Hafnium dissolved

Zirconium and hafnium dissolve rather less easily in these acids but all three metals dissolve in the presence of F ions, titanium giving salts of the Ti + ion, Zr and Hf salts of the ion. The metals are remarkably... 

The greatest problem in working with hafnium is finding a way to separate it from zirconium. Today, chemists know that compounds of hafnium dissolve more easily in some liquids than do compounds of zirconium. This method can be used to separate compounds of the two elements from each other. 

Hafnium carbide is inert to most reagents at room temperature, but is dissolved by hydrofluoric acid solutions which also contain an oxidising agent. Above 250°C, hafnium carbide reacts exothermically with halogens to form hafnium tetrahaUde, and above 500°C, with oxygen to form hafnium dioxide. At higher temperatures in a flow of hydrogen, hafnium carbide slowly loses some of its carbon. 

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. 

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). 

A simplified series of reactions between a hafnium salt and sulfuric acid is given in Fig. 4.3. The reactions showcase important facets of thin-film synthesis (but do not address the precise identities of intermediates or complexities of aqueous hafnium chemistry.) In the first step, a hafnium oxide chloride crystal hydrate is dissolved in water to disperse small hafnium-hydroxo molecular clusters. Sulfato ligands are subsequently added in the form of sulfuric acid. Since sulfato binds more strongly than chloro, hafnium-hydroxo-sulfato aqueous species are created. Under mild heating, these species readily poly-... 

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. 

Hafnium metal dissolves in HCI twarm) and slowly in H SOj, more rapidly if fluoride iun F is present, funning compounds of llfO . or fluttro complexes in the latter case. The metal resists the attack of weak acids and tbeir salts. 

In the TBP process, which was developed in the USA283 and in England,286 zirconium(IV) hydroxide (produced, for example, by the hydrolysis of the material obtained from the caustic fusion of zircon sand) is dissolved in nitric acid to give a solution containing 30-100 g of zirconium (plus hafnium) per litre and 5-8 M free nitric acid. The zirconium is extracted into a 50-60% solution of TBP in a suitable hydrocarbon diluent, the loaded organic phase is washed in 5 M nitric... 

To prepare the solution we used zirconium nitrate containing 1.5 to 2.1% of hafnium in a ratio to the sum of zirconium and hafnium oxides (2 Zr(Hf)02). Initial solutions of zirconium nitrate with different concentrations of salts and nitric acid have been prepared by dissolving a concentrated water solution of zirconium nitrate salts. TBF, dissolved by o-xylol has been used as extraction media. Trials have been done in batches. The extraction media, previously saturated by nitric acid, is mixed with water solution (20 ml at a time) for 15 minutes on a vibration apparatus. Re-extraction has been done by two-step vibration of the organic phase with equal volumes of distilled water. The water phase was, after extraction and reextraction, analyzed gravimetrically to find out the contents of the sum of zirconium and hafnium oxides. Hafnium contents were determined by spectral analysis. Nitric acid concentration has been determined by titration. 

Godfrey, L.V, White, W.M. and Salters, VJ.M. (1986) Dissolved zirconium and hafnium distributions across a shelf break in the Northeastern Atlantic Ocean. Geochim. Cosmochim. Acta, 60, 3995-4006. 

The zirconium tetrachloride product must then be purified before reduction to metal. In particular, hafnium must be removed to less than 100 ppm Hf Zr because of the high neutron absorption cross-section it exhibits, and phosphorus and aluminum must be removed to even lower specifications due to their deleterious metallurgical impact on the final zirconium alloys. The tetrachloride product is first dissolved in water under carefully controlled conditions to produce an acidic ZrOCl2 solution. This solution is complexed with ammonium thiocyanate, and contacted with methyl isobutyl ketone (MIBK) solvent in a series of solvent extraction columns. Advantage is taken of the relative solubilities of Zr, Hf, and Fe thiocyanate complexes to accomplish a high degree of separation of hafnium and iron from the zirconium. 

Zirconium and hafnium This separation must rank as one of the major achievements of inorganic paper chromatography. The metals are present as zirconyl and hafnyl nitrates basic nitrates must be absent since these are immobile. The mixture is best prepared by digesting the sample at 80°C with concentrated nitric acid and evaporating the excess acid at the same temperature under reduced pressure the product gives a clear solution when dissolved in water. 

Zirconium metal (mp 1855°C 15°C), like titanium, is hard and corrosion resistant, resembling stainless steel in appearance. It is made by the Kroll process (Section 17-A-l). Hafnium metal (mp 2222°C 30°C) is similar. Like titanium, these metals are fairly resistant to acids, and they are best dissolved in HF where the formation of anionic fluoro complexes is important in the stabilization of the solutions. Zirconium will burn in air at high temperatures, reacting more rapidly with nitrogen than with oxygen, to give a mixture of nitride, oxide, and oxide nitride (Zr2ON2). 

Zirconium and hafnium nitrides dissolve in cold 6 M HF but are stable in other acids except when heated (equation 4). 

With phosphorus, zirconium and hafnium form phosphides of composition M3P, MP, and MP2, which dissolve only in HF/HNO3 mixtures (see Phosphides Solid-state Chemistry). ZrP2 can be also obtained by reaction of ZrCLt with phosphine. 

The zirconium and hafnium complexes of trifluoroacetyl-acetone are white crystalline solids, insoluble in water but soluble in benzene, cyclohexane, and carbon tetrachloride. The hafnium complex melts at 128 to 129° and the zirconium complex at 130 to 131°. The complexes have been subjected to gas-phase chromatography and may be sublimed at 115° at a pressure of 0.05 mm. The proton magnetic resonance spectra of the compounds dissolved in carbon tetrachloride show single peaks in the methyl and methylene regions. The peaks appear at 2.20 and 6.00 p.p.m. (5) relative to tetramethylsilane (internal reference) for the zirconium complex and at 2.20 and 5.97 p.p.m. for the hafnium complex. 

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. 

The ZrF sample employed by McDonald et al. ( ) for enthalpy measurement was prepared by dissolving hafnium-free zirconium metal in 48% aqueous HF, and the resulting solution was evaporated to dryness. The crystalline product was heated slowly to 773 K in a platinum boat in a slow current of anhydrous HF. X-ray diffraction showed only crysalline ZrF. Wet analysis indicated 54.6% Zr (theory 54.55) and 44.9% F (theory 45.45). Due to the above facts we are uncertain whether the sample prepared is a mixture of a and B forms of a pure ZrF (B). Smith et al. (4) obtained their ZrF sample from the Oak Ridge National Laboratories, Oak Ridge, Tenn. Since the method of preparation of the compound is unavailable from the report, we do not know what kind of sample they used for measurement. 

Separation of hafnium from zirconium. In a zirconium-hafnium separation process developed by Eldorado Nuclear, a mixture of sodium zir-conate and hafnate can be obtained by fusing zircon sand with NaOH and dissolving the product in water. Acidification with nitric acid then gives aqueous zirconyl (ZrO +, or hydrolyzed Zr + ) and hafnyl (HfO " ") nitrates. Extraction with TBP then gives an extract containing mainly [Hf0(N03)2(TBP) ], but most of the ZrO " " remains in the aqueous phase and can be recovered on evaporation as essentially Hf-free Zr0(N03)2(s). The effectiveness of this process can be ascribed to compounding of factors... 

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