Upcycling end-of-life MgO-C refractories into functional anodes offers a sustainable pathway toward carbon-free aluminum electrolysis. Here, a metal-ceramic composite anode was manufactured from refractory-recyclate-derived MgO powder and 316L stainless steel by cold isostatic pressing and sintering. It was evaluated in laboratory-scale molten-salt electrolysis using potassium cryolite (KF-AlF3-Al2O3) at 800 degrees C. Post-electrolysis SEM-EDX, elemental mapping, Raman spectroscopy and ICP-OES were applied to quantify reaction-layer formation, electrolyte infiltration and element release into the electrolyte. Multi-layered reaction zone formed at the anode edge, with hematite (alpha-Fe2O3) as the dominant outer oxide and inward regions containing Mg-F-O oxyfluoride-related products and Ni-Cr-O phases. Electrolyte infiltration through open porosity reached 3.59 mm, whereas near-surface corrosion depth was limited to 0.27 mm. MgO depletion was most pronounced within a thin outer oxide-rich zone, which potentially acted as partial diffusion barrier. Dissolution of anode components into the electrolyte remained below the analytical detection limit (<0.01 wt.%). Cell voltage increased gradually and then rose sharply after similar to 50 min, prompting termination at 60 min, consistent with electrolyte evaporation and anode passivation. The results establish a benchmark for recyclate-based anodes in K-cryolite and guide optimization of electrolyte composition and anode microstructure.

Refractories are polycrystalline, porous, nonmetallic materials used as protective liners in high-temperature manufacturing. During service, rapid temperature changes generate stresses from temperature gradients and thermal expansion mismatches, leading to crack initiation and propagation, making thermal shock resistance (TSR) a critical property. TSR is strongly influenced by mesostructural features such as aggregate size distribution, phase volume fractions, aggregate-matrix interface properties, and toughening mechanisms, making its evaluation challenging. While analytical and experimental methods exist, numerical approaches remain limited. This study computationally evaluates TSR in MgO-C refractories using a 2D plane-strain model of a laboratory-scale brick, represented as a three-phase composite of MgO aggregates, graphite flakes, and an effective matrix. A thermo-mechanically coupled cohesive zone model captures interface debonding during thermal shock. A modified TSR parameter incorporating initial crack length and crack density is introduced, providing a mesostructure-sensitive and physically interpretable measure of TSR in the computational assessments. Thermal shock simulations are performed, with varying graphite volume fractions and aggregate size distributions, and results are compared with classical TSR parameters, demonstrating improved sensitivity of the proposed approach to mesostructural effects. This framework enables computational exploration of mesostructure-property relations in refractories where interface debonding influences TSR, supporting materials development by identifying promising design targets.

The high-temperature interaction between Al-killed Mn-B steel, desulfurization slag, and carbon-bonded magnesia refractories containing conventional, environmentally friendly binders and recyclate systems was systematically investigated using finger immersion testing at 1600 degrees C for 30 min. Chemical analysis of steel and slag, combined with SEM/EDX characterization of refractory/steel/slag interfaces and non-metallic inclusions (NMIs), was performed to interpret reaction mechanisms and the evolution of inclusions. Significant slag modification occurred during testing, characterized by MgO enrichment, CaO and Al2O3 depletion, and the formation of Fe- and Mn-bearing oxides. In the steel, pronounced decarburization, sulfur increase, boron depletion, and enhanced oxygen and nitrogen contents were observed, indicating reoxidation and interfacial reactions. Independent of refractory condition, MgAl2O4 spinel and Ca2SiO4 phases were detected in the infiltrated refractory zone. Refractories containing recyclates and alternative binders chemically modified the inclusion population and significantly increased NMI number density, particularly Mn-Si-Ti-based inclusions. The results demonstrate that refractory composition strongly influences slag chemistry, inclusion characteristics, and steel cleanliness. These findings provide a new understanding of refractory-slag-steel interactions during secondary metallurgy and highlight the need for optimized slag/refractory formulations to ensure inclusion control and steel quality under increasingly sustainable material strategies.

