EP0258012A2

EP0258012A2 - A centrifugal solids-liquids separator

Info

Publication number: EP0258012A2
Application number: EP87307415A
Authority: EP
Inventors: Richard Henry Mozley
Original assignee: Richard Henry Mozley
Priority date: 1986-08-22
Filing date: 1987-08-21
Publication date: 1988-03-02
Also published as: EP0258012A3; GB8620436D0

Abstract

A centrifugal solids-liquids separator comprises an outer drum (46) having a cylindrical part (47) and a conically tapered part (49) joined thereto at the wider end and housing a conveyor (45) likewise comprising a cylindrical part (44) of approximately the same axial length as the cylindrical part (47) of the centrifugal drum (46) and a conically tapered part (45) matching the conically tapered part (49) of the centrifuge drum (46). The conveyor (45) has radially extending deflector blades (53) either in the form of separate elements or as a continuous scroll and both conveyor and centrifuge drum are supported in bearings (38,39) at the end through which a solids-liquids mixture is introduced so that both the solid and liquid components flow in co-current at least over the cylindrical part of the interspace (61) between the circularly cylindrical parts of the conveyor (45) and drum (46) respectively.

Description

This invention relates to a centrifugal solids-liquids separator.
There is a frequent need to separate liquid from fine solids on an industrial scale. The solids are often of such low value per unit volume (e.g. wet coal and wet quarry waste) that the cost of dewatering using conventional equipment is likely to exceed the value of the project; in such cases, the potentially useful product is simply dumped, which is almost invariably an undesirable practice.
Centrifugal separators are known for various purposes, one type, known as the Bird solid-bowl centrifuge, consists essentially of two rotating elements, the outer being a solid-wall (that is imperforate) rotatable bowl having a generally conical shape with an inwards flange or lip at its wider end, and the inner comprising a helical-screw conveyor revolving at a speed slightly lower than that of the bow. Raw feed slurry is delivered through a stationary feed pipe to the conveyor where, urged by centrifugal force, it is transferred to the revolving bowl. A circumferential classifying pool is formed and contained by the flange of the conical bow. Ports for oversize material are located closer to one end of the axis of rotation than ports for the overflow to form a beach line and effect drainage.
Centrifugal force deposits the oversize particles against the bowl wall from which they are conveyed by the helix. The overflow fractions flow around the helix to the liquid-discharge ports. Size of separation is controlled by feed rate and degree of centrifugal force, which can reach 1800 g. Typical applications are desliming to upgrade cement rock, sizing of abrasives, fractionating for reagent control and classification of pigments. Such centrifuges are massive, rotate at a high speed and must be carefully balanced (weights from 11/2 to 15 tonnes are typical). For this reason installation costs are high. Moreover the fluid velocity at any given volumetric flow rate has to be relatively high as the fluid has to flow round and round the scroll to reach the discharge ports. Such a high fluid velocity is disadvantageous for settling out fine mineral as it causes turbulence and radial mixing.
A second known type of centrifuge, known as the Birtley-Humboldt vibrating screen centrifuge, consists of a conical wire-screen basket open at its upper (wider) end. The basket is rotated about its vertical axis and simultaneously shaken vertically. Wet coal is delivered to the bottom of the basket. Under the influence of the combined rotating and vibrating movements the coal forms an evenly distributed layer of relatively thin and uniform depth passing up the inner surface of the screen basket. This acts as a partial filter allowing the water to escape radially through the mesh of the basket. The dried coal then passes up and over the lip of the basket and is collected for disposal. The extracted water passing through the wire-screen basket enters a water collector. The vertical vibrations impose certain design drawbacks and complications on this centrifuge as a whole, which has to be supported on flexible rubber mountings and is much heavier than the basket itself, in order to absorb the out-of-balance forces which would otherwise be transmitted to the supporting structure. Its mass is of the order of several tonnes, and it consumes some 20 kW in operation. The screen-wire basket suffers abrasion and must be replaced from time to time.
The technical problem which the present invention seeks to solve is that of providing a centrifugal separator the design of which is such that an effective separation of the solid and liquid components of a solids-liquids mixture can be achieved with a relatively low power consumption and at a relatively low speed, using apparatus the mass of which is substantially less than that of conventional centrifuge apparatus.
According to one aspect of the present invention a centrifugal solids-liquids separator comprising a rotatable imperforate hollow centrifuge drum at least a part of which is tapered and the axis of which coincides with the axis of rotation of the drum, a conveyor coaxially located within the centrifuge drum and itself rotatable in the same direction as the centrifuge drum at a speed above or below that of the centrifuge drum such that the differential speed of the two is small in relation to the absolute speed of rotation of the centrifuge drum whereby to convey material in the annular space between the centrifuge drum and the conveyor axially thereof, as a result of the differential speed of rotation, towards the narrow end of the tapered part of the centrifuge drum, and means for introducing a solids-liquid mixture to be separated into the said annular space, in which both the centrifuge drum and the conveyor are driven to rotate by drive means operable to transmit the driving force thereto at the same end, and the means for introducing the solids-liquid mixture into the separator includes a duct opening into the interior of the centrifuge drum at, or adjacent, the end thereof to which the drive is transmitted.
In an alternative aspect the invention provides a centrifugal solids-liquids separator, comprising a hollow imperforate rotatable centrifuge drum having a right circularly cylindrical first part the axis of which coincides with the axis of rotation of the drum, and a conically tapered second part coaxial with the first and joined thereto at its wider end, a conveyor coaxially located within the centrifuge drum and having a body with a right circularly cylindrical first part, a conically tapered second part, and a deflector projecting radially therefrom and operative, when the centrifuge drum and the conveyor are rotated in the same direction, but with a speed differential therebetween, to cause axial displacement of the solids in the interspace between the centrifuge drum and the conveyor towards the narrow end of the tapered part of the centrifuge drum, and means for introducing a solids-liquids mixture into the interspace between the centrifuge drum and the conveyor body at or adjacent the axial end of the said first parts thereof remote from the second parts thereof, the conveyor body having openings therein in the vicinity of the junction between the first and second parts thereof, so that, in operation of the separator, both the liquid and the solid components of the mixture flow axially in the same direction along the first part of the centrifuge drum.
