Note: Descriptions are shown in the official language in which they were submitted.
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AGITATOR MILL
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to an agitator mill comprising a grinding receptacle,
which encloses a grinding chamber of a diameter D, which is closed
downwards by a bottom, and which has a top cover and a vertical central
longitudinal axis; an agitator, which has an agitator axis that is parallel to
the central longitudinal axis, and which is equipped with agitator imple-
ments inside the grinding chamber; an agitator drive for rotary actuation of
the agitator about the agitator axis; a grinding-stock supply which dis-
charges into the grinding chamber; a partial filling of the grinding chamber
with auxiliary grinding bodies of a diameter b which are movable in a bed
of grinding stock and auxiliary grinding bodies in a direction of flow; and a
device for grinding-stock suction and auxiliary-grinding-body separation,
which leads out of the grinding chamber, which comprises a plunge pipe of
an inside diameter d, which, by a bottom inlet, dips into the bed of grinding
stock and auxiliary grinding bodies and from which a suction pipe dis-
charges above the grinding receptacle, having a grinding-stock suction de-
vice.
Background Art
An agitator mill of the generic type is known from US 4 998 678; it com-
prises a rotarily drivable grinding receptacle, with a seal that serves as a
splash guard being provided between the grinding receptacle and a cover
which is non-rotatably mounted on the machine stand, serving as a lid.
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These agitator mills cannot be operated at overpressure. Grinding-stock
discharge is pressureless i.e., it takes place against atmospheric pressure.
In practice, numerous approaches towards separation of the auxiliary grind-
ing bodies from the grinding stock after a grinding job have been made and
published. Using screens and screen cartridges has become a familiar ap-
proach; however, they bear the risk of clogging and have a restricted sur-
face. Providing rotary separating devices has also been known; they are
comparatively complicated, tending to wear off in particular with abrasive
grinding stock.
US 2003/0116663 teaches an agitator mill of a design similar to the above,
with the grinding-stock/auxiliary-grinding-body separating device being
embodied in such a way that a plunge pipe is placed on an agitating disk
with a gap being left through which to suck grinding stock. Such a design
too tends to comparatively strong wear.
SUMMARY OF THE INVENTION
It is an object of the invention to embody an agitator mill of the generic
type in such a way that separating the auxiliary grinding bodies is put into
practice in a solid design, demanding but for minor constructional require-
ments.
According to the invention, this object is attained in that the plunge pipe,
at
its inlet, comprises a recess which - related to the direction of flow - is
formed on the downstream side of the plunge pipe; and in that an area of
the plunge pipe which - related to the direction of flow - is located on the
upstream side of the plunge pipe in direct vicinity to the inlet shields the
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recess in the direction of flow. The gist of the invention resides in that the
mixture of grinding stock and auxiliary grinding bodies is sucked out of the
agitator mill by way of a plunge pipe after the grinding process, segrega-
tion of the auxiliary grinding bodies taking place within the plunge pipe by
weight and inertia. By reason of gravity and entrained by the bed of auxil-
iary grinding bodies that passes below the plunge pipe, the auxiliary grind-
ing bodies directly return into the grinding chamber. This can be put into
practice very easily and at a low cost. The components that are used can be
protected from wear very easily and at almost no expense. Even tiny auxil-
iary grinding bodies can be segregated.
The agitator mill according to the invention is preferably employed for
grinding substances that will cause strong wear in the agitator mill. They
are in particular ceramic substances which are blended with water, forming
a comparatively low-viscosity slush of a grinding stock. Grinding stock of
this kind is of comparatively inferior value, restricting the cost of wear per
unit of weight of the grinding stock. As a result of the embodiment accord-
ing to the invention, the agitator mill can be safely operated for a long time
at almost no wear, which cannot be said for other auxiliary-grinding-body
separating devices. The design according to the invention permits the auxil-
iary grinding bodies to sediment from the grinding stock in the vicinity of
the device for grinding-stock suction and auxiliary-grinding-body separa-
tion. A sort of a pocket forms in the plunge pipe within the bed of auxiliary
grinding bodies, holding no or only very few auxiliary grinding bodies
which may rise in the plunge pipe together with the grinding stock.
