Partikeltechnologien, Rohstoffinnovationen und Ressourceneffizienz
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- Additive (1)
- Anhaftung (1)
- Caking (1)
- Design of experiment (1)
- Discrete Element Method (DEM) Dry stirred media mill Milling energy efficiency Grinding aids Grinding media motion Stress intensity (1)
- Eintreffwinkeln (1)
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- Mechanical stress types (1)
- Mechanochemistry Mechanical stress types Stress intensity Reaction kinetics (1)
Caking and powder adhesion are widespread challenges in dry powder processes. The influence of process parameters such as humidity and temperature on the adhesion behavior of dry powders has been extensively studied in numerous studies. Besides that, the impact of other process characteristics, such as additional process parameters or wall materials, has received little attention so far. In addition, existing methods to characterize caking behavior do not account for powders in a fluidized state. To address phenomena based on process and material behavior, a test rig was specifically designed to investigate the adhesion of dry particles to different metal walls at varying speeds at a 90° angle, representing the main novelty of this study. The deposition area, deposition mass, and maximum deposition thickness were evaluated, and the correlations were discussed. The investigations revealed that at low velocities (<12 m/s) and for smooth surfaces (Sq < 0.3–0.4 µm), wall materials with a high ratio of dispersive to polar surface energy components (D/P: 13–15.8) exhibit minimal powder adhesion. The test rig has demonstrated its effectiveness as a straightforward method for measuring adhesion across various powder–wall material pairs and could serve as a valuable preliminary test for industrial applications.
Feine Pulver neigen stark zur Anhaftungs- und Klumpenbildung. Selbst bei streng kontrollierten Umgebungsbedingungen und minimalen mechanischen Belastungen tritt dieses Problem weiterhin auf. Besonders ausgeprägt ist das Phänomen bei Partikeln kleiner als 100 μm, für die die Schwerkraft praktisch keine Rolle mehr spielt [1]. Wechselwirkungen zwischen Partikeln sowie zwischen Partikeln und Wandoberflächen führen zu Ablagerungen, die die Qualität und Sicherheit des Endprodukts beeinträchtigen und im Zeitverlauf Produktionsausfälle verursachen können [2 – 4]. Ein Beispiel für Caking in einem gasdurchströmten System zeigt Abb. 1 anhand einer Laborspiralstrahlmühle. Im Projekt „Fluid-Cake“ wird das Caking-Phänomen an fluidisierten Systemen untersucht. Für die praktische und schnelle Untersuchung des Anhaftungsverhaltens unterschiedlicher Pulver wurde ein Pulverversuchsstand konstruiert, in dem verschiedene Produkte direkt getestet werden können. Der Versuchsstand bietet den Vorteil, dass unterschiedliche Einflussfaktoren – wie Pulver- und Partikeleigenschaften, Wahl der Prozessparameter und Wandmaterialien – gemeinsam bewertet werden können. Während bisherige Untersuchungen die Abhängigkeit des Adhäsionsverhaltens von Wandmaterialien, Pulverbeladungen und gewählten Gasgeschwindigkeiten gezeigt haben, werden in aktuellen Studien zusätzlich der Einfluss von Pulvereintreffwinkeln, Beschichtungs-stoffen und Reinigungsansätzen sowie die Modifikation des Kohäsionsverhaltens durch den Einsatz flüssiger Additive untersucht. Parallel werden die Pulver- und Partikeleigenschaften charakterisiert und die relevanten Fluidisierungssysteme mittels CFD nachgebildet. Diese vielseitige Herangehensweise ermöglicht eine umfassende Analyse der Faktoren, die das Caking-Phänomen beeinflussen, und liefert praxisrelevante Empfehlungen für die Auswahl geeigneter Pulver und Prozessparameter bereits in der Planungsphase.
Dry operated stirred media mills are gaining increasing relevance in fine grinding applications due to their high energy densities and process flexibility. Nevertheless, the interrelation between operating parameters and stress
conditions remains insufficiently understood compared to wet milling systems. In this work, the grinding media motion within a vertically oriented batch stirred media mill was investigated using Discrete Element Method
(DEM) simulations, experimentally calibrated to incorporate the influence of powder flowability as a function of grinding aid formulation and applied to dry milling experiments of calcium carbonate. The results reveal that, analogous to wet operation, an optimum stress intensity exists at which the specific energy demand to achieve a target fineness is minimized. For dry milling, however, the stress intensity must be defined as the ratio of stress energy to the stressed product mass, since the captured product fraction which varies with powder properties is decisive for the stress intensity value. DEM analysis further indicates that the tip speed alone does not adequately represent the effective bead velocity distribution under dry conditions. The study demonstrates that both operating parameters and product formulation significantly affect the stress environment and comminution efficiency, implying that individual optimization strategies are required for each product system.
Mechanochemistry is considered a green alternative to conventional wet-chemistry. Several reactions have been reported to be feasible via the mechanochemical route, while detailed insight in the mechanistic background of mechanochemical reactions is still elusive. Certain is a significant role of the mechanical stress applied, but the influence of the stress type is rarely addressed. This study uses different simple setups to apply isolated impact,
compressive and shear stress to the reactants of the mechanochemical CaCO3-synthesis. Measuring of the energy inputs and modelling of the stressing conditions allowed correlation of the specific energies in the setups with the chemical conversion of the inorganic model reaction. For the same specific energy, impact stressing was found to be most successful in yielding product and the stress intensity could be identified to play a crucial role. A lower stress intensity was beneficial during initiation of the reaction, whereas a progressed reaction state could take advantage of higher stress intensities. This change is ascribed to the energy utilisation, which is limited in the
beginning, but rises in an advanced reaction state due to the formation of product layers and an increased local temperature. Based on the results, a hypothesis on the influencing factors and procedure of the mechanochemical
CaCO3-synthesis was formulated.
