Chemical Engineering And Processing 47 (2008) 1509–1519

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Chemical Engineering and Processing 47 (2008) 1509–1519

Coagulation and flocculation of laterite suspensions with
low levels of aluminium chloride and polyacrylamids
D´esir´e Dihang a,b , Pierre Aimar b,∗ , Joseph Kayem a , Sylv`ere Ndi Koungou a,b
a

TEFI Unit-ENSAI/IUT, University of Ngaoundere, P.O. Box 455, Ngaoundere, Cameroon
b LGC-CNRS-UMR 5503 Universit´
e P. Sabatier Toulouse Cedex 9, France
Received 28 July 2006; received in revised form 4 July 2007; accepted 4 July 2007
Available online 18 July 2007

Abstract
Laterite particles in suspension undergo auto-flocculation as the concentration increases from 160 NTU, and therefore, coagulation and
flocculation properties are affected.
The critical coagulant concentration of laterite by aluminium chloride increases when the initial turbidity is less than 160 NTU, but decreases
with the initial turbidity for more turbid ones. The maximum concentration is fourth of the standards for potable water. In all cases, the critical Zeta
potential for coagulation equals ca. −20 mV. The Zeta potential appears to be a more relevant parameter to study coagulation than the turbidity of
the supernatant.
Flocculation either by non-ionic (PAM-N), cationic (PAM-C), or anionic (PAM-A) high molecular weight polyacrylamids promotes turbidity
reduction of pre-coagulated laterite suspensions. This turbidity reduction is independent on the amount of polymer added when the suspension is
coagulated at the CCC. In the other cases, turbidity reduction depends on polymer concentration. For suspensions of high turbidity, flocculation does
not improve significantly the efficiency as compared to coagulation. At low concentration, PAM-N and PAM-A do not significantly modify the Zeta
potential of the particles, enabling it to remain a relevant parameter to monitor destabilisation by combined coagulation and flocculation. Laterite
particles are very sensitive to the presence of PAM-C, which induces charge reversal even at very low concentration. The critical concentration for
flocculation is lower than 0.1 mg/L.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Laterite; Coagulation; Flocculation; Potable water

1. Introduction
Laterite is the major component found in raw water in most
tropical regions in Africa, and its removal represents the main
objective of the drinking water processes. Laterite is a clay material that confers to raw water a red colour and hazy aspect. It is
also known to be the main vector of arsenic contamination in
ground water in many regions of the world. Because of the lack
of knowledge on this clay and/or inappropriate process, it is common to find suspended particles in tap water, especially during
the rainy season. Rather than the distribution system (Lehtola et
al. [1]), investigations reveal the inefficiency of the clarification
process, where particle removal is achieved by decantation and

∗

Corresponding author at: LGC-CNRS-UMR 5503 Universit´e P. Sabatier
Toulouse Cedex 9, France. Tel.: +335 6 15 58 304; fax: +335 6 15 56 139.
E-mail address: (P. Aimar).
0255-2701/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.cep.2007.07.002

filtration through sand beds. Particle removal is promoted by
coagulation with aluminium chloride and by flocculation with
polymers. In these tropical regions, where water plants are rather
old, but where it is necessary to comply with turbidity standards
for obvious health issues, it is important to use as little coagulant as possible, in order to fulfil both standards and economical
requirements. Moreover, the known implication of aluminium
in Alzheimer disease makes this issue of using low coagulant level a worldwide problem. In drinkable water application,
aluminium salt concentration depends on the physicochemical
properties influencing salt precipitation. So, the salt concentration varies with the pH, ionic force, nature and concentration of
ionic species, amount of organic matter, temperature, etc.

In practical, hydroxide precipitation occurs above 40 ␮M aluminium added but OMS recommended less than 200 ␮g/l of
aluminium in treated “potable” water.
However, using such low coagulant dosage makes it very
difficult to locate the concentration giving maximum elimi-


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D. Dihang et al. / Chemical Engineering and Processing 47 (2008) 1509–1519

nation of the suspended particles, unless jar test experiments
are carried out almost continuously (Gregory and Duan [2],
Gregory [3]). The literature abounds with studies on the
coagulation–flocculation of clay material, but most of them
focus either on concentrated suspensions as generally encountered in mining processes, or on waste water process with high
content in organic matter. Contrarily, this work investigates
destabilisation by coagulation and flocculation of laterite suspensions diluted to over 10-fold the usual cases studied in the
literature and where electrokinetics aspects of the suspended
particles are important. It is aimed in filling the gap in the literature on the destabilization characteristics of laterite suspensions
with regards to the classical drinkable water process.
2. Materials and experimental methods
2.1. Materials
Raw laterite clay was obtained from the banks of the river
Bini in Cameroon. The sample was washed with ultra pure water
and dried. The coagulant, aluminium chloride, was prepared
daily as 1 g L−1 solution to avoid polymerisation in solution.
Graciously supplied by FLOERGER, the flocculants, a cationic
(FO8990 SEP), an anionic (AH912 SEP) and a non-ionic (FA920
SEP) high molecular weight polyacrylamids, were used for the
flocculation of the pre-coagulated laterite suspensions. They

were prepared as 0.2 g L−1 solutions in distilled water and
kept for a week at room temperature. Sodium hydroxide and
hydrochloric acid, used for pH adjustments, were prepared as
1% (w/w) solutions. Potassium chloride was used to set the ionic
strength.
Laterite suspensions were prepared in two steps: first, a
dispersion–hydration of the dried powder in distilled water and
second a dilution of the obtained suspension to the required turbidity. The laterite powder (50 g) is added to water (5 L) under
vigorous agitation (750 rpm) and the pH raised to 10. Sodium
azide is added for preservation (0.02%, w/w) and mixing is prolonged for 12 h. The suspension is then allowed to settle for 4 h
and the supernatant is withdrawn. It is kept at room temperature
and can be used for a week. Knowing that raw water turbidity
ranges from 22 NTU in the dry season to around 350 NTU during the rainy season, we selected five turbidities (30, 90, 150,
180 and 300 NTU) for our study. For this purpose, the supernatant is diluted to the desired turbidity. 10−3 M in KCl was
added raising the conductivity to around 150 ␮S/cm and the pH
was adjusted to 7. All reagents used were of analytical grade.
Laterite suspensions exhibit negligible organic matter, around
1 mg/L of TOC measured on a TOC Analyzer model VCSN
from Shimadzu.
2.2. Experimental methods
2.2.1. Characterisation of laterite
The particle distribution measurements were carried out
using a Malvern Mastersizer 2000 analyser. It also gives the
measure of specific surface area of the particles in square meter
per gram. The characterisation of surface structure of the lat-

Table 1
Mean hydrodynamic radius and the Zeta potential of the floculants
Floculant

Mean hydrodynamic radius (nm)

Zeta potential (mV)

PAM-N
PAM-A
PAM-C

63.7
74.6
371.0

−5.71 ± 5
−30.0 ± 5
+82.9 ± 5

erite was carried out using transmission electron microscopy
(TEM), scanning electron microscopy (SEM) and X-ray diffraction (XRD) of the laterite suspension.
Zeta potential indicating surface charge of the particles was
measured with a Malvern Zetasizer 4. The measurement was
carried out in various electrolytes, (KCl, NaCl and CaCl2 ) at
various concentrations and pH.
Turbidimetry, light scattering using a Turbiscan-On-Line
(Formulaction, France), particle counting (Malvern coulter
counter) and dry mass determination by desiccation were used
to characterise the properties of laterite suspensions.
2.2.2. Characterisation of the floculants
We could not obtain the molecular weights from the manufacturer. However, we measured the hydrodynamic radius so
as to have an idea of the molecular size, as shown in the table
(Table 1).

