Zirconium and hafnium

Introduction.—The fifth ionization potential of zirconium has been re-evaluated as ca. 80.36 eV from spectroscopic studies. 3o. 131 describing the analytical 

Elinson and K. I. Petrov, Analytical Chemistry of Zirconium and Hafnium , Ann Arbor-Humphrey Ann Arbor, Mich, 1969. 

The chemistries of zirconium and hafnium are more nearly identical than for any other two congeneric elements. This is due in considerable measure the result of the lanthanide contraction having made both the atomic and ionic radii (1.45 and 0.86 A for Zr and Zr4 1.44 and 0.85 A for Hf and Hf4+) essentially identical. 

Oxidation state Coordination number Geometry Examples 

Hf° 6 Octahedral it complex [Zr(bipy)3], Zr(C4H6)2dmpe (arene)2HfPMe3 

d3 1 Zr11, d2 J 7 Pentagonal bipyramidal Complex sheet and [Zr(CO)5(SnMe3)2]2-[ cluster stmctures see text 

Hf11 8 Cp2M(CO)2, CpZrCl(dmpe)2, [Zr(CO)4(SnMe3)4]2- 

Introduction.—A text describing the co-ordination compounds of zirconium and hafnium with organic ligands has been published,and the history of the discovery of hafnium and its separation from zirconium has been described. There have been reviews of the structures of zirconium and hafnium compounds, of hydrozircona-tion, and of the organometallic chemistry of zirconium and hafnium.  

Introduction.—A text describing the chemistry of hafnium has been published.226 The structural,30,227 synthetic,3 and organometallic4 chemistry of zirconium and hafnium have been reviewed, and the compilation of the chemistry of organozirconium compounds has been revised.228 An account of the controversy following Urbain s claim, in 1911, to have isolated and identified element 72 has appeared.229 

Introduction.— The half-life of Hf has been determined as 1.9 0.3 years. The electronic spectrum of Zr has been re-analysed and the ionization potential of this ion estimated as 95.8 0.6 eV. The vapour pressures of solid and liquid zirconium and hafnium have been determined and used to calculate the enthalpies of sublimation at 298.15 K as 621 and 600 kJ mol, respectively.  

Sheka and K. F. Karlysheva, The Chemistry of Hafnium , Nauk Dumka, Kiev, 1972. 

F ure 3 A stereoscopic view of the (CHsj)s(C5H4)j Tia2 molecule (Reproduced by permission from J. Organometallic Chem., 1971, 30, 75) 

Zirconium (like niobium in the next group) appears to be receiving a considerable amount of attention chemically, and this interest is also apparent in the number of structural studies reported. With a few exceptions, however, the results obtained seem to follow established patterns. All the compounds of zirconium and hafnium which have been described contain the metals in the +4 oxidation state. 

The Hf-Cl distance of 2.466 A found in the anion in [(Bi+)(Bi9 +)-(HfCl8 )3], when compared with the Zr-Cl distances in the previous compound, implies a greater difference in the octahedral covalent radii of Zr and Hf than is usually accepted, although it is possible that the charge on the [HfClJ ion may have a significant lengthening effect. 

5 T/te ZrF polyhedron in (NjH,)ZirF, as a derivative of a square antiprism the mirror plane is sha d (Reproduced by permission from Acta Cryst., 1971, B27, 638) 

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Hafnium is a ductile metal with a brilliant silver luster. Its properties are considerably influenced by presence of zirconium impurities. Of all the elements, zirconium and hafnium are... 

Decomposition of Zircon. Zircon sand is inert and refractory. Therefore the first extractive step is to convert the zirconium and hafnium portions into active forms amenable to the subsequent processing scheme. For the production of hafnium, this is done in the United States by carbochlorination as shown in Figure 1. In the Ukraine, fluorosiUcate fusion is used. Caustic fusion is the usual starting procedure for the production of aqueous zirconium chemicals, which usually does not involve hafnium separation. Other methods of decomposing zircon such as plasma dissociation or lime fusions are used for production of some grades of zirconium oxide. 

Hafnium dioxide is formed by ignition of hafnium metal, carbide, tetrachloride, sulfide, boride, nitride, or hydrous oxide. Commercial hafnium oxide, the product of the separation process for zirconium and hafnium, contains 97—99% hafnium oxide. Purer forms, up to 99.99%, are available. 

J. Scheme , ASTM Manual on Zirconium and Hafnium, ASTM STP 639, American for Testing and Materials, Philadelphia, 1977. Covers safe handling of hafnium metal. 

D. J. Cardin, M. F. Lappert, and C. L. Raston, Chemistry of Organo-Zirconium and -Hafnium Compounds, Hasted Press, Division of John Wiley Sons,... 

Inc., New York, 1986. Excellent for organometaUic chemistry of zirconium and hafnium. 

P. C. Wailes, R S. P. Coutts, and H. Weigold, OrganometaUic Chemistry of Titanium, Zirconium, andHafnium, Academic Press, Inc., New York, 1974. Excellent for organometaUic chemistry of zirconium and hafnium. 

The manufacture of refractory metals such as titanium, zirconium, and hafnium by sodium reduction of their haHdes is a growing appHcation, except for titanium, which is produced principally via magnesium reduction (109—114). Typical overall haHde reactions are... 

R. J. H. Clark, D. C. Bradley, and P. Thornton, Chemistry of Titanium, Zirconium, and Hafnium, Pergamon Press, New York, 1975. 

