Abstract
Manganese ferroalloys are important alloying additives in steel and aluminium, produced efficiently by carbothermic reduction in Submerged Arc Furnaces (SAF). These processes emit 0.9-1.3 kg CO 2 /kg Mn ferroalloy [1]. Decarbonising the energy system is a first and very important step, but it will not be enough because carbon dioxide is also a product of the metal-producing reactions themselves. Some relatively mature technologies such as biocarbon reductants and carbon capture and storage or use (CCSU), may allow the industry to move away from fossil CO 2 emissions while continuing to use the carbothermic reduction route for ferroalloy production. However, there are many reasons to look beyond these carbon-based solutions and investigate the possibility to completely decarbonise the industry. Electrolytic production of Mn-alloys is the most direct way to electrify and decarbonize process, compared to e.g., hydrogen reduction and metallothermic reduction. Electrolytic manganese metal (EMM), a very pure manganese, is today produced electrochemically through an aqueous electrowinning process, but electrowinning processes producing alloys and with higher throughput is currently under evaluation for several Mn-alloy producers. In this work, a molten oxide electrolysis (MOE) process [2] at 1300-1400 °C with an inert anode for O 2 production is investigated as an alternative CO 2 -free production process of liquid Mn/FeMn/other Mn-alloys from MnO /Mn ores. The suggested process in the attached figure looks similar as aluminium primary production using traditional molten salts, and the MOE process for iron production have been demonstrated by Boston Metal [3]. Advantages of the process include the production of a liquid product (high throughput), no or low emission of halogenides, fines can be used as raw material, low carbon content alloys are produced directly, and less steps before metal production should give a lower capex. The electrowinning process is likely to require pre-reduction of the ore, but this could e.g. be accomplished with H 2 [4]. The main challenges are related to the typically high viscosity of the molten oxide melt compared to traditional molten salts, materials selection related to the melt composition and corrosion, the oxidizing atmosphere in the cell which may oxidize the Mn resulting in lower current efficiency, and the high energy consumption related to electrowinning of oxides without carbon. Electrolysis experiments with an electrolyte containing MnO, Al 2 O 3 , SiO 2 , MgO, and CaO with a metal alloy as the oxygen-evolving inert anode have been conducted, using a variation of cathode materials like Mo [5], Fe, and W, at 1250-1450 °C. The process of using MOE and oxygen evolving electrodes was confirmed to be able to produce Mn-alloys using the consumable Mo and Fe cathodes, which appeared easier than producing pure Mn with a more inert cathode material due to the evaporation of metal at the high temperatures. Acknowledgements This project has received funding from The Research Council of Norway: Contract number 344259 (ZeSiM). Partners in the project are SINTEF, NTNU, Elkem ASA, Eramet Norway AS, Finnfjord AS and Wacker Chemicals Norway. References [1] M. Sommerfeld and B. Friedrich (2021), Replacing Fossil Carbon in the Production of Ferroalloys with a Focus on Bio-Based Carbon: A Review. Minerals , 11 (11), pp.1286 [2] A. Allanore (2014), Features and Challenges of Molten Oxide Electrolytes for Metal Extraction. J. Electrochem. Soc. , 162 , pp. E13 [3] Boston Metal: https://www.bostonmetal.com/ [4] J. Davies et. al. (2023) Pre-reduction of United Manganese of Kalahari Ore in CO/CO 2 , H 2 /H 2 O, and H 2 Atmospheres, Metall Mater Trans B , 54 , pp. 515–535 [5] K.S. Osen et.al. (2023), CO 2 Free FeMn/Mn Production Through Molten Oxide Electrolysis, TMS 2023: Advances in Pyrometallurgy , pp. 267 Figure 1