Growth mechanism of metal iron particles with sulfur additives in direct reduction: First-principles calculations and experiments

Abstract

Sulfur additive can greatly promote iron particles growth in direct reduction. However, promotion mechanism for metallic iron particles growth is unclear. FeS was found as the thermodynamic stable sulfur phase in reduction, intensifying in metal iron particles growth. Therefore, microstructure evolution and phase transition of metallic iron growth with FeS was investigated by reduction experiments. There were three stages in reduction. Firstly, liquid FeS was found, and Fe was produced around pore. Secondly, Fe was wetted and wrapped by liquid phase (Fe-FeS). Thirdly, Fe migrated to even in liquid phase, then irregular fine Fe particles gradually transformed and aggregated to sphere. Furthermore, controlling mechanisms in each stage were identified. The first stage was driven by reduction reaction. For second stage, interfacial interaction was found as the driving force. As wettability is macroscopic manifestation of interfacial interaction, wetting experiments was used. The best wettability was found in Fe/FeS interface followed wetting order: Fe/FeS(1200 °C) > FeS/SiO2(1200 °C) > Fe/SiO2 (1570 °C) with contact angle of 1.86°, 71.5° and 100°, respectively. First-principles calculations further revealed that the strongest interfacial interaction between Fe/FeS drive second stage due to its combining ionic and covalent bonding, and the highest interfacial separation work in Fe(200)/FeS(110) which was 1.998 J/m2. Meanwhile, the well wettability of Fe/FeS also provides a well kinetic basis for third stage, and the huge Fe concentrations difference in FeS and irregular fine particles interfacial energy are combined to drive the stage. Summary, the growth mechanisms of metallic iron particles in direct reduction with sulfur is systematic established.