WO2013135956A2

WO2013135956A2 - A method and a system for treating liquid flows at a chemical pulp mill

Info

Publication number: WO2013135956A2
Application number: PCT/FI2013/050269
Authority: WO
Inventors: Katja Viitikko; Sami Saarela; Tuija Vartia; Virginia FERNANDEZ; Maria Jose AGUERREBERE; Matti Ristolainen; Olli RIMPINEN
Original assignee: Upm-Kymmene Corporation
Priority date: 2012-03-12
Filing date: 2013-03-12
Publication date: 2013-09-19
Also published as: RU2014136219A; BR112014022420B1; WO2013135956A3; FI128900B; BR112014022420A2; CN104169493B; RU2634894C2; UY34673A; FI20125264L; CN104169493A

Abstract

The invention relates to a method for treating liquid flows at a chemical pulp mill. The system comprises a cooking process for producing pulp, at least one washing stage for washing the pulp, at least one bleaching stage for bleaching the pulp, a waste water treatment plant (40), a raw water treatment plant (30), and a wood processing unit (50), and the method comprises conveying at least a portion of low chloride content process waters to the chemical pulp mill in order to be reused as process water. In addition, the invention relates a system for treating liquid flows at a chemical pulp mill and a use of low chloride content process waters at a chemical pulp mill.

Description

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

Effluents of chemical pulp mills are typically treated in waste water treatment plants in order to decrease the amount of the compounds that are harmful for the environment. Many motivations exist to decrease water use and effluent volume, such as the cost of supplying raw water, possible limited water supply, energy savings and the possibility to decrease fiber and chemical losses. Nowadays, mills are often 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 to a place that otherwise fulfills the 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, nowadays, it is important to look for solutions for finding a more closed 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. According to the present invention, a secondary water circulation is provided, which advantageously comprises or consists of low chloride content process waters.

At present, the whole effluent flow from the chemical pulp mill is typically treated together. The present invention gives a solution to reduce the total amount of effluents at the pulp mill by separating and circulating the purest fractions of the process waters. This will enable reuse of those process water flows without treatments in the waste water treatment plant and, hence, the solution can be cost efficient.

The target of an advantageous embodiment of the invention is to create a separate secondary water circulation with a low chloride content, which helps the usability of the water and makes it possible to purify this circulation with cost efficient purification methods if light purification is needed.

Advantageously, the system comprises

a digestion process for cooking pulp,

at least one washing stage in order to wash said pulp, at least one bleaching stage to bleach the pulp,

a waste water treatment plant, and

a raw water treatment plant.

In an advantageous example, the system according to the present invention comprises

at least one wood processing unit,

a brown stock preparation unit comprising cooking, washing, screening and oxygen delignification stages for producing pulp, a bleaching unit comprising bleaching stages to bleach the produced pulp, which bleaching unit preferably comprises Nash pump sealing waters and/or bleaching filtrates,

a chemical recovery unit, which may comprise jet condenser cooling waters and/or vacuum pump sealing waters,

a recovery boiler, which also generates boiler blow down water, and an evaporator, which generates condensates.

In addition, the system may comprise a pulp drying machine. However, it is also possible that the system does not comprise said pulp drying machine. Advantageously, the method according to the present invention comprises the following steps:

- conveying at least a portion of the low chloride content process waters from the pulp mill, and

- circulating the low chloride content process waters to the pulp mill to be used as process waters. Advantageously, at least a portion of the low chloride content process waters is conveyed to an equalization tank and/or a pond before it is reused as process water at the pulp mill . CI content of the low chloride content process waters is preferably less than 200 mg/l, more preferably less than 100 mg/l. In addition or alternatively, Ca content of the low chloride content process waters is preferably less than 200 mg/l, more preferably less than 100 mg/l. Advantageously, the pH of the low chloride content process waters is between 6.5 and 10.

Advantageously, the low chloride content process water flow comprises fiberline process waters from canals. Alternatively or in addition, the low chloride content process water flow preferably comprises sealing waters from black liquor evaporation and recovery boiler area. Alternatively or in addition, the low chloride content process water flow preferably comprises sealing waters from a demineralization plant. Alternatively or in addition, the low chloride content process water flow preferably comprises rejects from a drying machine. Alternatively or in addition, the low chloride content process water flow preferably comprises tank overflows from a drying machine.

