CA1192707A - Method of polysulphide treatment of lignocellulose material in alkalic pulp production - Google Patents

Abstract

ABSTRACT
The invention relates to the production of polysulphide for the treatment of wood chips so that the consumption thereof is reduced in sulphate digestion of paper pulp. The pulp is prefer-ably subjected to such a bleaching that implies access to oxygen in the pulp factory. In an impregnation stage the wood chips are caused to react with black liquor, or digestion liquor prepared from white liquor and containing black liquor, while introducing oxygen of appropriate concentration. The content of monosulphide, and to a certain extent other sulphur compounds also of the liquors, then forms polysulphide in contact with the wood chips which instan-taneously adsorb polysulphide. The generation of polysulphide is promoted by wood reactions. Black liquor having a reduced content of hydroxides is extracted from the wood chips before delignificat-ion thereof in digestion liquor prepared of white liquor and resi-dual of impregnation liquor from the wood chips. The extracted black liquor is conveyed to a plant for regeneration of hydroxide in white liquor and sulphide totally or partly in the form of white liquor and which is again polysulphide-oxidized. The increase of the yield of pulp from the digester as well as after the bleaching stage is greater in oxidative polysulphide production in the presence of wood chips than in separate production thereof in liquor before the charging into wood chips. The invention is well adapted for continuously as well as batchwise performed pulp di-gestion.

Description

It has been known for a long time in pulp production by .~1 k" 1, ,~ ~means of ~lk~lic delignification of wood and other cellulGse~con-taining materials, e.g. accordiny to the soda and sulphate pro~
cesses, that the yield of the cellulose portion of the pulp can be increased by adding polysulphide to the digestion liquor.
Methodical studies of polysulphide digestion have been published by René Berthler (A.T.I.P., Bull. No. 4:93-106, 1953). This pioneer work deals not only with increase of yield and polysul phide analysis, but also with the regeneration of pol~sulphide from monosulphide by carbonating for producing hydrogen sulphide which is to be converted into elementary sulphur. This sulphur is con-verted from monosulphide into polysulphide~ Berthier also dealt with the subject of recovering sulphur dioxide from fumes derived from the combustion of black liquor in ovens emitting sulphide-containing soda melt. The sulphur dioxide is then converted with hydrogen sulphide originating from the soda melt for the generation of elementary sulphur.
In Swedish patent No. 160 736 (1957), Venemark disclosed a method in which a sulphide li~uor fraction prepared of soda melt was used for the production of polysulphide, the liquors otherwise being charged to the sulphate digestion in a conventional manner.
Peckham and May developed Berthier's work further (Tappi 43, No.
1:45-48, 1960), and within a short time thereafter Scoppls United States paten-t No. 2,944,928 was also disclosed. In his article (Paper Trade Journal, October 3, 1960) Scopp mentioned the prepara-tion of polysulphide "...prior to, or during the reaction with the ~gi~7V~

