JP3276083B2 - Pulp bleaching with chlorine-free chemicals. - Google Patents

Abstract

The present invention is directed to a method of washing pulp in connection with bleaching, in which method the pulp is bleached in a first bleaching arrangement, washed in a first washer, bleached in a second bleaching arrangement, and washed in a second washer in such a manner that the washing liquid is recirculated from said second washer to said first washer countercurrent to the pulp flow, characterized in introducing washing liquid to said second washer for washing the pulp coming from said second bleaching arrangement, displacing with said washing liquid at least two different filtrates, collecting said at least two different filtrates separately, and using said at least two filtrates separately upstream of said second washer as washing liquids.

Description

Description: TECHNICAL FIELD The present invention relates to a new class of methods for producing pulp using chlorine-free chemicals. The invention relates in particular to ozone bleaching of pulp without prior removal of heavy metals.

(Background technology)

Due to various regulations and commercial conditions, there is an increasing demand from kraft pulp manufacturers to reduce or eliminate organochlorine compounds in pulp products and bleach effluents. To meet these requirements, chlorine gas or other chlorine-containing compounds (eg,
Chlorine dioxide) should not be used. Without the use of chlorine bleaching chemicals, obtaining the desired whiteness becomes extremely difficult, especially if the pulp produced is given acceptable strength conditions. Thus, for example, lignin must be removed with oxygen. By using multi-stage delignification with oxygen,
Benefits have been obtained with respect to delignification and selectivity, especially when chelating agents are added to limit the amount of harmful metal ions, and especially when washing is provided between the steps of the process (US Pat. No. 4,946,556). However, the real obstacle is that both delignification and the quality of the pulp produced solely by oxygen delignification are limited, especially if an ozone bleaching stage follows.

Heretofore, the typical and general purpose of chlorine-free bleaching processes has been to remove as much of the heavy metals from the pulp as possible before ozonation. Because heavy metals
This is because it is known to destroy ozone, as taught in EP-A-0512590. A typical series of bleaching steps that bleach pulp more easily is, for example, OOAZ
EZPZ, OAZEZPZ, OOAZEZP, and OAZEZP. These successive stages include one or more oxygen bleaching stages (O), A-stages (acid wash), ozone stages (Z), extraction stages (E), second ozone stages (Z), and peroxide stages (O). P), and sometimes a third ozone stage (Z). The acid step (A) prior to the first ozone step removes heavy metals, which are effluent off when removing a portion of the wash filtrate. The extraction step (E) may be a oxidative extraction with a peroxide or a conventional oxidative extraction step. Ozone bleaching stage is about 5-18
It is preferably carried out on pulp having a% consistency.

It includes at least five washing stages, in other words, the bleaching stage and the washing stage, i.e., the washing machine, are alternated, in which the chemicals are separated from the fiber as reaction product, or otherwise each. Removal of the drug from the suspension during the bleaching step is a feature of the continuous step. Washers account for a significant portion of the capital cost of a bleach plant,
The number of washers should, of course, be as small as possible, without risk of compromising the quality of the end product.

In that regard, the chemicals required for certain bleaching reactions, such as magnesium (Mg), are also removed from the pulp, requiring the addition of magnesium after ozonation.

On the other hand, the chlorine used so far has prevented screening steps after the bleaching step from being combined with the bleaching plant. This is because in chlorine bleaching, pulp screening and / or vortex cleaning treatments can cause severe corrosion problems. Therefore, swirl cleaning and / or
Screening had to be performed as a separate step. Thus, conventional bleaching + screening plants include four or five bleaching stages and one screening and / or vortex cleaning stage, separated from each other by a washing stage. The present invention integrates these five to six stages into three stages, thus nearly halving the capital cost of the bleaching and screening plants.

By using the present invention, the above-mentioned drawbacks caused by the elimination of conventional heavy metal removal and screening can be eliminated. At the same time, a bleach plant having only three washing stages is provided.

It is a feature of the process according to the invention that the pulp is bleached in a series of steps starting with ozone and without heavy metal removal before the ozone stage.

