GB1589389A - Method of operating a centrifuge - Google Patents

Description

PATENT SPECIFICATION

( 11) 1 589 389 ( 21) Application No 40719/79 ( 62) Divided Outof No 1589387 ( 22) Filed 26 Aug 1977 Convention Application No 720200 ( 32) Filed 3 Sep 1976 in United States of America (US)

Complete Specification Published 13 May 1981

INT CL 3 B 04 B 1/12 7/08 Index at Acceptance B 2 P 10 C 3 B 1 A 10 C 3 B 3 ( 54) IMPROVEMENTS IN OR RELATING TO A METHOD OF OPERATING A CENTRIFUGE ( 71) We, JOY MANUFACTURING COMPANY, a corporation of the State of Pennsylvania, United States of America, located at Oliver Building, 535 Smithfield

Street, Pittsburgh, State of Pennsylvania, United States of America, do hereby declare the invention for which we pray that a Patent may be granted to us and the method by which it is to be performed, to be particularly described in and by the following statement:-

Centrifugal separators or centrifuges for numerous applications are well known in the art for separating fluid mixtures into relatively higher density and lower density fractions with one particular application being to continuously centrifugally separate solids from a slurry having a high water content to permit the solids to be transported by pumping to a location at which the solids are to be used Thus, for example, coal-water slurries are dewatered at the discharge end of a transporting pipeline to provide relatively water-free coal The hydraulic transporting of mill tailings to provide a mine backfill material is also well known in the prior art, however, to date there are no mining systems which centrifugally dewater mill tailings slurries underground due to the inability of present separators to either handle such slurries, or provide a usable discharge, or to provide a sufficient quantity of discharge to accomplish backfilling in a practical manner.

As is known mill tailings result from processing ores and having a range of particle sizes therein with a range of 600 microns to sub-micron size being typical which fine particles can advantageously be utilized in backfilling Of the various types of separators presently available, screen type centrifuges are not satisfactory for dewatering mill tailings slurries as they are not designed to handle fine particles In order to obtain a large quantity of dewatered mill tailings underground the slurry has a solids content as high as practical, with typical prior high solids content slurry having a solids content of 50 to 65 percent by weight Solids content in such range are too high to permit separation by disc bowl or self-ejecting centrifuges Also for underground dewatering the mill tailings slurries it is necessary to use and transport the centrifuge within the confines of the mine passageways which eliminates using any large size centrifuges such as solids bowl or conveyor type centrifuges For the same reasons the use of a number of prior centrifuges in parallel or sequence is not practical underground.

According to one aspect of this invention there is provided a method of operating a bowl centrifuge in which a slurry consisting of solid particles entrained in a liquid is separated into a liquid portion which is discharged from said bowl centrifuge, and a portion consisting of a mixture of particles and liquid within a radially outer section of such bowl with such mixture having a liquid content which is sufficient to permit the substantially uninterrupted flow of the mixture through a discharge opening, said method comprising: maintaining a volume of said mixture within a predetermined range of volumes as an annulus within said outer section throughout a period of time while simultaneously discharging seriatim portions of said mixture portion in a substantially uninterrupted flow from radially outermost portions of said volume by replenishing said mixture portion by continuous centrifugal separation of supplied additional slurry.

Preferably the average rate of particle discharge is substantially equal to the average rate of particle replenishment in said volume during said maintaining.

Conveniently said maintaining of said volume is effected by varying the rate of discharge of said mixture from said volume.

According to another aspect of this invention there is provided a method of operating a centrifuge having an elongated shaft cl 00 M C 00 tn r 4 ( 31) ( 33) ( 44) ( 51) ( 52) ( 19) 1,589,389 member rotatable about the central longitudinal axis thereof, a pair of formed bowl members defining a centrifuge chamber rotatably carried by such shaft member and means in at least one of such members for admitting a slurry of solid particles entrained in a fluid vehicle into such a chamber over a selected period of time; comprising: separating portions of such a slurry, continuously admitted throughout an initial period of time interiorly of such bowl members during continuous rotation of such bowl members with peripheral portions of such bowl members radially outwardly of such axis being in continous engagement with each other, into a product component primarly consisting of a mass of such particles which increases in radial extent with respect to said axis throughout said initial period until said mass extends from the interior of said engaged peripheral portions to a position radially outwardly adjacent such shaft with the portion of said mass adjacent said shaft defining an elongated unobstructed passageway axially encompassing the full extent of such shaft within said bowl members and with said passageway having a lateral extent substantially less than the maximum radial extent of said mass; simultaneously separating portions of such slurry admitted throughout said initial period of time into an effluent component primarily consisting of such fluid vehicle, and discharging portions of said effluent component exteriorly of such bowl members from one axial end of said passageway; discharging portions of said mass exteriorly of said bowl members over an immediately subsequent period of time while maintaining the rotation of said bowl members with said peripheral portions being spaced from each other, while simultaneously separating additional portions of such slurry admitted on a continuous basis to the interior of the bowl members into said product component and said effluent component at a substantially uniform rate so that increments of said product component are added to said mass to maintain the radial extent of such mass within predetermined limits throughout said subsequent period of time; and simultaneously throughout said subsequent period of time substantially uninterruptedly discharging portions of said effluent component of said additional slurry from said one axial end of said passageway.

