Developing composite binders and optimizing fabrication process for cold-bonded pellets

With the ongoing advancement of low-carbon transition in the iron and steel industry, the development of novel green iron-containing burden materials has become an important strategy for energy conservation and carbon emission reduction in ironmaking processes. Cold-bonded pellets have attracted considerable interest owing to their avoidance of high-temperature roasting and low energy consumption. However, their industrial deployment remains limited by insufficient cold strength and high-temperature performance. This study focuses on overcoming the key technical challenge of enhancing the cold strength of cold-bonded pellets, while high-temperature behavior will be examined in future work. Using sinter return fines and iron concentrate as raw materials, this research systematically investigates the influence of molding parameters on the properties of cold-bonded pellets. A composite binder system was developed, primarily consisting of inorganic binder I₄ with the addition of organic binder O₅ and additives A₅ and A₆. The synergistic mechanisms among these components were elucidated. The effect of the drying regimen on binder curing and pellet mechanical properties was also studied. Results indicate that under the optimum processing conditions, which include a raw material moisture content of 7%, a molding pressure of 60 MPa, and pellet dimensions of 32 mm×20 mm×15 mm, the cold strength of pellets prepared with 4% inorganic binder I₄ reached 1 055 N/P. The introduction of a composite binder system containing O₅, A₅, and A₆ significantly enhanced the strength, raising it to 2 629. 5 N/P. Additionally, optimal pellet performance was obtained after drying at 100 ℃ for 3 h. Mechanistic studies show that suitable moisture enhances liquid bridging and improves particle packing. Appropriate molding pressure reduces porosity and promotes particle interlocking. Increased pellet size expands inter-particle contact area and improves binder distribution, leading to enhanced mechanical properties. Cross-linking between organic binder O₅ and inorganic binder I₄ improves the network structure, while additives A₅ and A₆ further strengthen the pellets by increasing the reactivity of binder I₄. A well-designed drying protocol facilitates uniform binder curing and controlled moisture release, thereby enhancing pellet integrity. Comparisons of softening and melting behavior revealed that the cold-bonded pellets possess a broader softening interval, lower maximum pressure difference, and superior gas permeability compared to acid pellets. This study offers valuable insights for advancing the industrial application of cold-bonded pellet technology and provides a novel technical approach and theoretical foundation for developing highperformance green ironmaking burden materials.