APERTURES OF A GRATE ELEMENT FOR A GRINDING MILL

Field of the Invention

The present invention relates grate elements for a grinding mill, in particular to the apertures of the 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 apertures of the discharge end screen are typically linear slots or circular holes, as these are easy shapes to design, mould and manufacture.

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.

Grate elements are cast separately and then assembled together in situ to form the discharge end screen for the grinding mill. Each of the grate elements has apertures provided therein.

Casting of the grate elements typically involves feeding molten metal from a riser or risers into a mould.

Usually, the riser (s) is positioned on top of or under the centre 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 portions. Such casting defects are susceptible to wear and breakage during manufacture and use of the grate element in the grinding mill.

After the grate elements have been cast, they may be subjected to further treatment in order to improve a particular property or properties of the casting. For example they may be subjected to heat treatment with hardening and/or tempering to improve the strength and/or ductility of the casting respectively.

During casting and also during the aforementioned further treatment of the cast grate elements, residual stress can build up in the grate element. If the residual stresses become sufficiently high, then they can ultimately cause fracture and failure of the cast elements. Such failures are unlikely to be repairable, and hence this problem can cause significant losses during manufacture (in casting and post-casting treatment) and use of the grate elements.

Summary of the Invention

According to a first aspect of the present invention, there is provided a grate element for a grinding mill comprising a body having a plurality of apertures therethrough, each aperture having an aperture axis extending through the body, wherein at least one of the apertures comprises a slot which has a longitudinal extent orthogonal to the aperture axis and is non-linear in cross-section through the aperture axis.

Because the slot has a longitudinal extent orthogonal to the aperture axis it is not circular or elliptical. In an embodiment, the slot has a curved portion in cross-section through the aperture axis.

In an embodiment, the slot may be entirely curved

in cross-section through the aperture axis.

In other embodiments, the slot comprises a plurality of straight portions in cross-section through the aperture axis. In an embodiment, the slot is bent in cross- section through the aperture axis.

The slot may have multiple bends in cross-section through the aperture axis in some embodiments.

In another embodiment, the slot comprises a number of straight and curved portions in cross-section through the aperture axis.

In an embodiment, the slot is parabolic in shape in cross-section through the aperture axis.

In another embodiment, the slot is sinusoidal in shape in cross-section through the aperture axis.

In another embodiment, the slot is V-shaped in cross-section through the aperture axis.

In another embodiment, the slot is M-shaped in cross-section through the aperture axis. In a further embodiment, the slot is N-shaped in cross-section through the aperture axis.

In an embodiment, the slot has curved ends. In another embodiment, the slot has straight ends. In an embodiment, most of the apertures through the body comprise slots having longitudinal extents orthogonal to their respective aperture axis, the slots being non-linear in cross-section through their respective aperture axis. In an embodiment, all of the apertures through the body comprise slots having longitudinal extents orthogonal to their respective aperture axis, the slots being non-linear in cross-section through their respective aperture axis. According to a second aspect of the present invention, there is provided a grate element for a grinding mill comprising a body having a plurality of

apertures therethrough, each aperture having an aperture axis extending through the body, wherein at least one of the apertures comprises a slot having a longitudinal extent orthogonal to the aperture axis, and wherein, the slot is manufactured with longitudinal side walls which are non-parallel to one another in cross-section through the aperture axis.

Because the slot has a longitudinal extent orthogonal to the aperture axis it is not circular or elliptical.

In an embodiment, the side walls of the slot are linear.

In this embodiment, the side walls of the slot are at an angle from the parallel with respect to one another in cross-section through the aperture axis.

One end of the slot may be wider than the other.

In an embodiment, the wider end of the slot is located towards the centre of the body of the grate element. In another embodiment, the wider end of the slot is located towards a side edge of the body of the grate element

In one variation, one of the side walls of the slot is curved in cross-section through the aperture axis. Preferably, the side wall is curved convexly with respect to the other side wall. However, it may be curved concavely.

In another embodiment, both side walls of the slot are curved in cross-section through the aperture axis.

Preferably, both side walls are curved convexly with respect to each other. However, they may be curved concavely with respect to each other.

