WO1995018884A1

WO1995018884A1 - Process for the recovery of chemicals and energy from pulp bleaching effluent

Info

Publication number: WO1995018884A1
Application number: PCT/SE1995/000002
Authority: WO
Inventors: Nils Mannbro
Original assignee: Nils Mannbro
Priority date: 1994-01-04
Filing date: 1995-01-04
Publication date: 1995-07-13
Also published as: SE9400016L; SE9400016D0

Abstract

A process for kraft paper fibre pulping and bleaching of lignocellulose material by delignification with alkaline liquors in various digestion and bleaching sequences. Black liquor is displaced through the wood chips, charged to the pulping digester, thereby enabling the use of bleaching stage filtrate or composite bleach plant effluent for displacement of cooking liquor rather than the conventional displacement through brownstock washing stages from which black liquor is discharged. Carry-over with pulp of cooking liquor alkali contributes to alkalinity in the processing steps of the effluent so that the inorganic combustion residue has an excess of soda as compared to sulphate and optional chloride. By withdrawing black liquor directly from charged wood chips material, no dilution occurs at this operation while carry-over of cooking liquor diluted with bleach plant effluent can be economically concentrated, e.g. by Mechanical Vapor Compression Evaporation (MVCE).

Description

PROCESS FOR THE RECOVERY OF CHEMICALS AND ENERGY FROM PULP BLEACHING EFFLUENT

General background

Known methods for abatement of pollution from delignification and bleaching stages of cellulose pulp production comprise combustion of effluent in the alkali recovery process of kraft pulp mills. In this respect the most cost intensive investments involve a furnace for recovery of alkali from black liquor and process effluent. Historically, at an early phase of pollution abatement, mill effluent was subjected to external treatment with energy consuming aeration. Meanwhile, introduction of alkaline oxygen delignification (oxynation) for bleaching of brownstock called for additional capacity of black liquor furnacing to accommodate spent liquor (oxyli- quor) reclaimed from washing of the treated stock (oxystock) . Cooking requires, for brownstock aimed for oxynation at acceptable cellulose viscosity, high sulfidity of white liquor or, better, high sulfide concentration in the early stage of cooking. When white liquor, i.e. at certain sulfidi¬ ty, is used as oxynation, alkali a pre-oxidation of sulfide to thiosulphate, is applied. Hence, there is an interrelation between on the one hand, valuable sulfide for selective cooking delignification and, on the other hand, elimination of sulphide harmful for selective oxynation (oxynation = alkaline oxygen delignification) delignification. The latter is actually depriving the white liquor of sulfide and effec¬ tive alkali and consumes additional heat for reduction of sulphate formed by charge of (additional) oxygen for bleaching.

With regard to environmental restrictions applied at present, even advanced bleaching sequences of oxynation and chlorine dioxide bleaching, e.g. Elemental Chlorine Free (ECF) bleaching, would be supplemented by some external effluent treatment or, alternatively, Totally Chlorine Free (TCF) bleaching. The latter is required to avoid any discharge of chlorate from chlorine dioxide which is marginally required for high brightness.

Problems deriving from TCF and closed-cycle process involve, besides COD of final effluent, handling of non-process ele¬ ments from raw material (wood) . These are nutrients and require oxygen-based bleaching chelating agents for the elimination of transition metals. Particular corrosion pro- blems are caused by the accumulation of potassium from raw material combined with chloride from combustion of bleaching effluent in the black liquor furnace.

As yet, many mills use brownstock chlorination bleaching and the effluent from such bleaching may be incinerated separate¬ ly but its combustion in the kraft process is out of the question. Among kraft mills, a great deal use oxygen-based bleaching agents in regular TCF-sequences. Oxygen-based agents are applied in alternating alkaline stages (oxygen, peroxide etc.) and acid stages (ozone) as well as peracetic acid, persulphate etc. The oxygen-based agents (besides oxy¬ gen) are costly which emphasizes the need for recovery capa¬ city for effluent at low original concentration of sodium compounds and for combustible substance, dissolved from wood/pulp.

