US3469795A - Material reduction mill - Google Patents

Description

Sept 30, 3969 D. WESTON MATERIAL REDUCTION MILL 4 Sheets-Sheet 1 Filed April 22, 1966 Arrow/[rs sept. 30, 1969 WESTON 3,469,795

MATERIAL REDUCTION MILL Filed April 22, 1966 4 Sheets-Sheet 1;

Sept. 30, 1969 D. WESTON 3,469,795

MATERIAL REDUCTION MILL I Filed April 22, 1966 4 Sheets-Sheet 5 Ahoy/vars Sept. 30, 1969 D. WESTON 3,469,795

MATERIAL REDUCTION MILL Filed April 22, 1966 4 Sheets-Sheet 4 3,469,795 MATERIAL REDUCTION NHLL David Weston, Toronto, Ontario, Canada, assignor to Aerofail Mills inc, Clarkson, Untario, Canada Filed Apr. 22, 1966, Ser. No. 544,471 Claims priority, application Canada, Feb. 25, 1966, 953,292 Int. Cl. BGZc 17/04, /08

US. Cl. 241-176 9 Claims ABSTRACT OF THE DISCLOSURE A material reduction mill of the cylindrical drum type characterized by keying liners on the end walls of the drum and a substantially cylindrical inner face, and having a large diameter in comparison to its length. With such a drum, having the proper size and peripheral speed, the keying action can be obtained without the use of highly upstanding crusher bars on the cylindrical surface of the drum.

This invention relates to material reduction mills of the type having a cylindrical drum, having a large diameter in comparison to its length and which is provided in the interior thereof with keying liners on the interior end walls of the drum having keying faces which together with the interior face of the cylindrical wall and the radially-most outward parts of the faces of the end walls form a keying zone within the mill. Such mills are described in my prior Canadian Patent 522,803 which discloses such a mill adapted for dry comminution and in my prior Canadian Patent 618,313 which discloses such a type of mill particularly adapted for wet comminution. Such mills have proved highly eificient in the reduction of run-of-mine or primary crusher product ores to a final milled product in a single stage. Depending upon the type of ore and the grindability characteristics thereof, such mills may be operated without a ball charge, or with a ball charge consisting of a relatively small number of large size balls as inertiabodies as disclosed in my prior Canadian Patent 489,563. In certain instances and particularly where a fine product is desired, such mills have been operated with a keying or synthetic charge consisting of a ball charge which is preferably seasoned in character and which contains a sufiicient number of balls effectively to fill the keying zone of the mill as described in prior Canadian Patent 604,874 to Weston and Turner.

The keying action of mills of the above type when used in the manner set forth in the above-mentioned prior patents depended upon the presence around the periphery of the interior of the mill drum of highly upstanding transverse crusher bars spaced widely enough apart to be able to engage the largest pieces of the intended feed material so that as each such crusher bat would enter the toe of the charge it would cause compaction of the charge as a whole and the development of keying pressures normal to the faces of the keying liners.

I have now found that it is possible when utilizing a charge according to the teachings of my prior Canadian Patent 604,874 where the keying zone is effectively filled with balls or with keyable feed materials and where the diameter of the mill is sufficient to provide, in operation, a keying zone of suflicient peripheral extent and of sufficient linear velocity of the shell relative to the material within such keying zone to obtain a much more efficient type of keying action than was heretofore thought possible. I have found that as the diameter of the mill increases and accordingly the total inertia of the whole charge increases (owing to the greater volume of the keying zone and the greater linear velocity of the mill atent shell relative to the charge) there comes a point where the internal pressures developed in the keying zone are sufiicient effectively to key the charge by direct engagement between the reduction media in the charge and interior peripheral wall of the mill. The effective attainment of this type of keying action depends upon the interior peripheral wall of the mill being essentially cylindrical and free of highly upstanding crusher bars which tend to interfere with the new type of keying action with which this invention is concerned. Accordingly, in the present invention it is the co-operation between the keying faces of the keying liners the ball charge and the interior peripheral walls of the mill which provides the effective comminution results rather than the co-operation of the faces of the crusher bars with ball charge and the keying faces of the keying liners which was the essential character of mills designed to accomplish the keying action described in prior Canadian Patents 522,- 803; 618,313 and 604,874. Whereas in mills designed to accomplish the keying action described in the said patents the main wear load was borne by the crusher bars with very little provision for wear resistance being necessary at the interior face of the peripheral wall, in mills designed in accordance with the present invention the principal wear load is borne by the face of the peripheral wall and it is necessary to make appropriate provision for it. Therefore, although theoretically the ideal peripheral wall for purposes of the present invention would be one which is perfectly cylindrical it is necessary as a practical matter to depart from a perfect cylinder or other surface of revolution to the extent necessary to provide for effective wear resistance to keep the wear on the peripheral liners within acceptable limits.

