Note: Descriptions are shown in the official language in which they were submitted.
1323015
METHOD OF PRODUCING FINE PARTICLES
This invention relates to a method of reducing the particle
size of solid particles and is applicable to the
production of very fine particles of a wide variety of
solids, including relatively hard solids.
Very fine inorganic particles, of median particle size of
2 microns or less, are used for various purposes~ One
application of such particles is as a filler material for
plastics compositions, for example in filled cable
sheathing compounds. Use of fine powders can also
accelerate reaction rates in chemical reactions involving
a solid reagent and accelerate dissolution of the solid,
metallic or ceramic powders of small particle size are
used for processing into components, and some solid
catalysts are more effective when of small particle size.
In many applications a superior solid product, or superior
process using the products may be obtained.
Reduction of solid particles, especially of hard
materials, to micron and sub-micron size is generally
achieved by means of an attrition mill, such as a bead
mill, fed with a dispersion of the coarse particles in a
liquid (usually water). The milled particles obtained
usually have a wide particle size distribution and to
obtain a reasonably uniform small particle size the
particles obtained have to be classified.
3o The Applicants' EP-A-0253635 describes and claims alumina
hydrate particles having a high surface area and a narrow
particle size distribution, optionally with a low soluble
~oda content. Such finely divided particles are useful as
fillers in paper, rubber and plastics compositions where,
not only can they improve the mechanical and electrical
properties of such compositions, but also can act both as
a fire/flame retardant and as a smoke supressant. Too
wide a particle size distribution can have deleterious
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effects on filled polymer mechanioal properties and
residual soda can adversely affect the alumina hydrate's
performance in many applications, particularly because of
water pick-up. ~-
In the course of developing the novel alumina hydrate
particles of that invention, a process was used for
producing the particles which appeared to have a very
favourable effect on the breadth of the particle size
distribution, and is described for this purpose in EP-A-
0253635 as the preferred preparative method for such
particles. Specifically, the preferred method of
producing alumina hydrate particles comprises milling a
liquid suspension of larger alumina hydrate particles in a
stirred media mill, subjecting the milled suspension to
continuous classification to separate the suspension into
a coarse fraction of greater particle size and a fine
fraction of smaller particle size, recycling the coarse
fraction to the mill input and recycling the fine fraction
to the continuous classification step, if required
subjecting the milled suspension to ion exchange to reduce
the content of the soluble soda in the particles, and
subsequently drying the suspension.
Because of the particular morphology of the coarse alumina
hydrate particles used, it had been considered that this
particular preparative method had applicability only to
alumina hydrate particles and would not have the same
beneficial effect on particle distribution width with
other materials. As a result of further work, however, it
ha~ now been found that this preparative method does yield
particles with a desirably narrow particle size
distribution with a wide range of differing materials.
Furthermore, in EP-A-0253635, no particular
classification system or device i5 described. It has now
been realised that the preparative method of EP-A-0253635
is particularly suited to classification devices which
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have a low separation efficiency, particularly
hydrocyclones.
Hydrocyclones are known for dividing a suspension of
milled particles into a coarse fraction and a fine
fraction, but it has not so far been possible to obtain
satisfactory particle size separation for particles
smaller than 2 or 3 microns in a single pass through the
classifying device. It has been necessary to pass the
suspension through a series of classifying devices, which
results in an inefficient process having a very poor
yield. Furthermore, conventional theory holds that
hydrocyclones have little useful separating capacity for
particles below about 4 microns, particularly using
relatively high slurry loadings.
It has been found, surprisingly, that very efficient
separation of particles having a median size of 2 microns
or less from larger median size particles and a high
overall yield may be obtained by the method of the present
invention, which can require for its performance only a
single mill and a single classification device, although
more than one mill and/or classification device can be
used if desired.
According to the present invention, there is provided a
method of producing solid particles of reduced median
particle ~ize, other than alumina hydrate as claimed in
EP-A-0253635, which comprises milling a liquid suspension
3 of solid particle~ in an agitated media mill, pumping the
milled suspension through a particle size classification
device to separate the slurry into a coarse fraction and a
fine fraction, the particles of the coarse fraction having
a greater median particle size than the particles of the
fine fraction, recycling the coarse fraction from the
particle size classification device to the input of the
mill, and recycling the fine fraction by pumping it to the
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classification device wherein recyoling of both coarse and
fine fractions are continued until solid particles of the
desired reduced particle size are produced.
The agitated media mill may be of known type and may be a
stirred media mill in wh,i,ch milling media, such as ceramic
balls or rods typically of size 0.5 to 3.0 mm are agitated
by means of a rotating shaft. The shaft may be provided
with agitating discs. Alternatively the mill may be a
vibro energy mill in which the milling medium is agitated
by vigorous movement of the milling chamber. In all cases
the milling medium reduces the average particle size of
the solid by attrition. The mill is preferably of a type
allowing continuous operation, in which the slurry can be
continuously fed into the mill, generally pumped into the
mill under pressure, and continuously removed at one or
more points.
