GRINDING MILL GRATE ELEMENT

Field of the Invention

The present invention relates to grinding mill grate elements .

Background of the Invention

Operation of a grinding mill typically involves feeding material and water into one end of the mill and discharging the ground product at the other end of the mill. The product is discharged through a discharge end screen which has apertures sized to allow only particles of the desired size to exit the mill as product.

The discharge end screen of a grinding mill typically comprises a number of grate elements arranged side by side in a number of concentric rings around the central axis of the mill. Each grate element is generally in the shape of a truncated circular segment or wedge. The grate elements are cast separately and then assembled together in situ to form the discharge end serpen for the grinding mill. Each of the grate elements has apertures provided therein.

Casting of the grate elements typically requires feeding molten metal from a riser or risers into a mould. Usually, the riser (s) is positioned on top of or under the central portion of the mould because it is generally central to the casting as well as being the section of the casting requiring the most metal.

As the casting process occurs, molten metal flows out to the side edges of the mould cavity. In some mould designs and conditions, the molten metal can cool and solidify first at the side edges and in the inner portions between the apertures. The uneven cooling of the molten metal can restrict the flow of molten metal in some portions of the mould cavity and as a result can lead to the formation of voids and other casting defects in the grate elements. Such casting defects are susceptible

to wear and breakage during manufacture and use of the grate element in the grinding mill .

A more significant problem for the manufacturing of the grate elements, however, is the build-up of residual stresses. These residual stresses are formed during casting as well as during subsequent treatment (such as heat treatment) of the grate elements. If these stresses become sufficiently high, then they can ultimately cause fracture and failure of the grate elements either during manufacture or use of the grate elements. Such failures are unlikely to be repairable and hence this problem can cause significant losses during manufacture and use of the grate elements .

Summary of the Invention

According to a first aspect of the present invention, there is provided a grinding mill grate element comprising a body with a plurality of apertures therethrough, the body being of substantially constant thickness across its length and width.

According to a second aspect of the present invention, there is provided a grinding mill grate element comprising a body with a plurality of apertures therethrough, the body comprising two wing portions extending from either side of a central portion, wherein the wing portions are non-parallel with one another.

In an embodiment, the body is of a substantially constant thickness across its width.

In an embodiment, the body is of substantially constant cross section across its length.

In an embodiment, the apertures are formed in the wing portions .

In an embodiment, the wing portions are angled downwardly from the central portion. In an embodiment, the wing portions are slightly curved.

In another embodiment, the wing portions are

substantially linear.

In this embodiment, the side edges of the wing portions may be bevelled so that the wing portions can snugly abut the wing portions of adjacent grate elements in a grinding mill grate.

In other embodiments, the wing portions comprise a number of linear and/or non-linear portions. The wing portions may be tapered. The wing portions may be thinner towards their side edges.

In an embodiment, the grate element also comprises a lifter bar, connected to the body.

In an embodiment, the lifter bar is connected to the centre portion of the body. In an embodiment, the lifter bar is integrally cast with the body. However, it may be cast separately and attached to the body after casting.

The grate element may comprise more than one lifter bar. The lifter bar(s) may be linear or non-linear or may comprise a number of linear and/or non-linear portions .

In an embodiment, the grate element also comprises at least one lifting lug.

In an embodiment, the at least one lifting lug is connected to the body.

In an embodiment, the at least one lifting lug is integrally cast with the body.

According to a third aspect of the present invention, there is provided a grate element for a grinding mill, the grate element comprising a body having a plurality of apertures therethrough, wherein the body is thicker in at least a central portion than in at least one thinner edge portion.

In an embodiment, the at least one thicker central portion extends the length of the body.

The body has a central axis extending centrally through the length of the body.

In an embodiment, the thicker central portion is centred on the central axis of the body.

However, in other embodiments, the at least one thicker central portion may be offset from the central axis of the body.

In an embodiment, the at least one thinner edge portion is on at least one side of the at least one thicker central portion.

In an embodiment, the at least one thinner edge portion extends the length of the body.

In an embodiment, the body has at least two thinner edge portions, located on either side of the at least one thicker central portion.

In an embodiment, the body has two thicker central portions.

In an embodiment, the two thicker portions are of approximately equal thickness.

In an embodiment, the body has a middle portion located between the two central portions which is thinner than both central portions.

