CA1064206A - Method of recovering substances in the extraction of spent liquors from the delignification of lignocellulosic material by means of chloride-producing bleaching agents - Google Patents

Abstract

Abstract of the Disclosure Lignocellulosic fibrous materials are delignified in two or more stages. An introductory stage of digestion discharges brown stock and a black liquor which contains i.a. sulphur compounds wherein sulphur has an oxidation number that equals either -II (e.g. Na2S) or IV (e.g.
Na2SO3). Optionally, the brown stock is further delignified by means of oxygen, whereby oxystock and oxyliquor are obtained. Stock from digestion, i.e. brown stock or oxystock, is subjected to delignifying oxidative bleaching using chlorine or chlorine compounds, the reac-tion products of which subsequently form chlorides in the bleaching effluent.
The non-chloride liquors, i.e. black liquor and oxyliquor, possibly also a low chloride fraction of bleaching effluents, are joined to form a lye which is recovered and combusted and includes the major part of the sulphur compounds used for digestion. The lye combustion furnace releases compounds comprising sulphur of oxidation number -II and IV.
The chloride forming effluents, optionally including a minor lye fraction, are combined to form a brine which is subjected to another combustion which principally produces chlorides and compounds of sulphur of oxidation number IV
(e.g. SO2) and VI (e.g. SO3 and Na2SO4). This makes it possible to eliminate sulphur compounds, e.g.
sulfidic compounds with oxidation number -II, from the brine recovery operations including separation of chloride, while the digestion chemical recovery sysstem only handles the lye.
By this means the digestion as well as the entire pulp mill system is relieved from chlorides, however, at least two combustion furnaces and two separate evaporation lines, one for lye and one for brine, are required. The combined water evaporation from lye and brine can be kept on the same level as for a conventional single-line black liquor operation because brine substitutes wash water.

Description

The presen~ invention relates to the recovery of substances in the extraction of spent liquors from delig-nification in order to reduce water pollution from pulp mills employing some of the previously known systems for digestion and subsequent delignification of cèllulose pulp in a sequence of bleaching stages of which the final stage of said delignification is effected with bleaching agents which form chlorides during the combustion of the effluent in the chemical regeneration process of the pulp mill.
According to this invention of the extracted spent liquor, the chloride-forming bleaching effluent is sub- -jected to a thermal oxidation or combustion, but averting -build-up of chloride in the recovery system for the digestion chemicals.
In cellulose pulp technology it is known to combust bleaching effluents in the alkali recovery system of a kraft (sulphate) mill, and then to separate ceystallized sodium chloride. The total amount of liquor which is to be used for alkaline delignification such as digestion, `
may be used for such chloride separation. The crystal-lization of the sodium chloride in kraft process liquors ~ ~
containing sodium sulfide, sodium carbonate and sodium `
hydroxide is also known. An embodiment relates to kraft ` ~-mills comprising two or more recovery units and to con- ~ -~; centrating the chloride to one single unit. A chloride-leaner fraction of the effluent from the bleach plant alkaline extraction stages is, for this embodiment pre-ferred to serve as a kraft process chemical make-up ~- 30 because this particular effluent is better converted for digestion to useful sodium compounds.

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~ 1064206 Another known pulping procedure comprises delignifica-tion of cellulose fiber raw materials with caustic soda or, as an alternative soda, and oxidative chemicals in the form of either elementary chlorine or chlorine compounds.
The regeneration of these delignification chemicals in the absence of sulphur compounds simpler than the dominant kraft- and sulfite pulping methods comprising digestion in one or more stages.
Recently an intermediate delignification stage between digestion and bleaching with chlorine compounds has been developed. -; According to the invention there is provided in the -~
manufacture of bleached cellulose pulp from lignocellu~
losic raw material a method to reduce water pollution from bleaching effluent comprising the steps of delignifying the raw material with useful non-chlorine delignification agents to form spent delignification liquor and unbleached stock, bleaching said stock with a chlorine-compound : containing bleaching agent to form a bleached stock and chlorine-compound substances containing bleaching effluent -and applying bleaching effluent for displacement of the spent delignification liquor from said unbleached stock, and thereby employ means for counteracting chloride :~
accumulation in plants for utilization of substances dissolved from the lignocellulosic raw material by said ~ -non-chlorine delignification agents of which agents on the most only a minor portion is generated from bleaching - :
effluent, the improvement comprising:
. (a) removing the major portion of the spent delignifi- -:~
cation liquor from the unbleached stock to form thereof a lye with on the most a minor portion of the chlorine-: ' , compound containing bleaching effluent entrained into thelye;
(b) removing in the unbleached stock remaining portion of the spent delignification liquor by displacement washing with the addition of a major portion of on said unbleached stock applied bleaching effluent in a counter-cuerent washing system, to form brine of the applied bleaching effluent and which brine contains a higher concentration of chlorine compounds than the lye does;
(c) passing the brine, discharged from step (b), directly to a separate brine treatment system which comprises a step for thermally oxidating organic substances of the major portion of said brine by combustion to such degree that therein contained chlorine compounds form chloride and that any together with said chloride occurring sulphur compounds of lower oxidation numbers than IV are oxidized to at least oxidation number IV; and (d) subjecting the lye, discharged from step (a), to ~- -utilization of the substances which have been dissolved by the delignification and which are contained in said lye.
In the following description of the invention all digestion methods employing sulphur compounds in some stage of digestion will be considered. Therefore it is found useful to give an introductory review of the sig-nificant properties of various digestion spent liquors.
The established course of delignification of cellulose material, e.g. wood chips, runs in two or more stages.
First brown stock (pulp) is produced and the brown stock is subsequently further delignified by means of oxidizing bleaching agents.
The brown stock digestion encompasses digestion with ;' ~ ' ' . .' solutions containing a base which supplies cations selected from the group of sodium, ammonium, magnesium and calcium. The cations exist in the digestion solution in various combinations with sulfite, bisulfite, sulfide, hydrogen sulfide ions etc. at certain hydroxide ion and hydrogen ion concentrations. Common digestion methods comprise 1) kraft pulping with essentially hydroxide and

