What is van Arkel process

Van Arkel de Boer method

Author: Hans Lohninger

The Van Arkel de Boer process was developed in 1924 by A.E. van Arkel and J.H. de Boer developed to be able to clean metals. The principle of this process is based on the reaction of iodine with the respective metal M to be cleaned. This reaction runs from left to right at low temperature (e.g. 600 ° C), but from right to left at high temperature (e.g. 1200 ° C):

M.(n) + n/2 I.2 M.(n)I.n

Now you put the metal in question with a little iodine in a vessel with a glowing tungsten wire in the middle (similar to a light bulb (1)), and if this is heated, the corresponding gaseous iodide will form and spread in the vessel. Iodide, which comes into contact with the much hotter tungsten wire, decomposes and leads to a deposition of the metal on the tungsten wire. The released iodine migrates back into the cooler areas of the vessel and there forms metal iodide again.

Overall, this process leads to a transport reaction in which impurities that do not form iodide (or do not deposit on the tungsten wire) remain and the cleaned metal grows on the wire.

High-purity titanium, manufactured using the Van Arkel de Boer process. The rod is approx. 14 cm long and has a diameter of around 25 mm.
Photo: Wikimedia Commons, User Alchemist-hp.
The Van Arkel de Boer process is normally carried out in a vacuum and can be used for the following elements: boron, chromium, hafnium, protactinium, rhenium, silicon, tantalum, thorium, titanium, vanadium and zirconium. The disadvantage of the process is that it can only be used for small quantities (on a kilogram scale).