Geology Reference
In-Depth Information
systems. Where vanadium slag is used as the raw material, the wastewater coming
from the leach contains arsenic that in turn needs to be eliminated. However, the
gravest concern relating to vanadium emissions derives from the burning of fossil
fuels 15 . Crude oil can contain up to 1,200 ppm of the metal whilst annual worldwide
emissions of vanadium to the atmosphere are estimated at 110,000 tonnes (Anke,
2004). This figure is comparable to the worldwide production of vanadium of 56,000
tonnes with 98% exploited from mines located in South Africa, China and Russia
(Magyar, 2011; Athanassiadis, 1969).
According to Kellogg (1977) the total energy required to produce ferrovanadium
is 517 GJ/t. The metal undergoes a process similar to that of molybdenum, in
which the ore is roasted and the ferrometal obtained, therefore it is assumed that
the vanadium concentration process likewise requires a similar amount of energy to
that of molybdenum - 136 GJ/t metal with the remaining 381 GJ/t of metal used
in the process of obtaining the ferrovanadium.
8.11.5 Molybdenum
Molybdenum's principal ore is molybdenite (MoS 2 ) but the metal is also recovered
as a byproduct of copper and tungsten mining. Molybdenite is first ground and
concentrated by a froth floatation process before being roasted at 700 o C in order to
oxidise the sulphide into molybdenum oxide (MoO 3 ). The off-gas contains sulphur
dioxide and dust. The latter is removed by cyclones and electrostatic precipitators
and recycled to the concentrate feed. The gases are wet scrubbed and SO 2 is
converted into sulphuric acid. The selenium and rhenium present in the raw material
become vaporised as metal oxides, which are in turn removed by ion exchangers in
the wastewater treatment plant or by solvent extraction (IPPC, 2009).
The molybdenum oxide may be sublimated when heated at 1100 o C, an activity
which allows for its separation. Another way to do it is via leaching it with ammonia
to form an ammonium molybdate (NH 4 ) 2 (MoO 4 ) that easily dissolves in water.
Hydrogen sulphide also helps to liberate the dissolved molybdenum compound from
its insoluble copper impurities.
Molybdenum, the metal, is produced either by reducing molybdenum trioxide
or ammonium molybdates with hydrogen at 600 o C to produce molybdenum dio-
xide. The second step consists of reducing the metal oxide, normally in a pusher
furnace at 1050 o C with hydrogen as a reducing agent. Hydrogen is impelled in a
countercurrent flow to the feed material converting it into powder molybdenum.
Compact molybdenum metal is then produced by melting its powder in an electron
beam or vacuum arc furnace. Such furnaces are necessary as molybdenum has the
sixth-highest melting point of any element.
Ferromolybdenum, meanwhile, is mostly produced by an aluminothermic reduc-
tion of molybdenum trioxide with iron oxide and iron scrap. In addition to the use
15 Vanadium traces in diesel fuels induce corrosion in engine components.
 
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