| The operating conditions of commercial chromium electrowinning plants are based on the United States Bureau of Mines reports published in the nineteen fifties. The effects of process parameters, such as pH, temperature, current density, impurities, etc. have not been thoroughly investigated yet. The successful operation of the electrolytic cells depends on the close control of the solution composition and in particular its pH.; Two electrochemical reactions proceed simultaneously at the anode: the evolution of oxygen and the oxidation of Cr(III) to Cr(VI). The Cr(VI) diffuses from the anode compartment through the diaphragm, and it reacts with Cr(II) in the catholyte. Since the current efficiency for chromium deposition depends on the Cr(II) to Cr(III) ratio, a better understanding of the rates of transport and reaction of Cr(VI) is required to establish a model for the process.; These rates and the parameters affecting the anodic processes in chromium electrowinning are inspected by carrying out experiments with chrome-alum electrolyte. The effects of temperature, current density, level difference between anolyte and catholyte ({dollar}Delta{dollar}h), and anode material-lead alloys and precious metal coated titanium anodes (DSA), were investigated without attempting to optimize the current efficiency for chromium electrodeposition.; A laboratory-scale cell for continuous electrowinning of chromium was designed and built. It consists of two compartments of about 50 ml each, separated by a diaphragm. A reservoir, equipped with a pH electrode and stirrer, is used to supply a circulating catholyte, while the anolyte is discharged from the cell. The electrolysis is carried out for 24 hours with 1 ml samples taken periodically from the anolyte and catholyte. A personal computer is dedicated to monitoring pH, anode and cathode potentials, as well as current and cell voltages. Total chromium and Cr(VI) concentrations are determined by means of a microprocessor-controlled, single-beam spectrophotometer.; Anodic oxidation of Cr(III) took place at current efficiencies higher than 85% on lead alloy anodes and showed an increasing trend with rising current densities. The changes in temperature and {dollar}Delta{dollar}h did not have a considerable effect on Cr(VI) concentration in anolyte. The current efficiencies for Cr(III) to Cr(VI) oxidation on DSA anodes, on the other hand, reached only 10 to 15% owing to their lower overvoltages for oxygen evolution than that of lead alloy anodes. Metallic chromium depositions with 25% current efficiencies could be reached with DSA anodes while about 10% current efficiencies were attained with Pb alloy anodes.; A mechanistic model, based on the physico-chemical principles, experimental conditions and material balances for the anodic and cathodic compartments, is presented. A good fit was observed between the mechanistically developed equation for the Cr(VI) concentrations in the anolyte and the experimental data. |
