Note: Descriptions are shown in the official language in which they were submitted.
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Grir~tdix~,g of Particulate Material
The present invention relates to grinding, and
particularly to an apparatus and a method for
attrition grinding of particulate mineral materials.
It is well known to grind a particulate solid
material suspended in a liquid medium, such as an
aqueous medium. Grinding such suspensions of
material is known in the art as 'wet' grinding and
the typical name of a type of apparatus employed to
perform such grinding is a stirred media mill or an
ultrafine grinding mill.
We have now devised an improved way of grinding
particulate solid materials.
According to a first aspect of the present
invention, there is provided apparatus for grinding a
suspension of a particulate solid material, which
apparatus comprises:
a grinding chamber for containing a particulate
grinding medium;
means for introducing a suspension of the
particulate solid material into the grinding chamber;
a rotatable impeller for agitating the
particulate grinding medium and the suspension of
particulate solid material in the grinding chamber,
the impeller having at least one blade, wherein the
or each blade of the impeller is pitched relative to
the plane of rotation of the impeller; and
means to drive the impeller, so as to produce a
suspension of a ground particulate solid material.
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According to a second aspect of the present
invention, there is provided apparatus for grinding a
suspension of a particulate solid material, which
apparatus comprises:
a grinding chamber for containing a particulate
grinding medium;
means for introducing a suspension of the
particulate solid material into the grinding chamber;
a rotatable impeller for agitating the
particulate grinding medium and the suspension of
particulate solid material in the grinding chamber,
means to drive the impeller; and
a baffle member arranged over the particulate
grinding medium and the suspension of particulate
solid material, so as to produce a suspension of a
ground particulate solid material.
According to a third aspect of the present
invention, there is provided a method of grinding a
suspension of a particulate solid material, which
method comprises:
providing apparatus as defined in the first
aspect of the invention;
introducing into the grinding chamber the
particulate grinding medium and the suspension of
particulate solid material; and
agitating the particulate grinding medium and
the suspension of particulate solid material by
rotation of the impeller, so as to produce a
suspension of a ground particulate solid material.
According to a fourth aspect of the present
invention, there is provided a method of grinding a
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suspension of a particulate solid material, which
method comprises:
providing apparatus as defined in the second
aspect of the invention;
introducing into the grinding chamber the
particulate grinding medium and the suspension of
particulate solid material; and
agitating the particulate grinding medium and
the suspension of particulate solid material by
rotation of the impeller, so as to produce a
suspension of a ground particulate solid material.
According to a fifth aspect of the present
invention, there is provided apparatus for grinding a
suspension of a particulate solid material, which
apparatus comprises:
a grinding chamber;
means for introducing a suspension of the
particulate solid material into the grinding chamber,
an impeller having at least one blade, the said
blade having a leading face in the direction. of
rotation of the impeller, which leading face is
offset relative to a plane of rotation of the
impeller. The angle of offset of the leading face
relative to the plane of rotation of the impeller is
preferably greater than 0° and less than 90°.
Preferably, the impeller rotates in a
substantially horizontal plane and an upper edge of
the said leading face is the leading edge and a lower
edge of the said leading face is the trailing edge,
so that the impeller acts to drive the material in
the grinding chamber downwards towards a base of the
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grinding chamber, thereby compressing the material in
the grinding chamber.
The configuration of the impeller provided in
the apparatus of the invention causes enhanced
grinding of the suspension of particulate material
introduced into the apparatus. The impeller may
comprise an annular body having a central opening
which may be disposed on a substantially vertically
orientated rotatable shaft. An upper end of the
shaft may be connected to a drive means to rotatably
drive the shaft and the impeller during operation of
the apparatus. It is preferred that the annular body
of the impeller may comprise a plurality of blades,
each of the blades projecting radially from the
annular body and preferably being equidistantly
spaced from the other around an exterior face of the
annular body, and each of the blades being pitched
relative to the plane of rotation of the impeller.
