Reduction of Manganese Ores in CO-CO 2 Atmospheres
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CKGROUND
MANGANESE ferroalloys are produced in submerged arc furnaces, where the carbon and energy consumption of the process will be largely dependent on the solid-state reduction of manganese ore at relatively low temperatures. Higher manganese oxides (MnOx, x > 1) in the ore will reduce to MnO by CO(g) present in the furnace gas when exposed to elevated temperatures. The reduction’s influence on the efficiency is two-fold. Firstly, the reduction reactions are exothermic, thus heating the charge material and minimizing the required electric energy feed. This implies that the heat production is dependent on the oxygen level of the ore. Secondly, CO2(g) produced from the reduction reactions may be consumed by the Boudouard reaction (CO2(g) TRINE ASKLUND LARSSEN and MERETE TANGSTAD are with the Norwegian University of Science and Technology (NTNU), 7034 Trondheim, Norway. DIETER SENK is with the RWTH Aachen University, 52062 Aachen, Germany Contact e-mail: [email protected] Manuscript submitted May 28, 2020; accepted October 18, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS B
+ C = 2CO(g)), which is both carbon consuming and highly endothermic. It is generally agreed that the Boudouard reaction obtains sufficient kinetics at temperatures exceeding 800 °C in a ferromanganese furnace.[1,2] Kinetics and mechanisms of the ore-gas reduction is dependent on the properties of the ore. Geology and geochemistry of different commercial manganese ores and minerals have been presented in a number of previous publications. Certain studies reporting on mineralogy of manganese ores provide the main constituting minerals,[3–5] whereas others provide quantitative measures of both major and minor phases.[6,7] In general, these studies have shown that characterization of manganese ores is not a straight-forward procedure. A great number of minerals is found to constitute the same ore, where both highly similar composition of several minerals, cation substitution, heterogeneity of the ore, as well as geometric effects, such as porosity and cracks, complicates the characterization process. Varentsov and Grasselly[8] presented a review of existing literature on characteristics of the most common manganese minerals, where it was stated that several of the minerals are frequently intergrown. Waldemar and Dressel[9] investigated the thermal behavior of various
manganese minerals in controlled atmospheres, reporting that the reduction/decomposition behavior of the minerals was affected by the presence of other minerals. Mineralogy of Nchwaning and Gloria manganese ores obtained through XRD were reported by Visser et al.,[6] who identified ten different constituting minerals, of which five were considered of minor abundance (< 5 pct). Nchwaning ore was mainly composed of braunite and braunite II, whereas Gloria contained braunite, as well as considerable amounts of calcite and kutnahorite. A low correlation between XRD and SEM/EDS analyses was presented by Sørensen et al.[7] According to XRD, Groote Eylandt ore was found to be fully composed
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