Tantalum mechanism

The lattice of vanadium expands approximately linearly with the addition of aluminum [64]. The aluminum intermetallic compound, V3AI (V-25 atom% Al), expands the lattice by about 1% from 0.3025 nm in unalloyed vanadium to 0.3054 nm [64]. Molybdenum, cobalt and titanium also expand the lattice of vanadium, whereas elements such as chromium and iron cause the lattice to contract [83]. Addition of these elements can increase the mechanical strength of alloys relative to unalloyed vanadium [85]. For niobium and tantalum, mechanical properties can also be improved by alloying [86]. Buxbaum has patented a number of alloys of niobium, tantalum and vanadium for membrane use, including Ta-W, V-Co, V-Pd, V-Au, V-Cu, V-Al, Nb-Ag, Nb-Pt, Nb-Pd, V-Ni-Co, V-Ni-Pd, V-Nb-Pt, and V-Pd-Au [45]. 

Titanium, tantalum and zirconium are used for construction in process plants. The principal physical and mechanical properties of these three metals are given in the Table 3.34. 

Plain tubes (either as solid wall or duplex) are available in carbon steel, carbon alloy steels, stainless steels, copper, brass and alloys, cupro-nickel, nickel, monel, tantalum, carbon, glass, and other special materials. Usually there is no great problem in selecting an available tube material. However, when its assembly into the tubesheet along with the resulting fabrication problems are considered, the selection of the tube alone is only part of a coordinated design. Plain-tube mechanical data and dimensions are given in Tables 10-3 and 10-4. 

It is not subjected to hydrogen embrittlement as is tantalum, niobium and nickel alloys, and thus is able to sustain thermal and mechanical shock after exposure to gaseous hydrogen at high temperatures. 

Niobium is always found in nature associated with tantalum and it closely resembles tantalum in its chemical and mechanical properties. It is a soft ductile metal which, like tantalum, work hardens more slowly than most metals. It will in fact absorb over 90% cold work before annealing becomes necessary, and it is easily formed at room temperature. In addition, welds of high quality can be produced in the metal. In appearance the metal is somewhat similar to stainless steel it has a density slightly higher than stainless steel and a thermal conductivity similar to 1% carbon steel. 

Niobium closely resembles tantalum in its mechanical properties and for more detailed information relating to the fabrication of niobium see Section 5.5 on tantalum. 

Chemical plant It has been reported from some plants producing hydrochloric acid that tantalum condensers are being replaced by ones of niobium, and in certain petroleum plant niobium is being specified for its corrosion resistance and mechanical properties. 

The mechanical properties of tantalum are dependent on the previous history of the material and the manufacturer should be consulted if these properties are likely to be critical. The physical and some typical mechanical properties are listed in Tables 5.21 and 5.22. The effect of the temperature on the strength and elongation of tantalum sheet in vacuum is shown in Figs. 5.8 and 5.9. 

Tantalum is severely attacked at ambient temperatures and up to about 100°C in aqueous atmospheric environments in the presence of fluorine and hydrofluoric acids. Flourine, hydrofluoric acid and fluoride salt solutions represent typical aggressive environments in which tantalum corrodes at ambient temperatures. Under exposure to these environments the protective TajOj oxide film is attacked and the metal is transformed from a passive to an active state. The corrosion mechanism of tantalum in these environments is mainly based on dissolution reactions to give fluoro complexes. The composition depends markedly on the conditions. The existence of oxidizing agents such as sulphur trioxide or peroxides in aqueous fluoride environments enhance the corrosion rate of tantalum owing to rapid formation of oxofluoro complexes. 

Tantalum has a high solubility for hydrogen, forming two internal hydrides, but the exact mechanism of their formation is not precisely known. 

