Carbothermic reactions involve the reduction of substances, often metal oxides (O^2-), using

carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent In chemistry, the valence (US spelling) or valency (British spelling) of an element is the measure of its combining capacity with o ...

as the

reducing agent In chemistry, a reducing agent (also known as a reductant, reducer, or electron donor) is a chemical species that "donates" an electron to an (called the , , , or ). Examples of substances that are commonly reducing agents include the Earth meta ...

. These

chemical reaction A chemical reaction is a process that leads to the IUPAC nomenclature for organic transformations, chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the pos ...

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 Ellin ...

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 simple ...

and sometimes

carbon dioxide Carbon dioxide (chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is transpar ...

. The facility of these conversions is attributable to the

entropy Entropy is a scientific concept, as well as a measurable physical property, that is most commonly associated with a state of disorder, randomness, or uncertainty. The term and the concept are used in diverse fields, from classical thermodynam ...

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 whi ...

(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, a common phosphate mineral, is: : 4 + 18 + 30C → 3 + 30CO + 18 + 2 Of historic interest is the Leblanc process. 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 Sodium carbonate, , (also known as washing soda, soda ash and soda crystals) is the inorganic compound with the formula Na2CO3 and its various hydrates. All forms are white, odourless, water-soluble salts that yield moderately alkaline solutions ...

, 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 Magnesium is a chemical element with the symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 of the periodic ta ...

process has restarted interest in its chemistry: : + ↔ + The reaction is readily reversible from its product vapors, and requires rapid cooling to prevent back-reaction.

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 Titanium is a chemical element with the symbol Ti and atomic number 22. Found in nature only as an oxide, it can be reduced to produce a lustrous transition metal with a silver color, low density, and high strength, resistant to corrosion in ...

from

ilmenite Ilmenite is a titanium-iron oxide mineral with the idealized formula . It is a weakly magnetic black or steel-gray solid. Ilmenite is the most important ore of titanium and the main source of titanium dioxide, which is used in paints, printing ...

, the main ore of titanium. In this process, a mixture of

and the crushed ore is heated at 1000 °C under flowing

chlorine Chlorine is a chemical element with the Symbol (chemistry), 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 betwee ...

gas, giving

titanium tetrachloride Titanium tetrachloride is the inorganic compound with the formula . It is an important intermediate in the production of titanium metal and the pigment titanium dioxide. is a volatile liquid. Upon contact with humid air, it forms thick clouds o ...

: : 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 the ...

. 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. I ...

is employed as the reducing agent.

References

Chemical reactions {{reaction-stub