Hafnium(IV) Chloride - Purification

Purification

The purification of HfCl₄ tends to be a difficult endeavor because Hf and Zr have very similar chemical and physical properties. Both metals have the same number of d electrons and they have a small difference in atomic radii (hafnium atomic radius is 156.4 pm while that of Zr is 160 pm). This means that these two metals undergo similar reactions and form the similar coordination complexes which makes the separation of these so formed complexes difficult.

The Tetrahalides can also be separated by selectively reducing the Zirconium tetrahalide to one or more lower halides or even Zirconium. The Hafnium tetrachloride remains substantially unchanged during the reduction and may be recovered readily from the zirconium subhalides. Hafnium tetrachloride is volatile and can therefore easily be separated from the involatile zirconium trihalide. This is based on the difference in the reducibility between the two tetrahalides.

There have been a number of proposed processes that could be used to purify HfCl₄ from ZrCl₄ and some of the proposed methods include fractional distillation, fractional precicpitation, fractional crystallization and ion exchange.