A DISSERTATION entitled THE ANALYTICAL CHEMISTRY OF ZIRCONIUM by P. R. S. SMITH Submitted in part fulfilment of the requirements for the Diploma of Imperial College. Imperial College S.W,7. June, 1961. ACKNOWLEDGEMENTS I wish to express my thanks to Assistant Professor L. S. Theobald for his assistance and advice in the writing of this dissertation. riz. s. s, ..„------ . CONTENTS Page No. 1. Introduction . o oe ow oo oo Os 04 so 1 2. Separation of Zirconium from Hafnium .. 3 3. Indirect Determination of Zirconium and Hafnium . • .. 11 4. Separation of Zirconium from most other Elements (a) General Methods .. .. .. .. 16 (b) "Selective" Reagents .. .. .. 18 (c) Separation of Zirconium from Niobium and Tantalum .. 42 5. Volumetric Determination of Zirconium 46 6. Instrumental Methods .. .. .. 60 7. Conclusion . .. 00 00 00 00 00 00 62 1. Zirconium is a transition metal, a member of sub— group IVA in the periodic table. The normal valency of Zirconium is 4, although valencies of 5,6,7 and 8 can be realised by co—ordination; valencies of 2 and 3 are known, but only in exceptional circumstances. Although normally quadrivalent, zirconium never exists as the Zr4+ ion but always in combination with some other element, usually oxygen or fluorine. for which it has great affinity. The most stable entity is the zirconyl ion, ZrO2+ , which can undergo normal ionic reactions. In sulphate solutions there is evidence that zirconium is present mainly as an ionic complex containing one or more zirconium atoms bound by oxygen linkings (1). In faintly acid solutions hydrolysis of the zirconium can give rise to some difficulty. However, the degree of complex formation or hydrolysis depends on both time and temperature, so that a freshly prepared solution of zirconium might give rise to different reactions from those of a solution that has been either set aside for some time or heated. This phenomenon must always be borne in mind, especially in the preparation of solutions for analysis (2). An exceptional feature of zirconium is the chemical similarity of hafnium, the next member of the sub—group. This is due to the lanthanide contraction; as a result, the difference in atomic volume between the two metals is only 2.2 per cent. M MO2 Molecular Volume Zr 13.97 21.50 Hf 13.66 21.70 Diff % —2.22 +0.93 Linear Diff % —0.74 +0.31 This means that these elements are more similar in character than two successive lanthanides (3). Zirconium has been shown by microscopical examina— tion to be one of the most constant of rock constituents, usually in the form of zircon (ZrSiO4) (4). It may be present up to a few per cent, but rarely reaches 0.2 per cent, and is normally less than 0.1 per cent. The ore minerals are zircon and baddeleyite (Zr02). Other minerals are: Oxides Silicates Brazilite Naegite Zirbelite Lavenite Uhligite Hainite Eudialyte Rosenbuschite G. Van Hevesy (5) found hafnium in all zirconium minerals, usually not greater than 2.0 per cent. except in alvite and cyrtolite, altered zircons. Only the rare mineral 3 thortveitite contains more hafnium than zirconium (6). Most analytical methods give results for zirconium plus hafnium. 2. Separation of Zirconium from Hafnium The separation of zirconium and hafnium is extremely difficult and, as yet, no analytical separation is known. Since pure zirconium is required for the prepara- tion of standards, methods of separation are of interest to the analyst. Most early work in this field was concerned with the extraction of hafnium; recently more interest has been shown in the preparation of "hafnium-free" zirconium metal to accommodate the needs of nuclear technology, as it has particularly suitable properties (5), i.o. "transparent" to neutrons (Zr : 0.18 barns cf. Hf 120 barns). The methods available may be conveniently grouped as follows : (a) Fractional Crystallization (b) Fractional Distillation (c) Ion-Exchange (d) Liquid-Liquid Extraction (e) Miscellaneous. (a) Fractional Crystallization The earliest attempt at a separation was made by fractionally crystallizing the ammonium or potassium double fluorides, e.g. KZrF6, or KHfF6. As the hafnium salt is more soluble it concentrates in the mother-liquors, while zirconium is left in the precipitates. Von Hevesy and his co-workers (7) had to make 650 crystallizations to produce hafnium substantially free from zirconium. Van Atkel and De Boer (8) with a very similar method, had to make 180 crystallizations to concentrate hafnium from 10 per cent. to 50 per cent. of the mother liquor. Von Hevesy noted that hafnium secondary phosphate was appreciably less soluble than zirconium secondary phos- phate but attempted no further work because of the difficult nature of the precipitate. De Boer (9) in 1926 managed to conduct a separation by dissolving the freshly precipitated phosphates in saturated oxalic acid solution and re-preci- pitating in the cold by adding hydrochloric acid. After only 26 precipitations, 