A new method for determining the optimum dispersant concentration

Prévia do material em texto

Ž .Powder Technology 123 2002 199–207
www.elsevier.comrlocaterpowtec
A new method for determining the optimum dispersant concentration in
aqueous grinding
R. Greenwood a,), N. Rowson a, S. Kingman b, G. Brown c
a School of Chemical Engineering, UniÕersity of Birmingham, Edgbaston, Birmingham B15 2TT, UK
b School of Chemical EnÕironmental and Mining Engineering, UniÕersity of Nottingham, UniÕersity Park, Nottingham, UK
c G. Brown Associates, Matlock Bath, Derbyshire, UK
Received 26 January 2001; received in revised form 11 June 2001; accepted 30 August 2001
Abstract
The use of electroacoustics as a method for determining the optimum dispersant dosage for the ultrafine grinding of limestone is
presented. The technique measures the zeta potential of concentrated suspensions, hence it was possible to study the adsorption of three
commercially available polyelectrolyte dispersants onto the limestone. The optimum dosage of the dispersants was determined and the
most suitable one chosen for the grinding experiments. By utilising this optimum dispersant dosage, the productivity and throughput of a
stirred vertical mill was greatly enhanced. q 2002 Elsevier Science B.V. All rights reserved.
Keywords: Electroacoustics; Calcium carbonate; Ultrafine grinding; Zeta potential; Dispersants
1. Introduction
Expenditure of energy in the commination of ultrafine
materials is prodigious, but its utilisation is very poor.
Therefore, any chemical additive that improves the process
efficiency is of utmost interest. A class of such materials
or ‘grinding aids’ is water-soluble surface-active polymers.
When used in small amounts, these chemicals can greatly
improve the efficiency of grinding mills by their effect on
the slurry rheology. The importance of the slurry viscosity
on the size reduction process has been widely reported,
w xe.g. by Fuerstenau et al. 1 . The rheology of concentrated
suspensions is a matter of considerable complexity, espe-
cially in the case of wet grinding as the rheological
properties of the suspension vary considerably with grind-
ing time. The main parameters that effect the viscosity of a
Ž . Žsuspension h are the solids concentration volumesuspension
.fraction, f , particle size, size distribution and the nature
of the interparticle forces. In a system of monodisperse
spherical particles, the increase in viscosity with solids
) Corresponding author. Tel.: q44-121-414-7234; fax: q44-121-414-
5324.
Ž .E-mail address: r.w.greenwood@bham.ac.uk R. Greenwood .
loading is generally accepted to be given by the Krieger
w xDougherty 2 equation:
w xy h fmf
h sh 1y , 1Ž .suspension medium
fm
where f is the maximum packing fraction obtainable,m
which for monodisperse spheres is equal to random close
w xpacking, i.e. 0.64, h is the intrinsic viscosity which for
spherical particles is 2.5, and h , which in this case ismedium
the viscosity of water. In other words, addition of particles
to a suspension increases the viscosity until at the maxi-
mum packing fraction the viscosity tends to infinity.
However, the state of dispersion of the particles is also
important and this depends on the nature of the interparti-
cle forces. Fine particles of -1 mm in size are attracted to
one another by van der Waals attractive forces to form
aggregates or flocs. In rheological terms, this means that a
concentrated suspension will demonstrate a high viscosity
due to the presence of the flocculated particles. However,
by addition of a small amount of a water-soluble surface-
active polymer, it is possible to change the nature of the
interparticle forces to be entirely repulsive, hence the fine
particles will not flocculate, but remain as discrete entities.
Hence, the viscosity will be lower than a flocculated
system at the same solids loading. The particle size distri-
bution is also an important factor as this determines the
0032-5910r02r$ - see front matter q 2002 Elsevier Science B.V. All rights reserved.
Ž .PII: S0032-5910 01 00457-0
( )R. Greenwood et al.rPowder Technology 123 2002 199–207200
maximum solids loading of a suspension. For mono-
disperse suspensions the maximum theoretical packing is a
volume fraction of 0.64. However, if smaller particles are
introduced to pack in between the gaps left by the larger
particles, then it is possible to increase the solids loading
of the suspension. If these gaps are then filled by smaller
particles again then the theoretical maximum packing frac-
tion can be extended again. Fortunately, most industrial
ores and powders are not monodisperse, therefore, high
solids loadings are possible.
In wet grinding the suspension viscosity is constantly
changing, as the aggregates are broken down altering the
Žparticle size distribution and hence the maximum packing
.fraction . The viscosity of the suspension is also changing
due the increased presence of fine colloidal particles,
which, if no surface-active polymer is present at the cor-
rect dosage, then flocs will form. So a meaningful knowl-
edge of grinding based on rheology alone is not feasible.
w xFuerstenau et al. 1 tried to circumvent this problem by
measuring the mill torque continuously and relating the
specific energy input with the extent of grinding.
