ES2525263T3 - Procedure and treatment system for paper pulp using cold caustic extraction with reuse of alkaline filtrate - Google Patents

Abstract

A pulp manufacturing process using cold caustic extraction for dissolution pulp production, comprising: delignification (221) of organic materials (218) in one or more digesters and treatment (222) of a pulp resulting brown (212) to obtain a semi-purified paste for use in the production of dissolution paste; the extraction of the semi-purified pulp with a caustic solution during a cold caustic extraction process (223) to obtain a purified pulp and a solution containing hemicellulose; separating the hemicellulose-containing solution from the purified pulp; washing (224) the purified pulp and collecting the spent washing liquid therefrom; combining the spent scrubbing liquid and the hemicellulose-containing solution to form an alkaline filtrate (216); the concentration (225) of the alkaline filtrate (216) to form a concentrated alkaline filtrate (217); and utilizing at least a portion of the concentrated alkaline filtrate (217) in at least one of said digesters in connection with the production of dissolution paste.

Description

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DESCRIPTION

Procedure and treatment system for paper pulp using cold caustic extraction with reuse of alkaline filtrate

1) Field of the invention

The field of the invention relates, in general, to the treatment of paper pulp and, more specifically, to an improved method and system for treating effluents from a cold caustic extraction in connection with a chemical kraft pulp production process. of paper.

2. Background

Paper pulp from wood and plant materials has a wide range of commercial uses. Although it is in the field of papermaking that they are used in the most widespread manner, the pulp can also be used to manufacture a variety of different products including rayon and other synthetic materials, as well as cellulose acetate and cellulosic esters, which are used, for example, in the manufacture of bast for filters, packaging wrappers and explosives.

There are a plurality of chemical and mechanical procedures for treating wood and plant materials in order to make pulp and paper pulp. The basic treatment steps include the preparation of the raw material (for example, debarking and chipping), the separation of the wood fibers by mechanical or chemical means (for example crushing, refining or cooking) to separate from the cellulose of wood fibers, lignin and extracts, remove coloring agents by bleaching and constitute the resulting treated pulp in paper and other products. In addition and in connection with the manufacture of pulp and paper, paper mills can also incorporate facilities to produce and recycle chemical agents, collect and process by-products for energy and dispose of and treat wastes to minimize their environmental impact.

"Pasta production" refers in general to the process to achieve defibration. Wood and other plant materials comprise cellulose, hemicellulose, lignin and other minor components. Lignin is a network of polymers dispersed between individual fibers, and functions as an intercellular adhesive to cement individual wood fibers together. During the pulp production process, lignin macromolecules are fragmented, thereby releasing the individual cellulosic fibers and dissolving impurities that can cause discoloration and future disintegration of paper or other final product.

The kraft process is a commonly used pasta production process. The paper obtained through the kraft pulp production process can be used, for example, to make white cardboard for boxes and corrugated cardboard used in the packaging industry. A conventional kraft process treats wood with an aqueous mixture of sodium hydroxide and sodium sulphide known as "white liquor." The treatment breaks the bond between lignin and cellulose and degrades most of the lignin and a portion of hemicellulose macromolecules into soluble fragments in strongly basic solutions. This process of releasing lignin from surrounding cellulose is known as delignification. The soluble portion is then separated from the cellulose pulp.

Figure 1 shows a flow chart of a conventional kraft 100 process. Process 100 involves feeding wood chips (or other organic raw material containing pulp) 118 and alkaline solutions in a high pressure reaction vessel called a digester to carry out the delignification in what is designated as the "cooking" stage 121 Wood chips are combined with white liquors 111, which can be generated by downstream processes or provided from a separate source. The delignification can take several hours and the degree of delignification is expressed as a "unitary" factor H ", which is generally defined so that cooking for one hour at 100 ° C is equivalent to a factor H of 1. Due to the high temperature, The reaction vessel is often pressurized due to vapor reduction. Towards the end of the cooking stage, the reaction vessel is reduced to atmospheric pressure thus releasing steam and volatiles.

The white liquor used in cooking can be, for example, a caustic solution containing sodium hydroxide (NaOH) and sodium sulphide (Na2S). The property of white liquor is often expressed in terms of effective alkali ("EA") and sulphidity. The effective alkali concentration can be calculated as the weight of sodium hydroxide plus half the weight of sodium sulphide and represents the equivalent of the weight of sodium hydroxide per liter of liquor, expressed in grams per liter. The effective alkali charge as sodium hydroxide represents the equivalent weight of sodium hydroxide per kiln-dried wood weight, expressed as a percentage. Sulfidity is the ratio of half the weight of sodium sulfide to the foam of the weight of sodium hydroxide and half the weight of sodium sulfide, expressed as a percentage.

