SE0400568A1 - Procedure for pre-treating chips before cooking by "cold" impregnation - Google Patents

Abstract

The strength properties of chemical cellulose pulp (especially kraft pulp) are improved by using a cold impregnation soak instead of conventional impregnation procedures. After basing, wood chips are soaked in an alkaline liquid at a temperature of about 80 - 110 ° C (preferably 80 - 100 ° C, or 90 - 105 ° C) for between half - 72 hours (usually about 2 - 4 hours) at a pressure. of about 0-15 bar (preferably about 1-5 bar) to dissolve at least about 8%> of the wood (preferably about 104-20%) and at least about 15% of the lignin. The alkaline liquid used preferably contains sulfide (eg, black liquor, green liquor, white liquor or mixtures thereof) but almost any alkaline liquid having an alkali concentration of about 1.0 mol NaOH / liter or less (usually about 0.75 m / l or less) is suitable. The wood chips are then raised to a boiling point of approximately 145 - 180 ”C and boiled to produce the cellulosic chemical pulp. Between soaking and raising the boiling temperature there may be an intermediate step of heating the wood chips to 110 - 150 ° C (preferably about 120 - 140 ° C) for about 10 - 90 minutes (preferably 10 - 30 minutes) - Alternatively or in addition the majority of the dissolved lignin (and / or other solids) is removed before the wood chips are raised to boiling temperature. The system for treating the wood chips need only comprise a low-pressure type soaking vessel (with an associated pump or other transfer device) in addition to conventional equipment.

Description

BACKGROUND AND SUMMARY OF THE INVENTION In the preparation of kraft cellulose pulp, especially by continuous cooking techniques, wood chips (or similar finely divided fibrous cellulosic material) are usually based on distant air from the chips and preheated to a temperature of about 100 ° C. Rydholm "Pulp Processes" lead a higher temperature to both carbohydrate hydrolysis and liquid condensation. After this, the chips are usually led to a single-channel chute connected to a high-pressure feeder in which it is enclosed in cooking liquid at a temperature of approximately 115-130 "Coch at the same time as it is transferred and either at the top of the decoction or in a separate impregnation vessel. usually for about 15 -30 minutes. Afterwards, the sludge is heated to a temperature of approximately 150 - 170 ° C for the actual boiling which takes place in the boiler. There are about 400 continuous cookers working around the world in this way, which has long been accepted as the appropriate way to prepare chips for cooking and then achieve continuous cooking. According to the present invention, it has been found that this conventional, widely accepted method of handling chip pre-cooking can in fact have an adverse effect on the strength of the pulp produced, especially when producing pulp with a kappa number lower than normal. Conventional knowledge in the art has failed to make us aware of the fact that, when the chips are at the temperature there during conventional impregnation, the acidic substances formed, including reactions by hydrolysis and during delignification, will often produce a sufficient build-up of acid - before neutralization with the alkaline liquid, E which the chips are impregnated - so that the acid can attack the cellulose in the chips and / or cause lignin condensation, both of which are disadvantageous for subsequent pulping operations and at least the acid attacking cellulose has an unfavorable effect on the pulp control . According to the present invention, it has been realized that the diffusion of alkali 000 000000 000 0- 00 00000 0 00 i0 000 0000 000 000000 000000000000 0030 into the wood chips is not fast enough to neutralize the acidic substances and that therefore another solution must be given to the maximum final mass strength must be obtained from a given source of cellulosic raw material.

According to the present invention, instead of heating the chips to the temperature range 110 - 130 ° C in which hydrolysis, delignification and other chemical reactions will start (and produce oxygen) the chips will instead be "cold" -impregnated, i.e. it is subjected to impregnation in such a way that all-chemical reactions which generate acid become slow enough that diffusion of alkali from the impregnation liquid is sufficiently fast enough to neutralize the acid formed inhalation can attack the cellulose and / or cause lignin condensation. If no chemical reactions take place, there is sufficient time for alkali to diffuse into the cells so that when the acid is formed, alkali is immediately available to neutralize it. It has surprisingly been found that according to the present invention the alkali concentration can be relatively low and finally a pulp is obtained with improved strength compared to pulp produced by conventional impregnation methods.

