US3056561A

US3056561A - Method and apparatus for grinding material to a fine degree

Info

Publication number: US3056561A
Application number: US858134A
Authority: US
Inventors: Hukki Risto Tapani
Original assignee: INSINOORITORMISTO ENGINEERING
Current assignee: INSINOORITORMISTO ENGINEERING; INSINOORITORMISTO-ENGINEERING BUREAU R T HUKKI
Priority date: 1958-12-13
Filing date: 1959-12-08
Publication date: 1962-10-02
Anticipated expiration: 1979-10-02

Description

R. T. HUKKl 3,056,561

ING MATERIAL TO A FINE DEGREE Oct. 2, 1962 METHOD AND APPARATUS FOR GRIND 2 Sheets-Sheet 1 Filed Dec. 8, 1959 Q/sro 7:4PA N/ /ggg/ WQKQ,

ATTORNEFY R. 'r. HUKKI 3,05

me MATERIAL TO A FINE DEGREE Oct. 2, 1962 METHOD AND APPARATUS FOR GRIND 2 Sheets-Sheet 2 Filed Dec. 8, 1959 PA /v/ HUKK/ INVENTOR.

BYWZFQQW A T TOR/V5 X 3,ll56,561 METHGH) AND APPARATUS FOR GRINDING MATERHAL TO A FINE DEGREE Risto Tapani Hulrlri, (ltakallio, Otaniemi, Finland, as-

signor to llnsinooritormisto-Engineering Bureau R. T. Hulklri, Helsingfors, Finland, a corporation of Finland Filed Dec. 8, 1959, Ser. No. 858,134 Claims priority, application Sweden, Dec. 13, 1958 6 Claims. (Cl. 241-26) The present invention relates generally to the grind ing or pulverizing art and has particular relation to an improved method and apparatus for grinding material to a high degree of fineness and utilizing a mill of the type having a drum rotating about a horizontal axis and containing a tumbling load consisting of the material to be ground and of grinding media.

The method and inventive organization are applicable for grinding mineral raw materials such as they occur in nature, industrial products such as cement clinker and organic materials such as wood fibres and seeds, to the degree of fineness necessary for further treatment or utilization of the said materials, and to illustrate the materials that may be employed with and the fineness of the grind obtainable with the invention it may be mentioned the ores for flotation processes are ground to a fineness of about 9099% through a 65 mesh sieve (208 microns), and the raw material in cement-making is ground to a fineness of at least 80-90% through a 200 mesh sieve (74 microns), while cement clinker is ground to the fineness of about 80-90% through a 325 mesh sieve (44 microns).

it is well known that in order to effectively and emciently grind material to a fine degree the grinding action must be based on friction. For example, a grinding member moved relative to a stationary support will subject any material of suitable initial fineness which is placed between these two elements to an effective pulverization, based on friction, with the fine grinding capacity of such a system being primarily dependent on the load of the grinding member and the speed difference in the friction zone between said member and the support.

In mills of conventional design employing a mill or grinding drum which rotates about a substantially horizontal axis a similar effect is obtained. The charge of grinding bodies corresponds to the grinding member referred to above and the mill lining corresponds to the support. The outer layer of the charge and the mill lining will rotate in the same direction, but the lining rotates at a greater angular velocity than the outer layers of the grinding charge so that a sliding occurs which brings about the grinding action at the location of the sliding, i.e. the so-called friction or attrition zone.

The resulting difference in velocity in the friction zone will, in fact, be substantial only if the speed of the mill, i.e. the grinding surface or lining, exceeds the so-called critical speed. The critical speed for a mill is the speed at which the force of gravity due to the weight of a grinding ball equals the centrifugal force required to maintain the same ball in a circular motion at the inner periphery of the mill. A mathematical expression for the critical speed is where n is measured in revolutions per minute, and D is the inside diameter of the mill in feet. While the mill proper is rotating at supercritical speed, i.e. a speed above the critical, the angular velocity of the outer layers of the grindin medium must correspond to a sub-critical speed, i.e. a speed below the critical speed. For further dis i gt? cussion on this subject reference is made to applicants co-pending application Serial #729,115 filed April 17, 1958, now Patent No. 2,991,617 and entitled Grinding Method in Ball, Tube and other Mills.

