EP0091457B1

EP0091457B1 - A method of polysulphide treatment in alkaline pulping of lignocellulose material

Info

Publication number: EP0091457B1
Application number: EP82903084A
Authority: EP
Inventors: Nils V Mannbro
Original assignee: Individual
Current assignee: Individual
Priority date: 1981-10-21
Filing date: 1982-10-13
Publication date: 1986-12-30
Also published as: SE8106203L; DE3274887D1; EP0091457A1; WO1983001469A1; ATE24556T1

Abstract

Preparation of polysulphide for treatment of wood chips so that the wood consumption is reduced in kraft digestion of paper pulp. In an impregnation stage the wood chips are caused to react with black liquor, or black liquor components containing digestion liquor prepared of white liquor, while introducing oxygen of appropriate concentration in an impregnation vessel. The content of monosulphide, and to a certain extent other sulphur compounds present in the liquors, thereby forms polysulphide in contact with the wood chips which instantaneously absorb the polysulphide. Generation of polysulphide is promoted by wood reactions. Black liquor having a reduced concentration of hydroxide is withdrawn from the wood chips before delignification thereof in digestion liquor prepared of white liquor and residual impregnation liquor from the wood chips. The withdrawn black liquor is conveyed to a plant for regeneration of hydroxide in white liquor and sulphide, which wholly or partly is recovered in the form of white liquor, and which sulphide is repeatedly polysulphide-oxidized. The yield increment of pulp from the digester, as well as after the bleaching stage, is greater in oxidative polysulphide processing in presence of wood chips than in separate processing thereof in liquor before the charging into wood chips. This is interpreted such that more polysulphide is formed in the presence of chips than without.

Description

It has been known for a long time in pulping by alkaline digestion of wood and other cellulose-containing materials, e.g. according to the soda and sulphate or kraft processes, that the yield of the cellulose portion of the pulp can be increased by the use of polysulphide in the digestion liquor. Methodical studies of polysulphide digestion have been published by René Berthier (A.T.I.P., Bull. No. 4:93-196, 1953). This pioneer work deals with not only increase of pulp yield and polysulphide analysis, but also the regeneration of polysulphide from monosulphide by carbonation for release of hydrogen sulphide which is converted into elementary sulphur. This sulphur is reacted with monosulphide to form polysulphide. Berthier also dealt with the subject of recovering sulphur dioxide in flue gas from combustion of black liquor in furnaces for recovery of sulphide-containing soda smelt. The sulphur dioxide is then converted with hydrogen sulphide orginating from the smelt for generating elementary sulphur.
In Swedish patent No. 160 736 (1957) Venemark disclosed a kraft pulping process in which a sodium sulphide liquor prepared of fraction smelt is used for the production of polysulphide, the white liquor from the other fraction is charged to the digestion in a conventional manner. Peckham and May developed Berthier's work further (Tappi 43, No. 1:45-48,1960), and within short time also Scopp's U.S. patent No. 2 944 928 was disclosed. In his article (Paper Trade Journal, October 3, 1960) Scopp mentioned the preparation of polysulphide "... prior to, or during the reaction with the wood chips". He also noted the easiness of bleaching of polysulphide pulps.
As far as is known no kraft mill has been provided with polysulphide regeneration of the Berthier and Venemark type, and the efforts have merely a make-up of elementary sulphur for reaction with sulphide. However, there is a balance between make-up sulphur and sulphur losses from the process. In order to make the best possible use of sulphide produced from any sulphur make-up it is proposed in Norwegian patent 102304 (Papirindustrins Forsknings- institutt-Research Institute of Paper Industry) to treat fresh kraft white liquor and recirculated black liquor charged to a digestion step in such a way that the sulphide content of 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 Corporation discloses in Swedish patent application No. 72 03993-6 (publication 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 entire volume of liquor and chips. After the delignification black liquor is recovered from the pulp in a conventional washing step which means that the portion of black liquor that is not recycled to the digestion plant for the polysulphide oxidation is conveyed from the pulp washing to the evaporation and combustion stages.
Norwegian patent No. 142 408 describes a method of treatment of cellulose material comprising certain stages such as a pretreatment stage, a digestion (delignification) stage, and stages for processing black liquor to regenerate white liquor hydroxide. The black liquor is withdrawn from digested raw material and a portion of this black liquor is passed outside the digester to the raw material in the pretreatment stage.
According to Swedish patent No. 71 14168-3 (publication No. 388 882) which describes the MOXY-process, white liquor monosulphide is transformed to polysulphide at high efficiency by means of oxygen and a certain catalyzer.
In traditional batch digestion of bleachable pulp normal liquor charge is (on moisture-free basis):

