A sensing arrangement for a stirred mill and a method of sensing in a stirred mill

Abstract

A sensing arrangement for a stirred mill is disclosed having a mill shell defining an internal grinding chamber, and a stirrer for rotation within the grinding chamber. The sensing arrangement may comprise a plurality of acoustic and / or vibration sensors mounted on an external surface of the mill shell that are responsive to sounds and / or vibrations within the grinding chamber to generate a signal input, and a processing arrangement for receiving the signal input from the at least one acoustic and / or vibration sensor. The plurality of sensors may be spaced apart in an axial direction of the stirrer. The processing arrangement may generate a processing output on the shape and / or dimension of the stirrer or a processing output on the dynamic height of grinding media within the mill.

Description

[0001] A SENSING ARRANGEMENT FOR A STIRRED MILL AND A METHOD OF SENSING IN A STIRRED MILL

[0002] FIELD

[0003] This invention relates generally to a sensing arrangement for a stirred mill and a stirred mill including the sensing arrangement. It also extends to a method of sensing in a stirred mill.

[0004] The invention relates particularly to a stirred mill that is a vertical mill or a vertical tower mill (VTM) and it will be convenient to hereinafter describe the invention with reference to this example application. However, it also relates to a HIGmill™ and a Stirred Media Detritor (SMD) mill. Yet further, it relates to a horizontal stirred mill, or an inclined stirred mill, and it is to be clearly understood that the invention applies to all forms of stirred mills.

[0005] DEFINITIONS

[0006] In this specification, the term ‘comprising’ is intended to denote the inclusion of a stated integer or integers, but not necessarily the exclusion of any other integer, depending on the context in which that term is used. This applies also to variants of that term such as ‘comprise’ or ‘comprises’.

[0007] BACKGROUND

[0008] One example of a known type of stirred mill comprises a mill shell defining an internal grinding chamber. A stirrer or agitator, which, in this example, is in the form of an agitator screw, is mounted within the grinding chamber for agitating contents within the grinding chamber. The agitator screw is driven by an external drive arrangement that is mounted co-axially with the agitator. In this example of a vertical mill, the drive arrangement is mounted in proximity to the mill shell.

[0009] In use, the mill typically contains a grinding media, e.g. in the form of a ball charge comprised of grinding balls. The mill with its contained grinding media is used to grind a slurry of mineralcontaining particles to reduce the size of the particles and thereby liberate valuable mineral within the particles. Sometimes dry grinding, rather than grinding a slurry of particles, is preferred or required. Rotation of the agitator screw lifts and moves the particles and the grinding media, and this progressively reduces the size of the mineral-containing particles within the mill. Once the mineral-containing particles have been reduced to fines, they tend to rise up through the grinding chamber and be drawn off.

[0010] The agitator screw comprises a shaft and a blade extending radially outward away from the shaft that makes contact with the particles, and the grinding balls. In one form, the blade may be in the form of a helical spiral although it must be appreciated that other types or configurations of blades may also be used. The blade has a wear surface that is exposed to significant wear during operation of the mill. In some stirred mills, the blade comprises an underlying blade base and a set of replaceable wear liners mounted over the blade base to deal with this wear. The replaceable wear liners wear down over time and need to be replaced periodically. Further, other stirred mills, instead of having a blade base with a replaceable wear liner mounted thereon, have a sacrificial stirrer. These stirrers are progressively worn down in use and replaced at their end of life. A H IGm ill™ is an example of such a mill. Stirrers, including sacrificial stirrers, may be of forms other than agitator screws. For example, some mills, including the HIG mill and the SMD mill, may have a stirrer configured with a central shaft and a plurality of posts (sometimes called ‘pins’) or other structures extending outwardly therefrom towards the sides of the internal grinding chamber.

[0011] One example of an existing stirred mill is the VERTIMILL® vertical mill manufactured and produced by the Applicant which is shown in Figures 1 and 2 of the drawings. This mill has a stirrer in the form of an agitator screw with a vertically extending axis driven by a co-axial drive arrangement mounted on top of the mill shell.

[0012] The VERTIMILL® is extremely effective at comminution and its utilization and uptake in mineral processing circuits has steadily increased over the past 10 years. The VERTIMILL® is primarily used for grinding, particularly fine grinding, to reduce the size of mineral containing particles to liberate minerals. The VERTIMILL® has also proved to be very efficient for regrinding, secondary and tertiary grinding, and lime slaking applications.

[0013] In Figures 1 and 2, a lower region of the wall of the mill shell has been removed to open up the mill shell and provide visibility of the internal grinding chamber. An agitator screw is received within the grinding chamber comprising an axial shaft and a blade that extends radially outward from the axial shaft and along the axial shaft with a helical spiral.

[0014] Figure 2 shows a grinding media in the form of a ball charge in the internal grinding chamber. The blade extends in a helical spiral around the screw axis as shown in Figures 1 and 3. During operation of the vertical mill, rotation of the blade lifts the grinding balls and the mineral particles, and it also moves the grinding balls and mineral particles around. This causes collisions between the balls and the mineral particles which reduces the size of the mineral particles. As the size of mineral particles is reduced, some of the smaller mineral particles rise in the chamber, and are drawn off through an outlet in an upper region thereof.

[0015] As discussed above, the wear liners on the blade of the vertical mill wear down over time during use. Once a wear liner has reached a limit of wear (and has basically been worn out or worn down), the blade will not work effectively. For example, as shown in Figure 3, the radius of a lower region of the blade is progressively reduced over time so the distance from the radially outer edge of the blade to the mill shell in a horizontal direction is increased. This results in a greater slippage of grinding media, e.g. balls past the blade and reduces their lifting ability, lowering the efficiency of the mill.

[0016] Consequently, an important factor in the management of a vertical mill is monitoring the wear of the wear liners and swapping out the wear liners once they reach their wear limit. At the same time, the wear liners represent a significant cost and mill operators would try and avoid swapping out a set of liners before they were fully worn out. Similar considerations apply to other sacrificial stirrers that need to be replaced when they are worn out.

[0017] Applicant is aware of some prior art techniques for measuring wear of the stirrer, but they are quite rudimentary and generally it is difficult to optimize operation of the mill using these techniques.

[0018] One such technique involves placing physical wear sensors on the stirrer inside the grinding chamber and periodically inspecting the sensors to measure wear of the stirrer or wear liner at each sensor. However, this technique is intrusive and invasive because a worker needs to gain physical access to the internal chamber of the mill periodically to inspect the wear on the physical wear sensors.