After cooking, a brown solid cellulose paste, also known as brown paste, is released from the digester used in the cooking step 121 and is then screened and washed in the washing and sieving process 122. Screening separates the paste from the agramiza (bundles of wood fibers), knots (uncooked chips) dirt and other debris. Separate materials from pasta are sometimes referred to as "rejected" and pasta as "accepted." Multistage cascade operations are often used to reduce the amount of

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Cellulosic fibers in the rejected flow while having a high purity of the accepted flow. Higher fiber recovery can be achieved by a downstream refiner or by reprocessing the agramizas and the nodes of the digester.

The brown paste can then be subjected to several washing phases in series to separate spent cooking liquors and dissolved materials from the cellulose fibers. The cooking liquor 112 spent from the digester used in the cooking phase 121 and the liquor 113 collected from the washing and sieving process 122 are both jointly designated as "black liquor" due to their coloration. Black liquor generally contains fragments of lignin, carbohydrates from fragmented hemicelluloses, and inorganic compounds. The black liquor can be used in addition to the white liquor in the cooking stage, as illustrated in the example of Figure 1 by the arrow representing the black liquor 113 produced in the washing and sieving process 122 and transferred to step 121 Cooking Black liquor 135 from an accumulator tank (not shown in Figure 1) can also be fed to the digester as part of the cooking step 121, if necessary to achieve adequate alkaline concentration or for other similar purposes.

The brown paste 131 cleaned from the washing and sieving process 122 can then be mixed with the white liquor 114 and fed to a reaction vessel to further remove dissolved materials such as hemicellulose and low molecular weight cellulose. An exemplary separation procedure is the so-called cold caustic removal procedure ("CCE"), and is represented by the reaction stage 123 of the CCE in Figure 1. The temperature at which the extraction is carried out may vary. but typically it is below 60 ° C.

The pulp 132 purified from the reactor used in the CCE reaction stage 123 is then separated from the spent cold caustic solution and the dissolved hemicellulose, and washed several times in a second washing and separation unit in a stage 124 CCE wash. The resulting purified brown pulp 133 may have a relatively high alpha cellulose content that still contains some lignin, continues to a bleaching unit downstream for further delignification. In some pulp manufacturing processes, bleaching is carried out before the 123 CCE reaction phase and the CCE washing step 124.

In various applications it is desirable, for example in the manufacture of synthetic materials or pharmaceuticals, to have a paste with a very high purity or quality. The quality of the pasta can be evaluated by various parameters. For example, the percentage of alpha cellulose content expresses the relative purity of the treated pulp. The degrees of delignification and cellulosic degradation are measured by the kappa index ("KN") and the viscosity of the pulp, respectively. A higher paste viscosity means a longer cellulose chain length and less degradation. The solubility of aqueous solutions of sodium hydroxide ("S18") with a weight percentage of 18 provides an estimate of the amount of residual hemiculose. The solubility of the paste in an aqueous solution of sodium hydroxide ("S10") with a weight percentage of 10 provides an indication about the total amounts of soluble materials in basic solutions that include the sum of hemicellulose and degraded cellulose . Finally, the difference between S10 and S18 determines the amount of degraded cellulose.

In a conventional process, the filtrate 116, also designated as CCE alkaline filtrate, from the wash and separation stage 124 comprises both the spent cold caustic solution and the spent wash liquid from the wash step 124 and separation. This filtrate 116 often contains substantial amounts of high molecular weight hemicellulose. When the filtrate with the high hemicellulose content is used as part of the cooking liquor in the digester of the cooking stage 121, the hemicellulose can precipitate the solution and deposit on the cellulosic fibers. This can prevent high paste quality from being achieved. On the other hand, certain applications - such as high-quality yarn or synthetic fibers, materials for liquid crystal displays, products made from derivatives of acetates, viscous products (for example thread for tires and special fibers), segments of bast for filters used in cigarettes, and certain food and pharmaceutical applications - desires pastes containing a minimum amount of redeposited hemicellulose and alpha cellulose content.

