Classifications

• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
  • D21C3/00—Pulping cellulose-containing materials
  • D21C3/22—Other features of pulping processes
  • D21C3/24—Continuous processes
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
  • D21C9/00—After-treatment of cellulose pulp, e.g. of wood pulp, or cotton linters ; Treatment of dilute or dewatered pulp or process improvement taking place after obtaining the raw cellulosic material and not provided for elsewhere
  • D21C9/02—Washing ; Displacing cooking or pulp-treating liquors contained in the pulp by fluids, e.g. wash water or other pulp-treating agents
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
  • D21C7/00—Digesters
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
  • D21C9/00—After-treatment of cellulose pulp, e.g. of wood pulp, or cotton linters ; Treatment of dilute or dewatered pulp or process improvement taking place after obtaining the raw cellulosic material and not provided for elsewhere
  • D21C9/10—Bleaching ; Apparatus therefor
  • D21C9/147—Bleaching ; Apparatus therefor with oxygen or its allotropic modifications

Definitions

• the present invention relates to a method and an arrangement for treating chemical pulp when the pulp has been cooked to a high kappa.
• the kappa number indicates the residual lignin content and it is used at all mills as a target value for cooking.
• a typical kappa number for pulp to be bleached is at present 14-20 for hard wood and 25-35 for soft wood. If the pulp is not bleached, the kappa number after cooking is clearly higher, typically 40-100.
• Soft wood pulp to be bleached is typically treated in an oxygen stage for decreasing the kappa number to 10-20.
• a high kappa pulp refers to kappa numbers exceeding 50.
• High kappa pulps (high yield pulps) are usually used in products, from which high strength properties and rigidity are required, but not brightness or printing properties.
• soft wood is a typical raw material for high yield pulps due to its long fibers.
• Soft wood has a high lignin content, for which it requires a relatively long chemical treatment.
• a combination of strength properties and rigidity is typical for soft wood pulp.
• the lignin content remains so high that in pri- or art technique the pulps can not be defibrated without mechanical treatment, but in this case the final product does not require printing properties or brightness.
• High yield pulps are typically produced in a sulfate process.
• the yield range of high yield pulps varies in the range of 50-70%. It is common knowledge that this kind of pulp is not produced using fiber line equipment designed for pulp with a normal kap- pa number, but additional technique is required.
• the most important difference between the fiber lines is that high kappa pulp after cooking is in such a strong chip form that the defibering thereof requires special defibering equipment, such as a blow line refiner (In-Line refiner), or alternatively hot refining.
• blow line refiner In-Line refiner
• the arrangements are of similar type. In the first-mentioned defibering arrangement, cooked pulp in the form of chips is discharged from the digester into a refiner located in the blow line upstream of washing stages.
• the aim of the defibration of cooked pulp is to separate the fibers from each other, since after cooking the fibers are still in the chip matrix.
• This defibration allows producing pulp to a high kappa and to high yield values. Even a slight defibration is mechanical treatment of the pulp, the effects of which in alka- line conditions decrease the quality of the pulp. High kappa pulp is produced to so-called brown stock. Thus, there is no aim to bleach this kind of pulp.
• the production line for this pulp comprises washing and screening of the pulp. If any special property requirements are set for the pulp quality, the line may comprise an oxygen stage for adjusting the properties of the pulp to a desired level.
• Vigorous development of oxygen delignification has provided an efficient method of decreasing the kappa of pulp to be bleached without substantial losses in yield.
• the present trend has been to stop reaching cooking kappa numbers of remarkably low level.
• Important in sulfate cooking is selectivity, i.e. when lignin is dissolved, a least possible amount of carbohydrates (cellulose and hemicelluloses) is split and dissolved.
• a problem is caused by chemical reactions, due to which a remarkable amount of hem- icellulose is dissolved at the finalizing stage of the cook.
• dissolving of carbohydrates also causes a decrease in the strength of chemical pulp. In the case of softwood pulp, the strength of chemical pulp is one of the most essential quality indicators.
• oxygen delignification is a direct continuation to lignin- removal taking place during cooking.
• the chemical pulp is not cooked to significantly below kappa number 25, since the yield loss would be essential.
• oxygen delignifi- cation is a more selective and gentler process. It disintegrates and oxidizes lignin to a form that is dissoluble in alkali, destroys colored compounds present in lignin and removes impurities (resin) from the pulp.
