FI93232C - Method for bleaching pulp 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

93232

Method for bleaching pulp with chlorine-free chemicals

The present invention relates to a new type of pulp bleaching process with chlorine-free chemicals. In particular, the invention relates to ozone bleaching of pulp without prior heavy metal removal treatment.

Various regulations and marketing have placed increasing demands on the production of kraft-10 pulp to reduce or completely avoid organic chlorine compounds in pulp products and bleaching effluents. In order to meet these requirements, the use of chlorine gas or any other chlorine-containing compound (eg chlorine dioxide) must be avoided. However, when chlorine-based bleaches are phased out, it is extremely difficult to achieve the desired brightness, especially if acceptable strength requirements are set for the pulp produced. In this case, lignin must be removed, for example by oxygen, and multi-stage oxygen delignification has advantages in lignin removal and selectivity, especially when chelating treatment is added to limit the amount of harmful metal ions, and especially if the process involves intermediate washing (see U.S. Patent 4,946,556). However, practical barriers limit both lignin removal and the quality of the pulp produced by oxygen delignification only.

Until now, however, it has been characteristic of all such methods 30 that efforts have been made to remove heavy metals from the pulp as accurately as possible prior to ozone treatment, as they have been known to deplete ozone. A typical bleaching sequence described in FI patent application NN, which has previously been used to bleach the pulp, is OOAZEZPZ, OAZEZPZ, 00ΑΖΕΖΡ and OAZEZP. The sequence thus comprises one or more oxygen bleaching steps (O), step A (acid washing), ozone step (Z), extraction step (E), second ozone step (Z) and peroxide step (P) and possibly a third 2,93232 ozone step. In the acid stage before the ozone stage, heavy metals are removed, which are leached out when part of the wash filtrate is removed. The extraction step (E) may be an oxidizing peroxide extraction step or a conventional oxidative extraction step. 5 The ozone bleaching steps are performed with a pulp having a consistency of about 5 to 18%.

The above sequences are characterized in that they comprise at least five washing steps, i.e. between the bleaching steps there are washing steps, i.e. scrubbers, in each of which the chemicals released as reaction products or otherwise from the fibers are removed from the suspension. As scrubbers represent a significant part of the investment costs of bleaching, the aim should, of course, be to keep the number of scrubbers to a minimum, if at all possible without compromising the quality of the final product.

At the same time, chemicals important for bleaching reactions, such as magnesium (Mg), are also removed from the pulp, which has naturally been followed by the addition of magnesium after ozone treatment.

On the other hand, the chlorine previously used has also prevented the post-bleach sorting step from being combined with the bleaching plant itself, because in chlorine bleaching, the pulp deposition and / or vortex cleaning treatment would lead to severe corrosion problems. Thus, vortex cleaning and / or sorting has to be performed in a different step. The conventional bleaching plant + sorting plant 30 thus consists of four or five bleaching steps plus one sorting and / or vortex cleaning steps, which are separated by a washing step. The present invention reduces said five to six steps to three steps and thus nearly halves the investment costs of bleaching and sorting.

The present invention has been able to overcome the above-mentioned disadvantages of the prior art, both in terms of the removal of heavy metals and the omission of sorting 3 93232. At the same time, a bleaching plant with only three washing steps is provided.

The process according to the invention is characterized in that the pulp is bleached with a three-step sequence (ZT) (EOP) (ZP) without the removal of heavy metals preceding said sequence, where - (ZT) is an ozone bleaching step, John is combined with a heavy metal treatment and followed by washing or precipitation, - (EOP) means a bleaching step under time conditions with peroxide or oxygen and peroxide followed by washing and / or precipitation, 15 - (ZP) means a bleaching step with ozone and peroxide without intermediate washing and followed by washing and / or precipitation.

The method of the invention will now be described in more detail with reference to the accompanying figures, in which Figure 1 shows a bleaching sequence according to a preferred embodiment of the invention, Figure 2 shows a bleaching sequence according to another preferred embodiment of the invention, and Figure 3 shows a bleaching sequence according to a third preferred embodiment, or the remainder of the bleaching sequence.

