US3541593A - Control apparatus for wet oreprocessing system - Google Patents

Description

Nov. 17, 1970 D. wEsTo'Ny 3,541,593

CONTROL APPARATUS FOR WET ORE-PROCESSING SYSTEM -f-ff 5-@w l A A Sm l 15H/131.1 l l' .5H SL $62 Sca 5c4 D. WESTON Nav. 17, 1970 CONTROL lPPRATUS FOR WET ORE-PROCESSING SYSTEM uGm @SG AQSSWQ CONTROL APPARATUS FOR WET OREPROCESSING SYSTEM Filed Feb. 13, 1967 D. w'sToN Nov. 17, 1970 5 Sheets-Sheet L .vhmi

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Ll Ll L.. nr i uw Nov. 17, 1970 CONTRGL APPARATUS FOR WET ORE-PROCESSING SYSTEM Filed Feb.` 13, 19s? /m/E/vrol? m//a Wfsm/v ggg; @mdf 5W Arme/vers United States Patent O 3,541,593 CONTROL APPARATUS FOR WET ORE- PROCESSING SYSTEM David Weston, Toronto, Ontario, Canada, assignor to Aerofall Mills Limited, Toronto, Ontario, Canada Filed Feb. 13,`1967, Ser. No. 615,755 Claims priority, application Canada, Nov. 18, 1966,

Int. Cl. B03b 13/00;104c 11/00; F15b 5/00 U.S. Cl. 209-211 9 Claims ABSTRACT OF THE DISCLOSURE The automatic control of slurry pressure in cyclone classifiers by sequentially opening or closing selected classifiers to ow of slurry therethrough.

BACKGROUND OF THE INVENTION In the processing of ore, it is common to mix the ore with water or another suitable liquid and, following one or more grinding operations, to separate coarse ore particles from fine ore particles using cyclone classifiers.

The cyclone classifier, by means of centrifugal force, separates fine particles from coarse particles in a wet slurry. The coarse particles are forced toward the outside wall of the cyclone classifier and proceed downward to the underflow while the fine particles tend to remain in the center of the classifier and rise with the overow output from the top of the classifier.

The manner in which a cyclone classifier behaves is affected by the rate of flow of slurry through the classifier and by the pressure of the slurry entering the classifier. If rate of flow is increased at constant pressure, or if pressure is increased at constant rate of flow, finer particles tend to be forced to the outside wall and to flow out of the underfiow discharge with the result that the overflow has proportionally fewer particles and a proportionally greater quantity of liquid so that the overow slurry density decreases. The converse of the above statement is also true. lf pressure or rate of flow decreases, coarser particles will enter the overfiow and the overfiow slurry density will increase.

The change in the pressure at the input of each classifier will vary as the square of the change of the rate of fiow through the classifier. The rate of flow in the system as a whole can be affected in many ways, some of which are:

(a) Variation in the output of a pump controlling a sump level;

(b) Variation in the amount of water entering a sump as an input;

(c) Variation in the feed rate of fresh unprocessed material into the system.

It has been found in practice that in order to maintain efficient cyclone classification, the pressure of the slurry input to the classifier shouldbe maintained within a predetermined range.

In many ore-processing operations, two or more grinding stages are used-a first grinding stage for reducing the size of the input raw materials, and one or more additional grinding stages receiving the fines from the first grinding stage and regrinding these to produce still finer particles. In such operation, cyclone classifiers may be employed to separate, in the output of the first grinding stages, the

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coarse particles from the fines. The fines, or the more coarse of the fines, are transported to the'secondary grinding stage or stages while the coarse particles are fed back into the input of the first grinding stage'so that they may be further reduced in size before being transmitted to the secondary grinding stages. The underflow of the cyclone classifiers is therefore fed as an input to the first grinding stage and the overfiow of the cyclone classifiers is usually fed into a collecting sump or surge tank. The slurry in the collecting sump is pumped as an input into the secondary grinding stages.

In such systems as the foregoing, it is common to pro- I vide a density control device for controlling the density of slurry fed into the secondary grinding stages. The density control device senses the density of the slurry flowing from the output of the pump and controls the amount of liquids (usually water) fed as an input into the collecting sump. As the slurry density increases, the amount of water input increases correspondingly to reduce the density of the slurry in the sump. Similarly, as the density of the slurry decreases, the rate of flow of water into the collecting sump decreases so that the slurry density will increase.

For efiicient operation of systems such as the foregoing, it has been found that it is desirable to have the secondary grinding stages operate at maximum efficiency and to adjust the operating parameters of the primary stage so as to make possible the efficient operation of the secondary grinding stage or stages. This implies that the level and density of slurry in the collecting sump should be more or less constant. As indicated above, the level of slurry in the collecting sump is determined partly by the rate of fiow of water input into the sump and partly by the overfiOw of the cyclone classifiers leading into the collecting sump. The water input into the collecting sump is, as stated above, controlled by the density control means. Accordingly, the problem remains of controlling the overfiow from the primary stage cyclone classifiers. If the quantity of ore in the cyclone classifier overfiow is greater than the amount of slurry that the secondary grinding system can handle, the level of the collecting sump will increase and the amount of ore in the overflow will have to be reduced. Conversely, if the overfiow from the cyclone classifiers contains too little ore, the level of slurry in the collecting sump could decrease so as to reduce the efficiency of the overall ore-processing operation.

In general, an efficient ore-processing system requires an efficiently-operated secondary (ne) grinding stage and a primary grinding stage that can act as a surge with respect to the secondary stage. Both primary and secondary stages must be provided with control devices to maintain system parameters within efficient operating limits. Prior to the present invention there has been no means for automatic overall control of the control devices in both primary and secondary stages to promote their efficient interaction.

SUMMARY OF THE INVENTION It is accordingly an object of the present invention to ,provide means for automatically controlling, within predetermined limits, the pressure of the slurry introduced into cyclone classifiers used in an ore-processing operation.

According to the present invention, the apparatus for controlling the pressure of a slurry introduced into a plurality of cyclone classifiers comprises a plurality of control valves each respectively associated with a corresponding one of said cyclone classifiers, permitting slurry to flow through any given one of the classifiers when its associated control valve is open and blocking fiow of the slurry through any one of the cyclone classifiers when its respective control valve is closed. A pressure transducer responsive to the pressure of the slurry at the input to the classifiers produces a signal representing the slurry pressure and a pressure comparator responds to this signal so as to produce a high pressure signal when the slurry pressure reaches a predetermined high pressure and produces a low pressure signal when the slurry pressure falls to a predetermined low pressure. Valve closing control means respond to the low pressure signal so as to close one of the valves when the low pressure signal is present, and valve opening control means respond to the high pressure signal so as to open one of the valves when the high pressure signal is present.

If, after a predetermined delay interval following closing of a valve, the low pressure signal is still present, the valve closing control means closes another one of the valves. This operation repeats itself as long as the low pressure signal remains and as long as there remain some valves to be closed. Similarly, if, after a predetermined delay interval following opening of a valve the high pressure signal remains, the valve-opening control means opens still another valve. This operation repeats itself until all the valves are open. It is usually necessary to provide delay means to render inoperative the valve control means immediately following opening or closing of the valves because the opening or closing of a valve gives rise to a transient pressure drop or rise, as the case may be, which could stimulate the pressure comparator into nullifying the previous valve opening or closing because of the transient presence of a low pressure or high pressure signal, as the case may be. In order to eliminate the undesirable efect of the transient pressure change, the delay means operates so that the system can adjust to steady state operation before any further valve operation is effected.

