SE516065C2 - Peroxide bleaching process for pref. chemical pulp - Google Patents

Abstract

In the process for bleaching cellulosic pulp with peroxide, using at least one pressurised bleaching vessel: (a) peroxide is blended with the pulp to form an intimate mixt.; (b) the mixt. is added to the pressurised bleaching vessel where the pulp is bleached in a superatmospheric pressure to form oxygen and reaction gases; (c) at least 20 % of the oxygen and reaction gases are removed from the mixt. before all the peroxide is used up; (d) the mixt. is added to a second vessel where the temp. is greater, pressure is lower and treatment time is longer than in the first vessel; and (e) the remaining oxygen and reaction gases are removed from the pulp.

Description

30 516 065 In an article entitled "Altematives for Achieving High Brightness TCF Pulps" by Basta (International Non-Chlorine Bleaching Conference, Amelia Island, Florida, March 6-10, 1994) et al. For example, it has been suggested that peroxide bleaching be performed first in a pressurized pre-reactor and then in an atmospheric, larger reactor. However, this has fl your disadvantages. If the larger reactor is atmospheric, the temperature is low and the peroxide reaction is not so efficient. Without overpressure, it can also be more difficult to discharge the pulp from the tower. Furthermore, there is no oxygen pressure, and without oxygen pressure the decomposition of the peroxide is accelerated. By maintaining a higher than atmospheric pressure in the second reactor according to the invention, an oxygen atmosphere is ensured.

Chemically speaking, the peroxide reaction is started with: Hzoz -> ooH * + H * The bleaching reaction then continues as follows: OOH '+ fi bres -> 02 + bleached fi bres This means that oxygen is generated in the suspension. An oxygen atmosphere prevents a detrimental decomposition of the peroxide, which is expressed as follows: OOH '-> Oz + H' so that the oxygen can easily react to induce the above-mentioned degradation realdione. According to the present invention, it has been found that a high overpressure is not necessary to prevent the decomposition reaction, but only a small overpressure (eg about 1.1 - 5 bar) so that the gas atmosphere can be controlled properly.

Basta o.a. describes the use of a hot pre-reaction, where the amount of chemicals used is large, as well as a cold "e-reaction" zone. According to the present invention, however, it has been found that the hot pre-reaction accelerates the reaction rate and the peroxide decomposition and that the bremine can be easily damaged due to the too rapid reaction. It is better to have a colder pre-reaction with a large amount of chemicals and a warm "after-reaction" with a smaller amount of chemicals.

According to the present invention, there is provided an advantageous method and system for performing peroxide bleaching of cellulosic pulp. Although the invention is particularly suitable for chemical pulps, it can also be used for mechanical pulps and recycled pulps. The invention can be applied within your different consistency areas, although pulp of medium consistency, with about 5 - 20% solids and preferably about 8 - 15%, is preferred.

According to an embodiment of the present invention there is provided a process for bleaching cellulose pulp with peroxide, using pressurized bleaching vessels, which process comprises the following steps: (a) Peroxide (eg 5 - 20 kg / ton of air dry pulp) is mixed with cellulose pulp to form an intimate mixture. (b) The intimate mixture is fed into the first pressurized bleaching vessel and peroxide bleaching of the pulp is performed in the first vessel while a pressure higher than atmospheric is maintained everywhere so that oxygen and reaction gases are formed during the peroxide bleaching of the pulp. (c) At least 20% (preferably at least the majority and most preferably substantially the entire amount - i.e. 90% or more) of the oxygen and the reaction gases are separated from the mixture before the peroxide has been completely consumed. (d) After step (c), the mixture is immersed in a second bleaching vessel and the peroxide bleaching is allowed to proceed under a second temperature, a second pressure, and a second treatment time and under overpressure conditions until substantially all of the peroxide has been consumed, whereby oxygen and reaction gases are formed again. and wherein the first pressure is higher than the second, the first temperature is lower than the second and the first treatment time is shorter than the second. And (e) the majority (preferably substantially the entire amount) of the newly formed oxygen and the reaction gases are separated from the pulp. Step (e) is typically performed substantially simultaneously with the pulp being discharged from the second vessel.

