Carbothermic reaction
redox reaction using elemental carbon as reducing agent
Carbothermic reactions are chemical reactions involving reduction of substances, often metal oxides (O2-), using carbon (C) as the reducing agent. Depending on the starting material, the process is usually carried out in an electric arc furnace or reverberatory furnace at temperatures of several hundred degrees Celsius. Carbothermal reactions produce carbon monoxide (CO) and sometimes carbon dioxide (CO2). The facility of these conversions is attributable to the entropy of reaction: two solid compounds—the (metal) oxide (and flux) and carbon—are converted to a new solid (metal) compound and a gas (COx), the latter having high entropy.
Carbothermic reactions are used in the production of the elemental forms of many elements. The ability of metals to participate in carbothermic reactions can be predicted from Ellingham diagrams.
Applications
A prominent and probably the best-known example is that of iron ore smelting. Many reactions are involved, but the simplified equation is usually shown as:
2 Fe2O3 + 3 C → 4 Fe + 3 CO2
On a more modest scale, about 1 million tons of elemental phosphorus is produced annually by carbothermic reactions. Calcium phosphate (phosphate rock) is heated to 1,200–1,500 °C with sand, which is mostly SiO2, and coke (impure carbon) to produce P4. The chemical equation for this process when starting with fluoroapatite, a common phosphate mineral, is:
4 Ca5(PO4)3F + 18 SiO2 + 30 C → 3 P4 + 30 CO + 18 CaSiO3 + 2 CaF2
Of historic interest is the Leblanc process.
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