High charge density on PAM-C enable it to adopt a more
extended configuration in suspension while PAM-A and PAMN, with their low charge density exhibit lower repulsion between
charged polymer segment thus a less extended configuration.
2.2.3. Coagulation and flocculation study
Coagulation and flocculation experiments were carried out in
a classical jar test apparatus. The flocculants were added after
coagulation as it has proven to give better results than other
combinations (Hogg [4], Somasundaran [5]). After 5 min of
agitation at 230 rpm, the coagulant was added drop by drop
and left to coagulate particles for 5 min. The stirring speed
was then reduced to 30 rpm for 15 min to allow flocs growth
and the suspension was allowed to settle for 30 min. For flocculation of coagulated suspension, the flocculant was added
under rapid mixing before the flocs are left to grow. The supernatant was further taken out at a fixed 2 cm below the air–liquid
interface for turbidity, Zeta potential, and TOC measurements.
The turbidity measurements were performed on a Hach 2000N
turbidimeter and through light scattering measurements with
a Turbiscan-On-Line. The Zeta potential was measured on a
Malvern Zetasizer 4. All experiments were performed at room
temperature. The experimental procedure is summarised in
Fig. 1.
The performance of coagulation and flocculation processes
is assessed through the turbidity reduction (%) given by (Ozkan
and Yekeler [6]):
Turbidity reduction (%) =

Ti − Tf
× 100
Ti

(1.1)

where Ti (NTU) is the initial turbidity of the suspension and Tf
(NTU) is the supernatant turbidity. Amounts of coagulant and


D. Dihang et al. / Chemical Engineering and Processing 47 (2008) 1509–1519

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Fig. 1. Protocol for coagulation (a) and coagulation/flocculation (b) of laterite suspension.

flocculant are expressed both in weight ratio (mass per mass of
laterite), associated to molar concentration for the coagulant and
mass per volume concentration for the flocculants.
3. Results and discussion
3.1. Properties of laterite
The investigation of the composition of the laterite by
whole-rock analysis and of the main components, using SEM/
microprobe, TEM and XRD, reveals in Fig. 2 the presence of
platelets of clay material and clusters of iron oxides, corresponding to gibbsite, goethite and hematite titanium oxide. The clay
composition indicates the presence of high amount of kaolinite
and traces of smectite.
Youngue-Fouateu et al. [7]) reported similar results on different samples of laterite from Cameroon.
In Fig. 3, the Zeta potential of the laterite suspensions in
absence and presence of different salts is plotted as a function of
pH. In the presence of CaCl2 , the Zeta potential is constant and
independent of the pH. Contrarily, in the absence, as well as in
the presence of added KCl and NaCl, laterite suspensions show a
pH dependence of the Zeta potential. The isoelectric point (IEP)
is located around pH 3, the Zeta potential is positive below the

IEP and negative above. Similar Zeta potential is obtained in the
absence and in the presence of 10−3 M KCl. At pH 7 where the
destabilisation tests are performed, the Zeta potential is −35 mV.
Silica and aluminium oxides have a more prominent influence on
the Zeta potential, as indicated by the significantly more negative
values measured under alkaline conditions in contrast to acidic
one.
This behaviour is in good agreement with the data on kaolinite
and oxides, but the Zeta potential of laterite in the absence of
salt is greater in magnitude than the values reported for kaolin
(Besra et al. [8–10], McFarlane et al. [11]) and lower than the
values for iron oxides (McGuire et al. [12]).
The particle size analysis (Fig. 4) shows a distribution with
an average diameter d50 of approximately 0.25 ␮m, which is
smaller than the value obtained by Besra et al. [8]. The specific
surface area calculated from this diameter equals 26.7 m2 /g, and
it is very high compared to the result on kaolin (Besra et al.
[8]) but closer to the one obtained on iron oxides (McGuire et
al. [12]). It is smaller than the BET area reported on laterite by
Blakey and James ([13]). These results suggest that our mode of

preparation promotes the maximum of individualization of the
suspended particles and a complete elimination of the coarse
phase.
Dispersion properties of laterite suspension examined
through the measurement of the turbidity, particle number,
light scattered and concentration are given in Fig. 5. Graph
(a) presents two regions of linear properties that separate at
160 NTU. Above this turbidity, the slope is lower. The same
behaviour is obtained when plotting the turbidity versus the light

scattered or the number of particles. This suggests that for turbidity smaller than 160 NTU, the suspension behaves as dilute
while for higher turbidities, it behaves as concentrated.
In concentrated suspensions, particles influence each other,
and several authors suggest that for clay suspension, the positive charges from the edge face orientate towards the negative
charges from the lateral faces in an auto-flocculation reaction
(Blakey and James [13]). This observation is important because
it can have a critical impact on the destabilisation behaviour
of these types of suspension, both on the mechanism involved
and the destabilisation results. According to our results, this
mechanism would be relevant for turbidities higher than 160
NTU.
3.2. Coagulation of stable laterite suspension
Zeta potential (mV), turbidity reduction (%) are plotted as a
function of the coagulant concentration expressed in mass ratio
(mg of AlCl3 /g of laterite) and in mass per volume of suspension
(␮M of AlCl3 ) for different initial turbidities (Fig. 6). As a consequence of particles removal, the turbidity reduction increases
with coagulant concentration until it reaches a maximum value,
then it decreases with the addition of more coagulant. This is
often called restabilisation of the system by addition of an excess
of coagulant. For dilute suspensions, the mass per volume concentration at the critical coagulation concentration CCC increase
with the initial turbidity while for concentrated suspension, it
decreases. On the other hand, for both dilute and concentrated
suspensions, the corresponding coagulant mass ratio diminishes
as the initial turbidity increases. These results confirm the suggestion of an autocoagulation of particles as the suspension
turbidity increases. As a consequence, the coagulant mass ratio
at the CCC at 30 NTU is three folds that at 300 NTU. For both
cases, the residual turbidity remains high and not acceptable for
drinking purposes.