[0019] To do this, the mill needs to be shut down and the grinding media needs to be emptied from the mill. Applicant has established that it typically takes 6 to 10 hours to empty a mill and about two hours to inspect the liners on the agitator. The mill then needs to be refilled with grinding media which takes 4 to 6 hours. Adding this up, about 12 to 18 hours are lost each time a worker physically inspects the wear liners on the agitator. This represents a substantial cost in downtime and lost production to mill operators. While stopping a mill takes time, restarting a mill again after it has been stopped strains the drive. This is because the media and ore need to be set in motion again when the mill is started again, and this imposes a large load on the drive.

[0020] Another issue that bears on the operation of a vertical mill is the level of grinding media charge contained in the mill. Persons skilled in the art will understand that the volume of grinding media charge within a grinding chamber decreases over time due to breakage, for example, of individual balls within the charge. However, the quantity of grinding media in the grinding chamber has an important influence on the productivity and efficiency of the mill. The level of grinding media charge therefore needs to be maintained at or close to an optimum level to maintain mill efficiency and the mill is periodically topped up with grinding media to do this. One current method involves making a rough estimate of the level of grinding media in the mill based on an operator’s experience of working on the mill, and then topping the grinding media up based on this estimate. Another method involves stopping the mill, then dropping a plumb line into the mill to get an indication of the level of grinding media charge within the stationary mill. This technique also has its shortcomings because it requires the mill to be shut down losing productivity. Further, this method provides only a measure of the ‘static charge’ when the stirrers and the charge are stationary, and not the ‘dynamic charge’ of an operating mill, which is a desired and more important measurement than static charge, and generally differs from the static charge.

[0021] Thus, the prior art technique of physically entering the mill to physically inspect wear liners and sensors on the wear liners has significant shortcomings, as does the technique of dropping a plumb line into the mill to measure the level of charge in the mill.

[0022] SUMMARY OF INVENTION

[0023] Applicant recognizes that it would be beneficial if a different technique for measuring wear of the stirrer, for example, a blade on an agitator screw within a stirred mill could be developed. In particular, the Applicant recognizes the benefit that would be conferred by being able to measure the wear of the stirrer within the mill accurately with a non-invasive or non-intrusive technique that was implemented from outside the mill. Applicant also appreciates the benefits that would be achieved if the wear could be measured without downtime of the mill, and if workers did not need to physically enter the mill.

[0024] Applicant further recognizes that it would be beneficial to measure wear of the stirrer on a regular and ongoing basis. This would help mill operators to increase their operational efficiency by reducing mill downtime. It would also help mill operators to better manage the utilization of stirrers and / or wear liners and extract greater use out of them before swapping them out.

[0025] Applicant also recognizes it would be beneficial to be able to measure the level of grinding media charge within the grinding chamber non-invasively without stopping the mill. This would help mill operators to keep the level of grinding media charge at or close to its optimum level and thereby improve operational efficiency of the mill.

[0026] It would therefore be desirable to provide a technique for measuring wear of the stirrer of a stirred mill and / or for measuring the level of the grinding media which, at least in preferred embodiments, provides one or more of the above benefits, or which at least provides a useful alternative to known techniques.

[0027] According to one aspect of the invention there is provided a sensing arrangement for a stirred mill comprising an external mill shell defining an internal grinding chamber, and a stirrer for rotation within the grinding chamber, the sensing arrangement comprising: at least one acoustic and / or vibration sensor mounted on an external surface of the mill shell in use, the at least one sensor being responsive to sounds and / or vibrations within the grinding chamber to generate a signal input; and a processing arrangement for receiving the signal input from the at least one acoustic and / or vibration sensor and utilizing a relationship, between the signal input sensed by the at least one acoustic and / or vibration sensor and a configuration and / or dimension of at least a part of the stirrer, to generate a processing output comprising information on the shape and / or dimension of the stirrer.

[0028] The at least one acoustic and / or vibration sensor may comprise a plurality of acoustic and / or vibration sensors.

[0029] The sounds and / or vibrations within the grinding chamber may be expressed in the form of sound and / or vibration and energy.

[0030] The acoustic and / or vibration sensors of the plurality of acoustic and / or vibration sensors may be spaced apart from each other on the external surface in a direction corresponding to an axial direction of the stirrer.

[0031] The sensing arrangement may be for use on a stirred mill in which the stirrer comprises a screw agitator having a screw axis and a blade for rotation within the grinding chamber.

[0032] The at least a part of the stirrer may comprise at least one radially outer part of, e.g. an outer edge, the blade.

[0033] The sensing arrangement may be for use on a stirred mill in which the stirrer comprises a shaft and a plurality of discrete structural elements, e.g. pins and / or frames, which extend outwardly from the shaft for rotation within the grinding chamber.

[0034] The processing output may generate information on a shape and / or dimension of one or more radially outer parts of the stirrer.

[0035] The processing output resulting from the signal input of the at least one sensor may provide information on a change in the shape and / or dimension of the stirrer resulting from use of the mill over a length of time, which provides a measure of wear over the length of time.

[0036] The at least one acoustic and / or vibration sensor may comprise a plurality of acoustic and / or vibration sensors, and the processing output resulting from the signal inputs of the plurality of sensors may provide a measure of wear along the length of the stirrer.

[0037] In an embodiment, the use of a plurality of sensors provides a measure of radial wear, e.g. wear in a radial direction, occurring along the axial length of the blade.

[0038] The processing output may comprise a measure of the distance between at least one part of the stirrer, e.g. a radially outer party, and an inner surface of the mill shell (R tank - R max stirrer).

[0039] In embodiments comprising a plurality of acoustic and / or vibration sensors, the processing output may comprise a measure of the distance between a plurality of parts of the stirrer, e.g. radially outer party, and an inner surface of the mill shell (R tank - R max stirrer) at a position of each acoustic and / or vibration sensor.

[0040] With the sensing arrangement, an increase in the distance between said part, e.g. the radially outer part, and the inner surface (R tank - R max stirrer) over the length of time provides a measure of wear of the stirrer.

[0041] Applicant has recognized that the sounds and / or vibrations, or sound and vibrational energy, sensed by acoustic and / or vibration sensors on the outside of a mill shell carry information about the stirrer and the level of grinding media within the mill. They reflect impacts of grinding media with the stirrer and the mill shell, and impacts of the grinding media with each other, that are proximate to or adjacent to the particular sensor.

[0042] Applicant has discovered that the sounds and / or vibrations generated by the mill and sensed by an acoustic and / or vibration sensor change as the distance from a radially outer part of the stirrer (for example, but not limited to, the edge of the blade of a screw agitator) to the internal surface of the mill shell increases. At least in some instances, there is a relationship, e.g. a correlation, between the sensed information and the radial distance at the vertical height of the sensor (which can be expressed as R tank - R max stirrer). This, in turn, enables wear of the stirrer to be measured.

