Hafnium oxychloride

The HfOClj-SHsO must be recrystallized to obtain a product which dissolves to give a clear solution. If a gelatinous precipitate appears when the hafnium oxychloride is dissolved, it may be removed by centrifuging before proceeding with the next step. This results in a corresponding lower yield of final product. 

A variety of methods (65, 228, 434) have been used to prepare soluble tetraoxalato salts. A suitable synthesis for potassium tetraxoalato-zirconate(hafnate) 267) free of hydroxy species is one in which a zirconium (hafnium) oxychloride solution is added to a buffered solution containing a stoichiometric amount of potassium oxalate and half the amount of oxalic acid. The potassium tetraoxalatometallate penta-hydrate is then precipitated upon slow addition of ethanol. 

Morozov, A.I., Karlova, E.V. Physico-chemical properties of anhydous Zirconium and Hafnium oxychlorides. Zh. Neorg. Khim. 16, 25 (1971)... 

Willi improved means to separate the compounds of these two elements, future research will yield more details of specific hafnium compounds. The methods of separation used effectively include ion exchange techniques, a particularly effective one using u column of silica gel. w ith a solution of the tetrachlorides in methanol as feed and a 1.9 N HCI solution as eluant for zirconium. Separations also have heen accomplished through the distillation of the phosphorus oxychloride addition products. 

Hafnium (IV) oxide is prepared by calcination of the hydroxide, oxalate, oxychloride or sulfate at 600-1000°C. The crystallization of the oxide starts at 400°C. 

To prepare low-hafnium zirconia, the mother liquor from the first fractional precipitation is treated with ammonium hydroxide to precipitate the hydroxides, which are then filtered and redissolved in a calculated amount of moderately concentrated sulfuric acid. This solution is then diluted with the amount of water required to give 2 N sulfuric acid solution that contains 5 per cent RO2. From a fraction containing 0.7 per cent hafnium, one fractionation in which about 60 per cent of the total oxide is precipitated as the phosphate will yield a product containing only 0.2 per cent hafnium. Additional fractionations of the mother liquor will reduce the hafnium content to a concentration below the sensitivity of the arc spectrographic method used (about 0.05 per cent Hf). Because the impurities concentrate in the most soluble fraction, a complete phosphate precipitation is made on the final solution, the precipitate is washed with 2 N sulfuric acid, and then converted to the peroxy compound. For final purification, the acid-soluble peroxy compound is dissolved in hydrochloric acid, and the oxychloride prepared according to the procedure of Young and Arch. The oxychloride may then be used as a starting material for the preparation of any other zirconium compound. 

Removal of the hafnium tetrachloride from the zirconium tetrachloride cannot easily be achieved in a direct manner. However, the addition compounds with phosphorus pentachloride and with phosphorus oxychloride (3Zr(Hf)Cl4.2POCls) have been separated by fractional distillation on a small scale.A proposal has also been made to separate the chloride by vapour phase dechlorination with a mixture of chlorine and oxygen, making use of the difference in equilibrium constant for the two reactions ... 

A small air vibrator is used occasionally, to loosen the product from the condenser walls and provide a new condensing surface. As in the case of zirconium tetrachloride, additional purification by sublimation is needed, immediately before metal production, to remove oxide and oxychloride. A rather lower sublimation temperature is used, however, for hafnium owing to the more volatile nature of its oxychloride. 

It is possible to separate zirconium and halfnium by cation-exchange, but they must both be first absorbed upon the resin and then subjected to elution analysis down a column of resin. This process was first described briefly by Street and Seaborg in 1948. They absorbed zirconium and hafnium from the oxychloride solution upon the cation-exchange resin... 

Dowex 50, by a special technique. In this, the resin in the ammonium form was suspended in 2M perchloric acid and the oxychlorides added slowly with air agitation. About 80 per cent of the zirconium and hafnium were absorbed and this resin slurry was placed on top of a column of resin for elution with 6M hydrochloric acid. The hafnium was eluted first and a fairly good separation of the two elements was obtained. 

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