WO2007124528A1

WO2007124528A1 - Method and apparatus for monitoring a mill

Info

Publication number: WO2007124528A1
Application number: PCT/AU2007/000378
Authority: WO
Inventors: Nenad Djordjevic
Original assignee: The University Of Queensland
Priority date: 2006-04-27
Filing date: 2007-03-23
Publication date: 2007-11-08

Abstract

A method of monitoring a charge including at least one phase within a mill, the method including gauging the resistivity of the phase that is immediately adjacent at least one region of an interior wall of the mill.

Description

METHOD AND APPARATUS FOR MONITORING A MILL

FIELD OF THE INVENTION

This invention relates to a method for monitoring a mill. This invention aiso relates to an apparatus for monitoring a mill. The method and apparatus may also advantageously be used for operating a mill.

This invention relates particularly but not exclusively to a method of monitoring a mill that is a SAG (semi-autogenous) grinding mill, or an AG (autogenous) grinding mill. The method of monitoring the mill involves developing an image of the position of the charge inside the mill in real time as it is tumbling. This information can then be used to operate the mill in a way to improve a mill's operating performance. It will therefore be convenient to hereinafter describe the invention with reference to this example application. However, it is to be clearly understood that the invention is capable of broader application.

BACKGROUND TO THE INVENTION

Mills are widely used in mineral processing operations around the world to reduce the size of ore particles that are generated by the basic mining operation.

For example, blasting with an explosive will result in breakage of the native ore body into pieces of rock. The size of rocks generated by blasting will differ from mine to mine and will depend on a number of factors such as rock type, the explosives used and the pattern of the explosives used.

Usually after blasting has taken place, the broken rock will have a mixture of large and small particles. That is, the size of the blasted rock pieces will vary greatly. The fragments of rock or ore are generally subjected to a particle size reduction operation, typically in a mill. One object of milling the fragments is to reduce the mean size of the fragments for further processing downstream of the mill. A further object is to narrow the size distribution of the fragments or particles so that the variation in particle size is not too great. Generally, a mill comprises a mill shell that is typically cylindrical and that rotates about a substantially horizontal axis. The particles are contained within the mill shell and tumble on rotation of the mill shell, while moving in a broadly longitudinal direction from an inlet of the mill to an outlet of the mill, in some mills the mill shell also contains steel balls that collide with the ore particles during tumbling and assist with breakage of the ore particles into smaller ore particles. In addition, in SAG and AG mills the ore particles themselves assist with breakage of the ore particles. In fact, in an AG mill the mill shell does not contain any mill balls and the rock and ore breakage occurs due to collisions between ore particles, and between ore particles and the inner wall of the mill shell.

Mills have a high capital running cost. Mill shells are generally driven to rotate by electrically powered motors and the electrical power required to run the mills is extremely high. The installed power of the largest mills is of the order of 10-12

MWatt. However, as a general statement, the milling process is very inefficient.

A small fraction of the mill input energy actually goes into breaking the ore particles. A large part of the energy is just dissipated in the mill. As energy prices rise a greater imperative is being placed on engineers to run mills more efficiently.

Current techniques for operating a mill rely largely on an operator's experience. Over time they learn which mill settings work best for certain sizes of ore particles and certain types of ore. However, the mill settings are crude and certainly do not go far towards optimising the operation of the mill.

Some mineral processing engineers recognise that in order to be in a position to improve or even optimise operation of the mill, engineers need to be able to monitor and/or visualise the position and motion of the ore particles and water (hereinafter called the charge) within the tumbling mill shell. The motion of the charge within the tumbling mill shell has a critical influence on the mode and intensity of the rock breakage in the charge, as well as wear of inner surface of the mill shell and mill balls.

Clearly, therefore, it would be advantageous if a method and apparatus could be devised that enables information about the position of the mill charge within the tumbling mill to be gathered, in particular the position of the toe and shoulder of the charge, which could then be used to improve operation of the mill.

It would be particularly advantageous if such a method could be devised that was non-invasive. A non-invasive method is one that involves measurement of a parameter of the mill from outside the mill rather than from within the interior of the mill. In that regard, it will be appreciated that the interior of a mill is a very harsh and aggressive environment with tumbling rocks and the like and internal measuring devices that project into, or that are located in the mill interior can be easily damaged.

Further, it would be advantageous if the information about the position of the mill charge could be provided in real time as distinct from in historical time. The provision of this information in real time would enable it to be used more readily to influence operation of the mill.

Yet further, it would be advantageous if precise and reliable information on the position of the charge within the tumbling mill could be provided. This would enable fine adjustments to be made to the operation of the mill in response to the information. This could then provide an environment for enabling the performance of the mill to be optimised.