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 charged to the digestion step is often of the same kind as that conveyed from the pulp wash to the evaporation plant. The total recovered black liquor volume from the pulp wash is 4 a 5 m³ per ton of wood, and thus only one fourth of the black liquor substance is as indicated above recycled to the digestion stage. Thus only about one fourth of the monosulphide present in the digestion stage would, at the most, be available for oxidation of black liquor to polysulphide according to known technique. It is known that polysulphide is reverted to monosulphide during the digestion. Sulphide absorbed from the liquor by the wood as organic compounds is solubilized towards the end of the digestion stage and some sulphide is split off. Thio-sulphate generated by non-desired reactions during the preparation of polysulphide may revert partially to monosulphide in the digestion stage.
In high yield digestion of pulp, e.g. for kraft liner, the supplying of various sulphides from charged white liquor will be less than has been set forth above, but more batch digester volume available for black liquor recycle.
The present invention relates to a method of preparing polysulphide of practically the whole volume of the used digestion liquor or black liquor, respectively, from the preceding digestion, the white liquor then being completely or partially charged to the lignocellulose material or pulp being delignified. Recovered black liquor is not diverted directly from the pulp to the stages of alkali regeneration, i.e. evaporation and combustion steps, but from stages for pretreatment of the wood chips to said alkali generation steps.
Polysulphide treatment of wood chips can be carried out batchwise or continuously when connected to batch digestion. In continuous digestion only continuous pre-treatment should be of interest. According to an appropriate embodiment, polysulphide treatment liquor is circulated through the chips via an external pipe system and oxygen is injected immediately before the liquor enters the treatment vessel. Since according to the invention ultimate black liquor is recovered after the polysulphide reaction with chips, other liquors used in the digester liquor circulation system subsequently to the polysulphide stage are referred to as digestion liquor or spent digestion liquor or just spent liquor.
A digestion method preferred for the invention is the digestion method described in Swedish patent No. 73 15894-1 (publication No. 381 897). The present invention is applicable in general for such an operation of a cooking liquor system for multiple digesters, irrespective of the presence of a possible subsequent bleaching. Sulphide- carrying white liquor is thus dosed to digestion liquor containing black liquor components and which cooking liquor being circulated through a multiple of digesters with various contents of partially delignified wood or pulp, respectively. A digester filled with fresh wood chips is charged with digestion liquor and then shut off from the common cooking liquor circulation system of the digesters. A mixture temperature of wood chips and digestion liquor is arrived at, and a rapid consumption of the alkali of the black liquor takes place. At an appropriate point of time during this stage oxygen or oxygen containing gas is injected and conveyed through the content of the digester, preferably into circulating liquor, whereby the generation of polysulphide will be at maximum. This takes place when about half of the sum of monosulphide and reactable organic sulphide compounds has been converted. If the oxidization is continued the polysulphide is oxidized further to thiosulphate.
Exact measurement of the degree of polysulphide generation is more difficult to carry out in presence of wood chips according to the present method than in previously known methods.
Black liquor is successively reused according to the invention and the carbohydrate components of the wood react directly with the generated polysulphide. Therefore it is not purposeful to perform analyses of separate liquor samples afterwards. A measure of the efficiency of the polysulphide generation is obtained by relating the increase of the yield of cellulose in the pulp to the content of the liquor of the polysulphide stage of 1) total amount of sulphur in various sulphur compounds of black liquor solids present in digestion liquor and 2) sodium. These substances have to a large extent passed previous digestion stages while a minor portion thereof may have been immediately transferred from white liquor charged to digestion liquor.
The wood chips rapidly absorb or consume, respectively, the content of hydroxide of the digestion liquor. For certain wood species it may be preferred to let the wood first react with the content of the hydroxide of the spent digestion liquor. During this reaction the pH of the spent - digestion liquor is lowered and during the polysulphide oxidation pH is further decreased. With certain wood species the yield gain is improved as compared to that normally obtained by polysulphide reaction alone. The comparison is based on pulps of the same degree of delignification from the same lot of wood chips.