RESUME OF THE DRAWINGS FIG. l is a schematic diagram of a cyclone header including its valves and the related control system according to the invention;

FIG. 2 is a schematic flow diagram showing two stages of an ore processing system employing control apparatus according to the invention;

FIG. 3 is a schematic diagram of a preferred arrangement of override controls according to the invention;

FIG. 4 is a block diagram of an ore processing system using control apparatus according to the invention;

FIG. 5 is a schematic plan view of a plurality of cyclone classifiers arranged around a central cyclone header, which may be used in the system of FIG. 2;

FIG. 6, found on the same sheet of drawings as in FIG. l, is a pressure graph used to explain the pressure control system and level control system according to this invention;

FIG. 7, on the same sheet of drawings as FIG. l, is a circuit diagram of a preferred embodiment of a programmed timer for use in the system of FIG. 4, according to this invention; and

FIG. 8 is a circuit diagram of another preferred embodiment of a programmed timer for use in the system of FIG. 4 according to this invention.

DETAILED DESCRIPTION REFERRING TO THE DRAWINGS FIG. l illustrates in schematic form the pressure control device according to the invention. A fiuid in an input conduit 10 is distributed via a distributor 11 to uncontrolled conduit 12 and controlled conduits 14, 16, 18 and respectively. The conduits 12, 14, 16, 18 and 20 are connected to a common input (namely the distributor 11). Only four controlled conduits and four uncontrolled con- 4 duits are shown in FIG. l, by way of example, but it is understood that any desired number of controlled and uncontrolled conduits may be provided.

The controlled conduits 14, 16, 18 and 20 have mounted therein valves V1, V2, V3, and V4 respectively. According to the invention, the pressure of the fiuid in input conduit 10 is controlled by opening or closing one or more of the valves V1 through V4 inclusive. Thus, if the pressure in input column 10 is too high, one or more of the valves V1, V2, etc., can be opened so as to provide additional routes for the escape of Huid from the distributor 11, thereby reducing the pressure in the input conduit 10. Likewise, if it is desired to increase the pressure in the input conduit 10, one or more of the valves V1, V2, V3, V4 may be be closed so as to decrease the number of routes through which fiuid may leave the distributor 11. It is contemplated that in the system shown in FIG. l, normal operation will require that two of the valves' be opened and two closed. It is further contemplated that the pressure variations in the input conduit 10, will, in almost all cases, be rectifiable by the opening or closing of no more than two additional valves. If the case were otherwise, more controlled conduits should be provided.

The valves V1 to V4 are controlled as follows:

A pressure sensor 22 is mounted on the input conduit 10 or distributor 11 at any convenient place. The pressure sensor 22 is adapted to produce an output signal representative of the pressure in input conduit 10. This output signal is received as an input by a pressure comparator 24 which is adapted to generate a high pressure signal when the pressure in the input conduit 10 rises above a predetermined pressure and is adapted to generate a low pressure signal whenever the pressure in the input conduit 10 falls below a certain predetermined pressure. The high pressure signal is applied via a delay device 30 to a valveopening device 28 and a low pressure signal is applied, also via the delay device 30, to a valve-closing device 26. It is to be understood that the devices 26 and 28 may be combined in a single valve control device, but are shown separately in FIG. 1 for explanatory purposes. The predetermined high and low pressure limits can be taken as predetermined pressure differences with respect to a pressure standard. For some purposes it is convenient to permit adjustment of these limits, and for such purposes pressure standard adjustment means 25 is provided for adjustment of the pressure standard, and thus for the adjustment of the high and low pressure limits. The specific details of the adjustment means 25 are not per se part of the present invention, and obviously may take many forms depending upon, e.g., the parameter chosen to govern the pressure standard adjustment. The means 25 may obviously be integral with comparator 25; in the simplest case, the pressure standard may be a dial setting in the comparator 24.

In response to a high signal, the valve-opening device 28 opens a predetermined one of the valves V1 through V4. This results in the provision of an additional conduit through which fiuid may fiow out of the distributor 11, and therefore the pressure in the input conduit 10 is reduced.

Similarly, if a low pressure signal is generated by the pressure comparator 24, the valve-closing device 26 closes a predetermined one of the valves V1 through V4 so as to block off one of the conduits leading from the distributor 11 and therefore to increase the pressure in the input conduit 10.

It can be seen that opening a valve has the effect of immediately or almost immediately reducing the pressure in the input conduit 10. In the absence of any delay means, the transient reduced pressure could be sensed by the pressure sensor 22, and passed on to the pressure comparator 24, which might then generate the low pressure signal having regard to the transient reduced pressure. (Under steady state conditions, however, the low pressure signal should not be produced-the system should be designed so that opening or closing a valve produces a steady state change not sufficient to bring the pressure to the opposite pressure limit). The transient W pressure signal, however, would result in the closing of the valve that had just been opened, cancelling out the desired opening operation. Accordingly, the delay device 30 is included in the system so that the foregoing undesired transient operation does not occurfTo this end the feed- back paths 32 and 34 leading from the valveclosing device 26 and the valve-opening device 28 respectively operate the delay means 30 whenever the valve-operating devices open or close one of the valves. The delay device 30 then interrupts the signal from the pressure comparator 24 to the valve-operating devices 26 and 28 so that notwithstanding the production by the pressure comparator 24 of a high pressure signal or low pressure signal, neither of the valve-operating devices will operate.

The time intervals during which the delay switch 30 is operable will depend upon the system Whose pressure is desired to be controlled. Basically, it is required that the delay interval be long enough so that the system has ample opportunity to return to its steady state. Then, if after having reached a steady state, the pressure in the system is still too high or too low, the pressure comparator will continue to generate either the high pressure signal or the low pressure signal, as the case may be, and a further valve may be opened or closed.

The valve-operating devices 26 and 28 adapted to operate the valves V1 through V4 sequentially. In steady state operation, let us suppose that the valves V1 and V2 are opened and the valves V3 and V4 are closed. Then, for example, too great an increase in pressure would result in the opening of the valve V3, and if after the delay interval had elapsed, the pressure were still too high, the valve V4 would be opened.

Likewise, if the pressure in the conduit 10 decreases, the valve-closing device 26 closes irst the valve V2 and, if the pressure decrease persists after the delay interval provided by the delay device 30, closes the valve V1 so as to increase the pressure in the input conduit 10.

As mentioned above, as many controlled conduits may be provided as are necessary to cope with the pressure changes in the system. While in most cases approximately the same number of valves will be open as are closed during normal operation, there may be instances in which the system pressure rises above normal by (say) 50% but never falls below normal by more than (say) 10%. In this case, there should be more closed valves than open 'valves during normal operation so that both the expected pressure increases and expected pressure decreases can be handled by the system.

Where the fluid whose pressure is desired to be controlled is the slurry input to cyclone classiers in an ore-processing system, the slurry input will be applied to the conduit -10 in FIG. 1, the distributor 11 in FIG. 1 will correspond to the cyclone header, and the conduits 12, 14, 16, 18 and 20 will lead to respective cyclone classifiers.

FIG. 2 is a flow chart illustrating the automatic control according to the invention, of two grinding stages of an ore-processing system.

The primary grinding stage includes primary grinder P14 to which ore is fed from an ore input P16. The output of the grinder is fed to one or more separators P18 which separate the waste products from usable ore. The usable ore is fed as an input to a collecting sump P12.

A pump P20 pumps the slurry from the collecting sump P12 through a density controller P22 to a plurality of cyclone classifiers P24. The cyclone classifiers may be arranged in the manner illustrated in FIG. 3 and comprise both uncontrolled classiers which pass slurry under all conditions and controlled classifiers provided withgvf'alves enabling slurry to be passed through selected ones of such classifiers having their respective valves open. The valves are opened and closed sequentially in response to pressure conditions prevailing at the cyclone header 56 (FIG. 5) in the manner previously described with reference to FIG. l.

The overflow output from the cyclone classifiers P24 contains the fines which are transmitted as an input to a collecting sump S12 of the secondary grinding stage. The underllow from the cyclone classifiers P24 containing the coarse particles is fed back to the primary grinder P14 for further grinding.

