The Effect of Solids Suspended in a Nickel Electrolyte on Deposit Morphology

"Nickel electroplating from sulphate electrolyte is an efficient process. To achieve good quality cathode, however, parameters such as temperature, nickel concentration, pH, electrode alignment, electrolyte flowrate and additives must be optimised, because these affect cathode morphology and current efficiency. It is also important to control the clarity and contamination of the electrolyte, as these have a considerable effect on the quality of the cathode surface. Delamination, surface roughness and pitting on the cathode surface can be caused by incorporation of solid particles or impurities form the electrolyte. Three types of solid particles were investigated: cuprous selenide, barium/ lead sulphate (co-precipitated) and ferric hydroxide. These particles can be found in industrial electrolytes, especially if there is a problem with the filtration step or contaminants have reacted to form insoluble particles. A rotating cylinder Hull cell was used for this investigation. It is found that the effects on morphology depend on the type of solid, concentration, particle size, current density and electrolyte flowrate. Cuprous selenide is the most detrimental, especially at higher concentrations, higher current densities and low flowrates. Increased rotational speed offers a uniform sheet of fine gas bubbles across the surface of the cathode which reduces the thickness of the diffusion layer of liquid adjacent to the cathode and increases the rate of mass transport to the cathode from the bulk liquid. INTRODUCTION Electrowinning operations are important when recovering associated by-products of nickel production, such as copper, cobalt and zinc, from pressure leach solutions. The base metal mineralization associated with platinum-group metal (PGM) production yields significant quantities of copper and nickel as by-products. The base metal refinery is vital in PGM processing as it increases the PGM concentration from mined ore by a factor of 400, in comparison with concentration by flotation (20 times) and pyrometallurgical processing (20 times) (Crundwell et al., 2011). Anglo American Platinum Ltd produces nickel cathodes by electrowinning and purges sodium from the base metal refinery in the form of sodium sulphate at the Rustenburg Base Metals Refinery (RBMR), as shown in Figure 1. This is the Sherritt acid leach process used in most base metal refineries to ensure no loss of PGMs and shows the subsequent purification steps for copper and nickel. Other refineries in Zimbabwe, such as Bindura and Empress, use the Outokumpu leach process (Pavlides, 2001)."