JP2013528715A - Method and system for processing pulp using cold caustic soda extraction by reuse of alkaline filtrate - Google Patents

Abstract

  A method of processing pulp includes a cold caustic soda extraction step in which spent cold caustic solution and waste liquid used to wash the extracted pulp is concentrated by an evaporation system. Using the concentrated liquid as part of the neutralization and cooking liquor in the pulp process can result in increased efficiency without significantly reducing pulp quality. The highly concentrated filtrate from the cold caustic extraction process can help reduce hemicellulose precipitation on the wood fiber in the cooking process.

Description

  The field of the invention relates generally to pulp processing, and more specifically to an improved method and system for treating effluent from cold caustic soda extraction associated with kraft chemical pulping processes.

  Pulp made from wood and plant material has many commercial uses. One of the most common uses is in papermaking, but pulp contains rayon and other synthetic materials, as well as cellulose acetate and cellulose esters used, for example, in the manufacture of filter tows, fabrics, packaging films and explosives. It can also be used to produce many other products.

  There are a number of chemical and mechanical methods for processing wood and plant materials to produce pulp and paper. The basic processing steps include preparing raw materials (eg, peeling and chipping) and separating the wood fibers by mechanical or chemical means (eg, grinding, to separate lignin and extracts from the wood fiber cellulose). Separation by beating or cooking), removing the colored material by bleaching, and shaping the resulting processed pulp into paper or other product. Paper mills generally remove, in addition to and in connection with pulp and paper production, facilities that produce and recycle chemicals, collect and process by-products to produce energy, and waste. And equipment to process and minimize environmental impact.

  “Pulping” generally refers to the process of achieving fiber separation. Wood and other plant materials contain cellulose, hemicellulose, lignin and other minor components. Lignin is a network of macromolecules interspersed between individual fibers and functions as an intercellular adhesion molecule that binds individual wood fibers. During the pulping process, the lignin macromolecules fragment, thereby liberating individual cellulosic fibers and dissolving impurities that can cause discoloration and subsequent disintegration of the paper or other final product.

  The kraft process is a commonly used pulping process. Paper produced by the kraft pulping process can be used, for example, to make bleached board and linerboard used in the packaging industry. In a conventional craft process, wood is treated with an aqueous mixture of sodium hydroxide and sodium sulfide known as “white liquor”. This treatment breaks the bond between lignin and cellulose, breaking down most of the lignin and part of the hemicellulose macromolecules into fragments that are soluble in strongly basic solutions. This process of releasing lignin from the surrounding cellulose is known as delignification. Thereafter, the soluble portion is separated from the cellulose pulp.

  FIG. 1 shows a flow diagram of a conventional craft process 100. Process 100 involves feeding wood chips (or other organic pulp-containing raw material) 118 and an alkaline solution to a high pressure reactor called a digester to perform delignification, which is referred to as “cooking” stage 121. The Wood chips are combined with white liquor 111 that can be produced by a downstream process or can be supplied from a separate source. Delignification can take several hours, and the degree of delignification is expressed as unitless “H factor”, which is generally defined as one hour cooking at 100 ° C. equals one H factor. Because of the high temperature, the reactor is often pressurized by the introduction of steam. At the end of the cooking process, the reaction tank is depressurized to atmospheric pressure to release vapor and volatile substances.

The white liquor used for cooking can be, for example, a caustic solution containing sodium hydroxide (NaOH) and sodium sulfide (Na ₂ S). White liquor properties are often expressed in terms of effective alkali ("EA") and sulfidity. The effective alkali concentration can be calculated as the weight of sodium hydroxide plus one half of the weight of sodium sulfide, which corresponds to the equivalent of sodium hydroxide per liter of liquid, grams per liter Expressed as a number. The effective amount of alkali added as sodium hydroxide corresponds to the equivalent of sodium hydroxide per wood oven dry weight and is expressed as a percentage. The degree of sulfidation is the ratio of one half of the weight of sodium sulfide to the sum of the weight of sodium hydroxide and one half of the weight of sodium sulfide, and is expressed as a percentage.

  After cooking, the brown solid cellulosic pulp, also known as “brown stock”, is released from the digester used in the cooking stage 121 and then screened and washed in a washing and screening process 122. Screening separates the pulp from bundled fibers (a bundle of wood fibers), knots (uncooked chips), contaminants and other residues. The material separated from the pulp may be referred to as “reject”, and the pulp may be referred to as “accept”. Multistage cascade operations are often used to reduce the amount of cellulosic fibers in the reject stream while maintaining high purity in the accept stream. Further fiber recovery can be achieved using a downstream refiner or by reprocessing the bundled fibers and knots in the digester.

  The brown stock can then be subjected to several successive washing steps to separate spent cooking liquor and dissolved material from the cellulose fibers. The spent cooking liquid 112 from the digester used in the cooking stage 121 and the liquid 113 collected from the cleaning and screening process 122 are generally referred to as “black liquor” because of their color. . Black liquor generally contains lignin fragments, carbohydrates derived from fragmented hemicellulose, and minerals. For example, as shown by the arrow representing the black liquor 113 produced in the cleaning and screening process 122 in FIG. Black liquor 135 from an accumulator tank (not shown in FIG. 1) is fed to the digester as part of the digestion stage 121 as needed to achieve a suitable alkali concentration or for other similar purposes. May be.

  The brown paper pulp 131 dedusted by the washing and screening process 122 can then be blended with the white liquor 114 and fed to the reaction vessel to further remove dissolved substances such as hemicellulose and low molecular weight cellulose. An exemplary separation method is the so-called cold caustic soda extraction (“CCE”) method, represented in FIG. The temperature at which the extraction is performed can vary, but a typical range is less than 60 ° C.

  The refined pulp 132 from the reactor used in the CCE reaction stage 123 is then separated from the spent cold caustic solution and dissolved hemicellulose and washed several times in the second washing and separation unit in the CCE washing stage 124. . The resulting refined brown pulp 133, which has a relatively high alpha cellulose content but still contains some lignin, is advanced to a downstream bleach unit for further delignification. In some pulp manufacturing processes, bleaching occurs before the CCE reaction stage 123 and the CCE washing stage 124.

