Carbothermic reaction
Encyclopedia
Carbothermic reactions use carbon
as reducing agent
, usually for metal oxides. These chemical reaction
s 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 diagram
s.
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.
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:
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
:
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.
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
as reducing agent
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
, usually for metal oxides. These chemical reaction
Chemical reaction
A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity...
s 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 diagram
Ellingham diagram
An Ellingham diagram is a graph showing the temperature dependence of the stability for compounds. This analysis is usually used to evaluate the ease of reduction of metal oxides and sulphides. These diagrams were first constructed by Harold Ellingham in 1944...
s.
Carbothermal reactions produce carbon monoxide
Carbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...
and sometimes carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
. The facility of these conversion is attributable to the entropy
Entropy
Entropy is a thermodynamic property that can be used to determine the energy available for useful work in a thermodynamic process, such as in energy conversion devices, engines, or machines. Such devices can only be driven by convertible energy, and have a theoretical maximum efficiency when...
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.
Applications
The most prominent example is that of iron ore smelting. Many reactions are involved, but the simplified equation is usually shown as:- 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 , and coke (impure carbon) to produce . The chemical equation for this process when starting with fluoroapatite, a common phosphate mineral, is:
- 4Ca5(PO4)3F + 18SiO2 + 30C → 3P4 + 30CO + 18CaSiO3 + 2CaF2
Variations
Sometimes carbothermic reactions are coupled to other conversions. One example is the chloride processChloride process
The chloride process is used to separate titanium from its ores. In this process, the feedstock is chlorinated at 1000 °C with carbon and chlorine gas, giving titanium tetrachloride...
for separating titanium
Titanium
Titanium is a chemical element with the symbol Ti and atomic number 22. It has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color....
from ilmenite
Ilmenite
Ilmenite is a weakly magnetic titanium-iron oxide mineral which is iron-black or steel-gray. It is a crystalline iron titanium oxide . It crystallizes in the trigonal system, and it has the same crystal structure as corundum and hematite....
, the main ore of titanium. In this process, a mixture of carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
and the crushed ore is heated at 1000 °C under flowing chlorine
Chlorine
Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...
gas, giving titanium tetrachloride
Titanium tetrachloride
Titanium tetrachloride is the inorganic compound with the formula TiCl4. It is an important intermediate in the production of titanium metal and the pigment titanium dioxide. TiCl4 is an unusual example of a metal halide that is highly volatile...
:
- 2FeTiO3 + 7Cl2 + 6C → 2TiCl4 + 2FeCl3 + 6CO
For some metals, carbothermic reactions do not afford the metal, but instead give the metal carbide
Carbide
In chemistry, a carbide is a compound composed of carbon and a less electronegative element. Carbides can be generally classified by chemical bonding type as follows: salt-like, covalent compounds, interstitial compounds, and "intermediate" transition metal carbides...
. This behavior is observed for titanium, hence the use of the chloride process
Chloride process
The chloride process is used to separate titanium from its ores. In this process, the feedstock is chlorinated at 1000 °C with carbon and chlorine gas, giving titanium tetrachloride...
. Carbides also form upon high temperature treatment of Cr2O3 with carbon. For this reason, aluminium
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
is employed as the reducing agent.