To achieve better process control of silicon (Si) alloy production using aluminum as a reductant of calcium silicate (CaO-SiO₂) slag, it is necessary to understand the reaction phenomena concerning the behavior of formed phases at the metal-slag interface during conversion. The interfacial interaction behavior of non-agitated melt was investigated using the sessile drop method for varying time and temperature, followed by EPMA phase analysis at the vicinity of the metal–slag interface. The most remarkable features of the reaction were the accumulation of solid calcium aluminate product layers at the Al alloy–slag interface and spontaneous emulsion of Si-alloy droplets in the slag phase. The reduction is strictly limited at 1550 °C due to the slow transfer of calcium aluminates away from the metal-slag interface into the partially liquid bulk slag. Reduction was significantly improved at 1600–1650 °C despite an interfacial layer being present, where the conversion rate is most intense in the first minutes of the liquid–liquid contact. A high mass transfer rate across the interface was shown related to the apparent interfacial tension depression of the wetting droplet along with a significant perturbed interface and emulsion due to Kelvin–Helmholtz instability driven by built-up interfacial charge at the interface. The increased reaction rate observed from 1550 °C to 1600–1650 °C for the non-agitated melt was attributed to the advantageous physical properties of the slag phase, which can be further regulated by the stoichiometry of metal–slag interactions and the composition of the slag.

We investigated the reaction kinetics and initial chemical conditions in the production of silicon alloys, employing aluminum as the reductant for calcium silicate slag, to enhance process economics and scalability to industrial levels. The apparent kinetics and transient chemical conditions were studied by immersing solid aluminum into molten slag, allowing the reaction to proceed for varying durations without external agitation, before quenching the reaction for chemical and microscopic analyses of the resulting silicon alloy and slag. The majority of the conversion was observed within the first 15 s at 1650 °C, driven by significant chemical interactions and interfacial turbulence introduced upon aluminum immersion. For Al-SiO₂ stoichiometries ranging from 0.5 to 1.2, the slag phase reaction conformed to first-order kinetics during the initial two minutes, when it approached equilibrium. The mass transfer coefficients for Al₂O₃ were estimated at 1–2 × 10⁻⁴ m/s, comparable to those for SiO₂ and CaO. A constant mass transfer coefficient could not be established for stoichiometries of 1.6 and 2, as these deviated from the standard slag mass transfer relationship and did not adhere to established relationships. Despite near-complete reactions, alloy–slag mixing was extensive, decreasing with lower stoichiometry values.

Three pilot-scale experiments have been conducted at SINTEF/NTNU in a 440 kVA AC electric furnace to demonstrate the process operation, energy requirements and CO2 emissions in the production of high carbon ferromanganese alloys. Comilog, UMK and Nchwaning (Assmang) ores blended with other materials, such as sinter and flux, thus achieving different charge mixtures have been utilized in the experiments. In the prereduction zone, higher manganese oxides in the ore are reduced to MnO through solid-gas exothermic reactions and at a temperature around 800oC, the unwanted endothermic Boudouard reaction is also active. As such, the total coke and energy consumption is highly dependent on if the prereduction occurs by CO gas or solid C. The pilot furnace has been excavated after each experiment and the extent of prereduction of the ore has been investigated by collecting samples from specific regions in the prereduction zone. In addition, material, and energy balance calculations for the three pilot experiments have been calculated using HSC Chemistry software. The HSC material and energy balance calculations have shown that the slag/alloy ratios, metal analyses, carbon consumption and the overall energy consumption are mainly affected by the composition of the charge mixtures. The relationship between the specific carbon consumption, the off-gas CO2/(CO2+CO) ratio and energy consumption to produce 1 tonne of HCFeMn alloy is discussed for the three different pilot-scale scenarios.

This study compares methodologies and provides insights on best approaches for quantifying EU-listed critical raw materials. Europe's strategy towards greater resilience involves valorization of secondary resources such as mine tailings or by-products of metallurgical production that are typically discarded as waste, despite containing valuable resources. Thus, identifying the best methods to determine elemental concentrations in various resources is crucial for evaluating their recovery, both environmentally and economically. Elemental content is usually determined using techniques such as inductively coupled plasma mass spectrometry (ICP-MS) by digesting solid samples into liquid solutions, although challenging for some refractory materials. Direct analysis of solids by laser ablation (LA)-ICP-MS is another alternative and can be used for both bulk analysis and spatially resolved mapping of critical elements throughout the supply value chain, from raw materials to refined products. Here, we compare ICP-MS/MS analysis on dissolved samples, obtained after microwave-assisted acid dissolution, with LA-ICP-MS analysis on solid samples for determining rare earth element (REE) compositions of manganese ores and slags. For LA-ICP-MS, we present two approaches, particularly useful in cases where dissolution proves challenging: (i) analysis of glass beads obtained through borate flux fusion of raw powders, and (ii) direct analysis on pelletized powders using a novel non-matrix matched calibration strategy. Each method's feasibility and strengths are considered for the specific chemical composition of the samples of interests. The study shows that different methods have advantages and disadvantages related to particle size distributions and preparation costs that need to be considered during characterization of secondary resources.

The sludge generated by wet scrubbing the off-gas from a silicomanganese plant in Norway represents a significant loss in silicon and manganese values. This work seeks to extract these values in the production of ferroalloys and slags that can be utilized by alternative industries. Carbothermic smelting the sludge together with iron or iron scraps at 1600 °C produced FeSiMn alloys consisting of 57–64 wt.% Mn, 16–22 wt.% Si and 18–25 wt.% Fe. The low level of phosphorus in the sludge allowed for beneficial phosphorus concentrations as low as 500 ppmw in the metal alloys. The addition of lime to the material mix resulted in increased evaporation of alkalis, capture of sulfur in the slags as calcium sulfides and slag compositions similar to conventional steel-making slags that can be recycled accordingly. Graphical Abstract