Carbothermic reactions involve the reduction of substances, often metal oxides (O2-), using carbon (C) as the reducing agent. The reduction is usually conducted in the electric arc furnace or reverberatory furnace, depending on the metal ore. These chemical reactions are usually conducted at temperatures of several hundred degrees Celsius. Such processes are applied for production of the elemental forms of many elements. The ability of metals to participate in carbothermic reactions can be predicted from Ellingham diagrams.[1]
Carbothermal reactions produce carbon monoxide (CO) and sometimes carbon dioxide (CO2). The facility of these conversions is attributable to the entropy of reaction: two solids, the metal oxide (and flux) and carbon, are converted to a new solid (metal) and a gas (COx), the latter having high entropy.
Applications
A prominent example is that of iron ore smelting. Many reactions are involved, but the simplified equation is usually shown as:
2Fe 2O 3 + 3C → 4Fe + 3CO2
On a more modest scale, about 1 million tons of elemental phosphorus is produced annually by carbothermic reactions.[2]Calcium phosphate (phosphate rock) is heated to 1,200–1,500°C with sand, which is mostly SiO 2, and coke (impure carbon) to produce P 4. The chemical equation for this process when starting with fluoroapatite, a common phosphate mineral, is:
The actual reaction given is more complex than it seems and includes several steps.[5]
Variations
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:
2FeTiO 3 + 7Cl 2 + 6C → 2TiCl 4 + 2FeCl 3 + 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 Cr 2O 3 with carbon. For this reason, aluminium is employed as the reducing agent.
