The coulombic efficiency of zinc electrowinning in high-purity synthetic electrolytes

Abstract

Measurements of coulombic efficiency (QE) for zinc electrodeposition were carried out under mass transfer-controlled conditions using a rotating disc electrode in synthetic acidic zinc sulphate electrolytes. At 25°C in 0.8 M ZnSO4+1.07 M H2SO4 prepared from reagent grade chemicals, the QE at an aluminium cathode was 95.7–97.6%. In order to study the influence of electrolyte purity on QE several preparation and purification techniques were employed. While different sources of chemicals produced different QEs, the main source of impurities seemed to be the zinc-containing reagent rather than the sulphuric acid. Improvements in purity either had a negligible effect or lowered the QE, indicating that some impurities are beneficial to electrolyte performance. In the purest solutions prepared, an effect due to residual impurities still seemed to be present. The maximum QE obtainable through variation of the three parameters, i.e. temperature, current density and electrode rotation rate, was determined for two electrolytes of different purities; the values of QE obtained were 98.4 and 98.8%, with temperature as the dominant factor. ‘Wark's Rule’ (the dependence of QE on zinc/acid ratio) was obeyed approximately in the purest electrolyte prepared, over a limited range of composition.