The coprecipitation of magnesium aluminium hydroxocarbonate powders from aqueous solution: Precipitate compositions and coprecipitation mechanisms

Magnesium aluminium hydroxocarbonate hydrates were coprecipitated from mixed metal nitrate solutions, at total C M = 0.2 M and Mg/Al 2 = 1 ratio, with four sodium hydrogen carbonate‐sodium carbonate solutions (of p H 8.1 to 11.5) at ambient temperature. The course of precipitation was monitored by potentiometric ( p H) titration, and the compositions of the primary and final precipitates were determined by chemical analysis, infrared spectrophotometry and X‐ray diffraction. Precipitation generally occurred through three stages, primary precipitation (of low CO 3 aluminium hydroxocarbonates) at low p H with evolution of carbon dioxide, their dissolution by complexing to form hydroxocarbonatoaluminate anions and then secondary precipitation of the final coprecipitate at higher p Hs. The final product from coprecipitation by sodium hydrogen carbonate solution ( p H 8.1) was mainly the magnesium hydroxocarbonatoaluminate ‘MAHC I’; the final products from coprecipitation by sodium hydrogen carbonate‐sodium carbonate solutions ( p H 9.4 and 10.3) were ‘MAHC I’/‘MAHC II’ mixture and ‘MAHC II’/‘MAHC I’ mixture whereas the final product from coprecipitation by sodium carbonate solution ( p H 11.5) was a complex mixture if ‘MAHC II’ with ‘MAHC I’ and ‘MAHC III’; ‘MAHC I’ was probably Mg 2 [Al 4 (OH) 10 (CO 3 ) 3 ] · h H 2 O, ‘MAHC II’ was probably Mg[Al 2 (OH) 4 (CO 3 ) 2 ] · h H 2 O whereas ‘MAHC III’ was probably Mg[Al 2 (OH) 6 CO 3 ] · h H 2 O.