WO2013135957A2

WO2013135957A2 - A method and a system for treating liquid flows at a chemical pulp mill

Info

Publication number: WO2013135957A2
Application number: PCT/FI2013/050270
Authority: WO
Inventors: Sami Saarela; Matti Ristolainen; Olli RIMPINEN; Katja Viitikko; Tuija Vartia; Maria Jose AGUERREBERE; Virginia FERNANDEZ; Esa Hassinen
Original assignee: Upm-Kymmene Corporation
Priority date: 2012-03-12
Filing date: 2013-03-12
Publication date: 2013-09-19
Also published as: RU2014136220A; WO2013135957A3; BR112014022539B1; CN104169494B; FI20125262A; RU2635040C2; FI126563B; UY34672A; CN104169494A

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 said pulp, at least one bleaching stage for bleaching the washed pulp, a waste water treatment plant comprising at least one purifying stage to purify effluents, and a raw water treatment plant comprising at least one purifying stage, and the method comprises conveying a liquid flow comprising a portion of purified effluents (45) coming from the waste water treatment plant (40) and/or a portion of process waters (35) to said at least one purifying stage of the raw water treatment plant (20) for purifying the liquid flow in said at least one purifying unit of the raw water treatment plant (20) to obtain recyclable water (29), and using said recyclable water as process water in at least one process and/or process stage at the pulp mill. In addition, the invention relates a system for treating liquid flows at a chemical pulp mill and a use of a raw water treatment plant of a chemical pulp mill for purifying process waters and/or effluents coming from the waste water treatment plant.

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. In the solution relating to a chemical pulp production process, at least a portion of process waters and/or effluents are conveyed to a raw water treatment plant. After at least one purification stage in the raw water treatment plant, at least a portion of the treated waters is returned to the pulp production process and reused as process water. In this application, said water that is purified in at least one purification stage of the raw water treatment plant and reused as process water is called recyclable water. The solution is advantageously implemented in such a way that substances disturbing the process are not accumulated in the process.

Aspects of the invention are characterized by what is stated in the independent claims 1 , 7 and 8. Various embodiments of the invention are disclosed in the dependent claims.

Advantageously, the chemical pulp mill according to the present invention comprises:

a cooking process for producing pulp,

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

a waste water treatment plant in order to purify effluents, and a raw water treatment plant comprising at least one purifying unit in order to purify water flows.

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 may comprise 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.

The method according to the present invention advantageously comprises the following steps:

conveying a liquid flow comprising

a portion of effluents coming from the waste water treatment plant and/or a portion of process waters

to said at least one purifying unit of the raw water treatment plant in order to purify the liquid flow in said at least one purifying unit of the raw water treatment plant to obtain recyclable water, and

- using said recyclable water as process water in at least one process at the pulp mill.

The system according to the present invention advantageously comprises:

- a first conveyor line that is placed between the waste water treatment plant and the raw water treatment plant and/or between the pulping system and the raw water treatment plant, and

- a second conveyor line that is placed between the raw water treatment plant and the pulping system.

The first and/or the second conveyor line preferably comprises pump(s) and pipe(s).

At least one purification step in the raw water treatment plant preferably comprises at least one filtration treatment. In this case, preferably at least one sand filter or at least one multimedia filter is used.

In an advantageous embodiment of the invention, the purifying treatments are carried out by means of existing process stages. The present invention can be implemented at a chemical pulp mill having a digestion process, at least one bleaching stage, chemical recovery, a waste water treatment plant, a raw water treatment plant, 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 and, in addition, the amount of raw water used by the chemical pulp mill, can be provided.

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 5 show some example embodiments in reduced schematic charts.