The invention can in particular be used to advantage when the grinding
receptacle too is rotarily drivable, enforcing a flow of auxiliary grinding
bodies in the grinding chamber. As a result of the eccentric arrangement of
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the plunge pipe, the auxiliary grinding bodies which sink downwards in the
plunge pipe are entrained by the rotating bed of grinding stock and auxil-
iary grinding bodies, the plunge pipe mouthing into a grinding-chamber
area of high-intensity motion of the bed of grinding stock and auxiliary
grinding bodies, which is still supported by the eccentric arrangement of
the at least one agitator and in particular by the arrangement of the plunge
pipe.
In one aspect, the invention provides an agitator mill, comprising
a grinding receptacle;
which encloses a grinding chamber of a diameter D;
which is closed downwards by a bottom;
which has a top cover; and
a vertical central longitudinal axis;
an agitator;
which has an agitator axis that is parallel to the central longitudinal axis;
and
which is equipped with agitator implements inside the grinding chamber;
an agitator drive for rotary actuation of the agitator about the agitator
axis;
a grinding-stock supply which discharges into the grinding chamber;
a partial filling of the grinding chamber with auxiliary grinding bodies of a
diameter b which are movable in a bed of grinding stock and auxiliary grinding
bodies in a direction of flow; and
a device for grinding-stock suction and auxiliary-grinding-body separation,
which leads out of the grinding chamber;
which comprises a plunge pipe which is laterally spaced apart from the
agitator to form an area where there is a strong compaction of the flow in the
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direction of flow, the plunge pipe having an inside diameter d which, by a
bottom
inlet, dips into the bed of grinding stock and auxiliary grinding bodies and
from
which a suction pipe having a grinding-stock suction device that discharges
above the
grinding receptacle;
wherein the grinding-stock supply is a grinding-stock supply line that
projects
into the grinding chamber;
wherein the plunge pipe is disposed between the agitator and the grinding-
stock supply line;
wherein the plunge pipe, at said bottom inlet, comprises a recess which-
related to the direction of flow-is formed on the downstream side of the
plunge pipe;
and
wherein an area of the plunge pipe which-related to the direction of flow is
located on the upstream side of the plunge pipe in direct vicinity to the
inlet shields
the recess in the direction of flow;
wherein the grinding stock and the auxiliary grinding bodies are sucked out of
the agitator mill into the plunge pipe and the auxiliary grinding bodies are
segregated
in the plunge pipe by weight and inertia; and
wherein after a grinding process the auxiliary grinding bodies that pass below
the plunge pipe are entrained by gravity in the bed and are directly returned
to the
grinding chamber.
Further features, advantages and details of the invention will become ap-
parent from the ensuing description of an exemplary embodiment, taken in
conjunction with the drawing.
BRIEF DESCRIPTION OF THE DRAWING
Fig. 1 is a view of a vertical central longitudinal section of an agitator
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mill; and
Fig. 2 is a horizontal cross-sectional view of the agitator mill of Fig. 1.
DESCRIPTION OF A PREFERRED EMBODIMENT
The agitator mill seen in Figs. 1 and 2 comprises a substantially circular
cylindrical grinding receptacle 1, the central longitudinal axis 2 of which is
vertical i.e., the grinding receptacle 1 is vertically upright. It is closed
downwards by a bottom 3 that runs crosswise of the axis 2. By way of a
rotary bearing 4 that is concentric of the axis 2, it supports itself on a ma-
chine stand 5 which is only roughly outlined, the grinding receptacle 1 thus
being rotatable about the central longitudinal axis 2. A stand-5-supported
grinding-receptacle driving motor 6 is provided as a rotary drive of the
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grinding receptacle 1; the shaft 7 of the motor 6 is parallel to the axis 2,
driving the grinding receptacle 1 in the direction of rotation 10 by way of a
gear drive 8 and a gear ring 9 which is mounted on the bottom periphery of
the grinding receptacle 1. A corresponding reduction ratio of the gear drive
8 relative to the gear ring 9 enables the grinding receptacle 1 to be driven
at
a comparatively low speed. Of course, a wheel-and-disk drive may be used
instead of a gear drive 8.
An agitator 11 is disposed in the grinding receptacle 1, substantially - and
in this regard conventionally - comprising an agitator shaft 12 and agitator
implements 13 that are disposed thereon and stand out radially. In the pre-
sent case, the agitator implements 13 are agitating disks with passages 14.