Grinding of a material by means of itself, i.e. the socalled autogenous grinding, offers great advantages, due primarily to the low cost of the grinding bodies. When autogenous grinding is involved it is customary instead of speaking of mill charge and grinding bodies to refer to the tumbling load as comprising the material to be ground and the grinding bodies which are usually, in fact, bigger lumps of the material proper.

in all cases where autogenous fine grinding has been adopted in practice, either in connection with subcritical or with supercritical mill speeds, it has been found that the relatively low specific gravity of the grinding bodies involves a heavy reduction in the grinding capacity of the mill compared with a corresponding grinding operation carried out with the use of metallic grinding media such as steel balls or cylinders.

It has now been found that in grinding by means of rotating drum mills the two most decisive limiting factors for fine grinding in general and autogenous grinding in particular, i.e. the specific gravity of the tumbling load and the speed of the mill, can be avoided by the provision of special means which on the one hand indirectly increases the active mass of the tumbling load and on the other hand increases the speed difference between the outer layer of the tumbling load and the mill lining. In this manner the grinding capacity of the mill will be approximately unaifected by the specific gravity of the tumbling load. Furthermore, the maximum mill speed that can be used will be equally independent of the specific gravity of the said load. Consequently it will be possible to use speeds higher than those ever used in practice before. However, a desired speed difference can be provided at a considerably lower mill speed than hitherto and so the fine grinding capacity of a rotating mill according to the invention can be very substantially improved, even increased several times as compared with the capacity of corresponding mills of conventional design.

According to the invention these aims are obtained by the provision of means inside the mill capable of producing a braking effect upon the movement of the tumbling load in relation to the mill drum so as to increase the pressure exerted by the tumbling load against the mill shell beneath the said means.

The mill drum is advantageously rotated at constant, preferably critical or supercritical speed.

A mill for carrying through the method of the invention is equipped with a special device, referred to hereinafter as a brake member or simply a brake, and arranged so as to produce the desired braking effect upon the movement of the tumbling load. While the mill is rotating at substantially uniform speed in the desired direction, the brake increases the active mass in the mill by a desired active mass, simultaneously, wholly or partly, reducing to the desired extent the angular velocity of a certain outer layer of the tumbling load relative to the inner surface or periphery of the mill drum. As a result, a friction grinding zone will be created between the retarded or stopped layers of the tumbling load and the rotating mill lining. The mill lining may be a lining made wholly or partially of metal or be a substantially autogenous lining separated from the charge in the mill by separate retarding means, so-called holding bars, provided at the mill shell or on a protecting shell lining and also, if desired, on a lining on the inner side of the mill ends.

For special purposes the lining on the .mill shell may consist of relatively wide sections of a metallic liner having a relatively uniform surface, alternating with an autogenous liner having a somewhat coarser surface and preferably equipped with holding bars.

The tumbling load is introduced into the mill in conventional manner, in batches or continuously.

It will be appreciated that autogenous fine grinding in these circumstances will take place under conditions resembling those referred to previously, i.e. a stationary autogenous grinding member (the tumbling load and the brake) will be dragged relative to a moving support (the mill lining) with a certain amount of material (part of the tumbling load) being introduced between the two members. The unavoidable and even heavy wear on the tumbling load and the lining will now be directed almost exclusively to the material to be ground by itself with resulting further improved quality of the final product.

The brake may be designed in a number of ways. lts suspension axis may be coinciding with the axis of the mill or it may be arranged eccentrically in relation to the latter. The brake may be rigidly suspended or movably suspended in such manner that it can freely adopt its angular equilibrium within the mill. Furthermore it may be arranged so as to be positively moved to and fro from an intermediate position, such movements resulting in regular or irregular cyclic variations in the difference between the velocity of the tumbling load and that of the lining.