Detailed Description of the Invention

In this application, reference is made to Figs. 1 to 5, in which the following reference numerals are used:

10 water system,

1 1 pre-treatment unit,

15 raw water from the water system,

20 raw water treatment plant,

21 raw water intake and cascade,

22 raw water clarifier,

22a first raw water clarifier,

22b second raw water clarifier,

22c third raw water clarifier,

23 sand filter or multimedia filter,

23a first sand filter or multimedia filter,

23b second sand filter or multimedia filter,

23c third sand filter or multimedia filter,

23d fourth sand filter or multimedia filter,

23e fifth sand filter or multimedia filter,

24 secondary purification unit,

29 recyclable water,

30 pulping system,

35 process water from the pulping system.

40 waste water treatment plant,

41 anaerobic reactor,

42 sludge treatment, and

45 purified effluent from the waste water treatment plant

In this application, the term "water system" refers to a raw water source (also called a fresh water source), such as a sea, a river, a lake or another water source. 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 "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 "Adt" refers to air dry metric ton of 90% dry chemical pulp. 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 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. Advantageously, the bleaching effluents do not contain fibers.

The term "purified effluents" refers to effluents that are purified in the waste water treatment plant.

The term "recyclable water" refers to process waters and/or purified effluents that are purified in the raw water treatment plant and are clean enough to be reused as process waters.

The term "EOP" refers to alkaline extraction stage in pulp bleaching using oxygen and peroxide as additional chemicals. 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 (wire) in wastewater treatment. Gravity table shower waters are used for washing the belt.

The term "low chloride content process waters" refers to process waters having a low chloride content. Advantageously, the chloride content of the low chloride content process waters is less than 200 mg/l, preferably less 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. In this application, if process waters are supplied to the raw water treatment plant, they are advantageously so called low chloride content process waters.

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. Figures 1 to 5 show some examples of the process water circulation in reduced schematic charts. The figures show a water system 10, a possible pre-treatment unit 1 1 placed before the raw water treatment plant 20, a possible secondary purification unit 24 placed after the raw water treatment plant, a pulping system 30, the raw water treatment plant 20 preferably comprising raw water intake and cascade 21 , raw water clarifiers 22, 22a-22c and sand and/or multimedia filters 23, 23a-23e, a waste water treatment plant 40 comprising sludge treatment 42 and a possible anaerobic reactor 41 .

The raw water treatment plant 20 preferably comprises at least a raw water intake and cascade 21 , raw water clarifier(s) 22 (22a-22c), sand and/or multimedia filter(s) 23 (23a-23e). There may be a pre-treatment unit 1 1 before the raw water treatment plant 20 and/or a secondary purification unit 24 after the raw water treatment plant 20.

The pulping system 30 advantageously comprises at least a digestion process for producing pulp, at least one washing stage in order to wash said pulp, and a bleaching unit comprising at least one bleaching stage as shown in Figure 1 .

The purified effluent flow 45 and/or the process water flow 35 are preferably directly combined with the raw water flow 15 if the quality of the purified effluent flow 45 and/or the quality of the process water flow 35 is sufficiently close to the quality of the raw water flow 15. On the other hand, an additional purification step is needed for low quality purified effluents and process waters. If good quality raw water is contaminated with low quality water flow, strong purification treatment(s) is needed for the whole water flow instead of purification of the smaller low quality water flow only. Thus, if the purified effluent 45 and/or the process water 35 is of low quality, it is preferably pre- treated in at least one purifying step in the pre-treatment unit 1 1 and/or in the raw water treatment plant 20 and/or in the secondary purification unit 24 before the purified effluent 45 flow and/or the process water 35 flow and the raw water 15 flow are combined with each other. The quality differences include, among other things, COD and conductivity levels of the waters.

Typically, the order from high quality process waters to the low quality process waters is as follows:

- Boiler water (highest quality), - Seal and fire water,

- Drying machine,

- Fibre line/bleaching,

- Fiberline/brownstock, and

- Log washing (lowest quality).

Thus, process waters comprising log washing waters typically need stronger purification than process waters comprising boiler waters.