In the mill, according to this invention, the movement of the balls is brought about by the keying of the balls with particles of feed material. The useful crushing work done by a ball is in fact a measure of the acceleration given the individual balls and material. It will be appreciated that in order for a keying action of this type to develop, there must be at least a certain peripheral length of charge within the keying zone; the peripheral speed must also be above a certain value. Otherwise. the forces generated by the keying action are not sufficient to prevent slippage. Consequently, there is a minimum diameter of mill for which the present new mill structure will exhibit benefits compared to mills where the balls are motivated by highly upstanding crusher bars. While this minimum size will vary to some extent, depending upon the nature of the material which is undergoing comminution, it is recommended that the new mill structure herein described be used in mills having a diameter of at least 8' measured between the innermost extremities of the keying faces of the keying liners.

It is a great advantage of the invention that it enables practically all the comminution to be done by crushing even where a very fine product is desired. Milled products produced in this mill are characterized by outs anding freedom from slimes and by outstanding mineral liberation. An example of one such product is given in Table I:

This ore contained a 22% magnetic iron head value with the magnetic iron finely disseminated. At the above noted medium-fine grind, a concentrate was produced containing 99% of the magnetic iron content of the ore and at a grade of 65.5% iron.

The development of an eflicient keying action in a mill according to the present invention depends upon an efficient transmission of the force developed at the interior peripheral wall through the charge of the outwardly facing walls of the keying liners. If the radical distance from the keying liners to the interior peripheral wall is too great, the force generated normal to the outwardly facing wall of the keying liners becomes relatively small compared to that generated normal to the interior peripheral wall and the result is that slippage Occurs at the outwardly facing wall of the keying liners. If the length of the mill is too great, there is likewise a less efiicient transmission of forces through the charge to the outwardly facing walls of the keying liners. Either the inward extremities of the keying liners must extend inwardly across a greater portion of the mill length to maintain a free space between them which can effectively be bridged by the charge, in which case, the area of the inwardly facing walls of the keying liners is enlarged, and the total force generated normal to the peripheral wall is exerted against these surfaces over a larger area so that the maximum unit force is accordingly decreased; or if the area of the outwardly facing walls of the keying liners is maintained the same, the distance between the inward extremities of the keying liners becomes too great to maintain an effective bridging action, with the result that the effectiveness of the keying action is lost, charge material escapes too readily inwardly towards the mill axis and the forces exerted against the outwardly facing walls of the keying liners become too small to prevent slippage at that point. Thus, as the diameter of the mill is increased, the ratio of its length to diameter will decrease in order to maintain the proper development of the keying action according to the invention. While a 12 diameter mill may suitably have a diameter length ratio of about 3 to 1, a 27' mill will suitably have a diameter length ratio of about 4.5 to 1.

While the free space between the inward extremities of the keying liners could :be increased and still be effectively bridged if larger diameter balls are employed, it has been found that there is a practical upper limit to the maximum size of ball which may efliciently be employed although this is governed to some extent by the friability of the feed material. It can in principle be stated that in the crushing action which takes place within the charge in operation, useful work results from compression forces generated normal to the surfaces of the balls and the interior surfaces of the keying zone, the force of any particular ball being momentarily concentrated upon the particle of feed material which happens at a particular moment to find itself along the developing line of force between the interior peripheral wall of the mill, the ball and the outwardly facing wall of the keying liner. As is well known for any given material, there is a critical force which must be applied to break the material down by crushing. A force below the critical limit will do little useful crushing work, while a force substantially in excess of the critical limit will tend to shatter rather than to crush. In the latter case energy is lost in friction and in impact between adjacent balls. In a large diameter mill it will be found that for most materials steel balls of about in diameter will give good results. Larger balls tend to develop such high maximum forces that shattering rather than crushing is accomplished with a subsequent loss of useful energy and loss of milling efficiency which shows up in a lower milling capacity and a higher charge temperature. Furthermore, since the most efficient keying action according to the present invention is accomplished with a seasoned ball charge, the gap between the keying liners which can effectively be bridged by the charge is not determined by the largest ball size but by the rate of breakdown of the material and the mean ball size in terms of the total weight of each size ball present in the mill at any particular time. Thus, the maximum gap that can effectively be bridged will not be dependent upon the maximum ball size. It will also be apparent from the foregoing that the maximum ball size to be used for optimum results will tend to be greater in the smaller diameter mills because of the lower peripheral velocity of the smaller milis when operating within the range of proportions of critical speed which are required to produce the desired mill action.

