An annular gap-type mill for continuously pulverizing in particular hard mineral substances comprises a grinding container (15) which accommodates an axially symmetrical inner body (13) whose outer surface defines with the inner surface of the grinding container (15) a grindimg gap (23). It is essential that the upper and lower region of the inner body (13) are tapered in opposite directions and adjoined to a common equatorial zone (24) of maxi diameter and that the outer surface of at least one of the regions is curved convexly. The grinding container (15) and/or the inner body (13) may be driven rotatingly. The annular gap-type mill is suited for wet and dry grinding and its quantitative and qualitative output is high.

The invention relates to an annular gap-type mill for continuously 
pulverizing in particular hard mineral substances comprising an outer 
grinding container accomodating an axially symmetrical inner body whose 
outer surface defines with the inner surface of the grinding container a 
grinding gap. 
Hard mineral substances (Mohs'hardness&gt;5) such as corundum, circonium 
dioxide, alumina, silicon carbide and similar substances have been 
pulverized predominantly hitherto by iron balls in ball mills. 
Considerable residence times of the material in the grinding chamber are 
involved therewith and all of the elements contacting the grinding stock 
and the iron balls are exposed to a very strong wear. Further, the noise 
developing with the grinding operation is very disturbing. Moreover, as an 
additionala disadvantage of such ball mills, the abrasion of the iron 
balls which gets into the grinding material must be washed out by 
complicated expensive means with the use of chemical processes. 
Annular gap-type mills of the above mentioned type comprising a cylindrical 
or frustoconical, straight-face, rotatable inner rotor (U.S. Pat. No. 
4,225,092) are supposed to incorporate an improvement over conventional 
ball mills. However, they are less suited for pulverizing hard mineral 
substances, and they are only economic in view of the comminution of 
considerably softer substances such as chalk and the like. This is 
particularly due to the behaviour of the grinding balls or grinding 
pellets in the grinding gap. While the grinding pellets pumped together 
with the grinding stock (slurry) from below into the grinding gap first 
are moved up in the latter by the pressure of the feed pump by which the 
grinding stock suspension is pressed into the annular gap-type mill, and 
by the rotational movement of the rotor, they sink down by gravity with 
decreasing pump pressure thus excluding a grinding operation in the upper 
part of the grinding gap. This may be avoided by increasing the feed pump 
pressure or the flow of the grinding stock to such an extent that the 
grinding pellets are held in the upper portion of the grinding gap. This 
involves the risk for the grinding pellets to be discharged together with 
the grinding stock, thus causing a reduction of the grinding output. 
Experience has shown that with an average flow rate of the grinding 
material, only the lower half of the grinding gap is more or less fully 
utilized for the grinding operation while the grinding output obtainable 
theoretically is only half-realised. Further, the high packing density of 
the grinding pellets in the lower part of the grinding gap causes a high 
wear of the surface of the rotor and of the grinding container. The rotor 
may be even blocked, in particular after a short rest period of the inner 
rotor or of the feed pump. Said risk shall be reduced in case of the 
foregoing annular gap-type mills in that the lower end of the rotor is 
provided with an impeller which, however, only will intensify another 
disadvantage of the annular gap-type mill to the effect that the grinding 
pellets which do not sink down are increasingly pumped together with the 
grinding stock to the discharge opening and thus are lost for the grinding 
operation. Moreover, the impeller is exposed to high wear caused by 
grinding pellets and grinding stock. Sometimes, screens are used to retain 
the grinding pellets in the grinding gap; however, they inhibit the 
discharge of the grinding material such has to even stop it if they are 
clogged by grinding stock and grinding pellets. 
