US3910509A

US3910509A - Mill which operates at an overcritical speed of rotation

Info

Publication number: US3910509A
Application number: US450239A
Authority: US
Inventors: Wilhelm Eirich
Original assignee: Maschinenfabrik Gustav Eirich GmbH and Co KG
Current assignee: Maschinenfabrik Gustav Eirich GmbH and Co KG
Priority date: 1973-03-14
Filing date: 1974-03-11
Publication date: 1975-10-07
Anticipated expiration: 1992-10-07
Also published as: FR2221186B1; ZA741645B; SE408140B; FR2221186A1; GB1460805A; CH558678A; CA1015726A; DE2410075A1; JPS5052660A; JPS5730536B2; BR7401981D0

Abstract

A mill which operates at an overcritical speed of rotation comprises a grinding container in the form of a rotational body, and one or more internal grinding members revolvable around the axis of the grinding container at an overcritical speed. The grinding member or members are rotatably mounted on a trailing arm which is mounted on a carrier that can pivot about an axis which is substantially parallel to the axis of rotation of the grinding container. An arrangement is provided for controlling the difference between the peripheral speed of rotation of the grinding member or members and the speed of rotation of the grinding container about the axis of the grinding container, and provides efficient grinding with low power consumption.

Description

United States Patent Eirich Oct. 7, 1975 MILL WHICH OPERATES AT AN 2,463,556 3/1949 Piper 241 129 OVERCRITICAL SPEED OF ROTATION L Primary ExaminerRoy ake [75] Inventor. gilhelm Enrich, Hardhelm, Assistant Examiner E' F Desmond ermany Attorney, Agent, or Firm.Toren McGeady and [73] Assignee: Maschinenfabrik Gustav Eirich, Stanger I Hardheim, Germany 22 Filed: Mar. 11, 1974 [57] ABSTRACT A mill which operates at an overcritical speed of rota- [21] Appl' 450239 tioncomprises a grinding container in the form of a rotational body, and one or more internal grinding [30] F i A li ti P i it D t members revolvable around the axis of the grinding Mar 14, 1973 Switzerland 3732/73 container at overcrihcal Speed- The grinding her or members are rotatably mounted on a trailing 52 us. or 241/120; 241/130 arm which is mounted carrier that can [51 Int. c1. 1302c 15/02 axis which is substantially Parallel the am [58] Field of Search 241/1O8 110 117, 118 of rotation of the grinding container. An arrangement 241/120, [21 123, 125 126 127431 is provided for controlling the difference between the peripheral speed of rotation of the grinding member [56] References Cited or members and the speed of rotation of the grinding container about the axis of the grinding container, and UNITED STATES PATENTS provides efficient grinding with low power consumpl,936,593 11/1933 Fraser 241/129 X tion 2,177,945 10/1939 Piper 241/118 2,303,319 12/1942 Beardsley et a1. 241/129 X 18 Claims, 8 Drawing Figures us. Patent (M11975 sheetlom 3,910,509

U.S. Patent 0a. 7,1975 Sheet 2 of4 3,910,509

U.S. Patent Oct. 7,1975 Sheet 3 of4 3,910,509

US. Patent 0a. 7,1975 Sheet4 Of4 3 910509 MILL WHICH OPERATES AT AN OVERCRITICAL SPEED OF ROTATION The present invention relates to a mill which operates at an overcritical speed of rotation, in which a grinding member rolls over the material to be ground, in a grinding container which rotates about its axis together with the material to be ground, at an overcritical speed of rotation.

When materials are being ground, in particular hard and medium'hard materials, the particles to be crushed are loaded until they break up, abrupt changes in force being conducive to crushing of the particles. Therefore the particles of the material to be ground should be subjected to loadings which fluctuate as rapidly as possible, and which can be produced by the action of pressure, impact, shearing and also due to the expansion effect which follows the aforementioned kinds of loading. The quite considerable power requirement necessary for crushing the particles rises sharply with decreasing particle size. Recent findings indicate that it is hardly possible to get below a given minimum particle size, which varies from one material to another, because in that region the particles can only be plastically deformed with the previous methods, or because the amount of energy required for crushing the particles cannot be transmitted to the particles to be crushed, or cannot be transmitted sufficiently quickly. The minimum particle size is also determined by the particles which agglomerate under pressure.

Ball mills are extremely widely used, in'particular in the cement industry. The degree of efficiency of such gravitational force ball mills is very low and is of the order of magnitude of 1 percent. Recent tests and attempts at increasing the level of efficiency have resulted in a considerable increase in efficiency in the case of so-called planet ball mills and swing ball mills. However, hitherto these more recently developed mills could only be constructed in relatively small sizes. Although they make it possible to achieve grinding results as are obtained in much larger gravitational force ball mills, they are not in a position, due to their relative smallness, to provide the desired increase in respect of grinding large amounts of material.