Preferably the method further includes maintaining such radial extent by restraining flow of said product component by the interior surface of said bowl members.

Conveniently said discharging is through the radial outermost portion of said bowl members.

Preferably during said subsequent period of time the mass of previously accumulated particles continuously flows towards an outermost portion of said product component at a controlled rate.

Advantageously said continuous flow is retarded at a greater rate as the distance from said axis radially increases 70 Preferably said discharging is of a continuous circumferential radial outermost portion of said outer portion.

Alternatively said discharging is of a variable portion of said outermost portion 75 Preferably the flow of deposited solid particles or mixture through said outer portion is at a uniform rate.

Alternatively the rate of said discharging is variable 80 Conveniently the location of the interface between said accumulated particles and said path radially shifts during said centrifugal accumulating.

In order that the invention may be more 85 readily understood, and so that further features thereof may be appreciated, the invention will now be described by way of example with reference to the accompanying drawings, in which: 90 Figure 1 is an axial cross-section of a centrifuge; Figure 2 is an enlarged cross-sectional view of the outer peripheral portion of the rotating portion of the centrifuge as shown in 95 Figure 1; Figure 3 is an enlarged schematic illustration of the interior configuration of the bowl of the centrifuge as shown in Figure 1; Figure 4 is a schematic illustration similar 100 to Figure 3 of the interior configuration of an alternate embodiment of a centrifuge bowl; Figure 5 is a partial perspective view of an alternate structure for the lower discharge ring of the centrifuge as shown in Figure 1; 105 and Figure 6 is a a diagrammatic representation of the various zones within the bowl of the separator as shown in Figure 1.

The centrifuge of Figure 1 is of the rotary 110 bowl type.

Centrifuges of the rotary bowl type are well known in the art The centrifuge of Figure 1 comprises a stationary formed housing having vertically spaced and aligned bear 115 ings 12 for rotationally supporting an elongated shaft 14 about vertical rotation axis X-X of the housing 12 Shaft 14 is rotationally driven as set forth herein by means of a controllable motor (not shown) having drive 120 belts 16 cooperable with a pulley 18 rigidly secured to the shaft 14 adjacent the housing Shaft 14 is provided with a central vertically extending passageway 20 therein with the upper end being open and cooperable 125 with a feed pipe assembly 22 to supply the material i e the slurry to be separated in the centrifuge A separation chamber or bowl 24, comprising upper and lower formed members 24 a and 24 b, respectively, suitably 130 1,589,389 rigidly secured to shaft 14 to rotate therewith, is located within housing 10 The lowermost bowl member 24 b is provided with an acceleration chamber 26 in an open communication with ports 28 extending laterally and downwardly from the inner end of passageway 20 The upper bowl portion 24 a has an uppermost central annular chamber 30 encompassing the upper portion of the shaft 14 within the bowl 24 with circumferentially spaced discharge ports 32 extending upwardly and outwardly of the chamber 30 and in open fluid communication therewith.

As described in detail hereinafter the upper and lower portions 24 a and 24 b form a material discharge opening therebetween at their outer periphery.

In operation the slurry to be subjected to centrifugal action of the centrifuge is introduced through the feed pipe assembly 22 and thereafter flows through passageway 20, ports 28, acceleration chamber 26 into bowl 24 in which the slurry is separated with the low density fraction thereof being discharged through ports 32 and the high density fraction thereof being discharged through opening 34 The construction of the feed pipe assembly, drive, bearings, shaft, acceleration chamber, discharge ports and housing may be of various well known forms such that further description thereof is not necessary in this Specification In the preferred embodiment illustrated the accelerator chamber 26 is of a structure as shown, described in the Specification of co-pending U K Patent

Application No 7226/78 (Serial No.

1589388) and accordingly the disclosure of such Specification is incorporated herein for the purpose of describing the preferred embodiment of this invention.