Advantageously, the low chloride content process water flow is at least partly used as washing water in a firewater line and/or as sealing water in a firewater line. In an advantageous embodiment, the low chloride content process water flow is purified before it is reused as process water at the pulp mill. Preferably, the low chloride content process water flow is purified by a chemical method and/or a mechanical method. Advantageously, the low chloride content process water flow is purified before it is reused as process water by at least one of the following techniques:

a chemical flocculation technique,

a clarification technique,

a sand filtration technique and

a combination of two or three of these.

Advantageously, the purification is implemented in a raw water treatment plant. Alternatively or in addition, the purification can be implemented by a purifying unit comprising a mechanical or chemical purifying unit. The present invention can be implemented 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. Thanks to the present invention, a system capable of decreasing

the environmental load of the chemical pulp mill,

the amount of raw water used by the chemical pulp mill, and the amount of effluents generated in the pulp mill

can be provided.

Aspects of the invention are characterized by what is stated in the independent claims 1 , 9 and 10. Various embodiments of the invention are disclosed in the dependent claims. Description of the Drawings

In the following, the invention will be described in more detail with reference to the appended drawings, in which Figs 1 to 2 show example embodiments in reduced schematic charts.

Detailed Description of the Invention

In this application, reference is made to Figs 1 to 2, in which the following references are used:

10 low chloride content process waters,

20 effluents comprising bleaching effluents,

30 raw water treatment plant,

40 waste water treatment plant,

41 gravity table,

50 wood handling unit,

60 fire and sealing waters, and

70 purifying unit comprising at least one mechanical purifying stage and/or at least one chemical purifying stage. The purifying unit may comprise for example a clarifier. The term "low chloride content process waters" refers to process waters having low chloride content. Advantageously, the chloride content of the low chloride content process waters is lower than 200 mg/l, preferably lower than 100 mg/l. In addition, the low chloride content process waters advantageously have a high solids content and, in addition, they preferably contain fibers.

The term "effluents" refers to waste waters. Effluents are conveyed to the waste water treatment plant for a purifying process. Effluents may comprise, for example, bleaching effluents and ash leaching purge. In addition, the effluents may contain wood processing effluents.

The term "bleaching effluents" refers to effluents from the bleaching unit and the washing steps therein. Advantageously, the bleaching effluents contain bleaching filtrates. Most advantageously, the bleaching effluents consist of bleaching filtrates. The bleaching filtrates contain acidic and/or alkaline flows that are generated during bleaching stages. Advantageously, the bleaching effluents do not contain fibers. 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.

The term "ECF" refers to elemental chlorine free.

The term "TCF" refers to totally chlorine free.

The term "ESP" refers to refers to electrostatic precipitator. The term "recyclable water" refers to process waters that are clean enough to be reused as process waters in the process. The recyclable water may be purified in a raw water treatment plant 30 and/or in at least one purifying unit 70. The term "EOP" refers to an alkaline extraction stage in pulp bleaching using oxygen and peroxide as additional chemical . The term "NPE" refers to non-process elements. NPEs are inorganic substances originating from process devices, some also from wood, raw water and chemicals, which are not wanted to process. The term "secondary water circulation" refers to a process water circulation comprising low chloride content process waters which are not conveyed to the waste water treatment plant.

The term "Adt" refers to air dry metric ton of 90% dry chemical pulp.

Firewater refers to waters supplied through a water supply line for washing various targets, and fire extinction in case of urgency.

Sealing waters are waters used in vacuum pumps for sealing.

Jet condenser cooling waters are waters that are led in direct contact with the steam in jet condensers or waters that result from such direct contact with steam. Gravity table is a device used for dewatering sludge through a permeable belt in wastewater treatment. Gravity table shower waters are used for washing the belt.

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 which dissolve as much lignin and as little cellulose as possible are typically used in the pulping process. Typically, the process for manufacturing bleached chemical pulp comprises pulping, washing, screening, bleaching, and cleaning stages. Nowadays sulfate cooking, also called as kraft cooking or pulping, which uses a mixture of sodium hydroxide (NaOH) and sodium sulfide (Na₂S), is the most commonly used pulp production method. The cooking process may be based on batch cooking or continuous cooking comprising a digester or several digesters. Brown stock treatment after the cooking process preferably includes a washing process, an oxygen stage, and a screening followed by washing. Typically, the last washing apparatus in the oxygen 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. When the filtrate is recovered from the first brown stock washing apparatus, it may be forwarded either directly to a black liquor evaporation plant or it may be used in digester plant processes for dilution and displacement, after which it ends up in the black liquor flow. After the last washing stage, the pulp may be led to bleaching.