2) sulfite pulping employing acid, neutral or even alka-line solutions. For common sulfite pulping SO2 is used in the presence of said cations but an extreme type of sulfite pulping using only SO2 and water (hydrogen serves as cation) is also known. Another extreme is sulphur-free caustic soda pulping with solely sodium hydroxide and possible residuals of non-causticized soda (sodium carbonate). Caustic soda pulping may be applied subsequently to pretreatment of the lignocellulose material with sodium carbonate liquor. Optionally, sulphur compounds, e.g. sulfite, sulfide, polysulfide or -~
hydrogen sulfide, may be used in a pretreatment stage of caustic soda pulping. Caustic soda digestion or sodium carbonate pretreatment is used for semi-chemical pulping, e.g. of hardwood, followed by mechanical defibration and optional bleaching. -~
The brown stock is obtained at a yield in the range of from about 35 to 90~ calculated on the moisture-free weight lignocellulose material. In the range above 90 yield, the pulp is usually not considered as brown stock but a sort of mechanical pulp, nevertheless the ligno-cellulose material, i.a. wood chips, is treated by 8ulfite or soda liquor.
The various brown stock pulping methods discharge B
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spent pulping liquors, which in the following description are denoted black liquor. Thus, even sulfite spent liquor is denoted as black liquor.
Brown stock is obtained in general at a yield in the range of same type of reducing recovery furnacès as used in the kraft process while Mg-, Ca- and NH4-base sulfite black liquors are combusted under entirely oxidative con-ditions. Mg-base sulfite pulping, e.g. according to the Magnefite method, gives a black liquor that on combustion produces MgO and Mg(OH)2 respectively from which chlor-ides and alkali salts may be leached. In the leaching some losses of other Mg-compounds occur. This loss has to be compensated for by a make-up of other Mg-compounds.
Ca-base sulfite pulping gives a black liquor which on ~` combustion produces an ash from which calcium cannot be regenerated except in a certain known cross-recovery system with the kraft process.
Corrosion caused by chlorides creates problems in combustion plants for sulfite black liquor and regenera-tion of sulfite. There are, however, systems availablefor flue gas scrubbing in which absorbed hydrogen chloride and sulphur dioxide is separated from regenerated S02 and sulfite, respectively. Therefore it was suggested in the Inventor's U.S. Patent No. 3,248,169 (col. 5, lines 5-7) and Canadian Patent No. 593,787 to, in a sodium base sulfite process, recover liquor combustion products for regeneration of chlorine dioxide to be used in brown stock bleaching.
Many problems arise in chloride accumulation in closed chemical recovery systems for all various pulp digestion methods. Most investigations in this field have been _ 5 _ ~3 :`~ . .

applied on the kraft process.
Kraft mills supplied entirely with fresh water have only approx. one percent of their sodium as sodium chloride. Ocean-floated wood for pulping as well as use of brackish water at some mills has proven that it is indeed possible to run kraft mill recovery processes at an equilibrium with 20% of the sodium as sodium chloride.
But when such chloride levels are further raised, the costs of corrosion and its prevention increase. There-fore a build-up of chloride would have to be counteracted or removed according to known methods, e.g. comprising separation of crystallized sodium chloride from white liquor or leaching of precipitated flue gas carryover dust.
The brown stock delignification is carried out in suc-cessive stages using oxidative bleaching agents and the resulting bleached stock is ultimately obtained at a yield ~-amounting to between 30 and 65 percent of the original lignocellulosic material.
By using chlorine based bleaching agents in lenient and especially well controlled delignification stages, it is possible to produce bleached stock wherein the cellu~
lose components, as for instance holocellulose, are preserved, whereby a yield of about 70~, calculated on original material, is obtained.
Oxygen is another oxidative bleaching agent which in recent years has been put into practice and it may be applied on the stock preferably under alkaline conditions as either concentrated oxygen gas or air, Oxygen deligni-fication stages, here denoted oxystages, convert brown stock to oxystock. An oxystage is used in sequences either prior to or subsequent to a stage using a chlorine . .

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bleaching agent. The great advantage of oxygen is that it replaces chloride-forming bleaching agents. Brown stock as well as oxystock releases, during its bleaching, a bleaching effluent containing dissolved colored substances and degraded and hydrolyzed cellulose components.
Alkaline extraction stages follow in sequences com-prising chlorine compound stages which do not terminate the sequence. The chloride compound is preferably selected from the group of chlorine dioxide or chlorine monoxide. The stock to be treated with chlorine compounds is chosen from brown stock and oxystock. The alkaline extraction may either involve pure alkali or alkali to-gether with an oxidant. With regard to the recovery of alkali from bleaching effluent it is preferred to choose said oxidant from oxygen or oxygen compounds such as ozone and peroxide. However, addition of hypochlorite to alka-line stages is still practised. The alkalinity in an alkaline extraction stage is accomplished by supply of either sodium hydroxide or, for gas phase operation, - ~
ammonia. To shorten a bleaching sequence to a few stages -~ -it is known that an alkaline oxystage following an initial chlorine/chlorine dioxide stage is highly efficient in ~ ~-making bleached stock of high brightness in combination with well-preserved paper sheet physical strength. To - -minimize the chlorine consumption in such a short sequence, this sequence may be preceded by an additional oxystage, alternatively using air with optional charge of ~ -alkali.
The bleaching effluents that have been dealt with here have different properties which determine their manner of utilization together with black liquor recovery B