Upper bowl member 24 a has a generally conical disc portion 36 which extends outwardly and downwardly from the portion of bowl member 24 a forming chamber 30 A formed ring member 38 is suitably removably secured to the lower end of the disc portion 36 and has an inner surface of a form to provide a smooth continuation of the inner curvature of disc portion 36, and a lower edge 40 (Figure 2) in a plane extending transversely of the vertical axis of shaft 14.

The lower bowl member 24 b also has a generally conical disc portion 42 which extends outwardly and upwardly from the portion of bowl member 24 b forming the accelerator chamber 26 A formed ring member 44 is movably supported by the disc portion 42 at the outer end thereof for controlled movement toward and away from the ring member 38 Ring member 44 has an inner surface of a form to provide a smooth continuation of the inner curvature of disc portion 36 and an upper edge 46 extending in a plane extending parallel to the lower edge 38 Ring member 44 is suitably selectively movable towards and away from the ring member 38 by suitable circumferentially spaced mechanisms 48, only one of which is shown in Figure 1.

One particular mechanism 48 for controlling the movement of ring member 44 is more 70 particularly shown, described in copending U.K Patent Applicationlower portion of ring member 44 so that axial movement of the piston assemblies 54 cause a corresponding axial movement of the ring member 44 75 within the limits of travel of ring member 44.

Piston assemblies 54 have an inwardly extending circumferentially continuous flange portion 58 upwardly overlying a radially outer circumferentially continuous 80 portion of the seal adaptor 50 to form a chamber 60 therebetween extending circumferentially around the lower disc portion 42 A hollow annular flexible control member 62 is located within chamber 60 and 85 is of a form to be closely received therein A suitable number of tubular supply lines 64 are in fluid communication with the interior of the control member 62 and extend radially inwardly and downwardly of chamber 60 90 through seal adaptor 50 along the outer surface of the lower bowl member 24 b, through the lower portion of bowl member 24 b into registry with the open end of a supply passageway 64 extending from the exterior of 95 the shaft 14 to the center thereof and then coaxially downwardly of shaft 14 into registry with a suitable air flow control device 66 having a suitable controllable supply line 68 connected thereto Although any suitable 100 fluid may be utilized to pressurize the chamber 60, pressurized air is preferred due to its availability, the known controls for air, and the minimal inertial effect thereof; accordingly, air will be described as the 105 operating fluid for the control member 62 herein The control means 66 may be of any suitable type to permit air to be controlled as described herein and suitable known fittings are employed between the described por 110 tions of the air supply system A suitable circumferentially continuous flexible seal 70, having one portion secured between the seal adaptor 50 and the disc portion 42 and another portion secured between the upper 115 portion of the piston assembly 54 and a lower surface of ring member 44, is provided to prevent material from within the bowl 24 from entering the chamber 60 upon movement of the ring member 44 upwardly away 120 from the seal adaptor 50 A suitable vent line 63 open to the atmosphere is provided within the piston assembly 54 to prevent any air pressure buildup within chamber 50 or below the seal 70 in any suitable manner 125 Although this invention can relate to various types of centrifuges the embodiment illustrated has been designed for use in dewatering mine or mill tailings as are commonly used in backfilling in mining opera 130 1,589,389 tions and accordingly the description is given with reference to such mill tailings In such a backfilling operation it is often desirable to have a continuous discharge of mill tailings from a centrifuge In order to achieve a continuous substantially uninterrupted discharge of mill tailings from an incoming mill tailings slurry when utilising the above described centrifuge it is necessary to establish as described hereinafter, a relatively impervious beach zone within the rotating bowl 24 of the centrifuge Such beach zone is accomplished by admitting the mill tailings slurry to the centrifuge, as described, and rotating, the centrifuge at a speed lower than the normally operational speed typically at 400 to 600 rpm, with the discharge gap between edges 40-46 closed.

