US2952414A

US2952414A - Electrically operated grinding mills

Info

Publication number: US2952414A
Application number: US679502A
Authority: US
Inventors: Williamson John Earlston
Original assignee: Union Corp Ltd
Current assignee: Union Corp Ltd
Priority date: 1956-08-31
Filing date: 1957-08-21
Publication date: 1960-09-13
Anticipated expiration: 1977-09-13

Description

J. E. WILLIAMsQN 2,952,414 ELECTRICALLY OPERATED GRINDING 4MILLS 4 sheets-sheet 1 nl" wml om Sept. 13, 1960 Filed Agg. 21. 1957 n, JQQLZOQ h. LESS .N

Sept. 13, 1960 1. E. WILLIAMSON ELECTRICALLY OPER'ATED GRINDING MILLS 4 Sheets-Sheet 2 Filed Aug. 2l, 195'? QSQ wutwwk Smm 'IAIII i2 QKQQ EBEE, I l I I I I I l l I I lll I. l I I I I I I I. I mm. Irv Q mm |r ummm@ m lll" nmN

Sept.4 13, 1960 .1.A E. wlLLlAMscN 2,952,414

ELECTRICALLY OPERATED GRINDING vMILLS filed Aug. 21, 1951 i Y 4 sheets-sheet s Sept, 13, 1960 J. E. WILLIAMSON ELECTRICALLY OFERATED GRINDING MILLS Filed Aug. 21, 1957 4 Sheets-Sheet 4 United States "atent f' John Earlston Williamson, Petersiield, Union of South Africa, assignor to Union Corporation Limited, Johannesburg, Union of South Africa Filed Aug. 21, 1951, ser. No. '679,502

Claims priority, application Union of South Africa Aug. 31, 1956 This invention relates to electrically operated tumbling grinding mills, such as for example, tube mills using a coarse component of the ore as the grinding medium.

It is a well-known fact that maximum grinding work will be accomplished in a tumbling grinding mill when the power drawn by the mill is at a maximum, provided that other factors remain constant. 'Ihe maintenance of maximum power input to the mill is directly dependent on maintaining the optimum grinding medium load in the mill and, insofar as grinding medium wear is a continuous process, the maintenance of the optimum grinding medium load calls for constant control of the addition of grinding medium.

The input power to the mill motor increases with the addition of grinding medium, until maximum power input is reached at the optimum grinding medium load. After the maximum is reached, power input decreases with further addition of grinding medium.

Manual control of the addition of grinding medium is a diiiicult operation for the following reasons:

(a) The best power meters are not suiiiciently sensitive to give a precise Visual indication as to when the input power is` approaching its maximum value.

(b) An ambiguity arises in power reading due to the decrease of input power if the addition of grinding medium continues after maximum power has been reached.

The mill input power can also be varied by varying the rotational speed of the mill.

An object of this invention is to provide improved operational control of tumbling grinding mills.

According to one feature of the invention there is provided a method of effecting operational control of a tumbling grinding mill driven by an electric motor, wherein variations in the volume of grinding medium in the mill cause variations in the input power to the mill motor, comprising the steps of generating a control signal which is responsive to the variations of the mill motor input power insofar as it concerns the rate and/or sense of change of the input power; translating the control signal; and utilizing the translated signal to determine the controlling operation to be exercised on the mill for reducing the rate of change of mill motor input power towards zero.

The translated signal may be utilized to actuate automatic control means for the mill for reducing the rate of change of mill motor input power towards zero. The automatic control means will determine and exercise automatically the required controlling operation on the mill and may be adapted to adjust the addition of grinding medium to the mill or to vary the rotational speed of the mill.

, The translated signal may also be utilized to actuate indicator means. If the mill is to be controlled manually an indication, such as for example visual, can be obtained of whether the input power to the mill motor is increasngor decreasing at any instant, and the indication obtained will determine the controlling operation to be exy, assaut Patented sept. 13, lese ICC ercised on the mill for reducing the rate of change of mill motor input power towards zero.

According to another aspect of `the invention there is provided means for effecting operational control of a tumbling grinding mill driven by an electric motor, wherein variations in the volume of the grinding medium in the mill cause variations in the input power to the mill motor, comprising a control signal generator in which an input signal derived from the power input to the mill motor and proportional to the instantaneous mill motor input power, is interacted with a continually and automatically variable independently generated reference signal, and in which, on unbalance of the two signals due to an increase or decrease of the mill motor input power, a control signal is generated which is indicative of the rate and sense of `change of the mill motor input power; control signal translating means; and means actuated `in accordance with theintelligence conveyed by the translated signal for determining the controlling operation to be exercised on the mill for reducing the rate of change of mill motor input power towards zero; and reference signal Varying means actuated in accordance with the intelligence conveyed by the translated signal for restoring the reference signal towards constant balance with the input signal.

