SE1550615A1 - A method and a system for treating liquid flows at a chemical pulp mill - Google Patents

Abstract

Abstract The invention relates to a method for treating liquid flows at a chemical pulp mill comprising a brown stock preparation unit (20), a bleaching unit (30), a pulp drying machine (80), and a waste water treatment plant (50). The method comprises conveying at least a portion of white waters from the pulp drying machine (80) to the oxygen delignification unit (25). The invention further relates to a system for treating liquid flows at a chemical pulp mill, and the use of white water as dilution water in the oxygen delignification unit (25).

Description

1 A METHOD AND A SYSTEM FOR TREATING LIQUID FLOWS AT A CHEMICAL PULP MILL Field of the Invention The invention relates to a method and a system for treating liquid flows at a chemical pulp mill.

Background of the Invention At present, chemical pulp mills are often being built in areas and surroundings with very strict environmental regulations. For example, the amount of water used by a chemical pulp mill may be strongly restricted. Therefore, it is possible that due to water resources it is not possible to build a mill in a place that otherwise fulfills demands. In many areas, a cleaner environment is desired in such a way that the mills produce substances that are less detrimental to the environment. Therefore, at present, it is important to look for new solutions for a water circulation process.

Summary of the Invention The present invention discloses a method and a system for treating liquid flows at a chemical pulp mill. Aspects of the invention are characterized by what is stated in the independent claims 1, 17, and 19. Various embodiments of the invention are disclosed in the dependent claims.

The system according to the present invention preferably comprises a brown stock preparation unit comprising a cooking unit, a brown stock washing unit, and an oxygen delignification unit, 30 -a pre-treatment stage for a bleaching unit comprising an acid treatment stage (A-stage), a bleaching unit comprising bleaching stages to bleach the produced pulp, and a pulp drying machine to dry the produced bleached pulp.

Preferably, the system further comprises: 2 at least one wood handling unit, an evaporator, a recovery boiler, and a chemical recovery unit.

The cooking and the brown stock washing steps are followed by the oxygen delignification stage. After the oxygen delignification stage, the pulp is preferably conveyed to the bleaching unit. Therefore, the oxygen delignification is typically a process stage between the brown stock washing and the bleaching. In addition, a pre-treatment stage comprising acid treatment to remove at least a portion of hexenuronic acids, and preferably also at least a portion of metals, from the pulp is preferably placed between the oxygen delignification and the bleaching.

The oxygen delignification unit is used to remove lignin from unbleached pulp. In the oxygen delignification stage, the pulp is treated with oxygen. The oxygen delignification is used because it may give, in addition to the additional delignification after the cooking, reduced consumption of bleaching chemicals and reduced emissions from the bleaching unit.

The method for treating liquid flows at a chemical pulp mill preferably comprises -conveying white waters from the pulp drying machine to the oxygen delignification stage.

In the oxygen delignification, the white water is preferably used as dilution water in the dilution step of the oxygen treatment, which dilution step is preferably situated after a post-washing of the oxygen treatment unit. The dilution step may be a separate process stage, or it may be a part of said post-washing stage.

The white waters preferably comprise or consist of at least a portion of white water overflow of the pulp drying machine.

The post-washing stage comprises a washing unit. The post-washing unit 35 preferably comprises a wash-press device and/or a DD washer. 3 In addition, the method for treating liquid flows at a chemical pulp mill preferably comprises a step in which at least a portion of waters from a second clarifier of the waste water treatment plant is conveyed to the pretreatment stage of the bleaching unit and/or to the bleaching unit. In the bleaching unit and/or the pre-treatment stage of the bleaching unit, said water is preferably used in Drum Displacement washer (DD-washer) plates.

Advantageously, at least a portion of the waters coming from the second clarifier of the waste water treatment plant is conveyed to the pre-treatment stage of the bleaching unit, i.e. the acid stage (A-stage). In addition or alternatively, at least a portion of the waters coming from the second clarifier of the waste water treatment plant is preferably conveyed to an alkaline extraction stage of the bleaching unit, i.e. E-stage, in which oxygen and peroxide can be used as additional chemicals, i.e. EOP-stage.

The system according to the present invention preferably comprises a first conveying apparatus for conveying at least a portion of the process waters coming from the drying machine of the pulp (i.e. white waters) to the oxygen delignification stage. In addition, the system preferably comprises a second 20 conveying apparatus for conveying at least a portion of the waters coming from the second clarifier of the waste water treatment plant to the pretreatment stage of the bleaching unit. In addition or alternatively, the system preferably comprises a third conveying apparatus for conveying at least a portion of the waters coming from the second clarifier of the waste water treatment plant to the bleaching unit. The conveying apparatus(es) preferably comprise(s) at least pipe(s) and pump(s).

Thus, according to the present invention, at least a portion of the raw water needed at the pulp mill is replaced by a circulation of process waters. The 30 white water from the drying machine is preferably circulated to the oxygen delignification unit. In addition, there are preferably at least some process water circulations within one process unit, such as bleaching unit, or between at least two process units, for example between the pre-treatment stage of the bleaching unit and the oxygen delignification unit. Therefore, thanks to the present invention, it is possible to significantly decrease the amount of the generated waste water. 4 Advantageously, the method for treating liquid flows at a chemical pulp mill comprising a brown stock preparation unit comprising a cooking unit, a brown stock washing unit and an oxygen delignification unit, a bleaching unit comprising at least one bleaching stage, a pulp drying machine, a waste water treatment plant comprising a second clarifier, and an optional pre-treatment stage of the bleaching unit, 10 comprises -conveying at least a portion of white waters from the pulp drying machine to the oxygen delignification unit.

Most preferably, the white water is collected as white water overflow from the drying machine.

Preferably, the method further comprises conveying at least a portion of waste waters from the second clarifier of the waste water treatment plant to the bleaching unit and/or to the pre-treatment stage of the bleaching unit.

Advantageously, the oxygen delignification unit comprises a dilution step, and at least a portion of the white waters is conveyed to the oxygen delignification unit to be used as dilution water in the dilution step of the oxygen delignification unit.

Preferably, at least a portion of the white waters is used as dilution water in the last dilution step of the oxygen delignification unit.

Preferably, the bleaching unit and/or the pre-treatment stage (30a) of the bleaching unit comprises a washing unit comprising a drum displacement washer (DD washer) and/or a wash press, and at least a portion of the waters from the second clarifier is used in said washing unit(s).

Advantageously, the pre-treatment stage comprises an acidic stage, and at least a portion of the waters coming from the second clarifier of the waste water treatment plant is conveyed to the pre-treatment stage.

Preferably, the bleaching unit comprises an alkaline extraction stage in which oxygen and peroxide are used as additional chemicals, and at least a portion of the waters coming from the second clarifier of the waste water treatment plant is conveyed to said alkaline extraction stage.

