CA1110787A - Thickener control system - Google Patents

Abstract

APPLICATION FOR UNITED STATES PATENT

THICKENER CONTROL SYSTEM

ABSTRACT OF THE DISCLOSURE
The operation of a continuous thickener, clarifier, or similar device having a feed well into which a feed slurry of suspended solids and a flocculant are continually introduced to effect separation of the slurry into a clari-fied solution which is withdrawn through an overflow launder and a thickened sludge which is transported into a discharge port at the bottom of the unit is controlled by establishing the slime level at a point near the bottom of the feed well, continuously monitoring the density of the downflowing slurry within the well, and regulating the rate of floccu-lant addition in response to small changes in the slurry density.

Description

1 BACKGROUN~ 0~ THE INV¢NTIO~

2 1. Field of the Invention: This invention

3 relates to continuous thic!;eners~ c',arif~ers and similar

4 gravitational settling devices for separating feed s]urries or pulps into clarified liquid and sludge and is particu-6 larly concerned with a method for controlling the operation 7 of such devices to irnprove their efficiency.
8 2. Descr~ ion of the P ior Art: Continuous 9 thickeners, clarifiers and similar gravitational settling devices are widely used in the chemical and metallurgical 11 industries for the removal of liquids from slurries, metal-12 lurgical pulps, sewage, and other liquid-solid suspensions.
13 Such devices generally include a circular tank having a 14 cylindrical center well which extends downwardly into the vessel and is open at the bottom. I'he incoming slurry or 16 pulp passes through a feed pipe or launder into the upper 17 part of this center well and is introduced into the sur-18 rounding liquid through the bottom of the well in a manner 19 designed to create a minimum of turbulence. This makes it possible to contain the bulk of the solids near the center~
21 of the unit. On leaving the well, the liquid entering with 22 the pulp or slurry tends to move outwardly in a radial di-23 rection and flow upwardly toward a peripheral overflow 24 launder. ~le solids suspended in the slurry or pulp settle downwardly through the slow-moving liquid and accumulate 26 on the bottom of the unit. These solids are compressed as 27 they accumulate and are slowly moved toward a bottom sludge 28 discharge opening by means of slowly revolving rakes sus-29 pended a short distance above the bottom. The rakes aid ln compressing the sludge and reduce its liquid content ~1 to a greater extent than is normally achieved in batch 32 sedimentatlon operations.

1 During the normal operation of a thickenerJ
2 clarifier or similar continuous gravity sedimentation device 3 of the type ref`~.~.ed tc~ above, a series of relatively well-4 defined, vertically-spaced zones e~ist within the unit.
The uppermost of these zones comprises a layer of clear 6 liquid or clarified solution from which most of the solids 7 have settled out. Below this is an intermediate layer con~
8 taining suspended solid particles which is generally re-9 ferred to as the "floc" layer. The interface between the clear solution and the floc layer is normally referred to 11 as the "slime level". At the bottom of the unit is a layer 12 of settled sludge. Such a system is a dynamic one charac-13 terized by the movement of liquid and solid particles 14 between these zones. The levels of the three zones may vary considerably, depending upon the feed stream, operat-16 ing conditions and other variables. During startup of the 17 unit and during periods in which significant changes in 18 feed rate, feed composition or other variables take place, 19 three distinct zones may not exist. To achieve ma~imum capacity with such a unit, it has generally been thought 21 that the slime level should be maintained as close to the 22 top of the unit as possible and that only a thin layer of 23 clarified solution should be maintained above the floc 24 layer.
It is conventional to add flocculants or coagu-26 lants to thickeners, clarifiers and similar devices to 27 increase their capacities. These materials cause the sus-28 pended particles in the slurry or pvlp to floc:culate or 29 agglomerate and thus settle more rapidly. The amount of flocculant or the like which is required at any particu-31 lar moment depends in part upon the slurry or pulp feed 32 rate, the solids content of the feed, the solids size range ?`~ a~

