Kroll process

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The Kroll process is a pyrometallurgical industrial process used to produce metallic titanium. It was invented in 1940 by William J. Kroll in Luxemborg. 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.

[edit] The process

First, titanium dioxide is "carbochlorinated" to produce TiCl₄.

Preferably the most pure ore of titanium, rutile (in contrast the more impure ilmenite), is combined with petroleum coke and chlorinated in a fluid bed reactor at 1000°C by first converting it to titanium-carbide in a sponge-like state. Then chlorine gas is passed through the charge making volatile titanium tetrachloride (TiCl₄, also known as 'tickle'). The resulting condensate liquid of TiCl₄ is purified by continuous fractional distillation until it is pure, the recovered chloride is reprocessed and fed back into the system. TiCl₄ sells for about $1.30/lb. Problems in this stage relate to lower chlorides forming (TiCl₂, and TiCl₃).

Next, the titanium tetrachloride (TiCl₄) is reduced with magnesium and is heated.

A clean dry stainless steel retort is vacuum-pumped, and filled with a slight over-pressure of argon (an inert atmosphere). Then enough magnesium to completely react the TiCl₄, plus 15-30% excess is put into the retort. It is then heated to 800°C-900°C and TiCl₄ is slowly fed in. The magnesium chloride is periodically tapped off.

The expensive magnesium is lost to a lower-value compound, contributing towards higher prices. (Although the MgCl₂ can be further refined back to magesium.)

2Mg(l) + TiCl₄(l) → 2MgCl₂(l) + Ti(s)

After several days reaction stops, the pressure rises, and the spongy titanium is crushed and melted into an ingot.

The resulting porous metallic titanium sponge, impurities, and unreacted magnesium are 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. The first melt adds about a $1/lb to the cost of titanium, and each additional remelt about $.50/lb.

This batch process takes several days to produce only a couple of tons per reactor vessel, and the titanium is about six times as expensive as stainless steel. The Kroll process has not been replaced by a molten electrolytic process because of redox recycling, diaphragm failure and dendritic deposition in the electrolyte solution. However another process, the FFC Cambridge Process, has been patented for a solid electrolytic solution, and there is a partial process for eliminating the titanium-sponge processing.

[edit] References

• Habashi, F. (ed.) Handbook of Extractive Metallurgy 1129±1180 Wiley-VCH, Weinham, 1997.
• G. Z. Chen, D. J. Fray, T. W. Farthing "Direct Electrochemical Reduction of Titanium Dioxide to Titanium in Molten Calcium Chloride" Nature 2000, Volume 407, pages 361-4.