In an embodiment, at least 50% of white water overflows of the drying machine is conveyed to the oxygen delignification unit. 10 The total content of the white water that is conveyed to the oxygen delignification unit is preferably at least 0.5 m3/Adt.

Preferably, the total content of the waste water that is conveyed from the second clarifier of the waste water treatment plant to the bleaching unit and/or to the pre-treatment stage is at least 0.5 m3/Adt.

Advantageously, at least some filtrates coming from the pre-treatment stage of the bleaching unit are conveyed from the pre-treatment stage of the bleaching stage to the dilution step of the oxygen delignification unit.

In an embodiment, the total content of chloride in the white water is between 5 and 40 mg/I.

In an embodiment, the total content of calcium in the white water is between 25 0 and 6 mg/I.

In an embodiment, the total content of wood extractives in the white water is between 0 and 12 mg/I.

Preferably, the bleaching unit comprises the following stages: a D1-stage, an EOP-stage, a D2-stage and a P-stage. 6 Advantageously, the system for treating liquid flows at a chemical pulp mill comprises a brown stock preparation unit comprising a cooking unit, a washing unit, and an oxygen delignification unit, a bleaching unit, a pulp drying machine, a waste water treatment plant, optionally an apparatus for a pretreatment stage of the bleaching unit, and 10 -an apparatus adapted to convey white waters from the pulp drying machine to the oxygen delignification unit.

Preferably, the oxygen delignification unit comprises a post-washing unit comprising a DD washer and/or a wash press.

Preferably, the pre-treatment stage of the bleaching unit comprises a post-washing unit comprising a DD washer and/or a wash press.

Advantageously, white water coming from a pulp drying machine is used as dilution water in the oxygen delignification unit.

Thus, thanks to the present invention, a system capable of decreasing the environmental load of the chemical pulp mill, and decreasing the amount of raw water used by the chemical pulp mill 25 may be provided. Moreover, energy and chemicals may also be saved, which may lead to a significant reduction in the operational costs.

The present invention can be implemented, for example, at a chemical pulp mill having a digestion process, at least one bleaching stage, chemical recovery and various reactors, vessels, pumps, mixers, filters etc. known per se.

Description of the Drawings In the following, the invention will be described in more detail with reference to the appended drawing, in which 7 Figs 1 to 3show example embodiments in reduced schematic chart.

Detailed Description of the Invention In this application, reference is made to Figs 1 to 3, in which the following reference numerals are used: brown stock preparation comprising digester, brown stock washing, and oxygen delignification stages, 21cooking, 22brown stock washing, oxygen delignification stage, 25X post-washing stage of the oxygen delignification stage, 25Y dilution step of the oxygen delignification stage, 30a pre-treatment for the bleaching unit, bleaching unit comprising bleaching stages and washing therein, waste water treatment plant comprising a second clarifier, and 80drying machine, The term "raw water treatment plant" refers to a fresh water treatment plant.

The term "effluent treatment plant" refers to a waste water treatment plant. 25 The term "ECF" refers to elemental chlorine free. The term "TCF" refers to totally chlorine free.

The term "NPE" refers to non-process elements. NPEs are inorganic 30 substances originating from process devices, some also from wood, raw water and chemicals, which are not wanted to be processed.

The term "Adt" refers to air dry metric ton of 90% dry chemical pulp. 35 The term "effluent" refers to waste waters. Effluents are conveyed to the waste water treatment plant for purifying process. Effluents may comprise, for 8 example, bleaching effluents and ash leaching purge. In addition, the effluents may contain wood processing effluents.

The term "bleaching effluents" refers to water fractions from the bleaching unit into the waste water treatment plant or the waste water stream. Thus, bleaching effluents comprise effluents from the bleaching unit and washing steps therein. Advantageously, bleaching effluents contain bleaching filtrates. Most advantageously, bleaching effluents consist of bleaching filtrates. The bleaching filtrates contain acidic and/or alkaline flows. Advantageously, bleaching effluents do not contain substantial amounts of fibers.

The term "EOP" refers to the alkaline extraction stage of the bleaching unit using oxygen and peroxide as additional chemicals.

The term "A-stage" refers to an acid stage, which is preferably a pre- treatment stage for the bleaching unit.

The term "P-stage" refers to a hydrogen peroxide stage of the bleaching unit.

The terms "D-stage", "D1-stage" and "D2-stage" refers to a chlorine dioxide stage of the bleaching unit.

The term "HexA" refers to hexenuronic acids. 25 The term "DD washer" refers to a drum displacement washer.

The term "MTBE/GC" refers to wood extractive measurements that are measured according to Orsa, F. and Holmbom, B., 1994. A Convenient Method for the Determination of Wood Extractives in Papermaking Process Waters and Effluents. Journal of Pulp and Paper Science, 20(12): 361-366.

Lignin content measurements "UV 280" are measured according to the following document: Matti Ristolainen, 1999. Analysis of the organic material dissolved during totally chlorine-free bleaching, Doctoral Thesis, University of Jyvaskyla, Finland. 9 Advantageously, the system according to the invention comprises a brown stock preparation unit 20 to produce brown stock, an apparatus to remove at least a portion of hexenuronic acids in the pre-treatment stage 30a, a bleaching unit 30 to bleach the brown stock, and a pulp drying machine 80 to dry the produced bleached pulp.

Advantageously, the brown stock preparation unit 20 comprises a cooking stage 21, - a washing stage 22, a screening step, and an oxygen delignification stage The oxygen delignification stage 25 preferably comprises, among other 15 things, an oxygen treatment and a post-washing stage(s) 25X after the oxygen treatment. In addition, the oxygen delignification stage 25 preferably comprises a dilution step 25Y in which the pulp is diluted after the post-washing.

In an embodiment, the dilution step 25Y is a part of the post-washing stage 25X. In another embodiment, the dilution step 25Y is a separate process step.

Preferably, the post-washing stage 25X of the oxygen delignification stage 25 25 is implemented by a wash press-device and/or a DD washer, most preferably by the wash press-device.

Brown stock treatments after the cooking process preferably include a brown stock washing process, a screening process, and an oxygen delignification 30 process comprising a washing step.

When filtrate is recovered from the first brown stock washing apparatus, it is preferably forwarded either directly to a black liquor evaporation plant, or -used in digester plant processes for dilution and displacement, after which it ends up in the black liquor flow.

The screening process may be located after digester blowing, in the middle of or after the washing process, and/or after oxygen delignification. Typically, brown stock treatments are followed by a bleaching process, preferably 5 based on ECF technique, which comprises a pulp bleaching plant with one or more bleaching stages based on the use of chlorine dioxide in addition to other possible stages using other known bleaching chemicals. Advantageously, the bleaching sequence comprises at least one alkaline extraction stage, wherein preferably at least oxygen and/or peroxide are/is used. Ozone stage(s), acid stage(s), and/or chelation stage(s) for removing heavy metals can also be used.

