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Settling velocities of particulate systems 18: Solid ux density determination by
ultra-occulation
F. Concha a,, N.N. Rulyovb, J.S. Laskowski c
a Department of Metallurgical Engineering, University of Concepcin, Chileb Institute of Biocolloid Chemistry, National Academy of Sciences of Ukraine, Kiev, Ukrainec Department of Mining Engineering, University of British Columbia, Vancouver, Canada
a b s t r a c ta r t i c l e i n f o
Article history:Received 3 August 2011
Received in revised form 21 December 2011
Accepted 27 December 2011
Available online 5 January 2012
Keywords:
Thickening
Settling
Flocculation
Ultra-occulation
Flocculant dosage
The scale-up of thickening parameters from laboratory to industrial plant is still an open problem. Severaltechniques have been in use but none is free of criticism. In order to clarify these issue, in this work otation
tailings from one of the major Chilean copper mines was subjected to occulation-settling tests with Orioc-
2010 polycarylamide in a Couette type reactor. By varying the shear rate from 100 to 2000 [s1] the solid
concentration from 1 to 15 [% by volume] and the occulant dosage from 0 to 20 [g/ton] it was shown that an
important interaction exists between these variables. At the optimal occulant dosage, the optimal suspension
concentration and the optimal occulation time, an increase by 50% in the solid ux density function is possible
when the shear rate of _ 100 s1
is changed to the optimum value of around _400 s1
.
2012 Elsevier B.V. All rights reserved.
1. Introduction
In many countries water has become a scarce commodity. The
necessity of recovering and recycling water has turned thickening into
an important process, requiring the best operational practice. This
objective requires improvement in the control system that maximizes
thickener underow concentration, minimize occulant consumption
and, in general, permits the stabilization of the whole operation. Several
feedback control systems have been proposed for thickening opera-
tions, unfortunately, due to the non-linearity of the process and to the
slow response to perturbations, thickening is difcult to control and
classical control systems fail to operate adequately. Even more impor-
tantly, current methods of thickener control are based on feedback to
change operational variables that do not take into consideration the
properties of the feed material. Two such parameters are the solidux-densitythat expresses, for any concentration of a suspension, the
product of the settling velocity and of the solid concentration (and, for
a material of constant concentration, represents the momentum of the
motion), and the solid effective stress that represents the compressibility
of the sediment produced by settling. These parameters permit a
phenomenological description of the thickening process. Models
using these two parameters permit improved operation and optimal
control strategies. Determination of these parameters is still an openproblem (Brger et al., 1999). In this paper we will discuss the
determination of one of these parameters, the solid ux-density
function.
Several techniques have been proposed to determinethe settling ve-
locity in laboratory experiments, thejar tests being the most common
(Coe and Clevenger, 1916; Richardson and Zaki, 1954; Michael and
Bolgers, 1962). The Jar test involves homogenization of suspensions
varying solid concentrations in settling cylinders, introduction of the
occulant and mixing by moving a plunger up and down in the cylin-
ders, or by inverting the cylinders several times. This procedure is
claimed not to be satisfactory because of the local over-dosing that
can occur when the relatively concentrated occulant solution meets
the slurry (Kitchener, 1978); but more important is that the agitation
obtained by this method does not produce the optimum occulation.
Farrow and Swift (1996)show that jar test has three main inconve-
nient: (1) the occulation efciency, measured by the sedimentation
results, depend strongly on the mixing method (numbers of inver-
sions), (2) that the diameter of the cylinders have a signicant effect
on the settling rate, diminishing as the diameter of the cylinder
increases and (3) the experimental reproducibility associated with the
process is low, with standard errors of the mean from 8 to 12% for the
settling velocity.
Flocculation is used to aggregate ne particle to maximize the solid
ux-density, which is directly related to thickener capacity. Hogg et al.
(1993) showedthatthe choiceof a occulant is determined by chemical
factors such as the mineral composition and solution chemistry. But
International Journal of Mineral Processing 104-105 (2012) 5357
Corresponding author.
