AU479225B2 - Process and device for operation ofa filter centrifuge - Google Patents

Description

62, 454n3 The present invention relates to a process for the

operation of a,filter centrifuge and also relates to a cen-

trifuge for the performance of the process.

Systems are known in which the performance of centri-

fuges of this kind is increased by utilizing the suctional

effect, i.e. increasing the pressure difference on the two sides of the filter medium. According to German Patent 904 999 the kinetic energy of the centrifuged liquid is to be used for this purpose. This kinetic energy results from a pressure reduction and causes a hydro-dynamic suction effect.

The physical limit for the possible pressure difference is the

vapour pressure of the liquid.

The present invention provides a new process which will

result in an increased filtering performance, despite the

simple construction of the filter centrifuge, and a filter

centrifuge for the performance of the process.

According to the present invention there is provided a

method of operating a filter centrifuge comprising a centri-

fuge drum rotatable about an axis and having a filter medium

upon which a filter cake forms during operation, a liquid-

collecting chamber arranged to rotate together with said drum

and having at least one discharge aperture, said method com-

prising the steps of rotating said drum about said axis at a

rate sufficient to induce passage of a liquid phase of a

suspension outwardly through said filter medium into said

liquid-collecting chamber; removing a from said liquid- collecting chamber and filling same with liquid prior to the

operation offiltering to establish a rotating liquid continuum

2 479,225

62, 4 5 473 in said liquid-collecting chamber extending to said filter

medium, producing a pressure differential between the oppo-

site sides of the at least cnz discharge apertureiby the

combination of the centrifugal force of the.rotatable drum

and a liquid column having one liquid level in the region of

the filter medium and the other liquid level radially further

outwards from the centrifuge axis than the filter medium, the

pressure differential causing a suctional effect on the filter

medium, and throughout the filtering operation preventing

entry of gas through the discharge aperture into said liquid-

collecting chamber by the liquid in said liquid-collecting

chamber blocking entry of'tk thereto via the at least one discharge aperture.

These measures no longer make use of the hydrodynamic

suction effect, in which liquid is passed through more or

other liquid in the manner of a jet pump, but of the hydro-

static suction effect, in which the forceLe,&- an amount of

liquid acts directly on another amount of liquid to move the

another amount of liquid. Hereinafter the term "suction

effect" will refer to the hydrostatic suction effect or

pressure unless otherwise stated. Owing to the utilization of the hydrostatic suction

effect a considerably higher yield of filtrate is obtained,

thanks to the fact that the suction effect, in relation to

atmospheric pressure, reaches a level of almost 10 m water

column, according to the vapour pressure of the liquid

employed. Such high suctional effect pressures are far in

excess of those reached with any filter centrifuge hitherto.

_3 479,225

62, 454n3 The filter cake may be largely dried, by the aid of

the hydrostatic suctional pressure.

According to another feature of the invention there is

provided a filter centrifuge comprising a rotatable drum, a

support mounted in the drum for supporting a filter medium

centred on an axis of rotation of the drum, a liquid collect-

ing chamber rotatable with and located adjacent the filter

medium in the region of the radially outermost periphery of

the drum and at least one discharge aperture from the collect-

ing chamber located radially outwardly of the filter by an

amount sufficient to establish a suctional pressure effect in

the filter medium with respect to said axis, wherein during

operation of the filter centrifuge liquid in the collecting chamber serves to block entry of air thereto through the at least one discharge aperture.

A particularly suitable embodiment of the filter centri-

fuge is one comprising a plurality of discharge apertures from

the collecting chamber each associated with a respective feed

aperture into an annular chamber concentric with the axis of

the centrifuge, the feed apertures being at the same or at a

greater distance from the axis of the centrifuge than the dis-

charge apertures, the annular chamber being provided with a

discharge dam of which the discharge edge is at a smaller

distance from the axis of the centrifuge than are the feed.

apertures.

In another embodiment of a filter centrifuge there is

additionally provided a pivotally mounted discharge tube ex-

tending into the annular chamber for removing liquid therefrom

-4- 479.225

S62, 454 n wherein the annular chamber is provided with a discharge dam

of which the discharge edge is approximately at that distance

from the axis which corresponds to the distance of the filter

medium from the axis of the centrifuge, and wherein the dis-

charge tube is pivotable within a range approximately extend-

ing from the radial distance of the filter medium to that of

the feed apertures. In these embodiments it is possible, inthe annular

chamber, to select a liquid level with a greater radius than that of the liquid level in the collecting chamber, thus

ensuring that a suction force will be exerted on the filter

cake. The difference between the radii of the levels of

liquid, multiplied by the centrifugal acceleration and the

density of the liquid, will thus correspond to the difference

between the pressure in the interior of the centrifugal drum

and the pressure in the collecting chamber. This latter pressure can be reduced to the value of the vapour pressure of

the liquid, due to the temperature prevailing, and automati-

cally maintained in the filter centrifuge to which the inven- tion relates, even when liquid is flowing in from the filter

cake. With a fixed liquid level in the discharge part of the annular chamber, e.g. by the adjustment of the end of a dis-

charge tube, the liquid level in the annular chamber, is

rendered constant, even with the entry of liquid which is

sucked into the collecting chamber through the screen casing

and flung off centrifugally, because this quantity of liquid

which enters the collecting chamber from the filter through

the screen casing and which would alter the level of the liquid

479.22

62, 454n3 in the discharge part of the annular chamber corresponds to

the quantity of liquid which is then extracted from the annu-

lar casing by the discharge tube. When liquid flows into the

collecting chamber and when a corresponding quantity of liquid

is then discharged from it into the annular chamber, there-

fore, the pressure in the collecting chamber is maintained

until, in place of the filtrate liquid, a flow of air occurs through the filter cake.