The place or process step in which the recyclable water 29 is reused may have an effect on the purification steps needed. Where water quality requirements are lower, only light purification of the purified effluents 45 and/or process waters 35 may be needed before the re-use of those waters, and when water quality requirements are higher, strong purification of the purified effluents and/or process waters may be needed before the re-use of those waters.

According to the present invention, at least a portion of the purified effluents 45 and/or process waters 35 is returned after the purification step(s) in the raw water treatment plant 20 as a source of process water. Advantageously, said water that is called recyclable water is conveyed to be reused, at least partly, as cooling water. Alternatively or in addition, the recyclable water 29 is advantageously conveyed to be reused, at least partly, for debarking. Alternatively or in addition, the recyclable water 29 is advantageously conveyed to be reused, at least partly, for log washing. Alternatively or in addition, the recyclable water 29 is advantageously conveyed to be reused, at least partly, as fire water. Alternatively or in addition, the recyclable water 29 is advantageously conveyed to be reused, at least partly, as boiler water. Alternatively or in addition, the recyclable water 29 is advantageously conveyed to be reused, at least partly, as shower waters of the pulp drying machine. Alternatively or in addition, the recyclable water 29 is advantageously conveyed to be reused, at least partly, for bleaching. Alternatively or in addition, the recyclable water 29 is advantageously conveyed to be reused, at least partly, for chemical recovery. Alternatively or in addition, the recyclable water 29 is advantageously conveyed to be reused, at least partly, as sealing waters. Alternatively or in addition, the recyclable water 29 is advantageously conveyed to be reused, at least partly, as shower waters for a wire in a gravity table.

Advantageously, a portion of the effluents is discharged from the mill, the portion being preferably 4 to 15 m³/ADt, more preferably 4 to 8 m³/ADt. In an example, the total amount of the purified effluent conveyed to the raw water treatment plant is between 2 and 15 m³/ADt, more preferably 6 m³/ADt at the most. Advantageously, the process water and/or the purified effluent to be conveyed to the raw water treatment plant 20 is supplied to a clarifier 22 of the raw water treatment plant 20, or to a sand or multimedia filter 23 of the raw water treatment plant 20. The purified effluent 45 and/or the process water 35 may comprise, among other things, bacteria, chlorides, inorganic compounds, high molecular mass lignin residues and/or low molecular mass compounds. Thus, there is advantageously at least one pre-treatment unit 1 1 in order to decrease the amount of impurities before the raw water treatment plant 20. In the case of the pretreatment unit 1 1 , the purified effluent 45 and/or the process water 35 to be conveyed to the raw water treatment plant 20 is preferably conveyed, after the pretreatment unit 1 1 , to the clarifier 22 of the raw water treatment plant 20 or to a sand or multimedia filter 23 of the raw water treatment plant 20.

The pre-treatment step implemented in the pretreatment unit 1 1 preferably comprises a chemical treatment, such as a precipitation stage to remove organic and inorganic substances. Alternatively or in addition, the pretreatment step implemented in the pretreatment unit 1 1 preferably comprises a biological treatment stage. Alternatively or in addition, the pre-treatment step implemented in the pretreatment unit 1 1 preferably comprises a mechanical treatment stage, such as a coarse filtration, flotation or clarification to remove separated particles. Alternatively or in addition, the pre-treatment step implemented in the pretreatment unit 1 1 preferably comprises a disinfection stage of the water flow. Most advantageously, the pretreatment unit 1 1 comprises one, two or three of the above mentioned stages. Advantageously there is at least one pre-treatment unit 1 1 in a system comprising a pond with plants in order to decrease the total amount of metals in the system, wherein the plants absorb metals. At least one pre-treatment stage can be implemented, for example, by using an oxidizing agent, such as peracetic acid, hydrogen peroxide, and/or ozone. Alternatively or in addition, at least one pre-treatment stage can be implemented, for example, by using an arrangement which can be used for disinfection, such as by plasma-corona treatment.