In this specification, the 'pitch' of a blade is
defined as the angle between the chord of the blade
(or the leading face of the blade in the direction of
rotation of the impeller) and the plane of rotation
of the blades of the impeller. The impeller may have
an annular body and a plurality of blades which may
be unitary, or, alternatively, the plurality of
blades may be fixedly secured to the annular body.
It is preferred that the pitched blades have a
pitch angle in the range of from about 30° to about
60°. Optimally, the pitch angle is about 45° which
during use of the apparatus of the invention
advantageously produces a ground particulate material
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product having the desired particle size properties.
It is also preferred that the plane of rotation of
the impeller may be substantially perpendicular to a
substantially vertically orientated rotatable
impeller shaft. Preferably, each blade of the
plurality of blades of the impeller has a
substantially equivalent pitch angle; in other
words, the chord of each blade may have a
substantially equivalent gradient. The preferred
orientation of the pitched blades provides enhanced
grinding of the suspension of particulate mineral
material, to produce a ground mineral product having
the required particle size with a reduce energy
input. Preferably, a front (leading) face and/or a
rear (trailing) face of each blade may be
substantially flat (planar) .
In an embodiment of the apparatus of the
invention, the impeller may comprise an arrangement
of one or more tiers. Preferably, there are two,
three or four tiers to further improve the
performance of the method of the invention. A two
tier impeller provided in such apparatus may be
called a double-pitched impeller. Preferably, the
two tiered impeller may be arranged to rotate in
unison and the upper tier of the impeller may be
upwardly spaced from the lower tier so as to maximise
the grinding effect of the impeller to comminute the
suspension of particulate material introduced into
the grinding chamber. Preferably, the impeller
blades of the upper tier may be pitched at
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substantially an equivalent pitch angle to the
impeller blades of the lower tier.
Known prior art configurations of 'wet' grinding
apparatus do not utilise a pitched impeller as
defined by the present invention and hence such known
prior art configurations do not provide the
advantages as described herein. Generally, the
effectiveness of a grinding process may be attributed
to the fineness of product produced and the yield of
product of the desired particle size. A comminuted
product mainly consisting of particles having a
particle size less than 2~m is generally recognised
in the art as being an aptly ground product. The
ground particulate product produced by the method of
the invention advantageously consists of a greater
yield of particles having a finer particle size than
a ground product produced by known prior art grinding
apparatus. Preferably, at least 90o by weight of the
ground particulate product comprises particles having
a particle size less than 5~m; and preferably at
least 70o by weight of the ground product comprises
particles having a particle size less than 2lZm.
Furthermore, use of the apparatus of the
invention provided with a pitched impeller is more
cost-effective and energy efficient than known prior
art grinding apparatus. The apparatus of the
invention uses less energy during operation and
achieves an improved level of grinding compared to
prior art configurations. The amount of energy used
by apparatus in accordance with the invention when
grinding materials such as kaolin may be in the range
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of from about 20kWh/t to 150kWh/t; optimally the
energy to be applied is about 55kWh/t to 150kWh/t.
The amount of energy to be applied when grinding
materials such as calcium carbonate is preferably up
to 300kWh/t.
In tests conducted by the applicant, the use of
an impeller having blades with a 45° pitch rather
than pin type (cylindrical) blades in a conventional
wet grinding test chamber decreased by approximately
half the energy input to achieve a ground product in
which 80% of the particles were smaller than 2~m.
The use of a pitched impeller as described
hereinbefore in the apparatus of the invention causes
improved mixing of the charge resulting in improved
grinding of the suspension of material introduced
into the apparatus. In this specification, the
'charge' is defined as the mixture of the particulate
grinding medium and the suspension of particulate
solid material contained in the grinding chamber of
the apparatus, which term is commonly known to
skilled people in the art. Such improved grinding is
believed by the applicant to be caused by grinding of
the mineral material suspension relatively near, or
in the region of, an interior face of the wall of the
grinding chamber. Grinding may be initiated by
rotation of the pitched impeller at a suitable
velocity so as to fluidise the (bed of) grinding
medium and cause mixing of the grinding medium with
the suspension of material to be ground. A suitable
rotational velocity of the pitched impeller may be in
the range of from 50rpm to 400rpm.
