NZ550495A - Grate system for grinding mill - Google Patents

Abstract

A grating System for a grinding mill comprising a base plate (2) and at least one wear plate (3), wherein said wear plate (3) is removably secured to said base plate (2), said wear plate (3) having a plurality of apertures (7) that substantially overlie a corresponding plurality of apertures (8) in said base plate (2) such that pulverised material may pass through said apertures and wherein said base plate (2) and said corresponding plurality of apertures are arranged such that said grinding mill may continue operation with portions of said wear plate (3) broken away.

Description

550495 1 GRATE SYSTEM TECHNICAL FIELD The present invention relates to a grate system for grinding (or tumbling) mills, and in particular for use in autogenous (AG) and semi-autogenous (SAG) grinding mills.

BACKGROUND A tumbling mill is a grinding plant that reduces a charged material by causing the charged 10 material to tumble onto itself. As the material tumbles within the mill, it generates substantial shearing forces. At the same time, as the charged material shifts it generates substantial grinding forces. Autogenous grinding (AG) mills and semi-autogenous grinding (SAG) mills are generally large-scale tumbling mills used for pulverising mined ore. The diameter of such mills is large, typically between three and twelve metres. SAG 15 mills differ from AG mills in that they utilise disintegrating members mixed- in with the charged material to supplement pulverising action. The disintegrating members are commonly hardened steel balls with a diameter on the order of 100mm. When the charged material is sufficiently reduced, the material and possibly the disintegrating members discharge through some form of grating and either re-enter the mill through a recycling 20 circuit for further reduction or are ready for further downstream processing.

The recycling circuit controls the process of selecting and transporting oversized ground material and/ or disintegrating members back into the mill. Recycling circuits may use magnets to retrieve discharged metallic disintegrating members and trommels, vibrating 25 screens, sieves or other mass distinguishing means to select oversize ground material that needs to re-enter the mill for further grinding.

It is well known in the art to increase the lifetime of the mills by protecting their interiors with some form of either resilient or hardened members often referred to as liners or grates. 30 Eventually, the continuous grinding action occurring inside the mill causes the grates to wear out and require replacement. Premature replacement may also occur when a small section of grating breaks away suddenly. Furthermore, cold working, or peening, of the 550495 2 grate typically causes reductions in aperture size which may decrease mill throughput by as much as 25% for a time early in the grates useful life. In other instances, the grinding action can wear the aperture size larger, reducing the mill's efficiency and eventually overwhelming the mill's recycling circuit, causing premature shutdown and repair. Where 5 aperture size wears too wide, the recycling circuit becomes unmanageable and less raw feed can be introduced. This is most problematic where recycling circuits do not have a secondary crushing means, or pebble crusher, to reduce the oversize discharge. However, even recycling circuits with a pebble crusher may be quickly overwhelmed. For example, when the apertures wear too wide, excessive amounts of metallic disintegrating members 10 discharge into the recycling circuit. The excessive volume of disintegrating members is greater than can be handled by any magnetic retrieval systems, and the members are introduced and overwhelm downstream processes such as pebble crushers.

Due to the physical size and weight of the grates, relining a mill is labour, cost, and time 15 intensive. Mill relining machines have been necessarily designed to handle grates on the order of 3,500kg. Furthermore, the fasteners required to mount such heavy grates are themselves difficult to install and remove. Not only axe large setting forces involved, the portions of the fasteners inside the mill are subject to cold working during the life of the liner and may become peened-over. As expected, the intense process to reline a mill 20 represents substantial time and cost to mining operations. As such, there is a continuing trend in the milling industry to minimise the frequency of relining a mill. Reducing the total number of grates per mill and reducing the number of fasteners involved, reduces the associated shutdown time when relining is necessary.

A particular problem of prior art devices occurs when a small section of grating breaks away, usually no more than two inter-aperture grating portions. The grate breakage combines several apertures to create one larger aperture. In these instances, too much insufficiently ground material and disintegrating members are discharged and quickly overwhelm the recycling circuit as well as other subsequent mill processes. The result is 30 that the mills must be immediately shutdown for repair. SAG mills run in particularly hostile environments, where grate breakage is the most prevalent cause of unscheduled shutdown and repair. 550495 Received 4 October 2005 Preferably, the present invention overcomes at least one of the deficiencies of the prior art liners by providing an improved grate system.

SUMMARY OF THE INVENTION According to a first aspect, the invention consists of a grating system for an autogenous or semi-antogenous grinding mill comprising a base plate and at least one wear plate, wherein said wear plate is removably secured to said base plate, said wear plate having a plurality of apertures in said wear plate that substantially overlie a corresponding plurality of 10 apertures in said base plate such that pulverised material may pass through said apertures, and wherein said apertures in said wear plate are substantially the same size as said corresponding apertures in said base plate and said base plate and said corresponding plurality of apertures are arranged such that said grinding mill may continue operation with portions of said wear plate broken away.