For special cases it is of advantage if the edge of the

annular chamber is formed by openings which can be sealed

selectively, in an end wall of the annular chamber, in which case the openings are situated at different radial distances

from the axis of the centrifuge with the innermost openings on a radial level with the filter medium. In all the embodi-

ments mentioned a filling tube extending into the annular

chamber can be provided for the supply of a liquid.

The filter centrifuge may also comprise siphon tubes of

generally U-shape cross-section which extend radially outwards

from the discharge apertures and which then lead radially in-

wards towards the axis of the centrifuge, and of which the

discharge ends of the siphon tubes serve as a discharge dam

and are spaced a greater distance from the centrifuge axis than-the filter medium. When the filter centrifuge is being started up, it is

of advantage if blocking devices are provided in the zone

between the discharge apertures and supply apertures. It is

advantageous to provide, for the filter centrifuges, a trans-

mission device, such as a system of rods, by which the dis-

-6- 479.225

62. 454n3 charge dam is adjustable during operation. Embodiments are also proposed in which the axial distance of the discharge

dam from said axis is displaceable in accordance with the

acceleration.

It is also advantageous when using pressure stage filtration, if in filter centrifuges of the type mentioned at

the beginning sealing devices enabling a throttle effect to

be obtained are connected up to the discharge openings.

For certain particular filtering operations it is

desirable for these sealing devices to be operable in accord-

ance with acceleration of the centrifuge or else capable of being actuated, during operation, by means of a transmission

device. The centrifuges just described offer a number of advantages by comparison with those already known, particu-

larly the fact that the suctional effect and hence the filtering process can now be influenced in many ways during

operation. This control of the filtering process is particu- larly important if in the course of the said process the

rotational speed of the drum is altered or a change takes

place in the filter cake which, for example, makes a variation

in the suction effect appear desirable in order to adapt the

operation of the centrifuge in the optimum manner to the

permeability of the filter cake, the tendency towards com-

pression and the resulting change in the capillary effect or

to other properties of the substance to be filtered.

The filter centrifuges covered by the invention advan-

tageously result in a lower expenditure and obviate many of the difficulties involved in the known filter centrifuge

-7 7 479,225

62, 454n3 already mentioned at the beginning, since various porous sub- stances can be kept in readiness for the filtration of the various mixtures and also the said porous substances can be easily purified. A further advantage is that at the appro- priate moments at the beginning and during operation of the centrifuge, by the simple supply or discharge of filtrate or some other liquid in the collecting and discharge chambers, by means of the siphon apparatus or the sealing devices, any desired liquid level, and thus the desired pressure difference between the two sides of the filter cake, can be selected and maintained rapidly and without difficulty. One embodiment particularly aimed at extending the sphere of application of the filter centrifuges according to the invention and described in the foregoing is characterized by the fact that the internal chamber of the centrifugal drum, preferably inside a housing surrounding the said drum, the annular chamber and the sealing devices in a sealed manner, can be subjected to a gas pressure higher than the pressure prevailing in the collecting chamber. Owing to the fact that in this embodiment of the filter centrifuges the desired pressure difference can be produced in different pressure ranges, it provides a very simple means of largely adapting the apparatus to the physical properties of the mixture components to be separated by filtration and to other technical conditions arising in the process. The inven- tion makes it possible to dispense with any known means of sealing off the rapidly rotating centrifugal drum from sta- tionary parts of the centrifuge; the sealing of the interior

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62, 454/73 of the housing against rotating parts can either be carried out in the zone of the driving shaft or dispensed with alto- gether, as the housing surrounds the entire apparatus. Any pressure increase which, in the event of a pressure increase inside the housing, initially occurs in the selected chamber likewise can be prevented, or reduced to the extent desired, as far as the level of the vapour pressure, by additionally increasing the radius of the liquid level in the annular chamber, possibly brought about by pivoting the discharge tube radially farther outwardly and by the resulting change in the liquid level in the annular chamber. The sealing devicesmust in any event be so constructed that they cannot lead to any reduction of the pressure in the annular chamber. For this reason the radius of the liquid level in the annular chamber, for example, must not exceed the radial distance of the discharge apertures from the centri- fuge axis, since air could then enter the collecting chamber from the annular chamber. Similar considerations apply to the sealing devices. In filter centrifuges equipped with siphon apparatus of the annular chamber type, the collecting chamber, for the purpose of washing the filter material or the solid substance filtered, can be fed with liquid from the outside of the centrifuge towards the inside thereof, via a filling tube ex- tending into the annular chamber, which said liquid is pressed through the discharge apertures into the collecting chamber and radially inwards through the screen casing to the radially innermost surface of the filter medium. This measure also

-9- 479, 225

62, 454n3 in tnt; enables the solid substance to be evenly distributed

filter medium, in order to reduce or avoid disequilibrium, up since any unevenly deposited substances are thus washed

before the pressure stage filtration begins. In filter centrifuges with a siphoning apparatus at the a discharge apertures and/or with sealing devices allowing throttle effect, it is also an advantage that the collecting

chamber has at least one suction removal aperture which

communicates with an air suction device. In a further devel- device opment of this characteristic the air suction removal

takes the form of a jet pump fed utilizing an extraneous the source of liquid or the filtrate liquid discharged from

collecting chamber. In another embodiment the suction removal and is aperture is situated in the zone of the filter medium via a connected with an external air suction removal device and suction removal conduit which is integral with the drum the which communicates with a suction conduit integral with housing.

