NZ231914A - Filtering pulp with drum type filter - Google Patents

Description

/ 2315314 Priority Date(s): Cc/iu^iuia Speclficclicn Fiic- Class: .BCI. P2»3/pfc?; .

Publication Dei?: P.O. Journal, No: Patents Form No. 5 Patents Act 1953 COMPLETE SPECIFICATION METHOD AND EQUIPMENT FOR FILTRATION OF SUSPENSION We, SUNDS DEFIBRATOR RAUMA OY, a Finnish corporation of PL 34, 28101 Pori, Finland, hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: 1 (to be followed by la) 231914 1/3 The present invention concerns a method for filtration 5 of a suspension so that the suspension to be filtered is fed from a feed part onto the filter face of a filter member which moves relative the feed part, whereby a layer of pulp is formed on the outer face of the filter member and the filtrate flows into the interior of the 10 filter member, and the pulp layer is removed from the filter member at the removing point. The invention also concerns an equipment for filtration of suspension, said equipment comprising a basin, a filter member moving in the basin, a feed part for feeding the suspension to be 15 filtered onto the outer face of the filter member, means for removing the pulp web from the filter face, as well as means for removing the filtrate out of the equipment.

For filtration or thickening and for washing of fibrous suspensions, in particular of cellulosic fibrous suspension 20 or pulp slurry, filters are used wherein the fibrous suspension is thickened either onto separate constantly circulating members, such as onto revolving drums., disks, or other filtering members, such as, e.g., onto an endless wire or equivalent.

The fibrous suspension is passed into the feed box, basin, or equivalent of the filter as diluted to a suitable consistency. The fibrous suspension is thickened by filtration by means of a difference in pressure onto a filtering face, which moves in relation to the stationary 30 feed part and carries the thickened suspension further.

At present the sealing between the moving filtering member and the edge of the stationary feed part has been solved by using dragging members or a small gap as seals.

Dragging members cause abrasion of the filtering member, 35 and a small gap is readily blocked and causes damage to the filtering member. Nor is it possible to make a gap small enough so that no leakage arises. If a resilient, dragging seal is used, a further problem is that all of the pulp that adheres to the face of the filtering member cannot be removed at the removing point, but some of it remains on the filter face as an uneven layer. Thereat, when it reaches the location of the seal during the next revolution, it raises the seal apart from the filtering member. When more pulp is sedimented on the filter face, the friction increases until the pulp layer is wedged between the seal and the filter layer and stops the movement of the filter member.

The object of the present invention is to eliminate the problem mentioned above. The present invention provides a method for filtration of a suspension in which the suspension to be filtered is fed from a feed part onto a filter face of a filter member which moves relative to the feed part, whereby a layer of pulp is formed on the outside-face side of the filter member and the filtrate flows through the filter member to the inside-face side thereof, and the pulp layer is removed from the filter member at a removing point; the filtrate passing again through the lower part of the filter member from the inside-face side to the outside-face side of the filter member into a basin, and the filter member rotating partly submerged in the filtrate in the basin, characterized in that after the pulp removing point, before a feed point, a cleaning point is provided wherein filtrate is made to flow through the filter member from the inside-face side of the filter member to the outside-face side, at a flow rate higher than the rate of the filtrate flow through the rest of the filter member from the inside-face side to the outside-face side, by removing the filtrate which passes through the filter at the cleaning point from the outside-face side at a rate greater than the rate at which filtrate passes through the filter at the cleaning point.

The present invention further provides equipment for filtration of suspension, said equipment comprising a basin, a filter member moving in the basin, partly submerged in filtrate, a feed part for feeding the suspension to be filtered onto an outside-face side of the filter member, means for removing the pulp from the filter face, and means for removing the filtrate out of the equipment, wherein between a pulp removing point and a pulp feed point, there is a cleaning point where the filtrate can be made to flow from the inside-face side of the filter member to the outside-face side, at a desired flow rate higher than the flow rate through the rest of the filter member, by providing means for removing from the outside-face side of the filter the filtrate which passes through the filter at the cleaning point at a rate greater than the rate at which the filtrate passes through the filter at the cleaning point.

The equipment may include more than one cleaning point, if desired.