The development of inert anodes for aluminum electrolysis remains challenging due to the high corrosivity of cryolite‐based melts at 950°C–1000°C. This study investigates the corrosion and process behavior of a carbon‐free MgO–steel cermet anode derived from refractory recycling during laboratory‐scale Na‐cryolite electrolysis at 1000°C, focusing on the effect of a pre‐oxidation treatment on its corrosion behavior. In the as‐sintered state, MgO grains are mechanically bonded to a 316L steel matrix with interfacial porosity that limits corrosion resistance. Pre‐oxidation at 900°C for 24 h forms a dense Mg–M–O (MFe, Cr) oxide‐rich surface layer, sealing pores and improving resistance to cryolite infiltration, albeit at the cost of reduced electrical conductivity, which translated into an overall higher cell voltage compared to the as‐sintered state. During electrolysis, this layer degrades under anodic polarization, O 2 evolution, and fluorination. The surface transforms into a Fe–O‐rich magnetite layer (Fe 3 O 4 ), while a Cr‐containing oxide and an in situ Mg–Fe–O transition zone form beneath, maintaining partial protection. Overall, pre‐oxidation effectively delays electrolyte penetration by converting MgO into new oxide phases but introduces conductivity losses and surface instability. The results highlight both the potential and limitations of MgO‐based cermet anodes for sustainable aluminum production.

A carbon-free MgO–316L cermet anode, suggested by an upcycling strategy for end-of-life MgO–C bricks, was evaluated with regard to its corrosion behavior in the intended application environment of aluminum electrolysis. The study investigates how the processing dependent microstructure affects chemical corrosion in a conventional Na-cryolitic melt at 1000 °C under current-free immersion conditions. Two manufacturing routes with identical nominal composition but highly different porosities were compared. Spark plasma sintering was used to produce a dense reference material, while granule-assisted cold isostatic pressing was used as the more scalable route for larger anode geometries. Thermodynamic modeling predicted favorable fluorination of MgO by AlF3 and low reactivity of the 316L steel phases with salt components. This was confirmed experimentally by ICP-OES, which identified Mg as the major element that was released from the specimens into the molten salt, whereas Fe, Cr, Ni and Mo remained quasi inert. The only exception was Mn, which also dissolved into the molten salt. EDX line scans and elemental mapping revealed a zonal degradation sequence with a Mg-depleted region, an Al-rich reaction zone, and a preserved bulk containing MgO grains. Degradation depended strongly on the processing route of the specimen, where the more porous CIP specimens showed greater corrosion depths. Molten salt related Na-signals extended beyond the region of complete Mg-depletion, showing that molten salt infiltration and corrosion progression were locally related but not identical. The results suggest that corrosion resistance depends not only on composition, but also on the microstructure resulting from processing.

This study presents a novel material concept utilizing pressed rice husk ash (RHA) substrates coated with a chemically inert ceramic barrier via flame-spraying for steel ingot casting. Runner-brick prototypes were successfully fabricated and internally coated by a flame-sprayed alumina layer. An Al2O3-C interface layer was introduced between the RHA substrate and the flame-sprayed coating to overcome the highly non-linear thermal expansion arising from the alpha- to beta-cristobalite transformation of the RHA substrate. This interface layer effectively accommodated thermal expansion mismatch and prevented cracking of the flame-sprayed coating during heating. The runner-brick prototypes were tested in proof-of-concept casting experiments with steel melt at 1600 degrees C and successfully withstood the casting process without significant damage or molten steel leakage. The solidified steel blocks were analyzed regarding their population of non-metallic inclusions (NMIs), revealing no drastic change in total NMI counts compared to two state-of-the-art fireclay-based references and showing no indications of coating detachment.