A distinction is made between mills which have a overcritical speed of rotation and mills which have a subcritical speed of rotation. It is known that the critical speed of rotation in the case of mills is that speed of rotation at which the centrifugal force exceeds the weight of the members, as determined by the force of gravity.

Now if a conventional gravitational force ball mill were to be driven at a speed of rotation which is excessively overcritical, the material being ground, together with the grinding balls, would remain stationary, and

there would not be any grinding action. In order to overcome this, it has already been proposed that deflector members should be incorporated into the grinding container, which again locally slow the material being ground and the grinding balls down to a subcritical peripheral speed, so that the material and the balls crash. Such mills are to a certain extent an agitator ball mill with a stationary agitator vane, which results in a high power requirement and relatively low speeds of rotation. The material being ground always'has to be accelerated again to a higher speed.

2 The principle of securing the material being ground against the cylindrical walls of a grinding material container, by means of overcritical speeds of rotation, has

also already been used in those centrifugal force mills in which a cylindrical grinding member which is disposed in the grinding container rolls overthe material which is held against the walls of the container. However, the material can only be crushed by squashing down to a predetermined grain size (corresponding to the fixed clearance between the grinding member and the internal surfaceof the grinding container). A plurality of such mills are arranged one above the other,

with a vertical axis. Therefore the fact that the material to be ground is held against the walls of the container by the high centrifugal forces generated is 'utilised in this apparatus in order better to be able to crush the material. The level of success is low. The clearance between the grinding member and the casing of the grinding container, and thus the grain size, is varied by wear on the grindingmembers and on the casing of y the grinding container. Success is attained only in the region in which there is a relative movement of the material being ground, with respect to the container wall, in the form of a friction force, as in autogenous mills. The grinding pressure is determined by the degree of filling of the container and the clearance between the container and the grinding member, the centrifugal force being intended only to hold the material against the wallof the container. 7 I

In the field of rnills which operate at a subcritical speed of rotation, there is known a ball or roll mill having a vertical axis of rotation, for crushing cocoa, chocolate, varnish pigments, enamels, and the like, havinga cylindrical container within which there is arranged a coaxial rotor. Disposed at the surface of the rotor are grinding rollers which eachhave their rotary mounting spindle guided in radially extending slots in the rotor. Axial passages are provided in the end. walls of the cylindrical container, for the delivery and discharge of the substance to be crushed, or the material to be ground. The material to be ground therefore moves parallel to the axis of the container along the cylindrical wall thereof, in a downward direction, and this excludes operation at an overcritical speed of rotation. In addition, the mounting of the grinding rollers is exposed to the material to be ground, so that the bearings are subjected to a high rate of wear.

Starting from this state of the art, the invention is based on the problem of providing a grinding principle which utilises the method, which is favourable per se, of holding the material to be ground against the wall of the grinding container, by centrifugal force, but which avoids thedisadvantages thereof, in particular the disadvantages regarding degree of efficiency, wear and limitation in respect of grain size. The invention seeks to permit grinding 'over wide ranges, with small, medium and large. machine units, while saving space and increasing the degree of efficiency. It is also intended to permit savings both as regards the apparatus and also the operating costs. I

In accordance with the invention, this problem is solved by using a grinding principle in which grinding members rotate in the grinding container about the axis thereof at an overcritical speed of rotation which is dif-' ferent from the speed and/or the. direction of rotation of the grinding container, while the grinding members roll over the material to be ground, rotating about their" 3 own axis, and lying against the material under the effect of centrifugal force.

By selecting suitable speeds of rotation, itis possible for the. pressure of the material to be ground, against the grinding surface, which is preferably armoured, of t the grinding container, and the pressure ofthe grinding members on the material to be ground, to reach values which can exceed 20 to several hundred times their weight.

The grinding principle according to the invention can be embodied both with substantially cylindrical grinding members alone, and also with grinding members which in particular are in the form of wheels of tapered cross-section, in conjunction with grinding balls. Very small balls can be used in this arrangement, which makes it possible to increase the number of contact points and a reduction .in the contact surface areas of the balls. When operating with grinding balls, the power required is in fact higher than when operating without grinding balls, but normally the power required does not reach the: value of the power required by agitator ball mills.

, A mill according to the invention is characterised in that a grinding container whichis in the form of a rotational body is mounted drivably at an ,overcritical speed of rotation about an axis which extends substantially parallel to its axis of rotation, and that eachgrinding member which revolves around the axis of theygrinding container at an overcritical speed is rotatably mounted in a trailing arm which is mounted on a carrier pivotably about an axis which is substantially parallel to the axis of rotation of the grinding container, the carrier being drivably rotated about the axis of the grinding container, while means are provided for controlling the difference between the peripheral speed of rotation of the grinding members and the speed of rotation of the grinding container about the axis of the grinding container. V

In operation the grinding members which rotate on the pivotal trailing arms roll on the grinding face of the grinding container or the material to be ground, with a corresponding speed difference.