[0043] The processing output generated by the processing arrangement may represent, e.g. graphically represent, an amount of acoustic and / or vibration and energy from the mill sensed by an acoustic and / or vibration sensor, and a decrease in the amount of acoustic and / or vibration energy over time indicates an increase in the distance between the radially outer part of the stirrer and the inner surface of the mill shell, for example, between the radially outer edge of the blade and the inner surface (R tank - R max blade).

[0044] Each sensor may sense sounds and / or vibrations generated within the grinding chamber of the mill, as often as desired, periodically, and / or substantially continuously, and each sensor may forward the signals from the plurality of sensors as often as desired, periodically and / or substantially continuously to the processing arrangement.

[0045] The plurality of acoustic and / or vibration sensors may be arranged substantially in a line along the external surface of the mill shell. Optionally, the plurality of acoustic and / or vibration sensors may be arranged in a straight line, e.g. a vertical or diagonal line, but this does not need to be the case. For example, the plurality of sensors may extend in a line that is non-linear, e.g. a curved line or a line made up of discrete segments.

[0046] Further, the plurality of acoustic and / or vibration sensors may extend in a line that is parallel to the stirrer axis. In one alternative, the plurality of acoustic and / or vibration sensors may extend at an angle to the stirrer axis.

[0047] The plurality of acoustic and / or vibration sensors may comprise at least four sensors arranged on the external surface of the mill shell at different positions along the stirrer axis. In one example, the arrangement comprises at least eight sensors extending in a line up along a length of the mill shell.

[0048] In one example form, the sensors may be spaced a distance apart from each other of approximately 0.3m (300mm) to 2m (2000mm). However, the spacing of the sensors may change with different applications. Further, the sensors do not need to be positioned at a regular or even spacing apart from each other.

[0049] The plurality of acoustic and / or vibration sensors may be mounted on a mounting element and the mounting element may be mountable on the external surface of the mill shell.

[0050] For example, the mounting element may comprise a planar element, which may comprise an elongated sheet.

[0051] The mounting element may provide two major surfaces, the sensors being mounted on one of the major surfaces, and the other major surface may be mounted on an external surface of the mill shell.

[0052] The plurality of acoustic and / or vibration sensors may comprise a plurality of acoustic cameras, and each acoustic camera may form a representation of the location of the origin of sensed sounds and vibrations.

[0053] In addition to or instead of an acoustic camera, the plurality of sensors may comprise at least one microphone and / or an accelerometer.

[0054] The processing arrangement may comprise a computer that is positioned in any relation to the sensor or sensors. For example, in some forms, the computer may be positioned in close proximate relation to the mill shell and may even be positioned on the mill shell. In other forms, the computer may be positioned spaced away from the mill shell, and it may even be quite remote from the mill shell, where the processing arrangement is physically spaced away from the mill shell, and the processing arrangement may be located on a control panel or in a control room.

[0055] The signal input may be transmitted wirelessly to the processing arrangement.

[0056] Conveniently, a radio-frequency arrangement, for example, a wireless UHF radio frequency such as a Bluetooth arrangement, may be used for transmitting the signal input wirelessly to the processing arrangement.

[0057] Instead, the signal input from the one or more acoustic and / or vibration sensors may be transmitted by actual wiring and / or hard wiring to the processing arrangement.

[0058] The processing output may be transmitted from the processing arrangement to a mill operator over a computer network and the processing output may be presented visually and / or audibly. In one example form, the processing output may be presented graphically on a screen. In another example form, the processing output may be presented visually in terms of words and numbers indicating wear. Alternatively, or additionally, the processing output may comprise an audible warning that a predetermined wear threshold has been reached or exceeded.

[0059] Conveniently, the computer network may be the internet.

[0060] Instead of or in addition to measuring wear of the stirrer, the measure of changes in shape and / or dimension of the stirrer may indicate some damage to the stirrer.

[0061] This is particularly the case where a radial distance from a radially outer part of the stirrer (for example, but not limited to, a radially outer edge part or radially outer edge of the blade) to an internal surface of the mill shell changes abruptly particularly at a position other than an end thereof.

[0062] A sudden change in the sounds and / or vibrations of the signal input generated by the sensor / s over a short space of time may indicate damage to the stirrer.

[0063] The sensing arrangement may include any one or more of the features or combinations of features described in any other aspect of the invention.

[0064] According to another aspect of the invention there is provided a method of sensing a change in a stirrer within a stirred mill comprising an external mill shell defining an internal grinding chamber, and a stirrer mounted for rotation within the grinding chamber, the method comprising: sensing acoustic and / or vibration energy within the grinding chamber, by means of at least one acoustic and / or vibration sensor mounted on an external surface of the mill shell, during operation of the mill, to generate a signal input; forwarding the signal input sensed by the at least one acoustic and / or vibration sensor to a processing arrangement; and processing the signal input received from the at least one acoustic and / or vibration sensor using a relationship between the signal input sensed by the acoustic and / or vibration sensor and a shape and / or dimension of the blade to generate a processing output comprising information on the shape and / or dimension of at least a part of the stirrer. The at least one acoustic and / or vibration sensor may comprise a plurality of acoustic and / or vibration sensors.

[0065] The acoustic and / or vibration sensors of the plurality of acoustic and / or vibration sensors may be spaced apart from each other in a direction corresponding to an axial direction of the stirrer.

[0066] The sensing arrangement may be for a stirred mill in which the stirrer comprises a screw agitator having a screw axis and a blade for rotation within the grinding chamber.

[0067] The at least a part of the stirrer may comprise at least one radially outer part of the blade.

[0068] Instead, in another form, the sensing arrangement may be for a stirred mill in which the stirrer comprises a shaft and a plurality of discrete structural elements which extend outwardly from the shaft for rotation within the grinding chamber.

[0069] Processing the signal input received from the plurality of acoustic and / or vibration sensors may comprise generating information on the shape and / or dimension of the stirrer along its full length.

[0070] Processing the signal input received from the sensors may comprise generating information on a change in the shape and / or dimension of the stirrer occurring over a length of time, through use of the mill, which provides a measure of wear over the length of time.

[0071] Processing the signal input received from the at least one acoustic and / or vibration sensor may further comprise generating a measure of the distance between a radially outer part of the stirrer and an inner surface of the mill shell (R tank - R max stirrer) at the position along the length of the stirrer of each acoustic and / or vibration sensor, and an increase in said distance (R tank - R max stirrer) over time provides a measure of wear of the stirrer.

[0072] Further, instead of providing a measure of wear, providing a measure of changes in shape and / or dimension of the stirrer may indicate there is damage to the stirrer.

[0073] The method may further include predicting when the blade will reach its wear limit based on the wear pattern over a period of time. The predicting may include extrapolating the rate of wear of the stirrer, e.g. as measured by the processing output into the future and determining a date of end use, e.g. when the wear limit is reached.

