(25c) Wet Classification of Fine Particles Using Crossflow Filtration
Due to the increasing use of particulate products with a fine and narrow particle size distribution, processes are needed to generate such particle systems. Particles below 10 µm are used for pigments, polishing compounds or pharmaceutics. Especially for these products, it is necessary that not a single coarse particle occurs. Conventional production processes like precipitation, crystallization or comminution of coarse particles cannot guarantee that all particles are smaller than a few µm or have a narrow particle size distribution at a small size. Additionally, conventional wet classification processes like hydro-sieves and centrifuges cannot separate fine particles in highly concentrated suspensions with a high selectivity. Disadvantages of these processes include the suspension concentration limit and high energy consumption.
In this work, the crossflow classification is investigated as a method to classify particles in a particle size range down to 1 µm. Crossflow classification utilizes the principles of the crossflow filtration. A suspension flows tangentially across the surface of a membrane. The pore size of the membrane is smaller than the particle size. Due to a higher pressure on the concentrate side, pure liquid permeates through the membrane (permeate flux). Meier et al.  suggest a new method of wet classification with the crossflow filtration. They recognized a selective deposition of fine particles. Altmann and Ripperger  developed a model for the particle deposition, which is based on a hydrodynamic model. The permeate flux creates a drag force on the suspended particles which transports them towards the membrane surface. The velocity gradient in the boundary layer of the crossflow creates a lift force on the particles, which transports them back into the bulk flow. In the present work, the model of the lift force by Rubin  is used. Both the drag and lift forces depend on the particle size. By regulating the crossflow rate and permeate flux, the separation size can be adjusted. The particles smaller than the separation size are deposited on the membrane, while the larger particles remain in the bulk flow. After rinsing of the system with a pure liquid, the particles deposited on the membrane can be resuspended by backwashing and discharged from the system. To find an optimal operating point, the influences of velocity and concentration were investigated.
- Experimental setup
For the initial experiments, a lab-scale system. containing a pipe module with tubular membranes, which have a good resistance against backflushing, was constructed. The polypropylene (PP) membranes used have a nominal pore size of 0.2 µm. The effective membrane area was adjusted with different modules between 0.008 and 0.086 m2. An automatic regulated pump produced a defined constant overflow velocity. By modulating the transmembrane pressure, the flux through the membrane was adjusted. To prevent decreasing flux caused by a particle layer on the surface, the transmembrane pressure was increased.
A study of crossflow mean velocity on the separation size and selectivity was performed. The overflow velocity was varied between 0.1 and 2.5 m/s. The models for the lift force were adjusted for non-spherical particles and the influence of particle concentration and validated by experiments with different particle systems at multiple concentrations. The resuspension and discharge of the deposited fine particles was optimized. Factors affecting the stability of the resuspended particles after the classification such as particle charge were investigated.
The experiments demonstrate successful classification of particles smaller than 5 µm. The influence of the pH on the stability of the suspension is shown. By adjusting the pH, resuspension of agglomerates into primary particles can be achieved.
 J. Meier, G.-M. Klein, Volker Kottke, Crossflow filtration as a new method of wet classification of ultrafine particles, Separation and Purification Technology 26 (2002) pp.43-50
 J. Altmann, S. Ripperger, Particle deposition and layer formation at the crossflow microfiltration, Journal of Membrane Sciense 124 (1997) 119-128
 G. Rubin, Widerstands- und Auftriebsbeiwerte von ruhenden, kugelförmigen Partikeln in stationären, wandnahen laminaren Grenzschichten, University of Karlsruhe, 1977