SE523084C2 - Pulping process with acidic bleaching and washing steps - Google Patents

Abstract

Conditions are controlled in the acidic bleaching step to give a final pulp pH of 3 or above, then acid is added to the pulp prior to washing to reduce pH to a value at least 0.3 units less than the previous final pH value, and then an acidic filtrate is removed in a first washing step phase to remove metal ions capable of forming poorly soluble compounds. A continuous method for the preparation of pulp for papermaking, comprises cooking a fibrous material, delignifying it with oxygen and bleaching. The bleaching process used comprises an initial acidic bleaching step, followed by a washing step, from which at least one filtrate is removed, and at least some of the filtrate is used as dilution liquid to reduce pulp consistency following the acidic bleaching step. Conditions are controlled in the acidic bleaching step to give a final pulp pH of 3 or above, then for a pre-determined period before the washing step the pulp is treated at a pH at leas t 0.3 units less than this final pH value, by adding acid. The washing step is carried out using at least one initial washing phase where the filtrate removed is acidic. An Independent claim is also included for the above bleaching process, where acid is added to bring into solution materials capable of forming poorly soluble salts, which are then removed in ion form with the acidic filtrate in the washing step.

Description

'30 35 åszs os4 Development is underway to gradually reduce emissions of organic matter from the bleaching process, which can be done with different techniques. The common starting point is that sewage volumes must be reduced. Typical sewage flows from the bleachers in 1996 are 20-30 ma / ton bleached pulp. In the future, flows down to 5 m3 / ton will be required, preferably the flows should be even lower.

Alkaline filters and complexing agents have a greater environmental impact than acidic filters and are therefore priority streams to exclude from the sewer. In the bleaching plant, released organic material is formed, which with regard to the environment should be avoided from being released. Especially in alkaline steps, large lignin fragments are released that have low biodegradability. Emissions of complexing agents, such as EDTA or DTPA, should also be regulated and done so in many factories.

Complexers are difficult to degrade and are suspected of interfering with the flora and fauna of the recipient. Other environmentally hazardous associations also occur.

This means that future bleaching takes place in an environment with significantly higher levels of released organic and inorganic substance compared with today. Precipitation of compounds with limited solubility is a consequence that has already been observed at several factories.

The most insoluble compounds in the processes include various calcium salts such as calcium oxalate and calcium carbonate. Calcium ions are carried with the wood into the process. Via the cooking chemicals, calcium compounds from the recycled lime cycle can also be introduced into the fiber line.

The possibility of reaching low sewage volumes and low environmental impact is limited by the presence of compounds with low solubility, such as calcium salts and barium sulphate. 10 15 20 25 30 35: S23 084 Low levels of calcium, barium and manganese are important prerequisites for tightly closed bleaching. The amounts that are introduced into the bleaching plant will be released mainly in the acid bleaching steps.

Upon bleaching, oxalate is formed by oxidizing lignin and hexenuronic acids. Hexenuronic acids are formed by xylan during sulphate boiling.

Oxalate and carbonate are formed from the pulp during the bleaching reactions. Techniques to limit the formation are to keep a low kappa number into the bleaching plant and to avoid extremely strong oxidizing agents. Peracetic acid has been shown to give low levels of oxalate compared to other bleaching chemicals.

A problem with precipitates is that they limit the degree of closure of a bleaching plant at optimal conditions in each individual bleaching stage. When precipitates form and adhere to sensitive equipment, such as filter cloths and press wires, production must be stopped and the equipment cleaned. Calcium oxalate forms e.g. a porcelain-like precipitate that is very difficult to remove from process equipment. Calcium oxalate has the lowest solubility in the pH range 4-7. Since the pH fluctuates in a bleaching plant between acidic and alkaline, it is difficult to avoid precipitation in a delignifying process with high calcium levels. Calcium carbonate, which is also a troublesome precipitate, is formed at about pH 9. This is because carbonate is present as bicarbonate and carbonic acid at lower pH.