Most advantageously, the process water 35 and/or the purified effluent 45 is treated at least in the sand or multimedia filter 23 of the raw water treatment plant 20. The sand or multimedia filter 23 should be washed at times in order to maintain the efficiency of said filter. If the water to be purified in the raw water treatment plant 20 is dirty, the sand or multimedia filter 23 should be washed more often, which increases the total cost of said treatment.

Therefore, at least one pre-treatment unit 1 1 is preferably implemented using chemical(s) in order to precipitate organic compound and/or inorganic compound from said purified effluent flow 45 and/or the process water flow 35. The term "flocculation of organic compound" refers to decreasing COD and/or color of the effluent using chemical precipitation method(s). COD can be too high due to, for example, lignin fragments. Thus, the pre-treatment unit 1 1 preferably comprises at least one flocculation stage. The flocculation process may be important in order to keep the sand and/or multimedia filter 23 of the raw water treatment plant 20 following the at least one pre-treatment unit 1 1 as clean as possible. Without the flocculation process or other pre-treatment stages in the pretreatment unit 1 1 , the sand and/or multimedia filter 23 may become quickly too dirty due to, for example, possible organic impurities in said water flows. Chemicals used in the flocculation stage can vary due to differences in waters to be cleaned.

Flocculants 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. Advantageously, 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(amido- amine) (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 in the pre-treatment stage 1 1 .

The 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 and their polymeric forms, such as polyaluminiumnitrate (PAN), polyaluminiumchloride (PAC), and/or polyaluminiumsulfate (PAS).

Alternatively or in addition, the coagulants comprise ferrous compounds, such as FeC , and/or FeSO₄. Alternatively or in addition, the coagulants comprise ferric compounds, such as FeCh, and/or Fe2(SO₄)3. The adsorbents may comprise, for example, clay, bentonite and/or talc. Advantageously, the raw water treatment plant 20 comprises at least two, more preferably at least three, and most preferably at least four separate treatment lines. The process water flow 35 and/or the purified effluent flow 45 to be treated in the raw water treatment plant 20 can be conveyed to at least one treatment line of the raw water treatment plant. Advantageously, one or two of said raw water treatment plant lines are used to clean the process waters 35 and/or the purified effluents 45. Advantageously, said at least one treatment line used to purify process waters 35 and/or the purified effluents 45 in the raw water treatment plant 20 is separated from the raw water sources, i.e. the treatment line(s) is preferably used only to purify process waters 35 and/or the purified effluents 45.

The cleaning process of the purified effluent 45 and/or the process water 35 in the raw water treatment plant 20 may be used in order to decrease the total amount of inorganic and/or organic impurities present in the effluent. This can be measured by using, for example, generally known methods to measure COD, TOC, BOD, AOX and/or percentage of solids.

The content of inorganic and/or organic impurities in a sample of the purified effluent from the second clarifier of waste water treatment plant, i.e. so called 2^nd clarifier effluent, to be conveyed to the raw water treatment plant 20 is disclosed in Table 1 .