Springs 56 normally bias the member 44 away from the member 38 so that the discharge gap is normally open The discharge gap is closed and maintained closed by controllably admitting pressurized air from supply 68 through passageway 64, and line 64 to the interior of control member 62 By proper supply pressure of the pressurized air the control member 62 is flexed within chamber to move the piston assembly 54, with the ring member 44 attached, so that edge 46 firmly engages the edge 40 Such air is supplied at a pressure to overcome the bias of the springs 56 and to provide the necessary contact pressure between edges 40-46 Any air within chamber 60 surrounding the control member 62 is vented upon the expansion of member 62 through line 62 to the atmosphere With the discharge gap closed the solids of the incoming and rotating slurry are separated by centrifugal action and accumulated in the radial outermost portions of the interior of the bowl 24 The liquid content of the slurry is discharged through ports 32 into discharge chamber 25 and is handled in any suitable manner such as by lines (not shown) discharging into a tank for permitting the pumping of the collected liquid to a location as desired Preferably the liquid content is pumped to the surface for use in forming subsequent mill tailings slurry The admission of slurry and the rotation of the centrifuge is continued until the buildup of the solid particles is of a depth extending radially inwardly from the discharge gap and of a consistency such that the discharge gap can be opened without discharging the deposited solids Such deposited solids occupy the greater portion of the bowl 24 and define a generally cylindrical surface 74 spaced radially outwardly of shaft 14 which is the innermost surface of the beach defined by said solids.

By so operating the centrifuge surface 74 occurs at a location radially outwardly of the accelerator portion 26 and extends generally vertically between the inner surfaces of the disc portions 38-42.

In order to provide for a continuous discharge of such despoited the discharge gap is opened to the desired width by suitably controlling the supply of air to the control member 62, and the operating speed of the 70 centrifuge increased to a speed at which initiation of the discharge of deposited solids occurs through the discharge gap The speed at which such discharge is initiated is hereinafter referred to as the threshold speed 75 and typically is 100 to 300 rpm higher than the rpm utilized to establish the beach The discharge gap is opened by reducing the air pressure in the control member 62 by suitable operation of the control 66 80 The threshold speed is not necessarily the normal operating speed of the centrifuge and to obtain a continuous discharge of deposited solids from a high solids content slurry after discharge is initiated the motor is controlled 85 to provide a centrifuge speed of between 1400 to 2500 rpm At such operating speed three distinct functional zones occur within the bowl 24 Referring to Figure 6, a separation zone 76 extends as an annulus around 90 shaft 14 and slurry is admitted to zone 76 from the acceleration chamber 26 and effluent is discharged from the upper portion of zone 76 through discharge ports 32; a beach zone extends from the separating zone 95 76 to the discharge zone, said beach zone defining the inner surface 74; and a discharge zone is located between the surfaces 40-46 and radially inwardly of such surfaces but adjacent thereto As the incoming slurry 100 enters the separation zone 76 the solids therein, due to their higher relative density with respect to the transporting water vehicle, move with respect to net direction of flow, radially of the separation zone 76 and 105 are deposited on the radially innermost portion of the beach zone The beach zone serves as an accumulating zone for such deposited solids and isolates the discharge zone from the separation zone whereby the 110 separating process can be accomplished independently of the discharge of material.

Since the separating and discharge are isolated the centrifuge can be operated with reference to the requirements of the separat 115 ing process provided that the requisite material discharge occurs Thus it is preferred that the discharge zone be adjustable to provide the rate of material discharge required for the desired separation process By control 120 ling operation of the centrifuge with reference to the requirements of the separating process it is envisaged that the smallest solid particles can be deposited in the beach zone and a substantially clear water can be dis 125 charged through ports 32 so that all the solids of the slurry are utilised in the material discharge As hereinafter discussed there are many interrelated factors which affect the deposition of solids and it has been found 130 1,589,389 practical to have an effluent discharge through ports 32 containing 5 % by weight of the smaller particle solids of the slurry.

When the centrifuge is operated with reference to the requirements of the separation process if the discharge of solids is to be uniformly continuous the rate at which solid particles are deposited in the beach zone should be the same as the rate at which the deposited solids are to be discharged Thus it is preferred that the discharge gap is of a configuration to permit deposited material to be discharged at the required rate, i e equal to the rate at which solids are deposited in the beach zone A discharge of deposited material at a rate higher than the rate at which solids are deposited would, over a period of time, cause the elimination of the beach A rate of discharge lower than the rate of deposition would cause surface 74 of the beach zone to shift radially towards the shaft 14 and reduce the volume of separation zone which reduction would cause more solids to be contained in the effluent discharge due to the higher axial velocity of water flow through the separation zone resulting from the decrease in volume of the separation zone However, it is to be understood that with the centrifuge described above some radial movement of the surface 74 can be tolerated without adversely affecting the operation of the centrifuge.

