Final stage of reduction of iron ores by hydrogen

A high degree of reduction or metallization of products, such as hot briquetted iron (HBI), used in steelmaking is desirable. For many ores it is difficult to achieve this goal because the rate of reduction slows significantly after 70-90% reduction. The present study shows that this happens for several types of ores and oxides because a shell of dense iron forms around wustite grains requiring atomic diffusion of oxygen through iron, for reduction to proceed. This rate is slow because of the low driving force and diffusion coefficient. Examination of the rate equations for gaseous reduction and oxygen diffusion indicate that if the wustite grains are about 1 mu m, gaseous reduction will govern the rate up to 95% reduction or greater, but for 2-mu m-sized grains the rate will slow significantly earlier when atomic diffusion controls the rate. The present work also indicates that if the equilibrium gas pressure at the Fe/FeO exceeds 1 atm, the iron shell may fracture allowing for faster gaseous reduction. Also, annealing or holding the ore at magnetite (Fe3O4) can also cause the rate of reduction to slow at lower degrees of reduction.