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in a closed system. Environmental requirements these days not only demand a closed system with regard to black liquor recovery but also a similar treatment of bleaching effluents, at least of the fractions deriving from the heavily polluting stages. Thus it has been proposed to use bleaching effluent either for displacing black liquor in the brown stock washing or for preparing digestion liquors, kraft or sulfite, from bleaching effluent, con-taining sodium, which is of value as make-up. However, previously mentioned difficulties caused by chlorides arise. To avoid such difficulties, it has been proposed ~ ;
(N. Mannbro, Svensk Papperstidning 1963, page 34) to extract only effluent fractions having low contents of chloride. On the basis of this concept, methods have later been developed for discharge of NaCl from the kraft process regeneration of NaOH and Na2S. Rapson and co-workers have made such great contributions to the tech-nique for crystallization of NaCl from the kraft process ; effluents that it is possible to direct almost all former bleaching effluents into the kraft mill black liquor system. The sodium deriving from bleach plant effluent -~
will be included in the soda smelt produced together with ~-Na2S from one or more of the black liquor furnaces. The presence of Na2S (sulphur with oxidation number -II) in kraft process liquors, however, necessitates particular procedures for the sodium chloride separation. When separated, the sodium chloride can be electrolyzed for manufacture of chlorine dioxide and chlorine according to Rapson's momentous methods.
In the pulp industry, the major portion of brown stock is kraft pulp. Kraft pulping also includes use of , ~

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predigestion to improve the pulping yield. These pre-digestion methods utilize sulfide compounds. One such compound is hydrogen sulfide and another is polysulfide which constitutes Na2S (wherein sulphur has oxidation number -II) and sulphur (with oxidation number 0). Kraft pulping is the most common form of pulping. Sulfite pulping twherein the sulphur has oxidation number IV), is common also and particularly magnesium base (Magnefite) pulping and neutral sulfite pulping (NSSC) using either sodium or ammonium as base. However, a number of calcium base sulfite mills still operate.
Combinations of kraft and sulfite digestion methods are also known. For special purposes, an acid prehydro-lysis is used fpr sulfate (kraft) pulping. In other instances, sulfite digestion is combined with a subsequent alkaline digestion. Modifications of alkaline digestion in the presence of either sulfite alone or sulfite ~
sulfide are also known. Brown stock obtained according to any of the methods described herein may be bleached in sequence featuring one or more of the oxidation agents chlorine, chlorine dioxide, chlorine monoxide, oxygen (optionally as air), peroxides, ozone etc.
Of prime importance to system closing in practice is whether the recovered black liquors and bleaching effluents are compatible in the various combustion and chemical regeneration systems. Offhand it is clear that kraft digestion as well as sodium base sulfite digestion (possibly mixed Ca- and Na-base digestion) are favorably matched with effluent from oxynation ~oxygen deligni-fication) in the presence of alkali, e.g. NaOH and/orNa2CO3. Oxynation with NH3 is, however, adaptable :
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to all digestion methods because ammonia in oxynation effluent as well as ammonia in other spent liquors is combusted with no solid residue. Alternatively, ammonia may be expelled from a mixture made alkaline by a base.
Consequently, sul~ite digestion using a base selected from a group consisting of Mg-, NH4- and Ca- base should be considered together with oxynation using NH3 as alkali supply.
Preferred embodiments of the invention are described in the following with reference to the accompanying drawings, in which:-Figure 1 is a schematic representation of a Rapsonsystem of countercurrent washing;
Figure 2 is a schematic representation of an oxygen delignification system;
Figure 3 is a schematic representation of an embodiment of the invention applied to conventional chlorine bleaching;
Figure 4 is a schematic representation of a further development of oxygen delignification;
Figure A is a diagrammatic scheme of a kraft mill employing the invention; and Figure B is a diagrammatic scheme of a sulfite pulping process using MgtHSO3)2 for digestion according to the Magnefite method.
The distinguishing features of some fundamental effluent systems are schematically shown in Figs. 1 to 4.
These schemes refer to kraft brown stock bleaching and also apply to a sodium base sulfite process.
The following symbols are used:
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1 _ , :~

: . ; . ,, K = dig~sting stage Bl; B2 = bleaching stages in sequences employing chloride-forming bleaching agents OXY = oxynation (oxygen delignification) stage ;_ T = stages for recovering black liquor by brown stock washing Bf = combustion of chloride-forming bleaching effluent R = regeneration of white liquor for digestion NaCl = separation of chloride.
The state of the art is shown in Figs. 1 and 2. Fig.
1 illustrates a Rapson system of countercurrent washing along the bleaching stages, and mixed black liquor and -bleaching effluent are combusted together in a black liquor furnace of the chemical regeneration system (R + Bf). This regeneration involves separation of NaCl from which chlorine is generated by electrolysis. If black liquor combustion involves a pyrolysis step according to the SCA-Billerud method, it is possible in step R to convert almost sulfide-free soda liquor to -caustic soda for bleaching.
Fig. 2 shows an oxynation system (oxygen delignifica-tion) which half replaces delignification with chloride forming bleaching chemicals. Products of these chemicals (Cl-residues) are in a traditional way discharged as bleaching effluent at the arrow marked "Cl-REST" in Fig. 2 while only alkali passed through "T" stages is regenerated for digestion and, optionally, for the oxynation(oxy).
The method according to this invention is disclosed with reference to Figs. 3 and 4.
Fig. 3 shows an embodiment of the invention applied on conventional chlorine bleaching and the effluent thus : .;