The top portion of the agitator shaft 12 that faces away from the bottom 3
is run on a bearing 15. This bearing 15 is mounted on a frontal cover 16
which is not rotatable and supports itself on the machine stand 5 in a man-
ner not illustrated. Located between the cover 16 and the top edge 17 of the
grinding receptacle 1 is a splash guard 18 which is concentric of the central
longitudinal axis 2 of the grinding receptacle 1. The splash guard 18 is not
joined to the edge 17 of the grinding receptacle 1, the grinding receptacle 1
being rotatable and the lid in the form of a cover 16 being stationarily,
though removably mounted on the machine stand 5. The cover 16 and the
splash guard 18 not closing the grinding receptacle 1 pressure-proof, at-
mospheric pressure will prevail in the grinding receptacle 1; air may pene-
trate into the grinding receptacle 1.
The agitator 11 is actuated by an agitator driving motor 19, which is con-
nected to the machine stand 5 and the shaft 20 of which is parallel to the
agitator axis 21, but displaced therefrom by an eccentricity e. A belt drive
22 provides for transmission of actuation to the agitator shaft 12. The driv-
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ing motor 19 actuates the agitator 11 in the direction of rotation 23, which
may be the same as the direction of rotation 10; however, the directions of
rotation 10 and 23 may as well be opposite.
A grinding-stock supply line 24 leads through the non-rotary cover 16,
there being secured; its outlet 25 is in vicinity of the bottom 3 of the grind-
ing receptacle 1. In the embodiment seen, this line 24 is embodied in the
form of a flow deflector 26. This deflector 26 can have a deflection surface
27, as a result of which any flow 29 of grinding stock and auxiliary grind-
ing bodies (shown only in Fig. 2) that impinges thereon is deflected ra-
dially inwards. The line 24 is arranged in proximity to the wall 28 of the
grinding chamber. A grinding-stock feed pump 31 supplies grinding stock
to the supply line 24, the grinding stock being fed to the grinding recepta-
cle 1 through the outlet 25 in the bottom area thereof i.e., in vicinity of
the
bottom 3.
A pressure-proof device 32 for grinding-stock suction and auxiliary-
grinding-body separation passes from outside through the cover 16. It is
designed in the form of a circular cylindrical plunge pipe 33 that projects
into the grinding chamber 34 provided in the grinding receptacle 1. An
inlet 35 is located at its bottom end inside the grinding chamber 34. The
inlet 35 dips into the level 36 formed by the grinding stock and auxiliary
grinding bodies 37 filling the grinding chamber 34. The plunge pipe 33
projects into the bed of grinding stock and auxiliary grinding bodies that is
defined upwards by the level 36. At the top end outside the grinding cham-
ber 34, the device 32 comprises a portion 38 that is closed all around, ta-
pering in the shape of a funnel. A suction pipe 39 discharges upwardly
therefrom, including a grinding-stock suction pump 40. The tapering por-
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tion 38 is further provided with a vibration exciter 41 that can set the de-
vice 32 vibrating.
As seen in Fig. 2, the plunge pipe 33 is disposed between the deflector 26
and the agitator 11 where there is strong compacting of the flow in the di-
rection of flow 29. The substantially cylindrical plunge pipe 33 has a com-
paratively great inside diameter d of a dimensioning as permitted by the
plunge pipe 33 at the place specified. In relation to the inside diameter D of
the cylindrical grinding receptacle 1 i.e., in relation to the inside diameter
D of the grinding chamber 34, the following applies: lOd >_ D 24d, with in
particular 8D >_ D >_5d. As seen in Fig. 1, the diameter d of the plunge pipe
33 distinctly exceeds the diameter a of the suction pipe 39. The diameter d
of the plunge pipe 33 and consequently the diameter d of the inlet 35 con-
siderably exceeds the diameter b of the biggest auxiliary grinding bodies 37
used, with l Ob <_ d, and preferably 20b <_ d, applying. As for the diameter b
of the auxiliary grinding bodies 37, b >_ 2.0 mm applies. The diameter b of
the fresh, non worn auxiliary grinding bodies 37 is in the range of 2 to
10 mm, preferably 4 to 7 mm. As a rule they are made of steel or -
preferably - of ceramics such as A12O3 or ZrO2 or other suitable materials.