The individual grinding members which are generally substantially cylindrical can be secured to a whip-like suspension means by way of which they can be drawn around the centre of the grinding container in a circular motion. As a result of centrifugal force, the grinding members are urged towards the grinding face of the grinding container, with a force equal to a multiple of their weight. The grinding members roll over the grinding face of the container or over the material to be ground which is disposed on the grinding face, and which is compressed and held firm thereagainst by the centrifugal force. Very high levels of centrifugal force can be attained. The number of times that the grinding members roll over the material in each unit of time may be controlled by adjusting the difference in travel from ,the grinding container to the grinding member, and by tions of rotation are the same and the speeds of rotation are almost the same, and a maximum grinding action can be attained when the directions of rotation are opposed and the speeds of rotation are high.

When grinding with the same directions of rotationof the grinding members and the grinding container, it is of advantage for the'grinding container to be rotated at,

a higher speed, for example twice as fast as the grinding member carrier. In this case the whole drive power flows through the plate drive into the machine. The

plate has a tendency to entrain the grinding members at the same peripheral speed. However; the grinding members are kept down to the lower peripheral speed by means of the grinding member suspension means.

Thus drive power flows from the plate through the layer of material to be ground, on the wall of the container, to the grinding members, and from the grinding members to the drive point of the grinding member carrier. Therefore mechanical energy is transmitted through the layer of material to be ground, and applies a tensile loading thereto, in addition to the high compression forces. Most materials to be ground have a substantially lower level of tensile strength than their compression strength. Therefore, the crushing action can be substantially increased, with a relatively low generating grinding action can even occur from one grain of material to another. The expansion effect which follows any compression effect has a similar effect on the material to be ground. Under, such conditions the rolling resistance of the grinding members is minimal.

As the rolling resistance can therefore be kept relatively low, it is possible (in this case also the increase in weight due to centrifugalforce is of use) for the indi-.

vidual grinding members to beef small diameters, so that very short compression ,zone 7 regions can be formed and the rolling speed attained by the grinding members can be high.

In order to increase their effect the grinding members can also be eccentricallymounted and eccentrically constructed, or can be made out-ofbalance in some other way (for example by means of weight distribution), so that the grinding members make. it possible easily to apply the out-of-balance effect which is known from swing mills, without the necessity of moving large out-of-balance masses. Owing to the pivotal mounting of the grinding members on their rotating carrier, the container is substantially protected from the material to be ground.

Owing to the. high pressing forces applied by the grinding members, and their high peripheral speeds, when there is a sudden interruption in current the grinding members tend to break-out of their path of movement, as the tensile loading previously applied by the grinding members is replaced by a strong shearing action. Account should be takenof this fact when constructing the. guide elements for the grinding members, for example by limiting the outward movementof the grinding members by a mechanical abutment or a resilient means.

The grinding members preferably comprise pairs of grinding rollers, one roller being mounted above the trailing arm and one roller being mounted below the trailing arm, on a shaft which is carried in the arm. In one embodiment the shaft is non-rotatably carried in the pivotal arm, and each grinding roller is independently rotatably mounted therein.

Small differences in the thickness of the layer of material lying against the wall of the grinding container can result in varying speeds of rotation of the grinding members, so that when sudden retardation occurs, for example due to an interruption in current, there is an increased tendency on the part of the grinding members to veer out of their line of movement. Therefore, in an advantageous embodiment of the invention, the grinding rollers of each pair are rigidly mounted on their shaft, while the shaft is mounted, in the form of a spindle, rotatably in the associated trailing arm. The above-mentioned danger of the grinding members veering out of their path of movement is overcome by this fixed connection between the grinding rollers of each pair and the common spindle.

The grinding members can also be constructed with an out-of-balance, in order that the impact action or the bounce effect of the grinding members against the material to be ground is varied. As vibration and oscillations are produced in this case owing to the high speeds of rotation, and these put a loading on the spindle and the bearings, it can be advantageous for the grinding members not to be solid in construction, but to be so constructed in an annular configuration that a layer of resilient material is carried on an internal hub, and a grinding ring comprising a material having a high resistance to wear is fitted over the layer of resilient material. With this construction the vibration forces which occur in the grinding ring are substantially damped, and this construction also has the additional disadvantage that the grinding rings which are subjected to wear can be relatively easily replaced.