Bleaching steps 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 any lignin, known as residual lignin, that remains in the pulp after the cooking and oxygen stages, and which could not 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 on the washer following the bleaching stage. For bleaching purpose,

- chlorine (CI₂), ozone (O3) and/or peroxide acid (Paa and Caa) can be used to react with aromatic lignin units, and/or

- chlorine dioxide(CIO2) and/or oxygen (O2) can be used to react in general with lignin structures that have free phenolic hydroxyl groups, and/or

- hypochlorite (H) and/or hydrogen peroxide (H₂O₂) 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 chlorine dioxide (CIO₂) is used for the bleaching purpose. Bleaching effluent is typically a significant source of both biological and chemical oxygen consumption. For example chlorine-containing inorganic compounds and 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 form of organic molecules. Additionally, sulfuric acid may be used in bleaching stage(s) for pH regulation and as main chemical in the hydrolysis of hexenuronic acids. 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, hydrochloric acid is used for pH regulation, and/or sulfur dioxide and/or other reductants are used for elimination of chemical residuals from the bleaching, i.e. for elimination of unreacted bleaching chemicals.

The pulp mill has not only bleaching effluents but also process waters, such as cooling waters, sealing waters, reject flows, channel waters, washing waters of the plant, and rain waters, as well as wood processing water. Said process waters have typically not been in contact with the pulping process with the exception of wood processing water and some channel waters that originate from process overflows and are therefore in contact with the pulping process. Thus, the emissions accumulated therein are mainly leakages and overflows, occasional emissions caused by apparatus breakages, washing waters of devices, textiles (wires and felts) or containers originating from continuous or batch washings, and leakages from the reject system.

As mentioned, brown stock treatment comprises at least one washing process. Advantageously, the brown stock treatment comprises

- a screening process,

- an oxygen delignification stage and

- at least one washing stage after the oxygen delignification stage. The screening process may be located after digester blowing, in the middle of or after the washing process, or after oxygen delignification. Typically, these process stages are followed by a bleaching process, preferably 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 stage, wherein preferably at least oxygen and/or peroxide is used. Ozone stage(s), acid stage(s), and chelate stage(s) for removing heavy metals can also be used.

A method according to the present invention comprises at least an alkaline cooking process for producing pulp, a bleaching plant advantageously using ECF bleaching in which chloride-containing effluents are formed, and an effluent purification plant (waste water treatment plant) for treating bleaching plant effluents and/or other effluents generated at the mill . Alternatively, TCF bleaching can be used instead of the ECF bleaching. Advantageously ECF bleaching is used, wherein said bleaching comprises both, acid 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. If chlorine dioxide is used in one bleaching stage, most typically the doses are between 5 and 15 kg act. Cl/adt pulp. The chlorine dioxide doses for softwood are typically between 25 and 35 kg/adt and for hardwood between 20 and 30 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 typically comprises a chemical recovery plant including an evaporation process typically with 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. Said systems are particularly intended 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, phosphorous, iron, and barium are some elements of concern in a bleached chemical pulp mill . 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 various wood-originating compounds, and part of the compounds is chlorinated and part of them is 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 solid and liquid fraction, 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. Figure 1 shows an example of a pulp mill. Typically at the pulp mill 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 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 typically conveyed to a paper mill and/or to a pulp drying machine. Figure 2 shows an example of the secondary water circulation in a reduced schematic chart. This can be implemented, for example, at the pulp mill according to Figure 1 . Figure 2 shows a low chloride content process water flow 10, effluents 20 comprising bleaching effluents, a raw water treatment plant 30, a waste water treatment plant 40, a gravity table 41 , a wood handling unit 50, fire and sealing water flow 60, and a possible purifying unit 70.

At least one purifying unit 70 may be placed before the raw water treatment plant 30 and/or after the raw water treatment plant 30.

