Zirconium thiocyanates

These results explain previous observations of the greater stability of zirconium thiocyanate and selenocyanate complexes compared with their hafnium analogues, and the greater stability of zirconium and hafnium complexes in MeCN compared with DMF in terms of competition between the ligand and solvent molecules for co-ordination sites on the metal. Zirconium alkoxides have been prepared from ZrCl4 and aliphatic alcohols158 but with salicylaldehyde a Meerwein-Ponndorf... 

Hafnium thiocyanate is more soluble than, and thus separable from, zirconium thiocyanate in, for example, 4-methyl-2-pentanone (methyl isobutyl ketone, hexone ). 

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. 

The zone elution method has been used for quantitative estimation or recovery of heavy metals in plants and vegetable juices [29], mercury (11) in river and waste waters [52], zinc in different environmental samples [46], nickel and copper in alloys [53], zirconium in Mg-Al alloys [22], cobalt, zinc, nickel, and copper in natural water and alloy samples [54], thiocyanate in spiked photogenic waste water [55], and aluminum in bauxite ores [42],... 

Ammonium thiocyanate is used in the manufacture of herbicides, thiourea, and transparent artificial resins in matches as a stabilizing agent in photography in various rustproofing compositions as an adjuvant in textile dyeing and printing as a tracer in oil fields in the separation of hafnium from zirconium, and in titrimetric analyses. 

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. 

Zirconium and hafnium cyanate, thiocyanate, and selenocyanate complexes that contain N-heterocyclic ligands are discussed in Sections 32.4.2.5.iii, 32.4.2.5.iv and 32.4.2.5.V. 

In aqueous solution, zirconium(IV) and hafnium(IV) form complexes M(NCS)4-", where n = 1-8.104 Selective extraction of hafnium thiocyanate complexes from acidic aqueous solution by methyl isobutyl ketone is a widely used industrial method for the separation of zirconium and hafnium. Separation methods have been reviewed by Vinarov.105... 

Solvent extraction has proved to be the most effective method for the separation of zirconium and hafnium, which invariably occur in nature in close association, owing to their almost identical chemical properties. These metals have found considerable use in the nuclear-power industry on account of their unusually high (hafnium) and low (zirconium) neutron-capture cross-sections. It is evident that the mutual separation of the two metals must be of a high degree to make them suitable for such applications. Two different solvent-extraction processes are known to be used on a commercial scale in one process, zirconium is selectively extracted from nitrate media into TBP in the second process, hafnium is selectively extracted from thiocyanate solutions into methyl isobutyl ketone (MIBK). 

The reasons for the selective extraction of hafnium over zirconium from thiocyanate solutions by solvating extractants are not well understood. Hence, a recent review of the chemistry of these metals described the separation process but offered no explanation for the observed selectivity.306 There is no evidence that differences in the stabilities of the thiocyanate complexes of hafnium(IV) and zirconium(IV) are responsible for the selective extraction of the former, since the formation constants of the respective complexes are essentially identical for both metals.307 However, there is some indication that the hafnium thiocyanates are more readily solvated by the extractant than are the corresponding complexes of zirconium. 

Most frequently, the number of thiocyanate moieties in the extracted complex is equal to two, although some authors consider that the extracted species changes progressively to the trithio-cyanato complex (x = 1) and tetrathiocyanato complex (x = 0) on the successive extraction of a given aqueous phase.312 IR data show that the thiocyanate groups are bound to the metal via the nitrogen atom,315 as would be expected from the distinct A-character of zirconium(IV). 

Fuels used include antimony sulfide (which also acts as a frictionator), gum arabic (which also acts as a binding agent), calcium silicide (which also acts as a frictionator), nitrocellulose, carbon black, lead thiocyanate, and powdered metals such as aluminum, magnesium, zirconium, or their alloys. 

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. 

