AU3741693A

AU3741693A - Grinding mill

Info

Publication number: AU3741693A
Application number: AU37416/93A
Authority: AU
Inventors: Brian Raymond Hudson
Original assignee: TRELLEX Pty Ltd
Current assignee: Metso Australia Pty Ltd
Priority date: 1992-06-18
Filing date: 1993-03-30
Publication date: 1994-01-04
Anticipated expiration: 2013-03-30
Also published as: AU656072B2

Description

GRINDING MILL AND LINING MEDIUM THEREFOR

Field of the Invention

This invention relates generally to grinding mills and to lining media for such mills.

Background Art

Rotary grinding mills are widely used for size reduction in various industries.

Typical types of rotary grinding mills are ball and semi-autogenous mills, in which a rotating drum drives a contained ball charge for effecting comminution of the material being treated. Semi-autogenous grinding (SAG) relies on a combination of balls and large particles of ore to effect comminution of the material being ground. Within the speed range applicable to grinding mills it is generally recognised that when, with all other factors remaining constant, the speed of the grinding mill is increased then a proportionate increase in power is drawn from the drive motor, allowing a greater tonnage of feed material to be ground down to the desired product size. In the mining industry, the balls are generally steel and the annular lining is often formed of circumferentially alternate replaceable -"^mponents respectively known as shell plates and lifter bars. The shell plates are commonly formed in an elastomeric rubber having a high impact resistance, while the lifter bars are formed wholly in such material or are themselves capped with a wear-resistant insert of metal, cemented carbide or ceramic. Examples of the latter construction are shown, for example, in Australian patent applications 23435/70 and 79998/87.

The lifter bars are designed to protrude above the level of the shell plates to provide the grip necessary to transmit the motor power to the ball charge in the mill.

It is generally accepted that the optimum ratio of lifter height B to shell plate width

A is expressed in the following formula: (1 - — ) x A = B 100

where:- "P" equals the speed of the mill in percentage of critical speed (100% critical speed being the speed at which material at the periphery of the mill will centrifuge). - "A' equals shell plate width (edge to edge)

"B' equals lifter height above shell plates It is also generally accepted that a ratio between shell plate width (A) and lifter height (B) of 4:1 provides the optimum grinding efficiency and liner life. The problem for a conventional all rubber mill lining is that as mill speed is progressively increased then lifter height above the shell plate must decrease to maintain the formula design guide detailed above. In practice this has resulted in conventional all rubber mill linings being restricted to mill speeds of up to 75% critical speed.

When a ball mill with a conventional rubber lining is operated in accordance with the above formula at a speed corresponding to 75% critical, the outermost layer of balls falls on the toe of the ball charge itself and not the rubber liner. This is highly desirable, from a wear-life point of view, and enables the rubber liner to operate cost-effectively. In maintaining the formula design guide detailed above, at speeds in excess of 75% critical, the lifter height above the shell plate becomes progressively too low to be considered practical or cost-effective since only a modest amount of wear need take place to the lower lifter height before its ability to provide adequate grip to the ball charge is compromised. Once this happens, slippage of the charge will take place, resulting in poor grinding efficiency and a rapid acceleration of the wear on the shell plates, resulting in early failure. Alternatively, should the ratio A/B be maintained at say 4:1 "when rotating the mill at higher speeds, than the lifter bar protruding into the charge functions as a paddle. The outermost layer of balls will now be thrown progressively further over the ball charge to directly impact the rubber lining itself. This results in premature failure of the rubber lining and inefficient grinding due to excessive turbulence of the ball charge.

US patent 3607606 to Beninga is concerned generally with composite rubber/ ceramic wear-resistant linings. This patent shows, in Figures 6 and 7, a lining for truck beds used to carry rocks or the like, in which an array of alumina-based ceramic spheres are embedded in an elastomeric substrate. This patent also shows a lining for a ball mill in Figures 8 to 9 in which square ceramic bodies are embedded in the rubber substrate but do not protrude therefrom, apart from occasional elongate ceramic bodies which serve as lifter bars.

US patent 4162900 to Judd discloses a wear-resistant composition in which a plurality of abrasive-resistant bodies are embedded within an elastomeric substrate.

