The Kroll process is a pyrometallurgical industrial process used to produce metallic titanium. It was invented by William J. Kroll in Luxembourg. After moving to the United States, Kroll further developed the method for the production of zirconium. The Kroll process replaced the Hunter process for almost all commercial production.
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 or sodium (15-20% excess) at 800-850 °C in a stainless steel retort to ensure complete reduction:
Complications 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.
Many methods have 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 the 1920s van Arkel had described the thermal decomposition of titanium tetraiodide to give highly pure titanium. Titanium tetrachloride was found to reduce with hydrogen at high temperatures to give hydrides that can be thermally processed to the pure metal. With this background, Kroll developed both new reductants and new apparatus for the reduction of titanium tetrachloride. Its high reactivity toward trace amounts of water and other metal oxides presented challenges. Significant success came with the use of calcium as a reductant, but the resulting still contained significant oxide impurities. Major success using magnesium at 1000 °C using a molybdenum clad reactor, as reported 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.
Other technologies are competing with the Kroll process. One process involves electrolysis of a molten salt. Problems with this process include "redox recycling," the failure of the diaphragm, and dendritic deposition in the electrolyte solution. Another process, the FFC Cambridge process, has been patented for a solid electrolytic solution, and its implementation would eliminate the titanium-sponge processing. Also in development is a pyrometallurgical route that involves the reduction of an intermediate form of titanium with aluminium. It combines the advantages of pyrometallurgy and a cheap reductant.