Table 1 . Second clarifier effluent, example

Treated

Description effluent

Aluminium, Al m g/l <0,005

Barium, Ba m g/l <0,05

Calcium, Ca m g/l 108

Chromium, Cr m g/l <0,002

Copper, Cu m g/l <0,002

Iron, Fe m g/l 0,097

Potassium, K m g/l 77,7

Magnesium, Mg m g/l 12

Manganese, Mn m g/l 0,324

Sodium, Na m g/l 857

Phosphorus, P m g/l 0,385

Sulphur, S m g/l 425

Silicon, Si m g/l 10,5

Chloride m g/l 243

Sulphate m g/l 1249

Total oxalate m g/l 0,4

Diss, oxalate m g/l 0,0

Phosphate m g/l <0,3

Carbonate % 0,1

Total organic carbon, TOC m gC/l 96

Total inorganic carbon, TIC m gC/l 122

After the purifying step(s) in the raw water treatment plant 20, there may still be compounds which need to be removed from the water comprising purified effluents 45 and/or process waters 35 before the waters can be called recyclable water 29. Therefore, after the treatment in the raw water treatment plant 20, the water flow comprising the process water 35 and/or the purified effluent 45 is preferably either conveyed to be used as process water, or cleaned further in a secondary purifying step by a secondary purifying unit 24. The secondary purifying step 24 advantageously comprises disinfection treatment. Alternatively or in addition, the secondary purifying step 24 preferably comprises ion exchange treatment. Alternatively or in addition, the secondary purifying step 24 preferably comprises electrodialysis treatment. Alternatively or in addition, the secondary purifying step 24 preferably comprises osmosis treatment. Alternatively or in addition, the secondary purifying step 24 preferably comprises dialysis technique. Alternatively or in addition, the secondary purifying step 24 preferably comprises reverse osmosis treatment. Alternatively or in addition, the secondary purifying step 24 preferably comprises chemical precipitation. Alternatively or in addition, the secondary purifying step preferably 24 comprises nano filtration technique.

Advantageously, the secondary purifying steps comprise one, two, three, four or five of the above mentioned steps. The process may be more cost effective with a smaller number of steps, but the recyclable water 29 may be cleaner with a greater number of steps.

Advantageously, a portion of the effluents is purified first in the second clarifier of the waste water treatment plant 40 and after that in at least one purifying unit of the raw water treatment plant 20. The waste water treatment plant 40 typically comprises a second clarifier, from which the effluent is typically conveyed to a water system 10. In an advantageous embodiment of the present invention, at least a portion of the effluent purified in the second clarifier of the waste water treatment plant is conveyed to the raw water treatment plant 20. Advantageously, at least a part of the effluent purified in the second clarifier of the waste water treatment plant 40 is conveyed to the clarifier 22 of the raw water treatment plant 20. Preferably at least 50 %, more preferably at least 75 %, and most preferably at least 90 % of filtrate of the second clarifier of the waste water treatment plant 40 is purified in at least one purifying unit of the raw water treatment plant 20 before the water is reused as process water at the chemical pulp mill . Alternately or in addition, the effluent to be conveyed to the raw water treatment plant 20 may comprise, for example, bleaching filtrates. Some process waters, such as sealing waters or rain waters, are typically very clean. Therefore, it is unnecessary to mix these flows with effluents from, for example, the bleaching or the debarking stages. Only when these waters are contaminated due to, for example, apparatus breakages etc., they should typically be collected and conveyed to waste water treatment plant 40. Therefore, advantageously a portion of process waters 35 is conveyed to the raw water treatment plant 20 without treatments in the wastewater treatment plant 40. Advantageously, process waters 35 comprising one, two, three or four of the following sources, i.e.

- sealing waters, for example, from recovery boiler area, Nash pumps and/or from demineralization plant,

- waters from a drying machine,

- rain waters, and/or

- fiberline waters from canals,

are at least partly recovered and conveyed to the raw water treatment plant 20 as such or after pre-cleaning in at least one stage of the pre-treatment unit 1 1 , i.e. without purifying steps in a waste water treatment plant. In an example, the amount of said process waters conveyed to the raw water treatment plant 20 is between 2 and 5 m³/t.

The purified effluent flow 45 and/or the process water flow 35 purified in the raw water treatment plant 20, i.e. recyclable water, is advantageously at least partly used in a process at the pulp mill . Preferably, the recyclable water is used in fiber line. Alternatively or in addition, the recyclable water is advan- tageously used in chemical recovery. Alternatively or in addition, the recyclable water is advantageously used as cooling water. Alternatively or in addition, the recyclable water is advantageously used as boiler water. Alternatively or in addition, the recyclable water is advantageously used in log washing. Alternatively or in addition, the recyclable water is advan- tageously used as fire water. Alternatively or in addition, the recyclable water is advantageously used as sealing water. Alternatively or in addition, the recyclable water is advantageously used as raw water for a pulp drying machine. Alternatively or in addition, the recyclable water is advantageously used as shower waters for a wire in a gravity table. In an example, the purified effluent 45 and/or the process water 35 is conveyed as recyclable water 29 to be reused as process water without being combined with the raw water. 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 advantageous embodiment of the present invention, wherein a portion of the purified effluents coming from the waste water treatment plant 40 is conveyed to the raw water treatment plant 20 and reused as process water for pulping device(s) 30.