According to one aspect of the invention, a method of impregnating malt, cellulosic fibrous material (such as softwood chips, but also comprising a wide variety of other cellulosic materials, such as hardwood, bagasse and agricultural materials) comprises the steps of sequentially: (a) continuously removing air from it; atomizing the cellulosic, fibrous material and preheating the material to an ambient temperature, (b) soaking the material in an alkaline liquid (preferably containing sulfide) at a temperature such that, in general, oxygen-forming chemical reactions take place slowly enough to produce the acid. must be neutralized by the alkaline liquid before the acid can damage the cellulosic material or cause lignin condensation, and that the alkali concentration of the liquid is sufficient to neutralize any acid produced; and for a period of time sufficient to substantially completely impregnate the cellulosic material with alkali, and (c) raising the alkali-impregnated cellulose-containing material to boiling temperature to provide continuous boiling thereof to produce cellulose pulp. The boiling is preferably to a kappa number lower than normal, i.e. for power cooking lower than kappa number 25, also lower than kappa number 20.

In practicing the method described above, step (a) is usually carried out by basing in a conventional manner, except that the maximum temperature at which the chips are based is about 10 ° C. Preferably, the basing for preheating the chips and for removing air takes place at about 80 - 110 ° C (more preferably 90 - 105 ° C), and possibly most desirably at about 90 - 100 ° C for a period of about 5 - 60 minutes (usually about 10 - 30 minutes). It is advantageous (but not necessary) that alkali is present during basing. In this way, condensate formed during basing will be alkaline. Alkali can be added to the chips before or during the base in the form of ash or powder, white liquor, black liquor or the like. Step (c) is preferably carried out in a continuous digester having a boiling temperature of about 145 DEG-180 DEG C.

Step (b), which is the main feature of the present invention, is carried out for most cellulosic fibrous materials at a temperature of about 80 DEG-110 DEG C., more desirably about 90 DEG-105 DEG C., or 80 DEG-100 DEG C. The treatment time is usually at least half an hour and the treatment time can be substantially unlimited as long as the pulp production plant can cause this part of the process to proceed very slowly and as long as there is no detrimental effect on the cellulosic material (which would not normally occur in a relatively mild alkaline liquid).

In practice, the time period for treatment would rarely be over 72 hours and most desirable is about 1 -_4 hours with approximately 2 - 3 hours as the optimum. The soaking does not need to be at high pressure; in fact, the only reason to use overpressure is to compress any gas bubbles to make it easier for the chips to sink. In conventional impregnation the pressure is approximately 10 - 20 bar, while according to the present invention step (b) would be performed at 0 - 15 bar, preferably 0 - 5 bar or 1 - 5 bar.

Step (b) is usually performed so that at least about 8% of the wood material (preferably about 10-20%) is dissolved and also to dissolve at least about 15% of the lignin, usually about 20-40%. Substantially all or at least most of the dissolved lignin (and other organic solids, such as hemicellulose), can be removed before step (c), which has advantages in the final overall treatment sequence. Between steps (b) and (c) there may be an additional step. preheating the cellulosic material to a temperature of about 110 - 150 "C for about 10 - 90 minutes (preferably about 120 - 140" C for about 10 - 30 minutes). During the performance of step (b), although high alkali concentrations may be used, it is not necessary to use an alkali concentration of about 1.0 mole of NaOH per liter (or the equivalent of other alkali substances) and in fact an alkali. concentration of about 0.75 moles of NaOH perlite, or less, very effective. According to another aspect of the present invention, there is provided a method of impregnating wood chips comprising the steps of: (a) Basing the chips to remove air from it and heating it to a temperature of about 80-100 ° C. (b) Soaking the chips in an alkaline liquid (preferably also including sulfide) for about 1 - 72 hours at a temperature of approximately 80 - 110 ° C (preferably about 80 - 100 ° C) to impregnate the chips with alkali and dissolve at least about 8% of the wood. And, (c) raising the wood chips to boiling temperature and effecting their continuous boiling to produce cellulose pulp.