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obtainecl, denoted brine, is subjected to separate combustion (Bf) which produces a combustion residue enriched on NaCl.
Fig. 4 represents a further development of oxynation succeeded by bleaching with chlorine compounds which are contained in the brine which is combusted separately (Bf) with no chloride contamination of the alkali regeneration system (R). The split of countercurrent washing is indicated by the dashed slope in the middle "T" washing stage.
To facilitate the description of the invention, the following terminology has been applied to the digestion ~.i spent liquors and bleaching effluents of various origins:
Black liquor = spent digestion liquor from brown stock, discharged from a pulp digester;
Oxyliquor = effluent recovered from oxystock (oxygen bleached stock) from an oxystage, where oxynation (oxygen delignification) is carried out. Oxyliquor also denotes effluent from a bleaching stage using a i peroxide or ozone; ~ -Lye = black liquor and mixture of black liquor and regenerable bleaching effluents, i.e.
either oxyliquor or bleaching effluents ~-containing either no chloride forming compounds or containing said compounds at a concentration that is lower than in brine;
Brine = bleaching effluent recovered from various bleach plant stock washing stages, said effluent containing substances formed by reactions of a spent liquor with chlorine . ~ .
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based bleaching agents and which substances by treatment comprising a combustion step, would produce chloride.
This invention relates to the bleaching of unbleached stock, such as brown stock and oxystock in sequences, where a chlorinating agent is used in at least one of the stages and brine, as indicated above a current mixture of bleaching effluents, is recovered from the bleach plant washers. The target is environmental protection and it is achieved by the effluent being recycled in a closed system wherein the organic delignification products are elimi-nated by oxidation. Such a system may be either preceded by or comprise an oxystage from which oxyliquor is dis-charged. In the case addition of alkali is made to the oxystage this alkali will be included in the oxyliquor.
The oxynation alkali is usually supplied by a hydroxide-forming liquor, i.e. NaOH or possibly Na2C03-liquor, but there is an increasing use of NH3 which forms NH40H in the liquid.
Particularly gas phase delignification with oxygen, is favorably combined with gas-phase application of NH3 as ~ s a supply of hydroxide to alkaline stages. It is possible to combine oxygen delignification and gas phase deligni- -~
fication with chlorine, chlorine dioxide, etc.
According to the invention, oxyliquor is used for displacement of black liquor from brown stock, and residues of lye remaining in the stock are displaced by means of bleaching effluents whereby, if desired, the addition of brine terminates the succession of stock washing steps. For most efficient recovery of the lye, a limited penetration of brine into the regeneration ~ ~ .
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. -, system may be tolerat~d in practical application of the invention. This tolerance for chloride as previously staged, has been demonstrated in coastal kraft mills with wood containing so much salt that soda smelt from the black liquor combustion comprise a substantial part of NaCl.
The tolerable input of chloride formers to the lye which as previously stated is obtained by black liquor or bleaching effluent displacement differs for various pulping methods and it is also dependent on the degree to which the entire process is closed. As a rule, in the kraft process not more than 20% of the volume of lye ~ -should comprise brine and preferably less than 10%. The actual limits will be influenced by other sources of chloride, e.g. wood and water.
The invention is primarily directed towards pulp digestion methods which produce black liquors containing residues and reaction products of the particular sulphur compounds in the digesting liquor which, during digestion, promote the delignification and prevent the degradation of - i;
cellulose- and hemicellulose compounds. This degradation lowers the yield of cellulose pulp.
The sulphur compounds contained in the fraction of black liquor which is left in the brown stock after washing, are transferred to the brine oxidation step. `- -According to this invention they are oxidized to products ~-with oxidation number IV or, alternatively, VI. For ~ -example sulphur in Na2SO4, SO3 and CaSO4 has oxidation number VI and sulphur in SO2 and thereof prepared Mg(HS03) (NH4)2SO3, NA2SO3 has oxidation number IV. Thus, according to the invention, no Na2S, i.e. sulphur with oxi- ;
dation number -II, can be produced in smelt containing NaCl B - ~

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deriving from brine, This is an important advantage of the invention, because it is otherwise hazardous to handle Na2S and s2 which readily emit toxic H2S from steps in generation of chlorine compounds as well as from purification of effluents discharged to a recipient. The termic oxidation of brine may be carried out in liquid phase with supply of oxygen to a more or less complete oxidation of the effluent's combined content of organic carbon. The oxidized effluent may subsequently be dis-charged or recycled to the process. In case oxygen isused for bleaching in oxystages, an additional quantity of oxygen for oxidation of brine may be produced at acceptable economic terms.
In case the combustion products of the brine can be processed in electrolytic plants for regeneration of oxidizing (active) chlorine, it would be of advantage to also supply the brine system with sodium chloride that has been separated from the kraft process according to Rapson. An optimized operation of an electrolytic plant in connection with this invention could utilize a chloride make-up to compensate for some acid chlorine and chlorine dioxide stage effluent and also less sodium-containing extraction stage effluents which may be discharged to the recipient either directly or after cleaning treatment.
In those cases where the kraft process is operated at the tolerable chloride level in the liquors, sodium chloride is enriched in the flue gas carry-over which ordinarily comprises what is called precipitator salt cake. This chloride can be extracted, e.g. leached, from the salt cake. Extracted chloride is, according to the invention, trans~erred to the brine system and treated together with the combustion products of the brine.

