Effect of Direct Reduced Iron Proportion on Softening–Melting–Dripping Behavior of Mixed Burdens

Charging direct reduced iron (DRI) into a blast furnace alters the softening–melting–dripping behavior of mixed burdens; however, the influencing laws and mechanisms remain unclear. In this study, DRI, pellets, and sinter were used as raw materials to systematically investigate the effect of DRI proportion (0–40%) on the softening–melting–dripping behavior of mixed burdens, and the underlying mechanism was revealed by combining XRD, SEM-EDS, and FactSage thermodynamic simulation. The results show that with increasing DRI proportion, the softening–melting behavior of the mixed burden exhibits a regular variation: the softening start temperature decreases, the softening interval widens, and the melting–dripping interval narrows due to an increase in melting start temperature and a decrease in dripping temperature, indicating an overall improvement in softening–melting–dripping performance. Meanwhile, the maximum pressure difference and total characteristic value decrease by 73.3% and 85.2%, respectively, indicating markedly improved permeability of the burden layer. Moreover, the addition of DRI promotes deep reduction of the mixed burden, leading to a decrease in FeO content. FactSage calculations show that the onset temperature of liquid slag formation decreases, and the equilibrium liquid slag fraction at the maximum pressure-difference temperature decreases. The decrease in basicity combined with FeO fluxing leads to a lower softening start temperature, while reduced primary slag formation and the supporting effect of the DRI metallic skeleton jointly improve bed permeability.