Effect of Mg2+ Ions on the Nucleation Kinetics of Calcium Sulfate in Concentrated Calcium Chloride Solutions

The precipitation kinetics of the metastable calcium sulfate hemihydrate (HH) from aqueous solutions has seldom been addressed. For the first time, the nucleation kinetics of calcium sulfate (probably in alpha-HH form) in concentrated CaCl2 solutions at 90 degrees C and the effect of Mg2+ ions were investigated by spontaneous precipitation experiments and a turbidity monitoring method. The results indicate that, at a moderate supersaturation with respect to calcium sulfate, Mg2+ ions retard the nucleation with 0.01 M Mg2+ ions showing the strongest effect; at a higher supersaturation, the inhibition effect decreases when the concentration of Mg2+ ions increases from 0.001 to 0.10 M, and 0.20 M Mg2+ ions show a promoting effect on the precipitation. The interfacial tension values increase with the concentration of Mg2+ ions, resulting in a decrease in nucleation rate in Mg2+ ion-containing solutions compared to Mg2+ ion-free solutions. This may indicate an involvement of Mg2+ ions in the surface reaction during calcium sulfate precipitation.