Figure 4 therein depicts the situation in which these bodies are exposed after wearing down of the substrate in use. The bodies are generally of elongate configuration.

Summary of the Invention

It has been realised, in accordance with the invention, that improved performance of a grinding mill, especially a ball mill or SAG mill, can be achieved by, in general, providing a lining in which a multiplicity of at least partially spherical wear-resistant bodies protrude from an elastomeric substrate, eg of rubber or polyurethane plastics material. In a preferred embodiment, these bodies are balls of a material of similar hardness or harder than, and preferably similar material to, the ball charge.

In one aspect, the invention provides a grinding mill comprising a rotatable drum with an internal lining comprising an impact absorbent substrate and having an inwardly facing working surface, and a ball charge for the interior of said drum, wherein said lining arther includes an array of wear-resistant bodies of a hardness similar to or greater than the hardness of the ball charge, which bodies are embedded in the substrate and protrude from said working surface, said bodies defining part-spherical impact surface portions above said working surface.

In another aspect, the invention provides a lining medium for a grinding mill comprising an impact absorbent substrate having a working surface and an array of wear-resistant bodies which are embedded in and protrude from said surface, said bodies presenting respective part-spherical impact surface portions above said working surface.

In a further aspect, the invention provides a grinding mill comprising a rotatable drum with an internal lining comprising an impact absorbent substrate and having an inwardly facing working surface, wherein said lining further includes an array of wear-resistant bodies which are embedded in and protrude from said surface, said bodies presenting respective part-spherical impact surface portions above said working surf ace.

In any of the aspects of the invention, the embedded wear-resistant bodies are preferably balls, which may conveniently have diameters of similar order to the balls of the ball charge. In a preferred embodiment, the embedded balls are of similar construction to those of the ball charge.

The lining medium is preferably a shell plate or lifter for a ball mill and preferably includes provision for replaceably attaching the medium on the interior surface of a drum of a ball mill. The lining of the mill preferably comprises an annular array of lining segments replaceably mounted to the drum. In one embodiment, these segments are of substantially uniform height inwardly from said wall, or at least the top surfaces of the segments are flush, ie there is no alteration of height from one segment to another. In this embodiment, there are therefore no lifter bars in the conventional sense and the arrangement relies on the embedded bodies to drive the ball charge. In an alternative embodiment, providing both shell plates and lifter bars, the embedded bodies may be provided in both the shell plates and lifter bars.

Brief Description of the Drawings

The invention will now be further described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a cross-section of a small segment of the drum of a ball mill in accordance with a first embodiment of the invention; Figure 2 depicts the shape of the ball charge when the mill is rotated at various speeds with different designs of liners;

Figure 3 is a similar view to Figure 1 but showing a second embodiment of the invention; and

Figures 4 and 5 are views similar to Figures 1 and 3 but respectively showing third and fourth embodiments of the invention.

Description of Preferred Embodiments

Figure 1 shows part of a ball mill 10 having a drum 12, an internal lining 14 of a suitable impact absorbent elastomeric rubber or polyurethane plastics material, and a charge 16 of steel balls. Means (not shown) is provided to mount this drum for rotation and to rotatably drive the drum. The drum may include a thin secondary rubber liner 12a under lining 14.

Lining 14 is essentially made up of axially extending substrate segments 22, 24 of two configurations which alternate circumferentially. Segments 22 correspond to the shell plates of conventional ball mills and have circumferentially extending base flange elements 26 which project under and are held in place by the other lining segments 24. Lining segments 24 occupy the location in conventional mills of the lifter bars, and are mounted to the drum wall in a similar fashion. Underlying metal inserts 28 vulcanised into the rubber retain the heads 29 of threaded studs 30 which project through apertures in the drum 12 and through rubber bushes 32 lining holes drilled in drum 12. These studs 30 are fastened by nuts 34 acting against external thrust washer assemblies 36.