From the above discussion and previous researcher’s
work, the grinding throughput can be increased greatly by
the use of dispersants. Of the numerous dispersants com-
mercially available, it can be difficult to select the most
appropriate at the correct dosage. Insufficient dispersant is
a problem as some particles will be flocculated and will
raise the viscosity; whilst an excess of dispersant is an
unnecessary extravagance and can cause destabilisation.
Previously optimum dosages of dispersant have been mea-
sured by sedimentation tests, adsorption isotherms, rheo-
logical experiments or electrophoresis. The first two are
extremely time consuming, whilst electrophoresis can only
measure zeta potentials in very dilute conditions. Hence,
there are significant errors in extrapolating the results to
the high solids loading that are used in many industrial
processes. Until fairly recently, therefore, the only quick
and reliable way of measuring the optimum amount of
dispersant was rheology.
Over the last few years, a technique called electroacous-
w xtics has been developed by O’Brien et al. 3–5 . This
technique allows zeta potentials to be measured in concen-
trated suspensions. This allows the adsorption of the dis-
persant to be followed if the dispersants are polyelec-
trolytes in nature, i.e. adsorption at the particle surface
alters the zeta potential. Polyelectrolyte-based dispersants
impart stability to colloidal particles via an electrosteric
mechanism. There are two methods for preventing parti-
w xcles aggregating 6 . Firstly, electrostatic stabilisation in-
volves varying the natural surface charge on the particles
by adjusting the pH of the suspension. Particles of a
similar charge then repel one another. The second method
is to adsorb polymers onto the particle surface such that
the particles cannot approach close enough to one another
for the van der Waals attractive force to dominate. A
combination of these two effects is called electrosteric
stabilisation and is the probable stabilisation mechanism
when polyelectrolytes are used.
Electroacoustics has the advantage of working at much
higher volume fractions than electrophoresis. An alternat-
ing electric field of a known frequency is applied to the
suspension, which then causes the particles to oscillate
with the same frequency. The liquid in the shear plane
around the particles also moves with the particle, but due
to differences in permittivity and density the movements
are out of phase. This results in a cyclic longitudinal
pressure variation in the suspending liquid, i.e. a sound
wave. The amplitude of this acoustic signal and its phase
difference is then recorded in the dynamic mobility spec-
Ž .trum frequency range 300 kHz to 11.5 MHz . From this
spectrum, a zeta potentialand a particle size can be
w xdetermined 4,5 . The zeta potential of a suspension is a
function of both the pH and the ionic strength of the
suspension, hence it is important to carefully monitorrcon-
trol these parameters.
w xGreenwood and Bergstrom 7 studied the adsorption of
a dispersant onto particle dispersions by comparing results
from adsorption isotherms, rheology and electroacoustics.
They demonstrated that adsorption isotherm plateaus corre-
sponded with viscosity minima and zeta potential maxima.
All three techniques effectively measure the coverage of
the polyelectrolyte on the particle surface. If insufficient
polyelectrolyte is added, then flocculation can occur by
bridging mechanisms in which the polyelectrolyte adsorbs
across two or more particles.
Another flocculation mechanism is possible when poly-
electrolytes are added at insufficient coverage: electrostatic
w xpatch or mosaic flocculation 8 . Here, the polyelectrolyte
adsorbs onto one particle causing a small patch to become
oppositely charged, this patch electrostatically attracts an-
other uncovered particle, such that the two particles floccu-
late. It is therefore vital to have the correct amount of
dispersant present in a system. Excess polyelectrolyte not
only increases the underlying medium viscosity, but causes
compression of the double layers, reducing the zeta poten-
tial and eventually causes flocculation
The zeta potential of a suspension is an indication of the
magnitude of the repulsive force between the particles.
Essentially, the greater the zeta potential the greater the
repulsive force, hence the lower the probability of aggre-
gating particles occurring. Once a dispersant has been
added to a suspension, mechanical energy is still required
to break down the aggregates and allow the dispersant to
adsorb onto the newly exposed surface. It is the objective
of this paper to firstly study the adsorption of various
commercially available dispersants onto calcium carbonate
using electroacoustics. Secondly, using this optimum
dosage data, the aim is to study the industrial scale grind-
ing of the calcium carbonate powder with the view of
optimising the process.
To summarise, the grinding efficiency of a system
depends on many parameters, many of which are interre-
( )R. Greenwood et al.rPowder Technology 123 2002 199–207 201
lated, but can be broken down into the characteristics of
the powder, the characteristics of the suspension, the na-
ture of the interparticle forces, the type of machinery and
the grinding media present. Hence, the problem may seem
intractable, however, by working with a well dispersed
calcium carbonate suspension with the optimum dosage of
dispersant, the effects of solids loading on the grinding
efficiency will be investigated. These results will be com-
pared to the power draw during grinding, the final particle
size distribution and the suspension viscosity.
2. Experimental
The work is divided into four parts. Firstly, sample
preparation; secondly, electroacoustic measurements to in-
vestigate if any material was dissolving from the powder
and to determine the optimum amount of a suitable disper-
sant; thirdly, the application of the data to pilot scale
grinding mill trials was carried out; finally, the viscosity of
the suspensions was investigated. Calcium carbonate was
selected as it is not too hard and is readily available in
sufficient quantities.
