Sulfate and Chloride Electrolytes for Nickel Electrowinning

Sulfate and Chloride Electrolytes for Nickel Electrowinning

Electrowinning of nickel is usually carried out from three types of electrolytes: typically sulfate, chloride, and sulfate-chloride mixtures, with the operating parameters and the cell configuration closely related to the nature of the electrolyte used.

Typically, for sulfate electrolytes, electrowinning is conducted in a range of pH 2 to 4, as lower pH values give excessive hydrogen evolution at the cathode surface, while higher pH values lead generally to the formation of hydroxide precipitates of metallic cations in the electrolyte that may be entrained in the deposit and spoil the deposit quality.

A current density of 200 - 240 A/m2 is usual and represents a balance between the rate of deposition and the deposit quality, as high current densities may lead to dendritic or powdery metal deposits. To improve the kinetics of reactions at the electrodes, the conductivity of the electrolyte, and the deposit quality, electrowinning from sulfate is usually carried out in the range of temperatures of 60 to 65oC. The anolyte is typically separated by a membrane from the catholyte to lower the transport of hydrogen ions generated at the anode to the cathode surface where it would compete with the nickel reduction and significantly reduce the current efficiency. A positive hydrostatic head is also typically maintained in the cathode bag relative to the anolyte to reduce the rate of mass transport of hydrogen ions to the cathode compartment by carefully controlling the flow of electrolyte. The types of additives and the dimensions of the electrodes differ from one plant to another.

The main advantages of sulfate solutions over chloride and sulfate-chloride solutions are less corrosion, the possibility to use less‐expensive lead or lead‐alloy anodes, and much simpler equipment as there is no gas to be collected for environmental reasons and the possibility to produce electrodeposits with acceptable internal stress levels.

However, the industrially optimal current densities attainable in sulfate solutions are significantly lower than those for chloride‐based electrolytes, mainly due to the relatively lower activity of nickel in sulfate electrolytes. A sulfate-chloride solution is usually used for electrowinning using the matte from traditional matte-smelting operations as anodes.