The more the sewage flow is lowered in a bleaching plant, ie. the more the bleaching is stopped, the higher the levels of COD, oxalate and calcium will be in the filtrates. This increases the risk of calcium oxalate encrustation in the equipment. 10 15 20 25 30 35 523 084 Production stoppages due to precipitation lead to large financial losses. It can also contribute to increased environmental impact, as washing leads to increased water flow and in some cases cleaning chemicals must be used.

The mass contains large amounts of calcium that is released during the process. Therefore, it is mainly calcium compounds that cause precipitation. Barium can also be dissolved in such high concentrations that it forms solid barium sulphate.

In the bleaching plant, it is in the first acidic delignifying bleaching step that the largest amount of barium and calcium is dissolved and the largest amount of oxalate is formed. In the wash after such a step, the pH usually changes to alkaline.

The solubility of calcium oxalates is adversely affected by the increase in pH. Therefore, it is also on the washing filter after such a bleaching step that the risk of clogging is greatest.

In a bleaching plant with low sewage flows, the washing will take place in some form of countercurrent. Filtrates from bleaching steps placed late in the process are then used for washing after the previous bleaching steps. These filtrates contain both calcium, oxalate and other substances with low solubility. The highest levels of cross-prone compounds will usually be reached in the first acidic step.

As mentioned above, precipitates limit the current degree of closure of bleachers. The availability of process equipment due to precipitation is regulated by the water flow. In cases where the water flow cannot be regulated, the pH in the process may be forced to be adjusted to undesirable levels, e.g. not optimal pH for the function of chemicals, the quality of the pulp, the durability of the process equipment and the economy of the process. Patent 74012842 B describes a technique for washing out calcium. With this method, the pH is adjusted to 0.5-1.5 with HCl. This is done through a whole chlorine step. Chlorine is not used as a conventional bleach today and this method cannot be applied to today's modern bleach.

The result is poorer delignification and poorer selectivity as shown above.

To dissolve already solid calcium oxalate from the wood, a medium with high ionic strength can be used. This is described in patent 76026522 B. This requires a long residence time and a subsequent acidic step. This is usable on e.g. mangroves with extremely high calcium oxalate levels already in the wood. Other wood does not contain such high levels and calcium can not be removed from the pulp in this way.

The pulp still contains high calcium levels which can be leached out later in the fiber line and in later positions precipitate as calcium oxalate or other calcium compound.

To avoid precipitation of calcium oxalate, a polymer can be added to the exposed process section.

The polymer dissolves solid precipitates and prevents further crystal formation, which is described in Swedish patent 0401818 A. With this technology, this additive chemical must be constantly doped into the process, which in the long run is not a sustainable solution. This solution only counteracts precipitation within a limited process section.

When bleached with chlorine dioxide at low pH, hexenuronic acids and thus also charged groups are broken down. The charged groups are good for the quality of the mass. The hexenuronic acids can, as previously mentioned, upon decomposition give oxalates.

The structures of hexenuronic acids have been known for a long time, but new analytical techniques make them easier to study. It is known that hexenuronic acids are formed during the sulphate cooking by rearrangement of the native uronic acids of the wood, for example 4-O-methyl-glucuronic acid. Hexenuronic acids are broken down in a strongly acidic environment and during strong oxidative treatment with, for example, ozone or chlorine dioxide. Relatively milder oxidizing agents such as oxygen and peroxide do not seem to affect the hexenuronic acids. The object of the present invention is to provide an improved method for producing pulp which substantially reduces the problems discussed above.

It is also an object of the invention to provide an improved method of producing pulp so that metals which can form sparingly soluble compounds are brought into solution before the wash, which follows an acidic bleaching step, to thereby be removed from the fiber line by said wash.

It is also an object of the invention to provide an improved method for producing pulp so that valuable hexenuronic acids are preserved in greater amount than before in the performance of the acid bleaching step.

Another object of the invention is to provide an improved method for producing pulp so that the degree of closure in the bleaching plant can be further increased in relation to the prior art.

