Effects of Re-ignition and Coke Breeze Reduction on Sinter Microstructure in Sintering Process

Abstract

REMO-tec, developed by Nippon Steel, is the sintering technique of re-igniting sintering packed bed at certain intervals after first ignition. This method improves sinter yield while maintaining high reducibility, thereby making REMO-tec a technology that contributes to reduce CO₂ emissions in both sintering and blast furnace operations. This study aims to investigate the effects of re-ignition and coke breeze reduction on the mineral phase and pore structure of sintered ore through pot tests, providing a comprehensive evaluation linked to sintering operational parameters. Sinter pot tests were conducted with a 130 mm layer of raw materials, focusing on the upper part of the sintering layer. Experimental conditions included a base condition with single ignition and REMO-tec with re-ignition under three levels of coke breeze blending ratios (4.1, 3.3, and 2.9%). Microstructure observation and pore structure analysis were conducted using an optical microscope and image processing, respectively. REMO-tec extended the high-temperature holding time while maintaining the maximum bed temperature, leading to the formation of an acicular calcium ferrite under low coke conditions (3.3 and 2.9%) with a peak temperature of approximately 1250℃. Additionally, pore structure analysis revealed that the reducibility of sintered ore correlates with the volume of pores smaller than 200 µm. Consequently, producing high-FeO sintered ore with an acicular calcium ferrite matrix and a large volume of pores smaller than 200 µm through REMO-tec under low bonding agent conditions is the most desirable approach for balancing sintering operational parameters and reducing CO₂ emissions in the ironmaking process.