This study presents the postmortem phase characterization of a metal-ceramic composite anode composed of 316L stainless steel and recycled MgO, sourced from spent refractory lining bricks, employed in aluminum molten salt electrolysis. The analysis focused on the immersed section of the anode, where direct exposure to the molten Na-cryolite melt promotes the formation of corrosion products. Raman spectroscopy was applied as a structural characterization technique, providing phase information that complements the morphological and elemental analyses obtained from SEM-EDX measurements. Due to its high spatial resolution, micro-Raman spectroscopy enabled the identification of local phases within the corrosion layer and the determination of their depth-dependent distribution. The local chemical analysis revealed an outer Fe-O-rich layer penetrating several tens of micrometers into the material, followed by a Fe-Al-O-containing zone. Raman spectroscopy identified the Fe-O layer as magnetite (Fe O) and the inner layer as hercynite (FeAlO), with a transition region consisting of Al-doped FeO. The results demonstrate the applicability of Raman spectroscopy for identifying corrosion products to provide contributions to the corrosion mechanisms of MgO-steel anodes under electrolytic conditions.

The fracture resistance is a design-relevant and material-specific feature. In ceramic materials, the fracture resistance can increase as a function of the crack length. Such behaviour is beneficial, e.g. in terms of the thermal shock resistance. It is termed R-curve behaviour and reflects the capacity of the material for intrinsic and/or extrinsic toughening during stable crack advance. Another design-relevant material-specific property is the sensitivity towards subcritical crack growth. The latter is a time-dependent phenomenon and commonly estimated by two crack growth parameters. The presented paper aims for a detailed review of sequential four-point bending experiments on V-notched beams, which are used to evaluate both phenomena from a single measurement. Three groups of materials (Al2O3, Si3N4, and Lithosil glass) were tested and analysed by this method in order to provide an experimental data background.

This study explores the thermo-mechanical behavior of reticulated alumina foam ceramics and the influence of structural characteristics. Industrial and laboratory specimens with varied ppi-numbers and strut thickness were used, the latter prepared via two coating processes to introduce inner/outer layer inhomogeneity. Cylindrical splitting tests revealed that increases in foam weight and strut thickness enhance mechanical strength, while inhomogeneity reduces load-bearing capacity due to uneven stress distribution. Heavier foams showed more concentrated cracking and energy release. The inner/outer homogeneity gradient provided structural support, reducing early energy dissipation. High-temperature tests, including creep and refractoriness under load, showed that composition differences affected softening behavior, with high-ppi foams exhibiting better pressure transfer and creep resistance. Inhomogeneity had limited impact on softening resistance. This study can offer theoretical guidance for optimizing structure and fabrication of ceramic foam filters.

MgO–316L cermets derived from end-of-life MgO–C refractories were evaluated for chemical compatibility with a conventional Na-cryolite melt. Specimens with identical nominal compositions were produced by spark plasma sintering (SPS) and granule-assisted cold isostatic pressing (CIP), resulting in markedly different processing-induced microstructures and open porosities. Current-free immersion tests were conducted at 1000°C for up to 24 h. Thermodynamic calculations indicated favorable MgO fluorination by AlF3, while ICP-OES identified Mg as the main released element. SEM-EDX revealed Mg-depleted regions, an Al–O-rich reaction zone, and preserved MgO in the bulk. After 24 h, normalized Mg loss reached 50.46% for SPS and 88.00% for CIP. The apparent Mg-depletion depth reached 1.82 mm for SPS, whereas no Mg recovery was detected within the full 10 mm specimen height of CIP from 16 h onward. Processing-induced microstructure strongly influenced degradation progression, but densification did not overcome the chemical instability of MgO in AlF3-containing cryolite.

This study explores the incorporation of ultra-fine coloured waste glass containing cadmium, sulphur, and selenium pigments into magnesium oxychloride cement (MOC) formulations for sustainable construction applications, including evaluating the preparation feasibility of the cementitious paste for subsequent 3D printing development. The glass waste, in the form of fine powder originating from red glass bead production, was added to MOC mixtures in varying amounts (3%, 6%, and 9%). The composites were evaluated for structural, mechanical, thermal, and environmental performance. Results showed that the addition of glass dust had minimal impact on porosity, thermal properties, and colour of the cement, while slightly improving compressive strength and water resistance. Microstructural analysis confirmed the presence of dense intergrown MOC phase 5 crystals, and leachability tests indicated negligible environmental risk. It was demonstrated that the material can be produced using 3D printing, and the setting time increases slightly with a higher glass dust content. The findings demonstrate the feasibility of reusing specialised industrial waste in high-performance, eco-friendly construction materials.

