METHOD AND APPARATUS FOR BLEACHING PULP

Abstract: The present invention relates to a method of and an apparatus for bleaching pulp. The method and apparatus in accordance with the invention are especially suitable for bleaching pulp with ozone at medium consistency, in other words when the consistency is 5 - 25 %, preferably 10 - 15 %. It is characteristic of the invention that ozone is mixed into the pulp to be bleached by means of two fluidizing mixers (14 and 16) and that the foamy suspension of gas and pulp thus obtained is discharged to a reaction vessel (30), in which the suspension is mixed, for example, by a paddle mixer.

Description: METHOD AND APPARATUS FOR BLEACHING PULP Field of invention The present invention relates to a method of and an apparatus for bleaching pulp. The method and the apparatus of the invention are particularly meant to be used for bleaching pulp with ozone at medium consistency, i.e. at the consistency of 5 - 25 %, preferably 10 - 15 %. The method of the invention is particularly well applicable in prosesses where the volume of the gas used for the bleaching is larger than in conven¬ tional ozone bleaching. Background art Many prior art publications on bleachinc of medium consistency pulp are already known, the first of which is European patent application no. 397308 of A. Ahlstrom Corporation. The invention has resulted in several mill-scale applications, already. Said EP application describes for the first time in detail how ozone bleaching can be effected at the medium consistency range in a way acceptable in industrial mill-scale operation. According to the publication, pressurized gas, consisting mainly of carrier gas and ozone, is mixed in a fluidixing mixer with pulp so as to produce a foamy suspension of liquid, gas and fibers. The suspension is transported from the mixer to a reaction vessel which may be a larger vessel or, for example, a portion of the flow channel subsequent to the mixer. After the ozone reaction, residual gas is separated from the suspension, said gas consisting mainly, as is known, of the carrier gas mixed with the unreacted ozone. If it is desirable to introduce a larger amount of ozone into the pulp, it is possible according to the publication to employ two or more fluidizing ozone mixers, for example, by introducing the pulp immediately after the first ozone reactor and the gas separation to a second fluidizing mixer in which another dose of pressurized mixture of ozone and carrier gas is mixed into the pulp and from which the pulp is directed further to a second reactor, etc. SUBSTITUTE SHEET Also WO publication no. 93/00470 is known which discloses a bleaching method slightly different from the method described above. Also according to this publication, the mixture of ozone and carrier gas is mixed in the pulp by a fluidizing mixer and the mixture of gas and pulp is introduced under pressure into a reaction vessel. In the upper section of the reaction vessel, gas is separated from the pulp and additional chemi¬ cals, such as sodium hydroxide, hydrogen peroxide or chlorine dioxide, are added to the pulp. After this, the pulp flow is introduced to a second reaction vessel in which the pressure is maintained at a clearly lower level than in the first vessel. Gas is separated from the pulp also in the upper section of the second vessel from which it is discharged to a separate further treatment or for other use. Even though the apparatus described above already function in mill-scale use they still have a few drawbacks, for example if it is desirable to improve the bleaching efficiency without increasing the size of the reaction vessel. For example, the method of the EP publication mentioned above requires a separate reaction vessel and a separate gas separation apparatus for each ozone mixing stage, etc. Disclosure of invention The object of the present invention is to provide a solution which reduces the equipment requirement and improves the bleaching efficiency of a pulp mill compared to conventional methods. The method of the invention allows the use of a larger gas dose, allowing thus either the use of larger ozone volumes or the use of weaker ozone mixtures in the bleaching. Brief description of drawings The method and the apparatus of the invention will be described more in detail, by way of example, with reference to the accompanying drawings of which SUBSTITUTE Sr.