SUMMARY OF THE INVENTION

According to one aspect of this invention there is provided a method of monitoring a charge including at least one phase within a mill, the method including gauging the resistivity of the phase that is immediately adjacent at least one region of an interior wall of the mill. The step of gauging the resistivity of the phase may comprise measuring the resistivity of the phase. For example this may be accomplished by passing a current over an external shell of the mill and obtaining a measure of the extent of current leakage from the external shell into the phase immediately adjacent the region of the interior wall of the mill.

As noted above, the method involves gauging resistivity of the phase that is immediately adjacent at least one region of the interior wall of the mill. In order to obtain a more complete picture of the movement of the charge within the mill, the method preferably includes gauging the resistivity of the phase that is immediately adjacent a plurality of regions of the interior wall of the mill. For example, the method may include obtaining a measure of the extent of current leakage from the external shell of the mill into the phase immediately adjacent a plurality of regions of the interior wall of the mill.

The regions may be positioned adjacent to each other and extending around the circumference of the mill shell, for example extending equidistantly around the circumference of the mill shell. That is, the regions may be positioned in turn as one travels around the circumference of the mill shell.

In a certain embodiment, the extent of current leakage may be measured in at least three regions of the mill shell. For example, there may be 4-8 regions, or5- 7 regions on the mill shell in respect of which current leakage is measured.

The step of obtaining a measure of the extent of current leakage may comprise measuring the voltage difference between spaced points on the mill shell. The step may include measuring the voltage difference between two spaced points in each said region on the mill shell.

A measured voltage difference between two points on the mill shell indicates leakage of current into the phase immediately adjacent the interior wall of the mill shell in that region. The voltage that is applied, for example across two electrodes, causes a current to flow across the mill shell between the two electrodes. The mill shell will generally be made of steel which is highly conductive. Consequently, if there is no current leakage there will be very little drop in voltage across the electrodes.

However, there may be some voltage drop between points on the shell depending on the resistivity of the material or phase immediately adjacent the interior wall of the mill shell between the electrodes. That is, a phase having a low resistivity can lead to current leakage into the phase and thereby a measurable voltage difference between the electrodes.

In one embodiment, the two electrodes are in electrical contact with the mill shell itself, for example at diametrically opposed points on the mill shell. The electrodes may be stationary and make contact by means of brushes that permit the mill shell to rotate past the stationary electrodes yet still make effective contact.

In another embodiment, one electrode is in electrical contact with the mill shell and the other electrode is positioned so as to make contact with liquid within the mill shell, or issuing from the mill shell in use. In this arrangement the current will flow over the mill shell and then through the liquid issuing from the mill shell to close the electrical circuit.

The step of measuring the leakage of electrical current into the phase immediately adjacent the interior wall of the mill shell may include measuring the voltage drop between a plurality of voltage electrodes. The voltage electrodes may be positioned on the mill shell so as to measure the voltage drop, if any, between points in each of the regions on the mill shell. The voltage drop, if any, between two spaced voltage electrodes in that region is a reflection of the resisitivity of the phase immediately adjacent the interior wall of the mill in any said region. By measuring the voltage difference between adjacent voltage electrodes a picture of the resistivity in each of the regions around the mill shell can be built up and from that the nature of the phase immediately adjacent the interior wall'of the mill, and therefore the position of the charge within the mill shell, can be deduced.

The current leakage into the phase immediately adjacent the interior wall of the mill is greatest in a given region when water is in contact with the interior wall of the mill shell. The current leakage into the phase immediately adjacent the interior wall of the mill is smallest when air is in contact with the interior wall of the mill shell. Further, the current leakage is at an intermediate level when a rock or ore is in contact with the interior wall of the mill shell.

The method may further include sending the measured voltage drops between adjacent voltage electrodes to a processor. If so, the voltage readings are preferably sent to the processor in real time.

The voltage electrodes may have means for transmitting the sensed voltage reading to the processor by wireless transmission, for example by telemetry equipment.

The step of determining which of the materials is in contact with the inner surface of the mill- at any given time may be carried out using the processor. The processor may be a central processing unit, for example a microprocessor, which may be incorporated in a computer.

The step of determining which phase, that is whether water, rock or air, is in contact with the interior wall of the mill shell at any given point may be carried out by comparing the voltage difference between adjacent voltage electrodes with data in the processor. For example, a nil voltage drop may indicate that air is in contact with the interior wall at that point; a voltage drop of at least 50 microvolt may indicate that water is in contact with the interior wall at that point; and a voltage drop of an intermediate amount, say 20-40 microvolt may indicate that solid rock from the charge, for example the ore, is in contact with the interior wall at that point.

The method may further include using the processor to build up an image of the charge within the mill while the mill is tumbling based on the determination of what phase is in contact with the interior wall of the mill shell in the regions being detected by the voltage electrodes.

The method advantageously enables the position of the charge within the mill at any one time to be determined. Over a period of time with repeated measurements at regular time intervals the method may include producing an image of the movement of the charge within the tumbling mill shell in real time.