According to another known annular gap-type mill (German laid-open print 
No. 28 11 899), a cone ring-shaped grinding stock container is used whose 
inner face confines with a cone ring-shaped rotatable displacement body a 
grinding chamber. In an annular plate carrying the displacement body, 
return channels for the grinding pellets are fitted to extend obliquely to 
the outside. Also in this case, said grinding pellets show the mentioned 
unfavorable behaviour, and, in spite of the circulation of the grinding 
pellets, the total height of both grinding gap portions practically is not 
used for the grinding operation. The grinding pellets present in the inner 
down-feed grinding gap portion are following rather than counteracting the 
grinding stock flow in discharge direction so that the operation performed 
in this part of the grinding gap is even less effective than in the other 
portion thereof in which gravity may cause a certain longer residence 
time. In a probable other embodiment, the grinding container may be 
adapted to be driven rotatably about the center axis. However this measure 
does not entail any advantages concerning an optimization of the degree of 
comminution, but, on the contrary, the grinding pellets are driven more 
quickly through the grinding gap down the inside and up the outside so 
that, by their shorter residence time in the grinding gap, the grinding 
effect is decreased. Besides, this known annular gap-type mill is only 
suited for wet grinding, while dry material may not be treated therewith 
at all. 
The prior filed but not yet published U.S. patent application Ser. No. 
766,111 offers a certain remedy in that the rotatable inner rotor and the 
stationary grinding container comprise a frustoconical, straight-faced 
lower part and an oppositely tapered, frustoconical, straight-faced upper 
part which include, within the range of the lower parts a grinding gap, 
and, within the range of the upper parts, an outlet cap whose lower end of 
maximum diameter ends in an annular chamber at the open upper end of 
maximum diameter of the grinding gap. Due to the annular chamber, a 
reduction of the amount of grinding pellets or of the grinding effect is 
avoided in that a predetermined grinding pellet surplus received by the 
chamber is adapted to form there a floating barrier layer to withold the 
active grinding pellets in the grinding gap. While the total grinding gap 
height is utilized this way in favor of the active grinding operation of 
the grinding pellets, which are prevented from sinking down in the 
grinding gap by hydrodynamics and centrifugal force, the height of the 
grinding gap is restricted to the lower part of rotor and grinding 
container thus resulting in an undesired decline of output. Further, said 
advantageous hydrodynamic effect is only inherent to wet grinding but not 
to the dry grinding operations which, however, is just frequently 
desirable in case of hard mineral substances, because their pulverized 
powders shall be further processed in dry so that wet grinding (with 
subsequent drying and disagglomeration) implies an energetic detour. 
Therefore, it is the object of the invention to improve an annular gap-type 
ball mill of the above mentioned type so that, by an increased effectivity 
in the grinding gap, an economically and technically perfect pulverization 
of hard mineral substances in wet and in dry conditions is possible. 
The problem is solved according to the invention in that the upper and 
lower region of the inner body are tapered in opposite directions and 
adjoined to a common equatorial zone of maximum diameter, the outer 
surface of at least one of the regions being curved convexly. 
Tests have shown that axially symmetrical bodies of the above mentioned 
shape allow to realise a relative optimum of the total of all requirements 
to be met in case of the performance of an annular gap-type mill: high 
ball filling degree in the grinding gap, high feed rate of grinding stock 
through the ball packing, high power receptivity of the balls from the 
driving source with a resultant high shearing effect of the balls from the 
qualitative (grinding fineness) and quantitative (grinding stock amount) 
viewpoint, no discharge of grinding balls by the flow (or transport) of 
the grinding stocks, said requirements being applicable to wet and to dry 
grinding operations as well. 
