Influence of pressure on the kinetics of synthetic ilmenite reduction in hydrogen

In-situ thermogravimetric measurements were used in the hydrogen reduction of poly-granular synthetic ilmenite discs at temperatures in the range 823 to 1173 K and at pressures in the range 1.2 to 13 atm. A symmetrical beam microbalance was used, coupled with twin reactors and twin furnaces, to minimize buoyancy and drag effects. Stable operation was achieved at high gas flow rates where gas film transport effects were negligible. Polishing the ilmenite discs prior to reduction eliminated the formation of dense surface metallic iron films that can impede gas diffusion into the discs. Macroscopically, the reduction reaction proceeded topochemically and a shrinking core reaction model was found to be appropriate to predict conversion-time relationships. It was necessary to allow for water vapor adsorption onto the reacting interface in order to model the effect of pressure on the reduction kinetics. The observed reduction rate increased sharply with pressure up to approximately 3 atm and then approached a plateau with further pressure increase. The porosity in the reduced ilmenite samples was very fine, with pore diameters of typically 0.05 to 0.3 mu m. Intragrain gas pressure buildup in the fine pores due to the influence of Knudsen diffusion was incorporated into the modeling of the kinetic data.