The blister copper is cast into large slabs which will be used as the anodes in the electrolysis apparatus (see page 4). The electrolytic refining of copper produces the high quality, high purity copper required by industry.

In industry this is carried out on a massive scale. Even the best chemical method cannot remove all the impurities from the copper, but with electrolytic refining it is possible to produce 99.99% pure copper.

The blister copper anodes are immersed in an electrolyte containing copper sulphate and sulphuric acid. Pure copper cathodes are arranged between the blister copper anodes and a current of over 200A passes through the solution.

At the anode: Cu(s) → Cu²⁺(aq) + 2e^-

At the cathode: Cu²⁺(aq) + 2e^- → Cu(s)

This is the same reaction as the one you may have tried in the school laboratory.

The cell pushes the ions from the anode to the cathode. At the same time, it pushes the free electrons around the wires (these electrons are already spread through the wires). Electrons in the cathode recombine with copper ions from the solution, forming a new layer of copper atoms.

Gradually, the anode is eroded and the cathode grows. Insoluble impurities in the anode fall to the bottom in a pile. This valuable biproduct is removed.

Soluble impurities of iron and nickel dissolve in the electrolyte, which has to be continually purified to prevent excessive deposition onto the cathodes, which would reduce the purity of the copper.

Recently, stainless steel cathodes have replaced copper cathodes. Identical chemical reactions take place. Periodically, the cathodes are removed and pure copper is scraped off.