Separation of Hafnium. Zirconium and hafnium always occur together in natural minerals and therefore all zirconium compounds contain hafnium, usually about 2 wt % Hf/Hf + Zr. However, the only appHcations that require hafnium-free material are zirconium components of water-cooled nuclear reactors. 

Zirconium and hafnium have very similar chemical properties, exhibit the same valences, and have similar ionic radii, ie, 0.074 mm for, 0.075 mm for (see Hafniumand hafnium compounds). Because of these similarities, their separation was difficult (37—40). Today, the separation of zirconium and hafnium by multistage counter-current Hquid—Hquid extraction is routine (41) (see Extraction, liquid—liquid). 

In the initial thiocyanate-complex Hquid—Hquid extraction process (42,43), the thiocyanate complexes of hafnium and zirconium were extracted with ether from a dilute sulfuric acid solution of zirconium and hafnium to obtain hafnium. This process was modified in 1949—1950 by an Oak Ridge team and is stiH used in the United States. A solution of thiocyanic acid in methyl isobutyl ketone (MIBK) is used to extract hafnium preferentially from a concentrated zirconium—hafnium oxide chloride solution which also contains thiocyanic acid. The separated metals are recovered by precipitation as basic zirconium sulfate and hydrous hafnium oxide, respectively, and calcined to the oxide (44,45). This process is used by Teledyne Wah Chang Albany Corporation and Western Zirconium Division of Westinghouse, and was used by Carbomndum Metals Company, Reactive Metals Inc., AMAX Specialty Metals, Toyo Zirconium in Japan, and Pechiney Ugine Kuhlmann in France. 

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. 

High molecular weight primary, secondary, and tertiary amines can be employed as extractants for zirconium and hafnium in hydrochloric acid (49—51). With similar aqueous-phase conditions, the selectivity is in the order tertiary > secondary > primary amines. The addition of small amounts of nitric acid increases the separation of zirconium and hafnium but decreases the zirconium yield. Good extraction of zirconium and hafnium from ca 1 Af sulfuric acid has been effected with tertiary amines (52—54), with separation factors of 10 or more. A system of this type, using trioctylarnine in kerosene as the organic solvent, is used by Nippon Mining of Japan in the production of zirconium (55). 

Zirconium and hafnium are separated by fractional distillation of the anhydrous tetrachlorides in a continuous molten solvent salt KCl—AlCl system at atmospheric pressure (56,57). Zirconium and hafnium tetrachlorides are soluble in KCl—AlCl without compound formation and are produced simultaneously. 

Boron forms B—N compounds that are isoelectronic with graphite (see Boron compounds, refractoryboron compounds). The small size also has a significant role in the interstitial alloy-type metal borides boron forms. Boron forms borides with metals that are less electronegative than itself including titanium, zirconium, and hafnium. 

Table 21.1 summarizes a number of properties of these elements. The difficulties in attaining high purity has led to frequent revision of the estimates of several of these properties. Each element has a number of naturally occurring isotopes and, in the case of zirconium and hafnium, the least abundant of these is radioactive, though with a very long half-life ( Zr, 2.76%, 3.6 x 10 y Hf, 0.162%, 2.0 X 10 5 y). 

The effect of the lanthanide contraction on the metal and ionic radii of hafnium has already been mentioned. That these radii are virtually identical for zirconium and hafnium has the result that the ratio of their densities, like that of their atomic weights, is very close to Zr Hf = 1 2.0. Indeed, the densities, the transition temperatures and the neutron-absorbing abilities are the only common properties of these two elements which differ... 

The chemistry of hafnium has not received the same attention as that of titanium or zirconium, but it is clear that its behaviour follows that of zirconium very closely indeed with only minor differences in such properties as solubility and volatility being apparent in most of their compounds. The most important oxidation state in the chemistry of these elements is the group oxidation state of +4. This is too high to be ionic, but zirconium and hafnium, being larger, have oxides which are more basic than that of titanium and give rise to a more extensive and less-hydrolysed aqueous chemistry. In this oxidation state, particularly in the case of the dioxide and tetrachloride, titanium shows many similarities with tin which is of much the same size. A large... 

Table 21.2 Oxidation states and stereochemistries of titanium, zirconium and hafnium...

The effect of the metals used was then examined (Table 5.4). When the group 4 metals, titanium, zirconium, and hafnium, were screened it was found that a chiral hafnium catalyst gave high yields and enantioselectivity in the model reaction of aldimine lb with 7a, while lower yields and enantiomeric excesses were obtained using a chiral titanium catalyst [17]. 

The liquid-liquid extraction (solvent extraction) process was developed about 50 years ago and has found wide application in the hydrometallurgy of rare refractory and rare earth metals. Liquid-liquid extraction is used successfully for the separation of problematic pairs of metals such as niobium and tantalum, zirconium and hafnium, cobalt and nickel etc. Moreover, liquid-liquid extraction is the only method available for the separation of rare earth group elements to obtain individual metals. 

The extraction of metals by liquid amines has been widely investigated and depends on the formation of anionic complexes of the metals in aqueous solution. Such applications are illustrated by the use of Amberlite LA.l for extraction of zirconium and hafnium from hydrochloric acid solutions, and the use of liquid amines for extraction of uranium from sulphuric acid solutions.42,43... 

The metallocene dichloride of zirconium and hafnium 20b and 20c were also prepared and underwent reduction with potassium to give monomeric metallocene monochloride complexes 21b and 21c (Eq. 8) [39b]. The structure of the zirconocene complex 21 b in the crystal showed a conformation which suggests a less steric strain as compared to 21a due to zirconium s larger atomic size. As a consequence of the coordinative unsaturation an unusually short Zr —Cl bond length was found. 

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