Thus, according to a particular aspect of the invention there is provided a method of determining the distribution of a charge having at least one phase within a mill including: gauging the resistivities of phases that are immediately adjacent regions of an interior wall of the mill; and interpreting the resistivities gauged to determine the phases that are immediately adjacent the regions of the interior wall of the mill.

According to another aspect of this invention, there is provided an apparatus for monitoring a charge including at least one phase within a mill, the apparatus including:

at least one sensor in at least one region of the mill for enabling the resistivity of the phase immediately adjacent an internal wall of the mill to be gauged.

Preferably a number of sensors are provided so that resistivity of phases immediately adjacent the internal wall of the mill in a plurality of regions can be measured. The regions may be arranged in turn around the circumference of the mill. Preferably, the regions are adjacent each other and arranged end to end around the circumference of the mill.

The sensors for enabling the resistivity of the phase immediately adjacent the interior wall of the mill to be gauged may comprise sensors for sensing the leakage of current from the mill shell into the phase immediately adjacent the interior wall of the mill.

Specifically, the sensors may include voltage electrodes spaced apart from each other for measuring a voltage drop, if any, between pairs of the electrodes.

The sensor may be associated with an energised electrical circuit having a pair of supply electrodes that are spaced apart from each other on the outer surface of the mill shell and that are in electrical contact with the shell in use. That way an applied voltage and current can pass over the mill shell between the supply electrodes.

In one form both supply electrodes are mounted spaced apart from each other in electrical contact with the mill shell. The electrodes may be substantially diametrically opposed from each other. IrTthis form the supply electrodes may be in the form of brushes that permit the electrodes to slide over the surface of the mill shell while maintaining electrical contact therewith.

In another form one supply electrode is positioned to make electrical contact with the outer surface of the mill shell and the other supply electrode is positioned so as to make contact with liquid emitted from the mill in use. The supply electrodes may be electrically connected to each other in the usual way by a conductor to complete the circuit.

In this embodiment, the supply electrode that is in contact with the outer surface of the mill may be in the form of a brush to enable it to make electrical contact with the outer surface of the mill shell while it is rotating. Further, the apparatus may include a support for supporting the supply electrode and associated electrical lead in position on the outer surface of the mill shell. The support may be in the form of an arm.

Again in this embodiment, the other supply electrode that contacts the water emitted from the mill in use may be in the form of a plate. The plate may be positioned outside of the mill shell, for example adjacent the outlet of the mill shell. It simply needs to make contact with water issuing from the mill shell and does not need any special structure. The apparatus may include a further support for supporting this supply electrode in its position adjacent the outlet. The further support may also be in the form of an arm.

Naturally, the circuit also includes an electrical power supply coupled in series with the supply electrodes.

Thus, the energised circuit can apply a voltage across the two supply electrodes to cause electrical current to flow over the outer surface of the mill shell. The mill shell is a conductor and thus most of this charge will pass from the first supply electrode, around the outer surface of the shell and through the other supply electrode. However, some of the current may leak into the interior space of the mill shell depending on what material or phase is immediately adjacent the interior wall of the mill shell. This can give rise to small but real and measurable voltage drops between points on the mill shell depending on the amount of current leakage.

The apparatus preferably includes a plurality of voltage electrodes spaced apart from each other around the circumference of the mill shell, as described above. Adjacent pairs of the voltage electrodes define a region in the mill shell. By measuring the voltage difference between these adjacent voltage electrodes the current leakage in each of the defined regions can be gauged. Preferably, each voltage electrode is mounted in a fixed and static position on the outer wall of the mill shell. Thus, each voltage electrode rotates with the rotating mill shell in use.

Each voltage electrode is preferably capable of measuring very small amounts of voltage, for example in the order of 20-60 microvolt.

Further, each voltage electrode preferably includes means for communicating a voltage reading that has been sensed to a receiver. The sensor may transmit the reading wirelessly to a receiver by means of a transmitter, for example telemetry equipment.

The apparatus may further include a processor for receiving and processing the sensed voltage readings from the voltage electrodes, for example a microprocessor. The processor may include means for determining whether water, rock or air is in contact with the interior surface of the mill based on any given reading from a sensor, for example readings between a pair of voltage electrodes in a given region.

Further, the processor may include means for building up an image of the charge within the mill from the data received from the sensors. Further, the processor may produce a dynamic image of the charge within the mill shell in real time.

The processor may include a microprocessor and a memory storage device, for example operatively coupled to the microprocessor.

Thus, the voltage drop between adjacent voltage electrodes, for example each of the pairs of adjacent voltage electrodes, may be sensed a plurality of times for each rotation of the mill shell. The voltage drop between each electrode pair provides an indication of whether it is a rock, liquid or air in contact with the interior surface of the shell. This invention also extends to a method otoperating a mill including monitoring a- charge within the mill in accordance with the method described above to yield an image of movement of the charge in the mill, and controlling operation of the mill based on the information obtained from the monitoring step.

