Processing Ferroalloys from Lean Ore and Fines using Thermal Plasma

Abstract

The depletion of high grade ore minerals and scarcity of fossil fuel reserves are challenging factors for metallurgical industries in the future. In addition, extensive mining for increased steel demand results in the generation of fines, often found unsuitable for use as direct feedstock for the production of metals and alloys. Apart from mines waste, the other major sources of fine minerals are leftover in charge burdens, sludges, and dust generated in the high-temperature process. Sludge and fines generated during beneficiation of ore add to this woe, as the outcomes of beneficiation plants for lean ores show better yield for fine particles. These are becoming an environmental hazard, and need utilization for economic and environmental aspects. The production of ferroalloys is dominated by the Submerged Arc smelting route and also is limited to lumpy ores or agglomerates. With the evolution of DC/plasma arc smelting, direct charging of fines for ferroalloys production, particularly ferrochrome, has been made feasible in recent years. However, continuous improvements in the process economy are essential. The utilization of lean ore and wastes in ferroalloys production requires wide research and adoption of new advanced technologies for quality production with time-saving operations. Thermal plasma has several advantages over conventional pyro-metallurgical processes viz. no constraint in size and composition of the feedstock, comparatively higher recovery rates, purity in alloy grade, and so on. Thus, the present research is focused on the use of thermal arc plasma for production of ferroalloys, i.e., ferrochrome, ferromanganese, and ferrotitanium from ore mineral fines and mines waste. Friable chrome ore of Indian origin is subjected to carbothermic reduction at 1450 ℃, where the chromium and iron oxides are partially reduced and metallized with the formation of slag phases. The addition of flux and the type of reductant affects the reduction rate as confirmed from characterization studies. The subsequent milling and magnetic separation result in the elimination of gangue up to 70% by weight. The fine magnetic materials are found suitable for plasma smelting with a lime-rich melilite composition, which is advantageous over the current industrial (cordierite slag) practice. The comparative study of smelting ore with/without slag chemistry and magnetic parts of partially reduced chromite evidences that the latter route i.e. smelting after magnetic separation, is more advantageous for minimizing processing cost and improving alloy quality as well as recovery rates. The chromium and silicon content in ferrochrome alloy is about 68% and less than 0.5% by weight, with overall metallic recovery rates exceeding 95%.The generated melilite slag can also be used in cement production after granulation. Phase, composition, and thermal analyses of different manganese ore fines are investigated. The minerals associated with ores exhibit various thermal transformations and inversions, and are quantified. The ores are of medium grade, ferruginous, alumina-rich ferruginous, and siliceous types. Reducibility, phase, and microstructure evolution of stiff extruded briquettes under vacuum made from above ores are studied in the temperature range of 1000 ℃-1400 ℃. Physico-mechanical properties are evaluated for cold and reduced briquettes and observed that beyond 1300 ℃ briquettes partially melted and deformed. XRD and SEM-EDS analyses evidence the presence, formation, and distribution of liquid silicate and or aluminate complex phases, might be responsible for retarding reduction rates. Further, smelting tests/works are carried out for studying different slag chemistry. The comparative study shows that high alumina in the final slag has less impact when basicity increases beyond 1.0. Alumina rich ferruginous ores are adjusted to the MgO-Al2O3-CaO-SiO2 and CaO-SiO2-Al2O3 slag systems, and it is observed that the increase in basicity ratios improves recovery rates. Siliceous ore upon smelting results in high silicon pick-up in the alloy, and the phase and composition satisfies ferro-silicomanganese alloy grade. Ilmenite on carbothermic smelting in thermal plasma resulted in pig iron and titania-rich slag as end products. Further, to produce ferrotitanium, combined carbothermic and aluminothermic reduction approaches are made in stages and varying compositions. It is seen that lime addition into charge mixture lowered the recovery rate of titanium, by forming calcium titanate (in varying CaO/Al2O3 ratios). However, the addition of carbon and aluminium in stages has the benefit of utilization of heat lost in between steps if carried out in separate processes. Phase analysis and the microstructural investigation reveals the presence of iron-rich FeTi and Ti phases in the produced alloy. The next approach made to use carbon, aluminium, and silicon as reductants in stages. This combined stage-wise reductant injections in an uninterrupted single process form ferrotitanium silicide of varying composition, which is of high value particularly while making RH degassing of steel. The direct use of ore fines and wastes avoiding agglomeration is cost-effective over lumpy high-grade ore minerals. The beneficiation of lean ores with subsequent change in slag chemistry fit into the requirement of thermal plasma.