Another object of the invention is to provide an improved method for producing pulp so that the amount (AOX) becomes low according to the regulations when bleaching with chlorine dioxide in the acidic delignifying bleaching step. chlorinated organic material The process for producing pulp according to the invention is characterized in that the bleaching in the acid bleaching step is carried out under such controlled conditions that the final pH is 3.0 or more; that for a predetermined period of time before the washing step, which is carried out after the initial acid bleaching step, the pulp is treated at a pH which is 0.3 pH units lower or more than said final pH by adding a required amount of an acid to the pulp ; and that the washing step is performed with at least one initial washing phase so that said withdrawn filtrate is acidic.

The method of increasing the degree of closure in a bleaching plant according to the invention is characterized in that the bleaching in the acid bleaching step is carried out under such controlled conditions that the final pH is 3.0 or above; that for a predetermined period of time before the washing step, which is carried out after the initial acid bleaching step, the pulp is treated at a pH which is 0.3 pH units lower or more than said final pH by adding a required amount of an acid to the pulp so that at least some of said substances are brought into solution; and that the washing step is performed with at least one initial washing phase so that said withdrawn filtrate is acidic and contains said substances in ionic form.

By the invention it is possible to avoid precipitation in a simple and inexpensive manner, without losing efficiency, quality and selectivity in bleaching and without the addition of foreign chemicals, and thereby achieving a tightly closed bleaching plant with a total effluent volume of less than 5 m fl / ton, where the main part of the drain comes from the first acid bleaching stage. This can be done with chemically optimized bleaching by lowering the pH only for a short time, e.g. 1-5 min., Over the first washing step in relation to the pH in the previous bleaching tower.

By means of the invention, it is possible to carry out bleaching of hexenuronic acid. u wave nu 000: o o 10 15 20 25 30 35 i 523 084 The invention avoids precipitation of calcium oxalate in the sensitive washing equipment.

With the invention, with the acidic washing filtrate, the metals can bleed out early in the process line so that the remaining bleaching can be stopped far without risk of clogging.

The higher pH values (2 3.0) have been shown to give a significantly lower amount of chlorinated organic material (AOX) when bleached with chlorine dioxide. Emissions of AOX from pulp mills are regulated by concession decisions and must be kept at a low level.

The technique of lowering the pH before washing the pulp after e.g. the chlorine dioxide step is very useful to be able to combine low amounts of AOX and good "runnability".

It has surprisingly been found according to the invention that when bleaching with a final pH above 3.0 (including 3.0) it is possible to preserve certain groups, namely the said hexenuronic acids, in the pulp which is decomposed during normal chlorine dioxide bleaching. These groups are hydrophilic due to their acid end group and help to give a stronger mass.

It has also been found that the hydrophilic hexenuronic acid groups help to provide easily swollen fibers with improved strength properties compared to conventional bleaching techniques. The grinding requirement to reach a certain tensile strength can be halved and the tensile strength at a certain dewatering ability ("drivability" on the paper machine) can be increased by 10%.

Through the invention, a high pH can be maintained in the chlorine dioxide step to improve bleachability, avoid AOX formation and avoid degradation of hexenuronic acids and reduction of charged groups, but at the same time avoid crust formation at e.g. first wash. 10 15 20 25 30 35 §523 084 To acidify after the first acid bleaching step to reduce the amount of e.g. calcium in the bleaching plant, is a technology that requires little extra equipment. It is also the placement that requires the least amount of additive chemicals for pH adjustment. The invention also makes it possible to avoid precipitation on the most exposed washing filter.

By applying the invention, transition metals are also washed out. This can lead to a reduction in chemical consumption in the bleaching plant. Complexing agents can be added in connection with the acidification to further wash transition metals from the pulp.

The primarily calcium-rich filtrate can be purified in several ways and then returned to the process. In this way, the process can be further closed.

Far-reaching delignification with oxygen is preferable from the precipitation point of view. In this way, the delignification in the bleaching plant does not have to be driven as far and the amount of oxalate formed in the bleaching plant is kept down.

It has surprisingly been shown that lowering the pH of a pulp suspension for a short period of time can halve the content of crust-forming and transition metals in the pulp. The time period can vary between 10 seconds and 1 hour.