The temperature in the first bleaching vessel is typically about 80 - 110 (preferably 80 - 100) ° C and the pressure 5 - 60 (preferably 5 - 15) bar and the treatment time 15 - 60 (preferably 15 - 45) minutes. In the second vessel the temperature is preferably slightly higher (eg about 10 ° C) and the pressure is about 1.1 - 10 bar, preferably 5 bar or below, and the treatment time is about 30 - 200 minutes, preferably about 60 - 150 minutes.

The pulp is preferably pretreated before the peroxide bleaching. The. can be treated, for example, to remove transition metals therefrom, and / or bleached with other alkaline or acidic bleaching chemicals.

According to another embodiment of the present embodiment, a cellulose pulp bleaching system is provided comprising the following elements: A first overpressure bleaching vessel provided with a top and a bottom with a pulp inlet at the bottom and a pulp outlet at the top. A first flowing dispenser at the top of the first vessel for discharging pulp from the outlet and for removing the bulk of the gas from the pulp. A second overpressure bleaching vessel provided with a top and a bottom with a pulp inlet at the bottom and a pulp outlet at the top. A second dispensing dispenser at the top of the second vessel for discharging pulp from the outlet and for removing the bulk of the gas from the pulp. Agents for mixing peroxide and other chemicals into the pulp before or while it is fed into the pulp inlet of the first vessel.

And a connection between the pulp outlet from the first vessel and the pulp inlet to the second vessel.

The mixing means may comprise a pump for simultaneous pumping of pulp and mixing of pulp with peroxide and chemicals and / or may comprise a medium consistency mixer. Additional blending agents may be present for mixing peroxide and other chemicals into the pulp in or after the first dispensing dispenser. If the additional mixing means are after the first flow dispenser, they may comprise one (or two) medium consistency mixers between the first flow dispenser and the mass inlet of the second vessel. The first vessel typically has a height of about 15-30 m and a volume of about 200 - 500 cubic meters, while the second vessel has a volume at least twice as large as the first vessel.

The main object of the present invention is to provide a simple and efficient process and a system for using this for bleaching cellulose pulp with peroxide. This and other objects of the invention appear from the following detailed description of the invention and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS 10 15 20 25 30 516 065 FIG. 1 schematically shows a first exemplary embodiment of a system according to the invention for bleaching cellulose pulp with peroxide; FIG. 2 is a view similar to that of FIG. 1, but for a different embodiment; FIG. 3 and 4 are views similar to that of FIG. 1, but for still other embodiments.

DETAILED DESCRIPTION OF THE DRAWINGS In the embodiment shown in FIG. 1, pulp is fed to a storage unit 10 from one or more of the pretreatment stages II. The step or steps 11 may consist of Og delignification, ozone bleaching or other acidic or alkaline bleaching or delignification steps and / or removal of transition metals with acidic detergents or chelating agents such as EDTA or DTPA [A- and / or Q steps] followed by washing or pressing (dewatering) or the like. The mass is transferred from the storage unit 10 by means of a pump 12 [which is preferably a degassing medium consistency pump] through a line or line 16 to a flowing mixer 14. The mixer 14 is preferably a high shear medium consistency mixer such as an MC® mixer marketed by KAMYR. Inc., Glens Falls, New York, or an AHLMIXTM mixer provided by A. Ahlström Osakeyhtiö, Finland.

The mixer 14 is provided with an inlet for the pulp suspension of medium consistency (eg 5 - 20%, preferably 8 - 15%), which is connected to the line 16, and an inlet opening 18 for pressurized feed of peroxide bleaching chemicals. In addition to the addition of peroxide (eg, typically H 2 O; in a liquid), the device is the same as in FIG. 1 of SE patent application 9403073-1. Alkali (eg NaOH), Mg, O 2 and / or steam are preferably added with the chemicals. Normally about 5 - 20 (eg about 10) kg hydrogen peroxide / tonne of air-dry pulp (or other equivalent type of peroxide) is added.

The pulp suspension is mixed intimately and evenly with the peroxide and discharged through an outlet in the mixer 14 to a line 20 and fed within about 2-3 seconds into the bottom 23 of a first, upright, pressurized reaction vessel 22. After a residence time of about 10 15 20 25 30 5-16,065 15-60 minutes (preferably about 15-45 minutes) the mixture of pulp and bleaching chemicals, which have not yet been completely consumed during the bleaching reaction, reaches the top 25 of the reaction vessel 22.

