AU704716B2 - Centrifuge - Google Patents

Description

WO 96/16744 PCT/AU95/00791 -1-

CENTRIFUGE

TECHNICAL FIELD The present invention relates to a centrifuge for continuously separating the components of a mixture according to particle mass and/or specific gravity.

BACKGROUND ART In a conventional centrifuge, separation of a heterogeneous feed material into fractions takes place in a generally frustoconical centrifuge bowl which is rotating about its axis. Particles in the feed mixture migrate radially outwards due to centrifugal settling as the material travels as a film up the inner surface of the bowl. The heavy components of the mixture are concentrated towards the outside of the film, so that the material can be separated into light and heavy fractions, which are separately discharged from the centrifuge.

In some types of centrifuge, a series of coaxial, frustoconical discs are positioned inside the centrifuge bowl. These discs provide a large surface over which the feed material is spread thinly, so that the separation can be achieved in a relatively compact centrifuge.

In U.S. Patent No. 4 976 678, a centrifuge is disclosed in which the heavy fraction is collected from the centrifuge bowl in a series of compartments about the circumference of the rotor.

U.S. Patent No. 5 188 583 discloses a centrifuge for batch operation for separating whole blood into plasma and red corpuscles, in which the corpuscles are collected in a series of peripheral compartments.

DISCLOSURE OF INVENTION There is disclosed herein a centrifuge for separating a continuous input mixed feed material into a plurality of fractions, including a rotor mounted for rotation about its longitudinal axis, the rotor including a plurality of compartments spaced about the axis, each compartment having a feed inlet and being formed to conduct a flow of the material through the compartment so as to allow centrifugal settling of the material to occur as the material flows through the compartment, means for rotating said rotor, feed means for the continuous distribution of the mixed feed material to said compartment feed inlets, and discharge means for discharge of the respective o separated fractions from the compartments, wherein each compartment changes in transverse cross-sectional shape or orientation between the inlet and a discharge 15 portion, having a first portion adjacent the inlet and a second portion adjacent the discharge portion, the second portion being greater in radial extent and less in circumferential extent than said first portion.

0 g oCo C -3- The compartments may be defined by radially inner, radially outer and a pair of opposite side walls, the inner and outer walls diverging and the side walls converging in the direction of the material flow through the compartment, so that the transverse cross-section of the compartment becomes more radially elongate towards the discharge end. Alternatively, and most desirably, thecompartments may be formed as a passage of elongate transverse cross-section, the passage being twisted so that its orientation changes from being circumferentially elongate near the entry to being radially elongate near the discharge.

r The invention also provides a method of separating a mixed feed material into fractions, comprising 20 continuously distributing the feed material into a plurality of separating compartments spaced about a rotational axis of a rotating centrifuge rotor, centrifugally settling the material while the material flows through the compartments and discharging the material as a plurality of fractions.

9 Preferably, the feed material is distributed initially into a circumferentially elongate form and is changed .9 9into a more radially elongate form while the 30 centrifugal settling is taking place.

A second form of the invention is characterised by the provision of filter means in a centrifuge, provides a centrifuge for separating a mixed feed material into light fluid fraction and a heavy fraction, including a RA( rotor mounted for rotation about its longitudinal WO 96/16744 PCT/AU95/00791 -4axis, the rotor being divided by filter means into an axial region and a peripheral region, means for rotating the rotor, feed means for supplying the mixed feed material to the peripheral region, light fluid discharge means for removing from the axial region the light fluid fraction which has passed through the filter, and heavy fraction discharge means for removing from the peripheral region the heavy fraction.

BRIEF DESCRIPTION OF THE DRAWINGS Further preferred embodiments will now be described with reference to the accompanying drawings, in which: Fig. 1 is an elevational cross-section of a first preferred embodiment; Fig. 2 is a plan view of the Fig. 1 centrifuge with one splitter assembly removed; Fig. 3 is an elevation of the compartment from radially outside; Fig. 4 is an elevation of the compartment from radially inside; Figs. 5 and 6 are schematic plan and elevation views showing an alternative compartment construction; Fig. 7 is an elevational cross-section of a centrifuge embodying the compartment shape of Figs. 5 and 6; Fig. 8 is a schematic plan view showing a variation on the compartment shape of Fig. Figs. 9 and 10 illustrate an arrangement for forming WO 96/16744 PCT/AU95/00791 the compartments in a moulded rotor block; and Fig. 11 is an elevational cross-section of a further embodiment in which the compartments include a filter.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The centrifuge illustrated in Figs. 1 and 2 comprises a rotor 10 having its main shaft 12 supported on a stationary structure 14 via bearings 16 to allow rotation of the rotor about its axis 18. The main shaft will be connected to drive means (not shown) for driving the rotation.

