The present invention relates to a method and apparatus for the filtering of the unfiltered residual content in a filtering apparatus for liquids, more in particular a discontinuous filtering apparatus, comprising a filter vessel with one or more filter elements, more in particular pressure candle filter elements or pressure leaf filter elements, whereby air or another gas under pressure is fed into the filter vessel.

After a completed filtration an unfiltered residual content remains left in the filter vessel, which is a disadvantage more in particular in the case of batch-processes.

It is known to feed air or another gas under pressure at the top into the filter vessel at the end of the filtration, in order to filter as much as possible of the liquid in the filter vessel through the filter elements.

This method has the drawback, that after a period of time an ever increasing part of the filtering surface of the filter elements is no longer in contact with the liquid.

Because of this the filtration surface becomes smaller and smaller, while an ever increasing part of the filtering surface does not come into contact with the liquid, but is confronted with the gas under pressure. This gas under pressure locally removes for the greater part the liquid in the pores between the filtered parts. This has as a result, that a large quantity of the gas at these places passes through the filtering surface, which results in a large non-functional consumptive use of gas. This gas consumption, with an increasing exposed filtering surface, can become so substantial, that the gas, being a compressible medium, causes a build-up of pressure behind the filtering surface, whereby the driving force, which consists in the pressure difference outside and within the filter element, becomes ever smaller for the residual filtration.

With this method it can at the most be achieved that the residual volume is reduced to the point where the liquid level reaches the bottom side of the filter elements. In practice one usually stops the filtration much earlier because of a non-economical gas consumption and because it takes too long to reach this minimum level as a

result of the available filtration surface becoming ever smaller and the decreasing driving force.

The invention aims to obviate this drawback of the known method and apparatus.

The method according to the invention thereto is characterized in that the air or the gas is fed into the filter vessel at the lower part thereof.

According to a characteristic of the method according to the invention the air or the gas is fed into the filter vessel in the space below the filter elements.

According to a further characteristic of the method according to the invention, the air or the gas is fed into the filter vessel at the bottom of the filter vessel.

According to yet another characteristic of the method according to the invention the air or gas is fed into the filter vessel from out of several places.

With the application of the method according to the invention the gas is not fed into the vessel in the upper part thereof, as is the case with the known method, but the air or gas is fed into the filter vessel at the bottom thereof.

By this measure it is achieved, due to the volume of the rising gas bubbles, that the level of the surface of the liquid, consisting of the residual content in the filter vessel, constantly is substantially higher, because of which, when compared with the known method, constantly more filtering surface is exposed to the residual liquid and less to the gas itself, whereby substantially less liquid is removed out of the pores, which results in a lower consumption of gas and whereby also a higher effective pressure difference is maintained. When the gas bubbles reach the surface of the residual liquid, they will take along a part of the residual liquid, whereby the filtering surface above the level of the liquid remains"wetter", which results in a lower flow through of the gas.

Further, the rising gas bubbles take along particles of solid matter from the bottom of the vessel. These will more in particular be the smallest particles; by this and by the concentration reduction the remaining residual volume can be filtered much better than the original residual volume.

With the invented method it can be achieved that the remaining residual content is less than the volume below the bottom side of the filter elements.

The apparatus for carrying out the method according to the invention is formed by a filter apparatus with a filter vessel that is provided with one or more filter elements, with a feed line for the supply into the filter vessel of air or another gas under a given pressure, and is characterized in that the feed line opens into the lower part of the filter vessel.

With an advantageous embodiment of the apparatus according to the invention the feed line opens into the filter vessel into a space below the filter elements.

With a further embodiment of the apparatus according to the invention the feed line opens into the filter vessel at the bottom of the filter vessel.

The invention will now be described further with reference to the drawing of an example of an embodiment.

Figure 1 shows schematic in cross section a side view of a filter apparatus according to the invention.

Figure 2 shows a detail of a side view of an embodiment of the filter apparatus according to the invention.

Figure 3 show a view corresponding to figure 2 of an another embodiment of the filter apparatus according to the invention.

As can be seen in figure 1 the filter apparatus 1 comprises a filter vessel 2 with one or more filter elements 3 and a valve 6. Further there are present (not shown in the drawing) a feed line and a manifold for the discharge of the filtrate. In the filter vessel 2 an unfiltered liquid content 7 is present. In figure 1 further is shown a feed line 8 for the supply into the filter vessel 2 of air or another gas under a pressure.

The feed line 8 opens into the lower part of the filter vessel 2 and the air or the gas is fed into the filter vessel 2 at the lower part. By the volume of the rising gas bubbles 9 the level of the liquid surface of the liquid content 7 as well as, at the end of the

filtration, the level of the residual volume 7a, become substantially higher, while the gas bubbles 9 take along a part of the residual content 7a to the filtering surface 10 of the filter elements.

With an advantageous embodiment of the apparatus according to the invention in the filter vessel below the filter elements horizontally a plate or disc is provided, which plate completely or in part has been provided with openings or perforations (for instance of a porous material consisting of metal, plastic or ceramic), while the feed line opens into the space underneath the plate, and by means of which perforations an equal spreading of the gas bubbles in the lower part of the filter vessel can be obtained.

In figure 2 an embodiment is shown, whereby the valve 6 has the shape of a double- walled plate or disc 4, the bottom side 5 of which is closed and the upper side 11 of which is provided with openings or perforations 12, while the feed line 8 opens into the inner space 13 of the double-walled plate 4. Air or another gas under pressure is fed through the feed line 8 into the inner space 13, whereby the air passes through the perforations 12 and forms air bubbles in the filter vessel 2. The perforations 12 bring about an equal spreading of the gas bubbles in the lower part of the filter vessel 2.

In figure 3 yet another embodiment is shown, whereby in the filter vessel 2 below the filter elements a double-walled ring 14 is provided, which, completely or in part, is provided with perforations or openings 15 (for instance of a porous material consisting of metal, plastic or ceramic), while the feed line 8 opens into the inner space 16 of the ring 14, and by which an equal spreading of the gas bubbles 9 in the lower part of the filter vessel can be achieved.

It is also possible to combine both systems.

The plate as well as the ring can also be used for the application thereon of a filter medium and to filter the last residual unfiltered liquid in the filter vessel, whereby the air/gas supply or otherwise a separate connection can serve as a filtrate discharge.