The raw water treatment plant 20 preferably comprises at least one sand filter and/or multimedia filter. In addition, the raw water treatment plant preferably comprises at least one clarifier.

Advantageously, at least a portion of biologically and/or chemically and/or mechanically purified effluents from the waste water treatment plant 40 is further purified in at least one purifying unit of the raw water treatment plant 20. The effluent coming from the waste water treatment plant 40 may be cleaned in treatment lines different from the raw water coming from the water system 10, i.e. the purified effluent is further purified in a treatment line separate from the raw water in the raw water treatment plant 20. Alternatively, the purified effluent coming from the waste water treatment plant 40 may be purified together with the raw water coming from the water system 10 in at least one of the purifying stages of the raw water treatment plant 20.

Alternatively or in addition, advantageously at least a portion of the process waters coming from the pulping system 30 is purified in at least one purifying stage of the raw water treatment plant 20. This portion of the process waters may be cleaned in a purifying treatment line different from the raw water coming from the water system 10, i.e. the process water coming from the pulping system 30 is purified in a treatment line separate from the raw water coming from the water system 10. Alternatively, the process water may be purified together with the raw water in at least one of the purifying stages of the raw water treatment plant 20.

Figure 3 shows an example, in which a portion of the effluents 45 purified in the wastewater treatment plant 40 and/or a portion of the process waters 35 coming from the pulping system 30 is conveyed to a first clarifier 22a of the raw water treatment plant 20, after which the water flow is preferably conveyed to a sand filter or a multimedia filter 23. According to this example, the purified effluent 45 and/or the process water 35 are purified in treatment lines separate from the raw water 15 coming from the water system 10. In other words, different clarifier(s) 22 and/or sand filter(s) 23 are used for the purified effluent flow 45 and/orthe process water flow 35 and for the raw water 15. In this example, after the purifying step(s) in the raw water treatment plant 20, the water flows are preferably combined with each other. Advantageously, the system comprises at least one pre-treatment unit 1 1 (shown in Figures 4 and 5) before the raw water treatment plant 20, preferably between the wastewater treatment plant 40 and the raw water treatment plant 20. Alternatively or in addition, a secondary purification unit 24 is placed after the raw water treatment plant 20 but before the purified water is used as process water in the pulping system 30. Figure 4 shows an example, in which a portion of the purified effluents 45 that is purified in the wastewater treatment plant 40 and/or process waters 35 coming from the pulping system 30 is conveyed to a sand or multimedia filter 23 of the raw water treatment plant 20. According to this example, the purified effluent 45 and/or the process water 35 is purified in a separate treatment line as the raw water 15, i.e. they use separate purifying apparatuses. Advantageously, the purified effluent 45 and/or the process water 35 is purified using a sand or multimedia filter 23 separate from the raw water 15, after which purification step in said filter the water flows are combined with each other.