As mentioned previously, the lining of the interior of the peripheral wall of the mill is of considerable importance in order to maintain the wear loss within permissible limits. Various designs of liners are known in the art for this purpose and many of these are acceptable for purposes of the present invention. However, I have found that it is possible to improve upon the previous designs and to substantially decrease the wear load and scrap loss by using a unique liner arrangement wherein the principal wear at the peripheral wall is borne by steel balls. One advantage of this arrangement is that, as is well known, it is possible to produce a much greater hardness in balls than it is in plate made from similar alloys. For instance Nihard alloy plate to withstand the forces herein involved can have a hardness of the order of 350 to 400 Brinell whereas Nihard balls say in a 2" diameter size can be made to have a hardness of up to 600 Brinell. Scrap loss of the relatively expensive Nihard alloy is completely eliminated by the fact that the balls which have worn to below a certain size simply become a part of the seasoned ball charge. In employing 'balls as the principal wear resistant component of the liners for the interior peripheral wall, I secure wedge shaped channels composed of relatively light section mild steel either transversely to the peripheral wall or longitudinally thereof. These channel sections have their bases secured to the interior of the peripheral wall of the mill and are of a dimension such that a selected size ball will just fit between the legs of the channel to become firmly wedged therein. Thus each channel will contain a row of balls, side by side, firmly wedged between its open legs. In the first instance the balls may be wedged in position by a sledge or power hammer. During operation the forces generated normal to the interior peripheral wall by the charge tend to keep these balls firmly wedged in position.

My invention and its operation will be described in greater detail in conjunction with the accompanying drawings wherein:

FIGURE 1 is a longitudinal cross-section taken through a 25 /2 foot diameter wet mill according to the invention;

FIGURE 2 is a smilar section taken through a 12' diameter wet mill according to the invention;

FIGURE 3 is a similar section taken through a 12 diameter dry mill according to the invention;

FIGURE 4 is an end view partly in section, of the wet mill of FIGURE 2 taken along the line 4-4 illustrating a typical arrangement of the liners and grates;

FIGURE 5 illustrates one suitable form of liner for the peripheral wall;

FIGURE 6 illustrates an alternative form of liner de sign for the peripheral wall employing balls as the wear elements;

FIGURE 7 is a fragmentary view illustrating a slightly modified design for a peripheral wall liner employing balls as the wear elements;

FIGURE 8 is a fragmentary view of an alternative form of liner employing balls as the wear elements wherein the channels run longitudinally around the peripheral wall;

And FIGURE 9 illustrates a further conventional form of liner which is suitable, wherein the interior face is given slight undulations for wear reducing purposes.

Referring now more particularly to the drawings, FIG- URE 1 is a section taken through the axis of a 25' peripheral discharge mill according to the invention. The mill is supported for rotation on trunnions 10 and 11 upon suitable bearings such for instance as those described in Canadian Patent 637,709, Cornford. The ends 12 and 13 of the mill drum may be cast from steel or fabricated from heavy plate. Normally in a drum of this size the ends 12 and 13, whether cast or fabricated, will be made in a number of sections which are then bolted or deep welded together. The peripheral wall 14 may be made from plate of relatively lighter section than the end walls 12 and 13. Mounted on the peripheral portions of the end wall 12 is the drive gear 15 which is adapted to mesh with a drive pinion (not shown) driven by the mill motor. As will be seen, the trunnion which is on the feed side of the mill is hollow to permit material to be fed to the mill through the space 16. The trunnion 11 on the discharge side is similar in design to the trunnion 10 but the hollow space within it is blocked by the plate 17. Mounted upon the end walls 12 and 13 in opposed position are the ring liners 18 and 19 having frusto- conical surfaces 20 and 21 which tend to direct feed material toward the central portions of the peripheral wall 14 in accordance with the teachings of prior Canadian Patent 474,899. Also mounted on the end walls 12 and 13 are the outer keying liners 22 and 23. These liners carry the frusto- conical surfaces 24 and 25 which face the peripheral wall 14 and co-operate with it and the ball charge to produce the keying action of the present invention.

Mounted on the end wall 12 between the liner 22 and the peripheral wall 14 is a conventional wear resistant liner 26. In the corresponding position on the end wall 13 is mounted a peripheral grate 27 containing a plurality of grate openings 28 through which charge material reduced sufliciently may leave the mill. The peripheral grate 27 is suitably fabricated in sections, each one of which has a peripheral outlet 29.