  In many applications, such as the production of synthetic materials or pharmaceuticals, it is desirable to obtain very high purity or quality pulp. Pulp quality can be assessed by several parameters. For example, the alpha cellulose content represents the relative purity of the processed pulp. The extent of delignification and cellulose degradation is measured by kappa number ("KN") and pulp viscosity, respectively. Higher pulp viscosity indicates longer cellulose chain length and less degradation. The amount of residual hemicellulose is estimated from the pulp solubility (“S18”) in an 18 wt% aqueous sodium hydroxide solution. Pulp solubility (“S10”) in a 10 wt% aqueous sodium hydroxide solution gives an indication of the total amount of solubles in the basic solution, including the sum of hemicellulose and degraded cellulose. Finally, the amount of decomposed cellulose is determined from the difference between S10 and S18.

  In conventional processes, the filtrate 116 from the CCE wash and separation stage 124, also referred to as CCE alkaline filtrate, includes both the used cold caustic solution and the used wash liquid from the wash and separation stage 124. This filtrate 116 often contains a substantial amount of polymeric hemicellulose. When a filtrate with a high hemicellulose content is used as part of the cooking liquor in the digester at the cooking stage 121, hemicellulose may precipitate from the solution and precipitate into cellulosic fibers. This may hinder the achievement of high quality pulp. On the other hand, certain applications such as high quality yarns or synthetic fabrics, materials for liquid crystal displays, products made from acetate derivatives, viscose products (such as tire cords and special fibers), filter tow parts used in tobacco, And certain food and pharmaceutical applications require pulps that contain a minimal amount of reprecipitated hemicellulose and that have a high alpha cellulose content.

  A portion of the CCE alkaline filtrate 116 can be reused in the cooking stage 121 and the remaining part is sent to the recovery zone 134 to reduce the risk of hemicellulose reprecipitation in the cooking stage 121. In the recovery region 134, the diverted CCE alkaline filtrate 116 can be combined with excess black liquor, concentrated, and combusted in a recovery boiler to consume organics and recover inorganic salts. For example, the CCE alkaline filtrate 116 is carried to another pulping line or a combination thereof. In order to maintain a proper alkali balance in the cooking stage 121, a new alkali source may then be required to replenish the CCE filtrate and black liquor sent to the recovery zone 134. The recovery process and the preparation of a new alkali source tend to increase manufacturing costs.

  What is needed is a method and system for processing pulp that results in a dissolving pulp with a very high alpha cellulose content. There is a further need for pulp processing methods and systems that provide increased efficiency and allow efficient use of CCE filtrate while minimizing hemicellulose precipitation during cooking.

  In one aspect, an improved method and system for pulp production, in particular, washing refined pulp obtained from a cold caustic soda extraction process, collecting the resulting alkaline filtrate, Concentrating by evaporation and utilizing at least a portion of the concentrated alkaline filtrate for upstream cooking processes.

  According to one or more embodiments, a method and system for pulp production using cold caustic soda extraction in conjunction with a kraft process includes delignifying organic pulp-containing material in a digester and resulting brown paper Treating the material to obtain a semi-refined pulp, treating, extracting the semi-refined pulp with a caustic solution to obtain a refined pulp and a hemicellulose-containing solution, and extracting the hemicellulose-containing solution and the refined pulp. Separating the refined pulp and collecting the alkaline filtrate obtained thereby, concentrating the alkaline filtrate, and utilizing at least a portion of the concentrated alkaline filtrate in the digester. Concentrated alkaline filtrate can gradually replace the different cooking liquor initially used to start the cooking process, thereby increasing efficiency.

  In certain embodiments, the alkaline filtrate is concentrated to obtain a solution containing more than 90 grams of effective alkali per liter, for example as sodium hydroxide. By using the concentrated alkaline filtrate as part of the cooking liquor, the purity of the brown stock and the purity of the resulting refined pulp can be increased.

  Further embodiments, alternatives and variations are also described herein or shown in the accompanying drawings.

FIG. 2 is a general process flow diagram of a conventional prehydrolyzed kraft pulping process used in connection with pulp manufacturing, as known in the art. 2 is a process flow diagram of a pulp manufacturing process according to one embodiment disclosed herein. FIG. FIG. 3 is a conceptual diagram of a system and related processes for evaporation after cold caustic soda extraction according to the general principle shown in FIG. FIG. 1 shows a conventional evaporation system and evaporation process that can be used in particular in connection with cold caustic soda extraction. FIG. 4 shows a system and related process for evaporation of filtrate by cold caustic soda extraction according to the general principle shown in FIGS. 2 and 3.

  According to one or more embodiments, a method and system for pulp processing includes a first caustic solution, such as white liquor, with an amount of wood or other organic material containing raw pulp, and suitable It involves combining in a tank or reaction vessel (digestion kettle) and cooking at a suitable temperature, for example 130 ° C. to 180 ° C. to obtain a brown stock. Brown paper washing and screening produces semi-refined pulp and derivatives (black liquor, etc.) that are returned to the digester. The semi-refined pulp can be extracted with another caustic solution (which can also be white liquor) at a suitable temperature, for example below 60 ° C., to obtain a refined pulp. By further washing, the hemicellulose-containing solution can be separated from the refined pulp to obtain another caustic solution in the form of an alkaline filtrate that can be collected and stored separately. This alkaline filtrate can be concentrated, for example, by evaporation or other means, and used alone or in combination with the first caustic solution in the digester to treat the organic material and restart the cycle.

  According to one aspect of one or more embodiments, wood chips or other pulp-containing organics are reacted with a caustic solution in a reaction vessel. When the reaction is complete, the reaction mixture contains free cellulosic fibers. These fibers are further extracted with a second caustic solution to dissolve the hemicellulose. The spent caustic solution is separated from the extracted pulp along with the dissolved hemicellulose and the pulp is subjected to further washing to remove residual caustic solution and hemicellulose. The washing solution containing hemicellulose and the spent caustic solution are combined and concentrated to obtain a concentrated CCE filtrate. The concentrated CCE filtrate can then be used alone or in combination with another caustic solution to treat the wood in the reaction vessel.

  All steps outlined above can be performed with conventional equipment. By following the steps outlined above by this specification, a concentrated CCE filtrate having an effective alkali concentration comparable to that of white liquor generally used for cooking can be obtained.

  A process according to one embodiment is shown in FIG. Process 200 begins with a cooking stage 221, where wood chips or other pulp-containing organic material 218 is fed to a digester capable of withstanding high pressure, similar to conventional craft processes. The digester can be of any suitable capacity, for example approximately 360 cubic meters. In a typical industrial situation, multiple digesters can be operated in parallel, and various digesters can be operated at various stages of the pulp production process.

