The present invention relates to method for filtering slurry or similar material in a continuously operating pressure filter, said filter being formed by a rotating vessel, in which method the slurry and the pressurizing gas are routed to the filter, and further in which method the filtrate and the solids matter accumulating on the vessel surface are thereafter removed from the filter. The invention also concerns an apparatus suitable for the implementation of the method.

Filters are used in the process industries for separating liquid from solids. Filters are categorized in two major types: batch filters and continuous filters.

In typical batch filters the slurry to be filtered is pumped through filter elements typically comprised of filter plates, cartridges or similar members. In these elements the pressurized slurry passes through the filter medium, whereby the solids contained in the slurry remains on the surface of the medium thus forming the filter cake, while the liquid passes through the filter element to the outside of the filter. In many filter types the cake formed in the filter can additionally be processed further, e.g., by pressing as well as by blowing compressed air through the cake in order to reduce its moisture content. Prior to the next batch, the filter is opened and the filter cakes are removed from the filter by manual or automatic means.

The duration of the batch process in such filters typically is in the order of 5...15 minutes, in some cases up to hours. In this kind of a filter, the slurry is processed under pressure (typically at 5...15 bar gauge pressure) above the barometric pressure, whereby

the formation of the filter cake and its further processing are speeded.

If the further processing of the filter cake takes place as a continuous process immediately after filtration, the filter must be followed by, e.g., a buffer silo in which the cyclic outflow of solids dumped into the silo is converted into a steady solids mass outflow.

Continuous filters are generally constructed as drum or disc filters in which the surface of a rotating drum or disc covered by the filter medium, conventionally a textile fabric, is brought into contact with the slurry. The inside of the drum or disc is subjected to vacuum, which causes the liquid contained in the slurry to pass through the fabric to the outlet manifold of the filtrate, while the solids remain on the fabric surface. When the filter cake travels away from the slurry along with the rotation of the drum or disc, the vacuum also forces air to flow through the cake, whereby the filter cake is dried. The filter cake is removed from the surface of the drum or disc before it can again make contact with the slurry.

Filters employing merely vacuum to improve filtration are hampered by the fact that the highest theoretical pres¬ sure differential across the two sides of the fabric and the cake that thus directly affects the filtration speed can maximally amount to the magnitude of the barometric pressure. In practice the pressure differential almost invariably remains below 0.9 bar.

To eliminate this drawback, several constructions have been developed in the art based on adapting a convention- al disc or drum filter into a pressurized vessel. The pressurized vessel is typically brought to a gauge pressure of 1...6 bar, whereby the formation and drying of the filter cake is improved. The filter cakes are

removed from within the pressurized vessel via an air lock constructed using, e.g., a gated feeder inlet or two sequentially operating hose valves, thus avoiding the escape of the inside pressure from the vessel.

Drawbacks of the above described implementation include its complicated construction and difficult maintenance.

Disc and drum filters must generally have an efficient mixing of the slurry, because the solids particles of the slurry tend to settle on the bottom of the slurry basin, while the slurry in the upper layer close to the filter fabric has a tendency to become clarified, whereby the filtration efficiency is reduced.

It is an object of the present invention to achieve a continuous drum-type pressurized filter in which the above described disadvantages are eliminated. The characterizing properties of the filter according to the invention are disclosed in the subsequent claims.

The filter according to the invention is comprised of a slowly rotating vessel, whose inside surface is covered by filter elements. Such a filter element can be formed by, e.g., a fabric-coated screen in which the screen facilitates the flow of the filtrate between the fabric and the wall of the pressurized vessel. Thence, the continuous pressurized filter obviates the use of a pressurized vessel necessary in a continuous pressurized filter of the prior-art technology, but rather, the rotating vessel according to the present invention itself acts as the pressurized vessel.

The inside surface of the pressurized vessel is divided by intermediate radial walls aligned parallel to the center axis of the pressurized vessel and typically having the height of the screen into zones which allow

the flow of the filtrate to take place only within the confined space of each zone.