Bleaching of the pulp in a series of three stages (ZT) (EOP) (ZP) without the removal of heavy metals prior to those processes is a feature of another embodiment of the process according to the invention, wherein -(ZT) refers to the stage of bleaching with ozone, including the treatment of heavy metals, followed by washing and / or enrichment;-(EOP) refers to peroxide or oxygen and peroxide under alkaline conditions. (ZP) refers to the ozone and peroxide bleaching step, followed by washing and / or thickening.
Refers to a stage in which washing and thickening follow without washing between stages.

In each of steps (ZT), (EOP) and (ZP), there is no intermediate wash.

[Brief description of drawings]

FIG. 1 is a diagram schematically illustrating a bleaching process according to an example of the first embodiment of the present invention.

FIG. 2 is a diagram schematically illustrating a bleaching process according to the second embodiment of the present invention.

FIG. 3 is a diagram schematically illustrating a portion after the bleaching process according to the third embodiment of the present invention.

FIG. 4 is a diagram schematically illustrating another bleaching process according to the present invention, particularly a portion where an acid and / or alkali is added.

FIG. 5 is a diagram schematically illustrating a method of recycling a washing filtrate according to still another embodiment of the present invention.

FIG. 6 is a diagram schematically illustrating a method of recycling the washing filtrate according to the second embodiment of the present invention illustrated in FIG.

Fig. 7 shows the DRUM DISPLAC
FIG. 2 schematically illustrates the results of a study performed using an ER) ™ washer.

[Detailed description of drawings]

According to the embodiment of FIG. 1, the high consistency pulp tower 10
Pulp is discharged from the column and fed into an ozone reactor 14 by, for example, an MC (trade name) (ie, fluidizing) pump 12 from which the pulp is discharged. The pulp is preferably discharged into the first reaction column 18 by the gas separator 16. The pulp is preferably discharged from column 18 by a MC pump 20 into a washer 22. The washer may be a conventional drum displacer washer or a conventional pressurized diffuser. The pulp is pumped from the washer 22 by the MC pump 24 into the oxygen reactor 26 and from the reactor 26 to the second reactor 28. From the column 28 the pulp is sent by the MC pump 30 to a second washer 32 (preferably a drum displacer washer), from which the pulp is further sent by the MC pump 34 to a second ozone reactor 36 and from there a separator It is sent to the third reaction column 40 through 38. MC pump 42 for pulp from tower 40
To a third washer 44 (preferably a drum displacer washer).

As can be seen from FIG. 1, ozone (in the carrier gas) is mixed with the pulp by the mixer 60 and then sent to the first ozone reactor 14. Similarly, oxygen may be mixed by a mixer 62 before the oxygen reactor 26, and the second ozone reactor
Before 36, it may be mixed with a mixture of ozone and a carrier gas. Mixers 60 and 62 are AHLMIXER
Preferably, it is a fluidized mixer of the trade name, which is capable of mixing very large amounts of gas into the fiber suspension, including the intermediate consistency suspension.

FIG. 1 also shows the pH of the pulp for the first ozone stage.
It teaches how the acid can be fed into the pulp, for example, into the pump 12, to adjust the value and subsequent heavy metal removal. Similarly, a complexing agent such as EDTA and / or an alkali may be added to the pulp prior to removing heavy metals in the first reaction column 18. If too much magnesium has been removed from the pulp by scrubber 22, for example during pump 24 and / or during discharge from oxygen reactor 26, or in any other suitable manner, with alkali Magnesium may be added. Another possible way to adjust the pH of the pulp for the second ozonation is to feed the acid in the pump 34 after the second washer 32 or in some other suitable manner. Before supplying the pulp into the third reaction tower 38, alkali, peroxide and / or magnesium may be added to the pulp as illustrated in FIG.

The reaction columns 18, 28 and 40 in FIG. 1 are all shown as being of the downward flowing type. Alternatively, they
As shown in the figure, a type that flows upward may be used. The only important difference between FIGS. 1 and 2 is the direction of flow in the reaction column. In the embodiment of FIG. 2, parts that are the same as or functionally equivalent to the parts of the embodiment of FIG. 1 are indicated by prefixing the same reference numerals with only a “1”. The pump 20 of FIG.
30 and 42 have also been replaced with 120 ', 130' and 142 '. 1 because they have been relocated to the other side of the reaction column, in other words, the scrubber 2 illustrated in FIG.
Instead of feeding to 2, 32, and 44, they are feeding pulp to the reactors 118, 128, and 140 of the embodiment of FIG.