The rotor 10 has a central feed tube 20, a feed impellor 22 with radial feed distribution passages 24 leading to respective of a plurality of separating compartments 26. Four compartments are shown, although any suitable number may be provided. A splitter blade assembly 28 is mounted on top of each compartment to split the material discharged from the compartment into fractions.

The shape of each compartment varies as the material flows up through the compartment, from being circumferentially broad but radially shallow at the inlet end 30 to circumferentially narrow but radially elongate at the discharge end 32. Further details of the shape of the compartments will be discussed below with reference to Figs. 3 and 4.

The structure 14 supports a stationary launder assembly 34 which surrounds the rotor 10. The top of each annular launder 34a,b,c communicates with a respective passage 36a,b,c of the splitter blade assembly 28 so that the fractions split off by the blades 29a,b are deposited into respective launders.

WO 96/16744 PCT/AU95/00791 -6- The number of fractions, and therefore the number of launders, will vary according to the application for which the centrifuge is to be used. Figs. 1 and 2 show two splitter blades to split the compartment discharge into three fractions a heavy fraction furthest from the rotor axis to be collected in launder 34a, a light fraction closest to the rotor axis for launder 34c, and a medium fraction for launder 34b.

The launder arrangement supports a stationary feed pipe 38, which communicates with the rotating feed tube 20 for supplying the feed material to the rotor.

The feed material contains a mixture of materials of different specific gravities. Typically, the feed material will be a slurry. This is sucked into the rotor through the feed pipe 38 and feed tube 20 by the negative head generated by the feed impellor 22 as it rotates as part of the rotor. The feed material then flows outwardly through the radial feed passages 24 to respective separation compartments 26.

As the slurry travels upward through the compartment, the heavy particles in the slurry migrate towards the outer wall 40 of the compartment due to the rotation about axis 18. The lighter materials such as water or fine, light particles will thus be concentrated closer to the inner wall 42 of the compartment while the heavy particles will be concentrated toward the outside.

Depending on the properties of the materials to be separated, the material may stratify as it passes through the compartment. The strata can be divided by the appropriate positioning and number of splitter blades 29a, 29b positioned at the compartment exit so that, for example, the stratified discharge can be WO 96/16744 PCT/AU95/00791 -7split into streams comprising (in order, from the radially outermost) coarse and heavy particles, medium size and small heavy particles, fine slime particles suspended in fluid, and clear fluid. These streams then pass to separate launders or are diverted for recycling or further processing.

Referring to Figs. 3 and 4, it can be seen more clearly that the compartment is defined by radially outer wall 40 and radially inner wall 42 joined by side walls 44, 46 so as to form a duct extending from the compartment inlet 30 to the compartment outlet end 32. The outer wall is angled outwardly and upwardly so that it diverges from both the inner wall and the rotor axis 18. The bottom of the compartment is circumferentially wide, with the side walls 44 and 46 being convergent so that the width of the compartment decreases as its depth increases.

From Fig. 4, it can be seen that the inlet 30 to the compartment is a slot extending across substantially the whole width of the bottom of the compartment.

Immediately above the inlet slot 30, the transverse cross-section of the compartment is circumferentially elongated. The circumferential width of the compartment decreases as the material travels up the compartment, balancing the increasing radial depth so that the cross-sectional area of the compartment at any height is substantially equal. Therefore, constant flow through the compartment is encouraged, over substantially the whole cross-section of the compartment, rather than retaining a thin radial film along the entire length of a centrifuge bowl as in prior art centrifuges.

The compartment profile results in efficient separation and splitting of the components of the feed WO 96/16744 PCT/AU95/00791 -8slurry. At the inlet, the material is spread out to form a thin layer, so that the heavy particles have only a short distance to traverse to the outer wall.

