Abstract:
Ferrochrome alloy is a critical raw material in the production of stainless steel, corrosion-resistant steels, and high-temperature alloys, with global demand continuously rising. However, its production is highly energy-intensive, primarily due to the high-temperature reduction requirement of Cr2O3 (>1600?°C) and the elevated melting point of chromium-containing melts (1900~2050?°C). Pre-reduction of chromite ore is an effective approach to lowering energy consumption and carbon emissions in ferrochrome production. At present, carbon-bearing pellet oxidation roasting remains the mainstream method, which results in high energy use and CO2 emissions. The advancement of hydrogen metallurgy offers a promising alternative, and under the “dual carbon” policy framework, developing a hydrogen-based pre-reduction process for chromite holds significant strategic value. In this study, pre-reduction experiments were conducted on chromite pellets using a horizontal tube furnace. A self-developed proton flow meter in combination with a multi-gas mixing system was employed to precisely control the furnace atmosphere. The pellets were first heated to the target temperature under an argon atmosphere, followed by isothermal reduction in a H2–CO mixed gas atmosphere. Upon completion of reduction, argon was reintroduced during cooling to room temperature to prevent reoxidation. The effects of reduction parameters—including the H2/CO ratio, temperature, and time—on iron metallization rate, Fe2+ conversion, and compressive strength were systematically investigated. Post-reduction, the compressive strength of pellets was tested using a universal testing machine in accordance with the national standard GB/T 14201-2018. Coupled with chemical analysis, XRD phase identification, and SEM microstructural characterization, the mineral phase transformation and microstructural evolution mechanisms of the pellets were clarified, and the synergistic mechanism between reduction and consolidation was revealed. The results show that the reduction temperature, time, and H2:CO ratio are positively correlated with the iron metallization and deoxygenation rates. Under pure H? at 1300?°C for 3 hours, the Fe metallization rate reached 85.9%. XRD and SEM analyses revealed that the chromite phase exhibits a complex spinel structure of the form (Mg, Fe)(Cr, Fe, Al)2O4. Fe atoms at Fe3+ sites were preferentially reduced, whereas only a limited amount of Cr was reduced, indicating the relatively weak reducing ability of H2 for Cr. In terms of mechanical performance, pellet strength first increased and then decreased with rising iron metallization. The decline in Fe2+ content in the bonding phase at higher metallization levels led to an increase in the bonding phase's melting point, resulting in poor fluidity and reduced softening and solidification ability. Strength enhancement was mainly attributed to the bonding effect between molten metallic Fe and the bonding phase at high temperatures. When the reduction temperature exceeded 1200?°C, the average compressive strength of the pellets remained above 1000?N, meeting the strength requirements for furnace charging. However, further increases in the metallization degree continued to reduce Fe2? content in the bonding phase, elevating its melting point, weakening interparticle bonding, and thus decreasing pellet strength. The reduction process of chromite pellets can be divided into four distinct stages: preheating, solid-state reduction, softening and consolidation, and over-reduction-induced weakening. This study is expected to provide theoretical support for the development of hydrogen-based pre-reduction processes for chromite ore.