Attrition mill

Abstract

An attrition mill that includes a grinding chamber having a plurality of grinding elements and an internal classification and separation stage. The mill also includes at least one grinding element providing a larger flow path therethrough, when compared to other of the grinding elements. In other embodiments, mill includes at least one grinding element having an open area in the grinding element created to allow a larger flow path as a proportion of the grinding element surface area without such allowance and in the range of from 15% to equal to or less than 100%.

Claims

1. An attrition mill that includes an internal structure and grinding media for grinding particles into coarse and fine particles and classifying and separating the fine particles from the coarse particles using a classification and separation stage in which fine particles are separated from coarse particles and removed from the mill along its length and coarse particles are recycled internally in the mill, the attrition mill comprising: a grinding chamber having an elongated cylindrical side wall, an inlet positioned at or near an upstream end of the grinding chamber, an outlet positioned at or near a downstream end of the grinding chamber, an axial shaft located in the grinding chamber and extending in a direction from the upstream end to the downstream end, the axial shaft being rotatably driven, a plurality of spaced grinding elements in the grinding chamber, each of the plurality of spaced grinding elements being concentrically mounted radially and in a perpendicular orientation relative to the axial shaft, each element being spaced from an adjacent element, each element having a front face that is perpendicular to the axial shaft and including one or more apertures or spaces therein to provide a flow path through each element for slurry and grinding media, each aperture having side walls extending from the front face of the element to a rear face of the element, the side walls being generally perpendicular to the front face, the axial shaft being exposed in between each of the elements thereby allowing passage of slurry and grinding media along and through the grinding chamber from the inlet to the outlet in a direction from the inlet to the outlet, and wherein the fine particles are passed to the outlet and wherein the coarse particles are internally recycled back towards an upstream end of the grinding chamber, wherein at least one of the plurality of grinding elements with the apertures therethrough provides a larger flow path therethrough for the slurry and grinding media compared to one or more of the other of the plurality of grinding elements; and wherein the total open area of the apertures of the at least one of the plurality of grinding elements providing the larger flow path is larger than the total open area of the apertures in one or more of the other of the plurality of grinding elements in the mill.

2. The attrition mill of claim 1, wherein the grinding elements comprise apertures wherein the total open area of the apertures of the at least one of the plurality of grinding elements providing the larger flow path is larger than the total open area of the apertures in one or more of the other of the plurality of grinding elements in the mill.

3. The attrition mill of claim 1, wherein the at least one grinding element that provides the larger flow path therethrough is positioned towards a downstream end of the grinding chamber.

4. The attrition mill of claim 3, wherein the plurality of the grinding elements include eight grinding elements wherein the grinding element that provides the larger flow path is positioned at element 5, 6 or 7 with element 1 positioned near the inlet end of the grinding chamber and element 8 is positioned near the outlet end of the grinding chamber.

5. The attrition mill of claim 1, wherein the mill comprises two or more grinding elements having the larger flow path therethrough.

6. An attrition mill that includes an internal structure and grinding media for grinding particles into coarse and fine particles and classifying and separating the fine particles from the coarse particles using a classification and separation stage in which fine particles are separated from coarse particles and removed from the mill along its length and coarse particles are recycled internally in the mill, the attrition mill comprising: a grinding chamber having an elongated cylindrical side wall, an inlet positioned at or near an upstream end of the grinding chamber, an outlet positioned at or near a downstream end of the grinding chamber, an axial shaft located in the grinding chamber and extending from the upstream end to the downstream end, the axial shaft being rotatably driven, and a plurality of spaced grinding elements in the grinding chamber, each of the plurality of spaced grinding elements being concentrically mounted radially and in a perpendicular orientation relative to the axial shaft, each element being spaced from an adjacent element, each element having a front face that is perpendicular to the axial shaft and including one or more apertures or spaces therein to provide a flow path through each element for slurry and grinding media, each aperture having side walls extending from the front face of the element to a rear face of the element, the side walls being generally perpendicular to the front face, the axial shaft being exposed in between each of the elements along and through the grinding chamber from the inlet to the outlet, wherein the fine particles are passed to the outlet and wherein the coarse particles are internally recycled back towards an upstream end of the grinding chamber, wherein at least one of the plurality of grinding elements comprises said one or more apertures or spaces therein to provide an open area in the grinding element to allow a larger flow path for the slurry and grinding media, the open area being as a proportion of the grinding elements surface area one that does not allow passage of the slurry and grinding media, the proportion being in the range of from 15% to equal to or less than 100%; and wherein the total open area of the apertures of the at least one of the plurality of grinding elements providing the larger flow path is larger than the total open area of the apertures in one or more of the other of the plurality of grinding elements in the mill.

7. The attrition mill of claim 6, wherein the open area in the grinding element created to allow the larger flow path is from 20% to equal to or less than 100%.

8. The attrition mill of claim 6, wherein the open area in the grinding elements created to allow the larger flow path is from 25% to equal to or less than 100%.

9. The attrition mill of claim 6, wherein the open area in the grinding elements created to allow the larger flow path is from 30% to equal to or less than 100%.