ΞET Fig. 1 illustrates a preferred embodiment of the apparatus according to the invention ; Fig. 2 illustrates a second preferred embodiment of the apparatus according to the invention; Fig. 3 illustrates a third preferred embodiment of the apparatus according to the invention;* Fig. 4 illustrates a fourth preferred embodiment of the apparatus according to the invention; Fig. 5 illustrates a fifth preferred embodiment of the apparatus according to the invention; and Fig. 6 illustrates a sixth preferred embodiment of the apparatus according to the invention. Detailed description of preferred embodiments The apparatus according to Fig. 1 substantially comprises a drop leg or a corresponding means 10 supplying pulp at medium consistency from a preceding treatment stage; a pump 12, preferably a fluidizing centrifugal pump pumping the pulp; two fluidizing mixers 14 and 16; a reaction vessel 18; a gas separator 20; and . conduit 22 in the mixer 14 for adding a mixture of gas and carrier gas; a conduit 24 for separated gas; and a conduit 26 for treated pulp discharged from the appara¬ tus. Further, the apparatus naturally comprises conduits for transport of pulp in the pump 12, in the mixers 14 and 16, in the reaction vessel 18 and in the gas separator 20. The separator 20 may be connected to the reaction vessel either directly in the discharge opening thereof or via a flow channel. The fluidizing mixers 14 and 16 are preferably of the type disclosed in CA patent no. 1,313,325 or WO publication no. 93/07961 by A. Ahlstrom Corporation, and the gas separators are preferably of the type disclosed in EP patent application no. 90302993.2 and WO publication no. 93/01875 of A. Ahlstrom Corporation, however, also apparatus of other types may be used. The apparatus illustrated in the figure operates as follows: a mixture of ozone and carrier gas (the carrier gas in the SUBSTITUTESHEET figure is oxygen but also other gases such as nitrogen or air can be used) is mixed into the pulp, which is pressurized by means of the pump 12, with a mixer 14 fluidizing the suspension of gas and pulp, the volume of the introduced gas mixture being clearly larger than in the method of the EP application mentioned above. Supplying a larger volume of ozone mixture into the pulp results in that the gas is no longer mixed properly with the pulp but large bubbles remain in the pulp. The second fluidizing mixer 16 is used to break up these gas bubbles and a foam of the type described in the above EP application is formed, in which form the mixture of gas and pulp is introduced into the reaction vessel. The operation of the second mixer 16 is facilitated by fact that a remarkable portion of the ozone has already reacted with the fibers both in the mixer 14 and in the subsequent flow channel whereby the total gas volume in the suspension has been reduced to some extent. Tests have proved that this kind of a reactor application allows efficient mixing of 3 - 5 m3/adt of gas into the pulp. If larger volumes of gas are to be used, the reaction vessel should be provided with a slowly rotating mixer such as a paddle mixer in order to mix the created gas bubbles and the pulp. This kind of a mixer should preferably be used in the reaction vessel when gas doses exceed 2 - 3 m 3 /adt. The reasons for using large total gas volumes are, for example, a high ozone dosage desired, possibly also a fairly low ozone content in the carrier gas. Figure 2 illustrates a second preferred embodiment of the invention in which the mixture of pulp and gas is discharged from a second mixer 16 to a reaction vessel 30 which in the figure has been illustrated as being horizontal but which may be also vertical or inclined. The reaction vessel 30 has been provided with a paddle mixer 32 which slowly mixes into the pulp gas bubbles which, despite the foaming in mixers 14 and 16, remain in the pulp or have been separated in the pulp after SUBSTITUTESHEET said mixers. Subsequent to the paddle mixer 32, gas is separated from the pulp and the pulp is allowed to drop in a drop leg 34 from which it is pumped further by a pump 36. It is advisable also in this embodiment to use a combination of two fluidizing mixers and a tumbling mixer located m the reaction vessel when the gas volume to be mixed exceeds 2 - 3 m 3 /adt pulp. Example Per " ->rmed tests have proved that even slight mixing with a pac _e mixer results in lower ozone content in the separated residual gas and also in slightly more uniform bleaching. Both of these observations confirm that tumbling of the pulp in the reactor intensifies ozone consumption. The most preferred retention time of the pulp in the reaction vessel has been found to be 30 - 150 seconds while the pressure is 6 - 15 bar, in order to reduce the gas volume. Further, the temperature should be 50 - 90°C and the pH between 3 and 5. Preferably, the conditions in the bleaching reactor are: the pressure 11 bar, the temperature 60°C, pH 3 - 5 and the retention time 120 seconds. Typically, the energy intensity of the mixer 32 rotating slowly in the vessel is only one tenth of the energy of the fluidizing mixer. The energy consumption of the tumbling mixer is approx. 