EP0520140A1

EP0520140A1 - Ozone-alkaline extraction bleaching without intermediate washing

Info

Publication number: EP0520140A1
Application number: EP92103706A
Authority: EP
Inventors: Jian Ridge Center Jiang; Brain Ridge Center Greenwood; Joseph R. Ridge Center Phillips
Original assignee: Kamyr Inc
Current assignee: Kamyr Inc
Priority date: 1991-06-28
Filing date: 1992-03-04
Publication date: 1992-12-30
Also published as: FI922945A; NO922526D0; BR9202472A; ZA921843B; NO922526L; FI922945A0; MX9203191A; AU1282692A

Abstract

Enhanced brightness pulp, decreased color, and decreased COD discharge, are obtained during the bleaching of chemical cellulose (e.g. kraft) pulp by using a combined (ZE) stage, that is without washing between ozonation and alkaline extraction. Ozonation takes place preferably at acidic conditions and proceeds rapidly, while caustic extraction is at a pH of about 10-13. The (ZE)stage (29, 33) is preferably used in a bleaching sequence containing no more than five stages (25, 29, 31, 33 and 35), preferably (ZE)P(ZE)P, or (ZE)D(ZE)D, which may be preceded by a C/D stage, an O stage, an acid washing stage, or a nitrogen dioxide stage. The pulp produced can have a brightness of greater than 90 ISO, and ozonation can take place at a high consistency (greater than 25%), a medium consistency (about 6-18%), or low consistency (about 3% or less). If ozonation (10, 17) is at low consistency, dewatering (at 18) is effected prior to alkaline extraction (13,21).

Description

BACKGROUND AND SUMMARY OF THE INVENTION

In the conventional bleaching of chemical pulp, an ozone bleaching (Z) stage is generally followed by an alkaline extraction (E) stage, with inter-stage washing. Inter-stage washing has conventionally been considered necessary in order to remove ozonated lignin products so as to decrease alkali consumption in the subsequent E stage. It has been proposed to provide a (PZE) stage without washing between the Z and E treatments solely to maintain the peroxide in the pulp so that a peroxide reinforced alkaline extraction stage (Ep) is effected. However this proposal is merely another way to provide an Ep or Epo stage.
According to the present invention, going specifically against conventional wisdom in the art, a single (ZE) stage is provided, without washing between the ozonation and alkaline extraction. It has been found according to the invention that when such a stage is utilized higher pulp brightness can be obtained for a given amount of chemical, and there is decreased color and decreased COD discharge in the E stage effluent. The increase in brightness is typically about 2-8 percentage points (ISO), and a lower kappa number is achieved, compared to exactly the same treatment only with washing between ozonation and alkali extraction. While it is not positively understood why this improved result is obtained, it is believed that it is attributable to the existence of some ozonated lignin degradation products which are themselves strong oxidants, and thus enhance the removal of lignin degraded product in the alkaline extraction stage. The single (ZE) stage according to the invention is in fact substantially equivalent to the two stage sequence ZEpo, yet is simpler and more economical to operate.
The invention also relates to a method of bleaching cellulose chemical pulp in multiple stages, providing a (ZE)P(ZE)P or (ZE)D(ZE)D bleaching sequence. Optionally, a pretreatment step is provided before the first (ZE) stage, which pretreatment may be an oxygen stage, a C/D stage, an acid washing stage, or a nitrogen dioxide stage. The use of an oxygen delignification stage is particularly desirable. According to this aspect of the present invention, the bleaching sequence includes not more than five stages. With an O(ZE)P(ZE)P sequence, it is capable of achieving a brightness of significantly greater than 90 ISO. Normally there is between stage washing in the practice of the invention.
The method of bleaching according to the invention is also quite flexible. Ozonation can take place either at high consistency (about 25-50%), medium consistency (about 6-18%), or low consistency (about 3% or less). If ozonation takes place at high or medium consistency, caustic, typically with some water, is added directly to the pulp discharged from the ozonation treatment. The consistency of the pulp is adjusted to medium consistency range by the amount of water added, either separately, or in solution with the caustic. Where low consistency ozonation takes place, the pulp is dewatered before alkaline extraction.
According to the invention there also is provided a multi-stage bleaching apparatus, having separate and distinct stages, for bleaching cellulose pulp. The apparatus comprises: In a first stage, means for subjecting the pulp to treatment with ozone at acidic conditions, immediately followed -- without any washing -- by alkaline extraction. In a second stage, means for effecting peroxide or chlorine dioxide bleaching of the pulp. In a third stage, means for subjecting the pulp to treatment with ozone at acidic conditions, immediately followed -- without any washing -- by alkaline extraction. And, in a fourth stage, means for effecting peroxide or chlorine dioixde bleaching of the pulp.
It is the primary object of the present invention to provide for the production of enhanced brightness, per unit of chemical consumed, chemical pulp utilizing a single (ZE) stage, without washing between ozonation and extraction, and a maximum five stage bleaching sequence making optimum utilization of the (ZE) stage. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGURE 1 is a schematic view showing the detail of a single (ZE) stage according to the invention when the pulp is high or medium consistency;
FIGURE 2 is a view like that of FIGURE 2 for when the pulp is of low consistency; and
FIGURE 3 is a schematic view showing an exemplary bleaching sequence according to the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

