Centrifuging process and apparatus

Feb. 10, 1970 w. h. craig etal 3,494,542

CENTRIFUGING PROCESS AND APPARATUS

Filed May 27, 1968

Sheets-Sheet 1

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INVENTORS.

WALTON H. CRAIG, ROBERT J. FINKELSTON, DEAN E. HUMPHREY, ANDRE C. LAVANCHY and LEONARD SHAPIRO

BY

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ATTORNEY.

Feb. 10, 1970

W. H. CRAIG ET AL

3,494,542

CENTRIFUGING PROCESS AND APPARATUS

Filed May 27, 1968 . 2 Sheets-Sheet 2

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INVENTORS.

WALTON H. CRAIG, ROBERT J. FINKELSTON, DEAN E. HUMPHREY, ANDRE C. LAVANCHY and LEONARD SHAPIRO

BY &lt;2.

<img file="US3494542A_D0004.tif" />

ATTORNEY.

3,494,542

Patented Feb. 10, 1970

United States Patent Office

3,494,542

CENTRIFUGING PROCESS AND APPARATUS Walton H. Craig, Winnetka, III., Robert J. Finkelston,

Philadelphia, Dean E. Humphrey, Lansdale, Andre C.

Lavanchy, Devon, and Leonard Shapiro, Upper Darby, 5 Pa., assignors to Pennwalt Corporation, Philadelphia, Pa., a corporation of Pennsylvania

Filed May 27,1968, Ser. No. 732,207

Int. CI. B04b 5/12, 5/06, 5/10

U.S. CI. 233—7 16 Claims _1Q

ABSTRACT OF THE DISCLOSURE

Solids and liquid are separated in a centrifuge having a helical conveyor coaxially mounted within an elongated bowl tapered at one end, by first feeding a solids-liquid slurry to the bowl, and selectively driving the conveyor and the bowl synchronously during and after feeding to promote sedimentation of the solids without agitation ₂θ by the conveyor, and then effecting a differential speed between the bowl and the conveyor so as to move the settled solids toward solids discharge means. Optionally, some residual clarified liquid may be removed by a skimming device prior to discharging solids. In addition, 25 an electric drive or brake can be employed to provide an infinite adjustment of the differential speed within a given range.

This invention relates to a method and apparatus for separating solids from liquid-solid mixtures, utilizing a centrifuge having a coaxially mounted screw conveyor within the centrifuge bowl. The invention is especially applicable when the feed mixture is difficult to separate _3gby centrifugation, for example when the solids and the somewhat lighter liquid have nearly the same specific gravity.

Various other factors may contribute to the difficulty of separating some solids from a solids-liquid mixture in a centrifuge of the general type, as shown in Reed Patent No. 3,148,145, wherein a solid bowl centrifuge with an axial screw conveyor is adapted to move solids toward one end of the bowl for discharge. As examples of such other factors, the solids may be gelatinous or tend to agglomerate. Still further, the solids may be of small par- 45 ticle size and become slippery when wet so as to form a soft cake which is difficult for the conveyor to scroll and/or is susceptible to remixing into suspension with the supernatant liquid when agitated by the screw conveyor.

When there is very little friction between slippery solids ⁵⁰and the surface of the screw conveyor, the solids, instead of being conveyed axially and inwardly to the solids discharge port, will move under the influence of centrifugal forces back into the main separating chamber, with the result that the efficiency of separation and the clarity of ⁵⁵the effluent are both reduced. In this connection reference is made to U.S. Patent No. 3,172,851, issued Mar. 9, 1965 to the present assignee in the name of C. M. Ambler, wherein this related problem is discussed in greater detail and a solution is disclosed which is most effective provided lower effluent rates and wetter solids are permissible. The apparatus of C. M. Ambler, however, is to be distinguished from that shown in the cited Reed patent, as well as the prior U.S. Patents Nos. 2,679,974 and 2,703,676 of F. P. Gooch, in that Ambler provides a solids discharge open- ³³ing at one end of the centrifuge located radially outward of a liquid discharge opening at the other end of the centrifuge so that settled solids form a relatively impervious dam extending inwardly of the solids discharge opening, whereby the dam blocks the escape of liquids ⁷⁰therethrough toward the solids discharge opening; and the force of the liquid behind the solids of the dam assists in the discharge of otherwise non-scrollable solids.