The present invention also comprehends in a further alternative aspect a centrifugal solids-liquids separator comprising a hollow, rotatable, centrifuge drum having a right circularly cylindrical first part and a conically tapered second part joined to the first part at its wider end and coaxial therewith and with the axis of rotation of the centrifuge drum, a conveyor coaxially located within the centrifuge drum and having a body with a right circularly cylindrical first part and a conically tapered second part with a deflector projecting radially outwardly therefrom into the interspace between the conveyor body and the centrifuge drum, and means for introducing a solids-liquids mixture into the interspace between the centrifuge drum and the conveyor body, the said first part of the centrifuge drum having a radially inwardly directed annular peripheral lip at the end thereof remote from the second part extending radially inwardly beyond the rim of the conveyor body, and the conveyor body having openings therein in the vicinity of the junction between the first and second parts thereof whereby to allow liquid from the mixture, from which the solid component has been separated by centrifugal action in use of the separator, to pass radially inwardly into the interior of the conveyor body to be discharged therefrom over the said radially inwardly directed lip of the centrifuge drum.
Preferably the conveyor's rotational speed is within 15% of that of the centrifuge drum, which may lie within the range from 300 to 750 r.p.m. In a preferred embodiment of the invention the said inlet duct extends axially and passes through the drive transmission members of the centrifugal drum and of the conveyor. The mixture introduction means preferably includes means for imparting a circumferential acceleration to the mixture before it is delivered to the interspace between the rotatable centrifuge drum and the conveyor. In this way a rotational speed is imparted to the mixture before it meets the surface of the centrifuge drum.
The mixture introduction means may include ducts extending substantially radially and leading from a central collection chamber at the outlet of the axial ducts to the said annular interspace. These substantially radial ducts may, in practice, be substantially straight or curved in the direction of rotation of the centrifuge drum. A further increase in the circumferential speed of the mixture delivered to the centrifuge drum may be achieved by providing the above mentioned radial ducts with deflectors at their radially outermost or outlet ends, such deflectors being shaped so as to deflect the mixture circumferentially in the direction of rotation of the centrifuge drum.
In a preferred embodiment of the invention the conveyor comprises a cylindrical body having a conically tapered part with the same cone angle as that of the centrifuge drum so that the annular interspace has substantially the same radial dimension throughout its axial and circumferential extent, and a radially projecting deflector the major surfaces of which are inclined to a plane perpendicular to the axis. The cone angle (half angle) preferably lies between 2° and 6°.
In one embodiment of the invention the conveyor has a plurality of discrete deflector plates or blades having a circumferential extent not greater than half of the circumference of the conveyor.
In an alternative embodiment the radially projecting deflector is formed as a helically extending uninterrupted scroll blade. Such blade may extend from one end of the conveyor to the other. To increase the working life of the separator it is preferred that the deflector has at least one major face lined with a wear-resistant material. Such material may be one of several different types: it has been found that a resin-based material such as polyurethane provides an excellent wear-resistance, although other materials, including ceramics or silicone carbide, may alternatively be employed. Such a lining may also be applied to the inner surface of a centrifuge drum.
In embodiments having discrete conveyor blades or elements, the conveyor is preferably so composed that a particle conveyed axially from one end to the other of the conveyor does so by contact with at least six and preferably at least 10 of the elements.
Conventional centrifuge apparatus having an internal conveyor is constructed so that the outer centrifugal drum is driven at one axial end whilst the internal conveyor is driven at the other. By providing for both the internal conveyor and the centrifuge drum to be driven at the same end the structure can be made lighter and is easier to dismantle. Further, the conveyor may be borne at the end to which drive is transmitted on a tubular element defining the inlet passage, which is supported by a rotary bearing on a fixed frame of the separator. The tubular element defining the inlet passage thus serves both as a guide for the solids-liquids mixture being delivered to the separator and as a support element, simplifying the structure. This arrangement also allows the centrifuge drum to be supported at the end to which drive is transmitted by a rotary bearing carried by the said mixture inlet tube. At the opposite end from that to which drive is transmitted the conveyor may have an axial projection supported by a rotary bearing carried on a part of the fixed frame, and the centrifuge drum may also be borne by a rotary bearing carried by the said axial projection. Alternatively, however, the centrifuge drum and the conveyor may be supported only at one end, that to which drive is transmitted, and left unsupported at the other end. This is only possible, of course, in a vertical axis configuration, but is considered to have some advantages, particularly in combination with a resilient suspension capable of absorbing some of the vibrations generated in use due to any asymmetry of the load.
In a preferred embodiment of the invention the wider end of the tapered part of the centrifuge drum is joined to a right circularly cylindrical part and the conveyor body is likewise formed with a conically tapered part and a right circularly cylindrical part joined at the wider end thereof. In such an arrangement the inlet feed ducts may open into the annular interspace between the centrifuge drum and the conveyor body at a point axially spaced from this junction. This allows a co-current flow of the liquid and the solid component of the mixture thereby enabling finer particles to be separated since less turbulence is experienced in such a configuration than is the case with counter current flow devices. Two embodiments of the present invention will now be more particularly described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is an axial section of a centrifugal separator formed as a first embodiment of the invention, shown in schematic form; and
Figure 2 is a schematic axial section of a second embodiment of the invention.

