Rotating disc liquid filtering means

The present invention relates to a rotating filter with a number of filter discs arranged on a rotor. More specifically, the filter is designed so that the liquid mixture to be filtered is supplied through a hollow shaft of the rotor to the inside of the filter discs, each filter disc comprising two side walls with filter means and a wall arranged around the periphery, preferably also provided with filtering means.

During the filtration, the liquid passes through the filter means outwards from the inside, whereby a filter layer composed of solid particles, a coarse phase, is formed on the inside of the filter means. The filtration takes place by means of a hydrostatic overpressure, which is achieved because the liquid level of the liquid mixture inside the rotor is arranged higher than the liquid level of the filtrate outside the filter discs. A disc filter of this kind is described e.g. in U.S. Patent No. US, A, 3163601.

The problem with filters having a direction of flow from the inside to the outside through the filter means is that a large part of the filter layer, when it has reached over the pool surface of unfiltered liquid mixture, falls off and back into the pool during the rotation. This creates a higher concentration in the liquid mixture that enters and thereby affects the technical performance of the filter in a negative direction, ie. its capacity in various aspects, such as the hydraulic capacity, the amount of solid particles separated and / or the purity of the filtrate.

The object of the present invention is to increase the capacity of the filter. This results in that, at a given incoming flow, a smaller filter area and thereby smaller filter units are required.

In accordance with the present invention, carrier elements for the filtered coarse phase are provided in the inner space of the filter discs as well as blocking means for preventing this coarse phase from falling back into the basin of incoming liquid mixture. These blocking means will hereinafter be called barrier elements. By preventing coarse phase from being filtered once more and thereby occupying the filter area, the capacity of the filter is increased.

A filter in accordance with the present invention is well adapted for purification of wastewater, raw water, so-called backwater in the paper industry and for fiber recycling in the paper52 264 industry.

The invention, having the features which appear from the appended claims, will be described in more detail in the following with reference to the accompanying drawings, in which

Fig. 1 shows a cross section (II in Fig. 2) of a filter,

Fig. 2 shows a longitudinal section (II-II in Fig. 1),

Fig. 3 shows the invention in a sector of a filter disc,

Fig. 4 shows an alternative embodiment of the invention,

Fig. 5 shows, in a cross-sectional view, a side surface of a filter disc (VV in Fig. 6),

Fig. 6 shows, on an enlarged scale, a section (VI-VI in Fig. 5) of the side surface,

Fig. 7 shows an alternative embodiment of the invention,

Fig. 8 shows a modification of the embodiment in Fig. 7.

Figures 1 and 2 show a basic embodiment of a disc filter. This comprises a filter rotor 1 placed in a filter container 2 for the filtrate. A hollow shaft 3, on which filter discs 4 with filter means are arranged, is rotated in the container by means of a drive means 5. The filter means in their simplest embodiment consist of a filter cloth attached to a supporting sublayer so that an inner space 6 is formed. The filter cloth encloses both sides of the filter discs and preferably also the outer periphery 7. The interior of the filter discs communicates with the hollow shaft 3. The liquid mixture to be filtered is introduced into the hollow shaft through the inlet 8 and thereby also to the inner space of the filter discs. not shown, ensures that a substantially constant level is maintained in the shaft and inside the filter discs. An advantageous arrangement for this purpose can be an automatic speed control so that, when the level tends to rise, the speed is increased and vice versa. The filtrate is removed through an outlet 9. The difference between the inlet and outlet levels is a measure of the effective hydrostatic pressure. According to the invention, the filtered coarse phase is to be transferred to a collecting vessel 10 inside the hollow shaft. The bottom of this stationary collecting vessel is inclined towards an outlet 11 for the coarse phase. Such coarse phase, which during the rotation of the filter means has not yet been released and fallen off from the filter cloth, as it approaches an upper position, is washed away by means of spray nozzles 12.

520 264 arranged on the outside. Part of the sprayed water thereby passes through the filter cloth and cleans it, whereby the coarse phase falls into the collecting vessel 10.

Fig. 3 shows in cross section a quartz sector of a filter disc 4 in an embodiment according to the invention. The space on the inside of the filter discs has been provided with plates, here called carrier elements 13, starting from the hollow shaft 3. They extend towards the periphery and are arranged between the sides of the filter discs in the axial direction. The extent is such that a gap 14, an opening, is left towards the periphery. The purpose of this opening is that during the movement of the carrier elements in the liquid mixture, the liquid mixture should be able to flow back opposite the rotation. Another function made possible by these gaps is the rapid removal of air from the cells formed by the carrier elements, as they rotate back down into the liquid pool, as well as the rapid supply of air below the carrier elements, as they move up over the pool surface. The carrier elements are oriented in such a way that, during the passage of the liquid level, they occupy a substantially horizontal position up and out of the bath.

