Kroll process
Encyclopedia
Kroll Process is a pyrometallurgical industrial process used to produce metallic titanium
. It was invented by William J. Kroll
in Luxembourg
. After moving to the United States, Kroll further developed the method for the production of zirconium
. The Kroll process replaced the Hunter process
for almost all commercial production.
(or ilmenite
) from the ore is reduced with petroleum-derived coke in a fluidized bed reactor
at 1000 °C. The mixture is then treated with chlorine
gas, affording titanium tetrachloride
TiCl4 and other volatile chlorides, which are subsequently separated by continuous fractional distillation
. In a separate reactor, the TiCl4 is reduced by liquid magnesium (15-20% excess) at 800-850 °C in a stainless steel
retort to ensure complete reduction:
Complications result from partial reduction of the titanium to its lower chlorides TiCl2
and TiCl3
. The MgCl2
can be further refined back to magnesium. The resulting porous metallic titanium sponge is purified by leaching
or heated vacuum distillation
. The sponge is jackhammered out, crushed, and pressed before it is melted in a consumable electrode vacuum arc furnace
. The melted ingot
is allowed to solidify under vacuum
. 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.
. In the 1920s van Arkel had described the thermal decomposition of titanium tetraiodide
to give highly pure titanium. Titanium tetrachloride was found to reduce with hydrogen
at high temperatures to give hydrides that can be thermally processed to the pure metal. With this background, Kroll developed both new reductants and new apparatus for the reduction of titanium tetrachloride. Its high reactivity toward trace amounts of water and other metal oxides presented challenges. Significant success came with the use of calcium
as a reductant, but the resulting still contained significant oxide impurities. Major success using magnesium
at 1000 °C using a molybdenum clad reactor, as reported to the Electrochemical Society in Ottawa. Kroll's titanium was highly ductile reflecting its high purity. The Kroll process displaced the Hunter process and continues to be the dominant technology for the production of titanium metal, as well as driving the majority of the world's production of magnesium metal.
Other technologies are competing with the Kroll process. One process involves electrolysis
of a molten salt
. Problems with this process include "redox recycling," the failure of the diaphragm, and dendritic deposition in the electrolyte solution. Another process, the FFC Cambridge process
, has been patented for a solid electrolytic solution, and its implementation would eliminate the titanium-sponge processing. Also in development is a pyrometallurgical route that involves the reduction of an intermediate form of titanium with aluminium. It combines the advantages of pyrometallurgy and a cheap reductant.
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....
. It was invented 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 landlocked country in western Europe, bordered by Belgium, France, and Germany. It has two principal regions: the Oesling in the North as part of the Ardennes massif, and the Gutland in the south...
. After moving to the United States, Kroll further developed the method for the production of zirconium
Zirconium
Zirconium is a chemical element with the symbol Zr and atomic number 40. The name of zirconium is taken from the mineral zircon. Its atomic mass is 91.224. It is a lustrous, grey-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, who worked in America....
for almost all commercial production.
Process
Refined rutileRutile
Rutile is a mineral composed primarily of titanium dioxide, TiO2.Rutile is the most common natural form of TiO2. Two rarer polymorphs of TiO2 are known:...
(or 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....
) from the ore is reduced with petroleum-derived coke in a fluidized bed reactor
Fluidized bed reactor
A fluidized bed reactor is a type of reactor device that can be used to carry out a variety of multiphase chemical reactions. In this type of reactor, a fluid is passed through a granular solid material at high enough velocities to suspend the solid and cause it to behave as though it were a fluid...
at 1000 °C. The mixture is then treated with 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, affording 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...
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 compounds by their boiling point by heating them to a temperature at which several fractions of the compound will evaporate. It is a special type of distillation...
. 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, also known as inox steel or inox from French "inoxydable", is defined as a steel alloy with a minimum of 10.5 or 11% chromium content by mass....
retort to ensure complete reduction:
- 2MgMagnesiumMagnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...
(l) + TiCl4Titanium tetrachlorideTitanium 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...
