AU739607B2 - Grinding mill - Google Patents

Description

P/00/0 11 Regulation 3.2

AUSTRALIA

Patents Act 1 990

ORIGINAL

COMPLETE SPECIFICATION STANDARD PATENT Invention Title: GRINDING MILL 0* 0 0 0 0 0 0 0 The following statement is a full description of this invention, including the best method of performing it known to us: GH REF: P50519C/PJW 4 2 GRINDING MILL FIELD OF THE INVENTION The present invention relates to grinding mills, including autogenous and semi-autogenous grinding mills.

BACKGROUND TO THE INVENTION A grinding mill typically takes the form of a horizontally orientated drum having a large diameter relative to its length (ie. height if standing on its base). For example, the diameter may be of the order of 1.5-3.5 times the length. The drum is typically mounted for rotation about a substantially horizontal axis on large bearings in the two end walls of the drum. The end walls may be planar and vertical but, in larger grinding mills, the end walls are typically of more complicated design to enhance the structural rigidity of the grinding mill. Typically, in larger grinding mills (for example, having a diameter of 6 metres and greater), the end walls take the form of truncated cones.

A grinding mill can be notionally divided into three portions which are typically separately manufactured and welded, bolted or otherwise secured together. Material to be ground and water are typically introduced into the mill through a feed portion and a slurry containing ground material is typically discharged from the mill through a discharge portion with a grinding portion typically located between the feed portion and the discharge portion. Flow of material through the mill is facilitated by the introduction of water and the discharge of slurry.

30 The interior of the cylindrical wall of the grinding S"portion is typically formed with a series of protrusions V....which are commonly referred to as lifter bars. The lifter bars typically protrude towards the substantially S"horizontal axis of the grinding mill and serve two functions. Firstly, the lifter bars enhance the lifting of material as the grinding mill rotates which, in the absence of lifter bars, occurs as a consequence of the centrifugal force acting on the material and friction S:50519C/700 3 between the material and the interior wall of the grinding portion. The lifter bars increase friction and thereby enhance lifting. Secondly, the lifter bars form part of a plating arrangement which overlies the interior wall of the grinding portion to protect the interior wall from wear. The plating arrangement (including the lifter bars) is arranged to be replaced as it wears over a period of time.

A grinding mill has a critical speed of rotation below which it must be operated. At the critical speed, the material which is lifted from the bottom of the grinding portion is pinned to the interior wall of the grinding portion by centrifugal force. At an appropriate operating speed, below the critical speed, the lifted material falls away from the interior wall. onto other material in the grinding portion to effect grinding of the material. In autogenous grinding mills, grinding is ,effected by the action of material striking other material and in semi-autogenous grinding mills the grinding action is enhanced by the grinding mill being *loaded with a charge of grinding media such as steel balls or steel rods.

Material and water are typically fed into a grinding mill through a channel in the feed end wall which forms 25 part of the feed portion. Slurry containing ground material is typically discharged through an annular channel surrounding the bearing in the discharge end wall which forms part of the discharge portion. Prior to discharge through the discharge end wall, the slurry is 30 typically required to pass through a screening Sarrangement to ensure that only material which has been ground to an appropriate size is discharged from the grinding mill.

S•A screening wall for screening material to be discharged from the grinding mill is typically located adjacent to and inwardly of the discharge end wall with a cavity formed between the screening wall and the discharge end wall. Portions of the cavity are solid to S:50519C/700 4 enhance structural rigidity and provide cavity channels between the solid portions. After passing through the screening wall, material to be discharged passes through the cavity channels on route to discharge through the discharge end wall. The screening wall is typically of the same configuration as the discharge end wall; ie. if the discharge end wall is a vertical planar wall, the screening wall is typically also a vertically orientated wall.

Irrespective of the configuration of the screening wall, the discharge portion is typically formed with lifter bars which are in addition to the lifter bars of the grinding portion. Discharge portion lifter bars serve the same two functions as the grinding portion ooooe: 15 lifter bars but their primary function is typically wear protection. Discharge portion lifter bars typically overlay the solid portions in the cavity between the screening wall and the discharge end wall and protrude from the screening wall towards the grinding portion.

20 The discharge portion lifter bars typically form a radial array on the screening wall with the cavity channels taking the form of truncated sectors which narrow towards the centre of the discharge end wall. Such an .2 arrangement can be problematic because centrifugal force tends to inhibit movement of material through the cavity channels towards the centre of the discharge end wall.

