The Kroll process is a
pyrometallurgical
Pyrometallurgy is a branch of extractive metallurgy. It consists of the thermal treatment of minerals and metallurgical ores and concentrates to bring about physical and chemical transformations in the materials to enable recovery of valuable ...
industrial process used to produce metallic
titanium
Titanium is a chemical element with the 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
titanium tetrachloride
Titanium tetrachloride is the inorganic compound with the formula . It is an important intermediate in the production of titanium metal and the pigment titanium dioxide. is a volatile liquid. Upon contact with humid air, it forms thick clouds o ...
. The Kroll process replaced the
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 the United States. The process involves reducing titanium tet ...
for almost all commercial production.
Process
In the Kroll process, the TiCl
4 is reduced by liquid magnesium to give titanium metal:
:
TiCl4 + 2Mg -> 25^oCi + 2MgCl2
The reduction is conducted at 800–850 °C in a
stainless steel
Stainless steel is an alloy of iron that is resistant to rusting and corrosion. It contains at least 11% chromium and may contain elements such as carbon, other nonmetals and metals to obtain other desired properties. Stainless steel's corros ...
retort.
Complications result from partial reduction of the TiCl
4, giving to the lower chlorides
TiCl2 and
TiCl3. The
MgCl2 can be further refined back to magnesium. The resulting porous metallic titanium sponge is purified by
leaching
Leaching is the loss or extraction of certain materials from a carrier into a liquid (usually, but not always a solvent). and may refer to:
* Leaching (agriculture), the loss of water-soluble plant nutrients from the soil; or applying a small amou ...
or
vacuum distillation
Vacuum distillation is distillation performed under reduced pressure, which allows the purification of compounds not readily distilled at ambient pressures or simply to save time or energy. This technique separates compounds based on differences i ...
. The sponge is crushed, and pressed before it is melted in a consumable carbon electrode vacuum
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 producing ...
. The melted
ingot
An ingot is a piece of relatively pure material, usually metal, that is cast into a shape suitable for further processing. In steelmaking, it is the first step among semi-finished casting products. Ingots usually require a second procedure of sh ...
is allowed to solidify under
vacuum
A vacuum is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or "void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often dis ...
. 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 earlier
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 the United States. The process involves reducing titanium tet ...
, which ceased to be commercial in the 1990s, the TiCl
4 from the chloride process is reduced to the metal by sodium.
[
]
History and subsequent developments
The Kroll process was invented in 1940 by William J. Kroll in Luxembourg
Luxembourg ( ; lb, Lëtzebuerg ; french: link=no, Luxembourg; german: link=no, Luxemburg), officially the Grand Duchy of Luxembourg, ; french: link=no, Grand-Duché de Luxembourg ; german: link=no, Großherzogtum Luxemburg is a small lan ...
. After moving to the United States, Kroll further developed the method for the production of zirconium
Zirconium is a chemical element with the symbol Zr and atomic number 40. The name ''zirconium'' is taken from the name of the mineral zircon, the most important source of zirconium. The word is related to Persian '' zargun'' (zircon; ''zar-gun'', ...
.
Many methods had 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 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 the United States. The process involves reducing titanium tet ...
. In the 1920s van Arkel had described the thermal decomposition of titanium tetraiodide
Titanium tetraiodide is an inorganic compound with the formula TiI4. It is a black volatile solid, first reported by Rudolph Weber in 1863. It is an intermediate in the van Arkel–de Boer process for the purification of titanium.
Physical prop ...
to give highly pure titanium. Titanium tetrachloride was found to reduce with hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, an ...
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 is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to ...
as a reductant, but the resulting mixture still contained significant oxide impurities. Major success using magnesium
Magnesium is a chemical element with the 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 of the periodic ta ...
at 1000 °C using a molybdenum clad reactor, as reported to the Electrochemical Society in Ottawa.[W. J. Kroll, “The Production of Ductile Titanium” Transactions of the Electrochemical Society volume 78 (1940) 35–47.] Kroll's titanium was highly ductile reflecting its high purity. The Kroll process displaced the 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 the United States. The process involves reducing titanium tet ...
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.
See also
* Chloride process The chloride process is used to separate titanium from its ores. The goal of the process is to win high purity titanium dioxide from ores such as ilmenite (FeTiO3) and rutile (TiO2). The strategy exploits the volatility of TiCl4, which is readily p ...
* {{Annotated link, FFC Cambridge process
References
Further reading
* P.Kar, Mathematical modeling of phase change electrodes with application to the FFC process, PhD thesis; UC, Berkeley, 2007.
External links
Titanium: Kroll Method
YouTube video uploaded by Innovations in Manufacturing at Oak Ridge National Laboratory
Oak Ridge National Laboratory (ORNL) is a U.S. multiprogram science and technology national laboratory sponsored by the U.S. Department of Energy (DOE) and administered, managed, and operated by UT–Battelle as a federally funded research and ...
Industrial processes
Chemical processes
Zirconium
Titanium