WO2009029962A1

WO2009029962A1 - Screens

Info

Publication number: WO2009029962A1
Application number: PCT/ZA2008/000078
Authority: WO
Inventors: Gavin Campbell Armstrong
Original assignee: Gavin Campbell Armstrong
Priority date: 2007-08-31
Filing date: 2008-08-29
Publication date: 2009-03-05
Also published as: ZA201000768B

Abstract

A vibratory screen of exceptional lightness and simplicity leading to relatively light, low power consumption, vibratory motors being effective for vibratory screening of a relatively large mass of material. The screen has a rectangular frame with a woven wire mesh, having transverse bracing struts extending across the width of the frame and located below the mesh and is characterised by vibratory motors attached to the outside of the structural members that define the length of the frame.

Description

FIELD OF THE INVENTION

This invention lies in the field of mineral recovery, with particular application to diamond recovery.

The application of the invention extends to the recovery of precious stones, minerals and any materials which have to be sized or classified for industrial or recovery purposes.

This invention relates particularly to vibratory screening as a preliminary process step in diamond recovery as its primary application but is applicable to the wider field of materials sizing and classification.

BACKGROUND

Diamond gravel or crushed primary kimberlite requires screening into specific size fractions for delivery to diamond concentrating plants, whether these are rotary pans, dense or heavy medium recovery plants, jig classification plants or other means of separation of materials of different specific gravity. The material concentrated by the above or any other means may require further separation into specific size fractions for the separation and/or recovery of diamonds over grease tables and/or through diamond X-ray machines.

Often diamond recovery operations are carried out in remote localities, where power supply, transport and other logistical requirements cannot be taken for granted and present significant problems or constraints.

An efficient, lightweight, durable and low energy demand screening apparatus and process or processes are desirable in the above applications.

THE INVENTION A vibratory screen in accordance with this invention includes a structural frame that is rectangular, fabricated from profiled structural members, defining a length and width of the frame, the frame having within its surround a woven wire mesh screen cloth that is supported between the structural members by suitable securing means, having transverse bracing struts extending across the width of the frame and located below the mesh, characterised by vibratory motors attached to the outside of the structural members that define the length of the frame, with the direction of the feed in use along the length of the frame.

Generally the length is longer than the width.

The vibratory motors are provided with their vibration axes oriented at an acute angle to the length of the screen, which is the feed direction of the screen. An acute angle means an angle that is not orthogonal to the length of the frame and not parallel to the length direction of the frame, i.e. an angle that lies between 90 degrees and 0/180 degrees. Preferably the angle lies within 10 or 20 degrees of 45 degrees to the longitudinal direction of the frame, that is, between 25 and 65 degrees, preferably between 35 and 55 degrees.

Though the angle of the motors is important for the performance of the screen and may be adjusted for the type of material screened, the feed rate, moisture level, nature of the material and other criteria during use, it is found that adjustment is not required for most applications. The advantage of the screen is its exceptional lightness and simplicity leading to relatively light, low power consumption vibratory motors being effective for vibratory screening of a relatively large mass of material.

In accordance with a preferred embodiment of the invention an inner frame is located inside the structural frame and carries the screen cloth.

Preferably the inner frame is fixed to the structural frame by bolting. This provides a reinforced structure with the screen cloth fixed to the inner frame plus the advantage that the inner frame may be easily detached from the structural frame by unbolting.

Preferably the vibratory motors are attached to the screen by bolting rather than welding, and preferably the bolting passes through both the structural frame and the inner frame of the screen. This gives the benefit that the vibratory action of the vibratory motors is transferred to the reinforced structure and directly to the inner frame and screen cloth. The structural members of the structural frame and the inner frame are in accordance with the invention preferably made of hollow tubing. Hollow tubing has a higher strength to mass ratio especially in bending action than solid bar, this gives effect to the goal of the invention which is a lightweight vibratory screen, that has been referred to above. Bolting not only affords the advantage of ease of disassembly but also avoids risk of fatigue fracture that is a tendency with welded joints subjected to vibratory action.

In accordance with a preferred embodiment of the invention a multiple deck screening assembly is created that has the abovementioned features applied to each deck, thus for example a four deck screening assembly is built, or any other number of decks within reason. The screen decks screen into separate fractions and these are collected separately for delivery to bins or to separation equipment. The decks are bolted together by means of fish plates on the sides.