1 and distribution~ the densities of the solid particles, 2 and the temperature and other operating conditions. Under 3 constant conditions, the amount of flocculant needed to 4 achieve maximum capacity in a particular thickener, clari-fier or the like can generally be determined by trial and 6 error. In actual practice, howeverJ the conditions may 7 change rapidly due to variations in the amount and composi-8 tion of the solids suspended in the feed stream and other 9 variables over which the operator of the unit may have relatively little or no control. Frequent adjustment of 11 the amount of flocculant or coagulant added to the system 12 is necessary to compensate for these variations and main-13 tain the desired capacity and degree of separation while 14 at the same time keepîng operating costs within acceptable bounds by eliminating overflocculation.
16 It is common practice to use the slime level 17 within a thickener or clarifier as a measure of the unit's 18 performance and ko monitor this level as a means for de- ;
19 termining the need for changes in the flocculant rate. In general, the higher the slime level, the more flocculant 21 that is needed. This measurement of the slime level has 22 generally been done manually by means of measuring sticks 23 lowered into the vessel near the outer edge of the unit.
24 It has also been proposed that floats, differential pressure cells, or radiation detectors be positioned in the outer 26 part of the tank to locate the step discontinuity in density 27 representing the slime level and that changes in this le-~el 28 be used to control the addition of flocculant to the system.
29 ~ne slime level is not a direct measure of the settling characteristics of the solids in the pulp or slurry, how-31 everJ and instead is the effect of a combination of vari-32 ables, inclu~i?-g flocclllant rate, solids feed rate, solids ~4-1 and liquid characteristicsJ and the like. mere is normally 2 a long time lag between changes in the floccula~t rate and 3 corresponding changes in the slime level and hence the 4 operator must estimate the amount of change in the floccu-lant rate which will be needed to produce a desired change 6 in slime level. If he overestimates or underestimates the 7 change in rate required~ the unit may become unstable and 8 eventually have to be shut down to avoid overloading or 9 the carryover of slimes. The slime level therefore pro-vides at best a visible means for assessing the state of 11 the thickener or clarifier operation and, if it increases 12 progressively, may serve as an advance warning that the -13 capacity of the thickener or clarifier has been exceeded.
14 It is virtually useless as a means for controlling the rate of flocculant addition.
16 Recognition that earlier methods for controlling 17 thickener operation by regulating the rate of flocculant 18 addition are ineffective has led to a suggestion that the 19 incoming feed slurry or pulp in the feed launder be sampled at regular intervals downstream of the point of flocculant 21 addition, that each sample thus collected be passed into a 22 special gravity separation vessel where representative 23 settling can take place, and that the separated liquid and 24 solids phases be separately withdrawn from this special separation vessel in a volume ratio determined by the con-26 ditions desired in the thickener or clarifier. By sensing 27 the interface level between the liquid and solids phases 28 in the separation vessel and adjusting the rate of floccu-29 lant or coagulant addition to the feed stream in accor- -~
dance with variations in the level of the interface during ~1 operation of the system, it has been said that the rate 32 of flocculant addition can be controlled automatically and that flocculant consumption can be reduced substantially. Experlence has demonstrated, however, that the system thus proposed is not effective. Its use has therefore been abandoned Efforts to develop other, more reliable methods for the automatic control of flocculan~ or coagulant addition in order to stabi-lize the operation of thickeners and clarifiers and reduce chemical consumption have in the past been largely unsuccessful.
SUMMARY OF THE INVEMTION
The present invention provides an lmproved method for controlling a thickener, clarifier or similar gravitational settling device by regulating the rate of flocculant or coagulant addition which largely avoids the difficulties outlined above. The method of the invention results in more stable thickener or clarifier operation and makes it possible to achieve higher sludge densit~es, lower clarified solution solids contents, decreased flocculant or coagulant con-sumption, increased slurry or pulp capacity, and reduced unit downtime. The overall effect of this is a significant reduction in the cost of thickening or clarification per unit volume of slurry or pulp handled.
In a more particular way, the invention provides a method for con-trolling the opera~ion of a continuous gravlty sedimentation unit having a feed well into which a slurry of suspended solids and a flocculating agent are introduced to effect separa~ion of said slurry into a clarified solution that is recovered fro~ sa$d unit as an overflow liquid and a sludge that is withdrawnfrom ~he bottom of said unit whereby consumption of flocculating agent is re-duced comprising the steps of establishing a slime level within a predetermined optimum distance from the bottom of said feed well by varying the flow rates to and from said unit, detecting changes in the density of the downflowing slurry over a vertical interval in said feed well, correlating s~id density changes in the solids settling rate ~ithin said well, and continuously varying the rate at which sai.d flocculating agent is introduced into said feed well in response to : . , : :

