Kroll process

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title: "Kroll process" type: doc version: 1 created: 2026-02-28 author: "Wikipedia contributors" status: active scope: public tags: ["industrial-processes", "chemical-processes", "zirconium", "titanium-processes", "metallurgical-processes", "materials-science", "20th-century-inventions", "luxembourgish-inventions"] description: "Pyrometallugrical industrial process" topic_path: "science/chemistry" source: "https://en.wikipedia.org/wiki/Kroll_process" license: "CC BY-SA 4.0" wikipedia_page_id: 0 wikipedia_revision_id: 0

::summary Pyrometallugrical industrial process ::

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: :{TiCl4} + 2{Mg} -[825~^{\circ}\mathrm{C}]{Ti} + 2{MgCl2} The reduction is conducted at 800–850 °C in a stainless steel retort. Complications result from partial reduction of the TiCl4, giving to the lower chlorides TiCl2 and TiCl3. The MgCl2 can be further refined back to magnesium.

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) TiCl4 is reduced to the metal by sodium.

In the 1920s Anton Eduard van Arkel working for Philips NV 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 these three ideas as background, Kroll in Luxembourg 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 mixture still contained significant oxide impurities. 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.

References

References

1. Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. {{ISBN. 0-12-352651-5.
2. Habashi, F. (ed.) Handbook of Extractive Metallurgy, Wiley-VCH, Weinheim, 1997.
3. Heinz Sibum. (2005). "Titanium, Titanium Alloys, and Titanium Compounds".
4. Potter, Brian. (7 July 2023). "The Story of Titanium".
5. W. Kroll "Verformbares Titan und Zirkon" (Eng: Ductile Titanium and Zirconium) Zeitschrift für anorganische und allgemeine Chemie Volume 234, p. 42-50. {{doi. 10.1002/zaac.19372340105
6. W. J. Kroll, "The Production of Ductile Titanium" Transactions of the Electrochemical Society volume 78 (1940) 35–47.
7. (1955). "How commercial titanium and zirconium were born". Journal of the Franklin Institute.

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