Mechanochemical methods enable sustainable and efficient chemical reactions. Mechanical stressing activates the reactants and triggers chemical transformations without the need for solvents. Previous investigations in simple setups using a model reaction (1) have shown that both the stress type and stress intensity have a significant influence on the reaction kinetics.
Na2CO3 + CaCl2 → CaCO3 + 2 NaCl (1)
These findings are now being transferred to grinding media mills, in which impact, compressive and shear stress occurs together. For the investigations, media mills with different movement patterns were selected (high-energy, planetary and mixer ball mill), whose grinding media introduce the mechanical energy into the reactants at different normal and tangential speeds. With the help of DEM simulations, the impact and shear influences can be analysed separately by quantification of the effective stress energies. Experiments in the respective mills allow a correlation of the stress conditions from the simulations with the experimentally determined reaction kinetics, whereby the stress type, number and intensity were in focus. It can be shown that the dominant stress mechanism varies as function of the mill and that a certain minimum stress intensity is beneficial for an efficient chemical conversion. During processing, significant caking of the powder on the grinding chamber and media was observed, which changes the amount of captured powder and the elasticity of the impacts. Both variables influence the stress intensity and, thus, potentially the reaction kinetics. By combining numerical and experimental data, new insights into the stress conditions of grinding media mills and their influence on the reaction kinetics of a mechanochemical model reaction were gained. PARTEC 2025 261
Punicines are phenolpyridinium compounds suitable for surface modification of lithium aluminate and other mineral particles due to their versatile chemical functionalization without losing switchability. They exist as cations in acidic solutions, neutral dipolar molecules around pH 7, and as anions or dianions in basic conditions. Light can reversibly convert punicines into radicals, enabling external control over their binding to particles and their surface properties via pH and light. For efficient separation of valuable mineral components from gangue materials, particles are ground to the micrometer range. This comminution creates new surfaces and induces chemical and structural changes and increases surface reactivity. Adding punicines during comminution ensures uniform surface binding, resulting in higher separation efficiencies compared to dry or wet mixing. Three separation mechanisms are considered: forced triboelectrification (FTC), selective wet agglomeration, and flotation. FTC involves applying high voltage to a conductive surface interacting with the material, neutralizing target material charge while others remain charged. This enables separation in an electrostatic separator. Selective wet agglomeration employs a suspension liquid, an oil-based binding liquid, and suspended particles. Hydrophobized particles form agglomerates via oil droplet adhesion. The influence of punicines on adhesion forces is studied using a FluidFM® system. Flotation separates valuable particles via selective adsorption of air bubbles. Punicines act as collectors, hydrophobizing particles and enhancing their affinity for air bubbles. A feasibility study with lithium aluminate demonstrated significantly higher yields when punicines were added during co-grinding.
The coal gangue (CG), a prominent industrial coal mining waste, for sustainable construction materials is a key focus of this research. We employed mechanical activation via a planetary ball mill to enhance coal gangue reactivity for geopolymer application. A systematic experimental design explored the influence of crucial rinding parameters: grinding media size dGM, rotational speed (rpm), and grinding time (tg). A primary objective was to optimize the specific grinding energy (Em), recognizing the substantial energy consumption associated with ball milling. The effectiveness of mechanical activation was rigorously evaluated by analysing changes in particle size distribution (PSD), stressing conditions (CF, SE and SI), phases analysis X-ray diffraction (XRD), morphological of powder by scanning electron microscopy (SEM), and chemical bonding by Fouriertransform infrared spectroscopy (FT-IR). Subsequently, the performance of the developed geopolymers was comprehensively assessed through visual observation, compressive strength measurements, and detailed analysis of reaction mechanisms. Our results demonstrate a significant improvement in geopolymer properties directly attributable to increased geometric specific surface area and reduced particle size of the mechanically activated coal gangue. This research elucidates a strong correlation between particle characteristics (size and geometric surface area), specific grinding energy Em and stressing intensity SI, and the overall mechanically activated coal
gangue, ultimately its efficacy in geopolymer applications.
As part of the so-called interfacial separation techniques, selective particle ag-
glomeration is one of the few options that are suitable for the separation of heterogeneous, multicomponent systems of particles smaller than 1 μm. In this regard, the component to be separated is exclusively transferred into a coarser size range, so that a material selective size separation by traditional mechanical methods can be achieved. In the presented study, this is demonstrated using heterogeneous suspensions of ceramic and organic particles, from which the separation of the inorganic material is pursued subsequent to the targeted control of the material-specific, electrostatic particle–particle interaction. Resulting from theoretical considerations on these interactions, favorable conditions for the selective agglomeration can be predicted. Experimental data reveal that for suitable param-
eters, resulting from variations in interfacial particle properties, particle size, and the composition of the dispersions, a separation efficiency of up to 97% can be obtained. Thereby, the importance of the particle-number fraction as an adjustable parameter needs to be clearly emphasized. Since a separation of the agglomerates can be achieved by simply using centrifugal forces, the shown technique is easy to apply and valuable for various industrial fields such as chemical and pharmaceutical engineering or recycling processes. In addition, no external additives are required for selective agglomeration, eliminating the risk of secondary contamination.
