Reduction Kinetics Study of Mill Scale Waste Using Low-Grade Thermal Coal for The Production of Pure Iron

Abstract

Mill scale, a by-product from hot rolling processes, offers high iron content but poses handling and recycling challenges due to its fine particle size and tendency to oxidize. This present study investigates the reduction behavior of mill scale pellets in both carbonaceous (low-grade thermal coal) and non-carbonaceous atmosphere (N2 and H2) using as reductant, emphasizes reduction efficiency, microstructural and phase evolution, and reaction kinetics. Mill scale, a major iron oxide-rich by-product of steel industries, was pelletized with suitable binders and reduced at temperatures ranging from 973 to 1273 K for durations between 30 and 120 minutes. The effect of two different binders: bentonite (1.5 wt.%) and molasses (5 wt.%) are focused on pelletization, appropriate binder selection for reduction processes. Inorganic bentonite ensures structural integrity, whereas organic molasses increases porosity, thereby potentially improving reducibility. Low-grade thermal coal, characterized through proximate analysis, is employed as the reductant in a non-coking route. The coal’s fixed carbon and volatile matter content are key parameters influencing the thermal decomposition and reduction kinetics. The performance of the pellets was assessed in terms of swelling index, porosity, percentage reduction, and degree of reduction. In another set of experiment, mill scale pellets were reduced in N2+H2 atmosphere keeping other parameters constant. It has been found that the reduction degree increased with temperature and time, reaching a maximum of 80–82 % reduction at 1273 K, accompanied by an apparent porosity of around 40 % and a swelling index of 32–34 %, indicating satisfactory structural stability. Kinetic evaluation revealed that the gas–solid interfacial reaction was the rate-controlling step at lower temperatures, while diffusion through the product layer dominated at higher temperatures. The apparent activation energy value suggests a mixed control mechanism. X-ray diffraction (XRD) confirmed the progressive transformation of Fe₂O₃ to metallic Fe through Fe₃O₄ and FeO intermediates, while scanning electron microscopy (SEM) revealed porous, metallic iron structures after reduction. Figure 1 shows the XRD patterns of molasses bonded mill scale pellets after reduction at 1273 K for 30, 60, 90 and 120 minutes in low-grade thermal coal. The spectrums show that intensity of iron peaks (I) increases and intensity of wustitte (W) and magnetite (M) decreases with increasing reduction time. Figure 2 (a-d) and (e-h) show the SEM micrographs of bentonite and molasses bonded mill scale pellets after reduction at 1273 K for 30, 60, 90 and 120 minutes respectively. The micrographs show porous structure after 120 minutes indicating reduction of pellets due to diffusion of Co and N2+H2 gas inside the pellets.