Niobium analysis

Spectroscopic methods for the deterrnination of impurities in niobium include the older arc and spark emission procedures (53) along with newer inductively coupled plasma source optical emission methods (54). Some work has been done using inductively coupled mass spectroscopy to determine impurities in niobium (55,56). X-ray fluorescence analysis, a widely used method for niobium analysis, is used for routine work by niobium concentrates producers (57,58). Paying careful attention to matrix effects, precision and accuracy of x-ray fluorescence analyses are at least equal to those of the gravimetric and ion-exchange methods. 

Solvent extraction techniques are useful in the quantitative analysis of niobium. The fluoro complexes are amenable to extraction by a wide variety of ketones. Some of the water-insoluble complexes with organic precipitants are extractable by organic solvents and colorimetry is performed on the extract. An example is the extraction of the niobium—oxine complex with chloroform (41). The extraction of the niobium—pyrocatechol violet complex with tridodecylethylammonium bromide and the extraction of niobium—pyrocatechol—sparteine complex with chloroform are examples of extractions of water-soluble complexes. Colorimetry is performed on the extract (42,43). Colorimetry may also be performed directly on the water-soluble complex, eg, using ascorbic acid and 5-nitrosahcyhc acid (44,45). 

Table 1 presents the distribution of tantalum and niobium processor shipments for 1999, 2000 and 2001. The values presented in Table 1 were estimated based on graphs given by Mosheim in a comprehensive statistical analysis [20]. Breakdown of the total worldwide amount of tantalum and niobium used in 2000 in main applications is given in Figs.l and 2, respectively. 

Deposits of niobium-tantalum ores are found in Australia, Brazil, Canada, China, Malaysia, Namibia, Nigeria, Russia, Rwanda, Spain, Thailand, Zaire, and Zimbabwe. A more detailed analysis of worldwide tantalum mineral raw material supply can be found in Linden s comprehensive overview [22,23]. 

The synthesis of tantalum and niobium fluoride compounds is, above all, related to the fluorination of metals or oxides. Table 3 presents a thermodynamic analysis of fluorination processes at ambient temperature as performed by Rakov [51, 52]. It is obvious that the fluorination of both metals and oxides of niobium and tantalum can take place even at low temperatures, whereas fluorination using ammonium fluoride and ammonium hydrofluoride can be performed only at higher temperatures. 

In the absence of water molecules in the system, similar compositions of fluoride complexes of tantalum and niobium are expected. An analysis of the 19F and 93Nb NMR spectra of solutions based on anhydrous hydrogen fluoride, performed by Buslaev et al., revealed the presence of NbF6 and NbF72 complexes [57]. 

According to crystal analysis performed by Stomberg [173], Na2NbOF5 is made up of sodium ions and isolated NbOF52 complex ions and is similar in structure to FeWC>6. NbOFs2" polyhedrons comprise slightly distorted octahedrons that are located in one of two equivalent positions. The niobium atom is shifted 0.234 A from the equatorial plane towards the oxygen atom. 

Compounds of the same stoichiometry type usually have the same type crystal structure within the row of alkali metals K - Rb - Cs rarely the same type structure with sodium-containing analogues and never ciystallize similarly with lithium-containing compounds. The crystal structure analysis of different fluoride and oxyfluoride compounds clearly indicates that the steric similarity between all cations and tantalum or niobium must be taken into account when calculating the X Me ratio. 

Three conceptual steps can be discerned in the definition of the ionic structure of fluoride melts containing tantalum or niobium. Based on the very first thermodynamic calculations and melting diagram analysis, it was initially believed that the coordination numbers of tantalum and niobium, in a molten system containing alkali metal fluorides, increase up to 8. 

Analysis of the melting diagrams led to the conclusion that fluoride, fluoride-chloride and oxyfluoride-chloride melts containing niobium contain the complex ions NbF83 NbF7Cl3 Nb02F43 ... 

The same analysis of the properties of niobium-containing melts [316, 317] shows that the KF - NbFs system exhibits behavior similar to that of the tantalum-containing melts. 

Based on an analysis of the initial dissolution rate in different solutions at different temperatures, several very useful conclusions and recommendations were made. It was found that the apparent activation energies for the dissolution of niobium and tantalum in 10 mol/1 HF solution are 56.5 and 65.5 kJ/mol, respectively for columbite, and 42.7 and 61.1, respectively, in the case of tantalite. It was also concluded that the mechanism of dissolution is the same for both columbite and tantalite. In addition, the initial dissolution rate of niobiuth (RNb) from columbite is controlled primarily by the activities of the... 

The results of a thermodynamic analysis of the interactions in Equations (127) and (128), as presented in [452], show that a coherent shell of tantalum and niobium hydroxides is formed on the surface of the columbite or tantalite during the interaction with sulfuric acid. The formation of the shell drives the process towards a forced thermodynamic equilibrium between the initial components and the products of the interaction, making any further interaction thermodynamically disadvantageous. It was also shown that, from a thermodynamic standpoint, the formation of a pseudomorphic structure on the surface of columbite or tantalite components is preferable to the formation of tantalum and niobium oxysulfates. Hence, the formation of the pseudomorphic phases catalyzes the interaction described by Equation (127) while halting that described by Equation (128). 

