Synergistic reduction mechanism of pellets by external biomass and internal carbon under rotary kiln conditions

To clarify the synergistic effect of externally added biomass on the reduction behavior of carbon-containing pellets under rotary kiln conditions, magnetite-biochar pellets were taken as the research object. In combination with the thermal characteristics of an industrial rotary kiln, the effects of preparation method, heat-treatment schedule, C/O molar ratio, and atmosphere on the metallization rate, cold compressive strength （CCS）, and microstructural evolution of the pellets were systematically investigated. The results show that the disc pelletizing method is more favorable than extrusion forming for developing a graded pore structure that balances gas transport and carbon-iron contact, and thus significantly improves both the deep reduction behavior and mechanical properties of the pellets. Under a reducing atmosphere, after being treated at 1 050 ℃ for 30 min, the pellets achieved a metallization rate of 97.52% and a CCS of 942 N, while the shatter strength met the requirement for industrial application. An increase in temperature accelerated the stepwise reduction from Fe₃O₄ to FeO and then to Fe, and promoted the formation of a metallic iron framework. However, when the temperature is too high or the holding time is too long, local liquid-phase formation and structural densification increased the risk of ring formation. The C/O molar ratio plays a decisive role in both reduction efficiency and structural stability. When the C/O molar ratio increases from 0.2 to 0.4, the metallization rate rose markedly, and a continuous intergrown metallic iron skeleton forms. When the C/O molar ratio further exceeded 0.6, excessive carbon gasification led to the formation of more large internal pores and aggravated residual carbon inclusion, resulting in a clear decrease in pellet strength. The comparison of atmospheres further indicates that the C/O molar ratio generated by external biomass gasification not only directly participates in the reduction of iron oxides, but also suppresses the overly rapid consumption of internal carbon and maintains the reducing potential inside the pellets at the later stage, thereby strengthening the synergistic effect between the external atmosphere and the internal carbon source. Considering metallization rate, strength, and ring formation risk together, disc pelletizing, a C/O molar ratio of 0.4, 1 050 ℃, and 30 min are the more suitable process conditions. This study provides an experimental basis for the application of biomass as a substitute for fossil carbon in the rotary kiln direct reduction process.