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D. Dihang et al. / Chemical Engineering and Processing 47 (2008) 1509–1519

Fig. 2. Scanning electron micrograph (SEM) (a), transmission electron micrograph (TEM) (b) and corresponding chemical composition (XRD) of global laterite
(c); platelet material (d); iron oxides clusters (e).
Elements

Carbon

Silica

Oxygen

Aluminium

Iron

Titanium

%

13.39

12.54

58.27

13

1.70

0.16

The Zeta potential increases with the amount of coagulant
until charge reversal, and stabilization around 30 mV. For dilute
suspensions, the point of charge reversal (PCR) corresponds to
restabilisation (30 and 90 NTU), while it is close to the optimum
turbidity reduction for higher turbidity (>150 NTU) suspensions
(Fig. 6). Turbidity reduction increases with the initial turbidity
for dilute suspensions, but it remains constant, however higher,

for concentrated suspensions. In all cases, the critical Zeta potential is constant and equal to ca. −20 mV.
In Fig. 7, the supernatant turbidity is given for various
decantation times.
The results show that, as decantation time increases, the
supernatant turbidity decreases and reaches the same maximum
value for 30 and 300 NTU suspensions. The curves also high-


D. Dihang et al. / Chemical Engineering and Processing 47 (2008) 1509–1519

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Fig. 3. Colloidal titration of a laterite suspension of initial turbidity 150 NTU.

lights that restabilisation of the suspension, due to excess of
coagulant disappears as decantation time increases. This suggests that what is called restabilisation would in fact be a kinetics
effect.
3.3. Discussion on coagulation

NMR spectra on the aluminium chloride solution used for
coagulation show that only the monomeric octahedral hexahydrate Al3+ (H2 O)6 is added to the laterite suspension. In all the
jar test experiments, this addition decreases the pH of the suspension to values between 5.5 and 6.5. In this region of pH,
there is an intensive formation of in situ Al13 polymers that
significantly improves the efficiency of AlCl3 . These polymers
coexist with lower amounts of monomeric and medium polymerised positively charged aluminium species (Chengzhi Hu et
al. [14]). Coagulation occurs by particle charge neutralisation
and depends closely on the quantity of added coagulant and
consequently on the Zeta potential of the suspended particles
(Lartiges et al. [15], Gregory [16], Wang et al. [17], Duan and
Gregory [18], Hu et al. [14]. At the CCC, we observed that the
Zeta potential is negative. This can be due to soluble silica that
lowers the Zeta potential of laterite clay, as it is more negative

Fig. 4. Particle size distribution of laterite suspension 90 NTU.

Fig. 5. Turbidity (NTU) vs. concentration (mg/L) (a) and transmission (%) (b)
of laterite suspension.

than hydrolysis products from aluminium at the same pH (Duan
and Gregory [19]).
These results suggest a great influence of kinetic aspects in
particle size growth due to collision. In fact, although the destabilisation process is effective at low initial turbidity, agitation is
unable to promote sufficient collision and particle growth, making sedimentation slow and leading to turbid supernatant. As
the initial turbidity increases (concentrated suspension), kinetic
limitations disappear and turbidity reduction becomes high and
constant.
The maximum turbidity removal is obtained at the critical
concentration of coagulation; it is around 60% for low turbidities
and 90% for high turbidities. When the settling time is increased,

the turbidity reduction is constant and greater than 95% and the
apparent restabilisation of the suspension by excess of coagulant
disappears. The point of charge reversal (PCR) corresponds to
the restabilisation of the system for low turbidity suspensions,
but it coincides with the optimum turbidity reduction for concentrated suspensions and long settling times. This PCR can
therefore not be used as a process control parameter. At CCC,
the value of the turbidity reduction varies, but the Zeta potential
is constant (−20 mV). Therefore, Zeta potential proves to be a
more relevant parameter to predict the coagulation of laterite suspensions than the turbidity of the supernatant or the PCR. Then,
using turbidity to control industrial plants can generate errors
as it is dependent on many parameters. It can be emphasised


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Fig. 6. Turbidity reduction after 30 min of decantation, TR30min (%) and Zeta potential vs. coagulant mass ratio (mg/g) and coagulant concentration (␮M) for laterite
suspensions of various turbidities: 30 NTU (a), 90 NTU (b), 150 NTU(c), 180 NTU (d) and 300 NTU (e).

that the maximum concentration of aluminium used, 40 ␮M,
is five-fold smaller than the standards allowed in processes for
drinking water while the minimum value, 9 ␮M is quite equivalent to the standards. Definitely, coagulation does not promote
sufficient particle removal for water to become potable. Table 2
summarises the coagulation characteristics of various laterite
suspensions.
3.4. Flocculation of coagulated laterite suspension
In Fig. 8, the turbidity reduction (%) for the flocculation
of pre-coagulated laterite suspensions of 30 NTU initial tur-

bidity is plotted as a function of the amount of non-ionic
PAM-N (a), cationic PAM-C (b), and anionic PAM-A (c) polyacrylamid polymers added. At CCC, all polymers promote
turbidity reduction, and there is no restabilisation of the suspension with excess of flocculant. This particle removal is
the maximum achievable, from around 50% for coagulation
to 90% for flocculation. In addition, PAM-N also induces turbidity reduction for salt concentration lower than CCC, and to
a lower extent, slightly above the CCC. PAM-C extends the
area of efficiency of PAM-N to coagulant concentration slightly
greater than CCC. Finally, PAM-A provides turbidity reduction at almost all concentrations, except at very low aluminium


D. Dihang et al. / Chemical Engineering and Processing 47 (2008) 1509–1519

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Fig. 7. Effect of settling duration on turbidity reduction for 30 NTU (a) and 300 NTU (b) laterite suspensions (turbidity reduction, TRX , where x is the time of
decantation).

dosage. Contrarily to PAM-N, PAM-C and PAM-A induce a
dependence of the turbidity reduction to polymer concentration.
For high turbidity laterite suspensions (Fig. 9), PAM-N
promotes turbidity reduction for almost all coagulant concentrations, from around 80% to nearly 100%, but has no effect in
the absence of coagulant.
The critical flocculation concentration (CFC) at CCC diminishes of at least 30-fold with suspension turbidity, from 5 mg/g
at 30 NTU to 0.15 mg/g at 300 NTU.
PAM-C and PAM-A are more effective than PAM-N as they
promote turbidity removal on a wider range of coagulant concentration. However, as PAM-C and PAM-A are not allowed
for drinking water processes, we present only flocculation with
PAM-N for laterite suspension of 300 NTU initial turbidity.
In Figs. 10 and 11, the Zeta potentials of the particles in the
supernatant of the previously flocculated systems are presented

