US3429439A

US3429439A - Slow magnetic drum ore separator control device

Info

Publication number: US3429439A
Application number: US684427A
Authority: US
Inventors: David Weston
Original assignee: Aerofall Mills Ltd
Current assignee: Aerofall Mills Ltd
Priority date: 1967-03-01
Filing date: 1967-11-20
Publication date: 1969-02-25
Anticipated expiration: 1986-02-25
Also published as: FI43573B; FR1602650A; SE321905B; DE1583772A1; GB1171093A; NO120113B

Description

Feb. 25, 1969 D WESTON 3,429,43

SLOW MAGNETIC DRUM ORE SEPARATOR CONTROL DEVICE Filed NOV. 20, 1967 I CONTROL CYCLONE Sheet of 2 m-k E 4 SIGNAL CLASSIFIER CYCLONE SIGNAL COLLECTOR "/08 22 VERTICAL CLASSIFIER COLLECTING TAILINGS 26 MOTOR 28 ogRR aE FAST GN /42 F/ G. COBBERW 3 4a 34 0 WW \W/ CONTROLLER y CCENTRATE I: .1 SLOW, 3

3 COBBER 2 OVERRIDE MOTOR CQLgEgTING 36 ISIGNAL\ 40 L 50 46 CONTROLLEE ON ENAEAT c c T E TAILiNGS DISPOSAL 44 SENSOR M|DDL|NGS\ SECONDARY F FROM C%TI\II\'IREOL' OVERRIDE m g-R 23 SIGNAL /02 SLOW 32 SECONDARY COBBER STAGE- P TI W W, o ERA N6 38 ,I 40 y /06 UNIT TAILINGS sscsaxlgm FAST 26 COBBER SENSOR 30w! 34 i, r' CONCENTRATE REGRIND INVENTOR DAVID WESTON F/ G 2 BY W P 6% ATTORNEYS Feb. 25, 1969 D. \INESTON 3,429,439

SLOW MAGNETIC DRUM ORE SEPARATOR CONTROL DEVICE Filed Nov. 20, 1967 Sheet 2 of 2 ORE INPUT I 78 m MOTOR fi INTERMEDIATE SPEED SPEED coNTRoLLER COBBER 88 SPEED CONTROLLER MOTO SPEED CONTROLLER 38 -ENSOR 52 a 'TAILINGS MIDDLINGS oRE CONCENTRATE \RECIRCULATING LoAD To 74 72 76 33 68 PRIMARY MILL LOW{ MASTER RYTAILINGS D.C VARIABLE IRON SET IRDN SPEED MoToR E SENSING KPQ 'Q E POINT coNTRoLLER CONTROLLER MOTOR COIL I L l L I K 66 SLAVE 70 SIGNAL lNvENToR DAVID WESTON BY W *(J ATTORNEYS.

United States Patent 3,429,439 SLOW MAGNETIC DRUM ORE SEPARATOR CONTROL DEVICE David Weston, Toronto, Ontario, Canada, assignor t0 Aerofall Mills Limited, Toronto, Ontario, Canada Filed Nov. 20, 1967, Ser. No. 684,427 Claims priority, application Canada, Mar. 1, 1967,

984,101 US. Cl. 209 223 6 Claims Int. Cl. B03c 1/30 ABSTRACT OF THE DISCLOSURE Control device for rotary magnetic ore separator, having a sensor for sensing magnetic material in the tailings separated out and a speed controller responsive to the sensor for regulating the speed of rotation of the separator to regulate the amount of magnetic material in the tailings, separated out.

Background of the invention This invention relates to control means for rotary magnetic separators in a magnetic ore processing system.

In the processing of magnetic ore, it is common to use rotary drum-type magnetic separators for classifying ground ore. Such separators are frequently arranged in series in the primary dry grinding stage of the ore processing system so as to yield an ore concentrate, tailings, and a middlings product for regrinding.

The rotary separators arranged in series generally include a faster separator for separating out a coarse ore concentrate separation product and a slower separator for separating out a tailings separation product. For economic operation of the system, it is usually desired that the quantity of ore concentrate separated out be relatively constant, and that the tailings separated out have an iron content lying within predetermined limits. To this end, in conventional ore processing systems, the speed of rotation of the faster rotary separator is preset to separate out the desired quantity of ore concentrate and the speed of the slower rotary separator is adjusted to separate the tailings having the desired iron content.