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Y, 1064Z06 Brine evaporation according to the invention, separate from lye evaporation, allows water introduced with bleach-ing effluents to be recycled with no contamination from black liquor vaporized digestion products and compounds of sulphur with oxidation number -II. Thus evàporation condensate from brine may be utilized for washing of finally bleached pulp, whereas the previous known closed systems produced one common evaporation condensate con-taminated by volatilized black liquor substances. These volatile substances effect the various digestion processes differently. Thus they produce evil odors in the kraft process while condensate for sulfite digestion acid pre-paration may jeopardize the stability of the digestion liquor to the degree that delignification is inhibited.
The utilization of brine evaporation condensate in said ` processes according to the invention will overcome such problems.
Treatment of condensates, e.g. by stripping or similar purification steps, will be considerably simplified when i only a limited amount of vapor condensate from black liquor or lye has to be treated. The common rules for connecting a series of evaporator units with regard to steam flow between brine and lye lines are applied to recover the largest possible amount of non-contaminated condensate.
Because all or almost all sulphur from digestion is contained in the lye, the brine combustion gas will emit only a small quantity of sulphur compounds. The forma-tion of HCl in the flue gas increases with decreasing content of ash forming base in the brine and simultao neously increases the input of HCl to flue gas scrubbers ~,'' -, :,' , .

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' 1064206 in systems for separation of chloride.
A brine recovery system combined with a sulfite pulping MgO-recovery plant will prevent corrosion in the lye chemical recovery system. Chlorides from ocean water floated wood introduced into the lye through the diges-tion system may also be discharged by means of the brine system. Thus, the passing of lye combustion flue gas containing chloride and/or chloride extracted thereof to a single flue gas cleaning system shared with the brine combustion lies within the scope of the present invention.
As examples on the application of the invention are a kraft mill with C/D E D-bleaching of the brown stock (Example A) and a magnesium base sulfite mill with O C/D E
D-bleaching (Example B) are described.
Example A
Fig. A on the attached drawing comprises a diagram-matic scheme of a kraft mill using the invention. In the digestion plant 1 produced brown stock and black liquor are separated by a disc filter 2. About 2/3 to 4/5 of the black liquor 3 is first drained off from the brown stock, and of remaining black liquor somewhat more than 50% is in the stock layer removed by displacement with bleaching effluent 4 and with essentially no intermixing of the liquids in the pulp layer.
Lye is by this means obtained from about 85 to 90 of the black liquor with no dilution caused by bleaching effluent. This lye is drawn off by pipe 3. Then more bleaching effluent 4b is used for displacement washing.
The brine produced by this washing is discharged at 5 and amounts to between 3 and 5 cubic meters per metric ton of brown stock, of which about 2 to 4 cubic meters derives , ~064206 from bleaching e~1uent.
Prior to stock discharge from the disc filter, bleaching alkaline effluent remaining in the stock layer is removed by displacement with an acid bleaching effluent supplied through lines 9 and 6 from the acid bieaching stages 7 and 13, respectively. Chlorinated stock from a hot chlorine/chlorine dioxide stage 7 (C/D-stage) is, on filter 8, washed free from dissolved, mainly oxidized delignification products by supply of acid bleaching ef-fluent and is then neutralized on the filter by applying alkaline bleaching effluent 4 to reduce the consumption of caustic 10. The caustic may be either a make-up or regenerated from NaCl 10b. The alkaline bleaching effluent 4 is recovered by washing stock from the alkaline extraction stage 11 on filter 12 with chlorine dioxide stage 13 effluent 9. The chlorine dioxide bleached stock is washed on the filter with process vapor condensate 15 or alternatively with stripped or otherwise treated process vapor condensate and optionally a supply of water 16 as white water or fresh water. ~
BOD-causing residues of black liquor are oxidized in ?~ -`
the C/D-stage 7 as stated in the Inventor's U.S. Patent No. 3,830,688 (col. 7, lines 13-19) and Canadian Patent No. 828,653. Effluent 17 from C/D-stage 7 may be dis-charged to the recipient while bleaching effluent passed through the brown stock washing 2 constitutes brine that is introduced to a plant 18 designed for its evaporation `~
and in turn cooperating with a fluid bed combustion furnace 19. This furnace is of the type which is manufactured by Copeland or Dorr-Oliver (Fluo-Solid) and its design makes it possible to perform a substantial part of the brine .
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evaporation directly witl- hot ~lue gas l9b.
The fluid bed delivers solid combustion products as ash in the form of a pelletized salt residue 20. It contains about 40% NaC1 by weight. The NaCl is in stage 21 separated from Na2CO3 and Na2SO4 alternatively aEter causticizing of Na2CO3 to NaOH accocding to methods recently published by Rapson et al. The NaCl is regenerated in an electrolytical plant such as a chlorine -alkali plant or chlorate plant 21b and the products 21c are used for bleaching. Electrolytic caustic 21d is used for delignification such as bleaching 10b or as make-up for the digester with white liquor 23b.
In case diaphragm cells are used to make caustic soda for the bleach plant, a partial recycling of chloride, contained in the caustic, to the bleach departments 10 may be possible because, according to the invention, this chloride will not burden the regeneration of alkali for digestion of pulp. This novel unloading of the chloride from black liquor and alkali regeneration system is con-sidered to be a major feature of the invention.
Subsequent to the separation of NaCl the residue of sodium compounds 22, containing i.a. Na2SO4, Na2CO3 and double salts etc., is passed to the alkali regeneration system 23 of the kraft mill. Concentrated lye 24b from the normal black liquor evaporation plant 24 is combusted ~
in the recovery furnace 25 and the soda smelt 25b, which ~ -contains sulfide, is in the regeneration plant 23 converted to white liquor 23b for the digestion 1.
The evaporation lines 24 and 18 are interconnected in a manner known in the art for multiple use of steam and vapour. The invention may be operated in such a way that '` ''' '~3~ ,:, . "
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: : , ~ ' ' :~' ,' .-the combined amount o~ evaporated water not exceed that of the previous kraft process with its discharge to sewer of all bleach plant effluents.
The recycled bleaching effluent contributes make-up Na and a substantial amount of combustible solids to the kraft process while chloride is controlled according to the present invention. The combustible solids comprise organic substance amounting to about 5 to 10% of the weight of the brown stock and corresponds to about 5 to 25% of the steam generated by the boilers of the furnaces 19 and 25. The actual amounts of steam depend on the relations between wood solids dissolved by digestion 1 and solids from delignification of the brown stock in the primary bleaching stages 7, 11.
The additional capacity of furnace 19 is of particular interest to kraft mills which otherwise would have their -capacities for evaporation 24 and combustion 25 overloaded. --Thus the amount of black liquor left with the brown stock to the brine 5 can be optimized for the economy of the `
process as a whole. - -Prior art combustion of bleaching effluent in the -~
black liquor furnace, e.g. according to Rapson, implies problems in the flue gas cleaning system caused by hydrogen chloride and sodium chloride.
Such problems are eliminated according to the present invention because chloride compounds in the flue gas l9b are treated by contact with just brine 5. Chlorides recovered from black liquor recovery 25 flue gas also may be passed to the chloride separation system 21. The suphuric acid that is used for acidulation of by-product -~
tall oil contibutes sulphur to the emission of odorous v ., .~ - . . . ..
.:; . .
- .
,: ' sulphur compounds from the conventional kraft process, but according to the present invention, hydrogen chloride can replace sulphuric acid. This change is made possible by passing also the soap splitting brine to the brine combut-tion plant.
Example B
Fig. B exhibits a scheme of a sulfite pulping pro-cess with Mg(HSO3)2 for digestion according to the Magnefite method. Brown stock from a digester 1 is blown to a dual blow and washing (diffuser) tank 2, whence lye 2a is recovered by displacement washing with oxyliquor Sa. Brown stock 3 is also flushed with oxyliquor to the screening operation 4. Coarse material in the stock is disintegrated in the screening department to facilitate the use of a press filter 5 for washing. Black liquor `
in the brown stock layer is on the filter removed by dis-placement with oxyliquor 11 and the operation concludes with pressing 5b.
. . .
Lye of mixed black liquor and oxyliquor 5a is passed through the vessel 5c to said washing tank 2, whence lye 2a is drawn off. (The diffuser washer, the filter and/or the press with their interconnections may advantageously be arranged as described by the Inventor in his Swedish patents 127,315 and 139,301 and in his article in Paper Trade Journal, February 24, 1958). ~
Brown stock is fed from the press 5b to the oxystage ~ -6 where it is reacted with oxygen 7 and NH3 8. Oxystock 9 is diluted with recycled oxyliquor lla and the stock suspension is passed to a multiple-zone wash filter 10 on which the oxyliquor 11 is successively removed by dis- ;
placement bleaching effluent 12 from subsequent stages of L~