Analysis of the volumetric effects indicates that as a result of such mechanical activation, iron and manganese are concentrated in the extended part of the crystal, while tantalum and niobium are predominantly collected in the compressed part of the distorted crystal structure. It is interesting to note that this effect is more pronounced in the case of tantalite than it is for columbite, due to the higher rigidity of the former. Akimov and Chernyak [452] concluded that the effect of redistribution of the ions might cause the selective predominant dissolution of iron and manganese during the interaction with sulfuric acid and other acids. 

Equations (141) and (142) describe the equilibrium between the hydrolysis of complex fluoride acids (shift to the right) and the fluorination of hydroxides (shift to the left). Near complete precipitation of hydroxides can be achieved by applying an excessive amount of ammonia. Typically, precipitation is performed by adding ammonia solution up to pH = 8-9. However, the precipitate that separates from the mother solution can be contaminated with as much as 20% wt. fluorine [490]. Analysis of niobium hydroxides obtained under different precipitation conditions showed that the most important parameter affecting the fluorine content of the resultant hydroxide is the amount of ammonia added [490]. Sheka et al. [491] found that increasing the pH to 9.6 toward the end of the precipitation process leads to a significant reduction in fluorine content of the niobium hydroxide. 

Analysis of some experimental results [527, 528, 532] suggests that niobium oxides are first formed as gaseous components resulting from the pure gas-phase hydrolysis ... 

Chemical analysis of niobium oxide indicated that the purity of the final product depends strongly on the purity of the initial solution. Account should be taken of about 0.02-0.03% wt. cationic impurities, introduced due to interactions with metal parts of the equipment. The main added impurities are Fe, Ni, Cr, which originate mostly from the stainless steel filter. The purity of the final product can be significantly increased by using a filter made of niobium or other appropriate material. Nevertheless, the material obtained using a stainless steel filter is sufficient for use in ceramic applications or as an initial material for carbide manufacture. 

Table 66 presents a comparative analysis of different methods for the production of tantalum and niobium oxides. 

Table 66. Comparative analysis of different possible methods of the production of tantalum and niobium oxide powders from strip solutions.

V.I. Konstantinov, V.A. Bessonova, O.A. Karpenko et al., Abstracts of reports on chemistry, analysis and technology of niobium and tantalum, Naukova dumka, Kiev, 1968 p. 21. 

The examination and analysis of minerals have provided x-ray emission spectrography with a challenge and an opportunity. This situation has arisen because of a great growth of interest in uranium and thorium minerals in the rare-earth oxides and in metals such as tantalum and niobium, or hafnium and zirconium. On the whole, x-ray emission spectrography has met the challenge successfully, and the investigations that prove this also demonstrate the versatility and the value of the method.70"72... 

For papers on the analysis and examination of tantalum and niobium minerals by x-ray emission, see the following references listed in Appendix VI 17, 65, 82, 89. 

Niobium coordination compounds classification and analysis of crystallographic and structural data. C. E. Holloway and M. Melnik, Rev. Inorg. Chem., 1985,7,162 (198). 

Just as, in Group VB, niobium, so, in this Group, molybdenum provides most of the examples of the chalcogenide halides. The occurrence and preparation of such compounds are described in numerous publications. In most cases, they have been obtained as powders, with the composition based on chemical analyses only. The presence of defined, homogeneous phases is, therefore, in many cases doubtful. In addition, some published results are contradictory. A decision is possible where a complete structure analysis has been made. As will be shown later, the formation of metal-metal bonds (so-called clusters), as in the case of niobium, is the most characteristic building-principle. Such clusters... 

Heated in molybdenum, nickel or niobium containers for times varying from 1 to 100 h before analysis. 

In order to confirm the formation of nanodots, the fabricated nanodot arrays on a substrate were examined using energy dispersive spectroscopy (EDS). The EDS analysis of niobium oxide arrays on Si film before etching (Fig. 2(a)) was shown in Fig. 3(a). The Si peak as well as Nb and O peaks was observed because niobium oxide on Si film was so thin. 

Fig. 3. EDS analysis of (a) niobium oxide arrays before etching, and of Si nanodot arrays etched for (b) 20 s and (c) 30 s at 20% CI2, 500 W coil rf power, 300 V dc-bias voltage and 5 mXorr gas pressure...

The niobium phosphate complex ( Pr0)4Nb[02P(0 Bu)2] 2 was produced by the reaction of Nb(0 Pr)5 with H0P(0)(0 Bu)2 in pentane [71]. Although crystals of sufficient quality for an X-ray crystallographic analysis were not obtained, spectroscopic evidence suggests that the phosphate ligands bridge the two Nb centers. 

Chong et al. [742] have described a multielement analysis of multicomponent metallic electrode deposits, based on scanning electron microscopy with energy dispersive X-ray fluorescence detection, followed by dissolution and ICP-MS detection. Application of the method is described for determination of trace elements in seawater, including the above elements. These elements are simultaneously electrodeposited onto a niobium-wire working electrode at -1.40 V relative to an Ag/AgCl reference electrode, and subjected to energy dispersive X-ray fluorescence spectroscopy analysis. Internal standardisation... 

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