Sustainable hot metal production by solid and liquid carbon from H2 direct reduced iron (H2-DRI) in electric smelting furnace (ESF) conditions

Abstract

Steel industry is under growing pressure to reduce carbon emissions, prompting increased interest in H2 direct reduced iron (H2-DRI) as a sustainable alternative to conventional iron sources. If low-grade H2-DRI can be melted in an electric smelting furnace (ESF) to produce hot metal, carbon emissions can be significantly reduced compared with the conventional blast furnace, thereby enabling the clean production of high-grade steels. Such integration of H2-DRI with ESF aligns with the broader goals of cleaner production by reducing the carbon footprint (CFP) of the process. However, effective utilization of the ESF for processing H2-DRI requires a more comprehensive understanding of the internal metallurgical reactions, particularly concerning the reduction behavior of FeO and the potential of hot metal production. In response to these needs, the current study presents the first systematic investigation of the FeO reduction behavior under ESF conditions, aiming to establish fundamental knowledge that can directly contribute to the development of carbon-neutral smelting technologies and the advancement of cleaner steel production. The formation behavior of slag derived from H2-DRI at 1823 K was investigated under two reduction conditions: using solid carbon alone and using both solid and dissolved (liquid) carbon in molten iron. Using solid carbon alone as a reductant, FeO reduction proceeded through three distinct stages: incubation, steady state, and degradation, forming a characteristic sigmoidal curve. The carbon requirement for complete FeO reduction and at least 3 wt% carbon in hot metal (HM) was calculated as 66 kg-carbon/ton-HM. Introducing a hot heel with dissolved carbon accelerated FeO reduction, lowering the FeO concentration to approximately 3 wt% in the slag and producing hot metal with 2.8 wt% C. However, density differences can cause carbon to float on the slag surface, leading to practical issues that reduce carburization efficiency. Therefore, carbon charging methods should be optimized to improve FeO reduction and carbon transfer into molten iron. Furthermore, thermodynamic calculations predicted significant SiO2 reduction; however, the experiment demonstrated only limited SiO2 reduction, resulting in insufficient slag basicity for effective desulfurization. Additionally, MgO dissolution from the crucible was observed, which potentially hinder slag recycling due to volume expansion caused by MgO hydration. To overcome this, additional fluxes to optimize slag basicity and refractory materials are essential for electric smelting furnace (ESF) operation.