In an embodiment, the at least one thinner edge portion is shaped with a taper of decreasing thickness towards its furthest edge from the at least one central portion. Thus, the at least one thinner edge portion is thinnest at its furthest edge from the at least one central portion.

In an embodiment, one surface of the at least one thinner edge portion is tapered. In this embodiment, the opposing surface is substantially level. In another embodiment, opposing surfaces of the at least one thinner edge portion are tapered.

It is noted that the taper of the surface (s) of the at least one edge portion may be a linear or curved taper. In an embodiment, the angle of the taper is between 0.5 and 15°, preferably between 0.5 and 10°, more preferably between 0.5 and 5°, even more preferably between

1 and 3° .

It is to be appreciated that different geometric definitions for the angle of the taper may be appropriate for different embodiments of the invention. In the embodiment where only one surface of the at least one thinner edge portion is shaped with a taper and the opposing surface is substantially level, the angle of the taper may be considered to be the angle of the tapered surface with respect to the opposing surface. However, this definition may not be appropriate in the embodiment where both surfaces of the at least one thinner edge portion are tapered. In this case, the angle of the taper may be considered to be the angle of a surface of the at least one thinner edge portion to a theoretical plane which is perpendicular to the face of the furtherest edge of the at least one thinner edge portion from the at least one central portion.

However, this definition may not be appropriate in some embodiments where the face of the furtherest edge of the at least one thinner edge portion from the at least one central portion is also shaped with a taper. This shaping may be employed to provide assistance in releasing the cast grate element from the mould once it has solidified. The angle of the taper (of the at least one thinner edge portion) may therefore also be considered to be the angle of a surface of the at least one thinner edge portion to a theoretical plane which is perpendicular to a central axial plane of the grate element about which the grate element is symmetrical. In an embodiment, the middle portion is shaped with tapers of decreasing thickness from the sides of the middle portion towards the middle of the middle portion.

In an embodiment, one surface of the middle portion is tapered. In this embodiment, the opposing surface is substantially level.

In another embodiment, opposing surfaces of the middle portion are tapered.

It is noted that the tapers of the surface (s) of the middle portion may be linear or curved tapers.

In an embodiment, the angle of the tapers is between 0.5 and 15°, preferably between 0.5 and 10°, more preferably between 0.5 and 5°, even more preferably between 1 and 3°.

It is to be appreciated that different geometric definitions for the angle of the tapers may be appropriate for different embodiments of the invention. In the embodiment where only one surface of the middle portion is shaped with tapers and the opposing, surface is substantially level, the angle of the tapers may be considered to be the angle of the tapered surface with respect to the opposing surface. However, this definition may not be appropriate in the embodiment where both surfaces of the middle portion are tapered. In this case, the angle of the tapers may be considered to be the angle of a surface of the middle portion to a theoretical plane which is perpendicular to a central axial plane of the grate element about which the grate element is symmetrical.

However, the above definition requires the tapers on both surfaces to be the same (so that the grate element is symmetrical) . If this is not the case, the angle of the tapers may also be considered to be the angle of a surface of the middle portion to a theoretical plane which is perpendicular to another theoretical plane extending through the thinnest part of the middle portion.

In an embodiment, the grate element also comprises at least one lifting lug, preferably two, connected to the body.

The at least one lifting lug is preferably integrally cast with the body.

In an embodiment, the grate element also comprises at least one, preferably two, lifter bars, connected to the body.

The lifter bar(s) may be linear or non-linear

(ie. curved) or may comprise a number of linear and/or non-linear portions.

The at least one lifter bar is preferably integrally cast with the body. However, it may be cast separately and attached to the body after casting.

In an embodiment, the at least one lifter bar extends the length of the body.

In an embodiment, the at least one lifter bar is located above the at least one thicker central portion of the body.

In an embodiment, the two lifter bars are located above the two thicker central portions of the body respectively.

In an embodiment, at least some of the plurality of apertures are formed in the at least one thinner edge portion of the body.

In an embodiment, at least some of the plurality of apertures are formed in the middle portion of the body.

According to a fourth aspect of the present invention there is provided a discharge end screen for a grinding mill comprising a plurality, of grate elements, wherein at least one of the grate elements is a grate element according to the first, second or third aspects of the present invention. The plurality of grate elements are arranged side by side in a number of concentric rings around the central axis of the mill to form the discharge end screen. Typically, the discharge end screen comprises three rings of grate elements; an inner, a middle and an outer ring. The at least one grate element according to the first aspect of the present invention may be located in any of the inner, middle or outer rings.