, '7 said changes in density and whereby control of both settllng rate and ~sllme level is obtained.
In accordance with the invention, it has now been found that the operation of a thickener~ clarifier or similar device having a feed well through which the incoming slurry or pulp and a flocculant or coagulant are introduced into the unit can be effectively controlled by establishing the sl~me level with-in a predeter~ined distance of the bottom of the feed well9 monitoring very small changes in the density of the slurry or pulp as it moves dowmwardly within the well, and adjusting the rate at which the flocculant or coagulant is added in response to these small changes in density. The invention is based in part upon - 6a -~,, , :~ , , .

1 the discovery that the position of the slime level with 2 reference to the bottom of the center well has a pronounced 3 ef'fect upon the settling rate within the well and that a 4 suitable slime level is a critical factor if ef'fective control is to be established and maintained. Once the 6 proper slime level has been obtainedJ changes in the den-7 sity of the downflowing slurry or pulp provide a direct 8 measure of the effect of flocculant or coagulant addition 9 upon particle settling velocities. The monitoring of such changes makes possible the stabilization of thickener or 11 clarifier operations and permits optimization in terms of 12 reagent consumption, sludge density, clarified solution 13 solids content, slurry or pulp volume, and unit downtime, 14 despite su~stantial fluctuations in the feed rate to the unit, variations in the characteristics of the ore or other 16 solids being handled, and changes in the amount of solids 17 present in the incoming slurry or metallurgical pulp.
18 ~s indicated above, it has been found that the 19 position of' the slime level with reference to the bottom of the feed well has a marked effect upon the settling rate 21 within the well. If' the level becomes too high, the slimes 22 concentration in the well will increase until a point is 23 reached at which effective control of the unit can no longer 24 be maintained. Similarly, if the level becomes too low, the well will become depleted of slimes and control will 26 again be lost. The zone within which the slime level must 27 be maintained depends in part upon the configuration and 28 dimensions of the particular thickener, clarifier or simi~
29 lar device and will vary sornewhat from one unit to another.
~0 In general, however3 it has been found that the slime ~1 level should normally be established and maintained within 32 a zone extendir~g fro~ about 18 inches above the bottom of 1 the feed well to about 18 inches below the bottom of the 2 well. The optimum position in a particular thlckener or 3 the like can readily be deter~nined experimentally by vary-4 ing the slime level while adding flocculant in response to changes in slurry density within the feed well and ob-6 serving the effect of such variations on control of the 7 unit. Further changes in slime level will normally occur, 8 although in some cases slowly, if the initial level is 9 too far above or below the bottom of the well~
The slime level in a thickener or similar device 11 can be adjusted to the required level by changing the flow 12 rates to and from the device. An increase in the rate 13 at which slurry is supplied to such a unit and a corre-14 sponding increase in the rate at which clarified solution is withdrawn, assuming there is no change in settling rate, 16 will normally result in an increase in the slime level;
17 while a reduction in rates will have the opposite effect.
18 By varying the rates in this manner, the level needed to 19 permit effective control in accordance with the invention can normally be established without difficulty.
21 The method of the invention requires the measur-22 ing of the density of the downflowing pulp or slurry at 23 two or more points or over an interval within the feed well 24 of the thickener or similar device. The density value near the top of the feed well will normally be less than near 26 the bottom of the well. As the pulp or slurry moves down-27 wardly within the well, the density will increase with 2~ flocculation. The variations in density which thus take 29 place wi]l depend in part upon the particular liquid-solids system being handled, the degree of flocculation which 31 occurs, and other factors. Although these variations are 32 normally small, they can be readily detected. In a 1 typical thickener operation handling a metallurgical pulp 2 containing mineral solids, for example, the density of 3 the incoming pulp in the upper portion of the feed well 4 may average about 1. o43 grams per cubic centimeter; while that of the heavily flocculated pulp in the lower part of 6 the well may average about 1.081 grams per cubic centimeter.