Typically, the last washing apparatus in the oxygen delignification stage receives the purest washing liquid for facilitating the bleaching of the pulp, and the filtrate obtained from this last washing apparatus is used in accordance with counter-current washing principles as washing liquid and in dilutions.

Advantageously, after the last washing stage of the oxygen delignification stage and washing unit therein, the pulp is preferably diluted and then conveyed to the bleaching unit. Advantageously, a pre-treatment stage of the bleaching unit comprising an acid stage is placed between the oxygen delignification unit and the bleaching unit. Advantageously, sulfuric acid is used as main chemical in the hydrolysis of hexenuronic acids.

According to the present invention, white water, preferably white water overflow, coming from the pulp drying machine is preferably used as dilution water in the oxygen delignification unit, most preferably it is used for dilution in the last washing step of the oxygen delignification stage.

Advantageously, the bleaching unit 30 comprises a D1-stage, an EOP-stage, a D2-stage and/or - a P-stage. 11 In an advantageous example, the first stage of the bleaching unit is the Dl-stage, the second stage of the bleaching unit is the EOP-stage, the third stage of the bleaching unit is the D2 stage and/or the fourth stage of the bleaching unit is the P-stage.

The general purpose of cooking in bleached chemical pulp production is to recover fibers from chips that are fed to the digester by using chemicals and heat to remove fiber binding lignin and, in addition, to remove wood extractives which can later cause foaming and precipitants in the process.

Therefore, chemicals that dissolve as much lignin and as little cellulose as possible are preferably used in the pulping process. Typically, the process for manufacturing bleached chemical pulp comprises pulping, washing, screening, bleaching, and cleaning stages. Advantageously, sulphate cooking, also called kraft cooking or pulping, which uses a mixture of sodium hydroxide (NaOH) and sodium sulphide (Na2S), is used pulp production method. The cooking process may be based on batch cooking or continuous cooking comprising a digester or several digesters.

The pulp mill has not only bleaching effluents but also process waters, such as cooling waters, white water from the drying machine, sealing waters, reject flows, channel waters, washing waters of the plant, and rain waters, as well as wood processing water.

Bleaching steps in the bleaching unit are used to improve the brightness, cleanliness, and brightness stability of pulp. Residual lignin is a major contributing factor in color, so it usually has to be removed or brightened. Generally, the aim of bleaching pulp is to continue delignification and, by using bleaching chemicals, to remove lignin, known as residual lignin, that remains in the pulp after the cooking and oxygen stages, and which could not 30 be broken down and dissolved in the cooking and oxygen stages without sacrificing pulp yield or fiber properties. Also, in bleaching, chromophorous compounds are removed and oxidized to colorless compounds. Typically, an important part of bleaching is washing dissolved lignin out of the pulp in the washer following the bleaching stage. 12 For bleaching purpose, for example, chlorine (0I2), ozone (03) and/or peroxide acid (Paa and Caa) can be used to react with aromatic lignin units, and/or - chlorine dioxide(C102) and/or oxygen (02) can be used to react in general with lignin structures that have free phenolic hydroxyl groups, and/or hypochlorite (H) and/or hydrogen peroxide (H202) can be used to react with some functional groups.

However, the chemical pulp industry desires to maintain a technique in which pulp is bleached in at least one stage with chlorine-containing chemicals in such a way that chlorine dioxide is the main chemical of the bleaching process of the mill. Thus, advantageously at least one, more preferably one or two chlorine dioxide (d02) stage(s) (D-stage) is (are) used for the bleaching purpose.

Bleaching effluent is typically a significant source of both biological and chemical oxygen consumption. For example chlorine-containing inorganic 20 compounds and/or organic chlorine compounds from the reactions of chlorine dioxide and/or chlorine may remain in the process. Bleaching separates various compounds of lignin from the fibers, which compounds remain in the effluent in the form of organic molecules. Additionally, sulphuric acid may be used in bleaching stage(s) for pH regulation. Sodium hydroxide may also be used for pH regulation and lignin extraction in alkaline stages. In addition to these, depending on the bleaching sequence, oxygen and/or peroxide may be used in bleaching. However, in elementary analysis, they are such substances that their contribution in, for example, purification processes is not noticed.

In an example, sulphuric acid is used for pH regulation, and/or sulphur dioxide and/or other red uctants are used for elimination of chemical residuals from the bleaching, i.e. for elimination of unreacted bleaching chemicals.

Advantageously, ECF bleaching is used at the pulp mill according to the present invention, wherein said bleaching preferably comprises both, acid 13 and alkaline, stages. ECF bleaching covers all such bleaching sequences, which comprise at least one chlorine dioxide stage and which do not use elemental chlorine in any bleaching stage. Modern ECF bleaching used for bleaching pulp typically consists of at least two, more advantageously of at least three bleaching stages comprising preferably at least three washing apparatuses. Advantageously, the amount of the bleaching stages in the bleaching unit is 1, 2, 3, 4, 5, 6, or 7, more preferably between 2 and 5, and most preferably between 3 and 4.

If chlorine dioxide is used in the bleaching stage, most typically the doses are between 5 and 15 kg of chlorine dioxide/adt pulp. The chlorine dioxide doses for softwood are preferably between 25 and 35 act. Cl kg/adt and for hardwood between 20 and 30 act. Cl kg/adt. If a mill is to further decrease the amount of organic chlorine compounds, the aim of the mills is typically to treat them within the mill rather than to decrease the use of chlorine dioxide.

The pulp mill preferably comprises a chemical recovery plant including an evaporation process, for example, an in-series connected evaporation plant, a chemical recovery boiler, removal of chlorides from the process, and a chemical production plant for producing cooking chemicals.

At least partly closed cycle systems for manufacturing bleached chemical pulp apply processes where at least part of the water and other chemicals is recycled and reused, which minimizes waste disposal. Advantageously, said systems are used to minimize aqueous effluent and, hence, to protect the environment from the impact of disposal of effluents without significantly jeopardizing the processing cost or the value of saleable products.

Chlorine, potassium, calcium, manganese, silicon, aluminum, phosphorus, iron, and barium are some elements of concern in a bleached chemical pulp mill.

The waste water treatment plant preferably comprises at least one effluent treatment line of the waste water treatment plant. The waste water treatment plant may comprise, for example, a pre-treatment, neutralization, a biological treatment by an anaerobic or an aerobic method, and/or a chemical 14 treatment. In addition, typically a clarification step comprising clarifier(s) is performed, in which sludge generated in the process is removed.

