Mill
United States Patent 1191 Eigner Apr. 29, 1975 [541 MILL FOREIGN PATENTS OR APPLICATIONS Inventor: Erich s Rademhein&#39; Austria 1.000.220 8/1965 United Kingdom 24l/l83 Assignee: oserreichisch-Amerkianische 188,190 &#34;I957 Austria i Magnesit Aktiengesellschait Primary E.raminerRoy Lake [22] I973 Assislam E.ram:&#39;nerE. F. Desmond Appl. 010.; 406,665  
 [30] Foreign Application Priority Data Oct. 23. I972 Austria 9023/72 July I9. 1973 Austria 0. 6354/73 [52] [1.8. CI. 24l/l82 [51] Int. Cl. B02c 17/22 [58] Field of Search 24l/l70 172, I76. 179, 24l/18l-l83 [56] References Cited UNITED STATES PATENTS 3.677.479 7/1972 Slcgten 241/183 Attorney, Agent, or Firm-Finnegan, Henderson, Farabow &amp; Garrett [57] ABSTRACT A mill is disclosed having a shell and a substantially square cross section with rounded corners and including a plurality of liner plates being aligned along the axis of the shell, interior surfaces of the plates defining annular courses at least a portion of which are inclined toward the axis and a portion of which are parallel to the axis, with adjacent courses being angularly displaced.  
 l7 Claims, 4 Drawing Figures PATENTED RZ IHYS 3.880885 SHEET 2 m 2 W W W FIG 3 FIG. 4  
 MILL  
  This invention relates to a mill, particularly a tube mill or ball mill, which has an interior polygonal, preferably square, cross section with rounded corners and straight or mildly curved sides, and in which the grinding action is performed by the falling and rolling motion of the contents or filling which consists of the grinding elements and the material to be ground. The size of the grinding elements, which usually are short cylinders or balls, is only a small fraction of the side length of the basic cross section of the mill interior. The interior of the mill contains liner plates arranged in a series of annular courses arranged one behind the other in the longitudinal direction of the mill, and individual annular courses formed by these plates are angularly displaced or offset relative to each other, preferably by equal angles. The invention relates particularly to a further development of the known mill ofthat kind having a square basic cross section with rounded corners and straight sides (Austrian Patent Specification 239,634).  
  For this reason the invention will be explained hereinafter mainly with reference to that mill although the explanation is applicable with corresponding modification also to the other mills of the kind described which have a polygonal basic cross section.  
  In that known mill having a substantially square cross section, the grinding action can be strongly influenced by the selection of a proper radius for the rounded corners. A decrease of the radius of the rounded portion relative to the diagonal of the cross section of the mill will result in a stronger impacting action on the contents and an increase of the radius of the rounded portions will result in a lighter impact and in a stronger frictional action.  
  Because the successive individual annular courses are angularly displaced or offset, a passage is formed in the mill which, owing to the square basic cross section of the mill, is similar in shape to a four-thread screw. Owing to this design, an additional impulse in the direction toward the axis of the mill is imparted to the grinding elements. Besides, the helical design also influences the flow of the material being ground in the longitudinal direction of the mill. The flow of the material being ground can be decreased or increased by a suitable selection of the lead or hand of the helical passage resulting from the offset of the annular courses. The lead of this helical passage is determined by the selection of the angle between successive annular courses.  
  The movement through the mill of the material being ground also depends strongly on the specific gravity of the material being ground. Easily flowing materials to be ground in a dense bed of grinding elements tend to remain on the surface and virtually float rather than enter the bed of grinding elements. On the other hand, a thorough mixing of the grinding elements and the material being ground is required particularly for a very fine grinding. If the individual annular courses are rela tively offset by up to 45 in the direction of rotation, material to be ground which has a higher specific gravity will move from the inlet of the mill to its outlet. If the annular courses are offset in a direction opposite to the direction of rotation of the mill, the material movement will be arrested, because the effect of each of these is to promote movement in opposite directions.  