The raw water treatment plant 30 preferably comprises at least raw water intake and cascade, raw water clarifier(s), sand and/or multimedia filter(s). Advantageously, the raw water treatment plant 30 comprises at least two, more preferably at least three, and most preferably at least four separate treatment lines. If the low chloride content process water flow 10 is treated in the raw water treatment plant 30, it can be conveyed to any of said purifying stages of the raw water treatment plant 30. The low chloride content process water flow 10 may be cleaned in a treatment line different from the raw water coming from the water system. In this case, the low chloride content process water flow 10 is purified in a treatment line of the raw water treatment plant 30 separate from the raw water. Alternatively, the low chloride content process water flow 10 may be purified together with the raw water coming from the water system 10 in at least one of the treatment lines of the raw water treatment plant 30. However, advantageously the treatment line that is used for purifying the low chloride content process water flow 10 in the raw water treatment plant 30 is separate from the raw water sources, i.e. said at least one treatment line is preferably only used for purifying the low chloride content process water flow 10.

According to the present invention, a portion of the process waters is separated and reused, advantageously after a light purification. Due to the secondary water circulation there is a circulation inside the process waters. In other words, not all process waters are conveyed to the waste water treatment plant 40 but a proportion of the process waters is reused in the process. Thanks to the secondary water circulation, it may be possible to reduce the raw water consumption at a pulp mill.

Advantageously, ash treatment reject flow is combined with acidic and alkaline flow from bleaching to form the effluent flow 20. If EOP filtrate is used in the wood processing 50, the liquid flow from the wood processing unit 50 is preferably conveyed to the effluent flow 20 comprising bleaching effluents. Otherwise, the liquid flow from the wood processing unit 50 is preferably conveyed to the low chloride content process water flow 10. The effluent flow 20 comprising bleaching effluents is preferably conveyed to the waste water treatment plant 40.

Fire and sealing water flow is preferably led to the low chloride process water flow 10. In addition or alternatively, water flow from fiberline comprising waters from canals is preferably led to the low chloride process water flow 10.

Advantageously, the system comprises a purifying unit 70, which comprises at least one mechanical purifying stage and/or at least one chemical purifying stage for purifying the low chloride process water flow 10. Advantageously, the purifying unit 70 is a clarifier that is used to remove suspended solids.

If the purifying unit 70 is used, the waters purified in the mechanical or chemical purifying device 70 are advantageously led at least partly to the raw water treatment plant 30. Alternatively or in addition, the waters purified in the mechanical or chemical purifying device 70 are advantageously led to the pulp mill to be reused as process waters. However, the process waters have to be recyclable, i.e., clean enough to be used as process waters, already after the purifying device 70 if the raw water treatment plant, or any other purifying device, is not used after said treatment in the purifying unit.

Advantageously, at least part of the low chloride content process waters 10 are purified in the purifying unit 70 and/or in the raw water treatment plant 30 in order to achieve recyclable water, after which said recyclable waters are reused in the process. Advantageously, the secondary water circulation is a separate water flow from substantially pure fractions, i.e. low chloride content process waters. Low chloride content process waters preferably comprise fiberline process waters from canals, including preferably Nash pumps sealing waters, the total amount of the fiberline process waters being, for example, between 0 and 5 m³/Adt, more preferably between 1 and 3 m³/Adt. Alternatively or in addition, the low chloride content process waters preferably comprise sealing waters and blow down waters from a recovery boiler and/or demineralization plant areas, the total amount being, for example, between 0 and 1 m³/Adt, more preferably between 0.1 and 0.5 m³/Adt. Alternatively or in addition, the low chloride content process waters preferably comprise sealing waters from a black liquor evaporation plant, the total amount being, for example, between 0 and 1 m³/Adt, more preferably between 0.1 and 0.5 m³/Adt. Alternatively or in addition, the low chloride content process waters preferably comprise tank overflows and/or rejects from a drying machine, the total amount being, for example, 3 m³/Adt at the most, more preferably 0.1 m³/Adt at the most. Alternatively or in addition, the low chloride content process waters preferably comprise causticizing jet condenser cooling waters, the total amount being, for example, 4 m³/Adt at the most, more preferably between 1 and 2 m³/Adt. Alternatively or in addition, the low chloride content process waters preferably comprise wood processing process waters, the total amount being, for example, 2 m³/Adt at the most, more preferably between 1 and 2 m³/Adt.

Advantageously, the low chloride content process waters comprise 2 or 3, more preferably 4 or 5 and most preferably 5 or 6 of the above mentioned sources. Most preferably, the low chloride content process waters comprise fiberline process waters and/or sealing waters.

