Partha Sarathy Mukherjee

A novel strategy has been developed for extracting value-added resources from iron-lean, high-alumina- and -silica-containing red muds (RMs). With little or no recycling, such RMs are generally destined for waste dumps. Detailed results are presented on the carbothermic reduction of 100% RM (29.3 wt.% Fe₂O₃, 22.2 wt.% Al₂O₃, 20.0 wt.% SiO₂, 1.2 wt.% CaO, 12.2 wt.% Na₂O) and its 2:1 blends with Fe₂O₃ and red mill scale (MS). Synthetic graphite was used as the reductant. Carbothermic reduction of RM and blends was carried out in a Tamman resistance furnace at 1650 °C for 20 min in an Ar atmosphere. Reduction residues were characterized using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), elemental mapping and X-ray diffraction (XRD). Small amounts of Fe₃Si alloys, alumina, SiC and other oxide-based residuals were detected in the carbothermic residue of 100% RM. A number of large metallic droplets of Fe–Si alloys were observed for RM/Fe₂O₃ blends; no aluminium was detected in these metallic droplets. A clear segregation of alumina was observed as a separate phase. For the RM/red MS blends, a number of metallic Fe–Si droplets were seen embedded in an alumina matrix in the form of a cermet. This study has shown the regeneration of alumina and the formation of alumina-based cermets, Fe–Si alloys and SiC during carbothermic reduction of RM and its blends. This innovative recycling strategy could be used for extracting value-added resources from iron-lean RMs, thereby enhancing process productivity, cost-effectiveness of alumina regeneration, waste utilization and sustainable developments in the field.

The present study developed a novel approach for transforming red mud (RM) into soft magnetic materials (SMMs) for applications in advanced electrical devices in the form of Fe-Si and Fe-Si-Al alloys. A total of ten blends were prepared based on two RMs, three iron oxide additives (Fe₂O₃, black and red mill scales), alumina and carbonaceous reductants in a range of proportions. Carbothermic reduction of the blends was carried out in a vertical Tamman resistance furnace at 1600–1650 °C for 30 min in an argon atmosphere; synthetic graphite was used as a reductant. Reaction products were characterized using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray fluorescence (XRF) and X-ray diffraction (XRD). Significant amounts of Fe-rich metallic droplets/regions of different grain sizes (0.5 to 500 μm) were produced in these studies. The formation of Fe-Si alloys with Si contents from 3.9 to 6.7 wt.% was achieved in 8 out of 10 blends; the optimal levels of Si for SMMs ranged from 3.2 to 6.5 wt.%. There was clear evidence for the formation of Fe-Si-Al (up to 1.8 wt.% Al) alloys in 4 out of 10 blends. In addition to lowering operating challenges associated with RM processing, blending of RMs with iron oxide additives and alumina presents a novel recycling approach for converting RMs into valuable SMMs for possible emerging applications in renewable energy, storage, electrical vehicles and other fields. Along with reducing RM stockpiles across the globe, this approach is expected to improve resource efficiency, mitigating environmental impacts while generating economic benefits.

Although several techniques have been developed to extract copper from waste printed circuit boards (PCBs), there remain several challenges regarding energy consumption, local area contamination and environmental damage. A novel technique has been developed for extracting copper foils from waste PCBs based on low temperature pyrolysis followed by exfoliation to overcome these issues. The standard pretreatment steps of removing electronic components from PCBs and mechanical processing/size-reduction/powdering, etc., were minimized in this study. Several unsorted ‘as received’ PCBs were heat treated in the temperature range 750–850 °C for 5–20 min. in an argon atmosphere. Brittle dark chars and other residues on the heat-treated specimens were scrapped off to separate copper foils and other residuals. Most of the electronic components mounted on PCBs had dropped off during the heat treatment. Good-quality copper foils were recovered in all cases; the purity of copper was in excess of 85 wt.%. Key impurities present were Pb, Sn and Zn with typical concentrations less than 4 wt.%. Key features of the technique include minimizing energy intensive pre-treatment processes and waste handling, low pyrolysis temperatures and short heating times. This energy-efficient approach has the potential to enhance resource recovery while reducing the loss of materials, local area contamination and pollution near e-waste processing facilities.

The influence of the parameters of high-energy, thermal, and magnetic-pulse processing on the formation of the phase composition and magnetic properties of nanocrystalline powders of the Fe–O system during their production from cheap problematic iron ore raw materials “blue dust” of the Indian deposit has been studied. The results of research showed that the GR-1 concentrate “blue dust” in its properties refers to waste, and not to natural ore material. The presence of an oxide-salt layer on the surface of the particles and the high dispersion of the material do not allow it to be used in its pure form as a feedstock for blast-furnace production, and the development of a processing method requires a separate approach. From the analysis of the results of measuring the magnetic properties, it follows that, as a result of the processing of iron ore material in a high-energy planetary ball mill, it was possible to obtain samples with properties characteristic of hard magnetic materials. The particle size is 1 μm and the values of the coercive force and remanent magnetization are 190 Oe and 0.5 kG, respectively. This makes it possible to recommend the use of this magnetic material as fillers for polymer films. The conditions of thermal and magnetic treatment are determined, which make it possible to increase the values of the coercive force by 27–35% and the remanent magnetization by 1.9–2.6 times.

