A METHOD FOR TREATING ACID AND ALKALINE WASTE STREAMS FROM A BLEACH PLANT SEPARATELY

Abstract: Acid and alkaline waste water from a bleach plant comprise different amounts of inorganic and organic materials and have varying salts contents. If the acid and the alkaline waste water are mixed, it becomes more difficult to separate the different substances from the process flow in a subsequent treatment and it also results in larger amounts of waste water, which is disadvantageous in methods where aiming at closing up the bleach plant. Also, problems arise due to the formation of incrustations on heat-transfer surfaces. This invention relates to an improved method for treating the process waste water from a bleach plant, where the acid and the alkaline process flow being separately concentrated in evaporators, whereupon the salt content of the acid process flow is reduced by electrodialysis and the alkaline process flow is combusted. This method enables the recirculated amount of water to be reduced and the problems regarding the processing equipment to be avoided, and also contributes to a higher degree of closing, even if the bleaching involves clorine dioxide (ECF).

Description: A method for treating acid and alkaline waste streams from a bleach plant separately. This invention relates to an improved method for treating lignocellulose-containing process waste water from a bleach plant. In the method according to the invention, process waste water, which comprises of at least one acid process flow and one alkaline process flow, is treated, the acid and the alkaline process flow being separately concen¬ trated in evaporators, whereupon the salt content of the acid process flow is reduced by electrodialysis and the alkaline process flow is combusted. Acid waste water and alkaline waste water from a bleach plant comprise different amounts of inorganic and organic material and have varying salt contents. Thus, the alkaline waste water comprises a great deal of organic material and alkali, whereas the acid waste water primarily comprises inorganic salts, such as chlorides and chlorate and so on. If the acid and the alkaline waste water are mixed, it becomes more difficult to separate the different substances from the process flow in a subsequent treatment, which is disadvantage- ous in methods aiming at closing up the bleach plant. Also, problems arise in the processing equipment for the evaporation of waste water comprising of a mixture of acid and alkaline process flows, inter alia as regards the formation of incru¬ stations on heat-transfer surfaces. Another problem is that such a mixture results in larger amounts of waste water, which of course is disadvantageous if one aims at closing up the bleach plant. The present invention now provides a method for treating lignocellulose-containing process waste water from a bleach plant, in which the recirculated amount of water is reduced and the problems regarding the processing equipment are avoided, which contributes to a higher degree of closing, regardless of whether the bleaching involves chlorine dioxide (ECF) or a completely chlorine-free bleaching method (TC7) is employed. Thus, the method according to the invention enables the evaporation of waste water comprising salt components of chlorides and/or chlorates, and pulp manufacturers may use existing bleaching chemicals, processing equipment and bleaching sequences, while the quality of the pulp produced is maintained. Thus, the invention relates to a method for treating lignocellulose-containing process waste water from a bleach plant, the process waste water comprising of at least one acid process flow and one alkaline process flow, which are separa¬ tely concentrated in evaporators, whereupon the salt content of the acid process flow is reduced by electrodialysis and the alkaline process flow is combusted. The method according to the invention reduces the amount of water recirculated, avoids the problems regarding the processing equipment and contributes to a higher degree of closing, even if the bleaching involves chlorine dioxide (ECF) . Owing to the reduction of the process flow undergoing electrodialysis, the demands placed on the processing equip- ment are lessened, resulting in a reduction of the investment costs. Since it is only the acid process flow that is sub¬ jected to electrodialysis, a higher salt concentration can be obtained resulting in a more effective process. Acid process waste water and alkaline process waste water from the bleach plant are brought to at least one stage, where they are concentrated by evaporation. In the method according to the invention, the evaporation is suitably performed in a falling-film evaporator with recirculation. Preferably, the falling-film evaporator works on the principle of mechanical vapour compression, so that no energy need normally be supplied to the evaporator from outside, in addition to the energy required by fans, pumps, and c-her processing equipment. The energy consumption is very low. In prior-art evaporators, the heat-transfer elements are made of metallic material. In the present invention, however, use is preferably made of a new principle, according to which the heat-transfer elements comprise partly of a thin and flexible sheet, of which at