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 temperatur es. A model for the solution thermodynamics has been developed to predict t he solution speciation, temperature, and concentration effects on the pH. I t was found from both the thermodynamic predictions and the pH measurements that the pH of zinc sulfate electrolytes, in the absence of free acid, dro ps with increasing temperature. The pH-temperature behavior was largely dom inated by zinc hydrolysis. The pH of zinc sulfate electrolytes with small a mounts of free acid both increased and then decreased in the temperature ra nge of interest. This was explained by taking into account the additional e ffects of bisulfate/sulfate equilibrium and/or ZnSO4 ion pairing on the ove rall pH behavior. Based on the correlation between the model and pH measure ments, it is evident that the dinuclear species Zn-2(OH)(3+) exists at a mu ch higher concentration than Zn(OH)(+) ions and dominates the pH-temperatur e behavior of the solution. Speciation and the acid/base composition of a Z nSO4 solution, against pH at 100 degrees C, were also predicted. The pH-tem perature 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 degrees C and then allowing the solutions to cool. The pH first increased slightly and th en dropped from a maximum pH of 4.2. By including species involving Al3+, M g2+, Mn2+, and Na+ in the zinc plant electrolytes in the solution model cal culation, model predictions of the pH-temperature were again correlated wit h the pH-temperature measurements on zinc plant electrolytes.