Tantalum

Introduction:

History and Development

Tantalum is one of the rarest elements found in the earth’s crust. It only comprises 0.00017 percent of the crust which makes it 15 times less abundant than gold. It was found as an oxide compound of an unknown metal in a sample taken from an area near Stockholm by A.G Ekeberg in 1802. It was named tantalum from King Tantalus of Lydia in Greek mythology (Wineski, 1976 as cited by Espinoza, 2012). In 1824, J.J Berzelius produced the first tantalum metal but with low purity (Lambert, 2007 as cited by Espinoza, 2012). Tantalum has very similar properties with niobium making them hard to differentiate from one another. It was only in 1844 when the two were successfully separated by H. Rose and a separation scheme was established by C.G. de Marignac in 1865 (Sitzmann, 2007; Wineski, 1976 as cited by Espinoza, 2012).
During the massive space explorations of the United States and Soviet Union in 1958, many laboratories focused on the development and large-scale production of tantalum for its high potential use in some space exploration systems due to its unique properties. Tantalum was also popularized as high-refractory metal with the development in vacuum systems, electron-beam melting, vacuum consumable electrode arc-melting equipment and associated metal processing and testing equipment in early 1960’s (Buckman, 2000).

Physical and Chemical Properties

Physical Properties

Tantalum is a high density (16.6 g/cm3) and high melting point (2996 °C) body-centred cubic (b.c.c) metal. It has a superior refractory property effectively influenced by its high melting point (Moser, 1999). Tantalum is soluble in fused alkalis and hydrofluoric acid while insoluble in
water, alcohol, and all other acids beside other than hydrofluoric acid and fuming sulphuric acids. Although considered as hard metal, tantalum has high ductility in which it can be transformed into fine wires as filaments for evaporating metals such as aluminium. Other physical properties of tantalum are presented in Table 1.

Chemical Properties

Tantalum usually reacts with oxygen creating an amorphous oxide film on the surface that is attributed to tantalum’s superior corrosion resistant properties similar to glass (Moser, 1999). The minimum ignition temperature is within the range of 300 °C to 630 °C. Tantalum is usually reactive with strong oxidizers at normal conditions. The excessive exposure to oxidizer may cause interstitial migration of oxygen that can further result to embrittlement of the material (Moser, 1999). The Table below shows the different element components of unalloyed tantalum.

Fabrication Considerations

The general fabrication of tantalum metal and alloy products presented by Moser (1999) in his journal includes several steps namely extraction, processing, melting and consolidating, material forming and final fabrication. According to him, the temperature during the processing should cautiously maintained at temperature not over 250 °C in atmospheric conditions to avoid excessive oxidation due to its reactive nature although most usual working method suits well with tantalum.
Extracting and Processing. In nature, tantalum does not exist in pure form. The ores are quarried from mining sites as titanium dioxide combined with niobium thus companies designs extractive processes that can recover both. The ore and tin slags are size-reduced to form powder. After, hydrofluotantalic acid and hydrofluoniobic acid are formed by reacting the powder with
hydrofluoric acid. The filtrate which contains the two compounds are separated from the insoluble impurities through filtration. The filtrate is then subjected to liquid-liquid extraction to further separate the two acid compounds from the impurities. The hydrofluotantalic stream is reacted with potassium to form the salt K2TaF7. This potassium salt will produce relatively pure tantalum along with the by-products potassium fluoride and water soluble sodium. The relatively pure tantalum will be recovered as powder and separated from other products by leaching with water.
Melting and consolidating. The powder from the first step is usually mixed with various recycled materials and the mixture is purified through melting operations. There are several melting methods that can be used to treat the mixture but the most commonly used are electron beam (EB) drip or vacuum-arc remelt (VAR) techniques. The operating temperatures with EB drip are usually greater than 3000 °C under vacuum conditions. These conditions will allow low-melting-point elements to be vaporized from the molten mixture and the metallic purity of tantalum can be raised up to 99.9 %. However, tungsten, molybdenum, and/or niobium cannot be removed with this method. On the other hand, VAR method is especially employed for the production of tantalum alloy since it provides excellent mixing.
Product Manufacturing. This production step is commonly started in thermomechanical processing by means of forging. Unlike other metals, the hammer and press forge technique are usually employed at low temperature to prevent excessive oxidation as discussed earlier. The process will produce flat slabs and round billets. These preliminary products can be annealed before or after specific methods are done required for a certain type of product. Annealing is done in an extremely tight furnace at temperatures of 950 °C to 1300 °C. For flat products, the common type 2- or 4-high rolling mills are used to cold roll the annealed ingots with starting size of 15 to 30 cm in diameter. The resulting bars usually have a thickness of 0.63 to 1.2 cm and a width 51 to
102 cm. Unlike flat products, round material and tubing are usually annealed after extrusion since softer tantalum feed is required for the material to be extruded or drawn. The manufacturers usually use lubricants during the moulding process to prevent scratches since the material is practically soft.
Final Fabrication. There are different methods employed in fabricating equipment parts such as loose liners in tanks pipes and valves and other material on demand. Generally, tantalum metal has high machinability since it is essentially ductile however the problem in working tantalum is its reactivity at high temperatures. For an instance, the welding process is worked under a glove box with positive pressure of argon or helium. The inert gases will cover the metal while the work is done and so until the temperature is cooled at 100 °C.