In the mill illustrated in FIGURE 1 the peripheral wall 14 carries a wear resistant liner consisting of transversely disposed mild steel trough elements 30 (see FIGURE 6) which are loaded with balls 31 which are firmly wedged in the trough elements 30 and act as the wear resistant elements. As illustrated in FIGURE 6, the troughs 30A which overlie a joint 32 in the plates forming the end wall 14 may be made slightly larger than the other troughs 30 in order to accommodate a slightly larger ball 31A.

In operation the mill of FIGURE 1 is rotated at a selected speed of from about 75 percent to 90 percent of critical speed with a ball charge of the type described in Canadian Patent 604,874 and feed material and water together with classifier oversize are fed to the mill continuously and in metered quantity through the space 16 in the trunnion 10. Mill product in the form of a slurry is continuously discharged through the outlets 29 at the periphery of the mill and is collected in suitable collection means (not shown) and passed to a classifier or other unit for further processing.

FIGURE 2 is a section taken through the axis of a Wet mill according to the invention. This mill differs from the mill illustrated in FIGURE 1 in that its diameter is nominally 12 feet, the discharge is through the trunnion on the discharge side of the mill and the peripheral wall liner (detailed in FIGURE 7) contains balls of uniform size.

Insofar as the parts of the mill correspond to parts already described in connection with FIGURE 1, the same reference numerals have been used. For simplicity in illustration, the drive gear has been omitted.

In the embodiment illustrated in FIGURE 2, the liners are generally similar to the liner described in connection with the embodiment illustrated in FIGURE 1 with the exception that the channel elements 30b and the balls 31b are uniform in size as originally installed.

The mill illustrated in FIGURE 2 differs from that illustrated in FIGURE 1, firstly in that it is adapted for trunnion discharge and secondly, in that its diameter to length ratio is smaller, the design illustrated being for a 12 foot diameter mill. As will be seen by reference to FIGURES 2 and 4, the reduced material discharges through grates 33 which are made in sections so as to provide radial flow paths 34 which extend towards the mill trunnion, the keying liners 23a and the deflector liners 19a being fashioned to provide a continuation of the flow paths 34 so that as the mill rotates material in these flow paths will flow by gravity towards the mill axis and be delivered to the discharge trunnion through the pathway 35 formed by the deflector liner 19a which in this case has flanged portion 36 extending towards the mill axis to provide a central opening which is masked by the plate 37 on which is axially mounted the conical deflector 38. Also secured within the trunnion 11a is a discharge chute 39 which is adapted to collect and deliver material passing downwardly through pathway 35 and along the surface 38 as illustrated by the dotted line with arrows in FIGURE 2.

In the 12 foot mill illustrated in FIGURE 2, the dimension B is approximately 3.4 feet. The dimension A which is the diameter of the mill measured between the innermost extremities of the keying surfaces is approximately 8.8 feet and the interior mill diameter (A+2C) is approximately 11.7 feet. This compares with similar dimensions on the 25 /2 foot diameter mill illustrated in FIG- URE 1 of 5.5, 17.8 and 25.5 feet, respectively.

The mill .drum illustrated in FIGURE 3 is a mill according to the invention, adapted for dry grinding, and of a nominal internal diameter of 12 feet. The mill is supported for rotation on trunnions 40 and 41 within which are mounted the trunnion liners 42 and 43. The ends of the mill 44 and 45 are either of fabricated steel plate or of cast steel and are similar in design to those already described in connection with FIGURE 2.

Secured to the end walls 44 and 45 of the mill are respectively the deflector liners 46 and 47 and the keying liners 48 and 49. In the embodiment illustrated these last mentioned keying liners have peripheral extensions 50 and 51 lying against the ends 44 and 45 and defining the sides of the keying zone.

The peripheral wall 52 is lined by the conventional line-r elements 53 (see FIGURE 9) which are secured in place by bolts 54 extending through the peripheral wall 52. As is best seen in FIGURE 9, the faces 55 of the liner elements 53 are so shaped as to give the liners as a whole a slightly undulating surface. This undulating surface has been found to have better wear characteristics than a simple cylindrical surface. An alternative form of liner is illustrated in FIGURE 8 and consists of channel elements 56 running peripherally around the peripheral wall and containing rows of balls 57 firmly wedged therein. A further alternative type of liner is that illustrated in FIGURE 5 which consists of liner elements 58 alternated with liner elements 59. These elements are bolted or otherwise secured to the peripheral wall 60 of the mill so that in section the ribs 61 of the liner elements 58 are slightly proud of the surfaces 62 of the liner elements 59. The slight proudness of the rib portions 61 reduces the wear on the surfaces 62.

