Thermodynamic modelling of magnesium fluoride precipitation in concentrated zinc sulphate environment

Most electrolytic zinc plants have to deal with dissolved magnesium in their process liquors as zinc sulphide concentrates contain small amounts of magnesium. Applied magnesium bleed method are generally expensive and environmetally unfriendly. Recently, a new approach was suggested and discussed in which magnesium fluoride is selectively precipitated from purified zinc sulphate solutions by addition of zinc fluoride solution. Magnesium fluoride can be converted into magnesium hydroxide with sodium hydroxide solution and regeneration of chemicals is accomplished by electrolysis of the resulting sodium fluoride solution in a membrane cell. In this paper, a thermodynamic model of the precipitating system is evaluated in order to predict the residual fluoride content of the solution as a function of pH and temperature. This is an important parameter as residual fluoride can cause detrimental effects in the electrowinning stage (pitting of the aluminium chatodes). The model predictions are compared with experimental data. The presented thermodynamic model is a tool that contributes to the determination of the optimum precipitation conditions.