Avaliacao economica de tantalite
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Avaliacao economica de tantalite

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W.L. Pohl:
Economic Geology, Principles and Practice: Metals, Minerals, Coal and Hydrocarbons
\u2014 an Introduction to Formation and Sustainable Exploitation of Mineral Deposits
(book published May 2011)
Sample Chapter: 
Economic Geology of Niobium and Tantalum (Update 1st July 2016)
	Common Ore Minerals:
	Density (g/cm3)
(niobium-rich member of the columbite series)
	31-79% Nb2O5
	min. 5.2
(tantalum-rich member of the columbite series)
	52 to 86% Ta2O5
	max. 7.9
+3-6% REE-oxides
	56-73% (Nb,Ta)2O5
(tantalum-rich end member of the pyrochlore series)
	max. 80% Ta2O5
	max. 70% Ta2O5
Note that pure niobite and tantalite are unknown in nature; they are theoretical end members of a solid solution series called columbite group minerals (CGM), or short, columbites. Typically, CGM form the recoverable part of a large number of minor Ta\u2013Nb oxides (TNO) in the ore (Melcher et al. 2015). Tantalite is defined by an Nb/Ta atomic ratio <1. Before discovery of pyrochlore in carbonatites of Brazil and Canada, columbite was the main source for both tantalum and niobium.
The chemical composition of columbites is highly variable (Melcher et al. 2015, Melcher et al. 2016). Often, the \u201ccolumbite quadrilateral\u201d is used to depict changes in the Fe/Mn and Nb/Ta ratios (e.g. Beurlen et al. 2008). Preponderance of either Fe or Mn is noted by a suffix, e.g. \u201ctantalite-Mn\u201d (Burke 2008). With increasing fractionation of pegmatites, columbites are enriched in tantalum and manganese. Columbites contain traces of numerous elements, including As, Bi, Sn, W, Ti, Hf, Sc, REE, V, Pb, Zn, Zr, Th and U (Lehmann et al. 2014). In concentrates, the latter may enforce radiation protection measures. Inclusions of columbite and other Ta-minerals in cassiterite of pegmatite deposits are frequent. As columbite is initially free of lead and because of its resistance to later alterations, the mineral is useful for U-Pb dating of granites and pegmatites (Romer et al. 2007). Scandium in columbite and related minerals may reach >6 wt. % (Kempe & Wolf 2005).
Scandium (density 3.0 g/cm3, melting point 1539°C) is a typical dispersed element. Its Clarke value (25 ppm) is higher than that of tungsten or lead (1.2 and 13 ppm, respectively) but concentrations and minerals of this element are extremely rare (e.g. thortveitite, a silicate of scandium and yttrium with the formula (Sc,Y)2Si2O7, traces of which occur in granitic pegmatites). Like Y, scandium is one of the \u201cpseudolanthanides\u201d (cf. chapter \u201cRare Earth Elements\u201d). In igneous rocks, trace contents of Sc3+ are positively correlated with and substitute Fe2+ in clinopyroxene, amphibole, biotite and ilmenite. High-silica magmatic rocks have low concentrations (3-5 ppm). Scandium may be enriched to an economically recoverable tenor of ca. 300 (100-500) ppm in laterite, or to by-product grades in bauxite, lateritic nickel ore, phosphorites, titanium placers, columbite concentrate, wolframite, cassiterite, uraninite and in zircon (Wood & Samson 2006, Kempe & Wolf 2005, Wiesheu et al. 1997). Cut-off grade is typically at 100 ppm. Among other applications, scandium is useful as a grain refiner in high-strength aluminium alloys (e.g. in sports equipment), and in solid oxide fuel cells (SOFCs), which convert stored chemical energy (such as hydrogen or hydrocarbons) to useable electrical energy. Annual world production sourced from China, Kazakhstan, Russia and Ukraine is estimated at 10-15 tonnes (USGS 2016).