The means actuated in accordance with the intelligence conveyed by the translated signal for determining the controlling operation to be exercised on the mill may comprise automatic control apparatus or may comprise indicator means.

Preferred embodiments of the invention will now be described purely by way of example and in no sense restrictive, with reference to the accompanying drawings in which:

Figure 1 is a schematic representation of one system of effecting operational control of a pebble mill.

Figure 2 is a circuit diagram of the apparatus for obtaining an output signal which is dependent on the change of input power to the mill motor.

Figure 3 is a front elevational view of the electrical switch means.

Figure `4 is a side elevational view of the electrical switch means.

Figure 5 is a circuit diagram of automatic control means for effecting automatic operational control of a pebble mill.

Figure 6 is a schematic representation of an alternative system of effecting operational control of a pebble mill.

Figure 7 is a circuit diagram of alternative control means for effecting automatic operational control of a pebble mill.

Figure 8 is a schematic representation of further alternative control means for effecting automatic operational control of a pebble mill.

As shown in Figure 1 an input signal which is proportional to the instantaneous input power to the mill motor is derived as 'an alternating signal from current transformer 1 which is connected in the mill motor power input circuit. After suitable step-up transformation by means of potential transformer 2, the alternating signal is rectified by rectier 3 and is then passed through an averaging network 4 with a long time constant in order to minimize the effect of transient power surges which are inherent in the operation of grinding mills.

The rectitied and averaged input signal is applied as a direct voltage to the signal generating bridge 5. Another direct voltage, obtained from a stabilized source and variable by means of potentiometer 10, is applied to the bridge 5 as a reference signal.

The bridge 5 interactsor compares the Iinput and reference signals with each other. The input signal varies with the mill motor input power and on imbalance of the two signals due to an increase or decrease of the input power a control signal is generated which is responsive to the variations of the mill motor input power, the magnitude and sense of the'control signal being indicative of the rate and sense of change of mill motor input power.

The control signal is amplified by voltage amplifier 6 and phase sensitive power amplier 7 and is then fed to the control phase windings of reversible two phase motor 8, the second phase windings of which are fed from an independent source. The reversible motor 8 translates the control signal and the sense of rotation of motor 8 at any instant will depend on the sense of the control signal at that instant and, therefore, on whether the input power to the mill motor is increasing or decreasing at that instant.

The potentiometer 10, for varying the reference signal is rotatably coupled to reversible motor 8 through reduction gearing. The setting of potentiometer 10 is varied automatically in accordance with the intelligence con- Veyed by the translated control signal in such a manner that the reference signal is restored continually towards constant balance with the input signal.

Magnetic clutch 9 is also rotatably coupled to the reversible motor 8 through reduction gearing. The magnetic clutch 9 is adapted to operate electrical switches 11 for indicator means 12 and/or automatic control means 13. Indicator means 12 provides a visual indication of whether the power input to the mill motor is increasing or decreasing at any instant, and where the mill is to be controlled manually, the indication obtained will determine the controll-ing operation to be exercised on the mill in order to restore the volume of grinding medium inthe mill towards the optimum value.

Control means 13 is arranged to determine and exercise automatically the required controlling operation on the mill in o-rder to restore the volume of grinding medium in the mill towards the optimum value.

The mill motor may be a 3 phase induction motor and as shown in Figure 2 current transformer 1 is connected in one line of the rnill motor power input circuit. Over the operative region of the mill, power changes are small and to all intents and purposes `voltage and power factor remain constant so that the input power to the mill motor can be lregarded as being proportional to the input current. Since the control signal is to be a function of the change of input power to the mill motor, actual instantaneous values of power or current are unimportant. The usual minor unbalances between the three phases of the input circuit will not have any adverse eliects on the operation of the system and it is sufficient to obtain the input signal from one phase only.

After suitable voltage step-up transformation in potential transformer 2 the alternating input signal is applied to potentiometer 15 which is adjustable to provide control of the magnitude of the input signal. The input signal from potentiometer 15 is rectied in rectifier 3 and passed through the averaging network 4 comprising resistor 16 and condenser I17.

The signal generating bridge comprises twin triode 18 the cathodes of which are connected together by means of the circuit containing condenser 19 and the primary winding of transformer 20. The rectiiied and averaged input signal is applied as a direct voltage to the one grid of twin triode 18. The reference signal is applied as a direct voltage to the other grid of twin triode 18.

The reference signal is `obtained from tube 21, to the plate of which an alternating voltage from an independent source is applied via lead 22. The alternating voltage is rectified by tube 21 and is smoothed by means of the iiltering circuit comprising choke 23 and

condensers

24, 25 and 26. The regulating tube 27 serves to stabilize the voltage which is variable towards constant balance with the input signal by means of potentiometer 10.