The classification device used may be a continuous
centrifugal device or a hydrocyclone which allow particle
size classification of the solid suspended in the slurry.
A suitable hydrocyclone typically has a maximum internal
diameter up to 10 cm.
The concentration of solid in the slurry may vary widely
and would normally be within the range of 5% to 65%,
preferably 35% to 50% by weight. The preferred
con¢entration generally depends on the use to which the
m~lled slurry is to be put. A high concentration is
3 normally favourable when the slurry is to be dried to
produce a dry solid. A viscosity modifier can be added if
desired.
In one method according to the invention the milled
su~pension discharged from the mill and the fine fraction
discharged from the classification device are both
conducted to a receptacle for receiving the de~ired milled
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product and the contents of the receptacle are recycled to
the input of the classifioation device by a pump
intermediate the receptacle and the classifiaation
device. In this embodiment the suspension may be pumped
from a container for the initial unmilled suspension,
passed into the mill at a typical pressure of up to
20 psi, and discharged to the receptacle where it is not
under pressure. When a hydrocyclone is used the pump
intermediate the receptacle and hydrocyclone may feed the
contents of the receptacle to the hydrocyclone at a
typical pressure of 50 psi. The coarse fraction is
discharged to the container for starting material, and the
fine fraction discharged to the receptacle, at
substantially zero gauge pressure. As the suspension is
repeatedly recycled through the apparatus the median
average size of the particles obtained in the receptacle
iB reduced, and the larger particles eliminated by
attrition, so that after a certain time the suspension may
have a substantially uniform particle size which is very
small.
In another embodiment of the invention the suspension
discharged from the mill is conducted, not to the final
receptacle for the product, but to an intermediate
reservoir, and the contents of the reservoir are pumped to
the classification device, from which the coarse fraction
is recycled to be passed again through the mill and the
fine fraction is delivered to the receptacle. The fine
fraction from the receptacle is brought, for example by
pumping, to the reservoir so that the fine fraction is
recycled through the classification device together with
the suspension discharged by the mill. Control of the
process of this embodiment is more complex than for the
embodiment described above, but the efficiency of the
process is greater as only the fine fraction from the
classification device is discharged to the receptacle in
which the desired suspension of finely divided product
eventually accumulates.
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In a further embodiment, instead of pumping the ~uspension
to be treated through the mill under positive pressure the
suspension is aspirated through the mill by a pump
arranged between the mill and the classification device,
the pump feeding the milled suspension from the mill to
the classification de~ice under positive pressure. With
th~s arrangement the pump can feed the milled suspension
to a hydrocyclone at the desired relatively high pressure,
typically about 50 psi, and the pressure difference across
the mill may approach atmospheric pressure (about 15 psi~
which may be sufficient to allow efficient operation of
the mill. The coarse fraction from the classification
device is again recycled to pass through the mill and the
fine fraction, discharged to the receptacle, may be
returned to the feed line for the classification device at
a point between the mill and the pump, so that the pump
aspirates the suspension from the receptacle also. With
this arrangement only one pump is required to operate the
process. In a variant of this embodiment, a further pump
is provided to pump the fine fraction from the receptacle
to the line feeding the classification device, the fine
fraction from the receptacle being delivered to the
classification device feed line at a point between the
classificat$on device and the pump feeding suspension from
the mill to the device. In this variant the efficiency of
the mill may be increased as the pump aspirating
suspension through it does not have the additional
fun¢tion of aspirating the fine fraction from the
receptacle. `
The mill used in the method of the invention may be a bead
mill of the known "Eiger" type, loaded with zirconia beads
of diameter about o.8 mm. The classification device may
be a hydrocyclone of a known type, such as the "Mozley"
hydrocyclone.
Methods of reducing the median particle size of particles
according to particular embodiments of the invention will
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now be described with referenced to the accompanying
drawings in whiah: A ,
Figure 1 is a flow diagram showing a method according
to the prior art,
Figures 2-5 are flow diagrams of methods according to
the invention,
Figure 6 is a diagram of a bead mill which may be
used in the invention.
In the prior art arrangement of Figure 1 a liquid slurry
of particles to be treated is fed from a container 1 to a
bead mill 3 which grinds the slurry and discharges the
ground slurry to hydrocyclone 4 which separates it into a
coarse and a fine fraction. The coarse fraction is
returned through line 5 to container 1 for recycling
through the mill and hydrocyclone and the fine fraction is
delivered through line 6 to receptacle 7.