The method may include forming an image of movement of the charge in real time and also controlling operation of the mill based on this information in real time.

The method may include adjusting the speed of rotation of the mill.

The method may also include adjusting the feed rate of solid material to the charge in the mill. Further, the method may also include adjusting the rate at which liquid, for example water is introduced to the mill.

Yet further, this invention extends to a mill assembly comprising a mill in combination with the apparatus for monitoring the movement of charge as described above mounted on the mill.

Typically, the mill comprises a support, a mill shell that is rotatably mounted on the support, and drive means for driving the mill shell to rotate. The drive means may be one or more electric motors.

The mill shell may have a substantially cylindrical configuration and may have an inlet end defining an inlet and an opposing outlet end defining an outlet.

The mill may further include a feed hopper adjacent and above the inlet and a discharge chute adjacent the outlet.

The mill may be a ball mill. Further, the mill may be a SAG mill or an AG mill.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS A method and an apparatus for monitoring the movement of a charge within a tumbling mil! may manifest itself in a variety of forms. It will be convenient to hereinafter describe in detail several embodiments of the invention with reference to the accompanying drawings. The purpose of providing this detailed description is to instruct persons having an interest in the invention how to carry the invention into practical effect. However, it is to be clearly understood that the invention is capable of broader application. In the drawings:

Fig 1 is a schematic three dimensional view of a mill used for the comminution of ore particles into smaller sizes for mineral processing;

Fig 2 is a schematic three dimensional view of a first embodiment of an apparatus for monitoring movement of charge within a mill mounted on the mill of Fig 1;

Fig 3 is a schematic front view of the mill and monitoring apparatus of Fig 2 showing the communication of voltage readings across the voltage electrodes by telemetry to a processing station;

Fig 4 is a front view of a second embodiment of an apparatus for monitoring the movement of charge within a mill;

Fig 5 is a schematic drawing of the voltage electrodes and associated voltmeter on the mill shell;

Fig 6 is a schematic drawing of a receiver and associated processing station for receiving voltage signals from the voltage electrodes;

Fig 7 is a front view of an experimental monitoring apparatus devised by the Applicant; Fig 8 to 13 are graphs of experimental work done by the Applicant to demonstrate the different voltages that are measured by the voltage electrodes depending on the material that is in contact with the inner surface.

In Fig 1 reference numeral 1 refers generally to a mill for use with the invention.

The mill 1 comprises broadly a mill shell 2 that has a cylindrical configuration. The mill shell 2 has an inlet 4 through which material to be milled enters the mill shell 2 and an opposing outlet 5. The mill shell 2 has its longitudinal axis extending in a substantially horizontal orientation. The mill shell 2 is mounted for rotation on a support (not shown). Further, the mill shell 2 is driven to rotate by a drive that is energised by an electric motor. The drive for rotating the mill shell 2 has not been shown in this illustration. The mill shell also has an inner surface 6 and an outer surface 7 (shown in Figure 2).

The mill 1 includes a feed hopper 8 above the inlet 4 from which ore to be processed can be charged under gravity into the mill shell 2.

The mill 1 also includes a discharge hopper 9 adjacent and below the outlet 5 through which material is discharged from the mill 1. The material that is discharged from the mill 1 includes particles of rock including ore and water.

The interior wall 6 of the mill shell 2 has an lining 10 of material that is resistant to wear when subjected to collisions with rocks and steel balls. This lining 10 is mounted on the interior wall of the mill shell 2 and forms an inner wear surface of the mill shell 2.

The outlet 5 is defined in a lower region of the outlet end of the mill shell 2. The outlet 5 may include a mesh 12 that permits water and undersize ore particles to pass therethrough but resists the passage of mill balls and oversize particles.

The broken rocks can then be temporarily held in the discharge hopper 9 and sent through a conduit to a downstream processing plant as required. When operational the mill shell 2 is rotated about its longitudinal axis. This causes mill charge within the mill shell 2 to be lifted and then tumble in the mill shell 2. The collisions between ore particles, the wall of the mill shell 2 and mill 5 balls progressively breaks up the ore particles to render them suitable for downstream processing. During the course of this process, the ore particles are progressively displaced from the inlet 4 to the outlet 5.

Fig 2 shows an apparatus for monitoring a tumbling mill charge within the mill 10 shell. This monitoring apparatus is indicated generally by the reference numeral 20.

The apparatus 20 comprises broadly an electrical circuit comprising a first electrical supply electrode 22 that makes contact with the outside surface 7 of

15 the mill shell 2 and a conductor 24 in the form of electrical lead that leads to a second electrical supply electrode 25 that is positioned adjacent the outlet 5 so as to contact water issuing from the outlet 5 of the mill shell 2 in use. The first supply electrode 22 is in the form of a brush that makes contact with the rotating outer surface of the mill shell 2. The circuit also includes a power supply in the

20 form of a battery 27 that is coupled in series through the electrical conductor 24.