According to Figure 1, the bleaching sequence 30 according to the invention consists, in this exemplary embodiment, of a thick pulp tower 10 from which the pulp is discharged and fed, for example by an MC pump 12, to an ozone reactor 14, from which the pulp is preferably discharged to a first reaction tower 18. by means of a pump 20 to a scrubber 22, which may be, for example, a so-called DRUM DISPLACER ™ washer or pressure diffuser. From scrubber 22, the pulp is preferably pumped by MC pump 24 to oxygen reactor 26 and then to another 4,93232 reaction tower 28. From tower 28, pulp is preferably fed by MC pump 30 to another scrubber 32 (preferably DRUM DISPLACER ™), from which pulp is further pumped by MC pump 34 to second ozone reactor 36 and further through the gas separator-5 to the third reaction tower 40. From the tower 40, the pulp is pumped by an MC pump 42 to a third scrubber 44 (preferably a DRUM DISPLACER ™).

The figure also shows how ozone, together with carrier gas 10, is mixed into the pulp by mixer 60 before the first ozone reactor 14. Similarly, oxygen can be mixed before oxygen reactor 26 by mixer 62 and a mixture of ozone and carrier gas by mixer 64 before the second ozone reactor 36. The mixers used are preferably used. AHLMIXER ™ type fluidizing mixers capable of mixing very large amounts of gas into a fiber suspension.

In addition, Fig. 1 shows how, in order to adjust the pH of the pulp, the acid can be fed to the first ozone stage and the subsequent heavy metal removal stage, for example in connection with the pump 12. Likewise, prior to the heavy metal removal step in the first reaction tower 18, complexing agents such as EDTA and / or alkali may be added to the pulp. If too much magnesium has been removed from the pulp by scrubber 22, it can be added, for example, with alkali either in the pump 24 and / or in the oxygen reactor outlet or in some other suitable manner. Further, to adjust the pH of the pulp for the second ozone treatment 30, the acid may be fed after the second scrubber 32 in connection with the pump 34 or in some other suitable manner. Alkali, peroxide and / or magnesium may also be added to the pulp prior to feeding the pulp to the third reaction tower 38.

35

As can be seen from the figure, all reaction towers 18, 28 and 40 in Figure 1 are downstream. They can also be equally upstream, as shown in Figure 2. Thus, the only difference between Figs. 1 and 2 is the flow direction of the reaction towers and the fact that when reference numerals are indicated in Fig. 2, the figure 1 is added to the reference numerals in Fig. 1. In addition, in Fig. 2, pumps 20, 30 and 42 are replaced by pumps 120 ', 130 'and 142' because they have moved to the other side of the reaction tower, i.e. instead of the feed of scrubbers 22, 32 and 44 shown in Figure 1, they feed pulp to the reaction towers 118, 128 and 140 in the embodiments of Figure 2.

In the process of the invention, the pulp is boiled, for example, by the Extended Modified Continuous Cooking (EMCC) method developed by Kamyr Inc. to a low kappa number, after which oxygen bleaching further reduces the pulp count to at least 14. Utilizing efficient cooking, hot alkali extraction and oxygen bleaching area 14-5 with both coniferous and birch wood. In general, however, the number of pieces 14 is sufficient to allow the final bleaching to be performed with chlorine-free bleaching chemicals and still achieve the full brightness specified by ISO 88. Thus, the method according to the invention can also be successfully applied to bleach a pulp pre-bleached up to a number of pieces 14.

After washing the pulp and, according to the invention, without a separate heavy metal removal step, the pulp is bleached, for example, with the sequence according to Figure 1, which will be described in more detail below. If necessary, the pulp can also be treated with enzymes before the sequence according to our invention. The pulp is introduced from the thick pulp tower 10 to the first 30 bleaching stages, which is the ZT stage. In step Z, the pulp is bleached with ozone at a dosage of 2 to 10 kg 03 / adt, a pH of 2 to 7 and a temperature of 40 to 70 ° C. The pH of the pulp is adjusted by adding acid to the bottom of the thick pulp tower 10, pump 12 (shown in the figure) or chemical-35 mixer 60. After the ozone has reacted, the residue is preferably removed from the pulp in a gas separator 16 and treated with heavy metals in the first reaction tower 18.

