Precipitation from alkaline aqueous aluminosilicate solutions

The longevity of metastable, highly alkaline Na and K aluminosilicate solutions has been examined over a wide range of Si/Al concentration ratios R (0.04 R 20) at 24°C and, in selected cases, at 50 and 75°C. In general, the longest solution lifetimes (tppt) (several weeks) were found at 24°C and for the highest alkalinities and (or) large R, but long tppt were also found for R = 1.0 at the lower alkalinities. Solution lifetimes generally shortened when the temperature was raised, but the opposite effect was observed with the smaller Al concentrations when 2 R 6. These results are interpreted in terms of the requirement for =Si-OH groups for precipitate growth by condensation and of the relatively high solubility (few nuclei for gel growth) of the aluminosilicate solutes when R = 1.0.Key Words: aluminosilicates, precipitation kinetics, aluminates, aqueous silicates, gelation, Bayer process, zeolite synthesis.