E-mail addresses:[email protected](F. Concha),[email protected](N.N. Rulyov),
[email protected](J.S. Laskowski).
0301-7516/$ see front matter 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.minpro.2011.12.007
Contents lists available at SciVerse ScienceDirect
International Journal of Mineral Processing
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / i j m i n p r o
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occulant adsorption and bridging occulation take place simulta-
neously and dynamic variables, such as the intensity of mixing and
the mixing time, affect particle-particle linkages (Keys and Hogg;,
1979; Hogg, 1999). Molecular weight of typical occulants fall in the
range of 1020 million Daltons. The transfer of the occulant from the
stock solution into thesuspension and distribution within the dispersed
systemtakes some time. An improved procedureto assessthe efciency
ofocculation is via the use of a shear vessel, which is similar to a rota-
tional Couette viscometers and has the advantage of quantifying the
mixing quality through the shear rate. Several investigators have used
the shear vessel in the past in coagulation experiments (Ives and
Bhole, 1977; Smith and Kitchener, 1978; Stein et al., 1986) and in oc-
culation studies (Muhle and Domasch, 1991; Farrow and Swift, 1996;
Rulyov, 1999, 2004; Rulyov et al., 2005a, 2005b; Rulyov et al., 2009).
Farrow and Swift (1996)designed their shear vessel with concen-
tric cylinders of 200 and 210 [mm] in diameter and 120 [mm] in
length (Fig. 1). At the bottom of the vessel a glass tube 14 mm indiameter and 220 [mm] in length was used to measure the settling
velocity. The experiments were carried out at a constant rotational
velocity of 200 [rpm]. The outow of the shear vessel was introduced
immediately in the settling column, depicted as A and B in the gure.
The authors conclude that the combination of shear vessel and set-
tling column overcame most of the problems associated with the jar
test, in particular the strong dependence of batch settling test on
mixing rate and cylinder diameter.
It was shown byRulyov (1999)and Rulyov et al. (2000) that the
mixing time in occulation can be reduced down from minutes to
56 s by the appropriate hydrodynamic treatment of the suspension
in a shear vessel at high shear rate _. This treatment, termed ultra-oc-
culation(Rulyov, 2004; Rulyov et al., 2005a, 2005b), ensures that, not
only occulant macro molecules are quickly and evenly distributed
within the suspension and adsorb onto the surface of the particles, but
also provides for the formation of large and dense ocs. Depending on
the size distribution and density of the solid particles in the dispersion,
as well as on their volume concentration, the optimum values of the
mean shear rate _ may vary in a wide range such as 300b _b5000
[s
1
]. The signi
cant advantage of ultra-
occulation is that it ensuresa good mix of small and large particles in ocs before they get into the
settler, thus providing for fast sedimentation and high degree superna-
tant clarication (Rulyov et al., 2009). It was recently shown that under
certain conditions intense agitation for short times may even change
the nature ofocculation, from total occulation to a selective occula-
tion of only some mineral constituents (Ding and Laskowski, 2007).
In this work an instrument called UltraocTester has been used,
which combines a shear vesselwith variable shear rateand an optoelec-
tronic device that measures the uctuation via intensity of the a light
beampassing normally through the transparent tube, while the formed
ocs pass through this tube, to (1) analyze the relationship ofoccula-
tion efciency (or mean ocs size) with solid concentration, occulant
dosage and shear rate, (2) the effect of the concentration and shear
rate on the settling velocity and solid ux density and (3) the effect of
the solid concentration on the optimum shear rate for occulation.
2. Material, experimental set-up and method
Flotation tailings from one of the major copper otation plants in
Chile were used in all the experiments. Solid concentration was varied
over the range from 1.8 to 15 [%] by volume] (4.7 and 32.3[%] by
weight); material density was 2700 [kg/m3]. An average particle size
of ,x50 20 m with size distribution characterized by x80 40 m
and x20 0:5 m , was determined using a Sympatec Helos-Rhodos
laser dispersion instrument. Orioc-2020 polyacrylamide was used as
a occulant with a molecular weight of 9.64106[g/mol].