6 93232 The T-step can be performed in the following ways, for example. The first option is to allow the pH of the pulp to drop to a range of 2-4, whereby most of the heavy metals are soluble in the filtrate phase and can be washed off in a subsequent precipitator or scrubber 22. The disadvantage is that a large proportion of magnesium (Mg) is also removed. it may be necessary to add magnesium to the pulp, usually in the form of magnesium sulphate, in the sequence for the subsequent oxygen and / or peroxide steps.

Another way to perform the T-step is to use complexing agents, for example EDTA. The T-step is then carried out in a pH range of 4 to 7, and it is also advantageous to keep the pH of the Z-step preceding the T-step above 4. In this case, the peroxide and oxygen-harmful manganese (Mn) can be kept in the filtrate phase without the oxygen and peroxide-preferred magnesium fibers. Thus, the manganese can be washed off without the magnesium being removed and clarified with less or no magnesium addition in the oxygen and peroxide steps of the final bleaching.

As described above, the actual metal treatment (T-25 step) is not performed before the Z step. In the past, this has been considered necessary. However, experiments with MC ozone treatment have shown that bleaching reactions with ozone are so rapid that heavy metals do not have time to deplete ozone to a substantial extent.

In the T-stage, enzymes can also be added if necessary. The filtrate SI of the scrubber 22 after the ZT step can be taken to the pulp wash before the ZT step, to the sewer or for the recovery of cooking chemicals.

35 The ZT phase is followed by the EOP phase as shown in Figure 1. Here, the oxygen dose is 2 to 6 kg O 2 / adt and the peroxide dose is 10 to 20 kg H 2 O 2 / adt. In some special cases, 7,93232 can also be driven completely without oxygen. The temperature in the EOP step is 60-95 ° C, the pH 9-12 and the time 2-8 hours. If necessary, magnesium is added as a protective chemical. The step is followed by washing to give a filtrate S2. The filtrate S2 can be taken to the washing of the pulp 5 before or after the ZT stage, to the sewer or to the recovery of cooking chemicals.

The EOP stage is followed by a second ozone bleaching stage, i.e. the ZP stage, as shown in Figure 1. Step Z is normally performed in prior art processes under acidic cold conditions to allow ozone to react efficiently. Correspondingly, the P-step is performed according to the teachings of the prior art under hot time conditions in order for the peroxide to react well. Thus, combining them in an economically meaningful way seems impossible according to the current perception. However, it is possible to carry out the ZP step, taking into account the following points: In the 20 Z step, the ozone dose is low, less than 3 kg 03 / adt and the purpose of ozone is only activation. Although unfavorable conditions are used and some of the ozone reacts poorly, the significance of this is small because the dose is small. Thus, ozone is mainly used to activate the bleaching stage 25. The temperature in the ozone step may be 50 to 80 ° C, preferably for example 60 to 70 ° C. The pH is between 4 and 10, preferably between 6 and 10.

The P-phase dose is also small, usually less than 10 kg H 2 O 2 / adt. 30 Normally 3 to 7 kg H 2 O 2 / adt is sufficient. Thus, in the peroxide step, the temperature can be reduced to 60 to 80 ° C, preferably 70 to 80 ° C. The pH is 9 to 11, preferably 10. The time is 1 to 6 hours.

35 Thus, the conditions of the Z and P phases are brought close to each other and intermediate washing and intermediate heating are avoided. In addition, a small amount of acid and alkali in the pH adjustment in the Z-8 93232 and P phases is determined. In some cases, no intermediate heating and / or acid is required at all.

After the ZP step, the pulp is washed to give a filtrate S3, which is useful for washing the pulp in connection with the previous bleaching steps, discharged to the sewer, or led to the recovery of cooking chemicals.

According to another preferred embodiment of the invention, shown in Figure 3, the process according to the invention has also made possible a significant change in terms of device placement. For example, in connection with the last stage of the bleaching plant, a vortex purifier 66 and / or a sorting plant can be added before the precipitator / washer 68, which in this case does not have to be an MC washer, as in previous embodiments. In this case, the pulp is diluted to a consistency range of 0.5 to 1.5% after P-tower 140 in the case of vortex cleaning or slit sorting. In the case of hole sorting, a dilution in the range of 2 to 4% is usually sufficient. 20 After vortex cleaning or sorting, the pulp is precipitated and washed - usually with a suction filter 68. Previously, the pulp had to be diluted to a sorting consistency after washing and again mixed to a medium consistency after sorting.

