Hafnium tetrachloride

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). 

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. 

Reduction. Hafnium oxide can be reduced using calcium metal to yield a fine, pyrophoric metal powder (see Calciumand calciumalloys). This powder contains considerable oxygen contamination because of oxygen s high solubility in hot hafnium, and caimot be consoHdated into ductile metal. To obtain low oxygen ductile hafnium, the feed must be an oxygen-free halide compound such as hafnium tetrachloride or potassium hexafluorohafnate [16871-86-6]. 

Most hafnium compounds requite no special safety precautions because hafnium is nontoxic under normal exposure. Acidic compounds such as hafnium tetrachloride hydroly2e easily to form strongly acidic solutions and to release hydrogen chloride fumes, and these compounds must be handled properly. Whereas laboratory tests in which soluble hafnium compounds were injected into animals did show toxicity, feeding test results indicated essentially no toxicity when hafnium compounds were taken orally (33,34). 

After the discovery of isotactic polymerisation of propylene using shconocene catalysts, stmcturaHy analogous hafnium catalysts produced from hafnium tetrachloride [13499-05-3] were found to produce high yields of high molecular weight polypropylene (55), but not enough to lead to commercial development. 

Hafnium Boride. Hafnium diboride [12007-23-7] HfB2, is a gray crystalline soHd. It is usually prepared by the reaction of hafnium oxide with carbon and either boron oxide or boron carbide, but it can also be prepared from mixtures of hafnium tetrachloride, boron trichloride, and hydrogen above 2000°C, or by direct synthesis from the elements. Hafnium diboride is attacked by hydrofluoric acid but is resistant to nearly all other reagents at room temperature. Hafnium dodecaboride [32342-52-2] has been prepared by direct synthesis from the elements (56). 

Hafnium Carbide. Hafnium carbide, HfC, is a dark gray brittle soHd. This carbide can be prepared by heating an intimate mixture of the elements or by the reaction of hafnium tetrachloride with methane at 2100°C, but is commonly produced by the reaction of hafnium oxide with lampblack... 

At room temperature, hafnium dioxide is slowly dissolved by hydrofluoric acid. At elevated temperatures, hafnium dioxide reacts with concentrated sulfuric acid or alkaU bisulfates to form various sulfates, with carbon tetrachloride or with chlorine in the presence of carbon to form hafnium tetrachloride, with alkaline fluorosiUcates to form alkaU fluorohafnates, with alkaUes to form alkaline hafnates, and with carbon above 1500°C to form hafnium carbide. 

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 interaction between anhydrous hafnium tetrachloride and tetrahydrofuran is particularly exothermic and violent. 

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... 

Hafnium metal, analysis of, 13 87 Hafnium nitride, 13 89—90, 93 Hafnium oxide, 13 89, 93—94 reduction of, 73 84 Hafnium sulfides, 73 94 Hafnium tetrabromide, 73 93 Hafnium tetrachloride, 73 92 26 631 vapor reduction of, 73 84-85 Hafnium tetrafluoride, 73 90, 91 Hafnium tetrahydridoborate, 73 90 Hagen-Poiseuille expression/law, 27 726, 729 Hagen-Poiseuille flow, in microfluidics, 26 961... 

Hafnium chloride Hf + 4C1 —> HfCl also known as hafnium tetrachloride. 

In the presence of hafnium tetrachloride/trimethylchlorosilane catalyst, alkynyl allylsilane compounds undergo an intramolecular allylsilylation reaction to give cyclic alkenylsilanes in good yields ... 

Hafnium dioxide reacts with chlorine in the presence of carbon at elevated temperatures to yield hafnium tetrachloride, HfCh. When ammonium hydroxide solution is added to an acid solution of hafnium dioxide, the hydrous oxide, Hf02 xH20 precipitates. 

Hafnium tetrachloride is an important intermediate in production of hafnium metal. It also is used to prepare many hafnium compounds. 

Hafnium tetrachloride can be prepared (i) by chlorination of hafnium dioxide in the presence of carbon ... 

Hafnium tetrachloride reacts with water at room temperature, forming hafnium oxide chloride, HfOCb, and hydrochloric acid ... 

Hafnium tetrachloride combines with molten sodium chloride, potassium chloride, or other alkali halides to form addition products such as 2NaCl HfCl4, which decompose at higher temperatures. 

Hafnium tetrachloride forms many octahedral complexes of structure HfCl4L2 with neutral donors. For example, with tetrahydrofuran, it forms HfCl4(THF)2. Mononuclear and dinuclear hafnium chloride ions have been reported, produced by reaction with triphenylchloromethane, CPhsCl (Pampaloni, G. O. 1996. J. Organomet. Chem. 518, 189). 

Elemental composition Hf 55.73%, Cl 44.27% The acid extract of hafnium tetrachloride may be analyzed for hafnium by AA or other instrumental methods (See Hafnium). 

Hafnium tetrafluoride may be prepared by passing anhydrous hydrogen fluoride over hafnium tetrachloride at 300°C ... 

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