Advances in Pyrometallurgy

Eramet produces materials useful for a low-emission society, in a resource-effective way, and for manganese (Mn) alloys, with a lower climate footprint than the industry average. Such high standards give increased competitivity because most stakeholders, in particular customers and investors, are interested in environmental-friendly production. This responsible strategy will eventually lead to improving profitability. Eramet has set goals for the reduction of emission of greenhouse gases from their production in the framework of the Science Based Target. In this paper, we present the strategy and a simplified roadmap to reach the target in Eramet Mn alloys activity. The roadmap is made a reality through actions and investments for industrial implementations. The reduction initiatives can be divided into four main areas, somewhat reflecting some sequences in time with overlap, from short term (2025) to long term (2040 and beyond): improvement of existing processes in resource and energy efficiency (2025), increase or introduction of biomass-based reductants to replace fossil carbonaceous materials (2030), carbon capture and usage (CCU) or storage (CCS) (2030), and development of innovative technologies (2040). All actions are rooted in scientific and techno-economic studies. Open innovation is necessary when developing technologies outside the core competence of the companies.

With the growing global focus on reducing the environmental and social impacts of modern society, many smelters and recyclers are moving rapidly to decarbonise their processes. While existing and new solutions are required to optimise for reduced emissions across all reporting greenhouse gas (GHG) emission scopes, advancement of existing solutions can hold greater emissions-reduction potential. With a 37-year operating history and 25 global installations, across both primary and secondary (recycling) applications, ISASMELT™ and ISACYCLE™ technology is a mature, modern, and efficient smelting solution. The technology is well situated as it currently stands to achieve Net Zero status. The ability to decarbonise existing and new ISASMELT™ and ISACYCLE™ operations follows an emerging ‘Emissions Optimisation’ approach, which builds on significant energy-saving and emissions-reduction advancements derived from traditional profit-driven innovations. Scope 1 onsite direct emissions, and Scope 2 indirect emissions associated with electricity, heating, and cooling, can be minimised through the goal of attaining a low energy-use smelting operation. This can be achieved via aspects of the smelting process itself, including proprietary refractory design, advanced temperature control, waste heat capture, feed profile modifications, and oxygen enrichment. Further, scope 1 emissions can be eliminated through improvements such as alternative fuel substitution (i.e., hydrogen and sulphides) and off-gas processing. Finally, scope 3 emissions can be minimised through reduced maintenance, reduced consumables, and reduced equipment wear—by advancements including refractory design, lance design, and furnace control (for long campaign life and consistent production of high-quality products).

Minimization of CO₂ emissions has become one of the main topics in our daily life. Especially industry, as one of the biggest emitters, has to contribute essentially to a global CO₂ reduction. When focusing on the metal production sector, one of the most promising solutions seems to be the utilization of hydrogen. Similar to carbon, hydrogen can act as both a reducing agent and an energy source. However, this looks easy only at the first glance. Various aspects such as efficient production, storage, transport, requirement for new processes concepts or a more difficult off-gas treatment underline that such a change will also be a future challenge. The paper gives an overview of hydrogen-production processes with some details about the ongoing development of methane pyrolysis as an effective way to generate hydrogen in huge volumes. Furthermore, it discusses where hydrogen can be used efficiently in metallurgical processes and in which areas alternatives have to be developed.

The use of H₂ as a reductant in the iron and steel industry is an obvious choice towards carbon neutrality. For pyrometallurgical processes like Mn and Si, H₂ cannot be the only solution, as the stability of the oxides of these elements is higher compared to Fe. H₂ can, however, be used in the Mn-process together with other low CO₂ emission mitigations. In the Mn-ferroalloy process, H₂ can be used to reduce higher manganese oxides to MnO, and the last part of the reduction to metallic Mn can be done with biocarbon or with electrolysis. In studies from NTNU, the reduction of higher manganese oxides to MnO has been investigated with pure H₂ or with CO/H₂ mixtures. It is shown that the H₂ containing gases will give higher reduction rate compared to CO/CO₂ gases with the same reduction potential. The reduction rate will be increased in the order of 20–100%, and H₂ has a larger impact on the reduction rate for high oxygen pressures. In H₂ gas, it is seen that the final degree of reduction is higher than using CO gas, and this is believed to be due to the formation of metallic iron. The reduction rate in H₂ gases varies for various ores, and as for CO containing gases, the reduction rate of Comilog ore is faster than the Nchwaning ore, and activation energies of 23 kJ/mol versus 68 kJ/mol are, respectively, found for isothermal experiments in CO, H₂, and CO₂ gases.