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_g_
In accordance with the second aspect of the
present invention, a baffle member is arranged over
the particulate grinding medium and the suspension of
particulate solid material. The baffle member may be
formed of an annular baffle plate having a central
opening which may receive therethrough a
substantially vertically orientated rotatable shaft.
The baffle plate may be made of a plastics material,
such as, for example a polyurethane material. Most
preferably, the baffle member is of rigid
construction and may be made of steel. It may be
lined with an antiwear component such as a
polyurethane coating or lining.
Preferably, the baffle member is removably
secured to an interior face of the wall of the
grinding chamber, or, alternatively, to a top end of
the grinding chamber. Preferably, the baffle member
lies in a plane which is substantially perpendicular
to a rotational axis of the impeller shaft.
Preferably, the baffle member is located at a preset
height over the charge, such that during operation of
the apparatus the baffle member ensures that the
charge is in a relatively compressed condition. Such
location of the baffle member optimises the energy
delivered to each unit volume of the charge (through
rotation of the impeller) thereby maximising the
grinding of the mineral material suspension. The
baffle member also serves to limit the extent to
which the charge rides up the interior face of the
wall of the grinding chamber.
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In tests conducted by the applicant, the
addition of a baffle member to a conventional wet
grinding test chamber decreased by approximately 40%,
the energy input to achieve a ground product in which
80% of the particles were smaller than 2um.
The provision of a baffle member in the
apparatus of the first and fifth aspects of the
invention also further enhances the technical
advantages which may be achieved by the provision of
a pitched impeller as hereinbefore described. For
instance, the yield of ground particulate product
having a fine particle size (of ideally less than
2um) is further improved and the amount of energy
applied to the apparatus is further reduced resulting
in a particularly preferred embodiment of the
invention.
The particulate grinding medium which may be
employed according to the method of the invention may
be of a natural or a synthetic material. The
grinding medium may comprise balls, beads or pellets
of any hard mineral, ceramic or metallic material;
such materials may include, for example, alumina,
zirconia, zirconium, silicate, aluminium silicate or
the mullite-rich material which is produced by
calcining kaolinitic clay at a temperature in the
range of from 1300°C to 1800°C. Alternatively,
particles of natural sand of a suitable particle size
may be used in the method of the invention.
Generally, the type of and particle size of
grinding medium to be selected for use in the
invention may be dependent on the properties, such
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as, eg. the particle size of and the chemical
composition of the feed suspension of inorganic
material to be ground. Preferably, the particulate
grinding medium comprises particles having an average
diameter in the range of from 0.lmm to 6.Omm and,
more preferably in the range of from 0.2mm to 4.Omm.
Preferably, the grinding medium (or media) may be
present in an amount of from 40o to 70% by volume of
the charge; and, more preferably in an amount of
about 60% by volume of the charge.
The suspension of particulate solid material to
be comminuted by the method of the invention may be
of an inorganic material and preferably a mineral
material. The mineral material may be used in its
naturally occurring form or its chemically
synthesised form. Such materials may include, for
example calcium carbonate, a kaolin clay and any
other suitable mineral material.
The particulate inorganic material may
preferably be present in an amount of from loo to 85%
by weight of the suspension; more preferably, the
inorganic material may be present in an amount of
from 20% to 80% by weight of the suspension. Most
preferably, the inorganic material may be present in
an amount of about 30% to 75% by weight of the
suspension.
