Reduction of Manganese Ores in CO-CO₂ Atmospheres

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) + 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 by pyrolusite, however, optical and SEM analysis showed that the ore contained pyrolusite, cryptomelane, iron alumo-silicates, and quartz. It was further observed that XRD identified bixbyite as main mineral of Wessels ore, whereas SEM and EDS investigations showed that the mineral identified as bixbyite was in fact braunite in two different modifications.

The reduction behavior of manganese ores is complex due to multistep and parallel reaction schemes. High oxygen ores, such as Comilog, contains large abundance of tetravalent oxides (MnO₂), and may reduce through oxidation sequence MnO₂-Mn₂O₃-Mn₃O₄-MnO, whereas semi-oxidized ores, such as Nchwaning, generally contains trivalent oxides (Mn₂O₃), thus omitting the first reduction step. On the other hand, these semi-oxidized ores often contains significant amount of iron oxides, which may reduce through sequence Fe₂O₃-Fe₃O₄-FeO-Fe. Several experimental studies have been reported regarding prereduction of manganese ores in CO-CO₂ atmosphere, both in isothermal[2,5,10,11] and non-isothermal[6,12‐16] temperature regimes. Non-isothermal studies have commonly been used to simulate the industrial furnace conditions, i.e. to evaluate whether the ores will continue to reduce in the active region of the Boudouard reaction, often defined as the CO-reactivity of the ore. These studies generally provide little or no information of kinetics and mechanisms. They do however provide an overview of the reduction extent obtained at a given temperature, commonly 800 °C, for various commercial manganese ores. The studies show that Comilog ore has a higher CO-reactivity compared to Nchwaning ore. According to Turkova, Slizovskiy, and Tangstad this may be explained by the higher porosity of Comilog ore compared to Nchwaning ore, as the authors reported a lower oxygen level at 800 °C, i.e. higher reduction degree, with increasing initial porosity of the ores.[13] Isothermal conditions are commonly used to simplify the mathematical interpretation of reaction mechanisms and kinetics by avoiding temperature as a variable. As such, all studies reporting on kinetics and mechanisms of manganese ores have been conducted in isothermal temperature regimes. Several studies have been performed where the ore has been calcined prior to the reduction experiments, as the aim was to investigate the reduction of Mn₃O₄ to MnO, which is the reduction step that may occur simultaneously as the Boudouard reaction. Thus, less information is available on the initial reduction steps, such as MnO₂ and Mn₂O₃. In addition, studies have shown that reduction behavior is dependent on the pretreatment of the ore, implying that relevant and representative data may not be obtained from calcined or synthetic materials.[2,12,17] Another aspect shared by the studies is the fairly high temperatures at which the experiments have been conducted, implying that the extent of information regarding the ores behavior as it is being fed to the furnace is low.

The literature review shows that the reduction of manganese ores have been investigated in both no-isothermal and isothermal experiments. Non-isothermal studies provide information on the extent at which the ore reduces at temperatures exceeding 800 °C, but provides no or little information on the reaction mechanisms. Isothermal studies have generally been conducted using pretreated (or synthetic) manganese materials, which have been observed to exhibit dissimilar reduction behavior compared to the untreated ores. Further, these studies have been conducted at fairly high temperatures, close to the melting point of the ores. As such, information on the behavior of the ore as it is fed to the industrial furnace is scarce.

This study aims to determine the reduction path of reduction of Comilog and Nchwaning manganese ores in CO-CO₂ atmosphere. The focus is on phase compositions of ores and changes observed when the ore is exposed to elevated temperatures and CO(g).

Comilog- and Nchwaning-ore were investigated in CO-CO₂ atmosphere under isothermal and non-isothermal temperature regimes. The majority of the experimental work was carried out at NTNU, whereas parts of the work (isothermal reduction experiments) were conducted at RWTH Aachen. All characterization of reduced samples were performed at NTNU.

The ores were ground and sieved into three different particle size fractions, i.e. 0.50 to 1.36, 3.33 to 4.00, and 11.20 to 15.00 mm. Chemical composition was determined by X-ray Fluorescence, except for MnO₂, CO₂, H₂O and LOI content. MnO₂ was determined by titration (ASTM 465-11:2017), whereas H₂O and LOI were determined by thermogravimetry. Eltra (combustion-IR) was used to determine amount of carbon, which was recalculated to CO₂. The content of light elements, such as lithium and hydrogen, is not quantified by XRF due to low energy levels. As such, the amount of chemically bound moisture in the ores was estimated by heating approximately 10 g of sample in air in a muffle furnace at 950 °C for 12 hours. It was assumed that the registered weight loss included removal of volatiles (surface and bound moisture, and CO₂), as well a contribution from manganese and iron oxide reduction. The extent of weight loss attributed to manganese and iron oxides was determined by titration. Hence, the content of chemically bound moisture may be calculated, as all remaining components of the total weight loss is accounted for.