The invention also relates to a new kraft pulp, wherein the pulp according to the present invention has improved strength compared with kraft pulps exactly produced in another way only by using conventional impregnation techniques in the opposite cold impregnation technique according to the invention. That is, according to a further aspect of the invention, there is provided kraft paper pulp which has improved strength produced by step night in succession continuously: (a) Remove air from the atomized, cellulosic, fibrous material and preheat or base the material to an ambient temperature. (b) Soaking or impregnating the material in an alkaline liquid (preferably also containing sulphide) at a temperature such that if acid-forming chemical reactions take place at all, these reactions take place slowly enough for the oxygen produced to be neutralized by the alkaline liquid before the oxygen may damage it. cellulosic material or cause lignin condensation; and wherein the alkali concentration of the liquid is sufficient to neutralize all the acid produced; and for a period of time sufficient to substantially completely impregnate the cellulosic material with alkali. And (c) Raising the alkali-impregnated cellulosic material to boiling temperature and performing continuous power boiling of the cellulosic material to produce chemical pulp which has increased strength compared to otherwise identically treated material only impregnated in a manner other than the impregnation according to step (b). Preferably, step (c) is performed at about 150-180 ° C to produce power pulp in a continuous digester.

The invention also relates to a system for treating cellulosic fibrous material to produce chemical pulp. The system comprises the following components: Means for removing air from the atomized cellulosic, fibrous material and preheating or basing the material to an ambient temperature. A first vessel for soaking or impregnating finely divided cellulosic material from the means for removing air therefrom and which can withstand a pressure of about 5 bar or less and an alkaline liquid at a temperature of less than about 110 ° C. A continuous boiler for boiling the cellulosic fibrous material from the first vessel. And means for transferring the cellulosic material from the first vessel to the digester. The boiler is preferably a continuous boiler and the transfer means also pressurize the material during the transfer (eg may comprise a conventional high pressure feeder). At least one impregnation vessel may be arranged between the first vessel and the digester, especially in retrofit operations, or cold impregnation may be performed at the top (eg the impregnation zone) in a single vessel hydraulic digester. Likewise, the can system further comprises means for removing a first liquid from the first vessel, which first liquid contains dissolved lignin / solids and that the first liquid in the first vessel is replaced by a second liquid (such as washing or bleaching filtrate) which has a lower content of dissolved lignin / solids than the first liquid. When using the washers according to the invention, it may also be possible to eliminate the conventional high pressure feeder in continuous power systems. The air removing means may, for example, comprise a chip pocket with base and the chip pocket and that first vessel is above the transfer means and the transfer means is above ground level and consists essentially of a pump (i.e. when high pressure feeders are missing).

The main object of the present invention is to simplify but effectively increase the strength of chemical pulp, especially produced by continuous boiling. These and other objects of the invention will become apparent from the following detailed description of the invention and the appended claims.

Brief Description of the Drawings Figure 1 shows a graph of alkali consumption as a function of time in experiments showing the strength of the method of the present invention. Figure 2 shows a diagram similar to that of Figure 1 but indicating absorbed alkali.

Figure 3 shows a diagram indicating the amount of wood material that is dissolved in experiments showing the strength of the method according to the invention.

Figure 4 shows a graph showing the effect of alkali concentration during pretreatment at about 100 ° C according to the present invention.

Figure 5 schematically shows a first example of an embodiment with different equipment parts that can be used in a system according to the present invention, and Figure 6 shows a view similar to Figure 5, which only shows an alternative embodiment of equipment that can be used.

