Precipitation from supersaturated aluminate solutions. III. Influence of alkali ions with special reference to Li +

Abstract

The precipitation of Al(OH) 3 from supersaturated aluminate solutions ( C Al = 4 × 10 −3 M) in the presence of Li +, Na +, and K + is compared. In the presence of Na + and K + at 25°C the same precipitation behavior is observed. The precipitation sequence, amorphous phase → pseudo-boehmite → bayerite, is dictated by the supersaturation (pH + pAl) and the growth rate of bayerite is found to depend on the available surface area and on the square of the supersaturation. In the presence of Li + ions a selective and drastic acceleration of the precipitation rate of bayerite is observed at 25 and 50°C. Pseudo-boehmite plays a minor role in the overall precipitation process because lithium not only selectively accelerate bayerite formation but also increase the rate of transformation of pseudoboehmite to bayerite. The discontinuous nature of the acceleration of bayerite formation strongly suggests that a fast nucleating lithium aluminate phase acts as a precursor for the precipitation and growth of bayerite. From an analysis of the measured relaxation times a variable composition of the precursor phase is deduced with a Li Al ratio ranging from 0.2 to 0.4. At high solution Li Al ratios and low supersaturations a distinct phase of composition LiA1 2(OH) 7 · 2H 2O is observed.