R20 Electrolytic extraction and purification of metals

Electrolysis is frequently used to extract a metal from its ore, or to purify an impure metal.

1. Electrolytic extraction of metals.
The commonest metal ores contain the metals as oxides or carbonates. The metal can in theory be obtained by the electrolytic reduction of the ores. However, in practice chemical reduction using carbon or sulphur is more often used because it is cheaper, but this produces an impure metal and can cause environmental problems. When product purity is important, electrolysis is the preferred method. Al, Co, Cr, Mn, Ta, and Mg are all extracted using electrolytic processes.
Illustration R20 schematically shows an electrolytic process for the extraction of Al from its ore (bauxite, an oxide of aluminium containing silicon and other impurities). This process is used to produce 25 million tons of aluminium per year. After chlorine, this is the most important product of the electrochemical industry.
Aluminium is in great demand for the automobile, shipbuilding, aircraft, electrotechnical and building industries and, since there are ample reserves of bauxite, the future for the aluminium industry seems bright.

The electrolytic extraction process of aluminium from bauxite was originally developed by Hall (USA) and Héroult (France) in 1886 and improved in 1887 by Bayer (Germany).

The first step in this process is the dissolution of bauxite in sodium hydroxide under pressure as sodium aluminate. This is a self-sustaining reaction, which does not involve electron transfer. The SiO₂ and Fe₂O₃ impurities are precipitated and removed at this stage.

The next step is the precipitation of aluminium oxide from the sodium aluminate solution. This is achieved by dilution with water, seeding with solid aluminium(III) hydroxide or treating with carbon dioxide.
The precipitated aluminium hydroxide is then separated off and heated to obtain aluminium oxide, which is then dissolved in molten cryolite (Na₃AlF₆).

where Q indicates the consumption of electricity.
Aluminium is discharged at the cathode and oxygen is evolved at the anode, which oxidises the graphite anode to carbon dioxide.

The appropriate half-reaction equations are:

The second equation contains a multiple of 3 in order to balance the electrons in the two half-equations.
This process is operated under the following conditions:

• Voltage = 4.5 V (cf. theoretical value of 2.2 V).
• Current = 150 kA
• Cell size = 3 x 8 x 0.7m, containing 8 graphite blocks (200 single cells, each 15 m³, in series).
• 15 MWh of electricity are required to produce 1 ton of aluminium, which is 5 times more electricity than is required to produce 1 ton of chlorine.

2. Electrolytic refining of metals.
Impure metals can be purified by electrolysis. In an electrolytic cell the anode is made from the crude metal needing to be purified, the cathode from the purified metal. The electrode potential is selected to ensure the very selective reduction of the metal at the cathode.
During the transfer of the metal from the impure metal of the positively charged anode to the negatively charged cathode the impurities remain behind in the electrolyte solution. The electrolyte is chosen according to which element is to be purified. For Cu, Ag, Au and Pt, aqueous solutions are used, whereas for Na, Mg, Ca and Al molten salts are employed.
Illustration R20 illustrates the electrolytic refining of copper. Impure copper is the anode, pure copper the cathode and an aqueous mixture of sulphuric acid and copper sulphate is the electrolyte. The electrode potential is carefully selected so that only copper is reduced at the cathode.

This is the most important method of copper purification, producing 100,000 tons of purified Cu per year. It is based on the following considerations:

• Ag, Au and Pt, all precious metals, have a lower reduction potential than Cu. They are not oxidised to ions at the anode. As the copper ions are formed, the anode crumbles away allowing the precious metal impurities to fall as sludge to the bottom of the cell.
• Sn, Bi and Sb have a larger reduction potential than Cu. They are therefore oxidised at the anode but their ions react with the electrolyte to form insoluble oxides and hydroxides. These are also deposited in the sludge.
• Pb is also oxidised, but forms insoluble PbSO₄, which again sinks into the sludge.
• Fe, Ni, Co and Sn are oxidised at the anode, but remain as ions in the electrolyte. This is a consequence of the careful choice of electrode potential, which ensures that only the copper ions are reduced at the cathode, the other metal ions needing a higher electrode potential to be reduced.

Electrolytic methods are also used to recover the precious metals from the anodic sludge.