Precipitation of Zirconium-Hafnium Phosphates

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. 

For large precipitations an earthenware crock is suitable (Fig. 14). In such cases, the acid liquor is heated to 75° by a Vitreosil immersion heater, B. 

The solution is kept in agitation by a motor-driven stirrer, shaft, and paddles, C, which are constructed of an acid-resistant material such as Hastelloy. 

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. 

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