FI122246B - Procedure for treating pulp at a pulp mill - Google Patents

Description

METHOD FOR HANDLING PULP AT THE PULP MILL

The size of the pulp mills has grown dramatically in recent years, with the current 1 million tonne pulp mill being the normal mill size and there is no indication that the mill size is stalling. As pulp mills grow in size, they are built in areas and environments where environmental regulations are very strict. For example, the amount of water used by the mill is severely limited. As the mill size increases with small reductions in the mill's water consumption per 10 tons of pulp, it does not absolutely reduce the mill's water consumption, but is compensated back to the same level as the mill's production size increases. This development is difficult, especially in countries where the plant simply does not have enough water or water resources need to be saved for people and farming. In such a situation, it is simply impossible to build the plant in a location 15 where other production requirements can be easily met, but due to water resources, the plant cannot be built. In addition, in many areas, a cleaner environment is desired, with plants producing less environmentally harmful substances. Therefore, it is essential that solutions are sought to find an increasingly closed process.

20

Chlorine-containing chemicals have been used throughout pulp production in many forms, of which elemental chlorine Cl2, chlorine dioxide ClO2 and hypochlorite NaOCl or CaOCl are the most well-known. Chlorine-containing chemicals have also been used e.g. in the form of a sub-chlorinated poke in bleaching, but no permanent applications have been used. On the other hand, the pulp industry has been keen to adhere to the technology of bleaching pulp with chlorine-containing chemicals so that chlorine dioxide is the main chemical in the mill's bleaching process. For years, the pressure to reduce organic chlorine compounds in bleaching wastewater has led to the first abandonment of chlorine and hypochlorite, and further reduced the pulp kappa number from 30 to 30 at 10-15 softwood and from 6 to 20 at 10-13 hardwoods. In the 1990s, chlorine dioxide was also discontinued and several plants switched to total chlorine-free (TCF) bleaching technology, which also replaced the use of chlorine dioxide with completely chlorine-free bleaching chemicals, such as ozone and peroxide. This technique eliminated 35 of all chlorine-containing chemicals, but on the other hand many paper manufacturers were dissatisfied with the properties of pulp without chlorine chemicals. Therefore, all marginal solutions for factory closure solutions are that chlorine dioxide remains the bleaching chemical.

Thus, the dominance of chlorine dioxide as a bleaching chemical has even increased in the last 5 years, and recent research or industrial experience has not been able to shake its position, but, with a few exceptions, the pulp industry has generally accepted the use of chlorine dioxide as a key chemical. Thus, if the mill is to further reduce the amount of organic chlorine compounds, the mills will aim primarily at eliminating and treating it within the mill, rather than reducing the use of chlorine dioxide.

Modern ECF bleaching to bleach pulp typically consists of at least three bleaching steps and three washers. In a special case, there may be only two devices, but such applications are rare. All bleaching sequences that have at least one chlorine dioxide step and do not use elemental chlorine in any of the bleaching steps are counted as ECF-15 bleaching. Because the use of hypochlorite for pulp quality reasons is limited to the production of some specialty pulps, such as glucose pulp, neither is hypochlorite calculated for the production of the ECF pulp used, nor is it strictly excluded from the definition. In addition, the bleaching sequence comprises a single alkaline step, which is currently typically utilized as either auxiliary chemicals, oxygen, peroxide or both. In addition, modern bleaching may use ozone, various types of acidic steps and a chelating step to remove heavy metals. In the literature, the bleaching steps are described by the letters: 25 D = chlorine dioxide step H = hypochlorite step C = chlorination step E = alkaline extraction step using oxygen EP = alkaline extraction step using peroxide as auxiliary chemical EOP (PO) = alkaline extraction step using oxygen = acid hydrolysis step, hexenuronic acid removal step 35 a = pulp acidification step Z = ozone step 3 PAA = peroxyacetic acid step, acid peroxide step

When bleaching is referred to as ECF bleaching, the amount of chlorine dioxide used in the bleaching sequence is greater than 5 kg act.Cl / adt pulp. If chlorine dioxide is used in one bleaching step, doses will typically be in the range of 5-15 kg act. Cl / adt. Doses refer to active chlorine, ie when converted to chlorine dioxide the dose has to be divided by a ratio of 2.63.

If the use of peroxide in bleaching is limited to doses of less than 6 kg and if chlorine dioxide is the 10 major bleaching chemicals, then the dose of chlorine dioxide in bleaching will increase from 25 kg / adt each time depending on pulp bleaching properties and how much pulp kappa is lowered prior to bleaching. Thus, the process of bleaching can be controlled relatively freely for different levels of consumption of chlorine dioxide so that the amount of chlorine-containing chemicals resulting from bleaching corresponds to the ability of the chemical cycle to absorb chlorides.

In practice, in the context of the present invention, it is advantageous to take the bleaching sequence A / D-EOP-D-P as the reference internal sequence for hardwood, carried out in four bleaching steps 20, and to exclude ozone from the alternatives. On conifers, the corresponding sequence is D-EOP-D-P. The quality of the pulp can then be considered to correspond to the properties of the pulp being applied to the ECF pulp and the pulp yield remains reasonable. Then doses of chlorine dioxide are typically in the range of 25-35 kg / ADT for softwood and 20-30 kg / ADT for hardwood. These values can be considered as design values and there is no need to invent any specific new techniques for bleaching. The theory of bleaching and the various coupling alternatives allow for countless different bleaching sequences, ranging from connections of two washers to six-step bleaching sequences. At the same time, the number of chlorine dioxide phases may vary from one to up to four and, where applicable, there are temporal phases.

30

When the amount of active chlorine is calculated as the chloride content as described above, it is found that even for coniferous wood, to obtain a good bleaching result, the bleaching line produces about 10 kg of chlorides per tonne of pulp and even less the hardwood bleaching line. If the plant is shut down with less and less than 35 fresh water bleaching, then up to 50% higher doses of chlorine dioxide may need to be prepared and on the other hand the chloride content in the bleaching effluent will increase to about 4 to 15 kg. kg / ADT. Higher values cannot be considered economically meaningful, but the basic bleaching solution follows these principles.

One technique for reducing the environmental impact of chlorine-containing chemicals has been proposed to close liquid circuits in bleaching plants, and modern bleaching plants have achieved 10-15 m3 waste water / adt pulp without loss of pulp quality. However, already decreasing bleaching effluent from 15 m3 / adt to 10 m3 / adt shows an increase in chemical consumption, thus leading to an even greater amount of organic chlorine compounds out of bleaching. Thus, it can be concluded that purely closing the water circulation of the bleaching process does not directly affect the amount of organic chlorine compounds, but on the other hand, a smaller amount of waste water and a higher concentration make them easier and more economical to purify.

15

Chloride-containing chemicals are used in bleaching so that the total addition of chloride to the chemical cycle is 5-10 kg chlorides per tonne of pulp. Since this amount must be allowed to migrate so that the amount of liquid to be evaporated in the process remains reasonable, the challenge is to find a process connection in which the chloride-containing nes-20 te replaces another fluid used in the mill process. Thus, there is no need for separate processing steps, new unproductive side processes to the factory, but processing can be done through existing process steps.

In order to optimize the treatment of chloride-containing liquid and, in practice, bleaching wastewater, it is first necessary to know the properties that the wastewater has. In the bleaching process, chloride-containing compounds and organic chlorine compounds remain in the reactions of chlorine dioxide or chlorine. Bleaching separates the various lignin compounds from the fibers that remain in the wastewater as organic molecules. In addition, sulfuric acid is used in bleaching to adjust pH and as the main chemical in hydrolysis of hexenuronic acids. Sodium 30 hydroxide is also used for pH adjustment and lignin extraction in the alkaline steps. In addition to these, oxygen and peroxide are used in the bleaching process, depending on the bleaching sequence, however, in Elemental Analysis, they are such substances that their contribution to, for example, purification processes is overlooked. In some special cases, hydrochloric acid may also be used to adjust pH and sulfur dioxide or other reducing agents to eliminate chemical residues in the bleaching step, i.e. unreacted bleaching chemicals.

5

The closure of bleaching is based on the subsequent bleaching steps being recycled to pre-filtration stages of the washing equipment. Bleaching is only designed to circulate filtrates between bleaching steps and pulp step-by-step with different bleaching chemicals in 5 countries. Thus, the idea of closing all bleaching is based on the fact that all the constituents separated in bleaching end up in the filtrates. The optimization of bleach closure is largely based on how the bleach reaction products interfere with the bleaching process. Although it is stated in many different contexts that a different degree of closure is possible, practical experience has shown that bleaching wash water connections where the filtrates are connected so that the amount of waste water is less than 12-13 m3 / adt increase the consumption of bleaching chemicals. Of course, it depends on the quality of the pulp and the structure of the bleaching plant how much additional bleaching chemicals are consumed when the amount of wastewater in the plant decreases below the above level.