Preferably only 1-5 minutes are used. The pH can preferably be lowered to 1-3.

In the first bleaching step, chlorine dioxide, peracetic acid or ozone was used as the bleaching chemical.

The lowering of the pH thus takes place after the first acid bleaching step. In this position the bleaching liquid is not alkaline and the pH ranges with the greatest risk of precipitation can be completely avoided. The cost of a pH decrease is also not so great in relation to acidifying after an alkaline bleaching step.

The primarily calcium-rich water is taken out of the bleaching process from the washing filter that follows the first acid bleaching step. In this way, calcium bleeds out and does not give rise to precipitates later in the bleaching plant.

Sectional washers that can separate filtrates are exemplary in this washing technique. In this way, the acidic wash stream can be separated from the next more alkaline wash water.

The pH can be lowered with a number of different acids, such as acetic acid, sulfuric acid, hydrochloric acid, nitric acid, sulfur dioxide, and carbon dioxide.

The invention will be described in more detail with reference to the drawings and some examples of experiments performed according to the invention and comparative experiments.

Figure 1 schematically shows an upstream part of a bleaching plant for carrying out the method according to the invention.

Figure 2 is a graph showing the solubility curves for calcium oxalate as a function of COD and pH and reported by Per Ulmgren and Rune Rådeström, Swedish Pulp and Paper Research Institute, at a symposium in the USA, October 23-24, 1997, "l997 TAPPI, Minimum Effluent Mills, San Francisco, Symposium ".

Figures 3-5 show a bleaching sequence with the flow of washing liquids including filtrate according to three different cases.

Figure 1 schematically shows the upstream part of a pulp bleaching plant according to the present invention. c o n »nous: a 00-0 3 I U Ü. The bleaching comprises a first bleaching tower 1 and a second bleaching tower 2. The pulp is fed through a line 3 to the first bleaching tower 1 via a lower inlet 4 and from the first bleaching tower 1 via an upper outlet. 5 through a line 6 to a washing plant 7, from which the pulp is fed through a line 8 to the second bleaching tower 2 via a lower inlet 9. The upstream part of the bleaching describes a fiber line which comprises a first bleaching step, a washing step and a second bleaching step. Washing liquid is supplied to the washing plant 7 through one or more lines 10, and acid filtrate is discharged via a line 11. The line 11 for the acid filtrate is connected to the line 6, which connects the first bleaching tower 1 to the washing plant 7, via a branch line 12. , which connects to the line 6 near or at the upper outlet 5 of the first bleaching tower 1. Furthermore, a branch line 13 is connected between the line 11 for the acid filtrate and the line 3 for feeding pulp to the first bleaching tower 1.

Ink chemicals are supplied through a line 14. Acid, e.g. sulfuric acid is supplied through a line 15, which connects to the branch line 12 for diluting the pulp so that the consistency of the pulp decreases, e.g. from 10% to 3%, while adjusting the pH of the pulp to a predetermined value in accordance with the present invention.

The residence time of the pulp during the passage through line 6 represents a predetermined period of time extending from the time when the lower pH of the pulp is set, and to the time when an initial washing phase of the washing step takes place in the washing plant 7 to obtain an acidic filtrate, which discharged through line 11. This initial washing phase is followed by a second washing phase which is carried out with alkaline washing liquid, this drain not having been drawn. Alternatively, the "initial" washing phase is the only one. 10 15 20 ._ 25 3 30 s2s 004 12 fi xemneLi Oxygen delignified softwood pulp of coniferous wood with coat 12 was bleached with chlorine dioxide in a first acid bleaching step with a final pH of 3.5 and coat 6. After washing with washing liquid with a pH of 7 was measured the levels of the metals in question, which were thus bound in insoluble compounds. In a first experiment, Experiment 1, no acid was added before washing, which experiment was thus a reference. Then four different experiments, Experiments 2-5, were performed with the addition of H 2 SO 4 after the first bleaching step in such an amount that the pH became 2.0, and an acid treatment at this lowered pH took place for a time of 1 min., 6 min. , 11 min. respectively 16 min. before washing. The results are reported in Table 1 below. LahelLl mrsck RH Iid1_min Ca M9 Mn 1 3.5 o 240 38 10 2 2.0 60 9 2.1 3 2.0 61 9 2.5 4 2.0 11 58 8 2 5 2 .0 16 70 6 1.6 As can be seen from the result, a treatment at pH 2.0 for 1 min. sufficient to bring a substantial portion of the sparingly soluble metal salts into solution to accompany the filtrate.

