Classifications

• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
  • D21C9/00—After-treatment of cellulose pulp, e.g. of wood pulp, or cotton linters ; Treatment of dilute or dewatered pulp or process improvement taking place after obtaining the raw cellulosic material and not provided for elsewhere
  • D21C9/10—Bleaching ; Apparatus therefor
  • D21C9/1026—Other features in bleaching processes
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D5/00—Condensation of vapours; Recovering volatile solvents by condensation
  • B01D5/0078—Condensation of vapours; Recovering volatile solvents by condensation characterised by auxiliary systems or arrangements
  • B01D5/009—Collecting, removing and/or treatment of the condensate
• • C—CHEMISTRY; METALLURGY
  • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F1/00—Treatment of water, waste water, or sewage
  • C02F1/02—Treatment of water, waste water, or sewage by heating
  • C02F1/04—Treatment of water, waste water, or sewage by heating by distillation or evaporation
  • C02F1/048—Purification of waste water by evaporation
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
  • D21C11/00—Regeneration of pulp liquors or effluent waste waters
  • D21C11/0021—Introduction of various effluents, e.g. waste waters, into the pulping, recovery and regeneration cycle (closed-cycle)
  • D21C11/0028—Effluents derived from the washing or bleaching plants
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
  • D21C11/00—Regeneration of pulp liquors or effluent waste waters
  • D21C11/0042—Fractionating or concentration of spent liquors by special methods

Definitions

• This invention relates to an improved method for treating lignocellulose-containing process waste water from a bleach plant.
• process waste water which comprises of at least one acid process flow and one alkaline process flow
• 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.
• the alkaline waste water comprises a great deal of organic material and alkali
• 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.
• 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.
• ECF chlorine dioxide
• TC7 completely chlorine-free bleaching method
• 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 electrodialysis
• 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.
• the evaporation is suitably performed in a falling-film evaporator with recirculation.
• 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.
• the heat-transfer elements are made of metallic material.
• 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 ⁇ m or less.
• the sheet has a thickness not exceeding about 50 ⁇ m, and most preferred a thickness not exceeding about 30 ⁇ m.
• the sheet may have a smallest thickness of about 20 ⁇ m, suitably about 15 ⁇ m.
• 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.
• the deposits accumulate on the heat-transfer surfaces, impairing the heat transfer and the flow pattern in the evaporator.
• 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.
• 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.
• Fig. 1 shows an example of an evaporator
• Fig. 2 shows an example of an electrodialysis device
• 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.
• 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.
• 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.
• 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.
• EDR electrodialysis reversal
• 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.
• 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.
• 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.
• green liquor or white liquor is used in order to avoid any formation of hydrogen sulphide.
• use is made of alkali from the combus ⁇ tion of the alkaline concentrate.
• 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.
• 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.
• 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.
• 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.
• the gas may be cleaned by prior-art methods in a gas- cleaning plant.
• the gas is cooled in a first stage, resulting in a cleaning effect.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• the concen ⁇ trated solution, preferably the alkaline process flow, from the evaporator is treated in an additional concentration stage, for instance by further evaporation.
• an additional concentration stage for instance by further evaporation.
• 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.
• 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.
• the pulp is at least bleached with chlorine dioxide as o ly chlorine-contain ⁇ ing bleaching agent.
• the last extraction stage may be dispensed with.
• other bleaching sequences such as D-Q-?
• 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.
• 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.
• 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.
• 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.

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• Chemical & Material Sciences (AREA)
• Life Sciences & Earth Sciences (AREA)
• Engineering & Computer Science (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Hydrology & Water Resources (AREA)
• Environmental & Geological Engineering (AREA)
• Water Supply & Treatment (AREA)
• Organic Chemistry (AREA)
• Wood Science & Technology (AREA)
• Water Treatment By Electricity Or Magnetism (AREA)
• Separation Using Semi-Permeable Membranes (AREA)
• Heat Treatment Of Water, Waste Water Or Sewage (AREA)

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Cited By (8)

Citations (4)

• 1995
  • 1995-10-03 WO PCT/SE1995/001121 patent/WO1996011299A1/en not_active Application Discontinuation
  • 1995-10-03 BR BR9509188A patent/BR9509188A/pt not_active Application Discontinuation
  • 1995-10-03 CA CA002201117A patent/CA2201117A1/en not_active Abandoned
  • 1995-10-03 EP EP95934908A patent/EP0784721A1/en not_active Ceased
  • 1995-10-03 JP JP8512515A patent/JPH10506964A/ja active Pending
  • 1995-10-03 AU AU37121/95A patent/AU3712195A/en not_active Abandoned
• 1997
  • 1997-04-03 NO NO971504A patent/NO971504D0/no unknown
  • 1997-04-16 FI FI971605A patent/FI971605A/fi active IP Right Revival

Patent Citations (4)

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