The brake preferably consists of a substantially horizontal suspension shaft supporting one or more separately independent or interconnected braking members such as a hammer, rails, rods, strips, rings, chains or any other type Whatever weighing from one to several hundred kilograms each. Outside the mill the suspension shaft may be provided with a counterweight in order to produce the desired torque inside the mill and in order to facilitate manipulations and repairs the brake is preferably manufactured as a separate unit which can be mounted or moved independent of the mill drum as by means of a crane. Normally, the brake proper is introduced through the outlet opening of the mill, but its supporting device may more conveniently be placed outside the mill.

Although the principal amount of Wear is concentrated on the tumbling load and on the lining, there will, of course, also be a certain amount of wear on the brake and the parts of the brake which are subject to wear are made of a wear resistant material with the material varying from scrap iron to cast iron, hardened carbon steel, cast manganese steel, a large number of alloy steels resistant to wear, sintered tungsten and vanadium carbides, alloys of cobalt, alloys of chrome and alloys of tungsten such as stellites etc. The braking member proper may be made solely of such materials or may be coated with wear-resistant sections prepared beforehand.

If a substantially autogenous lining is used, the wear on the lining will be concentrated on the actual material to be ground. In that case the retarding means necessary for creating the autogenous lining, i.e. the holding bars, are made of one of the previously named wear resistant materials.

The mill or mill drum used for carrying the invention into effect may be of substantially conventional design. If, however, the mill is used specifically for -autogenous grinding, a substantial economy is obtainable if the mill is supported and rotated by rollers covered by a heavy layer of rubber or having pneumatic tires and which engage the outer surface of the drum. By this method all the normally most expensive parts such as conventional supports and gear 'wheels are dispensed with. Furthermore, the reduced weight of the autogenous grinding medium, the reduced quantity of the medium and the mechanically more advantageous supporting of the mill from below instead of at the ends permits a reduction in the required strength of the mill drum and consequently in its weight unloaded. The mill speed adopted when using the present invention may be sub-critical, critical or supercritical in accordance with the definitions given hereinbefore.

The present invention can be utilized in connection with wet and dry grinding performed in an open or closed circuit and in combination with any outer screening or concentrating device whatever. However, in connection with wet grinding the conventional classification effected outside the mill may be eliminated because an effective classifying basin is produced within the mill proper by a suitable design of the outlet end. Such a classifying basin is created in wet grinding mills on an industrial scale when operating in accordance with the present invention and wherein the discharge of the material takes place at a point approximately 30 cm. or more above the lowest location of the effective lining of the mill. Since the cataracting and cascading effect of the tumbling load under the circumstances described is prevented by the brake, to a large extent, excellent conditions for preventing sedimentation will arise. However, as the coarser parts of the solid material contained in the mass deposit as sediments on the bottom of the basin, a considerable amount of the material will be introduced chiefly in the friction zone.

Accordingly it is an object of this invention to provide and improved method and apparatus for grinding or pulverizing materials to a high degree of fineness.

A further object is to effect this improvement in the pulverizing art in a relatively simple and expeditious manner utilizing a horizontally disposed mill drum rotating about its principal, horizontal axis.

A still further object is to provide such an improved method and apparatus which is extremely efficient in operation providing a substantially increased capacity over that heretofore obtained in the type of mill to which the invention pertains while at the same time maintaining Wear and operating problems at a minimum.

Further and more detailed objects will in part be obvious and in part be pointed out as the description of the invention, taken in conjunction with the accompanying drawing proceeds.

In that drawing:

FIG. 1 is a vertical, transverse, sectional view of a drum in accordance with the preferred embodiment of the invention, with this view depicting a wet grinding mill and;

FIG. 2. is a longitudinal vertical sectional view taken generally along line 2-2 of FEGURE 1.

Referring now to the illustrative organization of the drawing the substantially cylindrical mill or mill drum 1 as depicted therein is supported and rotated by means of two pairs of rollers 2 provided with pneumatic tires 3. The mill hell is provided with live rings 4 that engage these tires and one or both pairs of rollers 2 may be rotated directly or indirectly by any suitable driving means. In order to limit lateral movement of the mill vertically mounted rollers 5 are preferably provided adjacent each end of the mill for engagement with the mill end faces.