As the material travels up the compartment, the compartment gradually narrows in the circumferential direction but becomes more radially elongate. The radially outward settling of the heavy particles is exaggerated by this transition in profile, the heavy particles accumulating along the outer wall and squeezing the lighter components towards the inner wall. At the top the discharge is presented to the splitter blades along an elongate radial slot, thus providing improved accuracy and efficiency in splitting the discharge into fractions and allowing a larger number of fractions to be selected if desired.

Also, the discharge slot need not be regular in shape, but can be narrower in the regions of the boundaries between the strata so that even more accurate splitting of the fractions can be achieved.

Figs. 5 and 6 are schematic views of an alternative, preferred compartment configuration. In this embodiment, each compartment is formed as a tube supported between the rotor top plate 50 and bottom plate 52, with an additional support plate 54. The direction of rotation of the rotor is shown by the large arrow in Fig. As in the embodiment of Figs. 1 and 2, the compartments are disposed generally parallel to the rotor axis. However, in Figs. 5 and 6 the compartments are tubes of elongate, preferably rectangular, cross-section, which have a 900 twist between the inlet end 56 at the top and the discharge end 58 at the bottom. The inlets are circumferentially elongated, with the twist resulting in a gradual increase in overall radial length and WO 96/16744 WO 9616744PCT/AU95/00791 -9decrease in circumferential width as the material flows through the tube. The discharge end is radially elongated.

From Fig. 5, it can be seen that the direction of the twist is such that the trailing end 60 (relative to the direction of rotation) of the inlet 56 becomes the radially outer end 62 of the discharge. The direction of this twist assists the settling of the heavy particles, as the inertia of the particles will cause a time lag in the particles reaching the rotational velocity of the centrifuge. The heavy particles in the feed mixture will thus be travelling more slowly than the compartment and will tend to accumulate at the trailing end of the compartment. As this trailing end at the inlet becomes the radially outer end of the discharge, and hence the end at which it is desired to concentrate the heavy particles, this circumferential hysteresis effect is used to assist the settling of the particles. Also, the tube wall which forms the radially inner wall 64 of the passage at the inlet becomes a trailing wall 66 of the passage. Part way through the compartment, where the total twist from the inlet is not yet 900, this trailing wall is angled radially outwardly and rearwards relative to the direction of rotation. Due to the hysteresis effect described above, the particles come into contact with this trailing wall. The angle of the trailing wall has a vector component which is radially outwards, and thus the particles coming into contact with the trailing wall are urged radially outwards by the trailing wall, in addition to the centrifugal settling of the particles which also acts radially outwards.

The broad side walls of the twisted tubes can be formed from strips of metal plate, clamped at each end while one end is twisted through 900. Two such WO 96/16744 PCT/AU95/00791 twisted strips can then be welded together with a small gap between to form the twisted rectangular tube. As the exaggerated settling effect achieved by the compartment profile transition increases with increased aspect ratio of the passage cross-section, it is preferred that the tube cross-section has an aspect ratio of at least 5:1, preferably greater than 10:1, so that there is a corresponding increase in radial length between the inlet and outlet of the compartment.

Fig. 7 shows a centrifuge which embodies the compartment configuration of Figs. 5 and 6, but in which the compartment passages 26 are formed between a rotor central portion 68 and a rotor shell 70 formed from cast metal. The top of the rotor has an annular top plate 50 surrounding an opening 72 in the top of the shell, for introduction of the feed material and overflow of excess feed material from the feed distribution chamber 74, as described in more detail below. The rotor bottom plate 76 closes off the bottom of the compartments and carries outlet spigots 78a,b,c,d for the compartments.

The rotor central portion 68 is mounted on an axial shaft 80 by bearings 82, and the outside of the shell carries a drive pulley 84 for rotation of the rotor by motor 86 via drive belt 88.

A stationary top launder arrangement 90 surrounds the top of the rotor, supporting feed tube 38. Feed material is supplied to the feed distribution chamber 74 in excess of the amount of material which can flow through the compartments 26. This ensures that the compartments are at all times full of material, ensuring effective operation of the centrifuge. The excess feed material escapes through the opening 72 to

I

WO 96/16744 PCT/AU95/00791 -11an annular overflow launder 92 and is discharged through outlet 94 to be recycled.

