CENTRIFUGAL SCREEN APPARATUS This invention relates to centrifugal screen apparatus This invention has particular but not exclusive application to centrifugal wire screen apparatus for mineral processing, and for illustrative purposes reference will be made to such application. However, it is to be understood that this invention could be used in other applications, such as perforated centrifugal screens generally. PRIOR ART

Centrifugal screens are widely used in the mineral processing art. The traditional construction for a coal centrifuge basket (used to dry coal) has been to fabricate rolled wedge wire segments into a conical shape, supported by and welded to a series of hoops and stringers. The principal wear component is the rotating, commonly frusto-conical wedge wire basket. The basket has a wedge wire screening surface where the wires run in axial planes to provide for product shear in use. The basket is formed from wedge wire screen stock. To account for the fact that the wedge wires are parallel while the conical form is divergent, the screening surface is defined an assembly of trapezoidal sections cut from flat wedge wire screen stock and pressed to form a segment of a conical surface. Wedge wire screen stock generally comprises wedge wires interconnected by spaced cross bars welded to the back of the wires at the intersections. Alternatively the stock may be produced with no cross bars but a continuous MlG weld or back-weld in lieu of the cross bars. The cut sections have the wedge wires running between the prospective ends of the conical

structure with the cross bars or back welds in transverse orientation. The sections are then welded together at the points of contact at the back, generally at the butt join of the cross members or back welds and contact lines between corresponding tapered cuts on the wedge wires. A drive end terminal flange is generally welded to the narrow end of the conical form of the wedge wire screening surface, and a discharge end terminal flange is provided at the other end. In order to stiffen the structure against torque loading in use, there are generally provided welded-in stringers which stretch between the end flanges, and intersect steel hoops which conjointly define a support for the screening surface.

A drive plate is generally secured to the drive end flange within the housing whereby the basket may be rotated. The open end of the basket provides an inlet to admit material to be screened to the inside of the basket. The housing has an open face opposite the drive flange end to allow installation of the basket, the open face being selectively closed by a door that generally includes an inlet conduit adapted to extend into the basket to deposit material to be screened at the narrow end. The housing may have one or more drain outlets to allow egress of material passing through the screening surface from the housing. The housing at the open end of the basket may be provided with an outlet for material retained on the screen and migrating along the taper of the screening surface.

Maintenance involves replacement or refurbishment of the basket. The basket is a heavy item. In particular, the supporting frame is heavy to account for the significant centrifugal and driving loads, exacerbated by the anisotropy of

the material feed. Large baskets may weigh in excess of 300 kg. Maintenance accordingly requires the use of lifting gear. Customer research has indicated a need for a safer and more user friendly centrifuge basket change over during shut-downs. This monocoque design has a number of other undesirable attributes.

One alternative construction proposed is to manufacture the screening surface as a basket liner separable from its supporting basket cage of hoops and stringers. In this construction, the basket liner comprising the shaped and welded up screening surface is provided with welded lugs that enable the basket liner to be bolted to the basket cage. While this construction enables the worn out liner to be separated from the reusable cage, it still requires that the assembly be changed out as a heavy unit.

It has been envisaged to use a basket formed on multiple screening panel segments adapted to be resiliency mounted to a driven cage. The idea was to enable change-out of the panel elements while the cage remains in the machine, by unclipping the panel segments. The resilient mounting was done at close tolerances to prevent erosion at the segment abutments. However, it has been found that fines blocking reduces the need for close tolerance, and that the complexity of the clip in arrangement can be dispensed with. DESCRIPTION OF INVENTION

This invention in one aspect resides broadly in a centrifugal screen of the type including a screen support mounted for driven rotation within a housing and including axially spaced hoops interconnected to form a cage having an inner driven end and an outer end adapted to be selectably closed by a housing

closure, and a screen including a plurality of screen surface modules supported in the screen support between the inner and outer ends and forming a screening surface in assembly, characterised in that the hoops and modules have complementary lugs and recesses engaging axially and locating the modules circumferentially, each module having an inner edge adapted to locate to the inner end against radial displacement and an outer edge having securing means adapted to be secured to the outer end.

The screen surface modules may present a surface of a shape selected from substantially cylindrical, substantially polygonal, and substantially frusto- conical, and is rotatable about its major axis. The screen support hoops are preferably selected to conform generally to the shape of rotation of the screen surface modules in assembly. The invention will be described hereinafter with reference to conical screens.

The drive for the centrifugal screen may be via a drive plate that also serves to substantially close off the inner end of the screen assembly.

The hoops may be of flat bar or cast with flats perpendicular to the rotational axis, rather than as barrel hoops. This provides form stiffness to the cage to remove the requirement for the screening surface modules to stiffen the structure in assembly, as required by the integral baskets of the prior art. The hoops are generally interconnected by stringers lying in axial planes and disposed in the surface of rotation of the spaced hoops in assembly. By this means a rigid driven support cage is formed. The cage may be structurally stiff to be driven without relying particularly on the modules in assembly providing necessary structural stiffness.

