Crystal bar process

The van Arkel -de Boer process [ bu ː r ], also called growth method, was developed by Anton Eduard van Arkel and Jan Hendrik de Boer in 1924 transport reaction for the extraction or cleaning of metals, such B. titanium, zirconium, hafnium, vanadium, tantalum, thorium, chromium, rhenium, protactinium or compounds (for example, titanium carbide TiC, titanium nitride TiN, zirconium nitride ZrN ) and the metalloids boron, silicon.

Process

The metal to be cleaned is heated together with iodine at the bottom of the evacuated bell-shaped vessel at a reduced pressure of 0.1-20 Pa. For titanium, the reaction temperature is about 800 ° C.

In this case the forward reaction in the gaseous form at this temperature metal iodide, wherein the gaseous titanium as TiI4. This passes through diffusion or convection to a glowing tungsten wire on which it decomposes again. Here, the pure metal separates out in the reverse reaction. The released iodine again takes on metal. Any previously existing in the metal impurities remain in the residue.

The method takes advantage of the different equilibrium position of forward and reverse reaction, which, with respect to the Iodidbildung, is exothermic, that is, at high temperature the equilibrium shifts to the left side. This is entropically favorable, and therefore the reaction takes place under reduced pressure:

The reaction is exothermic. With AH = -427 kJ / mol

During operation of a halogen lamp, the method is exploited. Due to the high operating temperature sublimated tungsten atoms that have been deposited on the glass wall of the bulb, are transferred from the present in small amounts in the shielding gas iodine and chlorine in WCl4 or WI4. In the much higher temperature at the filament itself, the equilibrium shifts to the side of the reactants, so that the tungsten is deposited on the elementary filament, thus reducing the Sublimationsverlust.

Products

The photos show high-purity metal-to wax rods ( " Crystal Bar "), which were prepared by the Van Arkel -de Boer process.

Zirconium Crystal Bar, 99.97 %

Hafnium crystal bar, > 99.9 %

Vanadium Crystal Bar, 99.9 %