  The particular choice of wood type or other plant or organic material used in the digester may depend on the desired end product. For example, softwoods such as pine, fir and spruce are used in some derivatization processes and cellulose ethers (e.g. food, paint, oil recovery fluid or oil recovery mud, paper, cosmetics, pharmaceuticals, adhesives, printing products, Products having a high viscosity can be obtained, such as agricultural products, ceramics, textiles, detergents and building materials. Hardwoods such as eucalyptus and acacia may be suitable for applications that do not require pulp with very high viscosity.

  In one embodiment, the digester is heated to a first predetermined temperature during the cooking stage 221 using steam or other suitable means. This predetermined temperature can be set to 110 ° C. to 130 ° C., more specifically, for example, 120 ° C. Heating in this particular example occurs over a period of 15 minutes to 60 minutes (eg, 30 minutes), although other heating times may be used depending on the details of the equipment and the nature of the organic material being heated.

  The digester is then further heated, preferably by steam or other means, to a second temperature above the first predetermined temperature of the prehydrolysis stage. This second prehydrolysis temperature is preferably around 165 ° C., but again the exact temperature can depend on a number of variables including equipment and organic materials. The prehydrolysis heating can be performed for 30 minutes to 120 minutes (eg, 60 minutes), but again the heating time can vary as needed. After reaching the prehydrolysis temperature, the digester is held at that temperature for a suitable period of time, such as 35 minutes to 45 minutes, or any other time sufficient to complete the prehydrolysis.

  In a preferred embodiment, the neutralization solution 210 is added to the digester as part of the cooking stage 221. Neutralization solution 210 can consist of a freshly prepared white liquor followed by black liquor, or it can consist of a CCE filtrate followed by black liquor. The white liquor can take the form of, for example, a mixture of sodium hydroxide and sodium sulfide. In a preferred embodiment, the white liquor is from 85 grams to 150 grams of effective alkali per liter as sodium hydroxide (NaOH), more preferably from 95 grams to 125 grams of sodium hydroxide per liter, most preferably It has an effective alkali of 100 to 110 grams of sodium hydroxide per liter. The sulfidity of the white liquor can have a range of 10% to 40%, preferably 15% to 35%, most preferably 20% to 30%.

  The effective NaOH concentration in the black liquor can be between 10 grams and 50 grams per liter, but can vary depending on the particular process. In one embodiment, the neutralizing solution 210 includes both white liquor and black liquor, the effective alkali concentration for white liquor is 85 grams to 150 grams of sodium hydroxide per liter, and the effective alkali concentration for black liquor. Is 20 to 50 grams of sodium hydroxide per liter. In a preferred embodiment, neutralizing solution 210 containing both white liquor and black liquor has an effective alkali concentration of 95 grams to 125 grams per liter and 30 grams to 35 grams per liter, respectively, more preferably each. It has an effective alkali concentration of 100 grams to 110 grams per liter and 38 grams to 45 grams per liter. Neutralization solution 210 can have an effective alkali concentration of 38 to 48 grams of NaOH per liter for the combined liquid.

  Neutralizing solution 210 may be added to the digester at once, otherwise it may be added to the digester in several portions. In one embodiment, neutralizing solution 210 containing both white liquor and black liquor is added in two portions, whereby white liquor is first fed to the digester and subsequently black liquor is added. In one embodiment, the neutralization solution 210 is added at a temperature of 130 ° C to 160 ° C, more preferably 140 ° C to 150 ° C. The addition can be performed over a period of 15 minutes to 60 minutes, preferably 30 minutes. In a preferred embodiment, the neutralization solution 210 is added in two portions at a temperature of 140 ° C. to 150 ° C. over 15 minutes each.

  The neutralizing solution 210 can then be replaced by the first caustic solution 211, and the first caustic solution 211 can be used to digest the wood in the digester. The first caustic solution 211 may have the same composition as that of the neutralizing solution 210 or may have a different composition. The ranges and preferred ranges of sodium hydroxide and sodium sulfide in the first caustic solution 211 are the same as in the neutralization solution 210 and are known to those skilled in the art.

  The digester can be heated to cooking temperature using steam or other means. The cooking temperature can be in the range of 140 ° C to 180 ° C, preferably in the range of 145 ° C to 160 ° C. Heating can be from 10 minutes to 30 minutes or other suitable period. The digester is held at the cooking temperature for a period suitable for the cooking process, for example 15 minutes to 120 minutes. The temperature range and cooking time are preferably selected with a target H factor that is in the range of 130-250.

  A preferred technique for neutralizing and cooking is assigned to the assignee of the present invention and is incorporated herein by reference as if fully set forth herein. Co-pending US patent application Ser. No. 12 / 789,307 entitled “Method and System for High Alpha Dissolving Pulp Production” (Attorney Docket No. 161551-0003) It is described in.

  As a result of the cooking stage 221, a brown stock 212 is obtained. Brown stock 212 is fed to a washing and screening process 222, similar to conventional crafting procedures, where brown stock 212 is screened using various types of sieves or screens and centrifuged. The brown stock 212 is then washed with a washer in a screening and washing process 222. The washer can be any type commercially available, such as a horizontal belt washer, rotary drum washer, vacuum filter, wash press, compression baffle filter, atmospheric diffuser and pressure diffuser. The washing unit can use countercurrent between stages so that the pulp moves in the opposite direction to the washing water. In one embodiment, the brown stock 212 is washed with pressurized water. In another embodiment, the brown stock 212 is washed using a dilute caustic solution. The diluted caustic solution may have an effective alkali concentration of, for example, less than 5 grams of NaOH per liter, more preferably less than 1 gram of NaOH per liter. The spent cleaning solution is collected and used as black liquor 213 in other parts of the process 200. In one embodiment, the black liquor 213 is used as part of the cooking liquor or other caustic solution 211 that is fed to the digester in the cooking stage 221.

  The semi-refined pulp from the washing and screening process 222 is then pumped as a slurry to the reactor used in the cold caustic soda extraction (“CCE”) stage 223, again as in the conventional method, where the second caustic solution 214 (which may be the same as or different from the first caustic solution 211) to provide further separation of the hemicellulose from the desired cellulosic fibers. Cold caustic soda extraction is a process known in the art. Examples of cold caustic soda processing systems include, for example, Ali et al., US Patent Application Publication No. 2004/0020854, and Svenson et al., US Patent Application Publication No. 2005/0202931, both of which are fully described herein. As described above, which is incorporated herein by reference in its entirety).