Each of the above mentioned zones has a separate outlet pipe via which the filtrate can be discharged to the outside of the pressurized vessel. Each outlet pipe ends at a commutating valve, whereby the pipe remains closed over a certain angle of rotation to avoid the escape of overpressure from the vessel.

During the operation of the filter, the pressurized vessel which rotates about its longitudinal center axis is filled with the slurry to be filtered up to a height which is, e.g., 1/10 of the vessel diameter. The overpressure inside the vessel forces the slurry against the fabric thus forming the filter cake.

The formed filter cake rises during the rotation of the vessel up from the slurry, whereby the pressurized gas inside the vessel can pass through the filter cake thus drying the cake.

When said filter cake during the continued rotation reaches the point of cake removal that typically is situated at the highest point of the horizontal vessel or immediately after that point in the direction of rota¬ tion, however, prior to the reentry of the filter cake into the slurry, the commutating valve switches com¬ pressed gas onto the filter element being discussed at a gauge pressure which is slightly higher than that of the gas atmosphere prevailing in the vessel. Then, the cake formed onto said filter element is forced to fall onto a cake discharge conveyor. The cake discharge conveyor can be, e.g., a belt or screw conveyor, which transfers the formed filter cakes to a place wherefrom they are discharged to the outside of the conveyor.

The commuting valve shuts off the filtrate pipe of the filter element that has passed the filter cake removal point, thus preventing the pressure prevailing in the vessel from readily escaping to the outside of the vessel.

The commutating valve reopens the filtrate pipe as soon as the filter element being discussed again lands under the slurry surface, thus initiating the next filtration cycle.

In this manner the filter implementation according to the present invention achieves a continuous filter, in which operating pressure levels in the order of multiple barometric pressure are possible. Thereby, the formation, drying and other processing of the filter cake are improved manyfold with respect to continuous vacuum filters.

In comparison with a vacuum filter enclosed in a pressure vessel, the implementation according to the present invention offers a vastly simplified construction, since the vessel and the filter in the proposed implementation are integrated, thus disposing of need for the "conventional" disc or drum filter. Neither do cake pieces dripping off from the cake conveyor cause problems, because they fall into the slurry below. The construction according to the invention also manages without the slurry mixer. By contrast, the filtering process is improved by the omitted mixing, since solids particles in the slurry fed into the filter tend to settle onto the bottom of the slurry layer, thus bringing the thickest slurry close to the surface of the filter element and thereby improving cake formation.

The invention is next examined in greater detail with the help of an exemplifying embodiment by making reference to attached drawings, in which

Figure 1 shows the cross section of a filter according to the invention,

Figure 2 shows along the plane I-I the cross section of the filter illustrated in Fig. 1,

Figure 3 shows detail A of Fig. 2,

Figure 4 shows the phases of filtering process in a filter according to the invention.

According to Figs. 1...3, the principal element of the pressurized filter according to the invention is a horizontal rotatable pressure vessel (1) . Its rotation can be arranged by means of a set of rolls (2) as shown in Figs. 1 and 2, or alternatively, the pressure vessel can be rotatable in bearings adapted to its central axis. The inner surface of the vessel is divided into filter elements (3) , which are longitudinally aligned parallel to the center axis of rotation, said filter elements typically being comprised of a screen (4) and a filter fabric (5) coating it. The screen (4) can be comprised of, e.g. , a perforated plate (6) which makes the filtrate flow possible through the fabric and support pegs (7) which make the filtrate flow possible in a direction parallel to the wall of the pressure vessel (1) .

The filter elements are separated from each other by intermediate walls (8) aligned parallel to the center axis of rotation, said walls permitting the filtrate flow only within the space of each filter element.

Each filter element (3) is connected via a filtrate pipe (9) to a commutating valve (10) which closes and opens the filtrate pipe (9) of each filter element (3) at a desired point.