According to a fifth aspect of the present invention there is provided a grinding mill having a discharge end screen, the discharge end screen comprising a plurality of grate elements, wherein at least one of the grate elements is a grate element according to first,

second or third aspects of the present invention.

The grinding mill may be a single or bidirectional mill, that is, it may be able to rotate in one or both directions.

Brief Description of the Drawings

Preferred embodiments of the present invention will now be described, by way of example only, in which:

Figure 1 is an end view of a grinding mill grate element according to embodiments of the present invention;

Figure 2 is a bottom view of the grate element of Figure 1;

Figure 3 is an elevated perspective view of the grate element of Figure 1; and Figure 4 is an elevated end view of a grate element for a grinding mill according to another embodiment of the present invention;

Figure 5 is a top view of the grate element of Figure 4 ; Figure 6 is a perspective view of the grate element of Figure 4;

Figure 7 is an end view of a grate element for a grinding mill according to another embodiment of the present invention; Figure 8 is a schematic end view of different embodiments of a grate element for a grinding mill; and

Figure 9 is a partial schematic end view of further different embodiments of a grate element for a grinding mill .

Detailed Description of Preferred Embodiments

Referring to Figures 1 to 3, a grinding mill grate element 10 according to embodiments of the present invention is shown. The grate element 10 comprises a body 11 with a plurality of apertures 12 therethrough. The apertures 12 are sized to enable material of the desired particle size through the grate element 10 when it is

installed in a grinding mill.

A plurality of the grate elements 10 may be assembled together to form a discharge end screen for the grinding mill. The grinding mill may be a single or bi- directional mill (ie. can rotate in one or both directions) . When the grate elements 10 are assembled to form the discharge end screen, they are arranged in a number of concentric rings around the central axis of the grinding mill . Typically, the grate elements 10 are arranged in three rings; an inner, a middle and an outer ring. It is noted that in another embodiment the discharge end screen of a grinding mill may comprise a plurality of grate elements of which at least one is a grate element 10 according to embodiments of the present invention, the remainder being conventional grate elements. In this case, the at least one grate element 10 may be located in any of the inner, middle or outer rings of the discharge end screen. The body 11 comprises two wing portions 13, 14 extending from either side of a central portion 15. The apertures 12 are formed in each of the wing portions 13, 14. The wing portions 13, 14 are non-parallel to one another, angling downwardly from the central portion 15. Notably, the body 11, comprising the wing portions 13, 14 and the central portion 15, is of substantially constant thickness across its width. The body 11 is also of substantially constant cross section across its length.

These features of the grate element 10 provide a more robust grate element during manufacture and use when compared to conventional grate elements by providing a downward path for the molten metal to readily flow during casting, a more even volumetric distribution of molten metal through the casting as well as facilitating a more even distribution of heat through the casting. All of this reduces the likelihood of casting defects being formed.

Most importantly, however, the shape of the grate

element 10 also advantageously reduces the residual stress built up during casting and subsequent treatment (such as heat treatment for example) . This also significantly contributes to the production of a more robust grate element.

Another advantage of the grate element 10 is that it can provide an improvement in the discharge efficiency for the grinding mill. This is because, during operation of the grinding mill, the shape of the wing portions 13, 14 cause the material in the grinding mill to be more inclined to pass into the apertures 12 as opposed to rolling over the face of the body 11 of the grate element 10.

In embodiment shown in Figures 1 to 3, the wing portions 13, 14 are slightly curved. However, the wing portions 13, 14 may also be substantially linear. In this case, the side edges of the wing portions 13, 14 may be bevelled so that the wing portions can snugly abut the wing portions of adjacent grate elements in a grinding mill grate. In a further variation, the wing portions 13, 14 may comprise a number of linear and/or non-linear portions. In another variation, the wing portions 13 may be tapered such that they are thinner towards their side edges . The grate element 10 also comprises a lifter bar

16, connected to the body 11. The lifter bar 16 is connected to the centre portion 15 of the body 11. The lifter bar 16 is preferably integrally cast with the body 11, however, it may be cast separately and connected after casting to the body 11 by any suitable means. Although the Figures only show the grate element 10 as having one lifter bar 16, it may comprise more than one lifter bar. Furthermore the lifter bar(s) 16 may be non-linear or linear (as shown in Figures 1 to 3) or may comprise a number of linear and/or non-linear portions.