least the one surface is made of plastic and which preferably is shaped as a bag having a thickness of about 100 μm or less. Preferably, the sheet has a thickness not exceeding about 50 μm, and most preferred a thickness not exceeding about 30 μm. The sheet may have a smallest thickness of about 20 μm, suitably about 15 μm. A suitably sheet material may, for instance, comprise of a mixture of plastic material, such as e.g. polyethylene, optionally including fillers or carbon fibres. Heat-transfer elements of plastic involve a high corrosion resistance, a low weight and a low price. The solution to be evaporated is preferably conducted to the upper part of the evaporator, where a distributor dis¬ tributes the solution evenly, such that it flows downwards on the outside of the bag structure. Any vapour formed is suitably, by internal circulation, conducted to the inside of the bag structure and emits heat while condensating to liquid. Unevaporated liquid on the outside of the bag structure is recirculated in the system. The condensate formed is trans¬ ferred to the condensate cleaning stage. Concentrate is removed for further concentration. The waste water solution concentrated in the evaporator contains salts, primarily oxalates and sulphates, which precipitate in the evaporation and deposit on the machine equipment. It is especially inconvenient that the deposits accumulate on the heat-transfer surfaces, impairing the heat transfer and the flow pattern in the evaporator. By using for example a plastic construction as in the method according to the invention, the deposits can be gradually broken off from the heat-transfer surface, for instance by pressure changes in the evaporator causing the bag structure (from inside) to expand and to retract, whereupon the deposits can be separated by screening of the liquid flow. The deposits may also be removed by mechanical means. Fluidized solid particles which have a polishing action en the surfaces inside the evaporator may be used. The particles can for instance be made of glass, ceramic beads, chopped wire, metal shot, sand or gravel. The evaporation results in the formation of a condensate which mainly contains the volatile components of the effluent from the bleach plant, as well as water. The volatile compo¬ nents, which have been released during the bleaching and been evaporated during the evaporation, preferably consist of low- molecular organic matter, such as methanol and chloroform. In order to be able to reutilise this condensate as process water recycled to the bleach plant, the condensate usually has to be cleaned of most of the organic components. This cleaning, which is termed "flashing" or "stripping", can be performed by conventional methods, suitably in a stripping column. This cleaning operation may also be supplemented with and/or replaced with biological cleaning. One embodiment of the invention will now be described in more detail with reference to the accompanying drawings, in which Fig. 1 shows an example of an evaporator, Fig. 2 shows an example of an electrodialysis device, and Fig. 3 is a block diagram illustrating a processing plant. These illustrated devices are suitably for carrying out the method according to the invention. Fig. 1 is a schematic view of an evaporator, in which process waste water (1) is conducted to the upper part of the evaporator, where a distributor (2) distributes it evenly so that it flows downwards on the outside (3) of the bag con¬ struction. The vapour (4) formed is, by means of a fan (5), recirculated to the inside (6) of the bag construction and emits heat while condensating to liquid. Unevaporated liquid (7) on the outside of the bag construction is recycled (8) in the system. The condensate (9) formed is transferred to a condensate-cleaning stage, and the concentrate (10) is further concentrated. The degree of evaporation may range from about 0.5% to about 15% dry solids. Suitably, the degree of evaporation is at least about 1% dry solids, preferably at least about 3% dry solids, and most preferred at least about 5% dry solids. Higher concentrations can be obtained by stepwise evaporation, and about 15% dry solids may, for instance, be obtained in a last stage. Fig. 2 is a flow chart illustrating an electrodialysis device according to a preferred embodiment of the invention. The electrodialysis cell comprises at least one anion-selec- tive membrane (A) and one cation-selective membrane (K) arranged between an anode and a cathode. As a rule, the cell comprises multiple pairs of alternating anion-selective and cation-selective membranes arranged between an anode and a cathode. Between the anode and the cathode, pairs of membranes form chambers having inlets and outlets for the supply of liquids to and the withdrawal of liquids from the chambers. An anode solution (30) is supplied at the anode, and a cathode solution (31) is supplied at the cathode. When the evaporated acid process flow (32) is introduced into the cell, the anions will migrate through the anode-selective membranes towards the anode, and the cations will migrate through the cation- selective membranes towards the cathode. The aqueous solution will be depleted of salt and is therefore termed diluate (D) . The concentrate (C) is preferably produced in