According to the present invention the inter-stage washing normally provided between ozone bleaching and alkaline extraction stages is eliminated to provide a single (ZE) stage comprising both ozonation (typically under acidic conditions, e.g. at about pH 2-4), and alkaline extraction (e.g. at about pH 10-13). The single (ZE) stage provides improved brightness and other pulp properties compared to conventional ozone bleaching followed by alkaline extraction with between stage washing, in fact providing an enhancement substantially equivalent to the two stage sequence ZEpo, yet being simpler and more economical. The ozonation/extraction stage according to the invention can employ high (greater than 25%), medium (about 6-18%), and low (less than about 3%) pulp consistency during the ozone bleaching phase.
FIGURE 1 schematically illustrates a (ZE) stage according to the invention when the pulp being fed to the mixer 10 is high or medium consistency. Ozone is added to the pulp in the mixer 10, and after a short period of time is allowed for ozonation to take place, alkali is added at 11 either in a conduit 12, or in a vessel 13. Where it is necessary to dilute the pulp -- that is where the pulp initially has a concentration greater than the medium consistency that is desired for alkaline extraction -- water can be added in a line 14; or the water may be added merely in the form of a more dilute caustic in line 11. The pulp is maintained in the reactor 13 a conventional length of time for alkaline extraction, and then is discharged in line 15, typically to be washed before a subsequent bleaching stage.
The mixer 10 and reactor 13 are only exemplary structures. A tumbling device may alternately be utilized, as may be conventional ozone and alkaline extraction treatment devices.
FIGURE 2 schematically illustrates apparatus similar to that in FIGURE 1 only for the situation when the pulp being fed to the mixer 17 is of low consistency (e.g. about 3% or less). In this case, after ozonation, the pulp passes to a conventional dewaterer 18, and only after the pulp has been concentrated to medium consistency is alkali added in line 19 either to the conduit 20, or to the vessel 21, with the treated pulp being discharged from the vessel 21 in conduit 22 after an appropriate alkali extraction time. The filtrate in line 23 from the dewaterer 18 -- or a portion thereof -- can be used to dilute the pulp prior to the ozonation (e.g. prior to mixer 17) in actual mill practice.
An example of an (ZE) stage, in comparison to a conventional ZE sequence with between stage washing, will now be set forth:

