The Gilchrist–Thomas process or Thomas process is a historical process for

refining Refining is the process of purification of a (1) substance or a (2) form. The term is usually used of a natural resource that is almost in a usable form, but which is more useful in its pure form. For instance, most types of natural petroleum w ...

pig iron Pig iron, also known as crude iron, is an intermediate good used by the iron industry in the production of steel. It is developed by smelting iron ore in a blast furnace. Pig iron has a high carbon content, typically 3.8–4.7%, along with si ...

, derived from the

Bessemer converter The Bessemer process was the first inexpensive industrial process for the mass production of steel from molten pig iron before the development of the open hearth furnace. The key principle is removal of impurities and undesired elements, primar ...

. It is named after its inventors who patented it in 1877: Percy Carlyle Gilchrist and his cousin Sidney Gilchrist Thomas. By allowing the exploitation of

phosphor A phosphor is a substance that exhibits the phenomenon of luminescence; it emits light when exposed to some type of radiant energy. The term is used both for fluorescent or phosphorescent substances which glow on exposure to ultraviolet or ...

ous

iron ore Iron ores are rocks and minerals from which metallic iron can be economically extracted. The ores are usually rich in iron oxides and vary in color from dark grey, bright yellow, or deep purple to rusty red. The iron is usually found in the f ...

, the most abundant, this process allowed the rapid expansion of the

steel industry Steel is an alloy of iron and carbon that demonstrates improved mechanical properties compared to the pure form of iron. Due to steel's high elastic modulus, yield strength, fracture strength and low raw material cost, steel is one of the ...

outside the

United Kingdom The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain, is a country in Northwestern Europe, off the coast of European mainland, the continental mainland. It comprises England, Scotlan ...

and the

United States The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...

. The process differs essentially from the

Bessemer process The Bessemer process was the first inexpensive industrial process for the mass production of steel from molten pig iron before the development of the open hearth furnace. The key principle is steelmaking, removal of impurities and undesired eleme ...

in the refractory lining of the converter. The latter, being made of dolomite () fired with

tar Tar is a dark brown or black viscous liquid of hydrocarbons and free carbon, obtained from a wide variety of organic materials through destructive distillation. Tar can be produced from coal, wood, petroleum, or peat. "a dark brown or black b ...

, is

basic Basic or BASIC may refer to: Science and technology * BASIC, a computer programming language * Basic (chemistry), having the properties of a base * Basic access authentication, in HTTP Entertainment * Basic (film), ''Basic'' (film), a 2003 film ...

( MgO giving

anion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...

s), whereas the Bessemer lining, made of packed

sand Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is usually defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural ...

, is

acid An acid is a molecule or ion capable of either donating a proton (i.e. Hydron, hydrogen cation, H+), known as a Brønsted–Lowry acid–base theory, Brønsted–Lowry acid, or forming a covalent bond with an electron pair, known as a Lewis ...

ic ( accepting anions) according to the Lux-Flood theory of molten oxides.

Phosphorus Phosphorus is a chemical element; it has Chemical symbol, symbol P and atomic number 15. All elemental forms of phosphorus are highly Reactivity (chemistry), reactive and are therefore never found in nature. They can nevertheless be prepared ar ...

, by migrating from liquid

iron Iron is a chemical element; it has symbol Fe () and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, forming much of Earth's o ...

to molten

slag The general term slag may be a by-product or co-product of smelting (pyrometallurgical) ores and recycled metals depending on the type of material being produced. Slag is mainly a mixture of metal oxides and silicon dioxide. Broadly, it can be c ...

, allows both the production of a

steel Steel is an alloy of iron and carbon that demonstrates improved mechanical properties compared to the pure form of iron. Due to steel's high Young's modulus, elastic modulus, Yield (engineering), yield strength, Fracture, fracture strength a ...

of satisfactory quality, and of

phosphate Phosphates are the naturally occurring form of the element phosphorus. In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthop ...

s sought after as

fertilizer A fertilizer or fertiliser is any material of natural or synthetic origin that is applied to soil or to plant tissues to supply plant nutrients. Fertilizers may be distinct from liming materials or other non-nutrient soil amendments. Man ...

, known as "Thomas meal". The disadvantages of the basic process includes larger iron loss and more frequent relining of the converter vessel. After having favored the spectacular growth of the

Lorraine Lorraine, also , ; ; Lorrain: ''Louréne''; Lorraine Franconian: ''Lottringe''; ; ; is a cultural and historical region in Eastern France, now located in the administrative region of Grand Est. Its name stems from the medieval kingdom of ...

iron and steel industry, the process progressively faded away in front of the

Siemens-Martin An open-hearth furnace or open hearth furnace is any of several kinds of industrial furnace in which excess carbon and other impurities are burnt out of pig iron to produce steel. Because steel is difficult to manufacture owing to its high mel ...

Open-hearth furnace, which also used the benefit of basic

refractory In materials science, a refractory (or refractory material) is a material that is resistant to decomposition by heat or chemical attack and that retains its strength and rigidity at high temperatures. They are inorganic, non-metallic compound ...

lining, before disappearing in the mid-1960s: with the development of

gas liquefaction Liquefaction of gases is physical conversion of a gas into a liquid state (condensation). The liquefaction of gases is a complicated process that uses various compressions and expansions to achieve high pressures and very low temperatures, usin ...

and the cryogenic separation of O₂ from

air An atmosphere () is a layer of gases that envelop an astronomical object, held in place by the gravity of the object. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosph ...

, the use of pure

oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...

became economically viable. Even if modern pure oxygen converters all operate with a basic medium, their performance and operation have little to do with their ancestor.

Bibliographic sources

* G. Reginald Bashforth, ''The manufacture of iron and steel, vol. 2: Steel production'', London, Chapman & Hall Ltd, 1951, 461 p. * Thomas Turner (dir.), ''The metallurgy of iron: By Thomas Turner...: Being one of a series of treatises on metallurgy written by associates of the Royal school of mines'', C. Griffin & company, limited, coll. "Griffin's metallurgical series", 1908, 3rd ed., 463 p. * Walter MacFarlane, ''The principles and practice of iron and steel manufacture'', Longmans, Green, and Co, 1917, 5th ed. * R.W. Burnie, ''Memoir and letters of Sidney Gilchrist Thomas, Inventor'', John Murray, 1891 * William Tulloch Jeans, ''The Creators of the Age of Steel'', 1884, 356 p. * Hermann Wedding (translated from German by: William B. Phillips, Ph.D. & Ernst Prochaska), ''Wedding's basic Bessemer process'' Basische Bessemer - oder Thomas-Process" New York Scientific Publishing Company, 1891, 224 p. * Jean Duflot, ''Encyclopædia Universalis'', "Sidérurgie" British inventions Steelmaking Metallurgical processes Chemistry {{metallurgy-stub