The distance over which the interface between the separation zone and the beach zone, i e surface 74, can move radially is determined by the radius of surface 74 (hereinafter called the blow-out radius) at which the beach zone cannot be maintained since the volume of deposited solids is insufficient to contain the hydrostatic pressure being exerted upon the beach zone; and the radius of surface 74 (hereinafter called the washout radius) below which deposited solids at the interface of the beach zone and the separation zone are all re-entrained in the water vehicle and are discharged through ports 32 and/or some solid particles in the incoming slurry are not deposited in the beach zone and are discharged through ports 32 Since the blow-out radius is larger than the wash-out radius the surface 74 can safely shift radially between a radius slightly smaller than the blow-out radius and slightly larger than the wash-out radius.

The deposition rate of solids in a centrifuge of the type described above is dependent upon various factors with the principle factors being the speed of rotation of the centrifuge, the characteristics of the solids content of the slurry utilised when acted upon by centrifugal force, the rate at which the slurry enters the centrifuge, and the solids content of the slurry Although centrifuge speed can be controlled the other factors vary substantially for various backfilling operations so that as a result it is difficult to control the rate of solids deposition For a given backfill operation in which the mill tailings are produced in a constant manner and the amount of transport water and the 70 transport system is known for a particular location the rate of solids deposition can be fairly well controlled; however, as variations in the pipe system and location occur a given rate of solids deposition is not practical other 75 than for a specific mine location Consequently with this invention the balance required for continuous operation is achieved by controlling the rate at which the deposited solids are discharged The dis 80 charge rate is determined by various factors with the principal factors being the centrifuge speed, size and shape of the discharge opening, properties of the material being dischargedand the manner in which the depo 85 sited material flows through the beach zone.

The bowl 24 is provided with an inner surface configuration or profile specifically designed to control the flow of deposited material through the beach zone such that the resul 90 tant material discharge is uniform Since the flow resistance of the deposited solid particle is dependent upon the flow characteristics of the material and the forces, hydrostatic and centrifugal, acting upon the beach zone, the 95 bowl profile is thus selected with reference to the particular material that is to be within the beach zone and the operating conditions for the centrifuge.

In addition, the bowl profile is preferably 101 determined by the control functions to be provided in the bowl 24 Thus, the initial slope of the inner surface of the bowl 24 radially outward of surface 74 is selected to maintain equality between the rate of solid 10.

deposition and the rate at which the deposited material moves outwardly from shaft 14 in such initial area The final slope of the inner surface of the bowl 24 determines the rate of material discharge and the manner in 11 ( which the deposited material flows into the discharge gap The intermediate slope or transition slope of the inner surface of the bowl 24 between the initial and final slopes provides the control to accept the volume of 11 ' material from the initial slope and to provide the volume of material required by the discharge portion Consequently the interior profile of bowl 24 for a selected material and centrifuge operation provides a selected 121 resistance to material flow to ensure uniform and controlled material movement through the beach zone The terms uniform flow and controlled flow do not relate to having deposited particles move in a precise pattern or 12 ' manner within the beach zone but specify the overall resultant flow of the deposited particles through the beach zone in which individual particles may have entirely different movements As shown, Figure 3, the inner 131 D 1,589,389 surface of the bowl 24 is provided with a variable slope in which the slope of the bowl increases with respect to the increase in radius from the rotation axis X-X to impose a higher resistance to material flow as the centrifugal force acting upon the material increases Such profile imposes the optimum flow resistance on the beach zone at all bowl 24 radii and in practice the inner surfaces of the bowl member 24 a and 24 b are mirror images As shown, the inner surface of the lower member 24 b extends at an angle 0, to the radial direction at its radially inner extent adjacent the chamber 26 and an angle 02 to the radial direction at its radially outer extent adjacent discharge 34 An intermediate surface portion of member 24 b extends radially between the respective innermost and outermost portions along a curvalinear path such that at each succeedingly larger bowl radius in the surface of member 24 b forms a respectively larger angle with the radial direction such that the interior profile of the bowl 24 is of a concave form The increase of surface profile angle with respect to the radial direction with increasing bowl radius imposes progressively larger flow resistance on each material element in the beach zone as it flows radially outward thereby compensating for increased centrifugal force on the solids load at such larger radii and helping to establish and maintain uniform movement ofparticles through the beach zone.

The angle of bowl surface profile to radial direction need not increase continuously from the inner radii outward For example an alternative surface profile (Figure 4) comprises a plurality of contiguous linear profile portions with the portions located at respectively larger radii forming respectively larger angles 03, 04 and 05 to the radial direction.

Other profile configurations providing an increased surface profile angle to the radialdirection at successively larger bowl radii may also be used.

