Understanding the effect of roasting on vanadium speciation in steel slags and impact on leaching

Vanadium is an extremely useful element that is primarily sourced from steelmaking slags via hydrometallurgical processes that use an oxidative roasting step. This roasting step oxidises vanadium to soluble high valent states and influences vanadium extraction in the subsequent leach step. This study examines the structural and speciation behaviour of vanadium in vanadium-rich slag during roasting and investigate how it affects the extraction efficiency of vanadium during leaching. Vanadium-rich slag was roasted at a range of temperatures under air or argon, and was characterized via standard techniques: XRD, SEM/EDS and XAS. XRD and SEM/EDS analyses showed that increasing roasting temperature or time resulted in greater destruction of vanadium spinel, with the released vanadium migrating with manganese to form a vanadium‑manganese rich phase. X-ray absorption spectroscopy showed a clear increase in the presence of V(V) with increasing roasting temperature. Leaching these roasted slags with a mildly acidic sodium acetate buffer solution (pH = 4) gave greater vanadium extraction efficiencies for slag samples that had higher proportions of V(V). This work thoroughly describes the changes in structure and speciation that occur in vanadium-rich slag during the roasting step, and shows the impact that roasting has upon extraction efficiencies in the following leaching step, justified by testing the elemental accountability. • Vanadium-rich steel slags contain vanadium locked in an iron-containing spinel. • The vanadium cannot be effectively leached from as-received steel slags. • Air roasting converts the V 3+ to V 5+ as evidenced by X-ray absorption spectroscopy. • The V 5+ migrates to a new phase containing mostly Mn and V. • The vanadium can be selectively leached from air roasted slags.