Properties of carbonate-containing aluminum hydroxide produced by precipitation at constant pH.

Carbonate-containing aluminum hydroxide was precipitated at constant pH at intervals of 0.5 pH units from pH 6 to 10 by pumping 0.5 M AlCl3 into the reaction vessel at a constant rate of 2 mL/min and infusing 2 M Na2CO3 at a rate necessary to maintain the desired pH. The pH of precipitation affected both the particle size and the composition of the precipitate. The particle size of the precipitate decreased as the pH of precipitation was increased from 6 to 10. The precipitates formed between pH 7.5 and 9.5 contained crystalline sodium aluminum hydroxycarbonate (dawsonite) in addition to amorphous carbonate-containing aluminum hydroxide.