Equilibrium decomposition pressures of the compounds Na2ZrCl6 and Na2HfCl6

The chemical equilibria representing the reactions Na2ZrCl6 (s or l)>=2NaCl(s or l) + ZrCl4(vap, P, T) and Na2HfCl6 (s or l)>=2NaCl(s or l) + HfCl4(vap, P, T) were investigated by direct vapour pressure measurements and thermodynamic data such as free energies and enthalpies of reaction have been calculated. With reference to the separation of Hf from Zr by a recently developed process based on the selective reactivity of ZrCl4 and HfCl4 vapours with alkali chlorides, it is shown that the efficiency of alkali chlorides as separating agents increases in the order of LiCl→NaCl→KCl. (auth)