Iodide Process

a method of refining rare metals, involving the preparation of gaseous compounds of the metals with iodine (iodides) and their subsequent decomposition into pure metal and iodine. The first reference to the iodide method appeared in 1923, when it was used by the Dutch scientist Van Arkel to obtain pure tungsten. High-purity metals with a total impurity content of up to 10 −⁶ percent (titanium, zirconium) are obtained by the iodide process.

Refining is carried out in hermetically sealed containers, within which “low” (400’−700°C) and “high” (1300°−1700°C) temperature zones have been created. Ferrous crude metal in the form of powder, chips, or bits is mixed with a small quantity of iodine in the low temperature zone. The generated metal iodide vapor decomposes into iodine and free metal as it passes into the high temperature zone. Subsequently, iodine undergoes reverse diffusion into crude metal, again forming an iodide, while the vapor from the metal is precipitated out in a dense layer onto the incandescent filament of the heated element (the filament is usually made from the same pure metal). All the impurities that do not form iodides are isolated by iodide refining. Pure metal bars up to 0.04 m in diameter and up to 1 m long are produced in this way.

The iodide process is expensive and not very productive and is gradually being replaced by more promising methods (for example, zone melting); however, the purest metals are still obtained by the iodide process.