The illustrative mill is of the type having an autogenous lining so that immediately inside of the cylindrical shell are secured the longitudinally extending holding bars 8 while radially disposed bars 10 extend along the inner surfaces of the end faces and effectively form continuations of the bars 8.

The tumbling load comprising the material to be ground, the autogeneous grinding media or grinding bodies and water is continuously introduced into the mill through feeding trough 6. A part of the tumbling load constitutes, with the aid of the holding bars 8, the autogenous shell lining 7, whereas another part of the tumbling load constitutes autogenous grinding members 11 and still another part of th said load constitutes a mass basin 12 from the upper part of which a continuous discharge through an outlet opening 13 is taken. The autogenous lining at the end wall of the drum is shown at 9.

In order to retard the mo vement of the tumbling relative to the mill, as mentioned previously, a brake organization, designated generally 1 is mounted for disposition within the mill with this brake being illustratively disclosed as in eccentric relation to the axis of rotation of the mill with the brake being laterally displaced fro-m the axis of the mill drum in the direction of rotation relative to the lower region of the drum. This brake consists of a suspension shaft 15 which carries six uniform hammers 16 that extend downward therefrom into the tumbling load with each hammer consisting of a handle 17 and a head 18 fastened to the end thereof, with these heads preferably being made of wear-resistant steel alloys. The brake organization 14 is supported so as to be angularly displaceable and for this purpose the outer end of shaft 15 is round and is mounted in a bearing 19 so as to be movable about its axis. This bearing 19 is secured to and sup orted by a suitable support bracket Zti. Mounted on the outer end of the shaft 15 is a regulative weight 21 by means of which the torque inside the mill can be varied i.e. the torque that the brake member exerts against the tumbling load. The weight 21 is only diagrammatically shown and it will be understood that either the magnitude of the weight or its connection with the end of shaft 15 may be varied to vary its reaction upon the shaft.

The eccentric or off-center mounting of shaft 15 causes the hammers to move closer to the grinding surface of the mill drum as they are rotated in the counterclockwise direction as viewed in FIG. 1 with this being the direction of rotation of the mill drum. The hammers cause an increased retarding action on the tumbling load as they move closer giving a somewhat automatic operation increasing their effectiveness when the tumbling load is such that it tends with greater force to move with the rotation of the drum.

To facilitate the introduction and removal or regulation of the brake in horizontal, vertical or angular direction special mechanical devices or hydraulic systems may be provided on the foundation 20.

As shown in FIG. 1 a friction zone 22 will be established between the movable lining 7 and the grinding member 11. The grinding media passing through the space between the hammer heads 13 and the mill lining 7 will be circulated until is has been reduced to the fineness of th final product. In the embodiment illustrated the speed of the mill shell corresponds to a supercritical speed and as a result the autogenous lining is retained by means of the centrifugal force in its position in relation to the mill shell even in its uppermost position.

In a dry grinding mill the method is substantially the same as the illustrative wet mill although special provision may have to be made to check the formation of dust.

It will be understood that in lieu of the mill being cylindrical it may be conical or be made up of cylindrical and conical sections and instead of being mounted on rollers as illustratively disclosed it may be supported other ways, e.g. by means of end trunnions of slide shoe bearings as used in conventional mills and it may be driven by other mechanisms such as gear wheels of convenient design.

Though the invention has been shown and described from the standpoint of certain structural details in which it is embodied, it is not however to be considered as being limited by such showing. Rather, as will be apparent to those skilled in the art, certain modifications or variations may be made in the structure as shown and described, without departing from the spirit or scope of the invention. It is, accordingly, intended that all matter contained in the above description and shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

Having described my invention, what I claim as new and desire to secure by Letters Patent is:

1. The improved method of grinding material to a fine degree comprising introducing a tumbling load including material to be ground and grinding media, into a horizontally disposed mill drum, rotating the drum about its horizontal axis at a predetermined generally constant supercritical speed, mechanically retarding the movement of the tumbling load relative to the drum by introducing a retarding element part way into the tumbling load from the inner border thereof to a position spaced inwardly from the inner surface of the drum for a limited portion of the angular extent of the tumbling load and somewhat forward of the lowermost drum region relative to the direction of rotation of the drum thereby retarding the cascading and cataracting effects and promoting grinding action due to the attrition effect reducing the angular velocity of the tumbling load at the location relative to the inner surface of the drum, increasing the pressure exerted by the tumbling load against the mill shell and increasing the friction action between different portions of the material to be ground and between the material to be ground and the grinding media.