DETAILED DESCRIPTION OF THE DRAWINGS As described above in the Summary of the Invention, this primarily relates to a process for impregnating finely divided, cellulosic fibrous material by sequentially removing air from the finely divided cellulosic fibrous material and preheating or basing to an ambient temperature (usually by basing at a temperature of about 80 - 110 ° C, preferably about 90 - 105 ° C or 90 - 100 ° C, for a period of about 5 - 60 minutes, preferably about 10-30 minutes), cold soaking of the alkali material to impregnate the material with alkali and then raise the alkali-impregnated cellulosic material to boiling temperature and finally effect boiling thereof to produce chemical cellulose pulp. In the cold impregnation according to the invention, in which the material is soaked in an alkaline solution at a temperature such that, in the event of any oxygen-forming chemical reactions (such as hydrolysis) taking place at all, they take place slowly enough that any acid produced must be neutralized by the alkaline liquid in which the material is soaked. before the acid can damage the cellulosic material or cause lignin damage. If chemical reactions take place, there is sufficient time for alkali to diffuse into the cells so that when acid is produced IC OCI CO O00 etui un O anno Oo coon onI I I0 000 oo! 0 0 0 0 0lo 000 lo Oo 0000000Q, alkali is immediately available to neutralize the same. The alkali concentration of the impregnated liquid is sufficient to neutralize the acid produced and the impregnation takes place for a period of time sufficient to substantially completely impregnate the cellulosic material with alkali. The example below - and with reference to the diagrams in Figures 1-4 - describes a laboratory experiment demonstrating the effectiveness of the method of the present invention.

Example Softwood chips of various thicknesses (without first being base) were introduced into an alkaline solution having a strength of about 1.0 mol perl of NaOH and at a temperature of about 100 ° C and the consumption of alkali (NaOH) was measured. In this example, a liquid without sulfur is used, but preferably the alkaline liquid used for soaking / impregnation contains hydrosulfide, sulfide ions or sodium sulfide. The bars 10 illustrate the results (for varying times) when the chip thickness is approximately 3 mm, while the bars 11 illustrate the results with a chip thickness of about 4-6, and the bars 12 with a single chip thickness of about 6: 8 mm. As shown in Figure 1, before the particular chip thickness was shown, a time period of one hour was not sufficient for complete alkali impregnation, but after about two hours, and certainly after about 3 hours, consumption stabilizes during impregnation and diffusion indicating that after this time period (about 2 hours). - 3 hours) heating and boiling can be performed (if chips were damaged, it is likely that the time period for effective basing would be reduced slightly). From the results of the diagram in Figure 1 as well as from other available information, depending on the particular cellulosic material, the exact temperature, the material thickness and the like, the soaking in alkali according to the invention would be for about 1 to 72 hours, preferably about 1 to 4 hours and usually about 2-3 hours.

Figure 2 confirms the results described above. Figure 2 IQI O ^ O Du 0000I c nOI Go QIO; Ooo! shows a diagram similar to that in Figure 1 with bars 15 - 17 corresponding to the same wood chip thicknesses as for bars 10 - 12. Figure 2 indicates absorbed alkali, i.e. the spent alkali plus alkali in the liquid that passes into the chips.

