Effects and mechanism of composite binder on high-temperature consolidation of pellets

Iron ore pellets have several substantial advantages, such as high iron grade, low harmful elements, low smelting slag, and low pollution in the production process. Rapidly developing the pelleting process and improving the quality of pellet ores is crucial for achieving the goals of carbon peaking and carbon neutrality in the steel industry. Bentonite, a major binder in the pellet ore production process, can significantly improve the sphericity of raw materials and enhance the pellet quality; however, the higher SiO₂ and Al₂O₃ content will cause an increase in the slag volume of ironmaking production. An organic binder has the advantages of low dosage and less harmful impurities, which can improve the pellet ore grade and the expansion rate of pellets. Thus, adding a small amount of organic binder to replace part of the bentonite is essential to improving the pellet performance. This study investigates the effect of organic binder P replacing part of bentonite on the high-temperature strength of pellets. The laser flash and thermogravimetric methods were employed to investigate the effects of organic binder on the internal structure, heat transfer, and mass transfer of pellets. The results indicated that bentonite was beneficial in reducing the porosity and high-temperature consolidation of pellets because it could promote the generation of a low-melting-point liquid phase. With increasing bentonite addition, the strength of preheated and roasted pellets increased, and the porosity decreased from 21.82% to 15.68% when bentonite addition increased from 1.1% to 2.0%. Thus, the composite binder could replace a part of the bentonite and significantly improve the strength of preheated and roasted pellets with increasing addition. Moreover, the organic binder P was pyrolyzed at a high temperature and formed pores inside the pellet, resulting in a lower thermal conductivity and a slower internal heating gradient. The thermal diffusivity reduced from 0.321 to 0.266 mm²·s⁻¹, and the heat transfer coefficient decreased gradually from 0.551 to 0.454 J·g⁻¹·K⁻¹. Thus, the formation of a dense oxide layer on the surface of the pellets owing to rapid oxidation was avoided, and the oxidation of hematite inside the pellets was promoted, thus enhancing the pellet strength. Moreover, the tiny pores facilitated oxygen transfer to the interior of the pellets and promoted the oxidation of Fe₃O₄ to Fe₂O₃. The oxidation fraction f _TGA gradually increased from 90.80% to 92.17% with the addition of organic binder P.