A DEVICE AND METHOD FOR DEW ATERING AQUEOUS

SUSPENSIONS

Field of the Invention

The present invention relates to a device and a method for dewatering particulate materials. In particular the dewatering of sludge materials whereby an electrokinetic process of removing water, together with conventional hydraulic dewatering is employed.

Background to the Invention

Many industrial processes generate large volumes of aqueous particulate suspensions in the forms of sludges, slurries, pulps, tailings and other materials. Even where the constituents of the suspensions are not harmful, the difficulty of handling the suspensions lies in the volume required for transportation and for their disposal. Although the materials comprise a large proportion of water, the rate of loss of water therefrom is slow, partly due to the stability of the suspension. Moreover, water loss from the surface can lead to a semi-solid crust of material forming which can be dangerous should someone attempt to traverse the surface. Further, it would be advantageous if the water could be re-used, a particularly important feature where material is produced in an arid region, for example as waste in a mining operation.

It is therefore desirable to remove as much of the water from the suspension as possible. To this end, a number of processes are known in the prior art: both continuous and batch processes. Amongst the latter set include plate

filter presses and the use of electrokinetic materials disclosed in

PCT/GB 04/04181. The presses operate through application of a positive pressure to the waste material contained within a cell. The applied pressure causes drainage to occur through passage of water through a, usually, polymeric belt or sheet.

The materials disclosed in PCT/GB 04/04181, include electrodes woven into a sheet of the materials which can itself be a woven material. The electrodes operate to cause water to move from an anode sheet toward a cathode sheet/filter, thence out of the suspension, thereby drying out the suspension.

Both of the above-described methods are suitable for dewatering large volumes of inhomogeneous materials. They do however tend to require supporting and associated apparatus which occupies a large surface area in order to function. It would be of advantage therefore to devise a method or apparatus requiring a smaller area.

It is an object of the current invention to provide an apparatus which seeks to address the above problems and which could be used with both homogeneous and non-homogeneous materials. It is a further object of the invention to provide a method of dewatering aqueous particulate suspensions which seeks to address the problem.

Summary of the Invention

According to a first aspect of the invention there is provided a method of dewatering an aqueous suspension of particulate material, the method comprising selecting a bag having walls comprising a non-conductive base material the walls being permeable to water but not the particulate

materials, the bag defining an aperture at both ends to allow a suspension to be added and removed from the bag;

wherein the aperture at a first end of the bag is larger than that at a second end;

suspending the bag by the first end of bag such that the aperture is held open;

securing the second end within the first aperture, whereby the second aperture is held in an open configuration, the suspended bag forming thereby a bisected torus having a trough portion to receive a material to be dewatered;

adding a suspension to be dewatered to the trough portion and allowing water from the suspension to drain out of the bag

Preferably the base material is associated with at least one cathode element comprising a metallic or conductive element and engaging an anode with the material for dewatering and applying a current such that current flows between the anode and the cathode, removing water across the wall of the bag;

releasing the first end of the bag, allowing dewatered material to be removed.

The inside of the bag can be easily cleaned of material adhered to its sides, ready for re-use.

-A- Optionally, the first end of the bag is suspended prior to suspension of the second end. As the first end is larger, the distribution of the weight is spread over a large circumference.

Advantageously, the bag is suspended from straps secured to the side of the bag. The straps facilitate fixture of the bag to a supporting structure, which may be specially designed for the bag. Further advantageously, the support structure includes a conical support element over which the bag is draped.

The material from which the bag is made advantageously includes an ultraviolet resistant component to reduce degradation of the material when the bag is used in the open air.

The cathode preferably comprises a metallic wire or conductive element woven into the material of the bag. Further preferably, the width of wire is from approximately 0.1 - 2mm in thickness. Yet further optionally, the spacing between adjacent wires is from 5 - 50mm.

Optionally the diameter of the bag is selected to be from 0.3 - 2.0m. Further optionally, the height of the bag is selected to be from 0.3 - 4.0m.

Preferably, the bag is formed by bringing together in seaming engagement opposed edges of a suitably configured piece of material.

The cathode is advantageously connected to a current distribution means through bundles of metallic or conductive yarn within the bag material which may be disposed in either or both of the warp or weft direction. Further advantageously, the connection is achieved by means of a transfer strip in the complimentary weave direction of the main conducting elements. Yet further advantageously, connection is achieved by stapling cut edges of the conductive elements in the warp or weft of material,

following the seaming engagement of the material edges.

According to a second aspect of the invention there is provided a dewatering bag, the dewatering bag comprising a wall including a non- conducting polymeric base material in association with a cathode element comprising a metal or other conducting material;

the bag being substantially tubular in shape, the aperture at a first end of the bag being larger than that at a second end.