A base 1 carries rigid supports for a motor 2, an annular launder 3, idle rollers 4 and a solids discharge chute 5, and carries bearings 10 and 11. The motor 2 has a stepped shaft 2a with belt or chain drive (or other means, such as a gearbox) for simultaneously driving two components at slightly different rotational speeds. These two components are a body 12 which internally has the form of a tapered cylinder, and a shaft 13 coaxial within the body. Various bearings 10,11 and others not individually numbered, along with the roller 4, keep the body 12 and shaft 13 rotating steadily. To avoid splashing to and danger from the drive chains, the drive gear may instead be mounted generally above the separator.
In use, the body 12 is arranged to rotate at 500 r.p.m. and the shaft 13 in the same sense at 440 r.p.m. The shaft 13 carries radial arms 14 each with a distal scraper 14a spirally angled so as to transport material towards the narrower end of the body 12, i.e. downwardly. Viewed in plan, there are two sets of these radial arms 14 spaced 180° apart circumferentially. In the drawing, one set is mostly omitted, for clarity. Each set comprises seven (only four shown) equally axially spaced arms, the sets being axially staggered so that each point on the inner body surface passes a scraper 14a of one or other set. The circumferential extent of each scraper is however only a few degrees, and thus a sufficiently mobile particle on the inner body surface could miss every scraper 14a. The scrapers are ceramic tipped and spaced from the surface by a gap larger than the diameter of the largest grain of mineral to be fed to the separator. This minimises wear on said surface.
The shaft 13 carries also a rigid disc 15 about midway axially in the body, to distribute material to be separated, as will be described. The disc 15 may have an apertured circumferential upstand (not shown), for closer equalisation of the rotational velocities of incoming material with the body.
The conicity of the body 12 is drawn exaggerated; the half-angle of the cone (taper) of the body 12 is 4°. The axial height of the body is 50 cm and its wider diameter 40 cm. At its wider end, it carries an inwards radial 1 cm flange 16 to retain a certain volume of material. Excess material overlows into the fixed launder 3 for collection or disposal. The launders have eyelets 3a so that the separator can be suspended vertically on ropes as an alternative to standing on the base 1. This saves having heavy foundations as well as the need for careful balancing.
In an example of its use, the separator is stood so as to receive the underflow stream 20 of a process cyclone 21 which is treating, perhaps, sand washings from a quarry. The stream 20 thus consists of partly dewatered sand, but is still too wet to be saleable. The separator is sufficiently portable to be placed where needed even if the stream 20 is known to be only a temporary product, arising for maybe only a few hours in that location. The motor drives the body 12 and shaft 13/scraper 14 at 500 and 440 r.p.m. as described. The stream 20 lands on the disc 15 (which is rotating at 440 r.p.m.), and is centrifugally hurled outwardly, with rotational momentum, onto the inner surface of the body 12 (which is rotating at 500 r.p.m.).
The combined effect of the centrifugal force on the body surface and the body taper is for solid particles in the stream 20 to become urged against the inner body surface while water in that stream spreads to form a radially inner layer which tends to travel to the wider end of the body 12, as shown by the arrow L of Figure 1. The action of the scrapers 14a, which reach any part of the body inner surface at the relatively low frequency of once per second, is to convey those solid particles downwards, as shown by the arrow S of Figure 1. The scrapers have no such downwards-driving effect on the water, however, which simply flows round each scraper 14a in the course of its upwards journey.
Eventually, therefore, substantially dry solids of saleable quality emerge through the chute 5, assisted gravitationally by the vertical axis, while reasonably clear water is collected in the launder 3. Fine solids which remain in suspension have a chance to settle out centrifugally in the quiescent zone formed by the flange 16, and when so settled will be conveyed downwards towards the chute 5 by the scrapers 14a. The whole journey from flange 16 to chute 5 takes little over 14 seconds. In the 'drive from above' version, the chute 5 may be modified to comprise means for killing the circumferential velocity of the emerging dry solids, whereby to permit their better discharge, also assisted by the choice of vertical axis.
In an alternative embodiment, feed is introduced down the side of a shaft, thus allowing wash water to irrigate settled solids and remove feed liquid.
Referring now to Figure 2, an alternative, vertical axis centrifugal separator is illustrated. The separator shown in Figure 2 comprises a support frame 22 having a transverse lower frame member or base 23 and upper inclined frame members 24,25 which support a funnel-shaped solids-liquids mixture inlet chute 26 having a generally cylindrical delivery tube 27.
