The acid-base behavior of zinc sulfate electrolytes: The temperature effect

The pH of both synthetic zinc sulfate solutions of various compositions and commercial zinc plant electrolytes was measured over a range of temperatures. A model for the solution thermodynamics has been developed to predict the solution speciation, temperature, and concentration effects on the pH. It was found from both the thermodynamic predictions and the pH measurements that the pH of zinc sulfate electrolytes, in the absence of free acid, drops with increasing temperature. The pH-temperature behavior was largely dominated by zinc hydrolysis. The pH of zinc sulfate electrolytes with small amounts of free acid both increased and then decreased in the temperature range of interest. This was explained by taking into account the additional effects of bisulfate/sulfate equilibrium and/or ZnSO₄ ion pairing on the overall pH behavior. Based on the correlation between the model and pH measurements, it is evident that the dinuclear species Zn₂(OH)³⁺ exists at a much higher concentration than Zn(OH)⁺ ions and dominates the pH-temperature behavior of the solution. Speciation and the acid/base composition of a ZnSO₄ solution, against pH at 100 °C, were also predicted. The pH-temperature behavior of zinc plant electrolytes from Kidd creek (Falconbridge Limited, Timmins, Canada) and CEZinc (Noranda Limited, Valleyfield, Canada) was measured by saturating the electrolytes with ZnO at 100 °C and then allowing the solutions to cool. The pH first increased slightly and then dropped from a maximum pH of 4.2. By including species involving Al³⁺, Mg²⁺, Mn²⁺, and Na⁺ in the zinc plant electrolytes in the solution model calculation, model predictions of the pH-temperature were again correlated with the pH-temperature measurements on zinc plant electrolytes.