Carbothermic reactions involve the reduction of substances, often metal oxides (O^2-), using carbon as the reducing agent. These

chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the positions of electrons in the forming and breakin ...

s 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 diagram An Ellingham diagram is a graph showing the temperature dependence of the stability of compounds. This analysis is usually used to evaluate the ease of reduction of metal oxides and sulfides. These diagrams were first constructed by Harold Elli ...

s."Figure 8.19 Ellingham diagram for the free energy of formation of metallic oxides" p. 308
/ref> Carbothermal reactions produce

carbon monoxide Carbon monoxide (chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simpl ...

and sometimes carbon dioxide. The facility of these conversions 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.

Applications

A prominent example is that of iron ore smelting. Many reactions are involved, but the simplified equation is usually shown as: : 2 + 3C → 4Fe + 3 On a more modest scale, about 1 million tons of elemental phosphorus is produced annually by carbothermic reactions.

Calcium phosphate The term calcium phosphate refers to a family of materials and minerals containing calcium ions (Ca2+) together with inorganic phosphate anions. Some so-called calcium phosphates contain oxide and hydroxide as well. Calcium phosphates are white ...

(phosphate rock) is heated to 1,200–1,500 °C with sand, which is mostly , and coke (impure carbon) to produce . The chemical equation for this process when starting with

fluoroapatite Fluorapatite, often with the alternate spelling of fluoroapatite, is a phosphate mineral with the formula Ca5(PO4)3F (calcium fluorophosphate). Fluorapatite is a hard crystalline solid. Although samples can have various color (green, brown, bl ...

, a common phosphate mineral, is: : 4 + 18 + 30C → 3 + 30CO + 18 + 2 Of historic interest is the

Leblanc process The Leblanc process (pronounced leh-blaank) was an early industrial process for making ''soda ash'' ( sodium carbonate) used throughout the 19th century, named after its inventor, Nicolas Leblanc. It involved two stages: making sodium sulfate f ...

. A key step in this process is the reduction of sodium sulfate with coal: : Na₂SO₄ + 2 C → Na₂S + 2 CO₂ The Na₂S is then treated with calcium carbonate to give sodium carbonate, a

commodity chemical Commodity chemicals (or bulk commodities or bulk chemicals) are a group of chemicals that are made on a very large scale to satisfy global markets. The average prices of commodity chemicals are regularly published in the chemical trade magazines an ...

. Recently, development of the 'MagSonic' carbothermic magnesium process has restarted interest in its chemistry: : + ↔ + The reaction is readily reversible from its product vapors, and requires rapid cooling to prevent

back-reaction In theoretical physics, back-reaction (or backreaction) is often necessary to calculate the self-consistent behaviour of a particle or an object in an external field. Intuitive definition When a particle is considered to have no mass or to have a ...

Variations

Sometimes carbothermic reactions are coupled to other conversions. One example is the

chloride process The chloride process is used to separate titanium from its ores. The goal of the process is to win high purity titanium dioxide from ores such as ilmenite (FeTiO3) and rutile (TiO2). The strategy exploits the volatility of TiCl4, which is readily p ...

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 Chlorine is a chemical element with the symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between them. Chlorine is ...

gas, giving titanium tetrachloride: : 2 + 7 + 6C → 2 + 2 + 6CO For some metals, carbothermic reactions do not afford the metal, but instead give the metal

carbide In chemistry, a carbide usually describes a compound composed of carbon and a metal. In metallurgy, carbiding or carburizing is the process for producing carbide coatings on a metal piece. Interstitial / Metallic carbides The carbides of t ...

. This behavior is observed for titanium, hence the use of the

. Carbides also form upon high temperature treatment of with carbon. For this reason,

aluminium Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It has ...

is employed as the reducing agent.

References

Chemical reactions {{reaction-stub