The primary grinding stage includes three control devices, viz, the density controller P22, a pressure controller P26, and a pump controller P28.

The density controller P22 may be of a conventional type known in the art, using a sensing element, for example, a gamma gauge responsive to gamma radiation. The density controller P22 regulates a valve P30 on the water input P32 leading into the collecting sump P12. If the density sensed by the density controller P22 is too low, the controller P22 partially closes the valve P30, permitting less water to pass, thereby increasing the ratio of solids to liquids in the collecting sump P12. Similarily, if the density controller P22 senses a solids-to-liquids ratio which is too high, it opens the valve P30 to permit more water to fiow into the collecting sump P12 thereby tending to reduce the density of the slurry.

The pressure controller P26 comprises the pressure comparator, valve opening and valve closing control device, and delay unit described with reference to FIG. 1. Thus, the pressure controller P26 opens a valve to permit an additional cyclone classifier to pass slurry if the pressure in the cyclone classifier distributor becomes too high. Likewise, the pressure controller closes a valve thereby cutting off a cyclone classifier, whenever the pressure in the cyclone header becomes too low. Por example, assuming cyclone classifiers of 15 inches in diameter, it may be desired to regulate the pressure in the cyclone classilier distributor between 14 and 16 p.s.i. If the pressure reaches 16 p.s.i., the pressure controller opens a valve; if the pressure drops to 14 p.s.i., the pressure controller closes a valve.

It will be noted that in PIG. 5, eight uncontrolled and four controlled cyclone classifiers are shown. Assuming that all the valves are closed, only eight cyclone classiiers pass slurry. If the pressure in the cyclone header rises to 16 p.s.i., the opening of an additional valve will increase the number of conduits conducting slurry from eight to nine. The result is that a pressure drop of the order of 2 p.s.i. (and as a practical matter, somewhat less) occurs. The pressure drop will reduce the pressure prevailing in the system to something just greater than 14 p.s.i. It will be noted that the opening of closing of a valve does not tend to change the pressure in the system so much that the ensuing steady state operation would result in a pressure outside the 14 to 16 p.s.i. limits. If there were too few uncontrolled cyclone classiiers in the system, or if the pressure differential between the permitted high and permitted low pressures were too small, the opening of a valve could lead to a steady state condition in which the pressure was too low, which would result in the re-closing of the valve as soon as delay period had expired; similarly, the closing of a valve would, after steady state was reached, result in a pressure which would require the opening of a valve. This hunting oscillation of the system should generally be avoided; the designer of the system should ensure that the number of uncontrolled cyclone classiiiers is sufcient that the opening or closing of a valve on a controlled classifier Will not cause an ensuing oscillation.

If the number of uncontrolled classifiers is suiiiciently large, and if the pressure differential between the permitted high and permitted low pressures is sufficiently large, it may be possible in some systems to eliminate the delay device described with reference to FIG. 1 alto- 7 gether. However, in most applications, the transient effect caused by the opening or closing of a valve will be sufficient to warrant the inclusion of the delay device.

The pump controller P28 may be of a conventional type known in the art using, for example, an eddy current clutch to control the speed of the pump P20. The pump controller P28 is responsive to a high level sensor P34 and a low level sensor P36 on the collecting sump. If the level of the slurry in the sump falls below the level of the sensor P36, the pump controller P28 causes the pump P20 to slow down so as to permit the level of slurry in the collecting sump P12 to rise. The pump P20 speeds up in response to a rise in the slurry level above the sensor P34.

The sensors P34 and P36 may be of known design and may be combined into a single unit. Among known sensors are bubbler type sensors and resistance probes. In lieu of sensors producing a high level and low level signal, a sensor providing an error signal when the sump level deviates from a preset desired level may be used. The error signal may be positive, for example, when the level is above the preset desired level and may be negative when the level is below the preset level. The error signal would increase in magnitude with increasing deviation of the sump level from the preset desired level. The Pump controller would respond to the error signal so as to speed up or slow down the pump to correct the sump level.

The discussion will proceed on the assumption that discrete sensors P34 and P36 are used to produce a high level and low level signal, it being understood that other sensors could be used instead. The discussion will also proceed on the simplification that the controllers respond only to the presence and/ or magnitude of an input signal. In fact, controllers likely to be used would be more sophisticated and would respond additionally to delays and to rate of change of system parameters. However, the principles of the invention can be usefully and more easily discussed with the aid of the foregoing assumptions.

Returning now to the discussion of the pump controller,

it will be noted that when the pump P20 slows down, the

pressure at the cyclone classifiers P24 tends to drop. This may have the effect of closing a valve so as to permit the pressure in the cyclone classifiers to remain within the predetermined limits. Similarly, when the level of slurry in the sump rises above the level of the sensor P34, and the sump controller P28 sends a signal to the pump P20 causing it to increase its output, the pressure controller P26 may open a valve on one of the controlled classifiers.

The density controller P22 and the pump controller P28 may have a stepped operation analogous to that of the sequential opening and closing of valves caused by the pressure controller P26. In other words, the pressure controller P26 causes stepped increases or decreases in pressure as a result of the sequential closing or opening of valves. Likewise, the density controller P22 may regulate the valve P30 in stepped intervals in response to density information. If the density falls below a lower limit, the valve P30 may, for example, be turned in a closing direction through one revolution. Likewise, if the density controller senses that the density has risen above a certain predetermined density, the valve P30 may be opened through one complete revolution. There may be, say, eight or ten revolutions required to completely open the valve from a completely shutoff condition.

Similarly, the pump controller P28 may, in response to a high level signal from the sensor P34, speed up the pump fiow rate by, say, 10%. Likewise, in response to a low level signal from the sensor P36, the pump controller P28 may slow down the fiow rate of the pump P20 by, say, 10%.

It is clear that there is inter-action between the density controller P22, the pressure controller P26, and the pump controller P28. Basically, the system will be regulated so that first, pressure and second, density tend to remain within preselected optimum ranges. While the pressure and density remain within these optimum preselected ranges, the pump controller P28 tends to maintain the level in the collecting sump P12 within a desired range.

However, let us assume that, with pressure and density in the desired operating ranges, the collecting sump level rises so that the pump controller P28 has signalled the pump P20 to pump at its maximum output rate. Let it be further assumed that the level in the sump P12, notwithstanding the maximum output of pump P20, continues to rise and that a high level signal is transmitted from the high level sensor P34.

In this situation, the only satisfactory way of reducing the level of slurry in the collecting sump P12 is to reduce the fiow of water via the water input pipe P32. The water ow can only be reduced if the density controller senses that the density is too low. The density of the slurry can be made lower by having the cyclone classifiers P24 operate at lower pressure. At lower pressure, more and coarser particles will fiow out of the overflow, with the result that the underflow slurry density will decrease. Accordingly, the high level signals from the sensor P34 may be passed onto the pressure controller P26 to adjust the pressure limits to lower levels than would be desired for optimum operation. At these lower limits, a high pressure signal will be more readily produced, resulting in the opening of a valve to a further cyclone classifier, which has the effect of reducing the pressure at which the cyclone classifiers operate, so as to increase the overfiow slurry density and decrease the underflow slurry density fed to the primary grinder P14. The result is that the density of the slurry in the collecting sump P12 will tend to decrease, and the density controller P22 will cause less water to be fed into the collecting sump, which has the effect of tending to lower the sump level, which is the result desired.

In the foregoing example, the level of the collecting sump was regulated by overriding the optimum pressure limits and substituting lower pressure limits for the operation of the cyclone classifiers. It can be readily seen that other override connections between control units are possible in practice. The example just given would not be representative of optimum system design because operating inefficiencies are introduced when optimum pressure limits are overridden. It is preferred that the pressure limits be kept at optimum setting and that the pressure controller override the density controller when all controlled classifier valves are open or all such valves are closed and the pressure remains too high or too low, as the case may be. Further it is preferred that when the density controller has reached one of its operational limits (by completely opening or completely shutting off the water input to the collecting sump) an override control signal should be sent to the pump controller to speed up or slow down the pump. Thus, the pump capacity and pump speed range should b e designed to be sufficient to keep the sump level within its limits even when the pressure and density controllers are operating at their operational limits.