Brief Description of the Drawings

The invention is now described with reference to the accompanying drawings which show by way of example only, one embodiment of a dewatering bag. In the drawings:

Figure 1 illustrates a bisected torus; Figure 2 illustrates a pattern for a bag; Figure 3 illustrates a bag in an in-use configuration; Figure 4 illustrates release of dewatered material from the bag;

Figure 5 illustrates a conical framework to support a bag and Figure 6 illustrates the ratios of the radii to the height in the bag.

Detailed Description of the Invention

The bags as disclosed herein are intended for use in a number of technical fields where it is necessary or of advantage to be able to remove water from a material prior to, for example, disposal of the dewatered material in a landfill site.

Such uses include removing water from estuarine marine soils and dredgings, mine tailings, mineral sludges, agricultural and food wastes, industrial wastes, sewage sludges and gully emptyings, and the general operating principle is exemplified in International Patent Application No.

PCT/GB04/04181.

In general, a bag is provided having walls made of a material which allows water to pass therethrough, but which retains substantially all of the particulate material. Woven into the walls is a metal or other electric conductor capable of functioning as a cathode. When sludge or other suitable material to be dewatered is loaded into the bag, an anode is brought into contact with the sludge and an electric potential applied. A voltage gradient is applied across the sludge the result of which is that water is removed from the sludge to pass through the walls by electro-osmosis which water can thence drain away. The water content of the sludge is thereby substantially reduced, resulting in a material which is more easily handled, takes up less volume and is less hazardous.

The bag is provided in the form of a bisected torus, and has therefore a trough portion to retain the sludge. The advantage of the bisected torus design is that it provides, effectively, two surfaces (inner surface and outer surface) for filtration. The flow path length for water moving out of the sludge compared to a simple cylindrical bag shape is thereby halved. Further, having all surfaces of the bag acting as the cathode and having an anode placed equidistant from the cathode surfaces (Figure 3)means that an applied voltage will achieve double the voltage gradient, driving water in two directions and for half the distance compared to a simple cylindrical design with the anode placed in the centre of a cylindrical or similar bag. This has the potential to increase rapidly the speed of dewatering.

Once dewatering has taken place however, the problem then remains of removing the dry material from the bag. This difficulty is increased when the bag is intended for re-use as the material not only needs to be removed, without damage to the bag, but also cleaning needs to be carried out in order to prevent a layer of dried material adhering to the bag's inner surface and hindering or preventing operation of the cathode on a further batch of wet material.

The bag and the method of its use mitigate the above difficulties. Referring initially to Figure 1, this illustrates a bag 10 in a configuration to retain and dewater a sludge material. The configuration is referred to as a bisected torus having a central aperture 11 defined by in inner edge 12, and an outer edge 13. A trough 14 is thereby formed into which sludge can be added. In

Figure 2, a pattern is shown for material to be used in the construction of a bag 10. The basic material from which a bag 10 is formed is designed to allow the passage of water through its walls. Typically the walls are formed from a non-conductive polymeric base material and can include a component which absorbs ultra violet radiation and so reduces the degredation of the material when the bag is exposed to the sunlight. In order to assist the passage of water through the wall, an electric potential is set up which moves the water by electro-osmosis.

To this end a conductor, which in use functions as a cathode, is incorporated into the wall to create an electrokinetic geosynthetic (EKG) material. The conducting element can be either formed of a metal or other material known in the art. The conducting element is typically in wire or thread form, and can be woven, knitted or otherwise attached onto or within the material making up the wall of the bag. The conducting element is incorporated into the weft or warp direction of the material and is shown as

the set of horizontal lines 21 in Figure 2. In order to enable a charge to be transferred a circuit is formed through the use of warp or weft transfer strips 22 incorporated into the material and engaging the ends of bundles of wires or threads.

In order to form a bag therefore, the pattern of material shown in Figure 2 is taken and the edges of the pattern 23 and 24 are brought together. The edges 23, 24 are sealed to each other along a seam. The bag 10 thereby formed is essentially funnel-shaped. The size of the bag 10 made in accordance with the above description has a height of around 0.3 - 4.0m.

Typically, diameters are around 0.3 - 2.0m (giving a circumference of from 1.0 - 6.3m).

In order to enable the bag 10 to function therefore, the bag 10 is suspended from a suitable support by the upper support straps 25. The funnel portion

26 of the bag 10 also includes support straps 27. Once the bag 10 has been suspended from the support straps 25, the funnel portion 26 is drawn up through the centre of the bag 10 such that the edge 12 of the funnel portion lies within the circumference of the outer edge 13. The inner and outer edges 12, 13 are typically held at the same level to maximise the volume of the trough 14 formed, although where required due to other constraints, they may be at different levels. The straps 27 are secured in position to the support. The funnel portion 26 of the bag 10 is shown in the Figures to be open. However in certain embodiments, not illustrated, the funnel portion 26 is closed.