This study investigates sustainable binders based on lignin and collagen hydrolysate as substitutes for conventional binders in MgO-C refractories, with a particular focus on volume stability after thermal treatments. Collagen hydrolysate undergoes thermally induced decomposition of peptide chains while simultaneously forming a cross-linked network, accompanied by the release of H2O, CO2, and NH3. During coking, this network forms a partially ordered carbon network that provides high oxidation resistance, bulk density, and strength of the MgO-C material both at room and elevated temperatures, comparable to reference samples bonded by a conventional mixed binder based on novolak and Carbores P. However, the combination of gas formation and cross-linking leads, in small laboratory samples, to a volume increase without structural damage. In large-format bricks the longer diffusion paths cause an increase of internal gas pressure, resulting in macroscopic cracking. The variation of the graphite grade, the partial replacement of virgin fused MgO and graphite with MgO-C recyclate, and lignin addition reveals that porous channels facilitate gas release and thereby significantly improve volume stability.

In the present study, ring filters were applied directly in a steel casting ladle for the first time. The carbon-bonded alumina filter system was fitted to the existing stopper system enabling steel melt filtration directly above the shroud without risk of filter blockage. Two identical casting trials were performed, whereby one resulted in successful retrieval of the filter and one resulted in filter failure. Analysis of cast steel samples regarding their inclusion population revealed that the filter failure did not impair the steel quality. The intact filter was analyzed via digital light microscopy, SEM/EDS and XRD. Contact zones revealed location-dependent morphological and chemical differences indicating inclusion removal and limited damage by slag contact. Overall, the investigation showed that the new principle is feasible and the risk due to filter failure is limited, which makes room for future casting trials to quantify the filtration efficiency and the impact of filter geometry.

Magnesia-carbon (MgO-C) bricks partially made of recycled magnesia (MgO) are examined in hardened and coked state using Raman and photoluminescence (PL) spectroscopy, focusing on the MgO grains compared with nominally pure MgO powder for reference. Raman and PL signals in the visible to near infrared range are distinguished and interpreted by varying the excitation wavelength between 532, 633, and 785 nm. Besides a broad PL band centered at 565 nm characteristic of the brick-derived MgO, the origin of two sharp high-intensity PL signals (699, 871 nm) surrounded by symmetrical sidebands is clarified. The 699 nm luminescence reveals the presence of trivalent chromium (Cr3+) impurities in the MgO lattice of MgO-C bricks as well as MgO powder, while the latter, V2+-related PL signal is only observed for the brick-derived MgO. The nature of the symmetrical sidebands is investigated by temperature-dependent spectroscopic measurements between 100 and 295 K. The temperature dependence of the intensity ratio between higher energy (anti-Stokes) and lower energy (Stokes) sidebands confirms the phonon involvement in both the MgO:Cr3+ and MgO:V2+ signals. This study enhances the spectroscopic methods to apply to industrial refractory materials, while it reveals impurities on a defect level that is not accessible by any well-established analysis.

Nonmetallic inclusions (NMIs) are integral constituents of ferrous materials, but almost always have a negative impact on their properties. Therefore, it is best to implement strategies during production to avoid NMIs as much as possible or at least control their formation. To gain a better understanding of these mechanisms, this study investigated the population of nonmetallic inclusions in liquid high-silicon electrical steel (Si approximate to 3 mass-%) depending on the contact with different MgO-C refractories. Both conventional refractory materials and those containing MgO-C recyclate and environmentally friendly binder systems, such as collagen, fructose, and lignin, were considered. Immersion tests were carried out for 30 min at a temperature of 1600 degrees C in accordance with DIN CEN/TS 15 418. Surface phenomena, diffusions reactions, and corrosion mechanisms occurring in the refractory materials were characterized after the tests using SEM and EDX analyses. The change in the chemical composition of the steels was determined using spark spectroscopy, and the NMI population was evaluated after the tests using an automated feature analysis (AFA).

