Iron recovery from Waelz slag through biogenic carbothermic reduction

Waelz slag (WS) can be considered as one of the iron and steel related residues that have not yet been fully valorised. With more than 600-800 kg of WS generated per ton of electric arc furnace dust processed in the Waelz process, and a residual iron content of 10-40 wt.%, the iron recovery from WS by carbothermic reduction can represent an intriguing valorisation pathway. In this study, WS was mixed with an olive pomace derived biochar in three recipes with increasing C/(Fe2O3+ZnO) ratios and subsequently agglomerated. The reduction behaviour (mass loss, swelling, degree of metallization, mineralogy and chemistry) of the carbon composite agglomerates was evaluated at 1000, 1200 and 1400 °C under inert atmosphere and compared with the corresponding performance of coke as a reducing agent. The results highlighted the possibility of iron recovery from WS at 1400 °C and the comparable reduction behaviour in the case of using either a biogenic or fossil carbon-based reducing agent. The optimal C/(Fe2O3+ZnO) ratio was found to be 0.262, with a degree of metallization of 91 wt.% achieved at 1400 °C using biochar. The mineralogy of the slag produced, mainly composed of calcium aluminates and calcium silicates, showed high desulphurization properties, thus protecting the recovered iron particles from pollution. Nevertheless, the absence of slag-metal separation at each thermal treatment temperature suggests the need for pretreatment of the WS or the addition of fluxes to the agglomerate recipe.