The metal industry alone contributes to about 8% of the CO₂ emissions globally. For a sustainable and carbon-neutral metal industry, the use of H₂ and biocarbon for the substitution of fossil carbon is essential. Swerim has in recent years made great efforts to support the metal industry for a smooth C-neutral transition. This paper will highlight the projects using biocarbon and H₂ in different metallurgical processes including the installation of an H₂-electrolyser which could be connected to various pilot test facilities in Swerim including an EAF, a DC furnace, and a Universal Converter of 6–10 tons in scale. The Swerim H₂-electrolyser has the capacity to produce 100 Nm³ of H₂/h. Our unique gas processing equipment for the separation of CO₂ and production of H₂ in combination with our demo facilities enable us to develop high TRL demonstration of different CCUS opportunities for the steel and metal industry. The use of H₂ and biocarbon in metallurgical applications will also be discussed.

As both a significant contributor to the steelmaking process and the largest single source of CO₂ emissions in integrated steelmaking, the blast furnace is a critical area of focus for decarbonization efforts. Generally, the replacement of coke with injected fuels such as natural gas or syngas has been a key pathway for reducing emissions. These injectants provide higher concentrations of H₂ reducing gas, decreasing reliance on CO reactions, but they are limited by their endothermic impacts on flame and reducing gas temperature. One potential method of circumventing these limitations is the use of shaft-level tuyeres for high rates of hot reducing gas delivery, in a similar fashion to the direct-reduced ironmaking process. In this paper, the impacts of such a system are investigated with Computational Fluid Dynamics to predict the reaction rates, coke replacement ratios, and carbon emissions of the blast furnace process under the proposed operating scheme.

We present a study about the reduction of hematite ores by hydrogen plasma. The transformation kinetics and chemical composition are studied over several intermediate states. We find that the reduction kinetics depends on the balance between the input mass and arc powder. For an optimized input mass-arc power ratio, the reduction is obtained within 15 min exposure to the hydrogen plasma. Micro- and nanoscale chemical and microstructure analysis show that the gangue elements partition to the slag oxides, revealed by energy dispersive spectroscopy and atom probe tomography. Si-enrichment was observed in the interdendritic fayalite domains, at the wustite/iron hetero-interfaces and in the oxide particles inside iron. With proceeding reduction, however, such elements are gradually removed from the samples so that the final iron product is nearly free of gangue-related impurities.

Hydrogen is a candidate to replace carbon in metal production, as it can reduce some metal ores (e.g., iron ore). However, for other oxides, such as those of manganese and chromium, the situation is much more challenging. As the exotic species found in hydrogen plasma are much more reactive than molecular hydrogen, the use of hydrogen plasma can improve hydrogen reduction for all these oxides. Here, using a plasma arc melter, samples of Fe₂O₃, Cr₂O₃ and MnO have been exposed to hydrogen plasma. Reactions between oxides and hydrogen have been observed in all cases, producing metallic iron, chromium, and manganese, hinting that plasma technology can play a part in sustainable metal production, allowing for carbon free production of chromium and manganese. The paper also discusses possible reaction mechanisms.

To evaluate the effect of hydrogen-rich smelting on the softening and melting behaviors of the ferrous burden in a blast furnace, softening and melt-dripping performance tests were performed on the burden under different atmospheric ratios. The results showed that the softening start temperature decreased as the H₂ ratio increased, giving the blast furnace a broader softening zone. As less FeO-containing low-melting-point phase was generated, the melting start temperature increased, the temperature ranges of the melting zone narrowed and moved to the high-temperature zone, and the permeability of the burden significantly improved. Comparing the slag-iron-coke interface characteristics demonstrated that the carbon content in its metallic iron decreased as the ratio of H₂ increased.