In a multiple deck screen preferably each deck is provided with a guide bar that directs the fraction retained on the screen to a position in which the fraction falls into a bin or onto a further processing component or a position on a component separate to the other fractions. It may be desirable to separate a four deck screen for example into two two-deck screens so that vibratory motors that are suitable to the size ranges on each pair of screens can be used. Otherwise it can happen that the vibration is too much for the fine fraction on one screen but too little for a coarse fraction on another screen. Preferably the screening device feeds directly in this way into a separation device and not into temporary holding bins, for example a grease or X-ray separating screen. This is particularly important with diamond separation, for example, as in some chemical conditions the diamonds develop surface conditions that militate against later separation technologies.

Preferably the vibratory motors are mounted on flat plates that are bolted to the outer frame of the screen deck or each deck in the case of a multiple deck screen assembly, the plates serving also to secure the decks of a multiple deck screen together and to secure an outer frame to an inner frame of the screen deck.

The screens can be adapted to allow them to be inclined in the feed direction either upwardly or downwardly.

A technique to prevent blinding of a screen may be adopted which is to locate an additional screen below the screen and provide between the two screens a multiplicity of rings or short tubes of plastic material that vibrate between the two screens during use. The gap between the two screens is only slightly more than the dimension of the short tubes which are oriented with their axial direction orthogonal to the screen cloth. It is found that these tubes tend to knock out particles that are blinding the screen and the screen below is made with a larger aperture than the screen above so as to pass the material that passes the upper screen.

There are applications where the material being screened is so hot that the vibratory motors must be protected from the heat, for example where sand or aggregate has been dried it may enter the screen at 600 degrees centigrade. The motors may in such cases be mounted on extension plates so as to raise the motors or lower them to a position where they may be above or below respectively of a partition that closes off the very hot material on the screen.

THE DRAWINGS The invention is more fully described by way of example, with reference to the drawings, in which : -

Figure 1 is an oblique projection of a single deck screen,

Figure 2 is an oblique projection of a single deck screen raised on legs,

Figure 3 is an oblique projection of a two deck screen,

Figure 4 is an oblique view of a four deck screen,

Figure 5 is an oblique view of a five deck screen,

Figure 6 is an oblique view of a single deck screen with raised motors,

Figure 7 is an oblique view of a single deck screen with lowered motors,

Figure 8 is an oblique view of short tubes between a double mesh to prevent blinding,

Figure 9 is an enlarged scale transverse cross section of a single deck screen showing features of the screen,

Figure 10 is another enlarged scale transverse cross section showing other features of the screen,

Figure 11 is a transverse cross section of the single deck screen shown in figure 1.

THE PREFERRED EMBODIMENTS As shown in figures 1 to 5 of the drawings, the vibratory screen consists of a single deck shown in figure 1 , a two deck shown in figure 2, a three deck shown in figure 3, a four deck shown in figure 4 and a five deck assembly shown in figure 5. Each deck consists of an outer frame that is rectangular, fabricated from profiled structural members, longitudinal members 1 and transverse members 2 of the frame, the frame having within its surround a woven wire mesh 3 that is supported between the structural members by suitable securing means, having transverse bracing struts 4 extending across the width of the frame and located below the mesh, characterised by vibratory motors 5 attached to the outside of the longitudinal structural members. The outer frame is made of thick walled rectangular tubing, for example 25mm by 75mm with a wall thickness of 4mm. This is suitable for a screen up to about 2 meters in length, for longer lengths a stiffer frame is required to avoid a whip action towards the ends of the frame. For example the frame may be a rectangular section tube of 150mm by 50mm by 4mm or 6mm wall thickness. The reason to avoid a whip action is that this results in the amplitude of vibration of the screen at the ends that is too large and reduces the throughput of undersize fractions through the screen.