wood chips". He also noted the easiness of bleaching of the poly-sulphide pulp.
As far as is known, no sulphate factory has as yet been provided with polysulphide regeneration of -the Berkhier and Venemark type, and the efforts have been delimi-ted to the addition of elementary sulphur via make-up chemicals Eor reaction with the sulphide which defines the balance of the process. In order to make the best possible of this it is proposed in Norwegian patent 102,304 (Papirindustrins Forskningsinstitutt - Research Institute of Paper Industry) to -treat fresh white liquor and recirculated black liquor charged to a sulphate cooking or digestion in such a way that the mixture is partially oxidized with air oxygen. The oxidation is carried out such that a considerable portion of the (mono )sulphide of the mixture is transformed into polysulphide.
The liquor mixture is then charged to the wood.
Union Camp Corpora-tion discloses in Swedish patent Appli-cation No. 72 03993-6 (published specification No. 381,691) a method invented by Barker. Chips are impregnated with white liquor, and then black liquor is added. The sulphide of the liquor mixture is oxidized to polysulphide by means of a gas containing gaseous oxygen. The delignification of the wood then takes place with the generated volume of liquor. After the delignification black liquor is obtained from the pulp in a conventional way which means that the portion of black liquor that is not reused for the polysulphide oxidation is conveyed to the evaporation and combustion stages.
According to the Swedish patent No. 71 14168-3 which describes the MOXY-process, monosulphide is transformed to poly-sulphide at high efficiency by means of oxygen and a certain cata-lyzer.
In traditional, batchwise performed digestion of pulp intended for bleaching a normal charge is ton the basis of dry regarded wood).
per ton of softwood: 1.5 m3 of white liquor and 1.4 m3 of black liquor per ton of hardwood: 1.3 m3 of white liquor and 0.9 m3 of black liquor.
The black liquor is often of the same kind as that der-ived from the pulp wash to the evaporation plant. The total gain from the pulp wash is 4 to 5 m3 per ton of wood, and thus only one fourth of the black liquor substance is recirculated to the digest-ion stage. According to known technique, in addition to the fresh monosulphide of the white liquor about one fourth at the most of the monosulphide present in the digestion stage will be available for oxidation to polysulphide. Besides, it is known that poly sulphide is reverted to monosulphide during the digestion. Sulphide absorbed from the liquor by the wood as organic sulphiaes reverts to the liquor toward the end of the digestion stage. Thio-sulphate generated by non-desired reactions during the preparation of poly-sulphide also reverts partially to monosulphide in the digestion stage.
In high yield digestion of pulp, e.g. for kraft liner, the supplying of different sulphides from white liquor will be even ~3L9Z7~7 less than has been se-t forth above, but more black ~iquor can be recirculated.
The present invention relates to a method of preparirly polysulphide of practically the whole volume of the used diyestion liquor or black liquor, respectively, from the precediny diyestion, the white liquor then being completely or partially charged to the lignocellulose material or pulp being delignified. No black liquor is diverted to the stages of alkali regeneration, evapora-tion and combustion directly from the pulp, but from stages for pretreatment of the wood chips.
Thus, this invention provides a method of polysulphide treatment of lignocellulose material in alkalic pulp production comprising digestion with white liquor which contains sulphide, wherein used liquor with its content of black liquor substance consistiny of either diyestion liquor from injection diyestion or black liquor from conventional digestion is recirculated to fresh--supplied chips, the method relating to treatment of the chips with polysulphide which increases the yield of cellulose and is pro-duced by oxidation of the content of monosulphide of the liquors, characterized by the steps of: a) charging the chips with a volume of used liquor, said volume containing sulphide and being excessive with relation to the volume that can form part of the subsequent digestion stage, the surplus of liquor being then re-moved from the chips before said digestion stage in the form of black liquor and conveyed to a plant for regeneration of white liquor; b) charging white liquor to the chips in a stage sub~

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sequent to the removal of surplus liquor according to a) with regard to the content of remaining white liquor in the liquor used in the preceding digestion stage; c) oxidiziny of monosulphjde in the charge of liquor and chips according to a) with oxyyen or an oxygen-containing gas mixture to generate polysulphide heEore and/or after the removal of surplus liquor according to a); d) deligni-fication into pulp of the polysulphide-treated chips from c) with the white liquor in a subsequent digestion stage in which the poly-sulphide forms monosulphide in used liquor which completely or to a large extent is reacted with chips according to a) for reoxidat-ion to polysulphide.
A digestion method preferred especially for the inventionis the method of batchwise performed digestion described in the Swedish patent No. 73 15894-1. The present invention is applicable in general for such a multiple operation of batchwise operating digesters, irrespective of the presence of a possible subsequent bleaching. Sulphide-carrying white liquor is thus dosed to a digestion liquor containing black liquor components and being cir-culated through a digeste~ together with partially delignified wood or pulp, respectively. A digester filled with fresh wood chips is charged with digestion liquor and shut oEf from the common circulation of the digesters. A mixture temperature of wood chips and digestion liquor is then obtained, and a rapid consumption of the alkali of the black liquor takes place. At an appropriate point of time during this stage air is conveyed ~hrough the content of the digester, or is oxygen injected, preferably into circulatiny ~9~ 7 liquor, whereby the generation of sulphide will be at a maximum.
This takes place when abou-t half of the sum of monosulphide and reactable organic sulphide compounds has been converted. I~ the oxidization is continued the polysulphide is oxidized further to thio-sulphate. An exact measurement of the degree of polysulphide generation is more difficult to carry out according to the present method than in the previously known methods in which fresh white liquor i5 added.
Since the carbohydrate components of the wood react grad-ually with polysulphide from black liquor, which is gradually reused according to the in~ention, at the same rate as they are generated it is not purposeful to perform analyses of separate liquor samples afterwards. A measure of the efficie~y of the poly-sulphide generation is obtained by relating the increase of the yield of cellulose to the content of 1) total existing amount of sulphur in different sulphur compounds (substance of black liquor p~ in digestion liquor), and 2) n~trillm, of the liquor of the poly-sulphide stage. The major portion of these substances have passed previous digestion stages and a minor portion thereof have been immediately transferred from white liquor charged to diges-tion liquor.
The wood chips greedily absorb or consume, respectively, the content of hydroxide of the digestion liquor. For certain kinds of wood it may appear to be preferable to let the wood first absorb the content of hydroxide of the digestion ]iquor. During this stage the digestion liquor is transformed to black liquor with a '7~