Y A 50 cycle alternating voltage from an independent source is applied to the plates of the twin triode 18 via lead 28. By virtue of the connections of the twin triode 18 there is obtained from the secondary winding of transformer 20 an alternating output control signal, the magnitude and sense of which is indicative of the rate and sense of change of the mill motor input power.

It will be appreciated that whereas the input signal is proportional to the instantaneous Values of mill motor input power, the reference signal is in essence an integration of the input signal over a period of time stored and constantly compa-red with the instantaneous values of mill motor input power. In other words, the reference signal is proportional at any instant to the average mill motor input power over a period of time immediately preceding that instant.

The control signal is applied to the Vone grid of twin triode 30 of the Voltage amplifier 6 via lead 29. Twin triode 30 is connected to provide two stages of voltage amplification and the output from the' second' stage Vis applied via lead 31 to phase sensitive power amplier 7 comprising

tubes

32 and 33.

Tubes

32 and 33 are triodes connected with a common grid bias cathode resistance, the grids being tied together. Two more or less equal, 50 cycle, alternating voltages which are 180 out of phase with each other are applied to the plates of

tubes

32, 33 by means of transformer 34 which is connected via leads 35, 36 to an independent 50 cycle source of supply.

The output from

power ampliiier tubes

32, 33 is obtained from transformer 34 via lead 37. The output from

tubes

32, 33 will be in phase with the signal applied to them with the result that the amplified control signal in lead 37 will have a sense which will be indicative of the sense of the change of input power to the mill motor.

The amplified control signal is fed viav lead 37 to th control phase windings 38 of the reversible two phase motor 8. The second phase winding 39 is fed from an independent source via leads 40, 41. Condenser 42 is provided in lead 40, to cause a phase diierence between the power supplied to the two

windings

38, 39 so that two phase operation is possible.

The magnetic clutch 9 and the electrical switches 11 which together form switching means actuated by the. reversible motor 8, the potentiometer 10` and the reversible motor 8 are arranged to form a single unit as shown in Figures 3 and 4.

The magnetic clutch 9 comprises a ferro-magnetic disc 43 which is provided with a plurality of open slots 44 extending radially inwards from the periphery. The slotted disc 43 is secured to shaft 45 which is rotatably supported by bearing 46 on support frame 47 and which is driven from the internally geared reversible motor 8 through reduction gearing 48, 49.

Two pairs of

rods

50, 51 and 52, S3 which Vare prok vided with threaded end portions are secured to bearing members 54 yand 55 respectively. The bearing members 54 and 55 are supported on shaft 45 by means of bearings in a manner allowing shaft 45 to rotate freely in the bearings without any appreciable rotational effect on members 54, 55.

Permanent magnets

56, 57 and 58, 59 are secured together in pairs with

plates

60, 61 and 62, 63 respectively by means of nuts on

screws

64, 65 and 66, 67 respectively. The ends of

screws

64, 65, 66, 67 are secured to the threaded ends of

rods

50, 51, 52, 53 by means of nuts to form a rrn support member which supports ,the`

magnets

56, 57 and 58, 59 along the periphery of slotted disc 43 in positions which are more or less 180 apart.

The slotted disc 43 rotates in the field of the permanent magnets and due to magnetic influence rotation ofthe slotted disc in a certain sense will cause the support mem-1 ber to be pivoted in one or the other direction.

The electrical switches 11 are operated bythe magnetic clutch through mediumof Va pivoted member 68- provided with engaging portion 69 extending upwards between

nuts

70, 71 on screws 64, 65 and with

arm portions

72 and 73. Movement of the support member in the one direction or the other will cause engaging portion 69 to be engaged by either nut 70 or nut 71 as the case may be. This will cause the pivoted member 68 to pivot around its pivot 74 and assume one of two positions. Depending

portions

75 and 76 are provided on member 68 to act as stop members to limit the pivotal movement of member 68 and hence also to limit the pivotal movement of the support member.

Electrical switches which are generally denoted by reference numeral 11 are mounted on the pivoted member 68 and will open or close depending on the position assumed by pivoted member 68. Two

specific mercury switches

77 and 78 are shown mounted on

arm portions

72 and 73 respectively, in such a manner that when the pivoted member 68 assumes one of its two positions, the one mercury switch will close while the other will open. When the pivoted member 68 assumes the other position the closing and opening of the switches will be reversed.

It will be seen that the switching means described is so arranged that once the pivoted member 68 has assumed one of its two positions on generation of a control signal of a certain sense, the pivoted member 68 will remain in that position until a control signal of opposite sense is generated, which will then cause the pivoted member 68 to assume its other position.