The mill of the instant invention will comply with said requirements, it 
being also possible to determine the mill character by the selected drive, 
as follows: 
If the mill is to operate in wet, the inner body (as a rotor) is to be 
driven; a hydrodynamic effect which then develops in the grinding gap 
acts, as a consequence of the oppositely tapered upper and lower regions 
of inner body and grinding container and of the convex curvature of at 
least one of the regions, against the gravity of the grinding pellets and 
of the grinding stock thus inhibiting their sinking in the grinding gap 
while due to the centrifugal force in the region of the maxi diameter, the 
grinding pellets are not discharged with the grinding stock. Hence, a 
separation of grinding stock and grinding pellets is achieved without the 
use of screens. The rising speed of the grinding stock in the grinding gap 
being dependent, on the one hand, on the speed of the inner body, the 
grinding effect may be influenced by the speed control. Thus, by avoiding 
the discharge of grinding pellets, the grinding effect may be varied and 
the desired fineness may be adjusted. The residence time of the slurry in 
the grinding gap is dictated, on the other hand, by the grinding stock 
delivery rate which may be controlled by the feed pump so that, by 
influencing this parameter, the grinding effect is changeable in the 
desired manner. If the operation is performed at high peripheral speeds of 
the inner body, but at a low feed pump output, the grinding stock slowly 
moves upward towards the discharge end by the rotatingly driven grinding 
pellet ring, and, due to the long residence time, the grain spectrum of 
the slurry is close. 
If the mill is operated in dry, the grinding container is to be driven (as 
an outer rotor). The grinding pellets and grinding stock particles present 
in the grinding gap are subjected to the centrifugal force which, as a 
consequence of the oppositely tapered upper and lower regions of inner 
body and grinding container and by the convex curvature of at least one of 
the regions, counteracts the gravity of the grinding balls and grinding 
stock particles thus preventing them from sinking down in the grinding 
gap, on the one hand, and avoiding the discharge of grinding pellets by 
the grinding stock particles, on the other hand. Besides, in dry grinding, 
the possibilities of control of the grinding process are basically the 
same as in case of wet grinding. The slurry feed pump may be replaced by 
an air current feed. 
In case of both embodiments, the convex curvature of one region of the mill 
cross section tapered in opposite directions may be supplemented by a 
second convexly curved region or a conical, straight faced zone. 
Advantageously, a convex lower region may be combined with an upper region 
shaped concavely at least in part. Due to the concavity of the upper 
region of the cross section, the grinding pellets are hindered from 
drifting upwardly. 
It is favorable for the outer surface of the inner body to be spherically 
curved in a closed line. The inner face of the grinding container is 
correspondingly curved spherically, thus resulting in the formation of a 
ball cup-shaped grinding gap whose upper end, preferably beyond the inner 
body, is provided with the outlet for the ground material. Preferably, 
material to be ground is fed in the lower apex bottom of the grinding gap. 
The outer surface of the inner body and the inner face of the grinding 
container may be designed as an ellipsoid or hyperbolic body or the like. 
The shape of the outer surface of the inner body and of the inner face of 
the grinding container need not be identical. It is possible, for 
instance, to combine an elliptic inner body or a spherical inner body 
being somewhat flattened in the equatorial zone of maxi diameter with an 
absolutely spherical inner face of a grinding container. Due to such a 
variety of radii of the curvatures of the outer surface of the inner body 
and of the inner surface of the grinding container, in particular in the 
equatorial zone, the retention of grinding pellets in the equatorial zone 
is favored and the grinding operation is intensified because of the higher 
forces prevailing there. 
The central axis of the inner body may be inclined relative to that of the 
grinding container. Since, in operation of the mill, particles richest in 
mass, viz. the grinding pellets, as a rule, move on an orbit extending at 
right angles to the central axis of the driven mill component (inner body 
or grinding container), this will mean that, subject to the inclination of 
inner body or grinding container, the outlet may be transferred to the 
highest or lowermost point of the grinding gap. Said distance of the 
material outlet to the equitorial zone as the most operative region of the 
driven mill component additionally contributes to hinder a discharge of 
grinding pellets. 