lower pH than that of the digestion liquor, and duriny -the poly-sulphide oxidation pH is reduced further. With cer-tain kinds of wood a greater increase of the yield is obtained than that normally provided by the polysulphide alone. The comparison i5 based on the same rate of lignin release during the delignification to pulp of the woodO
In other cases it appeared to be more appropriate to inject air or alternatively oxygen at once and then let the wood react with hydroxide and polvsulphide. The polysulphide made use of by the wood is then reverted to monosulphide in the digestion/
blaGk liquor and can be partially oxidized again to polysulphide.
In order to be able to utilize the temperature of the digestion liquor in the stages of pretreatment of the wood to preserve heat for the subs uent digestion stages it is advantageous to use concentrated oxygen, preferably comprising at least 90% of

2' or the polysulphide oxidation. To obtain an even distribution thereof a certain volume of gas, possibly digestion exhaust gases with steam, may be recirculated through the liquor in contact with chips, and oxygen is in~ected therein as it is consumed. (This technique was used by the inventor to measure the absorption of air oxygen of the black liquor according to the Swedish patent No.
74 00366-6).
It is previously known that certain concentrations of polysulphide have a corroding effect on carbon steel and ordinary material in digesters and equipment for treatment of black liquor.
Nowadays resistive steel alloys are of~en used, but otherwise the ~Lg~7~

concentration of polysulphide should either be kept at a suffient-ly high level, or should the polysul.phide be removed. rrhus, it may be advantageous to further oxi.dize the sulphide compounds of the black liquor extracted from the chips to thio-sulphate and/or to let polysulphide react with sulphite, possibly from other sections of the process, to form thio-sulphate.
In Swedish patent No. 227,464, Swedish published speci-fication No. 369,611 and United States patent No. 4,155,806 the inventor discloses methods of continuous digestion in which washing liquor flows through the digester such that the wood chips are met by a digestion liquor containing black liquor, and a white liquor which has reacted more or less completely with the chips. It should then be advantageous to supply oxygen which is distributed in the liquor and the pulp of chips with a gas circulating in a closed circuit and possibly having about the same composition as Especially in continuous digestion it is possible to force the conswnption of alkali (hydroxide) of the wood chips so far that hydrogen sulphide is released which, however, is oxidized by sulphur and polysulphide reactions when oxygen is supplled.
Accordiny to the invention it is possible to achieve an additive yield-increasing effect between hydrogen sulphide and polysulphide.
Finally, hydrogen sulphide may in certain cases be cupelled from the black liquor depleted of hydroxide, and it is then recirculat-ed to the liquor system of the digester. Thio-sulphate reacts only to a small extent with the chips and will consequently be ~l~9~

removed with the black liquor to be regenerated to monosulphide in white liquor.
According to United States Patent 41155,806 pulp is subjected to further delignification with oxygen in oxy-,stayes in s~
the presence of hydroxide-generating-nat~iu-,, compounds, wherein is obtained oxy-pulp and oxy-liquor extracted therefrom. These methods may advantageously be applied to the present invention in such a way that oxy-liquor passes completely or partially through the polysulphide stage. The method has appeared to increase the yield of oxy-pulp as compared to that obtained if the oxy-liquor is conveyed to pass by the polysulphide stage. The surprising effect has not yet been explained but should be due to greater transformation of polysulphide and/or sorption of carbohydrates from the oxy-liquor. It is possible that oxidation products in the oxy-liquor cooperate in the oxidation of sulphide to poly-sulphide.
During the reaction of the sulphides with wood the proportion of oxidizable sulphur compounds in the surrounding black liquor decreases. Approximately between 1/5 and 1/2 of the sulphide compounds accompany the wood to the subsequent digestion st~ge in which they are again released in the digestion liquor.
According to the invention, with the reaction of digestion liquor and black liquor, respectively, with gradually supplied wood they will be recirculated in accordance with a certain state of balance in the digester. This balance i5 defined by the fact that even black liquor which is to be discharged to a regeneration plant, is _ g _ ~9~