Ezsemneli Oxygen-deligated softwood sulphate pulp with kappa 12 was bleached with chlorine dioxide in a first acid bleaching step with a final pH of 3.5 and kappa 6. Washing and measurements were performed according to Example 1. In a first experiment, Experiment 6, no acid was added after washing, which experiment was thus a reference. Thereafter, three different experiments were performed, Experiments 7-9, with the addition of HQSO 4 to the bleached pulp, which had a pH of 5, in different amounts so that pH 3, pH 2.5 resp. pH 2.3 was adjusted. The treatment time from the addition of acids to the laundry was the same, namely 1 minute. The results are reported in Table 2 below.

Iâb fi ll_2 ll JJ] J. E, H Eörsök QH Iidi_min Ca M9 Mn 6 3,5 0 220 110 15 7 3,0 1 160 96 13 s 2,5 F1 130 83 11 9 2,3 1 110 68 10 The results show that the more acidic the pre-treatment is more metal ions ferment in solution and follow with the filtrate.

The risk of precipitation thereby decreases to a corresponding degree throughout the bleaching plant.

Figure 2 shows the solubility curves for calcium oxalate in a bleaching filtrate at different levels of COD and at decreasing pH, where LS stands for solubility product. The content of e.g. calcium, which is present in a bleaching filtrate, is thus a function of the pH and the amount of organic matter. Oxalate formation is a function primarily of delignification, which can be expressed as kappa reduction. The delignification leads to the release of organic material, which can be quantified indirectly by determining the oxygen demand in the filtrate, COD.

By applying the invention, one can choose the closure alternatives found best without the risk of incrustation.

The sequence O OP D Q PO with washing presses can in several ways be concluded to a drain volume <S ms / ton, where each wash uses washing factor 2, which means washing volume 10 15 20 25 30 35 523 084 14 4 m = / ton. Each outgoing sewer thus also contains 4 m3 / ton.

Figures 3-5 illustrate the sequence with the flow of wash liquids including filtrates according to three different embodiments, designated Case 1, Case 2 and Case 3.

In Case 1, Figure 3, the total sewage volume will be very low about 4 mß / ton. All COD formed in the PO and Q steps is returned to the alkaline closed brown side. An advantage of this strategy is that no complexing agent accompanies the drain to the recipient. The risk of precipitation after stage D is relatively low, but the times you need to delignify hard in stage D, a lot of oxalate will also form in stage D. There is therefore a risk of precipitation in the washing equipment, when the pH in the D-stage or in the subsequent washing exceeds about 3.5. A short acidification before the wash according to the invention eliminates this risk and enables bleaching under optimal conditions also according to the invention in order to retain the hexenuronic acids and the charged groups. Also in Case 2, figure 4, the sewage volume is 4 m * / ton. The large amount of COD formed in the PO step is avoided. However, a large number of complexing agents flow out through the D-filtrate to sewage. The risk of calcium oxalate precipitation in the D-wash is greater than in Case 1, because the washing liquid coming from the Q-step contains calcium and oxalate carried over from the D-step, and the pH is optimal for precipitation in the washing step. Also in this case, a short acidification after the D-step according to the invention will prevent a precipitation in the washing equipment.