bleaching 1~, 18, 21. Oxyliquor 11 is recovered by only a negligible dilution of the bleaching effluent 12 which contains chloride forming substance.
The liquid phase of the oxystock at discharge from the first zone 10a of the washing filter constitutès a mixture of black liquor, oxyliquor and bleaching effluent. This liquid phase is in a following zone 10b gradually dis-placed by bleaching effluent 12 and finally discharged as brine 13. Oxystock 10c is pressed 14 to a consistency required for gas phase reaction in the vessel 15, in which the oxystock is further delignified by a mixture of chlor-ine and chlorine dioxide 15b. The press liquor 14a is ~ -~
passed to the brine 13. Chlorinated stock suspended in bleaching effluent 15a is washed in the diffuser 16 with chlorine dioxide stage effluent 17 and advantageously neutralized slightly with an alkaline bleaching effluent 18a from the ammoniacal extraction stage 18. For gas -phase alkaline extraction a suitable stock consistency is obtained by the concentrator 18b and then the stock is reacted with ammonia 19. At the discharge zone 18c of the extraction vessel the stock is diluted with recycled bleaching effluent 18a from the screening plant decker 18d.
Bleaching effluent, made up from vapor condensate from the process, is used in the wash apparatus 20 and the pulp is finally bleached in the chlorine dioxide tower 21.
Caustic bleaching effluents precipitate Mg-compounds in magnesium base sulfite black liquor, but the fact that ammonium hydroxide does not precipitate Mg-compounds makes this application of the invention possible. The brine 13 is passed to an evaporation plant 25, and the concentrated brine 13a is combusted in a fluid bed furnace 26, e.g. of .~
.

:

~ 064206 Copeland design, which furnace on demand may admit various combustible wastes 26a, e.g. slude containing waste sediment from settlers etc. The ammonia compounds are combusted in the furnace and release chlorides with the flue gas which, optionally after separatioh of parti-culate matter, can be recovered by scrubbing 26b with water. The ash 26c is optionally discharged. (In a gas scrubbing stage 26b produced chloride solution, e.g.
hydrogen chloride, can be utilized for production of chlorine/chlorine dioxide for the bleaching.) Lye 2a withdrawn from washing tank 2 is evaporated j--in a sulfite black liquor type evaporation plant 31 and concentrated liquor 2b is combusted in another fluid bed - furnace 32. Hereby Mg0-ash 33 is produced which is utilized for preparation of magnesium base digestion liquor. In this preparation, the ash passes a leaching stage 33a which separates easily soluble salts 33b and thereby causes some loss of, i.a., MgC12. But this loss of Mg will be low according to the invention because chloride, which makes magnesium soluble, is essentially kept away from the recovery plant for digestion chemicals 30. The plants for regeneration of Mg(HSO3)2 30 and S2 from flue gas 32b are considerably more complicated but well-known to those skilled in the art and therefore they are not included in detail in the drawings.
Evaporation condensates 31b and 25b are passed to pulp washing 24, 20 and to the sulfite acid making system 31c, 25c, 30.
A useful modification of Fig. B would involve sewering of the chlorine/chlorine dioxide stage (C/D-stage) 15 effluent 12 with low BOD as compared to the extraction .
" , :

.