It is noted here that past efforts to improve the separation of solids from some solids-liquid mixtures have included the addition of fibrous materials to the sludge-like solids-liquid mixture. Such fibrous materials act as matting agents to provide a stiffer cake formation which is more easily conveyed by the screw conveyor to the solids discharge port. Besides the cost of the fibrous additive, this procedure has a number of disadvantages including the inability to discharge solids which are preferably free of additives. In many instances, however, these disadvantages are justified, especially when the primary purpose of the procedure is to obtain an effluent with high clarity.

According to the present invention, many of the disadvantages of prior methods and apparatuses are eliminated by a method and apparatus now to be described. The invention proposes a method wherein feed is introduced to a zone of centrifugation for a period of time and then preferably shut off. During feeding, and for a limited period thereafter, the screw conveyor and the centrifuge bowl are operated at synchronous operational speeds in order to promote sedimentation toward the bowl wall of solids having a greater specific gravity than the liquids. Thus, when the screw conveyor turns at the same speed as the bowl there will be no agitation of the solids as they sediment outwardly; and the settling and compaction of the solids along the bowl wall precondition the solids for the scrolling operations to follow. As a compact solids cake develops as an annular formation inwardly of the inner surface of the bowl, a substantially clear body of separated liquid effluent is positioned inwardly of the cake; and it is therefore optional to skim clarified effluent from the zone of centrifugation at this time. Alternatively, skimming can be eliminated or it can be performed at other times during the separation procedure. Next, as by energizing an electric brake connected to a gear box, the screw conveyor and the bowl are operated at a differential speed optimum for the given separation conditions and the solids are conveyed to the solids discharge port in relatively dry condition.

This method of operation, consisting basically of separate feed and conveying periods, is then rapidly repeated to maintain a semi-continuous flow of the separated phases.

The various objects, features and advantages of the invention will appear more fully from the detailed description which follows, taken in connection with the accompanying drawings, in which:

FIGURE 1 is a view, partly in section and partly schematic, of apparatus embodying the invention together with the controls for the operation thereof; and

FIGURE 2 is a fragmentary view of the same incorporating an additional skimmer device as a modification thereof, with like reference numerals being employed to designate correspondingly similar parts.

Referring to FIG. 1 of the drawings, there is illustrated a centrifuge 10 having an imperforate solids bowl 11 disposed within a housing 12 and adapted to be belt-driven by a motor 14 for rotational speeds up to and beyond 3000 times the force of gravity. Suitable bearings 15 are employed at opposite ends of the housing 12 to provide journaled rotational support for the bowl 11. Within the bowl 11 is a helical screw conveyor 16 mounted on a hollow shaft or hub 18, the latter extending in axial direction through the bowl in position to receive an axially extending feed tube 20. The bowl 11 includes a major or left-hand, portion 21 of cylindrical shape and a minor, or right-hand, portion 22 of conical configuration tapering rightward toward the feed tube 20. In addition, the right-hand end of the bowl 11 is provided with one or more solids discharge ports 24 which are disposed closer

3,494,542 to the rotational axis than are liquid discharge ports 26 provided at the opposite, or left-hand end Of the bowl. By means of a suitable gear box 30 and braking device 32, connected together by universal coupling 33, the speed of the conveyor 16 relative to the speed of the bowl 11 may be controlled. Such control, as will be described hereinafter in detail is capable of effecting synchronous rotation of the conveyor 16 and the bowl 11. Such control is also capable of effecting a slight difference in speed between the conveyor 16 and the bowl 11, preferably while they rotate in the same direction.

In operation, the feed material or mixture to be separated is delivered through the feed tube 20 and passed into the hollow hub 18 from whence it is discharged radially outwardly through one or more feed ports 34 into a separating chamber 36 interiorly of the bowl 11 and exteriorly of the shaft 18.