Advantageously, at least one effluent treatment line of the waste water treatment plant is provided with biological treatment. Biological treatment is efficient specifically when the proportion of detrimental organic substances is decreased, which mainly comprise lignin compounds separated in bleaching, hemicelluloses and components originating from extractives, which constitute a significant portion of effluent coming from the bleaching plant. There are 10 various compounds of wood origin, and part of the compounds may be chlorinated and part of them may be low-molecular compounds of carbon and hydrogen. As microbes act so that they use as nutrition only the organic portion of effluent, all inorganic substances, at least inorganic elements, remain in the effluent. Thus, biologically treated water, after separation of the solids and liquid fractions, e.g. by sedimentation, has an organic load that makes it clearly cleaner than effluent treated in other ways, but due to the inorganic substances it typically has to be discharged from the process.

In addition to or instead of the biological treatment, the purification stage may be e.g. chemical, whereby the purpose is to remove, for example, metals by precipitating, whereby also part of the organic substances is removed. Alternatively or in addition to the above mentioned stages, a filtration technique can be applied, such as ultrafiltration and/or a method based on membrane technique and/or osmosis. In other words, in addition to or instead of the biological treatment, for example ultrafiltration membranes, ion exchange, chemical precipitation, sedimentation, flotation, and/or filtration may be used for treating effluents.

Figure 1 shows an example of a pulp mill.

Typically at the pulp mill wood chips and white liquor are supplied to a digester for the cooking of the pulp. After the cooking, the cooked pulp, so called brown stock, is fed to washing and oxygen stages. Typically, hot water is used for said washing stage(s). Weak black liquor coming from the cooking 35 is conveyed from the cooking to an evaporation stage, in which some water is removed from the weak black liquor and, therefore, strong black liquor is generated. The water removed from the weak black liquor, i.e. condensates from the evaporation, is preferably conveyed to the washing stages or the brown stock. The strong black liquor generated in the evaporation stage is treated in order to obtain green liquor. The green liquor is further treated in order to obtain white liquor that can be reused for the cooking.

From the washing and oxygen stages of the brown stock, the pulp is conveyed to the bleaching unit. The bleaching unit typically comprises several bleaching stages for bleaching the pulp. The bleaching unit typically generates acid and alkaline effluents while bleaching the pulp, which effluents are typically conveyed to the waste water treatment plant. The bleached pulp is preferably conveyed to the pulp drying machine 80. The acid and alkaline effluents from the bleaching unit are conveyed to the waste water treatment plant.

Figures 2 to 3 show some example process steps in a reduced schematic chart. These can be implemented, for example, at the pulp mill according to Figure 1. 20 The present invention preferably comprises the brown stock preparation unit 20 comprising an alkaline cooking unit 21, a brown stock washing unit 22 and an oxygen delignification unit 25 for producing pulp, the pre-treatment stage 30a of the bleaching unit 30 to remove at least a portion of HexA, the bleaching unit 30 that advantageously uses ECF bleaching in which chloride-containing effluents are formed, or alternatively uses TCF bleaching instead of the ECF bleaching, an effluent purification plant (waste water treatment plant 50) for treating bleaching plant effluents and/or other effluents generated at the pulp mill, and a pulp drying machine.

During the oxygen delignification stage 25, a part of the residual lignin left in 35 pulp after the cooking 21 and the brown stock washing 22 is removed using oxygen and alkali. Thus, the oxygen delignification stage 25 is used to 16 remove much of lignin from unbleached pulp. Delignification with oxygen is a gentler way of reducing the kappa number than extended cooking. It may also lower the content of bleach plant effluents, and may reduce the need for environmentally unfriendly chlorine-based bleaching agents in the bleaching.

Preferably, the pulp is washed in at least one washing step after it has reacted with oxygen. In the washing step, some of the filtrates are recovered. The pulp having an increased dry matter content is first diluted and then led to the bleaching unit 30, preferably via the pre-treatment stage 30a.

During the oxygen delignification stage 25 that is implemented in the oxygen delignification unit, alkali, oxygen and steam are preferably added to the pulp coming from the brown stock washing unit 22. During the oxygen delignification 25, oxygen gas is preferably brought into contact with the fiber material under alkaline conditions. Preferably, the fiber material has enough OH-ions to neutralize and dissolve the organic acids which are generated in oxygen-lignin reactions. After a predetermined retention time the substances formed in the reactions and residual chemicals are washed out in the consecutive washing and dilution step(s).

In the pulp drying machine, the pulp is dried by pressing and/or heat. The removed water and/or condensate form the process waters that are collected from the pulp drying machine.

Advantageously, at least a portion of process waters collected from the pulp drying machine 80, i.e. at least a portion of white waters, is conveyed to be reused as dilution water at the pulp mill, preferably as dilution water in at least one dilution step in the oxygen delignification unit 25, most preferably as dilution water in the last dilution step 25Y in the oxygen delignification unit 25. The purpose of the dilution water is to adjust the consistency of the pulp to an appropriate level before bleaching.

The washing steps of the oxygen delignification stage 25 preferably comprise at least one washing press device and/or at least one DD washer. 17 After the post-washing 25X, the pulp is preferably diluted 25Y using the white water and is conveyed to the bleaching unit 30, most preferably via the pretreatment stage 30a.

Preferably, the white water that is collected from the drying machine and conveyed to the oxygen delignification unit 25 has at least some of the following properties: In an advantageous example, the total content of wood extractives 10 (measured according to the method of MTBE/GC) in the white water that is collected from the drying machine 80 to be used in the oxygen delignification unit and the dilution step therein is preferably between 0 and 12 mg/I, more preferably between 0.5 and 11 mg/I, and most preferably between 1 and 10 mg/I. In an example, the total content of wood extractives (measured 15 according to the method of MTBE/GC) in the white water that is collected from the drying machine 80 to be used in the oxygen delignification unit and the dilution step therein is lower than 9 mg/I, or lower than 8 mg/I.

In an advantageous example, the total content of dissolved lignin (measured according to the "UV 280") in the white water that is collected from the drying machine 80 to be used in the oxygen delignification unit and the dilution step therein is preferably between 0 and 45 mg/I, more preferably between 3 and 40 mg/I, and most preferably between 5 and 35 mg/I. In an example, the total content of dissolved lignin (measured according to the "UV 280") in the white 25 water that is collected from the drying machine 80 to be used in the oxygen delignification unit and the dilution step therein is lower than 35 mg/I, more preferably lower than 30 mg/I, and most preferably lower than 27 mg/I.

Advantageously, the total content of manganese, Mn (according to SFS-EN ISO 11885, valid 2011) in the white water that is collected from the drying machine 80 to be used in the oxygen delignification unit and the dilution step therein is preferably between 0 and 0.02 mg/I. Advantageously, the total content of manganese, Mn (according to SFS-EN ISO 11885, valid 2011) in the white water that is collected from the drying machine 80 to be used in the oxygen delignification unit and the dilution step therein is lower than 0.0 18 mg/I, more preferably lower than 0.008 mg/I or lower than 0.006 mg/I, and most preferably lower than 0.005 mg/I or lower than 0.004 mg/I.