• Fl patent application 20115277 presents a method of treating pulp that is produced by chemically cooking soft wood chips to a kappa number of 50-100, preferably 60- 90, in which method
• the cooked pulp is led to washing without treament in a mechanical defibrator between the digester and the washing, the pulp is washed,
• the washed pulp is led into a first oxygen delignification stage where the pulp is treated in the presence of oxygen and alkali,
• the oxygen delignified pulp is washed and after the washing it is treated in a subsequent stage with an electrophilic chemical, after which
• Oxygen stage in connection of the present patent application refers to such an alkaline stage that takes place pressurized within a pressure range of 1-20, preferably 8-12 bar (abs.) at the mixing point, at a pH range of 8.5-14 and wherein at least dur- ing part of the reaction time oxygen is present around the fibers.
• the oxygen stage can comprise one, two or even several steps, whereby each reaction step comprises chemical mixing and a reaction vessel or a reaction retention accomplished by means of a tube.
• treatment step here refers to addition and mixing of a chemical used in the oxygen stage and the subsequent retention in a portion of the tube or the reactor.
• Reaction retentions are, depending on the practicing method, from 0.1 minutes up to 120 minutes, so that the reaction retention depends on the desired type of reactor.
• an oxygen stage is also known from a washing stage both upstream and downstream of the oxygen stage.
• oxygen and alkaline and possibly some inhibitor preventing fibers from getting damaged by metals are dosed into the oxygen stage, or metals entrained in the fibers are otherwise removed or treated to become inactive.
• Alkali is generally dosed in the amount of 1-60 kg/admt and oxygen 1-50 kg/admt.
• the alkali dose of the oxygen stage may be up to 90 kg/adt and the oxygen dose 70 kg/adt, when the kappa reduction in the oxygen stage is 60 units.
• the alkali that is used is most often sodium hydroxide or oxidized white liquor, but in principle all alkaline compounds containing OH-ion are alkalies that could be used in the oxygen stage under some conditions.
• the make-up sodium hydroxide of the chemical cycle is often advantageously dosed in the oxygen stage.
• the oxygen is dosed in gaseous form, where the content of oxygen is most usually 75-100% of specific gravity.
• the temperature of the oxygen stage is 70 - 120 °C and most usually 80 - 105 °C.
• the temperature can be raised using some suitable steam having a pressure of 0.5-20 bar, and hot water either via washing or diluting. Steam can be used for heating either directly, mixed in the pulp, or indirectly.
• the oxygen stage is typically connected in accordance with the counter-current washing principle so that the function of the washing of the pulp arranged between the cooking and the oxy- gen stage is to displace the liquor entrained in the pulp coming from the washing, which liquor can be separated as a filtrate of the washing stage of the cooked pulp, with a filtrate obtained from the oxygen stage washing.
• the COD of the washing liquid of the cooked pulp will be in the range of 30-60 g/l and that of the pulp suspension in the range of 200-500 kg/adt at a consistency of 12%.
• filtrates from pulp washing are led counter-currently in the fiber line, but treating an oxygen stage filtrate having a high COD-value this way is not necessarily advantageous.
• the present invention relates to a method of treating chemical pulp, in which method pulp is produced by cooking wood chips chemically to a kappa number of 50-120, preferably 60-100, the cooked pulp is washed, the washed pulp is led into an oxygen stage where the pulp is treated in the presence of oxygen and alkali for removing lignin, and the oxygen-treated pulp is washed, whereby lignin-containing washing filtrate is formed.
• the method is characterized in that
• the E 10 -value of the washing apparatus for cooked pulp is typically at least 8, preferably at least 10.
• the so-called E10-value defines the washing efficiency so that washers of different types can be compared to each other.
• the E10-value is a numerical value defined for a washer or a combination of several washers, which reveals how many ideal mixings the washer or washer combination reaches.
• An ideal mixing is un- derstood as a situation when the washing liquid is mixed into the pulp being washed so efficiently that the concentrations of both the liquid remaining inside the pulp and the liquid being withdrawn therefrom are equal.
• Number 10 in the E 10 value reveals the calculated consistency per cent, at which the pulp exits the washer.
