(340g) Systematic Optimization of Pig Iron Production During Red Mud Reduction: Enhancing Sustainability in the Aluminum Industry

Primary aluminium is produced from bauxite ore that which is converted into aluminium oxide, which is subsequently reduced to primary aluminium by means of electrolysis (Frank, 2005). Common industrial aluminium production practice and plants consist of two (2) distinct stages: (i) the production of metallurgical alumina (Al₂O₃) from bauxite, conducted according to the Bayer process and (ii) the electrolytic reduction of alumina to aluminium (Al), which is performed according to the Hall-Héroult process; both processes have been developed in the 19^thcentury. Although both have been extensively investigated and optimized via technological breakthroughs (Haupin, 2001), their scientific principles and environmental issues have remained unchanged. Alternative processes, such as carbothermic aluminium production, have been the focus of long and sustained industrial interest; production-scale progress has been recently achieved via many experimental campaigns and concurrent mathematical and CFD modeling (Gerogiorgis, 2004).

In order to improve significantly the Bayer process energy and exergy efficiency and reduce substantially its environmental footprint, an arsenal of innovative technologies is required so as to transform red mud into marketable products of sustainable industrial interest (Wang, 2008). Red mud is a visible, inevitable Bayer process by-product with environmental impact, containing heavy metals (Santona, 2006) as well as oxides (Maitra, 1994) worth recovering (Kumar, 2006). The novel process (Balomenos & Panias, 2013) uses a novel Electric Arc Furnace (EAF) technology (AMRT, 2013) to achieve the reductive smelting of red mud without laborious pre-treatment, thus producing pig iron of required purity standards and viscous slag suitable for mineral wool production. This EAF innovation is ideal for processing dust-like red mud produced by aluminium plants (mean particle size is less than 500 nm) without any pre-treatment and without substantial energy losses, providing a crucial industrial advantage.

The novel red mud treatment process which has been proposed under the auspices of the EU FP7 ENEXAL project comprises three distinct stages: feedstock preparation, reductive smelting and product handling. The reductive smelting stage (Stage 2) is the exclusive focus of this metallurgical process optimization study: therein, the mixture is fed automatically to the bowl of the AMRT Furnace, where a melt of 1590°C-1610°C is sustained throughout the reductive smelting process. Evidently, the extent of remnant iron oxides reduction and thus process efficiency strongly depends on the complex, multi-component thermodynamics of the molten inorganic mixture. Upon reductive smelting completion, two immiscible liquid phases (molten slag and pig iron) remain in the furnace. In the product handling stage (Stage 3), the two phases are separated via sequential decantation (by tilting the furnace bowl on its horizontal axis), due to a considerably high density difference. The lighter slag phase is channeled into a fiberization unit and finely dispersed to mineral wool, while the heavier metal phase is poured into refractory moulds and solidifies, producing pig-iron slabs. All off-gases generated in the AMRT Furnace are passed through a dryer, filtered in an adjacent baghouse and subsequently discharged (as soon as adequately purified) to the atmosphere.

A complex technical optimization problem of significant industrial importance forms the focus of the present paper: the added inorganic fluxes required in Stage 1 to control basicity and the prevailing process conditions in Stage 2 to achieve reduction both greatly influence the amount of the primary marketable product (pig iron) as well as the energy efficiency of the new furnace. A novel NTUA pilot plant which has recently been constructed and operated has indeed yielded pig iron confirming the potential of the new furnace, but its operation currently relies on largely empirical knowledge: a systematic process modeling and optimization endeavor which explicitly considers accurate thermodynamics (FactSage) can precisely pinpoint the optimal flux inputs and operating conditions towards maximizing pig iron production under efficient energy usage.

LITERATURE REFERENCES

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