Note: Descriptions are shown in the official language in which they were submitted.
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MILL WORKING BY WET PROCESSING AND EQUIPPED WITH A
PARTITION, AND OUTLET PARTITION INTENDED FOR THE MILL
Subject of the invention
The present invention relates to grinding mills
working by wet processing, either of the autogenous
type, also known as FAG (Full Autogenous Grinding)
mills, or of the semi-autogenous type, also known as
SAG (Semi Autogenous Grinding) mills, but also relates
to ball mills using grid discharge.
The present invention relates more specifically
to an outlet partition arranged in such mills.
Technological background
There are two main families of mills that exist
side by side; on the one hand, there are mills which
work by wet processing and, on the other hand, there
are mills which work by dry processing. In general
terms, the solutions put forward for mills that work by
dry processing cannot be read across directly to mills
which work by wet processing.
The various types of mill are fitted with an
outlet partition which allows the ground matter to be
discharged towards an outlet trunnion. In some
instances, the mills may be equipped with intermediate
partitions separating two successive grinding chambers.
Mills which discharge through a grid comprise a
partition generally made up of a frame, the upstream
face of which is covered with grids pierced with a
number of openings and which thus allow the
sufficiently-ground matter to pass through while at the
same time holding back the matter that is
insufficiently ground, and the grinding bodies. The
upstream face of which is some distance from the
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downstream face, which is preferably solid with the
exception of a central discharge opening.
There is thus defined within the partition a
small compartment which the matter which passes through
the openings enters. This matter is able to circulate
diametrically within the compartment. Lifting elements
or lifters are provided in this partition so that as
the mill rotates, the matter is lifted above the axis
of the mill, from where it falls back down, possibly
onto a deflector member, to be directed towards the
central discharge opening.
In the case of mills which use overflow
discharge (100% overflow), the mills have rather a
tendency to keep the matter inside the grinding
chambers for too long, which produces undesirable
overly ground matters, the retention time being too
long and the mill too full, which causes a certain
slipping of the charge and reduces the throughput of
the mill.
There is no deflector member in this type of
mill, and the ground matters are removed directly by
overflow via the central discharge opening towards the
outlet trunnion.
In the case of mills using 1000 grid discharge,
there is a deflector member present which forces the
matter to follow a favoured path towards the outlet
trunnion.
In this type of mill, it can be seen that these
mills are often underfilled because they discharge the
matter more and sometimes too quickly and therefore
have a higher throughput. However, given that too
little matter is held within the grinding chamber, the
mill does not operate optimally. Furthermore, in the
specific instance of ball mills, substantial wear of
the grinding bodies is observed, and this is due to an
increase in the direct (without matter in between)
metal/metal impact either between the grinding bodies
or between the grinding bodies and the lining. The loss
of energy without any grinding effect, which is caused
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by these impacts and especially by the lack of matter
to grind, should also be taken into account.
Several methods of regulating the passage of
matter through the number of openings in the upstream
wall of the partition have been proposed in order to
control the level of matter within the grinding
chamber, and therefore the retention time.
Document US-A-1787897 describes a partition
intended to be placed between two successive grinding
compartments and the frame of which consists of several
segments joined together, each segment comprising two
arms and a part corresponding to a portion of the
circumference which is intended to hug the lining of
the shell, the two arms being connected at the opposite
end of the portion of the circumference by an
essentially conical portion so that when the various
frame segments are joined together, they create a
conical deflector element. The upstream face of the
wall covering the frame of the partition has openings
(or apertures) in the conventional way, so as to allow
sufficiently ground matter to enter the partition,
whereas the rear face has no openings except for an
annular discharge passage placed at the periphery of
the conical element so as to remove the ground matters
from the partition.
Publication FR-A-2126051 describes a method and
a device which allow the amount of matter mixed with
the grinding bodies in a grinding compartment of a tube
mill of the ball type for example to be regulated, and
in which grinding can take place either working by dry
processing or working by wet processing. There is
described in this document a partition intended to
separate two successive grinding chambers or to be used
as an outlet partition, which comprises means of
regulating the level of matter within the grinding
chamber, this partition consisting of vanes pivoting
about a longitudinal axis so as to allow the useful
volume to be adjusted or comprising a telescopic part
also allowing this useful volume to be adjusted.