The lecture presents some recent progress in understanding the key mechanisms of hydrogen-based direct reduction. The kinetics of the solid-state reduction reactions strongly depend on mass transport kinetics, nucleation during the multiple phase transformations, the oxide’s chemistry and microstructure, and on plasticity, damage, and fracture associated with the phase transformation and mass transport phenomena occurring during reduction. Understanding these effects is key to produce hydrogen-based green steel and design corresponding direct reduction shaft or fluidized bed reactors, enabling massive CO₂ reductions.

To reach the goal of fossil free ferroalloy production within 2050, several alternative materials and processes are being investigated. Fast transition requires that the more mature technologies are implemented first. In parallel technologies with lower TRL levels must be further developed. An overview of different options is given and discussed. Substituting fossil carbon with biocarbon seems to be among the first to be implemented. How the differences in key properties between these two reductants may affect the operation of Mn-alloy and Si/FeSi furnaces will be discussed. Pretreatment and prereduction by sustainable energy sources are another way for reducing fossil CO₂ emission. Pretreatment of Mn-ores with CO-rich off-gases and with thermal solar heated air have been investigated and tested on a pilot-scale within the PreMa project. The effect of different temperatures and gas atmospheres has been studied. Effect on CO₂ emissions on integrated pre-treatment units and furnaces are evaluated by comparing various scenarios in an HSC Sim model adapted for this purpose.

The global community has set a path towards carbon neutrality by 2050. Almost one-fourth of the global emissions is attributed to direct emissions from industrial processes. Therefore, a zero-carbon alternative must be developed for each process, including the production of silicon. The silicon industry is exploring ways to efficiently capture CO₂ from the flue gases from the submerged arc furnaces, for example by increasing the CO₂ concentration. Replacing fossil reducing agents with biofuel is a carbon neutral alternative, while recycling waste streams from aluminium production as a reducing agent for silicon, is a more recent development. Electrochemical methods have also been explored at a laboratory scale. This paper gives a review of the efforts to date, from industry and academia, to decarbonize the production of silicon. The development of the largest part of the carbon footprint, arising from the production of the electrical energy used, is also discussed.

As environmental, energy saving, and legislation considerations are becoming ever more important, there is evidently a demand for further improvement in slag handling and metal recovery practice. The metallurgical industry is initiating its actions into minimizing and processing of slags to achieve sustainable development and circular economy. Reduction of copper oxide is an important step of metal recovery and slag cleaning in copper production. This can be achieved by slag treatment with injected reducing agents. In this paper, it is presented a computational fluid mechanics model of tuyere and purging plug injection of reductants into slag that considers a first-order heterogenic chemical reaction to model copper oxide reduction. Using the model, it is possible to compare and optimize tuyere and purging plug arrangements in terms of time needed to reduce copper oxide content to a given value. Copper oxide reduction accomplished with the injection of different reducing media (oil, natural gas, and hydrogen) for several tuyere/plug arrangements in a slag refining furnace was simulated and the reduction performance using different reduction media was compared. It was shown that purging plugs, which may be installed at the deepest position in the furnace, create an improved gas flow pattern that results in a higher copper oxide reduction rate and decreasing in CO₂ emission.

The unique structural versatility and chemical and thermophysical properties of carbon make it essentially irreplaceable for non-reductant uses in many high-temperature metallurgical processes. At present, bio-carbon substitutes are not technically feasible for large-scale use in electrode and refractory materials that are vital consumables in the steel, aluminum, and non-ferrous metal industries. Carbon electrodes of all types (including Soderberg, prebaked, and anodes/cathodes for Al) as well as carbon lining pastes are all similar in that they are comprised of a granular carbon aggregate and a carbon-based binder. Similarly, refractories such as MgO-C utilize both natural (mined) graphite and carbon-based binders. Replacements of fossil materials with equivalent bio-carbon substitutes have the potential to dramatically reduce the carbon footprints of these products. However, there are still considerable materials engineering challenges that must be surmounted. The properties of bio-carbon materials and technological obstacles are explored, including catalytic graphitization and development of bio-pitch materials.