In the process according to the invention, for example, a continuous EMCC digester, sold by Kamyr Inc. of Glens Halls, NY, is cooked to a low Kapper number, followed by oxygen bleaching. The kappa value is further reduced to a value of about 14 or less. By using effective steaming, hot alkali extraction, and oxygen bleaching, kappa numbers in the range of 14-5 are obtained for both softwood and birch wood. A kappa number of 14 is sufficient to achieve the final bleaching with chlorine-free bleaching chemicals and to reach the maximum whiteness defined by ISO 86 (preferably ISO 88). Therefore,
The process according to the invention can also be used successfully for the final bleaching of pulp prebleached to a kappa number of 14.

The kraft pulp is bleached after the pulp washes according to the invention without separate removal of heavy metals, for example in the sequence according to FIG. 1, which is described in more detail below. Prior to performing the sequence of steps according to the invention, the pulp may be treated with enzymes, if necessary. The pulp is taken from the high consistency pulp tower 10 to the (ZT) stage, the first bleaching stage. In the Z section of the (ZT) stage, the pulp is bleached with ozone, the ozone dosage is about 2-10 kg / adt, the pH is about 2-7, and the temperature is about 40-70 ° C. Pulp
The pH value is adjusted by adding acid to the bottom of the high consistency tower 10, the pump 12 (or the outlet to the pump 12 as seen in FIG. 1), or the chemical mixer 60. After the ozone has reacted, the residual gas is removed from the pulp, preferably in a gas separator 16, and the treatment of the heavy metals commences in the first reactor 18 in the T portion of the (ZT) stage.

The T portion of the (ZT) stage may be performed, for example, in the following manner. One method lowers the pH value of the pulp to the range of 2 to 4, thereby dissolving most of the heavy metals into the filtrate phase and washing away in a concentrator or scrubber 22 following that step. Is also good. The disadvantage is that most of the magnesium (Mg) is also discharged, so that magnesium must be added to the pulp, often in the form of magnesium sulfate, due to the oxygen and / or peroxide stages that follow the sequence. It is gone.

Another method of performing the T moiety of the (ZT) stage is a complexing agent,
For example, using EDTA. Next, T in the (ZT) stage
It is also advantageous to carry out the portion in a pH range of about 4-7, and to bring the pH of the Z portion before the T portion of the (ZT) stage to about pH 4. In this manner, manganese (which is harmful in the oxygen stage) can be washed away without discharging magnesium, thereby reducing the magnesium addition required in the oxygen and peroxide stages of the final bleach (or It may even be unnecessary to add magnesium).

As mentioned above, the actual metallization [T at the (ZT) stage
Part] is not performed before the Z part. Conventionally, this was considered necessary. Tests with intermediate consistency ozonation showed that the bleaching reaction with ozone was so rapid that heavy metals had no time to destroy significant amounts of ozone.

If necessary, an enzyme may be added to the T portion of the (ZT) stage. (ZT) of the filtrate S ₁ of scrubber 22 following the step, (Z
T) sent to pulp washing before stage or sent to sewage,
Or you may send to collection | recovery of chemicals for cooking.

In FIG. 1, an (EOP) stage follows the (ZT) stage. At this stage, the oxygen dose is about 2-6 kg / adt and the peroxide dose is about 10-20 kg / adt. In certain special cases, the process can be carried out without any oxygen.
The temperature of the (EOP) stage is about 60-95 ° C and the pH is about 9-1.
2. The time is about 2-8 hours. If necessary, magnesium may be added as a protective chemical. It was cleaned after (EOP) stage, thereby resulting filtrate S _2. The filtrate S ₂ is sent to a pulp wash before or after the (ZT) stage,
Or it may be sent to sewage or chemical recovery.

In FIG. 1, after the (EOP) stage, the second ozone bleaching stage,
That is, the (ZP) step follows. The ozone portion at the (ZP) stage is
Usually, it is carried out under a low temperature acidic condition by a method according to a conventional method so that ozone reacts appropriately. Correspondingly, (ZP)
The P portion of the step is performed under hot alkaline conditions in accordance with the teachings of the prior art to allow the peroxide to react properly. Thus, their combination in an economically advantageous manner according to the concept of the present invention has heretofore been considered impossible. In the (ZP) stage, the following conditions can be used.

