Niobium-Tantalum

Hafnium has been successfully alloyed with iron, titanium, niobium, tantalum, and other metals. Hafnium carbide is the most refractory binary composition known, and the nitride is the most refractory of all known metal nitrides (m.p. 3310C). At 700 degrees C hafnium rapidly absorbs hydrogen to form the composition HfHl.86. 

The reaction of finely ground ores and an excess of carbon at high temperatures produces a mixture of metal carbides. The reaction of pyrochlore and carbon starts at 950°C and proceeds vigorously. After being heated to 1800—2000°C, the cooled friable mixture is acid-leached leaving an insoluble residue of carbides of niobium, tantalum, and titanium. These may be dissolved in HF or may be chlorinated or burned to oxides for further processing. 

Another solvent extraction scheme uses the mixed anhydrous chlorides from a chlorination process as the feed (28). The chlorides, which are mostly of niobium, tantalum, and iron, are dissolved in an organic phase and are extracted with 12 Ai hydrochloric acid. The best separation occurs from a mixture of MIBK and diisobutyl ketone (DIBK). The tantalum transfers to the hydrochloric acid leaving the niobium and iron, the DIBK enhancing the separation factor in the organic phase. Niobium and iron are stripped with hot 14—20 wt % H2SO4 which is boiled to precipitate niobic acid, leaving the iron in solution. 

Ammonia and alcohol may be used instead of sodium alkoxides to manufacture alkoxides of titanium and other metals such as tirconium, hafnium, germanium, niobium, tantalum, aluminum, and tin. 

Borides are inert toward nonoxidizing acids however, a few, such as Be2B and MgB2, react with aqueous acids to form boron hydrides. Most borides dissolve in oxidizing acids such as nitric or hot sulfuric acid and they ate also readily attacked by hot alkaline salt melts or fused alkaU peroxides, forming the mote stable borates. In dry air, where a protective oxide film can be preserved, borides ate relatively resistant to oxidation. For example, the borides of vanadium, niobium, tantalum, molybdenum, and tungsten do not oxidize appreciably in air up to temperatures of 1000—1200°C. Zirconium and titanium borides ate fairly resistant up to 1400°C. Engineering and other properties of refractory metal borides have been summarized (1). 

The corrosion behaviour of amorphous alloys has received particular attention since the extraordinarily high corrosion resistance of amorphous iron-chromium-metalloid alloys was reported. The majority of amorphous ferrous alloys contain large amounts of metalloids. The corrosion rate of amorphous iron-metalloid alloys decreases with the addition of most second metallic elements such as titanium, zirconium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, cobalt, nickel, copper, ruthenium, rhodium, palladium, iridium and platinum . The addition of chromium is particularly effective. For instance amorphous Fe-8Cr-13P-7C alloy passivates spontaneously even in 2 N HCl at ambient temperature ". (The number denoting the concentration of an alloy element in the amorphous alloy formulae is the atomic percent unless otherwise stated.)... 

Niobium-Tantalum Niobium and tantalum form solid-solution alloys which are resistant to many corrosive media and possess all the valuable properties of the pure metals. This could have great practical value since in a number of branches of technology it might permit the replacement of pure tantalum by a cheaper alloy of niobium and tantalum. Miller" and Argent" reported data on the resistance of the niobium-tantalum system, but the tests were only carried out under mild conditions and the data have only limited significance. However, Gulyaev and Georgieva and Kieffer, Bach and Slempkowski carried out tests at elevated temperatures and their work indicated that the corrosion rates of the alloys are substantially that of tantalum provided the niobium content does not exceed 50%. 

Quarrel, A. G., Niobium, Tantalum, Molybdenum and Tungsten, Report of Conference, University of Sheffield (I960) ... 

Use stabilised steels, i.e. austenitic steels containing niobium, tantalum or titanium. 

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 niobium-tantalum weight ratio in North American pyrochlores is 100-150 1, while in Russian ores the ratio varies between 8 1 and 1 20 [21 ]. 

A.G. Babkin, B.G. Miorov, A.I. Nikolaev, Extraction of niobium tantalum and other elements from fluoride solutions, Nauka, Leningrad, 1988 (in Russian). 

The Group II problems listed are of particular interest because they siiow the importance of achieving high resolution without undue loss of intensity (4.8). They resemble each other closely enough that only one (niobium-tantalum) will be discussed. 

Fig. 9-3. Comparison of 50-kv, 40-ma (dotted line) spectra with 100-kv, 20-ma (solid line) spectra of hafnium and tantalum, showing efficient excitation of the K spectra at the higher voltage. The higher voltage is particularly useful in connection with analyses of niobium-tantalum mixtures and zirconium-hafnium mixtures which are difficult at the lower voltage because of the interference of the K lines of the lighter elements with the L spectra of the heavier elements. (Courtesy of M. L. Salmon, Fluo-X-Spec Analytical Laboratory, Denver, Colo.)...

Fig. 6.1b) in which twelve inner ligands bridge the edges of the Me octahedron, and six outer ligands occupy apical positions, predominate. These units are found in reduced zirconium, niobium, tantalum, and rare-earth halides, and niobium, tantalum, molybdenum and tungsten oxides [la, 6, 10]. 

Table 1.15 Chemical composition of the principal niobium-tantalum-bearing minerals.

C. K. Gupta, Extractive Metallurgy of Niobium, Tantalum and Vanadium, Int. Metals Reviews, Vol. 29,... 

Solvent extraction is often applied to separate two chemically similar metals such as nickel/ cobalt, adjacent rare earths, niobium/tantalum, zirconium/hafnium, etc. For the purpose of elaboration, the example of the separation of two chemically similar elements such as zirconium and hafnium from their nitrate solution, using TBP as an extractant is considered. The solvent extraction process in this case is chemically constant (K) is given by ... 

Figure 5.23 Processing flowsheet for various niobium-tantalum bearing sources.

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