Some aspects of hydrogen reduction of iron ore

Abstract

Fines of hematite and magnetite were studied in the fluidized bed using a pure hydrogen atmosphere in the temperature range of 768 to 888 K. Hematite pellets were studied based on Thermogravimetric Analysis (TGA) experiments using hydrogen atmospheres containing 0–15 % pH2O, in the temperature range of 873 to 1173 K. Thermocouples in contact with the pellet’s surface and embedded in the pellet’s center recorded the thermal gradient in the pellet during reduction. The fluidized bed and the TGA experiments used an alternative method to start the reaction. The furnace was pre-heated with a reactive atmosphere. After this, the reaction was initiated by introducing the sample to the hot furnace, to eliminate the effect of gas dilution during gas switching. After the experiments, Scanning Electron Microscopy (SEM)analyses were employed to study the reduction microstructures. Both types of fines showed similar reduction rates. Fines and pellets showed high initial reduction rates, which increased with increasing temperatures. The reduction rate in the last reduction stage was low for both fines and pellets. An increasing pH2O content in the atmosphere lowered the reaction rate, and theeffect decreased with increased temperature. A difference between the pellet's surface and center temperatures was observed during reduction. The pellet'smacro-pore structure was seen to be unaltered by changes in temperature or atmosphere. However, at 873 K, the iron product microstructure was found to be highly porous. Furthermore, increasing temperatures caused dense iron to form. In addition, when porous iron or iron oxides were observed, increasing pH2O contentsincreased the pore diameter but decreased the pore amount. Pellet properties with varied pellet compositions were also investigated using Cold Crushing Strength (CCS), reduction in a TGA setup, and melting experiments. The composition was not found to influence the mechanical or reduction properties but significantly affected the phosphorus refining during melting.The results showed that a mixed reaction rate control occurred during the early reduction stage for both the fines and the pellets. The temperature differences observed during this reduction stage resulted from a combined effect of heat transfer and an endothermic chemical reaction. The impact of water in the atmosphere influenced the reaction rate through the backward reaction and mass transfer. At 873 K, the retarding effect is mainly caused by the backwardreaction. The results show the late stage of reduction to be primarily diffusioncontrolled. In addition, it should be possible to alter the pellet composition while maintaining pellet properties to increase the usefulness of the pellet.

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