In practice for a centrifuge used for dewatering slurried mill tailings, an angle 0, of 13-1/2 degrees has proven to be very satisfactory; however, the angle 01 can vary within a range dependent upon the latitude in designing the centrifuge Changing the angle 01 varies the rate of flow of the particles within the beach; accordingly, the angle 0, is selected to provide for the uniform flow described with the maximum cross section of discharge gap 34 For such a centrifuge an angle 02 of 45 degrees has proven to be very satisfactory; however, the angle 02 can also be varied, as desired, with relation to the variations of the angle 01 or as desired independently of the angle 01 to provide such uniform flow and maximum cross section of the discharge gap 34.

Another important aspect of bowl geometry is the height to depth ratio of h /d of the separation chamber as indicated in the left half portion of the bowl 12 in Figure 1.

Generally a small height to depth ratio provides a more stable beach than a large ratio.

For example with a large ratio, that is rela 70 tively large bowl axial height h and small radial depth d, the beach zone exhibits instability in that relatively small changes in the operation of the centrifuge, such as bowl rotational speed, feed concentration, feed 75 rate may disrupt uniform beach flow as by precipitating a gradient of increasing beach flow rate toward the axially central portion of the beach zone Ultimately such disruption may cause non-uniform percolation of 80 effluent, destroy the uniformity of the interface 74, or cause a beach blowout or collapse as described The optimum height to depth ratio of course will depend largely on other parameters of the system, notably the physi 85 cal properties of the material to be separated, but generally will be in the range of 0 75 to 1.5 or more specifically in the range of 0 9 to 1.2 for a centrifuge to be used for processing a solids concentrated slurry of mill tailings 90 With the profile of the bowl 24 established with reference to the incoming slurry and operating speed, the balance required for continuous operation is obtained by selecting an appropriate width of the discharge gap 95 Accordingly, after the beach zone has been established and the operational speed obtained, the control member 62 is depressurized by discharging the pressurized air therein to atmosphere by the control 66 The 100 width of the discharge gap is adjustable throughout the axial extent of movement of the ring member 44 by varying the air pressure within the control member 62 Accordingly, the discharge gap is maintained at the 105 width to obtain the equality between the deposition rate and the discharge rate.

Should any of the operational parameters vary during the operation of the centrifuge, such as slurry feed rate or the solids content 110 of the slurry, the discharge gap can be varied by pressurizing or depressurizing the control member 62 as desired to maintain such equality For example, if the feed rate suddenly drops below the operational level less 115 solids will be deposited at the beach interface 74 and in order to maintain this interface 74 within the allowable radial distance, the material discharge rate is decreased by increasing the inflatable seal pressure to reduce the 120 gap width Conversely, if the discharge rate needs to be increased, the seal pressure is lowered which results in a wider gap Since the function of the control device 62 is to move the ring member 44 via the piston 125 assemblies 54 the control device 62 and chamber 60 may be of any suitable configuration to provide such movement.

Although the continuous peripheral material discharge gap as described is preferred, 130 1,589,389 if desired, other forms of discharge gap may be employed A satisfactory alternate discharge gap is obtained as shown in Figure 5, by providing the ring member 44 with integral upwardly extending square block portions 15 radially outwardly adjacent the discharge gap previously described and which block portions 15 are of uniform configuration and uniformly circumferentially spaced to provide uniform width and height slots 17 therebetween through which material is discharged Alternatively the block portions may comprise separate members rigidly secured to the ring member 44 With such structure the cross-sectional area of the discharge opening is reduced which causes a corresponding reduction in the material discharge rate Such lower material discharge rate is desired in instances where the solid particles content of an incoming slurry is not particularly high With this modification the circumferential extent of openings 17 are approximately the same as the circumferential extent of the individual blocks 15.

Housing 10 is provided with suitable interior annular radially spaced wall members 21 into which the material from the centrifuge is discharged and subsequently removed in any suitable manner through one or more suitable lowermost outlets 23 in communication with the annulus between the wall members 21 The radially outermost wall member 21 forms, in conjunction with the walls of housing 10, a formed annular chamber 25 for receiving discharge water from the discharge ports 32 Chamber 25 is connected by one or more suitable discharge ports 33 to permit the desired disposal of the discharged water effluent Ring members 38 and 44 are subject to wear due to the discharge of material therebetween and accordingly are replaceable If desired, ring members 38 and 44 can be made in suitable sections to permit their easy replacement.

Further, a suitable inlet means 27 can be provided to permit the addition of cement to the material discharged from the centrifuge to provide a suitable resultant material The centrifuge described has rotating components which rotate coaxially with respect to the axis X-X and are radially located to prevent rotational unbalance force from occurring within the centrifuge.

