Formation of X-ray amorphous and crystalline aluminium hydroxides

Abstract

Summary Various crystalline and X-ray amorphous forms of aluminium hydroxide prepared from sulphate and chloride solutions by varying the mole ratio of NaOH to Al 3+ have been characterized by chemical analysis and X-ray diffraction. When the NaOH/Al ratio is 3 or 3·3, crystalline Al(OH) 3 in the form of bayerite, nordstrandite, gibbsite, or mixtures is obtained in several hours. When the ratio is 2·75 or below, no Al(OH) 3 is evident and the products are X-ray amorphous and remain so even after ageing for six months. Chemical analysis indicates that the amorphous precipitates are basic aluminium sulphates or chlorides. In the 0 to 2·1 range of NaOH/Al, the composition of the products is constant, being approximately Al(OH) 2·2 X 0·8 . In the 2·1 to 2·75 range, a continuous series of basic salts, of composition ranging from Al(OH) 2·2 X 0·8 to Al(OH) 2·75 X 0·25 , is obtained.