Peroxide bleaching process

Abstract: Method and apparatus for peroxide bleaching a medium consistency cellulosic fiber suspension include feeding the fiber suspension and a pretreatment agent into a fluidizing mixer (14); intimately and uniformly mixing the fiber suspension with the pretreatment agent to achieve, for instance, a prebleaching reaction; passing the mixture into a pretreatment vessel (22) to permit the pretreatment to proceed and to consume a major part of the pretreatment agent; adding a bleaching chemical to the mixture; separating the excess pretreatment agent from the mixture in a second considerably larger vessel (32) and removing the fiber suspension from the second vessel after the effective bleaching reaction.

Description: Field of the invention The present invention relates to peroxide bleaching of a medium consistency suspension of cellulosic fibers such as paper pulp and, particularly, in accordance with a preferred embodiment of the invention, to a method of sequentially bleaching pulp by a pretreatment agent, preferably, an oxidizing agent, like ozone and thereafter by hydrogen peroxide to obtain a ZE P bleaching sequence. Background of the invention Bleaching of medium consistency paper pulp with ozone has only recently become possible. Previous drawbacks, e.g. the high cost of ozone, the known disadvantages associated with operating at either low consistency (less than about 5 %) or at very high consistency (above about 25 %) and the fact that ozone readily attacks the carbohydrates of the pulp, have now been overcome. Due to the present invention the efficiency of an ozone bleaching operation of medium consistency pulp, i.e. a cellulosic fiber suspension having a consistency of from about 5 to about 20 percent, is further increased by incorporation of an additional chemical feeding step into the ozone bleaching stage. Ozone bleaching of medium consistency pulp is known e.g. from a European patent application EP-0 426 652 A1. The publication discloses a method in which medium consistency pulp is introduced into a so called fluidizing mixer into which also ozone with carrier gas is introduced. From the mixer the mixture of ozone and pulp is discharged to a reaction tube from where the pulp is transferred to a bleaching tower via a pressure decreasing throttling. Between the throttling and the bleaching tower there has been arranged a degassing device and a feed conduit for dilution water. In other words, the dilution water is introduced into a pulp the pressure of which has already been decreased to equal with the atmospheric pressure. The specification also suggests that after the above described ozone bleaching stage the pulp may be treated, for instance, by means of an alkaline extraction stage. There is yet another publication disclosing ozone bleaching, namely US-A-4,450,044 which, however, teaches a totally different method of bleaching. The publication relates to a so called high consistency ozone bleaching in which the consistency of the pulp is between 25 and 60 %, preferably between 30 and 45 %. However, the above defined consistencies mean in reality that the pulp is substantially dry, in other words, there is no free water between the fibers. Due to the low, or in this case negligible, water content high consistency ozone bleaching has no problems with regard to water whereas in medium consistency ozone bleaching the biggest problem is the presence of free water between the fibers. Our present invention is directed to solving said problem relating to the presence of free water between the fibers and its effects on the behaviour of the fiber suspension. EP-A-0 087 553 discloses a method of peroxide bleaching a medium consistency cellulose fiber suspension (pulp) comprising a) adding hydrogen peroxide to the pulp (under overpressure of 3 or 2 bar according to said examples), b) introducing the mixture into a reaction vessel, c) maintaining said overpressure in said reaction vessel, d) allowing the bleaching reaction to take place, and e) removing the pulp from said reaction vessel. The bleaching is carried out under alkaline conditions (NaOH). Bleaching time is e.g. 2 hours. The present invention is directed to a method of efficient peroxide bleaching of pulp under pressure as a cellulosic fiber suspension having a consistency between 5 and 20% where said peroxide bleaching takes place in a pressurized reaction vessel 132 where the pressure is below 25 bar and thereafter feeding the pulp to a washer 150, which is characterized in a) feeding said fiber suspension under pressure into a mixer, introducing a pretreatment agent under pressure into said mixer, intimately and uniformly mixing said fiber suspension and said pretreatment agent in said mixer, and allowing a pretreatment reaction to take place, b) adding hydrogen peroxide to said pretreated fiber suspension, c) introducing said mixture of fiber suspension and said peroxide into a pressurized reaction vessel 132 where oxygen is present, d) maintaining the overpressure in said reaction vessel 132 where the pressure is preferably controlled by a pressure regulating valve 136, e) allowing the bleaching reaction to take place so that the time of the bleaching reaction is 1 to 3 hours, and f) removing the fiber suspension from said reaction vessel via an outlet 152 which is dimensioned to permit the pulp to be fed, due to the pressure head, into a suitable washer 150. Preferably, the