Speciation and reaction equilibrium constant modelling of aqueous hydrometallurgical systems at elevated temperatures: A review

Encrustation occurs in many processing fluids where high levels of dissolved solids are present, especially in processes that use heat transfer equipment. The deposition of these scales in the interior surfaces of an autoclave can cause major issues in the operation of industrial processes such as hydrometallurgy. The knowledge of the minerals' chemistry, distribution of the chemical forms of these minerals in the autoclave, and their solubility product can assist to inhibit these solid deposits. To model such systems, it is necessary to know the reactions involved and by extension their equilibrium constants. These electrolytic systems being strongly non-ideal, models of activity coefficients are necessary to deduce the concentration of each species. This review presents and compares various models for the calculation of activity coefficients and the thermodynamic equilibrium constants at temperatures above 25 °C. For model validity and comparison purposes, a case study on the speciation of the aqueous binary systems of H₂SO₄-Al₂(SO₄)₃ and H₂SO₄₋MgSO₄ is presented and compared with experimental data. From the results obtained and in the framework presented above, the Density equilibrium constant model coupled with the Truesdell-Jones activity coefficient model gave the best fit with experimental data at the studied temperatures of 235, 250, 270, and 300 °C.