Preparation of tantalum and niobium oxides

Preparation of tantalum and niobium oxides 8.4.1. General notes... 

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. 

Another promising method for the preparation of tantalum and niobium oxide precursors involves the precipitation of peroxoniobate and peroxotantalate compounds. 

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

Tantalum and niobium oxides dissolve very slowly in HF solutions. Thus, it is recommended to use a high concentration of HF or a mixture of HF and H2SO4 at a temperature of about 70-90°C. The best precursors for the preparation of fluoride solutions are hydroxides. Both tantalum hydroxide, Ta205 nH20, and niobium hydroxide, M Os-nHjO, dissolve well, even in diluted HF solutions. 

Optimal parameters for the extraction, washing and stripping of niobium were determined to be number of stages for all three processes - 4, volumetric ratios Vorg Vaqu are 1 1, 20 1 and 8 1, respectively. Additional fine purification of the extractant was recommended by stripping of tantalum and niobium remainders using a 0.5% wt. ammonia solution. This additional stripping leads to final concentrations of both tantalum and niobium in the extractant that are < 0.001 g/1. Table 62 shows the purity of niobium oxide prepared by the described method. 

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. 

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. 

Using metallic precursors, HF solutions with higher concentrations of tantalum or niobium can be achieved. It is possible to prepare solutions that have maximum concentrations of about 1000 g/1 tantalum oxide and about 600 g/1 niobium oxide (Me205). 

Niobium- and tantalum-containing mesoporous molecular sieves MCM-41 have been studied by X-ray powder diffraction, 29Si MAS NMR, electron spin resonance, nitrogen adsorption and UV-Vis spectroscopy and compared with niobium- and tantalum-containing silicalite-1. The results of the physical characterization indicate that it is possible to prepare niobium- and tantalum-containing MCM-41 and silicalite-1, where isolated Nb(V) or Ta(V) species are connected to framework defect sites via formation of Nb-O-Si and Ta-O-Si bonds. The results of this study allow the preparation of microporous and mesoporous molecular sieves with remarkable redox properties (as revealed by ESR), making them potential catalysts for oxidation reactions. 

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. 

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. 

The action of carbon tetrachloride or a mixture of chlorine with a hydrocarbon or carbon monoxide on the oxide.—H. N. Warren 9 obtained aluminium chloride by heating the oxide to redness with a mixture of petroleum vapour and hydrogen chloride or chlorine, naphthalene chloride or carbon tetrachloride was also used. The bromide was prepared in a similar manner. E. Demarpay used the vapour of carbon tetrachloride, the chlorides of chromium, titanium, niobium, tantalum, zirconium, cobalt, nickel, tungsten, and molybdenum H. Quantin, a mixture of carbon monoxide and chlorine and W. Heap and E. Newbery, carbonyl chloride. 

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... 

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. 

Progress in the preparative and structural fields of protactinium chemistry has been rapid during the past 6 years and there is now sufficient information available, particularly in the halide and oxide fields, to permit a more balanced comparison than has previously been possible with the properties of the actinide elements, on the one-hand, and those of niobium and tantalum, on the other. In this connection one must, of course, bear in mind the fact that the ionic radii of protactinium in its various valence states [Pa(V), 0.90 A and Pa(IV), 0.96 A] are appreciably larger than those of niobium or tantalum and this itself will have a considerable influence on the chemical and crystallographic properties of the elements. 

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