The vessel 22 preferably has a height of about 15-30 meters. Its cross-sectional area (typical diameter) is determined by the desired production. The vessel's 22 volume is typically about 200 - 500 cubic meters). If the desired production e.g. is 1500 tons of air dry mass / day and the treatment time is about 30 minutes, the volume of the reactor 22 should be at least 310 cubic meters, e.g. 310 - 400 m3. Thus, for a reactor 22 with a height of about 30 meters, the diameter would be about 3.5 meters and for a height of about 15 meters, the diameter would be about 5.0 meters.

The mass in the vessel 22 is constantly under overpressure and at an elevated temperature. The preferred temperature range is 80-110 ° C, preferably 80-100 ° C. To minimize the problems described above with the prior art, the pressure in the vessel 22 is not extremely high, but preferably about 5 - 15 bar, preferably 5 - 10 bar. During the bleaching reaction in the vessel 22, oxygen and reaction gases are formed.

According to the present invention, in at least one intermediate stage during the bleaching, a substantial part of this oxygen and the reaction gases generated during the bleaching reaction are removed, so that the pressure need not be unnecessarily high. This is achieved in the embodiments shown in FIG. 1 by removing at least 20% of the oxygen and the reaction gases (preferably at least 50% and most preferably substantially the entire amount - - i.e. at least 90% - of the oxygen and the reaction gases) in a line 28 by means of a id deodising feeder 26. preferably of such a type that it has a low flow resistance and is also able to raise the temperature and because of this the pressure, when additive bleaching chemicals, steam and the like are supplied in the line 24, is manageable (eg below 12 bar) . Normally about 75% of the bleaching chemicals are consumed in reactor 22 while 25% remains. In some circumstances, the addition of additional peroxide, alkali, steam or the like is not necessary, although typically at least steam is supplied to line 24 to slightly raise (eg about 10 ° C) or maintain the temperature of the pulp as it exits through a valve 10. 15 20 25 30 (fl -gln ON CI »On UI 30 to the second pressurized bleaching vessel 32.

Good bleaching results are obtained when the pressure in the first vessel 22 is stable. The pressure can be kept at a constant level by utilizing the pressure control valve 30, which is preferably located in the vicinity of the flow dispenser 26, and the conventional control circuit 31.

The mixture of pulp and peroxide bleaching chemicals is discharged from the vessel 22 to the expanded inlet portion 33 of the second vessel 32. The pressure in the vessel 32 is typically lower than in the vessel 22, but still higher than the atmospheric pressure. The temperature in the vessel 32 is e.g. preferably about 90 - 120 ° C, more preferably about 90 - 110 ° C, while the higher than atmospheric pressure is about 1.1 - 10 bar, preferably about 1.1 - 5 bar. In the second vessel 32, the bleaching reaction continues and oxygen and reaction gases are generated again.

Separated gas is removed from the second vessel 32 through a gas outlet line 34.

The pressure in the second vessel 32, which preferably has a volume at least twice as large as the vessel 22, is also kept constant in a known manner by means of a separate pressure control valve 36 and a control circuit 37. The mass forms a pillar in the second vessel 32 on or near a predetermined level by known level control means 38, a line 41, a pump 44 and a pressure control valve 47, and treated for about 30 - 200 minutes, preferably about 60 - 150 minutes (and preferably longer than in the vessel 22) to complete the peroxide bleaching. The mass is then discharged from the bottom 42 of the vessel 32 by means of a pump 44, which is preferably also a degassing medium consistency pump, through a valve 47 to a line 46 which leads to a washer or other suitable treatment (eg bleaching) step.

In the second vessel 32, the majority of the gas, mainly in the form of oxygen and reaction gases, is separated through the outlet 32, while an "oxygen atmosphere" is maintained in the vessel 32 as the reference numeral 39 indicates. Although it is preferred that two different bleaching vessels 22, 32 be used, it is possible that the intermediate gas separation and the subsequent continued peroxide bleaching (with or without the addition of additional bleaching chemicals) may take place in the same vessel; or that more than two vessels are used, preferably with removal of gas between each.

FIG. 2 shows another exemplary device according to the present invention. In FIG. 2 are the components comparable to the components of FIG. 1 denoted by the same reference numerals but by the number "1" added in front of the designation, and the common constructions are not described.