  The hemicellulose extraction in the CCE extraction process 223 is performed at a suitable temperature, generally 15 ° C. to 50 ° C., preferably around 30 ° C. The pH of the pulp slurry is generally above 13, and the effective alkali is 60 to 90 grams of NaOH per liter. The pulp is immersed in the cold caustic solution 214 for a sufficient amount of time to allow the hemicellulose to diffuse into the solution to the desired extent. An exemplary residence time for extraction at 30 ° C. and pH 13 is 30 minutes. Cold caustic soda extraction generally can result in refined pulps with an alpha cellulose content in the range of 92 percent to 96 percent, while maintaining purity at the upper end of the scale while maintaining other desirable properties (such as viscosity levels). Reaching purity beyond that has historically been extremely difficult. It was also difficult to reach high purity while maintaining high process efficiency.

  The caustic solution 214 used in the blending and extraction procedure of the CCE extraction process 223 is a freshly prepared sodium hydroxide solution, a recovery from a downstream process, or a by-product in pulp or paper mill operations, such as semi-caustic It may contain white liquor, oxidized white liquor and the like. Other basic solutions such as ammonium hydroxide and potassium hydroxide may be used.

  The caustic solution 214 used in the CCE extraction process 223 may have a suitable hydroxide concentration. For example, the caustic solution 214 may have a hydroxide concentration of 3 wt% to 50 wt%, more preferably a hydroxide concentration of 6 wt% to 18 wt%. The extraction can be performed at any suitable pulp consistency, for example from about 2% to 50% by weight, preferably from about 5% to 10% by weight. The term “viscosity” in this case refers to the concentration of cellulosic fibers in the extraction mixture.

  After the desired residence time, the pulp is separated from the spent cold caustic solution in a subsequent wash process 224. The spent cold caustic solution contains extracted hemicellulose. The pulp is washed in a CCE washing unit. Exemplary washers include horizontal belt washer, rotary drum washer, vacuum filter, wash press, compression baffle filter, atmospheric diffuser and pressure diffuser. The cleaning liquid may include, for example, pure water or a dilute caustic solution having an effective alkali concentration of, for example, 1 gram or less NaOH per liter. The spent cleaning solution can be collected in a conventional manner and combined with the used cold caustic solution to obtain another caustic solution 216, which in one aspect comprises an alkaline filtrate obtained from the cleaning process 224. In the meantime, the extracted and washed pulp 233 moves to the next bleaching stage.

  The third caustic solution 216 is preferably supplied to the concentration process 225, for example, can be supplied to an evaporation system for concentration. A typical evaporation system may have several units or effects installed in series. The liquid travels through each effect and is further concentrated at the outlet of the effect. A vacuum may be applied to facilitate evaporation and concentration of the solution.

  In connection with the concentration process 225, the dark black liquor 243 is evaporated into the dark black liquor 244 by gradually increasing the concentration of the thin black liquor 243 during the process, for example by evaporating using one or more effects in a continuous arrangement. It can also be concentrated. The dark black liquor 244 can be stored in a storage tank and used in a recovery area (recovery boiler) or for other purposes, thereby increasing the efficiency of reuse or recycle of by-products that result.

  The number of effects used for evaporation depends in part on the desired concentration level, plant capacity, and other factors. In one embodiment, the evaporation facility for the concentration stage 225 comprises six effects that can process, for example, 740 tons of liquid per hour. The effect may be of the same type used for the concentration of black liquor from the cooking stage 221, but this is not necessary. For example, it is common to use a series of effects to concentrate the thin black liquor remaining from the cooking stage and store it in a holding tank, in which case the black liquor is used for the cooking process Can be recirculated or otherwise sent to other processes for different purposes. In general, excess black liquor is produced and this excess black liquor is burned in an incinerator for power generation.

  In a preferred embodiment (shown in FIG. 3), the concentration of the alkaline extraction solution 316 from the CCE wash stage 224 is two of six effects (in this example, fifth effect 327 and sixth effect 328). To obtain a concentrated solution 330, ie a concentrated CCE alkaline filtrate. The concentration of the thin black liquor from the cooking stage 221 to the concentrated black liquor takes place at higher pressures in 4 of the 6 effects. In this example, the thin black liquor 313 is introduced into one effect (in this example, the fourth effect 326) and after preconcentration, pumped to another downstream effect 329 for further concentration. Concentration of the alkaline extraction solution 316 from the CCE cleaning stage 224, which can be a combination of a used cleaning solution 314 and a used cold caustic solution 315, is achieved in the fifth effect 327 and the sixth effect 328, for example 1 It can be carried out at a suitable pressure for a period sufficient to reach the desired concentration, which is in the range of about 85 grams to 110 grams of NaOH per liter, more preferably 95 grams to 105 grams of NaOH per liter. In one embodiment, the alkaline extraction solution 316 is under a negative pressure of approximately −0.84 bar (g) within the fifth effect 327 and under a negative pressure of approximately −0.50 bar (g) within the sixth effect 328. To obtain a concentrated solution 330 having an effective alkali concentration of, for example, approximately 95 grams to 105 grams of NaOH per liter.

  Advantageously, the processing plant can be configured to use the process of the present invention with little additional equipment costs. If the plant uses, for example, six effects to concentrate the thin black liquor remaining from the cooking stage, two of the effects are used to concentrate the alkaline filtrate produced in the CCE cleaning process. Can be rearranged. A reduction in the number of effects available for black liquor concentration reduces black liquor evaporation capacity by approximately 20-30%, but can maintain black liquor quality (final solids concentration) and has four effects. This is not a problem because the black liquor obtained from can be burned in the recovery boiler without any significant effect. However, the use of two of the effects for alkaline filtrate concentration and recirculation in accordance with the inventive techniques described herein can have a significant impact on plant efficiency. The same number of effects can be used for two different processes, so that the operator can choose to use the conventional process for evaporation of the thin black liquor for all effects, or much worse The plant can be configured so that some of the effects can be assigned to the concentration of the alkaline filtrate without any improvement and an improvement in efficiency can be obtained.

  Returning to FIG. 2, the concentrated alkaline filtrate solution 217 can be reused in whole or in part as neutralizing solution 210 and / or as part of cooking liquor 211. In one embodiment, the neutralization solution 210 consists entirely of the concentrated alkaline filtrate solution 217. In another embodiment, the neutralization solution 210 is a concentrated alkaline filtrate solution 217 and a white liquor that can be initially added to the digester and optionally used to enhance the concentrated alkaline filtrate solution 217. Includes both. In the third embodiment, the concentrated alkaline filtrate solution 217 is used as the cooking solution 211. In the fourth embodiment, the concentrated alkaline filtrate solution 117 is combined with the white liquor for use as the cooking liquor 211.