` The two

mercury switches

77 and 78 can be adapted to actuate the indicator means 12 to provide a visual indication of whether the input power to the mill motor is increasing or decreasing at any instant. The mercury switches 77 and 78 can also be adapted to actuate the automatic control means 13 It will be appreciated that the sense of rotation of the magnetic disc 43 provides a very convenient visual indication of the change of mill motor input power.

The potentiometer for varying the reference signal is driven from motor 8 through reduction gearing 48, 49, 79a, 79b, 79e.

In this specification the term pebble mill designates a tube mill using a coarse component of ore as grinding medium. Such a mill normally receives two components of feed, namely coarse ore pebbles acting as grinding medium and fine ore. No extraneous grinding medium need be used. The coarse ore pebbles and the tine ore may be fed separately to the mill. Alternatively, the feed to the mill may comprise essentially uncrushed runof-mine ore containing both coarse ore pebbles and fine ore.

The automatic control means 13 can be adapted to restore the mill load continually towards the optimum value in order to bring the mill motor input power towards maximum by arranging for a pebble feeding operation to the mill to be arrested when the intelligence conveyed by the translated control signal indicates a decrease of input power to the mill motor and for another pebble feeding operation to be initiated subsequently when the intelligence conveyed by the translated signal indicates a change in input power in the opposite sense. The decrease of mill motor input power will be due to the mill load exceeding the optimum value and with this type of control there will be a tendency for the mill to operate with a slight overload.

At or near maximum power input to the mill motor, changes in power for a given increment in load are small and in order to prevent ambiguity arising when the intelligence conveyed by a translated control signal indicates a decrease of mill motor input power, the control yapparatus 13 can be so arranged that during successive periods of time, the pebble feeding operation is alternately compulsory and optional. The arrangement can further be such that pebble feeding can be arrested at any time during the peri-od when pebble feeding is optional.

As shown in Figure 5 a D C. plate voltage is provided for thyratron tube by means of transformer 81, rectifier 82 and the smoothing circuit comprising resistor 83 and

condensers

84, 85. By means of the

circuit comprising resistors

86, 87, 88, rectifier 89 and condenser 90 the grid of thyratron 80 is held suliiciently negative to prevent thyratron 80 from conducting when the plate circuit is completed. Switch 77 is that mercury switch which is closed when the intelligence conveyed by a translated signal indicates a decrease of input power to the mill motor. When switch 77 is closed the grid of thyratron 80 is earthed and it will be possible for the thyratron 80 to conduct if its plate circuit is completed. Relay 9.1" is energized by the current in the plate circuit when the thyratron 80 conducts.

An automatic timer device 92 is provided to alternately complete and interrupt the plate circuit of thyratron 80 during successive predetermined periods of time by means of switch 93. The timer device 92 may comprise a cam which is driven by a synchronous motor. The cam can be so arranged that it interrupts the plate circuit during a predetermined part of its rotational cycle.4

Any other suitable timer device may be utilized.

At any time during the period when the plate circuit is completed, the thyratron 80 can start to conduct if switch 77 is closed to earth the grid. Once conduction has commenced it will continue until the plate circuit is interrupted again by the timer device.

Conduction of the thyratron 80 will energize relay 91 which will in turn energize a normally closed relay in the starting circuit 94 of the pebble feeder motor to arrest the pebble feeding operation. When the plate circuit of thyratron 80 is interrupted, the relays will deenergize and another pebble feeding operation will be initiated.

The duration of the alternate compulsory :and optional peeble feeding periods can be adjusted to suit the operational conditions ofthe mill.

In the system just described the control signal is translated by means of a reversible electric motor. Numerous other phase sensitive devices which will be responsive to the control signal can be used for the same purpose. A signal responsive relay, such as for example a differential relay provided with coils containing bucking windings, may be used and Figure 6 shows schematically an alternative system of eliecting operational control of a mill in which a signal responsive relay is incorporated.

The amplified control signal is obtained as described above and the reference signal is again varied automatically by means of the potentiometer 10 which is driven by reversible motor 8. The magnetic clutch 9 is, however, dispensed with a differential relay 95 which operates electrical switches is provided to translate the control signal. The electrical switches will actuate the indicator and/or automatic control means. In this embodiment of the invention the sense of rotation of the reversible motor will provide a convenient visual indication of the change of mill motor input power.

The operational control of the mill described above with reference to Figure 5 -adjusts the addition of pebbles to the mill. The addition of pebbles may be controlled in a variety of ways and it is also possible to effect operational control by varying the rotational speed of the mill.

