EP0940498A1

EP0940498A1 - Method for treatment of pulp

Info

Publication number: EP0940498A1
Application number: EP99103920A
Authority: EP
Inventors: Tapani Vuorinen; Sari Juutilainen; Aki Vilpponen
Original assignee: Andritz Oy
Current assignee: Andritz Oy
Priority date: 1998-03-06
Filing date: 1999-03-05
Publication date: 1999-09-08
Anticipated expiration: 2019-03-05
Also published as: CA2264423C; DE69931295D1; EP0940498B1; ES2263236T3; FI119383B; CA2264423A1; US20020056533A1; FI980519A0; PT940498E; FI980519A

Abstract

The present invention relates to a method for treatment of pulp in an alkaline pulping process of chemical pulp. The pulp is treated in a chlorine dioxide stage comprising successive preferably a chlorine dioxide step and an acid treatment step at a pH of 2 - 5 and a temperature of over 80°C. The conditions in the chlorine dioxide step are such that the hexenuronic acid groups in the pulp do not react with chlorine dioxide.

Description

The present invention relates to a method for treatment of pulp in an alkaline pulping process of chemical pulp during which process pulp is treated in a chlorine dioxide stage including an acid treatment at a temperature of over 80°C.
Pulp mills have recently attempted to abandon the use of elementary chlorine, and partly also chlorine dioxide, the reasons for this being both aspects of environmental protection and market factors. Disadvantages caused by elementary chlorine include both noticeable malodorous gaseous emissions and liquid effluents from chemical pulp mills into water systems. Liquid chlorine dioxide does not cause odour disadvantages on such a large scale, it affects primarily water systems. When comparing these chlorine chemicals with each other by means of the AOX number designating the loading they incur on water systems, it can be noted that elementary chlorine is many times more detrimental than chlorine dioxide. The AOX number of chlorine is of the order of 4 - 7 and that of chlorine dioxide is 1 - 1.5, or even below one.
However, sequences utilizing chlorine dioxide are still popular and environmentally feasible. There are many reasons for this. The price of chlorine dioxide is very competitive compared with that of other chemicals. Also the strength and brightness of pulp obtained by dioxide bleaching are good, in fact at least approximately of the same order as the ones obtained by using peroxide at the same consumption of chemical (kg/admt).
Chlorine dioxide bleaching (D) is a well-known and widely used bleaching method. Conventional chlorine dioxide bleach treatment is usually performed at a temperature of 50 - 80°C and a with a chlorine dioxide dosage of about 10 - 30 kg per ton of pulp. Sequences used are for example D_oED₁ED₂ and OD_oED₁ED₂ and various modifications of these. The treatment time in a D_o stage is usually shorter than in other D stages, being for example 30 - 90 minutes. The pH of the pulp decreases to the range of 1 - 3 towards the end of the D_o stage. In the D₁ and D₂ stages the treatment time is 2-3 hours and the pH a little higher than in the D_o stage. The chlorine dioxide stage is usually performed at a temperature of about 70°C while the treatment time in the D_o stage is 0.5 - 2 hours and in the D₁ and D₂ stages 2 - 3 hours. Higher temperatures have been avoided as the low final pH in the range of 1 - 3 in the D stage combined with a high temperature and long treatment time deteriorates the strength properties of the fibers.