The bag 10 is now in a bisected torus configuration, shown in Figure 3, with the open edge facing upwards. The cathode wall of the bag 10 now forms a trough 14 into which sludge can be added and dewatered. As an alternative to using support straps 27, the funnel portion 26 of the bag 10 may be supported from beneath by a structural conical framework 50

(shown in Figure 5), which may in addition act as a means of electrical transfer to the cathode bag.

It will be recognised that in the above construction, the funnel portion 26 could be first suspended from a support by means of the strap 27 and the walls then drawn up about the outside of the funnel portion 26 and suspended from the straps 25. This method of attachment has the advantage in that a user does not have to secure the bag 10, whilst standing underneath the bag 10 or by working above the bag 10 and lifting the funnel portion 26 by suitable means. The task is therefore made easier and safer, although the weight of the bag 10 is borne for a period by the smaller, funnel portion 26 which may cause undue strain in the bag material.

Once the bag is in position, a multi-fingered anode 31 (see Figure 3) is brought down into a position such that the anode's fingers 32 are below the surface of the wet material or sludge which is to be dewatered.

When the sludge is contained with the trough 14 and the anode 31 is in position, a potential is applied causing water to be removed across the walls of the bag 10, which water once on the outside of the bag 10 can drain away. The water drained off can be subject to further processing if required to remove any contaminants which also pass through the walls. When dewatering has been completed, the potential is removed and the bag 10 readied for removal of the solid material remaining therein.

The means of removing the solid is illustrated in Figure 4. In the upper section of Figure 4, a vertical section through the bag 10, containing the solid material 41 is shown. One or more anode fingers 32 remain within the solid 41. To release the solid, the straps 25 are released allowing the

cathode wall of the bag 10 to fall down beneath the funnel portion 26 which remains supported. The solid 41 therefore is deposited on the ground or on a suitable surface ready for transport to a disposal or further treatment site.

The inner surface of the bag 10 is now exposed to the outside and is in the ideal position to be cleaned. Cleaning can be carried out by conventional means for example by jet-washing or air drying.

The bag 10 is ready for re-use once the walls have been resupported by the straps 25 to return the bag 10 to the configuration shown in Figure 2.

The design of the bag 10 presented herein has several advantages for both electroosomotic and conventional or hydraulic dewatering. As regards conventional dewatering, the bag design provides two effective filtration surfaces (inner and outer) and thus halves the flow path length compared to a simple bag design. Conventional dewatering in geotextile bags or tubes can have the disadvantage that when attempting to dewater awkward or difficult to dewater materials, dewatering may cease and yet the material in the centre of the bag remain in a wet state with a large proportion of its original water content still present. It is a specific object of the present invention to provide an additional filtration surface in the centre of the bag to tackle exactly this problem.

In an alternative embodiment, not illustrated, the funnel portion of the bag 10 shown in Figure 2 can be formed of a mesh material of, for example, tubular construction. The mesh material is selected to be capable of functioning as an electrode and in particular, as a cathode. The mesh is secured, at one end, to the EKG material of the rest of the bag by conventional means.

As regards electrokinetic dewatering, the invention also has specific

advantages. By making all surfaces cathodic and adopting the bisected torus shape and placing the anode midway between the cathode surfaces (i.e. circularly disposed in the trough), electroosmotic flow will act in two directions relative to the anode and in sympathy with hydraulic dewatering, which will do the same i.e. towards the cathode or bag outer surface and towards the cathode or bag inner surface. It is the voltage gradient which drives electroosmotic flow. By reducing the distance over which the voltage acts the voltage gradient is increased.

Compared to simple cylindrical or tubular designs with the anode at the centre, the bisected torus design has positive implications for health and safety by permitting larger voltage gradients to be achieved without the necessity of raising actual applied voltages.

The reduction in volume contained in the bag as a consequence of the open centre of the bisected torus design is an acceptably small loss for the increased effectiveness of both hydraulic and electroosmotic dewatering offered by this invention.

It is noted that given the advantages conferred by the invention, to the process of hydraulic dewatering, the invention may be used without the application of a voltage gradient. As such and for certain types of material the bag may be constructed without metallic conducting elements, which would normally serve to facilitate electroosmosis. In these instances the advantages for hydraulic dewatering would permit dewatering of materials which would not normally dewater acceptably with hydraulic dewatering in a conventionally designed bag or tube.

In practice, depending on the sludge being dewatered, the parameters of the

bag such as the height h, and the values of the inner and outer radii R ₁ and

R ₀ together with their ratios R ₁ IR ₀ (as shown in Figure 6) are set to give the optimum dewatering. The values are adjusted, consideration being given to the material density, the ratios of electroosmotic and hydraulic permeabilities and material shear strength for bag emptying operations.

It is of course understood that the invention is not limited to specific details described herein, given as examples only, and that various modifications and alterations are possible with the scope of the appended claims.