15 Often, bleach closure research concludes that bleach closure is successful, but there should be a sink or kidney where difficult inorganic substances can be separated from the process. Such a kidney has often been described, for example, as a membrane technology or ultrafiltration process, which would again be a kind of new and separate side process at the factory. In addition, 20 processes are very new and their continued technical functionality has been suspected. With significant operating costs, technological advances have not yet progressed.

Thus, the so-called bleaching effluent partial closure of the bleaching and external cleaning of the resulting filtrates (10-15 m 3 / adt) using e.g. filtration, various known forms of biological purification, various techniques of chemical purification and clarification. Thereafter, the treated water is recycled back to the same or different channel from which the liquid is drawn into the mill process. This applies to both TCF and ECF pulp mills. Biological purification is particularly effective in reducing the proportion of harmful organic substances, which primarily include lignin compounds separated in bleaching, hemicelluloses and extractant-derived components, which represent a significant proportion of the waste water from the bleaching compartment. Various wood-derived compounds are abundant, some of which are chlorinated and some are small-molecule carbon and hydrogen compounds. Because the microbes function by consuming only the organic portion of the waste water 35, all inorganic substances, at least inorganic elements, remain in the waste water. Thus, the biologically purified water has a significantly higher organic load 6 than the wastewater typically treated in other ways, but due to the inorganic constituents it has only been able to be discharged from the process.

The present invention overcomes the above problems and provides a chlorine dioxide pulp manufacturing process in which waste water discharge is minimized and chloride is not collected in the process. It is an object of the present invention to provide a method for utilizing liquid streams generated at a pulp mill in a preferred application and efficiently recycling without interrupting the main process and minimizing mill emissions.

10

A public study was carried out at the University of Oulu on the pulp bleaching washing process and the efficiency of the intervening process steps compared to the previous washing step (Viirimaa, M., Dahl, O., Niinimäki, J., Ala-Kaila, K. and Perämäki, P. Identification of the Wash. loss compounds affecting the 15 EOF bleaching of Softwood kraft pulp. Appita Journal 55 (2002) 6, 484-488). Decrease in the efficiency of the bleaching step is observed either as a lower brightness development or as a higher kappa number after the bleaching step or steps. According to one of the relevant results of the study, the most important single bleach-inhibiting component in the filtrate is lignin. From this study, two conclusions can be drawn: The amount of inorganic substances in the bleaching step is not significant for the bleaching result, and specifically removing or significantly reducing the lignin content could significantly improve the bleaching result and ultimately bring the bleaching result to the same level as the unfiltered bleaching plant. This result provides the opportunity for significant optimization of the bleaching process. Since the effect of inorganic components on the consumption of chemicals is basically not significant, washing water containing significant amounts of inorganic compounds can be accepted as pulp wash water. The □ process of the present invention utilizes e.g. these things.

30

The present invention relates to a process for treating pulp at a pulp mill comprising at least a cooking liquor-utilizing alkaline cooking process, treating a substantially closed-circuit brown pulp, an elemental chlorine-free bleaching pulp blend with at least one chlorine includes a black liquor evaporator and a chemical recovery 7 boiler plant, and wastewater treatment. It is essential to the invention that the bleaching chloride-containing wastewater is conducted to a wastewater treatment process to reduce its organic content, that the purified wastewater is recycled to the pulp mill process, that at least 20 / adt pulp as washing or dilution water for bleaching washing steps to reduce the total amount of wastewater, and that a chemical separation process is provided for the chemical recovery boiler plant to reduce the chloride level in the chemical cycle.

10

According to a preferred embodiment, the condenser is used with a D0, Dr, D2 or other D stage scrubber. According to another preferred embodiment, the condenser is used in an alkaline bleach scrubber, preferably EO, EP or EOP. The condenser can also be used on a peroxide (P) scrubber or a peracid scrubber 15 or an ozone scrubber.

The condensates are preferably used in the washing and / or dilution water of the bleaching washing steps, preferably from 1 to 7 m3 / adt mass, typically from 2 to 4 m3 / adt.

20 The cooking process is based on batch cooking or continuous cooking, which includes a machine or several cookers. The brown pulp treatment includes a washing process, oxygen delignification, typically a sorting process, and an oxygen delignification wash, which can be done in one or more washers. The sorting may be after the cooking butt, in the middle or after the washing process or after oxygen delignification. These process steps are followed by a bleaching process based on ECF technology, which comprises a pulp bleacher having one or more chlorine dioxide-based bleaching steps in addition to other known bleaching chemicals. The plant also includes a chemical recovery plant, which includes a black liquor evaporation process typically at a serially connected evaporation plant, a chemical recovery boiler, a chloride removal process, and a chemical manufacturing plant for the production of cooking chemicals.

According to a preferred embodiment of the invention, the purified wastewater is also used in the final washing step of the brown pulp treatment. In the treatment of the brown mas-35, the liquid stream is transported upstream to the evaporation, from which it is led to 8 for a digestion boiler process in which the chlorides are provided with a separation process to control the chloride level of the liquor cycle.

According to a preferred embodiment of the invention, the wastewater to be returned is heated by the heat from the wastewater for purification and the heated wastewater is used in a pulp mill. Preferably, the coupling includes a heat exchanger arrangement in which the wastewater returned from the purification is heated with heat from the wastewater to the purification. Heated, purified waste water is used, for example, in the final washing stage of brown pulp treatment.

10

According to the invention, typically at least 20% of the purified waste water is returned to the pulp mill, preferably at least 40%, most preferably at least 60%. At least 20%, preferably more than 40%, of the recycled purified waste water is used for washing the brown pulp, most preferably being added to the last 15 washes after the oxygen phase.

Further, purified wastewater can be used in a causticizer typically as a diluent and / or washer fluid. Preferably, the purified effluent is used as a diluent in a causticizing process filter in which the leach liquor formed is introduced into a solvent in a soo-dacta boiler. Preferably, the purified wastewater is used for washing the lime slurry, where the leach liquor formed is introduced into the recovery boiler solvent.

Preferably, the condensers and hot water from the evaporation plant are further used as a dilution and / or washing liquid by a causticization process filter.

25

Because the technology presented here is based on solutions that affect plant-to-plant connections and the plant-wide balance sheet, it is not possible to define in detail all the processes affected by the new connection. However, in the literature, for example, well-known process descriptions of 30 plants are found throughout the plant, and the equipment and mass production methods disclosed in this patent application are substantially known per se. Furthermore, the application of the present invention is based on devices known per se. Thus, the development of more sophisticated technology will sometimes be unnecessary for the implementation of the present invention. The present invention can be applied to 35 pulp mills whose pulping process, bleaching, other pulping, chemical recovery and chemical production comprise various known reactors, vessels, pumps, mixers, filters, etc. For example, the invention is not limited to certain washing facilities but purified using a mass scrubber can be a Drum Displacer ™ (DD) scrubber, scrubber, drum scrubber, suction filter, pressure filter, disc filter, etc.

5

When the wastewater from the bleaching plant is treated in a state-of-the-art biological wastewater treatment plant, its chemical oxygen demand (COD) has been reduced by more than 70% and its organic chlorine content, measured by the AOX measurement method, has decreased by more than 50%. If the anaerobic purification step is added to the system, then the color of the water to be treated is also significantly reduced. Thus, this biologically treated water is clearly cleaner than conventionally recycled filtrates in the bleaching plant D0 and first alkali step. In addition, chemical purification methods based on precipitation or oxidation of oxidizable compounds may be used for waste water. The usability of this treated filtrate in the final oxygen scrubber, from which it is significantly transported with the pulp to the first bleaching step, is much better for organic matter than the use of filtrates from said bleaching steps, e.g. D0 step, for bleaching or even browning. For example, the European Union's definition of forest-20 industrial technology Bat, or Best Available Technology, defines the use of the first alkaline phase filtrate as post-oxygen washing. On the other hand, pulp manufacturers utilizing press technology have for many years diluted the pulp with D0 only filtration prior to D0. As a result of this coupling, the consumption of chemicals in the whole bleaching process has increased but still has remained at levels which in many cases have been acceptable in some cases.

If the last device before bleaching is a press or a washing press, then its water use is distributed so that 3-6 m3 / adt of liquid is used for washing and the mass is removed from the 30 devices by more than 20%, typically 25-35% consistency. Since this is followed by a situation where the pulp has to be diluted before bleaching to a pumping consistency of 8-16%, for which a dilution fluid of 3-6 m3 / adt is used. Now, if both liquids are purified wastewater from the treatment plant, the chlorides are allowed to enter the chemical cycle. Changing the diluent alone from the purification plant to the purified effluent from pe-35 provides significant benefits in the consumption of chemicals through the removal of lignin compared to the unpurified filtration of bleaching, but leaving the chemical cycle intact and the chlorides not being transported to the recovery boiler. This may be a recommended connection when the recovery boiler is not equipped with devices to control chloride levels. However, if a press-type washer is used, then cleaned wastewater from the purifier and dilution with clean water, bleaching filtrate or a mixture of both may be used.

