Zirconium-hafnium tetrachloride

In France, Compagnie Europnene du Zirconium (CEZUS) now owned jointly by Pechiney, Eramatome, and Cogema, uses a separation (14) based on the extractive distillation of zirconium—hafnium tetrachlorides in a molten potassium chloride—aluminum trichloride solvent at atmospheric pressure at 350°C. Eor feed, the impure zirconium—hafnium tetrachlorides from the zircon chlorination are first purified by sublimation. The purified tetrachlorides are again sublimed to vapor feed the distillation column containing the solvent salt. Hafnium tetrachloride is recovered in an enriched overhead fraction which is accumulated and reprocessed to pure hafnium tetrachloride. 

This reaction is endothermic and additional energy must be provided to sustain it, usually by induction heating, or by adding silicon carbide grain which chlorinates exothermically. The product gases are cooled below 200°C to condense and collect the zirconium—hafnium tetrachloride as a powder. The offgas stream then is refrigerated to obtain by-product silicon tetrachloride liquid. 

Molten Salt Distillation. Hafnium tetrachloride is slightly more volatile than zirconium tetrachloride, but a separation process based on this volatility difference is impractical at atmospheric pressures because only soHd and vapor phases exist. The triple point for these systems is at about 2.7 MPa (400 psia) and 400°C so that separation of the Hquids by distillation would necessarily require a massive pressurized system (13). 

Zirconium and hafnium are separated by fractional distillation of the anhydrous tetrachlorides in a continuous molten solvent salt KCl—AlCl system at atmospheric pressure (56,57). Zirconium and hafnium tetrachlorides are soluble in KCl—AlCl without compound formation and are produced simultaneously. 

Pure zirconium tetrachloride is obtained by the fractional distillation of the anhydrous tetrachlorides in a high pressure system (58). Commercial operation of the fractional distillation process in a batch mode was proposed by Ishizuka Research Institute (59). The mixed tetrachlorides are heated above 437°C, the triple point of zirconium tetrachloride. AH of the hafnium tetrachloride and some of the zirconium tetrachloride are distiUed, leaving pure zirconium tetrachloride. The innovative aspect of this operation is the use of a double-sheU reactor. The autogenous pressure of 3—4.5 MPa (30—45 atm) inside the heated reactor is balanced by the nitrogen pressure contained in the cold outer reactor (60). However, previous evaluation in the former USSR of the binary distiUation process (61) has cast doubt on the feasibHity of also producing zirconium-free hafnium tetrachloride by this method because of the limited range of operating temperature imposed by the smaH difference in temperature between the triple point, 433°C, and critical temperature, 453°C, a hafnium tetrachloride. 

The volatilities of both zirconium tetrachloride and hafnium tetrachloride are very similar to each other at normal operating temperatures, and their separation by a simple distillation or fractional distillation operation is not viable. However, when the mixed chloride vapor is contacted with an eutectic molten salt mixture of aluminum chloride and potassium chloride, zirconium chloride is preferentially absorbed. The vapor pressure difference between zirconium and hafnium tetrachlorides is greatly enhanced over the molten... 

See Hafnium tetrachloride Tetrahydrofuran Titanium tetrachloride Tetrahydrofuran Zirconium tetrachloride Tetrahydrofuran... 

Because of the relatively recent discovery of hafnium and the great cost of reasonably pure hafnium compounds, fewer authors report attempted syntheses of hafnium tetrachloride than for its congener, zirconium. De Boer and Fast11 report that the action of chlorine and carbon tetrachloride converts hafnium oxide to the tetrachloride. Fischer et al.12 prepared the tetrachloride by chlorinating a mixture of the oxide and carbon. 

Zirconium tetrachloride is a white crystalline solid with specific gravity 2.803.4 The solid sublimes under atmospheric pressure at 331° [log pBin. = — (26,000/4.57T) + 12.30] but melts at 437° under its own pressure, which is about 25 atm. at this temperature.18 The vapor density shows normal behavior up to 500°, at which point dissociation is suspected.16 Electron-diffraction results show tetrahedral symmetry with the Zr-Cl distance 2.33 A.17 Todd18 has recently determined the heat capacity of zirconium tetrachloride at low temperatures (extrapolated below —222°) and calculated the usual thermodynamic constants for the compound. The melting point of hafnium tetrachloride has been estimated as 432° under its own pressure, but the solid sublimes under atmospheric pressure at 317°.12 Both metal chlorides are extremely hygroscopic. They are easily soluble in water, although extensively hydrolyzed. All high-purity samples should be desiccated as well as possible. 

Both zirconium and hafnium tetrachlorides react with phthalonitrile at 170°C to give products which when crystallized from sulfuric acid were formulated as dihydroxyzirconium(IV) and -hafnium(IV) chloro-phthalocyanine dihydrates (284). Once again their formulation as oxy derivatives, perhaps polymeric, seems more reasonable. A sulfonated hafnium phthalocyanine has also been reported (119). 

U.S. Bureau of Mines plant. Figure 7.6 is a process flow sheet for the zirconium-hafnium separation portion of the U.S. Bureau of Mines zirconium plant at Albany, Oregon [Ml]. Commercial-grade zirconium tetrachloride containing about 2 w/o hafnium was dissolved in water together with ammonium thiocyanate (NH4CNS) and NH4OH, to make a feed solution... 

It has been observed that (110) zirconium tetrachloride reacts immediately with diarsine in THF, whereas the reaction with hafnium tetrachloride is much slower. A similar observation was made with the bromide. This is the basis for a separation process for zirconium and hafnium. Equimolar proportions of the tetrachlorides were dissolved in THF and enough diarsine was added to precipitate 86% of the zirconium. The white precipitate which was filtered after 10 minutes was found to contain 35% of the zirconium originally present. The cost of the ligand hardly makes this a practical method of separation, but the idea of a process based on differences in rates of reaction is an important idea to pursue in other systems. 

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