Generally, the most important reaction is that of tantalum with oxygen, since it tends to form oxides when heated in air. Reaction starts above 300°C and becomes rapid above 600°C . The scale is not adherent, and if the oxidised material is heated above 1000°C oxygen will diffuse into the bulk of the material and embrittle it. At 1200°C catastrophic oxidation attack takes place at a rate of about 150 mm/h Oxygen is not driven off by heating alone, but in vacuum above 2300°C it is removed as a suboxide. The first step of the conversion mechanism of tantalum into oxide was shown to occur by the nucleation and growth of small plates along the 100) planes of the BCC metaP. ... 

Our life would never be as advanced and comfortable as it is if not for the applications of tantalum and niobium. These materials unique properties ensure their increasing usage in electronic, optic, mechanical, aerospace, nuclear and other modem applications. 

Niobium dioxyfluoride, Nb02F, and tantalum dioxyfluoride, Ta02F, can be successfully used as precursors for the synthesis of many oxyfluoride compounds of niobium and tantalum. Systematic investigations performed on MeC>2F - M2CO3 systems, in which Me = Nb or Ta and M = alkali metal, provided necessary information on optimal synthesis procedures and imparted some conformity on the mechanism of the chemical interaction between the components. 

NMR, Raman and IR spectroscopy are most frequently used to investigate the complex structures of fluoride solutions containing tantalum and niobium. Most investigations of such solutions were performed on the liquid-liquid extraction of tantalum and niobium, with the objective of describing the mechanism of the process. These publications will be discussed separately. 

For a long period of time, molten salts containing niobium and tantalum were widely used for the production by electrolysis of metals and alloys. This situation initiated intensive investigations into the electrochemical processes that take place in molten fluorides containing dissolved tantalum and niobium in the form of complex fluoride compounds. Well-developed sodium reduction processes currently used are also based on molten salt media. In addition, molten salts are a suitable reagent media for the synthesis of various compounds, in the form of both single crystals and powdered material. The mechanisms of the chemical interactions and the compositions of the compounds depend on the structure of the melt. 

The scheme of the interaction mechanism (Equation 88) testifies to an electro-affinity of MeFe" ions. In addition, MeFe" ions have a lower negative charge, smaller size and higher mobility compared to MeF6X(n+1> ions. The above arguments lead to the assumption that the reduction to metal form of niobium or tantalum from melts, both by electrolysis [368] and by alkali metals, most probably occurs due to interaction with MeF6 ions. The kinetics of the reduction processes are defined by flowing equilibriums between hexa-and heptacoordinated complexes. 

The fluorination process aims to decompose the material and convert tantalum and niobium oxides into complex fluoride compounds to be dissolved in aqueous solutions. The correct and successful performance of the decomposition process requires a clear understanding of the oxygen-fluorine substitution mechanism of the interaction itself. 

A residual phase, usually consisting of insoluble fluorides and oxyfluorides of alkali earth and rare earth metals, is separated from the solution by filtration. The mechanism of the chemical decomposition of raw materials of the tantalum- and niobium-containing oxide type seems to be complicated, and unfortunately, the process has yet to be adequately investigated. 

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 ... 

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. 

A possible mechanism of the extraction of tantalum and niobium using different solvents, and their stripping into the aqueous phase, was reviewed comprehensively by Babkin, Maiorov and Nikolaev [458,459]. 

Investigations of the plasma chemical decomposition of tantalum-containing fluoride solutions indicated no significant differences in the process and product parameters compared to the corresponding decomposition of niobium-containing fluoride solution [529, 532]. The particle diameter, shape and specific surface area of both niobium oxide and tantalum oxide powders attest to a gas-phase mechanism of the interaction, with sequential condensation and agglomeration of the oxides. 

An ongoing debate continues among researchers regarding the mechanism of the electrochemical reduction of tantalum in fluoride melts. No unified model exists for the process. Espinola et al. [545] concluded, based on an investigation of the electrochemical reduction of tantalum from a molten LiF-NaF-KF eutectic solution, that the process takes place in two stages. The first, reversible step involves the transfer of two electrons ... 

Substitution of the fluoride ligands by ions that lead to higher covalence of the bonds can alter the mechanism of tantalum reduction. For instance, the investigation of pure chloride systems containing tantalum pentachloride,... 

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