• Dilution factor means the difference between used washing liquid and liquid exiting the washing apparatus with the pulp per a ton of pulp.
• the invention also relates to an arrangement for treating pulp produced from wood chips in a cooking apparatus wherein the wood chips are cooked chemically to a kappa number of 50-120, preferably 60-100, said arrangement comprising at least a cooking apparatus for cellulose fibrous material, washing devices for cooked pulp,
• filtrate lines for leading washing filtrates counter-currently to preceding washers as washing liquid. It is essential that a filtrate line downstream bf the oxygen stage is provided with devices for separating lignin from the filtrate and that the filtrate line is connected to the washing devices for cooked pulp for using the filtrate as washing liquid after lignin separation.
• the E10-value of the washing devices for cooked pulp is preferably at least 8, preferably at least 10.
• the washing devices for cooked pulp typically comprise more than one stage.
• the washing devices for cooked pulp typically comprise a so-called digester washing and at least one washing device between the digester and the oxy- gen treatment apparatuses.
• the washing devices for cooked pulp can comprise as one stage a digester wash conducted in the lower part of the digester, such as a hi-heat type of digester displacement or final displacement taking place in batch cooking.
• the washing devices for cooked pulp comprise downstream thereof one or several washers, such as pref- erably a single drum washer, for instance a Drum Displacer® (DD) washer (Andritz Oy) or a diffuser washer.
• DD Drum Displacer®
• the cooked pulp does not contain uncooked fraction in harmful amounts, such as knot
• a press can be used, under the precondition that the Ei 0 -level of the overall washing devices for cooked pulp is over 8.
• the dilution factor of the washing devices is to be 0-5, preferably 1-3. Adequate efficiency of washing is important in order to ensure adequate removal of lignin released from the pulp during cooking. More than 95%, preferably more than 97% of lignin is removed in the washing. Simultaneously, also sulfur can be removed from the pulp.
• the present invention complement the performance of bleaching pulp delignified at a high kappa so that it becomes industrially possible. Additionally, it allows pulp production to a significantly higher yield than in a conventional fiber line. Further, sulfur- poor lignin can be recovered from the process even though the pulp is produced by the sulphate method using a conventional fiber line and recovery concept.
• the invention is more preferably applied to soft wood in view of lignin amount and cook- ing/delignifying properties, but it can also be applied to hard wood, especially hard wood species having a high lignin-content.
• the pulp is produced by an alkaline cooking method, such as a sulphate cook, a soda cook or a soda-anthraquinone cook.
• the cooking method may also be modifications of these.
• a preferred example is a sulphate cook having a low sulfidity, i.e. 5-25%. Sulfidity values lower than that relate to soda cooking or modifications thereof.
• the cook may be a continuous cook or a batch cook.
• the pulp is cooked to a kappa number of 50-120, preferably 60-100.
• This kind of pulp has approximately 7-14 weight per cent lignin, which is an essentially significant amount.
• This lignin is removed in the oxygen stage so that the kappa number of the pu
• the oxygen stage is not carried out using oxidized white liquor, but preferably using sodium hydroxide, whereby the lignin is released in the solution, which is sulfur-poor compared to an oxygen stage carried out using oxidized white liquor.
• Sulfur from the magnesium sulphate Used as an inhibitor and from cooking chemicals enters the oxygen stage.
• Lig- nin that has less sulfur than normal is advantageous in many further use applications for lignin.
• the method according to the invention does not require or perform any separate chelate treatment prior to the oxygen stage. Thus it is possible to decrease the amount of effluent fractions and process costs.
• the pulp is washed normally so that reaction products are separated from the pulp in a liquid phase. It is possible to produce lignin in the amount of 40-120 kg, typically 40-80 kg, per ton of pulp.
• the alkali used in the oxygen stage is preferably sulfur-free or sulfur-poor, whereby, when efficiently washing the pulp both downstream and upstream of the oxygen stage using liquid essentially free of sulfur compounds, it is possible to receive a lignin-containing liquid having sulfur-poor lignin in the amount of 40-100 kg per ton of pulp.
• Low-sulfur lignin refers to lignin having a sulfur-content less than 1% of the dry matter in the lignin. According to a study, the sulfur-content of precipitated from black liquor was approximately 1.4% (sulfur) of the dry matter and the sulfur- content of lignin precipitated from the oxygen stage washing filtrate was approximately 0.66% of dry matter.