A slight slope downwards towards the gap 14 facilitates a reflux of the liquid mixture to the pool during the passage of the liquid surface, but this slope must not be so large that any coarse phase that has fallen down also slides down into the pool. The level of the liquid mixture should suitably be kept higher than the center of rotation in order to be able to achieve an effective filter surface which is as large as possible. The carrier elements 13 will thus be so oriented that the inner end thereof which abuts against the shaft during the rotation will lead in relation to the outer end.

In a preferred embodiment according to the invention, between each pair of carrier elements 13, one or more shorter, outer carrier elements 15 are arranged close to the periphery. As a result, the number of larger carrier elements 13 can be kept low.

Furthermore, these outer carrier elements 15 are suitably arranged in such a way that they occupy a substantially horizontal position with a slight inclination outwards at the passage up from the liquid basin. At the periphery there should be a gap 16 also for these outer carrier elements for the outflow of unfiltered liquid mixture and an air outlet, respectively.

According to the present invention, it should not be possible

520 264 for coarse phase 22, which has released the filter means, to be able to fall back into the liquid mixture. This risk exists from the time the blade has come out of the liquid basin until it becomes possible for the coarse phase to slide down into the collecting vessel 10. For this purpose, blocking means are provided, herein called barrier elements 17, so that the coarse phase cannot slide off the blades before rotation. reached the edge of the collecting vessel 10.

These barrier elements may be arranged on the innermost part of the blades and extend forward in the direction of rotation, i.e. they may comprise an upwardly directed bend seen on the side where the filter disc has an upward direction of movement, in the direction of rotation at or through the inner ends of the blades. The angle of the upwardly directed bend must be so large that the upwardly bent part, the barrier element, during rotation rotates a horizontal position only after the passage of the edge of the collecting vessel, whereupon the coarse phase must be able to slide down into the vessel. This upwardly directed bend of approximately the same size as described above can suitably also be found on the outer carrier elements at the periphery.

As can be seen from Fig. 4, the barrier element for the coarse phase 22 may alternatively be a plate 18, suitably attached to the collecting vessel 10 or constitute one of the walls of the vessel. The plate extends in the direction of rotation in a circular arc from the basin surface of the liquid mixture up to the edge of the collecting vessel along all the filter discs. In contrast to the barrier elements of the carrier elements described above, this plate is a stationary barrier element 18. This barrier element must be placed so that the inner edge of the carrier element sweeps tightly along the barrier element. As an additional certainty against any coarse phase entering the liquid mixture through the gap between the stationary barrier element 18 and the carrier elements, according to this embodiment the carrier elements can also be formed with the barrier elements 17 described above.

As can be seen from Figures 5 and 6, in certain applications carrier elements 19 may be arranged on the sides of the filter discs, hereinafter referred to as side carrier elements. These are advantageously given a radial extent corresponding to the extent of the filter member in the radial direction and can e.g. be flat bars or a grid. These side plates are dimensioned so that they at least transport coarse phase 22 which has fallen down from the sides

520 264 to such a level that the subsequent inner carrier elements (13 in Fig. 3) with their barrier elements on rotation have time to reach over the basin surface of the liquid mixture. The barrier elements 17 thus cause the coarse phase, which upon rotation after this position falls on the blades 13, to be transported up to the collecting vessel 10. Advantageously, these side carrier elements 19 can be arranged to function as support elements for the filter means, usually the filter cloth.

By creating the barrier elements, a greater flow of liquid mixture is achieved through the gap 14 (Fig. 3) between the carrier element and the periphery compared to what would be the case without the barrier elements. It has been found that this causes some disturbance at higher rotational speeds. Liquid mixture flowing back through the gap 14 has loosened the filter layer, the coarse phase, over a certain distance. This then reduces both the filter capacity with respect to the amount of coarse phase as well as the purity of the filtrate.

This problem is solved as shown in Figures 7 and 8, with a screen element 20 radially outside the driver element 13 towards the periphery. This screen is to protect the coarse phase layer 22 outside the carrier element from the liquid stream by guiding it from the periphery. The screen 20 may be given a straight shape as shown in Fig. 7 or it may be curved. An advantageous design is an angled plate 21 (Fig. 8), which provides better strength and prevents coarse phase from passing the gap. This design thus reduces the need or number of external carrier elements.

It has been found that the combination of barrier elements and screen elements outside the inner driver element 13 has made it possible to run the filters at a considerably higher rotational speed. This means that the filter units for a certain incoming flow of liquid mixture can be significantly reduced in size.

A number of modifications within the scope of the invention may be made. In some applications, from an economic point of view, it may be advantageous to eliminate the filter means extending around the periphery of the filter discs. In these cases, neither the outer carrier elements 15 nor the shielding elements 20, 21 are required.