Thermodynamic Modeling of Calcium Sulfate Hydrates in a CaSO4-H2SO4-H2O System from 273.15 to 473.15 K up to 5 m Sulfuric Acid

Abstract

To prevent scaling and to recycle aqueous solutions in industrial processes, the thermodynamic properties of the CaSO₄-H₂SO₄-H₂O 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 CaSO₄-H₂SO₄-H₂O 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 (CaSO₄·2H₂O), anhydrite (CaSO₄), and hemihydrate (CaSO₄·0.5H₂O), 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 H₂SO₄ concentration, the transformation temperatures of gypsum to anhydrite and hemihydrate will decrease. Thus, gypsum is stable at low temperatures in solutions of low H₂SO₄ concentrations and transforms to anhydrite at high temperatures and in concentrated H₂SO₄ 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.

Figure 1

Difference between calculated and experimental values of Gibbs energy for calcium sulfates in sulfuric acid solutions in the assessment. Error = (C_i–E_i)/U_i (solid symbol, adopted value; open symbol, unadopted in the assessment).

Figure 2

Comparison between set-1 and model D for the CaSO₄–H₂O system.

Figure 3

Assessed and experimental solubilities^,,− of gypsum in the CaSO₄–H₂SO₄–H₂O system at (a) 283.15 K, (b) 298.15 K, (c) 303.15 K, (d) 308.15 K, (e) 313.15 K, (f) 315.15 K, (g) 316.15 K, (h) 318.15 K, (i) 323.15 K, (j) 333.15 K, (k) 343.15 K, (l) 348.15 K, (m) 353.15 K, (n) 363.15 K, and (o) 368.15 K.

Figure 3

Assessed and experimental solubilities^,,− of gypsum in the CaSO₄–H₂SO₄–H₂O system at (a) 283.15 K, (b) 298.15 K, (c) 303.15 K, (d) 308.15 K, (e) 313.15 K, (f) 315.15 K, (g) 316.15 K, (h) 318.15 K, (i) 323.15 K, (j) 333.15 K, (k) 343.15 K, (l) 348.15 K, (m) 353.15 K, (n) 363.15 K, and (o) 368.15 K.

Figure 4

Assessed and experimental solubilities of anhydrite^,,,, in the CaSO₄–H₂SO₄–H₂O system at (a) 283.15 K, (b) 298.15 K, (c) 308.15 K, (d) 315.15 K, (e) 323.15 K, (f) 348.15 K, (g) 363.15 K, (h) 368.15 K, (i) 373.15 K, (j) 378.15 K, (k) 398.15 K, (l) 423.15 K, (m) 448.15 K, and (n) 473.15 K.

Figure 5

Assessed and experimental solubilities^,, of hemihydrate in the CaSO₄–H₂SO₄–H₂O system at (a) 298.15 K, (b) 323.15 K, (c) 348.15 K, (d) 368.15 K, (e) 373.15 K, and (f) 398.15 K.

Figure 6

Transformation of gypsum to anhydrite and hemihydrate in the CaSO₄–H₂SO₄–H₂O system.

Figure 7

Difference between calculated and experimental values of solubility of gypsum in sulfuric acid solutions at (a) 298.15 K, (b) 323.15 K, (c) 348.15 K, and (d) 363.15 K. Hollow symbol indicates this work; filled symbol is the work by Wang et al.

Figure 8

Difference between calculated and experimental values of solubility of gypsum in sulfuric acid solutions. (a) Schäfer and Hunger, at 298.15 K; (b) Beremzhanov and Kruchenko, at 298.15 and 323.15 K; (c) Zhang and Muhammed, at 298.15 K; and (d) Calmanovici et al., at 298.15, 323.15, and 343.15 K.

Figure 9

Refitted solubility of anhydrite at 423.15 K based on the data of Marshall and Jones only. The predicted solubility by the original model is also included.

Figure 10

Predicted solubility of gypsum by our model in dilute sulfuric acid in the temperature range of 273.15–298.15 K.