Browsing by Author "Hubálková, Jana"

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GRK 2802: Anodes Based On MgO-C Recyclates And 316L Steel For Prospective Aluminium Fused-Salt Electrolysis Application
(Technische Universität Bergakademie Freiberg, 2026-06-25) Yaroshevskyi, Serhii; Gehre, Patrick; Kaiser, Patricia; Brachhold, Nora; Hubálková, Jana; Aneziris, Christos G.
Every year, approx. 28 million tons of used refractories accumulate worldwide. The majority of them are used as aggregates for road construction (downcycling) or are deposited in landfills. For ecological and economic reasons, an increased research potential was identified in recent years, dedicated to increasing the recycling rate and finding new markets and application fields with a higher value of the refractory recyclates. The study presents the current findings of the development of inert electrodes for the aluminum fused-salt electrolysis composed of steel and MgO gained from MgO-C recyclates. To withstand the chemical attack of molten aluminum and cryolitic melts, the metal-ceramic composites were preoxidized (PO) at 800 °C, 900 °C, and 1000 °C. The microstructure after each PO thermal treatment was analyzed. After PO at 800 °C, a (Cr,Fe)3O4 spinel-like phase and Fe-O Mg-O solid solution form. After PO at 900 °C, a larger amount of the Fe-O Mg-O solid solution and a (Cr,Fe)2O3 solid solution around the steel grains was identified. Furthermore, the electrical conductivity of the metal-ceramic-composites preoxidized at 900 °C amounts to 1.49·10^2 S/cm and hence is in the range of carbon, which exhibits a value of 1.54·10^2 S/cm. Additionally, the impact of different preoxidation treatments at 900 °C by applying furnaces equipped with electrical heating, natural gas burner, and microwave plasma burner on the microstructure was investigated.
Open Access
GRK 2802: Anodes based on MgO-C Recyclates and 316L Steel for Prospective Aluminium Fused-Salt Electrolysis Application
(Technische Universität Bergakademie Freiberg, 2026-09-25) Gehre, Patrick; Yaroshevskyi, Serhii; Kaiser, Patricia; Brachhold, Nora; Hubálková, Jana; Aneziris, Christos
Every year, approx. 28 million tons of used refractories accumulate worldwide. The majority of them are used as aggregates for road construction (downcycling) or are deposited in landfills. For ecological and economic reasons, an increased research potential was identified in recent years, dedicated to increasing the recycling rate and finding new markets and application fields with a higher value of the refractory recyclates. The study presents the current findings of the development of inert electrodes for the aluminum fused-salt electrolysis composed of steel and MgO gained from MgO-C recyclates. To withstand the chemical attack of molten aluminum and cryolitic melts, the metal-ceramic composites were preoxidized (PO) at 800 °C, 900 °C, and 1000 °C. The microstructure after each PO thermal treatment was analyzed. After PO at 800 °C, a (Cr,Fe)3O4 spinel-like phase and Fe-O Mg-O solid solution form. After PO at 900 °C, a larger amount of the Fe-O Mg-O solid solution and a (Cr,Fe)2O3 solid solution around the steel grains was identified. Furthermore, the electrical conductivity of the metal-ceramic-composites preoxidized at 900 °C amounts to 1.49·102 S·cm-1 and hence is in the range of carbon, which exhibits a value of 1.54·102 S·cm-1. Additionally, the impact of different preoxidation treatments at 900 °C by applying furnaces equipped with electrical heating, natural gas burner, and microwave plasma burner on the microstructure was investigated.