However, as there is variation from time to time in the quality and characteristics of the input ore, and as the other parameters in the ore processing system vary, the speed of rotation of both the faster and the slower rotary separators may depart from time to time from what is optimum for separating out the desired quantity of ore concentrate and the desired quality of tailings.

Summary of the invention According to the present invention, in conjunction with the invention described in applicants co-pending application Ser. No. 684,466 filed concurrently herewith, the above mentioned problems created by non-uniformity of input ore and variation in operating parameters can be overcome by providing speed control means for both the faster and the slower rotary magnetic separators for adjusting the speed of rotation automatically in response to measurable characteristics of the separation products. In the present invention, which provides control means for the slower rotary magnetic separator, a sensor is provided for sensing the proportion of magnetic material in the tailings separated out by the slower separator, and the speed control means responds to this sensor and adjusts the speed of rotation of the slower separator so as to regulate the proportion of magnetic material in the tailings. In the invention defined in application Ser. No. 684,- 466, which relates to the faster separator, a sensor is provided for sensing the quantity of ore concentrate separated 3,429,439 Patented Feb. 25, 1969 out by the faster separator, and the speed control means responds to this sensor and adjusts the speed of rotation of the faster separator so as to regulate the quantity of ore concentrate separated out.

Brief description of the drawings FIGURE 1 is a schematic drawing illustrating an orcprocessing system utilizing control apparatus according to the invention;

FIGURE 2 illustrates a modification of the inter-relationship of the rotary separators used in the system of FIGURE 1;

FIGURE 3 is a schematic diagram of a further alternative arrangement of the rotary separators, together with control apparatus according to the invention, for use in the system of FIGURE 1; and

FIGURE 4 is a schematic diagram of a control arrangement according to the invention for a low speed cobber.

Detailed description with reference to the drawings In FIGURE 1, a rotary forced air grinding mill 12 of the type through which a stream of air is blown is fed iron ore via an input feed chute 14 attached to the mill 12.

An output duct 16 leading from the mill 12 conveys the ground are through a vertical classifier 18 and thence through one or more cyclone classifiers 20. Only one cyclone classifier 20 is shown by way of illustration in FIGURE l, but it is to be understood that a plurality of such classifiers may be fed from the rotary mill 12. The fines from the cyclone classifiers 20 are collected in a cyclone collector 22 and thence distributed to the secondary grinding stage of the ore processing system for finer grinding.

The coarse particles from the vertical classifier 18 and the cyclone classifier 20 are distributed to a collecting bin 24 and thence are fed onto the top of a dry rotary drumtype magnetic separator (often referred to in the industry a cobber) 26 driven by a motor 28. (The drive may be any convenient drive and the drawing is not intended to imply that a belt-type drive is appropriate.)

The cobber 26 typically comprises a cylindrical drum of the order of 3 feet in diameter and up to six feed in length, which is rotated about a stationary grouping of magnets which are generally located on the downward side of the drums rotation. The ore fed from the collecting bin 24 includes a mixture of magnetically susceptible and non-susceptible particles, of which the susceptible particles have varying degrees of susceptibility. As the drum rotates, the magnetically susceptible particles tend to be held to the drum surface by the force of the magnetic field; the non-susceptible particles fall away due to the force of gravity and are thrown away from the drum surface by centrifugal force. The magnetically susceptible particles tend to adhere to the drum until they pass out of the field of the magnets, at which time they fall or are removed from the drum surface. In this manner, a separation of the ore into two groups of differing susceptibility can be obtained.

Most natural ores contain some relatively large mineral particles that may be liberated from their associated gangue minerals using a relatively coarse grind, while the remainder of the mineral particles remain physically combined with other particles of gangue minerals and require further grinding. Further, a coarse grind will also produce (i.e. one cobber fed from one output of the other cobber) are arranged to yield the three desired separations.

In FIGURE 1, the cobber 26 is indicated as being a fast cobber, i.e. a cobber having a high speed of rotation. Accordingly, the bulk of the ore fed from the collecting bin 24 to the cobber 26 will not cling to the cobber but will be thrown off. If that portion of the ore which clings to the cobber is collected, such portion will yield a relatively high ore concentrate separation product. The larger portion of the ore i.e. that which does not cling to the cobber 26, can be fed to a second cobber rotating at much lower velocity which can be used to separate tailings having very low ore content from a middlings product which can be fed back to the rotary mill 12 to be reground.

