A meter for measuring the quality of zinc electrolytes (1972)

T.R. Ingraham was Head of Extraction, Metallurgy Research Section, Mines Branch and is now Director, Technology Develop~ ment Branch (Air), Environmental Protection Service, Department of the Environment. R.C. Kerby is a Scientific Officer with the Extraction Metallurgy Research Section, Mines Branch, Department of Energy, Mines and Resources~ Ottawa. The successful electrolytic production of zinc is dependent on several factors, one of which is the concentration of harmful impurities in the zinc electrolyte. (1) The concentration of impurities must be kept below some predetermined level or excessive amounts of hydrogen will be evolved during electrodeposition, with a consequent loss in current efficiency and a decrease in the quality of the zinc electrodeposit. (2) To keep the current efficiency of zinc electrodeposition at an economically acceptable level, many electrolytic zinc plants depend on the judgment of experienced operators and, to a lesser extent, on daily measurements of the concentration of certain impurities. The developmen t of a reliable method of automatically measuring and controlling the quality of zinc electrolyte would be of some advantage to the electrolytic zinc industry. Several methods.involving some degree of automation have been tested with varying degrees of sucess. (3-9) However, none of the methods has gained .wide acceptance with the electrolytic zinc industry. Three basic methods have been used to measure the current efficiency of zinc electrolysis. The earliest method was to compare the weight of zinc actually deposited with the amount expected from the quantity of electricity consumed: (10) this method ~oes not lend itself to the automatic measurement and control of zinc' electrolyte quality because the results are known only at the end of the electrolysis. Several groups have developed methods which involve the electrodeposition and anodic dissolution of zinc from a sample of the electrolyte under carefully controlled conditions. (3-5) The ratio of the current required for anodic dissolution to that required for cathodic deposi, tion was used as an indication of the current efficiency of the process. The method has been used for controlling zinc electrolyte quality. Most electrolytic zinc plants control the quality of the zinc electrolyte by continuously analyzing for certain metallic impurities such as iron, cadmium, lead, copper, and nickel. (1) This method presumes that when the concentration of these impurities is kept below certain levels, all other .. harmful impurities are likely to have also been removed during the purification process. Several methods of automatically analyzing samples of the zinc electrolyte have been developed. They involve instrumental techniques such as spectrophotometric analysis (6) and automatic titrations. (7) These methods have the disadvantages of (a) not measuring all the metallic impurities in the zinc electrolyte which could be harmful to zinc electrolysis and (b) not providing data on the cumulative effects of metallic impurities on the current efficiency of zinc electrolysis. A direct method of measuring the