EXAMPLE I

In a laboratory Rotovap reactor, oxygen-delignified pine pulp at about 30% consistency, pH 3, and room temperature, was charged with an ozone-carrying gas to deliver a desired ozone addition of about 0.10-0.75% on pulp. The pulp was mixed with the gas by adjusting the rotation speed of the reactor so as to cause the pulp to tumble. After the desired ozone charge was reached, the gas was turned off and the pulp was allowed to continue to tumble for about one minute. Upon completion of ozonation, the pulp was immediately subjected to alkaline extraction without washing. The extraction conditions were 1-2% NaOH, 10% consistency, 70°C, 60 minute treatment time, and end pH about 11. After extraction the pulp was washed, and its kappa number, viscosity, and brightness were measured.
A ZE two-stage bleaching sequence with inter-stage washing was conducted under otherwise identical conditions. It was found that the (ZE) single stage, without washing, gave about 2-8 percentage points higher in pulp brightness and slightly lower kappa number as compared to the ZE two-stage sequence with between stage washing.
A decreased color and COD discharge in the E-stage effluent is also expected in actual practice as a side benefit from the (ZE) stage. The decrease in color and COD should be comparable to that achieved by converting an E to an Eo stage.
An example of a low consistency (see FIGURE 2) ozone bleaching O(ZE) sequence compared to a prior art OZE sequence will now be described:

EXAMPLE II

Ozonation was carried out at 1.0% pulp consistency in a 10-liter glass reactor at 20-50°C. The pulp slurry was stirred vigorously with a Waring blender mounted at the bottom of the reactor. The ozone gas was introduced into the slurry in the center of the reactor, about 0.5 inch above the blender blades to effect uniform mixing and adequate contact between the ozone gas and the pulp. After reaching the desired ozone dosage, the ozone gas was shut off and the slurry was further stirred for about 10-30 min. The slurry was then dewatered to about 15% consistency. For the O(ZE) sequence, the dewatered pulp was immediately subjected to alkaline extraction with 1-2% NaOH at 60-70°C for about 60 minutes. For the OZE sequence, a washing stage was performed after dewatering prior to an alkaline extraction stage.
The results from this test for low consistency ozonation were found to be comparable to those in Example I (for high consistency ozonation).
The (ZE) single stage according to the invention is best utilized in a four or five stage bleaching sequence with peroxide. Such a bleaching sequence is illustrated in FIGURE 3. Where a pretreatment stage 25 is provided, chemical pulp (e.g. kraft, sulfite, sodium, or like pulp) is fed to a conventional reactor 26 suitable for the particular pretreatment involved, and the discharged pulp in line 27 may optionally be washed in washer 28, and then fed to the first (ZE) stage 29, which typically will be one of the structures schematically illustrated in FIGURES 1 and 2 (or described with respect to these FIGURES), depending upon the concentration of the pulp.
The pre-treatment stage 25 preferably is an oxygen delignification stage, which is practiced at typical oxygen conditions of time, temperature, consistency, chemical charge, and pH. However other pretreatment stages also may be provided. For example there may be a chlorine/chlorine dioxide (C/D) stage, such as described in U.S. patent 4,959,124, prior to the (ZE) stage 29, or a nitrogen dioxide pretreatment stage, or an acid washing stage (e.g. where the pH of the wash water is adjusted to 2-3 with sulfuric acid, with the temperature maintained at about 20-60°C, for about 30-60 minutes) to remove heavy metal ions.
After the first (ZE) stage 29, the pulp is fed in a conduit preferably to a conventional washer 30, and then from there to a conventional peroxide or chlorine dioxide reactor 31. From the conventional peroxide or chlorine dioxide reactor 31 the pulp is optionally fed to another washer 32 which removes the bleach chemical and other products, then to a second (ZE) stage 33. From the stage 33 the pulp is fed to the washer 34 and then to the second peroxide or chlorine dioxide stage 35. The bleached pulp discharged in line 36 typically has enhanced brightness (e.g. greater than 90 ISO) compared to pulp produced in exactly the same way if the six stage sequence ZEPZEP, or the four stage sequence ZEPP, is utilized given the same total ozone and peroxide or (chlorine dioxide) dosage.
The following example compares an O(ZE)P(ZE)P sequence according to the invention to a prior art OZEPZEP sequence.

