Crystal bar process

The crystal bar process (or iodide process) was discovered by Anton Eduard van Arkel and Jan Hendrik de Boer in 1925. It is also known as the van Arkel - de Boer process. This process was the first industrial process for the commercial production of pure ductile metallic zirconium. It is used in the production of small quantities of ultra-pure titanium and zirconium. It primarily involves the formation of the metal iodides and their subsequent decomposition to yield pure metal. This process was superseded commercially by the Kroll process.

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Impure titanium or zirconium is heated in an evacuated vessel with iodine at 50-250^oC. Titanium or zirconium iodide (TiI₄ or ZrI₄) is formed and is volatilized (leaving impurities as solid). At atmospheric pressure TiI₄ melts at 150^oC and boils at 377^oC, while ZrI₄ melts at 499^oC and boils at 600^oC. The boiling points are lower at reduced pressure. The gaseous metal tetraiodide is decomposed on a white hot tungsten filament (1400^oC). As more metal is deposited the filament conducts better and thus a greater electric current is required to maintain the temperature of the filament.