WO2013135958A2

WO2013135958A2 - A method and a system for treating liquid flows at a chemical pulp mill

Info

Publication number: WO2013135958A2
Application number: PCT/FI2013/050272
Authority: WO
Inventors: Matti Ristolainen; Katja Viitikko; Olli RIMPINEN; Tuija Vartia; Maria Jose AGUERREBERE; Virginia FERNANDEZ; Gáston CUBAS; Alvaro MORAGA; Milton LÓPEZ; David BROGGI; Diego CAVALLO; Juan MAITIA; Sami Saarela; Joaquin QUINTEROS; Richard Alonso
Original assignee: Upm-Kymmene Corporation
Priority date: 2012-03-12
Filing date: 2013-03-12
Publication date: 2013-09-19
Also published as: WO2013135958A3; RU2636560C2; BR112014022417A2; RU2621662C2; CN109811572A; UY34671A; RU2014136221A; RU2014136218A; BR112014022418A2; BR112014022417B1; CN104169492A; FI129114B; CN104160088A; BR112014022418B1; WO2013135959A2; WO2013135959A3

Abstract

The invention relates to a method for treating liquid flows at a chemical pulp mill. The method comprises conveying at least one predetermined water flow to at least one predetermined place at the system. In addition, the invention relates to a system for treating liquid flows. EOP bleaching filtrates from the bleaching unit (30) and/or waters from a second clarifier of the waste water treatment plant (50) are conveyed to wood handling in the wood handling unit (10) and/or to gravity table shower water of the waste water treatment plant (50).

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 compounds that are harmful for the environment. Many motivations exist for decreasing 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, the 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 to 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, 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. Aspects of the invention are characterized by what is stated in the independent claims 1 and 10. Various embodiments of the invention are disclosed in the dependent claims.

The system according to the present invention comprises

at least one wood handling unit,

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

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.

The system may comprise a pulp drying machine. However, it is also possible that the system does not comprise said pulp drying machine.

Advantageously at least a portion of process waters is conveyed to be reused as process water at the pulp mill .

Advantageously the method for treating liquid flows at a chemical pulp mill comprises conveying EOP bleaching filtrates from the bleaching unit to be reused as process water in at least one of the following process stages at the pulp mill: 1 ) wood handling and 2) gravity table shower water of the waste water treatment plant. Alternatively or in addition, the method for treating liquid flows at a chemical pulp mill comprises conveying waters from a second clarifier of the waste water treatment plant to be reused as process water in at least one of the following process stages at the pulp mill : 1 ) wood handling and 2) gravity table shower water of the waste water treatment plant. The method for treating liquid flows at a chemical pulp mill may also use other waters. The method for treating liquid flows at a chemical pulp mill may also comprise conveying Nash pump sealing waters from the bleaching unit to be reused as process water in at least one process stage at the pulp mill. Thus, the method for treating liquid flows at a chemical pulp mill may also comprise conveying white waters from a pulp drying machine to be reused as process water in at least one process stage at the pulp mill. The method for treating liquid flows at a chemical pulp mill may also comprise conveying condensates from the evaporator to be reused as process water in at least one process stage at the pulp mill .

The method for treating liquid flows at a chemical pulp mill may also comprise conveying jet condenser cooling waters from the chemical recovery unit to be reused as process water in at least one process stage at the pulp mill. The method for treating liquid flows at a chemical pulp mill may also comprise conveying vacuum pump sealing waters from the chemical recovery unit to be reused as process water in at least one process stage at the pulp mill. The method for treating liquid flows at a chemical pulp mill may also comprise conveying boiler blow down waters from the recovery boiler to be reused as process water in at least one process stage at the pulp mill .

Preferably one or two of the above mentioned other waters, more preferably three or four, and most preferably five or six of the above mentioned other waters is used in this invention.

EOP bleaching filtrates from the bleaching unit and/or waters from a second clarifier of the waste water treatment plant are led to be used in wood handling. They can be used in log washing of the wood handling unit. Alternatively or in addition, they can be used in chipping of the wood handling unit. Alternatively or in addition, they can be used in chip screening of the wood handling unit. Alternatively or in addition, they can be be used in chip conveyer of the wood handling unit.