The complementary lugs and recesses engaging axially and locating the modules circumferentially may comprise lugs on the modules and recesses in the hoops or vice versa. The modules and hoops may be provided with alternating lugs and recesses, and may be configured whereby particular modules may only be fitted to particular circumferential positions in the support cage. However, it is preferred that the modules are identical as to fitment position in the support cage.

The lugs may include spaced bars extending substantially from the inner to the outer edges. The spaced bars preferably include one disposed substantially adjacent each side edge of the module. This provides for stiffening of the edges. The bars may be adapted to engage simple recesses machined or otherwise formed in the inner edge of the hoops. The bars may be round bar in order that they may seat in a self centering manner in complementary U-shaped cut-outs in the hoop inner edges. The screen surface module inner edge adaptation to locate to the inner end against radial displacement may take any suitable form. For example the inner edge may seat to the preferred drive plate which may include a simple groove or channel adapted to receive the module inner edge or a channel engaging ridge or flange formed on the module inner edge. In addition or in the alternative, the inner end and inner edge may include complementary engagement means such as pin and socket or tab and slot inter-engagement means. The inner end may include a fabricated drive plate having an outer flange portion adapted to bolt up to an inner end hoop of the support cage and a drive centre plate welded outward of the outer flange

portion to provide a circumferential step against which the conical face of the module edge may bear. Alternatively or additionally, one or both of the outer flange portion and drive centre plate may be provided with circumferentially spaced slots or lugs adapted to engage corresponding locating tabs or recesses provided on the module inner edges. The drive plate may be domed into the drum form of the screening apparatus for dynamic control of material flows from the central material delivery pipe to the drive plate and thence to the screening surface.

In use, the ingress of slurries to be screened results in deposition on a drive plate secured to the inner end and rapid acceleration out to the inner portions of the screen module screening surfaces. The inner end portion of the cage may be adapted to support wear resistant inner edge portions of the screen module portions. For example, the cage may include an extension portion having an annular bar providing a shoulder to support the screening surface portion of the modules. The extension portion allows a sleeve portion of wear resistant material forming the inner edge of the module to extend a selected distance to engagement with the drive plate. The extension portion may be particularly braced and webbed to stay dimensionally stable under driving and load-up forces. The module outer edges have securing means adapted to be secured to the outer end. The outer end is usually defined by an outermost hoop, and this may be engaged by any suitable means. For example, the module outer edge may be closed by an outward depending flange adapted to overlay and be secured to the outer terminal hoop or other portion of the cage. The securing in

this embodiment may be by bolts. In one embodiment, fixings are reduced in number by adjacent outward depending flanges sharing a fixing at their abutments, each having one or more additional fixings between the flange ends. Preferably, the fixings are countersunk machine screws that both take the fixing out of the product exit path and also centre and align the modules about the notional centres. Where the fixings are shared, countersunk fixings assist in distributing the modules correctly to avoid undue point wear on particular modules.

In the alternative, the modules may be plain-edges and the retention may be by a retaining ring adapted to bolt to the outer terminal hoop and trap the outer edges of the modules there under. The retaining ring may include an annular bead, lip or flange adapted to trap the modules radially at the screening surface outer edge portion between itself and the outer terminal hoop inside edge. The extension of the retaining ring and annular bead, lip or flange into the product flow path may effect an interference acting to retard the product moving through, and may be selectable as to size to control this effect.

Prior art coarse coal and other centrifugal screens are generally formed of steel. The screen support of the present invention may be formed of an abrasion resistant material whereby said screen support is replaceable on a longer cycle than the screen surface modules. For example, the screen support may be formed of stainless steel or a composite material or may comprise a substrate covered with a wear resistant polymer such as polyurethane. The screen surface may be of a different material whereby the screen segments are

supported and secured by the basket cage such that the traditional problem of stainless steel failure, through work hardening, is overcome.

The drive end plate may be adapted to form a removable intermediate- wear component that may be replaced on a longer cycle than the screen surface member. The drive end plate may be adapted to accept the rotational driving force by any suitable means. For example the drive end plate may be adapted to be bolted through to the drive flange of a motor and gearbox assembly mounted coaxially through the end of the housing.

The module edges may be configured to resist the ingress of fines into the panel joins, and by doing so avoid or reduce abrasive wear of the profiles. For example, there may be a resilient insert entrapped between the respective panel edges, or the panel edges may be configured to cooperate to form an effective labyrinth. However, it has been noticed that particulate infill or blocking tends to reduce turbulent wear at the module joins. The number of panels may be selected having regard to the weight of the individual panels, with it being preferred to limit the individual panel weight to about 20kg and preferably down to a one-man safety limit of 16kg.

