Morphology of metallic iron during direct reduction with hydrogen

The morphology of the metallic iron product strongly affects the reduction kinetics of iron oxide with hydrogen. Under some circumstances – especially when the initial oxide phase is magnetite rather than hematite, and the temperature is around 700 °C – a dense iron layer forms and encapsulates unreduced wüstite, effectively stopping reduction. In this work, natural magnetite concentrates – from taconite deposits in the Great Lakes region of the United States – were used to test whether encapsulation always occurs upon reduction of magnetite. The results confirm that encapsulation can occur, but magnetite from at least one location did not encapsulate upon reduction. In all cases, pre-oxidation of the magnetite to hematite avoided encapsulation. A mechanism is proposed that the propensity for encapsulation depends on the balance between pore creation by reduction (since the molar volume of iron is much smaller than that of wüstite), and surface diffusion that tends to eliminate pores. In experiments to test whether encapsulation was linked to the density of iron nuclei, concentrate samples were examined by scanning electron microscopy after exposure to hydrogen for a short time.