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Document DE-477135 describes a deflector member
in the form of a mobile cone secured to the mill, which
allows the matter from the lifting elements (or
lifters), in this case paddles, to be returned
partially or in full towards the compartment upstream
of the partition so that insufficiently ground
particles which have entered the partition can be
reground. Using this system, the matter passes once
more through a grid, which in theory could allow to
control the time matter is retained within the mill.
However, since this cone is an integral part of the
mill, a substantial proportion of the regulating
mechanism operates inside the mill. In actual fact,
this device is particularly vulnerable because of wear
and seizure through the jamming of grains and steel
waste.
Various devices allowing some of the ground
matter to be sent back to the grinding chamber have
also been proposed.
Document FR-2261812 describes a very simple
system for sending back to the grinding chamber the
matters which are insufficiently ground (also known as
"pebbles") associated with already worn grinding
bodies, and which essentially comprises, in the central
region of the end wall, a discharge tube which is
associated with a two-way valve, so that the
insufficiently ground matters can be led back to the
grinding chamber while the sufficiently ground matter
leaves this grinding chamber.
Document GB-A-2064364 describes a drum mill
intended for a wet treatment of the autogenous or semi-
autogenous type, and which has a partition adj acent to
the outlet partition itself equipped with a trunnion
for discharging the matter which is separated from the
outlet partition by a space, the said upstream wall
comprising, on the one hand, a first group of
relatively small openings through which the finely
ground matter leaving the mill can pass and travel
towards the outlet trunnion and, on the other hand, a
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second group of relatively larger openings through
which matter containing both fine particles and coarse
particles can leave the mill. This second group of
openings communicates with a matter outlet via
deflection means which prevent this mixture from
leaving the mill. In this case, the deflector elements
act only on some of the matter, that is to say the
proportion which has come from the relatively large
openings, and not on all the matter from all the
openings. This means that it will be impossible to
correctly control the time for which the matter is
retained inside the mill.
Document US-3078050 describes a mill of the
autogenous type in which the matter is taken from the
grinding chamber by a "fall" that enters the grinding
chamber, the said fall having appropriate positioning
means for removing the matter from the mill in a
controllable way. This means that the mill described in
this document has neither grid nor discharge chamber.
Furthermore, sustaining the falling conditions in a
grinding chamber in a mill of the autogenous type is
relatively problematical. This becomes impossible in
the case of mill of the semi-autogenous type, or in the
case of a ball mill.
Document WO 86/04526 describes a device for
regulating the time that a matter is retained in the
mill, which may be an autogenous mill, a semi-
autogenous mill or a ball mill, and which comprises a
reservoir chamber upstream of the outlet end. This mill
is characterized in that it comprises a discharge duct
more commonly called a "spoon" more or less coaxial
with the mill, detached from the latter and
longitudinally movable. This spoon enters the reservoir
chamber through the outlet end of the mill and at its
upstream end has a first opening made in its upper
part. This spoon can be advanced as far as close to the
upstream face of the reservoir chamber. It should be
noted that this device known as a "spoon" is stationary
relative to the rotation of the mill, and will not be
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able to withstand the moving mass if used in large
mills. Furthermore, given that it is a stationary
element, a particularly large clearance needs to be
provided between the said spoon and the mill.
It should be noted that most of the regulating
methods aim to propose either means allowing the
insufficiently ground matter to be reinjected into the
grinding chamber if necessary, or propose devices for
partially blocking off the discharge region in order to
increase the retention time and/or the amount of matter
retained within the grinding chamber.
Nonetheless, last devices proposed are
adjustable and mobile devices that can only be
envisaged for small-sized mills. The present invention
particularly aims at large and very large mills, the
diameter of which may be as much as 12 metres or more.
