Abstract
The increasing costs of electric energy and thus the need to reduce the energy required to produce ferrochrome from
chromite ore have spurred the innovations in the technologies used for smelting chromite ore. These technologies
(Conventional smelting process, Outokumpu process, DC arc route, and Premus process) have been reviewed in this
work. Premus process has been found to be the lowest-cost and most energy-efficient ferrochrome smelting
technology. The process is designed to reduce electrical energy consumption during smelting 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. It also provides high recoveries of ferrochrome and utilizes low cost reductant
sources such as anthracite.
Keywords: Chromite ore, Ferrochrome, Submerged arc furnace, Outokumpo, Premuse
1. Introduction
The mineral chromite, with chemical composition FeCr2O4 (ferrous chromic oxide),
is a sub-metallic mineral
belonging to the spinel group (with a generic formula of R+2O.R+3O4). It is the only economic mineral mined for
chromium, a steel-gray, radiant, hard metal used mainly for making stainless steel. Because of the high heat stability
of chromite, it can also be used as a refractory material for high temperature vessels such as furnaces.
Two main products can be achieved from the refining of chromite namely: ferrochromium and metallic chromium.
Smelting operation must be carried out on the chromite ore in order to refine it into any of the two products
mentioned above. One of the major problems encountered during chromite smelting is the issue of energy or
electricity. Large amount of energy is required to smelt chromite to produce ferrochromium or metallic chromium.
According to Keesara (2009), up to 4,000 KWh of energy per ton material weight is required for smelting of
chromite. This intensive energy is as a result of the high melting temperature of chromium.
In terms of minerals economics, the revenue generated from a ferrochromium plant is a function of Cr/Fe ratio
(Buchanan, 2001).
The higher the ratio, the higher the revenue the plant stands to make. However, with the
increasing cost of electric energy, an economic chromite mine can become uneconomic if large amount of electric
energy is required to smelt and refine the ore into ferrochrome. There is no point smelting a chromite ore if
breakeven cannot be achieved. During chromite smelting, energy requirement and its cost depend to a large extent on
the technology used in smelting the ore. New process technologies have been developed to reduce energy required
for smelting chromite to ferrochrome. These technologies, together with the conventional chromite smelting
technique, are what this paper aims to discuss.
2. Chromite Smelting Technologies
Four primary processes are available for smelting chromite ore to produce ferrochrome. They are: Conventional
smelting process, Outokumpu process, DC Arc route, and Premus technology. These technologies are discussed
below:
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49
2.1 Conventional Smelting Process
The traditional chromite smelting technology involves charging the chromite ore into a submerged AC Electric Arc
Furnace (Figure 1) and reductants (coke, coal and quartzite) added to reduce the ore into ferrochrome. The
metal/ferrochrome and slag produced are tapped from the furnace for further processing. According to Naiker (2006),
the primary advantages of the conventional smelting process are low capital investment and flexibility in terms of
raw materials that can be used in the process. However, in terms energy requirement, the process is not efficient as it
is an energy intensive process, requiring up to 4,000 KWh per ton material weight (Figure 2a). Figure 2b shows the
indexed energy cost per ton of alloy. The conventional smelting process requires about 1.0 indexed energy cost per
ton of every ferrochrome alloy produced.