Suitably, the inner body or grinding container may be supported 
displaceably thus allowing to change the grinding gap width. The 
displacement is performed substantially transversely to the central axis 
of the inner body which is situated eccentrically in the grinding gap 
accordingly, of which one side is narrower than the opposite side with the 
result that, in operation, due to the narrow grinding gap portion of the 
mill, grinding stock and also grinding pellets are accumulated there and 
hindered this way from passing over into a merely tangential movement to 
the driven mill component, so that the working capacity of the mill is 
increased accordingly. Another increase in output of the annular gap-type 
mill is realised according to the invention in that not only the inner 
body but also the grinding container are supported rotatably and provided 
with a rotary drive. The sense of rotation of the rotating elements may be 
opposite or equal. In case of rotating elements moving in the same sense, 
the speeds or number of rotations are different so as to cause the 
required relative movement. The rotation of the inner body at the grinding 
gap inside and of the grinding container at the grinding gap outside 
ensures that, from two sides, the grinding pellets in the grinding gap are 
caused to rotate and are activated to work. In such a case, the grinding 
pellet layer in its total thickness in the grinding gap participates in 
the grinding operation. If the two mill components are rotating in 
opposite directions, higher shearing forces of the grinding pellets are 
caused, and, particularly in the zone of largest diameter, the output may 
be doubled as compared to the embodiment having one sole driven mill 
component only. If grinding container and inner body are rotating in the 
same direction, the behaviour of the grinding pellets in the mill is 
distinctive in that the separation of the grinding pellets and their 
prevented discharge from the mill still becomes more effective. 
Apart from said increase in output, the simultaneous drive of the inner 
body and grinding container is also accompanied by another substantial 
advantage: It is possible to optionally use the mill for wet or dry 
grinding operations without a necessary conversion. 
Should the grinding stock be ground in wet as a slurry, the inner body is 
driven. Ir the grinding container is allowed to rest, the normal grinding 
effect is realised, but in case of its drive in the opposite sense, the 
grinding effect may be increased considerably. 
For grinding the stock in dry (in powder form), the grinding container is 
driven. Should the inner body be left at rest, the normal grinding effect 
is performed, while, the case of its drive in opposite direction, the 
grinding output is increased. The simultaneous drive in opposite 
directions of inner body and grinding container entails another 
considerable advantage in that, by the increase of speeds of both mill 
components, the achievable peripheral speeds in the grinding gap are so 
high that the energy absorbed by the grinding stock particles will ensure 
their comminution upon their impact in the grinding gap. In other words, 
grinding pellets need not be used and a material-to-material grinding is 
taking place (autogeneous grinding). This may play an important part if 
the grinding pellet abrasion involves a contamination of the grinding 
stock. Also in case of an autogenous grinding, the output of the mill may 
be still increased by a unilateral grinding gap constriction. 
In view of an adaption of the mill to the material to be ground and to the 
desired fineness, it is possible to adjust and coordinate a number of 
parameters. 
Preferably, an automatic interval control system is provided for the inner 
body and the grinding container to first allow them to rotate in the same 
direction, and, upon reaching the maximum speed, to cause the inner body 
or grinding container to be displaced relative to each other until a 
unilateral grinding gap of about 1 mm is obtained, while one of the 
rotating components is changed over to countersense rotation, the 
displaced part being returned in the same sense of rotation into its 
initial position and the operations being repeated. 
The inner face of the grinding container and the outer surface of the inner 
body are of a finely rough nature, i.e. they should not be very smooth or 
very rough. The condition of fine roughness may be realised by a suitable 
coating of the surfaces in the form of a corrosion- and wear-resistant 
later. To exclude a heat accumulation, the grinding container may be 
surrounded by a coolant jacket or it may be cooled by air.

In an optional support 10, an annular gap-type mill 12 for wet or dry 
grinding is suspended at a support plate 11, said mill substantially 
consisting of a driven hollow inner body 13 having a mainly spherical 
shape and an axis of rotation extending vertically upwardly in the form of 
a hollow shaft 14, and of an outer grinding container 15 whose inner face 
is spherical and which is independently rotatable about its central axis 
being coaxial to the hollow shaft of the inner body 13. Due to the removal 
of a spherical segment of the ball, the lower end marked with 17 of the 
inner body 13 is flat. A straight passage 18 of the tubular hollow shaft 
14 ends in said flat region 17, the lower end 19 of the hollow shaft being 
screwed into an internal thread bore of a fitting piece 20 in the inner 
body 13 and its upper end having an inlet aperture 18a and carrying a 
driving pulley 48. The hollow shaft 14 is supported by a double bearing 16 
whose bearing casing 21 is integrally connected to an adjusting means 22 
whose purpose and design will be explained hereunder in more detail. 