(g) → 2MgCl2Magnesium chlorideMagnesium chloride is the name for the chemical compounds with the formulas MgCl2 and its various hydrates MgCl2x. These salts are typical ionic halides, being highly soluble in water. The hydrated magnesium chloride can be extracted from brine or sea water...
(l) + TiTitaniumTitanium 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....
(s) [T = 800-850 °C]
Complications result from partial reduction of the titanium to its lower chlorides TiCl2
Titanium(II) chloride
Titanium chloride is the chemical compound with the formula TiCl2. The black solid has been studied only moderately, probably because of its high reactivity. Ti is a strong reducing agent: it has a high affinity for oxygen and reacts irreversibly with water to produce H2. The usual preparation is...
and TiCl3
Titanium(III) chloride
Titanium chloride is the inorganic compound with the formula TiCl3. At least four distinct species have this formula; additionally hydrated derivatives are known...
. The MgCl2
Magnesium chloride
Magnesium chloride is the name for the chemical compounds with the formulas MgCl2 and its various hydrates MgCl2x. These salts are typical ionic halides, being highly soluble in water. The hydrated magnesium chloride can be extracted from brine or sea water...
can be further refined back to magnesium. The resulting porous metallic titanium sponge is purified by leaching
Tank leaching
In metallurgical processes tank leaching is a hydrometallurgical method of extracting valuable material from ore.Tank leaching is usually differentiated from vat leaching on the following factors:...
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 electric arc furnace 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 cast into a shape suitable for further processing. Non-metallic and semiconductor materials prepared in bulk form may also be referred to as ingots, particularly when cast by mold based methods.-Uses:...
is allowed to solidify under vacuum
Vacuum
In everyday usage, 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". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...
. 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
Many methods have been applied to the production of titanium metal, beginning with a report in 1887 by Nilsen and Pettersen using sodium, which was optimized into the commercial Hunter processHunter 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, who worked in America....
. In the 1920s van Arkel had described the thermal decomposition of titanium tetraiodide
Titanium tetraiodide
Titanium tetraiodide is an inorganic compound with the formula TiI4. It is a rare molecular binary metal iodide, consisting of isolated molecules of tetrahedral Ti centers. Reflecting its molecular character, TiI4 can be distilled without decomposition at one atmosphere; this property is the...
to give highly pure titanium. Titanium tetrachloride was found to reduce with hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
at high temperatures to give hydrides that can be thermally processed to the pure metal. With this background, Kroll developed both new reductants and new apparatus for the reduction of titanium tetrachloride. Its high reactivity toward trace amounts of water and other metal oxides presented challenges. Significant success came with the use of calcium
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
as a reductant, but the resulting still contained significant oxide impurities. Major success using magnesium
Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...
at 1000 °C using a molybdenum clad reactor, as reported to the Electrochemical Society in Ottawa. Kroll's titanium was highly ductile reflecting its high purity. The Kroll process displaced the Hunter process and continues to be the dominant technology for the production of titanium metal, as well as driving the majority of the world's production of magnesium metal.
Other technologies are competing with the Kroll process. One process involves electrolysis
Electrolysis
In chemistry and manufacturing, electrolysis is a method of using a direct electric current to drive an otherwise non-spontaneous chemical reaction...
of a molten salt
Molten salt
Molten salt refers to a salt that is in the liquid phase that is normally a solid at standard temperature and pressure . A salt which is normally liquid at STP is usually called a room temperature ionic liquid, although technically molten salts are a class of ionic liquids.-Uses:Molten salts have...
. Problems with this process include "redox recycling," the failure of the diaphragm, and dendritic deposition in the electrolyte solution. 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. It is thought that this process will eventually be capable of producing metals or alloys more efficiently than...
, has been patented for a solid electrolytic solution, and its implementation would eliminate the titanium-sponge processing. Also in development is a pyrometallurgical route that involves the reduction of an intermediate form of titanium with aluminium. It combines the advantages of pyrometallurgy and a cheap reductant.
Further reading
- P.Kar, Mathematical modeling of phase change electrodes with application to the FFC process, PhD thesis; UC, Berkeley, 2007.