Abstract:

A sustainable and holistic transition from fossil fuel to environmentally friendly metallurgical processes necessitates a reliable replacement of conventional reducing agents. Researchers have investigated various solutions, such as revamping the reduction process or partially substituting coke with greener reducing agents, to lessen the environmental impact of the steelmaking sectors. One way of looking at the ways to lower the carbon footprint could be the use of alternate sources of carbon for the metallurgical processes. The utilization of waste plastics in metallurgical industries has been undermined; nevertheless, these materials are a rich source of carbon and hydrogen, the primary reductants in pyrometallurgical processes. Waste plastics are available in large quantities and widely diversified. One effective way of managing end-of-life plastics could be exploiting them as a substitute for non-renewable resources in iron and steel processing technologies. Pyrolysis of plastics generate gaseous products, like CH₄, H₂, CO, CO_2, and hydrocarbons. In processing iron ore pellets, plastics take the role of fossilized resources as reductants, alloying components, and binders. This lowers the process temperature, conserves energy resources, promotes energy production and the hydrogen economy, and lowers hazardous emissions. Most plastics fulfill the most important criterion for a viable reducing agent, i.e., possessing sufficient chemical energy to drive the endothermic reduction of iron. The polymers have the desired enthalpy of formation Gibbs free energy of formation, giving it sufficient energy density for the purpose of reduction. Therefore, it is perfectly viable to use waste plastic instead of coal to reduce ores like hematite. Most end-of-life plastic wastes are disposed of in bulk from domestic and industrial sources, and it is important to understand their individual and collective ability to function as reducing agents. In many cases, plastic wastes are not disposed of responsibly and are sent to facilities to be sorted, recycled, or recovered. The processes of sorting and recovering them individually may incur more effort and energy. An attractive option would be to use the mixed plastic waste materials directly in the extraction and production of base metals, in which the plastic content can be used as a source of carbon and hydrogen for reduction while the metal content can be conveniently recovered. Therefore, a knowledge of the reducing potential of different plastics is required to assess the qualitative and quantitative characteristics of the plastic wastes, whether individually or mixed.

This study aims to investigate the reduction potentials of different plastics, namely polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), polycarbonate (PC), and acrylonitrile styrene-butadiene (ABS). The pyrolysis of different polymers, individually and combined, was studied. Thermogravimetric and off-gas analyses, were conducted to understand the decomposition behaviour of different polymers, and the temperatures wherein the gaseous constituents are released. The thermodynamic basis for the reduction of hematite (Fe₂O₃) using plastic wastes was derived, and the reducing potential of the polymers was experimentally studied by reacting the polymers at 1200°C. The knowledge gained from the study will offer a comprehensive understanding of using polymers from plastic wastes as reductants for a broader range of metallurgical applications.

Keywords: Hematite, Polymers, Plastic wastes, Hydrogen, Carbon, Recycling, Reduction.

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Smitirupa Biswal