as a function of the coagulant and flocculant concentration. For
initial high turbidity (300 NTU), all the Zeta potential curves
obtained with PAM-N are similar and aligned on the curve
obtained for coagulation (Fig. 10).
For the lowest turbidity suspension (30 NTU), Fig. 11 shows
that PAM-N and PAM-C tend to increase Zeta potential while
PAM-A decreases it. At low flocculant concentration (1.5 mg/g
of laterite), the Zeta potential curves for coagulation and flocculation are similar for PAM-N and PAM-A. Higher amounts
of PAM-N (≥5 mg/g) induce a rapid growth of Zeta potential at
low aluminium content, and just before the CCC the Zeta potential approaches a plateau value close to 5 mV. As a consequence

of this plateau value, the flocculation curve intercepts with the
coagulation curve at a point below the PCR. At this particular
point, the coagulated and flocculated particles have equal Zeta
potential.
PAM-C inverted the Zeta potential of the particles, indicating
that for the amount of polymer used, we are already overdosing
it. This behaviour confirms the high charge density of the PAMC, as indicated by the supplier. To compare the effect of the three
polymers on laterite particles, the turbidity reduction and Zeta
potential is plotted versus the flocculant dosage, for a laterite
suspension at 30 NTU, coagulated at the CCC (Fig. 12). The
results show that all polymers exhibit the same efficiency for
particle removal at CCC. This efficiency is independent of the
particle Zeta potential.
3.5. Discussion on flocculation
PAM-C adsorbs on the laterite particles surface via hydrogen bonding interactions between the silanol and aluminol OH
groups at the particle surface and polymer’s primary amide
functional groups. The electrostatic attractions between the positively charged polymer segments and the negatively charged
laterite particles promote adsorption and result in raising the particles Zeta potential. The amount of polymer adsorbed increases
with increasing polymer concentration, generating higher Zeta

potential. Similar results are reported for flocculation of kaolin
with PAM-C (Nasser and James [20]). High molecular weight
combined to segment repulsion enable particle bridging by the
adsorbed polymer.

Table 2
properties of coagulated laterite suspension
Initial turbidity (NTU)
AlCl3 (mg/g laterite)
CCC (␮M)
Zeta potential at CCC (mV)
Residual turbidity at CCC (NTU)
Turbidity reduction at CCC (%)
Coagulant mass ratio at the PCR (mg AlCl3 /g laterite)
Coagulant concentration at the PCR (␮M)
Turbidity reduction at the PCR (%)

30
89
9
−21
11
65
150
17
0

90
40
17

−21
26
71
59
24
5

150
38
27
−20
29
86
50
33
82

180
53
40
−21
20
90
60
52
86

300
23
33

−19
15
95
30
40
90


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D. Dihang et al. / Chemical Engineering and Processing 47 (2008) 1509–1519

Fig. 8. Turbidity reduction (%) vs. PAM-N (a), PAM-C (b) and PAM-A (c) concentration of coagulated laterite suspensions of 30 NTU initial turbidity.

Fig. 9. Turbidity reduction (%) vs. PAM-N concentration of coagulated laterite
suspension of 300 NTU initial turbidity.

Fig. 10. Zeta potential (mV) (%) vs. PAM-N concentration of coagulated laterite
suspensions of 300 NTU initial turbidity.


D. Dihang et al. / Chemical Engineering and Processing 47 (2008) 1509–1519

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Fig. 11. Zeta potential (mV) (%) vs. PAM-N (a), PAM-C (b) and PAM-A (c) concentration of coagulated laterite suspensions of 30 NTU initial turbidity.

Likewise, PAM-A adsorbed on laterite particles via hydrogen
bonding between the silanol and aluminol OH groups at the particle surface and polymer’s primary amide functional groups, but
the amount adsorbed is minimised by the electrostatic repulsion

between the particles negative charges and the negative polymer

segments. The polymer adsorbed results in a shift in position
of the plane of shear, hence generating a small decrease in the
magnitude of the Zeta potential (Nasser and James [20], Mpofu
et al. [21]). PAM-A can also adsorb on the positively charged
edges of the particles, creating extra negative charges that lower

Fig. 12. Turbidity reduction (%) and Zeta potential of flocculated laterite suspensions containing AlCl3 at the critical concentration for coagulation, 30 NTU initial
turbidity.


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D. Dihang et al. / Chemical Engineering and Processing 47 (2008) 1509–1519

the overall Zeta potential. Moreover, through adsorption on particle negative surface via a polycation (Al3+ , Ca2+ , etc.), PAM-A
can similarly lower the particle surface charges. The repulsive
forces between polymer segments allow the polymer molecules
to be extended and to produce loops and tails that promote bridging mechanisms and the formation of large open-structure flocs
(Gregory [3], McGuire et al. [12]).
For PAM-N concentration less than 5 mg/g, adsorption is
probably patchy due to a contracted conformation and to the
fact that particle surface coverage is less than the optimum. As a
consequence, there is a little change in the Zeta potential with the
amount of PAM-N added. This suggests that the adsorbed polymer layer thickness has a minor charge-shielding effect caused
by the shift in shear plane at which the Zeta potential is measured. Brooks ([22]) noticed that the Zeta potential of particles
should increase after adsorption of a neutral polymer, as long as
the shear plane is not shifted too far from the negative particle
surface, due to the change in the ion distribution in the diffuse

double layer. The adsorbed layer thickness of PAM-N on iron
oxides and kaolin plateaus at approximately 2.3 nm after the
addition of 3–5 mg/g of solid (Mpofu et al. [21], McGuire et al.
[12]). Further addition of flocculant can lead to chemisorptions
on particle surface (Besra et al. [10], Besra et al. [23]), that can
promote aggregation without having any effect on Zeta potential.
The results indicate a greater adsorption of PAM-N than PAMA, probably due to adsorption on iron oxides (McGuire et al.
[12]). Bridging mechanism is enhanced by the high suspension
concentration, which enables high molecular weight segments
of the PAM-N to bind towards many particles. As a consequence
of this high molecular weight, the amount of PAM-N used for
flocculation diminishes as the suspension turbidity increases.
4. Conclusion
This work consists in destabilising laterite suspensions by
coagulation and flocculation, as usually encountered in the
classical potable water process. The results pointed out the autocoagulation of laterite clay at turbidity greater than 160 NTU
and the influence on this phenomenon on destabilisation process. Another important result is the fact that coagulation can
be monitored through Zeta potential measurements, as turbidity
removal is always optimum at ca. −20 mV, this value being independent of the initial turbidity and other coagulation parameters.
For low polymer concentration, Zeta potential is also a relevant
parameter to study flocculation, as polymer adsorption does not
significantly modify the Zeta potential of the particles.
Coagulation happens by charge neutralisation, the positive
hydrolysis product of AlCl3 reducing the global negative surface
charge of the particles. Turbidity reduction increases with the
initial turbidity of the suspension.
Flocculation mechanism depends on the type and concentration of flocculant. All three polymers investigated here are
efficient in removing turbidity, and their efficiency is independent on flocculant type at the CCC, PAM-N being the only one
agreed for drinking water processes. At low flocculant concentration, the amount of flocculant adsorbed on the particles
surface is constant and independent on the degree of coagulation.