It is also suggested that the collecting chamber should with be surrounded on the one hand by boundary walls integral with the drum and on the other hand by a sealing wall integral boundary the housing, the latter wall connecting up wiih the

walls via rotary sealing devices, preferably of the annular

cup type, while the suction aperture is situated in the sealing wall. of These latter measures provide an advantageous means centri- increasing the filtration performance of the filter owing fuges operating by the process covered by the invention,

I0 -10- 479,225

62. 45473 to the fact that the removal of the air and the vapours via the suction aperture in the collecting chamber causes the hydrostatic suction pressure in the filter medium to occur at an earlier point and higher pressure differences to be reached more rapidly than possible with the air displacement method, which owing to the accumulation of filtrate liquid or extran- eous liquid in the collecting chamber takes place in such a way that the air is pressed through the filter cake in the opposite direction to the liquid, which flows out from the said filter cake in a radial direction. The process covered by the invention can also be carried out if the collecting chamber is provided with boundary walls connected to the filter drum and with sealing walls connected to the housing, between which walls rotary sealings, prefera- bly of the annular cup type, are situated, and if at least one sealing wall integral with the housing is provided with a dis- charge aperture to which the suction side of an external feed pump is connected. This makes it possible for both the air and liquid to be rapidly withdrawn from the collecting chamber through a feed pump and for the high suction pressure to be produced in the filter medium, which pressure can be kept below the pressure vapour, even with a sufficiently high per- formance of the feed pump. The process covered by the invention can also be carried out in such a way that a suction siphon device, with an addi- tional siphon device following its outlet, is connected up to the discharge part of the siphon device. This measure pro- vides a simple means of enabling the discharge part of the

11 47 9,225

62, 4547n3 siphon device to be automatically emptied to some extent, by

the aid of parts rigidly affixed to the centrifuged drum; for example, a discharge tube device can thus be dispensed with.

A suitable apparatus is also provided by the fact that a non-return valve, directed outwards for the emergence of

air, is installed on the collecting chamber, in the zone of

the filter medium. This likewise provides a simple and reli- able means of facilitating the removal of air from the

collecting chamber.

Embodiments of the present invention will now be des-

cribed, by way of example, with reference to the accompanying

drawings, wherein:- Figure 1 shows a longitudinal section through a filter

centrifuge in accordance with the invention;

Figure 2 shows part of the longitudinal section through

a further embodiment of a centrifuge of this invention;

Figures 3a to 3e show the longitudinal sections through a number of filter centrifuges, with characteristics merely

shown symbolically; Figure 4 shows a filter centrifuge with a siphon device

and with an air suction removal device attached to the drum;

Figure 5 shows a filter centrifuge with sealing devices on the collecting chamber, likewise with an air suction device

installed on the drum;

Figure 6 shows a filter centrifuge with a siphon device and with an air suction device installed on the housing;

Figure 7 shows a filter centrifuge with an air suction

device which can be used as an extraction device for the

12 479,225 liquid; 62.45 4i73 Figure 8 shows a filter centrifuge with a siphon device followed by a suction siphon;

Figure 9 shows a filter centrifuge with a siphon device

and with non-return valves for the removal of gas;

Figures 10 and 11 show different embodiments of the

filter centrifuge shown in Figure 7;

Figures 12 and 13 show filter centrifuges similar to

Figure 5, with suction devices in the form of liquid jet pumps; and

Figure 14 shows a filter centrifuge with a float-regu-

lated discharge aperture f.or the collecting chamber. Referring now more particularly to Figure 1 there is

shown a centrifugal drum 1, containing a cylindrical screen

casing 2, arranged concentrically, the drum being affixed to

a shaft 3. The centrifugal drum 1 has a drum casing 4 which

forms one wall of a collecting chamber 5. This collecting

chamber 5 must be regarded as extending from the drum wall to

the radially innermost surface of the filter medium, e.g. of a filter cloth and of any grid or screen which supports the

filter cloth. The filtered solid material deposits itself as

a filter cake 7 on the screen casing 2, which may be covered with a customary filter medium 6, e.g. a filter cloth. Annu-

lar wall 9 is parallel to a wall 8 of the centrifugal drum 1.

This annular wall 9, the drum wall 8 and a casing wall

interconnecting the two form an annular chamber 11 extending

for a short distance in the axial direction, its maximum dia-

meter exceeding the diameter of the screen casing 2.

13 479.225

62, 454n3 The collecting chamber 5 has individual discharge

apertures 12 in the drum wall 8 of the annular chamber 11 and

adjacent the casing wall

A pivotable discharge or siphon tube 14 extends into

the annular chamber 11, the end 15 of the tube being adjusta-

ble within the radial distance between the level of the inner

surface of the filter cake and the discharge apertures 12. A supply device 16 serves to introduce the suspension into the

centrifugal drum 1; a peeling or discharge apparatus 17 which

is to be actuated in the known manner by an adjusting device

18, enables the filtered solid substance to be discharged from

the drum via a chute

Furthermore, in the embodiment of a filter centrifuge

in accordance with the invention, shown in Figure 1, a filling

tube 19 for introducing a liquid extends downwards into the annular chamber 11. With the whole of this apparatus the liquid level in the annular chamber 11 can be adjusted as

desired, by the feed or discharge of liquid. The outlet tube

14 and the filling tube can be combined in a single tube which

will perform the function of both tubes.

