Kroll process

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.^[1]

Additional recommended knowledge

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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 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:^[2]

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 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.

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 The Kroll process has not been replaced by a molten electrolytic process because of "redox recycling," failure of the diaphragm, and dendritic deposition in the electrolyte solution.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, the FFC Cambridge process,^[3] has been patented for a solid electrolytic solution, and its implementation would eliminate the titanium-sponge processing.

See also

• Chloride process

References

1. ^ Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
2. ^ Habashi, F. (ed.) Handbook of Extractive Metallurgy 1129±1180 Wiley-VCH, Weinham, 1997.
3. ^ G. Z. Chen, D. J. Fray, T. W. Farthing (2000). "Direct Electrochemical Reduction of Titanium Dioxide to Titanium in Molten Calcium Chloride". Nature 407: 361-4. doi:10.1038/35030069.

Further reading

• P.Kar, Mathematical modeling of phase change electrodes with application to the FFC process, PhD thesis; UC, Berkeley, 2007.