Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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This invention relates to a mill, particularly a tube mill or ball
mill, which has a polygonal, preferably square cross-section with rounded
corners and straight or less strongly curved sides, and in which the grinding
action is performed by the falling and rolling motion of the filling consist-
ing of the grinding elements and the material to be ground, the size of the
grinding elements, e.g., short cylindrical shapes (cylpebses) or balls, is
only a small fraction of the side length of the basic cross-section of the
mill.
For this reason the invention will be explained hereinafter mainly
with reference to such a mill, although the explanation is applicable with
corresponding modification also to the other mills of the kind described
which have a polygonal basic cross-section.
In a known mill having a square basic cross-section, the grinding
action can be strongly influenced by the selection of a proper radius for
the rounded portions. A decrease of the radius of the rounded portion
relative to the diagonal of the cross-section of the mill will result in a
stronger impact, and an increase of the radius of the rounded portions will
result in a lighter impact and in a stronger frictional action. Since the
successive individual annular courses are angularly displaced or offset, a
passage is formed in the mill which, owing to the square basic cross-section
of the millJ is similar in shape to a four-thread screw. Owing to this
design, an additional impulse in the direction toward the axis of the mill
is imparted to the grinding elements. Besides, the helical design also
influences the flow of the material being ground in the longitudinal
direction of the mill because the flow of the material being ground can be
braked or accelerated by a suitable selection of the lead or hand of the
screw resulting from the offset of the annular courses. The lead of this
helical passage and the hand thereof can be determined by the selection of
the angle between successive annular courses.
A
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The movement of the material being ground through the mill also
depends strongly on the specific gravity of the material being ground.
Easily flowing materials to be ground in a dense bed of grinding elements
tend to remain on the surface and virtually to float rather than to enter
the bed of grinding elements. On the other hand, thorough mixing of the
grinding elements and the material to be ground is required particularly
for a very fine grinding. If the individual annular courses are relatively
offset by up to 45 to each other in the direction of rotation, material
to be ground which has a higher specific gravity will move from the inlet
of the mill to its outlet. If the annular courses are offset opposite
to the direction of rotation, the movement will be retarded and the
material will be retained because the screw promotes a movement in the
opposite direction.
Practical operations and experiments have shown that the helical
passage formed in the mill by the offset annular courses also has a certain
sorting action on the grinding elements, so that most of the larger grinding
elements collect at the inlet and most of the smaller grinding elements col-
lect at the outlet. This effect is not complete, however, because it is
obtained only with grinding elements, such as balls or short cylindrical
shapes (cylpebses), up to 40 mm. at most. For non-fine grinding it is
necessary, however, to use also grinding elements which are larger in
diameter than 40 mm., e.g., grinding elements which are 60 to 70 mm. in
diameter; this is the si7e of the grinding elements in the second chamber
of a multichamber mill.
In an economical grinding operation, large grinding elements must
be used to grind the coarse material, and the size of the grinding element
e.g., balls, must progressively decrease as the material becomes finer. So
far, this has been accomplished by the mill being divided in its longitudinal
direction into chambers, so that the different grinding elements, e.g.,
balls of greatly different si~es, cannot mix with each other, whereas
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the material being ground can pass through slots in the chamber walls to the
next chamber nearer to the outlet.
An additional impulse is re4uired to improve the sorting action on the
grinding elements in the above-mentioned known mill and particularly to provide
a sorting action also for larger grinding elements, so that they remain in the
desired region of the mill. This holds true for all mills of the present
kind which have a polygonal basic cross-section with rounded corners and
straight or less strongly curved sides. It is an object of the present
invention to provide such additional impulse.