Development of the ENVIM process

It is known that, irrespective of reactions of halogen-based or oxygen-based agents, bleaching effluent can be incinerated separately from the black liquor furnace. Such processing is denoted ENVIM and was disclosed by the present inventor in CA 1 064 206 (SE 7412638-4) . In this process total effluent from brownstock system and bleaching sequences is incinerated. In the following reference is made to ENVIM-type liquors as follows: Regeneration liquor = conventional mixture of oxyliquor (alternatively any non-halogen bleaching effluent) and black liquor processed in kraft process;

Black liquor = final spent liquor from cooking as passed from brownstock washing to the kraft recovery plant;

Chloride liquor = effluent from halogen bleaching which wholly or partly is passed to incinerator.

The example and figure of said patent exhibit successive liquor displacement zones through a moving layer of black- stock (brownstock plus black liquor) washed to brownstock and from which layer essentially non-diluted black liquor and chloride liquor are withdrawn separately. The latter compri- ses a volume of between 3 and 5 m 3/t of which 2-4 m3/t derive from actual bleaching steps (t = metric ton pulp) . It was mentioned in said patent that use of gaseous oxidative blea¬ ching agents and reclamation of effluent by pressing, lowered the effluent volume for ENVIM processing. Apparently, this technique failed to gain attention and an environmental study in 1980 on behalf of the Swedish pulp industry indicated that any separate incineration of bleach plant effluent was un¬ realistic.

Ozone for kraft pulp bleaching was given a favorable prog- nosis already in 1979 by R.PO. Singh (The Bleaching of Pulp, TAPPI Press, Atlanta, GA, USA) . Since then, novel technology has lowered power consumption of ozone generation and increa¬ sed the concentration thereof in reacted oxygen. A number of kraft mills have substituted ozone for halogen-based blea- ching of oxystock.

SE patent application 8008073-2 (Mannbro) - accepted for publishing - discloses ENVIM treatment of brownstock or oxystock respectively by applying acid bleach plant filtrate for de ineralization and removal of non-process elements essentially deriving from wood. Thus, Ca 2+ and Mg2+ may otherwise accumulate in alternate acid and alkaline stages and precipitate slimy, colored substance on fibre. It is by now known that such bleach plant filtrate supplies complex formers which as well as addition of chelating agents are required particularly for closed cycle oxygen-based bleaching using e.g. peroxide and ozone. Chelating agents are conside¬ red necessary for eliminating transition metals from TCF- bleaching, becasue said metals decompose catalytically perox¬ ide, ozone and other oxygen-based agents and thereby degrade the cellulose component. Acidification prior to regular brownstock oxynation is useful as such and can replace charge of Mg-compounds.

SE patent application 8102081-0 (Mannbro) - accepted for publishing - comprises modified ENVIM and claims separation of make-up sodium sulphate from spent acid product from chlo¬ rine dioxide generator. This acid product is neutralized by bleach plant effluent and evaporated for crystallization of sodium sulphate from chloride enriched mother liquor which is passed to the incinerator.

SE patent application 8102082-8 (Mannbro) - laid open - discloses oxygen reinforced alkaline extraction (EO) , here denoted oxtraction, and alternatively recycling of EO-filtra- te to a pre-treatment stage in order to reduce consumption of alkali as well as subsequent chlorine dioxide demand.

Articles published in Nordic Pulp and Paper Research Journal (1989, page 189 and 1990, page 134; Mannbro), entitled "Clo¬ sed-cycle Recovery and Combustion of Bleach plant Filtrate", are directed to ENVIM applied on halogen-based bleaching sequences whereby optional oxyliquor is combusted in the recovery furnace.

