Zirconium and Hafnium Tetrachlorides

Zirconium tetrachloride is produced on a large scale by a method developed by Kroll et al.1 in which the ultimate product is ductile zirconium. Zirconium oxide is converted to the carbide, which is chlorinated to yield the tetrachloride. This procedure is not easily adaptable to laboratory use, since an arc furnace is used to produce zirconium carbide. Furthermore, the chlorination of zirconium carbide is a strongly exothermic reaction requiring apparatus that will withstand temperatures of at least 1000°. 

The laboratory-scale preparation of. zirconium tetrachloride has been accomplished through the use of a variety of techniques. Several authors4 4 report that dry zirconium oxychloride may be thermally decomposed to yield zirconium oxide and zirconium tetrachloride. These authors 

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. 

Two impurities are apparent in the product, iron (III) chloride and hexachloroethane. Both are more volatile than zirconium tetrachloride. Nevertheless, if both are to be removed from the product, considerable product inevitably sublimes beyond K. The amount of hexachloroethane impurity is roughly a function of temperature, since runs made at 450° yield virtually none, whereas if the furnace is allowed to go much above 500°, appreciable quantities are formed. The amount of iron (III) chloride impurity depends on the purity of starting materials.  

In a typical preparation, 5.0 g. of hafnium oxide (0.025 mol) gave a yield of 6.7 g. of the tetrachloride (88%). Six-tenths of a gram of unreacted oxide was recovered. Small 

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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. 

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... 

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). 

FUN/UCH] Funaki, K., Uchimura, K., Sublimation pressures and some thermodynamic properties of zirconium and hafnium tetrachlorides, Report LA-TR-6763, (1965). Cited on pages 168, 170. 

DEN/SAF] Denisova, N. D., Safronov, E. K., Pustilnik, A. I., Bystrova, 0. N., Boundary liquid - vapour curve and saturated vapour pressure of zirconium and hafnium tetrachlorides, Russ. J. Phys. Chem., 41, (1967), 30-33. Cited on page 170. 

McLaughlin, D.R and Stoltz, R.A. (1988) Zirconium and hafnium tetrachloride separation by extractive distillation with molten zinc chloride lead chloride solvent. US Patent 4737 244. 

Sathiyamoorthy, D., Shetty, S.M., and Bose, D.K. (1999) Pyrochemical separation of zirconium and hafnium tetrachlorides using fused salt extractive distillation process. High Temp. Mater. Process, 18(4), 213. 

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