Tantalite, tantalum

Gr. Tantalos, mythological character, father of Niobe) Discovered in 1802 by Ekeberg, but many chemists thought niobium and tantalum were identical elements until Rowe in 1844, and Marignac, in 1866, showed that niobic and tantalic acids were two different acids. The early investigators only isolated the impure metal. The first relatively pure ductile tantalum was produced by von Bolton in 1903. Tantalum occurs principally in the mineral columbite-tantalite. 

Tantal, n. tantalum, -erz, n. tantalum ore, specif, tantalite. tantalig, a. tantalous, tantalisch, a. of tantalum, tantalic. Tantal-lampe, /. tantalum lamp, -ozyd, n. 

Niobium and tantalum are rare elements. The content of niobium and of tantalum in the Earth s crust is lxl0"3 and 2x1 O 4 wt. %, respectively [21]. Niobium and tantalum are encountered in nature together, mostly in the form of oxides that are derived from orthoniobic (orthotantalic), metaniobic (metatantalic) and pyroniobic (pyrotantalic) acids. The main minerals are listed in Table 2, which reveals that the most important source of tantalum and niobium is tantalite-columbite, (Fe,Mn)(Nb,Ta)206. 

The purpose of the decomposition of raw materials is to convert tantalum and niobium compounds into a soluble form and prepare the solution for use in subsequent procedures. Fig. 116 presents the process flow chart. The most typical and frequently used raw materials are columbite-tantalite concentrates with the general formula (Fe, Mn)(Nb, Ta Cfo. 

Results of the measurements were analyzed in the form of the initial dissolution rate (R ), which is the first derivative of the dissolution fraction of a said element, in this case, niobium or tantalum. The dissolution fraction is defined as the molar ratio between the amount of metal dissolved and its total concentration in the said sample, in this case, columbite or tantalite. Table 60 presents some relevant values of the initial dissolution rate taken from [451]. 

Table 60. Initial dissolution rates (RJ of niobium and tantalum dissolved from columbite or tantalite in different solutions, at 80% . (Compositions of solutions are given in mol/l)(qfter Majima et al. [415]).

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... 

Akimov and Chernyak [452] investigated and reported on the mechanism of the interaction between columbite and tantalite and sulfuric acid. The said interaction is presented as comprising two steps. The first step is related to the formation of iron and manganese sulfates and of tantalum and niobium hydroxides ... 

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). 

This approach explains the experimental fact that the complete decomposition of columbite or tantalite by sulfuric acid yields hydroxides of tantalum and niobium according to the first step of Equation (127). 

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. 

Hydrofluoric acid, at relatively high concentrations and at elevated temperatures, dissolves columbite-tantalite concentrates at a reasonable rate. The dissolution process is based on the fluorination of tantalum, niobium and other metal oxides and their conversion into soluble complex fluoride acids yielding complex fluoride ions. 

The digestion of columbite, tantalite and other raw materials containing tantalum and niobium using both hydrofluoric acid and a mixture of hydrofluoric and sulfuric acids is widely applied in the industiy. The main advantage of the method is its simplicity. The method has, nevertheless, several disadvantages that should be noted, as follows. 

Ta/Nb minerals often occur as impurities in ilmenite, rutile, cassiterite, wolframite and perovskite, most of which contain REE. Because tantalite and columbite have similar chemical properties, they often replace each other, and are usually found as isomorph mixtures. Tantalum and niobium can also be found as separate minerals. Tantalite and microlite are primary sources of tantalum. 

Ores of magnetic origin. Tantalum/columbite granites are of economic interest when the columbite content of the ore ranges from 0.001% to 0.01% and the tantalite up to 0.2%. These deposits are most common in Nigeria (Africa). Because they are a low-grade ore, they do not represent significant economic value. 

Tantalum is the 51st most abundant element found on Earth. Although it is found in a free state, it is usually mixed with other minerals and is obtained by heating tantalum potassium fluoride or by the electrolysis of melted salts of tantalum. Tantalum is mainly obtained from the following ores and minerals columbite [(Fe, Mn, Mg)(Nb, Taj O ] tantalite [(Fe, Mn)(Ta, Nb)jOJ and euxenite [(Y, Ca, Er, La, Ce, U, Th)(Nb, Ta, Tij OJ. Tantalums ores are mined in South America, Thailand, Malaysia, Africa, Spain, and Canada. The United States has a few small native deposits but imports most of the tantalum it uses. 

Tantalum is never found in nature in free elemental form. The most important mineral is columbite-tantalite (Ee,Mn) (Nb,Ta)206. Tantalum also is found in minor quantities in minerals pyrochlore, samarskite, euexenite, and fergusonite. The abundance of tantalum in the earth s crust is estimated as 2 mg/kg. 

Tantalum pentoxide is obtained as an intermediate in extracting tantalum from the columbite-tantalite series of minerals. Also, the oxide can be made by heating Ta metal in oxygen or air at elevated temperatures. 

Heinrich Rose, 1795-1864. German analytical chemist and pharmacist. Son of Valentin Rose the Younger. His comparative study of American columbite and Bavarian tantalite proved that columbium (niobium) and tantalum are two distinct metals. 

In 1809 Dr. Wollaston analyzed both columbite and tantalite (10). His conclusion that niobium and tantalum are identical was accepted by chemists until 1846, when Heinrich Rose (a grandson of Valentin Rose the Elder and son of the Rose whom Klaproth educated) questioned it. Rose had made a thorough study of the columbites and tantalites from America and from Bodenmais, Bavaria, and had extracted from them... 

The natural niobites and tantalites axe usually black, and form iso-morphous, prismatic crystals, belonging to the rhombic system. They are easily fusible and very brittle, presenting an uneven fracture. Their density increases from 5-2 to 8 2 with increase in tantalum content.6 When heated to redness in vacuo they evolve small quantities of gas, which consists of carbon dioxide, nitrogen and oxygen.7 Small quantities of helium have also been found occluded in them. 

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