  The concentrated alkaline filtrate solution 217 that is not reused in the cooking stage 221 can be used for other purposes. For example, the concentrated alkaline filtrate solution 217 can optionally be diverted for other purposes, such as use in an adjacent production line (as white liquor) as indicated by arrow 251 in the example of FIG. At the same time, the concentrated alkaline filtrate solution 217 allows the use of higher liquid concentrations in the cooking stage 221, thereby preventing reprecipitation of hemicellulose into the fibers.

  4 and 5 illustrate and compare a conventional system for an evaporation process associated with cold caustic soda extraction and one possible embodiment disclosed herein. FIG. 4 is a diagram of a conventional system 400 showing an evaporation process that can be used in particular with cold caustic soda extraction. As shown in FIG. 4, system 400 includes a number of effects 461A-461D and 462-466. The thin black liquor 413 from the cooking process is received by one of the effects, in this case the fourth effect 464, where the evaporation process begins. The pipes 441 and 442 connect the fourth effect 464 to the fifth effect 465 and the fifth effect 465 to the sixth effect 466, respectively. After processing in the sixth effect 466, the semi-concentrated black liquor is moved to the intermediate heat exchangers 450 and 452. The semi-concentrated black liquor is supplied from the heat exchanger 452 to the third effect 463, and the product therefrom is transferred to another intermediate heat exchanger 454.

  Then, the semi-concentrated black liquor is supplied from the heat exchanger 454 to the second effect 462 (one main body is divided into two liquid circulation units “A” and “B”). After evaporation in the second effect 462, part of the black liquor is pumped directly to the first effect (concentrator) and the other part is subjected to flash evaporation in the evaporator 459 under atmospheric pressure. And pump to ash mixture (432). The first effect can physically consist of four evaporators 461A-461D. The evaporator may be a cylindrical multi-tube falling film evaporator. All four of the evaporators 461A-461D can be operated at the same time, thereby enabling the production of a higher concentration of black liquor. Liquid containing ash is pumped from the ash mixing tank to the evaporator 461D. After evaporation in evaporator 461D, the concentrated heavy liquid is flushed in flash evaporator 459 and stored in a pressurized heavy liquid tank (not shown in FIG. 4).

  The output of the evaporation system 400 includes heavy (dense) black liquor 430 and condensate 431 that is sent to the wash liquor storage. The dark black liquor 430 can be used for the purposes previously described herein. In the condensate tank 440A, the vapor condensate from the second effect 462, the third effect 463, and the fourth effect 464 are combined to obtain a clean condensate (“A condensate”), and a sixth effect 466 is obtained. It can be flushed through several stages until it is subjected to a pressure similar to the steam inlet pressure. A condensate can be collected in a clean condensate tank (tank A of condensate tank 440) and used in other parts, such as fiber lines.

  The condensate from the clean side of the fourth effect 464 and the fifth effect 465 forms an intermediate condensate (“B condensate”) that has a pressure similar to the steam inlet pressure of the sixth effect 466. Flush or depressurize through several stages until received. The flushed B condensate is treated with other parts of the evaporation system, for example treated or untreated condensate from the main section of the separated surface condenser 470, which is the clean side of the sixth effect 466, and Combine with the treated condensate from the strip column. This combined condensate may generally contain more impurities than the A condensate. The B condensate can be collected in an intermediate condensate tank (tank B of condensate tank 440) and used in other parts of the pulp production process, such as a causticization plant.

  Contaminant condensate (“C condensate”), which generally contains more impurities than A condensate or B condensate, is removed from the contaminated side of the fifth effect 465 and the sixth effect 466 on the isolated surface condenser. It can be collected from secondary sections as well as vacuum systems. C condensate is stored in a contaminated condensate tank (tank C of condensate tank 440).

  FIG. 5 is a diagram of a system 500 showing the process of evaporation of filtrate by cold caustic extraction according to the general principles shown in FIGS. In this example, system 500 uses the same basic equipment configuration and the same number of effects as system 400 of FIG. 4, but need not be so in other embodiments. The dotted line in FIG. 5 shows additional connections (including pipes and valves) that can be added to the facility of FIG. 4 to achieve further functionality of CCE filtrate concentration. In FIG. 5, system 500 similarly includes a plurality of effects 561A-561D and 562-566. Effects 561A-561D, 562 and 563 are suitable for the same general purpose as the corresponding effects 461A-461D, 462 and 463 of FIG. However, in the system 500 shown in FIG. 5, the thin black liquor 513 is first concentrated in the fourth effect 564 and then via the bypass pipe 537 (controlled by an additional valve 536) (otherwise). (Similar to the heat exchanger 450 in FIG. 4). In this way, the thin black liquor concentration process bypasses the fifth effect 565 and the sixth effect 566.

  Unlike the system 400 of FIG. 4, in the system 500 of FIG. 5, the cold caustic soda extraction (CCE) filtrate 516 from the CCE cleaning process is fed to the fifth effect 565 via the connector pipe 541, where the concentration process. Receive the first part. A new valve 538 has been added to FIG. 4 that allows isolation of the fourth effect 564 from the CCE filtrate 516. For example, if a smaller amount of concentration is desired, an optional branch connector pipe 539 may be added to allow CCE filtrate 516 to be added to the sixth effect to provide the option of supplying CCE filtrate 516 directly to sixth effect 566. 566. Otherwise, after evaporation in the fifth effect 565, the semi-concentrated CCE filtrate is fed to the sixth effect 566 via the connector pipe 542 where it undergoes further concentration by the desired degree of evaporation.

  Concentrated CCE filtrate 560 can be directed to tank C of condensate tank 540 via line 591 or to tank B of condensate tank 540 via line 592. In connection with the crafting process described above, the concentrated CCE filtrate 560 can be mixed with white liquor, black liquor or other solution as part of the cooking stage. If desired, the semi-concentrated CCE filtrate may be sent from the fifth effect 565 to the heat exchanger 550 via another additional connector pipe 535 controlled by a valve 534. Connector pipe 535 also gives the option to use five effects for thin black liquor concentration and only a single effect (sixth effect) for CCE filtrate concentration. This configuration in particular provides significant flexibility with respect to various mixing and concentration of cooking and cleaning solutions. In this embodiment where the CCE filtrate is concentrated in the fifth effect 565 and the sixth effect 566, the flow of condensate can be changed by a valve switch. For example, condensate from the contaminated side of fourth effect 564 can be part of the contaminated condensate (C condensate), and condensate from the contaminated side of sixth effect 466 can be intermediate condensate (B The condensate from the main section of the separated surface condenser can be part of the clean condensate (A condensate).

