Mechanisms and elemental partitioning during simultaneous dephosphorization and reduction of Fe-O-P melts by hydrogen plasma
Ömer K. Büyükuslu, Leonardo Shoji Aota, Dierk Raabe, Hauke Springer, Isnaldi Rodrigues de Souza Filho
Abstract
The major obstacle to render steelmaking more sustainable is undoubtedly the decarbonization of its process chains. Accompanied by this challenge, the scarcity of high-grade iron ores to be exploited as feedstock is a near reality. This creates a massive dilemma that will force steelmakers to produce green steel from low-grade iron ores. The hydrogen plasma smelting reduction of iron ores (HPSR) emerges as an attractive CO2-lean pathway to produce iron, where the ore is exposed to a reducing hydrogen-containing plasma (Ar-10 % H2) to get simultaneously melted and reduced. Here, we investigate the reduction of low-quality and low-cost model hematite ore samples containing 0.79 wt.% P via HPSR using an electric arc furnace (EAF). The dephosphorization mechanism of the ore was monitored by inspecting the evolving microstructure of the fast-solidified samples, condensed gas on the surface of electrostatic filters installed inside the EAF, as well as quantifying the corresponding P concentration in all constituents (i.e., unreduced oxides, iron and gas). We found that most of the P evaporates already in the beginning of the process (2 min) where the global O concentration in the melt varies from ∼29 to 21 wt.%. The remaining quantities of P, together with other minor gangue-related impurities (especially Si), allocates within the interdendritic domains of the partially reduced samples, as revealed by high-resolution scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX) and atom probe tomography (APT). Complementary thermodynamic modelling underpins the experimental observations, revealing that the evaporation of P is a thermodynamically-driven process, and it is dependent on the oxygen content of the melt. About 97 % dephosphorization was achieved, and only residual amounts of P were found within metallic iron domains, as revealed by APT and chemical analysis of the bulk iron (0.07 wt.% P). Our work aims at providing novel scientific-based perspectives in iron and steelmaking, permitting the production of clean and sustainable iron without the need for secondary metallurgical steps to remove undesired impurities.