Abstract:

Direct reduced iron (DRI) combined with electric arc furnace steelmaking is expected to be a promising future fossil-free-fuel feedstock for steel production. Despite several years of research on DRI melting in the steelmaking process, the melting behavior in the molten steel bath as well as on the slag bath is not well understood; this becomes even more challenging while melting hydrogen-reduced DRI. In the present study, a set of experiments on the melting of DRI were conducted in molten steel (~0.01% C) and molten slag (25%CaO–20%SiO₂–50%Al₂O₃–5%FeO) bath at high temperatures under controlled atmosphere and intermediate samples were collected. The primary focus of the experiment is to capture the melting behavior of DRI pellet under industrial steelmaking temperatures. The pore structure of DRI was characterized using X-ray computed tomography and scanning electron microscopy combined with energy dispersive X-ray spectroscopy which revealed an average pore size of 0.74 µm and an overall porosity of 45% by volume. The highly porous nature of the DRI reduces the effective thermal conductivity, which strongly influences its melting behavior. The study suggests that the shell formation followed by shell dissolution was rapid while immersing a DRI in molten steel bath, whereas the melting process was very slow in the molten slag bath. The higher thermal conductivity of the bath, as well as higher convective heat transfer from the molten bath to the DRI, promoted faster dissolution in the metal bath compared to that in the slag bath. A mathematical model has been developed, capturing the convective heat transfer from the bath to DRI, phase transformation at the DRI/bath interface and conduction heat transfer within DRI. The model agrees with experimental observations and indicates slower thermal penetration and longer shell stability in the slag medium. Overall, the study shows that DRI melting kinetics are strongly influenced by pore structure and thermal properties of the surrounding medium.

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Prof. Jayasree Biswas