When treating treated wastewater with a brown pulp wash, some of the wastewater compounds are transported to the bleaching process, particularly to the first bleaching stage. As can be seen from these short definitions, the properties of the treated effluent are very advantageous for bleaching specifically for organic matter. However, inorganic substances, and in particular the various forms of the chlorine molecule in organic and inorganic forms, have prevented this waste water from being utilized in a bleaching plant, and in particular in brown pulp washing. However, ECF bleaching fiber-15 always produces chloride compounds because chlorine dioxide as such is a compound containing chlorine molecules.

The above described so-called. conventional combination of bleaching and brown pulping. However, the invention is also applicable to solutions in which fractions advantageous for bleaching are introduced into the oxygen phase scrubber, such as the filtrate from the ozone phase, preferably the Z / D phase, or from the acid phase. If the coupling is made in such a way that these streams are mixed with chlorides without water cut-off and separation, such streams can also be considered to be essentially chloride sources when these streams are led to brown mass washing and hence to the chemical cycle 25 and purified wastewater according to the invention.

Due to the chemical properties of the pulp, bleaching technology is in a situation where the mill's bleaching effluent discharges from 7-17 m3 / adt waste water with an AOX discharge from the bleaching line of 0.15-0.5 kg / adt and COD 20-40 kg / adt and after cleaning AOX 30 is 0 , 06-0.3 kg / ADT and COD 4-15 kg / ADT. Thus, if it is desired to achieve a lower emission level from this level in an economically sustainable manner, then this will not be the case with the traditional process of decommissioning. It is necessary to define a technology in which the whole system is understood in a new way, for example, as disclosed in the present invention.

U.S. Patent Application No. 60/913322 discloses possible techniques for treating bleaching effluents so that they are ultimately delivered to a recovery boiler for incineration and separation. An essential part of the application is that the treatment of chloride-containing liquids in the recovery boiler process does not result in more severe corrosion than normal, and the recovery boiler process is well suited for separating the chloride-containing compounds from the process to prevent chlorine accumulation. Therein, the concentration of chlorine in the flue gases is maximized by increasing the temperature of the combustion zone at which the chloride-containing liquor is burned. It defines the advantageous combustion conditions in the recovery boiler where the chlorides start to evaporate into the flue gases and the process site where the chloride can be removed from the process. Thus, in the new process, the recovery boiler can be made into a factory chloride sink and the entire chloride problem is treated where it was previously expected to be the most harmful. Should the chlorine content increase too high in this invention in view of the desired vapor temperature or vapor temperatures, the final or final results may be made as described in US2005 / 0252458 and US2006 / 0236696, utilizing corrosion-free combustion in the anterior chamber.

This process coupling results in a technique that allows filtrates or purified waste water from bleaching mills from a mill using ECF bleaching to be introduced into the chemical cycle so that there is no process step between the chloride-containing liquid addition site and the digestion process prior to the sludge digestion. Thus, the new techniques now presented are based on a plant entity in which the recovery boiler process is capable of handling the chlorine-25 contained in the normal known ECF process without separate separation technology. Such known sub-processes associated with the recovery boiler process include e.g. methods for leaching, or leaching and recrystallizing, fly ash from a recovery boiler. In sulfur-free soups, the removal of chlorine can also be effected from a solvent or usually from green liquor. It is a particular feature of the present invention to provide a clearly more closed process than previous pulp mill solutions and to demonstrate how to utilize the opportunities offered by the recovery boiler technology. All of the proposed solutions aim at: 1. Reducing the environmental load of the pulp mill 2. Maintaining the use of chemicals and consumables at the pulp mill at a minimum level of ny-35 12 3. Maintaining pulp mill pulp quality at substantially the same level as □ Existing processes 4. Reducing water use.

5 Of these objectives, points 1 and 4 could be made using the same techniques, but then objectives 2 and 3 are very difficult and difficult to achieve by the same means. Therefore, the technology presented now allows all four goals to be possible simultaneously.

10 ECF bleaching has both acidic and temporal phases. In a typical ECF bleaching coupling, waste water is removed from the filtrate from the first D stage and the first alkaline stage. Bleaching closure has been investigated from various points of view in several publications, and has generally reached a level where bleaching coupling is set so that bleaching effluent is generated at the modern ECF-15 pulp mill at 6-20 m3 / adt, most typically 7-16 m3 / adt. When wastewater is generated below 10 m3 / adt, it has been shown that low waste water volumes also increase the use of mill bleaching chemicals. Thus, it is essential that a sufficient amount of pure or purified water fractions which does not increase the consumption of bleaching chemicals is introduced into the bleaching plant.

20

Since bleaching will now be part of a closed water process, it is preferable to use oxidized white liquor as an alkaline source for the alkaline steps or to neutralize wastewater instead of pure technical sodium hydroxide. In addition, lime used to neutralize wastewater can be replaced by oxidized white liquor. This is because, in the present invention, the alkali liquor is recycled with the purified wastewater to wash the brown pulp and thereby to the chemical cycle.

The bleaching sequence defined by several in the literature, ranging from either biphasic sequences to historical seven-step sequences, such that after the first or first acidic combination steps, there is an alkaline phase and then presently an acidic and acidic or acidic and alkaline phase. The acidic steps include the chlorine dioxide steps, the ozone steps, the hexenuronic acid removal step, or some acid peroxide treatment step. The alkaline step is typically a treatment in which the pH is raised to more than 7 with one of the hydroxide compounds, most typically sodium hydroxide, and wherein the additional chemical is hydrogen peroxide, oxygen, hypochlorite or other oxidizing chemical. In such a coupling, the circulating water from the bleaching pulp drying process is supplied to the final washing machine after all bleaching steps, but may also be used in earlier steps. Because this water comes from the dewatering process of the drying machine, it is internal to the pulp mill and thus does not increase the amount of water used.

Treatment of the brown mass after the cooking process typically includes a washing process, an oxygen step, sorting, and an oxygen step and subsequent washing. It is known that this process entity is arranged so that the cleanest washing fluid is introduced to the final pe-10 melter of the oxygen stage so that the pulp is easily bleached, and the filtrate from this last washing unit is used for backwash and dilutions. When the filtrate is recovered from the first brown pulp washer, as is the case with the present invention, it is either delivered directly to the black liquor evaporator or used for dilution and displacement in the boiled processes, after which it ends up in the black liquor stream. Although the chloride content of this filtrate increases in the system of the present invention, its high alkaline content nevertheless converts the chloride-containing compounds to a salt and does not pose a significant risk of corrosion or process in the treatment of brown pulp.

20

The new solution has redesigned the use of the entire mill's water. Conventional coupling had to use air dry per tonne of pulp: 3-5 m3 of condensate or hot water for white liquor production.

25 4-10 m3 of condensate or hot water for brown mass washing. Hot water comes from the cooker.

1-3 m3 of liquid from bleaching chemicals mainly from chlorine dioxide. 1-5 m3 of hot water for bleach washing, either for drum or roll washing, and for example. For EOP washer water.

30 2-4 m3 of pure water for dry machine washing of felts 1-3 m3 of purified or raw water for condensation and cooling. About 60-80% of this water can be recycled inside the factory.

In addition, the boiler uses 0-6 m3 of pure water for cooling, which is the main source of hot water. Since the boiler has traditionally been considered as a source of hot water, it has been the aim to produce a certain amount of hot water, for example 2-5 m3.

As a result of this use of water it is possible to determine the outflows from the factory: 8-11 m3 with black liquor for evaporation. Thus, the condensate forms an internal circulation.

5 Black liquor dry matter consists of a variety of compounds derived from various organic, mainly lignin and carbohydrate-based compounds. The evaporation plant produces 7-10 m3 of condensate at different stages.

8-10 m3 of effluent out of bleaching to the treatment plant including bleaching chemicals, 1-5 m3 of effluent from the drying section of felts washing and sealing water, and from refrigeration.

1-3 m3 is generated from the flows of the sealing and cooling water, but under certain conditions these fractions can be recycled with the rainwater to the canals. Thus, the total amount of wastewater generated is 15-25 m3 per tonne of pulp, and in addition the waste water from the wood treatment is 15. Wood treatment can also use either bleaching filtrate or purified bleaching filtrate without process problems, but since woodworking equipment is traditionally made of carbon steel, the use of chloride-containing fluid would require a review of material specifications.

In the new connection, the water consumption per tonne of dry air pulp is mainly distributed as follows: 3-5 m3 Bleaching filtrate and / or purified wastewater and / or hot water for the production of white liquor.