Thus, in these embodiments, the ends of the slot may be wider or narrower than the middle of the slot

(depending on whether the side wall(s) are curved convexly or concavely) ;

In another embodiment, one of the side walls of the slot comprises a number of linear and/or curved portions in cross-section through the aperture axis.

In a further embodiment, both of the side walls of the slot comprise a number of linear and/or curved portions in cross-section through the aperture axis.

In these embodiments, the slot may be of variable width across its length which is orthogonal to the aperture axis. In an embodiment, the slot has curved end walls.

In another embodiment, the slot has straight end walls .

In an embodiment, most of the apertures of the grate element comprise slots having longitudinal extents orthogonal to their respective apertures axis, wherein each of the slots is manufactured with longitudinal side walls which are non-parallel to one another in cross- section through their respective aperture axis.

In an embodiment, all of the apertures of the grate element comprise slots having longitudinal extents orthogonal to their respective aperture axis, wherein the slots are manufactured with longitudinal side walls which are non-parallel to one another in cross-section through their respective aperture axis. According to a third 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 or second aspect 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 fourth 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 the first or second aspect 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 Drawing

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawing, in which:

Figure 1 is a top view of a grate element for a grinding mill according to an embodiment of the present invention;

Figure 2 is an elevated perspective view of a grate portion for a grinding mill according to an embodiment of the present invention; and

Figure 3 is an end view of the grate portion according to the embodiment shown in either Figures 1 or 2, indicating the direction of the aperture axis.

Detailed Description of Embodiments

Referring to Figure 1, a grate element 10 according to an embodiment of the present invention is shown. The grate element 10 comprises a body 11, the body having a plurality of apertures 12 therethrough, each aperture having an aperture axis 20 (as shown in Figure 3) extending through the body 11. The aperture axis 20 of each aperture can be considered to extend into the page of Figure 1, that is projecting perpendicularly through the upper and lower surfaces of the grate element 10 as shown in Figure 3. The apertures 12 of the grate element 10 shown in Figure 1 are all slots 14 having a longitudinal

extent orthogonal to their respective aperture axis and thus are not circular or elliptical. In addition, all of the slots 14 are non-linear in cross-section through their respective aperture axis. This cross-section through the aperture axis 20 is indicated as through A-A in Figure 3. In other embodiments (not shown) only some of the apertures may be non-linear slots in cross-section through their respective aperture axis.

The non-linear slots 14 have at least one bend in cross-section through their respective aperture axis. The grate element 10 shown in Figure 1 has non-linear slots with two bends in cross-section through their respective aperture axis. However, the slots may have more than two bends . The slots 14 shown are of sinusoidal shape in cross-section through their respective aperture axis.

However, the non-linear slots may be of any suitable shape such as parabolic, V-shaped, N-shaped or M-shaped in cross-section through their respective aperture axis. The sinusoidal and parabolic shaped slots are curved or comprise curved portions, whereas the V, N and M-shaped slots generally comprise a number of straight portions. In another variation, the slots comprise a number of straight and curved portions in cross-section through their respective aperture axis. The non-linear slots 14 shown in Figure 1 have curved ends 17, however, they may have straight ends.

A significant advantage of the apertures 12 being non-linear slots 14 in cross-section through their respective aperture axis is that the non-linear slots, in particular the bends in the slots, reduce the residual stress built up in the grate element 10 during casting and also during further treatment (such as heat treatment for example) of the grate element 10. This means that failure due to excessive residual stress through the grate element 10 is less likely to occur during its manufacture and use.

A plurality of the grate elements 10 may be assembled together to form a discharge end screen for a

grinding mill. The grinding mill may be a single or bidirectional 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.

Referring now to Figure 2, a grate element 110 according to another embodiment of the present invention is shown. Similar features of the grate element 110 to those of the grate element 10 shown in Figure 1 have been designated with the same reference number, but prefixed with the numeral 1. The grate element 110 comprises a body 111, the body having a plurality of apertures 112 therethrough, each aperture 112 having an aperture axis 120 extending through the body (as indicated in Figure 3) . The apertures 112 of the grate element 110 shown in Figure 2 are all slots having a longitudinal extent orthogonal to their respective aperture axis. The slots 113 shown on the right hand side of the grate element 110 are conventional slots, which are manufactured with parallel longitudinal side walls in cross-section through their respective aperture axis. The slots 114 on the left hand side of the grate element 110, however, are manufactured with longitudinal side walls 115 which are non-parallel to one another in cross-section (cross-section A-A as indicated in Figure 3) through their respective aperture axis according to embodiments of the present invention.