Addition of thiocyanate ions to chloride or perchlorate solntions of zirconium and hafnium yields complexes containing from one to eight isothiocyanate groups per metal atom. These systems are of interest because of the importance of thiocyanate complexes in the extraction and separation of the elements. IR spectroscopy indicates that M-N bonds are present in the violet (Zr) and pink (Hf) complexes [NEt4]2[M(NCS)6] analogous complexes have been obtained with alkali metal cations. In the presence of pyridine, the dodecahedral Zr(bipy)2(NCS)4 complex is produced see Ammonia N-donor Ligands). 

Derivation Extremely difficult to separate from zirconium. Most important methods are (1) solvent extraction of the thiocyanates by hexone, (2) solvent extraction of the nitrates by tributyl phosphate, (3) fractional crystalization of the double fluorides. 

The thiocyanate method has been utilised for determination of tungsten in ores and concentrates [7], molybdenum and its compounds [128], steels [6,129,131,134,178], refractory alloys [131] titanium, zirconium, and their alloys [136], nickel alloys [179], and ferromolybdenum [135]. 

Zr has been separated from Hf with the use of TBP in 4 M HNO3 [21]. These metals can also be separated in thiocyanate medium with the use hexanone as extractant. Ion-pairs of thiocyanate complexes of Zr and Hf with antipyrine and DAM were separated by extraction (isoamyl alcohol, 1,2-dichloroethane) from other metals [22]. Zirconium can be extracted first with mesityl oxide from a 4 M solution (in HNO3 and NaNOs), then hafnium is extracted from 0.4 M HNO3 and 2 M NH4SCN medium [23]. 

Zirconium (hafnium) can be determined with Arsenazo III directly in the extract of the thiocyanate complex with antipyrine in isoamyl alcohol [22]. The fluoride complex of zirconium (ZrFe ) has been extracted with TOA in benzene, then the extract has been shaken with Arsenazo HI solution [58]. Zirconium has also been determined after froth-flotation of the Zr compound with Arsenazo El and Zephiramine [59],... 

Extraction with triphenylarsine oxide in CHCI3 followed by the reaction with Arsenazo III (after stripping of the analytes with 4 M HCl) makes a basis of the determination of Zr and Hf (and Ti) in minerals and alloys [1], Sequential liquid-liquid extraction and spectrophotometric determination of zirconium(IV) with calixarene hydroxamic acid and thiocyanate has been described [2]. Colorimetric determination of Zr and Hf with Xylenol Orange and a liquid-liquid extraction of thenoyltrifluoroacetone complexes using H2O-IBMK has been studied [3]. 

An investigation of the extraction equilibria of zirconium and hafnium thiocyanates in mixed aqueous-MeCOBu media showed that the distribution coefficients change at higher concentrations, probably because of polymerization of the ions and formation of complexes other than M(NCS)4. 

Chlorination of zircon has been the process mainly used in the United States because it produces ZrCU, which is used in the Kroll process for making zirconium metal (Sec. 8.3), and because ZrCU was the feed material for the first process developed for separating hafnium from zirconium, using thiocyanate extraction (Sec. 7.3). 

History. In 1947, Fischer and co-workers [FI, F2] described a solvent extraction method for separating hafnium from zirconium in which an aqueous solution of sulfates containing ammonium thiocyanate was extracted with diethyl ether containing thiocyanic acid. Hafnium concentrates preferentially in the organic phase in one reported experiment zirconium in the aqueous phase contained 0 percent hafnium, while the organic phase zirconium contained more tlum 5 percent. Six to eight batch laboratory separations concentrated hafnium from 0.5 percent in zirconium to 70 to 90 percent. 

U.S. Bureau of Mines plant. Figure 7.6 is a process flow sheet for the zirconium-hafnium separation portion of the U.S. Bureau of Mines zirconium plant at Albany, Oregon [Ml]. Commercial-grade zirconium tetrachloride containing about 2 w/o hafnium was dissolved in water together with ammonium thiocyanate (NH4CNS) and NH4OH, to make a feed solution... 

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