25 4-10 m3 of wastewater from the treatment plant for washing brown mass.

1-3 m3 of liquid from bleaching chemicals, mainly chlorine dioxide. Now this can be exchanged e.g. evaporation condensate or filtrate from the purification plant.

1- 5 m3 evaporator condensate for bleaching washing processes. It is used for either washing 30 drums or rolls and bleaching washers as pulp wash water.

2- 4 m3 of condensate water for dry machine washing of blankets.

1-3 m3 of condensate or raw water from the evaporation plant for condensation and cooling. About 60-80% of this water can be recycled inside the factory.

In addition, the boiler uses 0-6 m3 of pure water for cooling, which is the 35 main source of hot water. Since the boiler has traditionally been regarded as a source of hot water, the aim has been to produce a certain amount of hot water, for example 2 to 5 m3. However, in a new connection, the digester may heat the wastewater of the treatment plant or the hot water will have to be cooled without utilizing heat.

5 Depending on the amount of lean black liquor, the condensate condensate generates a maximum of 6-8 m3 / adt at the pulp mill. Larger quantities occur only in special cases.

As a result of this water use, it is possible to determine the outflows from the mill: 9 to 11 m3 with black liquor for evaporation. Thus, the condensate forms an internal spiral.

6-9 m3 of condensate is generated from the different stages of the evaporation plant. These condensates are used in the process at different sites as described above.

10-15 m3 of effluent out of bleaching to treatment plant through treatment plant for brown pulp washing including bleaching chemicals.

15 2-5 m3 of wastewater from the drying section for felts washing, sealing water and cooling

1-3 m3 is generated from the stream and cooling water flows, but under certain conditions these fractions can be recycled with the rainwater into the canals.

Thus, the total amount of waste water generated is from 0 to 10 m3 per tonne of mass, preferably from 20 to 0 m3, most preferably from 0 to 4 m3. In addition, there will be waste water from wood treatment. A significant part of these flows, too, consists of sealed waters, sewerage canals or other process secondary sources.

This shows that a real improvement in technology can be achieved, whereby a target of up to 0 m3 of waste water / adt mass out of the process can be set at steady driving conditions.

Since pulping and white liquor production typically requires about 10-16 m 3 of liquid / adt pulp, it is considered advantageous to treat and produce such an amount of wastewater for these purposes. The most important environmental requirements for the whole plant are the bleaching waste water, which is both a major source of biological and chemical oxygen demand and, above all, the organic chlorine compounds produced by ECF bleaching. Other pulp mill waste streams include cooling waters, sealing waters, reject streams, canal waters; plant washing waters and 35 rainwater and wood treatment water. With the exception of wood treatment water, said waters have not been in contact with a process containing chlorine compounds.

16

Accumulated emissions are, as a rule, leakages and over-discharges, accidental emissions from equipment failures, washing water from equipment, textiles or tanks from continuous or intermittent washing, and leakage from the rejection system. The environmental hazards of such fractions of mill effluents are mainly based on oxygen-consuming compounds. It can thus be stated that only bleaching effluent contains e.g. chlorinated organic compounds which are generally considered to be the most environmentally harmful.

The advantages of the present invention are best exemplified by treating different waste water streams in different compartments at a wastewater treatment plant. Thus, the waste water from bleaching would be treated in different basins, separate from, for example, the bark waste water. On the other hand, waste water does not dilute as a result of cooling water or rainwater. In addition, if the mill has several different bleaching lines and possibly several chemical recovery lines, the streams containing the most chlorine chemicals 15 can be led to the purification unit from which the purified wastewater is returned to factory processes, such as primarily brown pulp washing or chemical production. Thus, the organic chlorine compounds would be concentrated in the stream returned to the mill, and less harmful streams would be purified and discharged into the river.

20

Separate purification lines also benefit from the management of non-process elements (NPE). When e.g. water from the wood field contains a lot of material from the bark and the surface of the wood, as well as sand and dust adhering to it during harvesting and transportation, these contaminants may become harmful substances in the chemical cycle of the pulp if such waste water were introduced there. When only one bleaching wastewater treatment plant is treated in one wastewater treatment line, the wastewater returned here contains only bleaching agents, chemicals needed for the purification process and chemicals used to adjust the pH.

In particular, the migration of organic chlorine compounds in bleaching and out-of-bleaching through purification to water may also be controlled through separate cleaning. Since many other exits from the plant, such as condensate or rainwater, are still very clean even when entering the wastewater collection system, it is unnecessary to mix these streams with, for example, bleaching or bark effluent.

35 Thus, for example, sealing water can be collected and reused, cooling water can be recycled in factory processes, etc. Only when equipment failures, etc.

As a result, these waters become contaminated and must be collected and purified. Since it is now advantageous to balance the amount of effluent from bleaching and the water reused in the process, this objective also requires both the more efficient recycling of pure water fractions and the treatment of different wastewater fractions on different purification lines.

An example of this is rainwater. It may take several days for the mill site to drain and several cubic meters per hour for rainwater to flow into the mill's effluent. Even though the water is mostly pure, it still dilutes 10 unnecessarily cleaned water. In addition, rain may rinse the mill site, such as chips and fibers, or black liquor from the mill in case of disruption. Thus, rainwater can also cause unexpected load peaks in the cleaning process. Because the mill process is only able to recycle a certain amount of purified waste water, e.g. rain fluctuations 15 could have a significant impact on the quantity and quality of emissions leaving the factory. If the bleaching effluent is treated separately, then the bleaching effluent volume will mainly be affected by the rainwater coming from the bleaching plant and the rainwater entering the clarifiers, aeration basins and other open structures. In this way, the catchment area can be minimized and the volume and load variation are also small.

20

Since one alternative is to use oxidized white liquor or white liquor to neutralize the wastewater plant, it is also advantageous to purify the bleaching wastewater in its own treatment line or basin. When the treated waste water is neutralized and recycled, it also recovers the spent white liquor and recycles the chemical. At the same time, it is ensured that compounds that could interfere with the process do not enter the chemical cycle through neutralizing agents. Thus, in most plants, the use of non-slaked lime (CaO) would be significantly more difficult in the process and would cause significantly more difficulties than white liquor compounds. As already mentioned, when using a separate purification line specifically for the bleaching wastewater, the components of the white liquor are recycled to the chemical cycle and the migration of foreign matter into the process can be minimized.

The amount of waste water now depends on how efficiently the condensate is used in the mill's processes. In addition, the boiler always produces a certain amount of hot water 35, which is either recycled to the process or, if the process does not have access to hot water, the water needs to be cooled.

18

Wood treatment can also use either bleaching filtrate or purified bleaching filtrate without process problems, but since woodworking equipment is traditionally made of carbon steel, the use of chloride-containing fluid would require a review of material specifications. In a normal mill process, wood treatment wastewater is led to a common purification process and from there, it is recycled as purified water to the mill processes.

In addition to the main streams mentioned, the pulp mill has secondary streams, depending on the location of the 10 mills, the processes selected, and the final mill purity levels required, which will require separate processing steps when closing the mill process. Such streams include various vapors containing mainly water vapor, such as molten solvent vapor, bleach gas scrubber vapor, flue gas vapor, pulp drying, or, in the case of an integrator, up to 15 paper machine dewatering whitewashes, oxygen phase gaseous effluents and water vapor, HCLV and LCHV gas concentrate water vapor and other such secondary streams. The combustion of hydrogen-containing organic matter also produces water, which in the mill's total balance becomes one of the mill's liquid streams. All of these have their own chemical characteristics, and if the aim is to have an increasingly closed pulp mill, then the existing so-called "pulp mills" may be introduced. in addition to traditional cleaning methods, microfiltration, membrane technology, ion exchange technology, advanced evaporation techniques, and / or other advanced purification techniques. These streams, too, can be utilized either directly or after suitable purification steps as process waters for the pulp mill. Thus, these secondary streams are comparable to evaporator condensates or purified bleaching wastewater.

The streams shown are just examples of some possible solutions. Because there are hundreds of pulp mills with different connections and processes, it is impossible to determine water use areas that would apply to all mills. Thus, the ranges and amounts presented are indicative and frame the use of water from modern pulp mills and illustrate the opportunities that the present technology will improve.

35 The waste liquor from the present sulphate pulp cooking process is supplied to an evaporator where its solids level is raised from 10-20% in a series-linked evaporation process, usually to over 75%, so that the combustion conditions of the liquor are such as to achieve sufficient combustion temperatures. Patent Application 60/913322 describes. The waste or black liquor contains both temporal compounds formed in the reactions of the white liquor of the soup and the cellulose, hemicellulose, lignin and extractant substances which have been liberated in the soup 5.

