AU737414B2

AU737414B2 - Lifter bars

Info

Publication number: AU737414B2
Application number: AU17626/00A
Authority: AU
Inventors: David Royston
Original assignee: Ani Corp Ltd
Current assignee: Bradken Resources Pty Ltd
Priority date: 1998-12-07
Filing date: 1999-12-07
Publication date: 2001-08-16
Anticipated expiration: 2019-12-07
Also published as: WO2000033963A1; AU1762600A; AUPP755998A0

Description

WO 00/33963 PCT/AU99/01085 1 LIFTER BARS FIELD OF THE INVENTION The present invention relates to lifter bars for use in grinding mills in which material is thrown to effect grinding on impact and to grinding mills incorporating the lifter bars. Subject grinding mills include autogenous (AG) and semi-autogenous (SAG) grinding mills.

BACKGROUND TO THE INVENTION SAG/AG grinding mills typically take the form of a horizontally orientated cylindrical shell 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 shell is typically mounted for rotation about a substantially horizontal axis on large bearings in the two end walls of the shell.

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.

Through rotation of the mill the charge inside the mill is displaced in the direction of rotation. The portion of the charge towards the base of the mill is commonly referred to as the toe of the charge. Flow of material through the mill is facilitated by the introduction of water and the discharge of slurry.

The interior of the cylindrical shell of the grinding portion is typically formed with a series of circumferentially spaced protrusions which are commonly referred to as lifter bars. The lifter bars typically WO 00/33963 PCT/AU99/01085 -2comprise elongate members which extend substantially for the length of the grinding portion, protrude towards the substantially horizontal axis of the grinding mill, and typically serve two functions. Firstly, the lifter bars enhance the lifting and throwing of material as the grinding mill rotates which, in the absence of lifter bars, occurs to an extent as a consequence of the centrifugal force acting on the material and friction 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 is thrown and falls away from the interior wall desirably 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.

Lifter bars typically have a base located at, or adjacent to, the interior wall of the grinding portion; a head located inwardly of the interior wall; and two faces spanning respective sides of the base and head. Hereafter, the faces are referred to as a leading face and a trailing WO 00/33963 PCT/AU99/01085 3 face; the leading face being that face which, in use, moves in advance of the trailing face.

As mentioned above, lifter bars typically form part of the plating arrangement which overlies the interior wall of the grinding portion. Typically, the base of a lifter bar abuts the interior wall with other plating elements located between adjacent lifter bars. The bottom of the other plating elements typically also abut the interior wall of the grinding portion with sides of the other plating elements abutting faces of the lifter bars with the result that only an upper portion of the lifter bar is exposed.

In an alternative arrangement, the entire inner wall of the grinding portion may be covered by plating elements with the lifter bars mounted with their bases atop the other plating elements thereby leaving the entirety of the faces of the lifter bars exposed. The exposed area of a face, be it a portion or the entirety of a lifter bar face, is hereafter referred to as the exposed face of the lifter bar.

Various configurations of exposed faces of lifter bars have been proposed over the years. Often the faces are symmetrical about a notional centre line running through the head and the base on a radius of the cylindrical shell, although asymmetrical arrangements have also been proposed.

The exposed faces may be parallel to the notional centre line or at an angle thereto. In more recent times, so-called wide angle lifter bars have begun to be used.

Typically, in wide angle lifter bars, the angle between the exposed leading face and the notional centre line of a lifter bar is in the order of 250-35'. Wide angle lifter bars are believed to be advantageous compared to more conventional lifter bars in which the exposed leading face angle is typically 15' or less because the "throw" of the WO 00/33963 PCT/AU99/01085 4 charge is reduced and the thrown material lands predominantly on the "toe" of the charge which results in improved impact effect and hence improved grinding efficiency. Over-throwing is undesirable because grinding efficiency is reduced and wear can be accelerated due to undesirable impact on the plating arrangement.

Notwithstanding the advantages of wide angle lifter bars, problems with their use have been identified. For example, firstly, they are considerably heavier than conventional narrow angle lifter bars which adds to construction cost and enhances handling and safety difficulties during installation. Secondly, as wide angle lifter bars are wider at their bases, the number of lifter bars that can be accommodated within a grinding mill is reduced. Thirdly, all lifter bars wear over time and in the case of wide angle lifter bars, it is believed that wear may result in under-throwing. Fourthly, by reducing the number of lifter bars to accommodate the wide angle type, the overall lifting action in the mill is also reduced and with it the associated grinding action.

SUMMARY OF THE PRESENT INVENTION In a first aspect, the present invention provides a lifter bar for a grinding mill, the lifter bar being arranged to be secured to, or adjacent to, an inner surface of a cylindrical shell of the grinding mill, the lifter bar having a head, a base, an exposed leading face and an exposed trailing face wherein the exposed leading face has a non-linear profile.

In a second aspect, the present invention provides a lifter bar for a grinding mill, the lifter bar being arranged to be secured to, or adjacent to, an inner surface of a cylindrical shell of the grinding mill, the lifter bar having a head, a base, an exposed leading face and an WO 00/33963 PCT/AU99/01085 5 exposed trailing face wherein the exposed leading face has a non-linear profile and is arranged, in use, to produce a reduced throw of a charge in the grinding mill as compared with a conventional narrow angle lifter bar.

