Conversion of Ores into Oxides

This section covers the two industrial methods used to convert a concentrated ore into the oxide of the metal of interest -- roasting and calcination -- which are chosen according to whether the starting ore is a sulphide or a carbonate/hydrate.

Roasting. Roasting is the method normally applied to SULPHIDE ores. The concentrated sulphide ore is oxidised by strongly heating it in a suitable furnace in the presence of EXCESS OXYGEN, at a temperature kept below the melting point of the metal (so the metal itself does not melt and the solid-state reaction proceeds cleanly). Typical roasting reactions convert the metal sulphide into its oxide, releasing sulphur dioxide gas:

2PbS + 3O2 →(Δ) 2PbO + 2SO2

2ZnS + 3O2 →(Δ) 2ZnO + 2SO2

2Cu2S + 3O2 →(Δ) 2Cu2O + 2SO2

Roasting also serves a secondary purifying role: impurity elements such as arsenic, sulphur and phosphorus present in the ore are converted, by the same excess-oxygen heating, into their own VOLATILE oxides and driven off, e.g. 4As + 3O2 → 2As2O3, S8 + 8O2 → 8SO2, and P4 + 5O2 → P4O10. Because the sulphur dioxide generated in bulk during roasting is environmentally harmful, modern metallurgical plants trap this SO2 by-product and convert it into sulphuric acid rather than releasing it to the atmosphere, turning a pollution problem into a useful co-product.

Calcination. Calcination is the method applied to CARBONATE and HYDRATED oxide ores: the concentrated ore is strongly heated in the ABSENCE of air (or with only a limited air supply), rather than in excess oxygen as in roasting. This drives off volatile components without oxidising the metal itself -- water of crystallisation escapes as moisture from hydrated ores, and any organic matter present is also expelled, leaving behind a porous form of the ore (the porosity itself is useful, since it exposes more surface area for the reduction step that follows).

For a carbonate ore, calcination expels carbon dioxide:

PbCO3 →(Δ) PbO + CO2

CaCO3 →(Δ) CaO + CO2

ZnCO3 →(Δ) ZnO + CO2

MgCO3.CaCO3 →(Δ) MgO + CaO + 2CO2

For a hydrated oxide ore, calcination expels the water of hydration as vapour: …