Abstract:
Ferrochrome alloy is a critical raw material in the production of stainless steel, corrosion-resistant steels, and high-temperature alloys, and its global demand is continuously rising. However, its production is highly energy-intensive, owing to the high-temperature required for reducing Cr
2O
3 (>
1600 °C) and the elevated melting point of chromium-containing melts (1900–2050 °C). Pre-reduction of chromite ore is an effective approach to lower the energy consumption and carbon emissions in ferrochrome production. Currently, carbon-bearing pellet oxidation roasting remains the mainstream method, which results in high energy use and CO
2 emissions. With the advancement of hydrogen metallurgy and the “dual carbon” policy framework, hydrogen-based pre-reduction processes for chromite are gaining strategic significance. In this study, chromite pellets were pre-reduced using a horizontal tube furnace. A custom-developed proton flow meter was employed in combination with a multi-gas mixing system to precisely control the furnace atmosphere. Pellets were first heated to the target temperature under an argon atmosphere, and isothermally reduced in a H
2–CO mixed gas atmosphere; subsequently, argon was reintroduced during cooling of pellets to room temperature for preventing reoxidation. The effects of reduction parameters—H
2:CO ratio, temperature, and time—on iron metallization rate, Fe
2+ conversion, and compressive strength were systematically investigated. Compressive strength of the reduced pellets was tested using a universal testing machine in accordance with the national standard GB/T
14201—2018. The mineral phase transformation and microstructural evolution mechanisms were analyzed using chemical analysis, X-ray diffraction (XRD), and scanning electron microscopy (SEM), revealing the synergistic mechanism between reduction and consolidation. The results show that the reduction temperature, time, and H
2:CO ratio positively correlated with the iron metallization and deoxygenation rates. Under pure H
2 at
1300 °C for 3 h, 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)
2O
4. Fe atoms at Fe
3+ sites were preferentially reduced, whereas Cr reduction was limited, indicating the relatively weak reducing ability of H
2 for Cr. Pellet strength initially increased with iron metallization and then declined. The strength enhancement was mainly attributed to the bonding 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 criteria for furnace charging. However, excessive metallization led to Fe
2+ depletion 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.