[0074] The method may include any one or more of the features, or combinations of features, of the mill and / or sensing arrangement described in any other aspect of the invention. According to another aspect of the invention there is provided a sensing arrangement for a stirred mill comprising an external mill shell defining an internal grinding chamber, a stirrer mounted for rotation within the grinding chamber and a grinding media charge contained within the internal grinding chamber in use, the sensing arrangement comprising: at least one acoustic and / or vibration sensor mounted on an external surface of the external mill shell, the at least one sensor being responsive to sounds and / or vibrations within the grinding chamber to generate a signal input; and a processing arrangement for receiving the signal input from the at least one acoustic and / or vibration sensor / s, and then using the signal input to determine whether grinding media charge is present at a height of the or each sensor and thereby generate a processing output relating to a level of the grinding media charge in the grinding chamber.

[0075] Applicant has discovered that the acoustic and / or vibrational output sensed by the acoustic and / or vibration sensors is influenced by the presence of grinding media charge within the grinding chamber adjacent to the sensor. For example, in at least one instance, the acoustic and / or vibration energy generated within the mill shell above the surface of the charge is lower than the energy generated below the surface. Consequently, the signals sensed by acoustic and / or vibration sensors at or below a surface of the acoustic and / or vibration charge are different to those sensed by sensors above the surface of the ball charge, providing a way of measuring the height of the ball charge within the grinding chamber.

[0076] The at least one acoustic and / or vibration sensors may comprise a plurality of acoustic and / or vibration sensors mounted on an external surface of the external mill shell that are spaced from each other in at least a vertical direction.

[0077] The sensing arrangement may include any one or more of the features, or combination of features, of a sensing arrangement as described in any other aspect of the invention.

[0078] According to yet another aspect of the invention there is provided a method of measuring a level of grinding media charge in a stirred mill comprising an external mill shell defining an internal grinding chamber, the method comprising: sensing acoustic and / or vibration energy within the grinding chamber during operation of the mill using at least one acoustic and / or vibration sensor mounted on an external surface of the mill shell to generate a signal input; transmitting the signal input, e.g. sensed by the least one acoustic and / or vibration sensor, to a processing arrangement; and processing the signal input received from the least one sensor to determine whether the grinding media charge is present at a height of the or each sensor, and to generate a processing output relating to the level of the grinding media charge, e.g. the ball charge, in the grinding chamber.

[0079] The sensing of acoustic and / or vibrational energy, which may be in the form of acoustic and / or vibration waves, within the grinding chamber may comprise sensing with a plurality of sensors mounted on an external surface of the mill shell.

[0080] In an embodiment, the plurality of sensors may be spaced from each other in a direction of a stirrer within the stirred mill, e.g. a vertical direction.

[0081] The method may include any one or more of the features, or combinations of features, of the mill and / or sensing arrangement and the method described in any other aspect of the invention.

[0082] The invention also extends to a stirred mill comprising an external mill shell defining an internal grinding chamber, a stirrer mounted for rotation within the grinding chamber, and a sensing arrangement as defined in any one of the preceding aspects of the invention.

[0083] The stirred mill may further comprise a grinding media charge received within the grinding chamber.

[0084] The grinding media charge may comprise grinding balls having a selected density.

[0085] The stirrer may comprise a screw agitator having a screw axis and a blade for rotation within the grinding chamber. The at least a part of the stirrer may comprise at least one radially outer part of the blade. The blade may comprise a blade base and a set of replaceable wear liners mounted on the blade base that are subject to wear during operation of the mill.

[0086] The stirred mill may be a vertical stirred mill and the stirrer may have a rotational axis which is substantially vertically extending. The vertical stirred mill may include a drive arrangement extending away from an upper end of the mill shell that is driveably connected to the stirrer.

[0087] Instead, the stirrer may comprise a shaft and a plurality of discrete structural elements, e.g. pins or frames, which extend outwardly from the shaft for rotation within the grinding chamber.

[0088] The mill and the sensing arrangement may include any one or more of the features or combinations of features described in any other aspect of the invention.

[0089] BRIEF DESCRIPTION OF DRAWINGS

[0090] A sensing arrangement for a stirred mill, and a stirred mill, that is a vertical mill in accordance with this disclosure may manifest itself in a variety of forms. It will be convenient to hereinafter describe several embodiments of the disclosure in detail with reference to the accompanying drawings. The purpose of providing this detailed description is to instruct persons having an interest in the subject matter of the invention how to carry the disclosure into practical effect. However, it is to be clearly understood that the specific nature of this detailed description does not supersede the generality of the preceding broad description. In the drawings:

[0091] Figure 1 is a schematic perspective view of a stirred mill that is a vertical mill known in the prior art;

[0092] Figure 2 is a schematic front view of a vertical mill similar to that in Figure 1 (with some parts omitted for clarity) showing part of a screw agitator within an internal grinding chamber and grinding balls within the chamber;

[0093] Figure 3a is a front view of a screw agitator for vertical mill like that in Figure 2;

[0094] Figures 3b and 3c each provide schematic drawings of a screw agitator with a blade prior to use and then after the blade has been worn down;

[0095] Figure 4 is a schematic drawing of a vertical mill fitted with a sensing arrangement in accordance with one embodiment of the invention and showing a line of sensors arranged on an external surface of the mill shell;

[0096] Figure 5 is a schematic drawing of various components of the sensing arrangement of Figure 4 illustrating how the various components are operatively connected to, and interact with, each other;

[0097] Figure 6 is a schematic drawing showing the processing output from each of the sensors over a period of time; and

[0098] Figure 7 is a schematic drawing showing how the signal input received from the sensor can be processed to generate three forms of graphical output. DETAILED DESCRIPTION

[0099] Figures 1 to 3 illustrate a stirred mill that is a vertical tower mill in accordance with the prior art. These figures illustrate some basic structural features of a vertical tower mill and help to facilitate an understanding of the detailed description below.

[0100] The basic vertical mill 10 comprises a mill shell 12 having a vertically extending axis 14, mounted on a support surface 16. A stirrer in the form of a screw agitator 18 is mounted in the mill shell 12 and extends into an interior grinding chamber 19 within the mill shell 12.

[0101] The screw agitator 18 has a blade 20 extending helically around a central shaft 22 that forms a central axis of rotation. The blade 20 extends from an operatively upper end adjacent its mounting to the mill shell 12 to a lower end that is freely suspended in the internal grinding chamber 19. The blade 20 is sized and configured to be received within the mill shell 12 with some clearance. As illustrated, more than one blade may be provided, and in an embodiment, as shown in Figure 3b, two helical blade parts may be provided in a double-helix configuration. Further, it will be appreciated that the mill shell 12 can be regarded as defining the internal grinding chamber (notwithstanding that internal wear liners (not shown) may be provided to avoid excessive wear of the mill shell).

