Niobium preparation from oxide

TaF5 and NbF5 are prepared from their respective oxides by fluorination with fluorine, F2, or with anhydrous hydrogen fluoride, HF [73-75]. Rakov et al. reported the interaction between niobium metal and anhydrous hydrogen fluoride as being the most effective way of preparing NbF5 [76]. 

Table 62. Typical purity of tantalum and niobium oxides prepared from strip solutions after extraction with 2-octanol. Impurity level is given in ppm.

Preparation of tantalum and niobium oxides based on the precipitation by ammonium solution of tantalum or niobium hydroxides from strip solutions is the most frequently used method in the industry and consists of several steps. Fig. 135 presents a flow chart of the process. 

Agulyanskaya et al. [507] investigated the impact of fluorine content on the particle size of niobium and tantalum oxides and powdered lithium niobate and tantalate prepared from the oxides. It was shown that fluorine concentrations lower than 10"2% wt. do not influence particle size and result in a set minimum particle size. This concentration range was referred to as being non-... 

In addition to the obvious preparative advantages inherent in the pentachloride preparation, e.g., the low temperature of reaction and the possibility of performing the reaction without precautions against atmospheric moisture because of the protection afforded by thionyl chloride, the product is obtained free from oxide chloride. The major losses occur during the hydrous oxide precipitation and the nitric acid washings. The latter are essential to remove adsorbed ammonium ion, since, if this is not done, the reaction products will be niobium (V) chloride, in solution in thionyl chloride, and the bright yellow insoluble ammonium hexachloroniobate(V). In fact, the high purity of these two products in instances where complete removal of ammonium ion is not achieved shows clearly that the reaction of hydrous niobium (V) oxide with thionyl chloride is virtually quantitative. 

The carbothermic reduction processes are usually strongly endothermic and require high temperatures. For example, carbothermic reduction of uranium (U), boron (B), zirconium (Zr), niobium (Nb), and tantalum (Ta) from their oxides requires 2000 000 K and, therefore, application of thermal plasma. In most plasma-chemical carbothermic reduction processes, an arc electrode is prepared from well-mixed and pressed oxide and carbon particles. The arc provides heating of the mixture, stimulating the reduction process on the electrode. Carbon oxides leave the electrode, finalizing the reduction process. 

Djega-Mariadassou and co-workers (67) reported that a long induction period preceded the simultaneous reduction and carburization steps in the synthesis of NbC from the TPRe of niobium oxide with CH4/4H2. To avoid the long induction period, different catalysts were incorporated into the initial Nb205 and were treated with CH4/4H2 mixtures at relatively low carburization temperatures. In the presence of catalysts of methane decomposition (Ni and Rh), they were able to eliminate the induction period and fee 5-NbC with superficial carbon contamination and surface areas of 11-18 m /g were obtained. When a NiMo catalyst was mixed with Nb205 and carburized with a CH4/4H2 mixture gas, 5-NbC with a surface area of 50 m /g and large amounts of carbon contamination was produced. They also synthesized 5-NbC with surface areas of 27-49 m /g from the carbonization of niobium oxynitrides, which were prepared from the nitridation of niobium oxide with pure ammonia at 923 K. 

Precipitation of fluoride compounds from solutions of hydrofluoric acid, HF, is performed by the addition of certain soluble compounds to solutions containing niobium or tantalum. Initial solutions can be prepared by dissolving metals or oxides of tantalum or niobium in HF solution. Naturally, a higher concentration of HF leads to a higher dissolution rate, but it is recommended to use a commercial 40-48% HF acid. A 70% HF solution is also available, but it is usually heavily contaminated by H2SiF6 and other impurities, and the handling of such solutions is extremely dangerous. 

An initial solution was prepared by dissolving metallic niobium powder in 40% hydrofluoric acid. The dissolution was performed at elevated temperature with the addition of a small amount of nitric acid, HN03, to accelerate the process. The completeness of niobium oxidation was verified by UV absorption spectroscopy [21]. The prepared solution was evaporated to obtain a small amount of precipitate, which was separated from the solution by filtration. A saturated solution, containing Nb - 7.01 mol/1, HF - 42.63 mol/1, and corresponding to a molar ratio F Nb = 6.08, was prepared by the above method. The density of the solution at ambient temperature was p = 2.0 g/cc. Concentrations needed for the measurements were obtained by diluting the saturated solution with water or hydrofluoric acid. 

