Reduction of magnetite to metallic iron in strong alkaline medium

• Direct in situ reduction of Fe3O4 to Fe was done in alkaline solution.
• Thick and dense Fe layer was grown at the surface of dense magnetite cathode.
• Reduction of porous Fe3O4 ceramics occurred mostly across the sample.
• Porosity promoted the current efficiency of iron electroextraction up to 85%.

This work focuses on assessing the feasibility of cathodic iron extraction from the magnetite based precursors. For this, electrochemical processes at Fe3O4/alkaline electrolyte interface were screened by cycling voltammetry. Based on these results, one obtained guidelines for selecting the conditions (i.e., potential and temperature) where efficient direct electrochemical reduction of magnetite ceramics to metallic iron occurs. Electrochemical conversion of relatively dense magnetite samples yields a polycrystalline Fe scale, formed at the surface of the magnetite pellet in direct contact with the bulk electrolyte. Still, the onset of slightly open porosity results in formation of intermediate layers with coexisting magnetite and metallic Fe; this is ascribed to gradual development of additional porosity, which promotes sample impregnation with the electrolyte, extends the effective electrochemically active area, and facilitates dissolution of soluble species in the inner pores. This is clearly demonstrated by transient response behavior, with remarkable increase in the current density. The key roles of porosity and effective Fe3O4/electrolyte area are also emphasized by the enhanced kinetics of electrochemical reduction observed for highly porous magnetite samples, with nearly homogeneous distribution of reactant (Fe3O4) and product (metallic Fe), without a clear surface scale of metallic iron. In this case, the final product is very porous and fragile. The conversion of highly porous magnetite samples also proceeds with much higher Faradaic efficiency compared to nearly dense ceramics.