7 In this particular instance, the density span is thus about 8 0.04 gram per cubic centimeter. r~hese values illustrate 9 the variation which may occur due to flocculation but should not be taken as limiting. The difference in density 11 values due to flocculation may be greater or less than this, 12 depending upon the particular liquid-solids system being 13 handled, the solid particle sizes, and the like. For most 14 applications of the invention, it is preferred to employ a system sufficiently sensitive to detect changes in density 16 of about 0.005 gram per cubic centimeter or smaller within 17 the range between about 1.0 and about 1.200 grams per cubic 18 centimeter.
19 Any of a variety of different instruments having the required sensitivity may be employed for the detection 21 of density changes in carrying out the invention. Suitable 22 instruments include optical devices based upon changes in 23 light transmissibility of the pulp or slurry such as sub 24 merged turbidimeters, sonic devices responsive to changes in sound transmissibility which accompany changes in pulp 26 density, and radioactive systems which detect variations in 27 the passage of energy from a radioactive source to a de- ~r 28 tector with variations in pulp density. As pointed out 29 earlier, the system employed should be capable of detecting density changes which take place over a vertical interval 31 within the feed well and hence the system selected will 32 normally include either rnultiple sources or multiple 1 detectors or both or will ut:ilize an elongated or colli-2 mai:ed source or detector or both such that the value 3 obtained represents changes in density over such a ver-4 tical interval, In general, it is preferred to employ a radioactive source and detector system havirlg a strip 6 source of gamma ray radiation several inches long. Such 7 radioactive-source detector systems are available from 8 commercial sources and will be familiar to those skilled 9 in the art. Suitable optical and sonic devices which may be employed for purposes of the invention are also commer-11 cially available.
12 The density measuring instrument employed for 13 purposes of the invention will normally be suspended in 14 the feed well of the thickener or clarifier below the pulp or slurry surface and will be supported on an adjustable 16 carriage above the well so that the position of the device 17 can be varied within the well. The output signal from 18 this device is passed through an electrical conductor to 19 a power supply unit where the signal is amplified and used to provide a corresponding voltage which can be re-21 corded and employed to drive a controller connected to an 22 electrically or hydraulically operated valve in the supply 23 line through which flocculant is introduced into the well.
24 It is generally preferred to introduce this flocculant into a line through which water is supplied or clarified 26 liquid is returned to the well to increase the efficiency 27 of flocculation within the thickener or clarifier. This 28 aids in securing proper distribution of the flocculant 29 in the well. The submerged instrument detects changes in density representing variations in the settling rates of 31 the solids in the pulp or feed slurry and emits an elec-32 trical signal which results in an instantaneous adjustment 1 of the flocculant rate to counteract the changes in 2 settling rate. Changes in density representing a reduction 3 in setl1ing raie produce an increase in flocculant addition ~ su~ficient to o~fset and compensate ~or the reduction.
Changes indicating an increase in settling rate, on the 6 other hand~ result in a decrease in flocculant addition 7 sufficient to offset and compensate for the increase. The 8 result is better control of the unit and much more uniform 9 operation than could otherwise be obtained Experience has shown that this system provides 11 precise control of flocculant addition, enables the unit 12 to respond to extreme changes in pulp or slurry volume and 13 composition, permits the maintenance of a substantially 14 constant slime level within the thickener or clarifier, results in increased thickener underflow densities, sub-16 stantially reduces thlckener downtime, and has numerous 17 other advantages. These and other advantages of the system 18 will become more readily apparent from the detailed descrip-19 tion of the preferred embodiments of the invention set forth below.
21 BRIEF DESCRIPTION OF ~HE DR~WI~
22 The drawing is a schematic representation of a 23 vertical section through a rotary thickener provided with 24 a flocculant addition control system constructed in accor-dance with the invention.

