Ametal, if it is not already free, must be obtained from one of its compound by reduction using their electrolysis or chemical reduction this is called open reduction.
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Electrolysis : open reduction
Thereactive main group elements lithium, sodium and magnesium are obtainedby electrolysis (open system) of their molten chlorides.The electrode half reactions are
Na+(l) + e- → Na(l)
Cl-(l) → 1/2 Cl2(g) + e-
The overall reaction involving the reduction of sodium ion to sodium metal, is
NaCl (l) → Na(l) + 1/2 Cl2 (g); ΔG >0
Thereaction is non spontaneous, and the free energy change is positive.Free energy supplied by the electrical current drives the reaction to the right.Aluminium is manufactured from the aluminium oxideobtained in the Bayer process using electrolysis(open reduction) .
The method for doing this was discovered in 1886 by Charles Martin Hall in the united state and independently, by Paul Heroult in France. Before this discovery, aluminium was precious metal, and in the 1850s it was sorare and exotic that the French court used aluminium table ware at royal functions in place of the usual silver and gold utensils.
The Hall-Heroult process is the commercial method for producing aluminium bythe electrolysis(open reduction) of a molten mixture of aluminium oxidein cryolite. Originally, the process used natural cryolite. The Hall-Heroult process uses synthetic cryolite, or sodium hexafluoroaluminate, produced by reactive aluminium hydroxide from the Bayer process with sodium hydroxide and hydrofluoric acid
3NaOH (aq) + Al(OH)3(s) + 6HF(aq) → Na3AlF6(aq) + 6H2O (l)
AHall-Heroult electrolytic cell consist of a rectangular steel shell lying first with an insulating material, then with carbon to form cathode.The anodes are also made from carbon. The electrolyte consists of molten cryolite, at about 1000 degree celcius, into which some aluminium oxide is dissolved.The Carbon anodes are consumed in the electrolysis and must be replaced periodically; aluminium oxide is continually added to the electrolyte bath.