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The Kroll process is a pyrometallurgical industrial process used to produce metallic titanium
Titanium

Titanium is a chemical element with the symbol Ti and atomic number 22. Sometimes called the ?space age metal?, it has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver colour....
. It was invented in 1940 by William J. Kroll
William J. Kroll

William Justin Kroll was a metallurgist from Luxembourg. He is best known for inventing the Kroll process in 1940, which is used commercially to extract metallic titanium from ore....
 in Luxembourg
Luxembourg

Luxembourg , officially the Grand Duchy of Luxembourg , is a small landlocked country in western Europe, bordered by Belgium, France, and Germany....
. After moving to the United States, Kroll further developed the method in 1945 for the extraction of zirconium
Zirconium

Zirconium is a chemical element with the symbol Zr and atomic number 40. It is a lustrous, gray-white, strong transition metal that resembles titanium....
. The Kroll process replaced the Hunter process
Hunter process

The Hunter process was the first industrial process to produce pure ductile metallic titanium. It was invented in 1910 by Matthew A. Hunter, a chemist born in New Zealand and who worked in America....
 for almost all commercial production.

ned rutile
Rutile

Rutile is a mineral composed primarily of titanium dioxide, titaniumoxygen2.Rutile is the most common natural form of TiO2....
 (or ilmenite
Ilmenite

Ilmenite is a weakly Magnetism 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....
) from the ore is reduced with petroleum-derived coke in a fluidized bed reactor at 1000 °C.






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The Kroll process is a pyrometallurgical industrial process used to produce metallic titanium
Titanium

Titanium is a chemical element with the symbol Ti and atomic number 22. Sometimes called the ?space age metal?, it has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver colour....
. It was invented in 1940 by William J. Kroll
William J. Kroll

William Justin Kroll was a metallurgist from Luxembourg. He is best known for inventing the Kroll process in 1940, which is used commercially to extract metallic titanium from ore....
 in Luxembourg
Luxembourg

Luxembourg , officially the Grand Duchy of Luxembourg , is a small landlocked country in western Europe, bordered by Belgium, France, and Germany....
. After moving to the United States, Kroll further developed the method in 1945 for the extraction of zirconium
Zirconium

Zirconium is a chemical element with the symbol Zr and atomic number 40. It is a lustrous, gray-white, strong transition metal that resembles titanium....
. The Kroll process replaced the Hunter process
Hunter process

The Hunter process was the first industrial process to produce pure ductile metallic titanium. It was invented in 1910 by Matthew A. Hunter, a chemist born in New Zealand and who worked in America....
 for almost all commercial production.

Process

Refined rutile
Rutile

Rutile is a mineral composed primarily of titanium dioxide, titaniumoxygen2.Rutile is the most common natural form of TiO2....
 (or ilmenite
Ilmenite

Ilmenite is a weakly Magnetism 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....
) from the ore is reduced with petroleum-derived coke in a fluidized bed reactor at 1000 °C. The mixture is then treated with chlorine
Chlorine

Chlorine...
 gas, affording titanium tetrachloride
Titanium tetrachloride

Titanium tetrachloride is the inorganic compound with the chemical formula TiCl4. It is an important intermediate in the production of titanium metal and the pigment titanium dioxide....
 TiCl4 and other volatile chlorides, which are subsequently separated by continuous fractional distillation
Fractional distillation

Fractional distillation is the separation of a mixture into its component parts, or fractions, such as in separating chemical compound by their boiling point by heating them to a temperature at which several fractions of the compound will evaporate....
. In a separate reactor, the TiCl4 is reduced by liquid magnesium (15-20% excess) at 800-850 °C in a stainless steel
Stainless steel

In metallurgy, stainless steel is defined as a steel alloy with a minimum of 10% chromium content by mass. Stainless steel does not stain, corrode, or rust as easily as ordinary steel , but it is not stain-proof....
 retort to ensure complete reduction:
2Mg
Magnesium