Lining segments 22, 24 define inwardly facing flush working surfaces 40. Embedded in and protruding from surfaces 40 is a uniform two-dimensional array of wear-resistant bodies in the form of steel balls 42. Balls 42 may be selected from commercially available steel balls normally employed for ball mill charges, and may be forged or cast steel balls. In other embodiments, cast iron balls may be employed. Balls 17, 42 may be geometrically similar, eg spherical, and may be of like diameter but this is not necessary. The balls 42 are vulcanised into the rubber of lining segments 22, 24 so that they protrude from working surfaces 40 to define part-spherical impact surface portions 43, at a height above working surfaces 40 less than half a ball diameter, preferably between a half and a quarter of a ball radius. By way of example, a common ball size used in ball mills is 50 mm and thus a typical height of protrusion would be between 12.5 and 25 mm.

The array of embedded balls is preferably so arranged in relation to protruding height and centre-to-centre separation of the balls, that a ball 17 of the ball charge neatly fits between adjacent embedded balls in contact with or almost in contact with surface 40 and/or the adjacent balls. This condition is illustrated in Figure 1 and is optimised if the balls are arranged in rows extending circumferentially of the drum. The array may be a simple square array or each alternate row may be offset from the adjacent rows by, for example, half a centre-to- centre spacing. It will be appreciated that the balls adjacent the edges of lining segments 22,

24 will need to be spaced from the edge by distance approximately equal to half of the uniform separation of the balls so that the uniformity of the array is maintained from segment to segment.

Figure 2A shows the application of the configuration of Figure 1 to a grinding mill designed to operate at higher rotational speeds than conventionally possible. Instead of relying on the principle of the lifter bar to provide the grip on the ball charge, the grip is achieved with the multiplicity of part-spherical protrusions provided by the embedded balls. It is found that the mill can be rotated at considerably higher speeds than were possible with conventional lifters, without creating excessive turbulence, and without cataracting the ball charge, both of which result in inefficient grinding. This is due to the shape of the part-spherical protrusion: while providing the required transmission of power, it allows the outermost layer of balls to be released far earlier from contact with the lining, limiting the resultant throw to the toe 45 of the ball charge. By way of contrast, Figure 2B and 2C depict the shape of the ball charge for conventional rubber linings respectively operating at 75% critical speed and at 80% critical speed while maintaining the shell plate to lifter height at 4:1. Figure 2B demonstrates the behaviour of the ball charge at the maximum speed obtainable with the conventional mill, while Figure 2C shows how, at high speeds, the ball charge drops behind its toe, causing direct impact on the rubber lining.

As mentioned, the embodiment of Figure 1 is intended to operate at 85% critical speed and above. Figure 3 depicts an alternative embodiment designed to operate at intermediate speeds such as 75% to 80% critical. Here, the lining segments 24' have an inclined working surface 41a and thereby project above the adjacent lining segment 22' at one end to effectively serve as lifter bars. This configuration is thus nearer the conventional in that the lining segments 22' and 24' are in essence respectively shell plates and lifter bars. The embedded balls 42' are arranged in a similar array and provided in both shell plates and lifter bars.

A further embodiment is depicted in Figure 4 in which the lining segments 24" are still more pronounced lifter bars having a flat top face 41b, an inclined forward face 41c and a normal rear face 41 d. The embedded balls 42" are provided in both the top and forward faces of the lifter bars as well as in the shell plates 22".

In a still further alternative embodiment, shown in Figure 5, each lining segment 24'" is formed integrally with a lining segment 22'", so that only one type of lining segment is provided. Figure 5 also shows a quite different wear surface shape to suit particular grinding conditions in the mill. The use for the embedded balls 42 of balls conventionally employed as charge balls has two advantages. Firstly, there is the simple matter of readily available supply and therefore economy of construction. Secondly, if and when a spherical ball finally parts company with the elastomeric substrate of the liner as the liner is worn away, the ball simply becomes an additional component of the ball charge. This situation is in sharp contrast to the experience with existing lifter bar caps of metal, ceramics and cemented carbides. Such caps are of large mass and weight and are known to frequently detach themselves from the elastomeric underlay, especially after some period of service. These large irregularly shaped lumps of metal, rotating freely in the mill, can result in substantial damage to more vulnerable components and, due to their irregular shape, also have a detrimental effect on the grinding efficiency of the mill. The provision of embedded wear-resistant balls is also preferred over lifter bar caps and inserts because the latter are typically heavy and difficult to handle.

