1 CENTRIFUGAL SCREEN APPARATUS FIELD OF THE INVENTION This invention relates to centrifugal screen apparatus. This invention has particular application to centrifugal screen apparatus for use in separating washed 5coal from water and fines, and for illustrative purposes the invention will be described with reference to this application. However we envisage that this invention may find use in other applications such as maintaining the integrity of centrifugally separated heterogenous materials having an air phase generally. BACKGROUND OF THE INVENTION The reference to any prior art in this specification is not, and should not be taken as, an acknowledgement or any form of suggestion that the referenced prior art forms part of the common general knowledge in Australia. 15Coal is mined, crushed and screened to produce a raw industrial coarse coal stream. The coarse coal is finished by washing to reduce clays and particles or fines. The water entrained coarse coal is substantially dewatered by continuous flow through a solids/particulates/water/air separating centrifugal screening apparatus including a rotating conical screen disposed in a housing. The screen is generally driven (by shaft passing through a wall of the housing) from its narrow inner end by a drive plate that doubles as a closure for the screen narrow inner end. The coarse coal/water stream is directed by an inlet assembly passing into the housing at the wider outer end of the screen and deposits the stream at the narrower inner end of the screen. The housing about the screen is divided into an 25underflow chamber generally corresponding to the outer conical surface of the screen and an overflow chamber accepting washed coarse coal passing out of the open wider end of the screen. The respective under and overflow chambers pass water/fines and washed coal to respective outlets. 30The industry regards residual water in the cleaned coarse coal as contamination, even if the coal is clean to standards. While methods of operation of centrifugal screens and pre- and post reduction of water content can be chosen to reduce drag through of excess moisture, these either adversely impact on throughput 2 and/or add to processing and freight costs. It is accordingly an object of preferred embodiments of the invention to minimise this phenomenon. The rotation of the screen imparts a centrifugal force on the material deposited on 5the inner screening surface. One would expect that a heterogenous mixture comprising a substantially continuous water matrix entraining fines and clays and encapsulating a discontinuous coarse coal phase will, when rotated by the screen, be separated when the continuous phase, fines and clays passes under centrifugal force through the screen to the underflow chamber while the 10discontinuous coarse coal phase is retained on the screen. The conical form means that a component of the centrifugal force applied to the coarse coal is along the axis in the direction of the outer annular edge, providing the impetus for discharge of the substantially clean and dewatered coarse coal through the outer annual end. 15 However, a vented arrangement has the effect, in the dynamic environment of the housing, of disrupting an ideal continuous water phase. Instead, air is entrained to form a complex and variable gas/liquid/solids heterogenous mixture. As a consequence not all of the liquid phase is retained as a continuous matrix to be 20accelerated through the screen. Gas/liquid mixtures are more influenced by pressure differentials induced by the dynamic environment and less affected by centrifugal force. Accordingly, water contamination may result from several influences, including surface retention on the coarse coal, and aerosols and other entrained materials passing out through the outer end of the screen. 25 The typical form of conical screen assemblies includes stringers that effectively turn the screen frame into a centrifugal fan tending to provide lower gas pressure within the screen and higher pressure outside the screen in the underflow chamber. This creates a pressure differential causing material to pass between 30the underflow and overflow chambers through a clearance annulus between the wall and the outer end of the screen to contaminate the coarse coal. In addition, air and entrained aerosol and fine material may pass through an outer portion of 3 the screen at the outer portion counter to the centrifugal passage of liquid water as a local effect. SUMMARY OF THE INVENTION 51n one aspect the present invention resides broadly in centrifugal screen apparatus including: a screen having an inner substantially conical screening surface truncated by and extending between inner and outer annular edges and mounted for driven rotation in a housing; 10 an inlet assembly passing into the housing and directing a material to be screened toward an end portion of the screen adjacent the inner annular edge; a wall dividing the housing substantially about the outer annular edge and defining an