An alternative way of controlling the addition of pebbles to the mill in order to bring the mill motor input power towards maximum, can be achieved by arranging for a pebble feeding operation to the mill to be initiated when the intelligence conveyed by the translated signal indicates a decrease of input power to the mill motor and for the pebble feeding operation to be arrested subsequently when the intelligence conveyed by the translated signal indicates a change of input power in the opposite sense. In this case the decrease of mill motor input power must be due to the mill load falling short of the optimum value and there will be a tendency for the mill to operate with a load slightly less than the optimum.

' The control apparatus 13 may be so arranged that during successive periods of time the pebble feeding operation is alternately possible and precluded and that pebble feeding can be initiated at any time during the period when the pebble feeding operation is possible.

The circuit shown in Figure can be used for this type of control. The operation is exactly the same as described before with the exception that a normally open relay is provided in the starting circuit 94 of the pebble feeder motor. A pebble feeding operation is precluded during the period when the plate circuit of thyratron 80 is interrupted and is possible during the period when the plate circuit is closed. If switch 77 is closed a pebble feeding operation will be initiated during a period when the plate circuit of the thyratron 80 is closed. Once peeble feeding has commenced it will continue -until the plate circuit is interrupted again.

The duration of the alternate periods during which [pebble feeding is possible and precluded can be adjusted to suit the operational conditions of the mill.

As a further alternative the control means 13 can be arranged to arrest a pebble feeding operation to the mill when the intelligence conveyed by the translated signal indicates a decrease of input power to the mill motor due to the mill load exceeding the optimum value, and subsequently to initiate another pebble feeding operation when the intelligence conveyed by the translated signal indicates a decrease of input power due to the mill load falling short of the optimum value.

It will be appreciated that in this embodiment, means must be provided for distinguishing between a decrease of input power due to the mill load exceeding the optimum value and a decrease of input power due to the mill load falling short of the optimum value. This can be achieved by arranging for a predetermined time interval to elapse after the pebble feeding operation has been arrested before a subsequent operation can be started, and similarly for a predetermined time interval to elapse after a pebble feeding operation has lbeen initiated before it can be arrested again.

The circuit shown in Figure 5 can be used for effecting this type of control. The timer device 92 can be arranged to provide the time intervals required and the relays in the feeder motor circuit 94 can be adapted to initiate and arrest feeding operations as required under the influence of relay 91.

As yet another alternative, the control means 13 may be arranged to vary the rate of pebble feed to the mill in accordance with the rate and sense of change of mill motor input power.

Where a D.C. motor is used to drive the pebble feeder, the circuit arrangement shown in Figure 7 may be used to effect this type of control. The control signal is utilized as before to cause rotation of reversible motor 8 which will in turn cause magnetic clutch 9 to operate. The mercury switches operated by the magnetic clutch are arranged to form a double pole, double throw changeover switch as represented by 96 in Figure 7. A control signal conveying the intelligence of an increase of input power to the mill motor will cause switch 96 to assume one switching position and a control signal conveying the intelligence of a decrease of input power will cause switch 96 to assume the other switching position.

By means of lead 97, the output control signal from the second stage of the twin triode 30 (see Figure 2) is applied to the grid of tube 98 which receives its plate supp-ly via lead 99. The signal is obtained as an amplified, iiuctuating7 direct voltage from the plate circuit of tube 98 and is fed to the change-over switch 96. The signal may be smoothed to reduce the fluctuations.

The signal is fed via the change-over switch 96 to auxiliary excitation windings A of the feeder `motor 94. Depending on the switching position of switch 96the excitation produced by winding 100 will either assist or oppose the excitation produced bythe main excitation winding 101. The extent of the vopposition or assistanceY will depend on the -rate and sense of change of mill motor input power, and the rate of pebble feed will be varied" accordingly.

yThe methods of controlling the feed of pebbles to a pebble mill as described above may be used for controlling the addition of grinding medium to any tumbling mill using a coarse component of the ore as grinding medium.A These methods may be used for the control of tumbling mills receiving feed consisting of essentially uncrushedv run-of-mine ore as well as for the control of the addition of fine feed to tumbling mills.

Under certain circumstances it might be more advantageous to vary the rotational speed of the mill and theY control means 13 can also be adapted for that purpose.