The set-up to perform the ultra-occulation tests is shown inFigs. 2
and 3. It consists of a small shear vessel, referred to as ultra-occulator
inFig. 2. This Couette reactor, with a rotating cylinder of 28 [mm] indiameter and a gap of 1.5 [mm] was fed continuously with the suspen-
sion of tailings by a measuring peristaltic pump. Before entering the
Couette reactor the pulp receives continuouslya diluted occulant solu-
tion, at a ow-rate to give a pre-determined dosage. After 6 s treatment
at a pre-determined shear rate in the Couette reactor, the occulated
suspension is discharged from the ultra-occulator through a 3 [mm]
inner diameter transparent tube equipped with an opto-electronic
sensor which registers the uctuation of intensity of the light beam
passing normally through the tube [in accordance with techniques
proposed by Gregory and Nelson (1984)]. The electronic signal is
processed and displayed in a three digital format, thus showing, in
Fig. 1.Illustration ofFarrow and Swift shear vessel (1996) .
Fig. 2.Illustration of the UltraocTester layout.
54 F. Concha et al. / International Journal of Mineral Processing 104-105 (2012) 5357
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relative units, the values ofocculation efciency (or mean ocs size)
and the mean shear rate _.
The operational conditions consisted in changing theocculant feed
rate and the shear rate while maintaining a constant treatment time of
the suspension in the Couette reactor (6 s). When the feed suspension
concentration exceeded the threshold of 6[%by volume]of its optical
analysis capacity, it was diluted by introducing clean water between
the shear reactor and the optoelectronic sensor (shown by a dash line
inFig. 1). In the tests designed to measure settling rate of the treated
suspension, dilution was not used. In this case the suspension from
the outlet of the tester was continuously fed to a 14 mm diameter
settling cylinder and, as soon as the suspension lled the cylinder, it
was one time inverted and the suspension allowed settling and the
initial settling velocity was measured.
3. Results and discussion
Table 1shows the operational conditions of the experiments and
the output of the instrument.(In Table 1, vopt and v100 are the initial settling velocity of the
suspension after treatment, at optimal shear rate _optand at shear rate
equal to _100 s1
, fbk(opt) and fbk100are the corresponding solid-
ux densities.)
3.1. Effect of the occulant dose on the efciency ofocculation
The occulation was carried out over 6 s at optimal values of the
mean shear rate _. (for the respective suspension concentrations see
Table 1).Fig. 4demonstrates that the occulation efciency increases
monotonically with occulant dosage, reaching the relative value of
90 with a dosage of 10 [g/ton] for the low range of particle concentra-
tion, and 20 [g/ton] for the higher range. The observed increase in theocculant dosage with the increase in the suspension concentration
can most likely be attributed to the slowdown of theprocess of theoc-
culant macro molecules distribution within the volume of the suspen-
sion with the increase in solid concentration.
3.2. Effect of the shear rate on the efciency ofocculation
Fig. 5shows the effect of shear rate _on theocculation efciency
(or mean ocs size) and clearly demonstrate that a maximum exist
between 400 and 600 [s1] withhigher values for increasing concentra-
tion. The shift of the maximum occulation efciency to higher shear
values for higher occulant dosages may be due to the increased
strength of the bridges bonding particles within a oc, as shown by
Rulyov et al. (2005a, 2005b). (The operational principle of the ultra-occulator is based on the optoelectronic measuring system proposed and
described by J. Gregory and D.W. Nelson (J. Gregory and D.W. Nelson
A new method for occulation monitoring in Solidliquid Separation
(J. Gregory / Ed)Ellis Horwood, Chichester, 1984, pp.172182). The reading
of the device is proportional to the average size ofocs and, since these
readings are not in length units, they were called occulation efciency).
3.3. Effect of the shear rate on the settling velocity
Since the shear rate inuences the occulation efciency (or mean
ocs size) in the way expressed in the previous sections, one would
expect a similar inuence on the settling velocity. This was conrmed
as shown inFig. 6. The results provided in this gure indicate that the
optimum shear rate corresponding to the maximum occulation ef-
ciency also corresponds to the maximum initial settling velocity of the
occulated suspension, thus the ultra-occulation test is an effective
method for identication of the optimalocculation conditions.