The first supply electrode 22 is supported in position above an uppermost point of the mill shell 2 by means of a first support 30. This first support 30 is in the form of a cantilever arm extending in from one side of the mill shell 2.

25

The second supply electrode 25 is similarly supported by a second support 32 that supports the supply electrode 25 in position in the path of liquid issuing from the mill shell 2. The second support 32, like the first support 30, may extend in from one side of the mill shell 2.

30.

Naturally, the supports 30, 32 can take many forms provided that they have sufficient mechanical strength to withstand the rigours of their use. The apparatus 20 further includes a plurality of sensors in the form of voltage electrodes 40 arranged at spaced intervals around the circumference of the mill shell 2. The voltage electrodes 40 may be arranged around the circumference of the mill shell 2 towards the discharge end thereof as shown in Figs 2 and 3.

In the illustrated embodiment there are over twenty said voltage electrodes 40 spaced apart around the circumference of the mill shell 2.

The function of the voltage electrodes 40 is to sense or measure the electrical potential difference or voltage between adjacent pairs of electrodes 40 around the circumference of the mill shell 2. Any measurable voltage differences between adjacent electrodes 40 can then be used to determine where current leakage is occurring around the circumference of the mill shell 2.

Fig 5 shows in schematic fashion the structure of the voltage electrodes 40 on the mill shell.

Basically the voltage electrodes 40 are each connected to a common connecting cable 42 such that they are electrically connectedJo each other. Each voltage electrode 40 branches off the connecting cable 42 at the point where it is positioned on the mill shell 2. The voltage electrodes 40 are thereby electrically connected to the steel mill shell 2.

Voltage measurements are performed between multiple pairs of electrodes 40, for example adjacent pairs of electrodes 40. Each voltage electrode 40 will be firmly coupled to the surface of the mill shell 2. As described above the position of each voltage electrode 40 on the mill shell 2 is fixed. Thus the spacing between adjacent voltage electrodes 40 is fixed and does not change. Each voltage electrode 40 is operatively connected to a microvolt meter, for example a battery operated microvolt meter, which is also mounted on the mill shell 2.

Finally a multiplexer/ modulator 46 is operatively connected to the operating cable 42 which in turn is coupled to a transmitter 48. The voltage readings are modulated by the multiplexer 46 and then transmitted to the receiver 52 using the transmitter 48.

The resistivity of the mill shell 2, for example of steel, can readily be calculated and moreover will be the same across all regions of the mill shell 2. It should not vary from one region to another because the material is the same across all regions. Similarly the shell lining 10 should be the same across all regions of the mill shell because it is made of the same material. Tests can be conducted using basic electrical equipment to check that the resistivity is the same across all regions of the mill shell 2.

Periodic testing may also be carried out to make sure the resistivity of the mill shell 2 and lining 10 is the same across all regions.

The extent of current leakage through the mill shell 2 at any particular point, or in any particular region, will depend on whether rock including ore particles, water or air is in contact with the interior wall 6 of that part of the mill shell. The greatest level of current leakage will be observed where water is in contact with the interior wall 6 of the mill shell 2. There will be an intermediate level of current leakage where ore particles and rock are in contact with the interior wall 6 of the mill shell 2. There will be little or no current leakage from the mill shell where air is in contact with the interior wall 6 of the mill shell 2.

The voltage electrodes 40 also include transmitters for transmitting their reading of sensed voltage to a processing station.

Figs 3 and 6 show a processing station indicated generally by the reference numeral 50.

The processing station 50 includes a computer 54. The computer 54 includes a microprocessor and memory storage devices. The processing station 50 also includes a receiver 52 for receiving the readings sensed and transmitted by the voltage electrodes 40 and also a demodulator 56.

These readings are then processed by the computer 54 to ascertain in real time what material is in contact with the interior wall 6 of the mill shell 2 at the position of the voltage electrode 40 at that particular time. This is then repeated for all the voltage electrodes 40 at the same time. By forming an aggregate picture of the current leakage at the various regions around the circumference of the mill shell 2, the computer 54 can build up a picture of the charge within the mill shell

2 as it is tumbling. This measurement of voltage drop between adjacent voltage electrodes 40 is then repeated at closely spaced time intervals providing a picture of changes in the shape of the charge within the mill shell 2 as a function of time.

This information can then be used in a method of operating the mill 1 to improve the performance of the mill 1.

In use, material is fed into the inlet 4 of the mill 1 on a continuous basis and material is discharged from the outlet 5 of the mill 1 on a continuous basis. Inside the mill 1 the charge, shown by numeral 55, moves progressively from the inlet 4 towards the outlet 5. The charge 55 comprises rock material including ore particles, possibly steel mill balls and water.