10. The attrition mill of claim 6, wherein the mill includes two or more grinding elements having said open area in the two or more grinding elements to allow the larger flow path as a proportion of the surface area of the elements that does not allow passage of the slurry and the grinding media, with each proportion being in the range of from 15% to equal to or less than 100%.

11. The attrition mill of claim 10, wherein other grinding elements in the mill have said open area but with a lesser flow path than the said two elements having the larger open area.

12. The attrition mill of claim 6, wherein the percentage open area is calculated from the equation:
% Open Area=(Area of Apertures)/(Area of Full DiscArea of Hub)100%.

13. The attrition mill of claim 6 wherein the attrition mill is a horizontal shaft attrition mill.

14. The attrition mill of claim 1, wherein each grinding element has a circumference that defines an outer boundary of the grinding element, wherein at least some of the elements retain portions of its circumference and outer boundary intact with each such portion forming an arm that extends from the circumference to the shaft with the arm having a non-uniform width.

15. The attrition mill of claim 14, wherein each element retains portions of its circumference and outer boundary intact with each such portion forming an arm that extends from the circumference to the shaft with the arm having a non-uniform width.

16. The attrition mill of claim 6, wherein each element is mounted on the shaft and has a circumference that defines an outer boundary of the element, wherein at least some of the elements retain portions of its circumference and outer boundary intact with each such portion forming an arm that extends from the circumference to the shaft with the arm having a non-uniform width.

17. The attrition mill of claim 16, wherein each element retains portions of its circumference and outer boundary intact with each such portion forming an arm that extends form the circumference to the shaft with the arm having a non-uniform width.

18. The attrition mill of claim 8 wherein each element is equidistantly spaced from adjacent discs elements.

19. The attrition mill of claim 1 wherein each element is equidistantly spaced from adjacent elements.

20. An attrition mill as claimed in claim 1 wherein the grinding element that provides a larger flow path therethrough comprises a plurality of radially-extending arms.

21. An attrition mill as claimed in claim 20 wherein the grinding element has two to six radially extending arms extending from a central portion.

22. An attrition mill as claimed in claim 20 wherein the grinding element has four radially extending arm extending from a central point.

23. An attrition mill as claimed in 1 wherein the plurality of grinding elements comprises a plurality of grinding discs.

24. An attrition mill as claimed in claim 6 wherein the grinding element that provides a larger flow path therethrough comprises a plurality of radially-extending arms.

25. An attrition mill as claimed in claim 24 wherein the grinding element has two to six radially extending arms extending from a central portion.

26. An attrition mill as claimed in claim 25 wherein the grinding element has four radially extending arm extending from a central point.

27. An attrition mill as claimed in claim 6 wherein the plurality of grinding elements comprises a plurality of grinding discs.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a schematic diagram, partly in cross-section, of an attrition mill in accordance with an embodiment of the present invention;

(2) FIG. 2 shows a front view of a conventional grinding disc suitable for use in an embodiment of the present invention;

(3) FIG. 3 shows a schematic diagram of a circulation pattern of media and slurry within the attrition mill in the vicinity of the grinding discs;

(4) FIG. 4 shows a front view of a grinding disc in the form of an iron cross suitable for use in an embodiment of the present invention;

(5) FIG. 5 shows a front view of another grinding disc having a larger flow area therethrough suitable for use in an embodiment of the present invention;

(6) FIG. 6 shows a front view of yet another grinding disc having a larger flow area therethrough suitable for use in an embodiment of the present invention;

(7) FIG. 7 shows a front view of another grinding disc having a larger flow area therethrough suitable for use in an embodiment of the present invention; and

(8) FIG. 8 shows a front view of a grinding disc used in the example of calculating the open area, as given above.

DETAILED DESCRIPTION OF THE DRAWINGS

(9) It will be appreciated that the drawings have been provided for the purposes of illustrating preferred embodiments of the present invention. Therefore, it will be understood that the present invention should not be considered to be limited side to the features as shown in the attached drawings.

(10) With reference to FIG. 1 there is shown schematically a prior art attrition mill comprising a grinding chamber 1 defined by a generally cylindrical side wall 2, an inlet end wall 4 and a discharge end wall 5. Chamber 1 is provided with an inlet port 3 and an outlet pipe 6. Chamber 1 is mounted to foundations by means not illustrated. An axial shaft 9 extends through inlet discharge end wall 5 at a sealing device 11. Shaft 9 is driven by a drive train (not illustrated) and is supported by bearing 12. Internally of chamber 1, shaft 9 is fitted with a series of radially directed grinding disks 14 each of which when viewed in plan is seen to be pierced by equiangularly-spaced openings 15 (shown in FIG. 2). In the present example grinding disks 14 are keyed to shaft 9 and each grinding disk 14 is equidistance spaced from adjacent grinding disks 14 to expose the shaft therebetween. As can be seen from FIG. 1, the mill is provided with eight grinding disks, respectively referred to by reference numerals 14A, 14B, . . . 14H.