SUMMARY OF THE PRESENT INVENTION The present invention provides a grinding mill having a substantially cylindrical body arranged to rotate about a substantially horizontal axis, wherein the body includes a grinding portion adjacent to a discharge portion; the discharge portion includes a discharge end wall arranged for the passage therethrough of material to be discharged from the mill, a truncated conical or a truncated pyramidal screening wall located inwardly of the S'P"Z/discharge end wall and which has its notional vertex sabelH\Speci\36784 .doc 5 remote from the grinding portion, and a cavity located between the discharge end wall and the screening wall; portions of the cavity are solid with cavity channels being located between adjacent solid cavity portions; the screening wall includes screening means which overlay at least portions of the cavity channels for the passage therethrough of screened material into the cavity channels for subsequent discharge from the mill through the discharge end wall; discharge portion lifter bars overlay at least portions of at least some of the solid cavity portions and protrude from the screening wall towards the grinding portion; and at least portions of the cavity channels and at *least portions of the discharge portion lifter bars are curved when projected onto a notional plane perpendicular to any notional plane which passes through the :substantially horizontal axis.

As mentioned above, at least portions of the cavity channels are curved when projected onto a notional plane perpendicular to any notional plane which passes 9ego through the substantially horizontal axis. That is to say 2 that at least portions of the cavity channels are curved when the screening wall is viewed from the grinding portion along lines parallel to the substantially horizontal axis. Additionally, it is to be noted that the cavity channels follow the conical or pyramidal shape of the screening wall. It is therefore to be appreciated that the cavity channels are also non-linear in this respect. The cavity channels are therefore of a complex shape which may be described as a non-planar curve or twisted curve.

Preferably, the screening wall is a truncated right circular conical screening wall having its notional vertex on the substantially horizontal axis.

H:\lsabelH\Speci\36784.doc The cavity channels may be curved through their length between their outer and inner extremities and they may be of a single radius curve configuration between their outer and inner extremities. Alternatively, the cavity channels may be of a compound curve configuration having a plurality of different radii.

o log H:\IsabelH\Speci\36784.doc 6 In an alternative embodiment the cavity channels are linear through a portion of their length extending outwardly from their inner extremities. Preferably, the linear portions extend for between 30% and 70%, more preferably about 50%, of the radial distance between the inner and outer extremities of the cavity channels. The cavity channels may be a single radius curve configuration between their linear portions and their outer extremities. Alternatively the cavity channels may be of a compound curve configuration having a plurality of radii between their linear portions and their outer extremities.

BRIEF DESCRIPTION OF THE DRAWINGS A prior art grinding mill and preferred embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a cutaway perspective view of a prior art autogenous grinding mill; Figure 2 is an elevation of part of the discharge portion of the prior art grinding mill of Figure 1; Figure 3 is a view of the discharge portion of a grinding mill (of the type illustrated in Figures 1 and 2) viewed from inside of the mill which has been 25 projected onto a plane in which a prior art embodiment is illustrated on the right of the Figure and an embodiment according to the present invention is illustrated on the left of the Figure; Figure 4 is a view similar to Figure 3 which further 30 illustrates the embodiment according to the present invention illustrated in Figure 3; Figure 5 is a view similar to Figure 3 in which a prior art embodiment is illustrated on the left of the "Figure and a further embodiment of the present invention is illustrated on the right of the Figure; and Figures 6 and 7 are depictions of solid models of a portion of the embodiment according to the present invention illustrated in Figures S:50519C/700 7 BEST MODE FOR CARRYING OUT THE INVENTION Referring firstly to Figures 1 and 2, the autogenous grinding mill 10 comprises a feed portion 12, a grinding portion 14 and a discharge portion 16 and takes the form of a substantially cylindrical body arranged to rotate clockwise (as viewed from the feed portion 12 towards the discharge portion 16) about horizontal axis A-A. The mill 10 is mounted for rotation on large bearings (not shown) in the end walls of the feed portion 12 and discharge portion 16. The mill 10 is formed from a multitude of cast steel components which are bolted together.

Ore to be ground and water are fed into the mill via hopper 18, chute 20 and a channel (not shown) in the feed portion bearing (not shown). The levelling of this constant influx of ore and water results in a flow towards the discharge portion 16. The ore is ground in the grinding portion 14 by rotation of the mill 10 at a speed below the critical speed of the mill. The speed of rotation is such that ore is lifted on the interior wall S-of the grinding portion 14 by centrifugal force acting on the ore and friction as the mill 10 rotates and subsequently falls away from the wall under action of gravity onto ore in the bottom of the mill thereby 25 effecting a grinding action. Lifting of the ore is enhanced by grinding portion lifter bars 22 which are bolted to the interior wall of the grinding portion 14 and protrude towards axis A-A. The grinding portion lifter bars 22 are spaced apart about the interior wall 30 of the grinding portion 14 and run parallel to axis A-A.