stage 18. Therefore also countercurrent washing with chlorine dioxide efluent 17 from stage 21 through stages 16 and 10 to sewer is another useful application of the invention. Simultaneously, extraction stage effluent 18a is passed directly to the oxystock washing 10 and from there is conveyed to the brine combustion 13, 25, 26.
In sulfite pulping technology it has been common practice to dissolve or prepare the sulfites in digestion spent liquor (sulfite black liquor) which allows recycle of used liquor to the digester. Thereby the solids con- ~
centration of the black liquor reaches an equilibrium -level that is expedient for evaporation. Similar re-use may, according to the invention, include oxyliquor 11 from ~ -which ammonium bisulfite or monosulfite is prepared also -~ -for other digestion stages than those indicated here. One such stage may use oxyliquor 11 at higher pH than what the solubility of Mg-sulfite would allow. This higher pH
stage may be applied either prior to or subsequent to a - - ~
main digestion stage with Mg-bisulfite. - -By-product recovery from sulfite black liquors has -attracted renewed interest and is indicated 31a on the -figure. When coniferous woods are digested, the liquor may be fermented so as to produce ethanol and independent of the raw material, i.e. wood species, protein producing microorganisms, e.g. certain types of yeast, can be cul-tivated in sulfite black liquor. These fermentation processes require addition of nitrogen nutrients. Thus the ammonia is first utilized for bleaching and possibly also digestion and the resulting nitrogen compounds are ultimately consumed by fermentation of the carbohydrate content of the black liquor in the by-product plant 31a.

,., .:

Expul~ion of N~3 from successive stages of evapora-tion in the plants 31 and optionally 25 can be effected by adding alkaline substance to the black liquor. This substance is preferably Mg0-ash or a slurry thereof. The regenerated NH3 is then used for either bleaching or other purposes. The ammonia compounds will also serve as nutrient in the fermentation of hydrolyzates.
Hydrolysis of cellulosic material, similar to that which takes place in acid sulfite pulping, can also be applied in alkaline pulping process by a prehydrolysis of the cellulosic raw material with an acid, e.g. sulphuric acid, sulphurous acid tsulphur dioxide) and so-called-wood acids released by water or steam hydrolysis. Total - hydrolysis of wood waste may also be considered. Hydro-lysis of hardwood employing hydrogen chloride produces furfural that is separated in connection with brine evaporation.
Sulphite pulping methods other than that dealt with in Example B will in the following be reviewed in view `~ -of the invention. Many sulfite mills still operate the -~
previously most common of the sulfite methods, using a `;
digestion acid containing calcium bisulfite and sulphur dioxide. These calcium base sulfite mills are often equipped with plants for spent liquor evaporation and manufacture of by-products from the sulfite spent (black) liquor and several show promise of profibability in the future.
Sulfite digestion acid containing both calcium and ammonium ions may be produced by using ammonia oxyliquor for absorption of SO2 from combustion of calcium base `
spent sulfite liquor. Operations according to the present B

- . . - -, ~ . ., ~.. , .. ~. . . . ..

invention can b~ supplemented with a calcium preclpitation of the digestion spent liquor to make it suitable for manufacture of various by-products, e.g. tanning, binding and dispergating agents.
All known methods for fermentaion of the carbohydrates of sulfite black liquor can be used within the scope of the invention. Any ammoniacal bleaching effluent, e.g.
oxyliquor, can be utilized for fermentation of Ca-base sulfite black liquor. Thus, according to the invention, either ammoniacal oxyliquor or a chloride-lean fraction of ammoniacal bleaching effluent can serve as nitrogen nutrient for fermentation of any sulfite black liquor which is combusted subsequent to the separation of the fermentation products. Alternatively brine may be sub-jected to separate fermentation, or its ammonia content `
can be expelled prior to separate combustion.
It has been pointed out that Ca- and NH4-ions in certain proportions can be utilized for single and multi-stage sulfite digestion followed by combustion of sulfite black liquor as a component of the lye according to the invention. However, combination of Ca- and Na-ions pro- ~`
duces an ash upon combustion of the lye, the properties of which ash make it difficult to handle. This ash pro-blem can be overcome by precipitation of Ca-ions by added Na2C03 followed by separation of the CaC03-precipitate prior to combustion. By this means the combustion of sodium hydroxide-containing oxy- and bleaching effluents as lye can be incorporated with a calcium base sulfite process that includes combustion of brine which thereby produces Na2C03 amd NaCl etc. According to established methods the NaCl can be separated and electrolyzed to chlorine B

. .

,............. . ~. . .. ..
~ . . . .
` . ~ . . .

bleaching chemiclls.
Cross-recovery between two processes, e.g. sulfite and kraft mills, with bleaching of some or all brown stock produced in the processes, can be arranged according to the principle of this invention while considering the chloride content of the various liquors or fractions thereof. Consequently, bleaching effluent, e.g. oxy-liquor, from one type of pulp can be utilized for black liquor displacement from another type of pulp while considering which combinations of treatment steps for chloride forming versus easily regenerable chemicals will be found to be most economical.

..'