[0102] In the illustrated embodiment, a drive arrangement 26 for driving rotation of the screw agitator 18 is mounted on an upper end of the mill shell 12 and projects up away from the mill shell 12. The drive arrangement 26 is typically directly coupled to the screw agitator 18 and drives rotation of the screw agitator 18 within the interior grinding chamber 19.

[0103] The mill shell 12 has an openable wall portion 28 towards its lower end that can be opened to provide access to the grinding chamber 19 within the mill 10. Figure 1 shows the wall portion 28 in an open position. Figure 2 omits the wall portion 28 providing a schematic view into the interior grinding chamber 19 of the mill 10.

[0104] Figure 2 shows the interior grinding chamber 19 filled with a grinding media in the form of ball charge 30 comprising grinding balls that are packed up to a certain height in the interior space 19 called the ‘static height’. It will be appreciated that the ball charge is shown as if the wall portion 28 is in a closed position. The grinding balls of the ball charge 30 are lifted and displaced by the agitator screw 18, and this causes the grinding balls to grind the particulate material to be broken up. The left-hand side of Figure 3a depicts an unworn liner forming a part of the screw agitator 18 and the blade 20.

[0105] The blade 20 comprises a blade base and a set of replaceable wear liners mounted over the blade base. The liners contact the ball charge 30 and the particulate material within the mill 10 and wear down over time. As the liners wear down, the external shape of the blade 20 changes and this is particularly pronounced towards a lower region 20B of the blade 20. In this description, we will refer simply to the blade, which shall be understood to be the liners and the blade base, and wearing down of the blade which shall be understood to be wearing down of the liners, recognizing that the underlying blade base is typically not worn during normal operation of a stirred mill. It is easier to refer to the blade simpliciter in this description while understanding that the blade does include the wear liners that wear down progressively over time. Similarly, a stirrer other than a screw agitator, may include one or more wear liners and, in this document, such liners are considered parts of the stirrer, and wear of the liners is considered wear of the stirrer.

[0106] In a worn screw agitator 18, the radius of an outer edge of the blade 20 in the lower region 20B is considerably smaller than that of the blade 20 in the upper region 20A. This is due to the additional work (of displacing and lifting grinding balls) done by the blade 20 in the lower region 20B, which wears away the wear liners on lower region 20B of the blade 20 more quickly than the upper region 20A. The weight of the grinding balls 30 above the blade 20 in the lower region 20B is greater than that in the upper region 20A and this causes significantly higher rates of wear. Figures 3a, 3b and 3c all show how the operatively lower region 20B of the blade 20 is worn down relative to an operatively upper region 20A of the blade 20.

[0107] In Figures 3b and 3c, the left drawing of each pair of drawings shows an unworn screw agitator 18 prior to use, and the blade 20 has the same radial extent in the operatively lower region 20B as the operatively upper region 20A thereof. By contrast, the drawing on the right of each pair depicts a worn liner, and the lower region 20B of the blade 20 has a considerably smaller radius than the upper region 20A. Further, these drawings also show why the reduction in size of the lower region 20B of the blade 20 causes reduced effectiveness in lifting and displacing the grinding balls 30. There is reduced surface area of the blade 20 to do this work and consequently a greater gap between the blade and the mill shell 12.

[0108] It should be appreciated that although the detailed description uses the example of a vertical tower mill with a screw agitator, the invention has utility in many other types of stirred mills, including stirred mills which have different types of stirrer, for example, as described elsewhere herein with reference to HIG mills and SMD mills and horizontal stirred mills.

[0109] Figures 4 and 5 illustrate a stirred mill that is a vertical mill 10 having a sensing arrangement installed thereon in accordance with one embodiment of the invention.

[0110] The sensing arrangement, which is indicated generally by numeral 50, comprises a plurality of acoustic and / or vibration sensors 52 which are mounted on an external surface 54 of the mill shell 12 and which are spaced from each other in a vertical direction. It also includes a processing arrangement indicated generally by reference numeral 60 for receiving signal input from the plurality of acoustic and / or vibration sensors 52 and processing the signal input to generate a processing output. The processing output generated from the signal input is reflective of conditions within the mill 10.

[0111] As shown in Figure 4, the plurality of acoustic and / or vibration sensors comprises six to ten sensors 52 arranged on an external or outer surface of the mill shell in a line substantially parallel to the axis of the stirrer or agitator 18, which in the illustrated embodiment extends in a vertical line up the height of the mill shell 12. Applicant reiterates that this is only an example embodiment and the number of sensors used in different applications may vary. Optionally, some or all the sensors 52 may be mounted on a mounting element, such as a board-like planar element, having two major surfaces. The sensors may be mounted on one major surface and the other major surface may be mounted on the external surface of the mill shell 12. The mounting element may be configured to be complementary to the external surface 54 of the mill shell 12, which may include irregularities, for example, to meet structural considerations for the mill shell.

[0112] Conveniently, each acoustic and / or vibration sensor 52 may comprise an acoustic camera. An acoustic camera (which is an item of off the shelf equipment) comprises a group of microphones (called a ‘microphone array’) and has an ability to determine the spatial location of sound sources. While the invention may utilize an application of an acoustic camera, the structure and function of an acoustic camera perse does not form part of the invention and will not be described in further detail in this detailed description. As with the acoustic camera, one other option may be to use a laser as a microphone, and it will be appreciated that use of a laser to detect sound at a surface is known per se.

[0113] In other embodiments, other forms of acoustic and / or vibration sensors may be used, such as a basic microphone or an accelerometer, either by themselves or in combination with one or more acoustic cameras.

[0114] As illustrated in Figure 5, the processing arrangement 60 comprises broadly a processor such as an industrial PC or edge computer. The sensors 52 may communicate wirelessly with the processing arrangement 60 by means of a Bluetooth communication arrangement indicated generally by reference numeral 64. As the structure and function of such a wireless Bluetooth communication arrangement is well known in the art, it will not be described further in this description.

[0115] The processing arrangement 60 generates a processing output which is then communicated to the mill operator or a monitoring service (for example, operated by a supplier of replacement stirrers or stirrer liners) optionally over a network, such as the internet, indicated by numeral 61 . In one embodiment, the processing output comprises a graphical output indicated by numeral 62 showing a measure of change in a shape and / or dimension of the screw agitator resulting from use of the mill. However, instead of, or in addition to, a graphical output, the processing output could be used to provide a rough or lump measurement or basic parameter that provides a simple, concise and / or quickly comprehensible measurement of wear. Alternatively, or additionally, the processing output may include an audible warning when a wear threshold has been reached or exceeded.