Condensates are produced from the evaporation plant, which are mainly distilled water and contain a number of organic small molecules known in the literature for evaporation, the most famous being methanol, and inorganic sodium and sulfur compounds. Since evaporator condensates have been used for many years in the brown pulp washing process to save fresh water, purification methods such as condensate segregation systems and external purification methods, such as condensation stripping, have been developed inside the evaporators themselves. It really depends on how the condensate is used, how much the plant should invest in cleaning the condensate 15. In addition, the oxidation of organic parts of condensates with, for example, ozone has been studied. The condensers are extremely clean and suitable for a wide range of applications at the bleaching plant and fiber line. Now in the new coupling it is necessary to use the condensate on the fiber line and other departments for new applications because the real savings and benefits in terms of chemicals and pulp quality will not be achieved at the same time if the condensate is not fully utilized.

In the present system, the condensate is no longer used solely and principally for the washing of brown pulp, but is used for pulp bleaching and the drying machine process. Thus, the new coupling will require adequate cleaning of the condensates so that they can be used in new applications that will ultimately bring the benefit of the new coupling. Since the brown pulp washing according to the invention is carried out with purified wastewater, sufficient liquid must be introduced into the bleaching plant to provide a sufficient amount of washing liquid for the brown pulp washing process and possibly for the lime mud washing process. Thus, it is advantageous to provide a water coupling to the bleaching process wherein a sufficient amount of condensate is supplied to the bleach cleaners so that 11-15 m3 of waste water can be supplied to the bleaching effluent treatment process.

In addition, it may be considered advantageous for the operation of the plant if after purification, not all wastewater is returned to the process but 0.5-5 m3 / adt of wastewater purified is recycled back to the water. From the point of view of plant operation, it can reduce the number of malfunctions, although there is no obstacle to using all the purified waste water at the plant.

In addition to bleaching, clean water is needed in the pulp drying section to clean blankets and textile dryers. When the condensate is sufficiently cleaned, for example to a very low COD and odor compound content, the condensate can also be used in dryer processes, such as felt cleaning water. In addition, the condensate is suitable for high-pressure washing of the fabrics used in the stripping process, but this typically requires that a significant amount of the odor compound is removed from the condensate. Because of the significant increase in the use of condensate, in addition to conventional cleaning of condensates, new purification methods may have to be used, such as e.g. ozonation to reduce the amount of odor compounds in the condensate.

The concentrate formed by evaporation, i.e. the black liquor, is incinerated in the soda ash process, most preferably as described in U.S. Patent Application 60 / 913,322. In this process, the lye is burned for energy but the chloride is also removed. Thus, in the arrangement according to the invention, the recovery boiler section consists of a so-called. the throat to which the chloride compounds must be delivered for removal. Bleaching effluents have been the subject of many discussions to find a sink or kidney before the recovery boiler process and typically on a fiber line, now this sink is placed in the recovery boiler itself.

The chemicals to be regenerated from the recovery boiler will be melted. The melt consists mainly of sodium carbonate, sodium sulfide in the form of cooking chemicals, and compounds of sulfur, sodium, carbon and oxygen, which are well known in the literature. The molten solution is dissolved under the recovery boiler in a so-called. in a molten solvent to which the filtrate is added as dissolving water, e.g. man wash.

Causticization is a plant which typically contains green liquor filtration, slaked lime and green liquor in a solvent, causticizing vessels for carrying out the causticizing reaction. In the reaction, the sodium carbonate reacts with the calcium oxide to give the sodium hydroxide and the calcium carbonate. The resulting white liquor is filtered through dedicated filters and the calcium carbonate or lime sludge is washed with a lime sludge filter so that it can be transferred to a lime kiln. In a honey oven, calcium carbonate reacts and is converted to calcium oxide by heat so that it can be reused in the solvent. In addition, causticization results in various precipitates, including metal and organic matter, collected from the causticizing 21 solvent and causticizing vessels and subsequent filters, and these are removed from a specially designed precipitate filter.

It is known that ca. 2.5-5 m3 / adt of fresh water, more typically 3-4m3 / adt, is used in the causticization section due to the species of wood and the need for alkali. This is distributed so that about 1 to 2 m3 is pure washing liquid to minimize TRS emissions and the remaining 2 to 4 m3 is condensate from the evaporator. These washing and dilution fluids are derived from e.g. from the lime filter, where the lime is washed before the lime kiln and thus some alkali has been transferred. Because of the desirability of finding new uses for bleaching wastewater, either purified or untreated, and the desire to obtain as much chloride-containing liquids as possible through the chemical cycle to the recovery boiler, the present invention directly uses unbleached wastewater from bleaching. As the waste water may contain compounds that are not suitable for washing lime mud, it is alternatively possible to use in this process also the liquid that has undergone the purification process for the same purpose. Thus, a chloride-containing liquid is supplied to the system and can be sufficiently removed in the recovery boiler process.

In addition, fibers and 20 solids have been removed from the effluent that has undergone the purification process in the clarification process, so that these materials will not become a nuisance in the causticization filters.

However, the chemicals manufacturing department has been developed to keep the water it uses as clean as possible and, above all, to keep the volatile sulfur compounds low. At a chemical plant, liquids are required in all plant filters, such as green liquor, sediment or lime filter for dilution and in some cases for washing. Since the crude bleaching filtrate contains mainly non-volatile constituents of lignin and cellulose, as well as chlorides, sodium and sulfur as sulphate, it does not emit, e.g. TRS emissions, no big risk. TRS emissions occur when substances used for washing lime enter the lime kiln and are released as odorous or other harmful compounds. In the disclosed solution, a significant portion of the water from the white liquor plant is introduced either directly from the bleaching as a bleaching filtrate or after biological purification, whereby the organic load is significantly reduced. Of course, in all cases, it is possible that the lime slurry is still washed with clean water, although the dilution of the filter uses the solution described, in which part of the liquid is replaced by liquid fractions from bleaching.

22

As the use of liquid fractions from bleaching has been redefined and will bring significant savings in either water use or wastewater volume, the installation of additional equipment or purification systems in the lime kiln may also be considered. In its various forms, the Mee-5 sauna releases a great deal of environmental-licensed material, the amount of which has been set by the authorities at the factory. Thus, if the plant is able to dramatically reduce the amount of wastewater exiting the plant by means of a new connection, it will eventually be possible for the whole plant to be affordably priced at 10 filters and purifiers for the lime kiln.

Because in the new connection, purified wastewater is delivered to different applications in the process, different types of quality requirements may arise for different fractions of wastewater. Thus, the wastewater purification process can be performed such that, for example, fractions containing more 15 lignins are divided into another purification line and less fractions containing lignin, but fractions containing more dye compounds are purified in another. Also, various wastewater fractions such as dirty filtrate of acidic filtrate, pure fraction of acidic filtrate, and alkaline filtrate can be purified in the post-bleaching process as separate fractions so that their properties in the reuse site are optimal.

Wastewater treatment processes typically include pre-treatment, neutralization, biological treatment with an aerobic or anaerobic process, and possible chemical treatment. It is possible that the treatment of wastewater is also a solution.

25 air-conditioned ponds, where the purification efficiency is more modest than the biological wastewater purification process. Finally, clarification is done to remove the sludge generated by bacterial activity. This sludge can be further transported to a recovery boiler for incineration with black liquor, and is already in use today in several factories. In chemical processes, harmful constituents 30 can be precipitated from the waste water, thereby improving the quality of the waste water. In addition, the waste water can be oxidized with, for example, ozone or oxygen. By these methods, a treatment plant solution can be found which provides the waste water with sufficient purity for the intended uses.

23

Various methods based on microfiltration and membrane technology have also been investigated and are not yet commercially available. However, their use is not excluded from the scope of the present invention.

5 There are several manufacturers of purification plants in the world that have their own connections to purification processes. Thus, the processes cannot be universally defined, but are characterized by the above. In addition, delays and the like vary in nature, so that the invention is not limited to one particular specification of a known purification plant.

10

It has been found in all purification methods that chloride-containing inorganic substances are transported out of the plant with the liquid, but significant amounts of organic materials either change or decompose as a result of the purification. Since the goal is to remove significant amounts of compounds that interfere with bleaching, it can be stated that biological wastewater treatment in particular fulfills this purpose very well.

Because biological wastewater treatment removes significant amounts of lignin, the water thus treated is, by its purpose, most suitable for use in the brown pulp washing process.

20 For waste water treatment, the waste water must be cooled so that the bacteria can function properly. Because the treated water is recycled to the process, preferably at process temperature, the system is provided with conventional heat exchangers such that the other side of the waste water cooler is reserved for the waste water to be cooled and the treated waste water acts as a cooling liquid. Here, the untreated waste water reaches a temperature typically below 40 ° C required for the treatment of waste water and the returning liquid is heated to 65-80 ° C so that when the liquid returns to the fiber line, a reasonable amount of steam is consumed. When sufficient heat exchangers are added to the system, it is most advantageous to dispense with cooling towers, for example, which have been extensively used for cooling the waste water from a pulp mill.