Beside chlorine dioxide bleaching, also bleaching processes utilizing chlorine-free chemicals, such as peroxide and ozone, have been introduced today. As pulp bleached with chlorine dioxide has good properties, the development of this process has been continued in addition to the new processes mentioned. One of the most important objects is to reduce the amount of chlorine dioxide needed in the bleaching and thus to make the method more acceptable in view of the environmental impact.
Finnish patent application no. 944808 and WO publication no. 96/12063 disclose an acid treatment method for pulp by means of which the consumption of chlorine dioxide can be reduced. This treatment removes from the pulp hexenuronic acids which can react with chlorine dioxide and thus increase chemical consumption. The acids may be removed by adjusting the pH of the pulp to the range of 2 - 5, preferably to 2.5 - 4, and the temperature to over 80°C, preferably to 90 - 110°C, and by retaining the pulp in these conditions typically for 30 - 300 minutes. During the treatment the kappa number of the pulp reduces typically by 2-9 units.
When the sequence O-A-D-E-D has been used, the consumption of chlorine dioxide in the bleaching of hard wood pulp has reduced by 30 - 40 % at the brightness of ISO 88 %. The corresponding consumption with soft wood pulp has reduced by 10 - 20 %. In both cases, the yield remained nearly unchanged compared with bleaching without an A stage.
The object of the present invention is to provide an alternative method of combining a hot acid stage with a bleaching sequence utilizing chlorine dioxide. A further object of the invention is to provide a method which may be applied in an economical way particularly in the bleaching plants of existing pulp mills. In other words, the object of the invention is to fit chlorine dioxide bleaching of pulp into a bleaching plant in an overall economical and environmentally acceptable way.
To achieve the objects mentioned, the method of the invention is characterized in that the chlorine dioxide stage comprises successively at least a first chlorine dioxide step and an acid treatment step and that the pH of the first chlorine dioxide step is adjusted so that the final pH of the step is over 4. The conditions during the first chlorine dioxide treatment are such that the hexenuronic acid groups in the pulp do not react with chlorine dioxide.
It has been disclosed above that hexenuronic acid groups are removed by hot acid treatment prior to the chlorine dioxide treatment. According to the invention we have now discovered that the ClO₂ treatment can be performed in the bleaching sequences also before the hot acid treatment if reactions between the hexenuronic acid groups of the pulp and the chlorine dioxide are prevented. Thus, the D stage comprises successively a D step and an A step. According to the method of the invention, the chlorine dioxide step of the DA stage is performed so as to have a final pH of over 4, preferably over 5. The highest pH value is normally 7. Conventionally, the final pH of the first or the second chlorine dioxide bleaching stage is less than 4, typically 1 - 3.5. Surprisingly, it has been found out that hexenuronic acids do not react with chlorine dioxide at the pH range of over 4 and thus no chemical is consumed by reactions with chlorine dioxide. Chlorine dioxide is reduced to chlorite but it does not decompose further. The chlorine dioxide dose in this step is preferably 0.1 1.5 % active Cl (1 - 15 kg active Cl/admt), preferably over 0.5 - 1.0 % active Cl (5 - 10 kg active Cl/admt).
In the acid treatment (A step), the conditions are typically as follows:

pH2 - 5, preferably 2.5 - 4
temperature over 80°C, preferably 90 - 110°C
time 30 - 300 minutes, preferably at least t minutes, t = 0.5 exp(10517/(T+273) - 24) (t = 0.5 e((10517/(T+273))-24), in which T (°C) is the temperature of the acid treatment.

According to the invention, the temperature in the D step of the DA stage is preferably over 70°C, preferably 80 - 100°C, which is higher than the temperature of a conventional D stage. Thus, the temperature in the D and the A steps is essentially the same and there is no remarkable need to cool or to heat the pulp between the steps which is advantageous for the energy economy. However, the invention is not limited to the high temperature but the D step may be performed also at the conventional D step temperature of below 70°C.
The treatment time in the D step of the invention is short, less than 10 minutes, preferably 30 seconds to 3 minutes. Conventionally, the chlorine dioxide treatment time is over 30 minutes, even 120 minutes depending on the temperature and therefore the treatment requires a reactor of its own. The DA stage according to the invention may be practiced by performing the acid treatment in the reactor tower but because of the short retention time the D step preceding the A step may be arranged to take place for example in the feed line of the A tower. The pulp flowing in the line is heated to the desired temperature, for example 90°C, and chemicals, such as chlorine dioxide, and alkali or acid if necessary to adjust the pH, are mixed into it. The pulp flows under these conditions for the required period, e.g. one minute, and subsequently, acid is added to the pulp and it is supplied into the acid tower.
By providing the chlorine dioxide treatment before the acid treatment, the acid demand in the A step reduces as the reactions of the chlorine dioxide with the lignin produce hydrochloric acid and organic acids as byproduct.
It is characteristic of a preferred embodiment of the invention that the chlorine dioxide stage as described above comprises in addition to the D and A steps described above also a second D step, i.e. the stage in question comprises a first chlorine dioxide step, a hot acid treatment step and a second chlorine dioxide step (DAD, could be marked also D/A/D). The second step is performed in acidic conditions, in other words the pH may be less that 2 but preferably it is 2 - 4. Thus, no essential adjustment of the pH is needed after the A step. Adjustment of the temperature is not necessary, either, but the entire chlorine dioxide stage may be effected essentially at the same temperature as the A step and no essential adjustments of the temperature are needed between the steps. Thus facilitates remarkably practicing the step as far as process technology is concerned.
The treatment time in the second D step is preferably also short, less than 10 minutes, preferably 1 - 5 minutes and it may be performed for example in the discharge line of the A step prior to the washer following the DAD stage. Chlorine dioxide is added to the pulp flowing in the discharge line and the temperature and the pH are adjusted if necessary. The required chlorine dioxide dose is typically 0.5 - 2.0 % active Cl (5 - 20 kg active Cl/admt). The acid treatment removes hexenuronic acids from the pulp and therefore the amount of chlorine dioxide chemical required is smaller than with pulp from which hexenuronic acids have not been removed.
In this (DAD) embodiment of the invention, the chlorine dioxide treatment may be considered to have been divided into two relatively short partial treatments. Both steps may be practised at a high temperature; therefore the temperature and the treatment time must be chosen so as to cause as little viscosity losses in the pulp as possible. Two partial treatments contribute to a more homogenous brightness of the pulp.
The DA stage may be practiced also by arranging for the pulp a chelating treatment with EDTA, DTPA or a corresponding compound after the A step in order to remove harmful metals; in this case the stage is DAQ. Providing a chelating treatment in connection with the DA stage is advantageous particularly if the bleaching sequence comprises later a bleaching stage with a chemical such as peroxide which would be disturbed by heavy metals. The Q step may be performed at essentially the same temperature as the preceding D and A steps. The pH may be in the range of 3-6.
The total amount of chlorine dioxide needed in the DA stage is not greater than the amount needed in an AD stage although the first D step is performed before the acid treatment. The final pH of over 4 in the first D step prevents reactions between the ClO₂ and the hexenuronic acids. A DA stage has produced pulp of at least as good quality as an AD stage. An advantage provided by a DA stage or a multistep DAD stage is that, compared with an AD stage, a DA or a DAD stage may be effected with simpler apparatus as each step does not require a tower of its own. However, the invention may be practised also so that one or both of the D steps are performed in towers of their own.
A disadvantage of the AD step may be the release of malodorous gases in the acid step if the pulp still after washing contains adequate amounts of sulfur compounds from the digestion. When the D step precedes the A step, malodorous gases are oxidized by the bleaching chemical during the DA stage.
