Pressure filter

Pressure filter for filtering pulp which pressure filter comprises a pulp container (1, 3) comprising a feed opening (12) of the pulp at its upper part, a cylindrical filtering surface (6) adjusted to the cylinder part of the container in which case the pulp, which has penetrated the mentioned surface, can be removed from the container through one removal aggregate or through several removal aggregates (13), (14) located at various altitudes, further the pulp container comprises a vertical rotor (8, 16, 17) which can be rotated in the pulp room of the container which rotor is equipped with wings (7) which clean the filtering surface (6) and comprises a feed pipe (11) of the dilution water for directing the dilution water into the pulp room of the container in order to avoid the thickening of the pulp and that the dilution water can be directed between the rotor and the filtering surface (6) to such height that beginning from the lower edge of the filtering surface (6) at least an essential part of the filtering surface, such as over 30% of the filtering surface (6) comes within the range of the mass which has been diluted with the dilution water. The rotor (8, 16, 17) is equipped with wings (9) in that part of the pulp room to which the dilution water is directed in which case the mentioned wings (9) are adjusted together with the gaps (10) to achieve an internal, vertical pulp cycle at the lower part of the pulp room when the rotor is rotating in which case the mentioned part of over 30% of the filtering surface (6) is within the range of the mentioned pulp cycle.

Invention relates to a pressure filter for filtering pulp which pressure filter comprises a pulp container comprising a feed opening of the pulp at its upper part, a cylindrical filtering surface adjusted to the cylinder part of the container in which case the pulp which has penetrated the mentioned surface are removed from the container through one removal aggregate or through several removal aggregates located at various altitudes, further the pulp container comprises a vertical rotor which can be rotated in the pulp room of the container which rotor is equipped with wings which clean the filtering surface and comprises a feed pipe of the dilution water along which the dilution water is directed to the pulp room of the container in order to avoid the thickening of the pulp.

With these pressure filters pulp which is needed for manufacturing paper and board is purified and their implementation area is very large and according to each implementation the structure and form of the filter, rotor and filtering surface are changed. The pulp which is to be purified in the filter is divided into an accept and a reject. The accepted reject continues forward in the process and the rejected will be retreated so that it can be usually purified more, grounded or it will be totally removed from the process. Sometimes the pressure filter is used to fractionate fibres, too, in which case the fibres of the accept are on average clearly shorter or even thinner and more elastic than the fibres of the reject.

When the pulp is being purified/fractionated, the thickness of the pulp tends to increase during the filtering because the water part of the pulp fraction has an easier access through the filtering surface than the fibres have. One has tried to decrease this thickening phenomenon with the design of the rotor, especially with the design of the purifying wings so that when they rotate, they suck water back to the inside of the filtering surface. This kind of action causes harm especially when one tries to fractionate the pulp because then also short and elastic fibres come back along the water and this phenomenon lessens the efficiency of the fractionating.

One has tried to solve the thickening phenomenon of the pressure filter at the filtering surface with the solutions according to the patent specifications U.S. Pat. No. 6,679,384 B1 and SE-526033. In both solutions the dilution water is brought with the help of special dilution rings built in the cylinder. In the solution according to the U.S. Pat. No. 6,679,384 B1 the parts 33 and 34 function as rings and in the solution according to the SE-526033 it is the ring 28. Of these in the solution according to the U.S. Pat. No. 6,679,384 the accepted pulp is taken out of three different pipes 60, 61 and 62, whereas in the solution according to the SE-526033 there is only one pipe fitting of the accepted pulp and a pipe 18. Also in the Finnish patent application FI 961569 three pipes9a-cof the accepted pulp are shown. In the solution according to the publication U.S. Pat. No. 6,679,384 there are dilution water canals at the location of the pipes 50 and in the FI-961569 solution there are ring-shaped dilution water canals12aand12bin the casing of the filter at the location of the pipes13aand13b.