The inside of the rotating pressure vessel (1) encloses a conveyor (11), e.g., a belt conveyor shown in the diagram, which is stationary with respect to its rotating environment and transfers the formed filter cake to the discharge point of the conveyor. The conveyor (11) can be fixed stationary at its one end to a support bearing (12), while its other end is fixed stationary to a discharge channel (13) to which also the pressure vessel (1) is connected by means of a seal (14) .

The discharge channel has an air lock (15) which facili¬ tates the discharge of the filter cakes but simultaneous¬ ly prevents the overpressure prevailing in the pressurized vessel (1) from escaping to the outside of the vessel. The air lock (15) can be comprised of, e.g. , a gated feed channel, or alternatively, as shown in Fig. 1, of two valves (16) which are opened and closed sequentially.

The slurry to be filtered is fed into the filter via a slurry inlet feed pipe (17) , while the pressurizing gas which conventionally is air is routed via a pipe (18) .

According to Fig. 4, the function of the filter can take place as described in the following example:

During the operation of the filter, the rotating pressur¬ ized vessel (1) is kept at an overpressure which typi¬ cally is 1...6 bar gauge pressure maintained by routing compressed air to the filter via the pipe (18) . The slurry (19) to be filtered is routed as a continuous flow to the inside of the filter via the slurry inlet feed pipe (17).

The commutating valve (10) allows the filtrate outlet pipes of sector A to stay open. Then, the slurry (19) settled to the bottom of the vessel (1) begins to filtrate, that is, the liquid contained in the slurry is

conveyed by gravity and the overpressure prevailing in the vessel via a filter fabric (5) to the filtrate outlet pipe (9) and therefrom further via the commutating valve (10) to the outside of the filter into a filtrate discharge pipe (20) simultaneously as the solids contained in the slurry is compacted onto the surface of the filter fabric (5) thus forming the filter cake (21) .

When the filter cake (21) formed in the above described process is conveyed away from the slurry with the rotation of the vessel (1) , the compressed air contained in the vessel can pass through the cake via the filtrate outlet pipes (9) and the commutating valve (10) to the outside of the vessel (1) simultaneously bringing about the drying of the formed filter cake (21) . The commu¬ tating valve (10) closes the filter outlet pipes (9) as they approach the end of sector A in the direction of the vessel's rotation.

When the filter cake (21) formed onto the filter element (3) has reached sector B, a short-duration impulse of compressed air is blown at a pressure level slightly higher than that prevailing in the vessel (1) against the filter element via a separating air pipe (22) , the commutating valve (10) and the filtrate outlet pipe (9) . Then, the compressed air impulse separates the filter cake (21) formed onto said filter element (3) allowing the cake to fall onto the conveyor (11) . The sides of the conveyor (11) can be provided with, e.g., guide walls (22) that prevent the filter cakes (21) from falling further back into the slurry (19) .

When the filter element (3) which is thus cleaned from the filter cake (21) progresses into sector C, the commu- tating valve (10) closes its filter outlet pipe (9) , because otherwise the overpressure prevailing in the vessel (1) might readily escape through the plain filter fabric (5) to the outside of the filter.

As the filter element reaches the slurry surface with the continued rotation of the vessel (1) , the commutating valve (10) again opens the filtrate outlet pipe (9) , thus restarting the filtering cycle for said filter element.

For those versed in the art it is evident that the diffe¬ rent implementations of the invention are not limited by the exemplifying embodiments described above, but instead, it can be varied within the claims of the invention. For example, the feed of compressed air which in the above described example took place via the pipe (18) , can be arranged to occur entirely or partially within sector C via the commutating valve (10) and the filtrate outlet pipes (9), whereby the air flowing into the pressurized vessel (1) through the filter fabric (5) simultaneously cleans the filter fabric from solids particles adhering to it. Furthermore, the separation point of the filter cakes (sector C) need not necessarily be located to the highest point of the pressurized vessel, but instead, it can be transferred closer to the slurry surface in the direction of the vessel's rotation, thereby increasing the drying area of the filter cake. The filter described in the example above can further be complemented with filter fabric wash in sector C, whereby the wash is implemented by means of high-pressure water jets.