•The grinding portion lifter bars 22 enhance lifting of the ore and hence grinding by increasing the friction between the ore and the interior wall of the grinding portion 14.

The discharge portion 16 comprises a squat truncated conical discharge end wall (not shown) and a squat truncated conical screening wall 24 located inwardly of and spaced apart from the discharge end wall. A cavity S:50519C700 8 (not shown) is formed between the discharge end wall and the screening wall 24. Both truncated conical walls are truncated right circular conical walls and their notional vertexes lie on axis A-A. The screening wall 24 comprises a cap 26 which overlies and protects the bearing in the discharge end wall and screening means in the form of grates 28 which are positioned between adjacent discharge portion lifter bars 30. The cavity between the discharge end wall and the screening wall 24 is not continuous, with those portions of the cavity below the discharge portion lifter bars 30 being solid.

The solid cavity portions (not shown) and discharge portion lifter bars 30 are linear and lie on generators of the truncated cone in a radial array. Cavity channels (not shown) in the form of truncated sectors are located between adjacent solid cavity portions. The cavity channels narrow towards the centre of the screening wall 24 where they join an annular channel which surrounds the bearing in the discharge end wall and facilitates removal of slurry containing ground ore through the discharge end ~wall. The grates 28 overlie the cavity channels with slurry passing through the grates 28 prior to moving through the cavity channels and annular channel. The :.cavity channels rotate with the rest of the mill with 25 slurry passing through grates 28 and into the cavity channels in lower portions of the mill 10 whereafter the slurry is carried toward the top of the mill 10 as the cavity channels rotate. Centrifugal force acts on the slurry in the cavity channels as they are rotated and at a point in the top half of the mill 10 gravitational force overcomes the centrifugal force resulting in the slurry moving down the cavity channel to the annular channel surrounding the discharge end wall bearing and out of the mill.

Referring now to Figure 3, the right hand side of Figure 3 is a view of the discharge portion 16 of a mill as described in Figures 1 and 2 in which the discharge portion 16 has been projected onto a vertical plane, ie.

S:50519C/700 9 a notional plane which is perpendicular to any notional plane which passes through horizontal axis A-A. The arrow indicates the clockwise direction of rotation. The grates 28 and discharge portion lifter bars 30 have been removed to expose the discharge end wall 32, radially arrayed solid cavity portions 34 which are upstanding from the end wall 32 and truncated sector cavity channels 36 located between adjacent solid cavity portions 34.

Each solid cavity portion 34 is formed from three castings; two of which are illustrated as 34a and 34b.

Similarly, the discharge end wall 32 is formed from a multitude of castings; sets of three of which form a sector. Two only of each set of three castings are illustrated as 32a and 32b. The solid cavity portions 34, grates (not shown), discharge portion lifter bars (not shown) and end wall 32 are secured together by bolts (not shown) which pass through bolt holes 38.

The projection of Figure 3 can be readily imagined as the face of a clock with the prior art arrangement depicted between 12 o'clock and 6 o'clock on the right l ~hand side of the Figure and an embodiment according to the present invention depicted between 6 o'clock and 12 o'clock on the left hand side of the Figure. Slurry cannot move towards the annular channel from cavity 25 channels 36 in the bottom of the discharge portion 16 because of the action of gravity and centrifugal force ~which forces the slurry to the outside of the discharge portion 16. Slurry in a cavity channel in the bottom of the discharge portion 16 is subjected to settlement 30 forces of up to twice gravity. This results in rapid segregation of the slurry with denser and larger particles settling into the periphery of the grinding mill with less dense material above. In the top of the •discharge portion 16 flow of slurry towards the annular channel is promoted by gravity and reduced by both centrifugal force and friction. The balance of these forces determines the timing and extent of movement of slurry in a cavity channel and its ultimate point of S:50519C/700 10 discharge through the annular channel. As a cavity channel rises to horizontal (9 o'clock) centrifugal forces dominate to hold material at the grinding mill periphery. Initiation of flow through the cavity channel towards the annular channel requires the gravity component to exceed the centrifugal force. Based on computer modelling and video imaging of full-scale grinding mill discharge, a prior art cavity channel 36 is believed to initiate flow at about 10 o'clock but, at this point, the nett force promoting inward flow is weak because centrifugal force remains very significant.