As can be seen from the above, a new and unprecedented method has been developed for bleaching pulp with chlorine-free chemicals in a short sequence without prior heavy metal removal treatment. The invention also includes a new way of arranging the sorting of the pulp after bleaching so that no separate washing is required between the last bleaching step and sorting / vortex cleaning, but only dilution to sorting / cleaning. It should also be noted that only a few preferred embodiments of the invention have been described above and that the true scope and coverage of the invention will be defined solely by the appended claims.

Claims (27)

A process for bleaching pulp into a high brightness in a short sequence of chlorine-free bleaching chemicals, in which the pulp is delignified by boiling and then by oxygen, by extraction or any other suitable delignification method to a kappa number of at least 14, characterized by that the pulp is delignified in a three-step sequence (ZT) (EOP) (ZP) without a prior heavy metal removal, where - (ZT) refers to an ozone bleaching step, sonically combined with heavy metal treatment and which step is followed by washing and / or Precipitation, (EOP) refers to an under-alkaline conditions bleaching step with peroxide or oxygen and peroxide and which step is followed by washing and / or precipitation, (ZP) refers to a bleaching step with ozone and peroxide without intermediate washing and which step is followed by washing and / or precipitation. 20 Process according to claim 1, characterized in that 2-10 kg ozone / adt for the first ozone (ZT) stage is mixed into the mass. 25 Process according to claim 1, characterized in that the pH of the pulp in the first ozone bleaching stage (ZT) is 2 - 7 and the temperature 40 - 70 ° C. Method according to claim 1, characterized in that the pH of the pulp in the first ozone bleaching step (ZT) of heavy metal removal stage is 2 - 4. Process according to claim 1, characterized in that the pH of the pulp is also in the first stage of the ozone bleaching step (ZT), which in its T-stage is 4 - 7. 93232 Method according to claim 5, characterized in that complexing agents, e.g. EDTA, is used in the heavy metal removal step. 5 Process according to claim 1, characterized in that enzymes are added in the heavy metal removal step (T) of the first ozone bleaching step (ZT). Method according to claim 1, characterized in that an EOP step, where the peroxide addition is 10-20 kg H 2 O 2 / adt, is performed in the second step of said sequence. 9. A process according to claim 1, characterized in that an EOP step, wherein the oxygen addition is 2 - 6 kg oxygen / adt and the peroxide addition is 10 - 20 kg H2 O2 / adt, is carried out in the second step of said sequence. Process according to claim 1, characterized in that the pH of the pulp in the EOP stage is poured in the range 9 - 12, its temperature in the range 60 - 95 ° C, the treatment time being 2-8 hours. 11. A process according to claim 1, characterized in that magnesium is added as a protective chemical in the EOP step. 25 Method according to claim 1, characterized in that in the third stage of said sequence a second ozone treatment (ZP stage) is carried out, wherein the ozone addition (in the Z stage) is below 3 kg ozone / adt and the peroxide addition is usually below 10 kg H2 O2 / adt. Process according to claim 12, characterized in that the peroxide addition is 3 - 7 kg H 2 O 2 / adt. 93232 Process according to Claim 12, characterized in that the temperature in the Z-stage is 50 - 80 ° C and the pH in the range 4-10. Method according to claim 12, characterized in that the temperature of the Z-stage is in the range 60 - 70 ° C. Method according to claim 12, characterized in that the pH of the Z-stage is in the range 6-10. 10 Method according to claim 12, characterized in that the temperature in the P-stage is 60 - 80 ° C and the pH in the range 9-11, while the treatment time is 1 - 6 hours. 18. A process according to claim 12, characterized in that the temperature in the P-stage is 70 - 80 ° C. Process according to claim 12, characterized in that the pH of the P-stage is 10. 20. A method according to claim 1, characterized in that the pulp is screened after the ZP step, but before the subsequent washing / precipitation. 21. Process according to claim 20, characterized in that the pulp is diluted to a concentration of 0.5 - 1.5% after the ZP step and is led to a vortex cleaning plant. 