  Practical operations and experiments have shown that the helical passage formed in the mill by the offset annular courses has also a certain sorting action on the grinding elements so that most of the larger grinding elements collect at the mill inlet end and most of the smaller grinding elements collect at the mill outlet end. This effect is not complete, however, because it is obtained only with grinding elements, such as balls or short cylindrical shapes, up to 40 millimeters, at most. For non-fine grinding it is necessary to use also grinding elements which are larger in diameter than 40 millimeters, e.g., grinding elements which are -70 millimeters in diameter; this is the size of the grinding elements in the second chamber of a multichamber mill.  
  in an economical grinding operation, large grinding elements must be used to grind the coarse material and the size of the grinding elements, e.g., balls, must progressively decrease as the material becomes finer. This has been accomplished in a mill which has been divided in its longitudinal direction into chambers by walls so that the different grinding elements, e.g., balls of greatly different sizes, cannot mix with each other whereas the material being ground can pass to the next chamber nearer to the outlet through slots in the chamber walls.  
  An additional impulse is required to improve the sorting action on the grinding elements in the abovementioned known mill and particularly to provide a sorting action also for larger grinding elements so that they remain in a desired region of the mill. This holds analgously true for all mills of the present kind which have a polygonal basic cross section with rounded corners and straight or mildly curved sides. It is an object of the present invention to provide such additional impulse.  
  To achieve the foregoing objectives and in accordance with the purpose of the invention, as embodied and broadly described herein, the mill of this invention is composed of a circular shell with a polygonal interior cross section, preferably of square cross section with rounded corners and straight or mildly curved sides, and in which the grinding action is performed by the falling and rolling motion of the filling consisting of the grinding elements and the material to be ground, the size of the grinding elements being only a small fraction of the side length of the basic cross section of the mill. The interior of the mill is provided with liner plates arranged in a series of annular courses arranged one behind the other in the longitudinal direction of the mill. Individual annular courses formed by these plates are angularly displaced or offset relative to each other, preferably by equal angles. Annular courses of the mill are formed by plates (f, .f or f, .f which have at least a portion of their inside surface inclined toward the axis of the mill and in the direction of travel of the material being ground. Such plates are referred to hereinafter as inclined plates. The inside surfaces of the inclined plates may be inclined throughout their width or they may be inclined only throughout part of their width whereas the remainder of the inside surfaces of the inclined plates extend parallel or almost parallel to the longitudinal axis of the mill.  
  In one embodiment of the invention, an annular course comprising plates having inside surfaces which are parallel to the axis of the mill and an adjacent annular course which comprises inclined plates may be co mbined in a unit and each of said units may be angularly offset from the adjacent unit. The angle (a) by which the annular courses or units are angularly spaced or offset relative to each other may preferably be between and 50.  
  In general. in a mill of the present kind the balls or other grinding elements are sorted more quickly if a larger number of annular courses are composed of inclined plates. The fastest ball-sorting action is obtained if all annular courses comprise inclined plates and in this case the grinding elements above 60 millimeters will be more accurately sorted into several fractions. The use of plates whose inside surfaces have the same inclination in all annular courses affords the additional advantage that only one type of plate is required to line the mill. It is known, however, that grinding elements in different sizes are mostly desired in the coarse grinding region of the mill. The optimum lining can be selected for any particular material to be ground.  
  Best results will be obtained in all cases with those mills according to the invention which have a square basic cross section with rounded corners and straight sides and in which the radius of the rounded corners is preferably approximately one-third of the side length of the basic square. To minimize the loss of grinding space in mills which are larger in diameter, a polygonal basic cross section rather than a square one may be used in specific cases although best results will be obtained only if the geometric requirements relating to the ratio of the side iength to the curvature of the corners are complied with and inclined plates are used or co-used in accordance with the invention.  
  Annular courses of plates having inclined inside surfaces are known per se but only in mills having a circular cross section (see, e.g., German Patent Specification 462,188 and the article in the periodical Zement- Kalk-Gips&#34; 9( l956), pages 228-231). In the previous mills either each annular course has an inclined inside surface or the inclined surfaces are steep and high and are formed by separate internal fixtures (as in the German Patent Specification 462,l88) rather than by inclined linear plates. That system is subject to heavy wear. However, a mill formed in accordance with this invention and particularly the combination of annular courses wihch consist of plates having inside surfaces that are parallel to the mill axis to provide for a square cross section of flow, and annular courses of plates whose inside surfaces have a relatively small inclination in such a manner that every other or every third annu lar course is composed of inclined plates, whereas adjacent annular courses are angularly offset, will provide satisfactory grinding as well as sorting of the grinding element and also a small wear of the liner plates. It may be mentioned here that prior mills having a circular cross section provide for a sorting of grinding elements above 60 millimeters or above 40 millimeters but it is very difficult in such mills to sort small grinding elements below 40 millimeters and particularly below millimeters. Experiments have shown that only the design ofthe mill liner according to this invention enables an entirely satisfactory sorting of all grinding elements without a division of the mill into chambers.  