Such an arrangement is shown in FIGURE 1. If the fast cobber 26 is rotating counter-clockwise, as shown, the bulk of the ore fed onto the cobber will fall into a collecting trough 30, whose output can be applied to the top of a slow speed cobber 32. The ore fraction that clings to the fast cobber 26 will be released into a collecting trough 34 and can be fed to a collecting bin 36, for example, and thence to a classification system, whose coarser product would be ground in a secondary grinding stage.

(In the drawings, the position, shape, size and configuration of the collecting troughs relative to the cobbers are not intended to be accurate; the drawings are simply intended to convey the idea that two separation products are collected from each cobber. In general, however, there should be no horizontal separation between the two collecting troughs through which ore particles may be lost.)

The ore fed onto the slow cobber 32 from the trough 30 is again divided into two fractions by means of collecting

troughs

38 and 40. The trough 38 is arranged to collect that fraction of the ore fed onto the top of the slow cobber 32 that has little or no iron content. The remainder of the particles are fed into the trough 40 as a middlings product which can be fed back via an appropriate feed back conduit or conveyor 42 to the input feed chute 14 of the rotary air mill 12.

It will be noted that as the speed of the fast cobber 26 is increased a greater percentage of the ore applied to it from the collecting bin 24 will be thrown into the trough 30 while a smaller portion with an increased proportion of iron, will be fed into the collecting trough 34. Similarly, as the speed of the slow cobber 32 is increased, a greater percentage of the ore applied to it will fall into the trough 38 and will be rejected as tailings. In the absence of a variable control arrangement, the speed of the fast cobber 26 is ordinarily adjusted to provide a quantity of concentrate which will be appropriate to the capacity of the processing unit which receives such concentrate as an input. The speed of the slow cobber is usually set so that only that portion of ore containing a very small percentage of iron (say, 1.5 percent) is rejecting as tailings.

However, as the input ore is not of consistent quality, and as the operating parameters prevailing in the rotary forced air mill 12 are not constant, the

cobbers

26 and 32 will not achieve an optimal separation of the ore into concentrate, tailings, and middlings, if the speed of the fast and slow cobbers is fixed.

Only one fast cobber 26 and one slow cobber 32 are shown by way of example in the drawings, but it is to be understood that there are often batteries of such cobbers.

According to the invention, the speed of the slow cobber is regulated by a speed control device that responds automatically to an appropriate sensor.

In applicants co-pending application Ser. No. 684,466, a fast cobber control means is described. In the case of the fast cobber, the sensor may take the form of a load cell or load cells 44 located underneath the collecting bin 36 and responsive to the weight of ore concentrate col lected in the collecting bin 36. In this arrangement, it is assumed that the collecting bin 36 is provided with an output port 46 out of which ore concentrate is fed to a further processing plant or to some other collecting device at a constant rate or at a rate governed by the plant receiving the output.

The sensor 44 sends a signal representative of the weight of material in the collecting bin to a controller 48 which is connected to the motor 28 and regulates the speed of the cobber 26. The controller 48 may be any conventional motor speed controller which can respond to an electrical analogue of an input parameter. In this case, the controller 48 will regulate the speed of the motor 28 to keep the weight of the material in the collecting bin 36 within predetermined limits. If the weight of material in the collecting bin 36 increases beyond a preset limit, the controller 48 will cause the motor 28 to speed up, causing more particles to fall into the trough 30 and fewer particles to fall into the trough 34, thus reducing the amount of concentrate fed into the bin 36. Similarly, if the quantity of material in the collecting bin 36 falls below some preset limit, the controller 48 in response to the analogue of this information fed by the sensor 44 will cause the motor 28 to slow down thus causing more material to find its way into the trough 34 and fill up the collecting bin 36.

Turning now to the slow speed control means provided by the present invention, the slow cobber 32 driven by the motor 33 is regulated by a speed controller 50 in response to a sensor 52 which detects the proportion of iron in the tailings output. The sensor 52 may be, for example, a Ramsay coil which produces an electrical signal whenever magnetic particles fall through it, the electrical signal being related in a predetermined manner to the proportion of magnetic material falling past the coil in a given time.