The frame 22 supports a bracket 28 carrying an electric motor 29 by way of a pivoted connection 30. The motor 29 has an output shaft with two sets of drive sprockets 31, 32 each of which receives a respective pair of drive chains 33,34 passing over respective pairs of driven sprockets 35,36. Although a chain drive transmission is illustrated here, it will be appreciated that a flexible belt drive transmission may alternatively be employed and it is envisaged in other embodiments that a drive motor mounted directly in place of the driven sprockets 35,36, with an appropriate gearbox, may be employed.
The driven sprockets 35 are mounted on a tubular support 37 carried by a rolling element bearing 38 secured to a mounting 39 on an annular support 40 carried by the arms 24,25 of the support frame. This annular support 40 also carries a cylindrical casing 41 in which the funnel-shape inlet chute 26 is carried. The delivery tube 27 of the inlet chute 26 has a radial dimension less than the inside diameter of the tubular support 37 so that this latter can rotate out of contact with the delivery tube 27. The tubular support 37 carries a further tubular member 65 which carries a rolling element bearing 66 on which the pair of sprockets 36 is borne. At its lower end the tube 65 is connected to a disc 68 which constitutes the upper wall of a collection chamber 69 into which the solids-liquids mixture introduced into the funnel-shape chute 26 can be introduced upon passing through the delivery tube 27 and the tubular supports 37 and 38. The delivery chamber 41 is secured to a central shaft 42 borne at its lower end in rolling element bearings 58, 59 as will be described in more detail below.
Extending radially from the delivery chamber 69 are a plurality of radial ducts 43 which are connected at their radially outer ends to a right circularly cylindrical part 44 of a conveyor generally indicated 45 housed within a centrifuge drum generally indicated 46, which is carried on the rolling element bearing 39. The centrifuge drum 46 comprises a right circularly cylindrical part 47 joined at its upper end to a shallow conical cover 48 and at its lower end to a narrowly tapering conical part 49 which is open at its lower end 50. The conveyor 45 likewise has a narrowly tapering conical part 51 the cone angle of which matches that of the tapering part 49 of the centrifuge drum 46 and is joined at its wider end to the right circularly cylindrical part 44 of the conveyor at an axial point corresponding to the junction between the tapering part 49 of the centrifuge drum and the right circularly cylindrical part 47 thereof.
The circularly cylindrical part 44 of the conveyor 45 has openings 52 in register with the radially outer ends of the radial ducts 43 leading from the delivery chamber 41 to allow the liquids-solids mixture introduced into the chute 26 to be delivered into the annular interspace between the conveyor body 45 and the centrifuge drum 46.
In use of the separator of the present invention, as in the embodiment of Figure 1, the centrifuge drum 46 is rotated by the sprockets 36 driven by the chains 34 from the drive sprockets 32 on the output shaft of the motor 29. The speed of rotation of the centrifuge drum is slightly different from that of the conveyor 45, which is driven by the rotation of the sprockets 35 in turn driven by the chains 33 from the drive sprockets 31 also on the output shaft of the motor 29. Slight differences in the diameters of the sprockets 31,32 and/or the sprockets 35,36 cause the speed difference between the conveyor 45 and the drum 46. In this context, it has been found preferable that the speed of rotation of the conveyor 45 be greater than that of the drum 46, although it is possible that the drum may be driven at a greater speed and the conveyor at a slightly slower speed.
Around the outside of the conveyor 45 there are arranged a plurality of inclined scraper blades 53 arranged in a generally helical array. The surface of the scraper blades 53 facing the narrow end of the conical part 51 of the conveyor 45 is coated with a wear-resistant polyurethane layer bonded thereto, and likewise the interior face of the centrifuge drum 46 is coated with a similar polyurethane wear-resistant layer. In alternative embodiments, not illustrated, the discrete scraper blades 53 may be replaced by a continuous helical scroll extending over the whole of the circularly cylindrical part 44 and the conically tapered part 51 of a conveyor 45.
Adjacent the junction between the circularly cylindrical part 44 and the tapered part 51 the conveyor body 45 has a plurality of apertures 55 the purpose of which will be described in more detail below. At its axially uppermost end the cylindrical part 47 of the centrifuge drum 46 has an inwardly directed lip 56 which extends radially inwardly of the rim 57 of the cylindrical part 44 of the conveyor 45.