The method of the invention is applicable for pulps manufactured by the sulfate process and other alkaline methods which pulps contain hexenuronic acid compounds. The pulp to be treated has preferably been oxygen delignified. The DA stage according to the invention may be at the beginning of the bleaching sequence for example after the oxygen delignification or later in the sequence for example after a peroxide stage. Therefore the invention is advantageously applied for example in connection with the following bleaching sequences or partial bleaching sequences:
digestion-O- DAD-E;
digestion-O- DAD-E-D_N-D; or
digestion - O - Q - OP -D/A/Q - PO, in which
- indicates a wash between the stages,
O indicates a delignification stage utilizing at least oxygen,
Q indicates removal of metals by chelating,
A indicates a hot acid treatment,
E indicates an alkaline stage, and
OP or PO indicate a bleaching stage based on oxygen and/or peroxide, possibly at least part of the steps being pressurized and O indicating oxygen chemical and P peroxide, and the first letter in each step signifying the main effective bleaching chemical and the second letter possibly used the bleaching chemical supporting the bleaching reaction.
The method of the invention will be described more in detail in the following with reference to the enclosed drawing figure which illustrates by way of example a preferred apparatus for carrying out the method of an embodiment of the invention. Pulp is treated in a DAD stage.
Pulp is transferred from a preceding treatment stage 10 by means of a high consistency pump 12 to an acid tower 18. The consistency of the pulp is 6 - 25 %, preferably 8 - 18 %. The preceding stage may be a wash following digestion or more often an oxygen delignification following digestion or a wash following oxygen delignification.
Alkali or acid is added to the pulp in order to adjust the initial pH for the chlorine dioxide treatment or the first D step so that the final pH is over 4, preferably over 5, in order to prevent reactions between chlorine dioxide and hexenuronic acids. The alkali and the chlorine dioxide may be added directly to the pump 12, it may be injected into duct 15 between the pump 12 and the tower, or to a particular mixer 14 provided for this purpose. The typical chlorine dioxide amount added is 0.5 -1.5 % active Cl.
Steam is added to the pulp in order to raise the temperature typically up to 80 - 100°C. Steam may be added prior to the pump 12 in a steam mixer (not illustrated) or mixed in the duct 15 following the pump 12. Alternatively, pulp may be heated indirectly in the duct 15.
Thus, the first D step is performed in the feed line 15 of the acid tower while the pulp flows in the line. The treatment time in this step is typically 30 seconds to 3 minutes.
The D step is followed by an acid treatment in the tower 18. After the chlorine dioxide treatment the pH of the pulp is decreased to the level 2 - 5 required by the acid treatment by adding acid (sulfur acid, hydrochloric acid, waste acid from the production of chlorine dioxide or any suitable organic acid) to the pulp in the mixer 14. Also the temperature may be adjusted if necessary but according to the invention the steps of the DAD stage are preferably effected at the same temperature, for example 95°C, whereby no essential temperature adjustment is required between the steps.
The feed and flow of pulp to the acid tower should be as even as possible by using a distributor 16 or a doctor. The distributor mentioned is disclosed for example in U.S. patent no. 4,964,950, and its use for the above purpose is described in Finnish patent publication no. 94442.
If the device 16 has adequate mixing properties and if desired, acid may be added through it and thus even avoid the use/purchase of the device 14. Pulp flows from the device 16 to the reactor tank 18 dimensioned for the treatment time required by the acid treatment, for example 120 minutes at 95°C. Using the distributing feeder described in the FI publication no. 94442 mentioned ascertains that all portions of the tower are filled evenly and that the pulp column rises up evenly in the tower so that harmful channelling can not occur. Correspondingly, the top of the tower has been provided with a discharger 20 or a discharge doctor to guide pulp to the discharge line 24 of the tower.
The second D step is performed in the discharge line 24 in order to bleach the pulp. Chlorine dioxide may be added to the pulp via the acid tower discharger 20 of a pump 22 in the discharge line. Also a separate mixer (not illustrated) may be arranged in the line 24 for addition of chemical which is about 0.5 - 2.0 % actie C1
The addition of chemicals to the pulp is fitted so that a proper treatment time for carrying out the D step in the discharge line 24 before a washer 26 is reached. The retention time is less than 10 minutes, preferably 1 - 5 minutes: The treatment temperature of this D step is preferably the same as the one in the previous D and A steps; thus no essential adjustment of the temperature after the A step is needed here either. If desired, pulp may be heated or cooled in a heat exchanger or by direct feeding of steam. A higher temperature than conventionally is, however, required in order to obtain adequate bleaching during the short treatment time mentioned.
The pH of the second D step is lower than that of the first D step. Typically the pH is about 2 - 4; thus adjustment of the pH is usually not needed after the A step. If necessary, the chemical adjusting the pH may be added in the same stage as the chlorine dioxide.
Pulp has been bleached in a DAD stage the way described above and subsequently the pulp is treated in the washer 26 and is transported to further treatment. Typically the subsequent stage is an E stage.