Because the slots 114 have a longitudinal extent orthogonal to their respective aperture axis, they are not circular or elliptical.

A significant advantage of the apertures 112 being slots 114 which are manufactured with non-parallel longitudinal side walls 115 in cross-section through their respective aperture axis, is that it reduces the temperature differential across the grate element 110 as it cools during casting. This in turns means that there is a reduction in the likelihood that the flow of molten metal in some portions of the mould cavity will be restricted and hence less formation of voids and other casting defects in the grate element 110 as it is cast.

Some additional benefits are also provided by the slots 114 in reducing the residual stress built up in the grate element 110 during casting and also during further treatment, (such as heat treatment for example) of the grate element 110. These two advantages mean that failure due to excessive residual stress through the grate element 110 is less likely to occur during its manufacture and use.

A further advantage provided by the slots 114 is that during use of the grate element 110 in a grinding mill, pegging is less likely to occur. Pegging is a problem which occurs in ball mills, where grinding balls used to assist in the grinding of the material in the mill impact and become stuck or pegged in an aperture of a grate portion in the discharge end screen of the grinding mill. Pegging can cause significant damage to grate portions in the discharge end screen of a grinding mill as well as leading to a reduction in the grinding efficiency of the mill and the material throughput of the mill.

It is noted that although the embodiment of Figure 2 only shows some of the apertures 112 in the grate element 110 comprising slots 114 manufactured with non- parallel longitudinal side walls 115 in cross-section through their respective aperture axis, that in other

embodiments of the present invention fewer or more (even all) of the apertures 112 the body 111 of the grate element 110 may comprise slots having a longitudinal extent orthogonal to their respective aperture axis and which are manufactured with longitudinal side walls which are non-parallel to one another in cross-section through their respective aperture axis .

A number of different types of slots 114 having non-parallel longitudinal side walls 115 in cross-section through their respective aperture axis are shown in Figure 2. It is noted that, although the grate element 110 is shown with these different types of slots 114, in other embodiments the grate portion may only have one type. One of the type of slots 114 shown comprises linear side walls which are at an angle from the parallel with respect to each other in cross-section through the aperture axis. Thus, one end of the slot is wider than the other.

For some of these slots, the wider end is located towards the centre of the body 111 of the grate element 110, whilst for other slots the wider end is located towards the side edge of the body 111. For the slot which is wider towards the side edge of the body 111 and hence narrower towards the centre of the body 111 a further advantage arises in that this shaping provides greater strength to the members or "fingers" 116 located between the apertures 112 which comprise such slots. This is because they are thicker towards the centre of the body 111 which reduces the likelihood of a full breakout occurring for one of the fingers 116 between these slots . If a breakout of a finger 116 were to occur, it would lead to a greater amount of oversized material exiting the grinding mill .

Another type of slot 114 shown in Figure 2 has side walls which are both curved convexly with respect to one another in cross-section through the aperture axis. In other embodiments (not shown) only one of the side walls 115 of the slot 114 may be curved. In further

embodiments, the side wall(s) 115 is curved concavely in cross-section through the aperture axis. For these types of slot 114, the ends 117 of the slot 114 are wider or narrower than the middle of the slot (depending on whether the side wall (s) 115 are curved convexly or concavely) .

In further embodiments not shown in Figure 2 , one or both of the side walls 115 of the slot 114 may comprise a number of linear and/or curved portions in cross-section through the aperture axis. In these embodiments, the slot 114 is of variable width across its length which is orthogonal to the aperture axis.

The slots 114 shown in Figure 2 have curved end walls 117, however, they may have straight end walls.

A plurality of the grate elements 110 may be assembled together to form a discharge end screen for a grinding mill. The grinding mill may be a single or bidirectional mill (i.e. can rotate in one or both directions) . When the grate elements 110 are assembled to form a 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 the grinding mill may comprise a plurality of grate portions 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.

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.