It will be appreciated that the foregoing description relates to a centrifuge and method of operating a centrifuge for continuously removing solid particles from a continuous flow of incoming slurry by isolating the centrifuge separation zone from the centrifuge discharge zone whereby the deisgn of the centrifuge is made with reference to the physical requirements for centrifugal separation of such particles Such isolation is achieved by retaining and controlling the continuous movement of deposited particles through a zone, identified as the beach zone, between the separation zone and the discharge zone for a period of time such that the forces on the deposited particles do not adversely effect the movement of the 70 particles through the discharge zone.

In particular the above description shows that the hydrostatic pressure on the interface between the separation zone and the beach zone provides a force to cause the movement 75 of the deposited particles through the beach zone and the beach zone is of a composition to withstand the pressure gradient between the interface (as described herein) and the discharge zone Further, the hydrostatic 80 pressure of the water content of the incoming slurry is sufficiently high to cause percolation of a limited quantity of water through the beach zone to obtain the described flow of the deposited particles Flow of deposited 85 material in the beach zone is also controlled by means of the inner surface configuration or profile of the separation bowl of the centrifuge which configuration can be varied as described while still maintaining the control 90 function of such profile.

A centrifuge as described above is capable of discharging generally the same consistency of material throughout a wide range of solid particle content, by weight, of the 95 incoming slurry Mill tailings slurries having solid particle contents from between 30 and percent by weight can be handled to provide the same general consistency of discharged material The flexibility of the 100 described centrifuge with relation to the variable operating parameters of the centrifuge is obtained by providing both an interface which is locatable within a range of radial locations and a wide material discharge 105 Thus, for a given discharge opening various changes in operating parameters can be accommodated for by the radial shifting of the interface and conversely, for a given proper radial location of the interface varia 110 tions in the operating parameters can be accommodated for by varying the crosssection of the material discharge opening.

Should the radial location of the interface and the cross-sectional opening of the dis 115 charge gap be effective upon the same operational parameter of the centrifuge the described centrifuge permits the interface to shift radially or the gap to be varied in crosssection or a continuation thereof to accom 120 modate for any variation in such parameter.

Reference is made to co-pending Application No 35829/77 (Serial No 1589387) from which this Application is divided and which relates to a centrifuge apparatus for 125 separating a slurry of particles entrained in a liquid into a liquid portion and a mixture or product portion of particles and liquid, with such mixture or product portion being dischargeable from the centrifuge, said 130 1,589,389 apparatus comprising: at least one bowl member defining a centrifuge chamber having a central axis of rotation; a structural member supporting said at least one bowl member for rotation about said axis; means for supplying said slurry to said chamber; means for permitting discharge of said liquid portion from the chamber; said centrifuge chamber having a peripheral discharge means located around the periphery of the chamber and located at the outermost radial portion from said axis for discharging said mixture or product portion; said bowl member having an inner configuration comprising profiled surfaces the angular inclination of each of which to a plane perpendicular to said central axis increases with increasing radial distance from said central axis to control the flow of said mixture or product portion at points spaced successively radially outwardly from the axis of rotation so that, in use of the centrifuge, a volume of the mixture or product portion may be maintained in said chamber to provide a substantially continuous uninterruped discharge of said mixture or product portion.

The said co-pending application also relates to a centrifuge apparatus comprising, a housing member, an elongated shaft member rotably supported by said housing member for rotation about the central longitudinal axis thereof, a pair of formed bowl members defining a centrifuge chamber rotatably carried by said shaft member, passageway means to permit continuous supply of a slurry of solid particles entrained in a fluid vehicle into said chamber and to permit continuous discharge of an effluent portion of such a slurry from said chamber, said bowl members having peripheral portions spaced radially outwardly of said axis, means for rotating said shaft member whereby such a slurry as supplied to said chamber is continuously separated into a product portion and an effluent portion, means carried by at least one of said members for selectively axially moving at least a part of one of said bowl members with respect to said axis between an axial position whereat said peripheral portions are in continuous engagement with each other to permit an initial selected volume of said product portion to accumulate in said chamber and at least one axial position whereat said peripheral portions are axially spaced from each other to permit continuous discharge of said product portion as initially accumulated from said chamber throughout a period of time, and said bowl members having an inner configuration such that the inner surface of each of the bowl members has an angular inclination to a plane perpendicular to said central axis of rotation which increases with increasing radial distance from said central axis to control the flow of said product portion at points spaced successively radially outwardly from the axis of rotation so that, in use of the centrifuge, a volume of the product portion may be maintained in said chamber to provide a substantially continuous uninterrupted discharge of 70 said product portion.

