Hydrogen reduction of preoxidised ilmenite in fluidised bed and packed bed reactors

Abstract

The kinetics of fluidised bed hydrogen reduction of preoxidised ilmenite concentrates was studied as a function of the different operating parameters of gas velocity, pressure and temperature. The reduction kinetics is controlled by gas starvation at typical fluidised bed flowrates and pressures. Gas starvation was found to dominate at H₂ flowrate to ilmenite ratios less than 2 L min⁻¹ g⁻¹. At higher ratios, a form of attenuated gas starvation exists due to the small equilibrium constant for ilmenite reduction. This leads to large variations in the kinetic driving force for small variations in water vapour in the bed. Within the gas starvation region, there is an apparent linear dependence of reduction rate on pressure when a constant fluidising velocity is used. However if the supply of hydrogen to the reactor is kept constant, then the reduction rate is independent of gas pressure above 1 bar. At hydrogen supplies above 2 L min⁻¹ g⁻¹ the reduction rate increases with pressure but the relationship is not linear. An important contributing factor to reduction mechanisms is the very fine intrapore diameters (median pore diameter <0·15 μm) in reduced ilmenite grains, which lead to Knudsen gas diffusion in the pores and a build up of water vapour pressure within the grains.