  The process of embodiments of the present invention is demonstrated in the following examples. The analysis results described in the examples are obtained using the following method.

The method used to measure the S10 and S18 solubilities of pulp at 25 ° C. is TAPPI standard T235 cm-00 (herein as if fully described herein). Based on). The pulp is extracted with 10% and 18% sodium hydroxide (NaOH) solutions, respectively. Carbohydrate dissolution is determined by oxidation with potassium dichromate. Low molecular weight carbohydrates such as hemicellulose and degraded cellulose can be extracted from the pulp using sodium hydroxide solution. Thus, the solubility of pulp in alkali gives information about cellulose degradation and hemicellulose loss or retention in pulping and bleaching processes. In a typical procedure for measuring S10 solubility, a 10 gram oven-dried pulp sample is placed in a beaker and 75 mL of 10 wt% NaOH solution is added to the pulp. The mixture is stirred with a dispersing device for a sufficient time until the pulp is completely dispersed. An example of a dispersing device may have a variable speed motor and a stainless stir bar with a shell. The motor speed and blade angle are adjusted so that no air is trapped in the pulp suspension during agitation. After the pulp is completely dispersed, an additional 25 mL of 10% NaOH is added to the mixture to ensure that all pulp fibers are covered with the alkaline solution. The beaker containing the mixture is kept in a 25 ± 0.2 ° C. water bath for 60 minutes from the time of the first addition of NaOH reagent. After this time, approximately 50 ml of filtrate is collected in a clean dry filter flask. A 10.0 mL aliquot of the filtrate is mixed with 10.0 mL of 0.5 N potassium dichromate solution in a 250 mL flask. When 30 mL of concentrated sulfuric acid is added to this with stirring, the solution becomes hot due to the chemical reaction. The solution is stirred for 15 minutes while still hot. 50 mL of water is then added to the mixture and the mixture is cooled to room temperature. Two to four drops of ferroin indicator are added to the mixture and the mixture is titrated with 0.1 N ferrous ammonium sulfate solution. Repeat titration with 10 mL of 10% NaOH solution. Calculate S10 solubility using the following formula:
S (%) = [(V ₂ −V ₁ ) × N × 6.85 × 10] / (A × W)
Where V ₁ (in milliliters) is the volume of the ferrous ammonium sulfate solution used for the titration of the filtrate and V ₂ (also in milliliters) is used for the titration of the pure 10% NaOH solution. The volume of ammonium sulfate solution, N is the normality of ferrous ammonium sulfate solution, A (unit is milliliter) is the volume of pulp filtrate used for oxidation, and W is the oven dry weight of the pulp sample (grams). ).

  The procedure is the same for determining S18 solubility, except that the 10% NaOH solution used above is replaced with an 18% NaOH solution.

Pulp viscosity in capriethylenediamine (CED) solution is measured using a method based on SCAN standard CM 15-99 (incorporated herein by reference as if fully set forth herein). decide. This method determines the intrinsic viscosity of the pulp in the diluted CED solution. In a typical procedure, a sample of pulp is dissolved in a CED solution. The amount of pulp is selected with respect to the expected intrinsic viscosity. The weighed pulp sample is placed in a polyethylene bottle (capacity approximately 52 mL) and the residual air is driven off by crushing the bottle. Add 5-10 copper wires and 25 mL of deionized water to the pulp and shake the mixture on a suitable shaker until the pulp is completely disaggregated. A typical time interval until disaggregation is 10 to 30 minutes. An additional 25.0 mL of CED solution is added to the mixture. After expelling the residual air, seal the bottle and shake again for approximately 30 minutes or until the pulp sample is completely dissolved. Adjust the temperature of the test solution and viscometer to 25 ° C. A portion of the test solution is drawn into the test viscometer by aspiration. The outflow time, i.e., the time it takes for the meniscus to descend from the upper gauge line to the lower gauge line is measured. The relative viscosity is calculated using the following equation:
(Η _rel ) = (F / Tced) × T
( _Where F is the calibration factor of the viscometer, T _ced (seconds) is the outflow time for the 50% CED solution, and T (also seconds) is the outflow time for the test solution). The equivalent (η × c) value can be found in the table attached to the SCAN standard, where η is the intrinsic viscosity of the pulp (unit: mL / g), and c is the dry weight of the pulp The concentration of the test solution calculated as being divided by the volume of 50 ml in this example.

The kappa number (KN) is measured using a method similar to that of TAPPI standard T236 om-99. KN corresponds to the volume (mL) of 0.1 N potassium permanganate solution used to oxidize 1 gram of oven-dried pulp. In a typical procedure, a pulp sample is disaggregated or dissolved in approximately 300 ml of distilled water. Transfer the disaggregated or dissolved pulp sample to a beaker and add enough water to the pulp mixture to bring the total volume of the mixture to about 795 mL. 100 mL of 0.1 N potassium permanganate solution and 100 mL of 4 N sulfuric acid 4 N are mixed in a separate beaker and the mixture is quickly adjusted to 25 ° C. The acidified potassium permanganate solution is immediately added to the test pulp. After the addition, the total volume of the mixture is approximately 1000 ± 5 mL. The mixture is allowed to react for 10 minutes, after which 20 mL of 1N potassium iodide solution is added to quench the reaction. Immediately thereafter, the free iodine content of the mixture is determined by titrating the pulp mixture with a 0.2N solution of sodium thiosulfate. The end point of the titration is indicated by adding a starch indicator around the end of the reaction. Titration is performed without removing pulp fibers. Another titration is performed with a blank solution without pulp. KN is calculated using the following formula:
KN = (p × f) / w
(Wherein p is the amount of 0.1 N potassium permanganate consumed by the test sample (in milliliters), f is the correction factor for a 50% permanganate volume, see Tapfi standard) Depending on the "p" that can be, w is the oven dry weight of the pulp sample, and "p" is determined as follows:
p = [(ba) × N] /0.1
Where b is the amount of thiosulfate consumed in the titration of the blank solution (in milliliters), a is the amount of thiosulfate consumed in the titration of the pulp sample, and N is the thiosulfate specification. Degrees).

