Christos Georgios Aneziris

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.

Industrial MgO–C refractories fabricated from either pure fuzed magnesia (R0) or 50 wt% recycled MgO–C (R50) were exposed to ladle slags (C/A = 1) with varying SiO ₂ content (1–20 wt%). Wetting and infiltration behaviors were investigated using the sessile drop method at 1600°C under an Ar atmosphere for different dwell times. Quenched slag–refractory interfaces were characterized by SEM‐EDX and compared with phase equilibrium simulations using FactSage 8.1. Increasing SiO ₂ content at 1600°C reduced slag density and surface tension, increased viscosity, and shifted saturation from MgO to MgAl ₂ O ₄ spinel. During heating, R50 samples exhibited more intense gas formation. This delayed wetting and significantly slowed the initial stage of slag infiltration. Upon contact with recycled‐based refractories, the high‐SiO ₂ slag formed a dense, 195 µm‐thick MgAl ₂ O ₄ ‐rich layer, entrapping residual calcium‐silicate slag and serving as an in situ protective barrier against further oxidation and corrosion. These results demonstrate that well‐processed MgO–C recyclate can inhibit slag attack, first by delaying infiltration via transient gas release, and subsequently by promoting a protective spinel layer under suitable slag compositions. Furthermore, the findings highlight the influence of substrate carbon content and open porosity on the slag's melting and infiltration behavior.

This work investigates the high‐temperature interactions between low‐sulfur Al‐killed Mn–B steel and carbon‐bonded magnesia (MgO–C) refractories containing 0 and 50 wt% recycled material (recyclates). Finger immersion tests are conducted at 1600 °C under an argon atmosphere, with variations in boron (B) and sulfur (S) content. Microstructural and chemical analyses via scanning electron microscopy/energy‐dispersive X‐ray spectroscopy reveal the formation of protective MgO and CaS layers at the steel–refractory interface, alongside spinel (MgAl ₂ O ₄ ) and calcium silicate phases. B and S jointly influence wetting and infiltration behavior, while increased S promotes thicker CaS layers. Recyclates introduce microstructural heterogeneity (distributed dissolution sites) but do not significantly alter the interfacial reaction mechanisms. Both refractories exhibit comparable performance in forming protective layers, suggesting that recyclate‐containing MgO–C materials can serve as sustainable alternatives without compromising thermochemical stability or steel cleanliness. These findings support the use of recycled materials in refractory manufacturing, contributing to waste reduction and circular economy practice in steelmaking.

Given the growing emphasis on refractory recycling, the performance characteristics of commercial MgO‐C bricks incorporating MgO‐C recyclates are of considerable economic and environmental relevance. In the present study, four‐point bending tests are conducted in an argon atmosphere on three commercial MgO‐C brick grades over a temperature range from room temperature to 1300 °C. One MgO‐C brick grade is produced exclusively from virgin raw materials, whereas the other two grades incorporate MgO‐C recyclate at a high mass fraction of 47.5 wt%. Due to the different sources of the recyclates, their carbon contents vary, resulting in different total carbon contents in the two MgO‐C brick grades containing recyclate. Post‐mortem SEM investigations show predominantly transgranular fracture through the MgO‐C recyclates. The evaluation of the four‐point bending tests reveals no detrimental effect of the MgO‐C recyclates on bending strength. Instead, a decisive influence of the total carbon content is observed, with lower total carbon contents correlating with higher bending strength values. Furthermore, the heterogeneous structure of the MgO‐C recyclates contributes to enhanced stress dissipation caused by anelastic deformation processes in the respective MgO‐C brick grades, despite the accompanying decrease in virgin flake graphite content.

On the rising demand for eco‐friendly refractories, reducing the use of likely toxic magnesia‐chromium aggregates remains a challenge. Previous studies by some of the authors have proposed Cr‐free alternative compositions, although the morphology of the spinel precipitates has varied across the different suggested systems. The mechanisms involved in the formation of these distinct morphologies were unclear and, therefore, are the focus of this work. In all compositions, SEM/electron backscatter diffraction revealed cube–cube orientation relationships between matrix and precipitates, indicating that their formation is influenced by the lattice parameter misfit ( δ ), which was measured using synchrotron X‐ray diffraction. It could be concluded that coarser and spherical precipitates form to minimize their surface‐to‐volume ratio in compositions with high absolute δ ‐values. Conversely, low‐misfit systems enable the spinel to form a 3D‐network. The potential use of this knowledge to tailor the microstructure of novel compositions was demonstrated by a small Nb ₂ O ₅ addition into one of the proposed compositions.

In sintering simulation, there are basically two approaches: microscale simulation, in which distinct particles or pores are regarded, and macroscale, where the porous body is regarded as continuum with variable density.Material parameters of the latter can be determined by experiment or by microscale models.Current microscale sintering models mainly use circular resp.~spherical particle geometries to represent the actual shape of real particles.However, sintering behavior is heavily dependent on the morphology of the powder particles, since sintering progress is driven by reduction of the bound surface energy.So current models neglect the influence of local contact morphology.Here, a finite differences based microscopic sintering model is presented, which is capable to work with irregular particle geometries.Asymmetric particle contacts in shape and substance are possible within.The differences between circular particle contacts and asymmetric ones are investigated.Furthermore, a statistical way of describing the morphology of powder particles and its inclusion into sintering simulation using Monte Carlo techniques are shown.Morphology data are obtained from microscopic imaging by extracting the 2D contours.The particles' contour lines are fitted to a parameterized shape function including ovality and first order waves to obtain a description of the particles' fine shapes.From the statistical distribution of the shape parameters, randomized particle groupings are sampled as input for microscopic sintering simulation.Statistical analysis of the samples' sintering behaviors leads to statements about the powder's.Comparisons to classical spherical modelling are given.

This study focuses on the fabrication of metal hollow spheres using an alginate‐based sol–gel technology in combination with a freeze‐drying process. The mixing routine for the metal slurry is investigated. Critical processing parameters, influencing the yield of intact spheres such as the aging time in the reaction solution and in water as well as the freezing time and the rotation of the spheres during freezing are evaluated with statistical design of experiments. A customized debinding regime is developed with a heating rate of 0.1 K min⁻¹ in critical temperature ranges and a maximum temperature of 350 °C. For the fabrication of hollow metal spheres with additions of 1.0 wt% potassium alginate, the optimum aging time in Ca(OH)₂‐reaction solution is 4 min (4C) and the spheres are only to be rinsed with purified water (0 W). Furthermore, the rotation of the spheres (+R) during freezing significantly contributes to the increased yield of 80.2%, which is significantly higher as compared to the previous studies. Sintered spheres have a compressive strength of 159 MPa with a corresponding strain of 42%.