The invention provides a mill having at each position along its
length an internal cross-section which is substantially polygonal and is de-
fined by rounded corners between sides which are straight or less curved than
the corners, and in which the grinding action is performed by the falling
and rolling motion of a filling consisting of grinding elements and the
material to be ground, the dimensions of the grinding elements being only a
small fraction of the length of any of said sides of the mill, wherein the
interior of the mill is defined by liner plates forming a series of annular
courses arranged adjacent each other in the longitudinal direction of the
mill, each of said annular courses consisting of a plurality of liner plates
and at least some of said annular courses being angularly displaced or offset
relative to adjacent annular courses, and wherein plates of at least some
annular courses have inside surfaces that are inclined to the longitudinal
axis of the mill and are convergent in the direction of travel of the material
being ground, and remaining plates, if any, have inside surfaces which are
parallel to the longitudinal axis of the mill. Such plates having inside
surfaces which are inclined toward the axis of the mill and in the direction
of travel of the material being ground will be referred to hereinafter as in-
clined plates. The inside surfaces of the inclined plates may be inclined
throughout the total width of the respective annular course, or they
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may be inclined only in part of the width of the annular course whereas they
e-xpand parallel or almost parallel to the longitudinal axis of the mill in the
other part of the width of the annular course. In a specific embodiment, at
least every third annular course, and preferably every other annular course
comprises inclined plates. It will be particularly desirable if the mill
has a square basic cross-section with rounded corners and straight sides and
at least every third annulsr course and preferably every other annular course
is composed of inclined plates having inside surfaces which are inclined to-
ward the interior of the mill and in the direction of travel of the material
being ground, and the plates are preferably so arranged that plates which
have inside surfaces extending in the longitudinal direction of the mill paral-
lel to the axis of the mill alternate with plates which have inside surfaces
which are inclined to the axis of the mill, the annular courses composed of
plates having inside surfaces that are parallel to the axis of the mill are
mutually offset and the annular courses composed of the inclined plates are
also mutually offset.
The angle of inclination (angle ~) of the inside surfaces of the
inclined plates may be S to 30, preferably 5 to 15. The inclined surface
of the inclined plates need not be flat but may be concaVely curved. Besides,
i* is not necessary to use inherently inclined plates but the inclination may
be provided in that plates having inside surfaces extending parallel to the
axis of the mill are provided with protruding strips which extend at a de-
sired angle other than 90 to the axis of the mill.
In a development of the invention, an annular course comprising
plates having inside surfaces which are parallel to the axis of the mill and
an adjacent annular course which comprises inclined plates may be combined in
a unit, and each of said units may be offset from the adjacent unit.
The angle (~) by which the annular courses or units are angularly
spaced or offse~ relative to each other may preferably be between 15 and
50.
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In general, in a mill of the present kind the balls or other grind-
ing elements will be sorted more quickly if a larger number of annular courses
are composed of inclined plates. The fastest ball-sorting action will be ob-
tained if all annular courses comprise inclined plates, and in this case the
grinding elements above 60 mm. will be more accurately sorted into several
fractions. The use of plates whose inside surfaces have the same inclination
in all annular courses affords the additional advantage that only one type of
plate is required to line the mill. It is known, however, that grinding el-
ements in different sizes are mostly desired in the coarse grinding region of
the mill. The optimum lining can be selected for any material to be ground.
Best results will be obtained in all cases with those mills accord-
ing to the invention which have a square basic cross-section with rounded
corners and straight sides and in which the radius of the rounded corners is
preferably approximately one thirt of the side length of the basic square.
To minimize the loss of grinding space in mills which are larger in diameter,
a polygonal basic cross-section rather than a square one may be used in spe-
cific cases although best results will be obtained only if the geometric re-
quirements relating to the ratio of the side length to the curvature of the
corners are complied with and inclined plates are used or co-used in accord-
ance with the invention.
Annular courses of plates having inclined inside surfaces areknown per se but only in mills having a circular cross-section. In the known
embodiments either each annular course has an inclined inside surface or the
inclined surfaces are steep and high and are formed by separate internal fix-
tures rather than by inclined liner plates. That system is subject to heavy
wear. ~nly the design according to the invention, and particularly the com-
bination of annular courses which consist of plates having inside surfaces
that are parallel to the axis of the mill to provide for a square cross-sec-
tion of flow, and annular courses of plates whose inside surfaces have a re-
latively slight inclination in such a manner that every other or every third
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annulsr course is composed of inclined plates, whereas annular courses of the
same kind, or units consisting of an annular course of plates having parallel
inside surfaces and an annular course of inclined plates, are mutually offsetJ
will result in a satisfactory grinding as well as a sorting and also a small
wear of the liner. It may be mentioned here that the known mills having a
circular cross-sectiOn provide for a sorting of grinding elements above 60 mm.
or above 40 mm. but it is very difficult in such mills to sort small grinding
elements below 40 mm. and particularly below 20 mm. Experiments have shown
that only the design of the mill lining according to the invention enables an
ent1rely satisfactory sorting of all grinding elements without a division of
the mill into chambers.