To prepare the material, approximately 1500 kg of
Baildon limestone was ground using a Pendulum mill to
approximately 80% passing 45 mm. This material was then
dry-ground in closed circuit using a Svedala 7.5 kW SAM
Ž .mill stirred vertical mill to 80% passing 10 mm and with
a median size of 3.9 mm. Once sample preparation was
complete, representative samples were taken for the elec-
troacoustic measurements.
2.1. Electroacoustic experiments
Each time 400 ml of suspension was prepared and
Ž .inserted into the Acoustosizer Colloidal Dynamics, USA .
The density of the powder was 2650 kgrm3 and its
dielectric constant was 6.85. In all cases, in order to mimic
the exact conditions of the mill, the same tap water was
used for the grinding experiments as well as the electroa-
coustic and rheological experiments. The machine was
calibrated daily using the standard solution at 25 8C. The
pH and conductivity meters were calibrated weekly again
using known standards.
For the dissolution experiments a suspension was made
with 300 g of powder and 400 ml of tap water, i.e. 43% by
weight of powder; 400 ml of this suspension was poured
into the Acoustosizer measuring vessel and stirred at 450
r.p.m. for 15 min to allow the suspension to come to
equilibrium. Measurements of the zeta potential, particle
size distribution, pH and conductivity were then recorded
every 15 min for about 150 min. The Acoustosizer auto-
matically calculates the d , d and d values of the16 50 84
particle size distribution.
In order to select a good dispersant for the calcium
carbonate powder, a fresh 43% by weight suspension was
prepared. The automatic titration software was then pro-
grammed to add a fixed amount of dispersant in multiples
of 0.3 ml up to a total volume of 4–5 ml. The three
Ždispersants chosen were Dispex A40 a 40% solution of
.ammonium polyacrylate, molecular weight 10,000 Da ,
ŽDarvan 821A R.T. Vanderbilt, USA, a 40 % solution of
.ammonium polyacrylate, molecular weight 6000 Da and
ŽDolapix PC21 Zschimmer and Scharz, Germany, a 25%
solution of a synthetic polyelectrolyte free from alkalis,
.molecular weight 56,000 Da . All experiments were car-
ried out at 25 8C and took approximately 50 min to
complete. All the dispersants used were utilised as deliv-
ered and without further dilution. The zeta potential was
Ž .then plotted against the mass of dispersant grams added
per kilogram of powder. It is usual to express dispersant
concentrations in mass of dispersant per unit surface area,
but here the surface area of the calcium carbonate is
constantly changing during grinding. To confirm that the
optimum amount of dispersant was not a function of the
solids loading of the suspension, a second experiment was
carried out at 50% by weight. Again, the zeta potential was
plotted against amount of added dispersant.
A second confirmation experiment was carried out to
check the optimum amount of dispersant by measuring the
viscosity of the suspensions as a function of the amount of
dispersant added. Suspensions of 50% by weight were
prepared from 25 g of powder, 25 g of tap water and a
known amount of dispersant. A Bohlin Visco 88 was
utilised, using the C30 geometry at 25 8C. The shear rate
was increased from 366 to 1122 sy1.The viscosity at a
shear rate of 530 sy1 was then plotted against the amount
Ž .of added dispersant in g per kilogram of powder.
2.2. Grinding experiments
Grinding tests were carried out to determine the influ-
ence of optimum dispersant dosage on grinding mill per-
formance. All grinding tests were carried out in a Svedala
Ž .7.5 kW SAM Sala Agitated Mill mill; see Fig. 1. The
SAM mill was developed for grinding middlings concen-
trates in mineral beneficiation processes as well as for fine
or ultrafine grinding of industrial minerals. The motor
drives the vertically mounted shaft, which moves the
grinding media against one another and the vessel walls
breaking down the particles. All tests were carried out with
5-mm-diameter alumina media to minimise iron contami-
nation.
Mill performance was quantified in two ways: firstly by
logging the power draw with time and secondly by deter-
mination of the particle size distribution, specifically the
d and d values after 3.5 volume changes in the mill50 90
Ž .cavity. Tests were carried out at 40% by weight with no
Ždispersant present and then 40%, 60%, 70% and 80% by
.weight with the optimum dose of dispersant as determined
by the previous electroacoustic experiments. Sufficient
slurry was made to allow 3.5 volume changes in the mill
( )R. Greenwood et al.rPowder Technology123 2002 199–207202
Fig. 1. Schematic of the SAM mill.
cavity. Slurry was fed into the mill at the bottom inlet in
accordance with the manufacturer’s recommended operat-
ing conditions of 6 lrmin, and the product collected at the
top inlet. The feed rate for all tests was kept identical. All
tests were operated in open circuit. During each test the
power draw from the mill motor was recorded every
minute and the particle size distribution determined imme-
diately, after 3.5 volume changes, by a Malvern Laser
Ž .Diffraction equipment Mastersizer . Each test was re-
peated three times and the values reported are a mean
average.
