FORMATION AND CHARACTERIZATION OF NEW MAGNESIUM ALUMINUM HYDROXYCARBONATES

The removal of aluminum from strong caustic soda solutions (simulated paper pulp liquors) was studied using the addition of calcined magnesite to precipitate magnesium aluminum hydroxycarbonate phases. For aluminum concentrations higher than about 150 ppm the precipitated phase was hydrotalcite, whereas for lower aluminum concentrations, new hydroxycarbonate phases formed. For solution temperatures above 70°C the phase which formed was characterized as a 1:1 unit cell intergrowth of brucite and hydrotalcite. The new phase is isostructural with coalingite, having a rhombohedral unit cell, R3m, a = 3.108(4) Å, c = 38.3(1) Å, and a composition of [Mg0.88Al0.12(OH)2]2 · [(CO3)0.12(H2O)0.64]. With variation of solution temperature and concentration, other phases responsible for aluminum removal were formed and identified, including higher order brucite-hydrotalcite intergrowths and aluminum-containing brucite. © 1991.