How it began
William Justin Kroll, a Luxembourgian chemist, worked in Luxembourg on new reducing agents and new apparatus for turning titanium tetrachloride into metal. In 1932 he produced significant quantities of ductile titanium by combining titanium tetrachloride with calcium, though the product still held oxide impurities. He then succeeded with magnesium at about 1,000 degrees Celsius in a molybdenum-clad reactor. By 1938 he had produced 20 kilograms of titanium and was convinced of its corrosion resistance and strength. He reported the magnesium results to the Electrochemical Society at a meeting in Ottawa.
Before it existed
An 1887 report by Nilsen and Pettersen applied sodium to the production of titanium metal, which was optimized into the commercial Hunter process, in which titanium tetrachloride is reduced with sodium. In the 1920s Anton Eduard van Arkel, working for Philips, described making very pure titanium by decomposing titanium tetraiodide with heat. The difficulty in every route was that titanium tetrachloride reacts readily with traces of water and metal oxides, which spoil the metal.
How it works
Titanium tetrachloride is reduced by liquid magnesium at 800 to 850 degrees Celsius in a sealed stainless steel retort, giving titanium metal and magnesium chloride. The magnesium chloride can be refined back to magnesium and reused. The titanium forms as a porous mass called sponge, which is purified by leaching or vacuum distillation. The sponge is crushed, pressed and melted in a consumable electrode vacuum arc furnace, and the ingot is often remelted to remove inclusions.
How it spread
After Kroll moved to the United States he developed the method further for the production of zirconium at the Albany Research Center. Britannica records that in the early 1940s he worked out a cheaper process for zirconium based on reducing zirconium tetrachloride with magnesium. The titanium process was taken up for industrial production, and Britannica calls Kroll the father of the modern titanium industry.
Where it stands today
The Kroll process displaced the Hunter process, which stopped being used commercially in the 1990s, and it remains the dominant technology for making titanium metal. The steps that turn sponge into usable metal have changed little since the 1950s. It is a batch process with several melting steps, which is one reason titanium costs about six times as much as stainless steel. Electrolytic alternatives such as the FFC Cambridge process have been proposed. Titanium alloys made this way are used in aerospace for their strength-to-weight ratio and corrosion resistance.