Example:

Oxygen-bleached pulp was tested in a laboratory, the kappa number of which was 11.9, viscosity 1061 ml/g and the ISO brightness 49.2 %. The pulp was treated with the following sequences:

1. DAD-E_o-D_N-D

D:: - ClO₂ dose 1.0% as active Cl
- time 1 minute
-final pH 5.3
- temperature 95°C
A:: -time 180 minutes
-pH 3.5
- temperature 95°C
D:: -ClO₂dose 1.25% as active Cl
- time 2 minutes
-final pH 2.5
- temperature 95°C
E_o: - 85°C, 60 minutes, 1.25 %NaOH, O₂ 4 bar, final pH 11.7
- kappa number after treatment 2.3
- viscosity after treatment 890 mg/l
- brightness after treatment 70.2% ISO
D_N: -75°C, 180 minutes, 1.8% ClO₂ as active Cl, 0.3 % NaOH, final pH 3.3
- N (neutralization) pH 8.3
D: - 75°C, 180 minutes, 0.8% ClO₂ as active Cl, 0.1 % NaOH, final pH 5.1
Bleached pulp:: viscosity 868 ml/g, brightness 89,9 % ISO

In the second test the same pulp was treated

2. AD-E_o-D_N-D

A:: -time 180 minutes
-pH 3.5
- temperature 95°C
D:: - ClO₂ dose 2.25 % as active Cl
- time 1 minute
-final pH 2.1
- temperature 95°C
E₀: - 85°C, 60 minutes, 1.25 %NaOH, O₂ 4 bar, final pH 11.6
- kappa number after treatment 2.7
- viscosity after treatment 890 mg/l
- brightness after treatment 69.4% ISO
D_N: -75°C, 180 minutes, 1.8 % ClO₂ as active Cl, 0.3 % NaOH, final pH 3.4
- N (neutralization) pH 8.3
D: - 75°C, 180 minutes, 0.8% ClO₂ as active Cl, 0.1 %NaOH, final pH 5.1
Bleached pulp:: viscosity 866 ml/g, brightness 89,9 % ISO.

Based on the test we may state that a DA stage and an AD stage according to the invention produce pulp of the same quality. Some practical advantages described above may be obtained by effecting the chlorine dioxide and the acid treatment according to the invention.
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 methods.

Claims

A method for treatment of pulp in an alkaline pulping process of chemical pulp during which process pulp is treated in a chlorine dioxide stage including an acid treatment at a pH of 2-5 and at a temperature of over 80°C, characterized in that the chlorine dioxide stage comprises successively at least a first chlorine dioxide step and an acid treatment step and that the pH of the first chlorine dioxide step is adjusted so that the final pH of the step is over 4.
A method as recited in claim 1, characterized in that the pH of the chlorine dioxide step is adjusted so that the final pH of the step is over 5.
A method as recited in any of claims 1 - 2, characterized in that the temperature in the chlorine dioxide step is over 70°C.
A method as recited in claim 3, characterized in that the temperature in the chlorine dioxide step is 80 - 100°C.
A method as recited in any of the preceding claims, characterized in that the treatment time in the chlorine dioxide step is less that 10 minutes, preferably 30 seconds to 3 minutes.
A method as recited in any of the preceding claims, characterized in that the chlorine dioxide stage comprises in addition to the first chlorine dioxide step also a second chlorine dioxide step after the acid treatment step.
A method as recited in claim 6, characterized in that the treatment temperatures in the first chlorine dioxide step, the acid treatment step, and the second chlorine dioxide step are essentially the same.
A method as recited in any of the claims 1 - 4, characterized in that the chlorine dioxide stage comprises a treatment with chelating agent after the chlorine dioxide step and the acid treatment step.