EXAMPLE III

The (ZE) stage was conducted as described above in EXAMPLE I, and after the pulp from the first (ZE) stage was washed it was bleached with 1.0-2.0% peroxide, 0.05% MgSO₄ (as Mg), and 0.2-1.0% NaOH at 10% consistency, 70°C, for 180 minutes in a lab reactor. After washing the (ZE) second stage was practiced in the same manner as the first stage, and then after washing the second peroxide stage was practiced in the same manner as the first peroxide stage.
In the prior art sequence, the ZE two stage bleaching sequences were conducted identically to those described with respect to EXAMPLE I, and the peroxide stages were identical to those described earlier in this example. It was found that the bleached pulp according to the invention had 2-5 percentage points higher pulp brightness and a 5-10% reduction in both effluent color and COD loads as compared to the prior art sequence (with washing between the Z and E stages).
It will thus be seen that according to the present invention a method of bleaching cellulose chemical pulp, a multi-stage bleaching apparatus, and a new bleaching stage ((ZE) with no washing between ozonation and alkaline extraction) are provided, with subsequent advantages in pulp brightness, decrease in color and COD, etc. While the invention has been herein shown and described in what is presently conceived to be the most practical and preferred embodiment it will be apparent to those of ordinary skill in the art that many modifications may be made thereof within the scope of the invention, which scope is to be accorded the broadest interpretation of the appended claims so as to encompass all equivalent structures and methods.

Claims

A method of bleaching cellulose chemical pulp in multiple stages, comprising the steps of sequentially: (a) subjecting the pulp to treatment with ozone, followed by alkaline extraction; (b) effecting peroxide or chlorine dioxide bleaching of the pulp (31, 35); (c) repeating step (a); and (d) repeating step (b); characterized in that step (a) is a single stage (29) in which alkaline extraction immediately follows ozone treatment, without any washing between them.
A method as recited in claim 1 further characterized in that there are no other bleaching steps except steps (a) - (d).
A method as recited in claim 1, further characterized by the step of preceding step (a) by: (e) oxygen delignifying in a separate stage (25); and further characterized in that the bleaching comprises only steps (a)-(e).
A method as recited in claim 1 further characterized in that the ozonation (10, 17) of steps (a) and (c) is practiced at a pH of about 2-4, and the alkaline extraction of steps (b) and (d) is practiced at a pH of about 11-13.
A method as recited in claim 1 further characterized in that the pulp has a solids consistency of about 25%-50% during ozonation (10, 17), and the pulp solids consistency between ozonation and alkaline extraction is diluted to about 6-18%.
A method as recited in claim 1 further characterized in that the pulp has a solids consistency of about 6-18% during ozonation (10, 17).
A method as recited in claim 1 further comprising a pretreatment stage (25), before the stage of step (a), comprising one of a C/D stage, an O stage, an acid washing stage, and a nitrogen dioxide stage.
A method of bleaching essentially peroxide free cellulose pulp in a bleaching stage, comprising the step of treating the pulp with ozone (10, 17), and characterized by the step of then -- without washing -- subjecting the pulp to alkaline extraction (13, 21).
A method of bleaching cellulose kraft pulp comprising a bleaching sequence which includes not more than five stages, including: a P or D stage as the last stage (35); and a P or D stage as the third to last stage (31); and characterized by the steps of a combined ZE stage, without washing between the Z and E treatments, as the fourth to last stage (33), and a combined ZE stage, without washing between the Z and E treatments, as the second to last stage (29).
A method as recited in claim 9 further characterized in that the first stage (25) is an acid washing stage in which the pulp is treated with a wash liquor having a pH adjusted to about 2-3 with sulfuric acid, at a temperature of about 20-60° C for about 30-60 minutes.