Where an A.C. motor with a wound rotor is used as a mill motor, the rotational speed may, for example, be varied by using a stepped resistance for controlling the motor. A translated control signal conveying the intelligence of either an increase or decrease of input powerl to the mill motor can then be utilized to adjust the steps of the resistance in order to increase or decrease the rotational speed of the mill motor as required. i Y It is possible to effect operational control of a mill by controlling both the addition of feed to the mill and the rotational speed of the mill. Y It may be advantageous to arrange the control meansV 13 so that the rotational speed of the mill motor willV not be varied when casual changes of the mill motor? input power occur, but will be varied only when a sustained change occurs. An example of this type of control will be described with reference to a pebble mill in which the rotational speed is variable and the pebble feeding operation is controlled by arresting the pebble feeding operation when the intelligence conveyed by the translated control signal indicates a decrease of inputpower due to overloading of the mill. Figure 8 shows the arrangement schematically. A stepped relay 103 which is adapted to performl successive operations on receipt of successive actuating signals is provided in circuit with the control resistance 104 of the mill motor 105 and is also associated with the'v plate circuit of the thyratron tube 80 in circuit 102 which is similar to the circuit shown in Figure 5. When the translated control signal conveys the intelligence of av decrease of input power to the mill motor, the pebblefeeding operation is arrested during the first period when pebble feeding is optional and an actuating signal of a certain sense is transmitted to the stepped relay 103. An actuating signal of similar sense will be transmitted to the stepped relay 103 each time the pebble feeding operation is arrested. Generation of a control signal which indicates an increase of mill motor input power and which will result in the pebble feeding operation Ibeing maintained during at least part of the optional period, will result in an actuating signal of opposite sense being transmitted to the stepped relay 103. The stepped relay 103 is so arranged that two successive actuating signals of the same sense are required before it can actuate the variation of the setting of the control resistance 104 of the mill motor 105. Each successive actuating signal of the same sense after the first causes the control resistance 104 to be adjusted one step andL the rotational speed of the mill motor 105 tobe varied accordingly. After a number of actuating signals of the same sense have been received in succession, an actuating signal of opposite sense will cancel the preceding signals. but will not cause the setting of control resistance 104 to be varied. Only upon receipt of a second successive signal of the same sense will the control resistance 104v be adjusted one step in the direction opposite to the previous direction of adjustment.

In the embodiments described above the control signal is electrical in its nal form, but it may also be converted into either a pneumatic or hydraulic signal for eifecting the operational control of the mill.

I claim:

1. Means for effecting automatic operational control of a tumbling grinding mill driven by an electric motor, wherein variations in the volume of the grinding medium in the mill cause variations in the input power to the mill motor, comprising a control signal generator in which an input signal derived from the power input to the mill motor and proportional to the instantaneous mill motor input power, is compared with an independently generated reference signal Awhich is continually and automatically variable towards constant balance with the input signal, and in which control signal generator, on unbalance of the two signals due to an increase or decrease of the mill motor input power, a control signal is generated which is indicative of the rate land sense of change of the mill motor input power; control signal translating means; automatic control apparatus for the mill actuated in accordance with the intelligence conveyed by the translated signal for `bringing the power input to the mill motor continually towards a maximum; and reference signal varying means actuated in `accordance with the intelligence conveyed by the translated signal for restoring the reference signal towards constant balance with the input signal.

2. Means for effecting automatic operational control of a tumbling grinding mill driven by an electric motor, wherein variations in the volume of the grinding medium in the mill cause variations in the input power to the mill motor, comprising a control signal generator in which an input signal derived from the power input to the mill motor and proportional to the instantaneous mill motor input power is compared with an independently generated reference signal which is continually and automatically variable towards constant balance with the input signal, and in which the control signal generator, on unbalance of the two signals due to an increase or decrease of the mill motor input power, a control signal isl generated which is indicative of the rate and sense of change of the mill motor input power; control signal translating means; automatic control apparatus for the mill actuated in accordance with the intelligence conveyed `by the translated signal for bringing the power input to the mill motor continually towards a maximum; and reference signal varying means actuated in accordance with the intelligence conveyed by the translated signal for restoring the reference signal towards constant balance with the input signal, the automatic control apparatus comprising an electronic circuit including a thyratron tube, an electrical control switch actuated in accordance with the intelligence conveyed by the translated signal, and provided in the grid circuit of the thyratron tube; a timer provided in circuit with the plate of the thyratron tube and adapted to cause the thyratron `plate circuit to be alternately .completed and interrupted during successive predetermined periods of time; and a relay provided in the thyratron plate circuit and adapted to cause interruption and completion of the starting circuit of an electric motor driving the grinding medium `feeder of the mill.

3. Means for effecting automatic operational control of a tumbling grinding mill driven by an electric motor, wherein variations in the volume of the grinding medium in the mill cause variations in the input power to the mill `motor, comprising `a control signal generator in whichlan input signal derived from the power input to the mill motor and proportional to the instantaneous mill motor input power is compared with an independently generated reference signal which is continually and automatically variable towards constant balance with the if) input signal, and in which the control signal generator, on unbalance of the two signals due to an increase or decrease of the mill motor input power, a control signal is generated which is indicative of the rate :and sense of change of the mill motor input power; control signal translating means; automatic control apparatus for the mill actuated in accordance with the intelligence conveyed by the translated signal for bringing the power input to the mill motor continually towards a maximum; and reference signal varying means actuated in accordance with the intelligence conveyed by the translated signal for restoring the reference signal towards constant balance with the input signal, the automatic control apparatus comprising an electronic circuit including a thyratron tube, an electrical control switch actuated in accordance with the intelligence conveyed by the translatedV signal and provided in the grid circuit of the thyratron tube; a control resistance for the mill motor; and a stepped relay connected to the control resistance and associated with the plate circuit of the thyratron tube.