Document US-A-2344162 describes a device for
deflecting the ground matter towards the outlet
trunnion and to send back to the grinding chamber the
unground matter and the worn balls, whereas the aim of
the device according to the present invention is also
to deflect the ground matter towards the outlet
trunnion but to eliminate the unground matter and the
worn balls towards the outside of the mill so that they
do not enter the grinding chamber because they would
alter its effectiveness.
Document DE-A-549694 describes a mill working
by dry processing in which an intermediate partition
between two successive grinding chambers is present.
Nowhere can it be seen that there is regulation of the
level. Only a stream of air is used to separate the
fine particles from the matter that is to be ground.
Furthermore, the intermediate partition described has
no grid.
Document US-A-2815176 again describes a mill
working only by dry processing and intended for
applications in which there is a desire to mix fine and
coarse matters in a given percentage for producing
graphite electrodes. Once again, use is made of a
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stream of air to eliminate the fine particles. Finally,
this device has no grid and will not be able to
separate the pebbles and there is no envisaged
adjustment of the grinding level in the said mill.
Document DE-A-2848343 describes a device
comprising an intermediate partition between two
successive grinding chambers. It will be seen that a
spindle or linkage passes through the various grinding
chambers . The fact that such a spindle is present will
not allow this to be used in large mills, given that
this spindle is incapable of withstanding the
significant masses in movement in this case. Finally,
it can be seen that all the matter without distinction
can pass from one chamber to the other.
Objects of the invention
The present invention relates to a mill working
by wet processing, either of the autogenous type or the
semi-autogenous type, or also operating essentially by
a grid discharge but which at the same time combines
the advantages of a mill with grid discharge and a mill
with overflow discharge.
More specifically, the present invention aims
to propose novel means present in the partition which
make it possible to influence the amount of matter in
the grinding chamber by preventing this matter from
leaving the mill too fast or in excessive quantity.
The present invention aims to propose a
solution which is particularly simple from a mechanical
point of view and which does not require the fitting of
moving parts, as this would be unacceptable in certain
applications, particularly in the case of large or very
large mills.
Furthermore, the present invention aims to
propose a solution which also makes it possible to
avoid the discharge chamber becoming blocked with the
insufficiently ground matter ("pebbles") and with the
already worn grinding bodies.
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Main characteristic features of the present invention
The present invention relates to a mill working
by wet processing, of the autogenous type, semi-
autogenous type or a ball mill, using grid discharge
and comprising an outlet partition between the grinding
chamber and the outlet trunnion. The said partition has
two walls, the volume between the upstream wall and the
downstream wall forming a compartment which the matter
enters. This matter can circulate diametrically across
the said compartment, which is called "discharge
chamber".
The upstream wall is a wall essentially covered
with pierced grids comprising a number of openings
which allow the sufficiently ground matter to pass into
the said compartment inside the partition.
The downstream wall, which is separated from
the upstream wall, is preferably solid with the
exception of the central part which remains open and
faces towards the discharge trunnion.
The present invention is characterized by the
fact that the said partition comprises means secured to
this partition allowing some of the matter to pass
(partially) into the central region of the compartment
made between the upstream wall and the downstream wall
of the partition and therefore towards the grinding
chamber upstream of the said partition, the remainder
of the matter being removed towards the outlet
trunnion. This then makes it possible for the mill to
run as a hybrid partly by discharge and partly by
overflow.
The fact that the means are secured to the
partition has the effect that these means will be
subject to the rotation of the mill and can thus be
used even in very large mills.
According to a first preferred embodiment of
the present invention, the means allowing partial
passage through the central region of the partition
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consist of deflectors in the form of metal sheets
connecting the upstream wall to the downstream wall in
the central region of the compartment. If all the
deflectors are fitted, a discharge cone or pyramid and
100% discharge operation are obtained.
To make it possible to obtain a modulate
operation of the removal of the matter from the
partition between 0 and 1000 overflow, only some of
these deflectors will be fitted, the others being
removed.
The fact of fitting deflectors without openings
in a certain number of sectors of the partition allows
some of the ground matter to be deflected towards the
outlet trunnion, the remainder dropping into the
discharge chamber in such a way as to keep a
practically constant level in the mill.
