Effect of Mg2+, Al3+, and Fe3+ ions on crystallization of type α hemi-hydrated calcium sulfate under simulated conditions of hemi-hydrate process of phosphoric acid

The present study investigated the effects of Mg2+, Al3+, and Fe3+ ions, as additives, on crystallization of type α hemi-hydrated calcium sulfate (α-HH) under simulated conditions of hemi-hydrate wet-process phosphoric acid production. Results showed that induction time of α-HH without additives reduces with increasing supersaturated ratios, but increases in the presence of Mg2+, Al3+, and Fe3+ ions. Under the same concentration of impurity ions, the order of extending induction time of α-HH is as follows: Fe3+＞Mg2+＞Al3+, and surface energy and critical nucleus radius of α-HH increase in the presence of Mg2+, Al3+, and Fe3+ ions. Accordingly, nucleation rate and growth efficiency of α-HH crystals reduce. Length of α-HH crystals decreases and its diameter thicken in the presence of Mg2+, Al3+, and Fe3+ ions. The crystals of α-HH easily grow in the direction of [0 1 0], and feature long-needles. On the other hand, Mg2+, Al3+, and Fe3+ ions are mainly adsorbed on the exposed surface {1 −1 0} of α-HH crystals. Thus, crystal shapes of α-HH eventually show wedge or short columns.