In other cases it appeared to be more appropriate to inject oxygen containing gas or, alternatively, pure oxygen and then let the wood react with hydroxide and polysulphide. The polysulphide utilized by reactions with wood reverts to monosulphide in the digestion stage and the spent liquor therefrom is essentially reoxidized to polysulphide.
To preserve heat from the subsequent digestion stages and reduce steam consumption it is advantageous to utilize the temperature of the digestion liquor in the stages of pretreatment of the wood. Therefore concentrated oxygen, preferably comprising at least 90% of 0₂ should be applied for the polysulphide oxidation. To obtain a uniform distribution thereof an appropriate volume of gas, advantageously digester relief gases with water may be recirculated through the polysulphide producing liquor in contact with chips, and oxygen is injected therein in the same rate as it is consumed. This technique was used by the inventor to measure the absorption of air oxygen of black liquor according to Swedish patent No. 74 00366-6 (publication No. 386 693).
It is previously known that certain concentrations of polysulphide cause corrosion of carbon steel and ordinary material in digesters and equipment for treatment of black liquor. Resistant steel alloys are nowadays used but otherwise the concentration of polysulphide should either be kept at a sufficiently high level, or should the polysulphide be removed. Thus the black liquor extracted from the chips may be advantageously further oxidized to convert the sulphidic compounds into thiosulphate. Alternatively polysulphide can be reacted with sulphite, possibly from other sections of the process, to form thiosulphate.
In Swedish patent No. 227 464, Swedish patent No. 369611 and Swedish patent application No. 7406537-6 (U.S. patent No. 4155806) this inventor disclosed methods of continuous digestion with in-digester washing. The flow of the washing liquor through the digester is in counter-current fashion such that the bed of wood chips is impregnated by a spent digestion liquor which has reacted more or less completely with the chips. It should in such case be advantageous to supply oxygen which is dispersed in the polysulphide reaction liquor and the chips material with a gas that is circulated in a closed circuit. Such gas may possibly have about the same composition as air.
It is according to the invention possible to continue the consumption of alkali (hydroxide) of the wood chips so far that hydrogen sulphide is released which, however, when oxygen is supplied, is oxidized by sulphur and polysulphide reactions. Especially continuous digestion is suited for this process. The invention features an additive yield-increasing effect from hydrogen sulphide and polysulphide. In certain cases hydrogen sulphide gas may be expelled from black liquor that is depleted of hydroxide, and the gas is then recirculated to the liquor system of the digester. Thio-sulphate reacts only to a small extent with the chips and will consequently be removed with the black liquor to be generated to monosulphide in white liquor.
According to Swedish patent No. 7315894-1 (publication No. 381 897) and Swedish patent application No. 74 06537-6 (& US-A-4155806 and SE-B-394466) chips are reacted with black liquor which is removed prior to digestion to produce pulp which is subjected to further delignification with oxygen in oxy-stages in the presence of hydroxide-generating sodium compounds. Oxy-liquor is extracted from the obtained oxy-pulp. This delignification method may advantageously be applied to the present invention in such a way that oxy-liquor is passed completely or partially through the polysulphide stage. Thereby a yield increment of oxy-pulp was indicated as compared to that obtained in the case wherein oxy-liquor was not conveyed to the polysulphide stage. The surprising effect has not yet been explained but should be due to higher conversion to polysulphide and/or sorption by pulp of carbohydrates from the oxy-liquor. It is considered that oxidation products in the oxy-liquor may cooperate in the oxidation of sulphide to polysulphide.
The concentration of oxidizable sulphur compounds in the surrounding black liquor decreases in the course of reaction of the various sulphides with wood. Approximately between 1/5 and 1/2 of the reacted sulphide compounds accompany the wood to the subsequent digestion stage in which they are re-dissolved in digestion liquor. They will recirculate according to a liquor volume equilibrium in the digester system which, according to the invention, is determined by the reactions of digestion liquor and black liquor, respectively, with charged wood material. This equilibrium is governed by the fact that black liquor which is subjected to regeneration of its chemicals is first utilized for polysulphide reactions with the chips. The known methods, however, utilize only the fraction of black liquor required for subsequent digestion.
It is known that polysulphide treatment of chips affects increased hydroxide demand. According to the invention, this demand is compensated for by the fact that the hydroxide that remains in the black liquor portion which finally is discharged to regeneration of chemicals is first utilized in neutralizing reactions with wood. When considering the consumption of hydroxide it should, however, be reminded that a yield increasing pretreatment stage, e.g. with sulphide, hydrosulphide and/or oxidation to polysulphide, results in less formation or alkaline degradation products of cellulose.