A certain portion of the CCE alkaline filtrate 116 can be reused in the cooking step 121 while the rest is sent to a recovery area 134 in order to control the risk of redeposition network of the hemicellulose in the cooking step 121 . In the recovery area 134, the separated CCE alkaline filtrate 116 may be combined with the excess black liquor, concentrated and burned in a recovery boiler to consume the organic waste and recover the inorganic salts or, otherwise, be conducted to another pasta production line or a combination of both systems. A new alkaline source may then be required to replace the CCE filtrate and the black liquor sent to the recovery area 134, in order to maintain the proper alkaline balance in the cooking step 121. The process of recovery and the provision of a new source of alkali tends to translate into increased production costs.

There is a need for a pulp treatment process that results in a dissolving pulp with a very high alpha cellulose content. Likewise, a pulp treatment process and system is needed that provides increased efficiency and allows efficient use of CCE filtering while minimizing hemicellulose deposition during cooking.

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US 2004/020854 refers to the alkaline treatment of cellulosic fibers. A portion of the spent wash water flow, known as the hemichaustic flow, is transported to a nanofilter system to remove a portion of the hemicellulose content. The desired components of the hemicáustico flow pass through the nanofiltration membrane and exit through the nanofiltration system as permeate material. Unwanted components existing in the hemicáustico flow, for example hemicellulose, are rejected by the nanofiltration membrane. An evaporation system increases the concentration of permeate flow. The permeate flow can be recycled back into a soaking liquor supply system.

Summary of the invention

According to the invention, a method is provided in accordance with what is defined by claim 1.

In one aspect, an improved process and system for pulp manufacturing involves, among other things, the washing of the purified pulp obtained from a cold caustic extraction process, the collection of an alkaline filtrate resulting therefrom, the concentration of the alkaline filtrate by, for example, evaporation, and the use of at least a portion of the alkaline filtrate by filtration in an upstream cooking process.

A procedure and a system for the manufacture of pulp that uses cold caustic extraction in combination with a kraft process includes the steps of delignifying the organic pulp-containing materials in a digester, treating a brown paste to achieve a semi-purified paste, extracting the semi-purified paste with a caustic solution to obtain a purified paste and a solution containing hemicellulose, separate the solution containing hemicellulose with the purified paste, wash the purified paste and collect the resulting alkaline filtrate, concentrate the alkaline filtrate, and use at least a portion of the filtered alkaline filtrate in the digester. The concentrated alkaline filtrate can gradually be replaced by a different cooking liquor that is initially used to start the cooking process, which translates into increased efficiency.

In certain embodiments, the alkaline filtrate is concentrated to form a solution containing, for example, 90 grams or more per liter of effective alkali, such as sodium hydroxide. By using the concentrated alkaline filtrate as part of the cooking liquor, the purity of the brown paste and the resulting purified paste can be enhanced.

Additional embodiments, alternatives and variants are also described herein or illustrated in the accompanying figures.

Brief description of the drawings

FIG. 1 is a flow chart of the general process of a conventional prehydrolytic pulp making process used in connection with pulp production, as is known in the art.

FIG. 2 is a process flow diagram of a pulp production process according to an embodiment as disclosed herein.

FIG. 3 is a conceptual diagram of a system and a related process for evaporation after cold caustic extraction in accordance with the general principle illustrated in FIG. 2.

FIG. 4 is a diagram of a conventional evaporation system and process as it can be used in connection with, among other things, cold caustic extraction.

FIG. 5 is a diagram of a system and a related process for evaporation of the filtrate from cold caustic extraction in accordance with the general principle illustrated in FIGS. 2 and 3

Brief Description of Preferred Embodiments

According to one or more embodiments, a process and a system for the treatment of pulp involves the connection of a first caustic solution, such as a white liquor with an amount of wood or other caustic material containing raw pulp. within an appropriate tank or container (a digester) for cooking at an appropriate temperature of, for example, between 130 and 180 ° C to obtain a brown paste. The washing and sieving of the brown paste determines as a result a semi-purified paste as well as derivatives (for example black liquor) that are fed back to the digester. The semi-purified paste can be extracted with another caustic solution (which can again be white liquor) at an appropriate temperature of, for example, below 60 ° C to obtain a purified paste. By further washing, the solution containing hemicellulose can be separated from the purified paste, producing another caustic solution in the form of an alkaline filtrate that can be collected separately and stored. This alkaline filtrate can be concentrated by, for example, evaporation or other means, and used by itself or in combination with the first caustic solution inside the digester to treat the organic materials and restart the cycle.

