The retardation kinetics of magnetite reduction using H₂ and H₂-H₂O mixtures

Cylindrical compacts of magnetite were isothermally reduced at 773-1273 K with pure H₂ or H₂-H₂O mixtures. The initial reduction rates increased with temperature and partial pressures of H ₂ in the H₂-H₂O mixtures. However, with progressing reduction, a dense iron layer formed around the wüstite grains and the rate significantly reduced. In this regime, solid state oxygen diffusion through the dense iron layer was rate limiting. This retardation of reduction occurred at degrees of reduction of 51-89%, depending on the temperature and H₂ partial pressure, which has a linear relationship with the dimensionless kinetic parameter, k₁^mixed/k ₂^mixed, (k₁^mixed, k₂ ^mixed: contribution of gaseous mass transport (GMT) and interfacial chemical reaction (ICR) to the reduction rate, respectively) in the reaction-regime controlled by a combination of both mechanisms. However, under certain conditions (H₂, H₂-10%H₂O, 773 K//H₂-10, 20%H₂O, 873 K//H₂-20%H₂O, 973 K) the retardation was absent because of the formation of a microporous iron layer product.