As the mill 1 rotates a leading portion of the charge 55, when considered from the direction of rotation, is lifted up by the rising mill shell 2 and then dropped down onto other charge material positioned below it. Thus, around any given circumference of the mill there will be liquid in contact with part of the internal wall 6, ore particles in contact with other parts of the internal wall 6 and air in contact with yet other parts of the internal wall 6 of the mill shell.

The electrical circuit of the monitoring apparatus 20 is energised by means of a switch, or the like. This causes current to flow through the first supply electrode 22 onto the outer surface 7 of the mill shell 2. The mill shell 2 is made of steel which is a good conductor of electricity and, as such, there is little or no voltage drop across the outer surface 7 of the mill shell 2. It is at or close to the supply voltage.

However, depending on the material in contact with the internal wall 6 of the mill shell 2 there can be leakage of current into the mill charge in the interior space of the mill.

The voltage drops between adjacent voltage electrodes 40 around the circumference of the mill shell 2 at a given time are sensed and transmitted through to the processor 54. A voltage difference between adjacent voltage electrodes indicates current leakage into the charge between the electrodes. Depending on the position of the solid charge, the liquid and air within the mill the voltage readings will be different.

This enables the computer 54 to build up a picture of the position of solid charge, liquid and air around the circumference of the mill shell 2. This thereby provides a picture of the various components within the mill shell 2.

The voltage electrodes 40 repeat these measurements on a regular time basis, for example at very short time intervals, and each time transmit the readings back to the processing station 50. This enables a measure, or insight, of the movement and shape of the charge and other components within the tumbling mill to be maintained. This broad process is shown schematically in Fig 3.

As discussed above, there is only a small amount of leakage of current from the mill shell 2 through the charge and to the second supply electrode 25. However, using sensitive voltage electrodes 40 the variation in voltage due to the leakage of small amounts of current into the charge within the mill shell 2 can be measured. This is despite the fact that it is many, many times smaller than the electrical potential difference applied across the two supply electrodes 22, 25.

The Applicant has recognised that this small variation in voltage can be measured by sensitive voltage electrodes and an associated voltmeter. The Applicant has also recognised that this measurement of variation in voltage due to current leakage is diagnostic of what component of the mill charge, be it water, rock or air, is in contact with the interior wall of the mill shell in that region.

By measuring the voltage at the various points around the circumference of the mill shell at repeated and very short intervals, a dynamic picture of the movement of charge in the mill shell can be built up.

This can be used to establish settings for the controllable parameters of the mill that can lead to improved operation of the mill. Parameters that might be adjusted based on the information of the charge within the tumbling mill may be mill speed, and the rate of introduction of charge into the mill.

Fig 4 illustrates an apparatus in accordance with a second embodiment of the invention.

As this embodiment has a number of similarities to the first embodiment described above with reference to Figs 2 and 3 the same reference numerals will be used to refer to the same components unless otherwise specified. Further the following description will focus on the differences between the two embodiments.

In the Fig 4 monitoring apparatus the supply electrodes 22 and 25 are both in contact with the surface of the mill shell 2. They are diametrically spaced apart from each other on the mill shell 2. As a result when a voltage difference is applied across the electrodes current will flow over the surface of the mill shell from one supply electrode to the other electrode.

In this embodiment both supply electrodes 22 and 25 will be in the form of brushes that permit the surface of the mill shell 2 to rotate while still maintaining electrical contact with the surface of the mill shell. Fig 4 shows the .. components of charge 55 within the mill when the mill is stationary. It shows a depth of liquid within the mill shell 2 that is a bit less than half. It also shows ore particles and steel balls within the charge.

Applicant has conducted some experiments to ascertain that the variations in voltage are measurable and also to demonstrate that these small differences in voltage can be used to obtain information on the position and shape of the charge within the mill.

The apparatus used to conduct these experiments is shown in Fig 7. These experiments are described below.

The mill shell is shown generally by numeral 60. The mill shell has some salt water and mill charge within its interior space. The water and charge rests in a bottom region of the mill shell as the mill shell is stationary. An electrical circuit with two voltage supply electrodes 61 and 62 was placed in electrical connection with the mill shell 60. One supply electrode 62 wa placed in contact with the mill shell 60 . The other supply electrode 61 was placed in contact with water within the interior space of the mill shell 60. An electrical potential difference was applied across the supply electrodes 61 and 62 such that a current of 1.5 Amps flowed through the circuit. The current flowed over the surface of the mill shell between the two supply electrodes with very little voltage drop. This was because of the low resistivity of the mill shell which was made of steel.

A plurality of voltage (sensing) electrodes were mounted on the mill shell 60 at spaced intervals around a half circumference of the mill shell. The voltage electrodes were basically circumferentially aligned with each other around the mill shell 60. As shown in the drawings there are 7 voltage electrodes spaced apart from each other around the half circumference. These electrodes are given the reference numerals 2 to 8.