every other chamber. The diluate can be at least partly recycled (33) to the evaporator in order to reduce the salt concentration therein, enabling a more energy-efficient evaporation and reducing the formation of incrustations and, hence, the need of cleaning the evaporator. Further, the diluate enables evaporation at a lower temperature of the heating medium, as well as recycling to the washing stages in the bleaching sequence or to washing stages (scrubbers) or other sites in the pulp mill where additional water is required. The diluate may also undergo one or more electrodialysis treatments in order to further reduce its salt content. The electrodialysis can be carried out in electrodialysis stacks, which operate in parallel and/or in series, and with liquid flows that are parallel and/or connected in series. It is also possible to operate the stacks in an electrodialysis reversal (EDR) mode. EDR means that polarity is switched and the compartments are interchanged at a regular time interval, up to two to three times per hour. In this way the anode becomes cathode, the previous diluate compartments then operates as concentrate compartments and vice versa. By operating the stacks in an electrodialysis reversal mode, the problems with scale formation on the membranes are diminished and a higher level of impurities such as calcium, sulphate, oxalate etc., can be tolerated. The membranes in the electrodialysis cell can be equipped with monoion selective membranes. The electrodialysis results in a concentrate of the inorganic salts in an aqueous solution. The salt concentrate comprises, inter alia, chlorate, chloride and sulphates. After cleaning and processing, these salts can be recovered and used in e.g. a plant for producing fresh bleaching chemicals or be used as road salt. Fig. 3 is a block diagram illustrating a preferred embodiment of the invention. After evaporation, the acid process flow (1) can be concentrated further (2) , preferably by evaporation, whereupon the process flow may be brought to a separate treatment (3) for the precipitation of the dis¬ solved metals (4) , which are precipitated as sparingly-soluble salts by alkalisation/carbonate addition to the resulting concentrate. The metal salts precipitated may, for instance, be calcium carbonate, magnesium hydroxide, other metal hydroxides and metal sulphides. After the separation of the precipitates, the salt content, for instance sodium chloride, of the concentrate is reduced by electrodialysis (5) . The salt concentrate is separated (6) . The desalinated process flow (7) is suitably recycled to the evaporation/concentration (2;9) of the alkaline and acid process flow (1,*8) . Alkalisaticn/- carbonation ir. the precipitation stage (3) preferably involves an alkali source available at the mill, for instance green liquor or white liquor. Preferably, green liquor or white liquor is used in order to avoid any formation of hydrogen sulphide. Preferably, use is made of alkali from the combus¬ tion of the alkaline concentrate. Thus, the alkaline process flow can be further concentrated (9) , resulting in a concen¬ trate (10) which preferably is combusted (11) to ashes (12) . The alkaline ashes (12) from the combustion are suitably employed as alkali source in the alkalisation/carbonatior. of the precipitation stage (3) . The acid process flow may, for instance, have zhe following composition: 0-2.5 g/1 Na * , 0-2 g/1 Cl " , 0-0.5 g/1 C10 3 ", and other anions, such as S0 4 2" . The alkaline process flow may, for instance, have the following composition: 0-5 g/1 Na * , 0-2 g x l Cl " , 0-0.5 g/1 Cl0 3 " , and other anions, such as OH " , HC0 3 :" , C0 3 2" , and S0 4 2" . Both process flows may also comprise " Ca and Ba ions. In addition, there are a great number of organic anions present. The alkaline process flow which, apart from water, essentially comprises organic material and alkali, is combusted in the soda recovery boiler, the lime sludge reburning kiln, the bark boiler or a special furnace or kiln intended for this purpose. If the alkaline process flow has high salt contents, it may, after evaporation, undergo a treatment for the precipitation or separation of salts, the process flow thus cleaned of organic material may be recycled to the evaporator in order to be concentrated. Then, the concentrate is combusted in a furnace allowing the handling of ash having a low melting point. The tempera- ture suitably is above 800°C, and the residence time is at least 1 second, such that any dioxins present are destroyed. The temperature is preferably above 900°C and most preferred above 1000°C. Mainly all the organic substances present are combusted. The flue gases are suitably cooled to prevent any formation of chlorine-organic substances. Gas and ash are formed upon the combustion. The formed ash may contain inorganic salts such as chlorides or sulphates of sodium and/or potassium, as well as a certain amount of heavy metals primarily originating from the wood. Preferably, the ash thus has to be subjected to an ash or slag treatment, resulting in the precipitation of heavy metals. The remaining salts may be recovered and used in, for instance, a plant for chlorate production. After cleaning, the salt may also be used as road salt. Since most of the