  The invention consists in the novel parts, constructions, arrangements, combinations, and improvements shown and described. The accompanying drawings. which are incorporated in and constitute part of this specification, illustrate three embodiments of the invention and together with the description, serve to explain the principles of the invention.  
 Of the drawings:  
  FIG. 1 is a transverse schematic sectional view showing a mill formed in accordance with this invention;  
  FIG. 2 is a longitudinal sectional view taken along the line A-B of FIG. 1 showing a portion of a mill having straight and inclined liner plates in accordance with one embodiment;  
  FIG. 3 is a longitudinal sectional view taken along the line AC of FIG. 1 showing a portion of a mill having combined straight and inclined liner plates in accordance with a second embodiment of this invention; and  
  FIG. 4 is a longitudinal sectional view taken along the line AB of FIG. 1 showing a portion of a mill having only inclined plates in accordance with a third embodiment.  
  Reference will now be made in detail to the preferred embodiments of the invention, an example of the first embodiment of which is illustrated in FIGS. 1 and 2.  
  In accordance with the invention, the mill 10 has a cylindrical shell 12, in which annular courses 14 are formed by annular liner plates 15. These courses 14 are arranged one behind the other in the axial or longitudinal direction of the mill 10. The individual annular courses which are accommodated in the portion of the mill 10 shown in FIG. 2 are designated e f e j}. Reference characters e designate annular courses of plates which have inside surfaces 16 that are parallel to the axis of the mill. The plates of the annular courses designated with the reference character fhave inside surfaces 18 which are inclined at an angle of inclination B toward the interior or axis of the mill l0 and in the direction of flow, indicated by the arrow, of the material being ground. The angle of inclination of the inside surfaces of these inclined plates depends upon the specific use of the mill, namely. the nature of the material being ground, etc., and to a certain degree also on the width of each plate. It has been found that it is generally desirable to select an angle [3 in the region of 5l 5 although good results may be obtained in special cases with different angles B up to and above 30. Alternatively, different inclinations may be selected in successive annular courses for some of them or groups of them compared to the inclination in preceding or succeeding annular courses f or groups thereof. The width of each plate and of each annular course may vary; however, widths of, e.g., -250 millimeters have produced satisfactory results.  
  As can be seen in FIG. 3, each annular course con sisting of one axial course e and one inclined course f may be combined and formed by a plate 20 which is bent so that its inside surface is axially oriented in part and inclined in part.  
  In order to simplify the description, the combination of the axial and inclined annular courses. e,fin a group or unit. are designed E E in FIGS. 2 and 3. Each combined annular course or unit E E is offset or angularly spaced relative to the succeeding and preceding annular courses by an angle a (see FIG. 1) whereas there is no offset within such combined annular courses or units.  
  To clarify the offset in FIG. 2, the parting lines 22 betweeen each annular course e and the adjacent annular course fare represented by a thinner line than the parting lines 24 between successive units E,-E E -E etc. For the sake of clearness, the parting lines are represented in exaggerated width; they have actually only a width of 2-3 millimeters. The thicker lines are intended to indicate that the annular courses on opposite sides of the respective parting line are offset or angularly spaced. The extent offset will depend on the conditions, such as the material to be ground, the desired velocity of travel thereof through the mill, etc. Good results have been obtained with an offset (angle a) of l5-45 although this statement is not intended to impose a limitation on the selection of the offset.  