reacted for polysulphlde reactions with the chips; unlike the known methods in which only the recirculated black liquor fraction required for the digestion is utilized.
It is known that generation of polysulphide results in a certain increase of the requirement of hydro~ide for the digest-ion. According to the invention, -this consumpt:ion of hydroxide is compensated for in that the remaining hydroxide even in the black liquor portion finally discharged to regeneration of chemicals is caused to react with wood. When considering the consumption of hydroxide it should, however, be regarded that a pretreatment stage which increases the yield, thus with sulphide or alternatively hydrosulphide and/or oxidation to polysulphide performed before or after said stage, results in less release of cellulose de-gradation products.
Polysulphide treatment of chips can be carried out batch-wise or continuously when conveyed to batchwise performed digest-ion. In continuous digestion only continuous pre-treatment should be of interest. According to an appropriate embodiment, treatment liquor is circulated through the chips via an outer system and oxy-gen is injected immediately before the liquor enters the treatment vessel. Since according to the invention final black liquor is extracted after the reaction with chips, digestion liquor or spent liquor is referred to as used liquor.
Example 1 White liquor containing 112.5 kgs of EA per m (EA =
effective alkali - NaOH + 1/2 Na2S calcula-ted as equivalent NaOH

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with regard to Na) and having a sulphidity Na2S x 100 Na2S -~ NaO~I

(calculated as equivalent NaO~I with regard to Na) of 31.4% was used. Per ton of wood regarded as dry, 174.8 kg~ of EA and 2~.1 kgs of S in the state of sulphide compounds and in total 31.8 kgs of sulphur were charged.
An amount of 4.5 m3 of black liquor was formed of white liquor, wood moisture, steam condensate, oxy-liquor used as washing liquid, and released substance. Of this amount 1.3 m3 of black liquor was charged to the subsequent digestion stage, whereby a wood: liquid ratio of 1:3.5 was obtained.
~ hite and black liquor were mixed in a separate vessel which was blown through b~ air bUbbleS.Analyse~ showed that 12 kgs of polysulphide sulphur per ton of wood has been formed. In order to maintain the previous charge of alkali at a kappa-number of 36 the maximum temperature was raised from 170 to 173C. During a period of test operation the wood consumption and the produced amount of pulp were measured and it appeared that the quantity of pulp had increased by 2% as compared to previous results of con-sumption of the same weight o~ wood regarded as dry.
~xample 2 The method was modified in such a way that white and ~lack liquor was first charged to the wood, and during the mixing circulation of the liquors air was suppli~d into the digester from 7~

below for one hour, and during this period the temperature dropped to about 80C. The capacity of the digestion plan-t was reduced to 85 of normal in spite of the fact that the maximum temperature was raised from 170 to 176C and the charye of white liquor was increased to 190 kgs of EA per ton of wood. The charc~e of black liquor was consequently reduced to about 1.1 m3 whereby the liquor mixture contained 1.7 x 100 = ~0% of white liquor. The available 1.1 ~ 1.8 amount of sulphide was equivalent to 28 + 284x51.1 = 35 kgs of sulphur per ton of wood. The generation of polysulphide could not be determined by analysis of the liquor mixture, due to its instan-taneous reaction with the chips. The amount of oxy-pulp increased by 1% as calculated on the obtained pulp from the same weight of consumed wood according to Example 1.
Example 3 The boiling or digestion plant was equipped with a digestion liquor accumulator as described in the Swedish patent 73 15894-1. After filling with chips used liquor in the form of digestion liquor from a liquor accumulator was introduced, whereby the digester was completely filled with liquor and finally reached the same pressuxe as that in the digestion liquor accumulator. Gas containing oxygen was introduced from a gas circulation system through nozzles in the bottom portion while the liquor was circu-lated through the digester. Gas and foam were removed at the top and recirculated. The consumed volume of oxygen was replaced by 7~7 concentrated oxygen. The circulatiny gas contained about 25% by volume of 2 which was favourable with regard to even generation of polysulphide during the period of 30 minutes devoted thereto.
The temperature was kept at about 120C. When the through flow of gas bubbles haa been terminated, the content was exposed to two hydrostatic pressure changes, and after that a degassing was carried out. A volume of black liquor was extracted from the digester to the regeneration plant, said volume corresponding to the compensation for the volume added to the liquor accumulator by supply of white liquor and washing liquid, etc. Samples of wood chips drained of black liquor, and samples of surrounding black liquor were analysed with regard to their content of sulphur compounds. The result was that 20 kgs of sulphur per ton of wood was obtained in addition to the content of the surrounding black liquor. During the digestion the polysulphide is released and again forms monosulphide which together with monosulphide of the recirculated black liquo~ is again reacted with new wood to poly sulphide.
During the process thio-sulphate is enriched which only to a small extent reacts with the wood but can revert to sulphide via reactions in which a number of other sulphur compounds partici~
pate. I'he sulphur compounds which do not increase the yield were grad~ally removed with the black liquor which also contained a certain amount o~ polysulphide and regeneratable sulphide.
In the digestion trials the wood chips were weighed and their content of water was analysed, and the quantity of bleached pulp obtained therefrom was defined in the same way. By comparison - 13 ~

