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Plasma-arc technology provides an alternative method for the treatment of ferro-alloys, in which disadvantages can be obviated to some extent. The three basic processes that are applicable to the production and treatment of manganese ferro-alloys are: the smelting of the ore, the remelting of ferro-alloy fines or other physically unacceptable forms of the alloys, and the refining of the alloys to yield a product with a lower carbon or silicon content than that of the alloy produced initially. In the present study, plasma technology was applied to the production of manganese ferro-alloys, and its suitability was evaluated in the context of these three processes. Two furnaces were used in the tests: a 1400 kVA furnace manufactured by Tetronics Research and Development Limited (TRD) and a 100 kVA furnace at Mintek. Both furnaces employ direct current (dc), and operate with a single electrode as the cathode and the molten bath as the anode. The molten pool of process material forms an integral part of the electrical circuit, and the furnaces used in the experiments can therefore be classified as dc transferred-arc plasma furnaces. In the 1400 kVA furnace, a water-cooled tungsten cathode (plasma gun) was used; in the 100 kVA furnace, graphite or water-cooled tungsten was used as the cathode material. High-carbon ferromanganese fines were successfully remelted in the Mintek 100 kVA and the TRD 1.4 MVA furnaces. The TRD furnace was operated at a power level of 450 kW, and about 4.5 t of metal was tapped continuously over a period of 8 hours. The specific energy consumption was 795 kW.h per ton of metal produced, and the losses of manganese in the baghouse represented less than 1 per cent of the manganese in the feed. Silicomanganese was produced in the Mintek 100 kVA furnace by the simultaneous remelting of high-carbon ferromanganese and silicon fines. A sufficiently low level of carbon could be reached to meet the chemical requirements of a regular-grade silicomanganese, and the recoveries of manganese and silicon were 97 and 82 per cent respectively. In some instances, the carbon content of the product was even lower than its initial level in the silicon fines, indicating that the conditions for the removal of the carbon in the plasma system are very favourable. Experiments in the Mintek 100 kVA furnace demonstrated that, when high-carbon ferromanganese is refined by manganese ore at temperatures around 1600 degrees C, ferromanganese containing about 2 per cent carbon and 0.1 per cent silicon can be produced. Satisfactory results were obtained on synthetic ores with basicities of 1.4 and 2.4 at metal-to-ore ratios of 2:1. The overall losses of manganese were between 10 and 15 per cent. High-carbon ferromanganese was produced from manganese-ore fines in the Mintek 100 kVA and the TRD 1.4 MVA furnaces. The manganese oxide contents of the resulting slags were significantly lower than those reported for submerged-arc furnaces. The losses of manganese to the dust stream, however, were substantial and, at the least, about 10 per cent of the manganese in the feed was lost by evaporation and entrainment. The relatively small scale of operation and the use of finely sized feed materials probably enhanced the losses of manganese, which were substantially higher than in conventional submerged-arc furnaces. It would appear that, because of the open-bath configuration and the absence of a burden of charge material, the areas over which the feed materials and power are conveyed to the reaction zone should be optimized and controlled more carefully if the recoveries obtained by industry are to be attained. Further work is required so that losses to the dust can be reduced and the benefits that would result from the utilization of finely sized manganese ores and reducing agents can be realized. This is especially important to the possible future production of upgraded ore fines with high manganese-to-iron ratios from relatively low-grade ore deposits, and because fine reducing agents will be available from coal deposits where the beneficiation process necessitates reduction of the coal ore to fine particles.
Plasma-furnace technology was first applied in Africa in the mid- to late 1970s, when it was realized that advantages could be obtained in the processing of fines for the production of ferro-alloys. A number of processes have been implemented on an industrial scale, including the four 105 MVA ilmenite-smelting furnaces (AC transferred-arc) at Richards Bay Minerals, the 40 MVA ferrochromium furnace (DC transferred-arc) at Palmiet Ferrochrome, Krugersdorp, and the 11 MVA ferromanganese 'boot' furnace (DC transferred-arc) at Metalloys, Meyerton. Well-developed plasma-furnace research facilities are in place in South Africa, and include the 3,2 MVA DC transferred-arc plasma furnace at Mintek, Randburg. This paper highlights some of the applications of plasma technology to a variety of ores, minerals, concentrates, metals, and chemicals. The development of successfully-implemented plasma systems is described, as well as the problems that have been experienced with some of the less-successful activities.