The speed of the slow cobber 32 is preferably regulated so that only a very small amount of magnetic material, say 1.0 to 1.5%, escapes in the tailings. Thus, when the Ramsay coil or other appropriate sensor 52 signals the controller 50 that the amount of magnetic material in the tailings is too high, the controller 50 will slow down the motor 33 so that more material is fed into the collecting trough 40 and less material, with a lower iron percentage, is fed into the collecting trough 38. Conversely, if the sensor 52 detects a proportion of iron rejected in the tailings below some predetermined limit, say one percent this indicates that the grinding mill 12 is regrinding too much low-grade ore and therefore more of the ground material should be rejected as tailings. Accord ingly, the speed controller 50 will speed up the motor 33 so that a greater percentage of the separation product of the slow cobber 32 will fall into the trough 38, and such percentage will have a slightly higher iron content.

FIGURE 4 illustrates in block diagram form a control system for the slow speed cobber 32. The system is analogous to the one described with reference to FIG- URE 4 of applicants co-pending application Ser. No. 684,466. In the present case, the output of the slow speed cobber includes a recirculating load portion 64 fed back to the primary mill and a tailings portion 66. The tailings portion passes through an iron sensing coil 68 which produces a slave signal 70 fed as one input into a controller 72 whose other input is again a fixed signal preset by the operator as a master set point 74. The two signals fed into the controller 72 are compared and an error signal regulates a motor controller 76 which directly regulates the low speed cobber motor 33. Feed back means from the cobber motor 33 are provided so that the error signal produced by the controller 72 tends to be cancelled out.

If the iron sensing coil 68 indicates that too high a percentage of iron is being sent to the tailings disposal, the slave signal 70 will increase (say) and the controller 72 will produce an error signal which causes the motor controller 76 to slow down the cobber motor 33 so as to reduce the quantity of ore rejected as tailings and so as to reduce the quantity of iron 10st in tailings disposal.

Precisely the opposite action occurs if the iron sensing coil 68 produces a slave signal 70 indicative of too low a percent of iron in the tailings output.

Again similarly to the control system described in application Ser. No. 684,466 the control units of FIG- URE 4 may be provided with delay devices and may provide stepped rather than continuous speed increments for the cobber motor 33 if desired.

The foregoing discussion has not taken into account the fact that many present day control devices include means for responding not only to the absolute magnitude of an analogue signal but also to the rate at which such signal is increasing or decreasing. However, the addition of further control features does not exclude the applicability of such more sophisticated control devices to the cobber speed control system described above.

Instead of the arrangement of the fast and slow cobbers shown in FIGURE 1, the series relationship of the two cobbers can be reversed as indicated in FIGURE 2. In FIGURE 2, the slow cobber 32 receives the output from the collecting bin 24 and produces a tailings fraction which falls into collecting trough 38 and the middlings fraction which falls into a collecting trough 40. The middlings fraction 40 is directed to the fast cobber 26 which separates out from the middlings portion a coarse ore concentrate which falls into the collecting trough 34. The middlings portion which is to be reground by the mill 12 is collected in the collecting trough 30. Exactly the same control arrangement can be made for the fast cobber and slow cobber as described with reference to FIGURE 1.

A further alternative arrangement called a delta arrangement of cobbers is illustrated in FIGURE 3. In this case the ore input from the collecting bin 24 is fed first to an intermediate speed cobber 78 which separates the ore into two fractions, one containing a relatively high percentage of magnetic material collected in collecting trough 80 and the other containing a relatively low percentage of magnetic material collected in the collecting trough 82. The output of the collecting trough 80 is fed as an input to the fast cobber 26 which is driven by motor 28 whose speed is controlled by a speed controller 84. The fast cobber 26 produces a coarse ore concentrate portion collecting in the collecting trough 34 and a middlings portion collected in the collecting trough 30.

The ore containing the relatively low percentage of magnetic material is fed from the collecting trough 82 as an input to the slow cobber 32 which is driven by the motor 33. The slow cobber 32 produces a tailing portion collected in the collecting trough 38 and a middlings portion collected in the collecting trough 40. The ore collected in

troughs

30 and 40 may be combined and recirculated to the mill 12 for further grinding.

The tailings collected by the trough 38 pass through a sensor 52 which may be identical to that shown in FIG- URE .1, and the output of the sensor 52 is fed to a speed controller 50 again identical to that shown in FIGURE 1 which regulates the speed of the motor 33 in exactly the same way as has been described with reference to FIG- URE 1.