Advantageously, the total content of iron, Fe (measured according to SFS- EN ISO 11885, valid 2011) in the white water that is collected from the drying machine 80 to be used in the oxygen delignification unit and the dilution step therein is preferably between 0 and 0.5 mg/I. Advantageously, the total content of iron, Fe (measured according to SFS-EN ISO 11885, valid 2011) in the white water that is collected from the drying machine 80 to be used in the oxygen delignification unit and the dilution step therein is lower than 0.4 mg/I, more preferably lower than 0.3 mg/I or lower than 0.25 mg/I, and most preferably lower than 0.23 mg/I or lower than 0.22 mg/I.

Advantageously, the total content of calcium, Ca (measured according to SFS-EN ISO 11885, valid 2011) in the white water that is collected from the drying machine 80 to be used in the oxygen delignification unit and the dilution step therein is preferably between 0 and 8 mg/I, or between 0 and 6 mg/I. Advantageously, the total content of calcium (measured according to SFS-EN ISO 11885, valid 2011) in the white water that is collected from the drying machine 80 to be used in the oxygen delignification unit and the dilution step therein is lower than 5 mg/I, lower than 4 mg/I, lower than 3 mg/I, lower than 2 mg/I or lower than 1 mg/I, more preferably lower than 0.5 mg/I, lower than 0.4 mg/I, lower than 0.2 mg/I or lower than 0.1 mg/I, and most preferably lower than 0.05 mg/I or lower than 0.01 mg/I.

Advantageously, the total content of barium (measured according to SFS-EN ISO 11885, valid 2011) in the white water that is collected from the drying machine 80 to be used in the oxygen delignification unit and the dilution step therein is preferably between 0 and 0.05 mg/I. Advantageously, the total 30 amount of barium (measured according to SFS-EN ISO 11885, valid 2011) in the white water that is collected from the drying machine 80 to be used in the oxygen delignification unit and the dilution step therein is lower than 0.05 mg/I, more preferably lower than 0.04 mg/I, and most preferably lower than 0.03 mg/I. 19 Advantageously, the total amount of aluminium (measured according to SFSEN ISO 11885, valid 2011) in the white water that is collected from the drying machine 80 to be used in the oxygen delignification unit and the dilution step therein is preferably between 0 and 0.2 mg/I. Advantageously, the total 5 amount of aluminium (measured according to SFS-EN ISO 11885, valid 2011) in the white water that is collected from the drying machine 80 to be used in the oxygen delignification unit and the dilution step therein is lower than 0.1 mg/I, or lower than 0.04 mg/I, more preferably lower than 0.02mg/I, or lower than 0.01mg/I, and most preferably lower than 0.005 mg/I.

Advantageously, the total content of oxalate (measured according to modified SCAN-N 39, valid 2011) in the white water that is collected from the drying machine 80 to be used in the oxygen delignification unit and the dilution step therein is preferably between 1 and 10 mg/I. Advantageously, the total content of oxalate (measured according to modified SCAN-N 39, valid 2011) in the white water that is collected from the drying machine 80 to be used in the oxygen delignification unit and the dilution step therein is lower than 9 mg/I, more preferably lower than 8 mg/I, lower than 7 mg/I, or lower than 6 mg/I, and most preferably lower than 5 mg/I or lower than 4 mg/I.

Advantageously, the total content of sodium (measured according to SFS-EN ISO 11885, valid 2011) in the white water that is collected from the drying machine 80 to be used in the oxygen delignification unit and the dilution step therein is preferably between 10 and 135 mg/I. Advantageously, the total content of sodium (measured according to SFS-EN ISO 11885, valid 2011) in the white water that is collected from the drying machine 80 to be used in the oxygen delignification unit and the dilution step therein is lower than 100 mg/I, or lower than 90 mg/I, more preferably lower than 80 mg/I or lower than 70 mg/I, and most preferably lower than 60 mg/I or lower than 50 mg/I.

Advantageously, the total content of silicon (measured according to SFS-EN ISO 11885, valid 2011) in the white water that is collected from the drying machine 80 to be used in the oxygen delignification unit and the dilution step therein is preferably between 5 and 30 mg/I. Advantageously, the total content of silicon (measured according to SFS-EN ISO 11885, valid 2011) in the white water that is collected from the drying machine 80 to be used in the oxygen delignification unit and the dilution step therein is lower than 30 mg/I, more preferably lower than 25 mg/I, and most preferably lower than 20 mg/I or lower than 10 mg/I. 5 Advantageously, the total content of Phosphorus (measured according to SFS-EN ISO 11885, valid 2011) in the white water that is collected from the drying machine 80 to be used in the oxygen delignification unit and the dilution step therein is preferably between 0 and 0.2 mg/I. Advantageously, the total content of Phosphorus (measured according to SFS-EN ISO 11885, valid 2011) in the white water that is collected from the drying machine 80 to be used in the oxygen delignification unit and the dilution step therein is lower than 0.2 mg/I, more preferably lower than 0.17 mg/I, and most preferably lower than 0.15 mg/I or lower than 0.1 mg/I. 15 Advantageously, the total content of sulphur (measured according to SFS-EN ISO 11885, valid 2011) in the white water that is collected from the drying machine 80 to be used in the oxygen delignification unit and the dilution step therein is preferably between 50 and 200 mg/I. Advantageously, the total content of Sulphur (measured according to SFS-EN ISO 11885, valid 2011) in the white water that is collected from the drying machine 80 to be used in the oxygen delignification unit and the dilution step therein is lower than 200 mg/I, or lower than 170 mg/I, more preferably lower than 150 mg/I, or lower than 130 mg/I, and most preferably lower than 125 mg/I or lower than 120 mg/I.

Advantageously, the total content of chloride (measured according to SFSEN ISO-10304, valid 2011) in the white water that is collected from the drying machine 80 to be used in the oxygen delignification unit and the dilution step therein is preferably between 5 and 40 mg/I. Advantageously, the total content of Chloride (measured according to SFS-EN ISO-10304, valid 2011) in the white water that is collected from the drying machine 80 to be used in the oxygen delignification unit and the dilution step therein is lower than 40 mg/I, more preferably lower than 30 mg/I, and most preferably lower than 25 mg/I. 21 Advantageously, the total content of sulphate (measured according to SFSEN ISO-10304, valid 2011) in the white water that is collected from the drying machine 80 to be used in the oxygen delignification unit and the dilution step therein is preferably between 150 and 600 mg/I. Advantageously, the total 5 content of Sulphate (measured according to SFS-EN ISO-10304, valid 2011) in the white water that is collected from the drying machine 80 to be used in the oxygen delignification unit and the dilution step therein is lower than 600 mg/I, or lower than 550 mg/I, more preferably lower than 500 mg/I, or lower than 450 mg/I, and most preferably lower than 400 mg/I or lower than 3 ring/I.