pressure in the pretreatment vessel is maintained at a predetermined level by a suitable valve and control loop. The pressure in the reaction vessel is also controlled with known means, albeit at a lower level relative to the pressure in the pretreatment vessel. In addition, a suitable known control device is provided to maintain the level of the paper pulp within the second reaction vessel at least within a predetermined range. In accordance with a preferred embodiment of the invention the pretreatment agent is ozone which is used as a mixture of ozone itself in a carrier gas. Thereby the medium consistency fiber suspension is pretreated, in this case prebleached by (a) feeding said fiber suspension and an ozone/carrier gas mixture under pressure preferably at about 3 to about 25 bar, more preferably at about 5 - 14 bar, to a mixer effecting high shear mixing for intimately and homogeneously intermixing the gas mixture with the medium consistency fiber suspension. As a carrier gas oxygen, air and nitrogen may be used, with oxygen being presently preferred as it contains the greatest amount of ozone, namely, about 3-16 % at the most. Thus, for example, an ozone carrier gas mixture may contain for instance about 10 kg ozone and 90 kg oxygen. At a pulp suspension consistency of 10 %, the water/gas ratio is preferably between about 1:10 and 2:1 depending on the pressure which varies between 3 to about 25 bar. (b) In the high shear mixer, which is preferably a commercially available MC- mixer, the ozone in carrier gas and the paper pulp are thoroughly mixed so that an adequate mass transfer and contact between the ozone and the fibers is achieved resulting in high bleaching efficiency. (c) From the mixer, the intimate and uniform paper pulp/ozone mixture is, in accordance with a preferred embodiment, passed into a pretreatment vessel for allowing the prebleaching process to proceed until a major part of the ozone is consumed. The residence time of the mixture in the fluidizing mixer may then be less than 1 second and the residence time of the mixture of paper pulp and ozone in carrier gas in the pretreatment vessel is about 0. 1 to 5. 0 minutes. This permits about 99 % of the ozone to be consumed and the bleaching process to be substantially completed. Preferably, the top of the pretreatment vessel is provided with a known fluidizing device which fluidizes the fiber suspension for discharging the mixture into a reaction vessel for permitting the excess ozone, the carrier gas and a minor amount of possible additional reaction gases to separate from the mixture and also to permit the bleaching reaction to proceed. Excess gas may be vented at various locations, such as, for example, from the pretreatment vessel through the fluidizing device at the top of the pretreatment vessel. In this connection it should be kept in mind that a constant pressure should be maintained in the pretreatment vessel to achieve maximum ozone bleaching effect. Also, the injection under pressure of the liquid bleaching agent is performed at or in close proximity to the fluidizing operation to intimately and uniformly mix the preferably alkaline bleaching agent with the pulp. Finally, both the pretreatment and reaction vessels are preferably upright reactors, whereby the pulp is passed through the pretreatment vessel in an upward direction of flow while the pulp passes through the reaction vessel in a downward flow. Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for the purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. Brief description of the drawings The present invention is further described in detail below with reference to the accompanying drawings in which: FIG. 1 is a schematic illustration of a reference embodiment; and FIG. 2 is a schematic illustration of an embodiment of the present invention. Detailed description of the presently preferred embodiments In the figures the same elements are given the same numerals with the exception that all numbers in Fig. 2 are preceded by the numeral 1. Referring now to Fig. 1 in detail, pulp is transferred from a storage unit 10 by a pump 12 which is preferably a degassing medium consistency pump through a line or conduit 16 to a fluidizing mixer 14. Mixer 14 is preferably a high-shear, medium consistency mixer commercially available from A. AHLSTROM CORPORATION of Xarhula, Finland and Kamyr Inc. of Glens Falls, N.Y., USA. The mixer has an inlet for the medium consistency pulp suspensions connected to line 16, and an inlet port 18, for the pressurized feed of the pretreatment or prebleaching agent, like ozone-containing carrier gas such as air, nitrogen and preferably oxygen. The pulp suspension is intimately and uniformly mixed with the ozone-containing carrier gas and discharged through a mixture outlet into a conduit 20 and passed within about 2 - 3 seconds into the bottom 23 of an upright pretreatment vessel 22. After a residence time of about 0.5 to 5 min. the mixture of pulp, carrier gas and ozone, which has not yet been entirely consumed during the bleaching reaction, arrives at the top 25 of the pretreatment vessel 22. Into the reaction mixture is now fed through a conduit or line 24, at the top 25 of the pretreatment vessel 22 another bleaching chemical, for example, hydrogen peroxide, preferably in liquid form under pressure. Preferably, the other bleaching chemical is intimately mixed with the paper pulp to effect the alkaline