The grate element 10 also comprises two lifting lugs 17. It is noted that the grate element 10 may

comprise more or less lifting lugs than the two shown in the Figures. The lifting lugs 17 are connected to the body 11 via the lifter bar 16. The lifting lugs 17 are preferably integrally cast with the body 11, however, it may be cast separately and connected after casting to the body 11 by any suitable means.

Referring now to Figures 4 to 6, a grate element 110 for a grinding mill is shown. A plurality of the grate elements 110 may be assembled together to form a discharge end screen for the grinding mill, which may be a single or bi-directional mill (ie. can rotate in one or both directions) . The grate element 110 comprises a body 111 having a plurality of apertures 112 therethrough. The apertures 112 are shaped and sized to allow only product particles of the desired size through the grate element

110. The body 111 of the grate element 110 is a truncated circular segment in shape. Thus, when the grate elements 110 are assembled to form the discharge end screen, they are arranged in a number of concentric rings around the central axis of the grinding mill.

Typically, the grate elements 110 are arranged in three rings; an inner, a middle and an outer ring. It is noted that in another embodiment the discharge end screen of a grinding mill may comprise a plurality of grate elements of which at least one is a grate element 110 according to embodiments of the present invention, the remainder being conventional grate elements. In this case, the at least one grate element 110 may be located in any of the inner, middle or outer rings of the discharge end screen.

The body 111 of the grate element 110 has a central portion 113 which is thicker than edge portions 114 of the body 111. The edge portions 114 are located on either side of the central portion 113. The thicker central portion 113 extends the length of the body 111, as do the edge portions 114. The plurality of apertures 112 are formed in the edge portions 114. The central portion

113 is aligned with a central axis of the body (but need not be) , which extends centrally through the length of the body 111.

The thinner edge portions 114 are shaped with a taper of decreasing thickness towards their furthest edge from the central portion 113. Thus, the edge portions 114 are thinnest at their furthest edge, being the side edge of the body 111, from the central portion 113. One or both opposing surfaces of the edge portions 114 may be tapered. In the embodiment shown in Figures 4 to 6, only the top surface is tapered, with the bottom surface remaining substantially level. The taper is also shown as being a linear taper, however, in other embodiments it may be a curved taper. The angle of the taper across the edge portions 114 is between 0.5 and 15°, preferably between 0.5 and 10°, more preferably between 0.5 and 5°, even more preferably between 1 and 3°.

It is to be appreciated that different geometric definitions for the angle of the taper may be appropriate for different embodiments of the invention. Figure 8 provides examples of how the angle of the taper (θ) of the thinner edge portion 114 may defined geometrically. In the embodiment (as shown in Figures 4 to 6) where only one surface 120 of the edge portions 114 is shaped with a taper and the opposing surface 21 is substantially level, the angle of the taper may be considered to be the angle of the tapered surface 120 with respect to the opposing surface 21.

However, this definition may not be appropriate in other embodiments where both surfaces 120, 121a of the edge portions 114 are tapered. In this case, the angle of the taper may be considered to be the angle of a surface 120, 121a of the edge portions 114 to a theoretical plane 122 which is perpendicular to the face 123 of the furtherest edge of the thinner edge portions from the central portion 113.

However, this definition may also not be

appropriate in some embodiments where the face 123a of the furtherest edge of the edge portions 114 from the central portion 113 is also shaped with a taper. This shaping is typically employed to provide assistance in releasing the cast grate element 110 from the mould once it has solidified. The angle of the taper (of the edge portions 114) may therefore also be considered to be the angle of a surface 120 of the edge portion 114 to a theoretical plane 122 which is perpendicular to a central axial plane 124 of the grate element 110 about which the grate element 110 is symmetrical .

A significant advantage provided by the shaping of the body 111 of the grate element 110 is that during casting, less metal is required at the edges of the body 111 because they are thinner. This means that cooling across the casting is more even and hence better flow of molten metal to the extremities of the casting is achieved. This in turn means that there is less chance of voids or other casting defects being formed in the grate element 110. This advantage is further enhanced by the taper of the edge portions 114, because the angled mould walls used to produce the taper during casting encourage the molten metal to flow under gravity to the extremities of the mould. Most importantly, the requirement for less metal at the extremities of the body 111 of the grate element 110 also reduces the residual stress built up during casting and subsequent treatment, such as heat treatment, of the grate element 110. Thus, the manufacture of the grate element 110 with thinner edge portions 114 in the body 111 produces a more robust grate element 110.