Biocarbon will play an important role to achieve a carbon neutral and sustainable steel industry. In this study, three hydrochars (one type of biocoal produced via the hydrothermal carbonization process) derived from orange peel, green waste, and rice husk were tested in a 10-ton test-bed EAF (electric arc furnace). These hydrochars were added to EAF via injection and top-charge as carburizer to substitute anthracite. The obtained liquid slag composition after scrap meltdown is favorable for the desulphrization process. Moreover, a higher carburization yield was achieved by top charging of hydrochar into EAF at the beginning of the heat. The final P and S of liquid steel with addition of hydrochars were controlled to acceptable levels. Some perspectives of using hydrochar for EAF steelmaking are also presented.

The silicon, ferroalloy, and aluminum industries have mostly been dependent on fossil carbons for their respective process. However, efforts to reduce their fossil CO₂ emissions, the switch to biocarbon has already begun and targets of 25–40% biocarbon by 2030 have been set by various producers in Norway. To achieve this transformation a better understanding of the effects of physical properties of the carbon on the process must be obtained so that the transformation can occur with minimal process interruptions. For the silicon, ferrosilicon, and ferromanganese industries the effects of biocarbon reductants are the primary interest whereas for the aluminum industry, use of biocarbon to replace packing coke used in anode baking is desired. In this work, an overview over relevant carbon properties and methods to characterize these are presented together with an evaluation of how these properties may affect the different processes when introducing biocarbon.

An important path to the goal of reducing the metal producing industries’ CO₂ footprint is to replace fossil carbon sources with bio-based carbon sources for the electrodes and reductant agents. Since the structure of bio-carbon is substantially different from fossil carbon, characterizing the bio-carbon structure and examining its behaviour during the relevant processes are important. Focusing on the silicon and ferroalloy industries in Norway, micro X-ray computed tomography (µCT) has been used to analyse and compare single grains of bio-carbon before and after various experimental procedures. These procedures consist of high temperature treatment under different conditions for CO/CO₂ and SiO gas reactivity test, K-impregnation and CH₄-based carbon deposition. This paper shows examples on results from µCT measurements before and after the experiments, and describes briefly the data processing methods applied. The relevance to the experiments and industrial applications is also discussed.

Concentrating solar energy can deliver high temperature process heat to metallurgical processes. An overview of mineral resources in South Africa and Australia and the possibilities of using concentrating solar thermal energy for thermal decomposition as novel low-carbon pre-treatment processes are investigated. The paper will consider the thermodynamics of oxide and carbonate minerals and evaluate the potential carbon emission reductions as well as changes in energy demand of such processes. The state of the art of concentrating solar technologies for materials treatment as applied to the thermal decomposition reactions are reviewed.

In primary and secondary nonferrous sectors, industrial gases and associated injection equipment are applied to oxyfuel and pneumatic injection technologies. The former is applied to decarbonization through the improved utilization of alternative fuels including a growing interest in hydrogen. The latter is concerned with gas jetting for metal mixing and refining. CFD is used to support the design, installation and start-up of oxyfuel burners and gas injection equipment. Verification and validation (V&V) in CFD output can be enhanced via experimental efforts; however, the extraordinary difficulty in taking measurements in production furnaces under hot conditions is fully appreciated. We present an approach on a partial V&V method relying on basic assumptions in the computational and experimental domains, wherein posteriori error estimation is partially fulfilled through experiment. We draw upon industrial installations in the following reactors: fluidized bed roaster (g-g), cylindrical-horizontal furnaces (g-l) top-blown and submerged) and straight grate/rotary kilns (g-s).

Highly exothermic reactions in a Sulphuric Acid Plant are sources of energy that can be used effectively in metallurgical facilities. Unfortunately, this aspect does not receive the attention that it deserves while designing smelting and/or hydrometallurgical plants. High temperatures in gas circuits allow heat recovery as steam which is a very valuable source of energy. Over the last two decades, development of higher quality materials of construction has allowed the operation of acid circuits at high temperatures, resulting in increased opportunity for heat recovery as steam. This paper discusses various options for recovering heat and utilizing it effectively in metallurgical facilities.