Finally, to enable the filter centrifuge to be operated

with an increased gas pressure in the internal chamber of the

centrifugal drum 1, by comparison with the pressure prevailing

in the collecting chamber 5, a housing 20 is provided which

surround the drum. The tube of the feed device 16, chute

the discharge tube 14, the filling tube 19 and the shaft-3

extend in a known sealing manner through corresponding places in the walls of the housing

14 479,225

62.454/73 A device with which the gas pressure inside the

housing 20 can be influenced, e.g. a compressor or pressure

gas container, is not shown. This device is connected to a pipe socket 21 on the housing 20. Other details and devices on the centrifuge, e.g. the design of the driving device of

the centrifugal drum 1, have likewise been omitted from the

drawing, as being inessential to the explanation of the

invention. The manner in which the driving forceisit-eRi- milttd through the wall of the housing 20 is provided by the

shaft packing 22.

In another embodiment of a centrifuge according to the

invention, partly shown in Figure 2, the collecting chamber

merely comprises the narrow space between the filter cloth 6

and the drum casing 26, through which space passes a support-

ing grid 23. The drum casing 26 contains, in its turn, the

discharge apertures 12, which in this case communicate via

short discharge pipes 24 with supply apertures 13 for the

annular discharge chamber 11.

The method of operation of the filter centrifuge of

Figures 1 and 2 is as follows: At normal pressure inside the housing 20, and with the centrifugal drum 1 in operation, that

quantity of liquid is fed through the filling pipe 19 which

is required to ensure that the level in the collector chamber

reaches to the filter medium 6 or comes to rest by just

covering the filter medium. All of the air is thus expelled

from the collecting chamber 5. The tube 14 is pivoted so that

the end 15 thereof is radially inwards as far as possible.

By means of the feed device 16 the suspension is then

479,225

62 454/73 introduced into the centrifugal drum 1. The filtrate liquid-,

attempts to expel the liquid from the collecting chamber 5 or

flow out of the chamber 5 together with it, until an equal

level has come about in the internal chamber of the centrifu-

gal drum 1 and in the annular chamber 11.

This discharge is assisted very efficiently by the fact from that the discharge tube 14 is operative to remove liquid to the annular chamber 11 when the filtrate liquid -s applied

the filter whereupon the level of liquid in the annular the two chamber moves radially outwardly, so that between apertures sides of the-discharge apertures 12 or the discharge forms because 12 and the feed apertures 13 a pressure gradient and of the difference in level of liquid in the collecting operation discharge chambers thereby resulting in a filtering the which is considerably accelerated. By comparison with pressure, inner chamber of the centrifugal drum 1, a lower alia, on producing the suctional effect, and dependent, inter in the the difference between the liquid level prevailing collecting annular chamber 11 and that prevailing in the 5, radially chamber 5, now prevails in the collecting chamber

outside the filter medium 6. This pressure, as in the case

of a riser pipe pressure gauge; is kept in equilibrium with

in the drawings. In a a liquid column which is marked depends on the known manner this pressure difference thus density of the fil- relevant centrifugal acceleration, on the the liquid levels. trate liquid and on the difference h between 7 into the collect- Each entry of liquid from the filter cake

ing chamber 5 is immediately balanced out6 by the removal,

16 479,225

62, 45 4n3 through the discharge tube 14, of an equal quantity of liquid.

The lower pressure on the one hand must not fall below the

level of the vapour pressure to which the liquid is subjected

at the temperature prevailing, but on the other hand it can

reach this level if the radii of the collecting chamber 5 and

of the annular chamber 11 are appropriately designed and if

the liquid is largely removed in a suitable manner from the annular chamber 11. In order to ensure a particularly effective suctional

effect on the liquid present in the filter cake 7, it is .necessary to introduce via the filling pipe 19, enough liquid

into the annular chamber 11 and thus into the collecting

chamber 5 to ensure that the screen casing 2 and the filter

medium 6 will be permeated by this level of liquid. When the

discharge tube 14 is pivoted radially inwards the suspension is fed in to the level required to cover the filter medium 6

and the screen casing 2 completely. The internal chamber of

the filter centrifuge is now subjected, at the beginning of the filtration, and via the pipe sockets 21, to an increased

gas pressure, the liquid being at the same time largely

removed from the annular chamber 11 by pivoting the tube 14

so that its end 15 moves outwards in the radial direction.

With the filter centrifuge shovnm in Figure 1, it is true that

not only the internal or central chamber of the centrifugal

drum 1 but also the annular chamber 11 are subjected to the

same high gas pressure which attempts to drive the liquid

through the discharge apertures 12 into the collecting chamber

which is still approximately at normal pressure. With the

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62, 454/73 drum rotating at a sufficiently high speed, however, a liquid

column h is maintained, of which the centrifugal force gene-

rates a counter-pressure capable of standing up to the pressure

difference between the pressure in the collecting chamber

and the gas pressure in the housing

The filter centrifuge may be operated with an inert gas

atmosphere therein.

In Figures 3a to 3e the longitudinal sections of various

filter centrifuges according to the invention are shown sche-

matically. The centrifuge illustrated in Figure 3a consists of the collecting chamber -30 and of the discharge apertures 31

from the collecting chamber into the annular chamber 33. The discharge edge 35 of the discharge dam 34 has radius greater

than the radius of the filter medium 36 and smaller than the 3' radius of a circle on which the discharge apertures/4 lie. In the embodiment shown in Figure 3b a pivotable dis-

charge tube 37 is provided of which the location of the end

38 thereof enables the liquid level in the annular chamber 33

to be set, the annular chamber 33 of this centrifuge being

defined by at least an annular wall 39 of which the internal

edge has about the same radius as the filter medium 36.