An example of the white water that is collected from the drying machine 80 to be used in the oxygen delignification unit is presented in Table 1. 15 Table 1. An example of some elements/compounds in the white water Chemical description Unit Measuring method Whitewaterfromthedrying machine Example value(s) Example(s)ofan advantageous concentration range Dissolve lignin mg/I Ristolainen, 1999 23 5- Totalwood Orsa&Holmbo extractives mg/I m, 1994 8.1 1- Modified 4.5, Total oxalate mg/I SCAN-N 39 3.1- SFS-ENISO 39.9, 10-140, preferably Sodium mg/I 1188110-100 SFS-ENISO <0.005, Aluminium mg/I 1188<0.002 0-0.01 SFS-ENISO 17.3, Silicon mg/I 11888.3 5- SFS-ENISO 0.134, Phosphorus mg/I 11880.004 0-0.2 Sulphur mg/I SFS-ENISO 117, 50-200 22 11881 SFS-ENISO <0.59, 0-8, preferably Calcium mg/I 1188<0.01 0-6, or 0-1 SFS-ENISO <0.017, 0-0.02, preferably Manganese mg/I 1188<0.000-0.01 SFS-ENISO Iron mg/I 11880.20-0.

SFS-ENISO 0.035, 0-0.4, preferably Barium mg/I 11880.2 0-0.3 SFS-EN ISO- 25.3, Chloride mg/I 10304 20.1 5- SFS-EN ISO- 373, Sulphate mg/I 10304 576 150-600 SFS-EN ISO- Magnesium mg/I 10304 2.39 0.1-3 SFS-EN ISO- Potassium mg/I 10304 2.0.1-4 Advantageously, at least 60% or at least 70%, more preferably at least 80% or at least 90%, and most preferably at least 95% or exactly 100% of the white water overflow of the drying machine 80 is circulated to the oxygen delignification unit 25, preferably to be used as dilution water in the dilution step of the oxygen delignification unit 25, most preferably to be used as dilution water in the last dilution step 25Y of the oxygen delignification unit 25. 10 Advantageously, the white waters that are coming from the drying machine 80 and are conveyed to the dilution step of the oxygen delignification stage 25, preferably at least to the last dilution step 25Y of the oxygen delignification stage, forms preferably between 50 and 85%, more preferably between 60 and 80%, and most preferably between 70 and 75% of all waters that are conveyed to said dilution step of the oxygen delignification stage, excluding waters that are circulating inside the oxygen delignification stage or coming with the pulp from the earlier process step. 23 Advantageously, the consistency of the pulp decreases in the last dilution step 25Y of the oxygen delignification unit by at least 15 percentage units, more preferably by at least 20 percentage units, and most preferably by at least 25 percentage units due to the dilution implemented by white water flow from the drying machine 80. However, the consistency of the pulp preferably decreases in the last dilution step 25Y of the oxygen delignification unit by less than 40 percentage units, or by less than 35 percentage units due to the dilution implemented by white water flow from the drying machine.

Advantageously, the consistency of the pulp before the dilution step 25Y of the oxygen delignification unit that is conveyed to the bleaching unit is between 25 and 45%, or between 26 and 42%, more preferably between 27 and 41%, or between 28 and 40%, and most preferably between 29 and 15 39%, or between 30 and 38%.

Advantageously, the consistency of the pulp after (the last) dilution step 25Y of the oxygen delignification unit that is conveyed to the bleaching unit is between 6 and 20%, or between 7 and 18%, more preferably between 7. 20 and 15%, or between 8 and 14%, and most preferably between 8.5 and 13%, or between 9 and 12%.

Advantageously, the pulp mill according to the present invention comprises a pre-treatment stage 30a for the bleaching unit 30. The pre-treatment stage 30a preferably comprises an acid-stage (i.e. A-stage), in which at least a portion of hexenuronic acids (HexA) and metals are removed.

After the A-stage, the pulp is preferably washed. Advantageously, the washing step(s) of the A-stage comprises the use of a wash-press and/or DD 30 washer, most preferably at least the use of the DD washer.

Hexenuronic acids compounds are often formed in wood during kraft cooking, typically from Xylan. Hexenuronic acids have an effect on chemical pulp production, especially on pulp bleaching operations in a kraft pulp mill. 35 An acid hydrolysis stage prior to a bleaching may lead up to a 50% reduction in bleaching costs, especially with hardwood kraft pulps. 24 Hexenuronic acids are unsaturated sugars; hence, they readily consume many reagents such as ozone, peracetic acid, and chlorine dioxide. Therefore, HexA consumes huge amounts of chemical, for example, in peracetic acid bleaching stages and/or chlorine dioxide bleaching stages. For example in peracetic acid bleaching, HexA reacts faster than lignin, thus, peracetic acid bleaching is uneconomic if HexA content is high. However, hexA is relatively inert to alkaline peroxide and oxygen delignification.

Preferably at least 50% or at least 60%, more preferably at least 70% or at least 80%, and most preferably at least 90% of the hexenuronic acid is removed during the pre-treatment stage 30a of the bleaching unit 30.

Temperature in the pre-treatment stage 30a of the bleaching unit 30a is 15 preferably between 80 and 180°C, more preferably between 90 and 120°C. pH of the pre-treatment stage 30a of the bleaching unit 30 is preferably between 2 and 5, more preferably between 2.5 and 4.

Advantageously, sulphuric acid is used in the pre-treatment stage 30a of the bleaching unit 30.

Advantageously, at least a portion of cleaned waste waters coming from a second clarifier of the waste water treatment plant is preferably conveyed to be reused as process water at the pulp mill, most preferably to be reused in said pre-treatment stage 30a of the bleaching unit 30 and/or in the bleaching unit 30.

Advantageously, the portion of the waters coming from a second clarifier of the waste water treatment plant 50 that is conveyed to the bleaching unit 30 and/or to the pre-treatment stage 30a of the bleaching unit comprises second clarifier filtrate of the waste water treatment plant. Advantageously, waters coming from the second clarifier of the waste water treatment plant are preferably used as washing liquid in the DD-washer plates (Drum Displacement washer) and/or in the wash press device.

Most advantageously, the A-stage is used as the pre-treatment stage 30a of the bleaching unit, and the pulp is conveyed from the oxygen delignification unit 25 to the A-stage 30a. 5 The bleaching unit 30 preferably comprises an EOP-stage and/or a D-stage. Most advantageously, the pulp is conveyed from the A-stage (pre-treatment stage 30a of the bleaching unit 30) to the EOP-stage of the bleaching unit 30 or to the (first) D-stage of the bleaching unit 30.

In addition, the bleaching preferably comprises a second D2-stage (chlorine dioxide stage) and/or a P-stage (peroxide-stage). Preferably said D2 —stage and/or the P-stage are placed after the FOP-stage.

