Sedimentation and Consolidation behaviour of Suspensions in Centrifugal
Fields.
D.Lerche
LUM GmbH, Rudower Chaussee 29, 12489 Berlin, Germany
(E-mail: info@lum-gmbh.de)
Abstract
This paper focuses on separation phenomena of suspensions in centrifugal fields. After a short review of the
current knowledge regarding physical simulations of sedimentation and consolidation phenomena we
describe experimental results obtained by in-situ analysis of concentration changes during centrifugation.
Concentration changes in the supernatant, evolution of sediment height and packing density of the sediment
were determined by STEP-technology employing x-ray and/or NIR attenuation. Experimental results are
summarized for monodisperse, polydisperse and interacting (flocculating) dispersed particulates. We focus
ourselves on the dependence on the volume concentration and the state of flocculation. Finally, we propose
a procedure, which allows classifying the state of flocculation of dispersion without any dilution by analysing
the separation kinetics due to a stepwise increase of the centrifugal force (rpm).
Keywords Analytical centrifugation, sedimentation velocity, hindrance function, sediment packing,
agglomeration, flocculation
INTRODUCTION
Dispersions are widely used in research, industry, agriculture, food, pharma and everyday life. Despite the
pioneering work of Kynch in 1952 and numerous theoretical papers, the sedimentation and consolidation of
concentrated suspensions especially in the case of flocculation and agglomeration is not yet
comprehensively understood. New challenges rose with decreased particle size as for nanodispersions. In
this paper we are going to focus on separation phenomena of suspensions in centrifugal fields, but
conclusions may be of interest too for separation by filtration.
Current knowledge regarding physical simulations of sedimentation and consolidation phenomena of noninteracting particles are well described by the group of Bürger [1, 2] or Ungarish [3] and experimentally
validated e.g. by Detloff [4]. The separation and consolidation behaviour of flocculated dispersions is far
more complex.
We investigated the separation behaviour of monodisperse, polydisperse and interacting (flocculating)
dispersed particulates in the dependence on the volume concentration and the state of flocculation by in-situ
analysis of particle concentration alterations over the entire sample height during centrifugation. Based on
these results, we propose a procedure to classify the state of flocculation of a suspension without any
dilution analysing the separation behaviour due to a stepwise increase of the centrifugal force (rpm).
METHODS
Aqueous silica, quartz and lime stone suspensions at different volume concentrations were used in this study.
Flocculation state was tuned by pH, flocculants or solvent type.
Separation and consolidation were analysed by a multisample (n = 12) analytical photocentrifuge, obtaining
in situ highly resolved photometric concentration profiles over the entire sample height, i.e., from the cell
bottom up to the meniscus, in short time intervals (STEP-Technology, LUMiFuge/LUMiSizer, LUM GmbH,
Germany). A NIR-light flash of a few microseconds illuminates each sample cell instantaneously and sensors
with a micrometer scale spatial resolution depict the transmitted light in virtually every position along the
sample cell. The progression of these profiles with time quantifies in a direct way the separation due to
centrifugation. Concentration changes in the supernatant, evolution of sediment height and packing density
of the sediment were determined by the Software SEPView 6.0. Results based on light transmission were
partially validated by x-ray attenuation.
Rectangular cells of an optical path of 2mm or 10mm were used. Two types of experiments were done:
centrifugation at a constant rpm (centrifugal excess pressure) or increasing the rpm stepwise.
RESULTS
The main results are summarized on the fig. below.
super
112
de-
[mm]
110
position
top
108
indi-
4.6 % quartz
4 % limestone pH 9
5 % limestone pH 12
106
Figure 1:
Position of the interface
supernatant/dispersion
in
dependence on centrifugattime. RPM was ramped (see
centrifugal acceleration and
excess pressure indicated.
Cell bottom (black line at the
top at 113.5 mm. Arrows
indicate the sediment height.
Quartz: non-interacting
Limestone:
pH = 9, weakly flocculated
pH = 12, strongly flocculated
104
102
11 g
32 g
140 g
200
600
2700
200N/m²
390 g
1100 g
7000
20000
140 g
11 g
2700 N/mm²
100
0
1000
2000
time
3000
4000
[s]
In general, sedimentation velocity of non-interacting particles increases during the centrifugation and form a
sediment of high sediment density. At lower volume concentration (< about 15%) sediment formation ends
abruptly as predicted by theory. Above a critical initial volume concentration sediment formation slows down
before reaching its equilibrium height. Increasing the centrifugal force after the sediment formation comes to
its end at a given rpm exhibits only in a slight compression of the sediment. Polydisperse dispersions show,
in general, a higher sediment packing than monodisperse ones and the packing density depends on the
initial volume concentration.
Separation behaviour of interacting particles differs distinctly. Firstly, initial sedimentation velocity is higher,
but exhibits a pronounced decrease after a very short centrifugation time. It takes several times longer to
reach the final, equilibrium sediment height and the packing concentration of the sediment is always lower
compared to non-interacting particles at equal initial volume concentrations. Increasing the centrifugal force
after reaching the equilibrium sediment height (ramping rpm) is marked by a pronounced reduction of the
sediment packing concentration. X-ray studies reveal that especially at lower centrifugal forces a significant
packing density gradient exists, which reduces increasing the excess pressure.
Based on these results a standard operational procedure was proposed to classify flocculated and nonflocculated dispersions at original concentrations [5]. It contrast to the zeta-potential, this procedure monitors
in a direct way the flocculation state.
REFERENCES
1. Berres, S., Bürger, R., Karlsen, K.H., Tory, E.M.: Strongly degenerate parabolic - hyperbolic systems
modelling polydisperse sedimentation with compression. SIAM J.
Appl. Math. 64, 41-80 (2003)
2. Berres, S., Bürger, R.: On gravity and centrifugal settling of polydisperse suspensions forming
compressible sediments. Int. J. Solids Struct. 40, 4965-4987 (2003)
3. Ungarish, M.: Hydrodynamics of Suspensions. Springer, Berlin (1993)
4. Detloff, T., Lerche, D.: Centrifugal separation in tube and disc geometries: experiments and theoretical
models, Acta Mech 201, 83-94 (2008)
5. Lerche, D.: Sedimentation and consolidation of concentrated dispersions: Determination of the state of
flocculation without dilution. In: Proceedings WCPT,6 Nuremberg, HH 1000436.pdf (2010)