bleaching step. To assist the removal of the paper pulp from the pretreatment vessel, vessel 22 is preferably provided at the top portion 25 thereof with a known fluidizing device or fluidizing discharger 26 which preferably has an integral injection port for the bleaching chemical at or near the fluidizing rotor or fluidizing device or discharger 26 so as to effect the proper mixing of the bleaching chemical with the fluidized paper pulp. The fluidizing discharger 26 is preferably provided with means 28 for discharging preferably pressurized gas from the vessel to be used, for example, in another pressurized treatment or bleaching stage. To achieve good bleaching results and stable conditions the pressure in the pretreatment vessel 22 should be maintained at a constant level which is achieved with a pressure regulating valve 30, preferably located closely adjacent fluidizing discharger 26, and control loop 31 in known manner. The bleached pulp, which now contains the other bleaching chemical, for example, hydrogen peroxide, is now discharged from pretreatment vessel 22, preferably into the enlarged inlet portion 33 of a, preferably upright, reaction vessel 32 to assist in the separation of the gases from the pulp mixture. Separated gas is then removed from said reaction vessel 32 through a gas discharge line 34. The pressure in the reaction vessel 32 is also maintained steady at overpressure, by a separate pressure regulating valve 36 and a control loop 37 in a known manner. Preferably, the pressure is maintained in relatively substantially lower level than in the pretreatment vessel, generally only at slight overpressure. The pulp is now collected in the reaction vessel 32 at or near a predetermined level, through known level control means 38, line 40, pump 44, and pressure regulating valve 47 for up to 1 to 3 hours to complete the alkaline (E) bleaching stage and thereafter is discharged at the bottom 42 of vessel 32 by pump 44, which is preferably also a degassing medium consistency pump, through valve 47 into a conduit 46 leading to a washer or other suitable treatment stage. The elements, structure and operation of the embodiment of the invention illustrated in Fig. 2 are substantially the same as the reference embodiment described above in connection with Fig. 1, except that reaction vessel 132 is provided at its bottom 142 with an outlet 152 which is dimensioned to permit the bleached paper pulp to be fed, due to the pressure head thereof, into a suitable washer 150, preferably a drum diffusion washer as sold by assignee A. Ahlstrom Corporation, with pressurized inlet or diffuser available from Kamyr Inc. of Glens Falls, N.Y. A level control mechanism 138 cooperates through line 148 with an rpm regulator of washer 150 in known manner to maintain the paper pulp level in the reaction vessel 132 at a predetermined level. Finally, the washed pulp is discharged from washer 150 by a pump 144, preferably a degassing medium consistency pump through a conduit 146 for further treatment.

Claims: A method of efficient peroxide bleaching of pulp under pressure as a cellulosic fiber suspension having a consistency between 5 and 20% where said peroxide bleaching takes place in a pressurized reaction vessel (132) where the pressure is below 25 bar and thereafter feeding the pulp to a washer (150), is characterized in a) feeding said fiber suspension under pressure into a mixer, introducing a pretreatment agent under pressure into said mixer, intimately and uniformly mixing said fiber suspension and said pretreatment agent in said mixer, and allowing a pretreatment reaction to take place, b) adding hydrogen peroxide to said pretreated fiber suspension, c) introducing said mixture of fiber suspension and said peroxide into a pressurized reaction vessel (132) where oxygen is present, d) maintaining the overpressure in said reaction vessel (132) where the pressure is preferably controlled by a pressure regulating valve (136), e) allowing the bleaching reaction to take place so that the time of the bleaching reaction is 1 to 3 hours, and f) removing the fiber suspension from said reaction vessel via an outlet (152) which is dimensioned to permit the pulp to be fed, due to the pressure head, into a suitable washer (150). The method of claim 1, characterized in that the pulp is pretreated with an oxidizing pretreatment agent which preferably is ozone. The method of claims 1 or 2, characterized in that the peroxide bleaching is effected immediately after the pretreatment of the pulp. The method of claims 1 or 3, wherein the suitable washer (150) is a drum diffusion washer or a diffuser. The method of claim 1, characterized in that the bleaching with peroxide is performed under alkaline conditions. The method of claim 1, characterized in that said pressure in said reaction vessel is below 14 bar. The method of claims 1 to 6, characterized in that said mixture containing ozone and fiber suspension is moved through a pretreatment vessel in an upward flow direction, excess ozone and carrier gas is removed from said suspension and said mixture is moved through said reaction vessel in a downflow direction. The method of claim 5, characterized in that sodium hydroxide is added in connection with hydrogen peroxide.

Inventors: GREENVOOD BRIAN, HENRICSON KAJ, GREENVOOD, BRIAN, HENRICSON, KAJ
Applicants: AHLSTROM MACHINERY OY
Application year: 1992
Main classification: D21C9
IPC codes: D21C9