underflow chamber surrounding the rotating screen and an overflow chamber accepting material passing the outer annular edge; and 15 a plurality of vanes located on the screen in the underflow chamber and selected to impel material away from the wall. It has been found that the use of a plurality of vanes located on the screen in the underflow chamber and selected to impel material away from the wall overcomes 20a tendency for aerosols and other entrained fines and water being impelled by backflow through a screen outer portion and through any clearance gap between the wall and the outer annular end. The screen may be any known screen of the type having an inner substantially 25conical screening surface truncated by and extending between inner and outer annular edges and mounted for driven rotation in a housing vented to the atmosphere. Such screens are generally formed of a supporting structure of stainless or other steel and comprising at least two spaced ring frames and interconnected by a plurality of stringers. The supporting structure may be 30fabricated integrally with a screening surface comprising a plurality of wire elements. These may be longitudinal or a spiral wind about the conical axis. Alternatively the screening surface may be comprised of a screening surface insert 4 or panel segments secured to the supporting structure. Typically the screening surface is defined by a plurality of wedge wires. The supporting structure may be driven by any suitable means. For example, the 5open end formed by the inner end ring frame may be sealed by a drive plate adapted to be bolted to a flange on a drive shaft. The shaft and/or drive plate may be associated with bearing means that supports the screen assembly for rotation. The housing may be a conventional coarse coal screen assembly housing. The 10housing may be vented to the atmosphere. Venting to atmosphere may be by a vent assembly or be a consequence of an outlet from one or both of the overflow and underflow chambers being in communication with the atmosphere to the effect of venting one or both chambers. 15The housing is preferably configured whereby the screen assembly may be installed and withdrawn for servicing through a selectively closable end wall of the housing. For example, the housing may include a closure portion forming a wall of the overflow chamber and closing an access opening disposed in a plane parallel to the outer annular end of the screen. By this means, a screen extractor may 20pass through the access opening and axially into the screen inner space to attach to and support the screen assembly, which may then be released from the drive flange and withdrawn axially from the housing through the wall and the access opening. The closure may be hinged to the rest of the housing. The closure may support the inlet assembly whereby opening or removal of the closure withdraws 25the inlet assembly. The inlet assembly must pass necessarily through the open outer end of the screen assembly and so must form an effective conduit through the overflow chamber. For example, the inlet assembly may comprise a conduit secured 30through a top or side wall of the housing ands passing through the overflow chamber with an extension portion extending in to the region of the narrow end of the screen inner volume. Alternatively the inlet assembly may comprise a conduit 5 secured through a closure portion of the housing and passing through the overflow chamber and extending to the region of the narrow end of the screen. The wall is preferably a substantially vertical septum dividing the housing about 5the outer annular edge and effectively separating the underflow chamber surrounding the rotating screen from the overflow chamber accepting material passing the outer annular edge. The wall portion containing the opening for the screening assembly is preferably substantially coplanar with the outer annular edge. The screen assembly preferably has minimal run-out in its rotation so the 10clearance between the outer annular edge and the periphery of the opening in the wall may be kept to a minimum. In view of the potential for jamming by solids, there is preferably an unobstructed radial clearance between the outer annular edge and the periphery of the opening in the wall. 15The plurality of vanes located on the screen in the underflow chamber may take any suitable form. For example, the vanes may be arranged to form an axial impeller. To this end the individual vanes may be straight or curved to present a selected angle of attack. 20The vanes may be formed of a compatible metal and be welded to the screen assembly. For example the vanes may each extend between adjacent ring frames and we welded into position at the vane ends. The vanes may be welded to the back of the screening surface assembly. 25The vanes may scroll along the back of the screening surface assembly at constant span, thus tapering in concert with the screen assembly. Alternatively, the vanes may comprise a constant-chord section spanning out from the back of the screening surface assembly with the span progressively increasing along the chord to describe a cylindrical spool. 