• filtrate of the oxygen stage is separated from the pulp and lignin is recovered therefrom e.g. to be used as raw material for chemical industry.
• the liquid, wherefrom the lignin has been separated can be re-used in the oxygen stage such that it is led to washing upstream of the oxygen stage, without harmful effect of lignin.
• removal oxygen reaction products of the oxygen stage from a counter-currently connected washing cycle and a separation technique that is in any case to be connected to the oxygen stage of high-kappa pulp can be regarded as investments having a clear economical target, and not only as additional cost for ensuring the functioning of the process.
• the alkali of the oxygen stage is to be either pure sodium hydroxide (NaOH) or some other hydroxide that enables main- taining an adequately high pH-level in the oxygen stage. Pure here means that the sodium hydroxide solution does not contain any sulfur, or contains very small amounts of sulfur.
• the oxygen stage chemical is not a chemical that contains sulfur compounds.
• One chemical typically used in the oxygen stage is magne- sium sulfate as an inhibitor. The use thereof can be optimized according to the pulp quality requirements and according to how low the sulfur-content is to be.
• oxidized white liquor is preferably not used, it is still possible in connection with chemical regeneration at a chemical pulp mill to separate the alkali so that one fraction contains the sulfur compounds and another the pure NaOH.
• the chemical proportion is formed so that the sulfidity in the cook is over 40%.
• the sulfidity returns to "normal" in the stream going to black liquor evaporation plant, when in accordance with the counter-current principle the cooked pulp is washed using low- sulfur washing liquid from lignin separation.
• the sulfidity of the cook is approximately 25-40%, but that of liquor in recovery it is clearly lower, e.g. 25-30%.
• the lignin separation method as such is not essential in view of the invention, but before all the fact that lignin separation becomes profitable and that according to a preferred embodiment the lignin can in a kraft-process be separated in low-sulfur form. Then the process is to comprise the following properties: Kraft-cook to kappa 50-120. Efficient washing of pulp after cooking and the oxygen stage. The E10-value of the washing devices for cooked pulp at least 8, preferably over 10. The E10-value of the oxygen stage washer over 4, preferably over 7.
• the required washing efficiency is also dependent on how accurately the cook-generated lignin and the oxygen stage lignin are separated in the washes. The more efficient the washing of the cooked pulp, the better the cook-generated lingnin is separated and the more oxygen stage - generated lignin can be separated in the oxygen stage washing in low-sulfur form.
• the washing liquid introduced to the last stage of the wash upstream of the oxygen stage is liquid devoid of essential amounts of sulfur.
• the alkali during the oxygen stage is preferably sulfur-free, when very low-sulfur lignin is desired.
• the kappa is decreased by at least 30 units, preferably by more than 35 units, for releasing from the pulp an economically significant amount of lignin.
• a significant portion of the lignin of the pulp is dissolved in the oxygen stage into the liquid traveling with the fibers and it is thus possible to separate from the filtrate of the washers or the presses.
• the thus purified liquid can be used in the washing of cooked pulp.
• the washed pulp is taken either to board production as such or it can be further bleached in a method presented also in this patent application.
• pulp that has been cooked to a significantly high lignin-content can be processed for the market to pulp that can also be washed to a desired cleanliness level.
• possible embodiments are often limited to very affordable chemicals, because the process is to be made profitable, but still several various methods exist that can be applied for lignin separation.
• lignin can also be used as fuel in the boilers and kilns of a chemical pulp mill.
• wood processing industry and also the chemical industry have awakened to search substitute raw materials for producing polymers and other carbon-based materials.
• the most important raw material of polymer in- dustry is crude oil, the price and availability of which in the long run will make wood- based raw materials for polymers competitive.
• lignin based on a chemical pulping process has market where it can replace crude oil -based products.
• the oxygen stage is the simplest and best known method for delignifying high kappa pulp. Then the pulp is treated with a method in which fiber properties are the priority and the objects of use of the fiber raw material are known paper and fiber products. It is also possible to use other lignin dissolving and separation method, replacing the oxygen stage, which are used to produce e.g. special lignins or to separate e.g. hemicelluloses.
