GB2325177A - A composite filter screen - Google Patents

Abstract

A composite filter screen suitable for use in a vibratory screening apparatus comprises first fine mesh (screening) layer 1 laminated to second coarse mesh (backing) layer 2 so that the first mesh layer is held in a state of tension by the inherent rigidity of the second mesh layer. The screen is prepared by applying heated platen 5 to first mesh layer 1, which is held in tension in the directions indicated by the arrows in Fig. 2, second mesh layer 2 and plastic layer 3 which are stacked upon each other on metal surface 4. This pressure is applied to the stack for a fixed period of time and then platen 5 is raised. This causes plastic layer 3 to melt and flow into mesh layers 1 and 2. Thus, upon cooling, mesh layer 1 will bonded to mesh layer 2 with the required degree of tension. A composite filter screen (6) prepared using this method can be incorporated into a vibratory screening apparatus using the constructions of Figs 3 and 4 (not shown). The mesh layers are preferably formed from woven wires, particularly metal wires such as stainless steel and they may be coated in plastic.

Description

A FILTER SCREEN This invention relates to a composite filter screen suitable for use in a vibratory screening apparatus such as a shale shaker, to a method of making the composite screen and to a vibratory screening apparatus incorporating the composite screen.

Vibratory screening apparatus in which a filter screen is mounted are well known for the separation of liquids and solids such as drilling fluid from drill cuttings (to allow the drilling fluid to be recycled), water from coal, water from sewage, etc. The filter screen is generally formed of at least two wire mesh layers which are held under tension in a frame which is received in a suitable opening in the apparatus. The filter screen is periodically removed and replaced by a fresh screen.

This invention relates to a composite filter screen which overcomes the need to provide a frame around the filter screen to hold the meshes in tension.

Thus, in accordance with a first aspect of the present invention, there is provided a composite filter screen suitable for use in a vibratory screening apparatus which comprises a first, relatively fine, mesh laminated to a second, relatively coarse, mesh layer, wherein said first mesh layer is held in a state of tension by the inherent rigidity in other layers of the filter screen.

The mesh layers are normally, but not necessarily, formed of woven wires, particularly metal wire, such as stainless steel. The wire may be plastic coated.

The size of the openings in the first mesh layer are normally determined by the performance characteristics which are required of the final screen, because it is the layer of fine mesh which determines the size of particles retained on the filter.

Generally, for a shale shaker, the size of the openings in the fine mesh will be such that the screen is able to pass particles of at least 75ym in diameter.

Typically, the openings in the screen are such that particles up to about 340m in diameter, and normally up to 215ym in diameter, are able to pass through it.

The second mesh layer is provided to support the fine mesh. Its mesh size is therefore relatively coarse compared to the first mesh layer. The second mesh layer may be chosen to have a degree of rigidity which in itself will be sufficient, in the resultant composite screen, to hold the first mesh layer in the desired state of tension. The rigidity of the second mesh layer will be dependent on a number of factors, including the material from which the mesh is made (normally a metal wire), the thickness of the material used and the size of the openings in the mesh.

Generally, using a stainless steel mesh, and with the size of openings in the second mesh layer such that particles of the order of between about 500m and 2000m are able to pass through it, the thickness of the wire should be between about 0.1 and l.Omm, typically about 0.25mm. In general, however, it is for the skilled person to determine by routine experimentation the precise characteristics of a mesh layer which is to be used as the second mesh layer in the present invention, depending on the nature of the first mesh layer, and any other layers which may be present in the screen.

The first and second mesh layers may be laminated, or otherwise bonded together by any suitable means.

One particularly suited material for bonding together the mesh layers is a thermoplastic material which can be heated to an elevated temperature at which it is capable of flowing over and around neighbouring meshes in adjacent layers, and then solidifying. However, other methods for bonding together the mesh layers may be used including adhesives, such as curable adhesives well known in the art. It would even be possible to weld the mesh layers together, for example using spot welds.

In order that the composite screen retains a filtering function, it is important that the laminating or bonding together of the layers of the screen does not lead to a significant amount of the openings in the screen becoming obscured with the material used to bond the layers together. Typically, the mesh layers may be laminated together along lines of bonding which have a grid pattern. This may be achieved by using, in the production process (see below) a layer of a thermoplastic material, for example pvc, polythene or polypropylene, typically polypropylene, having a grid pattern which is superimposed on the mesh layers after lamination. The spacing of the lines of bonding should be significantly greater than the mesh spacing of the second, relatively coarse mesh layer. The openings in the plastic matrix are dependent on the wire diameter and spacing of the coarse mesh. The greater the stiffness of the coarse mesh, the greater can be the spacing of the plastic matrix.

According to a second aspect of the present invention, there is provided a vibratory screening apparatus in which the screening element or at least one of the screening elements thereof is a composite filter screen in accordance with the first aspect of this invention.

Typically a vibratory screening apparatus has the following components: a basket or frame into which the filtering mesh support system is inserted and carried; a drive system to vibrate the basket; a suspension system (normally large coli springs on which the shaker basket is mounted); a sump to contain the liquid passing through the screen; and a system to control the flow of fluid onto the shaker.