A further modification of the mill according to the invention provides that the axis of rotation of the grinding member carrier is arranged eccentrically relative to the axis of rotation of the grinding container. 'When operating such a mill, the radius as between the axis of the grinding members and the axis of the container varies twice in each revolution, that is to say, the rolling travel of each individual grinding member is increased and shortened during one revolution, thus producing a continuous alternate succession of acceleration and braking at the running surface of each grinding member, so that, in addition to the rolling and impact action of the grinding members, there is also a friction action on the material, which is conducive to grinding the material.

The above-mentioned incorporation of a resilient means for limiting the outward movement of the grinding members towards the wall of the grinding container also provides the possibility of adaptation to a material of a different nature, for example of a different degree of hardness. When such a variation occurs, changing the weight of the grinding members or the peripheral speed of the grinding member carrier involves expensive operations. If however an adjustable spring force is interposed between the grinding member carrier and the grinding member mounting, for example in the respective pivotal arm, the pressure of the grinding members against the material to be ground can be reduced or increased, depending on the nature of the material, by adjustment to the spring force. In a practical construction a suitable traction spring can be fitted between the trailing arm and the grinding member carrier. Alternatively a pneumatic tyre can be disposed at the periphery of the grinding member carrier. Against the pneumatic tyre lies for example an angle lever which acts on the respective grinding member in question. All the pairs of grinding member can be loaded or relieved of load to an absolutely uniform extent, by varying the air pressure in the pneumatic tyre.

The material to be ground can also be circulated in the grinding container, due to the grinding members having the effect of swirling up the material, in a manner not dissimilar to the action of a motor vehicle wheel on an ordinary road surface. Obviously the coarser particles in the atmosphere in the grinding container settle more quickly than the fine particles, so that the fine particles can be discharged from the container by means of a suitably directed air current. In this way it is also possible to provide a precautionary action (grading) by means of which the coarse particles can, so-tospeak, be concentrated the grinding container, which makes it possible .to increase the level of efficiency of the method and the mill.

Consequently, the fine material can be discharged through a discharge channel if the discharge apertures of the grinding container lie in the region of a subcritical speed of rotation. If the speed of rotation in the discharge region is overcritical, it is possible to use a collecting pipe in which a screw with a flexible shaft is movably disposed. This screw is charged with material at the collecting means, and then conveys the finely ground material through the screw tube into the vicinity of the mounting hub, and empties it into an annular collector. The flexible screw has its housing secured to the grinding member carrier, and extends through a hole in the bottom of the container. Below the bottom of the container the screw has a friction wheel drive which rolls against the stationary mounting hub. As the discharge end of the screw rotates with the grinding member carrier, it is necessary to provide an annular collector around the hub, in order to collect the fine material discharged from the container.

At least a part of the movement of air in the interior of the container can be derived from the rotary movements of the components. It is particularly advantageous for a blade wheel to rotate with the grinding members about the axis of the grinding container, whereby a desiredflow of air is produce in the container. The amount of air displaced and/or the direction of displacement of the air can be determined by adjustment of the blades of the blade wheel. The reduced pressure which is normally maintained within the container, in order to avoid the formation of dust, is usually produced by suction fans. However, the rotational movement of the grinding container can also be used for this purpose.

The axis of the grinding container can in principle be inclined at any desired angle relative to the ground surface, but in particular in larger machines the axis will preferably extend vertically or'at an angle. In that case the mounting can be at the top or at the bottom, or at both ends, or at the periphery.

Where it is desired to produce shearing forces, these can be obtained by inclining the axis of rotation of the individual grinding members relative to the axis of their path of rotary movement (that is to say, the axis of the 7 grinding container), although this obviously involves an increased level of resistance to their movement.

' In principle the following steps, which canbe combined together, are available for controlling the pres-- sure force applied by the grinding members:

a. Variation in the speed of rotation and thus the cen' trifugal force, i b. Variation in the weight of the grinding members,

c. Variation in the contact surface (shape) of the e. Fitting additional weights (acting on the grinding member shaft),

f. Controllable spring force which acts substantially radially on the grinding members.

The above-mentioned steps (a) to leave the grinding member mounting virtually unaffected. The above step (d) usually involves only a slight change in loading on the grinding member mounting. In both cases this is because the grinding member is supported in operation against the grinding face of the container,

by way of the material being ground. The additional weights which engage the grinding member shaft, or which are transmitted to the grinding member by way of the shaft mounting thereof, obviously involve an increase ,in the loading on the mounting and the shaft.

mill. The good cooling actionalso makes it possible to carry out low-temperature grinding.

It is possible, while the mill isoperating, to carry out a per se known automatic balancing operation.

The armouring on the grinding surface of the grinding container, and the grinding members, can :be formed from composite materials, with a permanently rough surface. Grinding members of a stratified structure can be used (plates of material of varying hardness, bonded together by a bonding agent and reinforcing).

High specific'grinding efficiency per, unit of surface area can be achieved.

The grinding members do notlift away from the ma- 'terial being ground, which can also apply in respect of balls.

