The coprecipitation of calcium aluminium hydroxide (calcium hydroxoaluminate hydrate) powders from aqueous solution with sodium hydroxide: Precipitate compositions and percipitation mechanisms

Calcium aluminium hydroxides were coprecipitated from different mixed metal cation solutions — at total C M = 0.1 M and Ca/Al 2 ratios from 1 to 4 — with sodium hydroxide solution at ambient temperature. The coprecipitations were monitored by potentiometric ( p H) titration and the final coprecipitate compositions were examined by chemical analysis, infra‐red spectrophotometry and thermal analysis Generally, microcrystalline aluminium hydroxide was first precipitated at p H about 4; this then redissolved on further addition of sodium hydroxide to form hydroxoaluminate anion and polyanion and calcium aluminium hydroxide coprecipitates were formed continuously at p Hs from about 9 to above 12. Their compositions were similar to the calcium hydroxoaluminate hydrates formed by direct precipitation from high p H sodium hydroxoaluminate solutions. At Ca/Al 2 ratio = 1, the main phase was probably Ca 2 (H 2 O) h [Al 2 (OH) 4 ] 2 with some Al(OH) 3 ; At Ca/Al 2 ratio = 2, the main phase was probably Ca 2 (H 2 O) h [Al 2 (OH) 10 ] dehydrating to Ca 2 [Al 2 O(OH) 8 ]; At Ca/Al 2 ratios = 3–4, the main phase was Ca 2 (H 2 O) h [Al 2 (OH) 10 ] with increasing amounts of Ca 4 (H 2 O) h (OH) 4 [Al 2 (OH) 10 ] and 5–10 percent adsorbed or post‐precipitated Ca(OH) 2 .