In Case 3, Figure 5, the sewage volume is as low as in the two previous cases. However, the amount of COD and also complexing agents is only slightly less than in a completely open case with approx. 12 m1 / ton of sewage volume. The risk of precipitation is 10 15 20 25 30 35 f523 084 15 greatest in this case compared with the two previous ones. Here, oxalate, which is formed in the PO step, is returned to the Q step and then on to the D-wash. The washing water then has a pH 5-7 which is optimal in a Q-step. In stage D, large amounts of calcium are released, which is given the opportunity to form calcium oxalate in the sensitive washing equipment because the solubility is at its lowest at pH 4-8. Also in Case 3, a short pH decrease before the wash according to the invention will mean that the solubility increases drastically and salt precipitation is avoided. Since the pulp is leached on calcium, the calcium content will not accumulate in the Q-step and precipitation of calcium oxalate or calcium carbonate in the next wash is also avoided.

What has been said above in the three cases of the illustrated bleaching sequence can also be applied to other sequences such as o D (Eo) D (Po), DEDED, D (EOP) D (EP) D, D (EOP) D (PO) / P, D (EOP) D, Paa Q (PO) / P, ZQ (PO) / P, m.fl.

The acidic bleaching effluent can be handled in several ways.

A large part of the acidic effluent can be used to collect and dilute the pulp into this first delignifying bleaching step. Depending on the bleaching chemical and the optimal pH in the bleaching step, acid usually needs to be added to the alkaline mass. In this case, when the pulp is collected and diluted with a more acidic filtrate, this acid addition can be reduced or completely eliminated.

The acidic effluent can be released for biological purification, before being released to the recipient.

For further closure, the acidic effluent can be purified from calcium and then reused in the process as washing water in, for example, the first acidic step. Purification can be done by a number of known methods.

The acidic effluent can be alkalized with a suitable alkali to pH 10-12 to precipitate organic and inorganic compounds, mainly calcium carbonate. Suitable alkali may be sodium hydroxide, magnesium hydroxide, mesa or lime.

The calcium-containing solid compounds can then be separated from the process water and the water can then be recycled. Separation of solid calcium compounds can be done by known techniques.

The acidic effluent can be evaporated and the condensate can be returned as washing water to the process.

The solid compounds from the alkalization or evaporation can then be mixed in the black liquor and burned in the recovery boiler.

The acidic effluent can also be purified from calcium and other metals with Champions BFR technology (Champion moves toward closure with BFR Start up at Canton, N.C. Pulp and Paper April 1996). With BFR technology, process water is purified from e.g. metals by first filtration and then ion exchange.

The purified water can then be returned to the process.

The acidic effluent can be ultrafiltered and then released to the recipient or used as a washing liquid.

The magnitude of the lowering of the pH that is to take place after the initial bleaching step and before washing is determined from case to case and depends on a number of factors, which i.a. includes the size of degree of closure, final pH at the first bleaching step, the kappa number, which indicates oxalate content and COD, calcium content, barium content. 10 15 20 25 30 35 523 084 17 At a high degree of closure, it is necessary to release calcium to be removed with the filtrate from the first wash and a large pH drop is therefore justified. At a high degree of closure, precipitates of calcium oxalate can form immediately after the first bleaching step and the pH drop should then be so great that the solubility of calcium oxalate is never exceeded in the position before washing to thereby avoid precipitation on the washing equipment after the bleaching step.

It will be appreciated that in order to achieve the object of the invention with respect to the reduction of precipitating substances in the pulp and the consequent possibility of increasing the degree of closure in the bleaching plant, a lowering of the pH value is always carried out after the first acidic bleaching step and before the subsequent washing. more of the precipitating substances in solution. The pH lowering is thus 0.3 pH units or more, preferably 0.5 pH units or more. 980311 Pl326SE.T0l

Claims (16)