:~9;~7~'7 based on the consumed amount of wood it appeared that the method according to the inventlon produced 2.5% more pulp than when only the black liquor recirculated to the digestion stage ~as reacted with the chipso It appeared that by the polysulphide treatment according to the invention the pulp was bleached more easily to the desired brightness of 92 SCAN, and a certain digestion delignification could be transferred to the oxy-stage. In this way it was possible to revert to the maximum temperature of 170~C used in conventional 0 digestion and a charge of 160 kgs of EA per ton of wood.
Example 4 The process was carried out witha practically closed system in regard to both gaseous sulphur compounds and those found in liquids. Chlorine dioxide for the bleaching of the pulp was p~oduced electrolytically with supply of hydrochloric acid. Sodium hydrate for the bleaching was reyenerated according to the Swedish patent No. 59 09674-1.
Losses of sulphur and natrium were met generally with Na2SO4 and oil having a high content of sulphur. Generated H2S and SO2 etc~ were converted to elementary sulphur which was charged to the polysulphide-oxidized black liquor in the chips pretreatmen-t stage after the removal of black liquor necessary for the liquid balance had taken place. The sulphidity of the white liquor intended for digestion was maintained at about 50% by diverting some recovered SO2 i.e. to the washing stage for chlorine dioxide bleached pulp of the bleachery. Digestion took place in a multi-7~7 ple digestion plant according to the Swedish patent No. 73 15894-1.
By these measures 10~ more pulp was obtained than :in conven-tional sulphate digestion without polysulphide. (End of examples).
During recent development work on the invention it has appeared that the measuring of redox potential in batchwise respectively continuously operating polysulphide reaction stages provides the possibility of optimizing the reac-tion to ob-tain the greatest increase of pulp yield. It has also appeared possible according to Ants Teder (Svensk Papperstidning - Swedish Paper Journal - 1968 p. 149-160) to follow the relationship between zero-sulphur (by Teder denominated surplus sulphur) respectively two--sulphur (sulphide) in polysulphide Na2Sl+n in relation to alkality. It has appeared in liquor oxidation in the presence of wood that values of zero-sulphur of n=1.5-2.5 may be achieved by participation of Na2S that has been recirculated via preceding digestion of polysulphide treated chips.
Studies have been made concerning the production of sulphite which is included in the white liquor for completion of the digestion stage, from black liquor combustion gas containing SO2. The reoxidation to polysulphide, however, is hampered by reactions with sulphite. Sulphite promotes the oxidation of final-ly removed black liquor of sulphide to thio-sulphate and sulphate by means o~ air or oxygen. It is advantageous to oxidize sulphide and polysulphide already in connection with the removal of -the surplus of black liquor from the chips treatment stage, whereby dis-charge of volatile and evil-smellirlg sulphur compounds from ~vd~ul~tion and combustion is prevented as far as possible. Fones and Sapp have dealt with the oxldation of black liquor with oxyyen in connection with sulphate pulp digesters (Tappi, April 1960, Vol.
43, 369). If the oxidation of black liquor takes place under pressure the oxidation heat can be utilized as yenerated steam for the e~aporation (Rydholm, Swedish Paper Journal, 31 May 1968, 417, respectively Magnotta et al, Pulp ~ Paper, August 1981, 150).
In the Swedish patent No. 74 07539-1 some oxidized black liquor is used for adsorption of evil-smelling gases containing sulphur, and the liquor enriched oE sulphur is subsequently oxidized in a closed system in connection with the evaporation of all the black liquor.
According to the inven-tion, the polysulphide reaction of the chips can be carried out with liquor used for digestion and consisting of - either black liquor extracted from the pulp, a surplus of said black liquor being conveyed to a plant for regeneration nf the alkali of the white liquor - or respectively spent digestion liquor which is recircu-lated from digested pulp to the digestion stage and is then rein-forced with white liquor while a surplus of (spent) digestion liquor is reacted with chips to form a black liquor which is sub-sequently conveyed to the plant for regeneration of alkali.