22. A method according to claim 20, characterized in that the pulp is diluted to a concentration of 0.5 to 4% after the ZP step and is led to a sieving / vortex cleaning plant. 23. A method according to claim 1, characterized in that the pulp has been treated with enzymes prior to said sequence. 35 Process according to claim 1, characterized in that the 93232 pulp is irradiated in average consistency from a preceding sequence of processing steps in at least one MC mixer, where - in the pulp there is a mixture of ozone and a carrier gas such that the ozone addition is 2-10 kg ozone / adt, 5. the mass flows out of the mixer into an ozone reactor, - the mass of the mass is poured in the range 2-4, - the mass of the mass is poured in the range 40 - 70 ° C, - the mass flows out of a first reaction tower, - the mass is subjected to a heavy metal removal treatment at a pH of 2 - 7, the pulp flows into a washing device, where both the substances released in the ozone bleaching step and the heavy metal removal step into the filtrate Sj₂ are washed out of the pulp - the pulp is fed into an MC mixer, where the pulp is mixed with oxygen, wherein the additive is 2 - 6 kg oxygen / adt, - the mass flows out of an oxygen reactor, - the mass flows out of the oxygen reactor and peroxide is mixed into the mass, whereby the addition one is 10 - 20 kg H 2 O 2 / adt, - the mass flows into a second reaction tower, 20. the reaction tower temperature is poured in the range 60 - 95 ° C and the pH in the range 9-12, - the mass is removed from the reaction tower into a washer, where the preceding oxygen and peroxide step in the filtrate S2 dissolved reaction products are separated from the pulp, 25. the pulp is fed into an MC mixer, where a pulp of ozone and a carrier gas is mixed into the pulp, the addition being below 3 kg ozone / adt. to an ozone reactor and the reaction conditions are regulated so that the pH is 4 - 10 and the temperature 50 - 80 ° C, 30. the pulp is removed from the ozone reactor via a gas separator, where residual gas is separated from the pulp - the peroxide is mixed, the addition is below 10 kg H 2 O 2 / adt and the mass exits in a third reaction tower, where the reaction conditions are regulated such that the pH is 9 - 11 and the temperature is 70 - 80 ° C, 93232 - the mass exits from the third reaction tower in a TV. ttare, wherein the third bleaching stage reaction products separated from the pulp Tili filtrate S3. 5 25. A method according to claim 1, characterized in that - the mass is fed in average consistency from a preceding sequence of processing steps in at least one MC mixer, where - in the mass a mixture of ozone and a carrier gas is involved, such that the ozone addition is 2 - 10 kg ozone / adt, 10. the mass flows from the mixer into an ozone reactor, - the pH of the mass is poured in the range 4-7, - the temperature of the mass is poured in the range 40 - 70 ° C, - in the mass complexes are formed and the mass flows out in a first reaction tower, 15. the pulp is subjected to a heavy metal removal treatment at a pH of 4 - 7, - the pulp flows into a washing device, where both the substances discharged at the ozone bleaching stage to the filtrate Si are washed out of the pulp, 20. the pulp is fed into a mass mixer, where oxygen is mixed, the additive being 2 - 6 kg oxygen / adt, - the mass flows out in an oxygen reactor, - the mass flows out of the oxygen reactor and into the mass breath the peroxide, the additive being 10 - 20 kg H 2 O 2 / adt, 25. the mass exits in a second reaction tower, - the reaction tower temperature is poured in the range 60 -95 ° C and the pH in the range 9 - 12, - the mass is removed from the reaction tower to a washer. wherein the reaction products dissolved in the foregoing oxygen and peroxide step of the filtrate S2 are separated from the pulp, - the pulp is fed into an MC mixer, where a mixture of ozone and a carrier gas is mixed, the addition being below 3 kg ozone / adt. - the mass is conducted to an ozone reactor and the reaction conditions are regulated so that the pH is 4 - 10 and the temperature 50 - 80 ° C, 93232 - the mass is removed from the ozone reactor via a gas separator, where residual gas is separated from the mass, - peroxide is added, is below 10 kg H 2 O 2 / adt and the mass flows out into a third reaction tower, where the reaction conditions are regulated such that the pH is 9 - 11 and the temperature 70 - 80 ° C - the mass flows out of the In the third reaction tower in a washer, the reaction products of the third bleaching step are separated from the mass of the filtrate S3. 