At least one of the above mentioned waters can also be led to be used in oxygen delignification washing of the brown stock preparation unit. At least one of the above mentioned waters can also be led to be used in dilution water in the brown stock preparation unit. A least one of the above mentioned waters can also be led to be used in Drum Displacement washer (DD-washer) plates of the bleaching unit. At least one of the above mentioned waters can also be led to be used in washing step(s) of the bleaching unit. At least one of the above mentioned waters can also led to be used in dilution water of the bleaching unit. At least one of the above mentioned waters can also be led to be used in drying machine shower water of a pulp drying machine. At least one of the above mentioned waters can also be led to be used in gravity table shower water of the waste water treatment plant. At least one of the above mentioned waters can also be led to be used in weak liquor tank of the chemical recovery unit. At least one of the above mentioned waters can also be led to be used in white liquor condensate tank of the chemical recovery unit. However, "oxygen delignification washing of the brown stock preparation unit" and "dilution water in the brown stock preparation unit" are preferably excluded from the above as they may not be as advantageous as the others.

The method advantageously comprises conveying at least a portion of the process waters coming from the Nash pump sealing waters to one, two or three of the following stages:

to at least one brown stock preparation stage,

to at least one bleaching stage, and

to at least one chemical recovery stage.

The method advantageously comprises conveying at least a portion of the process waters coming from the causticizing jet condenser cooling waters and/or boiler blow down waters to one, two, three, four or five of the following:

the wood handling,

the brown stock preparation,

the bleaching, and/or

the waste water treatment plant for gravity table shower waters, and/or the chemical recovery.

In an example, the "waste water treatment plant for gravity table shower waters" is excluded from the above list.

The method advantageously comprises conveying at least a portion of lime mud filter vacuum pump waters and/or jet condenser cooling waters to a weak white liquor tank of the chemical recovery. This may Increase liquor inventory and use of more secondary condensate at post O2 washing. The method advantageously comprises conveying at least a portion of boiler blow down waters to a white liquor condensate tank of the chemical recovery. This may increase liquor inventory and use more secondary condensate at post O2 washing. The method advantageously comprises conveying at least a portion of jet condenser cooling waters and/or dregs filter vacuum pump waters to a washing step of a sludge treatment area.

Advantageously the system for treating liquid flows at a chemical pulp mill comprises:

a wood handling unit, brown stock preparation unit comprising cooking (including a digester), washing, screening and oxygen delignification stages for producing pulp,

a bleaching unit,

- a chemical recovery unit, and

an evaporator.

The system comprises a conveying apparatus for conveying at least a portion of the process waters coming from the bleaching unit and adapted to convey EOP bleaching filtrates to wood handling and/or to gravity table shower of the waste water treatment plant. The system also comprises, additionally or alternatively, a conveying apparatus adapted to convey waters from a second clarifier of the waste water treatment plant to wood handling and/or to gravity table shower of the waste water treatment plant.

The water conveyed to the wood handling unit is conveyed to at least one of the following:

log washing of the wood handling unit,

chipping of the wood handling unit,

- chip screening of the wood handling unit,

chip conveyer of the wood handling unit,

In addition, the system advantageously comprises a conveying apparatus for conveying at least a portion of the process waters coming from the evaporator to the wood handling unit, and/or to the brown stock preparation unit, and/or to the bleaching unit, and/or to a drying machine of the pulp, and/or to the chemical recovery unit.

In addition, the system advantageously comprises a conveying apparatus for conveying at least a portion of the process waters coming from the recovery boiler to at least one chemical recovery stage of the chemical recovery unit and/or to the pulp drying machine.

In addition, the system advantageously comprises a conveying apparatus for conveying at least a portion of the causticizing jet condenser cooling waters and/or boiler blow down waters to the wood handling unit, and/or to the brown stock preparation unit, and/or to the bleaching unit, and/or to a gravity table shower waters of the waste water treatment plant, and/or to the chemical recovery stage of the chemical recovery unit.

In addition, the system advantageously comprises a conveying apparatus for conveying at least a portion of the process waters coming from the bleaching unit as Nash pump sealing waters to the brown stock preparation unit, and/or to the bleaching unit, and/or to the chemical recovery unit.