The system according to Figure 4 advantageously comprises a pre-treatment unit 1 1 comprising at least one process stage, which pre-treatment unit 1 1 is placed before the raw water treatment plant 20. Preferably the pre-treatment unit 1 1 is placed between the wastewater treatment plant 40 and the raw water treatment plant 20. Alternatively or in addition, there may be at least one secondary purification unit 24 that is placed after the raw water treatment plant 20. Figure 5 shows an example wherein a portion of the purified effluents 45 purified in the wastewater treatment plant 40 and/or a portion of the process waters 35 coming from the pulping system 30 are conveyed to a raw water clarifier 22 of the raw water treatment plant 20. In this example, at least a portion of the raw water 15 is purified in the same treatment line of the raw water treatment plant 20 as the purified effluent 45 and/or the process water 35 . Advantageously, at least the portion of the purified effluents 45 and/or the portion of the process waters 35 are treated by the same clarifier 22 and/or the same sand or multimedia filter 23 as at least a portion of the raw waters 15. Thus, the water flows are combined with each other before said at least one purification device or in the at least one purification device of the raw water treatment plant 20.

The system according to Figure 5 may comprise the pre-treatment unit 1 1 comprising at least one pre-treatment stage that is placed before the raw water treatment plant 20, preferably between the wastewater treatment plant 40 and the raw water treatment plant 20. In addition or alternatively, there is advantageously at least one secondary purification unit 24 after the raw water treatment plant 20.

One skilled in the art will readily understand that the different embodiments of the invention may have applications in environments where optimization of 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 said pulp,

at least one bleaching stage for bleaching the washed pulp,

a waste water treatment plant comprising at least one purifying stage to purify effluents, and

a raw water treatment plant taking a raw water flow from a water system and comprising at least one purifying stage,

wherein the method comprises:

conveying a liquid flow comprising a portion of purified effluents (45) coming from the waste water treatment plant (40) of the chemical pulp mill and/or a portion of process waters (35)

to said at least one purifying stage of the raw water treatment plant (20) for purifying the liquid flow in said at least one purifying unit of the raw water treatment plant (20) to obtain recyclable water (29), purifying the liquid flow in a purifying line in the raw water treatment plant separate from the raw water flow (15) and

using said recyclable water as process water in at least one process and/or process stage at the pulp mill .

2. The method according to claim 1 , characterized in that the raw water treatment plant (20) comprises at least one sand filter and/or at least one multimedia filter and the method comprises

- purifying the liquid flow in the sand filter and/or the multimedia filter of the raw water treatment plant (20).

3. The method according to claim 1 or 2, characterized in that at least a portion of the purified process waters is re-used for at least one of the following:

- in a fiber line,

- in a chemical recovery,

- as cooling water, - as boiler water,

- in log washing,

- as fire water,

- as sealing water,

- as raw water for a pulp drying machine, or

- as shower waters for a wire in a gravity table.

4. The method according to any of the preceding claims, characterized in that the process water flow (35) comprises at least one of the following waters:

- sealing water,

- water from drying machine,

- rain water, and

- fiberline waters from canals.

5. The method according to any of the preceding claims, characterized in that the liquid flow (45) comprises purified effluents from a clarifier of the waste water treatment plant (40) of the chemical pulp mill .

6. The method according to any of the preceding claims, characterized in that the liquid flow (45) comprises bleaching filtrates purified in the waste water treatment plant (40) of the chemical pulp mill.

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

a digestion apparatus for producing pulp,

a washing apparatus for washing the pulp,

a bleaching apparatus for bleaching the pulp,

a waste water treatment plant for purifying effluents, and

- a raw water treatment plant for taking raw water flow from a water system to a treatment line and for purifying a portion of process waters and/or purified effluents coming from the waste water treatment plant of the chemical pulp mill,

the system further comprising: - a first conveyor line from the waste water treatment plant (40) of the chemical pulp mill to the raw water treatment plant (20) and/or from the pulping system (30) to the raw water treatment plant (20), and

- a second conveyor line from the raw water treatment plant (20) to the pulping system,

said first conveyor line being connected to a treatment line that is separate from the treatment line of the raw water flow in the raw water treatment plant (20) for purifying the portion of process waters and/or purified effluents separately from the raw water flow.

8. A use of a raw water treatment plant of a chemical pulp mill for purifying process waters and/or effluents from the waste water treatment plant of the chemical pulp mill separately from the treatment of the raw water.