3 Dc Arc Route
DC arc furnace uses a single solid carbon electrode as the cathode, and produces a DC arc to an anode in the bottom
of the furnace (Figure 4). The arc is normally an open or semi-submerged one (Naiker, 2006). Raw materials can be
charged either directly into the furnace, or by using a hollow electrode. The DC arc route was designed mainly to
overcome the problem of ore fines encountered in conventional chromite smelting process. In terms of energy, the
process consumes large amount of energy (Figures 2a and 2b) and therefore may render a chromite mine uneconomic.
However, in terms of coke consumption, the technology has been designed to work without using coke as a reductant
(Figure 5). Therefore to use this technology for producing ferrochrome, there is the need to reach a compromise
between coke consumption and energy requirement.
2.4 Premus process
Premus process, Xstrata’s proprietary technology in use at the Lion Ferrochrome plant in South Africa, is the most
sophisticated, competitive and technological advanced process available for the production of ferrochrome from
chromite ore (Xstrata, 2012).
The technology involves three stages: sintering, pre-reduction, and smelting. Energy
reduction, just as was observed in Outokumpu process, is achieved in the pre-reduction stage of the process. As it
can be seen in Figure 6, the chromite pellets from the sintering stage are pre-reduced in a rotary kiln by roasting
operation before being charged into the closed submerged arc furnace for final smelting. The pre-reduction process
results to the reduction of energy required in the closed submerged arc furnace for final smelting of the ore into
ferrochromium. It is worth noting that the energy used in the pre-reduction stage is obtained from the hot gas
generated from the closed submerged arc furnance and also from coal pulverization, and thus is not paid for. With
this technology, Lion Ferrochromium is now in the forefront of chromite smelting in South Africa.
Innovative Systems Design and Engineering www.iiste.org
ISSN 2222-1727 (Paper) ISSN 2222-2871 (Online)
Vol 3, No.12, 2012
52
Figure 4 DC arc route process (Curr, 2009)
Figure 5 Coke consumption comparison chart (Lion Ferrochromiun, 2009)
As it can be seen in Figure 2a and Figure 2b, Premus technology is the lowest-cost and most energy-efficient
ferrochrome technology. The technology also uses low cost reductant material (anthracite) and thus as can be seen in
Figure 5, coke consumption is low in Premus technology.
Another important feature of this technology, which is also a part of the DC arc route and Outokumpu process, is that
it is designed for smelting fine chromite ores. The convectional smelting process is designed to handle only lumpy
ores which are ideal for ferrochromium production as the fine ores tend to form a sintered layer at the top of the
charge, preventing gas escape and thus causing operating difficulties (Buchanan, 2001). With this problem,
it
means that fine chromite ores are not of economic value. However, with the development of the Premus technology,
DC arc route, and Outokumpu process, fine chromite ores can now be smelted as the technologies have been
Innovative Systems Design and Engineering www.iiste.org
ISSN 2222-1727 (Paper) ISSN 2222-2871 (Online)
Vol 3, No.12, 2012
53
designed to convert the fine ores into pellets during the pre-reduction stage. These technologies can therefore be
used to generate more revenues in localities having large amount of fine chromite ore.
Figure 6 Schematic illustration of Premus technology (modified from Naiker, 2006)
3. Conclusion
The development in the technology used for smelting chromite ore to produce ferrochrome is as a result of the need
to reduce chromite smelting energy in order to cushion the effect of increasing costs of electric energy.
By partly
reducing pelletized chromite ores in a rotary kiln using energy obtained from coal pulverization and hot gases
generated from the closed submerged arc furnace, the electric energy required for chromite smelting is reduced. Out
of the four primary technologies (Conventional smelting process, Outokumpu process, DC arc route, and Premus
process), Premus process is the lowest-cost and most energy-efficient ferrochrome technology. The technology uses
low cost reductant material (anthracite) for preheating chromite pellets.
Large amount of energy is required in the DC arc route process when compared to other technologies; however, in
terms of coke consumption, the technology has been designed to work without using coke as a reductant. A
compromise can be reached in terms of energy requirement and coke consumption, thereby making the process
economic for chromite ore smelting.
During smelting, fine chromite ores form a sintered layer at the top of the charge, preventing gas escape and
therefore causing operating difficulties. However, with the development of Outokumpu, DC arc route, and Premus
technologies, this problem is now a thing of the past as the technologies have been designed to handle fine ores. The
fine ores are converted into pellets before smelting. Thus these technologies can be used to generate more revenues
in localities having large amount of fine chromite ore