Hafnium hydroxide, precipitation

Assay of beryUium metal and beryUium compounds is usuaUy accompHshed by titration. The sample is dissolved in sulfuric acid. Solution pH is adjusted to 8.5 using sodium hydroxide. The beryUium hydroxide precipitate is redissolved by addition of excess sodium fluoride. Liberated hydroxide is titrated with sulfuric acid. The beryUium content of the sample is calculated from the titration volume. Standards containing known beryUium concentrations must be analyzed along with the samples, as complexation of beryUium by fluoride is not quantitative. Titration rate and hold times ate critical therefore use of an automatic titrator is recommended. Other fluotide-complexing elements such as aluminum, sUicon, zirconium, hafnium, uranium, thorium, and rate earth elements must be absent, or must be corrected for if present in smaU amounts. Copper-beryUium and nickel—beryUium aUoys can be analyzed by titration if the beryUium is first separated from copper, nickel, and cobalt by ammonium hydroxide precipitation (15,16). 

Precipitation of Hafnium Hydroxide. In order to interpret the adsorption data it was necessary to determine the conditions which lead to the precipitation of hafnium hydroxide. It is not usually advisable to depend on the solubility product because the information on this quantity is often unreliable for hydroxides of polyvalent metal ions. In addition, "radiocolloids may apparently form much below saturation conditions in radioactive isotope solutions. In the specific case of hafnium hydroxide only two measurements of the solubility seem to have been reported. According to Larson and Gammill (16) K8 = [Hf(OH)22+] [OH ]2 — 4 X 10"26 assuming the existence of only one hydrolyzed species Hf(OH)22+. The second reported value is Kso = [Hf4+] [OH-]4 = 3.7 X 10 55 (15). If one uses the solubility data by Larson and Gammill (Ref. 16, Tables I and III) and takes into consideration all monomeric hafnium species (23) a KBO value of 4 X 10 58 is calculated. 

Figure 8 illustrates the correlation between the apparent adsorption increase and the onset of precipitation of hafnium hydroxide. 

The results presented in Figures 7 and 8 and the centrifugation experiments in Figure 1 show that saturation adsorption of hafnium on glass is reached before the onset of hydroxide precipitation. Therefore, the adsorption results must be explained in terms of soluble hafnium... 

The quadridentate ligands A,iV-dihydroxyethylglycine and N-hydroxyethyliminodiacetate(HIMDA) form 2 1 chelate zirconium complexes which are stable with respect to hydroxide precipitation even up to pH 10. These quadridentate ligands involve the bonding of the alkoxide groups at the higher pH values. The formation constant for the 1 1 HIMDA-hafnium complex in 0.123 M HCIO4 is log A = 14.6 (170, 305). 

The precipitability by polyhydroxyanthraquinones from weakly acid solution is specific for zirconium (and hafnium). The precipitate is an adsorption compound (lake) of zirconium hydroxide and alizarin (compare Al-alizarin lake, page 97). The production of the lake involves the binding, through chemical adsorption, of alizarin on the surface of the Zr(OH)4 sol particles, which are present in solutions of zirconium salts as a result of the hydrolysis Zr+ -f- 4 HgO- Zr(OH)4 -f 4 H+. The hydrolysis equilibrium is constantly disturbed by the removal of Zr(OH)4, so that, in a not too acid solution, there is extensive precipitation of zirconium in the form of the alizarin lake. This lake is also produced by precipitating solutions of zirconium salts with ammoniacal solutions of alizarin. The lake is stable against dilute hydrochloric acid. In strong hydrochloric acid solutions of zirconium salts, alizarin produces a fairly stable hydrosol of the lake (compare the test for fluoride, page 221). 

Analyses of alloys or ores for hafnium by plasma emission atomic absorption spectroscopy, optical emission spectroscopy (qv), mass spectrometry (qv), x-ray spectroscopy (see X-ray technology), and neutron activation are possible without prior separation of hafnium (19). Alternatively, the combined hafnium and zirconium content can be separated from the sample by fusing the sample with sodium hydroxide, separating silica if present, and precipitating with mandelic acid from a dilute hydrochloric acid solution (20). The precipitate is ignited to oxide which is analy2ed by x-ray or emission spectroscopy to determine the relative proportion of each oxide. 

Hafnium Acetate. Hafnium acetate [15978-87-7], Hf(OH)2(CH2COO)2, solutions are prepared by reacting the basic carbonate or freshly precipitated hydroxide with acetic acid. The acetate solution has been of interest in preparing oxide films free of chloride or sulfate anions. 

In the drying of compound intermediates of refractory and reactive metals, particular attention is given to the environment and to the materials so that the compound does not pick up impurities during the process. A good example is the drying of zirconium hydroxide. After the solvent extraction separation from hafnium, which co-occurs with zirconium in the mineral zircon, the zirconium values are precipitated as zirconium hydroxide. The hydroxide is dried first at 250 °C for 12 h in air in stainless steel trays and then at 850 °C on the silicon carbide hearth of a muffle furnace. 