The exposed leading face of the lifter bar may have a curved profile or have at least one portion having a curved profile. Preferably, the exposed leading face has a profile comprising a plurality of linear portions.

Desirably, there are two or three linear portions.

Preferably, there are three linear portions; an upper portion towards the head of the lifter bar, a lower portion towards the base of the lifter bar, and a middle portion between the upper and lower portions.

Preferably, the middle portion has a face angle in the order of 20o-35', more preferably about 28°-30'; the upper portion has a face angle in the order of and the lower portion has a face angle in the order of 0-15'.

Accordingly, it is preferred that the upper and lower portions approximate a conventional narrow angle lifter bar with the middle portion approximately a wide angle lifter bar.

The exposed leading face of the lifter bar may have a non-linear profile resulting from a protuberance on the exposed leading face. Typically, the protuberance will take the form of a bulge in the exposed leading face and will be located towards the base of the exposed leading face. Where the non-linear profile results from a protuberance, the remainder of the exposed leading face will typically be of conventional narrow angle design.

Typically, the lifter bar will have a height in the order of 120-350mm and will be utilised in a semi- WO 00/33963 PCT/AU99/01085 6 autogenous mill which is charged with grinding balls having diameters in the order of 75-150mm.

In a third aspect, the present invention provides a grinding mill having lifter bars according to the first or second aspects of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS A prior art grinding mill, prior art lifter bars 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 a cross-section of a prior art lifter bar of Figure 1; Figure 3 is a cross-section of an alternative prior art lifter bar; Figure 4 is a cross-section of a lifter bar in accordance with the present invention; Figure 5 is an output of a proprietary program which modeled performance of the lifter bar of Figure 3 and Figure 6 is an output of the proprietary program modeling performance of the lifter bar of Figure 4.

BEST MODE FOR CARRYING OUT THE INVENTION Referring firstly to Figure i, 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.

WO 00/33963 PCT/AU99/01085 7 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 leveling 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. The speed of rotation is such that ore is lifted on the interior wall 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 effecting a grinding action. Lifting and throwing 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 of the grinding portion 14 and run parallel to axis A-A. The grinding portion lifter bars 22 enhance lifting and throwing of the ore and hence grinding by increasing the friction between the ore and the interior wall of the grinding portion 14.

The lifter bar 22 is illustrated in cross-section in Figure 2. The lifter bar 22 has a head 24, a base 26, a leading face 28 and a trailing face 30. The leading face 28 has an exposed portion, the exposed leading face 32, which is of linear profile and which has a face angle A of 150.

An alternative prior art lifter bar 34 is illustrated in Figure 3. Lifter bar 34 is similar to lifter bar 22 and like numerals have been used to indicate equivalent parts.

Lifter bar 34 is a so-called wide angle lifter bar and differs from lifter bar 22 in that the exposed leading face angle A is 300. Notwithstanding the wide angle nature of WO 00/33963 PCT/AU99/01085 8 lifter bar 34, its exposed leading face 32 is of linear profile. The leading half of lifter bar 34 has a calculated mass of 27.7 units.

Figure 4 is a view of a lifter bar 40 according to the present invention. Again, like numerals have been used to indicate equivalent parts. In contrast to lifter bars 22 and 34, the exposed leading face 32 of lifter bar 40 is of non-linear profile. The exposed leading face 32 consists of an upper linear portion 42, a middle linear portion 44 and a lower linear portion 46. The portions 42, 44 and 46 have face angles X, Y and Z of 150, 300 and 150 respectively. The leading half of lifter bar 40 has a calculated mass of 23.56 units which is 85% of that of the leading half of lifter bar 34.

The applicant has developed a program called "Mill Throw" which models lifter bar performance and Figures and 6 are outputs of the program modeling lifter bars 34 and 40 respectively. It is to be noted that the impact points marked in Figures 5 and 6 are very similar and both deposit impact on the toe of the charge.

Claims

2. A lifter bar for a grinding mill, the lifter bar being arranged to be secured to, or adjacent to, an inner surface of a cylindrical shell of the grinding mill, the lifter bar having a head, a base, an exposed leading face and an exposed trailing face wherein the exposed leading face has a non-linear profile and is arranged, in use, to produce a reduced throw of a charge in the grinding mill as compared with a conventional narrow angle lifter bar.
3. A lifter bar as claimed in claim 1 or claim 2 wherein the exposed leading face has a profile comprising a plurality of linear portions.
4. A lifter bar as claimed in claim 3 wherein the exposed leading face has three linear portions; an upper portion towards the head of the lifter bar, a lower portion towards the base of the lifter bar, and a middle portion between the upper and lower portions. A lifter bar as claimed in claim 4 wherein the middle portion has a face angle of 200-35O.
6. A lifter bar as claimed in claim 4 wherein the middle portion has a face angle of 28o-30'.
7. A lifter bar as claimed in claim 5 or claim 6 wherein the upper portion has a face angle of 5-15o and the lower portion has a face angle of 0-15'.
8. A lifter bar as claimed in any one of the Preceding claims having a height of 120-350mm. WO 00/33963 PCT/AU99/01085 10
9. A grinding mill having lifter bars as claimed in any one of the preceding claims. A grinding mill as claimed in claim 9 being a semi-autogenous grinding mill arranged to be charged with grinding balls having diameters of 75 -150mm.