Preferably, said base plate and said wear plate are fabricated from dissimilar materials.

Preferably, said grating system has at least one pad disposed on a wear surface of said wear plate and adjacent to at least one of said apertures of said wear plate.

Preferably, said pad is an anti-peening pad.

Preferably, said wear plate and at least one of said pads are fabricated from dissimilar materials.

In a first preferred embodiment, at least one of said pads is affixed to said wear plate by fasteners. In a second preferred embodiment, at least one of said pads is previously fabricated and then subsequently integrated into said wear plate during the fabrication of said wear plate.

Preferably, said base plate is at least as thick as said wear plate.

Amended Sheet IPEA/AU 2 550495 Received 4 October 2005 4 Preferably, said base plate has a Brinell hardness between 500 - 700 bn.

Preferably, said wear plate has a Brinell hardness between 300 - 400 bn.

Preferably, said wear plate includes at least one lifter comprising a longitudinally elongate raised portion.

Preferably, said wear plate includes at least one lifting lug.

Preferably, a layer of resilient material is disposed between said wear plate and said base plate.

According to a second aspect, the invention consists of a grating system for an autogenous or semi-autogenous grinding mill comprising a wear plate, wherein a plurality of apertures 15 allow ground material to pass through said wear plate, wherein at least one anti-peening pad is disposed on a wear surface of said wear plate adjacent to at least one of said apertures, and said wear plate and said anti-peening pad are fabricated from dissimilar materials.

In a first preferred embodiment, said anti-peening pad is affixed to said wear plate by fasteners. In a second preferred embodiment, said anti-peening pad is fabricated separately and then subsequently integrated into said wear plate.

BRIEF DESCRIPTION OF THE DRAWINGS A preferred embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: Fig 1 is an exploded perspective view of a split grate system in accordance with an embodiment of the present invention.

Fig 2 is an enlarged perspective view of the split grate system shown in Fig 1.

Fig 3 is an enlarged partial perspective view of the corner of the split grate system of Fig. 1 in an assembled configuration.

Amended Sheet IPEA/AU 550495 Fig 4 is a cross-sectional schematic view of the split grate system through lines IV — IV7 of Fig. 3.

BEST MODE OF CARRYING OUT INVENTION Referring to Figs 1-3, split grate 1 comprises base plate 2 and wear plate 3. Base plaXe 2 is removably secured to a grinding mill (not shown) through base holes 4. Wear plate 3 is removably secured to base plate 2 through wear holes 6. Split grate 1 allows pulverised material to discharge out of the mill through apertures 7 in wear plate 3 and then through corresponding apertures 8 in base plate 2. Apertures 7 of wear plate 3 are substantially 10 aligned with apertures 8 in base plate 2.

Wear surface 9 preferably has at least one lifter 12 that acts to lift the charged material as the grinding mill rotates. Lifter 12 preferably consists of a longitudinally raised portion along wear surface 9 of wear plate 3. Wear plate 3 may also have at least one lifting lug 13 15 disposed centrally on wear surface 9 for handling by a relining machine not shown.

Wear plate 3 and base plate 2 are preferably constructed from dissimilar materials, whexein base plate 2 has a higher hardness than wear plate 3. Preferably, base plate 2 has a Brimell hardness in the order of 500 - 700 bn, whereas wear plate 3 may preferably be in the oarder 20 of 300 - 400 bn. Base plate 2 functions as a structural support and is less subject to impact fracturing and erosion than wear plate 3. Thus, hardness of base plate 2 may be maximised with less concern for decreases in ductility. Conversely, some degree of ductility is desirable in wear plate 3 to minimise fracturing and erosion of the wear plate and the disintegrating members whilst the grinding mill operates. Suitable materials would be IF34 25 - 38 for base plate 2 and chrome molybdenum for wear plate 3. In some applications., for example where frequent grate breakages are expected, it may be preferable to fabricate base plate 2 and wear plate 3 from materials having similar characteristics because base plate 2 will be expected to withstand impacts after portions of wear plate 3 break away.

Referring to Fig 3, thickness of base plate 2 is preferably equal to or greater than wear plate 3. Relative thicknesses of the plates may be varied to improve performance according to wear rate information of a particular milling process. Apertures 7 in wear 550495 6 plate 3 substantially overlie apertures 8 in base plate 2. The thickness of base plate 2 is advantageously chosen such that the plate is strong enough to withstand the impact of operating the grinding mill (not shown) without wear plate 3, as is the case when portions of wear plate 3 break away. Apertures 8 in base plate 2 are fabricated with substantially 5 similar sizes to that of apertures 7 in wear plate 3, so that the discharge characteristics of split grate 1 are substantially constant over its life, even when apertures 7 wear wider due to erosion or when portions of wear plate 3 break away suddenly.