For some reasons ENVIM incineration of halogen-based effluent is not practised. On the contrary, an impressive development of oxynation of low kappa brownstock obtained by selective digestion methods has facilitated the elimination of elemen- tal chlorine (ECF-bleaching) . Moreover, it is predicted elsewhere that pulp of required quality can be produced according to TCF and all effluent will eventually be TEF- processed in the black liquor furnace for total alkali rege- neration. The cost of oxidizers increases, however, the cost per unit produced pulp.

Selective digestion for less bleaching delignification

It is known that delignification selectivity, i.e. with regard to cellulose viscosity of bleached pulp, is improved by latter phase delignification at proper hydroxide concen¬ tration but rather low Na-concentration. This is attained by distributed charge of white liquor containing reactive sul¬ phur compounds but preferably at low sulfidity in the final phase. In the introductory phase, which comprises the impreg¬ nation of the chips, a high concentration of reactive sulphur as sulfide or hydrosulfide respectively is rather preferred. Although it is commonly accepted to use such selective diges¬ tion for low kappa, the black liquor with a content of more or less utilized reactive sulphur is, according to known schemes, left to be withdrawn or variously washed from diges¬ ted stock by the use of oxyliquor as a displacement washing medium. Several published investigations prove the importance of outmost low black liquor carry-over into oxynation and bleaching sequences.

When, according to the present invention, selective cooking is applied, a minor part of the latter phase cooking liquor will be occluded with washed digested stock and some other part will be included with oxyliquor and directly passed to the bleaching department effluent. Both parts join the final effluent and are together denoted Residual Cooking Liquor = RCL. This RCL features low content of reactive sulphur and is consequently, a low contributor of COD substance which im- pairs delignification bleaching by means of oxidation agents. Among novel or reconsidered alkaline cooking methods, besides original kraft cooking, are alkaline sulfite cooking and versions thereof with sulfid as such or reached to polysulfid of alternative addition of polysulfide and anthraquinon. Improved pulp yield is similarly reported with additions of ethanol and methanol respectively which are recovered in connection with black liquor evaporation. Said versions are mainly employed in order to facilitate bleaching with che¬ micals which are either environmentally acceptable or which do not produce non-processible elements by concentration and combustion together with black liquor.

Impressive development of cooking delignification often together with oxynation to low kappa for bleaching, enabled ECF bleaching and further application of novel, but probably long-term-costly, oxygen-based oxidizers which could facili¬ tate joint combustion of all delignification products in the black liquor furnace, although with reservation for filtrate from some final chlorine dioxide stage. An additional marke- ting price for "chlorine-free" pulp may hopefully be accep¬ ted. But in any case, black liquor furnace capacity is taken up by bleach effluent and the yield of pulp in relation to wood consumed is consequently impaired.

Development on spent liquor processing

Pulp and paper mills which consume power and have no or little use of pulping chemicals, e.g. thermo- echanical (TMP, CTMP and BCTMP) pulp producing mills, have found effluent evaporation competitive to external treatment. An amazingly huge Mechanical Vapor Compression Evaporation (MVCE) is run¬ ning in Canada. Previously, several sulfite pulp mills with good access to power, evaporated calcium base spent liquor successfully. As known, MVCE is performed within limits for BPR and an operating difference of at least 5°C is required.

ENVIM technique renders improved conditions for use of advan¬ ced black liquor recovery processes with no interference by foreign elements and no scaling or boiling point rising (BPR) components are discharged from bleaching. Pre-evaporation using secondary digester heat for multiflash evaporation, e.g. according to Lockman, is common. Concentration of black liquor to about 75-85 (90)% solids is practised. It is of no particular advantage to include bleach plant effluent in such a concentration. High solids liquor improves combustion efficiency with regard to heat and recovery of sulfide com¬ pounds. These are, in conventional kraft processes, contained in soda smelt from combustion furnace. For some time prac¬ tical progress has been made in combustion featuring separa¬ tion of sodium carbonate from gaseous sulphur compounds.