30

Another option for heating treated waste water is the boiler circuit. The boiler requires a coolant temperature of about 20-60 ° C for cooling and generally uses hot water or some unheated factory fraction of water. If the material of the heat exchanger is properly selected, the cooling 35 can then be treated with treated waste water. Sure, the treated waste water contains chlorides, but because the pH of 24 is neutral or even slightly alkaline, the material does not entail excessive costs.

Due to the presence of bacteria, significant amounts of microbial activity can be expected in the return treated wastewater, which can cause dirt or odor.

However, if the conditions of ECF bleaching are further analyzed, it can be stated that the chlorine dioxide is a strong oxidant and under the conditions of chlorine dioxide bleaching the activity of the bacteria is very low. In addition, temperatures above 80 ° C and a change in pH between the bleaching steps from acid to alkaline so that also the peroxide typically present at the stage results in virtually no significant biological activity of the treated waste water entering the bleaching step.

Wastewater can be brought to one treatment plant from several sources. If other wood processing industries, typically 15 paper machines, mechanical pulp mills or sawmills are located in or near the same mill site, these wastewater can still be treated in the same treatment plant. In addition, the treatment plant may treat municipal wastewater in nearby cities and, in some cases, the water of other production facilities. In case the treatment plant also processes non-pulp mill wastewater, it is necessary to investigate the quality of the elements from the 20 non-pulp mills accompanying the effluent before using the water from such a purification plant at the pulp mill. For example, it may be difficult to use calcium-containing purified wastewater on a fiber line due to precipitation, but its use in causticization may be quite possible.

25 Since the present invention affects all fluid streams at the factory level, the basic concepts to which the invention responds must be seen in the overall picture. First, technically, for example, U.S. Patent Application No. 60 / 913,332 describes how to advantageously obtain a sink for chlorine-containing liquids in ECF bleaching, which provides a significant amount of chlorides out of the process.

30

The now purified waste water, which has a certain chemical oxygen demand level and organic halogen level (AOX), has been recycled to the chemical cycle where it is practically concentrated by evaporation to be burned in a recovery boiler. If 90% of the wastewater is returned to the chemical cycle after purification, then about 90% of the AOX level in the water will also be reduced. Thus, if the amount of AOX discharged into the water after purification is 0.2 kg / adt, then a new 25 circuit, whereby 90% of the treated wastewater is returned to the mill, will achieve a level of 0.02 kg / adt. The same reduction can also be observed with chemical oxygen demand. For these reasons, the use of purified wastewater constitutes a real step towards a closed pulp mill process and enables a virtually pollution-free process. However, it must be accepted that there are some exceptional situations in which waste water cannot be returned from the purification process and must be temporarily discharged into the water system.

Once the chlorides have been thawed, the process must be arranged so that significant amounts of chloride-containing fluid streams can be fed to the throat 10 so that the throat sufficiently discharges the chlorides and does not accumulate in any mill cycle. According to the present invention, two liquid streams can be found through which significant quantities of chlorides can be fed into the liquid circulation into the recovery boiler: 1. Washing of the brown pulp and the chloride entering the chemical cycle; and 15 2. Making white liquor and washing lime.

Of these, lime sludge may be partially or completely successful without bleaching waste water treatment, but in order to perform bleaching economically without major additions of chemicals, it is preferred that the bleaching liquid be purified from substances that cause quality and lightness in bleaching. Thus, the effluent of bleaching 20 into which the lignins have been dissolved is purified by external purification either by mechanical, chemical, biological or oxidative methods or by any combination of methods in which the COD of the effluent is reduced without dilution by at least 30%, preferably more than 40%, most preferably above 60%, and / or the lignin content of the effluent is reduced without dilution by at least 30%, preferably by more than 40%, most preferably by more than 60%.

The consequence of this is that it is advisable to use significant amounts of condensate from the evaporator on the fiber line, i.e. 1-5 m3 / adt, in order to keep the pulp sufficiently clean and to provide sufficient fluid to prevent the accumulation of inorganic substances. In the new connection, there is 30 real need for this because the traditional condensate application is no longer available. Thus, new uses for the mill condensates will be the liquid water streams of the drying machine, for example, so that the washing of the felts and the wires will continue to be carried out on the evaporator condensates. In this case, the condensates must be cleaned so that no harmful or odorous compounds can enter the air through the dryer or dryer.

35 26

If the factory is equipped with a bleaching sequence with three washers, then the sequence alternatives could be:

A / D-EOP-D 5 A / D-EOP-DnD A / D-EOP-P D-EOP-D Z / D-EOP-D D-EOP-P 10 A-EOP-D A / D-EOP -P

Typically, the fluid circulation in these cases is solved by: - the final or D or P stage scrubber entering the pulp dry machine circulation water and 15 little hot water, - the middle bleaching step which is the EOP step in the examples (meaning the alkali-leaching step oxygen to enhance bleaching) use filtration from last bleaching washer and pure water, 20 - the first bleaching step, which is in Examples A, A / D, Z / D or D (meaning acid, ozone or dioxygen or a combination thereof without intermediate washing) ), the final bleach filtrate and EOP-phase filtrate are used for subsequent washing.

Although a significant number of the world-wide three-scrubber bleaching sequences are close to these or variants thereof, other possible sequences can be generated by the three-scrubber. In addition, it is not essential in the combination which other bleaching chemicals are used, but it is essential that there is one step in the sequence using chlorinated chemicals. In addition, clean water can also be brought to the en-30 first bleacher. In addition, the washing devices may be washing presses or mere presses, whereby not all pure water is introduced into the process by displacement, but pure liquid is mixed into the process by dilution.

Bleaching may also comprise from four to seven bleaching steps, all of which utilize the aforementioned bleaching steps or sequences having at least one chlorine dioxide step.

27

If the factory is equipped with a bleaching sequence with four washers, then the sequence alternatives could be:

5 A / D-EOP-D-D A / D-EOP-D-P D-EOP-D-D Z / D-EOP-D-P D-EOP-D-P 10 A-EOP-D-P A / D-EOP-Dn-D

Typically, the liquid circulation in these cases is solved by: - supplying the last or D or P stage scrubber circulating water and a little lunar groundwater, - the second last scrubber either flowing upstream of the last scrubber or partially upstream by adding some hot water, condenser or dryer circulation water from the evaporator, - after the second bleaching step, which is the EOP step in the examples, a filtrate of third or fourth bleach washer and clean water are used. The amount of pure water can vary and is not used at all in some applications. In some cases, circulating water from a dry machine is used instead of clean water, - after the first bleaching step, which is in Example A, A / D, Z / D or D, the third or fourth bleach washer filtrate and the 25 EOP step filtrate are used.

These examples illustrate the basic principles of how the water cycle of bleaching is typically organized, but there are many different modifications and connections in industrial plants depending on the materials used for bleaching, the temperature, the quality of the raw water, etc. Thus, the options presented here are examples of design solutions the most appropriate and workable solution.

Furthermore, it is not essential to the invention for how many washes and how much clean liquid is supplied. It is essential, and most important for the invention, that when a significant part or all of the condensing plant 28 previously used for brown pulp washing is not used in the previous coupling, it will be used with bleach scrubbers to replace the previously used hot water. The use of condensing in bleaching as such is not new, but in prior art arrangements, if the condensate was used in bleaching, the condensate fraction used in the brown pulp washing or in the oxygen phase with hot water had to be added. Thus, the use of condensing in bleaching did not result in a real reduction in waste water volume, nor did it reduce total mill emissions for eg AOX or COD. Now that water use according to the invention is reduced by adding purified wastewater as a source of process water for the mill, then the evaporator condensate comes in a key-10 position in reducing or optimizing pulp production and mill-wide water use.

Condensate is formed from the black liquor going to the evaporator in the pulp mill processes as follows. The black liquor from the cooking process is washed away, so that the pulp 15 after the cooking process contains about 6-101 black liquors per ton of pulp. While the washing process typically uses 0.5-41 excess of water to ensure black liquor recovery, 6.5-141 black liquor per tonne of pulp is transported to the evaporation plant. To this black liquor stream, 1.2-2 t / dry of dry matter has been transferred, some of which is organic matter dissolved in the soup and some is inorganic components of the cooking liquor. Thus, the dry liquor content of the black liquor going to evaporation is usually adjusted to a level of 10-20%, most preferably 14-19%.

