Evaluation of the solubility constants of the hydrated solid phases in the H2O-Al2O3-SO3 ternary system

During the acid processing of aluminosilicate ores, the precipitation of a solid phase principally consisting of hydrated aluminium hydroxysulfates may be observed. The experimental study of the H 2 O-Al 2 O 3 -SO 3 ternary system at 25 ∘ C and 101 kPa enabled to describe the solid-liquid equilibra and to identify the nature, the composition and the solubility of the solid phases which may form during the acid leaching. To predict the appearance of these aluminium hydroxysulfates in more complex systems, their solubility constants were calculated by modelling the experimental solubility results, using a geochemical reaction modelling software, CHESS. A model for non-ideality correction, based on the B-dot equation, was used as it was suitable for the considered ion concentration range. The solubility constants of three out of four solid phases were calculated: 10 4.08 for jurbanite (Al(SO 4 )(OH).5H 2 O), 10 28.09 for the solid T (Al 8 (SO 4 ) 5 (OH) 14 .34H 2 O) and 10 27.28 for the solid V (Al 10 (SO 4 ) 3 (OH) 24 .20H 2 O). However the activity correction model was not suitable to determine the solubility constant of alunogen (Al 2 (SO 4 ) 3 .15.8H 2 O), as the ion concentrations of the mixtures were too high and beyond the allowable limits of the model. Another ionic activity correction model, based on the Pitzer equation for example, must be applied to calculate the solubility constant of alunogen.