Effect of Potassium Sodium Tartrate and Sodium Citrate on the Preparation of α-Calcium Sulfate Hemihydrate from Flue Gas Desulfurization Gypsum in a Concentrated Electrolyte Solution

Flue gas desulfurization (FGD) gypsum mainly composed of calcium sulfate dihydrate (DH) was used as a raw material to obtain alpha-calcium sulfate hemihydrate (alpha-HH) through dehydration in a Ca-Mg-K-Cl-solution medium at 95 degrees C under atmospheric pressure. The effects of potassium sodium tartrate and sodium citrate on the preparation of alpha-HH in the electrolyte solution were investigated. The results revealed that the addition of potassium sodium tartrate (1.0 x 10-2 -2.5 x 10-2M) decreased the dehydration rate of FGD gypsum and increased the length/width (l/w) ratio of alpha-HH crystals, which could yield unfavorable strength properties. Addition of sodium citrate (1.0 x 10-5 - 2.0 x 10-5M) slightly increased the dehydration rate of FGD gypsum and decreased the l/w ratio of alpha-HH crystals, which could be beneficial to increase strength. However, it also led to a partial formation of anhydrite (AH) crystals. AH was also the only dehydration product when the concentration of sodium citrate increased to 1.0 x 10-4M. Therefore, sodium citrate rather than potassium sodium tartrate could be used as an additive in Ca-Mg-K-Cl electrolyte solutions if alpha-HH with a shorter l/w ratio is the desired product from FGD gypsum dehydration. The concentration of sodium citrate should be properly controlled to reduce the formation of AH.