[0116] As described above, in a particular embodiment, the stirrer is in the form of a screw agitator 18 comprising a blade 20 extending helically around a central axis 22. The screw agitator 18 may have a free end (e.g. an operatively lower end) remote from its mounting to the mill shell 12 and the blade may extend from the lower end up to a desired height. In one example form, the blade comprises a set of replaceable wear liners mounted on a blade base. However, as also described above, the sensor system does have utility with other forms of stirrer.

[0117] In another embodiment very similar to that illustrated in Figure 5 above that has not been illustrated, the sensors 52 may be connected to the sensor receiver and / or the edge computer 60 by a wired, rather than wireless, arrangement. If desired, the sensors may be physically connected to the sensor receiver and / or the edge computer. This embodiment functions equally efficaciously with the embodiment illustrated in Figure 5 and described above.

[0118] In use, in the illustrated embodiment, as the wear liners on the blade 20 are worn down, the radius of the outer edge of at least part of the blade 20 decreases as illustrated in Figures 3a to 3c and the radial distance from the worn radially outer edge of the blade 20 to the mill shell 12, e.g. an internal surface of the mill shell, increases. The increase is more pronounced in an operatively lower region 20B of the blade 20 where the weight of the ball charge 30 on the blade 20 is greater than other regions.

[0119] The processing output for each sensor 52 provides information on the blade 20 at the height of that specific sensor 52. In this embodiment, this provides a measure of the distance from a radially outer edge of the blade 20 to an inner surface of the mill shell 12 at the height of that specific sensor 52.

[0120] In this embodiment, processing the signal input from the sensors 52 provides a measure of a distance from the radially outer edge of the blade 20 to the mill shell 12. This, in turn, can be developed to obtain a measure of the wear of the blade 20 at the height of each sensor 52 over an extended length of time. This can be further extended to develop a full picture of wear of the blade 20 along the length of the blade. This can also be used to detect or predict when the blade 20 has reached its wear limit.

[0121] Figure 6 shows how the signal input from each of the sensors 52 mounted on the mill shell 12 is processed by the processing arrangement to produce a processing output. It graphically depicts how the output derived from each sensor 52 changes from one week to the next week over an extended period of many such weeks which, in turn, indicates changes in a shape and / or dimension of the blade 20.

[0122] Figure 6 plots vibrational and / or sound energy changes from each of the sensors 52 (which occupy different vertical heights) against the length of time of use of the mill 10. The graph shows how the vibrational energy or frequency changes over time during operation of the mill 10. A decrease in vibrational energy being sensed by a sensor 52 indicates liner wear at that height within the mill shell 12.

[0123] In Figure 6, the eight (by way of example) sensors 52 are designated by letters A to H, with A being the highest sensor and H being the lowest sensor 52. Figure 6 shows the graph plotted for sensor H at the bottom of the shell 12 which decays markedly over time, providing an indication of a relatively high rate of wear of the blade 20 at the height of sensor H. By contrast, the graph plotted for sensors A to D which are located on an upper region of the mill shell 12 does not show much change over the same length of time. This indicates that the blade 20 did not wear much at the height thereof corresponding to the height of these sensors. Further, the graph plotted for sensors E, F and G that are more centrally positioned, about midway up the height of the stirrer, shows an intermediate level of wear over the same length of time. With appropriate calibration, which may, for example, be determined empirically, experimentally, or by any desired and suitable modelling approach, this can be used to provide a useful measure of wear of the blade or stirrer, most importantly in the high-wear region that is proximate to and measured by sensor H.

[0124] In one example form in which the method can be implemented, the signal input from a sensor can be integrated over a full day to determine a daily average of the signal which can then be used in the graph that plots the daily average against time for a number of days and weeks.

[0125] Figure 7 illustrates schematically how a signal input received from the sensor can be processed to generate three forms of graphical output.

[0126] In the first, top, graph, vibration power spectral density (Vib PSD) in g2 / Hz is plotted against frequency. In a colour representation, the stripes shown at the right-hand side of the graph may be represented by the colours of the spectrum, from red (at the bottom) through the colours of the visible spectrum, to violet (at the top).

[0127] In the second, middle, graph, vibrational energy (or average vibrational energy, as described above) is plotted against a length of time since measurement started expressed in days. This shows a steady decrease in vibrational energy over time indicating wear.

[0128] In the third, bottom, graph, the central frequency is plotted against a length of time since measurement started expressed in days.

[0129] In another application different to that described immediately above for measuring the wear of a stirrer, the sensing arrangement shown in Figures 4 and 5 can be used to identity damage to the stirrer and notify operators of the mill 10 of the damage to the stirrer.

[0130] When the stirrer, which may for example be a blade gets damaged, the damage often changes the shape and configuration of the stirrer at a position along its length. In particular, the processing output may detect a change in configuration of a part of a stirrer, e.g. a blade of a screw agitator if it is broken or damaged. The sensing arrangement in Figures 5 can identify this change in the stirrer through the processing output from the sensors. Further, this method can also identify a position along the length of the blade 20 where the damage has occurred. An ability to detect an abnormality on the stirrer soon after the abnormality develops without having to shut down the mill for an internal inspection is naturally advantageous. As stated above, the sensing arrangement can also recognize abnormalities in other types of stirrers.

[0131] In yet another embodiment with a different application, the sensing arrangement can be used to provide a measure of the level of grinding media charge, e.g. a ball charge, within the grinding chamber. A way in which the sensing arrangement can be used to provide a measure of the level of ball charge within the grinding chamber of a vertical mill will now be described below.

[0132] Applicant has established that the signal input provided by a sensor 52 positioned at a height above a surface of the grinding medium charge 20 is different to that provided by a sensor 52 below the surface thereof. More specifically, there is a significantly lower level of energy detected by sensors 52 positioned above the surface of the charge of grinding media and mineral material, than the level of energy sensed by sensors below the surface. This provides a basis for determining the dynamic height of the grinding media charge within the mill shell when it is in use. This is different to and more useful than the static height of the grinding media when the mill is at rest.

[0133] The processing means 60 processes the signal input from the plurality of sensors 52 and determines an uppermost sensor 52 that senses its proximity of the grinding media charge 30 and / or a lowermost sensor that does not sense a proximity to the grinding media charge 30 at that height. This information provides a means for inferring the height of the level of grinding media charge in the mill 10. Further, it will be appreciated this information can then be provided to operators on a continuous basis to allow them to fine tune the addition of grinding media to the mill and thereby optimize operation of the mill.