Conditions are advantageous for using electrostatic and magnetic polarisation fields, and for the magnetic and pneumatic separation of material in the upper part of the grinding chamber.

Whole groups of grinding members canbe combined 7 together for mounting purposes.

This means that the last-mentioned step is less preferred, and that, when it is not utilised, the grinding member mountings are only exposed tothe relatively minimal drive loadings, which has the effect of reducing the amount of wear.

When using grinding balls it is possible for the move-. ment of the grinding balls to be taken over by suitable grinding members, for example which are in the shape of wheels of tapered cross-section. The power required in this arrangement is lower than in the case of agitator ball mills, but higher than in a mill without any grinding balls. Grinding balls also reduce the effective capacity of the mill and the possible operating speed. A high peripheral speed however should have an enhancing effeet on the production of material, which can increase in proportion to the third power of the speed of rota-, tion.

The rate of wear appears similarly low to thatencountered in autogenous mills, but without the disadvantages of the autogenous mill being encountered.

Some further advantages can be summarised as fol lows:

The rotating systems act as a stabilising gyroscope. Therefore little natural oscillationoccurs. The remaining oscillation effect can be prevented from affecting the ground below the mill, by suitable structuring.

The mill can be loaded in dependence on the motor loading (sensing of the motor output) Not only the shape of the grinding members but also their materials can be varied within wide limits. Grinding can be effected wet or dry, with or without balls.

The fact that the material being ground lies firmly against the grinding surface of the grinding container, under the effect of centrifugal force, gives the best con ditions for fine grinding of the material, with a low power consumption, by rolling over the material.

The invention will now be described in greater detail by way of example with reference to the drawings, in

which: a

FIG. 1 shows a diagrammatic view in vertical crosssection through a mill according to the invention,

FIG. 2 shows a diagrammatic plan view of half of the millshown in FIG. 1,,

FIG. 3 shows a diagrammatic view in horizontal,

cross-section through the mill shown in FIG. 1,

FIGS. 4 to 6 show alternative embodiments of the grinding members,

FIG. 7 shows an embodiment in which the grinding member shaft is guided and in addition the grinding member carrier rotates about an axis which is eccentric relative to the axis of the grinding container,

. FIG. 8 is a view, similar to that of FIG. 1', of another embodiment of the mill. 3

Reference is firstly made in particular to FIGS. 1 to 3.

The grinding container 1 is a substantially cylindrical vessel whose casing is provided with strengthening rings (not shown in greater detail) and which forms on its inward surface the grinding track 2 upon which the material (not shown) to be ground lies. The grinding members 3 roll over the materialto be ground on .the grinding track 2, in operation of the mill.

The grinding container 1 is rotatably mounted on the frame R on a hollow shaft member Z and is fixedly connected to a Vbelt pulley combination S which provides for the drive by means of a motor. The grinding container however could also be driven in another manner,

for example by one or more linear motors. Itcould also be supported on air cushions.

I The central shaft A of the grinding member system extends through the shaft member Z and is supported therein. The shaft A is concentric to the axis of rotation of the grinding container 1 and at its bottom end carries a V-belt wheel 8 for driving the shaft A.

The shaft A has an enlarged portion A in the interior of the grinding container 1, which carries rigid radial arms 30 upon which are mounted trailing levers 32 which are pivotal about shafts 31 parallel to the shaft y A. The shafts 33 of the grinding members are mounted on the levers 32. Thus, when the grinding member system is driven, the grinding member system with the grinding members 3 rotates in the direction of the arrow K which is shown extending through the shafts 31. The greater the distance of the shaft 31 of each grinding member from the centre 0, and the closer the shaft is to the grinding track 2, the better can the grinding member follow the centrifugal force which tends to press it against the grinding face or track 2.

The grinding container can in turn be driven in rotation either in the direction of the solid-line arrow P, in the opposite direction tothe direction of the arrow K, or in the same direction as the direction of the arrow K, in the direction indicated by the broken-line arrow P. The grinding container and the grinding members are always driven at a speed of rotation which is substantially above the critical speed, so that the material (not shown) to be ground is pressed against the grinding track 2, and the roller-shaped grinding members 3 are pressed against the material to be ground. In this case the grinding members 3 act with a multiple of their weight on the layer of material to be ground on the track 2, and swirl this material up. If the pressure is to be increased without raising the speed of rotation, additional weights 34 can be supported for this purpose on the grinding member shafts 33, which additional weights 34 can be shaped in accordance with flow principles, as illustrated.