An inner frame of square tubing has longitudinal members 6 and transverse members which constitute the transverse bracing struts 4 and in this example longitudinal bracing struts 7 which are optional. If longitudinal struts are used it is necessary to have blocking strips 8 on top of the screen positioned above these struts, so as to avoid material that is located above the longitudinal struts travelling the length of the screen without being screened (see also figure 10). The inner frame is bolted to the outer frame using bolts that pass through bushes 9 welded into the outer frame and are screwed into threaded bushes 10 which are welded into the inner frame square tubing (see figure 9). The screen frame is mounted on a base frame 11 by means of springs 12 on brackets 13. Instead of springs use may be made of "Roster" (trade mark) fittings which are articulated spring devices using rubber bushes in square tubes arranged to act on the rubber in torsion. The benefit of the inner frame square tubing frame to which the woven wire screen cloth 3 is attached being bolted into the outer frame using bolts and threaded bushes that are welded to the inner frame and normal bushes in the outer frame, is that the inner frame may be quickly removed. The securing of the inner frame to the outer frame gives the whole screening device added rigidity. The screen cloth is attached to the inner frame using "Posidrive" (trade mark) or other types of screws and rubber lined washers. In the case of light screen cloths, i.e. those that are made of thin wire for small aperture screens, it is effective to attach the screen cloth to the inner frame members by means of an epoxy or other suitable glue, rather than screws. Light screen cloths are also preferably stretched in a jig so that they are taught when fixed to the inner frame. Light screen cloths may be considered to be those below 2 mm aperture size. This jig may be a rectangular frame that is placed around the screen frame, the screen cloth connected to the jig around the edges of the cloth in a manner that allows the cloth to be tensioned in both longitudinal and transverse directions. The goal is generally that the amplitude of vibration of the screen cloth in use in the direction orthogonal to the screen surface is around 3 mm, for example. An amplitude larger than this tends to cause too much bouncing of the material so that the undersize material does not pass through the screen as promptly as it could.

In the case of light screen cloths that are flexible, which are those have a fine wire and used for small apertures, the longitudinal bracing struts 7 combined with the transverse struts are important as they define squares of support 14 for such cloths which increases the effective rigidity of the areas cloth inside each square. This avoids floppiness or flexibility of the screen cloth, which improves the screening efficiency. This is not essential in the case of screen cloths that have thick wire, which is used for large apertures as the cloth itself has sufficient rigidity.

Figure 2 shows adjustable leg mountings by which the angle of the screen may be adjusted so that flow of material is "uphill" or "downhill" if required. In cases where blinding of the screen tends to arise a solution may be adopted which is to fix a second screen cloth 15 to the underside of the inner frame. The second cloth has a larger apertures than the screen cloth 16 above the inner frame. Between the two screen cloths are located loosely a large number of short tubes 17, their length slightly less than the space between the cloths and oriented so that the axes of the tubes are orthogonal to the screen cloths. These tubes vibrate between the cloths in use and are effective in preventing the blinding by knocking the material that tends to blind from underneath the upper screen cloth. The tubes cannot fall over and so remain in the orientation shown. The tubes are made of a plastic like PVC or polyurethane for example. Several tubes are located concentrically inside each other and are packed fairly closely over the whole screen area.

The two vibrating motors which drive the screen are each attached at the centre of the long sides of the screen frame and are inclined to the direction of feed, which is longitudinally along the screen, at 45 degrees. The inclination is forward in the direction of the feed. The mounting is done by bolting a metal plate 18 of suitable size and thickness to each side of the screen using threaded bushes welded into the screen outer frame. The vibratory motors are then attached to the metal plate by bolting using threaded holes in the metal plates at the required positions. Alternatively the bolts that hold the motors may be passed through the plate and both the outer and inner frame members, using bushes welded into the outer frame and threaded bushes welded into the inner frame. The weight settings of the motors may be altered to change the vibration performance and also the motor power rating is selected according to the required size and performance of the vibratory screen. The motor may be mounted on a circular disc to allow the angle of the motor to be adjusted. However, it is found that a 45 degrees angle is suitable for most applications.

The single deck screen can be duplicated to configure a double or multiple deck screen simply by extending the size of the side securing plates 19 which bolt all the decks together and altering the positions of the threaded motor securing holes to accommodate the various size motors required to drive the screens of different deck multiples and screen weights.

The side securing plates are held by the bolts on either side of each screen frame which secure the inner frame to the outer frame, thus being used to further secure the multi-deck assembly. The bolts hold the inner screen to the outer screen and the various screen decks together.

The inner screen frames can be easily removed simply by removing the four securing bolts on each side of the securing frame and removing the frame from the top or the bottom of the multiple screen assembly, whichever is applicable. A screen in between other screens can be removed by sliding it out like a drawer after releasing the necessary few bolts; this is of great practical advantage.