Figure 3 indicates that during the cold impregnation according to the present invention a considerable amount of wood material dissolves. The stacks 21-23 correspond to the stacks 10 to 12 in Figure 1 and are the same wood chip thickness. The diagram in Figure 3, as well as other available information, indicates that during the first treatment according to the present invention approximately 15-20% of the wood material dissolves. The lignin content of the impregnation liquid increases due to dissolved lignin. From the information of Figure 3, as well as from other available information, it is desirable that at least about 8% of the wood be dissolved during the impregnation according to the invention and preferably about 10-20%, and typically about at least 15% of the lignin is dissolved, preferably about 20 - 40%. This amount of wood and lignin can be dissolved during the "cold" impregnation according to the invention without risk of fiber damage; The diagram in Figure 4 shows the effect of alkali concentration during pretreatment at approximately 100 ° C. The bars 26 - 28 correspond to chips of the same thickness as in diagrams 10 - 12. The alkali strength of the left-hand bars is approximately 0.75 molar liters ( all alkali strength is expressed in NaOH, or equivalent), while for the middle columns the alkali strength is about 1.5 m / l. and for the right-hand bars approximately 1.5 m / l. The graphical results in Figure 4 indicate that the alkali strength has no appreciable effect and therefore it is only necessary to have as much alkali present as is needed to neutralize any acidic by-products from forming acid regions which may damage the fine properties and are available in material cells for to effectively neutralize. Thus, the alkali required during actual cooking may be added after this pretreatment, and / or during cooking, i.e. a high alkali level is not required during impregnation. When using the information from Figure 4, as well as any other available 1%. In fact, according to the present invention, it is desirable according to the present invention that the alkali concentration during the cold impregnation step of the invention is only 1.0 mole of NaOHper liter or less, in fact preferably about 0.75 moles per liter or less.

Using data from the example given above, as well as any other available information of the present invention, it has been determined that it is desirable to effect impregnation of the chips when they are at a temperature below about 110 ° C. A typical temperature at which cold impregnation according to the invention takes place is about 80¿-110 ° C, more desirably about 90 - 105 ° C or about 80 - 100 ° C. The time period is preferably half an hour and most desirably about 1 - 4 hours, optimally about 2 - 3 hours. The pressure can be 0 - 15 bar, preferably 0 - 5 bar, and more desirably 1 - 5 bar only because a light pressurization compresses any gas bubbles inside and makes it easier for the chips to sink; however, pressurization is not necessary except to reduce the size of the gas bubbles. Thus, according to the present invention, less expensive equipment and vessels can be used because they do not need to be subjected to high pressures. According to the present invention, it may also be desirable to add additional alkali to start the boiling reaction and gradually rise the chips to boiling temperature by providing a step between the cold impregnation and the steps in which the boiling temperature is reached in which the cellulosic material is raised to a temperature of about 110 - 150 ° C (preferably 120 - 140 ° C) for about 10 - 90 minutes. (preferably about 10 to 30 minutes.) The invention is particularly applicable to continuous power and soda cooking processes but can also be applied to other chemical cooking processes.

According to one aspect of the invention, it is also advantageous to remove - at least in part - the dissolved lignin and / or other organic solids (eg hemi-cellulose) after the pre-treatment but before boiling to reduce the concentration of dissolved material during the actual cooking (of the advantageous reasons described in the co-pending application Serial No. 08 / O56,21l filed May 4, 1993 [agent's reference 10-846], the contents of which are hereby incorporated by reference). The displacement of the liquid containing lignin / solids from the first concentration of lignin with liquid having a lower concentration of lignin / solids can take place with white liquor, dilute white liquor, black liquor (eg from the lower screen 52 in figure 6), effluent from bleaching plant, effluent from mass washing or the like. If at least a majority of dissolved lignin / solids are removed, it is very possible according to the present invention that the amount of dissolved lignin / solids during the actual cooking will be 30-50% less than in conventional processes, which reduces the risk of lignin condensation during boiling. The actual source of alkaline liquid for carrying out the invention is not particularly significant, but it contains at least some sulfide. Black liquor, green liquor, white liquor, effluents from alkaline bleaching plants or a mixture of these (especially a mixture of black liquor and white liquor), as well as other readily available alkaline liquids can be used.

These liquids may be provided with additives, such as polysulfide, anthraquinone, or complex chelants. It is desirable to use a liquid which has a high concentration of sodium sulfide (eg from the strainers 51 in Figure 6) to which liquid-containing liquid can be added during soaking or between soaking and boiling.

