Novel approaches for manufacturing of carbon-bonded alumina filters with improved environmental sustainability

Zusammenfassung:

Ziel der Promotion war es, die Umweltverträglichkeit von kohlenstoffgebundenen Alumina-filtern für die Stahlschmelzefiltration hinsichtlich ihrer Zusammensetzung und Herstellung zu verbessern. Dafür wurde die schrittweise Substitution des herkömmlich eingesetzten Pechbinders durch ein umweltfreundlicheres Bindemittelsystem basierend auf Laktose und Tannin anhand von konventionellen Replikafiltern untersucht. In einem weiteren Schritt wurde ein neuartiges Filterherstellungsverfahren auf Grundlage von additiv-gefertigten, wasser-löslichen Filterschaumvorlagen und alginathaltigen Beschichtungsschlickern entwickelt, um die Filterschaumvorlage vor der thermischen Behandlung mittels Wasser zu entfernen und somit die Freisetzung von umweltschädlichen Gasen zu vermeiden. Die Analysen der Filterstrukturen nach der Verkokung und nach Kontakt mit einer Stahlschmelze bei 1650 °C zeigten, dass der ausschließliche Einsatz des umweltfreundlicheren Bindemittelsystems und das neuartige Filterherstellungsverfahren geeignet sind, um kohlenstoffhaltige Aluminafilter mit hinreichenden (thermo-)mechanischen Eigenschaften für die Stahlschmelzefiltration und erhöhter Umweltverträglichkeit herzustellen.

Advanced composites based on magnesium oxychloride and layered nanomaterials

Summary:

This dissertation investigates high-performace environmentally sustainable building composites based on highly reactove magnesium oxide. The presented work deals with the synthesis and chracterisation of composite materials based on a magnesium oxychloride cement phase 5 (Mg₃(OH)₅Cl.H₂O, MOC) matrix. Two approaches in the design and development of thesse composites are presented. In the first approach, MOC is modified with several types of layered nanomaterials in order to achieve the best possible material properties of the resulting composite material. Graphene, nanostructured alumina and molybdenum disulfide were used. The second approach, taken in order to highlight the environmental benefits of MOC-based building composites, involved using three types of waste fillers as substitues for standard silica sand, demonstrating the possibility of avoiding landfilling and giving them a secondary use. The waste materials used invluded wste carbon-bonded magnesia, waste MOC-based composites, and waste refrctory silicate bricks. All the prepared samples were thoroughly characterised in terms of their composition )phase and chemical), microstructure and morphology, structural properties (density, porosity), machenical properties (flexural and compressive strength, dynamic Young´s modulus) and properties related to water and moisture exposure (water absorption, softening coefficient, overall water resistance).

Advanced composite construction materials doped with partially oxidized carbon nanomaterials

Summary:

The construction industry is currently facing a fundamtenal challenge: to develop materials that combine high performance with a reduced environmental footprint. Traditional Portland cement (PC), although widely used, is associated with high CO2 emissions and limited chemical resistance. A promising low-carbon alternative is magnesium oxachloride cement (MOC), which provides high strengt and other advantageous properties; however, its broader application is hindered by poor water resistance and uncertain long-term stability. This dissertation, therefore, focuese on overcoming these limitations through the use of carbon nanomaterials with a partial degree of oxidation - specifically oxidized carbon nanotubes, graphene oxide , and carbon spheres prepared from waste plastics. The chosen approach combined a systematic study of the influence of these additives on hydration, microstructure, and the resulting physical and mechanical properties of cement matrices with an assessment of their environmental safety. The results demonstrated that the effect of nanomaterials is strongly dependent on the type of binder: in PC, they primarly improve the bonding of hydration products and the compactness of the microstructure, weheras in MOC, they significantly influence the crystallization of major phases and the porosity of the composite. An important contribution was also the inclusion of low-cost carbon spheres derived from waste plastics, which proved capable of enhancing MOC strength, thereby opening a pathway toward circular waste utilization. The research also incorporated experimental ecotoxicological evaluations, which showed that the releaseof carbon nanoparticles from MOC has a relatively low impact on aquatic organisms. However, toxiciity depends on the type and chemical modifications of the additive. Overall, this work provides a new perspective on the sustainable development of cement composites: it highlights that the succesul implementation of nanotechnologies in construction requires linking material performance improvements with both circular and environmental considerations. 

Effective properties of oxide and silicate ceramics and their dependence on composition, microstructure, and temperature

Summary:

This dissertation is a collection of nine original research papers and highlights the key findings on the relationships between processing, microstructure, and effective elastic properties of selected oxide and silicate ceramics, with a focus on SnO₂-ZnO based oxide ceramics and kaolin-based ceramics. Both types of ceramics are prepared using conventional sintering, and their microstructure, phase composition, and elastic properties are thoroughly characterized. Additionaly, microwave sintering is applied to SnO₂-ZnO based ceramics to compare its effects with conventional sintering.

The first part of the thesis focuses on SnO₂-ZnO based oxide ceramics. SnO₂ ceramics have emerged as an interesting material to investigate since their behavior differs from that of usual oxide ceramics in terms of microstructure evolution during sintering. There is no change in the overall prorosity, only in the shape and size of pores. Therefore, changes inits effective properties, such as Young´s modulus, have to be attributed to other microstructural parameters. On the other hand, ZnO ceramics do not exhibit such exotic sintering behavior and changes in its effective properites are mainly determined by changes in porosity. Composite SnO₂-ZnO based oxide ceramics contain spinel-type Zn₂SnO₄ as an additional phase, and the evolution of their effective properties is intermediate between SnO₂ and ZnO ceramics.