97 per cent. hafnium was produced. In oxalic acid, the phosphates form soluble com- plexes that are easily decomposed by mineral acid. The hafnium complexes are less stable and are decomposed more easily. This factor helps in the separation. De Boer (10) studied many reagents that form similar complexes including sulphuric acid, orthophosphoric acid, hydro- fluoric acid, dicarboxylic acid, o( -hydroxy carboxylic acids and polyhydroxy alcohols. By dissolving the precipitated phosphates concentrated sulphuric acid and re- precipitating by dilution with water, De Boer was able to obtain substantially zirconium-free hafnium in 12-15 precipitations. However, such a process is very expensive because of the large amounts of sulphuric acid required. By precipitating with sodium ferrocyanide from a mixture of zirconium and hafnium sulphates in the presence of dilute sulphuric acid, oxalic acid, and ammonium sulphate, Prandtl (11) obtained a precipitate enriched with hafnium. After three precipitations a product containing 95 per cent. of hafnium was obtained. Schumb and Pittman (12) in 1943 were unable to repeat Prandtl's work and with a revised procedure enriched a mixture containing 20 per cent. of hafnium to 80 per cent. of hafnium in four precipitations. To effect a separation using cheap materials, Larsen, Fernelius and Quill (13) used fractional precipita- tion of the phosphates from dilute sulphuric acid solution. To obtain a readily filterable precipitate they sprayed, similtaneously, zirconyl/hafnyl solutions and dilute ortho- phosphoric acid solution into 10 per cent. v/v sulphuric acid at 70°C. The action of an icc-cold solution of sodium hydroxide and sodium peroxide on the cold phosphate slurry yielded an acid-soluble hydrate which was reprecipi- tated as before. In seven operations the hafnium content - 6 - of the precipitate changed from 13 per cent. to 93 per cent. By treating the mother-liquors, zirconium substantially free from hafnium was obtained. Willard and Freund (14) precipitated the zirconyl and hafnyl phosphates by hydrolysis of trimethyl phosphate. This yielded an easily filterable precipitate. Separation of the two elements was achieved after 5 to 6 precipitations. However, Hillebrand et al. (4) have been unable to obtain clear-cut separations. Today, the fractional precipitation of potassium fluorzirconate is used in Russia on an industrial scale to produce hafnium-free zirconium (6). In 16 to 18 recrystal- lizations zirconium containing less than 0.01 per cent. of hafnium is obtained. By re-cycling the mother-liquors an 80 per cent. yield of the zirconium added is obtained. (b) Fractional Distillation The tetrachlorides of zirconium and hafnium form addition compounds with phosphorus pentachloride and phos- phorus oxychloride. Van Arkel and De Boer (15) fraction- ally distilled a mixture of crude zirconium/hafnium tetra- chloride and phosphorus pentachloride repeatedly at 340 to 410°0 in a stream of carbon dioxide and obtained a fraction, b.p. 416°, to which they assigned the formula 2ZrO14.FC15 and a fraction, b.p. 363, which was 2ZTC14.P0C13. The 7 PC1 residue 2ZrOl4 4 containing some hafnium was collected in several fractions that were analysed for hafnium by X-ray spectroscopy. They found that hafnium concentrated in the lower fractions. Gruen and Kratz (16) using phosphorus oxychloride obtained compounds of the type 3MC14.2P0C13. As the hafnium compound boiled at 355°C and the zirconium compounds at 360°C it was possible by careful fractionation through an efficient column (a glass perforated-plate column with 50 plates) to separate the two compounds fairly completely in relatively few fractions. For example, in a typical experiment, a zirconium compound containing 2.5 per cent. of hafnium yielded a first fraction (5 per cent.) containing 10 per cent of hafnium; the residue after distilling 40 per cent. away contained less than 0.2 per cent. of hafnium. Fractional distillation has not found any techni- cal applications. (c) Ion-exchange Several papers on ion-exchange separations of the two elements appeared in the late forties and early fifties but very little has appeared recently. Solvent extraction appears to be more popular on an industrial scale. Street and Seaborg (17) were among the first to show that a cation—exchange separation was possible. A mixture of oxyehlorides (mg. quantities) was absorbed on to a column of Dowex - 50 (H+ form) and eluted with 6M hydro- chloric acid. Hafnium was eluted first and a 66 per cent. yield of approximately 99.5 per cent. of hafnia was obtained. Newnham (18) applied the method to gram amounts and obtained a 42 per cent, yield of 99.9 per cent. pure hafnia. Kraus and Moore (19) obtained a partial separation of tracer amounts by adsorbing the fluoride and oxalate complexes of the elements on to the anion-exchange resin Dowex - 1. A mixture of 0.5Y hydrofluoric acid and 1.0M hydrochloric acid was used as eluant.