ash/slag is easily soluble in water, it may not be deposited without pretreatment. In this treatment, which suitably is carried out by dissolution in water, easily-soluble and natural salts, preferably chlorides or sulphates of sodium and/or potassium, will be separated from sparingly-soluble salts and impurities. Heavy metals are less soluble and may, after treatment, be deposited. The treatment can be made more selective by chemically rendering the heavy metals sparingly soluble. The gas may be cleaned by prior-art methods in a gas- cleaning plant. Suitably, the gas is cooled in a first stage, resulting in a cleaning effect. In a second stage, the gas can be further cleaned. The gas cleaning may result in a minor amount of slag or ash. The energy developed during the combustion may be recovered in the flue-gas cleaning and be used at a previous stage, for instance when concentrating the solution. In a preferred mode of carrying out the invention, at least one process flow from the bleach plant, which comprises acid or alkaline process waste water containing organic and inorganic, undissolved and/or dissolved material, is prefera¬ bly separated and treated in a preliminary cleaning treatment. Such a treatment can be carried out, before and/or after the evaporation, on the acid process flow in order to avoid problems with contaminated membranes in the following electrodialysis. Particulate material, such as fibres, resins, lignin, oxalates and sulphates, which may cause problems in the concentration treatment, is removed from the solution by suitably separation methods, such as precipitation, filtra- tion, centrifugation, flotation, membrane filtration, ultra- filtration, sedimentation, nanofiltration or other mechanical, chemical or combined separation methods. For instance, dissolved material can be treated and precipitated by adding pH-adjusting chemicals and complexing agents. Treatment is suitably carried out by sedimentation in a lamella thickener with addition of an acid. It comprises of a number of inclined plates, preferably filter-plates, stacked closely together. The process flow is fed from the side with addition of an acid such as sulphuric acid. The flow moves upward between the plates where the thus cleaned and treated flow is drawn off, suitably in an overflow, while the solids settle onto the plate surface and slide down ir- o the bottom of the lamella thickener. Lamella thickeners are known as such and are described in the "Kirk-Othmer Encyclopedia of Chemical Technology", third edition (1352), vol. 20, pages 572-573, which hereby is incorporated by reference. The preferred nanofiltration is carried out by filtering the solution, at high pressure, through a filter, which is more selective for small ions such as Cl " and K", than larger ions e.g. sulphate. Thus, a chloride and potassium enriched concentrate is separated from particulate material such as a concentrate of sulphate, depleted of chloride. The concen¬ trated solution of particulate material may be brought to incineration. Calcium and oxalate are preferably separated before the electrodialysis by filtration and a subsequent addition of sodium carbonate in order to precipitate calcium carbonate. Low molecular organic material may be destroyed with hydrogen peroxide and ultraviolet light. The organic material is separated easier if oxidized with air. The pH at the precipitation with acid is preferably adjusted within the range of from about 2 up to about 7. The pH selected depends on the pH of the water to be treated. Suitably precipitating chemicals are, for instance, sulphuric acid, hydrochloric acid or an organic acid, such as formic acid, optionally combined with organic and/or inorganic flocculating chemicals. Chemicals, for instance complexing agents, may be added in the electrodialysis step in order to avoid precipitation of harmful materials such as Mn, Ca, oxalates. In a preferred mode of carrying out the present inven¬ tion, the waste water from the bleach plant can be "flashed" before the evaporation stage. Evaporation by vacuum-vapour recompression in accordance with the invention is a suitably evaporation method, which may be carried out at a temperature that is lower than that of the waste water from the bleach plant, the heat content of the waste water being utilised to strip the volatile components even before the evaporation. The temperature of the waste water from the bleach plant may, for instance, be up to about 70°C, and the temperature during the evaporation may be about 50°C. By reducing the pressure to -he vapour pressure of the lower temperature, the volatile sub¬ stances will be stripped, while cooling is brought about to a temperature level suitably for the evaporator. Conveniently, cooling takes place to a temperature below 65°C. This makes it possible to reduce the extensive amount of processing equip¬ ment required for the condensate-cleaning stage. Also, this method is much used for cleaning various other liquid flows in the pulp mill. "Flashing" may also be carried out directly to a column for stripping according to a prior-art method with a certain reflux, in order to obtain an additional cleaning effect without consuming external energy. Volatile substances such as methanol may be used as reducing agent in