Thus, according to the invention, the problem arising from the pulp that adheres to the face of the drum is eliminated by arranging a filter-member cleaning stage before the filter member arrives at the feed point during its following revolution. According to the invention, a controlled liquid flow is achieved from the space below the liquid level inside the filter member into the separation space outside the filter member, and this flow cleans the filtering face. This solution in accordance with the invention permits such an efficient cleaning that in most cases no separate cleaning of the filter face is needed.

When the invention is employed, the sealing point may be chosen freely at a suitable location.

It is also possible to fit two or more separation spaces one after the other in the direction of running of the filter member. In such a case, the solution in accordance with the invention may also be used for fractioning of the filtrates, e.g., in accordance with air content, fibre content, or concentration of chemicals. From among the separated fractions, it is possible to choose the one that is, in respect of its properties, best suitable for recirculation into the process. On the same basis, it is also possible to choose the fraction to be removed from the process.

The invention will be described in more detail in the following as applied to a washing drum, reference being made to the accompanying drawings, wherein Figure 1 illustrates a flow diagram of the connections in the method in accordance with the invention, Figure 2 is a sectional illustration of principle of the carrying into effect of the method described above in a thickening washer, and Figure 3 is a sectional view along line A-A in Fig. 2. According to Fig. 1, the fibrous suspension, e.g. pulp slurry, is passed, e.g., from a buffer tank at a consistency that is higher (e.g., 5 %) than the feed consistency of the washing filter (e.g., 3 %). The pulp slurry is diluted in the feed pipe system between the points 1 and 2 before the pulp reaches the feed box 4. From the feed box the pulp slurry is pressed by means of the difference in pressure between the inside and the outside of the drum 5 and by means of liquid pressure against the filtering wire-faced drum, whereby the filtrate enters into the drum interior. The pressure prevailing outside the top portion of the drum inside the hood is higher than the pressure in the interior of the drum. On the other hand, the pressure prevailing in the basin 6 outside the bottom portion of the drum is equal .to the pressure inside the drum. The direction of rotation of the drum 5 is clockwise in Figs. 1 and 2. A separation space 14 is provided . below the gap between the lower wall of the feed part 4 and the drum 5. The gap between the lower wall of the feed part and the drum is sealed by means of a seal 15, and the gap between the lower wall of the separation ^ 5 space 14 and the drum is sealed by means of a seal 16.

At the cleaning point 12, between the seals 15 and 16, filtrate flows from the interior of the drum into the space 14. From the space 14, filtrate is passed further through the pipe 7, which joins the pipe 7'. The filtrate ^ 10 is passed out of the space 14 rapidly enough so that the liquid level in the space 14 remains below the sealing point 16.

The filtrate present in the basin 6 always contains some extent of fibres that have passed through the filter 15 faces and that, having a weight lower than the weight of the filtrate, attempt to rise, whereby they adhere to the outer face of the drum. The filtrate that flows through the cleaning point 12 from the interior of the drum into the separation space 14 washes these fibres efficiently 20 away from the drum face right before the feed point. At the same time, any leakages that may enter from the high-pressure feed part 4 beyond the seal 15 are also washed into the space 14.

The filtrate that comes from the interior of the drum 25 5 into the basin 6 and into the space 14 is passed through the pipes 7 and 7' to the suction side of the pump 8 and further through the dilution pipe 9 to dilute the pulp that is being fed in. The filtrate free from fibres is ^ passed along a separate pipe 10 from the interior of the drum into a filtrate tank 11, from which it can also be pumped further to such a process point where a content of fibres in the filtrate is detrimental, e.g. evaporation plant, to the washing jets of the preceding washing filter, or equivalent.

The drum interior and the basin 6 are isolated from each other in a way known in prior art by means of sealing cords 13, the filtrate having access through the wire into the basin, but the fibres not into the drum.

The pulp web gathered on the drum face is removed in a way in itself known at the point 17.

