The Kroll process is a pyrometallurgical industrial process used to produce metallic titanium from titanium tetrachloride. As of 2001 William Justin Kroll's process replaced the Hunter process for almost all commercial production.
Process
In the Kroll process, titanium tetrachloride is reduced by liquid magnesium to give titanium metal:
:<chem>{TiCl4} + 2{Mg} ->[825~^{\circ}\mathrm{C}]{Ti} + 2{MgCl2}</chem>
The reduction is conducted at 800–850 °C in a stainless steel retort. Complications result from partial reduction of the TiCl<sub>4</sub>, giving to the lower chlorides TiCl<sub>2</sub> and TiCl<sub>3</sub>. The 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.-->
Appurtenant processes
The resulting porous metallic titanium sponge is purified by leaching or vacuum distillation. The sponge is crushed, and pressed before it is melted in a consumable electrode vacuum arc furnace, "backfilled with pure gettered argon of a pressure high enough to avoid a glow discharge".
History and subsequent developments
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. In this process (which ceased to be commercial in the 1990s) TiCl<sub>4</sub> is reduced to the metal by sodium. Major success using magnesium at 1000 °C using a molybdenum clad reactor, was reported by Kroll to the Electrochemical Society in Ottawa. Kroll's titanium was highly ductile reflecting its high purity.
The Kroll process displaced the Hunter process 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.
After moving to the United States, Kroll further developed the method for the production of zirconium at the Albany Research Center.
See also
- Chloride process