This preferred system of override controls has the advantage that only one override control is provided to any control unit, so that there is no possibility of any unit receiving conflicting override signals from different units. Further, optimum pressure limits and optimum density limits tend to prevail, promoting economy. Finally, the preferred system enables a convenient override connection from the pump speed controller of the second grinding stage to the pressure controller of the primary stage, as illustrated in FIG. 3 (which also shows the preferred override control arrangement within each of the first and second stages).

Referring again to FIG. 2, the secondary grinding stage includes a secondary grinder S14 whose output is fed into a collecting sump S12. Also fed into the collecting sump S12 is the output of the fines overflow from the cyclone classifiers P24, and a water input S32 adjusted by a valve S30 regulated by a density controller S22.

The slurry in the collecting sump S12 is pumped by a pump S through a density controller S22 to a plurality of cyclone classifiers S24. These cyclone classifiers S24 include uncontrolled classifiers and controlled classifiers and operate in exactly the same manner as the cyclone classifiers P24 in the primary grinding stage. A pressure controller S26 regulates the pressure in the cyclone header of the cyclone classifiers S24 between preselected upper and lower limits, which may be the same as those established for the cyclone classifiers P24 or which may differ, depending upon the size of the cyclone classifiers used and the operational requirements of the system.

The overflow output of the cyclone classifiers S24 is fed as a fines output to final separators (not shown) which separate the final product from the waste. The coarse l output of the cyclone classifiers S24 is fed back to the secondary grinder S14.

In optimum operation of the system, the secondary grinding stage will be operated at full capacity and the pressure and density of the secondary grinding stage will be maintained within preselected limits. Internal override controls for the secondary stage are preferably analogous to those of the primary stage, as shown in FIG. 3. The primary grinding stage should for economy be designed to act as a surge with respect to the secondary grinding state and accordingly should have a capacity somewhat in excess of the capacity of the secondary grinding stage of the operation.

Referring specifically to the internal regulation of the secondary grinding stage which is similar to the internal regulation of the primary grinding state, the density controller S22 and pressure controller S26 maintain the density and pressure of the secondary system within certain predetermined limits subject to an override connection from the pressure controller S26 to the density controller S22. The pump controller S28 maintains the level of the collecting sump within predetermined limits subject to an override connection from the density controller S22. (In lieu of a collecting sump S12, there may be a surge tank in which case the weight of material in the surge tank would be maintained within certain predetermined limits.)

In addition to the internal control of the secondary grinding stage there is an override connection from the secondary grinding stage to the primary grinding stage control units so that the conditions prevailing in the primary grinding stage can be adjusted to make possible the steady state operation of the secondary grinding stage in its optimum pressure, density and level conditions. This can conveniently be accomplished by adjusting the pressure standard for controller P26 via adjustment means P25, corresponding to means in FIG. l. There are two situations in which feed back from the secondary grinding stage to the primary grinding stage control units will occur.

These are as follows: i

(l) With the secondary pump S20 operating at maximum output, the sump level in the sump S12 nevertheless remains too high and the high level sensor S34 transmits a high level signal to the pump controller S28. To correct this condition, the high level signal from the sensor S34 is sent to the pressure controller P26 to increase limits in the primary pressure controller P26. This tends to cause the closing of a valve in one of the controlled cyclone classifiers P24. The result is that the pressure in the cyclone classifiers P24 will increase so as to force finer particles to the outside of the primary cyclone classifiers P24 with the result that the overflow has fewer finer particles and the overflow slurry density decreases. The slurry fed into the collecting sump S12 therefore has a lower density and this lower density will be sensed eventually by the density controller S22 which thereupon decreases the amount of water fed by Water input S32 into the collecting sump. Because less water now enters the collecting sump S12, the sump level tends to decrease, thereby correcting the adverse condition that prevailed prior to the transmission of the signal.

(2) Similarly, if the pump S20 were operating at its lowest permitted speed, and a low level signal were transmitted by the sensor S36, this low level signal would be passed onto the pressure controller P26 so as to decrease the pressure limits at the primary pressure controller, which would tend to lead to the opening of a valve leading to one of the controlled cyclone classifiers P24 so as to permit reduction of pressure in the primary cyclone classifiers. The result would be that coarser particles would flow out of the overflow and the overflow slurry density would increase. This would be sensed eventually by the density controller S22 which would therefore add water via water input S32 and the level of the sump S12 would increase.

Situation (2) is somewhat unrealistic, and should not occur in a properly designed system. Optimally, the secondary grinding stage capacity will be so related to the primary grinding stage capacity, and the flow-through will be so regulated, that the secondary grinding stage tends to operate at full capacity at a relatively fast pump speed and With most or all controlled valves of the cyclone classifiers S24 open.

In FIG. 2, the signals sent by high level sensor S34 and low level sensor S36 to the pressure controller P26 of the primary grinding stage must pass through AND gates S38 and S40 respectively. Also fed to the AND gates S38 and S40 is a pump limit signal from the pump controller S28, which signal is present only when the pump controller S28 indicates that the' pump speed cannot be further adjusted, i.e., that the pump has reached its operational limits. Thus, the high level signal from the sensor S34 is transmitted to the pressure controller P26 only in the presence of a pump limit signal from pump controller S28 indicating that the pump S20 is functioning at maximum speed. Similarly, the low level signal from the sensor S36 appears in the pressure controller P26 only if at the AND gate S40 there is also a pump limit signal from the pump controller S28 indicating that the pump is operating at its slowest speed. Only a single line is shown transferring the pump limit signal from the pump controller S28 and to the AND gates S38 and S40; in practice it may be necessary to have two discrete lines, or it may be possible to have a positive limit signal when the pump is operating at its maximum rate of speed and a negative limit signal when the pump is operating at its minimum rate of speed.

Alternatively, the low level and high level signals can be fed directly to the pressure controller as well as to the pump controller, and the pump controller can send an inhibit signal to the pressure controller which cancels out the high level and low level signals. The inhibit signal itself would be cancelled when the pump reaches its operational limits, in which case the high level signal and low level signal would be operative at the pressure controller to increase or decrease the pressure limits by a preset amount.

The pressure limits prevailing at the pressure controller P26 can thus be varied upwardly or downwardly through stepped increments as a result of persistence of a high level or low level signal at the secondary collecting sump and in the circumstance that the pump S20 has reached an operational limit. Preferably, a delay device is provided to prevent successive adjustments of the pressure limits except after a time delay chosen to permit the system to achieve a steady state condition. The delay device can operate in exactly the same manner as the delay device described with reference to FIG. 1.

If the pressure limits prevailing in the pressure controller P26 have been adjusted to a predetermined maximum or minimum, the density controller P22 may be required to alter the density standard in order to prevent completely unsatisfactory pressure conditions in the primary cyclone classifiers. To this end, once the pressure controller P26 has reached its operational limits, the density controller P22 can be controlled to vary the permitted density limits so as to regulate the water input through input pipe P32 and thus to correct the undesired pressure condition. For example, if the cyclone pressure is too low, the density controller may be adjusted to lower the density limits. The result will be that the density of the slurry will appear to be too high to the controller and extra water will be admitted via the pipe P32. This tends to raise the sump level, and thus the pump P will speed up, raising the pressure at the cyclone classifiers P24.