The black liquor from the scrubber is evaporated so that its dry matter rises sufficiently high for the incineration process. for. In modern recovery boilers, black liquor with a dry solids content of 65-85% is burned, leaving only 0.53-0.181 water per kg of solids. Thus, 4 to 12.51 water per tonne of pulp has been evaporated from the black liquor. Since the water is basically separated by a distillation process, the water is free of minerals and solids, so it is very pure. Further, when condensate cleaning equipment such as strippers is used at the evaporation plant, the organic volatile compounds mixed in the condensate stream are removed and the condensates become odorless and clean for re-use in the mill processes such as bleaching. In addition, the chosen technique can influence how much impurities can be removed from the condensate and, at the same time, how clean the condensates are.

35 29

After purification, the condensates have properties similar to chemically purified water, which, for example, are essentially free of transition metals, which can be harmful e.g. during the peroxide steps. Several studies have been conducted on the use of condensers over the years. It has been found that when condensates are purified according to normal industrial practice and when no black liquor residues are mixed with them, e.g. lignin, they are well suited for use in bleaching processes.

When the evaporator condensate was used in accordance with the traditional factory concept) because of the brown pulp washing and the causticization department for the washing of lime, dilutions and finally the melting, the quantity of condensate alone was not even sufficient for these applications. However, in the ECF mill, the use of condensate in bleaching has increased the need for fresh water for washing and causticising the brown pulp, and this use of condensate did not actually reduce the mill's use of fresh water. Thus, the use of condensates in bleaching has not, in fact, produced clear reductions in, for example, the amount of waste water. In a few TCF mills, the condenser was also used for bleaching, but in these cases, the entire chemical cycle was organized based on the fact that no chlorides were added to the chemical recovery process.

20 In the present invention, the entire water cycle is redesigned, and the use of condensing in bleaching becomes a key technology in the new process. Because purified wastewater is led to both causticization and brown pulp washing, a significant portion or even all of the condensate remains for use in bleaching. It does not make sense to put the whole condensate volume in the bleaching, mainly 1-6 m3 liquid / adt 25 pulp. Thus, it can be stated that the amount of pure water that can be replaced by condensate in the bleaching washing steps reduces the total mill water requirement by the same volume. Since chemical fabrication and brown pulp washing require a total of 7-15 m3 of purified waste water / adt depending on the scrubber, it is sensible to use the required amount of condensate in bleach washing so that the condensate drains into and out of the chemical cycle. This coupling enables a very advantageous process for the mill's overall balance, since very little contaminated water is introduced into the bleaching process and the bleaching wash water comes from the plant's internal liquid circulation.

35 According to the invention, as a first step, it is achieved that virtually no significant process streams 30 which require the use of raw water supplied to the mill need be connected to the mill's pulping process. In other words, pulping, including cooking, washing, bleaching, drying, evaporation, black liquor incineration, and chemical production, can be accomplished so that all the liquid supplied to the process comes from either the condensing system of the evaporation plant or the plant wastewater treatment plant. This results in a closed liquid cycle comprising bleaching, external cleaning, brown pulping, cooking and evaporation, which produces sufficient condensate as a source of clean water for bleaching. This requires that the purified waste water be used in the treatment of brown pulp, e.g. washing and dilution.

10

In addition, condensates can also be used as condensate water. Since one object that requires clearly clean water at pulp mills is the sealed water in rotary equipment and pumps, one object for evaporator condensates is the use as sealed water. At present, the purified water from the mill is mainly used as concentrated water. In many mills, sealing water 15 is a significant water use and thus a significant cost. Since the condensate condensate contains no minerals, humus and no solid particles, the condensate as such is ideal for use in mechanical equipment.

In rotary devices, seals are nowadays typically mechanical seals, whereby the seal is either single-acting or double-acting. In a single-acting seal, the seal water is led into the process and thus no water is recovered. In double-acting seals the water comes out and can be recovered for reuse or used for waste water treatment. Mechanical seals are used in pumps, decompressors, mixers, sorters and stencil devices. In addition, packed seal solutions are used in applications with large shaft diameters.

Some other appliances, such as washing machines, require sealing water. Even there, it is essential for the quality of the water that the seal water does not contain 30 humus or particles, but small amounts of organic compounds do not prevent the use of condensate as condensate. From known washing equipment, seal water is used in some form for example. DrumDisplacer ™ (DD), vacuum drum filters, disc filters, pressure diffusers and diffusers. In addition, sealing water is used in certain presses and washing presses. The stove, evaporator, drying section, soda-35 boiler, and all other plant-related departments have rotating or other equipment that requires condensation water for condensation.

31

If the seals are so-called. double acting, then the sealing water comes out of the device approximately as clean as when it went there. Therefore, the seal water can be further recovered and recycled either to the seal water without any purification treatment or to the water before purification in the seal being purified by some filtration method or other means.

Care must be taken to ensure that organic matter in condensers does not cause premature wear, corrosion, dissolution or other damage to the seals. This is particularly true if the materials are, for example, plastics, rubbers or other volcanic or polymeric compounds.

When condensed water is condensed, it can also be used elsewhere in the process instead of clean water, such as washing water, dilutions, equipment cleaning water, and any other site for which pure water is commonly used in pulp mill conditions.

The present solutions also allow the use of condensates or waste water, for example, for the production of chlorine dioxide water. When chlorine dioxide water is typically made into raw water from 20 mills, the raw water may at some point be replaced by even purified wastewater or condensate. What is important in these streams is that the liquid is cold enough. Cooling the condenser to below 20 ° C consumes a lot of energy, but on the other hand it is possible in cold conditions. Economic considerations and the need for energy for cooling determine whether or not such water use is advisable 25.

Since these arrangements already result in a variety of redefinable process conditions, they can be considered as solved at least: The use of liquor such that oxidized white liquor functions to neutralize the entire bleaching area and to neutralize wastewater. Very high quality standards can be set for this oxidized white liquor. Since it is known that thiosulfate causes reduction of oxidizing chemicals, the quality requirements for oxidized white liquor must be set: residual sulfide is less than 2g / l, preferably less than 1g / l, and at least 35 50%, preferably more than 80% of thiosulfate is oxidized to its starting level. This is equally true for the neutralization of waste water, since a significant part of the waste water is thereby returned to the brown pulp for washing and whitening.

Heat exchanger arrangements for cooling the waste water and heating the treated waste water 5 with cross-linked heat exchangers or heating the treated waste water in boiler cycles.

The wastewater treatment process should in the future produce a liquid that is well suited for use mainly in two sites, washing the brown pulp 10 and making white liquor. Their quality requirements may differ to such an extent that it is advantageous to treat them in a purifier even as separate fractions.

When the pulp mill is tracked as described above, the process of inventing a near-closed pulp mill without the addition of any new compartments beyond those already in use can be observed.

The pulp mill can continue to use chlorine dioxide to ensure pulp quality even in a closed process.

20 Consumption of bleaching chemicals will remain substantially at the same level as the best current mill solutions and all target levels of lightness will be achieved for the pulp.

It is a primary object of the present invention to enable the production of a chemical pulp with substantially no liquid environmentally harmful emissions and very low gaseous and solid emissions. The invention will be described in more detail with reference to the accompanying drawings in which

Figure 1 is a schematic representation of the connections of subprocesses of the prior art pulp mill, and

Figure 2 is a schematic representation of a preferred embodiment of the present invention for carrying out the method of the invention.

In the known system shown in Fig. 1, reference numeral 10 35 denotes a conventional digester, for example a saucepan, fed with hardwood or softwood chips 11 or other comminuted cellulosic material. In digester 33, wood chips are treated with cooking chemicals under conventional temperature and pressure conditions to produce a chemical pulp, e.g., kraft pulp, after which the resulting brown mass 13 is preferably delignified with oxygen in step 12. After the oxygen step, the pulp is washed with hot water 14, e.g. 5 The oxygen phase typically also includes sorting. The washing solution 9 is conducted upstream of the pulp.

After oxygen delignification, the washed and oxygen treated pulp 15 is conducted to ECF bleach 16 where it is subjected to various bleaching steps, but at least 10 together use chlorine dioxide. Other bleaching steps used may vary and also depend on the type of pulp being treated. After the bleaching steps, the pulp 17 can be dried in the pulp imaging device 18 and further transported to a paper mill. Hot or warm water 19 is supplied for drying and the circulating water 20 of the drying machine is taken to bleaching 16 for use as clean washing water.

15

For example, the bleaching sequence is A / D-EOP-D-P or D-EOP-D-P. One bleaching chemical is introduced with 21 mm of dioxide. from the chlorine dioxide plant 22. Between the steps, the pulp is washed to use washing machine circulation water and / or fresh water 23. The washing filtrates are recycled upstream, but eventually 20 results in both acidic 24 and alkaline 25 bleaching filtrates which are removed from the waste water treatment process. to nearby watercourses.

In accordance with conventional practice, the weak black liquor 28 is removed from the digester 10 25 (or associated brown pulp washer) and fed to the evaporators 29. The condensate 30 produced in the evaporator is used as a washing liquid 12 in the brown pulp.