Additionally, the denser and larger particles at the periphery are subject to a significant frictional resistance to flow with the result that initial flow tends to act on the lighter components of. the slurry which are closer to the annular channel. The lighter components are subjected to less centrifugal force because of the reduced effective radius and, by their nature, are subjected to less frictional resistance. At about 12 o'clock the peripheral component begins to move *and is believed to enter a period of ballistic motion before hitting a trailing face of the cavity channel whereafter movement towards the annular channel is slightly impeded by friction. The peripheral material 25 tends to reach the annular channel at about 3 o'clock with some discharge possible slightly after 3 o'clock due to momentum.

".The above described flow mechanics is believed to result in residual particles being retained in the cavity oo 30 channels 36 which reduces the efficiency of the mill It is desirable to remove these residual particles, which are believed to consist of broken steel grinding media and larger particles of ground ore, from the mill The larger particles of ore are desirably returned into the mill 10 for further grinding or directed into a separate crushing circuit.

The left hand side of Figure 3 is an equivalent projection onto a vertical plane as for the right hand S:50519C/700 11 side. The screening wall 24 and end wall .32 remain truncated conical walls as in Figures 1 and 2 but, in accordance with an embodiment of the present invention, the arrangement of cavity channels is different. As with the right hand side of the Figure, the grates and discharge portion lifter bars have been removed to expose the discharge end wall 32. Cavity channels 40 are similarly located between adjacent solid cavity portions 42 but in this case the cavity channels 40 are curved when projected onto the notional plane (vertical plane) which is perpendicular to any notional plane which passes through horizontal axis A-A. The solid cavity portions 42 and the portions of the discharge end wall 32 are similarly formed from a multitude of ferrous castings which are bolted together.

As with the prior art arrangement, centrifugal force and gravity force the slurry in the cavity channels 40 to the outside of discharge portion 16 when a cavity channel is in the bottom of discharge portion 16. The arrangement of the cavity channels 40 however enables *.***gravity to begin to oppose centrifugal force earlier in

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*rotation between 6 o'clock and 12 o'clock than does the arrangement of cavity channels 36. This is believed to facilitate more efficient "emptying" of the cavity 25 channels 40 with avoidance of residual particles being retained in the cavity channels In the vertical plane depicted in Figure 3, the solid cavity portions 42 (between which are located curved cavity channels 40) are single radius curves. In nOO 30 addition it is to be noted that to maintain o perpendicularity with the conical surface of the discharge end wall 32 upon which they are formed, they twist in a clockwise direction while traversing from the centre of the discharge portion 16 to the outside of the discharge portion 16.

Referring now to Figure 4 which is very similar to the left hand side of Figure 3 but constituted by a different arrangement of component castings, some of the S:50519C/700 12 solid cavity portions 42 are illustrated with discharge portion lifter bars 44 atop. It is to be noted that every second discharge portion lifter bar 44 (designated 44a) is shorter than its adjacent lifter bars 44 to avoid overcrowding at the centre of the discharge portion 16.

Referring now to Figure 5, a projection similar to Figures 3 and 4 has a prior art arrangement depicted on the left hand side of the Figure and an alternative embodiment of the present invention depicted on the right hand side of the Figure with the arrow again indicating the direction of rotation. In contrast to the single radius solid cavity portions 42 of Figure 3, the solid cavity portions 46 and the cavity channels 48 are of a more complicated configuration. For just over half of the length of the solid cavity portions 46 from the inside towards the outside of the discharge portion 16, the solid cavity portions 46 are linear. Thereafter, the solid cavity portions 46 assume a compound curve configuration involving a curve having two different radii. This arrangement of solid cavity portions 46 and cavity channels 48 enhances the emptying of the cavity channels described in relation to Figure 3 by enabling gravity to begin to oppose centrifugal force earlier in the rotation between 6 o'clock and 12 o'clock than in the inventive embodiment described in relation to Figure 3.

Computer modelling and video imaging indicate that all of the slurry in cavity channels 48 of Figure reaches the annular channel between 12 o'clock and 1 o'clock. It is to be noted that the arrangement of the 30 solid cavity portions 46 in Figure 5 are such that retrofitting to an existing discharge end wall 32 is possible without the need to drill further bolt holes in the discharge end wall 32.