The second solution that results from the liquid-liquid extraction process is a high-purity niobium-containing solution. This solution is used in the preparation of niobium oxide, Nb205. The process is similar to the above-described process of tantalum oxide preparation and consists of the precipitation of niobium hydroxide and subsequent thermal treatment to obtain niobium oxide powder. 

Modem refining technology uses tantalum and niobium fluoride compounds, and includes fluorination of raw material, separation and purification of tantalum and niobium by liquid-liquid extraction from such fluoride solutions. Preparation of additional products and by-products is also related to the treatment of fluoride solutions oxide production is based on the hydrolysis of tantalum and niobium fluorides into hydroxides production of potassium fluorotantalate (K - salt) requires the precipitation of fine crystals and finishing avoiding hydrolysis. Tantalum metal production is related to the chemistry of fluoride melts and is performed by sodium reduction of fluoride melts. Thus, the refining technology of tantalum and niobium involves work with tantalum and niobium fluoride compounds in solid, dissolved and molten states. 

Several studies have been concerned with the chemistry of the + ni oxidation state of these elements, and the characterization of the first tantalum(iii) compounds has been claimed. The diamagnetic dimer [TaCl3(MeCN)2]2 has been prepared and used to obtain [TaClafphen)], [TaCljfbipy)], and tris-(dibenzoylmethanato)tantalum(ni). NbFa has been characterized as the product of the reaction of Nb and NbF (1 1) at 750 °C under pressure. Electrolytic reduction of niobium(v) in ethanol,formamide, and dimethylformamide can afford preparative concentrations of niobium(iii) and the new compound niobium(iii) trilactate has been obtained from ethanol. 

Materials. Niobium metal of low oxygen content was prepared by carbon reduction of high purity niobium(V) oxide obtained from Fansteel Metallurgical Corp. The metal then was arc-melted, cut into fine turnings, and outgassed at a pressure of 8 X 10 5 mm. of Hg and a temperature of 2050° for 2 hours. 

A method of separation which avoids the preparation of the double fluorides consists in fusing the mixed niobic and t an tali c acids with sodium carbonate and nitrate, the product is digested with warm water and a current of carbon dioxide is passed through the solution. It is claimed that only tantalic acid is precipitated.5 This process has, however, been the subject of adverse criticism.6 Partial separation of niobium from tantalum can be effected by warming the mixed, freshly precipitated, hydrated oxides with a mixture of hydrogen peroxide and hydrochloric acid the niobium dissolves readily, while the tantalum dissolves only sparingly.7... 

Niobium Pentafluoride, NbFs, is the only known compound of niobium and fluorine, and even this cannot be obtained in the free state by a wet method because of the extreme readiness with which it hydrolyses. Niobium pentoxide dissolves readily in hydrofluoric acid, but evaporation of the solution leaves a residue of the unchanged oxide. Niobium pentafluoride has been prepared synthetically 1 by passing dry fluorine over the gently heated metal contained in a boat in a platinum tube. The product is freed from platinum tetrafluoride, a little of which is formed at the same time, by distillation in vacuo at 100° to 110° . An alternative method consists in treating niobium pentachloride with anhydrous hydrogen fluoride in a freezing mixture and purifying by redistillation.3... 

Recently, the anodic oxidation of metals (method 2) was also applied for the preparation of the niobium and tantalum derivatives of M(OR)5 series (R= Me, Et, Pr, Bu ) [1478, 1616, 1639]. It should be mentioned that a crystalline oxoisopropoxideTa20(OPri)g,iPrOH (Fig. 4.1 c), was isolated from the PrOH-based electrolyte. It is destroyed on heating in vacuo, yielding Ta(OPri)J. 

Oxoalcoxides of niobium of the NbO(OR)3 series were obtained on the interaction ofNbOCljwith solutions ofNHjorNaOR in alcohols [791, 911] and also via alcohol interchange from NbO(OEt)3 they are even formed on the oxidation of Nb(OR)4 (the side products in the electrochemical preparation of Nb(OR)j) by oxygen [911]. 

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