Example 1
Concentration of CCE Filtrate According to a first example, a highly diluted caustic solution stream with an effective alkali concentration of 5.6 grams of NaOH per liter was introduced into the fifth effect 327 shown in FIG. Start the plant and observe its behavior with various alkali concentration levels. Remove water from the solution at a reduced pressure of -0.73 bar and a temperature of 51.5C to 56.8C. After 4 hours 30 minutes, as with the crude CCE filtrate, a caustic solution having an effective alkali concentration of about 50 grams of NaOH per liter is fed to the fifth effect and an inlet filtrate concentration of about 50 grams of NaOH per liter. To achieve an exit concentration of the sixth effect. The flow rate, temperature, effective alkali concentration and vacuum level as a function of time are listed in Table I.

Example 2
Conventional Craft Process According to a second embodiment, an experimental craft process is performed in a bench scale digester (approximately 20 liter capacity) that simulates industrial processing. A 20 liter bench scale digester is preheated to 120 ° C. with steam for 30 minutes. Add a suitable amount (eg 4.7 kg on a furnace dry basis) of eucalyptus wood chips to the digester. The digester is heated to 165 ° C. over 60 minutes and held at 165 ° C. for an additional 40 minutes to complete the prehydrolysis stage. For the conventional kraft process (without using the filtrate from CCE), 4.51 liters of the first white liquor (“WL1”) having an effective alkali concentration of 124.7 g NaOH per liter was added at 152 ° C. Add to digester over 15 minutes at temperature. A typical alkali loading for neutralization is about 12% effective alkali (EA) as NaOH based on dry chip weight. The digester is then charged with 10.8 liters of hot black liquor (“HBL1”) having an effective alkali concentration of 25.3 g NaOH per liter, added over 15 minutes at a temperature of 140 ° C., Complete the neutralization step. The same concentration of 10 liters of second hot black liquor (“HBL2”) is added to the digester to displace the neutralized liquid at a temperature of 146 ° C. for 23 minutes, followed by 1.0 liter of heat. A cooking liquor consisting of a mixture of black liquor ("HBL2") and 4.16 liters of second white liquor ("WL2") having an effective alkali concentration of 124.7 g NaOH per liter is 10 bar and Add over 12 minutes at 152 ° C. A typical alkali loading for the cooking phase is about 11% effective alkali (EA) as NaOH based on dry chip weight. The cooking liquor is circulated at a rate of 3 liters per minute for 3 minutes under a pressure of 9.1 bar. The digester is then heated to 160 ° C. over 14 minutes and held at 160 ° C. for an additional 23 minutes. The digester is then cooled and the reaction mixture is washed twice with dilute caustic solution. For each wash, 15 liters of an aqueous solution containing approximately 0.2 g NaOH per liter of solution is used. The resulting brown stock exhibits a kappa number of 10.3, a viscosity of 988 ml / g, an S10 solubility of 3.6% and an S18 solubility of 2.7%. The reaction has a yield of 39.3%. The mixture, when screened, has a rejection rate of 0.13%, resulting in a screening yield of 39.1%.

Example 3
Use of Thin Concentrated CCE Filtrate as Neutralization Solution and Cooking Solution According to a third example, the white liquor for the neutralization and cooking steps is taken from a CCE process with an EA of 54 g NaOH per liter. The same pulping process as described in Example 2 is repeated except that the filtrate (“CCE54”) is replaced. The neutralized product has a pH of 11.0 and the cooking mixture has an EoC of 18.5 g NaOH per liter. The P-factor for prehydrolysis is 297 and the H-factor for cooking reaction is 419. In this example, the total equivalent effective alkali addition to the wood is 12% EA as NaOH for the neutralized phase and 11% EA as NaOH for the cooking phase, respectively.

  The resulting brown stock exhibits a kappa number of 10.8, a viscosity of 1118 ml / g, an S10 solubility of 4.5% and an S18 solubility of 3.6%. The reaction has a yield of 40.4%. The mixture has a rejection rate of 0.09% when screened, resulting in a screening yield of 40.3%.

Example 4
Use of highly concentrated CCE filtrate as neutralizing solution and cooking solution According to a fourth embodiment, two thirds of WL1 and WL2 are concentrated CCE having an effective alkali concentration of 110 g NaOH per liter. The same pulping process as described in Example 2 is repeated except that the filtrate is replaced. The resulting brown stock exhibits a kappa number of 9.5, a viscosity of 990 ml / g, a S10 solubility of 4.1% and a S18 solubility of 3.0%. The reaction has a yield of 39.5%. The mixture has a rejection rate of 0.10% when screened, resulting in a screening yield of 39.43%.

  Compared to the conventional kraft process, in the process of replacing two-thirds of the white liquor with concentrated CCE filtrate, the viscosity and kappa number similar to the viscosity and kappa number of the conventional kraft process (about 990 mg / l in this example) Of pulp is produced. It is expected that a similar technique, for example replacing 60% -75% white liquor with concentrated CCE filtrate, can work over a wider range. A slightly lower kappa number is achieved with the concentrated CCE filtrate, suggesting that replacing the white liquor with the concentrated CCE filtrate does not adversely affect delignification. The viscosity to kappa number ratio (a measure of selectivity in the cooking process) is higher for processes using concentrated CCE filtrates (104 vs. 96 in conventional processes), which means cooking with concentrated CCE filtrates. It shows that the selectivity is better.

  When part of the white liquor was replaced with concentrated CCE filtrate, the S18 solubility increased from 2.7% to 3.0% and the S10 solubility increased from 3.6% to about 4.1%. This indicates that some hemicellulose reprecipitation occurs. If desired, the S18 solubility level can be further controlled by other means.

  It should be possible to further optimize the process by slightly lowering the cooking temperature to achieve the same kappa number (around 10.8) and higher viscosity. Based on various experiments, it can be determined from routine calculations or optimizations based on the principles and techniques described herein, while maintaining the quality of the resulting brown stock in a potentially desirable range, For example, it is expected that slight changes in alkalinity, relative amounts of white liquor and concentrated CCE filtrate, cooking temperature and cooking time can be added to the process. For example, the resulting brown stock can provide a kappa number less than 10.0, a viscosity less than 1000 ml / g, an S18 solubility not exceeding 3.0%, and / or a viscosity to kappa number ratio exceeding 100. Be expected.

  According to certain embodiments disclosed herein, the concentrated CCE filtrate is used to digest at the same or similar viscosity and kappa number level as a conventional kraft process using only fresh white liquor. This can result in increased efficiency.