Gasification of black liquor and any spent pulping liquor to process reactive sulphur in gas separate from a residue of mainly Na_C0- is for the time being manifoldly developed, e.g. using plasma-technique according to one method trademar- ked "CHEMREC". It is known to use black liquor gas for var¬ ious heating and gas turbine combi/power generation. This is of particular interest for powering the mechanical vapor compression evaporation of bleach plant effluent. One comp¬ ressor vapor cycle may serve said effluent and pre-evapora¬ tion of black liquor. Multi effect concentration of black liquor as well as additional concentration of effluent in dual evaporator trains can be arranged for common vapor flows.

It is known that by closed water cycling increased temperatu¬ re level affects the heat/power balance of mill operation and i.a. drum washing of stock. With regard to disposal of hot vapor condensate heat it is recalled that NH_-turbine cycle may be applied according to US patent 4,279,126 (G. Rosen- blad) . In this context it is noted that experience from wet combustion soda mill black liquor is applicable on oxidative combustion of alkaline bleach effluent. Subsequent reduction of Na_S0₄ to Na_S and other reactive sulphur compounds is provided by the kraft process. The kraft process operation suffers from admixture to black liquor of bleach effluent containing solids of low heat value. As noted above, the mere existence of sulfidic kraft recovery furnace has led waste streams of the pulp mill to this furnace. As known, the potassium content of lignocellu¬ losic material accumulates in closed-cycle process liquors and normal kraft process value of the molar ratio K/Na + K is significantly increased. When simultaneously Cl from various sources accumulates as chloride in the kraft process, grave corrosion on recovery boiler tube ensues. However, according to ENVIM, the chloride concentration is permanently low. Consequently, K can be considered as useful as Na and all compounds thereof are in the following simply written as Na. Eventually, effluent free mills may use pulping liquors wherein K dominates.

Features of the invention

Due to the magnitude of the cost in immediate investment for an ENVIM installation, it has only recently been put on the industry agenda. The articles mentioned by the present in¬ ventor (Mannbro 1989 and 1990) hold that oxynation in con¬ ventional sequence with oxyliquor recovered together with black liquor would benefit ENVIM processing of effluent from subsequent bleaching stages. However, a deviating view is put forth when the use of oxygen-based bleaching agents is signi¬ ficant. These agents are relatively costly and demand low kappas of brownstock and oxystock under close observation on final pulp cellulose viscosity as criterion of potential paper strength properties. The present invention creates freedom of process conditions by simultaneous release of recovery capacity for digestion and combustion of bleaching department effluent with no kappa restrictions.

The present invention comprises an improved ENVIM process for the production of bleached pulp from lignocellulosic material by multistage alkaline delignification with oxygen as the major oxidation delignification agent. One or more cooking stages are performed with wood chips and pulp thereof in one and the same digester with successive change of liquor through connected pressurized vessels in a batch digester system. Alternatively, the material is moving through the vessels with flows of liquors characterized by a certain concentration of alkali chemicals and dissolved delignifica¬ tion products.

Certain process steps known to the art are applied as fol¬ lows:

- Cooking delignification is adapted to produce brown¬ stock at kappa for effluent-free bleaching sequence. Delignification selectivity is controlled by cellulose viscosity at successively lowered kappas through cooking and bleaching stages wherein regenerated alkali is applied. Alkali regeneration is performed by separate combustion stages of which one handles practically all of the major part of black liquor, while to another there is passed bleach plant filtrate as an effluent containing certain brownstock carry-over, RCL, of cooking liquor.