Most preferably the order of said stages in the bleaching unit 30 is 1. D1-stage, EOP-stage, D2-stage, and P-stage. 20 At least a portion of the waste water coming from the second clarifier of the waste water treatment plant 50 is preferably conveyed to the pre-treatment stage 30a, preferably comprising the acid-stage, most preferably to the drum displacement washer (DD washer) or wash press therein, and/or 25 -the EOP-stage of the bleaching unit, most preferably the DD washer or wash press therein.

Most preferably, the waste waters coming from the second clarifier of the waste water treatment plant is conveyed, at least, to the pre-treatment stage 30a.

The DD washer preferably comprises 1 to 4 washing steps, most preferably 2, 3 or 4 washing steps. Advantageously, the waste waters coming from the second clarifier of the waste water treatment plant are conveyed to the first washing step of the drum displacement washer of the A-stage (pre-treatment 35 stage) and/or to the first washing step of the drum displacement washer of the EOP-stage (of the bleaching unit). 26 The waste waters coming from the second clarifier of the waste water treatment plant that are conveyed to the pre-treatment stage form preferably between 15 and 50%, more preferably between 20 and 40%, and most 5 preferably between 25 and 35% of the waters coming to the pre-treatment stage 30a of the bleaching unit, excluding waters that are circulating inside the bleaching stage or coming with the pulp from the previous process stage.

The waste waters coming from the second clarifier of the waste water 10 treatment plant that are conveyed to the EOP-stage of the bleaching stage preferably form between 0 and 44% or between 1 and 40%, more preferably between 1.5 and 35% or between 2 and 30%, and most preferably between 2.5 and 25% or between 3.0 and 20% of the waters coming to the EOP-stage of the bleaching unit, excluding waters that are circulating inside the bleaching stage or coming with the pulp from the previous process stage.

Advantageously, the total amount of waste waters coming from the second clarifier of the waste water treatment plant to the bleaching unit 30 and to the pre-treatment stage of the bleaching unit is between 5 and 45% or between 20 10 and 40%, more preferably between 15 and 35%, and most preferably between 20 and 30% of the total amount of effluents discharged from the bleaching unit 30 and the pre-treatment stage of the bleaching unit.

Advantageously, the total amount of the waste water coming from the second clarifier of the waste water treatment plant to the bleaching unit and/or to the pre-treatment stage 30a is at least 0.5 m3/Adt, or at least 1.0 m3/Adt, more preferably at least 2 m3/Adt or 2.5 m3/Adt, and most preferably at least 3.0 m3/Adt or at least 3.5 m3/Adt. However, preferably the amount of the waste water coming from the second clarifier of the waste water treatment plant to 30 the bleaching unit and/or to the pre-treatment stage 30a is not more than 7 m3/Adt, more preferably not more than 5 m3/Adt.

Advantageously, the total amount of the waste water coming from the second clarifier of the waste water treatment plant to the pre-treatment stage 30a of the bleaching unit is at least 0.5 m3/Adt, or at least 1.0 m3/Adt, more preferably at least 1.5 m3/Adt or at least 2.0 m3/Adt, and most preferably at 27 least 2.5 m3/Adt or at least 3.0 m3/Adt. However, preferably the amount of the waste water coming from the second clarifier of the waste water treatment plant to the pre-treatment stage 30a of the bleaching unit is not more than 5 m3/Adt, more preferably is not more than 4.5 m3/Adt.

Advantageously, the total amount of the waste water coming from the second clarifier of the waste water treatment plant to the EOP-stage of the bleaching unit is at least 0.1 m3/Adt, or at least 0.2 m3/Adt, more preferably at least 0.3 m3/Adt, and most preferably at least 0.4 m3/Adt. However, preferably the 10 amount of the waste water coming from the second clarifier of the waste water treatment plant to the bleaching unit is less than 5 m3/Adt, more preferably less than 4.5 m3/Adt.

Advantageously, the total amount of effluents that is discharged from the bleaching unit 30 and the pre-treatment stage 30a is less than 12.5 m3/Adt, more preferably less than 12 m3/Adt, or less than 11 m3/Adt, and most preferably less than 10.7 m3/Adt, or less than 10.5 m3/Adt.

Advantageously, the total amount of effluents that is discharged from the bleaching unit and the pre-treatment stage 30a is less than 10.5 m3/Adt, more preferably less than 10 m3/Adt or less than 9 m3/Adt, and most preferably less than 8.5 m3/Adt or less than 8 m3/Adt more than the total amount of waste waters coming from the second clarifier of the waste water treatment plant to the bleaching unit 30 and the pre-treatment stage 30a.

Advantageously, the total amount of the white water coming from the drying machine 80 to the dilution step of the oxygen delignification unit is at least 0.5 m3/Adt or at least 1.0 m3/Adt, more preferably at least 1.5 m3/Adt or at least 2.0 m3/Adt, and most preferably at least 2.5 m3/Adt or at least 3.0 m3/Adt. 30 However, the amount of white water coming from the drying machine to the dilution step of the oxygen delignification unit is preferably less than 10 m3/Adt, less than 8 m3/Adt, or less than 6 m3/Adt.

Advantageously, some filtrates are conveyed from the pre-treatment stage 30a of the bleaching unit 30 to the dilution step of the delignification unit 25.

The total amount of the waters that are conveyed from the pre-treatment 28 stage 30a of the bleaching unit 30 is preferably between 0 and 3 m3/Adt, more preferably between 0.5 and 2 m3/Adt, and most preferably between 0.7 and 1.6 m3/Adt.

There may be a filtration device within the process water circulation, i.e. a filtration device for the white water flow that is placed between the drying machine 80 and the oxygen delignification stage. However, this is typically not necessary. In addition, preferably no other purifying device is used. Therefore, the white water that is used according to the present invention typically does not need any purifying step before it is reused as process waters.

Advantageously, at least a portion of the process waters coming from the pulp drying machine 80 is conveyed to the oxygen delignification unit 25. 15 Therefore, the total amount of the generated effluents and the total amount of the needed raw water are decreased because the process water coming from the pulp drying machine is used as raw water in the oxygen delignification unit. Therefore, due to said circulation the total amount of effluents to be purified in the wastewater treatment plant can be decreased.

In addition, at least a part of waste water coming from the second clarifier of the waste water treatment plant is preferably used as wash water in the bleaching unit; thus, the amount of the needed raw water is decreased therein.

Example 1 One kind of process for wastewater reduction according to the present invention is disclosed in this example.