It will be appreciated that the invention is applicable to any grinding mills in which there is a ball charge in the interior of the drum. Such mills include the aforementioned SAG mills as well as ball mills.

Claims

CLAIMS:-

1. A grinding mill comprising a rotatable drum with an internal lining comprising an impact absorbent substrate and having an inwardly facing working surface, and a ball charge for the interior of said drum, wherein said lining further includes an array of wear-resistant bodies of a hardness similar to or greater than the hardness of the ball charge, which bodies are embedded in the substrate and protrude from said working surface, said bodies defining part-spherical impact surface portions above said working surface.

2. A grinding mill according to claim 1, wherein the embedded wear-resistant bodies are of a hardness similar to or greater than the balls of the ball charge.

3. A grinding mill according to claim 1 or 2, wherein the embedded wear- resistant bodies are balls.

4. A grinding mill according to claim 3, wherein said balls have diameters of similar order to the balls of the ball charge.

5. A grinding mill according to claim 3 or 4, wherein the embedded balls are of similar construction and method to those of the ball charge.

6. A grinding mill according to claim 3, 4 or 5, wherein each ball protrudes from the working surface to a height less than half the diameter of the ball.

7. A grinding mill according to any one of claims 3 to 6, wherein the balls are steel balls.

8. A grinding mill according to any one of claims 3 to 6, wherein the balls are cast iron balls.

9. A grinding mill according to any preceding claim, wherein the lining comprises a shell plate or lifter, and further includes means replaceably attaching the lining on the interior surface of said drum.

10. A grinding mill according to any preceding claim, wherein said lining comprises an annular array of lining segments replaceably mounted to the drum.

11. A grinding mill according to claim 10, wherein said segments are of substantially uniform height inwardly from said drum, or at least the top surfaces of the segments are flush.

12. A grinding mill according to any preceding claim, wherein said lining is shaped to include a portion of greater thickness adapted to form a lifter bar within said drum.

13. A grinding mill according to claim 13, wherein at least some of said wear resistant bodies are embedded in said portion of greater thickness.

14. A lining medium for a grinding mill comprising an impact absorbent substrate having a working surface and an array of wear-resistant bodies which are embedded in and protrude from said surface, said bodies presenting respective part-spherical impact surface portions above said working surface.

15. A lining medium according to claim 14, wherein the embedded wear-resistant bodies are balls.

16. A lining medium according to claim 15, wherein each ball protrudes from the working surface to a height less than half the diameter of the ball.

17. A lining medium according to claim 15 or 16, wherein the balls are steel balls.

18. A lining medium according to claim 15 or 16, wherein the balls are cast iron balls.

19. A lining medium according to any one of claims 14 to 18 comprising a shell plate or lifter for a ball or SAG mill, and further including means for replaceably attaching the medium on the interior surface of a drum of a mill.

20. A lining medium according to any one of claims 14 to 19, wherein said medium is shaped to include a portion of greater thickness adapted to form a lifter bar in a grinding mill.

21. A lining medium according to claim 20, wherein at least some of said wear resistant bodies are embedded in said portion of greater thickness.

22. A grinding mill comprising a rotatable drum with an internal lining comprising an impact absorbent substrate and having an inwardly facing working surface, wherein said lining further includes an array of wear-resistant bodies which are embedded in and protrude from said surface, said bodies presenting respective part- spherical impact surface portions above said working surface.

23. A grinding mill according to claim 22, wherein the embedded wear-resistant bodies are balls.

24. A grinding mill according to claim 23, wherein each ball protrudes from the working surface to a height less than half the diameter of the ball.

25. A grinding mill according to claim 23 or 24, wherein the balls are steel balls.

26. A grinding mill according to claim 23 or 24, wherein the balls are cast iron balls.

27. A grinding mill according to any one of claims 22 to 26, wherein said lining comprises a shell plate or lifter, and further including means for replaceably attaching the medium on the interior surface of said drum.

28. A grinding mill according to any one of claims 22 to 26, wherein said lining is shaped to include a portion of greater thickness adapted to form a lifter bar within said drums.

29. A grinding mill according to claim 28, wherein at least some of said wear resistant bodies are embedded in said portion of greater thickness.