  The number and order of plates 15 having inside surfaces 16 which are not inclined but which are parallel to the axis of the mill and of plates 15 having inside surfaces l8 which are inclined toward the interior of the mill may be modified. For example, every third instead of every other course may be defined by inclined plates. The length of annular courses e of plates having inside surfaces which are parallel to the axis of the mill and annular courses fof inclined plates can be varied as required so that units of longer annular courses e of axial plates and shorter annular courses fof inclined plates are installed in the shell 12 of the mill 10. It is particularly desirable if the mill has a square basic cross section with rounded corners and substantially straight sides. At least every third annular course and preferably every other annular course (f, .f, is composed of plates 15 which have inside surfaces 18 which are inclined toward the interior of the mill l0 and in the direction of travel of the material being ground and the plates 15 are preferably so arranged that plates (e, e which have inside surfaces [6 which extend in the longitudinal direction of the mill and parallel to the axis of the mill alternate with the inclined plates (f, .f, The annular courses e e, are mutually offset and the annular courses f .f are also mutually offset.  
  The mill 10 shown in FIG. 4 is composed solely of inclined platesf .f4; in other respects this embodiment is similar to the one shown in FIG. 2.  
  FIG. 1 illustrates the offset of annular courses 14 or units. To avoid overcrowding the drawing, only the inside contours of the first two units E and E of FIGS. 2 and 3 are shown; these are the inner edge contours of the inclined plates of the annular courses f and f Different angles of offset a could be selected between successive annular courses e orfor units E over the length of the mill. The variation of the local lead angle of the resulting helix defined at the beginning of the specification will be selected in view of the nature of the desired grinding operation and the desired distribution or sorting of the different sizes of grinding elements over the length of the mill.  
  The angle of inclination (angle B) of the inside surfaces 18 of the inclined plates may be &#34;-30. preferably 5-l5. The inclined surface of the inclined plates need not be flat but may be concavely curved. Besides, it is not necessary to use inherently inclined plates; the inclination may be provided by plates having inside surfaces extending parallel to the axis of the mill which are provided with protruding strips which extend at a desired angle other than 90 to the axis of the mill.  
  To maintain the required sorting of the grinding elements for a long period of operation, it is not necessary in a mill designed according to this invention to provide a plurality of chambers separated by partition walls. Such partition walls can usually be entirely omitted so that the mill consists only of a single chamber. Even if grinding elements of extreme sizes are used. only a single partition for the entire mill will be required so that the mill then comprises two chambers.  
  The result as regards the sorting of the grinding elements in a mill according to the invention having a square basic cross section and rounded corners and straight sides will now be explained with reference to a practical test made with such mill.  
  A tube mill having a length of 9 meters and 2.4 meters in diameter was fed with balls in sizes of 20-60 millimeters in the following proportions in percent by weight:  
 Size of ballsmm 20 25 30 40 50 60 1&#39; by weight 20.2 20.2 16.8 16.85 14.30 11.65  
  The mill 10 was rotated clockwise. Each annular course 14 had a width of 250 millimeters. Each pair of annular courses e, f were combined in a unit. The first annular course of each unit was offset in the direction of rotation by an angle of l5 from the first annular course of the adjacent unit. Every other annular course had an inclination of 15.  
  The balls were fed as an unsorted mixture. The degree of fullness was 26 percent. The mill was then operated for 24 hours at a speed of 0.7 times its critical speed so that a stationary equilibrium had then been obtained as regards the distribution of the balls. The distribution in the mill length portions 0-1.5 meters. l.5-3.0 meters, 3.0-4.5 meters, 4.5-6.0 meters. 6.0-7.5 meters, 7.5-9.0 meters, in the direction of flow of the material was then examined with the following results:  
 Length portion 0 l.5 meters Ball diameter, mm  
 Proportion. if by weight Length portion 1.5 3.0 meters Ball diameter. mm  
 Proportion, k by weight Length portion 30 4.5 meters Ball dimatcr. mm  
 Proportion Z by weight For a check. the direction of rotation was reversed in some tests so that the mill rotated counterclockwise. To cause the material being ground to flow in the same direction, the offset of the annular courses (in this case too every other course was offset in the counterclockwise direction of rotation) was changed. The ball mixture and the other conditions were entirely the tial degree.  
