Grinding mill control

The level of a grinding charge in a grinding mill is monitored by comparing signals which are generated by means of two sensors which are located on opposed sides of an impact point of the material in the mill. A signal produced in the comparison step may be used to control the feeding of material to the mill.

BACKGROUND OF THE INVENTION 
This invention relates to a method of and apparatus for monitoring a level 
of a grinding charge in a grinding mill. The invention is particularly 
concerned with autogenous and semi-autogenous mills and finds application 
in run of mine milling processes employed on gold and platinum mines. 
In run of mine milling it is necessary to maintain the feed rate of 
uncrushed ore into a mill at an optimum level in order to produce the 
desired fineness in the end product. 
If the feed rate is too high the mill overloads and if the feed rate is too 
low the mill becomes underloaded. In both cases the mill efficiency 
deteriorates rapidly. 
A mill of the kind referred to includes a cylinder which is rotated by 
means of a motor. The mill load within the cylinder is caused to rotate 
and cascades onto an impact point inside the cylinder. The position of the 
impact point is related to the level of the charge and a microphone has 
been used in the past to establish the location of the impact point. The 
microphone detects the sound level caused mainly by the impacting load and 
as the sound level varies when the location of the impact point changes an 
operator is able, through experience, to alter the feed rate of the ore 
into the mill accordingly. Thus as the loading of the mill is increased 
the point of impact rises and conversely if the charge level drops so does 
the impact point. Clearly if use is made of a microphone to detect the 
sound level at the point of impact then the microphone will provide an 
indication of optimum operating conditions. However if there is a 
reduction in sound level then the microphone is not able to indicate 
whether the rate of feed of ore should be increased or reduced. 
Sound-based systems of this kind are described for example in the 
specifications of U.S. Pat. Nos. 2,766,941 and 2,235,928. The 
specifications of UK Pat. No. 1105974 and U.S. Pat. No. 3314614 relate to 
the use of separate microphones for separate compartments in a 
multi-chamber mill, while the specification of U.S. Pat. No. 2,833,482 
discloses the use of a first microphone at the "solids" end of the mill 
and a separate microphone at the "water" end of the mill. 
U.S. Pat. No. 2,405,059 is concerned with a mill control system which makes 
use of multiple sensors which are in physical contact with the rotating 
mill shell. The objective is to eliminate errors which are present in 
devices which are responsive to air-borne vibrations. The sensors are 
symmetrically positioned around the shell to give "average values of 
grinding performance". 
Russian Pat. No. 869 809 shows a sonic method of diagnosis of the state of 
a ball mill and grinding process which uses at least three inductive 
sensors disposed around the periphery of the ball mill. A gradient signal 
which is produced by the sensors is used to define the dynamics of the 
process. Signals are also obtained for the mill content, and the degree of 
filling of the mill. 
A more recent approach to the problem has been to incorporate a load cell 
in the foundations of a grinding mill. The cell monitors the mill mass and 
this, in conjunction with data on the power drawn by the mill motor, is 
used to control the rate at which ore is fed to the mill. This technique 
however does not lend itself to incorporation in existing mills which do 
not have the facility for inclusion of a load cell. 
SUMMARY OF THE INVENTION 
The invention provides a method of monitoring a level of a grinding charge 
in a grinding mill which rotates and thereby causes the charge to cascade 
on to an impact point within the mill, the location of the impact point 
being dependent at least on the grinding charge level, the method 
including the step of detecting the prevailing sound level at least at two 
positions, generating signals which are respectively dependent on the 
detected sound levels, and comparing the signals. The positions may be 
spaced from one another in the direction of rotation of the mill. 
Preferably the positions are respectively on opposed sides of the impact 
point. 
In this way an indication is obtained of the position of the impact point 
or of the direction of movement of the impact point away from an potimum 
location which corresponds to an optimum charge level within the mill. 
A control signal may be produced in the comparison step. The control signal 
may be used to provide a display of the impact point position or to 
regulate the feed rate of ore into the mill, in both cases relatively to 
the optimum location of the impact point i.e. the optimum charge level. 
The invention also provides apparatus for monitoring a level of a grinding 
charge in a grinding mill which rotates and thereby causes the charge to 
cascade on to an impact point within the mill, the location of the impact 
point being dependent on the grinding charge level, the apparatus 
including at least two sensors for detecting the prevailing sound level, 
the sensors being spaced from each other in the direction of mill rotation 
with the impact point between the sensors, and means for comparing signals 
which are produced by the sensors. 
The sensors are preferably positioned so that they are equidistant from an 
impact point which corresponds to an optimum charge level. 
The comparison means may generate a control signal which is used for 
regulating the rate of feed of ore into the mill. The apparatus may also 
include a display which is indicative of the position of the impact point.