Primary and secondary mill scales (MSs) are waste products produced by the surface oxidation of steel during the hot (800 to 1200 °C) rolling process in downstream steelmaking. While the primary MS is comprised of FeO, Fe₃O₄, and Fe₂O₃ in a range of proportions, the secondary MS primarily contain red ferric oxide (Fe₂O₃) (red MS). We report a novel route for extracting iron from red MS and transforming it into ferro-aluminium alloys using carbothermic reduction in the presence of alumina. The red MS powder was blended with high-purity alumina (Al₂O₃) and synthetic graphite (C) in a range of proportions. The carbothermic reduction of red MS-Al₂O₃-C blends was carried out at 1450 °C and 1550 °C under an argon atmosphere for 30 min and then furnace-cooled. The red MS was completely reduced to iron at these temperatures with reduced iron distributed around the matrix as small droplets. However, the addition of alumina unexpectedly resulted in a significant increase in the number and sizes of iron droplets generated, much higher reactivity, and the formation of ferrous alloys. A small amount of alumina reduction into metallic aluminium was also observed at 1450 °C. There is an urgent need to identify the true potential of industrial waste and the materials within it. This study showed that red MS is a valuable material source that could be transformed into ferro-aluminium alloys. These alloys find application in a range of industrial sectors such as construction, automotive, infrastructure, etc.

Managing red mud (RM), a solid waste byproduct of the alumina recovery process, is a serious ecological and environmental issue. With ~150 million tons/year of RM being generated globally, nearly 4.6 billion tons of RM are presently stored in vast waste reserves. RM can be a valuable resource of metals, minor elements, and rare earth elements. The suitability of RM as a low-grade iron resource was assessed in this study. The utilization of RM as a material resource in several commercial, industrial operations was briefly reviewed. Key features of iron recovery techniques, such as magnetic separation, carbothermal reduction, smelting reduction, acid leaching, and hydrothermal techniques were presented. RMs from different parts of the globe including India, China, Greece, Italy, France, and Russia were examined for their iron recovery potential. Data on RM composition, iron recovery, techniques, and yields was presented. The composition range of RMs examined were: Fe₂O₃: 28.3–63.2 wt.%; Al₂O₃: 6.9–26.53 wt.%; SiO₂: 2.3–22.0 wt.%; Na₂O: 0.27–13.44 wt.%; CaO: 0.26–23.8 wt.%; Al₂O₃/SiO₂: 0.3–4.6. Even with a high alumina content and high Al₂O₃/SiO₂ ratios, it was possible to recover iron in all cases, showing the significant potential of RM as a secondary resource of low-grade iron.

The widespread generation of, ever increasing volumes of and the sustainable management of solid wastes are global issues of great concern. Due to wide variations in composition and associated complexities, significant efforts are required for their collection, processing and environmentally safe disposal in a cost effective manner. An overview of solid wastes is presented in this article with a specific focus on municipal solid wastes and industrial waste from the iron/steelmaking and aluminium industries. Key waste issues such as its sources, compositions, volumes, the factors affecting waste generation and waste processing are first discussed, followed by a further discussion regarding recycling, resource recovery, disposal and the associated environmental impacts. In a special case study, waste generation and management in Chile is presented in greater detail. Detailed information is provided on government initiatives and legislation for integrated solid waste management and its movement towards a circular economy. Measures include regulations on waste management framework which concerns the transboundary movements of hazardous wastes, persistent organic pollutants, the closure of mining activities and installations and restrictions on plastics disposal. With Chile being world’s largest producer of copper, significant efforts for mining waste management, its infrastructure and procedures are being put in place to reduce the environmental impact of the mining sector and its associated waste generation.

Light-weight ferro-aluminium alloys are finding increasing application in the transport sector to reduce overall weights, energy costs and CO₂ emissions. As the primary production processes of both Fe and Al are among the most energy-intensive industrial processes in the world, there is an urgent need to develop alternate routes for producing Fe-Al alloys. Our group has successfully produced these alloys in the Fe₂O₃-Al₂O₃-C system by producing molten iron in situ, followed by the reduction of alumina at 1550 °C and pick-up of Al by Fe. In this article, we report on the influence of silica, a typical impurity present in iron oxide and reductant carbon, on the reduction reactions in this system, and on the formation of ferroalloys. In-depth investigations were carried out on the Fe₂O₃-SiO₂-C and Fe₂O₃-Al₂O₃-SiO₂-C systems at 1550 °C for times of up to 60 min. Detailed HRSEM/EDS and XRD analysis was carried on the quenched reaction products recovered after various heat treatments. A complete reduction of silica and alumina was observed in the Fe₂O₃-SiO₂-C system, along with the formation of FeSi and SiC. The reduction reactions were relatively slow in the Fe₂O₃-Al₂O₃-SiO₂-C system. While the formation of SiC, FeSi and mullite (Al₆Si₂O₁₃) was observed, even small amounts of Fe-Al alloys could not be detected. The presence of silica impurities reduced the formation of Fe-Al to negligible levels by depleting molten iron from the reaction zone, a key ingredient for the low-temperature carbothermic reduction of alumina. This study shows that some impurities can be highly detrimental to the reaction kinetics and the formation of ferroalloys, and great care needs to be exercised during the choice of reaction constituents.