The bottom end, allocated to the inlet 35, of the plunge pipe 33 is provided
with a recess 42 that is located inside the flow 29 of auxiliary grinding bod-
ies seen only in Fig. 2. This is also where the recess 42 is shown in its cor-
rect position relative to the flow 29, whereas Fig. 1, for reasons of clarity,
illustrates the plunge pipe 33, inclusive of the recess 42, in a position ro-
tated by 90 about its longitudinal axis. Seen in the direction of flow 29,
the recess 42 is located on the downstream side of the plunge pipe 33 so
that, in relation to the flow 29, the recess 42 is covered or shielded by the
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bottom rear area 44 of the plunge pipe 33 that stretches as far as to the bot-
tom rear edge 43. Consequently, no or only few auxiliary grinding bodies
37 arrive in the recess 42. In the vicinity of the recess 42, a certain clear-
ance or free zone is produced below the level 36 in the plunge pipe 33,
holding no or only very few auxiliary grinding bodies 37.
As seen in Figs. 1 and 2, the plunge pipe 33 inclines counter to the direc-
tion of flow 29 of the grinding stock and the auxiliary grinding bodies 37 in
such a way that, seen from top to bottom, the inlet 35, in the direction of
flow 29, is in lead of the top end that comprises the tapered portion 38 and
the suction pipe 39. In this way, the bottom edge 43 and the bottom rear
area 44 of the plunge pipe 33 reaches even deeper into the bed of grinding
stock and auxiliary grinding bodies below the level 36, the upper area 45 of
the recess 42 being level therewith so that any air entering the grinding re-
ceptacle 1 in accordance with the arrow of flow direction 46 can as well be
sucked into the plunge pipe 33 whenever the level 36 drops below the top
edge of the recess 42.
The mode of operation is as follows:
Grinding stock in a purnpable condition i.e., as a rule in the form of slush,
is supplied through the grinding-stock supply line 24 by the grinding-stock
feed pump 31, so-called wet grinding taking place. The grinding chamber
34 holds a bed of auxiliary grinding bodies 37 in the form of the grinding
chamber 34 being partially filled with auxiliary grinding bodies 37, which
is defined upwards by a level 36. The agitator I 1 is actuated in the direc-
tion of rotation 23; the grinding receptacle I is driven in the direction of
rotation 10. The speeds are selected such that the bed of auxiliary grinding
bodies 37 is maintained as a compact bed, the auxiliary grinding bodies not
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being fluidized within the grinding stock. The auxiliary grinding bodies
start moving in the direction of the flow 29 in the grinding chamber 34, this
motion leading to great strain on the grinding stock accompanied with si-
multaneous comminution and dispersion of the grinding stock. In a station-
ary condition of the agitator mill, grinding stock is sucked off by way of
the device for grinding stock suction and auxiliary-grinding-body separa-
tion 32 i.e., grinding stock is sucked by the suction pump 40 in the device
32, with the pump 40 always being run at its nominal load. If the suction
rate of the pump 40 exceeds the feed rate of the grinding-stock feed pump
31, the level 36 will set automatically at the upper edge of the recess 42. If
the level 36 drops below the upper edge of the recess 42, air is sucked in
additionally, which reduces the liquid-suction rate of the pump 40. At a
reduced suction rate of the pump 40, the level 36 will again rise beyond the
upper edge of the recess 42, closing it air-tight. With no air penetrating any
more, the suction pump 40 will run at nominal load until the level 36 again
drops below the upper edge of the recess 42. In this way level regulation
takes place in the grinding chamber 34. With at best few auxiliary grinding
bodies 37 penetrating into the area of the plunge pipe 33 that is located in
the bed of grinding stock and auxiliary grinding bodies 37, these few auxil-
iary grinding bodies 37 do not move upwards in the plunge pipe 33 along
with the pumped stream of grinding stock; rather they will sediment down-
wards. This is still supported by the fact that the flow rate of pumped grind-
ing stock is very low in the plunge pipe 33 because of the great diameter d
thereof and because the density of auxiliary grinding bodies 37 is high
compared to the density of grinding stock. In addition, the grinding stock
has a very low viscosity, similar to that of water. Preferably, the grinding
stocks used are ceramic substances which are suspended in water, conse-
quently being comparatively mobile. Grinding of pasty or high-viscosity
liquids does not take place.
