Reduction of Kemi chromite with methane

Kemi chromite ore from Finland was reduced in CH₄-H₂ gas mixtures at temperatures from 1100° to 1350°C. Experimental variables were time, temperature, and the CH₄ content of the gas mixture. The phases in reacted samples were determined by XRD and quantitative phase analysis was performed using the Rietveld method. Particle morphology and phase compositions were determined using SEM and EDS analysis. Reduction proceeded through a shrinking core model in two stages. In the first stage, hydrogen and carbon from the cracking of methane reduced the iron and some chromium to carbides. The chromium in the remaining spinel was reduced during the second stage. Iron was essentially reduced in full after 30 minutes when the temperature reached 1300°C, and at 1350°C it was completely reduced within 20 minutes. At this highest temperature, all of the chromium was eventually reduced, the residue consisting of aluminum and magnesium oxides with varying amounts of silica. At the surface of the particle iron and chromium, together with carbon, formed two alloys, an iron-dominated and a chromium-based one. The iron-based phase was partially molten at the higher temperatures. At 1300°C and 1350°C the metallization was complete. Reduction of Kemi chromite with a CH₄-H₂ mixture is judged as highly efficient, since high of reduction extents could be reached faster at lower temperatures compared to carbothermic reduction. This is attributed to the very high activity of carbon (way above 1.0) due to the cracking of methane into hydrogen and carbon at around 550°C in the presence of a solid phase.