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

carbon Carbon () is a chemical element; it has chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—meaning that its atoms are able to form up to four covalent bonds due to its valence shell exhibiting 4 ...

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 common reducing agents include hydrogen, carbon ...

. The reduction is usually conducted in the

electric arc furnace An electric arc furnace (EAF) is a Industrial furnace, furnace that heats material by means of an electric arc. Industrial arc furnaces range in size from small units of approximately one-tonne capacity (used in foundry, foundries for producin ...

or reverberatory furnace, depending on the metal ore. These

chemical reaction A chemical reaction is a process that leads to the chemistry, chemical transformation of one set of chemical substances to another. When chemical reactions occur, the atoms are rearranged and the reaction is accompanied by an Gibbs free energy, ...

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 diagrams."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 poisonous, flammable gas that is colorless, odorless, tasteless, and slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the si ...

(CO) and sometimes

carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...

(CO₂). The facility of these conversions is attributable to the

entropy Entropy is a scientific concept, most commonly associated with states of disorder, randomness, or uncertainty. The term and the concept are used in diverse fields, from classical thermodynamics, where it was first recognized, to the micros ...

of reaction: two solids, the metal oxide (and flux) 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 + 3 C → 4 Fe + 3 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 , and coke (impure carbon) to produce . The chemical equation for this process when starting with fluoroapatite, a common phosphate mineral, is: : 4 + 18 + 30 C → 3 + 30 CO + 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 Calcium carbonate is a chemical compound with the chemical formula . It is a common substance found in Rock (geology), rocks as the minerals calcite and aragonite, most notably in chalk and limestone, eggshells, gastropod shells, shellfish skel ...

to give sodium carbonate, a commodity chemical. Recently, development of the 'MagSonic' carbothermic

magnesium Magnesium is a chemical element; it has Symbol (chemistry), 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 ...

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

Silicon

Metallurgical grade silicon may also be obtained by carbothermic reaction. The overall reaction is following: : + ↔ + The actual reaction given is more complex than it seems and includes several steps.

Variations

Sometimes carbothermic reactions are coupled to other conversions. One example is the chloride process for separating

titanium Titanium is a chemical element; it has 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, 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; it has 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 between ...

gas, giving titanium tetrachloride: : 2 + 7 + 6 C → 2 + 2 + 6 CO 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 with carbon. For this reason,

aluminium Aluminium (or aluminum in North American English) is a chemical element; it has chemical symbol, symbol Al and atomic number 13. It has a density lower than that of other common metals, about one-third that of steel. Aluminium has ...

is employed as the reducing agent.

References

{{Reflist Chemical reactions