2.3. Viscosity measurements
The viscosity of the suspensions was measured using a
ŽTA AR1000 rheometer TA Instruments, Leatherhead,
.UK . The suspensions were prepared from 100 g of pow-
der and varying amounts of tap water. The viscosity of the
suspensions was then measured at 25 8C using a cone and
plate geometry and using the rheometer in constant stress
mode. For the less viscous samples the initial stress was
0.002 Pa, which was increased to 2 Pa over a 5-min
period. For the more viscous samples the initial stress was
0.02 Pa, increased to 2 Pa over 2 min. The viscosity was
then plotted against shear stress for all the suspensions.
ŽExperiments were carried out at 40% by weight with and
.without optimum dosage of dispersant , 50%, 60% and
70% by weight with the optimum amount of dispersant.
These weight fractions correspond to volume fractions of
0.201, 0.274, 0.361 and 0.468, respectively.
3. Results
Again, this section is divided into a discussion of the
electroacoustic experiments, then the industrial grinding
experiments and finally the viscosity measurements.
3.1. Electroacoustic results
w xPrevious experiments on zirconia 7,9,10 and spinel
w x11 powders have revealed a gradual increase in the zeta
potential due to ceriaryttria and sodium dissolution, re-
spectively, which could lead to potential processing prob-
lems. However, here the initial dissolution experiment
revealed that there was no problem with material dissolv-
ing out of the powder as the zeta potential remained
reasonably consistent with a mean zeta potential of q32.9
"0.8 mV; see Fig. 2. Both the initial and final conductivi-
ties were 0.028 Srm, whilst the pH decreased slightly
from 8.29 to 8.20. Again, this indicates that there is no
dissolution of material from the powder under these pH
conditions.
ŽFig. 3 shows the particle size distribution d , d and16 50
.d values of the suspension with time. The d and d84 16 50
values are about 2 and 5 mm, respectively, and remain
fairly constant with time. However, the d increased from84
about 17 to 22 mm over the time period. A zeta potential
of "30 mV, is usually considered sufficient to prevent
w xflocculation, 6 . The increase in the d value could be84
due to sedimentation of the larger particles during the
Ž .experiment. The rate of sedimentation Õ for dilute sus-0
pensions of rigid, noninteracting spheres can be calculated
by equating the buoyancy and drag force to the gravita-
tional force:
2 a2
Dr g
Õ s , 2Ž .0 9h
where Dr is the density difference between the particles
and the medium, h is the viscosity of the medium, a is the
particle radius and g the acceleration due to gravity. So, a
17-mm-diameter particle of calcium carbonate sediments at
y 4 w xa rate of approximately 3=10 mrs 6,12 . So, over a
Fig. 2. Effect of time on the zeta potential of a calcium carbonate
Ž .suspension 43% by weight .
( )R. Greenwood et al.rPowder Technology 123 2002 199–207 203
Fig. 3. Effect of time on the particle size distribution of a calcium
Ž . Ž .carbonate suspension 43% by weight . Diamondssd value mm ;16
Ž . Ž .squaressd value mm ; and trianglessd value mm .50 84
150-min period there is considerable opportunity for the
larger particles to collide with other particles as they
sediment and form small flocs, which could explain the
increase in the d value.84
The curves of zeta potential against the amount of
added dispersant are shown in Fig. 4. The x axis shows
Ž .the amount of dispersant g added per kilogram of pow-
der. All three graphs show the same trends. With no
dispersant present the initial zeta potential is high and
positive, varying between 27 and 38 mV, so of a similar
magnitude to the previous results in Fig. 2. The wide range
of zeta potentials is due to the slightly different natural pH
values the suspensions achieved. Table 1 reveals that the
initial pH for the suspension where Dolapix PC21 was
added was 8.72, which is greater than the pH range of
8.2–8.3 measured in the dissolution experiment. With this
slightly higher pH a slightly lower zeta potential of q27
mV is reasonable. Similarly, the suspension used for the
experiment in which the Darvan 821A was added, had a
slightly lower pH value, hence, a slightly larger zeta
potential is not unexpected.
Addition of a small amount of dispersant reduces this
initial zeta potential slightly as the anionic dispersant
neutralises the positive charge on the particles. Addition of
further dispersant causes the zeta potentials to be zero at
Fig. 4. The effect of dispersant on the zeta potential of a calcium
Ž .carbonate suspension 43% by weight . DiamondssDolapix PC21;
squaressDispex A40; and circlessDarvan 821A.
Table 1
Initial and final characteristics of the calcium carbonate suspensions on
addition of the three dispersants
Dispersant Initial Final Initial Final
pH pH conductivity conductivity
Ž . Ž .Srm Srm
Dispex A40 8.23 8.67 0.027 0.264
Darvan 821A 8.18 8.51 0.028 0.329
Dolapix PC21 8.72 8.77 0.027 0.174
about 1.5 grkg. Addition of more dispersant causes the
particles to now become negatively charged. With the
addition of further dispersant the zeta potential increases in
magnitude until the graph starts to plateau off. This is
because no more dispersant can now adsorb onto the
particles; this means that the particles are now completely
covered in dispersant and the point at which this plateau
region begins is taken to be the optimum amount of
dispersant required. From the previous results in the disso-
lution experiments the error in the zeta potential was "0.8
mV, so it is not unexpected to see some scatter in the data
once the particles are coated in polyelectrolyte. From Fig.