0.05 - 0.25 kWh/1. At a production rate of 40 t/h, a motor of 50 - 200 kW is required to drive the mixer. Acquiring and using this kind of a motor and a mixer is very economical, compared to acquiring a second reaction vessel and the pipe lines and gas separators connected with it. Figure 3 illustrates a third preferred embodiment of the invention which does not employ fluidizing mixers but ozone is supplied to the outlet end of the reactor or of a tumbling mixer 40 via a conduit 42 which may be either in a discharge conduit 44 extending from the reaction vessel or in the vicinity of the discharge end of the reactor. In other words, the process is in a way counter-current bleaching in which the ozone-containing gas is caused to flow against the direction SUBSTITUTESHEET of the pulp flow so that the residual gas is separated via a connection 46 of the gas separator 48 at the inlet end of the tumbling mixer. The remaining reference numbers in Figure 3 are the same as in Figure 2. This kind of an application can be used only in special conditions because the mixing result achieved with it is poor. However, it can be used for mixing gas doses of almost unlimited volume which is not necessarily possible with fluidizing mixers. Typically, this kind of a reactor is employed when the volume of gas to be mixed is larger than 10 m 3 /adt and the ozone content of the gas is below five per cent. If the reactor vessels or reactors illustrated in Figures 2 and 3 are in fact horizontal it is advantageous to provide them with partition walls so as to prevent the gas collected against the upper surface of the reactor from flowing directly to the gas discharge. For example, said partition wall may cover approx. the upper half of the cross sectional area of the reactor whereby the gas must, in order to proceed to the discharge, flow around the edge of the wall and is thus unavoidably mixed with the pulp. In Fig. 4 there is illustrated yet another embodiment of the invention. Basically, the arrangement is the same as in Fig. 1 the only exception being the third fluidizing mixer 52 in the pipeline leading from the pump 12 to the reaction vessel 18. By providing the third mixer 52, use it is possible to introduce a larger amount of ozone and carrier gas mixture into the pulp in mixer 14 without a need to use a tumbling mixer in the reaction vessel 18. Also it is possible to add ozone in the second mixer 16, if such is found applicable. The ozone consumption between the mixers may, naturally, be ensured by enlarging the diameter of the flow channel between the mixers or by extending the flow paths to make sure that there is sufficient retention time for the ozone to react with the fiber material. SUBSTITUTESHEET In Fig. 5 there is shown a further embodiment of the arrange¬ ment of Fig. 2 where the reaction vessel has been replaced with somewhat longer, extended pipelines between the mixers 14, 16 and 52 and especially between the last mixer 52 and the gas separator 20. Also, the diameter of the pipelines between the mixers 14, 16, 52 and between the mixer and the gas separator 20 may be somewhat larger than needed for the system of Fig. 2. In spite of the fact that three mixers are shown in this emdodiment it is also possible to apply the idea of replacing the reaction vessel with a mere pipeline in cases where there are only two mixers or even more than three mixers. If desired or necessary, the mixers 16 and 52 can be pumping and/or degassing mixers prosupposing that the lenght of the preceding reaction zone is adequate. In Fig. 6 there is shown a further embodiment of the invention where the arrangement is basically the same as shown in Fig. 1 with the exception that after the pulp has been discharged from the reaction vessel 18 and the gas separator 20 it is once more subjected to an ozone bleaching stage by means of mixing the mixture of ozone and carrier gas into the pulp in a fluidizing mixer 54. After this, the pulp is introduced into a gas separator 56, which may be one of those cited earlier or also a centrifugal separator like, for instance, a hydrocyclo- ne. The gas is discharged from the separator via a duct 58 and the degassed bleached pulp via a conduit 60. As can be concluded from the above a method and apparatus have been developed, which are better than prior art bleaching methods, for use in ozone bleaching or any other bleaching requiring large volumes of gas. Thus, even though ozone bleaching, with a mixture of ozone and oxygen has been described in the embodiments, above it is evident that the ozone can be supplied with any suitable carrier gas. Furthermo- re, the apparatus is applicable to the mixing of any bleaching chemical, but it is particularly suitable for mixing bleaching chemicals in the form of large gas volumes. Further, it is SUBSTITUTESHEET clear that even though many patented apparatus alternatives have been referred to above, other fluidizing mixers and gas separators can be used. Further, it is also possible in the embodiments of Figures 1 and 2 to supply bleaching chemical, such as ozone, via the second or third mixer to the pulp. SUBSTITUTESHEET