4. Means for effecting automatic operation control of a tumbling grinding mill driven by an electric motor, wherein variations in the volume of the grinding medium in the mill cause variations in the input power to the mill motor, comprising a control signal generator in which an input signal derived from the power input to the mill motor and proportional to the instantaneous mill motor input power, is compared with an independently generated reference signal which is continually and automatically variable towards constant balance with the input signal, and in which the control signal generator, on unbalance of the two signals due to an increase or decrease of the mill motor input power, a control signal is generated which is indicative of the rate and sense of change of the mill motor input power; control signal translating means; automatic control apparatus for the mill actuated in accordance with the intelligence conveyed by the translated signal for bringing the power input to the mill motor continually towards a maximum; and reference signal varying means actuated in accordance with the intelligence conveyed by the translated signal for restoring the reference signal towards constant balance with the input signal, the automatic control apparatus comprising a circuit including a change-over switch actuated in accordance with the sense of change of mill motor input power conveyed by the translated signal; and auxiliary excitation windings provided in an electric moto-r driving the grinding medium feeder of the mill and adapted to be energised via the change-over switch in accordance with the rate and sense of change of mill motor input power.

5. Means for effecting automatic operational control of a tumbling grinding mill driven by an electric motor, wherein variations in the volume of the grinding medium in the mill cause variations in the input power to the mill motor, comprising a control signal generator in which an input signal derived from the power input to the mill motor and proportional to the instantaneous mill motor input power, is compared with an independently generated reference signal which is continually and automatically variable towards constant balance with the input signal, and in which the control signal generator, on unbalance of the two signals due to an increase or decrease of the mill motor input power, a control signal is generated which is indicative of the rate and sense of change of the mill motor input power; control signal translating means; automatic control apparatus for the mill actuated in accordance with the intelligence conveyed by the translated signal for bringing the power input to the mill motor continually towards a maximum; and reference signal Varying means actuated in accordance with the intelligence conveyed by the translated signal for restoring the reference signal towards constant balance with the input signal and in which the control signal translating means comprises a reversible, signal 11 responsive electric motor including control windings to which the control signal is applied; and comprising electric switching means responsive to the reversible motor, the electric switching means including a circumferentially slotted disc of ferro-magnetic material, which is rotatably driven by the reversible electric motor; a support member adapted for limited pivotal movement co-axially with the slotted disc; at least one magnet secured to the support member and adapted to produce a magnetic field in which the slotted disc rotates to cause pivotal movement of the support member in one or the other direction depending on the sense of rotation of the slotted disc; a pivoted member provided with a portion adapted to engage with the support member, movement of the support member in one or the other direction causing the pivoted member to assume one of two positions; and at least one electrical control switch which is mounted on the pivoted member, is connected in the circuit with the automatic control apparatus and is arranged to close when the pivoted member assumes one position and to open when the pivoted member assumes the other position.

6. A method of effecting automatic operational control of a tumbling grinding mill driven by an electric motor, wherein variations in the volume of grinding medium in the mill causes variations in the input power to the mill motor, comprising producing an input signal which is proportional to the instantaneous input power to the mill motor, producing a reference signal which is proportional at any instant to the average mill motor input power over a period of time immediately preceding that instant; comparing the two signals with each other to produce a control signal which is indicative of the rate and sense of change of the mill motor input power, when unbalance occurs between the input and reference signals due to an increase or decrease of the mill motor input power; translating the control signal and controlling the mill in accordance with the intelligence conveyed by the translated signal to bring the power input to the mill continually towards a maximum.

7. A method of effecting automatic operational control of a tumbling grinding mill driven by an electric motor, wherein variations in the volume of grinding medium in the mill causes variations in the input power to the mill motor, comprising producing an input signal which is proportional to the instantaneous input power to the mill motor, producing a reference signal which is continually and automatically variable towards constant balance with the input signal; comparing the two signals with each other to produce a control signal which is indicative of the rate and sense of change of the mill motor input power, when unbalance occurs between the input and reference signals due to an increase or decrease of the mill motor input power; translating the control signal; restoring the reference signal towards constant balance with the input signal in accordance with the intelligence conveyed by the translated signal; and controlling the mill in accordance with the intelligence conveyed by the translated signal to bring the power input to the mill continually towards a maximum, said method further comprising adjusting the addition of grinding medium to the mill in -accordance with the intelligence conveyed by the translated signal to reduce the rate of change of mill motor input power towards zero, the addition of grinding medium to the mill being alternately compulsory and optional during successive periods of time; the operation of addition of grinding medium to the mill being continued during an optional period it during that period the intelligence conveyed by the translated signal indicates that the mill motor input power is increasing, the operation of addition of grinding medium being arrested at any instant during an optional period and being precluded from being initiated again during the remainder of that period if the intelligence conveyed by the translated signal indicates that theuinput power is decreasing.