2. The improved method of grinding material as in claim 1 and including, providing an autogenous grinding surface on said drum and effecting grinding between said tumbling load and said autogenous grinding surface.

3. In a grinding mill of the type described, a horizontally disposed grinding drum into which a tumbling load is fed and from which finely ground material is dis charged, means for rotating said drum about its horizontal axis at supercritical speed, arm means extending from a position in said drum outside of said tumbling load down into said tumbling load generally throughout the length of the drum and said arm means terminating in substantial spaced relationship with respect to the inner surface of said drum, said arm means, adjacent the terminating portion thereof, being provided with wear resistant members on the side of said arm means opposed to the direction of movement of said drum, said arm means being of limited extension with respect to the direction of rotation of said drum so as to act as a brake to retard movement of the tumbling load resulting from the rotation of the drum, means for mounting said arm means in the drum and tending to maintain said arm means generally stationary while said drum rotates.

4. In a grinding mill of the type described, a horizontally disposed grinding drum into which a tumbling load is fed and from which finely ground material is discharged, means for rotating said drum about its horizontal axis at supercritical speed, arm means extending from a position in said drum outside of said tumbling load down into said ttunbling load generally throughout the length of the drum and said arm means terminating in substantial spaced relationship with respect to the inner surface of said drum, said arm means, adjacent the terminating portion thereof, being provided with wear resistant members on the side of said arm means opposed to the direction of movement of said drum, said arrn means being of limited extension with respect to the direction of rotation of said drum so as to act as a brake to retard movement of the tumbling load resulting from the rotation of the drum, and a support for said arm means, said support including yieldable restraining means for restraining said arm means from movement by the action of the moving tumbling load thereon.

5. In a grinding mill of the type described, a horizontally disposed grinding drum into which a tumbling load is fed and from which finely ground material is discharged, means for rotating said drum about its horizontal axis at supercritical speed, a support shaft extending into the mill drum longitudinally thereof and generally throughout the length thereof, a plurality of arms carried by said shaft and extending laterally of said shaft from a position outside of the tumbling load within said drum, down into said tumbling load, said arms terminating in substantial spaced relationship with respect to the interior of said drum, said arms being of limited extension References Cited in the file of this patent with respect to the direction of rotation of said arms so UNITED STATES PATENTS as to act as brakes to retard movement of said load as a result of rotation of the drum and at the location of said 478,252 culllngwofth y 5, 1392 arms and accordingly increase the pressure exerted by the 5 639,405 Krelss 1899 tumbling load on the inner surface of the drum, enlarged 926441 Shaffer June 29, 1909 wear resistant portions carried by said arms adjacent the 2,282,887 Rober'ts y 1942 terminating portions thereof on the side thereof opposed to the direction of rotation of said drum and means yield- FOREIGN PATENTS ably restraining said shaft and arms from rotating with 10 1,013,015 France Apr. 23, 1952 and in response to the movement of said tumbling load resulting from rotation of said drum. OTHER REFERENCES 111 a grinding mill as in Claim 5, Said Shaft being Hukki: Mining Engineering, May 1958, pages 581-591.

parallel with but laterally offset With respect to the axis of rotation of said drum, said ofiset being in the direction 15 of movement of the lower region of said drum.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent N00 3,056,561 October 2, 1962 Risto Tapa ni Hukki It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 1, lines 63 to 65, the formula should appear as shown below instead of as in the patent:

C V D Signed and sealed this 25th day of June 1963,

( 5 EA L) Attest:

ERNEST w. SWIDER DAVID LADD Attesting Officer Commissioner of Patents