• This kind of methods could comprise an acid pulping method, such as a defibration suitable for grass pulp presented by Chempolis, a formic acid pulping known as the CIMV-process, or a combination of acid hydrolysis and an oxygen stage.
• lignin is not the only possible product, but in addition to it, the products may comprise e.g. hemicelluloses.
• a solution that is separated from the pulp in the oxygen stage can be treated in many ways for separating lignin.
• the most conventional chemical separation method is precipitation, where an alkaline solution is neutralized or acidified, whereby alkali is precipitated from the solution and it can be clarified out of the liquid. This requires an abundant amount of acid, but on the other hand it is an efficient and simple way of recovering lignin.
• About 15% of the lignin is precipitated at a pH of 10 and 50% at a pH of 8.
• the pH of the oxygen stage is 10-13, typically 10.5-12. In lignin separation there is no need to aim at complete separation.
• lignin separation ways comprise various filtration methods, in which a filter can remove large chained molecules, but lets ions and water go through.
• This kind of methods comprise various ultra- or nanofiltrations, osmosis and reverse osmosis, dynamic cross flow -filtration, and other methods by means of which molecules can be separated from a solution.
• the cleaned solution can be utilized in the washing of cooked pulp, because the organic loading has been removed.
• the invention is especially applicable to pulp that is produced by a sulfate method or its modification.
• the use of e.g. yield enhancing chemicals, such as anthraquinone, is possible in connection with cooking.
• the pulp is cooked in a continuous or a batch cook so that the delignification is terminated at a high lignin-content level (kappa number 50-120); but so that the defiberization point of the chips is reached.
• This kidn of method is described e.g. in WO Patent Application PCT/US10/57417.
• the present method comprises as a preferred embodiment a process coupling, in which the mechanical defibration of the complete pulp stream after cooking can be omitted.
• the cooked pulp is discharged from the digester into washing and then into the oxygen stage without mechanical treatment in a mechanical defibrating device, e.g. a blow line refiner.
• a mechanical defibrator mechanical work is applied for releasing fibers from a chips matrix, which in the method according to this embodiment is not needed.
• the fiber line naturally comprises pumps, mixers and corresponding devices that are used for pulp transfer and for mixing chemicals and other substances into the pulp. Eliminating the mechanical defibrator is advantageous in view of energy economics, since the energy dissipation of the washer, pumps and mixers between the digester and the first oxygen stage is below 30 kW/adt, while a defibrator doubles that.
• the screen room is located downstream of the first oxygen stage, when there are several oxygen stages. According to an embod- iment, the screen room is located between the first oxygen stage and the subsequent wash.
• the pulp can be screened also later, but preferably prior to washing following the second oxygen stage.
• the screening can also be divided into coarse screening and fine screening, whereby the coarse screening takes place e.g. downstream of the first oxygen stage and the fine screening downstream of the se- cond oxygen stage.
• the pulp discharged from the digester is led, without mechanical defibration and without screening, via washing into the first oxygen stage.
• knot removal from the pulp is performed after dis- charging the pulp from the digester prior to the first wash.
• a knot reject separated from the main pulp stream is subjected to a gentle defibration.
• the defiberized reject stream can be returned to the main pulp stream either to the knot separation feed or co-currently downstream of knot separation. It is essential that the main pulp stream is not defibrated mechanically, but only a side stream sep- arated therefrom (e.g. preferably less than 30%, most preferably less than 20%) is defibrated such that the knot fraction is in a suitable condition for washing and subsequent treatment.
• Washing refers to one or more washing stages and thus it takes place in one or more washing apparatuses.
• the pulp discharged from the digester is washed prior to the first oxygen stage, preferably using a single-drum washer, such as a pressure drum washer produced by Andritz, i.e. a Drum DisplacerTM (DD)-washer, or a vacu- urn drum washer or a diffuser.
• a single-drum washer such as a pressure drum washer produced by Andritz, i.e. a Drum DisplacerTM (DD)-washer, or a vacu- urn drum washer or a diffuser.
• a washer in which the pulp gets pressed between drums and/or rolls in a so-called nip found e.g. in washing presses, is not suitable for pulp washing between the cook and the first oxygen stage.