Between the spherical inner face of the grinding container 15 and the outer 
surface of the nearly spherical inner body 13, there is a ball cup-shaped 
grinding gap 23 of unequal width and of a symmetrical shape in the upper 
and lower region. By flattening the inner body 13 in its equatorial zone 
24 of maximum diameter but by maintaining a complete spherical shape at 
the inner face of the grinding container 15, a partial enlargement of the 
grinding gap 23 is performed in the equatorial zone which changes upwardly 
and downwardly into grinding gap portions becoming gradually narrower. The 
lower narrower grinding gap portion ends, due to the flattening 17 of the 
inner body 13 in an enlarged chamber 25 of passage 18 of the hollow shaft 
14, while the upper grinding gap portions is open towards a crown of 
radial outlet openings 26 inclined peripherally and situated in a 
cylindrical driving housing 27 firmly connected to the grinding container 
15 to cause its rotation if a belt placed into a groove 32 transmits the 
driving force to the driving housing 27. The outlet openings 26 are radial 
and inclined in the same sense, their inner shaft-near end being 
confronted with a cylindrical attachment 28 of the inner body 13 which is 
covered by a plate 29 and which reinforces the exit of the hollow shaft 14 
out of the inner body 13. 
At a distance 30a, the hollow shaft 14 is surrounded by a bushing 30 whose 
upper end projecting through support plate 11 is clamped to the latter by 
a secured nut 41, and which comprises on its outer circumference the inner 
races of a double ball bearing 31 which pivotally supports driving housing 
27 of the grinding container 15. The driving housing 27 rotating with 
grinding container 15, outlet openings 26 are rotating as well thus 
throwing the upwardly fed pulverized grinding stock from the grinding gap 
23 radially to the outside into a box 33 from which it flows through a 
downwardly directed collecting channel 34 into a recipient. Due to the 
centrifugal force, the grinding pellets are retained in the equatorial 
zone 24 and kept off the discharged product. Without a special control, 
the grinding pellets are withheld at the desired point and, as a result of 
the opposite taper of the mill, there is no zone free from grinding 
pellets through which particles may pass without being ground. Speed 
(shearing gradient), rotation direction and flow rate are independently 
adjustable parameters in all embodiments and allow to further optimize the 
grinding output. 
The inner body 13 including its cylindrical attachment 28 and the passage 
18 of the hollow shaft 14 are equipped with a corrosion- and 
wear-resistant protective layer 35 provided preferably with a finely rough 
surface. This is also applicable to the inner face of the grinding 
container 15 also having such a finely rough lining 36 extending as far as 
to the zone of the outlet openings 26 at the inner face of the driving 
housing 27. The grinding container 15 is centrally divided in a horizontal 
plane. The upper and lower half of the grinding container 15 are screwed 
together by fitting flanges 37, 38. In the orifice region 25 in the center 
of the lower half of the grinding container 15, there is an opening 39 
adapted to be closed by a screw cap 40 and to serve as an outlet, for 
instance for a cleaning fluid. 
