Reduction of Metal Oxides

Once the metal oxide has been obtained (Section 1.3.1), it must be REDUCED to the crude, elemental metal. A range of reducing agents is available -- carbon, carbon monoxide, hydrogen, aluminium, and other reactive metals such as sodium -- and the choice of which one to use depends on the nature of the metal being extracted. As a general rule, carbon CANNOT be used to reduce the oxides of very reactive metals such as sodium, potassium or aluminium (their oxides are thermodynamically too stable for carbon to out-compete, as the Ellingham diagram in Section 1.4 will make quantitative); similarly, carbon MONOXIDE specifically cannot reduce oxides such as ZnO or Al2O3. Five distinct reduction routes are used in practice, covered below.

Smelting. In smelting, a FLUX (a chemical substance that combines with the gangue to form an easily fusible slag) is added to the concentrated ore together with a reducing agent -- carbon, carbon monoxide, or aluminium -- and the mixture is melted by heating at an elevated temperature, above the melting point of the metal, in a smelting furnace. For iron, the oxide is reduced by carbon monoxide:

Fe2O3(s) + 3CO(g) → 2Fe(s) + 3CO2(g)

Since the silica gangue present in iron ore is acidic, a BASIC flux -- quicklime, CaO -- is used; it combines with the silica gangue to form calcium silicate (slag): CaO(s) + SiO2(s) → CaSiO3(s) (Flux + Gangue → Slag).

For copper, extracted from copper pyrites (CuFeS2), the concentrated ore is heated in a reverberatory furnace with silica added as an ACIDIC flux (the reverse choice from iron, because the ferrous oxide by-product here is basic in nature): 2CuFeS2(s) + O2(g) → 2FeS(l) + Cu2S(l) + SO2(g); the FeS is then oxidised further, 2FeS(l) + 3O2(g) → 2FeO(l) + 2SO2(g), and the resulting basic FeO combines with the acidic silica flux to give ferrous silicate slag, FeO(s) + SiO2(s) → FeSiO3(s). The remaining Cu2S and FeS, being mutually soluble, form a liquid COPPER MATTE, which is separated from the slag and fed to a converting furnace. There, the FeS still present in the matte is oxidised to FeO and removed as slag with more silica, while the remaining copper sulphide is oxidised in stages to metallic copper: 2Cu2S(l,s) + 3O2(g) → 2Cu2O(l,s) + 2SO2(g), then 2Cu2O(l) + Cu2S(l) → 6Cu(l) + SO2(g). The metallic copper obtained solidifies with a blistered surface texture caused by the SO2 gas evolved during this last step, and is accordingly called BLISTER COPPER.

Reduction by carbon. The oxide ore is mixed with coal/coke and strongly heated, usually in a blast furnace; this method is used for metals that do NOT form stable carbides with carbon at the reduction temperature (which rules it out for very reactive metals). Examples: ZnO(s) + C(s) → Zn(s) + CO(g); Mn3O4(s) + 4C(s) → 3Mn(s) + 4CO(g); Cr2O3(s) + 3C(s) → 2Cr(s) + 3CO(g).

Reduction by hydrogen. Applicable to the oxides of metals -- such as Fe, Pb, Cu -- that are LESS electropositive than hydrogen itself. Examples: Ag2O(s) + H2(g) → 2Ag(s) + H2O(l); Fe3O4(s) + 4H2(g) → 3Fe(s) + 4H2O(l). Nickel oxide can similarly be reduced using a mixture of hydrogen and carbon monoxide, known as water gas: 2NiO(s) + CO(g) + H2(g) → 2Ni(s) + CO2(g) + H2O(l). …