Claims (10)

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

1. A method of polysulphide treatment of lignocellulose material in alkaline pulp production comprising digestion with white liquor which contains sulphide, wherein used liquor with its content of black liquor substance consisting of either digestion liquor from injection digestion or black liquor from conventional digestion is recirculated to fresh-supplied chips, the method relating to treatment of the chips with polysulphide which increases the yield of cellulose and is produced by oxidation of the content of mono-sulphide of the liquors, characterized by the steps of:
a) charging the chips with a volume of used liquor, said volume containing sulphide and being excessive with relation to the volume that can form part of the subsequent digestion stage, the surplus of liquor being then removed from the chips before said digestion stage in the form of black liquor and conveyed to a plant for regen-eration of white liquor;
b) charging white liquor to the chips in a stage subsequent to the removal of surplus liquor according to a) with regard to the content of remaining white liquor in the liquor used in the preceding digestion stage;
c) oxidizing of monosulphide in the charge of liquor and chips according to a) with oxygen or an oxygen-containing gas mixture to generate polysulphide before and/or after the removal of surplus liquor according to a);
d) delignification into pulp of the polvsulphide-treated chips from c) with the white liquor in a subsequent digestion stage in which the polysulphide forms monosulphide in used liquor which complete-ly or to a large extent is reacted with chips according to a) for reoxidation to polysulphide.

2. A method according to claim 1 in which the pulp digestion is performed batch-wise, characterized by charging the white liquor to and/or during the digestion of the chips which have reacted with polysulphide, the used liquor charged in step a) containing white liquor products.

3. A method according to claim 1 in which the pulp digestion is performed continuously, characterized by charging the major portion or all of the white liquor after removal of black liquor from the chips when the latter, together with polysulphide-black liquor enclosed therein or adhering, have reached the digestion zone of the digester in counter-current with freely flowing digestion liquor/black liquor.

4. A method according to claim 1 characterized by charging a white liquor having a small content of sulphide to the digestion stage, a fraction of white liquor respectively sulphide liquor separated during the production of this white liquor and having a higher percentage of sulphide being charged to the liquor which contains black liquor and is recirculated for the production of polysulphide.

5. A method according to claim 2 characterized by charging a white liquor having a small content of sulphide to the digestion stage, a fraction of white liquor respectively sulphide liquor separated during the production of this white liquor and having a higher percentage of sulphide being charged to -the liquor which contains black liquor and is recirculated for the production of poly-sulphide.

6. A method according to claim 3 characterized by charging a white liquor having a small content of sulphide to the digestion stage, a fraction of white liquor respectively sulphide liquor separated during the production of this white liquor and having a higher percentage of sulphide being charged to the liquor which contains black liquor and is recirculated for the production of polysulphide.

7. A method according to claim 4, 5 or 6, characterized by charging the white liquor enriched of sulphide after the removal of black liquor from the chips after a first polysulphide oxidation, the oxidation being repeated after the addition of sulphide and the oxidized liquor being subsequently conveyed to the digestion stage together with the chips.

8. A method according to claim 1, characterized in that alkali of used liquor for step a) as far as possible is consumed by the acid components of the chips, and that the alkali-impover-ished black liquor formed thereby is removed and conveyed to an alkali regeneration plant.

9. A method according to claim 1, characterized in that polysulphide is prepared in the white liquor before the chips is charged therewith, as in the Moxy process.

10. A method according to claim 1, characterized in that polysulphide is prepared in the white liquor before the chips is charged therewith, by adding either elementary sulphur or by oxidation of the white liquor by means of oxygen or a gas mixture containing oxygen, as in the Moxy process.