Reduction of Pelletized Tyssedal Ilmenite and the Effect of Changing Gas Composition and Flow, Pellet Size and Pre Oxidation Condition
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This master thesis is based on reduction experiments of pelletized Tyssedal ilmenite. The focus is set on the effect of changing parameters such as gas composition, gas flow, pellet size and pre-oxidation condition. The results are interpret regarding values of conversion, degree of metalization, XRF-analysis and microstructure images. Based on these results the aim was to find out how these parameters influenced reduction. Another goal was to reveal what hampered reduction inside both grains and pellets and caused the two stage reduction behaviour. EPMA was used to examine pellets and grains and to look for explanation for the slow reduction. XRD-analysis was decisive for explaining the importance of optimum pre-oxidation for the subsequent reduction. Pre-oxidized and green pellets with a size fraction of 8-10 mm were heated in a thermogravimetric analyzer. Ar was used during heating and cooling. Reduction occurred at 930-940 °C, with either CO, H2 or both for up to 45 minutes. Gas flows used were 4.8, 7 and 9 Nl/min. In total 18 experiments were carried out. Weight before and after reduction was measured, calculations of degree of metalization, XRF- and XRD-analysis was done. Microstructure images, point analysis, line scan and mapping were obtained by EPMA. Highest final values of conversion, c. 0.9, were obtained for pre-oxidized pellets from 2011 reduced with H2. Maximum degree of metalization, 94-99 % were obtained after reduction of pre-oxidized pellets from 2010. Final value of conversion and degree of metalization were 0.23 and c. 24 % higher for pre-oxidized pellets from 2010 than from 2011. Only green pellets from 2011 was harder to reduce with CO. Reducibility was not increased with the pre-oxidation conditions at ETI Tyssedal in January 2011. In spite of that the pre-oxidized pellets from 2010 and 2011 are made in the same manner and from the same raw material they react differently during reduction. This should be kept in mind when comparing results from different sources. Reduction of pre-oxidized pellets occurred in two distinct linear stages separated by a clear bend. A normal behaviour with steadily decreasing oxygen removal rate was observed with green pellets. An increased flow of CO from 7 Nl/min to 9 Nl/min resulted in decreased reduction. Examination by EPMA confirmed the presence of the barrier effect and higher amount of oxygen 0.6-1.0 mm from the surface. This indicates hampered reduction in grains and pellets. This combined with slow migration of CO/CO2 in the pellets can partly explain the unusual reduction behaviour. Pre-oxidized pellets from 2010 contains higher amount of the easily reducible M3O5, and less M2O3 compared to pre-oxidized pellets from 2011. The less reducible M3O4 was also found in the pre-oxidized pellets from 2011, making them harder to reduce.