Effect of Coatings and Coating Methods on Cluster Index in Iron Oxide Pellets for Direct Reduction Shaft Furnaces

Coatings have been used for a significant number of years in the Direct Reduction industry, and the ISO Cluster Index plays an important role in this context since it allows a better predictability of pellet performance in the industrial process. The present work developed a series of tests and characterization in order to better understand which materials, methods, and characteristics would be most relevant to decrease the clustering index. It was noticed that coating on green pellets is worse as compared to the conventional application method which coats the pellets after the hardening process in the pelletizing furnace. In an evaluation of predictive parameters, it was observed that the adhesion index and the sum of SiO2, Al2O3, CaO, and MgO content influence the clustering index response variable. Consequently, it was observed that the adhesion index is proportional to the microporosity and mesoporosity of the coating materials. Bauxite was the material that showed the best performance. Its differentials were the high content of Al2O3 combined with its high microporosity/mesoporosity, i.e., pores with diameter up to 50 nm.