Geology Reference
In-Depth Information
columbite (Fe;Mn) (Nb;Ta)
2
O
6
and coltan, (Fe;Mn)(Ta;Nb)
2
O
6
. Columbite
commonly has a Nb
2
O
5
: Ta
2
O
5
ratio ranging from 10:1 - 13:1. If the columbite
considered contains at least 30% Ta
2
O
5
, it is referred to as tantalite. Tantalum
mineral concentrates typically range from 20% to 60% Ta
2
O
5
content. Tantalum
is also a byproduct of the tin production from the cassiterite ores of Thailand and
Malaysia. Specifically, it is found in the tin-slag derived from the smelting process.
In addition, recycled tantalum streams account for about 20% of the total process
input
18
.
Any production of ferroniobium from pyrochlore essentially consists of reducing
niobium oxide in a mixture of pyrochlore with iron scrap and iron oxide through an
aluminothermic process with lime or fluorspar used as fluxing agents (Stuart et al.,
1980). A typical ferroniobium composition is Nb, 62-67%; Fe, 28-32% and to a lesser
extent varying amounts of Si, Al, Ti, P, S and C. The recovery of niobium in a
molten ferroniobium is around 87-93% with most of the impurities present flowing
into the floating slag. Ferroniobium can also be produced by reduction under a
vacuum in a semi-continuous electric arc furnace, albeit that niobium recovery is
lower than it would otherwise be in the alluminothermic process. As the furnace
imparts additional energy to the reduction process, the aluminium reducing agent
can be substituted in part by ferrosilicon, producing a low oxygen and carbon free
niobium metal. Nickel-niobium can also be obtained using this procedure. As
pyrochlores contain traces of the long-lived radioactive polonium which volatilises
at the processing temperatures, additional safety and environmental procedures are
required.
Niobium oxide can be reduced to its metal through a carbothermic or alu-
minothermic process in a vacuum furnace at 1950
o
C. The metal ingot produced
is later refined in an electric arc furnace, where most of the impurities volatilise.
A further second refining uses an electron beam furnace which produces the highly
purified niobium employed in superconductors (IPPC, 2009; Schlewitz, 2000).
The extraction of tantalum from columbites first requires the separation of tan-
talum from niobium. The ore is treated with a mixture of hydrofluoric and sulphuric
acid at elevated temperatures producing a slurry which contains various fluorides of
niobium and tantalum, not to mention all the other metals present in the ore such
as Fe, Mn, Ti, U, Th and the rare earths. The separation of Nb and Ta is then
carried out straightforwardly by solvent extraction with methyl isobutyl ketone or
by an ion exchange using an amine extractant in kerosene. Potassium heptafluo-
rotantalate (K
2
TaF
7
) is obtained through the addition of potassium fluoride and
can be precipitated with ammonia and then calcined to obtain tantalum oxide. In a
process occurring simultaneously, niobium fluoride is neutralised with ammonia to
form niobium hydroxide which is subsequently calcined to obtain niobium pentoxide
18
See Tantalum-Niobium International Study Center http : ==tanb:org=niobium. Accessed
Nov. 2011; http : ==minerals:usgs:gov=minerals=pubs=commodity=niobium=. Accessed Nov.
2011; and British Geological Survey (BGS). Mineral profiles:
Niobium and Tantalum.
www:bgs:ac:uk=downloads. Accessed April 2011.
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