3.4. Effect of the solid concentration on the optimal shear rate
It is important to establish the optimum solid concentration for
occulation in a commercial thickener. In the majority of industrial
thickeners occulation is perform in the feedwell where the feed is
diluted with up-coming and circulating water. Knowing the solid
concentration that gives the bestocculation should permit calculation
of the water dilution ow rate.
Fig. 7shows the effect of the suspension volume concentration onthe optimum shear rate for a given occulation. Interestingly, at a
certain solid concentration value, the dependence of optimal shear
rate on solid volume concentration has a minimum, which shows that
for much diluted and very concentrated suspensions higher shear rate
must be used. This relationship between shear rate and solids concen-
tration can be explained using Smoluchowski theory because, at a
given suspension concentration, the oc size increases to a maximum
within a short time interval. On the other hand, with the increase in
suspensionconcentration the distributionofocculant macromolecules
within the volume of suspension slows down. In particular, this is
conrmed by the increased consumption of the occulant with the
increased suspension concentration at a constant time interval.
However with increasing shear rate, due to convective diffusion, the
rate ofocculant molecules dissemination in the suspension signicantlyincreases, leading to the growth of the dependence of theoptimum shear
rate on concentration in the region of large concentration values. This
may also lead to some decrease in the required occulant dosage as
shown byRulyov et al. (2005a, 2005b).
Fig. 3. Photograph of the UltraocTester: (Model: UFT-TFS-029 Turbootservice
Company).
Table 1
General data.
Solid concentration
[g/l]
Concentration [%] solid
by volume
Settl. veloc. vopt/v100[mm/s]
Shear rate _
[s1]
Flocculant dose
[g/ton]
Csvopt 104
[g/cm2s]
fbkopt/fbk100104
[m/s]
50 1.8 20/14 600 10 1000 3.72/2.60
100 3.7 13.6/9.0 500 8 1360 5.03/3.51
200 7.4 2.26/1.50 350 16 452 1.67/1.11
300 11.1 0.50/0.24 300 10 150 0.51/0.27
405 15.0 0.15/0.07 600 20 60 0.22/0.10
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3.5. Effect of the shear rate on the ux-density function
The best way to show the effect of shear rate on sedimentation is
to plot theux-density function versus the suspension concentration
for several shear rates at the optimum occulant dosage. This is
plotted inFig. 8.
If one takes into account that occulation in a commercial thickener
is generally performed at shear values lower than _ 100 s1
, the
results of this work reveals an interesting improvement that could be
obtained by ultra-occulating the feed before entering the thickener.
3.6. Effect of the shear rate on the supernatant clarity
It is well established and known that in the process of ultra-
occulation treatment supernatant clarity is directly related to
occulation efciency, so in the context of the paper this information
has been regarded as redundant.
4. Application to industrial thickeners
Solidliquid separation of tailings in thickeners relies on the ef-
cient mixing of slurry and dilute occulant solution in the tube
conveying the pulp to the thickener or within the feedwell (Owen
et al., 1999). In both cases the mixing, measured in shear rates around
10 to 20[s1], is too low for efcient occulation. One of the major
resent improvements is thickener feedwell design, for example,
Outotec company on its webpage claims that in the Vane Feedwell
the upper zone, into which feed, dilution water and occulant are
added, provides enhanced mixing and energy dissipation. This maximizesocculant adsorption, eliminates the possibility of coarse-nes segrega-
tion and ensures all particles are aggregated together by the occulant.
Efcient operation is maintained in this upper zone over varying feed
rates. The lower zone promotes gentle mixing for continued aggregate
growth, with the option for secondaryocculant dosing. This zone also
enables aggregates to uniformly discharge under low shear conditions.
Unfortunately there is no indication of the shear rate in the feedwell.