With apparatus of the type described, the left-hand portion 21 of the bowl serves as a continuous solids bowl centrifuge which effects withdrawal of the lighter phase liquid (also termed “effluent” and “supernatant”) to the left, as shown, for discharge through the liquid discharge ports 26. When the bowl 11 and the conveyor 16 are operating at different speeds, as aforesaid, the solids are moved axially to the right and also inward toward the rotational axis within the right-hand portion 22 of the separating chamber 36 for eventual discharge through the solids discharge port 24. This conveying action is also referred to as scrolling the solids up the so-called beach defined by the tapered portion 22. It is to be noted that since the liquid discharge ports 26 are disposed radially outwardly of the solids discharge ports 24 the liquid level (also termed “pond surface”) is such that it intersects with the inner surface of the tapered portion 22 of the bowl at a radial location outwardly of the solids discharge ports 24 and at a radial location equal or inwardly of the outermost portion of the liquid discharge ports 26. Such liquid level, or pond surface, can therefore be seen to define a so-called wet beach for the submerged part of the tapered portion 22, and also a so-called dry beach for the remaining part of the tapered portion 22 disposed inwardly of the liquid level.

By providing a dry beach and eliminating turbulence during the settling phase, in the terms set forth, the separated solids are in drier condition than they otherwise would be; and the present invention is therefore to be distinguished from the Ambler patent previously cited herein.

Gear boxes 30 have long been in use in centrifuge apparatus of the general type described herein; and for the sake of brevity reference is made to U.S. Patent No. 2,703,676, issued in the name of Fred P. Gooch on Mar. 8, 1955 and assigned to the assignee of the present invention, for a detailed description of a gear box employed for achieving differences in speed of rotation of the conveyor 16 and the bowl 11, e.g. 10 to 60 r.p.m. It is to be further understood, however, that by holding an extension 38 of a pinion in the planetary system against rotation the conveyor 16 and the bowl 11 will operate at increased differential speed. Similarly when the pinion is rotating in synchronism (zero differential speed), as more and more braking torque is applied for resisting rotation of the pinion extension 38 the greater the differential speed will be. Accordingly, it is the function of the braking device 32 to apply a torque-resisting force to the pinion extension 38 through the universal coupling 33, and the greater the torque resistance the greater will be the speed differential so that with zero slippage the maximum speed differential is obtained.

It is a feature of the present invention that a so-called eddy-current braking device 32 is employed. Such devices are commercially available from the Dynamatic Division of Eaton Yale &amp; Towne, Inc. The braking device 32 comprises a pole ring 40 having an exciter coil 42 surrounding a drum assembly 44, the space therebetween de4 fining an outer parasitic gap. Within the drum assembly 44 is a polarized rotor 46, spaced therefrom to provide an inner working gap through which coolant is circulated. By energization of the exciter coil 42 a magnetic field is produced which generates eddy-currents on the smooth surface of the rotor 46 rotating therethrough. The interconnection between the eddy-current field and the poles of the field assembly produces a braking torque on the rotor 46. The torque, which varies directly with coil current and the speed of the rotor 46, may be infinitely and accurately adjusted by the control system to provide the required braking action. It is an advantage of this kind of braking device 32 that braking torque is developed by an electromagnetic reaction between separated members, thus eliminating wear and the requirement for replacement and adjustment of parts. It can be seen, therefore, that the greater the amount of electrical energy applied to the coil 42 of the device 32, the greater will be the braking action and the greater will be the speed differential between the conveyor 16 and the bowl 11.

Further provided is a solenoid-operated valve 50 in the supply line or feed tube 20. Valve 50 is normally open but when energized stops the flow of feed through the tube 20. In addition, there is further provided a control 52 driven by a timing motor 54. As will be developed more fully, it is the function of the control 52 to effect energization and deenergization of the braking device 32 for controlling differential speed as aforesaid according to a selected program, said program being related to a selected period of time for feeding the mixture to the centrifuge bowl 11. The conveyor 16 and bowl 11 are operated at synchronous speed during feeding and also during any residence period which follows to effect separation of solids from liquid by sedimentation of the former toward the inner surface of the bowl 11, and then scrolling the solids toward and out the solids discharge port 24 by effecting the differential speed operation of the conveyor 16 and bowl 11 by energization of the braking device 32 for a selected period of time.

By operating the centrifuge apparatus in this sequence, a quiescent condition is created inside the bowl 11 during the first or feed period and the second or residence (cakecompacting) period. This quiescent condition, brought about by synchronizing the conveyor rotational speed with that of the bowl, serves during these periods to prevent disturbances, to soft solids, which otherwise and heretofore were generated by continuous scrolling. During the third or scrolling period of operation, the conveyor 16 is rotated with optimum differential speed for discharging relatively dry solids. It is an advantage of this method that improved clarification takes place along with undisturbed solids compaction as a beneficial effect of producing a deep solids layer during a long residence time in the centrifugal field.

