Abstract
A more thorough understanding of the properties of bulk material structures in solid–liquid separation processes is essential to understand better and optimize industrially established processes, such as cake filtration, whose process outcome is mainly dependent on the properties of the bulk material structure. Here, changes of bulk properties like porosity and permeability can originate from local variations in particle size, especially for non-spherical particles. In this study, we mix self-similar fractions of crushed, irregularly shaped Al2O3 particles (20 to 90 µm and 55 to 300 µm) to bimodal distributions. These mixtures vary in volume fraction of fines (0, 20, 30, 40, 50, 60 and 100 vol.%). The self-similarity of both systems serves the improved parameter correlation in the case of multimodal distributed particle systems. We use nondestructive 3D X-ray microscopy to capture the filter cake microstructure directly after mechanical dewatering, whereby we give particular attention to packing structure and particle–particle relationships (porosity, coordination number, particle size and corresponding hydraulic isolated liquid areas). Our results reveal widely varying distributions of local porosity and particle contact points. An average coordination number (here 5.84 to 6.04) is no longer a sufficient measure to describe the significant bulk porosity variation (in our case, 40 and 49%). Therefore, the explanation of the correlation is provided on a discrete particle level. While individual particles < 90 µm had only two or three contacts, others > 100 µm took up to 25. Due to this higher local coordination number, the liquid load of corresponding particles (liquid volume/particle volume) after mechanical dewatering increases from 0.48 to 1.47.
Highlights
A detailed understanding of permeability and capillary effects has been of particular interest in fundamental and applied research of mechanical separation techniques (Iglauer et al 2013)
Due to the simple determination in a laboratory experiment, the bulk porosity often serves as a structural key parameter for describing porous media or filter cakes, respectively
These values are the only representative for the entire filter cake as the porosity is measured for the total bulk
Summary
A detailed understanding of permeability and capillary effects has been of particular interest in fundamental and applied research of mechanical separation techniques (Iglauer et al 2013). Cake-forming filtration represents the basic principle of many industrial applications where liquids are to be separated from solids. The filter medium allows the continuous phase to pass through almost unhindered, while at the beginning individual solid particles build up a rising structure on the filter cloth via blocking mechanisms, which increasingly retains the solids themselves. As the filter cake height increases, the continuous phase’s pressure drop increases, which is why the filter cake must be removed when a certain height is reached. This pressure drop, which determines the process throughput, is determined by the bulk material properties. In contrast to the pressure drop, permeability is often cited in this context, which unites the bulk material properties, such as porosity, as a macroscopic quantity
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