February 27 - March 1, 2007
Wiesbaden Germany
paper
Characterization of interparticle forces for solid-liquid separation processes using multisample analytical centrifugation
T. Sobisch, D. Lerche
Dispersion Stability & Particle Analysis, L.U.M. GmbH
Rudower Chaussee 29 (OWZ), 12489
info@lum-gmbh.de, www.lum-gmbh.com
ABSTRACT
The present work reports on the use of multisample analytical centrifugation for investigation of the kinetics and polydispersity of the sedimentation process as well as of the packing and compression behaviour. The method was applied for two different systems of practical importance, namely, polydisperse fine particle suspensions with varying degree of interparticle attraction and for pigment dispersions modified by milling.
The shape and evolution of these extinction profiles allow conclusions about the nature of the sedimentation process (free sedimentation of individual particles, hindered settling, zone sedimentation). Strong repulsive particle interactions lead to a polydisperse sedimentation behaviour, i.e. particles move with different sedimentation velocities according to their particle size. In case of strong attractive particle interactions all particles are moving with identical velocity. If only weak attractive forces act between the particles a sedimentation front of the main fraction of particles evolves, however, a part of the fine fraction still exhibits polydisperse sedimentation behaviour.
Packing density, obtained after compression, is related to the total interparticle potential energy. The packing behaviour further is directly related to the dewatering (deliquoring) behaviour of the sediment. For colloidal stable dispersions the equilibrium packing density is reached almost instantaneously after applying the centrifugal force and a virtually incompressible sediment is formed. For polydisperse suspensions segregation will occur especially at lower initial volume fractions. Increasing the degree of interparticle forces a pronounced compressional behaviour was experienced, packing density becomes very sensitive to changes in centrifugal pressure. However, for strongly flocculated particle dispersions even at high centrifugal pressures only moderate deliquoring can be achieved.
The sedimentation of pigment dispersions investigated is characterized by zone sedimentation. Packing density increases markedly as deagglomeration proceeds during wet milling. Sedimentation analysis allows for direct measurement of the dispersion degree without dilution.
KEYWORDS
Physicochemical aspects, centrifugation, particle interactions and characterization, solid-liquid separation, clarification, classification, dewatering
Introduction
The behaviour of dispersions in liquid media, i.e. dispersion stability, flow, separation and packing behaviour, is determined by the nature and degree of interparticle forces. This is of fundamental importance for their application in diverse fields but also for solid-liquid separation processes.
For characterization of dispersions techniques are preferable, which avoid dilution, thus don’t modify dispersion properties. To this end analytical centrifugation proved as an efficient tool [1-5]. Both, sedimentation kinetics and packing behaviour cover information on particle porosity [6], particle interaction and particle aggregation [7-11]. Fig. 1 shows a schematic overview of the relations between the nature of particle interactions and the sedimentation/packing behaviour.
Fig.1 Schematic overview of relations between the nature of particle interactions and sedimentation/packing behaviour
If strongly repulsive interparticle forces dominate, in dilute systems particles are fully dispersed and sediment individually (colloidal stable dispersions). As the centrifugal acceleration increases with the distance from the centre of rotation sedimentation speeds up until the final sediment height is reached and dense packing is obtained. In case of weakly attractive interactions particles are weakly flocculated. As long as individual flocs sediment the shape of sedimentation curves is similar to that of colloidal stable dispersions. However after formation of a space filling network sedimentation gradually slows down until the centrifugal pressure cannot compress the dense network further (consolidation). Strongly attractive interactions cause strong flocculation. Large flocs sediment very fast and form loose irregular particle networks, which show high resistance against consolidation. The final sediment height is reached only slowly.
The present work reports on the use of multisample analytical centrifugation for investigation of the kinetics and polydispersity of the sedimentation process as well as of the packing and compression behaviour. The method was applied for two different systems of practical importance, namely, polydisperse fine particle suspensions with varying degree of interparticle attraction and for pigment dispersions modified by milling.
paper
Comparison of particle separation in centrifuges with cylindrical tubes or disc rotors .
Experiments and theoretical models
Dispersion Stability & Particle Analysis, L.U.M. GmbH
Rudower Chaussee 31, 12489
info@lum-gmbh.de, www.lum-gmbh.com
ABSTRACT
Separation and purification of nano- and microparticles becomes essential for technical applications in manifold industrial fields such as biotechnology, drug development, coating and pigment to name only a few. Despite numerous papers since the pioneering work of Kynch, the separation process of concentrated dispersions driven by gravity or a centrifugal field, are not completely understood yet, especially, if particles interact with each other. In the centrifugal case additional difficulties emerge from the Coriolis coupling between the radial and the azimuthal velocity and the geometry of the "containers". Batch centrifuges operate either with containers of constant cross section (CCS geometry) or with disc/cylindrical rotors, where the cross sectional area changes with the distance from the axis of rotation (CSAC geometry). In this paper we shall focus on the sedimentation in these different cell geometries. Experimentally the separation process of dispersions of monodisperse silica and polydisperse quartz at different volume concentrations was investigated by analytical centrifugation (LUMiSizer®). Space and time resolved extinction profiles (STEP-Technology) during the centrifugation were measured using cells with CCS or CSAC geometries. In general, the sediment height in the CSAC cells is smaller compared to CCS ones, but the alteration of the concentration in the vicinity along the radius is similar for both cell types. Experiments were compared with model calculations based on the kinematic Kynch model advanced by Buerger. It will be shown that from results of tube centrifugation (CCS cells) the sedimentation behavior in a disk or cylinder centrifuge (CSAC cells) may be well predicted. This allows to use analytical tube centrifugation to optimize the separation and purification processes of nano- and microparticles by industrial cylinder.
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