Cyclic operation as described was performed experimentally for the purpose of separating electrolyte liquid from sludge carried therein as a result of electrochemical machining. Such a feed mixture is well known to be difficult to separate by centrifugal methods. The experimental results indicated, however, that the separated solids could be scrolled on a dry beach while giving 2 to 6% solids content in the effluent, on a volume basis, at average effluent rates up to 1.5 g.p.m. It was also found that quite dry solids were produced even with high differential speeds.

Optionally, the automatic programming of the steps in the process can provide for distinct time periods for each step, or they can overlap one another to some extent. It is preferred, however, that the conveyor 16 be operated at synchronous speed while the feed valve 50 is open, and that synchronous speed be maintained during a cake residence period initiated by shutting off the feed through energization of the solenoid-controlled feed valve 50. At the end of the cake residence period, the feed valve 50 remains closed but the conveyor 16 and the bowl 11 are

3,494,542 operated at the desired differential speed to discharge solids, after which the conveyor 16 and the bowl 11 are returned to synchronous speed to await the start of a new cycle by the opening of valve 50.

It is well understood that the elapsed time for each step or period can be adjusted to provide optimum performance and results for the particular feed mixture and process conditions encountered.

MODIFICATION

The method and apparatus of the invention is susceptible of further improvement by providing a skimming device 60 capable of discharging effluent immediately prior to the scrolling operation. As shown in FIG. 2, the skimming device 60 is positioned in the left-hand portion 21 of the bowl 11 adjacent the liquid discharge port 26, and it is further provided with a traversing mechanism 62 capable of moving the skimming device radially inwardly. The dynamic pressure of the rotating liquid coming into contact with the open inlet of the stationary skimming device 60 develops a hydrostatic pressure head which promotes the flow of effluent through the passageway 64 of the skimming device 60 and thus reduces the liquid level or pond depth. Obviously the greater the inward movement of the skimming device 60 effected by the traversing mechanism 62, the lower will be the pond depth. It is an advantage of this feature that solids will be discharged with greater dryness because of the greater length of dry beach. Furthermore, certain solid materials may exhibit better scrollability in a drier state. Moreover, where the solids consist of very fine particles having a tendency of remixing with the liquid while scrolling, there will be less chance for such solids to be discharged with the effluent if a greater percentage of the effluent is removed prior to scrolling.

The apparatus of FIG. 2 is capable of operating with a relatively deeper pond during the sedimentation part of the cycle, so that there is more room for the collection of solids. And skimming prior to discharging collected solids increases solids dryness since it is not necessary to discarge solids along a short dry beach, as would be the case if skimming were not performed after effecting sedimentation with a deep pond.

During operation of the conveyor 16 effluent is preferably prevented from being discharged through the skimmer passageway 64 by retracting the skimming device 6®.

In other words, by operating with a deeper pond during the sedimentation part of the cycle there is more room for the collection of solids; and skimming to decrease pond depth prior to discharging solids makes it unnecessary to compromise on solids dryness, that is, accept wetter discharged solids because of a shorter dry beach.

The traversing mechanism 62 can be of the manual type or, as shown, it may be operated by energizing a solenoid-actuated valve 65 to admit pressurized air to a cylinder 66, thus acting against a piston 67 within the cylinder and overcoming spring resistance to transmit motion through a piston rod 68 to the skimming device 60.

The control 52 will now be described in greater detail with reference to FIG. 1, it being understood that the invention can be practiced with other devices, such as those which signal for a completion of the sedimentation phase upon measuring a predetermined thickness of accumulated solids, or a certain degree of liquid, turbidity of the effluent discharge as indicated by a reduction in light transmissibility. According to the present invention, the timer 54 unidirectionally turns a shaft 70 on which is mounted a first drum 72 including an arcuate conductive portion 74 (shown cross-hatched) adapted to engage brushes 76, and thus complete a circuit to the coil 42 of the braking device 32 during a portion of one turn of the drum 72. In this coil-energizing circuit is a variable resistor 78 which can be preset to obtain the desired degree of energization, consistent as aforesaid with the amount of braking torque and corresponding speed differential desired for scrolling. A similar drum 80 is similarly mounted for rotation simultaneously with the drum 72; and it has an arcuate conductive portion 84 (shown cross-hatched) adapted to engage brushes 86 for completing a circuit to energize the solenoid of the valve 50 and thus close valve 50 and prevent the introduction of feed during the residence and scrolling periods.