The annular gap-type mill shown in FIG. 1 may operate with an inner body 13 
positioned centrally in the grinding container 15. However, it may be 
advisable, in view of a pulverization of particular hard substances, to 
place the inner body 13 eccentrically in the grinding container 15, in 
other words, to displace it coaxially or preferably transversely to its 
hollow shaft 14. Such a transverse displacement of the inner body 13 is 
possible within the range of oversize 30a of the bore of bushing 30 
relative to the external diameter of the hollow shaft 14. The mentioned 
adjusting means 22 of which a plan view is illustrated in FIG. 2 is used 
to this purpose and substantially consists of a double-track slide 42 
having a dovetail profile and being connected via a mounting 43 to the 
bearing casing 21 of the ball bearing 16 clamped by bushings intermediate 
an annular shoulder 44 on the hollow shaft 14 and a secured nut 45 screwed 
on an outer thread on hollow shaft 14. The two parallel side portions of 
slide 42 are displaceable in one corresponding parallel guide 46 fixed to 
the support plate 11. The position of the slide 42 in the parallel guide 
system 46 is ensured by transverse thread bolts 47 (FIG. 2) engaging 
through parallel guide 46 the inclined profile of each side portion of 
slide 42. Due to the displacement of the inner body 13 effected by the 
adjusting means 22 in transverse direction to the axis of rotation, the 
vertical central axis of the inner body 13 is transversely shifted 
relative to the central axis of the grinding container 15 by a distance a 
shown in FIG. 2, thus imparting a constriction 23a to one side of the 
grinding gap 23, while its opposite side shows an enlargement 23b. Upon 
rotation of the inner body 13, and of the grinding container 15, the 
grinding stock introduced with grinding pellets through the upper coaxial 
opening 18a of passage 18 into the orifice chamber 25 and into the 
grinding gap 23 is accumulated in the constriction 23a which, in practice, 
may be as broad as about 1 mm. The grinding stock which by the grinding 
pellets is urged through said constriction 23a is pulverized still more 
intensely. The efficiency of the grinding operation further may be nearly 
doubled if the inner body 13 and the grinding container 15 are rotated in 
opposite directions thus causing an increase of shearing forces of the 
grinding stock and of the grinding pellets. 
FIG. 3 shows a schematic view of an annular gap-type mill for dry grinding, 
the basic operating principle corresponding substantially to that of FIG. 
1. A cylindrical bushing 52 secured to support plate 50 of frame 51 is 
designed to suspend rotatably, via a double ball bearing 53, a grinding 
container 54 having an exactly spherical inner face, the grinding 
container being firmly joined to a driving housing 55 provided with a 
peripheral groove 65 for a drive belt. Said driving housing 55 contains a 
crown of radial outlet apertures 56 ending in an annular suction channel 
57 which has a tangential outlet 58 through which the dry pulverized stock 
is discharged as shown by the arrow. The grinding container 54 is divided 
horizontally so that, upon its opening, a nearly spherical inner body 58 
may be introduced from below into the cavity. The inner body 58 is 
provided with a coaxial passage 59 changing over into a coaxial hollow 
shaft 60 having at its upper end an inlet 59a for the introduction of 
grinding stock and grinding pellets. By a drive pulley 49 at its upper 
end, the hollow shaft 60 may be connected to a drive rotating the inner 
body 58 in direction of the arrow drafted within the range of a double 
bearing 61, the arrow pointing to a direction opposite to the indicated 
sense of rotation of grinding container 54. 
Due to an adjusting means 62, the inner body 58 may be displaced radially 
relative to the inner space of the grinding container 54 so that it is 
shifted eccentrically relative to the vertical central axis of the 
grinding container 54, the grinding gap 63 on the left side in the drawing 
(63a) being narrower than that on its right side (63b). The adjusting 
means 62 may contain a usual spindle drive 64 which permits a millimeter 
setting of the inner body 58 during the rotation of the elements, if 
necessary, i.e. during the operation of the annular gap-type mill. 
Otherwise, the configuration of the inner body 58 and of the grinding 
container 54 with the associated constructional elements substantially 
corresponds to the embodiment of FIG. 1. 