Pyrochlore is chemically quite variable; for example, calcium can be substituted by Ba, LREE and U. Therefore, pyrochlore tends to be radioactive and metamict, i.e. its crystal structure is damaged by radiation. Often, its paragenesis includes minerals of Ti, U, Th and REE. Bariopyrochlore is the main ore mineral at Araxá (see below). Formerly in Russia, niobium was produced from loparite concentrates, derived from nepheline syenites of the Kola Peninsula. Loparite is a perowskite (CaTiO3) containing Nb and Ce. Compared with pyrochlore, however, loparite is economically not competitive. Exploitable niobium ores haveminimal grades of about 0.3%, but ore at Araxá contains 2.5% Nb2O5. Tantalum ore has characteristically grades of about 0.03% Ta2O5, generally as a co-product of cassiterite and other minerals. Note that because of facile overgrinding, metallurgical recovery of columbite by crushing and milling hard-rock ore is often as low as 50%.
Tantalum is silvery grey, heavy and very hard but malleable and ductile. It has a high melting point exceeded only by tungsten and rhenium. Tantalum alloys with other elements have great strength, good ductility and high melting points. Niobium is mainly noted for its enhancement of strength and toughness in stainless steel (Tantalum-Niobium International Study Center).
	Table 2.2 - Properties of Tantalum and Niobium Metal
	Melting Point
	Boiling Point
	5429 °C
Most niobium is consumed for production of micro-alloyed steel (with ~0.03% Nb) for manufacturing high-pressure pipeline tubes, offshore petroleum drilling and exploitation platforms, and automobiles. Nickel-free stainless steel contains 0.5, and superalloys ca. 5% niobium (e.g. for jet engines). Niobium-titanium (-zirconium) alloys are used in supraconducting magnets. Some niobium, but 65% of tantalum supply is destined for the electronic sector. Most electronic-grade tantalum metal is used as a capacitor core, together with some Ta-pentoxide as a dielectric barrier. Tantalum is essential for portable electronic devices such as mobile phones, laptop computers, digital cameras and navigation systems in cars and aeroplanes. The remainder of tantalum production serves manufacturing cutting tools (tantalum carbide), high-temperature alloys (e.g. turbines of gas-fired power stations), and of extremely corrosion-resistant equipment for the chemical industry.
There are also medical applications (e.g. implants), based on tantalum\u2019s total inertness to body fluids (tantalum, similar to niobium, only dissolves in hydrofluoric or fuming sulphuric acid). Tantalum has no role in biochemical processes nor is it hazardous, with the possible exception of occupational exposure. Environmental problems of Ta-Nb mining concern foremost physical disturbance of land; toxic hazards are limited (Lehmann et al. 2014). Among them are somewhat elevated trace contents of arsenic (104 ppm in Greenbushes\u2019 albite zone which hosts the Ta ore: Partington et al. 1995) and uranium (16.3 ppm). Spring water seeping from tailings should be controlled. Landscaping and restoring ecosystems (cf. Chapter 5.4) is the main task.
Figure 1.18: Ta/TiO2 variation of granites from the northern French Massiv Central and the Saxo-Bohemian Erzgebirge (modified from Lehmann 1990). Tantalum (an incompatible element) is enriched to varying tenors that depend on the degree of fractionation of individual intrusions. Some samples from Beauvoir reach exploitable grades. Concurrently, contents of compatible titanium decrease. Derivation of tantalum from geochemically ordinary crust via partial melting and mineral fractionation appears possible.
The geochemical behaviour of niobium and tantalum in petrogenetic processes is nearly identical. Tantalum was discovered in 1802 by A. G. Ekeberg in Stockholm and named after the Lydian king Tantalos, father of Niobe (see wikipedia.org/wiki/Tantalos) because of its resistance to the most elaborate chemical digestion methods. Only about 50 years later, tantalum and niobium could be clearly discerned. Both are incompatible and lithophile high-field-strength elements (HFSE). Remember that HFSE form relatively small cations (Ta(V) = 88 compared to LIL Cs(I) = 188 and tiny lithophile Be(II) = 41 pm; 1 pm