In some ore-processing systems, it may be advantageous to maintain both pressure and density limits in the secondary stage constant, without having an override connection from the secondary pressure controller to the secondary density controller. In such systems, this may be accomplished by having the secondary pressure controller S26 send an override signal directly to the primary pressure controller P26, as well as having an override connection from the secondary pump controller S28 to the primary pressure controller P26. There would thus be a third and fourth situation in which an override signal would be sent from the secondary stage to the primary stage, viz,

(3) With all the valves of the controlled cyclone classifiers S24 opened, a high pressure signal persists in the pressure controller S26. In this case, the secondary grinding stage is operating at full capacity and therefore it is desired that the primary stage do more grinding. It is therefore desired to decrease the density of the primary overfiow and accordingly the primary pressure limits should be increased. Increasing these limits has the effect of decreasing the primary overflow density as desired, which will have the effect of lowering the density of the slurry fed into the collecting sump S12. The density controller S22 will therefore require less water to be fed in by supply pipe S32 into the collecting sump S12, as a result of which the level in the collecting sump S12 will drop. The drop in level tends to slow down the pump S20, which reduces the flow to the cyclone classifiers S24. The slow down in flow results in a decrease in pressure, as a result of which the high pressure signal will disappear in the pressure controller S26, which is what was desired initially.

(4) In the case that all valves of the controlled cyclone classifiers S24 are closed, and a low pressure signal persists in the pressure controller S26, the pressure controller P26 will be required to operate at decreased pressure limits. The operation will be the reverse of that described immediately above in situation No. (3), and the result will be the eventual elimination of the low pressure signal in the pressure controller S26.

Again situation (4) should not occur in a properly designed system-the secondary stage should be designed to operate at relatively fast pump speed with most of the controlled valves of the classifiers S24 open.

It should be noted that the signal permitting the secondary stage high level signal or low level signal to pass to the primary grinding stage pressure controller P26 may be derived from the pump controller S28 or can be derived from the pump S20 directly. This principle applies equally to the other instances referred to above in which one unit passes on a signal to another unitfor example, the all valves open or all valves closed signal can be derived from the pressure controller S26 or may be derived from limit switches attached directly to the valves.

Instead of having a fully automated feed-back operation as described above, it may be preferable in some circumstances to have only enunciators advising personnel supervising the process that certain physical limitations have reached, in which case the personnel could make appropriate adjustments of limits. For example, instead of having pump controller S28 pass on the high level or low level signal directly to the pressure controller P26 when the pump S20 has reached an operational limit, an operator could be signaled that the pump S20 was operating at an operational limit. The operator could then make the decision to adjust the limits in the pressure controller P26 or to take such other action as might rectify the undesirable situation.

The foregoing discussion has proceeded on the basis that an undesirable situation in the secondary grinding stage has been corrected firstly by adjustment of the pressure limits in the primary grinding stage pressure controller P26. It will be noted that instead of transmitting the override signal or signals from the secondary grinding stage to the primary pressure controller P26, these signals might have been fed back instead to the density controller P22 or to the pump controller P28. However, in most instances it will be found best to adjust primary pressure limits rather than primary density or pump speed.

One reason for this is that the cyclone classifiers P24 can be made to work reasonably satisfactorily over a fairly wide range of pressures, notwithstanding the fact that there is an optimum range to which the pressure limits initially correspond. Secondly, such arrangement permits the preferred override system within the primary grinding stage. Further, feed-back of the secondary grinding stage information to the pump controller P28 tends not to have a completely desirable result, because the initial effect on the secondary grinding stage does not coincide with the effect on the secondary grinding stage following the readjustment of the primary grinding stage to the difference in the speed of the pump P20. This can be best illustrated by an example. Suppose that with the pump S20 operating at maximum output, the sump level in the collecting sump S12 is still too high. Then the high level signal produced by the sensor S34 would be passed on to the pump controller P28. The pump controller P28 could be ordered to speed up or to slow down the pump P20. Let us suppose that it is ordered to speed up. This will have the immediate effect of passing a greater flow of fluid through the cyclone classifiers P24 which will result in a greater volume of slurry being introduced into the collecting sump S12. This has the immediate effect then of worsening the situation at the collecting sump S12. On the other hand, if the pump P20 is ordered to slow down, the immediate effect on the collecting sump S12 will be beneficial but the steady state effect may not be satisfactory. When the pump P20 slows down, the pressure drops at the cyclone classifiers P24. This has the effect of increasing the overflow slurry density. The increased density of the overliow could, when sensed by the density controller S22 lead to the introduction of more water via input S32 into the collecting sump S12. The greater volume of water input may be sufficient to offset the reduced input flow of slurry and the result may be that the situation in the collecting sump S12 has not been improved.

It is diflicult to predict, without reference to an actual physical plant, the optimum stepping operation of the limits in the pressure controller, density controller, or pump controller. The amount by which the limits set in any one of these control units varies in response to a feedback signal will depend upon many factors, including the relative capacities of the two systems; the flow rate, the length of time required for a change in operation of the primary stage to affect the conditions prevailing in the secondary stage, etc. However, as a general rule, the steps should not be too great with respect to the optimum desired conditions. For example, adjustments in pressure limits should be of an order of magnitude comparable to and preferably less than the change in operation which results from the opening or closing of a single valve of a controlled cyclone classifier.

Complete control of the system is effected by the provision of an overflow pipe S42 leading from the collecting sump S12 to the collecting sump P12. Thus, if conditions at the secondary grinding stage are such that the secondary grinding stage cannot handle the slurry fed to it, the overflow slurry will be simply fed back to the collecting sump P12 and will not escape as waste. The slurry can then be reprocessed through the primary grinding stage.

FIG. 4 illustrates the manner in which the control devices of FIG. 2 interact with conventional ore-processing apparatus in a complete ore-processing system.

Referring to FIG. 4, an ore processing system utilizing the inventive apparatus is shown in block diagram form. The particular process to be described is an iron-ore process using magnetic concentrators and separators, but it is to be understood that this is by way of example and that details of the method and apparatus may be modified to meet the requirements of different processes.

This system embodies a two-stage grinding process, the first stage utilizing a wet mill 42 and the second a ball mill '82.

The mill 42 is fed by a plurality of cyclone classifiers 60 and the mill 82 by cyclone classifiers 80.

The mill 42 grinds crude ore in slurry fed from an ore input 40 and regrinds the relatively coarse particles in slurry fed from the underflow of the cyclone classifiers 60. The slurry, after passing through the wet mill 42, is then passed through a screen classifier 44. The coarser material is returned via conduit 48 to the ore input 40 for reprocessing and the fines are fed into a sump 46 and then pumped by a pump 47 to magnetic separators 49. The tailings from the magnetic separators 49 are dumped into a tailing sump 50.

The slurry containing the ore is fed from the separators 49 to a magnetic concentrate sump 52 and then pumped by a pump 53 via a density controller 55 to a cyclone header 56 which distributes the slurry to the plurality of cyclone classifiers 60. Some of these classifiers, as is shown in FIG. 5, are uncontrolled and therefore in use at all times while others, the controlled classifiers, are in use only when their associated control valves V1, V2, V3, V4 are open. FIG. 4 shows only a single block representing the cyclone classifiers 60 but it is understood that it refers both to controlled and uncontrolled cyclone classifiers.

The overflow (the fines) from the cyclone classifiers 60 are fed into a collecting sump 74. The slurry from collecting sump 74 is pumped by a pump 75 through a density controller 78 to the plurality of cyclone classifiers 80. These classifiers 80 send their underfiow (containing the coarse material) through the ball mill 82 for fine grinding. The output of the ball mill 82 is fed back into the collecting sump 74.

The overflow fines from the cyclone classifiers 80 are passed through a hydro separator 90 and thence via a pump 92 to a final magnetic separator 94 whose output containing the ore concentrate is deposited in the final concentrate sump 96. Tailings from both the hydro separator 90 and the magnetic separator 94 are dumped into the tailing sump 50.

The description of FIG. 4 to this point is a description of a conventional ore processing system of a type known in the art, except the use of controlled and uncontrolled cyclone classifiers.