Example 1

White liquor containing 112.5 kg of EA per m³ (EA=effective alkali=NaOH+1/2 Na₂S calculated as equivalent NaOH based on Na) and having a sulphidity
of 31.4% was used. Sulphidity is calculated as equivalent NaOH based on Na. The chemical charges to the digestion are reported per ton of wood chips moisture-free basis, 174.8 kg of EA and 26.1 kg of S in the state of sulphide compounds and in total 31.8 kg of sulphur present in various sulphur compounds were charged.
An amount of 4.5 _M ³ of black liquor was formed of white liquor, wood moisture, steam condensate, oxy-liquor used as washing liquid and substance of digestion products. 1.3 m³ of preformed black liquor was charged for re-use in the subsequent digestion stage, whereby a ratio wood:liquor of 1:3.5 was obatined.
White and black liquor were mixed in a separate vessel into which air was injected to form air bubbles. Analyses showed that 12 kg of polysulphide sulphur per ton of wood had been formed. In order to maintain the previously applied charge of alkali for a target kappa-number of 36 the maximum temperature was raised from 170 to 173°C. During a period of test operation the wood consumption and the produced amount of pulp were measured and it was found that the quantity of pulp had increased by 2% as compared to previous results of consumption of the same wood quantity (moisture-free basis).

Example 2

The method was modified in such a way that white and black liquor was first charged to the wood chips. The mixed liquor was circulated through the batch of chips and simultaneously air was supplied into the digester from below for one hour. During this period the temperature dropped to about 80°C. The capacity of the digestion plant was reduced to 85% of normal in spite of the fact that the maximum digestion temperature was raised from 170 to 176°C and the charge of white liquor was increased to 190 kg of EA per ton of wood. The charge of black liquor was consequently reduced to about 1.1 m³ whereby the liquor mixture contained
of white liquor.
The available amount of sulphide was equivalent to
per ton of wood. The generation of polysulphide could not be determined by analysis of the liquor mixture, due to its instantaneous reaction with the chips. The pulp was alkali-oxygen delignified. The amount of oxy-pulp increased by 1% as calculated on the obtained pulp from the same quantity of wood as according to Example 1.

