Hafnium phosphate, precipitation

Hafnium chloride, 4 121 Hafnium phosphate, precipitation of, with zirconium phosphate, 3 71... 

A sandy, easily filtered and washed phosphate precipitate is obtained by dispersing dilute solutions of the reactants into a large volume of hot (70 to 90°) dilute sulfuric acid. Three solutions are required (1) a 2 iV sulfuric acid solution, which sierves as the reaction medium (2) a zirconium hafnium oxysulfate solution containing 2 to 5 per cent ROz in 2 A sulfuric acid, and (3) a 2 to 5 per cent phosphoric acid solution in 2 iV sulfuric acid. 

The reagents (2) and (3) are added simultaneously by means of an atomizer arrangement (Fig. 15) into the hot 2 N sulfuric acid reaction medium. To obtain a satisfactory precipitate the reagents must be added at a rate not to exceed about 1 1. per hour. Care must be taken to add both reagents at an equivalent rate. The rate of flow is checked by means of a drop counter inserted below the stopcock of the separatory funnel, D. Since the rate of evaporation of the solution at 75° almost equals the rate of addition of the reagents, a constant volume of solution is maintained with some increase in the acid concentration. Inasmuch as zirconium and hafnium phosphates are among the most insoluble phosphates known, one complete precipitation serves to remove most of the impurities associated with the zirconium and hafnium. The precipitated phosphates are separated from the mother liquor by filtration. 

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. 

In the tributyl phosphate extraction process developed at the Ames Laboratory, Iowa State University (46—48), a solution of tributyl phosphate (TBP) in heptane is used to extract zirconium preferentially from an acid solution (mixed hydrochloric—nitric or nitric acid) of zirconium and hafnium (45). Most other impurity elements remain with the hafnium in the aqueous acid layer. Zirconium recovered from the organic phase can be precipitated by neutralization without need for further purification. 

Good separation is obtained by precipitation of the phosphates the hafnium concentrates in the less soluble fractions. A detailed description of the recovery of Hf from cyrtolite, a silicate of very high Hf content (5.5% HfOg), is given in E. M. Larsen, W. C. Fernelius and L. L. Quill in L. F. Audrieth, Inorg. ntheses, Vol. Ill, New York-Toronto-London, 1950, p. 67. 

Extraction of columbate-tantalates, titanocolumbates, and titanosilicates may also be initiated by treatment of the mineral with hydrofluoric acid. The procedure has the advantage that columbium, tantalum, uranium(VI), scandium, titanium, zirconium, and hafnium are dissolved, while silica is volatilized as silicon tetrafluoride and the rare earth elements, together with thorium and uranium(IV), remain as slightly soluble fluorides. The residue is then heated with concentrated sulfuric acid to remove hydrogen fluoride and to oxidize uranium (IV), the thorium is separated by precipitation of the phosphate (synthesis 12), and the rare earths are precipitated as oxalates. 

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