• knot removal is effected prior to washing downstream of the digester, the use of a press is possible, too. Washing downstream of the first oxygen stage can be accomplished using conven- tional washing technique, which enables using the above mentioned washing apparatuses, and thus also a
• the first oxygen stage is a clear continuation to cooking, the purpose of which is to lower the kappa by 30-70 kappa units. Simultaneously also the reject content of the pulp decreases considerably, whereby in pulp screening at some treatment step after the first oxygen stage no remarkable amounts of reject are removed, since removal would end the yield benefit.
• the kappa number of the pulp is lowered to a level of 10-20, preferably to a value of 10-15 or less. Studies have revealed that the reject content in the first oxygen stage decreases even more than 95%, meaning that this coupling is especially efficient in lowering the reject content.
• This phenolic lignin is removed in oxygen delignification in accordance with oxygen delignification kinetics, but oxygen delignification does not have any notable effect on the amount of non-phenolic lignin.
• the delignification level in the oxygen stage stops at a level of 75-80%, even if oxygen treatments with the same chemical conditions were coupled several in series and the treatment period continued. Additionally, harshening of the conditions is clearly seen, above all as decreased quality of pulp and low yield.
• continuing delignification after the oxygen stage requires chemicals that also react with other lignin compositions and /or create new free phenolic groups.
• Chlorine dioxide treatment is typically performed at a temperature of 40 - 90 °C, at a pH of 1.5-5.5 and during 1-10 minutes, but longer periods are also possible.
• a typical chlorine dioxide dose in the treatment is 2 - 20 kg/adt.
• Typical conditions for peracid treat- ments are: temperature 50-90 °C and time 30-120 minutes.
• the amount of chemical added into the pulp depends on the peracid used, but typically it is 2-20 kg/adt.
• Treatment with ozone is typically effected pressurized under a pressure of 5-15 bar, preferably at a consistency of 5-35%, more preferably at a consistency of 7-18%, so that ozone-containing gas having an ozone-content of 8-18 weight per cent, preferably 10-15%, is introduced into the pulp at one or several points, preferably via one or more mixers.
• Said gas is a mixture of ozone and oxygen that acts as carrier gas.
• the amount of ozone dosed into the pulp is 2 - 8 kg/admt.
• the pH is in the acid range, typically 1-5.
• Ozone bleaching is performed either only in mixers allowing a long retention or so that the mixture of pulp and ozone with carrier gas is introduced into a reaction vessel, wherein the desired reaction retention is arranged.
• the pulp is delignified a second time with oxy- gen, the purpose of which is lowering the kappa number to a targeted kappa number of 10-20 or in some cases even lower than 10.
• oxy- gen the purpose of which is lowering the kappa number to a targeted kappa number of 10-20 or in some cases even lower than 10.
• TCF total chlorine free
• ECF elementary chlorine free
• Fig. is a schematic illustration of a preferred oxygen delignification of high kappa pulp without lignin separation
• Figure 2 is a schematic illustration of a preferred exemplary embodiment of the method according to the present invention.
• the digester equipment comprises one or more pulp digesters. Wood pulp cooked to a kappa number of 50-120, typically 60-100, is taken from the bottom of the digester 1 via blow line 2 into a blow tank 3, from where the pulp is pumped via line 4 into a washer 5.
• the washer is typically a single drum washer, such as e.g., a pressure drum washer produced by Andritz, i.e. a Drum DisplacerTM (DD)-washer, or a diffuser or a suction drum washer.
• the washed pulp is led into an oxygen stage.
• the pulp discharged from the digester is not defibrated mechanically and therefore the blow line is devoid of a mechanical defibrator downstream of the digester. Also, the pulp is not screened here prior to the first oxygen stage.
• the blow tank 3 is not inevitable, but the pulp can also be taken directly into the washer 5, depending on the type of washer and equipment coupling, as known per se.
• Black liquor is discharged from the digester via line 33.
• the first oxygen stage comprises two reactors 7 and 8, i.e. it is a two-step stage, but it can also be a one-step or multi-step stage, as described above.
• this oxygen stage the kappa number of the pulp typically decreases by 30-70 units.
• the pulp is led via line 9 to a screening room 10.
• the pulp is led further via line 11 to washing 12 downstream of the oxygen stage, where the washer can instead of a single drum washer or a diffuser be also a press.
• the pulp is treated with an electrophilic chemical in a reaction vessel 14, wherein the washed pulp is discharged from the washing apparatus 12 via line 13.