According to the illustration in Figure 3c the annular

wall 39 may be provided with apertures 40 at different radial

distances, which can be closed as desired. For preference a system of rods 41 is provided to enable these apertures to be

closed or opened during the rotation of the centrifugal drum.

A filling tube 42 for the supply of the liquid for the purpose

of displacing the air from the collecting chamber 30 at the

18 479.225

62, 454n3 beginning of the filtration, as explained previously, extends

into the annular chamber 33.

Another embodiment of a filter centrifuge covered by

the invention is illustrated in Figure 3d. In a similar fashion to the filter centrifuge shown in Figure 3a siphon

tubes 43 operate in this case likewise and may be additionally

provided with blocking devices or seals 44 inserted between

the discharge apertures 31 and in this case the discharge

parts 45 of the siphon tubes 43. By means of these blocking

devices 44 the premature discharge of the liquid, before the

aforementioned complete displacement of the air from the

collecting chamber, can be prevented. Figure 3e shows symbolically the arrangement of a

sealing device 46 which is coupled with the discharge aper-

tures 31 and which provides a throttling effect. The sealing

or throttling effect is preferably adjustable, as indicated

by the oblique arrow. This adjustability can be obtained by a suitable system of rods, on similar lines to those shown in

Figure 3c, or by other centrifuge regulating devices which are

already known Der se. This is particularly advantageous if

the output side of the sealing devices 46 is followed by a

siphon device 47. This provides a simple means of preventing

the ingress of gas into the collecting chamber 30. This

siphon device 47 consists of a discharge part of rectangular

cross-section in which the sealing device 46 is located. Figure 4 provides a schematic diagram of a drum 51

belonging to a filter centrifuge and rotatable about its

shaft 52. The drum 51 contains a filter medium 53, such as

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62. 45 413 a supporting grid bearing a filter cloth, on which the filter

cake 54 deposits itself. Outside the filter medium 53 in the radial direction is a collecting chamber 55 having discharge

apertures 56 for the filtrate liquid. By the aid of an annular chamber 57, which connects up in the manner shown to

the discharge apertures 56, and by the aid of a discharge

tube 58 which can be pivoted into this annular chamber 57, a

liquid column represented by the difference between the level

59 in the collecting chamber 55 and the level 60 in the annu-

lar chamber 57 can be built up and maintained, and it ensures

a considerable lower pressure in the collecting chamber 55 in

relation to the pressure in the'interior of the drum. This ensures that a considerable suction effect is exerted on the

filter cake 54.

In similar fashion, and as shown in Figure 5, the filter

centrifuge schematically shown therein as having the drum 101,

a filter medium 103 and a collecting chamber 105, is able to

exert a suctional effect on the filter cake 104. In this filter centrifuge, instead of the siphon device used with the

centrifuge shown in Figure 4 and consisting of part of the

collecting chamber 55, the discharge apertures 56 and the

annular chamber 57, providing a throttling effect, a sealing

device 111 is fitted to each discharge aperture 106. Here again the suction effect of a liquid column can be exerted on

the filter cake 104.

Any remaining quantities of air in the collecting

chamber 55 or 105 according to Figures 4 and 5 prevent this

suction effect from occurring to the optimum extent, because

20 479,225

62, 454n3 of the vapour pressure of the liquid. For this reason a suction aperture 62 or 112 is provided on the collecting chamber 55 or 105 and is in communication with the suction withdrawal device 63 or 113 respectively.

As may be seen in Figure 6, which shows a filter

centrifuge somewhat similar to that in Figure 4, it is also possible to provide an external air suction withdrawal device 64 .in which the suction apertures 62 are in communication with an external air suction withdrawal device 64 via a suction

withdrawal conduit 65 affixed to the drum and via a suction

conduit 66.

A filter centrifuge with which both the air and the liquid can be removed from a collecting chamber 205, is shown in Figure 7. The collecting chamber 205 is in this case surrounded by boundary walls 217 and 218 which are integral

with the drum and with the housing respectively and between

which rotary sealings are provided in the form of annular cup sealings (not shown). By the aid of a discharge device 221 which corresponds to a siphon device as shown in Figure 4 or

a sealing device as shown in Figure 5, the aforementioned

suctioned effect can be controlled. In addition, a discharge device 222 is provided in the boundary wall 218 integral with

the housing and is connected to the suction side of an exter-

nal feed pump 223. This discharge device enables the suctioned effect exerted on the filter cake 204 to be assisted by means

of the centrifugal force of the liquid column and also enables

the filter cake 204 to be dried after the termination of the

main filtration.