30 The vanes may be located adjacent the wall or at any other selected axial position along the screening assembly. Preferably the vanes are locates at a position on 6 the screening assembly selected from positions adjacent the wall to positions approximately mid way in axial extent from the wall to the inner annular end. BRIEF DESCRIPTION OF THE DRAWINGS 5The invention will be described with reference to the following non-limiting embodiment of the invention as illustrated in the drawings and wherein: Fig. 1 is vertical section containing the axis of apparatus in accordance with the present invention; 10 Fig. 2 is a side view of a screen for use in the apparatus of Fig. 1; and Fig. 3 is a rear perspective view of the screen of Fig. 2. In the figures there is provided a centrifugal screening apparatus including a mounting base 10 supporting a drive assembly 11 driving a shaft 12. The 15mounting base 10 and drive assembly 11 mutually support a screen housing 13 which is closed by a removable closure 14 and is divided by a dividing wall 15 into an underflow chamber 16 and an overflow chamber 17. The underflow chamber 16 passes water and fines to an underflow outlet 18 passing through the mounting 10. The overflow chamber 17 passes screened coal to a delivery outlet 19 20passing out of the housing 13. The shaft 12 has a drive plate 20 secured to its end, the drive plate forming an inner closure for a frusto-conical screen assembly 21. The screen assembly 21 comprises an inner frame ring and flange 22 bolted to the drive plate 20. The 25inner frame ring and flange 22 is axially spaced from intermediate ring frames 23 and an outer frame ring and flange 24. The respective ring and flanges 22, 24 and ring frames 23 are interconnected by circumferentially spaced stringers 25 to describe a generally part conical cage. The cage supports a wedge wire screening surface 26. The closure 14 and housing 13 mutually support an inlet 30assembly. The ring and flanges 22, 24, ring frames 23 and stringers 25 are all formed from steel flat bar and plate as appropriate.
7 The dividing wall 15 has a large circular opening in which the flange of the outer frame ring and flange 24 runs at a minimum clearance annulus indicated at 27. Welded between adjacent stringers 25 are a plurality of fluid-dynamically active 5steel vanes 30 formed from steel flat-bar. The vanes are cupped and presented at an angle of attack to provide for generation of air flow from the region of the underflow chamber 16 adjacent the outer frame ring and flange 24 to the underflow outlet 18. 1OAn inlet assembly 31 comprises a chute portion 32 receiving water and coarse coal from crushing and washing and an injector portion 33 feeding the water and coarse coal to the inner end of the screen assembly 21. In use, the screen assembly 21 is rotated by the drive assembly 11 and a stream 15of water and coal passes from the chute 32 through the injector portion 33 to impinge on the drive plate 20. Centrifugal force drives water and fines through the wedge wire screen surface 26 and into the underflow chamber 16 and thence by gravity to the underflow outlet 18. 20Meanwhile screened coarse coal is displaced from the inner end of the screen assembly 21 by the inclination and axial agitation (induced via out-of-balance motors) of the conical wedge wire screening surface 26 to the axis of the screening assembly 21. The inclination causes centrifugal force to resolve into a radial component and an axial component that is further aided via the axial force 25induced via the out-of-balance motors. The axial component causes the screened coarse coal to pass from the inner end of the screen assembly 21 to the open outer end of the screen assembly 21. In doing so the coal passes though the opening in the dividing wall 15 to pass into the overflow chamber 17 and thence under gravity to the delivery outlet 19. 30 The effect of volumes of water, fines and entrained air passing under centrifugal force into the underflow chamber 16 is to cause a slight pressurization of the space relative to the inside volume of the screen assembly 21 and the overflow 8 chamber 17. This has the effect of allowing water and fines to pass through at least the minimum clearance annulus 27 to contaminate the washes and screened coarse coal. In the present case, the dynamic effect of the vanes 30 is to reverse this flow in the region of at least the minimum clearance annulus 27 as well as to 5act as a slinger urging water and fines away from the minimum clearance annulus 27. It will of course be realised that while the above has been given by way of illustrative example of this invention, all such and other modifications and 10variations thereto as would be apparent to persons skilled in the art are deemed to fall within the broad scope and ambit of this invention as is set forth in the claims appended hereto.