From the evaporation plant, the strength black liquor 31 is eventually passed to a recovery boiler 32 where the flue gas 33 generated is led to further treatment for purification.

Melt 34 from recovery boiler 32 is introduced into solvent 35 to form a green flap. The green sash 36 is further used in the caffeine for the production of white liquor, referred to in figure 1 as 37. The insoluble precipitate 35 is removed from the green sash, for example by filtration, and the separated precipitate is further processed in a so-called. tab filter (not shown). The green flap thus clarified is treated with lime to effect the causticization reaction and to produce white liquor and lime mud. The honey is separated from the white liquor by filtration and washed. The thickened lime slurry is burned in a lime kiln.

The white liquor is led along the compound 38 to the digester 10. Typically, hot water 39 is supplied to wash the lime sludge 5 separated from the white liquor to form a lean white liquor for use in solvent 35.

Figure 2 shows a preferred embodiment of the present invention. Where applicable, the same reference numerals as in Figure 1 are used.

10

In the process of the invention, effluents from ECF bleaching, typically acidic wastewater 24 and alkaline wastewater 25, are taken to a wastewater treatment plant to reduce their organic content. When the wastewater from the bleaching plant is treated in a biological wastewater treatment plant, its chemical oxygen demand (COD) 15 has been reduced by more than 70% and its organic chlorine content, as measured by the AOX measurement method, has decreased by more than 50%. If an anaerobic purification step is added to the system, then the color of the water to be treated is also significantly reduced. In addition, chemical purification methods based on either precipitation or oxidation of oxidizable compounds may be used for waste water. The chloride-containing organic material-purified waste water 43 is conducted according to the invention to the post-oxygen washing step so that at least 20% of the amount of liquid used for the brown pulp washing is the purified waste water. If there are two or more scrubbers, the purified effluent 43 is discharged from the latter to the last of the pulp. From this scrubber, the filtrate is conducted in a manner known per se in counter-current to the brown pulp treatment, whereupon the filtrate is recovered from the first brown pulp scrubber. The chloride-containing filtrate is either delivered directly to the evaporation plant 29 or used in the cooking processes for dilution and displacement, after which it ends up in a black liquor stream 28. Although the chloride content of this filtrate is increased in the system of the present invention, its sulfate or soda and does not pose a significant risk of corrosion or process in the treatment of brown pulp. As chlorides are now added to the system at different locations than they have been in the past, the material specification of the entire plant will need to be checked for equipment, piping, valves, and other surfaces that come into contact with process agents. This applies to all compartments of the chemical cycle 35, fiber line compartments, and those auxiliary compartments in which, according to the invention, pure water is now converted into a chloride-containing liquid.

The condensers 30 of the evaporator are used in the process according to the invention in Fig. 2 5 as washing water with bleach 16 where the condensate is fed through line 41. Condensers are used as a washing or dilution water of at least 1 m3 / adt mass of bleaching washing steps to reduce the total amount of waste water from the mill.

The condensate can also be used instead of fresh water to dry the pulp where the condensate 10 is fed through line 42.

The process of the invention also allows purified chloride-containing waste water from the bleaching plant to be used in the preparation of cooking chemicals. Purified wastewater is used in caustic soda filters, such as green liquor, sludge and / or lime sludge filters, as washing liquid. The filtrates separated by the filters, or some of them, are then introduced into the molten solvent 35. This also provides the chloride-containing liquid supplied to the system, which can be sufficiently removed in the recovery boiler process.

If required by the process fluid balance, purified wastewater may be removed from the process as required.

The black liquor from the evaporation plant is incinerated in a recovery boiler or (if necessary, the filtrate from the brown pulp wash is evaporated separately and taken to the recovery boiler alone or in combination with the black liquor). U.S. Patent Application 60/913322 25 describes a preferred method for treating a chloride-containing liquor in a recovery boiler. Thus, the treatment of chloride-containing liquids in the recovery boiler process does not result in stronger than normal corrosion and the recovery boiler process is well suited for separating the chloride-containing compounds from the process to prevent the accumulation of chlorine. Therein, the concentration of chlorine in the flue gases is maximized by increasing the temperature of the combustion zone 30 where the chloride-containing liquor is burned. It defines the advantageous combustion conditions in the recovery boiler where the chlorides start to evaporate into the flue gases and the process site where the chloride can be removed from the process. The introduction of chlorine into the flue gas can advantageously be enhanced by using oxygen or oxygen-enriched air. Thus, in the new process, the recovery boiler can be made into a factory chloride sink. Chloro-35 compounds are enriched in the ash from the flue gas mainly as sodium chloride and potassium chloride, from which chlorine can be separated and removed from the process, e.g.

36 of the aforementioned US application, or the like. Thus, the recovery boiler process which includes e.g. Reduction incineration, molten leaching, steam generation for energy and heat production, flue gas treatment and several ancillary processes are considered to be a sub-process of the recovery boiler process.

5

As can be seen from the foregoing, the method and apparatus of the present invention enable the pulp mill to reduce emissions to an absolute minimum. Although the most preferred embodiment has been described above in light of the present knowledge, it will be apparent to one skilled in the art that the invention may be modified in many ways within the broadest possible scope, which is limited by the appended claims.

Claims (22)

A method of treating pulp in a pulp mill comprising at least one alkaline boiling process utilizing pulp liquor to produce pulp, a brown pulp treatment with substantially closed liquid circulation, a pulp bakery utilizing elemental chlorine-free bleaching with at least one chlorine dioxide stage, chemical recovery plant with a black liquor evaporation plant and a boiler plant for chemical recovery, and wastewater treatment, characterized in that the chloride-containing wastewater is led to wastewater treatment, where they are treated with the aim of reducing their content of organic waste back to the wastewater of organic matter. 20% of the amount of liquid used for washing pulp consists of purified wastewater and the condensate derived from the evaporation plant is used in an amount of at least 1 m3 / adt mass. said as washing and / or dilution water in the bleaching washing step to reduce the total amount of wastewater, and that a chloride separation process is provided at the chemical recovery boiler to lower the chloride level in the chemical circulation. Process according to Claim 1, characterized in that condensate is used in the washing after a D0, Dr, D2 stage or some other D stage. 25 Process according to Claim 1 or 2, characterized in that condensate is used in the washing after an alkaline bleaching step, preferably an EO, EP or EOP step. Process according to claim 1, 2 or 3, characterized in that condensate is used in the washing after a peroxide (P) step. 30 Process according to claim 1, characterized in that condensate is used in the washing after a peracid step. Process according to Claim 1, characterized in that condensate is used in the washing after an ozone step. Process according to any of the preceding claims, characterized in that the wastewater is purified to lower its lignin content. Process according to claim 1 or 7, characterized in that the wastewater is biologically purified. 10 Method according to claim 7 or 8, characterized in that the purification of wastewater comprises a further chemical treatment. Process according to any of the preceding claims, characterized in that purified wastewater is used in the last washing step included in the pulp treatment, and the liquid flow flows countercurrent in the pulp treatment to evaporation, from where it is led to treatment in a soda pan process, where for checking the chloride level of the liquor circulation. Method according to any of the preceding claims, characterized in that the last washing step included in the pulp treatment is the washing of oxygen delignified pulp. Method according to claim 11, characterized in that purified wastewater is used as a diluent in some process filter during causticization and the thin liquor formed in this way is supplied to the solvents in the soda boiler. Process according to any of the preceding claims, characterized in that condensate from the evaporation plant is used as a main fresh water source for bleaching. 30 Process according to any of the preceding claims, characterized in that purified condensate from the evaporation plant is further used on the pulp dryer. 15. A process according to any preceding claim, characterized in that condensate from the evaporation plant is further used as sealing water in the rotating devices of the factory. 5 Process according to any of the preceding claims, characterized in that the wastewater is purified as separate fractions so that different kinds of fractions are formed, the properties of which are optimal for the respective ether use purposes. 17. A process according to any of the preceding claims, characterized in that oxidized white liquor is used as a major alkaline source in the bleaching and neutralization of the wastewater. 18. A process according to any of the preceding claims, characterized in that at least 20%, preferably at least 40%, of the wastewater is returned to the pulp mill. Process according to any one of the preceding claims, characterized in that condensate from the evaporation plant is used on the dryer section for washing blankets. 20 20. A process according to any of the preceding claims, characterized in that the wastewater that is fed is heated with the heat extracted from the wastewater which is led to purification and heated wastewater is used at the pulp mill. 25 21. A method according to claim 20, characterized in that the temperature of the wastewater which is led to the purification and fed from there is controlled in a cross-flow heat exchanger. 30 22. A process according to any of claims 1-19, characterized in that the purified wastewater is heated with the heat extracted from the cookery liquid circulation.