10 Process according to claim 1, characterized in that - the pulp is fed in average consistency from a preceding sequence of processing steps in at least one MC mixer, where - in the pulp there is mixed a mixture of ozone and a carrier gas such that the ozone addition is 2-10 kg ozone / adt, - the mass exits from the mixer in an ozone reactor, - the pH of the mass is poured in the range 2-7, - the temperature of the mass is poured in the range 40 - 70 ° C, - the mass exits in a first reaction tower, 20. the mass is subjected to a heavy metal removal treatment at a pH of 2 - 7, the pulp flows into a washing device, where both the substances released at the ozone bleaching stage as in the heavy metal removal step to the filtrate Sx are washed out of the pulp, 25. the pulp is fed into an MC mixer, where the pulp is mixed with oxygen, constitutes 2 - 6 kg oxygen / adt, - the mass flows out of an oxygen reactor, - the mass flows out of the oxygen reactor and peroxide is mixed into the mass, the additive being exhausted The mass exits in a second reaction tower, - the temperature of the reaction tower is poured in the range 60 - 95 ° C and the pH in the range 9-12, - the mass is removed from the reaction tower into a washer, where the precursor oxygen and peroxide step dissolved reaction products are separated from the pulp into the filtrate S2, 93232 - the pulp is fed into an MC mixer, where a pulp of ozone and a carrier gas is mixed, the addition being below 3 kg of ozone / adt. to an ozone reactor and the reaction conditions are regulated so that the pH is 4 - 10 and the temperature 50 - 80 ° C, - the pulp is removed from the ozone reactor via a gas separator, where residual gas is separated from the pulp, - the peroxide is mixed, the addition is below 10 kg H / adt and the mass exits in a third reaction tower, where the reaction conditions are regulated such that the pH is in the range 9-11 and the temperature in the range 70 - 80 ° C - the mass exits from the third reaction tower. in a washer, where the reaction products of the third bleaching step are separated from the mass of the filtrate S3. 15 The method of claim 1, characterized in that - the pulp is fed in average consistency from a preceding sequence of processing steps in at least one MC mixer, where - in the pulp a mixture of ozone and a carrier gas is mixed, so that the ozone addition is 2-10 kg ozone / adt, - the mass exits from the mixer in an ozone reactor, - the pH of the mass is poured in the range 2-7, - the temperature of the mass is poured in the range 40 - 70 ° C, - the mass exits in a first reaction tower, 25. the mass is subjected to a heavy metal removal treatment at a pH of 2 - 7, the pulp flows into a washing device, where both the substances discharged at the ozone bleaching stage during the heavy metal removal step to the filtrate S2 are washed out of the pulp, 30. in the pulp the peroxide is mixed, the addition being 10 - 20 kg H 2 O 2 / adt. - the mass exits in a second reaction tower, - the temperature of the reaction tower is poured in the range 60 - 95 ° C and the pH in the range 9-12, 93232 - the mass is removed from the reactor. the reaction tower to a washer, in which the reaction products dissolved in the preceding oxygen and filtrate S2 solution are separated from the pulp, - the pulp is fed into an MC mixer, where the pulp is mixed with a mixture of ozone and a carrier gas, the addition being below 3 kg of ozone / adt, - the mass is led to an ozone reactor and the reaction conditions are regulated so that the pH is 4 - 10 and the temperature 50 - 80 ° C, - the mass is removed from the ozone reactor via a gas separator, where 10 residual gas is separated from the mass, - peroxide is mixed in the mass. wherein the additive is below 10 kg H 2 O 2 / adt and the mass flows into a third reaction tower, where the reaction conditions are regulated such that the pH is in the range 9-11 and the temperature in the range 70 - 80 ° C, 15. the mass flows out of the third reaction tower in a washer in which the reaction products of the third bleaching step are separated from the pulp of the filtrate S3.