Any of the liner arrangements illustrated in FIGURES 5, 6, 7, 8 or 9 may suitably be employed in either a wet or dry mill according to the invention regardless of the actual dimensions of the mill itself.

In operation the mills of the invention, whether wet or dry, are first of all loaded with a keying charge of reduction media in accordance with the teachings of prior Patent 604,874 which may, for instance, consist of a seasoned charge of balls calculated to co-operate with the keyable pieces of the material to be reduced to effectively fill the keying zone of the mill. The mill is then rotated at a selected speed approximately within the range of to percent of critical speed and feed material together with the appropriate quantity of water or make-up liquid if the mill is a wet mill are continuously fed to it in controlled quantities. When the mill is in balance, that is to say, after it has been operated for a suflicient length of time for the mill conditions to have become uniform, additional balls of the maximum size which has been utilized in the seasoned ball charge, are added from time to time in amounts which are calculated to make up for the wear loss in the seasoned charge. If the mill being operated is equipped with liners which employ balls as the wearing surface, additional balls will from time to time be added to compensate for the wear loss on the balls wedged in the channel elements. If the balls in the channel elements wear beyond a certain point, they will tend to drop out of the channels and other sized balls will tend automatically, by virtue of action of the mill, to replace the worn balls.

What I claim as my invention is:

1. A material reduction mill of the type having a cylindrical drum, the length whereof is less than half diam eter thereof, keying liners on the interior end walls of said drum with keying faces which, together With the interior face of the cylindrical Wall and the radically most out ward parts of the faces of the end walls form a keying zone within said mill, said mill being characterized by the keying zone having a diameter between the innermost extremities of said keying faces of at least eight feet, and the face of the peripheral wall being provided with wear resistant means adapted to present a generally cylindrical surface to said keying zone with the only departures from the cylindrical surface being minor in character for the purpose of improving the wear characteristics thereof.

2. A material reduction mill as defined in claim 1 wherein the peripheral wall is lined with wear plates having transverse undulations therein.

3. A material reduction mill as defined in claim 1 wherein the interior diameter of the mill is from about three to about 4 times the interior length thereof.

4. A material reduction mill as defined in claim 1 wherein the diameter of the mill measured between the innermost extremities of the keying faces of the keying liners is about 8 feet, the interior diameter of the mill is about 12 feet, and the interior length of the mill is about four feet.

5. A material reduction mill as defined in claim 1 wherein the interior diameter of the mill is between 20 and 30 feet, and the interior length thereof is between 4 and 6 /2 feet.

6. A material reduction mill of the type having a cylindrical drum, the length whereof is less than half diameter thereof, keying liners on the interior end walls of said drum with keying faces, which together with the interior face of the cylindrical wall and the radially most outward parts of the faces of the end walls form a keying zone Within said mill, said mill being characterized by the keying zone having a diameter between the innermost extremities of said keying faces of at least eight feet, and a plurality of transverse channel members secured side by side to the interior of the peripheral wall, said channel members being filled with balls securely wedged therein, whereby to present a generally cylindrical wearing surface composed of the inwardly facing surfaces of said balls.

7. A material reduction mill as defined in claim 6 wherein selected ones of said channel members are adapted to receive and hold wedged therein larger balls than can be held wedged in the others of said channel members.

8. A material reduction mill as defined in claim 6 wherein said channel members lie parallel to the mill axis.

9. A material reduction mill as defined in claim 6 wherein said channel members are longitudinally curved to conform to the curvature of the peripheral wall of the mill and are disposed with their channels running peripherally.

References Cited UNITED STATES PATENTS 939,637 11/ 1909 Rotherham 241-182 1,249,764 12/ 1'9 17 JOhnS0n 241-182 1,921,672 8/ 1933 Haushalter 241-183 2,066,138 12/1936 McKay-Clements 241-183 2,555,171 5/1951 Weston 241-183 2,560,972 7/1951 Martin 241-183 X 2,704,636 3/1955 Weston 241-183 3,010,661 11/1961 Weston 241-176 X 3,058,675 10/1962 Turner 241-183 X 3,125,302 3/1964 Henricot 241-183 FOREIGN PATENTS 676,829 12/1963 Canada.

FRANK T. YOST, Primary Examiner U.S. Cl. X.R. 241-181, 183, 284

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