Carbothermic reactions use carbon as reducing agent, usually for metal oxides. These chemical reactions are usually conducted at several hundreds of degree Celsius. Such processes are applied for production of the elemental forms of many elements. Carbothermic reactions are not however useful for some metal oxides such as those of sodium and potassium. The ability of metals to participate in carbothermic reactions can be predicted from Ellingham diagrams.
Carbothermal reactions produce carbon monoxide and sometimes carbon dioxide. The facility of these conversion is attributable to the entropy of reaction: two solids, the metal oxide and carbon, are converted to a new solid (metal) and a gas (CO), the latter having high entropy. Heat is required for carbothermic reactions because diffusion of the reacting solids is slow otherwise.
- 2Fe2O3 + 3C → 4Fe + 3CO2
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:
- 4Ca5(PO4)3F + 18SiO2 + 30C → 3P4 + 30CO + 18CaSiO3 + 2CaF2
Sometimes carbothermic reactions are coupled to other conversions. One example is the chloride process for separating titanium from ilmenite, the main ore of titanium. In this process, a mixture of carbon and the crushed ore is heated at 1000 °C under flowing chlorine gas, giving titanium tetrachloride:
- 2FeTiO3 + 7Cl2 + 6C → 2TiCl4 + 2FeCl3 + 6CO
For some metals, carbothermic reactions do not afford the metal, but instead give the metal carbide. This behavior is observed for titanium, hence the use of the chloride process. Carbides also form upon high temperature treatment of Cr2O3 with carbon. For this reason, aluminium is employed as the reducing agent.
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