A further advantage of the shaping of the grate element 110 is that it provides greater strength to the members or "fingers" 119 located between the apertures 112 because they are thicker towards the central portion 113. This reduces the likelihood of a full break out occurring for any one of the fingers 119, which if it were to occur.

would lead to a greater amount of oversized material exiting the grinding mill.

Another advantage of the grate element 110 is that it can provide an improvement in the discharge efficiency for the grinding mill. This is because, during operation of the grinding mill, the taper of the edge portions 114 and the middle portion 15 cause the material in the grinding mill to be more inclined to pass into the apertures 112 as opposed to rolling over the face of the body 111 of the grate element 110.

The grate element 110 also comprises two lifting lugs 117, connected to the body 111. The grate element 110 may, however, comprise more or less lifting lugs. The lifting lugs 117 enable the grate element 110 to be lifted and moved around, in particular in order to install or remove it from the grinding mill. The lifting lugs 117 are also integrally cast with the body 111, although they may be cast separately from the body 111 and connected later.

The grate element 110 also comprises a lifter bar 118, connected to the body. The grate element 110 may, however, comprise more lifter bars. The lifter bar 118 is integrally cast with the body 111, although it may be cast separately from the body 111 and connected later by any suitable means. The lifter bar 118 shown in Figures 1 to 3 is linear. However, in other embodiments, the lifter bar may be non-linear (ie. curved) or may comprise a number of linear and/or non-linear portions.

Referring now to Figure 7, a grate element 210 according to another embodiment of the present invention is shown. Similar features of the grate element 210 to the grate element 110 shown in Figures 4 to 6 have been designated with the same reference number, but are prefixed with the numeral 2.

The grate element 210 comprises a body 111 having a plurality of apertures therethrough. The body 211 of the grate element 210 has two central portions 213 which are thicker than edge portions 214 of the body 211. The two

central portions 213 are of approximately the same thickness. The body also has a middle portion 215, located between the two central portions 213, which are thinner than the two thicker central portions 213. The edge portions 214 are located on either side of the respective central portions 213. The thicker central portions 213 extend the length of the body 211, as do the edge portions 214 and the middle portion 215. Some of the plurality of apertures are formed in the edge portions 214 as well as in the middle portion 215.

The middle portion 215 is shaped with tapers of decreasing thickness from the sides of the middle portion towards the middle of the middle portion 215. One or both opposing surfaces of the middle portion 215 may be tapered. In the embodiment shown in Figure 7, only the top surface is tapered, with the bottom surface remaining substantially level. The tapers are also shown as being linear tapers, however, in other embodiments they may be curved tapers. The angle of these tapers is between 0.5 and 15°, preferably between 0.5 and 10°, more preferably between 0.5 and 5°, even more preferably between 1 and 3°. It is to be appreciated that different geometric definitions for the angle of the tapers may be appropriate for different embodiments of the invention. Figure 9 provides examples of how the angle of the tapers (θ) of the middle portion 215 may defined geometrically. In an embodiment (as shown in Figure 7) where only one surface 230 of the middle portion is shaped with tapers and the opposing surface 131 is substantially level, the angle of the tapers may be considered to be the angle of the tapered surface 230 with respect to the opposing surface 131.

However, this definition may not be appropriate in the embodiment where both surfaces 230, 231a of the middle portion 215 are tapered. In this case, the angle of the tapers may be considered to be the angle of a surface 230, 231a of the middle portion 215 to a theoretical plane

232 which is perpendicular to a central axial plane 233 of the grate element 210 about which the grate element 210 is symmetrical .

However, the above definition requires the tapers on both surfaces to be the same (so that the grate element 210 is symmetrical) . If this is not the case, the angle of the tapers may also be considered to be the angle of a surface 230, 231b of the middle portion 215 to a theoretical plane 232 which is perpendicular to another theoretical plane 234 extending through the thinnest part of the middle portion 215.

In the claims which follow and in the preceding description of the invention, except, where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as

"comprises" or "comprising" is used in an inclusive sense, ie. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.