Example 3

The batch digestion plant was equipped with a digestion liquor accumulator as described in Swedish patent No. 7315894-1 (publication No. 381 897). Used liquor in the form of digestion liquor from the liquor accumulator was introduced after filling the digesters with chips. Each digester was completely filled with liquor and finally the same pressure was reached as that in the digestion liquor accumulator. Oxygen containing gas was introduced from a gas circulation system through nozzles in the bottom zone of the digester while the liquor was circulated through the digester content. Gas and some foam were removed at the top and recirculated. The consumed volume of oxygen was replaced by concentrated oxygen. The circulating gas contained about 25% by volume of O2 which was favourable with regard to even generation of polysulphide during the period of 30 minutes devoted thereto. The temperature was kept at about 120°C. When the through flow of gas bubbles had been terminated, the digester content was exposed to two hydrostatic pressure changes, and after that a gas relief. A surplus volume of black liquor was withdrawn from the digester to the chemical regeneration plant and the surplus derived from the volumes added to the liquor accumulator by supply of white liquor, wood moisture 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 showed that 20 kg more sulphur in various compounds per ton of wood was taken up by the chips than what corresponded to the concentration of surrounding black liquor. During the digestion the sulphur in the polysulphide is released from the wood and again forms monosulphide which together with monosulphide of the recirculated black liquor is again reacted with new wood to polysulphide.
During the liquor recirculation in the process thiosulphate is enriched which only to a small extent reacts with the wood but can to some extent revert to sulphide via reactions in which a number of other sulphur compounds participate. Sulphur compounds which do not affect the pulp yield were gradually passed to the chemical generation plant with the removed black liquor which also contained a certain amount of polysulphide and regenerable sulphide.
In the digestion trials the wood chips were weighed and moisture content measured. The quantity of bleached pulp obtained therefrom was determined in the same way. On basis of consumed wood quantity it was calculated that the method according to the invention produced 2.5% more pulp than when the chips only were reacted with the black liquor portion which was recirculated to the digestion stage.
Polysulphide treatment according to the invention produced pulp which was bleached more easily than conventional pulp to the target brightness of 92 SCAN. Therefore more delignification could be transferred from digestion to the oxy-stage. Thus it was found advantageous to adjust to the maximum digestion temperature down to 170°C that was used in conventional digestion and reduce the charge of EA to 160 kg per ton of wood.

Example 4

The process was applied on a practically closed system in regard to both gaseous and liquid emissions of sulphur compounds. Chlorine dioxide for pulp bleaching was produced electrolytically with supply of hydrochloric acid. Sodium hydroxide liquor for bleaching was regenerated according to Swedish patent No. 6909674-1 (publication No. 360129).
Losses of sulphur and sodium compounds were met with make-up Na₂SO₄ and oil having a high content of sulphur. In the process generated H₂S and S0₂ etc. were converted to elementary sulphur which was charged to the polysulphide-oxidized black liquor in the chips pre- treatment stage after the removal of excess black liquor. The sulphidity of the white liquor for digestion was maintained at about 50% by diverting some recovered S0₂ to the bleachery, i.a. to the washing stage for chlorine dioxide bleached pulp. Digestion took place in a multiple batch digestion plant according to Swedish patent No. 73 15894-1 (publication No. 381 897). By these measures 10% more pulp was obtained than in conventional sulphate digestion without polysulphide.

(End of examples)

Recent development work on the invention has demonstrated that the measuring of redox potential in batchwise respectively continuously operating polysulphide reaction stages provides proper means of optimizing the reaction to obtain the greatest increase of pulp yield. It has been proposed by Ants Teder (Svensk Papper- stidning-Swedish Paper Journal-1968, p. 149-160) to record the relationship between zero-sulphur (by Teder denominated surplus sulphur) respectively two-sulphur (sulphide) in polysulphide Na₂S_l,, in relation to alkalinity. Separate analyses have shown that spent digestion liquor oxidation in presence of wood produces polysulphide with values of zero-sulphur of n=1.5-2.5 which comprises Na₂S that has been consentively recirculated via preceding digestion batches of similarly polysulphide treated chips.
Proposals have been presented concerning recovery of sulphite from black liquor combustion gas containing S0₂ which sulphite is included in the white liquor supplied for completion of the digestion stage. The reoxidation of monosulphide to polysulphide, however, is hampered by reactions with sulphite. Because sulphite forms thio- sulphate it promotes ultimate oxidation of finally removed black liquor of sulphide to thio-sulphate and sulphate by means of air or oxygen. It is advantageous to ultimately oxidize sulphide and polysulphide already in connection with the removal of the surplus of black liquor from the chips treatment stage. Such oxidation, so called polishing, prevents discharge of volatile and evil-smelling sulphur compounds from evaporation and combustion at high efficiency. Fones and Sapp have dealt with the oxidation of black liquor with oxygen in connection with discharge of kraft pulp digesters (Tappi, April 1960, Vol. 43, 369). When the oxidation of black liquor takes place in a pressurized vessel the oxidation heat can be utilized as from the black liquor generated steam in the evaporation effects (Rydholm, Swedish Paper Journal, 31 May 1968, 417, respectively Magnotta et al, Pulp & Paper, August 1981, 150). In Swedish patent No. 74 07539-1 (publication No. 393 824) some with oxygen gas in an evaporation plant oxidized black liquor is used for absorption of evil-smelling gases containing sulphur, and the liquor enriched of sulphur is ultimately oxidized in a closed system in connection with final evaporation of all black liquor conveyed to the combustion furnace.
According to the invention, the polysulphide reaction of the chips can be carried out with spent digestion liquor, i.e. in the digestion used liquor, and consisting of