The grinding container 1 is open at the top. Its opening is covered by a stationary hood 4 in which there is a pipe 40 or 40' for the supply of material to be ground into the container 1, while in the centre is a passage 41 for the supply of air. Around the passage 41 is an annuduced through the pipe 41. Disposed below the pipe I 41, and fixedly connected to the shaft A of the grinding member drive arrangement, is a blade wheel 5 with adjustable blades 51. The amount of air impelled by the wheel 5 into the interior of the container 1 can be controlled by adjustment of the position of the blades 51. The blade wheel 5 could also be replaced by a control plate which is stationarily connected to the rotor 41, to provide a similar function, for passive control of the air introduced into the container. By suitably extending the air supply pipe to the bottom of the container 1, it is possible for the air to be introduced below the grinding member arms 30 so that, rising to the hood 4.and .to the suction pipe 43, the introduced air is directly charged with fine material. In order to separate out the fine material, it may be advantageous to operate with circulating air and dust separators, for example cyclone separators, from which the air, even if it still contains some dust component, can be re-cycled back into the pipe 41.

As can be seen from FIG. 4, each grinding member is in the form of a single large roller 30 which is mounted on a lever 30a corresponding to the lever 30 in FIG. 3. In FIGS. 5 and 6, three grinding

members

3b and 30 respectively, are mounted on each lever 30b or 300 respectively. The grinding members 3b are of a tapered configuration, so that they permit particularly high pressures on the material to be ground, and are particularly suitable when used in conjunction with grinding balls. The grinding members 30 act upon a much smaller surface area than the grinding member 3a, so that they also make it possible to produce relatively high pressures. They also have out-of-balance weights or. out-of-balance bores 3c, so that they each rotate individually eccentrically. However, as is particularly clearly shown in FIG. 3, as the grinding members 3 in FIG. 3 are each combined in pairs and lie opposite to each other, such eccentric forces substantially compensate for each other. This compensation of forces also makes it possible to use grinding members of different sizes from one pair to another (in FIG. 3, for reasons of space, only two pairs are shown), and such grinding members of different sizes roll over the material to be ground with a different pressure and at different rolling speeds. This provides for a variation in the grinding action.

The embodiment shown in FIG. 5 is particularly suitable for using the machine as a ball mill. In this case grinding balls of small diameter are used, which give a much greater number of point contacts than grinding balls of larger diameter. In spite of their small size and thus their relatively low inherent weight, because of the increase in weight produced by the action of centrifugal force, in the present case such balls have a substantially stronger effect than when using balls in equal size in a normal gravitational force ball mill. The speed of the grinding member carrier must be so determined in this case that the loading is adapted to the material of the balls.

FIG. 7 illustrates the above-mentioned eccentric arrangement of the grinding member carrier 5 relative to the shaft A of the container 1. For the sake of clarity in the drawings, the degree of eccentricity is shown in exaggerated form, whereas in actual fact it is about 5 mm, so that the difference in the lengths of travel is from 1 to 6 percent.

FIG. 7 also shows the annular construction of grinding members, namely, the hub 3a is covered with a layer 3b of resilient material, for example hard rubber, upon which is fitted, so as to lie closely thereagainst, a grinding ring 3c which comprises material having a high resistance to wear, so that the transmission to the spindle 3a of vibration occurring in the grinding ring 30 is damped. In addition the swing arms 32 are extended beyond the rotary shaft 33a in the manner of a wagon pole 32. Rigidly mounted on the radial arm 30 is a strut 52 which ends in a fork or a slotted member 53. The pole 32' is guided in the slot of the fork or in the slotof the member 53. A traction spring 54 which connects the pole 32' to the enlarged portion A of the shaft A, as shown in'FIG. 1, ensures that the grinding member is not pressed. against the material to be ground in such a way as-to produce a strong shearing action, for example in the event of an interruption in current or some other mishap. The springs 54 are only shown diagrammatically in FIG. 7, but they can also be of a different construction with an adjustable spring force, in order in this way that the pressure of the grind ing members against the material to be ground can be adapted in .accordance with the conditions of various materials to be ground.

FIG. 8 shows a mill according to the invention in which the container 1 and the grinding member carrier A rotate in the same direction of rotation, the container preferably rotating at a faster speed of rotation. In this embodiment the-shaft A" is extended through the container and the hood 4', and is mounted at its lower end in a support frame T fitted on the frame. R. The hood 4 is sealed relative to the container 1' by means of a resilient lip 55 which lies on an annular bead 56 secured to the top side of the container 1. The lip 55 is also pressed resiliently against the bead 56 by a tube member 57 which is filled with compressed air and which bears against a ring 58. This sealing action is particularly suitable if the grinding operation is carried out under a reduced pressure or in an atmosphere "of low-temperature gas. a

When the mill operates in the same direction of rotation, with the container moving at a faster speed of rotation, the container 1' tends to entrain the grinding.