Surrounding the bottom of the rotor is a stationary fraction discharge launder, comprising a number of annular discharge launders 34a,b,c,d aligned with respective discharge spigots 78a,b,c,d of the compartments. Each discharge launder has an outlet 96a,b,c,d.

The compartments are formed as twisted rectangular passages similar to those described with reference to Figs. 5 and 6.

Feed material entering the compartment inlets undergoes settling due to a combination of centrifugal settling and the hysteresis effect described above, resulting in the heavies being concentrated towards the outer part of the compartments and being discharged via spigot 78a and launder 34a.

Successively lighter fractions are removed by spigots 78b, 78c and 78d.

Fig. 8 shows a variation of Fig. 5, in which the twist of the passages is assymetric so that the radially elongate discharge 58' is located rearwards, compared to the direction of rotation as shown by the large arrow, of the compartment inlet 56'.

Figs. 9 and 10 show an arrangement by which the rotor can be formed as a moulded block with the compartments formed as passages in the block. The mould is formed by a mould top plate 98, mould bottom plate 100 and a frustoconical outer member 102. The top and bottom plates have apertures for closely receiving rectangular strips 104 of rubber or the like. The top plate additionally has filling holes 106 for WO 96/16744 PCT/AU95/00791 -12introducing a settable material such as liquid monomer or molten polymer.

Each rubber strip 104 is held in a twisted form by clamps 108a, 108b which abut against the top and bottom plates. The polymer or monomer is poured into the filling holes 106 and allowed to solidify. After solidification, the strips are removed by releasing the clamps and pulling the strips out. The strips will reduce in cross-section as they stretch axially when being pulled out, facilitating their removal.

It is believed that centrifuges according to the invention will find application for dewatering slurry and for separating particles in a slurry according to specific gravity, and may also be useful for separating immiscible liquids such as oil and water.

The shape of the compartments may be modified from that shown to take into account the properties of the feed material, including the difficulty in separating the components. For example, it will generally be more difficult to separate fine slime particles from a suspension than to separate coarse particles.

Therefore, a centrifuge for a feed with coarse particles can have a less vertical compartment outer wall, allowing a higher throughput. In contrast, for slimes the compartment outside wall will be more vertical and/or the compartment can have a greater length, thus increasing the residence time in the separating compartment and holding the feed in a thin film for longer to achieve better separation.

In Fig. 11 each compartment is divided into a peripheral region 110 and axial region 112 by a filter 114. The feed enters the peripheral region of the compartment, near the outer wall, where the centrifugal force is highest. This assists in rapid WO 96/16744 PCT/AU95/00791 -13settling of the heavy components towards the outer wall, and helps to prevent entrainment of fine particles in the fluid which flows radially inwardly as the radial depth of the compartment expands. The particulates are retained along the outside wall where they separated into two fractions and are discharged from the top of the peripheral region. The clear fluid is forced radially inwardly through the filter under the influence of the head H of feed material in the feed passages 24 radially outside the zero pressure line.

The arrangement of Fig. 11 has advantages over conventional filtering arrangements as the particulate matter is being forced away from the filter by centrifugal settling, counteracting the tendency for particles entrained in the fluid flow to clog the filter.

The principles described above in relation to Fig. 11, according to one aspect of the invention, also have application to centrifuges of types other than the multiple compartment centrifuges described herein.

In an unillustrated embodiment of Figs. 1 and 2, one or both of the side walls of each compartment may be perforated, either to allow the centrifuge to act as a centrifugal filter in which the fluid is forced through the accumulated particulate matter and is expelled through the centrifuge walls or to allow fluid to be injected through the side walls to elutriate the particulate material and thus achieve improved separation of components. The injected fluid may be pulsed into the compartment.

Because the side walls are not normal to the radius of rotor, the force compacting the particles against the WO 96/16744 PCT/AU95/00791 -14perforated wall is not as high as in conventional centrifugal filters. Also, the rotation will cause the particles to accumulate more on the trailing side wall than on the leading side wall. Where the unit is to be used as a centrifugal filter, these factors allow the fluid readily to escape the compartments.

