What is the Kroll process?

Titanium can be used in medical implants.

The Kroll process is a method used to convert ore into metallic titanium. Engineers, manufacturing firms and medical companies use titanium for a variety of different purposes because it is as strong as steel but lighter. Traces of titanium can be found in minerals such as rutile and ilmenite, but the Kroll pyrometallurgical process removes impurities and produces a metal that can be used in medical implants, construction, and aircraft design.

Titanium was discovered in Great Britain in 1791 by a man named William Gregor. A German scientist named Martin Heinrich Klaproth named it after the Greek god Titan during the same year. Scientists began to develop ways to extract the newly discovered element from rutile and ilmenite, and in 1910 a chemist named Matthew Hunter developed a method for producing metallic titanium by mixing rutile with coke and chlorine. The Hunter process became the first industrial-scale process for the production of this metal.

During the 1930s, a scientist from Luxembourg named William Kroll began experimenting with titanium. In 1938, he developed what later became known as the Kroll method. Kroll moved to the United States after the outbreak of World War II, and his process of producing metallic titanium was patented in the United States in 1940. The patent was later revoked by the federal government because Kroll was not a citizen of the United States. He entered a seven-year legal battle that culminated in the reinstatement of the patent.

The Kroll process begins by passing chlorine gas through rutile in a chlorinator. During the first step of the process, titanium tetrachloride and chlorides are produced from the ore. Oxygen is removed from the titanium tetrachloride through a distillation process, leaving titanium tetrachloride in liquid form. Scientists add liquid magnesium or sodium to titanium tetrachloride and the end result is a metallic sponge.

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This titanium sponge is crushed and then placed in a consumable electrode vacuum arc furnace. The sponge melts inside the oven, but, unlike other metals, it is not spilled because it solidifies in a vacuum. A titanium ingot produced during the Kroll process can weigh over 5,000 kilograms (5.51 tons). The multiple steps of the Kroll process mean that titanium is much more expensive to produce than similar types of metal, such as steel.

After developing the Kroll process, William Kroll used a similar technique to create zirconium metal. Both titanium and zirconium are now used to make parts for spacecraft. Titanium, unlike zirconium, is not a health hazard and is therefore also used for medical implants.

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