The Electrodeposition of High Purity Chromium

SummaryA pilot plant for the electrowinning of high purity chromium has been described earlier. The present paper provides further details and developments together with the results of several years of operation during which more than two tonnes of chromium with a purity often better than 99·99% (including oxygen but excluding hydrogen) has been produced. Hydrogen was not an important impurity because the metal was melted under vacuum in the associated alloy research project. Oxygen was the major impurity because it remains as oxide inclusions in ingots and alloys and the aim, therefore, was to keep this to a minimum. The influence of each process variable on the oxygen content of the chromium has been established. The electrolyte temperature was the most important, followed by the chromic acid and sulphate contents of the solution. Current density affects the oxygen content of the metal but the current values were always a compromise related to the run duration and the cathodic (electrowinning) efficiency. The chromic acid/sulphate ratio in the electrolyte is generally considered to be a control variable in the deposition of chromium for decorative and engineering applications. In the present work it is shown that each component functions independently, and that large changes in the ratio of the two have little effect on either the process or the product.