In the Z part of the (ZP) stage, the ozone dose is small, 3k
Lower than g / adt, the purpose of ozone is only to activate. With disadvantageous conditions, some of the ozone reacts only slightly, but this is not a big deal. This is because the dose is small. Thus, ozone is used primarily to activate the bleaching stage. The temperature of the ozone stage is between 50 and 80C, preferably between 60 and 70C.

The pH is between 4 and 10, preferably between about 6 and 10.

The dose in the P part of the (ZP) stage is also small, usually 10 kg / a
Less than dt. Usually about 3-7 kg / adt is sufficient. Therefore, the temperature of the peroxide stage is 60-80 ° C., preferably 70-80 ° C.
Can be lowered to 80 ° C. The pH is between 9 and 11, preferably about 10. The time is about 1-6 hours.

(ZP) The conditions of the Z and P portions of the stage are brought closer to each other,
Eliminates the need for cleaning and heating between the Z and P portions of the (ZP) step. In addition, small amounts of acid and alkali are sufficient to control the pH of the Z and P portions of the (ZP) stage. In some cases, heating and / or acid between stages is unnecessary.

(ZP) stage after the pulp is washed and the filtrate S ₃ is obtained.
The filtrate S ₃ may be used for washing before the bleaching step in combination with pulp or waste sewage, it may be sent to recover the cooking chemicals.

According to a further embodiment shown in FIG. 3, the method according to the invention has changed considerably due to the rearrangement of the apparatus. For example,
A vortex cleaner 66 and / or a screening plant is added to the last stage of the bleaching plant according to FIG.
Place before thickener / washer 68. In this case, the washer need not be the same MC washer as in the previous embodiment. If vortex cleaning or screening with a slot screen is used, the pulp is diluted after P-tower 140 to a consistency range of about 0.5-1.5%. On the other hand, when screening using a perforated screen, a dilution of about 2-4% is usually sufficient. After vortex cleaning or screening, the pulp is thickened and washed. Normal suction filter
Use 68. Conventionally, the pulp had to be diluted to a screening consistency after washing and then concentrated again to an intermediate consistency after screening.

In the embodiment illustrated in FIG. 4, EDTA is not used, but the metal is removed by the acid in the Z step and by the addition of magnesium to the (EOP) and (ZP) steps. The addition is
In the (EOP) stage, the operation can be sufficiently performed by the MC pump 224 or the oxygen mixer 226. This is the MC pump in the (ZP) phase
234, an ozone mixer 264, or a peroxide mixer 70 can also be used. The total amount of chemical required is given in Table 1. The initial κ value before bleaching is estimated to be 10.

When carrying out the method described in Table 1, about 20-30 kg of NaOH
/ The adt and H ₂ SO ₄ 15~25kg / adt, consumed by water usage.

In addition to the chemicals in Table 1, bleaching may be enhanced using enzymes. Appropriate locations for enzyme treatment are as follows.

-HD-tower 210 before the (ZT) stage,-drop leg 21 following the Z reactor
8, the drop leg between the washer 222 and the (EOP) stage
72, the drop leg 7 between the washer 232 and the (ZP) stage
Four.

The effluent stream from the bleach plant may be reduced by recirculating the filtrate in the process according to FIG. FIG. 5 illustrates an oxygen delignification step 80, followed by two washing steps 82. The pulp is transferred from the washing stage to the (ZT) stage 83, from there to the washing stage 84, to the (EOP) stage 85, from there to the washing stage 86 to the
P) Send to step 87, which is followed by a wash step 88. Effluent clarification, the amount of effluent to bring to the discharge channel 90 is a 0~5m ³ / adt. Alternatively, a portion of the effluent may be sent to the cooking chemicals production section via the discharge channel 92 and used in place of fresh water. Thus, the amount of effluent that must be treated is minimized.