It is found that the method of Figure 1 is incapable of
producing a fine fraction having a very low average
particle size, as the slurry delivered to receptacle 7
still has a hlgh proportion of relatively coarse
particles. When a slurry of solid particles is treated
with this arrangement it has not been found possible to
obtain a fine fraction of median particle having a size of
2 microns or less.
3o
In the arrangement of Figure 2, the slurry containing the
80lid particles is fed from container 11 to pump 12 whioh
delivers the slurry at a pressure of up to 20 psi to the
input of bead mill 13, which is of the type described
below with reference to Figure 6. The slurry is ground in
the mill and discharged to receptacle 14.
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The slurry in receptacle 14 is then fed to pump 15 which
feeds it at a pressure of about 50 psi to hyqrocyclone 16,
which separates the slurry into a coarse fraction which is
returned by line 17 to container 11, and a fine fraction
which is sent by line 18 to the receptacle 14.
When the embodiment of Figure 2 is used, a batch of slurry
is supplied, one half to-container 11 and the other half
to receptacle 14, and the pumps, mill and hydrocyclone are
run until the median particle size of the product batch
which accumulates in receptacle 14 has the desired value.
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The method described with reference to Figure 2 allows the
mill to be operated under favourable grinding conditions,
that is with a slurry having a relatively high solids
content (up to 65% by weight~ and a high flow rate. The
slurry is fed to the mill under positive pressure. The
rate of flow is easily adjusted by adjusting the rate of
operation of pump 12 so that the rate of flow of the
slurry through the mill is matched to the requirements of
the hydrocyclone. Pump 15 may be used simply to maintain ` `~
the feed pressure for hydrocyclone 15; thus the method is
simply controlled by adjusting pump 12 according to the
respective levels of the slurry in container 11 and in
receptacle 14. When operated with an aqueous slurry of a
solid particles the method is capable of yielding
particles of a median particle size of 0.3 microns or
less, u~ing only one mill and only one hydrocyclone.
The method illustrated by Figure 3 is similar to that of
Flgure 2 and common components are shown by the same
reference numerals. Pump 12, mill 13, pump 15, and
hydrocyclone 16 operate in the same way as in Figure 2 and
the coarse fraction from the hydrocyclone is again
recycled to container 11 throu~h line 17, the fine
fraction being delivered to receptacle 14 through line 18.
However in this arrangement the output of slurry from the
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mill 13 is fed not to receptacle 14, but to a reservoir 20
from which it is fed by pump 15 to the hydrocyclone 16,
and a further pump 21 returns the fine fraction from
receptacle 14 to reservoir 20.
This arrangement is more complex than that of Figure 2 in
that an extra container (reservoir 20) is required and an
extra pump (21) is needed to transfer the fine fraction
from receptacle 14 to the reservoir 20. However the
efficiency of this embodiment is rather greater as the
coarse fraction from the mill 13 is fed to the
hydrocyclone 16 without passing through the receptacle 14
which receives the fine fraction.
Figure 4 shows an arrangement in which only one pump is
required. In this case the slurry from container 11 is
again fed to bead mill 13 and passes from the mill 13 to
hydrocyclone 16 whic,h divides it into a coarse fraction
which is returned to container 11 through line 17 and a
fine fraction which is sent through line 18 to receptacle
14. However in this case a single pump 20 both delivers
the slurry to the hydrocyclone 16 at a pressure of about
50 psi and draws the slurry through mill 13 by suction.
The pressure difference urging the slurry through mill 13
is thus generated by aspiration by pump 20 and it may
correcpond substantially to atmospheric pressure, that is
about 15 psi. If a higher input pressure for the mill 13
is required, container 11 may be a closed tank and the
tank may be pressurised. In this arrangement the slurry
discharged to receptacle 14 is recycled through line 21 to
a point between mill 13 and pump 20, and the slurry is
drawn through line 21 by the aspiration of the pump 20. A
valve 22 is in~erted in line 21 to control the rate of
recycling of the slurry from receptacle 14 and the process
is controlled by adjustment of pump 20 and valve 22 as
required.
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Figure 5 shows a variant of the process of Figure 4. In
this variant the slurry is again aspirated through mill 13
and fed to hydrocyclone 16 by pump 20, the ooarse fraction
is again recycled through line 17 and the fine fraction of
the slurry is recycled from receptacle 14 to the
hydrocyolone 16. However in this instance line 21 returns
the fine fraction to a point between the pump 20 and the
hydrocyclone 16 and is impelled by a further pump 23
provided in line 21. Pump 23 delivers the recycled fine
fraction to the hydrocyclone 16 at a pressure of about
50 psi and the process is controlled by adjusting both
pumps 20 and 23. This variant allows pump 20 to aspirate
slurry from container 11 through mill 13 more efficiently.