DESCRIPTION OF PREFERRED EMBODIMENT 
FIG. 1 illustrates schematically a cylinder 10 of a grinding mill which is 
charged in a conventional manner with ore. The cylinder 10 rotates in the 
direction of an arrow 12 and, due to the rotation, the load inside the 
cylinder travels along a path designated 14. The load travels with the 
cylinder for a substantial part of each revolution but as the load reaches 
an upper region it falls free and cascades on to an impact point 16. 
The position of the impact point is dependent on the level of the load 
inside the cylinder. As the load level increases the point 16 rises and 
when the load level drops the point 16 drops as well. There is an optimum 
position for the impact point which corresponds to optimum operating 
conditions of the mill. 
In accordance with the invention two microphones 18 and 20 respectively are 
employed as sound level sensors and are positioned spaced from one another 
in the direction of rotation of the cylinder on opposed sides of the 
impact point 16. Each microphone produces an electrical signal which is 
dependent on the sound level detected by the microphone and the signals 
are applied to a comparator 22. An output signal from the comparator is 
connected to a visual display 24 and to a control module 26. The control 
module produces control signals which are used to vary the rate at which 
ore is fed to the cylinder 10. 
In use of the mill the load cascades on to the impact point 16 in the 
manner described. If the mill is charged to its optimum level and the 
microphones 18 and 20 are positioned equidistantly from the impact point 
16 then the signals produced by the microphones are substantially equal 
and the control signal output by the comparator 22 reflects this. On the 
other hand if the mill carries too high a load then the impact point 16 
moves upeardly towards the microphone 18 and the signal generated by this 
microphone exceeds that generated by the lower microphone 20. The 
comparator 22 detects the imbalance between the signals and the display 24 
indicates that the impact point is moved away from the optimum position. 
On the other hand if the mill is undercharged then the impact point 16 
advances towards the microphone 20. The signal from this lower microphone 
then exceeds the signal from the upper microphone and in the manner 
described the control module 26 is actuated to cause the feed rate of ore 
to be increased 
In its simplest form the display 24 is a meter, with a centre zero point, 
and an indicator which departs from the centre point, in either direction, 
depending on the under-, or over-, loading of the mill as the case may be. 
Appropriate action could therefore be taken manually. Alternatively the 
control signal that is used to regulate the operation of the control 
module 26 so that, for example with the aid of a suitably programmed 
microprocessor, appropriate action is taken automatically to vary the feed 
rate of the ore. 
The apparatus of the invention indicates whether a change in the power 
draft of a motor driving the mill is due to an increase, or decrease, in 
the load level of the mill. By means of a suitable control device e.g. a 
microprocessor, the information is used to regulate the feed rate of 
material to the mill to maximise the power draft. Thus the apparatus is 
suited specifically to be part of a system which varies the rate of feed 
of run of mine ore to an autogeneous, or semi-autogeneous, mill to 
maintain the optimum milling state. As variations in the composition of 
the run of mine ore cause the feed demand and maximum power draft to vary, 
a computer based control technique will normally be required to monitor 
the mill performance and to regulate the feed supply rate in the optimum 
way. 
The control device, in effect, monitors the amplitude, and sense, of the 
control signal. For example if the control signal is positive the mill is 
overloaded. If the signal is negative the mill is underloaded. The 
amplitude of the signal indicates the degree of departure from the optimum 
loading position. It is thus straightforward to use the signal to control 
the feed rate of the ore to achieve a desired load level. 
One benefit which arises through the use of the comparator, which 
essentially substracts one microphone signal from the other, is that 
compensation is automatically achieved for variations in the sound level 
in the mill which arise due to fluctuations in the density of the material 
in the mill. In other words a degree of auto-correlation is achieved which 
enhances the noise-immunity of the system. 
The output signal of each microphone may be applied to an amplifier before 
being connected to the comparator. Initially the output signals from the 
amplifiers are balanced, under controlled conditions, to ensure that the 
apparatus is effectively calibrated for the particular installation. 
FIG. 2 illustrates portion of a chart recording which carries a signal 
trace 30 produced by the comparator 22, and a trace 32 produced by load 
cells which were fitted to a test mill. The pens used for recording the 
traces were not in line, and this accounts for an offset between the 
traces. It is nonetheless quite clear that a very strong correlation 
exists between the two signals which demonstrates that the apparatus of 
the invention gives an accurate indication of the mill content.