Claims: We claim : 1. A method of bleaching medium consistency pulp with a gaseous bleaching chemical having a large volume, characterized by the following steps: a) mixing said pressurized bleaching chemical into pulp in a fluidizing mixer in order to produce a foamy suspension; b) discharging said foamy suspension to a second fluidizing mixer, in which the mixture of gas and pulp is refluidized in order to maintain the foamy state in the suspension; c) discharging said foamy suspension into a reaction vessel for giving the bleaching chemical sufficient time to react with the fiber material. 2. A method as recited in claim 1, characterized by step d) tumbling the suspension in said reaction vessel so that the gas bubbles remaining in the suspension, or separated therein, break and the bleaching chemical in them is able to react better with the fiber material. 3. A method as recited in claim 1, characterized in that the pressure in the reaction vessel is 6 - 15 bar and the treatment time 30 - 150 seconds. 4. A method as recited in claim 1 or 3, characterized in that said reaction vessel is a tanx having an enlarged diameter to allow the foamy suspension sufficient retention time for the bleaching reaction. 5. A method as recited in claim 1 or 3, characterized in that said reaction vessel is a flow channel having an extended length to allow the foamy suspension sufficient retention time for the bleaching reaction. 6. A method as recited in claim 3, 4 or 5, characterized in that the treatment temperature is approx. 50 - 90°C and the pH 3 - 5. SUBSTITUTE SHEET 7. A method as recited in claim 1, characterized in that bleaching chemical is supplied to the second mixer. 8. A method as recited in claim 1, characterized in that excess gas is separated from the suspension after the bleaching reaction. 9. A method as recited in claim 8 characterized in mixing said pressurized bleaching chemical into pulp in a fluidizing mixer after said gas separation. 10. A method as recited in claim 9 characterized in allowing the bleaching reaction to take place after the initial gas separtion and remixing and again separating gas thereafter from said suspension. 11. A method for bleaching medium consistency pulp with a gaseous bleaching chemical having a large volume, characterized in that pulp is supplied into a bleaching reactor in which pulp is tumbled slowly with a rotatable mixer and into which reactor gaseous bleaching chemical is supplied. 12. A method as recited in claim 11, characterized in that the bleaching chemical is supplied to the discharge end of the reactor in such a way that it is forced to flow in a direction opposite to the flow direction of the pulp in the reactor and gas is discharged from the inlet end of the reaction vessel. 13. A method as recited in claim 1 or 11, characterized in that said gaseous bleaching chemical is a mixture of ozone and carrier gas. 14. An apparatus for bleaching medium consistency pulp with a gaseous bleaching chemical having a large volume, comprising a first fluidizing mixer (14), and thereto a conduit (22) for the chemical to be mixed, and an inlet conduit and an outlet conduit for pulp, a reaction vessel (18, 30), and therefrom SUBSTITUTESHEET conduit for pulp, a gas separator (20) and a conduit (24) therefrom for the gas being discharged, characterized in that the outlet conduit of the first fluidizing mixer (14) is connected to the inlet conduit of a second fluidizing mixer (16) and that the outlet conduit of the second fluidizing mixer (16) is connected to the inlet conduit of the reaction vessel (18). 15. An apparatus as recited in claim 14, characterized in that the reaction vessel (18, 30) is a flow channel having an enlarged diameter for giving the foamy suspension sufficient retention time for the bleaching reaction. 16. An apparatus as recited in claim 14, characterized in that the reaction vessel is a flow channel having an extended length for giving the foamy suspension sufficient retention time for the bleaching reaction. 17. An apparatus as recited in claim 14, characterized in that the reaction vessel is provided with a mixer (32) for tumbling the pulp. 18. An apparatus for bleaching medium consistency pulp with■ a gaseous bleaching chemical having a large volume, characterized in that it comprises a reaction vessel (50) provided with a tumbling mixer (40), said vessel having an inlet end with an inlet conduit and an outlet end with an outlet conduit (44) said outlet end being provided with a feed conduit (42) for bleaching chemical. 19. An apparatus as recited in claim 14 or 18, characterized in that the reaction vessel (30, 50) is horizontal. 20. An apparatus as recited in claim 19, characterized in that the reaction vessel (30, 50) is provided in its upper part with partitions to prevent a direct gas flow between the gas inlet and the gas separation. SUBSTITUTESHEET 21. An apparatus as recited in claim 18, characterized in that a gas separator (48) is provided in the inlet end of the reaction vessel (50) or in the vicinity thereof. SUBSTITUTESHEET

Inventors: DUNN STEPHEN, FUNK ERWIN, GREENWOOD BRIAN, HENRICSON KAJ, PHILLIPS JOSEPH, DUNN, STEPHEN, FUNK, ERWIN, GREENWOOD, BRIAN, HENRICSON, KAJ, PHILLIPS, JOSEPH
Applicants: AHLSTROEM OY, DUNN STEPHEN, FUNK ERWIN, GREENWOOD BRIAN, HENRICSON KAJ, PHILLIPS JOSEPH, A. AHLSTROM CORPORATION, DUNN, STEPHEN, FUNK, ERWIN, GREENWOOD, BRIAN, HENRICSON, KAJ, PHILLIPS, JOSEPH
Application year: 1993
Main classification: D21C9
IPC codes: D21C9