This amount is lower than the adsorption capacity of the particle
surface making charge neutralisation limited by the amount and
sign of charge carried by the polymer. Therefore, the magnitude
of the particle Zeta potential is governed by coagulant dosage.
At higher concentration, PAM-C induces charge reversal due
to excess charge adsorb at the surface. Contrarily, adding more
PAM-A or PAM-N has little effect on the Zeta potential, as
PAM-A exhibit little adsorption capacity on the negative surface
and PAM-N can chemisorbs on the particle surface, resulting in
negligible effect on the Zeta potential.
Acknowledgements
The authors wish to thank Formulaction (France) for
providing access to the Turbiscan on-line equipment, SNF FLOERGER, (France), for free samples of polyacrylamides and the
French Ministry of Cooperation and CNRS for financial support
to D.D. and S.N.K.
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Int. J. Miner. Process. 67 (2002) 123 – 144
www.elsevier.com/locate/ijminpro

The impact of polyacrylamide flocculant solution age
on flocculation performance
A.T. Owen1 , P.D. Fawell *, J.D. Swift1 , J.B. Farrow1
A.J. Parker Cooperative Research Centre for Hydrometallurgy (CSIRO Minerals),
PO Box 90, Bentley, WA 6982, Australia
Received 20 July 2001; received in revised form 25 February 2002; accepted 9 April 2002

Abstract
Aqueous solutions of high molecular weight polyacrylamides used to flocculate mineral slurries
undergo time-based changes in their properties. Previous studies of the impact of ageing on
flocculation performance have focused on time-scales of weeks or months, which has little relevance
to industrial practice. In this study, ageing times from 1 h to 6 days were examined. Flocculation was
achieved continuously in a Couette mixing device (stationary outer cylinder, rotating inner cylinder).
The extent of aggregation was assessed from batch settling tests and in situ size characterisation with
a focused beam reflectance measurement (FBRM) probe. The polyacrylamide dosages required to
achieve measurable flocculation decreased as the flocculant age was increased, with optimal
performance attained at 72 h. Flocculation using a 1-h-old flocculant solution consumed 75% more
polymer than with an optimally prepared stock solution. The relationship between hindered settling
rate and FBRM chord length measurements was found to be independent of flocculant age, but was
sensitive to shifts in aggregate density caused by variations in mixing intensity. It is proposed that the
early stages of flocculant dissolution involved the release of discrete polymer chains from highly
agglomerated species, the former dominating flocculation activity. Optimal ageing maximised the
discrete polymer concentration available for flocculation, leading to a significant increase in the
aggregate size distribution but did not appear to impact upon the aggregate packing structure
(density).
D 2002 Elsevier Science B.V. All rights reserved.
Keywords: flocculation; polyacrylamide; flocculant ageing; kaolin; aggregate size; settling rate

*

Corresponding author. Fax: +61-8-9334-8001.
E-mail addresses: (A.T. Owen), (P.D. Fawell),
(J.D. Swift), (J.B. Farrow).
1
Fax: + 61-8-9334-8001.
0301-7516/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved.

PII: S 0 3 0 1 - 7 5 1 6 ( 0 2 ) 0 0 0 3 5 - 2


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A.T. Owen et al. / Int. J. Miner. Process. 67 (2002) 123–144

1. Introduction
1.1. Flocculation in hydrometallurgy
The efficient solid –liquid separation of mineral suspensions is of critical importance to
most hydrometallurgical processing operations. A variety of reagents may be added to
slurries to enhance sedimentation by inducing aggregation. Salts of multivalent cations
that effectively reduce the surface charge of the solids are termed coagulants while high
molecular weight water-soluble polymers that are sufficiently large to bridge between
particles are termed flocculants.
The most frequently used flocculants are polymers derived from the acrylamide
monomer (Mortimer, 1991). The nonionic homopolymer (100% acrylamide) is an
effective flocculant; however, its activity can be enhanced by copolymerisation with other
monomers. This can introduce functional groups that have a high affinity for a particular
mineral phase (e.g. hydroxamate for the iron minerals within bauxite residue), or simply
provide charged groups that allow the polymer to take on an extended conformation in
solution (e.g. carboxylate or sulphonate).
The performance of a flocculant in any application (measured in terms of settling rate,
clarity, sediment volume or flocculant consumption) is decided by the complex interplay
between a number of factors, many of which have been reviewed in detail (La Mer and
Healy, 1963; Mortimer, 1991; Farrow and Swift, 1996b; Hocking et al., 1999; Hogg,
2000). These factors may include:


Slurry properties such as particle size, surface area, surface charge, solution

composition, pH and ionic strength.
 Physical properties of the flocculant, such as molecular weight, charge density and
functionality.
 Dynamic aspects of aggregate rupture and formation under applied shear.
The flocculation process is strongly influenced by the solution properties of the
polymer molecules. Flocculant adsorption and the bridging process depend not only upon
the functional interactions with the surface, but also the solution dimensions of the
polymer chain. The latter is determined by both the molecular weight of the polymer and
the conformation it takes in a particular electrolyte solution. The application of excessive
shear to long chain high molecular weight polymers during make-up or flocculation can
lead to chain rupture, substantially reducing the capacity for bridging (Abdel-Alim and
Hamielec, 1973; Nagashiro and Tsunoda, 1977; Nakano and Minoura, 1978; Henderson
and Wheatley, 1987; Scott et al., 1996).
1.2. Flocculant solution ageing
The change in polymer properties as a function of aqueous solution age is also an
important consideration. Although such changes may be far less dramatic than those
achieved as a consequence of shear degradation, they may still lead to significant
reductions in flocculation performance and increased flocculant dosages.


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125

The ageing of polyacrylamide solutions was first reported as a change in solution
viscosity over a period of weeks or months. Narkis and Rebhun (1966) explained this in
terms of a disentanglement of polyacrylamide molecules that were agglomerated during
the polymerisation process. They stated that the viscosity decrease on ‘‘ageing’’ was not
the result of molecular weight degradation, but rather a change in the polymer’s solution
configuration.