According to a second embodiment of the present
invention, the means allowing a partial passage through
the central region of the partition to be achieved
consist in fitting a truncated cone in this region. To
allow the removal of matter from the partition to be
altered between 0 and 100% overflow, use is again made
of a truncated cone which does not cover all the way
across the discharge chamber. In this case, there will
be an annular passage on the upstream side of the cone,
which will allow a partial drop of the matter into the
discharge chamber.
The fact of placing a more or less complete
truncated cone in the internal compartment of the
partition makes it possible to determine the amount of
matter directed towards the outlet trunnion and the
amount of matter dropping into the discharge chamber,
so as to keep a practically constant level in the mill.
Particularly advantageously, deflector grids
have also been placed in the compartment inside the
partition in the most diametrical part thereof, that is
to say towards the periphery. These grids allow matter
or worn or broken grinding elements to be removed
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directly to the outlet trunnion without allowing them
to be sent back to the discharge chamber.
Openings which allow the finer matter to pass
through, possibly with return to the grinding chamber,
may optionally be provided in these deflector grids.
To allow matter to be removed from the
partition, provision is also made for the downstream
wall to have an annular opening arranged around the
region of attachment of the deflectors or of the cone.
Brief description
of the figures
Figure 1 depicts a view in longitudinal section of a
ball mill with just one grinding chamber and
fitted with a customarily-used outlet
partition.
Figure 2 depicts a view in longitudinal section taken
from Figure 1 and corresponding to a first
embodiment of a partition fitted with means
of regulating the level of matter within the
grinding chamber.
Figures 3 depict several views in cross section on the
axis X-X' of the partition depicted in Figure
2 seen from the trunnion end, for various
mill charge conditions.
Figures depict several views in longitudinal section
4
on section A-A according to a second
embodiment of a partition equipped with means
for regulating the level of matter within the
grinding chamber for various mill charge
conditions.
Figure 5 depicts a view in cross section on the axis
X-X' of the partition of Figure 4D seen from
the trunnion end in various mill charge
conditions.
Figure 6 depicts a detail view of a grid used in a
partition like the one described in Figure 5.
Figures 7 depict the options for the regulating means
used in the second embodiment.
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Description of several preferred embodiments of the
i n « o n t i n n
In the figures, identical references relate to
identical or similar elements.
The mill in Figure 1 is a mill which works by
wet processing and which is formed of a shell 1 borne
by two conical ends 2 and 3. The outlet conical end 3
is secured to an outlet partition 15 of which an outlet
trunnion 4 forms an integral part. On the inlet side,
there is a trunnion 5 which also forms an integral part
of the end 2. The trunnions are borne in bearings (not
depicted) in Figure 1. The mill is driven by an annular
gear and a pinion (not depicted). To protect the mill
from wear, the shell 1 and the inlet and outlet ends 2
and 3 are covered with lining 8 and 9. Furthermore, the
shell 1 has a manhole 7, the cover of which is formed
of a part of the lining attached with clamps 10 and
bolts 11.
The mill depicted in Figure 1 has just one
grinding chamber 6 which is partially filled with
grinding bodies 17 which in this instance are metal
balls, and with matter to be ground.
Specific examples of the partition are depicted
in greater detail in Figures 2, 3, 4, 5 and 7.
Customarily, the partitions essentially consist
of a frame, itself comprising a certain number of
sectors. In the various embodiments depicted in the
figures, twelve steel sectors are joined together to
create a partition. In the case of an outlet partition,
these sectors directly hug the end 3 of the mill and
are bolted to it by bolts (not depicted in the
figures).
The partitions depicted in the various
embodiments have an upstream face and a downstream face
which define a volume 12 called the discharge chamber.
Customarily, the upstream face of the various sectors
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to separate coarse or insufficiently ground parts and
worn balls from the fine particles. These deflector
grids 18 also allow the insufficiently ground matters
(pebbles) to be directed directly and selectively
towards the outlet trunnion so that they can be
removed.
