Precipitation from supersaturated aluminate solutions. IV. Influence of citrate ions

Abstract

The influence of citrate on the precipitation in unseeded and seeded supersaturated aluminate solutions is reported. At high Cit/Al ratios (>0. 1) extensive complex formation takes place but already at low ratios (10−4–10−2), where this effect hardly changes the supersaturation, the precipitation rate is strongly retarded. The formation of bayerite appears to be more sensitive to citrate concentration than that of pseudoboehmite and can even be prevented at rather low Cit/Al ratios (0.01). Transformation of pseudoboehmite into bayerite is also inhibited at these low Cit/Al ratios. Citrate may therefore be used to selectively precipitate pseudoboehmite. A simple adsorption model is introduced to explain the role of citrate in retarding growth. Evidence is presented to show the dual role played by citrate in the nucleation of bayerite as revealed by long time lags in relaxation experiments. More nuclei are generated with increasing citrate concentration but the growth of these particles is slowed down.