The industrial sector is expected to be a large greenhouse gas producer in the future, with the majority of emissions being attributed to fossil fuel-based heat generation. By directly applying solar thermal energy to a high-temperature industrial process, the reliance of the industrial processes on fossil fuel-based energy sources can be lessened or completely removed. Preliminary investigations into the production and beneficiation of zinc using concentrated solar power (CSP), in a South African environment, show promise and even more so with a focus only on the cathode casting operations. To evaluate the practical implications of directly applying CSP to a high-temperature materials processing application, a study was launched into the direct solar melting of zinc metal. In this study, the setup and performance of a novel cavity receiver for zinc melting using CSP are discussed.

The EU-funded PreMa project investigated the potential for a preheating stage to reduce the electrical energy requirement and CO₂ emissions produced during the production of high-carbon ferromanganese in a submerged arc furnace. Pilot-scale test work was conducted at Mintek in South Africa to demonstrate the potential effect of preheating on furnace operation. The A 300 kVA AC-furnace facility at Mintek was upgraded extensively for this purpose. The paper reports on the lessons learnt from the pilot-scale test work.

Waste slag sources from South African pyrometallurgical smelters are evaluated to determine their potential for use as sensible heat storage media in Concentrated Solar Power applications. This offers the possibility of repurposing slag waste materials while tackling their environmental and handling problem. This work studies the key performance indicators of thermal energy storage systems to assess the suitability of pyrometallurgical slag for use as filling materials in packed bed thermal energy storage. The thermophysical properties of selected slags and their potential for long-term thermal stability is studied through the equilibrium module of FactSage 8.0 from 100 to 1000 °C. A validated model is employed to study the heat transport process in the fluid–solid interface with air as the heat transfer fluid. The maximum temperature of the studied thermal storage cycles is 750 °C. The overall system performance shows that pyrometallurgical slags have potential as energy storage materials.

Molten Oxide Electrolysis (MOE) is a new CO₂-free technology that could alleviate the environmental impact of ferroalloy production. In this work the objectives were to perform (i) experimental lab scale studies of MOE for Mn from MnO using synthetic raw materials and a commercial Ni–Cr–Fe alloy as oxygen-evolving anode, and (ii) theoretical mass and energy balances for different cases of Mn ore purification for a hypothetical industrial MOE FeMn process. Low carbon Mn metal was recovered at lab scale, and the feasibility of MOE manganese production proven. While the Ni–Fe–Cr anode performed satisfactory during the electrolysis, some dissolution of Ni, Fe and Cr was detected. Theoretical energy and mass balance calculations gave a total energy consumption of 6.5–7.6 MWh/tonne Mn depending on the purification scenario. The results showed that the purified ore greatly improves the process. Non-purified ore requires large amounts of flux and more hydrogen in the pre-reduction step, both aspects increasing the need for energy. Non-purified ore also generates large amounts of spent electrolyte. Despite the above, it is an open question whether the benefits of purifications would outweigh the costs.

Nickel and ferronickel are critical components for production of products ranging from commodity metals to lithium-ion batteries. Current and future production rely on processing lateritic sources of nickel, which exhibit a range of technical and sustainability challenges. New sulfide-based process chemistries have been developed as platform technologies to decarbonize mining, materials separations, impurity management, and metals production. Herein, we utilize sulfide chemistry to produce carbon-free ferronickel. We first demonstrate selective sulfidation of mixed laterite feedstocks to form an iron-nickel sulfide matte. We then explore the thermodynamics of vacuum thermal treatment processes to enrich the matte in nickel via selective oxidation of iron. Finally, we employ aluminothermic reduction via reactive vacuum distillation to produce ferronickel from iron-nickel sulfide. These results lay the groundwork for an autothermal pathway to manufacture ferronickel from lateritic ore without direct greenhouse gas emissions.