.
27 The drawing illustrates a continuous rotary 28 thickener including a cylindrical tank of steel) concrete 29 or wood having a bottom 11 which slopes downwardly from the tank periphery to a sludge discharge opening at the 31 center of the bottom. The discharge opening includes a 32 sludge cone 12 through which sludge produced in the unit 1 moves into discharge line 1~. Overflow launder 14 is pro~
2 vided at the upper periphery of the tank for the wi-thdrawal 3 of clarified liquid from the unit. A revolving rake 4 assembly including a central shaft 15 driven ~y a motor and gear assembly not shown in the drawing is provided at 6 the center of the tank~ Supported on shaft 15 is a rake 7 assembly including rake arms 16 and 17 which are attached 8 to the shaft by means of spider 18. Tie rods 19 and 20 9 connected to the shaft by tie rod spider 21 support the rakearms. Braces 22, 2~, 24 and 25 are provided to 11 strengthen the rake assembly. 31ades 26 are mounted on 12 the lower part of the rake arrns and are set at an angle 1~ to the arms so that sludge accumulating on the bottom of 14 the tank is pushed toward the discharge opening as the rake assembly revolves in the tank. In lieu of a sloping 16 bottom as shown, the unit may be provided with a flat bottom, 17 in which case solids will accumulate to form a sloping con-18 figuration similar to that shown.
19 The upper part of tank 10 is provided with a feed well 27 supported by structural members which do not appear 21 in the drawing. This feed well is a cylindrical member 22 which is open at the top and bottom and has a diameter of 2~ from about 10 to about 30~ of the tank diameter. The upper 24 end of the feed well may or may not extend above the inner edge of overflow launder 14 and thus be higher than the 26 liquid level in the tank. At its lower end, the feed well 27 may be provided with an adjustable skirt or shroud 28 28 which permits the length of the well ko be increased or 29 decreased as desired. ~ne skirt is attached to the outer ~0 part of the feed well by means of clamps or other devices ~1 not shown in the drawing. The feed pulp or slurry is ~2 introduced into the unit through feed launder or pipe 29 1 which discharges into the upper part of the well. An 2 overflow line 3Q with one or more outlets 31 in the upper 3 part of the feed we:ll is prc~ide.l for the introduction of 4 water or the return of clear solution to the unit. The device shown will normally have a center scrapper 32 6 located at the lower end of shaft 15 and a mechanism, not 7 shown in the drawing, for raising the shaft to liff the 8 rake assembly above the bottom to prevent overloading of 9 the unit.
The apparatus employed for purposes of the inven-11 tion has been described up to this point in terms of a 12 specific type of thickener but it will be understood that 13 the invention is not restricted to this particular appara- ~
14 tus. It may be employed with a variety of other thickeners, ;
clarifiers, and similar gravity sedimentation devices pro-16 vided with revolving rake assemblies~ including torque 17 thickeners, tray thickeners, traction thickeners, upflow or 18 siphon feed thickeners and clarifiers, and the like. It 19 can also be utilized in conjunction with horizontal, rather than rotary, thickeners, clarifiers and similar devices.
21 Such devices have been described at length in the technical 22 literature and will therefore be familiar to those skilled 23 in the art.
24 The instrument for detecting density changes in the system shown in the drawing may be an optical or sonic 26 source-detector unit but will preferably comprise a radio-27 active source and detector unit provided with a 30-inch 28 ac~ive length collimated strip source assembly 35 and a 29 temperature-controlled detector cell 36 of e~ui~alent active length, both mounted in rubber-covered waterproof 31 housings. A 100 mi]licurie cesium-137 strip source or 3? other equivalent radioactive source available from 1 commercial suppliers may be employed. Similarly, the 2 detector cell may be of conventional design. r~he length 3 of the sensor shoulc. normally be approximately the same 4 as the length of the center well. For maximum sensitivity~
the sensor should be as long as possible but in general 6 should not extend above or below the center well. The 7 source and detector units are mounted on an adjustable 8 frame 37 which permits variation of the horizontal distance 9 between the units from about 6 to about 24 inches. me frame is supported by means of a cable 38 or similar member 11 and suspended from an overhead traveling crane or the like 12 39 on rail 40 to permit variation of the horizontal and 13 vertical position o~ the assembly within the feed well o~
14 the thickener.
~e source-detector assembly is connected by cable 16 41 to a conventional power supply unit 42 of commercial 17 design which converts the signal from the detector into a 18 voltage which is transmitted through cable 43 to an electri-19 cal recorder 44 and an electrical controller 45. The output signal from the controller is used to actuate automatic 21 valve 46 in flocculant supply line 47 so that the amount of 22 flocculant supplied is increased in response to a decrease 23 in settling rate and reduced in response to an increase in 24 settling rate. me valve selected may be electrically or hydraulically actuated and hence the controller employed 26 will depend in part upon the particular type of valve used.
27 Any of a variety of commercially available power supply 28 units, recorders~ controllers, and valves compatible with 23 one another may be employed. Such equipment will be familiar to those skilled in the instrumentation art. The flocculant 31 sllpply line 47 downstream of valve 46 is connected into 32 overflow line 30 so that the flocculant solution can be ~$~ ~n ~ 7 1 injected with the water or clear solution :introduced into 2 the feed well through line 30. The diluted flocculant 3 concentration will generally be in the range between about 4 0.002 and about 0.2~ by volume. me f]occulant rate will