Advantageously at least 50% or at least 60%, more preferably at least 70% or at least 80% and most preferably at least 90% or at least 95% of the all above mentioned low chloride content process waters is reused as process waters without a purifying treatment in the waste water treatment plant. Advantageously, there is at least one storage unit, such as an equalization tank and/or a pond, for the low chloride content process waters 10. Fire and sealing waters 60 typically do not need a tank but the waters can be conveyed without any storage unit, for example, in pipes.

The low chloride content process waters preferably have a low NPE-content already before the possible purification step in the purifying unit 70 and/or in the raw water treatment plant 30.

Advantageously, the low chloride content process waters flow has, before any purification step,

a CI content lower than 200 mg/l, more preferably CI less than 100 mg/l,

a Ca content lower than 200 mg/l, more preferably Ca less than 100 mg/l,

a sulphate content lower than 1500 mg/l, more preferably lower than 1000 mg/l,

pH of the water between 6.5 and 10, and

a COD value lower than 1200 mg/l or lower than 1 100mg/l, more preferably lower than 1000 mg/l or lower than 800 mg/l, and most preferably lower than 500 mg/l.

A purification step may be needed, for example, in order to decrease the sulphate content and/or the COD value of the low chloride content process waters flow because, advantageously, the low chloride content process waters flow has, when reused as process water,

a CI content lower than 200 mg/l,

a Ca content lower than 1 10 mg/l,

a sulphate content lower than 200 mg/l,

a COD value lower than 100 mg/l,

pH of the water between 6.5 and 10, and

potassium-permanganate-usage less than 25 mg/l.

The CI, Ca and COD values differ significantly between the low chloride content process waters 10 and the effluents 20 comprising bleaching effluents and, in addition, between the low chloride content process waters 10 and the purified waste water of the waste water treatment plant. Especially the chloride content of the low chloride content process waters 10 differs significantly from the effluents 20 and, also, from the waste water purified in the waste water treatment plant. The content of the NPE's may be, for example at a pulp mill where waste water amount is 15 m³/Adt, for the bleaching effluents as follows:

- a CI content of approximately 350 mg/l, and

- a Ca content of approximately 1 10 mg/l,

and for waste water purified at the waste water treatment plant as follows:

- a CI content of approximately 440 mg/l, and

- a Ca content of approximately 150 mg/l.

The COD value of the bleaching effluents is typically around 2500 to 3000 mg/l, hence it needs to be decreased, typically by biological purification. Also the pH of the bleaching effluents differs significantly from the low chloride content process waters 10, the pH of the bleaching effluents being typically around 4. However, the pH value naturally varies, depending on the alkalinity or acidity of the effluents. Usually, the combined effluent flow is acidic, because the amount of acidic bleaching filtrate is typically higher.

The low chloride content process waters 10 are divided into at least two flows as regards their COD content. In lower fractions, the COD content is advantageously 600 mg/l at the most, for example between 40 and 600 mg/l. In an example, the low chloride content process waters 10 with higher COD than others, for example coming from a wood handling where the COD may be 1300 mg/l and/or a pulp drying units where the COD may be 800 mg/l, are conveyed to an additional purifying treatment.

Advantageously, to achieve the low chloride content in the low chloride content process waters 10, the following fractions are not included in the low chloride content process waters but are directed to the effluents 20 and/or to the waste water treatment plant 40:

- ESP ash leaching purge and/or other ash treatment reject, comprising typically extremely high chloride content, and

- wood processing process waters, if EOP bleaching filtrate is used in the wood processing. The low chloride content process water flow 10 is advantageously circulated to be reused, at least partly, for firewater line for washing targets, the amount being, for example, 2 m³/Adt, and/or for the firewater line for sealing targets. Alternatively or in addition, the low chloride content process water flow is advantageously circulated to be reused for gravity table showers of the waste water treatment plant, the amount being, for example 0.7 m³/Adt.

Alternatively or in addition, the low chloride content process water flow is advantageously circulated to be reused in wood processing, the amount being, for example, 1 .2 m³/Adt. Alkaline pH of this flow is preferably used in the wood processing unit while the components solubilized from wood may make the environment there otherwise too acidic. Alternatively or in addition, the low chloride content process water flow 10 is advantageously used in causticizing cooling waters in Jet Condensers.