The invention is not confined to the embodiment described above alone, but it may show variation in different 5 ways within the scope of the patent claims. On the circumference of the drum, it is possible to arrange several separation spaces, and the fractions passed into these can be passed further to the desired object, e.g. some fraction be recirculated to the feed box and some fraction 10 be removed completely. The separation spaces may be placed at any location whatsoever on the drum circumference between the pulp removing point and the feed point. However, their effect is most efficient if placed directly before the feed point. To increase the working rate, the 15 separation space 14 may also be divided into parts in the axial direction of the drum, e.g. into four parts. In these it is possible to open or to close areas, e.g., alternatingly or simultaneously by means of a slide or valve device. Nor does the separation space necessarily 20 have to extend across the entire width of the drum, but it may be provided at the points that are considered necessary only, e.g. at the ends of the drum.

The seals between the separation space 14 and the J drum may be arranged in different ways. The seal 15 between the feed part 4 and the separation part 14 may also be omitted completely, in which case the leakage arriving from the feed part 4 into the separation space 14 is recirculated, together with the washing flow used O for cleaning, back into the feed part.

Above, a drum filter has been described, positive pressure prevailing in the encapsulated space surrounding the drum. The invention may also be applied in connection with a filter member revolving in a basin in an open space, in which case suction is provided inside the filter 35 member by means of negative pressure. It is also suitable in connection with a circulating endless wire filter. 23ini- 7

Claims (10)

WHAT WE CLAIM IS:-

1. A method for filtration of a suspension in which the suspension to be filtered is fed from a feed part onto a filter face of a filter member which moves relative to the feed part, whereby a layer of pulp is formed on the outside-face side of the filter member and the filtrate flows through the filter member to the inside-face side thereof, and the pulp layer is removed from the filter member at a removing point; the filtrate passing again through the lower part of the filter member from the inside-face side to the outside-face side of the filter member into a basin, and the filter member rotating partly submerged in the filtrate in the basin, characterized in that after the pulp removing point, before a feed point, a cleaning point is provided wherein filtrate is made to flow through the filter member from the inside-face side of the filter member to the outside-face side, at a flow rate higher than the rate of the filtrate flow through the rest of the filter member from the inside-face side to the outside-face side, by removing the filtrate which passes through the filter at the cleaning point from the outside-face side at a rate greater than the rate at which filtrate passes through the filter at the cleaning point.

2. A method as claimed in claim 1, wherein pulp is fed onto the filter member at the point at which the face of the filter member moves upwards, characterised in that the cleaning point is arranged underneath the feed part at a point at which the face of the filter member rises upwards from the basin.

3. A method as claimed in claim 2, characterised in that in the space placed outside the cleaning point, the ^3ini4 8 liquid level is kept at a lower level than the topmost point of the cleaning point.

4- Equipment for filtration of suspension, said equipment comprising a basin, a filter member moving in the basin, partly submerged in filtrate, a feed part for feeding the suspension to be filtered onto an outside-face side of the filter member, means for removing the pulp from the filter face, and means for removing the filtrate out of the equipment, wherein between a pulp removing point and a pulp feed point, there is a cleaning point where the filtrate can be made to flow from the inside-face side of the filter member to the outside-face side, at a desired flow rate higher than the flow rate through the rest of the filter member, by providing means for removing from the outside-face side of the filter the filtrate which passes through the filter at the cleaning point at a rate greater than the rate at which the filtrate passes through the filter at the cleaning point.

5. Equipment as claimed in claim 4, characterised in that said cleaning point is placed underneath the feed part directly before the feed part.

6. Equipment as claimed in claim 4 or claim 5, characterised in that said cleaning point extends across the entire width or across part of the width of the filter member.

7. Equipment as claimed in any one of the claims 4 to 6, characterised in that said cleaning point is divided into two or more parts in the direction of width of the filter member.

8. Equipment as claimed in any one of the claims 5 to 7, characterised in that the space placed outside said cleaning point is sealed relative to the remaining space that defines the filter member fully or partly by means of seals fitted against the filter member.

9. A method for the filtration of a suspension, as claimed in claim 1 and substantially as hereinbefore described with reference to the accompanying drawings.

10. Equipment for the filtration of a suspension, substantially as hereinbefore described with reference to and as shown in the accompanying drawings. SUNDS DEFIBRATOR RAUMA OY by their authorised agents P.L. BERRY & ASSOCIATES ~r 13 A Liu IS 92