5 depend upon the sie of the unit, the material being han-

6 dled, and other factors but in a typical installation may

7 range between about 100 and about 200 gallons per mirlute.

8 A venturi-type injector not shown in the drawing will

9 preferably be used for introduction of the flocculant

10 solution into line 30~

11 In utilizing the system of the invention, the ;

12 apparatus must be carefully calibrated to insure the re-

13 quired sensitivity over the requisite range of operating

14 conditions. To permit proper calibration, the horizontal

15 space or gap between the source and detector in the sensor

16 should be variable, the vertical position of the sensor

17 within the center well should be capable of adjustment,

18 the sensor should be of sufficient length to make density r

19 differences between the top and bottom of the sensor readily

20 detectable, the detector should be equipped with a set of

21 variable resistors which permit ultrasensitive adjustment

22 for the zero and calibration readings, and a test calibra-

23 tion unit for determining suppression and range resister

24 selection may be used. A suitable test unit can be readily

25 prepared by mounting a plurality of parallel resistors

26 having different resistance values on a suitable chassis

27 so that individual suppression and range resistance values

28 can be varied until the desired readings are obtained.

29 Prior to calibration of the system, the flow

30 rates to and from the thickener should be adjusted to posi-

31 tion the s]ime level near the bottom of the center well.

32 As pointed out earlier, there is a critical zone within 7 i~ ~

1 which the slime level should be located to permit eff`ective 2 control in accordance with the invention. In general, this 3 zone will extend about eighteen inches above and below the 4 bottom of the feed well or, if a skirt or shroud 28 is used~
the bottom of the skirt or shroud. If the slime level is 6 more than about eighteen inches above the botto~l of the 7 well, the concentration of solids in the center well may 8 become too high to permit adequate control of the unit by 9 monitoring changes in density within the center well.
Similarly, if the slime level is more than about eighteen 11 inches below the bottom of the well, the solids concentra~
12 tion within the well may be too low to permit adequate 13 control through the measurement of density changes. The 14 use of an adjustable skirt or shroud as shown permits varia-tion of the length of the center well and may in some cases 16 facilitate operations under adverse or difficult conditions~
17 It is generally preferred that the slime level be located 18 as near the bottom of the well as possible. During calibra-19 tion, changes in the composition of the feed stream entering -the thickener, the feed rate, and other operating variables 21 should be avoided. At a constant settling rate, the slime 22 level normally tends to remain substantially constant.
23 Once the slime level has been established at the 24 proper posltion with reference to the bottom of the feed well, the control system of the invention is calibrated by 26 first adjusting the vertical and horizontal position of the 27 source-detector unit within the center well until an on- ;
28 scale reading is obtained on the recorder 44~ Experience 29 has shown that there are certain areas within the center well where the settling ra~e and density changes are propor-31 tional and that in other areas this may not always be the 32 case. By moving the source-detector unit within the well 1 until a constant density reading is obtained on the re-2 corder~ under constant thickener operatirlg conditions, 3 the optimum position for the source-detector can normally 4 be found without difficulty. In some cases, however~ it may be found that the recorder is initially off scale at 6 all source-detector unit locations within the center well.
7 If this is the case~ it will be necessary to adjust the 8 output of the detector 36 until an on-scale signal is 9 obtained. This can normally be done by changing the in-ternal resistance of the detector unit to obtain greater 11 or reduced amplification of the signal from the detector.