Based on its Laboratory Ultra-Flocculator,Rulyov et al. (2009)de-
veloped industrial sized Ultra-occulators, such as the one depicted
in Fig. 9. Fig. 10 shows the occulation efciency versus the shear
rate of a 200 [m3/h] for the quartz suspension treated by this utra-
occulation.
5. Conclusions
Proper addition and mixing of a occulants with the pulp is one of
the essential steps in the occulation process. Optimal dosage ofoc-
culant and the correct suspension concentration are not sufcient to
yield a good occulation. The shear rate at which occulation is per-
formed may decide on the quality of the process. Up to recently it
has been believed that mild mixing for a long time was the preferred
mode ofocculation, but work byRulyov (2004)demonstrate that an
intense mixing for a short time gives a better performance.
0
20
40
60
80
100
120
0 5 10 15 20 25
Flocculant dose, g/ton
Flocculationefficiency,relativeunits
1.8%
3.7%
7.4%
11.0%
15.0%
Fig. 4. Flocculation efciency versus occulant dosage with solid concentration as
parameter.
0
20
40
60
80
100
0 400 800 1200 1600 2000
Shear rate, 1/s
Flocculationefficiencyrelativ
,units
1.8%/1 0 g/t
3.7%/1 0 g/t
15.0%/20 g/t
Fig. 5. Effectof shear rate onocculation efciency for concentrations from 1.8 to 15 [%]
solid by volume, with occulant doses between 10 and 20 [g/ton].
0.1
1
10
100
0 400 800 1200 1600 2000
Shear rate, 1/s
Initialse
ttlingvelocity,mm/s
1.8/ 8 g/ton
1.8/ 4 g/ton
3.7/16 g/ton
3.7/10 g/ton
11.0/ 6 g/ton
11.0/16 g/ton
Fig. 6.Initial settling velocity versus shear rate for suspension volume concentrations
from 1.8[%] to 11 [%] by volume and occulant dose from 4 to 16 [g/ton].
200
300
400
500
600
700
0 2 4 6 8 10 12 14 16
Solid volume concentration [%]
Optimalshearrate,
[1/s]
Fig. 7.Optimal average shear rate _versus suspension solid [%] volume concentration.
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In this work copper otation tailings froma major Chilean minewas
occulated with an ultra-occulator yielding the following result:
1. Flocculation at the optimal occulant dosage and optimal time
permitted an increase settling rate by 50% when changing from a
shear rate of _ 100 s1
to the optimum value of around_400 s1
depending on the suspension concentration.
2. The best occulation was obtained at the suspension volume
concentration in the range of 34 [% by volume] (75115 [g/l]).
3. An increase of the suspension concentration above the optimum
value, for example to 15 [% by volume] (400 [g/l]), increased the
necessary occulant dosage from 10 to about 20 [g/ton].
4. The suspension concentration and necessary mixing intensity are
interrelated. For example, an increase in the suspension concentra-
tion from 1.8 to 10[% byvolume] (50 to 280 [g/l])requiresa decrease
in the shear rates from 600 to 280 [s1] to maintain equal occula-
tion efciency, but with further increase in the suspension concen-
tration from 10 to 15 [% by volume] (280 to 400 [g/l]) the optimal
value of the shear rate increases from 280 to 600 [s1].5. The UltraocTester: UFT-TFS-029 used in this work allowed to
dene explicitly theoptimumocculationconditions for maximizing
theux density function for a given thickening operation.
Acknowledgment
This work was nanced by the AMIRA Project P996/CORFO Project
08 MC01-17. This nancial support is gratefully acknowledged.
References
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240.
-2.0
-1.0
0.00.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0
Solid volume concentration, %
Flux-den
sityfunction,m/h
Shear rate=opt
Shear rate=100
Fig. 8. Flux-density functions versus solid volume concentration [%] for shear rates of
100 [s1] and the optimum shear rate.
Fig. 9.Ultra-occulator used for the industrial occulation of coal otation tailings in
Ukraine and Russia;Rulyov et al. (2009).
0
20
40
60
80
100
0 2000 4000 6000 8000 10000 12000 14000
Shear rate,1/s
FloculationEfficiency,relativeunits
size