Magnesium is a chemical element with the symbol Mg, atomic number 12, atomic weight 24.3050 and common oxidation number +2.Magnesium, an alkaline earth metal, is the ninth most abundance of the chemical elements in the universe by mass....
(l) + TiCl4
Titanium tetrachloride

Titanium tetrachloride is the inorganic compound with the chemical formula TiCl4. It is an important intermediate in the production of titanium metal and the pigment titanium dioxide....
(g) ? 2MgCl2
Magnesium chloride

Magnesium chloride is the name for the chemical compounds with the chemical formulas MgCl2 and its various water of hydrations MgCl2x....
(l) + Ti
Titanium

Titanium is a chemical element with the symbol Ti and atomic number 22. Sometimes called the ?space age metal?, it has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver colour....
(s) [T = 800-850 °C]
Complications for the process result from partial reduction of the titanium to its lower chlorides TiCl2
Titanium(II) chloride

Titanium chloride is the chemical compound with the Chemical formula TiCl2. The black solid has been studied only moderately, probably because of its high reactivity....
 and TiCl3
Titanium(III) chloride

Titanium chloride is the inorganic compound with the formula TiCl3. At least four distinct species have this formula; additionally water of crystallization derivatives are known....
. The MgCl2
Magnesium chloride

Magnesium chloride is the name for the chemical compounds with the chemical formulas MgCl2 and its various water of hydrations MgCl2x....
 can be further refined back to magnesium. The resulting porous metallic titanium sponge is purified by leaching
Leaching

In general, leaching is the extraction of certain materials from a carrier into a liquid . Specifically, it may refer to:*Leaching *Leaching ...
 or heated vacuum distillation
Vacuum distillation

Vacuum distillation is a method of distillation whereby the pressure above the liquid mixture to be distilled is reduced to less than its vapor pressure causing evaporation of the most volatile liquid ....
. The sponge is jackhammered out, crushed, and pressed before it is melted in a consumable electrode vacuum arc furnace
Electric arc furnace

An is a furnace that heats charged material by means of an electric arc.Arc furnaces range in size from small units of approximately one ton capacity up to about 400 ton units used for secondary steelmaking....
. The melted ingot
Ingot

An ingot is a material, usually metal, that is Casting into a shape suitable for further processing. It requires a second procedure of shaping, by means of cold/hot working to produce the final product....
 is allowed to solidify under vacuum
Vacuum

A vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty," but in reality, no volume of space can ever be perfectly empty....
. It is often remelted to remove inclusions and ensure uniformity. These melting steps add to the cost of the product. Titanium is about six times as expensive as stainless steel.

History and subsequent developments

The Hunter process
Hunter process

The Hunter process was the first industrial process to produce pure ductile metallic titanium. It was invented in 1910 by Matthew A. Hunter, a chemist born in New Zealand and who worked in America....
 was replaced by Kroll's process. The Kroll process has not been replaced by a molten electrolytic
Electrolysis

In chemistry and manufacturing, electrolysis is a method of separating Chemical bond chemical compound by passing an electric current through them....
 process because of "redox recycling", failure of the diaphragm, and dendritic deposition in the electrolyte solution. However another process, the FFC Cambridge process
FFC Cambridge Process

The FFC Cambridge Process is an electrochemical method in which solid metal compounds, particularly oxides, are cathodically reduced to the respective metals or alloys in molten salts....
, has been patented for a solid electrolytic solution, and its implementation would eliminate the titanium-sponge processing. There is also a new method based on powder metallurgy that is currently being tried by angloresearch. It pyrometallurgical and involves the reduction of an intermediate form of titanium with aluminium. It combines the advantages of pyrometallurgy and a cheap reductant.

See also

  • Chloride process
    Chloride process

    The chloride process is used to purify titanium dioxide from minerals containing at least 90% titanium dioxide. In this process, the feedstock is heated at 900 ?C with carbon and chlorine gas, giving titanium tetrachloride....


Further reading

  • P.Kar, Mathematical modeling of phase change electrodes with application to the FFC process, PhD thesis; UC, Berkeley, 2007.