12 After an on-scale reading is obtained~ the power supply 13 and controller calibration circuitry can be adjusted to 14 center the recorder reading obtained. Thereafter, the amount of flocculant fed to the thickener can be altered 16 to vary the settling rate within the center well. As 17 pointed out previously, the density reading obtained is 18 proportional to the settling rate. Commercially available 19 density measuring equipment using radioactive sensors and detectors or the like is normally intended for applications 21 requiring considerably less sensitivity than is needed 22 for purposes of this invention and hence it will generally 23 be necessary to change the suppression and feedback resis-24 tors in the amplification section of the system and make other adjustments to obtain a significant range of recorder 26 values with changes in the amount of flocculant added.
27 Once the required degree of sensitivity has been secured, 28 the controller 45 can be adjusted to regulate the operation 29 of valve 46 so that the quantity of flocculant needed to control the settling rate and maintain a constant slime ~1 level is added with the overflow liquid supplied through 32 line ~0. This calibration of the system may require 1 continued adjustment of the apparatus by trial and error 2 over a considerable period of time until the proper 3 response to changes in feed rate, feed composition and ~ other operating variables is obtained. The precise calibration technique employed will depend to a large 6 extent upon the particular density measuring system, 7 recorder, controller and valve used and may be varied 8 as necessary.
9 Once the system of the invention has been 10 properly calibrated as described above, the system will ~;
11 automatically regulate the flocculant addition rate to 12 maintain the slime level within the thickener relatively 13 constant. Any variation in settling rate within the 14 center well due to a change in feed rate, feed composition, 1~ solids particle size or density, slurry temperature or 16 the like which would normally produce a change in the 17 settling rate and an alteration in the slime level will 18 be compensated for and offset by a corresponding change 19 ln the flocculant addition rate. This makes lt possible to obtain higher sludge densities, lower clarified solu-21 tion solids contents, decreased flocculant consumption, 22 increased slurry capacity, substantially constant slime 23 levels, and reduced unit downtime. The overall effect of 24 this is a substantial improvement in thickener operation and a reduction in thickening cost.
26 The advantages of the system of the invention 27 are illustrated by the results obtained in a multiunit 28 thickening system used for the separation of solids from ; 29 a slurry of spent uranium oreparticles following the ex-traction of uranium values with sulfuric acid. This par-31 ticular installation included a series of rotary thickeners 32 similar to that shown in the drawing. Each thickener was , 1 equipped with a variable gap source-detector unit including 2 an Ohmart ~orporation SHRM-~O source holder containing a 3 thirty-inc~ strip source of 100 millicurie cesium 137, an 4 Ohmart Corporation thirty-inch strip source-detector Model LJ30-W/FA with an ~A amplifier, and an Qhmart Corporation 6 EDS-l power supply unit having an output of from O to -50 7 millivolts or from O to -10 volts~ and from 4 to 20 milli-8 amps. m e source and detector were enclosed in special 9 rubber-covered housings and connected by an adjustable frame so that the space between them could be varied from 11 6 to 20 inches. This unit was supported in the center well 12 of the thickener by a system which permitted adjustment of 13 the horizontal and vertical position within the well. The 14 suppression and feedback resistors in the amplifier section of the detector system were replaced during calibration to 16 secure proper density readings. me recorder controller 17 unit employed was a ~estinghouse recorder-controller Model 18 75RC2110/203-111/111 with proportional plus reset control 19 modes.
me effect of the system of the invention upon 21 flocculant consumption is shown in the table by a compari-22 son of the consumption over a 31-day period in which the 23 operation was controlled manually, during a similar period 24 in which the operation was controlled in accordance with the invention, and during a later period in which the method 26 of the invention was used. It will be noted from the data 27 set forth in the table that the amount of flocculant re-28 quired per ton of ore processed during manual operation 29 was relatively high, about one-ha]f pound per ton, and that the use of the system of the invention permitted a 31 substantial reduction in flocculant consumption, in excess 32 of ~0~. l~e figures shown in the table take into 1 consideration variations in the s:Lime content of ihe ore 2 being processed and are thus comparable. The reduction in 3 flocculant consumption thus achieved represents a substan-4 tial saving in operating costs.