- either black liquor removed from the pulp, a surplus of said black liquor being directly or indirectly conveyed to a plant for regeneration of the alkali of the white liquor,
- or digestion liquor used for delignificating reaction of polysulphide treated chips, respectively spent digestion liquor, which from digested pulp is recirculated to the digestion stage and is then reinforced with fresh white liquor while a surplus of such (spent) digestion liquor is reacted with fresh chips to form an ultimate spent liquor to the chemical recovery plant for regeneration of alkali.

Claims

1. Method of polysuphide treatment of lignocellulose material in alkaline pulp production by digestion of the material, preferably wood chips, with a digestion liquor prepared of white liquor containing sulphide, wherein

A) polysulphide is produced by oxidation of liquor containing monosulphide in a pretreatment stage in the presence of newly charged lignocellulose material by reaction with oxygen or a gas mixture containing oxygen;

B) the material reacted with polysulphide for increasing the yield of delignified cellulose is in a subsequent stage charged with fresh white liquor for delignification with digestion liquor;

C) monosulphide-containing liquor which has been used in a subsequent delignification stage with contents of substance dissolved from the material is withdrawn from said stage and reacted according to A);

D) used monosulphide-containing liquor according to C) is obtained from a known group of digestion methods comprising continuous as well as batch digestion of material during injection of white liquor with either succession injection of white liquor during the digestion or traditional initial charge of white liquor for digestion and removal from the formed pulp of used digestion liquor in the form of black liquor, characterized in that

a) the material is reacted with a volume of used liquor in the pretreatment stage with supply of oxygen and in-line recycling of any gas mixture, said volume being maintained during the reaction with the material at a surplus in relation to the volume which remains with the pretreated material and is included in a subsequent delignification stage;

b) the surplus liquor from the polysulphide reaction a) is withdrawn to a plant for regeneration of hydroxide in white liquor, said surplus liquor preferably comprising black liquor which is conveyed to an evaporation and combustion plant;

c) the surplus liquor according to b) is withdrawn from newly-charged material, the demand of hydroxide of which being partly met by the content of hydroxide of the surplus liquor and thereby partly compensating the consumption of hydroxide of the reaction liquor which remains with the material subjected to subsequent delignification stage, wherein polysulphide is reverted to monosulphide;

d) the major portion of monosulphide formed in the delignification stage is recirculated in the form of used digestion liquor to the pretreatment stage according to a).

2. Method according to claim 1, in which the pulp digestion is performed batchwise, 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 pulping reaction products.

3. 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 treated chips, 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 and black liquor respectively.

4. Method according to any of claims 1-3, characterized by charging a fraction of white liquor having a low content of sulphide to the digestion stage, while a fraction of high sulphide white liquor respectively sulphide liquor, separated during the production of the low sulphide white liquor, and being charged to the used liquor which contains black liquor forming solids and which used liquor is recirculated for the production of polysulphide.

5. Method according to claim 4, characterized by charging the white liquor enriched of sulphide after the removal of black liquor from the chips subjected to a first polysulphide oxidation, a second oxidation being performed after the addition of sulphide enriched liquor and the oxidized liquor mixture being subsequently conveyed to the digestion stage together with the chips.

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

7. Method according to claim 1, characterized in that polysulphide is prepared in the white liquor before it is charged to the chips, preferably by adding either elementary sulphur or by oxidation of the white liquor by means of oxygen or a gas mixture containing oxygen.