This invention relates to a method and apparatus for controlling the electrodeposition process of zinc and, more particularly, to a method for controlling the purification of zinc sulfate electrowinning solutions and the zinc electrowinning process by measuring the activation over-potential which is a measure of the solution purity and of the ratio of concentration of polarizing additive to concentration of impurities in zinc sulfate electrolyte, and an apparatus to carry out the method.
In the process for electrowinning zinc from zinc sulfate solutions impurities such as antimony, germanium, copper, nickel, cobalt, iron, cadmium and lead, when present above certain critical concentrations, cause resolution of deposited zinc and a corresponding decrease in the current efficiency of the zinc deposition. To reduce the concentration of impurities in electrolyte to the desired low levels, thereby to reduce these effects to a minimum, a complex purification procedure, which generally includes an iron precipitation and a zinc dust treatment, is employed prior to electrolysis. In addition to the purification, polarizing additives such as glue are added to the electrolyte to reduce the effects of the remaining impurities, as well as to provide smooth and level deposits, and, to some extent, to control acid mist evolution.
The procedures presently used for determining the purity of electrolyte are based on chemical analyses and determinations of current efficiencies as a measure of impurity content, while those for the additions of polarizing additive such as animal glue and the like are based on maintaining a constant concentration of additive in the electrolyte despite variations in the concentration of impurities. These procedures result in variations in the quality of the deposited zinc and the current efficiency of the electrowinning process. A more desirable system would be to control the purification process and, in the electrowinning process, to control the additive concentration in the electrolyte relative to the impurity concentration. These controls would result in a reduction of the effects of impurities with a corresponding increase in the current efficiency of zinc production.