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According to one aspect of one or more embodiments, the wood chips or other organic elements containing the pulp are reactivated with a caustic solution inside a reaction vessel. At the end of the reaction, the reaction mixture contains released cellulosic fibers. These fibers are in turn extracted with a second caustic solution to dissolve the hemicellulose. The spent caustic solution together with the dissolved hemicellulose is separated from the extracted pulp and the pulp is subjected to a new wash to remove the residual caustic solution and the hemicellulose. The washing liquids and the spent caustic solution containing hemicellulose are combined and concentrated to form a concentrated CCE filtrate. The concentrated CCE filtrate can then be used in isolation or in combination with another caustic solution to treat the wood inside the reaction vessel.

All stages outlined above can be carried out with traditional equipment. After the steps outlined above according to the specification, a concentrated CCE filtrate having an effective alkaline concentration comparable to that of the white liquor generally used for cooking can result.

A process according to an embodiment is illustrated in Figure 2. The process 200 begins with a cooking stage 221 in which, similar to a conventional kraft process, wood chips or other organic materials 218 containing pasta They are fed to a digester capable of withstanding high pressure. The digester can have any relevant volume, such as approximately 360 cubic meters. In a typical industrial installation a plurality of digesters can be operated in parallel, with different digesters operating at different stages of the pulp production process.

The specific choice of the type of wood or other plant or materials used in the digesters may depend on the desired terminal products. For example, softwoods such as pine, spruce and spruce can be used in some derivatization processes to obtain products with high viscosity, such as cellulosic ethers (which can be used, for example, as feed additives, paint, fluids and oil, paper, cosmetics, pharmaceuticals, adhesives, recovery sludges in the world of printing, agriculture, textile materials, detergents, and building materials). Hardwoods, such as eucalyptus and acacia, may be preferred for applications that do not require a paste with a very high viscosity.

In one embodiment, the digester is heated during cooking step 221 to a first predetermined temperature with steam or other appropriate means. This predetermined temperature may range between 110 and 130 ° C and more specifically, for example, it may be 120 ° C. Heating in this particular case is carried out for a period of time between 15 and 60 minutes (for example, 30 minutes), although other heating times may be used depending on the particularities of the equipment and the nature of the organic materials. They are being heated.

The digester, preferably, is then reheated by steam or other means at a second temperature above the first predetermined temperature during a prehydrolysis stage. This second prehydrolysis temperature is preferably about 165 ° C, although here also the precise temperature may depend on a plurality of variables that include equipment and organic materials. Heating for prehydolysis can be carried out over a period of 30 to 120 minutes (for example, 60 minutes), although again the heating time may vary according to needs. Once the prehydrolysis temperature has been reached, the digester is maintained at that temperature for an appropriate period of time, for example 35 to 45 minutes, or any other time sufficient to complete the prehydrolysis.

In a preferred embodiment, a neutralization solution 210 is added to the digester as part of the cooking step 221. The neutralization solution 210 may consist of a freshly prepared white liquor followed by a black liquor, or it may consist of a CCE filtrate followed by a black liquor. A white liquor may take the form of, for example, a mixture of sodium hydroxide and sodium sulphide. In a preferred embodiment, the white liquor has between 85 to 150 grams per effective liter of alkali, such as sodium hydroxide (NaOH), more preferably between 95 and 125 grams per liter of effective alkali such as sodium hydroxide and , as a maximum preference between 100 and 110 grams per liter of effective alkali, such as sodium hydroxide. The sulphidity of the white liquor can range between 10% and 40%, preferably between 15 and 35% and, as a maximum preference, between 20 and 30%.

The concentration of effective NaOH in the black liquor can range between 10 and 50 grams per liter, although it can vary according to the specific process. In one embodiment, the neutralization solution 210 comprises both white liquor and black liquor, with an effective alkali concentration of 85 to 150 grams of sodium hydroxide per liter for the white liquor and an effective alkali concentration of 20 to 50 grams of sodium hydroxide per liter for black liquor. In a preferred embodiment, the neutralization solution 210 comprising both white liquor and black liquor has an effective alkali concentration, respectively, between 95 to 125 grams per liter and 30 to 35 grams per liter and, so more preferably, it has an effective concentration of between 100 and 110 grams per liter and 38 to 45 grams per liter, respectively. The

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Neutralization solution 210 may have an effective alkali concentration of 38 to 48 grams of NaOH per liter for the combined liquors.