According to a third aspect of the present invention, there is provided a method of making a composite filter screen suitable for use in a vibratory screening apparatus, which comprises laminating together a first, relatively fine, mesh layer and a second, relatively coarse, mesh layer, wherein, during the lamination step, the first mesh layer is held under tension whereby, in the resultant screen after completion of the lamination step, the first mesh layer is held in a state of tension by the inherent rigidity in other layers of the filter screen.

Lamination may be accomplished by providing a solid layer of a bonding material adjacent to the mesh layers and heating the layer of bonding material so that it melts and flows into the mesh layers, under pressure if necessary, and subsequently cooling or allowing to cool the resultant screen. Good results have been achieved where the layer of bonding material is a grid of a thermoplastics material, such as a polypropylene, in which case the mesh layers become encapsulated in the plastic grid.

Other methods of laminating the layers together may be used, as described above.

For a better understanding of the present invention, and to show how it may be put into effect, reference will now be made to the accompanying drawings in which: Figure 1 illustrates the constituent layers of a filter screen in accordance with the present invention; Figure 2 illustrates a process for manufacturing a composite filter screen in accordance with the present invention; Figure 3 illustrates a filter screen in accordance with the present invention installed in a vibratory screening apparatus; and Figure 4 illustrates an alternative arrangement to that shown in Figure 3.

A composite filter screen in accordance with the invention is made by laminating together a fine (or screening) mesh layer 1 and a coarse (or backing) mesh layer 2 which supports and gives integrity to the fine mesh layer 1 (Figure 1). These mesh layers 1,2 are, in the embodiment shown, laminated together by a thermoplastics material which is initially provided in the form of a separate grid 3 disposed adjacent the mesh layers 1,2. In a method of manufacture of the composite filter screen (Figure 2), the two meshes 1,2 and the plastic grid 3 are heated and pressed together whilst the fine mesh 1 is held under tension in both directions. The combination of heat and pressure causes the plastics material of the grid 3 to flow into the mesh layers 1,2. When the assembly is allowed to cool, the mesh layers 1,2 are encapsulated in the plastic grid 3, and, owing to the rigidity of the coarse mesh 2, the fine mesh 1 is held in the state of tension which was applied during lamination.

More specifically, in the method of manufacture illustrated in Figure 2, the plastic grid 3 and the mesh layers 1,2 are placed on a metal surface 5 the plastic grid 3 first, followed by the support mesh layer 2. The fine mesh layer 1 is then placed over the support mesh layer 2 and placed under tension in both directions. A heated platen 5 is lowered onto the filter screen 1 and pressure is applied. After a predetermined dwell time, the heated platen 5 is raised. By accurate control of the platen temperature, the applied pressure and the dwell time, the plastic flows into both layers of mesh 1,2. The composite screen is then allowed to cool, the applied tension in the filter screen 1 released and the completed screen removed from the machine.

Figures 3 and 4 show the composite filter screen of the invention in use in an assembly which can be received in a vibratory screening apparatus in a manner which is known per se. The composite filter screen 6 is placed on and supported by a steel grid plate 7 which is the top surface of a substantial fabricated steel frame. The top surface of this steel plate may be either flat or radiused with the highest point being along the centre line. The screen is then clamped along the sides 8 as shown by arrows Y. In addition to the clamping, tension may also be applied to the filter screen by distorting the filter screen along its edge regions as shown by arrows Z in Figure 4. For example, this may be achieved by inflating an inflatable layflat hose into a cavity while the outer edge of the screen is securely clamped or otherwise fixed.

Alternatively, the edge of the filter screen may be attached to a "C" strip and tensioned by a moving blade within the support cradle, such as is described in British Patent Application No. 9623831.6.

Claims (10)

CLAIMS:

1. A composite filter screen suitable for use in a vibratory screening apparatus which comprises a first, relatively fine, mesh laminated to a second, relatively coarse, mesh layer, wherein said first mesh layer is held in a state of tension by the inherent rigidity in other layers of the filter screen.

2. A composite filter screen according to claim 1, wherein the first and second mesh layers are laminated together by a plastics material.

3. A composite filter screen according to claim 1 or 2, wherein the first and second mesh layers are laminated together along lines of bonding which have a grid pattern.

4. A composite filter screen according to claim 1, 2 or 3, wherein the spacing of the lines of bonding is significantly greater than the mesh spacing of the second, relatively coarse mesh layer.

5. A vibratory screening apparatus in which the screening element or at least one of the screening elements is a composite filter screen as claimed in any preceding claim.

6. A method of making a composite filter screen suitable for use in a vibratory screening apparatus, which comprises laminating together a first, relatively fine, mesh layer and a second, relatively coarse, mesh layer, wherein, during the lamination step, the first mesh layer is held under tension whereby, in the resultant screen after completion of the lamination step, the first mesh layer is held in a state of tension by the inherent rigidity in other layers of the filter screen.

7. A method of making a composite filter screen according to claim 6, wherein lamination is accomplished by providing a layer of bonding material adjacent the mesh layers and heating the layer of bonding material so that it melts and flows into the mesh layers, and subsequently cooling or allowing to cool the resultant screen.

8. A method of making a composite filter screen according to claim 7, wherein the layer of bonding material is a grid of a thermoplastics material.

9. A composite filter screen, substantially as herein before described, with reference to the accompanying drawings.

10. A method of making a composite filter screen, substantially as herein before described, with reference to the accompanying drawings.