At its lower end the conveyor 45 has a plurality of radial support struts 57 secured to the central shaft 42. The shaft 42 has two rolling element bearings 58,59 mounted thereon, the latter of which is secured to the transverse support 23 of the frame 22, whilst the former is attached to radial struts 60 connected to the lower rim 50 of the centrifuge drum 46.
In use of the separator described above the motor 29 drives the sprockets 31,32 which, via the chains 33,34 drive the sprockets 35,36 at a differential speed of rotation. As mentioned above it has been found convenient for the speed of rotation of the conveyor 45, driven by the sprockets 35, to be greater than that of the centrifugal drum 46 so that the scraper blades 53 "lead" the drum. A mixture of liquids and solids to be separated is introduced through the funnel shape chute 26 and passes through the delivery tube 27 into the axial passage defined by the tubular supports 27,38, entering from these into the delivery chamber 41. From this chamber the mixture is displaced radially outwardly by the centrifugal force along the radial arms 43, passing through the openings 62 in the cylindrical part 44 of the conveyor body 45 to enter the annular interspace 61 between the conveyor and the drum. In doing so, the mixture is given a circumferential component of motion which, if desired, can be increased by the provision of deflector guides, illustrated in broken outline and identified with the reference numeral 62, at the outlet ends of the radial ducts 43. The direction of the deflection imparted by the guides 62 will, of course, depend not on the direction of relative rotation, which is always the same, but on the direction of absolute rotation which depends on whether the conveyor 45 or the drum 46 is driven fastest. The mixture entering the annular interspace 62 forms a classifying pool as in the embodiment of Figure 1 which spreads along the inner surface of the cylindrical part 47 of the drum 46. As mixture continues to arrive into the interspace 62 the "depth" of the classifying pool will increase (radially inwardly) until it entirely fills the annular interspace 61 and extends axially into the conically tapered part of the interspace 61 defined by the parts 49 and 51 of the drum 46 and conveyor 45 respectively. The denser, solid particles in the mixture are driven by the centrifugal force radially outwardly to line the inner surface of the centrifugal drum 46 whilst the less dense liquid component occupies the radially inner part of the annular space 61 adjacent the outer face of the conveyor body 45. The liquid can thus pass through the openings 55 into the interior of the conveyor body 45 and, because the inclination of the conically tapered portion 51 of the conveyor 45 is effectively "uphill" the liquid flows axially towards the upper end of the cylindrical part 44 of the conveyor 45 where it flows over the radially inwardly projecting lip 56 of the centrifuge drum 46 and escapes radially into a launder 63 from where it flows out through a discharge port (not illustrated) as in the embodiment of Figure 1.
The differential rotation between the drum 46 and conveyor 45 causes the blades 53 to scrape the inner surface of the drum 46 and urge the solid components pressed against the inner surface of the centrifuge drum 46 by the centrifugal force axially towards the narrow end 50 of the drum 46 from which they are discharged. During the passage of the solids component from the junction line between the cylindrical part 47 and the conically tapered part 49 of the drum 46 the centrifugal separating action will continue with the denser solid particles being urged radially outwardly allowing the liquid component to "rise" (radially inwardly) to the free surface from which it can drain "down" (radially outwardly) towards the junction between the conically tapered part 49 and the cylindrical part 47 of the centrifuge drum 46 to join separated liquid passing through the apertures 55 into the interior of the conveyor 45.
It will be appreciated that with this configuration the solid material and liquid material both flow axially together from the outlets 62 from the radial ducts 43 along the cylindrical part of the annular interspace 61 defined by the cylindrical part 44 of the conveyor 45 and the cylindrical part 47 of the centrifuge drum 46. This co-current flow is particularly effective in limiting the turbulence within the mixture making it possible for finer particles to be separated therefrom before the liquid is transferred through the port 55 into the interior of the conveyor. This, in turn, makes it possible for the separator to be operated at the slower speeds indicated above, and typically in the region of 700 r.p.m. (with a differential speed in the region of 40-50 revolutions per minute) which is very much less than the high speed centrifuges of the prior art. The adoption of a configuration allowing lower speeds to be used in turn makes it possible for the structure to be made lighter in weight using pressed steel components in place of cast and milled components which consequently reduces the power requirement for driving the separator.

