Measurement and modelling of solubility for calcium sulfate dihydrate and calcium hydroxide in NaOH/KOH solutions

The solubility of calcium sulfate dihydrate (CaSO4·2H2O) and calcium hydroxide (Ca(OH)2) in alkali solutions is essential to understand their desilication behavior from Bayer liquor. In this work, solubilities of calcium sulfate dihydrate and calcium hydroxide for the ternary systems of CaSO4·2H2O–NaOH–H2O, CaSO4·2H2O–KOH–H2O, and Ca(OH)2–NaOH–H2O were measured by using the classic isothermal dissolution method over the temperature range of 25–75 °C. The Pitzer model embedded in Aspen Plus platform was used to model the experimental solubility data for these systems. The experimental solubility data was employed to obtain the new binary interaction parameters for Ca(OH)+–OH−, Ca(OH)+–Ca2+ and Ca(OH)+–K+, suggesting that the species Ca(OH)+ is a dominant species in simulated solubility for alkali systems. Validation of the parameters was performed by predicting the solubility for the ternary systems of Ca(OH)2–NaOH–H2O, CaSO4·2H2O–NaOH–H2O and CaSO4·2H2O–KOH–H2O with the overall average relatively deviation (ARD) of 2.12%, 0.75% and 1.63%, respectively.