The elements, constructions and function of the dehumidifier shown in FIG. 2 are substantially the same as in the embodiment described in connection with FIG. 1, except that the second vessel 132 is provided with an outlet 152 at its bottom, which is dimensioned to allow the bleached pulp, due to the liquid pressure, to be fed into a suitable washer 150, preferably a drum diffusion washer sold by the patent holder A. Ahlstrom Corporation, with a pressurized inlet or diffuser provided by Kamyr, Inc., Glens Falls, New York.

A level control mechanism 38 cooperates in a known manner through the conduit 148 with the speed controller of the washer 150 to maintain the level of the pulp in a second vessel 132 at a predetermined height. The washed pulp is finally discharged from the washer 150 by means of a pump 144, preferably a degassing medium consistency pump through a line 146 for further treatment.

FIG. 3 and 4 show two other embodiments of systems which can be used to carry out the method according to the present invention. In FIG. 3 and 4 are the constructions which are comparable to the constructions in FIG. 1 and 2 denoted by the same two-digit reference numerals, but with the digit "2" added in front of the denomination.

In the embodiments of FIG. 3, the peroxide, alkali, steam and / or oxygen are fed directly into the medium consistency pump 212, the pulp flowing in the upflow reactor 222 to gas separation efficiency is typically about 50-90%, the gas being discharged as the flowing discharge device 226. and alkali is used, these can be supplied directly into the dispenser 226, while steam is supplied in the line 29, which connects the dispenser 226 to the bottom of the second vessel 232. The vessel 232 is an upflow reactor, where the discharge mechanism 244 is constituted by a flowing dispenser such as the mechanism 226. The gas being removed is again typically discharged at least 50-90%, as shown at 45, while the pulp is discharged at 246 and dried into a blowing tank, a storage container, a scrubber and The temperatures and pressures are preferably those described in connection with the embodiments according to Fig. 1, more specifically the pressure in the vessel 22 is typically about 5 - 15 bar, typically less than 10 bar, while the pressure in the vessel 232 is 1.1 - 10 bar, typically less than 5 bar. It is desirable that the temperature in reactor 232 be slightly higher than in reactor 222, e.g. about 10 ° C in a typical situation, and the maximum temperature in the vessel 222 is relatively low, e.g. 120 ° C or less. The mass in the vessel 222 has a temperature of about 100 ° C and a pressure of about 10 bar, while the temperature is raised (by supplying steam in the line 29) in the rectum 232 to about 110 ° C and the pressure is below 5 bar (but however, higher than the atmospheric). In this way a high pressure is maintained during the gas formation but a low pressure during the steam injection.

FIG. 4 shows an exhaust manifold which is only slightly different from that shown in FIG. 3. In the embodiments of FIG. 4, a medium consistency mixer 214 is used for intimate mixing of hydrogen peroxide and other chemicals instead of supplying the chemicals directly to the pump 212 or in addition to being supplied to the pump 212. In this embodiment there are also mixers 49 and 58 in which hydrogen superoxide and alkali and / or steam can be added. Although in this embodiment it is shown that steam can also be supplied in the line 29, the addition of steam typically takes place only in one or both medium consistency mixtures 49, 58. Although in this embodiment the supply of hydrogen peroxide and alkali is further shown in the oxidizing dispenser 226 , this can be omitted and instead optionally additional peroxide etc. is added to one or both medium consistency mixers 49, 58.

Thus, it can be seen that according to the present invention there is provided a process and a peroxide bleaching system which have the advantages of requiring lower pumping pressure, lower vapor pressure and not so expensive reactor designs, since a lower pressure is used in the second reactor 32. 132, 232. Although the invention has been described particularly in connection with medium consistency pulp, which typically has a consistency of 5 - 20%, preferably about 8 - 15%, and chemical pulp, it is understood that it is also useful for other textures and also for other types of pulp such as mechanical pulp and return fi berrnassa. The invention is also an improvement of a configuration, where the second vessel is under atmospheric pressure, whereby the higher than atmospheric pressure has your advantages. Although the invention has been shown and described above in connection with what may at present be considered as its most practical and preferred embodiments, it will be apparent to those skilled in the art that many modifications may be made within the scope of the invention, which should be construed as they cover all equivalent procedures and systems.

