12 - 13 October 2006 - Compiègne - France
In-situ investigation of separation instabilities during sedimentation in the centrifugal field
Titus Sobisch, Dietmar Lerche, Torsten Detloff, Raúl Montalvo, Nelson Quintas
LUM GmbH, Rudower Chaussee 29 (OWZ) 12489 Berlin info@lum-gmbh.de, www.lum-gmbh.com
Abstract:
Only in some selected cases classification and clarification processes in thickeners and centrifuges can be described fairly well based on Stokes law. Normally, accelerating and hindering effects have to be accounted for. Furthermore, in real systems other irregular phenomena have been observed as sedimenting clusters, stratification of dispersions and the formation of an interlayer between supernatant and sedimenting particle collective. The latter is caused by the preferential floatation of fines and light particles due to liquid displaced by sedimenting particles. The demixing of dispersions containing lose agglomerates is even more complex.
These phenomena lead to an undesirable remixing and a loss of separation efficiency.
Instability phenomena during separation in a centrifugal field were characterized in-situ using multisample analytical centrifugation with photometric detection. In more detail instability phenomena in carbon black dispersions are investigated as function of centrifugal acceleration, temperature, viscosity and density of the dispersion media and of surfactant concentration during preparation.
separation instabilities, analytical centrifugation, carbon black dispersions, asymmetric aggregates
Results
As a general feature it was noticed that the more instable the dispersion or the faster separation occurs (due to centrifugal acceleration, preparation conditions, viscosity or density difference) the earlier the appearance of back folding.
A possible explanation for the back folding is a change in orientation of asymmetric aggregates. At high particle concentration asymmetric particle aggregates are supposed to have a preferential orientation ‘broadsideon’, which is the orientation leading to minimal light attenuation. After reaching a critical lower concentration larger aggregates are settled out and small asymmetric aggregates will be able to rotate resulting in an increased light attenuation.
A close link between time of appearance of back folding and quality of carbon black dispersions exists. Namely, the more stable the dispersion the less pronounced and later the back folding effect. Varying the surfactant concentration during dispersion preparation a considerable termination of the effect was observed when increasing the surfactant concentration from 3 to 5 %. However, a further increase in surfactant concentration did not change the quality of dispersion.
Titus Sobisch, Dietmar Lerche, Torsten Detloff
LUM GmbH, Rudower Chaussee 29 (OWZ) 12489 Berlin info@lum-gmbh.de, www.lum-gmbh.com
Abstract:
Unit processes using centrifugal fields are often applied for separation and classification of fine grained materials. Modelling of these processes requires information about the separation behaviour of the suspensions to be processed. To this end direct measurements in centrifuges are obligatory for the dimensioning of the separation units to obtain the desired cut-off size and residual effluent turbidity. Moreover, these measurements have to gather kinetic information as function of the processing conditions, particle interaction and particle concentration. Results of measurements at earth gravity cannot be scaled up to reflect the situation in a centrifugal field.
A multisample analytical centrifugation technique with photometric detection is presented which allows a comprehensive characterization of separation processes in the centrifugal field with high sample throughput. In particular the type of sedimentation behaviour (free sedimentation of individual particles, hindered settling, zone settling) can be deduced from the progression of the transmission profiles. Even more it can determine the critical residence time as function of feed concentration for a desired effluent clarity and allows measuring the velocity distribution of particles inside the centrate in-situ.
The photometric detection enables to determine the velocity distribution of dispersions down to very low feed concentrations. This is especially important for clarification processes.
Colloidal stable quartz suspensions revealed a broad velocity distribution as expected.
fine particle slurries, analytical centrifugation, sedimentation, velocity distributions, clarification
Conclusions
Multisample analytical centrifugation with photometric detection allows a comprehensive characterization of separation processes in the centrifugal field with high sample throughput. In particular the type of sedimentation behaviour (free sedimentation of individual particles, hindered settling, zone settling) can be deduced from the progression of the transmission profiles. Even more it can determine the critical residence time as function of feed concentration for a desired effluent clarity and can measure the velocity distribution of particles inside the centrate in-situ.
The separation behaviour of quartz suspensions as function of concentration of solids and centrifugal acceleration was characterized and the velocity distributions inside the centrate were determined time-dependently (i.e. the composition after different times of centrifugation) and as function of location (i.e. the composition at different distances from the centre of rotation). Both informations are essential for design of separation processes (residence time, weir height).
The photometric detection enables to determine the velocity distribution of dispersions down to very low feed concentrations. This is especially important for clarification processes.
Colloidal stable quartz suspensions revealed a broad velocity distribution as expected.
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