The following diagram illustrates a typical material flow:
Chrome ore in various sizes is typically charged into a submerged AC Electric Arc Furnace and reductants (coke, coal and quartzite ) are added. The smelting process is energy intensive requiring upt to 4,000 kWh per tonne material weight. Slag is separated from the liquid ferrochrome and tapped into ladles for further processing. Liquid ferrochrome is then poured into moulds and after cooling crushed into sizes as required by the customers. Crushed ferrochrome is railed to final customers or harbours for shipment.
Initial processing of chromite ores can be by hand sorting of lumpy ores, and by heavy media or gravity separation of finer ores, to remove gangue or waste materials and produce upgraded ores or concentrates. Magnetic separation and froth flotation techniques have also been applied in some cases.
Most of the world's production of chromite (95%) is used in the metallurgical industry in the form of ferrochromium alloys.
The alloys are produced by high temperature reduction (smelting) of chromite. They are essentially alloys of iron and chromium with much lesser amounts of carbon and silicon, the amounts depending upon the grade or type of alloy, and impurities such as sulphur, phosphorous and titanium. The conversion of chromite to ferrochromium alloys is dominated by electric submerged arc furnace smelting with carbonaceous reductants, predominantly coke, and fluxes to form the correct slag composition. The electric current is 3-phase Alternating Current (AC) and the furnaces have three equally spaced consumable graphite electrodes in a cylindrical, refractory-lined container with a bottom tap-hole.
Characteristics of the submerged arc furnace for smelting chromite include:
1. Relatively easy to control provided the charge is well sorted to maintain a permeable overburden which will allow easy escape of the gases produced.
2. Self-regulating with power input determining the rate of consumption of charge (overburden)
3. Some pre-heating and pre- reduction of the overburden by the hot ascending gases.
Submerged arc furnaces can be open, semi-closed or closed with correspondingly better thermal efficiency and the ability to make use of the energy in the off-gases from the closed furnaces.
In the early days of high-carbon ferrochromium production, the furnaces were supplied with high- grade, lumpy chromite from countries such as Zimbabwe but with the increasing demand from the 1970s, other countries, South Africa in particular, commenced production from their lower-grade ores. The alloy produced from these ores became known as charge chrome because the chromium content was lower and the carbon content, and in particular the ratio of C:Cr, was very much greater than in high-carbon ferrochromium. This did not suit the stainless steelmakers who required as little carbon as possible entering their melts for each chromium unit and who were, therefore, having to use larger amounts of the more costly low-carbon ferrochromium to compensate. However, the situation changed radically with the advent of the argon-oxygen decarburising (AOD) and vacuum-oxygen decarburising (VOD) processes. These processes enabled the steelmakers to remove carbon from the stainless melts without excessive oxidation and losses of chromium.
A more advanced attempt to overcome the problem of ore fines was the introduction of DC arc, or plasma, furnace technology. The DC arc furnace uses a single, central hollow graphite electrode as the cathode, with an electrically conducting refractory furnace hearth as the anode. The furnace operates with an open bath, so there is no problem with overburden, and the chromite fines, together with coal and fluxes, are fed directly into the bath through the hollow electrode. The furnace has a closed top. Some of the advantages of DC arc furnace operation are: use of fine ores without agglomeration, use of cheaper reductants and greater choice of reductants, higher chromium recoveries, deliberate changes in the charge composition are reflected rapidly in the slag or metal, and closed top operation allows furnace off-gas energy to be used.
Another approach to friable ores has been to pelletise them, after further grinding if necessary, with binder, reductant and fluxes and pass them through a rotary kiln where they are hardened (sintered), pre-heated and pre-reduced to a degree before charging to a submerged arc furnace.
A further development in treating ore fines by kiln pre-reduction used unaglomerated chromite fines and low cost coal, with fluxes, as the feed to the kiln. Self agglomeration of the fines was achieved close to the discharge from the kiln where the charge becomes pasty in a high temperature zone of approx. 1,500oC. Very high degrees of reduction were achieved (80-90%) so that the downstream electric furnace (DC arc) became essentially a melting furnace.
A more recent approach, and one which is being installed by more plants, is again by pelletising. Pellets are produced with coke included and these are sintered and partly pre-reduced on a steel belt sintering system. From there, the pellets are delivered to pre-heating shaft kilns that are sited above submerged arc furnaces and which operate as direct feed bins, making use of the off-gas heat from the furnaces. Lump ore, coke and fluxes are also directed to the feed bins.
In addition to the technologies already discussed, there have been various other approaches to smelting chromite. These include rotary hearth sintering and pre-reduction of pellets, and fluidised bed pre-heaters for chromite fines.
Some intensive development work has been carried out in Japan upon entirely coal/oxygen based smelting processes using no electrical energy, sometimes referred to as smelt-reduction processes.
Source: ENRC Marketing AG