As mentioned previously, it is desirable for efficient operation of the cyclone classifiers to keep the pressure at the input of the cyclone classifiers within a set range.

To this end, the differential pressure across the cyclone classifiers 60 is measured in the pressure transducer 54 conveniently attached to the top of the cyclone header 56. The differential pressure between the header 56 and the atmosphere is converted to an electrical signal which is transmitted from the pressure transducer 54 to a comparator 64. The pressure drops due to the cyclone header outlet and cyclone input valves must be taken into consideration, in order to obtain the actual cyclone differential pressure.

The comparator 64 receives the signal from the pressure transducer 54 as an input, compares said signal with a variable pressure standard. If said pressure signal indicates a pressure falling outside predetermined low and high dead band limits on either side of the variable pressure standard, the comparator 64 sends a low or high pressure signal, depending on whether the pressure is below or above the limits, to a timer 62.

An increase in the variable pressure standard increases the predetermined pressure limits at which the high and low pressure signals are produced by comparator 64. Conversely, when the variable pressure standard decreases, the predetermined pressures at which the high and low pressure signals are produced by the comparator 64 are decreased. The dead band limits remain at a uniform pressure differential on each side of the pressure standard notwithstanding changes in the pressure stadard, and thus the pressure differential between the predetermined pressures producing the high and low pressures signals is constant. The dead band limits are designed to accommodate the pressure range over which the cyclone classifiers operate efficiently during normal or optimum operation.

A pressure transducer, comparator, and timer must be provided also for the cyclone classifiers 80. These devices are shown simply as a single pressure controller 83 for simplification of description. However, while the pressure controller for the primary grinding stage will be designed to permit adjustment of the pressure standard, the pressure controller 83 is preferably designed to permit only manual adjustment of the pressure standard. The reason for the difference between the two pressure controllers as explained above with reference to FIG. 2, is that for efficient operation of the system, the secondary grinding unit must always be operating at full capacity and at optimum operational conditions. To this end, the pressure and density in the secondary cyclone classifiers are kept within predetermined optimum limits, and any variation from secondary optimum operation is corrected in th primary stage rather than in the secondary stage.

Two proposed embodiments of the timer 62 are described in detail later with reference to FIGS. 7 and 8. Essentially the timer includes a device which controls the sequential opening and closing of the cyclone control valves (e.g., V1, V2, V3 and V4, shown in FIG. l) and a delay device which will not permit a second opening or closing of a control valve within a specified period of time following a first opening or closing of a valve. This delay interval prevents immediate reaction to the transient pressure changes in the system following the opening or closing of a cyclone classifier control valve so that the system has time to stabilize itself.

The timer 62 responds to the low pressure signal from the comparator 64 to cause the closing of a predetermined open control valve (i.e., one of the valves shown in FIGS. 1 and 3 as V1, V2, V3, and V4). Similarly a high pressure signal from the comparator 64 will be converted by the timer 62 into a valve-opening operation a predetermined closed control valve. Thus the input pressure at the cyclone header is kept within certain predetermined limits rendering the operation of the system more efiicient.

As stated previously with reference to FIG. 2, it is necessary to keep the levels of the slurry in the magnetic concentrate sump 52 and in the collecting sump 74 within a specified range. To accomplish this, each of the sumps is provided with high level and low level sensors which produce a high level signal when the sump level is too high and produce a low level signal when the sump level falls below the predetermined low level. In the first instance, the sump level information is transmitted to pump controller 51 and pump controller 73 respectively which vary the speed of the pumps 53 and 75 so as to tend t0 maintain the slurry level in the sump within the predetermined limit.

Also as stated above, in certain instances the sump level information from the sump 74 may be fed back as an information input to the controllers of the primary stage. This is accomplished as follows:

The collecting sump 74 is tted vwith a high level sensor 86 which produces a high level signal, and a low level sensor 84 which produces a low level signal. The signals from the level sensors 84, 86 are transmitted first to the pump controller 73 and, when the speed of the pump 7S reaches an operation limit, the high level or low level signals may be passed onto the comparator 64. The signals from the level sensors 84, 86 vary the variable standard pressure in comparator 64, in a manner which can be understood more readily when explained with reference to FIG. 6, which graphically illustrates the operation of comparator 64.. The base line XY represents a pressure scale lwith pressure increasing from left to right. Points A and B on the pressure scale XY represent the lower and upper dead band limits respectively of the preferred variable pressure standard C. The initial pressure at the input of the cyclone classifiers 60 will `be assumed to be at the point P.

Assume that the cyclone input pressure drops from the point P to A. The pressure is now at the lower dead band limit A of the variable set point C so that the comparator 64 sends a low pressure signal to the timer 62. The timer 62 closes a valve and thus increases the cyclone input pressure.

Similarly if the pressure increases above the upper limit B the comparator 64 will send a high pressure signal to the timer `62 which will then open a control valve and reduce the pressure in the cyclone header 56.

Now assume, with the initial pressure at P, that a high level signal is sent from the high level sensor 86 to the comparator 64, with the pump 75 operating at maximum speed. The high level signal moves the variable pressure standard from C to C so that the new lower dead band limit A lies above the original pressure P.

The pressure P now falls outside the range AB so that the comparator 64 sends a low pressure signal to the timer 62. The timer 62 closes a valve to increase the pressure at the inputs to the classifiers. The resulting steady-state pressure indicated by the pressure transducer signal to the comparator 64 is, say, at P on the pressure scale. As stated previously an increase in pressure forces more lines to the outside of a cyclone classifier. Therefore the overflow coming from cyclone classifiers 60 to the collecting sump 74 contains fewer lines per volume of water and thus the density of slurry entering the collecting sump 74 is lower. As a result, the density controller 78, which controls the water valve 79 on the water input 81 to the sump 74, allows less water to pass into the collecting sump, in order to increase the density. Thus, the level of collecting sump 74 drops and the high level sensor 86 ceases to send a high level signal.

Conversely, at pressure P and dead band limits A, B, if when the pump 75 is running at its lowest speed a low level signal is sent from low level sensor 84 to comparator 64, then the variable pressure standard is moved from C to C so that new dead band limit B falls below the pressure P. Thus the comparator 64 sends a high pressure signal to the timer 62 which then opens one of the cyclone classifier control valves V1, etc. As a result, the pressure indicated by the pressure transducer 54 decreases to a point on the pressure scale, say P", the density of the fines in the overow from the cyclone classifier 60 increases and thus the density of slurry in collecting sump 74 increases. The density controller 78 accordingly adjust the valve 79 to add more water, increasing the level in the sump 74, whereupon the low level sensor 84 stops sending a low level signal.

The variation of the pressure standard in the pressure comparator 64, initiated by either the high level signal or the low level signal, should in general be sufficient to operate a valve. The means for varying the pressure CTI standard may be, for example, a potentiometer whose movable contact is operated by a servo-motor which is caused to move in one direction by the high level signal and in the other direction by the level signal. The servomotor may be turned off, for example, by the opening or closing of a valve, by the operation of the delay device, or in any other convenient manner following the desired actuation of a valve. The servo-motor then remains off until the next following high level signal or low level signal, after the expiration of the delay period. Instead of a potentiometer of servo-motor, other desired known means for varying the pressure standard could be used.

As discussed with reference to FIG. 2, the high level, maximum pump speed situation in the secondary stage may occur from time to-time, but with proper design of the units, the low level, minimum pump speed situation should rarely, if ever, occur.

The inter-relationship of the other control units, already described with reference to FIG. 2, :will not be described with reference to FIG. 4, as it is believed that the manner in which these control units interact with the other apparatus in an ore processing system will be clear to the reader who has followed the discussion so far.