The neutralization solution 210 may be added to the digester in one portion or if it cannot be added to the digester in several portions. In one embodiment, the neutralization solution 210 comprising both white liquor and black liquor is added in two portions, so that the white liquor is first supplied to the digester then followed by the addition of the black liquor. In one embodiment, the neutralization solution 210 is added at a temperature between 130 to 160 ° C and, more preferably, between 140 and 150 ° C. The addition can be carried out for a period of 15 to 60 minutes, preferably, for a period of 30 minutes. In a preferred embodiment, the neutralization solution 210 is added in two portions, every 15 minute period at a temperature between 140 and 150 ° C.

A first caustic solution 211 can then replace the neutralization solution 210 and is used to cook the wood inside the digester. The first caustic solution 211 may have the same composition as that of neutralization solution 210 or may have a different composition. The preferred range of sodium hydroxide and sodium sulphide in the first caustic solution 211 are the same as those of the neutralization solution 210 and are well known to those skilled in the art.

The digester can be heated to the cooking temperature with steam or other means. The cooking temperature may range between 140 and 180 ° C and preferably ranges from 145 to 185 ° C. The heating may extend over a period of 10 to 30 minutes or some other appropriate. The digester is maintained at the cooking temperature for a period relevant to the cooking process, for example between 15 to 120 minutes. The temperature range and cooking time are chosen for the target H factor, which preferably ranges from 130 to 250.

Preferred techniques for neutralization and cooking are described in US Patent Application with serial number 12 / 789,307 deposited jointly with it and entitled "Procedure and System for the Production of Alpha Delta Dissolution Paste" ["Method and System for High Alpha Dissolving Pulp Production ”], transferred to the assignee of the present invention.

As a result of the cooking step 221, a brown paste 212 is obtained. The brown paste 212 is supplied to a washing and sieving process 222, similar to a conventional kraft process, after which the brown paste 212 is sieved by use of different types of sieves or screens and centrifugal cleaning. The brown paste 212 is then washed in a washing machine of the screening and washing process 222. The washing machine can be of any commercial type, including horizontal belt washers, vacuum drum washing machines, vacuum presses, washing presses, compacting baffle filters, atmospheric diffusers and pressure diffusers. The washing unit can use a countercurrent flow between the stages so that the pulp moves in the opposite direction to the washing waters. In one embodiment, the pressurized needle is used to wash the brown paste 212. In another embodiment, a diluted caustic solution is used to wash the brown paste 212. The diluted caustic solution may, for example, have an alkali concentration. effective of less than 5 grams of NaOH per liter, more preferably less than 1 gram of NaOH per liter. The spent wash liquor is collected and used as black liquor 213 in any other part of the process 200. In one embodiment, the black liquor 213 is used as part of a cooking liquor or other caustic solution 211 used in the digester. in step 221 of cooking.

The semi-purified pulp from the washing and sieving process 222 is then pumped in the form of sludge onto a reactor that is used in the cold caustic extraction stage ("CCE"), also similar to the conventional procedure, in which the pulp semi-purified is mixed with a second caustic solution 214 (which may be the same or different from the first caustic solution 211 to carry out an additional separation of the hemicellulose from the desired cellulosic fibers. Cold caustic extraction is a process excessively known in the art Examples of the cold caustic treatment system are described in greater detail, for example, in Ali et al., US Patent Application Publication No. 2004/0020854, and Svenson et al., Application Publication U.S. Patent No. 2005/0203291.

Hemicellulose extraction in the CCE extraction process 223 is conducted at an appropriate temperature, typically between 15 and 50 ° C and, preferably, around 30 ° C. The pH of the sludge is typically greater than 13 with an effective alkali between 60 to 90 grams of NaOH per liter. The paste is soaked in the cold caustic solution 214 for a period of time sufficient to achieve the desired degree of diffusion of hemicellulose in the solution. An exemplary residence time for an extraction at 30 ° C with a pH 13 is 30 minutes. Cold caustic extraction can generally result in a purified pulp with an alpha cellulose content ranging from 92 to 96 percent, although historically it has been quite difficult to reach purities at the upper end of that scale or beyond, in particular while maintaining other desirable characteristics of the paste (for example its viscosity level). It has also been difficult to achieve high purities while maintaining high process efficiency.