A mill according to the invention having a square basic cross-sec-
tion with rounded corners and straight sides will now be described more fully
with reference to two embodiments shown by example in the drawing.
Figure 1 is a transverse sectional view showing such a mill.
Figure 2 is a longitudinal sectional view taken on line A-B and show-
ing a portion of the mill ccnsisting of straight and inclined plates, and
Figure 3 is also a longitudinal sectional view taken on line A-B and show~ a
portion of a mill consisting only of inclined plates.
The mill has a cylindrical shell M, in which annular courses of
liner plates are installedO These courses are arranged one behind the other
in the axial or longitudinal direction of the mill. Those annular courses
which are accommodated in the portion shown in Figure 2 are designated el,
fl...e4,f4 in Figure 2. Reference characters e designate annular courses of
plates which have inside surfaces that are parallel to the axis of the mill.
The plates of the annular courses designated with the reference character f
have inside surfaces which are inclined at an angle of inclination ~ toward
the interior of the mill and in the direction of flow, indicated by the arrow,
of the material being ground. The angle of inclination of the inside surfaces
of these inclined plates will depend on the requirements, namely, the nature
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of the material being ground, etc., and to a certain degree also the width
of each plate. It has been found that it is generally desirable to select an
angle ~ in the region of 5 to 15, although good results may be obtained in
special cases with different angles ~ up and above 30. Alternatively, dif-
ferent inclinations may be s01ected in successive annular courses f or some
of them or groups of them compared to the inclination in preceding or succeed-
ing annular courses f or groups thereof. The width of each plate and of each
annular course may be, e.g., 250 mm. or one-half that distance. In a special
embodiment, one annular course e and one annular course f may be combined in
a unit in that plates are used which extend continuously over the correspond-
ing width and are bent so that their inside surfaces are inclined in part of
their dimension extending in the longitudinal direction of the mill. To clar-
ify the combination of two annular courses e, f in a group or unit, these
pairs of annular courses are designated El...E4 in Figure 2. These combined
annular courses or units are offset or angularly spaced relative to the suc-
ceeding and preceding annular courses by an angle a (see Figure 1) whereas
there is no offset within such combined annular courses or units.
To clarify the offset in Figure 2, the parting lines between each
annular course e and the adjacent annular course f are represented by a thin-
ner line than the parting lines between successive units El-E2, E2-E3, etc.
For the sake of clearness, the parting lines are represented in exaggerated
width; they have actually only a width of 2 to 3 mm. The thicker lines are
intended to indicate that the annular courses on opposite sides of the respec-
tive parting line are offset or angularly spaced. This offset will depend on
the conditions, such as the material to be ground, the desired velocity of
travel thereof through the mill, etc. Good results have been obtained with
an offset (angle a) of 15 to 45, although this statement is not intended to
impose a limitation on the selection of the offset.
The succession of annular courses of plates having inside surfaces
which are not inclined but parallel to the axis of the mill, and of plates
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having inside surfaces which are inclined toward the interior of the mill may
be modified in that only every third annular course consists of inclined plates.
The length of annular courses e of plates having inside surfaces which are
parallel to the axis of the mill, and annular courses f of inclined plates
can be varied as required so that units of, e.g., longer annular courses e and
shorter annular courses f of inclined plates are installed in the shell M of
the mill.
The mill according to Figure 3 is composed only of inclined plates
fl...f4; in other respects that embodiment is similar to that one of Figure 2.
Figure 1 illustrates the offset of annular courses or units. To a-
void an overcrowding of the drawing at the expense of clarity, only the inside
contours of the first two units El and E2 in Figure 2 (plates fl and f2 in
Figure 3) are shown; these are the inner edge contours of the inclined plates
of the annular courses fl and f2.
Different angles of offset could be selected between successive
annular courses e or f (or f') or units E over the length of the mill. The
_
variation of the local lead angle of the resulting helix defined at the begin-
ning of the specification will be selected in view of the nature of the desir-
ed grinding operation and the desired distribution or sorting of the different
sizes of grinding elements over the length of the mill.