The embodidment of FIG. 4 is distinctive over that of FIGS. 1 and 3, in 
that the grinding container 74 is nonrotatingly connected to a support 
plate 70 of a frame 71 so that only the inner body 73 supported by a 
double ball bearing 72 is rotated. In this annular gap-type mill, only one 
rotating element may be used because--as evident from the discharge 
channel 75 and from box 77 surrounding the radial outlet openings 76--it 
is preferably employed for wet grinding, i.e. for processing slurry. The 
inner body 73 is nearly pear-shaped, its lower portion 73a being curved 
spherically convexly while its upper region 73b may be spahed conically or 
slightly concavely. The upper region 73b of the inner body 73 is continued 
by a shaft 79 having no passage and whose end extends through the support 
plate 70 and is pivotally supported in a ball bearing 72. By a drive 
pullet 83 at the upper end of shaft 79, the inner body 73 is caused to 
rotate in direction of the arrow. The lower region of the inner face of 
the grinding container 74 is also nearly spherical, its upper region being 
substantially adapted to the tapered inner body 73. Between both elements, 
a grinding gap 81 is left, which, in the equatorial zone, may be broadened 
so that the centrifugal force in this region is intensified and the 
retention of the grinding pellets away from the outlet openings 76 is 
improved. This purpose may be supported by a concave curvature to be 
provided, if necessary, in the upper region of the inner body 73 and of 
the grinding container 74. A passage 78 for feeding slurry and grinding 
pellets is situated centrally in the lower apex zone of the grinding gap 
81, said passage 78 being open towards an orifice chamber 80 formed 
between a flat portion of the inner body 73 and the spherical inner face 
of the grinding container 74. The vertical inner body 73 is radially 
displaceable relative to the central axis of the grinding container 74. To 
this effect, an adjusting means 82 may be used that may correspond to the 
adjusting means 62 of the embodiment of FIG. 3. 
The embodiment of FIG. 5 is different from the preceding embodiments in 
that a nearly spherical inner body 90 with vertical hollow shaft 91 is 
combined with a grinding container 92 spherically shaped at least 
internally and whose central axis 93 is inclined at an angle .alpha. 
relative to the vertical central axis of hollow shaft 91. The grinding 
container 92 is pivotally supported on an oblique base 94 by means of a 
double ball bearing 95, the rotary drive being transferred by a belt in a 
groove 96 of a driving housing 97. The rotation of the grinding container 
92 with spherical inner face shall be performed in direction of the arrow 
assigned thereto. A cylindrical neck portion 98 of the grinding container 
92 contains a crown of radial outlet openings 99 which ensure the feeding 
into a suction channel 100 having a tangential outlet 101. The oblique 
neck portion 98 has a relatively large clear diameter covered by a 
stationary oblique cover 102 suspendingly secured to a support plate 103 
of a frame 104. Between the underside of cover 102 and the end face of the 
neck portion 98, there is seated a slide ring seal 105. By a driving belt 
engaging at the upper end of hollow shaft 91 a driving pullet 106, the 
inner body 90 is caused to rotate in the direction of the arrow and 
oppositely to the grinding container 92. The hollow shaft 91 is supported 
by a double ball bearing 107 situated in a bearing casing 108 that is 
connected to an adjusting means 109 which permits an eccentric 
displacement of the inner body 90 in a transverse direction to its axis of 
rotation in the spherical cavity of the inclined grinding container 92 so 
that one side of the grinding gap 110 will be narrower than the opposite 
side. Since the grinding container 92 is inclined by angle .alpha. 
relative to the vertical, the outlet openings 99 situated in a plane 
parallel to the transverse plane A--A of the grinding container 92 consist 
of higher and lower portions. Since, in case of operation of the mill, the 
mass-rich particles, i.e. as a rule, the grinding pellets move on an orbit 
which is at right angles to the central axis of the driven mill element 
(inner body 90 or grinding container 92), the outlet for the ground stock 
may be shifted to the highest or the lowest position of the grinding gap 
110, subject to the inclination of the inner body 90 or the grinding 
container 92. Such a distance of the material outlet to the most operative 
equatorial zone of the driven mill element additionally contributes to 
avoid discharging of grinding pellets. The pulverized grinding stock is 
moved upwardly more or less slowly, subject to the feeding pressure by 
which, inside the hollow shaft 91, it is urged into the grinding gap 110, 
and it escapes free of grinding pellets into the suction channel 100. The 
effect improved by the inclination of the grinding container with respect 
to the reduced amount of grinding pellets drifting away is also realised 
if the grinding container is at rest. 