• the pulp is treated after the first oxygen stage with such a bleaching chemical that reacts with non-phenolic lignin. Suitable for this are so-called electrophilic bleaching chemicals. These typically comprise peracids, chlorine dioxide under acid conditions, ozone or chlorine. This treatment adequately promotes the delignifiction of pulp cooked to a high kappa for the actual bleaching process.
• the pulp After treatment in the reaction vessel 14 the pulp is discharged via line 15 into a washer 16, which can be a similar apparatus as the washer 12. After this the washed pulp is taken via line 17 to a second oxygen stage, which similar to the first oxygen stage comprises two steps 18 and 19. In this oxygen stage the kappa number of the pulp is lowered to a level of 10-20, preferably 10-15 or less.
• the pulp from the second oxygen stage is led via line 20 into a washer 21 , which can be single drum washer or a diffuser or also a press.
• the washed pulp is taken via line 27 into a bleaching process.
• the pulp that has been cooked to a high kappa has after the cook been treated so that its kappa number has been decreased to a level advantageous for bleaching, but simultaneously the quality of the pulp has remained at a good level, since no mechanical defibration has been needed.
• FIG. 1 illustrates that the pulp is washed as known per se in accordance with counter-current principle.
• the filtrate from the latter oxygen stage washing 21 is led counter-currently via line 22 into a washer 16 as washing liquid.
• treatment 14 with an electrophilic chemical is carried out under acid conditions, a portion of the filtrate from the downstream washer is discharged from the process via line 24, but a portion is taken via line 23 to the first oxygen stage washer 12 as washing liquid.
• the filtrate discharged from the washer 12 is led via line 25 to a washer 5 downstream of the digester, the filtrate from which washer is led via line 26 to digester washing into digester 1. Additionally, water or other washing liquid is introduced into the washers 21 and 12 via lines 32.
• Wood pulp cooked to a kappa number of 50-120, typically 60-100 is taken from the bottom of the digester 1 via blow line 2 into a blow tank 3, from where the pulp is pumped via line 4 into one or several washers 5.
• the washer is typically a single drum washer, such as e.g., a pressure drum washer produced by Andritz, i.e. a Drum DisplacerTM (DD)-washer, or a diffuser or a suction drum washer.
• DD Drum DisplacerTM
• the cooked pulp is washed at the bottom of the digester by a so- called digester wash.
• the E 10 -value of the washing equipment for cooked pulp i.e.
• digester wash and the washer 5 is at least 8, preferably at least 10.
• the washed pulp is led into an oxygen stage.
• the first and the second oxygen stage have one reaction vessel 8 and 19, respectively, but there may be more vessels, as in Figure 1.
• a further difference compared to the arrangement of Figure 1 is the treatment of the filtrate of the washer downstream of the oxygen stage.
• the oxygen stage especially in the first treatment, a large amount of lignin is released.
• the amount of removed lignin is e.g. 40 kappa units, the amount of organic material being released from the pulp becomes high. With said kappa reduction, the proportion of e.g. lignin only is approximately 7% of the dry matter of the pulp, and thereto carbohydrate losses.
• the COD-amount of the solution which characterizes the organic substance in the filtrates, may rise to 100-200 kg/adt as a result of lignin.
• the filtrate of the washer 12 downstream of the first oxygen stage 8 contains abundantly lignin, especially if the kappa number in the reaction vessel 8 has been decreased by 30-70 units. Therefore, lignin is separated from the filtrate 25 of the washer 12 in stage 29 by a method that may be known as such, e.g. by precipitating or filtering. Various lignin separation methods can be ap- plied here. It is advantageous that the first oxygen treatment uses sodium hydroxide as alkali instead of oxidized white liquor for avoiding sulfur compounds.
• the lignin fraction is recovered in low-sulfur form, and thus it is advantageously used as raw material for chemical industry.
• the filtrate fluid is cleaned, and the filtrate is used at the washer 5 downstream of the digester and upstream of the oxygen treatment, from which washer the filtrate is taken via line 32 into digester washing.
• the lignin-containing filtrate 25 is in Figure 1 used as washing liquid for pulp at the washer 5 without separating lignin.
• the present method can provide an efficient washing and delignifying method for producing pulp from the high kappa pulp so that the quality of the pulp is not deteriorated and that the yield is good.

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