Shyluk and Stow (1969) observed that there was a rapid and a slow stage in the
decrease of viscosity of a polyacrylamide solution with time. In proposing a mechanism
involving both disentanglement and chemical degradation, they also reported for the first
time that ageing reduced the ability of the polymer to flocculate a kaolin suspension.
Studies into the instability of polyacrylamide solutions have either detected no ageing
(Ma¨chtle, 1982; Henderson and Wheatley, 1987) or attributed change to microbial attack
(Chmelir et al., 1980), radical attack from residual catalyst (Haas and MacDonald, 1972),
disentanglement (Gardner et al., 1978) or conformational changes (Klein and Westerkamp,
1981; Kulicke and Kniewske, 1981; Kulicke, 1986). The latter is generally accepted, with
the polymer initially taking an extended conformation and water attacking intra-polymer
hydrogen bonds, leading to a more stable and compact coil over time.
Almost all of the above studies have focused on long-term ageing over periods of up to
100 days. Changes over such periods are of no practical interest in hydrometallurgical
applications, where the residence time for a flocculant before dosing is rarely more than a
day, and in some instances, may be less than an hour.
Gardner et al. (1978) did include short ageing times when examining the behaviour of
four different polyacrylamide solutions. A rapid increase in reduced viscosity was initially
observed, reaching a maximum between 5 and 24 h, followed by a much slower decrease.
This strongly suggests that there may be two distinct processes occurring, the first
involving the dissolution of the powder polymer and disentanglement of the chains to
give a homogeneous solution, the second involving more subtle conformational changes.
Clearly, this first stage is of much greater relevance industrially, with this short-term
ageing required to ensure maximum activity. Despite this, little has been published on the
effect of this process on flocculation performance.
1.3. Flocculation performance testing
Hindered settling rates, measured from the rate of descent of a mudline, provide a
qualitative indication of the aggregate dimensions, and serve as a guide to the throughput
of gravity thickeners. They are normally obtained from cylinder tests, where dilute
polymer is dosed into a standard cylinder of slurry and mixed, either by inversion or
with plungers. While such tests are simple and convenient, they suffer from ill-defined

mixing. The poor mixing can leave polymer-rich and polymer-poor regions, resulting in
irreproducible flocculation (Farrow and Swift, 1996a). Batch flocculation measurements
are also strongly dependent on variations in solids concentration and sampling procedures.
Farrow and Swift (1996a,b) developed a new tool to continuously characterise
flocculation, which they termed the Shear Vessel. An inner-rotating cylinder generates
an annular (Couette) shear zone for reproducible mixing. Flocculant may be added at a
number of points to change the mixing time, while varying the rotation rate changes the


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agitation intensity. The Shear Vessel has been shown to provide highly reproducible results
and allows measurement of both hindered settling rate and turbidity through isolation of
the flocculated product in a settling tube. It flocculates continuously under controlled
agitation conditions, and is more appropriate for modelling the operation of a thickener
feedwell than cylinder testing.
Hecker et al. (1999) substantially increased the information obtained from the Shear
Vessel by inserting a focused beam reflectance measurement (FBRM) probe in-line. The
principle of FBRM has been described in detail previously (Williams et al., 1992; Fawell
et al., 1997). The FBRM probe produces a rotating laser beam highly focused at a point
near the probe window. When the moving beam intersects the path of a particle or
aggregate, some of the light is reflected back to a detector. As the tangential velocity of the
beam is known, the duration of the reflected light pulse is directly proportional to the
intersected chord. Thousands of reflected chords are measured each second, generating
chord length distributions between 1 and 1000 Am. Such in situ distributions not only
relate to the size of the aggregates formed, but also indicate the efficiency of flocculation.
1.4. Objective
This paper describes investigations into the effect of flocculant solution ageing on the

resultant flocculation performance with a standard substrate. Continuous flocculation was
achieved in a Shear Vessel fitted with an FBRM probe, using hindered settling rates and
chord length distributions as measures of performance. Flocculant solution ages ranged
from 1 h to several days, representing the time period of greatest relevance to most mineral
processing operations.

2. Experimental
2.1. Materials
Farrow et al. (2000) found that the particle size in kaolin slurries slowly decreased with
continual stirring, due to rupturing of pre-existing kaolin aggregates. Such changes in
effective particle size with time substantially alter the flocculation characteristics of the
slurry.
Kaolin (12.5 kg, density 2.68 g cm À 3, RF grade Commercial Minerals, Perth,
Australia) was soaked overnight in 60 l of deionised water. The particle size for such
slurries was highly irreproducible, with the d50 (as measured by laser diffraction) varying
from 7 to 15 Am. To reduce the extent of aggregation, the concentrated slurry was
recirculated through an in-line mixer (Sulzer SMV4DM25, Wohlen, Switzerland) by
means of a self-priming centrifugal pump. This was found to reduce the d50 to a fairly
constant value of 6 Am following recirculation times of over 3 h. For this study, standard
recirculation times of 5 h were used. The concentrated slurry was then pumped into a
stirred 375-l baffled tank and diluted to 45 g l À 1 solids.
The kaolin size achieved by this method was still greater than that expected for fully
dispersed primary particles (d50 f 2 Am). It was not practical to fully disperse the solids,


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127

and indeed some degree of natural aggregation is observed in all kaolin-based mineral

tailings. While every effort was made to achieve a reproducible preparation procedure, the
large volumes of slurry required meant that slight variations in size between batches were
often unavoidable. Such variations could result in small shifts in the required flocculant
dosages. It was therefore necessary to ensure that all direct comparisons of activity for
aged flocculants were performed with the same slurry batch.
A commercial high molecular weight nonionic polyacrylamide, well known in mineral
processing applications, was used in this study. No evidence of acrylate moieties could be
detected by FTIR, 13C NMR or elemental analysis. The weight-average molecular mass
Mw of this polymer as measured by multi-angle laser light scattering (MALLS) was
20 Â 106. Polymer (1 g) was ‘‘wet’’ with absolute ethanol (2 g) and gently shaken by hand
for 30 s and then let stand for 2 min. Deionised water was then added to give a 0.50-wt.%
stock solution. This solution was shaken vigorously by hand for 2 –3 min to ensure the
powder was properly dispersed and then mixed gently on a tabletop shaker at 145 rpm for
the entirety of the ageing experiments. Working solutions (0.005 wt.%) were prepared at
the pre-determined ageing times by diluting the stock solution in water. The ageing times
studied were 1, 3, 5, 24, 48, 72 and 144 h.
2.2. Viscosity measurements
Kinematic viscosities were determined using an Ubbelohde-type capillary viscometer
#0C-C96 with a calibration constant of 0.002882 cSt s À 1 at 40 jC (Canon Instrument,
USA). 0.04 wt.% flocculant solutions were used, with all testing carried out at 40 jC. Each
diluted solution was passed through a 1.2-Am syringe filter before being transferred into
the viscometer.
2.3. Focused beam reflectance measurement
The particle and aggregate dimensions of suspensions at different stages of flocculation
were examined in situ by FBRM. All such measurements were carried out with an M500
field unit fitted with a laboratory probe (LasentecR, Redmond, WA, USA). The probe had
a 12-mm-diameter flat window located at the base of the probe body (318 mm long, 25
mm diameter). The probe body joins to a larger casing that contains the electric motor for
the scanning drive, which is connected to the controlling electronics in the field unit by a
10-m fibre optic cable. In all experiments, the focal point of the laser was set to be at the

surface of the probe window.
Using version 6.0 acquisition software, scans (10 s duration) were continuously
measured over the full range of chord lengths (1– 1000 Am), split into 90 channels in a
logarithmic progression. The unweighted chord length distribution is a number-sensitive
distribution and provides the best indication of the presence of fines. From this distribution,
the total counts and mean chord length may be calculated. Selected chord length ranges
may also be monitored to provide an indication of trends in the fine and coarse regions.
Applying a length square-weighting (i.e. volume weighting) to the chord length
distribution accentuates the larger particles within the slurry. The mean square-weighted
chord length was also calculated.