The reuse of the filtrates illustrated in FIG. 5 can be further improved by dividing them from the washer into two parts according to FIG. 6 having different pH. FIG. 6 uses the reference numbers of FIG. 5, but is preceded by a "1". The washer used in FIG. 6 is manufactured and marketed by Ahlstrom Corporation and is known as a drum displacer washer. For example, the pulp for the last washer 188 comes from the alkaline (ZP) stage. The filtrate that is first washed and then comes out is 1881 that is obviously alkaline, and the filtrate that comes out later 1882 can be less alkaline or even neutral. This is because the water 1880 flowing to the last washer 188 is generally neutral or slightly acidic.

In this manner, two circulating waters with different pH values 1881
And 1882, which can be used to properly adjust the pH of the pulp and then be sent to a bleaching continuous stage or to a particular stage of a single bleaching stage. The filtrates 1881 and 1882 of the last washer 188 of FIG. 6 are taken to the previous washer 186 in a manner that reduces the pH of the pulp before the (ZP) step. In this way, acids and alkalis are saved in the bleach plant. In the embodiment of FIG. 6, the washer before the (ZP) stage
From 186, two alkaline filtrates 1861 and 1862 are obtained, which are fed to a washer 184 before the (EOP) step. According to FIG. 6, it is preferred to add an acid (H ₂ SO ₄ ) 1840 to the filtrate 18 62, whereby the first filtrate obtained from the washer becomes acidic and a portion 1841 of which is passed to effluent clarification. Send the remaining 1842 to the washer 182 before the (ZT) stage,
The second filtrate 1843 becomes alkaline.

The pH of the filtrates may be adjusted by adding an acid or alkali to them before using them again. In some cases, for example, the filtrate is taken from brown stock (browm
stock) It may be necessary to add alkali to the filtrates before taking them to the wash, or to add an acid to keep the pH low while removing metals in the (ZT) step ( 1840). If the pH rises during washing, the heavy metals will thicken again and may even adhere to the fibers. The heavy metal is then carried to the P portion following the Z portion of the (ZP) stage, disturbing the peroxide stage. During the removal of heavy metals, the pH value is kept below 4, preferably at least 4.

Example In an experiment, using a drum displacer washer,
The incoming 100 ° C pulp was washed with 60 ° C water. The washed pulp was discharged from the washer at a temperature of 65 ° C. The temperature of the exiting filtrate was 95 ° C and 75 ° C. Thus, two significantly different filtrates were obtained from one washer. This was used in a bleaching plant to obtain two filtrates with different pH values. This is because the pH value of the filtrate is to some extent related to the temperature of the filtrate.

The experiment does not follow only the pH value and the temperature, but the expected pH value based on the temperature distribution is shown in each place in FIG.

pH-incoming pulp 10 pH-outgoing pulp 7.5 pH-filtrate I 9.5 pH-filtrate II 8.0 Such differences in pH values between filtrates are crucial when optimizing the chemical consumption of the bleaching plant. It is. The pH of the filtrate is close to the pH of the incoming and outgoing pulp,
Closer to these are more preferred than closer to each other.

As can be seen from the above description, a new method has been developed for bleaching pulp with chlorine-free chemicals in a short continuous process without removing heavy metals before the continuous bleaching stage. The present invention eliminates the need for a separate wash between the last wash step and the screening / vortex cleaning, and in a manner that only dilutes to the screening / cleaning consistency.
It also includes a new way to arrange screening following pulp bleaching.

Although the present invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, the invention is not limited to the described embodiment, but rather is defined by the following claims and their essence. It should be understood that the invention includes various modifications included therein and equivalent configurations.

──────────────────────────────────────────────────続 き Continuation of front page (72) Inventor Stromberg, Bertil Edison Road, Glens Falls, NY 12801 United States of America 4 (56) References JP-A-3-152286 (JP, A) JP-A-3-27191 (JP, A) JP-A-2-145,884 (JP, A) JP-A-5-148784 (JP, A) JP-A-63-256786 (JP, A) JP-T-62-502553 (JP, A) United States Patent 4450044 (US, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) D21C 9/00-9/18

Claims (26)