members 3 which are pressed strongly against the container. In this case the drive of the grinding member is then transmitted in part to the grinding members 3 by way of the layer of material to be ground, with the result that the drive motor of the grinding member carrier operates as a braking asynchronous generator. A part of the power supplied therefore flows back into the Plate diameter Plate depth carrier Speed of rotation of the grinding rings Drive motor: container Drive motor: grinding member carrier Pressure of centrifugal'force for each grinding member,

kind can also be incorporated in the embodiment of the machine shown inFlGS. l to 3, as indicated in FIG. 2.;

Finally, in order to promote the circulation of air,

ventilation ribs (not shown) can be fitted on the end faces of the grinding members 3, so that the air flow produced in the grindingchamber in the container can with the invention are given hereinafter, but it should.

be noted that suchdata are not given in a limiting sense, for the speed of rotation and the ratio of speed of rotation as between the container and the grinding members can be extensively varied according to the conditions of grinding. All the values given hereinafter relate to a' container. and grinding members rotating. in opposite directions. When the. container and the grinding members rotate in the same direction, the relative speed can be greatly reduced, while thepressure applied to the material to be ground is maintained at the level of pressure obtaining in the. counter-running mode of operation.

1400 mm 500 mm 300 mm 100 mm l 10 kg approximately I00 400 r.p.m. about I00 400 r.p.m.

at I00 r.p.m.: 933

400 r.p.m. 3732 I500 r.p.m. 90 kW 1500 r.p.m. 90 kW comprising 2 rollers, is, at a speed of rotation I00 for the grinding member carrier:

for the grinding member carrier: Peripheral speed of container at I00 r.p.m.

Peripheral speed of the grinding member carrier Peripheral speed of the grinding member rollers kg 740 at the speed of rotation 400 kg I2000 7.3 m/sec at 400 r.p.m. 29.0 m/sec at I00 r.p.m. 5.5 m/sec at 400 r.p.m. 22.0 m/sec at I00 r.p.m. 14.6 m/sec. at 400 r.p.m. 58.5 m/sec Number of times that all points on the grinding surface of the container are rolled over:

at I00 r.p.m. 800 per minute I3 per second at 400 r.p.m. 3200 per minute 53 per second current network. As shown in the drawing, this grinding operation can be carried out with an even greater saving of power if an electric motor 59 with two shaft ends 60 and 61 is used for the whole drive for the machine. As in the embodiment shown in FIG. 1, the shaft end 60 drives the container 1, while the second shaft end 61 is connected by way of the V-belt wheel S to the grinding member carrier A, and acts as'a brake. The power which is put out by way of the grinding member carrier A is therefore supplied to the same motor shaft, so that there is no necessity for conversion into electrical power.

FIG. 8 also shows a discharge channel 62 for the fine material. The channel 62 .is connected to a hood 63 with a downwardly directed aperture, the hood 63 being secured to the grinding member carrier A and consequently rotating therewith. The hood 63 extends through a circular opening in the bottom of the container. l, and its end opens into a stationary hopper 64 which encloses the shaft A" and into which the ground fine material is discharged. A discharge channel of this These figures show very remarkable average performances. However, they only provide some datum points. a i

The invention is equally well suited for the construction of small, medium and large mills, for high levels of throughput are achieved with relatively low weight and building material cost, and a relatively small amount of space.

What I claim is: 1. A, mill comprising a. a grinding container arranged to contain material to be ground and having an axis ftherein about which the container is drivably rotatable at an overcritical speed of rotation and an internal cylindrical material grinding face,

b. a grinding member carrier positioned within the I grinding container and rotatable therein at an overd. grinding members rotatably mounted on said trailing arms within said container inwardly from said grinding face at a position on said trailing arms spaced from the point of pivotal attachment to said carrier,

e. means for controlling the difference between the peripheral speed of rotation of the grinding members and the speed of rotation of the grinding face of said grinding container, so that the grinding members are forced outwardly by centrifugal force to roll over the material to be ground which is distributed within the container over the material grinding face by centrifugal force of said container, and the normal speeds of rotation of the grinding container and the grinding members are in a range in which the centrifugal forces in the region of the material to be ground and at the grinding member are a multiple of their weight.

2. A mill according to claim 1, wherein the pivot axes of the trailing arms are at a smaller spacing from the material grinding face than the grinding member diameter. f

3. A mill according to claim 1, wherein grinding members of identical configuration are mounted diametrically opposite each other with respect to the axis to form pairs of grinding members and there are differences of configuration as between said pairs;

4. A mill according to claim 1, wherein at least one grinding member is out-of-balance.

5. A mill according to claim 1, wherein the normal speeds of rotation of the grinding container and the grinding members are in a range in which the centrifugal forces in the region of the material to be ground and at the grinding members have a value which exceeds to several hundred times their weight.

6. A mill according to claim 1, wherein at least one grinding member is mounted on a shaft which is inclined with respect to the axis of the grinding container.