The delivery end 21 of the screen may be made to simply omit the transverse frame member 2, as shown in figures 1 and 2. Alternatively deflector bars 22 may be provided with a drop chute 23 for collection of material that does not pass the screen, as shown in figures 3 to 7. In multiple deck screens these deflectors will deflect in differing directions so as to provide for separate collection of the fractions.

This screen is then easily cleaned for replacement or immediately replaced by a spare screen.

The component parts of each individual outer frame or inner screen frame can easily be disassembled by removing the relevant securing bolts. All components are manufactured to be interchangeable.

Wherever sizing of material is of importance during mineral or material processing this screen system has distinct advantage over conventional screens. This is particularly the case in the diamond recovery industry in the following two recovery applications.

1) X-ray recovery where sizing is important as large particles can mask small particles from the X-ray beam and thus lead to diamond loss;

2) Grease recovery where smaller particles can be displaced after sticking to the grease by larger particles and lead to diamond loss.

The screens made according to the invention are 10% to 20% of the weight of conventional screens, they use 10% to 20% of the power required by the heavy conventional screens and being low profile can fit into spaces not possible for conventional screens. Prejudices of current practice are that heavy screen structures are necessary for durability, influenced by the development of polymeric screen panels which require heavy supporting structure. Most of the power goes into vibrating these heavy structures, whereas with the invention most of the power goes into the material being screened, because the screen is relatively so light.

REFERENCE NUMERALS

1 longitudinal frame member

2 transverse frame member

3 wire mesh screen cloth

4 transverse bracing strut

5 vibratory motor

6 inner frame

7 longitudinal bracing struts

8 blocking strips

9 bushes

10 threaded bushes

11 base frame

12 springs brackets square spaces second screen cloth screen cloth short tubes metal securing plates securing plates legs delivery end deflector bars drop chute

000

Claims

1. A vibratory screen which includes a structural frame that is rectangular, fabricated from profiled structural members, defining a length and width of the frame, the frame having within its surround a woven wire mesh screen cloth that is supported between the structural members by suitable securing means, having transverse bracing struts extending across the width of the frame and located below the mesh, characterised by vibratory motors attached to the outside of the structural members that define the length of the frame, with the direction of the feed in use along the length of the frame.

2. A vibratory screen as claimed in claim 1 , in which the vibratory motors are mounted with their vibration axes oriented at an acute angle to the length of the screen.

3. A vibratory screen as claimed in claim 2, in which the angle lies between 35 and 55 degrees.

4. A vibratory screen as claimed in any one of claims 1 to 3, in which the motors are mounted on extension plates that raise or lower the motors out of a very hot environment oat the screen cloth level.

5. A vibratory screen as claimed in any one of claims 1 to 4, in which an inner frame is located inside the structural frame and carries the screen cloth.

6. A vibratory screen as claimed in claim 5, in which the inner frame incorporates the bracing struts and also has longitudinally extending struts so producing squares of support for the cloth with blocking strips located above the cloth aligned with the longitudinally extending struts.

7. A vibratory screen as claimed in either one of claims 5 or 6, in which the inner frame is fixed to the structural frame by bolting to provide a reinforced structure with the screen cloth fixed to the inner frame.

8. A vibratory screen as claimed in claim 7, in which the inner frame is bolted to the outer frame using bolts that pass through bushes welded into the outer frame and screwed into threaded bushes which are welded into the inner frame.

9. A vibratory screen as claimed in any one of claims 4 to 8, in which the vibratory motors are attached to the screen by bolting and the bolting passes through both the structural frame and the inner frame of the screen.

10. A vibratory screen as claimed in any one of claims 1 to 9, in which an additional screen is located below the screen and a multiplicity of rings or short tubes of plastic or other material are provided between the two screens that vibrate between the two screens during use, the gap between the two screens being only slightly more than the dimension of the rings or short tubes which are oriented with their axial direction orthogonal to the screen cloth, the screen below being made with a larger aperture than the screen above so as to pass the material that passes the upper screen.

11. A multiple deck screening assembly in which each screen has the features claimed in any one of claims 1 to 10 applied to each deck, the structural frames of each deck being contiguous with adjacent decks.

12. A multiple deck screening assembly as claimed in claim 11 , in which at least some of the screens have guide bars that deflect the separate fractions so that these are collected separately for delivery to bins or to separation equipment.

13. A vibratory screen as herein described and as illustrated in the drawings.

14. A multiple deck vibratory screen as herein described and as illustrated in the drawings.

000