4 the optimum amount of each dispersant was estimated
from the graph to be:
Dispex A40 6"0.5 grkg of powder
Dolapix PC21 9"0.5 grkg of powder
Darvan 821A 9.3"0.6 grkg of powder.
The start position of the plateau region is not very well
Ždefined due to the polydispersity of the particle size and
.the dispersant molecular weight , hence there is a small
error associated with estimating the optimum amount. This
error was estimated to be plus or minus the concentration,
which corresponded to one experimental point. The best
dispersant is one that imparts a large final zeta potential
when it covers the particles. All three dispersants are
effective in that the plateau zeta potential is about y80
mV, with Darvan 821A giving a slightly higher value than
the other two. This makes it difficult to determine which is
the best dispersant. However, as considerable quantities
are required for the industrial trials, it was decided to
select Dispex A40 as this used the minimum amount of
dispersant to achieve stability, plus it could be readily
supplied in large amounts. Additionally, as grinding is
carried out on the thousands-of-tonnes scale, the relative
cost of each of the dispersants will be vital.
In all cases, dispersant addition increased the pH and
conductivity of the suspensions; see Table 1. The final pH
of the suspensions was such that the dispersant is almost
fully dissociated, implying that it can impart maximum
stability to the particles on adsorption. A polyelectrolyte is
essentially a polymer with dissociable groups. The degree
to which these groups dissociate depends on the suspen-
sion pH. For example, many dispersants are based on
( )R. Greenwood et al.rPowder Technology 1232002 199–207204
polyacrylic acid, which is undissociated at ;pH 3 and
forms a tightly bound coil in solution. However, increasing
the pH causes the groups to dissociate and repel one
another such that the coil unwinds, until at pH 8.5–9.0, the
w xgroups are fully dissociated 13 . Hence, it is more effec-
tive as a dispersant under basic conditions.
Fig. 5 confirms that the optimum amount of Dispex
A40 required to cover the particles was independent of the
volume fraction at which the experiment is carried, i.e. 6 g
of dispersant was required for every 1 kg of powder in the
suspension. In this case the initial starting pH value was
8.29, the conductivity was 0.027 Srm and the initial zeta
potential q30 mV, so in excellent agreement with the
previous result for Dispex A40 in Fig. 4. The final plateau
values of the zeta potential were also similar, y78 mV
compared to y80 mV, given the error of "0.8 mV.
However, the amount of dispersant required to achieve
zero zeta potential is different. In Fig. 4 it was 1.2 grkg,
whereas for the more concentrated suspension in Fig. 5 it
was 1.7 grkg. This discrepancy may be either due to
errors in weighing out the powder or some variation in the
composition of the dispersant, i.e. more dilute. Recently,
w xDukhin et al. 14 have questioned the validity of O’Brien’s
theory for the Acoustosizer. The zeta potential is theoreti-
cally independent of the volume fraction at which the
experiment is performed. However, they demonstrated that
for silica and rutile particles the zeta potential decreased
significantly with increasing volume fraction. However,
here we are not interested in the absolute values of the zeta
potential, but in the effect of the dispersant and the shape
of the curve. It is important to keep this in mind when
comparing zeta potentials in suspensions of different vol-
ume fractions and this may explain the discrepancy be-
tween the amount of dispersant required to cause zero zeta
potential in Figs. 4 and 5.
This optimum amount may seem high when converted
to a mass per tonne of material, but the optimum amount
of dispersant depends on many parameters especially the
surface chemistry of the powder and the salt concentration
of the medium; plus the dilution of the dispersant has not
Ž .been allowed for 40% . At low ionic strengths and high
Fig. 5. Effect of adding Dispex A40 to a calcium carbonate suspension
Ž .50% by weight .
Ž .Fig. 6. Viscosity of calcium carbonate suspensions 50% by weight as a
function of the Dispex A40 concentration. Striped barsd values and50
grey barsd values.90
pH, polyelectrolytes adsorb as trains along the particle
surface, however, as the salt concentration is increased the
polyelectrolyte adsorbs more as loops and tails, hence
w xmore dispersants can adsorb onto the same surface 15–18 .
Fig. 6 shows the corresponding viscosity of a 50% by
weight suspension as a function of the amount of added
Dispex A40. With no dispersant present the suspension has
a high viscosity of about 30 mPa. Addition of the disper-
sant reduces the suspension viscosity as the aggregates are
Ždestroyed. The minimum viscosity corresponding to com-
.plete coverage of the particles by the dispersant occurred
at 6.7 g dispersantrkg of powder, which is in excellent
agreement with the optimum amount determined by elec-
troacoustic measurements, clearly demonstrating the suit-
ability of the technique. The increase in suspension viscos-
ity after the optimum dispersant concentration is probably
due to some flocculation or an increase in the underlying
medium viscosity due to the unadsorbed polyelectrolyte in
solution. Again, the experiment demonstrates the impor-
tance of obtaining the optimum dispersant dosage.