The foregoing discussion has proceeded on the premise that both the primary and secondary grinding stages are wet stages. However, control of a secondary wet stage in accordance with the invention may be effected in the manner described above, in the case where the primary stage is a dry stage using, for example, a rotary forcedair type of grinding mill, whose lines output is fed into a collecting sump or surge tank and fed to the secondary cyclone classifiers as a slurry.

In the case of a dry primary stage, any override controls from the wet secondary stage to the dry primary stage must of necessity be adapted to the different operational parameters and control devices utilized in the primary stage. For example, the override control from the secondary pump controller might be applied to the main fan damper of the primary forced-air rotary mill, or to the speed controller for a magnetic drum-type rotary separator, in order to increase or decrease the fines output portion of the primary grinding mill to meet secondary stage requirements.

FIG. 7 is a schematic drawing of one proposed embodiment of the timer 62 shown in FIG. 4.

The timer of FIG. 7 employs a reversing motor MT, driving cams CA, CB, CC, C1, C2, C3 and C4, mounted on a single shaft, three auxiliary timers TH, TL and TX, and relay switches associated with said cams and timers. The timer controls solenoids RV-l, RV-Z, RV-3 and RV-4 which open and close valves V1, V2, V3, and V4 respectively. When the solenoids are energized, the valves are open; when the solenoids are deenergized, the valves are closed.

The cams are notched so that when a notch corresponds to the contact of the associated switching lmechanism the switch is opened. At all other times it is closed.

In FIG. 7, the 0 position for all the cams is, the lower end of the vertical axis, and the angle of rotation increases as the cams move in the clockwise direction. Carn CA has notches at the 0, 90, 180 and 270 positions. Cam CB has a single notch immediately before the 0 position. Cam CC has one notch immediately after the 0 position. Cam C1 has one notch extending from the 0 to the 90 positions, cam C2 has one notch extending from the 0 position, to the 180 position, cam C3 has one notch extending from the 0 position to the 270 position and cam C4 has a single bump at about the 355 mark, Le., it is notched from 0 almost to 360.

A high pressure signal from the comparator 64 shown in FIG. 4 closes switch SH and holds it closed until the pressure is reduced to a pressure below the upper dead band limit. The closing of switch SH closes a circuit through timer TH and normally-closed switch SX from a potential terminal P to ground. Timer TH is energized thus closing the two switches it controls, viz., switches SH1 and SH2. These switches will remain closed as long as TH is energized and for a short interval, say 3 seconds, after timer TH is denergized. With the closing of the switch SH1 two more circuits are closed between the potential line P and ground. One circuit leads through switch SH1 and running coil J of motor MT and the other through switch SII-I1 and timer TX which is in parallel with running coil J. Timer TX, when energized, opens normally-closed switch SX. This switch will remain open while timer TX remains energized and additionally remains open following deenergizing of timer TX for the specified delay interval required for stabilization of the system after thc opening or closing of a valve, say minutes. The opening of switch SX causes the deenergization of timer TH and the subsequent opening of switches SH1 and SH2 three seconds later (say).

When switch SHZ is closed, through switch SCB, a circuit is closed between the potential line P` and ground through the one-half K of the split eld coil in motor MT. When the running coil J is conducting current the energized half coil K will start the motor MT in, say, the clockwise direction. The motor MT will have its running coil J activated through switch SH1 until, say, 3 seconds after timer TH is deenergized. By that time the shaft supporting the cams will have rotated suciently so that cam CA will have closed a normally-open switch SCA. Switch SCA will remain closed and the cams will continue to rotate until cam CA reaches the 90 notch which opens switch SCA and stops the motor MT.

Assume that all valves V1 to V4 shown in FIG. l and F IG. 3 are closed. Then, after rotating through 90, cam C1 will close the switch SC1. Thus, there is a closed circuit between the potential line P and ground through switch SG1 and coil RV1. Coil RV1 is energized, opening valve V1.

It can be seen that another high pressure signal after the delay interval would tuin the shaft through another 90 to the next notch on cam CA, (there being one notch that will open switch SCA every 90) the cam CZ closing switch SC2 (switch SC1 remaining closed), thereby energizing coil RVZ and thus allowing another valve V2 to open.

It a similar high pressure signal persisted after the prescribed delay interval then switch SC3 and, ultimately, switch SC4 would be closed by cams C3 and C4 respectively, thus opening the valves V3 and V4 referred to above.

When all the valves have been opened (i.e., the cams have been rotated through almost 360, say 355) cam CB will open the switch SCB, which will interrupt the circuit to eld coil K thus preventing the motor MT from turning any farther in the clockwise direction. Means (not shown) responsive to the opening of the switch SCB may signal the operator that he has no more valves to open.

Conversely, when a low level pressure signal is sent from the comparator 64 to the timer 62 (both of which are shown in FIG. 4) switch SL is closed completing the circuit through switches SL and SX and timer TL from the potential line P to ground. Thus the timer TL is energized closing switchesl SL1 and SLZ for an interval extending a short time after the timer TL is deenergized, say 3 seconds. With switches SL1 and SLZ closed, running coil J of motor MT will be energized through switch SCC and the switch SLZ in the opposite direction to that of current ilow through half coil K and therefore the starting impulse is in the opposite direction, i.e., counterclockwise. As before, timer TX is energized, opening switch S-X which opens a circuit through timer TL causing it to become deenergized and thus it allows switches SL1 and SLZ to open. Running coil I is energized by electric current first through switch SL1 and then switch SCA until cam CA has rotated back through 90, opening the switch SCA. When the motor MT stops, one of cams C1 to C4 (depending upon the initial conditions of the circuit) will have reached the notch which allows the corresponding closed 18 switch SC1 to SC4 to open thereby deenergizing the associated coil RV1 to RV4 and closing the associated valve V1 to V4.

When the last valve V1 in the sequence closes, cam CC has rotated far enough to open the switch SCC thus preventing the motor MT from turning any farther in the counterclockwise direction and also may send a warning signal to the operator by means (not shown) responsive to the switch SCC, that he has no further control valves to close.

Cam CA has an extended notch at the 0 position so that the motor when turning clockwise will be stopped at the same time that cam CC opens switch SCC and, when turning counterclockwise, will be stopped at the same time that cam CB opens switch SCB. This prevents the shaft from actually reaching the 0 mark, from either direction. If the shaft could reach the 0 mar-k, no further operation would be possible because of the positions of the notches on cams CB and CC.

Although only four cyclone feeder control valves are shown in FIGS. 1 and 5 and discussed in the description of FIG. 7, it may be seen that more control valves may be added by adjusting the cam angles of cams C1, CZ, etc. and by adding notches to the cam CA. Thus for six valves the cam CA would be notched every 60. For the cyclone classifiers 60 in normal operation, valves V1 and V2 may be open and valves V3 and V4 mya be closed. Under all but extreme circumstances it is assumed that for the system described, the opening or closing of two more valves will be suicient to correct any pressure or level problems. If this is not the case more controlled conduits should be provided.

FIG. 8 is a schematic drawing of another proposed embodiment of the timer 62 shown in FIG. 4.

This device comprises two timers and pluralities of latching relays, unlatching relays and relay switches, which control solenoids RV1, RVZ, RV3 and RV4 that open associated valves V1, VZ, V3, V4 when energized and close the valves when deenergized.

A high pressure signal from the comparator 64 closes a switch RH which will stay closed until the high pressure signal stops. If the normally closed switches RA1 and RB1 are closed, there will now be two closed circuits be tween a potential line Q and ground, one through switches RH, RA1 and RB1 and a timer TA and the other through said switches and a latching coil L1. When latching coil L1 is energized, it closes and latches switch RL1 responsive to coil L1 thus closing the circuit between potential line Q and ground through coil RV1 and activating coil RV1, thereby opening control valve V1 (shown in FIGS. l and 3). When valve V1 opens it causes a limit switch SIA to close and another limit switch S1B to open.