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1 The advantages of the system of the invention are 2 further illustrated ~y the variations in slime level before 3 and after installation of the system. With manual regula 4 tion of the amount of flocculant added~ slirne level varia-tions of as much as ~ or - 16 inches were incurred. Follow-6 ing installation of the system of the invention~ slime 7 level variations were routinely held to about plu5 or minus 8 2 to 4 inches. Similarly, thickener downtime following 9 installation of the system was reduced from an average of about 7 to 8 hours per month to an average of about one 11 hour per month or less, a significant reduction. The 12 average density of the underflow sludge from the system 13 also increased as the result of use of the system of the 14 invention. It will be apparent that these improvements in thickener operation significantly reduce operating costs 16 and increase thickener efficiency.
17 Although the invention has been discussed above 18 primarily in terms of the treatment of metallurgical pulps 19 and similar slurries, it should be recognized that it is 2~ not limited to such applications and can also be utilized 21 in sewage disposal plants, chemical plants, and a variety 22 of other facilities requiring the use of thickeners and 23 similar continuous gravity sedimentation devices for the 24 separation of a liquid suspension of solid particles into clarified liquid and sludge phases.

-2~~

Claims (11)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY OR
PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method for controlling the operation of a continuous gravity sedimentation unit having a feed well into which a slurry of suspended solids and a flocculating agent are introduced to effect separation of said slurry into a clarified solution that is recovered from said unit as an overflow liquid and a sludge that is withdrawn from the bottom of said unit whereby consumption of flocculating agent is reduced comprising the steps of establishing a slime level within a predetermined optimum distance from the bottom of said feed well by varying the flow rates to and from said unit, detecting changes in the density of the downflowing slurry over a vertical interval in said feed well, correlating said density changes in the solids settling rate within said well, and con-tinuously varying the rate at which said flocculating agent is introduced into said feed well in response to said changes in density and whereby control of both settling rate and slime level is obtained.

2. A method as defined by claim 1 wherein said changes in density are detected by passing energy from a radioactive source to a detector located within said feed well.

3. A method as defined by claim 1 wherein said slime level is established at a point within about eighteen inches of the bottom of said feed well.

4. A method as defined by claim 1 wherein said changes in density are detected by means of a collimated radioactive source and detector.

5. A method as defined by claim 1 wherein changes in density of about 0.005 gram per cubic centimeter within the range between about 1.0 and about 1.200 are detected.

6. A method as defined by claim 1 wherein optical means are em-ployed to detect said changes in density.

7. A method as defined by claim 1 wherein sonic means are employed to detect said changes in density.

8. A method for controlling the operation of a continuous rotary thickener having a center well into which a flocculant and a slurry of suspended solids are introduced to effect separation of the slurry into a clarified solu-tion which is withdrawn from the thickener as overflow and a thickened sludge which is recovered from the bottom of the thickener comprising the steps of adjusting the flow rates to and from said thickener until a slime level within about eighteen inches of the bottom of said center well has been established, measuring the density of the downflowing liquid and suspended solids over a vertical interval within said center well with sufficient sensitivity to detect changes in density due to changes in the solids settling rate in said center well, and continuously varying the rate at which said flocculant is introduced into said center well in response to said changes in density to compensate for said changes in said solids settling rate whereby control of both settling rate and slime level is obtained.

9. A method as defined by claim 8 wherein said density is measured by radioactive means.

10. A method as defined by claim 8 wherein said density is measured with sufficient sensitivity to detect density changes of about 0.005 gram per cubic centimeter within the range between about 1.0 and about 1.200 grams per cubic centimeter.

11. A method as defined by claim 8 wherein said density is measured by means of a collimated radioactive source and detector immersed in said down-flowing liquid and suspended solids within said center well.