The present invention relates to vibrating screen devices used for the separation of ore particles and the like into different size groupings and, more particularly, to such devices particularly structured to improve efficiency and throughput. BACKGROUND ART

Large screens have been used for many years in the mining and minerals processing industry for separating ore and rock particles on the basis of size. Such separations are achieved by using a uniformly perforated surface which acts as a multiple go-no-go gauge. Ideally particles larger than the apertures are retained on the surface while particles smaller than the apertures pass through them.

Such screens can be classified into three basic types, declined circular motion screens, horizontal linear motion screens and banana deck linear motion screens.

These circular motion screens consist of a declined rectangular screening surface arranged in one, two of three decks which allows material to flow with the aid of vibration. The mechanical vibrations imparted to the screening surfaces give a circular motion to the screen surface. Declined circular motion screens have relatively high efficiency and are widely used in the quarrying and mining industry. However, they are not particularly efficient in screening damp and sticky bulk solids.

Horizontal linear motion screens consist of a horizontal rectangular screening surface arranged with one or two decks, which require a linear vibration motion to vibrate the screening surfaces with a vertical component to provide lift and a horizontal component to convey the material along the screen surface.

Horizontal linear motion screens are usually highly efficient and are particularly advantageous where low head height is required. This type of screen is not particularly efficient when screening damp and sticky bulk solids.

The linear motion banana deck screen is a relatively new concept in screening which seeks to combine the advantages of the declined screen with the horizontal screen. These screens may be arranged with one or two decks which consist of a number of surfaces of differentially declining stages usually beginning at 35 deg and finishing at 5 deg. The machine is usually vibrated with a linear motion similar to the horizontal screening machines.

The banana deck screen is generally superior in screening efficiency and capacity to the conventional declined and horizontal types and is usually engineered to suit the customer's requirements. It is relatively efficient in screening some types of damp and sticky bulk solids. There is also a type of screen known as a "non- blinding" screen. One well known manufacturer of this type of screen is Bivitec. In this type of screen the screening element is supported on a frame where alternating frame

members are arranged to move relative to each other so that the screening element "flaps" or oscillates in operation whereby the screening elements, in addition to imparting motion to all particles on their surface, are alternately stretched and relaxed so that the apertures in the screening elements cycle through different sizes.

Because the aperture size is continually changing it is relatively difficult for these apertures to become clogged with ore particles i.e. they tend not to become "blind": hence the name "non-blinding" screen. These screens are known for their ability to classify difficult (e.g. sticky) materials comprised of particles in the range of 1 to 30 mm. These non-blinding screens heretofor have been driven by circular amplitude exciters which impart a cyclical, predominantly elliptical motion to the screen element surface. DISCLOSURE OF THE INVENTION

According to one broad form of the invention, there is provided a composite screen comprising a first screen section followed by a second screen section; said first screen section being of a different nature to said second screen section; said first and second sections being selected to complement the processing characteristics of each other.

Preferably the first screen section comprises a banana screen. The second section may comprise a non-blinding screen section.

The composite screen may be excited by a linear motion exciter. Preferably, the exciter comprises adjacent, counter

rotating eccentric weights driven by a common drive source and all connected and supported from a predetermined location on a frame supporting said composite screen.

In a further broad form of the invention there is provided a non-blinding screen excited by a linear motion exciter. Preferably, the vector of excitation of the surface of the screening elements comprising said non-blinding screen is inclined forwardly in the direction of material flow and lies between 30° and 45° from the horizontal. BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described with reference to the drawings wherei :-

Fig.l is a side section view of a first embodiment of the invention incorporating a banana screen and a non-blinding screen in series,

Fig.2 is a detailed side section view of the interconnection arrangement between the banana screen deck and the non-blinding screen deck of Fig. 1, and Fig.3 is an end section view designated AA in Fig. 1 which shows detail of the alternating fixed and floating beams comprising the non-blinding screen of Fig. 1, Fig. is an end section view designated BB in Fig. 1 which shows detail of the alternating fixed and floating beams comprising the non-blinding screen of Fig. 1.

Fig.5 is a side section view of a second embodiment of the invention where linear motion is used to excite a non-blinding screen. MODES FOR CARRYING OUT THE INVENTION The double deck screen structure 10 shown in Fig. 1 comprises a banana screen top deck 16 and a composite lower deck comprising a lower banana screen portion 11 connected in series and feeding onto a non-blinding screen 13.

The entire screen structure is supported on four symmetrically placed legs 14, two of which are illustrated in Fig. 1. These legs are connected to a supporting building structure by means of springs 15 which are utilised to damp out communicated oscillations to the building structure, but which also form part of the tuned structure of the screen and linear motion exciter combination.

