Measurement of pH in the vicinity of a cathode during the chloride electrowinning of nickel

Abstract

An antimony microelectrode was prepared by quenching a molten Sb-Sb₂O₃ mixture (2 pct Sb₂O₃). The local pH in the vicinity of a nickel-plated copper cathode was directly measured using the microelectrode during the chloride electrowinning of nickel for a MCLE (matte chlorine leach electrowinning) process, where nickel metal is electrodeposited with a high current efficiency, 94 to 97 pct, from low-pH baths. The local pH at 328 K was increased by proton consumption during the electrolysis of aqueous electrolytes containing NiCl₂ (1.20 mol dm⁻³) and NaCl (0.43 mol dm⁻³) with the same concentrations as employed for the MCLE process. The difference in pH between the cathode surface and bulk solution increased with increasing cathodic current density. Nickel deposits with a metallic luster were obtained when the difference was not more than 1.2 pH units. The current efficiency was a maximum for electrolysis with a current density of 265 A m⁻² and bulk pH of 1.0 to 1.5; these optimal conditions coincided with those reported for the MCLE process: temperature 328 to 333 K, bulk pH 1.1 to 1.5, and current density 230 to 260 A m⁻². Electrolytes with lower NiCl₂ and NaCl concentrations resulted in a drop in current efficiency.