Figure 5 schematically shows various equipment details that can be used in an example of a system according to the invention in order to practice the method described above and achieve the mass with increased strength according to the invention. In Figure 5, reference numeral 30 denotes a conventional chip pocket in which basing takes place or, alternatively, the chip pocket 30 may be a DIAMONDBACKW type chip pocket sold by Ahlstrom Kamyr, Glens Falls, New York, as described in co-pending application. ilcøu 00nu 12 serial no. 08 / 189,546, filed February 1, 1994 [ombuds reference 10-926], the contents of which are hereby incorporated by reference. The pocket 30 can be at atmospheric pressure or can have a low overpressure of approximately 0.1 - 3 bar and the base usually has room to heat the chips to 80 - 100 ”C for about 10 - 30 min.

The chipped chip from the chip pocket 30 is then fed - such as while using a conventional chip pump 31 (or low / high pressure feeder or the like) via line 32 - to the top of the soaking / impregnating vessel 33 according to the invention. The vessel 33 need not be a pressure vessel - G.v.s. The impregnation according to the invention can take place at atmospheric pressure in the vessel 33. However, it is desirable that the pressure in the vessel 33 be a slight overpressure, usually about 1 - 5 bar to compress gas bubbles and make it easier for the chips to sink. Thus, the vessel 33 may be less expensive than conventional impregnation vessels, which require about 10-20 bar pressure. The chip pocket 30 and the pretreatment vessel 33 can be combined into a single vessel. Atmospheric vapor treatment can take place in the top of the vessel and a liquid level is maintained in the vessel to begin the pretreatment. If necessary to keep the chips immersed in the liquid, the vessel may be pressurized. The soaking / impregnating vessel 33 may have a first zone 34 and a second zone 35 or even several zones. Different liquids can be used during the impregnation in these different zones 34, 35, which are described in the co-pending application series no. 08 / 403,932, filed March 14, 1995 [agent's reference 10-1044], the contents of which are incorporated herein by reference. The liquid used in the second zone 35 may be warmer and more sulfur-rich than the liquid in the zone 34. The temperature in the zone 34 is usually between 80 - 110 ° C, preferably 80 - 100 ° C, and the material is retained in the zone 34 for at least half an hour. , usually about 1 - 4 hours. In zone 35, the temperature can be raised to about 110-150 ° C, such as 120-140 ° C, and the material can be maintained in zone 35 for about 10-90 minutes.

O 0010I0000 00 0 0cl 00 000 Ino 0 0 Q00 Qcc av0 0c0ooaoo00000n 13A usually 10 - 30 min. Alternatively, both zones 34, 35 may be less than 110 ° C, and the total impregnation time in both zones 34, 35 may be about 2-4 hours.

Figure 5 also indicates a conventional screen arrangement - shown generally with reference numeral 36 - for removing a portion of the liquid surrounding the impregnated chip after zone 34, which contains a high concentration of dissolved lignin / solids. This liquid may be replaced by replacement liquid. , which is added to any desired point in the vessel 33, which exchange liquid has a markedly lower content of lignin / solids "than the liquid from the strainers 36 in the line 37 so that the concentration of soluble lignin / solids in the pulp when it reaches the continuous digester 40 is about 30 - 50% less than in conventional continuous cooking systems.

The pulp discharged from the bottom of vessel 33 to line 41 is then transferred - preferably by the conventional high pressure feeder 42 (which both transfers and raises the pressure to the cellulose pulp) - to the top of the continuous digester 40. In some circumstances, according to the present invention, a conventional high pressure feeder 42 may be replaced. with single chip pump. In any case, the pressure at the top of the digester 40 must be at least about 5 to 10 bar to prevent boiling of the decoction liquid which is at a boiling temperature of about 145 F 180 ° C. Since two-stage pumping is used according to the invention (pump or steps 31, 42) in vessel 33, the pressure at the top can be 3 -18 bar if a chip pump is used at step 42 instead of the pre-pressure feeder.