FIG. 6 shows an annular gap-type mill in which the axis of rotation of an 
inner body 111 also forms an angle .beta. with the central axis of a 
rotatable grinding container 112. However, in this embodiment, the 
grinding container 112 is positioned vertically and the inner body 111 is 
inclined. Each of them is rotating in double ball bearings 113 and 114. 
Their drives are transmitted by motors which, via belts, engage a driving 
pullet 115 at the upper end of a hollow shaft 129 of the inner body 111 
and via a driving gear 116, the grinding container 112 which is mounted 
vertically on a straight base 177, while the inner body 111 is arranged to 
be suspended obliquely in an inclined bearing casing 118 that is provided 
on a support plate 119 of a frame 120. A crown of radial outlet openings 
121 surrounding a cylindrical neck portion 122 of the grinding container 
112 ensures that the milled slurry obtained in the wet grinding process is 
discharged through a collecting channel 123 which extends into a 
container. Also in this case, the discharge of grinding pellelts from the 
grinding gap 94 is excluded more satisfactorily because, relative to the 
effective equatorial zone B--B of the inner body 111 inclined to the 
vertical line, and in which the highest centrifugal forces prevail, the 
outlet openings 121 are divided into a lower left-hand portion and a 
higher right-hand portion which practically are not reached by the 
grinding pellets. 
FIG. 7 shows an annular gap-type mill in which a bearing casing 132 for the 
double ball bearing 133 of a vertical hollow shaft 134 of an inner body 
135 is mounted on a support plate 131 of a frame 130. The shape of the 
inner body 135 is nearly elliptical with a slight flattening 136 in the 
equatorial zone of maximum diameter. The lower section of the elliptic 
inner body 135 is flat near 137 thus forming an orifice chamber 138 
between the flattening 137 and the vault of the completely elliptic inner 
face of a grinding container 139. In the orifice chamber 138 ends the 
straight passage 140 of hollow shaft 134 through which dry material to be 
ground as well as the grinding pellets are fed from above. Between the 
outer face of inner body 135 and the inner face of the grinding container 
139, there is a grinding gap 141 being contracted uniformly in upward and 
downward direction. The grinding container 139 is firmly connected to a 
driving housing 142 accomodating a double ball bearing 143 and 
transmitting the drive of a motor to the grinding container 139 which 
rotates independently of the inner body 135, the axes of rotation of both 
rotating elements being provided coaxially. Through a crown of radial 
outlet openings 144, the pulverized stock gets into a suction channel 145. 
A driving disk 146 at the upper end of hollow shaft 134 is adapted to 
transmit the drive of a motor to the inner body 135. 
The embodiments of FIGS. 1 to 7 only are examples whose constructional 
elements are interchangeable, thus permitting to dispose of annular 
gap-type mills for wet or dry grinding hard substances of different kinds 
by operating with rotatable or stationary grinding containers or inner 
bodies and with the use of grinding gaps contracted unilaterally or being 
dimensioned uniformly. The inner body and grinding container speeds as 
well as the directions of rotation can be adapted to the kind of material 
to be ground, and may be equal or different. By the use of an automatic 
interval control system, it is possible, to first drive the grinding 
container and the inner body in the same sense, thereafter, upon reaching 
the maximum speed, to displace relative to each other the inner body or 
the grinding container until a unilateral grinding gap of about 1 mm is 
obtained, then, to simultaneously change the driving sense of the grinding 
container or the inner body, whereupon the grinding container or inner 
body is returned to its initial position with the same sense of rotation 
and the operations will be repeated.