Interactions between iron oxides and the additives quartzite, calcite and olivine in magnetite-based pellets

Abstract: In the present study, magnetite pellets with large amounts of the additives olivine, calcite and quartzite were isothermally reduced in a tubular furnace to study the interaction between iron oxides and the additives. A first attempt at using exaggerated amounts of additives was made in order to enable analyses of phases that do not otherwise occur in sufficient amounts for Xray diffraction and EDS-analyses. The reduction was thermodynamically set to yield either magnetite or wüstite at three different temperatures, 900, 1000 and 1150°C. For olivine, reduction tests were also performed at 500, 600, 700 and 800°C. The mineralogical phases that had formed were studied after oxidation as well as after reduction. The results showed that it was possible to identify, by X-ray diffraction, the main phases formed by the additives in all samples, after oxidation as well as reduction.The quartzite particles were shown to have remained quite intact after the oxidation treatment, except for small particles in the presence of impurities that formed melts. During reduction the quartzite particles reacted with iron so that fayalitic melts were formed already at 1000°C. After reduction at 1150°C all quartzite had transformed into a fayalitic melt so that most of the small pores had disappeared through sintering or had been filled by fayalite.In the sample with calcium oxide the additive particles had reacted during the oxidation treatment and formed calcium ferrites and calcium silicates. Upon reduction, the ferrites that formed during oxidation reduce, so that a porous calciowüstite becomes the primary phase already at 900°C. Calcium silicates that were formed during oxidation also remain in the sample as silicates during reduction.The results showed that the olivine after oxidation had reacted along the particle boundary and turned into magnesioferrite crystals and pyroxene/vitreous silica. Magnesium is liberated when the olivine particle breaks down, and finally ends up as islands of magnesioferrite surrounded by hematite in the original magnetite particles. In the pellet core the magnesium has diffused relatively long distances so that the magnesioferrite islands are not just found close to-, but also further away from the olivine particles. Upon reduction, the hematite converts to magnetite already at 500°C and in the tests carried out at 500-700°C, cracks were observed along the hematitemagnesioferrite boundary. At 800°C, temperature is enough to allow slow diffusion of magnesium from the magnesioferrite to the surrounding magnetite or wüstite, and at 900°C the cracks around the magnesioferrite phase disappear. The Mg stored in the wüstite then reacts with the silica slag in the sample when it approaches its melting point at 1000°C. The magnesium level in the wüstite then approaches a background level which was found to be about 2% after reduction for 2 hours at 1150°C.