The boiler 40 is preferably a conventional continuous boiler, such as those sold by Kamyr Inc, Glens Falls, New York, such as an MCC® boiler, EMCC® boiler, or Lo-Solidsm boiler. The boiling takes place in zone 44 while washing. takes place the zone 45 before the produced chemical mass with high strength (preferably kraft mass) is drawn from the line 46; [Kokaren40 can be completely downstream]. Deduction strainers are arranged as generally indicated by the reference numeral 47. The sludge up to the 00: 00 is heated between vessels 33 and 40 by arranging indirect heat transfer in the transfer system between vessels 33 and 40. The heat economy can be improved by using the heat in the extraction liquid from strainer 47 to preheat the white liquor (or other cooking liquid) before adding it to or before the boiler40.

The system according to Figure 6 is similar to the system according to Figure 5, but only for an already existing (i.e. not newly constructed) installation which already has an impregnation vessel. In this embodiment, structures comparable to those in the embodiment of Figure 5 with the same reference numerals are preceded by "1" only.

In the embodiment according to Figure 6, the already existing impregnation vessel for the pulp mill is vessel 50. The cold impregnation method according to the invention is practiced in vessel 133 (i.e. the maximum temperature of about 110 ° C) while in vessel 50 an intermediate treatment is provided in which the temperature of the chips is gradually raised , e.g. up to about 110 -150 ° C, preferably 120-140 ° C, so that the boiling conditions may be milder in the digester 140 and even less loose wood material will be present during cooking if liquid is removed or displaced in zones 51, 52, or removed before enters the digester 50. Another chip pump 53, connected to the vessel 50, is provided although the chip can be slurried by the liquid in the return line from the top of the digester 140. The existing high pressure feeder 142 is arranged between the vessels 133, 150.

As an alternative to the systems shown in Figures 5 and 6, such an apparatus as shown in co-pending application Serial No. 08 / 267,171 (the contents of which are incorporated herein by reference) may be used [agent's reference 10-9611, or as shown in co-pending application series No. 08 / 428,302 filed April 25, 1995 (the contents of which are hereby incorporated by reference) [agent's reference 10-1051], For example, the canning systems shown in Figures 4-9 of No. 08 / 267,171 or Figure 1 of No. 08 / 428,302 are used, the chips being retained in front of the high pressure feeder or pump for a sufficient period of time according to the present invention (eg 1 to 4 hours). In this way and according to the teachings of the present invention it may be possible to eliminate the high pressure feeder normally used for continuous power systems, especially where an overpressure in the chip pocket and a stack of sludge is maintained above the pump and where the digester is completely downstream or at least has no countercurrent washing zone and / or where the chip pocket and the soaking vessel are raised so that they are closer to the top of the digester than the ground level. It can thus be seen that according to the present invention an advantageous method and apparatus for producing chemical pulp (especially kraft pulp) which has increased strength is described. This increased strength is especially important when using kraft cooking methods. Although the invention has been described and shown herein which is presently considered to be the most practical and preferred embodiment, it will be apparent to those skilled in the art that many modifications may be made within the scope of the invention as determined by the broadest interpretation of the appended claims. , masses and systems are covered.

Claims (4)

A method of treating solid wood chips to produce chemical pulp, comprising the steps of sequentially (a) basing the wood chips to remove air therefrom and heating the wood chips to a temperature of about 80-100 ° C; (b) soaking the chips in an alkaline liquid having an alkali concentration of approximately 0.75 mol per liter or less, at a temperature of between approximately 80 and 100 ° C, for a period of 30 minutes. to 72 hours. so that about 10-20% of the wood material and about 20-40% of the lignin are dissolved; (c) displacing at least portions of said dissolved lignin and / or other organic solids after step (b) while treating white liquor, diluted white liquor, effluent from bleaching plant or effluent from pulp water; and (d) boiling the material from step (c) of 25. into a kappa layer A process according to claim 1, wherein the alkaline liquid in step (b) is black liquor or a mixture of black liquor and white liquor. A process according to any one of the preceding claims, wherein in step (c) at least a majority of lignin / solids are displaced. A method according to any one of the preceding claims, wherein steps (a) and (b) are performed in one and the same vessel.