1.10 Methods

Lustman and Kerze [LI], pp. 115-129, list a number of processes that have been used on the laboratory scale for separation of zirconium and hafnium. Of these, three that have been used on an industrial scale are

1. Fractional crystallization of double fluorides

2. Solvent extraction of the thiocyanates by hexone

3. Solvent extraction of the nitrates by tributyl phosphate, TBP

These processes are described in Secs. 7.2 through 7.4. Another recently patented [М2, М3] process,

4. Selective reduction of the molten double fluorides by aluminum dissolved in molten zinc seems promising and is described in Sec. 7.5.

1.11 Fractional Crystallization

Solubilities of corresponding salts of hafnium and zirconium are compared in Table 7.11.

Fractional crystallization of the double potassium fluorides was the method originally used to separate hafnium from zirconium. Because these salts form solid solutions and the ratio of solubilities is close to unity (1.54 at 20°C), multiple recrystallizations are necessary for the necessary completeness of separation. In the United States, Kawecki [Kl] has found that 10 recrystallizations of the double potassium fluorides reduced the hafnium content of zirconium from 2.0 to 0.1 percent.

A similar process has been used in the Soviet Union [SI], where the operating temperatures and solubilities were as follows:

After 16 to 18 recrystallizations, the hafnium content of zirconium was reduced to 0.003 percent. The yield of zirconium was about 80 percent.

Because of the large number of independent steps, this fractional crystallization process has been superseded by the solvent extraction processes next to be described.