The advantages of plasma systems over three-phase submerged-arc furnaces in their application to the production of ferro-alloys are examined briefly. The interaction between an arc and the metallurgical conditions in a plasma furnace is different from that in a submerged- arc furnace, particularly in the utilization of the energy of an arc. Experiments on the dissipation of energy in a transferred plasma arc are described, and the results, which give rise to the following conclusions, are given. There is a large dissipation of energy at the anode (up to 80 per cent), largely because the energy developed in the arc column is directed downwards onto the anode. The major factors affecting the relative proportion of energy dissipated at the anode are: the type and geometry of the cathode, since a strong cathode jet increases the proportion of energy directed at the anode; the arc current, since an increased arc current increases the proportion of energy at the anode; the arc length, since an increased arc length decreases the proportion of energy at the anode, although a large proportion of the energy is still directed through the anode region; the arc-supporting gas, since the use of nitrogen, a diatomic gas, results in a higher transfer of energy to the anode than for argon, a monatomic gas. The dissipation of energy from the arc column, (radiation energy) is relatively small. As long as the arc column is well collimated and directed downwards onto the anode (or the metal bath of the furnace), the exposed arc column does not cause extensive wear of the refractories. The major problems relating to the refractories in a transferred-arc furnace are slag attack on the walls of the furnace, refractory wear caused by convection energy generated in the arc column, and radiation to the roof of the furnace from the hot anode region and the molten bath. It is important for the reduction reactions to be carried out in the hot anode region, or as close as possible to it, so that the energy at the anode will be quenched. This is particularly important in ferro-alloy production in the open-bath mode.
This paper describes the development of large-scale thermal-plasma systems, which was motivated, in general, by the potential cost savings that could be achieved by their use as a replacement for the more conventional methods used in the generation of thermal energy. The anticipated cost savings arise not only from the use of plasma-generating devices but from the manner in which they have been interfaced with a furnace to process particular materials, mostly as fines. Thermal-plasma systems fall into two categories: non-transferred-arc and transferred-arc devices. In general, transferred-arc devices have been interfaced with open-bath furnaces in which melting or smelting processes are carried out, while non-transferred-arc devices have normally been applied to shaft furnaces. Water-cooled transferred-arc devices are somewhat limited in power (about 5 MW) because of the relatively low voltages (300 to 500 V) that can be attained in open-bath furnaces, where very long arcs are undesirable, and because only relatively low levels of current can be carried. Graphite electrodes can overcome the restriction of current, and power levels of 30 to 50 MW seem feasible, even with one electrode, if direct current is used. Multiple water-cooled devices are capable of attaining similar power levels, but the capital costs are much higher. Costs due to electrode wear are lower for water-cooled systems, but expensive gases are needed for transferred-arc devices. Mintek conducted extensive pilot-plant work in which water-cooled devices were used initially but graphite electrodes were used subsequently to produce ferrochromium from fines. Transferred-arc open-bath configurations were used. This work led to a decision by Middelburg Steel & Alloys (MS&A) to install a 16 MVA furnace of semi-industrial scale to produce ferrochromium alloys based on the ASEA dc arc furnace developed for the Elred process. Non-transferred-arc devices have attained reasonable scale-up to the 6 to 8 MW power level, and high-voltage operation, which is inherent in such devices, has enabled lower currents to be used. Nevertheless, multiple systems are still necessary to accommodate large-scale applications, and this can be costly from a capital point of view. The cooling requirements are large, and can represent a considerable loss of electric energy. Shaft furnaces equipped with non-transferred-arc devices are suitable for the processing of materials that have volatile species, eg, silica or manganese, or where the shaft is used to prereduce oxides that are amenable to gas-solid reactions. It is probably in the treatment of light and refractory metals that plasma technology will achieve its greatest development in the years to come. The energy requirements for the production of these metals are high, and very low oxygen potentials are necessary. These are factors that favour thermal plasma. Much developmental work is still needed in this interesting field. It should be remembered that electrically generated thermal energy is a unique temperature source that, in many instances, cannot be replaced technically or economically by the combustion of a fuel.
High carbon ferrochrome is one of the most common ferroalloys produced and is almost exclusively used in the production of stainless steel and high chromium steels. Production takes place primarily in countries with substantial chromite ore supply. Relatively cheap electricity and reductants also contribute to the viability of high carbon ferrochrome. The most common production technology utilized is submerged arc smelting in AC furnaces, although open arc smelting in DC furnaces is becoming increasingly common. A more advanced technology route that includes a prereduction step is only utilized at significant scale by one producer. Production processes have become more energy and metallurgically efficient by utilizing advanced processes such as prereduction, preheating, agglomeration of ore, and CO gas utilization. Recently installed plants display manageable risks in terms of environmental pollution and occupational health.