- Cooking is performed in the presence of one or more reactive sulphur compounds from the group consisting of Na_ 2S, Na_2SnS and Na2_S03_ as well as Na2_S2_0_3 formed thereof and contained in causticized white liquor. Two or more stages of cooking are used, one of which, preferably an impregnation stage, features higher concentration of reactive sulphur than another. Causticizing is also per¬ formed of alkali liquor for bleaching and for which no reactive sulphur is allowed besides optionally oxi¬ dized white liquor Na.S-O- for oxynation. Preferably, bleaching alkali is generated from Na_CO_ , free from reactive sulphur but possibly in the presence of Na.SO. or NaCl which may be allowed in the bleaching department system and its effluent.

- Bleaching is performed in sequences of multiple stages some of which are charged with oxidating bleaching agent together with either acid or alkali and essentially without admittance of the above mentioned Na-compounds of reactive sulphur, while SO_ may be charged to the bleached stock suspension.

- Filtrates reclaimed as bleaching department effluent are combusted to produce a residue partially containing Na_CO- and free of reactive sulphur compounds and optionally in- eluding NaCl, depending on presence of halogen-based stages in the bleaching sequence. Combustion products are success¬ ively treated for purging of substance containing Ca- and Mg-compounds and a concentrate of non-process elements including nutrients and transition metals.

Distinguishing features of the invention

I Oxyliquor (spent liquor from alkaline oxygen delignification of brownstock) is reclaimed from oxystock and included in composite effluent from the bleaching sequence with optional oxygen-based stages. Charges of agents are balanced to give, by effluent combustion, a residue free of reactive sulphur. The balance is controlled relative to any halogen-based agents such that NaCl molality will be lowest as compared to Na_C0_ and any

Na₂S0. which variously is utilized for the gene¬ ration of alkali.

II Water for bleaching operations is essentially supplied by water separated from the bleaching department effluent, e.g. evaporation condensate of for bleaching desirable low COD, while black liquor is reclaimed at optimal concentration of COD substance and with no water dilution other than a part of alkaline bleaching department filtrate or effluent, e.g. oxyliquor used for displacement of cooking liquor. III Acid, preferably H_SO., which is charged to stages using oxygen-based bleaching agent, e.g. ozone, 0_, is allowed in the effluent combustion.

IV Alkali charged to 0_ or any oxygen-based agent comprises regenerated alkali, preferably from effluent combustion residue free from reactive sulphur.

V ^H ₂°₂ ^^{s added to sta<}?^{e of} reactive sulphur-free alkali and oxygen.

VI Brown stock is treated with acid filtrate prior to oxynation and removed minerals, particularly tran- sition metals, are passed to bleach plant effluent.

VII Displacement of oxyliquor and RCL contained therein (carry-over of cooking liquor) uses filtrate from subsequent bleaching and part of it may be recycled with washed oxystock. It is of some importance that carry-over of oxyliquor is of low COD while any un¬ desirable elements ultimately are contained in bleaching effluent. Combustion residue of bleaching effluent constitutes Na CO. from RCL.

VIII NaCl is separated from crystallized carbonate and any sulphate of the effluent combustion residue prior to or after optional causticizing. It may be found useful for regeneration of active chlorine by electrolysis, e.g. to Cl_ and NaOH, or by reaction with NaC10_, to generate C10₂.

IX Na_C0_ in excess of alkali generated for bleaching, is passed to the plant for causticizing white liquor from which inactive Na-compounds preferably have been removed. Any Mg-compound etc. present in the oxyliquor has previously been discharged with final effluent and contained in the combustion residue.

X When white liquor is reacted with oxygen, e.g. air, to oxidize Na_S to Na-S-O-, this latter is further oxidized during the oxynation to Na₂S0₄ which together with some other substance of the same origin impairs black liquor evaporation, e.g. by scaling.

Example

Kraft mill operation of the OZ(EO)D bleaching sequence is reported by W.E. Nutt et al Union Camp Technology (TAPPI Journal 1993, page 115). From brownstock washing stages and subsequent stage reclaimed filtrates were, in this operation, admixed and as composite effluent evaporated and combusted using established technique.