21 479,225

62, 454/73 Figure 8 shows a simplified embodiment of a filter centrifuge in which a siphon device 326 is connected to the

discharge part of the siphon device 325. This construction

provides a simple means of enabling the automatic formation

of the liquid column exerting the aforementioned suctional

effect, this being done by the fact that at the beginning of

filtration the collecting chamber 305 and the annular chamber

307 are first filled with liquid, until the latter flows over

the overflow edge 327. Owing to the overflow edge 327 being

situated at the same radial distance from the axis 302 as the

filter medium 303, air is displaced from the collecting

chamber 305 in this process (in this case radially inwards

from the filter cake 304); that tube of the siphon device 326

which is on the discharge side fills with filtrate liquid until

the latter has flowed radially outwards and reaches the radial

distance of the discharge aperture 306. From this point on-

wards the siphon device 326 comes into operation and attempts

to suck the annular chamber 307 empty. This suctional effect

transmits itself to the collecting chamber 305, with the

formation of a corresponding liquid column. Therefore, the

centrifugal force, as described above, results in the desired

suctional effect on the filter medium. An additional siphon

device 328 at the outlet of the suction siphon device 326

serves to prevent the ingress of external air into the suction

siphon device 326. The removal of air from the collecting chamber 405 can

be effected in an advantageous manner by a non-return valve in 429 which is situated, in accordance with the embodiment

22 479,225

62. 454n3 Figure 9, on the collecting chamber 405, in the region of

the filter medium. This enables the air to be displaced from

the collecting chamber 405 practically without resistance.

The filter centrifuge illustrated in Figure 9 is provided with

a siphon device, the discharge apertures 406 and outlet tube

408. In Figure 10 the filter centrifuge is provided with a

drum 501, rotatable about its axis 502. The filter medium

503 is provided as in Figure 7; the end walls of the drum are prolonged radially beyond the cylindrical part of the drum 501

(dam rings 530) and combine with the collecting chamber 505

to form annular cups 520. Between the dam rings 530, on the

outer periphery of the drum 501, are a certain number of

driving blades 531. The centrifugal housing 532 is mounted

in a rigid position.

At the beginning of the separating process discharge

orifices 506 of the collecting chamber 505 are closed by means

of shut-off valves 533.

When the drum is rotating a liquid ring can form in the

collecting chamber 505, the height of the ring depending on

the arrangement of an overflow aperture 534. If the discharge

apertures 506 are now opened by means of the shut-off valves

533, the liquid ring is lowered until a liquid column occurs

defined between the levels of the filter 503 and the discharge

apertures 506, the height of the column combining with the

rotation speed of the drum 501 to form a hydrostatic suctional

force on the filter 503.

The filter centrifuge shown in Figure 11 operates in a

23 479,225

62, 4 5 4/73 similar manner to that sho.rn in Figure 10. In order tu reduce the liquid friction the annular cup 620 is made narrower

than that shown in Figure 10. Consequently, the dam rings 630 are moved close together and the collecting chamber 605 is

kept smaller. Tne discharge apertures 606 and the shut-off valves 633 are likewise provided. The overflow device con- sists of the radially innermost surfaces of the annular cups

620 and the borings 634.

Figures 12 and 13 show two embodiments of air suction devices in detail, which are more practical forms of the

embodiments shown schematically in Figures 5 and 6. In place of the suction device 64 shown in Figure 6, in the form of a

pump, Figures 12 and 13 show liquid jet pumps 838 and 839

respectively.

Referring particularly to Figure 12, liquid flowing out of the collecting chamber 805 flows via the discharge aper-

tures 806 into the annular chamber 807 of the siphon device.

An initial pump effect in the Figure 12 embodiment is caused

by an extraneous liquid fed through a pipe 848 into a channel

849 and a pump tube 840. Owing to the reduction in the cross- section of the tube 840 by the nozzle 842 a liquid jet is

provided into the tube 844, in which jet the liquid has a

greater speed and therefore a lower pressure. Any surrounding air in the tube 844 is thus sucked in and carried along into

the annular chamber 807. Similarly any air in the collecting chamber 805 will be sucked into the tube 844 via apertures 812

and again removed to the annular chamber 807.

After the initiation of the separation process the feed

24 479,225

62, 454n3 of the extraneous liquid according to Figure 12 can be termi-

nated. After the completion of the separating process the

water jet pump shown in Figure 12 can be re-started, in which

process the liquid level in the annular chamber 807 is lowered.

This ensures that the collecting chamber 805, largely filled

with air, will be subjected to a lower pressure, so that the

filter cake on the filter medium 803 is dried by the air

flowing through. Referring now more particularly to Figure 13, liquid

flowing out of the collecting chamber 805a flows under pressure

into a conically tapering tube 841, from the nozzle 843 of

which it is transferred in a short jet into a further tube 845.

At the lower end of the tube 845 the liquid is fed directly

into the annular chamber 807a of the siphon device. In the

embodiment shown in Figure 13 the entire liquid flowing through

the discharge aperture 806a from the collecting chamber 805

flows through the pump 839 which is constructed as a Venturi

tube. Owing to the reduction in the cross section of the

nozzle 843 a jet occurs in which the liquid has a greater

speed and therefore a lower pressure. The surrounding air in

the tube 845 is thus sucked in and carried along. As the tube

845 communicates, via suction aperture 812a and a conduit 847,

with the radially innermost part of the collecting chamber

805a, air present therein will be sucked off. The liquid causing the pump effect in the pump 841

consists of the filtrate flowing out of the collecting chamber

805a. In connection with the embodiment shown in Figure 13 it

should be mentioned that the filtrate liquid flows through

25 479.225

62, 454n3 pieces of piping 850 into the collecting chamber 805a, which extend outwards beyond the drum wall 801 and beyond the height

of the inlet aperture 812a of the conduit 847. An air buffer

is thus left in the collecting chamber and serves to enable

the centrifuge to operate by the hydrodynamic suction princi-

ple likewise.

The embodiments shown in Figures 12 and 13 are also

provided with siphon devices such as already described.