Claims (17)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In the manufacture of bleached cellulose pulp from lignocellulosic raw material a method to reduce water pollution from bleaching effluent comprising the steps of delignifying the raw material with useful non-chlorine delignification agents to form spent delignification liquor and unbleached stock, bleaching said stock with a chlorine-compound containing bleaching agent to form a bleached stock and chlorine-compound substances con-taining bleaching effluent and applying bleaching effluent for displacement of the spent delignification liquor from said unbleached stock, and thereby employ means for counteracting chloride accumulation in plants for utili-zation of substances dissolved from the lignocellulosic raw material by said non-chlorine delignification agents of which agents on the most only a minor portion is generated from bleaching effluent, the improvement comprising:
(a) removing the major portion of the spent delignifi-cation liquor from the unbleached stock to form thereof a lye with on the most a minor portion of the chlorine-compound containing bleaching effluent entrained into the lye;
(b) removing in the unbleached stock remaining portion of the spent delignification liquor by displacement washing with the addition of a major portion of on said unbleached stock applied bleaching effluent in a counter-current washing system, to form brine of the applied bleaching effluent and which brine contains a higher concentration of chlorine compounds than the lye does;

(c) passing the brine, discharged from step (b), directly to a separate brine treatment system which comprises a step for thermally oxidating organic substances of the major portion of said brine by combustion to such degree that therein contained chlorine compounds form chloride and that any together with said chloride occurring sulphur compounds of lower oxidation numbers than IV are oxidized to at least oxidation number IV; and (d) subjecting the lye, discharged from step (a), to utilization of the substances which have been dissolved by the delignification and which are contained in said lye.

2. A method according to claim 1 which comprises removal of lye by concentrating the unbleached stock, i.e. by draining or pressing, in step (a) whereafter the thusly concentrated stock is supplied with bleaching effluent in step (b).

3. A method according to claim 1 which comprises removal of lye by displacement washing of the unbleached stock in step (a), whereby a mixture of lye and brine that is formed in the displacement washing front is allowed to accompany the lye in a counter current system to the extent that a tolerable chloride concentration can be maintained in the delignification liquors in step (d).

4. The method according to Claim 1, including the steps of delignificating the unbleached stock in two main steps of which the first step is done in a pulp digestion plant with useful pulping agents and the second one in an oxystage with oxygen to form oxystock and oxyliquor and introducing the oxyliquor into the spent delignification liquor of which the lye in step (a) is formed.

5. The method according to Claim 4, including the steps of delignificating the unbleached stock with oxygen in presence of alkalizing or buffering agents and introducing the oxyliquor into the spent delignification liquor of which the lye in step (a) is formed.

6. The method of Claim 1, wherein separation of chlorine from the chlorides produced in step (c) by thermal oxidation of the brine is accomplished by treating the chloride either by electrolysis or by reaction with chlorate.

7. The method of Claim 1, including the step of adding an ash forming base to the brine to react with the chlorides during thermal oxidation of the brine in step (c).

8. A method according to Claim 1, which comprises recovering bleaching effluent from bleach plant washing steps by using sea water while controlling precipitation of magnesium hydroxide in alkaline solutions and then transfer of its content of sodium chloride to the brine, which is combusted in step (c) to form ash of which a treated solution (purified brine) is electrolyzed to chlorine and alkali products, of which an excess produced from the sea water, can be utilized elsewhere.

9. The method according to Claim 8, wherein brine is evaporated in step (c) to produce fresh water.

10. The method according to Claim 1, including the step of utilizing lye in step (d) by means of reductive combus-tion of compounds of sodium and sulfur to produce sodium sulfide in a furnace discharging soda smelt.

11. The method according to Claim 1, including the steps of applying a bleaching effluent of one chlorine compound concentration for displacement of the spent delignifica-tion liquor in step (b) and thereafter continuing the displacement in said step using another bleaching effluent of a different chlorine compound concentration.

12. The method according to Claim 1, including the step of controlling formation of chloride in steps (c) and (d) by introducing an oxygen or oxygen compound bleaching stage in the sequence of bleaching with chlorine-compound con-taining bleaching agents.

13. The method according to Claim 1, including the steps of separating chlorine compounds from the lye utilization in step (d) system and then passing them to the brine treatment system, in step (c).

14. The method according to Claim 1 including the step of utilizing lye in step (d) by oxidative combustion of lye from magnesium base sulphite pulping black liquor to pro-duce magnesia ash useful to prepare magnesium bisulphite for delignification in the digestion plant and to prepare magnesium hydroxide for subsequent delignification of the digested stock and preferably in a step employing oxygen.

15. The method according to Claim 1, wherein in the pulp mill available bleaching effluents by useful means are either concentrated on substances or from the effluents separated fractions of substances are added to the thermal brine oxidation step (c).

16. In a method for counteracting chloride accumulation in a kraft process chemical recovery system for substances from liquor utilized in the delignification of ligno-cellulosic raw material comprising the steps of digesting said raw material with sodium hydroxide solutions (white liquor) to form spent alkaline delignification liquor (black liquor) and unbleached stock (brown stock), bleaching the brown stock with chlorine-containing bleaching agent to form bleached stock and bleaching effluent and recovering alkali and chlorine compounds from said liquors and effluent, the improvement comprising:
(a) removing black liquor from brown stock layers to form lye with a dilution limited to not more than about 20%
bleaching effluent, based on the volume of the black liquor;
(b) removing from the brown stock therein remaining portion of the black liquor by displacement washing with the addition of a major portion of said bleaching effluent in a counter-current washing system to form brine which brine has a higher concentration of chlorine compounds than the lye;
(c) passing the brine from the brown stock washing step (b), kept separated from the black liquor alkali recovery system, to a step for thermally oxidating organic sub-stances of said brine by combustion to such a degree that their chlorine compounds form chloride and that their sulfide compounds, deriving from the black liquor which remains in the unbleached stock and wherein sulfur has oxidation number II, form sulfate wherein the sulfur has oxidation number IV; and (d) regenerating from the lye, a sulfide containing white liquor (sodium hydroxide solution) for the digestion step which white liquor has an equilibrium sodium chloride concentration of at most 20% calculated on the basis of the sodium content.

17. The method according to Claim 16, including the steps of treating brown stock with oxygen to further delignify the brown stock to form still unbleached oxystock and oxyliquor and introducing the oxyliquor into black liquor and thereby produce lye for regeneration of white liquor in step (d).