Kroll process 1606353 221079769 2008-06-22T23:37:58Z DOI bot 6652755 Citation maintenance. Initiated by [[User:Fconaway|Fconaway]]. You can [[WP:DOI|use this bot]] yourself! Please [[User:DOI_bot/bugs|report any bugs]]. The '''Kroll process''' is a [[pyrometallurgical]] industrial process used to produce metallic [[titanium]]. It was invented in [[1940]] by [[William J. Kroll]] in [[Luxembourg]]. After moving to the United States, Kroll further developed the method in [[1945]] for the extraction of [[zirconium]]. The Kroll process replaced the [[Hunter process]] for almost all commercial production.<ref>Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.</ref> ==The process== Refined [[rutile]] (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]] TiCl<sub>4</sub> and other volatile chlorides, which are subsequently separated by continuous [[fractional distillation]]. In a separate reactor, the TiCl<sub>4</sub> is reduced by liquid magnesium (15-20% excess) at 800-850 °C in a [[stainless steel]] retort to ensure complete reduction:<ref>Habashi, F. (ed.) Handbook of Extractive Metallurgy 1129±1180 Wiley-VCH, Weinham, 1997.</ref> :2[[magnesium|Mg]](l) + [[titanium tetrachloride|TiCl<sub>4</sub>]](g) → 2[[magnesium chloride|MgCl<sub>2</sub>]](l) + [[titanium|Ti]](s) [T = 800-850 °C] Complications for the process result from partial reduction of the titanium to its lower chlorides [[Titanium(II) chloride|TiCl<sub>2</sub>]] and [[Titanium(III) chloride|TiCl<sub>3</sub>]]. The [[magnesium chloride|MgCl<sub>2</sub>]] can be further refined back to magnesium. <!-- seems too detailed an inexplicable as well given the subsequent text "After several days, so commented out: reaction stops, the pressure rises, and the spongy titanium is crushed and melted into an [[ingot]].--> 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 [[Electric arc furnace|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. ==History and subsequent developments== The [[Hunter process]] was replaced by Kroll's process. The Kroll process has not been replaced by a molten [[Electrolysis|electrolytic]] process because of "redox recycling", failure of the diaphragm, and dendritic deposition in the electrolyte solution. However another process, the [[FFC Cambridge process]],<ref>{{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 (journal)|Nature]] | year = 2000 | volume = 407 | pages = 361–4 | doi = 10.1038/35030069}}</ref> has been patented for a solid electrolytic solution, and its implementation would eliminate the titanium-sponge processing. ==See also== * [[Chloride process]] ==References== <references/> ==Further reading== * P.Kar, Mathematical modeling of phase change electrodes with application to the FFC process, PhD thesis; UC, Berkeley, 2007. <!-- Categories --> [[Category:Industrial processes]] [[Category:Chemical processes]] [[Category:Zirconium]] [[Category:Titanium]] [[de:Kroll-Prozess]] [[pl:Proces Krolla]] [[pt:Processo Kroll]]