The caustic solution 214 used in the mixing and extraction processes of the CCE extraction process 223 may comprise finished sodium hydroxide solutions, recovery from the process

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downstream or by-products in an operation of a pulp or paper mill, for example hemicáustico white liquor, oxidized white liquor and the like. Other basic products, such as ammonium hydroxide and potassium hydroxide, can also be used.

The caustic solution 214 used in the CCE extraction process 223 may contain an appropriate hydroxide concentration, for example, the caustic solution 214 may contain from 3% to 50% by weight of hydroxide concentration and, more preferably , from 16 to 18 percent by weight of hydroxide concentration. The concentration can be carried out with any consistency of the appropriate paste, such as from about 2% to 50%, but more preferably from about 5% to 10% by weight. In this context, the term "consistency" refers to the concentration of cellulosic fibers in the extraction mixture.

After the desired residence time, the paste is separated from the cold caustic solution spent in the following washing process 224. The spent cold caustic solution contains extracted hemicellulose. The paste is washed in the washing unit of CCE. Exemplary washers include belt washers, rotary drum washers, vacuum filters, wash presses, compaction baffle filters, atmospheric diffusers and pressure diffusers. The washing liquid may comprise, for example, pure water or a diluted caustic solution with an effective alkali concentration of, for example, below 1 gram of NaOH per liter. The spent washing liquid is collected in a conventional manner and can be combined with the spent cold caustic solution to form another caustic solution 216 which, in one aspect, comprises an alkaline filtrate resulting from the washing process 225. The pulp 233 extracted and washed is, meanwhile, transported to the next bleaching stage.

The third caustic solution 216 is preferably supplied to a concentration process 225 and can, for example, be fed to an evaporation system for concentration. A typical evaporation system may contain several units or effects installed in series. The liquid travels through any effect and is more concentrated at the exit of the effect. A vacuum can be applied to facilitate evaporation and concentration of the solutions.

In connection with the concentration process 225, a soft black liquor 243 can be concentrated in a strong concentration of soft black liquor 243 during the process. The strong black liquor 244 can be stored in an accumulation tank and used in the recovery area (recovery boiler) or for other purposes, thus increasing efficiency through the reuse or recycling of the by-products outlet.

The number of effects used for evaporation depends in part on the level of concentration desired, the capacity of the plant and other factors. In one embodiment, the evaporation equipment of the concentration stage 225 comprises six effects capable of treating, for example, 740 tons of liquor per hour. The effects may, but is not necessary, be of the same type used to concentrate black liquor from cooking step 221. It is typical, for example, to use a series of effects to concentrate the soft black liquor that remains from the cooking stage and store it in the containment container, where it can be recycled for use in the process of cooking or be sent to other processes for different purposes. Generally, an excess of black liquor is produced, and the excess of black liquor is burned in an incinerator for power generation.

In a preferred embodiment (as illustrated in Figure 3) the concentration of the solution 316 of alkaline extracts from the washing step 224 of CCE takes place in two of the six effects (in this example, the fifth effect 327 and the sixth effect 328) under a reduced pressure to enable a concentrated solution 330, that is, a concentrated CCE alkaline filtrate. The concentration of the soft black liquor from the stage 221 of cooking in concentrated black liquor takes place in four of the six effects at a higher pressure. In this example, the soft black liquor 313 is introduced into one effect (in this example, the fourth effect 326), and after preliminary concentration, it is pumped for subsequent concentration in other effects 329 downstream. The concentration of the alkaline extract solution 316 from the CCE wash step 224, which may be a combination of the spent wash liquid 314 and the spent cold caustic solution 315, can be supplied in the fifth and sixth effects 327 and 328 at an appropriate pressure and with sufficient duration to obtain the desired concentration, which, in one example, ranges between about 85 and 110 grams of NaOH per liter and, more preferably, ranges from 95 to 105 grams of NaOH per liter. In one embodiment, the solution 316 of alkaline extract remains in the fifth effect 317 under a negative pressure of approximately -84 kPa, and in the sixth effect 328 under a negative pressure of approximately -50 kPa, to achieve a concentrated solution 330 which has an effective alkali concentration of, for example, between about 95 to 105 grams of NaOH per liter.