The present invention aims to provide means
which will allow the regulating of the level of matter
in the compartment 12 of the partition and hence in the
grinding chamber 6 which communicates with the
compartment 12 via the grids 9, by deflecting a greater
or lesser amount of matter towards the outlet trunnion
while the undeflected matter falls into the compartment
12.
According to the embodiment depicted in Figures
2 and 3, all that is required is for deflectors 19 not
to be fitted to a certain number of sectors of the
partition, thus not forcing some of the matter
contained in this sector to move towards the outlet
trunnion.
This embodiment is depicted in detail in
Figures 3a, 3b, 3c and 3d. In the case of Figure 3a,
all the deflectors 19 are fitted, which forces all of
the matter to move towards the outlet trunnion and thus
prevents the mill from operating in overflow mode.
Figure 3d, in the other hand, depicts an arrangement in
which none of the deflectors 19 is fitted, which allows
the mill to operate in 100% overflow mode. Figures 3b
.and 3c depict situations which are part way between the
100% grid (0% overflow) and 100% overflow situations,
namely 50% and 75% overflow.
According to the embodiment depicted in Figures
4, the frustoconical part is produced using a cone.
According to the present invention, the cone is
a cone which is truncated at the base, that is to say
that it makes it possible to open a passage at the base
of the cone on the upstream side of the partition. This
allows some of the matter to pass towards the outlet
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joined together is covered with grids 14, also made of
steel, which rest on the corresponding sectors.
These various grids, an example of which is
depicted in greater detail in Figure 6, are pierced
with a number of openings or apertures 20 which allow
sufficiently ground matter to pass through. Wider
openings 21, known as "pebbles ports" and located at
several places on these grids make it possible to avoid
the build-up of matter very difficult to grind and worn
grinding elements in the grinding chamber, as this
would reduce the efficiency of the latter.
The openings 20 in the grid widen from their
inlet at the upstream face of the grid towards their
outlet at the downstream face of the grid so that the
particles which enter them can easily get out. The
grids 9 may possibly be fitted with ribs (not depicted
in the figures) on the upstream face side.
The centre of the upstream wall of the
partition is in the form of an opening closed by a
metal sheet covered with rubber on both sides and fixed
to the frame so as to force the ground matter to pass
through the grids.
The downstream wall of the frame has a central
opening in the embodiment depicted in the various
figures, which communicates directly with the outlet
trunnion.
Customarily, the central part of the partition
is fitted with a discharge cone or deflector, the
vertex of which faces towards the outlet from the mill.
This discharge cone is generally in the form of a
frustoconical central part so that the matter present
in the compartment of the partition can be directed
towards the outlet trunnion. To achieve this, the base
of this frustoconical part faces in the upstream
direction, while its vertex faces towards the
downstream wall so as to cause the matter to slide
directly towards the outlet trunnion.
Furthermore, .this partition is fitted with
elements known as "deflector grids" 18 whose purpose is
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trunnion, the other part falling into the compartment
12.
More specifically, Figure 4a depicts a cone
which does not allow operation in overflow mode
(0% overflow) and occupies the whole of the central
region of the compartment 12. Figure 4d depicts the
conditions in which there is no cone fitted, allowing
operation in overflow mode (100% overflow). Figures 4b
and 4c depict intermediate situations.
Figure 7 depicts a partition in which the
second embodiment has been used. As already mentioned,
it can be seen that the upstream face is covered at the
periphery with grids which have openings, whereas the
central part is covered with a metal disc 16. The
downstream face consists at the periphery of the lining
or of a solid wall in the case of an intermediate
chamber, whereas the central part is free.
The two walls define a compartment, the central
part of which is fitted with radial elements which are
arranged in such a way as to hold either the cones or
the deflectors in place.
Figures 7b and 7c depict an exploded view of
the various elements that make up the central region of
the wall, wherein the various possibilities for the
height of cones are proposed. The partial attachment of
the cones is anticipated to be achieved by means known
per se, of the bolts, nuts, etc. type.
The choice of cone type is governed by the type
of matter to be ground and the desired fineness of the
matter, with a view to obtaining a correct level of
matter to be ground in the grinding chamber upstream.
