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To prevent scaling and to recycle aqueous solutions in industrial processes, the thermodynamic properties of the CaSO 4 -H 2 SO 4 -H 2 O system are studied by thermodynamic modeling with the Pitzer model. The published solubility data of calcium sulfate hydrates in sulfuric acid solutions were collected and reviewed critically. Then, the CaSO 4 -H 2 SO 4 -H 2 O system was modeled using the Pitzer activity coefficient approach from critically selected experimental data to obtain optimized parameters. The model reproduces the solubility data with good accuracy up to 5 m sulfuric acid at temperatures of 283.15-368.15, 283.15-473.15, and 298.15-398.15 K for gypsum (CaSO 4 ·2H 2 O), anhydrite (CaSO 4 ), and hemihydrate (CaSO 4 ·0.5H 2 O), respectively. However, at temperatures above 398.15 K and sulfuric acid concentration above 0.5 mol/kg, the solubility of anhydrite predicted by our model deviates significantly from the literature data. Our model predicts that the solubility of anhydrite would first increase but then decrease in more concentrated sulfuric acid solutions, which is in disagreement with the experimental data showing constantly increasing solubilities as a function of increasing sulfuric acid concentration. This discrepancy has been discussed. The transformations of gypsum to anhydrite and hemihydrate were predicted in sulfuric acid solutions. With increasing H 2 SO 4 concentration, the transformation temperatures of gypsum to anhydrite and hemihydrate will decrease. Thus, gypsum is stable at low temperatures in solutions of low H 2 SO 4 concentrations and transforms to anhydrite at high temperatures and in concentrated H 2 SO 4 solutions, while hemihydrate is always a metastable phase. Furthermore, the predicted results were compared with previous experimental studies to verify the accuracy of the model., Competing Interests: The authors declare no competing financial interest., (Copyright © 2020 American Chemical Society.)

Calcium sulfate is one of the most common inorganic salts with a high scaling potential. The solubility of calcium sulfate was modeled with the Pitzer equation at a temperature range from 273.15 to 473.15 K from published solubility data, which was critically evaluated. Only two Pitzer parameters, β (1) and β (2) , with simple temperature dependency are required to model the solubility with excellent extrapolating capabilities up to 548.15 K. The stable temperature range for gypsum is 273.15-315.95 K, whereas above 315.95 K the stable phase is anhydrite. Hemihydrate is in the metastable phase in the whole temperature range, and the obtained metastable invariant temperature from gypsum to hemihydrate is 374.55 K. The obtained enthalpy and entropy changes at 298.15 K for the solubility reactions are in good agreement with literature values yielding solubility products of 2.40 × 10 -05 , 3.22 × 10 -05 , and 8.75 × 10 -05 for gypsum, anhydrite, and hemihydrate, respectively. The obtained Pitzer model for the CaSO 4 -H 2 O system is capable of predicting the independent activity and osmotic coefficient data with experimental accuracy. The mean absolute average error of activity coefficient data at 298.15 K is less than 2.2%. Our model predicts the osmotic coefficient on the ice curve within 1.5% maximum error., Competing Interests: The authors declare no competing financial interest., (Copyright © 2019 American Chemical Society.)