7. A mill according to claim 1, comprising pairs of disc-shaped grinding members of conical cross-section, and grinding balls arranged between the grinding discs.

8. A mill according to claim 1, comprising air impeller blades positioned within and arranged to rotate within the grinding container.

9. A mill according to claim 1, wherein the grinding container and the grinding member carrier are arranged to rotate in the same direction and the grinding 14 container rotates at a higher speed of rotation whereby drive power flows from the grinding container through the layer of material being ground, to the grinding members, and from there to the grinding member carrier.

10. A mill according to claim 9, comprising a single drive motor with two shaft ends, of which one shaft end drives the container and thesecond shaft end is connected to the grinding member carrier whereby the grinding member carrier is braked.

11. A mill according to claim 1, wherein the movement of the grinding members in an outward direction under the effect of centrifugal force is limited by a mechanical abutment.

12. A mill according to claim 11, wherein the movement of the grinding members in an outward direction under the effect of centrifugal force is limited by resilient means. 7

13. A mill according to claim 12, wherein a spring for limiting the outward movement of the grinding members is constructed with an adjustable spring force.

14. Apparatus according to claim 1, wherein the axis of rotation of the grinding member carrier is arranged eccentrically with respect to the axis of rotation of the grinding container.

15. A mill according to claim 1, comprising discharge means from said container for the fine material produced in the grinding operation, which discharge means rotates with the grinding member carrier..

16. A mill according to claim 15, wherein the discharge means comprises a substantially radially extending discharge channel whose outlet aperture co-operates with anannular stationary hopper which surrounds the axis of the grinding container.

17. A mill according to claim 1, wherein the grinding members comprise a plurality of rings having hubs which carry at least one layer of resilient material and grinding rings comprising a material which has a high resistance to wear comprises the grinding surface of the members.

18. A mill according to claim 1, wherein the grinding members comprise pairs of grinding rollers,-in which the grinding rollers are rigidly mounted on both sides of the centre of their axis which is formed as a spindle, and the spindle is rotatably mounted at its centre part in the associated trailing arm.

Claims

1. A mill comprising a. a grinding container arranged to contain material to be ground and having an axis therein about which the container is drivably rotatable at an overcritical speed of rotation and an internal cylindrical material grinding face, b. a grinding member carrier positioned within the grinding container and rotatable therein at an overcritical speed, c. a plurality of trailing arms pivotally mounted on said carrier within said container and extending from the point of pivotal attachment in a direction opposite to the rotational direction of said carrier, d. grinding members rotatably mounted on said trailing arms within said container inwardly from said grinding face at a position on said trailing arms spaced from the point of pivotal attachment to said carrier, e. means for controlling the difference between the peripheral speed of rotation of the grinding members and the speed of rotation of the grinding face of said grinding container, so that the grinding members are forced outwardly by centrifugal force to roll over the material to be ground which is distributed within the container over the material grinding face by centrifugal force of said container, and the normal speeds of rotation of the grinding container and the grinding members are in a range in which the centrifugal forces in the region of the material to be ground and at the grinding member are a multiple of their weight.

2. A mill according to claim 1, wherein the pivot axes of the trailing arms are at a smaller spacing from the material grinding face than the grinding member diameter.

3. A mill according to claim 1, wherein grinding members of identical configuration are mounted diametrically opposite each other with respect to the axis to form pairs of grinding members and there are differences of configuration as between said pairs.

5. A mill according to claim 1, wherein the normal speeds of rotation of the grinding container and the grinding members are in a range in which the centrifugal forces in the region of the material to be ground and at the grinding members have a value which exCeeds 20 to several hundred times their weight.

9. A mill according to claim 1, wherein the grinding container and the grinding member carrier are arranged to rotate in the same direction and the grinding container rotates at a higher speed of rotation whereby drive power flows from the grinding container through the layer of material being ground, to the grinding members, and from there to the grinding member carrier.

10. A mill according to claim 9, comprising a single drive motor with two shaft ends, of which one shaft end drives the container and the second shaft end is connected to the grinding member carrier whereby the grinding member carrier is braked.

12. A mill according to claim 11, wherein the movement of the grinding members in an outward direction under the effect of centrifugal force is limited by resilient means.

15. A mill according to claim 1, comprising discharge means from said container for the fine material produced in the grinding operation, which discharge means rotates with the grinding member carrier.

16. A mill according to claim 15, wherein the discharge means comprises a substantially radially extending discharge channel whose outlet aperture co-operates with an annular stationary hopper which surrounds the axis of the grinding container.

18. A mill according to claim 1, wherein the grinding members comprise pairs of grinding rollers, in which the grinding rollers are rigidly mounted on both sides of the centre of their axis which is formed as a spindle, and the spindle is rotatably mounted at its centre part in the associated trailing arm.