In this arrangement,, feed 20 is fed as indicated in Fig. 1 onto the top deck 16 which acts as a "scalping" screen for the removal of particularly large oversized particles. In normal operation, the majority of the feed 20 will ultimately fall through the top screen 16 onto the lower banana screen portion 11 comprising banana screen segments 12A, 12B, 12C and 12D.

The inclination of the segments 12A to 12D varies along the length of the screen as illustrated in Fig. 1. The banana screen segments 12A to 12D are supported on supports

24. The screen segments of the top of the banana screen deck 16 are supported on a plurality of supports 23 as illustrated in Fig. 1.

Lifting lugs 19A and 19B are used to lift the entire screen structure 10 into place on site.

Vibratory motion is imparted to the entire screen structure by means of a linear motion exciter 17 which comprises counter rotating eccentric weights 18A and 18B. The motion imparted is a straight line movement angled slightly forwardly as illustrated by arrows 25 in Fig. l.

The second portion of the lower screen comprises a non- blinding screen 13 which comprises a plurality of screen elements 26A alternately disposed between a plurality of screen elements 26B as illustrated in greater detail in Fig. 2. These screen elements 26A, 26B are anchored to alternating fixed beam components 27A and floating beam components 27B. Fig. 3 shows a cross section AA from Fig. 1 through a typical floating beam structure. Fig. 4 shows a section BB from Fig. 1 through a typical fixed beam section.

The upper and lower screen frame components 28A and 28B are directly attached to the frame of the screen structure 10. These same frame components 28A, 28B also support, through rubber blocks 29 left and right main support channels 31A, 31B respectively for the non-blinding screen floating beams 27B.

As illustrated in section AA in Fig. 3, the floating beams 27B are indirectly supported from the screen frame components 28A, 28 B by a link to the main support channels 31A, 31B which themselves are connected to components 28A, 28B by means of the rubber blocks 29.

The blocks 29 are pre-compressed in such a manner that under the influence of the linear motion of the exciter 17 so as to cause the previously described relative motion between the fixed beam elements 27A and the floating beam elements 27B.

The linear vibratory motion imparted by the linear motion exciter 17 acts on the non-blinding screen frame 28A, B and the screen elements 26 which it and support channels 31A, 3IB support so as to cause screen elements placed between alternating beams to either stretch or sag in sequence whereby elements 26A are in the sag condition when elements 26B are in stretched condition and vice versa.

Fig. 2 also shows detail of the interconnection between the end of the lower banana screen portion 11 and the beginning of the non-blinding screen portion 13 of the lower deck. Particularly, left most floating beam 27B is attached to one edge of a left most screen element 26A with the opposite edge of the left most screen element 26A attached to support girder 30. Support girder 30 also supports the right most of the lower banana screen deck elements 12E by way of support 24. Beam 30 is connected to the frame of the screen structure 10.

The incoming flow of feed 20 enters and moves across the top of top screen 16. With oversized particles removed the ore or other particulate matter to be separated falls generally onto the left most portions of the lower banana deck screen 11 where oversized particles continue to be

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vibrated down the surface of the banana deck structure whilst undersized particles fall through apertures in the banana deck structure. Oversized particles reaching the rightmost (Fig. 1) end of the lower banana deck portion 11 pass onto the non-blinding screen 13 which is more efficient in separation of certain types and consistency of particulate matter than the banana screen structure.

Undersized material 21 falls through the screen whilst the remaining oversized material 22 exits from the right most end of the non-blinding screen structure 13.

Use of the non-blinding screen structure 13 excited by linear motion exciter 17 provides efficient separation (up to 98*) of ore particles on the non-blinding screen 13.

Combining the banana screen structure 11 serially with the non-blinding screen structure 13 allows the utilisation of complimentary features of the two screen structures in an advantageous manner within the one overall screen structure 10 whereby efficiency of ore separation can be maintained or improved for a given volume throughput of ore or any particulate material as compared with other known screen arrangements, particularly in the case of what are known as "sticky" materials.

A second embodiment of the invention is shown in Fig. 5, which includes a dual deck screen assembly 32 that comprises an upper scalping deck 33 from which undersize particles are ed onto a non-blinding screen 3 . The non- blinding screen construction is the same as that described

above in respect of the first embodiment. Oversized particles from the screen 34 fall from the top of the lower end of the screen 34 whilst undersize particles fall through the screen elements before reaching the end of the screen 34. The screen assembly 32 is excited by linear motion exciter 35 operating in the direction indicated through the centre of gravity of the structure 32.

The entire structure is supported on spring supports 36 of similar construction to spring supports 14 of the first embodiment.

The linear exciter 35 in combination with the specific construction of the non-binding screen (including block element 37 selection) operates so as to provide the previously described behaviour of the non-blinding screen elements.

The above describes only some embodiments of the present invention and modifications, obvious to those skilled in the art, can be made thereto without departing from the scope and spirit of the present invention.