(11) With reference to FIG. 3 there are shown schematic flow patterns (indicated by arrowed lines) believed to occur in and around adjacent grinding discs 14 of the mill of FIG. 1. Slurry circulates through apertures 15 in grinding discs 14 and particles also enter between facing surfaces of grinding discs 14 and are flung against other particles, against the shaft between grinding discs, against the disc surfaces, and against the mill walls. Slurry circulates in a radial direction between the dics and preferably to adjacent shaft 10. As a result, attrition of the particulate matter fed to the attrition mill occurs, resulting in a size reduction of the particulate material. The mill will also be typically provided with a grinding media to facilitate size reduction. The grinding media may comprise steel balls, ceramic particles, sand or indeed any other grinding media known to be suitable to a person skilled in the art. If the mill is an autogenous mill, a separate grinding media will not be present.

(12) The mill shown in FIG. 1 also includes a classification and separation stage 16 which provides an internal classification of particles. The classification and separation stage 16 may be as described in U.S. Pat. No. 5,797,550, the entire contents of which are herein incorporated by cross reference. The classification and separation stage 16 classifies and separates relatively coarse particles in the mill from relatively fine particles. The fine particles are sent to the mill outlet and exit the mill whilst the coarse particles are effectively recycled internally in the mill and move back towards the inlet end of the mill, so that they may be subject to further grinding or attrition.

(13) The mill shown schematically in FIG. 1 is commercially available from the present applicant and is sold under the trademark ISAMILL. Persons skilled in the art of attrition or grinding will readily understand how such a mill is constructed and operates.

(14) In presently available ISAMILL, each of the grinding discs 14A to 14H are essentially identical to each other. However, the present inventors have found that attrition mills having this configuration may be susceptible to significant movement of the media within the mill if the flowrate of material being fed to the mill varies. To overcome this difficulty, the present inventors have found that replacing one or more of the grinding discs with grinding discs having a larger flow area therethrough (than grinding discs presently being used in such mills) achieves a reduction in movement of media through the mill.

(15) FIG. 4 shows a schematic diagram of one possible replacement grinding disc suitable for use in an embodiment of the present invention. The grinding disc 20 in FIG. 4 includes a central aperture 10 that is similar to the disc shown in FIG. 2. This aperture allows the disc 20 to be mounted onto the shaft 9. The disc includes a central portion 21 that surrounds the central aperture 10. The disc has four arms 22, 23, 24 and 25 extending radially outwardly from the central portion 21. The disc 20 shown in FIG. 4 has a flow path therethrough that is defined by the spaces 26, 27, 28 and 29 between the adjacent arms 22 to 25. As can be seen by comparing FIG. 4 with FIG. 2, the spaces provide a much larger combined area than the open area provided by the apertures 15 in FIG. 2.

(16) FIG. 5 shows a schematic view of another disc that may be used in embodiments of the present invention. The disc 30 shown in FIG. 5 includes a central aperture 10. However, this disc also includes a plurality of apertures 31, 32, 33, etc. The disc 30 shown in FIG. 5 has more apertures than the shown in FIG. 2. Furthermore, the apertures of the disc 30 in FIG. 5 are larger than the apertures 15 in the disks 14 of FIG. 2. Therefore, the disc 30 of FIG. 5 provides a disc having a larger flow path for slurry therethrough when compared with the disc 14 shown in FIG. 2.

(17) FIG. 6 shows a schematic view of another disc suitable for use in an embodiment of the present invention. In the embodiment shown in FIG. 6, the disc 40 includes a plurality of apertures 41, 42, 43, etc. Each of these apertures 41, 42, 43 is largely identical to the apertures 15 of the disc 14 shown in FIG. 2. However, the disc 40 shown in FIG. 6 has a larger number of apertures than the disc 14 shown in FIG. 2.

(18) In embodiments of the present invention, the disc that provides a larger flow path therethrough may be placed at the position of disc 14G, as shown in FIG. 1. In other embodiments the disc that provides a larger flow path therethrough may be placed in any other position from disc 14A to 14H. Alternatively, two or more of the discs shown in FIG. 1 may be replaced by discs as shown in any of FIGS. 4 to 6. Indeed, in some embodiments, all of the discs 14A to 14H shown in FIG. 1 may be replaced with the discs as shown in any one of FIGS. 4 to 6.

(19) FIG. 7 shows a schematic diagram that is similar to that shown in FIG. 4 but with 5 arms instead of 4 arms. The grinding disc 120 in FIG. 7 includes a central aperture 110 that is similar to the disc shown in FIG. 2. This aperture allows the disc 120 to be mounted onto the shaft 9. The disc includes a central portion 121 that surrounds the central aperture 110. The disc has five arms 122, 123, 124, 125 and 126 extending radially outwardly from the central portion 121. The disc 120 shown in FIG. 7 has a flow path therethrough that is defined by the spaces 127, 128, 129, 130 and 131 between the adjacent arms 122 to 126. As can be seen by comparing FIG. 7 with FIG. 2, the spaces provide a much larger combined area than the open area provided by the apertures 15 in FIG. 2.

(20) Those skilled in the art will appreciate that the present invention may be susceptible to variations and modifications other than those specifically described. It will be understood that the present invention encompasses all such variations and modifications that fall within its spirit and scope.