The modules are preferably formed from a wedge wire panel stock in the conventional manner of cutting substantially trapezoidal panel forms and curving to a substantially conical surface by pressing or rolling. Alternatively the panels may be made flexible transverse the wire direction. Thereafter the preferred round bar or other lugs are welded on, or the recesses cut in the wedge wire stock cross members, as required.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that this invention may be more readily understood and put into practical effect, reference will now be made to the accompanying drawings which illustrate a preferred embodiment of the invention and wherein:

FIG. 1 is an exploded inner-end perspective view of screen apparatus in accordance with the present invention;

FIG. 2 is a partially disassembled outer-end perspective view of the cage assembly of FIG 1;

FIG. 3 is the outer-end orthogonal view of the cage assembly of FIG.1 ;

FlG. 4 is a detail section through the module mounting portions of the cage assembly of FIG. 1 ; and

FIG. 5 is the section G-G of FIG. 3.

DESCRIPTION OF THE EMBODIMENT

In the figures there is provided a centrifugal screen 10 including a screen support 11 mounted on a drive plate 12 for driven rotation within a housing. The screen support 11 is formed from five axially-spaced, steel hoops 13 interconnected by twelve welded stringers 14 to form a frustoconical cage. The inner driven end 15 of the screen support 11 is stiffened against drive distortion by bracing struts 16. The inner driven end includes an inner terminal hoop 17 adapted to receive the drive plate 12 in bolted relation via tapped holes 20. The outer end of the screen support 11 is defined by an outer terminal hoop 21 having tapped holes 22 spaced at 30° intervals about the terminal hoop 21. The screen support outer end portion is provided by a barrel-type hoop 23 welded both to tapered end portions of the stringers 14 and circumferentially to the outer terminal hoop 21. The barrel-type hoop or return ring is designed to

prevent material that has passed trough the basket from tracking along the outer basket surface and contaminating the product being discharged from the large diameter of the basket with fines.

The drive plate 12 includes an outer flange 24 having holes 25 in index with the tapped holes 20 to provide for the bolting-up of the drive plate to the inner terminal hoop 17. The outer flange 24 is integrally cast with a domed body portion 26, having an integrally cast flat inner drive flange 27 defining a central aperture, the inner drive flange 27 having twelve bolt holes adapting the flange to be bolted up to the output drive flange of a motor and gearbox assembly (not shown), and four jacking holes to assist in changing-out.

A screening surface is formed from six modular, part-conical screen modules 30. The screen modules 30 are formed from a flat stock produced by welding wedge wire 31 to inner, outer and intermediate transverse bars 32, cutting the flat stock to shape and rolling the shapes to part conical curvature with the wire direction in a plane containing the conical axis. Longitudinal, round-bar locating lugs 33 are welded to the transverse bars 32 adjacent the side edges of the screen modules 30.

The narrower, inner end 34 of the screen module 30 is provided with a welded-on sleeve portion 35 of wear resistant material forming an extended inner edge 36 of the module 30 which in use engages the step formed on the drive plate 12 at the junction of the outer flange 24 and the domed body portion 26. The wider, outer end 37 of the screen module 30 is provided with a welded peripheral flange 40 having a half-hole 41 for a countersunk fixing at each end and an intermediate countersunk fixing hole 42.

The axially-spaced, steel hoops 13, except for the inner terminal hoop 17, are each provided with pairs of slots 37 either side of every second one of the welded stringers 14. The slots 37 are adapted to receive the round-bar locating lugs 33 adjacent the side edges of each of the screen modules 30. The tolerances are selected whereby the modules 30 can be placed in engagement with the slots 37 in an axially withdrawn position and the module 37 then pushed axially into location with the peripheral flange 40 engaged with the outer terminal hoop 21 as the extended inner edge 36 of the module 30 engages the step formed on the drive plate 12 at the junction of the outer flange 24 and the domed body portion 26. Thereafter, countersunk metal threads 41 are installed to tapped holes 22 through the adjacent half-holes 41 and the intermediate countersunk fixing holes 42. The metal threads 41 are tightened in sequence to provide final centering of the modules 30.

The segmented basket in accordance with the foregoing embodiment has the specific advantages of occupational health and safety through ease and lightweight nature of small replacement segments as opposed to replacing a complete basket. Cost reductions are achieved because worn segments can be replaced independently of the cage, which is designed for re-use. Reduced transportation costs are achieved because the segments are lightweight and can be nested for easy transportation. Reclamation of used segmented basket componentry is easier and more cost-effective given that the design incorporates a natural divide between stainless steel and mild steel componentry. Less down time is required installing basket segments - i.e. removal of the basket cage is not required unless the cage is to be retired.

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 variations 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 defined in the claims appended hereto.