Claims

1. A centrifugal solids-liquids separator comprising a rotatable imperforate hollow centrifuge drum (12; 46) at least a part (49) of which is tapered and the axis of which coincides with the axis of rotation of the drum, a conveyor (13, 14; 45) coaxially located within the centrifuge drum (12; 46) and itself rotatable in the same direction as the centrifuge drum (12; 46) at a speed above or below that of the centrifuge drum (12; 46) such that the differential speed of the two is small in relation to the absolute speed of rotation of the centrifuge drum (12; 46) whereby to convey material in the annular space (61) between the centrifuge drum (12; 46) and the conveyor (13, 14; 45) axially thereof, as a result of the differential speed of rotation, towards the narrow end (50) of the tapered part (49) of the centrifuge drum (12; 46), and means (15, 21; 26, 27, 46, 62, 65, 69) for introducing a solids-liquids mixture to be separated into the said annular space (61), characterised in that there are provided drive means (29, 32, 33, 34, 35, 36) for driving both the centrifuge drum (12; 46) and the conveyor (13, 14; 45) to rotate, the said drive means (29, 32, 33, 34, 35, 36) being operable to transmit the driving force to the said centrifuge drum (12; 46) and the said conveyor (13, 14; 45) at the same end thereof, and in that the means (15, 21; 26, 27, 46, 62, 65, 69) for introducing the solids-liquid mixture into the separator includes a duct (43, 65) opening into the interior of the centrifuge drum (12; 46) at, or adjacent, the end thereof to which the drive is transmitted.

