Influence of reduction process on structural evolution and mechanical properties of biochar-containing briquettes

Abstract

This study systematically investigates the reduction behavior of biochar-containing briquettes under varying heat-treatment parameters (temperature, time, and C/O ratio), with a focus on metallization rate, mechanical strength evolution, and microstructural transformations. The results indicate that with increasing heat-treatment temperature, the strength of the biochar-containing briquettes first decreases then increases. Within the 600-800 °C range, the Fe₃O₄ phase remains stable, wherein carbonaceous networks derived from binder carbonization effectively preserve structural integrity. The critical 900-1100 °C interval exhibits significant strength deterioration, attributed to attributed to lattice distortion induced by the Fe₃O₄→Fe_xO phase transition and proliferation of iron whiskers. Above 1100 °C, the onset of metallic iron nucleation enhances strength (up to 0.68 MPa  ) and achieves a metallization rate of 91.97%. A 40-minute holding time at 1100 °C further increases reduction efficiency from 69.36% to 87.64%, driven by the formation of a dense metallic iron network. The C/O ratio exerts a pronounced influence: high-C/O-ratio (0.8) briquettes develop porous architectures due to gas escape, yielding 40% lower strength than low-C/O-ratio (0.4) specimens. Conversely, the limited reducing agents at C/O = 0.4 restrict metallization to even 68.7% at 1150°C. These findings validate potential of biochar as a sustainable reductant substitute, establishing technical prerequisites for low-carbon ironmaking through optimized biochar-containing briquettes production.