However, the control of the opposite side of the separation arrangement is preferably different from that shown in FIGURE 1. While it is possible to have the load sensor 44 provide its output signal to the speed controller 84 which controls the motor 28, it is preferable to provide instead a speed controller 86 for the motor 88 which drives the intermediate speed cobber 78. The reason for this is that the fast cobber 26 is preferably driven at a constant speed so that iron ore concentrate having a relatively constant percentage of iron content is obtained as a separation output collected by the trough 34. Accordingly, the speed controller 84 operates the motor 28 at a constant speed, but could, if necessary, be adjusted from time to time to take into account differences between different qualities of input ore. The fast cobber 26 therefore being regulated to produce an ore concentrate of a given purity, the quantity of ore of such purity produced may be regulated by having the intermediate speed cobber 78 feed more or less ore into the high iron content ore collecting trough 80.

The inter-action of the sensor 44, speed control 86, and motor 88 can be identical to the interaction of the sensor 44, the controller 48, and the motor 28 described with reference to FIGURE 1, and the speed controller 86 can be of exactly the same type as the controller 48.

An alternative arrangement, not shown, for use with the delta cobber arrangement of FIGURE 3 could involve an initial speed control of the speed controller 86 and an override control from that speed controller to the speed controller 84 in the event that the speed controller 86 were operating the motor 88 above or below a preset maximum or minimum limit. The reverse of this operation is also possible, in which the sensor signal is sent first to the speed controller 84, which would send an override control signal to the speed controller 86 in the event that the motor 28 were operating at a preset maximum or minimum limit.

The override signal operates to remove the condition creating it. Thus, for example, if the speed controller 84 responds to the sensor 44 and sends an override signal to the speed controller 86; and if the motor 28 is operating at maximum speed but there is still too much ore concentrate being separated out, the then override signal from the speed controller 84 to the speed controller 86 causes the speed controller 86 to increase the speed of the motor 88, thus reducing the quantity of separation product fed into collecting trough which has the effect of reducing the quantity of separation product fed to collecting trough 34, which is the result desired.

Referring again to FIGURE 1, the fines output from the cyclone collector 22 may be subjected to a separation prior to having the ore passed on to a secondary grinding stage. To this end a slow cobber 90 may be provided, driven by motor 92, to separate out a tailings separation product from the fines product sent on to the secondary grinding stage. The cobber 90 may be a Wet cobber if the cyclone collector output is fed into a slurry to avoid unnecessary dust. The tailings product is collected in a collecting trough 94 and the fines to be sent to the secondary grinding stage is collected in a collecting trough 96. Again a Ramsay coil or other appropriate sensor 98 is used to sense the percentage of iron in the tailings and the output signal is fed to a controller 100 which regulates the speed of the motor 92 and thus of the slow cobber 90 to ensure that the tailings product contains an iron portion kept within acceptable limits. The structure and operation of the coil 98, the controller 100 and the motor 92 may be identical to that of the sensor 52, the controller 50 and the motor 33.

The primary grinding stage of the ore processing system preferably acts as a surge with respect to the secondary grinding stage fed from the collecting bin 36 and the trough 96 so as to maximize the efficiency of the secondary grinding stage. In other words, it is desired that the operating parameters prevailing in the secondary grinding stage be more or less constant to utilize the secondary grinding stage at optimum eificiency. To this end, each of the various operating units in the secondary stage is subjected to control by an appropriate secondary stage control device. In FIGURE 1, an exemplar secondary stage operating unit 102 is controlled by a corresponding secondary stage control device 104 which responds to a secondary stage sensor 106 responsive to the operation of the unit 102. If, for example, the secondary grinding stage is a wet grinding stage, there may be provided pressure control devices for controlling the pressure of slurry entering secondary stage cyclone classifiers, density control devices for controlling the density of slurry fed into the secondary stage cyclone classifiers from a collecting sump fed from the primar grinding stage as well as from the secondary grinding unit, and a pump speed controller for controlling the rate of flow of slurry out of the collecting sump into the cyclone classifiers of the secondary grinding stage. An overall control arrangement for such a secondary grinding stage may be of the type described in copending application Ser. No. 615,755 filed Feb. 13, 1967, in the name of David Weston.

Secondary grinding stage demand may be met by ad justing (either manually or automatically) the output feed rate from the collecting bin 36. The primary stage will thus automatically adjust itself in response to the secondary demand by maintaining a given quantity of ore in the collecting bin 36.