The second part of the thesis investigate kaolin-based silicate ceramics. In this case, the focus is not only on the evolution of microstructure but also on understanding the relationship between phase composition and elastic properties. A key part of the research involeves determining the phase composition by X-ray diffraction, including the glass phase content, which plays a crucial role in the final elastic properties of these ceramics.

In addition to the experimental investigation, which focuses on the impulse excitation technique, the dissertation provides insight into the prediction of effective properties, particularly elastic properties and the porosity dependence of sound velocities, for which new models have been proposed.

Development of advanced eco-friendly spinel-containing materials to replace chromite-based refractories

Summary: 

Until the 1990s, the use of electrofused magnesia-chrome aggregates (EMCA) was widespread in refractory linings for the steel industry. Despite their efficiency and low cost, it was discovered that these materials generate toxic byproducts. Consequently, MgAl₂O₄-containing refractories emerged as the main alternative, though their performance has not yet reached equivalent levels. This thesis combined computational and experimental tools to design and evaluate non-toxic compositions, focusing on their thermomechanical, chemical and microstructural properties. Initially, the substitution of MgO with ZnO as a spinel-forming additive in alumina-based refractory castables was investigated. Additionally, introducing ZnO enabled microwave-assisted sintering of these materials. Nonetheless, zinc volatilization above 1300 °C revealed its limitations. Therefore, the study focused on more deeply characterizing the EMCA. Inspired by the findings, four chromium-free compositions were designed and produced via fusion or sintering, both at a laboratory scale. Dilatometry and thermal conductivity tests demonstrated the promising performance of the novel compositions and the potential to tailor their properties through minor compositional adjustments. Finally, the industrial interest in the results of this thesis enabled a partnership with a refractory manufacturer. In collaboration, pilot-scale tests are currently being conducted to develop industry-applicable technology capable of reducing the environmental impact associated with chromium-containing refractories.

Hydrogen-based reduction of iron ore pellets and recycling of metallurgical dusts 

Summary:

The use of hydrogen in metallurgical processes holds significant promise for enhancing sustainability and reducing environmental impact. Based on the literature, the direct reduction of iron ore can be characterized by the reduction degree and reduction swelling index (RSI), which significantly depend on parameters such as temperature, pressure, type of reducing agent, porosity and material chemistry. This thesis enabled the assessment of the influence of temperature, reducing atmosphere and material chemistry on the reduction degree and RSI. Three types of iron ore pellets were reduced and studied for their reduction degree and RSI at different temperatures. The effect of a reducing atmosphere consisting of 80 % nitrogen and 20 % hydrogen was studied and compared with the reduction in a hydrogen atmosphere under the same conditions. The high iron-rich metallurgical dust was also studied using hydrogen-based reduction. The results showed a possibility of the use of metallurgical dust during the direct reduction process, thereby reducing waste and conserving resources.

Development and characterization of MgO and TiO₂ reinforced Steel Ceramic Composites resistant to long-term contact with liquid aluminum alloys

Summary:

The PhD thesis provides detailed description of a successful development of MgO and TiO₂ particle reinforced Steel Ceramic Composites (SCC) for molten aluminum alloy applications. For this purpose, the influence of MgO and TiO₂ addition and subsequent pre-oxidation surface treatment on the structure of SCCs and their corrosion resistance against long-term contact with liquid aluminum alloys was investigated. The initiation and progression of corrosion processes were thoroughly analyzed by means of newly developed DSC-aided corrosion tests, high temperature electrochemical studies and adapted wettability measurements. The gained insights led to the recognition of most important factors contributing to the corrosion, including both the electrochemical and the chemical driving forces arising between the SCCs and aluminum alloy. The evaluation of long-term corrosion resistance was performed with the help of finger immersion tests, crucible corrosion tests and subsequent SEM/EDS/EBSD and XRD analyses aiming at the determination of elements most prone to the dissolution in the liquid aluminum alloy and formation of corrosion phases. The pre-oxidized MgO reinforced SCC revealed superior corrosion resistance, being capable of withstanding more than 168 h of contact with liquid aluminum alloy.

Reciprocal influence between MgO-C refractory materials with different MgO grade and a steel melt and the resulting effect on non-metallic inclusions

Summary:

The thesis addressed the effect of a varying MgO grade in MgO-C refractories on both their behavior in contact with a steel melt and the resulting effect on the non-metallic inclusion (NMI) population in the solidified steel. For this purpose, immersion tests were conducted in a semi-industrial steel casting simulator. In addition, the effect of the steel melting process parameters on the NMI population was thoroughly investigated, providing a guideline for the result interpretation for future experiments in the steel casting simulator. Here, a fundamental concept of data evaluation for the NMI characterization in a steel matrix using automated feature analysis was developed. The main NMI types detected in the solidified steel samples were Al₂O₃ and MnSi-based inclusions. Their number density depended on the steel melt’s temperature and amount of dissolved oxygen. A lower MgO grade refractory specimen in contact with the steel melt resulted in a higher proportion of low melting phases on its surface compared to a higher MgO grade specimen. These low-melting phases promoted the formation of MnSi-based inclusions and triggered NMI agglomeration leading to the formation of large Al₂O₃ inclusions.