(57) [Claims] 1. The method according to claim 1, wherein at least about
Pulp bleaching comprising the steps of steaming and delignifying ground cellulose fiber material to produce a pulp having a κ value of about 14 or less in a manner of bleaching kraft pulp containing heavy metals such as manganese to an ISO brightness of 86. By using 2 to 10 kg of ozone as the main bleaching chemical per ton of air-dried pulp, using a (Z) part of the pulp having a pH of 2 to 7, then using a chelating agent at a pH of 4 to 7 Bleaching of the pulp in a (ZT) stage consisting of a (T) part meaning the actual metal removal, but without interstage washing in the (ZT) stage,-washing and / or thickening the pulp,-peroxidation A kraft pulp bleaching method, comprising the steps of: bleaching pulp with a product alone or with a combination of ozone or oxygen and peroxide. 2. The process according to claim 1, wherein the bleaching is carried out in a continuous stage (ZT) (EOP) (ZP). 3. The method according to claim 2, wherein the temperature during the (ZT) stage is about 40-70 ° C. 4. The method according to claim 2, wherein the pH during the whole (ZT) stage is about 4-7. 5. The method according to claim 1, wherein a heavy metal chelating agent is added to the pulp during or before removing the metal in the (ZT) stage. 6. During the (EOP) stage, the amount of peroxide added is about 10%.
The method according to claim 2, characterized in that it is ~ 20 kg / adt. 7. During the (EOP) stage, the amount of peroxide added is about 1
7. The method according to claim 6, characterized in that it is between 0 and 20 kg / adt and the oxygen addition is between about 2 and 6 kg / adt. 8. The pulp pH is maintained in the range of about 9-12, the temperature is maintained in the range of about 60-95 ° C., and the treatment time is about 2-8 while performing the (EOP) step. 3. The method of claim 2, wherein the time is time. 9. The method of claim 2, further comprising the step of adding magnesium to the (EOP) stage to provide protection to the pulp. 10. During the implementation of the (ZP) step, the amount of ozone added is less than about 3 kg / adt and the amount of peroxide added is about 10 kg / adt.
3. The method according to claim 2, wherein there is less. 11. The method according to claim 10, wherein the amount of peroxide added in the (ZP) stage is about 3 to 7 kg / adt. 12. The method according to claim 10, wherein the temperature during the contacting of the ozone with the pulp during the (ZP) stage is about 50-80 ° C. and the pH is 4-10. the method of. 13. The method according to claim 10, wherein the temperature of the pulp during contact with ozone in the (ZP) stage is about 60-70 ° C. 14. The method of claim 10, wherein the pH during the (ZP) step is in the range of about 6-10. 15. The method according to claim 2, wherein the pulp is screened immediately after the (ZP) step and the screening is followed by washing. 16. The method according to claim 15, wherein the screening is achieved by diluting the pulp to a consistency of about 0.5-1.5% and then treating it with a vortex cleaner (66). the method of. 17. The method of claim 1 wherein the pulp is screened for about 0.1 before screening.
16. The method according to claim 15, characterized in that it is diluted to a consistency of 5-4% and the screening is carried out at that consistency. 18. The method according to claim 1, wherein an enzyme is added to the pulp before the (ZT) step. 19. The method according to claim 1, wherein during the performance of the at least one bleaching step, at least one filtrate is formed, and the pH of the filtrate is adjusted by adding an alkali or an acid thereto. the method of. 20. The method of claim 1 wherein, during the (ZT) step, the pulp is treated with ozone and then treated with a filtrate having an acidic pH sufficient to prevent heavy metal re-concentration.
The method described in. 21. The process of claim 20, wherein the filtrate is maintained acidic by adding an acid thereto to prevent acid re-enrichment of heavy metals. 22. The pH of the filtrate remains below 4, and the pulp is treated with a pH below 4 while treating the pulp with the acidic filtrate.
21. The method according to claim 20, comprising: 23. The method according to claim 1, wherein the pulp is oxygen delignified before the (ZT) step. 24. During the implementation of the (ZP) step, the amount of added ozone
24. The method of claim 23, wherein less than 3 kg / adt and the amount of peroxide added is less than about 10 kg / adt. 25. During the (EOP) stage, the amount of peroxide added is about 1
24. The method according to claim 23, wherein the amount is 0 to 20 kg / adt and the amount of added oxygen is about 2 to 6 kg / adt. 26. The method of claim 1, wherein the pH for the chelating agent treatment is adjusted to between 4 and 7 by adding an alkali between the ozone stage and the complexing treatment.