The effect of thermal pre-oxidation on the microstructure and molten-salt interaction of MgO–316L composite anodes produced from decarbonized MgO–C refractory recyclate was investigated. Composite anodes containing 40 vol.% recycled MgO and 60 vol.% AISI 316L steel were fabricated by cold isostatic pressing and pre-oxidized in flowing air at 800, 900, or 1000 °C for 24 h. The formation of Mg-containing spinel phases predicted by thermodynamic calculations was confirmed by Raman spectroscopy and SEM–EDX analysis. Pre-oxidation converted the initially physical MgO–steel contact into chemically bonded oxide interfaces and reduced molten-salt infiltration. The most pronounced reduction in Al-bearing salt penetration was observed for PO900, where infiltration was largely confined to a thin near-surface region due to its refined pore structure. Increasing the pre-oxidation temperature promoted outward oxide-scale growth while decreasing the depth of inward oxidation. Although Mg-containing spinels remained susceptible to interaction with the molten salt, pre-oxidation could substantially improve corrosion resistance compared with the as-sintered condition.

In the present study, various thermo-mechanical tests under compressive load were performed on commercial MgO-C refractory brick grades over a wide temperature range up to 1600 degrees C in an argon atmosphere. The focus of the present study was the assessment of the influence of the incorporation of MgO-C recyclate on the thermomechanical behavior. Therefore, one MgO-C brick grade contained only fresh raw materials, while the other grade included MgO-C recyclate with a high mass fraction of 47.5 wt%. Subsequent investigations, including scanning electron microscopy and porosity analyses, provided insights into the mechanisms occurring within the material during the thermo-mechanical tests. The incorporation of MgO-C recyclate led to an improvement in the thermo-mechanical properties, as increased compressive strength values were observed within the tested temperature range, along with enhanced creep resistance and improved behavior in the refractoriness under load tests.

This study investigates a novel approach to powder bed pre-debinding, focusing on the influence of the heating rate on the structural integrity of 3D-printed alumina parts. Cuboidal specimens were additive manufactured using fused granule fabrication, with a thermoplastic feedstock composed of a polyethylene-based binder and alumina ceramic solid phase. After printing, specimens were subjected to various pre-debinding conditions, followed by debinding and sintering in air atmosphere.The developed powder bed pre-debinding procedure proved advantageous compared to air-only debinding, significantly reducing bloating and structural damage. The powder bed not only stabilized the parts mechanically but also facilitated more uniform binder removal. Additionally, a higher heating rate during pre-debinding improved part quality, possibly because the faster onset of binder decomposition reduced the time available for time-dependent channel closure, thereby preserving pathways for volatile products to escape. Based on these findings, a prototype burner nozzle was successfully fabricated and thermally treated using the most favorable debinding procedure. The sintered nozzle withstood the applied non-standardized thermal shock test with only minor damage, indicating promising structural stability of the 3D-printed alumina component under severe localized thermal loading.

How would it be possible to functionalize ceramic aggregates for use in refractories? In this work, we demonstrate how paste extrusion can be used to fabricate layered and porous Nb-Al2O3-based composite refractories for adjusting thermal and electrical conductivity. Additive manufacturing is used to generate a specific sequence of alumina and composite layers. After drying, the samples were sintered at 1600 °C, crushed, and sieved into particle sizes up to 3150 µm. The rheology of the paste revealed the intended shear-thinning behavior with microcrack formation between the yield and flow strain. The sintered material showed promising thermal-shock characteristics reaching plateau values after the third cycle without signs of further structural damage up to the fifth thermal shock. The layered microstructure was retained after crushing the composites, establishing functionalization of the refractory granules for all particle sizes.