In this example, according to the invention, with separate processing of effluent from said bleaching sequence the effluent is reclaimed at 7-8% solids content and passed to a plant for mechanical vapor compression evaporation. The quantity of effluent passed for evaporation amounted to 6 t/tp (t = metric ton, p = bleached pulp, produced as air- dry) . Boiling point rise is 7°C and evaporation was performed to about 50% solids.

Black liquor is, without dilution with water from brownstock washing, passed to existent evaporation plant. Thereby there is a marginal increase in carry-over of solids contained in the final phases cooking liquor - to be distinguished from black liquor solids - and said carry-over RCL (Residual Cooking Liquor) entrains the bleaching filtrate and effluent respectively which is then passed to incineration.

Combustion of spent bleach liquor produces, according to the present invention, a residue from which NaOH for bleaching is regenerated. The reference method described in the above stated article is confined to oxidation of Na_S in white liquor to Na_S_0_. Such sodium thiosulphate liquor which is recovered in bleaching alkali cycle is indicated as OWL. Acid filtrate from final D stage is according to the reference article sewered - unless sophisticated means are used for its treatment. Tabel 1 shows OWL for reference while for versions of invention OWL and NaOH respectively are shown.

Brownstock for bleaching according to Tabel 1 is selectively delignified to kappa 10. The table exhibits that installation of an effluent incineration plant will enable increased pulp production by 19 or 11% respectively, provided that cooking capacity and supply of chips allow this.

Table 1

Process Reference Invention Effluent from sequence OZ(EO)- OZ(EO)- OZ(EO)D Alkali OWL OWL NaOH

Heat value of dry solids to combustion, GJ/tp: kraft recovery furnace 20.8 16.8 16.8

ENVIM incinerator 4.1 4.2 total 20.8 20.9 21.0

Netto energy in steam GJ/tp after deduction for evapora¬ tion and various losses: kraft process 10.2 9.2 9.2 ENVIM process - 1.6 1.6 total 10.2 10.8 10.8

Na kmol/tp to kraft recovery furnace 12.0 10.8 10.7 effluent incinerator - 2.5 2.7 total 12.0 13.3 13.4

Incinerator residue, mole % Na: Na_S0. 33 29 Na,CO^ - 67 68 NaCl ^J - - 3

Claims

1. Recovery process for bleaching of kraft pulp produced by successive alkaline delignification stages controlled by the lowering of kappa number or by similar test on ligno-cellulo- sic fibre material, produced from wood chips, while other tests indicate that cellulose viscosity is most selectively preserved at a kappa number chosen for oxidative delignifica- tion, said stages being initiated by steps of wood chips impregnation and cooking respectively, batchwise or in conti¬ nuous digesters, using by various means, e.g. spent liquor recycle, attained higher concentration of reactive sulphur compounds, optionally together with anthraquinon and/or methanol, whereby:

- the charge of chips is digested in a cooking plant system of one or more steps using cooking liquor to convert chips to brownstock while black liquor is withdrawn from chips impregnation step to kraft recovery process where the black liquor is combusted for generation of alkali and reactive sulphur which, preferably together in white liquor of certain sulphidity, are included in cooking liquor;

- the digester system for successive lowering of brownstock kappa number employs cooking liquor containing a minor part of black liquor, occluded with impregnated chips, and from brownstock at target kappa number cooking liquor is removed and replaced by filtrate or composite filtrate from some subsequent stock displacement washing and/or press stages; - continued delignification to lower kappa number and bleaching respectively is performed in sequence comprising alternating acid and alkaline steps some of which with oxidative bleaching agents and with inter-stage washing steps, from which filtrates are reclaimed and combined to form bleach plant effluent which, at pertinent content of water, is combusted for generation of heat and a residue of chemicals, essentially Na-salts or possibly an aqueous solution thereof; - oxidizing bleaching agents comprise one or more within the group of halogen-based agents, e.g. (elemental) chlorine and chlorine dioxide, and oxygen-based agents, e.g. (elemen- tal) oxygen, ozone (mostly used in H_SO.-acid stages) , peroxide, peracetic acid, persulphate etc.; characterized in that