chlorine dioxide production, or in the production of formic acid which can be used as a precipitation chemical in a treatment step before electrodialysis. Volatile substances such as methanol may be separated practically completely due to the relatively low temperature in the evaporation, thus resulting in a very pure product and which therefore can be used directly in the chlorine dioxide production. Volatile substances may also be brought to a furnace, preferably in the combustion step according to the present invention, where they constitutes as fuel. In one mode of carrying out the invention, the concen¬ trated solution, preferably the alkaline process flow, from the evaporator is treated in an additional concentration stage, for instance by further evaporation. Use can be made of a prior-art evaporation method, such as evaporation in a plate heat exchanger or a crystalliser. Suitably, the solution is concentrated to at least about 20% dry solids, preferably to at least about 30% dry solids, and most preferred to at least about 50% dry solids. The upper limit is not critical, but is motivated primarily by reasons of process technique. Any bleaching sequence may be used in the method according to the invention. Thus, the bleaching may involve chlorine dioxide as bleaching agent, so-called ECF bleaching, or use can be made of a completely chlorine-free process, so- called TCF bleaching, using such bleaching agents as ozone, hydrogen peroxide or peracetic acid. Suitably, the pulp is at least bleached with chlorine dioxide as o ly chlorine-contain¬ ing bleaching agent. Use can be made of such bleaching sequences as D-E-D-E-D (D=chlorine dioxide, E=alkaline extraction) . Optionally, the last extraction stage may be dispensed with. Also other bleaching sequences, such as D-Q-? (Q=complexing agent and P=peroxide-containing compound) , may be used. The first D-stage, in the prebleaching, can be replaced by a (C + D) stage (C=chlorine) . The amount of active chlorine used/required in the (C + D) stage is defined as the charge factor CF according to: CF = total active chlorine in kg/ton of pulp/ kappa number before the (C + D) stage. According to a preferred embodiment of the present invention, treatment is carried out with technical chlorine dioxide at a charge factor of up to 2.0, preferably within the range of from 0.6 to 1.8. The most preferred range for the charge factor is from 0.75 to 1.25. In order to be able to close up the bleach plant, it is necessary, for reasons of practicality as well as economy, to reduce the volume of waste water. Previously, fresh water was used when washing and dewatering the pulp, the waste water volumes being in the order of 50 m 3 /tonne of pulp. The fresh water has now mainly been replaced with recirculated white water from one or more bleaching or extraction stages, as well as redistribution and recycling of filtrate. In washing or dewatering according to the present invention, one may be careful with the water consumption, use filter presses and/or e.g. countercurrent washing. In a preferred method, no washing takes place between the first chlorine-dioxide stage and the first extraction stage. Neither is any washing performed between the second chlorine-dioxide stage and the subsequent extraction. By carrying out e.g. this method and/or being careful with the water consumption and/or using e.g. filter presses in the washing stages, the water consumption is reduced, suitably to below about 20 m 3 /tonne of pulp, prefera¬ bly to below about 12 m 3 /tonne of pulp, and most preferred to below about 10 m 3 /tonne of pulp. According to one embodiment of the invention the acid waste water is obtained by collecting white water from -he bleaching or treatment steps during the bleaching which are carried out under acidic conditions, while the alkaline waste water is obtained by collecting waste water from corresponding alkaline steps. The acid waste water is evaporated and treated by electrodialysis while the alkaline waste water is combusted in the soda recovery boiler, with or without preceding evaporation. The treatment of waste water according to the present invention can be a part of an overall process for closing up bleach plants. Such a process, may comprise the following steps: a) preliminary cleaning, b) concentration by evapor¬ ation in a falling-film evaporator with internal circulation to form an evaporation residue having a dry solids content of at least 1%, as well as a condensate, c) increasing the dry solids content of the evaporation residue from stage b) to at least 20% to form a concentrate, d) combusting the concentrate to ash and gas, e) treatment of the ash in order to form a salt and separate impurities, f) cooling and cleaning of the gas from stage d) ; or g) treating the evaporation residue from step b) with precipitating chemicals and the resulting precipitate is separated from the solution, h) electrodialytic treatment of the solution separated from the precipitate, in order to form an electrodialysis concentrate containing salts, as well as a diluate depleted of these salts. The invention is not restricted to the embodiments described above. The figures in % and parts given in the description and the appended claims are all by weight, unless otherwise stated.

