Mineral carbonation is a method in carbon capture and utilization (CCU) in which carbon dioxide reacts with natural or synthetic mineral phases to form carbonates. In this study, BOF slag consisting of alite, Ca₃SiO₅, belite, Ca₂SiO₄, melilite, (Ca,Na)₂(Mg,Al)^[4][Si₂O₇], brownmillerite, Ca(Fe,Al)₂O₅, calcium ferrite, Ca₂FeO₄ and Ca-, Mg- and Mn-bearing wuestite, (Mg,Ca,Mn,Fe)O, was crushed into different particle size fractions and exposed over various durations (1 d, 3 d, 9 d, 14 d, 24 d) at a grate to 120 °C hot off-gas with a CO₂ content of 25%. However, the total inorganic carbon (TIC) content never increases above the detection limit of 0.5% throughout the experimental duration. The determination of the carbonation depth using phenolphthalein does not reveal a homogeneous carbonation front, but an irregular carbonation. This observation was confirmed by microprobe analyses using elemental mapping. The solubility of the slag increases with increasing carbonation, e.g., the leachability of sulfate increases from 7.8 to 8270 mg/kg dry matter (DM), and of calcium from 940 to 3860 mg/kg DM. The leaching of environmentally relevant element varies: the leachable concentration of molybdenum increases from 0.017 mg/kg DM to 0.089 mg/kg DM, that of chromium remains constant (ca. 0.05 mg/kg DM) whereas that of vanadium decreases from 1.1 to 0.45 mg/kg. In summary, the chosen carbonation technology must be improved to enhance carbonate yield.

Different mixed cultures of extremely thermoacidophilic microorganisms were used for bioleaching of metalliferous industrial dust waste derived from the basic oxygen furnace (BOF) steelmaking process. Such mixed cultures can extract various metals from multi-metallic BOF-dust waste, improving the metal dissolution and bioleaching performance in frames of metal recycling processes to assist circular economies and waste management. The results of the investigation showed that mixed cultures of thermoacidophilic archaea of the order Sulfolobales (Acidianus spp., Sulfolobus spp., and Metallosphaera sedula) during their growth in laboratory glass bioreactors provided a superior bioleaching system to Acidianus manzaensis alone. Depending on the composition of mixed thermoacidophilic cultures, extraction of various metals from BOF-dust could be achieved. Among the three different types of mixed cultures tested, the mixed culture system of A. manzaensis, A. brierleyi, and S. acidocaldarius was most effective for extraction of major elements (Fe, Ca, Zn, Mn, and Al). The mixed culture of A. manzaensis, A. brierleyi, and M. sedula showed high performance for bioleaching of most of the minor elements (Cu, Ni, Pb, Co, Mo, and Sr). The efficient ability of mixed cultures to colonise the mineral matrix of the metal waste product was observed via scanning electron microscopy, while their metal extraction capacities were analysed by inductively coupled plasma mass spectrometry. These investigations will promote the further design of microbial consortia in order to break down the solid matrix and efficiently extract metals from metalliferous waste materials.

Advancing material efficiency in the steel and cement industry is essential for achieving climate goals. One approach to addressing this is to increase the provisioning of alternative reactive binder materials from residues, in this case, from the steel industry. Different mixtures of identified residues are evaluated for metal recovery and suitability as supplementary cementitious material. For this purpose, suitable combinations are modeled according to specified quality requirements from the cement industry. These mixtures are heated up to 1600 °C for a targeted reduction of predominantly transition metal oxides and a separation into a mineral fraction. Subsequently, controlled cooling of the molten material is implemented through water granulation. The produced granulate is crushed and sieved, and finally, the metallic and mineral fractions are magnetically separated. The chemical modification, reduction, and phase modification are tested to prevent landfilling and provide alternative secondary resources for the steel and cement industry. According to the results, it is possible to recover metals from metallurgical residues and simultaneously separate the modified mineral fraction as an alternative cement constituent. These findings will be further investigated through additional research to identify the variables that influence and impact/affect the reduction efficiency.

Sustainable mechanisms for efficient and circular metal recycling have yet to be uncovered. In this study, the metal recycling potential of seven metal-resistant bacterial species (Deinococcus radiodurans, Deinococcus aerius, Bacillus coagulans, Pseudomonas putida, Staphylococcus rimosus, Streptomyces xylosus and Acidocella aluminiidurans) was investigated in a multi-step strategy, which comprises bioleaching of industrial waste products and subsequent biosorption/bioaccumulation studies. Each species was subjected to an acidic, multi-metal bioleachate solution and screened for potential experimental implementation. Bacterial growth and metal acquisition were examined using scanning transmission electron microscopy coupled to electron dispersive X-ray spectroscopy (STEM-EDS). Two of the seven screened species, D. aerius and A. aluminiidurans, propagated in a highly acidic and metal-laden environment. Both accumulated iron and copper compounds during cultivation on a multi-metallic bioleachate. Our findings suggest that extremotolerant bacteria should be considered for waste recycling operations due to their inherent polyextremophily. Furthermore, STEM-EDS is a promising tool to investigate microbial–metal interactions in the frames of native industrial waste products. To develop further experimental steps, detailed analyses of adsorption/accumulation mechanisms in D. aerius and A. aluminiidurans are required to design a circular metal recycling procedure.