- Kroll process
The Kroll process is a
pyrometallurgical industrial process used to produce metallictitanium . It was invented in1940 byWilliam J. Kroll inLuxembourg . After moving to the United States, Kroll further developed the method in1945 for the extraction ofzirconium . The Kroll process replaced theHunter process for almost all commercial production. [Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.]Process
Refined
rutile (orilmenite ) from the ore is reduced with petroleum-derived coke in a fluidized bed reactor at 1000 °C. The mixture is then treated withchlorine gas, affordingtitanium tetrachloride TiCl4 and other volatile chlorides, which are subsequently separated by continuousfractional distillation . In a separate reactor, the TiCl4 is reduced by liquid magnesium (15-20% excess) at 800-850 °C in astainless steel retort to ensure complete reduction: [Habashi, F. (ed.) Handbook of Extractive Metallurgy 1129±1180 Wiley-VCH, Weinham, 1997.] :2Mg(l) + TiCl4(g) → 2MgCl2(l) + Ti(s) [T = 800-850 °C] Complications for the process 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 byleaching or heatedvacuum distillation . The sponge is jackhammered out, crushed, and pressed before it is melted in a consumable electrode vacuum arc furnace. The meltedingot , is allowed to solidify undervacuum . 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 was replaced by Kroll's process. The Kroll process has not been replaced by a molten electrolytic process because of "redox recycling", failure of the diaphragm, and dendritic deposition in the electrolyte solution. However another process, theFFC Cambridge process , [cite journal | author = G. Z. Chen, D. J. Fray, T. W. Farthing | title = Direct Electrochemical Reduction of Titanium Dioxide to Titanium in Molten Calcium Chloride | journal = Nature | year = 2000 | volume = 407 | pages = 361–4 | doi = 10.1038/35030069] has been patented for a solid electrolytic solution, and its implementation would eliminate the titanium-sponge processing.There is also a new methos 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.ee also
*
Chloride process References
Further reading
* P.Kar, Mathematical modeling of phase change electrodes with application to the FFC process, PhD thesis; UC, Berkeley, 2007.
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