| Popis: |
In countercurrent furnaces such as blast furnaces and gas-based shaft furnaces, ensuring smooth operation requires the rising reducing gas flow to pass through the charging materials promptly and uniformly. Two main factors lead to the permeability deterioration of the furnace charge: low-temperature reduction disintegration and reduction swelling of iron ore pellets. Generally, iron ore pellets exhibit good performance in low-temperature reduction disintegration. However, owing to the shortage of high-grade iron ore concentrates in China in recent years, the quality of pellet feeds has considerably declined, leading to a deterioration in the low-temperature reduction disintegration performance of certain types of iron ore pellets. Therefore, this paper comprehensively reviews the main factors affecting the low-temperature reduction disintegration performance of iron ore oxidized pellets under blast furnace and gas-based shaft furnace conditions. The mechanism behind the reduction disintegration of iron ore pellets at low temperatures, and the measures to improve their low-temperature reduction disintegration performance. The low-temperature reduction disintegration of iron ore pellets can be attributed to lattice expansion caused by the phase transformation from rhombohedral hematite to cubic magnetite during the reduction process. Additionally, carbon deposition occurs in the fine pores and crystal cracks of pellets, further increasing internal stress and exacerbating disintegration. As hematite is the dominant phase in iron ore oxidized pellets, phase transformation and internal stress generation during low-temperature reduction are unavoidable. The main factors influencing the low-temperature reduction disintegration performance of pellets include the type of pellet feeds, their chemical composition (e.g., basicity and SiO2, MgO, and Al2O3 content), phase composition, microstructure, pore characteristics of fired pellets, and carbon deposition reaction. In practical production, enhancing the low-temperature reduction disintegration performance of iron ore pellets involves optimizing preheating and roasting parameters based on the characteristics of raw materials, adjusting chemical composition by adding flux (such as increasing basicity or MgO content), modifying consolidation form (generating an appropriate amount of liquid phase consolidation), and regulating pore structure (by spraying or coating to reduce porosity) of iron ore pellets. The underlying principle of these measures is to strengthen the iron ore pellets matrix, increase the content of stable bonding phases during low-temperature reduction, and reduce the reaction rate during the initial reduction stage to mitigate severe expansion stress. |