3.2. Grinding results
The average power draw of the mill without any sus-
pension present was about 1.5 kW, whilst the average
power draw was 2.44 kW for the 40% solids by weight
with no dispersant. This compares to 2.19 kW with 40%
solids and the optimum dose of dispersant, again demon-
strating the usefulness of adding a dispersant; not only are
the aggregates eliminated, but slightly less power is re-
quired to grind the suspension. The power draw was 1.98
and 2.08 kW for the 70% solids and the 60% solids
Žsuspension, respectively when containing the optimum
.dose of dispersant . The results for the 80% solids suspen-
sion containing the optimum dose of dispersant were not
collected as it was found that excessive amounts of the
grinding media floated from the mill product port. Hence,
with the dispersant present at optimum dosage it is possi-
ble to obtain higher solids loading, which grinds at a
( )R. Greenwood et al.rPowder Technology 123 2002 199–207 205
Fig. 7. Influence of solids loading of the suspension on the product mean particle size.
similar power draw, i.e. a greater throughput can be
achieved for the same energy requirements.
Fig. 7 shows the influence of dispersant dosage on the
product size distribution. It can be seen that both the mean
d and mean d values were reduced with increasing90 50
solids loading. Data obtained for the 40% solids suspen-
sion with no dispersant gave a mean d of 3.1 mm and a50
mean d of 12.7 mm. This compares to a mean d of90 90
11.0 mm and a mean d of 2.9 mm for 40% solids with50
the optimum dose of dispersant, clearly showing the effec-
tiveness of the dispersant in destroying aggregates. As the
percentage solids was increased further to 70%, a value of
7.8 mm was obtained for the mean d and 2.3 mm for the90
mean d . It was noted, however, that as the percentage50
solids was increased so did the amount of media that
Žfloated from the mill until at 80% solids almost all the
.media floated from the mill . So it can clearly be seen that
more effective grinding occurred at the higher solids load-
ing. This is more apparent in the mean d values as it is90
the tail of the distribution, i.e. the aggregates that are
destroyed by a combination of the optimum amount of
dispersant and the mechanical grinding of the mill.
3.3. Viscosity results
Although the initial starting stress for the viscosity
measurements was either 0.002 or 0.02 Pa, the initial data
were noisy and below the sensitivity of the machine, hence
only data obtained from a shear stress greater than 0.1 Pa
are presented in Fig. 8. The viscosity experiments revealed
the expected trends. With no dispersant present the 40% by
Ž .weight suspension volume fraction 0.201 contained some
aggregates and hence demonstrated a shear thinning be-
haviour, i.e. the viscosity decreased with increasing shear
stress. On addition of the optimum dosage of dispersant to
Žthis suspension the system became Newtonian over the
.range studied , again demonstrating that the dispersant had
destroyed the aggregates and hence reduced the viscosity.
ŽIncreasing the solids loading to 50% by weight 0.274
.volume fraction with the optimum dispersant dosage in-
creased the viscosity slightly as would be expected and the
system was still Newtonian. Increasing the solids loading
Ž .further to 60% by weight volume fraction 0.361 with the
optimum dosage again increased the viscosity as compared
to the 50% suspension by weight, but the viscosity was
Fig. 8. Viscosity of the suspensions used in the grinding testwork. Empty squares40% by weight with no dispersant; full squares40% by weight with
optimum dispersant dosage; empty triangles50% by weight with optimum dispersant dosage; full circles60% by weight with optimum dispersant
dosage; and full diamonds70% by weight with optimum dispersant dosage.
( )R. Greenwood et al.rPowder Technology 123 2002 199–207206
still much lower than the viscosity of the 40% suspension
by weight with no dispersant present. This showed that
adding a dispersant at the correct dosage not only reduces
the viscosity of the suspension, but also allows a large
increase in the solids loading. Increasing the solids content
Ž .now to 70% by weight volume fraction 0.468 there was a
much larger increase in the viscosity than before,as the
particles are now much more closer to one another and the
maximum packing fraction is rapidly being approached.
The viscosity for this suspension was now greater than that
of the 40% suspension without any dispersant, but the
system was still Newtonian so there are no aggregates
present or they have been destroyed under shearing.
The increments in viscosity on increased solids loading
reflect the trend predicted by the Krieger–Dougherty equa-
tion. The slight increase in viscosity with increasing shear
stress is due to the sedimentation of particles in the
rheometer rather than any shear thickening phenomena.
We now try and relate the viscosity results to the power
draw data. It might be expected that as the viscosity of the
suspension increases then more power would be required
to rotate the shaft at the same speed. However, the power
draw for all the suspensions were quite similar. One
possible explanation is that as the solids loading of the
suspension is increased so is its effective density. This
means that an increasing amount of the grinding media is
supported by the suspension such that the actual resistance
to the rotating shaft is actually reduced. At 80% solids
loading the density is now so great that the grinding media
floated on the suspension, although the power draw was
similar to that for the 70% by weight suspension.
