One-, Two-, Three-Step Processing of Waelz Slag via Sequential Hydro- and Pyrometallurgical Treatments for the Iron Recovery Enhancement

Waelz slag (WS) is a critical waste closely tied to the iron and zinc recycling cycles, which, thanks to its chemical composition, represents a potential secondary source of Fe. However, the presence of undesired elements (e.g., Zn, Pb, and S) poses major constraints for its direct pyrometallurgical processing (one-step treatment). To decrease the amount of these elements and increase the suitability for the subsequent pyrometallurgical processing, this study investigates the pretreatment of WS via caustic leaching (two-step treatment) and alkaline roasting followed by leaching (three-step treatment). Overall, the first pretreatment resulted in extraction degrees of over 60 wt% for Zn and 45 wt% for Pb and S, whereas the second pretreatment led to extraction degrees greater than 85 wt% for each of the three elements. Concurrently, both pretreatments preserved over 95 wt% of the initial Fe content. The pretreated WS samples were pelletized and carbothermically reduced at 1500 °C. To evaluate the effect of the alkaline treatments, the reduced pellets were characterized metallurgically and compared with the properties of reduced untreated WS-based pellets. After reduction, the pretreated WS-based pellets contained two to three times the amount of metallic iron observed in the untreated WS-based pellets (19.78–28.80% vs. 9.52%), with a consistent metallization degree of over 90 wt%. Calcium aluminates were the main constituents of the slag phase of each reduced WS-based pellet. Despite the absence of slag–metal separation, the WS processing via hydrometallurgy followed by pyrometallurgical recovery of Fe proved to be an effective waste-to-resource pathway for Fe- and Zn-bearing residues.