Figures 6 and 7 further illustrate the inventive embodiment of Figure 5. In both Figures, the depicted portion is of the outside of discharge portion 16. A cavity channel 48 is depicted in Figure 6 between adjacent solid cavity portions 46. It is to be noted S:50519C/700 13 that the floor of the cavity channel 48 is not the discharge end wall 32 but a combination of parts of the castings which make up the solid cavity portions 46.

This arrangement protects the discharge end wall 32 from wear. A casting 50 is also depicted in Figure 6 which contains parts of two discharge portion lifter bars 52 which are arranged to sit atop adjacent solid cavity portions 46. The casting 50 is also depicted in Figure 7, together with two further castings 54 and 56 containing further parts of the two discharge portion lifter bars 52. Parts of screening grates 58 are integrally formed with the discharge portion lifter bars 52 in the castings 54 and 56. Alternatively, the screening grates 58 could be formed from a casting which spans adjacent solid cavity portions 46 with the discharge portion lifter bars 52 separately cast and arranged to secure the screening grate castings. It is believed that the curved shape of the discharge lifter bars 52 will assist in scooping slurry up from the bottom of the mill 10 adjacent to the discharge portion 16 and in presenting the slurry to the screening grates 58.

As compared with the prior art, the gap between adjacent screening grates 58 is relatively large and is believed to function to enhance movement of slurry into 25 the cavity channel 48 by functioning as a screening aperture. The scooping effect of the discharge lifter bars 52 is believed to direct slurry and larger particles :in a direction more along the discharge lifter bars 52 than in prior art arrangements which tend to pass slurry 30 over the screening wall in paths which follow the •circumference of the mill 10. The arrangement of discharge lifter bars 52 and the gap between adjacent S•screening grates 58 is therefore believed to enhance screening.

S:50519C700

Claims (1)

14- CLAIMS 1. A grinding mill having a substantially cylindrical body arranged to rotate about a substantially horizontal axis, wherein the body includes a grinding portion adjacent to a discharge portion; the discharge portion includes a discharge end wall arranged for the passage therethrough of material to be discharged from the mill, a truncated conical or a truncated pyramidal screening wall located inwardly of the discharge end wall and which has its notional vertex remote from the grinding portion, and a cavity located between the discharge end wall and the screening wall; portions of the cavity are solid with cavity channels being located between adjacent solid cavity portions; the screening wall includes screening means which overlay at least portions of the cavity channels for the passage therethrough of screened material into the cavity channels for subsequent discharge from the mill through the discharge end wall; discharge portion lifter bars overlay at least portions of at least some of the solid cavity portions and "'"protrude from the screening wall towards the grinding S 25 portion; and at least portions of the cavity channels and at least portions of the discharge portion lifter bars are curved when projected onto a notional plane perpendicular to any notional plane which passes through the substantially horizontal axis. 2. A grinding mill as claimed in claim 1 wherein the screening wall is a truncated right circular conical screening wall having its notional vertex on the substantially horizontal axis. 3. A grinding mill as claimed in claim 1 or claim 2 wherein the cavity channels are curved through >SM their length between their outer and inner extremities. H:\fsabelH\Speci\36784.doc 4. A grinding mill as claimed in claim 3 wherein the cavity channels are of a single radius curve configuration between their outer and inner extremities. A grinding mill as claimed in claim 3 wherein the cavity channels are of a compound curve configuration having a plurality of different radii. 6. A grinding mill as claimed in claim 1 or claim 2 wherein the cavity channels are linear through a portion of their length extending outwardly from their inner extremities. 7. A grinding mill as claimed in claim 6 wherein the cavity channels are linear extending outwardly from their inner extremities for between 30% and 70% of S" the radial distance between their inner and outer 15 extremities. 8. A grinding mill as claimed in claim 7 9oo wherein the cavity channels are linear for about 50% of the radial distance between their inner and outer extremities. 9. A grinding mill as claimed in any one of claims 6-8 wherein the cavity channels are of a single radius curve configuration between their linear portions and their outer extremities. 1 A grinding mill as claimed in any one of 25 claims 6-8 wherein the cavity channels are of a compound curve configuration having a plurality of radii between their linear portions and their outer extremities. 11. A non-prior art grinding mill substantially as herein described with reference to the accompanying drawings. Dated this 2 4 th day of August 2001 THE ANI CORPORATION LIMITED By its Patent Attorneys GRIFFITH HACK H:\1sabelH\Speci\36 7 84 .doc