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2.4. Shear Vessel flocculation assessment
2.4.1. Equipment
The Shear Vessel (Fig. 1) consists of an external fixed cylinder (I.D. 110 mm) and an
internal rotating cylinder (O.D. 100 mm) driven by a variable speed motor. The annular
gap width is 5 mm. The inner cylinder is constructed from stainless steel, while for this
work, an acrylic outer cylinder was used (for work at high temperatures, a stainless steel
outer cylinder is used). The conical section at the base of the cylinders has a pitch of 45j,
maintaining the gap of 5 mm.
Flocculant inlets (Ports 1 –4) are positioned on one side of the outer cylinder, while
the slurry inlet port is positioned on the opposite side. Introducing the flocculant through
different ports alters the ‘‘residence time’’ of the flocculant/feed contact, that is, the
duration that the flocculant is mixed with the suspension (this correlates in a general
sense with the residence time of the feed suspension within a feedwell). A column
is fitted to the base of the cylinders to allow the through-flow of the suspension, but

it is isolated (through the closure of two valves) for analysis of settling rates and
turbidity.

Fig. 1. The Shear Vessel system for continuous flocculation.


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129

2.4.2. Procedure
Slurry was pumped from the feed tank to the Shear Vessel using a peristaltic pump
(‘‘Feed slurry pump’’, MasterflexR 7550-62 L/S: Tygon tubing size 16) at a constant
delivery rate of 200 ml min À 1. Dilute flocculant delivery was via a second pump
(‘‘Flocculant pump’’, MasterflexR 7550-92 L/S: tubing size 14) through the flocculant
ports at rates between 6.9 and 12 ml min À 1. The dosages obtainable through such a
delivery were from 38 to 70 g of flocculant per tonne (g t À 1) of (dry) slurry. The slurry
passed through the vessel and the settling tube, with an underflow peristaltic pump located
after the analysis column ensuring a controlled flow rate, set at 95% of the total incoming
flows. This allowed for f 5% of the flow to report to a levelling outlet, thus maintaining a
constant volume of slurry within the vessel.
The agitation conditions experienced by the suspension in the annular gap are
controlled by the rotation of the inner cylinder, the agitation intensity being directly
proportional to the rotation speed. The Shear Vessel was used at a range of rotation speeds
from 75 to 400 rpm.
Hindered settling rates were determined by measuring the rate of fall of the mudline of
a sample isolated in the Shear Vessel’s analysis column. The residual turbidity (in units of
NTU) of the isolated sample after set time periods was measured with an Analite
nephelometer. The FBRM probe was inserted at an angle of 70j to the flow, allowing
material to be suitably presented to the probe window but preventing any solid build-up.

The flocculation characteristics of the 45 g l À 1 kaolin suspension were assessed with
the Shear Vessel over a range of rotation speeds (i.e. the agitation intensity used to mix the
flocculant into the feed suspension). In all cases, flocculant was added through Port 2.
2.5. Aggregate density measurements
Aggregate density is one of the most important properties to be determined in the
evaluation of flocculation performance (Ayyala et al., 1995). While direct measurement of
density is not possible, it can be estimated from sedimentation and size data, using a
modified version of the Stokes equation for ellipsoidal bodies (Happel and Brenner, 1973).
The floc density analyser (FDA) is a device developed for determining microscopic
properties of individual aggregates, such as size, shape, settling velocity and density,
without removing them from their process liquor. Details of the procedure have been
described previously by Farrow and Warren (1993). A video camera, coupled to selected
high magnification optical lenses, is used to record the sedimentation characteristics of
individual aggregates as they settle under the influence of gravity within a thermostated
analysis cell. To ensure that the aggregates achieve free settling within the cell, dilution of
the flocculated suspension by at least a factor of 10 is required. The recorded images are
then played back to allow the measurement of size and settling rate of the individual
aggregates with custom-designed image analysis software. The maximum horizontal and
vertical dimensions were recorded, with the third dimension of the ellipsoidal aggregates
assumed to be on average the same as the horizontal dimension. The diameter of the
sphere was then determined which had an equivalent Stokes settling velocity to the
ellipsoid with these three dimensions. A statistically representative number of aggregates
(usually 200– 300) are measured, covering a broad size range.


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Kaolin was flocculated in the Shear Vessel at 100 and 200 rpm with flocculant aged for

72 h at dosages that gave bulk settling rates of 1.7 m h À 1. This low settling rate was
selected to give a wide range of aggregate sizes while preventing too many large ( > 400
Am) aggregates forming (the fast settling rates of such large species cause them to impact
upon smaller aggregates, interfering with free settling). The flocculated slurry was sampled
for FDA measurements from below the analysis column and before the underflow pump
(Fig. 1).

3. Results and discussion
3.1. Viscosity
Kinematic viscosity measurements were conducted at 40 jC using 1.2 Am filtered
flocculant solutions that had been aged from 1 to 144 h (Fig. 2). After 1 h, the viscosity of
the solution was very close to that of water (0.7414 cSt compared to 0.6529 cSt for water).
At this age, the amount of polymer dissolved is small and does not alter the viscosity to a
great degree, while any undissolved polymer is filtered out to prevent blocking of the
capillary.
At 3 h, there is a rapid increase in the viscosity followed by a sharp decrease to 5 h. At 3
h ageing, the dissolved polymer is expected to be highly tangled producing ‘‘agglomerated
polymer’’; as the agglomerates are disentangled, more discrete polymer chains are
observed leading to a decrease in the viscosity. A further slight decrease in viscosity is
observed up to 7 h after which only minimal changes in viscosity occur up to the last
measurement time of 144 h. After 7 h, it is most likely that only subtle conformational
changes of individual polymer chains occur which do not affect the overall solution
viscosity but may very well have a large effect on flocculant activity.

Fig. 2. Kinematic viscosity at 40 jC as a function of age for flocculant stock solutions (diluted to 0.04 wt.% and
1.2 Am filtered before measurement).