Claims (23)

10 20 25 30 516 065 11 PATENT REQUIREMENTS A method of bleaching cellulose pulp with peroxide, wherein pressurized bleaching vessels are used, the method comprising the steps of: (a) mixing peroxide with cellulose pulp to form an intimate mixture; (b) the intimate mixture is fed into the first pressurized bleaching vessel and peroxide bleaching of the pulp is performed in the first vessel under a first temperature, a first pressure, and a first treatment time while the first pressure higher than atmospheric is maintained everywhere so that oxygen and reaction gases are formed during the peroxide bleaching of the pulp; (c) at least 20% of the oxygen and reaction gases are separated from the mixture before the peroxide has been completely used up; (d) after step (c) the mixture is fed into a second bleaching vessel and the peroxide bleaching is allowed to proceed under a second temperature, a second pressure, and a second treatment time and under overpressure conditions until substantially all of the peroxide has been consumed, whereby oxygen and reaction gases are formed and wherein the first pressure is higher than the second, the first temperature is lower than the second and the first treatment time is shorter than the second; and (e) the majority of the re-formed oxygen and the reaction gases are separated from the pulp. A method according to claim 1, in which step (c) is typically performed substantially simultaneously with the mass being removed from the first vessel. The method of claim 1, wherein step (e) is typically performed substantially simultaneously with the pulp being discharged from the second vessel. The method of claim 1 comprising the further step of feeding additional peroxide into the pulp and mixing with the pulp while performing step (c). The method of claim 1 comprising the further step of feeding additional peroxide into the pulp and mixing with the pulp just before it is introduced into the second vessel. 10 15 20 25 30 516 065 12 A method according to claim 1, in which the temperature of the pulp during the execution of step (b) is maintained at about 80 - 110 ° C and the pressure at about 5 - 20 bar and in which the treatment time in the first vessel is about 15 - 60 minutes. A method according to claim 6, in which the temperature of the pulp during the execution of step (d) is maintained at about 90 - 120 ° C and the pressure at about 1.1 - 10 bar and in which the treatment time in the second vessel is about 30 - 200 minutes . A process according to claim 6, in which the temperature of the pulp during the execution of step (d) is maintained at about 90 - 110 ° C and the pressure at about 1.1 - 5 bar and in which the treatment time in the second vessel is about 60 - 150 minutes. A method according to claim 1, in which the temperature of the pulp during the execution of step (b) is maintained at about 80 - 100 ° C and the pressure at about 5 - 10 bar and in which the treatment time in the first vessel is about 15 - 45 minutes. A process according to claim 7, in which the amount of peroxide supplied in step (a) is about 5 - 20 kg of hydrogen peroxide / ton of air dry mass or equivalent therewith. A process according to claim 1, in which the amount of peroxide added in step (a) is about 5 to 20 kg of hydrogen peroxide / ton of air dry mass or equivalent therewith. The method of claim 1, wherein step (a) is further performed by feeding alkali, oxygen, magnesium and steam with the peroxide. The method of claim 1 comprising the further step of introducing additional peroxide, alkali and steam into and mixing with the pulp, substantially simultaneously with the step (c) being performed or just before the pulp is fed into the second vessel. A process according to claim 3, in which step (c) is carried out to separate substantially all oxygen and all reaction gas from the mixture. 10 20 25 30 516 065 13 The method of claim 1 comprising the further step of treating the pulp prior to step (a) fi to remove transition metals therefrom. A method according to claim 15, in which during the pretreatment of the pulp to remove transition metals from this pulp is treated with at least one of the substances oxygen, EDTA and DTPA in combination with washing or pressing. The method of claim 1 comprising the further step of bleaching or delignifying the pulp with ozone or oxygen prior to step (a). A method according to claim 1, in which the first vessel is an upflow reactor and in which step (b) takes place when the gas flows upwards in the vessel and in which the spirit vessel is a downflow reactor and in which step (d) takes place when the mass flows downwards. A method according to claim 1, in which the first vessel is an upflow reactor and in which step (b) takes place when the gas flows upwards in the vessel and in which the second vessel is an upflow reactor and in which step (d) takes place when the mass flows upwards. A method according to claim 1, in which the pulp during the execution of steps (a) - (d) has a consistency of about 8 - 15%. The method of claim 20, wherein the cellulosic pulp treated is chemical pulp. A method according to claim 1, in which the first vessel has a height of about 15 - 30 meters and a volume of about 200 - 500 cubic meters and in which the second vessel has a volume at least twice as large as the first vessel. The method of claim 1, wherein step (e) is performed to remove substantially all of the oxygen and all of the reaction gas from the pulp.