2. A centrifugal solids-liquids separator according to Claim 1, characterised in that the said inlet duct (65) extends axially and passes through drive transmission members (35, 36) of the centrifuge drum (46) and of the conveyor (45).

3. A centrifugal solids-liquids separator according to Claim 1 or Claim 2, characterised in that the mixture introduction means (15, 21; 26, 27, 46, 62, 65, 69) includes means (15; 43) for imparting a circumferential acceleration to the mixture before it is delivered to the interspace (61) between the rotatable centrifuge drum (12; 46) and the conveyor (13, 14; 45).

4. A centrifugal solids-liquids separator according to any of Claims 1 to 3, characterised in that the mixture introduction means (26, 27, 46, 62, 65, 69) includes at least one radially extending duct (43) leading from a central collection chamber (69) at the outlet of the axial duct (65) to the annular interspace (61) between the centrifuge drum (46) and the conveyor (45).

5. A centrifugal solids-liquids separator according to Claim 4, characterised in that the, or each, radial duct (43) has a deflector (64) at its radially outermost or outlet end, shaped to deflect the mixture circumferentially in the direction of relative rotation of the centrifuge drum (46) with respect to the conveyor (45).

6. A centrifugal solids-liquids separator according to any preceding Claim, characterised in that the conveyor (45) comprises a cylindrical body (44) having a conically tapered part (57) with the same cone angle as the rotatable centrifugal drum (46) so that the annular interspace (61) between them has substantially the same radial dimension throughout its axial length, and a radially projecting deflector (53) the major surfaces of which are inclined to a plane perpendicular to the axis of the conveyor (45).

7. A centrifugal solids-liquids separator according to Claim 6, characterised in that the conveyor (45) includes a plurality of discrete deflector blades (53).

8. A centrifugal solids-liquids separator according to Claim 7, characterised in that the deflector blades (53) have a circumferential extent not greater than one half of the circumference of the conveyor (45).

9. A centrifugal solids-liquids separator according to Claim 6, characterised in that the radially projecting deflector is a helically extending uninterrupted scroll blade passing substantially over the whole of the axial length of the conveyor (45).

10. A centrifugal solids-liquids separator according to any of Claims 6 to 9, characterised in that the deflector (53) has at least one major face coated or lined with a wear-resistant resin material such as polyurethane.

11. A centrifugal solids-liquids separator according to any of Claims 6 to 10, characterised in that the internal surface of the centrifuge drum (46) is coated or lined with a wear-resistant resin material such as polyurethane.

12. A centrifugal solids-liquids separator according to any preceding Claim, characterised in that the conveyor (45) is borne at the end to which drive is transmitted on a tubular element (65) defining the said inlet passage, which element is supported by a rotary bearing (68) on a fixed frame of the separator.

13. A centrifugal solids-liquids separator according to Claim 12, characterised in that the rotatable centrifugal drum (46) is supported at the end thereof to which drive is transmitted by a rotary bearing (66) carried by the said tubular element (65) constituting the mixture inlet passage.

14. A centrifugal solids-liquids separator according to Claim 12 or Claim 13, characterised in that the conveyor (45) has an axial projection (42) at the end thereof remote from that to which drive is transmitted, supported by a rotary bearing (59) carried on a part (23) of the fixed frame (22) of the separator, and in that the centrifugal drum (46) is also borne at the end (50) remote from that to which drive is transmitted by a rotary bearing (58) carried by the said axial projection (42a).

15. A centrifugal solids-liquids separator according to Claim 12 or Claim 13, characterised in that the conveyor (45) and the centrifugal drum (46) are unsupported at the end remote from that to which drive is transmitted.