A cooking liquor is held at a significantly lower COD concentration than black liquor and with brownstock occluded Residual Cooking Liquor = RCL is to a major part included with bleaching effluent while a minor part of the RCL will constitute COD carry-over with brownstock that is reacted with oxidizing bleaching agent; B bleaching plant effluent containing acidic filtrate, e.g. from acid stage, e.g. H-SO. in ozone stage, acid halogen stages and generation of chlorine dioxide, is processed neutral/alkaline in the presence of RCL carry¬ over and admittance of filtrate from any alkaline oxygen- based stage and primarily from oxynation of brownstock; C volume of liquid forming the black liquor is displaced through digester vessel volume with no further addition of water through brownstock than filtrate or effluent respectively, of which the water component entirely or to a major part derives from low COD water as required for bleaching, said water being separated from bleaching effluent, preferably as vapor condensate; D effluent from the bleaching department is combusted for joint regeneration of valuable Na-compounds wherefrom the reactive sulphur is eliminated which otherwise contributes to COD substance that increases consumption of oxidative agents and impairs cellulose viscosity in bleaching of selectively delignified pulp and partially recycles with water, separated from effluent.

2. Method according to Claim 1 for elimination of reactive sulphur by gasification of concentrated bleaching department effluent to produce a residue of Na-compounds and other elements the major part of which being Na.CO- and whereby NaOH is regenerated from said residue.

3. Method according to Claim 2 which comprises recovery of gasified reactive sulphur to the kraft process, preferably by scrubbing gas with alkaline liquor for subsequent combustion of gas for energy generation, e.g. as a component of black liquor or in connection with combustion of black liquor.

4. Method according to Claim 1 which comprises oxidative combustion of any bleaching department effluent, with or without reactive sulphur, and preferably effluent containing halogen bleaching products.

5. Method according to Claim 4 which comprises treatment of combustion residue with regard to Na_SO. from sulphur com¬ pounds of various sources included in the bleaching depart¬ ment effluent, whereby said residue, with or without Na_C0₃, after separation of possible NaCl at no consideration of any K-compounds is passed to the kraft process in relation to any other make-up required for control of S/Na-balance.

6. Method according to one or more of the previous claims, comprising charge of alkali regenerated from effluent to 0_ and/or H_0₂ stage.

7. Method according to one or more of the previous claims, comprising treatment of brownstock at essentially maintained alkalinity followed by a stage for acid treatment using filtrate from subsequent acid bleaching stage and, if neces¬ sary, addition of acid, e.g. spent solution from chlorine dioxide generator and, optionally, chelating agents for transition metals, whereafter resulting filtrate containing nutrients, but regardless of K-compounds, is removed from the prior to next alkaline stock treatment stage.

8. Method according to one or more of the previous claims, comprising the use of pre-oxynation filtrate to displace cooking liquor from brownstock, whereafter a suspension of treated stock and further oxyliquor is reacted with 0₂, preferably with no additional charge of alkali.

9. Method according to any one of Claims 4-8, comprising regeneration of active chlorine from NaCl, which primarily is separated by crystallization of sodium carbonate and/or sulphate before or after causticizing, said regeneration being performed by electrolysis to produce Cl₂ and NaOH or alternatively, NaC10_ which may be reacted with NaCl in the generation of CIO .

10. Method according to any one of Claims 4-9, comprising utilization of excess Na₂C0_, obtained after regeneration of alkali for bleaching, by passing it, suitably together with Na₂S0., to kraft process green liquor for causticizing to white liquor, from which subsequently in cooking non-active Na-compounds are separated.

11. Method according to Claim 4, comprising purifying spent liquor steam condensate, preferably after methanol stripping, by reacting it with ozone containing gas in connection with ozone bleaching.