Prediction of micro processes, filter cake build-up and porous media flow is a key challenge to describe macroscopic parameters like filter cake resistance. This is based on a precise description, not only of the disperse solid fraction, but the distributed properties of the voids between the particles. Lab-experiments are carried outwith alumina and limestone,which differ in particle size distribution (PSD) and resulting filter cake structure. Filter cakes of bothmaterials are characterized by standardized lab tests and additionally, alumina cakes aremeasured with X-ray microscopy (XRM). Focusing on distributed process key parameters, the data gives a deeper understanding of the laboratory experiments. The solid volume fraction inside the feed strongly influences the particle sedimentation and leads typically to a top layer formation of fine particles in the final filter cake,which has a negative influence on subsequent process steps. The top layers seal the filter cake for washing liquid and increase the capillary entry pressure for gas differential pressure de-watering. The influence on cake structure can be seen in a change of porosity, particle size and shape distribution over the height of the filter cake. In all measurements, homogenous filter cake structures could only be achieved by increasing the solid volume fraction inside the suspension above a certain percentage, at which particle size related sedimentation effects could be neglected and only zone sedimentation occurred. XRM offers the chance to quantify these effects, which previously could only be described qualitatively.

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.

In recent years, non-destructive X-ray microscopy (XRM) has become a common method to characterize particle systems in various scientific fields: Besides the size and shape of particles in bulk powders, the insight into filter cake structures provides additional information about micro processes during filtration and dewatering. Distributed particle properties mainly influence the porous network build-up with possible local deviation in vertical and horizontal alignment. This article focusses on the model-based correlation between the distributed particle properties and characteristic network parameters like tortuosity, pore radii and preferred capillaries for dewatering, using tomography data as model input. Therefore, cake-forming filtration experiments were carried out with a down-scaled, self-constructed in-situ pressure nutsch. The entire tomographic dataset consists of seven individual scans at certain desaturation steps at different pressure levels. For the experiments, a lognormal distributed particle system (crushed Al2O3) in the range of 55 to 200 μm inside an aqueous suspension was used, containing additives for contrast enhancement. Image data processing based on reconstructed 360° projections allows the identification of the background, solid particles and liquid phase by a two-step segmentation. The subsequent modelling uses experimentally verified particle size distributions from laser diffraction measurements (integral value), 2D- (limited number of particles) as well as tomographic analysis, based on calculated single-particle volumes given by the voxel-dataset (all particles within the scanned volume). To characterize the porous network, a developed tetrahedron model is first applied to follow the shortest way through the porous matrix, then again to calculate the widest capillary related to the pore entrance. Furthermore, with information about the pore throat distribution and the wetting line from the tetrahedron side faces, the force balance is evaluated. This results in an entrance pressure distribution, the capillary pressure curve. Experimental data according to VDI 2762 built filter cakes and mercury intrusion tests are taken as reference for validation.

The paper takes up the old ideas of describing porous media with several tube and network models. The wellknown models received from literature gave a good concept of dewatering equilibria resulting in capillary pressure curves and pore size distributions (PoSD). However, numerical methods and measurement techniques were not sophisticated allowing to evaluate the models appropriately. In this work, a numerical method based on statistics is introduced to validate the network model of FATT from 1956: The porous filter cake structure is implemented as a matrix, which elements represent the pore size correlating with the capillary entrance pressure for each pore. The input for the calculations can be any mathematical approximation of a PoSD, which can be derived from capillary pressure tests or micro computer tomography (μCT) analysis of the filter cake. A procedure based on the concept of FATT is presented to generate dewatering equilibria for different applied pressures. Therefore, the elements of the matrix are checked to be ‘dewatered’ regarding to their size, position, the applied pressure level and the progress of dewatering. The network model known from literature is improved by implementing additional conditions for the description of physical phenomena, such as the formation of residual bridge liquid or hydrodynamic isolated areas. X-ray microscopy, mercury intrusion tests and laboratory desaturation experiments by using semipermeable membranes for capillary pressure tests are used to validate the pore size distribution. The different results are integrated into the matrix model as starting parameters. For the laboratory experiments, the PoSD is calculated from the measured capillary pressure curve, using the distributed tube model and the YOUNG-LAPLACE-equation on an equal basis to the established mercury intrusion analysis. However, with the tomography measurements, it is possible to determine PoSD using different defined geometry elements fitting inside the pore space. The force balance is evaluated at the pore entrance by using the wetting line of the pore throat. The direct measurement of the void geometry allows the calculation of the pressure distribution without the LAPLACIAN assumptions. In this way, the difference between experimental, measured and modelled PoSDs is emphasised to validate the old (and improved) ideas of network models describing porous media

The wetting behavior of remaining isolated liquid bridges between particle interfaces determines the efficiency of filter cake dewatering. Micro-processes during and after dewatering can be traced by means of direct X-ray microtomography (ZEISS Xradia 510 Versa) providing insights into the filter cake structure. We measure the local contact angle between the immiscible phases on the pore scale after in-situ filter cake dewatering. By tracing the three-phase contact line and the two perpendicular vectors belonging to the solid and liquid surface, the contact angle is obtained from their scalar product at every mesh-node. The range of the resulting distribution and curvature increases with the degree of roughness, becoming more obvious for larger contact angles. The occurring roughness causes a naturally water-repellent surface and leads to low liquid saturations. The resulting angular distribution serves for a more accurate prediction of multiphase flow in pore networks as input for further pore model enhancement.

Data sets for a seismic P-wave velocity model from a 3D VSP survey at the Alpine Fault DFDP-2 drill site in the Southern Alps in New Zealand containing the used first-arrival travel times and the final P-wave velocity model.

Supplement Texts S1, S2, and S3 Figures S1, S2, S3, and S4 Tables T1 and T2

Tables 1 to 3 giving Raman data of annealed zircons

Contents of this File Supporting information S1 Tables S1 to S3 Figure S1 to Figure S4 Dataset S1 with Tables S4 to Table S6