The Kroll process is a pyrometallurgical industrial process used produce metallic titanium and was invented in 1940 by William J. Kroll . It replaced the Hunter process for almost all commercial production. In 1945 the Kroll process was modified to make zirconium.

The most pure ore of titanium, rutile (contrast the more impure ilmenite), is combined with petroleum coke and chlorinated in a fluid bed reactor at 1000°C converting it to a sponge-like state. Then chlorine gas is passed through the charge making titanium tetrachloride (TiCl₄, also known as 'tickle'). The resulting liquid form 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).

Kroll Process

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 low-value compound, contributing towards higher prices.

2Mg(l) + TiCl₄(l) → 2MgCl₂(l) + Ti(s)

After several days reaction stops and the pressure rises. The resulting porous metallic titanium sponge, impurites, 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 insure 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 The Kroll process has not been replaced by a molten electrolytic process because of redox recycling, diaphragm failure and dendritic deposition in the electrolyte solution.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 has been patented for a solid electrolytic solution, and there is a partial process for eliminating the titanium-sponge processing.