The conveying apparatuses preferably comprise 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. In other words, a portion of the process waters is circulated between different process stages and/or within a process stage, most advantageously without any treatment in the waste water treatment plant. Therefore, thanks to the present invention, it is possible to significantly decrease the amount of the generated waste water. EOP filtrate conveyed to the wood handling can be used for log and chip washing and to wet chips for conveying (prevention of dusting). The alkaline filtrate from the EOP bleaching stage neutralizes organic acids coming from the wood that is handled, especially in case of eucalyptus, and it thus prevents acid corrosion of conveyors. Further, waters from a second clarifier of the waste water treatment plant have a purity that is suitable for wood handling. When waters from a second clarifier of the waste water treatment plant are used to gravity table shower water in the same plant, short piping can be used. Thus, thanks to the present invention, a system able to

decrease the environmental load of the chemical pulp mill, and decrease the amount of raw water used by the chemical pulp mill can be provided. Moreover, also energy and chemicals may be saved, which may lead to significant reduction in the operational costs. 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.

Description of the Drawings

In the following, the invention will be described in more detail with reference to the appended drawing, in which

Figs 1 to 2 show example embodiments in reduced schematic chart.

Detailed Description of the Invention

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

10 wood handling,

20 brown stock preparation, comprising digester, brown stock washing, oxygen delignification and washing stages therein,

30 bleaching, comprising bleaching stages and washing therein,

40 evaporator,

50 waste water treatment plant, comprising a second clarifier,

60 chemical recovery, comprising melt dissolving, green liquor filtration, causticizing and lime cycle stages, and

70 recovery boiler, and

80 drying machine,

The term "raw water treatment plant" refers to fresh water treatment plant The term "effluent treatment plant" refers to waste water treatment plant. 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 substances originated 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.

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.

Drum displacement washer is a rotating drum where a pulp mat formed on the outer filter face of the drum from a pulp surry is washed to displace the liquid in the mat to the inside of the drum.

Boiler blow down waters are waters blown out of the boiler to avoid concentration of impurities during continuing evaporation of steam.

White liquor condensate tank of the chemical recovery unit is a tank in the white liquor plant containing the secondary condensate from the evaporator. Process water conveyed to this point can be used together with the condensate for washing operations in the causticizing plant.

Weak liquor tank is a weak white liquor tank in the white liquor plant. Secondary condensate from the evaporator is brought to this tank for dilution.

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

The term "EOP" refers to alkaline extraction stage in pulp bleaching using oxygen and peroxide as additional chemicals. 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.

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 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 some examples of the process water circulations in reduced schematic chart. These can be implemented, for example, at the pulp mill according to Figure 1 . At least a portion of the process waters is conveyed to wood handling 10. The process water is preferably used for log washing. Alternatively or in addition the process water is preferably used for chipping. Alternatively or in addition, the process water is preferably used for chip screening. Alternatively or in addition, the process water is preferably used for a chip conveyer. The process waters conveyed to the wood handling comprises EOP bleaching filtrates (bold line 1 in Fig. 2).

Advantageously, in addition, at least a part of the process waters conveyed to the wood handling 10 comprises secondary condensate waters of the evaporator 40. Alternatively or in addition, at least a part of the process waters conveyed to the wood handling 10 comprises Nash pump sealing waters of the bleaching 30. Alternatively or in addition, at least a part of the process waters conveyed to the wood handling 10 comprises the causticizing jet condenser cooling waters. Alternatively or in addition, at least a part of the process waters conveyed to the wood handling 10 comprises boiler blow down waters.

A portion of the effluent to be conveyed to the wood handling 10 comprises second clarifier filtrate of the waste water treatment plant (bold line 2 in Fig. 2).

Advantageously at least a portion of the process waters is conveyed to brown stock preparation unit 20. Preferably the process water is used for oxygen delignification washing and/or dilution liquids. Alternatively or in addition, the process water is preferably used for washing liquid of the Drum Displacement (DD) washer plate.

The process water circulated to the brown stock preparation unit 20 preferably comprises the causticizing jet condenser cooling waters and/or the boiler blow down waters. Alternatively or in addition, the process water circulated to the brown stock preparation unit 20 preferably comprises the bleaching Nash pump sealing waters and/or the condensate waters of the evaporator. Advantageously at least a portion of the process waters is circulated to at least one bleaching stage 30, and is preferably used for one, two or three of the following:

- washing liquid of the DD-washer plates (Drum Displacement washer), - washing liquid of a press washing device, and

- washing stages and dilution of liquids therein.

