate concentration on both silica solubility and the crystallisation of sodium aluminosilicates. At both 90 and 160°C cancrinite (generically defined as a sodium aluminosilicate of space group P6 3) is the stable solid phase. Sodalite (generically defined as a sodium aluminosilicate with space group P4̄3n seed transforms to cancrinite at both these temperatures. A high concentration of sodium carbonate in the synthetic liquor causes a decrease in the rate of conversion of sodalite to cancrinite. The solubility of both cancrinite and sodalite decreases as the concentration of sodium carbonate in the synthetic liquor is increased. For instance at 90°C and with 40.0 g dm −3 sodium carbonate in the synthetic liquor after 13 days the sodium aluminosilicate concentration is 0.52 g dm −3 compared to 0.85 g dm −3 with 4.6 g dm −3 of sodium carbonate in solution. At 160°C the sodium aluminosilicate concentration is 0.47 g dm −3 with 40.0 g dm −3 sodium carbonate in solution after 13 days and 0.79 g dm −3 with 4.6 g dm −3 sodium carbonate in solution. Throughout all these experiments a progressive loss of carbonate from the sodium aluminosilicate crystallisation products was observed as a function of time.