Towards solar iron metallurgy: Complete hydrogen reduction of iron ore pellets under a concentrated light flux

Abstract

Current ironmaking process leads to large CO₂ emissions due to the use of fossil fuels as both heating agent and reducer. An alternative ironmaking process based on the reduction of iron ore by hydrogen under a concentrated light flux, simulating a direct solar heating reactor, is studied here. Experiments were performed in batch mode on the iron ore pellets used in industry, which consist in spherical agglomerates of iron oxide with a diameter of ca. 2 cm. Quantitative analysis of the reduction yield and kinetics were deduced from the Rietveld refinements of X-ray diffraction patterns as well as optical and scanning electron microscopy. It is shown that hydrogen pressure has a significant influence on the time evolution of the reaction, probably by its influence on re-oxydation. Observations and analysis of cut pellets show that reduction starts from the illuminated surface towards the shadowed side, due to a large temperature gradient inside the sample. This conducted us to perform experiments in which pellets were rotated, which significantly reduce reduction time. On single pellets, a reduction yield of 96 % was reached in 12 min by turning them three times during exposure. Samples under the form of gravels and flat disks were also tested. The former did not lead to significant improvement, but a 96 % reduction yield was measured on 2-mm-thick disks after only 2 min of exposure. An analysis of the energy efficiency of the process is provided. These results show that hydrogen-based solar metallurgy could meet industrial requirements in terms of reduction yields and might be envisaged as a low-carbon ironmaking process.