  While preferred embodiments of the invention have been described herein, many variations are possible which still fall within the concept and scope of the invention. Such variations will become apparent to those skilled in the art after reviewing the specification and drawings. Accordingly, the invention is not limited except as by the spirit and scope of any appended claims.

Claims (28)

A method of producing pulp using cold caustic soda extraction,
Delignifying the organic material in a digester and treating the resulting brown stock to obtain semi-refined pulp, delignifying and treating,
Extracting the semi-refined pulp with a caustic solution to obtain a refined pulp and a hemicellulose-containing solution; and
Separating the hemicellulose-containing solution and the purified pulp;
Washing the refined pulp and collecting the resulting alkaline filtrate;
A concentration step of concentrating the alkaline filtrate to obtain a concentrated alkaline filtrate;
Utilizing at least a portion of the concentrated alkaline filtrate in the digester;
A method for producing pulp using cold caustic soda extraction.   The method of claim 1, wherein the concentration of the alkaline filtrate is performed by an evaporation process.   The method of claim 2, wherein the evaporation process is performed in a plurality of effects connected in series.   The method of claim 2, wherein the evaporation process is performed in a temperature range of about 50C to 60C.   The process according to claim 2, wherein the evaporation process is carried out at a pressure of -0.6 bar to -0.84 bar.   The process of claim 2 wherein the concentrated alkaline filtrate has an effective alkali concentration of about 95 grams to 125 grams NaOH per liter.   The method of claim 2, wherein the concentrated alkaline filtrate has an effective alkali concentration of about 100 to 110 grams of NaOH per liter. The alkaline filtrate,
Separating the hemicellulose-containing solution and the purified pulp;
Washing the refined pulp and collecting the crude alkaline filtrate obtained thereby;
The method of claim 1 obtained by:   The method of claim 1, further comprising adding white liquor to the concentrated alkaline filtrate used in the digester.   The process according to claim 9, wherein the ratio of white liquor to concentrated alkaline filtrate is approximately 1: 1.5 to 1: 2.5. The method of claim 1, wherein the caustic solution comprises NaOH and Na ₂ S. A method of producing pulp using cold caustic soda extraction in a craft process,
Cooking the first batch of organic material in a digester to produce brown stock;
Washing and screening the brown stock to obtain semi-refined pulp, washing and screening;
Extracting the semi-refined pulp using cold caustic soda extraction to obtain a refined pulp and a hemicellulose-containing solution; and
Separating the hemicellulose-containing solution produced by cold caustic soda extraction and the purified pulp;
Washing the refined pulp and collecting the resulting alkaline filtrate;
Evaporating the alkaline filtrate in a controlled environment to obtain a concentrated caustic solution; evaporating;
Cooking a second batch of organic material using at least a portion of the concentrated caustic solution as at least one cooking liquor;
A process for producing pulp using cold caustic soda extraction in a craft process.   13. The method of claim 12, further comprising using the second portion of the concentrated alkaline filtrate in a different pulp processing production line.   The method of claim 12, wherein the evaporation process is performed in a plurality of effects that are serially combined.   The method of claim 12, wherein the evaporation process is performed in a temperature range of about 50 ° C. to 60 ° C.   13. A method according to claim 12, wherein the evaporation process is carried out at a pressure of -0.6 bar to -0.84 bar.   13. The method of claim 12, wherein the concentrated alkaline filtrate has an effective alkali concentration of about 95 grams to 125 grams NaOH per liter.   13. The method of claim 12, wherein the concentrated alkaline filtrate has an effective alkali concentration of about 100 grams to 110 grams of NaOH per liter.   13. The method of claim 12, further comprising adding white liquor to the concentrated alkaline filtrate used in the digester.   20. A process according to claim 19, wherein the ratio of white liquor to concentrated alkaline filtrate is approximately 1: 1.5 to 1: 2.5. The method of claim 12, wherein the caustic solution comprises NaOH and Na ₂ S. Producing a second brown stock by digesting a second batch of organic material in the digester;
Washing and screening the second brown stock to obtain a semi-refined pulp, washing and screening;
Extracting the semi-refined pulp derived from the second brown stock using cold caustic soda extraction to obtain a second refined pulp and a second hemicellulose-containing solution;
The method of claim 12, further comprising:   23. The method of claim 22, wherein the second brown stock has an S18 solubility not exceeding 3.0% prior to cold caustic soda extraction.   24. The method of claim 23, wherein the second brown stock has a kappa number of less than 10.0 prior to cold caustic soda extraction.   24. The method of claim 23, wherein the second brown stock has a viscosity of approximately 1000 milliliters per gram prior to cold caustic extraction.   24. The method of claim 23, wherein the second brown stock exhibits a viscosity to kappa number ratio of 100 or greater prior to cold caustic soda extraction. Digesting the organic material in a digester to produce brown stock, cooking, washing and screening the brown stock to obtain a crude pulp, washing and screening; and An improved method of producing pulp using cold caustic soda extraction in a craft process, comprising extracting the pulp using cold caustic soda extraction to obtain a refined pulp and a hemicellulose containing solution, wherein the extraction step comprises: Improvement,
Separating the hemicellulose-containing solution produced by cold caustic soda extraction and the purified pulp;
Washing the refined pulp and collecting the resulting alkaline filtrate;
A concentration step of concentrating the alkaline filtrate to obtain a concentrated alkaline filtrate;
Utilizing at least a portion of the concentrated alkaline filtrate for further neutralization or cooking of organic material in the digester;
Improved process for producing pulp using cold caustic soda extraction in a craft process. A method of producing pulp using cold caustic soda extraction in a craft process,
A cooking process, wherein at least a portion of the concentrated caustic solution derived from the downstream cold caustic soda extraction stage is used to digest the organic material in a plurality of batch digesters, thereby producing a brown stock;
Washing and screening the brown stock to obtain semi-refined pulp, washing and screening;
Extracting the semi-refined pulp using cold caustic soda extraction to obtain a refined pulp and a hemicellulose-containing solution; and
Separating the hemicellulose-containing solution produced by cold caustic soda extraction and the purified pulp;
Washing the refined pulp and collecting the resulting alkaline filtrate;
Evaporating the alkaline filtrate in a controlled environment to obtain a concentrated caustic solution; evaporating;
Returning at least a portion of the concentrated caustic solution to the batch digester as at least one digester;
A process for producing pulp using cold caustic soda extraction in a craft process.

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