It is also possible to provide further override control arrangements within the primary ore processing stage itself. For example, the controller 48 might provide an override signal to the mill control device 108 (say, the main fan damper) so as to increase or decrease the quantity of ore fed into the collecting bin 24 by the vertical classifier 18 and the cyclone classifier 20. If for example, too much ore concentrate were being delivered to the collecting bin 36 notwithstanding that the fast cobber 26 was operating at its maximum permissible speed, an override signal could be sent from the controller 48 to the main fan damper of the mill 12 to partially close the damper and thereby reduce the volume of air flowing through the mill 12, which will result in finer particles of ore being picked up from the mill 12. The vertical and cyclone classifier outputs will decrease giving a better separation at the fast cobber, yielding a lower volume, higher quality product in the collecting bin 36. Similarly, an override function from the slow speed controller 50 could be fed back to the air volume controller for the mill 12. This override to the air volume controller would close off the main fan damper and reduce the size and quality of the material coming from the fast cobber 26. The operation at the minimum speed of the slow speed cobber 32 would result in an increase in the mill recirculating load collected by the trough 40. This increase would be sensed by the material weighing system at the input to the mill 12. If this increase reached the preset limit of this quantity of recirculating load, an override control function would normally be fed back to the air volume controller. This would result in a reduction of the main fan damper opening and a subsequent decrease in air volume. The size of particle picked up in the air stream would decrease and the quantity and size of material discharged from the vertical classifier 18 and cyclone classifier 20 would decrease. The fast cobber 26 would be able to pick up a greater percentage of the susceptible particles, and the non-susceptible particles from the fast cobber 26 would have a higher non-susceptible tailings product. The slow speed cobber 32 would reject only a poor iron content tailings and thus bring the slow speed cobber controller 50 back within its limits.

The operation of the mill should be controlled by the usual sound and power sensors for optimum grinding efiiciency. No override functions from other controllers should enter into the mill feed controller.

Similar override signals could be provided from the

speed controllers

50, 84, and/ or 86 illustrated in FIGURE 3, if the delta arrangement of cobbers were used.

In accordance with conventional control system design, delay devices may be provided to delay the operation of a control unit by an override signal for a predetermined period following the operation of a control device by any override signal. Control via override signals may be eftected in stepped increments if desired, again in accordance with conventional control systems design practice. The particular choice of override control signals will depend upon the relative importance of the operating parameters of the system. For example, if the economics of the overall plant operation depend primarily on efiicient operation of the secondary grinding stage, then override signals from the secondary grinding stage to the primary grinding stage must be given priority in terms of system design and operation. If, on the other hand, it is more important to maintain a constant rate of output of coarse ore concentrate of a given purity, then the override controls from the controllers of the quantity of coarse ore concentrate will be given preference over the requirements of the secondary grinding stage.

What I claim as my invention is:

1. In a magnetic ore processing system having a rotary magnetic separator yielding a tailings product of low magnetic susceptibility and a product of higher magnetic susceptibility, a tailings sensor for sensing the proportion of magnetic material in the tailings separated out by the separator, and speed control means for the separator responsive to the tailings sensor and adjusting the speed of rotation of the separator so as to regulate the proportion of magnetic material in the tailings product separated out by the separator.

2. Apparatus as defined in claim 1, wherein the ore processing system is a dry ore processing system.

3. In a magnetic ore processing system having a fast rotary magnetic separator and a slower rotary magnetic separator in series relationship so as to yield an ore concentrate, tailings, and a middlings product;

a tailings sensor for sensing the proportion of magnetic material in the tailings separated out by the slower separator, and

speed control means for the slower separator responsive to the tailings sensor and adjusting the speed of rotation of the slower separator so as to regulate the proportion of magnetic material in the tailings separated out by the slower separator.

4. Apparatus as defined in claim 3, wherein the ore processing system is a dry ore processing system.

5. Magnetic ore classification apparatus comprising:

a faster rotary magnetic separator and a slower magnetic separator in series relationship for yielding an ore concentrate separated out by the faster separator, tail ings separated out by the slower separator, and a middlings product separated out by at least one of said separators;

a tailings sensor for sensing the proportion of magnetic material in the tailings separated out by the slower separator; and

6. Apparatus as defined in claim 5, wherein the separators are dry ore separators.

References Cited UNITED STATES PATENTS 2,078,513 4/1937 Stearns 209232 X 2,965,316 12/1960 Henderson et al 24134 3,045,822 7/1962 Cavanagh 209-231 X 3,291,398 12/1966 Weston et al 24124 FOREIGN PATENTS 772,958 4/1957 Great Britain.

TIM R. MILES, Primary Examiner.

US. Cl. X.R.