At the same time as coil L1 is energized timer TA is energized opening the two normally closed switches RA1 and RAZ which it controls, thereby cutting oi current to timer TA and coil L1. These switches RA1 and RAZ stay open for a specified delay interval (say 5 minutes), surticient for stabilization of the system after opening a valve, thus prevent the opening or closing of any valves during said delay interval.

IIf a high pressure signal persists after the delay interval has elapsed, switch RA1 becomes closed again and once more circuits will be closed through timer TA, through latching coil L1 (which has no effect because contacts RL1 are still locked shut), and through limit switch SIA and a latching coil LZ. When timer TA is energized it will again open RA1 and RAZ for said delay interval. When latching coil L2 is energized it closes and latches the switch RLZ which it controls thus activating coil RVZ and thereby opening valve V2. Valve VZ on opening closes a limit switch SZA and opens a limit switch SZB.

Similarly if the high pressure signal persists after the specified delay interval a latching coil L3 is activated closing and latching a switch RLS which it controls, activating a coil RV3, thus opening valve V3 which closes a limit switch S3A and opens a limit switch S3B.

If the high pressure signal still persists a latching coil L 4 is activated closing and latching a switch RL4 which it controls, activating coil RV4 which opens control valve V4. Valve V4 opens a limit switch S4B and closes a limit switch S4A which closes an indicating circuit (not shown) which warns the operator that he has no more valves to open.

Now assume, with all the valves open and switches RAZ and RB2 closed, that a low pressure signal is sent from the controller 64. The signal will cause switch RL to close thus closing a circuit between potential line Q and ground through a timer TB and an unlatching coil UL4. As soon as timer TB is energized it opens switches RBI and RBZ and holds said switches open for a specified delay interval that may be the same as the delay interval for timer TA. Unlatching coil UL4, which is energized at the same time as timer TB, unlatches the contacts RL4 thereby denergizing coil RV4 which causes control valve V4 to close. When valve V4 closes, it causes limit switch S4B to close. This allows an unlatching coil UL3 to be energized when the next low pressure signal is transmitted, and causes limit switch S4A to open, thereby discontinuing the warning signal.

If, after the delay interval, there is a low pressure signal, switch RL will close, timer TB will be energized and will respond as above, and unlatching coil UL3 will be energized through switch S4B. Unlatching coil UL3 will unlatch switch RL3 thereby, deenergizing coil RV3 and causing control valve V3 to close. The closure of valve V3 closes limit switch S3B and opens limit switch S3A.

Note that in this state of the system with switch S3A latching contacts RL1 and RL2 closed, the only latching circuit that would respond to a high pressure signal would be that associated with latching coil L3. With unlatching coils UL4 and UL3 already activated, switch S3B closed and switch S2B open, the only unlatching circuit that could respond to a low pressure signal would be that associated with unlatching coil ULZ. Therefore, in response to a low pressure signal the last opened valve (which is still open) is the only one that can be closed and in response to a high pressure signal the last closed valve (which is still closed) is the only valve that can be opened. It can be seen that the above is applicable mutatis mutandis, no matter how many valves are opened or closed.

Assuming that a low pressure signal persists, the closing of switch RL activates unlatching coil UL2 which will unlatch switch RLZ thereby deenergizing coil RVZ and causing valve V2 to close. When valve V2 closes it causes the limit switch S2B to close and the limit switch ,52A to open.

A further low pressure signal causes R1 to close, energizing unlatching coil UL1 thereby unlatching switch RL1 which deenergizes coil RVl thus closing valve V1. Valve V1, when it closes, causes switch S1A to open and switch S1B to close an indicating circuit (not shown) which indicates to the operator that he has no more valves t close.

Using the timer shown in either FIG. 7 or FIG. 8 it may be seen that in the event of a power failure all coils RV1 to RV4 would be deenergized thus closing all the cyclone classifier control valves. This tends to prevent overflow of the sumps.

I claim:

1. Apparatus for controlling the pressure of slurry introduced from a common input into a plurality of cyclone classifiers, comprising a plurality of control valves each respectively associated with a corresponding one of said cyclone classifiers permitting the slurry to flow through the corresponding cyclone classifier when the respective associated control valve is open and blocking flow of slurry through the corresponding cyclone classifier when the respective associated control valve is closed, a pressure transducer in the vicinity of the common input and responsive to said slurry pressure and producing an output Signal representing said slurry pressure, a pressurecom! parator responsive to the output signal of the pressure transducer and producing a high pressure signal when said slurry pressure reaches a predetermined high pressure limit and producing a low pressure signal when said slurry pressure falls to a predetermined low pressure lirriit, valve-closing means responsive to said low pressure signal and closing said valves continually and sequentially when the low pressure signal is present, valve-opening means responsive to the high pressure signal and opening said valves continually and sequentially when the high pressure signal is present, and delay means responsive to actuation of both said valve-operating means and preventing said valve-operating means from operating for a predetermined time interval following the last opening or closing of a valve notwithstanding the production of a low pressure signal or a high pressure signal during said interval.

2. Apparatus as defined in claim 1, wherein the valveopening means and the valve-closing means for each Valve are combined in a single valve-operating means capable of opening and closing the valve.

3. Apparatus as defined in claim 1 wherein the comparator is responsive to a variable pressure standard input thereby to increase the predetermined high pressure and low pressure limits at which the high pressure signal and low pressure signal are produced when the pressure standard increases, and to reduce the predetermined pressures at which the high pressure and low pressure signal are produced when the pressure standard decreases.

4. Apparatus as defined in claim 3, wherein the pressure differential between the predetermined high pressure and low pressure limits producing said high pressure signal and said low pressure signal is constant notwithstanding variation of the pressure standard.

5. Apparatus as defined in claim 4, wherein the pressure sensed by the pressure transducer is the pressure of the slurry in a distributor comprising said common input to said cyclone classifiers.

6. ln the ore-processing art, apparatus for controlling the pressure of slurry fed into a distributor for a plurality of cyclone classifiers, comprising a plurality of classifiers fed by this distributor and uncontrolled as to fiow of slurry therethrough, a plurality of classifiers each having an associated valve controlled so as to be openedor closed to fiow of slurry therethrough and fed by the distributor, and a pressure controller responsive to the pressure of the slurry in the distributor and sequentially opening said controlled classifiers when the slurry pressure exceeds a predetermined high pressure limit and sequentially closing said controlled classifiers when the slurry lpressure lies below a predetermined low pressure limit, the pressure controller including delay means interrupting the sequential operation of the controlled classifiers for a pre-set delay period following the opening or closing of any of the controlled classifiers, the pressure controller including (1) a pressure transducer sensing the pressure'of the 'slurry in the distributor and producing an electrical signal representative of the slurry pressure, (2) a pressure comparator receiving as inputs the transducer output signal and a signal representative of a pressure standard and sensing the difference between the two inputs,l and (3) valveoperating means responsive tothe difference sensed by the comparator and opening the valves sequentially when the comparator senses that the slurry pressure exceeds the pressure standard by more than a first predetermined amount therebyveXceeding the high pressure limit and closing the valves sequentially when the comparator senses that the slurry pressure lies below the pressure standard by more than a second predetermined amount thereby lying below the low pressure limit.

7. Apparatus as defined in claim 6, wherein the sum of the first and second predetermined amounts is of the order of the pressure standard divided by the average number of classifiers passing slurry therethrough.

21 22 8. Apparatus as dened in claim 7, wherein the rst FOREIGN PATENTS and second predetermined amounts are the same. 667,142 2/1952 Great Britain 9. Apparatus as defined in claim 6, additionally including means for adjusting the pressure standard in response FRANK W LUTTER Primary Examiner to one or more parameters of an ore-processing system. 5 ROBERT H ALPER, Assistant Examiner U.S. Cl. XR.

References Cited UNITED STATES PATENTS 1,890,070 12/1932 Whiton 55-344 X 2,119,478 5/1938 Whiron 55-344 X 10

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