Advantageously, a treatment plant can be configured to employ the inventive process without requiring a considerable additional deployment of equipment. When a plant has been used, for example, six effects to concentrate the soft black liquor remaining from the cooking stage, two of the effects can be re-deployed for use in the concentration of the alkaline filtrate produced in the process of CCE wash. The reduced number of effects available for the concentration of the black liquor is not significant because, although the evaporation capacity of the black liquor is reduced by approximately 20 to 30%, the quality of the black liquor (the concentration of the final solids ) can be maintained, making liquor possible

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black resulting from the four effects intended to be burned in the recovery boiler without relevant impact. However, the use of two of the effects for the concentration of the alkaline filtrate and for recycling, in accordance with the inventive techniques described herein, can have a significant impact on the efficiency of the plant. Because the same number of effects can be used for two different processes, a plant can be configured so that the operator can choose between using a conventional process for the production of soft black liquor in all effects, or can assign some of the effects for the concentration of the alkaline filtrate without appreciable negative consequences, while maintaining improvements in terms of efficiency

Returning to Figure 2, the solution 217 of the concentrated alkaline filtrate can be used, in whole or in part, either as a neutralization solution 210 and / or as part of the cooking liquor 211. In one embodiment, the neutralization solution 210 is composed entirely of the concentrated alkaline filtrate solution 217. In another embodiment, the neutralization solution 210 comprises both the solution 217 of the concentrated alkaline filtrate. In a third embodiment, solution 217 of the concentrated alkaline filtrate is used as cooking solution 211. In a fourth embodiment, the solution 117 of the concentrated alkaline filtrate is combined with a white liquor for use as cooking liquor 211.

Solution 217 of the concentrated alkaline filtrate that is not reused in cooking step 221 can be used for other purposes. For example, it can optionally be separated for other purposes, such as for use on an adjacent production line (such as white liquor), as illustrated by arrow 251 in the example of Figure 2. At the same time, the solution 217 of the concentrated alkaline filtrate may also make it possible to use higher liquor concentrations in the cooking stage 221, thus preventing the redeposition of hemicelluloses on the fibers.

Figures 4 and 5 illustrate and compare a conventional system for an evaporation process in connection with a cold caustic extraction, a possible embodiment being disclosed herein. Figure 4 is a diagram of a conventional system 400 that generates an evaporation process and can be used with, among other things, a cold caustic extraction. As shown in Figure 4, system 400 includes a plurality of effects 461A-D and 462-466. A soft black liquor 413 from a cooking process is received in one of the effects, in this case the fourth effect 464, where the evaporation process begins. Tubes 441 and 442 respectively connect the fourth effect 464 with the fifth effect 465 and the fifth effect 465 with the sixth effect 466. After treatment in the sixth effect 466, the semi-concentrated black liquor is displaced inside the changers 450 and 452 of intermediate heat. From the heat exchanger 452, the semi-concentrated black liquor is supplied to the third effect 463, whose product is displaced into another intermediate heat exchanger 454.

From the heat exchanger 454, the semi-concentrated black liquor is then supplied with the second effect 462 (a body divided into two liquor circulation units "A" and "B"). After evaporation in the second effect 462, one part of the black liquor is pumped directly to the first effect (concentrator) and the other is subjected to a sudden evaporation in the evaporator 459 under atmospheric pressure and pumped 432 for the cecina mixture. The first effect may physically consist of four 461A-D evaporators. The evaporators can be down-film evaporators of the tube and shell type. All 461A-D evaporators can be in operation simultaneously, which can make the production of black liquor with higher concentrations possible. The ash containing the liquor is pumped from the ash tank to the 461D evaporator. After evaporation in the evaporator 461D, the concentrated heavy black liquor is evaporated under vacuum in the abrupt expansion evaporator 459 and stored in a pressurized heavy liquor tank (not shown in Figure 4).

Among the outputs of the evaporation system 400 are a heavy (strong) black liquor 430, as well as a condensate 431 that is sent to a wash liquor storage. The strong black liquor 430 may be used for certain purposes as described hereinbefore. In the condensate tank 430A, the steam condensate from the second, third and fourth effects 462, 463 and 464 is combined to form a clean condensate ("condensate A") and can be evaporated under vacuum at various stages until it is subjected to a pressure similar to that of the vapor inlet pressure of the sixth effect 466. A condenser -A is collected in the clean condensate tank (Tank A of the condensate tank 440) and can be used anywhere else, by example, in the fiber line.