Mineralogy was determined by X-ray diffraction (XRD), where the specific instrument was a Bruker D8 A25 DaVinci X-ray Diffractometer with CuKα-radiation. The instrument has a LynxEye™ SuperSpeed Detector. Mineral identification was performed using the PDF database in the BRUKER EVA program. Quantitative results were obtained using Rietveld analysis in Topas 5 software. For some of the identified phases, no information on the crystal structure was available in the utilized database. In these cases, values were retrieved from the AMS Crystal Stucture Database. This enables data for all identified phases, except for nsutite, a mineral present in untreated Comilog ore, which does not have a defined crystal structure. Samples were prepared for scanning electron microscope by mounting in epoxy resin mold, which were subjected to a vacuum chamber before hardening. The samples were ground and polished, washed with water and soap, and lastly rinsed with ethanol. Hardened samples were coated with carbon and investigated by SEM/EDS. The specific instrument was a FE-SEM (Zeiss Ultra 55 LE) with a Bruker EDS/NORDIF EBSD system. An acceleration voltage of 15 kV was applied. It is known that light elements (Z < 11), such as oxygen and hydrogen, are not accurately quantified by EDS. Further, representative carbon contents are not obtained when the sample has been carbon coated. Another aspect of quantitative analysis of manganese ores is the geometric effects encountered due to concentration of porosity and cracks, which will lead to unreliable data. According to Newbury and Ritchie,[18,19] a strong indicator of the severity of geometric effects is the raw analytical total that is obtained. They observed that the deviation of the obtained normalized concentration was strongly correlated to the analytical total, where the relative errors fell within 20 pct or less of the correct value provided that the raw analytical total was within range 80 to 120 pct. As such, less attention is paid to the specific quantified concentration of different elements by EDS in this study. Rather, the focus has been the relation between different elements identified in the same analysis.

Non-isothermal reduction experiments were conducted in a vertical retort tube furnace, connected to a thermobalance and off-gas analyzer. The sample (75 g) was located in the lower parts of the crucible, placed on top of a grid in order to ensure an even distribution of the gas. The gas inlet is at the top of the crucible, from where the gas moves through the double walls, meeting the sample from the bottom. The exiting gas leaves through a gas-analyzer (NDIR), continuously measuring the CO-CO₂ concentrations in the gas. A schematic and description of the furnace may be found elsewhere.[13] A heating rate of 6 °C/min was applied for all experimental runs. The majority of the experiments were terminated at 1000 °C, however certain repeated experiments were terminated at a lower temperature in order to examine developing phases and structure. Samples were exposed to a gas flow of 4 Nl/min containing 50 or 80 pct CO, with remainder being CO₂. When target temperature was reached, the power supply was disconnected and the gas was replaced by a 4 Nl/min flow of argon during cooling. It was observed that the thermobalance was subjected to drifting, which was inconsistent both in terms of extent and duration. As such, it was decided to rely on the off-gas analysis, as the extent of prereduction of manganese ores is reflected by the CO/CO₂ ratio. The weight behavior obtained from the off-gas was normalized to the weight loss found from weighing of sample prior and subsequent to the reduction experiments.

Isothermal reduction experiments were conducted in the Tammann furnace facility at IEHK RWTH Aachen. The furnace is a thermogravimetric open tube furnace. Comilog and Nchwaning ore in size fraction 11.20 to 15.00 mm were reduced in CO-CO₂ atmosphere, where CO-concentration were 50 or 70 pct. A tungsten or Kanthal mesh wire was used to construct a sample holder from which the sample was suspended from an electronic mass balance during the experiments. The gas mixture was adjusted by mass flow controllers before entering the furnace chamber from the lower parts. A thermocouple was located in the chamber at the same height as the sample. A schematic of the furnace is found elsewhere.[20] 4 to 5 ore particles were evaluated in each experimental run at temperatures 400 °C to 900 °C. The smaller sample size was used mainly due to limitations of furnace geometry. It is further believed that a smaller sample size provides a more stable temperature, as less heat is evolved. Following 20 to 60 minutes holding time, the samples were removed from the chamber and cooled in a separate container while flushed in argon. Due to small sample size and furnace geometry, the weight reduction data was subjected to a high degree of fluctuations. As such, the results from isothermal reduction are discussed in terms of the composition and mineralogy of the reduction products.

The reduction of Comilog and Nchwaning manganese ores was investigated in isothermal and non-isothermal temperature regimes in CO-CO₂ atmosphere. The results were evaluated through thermogravimetric curves, chemical composition, XRD, and SEM analyses.

Comilog ore was found to be constituted by various MnO₂-minerals, including cryptomelane, nsutite, and pyrolusite. At temperatures below 550 to 580 °C, these MnO₂ oxides were reduced to MnO in an overall single step. At temperatures 550 to 600°C, any present MnO₂ was subjected to rapid conversion. The minerals formed bixbyite(Mn₂O₃) upon the rapid reduction, which continued to reduce to MnO. Manganosite was the main reduction product at all evaluated temperatures and holding times. It is believed that a more dense structure is formed when the ore is subjected to isothermal heating, as a reaction front was observed. This was not observed in any non-isothermal experiments.

Nchwaning ore was found to be mainly constituted by Mn₂O₃-oxides (braunite, braunite II and bixbyite), calcite and hematite. Two distinct particle types were observed, i.e. particles rich in braunite and carbonate, and particles rich in braunite II, respectively. Manganese and iron oxides reduce at highly similar temperature ranges, however the manganese oxide reduction was initiated prior to iron oxides in smaller particle sizes (< 4 mm). Reduction of trivalent oxides to MnO appeared to be a single-step reaction, as the intermediate oxide hausmannite was not detected. The reduction of trivalent oxides was initiated at 280 °C to 400 °C, depending on particle size. Investigations of microstructure showed no indications of a topochemical reaction mechanism.