4. Discussion
Previous work on the grinding of calcium carbonate
w xwas carried out by Bernhardt et al. 19 , also using sodium
polyacrylate as the dispersant. They too noted that the
grinding efficiency was strongly dependent on the solids
w xconcentration. Hann and Gamal 20 obtained higher solids
loadings of their suspensions with sodium carbonate and
sodium tripolyphosphate dispersants. The former re-
searchers also carried out some zeta potential measure-
ments, but not by the electroacoustic method. Kapur et al.
w x21 working with a base metal ore concluded that the
rheology of the suspension played an important role in
terms of both the throughput and energy consumption of a
mill. Addition of suitable dispersants resulted in the drastic
reduction or elimination of the yield stress and allowed
higher solids loadings to be obtained. They did not, how-
ever, affect the grinding mechanism, but under favourable
conditions enhanced the productivity and throughput.
From our results it has been clearly demonstrated that
as the percentage solids of the suspension was increased
the resulting particle size distribution narrowed signifi-
cantly. The reason for this is not specifically quantified in
the literature, however, it is thought to be as a result of
drastic reductions in the yield stress. Unfortunately, our
rheology experiments were not able to detect a yield stress
for the suspension without any dispersant. In the absence
of dispersants the typical maximum percentage solids by
weight in a suspension is approximately 50%. The most
Ž .efficient breakage occurs in a mill for dry grinding when
the powder fills about 80% of the spaces between the balls
w x22 . Too low a filling means that the balls collide more
often, increasing the rate of media wear and wasting
energy by increasing the alumina-on-alumina contact. Too
high filling gives a cushioning effect, i.e. if the layer of
powder is too deep it will absorb energy and fracture will
w xnot occur 22 . It is unlikely that this cushioning effect has
Žbeen reached for the solids loadings reported here with
.dispersant as the grinding performance is seen to improve.
It is likely, however, that if the solids loading is increased
the grinding performance would tail off. It therefore fol-
lows that an advantage of adding dispersant would be
reduced wear on the grinding media. In fact, it was shown
w xby Kapur et al. 21 that media wear could be reduced from
0.051 to 0.064 kgrkW h energy input for a 47 wt.% slurry
without dispersant, to 0.015 kgrkW h energy input for 65
wt.% slurry with dispersant. With increased solids loading
the distance between particles decreases, hence the in-
creased probability of the particles colliding with one
anotherrgrinding media. More collisions mean a reduced
particle size distribution. However, above a certain viscos-
ity the motion of the grinding media will be damped by the
viscosity of the suspension.
ŽIt is clear, however, that an upper limit also exists in
.terms of the suspension density as at 80% solids loading
by weight, the media floated from the mill giving a serious
reduction in grinding performance. The results of this
study agree with those of previous workers. However, in
many cases these workers stated that difficulties lie in
determining the optimum dosage of dispersant required
quickly and accurately. The required dose of dispersant
will vary depending on the surface chemistry of the parti-
cles and the surrounding medium, i.e. ionic strength and
pH. Currently, electroacoustics is the only satisfactory way
of measuring this optimum dosage. Further work is cur-
rently being carried out to apply this technique to the fine
Žgrinding of middlings concentrates in which the required
.dosage changes frequently rather than relatively homoge-
nous feed materials such as the limestone used here. With
the advent of the Acoustosizer II it is possible to measure
zeta potentials online.
5. Conclusions
Initial electroacoustic experiments determined that no
material was dissolving out of the calcium carbonate pow-
der with time and that the particles carried a large positive
charge of approximately q33 mV at a pH of 8.2. A small
( )R. Greenwood et al.rPowder Technology 123 2002 199–207 207
increase in the d value of the particle size distribution84
was attributed to sedimentation during the timescale of the
experiment. By using electroacoustics to follow the adsorp-
tion of the polyelectrolytes onto the powder via the change
in zeta potential, it was possible to determine the optimum
dosage of each dispersant required to cover the particles,
thus, demonstrating that electroacoustics is a powerful
technique for investigating concentrated aqueous suspen-
sions of minerals and ores.
All three dispersants were able to impart large zeta
potentials to the suspensions, hence, were likely to be
extremely good electrosteric stabilisers; but on economic
and availability grounds, 6 g of Dispex A40 per kilogram
of powder was selected as the optimum amount of disper-
sant required to stabilise the calcium carbonate powder.
This amount was shown to be independent of the solids
loading of the dispersant. Viscosity measurements also
confirmed that when the optimum amount of dispersant
was used the suspension had a viscosity minimum.
By using the optimum dosage of this dispersant, it was
possible to run the grinding mill at a higher solids content
than without any dispersant present. An upper limit of 70%
by weight was determined to be the most suitable weight
fraction for grinding, as above this level the grinding
media floated out of the mill. At this weight fraction the
d and d values were at a minimum, but the average50 90
power draw was similar to the 40% by weight suspension.