The suspension of solid material to be
comminuted may be of a relatively high viscosity, in
which case a dispersing agent may preferably be added
to the suspension prior to comminution by the method
of the invention. The dispersing agent may be, for
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example, a water soluble condensed phosphate, a water
soluble salt of a polysilicic acid or a
polyelectrolyte, for example a water soluble salt of
a poly(acrylic acid) or of a poly (methacrylic acid)
having a number average molecular weight not greater
than 20,000. The amount of the dispersing agent used
would generally be in the range of from 0.1 to 2.0%
by weight, based on the weight of the dry particulate
solid material. The suspension may be introduced
into the grinding chamber of the apparatus at a
temperature in the range of from 4°C to 100°C.
An embodiment of the invention will now be
described by way of example only and with reference
to. the accompanying drawings, in which:
Figure 1 is a sectional view in schematic form
of an apparatus embodying the invention which is
arranged to grind a suspension of a particulate solid
material by a method embodying the invention.
Figure 2 is a perspective view from above of a
pitched impeller for use in an apparatus embodying
the invention as illustrated, for instance, in Figure
1.
Referring to Figure l, there~is shown a grinding
apparatus generally designated by reference numeral 1
comprising a grinding chamber 2 containing a mixture
3 of a particulate grinding medium and a suspension
of a particulate solid material. A rotatable
vertically orientated shaft 4 is suspended in the
grinding chamber 3, an upper end of the shaft 4 being
supported above the grinding chamber. A motor 5 is
connected to the shaft by means of a suitable gear
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arrangement. A double pitched impeller 6 having two
tiers 6a, 6b is secured to the shaft 4. A conduit 7
having an inlet opening 8 is provided at the base of
the grinding chamber 2 to supply a feed suspension of
particulate solid material to the chamber 2 to be
comminuted. The inlet opening 8 is centrally located
at the base of the grinding chamber 2 under a free
end of the vertically orientated shaft 4.
An annular baffle plate 16, located at a preset
height above the impeller 6, is fixed to a side wall
18 of the grinding chamber 2. The drive shaft 4
extends through an opening 20 in the baffle plate 16.
Referring to Figure 2, there is shown a tier
(6a, 6b) of the pitched impeller 6 for use in the
apparatus of Figure 1. The pitched impeller 6
comprises an annular body 11 having a central opening
for disposing the impeller 6 onto the shaft 4, and,
six blades 12 radially projecting from the annular
body 11. Each blade 12 is pitched at an equivalent
angle of about 45° relative to the plane of rotation
of the impeller 6, such that a leading face 14 of
each blade in the direction of rotation R of the
impeller faces in a direction with a downward
component.
As the feed suspension is supplied to the
grinding chamber 2, the shaft 4 and the pitched
impeller 6 rotate to fluidise the particulate
grinding medium and cause mixing of the grinding
medium with the suspension of particulate solid
material, resulting in grinding of the particulate
solid suspension.
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The action of the pitched blades 6 and baffle
plate 16 combine to compress the particulate solid
suspension and improve the grinding efficiency.
The comminuted solid suspension comprising
particles of a desired particle size exits the
grinding chamber 2 through a mesh 9 and is carried
along a conduit 10 for further refining if necessary.
The invention will now be described with
reference to the following example.
Example
A kaolin suspension containing 30% by weight
solids was introduced through inlet conduit 7 into a
grinding apparatus of the type illustrated in Figure
1. Silica sand present in an amount of 60% by volume
was used as the grinding medium in the apparatus. The
magnitude of energy applied to the apparatus was
75kWh/t. The comminuted suspension obtained from the
apparatus through conduit 10 was found to contain 92%
by weight of particles having a particle size less
than 2~m.
The above experiment was also carried out using
conventional prior art apparatus and a ground kaolin
suspension containing 88% by weight of particles
having a particle size less than 2~Zm was obtained.
These results show that an improved comminuted
product suspension having a finer particle size can
be produced by use of the apparatus and method of the
invention.
