How electrowinning cobalt from chloride solutions?

How electrowinning cobalt from chloride solutions?

Electrowinning of cobalt from chloride solutions has several advantages over sulphate, like higher electrolyte conductivity, lower electrolyte viscosity, lower anodic and cathodic overpotentials, and more uniform and ductile metal deposition at high current densities and higher activity of the cobalt ion, resulting in less sensitivity towards fluctuations in electrolyte pH. Higher solubility also makes it possible to operate with more concentrated solutions. However, more advanced cell designs are needed for the safe handling of toxic chlorine liberated at the anodes, and chloride solutions are more corrosive than sulphate solutions. The fact is that smooth and coherent cobalt cathodes can be produced from pure chloride electrolytes without additives.  

Today, nickel and cobalt are leached from the raw materials using chlorine, and later separated by solvent extraction in the chlorine leach process. Extracted cobalt is stripped from the organic phase with cobalt anolyte and water, giving a cobalt chloride electrolyte that is subjected to solution purification before entering the cobalt tankhouse. Here cobalt is deposited on titanium mother blanks or cobalt starting sheets. Chlorine gas is produced on DSA type anodes, and each anode is encapsulated in bags of permeable synthetic cloth to collect the chlorine. Catholyte is sucked into the bags and leaves the cells via hoods as an anolyte along with the anode gas. The anolyte is dechlorinated and returned to the electrowinning tanks or the stripping section, while the chlorine gas is sent to the leaching department to dissolve more matte.

Industrial electrowinning of cobalt is carried out in rectangular open cells with alternate anode and cathode vertical plate electrodes connected in parallel within each cell, while the cells are connected in series. In sulphate solutions, permanent stainless steel cathodes are mostly used, while in chloride solutions cobalt starting sheets are produced on titanium blanks. Lead alloyed with antimony is the dominating anode material in sulphate solutions, although inert cobalt alloy anodes have been used as well. Lead contamination of the cobalt product is thus avoided, but these expensive anodes are affected by higher electrical resistivity and brittleness. Graphite anodes were used in chloride media, but they have now been replaced by DSA anodes.

When operating in sulphate media, anodic production of oxygen results in increased acidity of the electrolyte, which has a negative effect on cobalt current efficiency. To counteract this, the pH of the feed electrolyte is adjusted to high levels combined with a high circulation rate.