Electrochemical Principles of Electrorefining and Electrowinning

Electrochemical Principles of Electrorefining and Electrowinning

In electrolytic refining, the plates of crude metal are anodically dissolved in a suitable electrolyte, while "pure" metal is deposited on the adjacent cathodes. A process closely related to electrorefining is electrowinning with insoluble anodes. In this process, the metal is dissolved chemically, e.g., by leaching calcined ore, etc. The pure metal is then "electrowon" using insoluble anodes, e.g., lead anodes, producing oxygen:
2H2 0 -+ O2 + 4H+ + 4e

Electrochemical Selectivity

In an ideal electrorefining process, the metal would dissolve "nearly reversibly" on the anode and plate "nearly reversibly" at the cathode. The impurities which are more electropositive (noble) than the electrorefining metal will not dissolve at this potential and will remain in the anode slime. The impurities which are more electronegative (common) than the refined metal will dissolve anodically but will not deposit at the cathode, hence the accumulation in the electrolyte.

As could be expected, it reflects reality reasonably well only with the metals which exhibit low dissolution and deposition overpotentials (Ag, Cu, Pb) and with impurities whose standard potentials are sufficiently different from that of the refined metal.

Anodic Process
The selectivity of the anodic process is often helped by the refractory nature of some impurity-containing compounds present in the anode metal (Ag, Se, and Te are present in copper anodes as quite refractory selenides and tellurides of silver and copper). In spite of this, a very small amount of silver still tends to dissolve from copper anodes and plate onto cathodes. This partial dissolution is suppressed by maintaining -30 mg/liter of Cl- ion in the copper refining electrolyte so that Ag + concentration in the electrolyte will not exceed the value given by the solubility product of AgCl.

The "common" impurities which normally dissolve from the anode can also be present in the anodes in a poorly soluble form. (Nickel is often present in copper anodes as the refractory form of NiO, thus appearing at least partially in the anode slime.) In sulfate-ion-containing electrolytes lead is insolubilized as PbS04 • One component which, is always found as a major component of anode slimes is the refined metal itself.