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. 

The crude tetrachloride mixture of zirconium and hafnium is dissolved in ammonium thiocyanate solution. The solution is extracted with methyl isobutyl ketone (MIBK). MIBK is passed countercurrent to aqueous mixture of tetrachloride in the extraction column. Halhium is preferentially extracted into MIBK leaving zirconium in the aqueous phase. Simultaneously, zirconium tetrachloride oxidizes to zirconyl chloride, ZrOCb. When sulfuric acid is added to aqueous solution of zirconyl chloride, the chloride precipitates as a basic zirconium sulfate. On treatment with ammonia solution the basic sulfate is converted into zirconium hydroxide, Zr(OH)4. Zirconium hydroxide is washed, dried, and calcined to form zirconium oxide, Zr02. 

Because of the inconsistency of these results, experiments were carried out to establish the precipitation boundaries, as described earlier. Figure 1 gives as an example four curves in which the fraction of hafnium removed by precipitation as hydroxide is plotted against the pH for four different concentrations of HfCL. Open and blackened symbols are for experiments in which systems were equilibrated before centrifugation 1 hour and 70 hours, respectively. In all cases insoluble precipitates are... 

Purified zirconyl chloride solution is then reacted sequentially with surfuric acid and ammonium hydroxide to precipitate a complex zirconium oxysulfate. The precipitate is washed, filtered, and stripped to remove traces of MIBK, and then calcined to drive off sulfur and convert the product to Zr02. The precipitation process leaves behind most of the aluminum and phosphorus. The hafnium stream leaving solvent extraction is treated similarly, producing a Hf02 byproduct. 

Zirconium hydroxide is precipitated by bases at lower pH than the hafnium compound. Zr and Hf are obviously unable to form true hydroxides, and these compounds are more correctly formulated as MO2 XH2O. Amorphous hydrous zirconia and hafhia (a-phase) transform to microcrystalline forms (/f-phase) with noticeable heat evolution. They lose water up to the composition MO2 H2O at 140 °C (Zr) or 155 °C (Hf). Hydrous zirconia has excellent absorptive capacity, particularly for oxygen-containing anions. For example, the concentration of S04 anions over hydrous zirconia is so low that no precipitate forms on the addition of barium salts to the filtrate. While the hydroxides of composition M(OH)4 are not stable, in alkaline solutions, M(OH)s are present and even M(0H)6 anions have been reported in very concentrated alkalis. Salts of these anions, such as Na2Hf(OH)6, can be isolated. 

The thorium compound (10 g.) reacts with water (100 ml.) to give an insoluble portion for which the oxalate/thor-ium mol ratio is 2.67 and a soluble portion which can be recovered by precipitation with alcohol and which has an oxalate/thorium mol ratio of 2.99. The remainder of the oxalate is lost by replacement by hydroxide ion and water and stays in solution. The zirconium and hafnium compounds, on the other hand, can be recrystallized from water with very little loss of oxalate. 

Zirconium purification and conversion to zirconium dioxide. The zirconium-product stream leaving the HCNS recovery column D of Fig. 7.6 contained most of the metal impurities in the ZrCl4 feed other than hafnium. Purified zirconium was obtained by precipitating Zr(OH)4 at a pH low enough to prevent precipitation of other metal hydroxides. The precipitation procedure used by the Bureau of Mines was as follows. Zirconium content of the raffinate was diluted to 19 g/liter. To every cubic meter of diluted raffinate were added 5.7 liters of concentrated 33 N sulfuric acid, followed by sufficient 28% ammonium hydroxide to bring the pH to 1.2 to... 

Hafnium Content. The hafnium concentration is best determined by the method of Claasen as modified by Schumb and Pittman, Willard and Freund, and the checkers of this synthesis. An aliquot of. the zirconium hafnium solution that will provide about 1 g. of the oxide is diluted with water in a 250-ml. beaker. An excess of ammonium hydroxide is added to precipitate the hydroxide. The precipitate is filtered, washed free of sulfate, and then dissolved in 10 ml. of hot concentrated hydrochloric acid. The solution is placed in a 50-ml. beaker and is evaporated almost to dryness to dehydrate the silicic acid. The residue is treated with 15 ml. of hot 8 N hydrochloric acid and the insoluble material removed by filtration on a coarse paper. 

Total Oxide, RO2. An aliquot of the zirconium hafnium solution that will provide approximately 0.5 g. of the ignited oxide is diluted with water to 100 ml. and treated with ammonium hydroxide to precipitate the mixed hydroxides. The precipitate is filtered on quantitative paper, washed free of sulfate, placed in a weighed crucible, and ignited (900°) to constant weight in a muffle furnace or over a Fisher burner. 

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. 

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