This resulted in a greater grinding efficiency, i.e. more
powder of finer particle sizes can be processed per day,
with a similar energy input. In other words, with the
correct dosage of dispersant present, more efficient, effec-
tive grinding of a suspension can be achieved. Finally, the
rheological experiments revealed just how effective a dis-
persant can be. With the optimum amount of dispersant
present, the aggregates were destroyed and higher weight
fractions can be utilised whilst keeping a working viscos-
ity.
Acknowledgements
The authors would like to thank Svedala Grinding for
use of equipment in carrying out this work.
References
w x1 D.W. Fuerstenau, P.C. Kapur, P.C. Velamakanni, A multi torque
model for the effects of dispersants and slurry viscosity on ball
Ž .milling, Int. J. Miner. Process. 28 1990 81–98.
w x Ž .2 I.M. Kriger, T.J. Dougherty, Trans. Soc. Rheol. III 1959 137–152.w x3 R.W. O’Brien, The electroacoustic equations for a colloidal suspen-
Ž .sion, J. Fluid Mech. 212 1990 81–93.
w x4 R.W. O’Brien, B.R. Midmore, A. Lamb, R.J. Hunter, Electroacous-
tic studies of moderately concentrated suspensions, Faraday Discuss.
Ž .90 1990 301–312.
w x5 R.W. O’Brien, D.W. Cannon, W.N. Rowlands, Electroacoustic de-
termination of particle size and zeta potential, J. Colloid Interface
Ž .Sci. 173 1995 406–418.
w x6 R.J. Hunter, Foundations of Colloid Science, vol. 1, Science Publica-
tions, Oxford, 1995.
w x7 R. Greenwood, L. Bergstrom, Electroacoustic and rheological prop-
Ž .erties of aqueous Ce–ZrO Ce TZP suspensions, J. Eur. Ceram.2
Ž .Soc. 17 1997 537–548.
w x Ž .8 J. Gregory, in: Th.F. Tadros Ed. , SolidrLiquid Dispersions: Floc-
culation by Polymers and Polyelectrolytes, Academic Press, London,
1987, Chap. 8.
w x9 R. Greenwood, K. Kendall, Selection of suitable dispersants for
aqueous suspensions of zirconia and titania using acoustophoresis, J.
Ž .Eur. Ceram. Soc. 19 1999 479–488.
w x10 R. Greenwood, K. Kendall, Acoustophoretic studies of aqueous
suspensions of alumina and 8 mole % yttria stabilised zirconia
Ž .powders, J. Eur. Ceram. Soc. 20 2000 77–84.
w x11 R. Greenwood, K. Kendall, Acoustophoretic investigation of aque-
ous suspensions of three different spinel powders, Br. Ceram. Trans.
Ž . Ž .97 4 1998 174–179.
w x12 M. Burke, R. Greenwood, K. Kendall, Experimental methods for
measuring the optimum amount of dispersant for seven Sumitomo
Ž .alumina powders, J. Mater. Sci. 33 1988 5149–5156.
w x13 J. Cesarano III, I.A. Askay, Processing of highly concentrated
aqueous alumina suspensions stabilized with polyelectrolytes, J. Am.
Ž . Ž .Ceram. Soc. 71 12 1988 1062–1067.
w x14 A.S. Dukhin, V.N. Shilov, H. Ohshima, P.J. Goetz, Electroacoustic
phenomena in concentrated dispersions: new theory and CVI experi-
Ž .ment, Langmuir 15 1999 6692–6706.
w x15 T. Afshar-Rad, A.I. Bailey, P.F. Luckham, W. MacNaughtan, D.
Chapman, Forces between poly L lysine of molecular weight 4000–
Ž .75,000 adsorbed on mica surfaces, Colloids Surf. 25 1987 263–277.
w x16 M.A.G. Dahlgren, P.M. Claesson, R. Audebert, Highly charged
cationic polyelectrolytes on mica: influence of polyelectrolyte con-
Ž .centration on surface forces, J. Colloid Interface Sci. 166 1994
343–349.
w x17 P.M. Claesson, M.A.G. Dahlgren, L. Erikson, Forces between poly-
electrolyte coated surfaces: relations between surface interaction and
Ž .floc properties, Colloids Surf., A 93 1994 293–303.
w x18 K.E. Bremmel, G.J. Jameson, S. Biggs, Polyelectrolyte adsorption at
the solidrliquid interface. Interaction forces and stability, Colloilds
Ž .Surf. 139 1998 199–211.
w x19 C. Bernhardt, E. Reinsch, K. Husemann, The influence of suspen-
sion properties on ultra fine grinding in stirred ball mills, Powder
Ž .Technol. 105 1999 357–361.
w x20 K.M. Hann, A.E. Gamal, The effect of dispersing agents on fine
Ž .grinding of limestone, Powder Technol. 17 1977 19–25.
w x21 C.K. Kapur, W.H. Healy, P.J. Scales, D.V. Boger, D. Wilson, Role
of dispersants in kinetics and energetics of stirred ball mill grinding,
Ž .Int. J. Miner. Process. 47 1996 141–152.
w x22 L.G. Austin, Concepts in process design of mills, Mining Eng. 36
Ž .June 1984 628–635.