Valorization of an industrial iron-rich residue by electroreduction for steel production

• Autor(a) principal: Duarte, Francisco Henriques
• Data de Publicação: 2023
• Tipo de documento: Dissertação
• Idioma: eng
• Título da fonte: Repositório Científico de Acesso Aberto de Portugal (Repositórios Cientìficos)
• Texto Completo: http://hdl.handle.net/10773/38725
• topic: Alkaline electrolysis; Akaganeite; Electroreduction; Reduction; Residue

The development of energy-efficient, carbon-free, and environmentally friendly routes to produce iron and steel is critical for the climate change mitigation. Electrochemical reduction of iron oxides has been gaining attention as a process allowing in-situ reduction, under strong alkaline media with hydrogen and oxygen as by-products. This technology also considers the integration of renewable energies and reduces 87% of CO₂ emissions. In this scope, the present work consists the study of novel electrochemical reduction of akaganeite (β-FeOOH) particle suspensions into iron, under a strongly alkaline solution at low temperature, to study the prospects of using a metallurgical residue as an iron oxide source. The by-product of nickel manufacture is also studied in the frame of this work. Moreover, it was studied other synthetic oxide composition, such as hematite (Fe₂O₃) for comparison purposes. The iron deposition was performed in both galvanostatic and potentiostatic conditions for 16 hours. This allowed the microstructural iron optimization. The synthetic β-FeOOH and Fe₂O₃, as well as the industrial residue suspensions were added to 10 M of NaOH electrolyte at low temperatures of 90 ºC and 18 M of NaOH at 105, 120 and 130 ºC. The selected experimental conditions ensure efficient deposition of iron without significant evolution of hydrogen, allowing an efficiency increase in comparison to the deposition from dissolved iron salts or conventional acidic suspensions. Cyclic-voltammetry was used to study the electrochemical mechanism of reduction. Iron depositions from the β-FeOOH synthetic composition were accessed yet due to the high viscosity of the electrolyte, very low Faradaic efficiencies were achieved, reaching ~4%. Despite the low reproducibility of the synthetic β-FeOOH, promising results were obtained with the residue (β-FeOOH based composition), being not significantly less efficient when compared with the Fe₂O₃ suspensions. The galvanostatic deposition at 90 ºC reached 59% Faradaic efficiency, mere ~2% above the Faradaic Efficiency of Fe₂O₃ suspensions in the same conditions. At higher temperatures with 18 M of NaOH generally, the Faradaic efficiency decrease as the temperature increases, such as for the Fe₂O₃ suspensions, only to reach higher Faradaic efficiencies. The potentiostatic deposition at -1.075 V reached a maximum of 91% of Faradaic Efficiency and 83% at -1.15 V. Combined studies of XRD/SEM/EDS proved the presence of Fe crystals in all electrochemical tests. Certain differences in iron microstructure were found. It is shown that Fe production can be achieved in both synthetic β-FeOOH and Fe₂O₃ as well for the industrial residue. The used industrial residue proved to be a suitable alternative feedstock for the electrolytic production.