While particular embodiments of this invention have been described, it will be evident to those skilled in the art that the present invention may be embodied in other specific forms without departing from the essential characteristics thereof. The present embodiments and examples are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

Claims (20)

1. A centrifuge for separating a continuous input mixed feed material into a plurality of fractions, including a rotor mounted for rotation about its longitudinal axis, the rotor including a plurality of compartments spaced about the axis, each compartment having a feed inlet and being formed to conduct a flow of the material through the compartment so as to allow centrifugal settling of the material to occur as the material flows through the compartment, means for rotating said rotor, feed means for the continuous distribution of the mixed feed material to said 10 compartment feed inlets, and °•discharge means for discharge of the respective separated fractions from the compartments, S• wherein each compartment changes in transverse cross-sectional shape or orientation between the inlet and a discharge portion, having a first portion adjacent the inlet and a second portion adjacent the discharge portion, the second portion being greater in radial extent and less in circumferential extent than said first portion. p. A centrifuge according to claim 1 wherein the compartments extend generally axially between the inlet and the discharge means.

S,

3. A centrifuge according to claim 1 wherein the first portion is circumferentially elongate and the second portion is radially elongate.

4. A centrifuge according to claim 3 wherein the transverse cross-section of the compartment changes gradually between the first and second portions.

A centrifuge according to claim 4 wherein the compartment is defined by radially 4 inner, radially outer and a pair of opposite side walls, the inner and outer walls diverging and the side walls converging in the direction of the material flow through the compartment.

6. A centrifuge according to claim 3 wherein the compartment forms a passage which is elongate in transverse cross-section, said passage being twisted so that the elongation in transverse cross-section is oriented circumferentially at said first portion and oriented radially at said second portion.

7. A centrifuge according to claim 6 wherein the compartment is a tube which is elongate in transverse cross-section.

8. A centrifuge according to claim 7 wherein the tube is rectangular in transverse cross-section. o. oo*o 15

9. A centrifuge according to claim 8 wherein the tube has a twist of90 between the first and second portions. S0

10. A centrifuge according to claim 6 wherein the passage is formed in a moulded block. 0009

:11. A centrifuge according to claim 7 wherein the passage is formed in the moulded 0009 °0 block by moulding the block about a mould insert which is a twisted strip of flexible material and subsequently removing the strip from the block. 90

12. A centrifuge according to claim 11 wherein the strip is formed from elastomeric material. @°0609

13. A centrifuge according to claim 12 wherein the strip is released from the block by axial stretching to thin the strip.

14. A centrifuge according to claim 10 wherein the compartments are formed as passages in a single block.

A centrifuge according to claim 6 wherein the direction of the twist is such that a wall of the passage which forms a radially inner boundary of the passage at the first portion forms a trailing boundary wall, relative to the direction of rotation of the centrifuge, of the passage at the second portion.

16. A centrifuge according to claim 1 wherein the compartment has a trailing wall, relative to a direction of rotation of the centrifuge, the trailing wall being angled radially outwards and rearwards relative to the direction of rotation.

17. A centrifuge according to claim 1 wherein each compartment is divided by filter means into a peripheral region and an axial region, the feed inlet being in the peripheral region, heavy fraction discharge means for removing a heavy fraction from .00 •the peripheral region and light fluid discharge means for removing a light fluid °fraction from the axial region.

18. A method of separation of a mixed feed material into a plurality of fractions, comprising continuously distributing the feed material to feed inlets of a plurality of separating compartments spaced about a rotational axis of a rotating centrifuge rotor, distributing the feed material into a circumferentially elongate form, commencing centrifugal separation of the feed material in said circumferentially elongate form and 0 continuing centrifugal separation of the feed material while changing the material into 6o#o a form which is more radially elongate than circumferentially elongate. S00

19. A centrifuge substantially as hereinbefore described with reference to the 25 accompanying drawings. :.:09 0o0 9

•20. A method of separation of a mixed feed material into a plurality of fractions, the method being substantially as hereinbefore described with reference to the accompanying drawings. Dated this 15th day of February 1999 '0 PULP TECH PTY LIMITED Patent Attorneys for the Applicant HALFORD CO "V ro'<.