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GRK 2802: Characterization of MgO-C recyclate and commercial MgO-C refractory bricks with and without Recyclate
(Technische Universität Bergakademie Freiberg, 2025-11-13) Schramm, Alexander; Stadtmüller, Till M. J.; Hubálková, Jana; Schimpf, Christian; Wüstefeld, Christina; Schemmel, Thomas; Aneziris, Christos G.; Weidner, Anja; Biermann, Horst
In light of the increasing focus on the recycling of refractories, the properties of commercial MgO-C refractory bricks containing MgO-C recyclate are of significant economic and ecological importance. In the present study, MgO-C recyclate and two commercially available MgO-C bricks – one grade consisting exclusively of fresh raw materials while the other containing 47.5 wt.% MgO-C recyclate – were comprehensively characterized. This involved the use of X-ray computed tomography, scanning electron microscopy, X ray fluorescence analysis, X-ray diffraction, as well as density and porosity measurements. The MgO C recyclate exhibited a heterogeneous composition consisting of MgO aggregates and contained an increased content of impurities compared to fresh MgO raw materials. The incorporation of MgO C recyclate as a raw material for commercial MgO-C bricks resulted in a decrease of the average MgO aggregate size, a higher porosity with a decrease in the median pore size and a reduced CaO/SiO2 ratio of the corresponding MgO-C brick grade. Furthermore, the MgO-C grade with 47.5 wt.% MgO-C recyclate exhibited a higher cold crushing strength, but a reduced oxidation resistance.
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GRK 2802: IMPACT OF SUSTAINABLE BINDERS BASED ON LIGNIN AND COLLAGEN ON THE VOLUME STABILITY AND MECHANICAL PROPERTIES OF MgO-C RECYCLATES
(Technische Universität Bergakademie Freiberg, 2026-09-21) Stadtmüller, Till Manon Jannis; Kerber, Florian; Moritz, Kirsten; Dudczig, Steffen; Kraushaar, Konstantin; Schemmel, Thomas; Hubálková, Jana; Gehre, Patrick; Aneziris, Christos G.
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.
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GRK 2802: Influence of MgO-C recyclate on the thermo-mechanical compression behavior of commercial MgO-C refractory bricks
(Technische Universität Bergakademie Freiberg, 2026-02-02) Schramm, Alexander; Hubálková, Jana; Roungos, Vasileios; Schemmel, Thomas; Aneziris, Christos G.; Weidner, Anja; Biermann, Horst
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 °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 thermo-mechanical 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.
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GRK 2802: Investigation of Thermo-Chemical Processes in Commercial MgO-C Refractory Bricks With and Without MgO-C Recyclate at Elevated Temperatures
(Technische Universität Bergakademie Freiberg, 2026-07-10) Schramm, Alexander; Hubálková, Jana; Schimpf, Christian; Schemmel, Thomas; Aneziris, Christos G.; Weidner, Anja; Biermann, Horst
In the present study, the chemical processes occurring during high-temperature testing are investigated for two commercial MgO-C brick grades. One grade consists exclusively of virgin raw materials, while the other contains 47.5 wt.% MgO-C recyclate. High-temperature testing is conducted under argon atmosphere at ambient pressure using induction heating. To evaluate the effect of MgO-C recyclate incorporation on thermo-chemical processes, X-ray diffraction (XRD), electrothermal vaporization (ETV), differential thermal and thermogravimetric analysis (DTA/TG) coupled with mass spectrometry (MS) are applied. Scanning electron microscopy (SEM) combined with energy-dispersive X-ray spectroscopy (EDS) and electron backscatter diffraction (EBSD) provide phase characterization after exposure at 1300 °C and above. The results show that mechanisms typically occurring during the service of refractory materials such as the carbothermal reduction of magnesia, the incorporation of impurity phases into the newly formed MgO surface layer, and the deposition of calcium-rich phases and whisker-like structures also occur when MgO-C is exposed to the applied gas atmosphere. This confirms the relevance of the observed thermo-chemical processes and surface phase formation for real contact with molten steel and slag. Under the present test conditions, however, no significant influence of MgO-C recyclate incorporation on the thermo-chemical behavior of the coked MgO-C materials is detected.