To maintain the required sorting of the grinding elements also for
a long period of operation, it is not necessary in a mill designed according
to the invention to provide a plurality of chambers separated by partition
walls. Such partition walls can usually be entirely omitted, so that the mill
consists only of a single chamber. Even if grinding elements of extreme sizes
are used, only a single partition for the entire mill will be required, so
that the mill then comprises two chambers.
The result as regards the sorting of the grinding elements in a mill
according to the invention having a square basic cross-section and rounded cor-
ners and straight sides will now be explained with reference to a practical
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test made with such a mill.
A tube mill having a length of 9 m and 2.4 m, in diameter was fed
with balls in sizes of 20 to 60 mm. in the following proportions in percent by
weight:
Size of
balls, mm. 20 2S 30 40 50 60
% by weight 20.2 20.2 16.8 16.8S 14.30 11.6S
The mill was rotated in the clockwise sense. Each annular course
hat a witth of 250 mm. Each pair Gf annular courses were combined in a unit.
The first annular course of each unit was offset in the direction of rotation
by an angle of 15 from the first annular course of the adjacent unit. Every
other annular course had an inclination of 15.
The balls were fed as an unsorted mixture. The degree of fullness
was 26%. The mill was then operated for 24 hours at a speed of 0.7 times its
critical speet, so that a stationary equilibrium hat then been obtained as re-
gards the distributian of the balls. The distribution in the mill length por-
tions 0 to 1.5 m., 1.5 to 3.0 m., 3.0 to 4.5 m., 4.5 to 6.0 m., 6.0 to 7.5 m.,
ant 7.5 to 9.0 m., in the direction of flow of the material was then examined
with the following results:
Length portion
0 to 1.5 m.
Ball diameter, mm. 20 25 30 40 50 60
Proportion,
% by weight -- -- 0.5 10.9 30.4 58.2
Length portion
1.5 to 3.0 m.
Ball diameter, mm. 20 25 30 40 S0 60
Proportion,
% by weight -- 0.1 6.2 40.2 45.2 8.3
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Length portion
3.0 to 4.5 m.
Ball diameter, mm. 20 25 30 40 50 60
Proportion,
% by weight -- 2.1 41.0 47.0 9.9 --
Length portion
4.5 to 6.0 m.
Ball diameter, mm. 20 25 30 40 50 60
Proportion,
% by weight 0.5 38.2 56.5 4.6 0.2 --
Length portion
6.0 to 7.5 m.
Ball diameter, mm. 20 25 30 40 50 60
Proportion,
by weight 29.5 67.2 3.3 -- -- --
Length portion
7.5 to 9.0 m.
Ball diameter, mm. 20 25 30 40 50 60
Proportion,
% by weight 89.9 10.1
Por a check, the direction of rotation was reversed in some tests,
so that the mill rotated in the counterclockwise sense. To cause the mate-
rial being ground to flow in the same direction, the offset of the annular
courses (in this case too every other course was offse~ 15 in the counter-
clockwise direction of rotation) was changed. The ball mixture and the other
conditions were entirely the same. The following distribution of balls was
stated after an operation for 24 hours.
Lenth portion
O to 1.5 m. _
Ball diameter, mm. 20 25 30 40 50 60
Proportion,
~ by weight -- -- 0.13 5.77 30.8 63.3
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Length portion
1.5 to 3.0 m.
Ball diameter, mm. 20 25 30 40 50 60
Proportion,
% by weight -- -- 1.7 41.8 54.8 1.7
Length portion
3.0 to 4.5 m.
. ~
Ball diameter, mm. 20 25 30 40 50 60
Proportion,
% by weight -- 1.6 31.4 45.8 21,2 --
Length portion
4.5 to 6.0 m.
Ball diameter, mm. 20 25 30 40 50 60
Proportion,
by weight 1.3 38.8 56.1 3.8 -- --
Length portion
6.0 to 7.5 m.
Ball diameter, mm. 20 25 30 40 50 60
Propostion,
by weight 29.1 67.10 3.90 -- -- --
Length portion
7.5 to 9.0 m.
Ball diameter, mm. 20 25 30 40 50 60
Proportion,
% by weight 91.3 8.70 -- -- -- --
The same results as with balls are obtained with short cylindrical
shapes ~cylpebses) or other grinding elements and with any desired degree of
fullness. The liner designed according to the invention eliminates the need
for a subdivision into several chambers entirely or to a substantial degree.
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