The process water circulated to at least one bleaching stage 30 and washing steps therein preferably comprises one, two, three, or four of the following:

- secondary condensate waters of the evaporator, and/or

- causticizing jet condenser cooling waters, and/or

- bleaching Nash pump sealing waters, and/or

- boiler blow down waters. Advantageously at least a portion of the process waters is conveyed to a drying machine 80, and is preferably used as shower waters therein. The process water circulated to the drying machine 80 preferably comprises one two, three or four of the following:

- secondary condensate waters of the evaporator,

- boiler blow down waters,

- bleaching Nash pump sealing waters, and

- causticizing jet condenser cooling waters.

Advantageously at least a portion of the process waters is conveyed to a waste water treatment plant 50, wherein the process water is preferably used for the shower waters of the gravity table. The water used for the shower waters is waste waters after the second clarifier (bold line 3 in Fig. 2). The process water used as gravity table shower waters of the waste water treatment plant may also comprise one, two, three or four of the following:

- Nash pump sealing waters of the bleaching unit 30,

- jet condenser cooling waters from causticizing,

- boiler blow down waters, and

- evaporation secondary condensates. Advantageously at least a portion of the process waters is circulated to the chemical recovery unit 60, and is preferably used in a weak liquor tank and/or a white liquor condensate tank therein. The process water used in said chemical recovery 60 preferably comprises secondary condensate waters of the evaporator and/or jet condenser cooling waters and/or boiler blow down waters and/or bleaching Nash pump sealing waters. Advantageously at least a portion of the process waters to be circulated at the chemical pulp mill comprises one, two, three, four or five of the following water sources:

- the bleaching stage(s) 30, comprising preferably EOP bleaching filtrates and/or Nash pump sealing waters,

- the drying machine, comprising preferably white waters,

- the evaporator 40, comprising preferably condensates,

- the recovery boiler 70, comprising preferably boiler blow down waters, and

- the chemical recovery, comprising preferably jet condenser cooling waters and/or vacuum pump sealing waters.

Advantageously at least a portion of the process waters is circulated within the bleaching stage(s) 30. Alternatively or in addition, advantageously at least a portion of the process waters is circulated within the chemical recovery stage(s) 60. Thus, the total amount of the generated effluents and the total amount of the needed raw water are decreased therein. Alternatively or in addition, in an example at least a portion of the process waters is circulated within waste water treatment stage(s). However, the latter may not be an as advantageous embodiment as the others.

EOP bleaching filtrates from the bleaching unit 30 used to gravity table shower water of the waste water treatment plant 50 is shown by bold line 4 in Fig. 2. Due to the circulation of at least a portion of the process waters, the total amount of effluents to be purified in the wastewater treatment plant can be decreased. In addition, the total amount of needed raw water (also called fresh water) is typically decreased. Advantageously, at least a part of water coming from the Nash pumps is used as wash water of the screen plate of a DD washer; thus, the total amount of the generated effluents and the amount of the needed raw water is decreased therein.

There may be a filtration device within the process water circulation. In this case, the filtration device is preferably used for the gravity table shower waters. Typically, any other purifying device is not needed. Therefore, if said gravity table shower waters are not reused water, the process waters that are circulated according to the present invention do not typically need any purifying step before they are reused as process waters.

Waters of different purity levels can also be combined so that they match the purity level required at the point of use.

Tables 1 to 3 show some examples of wastewater reduction according to the present invention. "Realized Wastewater [m³/t] reduction" is the situation in an example pulp mill having in the beginning an amount of wastewater of approximately 22.5 m³/t.

Table 2 Wastewater reduction in chemical recovery and raw water treatment plant. CHEMICAL RECOVERY AND RAW WATER TREATMENT PLANT

Realized

Water reduction action Wastewater [m³/t]

reduction

Jet condenser cooling water and/or lime mud filter

vacuum pump water is led to a weak white liquor 1 .4

tank. This can increase liquor inventory and use of

more secondary condensate at post O₂ washing.

Jet condenser cooling water and/or dregs filter

vacuum pump water are led to washing step at a 0.7

sludge treatment area.