The voltage differences were then measured across adjacent voltage electrodes with the liquid levels at different levels in the mill shell 60. In the first test the water level was set at a level just above the voltage electrode 7. The voltage difference (or absolute voltage) between adjacent voltage electrodes was then measured in microvolts.

Fig 8 is a graph showing the voltage readings between the adjacent voltage electrodes under these conditions. There was little or no voltage difference between electrodes 2 and 3 suggesting that there was air in the mill shell at this level. However between electrodes 5 and 6 and electrodes 6 and 7 there is a measurable and noticeable voltage. This indicates that there is current leakage into the mill shell around these levels. This indicates the presence of a good conductor against the inside surface of the mill shell such as water. This broadly correlates to the level of water in the mill shell.

The voltage measured between voltage electrodes 4 and 5 is at an intermediate level. This indicates some current leakage around these levels but less than that caused by water. This therefore suggests the presence of solid charge in the mill shell at these levels which correlates with the actual position.

In the second test that was carried out the water level was raised to a level above electrode 6. The voltages between the various electrodes were then measured and the results were plotted as a graph which is shown in Fig 9. The results show a greater voltage between electrodes 4 and 5 than in the first test. This is due to the higher water level in the second test. It also shows little or no voltage between electrodes 6 and 7 and electrodes 7 and 8. This is because all three electrodes are submerged in water. Again this demonstrates the working of the Applicant's invention.

In the third test that was carried out the water level was raised to a level above electrode 5. The voltages between the various electrodes were then measured and the results were plotted as a graph which is shown in Fig 10. The greatest absolute voltage measured between adjacent electrodes was between electrodes 4 and 5. Again this is the level of the water showing that the voltage readings can point out the level of the water.

In another test that was carried out the water level was again placed above electrode 7. The voltage differences, measured in micro-volts, between adjacent pairs of electrodes were then plotted in a general sequence from electrode 2 to electrode 8. For example the first voltage difference was that between electrodes 3 and 4 was subtracted from that between electrodes 2 and 3. The next voltage difference was the voltage between electrodes 4 and 5 subtracted from that between 3 and 4. This process was then repeated up to electrode 8.

The graph shows the biggest voltage difference occurs between electrodes 6 and 7 and electrodes 7 and 8. This correlates with a water level above electrode 7 and represents a reduction to practice of the Applicant's invention.

In another test the experiment above was repeated for a water level within the mill shell above that of electrode 6. These results are shown as a graph in Fig 12. Again the results clearly show a greatest voltage difference between electrodes 5 and 6 and electrodes 6 and 7 that correlates with the actual height of the water.

In yet another test the experiment above was carried out with the water level above electrode 5. Again the results shown in Fig 13 clearly point out the height of the water within the mill shell.

Another experiment was carried out to test the influence of a wet surface on the inside of the mill shell above the water level. For this experiment the inside surface was deliberately wetted with water and the results compared with the results for a dry wall. The results for both wet and dry wall were basically the same. This demonstrates that a wet wall will not distort the results or produce inaccurate results. A wet wall behaves differently to a body of water in contact with the inside surface of the wall. An advantage of the method for monitoring the movement of the charge within a mil! described above is that it is able to provide a picture of the charge in real time, which can then be used to control the operation of the mill. Further, the information on the movement of the charge within the mill is provided in real time rather than in historical time. This enables the control of the mill also to be effected in real time.

A further advantage of the method for monitoring described above is that it is non invasive. It does not have electrodes or probes that project into the interior of the mill shell, for example through the wall of the mill shell. This is advantageous because the interior space of a tumbling mill is a very harsh environment where electrodes or probes would be prone to being damaged.

The Applicant's invention is able to use a measurable parameter or property, in this case electrical potential difference across a plurality of points on the surface of the shell, as a measure of current leakage and from this deduce the position and movement of the charge within the mill shell during operation of the mill.

Applicant has made the unexpected discovery that even with a highly conductive mill shell made of steel there is a small amount of current leakage into the charge at points on the shell where liquid or solid ore is in contact with the internal wall of the shell. The Applicant has further recognised that even though this leakage is small relative to the overall current flowing over the surface of the mill shell, it can be measured with sensitive micro-voltage meters that are positioned at spaced apart points on the mill shell. Sensors, such as voltage electrodes are positioned around the circumference of the mill shell to sense the voltage across adjacent sensors on the shell. While modern and accurate voltage sensors are necessary to detect the relatively small but real differences in voltage at the various points on the mill shell such voltage sensors are available as off the shelf items.

A further advantage of the method and apparatus is that it provides precise and accurate measurements of the current leakage and therefore the position of the various components of the charge in real time. A yet further advantage of the method and apparatus is that it is not technically compiicated to implement on a new mill. Further it can readily be retrofitted to existing mill equipment where it is essentially just bolted on to the basic mill structure.

It will be recognised that the above invention has been described by reference to one exemplary embodiment and that many modifications and alterations to the invention are possible. Accordingly, any such alterations and modifications that would be apparent to those skilled in the art are considered to fall within the general concept of the invention.