 What is claimed is:  
  l. A mill having a cylindrical shell and a plurality of liner plates mounted within said shell and forming the interior of the mill. said interior having a polygonal cross section formed of sides with rounded corners. a  
 same. The following distribution of balls was stated 25 plurality of grinding elements within Said i er the after an operation for 24 hours.  
 maximum dimension of each grinding element being a Length portion 0 1.5 meters Ball diameter. mm 4O Proportion, Z by weight 0.13 5.77  
 Length portion 1.5 3.0 meters Ball diameter. mm 20 25 30 Proportion. I by weight 1.7 41.8  
 Length portion 3.0 4.5 meters Ball diameter. mm 20 25 30 4O Proportion, Z by weight 1.6 31.4 45.8  
 Length portion 6.0 meters Ball diameter. mm 20 25 3O 40 Proportion. i by weight 1.3 38.8 56.1 3.8  
 Length portion 6.0 7.5 meters Bull diameter. mm 20 25 30 40 Proportion. by weight 29.1 67.10 3.90  
 Length portion 7.5 9.0 meters Bull diameter. mm 20 25 30 40 Proportion. 91 by weight 91.3 8.70  
 small fraction of the length of each of said sides. said interior formed solely by said liner plates which are arranged in a plurality of adjacent annular courses aligned one behind the other in the longitudinal direction of the shell. at least a portion of said annular courses being angularly displaced about the shell axis from the annular courses adjacent thereto, and at least some of said liner plates having inside surfaces being inclined toward the axis of the shell and in the direction of flow of the material to be ground.  
  2. A mill as defined in claim 1, wherein the inclined inside surface of said liner plates is inclined throughout its width.  
  3. A mill as defined in claim 1 wherein at least some of the annular courses are comprised of liner plates the inside surfaces of at least some of which are inclined in an axial direction and the inside surfaces of at least some of which are substantially parallel to said axis of the shell.  
  4. A mill as defined in claim 1 wherein at least every third said annular course is comprised solely of at least one liner plate having an inside surface inclined toward said axis of the shell and in the direction of the material flow.  
  S. A mill as defined in claim 4 wherein the angle of inclination of the annular surface of said liner plates is substantially within the range of 530.  
  6. A mill as defined in claim 1 wherein every second said annular course is comprised solely of at least one liner plate having an inside surface inclined toward said axis of the shell and in the direction of the material flow.  
  7. A mill as defined in claim 6 wherein the annular surface of the inclined liner plates has a concave curva ture.  
  8. A mill as defined in claim 1 wherein the angle of inclination of the annular surface of said plurality of liner plates is substantially within the range of 530.  
  9. A mill as defined in claim 8 wherein the amount of angular displacement of adjacent annular courses is substantially within the range of l5-50.  
  10. A mill as defined in claim 8 wherein the annular surface of the inclined plates has a concave curvature.  
  ll. A mill as defined in claim 1 wherein the angle of inclination of the annular surface of said plurality of liner plates is substantially within the range of 5l5.  
  12. A mill as defined in claim 1 wherein at least some of said annular courses are defined by at least some liner plates a portion of the annular surfaces of which are substantially parallel to said axis and a portion of the surfaces of which are inclined.  
  13. A mill as defined in claim 1 wherein the amount of angular displacement of adjacent annular courses is substantially within the range of l550.  
  14. A mill as defined in claim 1 wherein the annular surface of the inclined liner plates has a concave curvature.  
  15. A mill as defined in claim 1 wherein said inclined surfaces are formed by at least one strip protruding at an angle other than relative to said axis from the surface of said liner plates which are parallel to said axis.  
  16. A mill having a cylindrical shell having a plurality of liner plates mounted within said shell and forming the interior of the mill, said interior having a substantially square cross section with rounded corners. the radius of said corners being approximately one-third the length of each side of said square cross section, a plurality of grinding elements contained within said shell, the maximum dimension of each grinding element being a small fraction of the length of said side, said interior of the mill formed solely by said liner plates which are arranged in a plurality of adjacent annular courses aligned one behind the other in the longitudinal direction of the mill, each said annular course containing at least one said liner plate. at least a portion of the annular surface of a plurality of said liner plates being inclined toward the axis of the shell and in the direction of travel of the material to be ground, and at least a portion of said annular courses being angularly displaced about the shell axis from the annular courses adjacent thereto.  
  17. A mill as defined in claim 16 wherein the annular surface of at least some of said plurality of said liner plates is in part inclined in an axial direction and in part substantially parallel to said axis.