Finally, Figure 14 shows a filter centrifuge in which

the collecting chamber 905 is in communication with the interior of the drum 901 via pieces of piping 950 and with the

atmosphere via a throttling device 951. The throttling device

951 comprises a conical part 952 which serves to close dis-

charge aperture 906 and which is connected via a rod 953 with a float 954. The float 954 situated approximately on a level

with the radially outermost end of the piping sections 950 is

spring loaded so that a spring 955 resting on an abutment 956

will tend to open the throttle device. That part of the

collecting chamber 905 which is situated radially inwards is provided with a non-return valve 929 by which air tending to

emerge from the collecting chamber 905 is removed therefrom.

The method of operation of the centrifuge is as

follows: The suspension is fed into the rotating drum 901.

Since when the drum 901 is rotating the force of the spring

955 is overcome by the centrifugal force of the throttling device 951, this latter closes and prevents the re-emergence

of the liquid passing through the filter medium 903 and the

-26- 479,225

62. 454//; pipe section 950 into the collecting chamber 905. 'The air. displaced from the collecting chamber 905 can escape via the non-return 929. The liquid level rises radially inwards in the collecting chamber 905, first of all reaching the radially outward .ends of the pipe sections, then causing the float to be taken along with it in such a way that the throttle device 951 opens. If more liquid is flowing out of the collecting chamber 905 than is flowing in, the throttle device 951 will first of all re-close. This leads to an equilibrium in which, on the one hand, a liquid column closed in the radial direc- tion is maintained between the filter medium 903 and the dis- charge aperture 906, while on the other hand an air buffer closed off from the outside by means of a valve 929 is provided in the radially innermost department of the collect- ing chamber. The magnitude of the air buffer can be kept small. The possibility of a small air buffer reduces the expenditure of apparatus involved in the complete removal of the air. When the centrifuge is shut off the throttle device 951 opens and enables the liquid to flow out of the collecting chamber 905. It should be added that in the foregoing, for the sake of simplicity, the gas mentioned has consisted of air, what has been said regarding air will naturally apply to any other gas likewise.

-27- 479.225

Claims (1)

1. 62, 454n3 The Claims defining the invention are as follows:-

   1. A method of operating a filter centrifuge compris-

   ing a centrifuge drum rotatable aoout an axis and having a

   filter medium upon which a filter cake forms during oper-

   ation, a liquid-collecting chamber arranged to rotate

   together with said drum and having at least one discharge

   aperture, said method comprising the steps of rotating

   said drum about said axis at a rate sufficient to induce

   passage of a liquid phase of a suspension outwardly through

   said filter medium into said liquid-collecting chamber;

   removing gas from said liquid-collecting chamber and

   filling same with liquid prior to the operation of

   filtering to establish a rotating liquid continuum in

   said liquid-collecting chamber extending to said filter

   medium producing a pressure differential between the

   opposite sides of the discharge aperture or the discharge

   apertures by the combination of the centrifugal force

   of the rotatable drum and a liquid column having one liquid

   level in the region of the filter medium and the other

   licuid level radially further outwards from the centrif-

   uge axis tat- the filter medium the pressure differen-

   tial causing a suctional effect on the filter medium,

   and throughout the filtering operation preventing entry

   of gas through the discharge aperture into said liquid-

   collecting chamber by the liquid in said liquid-collecting

   chamber blocking entry of gas thereto via the at l-at

   discharge apertureX "Ia T' s4ud'

   -28- 479.225

   62, 454/73 2. A method as claimed in claim 1, wherein removal of the air from the collecting chamber is effected by filling the collecting chamber with liquid via an annular chamber concentric with the axis of the centrifuge and in coriunicat ion with said collecting chamber through said discharge/Iapertures mounted for rotation with said collecting chamber and communicating with the collecting

   chamber through said discharge aperture.

   3. A method as claimed in claim 2, wherein the suction pressure is formed by the difference in levels of liquid in the collecting and annular chambers relative to -the centrifuge axis.

   4. A method as claimed in claim 3, wherein the levels of the liquid in the collecting and annular chambers are adjustable in relation to each other.

   A method as claimed in claim 4, wherein the liquid levels are adjustable so that the pressure differential reaches the zone of the vapour pressure of the liquid.

   Ol 6. A method as claimed in any40of claims 3 to 5, wherein a controllable increased pressure is produced in the drum with a gaseous medium, the effective filtration pressure then being the sum of the increased pressure and of the pressure differential resulting from the liquid levels.

   ant 7. A method as claimed in any~of claims 1 to 6, wherein

   -29- 479, -29- 225

   62,454/3 during filtration only that quantity of liquid is drawn off through the at Jst cno discharge aperturekthat is

   required to ensure that the level of liquid will be main-

   tained in the collecting chamber within the filter medium.

   8. A method as claimed in claim 5, wherein the liquid

   levels are adjustable so that the pressures are approx-

   imately equal on each side of the radially outermost

   surface of a filter cake formed on the filter medium.

   9. A method as claimed in claim 1 or 4, wherein on the

   termination of filtration the pressure radially outside

   the filter medium is so regulated that the liquid level

   in the filter medium is lowered to the region of its

   radially outermost surface.

   A method as claimed in claim 1, substantially as

   hereinbefore described with reference to the accompany-

   ing drawings.

   11. A filter centrifuge comprising a rotatable drum,

   a support mounted in the drum for supporting a filter

   medium centred on an axis of rotation of the drum, a

   liquid collecting chamber rotatable with and located

   adjacent the filter medium in the region of the radially

   outermost periphery of the drum and Lat Is:at one discharge

   apertureAfrom the collecting chamber located radially

   outwardly of the filter by an amount sufficient to

   479.225

   62, 45473 establish a suctional pressure in the filter medium,

   wherein during operation of the filter rugo-

   liquid in the collecting chamber serves to block

   entry of air thereto through each discharge aperture.