In an example, the low chloride content_process water to be circulated is treated with cost efficient purification method(s) in order to achieve recyclable water. Advantageously, the purification of the low chloride content_process waters comprises a chemical flocculation stage, a clarification stage, a sand filtration stage and/or a disinfection stage such as UV, H2O2 or O3 treatment.

Alternatively or in addition, at least one purification stage of the low chloride content process water comprises a flocculation stage with magnetite by so called Johnson Screens method. Alternatively or in addition, at least one purification stage of the low chloride content process water comprises a microfiltration stage. Alternatively or in addition, at least one purification stage of the low chloride content process water comprises an oxidation stage. Alternatively or in addition, at least one purification stage of the low chloride content process water comprises an ultrafiltration stage. Alternatively or in addition, at least one purification stage of the low chloride content process water comprises a nanofiltration stage. Alternatively or in addition, at least one purification stage of the low chloride content process water comprises another chemical and/or mechanical purification stage. Advantageously, the purification of the low chloride content process water comprises one, two or three of the above mentioned stages. Flocculants used in the chemical flocculation stage may comprise, for example, long chain organic polymeric compounds with or without electrical charges (cationic, anionic or non-ionic polymers or their salts). Preferably, the flocculant comprises at least one, such as one, two, three, four, five or six, of the following compounds:

- polyamine,

- poly(allylamine),

- poly(diallyldimethylammonium-chloride),

- poly(amidoamine),

- poly(vinylformamide),

- poly(vinylpyrrolidone),

- poly(vinylacetamide),

- poly(vinylmethylformamide),

- poly(vinylmethylacetamide),

- poly(dimethylaminopropylmethacrylamide),

- poly(dimethylaminoethylacrylate),

- poly(diethylaminoethylacrylate),

- poly(acryloylethyltrimethylammoniumchloride),

- poly(acrylamidopropyltrimethylammoniumchlorid),

- poly(methacrylamidopropyltrimethylammoniumchloride),

- poly(acrylamide), poly(ethylene-imine),

- poly(ethylene-oxide),

- poly(vinylpyridine),

- hexadimethrinbromide,

- poly(dimethylamine-co-epichlorhydrin),

- poly(dimethylamine-co-epichlorhydrin-co-ethylendiamine),

- poly(amidoamine-epichlorhydrin) or copolymer,

- N-vinylformamide,

- allylamine,

- diallyldimethylammonium-chloride,

- N-vinylacetamide,

- N-vinylpyrrolidone,

- N-methyl-N-vinylformamide,

- N-methyl-N-vinylacetamide,

- dimethylaminopropylmethacrylamide,

- dimethylaminoethylacrylate,

- diethylaminoethylacrylate, - acryloylethyltrimethylammoniumchloride or

methacrylamidopropyltrimethylammoniumchloride in a

homopolymerized form and/or carrying branched structures, and

- other flocculants in which the functionality of the polymer is based on quaternary ammonium groups, such as in starch based derivatives.

In an advantageous example, acrylamide and/or polyacrylamide is used together with so called DADMAC (diallyldimethylammonium-chloride). Alternatively or in addition, acrylamide is advantageously reacted together with another acrylamide to form polyacrylamide.

Preferably, the flocculant comprises polyamine. Alternatively or in addition, the flocculant comprises poly(diallyldimethylammonium-chloride) (i.e. poly- DADMAC). Alternatively or in addition, the flocculant comprises poly(amidoamine) (i.e. PAA). Alternatively or in addition, the flocculant comprises poly(vinylformamide) (i.e. PVAm). Alternatively or in addition, the flocculant comprises poly(ethyleneimine) (i.e. PEI). Alternatively or in addition, the flocculant consists of homopolymerized form and/or carrying branched structures in which the functionality of the polymer is based on quaternary ammonium groups, such as in starch based derivatives. In an advantageous embodiment, two or three, more preferably four, and most preferably five of the above mentioned flocculants are used together.

Alternatively or in addition to the flocculants, other compounds such as coagulants and/or adsorbents may be used for the purifying stage.

Coagulants may comprise aluminium compounds. Advantageously, the aluminium compounds comprise aluminium nitrate. Alternatively or in addition, the aluminium compounds comprise aluminium chloride. Alternatively or in addition, the aluminium compounds comprise alum andpolymeric forms, such as polyaluminiumnitrate (PAN), polyaluminiumchloride (PAC), and/or polyaluminiumsulfate (PAS), Alternatively or in addition, coagulants comprise ferrous compounds, such as FeC , and/orFeSO₄. Alternatively or in addition, coagulants comprise ferric compounds, such as FeCh, and/or Fe2(SO₄)3. Adsorbents may comprise, for example, clay, bentonite and/or talc.

