TANTALUM
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Tantalum ores are found primarily in Australia, Canada,
Brazil, and central Africa, with some additional quantities originating in
southeast Asia. The average yearly growth rate of about 8 to 12% in tantalum
demand since about 1995 has caused a significant increase in exploration for
this element. Tantalum minerals with over 70 different chemical compositions
have been identified. Those of greatest economic importance are tantalite,
microlite, and wodginite; however, it is common practice to name any
tantalum-containing mineral concentrate as 'tantalite' primarily because it
will be processed for the tantalum values and is sold on that basis.
Tantalum mineral concentrates may contain from two to more than five
different tantalum-bearing minerals from the same mining area. The sale of
tantalum mineral concentrates is based on a certified analysis for the
tantalum oxide they contain, with a range from 10 or 15 to over 60%
depending on the mine source.
The single largest source of tantalum mineral concentrates
is the production by Sons of Gwalia Ltd. from its Greenbushes and Wodgina
mines in Western Australia. These two mines combined produce between 25 and
35% of the world's supply, with production in 2001 reported at approximately
1.8 million pounds. Additional operating mines are the Tanco Mine (Cabot) in
Manitoba, Canada, the Kenticha Mine (Ethiopia Minerals Development Authority)
in Ethiopia, the Yichun Mine in China, and the Pitinga Mine (Paranapanema)
and Mibra Mine (Metallurg) in Brazil. Additional quantities are available
from Brazil through the processing of small alluvial deposits by prospectors
and in numerous countries in Africa, such as Rwanda, Namibia, Uganda,
DRC-Kinshasa, Zaire, Gabon, Nigeria, South Africa, and Burundi. Mining
investment in Africa has been curtailed due to political instability and
associated risk.
The central African countries of Democratic Republic of the
Congo (DRC-Kinshasa) and Rwanda and their neighbours used to be the source
of significant tonnages. But civil war, plundering of national parks and
exporting of minerals, diamonds and other natural resources to provide
funding of militias has caused the Tantalum-Niobium International Study
Center to call on its members to take care to obtain their raw materials
from lawful sources. Members should refrain from purchasing materials from
regions where either human welfare or wildlife are threatened.
The downsizing of the tin industry in southeast Asia and
elsewhere over the period of 1980 through about 1990 has led to the
reduction of tantalum oxide units available from tin slags, a by-product of
the smelting of cassiterite ore concentrates for tin production. Although
some tin slags are available from new tin production, the primary source
today is from the digging up of old dump areas containing 1.5 to about 4.0%
tantalum oxide. It should be noted that struverite concentrates have been
available from this general area containing 9-12% tantalum oxide.
Scrap recycling generated within the various segments of the
tantalum industry accounts for about 20 to 25% of the total input each year.
Extraction/refining
The extraction and refining of tantalum, including the
separation from niobium in these various tantalum-containing mineral
concentrates, is generally accomplished by reacting the ores with a mixture
of hydrofluoric and sulfuric acids at elevated temperatures. This causes the
tantalum and niobium values to dissolve as complex fluorides, and numerous
impurities that were present also dissolve. Other elements such as silicon,
iron, manganese, titanium, zirconium, uranium, thorium, rare earths, etc.
are generally present. The filtration of the digestion slurry, and further
processing via solvent extraction using methyl isobutyl ketone (MIBK) or
liquid ion exchange using an amine extractant in kerosene, will produce
highly purified solutions of tantalum and niobium. Generally, the tantalum
values in solution are converted into potassium tantalum fluoride (K2TaF7)
or tantalum oxide (Ta2O5). The niobium is recovered as
niobium oxide (Nb2O5) via neutralization of the
niobium fluoride complex with ammonia, forming the hydroxide, followed by
calcination to the oxide.
The primary tantalum chemicals of industrial significance,
in addition to K2TaF7 and Ta2O5
are tantalum carbide (TaC), tantalum chloride (TaCl5), and
lithium tantalate (LiTaO3).
Tantalum metal powder is generally produced by the sodium
reduction of the potassium tantalum fluoride in a molten salt system at high
temperature. The metal can also be produced by the carbon or aluminum
reduction of the oxide or the hydrogen or alkaline earth reduction of
tantalum chloride. Capacitor grade powder is produced by the sodium
reduction of potassium tantalum fluoride. The choice of process is based on
the specific application and whether the resultant tantalum will be further
consolidated by processing into ingot, sheet, rod, tubing, wire, and other
fabricated articles.
Capacitor grade tantalum powder provides about 60% of the
market use of all tantalum shipments. Additional quantities are consumed by
tantalum wire for the anode lead as well as for heating elements, shielding,
and sintering tray assemblies in anode sintering furnaces.
The consolidation of metal powder for ingot and processing
into various metallurgical products begins with either vacuum arc melting or
electron beam melting of metal feedstocks, comprised of powder or high
purity scrap where the elements with boiling points greater than tantalum
are not present. Double and triple melt ingots achieve a very high level of
purification with regard to metallics and interstitials. Ingots are used to
produce the various metallurgical products named earlier. Ingot stock is
also used for the production of such alloys as tantalum-10% tungsten. Ingot
and pure scrap are used in the production of land and air-based turbine
alloys.
Applications for Tantalum
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