At a chemical mill according to this example, 3,700 tons of pulp are manufactured in one day. Figure 3 shows some process steps of this pulp mill in reduced schematic chart. 35 White water from the drying machine is conveyed to the dilution step of the oxygen delignification step, which dilution step is after wash press of the 29 post-washing stage of the delignification unit. The amount of the white water that is conveyed to the dilution step is approximately 3.1 m3/Adt. In addition, some filtrates from the first bleaching step of the bleaching unit are conveyed to the dilution step of the oxygen delignification unit. The total amount of the 5 waters that is conveyed from the pre-treatment stage of the bleaching unit to the oxygen delignification unit is 1.15 m3/Adt. The pre-treatment stage is, in this case, A-stage.

Example 2 In this example 2, white water from the drying machine is conveyed to the dilution step of the oxygen delignification step, as described in example 1.

In addition, some waste waters coming from the second clarifier of the waste water treatment plant are circulated to the first washing step of the drum displacement washer of the EOP stage of the bleaching unit. The amount of this water is 0.3 m3/Adt. In addition, some waste waters coming from the second clarifier of the waste water treatment plant is circulated to the first washing step of the Drum Displacement washer of the pre-treatment stage of the bleaching unit. The amount of this water is 2.3 m3/Adt. Therefore, the total amount of waste waters from the second clarifier of the waste water treatment plant to the bleaching unit and pre-treatment stage is 2.6 m3/Adt.

Some effluents are discharged from the pre-treatment stage of the bleaching unit, the amount of these effluents being 6.4 m3/Adt. In addition, 4.1 m3/Adt effluents (bleaching filtrates) coming from the D2-stage of the bleaching unit are discharged. Therefore, the total amount of the effluents discharged from the bleaching unit and pre-treatment stage is 10.5 m3/Adt. 30 Before these amendments, the amount of waste water was 9 m3/Adt from the pre-treatment stage and 3.8 m3/Adt from the D2-stage; thus, the total amount of the effluents was 12.8 m3/Adt. Thanks to the present invention and said white water and effluent circulations, the amount of the effluents comprising bleaching effluents and effluents from pre-treatment stage of the bleaching unit from this mill decreased by 2.4 m3/Adt. In addition, the circulation of the waste waters coming from the second clarifier of the waste water treatment plant decreases the total amount of effluents by 2.6 m3/Adt. Therefore, the effect on the total amount of effluents according to this Example was 5 m3/Adt.

One skilled in the art will readily understand that the different embodiments of the invention may have applications in environments where optimization of a liquid flow at the pulp mill is desired. Therefore, it is obvious that the present invention is not limited solely to the above-presented embodiments, but it can be modified within the scope of the appended claims. 31

Claims (19)

1. Claims

   1. A method for treating liquid flows at a chemical pulp mill comprising -a brown stock preparation unit (20) comprising a cooking unit (21), a brown stock washing unit (22) and an oxygen delignification unit (25), a bleaching unit (30) comprising at least one bleaching stage, a pulp drying machine (80), a waste water treatment plant (50) comprising a second clarifier, and an optional pre-treatment stage (30a) of the bleaching unit (30), wherein the method comprises -conveying at least a portion of white waters from the pulp drying machine (80) to the oxygen delignification unit (25).

   2. The method according to claim 1, wherein the method further comprises conveying at least a portion of waste waters from the second clarifier of the waste water treatment plant to the bleaching unit (30) and/or to the pre-treatment stage (30a).

   3. The method according to claim 1 or 2, wherein the oxygen delignification unit (25) comprises a dilution step, and at least a portion of the white waters is conveyed to the oxygen delignification unit to be used as dilution water in the dilution step of the oxygen delignification unit.

   4. The method according to claim 3, wherein at least a portion of the white waters is used as dilution water in the last dilution step (25Y) of the oxygen delignification unit.

   5. The method according to any of the preceding claims, wherein the bleaching unit (30) and/or the pre-treatment stage (30a) comprises a washing unit comprising a drum displacement washer and/or a wash press, and at least a portion of the waters from the second clarifier is used in said washing unit(s). 32

   6. The method according to any of the preceding claims 1 to 5, wherein the pre-treatment stage (30a) comprises an acidic stage, and at least a portion of the waters coming from the second clarifier of the waste water treatment plant is conveyed to said pre-treatment stage.

   7. The method according to any of the preceding claims 1 to 6, wherein the bleaching unit comprises an alkaline extraction stage in which oxygen and peroxide are used as additional chemicals, and at least a portion of the waters coming from the second clarifier of the waste water treatment plant is conveyed to said alkaline extraction stage.

   8. The method according to any of the preceding claims, wherein the white water is collected as white water overflow from the drying machine.

   9. The method according to claim 8, wherein at least 50% of white water overflows of the drying machine is conveyed to the oxygen delignification unit.

   10. The method according to any of the preceding claims, wherein the total amount of the white water that is conveyed to the oxygen delignification unit is at least 0.5 m3/Adt.

   11. The method according to any of the preceding claims 1 to 10, wherein the total amount of the waste water that is conveyed from the second clarifier of the waste water treatment plant to the bleaching unit and/or to the pre-treatment stage is at least 0.5 m3/Adt.

   12. The method according to any of the preceding claims, wherein at least some filtrates coming from the pre-treatment stage of the bleaching unit are conveyed from the pre-treatment stage of the bleaching stage to the dilution step of the oxygen delignification unit.

   13. The method according to any of the preceding claims, wherein the total amount of chloride in the white water is between 5 and 40 mg/I. 33

   14. The method according to any of the preceding claims, wherein the total amount of calcium in the white water is between 0 and 6 mg/I.

   15. The method according to any of the preceding claims, wherein the total amount of wood extractives in the white water is between 0 and 12 mg/I.

   16. The method according to any of the preceding claims, wherein bleaching unit (30) comprises the following stages:

   1. a Dl-stage,

   2. an FOP-stage,

   3. a D2-stage and

   4. a P-stage.

   17. A system for treating liquid flows at a chemical pulp mill, the system cornprising a brown stock preparation unit (20) comprising a cooking unit (21), a washing unit (22), and an oxygen delignification unit (25), a bleaching unit (30), optionally, an apparatus for a pretreatment stage of the bleaching unit, a pulp drying machine (80), and a waste water treatment plant (50), wherein the system further comprises an apparatus adapted to convey white waters from the pulp drying machine (80) to the oxygen delignification unit (25).

   18. The system according to claim 17, wherein the oxygen delignification unit comprises a post-washing unit comprising a DD washer and/or a wash press.

   19. Use of white water coming from a pulp drying machine (80) as dilution water in the oxygen delignification unit (25). oc wawm4 waWki buizormea ,g.redE 08 drd rg„;sis ui-46140 gtZZ Aelestes • awM pt 511-r,vd;N:

   64. p.ealE '411 PPV „ veig .rapori Alavx.3.9.8 CZ Wfie.a.°3dFik3 ucTi.a.z.,:deA- WOJ ale:z..v4pooicrIN I:',q4ufeckM MW 2/3 Wood material 212230a Pulp