Claims (14)

WHAT WE CLAIM IS:-

1 A method of operating a bowl centrifuge in which a slurry consisting of solid particles entrained in a liquid is separated 75 into a liquid portion which is discharged from said bowl centrifuge, and a portion consisting of a mixture of particles and liquid within a radially outer section of such bowl with such mixture having a liquid content which is suf 80 ficient to permit the substantially uninterrupted flow of the mixture through a discharge opening, said method comprising:

maintaining a volume of said mixture within a predetermined range of volumes as an 85 annulus within said outer section throughout a period of time while simultaneously discharging seriatim portions of said mixture portion in a substantially uninterrupted flow from radially outermost portions of said vol 90 ume by replenishing said mixture portion by continuous centrifugal separation of supplied additional slurry.

2 A method according to claim 1 in which the average rate of particle discharge is 95 substantially equal to the average rate of particle replenishment in said volume during said maintaining.

3 A method according to claim 1 or claim 2 wherein said maintaining of said vol 100 ume is effected by varying the rate of discharge of said mixture from said volume.

4 A method of operating a centrifuge having an elongated shaft member rotatable about the central longitudinal axis thereof, a 105 pair of formed bowl members defining a centrifuge chamber rotatably carried by such shaft member and means in at least one of such members for admitting a slurry of solid particles entrained in a fluid vehicle into such 110 a chamber over a selected period of time; comprising: separating portions of such a slurry, continuously admitted throughout an initial period of time interiorly of such bowl members during continuous rotation of such 115 bowl members with peripheral portions of such bowl members radially outwardly of such axis being in continuous engagement with each other, into a product component primarily consisting of a mass of such parti 120 cles which increases in radial extent with respect to said axis throughout said initial period until said mass extends from the interior of said engaged peripheral portions to a position radially outwardly adjacent 125 such shaft with the portion of said mass adjacent said shaft defining an elongated unobstructed passageway axially encompassing the full extent of such shaft within said bowl members and with said passageway having a 130 1,589,389 lateral extent substantially less than the maximum radial extent of said mass; simultaneously separating portions of such slurry admitted throughout said initial period of time into an effluent component primarily consisting of such fluid vehicle, and discharging portions of said effluent component exteriorly of such bowl members from one axial end of said passageway; discharging portions of said mass exteriorly of said bowl members over an immediately subsequent period of time while maintaining the rotation of said bowl members with said peripheral portions being spaced from each other, while simultaneously separating additional portions of such a slurry admitted on a continuous basis to the interior of the bowl members into said product component and said effluent component at a substantially uniform rate so that increments of said product component are added to said mass to maintain the radial extent of such mass within predetermined limits throughout said subsequent period of time; and simultaneously throughout said subsequent period of time substantially uninterruptedly discharging portions of said effluent component of said additional slurry from said one axial end of said passageway.

5 A method according to claim 4 further including maintaining such radial extent by restraining flow of said product component by the interior surface of said bowl members.

6 A method according to claim 4 or 5, wherein said discharging is through the radial outermost portion of said bowl members.

7 A method accordiing to claim 4, 5, or 6, wherein during said subsequent period of time the mass of previously accumulated partides continuously flows towards an outermost portion of said product component at a controlled rate.

8 A method according to claim 7 wherein said continuous flow is retarded at a greater rate as the distance from said axis radially increases.

9 A method according to claim 6 wherein said discharging is of a continuous circumferential radial outermost portion of said outer portion.

A method according to claim 6 wherein said discharging is of a variable portion of said outermost portion.

11 A method according to any one of the preceding claims wherein the flow of deposited solid particles or mixture through said outer portion is at a uniform rate.

12 A method according to any one of claim 1 to 10 wherein the rate of said discharging is variable.

13 A method according to claim 4 or any claim dependent thereon wherein the location of the interface between said accumulated particles and said path radially shifts during said centrifugal accumulating.

14 A method of operating a centrifuge substantially as herein described with reference to the accompanying drawings.

FORRESTER, KETLEY & CO, Chartered Patent Agents, Forrester House, 52 Bounds Green Road, London N 11 2 EY.

and also at Rutland House 148 Edmund Street Birmingham B 3 2 LD and Scottish Provident Building, 29, St Vincent Place Glasgow G 1 2 DT Agents for the Applicants Printed for Her Majesty's Stationery Office, by Croydon Printing Company Limited, Croydon, Surrey 1981.

Published by The Patent Office, 25 Southampton Buildings, London, WC 2 A IAY, from which copies may be obtained.