Wear surface 9 of wear plate 3 bears the majority of impact when the mill is in operation. 10 As such, pads 11 are disposed adjacent to apertures 7 on wear surface 9. The advantages of pads 11 are two fold. First, they act as anti-peening pads to eliminate problems that occur with prior art grate systems where the apertures peen over after a period of operation, normally by the fourth week. This reduces the aperture size by 25%, thus restricting mill throughput. In the prior art, restricted throughput typically lasts from the fourth through 15 the eighth week before the aperture wears large enough to correct for the peening. In other instances, pads 11 act to prevent the apertures from peening wider. Secondly, the extra material of pads 11 is sacrificial, thereby lengthening the overall amount of mill operating time before eroding the effective thickness of wear plate 3 and size of apertures 7. By preventing or minimising the occurrence of peening and erosion of apertures 7, the 20 effective shape and size of apertures 7 are less susceptible to deformations. As such, wear plate 3 has a longer useful life and exhibits more uniform discharge characteristics over that life.

In a second, not shown embodiment, pads 11 may comprise anti-peening pads that are cast 25 separately to wear plate 3. The separately cast anti-peening pads may then be affixed to, or alternatively cast into, wear plate 3. The separately cast anti-peening pads may be cast from a material dissimilar to that of wear plate 3. This allows the anti-peening pads to be made of materials more suitable, for example harder and more wear resistant, than what is suitable or possible for the larger wear plate 3. Further embodiments may vary the 30 characteristic of pads 11 to suit the particular milling process and may also vary with location on wear plate 3. For example, height, width, curvature and relief angle of pads 11 550495 7 may be adapted for different ground material types, whether disintegrating members are used, whether there is a recycling circuit, and the relative size and shape of apertures 7.

In a further, not shown embodiment, the wear plate may he removably affixed to the base 5 plate by a fastening means other than bolting through wear holes. Similarly, the wear holes and the base holes may vary in number and arrangement. Furthermore, a plurality of wear plates may be affixed to a single base plate, each of which may liave varying characteristics. In a further not shown embodiment, the base plate may vary in thickness relative to the wear plate. Preferably, the thickness of the base plate would be equal to or 10 greater than the thickness of the wear plate. In a further not shown preferred- embodiment, a layer of resilient material, for example a 6 mm rubber gasket, may be disposed between the wear plate and the base plate. One advantage of such a resilient layer is that wider mating surface tolerances are allowed, thereby reducing manufacturing costs.

Materials, configuration, and thicknesses of the plates may be optimised to the specific characteristics and requirements of a particular milling process.

The foregoing describes only a preferred embodiment of the present invention and modifications, obvious to those skilled in the art, can be made thereto withiout departing 20 from the scope of the present invention.

The term "comprising" (and its grammatical variations) as used herein Ls used in the inclusive sense of "having" or "including" and not in the exhaustive sense of "consisting only of'. 550495 Received 4 October 2005 8

Claims (13)

1. A grating system for an autogenous or semi-autogenous grinding mill comprising a base plate and at least one wear plate, wherein said wear plate is removably secured to said base plate, said wear plate having a plurality of apertures that substantially overlie a corresponding plurality of apertures in said base plate such that pulverised material may pass through said apertures and wherein said apertures in said wear plate are substantially the same size as said corresponding apertures in said base plate, and said base plate and said corresponding plurality of apertures are arranged such that said grinding mill may continue operation with portions of said wear plate broken away.

2. A grating system as claimed in claim 1, wherein said base plate and said wear plate are fabricated from dissimilar materials.

3. A grating system as claimed in claim 1, wherein at least one pad is disposed on a wear surface of said wear plate and adjacent to at least one of said apertures of said wear plate.

4. A grating system as claimed in claim 3, wherein said pad is an anti-peening pad.

5. A grating system as claimed in claim 3, wherein said wear plate and at least one of said pads are fabricated from dissimilar materials.

6. A grating system as claimed in claim 3, wherein at least one of said pads is affixed to said wear plate by fasteners.

7. A grating system as claimed in claim 3, wherein at least one of said pads is previously fabricated and then subsequently integrated into said wear plate during the fabrication of said wear plate. Amended Sheet IPEA/AU 04/05/2010 16:54 +61-2-9252-0133 PAPPAS IP PAGE 04/06 550495 9

8. A grating system as claimed in claim 1, wherein said base plate is at least as thick as said wear plate.

9. A grating system as claimed in claim 1, wherein said base plate has a Brinell hardness between 500 - 700 bn.

10. A grating system as claimed in claim 1, wherein said wear plate has a Brinell hardness between 300 - 400 bn.

11. A grating system as claimed in claim 1, wherein said wear plate includes at least one lifter comprising a longitudinally elongate raised portion.

12. A grating system as claimed in claim 1, wherein said wear plate includes at least one lifting lug.

13. A grating system as claimed in. claim 1, wherein a layer of resilient material is disposed between said wear plate and said base plate.