   12. A centrifuge as claimed in claim 11, comprising a

   plurality of discharge apertures from the collecting

   chamber each associated with a respective feed aperture

   into an annular chamber concentric with the axis of the

   centrifuge, the feed apertures being at the same or at

   a greater distance from the axis of the centrifuge than

   the discharge apertures, the annular chamber being provided

   with a discharge dam of which the discharge edge is at

   a smaller distance from the axis of the centrifuge

   than are the feed apertures.

   13. A centrifuge as claimed in claim 11, comprising a

   pivotally mounted discharge tube extending into an annul-

   ar chamber for removing liquid therefrom, said annular

   chamber being concentric with the axis of the centrifuge

   and communicating with the said collecting chamber

   through said discharge apertures, wherein the annular dis- chamber is provided with a discharge dam of which the

   charge edge is approximately at that distance from the

   axis which corresponds to the distance of the filter the medium from the axis of the centrifuge, and wherein

   discharge tube is pivotable within a range approximately

   -31- 479,225

   62, 4 5 4 3 extending from the radial distance of the filter medium

   to that of the feed apertures.

   14. A centrifuge as claimed in claim 12, wherein the

   discharge edge is defined by openings which are provided

   in an annular wall defining one side of the annular

   chamber which openings can be closed as desired, the

   openings being at different distances from the axis of

   the centrifuge and the radially innermost openings being

   situated approximately in the region of the radial

   distance of the filter medium from the axis.

   A centrifuge as claimed in anylof claims 12 to

   14, comprising a filling tube extending into the annular chamber for feeding liquid into said collecting

   chamber via the annular chamber prior to the operation

   of filtering.

   479.22

   62,454/73 Lf ISZ 16. A centrifuge as claimed in any/of claims l4 to comprising siphon tubes of substantially U-shaped cross-sec- tion which extend radially outwards from said discharge aper- tures, the ends of the said siphon tubes remote from those connected to the discharge apertures serving as a discharge edge, and are at a greater radial distance from the axis than the filter medium.

   17. A centrifuge as claimed in claim 16, comprising blocking devices located in the siphon tubes in the zones between the discharge apertures and the feed apertures.

   18. A centrifuge as claimed in anylof claims 12 to wherein the discharge dam is adjustable during operation by

   means of a system of transmission rods.

   19. A centrifuge as claimed ihAclaims 12 to 15, com- prising a centrifuge regulating device for adjusting the radial distance of the discharge dam from the axis in accor- dance with the acceleration.

   ne& 12 A centrifuge as claimed in any~of claims (k to comprising sealing devices connected to the discharge aper- tures, which sealing devices enable a throttling effect to take place.

   21. A centrifuge as claimed in claim 20, wherein the sealing devices are designed so that they can be operated in accordance with the acceleration. 33 479,225

   62, 454/73 22. A centrifuge as claimed in claim 20, wherein the

   sealing devices are arranged so that they can be operated

   during operation by means of a system of rods. CMo 23. A centrifuge as claimed in any~of claims 20 to 22,

   wherein the annular chamber is connected up to the outlet side

   of the sealing devices. 12 24. A centrifuge as claimed in any one of claims -i to

   23, wherein the internal space within said drum, the annular

   chamber and the sealing devices, are subjected to a higher

   gas pressure than the pressure prevailing in the collecting

   chamber.

   A centrifuge as claimed in any~of claims 20 to 24,

   wherein the sealing devices are non-return valves. On& 26. A centrifuge as claimed in any~of claims 11 to 19,

   wherein the collecting chamber is provided with a suction

   aperture which is in communication with an air removal suc-

   tion device.

   27. A centrifuge as claimed in claim 26, wherein the a air suction device is constructed as a jet pump fed from or from liquid discharged from an extraneous source of liquid

   the collecting chamber.

   28. A centrifuge as claimed in claim 26 or 27, wherein of the filter the suction aperture is situated in the zone which is medium and communicates, via a suction conduit with a u tion mounted on the drum and which communicates 34 479,.2

   62.454/n3 conduit provided on the housing, with an external air suction device.

   29. A centrifuge as claimed in claim 26, wherein the collecting chamber is surrounded by boundary walls integral with the drum and by a sealing wall integral with the housing, the latter wall connecting up with the boundary walls via rotary sealing devices and the suction aperture being located in the sealing wall.

   A centrifuge as claimed in claim 29, wherein at least one of the sealing walls is provided with a discharge outlet to which is connected the suction side of an external feed pump.

   31. A filter centrifuge substantially as described herein with reference to any one of the embodiments shown in the accompanying drawings.

   DATED THIS ninth day of June, 1976.

   KRAUSS-MAFFEI AKTIENGESELLSCHAFT

   Applicant.

   R.C. WRAY ASSOCIATES, Perth, Western Australia, Patent Attorneys for Applicant.

   479.225

   62,454R73

   479.225S

   62, 454/73

   Fig.3a

   Fig.3b

   Fig.3 c

   47 9,225

   62, 454/73

   Fig.3d M

   Fig.3e

   479.22

   62,45 4/73

   Fig.4

   Fig.6

   479,225

   62, 45 4m

   Fig.7

   Fig.8

   Fig.9

   479,225

   62, 454/173

   520

   Fig.ll

   630 634

   479,225

   62, 454,73

   Fig.12

   Fig.14