The alkali-fusion process was developed by the Ames Laboratory of the U. S. Atomic Energy Commission [B2] to provide a method for producing zirconium salts that did not need the high temperature of an electric furnace. A flow sheet for this process is shown in Fig. 7.5. In this process, zircon sand is mixed with from 1.0 to 1.5 times its weight of sodium hydroxide, and the mixture is heated in a furnace at 565°C. The sodium hydroxide melts at 318°C, and as its temperature rises it reacts with the zircon:

4NaOH + ZrSi04 -*■ Na2Zr03 + Na2Si03 + 2H20

Figure 7.4 Production of Z1CI4 from zircon.

Steam is evolved, the mix becomes viscous, and finally is converted to a fragile, porous solid (“frit”) when the temperature reaches 530°C. After cooling, this solid is ground and leached with water, which extracts the Na2Si03. The residue then is leached with acid, which dissolves the Na2Zr03. The final residue consists of unreacted zircon, which may be recycled. Any desired zirconyl salt can be made by using the appropriate acid in the final leaching step.

Ibis process would appear to be especially suitable for preparing feed for separating hafnium from zirconium by solvent extraction from an aqueous solution.

1.9 Fluosilicate Fusion

Fhiosilicate fusion has been used in the Soviet Union [SI] to produce feed for separation of hafnium from zirconium by fractional crystallization of K5MF6. Zircon is ground to pass 200 mesh and mixed with potassium fluosilicate and potassium chloride (to act as promoter). The mixture is sintered in a rotary furnace at 650 to 700°C. The following reaction takes place:

ZrSi04 + K2SiF6 -*■ KjZrF6 + 2Si03

The sinter is cooled, crushed to pass 100 mesh, and leached at 85°C with 1 percent HC1. The product is filtered at 80°C, then cooled, to crystallize K2ZrF6(+K2HfF6), which are filtered off and washed with water.