Reduction Kinetics of Oxidized New Zealand Ironsand Pellets in H 2 at Temperatures up to 1443 K
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Direct reduction of iron ore pellets using hydrogen gas has the potential to significantly reduce CO2 emissions from the ironmaking process. In this work, green pellets of titanomagnetite ironsand from New Zealand were oxidatively sintered to form titanohematite. These sintered pellets were then reduced by H2 gas at temperatures ‡ 1043 K, and a maximum reduction degree of ~ 97 pct was achieved. Fully reduced pellets contained metallic Fe as the main product phase, but several different (Fe, Ti) oxides were also present as minor inclusions. The phase distribution of these oxides depended on the reduction temperature. With increasing temperature, the relative proportion of pseudobrookite in the final product increased, while the proportion of residual ilmenite and rutile decreased. The reduction kinetics were found to be well described by a pellet-scale single-interface shrinking core model, for reduction degrees up to 90 pct. At temperatures above 1143 K, the rate-limiting step was found to be solely an interfacial chemical reaction process, with a calculated apparent activation energy of 31.3 kJ/mol. For pellet sizes from 5.5 to 8.5 mm, the reaction rate was observed to increase linearly with decreasing pellet diameter, and this linear correlation extrapolated to intercept the axis at a pellet diameter of 2.5 mm. This is interpreted as the minimum length required for a shrinking core interface to develop within the pellet. https://doi.org/10.1007/s11663-020-01790-3 The Minerals, Metals & Materials Society and ASM International 2020
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INTRODUCTION
THERE is increasing interest in the use of titanomagnetite (TTM) iron ore as a potential cheap source of iron,[1] as well as the possible co-production of other valuable minerals such as titania (TiO2) and vanadium pentoxide (V2O5). In New Zealand (NZ), large deposits of TTM ironsand are found along > 400 km of the west coast of the North Island.[2] The NZ ironsands typically contain ~ 8 wt pct TiO2. This is a significantly lower TiO2 content compared to other TTM deposits from around the world (e.g., 13 wt pct in China,[3] 10 wt pct in Indonesia,[4] and 14 wt pct in South Africa[5]). However, the presence of TiO2 means that the conventional blast furnace process is not suitable for reduction of these ironsands.[6] Instead, TTM ironsand is currently processed in NZ using a two-stage process. The TTM is
AO ZHANG, MOHAMMAD NUSHEH, and CHRIS W. BUMBY are with the Faculty of Engineering, Robinson Research Institute, Victoria University of Wellington, Lower Hutt 5046, New Zealand. Contact e-mail: [email protected] BRIAN J. MONAGHAN and RAYMOND J. LONGBOTTOM are with the Pyrometallurgy Group, School of Mechanical, Materials, Mechatronic and Biomedical Engineering, University of Wollongong, NSW 2522, Australia. Article published online February 18, 2020. 492—VOLUME 51B, APRIL 2020
initially partially reduced in a rotary kiln via a solid-state carbothermic reduction process, and then fully reduced and smelted in an electric furnace. This process is both energy and CO2 i
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