Value Generation by Recovering By-Products from Steelmaking Processes: Dezincification of Basic Oxygen Steelmaking Slurry

The production of steel through the blast furnace/basic oxygen steelmaking route generates approximately 10kg of ferrous dusts and sludges per tonne of hot metal produced. This ferrous material is readily recyclable via a sinter plant to recover iron units but is difficult to recycle when the material is contaminated with zinc. Historically, excess material has been stockpiled or landfilled which presents an environmental issue but also a commercial opportunity to recover huge volumes of ferrous material from operating and former steelmaking sites. A review of best available technology for recovery of zinc from steelmaking by-product dusts identified that the Rotary Hearth Furnace (RHF) is the most attractive commercialized option for processing of material at an integrated plant such as Tata Steel Port Talbot, but due to the relatively low value of the ferrous direct reduced iron (DRI) product is still not commercially viable. A characterization study of basic oxygen steelmaking dust from Port Talbot, Redcar and Scunthorpe steel plants in the United Kingdom was undertaken using a variety of techniques and indicated pyrometallurgical recovery would be the preferred recovery route due to the high metallization and variability of the morphology of the dusts. Benchmarking of material from Port Talbot material processed in laboratory furnace trials under conditions mimicking the RHF showed that while volatile metal removal and iron reduction performance is adequate for recycling at realistic RHF temperatures and hold times, the high sulfur and gangue content mean the produced DRI would not be valuable enough to offset production costs. A computational thermochemical study was undertaken using FactSage and identified the possibility of applying next generation RHF technology to produce high value pig iron nuggets from steelmaking by-products. This possibility was confirmed experimentally, producing pig iron nuggets from BOS dust with complete removal of zinc from the material, at feasible process conditions, by addition of SiO2 and MgO fluxes. Alternative reductants for the RHF were also explored, specifically adapting the RHF process to utilize the enormous volume of plastic waste generated from facemasks in the wake of the COVID-19 pandemic. The addition of facemask plastic to a low-volatile coal was found to increase its CO2 gasification reactivity, which has associated process benefits for direct reduction.