Distribution Of Fluorine And Arsenate In The Transition Of Calcium Sulfate Dehydrate To α-Hemihydrate In Salt Solution

A large quality of flue gas desulfurization (FGD) gypsum (dehydrate, DH) is produced when wet FGD process is employed to remove SO2 from flue gas. The preparation of a-calcium sulfate hemihydrate (a-HH) is one of the promising ways for FGD gypsum utilization. However a-HH runs the risk of being polluted by fluorine and arsenate from FGD gypsum. Fluorine and arsenate distribution were exploited to evaluate its transfer during the phase transit of gypsum to a-HH in the concentrated salt solution.It was found that during phase transition the increasing temperature resulted in an inhibition of the fluorine distribution into a-HH, while the increase in initial fluorine content caused a rise in the distribution. The apparent distribution coefficients revealed that a-HH cannot enrich F from solution but the substitution of F for sulfate in a-HH is very difficult, indicating F exists in a-HH by adsorption. XPS and SEM gave strong indications to a chemical adsorption of fluorine by the top pentagonal planes of a-HH rather than side planes. The distribution can be controlled by adjusting temperature, F content and pH of solution so as to reduce the F transfer from FGD gypsum to a-HH.The increase in temperature as well as initial pH resulted in an inhibition of arsenic transfer to a-HH, while a higher initial arsenic content in DH caused a rise of arsenic in a-HH. The phase transit blocked arsenate moving to a-HH, which was illustrated by the apparent distribution coefficient. Arsenate transfer to a-HH during the phase transformation was dominated by its distribution between solid and solution through the adsorption on the crystal faces. The process can be controlled by raising temperature and pH for the transformation to reduce arsenate transfer to a-HH.Some important parameters such as temperature, impurity content and pH, should be controlled to obtain high pure a-HH. The crystallization process of a-HH can separate and purify the impurity. This work provides the basis for the distribution of impurity between a-HH and high salt solution and will offer great opportunities to explore the distribution in the transition of calcium sulfates.