Open Access
GRK 2802: MgO-C Ladle Bricks based on Recyclates and Lignin-Collagen-Fructose Binder System
(Technische Universität Bergakademie Freiberg, 2026-09-25) Gehre, Patrick; Gunasekar, Dinesh Kumar; Hubálková, Jana; Aneziris, Christos
Every year, approx. 28 million tons of used refractories accrue worldwide. After their break out, the majority of them are used in subordinate application fields such as aggregates for road construction (downcycling) or deposited in landfills. Their recycling in the field of refractory materials played an underpart so far. For ecological and economic reasons, an increased research potential was identified in the last years consecrated to increasing the recycling rate of refractories and finding new markets and application fields with higher value for them. The study presents current findings of a long-term research training group addressing (amongst others) the development of MgO-C ladle bricks containing MgO-C recyclates and being bonded with pitch- and resin-free environmentally friendly binders. So far, the favored batches contained an oxidic grain fraction composed of 40.5 wt.-% fused MgO raw material, 40.5 wt.-% MgO-C recyclates, 14 wt.-% sintered magnesia (DBM) raw material together with 5 wt.-% graphite and optional antioxidants (TiO2, Al, SiO2). A binder system composed of a mixture of lignin, collagen, and polyethylene glycol (PEG) was developed. The latter served as a pressing aid and was necessary due to the absence of liquid resin in the batch. Even after a considerable and auspicious optimization during previous studies, this basic batch containing recyclates and no pitch or resin does not meet the physical and mechanical properties of standard industrial bricks. Hence, in the present study, the binder system was enhanced by using varying amounts of lignin, collagen, and tannin together with polyethylene glycol, fructose, and citric acid addition. The new developed MgO-C mixtures were pressed uniaxially, dried, cured, and carbonized at 1000 °C. It has been found that polyethylene glycol, which impairs the material properties, can be omitted using sufficient amounts of fructose. The addition of citric acid leads to a further increase in CCS. Consequently, actual optimized environmentally friendly MgO-C batches show a CCS of 85 % of the value of industrial pitch- and resin-bonded bricks without recyclates.
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GRK 2802: MgO-C REFRACTORIES BASED ON REFRACTORY RECYCLATES AND ENVIRONMENTALLY FRIENDLY BINDERS
(Technische Universität Bergakademie Freiberg, 2026-02-16) Stadtmüller, Till Manon Jannis; Storti, Enrico; Brachhold, Nora; Lauermannová, Anna-Marie; Jankovský, Ondřej; Schemmel, Thomas; Hubálková, Jana; Gehre, Patrick; Aneziris, Christos
This study focused on the development of an environmentally friendly binder system based on lignin and collagen for uniaxial pressed MgO-C refractories as an alternative to commonly used resin or pitch binders. Additionally, recycled MgO-C refractories from steel plants were partially utilized as raw material, investigating their influence on the resulting physical and mechanical properties. The binder system showed reliable binding properties, although the recyclate-containing MgO-C exhibited higher porosity, slightly lower density, and lower strength compared to the reference batches without recyclates. However, antioxidants significantly improved the properties of the recyclatecontaining MgO-C samples. Scanning electron microscopy analysis with energydispersive X-ray spectroscopy revealed the formation of whiskers, as well as oxidation and nitridation of aluminum particles. This research highlights the potential of environmentally friendly binders and the utilization of recycled materials in MgO-C refractories to mitigate their environmental impact and enhances the environmental performance of carbon containing refractory materials.
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SPP 2419: Investigation of the (thermo-)mechanical properties and the chemical resistance towards ammonia combustion atmosphere of alumina-based model structures manufactured via Fused Granulate Fabrication
(Technische Universität Bergakademie Freiberg, 2026-06-17) Brachhold, Nora; Bock-Seefeld, Benjamin; Kretzler, Daniel; Heuer, Claudia; Neumann, Marc; Hubálková, Jana; Stelzner, Björn; Stein, Oliver T.; Trimis, Dimosthenis; Aneziris, Christos G.
This study focuses on the fabrication of alumina-based model samples by fused granulate fabrication for the application in burner components for ammonia combustion. Three compositions were investigated: Two alumina systems with different initial grain size (A1-tabular alumina, A2-reactive alumina) and alumina (AZT) with MgOstabilized ZrO2- and TiO2-additions. Young’s modulus and flexural strength were determined after sintering and after up to five thermal shocks. A1 exhibited the best suitability for repeated thermal shocks, followed by A3. The high interparticle porosity of A1 limited crack propagation and resulted in a slow, gradual loss in elastic modulus (maximum loss of 8.6%) and strength (maximum loss of 35.7%) with increasing number of thermal shocks. Microstructural features due to the additive manufacturing process could enhance thermal shock performance. The materials were tested in ammonia combustion atomosphere at approx. 1500 ◦C for 1 h and showed no phase alterations. Model components with application-near geometries were successfully realized.