Economic Considerations

Availability of the raw material. Tantalum is one of the rarest metal on earth’s crust. Based on the abundance, it is ranked 49th among all the metals that can be found on the crust. Commercial scale tantalum-depositing ore can be seen in Australia, Brazil, Canada, and Africa while few in the United States (Moser, 1999).
Availability of tantalum products. According to Espinoza (2012), the European tantalum industry holds one of the largest share in the global supply chain of tantalum products. It produces around 250 to 300 tons of tantalum per year. All the raw material needed for the production except the scraps are obtained by Europe from other countries usually from those with major tantalum deposits (Espinoza, 2012). Figure 1 shows the schematic diagram of European tantalum industry.
Figure 1. The schematic diagram of European tantalum industry.
Other major suppliers of tantalum products are based on the United States and Canada.
Demand for Tantalum Products. Around 900, 000 kg of tantalum is consumed yearly around the world. The electronics industry is considered as the biggest consumer which covers the 66 % of the total global demand (Buckman, 2000). From this data in 2000, based on the study of Espinoza in 2012, as shown in Figure 2, the average demand increase to approximately 1700 metric tonnes annually from 2005 to 2009.
Figure 2. The demand structure in metric tonnes from 2005 to 2009
Price of Tantalum Products in Market. As described by Espinoza (2012), “Tantalum is an example for a high-tech metal with prices having shown a long-term stability interrupted by very sharp, though short-lived price increases. The price of tantalum depends on the supplier and type of product, and it is relatively higher than other metal due to its scarcity in earth’s crust. In a report of Philippon and Lambert (n.d.), the price of tantalum 2.5 % alloy is compared with other metals used in industries.
Table 4. Price of tantalum in US $ compared to other metals Material Price per pound Price per sq. ft. 0.25” 304 SS 3 31 316 SS 4 42 Ti-Gr 2 13 76 Ti-Gr 7 35 205 C-276 26 300 Zr-702 37 312 Ta 500 10 800

Applications in Chemical Engineering

In general, the uses of tantalum in different fields are primarily influenced by its properties. According to Espinoza (2012), the application of tantalum as material of construction in process equipment and machinery in the chemical industry depends on the excellent inertness of its oxide layer coating tantalum as well as its hardness as fabrication tool. Specifically, tantalum is a very good material of construction of piping systems, heat exchangers, columns, reactors, bayonet heaters, crucibles, furnace liners and thermocouple sheaths. In addition, tantalum is also used in repairing glass lined containers. Its corrosion resistant property is beneficial with its consumers through longer equipment life, lower operating downtime and increased operating temperatures (Allegheny Technologies Incorporated, 2013).
References
ALLEGHENY TECHNOLOGIES INC. 2013. Tantalum alloys: technical data sheet. Version 1 (3/12/13): page 1. 1000 Six PPG Place. Pittsburgh, PA 15222-5479 U. S. A.
BUCKMAN, R. W. JR. 2000. New applications for tantalum and tantalum alloys. Refractory Metal Markets. Journal of Microstructure.
ESPINOZA, L. A. T. 2012. Case study: tantalum in the world economy: history, uses and demand. EU Policy on Natural Resources Consortium n. 28. Version 1.
H. CROSS COMPANY. 2015. Tantalum: history. 150 West Commercial Avenue. Moonachie, N. J. 07074 U. S. A.
HAMILTON PRECISION METALS. N. D. Technical data sheet: tantalum. 1780 Rohrerstown Road. Lancaster. PA 17602.
ISOFLEX. N. D. Stable isotopes of tantalum available from ISOFLEX. Retrieved September 14, 2015 from http://www.isoflex.com/index.php/docman-all-categories/general/pt-element-pdfs/112-tantalum-ta-pdf/file.
MOSER, K. D. 1999. The manufacture and fabrication of tantalum. Refractory Metals Overview. Journal of Microstructure. Retrieved September 14, 2015 from download.springer.com.
PTABLE. N. D. Properties: tantalum. Retrieved September 14, 2015 from http://www.ptable. com/#Property/Abundance/Crust