In these above mentioned publications there are opposite gaps in both the dilution water rings and in the thrust ring of the cylinders from which gaps the dilution water streams inside the cylinder. In both solutions there are three successive cylinders. In the U.S. Pat. No. 6,679,384 solution X, Y and Z from below to upwards and in the FI-solution7a,7band7cfrom above to downwards. In the solution according to SE-526033 the sorting surface is divided into two pieces at the location of the dilution ring 48. The above mentioned solutions are expensive to build regarding both the frame of the filter and the cylinder. Also the impact of the dilution water fades fairly soon and it is directed to a fairly small pulp part immediately next to the filter surface.

With the pressure filter according to the invention the above mentioned problems can be solved and a considerable improvement regarding the efficient utilization of the dilution water can be achieved. It is characteristic of the invention that the dilution water is directed between the rotor and the filtering surface to such height that beginning from the lower edge of the sorting surface at least an essential part of the filtering surface, such as over 30% of the filtering surface comes within the range of the pulp which has been diluted with the dilution water when the rotor is rotating.

The advantage of the invention is the fact that the dilution water rings can be abandoned. Although the dilution water is directed along only one pipe to one location between the rotor and the filtering surface, the mixing of the dilution water to the whole pulp surrounding the rotor occurs nevertheless in a balanced way. Also the internal pulp cycle makes sure that the dilution water circulates partly the same cycle, too, in which case it has time to give a dilution effect to the pulp before it penetrates the filtering surface.

In the following the invention is described more detailed by referring to the accompanying drawing,FIG. 1. The pressure filter of theFIG. 1has parts that can be seen from outside, such as the lid 1 of the container, its frame part 3 which is a cylindrical casing or chamber, a control unit 4, feet 5 and aggregates 11-15. The pressure filter is equipped with a cylindrical cylinder 6 which functions as the filtering surface and in the middle of which cylinder there is a rotating rotor 8, 16, 17. Wings 7 that clean the filtering surface 6 are attached to the rotor by arms and there are at least 2 wings, the bigger filters have even 10 wings. In theFIG. 1they are uniform bars which have the height of the filtering surface 6.

There is a feed aggregate 12 of the pulp to be purified at the upper part of the filter, a removal aggregate 13 of the purified pulp and also another removal aggregate 14 located farther down for the accepted pulp. A ring-shaped collection room of the accepted pulp outside the filtering surface 6 is divided into two parts with the help of a flange 18 by using a supporting ring. The pulp reject is removed from the bottom of the container from the aggregate 15.

Enough space is left between the supporting structure17of the bearing of the rotor and the cylindrical lower part or wall16of the rotor so that the dilution water pipe11can be extended and can be turned till this space. There are gaps10at the upper part of the cylindrical part16of the rotor from which gaps the pulp together with the dilution water can stream out. There are pumping wings9that rotate the pulp at the inside and at the upper part of this cylindrical part16which pumping wings are sized in such a way that they circulate clearly more mass than what the stream of the dilution water is out of the pipe11. A cycle for the pulp is created in which cycle the dilution water becomes well mixed with it and the same mass may bypass the filter surface6several times. This kind of pulp cycle occurring in a reject mode is not known in the above mentioned, known solutions.

With this arrangement the dilution water is moved to rise in the pulp room to the height of the flange18. The dilution water dilutes the sorting surface6beginning from the flange18at least downwards, in other words dilutes the pulp coming from the aggregate14of the accepted pulp and dilutes the pulp coming from the aggregate15of the rejected pulp. In this case approximately 50% of the filter surface6is impacted by the dilution water, in other words the surface downwards from the flange18. It is advantageous for the invention that of the filter surface6at least 30% is impacted by the dilution water. The solution can be modified only to filters in which the rotor rotates inside the cylindrical sorting surface. The accepted pulp is taken out of the aggregates13and14. Only one cylinder is shown in theFIG. 1to which cylinder one has had to install a flange18and a supporting ring19. If in the pressure filter there is only one exit pipe of the accepted pulp, then neither the flange18nor the part19is needed.