Claims

Claims:

1. ^• A method of monitoring a charge including at least one phase within a mill, the method including gauging the resistivity of the phase that is immediately adjacent at least one region of an interior wall of the mill.

2. A method according to claim 1 , wherein the step of gauging the resistivity of the phase includes measuring the resistivity of the phase.

3. A method according to claim 2, wherein resistivity of the phase is measured by passing a current over an external shell of the mill and obtaining a measure of the extent of current leakage from the external shell into the phase immediately adjacent the region of the interior wall of the mill.

4. A method according to claim 1 , including gauging the resistivity of the phase that is immediately adjacent a plurality of regions of the interior wall of the mill.

5. A method according to claim 4, including obtaining a measure of the extent of current leakage from an external shell of the mill into the phase immediately adjacent the plurality of regions of the interior wall of the mill.

6. A method according to claim 5, wherein the regions are positioned adjacent to each other and extend equidistantly around the circumference of the mill shell.

7. A method according to claim 6, wherein the extent of current leakage is measured in at least three regions of the mill shell.

8. A method according to claim 7, wherein the step of obtaining a measure of the extent of current leakage includes measuring the voltage difference between spaced points on the mill shell.

9. A method according to claim 8, wherein the step of measuring the voltage difference between spaced points on the mill shell includes measuring the voltage drop between a plurality of voltage electrodes.

10. A method according to claim 9, including sending the measured voltage drops between adjacent voltage electrodes to a processor in real time.

11. A method according to claim 10, including using the processor to build up an image of the charge within the mill while the mill is tumbling based on the determination of what phase is in contact with the interior wall of the mill shell in the regions being detected by the voltage electrodes.

12. A method of determining the distribution of a charge having at least one phase within a mill including: gauging the resistivities of phases that are immediately adjacent regions of an interior wall of the mill; and interpreting the resistivities gauged to determine the phases that are immediately adjacent the regions of the interior wall of the mill.

13. An apparatus for monitoring a charge including at least one phase within a mill, the apparatus including: at least one sensor in at least one region of the mill for enabling the resistivity of the phase immediately adjacent an internal wall of the mill to be gauged.

14. An apparatus according to claim 13, wherein a number of sensors are provided so that resistivity of phases immediately adjacent the internal wad of the mill in a plurality of regions can be measured.

15. An apparatus according to claim 14, wherein the regions are adjacent each other and arranged end to end around the circumference of the mill.

16. An apparatus according to claim 15, wherein the sensors are voltage electrodes spaced apart from each other for measuring a voltage drop, if any, between pairs of the electrodes.

17. An apparatus according to claim 16, wherein each voltage electrode is mounted in a fixed and static position on the outer wall of the mill.

18. An apparatus according to claim 17, wherein each voltage electrode is capable of measuring voltages in the order of 20-60 microvolt.

19. An apparatus according to claim 18, wherein each voltage electrode includes a transmitter for communicating a voltage reading that has been sensed to a receiver.

20. An apparatus according to claim 19, including a processor for receiving and processing the sensed voltage readings from the voltage electrodes.

21. An apparatus according to claim 20, wherein the processor includes means for building up an image of the charge within the mill from the data received from the voltage electrodes.

22. An apparatus according to claim 21 , wherein the processor produces a dynamic image of the charge within the mill in real time.

23. An apparatus according to claim 13, wherein the sensor is associated with an energised electrical circuit having a pair of supply electrodes that are spaced apart from each other on the outer surface of the mill and that are in electrical contact with the mill in use.

24. An apparatus according to claim 23, wherein the supply electrodes are mounted such that they are substantially diametrically opposed from each other.

25. An apparatus according to claim 24, wherein the supply electrodes include brushes that permit the electrodes to slide over the surface of the mill while maintaining electrical contact therewith.

26. An apparatus according to claim 13, wherein the sensor is associated with an energised electrical circuit having a pair of supply electrodes, one supply electrode being positioned to make electrical contact with the outer surface of the mill and the other supply electrode being positioned so as to make contact with liquid emitted from the mill in use.

27. An apparatus according to claim 26, wherein the supply electrode that is in contact with the outer surface of the mill includes a brush that enables electrical contact with the outer surface of the mill while it is rotating and the supply electrode that contacts the liquid emitted from the mill in use is in the form of a plate.

28. A method of operating a mill including monitoring a charge within the mill in accordance with the method of claim 1 to yield an image of movement of the charge in the mill, and controlling operation of the mill based on the information obtained from the monitoring step.

29. A method according to claim 28 including adjusting the speed of rotation of the mill and/or adjusting the feed rate of solid material to the charge in the mill and/or adjusting the rate at which liquid is introduced to the mill.

30. A mill assembly comprising a mill in combination with the apparatus for monitoring the movement of charge within the mill according to claim 13.