8. A method as claimed in claim 7, in which the grindring medium forms part of the feed to the mill whichl comprises essentially uncrushed run-of-mine ore.

9. A method of effecting automatic operational control of a tumbling grinding mill driven by an electric motor, wherein variations in the volume of grinding medium in the mill causes variations in the input power to the mill with each other to produce a control signal which is indicative of the rate and sense of change of the mill motor input power, when unbalance occurs between the input; and reference signals due to an increase or decrease of the mill motor input power; translating the control signal; restoring the reference signal towards constant balance with the input signal in accordance with the intelligence conveyed by the translated signal; and controlling the mill in accordance with the intelligence con-` veyed by the translated signal to bring the power input to the mill continually towards a maximum, the addition' of grinding medium to the mill being adjusted in accordance with the intelligence conveyed by the translated signal to reduce the rate of change of mill motor input power towards zero, the addition of grinding medium to the mill being alternately possible and precluded during successive periods of time; preclusion of an operation of addition of grinding medium to the mill being continued during a period when an operation of addition is possible if during that period the intelligence conveyed by the translated signal indicates that the mill motor input power is increasing, an operation of addition of grinding medium `being initiated at any instant during a period when an operation of addition is possible and being maintained for the remainder of that period if the intelligence conveyed by the translated signal indicates that the input power is decreasing.

10. A method of effecting automatic operational control of a tumbling grinding mill driven by an electric motor, wherein variations in the volume of grinding medium in the mill causes variations in the input power to the mill motor, comprising producing an input signal which is proportional to the instantaneous input power to the mill motor, producing a reference signal which is continually and automatically variable towards constant balance with the input signal; comparing the two signals with each other to produce a control signal which is indicative of the rate and sense of change of the mill motor input power, when unbalance occurs between the input and reference signals due to an increase or decrease of the mill motor input power; translating the controll signal; restoring the reference signal towards constant balance with the input signal in accordance with the intelligence conveyed by the translated signal; and controlling the mill in accordance with the intelligence conveyed by the translated signal to bring the power input to the mill continually towards a maximum, the additiony of grinding medium to the mill being adjusted in accordance with the intelligence conveyed by the translated signal to reduce the rate of change of mill motor input power towards zero, an operation of laddition of grinding medium to the mill being arrested if the intelligence conveyed by the translated signal indicates that the mill motor input power is decreasing, and another operationY of addition of grinding medium being subsequently in? itiated if the intelligence conveyed by the translated signal again indicates that the power is decreasing. A

11. A method as claimed in claim 10, in which a pre? determined time interval is arranged to elapse after anY operation of addition of grinding medium is arrested before a subsequent operation of addition can be in? 13` itiated, and in which a predetermined time interval is arranged to elapse after an operation of addition is initiated before it can be arrested again.

12. A method of eiecting automatic operational control of a tumbling grinding mill driven by an electric motor, wherein variations in the volume of grinding medium in the mill causes variations in the input power to the mill motor, comprising producing an input signal which is proportional to the instantaneous input power to the mill motor, producing a reference signal which is continually and automatically Variable towards constant balance with the input signal; comparing the two signals with each other to produce a control signal which is indicative of the rate and sense of change of the mill motor input power, when unbalance occurs between the input and reference signals due to an increase or decrease of the mill motor input power; translating the control signal; restoring the reference signal towards constant balance with the input signal in accordance with the intelligence conveyed by the translated signal; and controlling the mill in accordance with the intelligence conveyed by the translated signal to bring the power input to the mill continually towards a maximum, the feed to the mill comprising essentially uncrushed run-of-mine ore and the coarse component acting as the grinding medium, the feed being added to the mill continuously and the rate of addition being varied in accordance with the intelligence conveyed -by the translated signal to reduce the rate of change of mill motor input power towards zero.

References Cited n the le of this patent UNITED STATES PATENTS 1,395,089 Burhans Oct. 25, 1921 1,619,807 Blomeld Mar. 8, 1927 2,766,939 Weston Oct. 16, 1956 2,766,940 Weston Oct. 16, 1956 2,766,941 Weston Oct. 16, 1956 FOREIGN PATENTS 475,421 Great Britain Nov. 18, 1937