16. A centrifugal solids-liquids separator according to any of Claims 6 to 15, characterised in that the wide end of the tapered part (49) of the centrifugal drum (46) is joined to a right circular cylindrical part (47) and the wide end of the tapered part (51) of the conveyor (45) is joined to a right circular cylindrical part (44) having an axial length substantially the same as the right circular cylindrical part (47) of the centrifugal drum (45), and the means (26, 27, 41, 43, 62, 65, 69) for supplying solids-liquids mixture to the annular interspace (61) between the conveyor (45) and the centrifugal drum (46) opens into this interspace (61) at a point axially spaced from the 3unction of the tapered parts (49, 51) with the right circular cylindrical parts (44, 47).

17. A centrifugal solids-liquids separator according to Claim 16, characterised in that the right circular cylindrical part (47) of the centrifugal body (46) and/or that (44) of the conveyor body (45), has an inwardly directed radial lip or flange (56, 57) at the axial end thereof remote from the tapered part (49, 51), acting to form a barrier to axial flow of liquid upon separation from the solids component of the mixture, and in that the conveyor (45) is provided with openings (55) in the body thereof in the vicinity of the junction between the tapered part (51) and the right circularly cylindrical part (44) thereof to allow liquid separated from the solids component of the mixture to enter the interior of the conveyor body (45).

18. A centrifugal solids-liquids separator according to Claim 17, characterised in that the axial separation of the exit end of the radial mixture feed ducts (43) and the said openings (55) in the body of the conveyor is such as to cause co-current flow of the solids and the liquids in the mixture axially along the right circularly cylindrical part of the interspace (61) between the centrifuge drum (46) and the conveyor body (45).

19. A centrifugal solids-liquids separator, comprising a hollow imperforate rotatable centrifuge drum (46) having a right circularly cylindrical first part (47) the axis of which coincides with the axis of rotation of the drum (46), and a conically tapered second part (49) coaxial with the first and joined thereto at its wider end, a conveyor (45) coaxially located within the centrifuge drum (46) and having a body with a right circularly cylindrical first part (44), a conically tapered second part (51), and a deflector (53) projecting radially therefrom and operative, when the centrifuge drum (46) and the conveyor (45) are rotated in the same direction, but with a speed differential therebetween, to cause axial displacement of the solids in the interspace (61) between the centrifuge drum (46) and the conveyor (45) towards the narrow end (50) of the tapered part (49) of the centrifuge drum (46), characterised in that there are provided means (26, 27, 43, 62, 69) for introducing a solids-liquids mixture into the interspace (61) between the centrifuge drum (46) and the conveyor body (45) at or adjacent the axial end of the said first parts (44, 47) thereof remote from the second parts (49, 51) thereof, and in that the conveyor body (45) has openings (55) therein in the vicinity of the junction between the first and second parts (44, 51) thereof, so that, in operation of the separator, both the liquid and the solid components of the mixture flow axially in the same direction along the first part (47) of the centrifuge drum (46).

20. A centrifugal solids-liquids separator according to Claim 19, characterised in that the cone angle of the said second part (51) of the conveyor (45) is substantially the same as that of the second part (49) of the centrifuge drum (46) such that the interspace (61) between the conveyor (45) and the drum (46) has a substantially constant radial dimension throughout its circumferential and axial extent.

21. A centrifugal solids-liquids separator comprising a hollow, rotatable, centrifuge drum (46) having a right circularly cylindrical first part (47) and a conically tapered second part (49) joined to the first part at its wider end and coaxial therewith and with the axis of rotation of the centrifuge drum (46), a conveyor (45) coaxially located within the centrifuge drum (46) and having a body with a right circularly cylindrical first part (44) and a conically tapered second part (51) with a deflector (53) projecting radially outwardly therefrom into the interspace (61) between the conveyor body (45) and the centrifuge drum (46), and means (26, 27, 41, 43, 62, 65, 69) for introducing a solids-liquids mixture into the interspace (61) between the centrifuge drum (46) and the conveyor body (45), characterised in that the said first part (47) of the centrifuge drum (46) has a radially inwardly directed annular peripheral lip (56) at the end thereof remote from the second part (49) extending radially inwardly beyond the rim (67) of the conveyor body (45) in that the conveyor body (45) has openings (55) therein in the vicinity of the junction between the first and second parts (44, 51) thereof whereby to allow liquid from the mixture, from which the solid component has been separated by centrifugal action in use of the separator, to pass radially inwardly into the interior of the conveyor body (45) to be discharged therefrom over the said radially inwardly directed lip (56) of the centrifuge drum (46)