[0134] It should be appreciated that although the illustrated embodiment relates to a system with a plurality of sensors, a system in accordance with the present disclosure can be implemented using a single acoustic and / or vibration sensor. For example, if a part of the stirrer which is subject to most rapid rate of wear is known, a single sensor may be provided at a corresponding position on the mill shell to monitor the wear of that part. While this may forego some of the information provided by the illustrated embodiment, including the monitoring of low-wear regions of the stirrer for unexpected damage, this may nonetheless provide valuable, non-intrusive, continuous monitoring of a highest-wear part of the stirrer. In a system intended only to monitor the height of the grinding media charge, it may be sufficient to provide a single acoustic and / or vibration sensor at the desired dynamic charge height, enabling monitoring of whether the height of the grinding media charge is below or above that desired height. Further, an arrangement with two sensors, one adjacent the highest-wear region of the stirrer (to monitor wear of that region of the stirrer) and one at the desired dynamic charge height (to monitor whether the dynamic charge height falls below that desired height) is also within the scope of the present disclosure.

[0135] In yet another embodiment, the stirred mill comprises a stirrer with a central shaft and radially extending structures such as pins, or frames arranged along its length, the sensor or sensors may be provided at positions on the outside of the mill shell which are close to the part or parts of an internal wall of the mill shell closest to the radially outer parts of the radially extending structures. Thus, in this embodiment, the sensors should be positioned at a (lengthwise) point on the mill shell that is close to the discrete elements of the stirrer.

[0136] A working advantage of the sensing arrangement described above with reference to the drawings, or of a vertical mill equipped with such an arrangement, is that it is capable of sensing wear of the blade (or other stirrer), and more specifically, in the illustrated embodiment, the wear liners on the blade, non-invasively. That is, it can sense the wear on the blade (or other stirrer) at different points along its length, from outside the mill shell. Further, it does this while the mill is operational and is processing material. This is a huge advantage because the mill does not need to be shut down and there is no lost production.

[0137] Another working advantage of the vertical mill described above with reference to the drawings is that it can detect when an operatively lower region of the blade (i.e. towards its terminal free end and remote from the drive arrangement) is worn down to the extent that it is adversely affecting operational efficiency of the mill and needs to be replaced. Further, the mill can provide continuous information on wear of the blade along its length and this enables wear to be continuously monitored or monitored periodically with any desired frequency.

[0138] Further, yet another working advantage is that the sensing arrangement and the sensing method can also be used to predict the time when the blade will reach its wear limit based on the wear trend.

[0139] Another working advantage of the vertical mill described above is that an abnormality in the blade can be detected quickly during operation of the mill without shutting the mill down. For example, if part of a blade (or other stirrer) is damaged, there will be a change in configuration of the blade which will lead to a change in the acoustic and / or vibrational energy which will be picked up the sensors, and as a result remedial action can be taken at an early stage. Another working advantage of the vertical mill described above with reference to the drawings relates to its ability to work on mills having variable speed motors. Traditionally, vertical mills have operated with constant speed motors which rotated the stirrer at a constant speed. Traditional prior art techniques or correlations for estimating wear of the blade developed over time assume that a screw agitator is rotating at a constant speed. However, recent developments in vertical mills have introduced variable speed motors which enable the operator to vary the power draw and save energy. It is therefore important that a modern system for measuring blade wear is suitable for use with variable speed motors. The system and method developed by the Applicant and described in this application with reference to the drawings is not dependent on a constant speed motor being used to drive the stirrer. Rather, it is based on the acoustic output generated directly by the rotating blade, which is equally efficacious when the stirrer rotates at different speeds. It is thus suitable for use on vertical mills having variable speed motors.

[0140] Yet another working advantage of the sensing arrangement and vertical mill described above is that the level of charge and grinding media in the mill can also be measured without the mill being stopped. Further, this information on the level of grinding media charge (for example, ball charge) can be provided to operators frequently, or even continuously if desired. This technique helps mill operators to respond quickly and effectively to a drop off in volume of grinding media charge, e.g. ball charge by adding a suitable amount of grinding media to top up the grinding media charge and operate the mill at a high efficiency.

[0141] Yet another working advantage of the vertical mill described above with reference to the drawings is that the sensing arrangement can be retrofitted to existing stirred mills. It is basically mounted on an external surface of the shell of the mill and is non-invasive. It can be speedily installed on an existing vertical mill and be operational straight away. It does not require any modification of the stirred mill and additionally it is not capital intensive to install.

[0142] It will of course be realized that the above has been given only by way of illustrative example of the invention and that all such modifications and variations thereto, as would be apparent to persons skilled in the art, are deemed to fall within the broad scope and ambit of the invention as is herein set forth.

Claims

CLAIMS:1 . A sensing arrangement for a stirred mill comprising a mill shell defining an internal grinding chamber, and a stirrer for rotation within the grinding chamber, the sensing arrangement comprising: at least one acoustic and / or vibration sensor mounted on an external surface of the mill shell in use, the at least one sensor being responsive to sounds and / or vibrations within the grinding chamber to generate a signal input; and a processing arrangement for receiving the signal input from the at least one acoustic and / or vibration sensor and utilizing a relationship, between the signal input sensed by the at least one acoustic and / or vibration sensor and a configuration and / or dimension of at least a part of the stirrer, to generate a processing output comprising information on the shape and / or dimension of the stirrer.

2. A sensing arrangement for a stirred mill according to claim 1 , wherein the at least one acoustic and / or vibration sensor comprises a plurality of acoustic and / or vibration sensors.

3. A sensing arrangement for a stirred mill according to claim 2, wherein the acoustic and / or vibration sensors of the plurality of acoustic and / or vibration sensors are spaced apart from each other on the external surface in a direction corresponding to an axial direction of the stirrer.

4. A sensing arrangement for a stirred mill according to any one of claims 1 to 3, wherein the sensing arrangement is for a stirred mill in which the stirrer comprises a screw agitator having a screw axis and a blade for rotation within the grinding chamber.

5. A sensing arrangement for a stirred mill according to claim 4, wherein said at least a part of the stirrer comprises at least one radially outer part of the blade.

6. A sensing arrangement for a stirred mill according to any one of claims 1 to 3, wherein the sensing arrangement is for a stirred mill in which the stirrer comprises a shaft and a plurality of discrete structural elements which extend outwardly from the shaft for rotation within the grinding chamber.

7. A sensing arrangement for a stirred mill according to any one of claims 1 to 6, wherein the processing output resulting from the signal input of the at least one sensor provides information on a change in the shape and / or dimension of the stirrer resulting from use of the mill over alength of time, which provides a measure of wear over the length of time.

8. A sensing arrangement for a stirred mill according to any one of claims 1 to 7, wherein the at least one acoustic and / or vibration sensor comprises a plurality of acoustic and / or vibration sensors, and the processing output resulting from the signal inputs of the plurality of sensors provides a measure of wear along the length of the stirrer.