Recovery boiler continuous blow down waters are

led to white liquor plant condensate tank. Increase 0.5

liquor inventory and use more secondary

condensate at post 02 washing.

Table 3 Wastewater reduction in wood handling

WOOD HANDLING

Realized

Water reduction proposal Wastewater [m³/t]

reduction

EOP filtrates are led to log washing. Also second

1

clarifier effluent can be utilized therein.

Use re-circulated water (e.g. wastewater, EOP or

condensate, low chloride content process waters) 0.2

for washing in screening, chipping and tunnels.

One skilled in the art understands readily 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.

Claims

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

- a wood handling unit (10),

a brown stock preparation unit (20) comprising cooking, washing, screening and oxygen delignification stages for producing pulp, a bleaching unit (30) to bleach the produced pulp,

a chemical recovery unit (60),

- a pulp drying machine (80),

a recovery boiler (70) generating boiler blow down water,

an evaporator (40) generating condensates,

a waste water treatment plant (50), and

a raw water treatment plant,

wherein the method comprises conveying at least one of the following water sources:

EOP bleaching filtrates from the bleaching unit (30),

waters from a second clarifier of the waste water treatment plant (50), to at least one of the following:

- wood handling in the wood handling unit (10),

gravity table shower water of the waste water treatment plant (50).

2. The method according to claim 1 , characterized by conveying EOP bleaching filtrates from the bleaching unit (30) and/or gravity table shower water of the waste water treatment plant (50) to the wood handling unit to at least one of the following:

- log washing of the wood handling unit (10),

- chipping of the wood handling unit (10),

- chip screening of the wood handling unit (10),

- chip conveyer of the wood handling unit (10).

3. The method according to claim 1 or 2, characterized by conveying at least a portion of the process waters to the pulp drying machine (80) and using said portion as shower water of the drying machine.

4. The method according to any of the preceding claims, characterized by conveying at least a portion of Nash pump sealing waters coming from the bleaching unit to an alkaline extraction stage in which oxygen and peroxide are used as additional chemicals.

5. The method according to any of the preceding claims, characterized by conveying at least a portion of Nash pump sealing waters coming from the bleaching unit to a screen plate of a Drum Displacement washer in the brown stock preparation (20).

6. The method according to any of the preceding claims, characterized in that the chemical recovery unit comprises a weak white liquor tank and a lime mud filter vacuum pump, and the method comprises

conveying at least a portion of the lime mud filter vacuum pump waters and/or the jet condenser cooling waters to the weak white liquor tank.

7. The method according to any of the preceding claims, characterized in that the chemical recovery unit comprises a white liquor condensate tank, and the method comprises

conveying at least a portion of the boiler blow down waters to the white liquor condensate tank.

8. The method according to any of the preceding claims, characterized in that the system comprises a dregs filter vacuum pump and a sludge treatment area of the waste water treatment plant, and the method comprises - conveying at least a portion of the jet condenser cooling waters and/or the dregs filter vacuum pump waters to a washing step at the sludge treatment area.

9. The method according to any of the preceding claims, characterized in that at least a portion of the process waters is circulated within the bleaching unit.

10. The method according to any of the preceding claims, characterized in that at least a portion of the process waters is circulated within the chemical recovery unit (60).

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

a wood handling unit (10),

a chemical recovery unit (60),

a recovery boiler (70),

an evaporator (40),

- a pulp drying machine (80),

a waste water treatment plant (50), and

a raw water treatment plant,

wherein the system further comprises at least one of the following

an apparatus adapted to convey EOP bleaching filtrates from the bleaching unit (30),

an apparatus adapted to convey waters from a second clarifier of the waste water treatment plant (50),

to at least one of the following

wood handling unit (10)

- gravity table shower water of the waste water treatment plant (50).

12. The system according to claim 1 1 , characterized in that the apparatus adapted to convey EOP bleaching filtrates from the bleaching unit (30) and/or the apparatus adapted to convey waters from a second clarifier of the waste water treatment plant (50) is connected to the wood handling unit (10) to at least one of the following:

log washing of the wood handling unit (10),

chipping of the wood handling unit (10),

chip screening of the wood handling unit (10),

chip conveyer of the wood handling unit (10).