Crystallization of Nordstrandite in Citrate Systems and in the Presence of Montmorillonite

Abstract

Nordstrandite, one of three polymorphs of A1(OH) 3 , is always accompanied by bayerite and/or gibbsite when samples are crystallized in the presence of chloride, nitrate, perchlorate or sulfate in the range of pH from slightly acid to highly alkaline. Pure nordstrandite (or a very high percentage of it) is obtained by precipitation of A1(OH) 3 in citrate systems with an A1/citrate molar ratio of 10 or more in the presence or absence of montmorillonite in the range of pH 9 to 11. Montmorillonite at high pH values in the presence of citrate, favors the crystallization of nordstrandite by inhibiting the formation of bayerite and pseudoboehmite. In A1(OH) x -montmorillonite complexes at higher than pH 9 and at different A1/citrate molar ratios, pure nordstrandite forms. Plots of A1/citrate ratio against pH show areas of stability of the various aluminous species, including pseudoboehmite, amorphous alumina, and interlayered montmorillonite. Electron micrographs of those complexes show many ovoidal particles or characteristic flamboyant clusters when the complexes are made at elevated temperatures. In citrate systems without the presence of clay, pure nordstrandite occurs mainly as rectangular particles.