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131

3.2. Activity measurements
3.2.1. Effect of agitation intensity
In addition to the viscosity measurements shown in Fig. 2, the activity of each aged
flocculant stock solution was assessed with a 45 g l À 1 kaolin suspension in the Shear
Vessel. Each stock solution was diluted to an appropriate level before testing. To examine
the impact of agitation intensity, the applied dosages were chosen to ensure that
measurable aggregation could be detected at settings below 300 rpm, without the onset
of over-flocculation at 100 rpm. In most instances, the selected dosages produced similar
settling rates at 200 rpm.
From Fig. 3, it can be seen that the applied agitation had a major effect on the extent
of aggregation achieved. Under mild mixing (100 rpm), large, voluminous aggregates
were readily formed and fast settling rates were measured. As the agitation intensity
increased, the measured settling rates decreased sharply, indicating a substantial
reduction in the size of the aggregates achieved. The maximum aggregate size that
can survive under the applied shear conditions is expected to diminish at higher agitation
intensities. While it is possible that some degree of densification may also occur for
aggregates formed under stronger mixing, this cannot be ascertained from the settling
data alone.
The settling rate diminished rapidly up to an agitation intensity of 200 rpm. At this
point, the settling rate was low, typically in the range 2– 3 m h À 1, but still in excess of that
for the unflocculated suspension ( f 0.3 m h À 1). Any further increase in agitation
intensity led to only a minor additional deterioration in flocculation performance.

Fig. 3. Effect of Shear Vessel agitation intensity and flocculant stock solution age on hindered settling rates for the
flocculation of kaolin (45 g l À 1).


The distinct change in behaviour between 100 and 200 rpm is considered indicative of a

low aggregate strength. Farrow and Swift (1996b) found that the flocculation performance
profile observed as a function of agitation intensity may vary for different flocculant products, with the aggregates formed by some polymers able to tolerate more intense mixing.
A clearer picture of the impact of agitation intensity may be obtained from examination
of the FBRM chord length distributions from flocculation at both 100 and 200 rpm for the
same dosages applied in Fig. 3. Flocculation led to a significant drop in the total number of
unweighted counts, with the formation of large aggregates indicated by a higher fraction of
longer chords (Fig. 4). While such aggregates contribute far fewer counts to the overall
distribution than their unflocculated primary particles, they represent the dominant fraction
of the slurry volume. Fig. 4a shows that at 100 rpm, the flocculated distribution was
typically broad and very low in counts. Large aggregates were readily formed; however,
the presence of bimodal character in the distributions, with a distinct fraction < 10 Am,
suggests that the capture of fines during flocculation may not be fully efficient.
At 200 rpm, the total counts were higher, a consequence of smaller aggregates being
formed (Fig. 4b). However, the proportion of counts < 10 Am was less than that at 100
rpm, which may be indicative of more efficient fines capture under the stronger mixing,
perhaps due to more efficient mixing of the flocculant with the slurry.
3.2.2. Effect of stock solution age
This general behaviour described in Section 3.2.1 was observed across the range of


A.T. Owen et al. / Int. J. Miner. Process. 67 (2002) 123–144

133

Fig. 4. Effect of flocculant stock solution age on FBRM unweighted chord length distributions for the flocculation
of kaolin (45 g l À 1) at Shear Vessel agitation intensities of (a) 100 rpm and (b) 200 rpm (dosages as given in Fig. 3).

h were identical to that obtained for 72 h and are not presented in Fig. 4 for simplicity). At
144 h, the increase in counts relative to the 72-h-old stock solution was evidence of a
reduction in flocculant activity.

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Bimodal character was observed at 100 rpm for flocculant ages of 5 h and over, but not
for the 1- and 3-h-old solutions (Fig. 4a). It may be that at the faster settling rates
measured for the former solutions, sedimentation was not fully hindered, that is, the
settling network was less able to capture and ‘‘drag down’’ residual fine, unflocculated
particles. This was confirmed by examining the equivalent distributions acquired at lower
flocculant dosages, for which bimodal character was far less evident (shown in Fig. 6a for
the 3-h-old stock).
3.2.3. Effect of dosage
A series of experiments was conducted to establish the full dosage response for each
flocculant age at agitation intensities of 100 and 200 rpm. In view of the large volume of
kaolin slurry and time required for Shear Vessel testing, it was not possible to examine
dosage effects concurrently with the agitation studies described in the previous section. A
separate set of aged flocculants was therefore examined, using another batch of kaolin
slurry. Slightly higher dosages were required for this slurry, reflecting a reduced degree of
natural aggregation (Section 2.1), but otherwise, the general trends were unchanged.
Fig. 5a confirms that fast settling rates may be achieved at 100 rpm for any flocculant
age if a sufficiently high flocculant dosage is applied. The slopes of the dosage-response
curves were similar, but shifted to higher dosages for flocculant ages below 24 h. For
example, 75% and 25% more flocculant were required to achieve a settling rate of
15 m h À 1 for 1- and 3-h-old solutions relative to solutions aged for 24 h or more.
Even under the more intense mixing at 200 rpm, fast settling rates could be achieved at
the expense of higher flocculant dosages (Fig. 5b). In the case of the 3-h-old solution, the
dosage required for a settling rate of 10 m h À 1 increased by 40%.
It can be seen from Fig. 5 that the addition of flocculant has effectively no impact on the

settling rate up to dosages in excess of 30 g t À 1. Above this point, the settling rates
typically rise sharply over a narrow range of dosages. A better understanding of this
behaviour is gained from examining the equivalent chord length distributions, shown in
Fig. 6 for a 3-h stock solution at 100 rpm. At a dosage of 36 g t À 1, the measured settling
rate was only 1 m h À 1, but Fig. 6a shows that substantial aggregation had actually taken
place, with the total counts dropping from f 35 000 to 16 000. This suggests that the
initial stages of flocculation may progress through a fines capture process, forming small,
slow settling aggregates. Once fines capture has achieved an equilibrium level, further
addition of flocculant may lead to ‘‘cluster’’ flocculation of such aggregates into larger
bodies (Farrow and Warren, 1993). As the external surface area of these aggregates is low
relative to their volume, the formation of cluster aggregates with much higher settling rates
may only require a small increase in flocculant dosage.
The application of a length square-weighting to the FBRM chord length distribution
effectively represents a volume-based weighting, emphasising the contribution of the
larger chords, which in this system are associated with aggregation. Fig. 6b shows the
effect of flocculant dosage on the square-weighted chord length distribution, with
higher dosages leading to increases in aggregate size. While the aggregate size at a
dosage of 36 g t À 1 was significantly larger than that of the unflocculated slurry, the
volumetric contribution of aggregates at dosages in excess of 50 g t À 1 was clearly
much greater.


A.T. Owen et al. / Int. J. Miner. Process. 67 (2002) 123–144

135

Fig. 5. Effect of flocculant dosage and stock solution age on hindered settling rate for kaolin (45 g l À 1)
flocculated at Shear Vessel agitation intensities of (a) 100 rpm and (b) 200 rpm.

136

A.T. Owen et al. / Int. J. Miner. Process. 67 (2002) 123–144

Fig. 6. Effect of flocculant dosage (3-h-old stock solution) on (a) unweighted and (b) length square-weighted
chord length distributions for the flocculation of kaolin (45 g l À 1) at a Shear Vessel agitation intensity of 100 rpm.