In the arrangement of FIG. 2, a third drum 87 with an arcuate conductive portion 88 is mounted on shaft 70 and adapted to engage brushes 89 for completing a circuit to energize the solenoid-actuated valve 65 of the traversing mechanism. As mentioned previously, the energization of valve 65 effects inward traversing of the skimming device 60 whereby the inlet of passageway 64 receives liquid from the bowl 11. The valve 65 can be optionally adjusted to admit air at a selected pressure which exerts a pressure force on the piston 67 sufficient to overcome the opposing bias of the coil spring within cylinder 66. The extent of inward traverse can thus be preselected in relation to air pressure on the piston and the spring constant of the opposing spring; and when the valve 65 is deenergized pressurized air is released from the cylinder 66 and the spring force returns the skimming device to original outward position.

It is to be understood that, in the broad sense of the invention, synchronous operation which minimizes agitation of the material within the separating chamber 36 embraces minor speed differentials which are substantially less than those normally employed for conveying solids toward the solids discharge ports. Such normal differential speeds vary, it will be appreciated, with the particular feed and apparatus. Furthermore, it is known that some speed differential may be encountered even when synchronous operation is intended, such as when a resisting torque is imposed on the conveyor 16 by an exchange of momentum between the conveyor hub and the feed material accelerated thereby. In order to compensate for the resisting torque imposed on the conveyor 16, an auxiliary motor 90 may be provided to drive the pinion extension 38 through the rotor 46 and a belt and pulley system generally designated 92, as shown in phantom in FIG. 1.

The additional motor 90 thus may be selectively energized, by applying electric current to leads 94 of motor 90, to drive the pinion shaft extension 38 and ensure positive synchronism between the conveyor 16 and the bowl 11, as when the torque developed between the bowl and the conveyor is so great as to prevent synchronous operation.

Preferably, the motor 90 and the braking device 32 are housed within a single enclosure (not shown).

Although the method and apparatus of the present invention provides for cyclic operation wherein feeding of the mixture to be separated is interrupted during sedimentation or compaction of solids and wherein effluent is discharged intermittently, it has been found that better results are obtained for mixtures difficult to separate as described herein than has been obtainable by continously operating similar apparatus.

For example, in one actual test the feed consisted of a 10% concentration of clay solids in a water-clay mixture and the apparatus was operated first continuously and then according to the invention in order to discharge effluent with about 15% of the solids unsedimented, that is, effluent with 1.5% residual clay solids. On a continuous basis the effluent discharge rate was 0.4 gallon per minute (g.p.m.), and with cyclic operation according to the invention the mean effluent discharge rate was 0.9 g.p.m., with comparable results.

In a similar test, using like feed material and clay solids concentration, effluent was continuously discharged at the rate of 0.27 g.p.m. with 5% of the solids unsedi3,494,542 merited, or 0.5% residual clay solids. An equivalent rate of 0.78 g.p.m. was obtained in accordance with the invention by feeding the material for 40 seconds at a rate of 1.75 g.p.m., then compacting solids for 10’ seconds, and discharging for 40 seconds, with no feed introduced while compacting and discharging, obtaining the same 5% of solids unsedimented or 0.5 % residual clay solids.

In the course of the same testing, using the same feed, or continuous operation, with an effluent discharge rate of 0.3 g.p.m., the effluent contained 12.8% unsedimented solids, that is, effluent with 1.28% residual clay solids, with solids concentration (dryness) in the solids discharge of 55.3%. During cyclic operation according to the invention with a mean effluent discharge rate of .78 g.p.m., the effluent contained 4.97% unsedimented solids, that is, effluent with 0.497% residual clay solids, and the solids were discharged at 62.4% solids concentration (dryness). Thus, even with a greater mean solids discharge rate the cyclic operation produces drier solids.

It can be seen, therefore, that improved results are obtained with the present invention.