Advantageously, an ash treatment reject flow, advantageously comprising ash leaching purge, is conveyed to the effluent flow 20. Preferably also acidic and alkaline flows from bleaching are conveyed to the effluent flow 20. The effluent flow is preferably led to the waste water treatment plant 40. In an example, due to the amount and quality of the effluents to be conveyed to the waste water treatment plant, for example, due to the concentrated load in lower volume and especially with high COD containing low molecular components, the existing aerobic treatment in the waste water treatment plant is changed. Advantageously, the waste water treatment plant according to the present invention comprises:

- Anaerobic reactor + aerobic treatment, or

- Anaerobic reactor + membrane bioreactor.

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 flows 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.

Claims

Claims:

1 . A method for treating liquid flows at a chemical pulp mill, the system comprising

- a cooking process for producing pulp,

at least one washing stage for washing the pulp,

at least one bleaching stage for bleaching the pulp, a waste water treatment plant (40),

a raw water treatment plant (30), and

- a wood processing unit (50)

wherein the method comprises:

- dividing low chloride content process waters into at least two flows according to their COD content,

- conveying at least a portion of low chloride content process waters to the chemical pulp mill in order to be reused as process water.

2. The method according to claim 1 , characterized in that the method comprises: conveying a low chloride content process water with a higher COD content to additional purification treatment.

3. The method according to claim 2, characterized in that in the low chloride content process water with a higher content, the COD content is over 600 mg/l.

4. The method according to any of claims 1 to 3, characterized in that the low chloride content process waters comprises at least one of the following:

- fiberline process waters from canals,

- sealing waters from a recovery boiler area,

- blow down waters from the recovery boiler area,

- sealing waters from a demineralization plant,

- sealing waters from an evaporation plant,

- rejects from a drying machine,

- tank overflows from the drying machine,

- causticizing jet condensers cooling waters, or

- wood processing process waters.

5. The method according to any of the preceding claims, characterized in that the CI content of the low chloride content process waters is lower than 200 mg/l, preferably lower than 100 mg/l.

6. The method according to any of the preceding claims, characterized in that Ca content of the low chloride content process waters is lower than 200 mg/l, preferably lower than 100 mg/l.

7. The method according to any of the preceding claims, characterized in that the pH of the low chloride content process waters is between 6.5 and

10.

8. The method according to any of the preceding claims, characterized in that at least a portion of the low chloride content process waters is conveyed to be used at the pulp mill in:

a firewater line for washing targets,

a firewater line for sealing targets,

the waste water treatment plant for gravity table showers,

the wood processing unit, or

- jet condensers as causticizing cooling waters.

9. The method according to any of the preceding claims, characterized in that the method comprises

conveying an ash treatment reject flow of the chemical pulp mill to an effluent flow (20) to be treated in the waste water treatment plant (40).

10. The method according to any of the preceding claims, characterized in that the low chloride content process water flow is purified in at least one purifying stage before it is reused as process water by at least one of the following technique:

a chemical flocculation technique,

a clarification technique,

a filtration technique,

- a disinfection technique,

a microfiltration technique,

an oxidation technique, an ultrafiltration technique,

a nanofiltration technique.

1 1 . A system for treating liquid flows at a chemical pulp mill, the system comprising

a digestion apparatus for cooking pulp,

a washing apparatus for washing pulp,

a bleaching apparatus,

a waste water treatment plant (40),

- a raw water treatment plant (30), and

a wood processing unit (50),

wherein the system further comprises

- an apparatus adapted to convey at least a portion of low chloride content process waters to at least one process stage at the chemical pulp mill.

12. A use of low chloride content process waters at a chemical pulp mill comprising

a digestion apparatus for cooking pulp,

- a washing apparatus for washing pulp,

a bleaching apparatus,

a waste water treatment plant (40),

a raw water treatment plant (30), and

a wood processing unit (50)

in at least one of the following:

- a firewater line for washing and/or sealing purposes,

- a gravity table of the waste water treatment plant for shower waters,

- a wood processing unit for washing purposes, or

- jet condensers for causticizing cooling waters.