SOLUBILITY OF ALUMINOSILICATES IN ALKALINE-SOLUTIONS AND A THERMODYNAMIC-EQUILIBRIUM MODEL

The solubility of aluminum and silicon in aqueous, alkaline solutions was investigated at 95-degrees-C for solutions containing 0.1-4 mol/L NaOH and NaCl to ionic strengths of 4. The apparent solubility product, [Al][Si], increased with increasing [OH-] and decreasing ionic strength. It remained constant over a wide range (0.076-270) of [Al]/[Si] ratios at equilibrium. In this range, the solid phase consisted of sodalite or, at higher [Al]/[Si] ratios, of sodalite and Al(OH)3. At lower equilibrium [Al]/[Si] ratios, both sodalite and an amorphous product which contained higher ratios of Si/Al were found. An analysis of the chemical characteristics of aluminum and silicon in alkaline solutions in the solution composition range of interest showed that Al(OH)4- and HSiO43- are the predominant aluminum and silicon species, respectively. In addition, precipitates chemically similar to sodalite (hydroxysodalite, cancrinite) were reported by others when the anion matrix differed from that in this study but other conditions were similar. The solubility of aluminosilicates does not appear to be affected greatly by the composition of the anion matrix. A chemical model, employing Pitzer's activity coefficient equations, was developed to describe the equilibrium of sodalite over the range of composition of interest. Values for the apparent solubility product at 95-degrees-C and interaction parameters for the aluminum and silicon species were obtained by fitting the model to experimental solubility data. The predictions from the model fell well within the scatter of the experimental data. A concise graphical method was developed for estimating the apparent solubility product from gross solution composition. It gives reasonable predictions for solutions which contain other anions as well.