In all the arrangements of ~igures 3, 4 and 5 all the
material present in receptacle 14 has been passed through
the hydrocyclone at least once, and in practice often many
hundreds of times, thereby increasing the overall
efficiency of the process. When starting up, the
suspension to be treated is generally divided up equally
between the various containers and receptacles.
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One type of attrition mill which may be used is the
"Eiger" bead mill shown diagrammatically in Figure 6. The
mill comprises a tubular vessel 31 containing an agitator
32 comprising paddles extending radially from a shaft
which is driven in rotation by motor 33. The vessel
contains a screen 34 to prevent discharge of gross
oversize particles from the mill and the vessel contains,
3 around agitator 32, beads of hard material which grind the
l~quid suspension. The suspension is fed into the mill at
~nlet 35, the suspension passes through the mill and is
discharged at 36 after passing through the screen 34.
It has been found, surprisingly, that when a slurry is
milled and classified by the methods described above the
classification device can yield a fine fraction of narrow
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particle size distribution down to a very small average
particle size, down to 0.4 microns or even lower. With
hitherto known ~illing and classifying methods, a
hydrocyclone classifying device does not produce any
useful separation of particle size fractions at particle
sizes as small as this.
The invention may be applied to a very wide variety of
solids which may be slurried with a wide range of liquids.
Solids which may be milled include iron oxide, talc,
silica and other minerals like chalk, zinc oxide, boric
oxide, borax, zinc borate, pigments, carbon black, various
metals, solid organic compounds, e.g. terephthalic acid
and mixtures thereof. The liquid may be chosen ~rom
water, volatile non-aqueous liquids such as hydrocarbons,
tetrahydrofuran, dioxan, alcohols and esters, and non-
volatile solvents such as phthalates, polyvinylchloride
plastisols and waxes. Non-volatile liquids may be used
when the slurry is to be used subsequently in liquid form,
without drying, for example as plastisols or in certain
pharmaceutical preparations. The slurry may include one
or more additives to aid milling, such as a dispersant, or
to assist later processing, for example a stearate which
forms a coating on the particles.
Possible applications for thè milled solid include
ceramics, catalysts, plastics fillers, fire/flame
retardant~, smoke supressants and powder metallurgy.
In the methods mentioned above the classification device
may be operated continuously or it may be operated
intermittently to give quasi-continuous operation, so as
to balance the flow of coarse fraction from the
classification device with the mill input. The overall
process is generally operated as a batch process, i.e.
with 100% recycle of both coarse and fine particle
fractions since generally the efficiency of the separation
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devi~e is too low for continuous operation to yield
particles of the d~sired average particle size and breadth
of particle size distribution.
The method of the invention may be operated at a range of
temperatures accordîng to the nature of the solid and/or
liquid being processed. An operating temperature down to
-20C is generally feasible.
Example 1
49.5 kg of zinc borate (crystalline form 2335) available
from U.S. Borax were dispersed into 150 litres of water.
This material had a median particle size of 6 microns. It
was processed in accordance with the preferred process of
this invention as shown diagramatically in Fig. 2 for
three hours. The grinding device was a 20 litre capacity
"Eiger" bead mill and the classif~cation device was a
"Mozley" hydrocyclone of 2 inch (5.08 cm~ nominal
diameter.
After three hours the product taken from receptacle 14 was
then analysed using the "Malvern" laser photon correlating
spectrometer and was found to have a median particle size
of 0.28 microns and a polydispersity of 0.23.
Polydispersity can be measured in a number of different
ways, but for the purposes of the present invention it
iB based on the light scattering analysis technique
utilized in the Model 4600 and 4700 series photon
correlation spectroscopes manufactured by Malvern
Instruments Limited of Malvern, England. In this
technique a scattered light auto-correlation function is
generated and a cumulant analysis performed thereon.
Polydispersity is then defined as being equal to the
normalized second moment of this cumulant analysis.
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Further information about polydispersity can be found in
the reference "The Coulter Nano-Sizer" published by Coulter
Electronics Limited in January 1980.
Example 2
:
50 kg of terephthalic acid available from ICI were
dispersed into 100 litres of water and milled as Example 1
above for a total of 15 hours. In order to maintain a
working viscosity during grinding a further 400 litres of
water were added at intervals during the 15 hours together
with 2 litres of "Teepoln qurfactant available from Shell.
The ~eed material prior to grinding had a specific surface
area of 0.18 m2/g as determined by the standard Strohlein
method as described in "Particle Size Measurement", p.
390, Terence Allen, Chapman and Hall Ltd. 1975, a median
particle size of 83 microns as determined by Coulter
counter, and a particle size mode of 90 microns as
determined by Coulter counter. After completion of
grinding the product taken from receptacle 14 had a
surface area of 3.9 m /g, a median particle size of less
than 1.2 microns, and particle size mode of 1.1 microns as
evaluated by the same methods.
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