10 15 20 25 30 35 få 523 084 = jj¿ = j '; _ = "= a.. 18 P A T E N T K R A V A method of continuously producing pulp from a cellulosic fibrous material, in which fibrous material is boiled, oxygen delignified and bleached in a bleaching sequence comprising an initial acid bleaching step with chlorine dioxide as bleach, which acid bleaching step with chlorine dioxide is followed by a washing step, from which at least one filtrate is taken out, at least a part of the filtrate being used as diluent to lower the consistency of the pulp after said acidic bleaching step, characterized in that the bleaching in the acidic bleaching step is carried out under such controlled conditions that the final pH is 3.0 or so. over; that for a predetermined period of time before the washing step, which is carried out after the initial acid bleaching step, the pulp is treated at a pH which is 0.3 pH units lower or more than said final pH by adding a required amount of an acid to the pulp so that metals, such as calcium and barium, which form sparingly soluble compounds are brought and kept in solution before the washing step; and that the washing step is performed with at least one initial washing phase so that said withdrawn filtrate is acidic and contains said metals in ionic form so that they can be removed from the fibrous material in said washing step. A method of increasing the degree of closure in a bleaching plant in which boiled and oxygen-delignified pulp of cellulosic fibrous material is bleached in a bleaching sequence comprising an initial acid bleaching step with chlorine dioxide as bleach, which acid bleaching step with chlorine dioxide is followed by a washing step, from at least one filtrate is taken, at least a part of the filtrate being used as diluent to lower the consistency of the pulp after said acidic bleaching step, characterized in that the bleaching in the acidic bleaching step is carried out under such controlled conditions that the final pH is 3.0 or above ; that for a predetermined period of time before the washing step, which is carried out after the initial acid bleaching step, the pulp is treated at a pH which is 0.3 pH units lower or more than said end. -pH by adding a required amount of an acid to the pulp so that metals, such as calcium and barium, which form sparingly soluble compounds are brought into solution before the washing step; and that the washing step is performed with at least an initial washing phase so that said withdrawn filtrate is acidic and contains said metals in ionic form so that they can be removed from the bleaching line in said washing step. A method according to claim 1 or 2, characterized in that said acid is added to the pulp before or after said dilution or in connection with the dilution added to said diluent. Method according to any one of the preceding claims, characterized in that said time period is from 10 seconds up to 1 hour. Method according to claim 4, characterized in that said time period is preferably from 1 minute up to 5 minutes. Method according to any one of the preceding claims, characterized in that the pH during said time period is preferably in the range 1-3. A method according to any one of the preceding claims, characterized in that said acid is acetic acid, sulfuric acid, hydrochloric acid, nitric acid, sulfur dioxide or carbon dioxide or a desired combination thereof. A method according to any one of the preceding claims, characterized in that a part of said acidic filtrate is fed to the pulp before said acidic bleaching step. 10 15 20 25 30 35 523 084 pppp o. 20 A method according to any one of the preceding claims, characterized in that the remaining part of said acidic filtrate is subjected to biological purification before discharge to the recipient. A method according to any one of claims 1-8, characterized in that the remaining part of said acidic filtrate is purified from metal ions, such as calcium, for reuse in the process as washing liquid at a suitable place. A method according to claim 10, characterized in that the acidic purified filtrate is used as washing water for said initial washing phase of said washing step. A method according to any one of claims 1-8, characterized in that the remaining part of said acidic filtrate is treated for precipitation in particular with alkali, preferably at pH 10-12, the addition of organic and inorganic compounds, calcium carbonate, after which it The purified filtrate is returned to the process for use as a washing liquid in a suitable place. A method according to any one of claims 1-8, characterized in that the remaining part of said acidic filtrate is evaporated to obtain a condensate which is returned to the process for use as a washing liquid at a suitable place. A method according to claim 12 and claim 13, respectively, characterized in that the solid compounds obtained by said alkalization and evaporation, respectively, are mixed with black liquor and burned in a recovery boiler. A method according to any one of claims 1-8, characterized in that the remaining part of said acidic filtrate is purified from metal ions by first filtration and then ion exchange, after which the purified filtrate is returned to 35 ~ 523 084. u o - o u now 21 the process of being used as a washing liquid in a suitable place. Method according to any one of claims 1-8, characterized in that the remaining part of said acidic filtrate is ultra-filtered, after which the purified filtrate is discharged to the recipient or returned to the process for use as washing liquid at a suitable place. 990426 Pl326SE.NKl