Anode effects in electrowinning

Abstract

In traditional electrowinning operations, metal particles from commercial insoluble anodes flake off the immersed surfaces under high current densities and thus become occluded on the adjacent cathode surface thereby impairing the cathode product quality and market grade. This results in an undersirable physical appearance of the cathode due to irregular protrusions which impede subsequent material handling operations such as packaging, weighing and shipping. The paper serves to highlight the existing problems inherent with permanent anodes and suggests possible areas for further research and development studies. At present, permanent anodes are employed in the worldwide electrolytic industry for the recovery of base metals such as copper, nickel, cobalt, and zinc while smaller scale operations exist to recover precious metals such as gold and silver from leach solutions. The coupling of solvent extraction with electrowinning technology (SX-EW) is now a widespread practice. Unfortunately, the usage of permanent anodes has the disadvantages of ongoing maintenance costs to clean the anode surfaces and refurbish the wetted area, results in lower anode useful life, the expense of anode replacement, lower current efficiency, higher power consumption as compared to electrorefining, and inferior cathode quality which becomes contaminated due to deterioration of the metal or metal alloy used to fabricate the insoluble anodes. Apart from improvements in productivity and cathode product quality, and lower labour requirements, the driving force to remedy present electrowinning performance is mainly a reduction in energy. Another associated problem area is the usage of mother blanks fabricated from either aluminum, copper, titanium, or stainless steel as employed to enable full deposit stripping. It is recognized that the construction of the different hanger bars used to support both permanent anodes and mother blanks presents another problem area for study which is addressed within the paper. The writers advocate investigations to improve non-ferrous metal stripping practice, increase current efficiency and extend the useful life of permanent electrodes. The anticipated improvement in full plate cathode chemical and physical quality will benefit downstream operations such as copper wirerod production and subsequent fine wire drawing.