Claims: CLAIMS 1. A method for treating lignocellulose-containing process waste water from a bleach plant, the process waste water comprising of at least one acid process flow and one alkaline process flow, c h a r a c t e r i s e d in that the acid and the alkaline process flow are separately concentrated in evaporators, whereupon the salt content of the acid process flow is reduced by electrodialysis and the alkaline process flow is combusted. 2. A method as claimed in claim 1, c h a r a c t e r i ¬ s e d in that the evaporated acid process flow is, for pur¬ poses of precipitation, subjected to a treatment with precipi¬ tating chemicals. 3. A method as claimed in claim 1 or 2, c h a r a c t e- r i s e d in that the electrodialysis is carried out in one or more electrodialysis devices which, electrically and/or with regard to liquid flow, are connected in series and/or in parallel, each device comprising at least one anion-selective membrane and one cation-selective membrane, a number of unit cells formed of a number of anion-selective and cation- selective membranes that are alternatingly arranged in the form of a stack between an anode and a cathode, the membranes forming between them chambers with inlets and outlets for the supply of an evaporation-residue saline solution to these chambers, and that, with the aid of electric current between the anode and the cathode, an ion migration is produced from the evaporation-residue saline solution through the ar.ion- selective and cation-selective membranes, solvent flows passing through the adjoining chambers, to form at least one diluate flow of the thus-depleted evaporation-residue saline solution, as well as at least one first electrodialysis concentrate flow comprising ions that have migrated from the evaporation residue, the diluate being partly circulated to an electrodialysis process and/or to the evaporation stage and/or to washing stages in the bleach plant, and the electrodialysis concentrate being, after cleaning and processing, recovered and used for producing fresh bleaching chemicals or used as road salt. . A method as claimed in any one of the preceding claims, c h a r a c t e r i s e d in that salt formed in the process is used for producing chlorate. 5. A method as claimed in any one of the preceding claims, c h a r a c t e r i s e d in that salt formed in the process is treated in order to separate chlorate, which is used for the production of chlorine dioxide. 6. A method as claimed in claim l, c h a r a c t e r i - s e d in that after evaporation, the alkaline process flow is subjected to a treatment in order to precipitate or separate salts, the process flow thus cleaned of organic material being recycled to the evaporator in order to be concentrated. 7. A method as claimed in any one of the preceding claims, c h a r a c t e r i s e d in that the process waste water comprises chloride- and/or chlorate-containing salts. 8. A method as claimed in claim 1, c h a r a c t e r i ¬ s e d in that the concentration is carried out in a falling- film evaporator with recirculation. 9. A method as claimed in claim ^ c h a r a c t e r i ¬ s e d in that the degree of evaporation is at least about 5% dry solids. 10. A method as claimed in any one of the preceding claims, c h a r a c t e r i s e d in that the acid process flow is concentrated before electrodialysis, whereupon dissolved metals are precipitated and separated by alkalisa- tion/carbonate addition to the concentrated, acid process flow.

Inventors: ENGSTROEM MIKAEL, JOHANSSON HANS, LANDFORS JOHAN, ENGSTROEM, MIKAEL, JOHANSSON, HANS, LANDFORS, JOHAN
Applicants: EKA NOBEL AB, ENGSTROEM MIKAEL, JOHANSSON HANS ARNE VALENTIN, LANDFORS JOHAN, ENGSTROEM, MIKAEL, JOHANSSON, HANS, LANDFORS, JOHAN
Application year: 1995
Main classification: D21C11
IPC codes: B01D5, C02F1, D21C9, D21C11