The present invention concerns a method of separating a solid and a liquid and an apparatus, in the form of a filter press . for carrying out the method .

Filter presses are used in a variety of industrial processes where separation of a liquid from a mixture of liquid and solid material must be achieved, such as in the dewatering of a washed solid effluent, for example. One method of separating such a mixture is to place the mixture to be separated between two porous belts which are then passed under tension around a set of rollers to squeeze the liquid out from the mixture. The apparatus for performing this operation is known as a pressure-belt filter press .

The separation of solids may sometimes be assisted by the addition of a flocculating agent to the liquid/solid mixture. However, although

the use of such material may greatly improve the recovery of solids, it also increases the cost of the process. If, as is often the case, the separation is merely being done as an effluent treatment, the resulting separated products have no value and so it is desirable to keep the use of a flocculating agent to a minimum.

It is an object of the present invention to provide an improved method of separating a mixture of a liquid and a solid material and an apparatus in the form of a filter press for carrying out the improved method.

The method of separation according to the invention comprises feeding at least one mixture comprising a solid and a liquid onto a porous belt, driving said belt over a drainage area comprising vacuum extraction means and then pressing said mixture between said belt and either a second belt or a roller. The apparatus according to the invention comprises a porous belt, at least one means to feed a mixture to be separated onto said belt, drive means for said belt, vacuum extraction means disposed below said belt over at least a part of the path of said belt and either a second belt or a roller arranged to contact and compress material between said first belt and

said second belt or roller over a different part of the path of said first belt.

The feed means which supplies the mixture to be pressed to the belt may comprise a direct feed from the process in which the mixture to be filtered is produced. More preferably, however, it comprises a feed box in which the mixture is agitated to maintain the composition and dispersion of the components of the mixture and which is provided with means such as a weir to feed an even layer of the mixture across the width of the belt. Use of a feed box is also advantageous if an additive such as a flocculating agent is to be used to aid the filtering process because the additive may be introduced to the mixture in the feed box and dispersed by the agitators therein.

More than one feed means may be supplied, enabling more than one mixture or type of mixture to be fed to the same press. There are a number of advantages to be gained by feeding more than one mixture, in addition to the increase in throughput achieved by so doing. For example, three feed systems may be used which are arranged to place successive layers of mixture onto the belt of the filter press. In this way a toxic material may be fed, and ultimately pressed, h to h XJo an

two non-toxic materials, thus reducing exposure of the toxic material to the environment. In a preferred method according to the invention a mixture is fed onto the belt first, then a second mixture, is fed on top of the layer of the first mixture on the belt. Optionally, a third layer, may then be fed on top of that second layer. This preferred method may be especially advantageous if the first mixture contains an additive such as a flocculating agent. In this way the intial layer containing flocculating agent may act as an additional filter for the subsequent layer or layers of material, thus increasing the cost- efficiency of the filtering process by using less additive.

The belt carrying the mixture to be separated preferably passes over an area which allows free drainage of liquid from the mixture on the belt. In this area liquid may drain through the belt and into a sump in which the liquid is collected.

The vacuum extraction means comprises an area over which suction is applied and a suction system.

Preferably the area over which suction is applied is the area provided by a suction pad in the form of a porous mat which is partially

contained within a pan of non-porous material which has means for connecting the pan to a suction system such as a vacuum pump. The application of suction to the pan tends to draw air or liquid through the suction pad and into a receiving vessel. Most preferably the suction pad extends above the vacuum pan by a few millimetres. The vacuum pan is situated below the belt in such a way that the belt is in contact with the suction pad as it passes over the vacuum pan. In this way the drainage of liquid from the mixture on the belt is assisted by the suction applied to it through the belt as the belt passes over . the suction pad. Preferably more than one suction pad is provided, and these may be adjacent to each other or placed in different suction areas on the path of the belt.

Where more than one feed means is provided the vacuum extraction means is preferably arranged so that the belt passes over at least one suction area after the first mixture has been fed to the belt and before a second mixture is fed on top of the first .

The porous belt is preferably an endless belt of woven fibres or strands of plastics material, such as polyester. When the belt

carrying the mixture to be pressed has passed over a vacuum extraction means it then passes around or between rollers arranged in such a way that the mixture on the belt is compressed so as to remove the greater part of the liquid which is remaining in the mixture after it has passed over the vacuum extraction means. Preferably a second porous belt is arranged to contact the mixture before it passes between compression rollers so that it is pressed between two porous belts. The resulting filter cake is then removed from the belt or beIts .

A preferred embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings which are : -

Fig.l a schematic sectional view from the side of a pressure- belt filter press according to the invention,

Fig.2 a schematic sectional view from the side through the longest dimension of a vacuum pan.

The filter press shown in Fig.l has two endless belts 10,11 between which an effluent sludge can be pressed into a filter cake. The

top belt 10 is driven through the press in a clockwise direction and the lower belt 11 is driven in an anti-clockwise direction with reference to the drawing. The press is c supplied with sludge by three feed boxes 12, 13 & 14 which have internal mixing paddles and which supply an even layer of sludge on to a belt across the width of the belt via a weir

15. 0 The three feed boxes are arranged so that three layers of sludge are built up on the belt 11 prior to pressing between the belts as they pass around rollers 16-19. The feed box 14 supplies the bottom layer and feed boxes 13 and 5 12 supply the middle and top layers respectively. Advantageously, feed boxes 12 and 14 may supply sludge which contains a flocculating agent. The belt carrying the sludge passes over a free drainage area 20 where water can drain through the belt and into a sump .

Vacuum pans 21 are provided at various locations below the belts to suck liquid out from the sludge. The vacuum pump and hoses which connect the vacuum pans to the vacuum 5 pump are not shown, for clarity. A vacuum pan

is shown in Fig.2. The pan is made of stainless steel and comprises two inverted elongated pyramidal shaped sections arranged end to end logitudinally across the width of the belt. A small part 23 of the upward facing opening of the pan is blocked at each end to provide a good seal to the belt at its edges The pan has an outlet 24 at the bottom of each section leading to a connection piece to which a hose is attached. The other end of the hose is connection to a vacuum pump.

A suction pad 25, made up of polyurethane wedge-wire screen mats, is placed into the upwardly facing opening of the pan. Liquid is drawn away from the sludge through the belt and the slots in the suction pad by the suction applied by the vacuum pump.

In use, the sludge is fed onto belts 10 and 11 which are then squeezed together as they pass around perforated drainage rollers 16. The belts continue together around successive rollers 17, 18 and finally slatted drainage rollers 19 before being parted to release the filter cake and then continue separately along their paths through the press .