The Effect of High-Pressure Roll Grinding and Damp-Milling Pretreatment on the Reduction Performance of Fired Pellets

The growing demand for direct reduced iron (DRI) in green steel production requires high-quality fired pellets as the burden for the gas-based shaft furnace direct reduction process. However, the properties of magnetite concentrate as pellet feed present a significant impact on the quality of fired pellets, especially the metallurgical performance. A systematic study of the effect of pretreating the magnetite concentrate on the properties of fired pellets was conducted using two pretreatment technologies, i.e., damp-milling and high-pressure roll grinding (HPRG). Green balls were made from pretreated magnetite concentrates and fired under optimal conditions. Their performance was then evaluated in a laboratory-scale setup simulating the HYL shaft furnace environment. Key metrics included cold compressive strength (CCS), reducibility index (RI), reduction swelling index (RSI), and dynamic low-temperature reduction degradation (LTD). The pretreatment of magnetite concentrates with HPRG twice showed significant benefits. The fired pellets not only have a CCS of 2500 N/p at a roasting temperature 150 °C lower, but also achieve an RI of 3.37 and an RSI of 3.15%, respectively. Furthermore, the reduction degradation tendency was markedly reduced; the +6.3 mm fraction reached 94.72% with a whole pellet ratio of 75.49%. Conversely, while damp-milling improved the LTD, it required a 100 °C increase in preheating temperature and yielded a whole pellet ratio of only 49.15%, failing to meet industrial requirements. The improvement in metallurgical performance is attributed to the intense micro-cracking induced by the two-pass HPRG process, which optimizes the particle size distribution, specific surface areas and improves the microstructure and pore properties of the fired pellets.