9. A sensing arrangement for a stirred mill according to any one of claims 1 to 8, wherein the processing output comprises a measure of the distance between at least one part of the stirrer and an inner surface of the mill shell (R tank - R max stirrer).

10. A sensing arrangement for a stirred mill according to any one of claims 1 to 9, wherein the at least one acoustic and / or vibration sensor comprises a plurality of acoustic and / or vibration sensors, and the processing output comprises a measure of the distance between a plurality of parts of the stirrer and an inner surface of the mill shell (R tank - R max stirrer) at a position of each acoustic and / or vibration sensor.

11. A sensing arrangement for a stirred mill according to claim 9 or claim 10, wherein an increase in the distance between said part of the stirrer and the inner surface (R tank - R max stirrer) over the length of time provides a measure of wear of the stirrer.

12. A sensing arrangement for a stirred mill according to any one of claims 1 to 11 , wherein the processing output generated by the processing arrangement represents an amount of acoustic and / or vibration energy from the mill sensed by an acoustic and / or vibration sensor, and a decrease in the amount of acoustic and / or vibration energy over time indicates an increase in the distance between a radially outer part of the stirrer and an inner surface of the mill shell.

13. A sensing arrangement for a stirred mill according to any one of claims 1 to 12, wherein the at least one sensor senses sounds and / or vibrations generated within the grinding chamber periodically, and / or substantially continuously, and the signal inputs from the at least one sensor are transmitted periodically, and / or substantially continuously, to the processing arrangement.

14. A sensing arrangement for a stirred mill according to any one of claims 1 to 13, wherein a plurality of acoustic and / or vibration sensors are arranged substantially in a line along the external surface of the mill shell.

15. A sensing arrangement for a stirred mill according to any one of claims 1 to 14 whereinthe at least one acoustic and / or vibration sensor comprises at least four acoustic and / or vibration sensors.

16. A sensing arrangement for a stirred mill according to any one of claims 1 to 15, wherein the processing arrangement comprises a computer that is positioned spaced away from the sensor.

17. A sensing arrangement for a stirred mill according to any one of claims 1 to 16, wherein the signal input is transmitted wirelessly to the processing arrangement.

18. A sensing arrangement for a stirred mill according to any one of claims 1 to 17, wherein the processing output is transmitted from the processing arrangement to one or more operating or monitoring personnel over a computer network, and wherein the processing output is presented visually or audibly.

19. A method of sensing a change in a stirrer within a stirred mill comprising a mill shell defining an internal grinding chamber, and a stirrer mounted for rotation within the grinding chamber, the method comprising: sensing acoustic and / or vibration energy within the grinding chamber, by means of at least one acoustic and / or vibration sensor mounted on an external surface of the mill shell, during operation of the mill, to generate a signal input; forwarding the signal input sensed by the at least one acoustic and / or vibration sensor to a processing arrangement; and processing the signal input received from the at least one acoustic and / or vibration sensor using a relationship between the signal input sensed by the sensor and a shape and / or dimension of the blade to generate a processing output comprising information on the shape and / or dimension of at least a part of the stirrer.

20. A method of sensing a change in a stirrer within a stirred mill according to claim 19, wherein the stirrer comprises a screw agitator having a screw axis and a blade for rotation within the grinding chamber.

21. A method of sensing a change in a stirrer within a stirred mill according to claim 19 or claim 20, wherein the at least one acoustic and / or vibration sensor comprises a plurality of acoustic and / or vibration sensors.

22. A method of sensing a change in a stirrer within a stirred mill according to claim 21 ,wherein the plurality of acoustic and / or vibration sensors are spaced apart from each other in a direction corresponding to an axial direction of the stirrer.

23. A method of sensing a change in a stirrer within a stirred mill according to claim 21 or claim 22, wherein said processing the signal input received from the sensors comprises generating information on the configuration and / or dimension of the stirrer at a plurality of positions along a length of the stirrer.

24. A method of sensing a change in a stirrer within a stirred mill according to claim 20, wherein said processing the signal input further comprises generating information on a change in configuration and / or dimensions of the blade occurring over a length of time, through use of the mill, which provides a measure of wear over said length of time.

25. A method of sensing a change in a stirrer within a stirred mill according to any one of claims 19 to 24, wherein said processing the signal input further comprises generating a measure of the distance between a radially outer part of the stirrer and an inner surface of the mill shell (R tank - R max stirrer) at the position of each acoustic and / or vibration sensor, and an increase in said distance (R tank - R max stirrer) over time provides a measure of wear of the stirrer.

26. A sensing arrangement for a stirred mill comprising a mill shell defining an internal grinding chamber, a stirrer mounted for rotation within the grinding chamber and a grinding media charge contained within the internal grinding chamber in use, the sensing arrangement comprising: at least one acoustic and / or vibration sensor mounted on an external surface of the mill shell in use, the at least one sensor being responsive to sounds and / or vibrations within the grinding chamber to generate a signal input; and a processing arrangement for receiving the signal input from the at least one acoustic and / or vibration sensor / s, and then using the signal input to determine whether grinding media charge is present at a height of the or each sensor and generate a processing output relating to a level of the grinding media charge in the grinding chamber.

27. A sensing arrangement for a stirred mill according to claim 26, wherein the at least one acoustic and / or vibration sensor comprises a plurality of acoustic and / or vibration sensors, mounted on an external surface of the mill shell, that are spaced from each other in at least a vertical direction.

28. A method of measuring a level of a grinding media charge in a stirred mill comprising amill shell defining an internal grinding chamber, the method comprising: sensing acoustic and / or vibration energy within the grinding chamber during operation of the mill by at least one acoustic and / or vibration sensor mounted on an external surface of the mill shell, wherein the sensor / s generate a signal input; transmitting the signal input to a processing arrangement; and processing the signal input received from the sensor / s to determine whether the grinding media charge is present at the height of each sensor, and to generate a processing output indicating the level of the grinding media charge in the grinding chamber.

29. A method of measuring a level of a grinding media charge in a stirred mill according to claim 28, wherein said sensing of acoustic and / or vibration energy within the grinding chamber comprises sensing with a plurality of acoustic and / or vibration sensors mounted on an external surface of the mill shell, wherein the sensors are spaced from each other in at least a vertical direction.30 A stirred mill comprising a mill shell defining an internal grinding chamber and a stirrer mounted for rotation within the grinding chamber, and a sensing arrangement as defined in any one of claims 1 to 18, 26 and 27.31 . A stirred mill according to claim 30, that is a stirred mill comprising a stirrer, wherein the stirrer comprises a screw agitator having a screw axis and a blade for rotation within the grinding chamber.

32. A stirred mill according to claim 31 , wherein the blade comprises a blade base and a set of replaceable wear liners mounted on the blade base that are subject to wear during operation of the mill.

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