Effect of Particle Size and Starch Gelatinization on the Mechanical and Metallurgical Performance of Jarosite Plus Blast Furnace Sludge Self-Reducing Briquettes

Jarosite and blast furnace sludge (BFS) are two of the main wastes from hydrometallurgical zinc production and iron production by blast furnace, respectively. Jarosite is a hazardous material that can, however, be reused in the steel industry after the recovering of the iron contained within it through carbothermal reduction in which BFS is exploited as a reducing agent. Yet, both wastes have a powdery nature that makes it necessary to agglomerate them for industrial use. On the other hand, despite the advantages of producing a self-reducing product, the particle size of the starting powders and the level of gelatinization of the binder could play a crucial role on the mechanical and metallurgical performance and, consequently, on the industrial applicability of the briquettes. Accordingly, two powder particle sizes (very fine sand vs. coarse silt) and three degree of corn starch binder retrogradation (10%, 30% and non-gelatinized starch) were used to produce briquettes, and their influence was studied by experimental and statistical investigation. The results showed that gelatinization plays the main role on the mechanical properties of briquettes, while particle size affects both density and reduction behavior; in particular, although all the mixtures were able to recover iron at 950 °C the most optimal mixture were obtained by using a granulometry of 63–125 µm for jarosite and less than 63 µm for BFS, while the local maximum of mechanical performance was obtained for a 30% starch retrogradation level.