The Manufacture of Aluminium

Although aluminium is an abundant element in the Earth's crust, it is usually found in combination with oxygen, in stable compounds from which the metal is very difficult to extract. As a result, aluminium was largely unknown until the end of the nineteenth century, when Charles Hall, in the USA, and Paul Héroult, working independently in Europe, developed an electrolytic method of manufacturing aluminium, using a solution of aluminium oxide in molten cryolite, K3AlF6, as the electrolyte.

Copper Purification

After it has been extracted from its ore, copper is refined electrolytically to produce the very pure metal which is required for use as an electrical conductor. The circuit which is used commercially to refine the copper is similar to that shown in Part 2 of the programme. The anode is a slab of 99%-pure copper; the cathode is a thin sheet of pure copper; and the electrolyte is a solution of copper sulphate. Some of the impurities in the crude copper are valuable, unreactive metals, which collect as a sludge around the anode during the electrolysis.

Electroplating

The last programme sequence shows how copper can be electroplated onto a metallic object. Other metals can be electroplated in a similar way. Chromium, for example, can be plated onto the cathode if a chromium anode is used in the electrolysis cell and the electrolyte contains a high concentration of a chromium compound (often chromic (VI) acid). Note that chromium-plated objects are usually nickel-plated first.

The application of a thin layer of tin to a steel surface, producing tin plate, is the single most important application of electroplating. Tin plate is used to manufacture food and drinks cans.

Steel can also be galvanised to reduce corrosion, and this treatment is often carried out electrolytically. The process of galvanising involves the electro-deposition of a thin layer of zinc onto a steel surface which has been pickled in sulphuric acid.