Influence of green pellet properties on pelletizing of magnetite iron ore
Sammanfattning: Magnetite iron ore green pellets are produced by balling moist concentrates to green pellets, which are then dried, oxidized to hematite, sintered, cooled and transported to steelmaking plants. The existing theory for balling is based on the capillary theory, but its applicability under industrial balling conditions is unclear. The aim of this study has been to clarify the principal mechanisms controlling the properties of iron ore green pellets. Special attention has been paid to studying how variations in raw material fineness influence green pellets behaviour during balling, oxidation and sintering. This knowledge of the principal mechanisms is needed to provide a sound basis for a successful process control strategy. The applied approach was to further develop the laboratory methods used in green pellet characterization. Oxidation in green pellets was measured by thermogravimetry and sintering was followed by dilatometry. A new measuring device for the characterization of green pellet strength was built and a new measuring method for green pellet plasticity was developed. The optimum moisture content in balling was defined as the moisture content resulting in a given degree of plasticity in green pellets. Pellet feeds with steeper particle size distributions required a higher moisture content in balling. Properties of green pellets prepared from different raw materials should be compared at constant plasticity (under realistic balling conditions), not at constant moisture content, as has been done earlier. At constant plasticity and with 0.5% bentonite binder, variations in the fineness of the magnetite concentrate did not influence the green pellet wet strength, within the limits studied in this work. This is because in the presence of the bentonite binder, green pellet wet strength was mainly controlled by the viscous forces of the binder liquid. A marked degradation in green pellet mechanical strength both in wet and dry states was found in the presence of a surface-active flotation collector reagent. This loss in green pellet quality was explained by a strong attachment of air bubbles in the green pellet structure. High-speed camera images showed multi-breakage patterns due to crack propagation between the air bubbles. This explains the increased generation of dust observed at the pellet plant. The negative effects of the flotation collector reagent on balling diminished during storage of the pellet feed. The results emphasize the importance of minimizing the reagent dosages in flotation and maximizing the residence time of the pellet feed in the homogenizing storage before balling. When a pellet starts to oxidize, a shell of hematite is formed while the pellet core is still magnetite. Thermal volume changes in these two phases were studied. Sintering in the magnetite phase started earlier (950°C) compared to the hematite phase (1100°C). Therefore, the difference in sintering rates between the magnetite and hematite phases was largest at around 1100°C. The sintering rate increased in both phases with increasing fineness in the magnetite concentrate. A finer grind in the raw material would, therefore, promote the formation of the unwanted duplex structures with a more heavily sintered core pulling off from the shell. At constant original porosity in green pellets, the oxidation rate decreased as the magnetite concentrate became finer, because of the enhanced sintering. However, in practical balling, finer raw materials would necessitate the use of more water in balling, which results in an increase in green pellet porosity. These two opposite effects levelled out and the oxidation time became constant. Under process conditions, differences in the duplex structure would still be expected. This is because only partial oxidation takes place before sintering in the kiln. Olivine, which is used as an additive in LKAB blast furnace pellets, was found to initiate the dissociation of hematite back to magnetite already at temperatures that can occur during oxidation in the PH zone. The rate of dissociation was largely influenced by the olivine fineness. If the dissociation temperature is exceeded, the resulting decrease in the oxidation rate increases the size of the un-oxidized core exposed to sintering before oxidation. Also, dilatometer measurements showed opposite thermal volume changes in the oxidized hematite shell and in the magnetite core in the presence of olivine. Dissociation caused a large volume increase in the oxidized hematite shell, while the olivine addition further enhanced the sintering of the magnetite core. These mechanisms lead to increased structural stress between the hematite shell and the magnetite core. This knowledge was applied at the LKAB Svappavaara pelletizing plant. Coarser grinding of the olivine additive resulted in a marked improvement in the low- temperature reduction strength (LTD) in pellets. The final conclusion, then, is that excessive grinding of the pelletizing raw materials, both the magnetite concentrate and the additives, can cause severe problems and step-wise changes in the oxidation and sintering mechanisms without resulting in any additional gain in terms of green pellet mechanical strength. The capillary theory failed to describe the properties of wet green pellets under industrial balling conditions. The results also clearly point out that continuous variations in raw material properties would cause complex fluctuations in both balling and induration.
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