The condensate from the clean side of the fourth and fifth effects 464 and 465 form an intermediate condensate ("condensate -B") that can be evaporated in vacuo or reduced in terms of the pressure in stages until it presents a pressure similar to that of the inlet pressure of the sixth effect 466. The condensate -B evaporated under vacuum is combined with treated or untreated condensates from other parts of the evaporation system, for example from the clean side 466 of the sixth effect. The primary section of the segregated surface condenser 470 and / or the condensate treated from the distillation column. This combined condensate generally contains more impurities than -A condensate. The condensate -B is collected in the intermediate condensate tank (Tank -B of the condensate tank 440) and can be used in other parts of the pulp manufacturing production such as in the causticification plant.

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The defective condensate ("condensate -C"), which generally contains more impurities than the condensate -A or the condensate -B, can be collected from the defective side of the fifth and sixth effects 465 and 466, the secondary section of the condenser segregated surface and vacuum system. The condensate -C is stored in the defective condensate tank (Tank C of the condensate tank 440).

Figure 5 is a diagram of a system 500 that reflects a process for evaporation of filtrate from cold caustic extraction in accordance with the general principles illustrated in FIGS. 2 and 3. In this example, the system 500 uses the same basic equipment configuration and the same number of effects as the system 400 of Figure 4, although it is not necessary that this be the case in other embodiments. The dotted lines of Figure 5 show additional connections (including tubes and valves) that can be added to the equipment of Figure 4 in order to achieve the additional functionality of the concentration of the CCE filtrate. In Figure 5, system 500 again has multiple effects 561A-D and 562-566. Effects 561A -561D, 562 and 563 serve the same general purpose as the corresponding effects 461A -D, 462 and 463 of Figure 4. However, in system 500 shown in Figure 5, after black liquor 513 Soft has initially concentrated on the fourth effect 464, is supplied by means of a bypass tube 537 (controlled by an added valve 536) to the heat exchanger 550 (which on the other hand is similar to the heat exchanger 450 of the Figure 4). In this way the process of concentration of soft black liquor bypasses the fifth and sixth effects 565, 566.

Unlike the system 400 of Figure 4, in the system 500 of Figure 5, a cold caustic extraction (CCE) filtrate 516 from the CCE wash stage is supplied by means of a connecting pipe 541 with the fifth effect 565, after which he experiences the first part of the concentration process. A new valve 538 has been added in Figure 4 to make it possible to isolate the fourth effect 464 from the CCE filtrate 466. An optional branching connection tube 539 can be added to the CCE filtrate link 516 to the sixth effect 566, to make it possible to filter the CEE directly to the sixth effect 566 if, for example, a smaller amount of concentration is desired. Otherwise, after evaporation of the fifth effect 565, the semi-concentrated CCE filtrate is supplied to the sixth effect 566 by means of a connecting tube 542, after which it experiences an additional concentration by means of evaporation to the desired point.

Concentrated CCE filtering 560 can be directed through line 591 to Tank C in condensate tank 540, or via line 592 to Tank B of condensate tank 540. In connection with the stages of the kraft process described above, the filtered CCE 560 filtrate can be mixed with white liquor, black liquor or other solutions as part of the cooking stage. If desired, the semi-concentrated CCE filtrate can be sent to the heat exchanger 550 from the fifth effect 565 by means of another added connection tube 535, controlled by the valve 534. The connection tube 535 also provides the option of using five effects for the concentration of soft black liquor and only a single effect (the sixth effect) for the concentration of the CCE filtrate. This configuration provides, among other things, considerable flexibility in terms of diversity of mixtures and concentrations of cooking and washing solutions. In this embodiment, in which the CCE filtering is concentrated in the fifth and sixth effects 565 and 566, the condensate flows can be modified by valve switches: for example, the defective side of the fourth effect 564 can be part of the defective condensate (condensate -C); the condensate on the defective side of the sixth effect 466 may be part of the intermediate condensate (condensate -B); The condensate from the primary section of the segregated surface condenser can be part of the clean condensate (condensate -A).

Examples

The processes of the embodiments of the present invention are demonstrated in the following examples. The analytical results described in the examples are obtained using the following procedures.

The procedure used to measure the solubility S10 and S18 of the paste at 25 ° C is based on the TAPPI Standard T 235 cm -00. The paste is extracted with a solution of sodium hydroxide (NaOH) of 10% and 18%, respectively. Dissolved carbohydrates are determined by oxidation with potassium dichromate. Low molecular weight carbohydrates, such as hemicelluloses and degraded cellulose, can be extracted from pastes with sodium hydroxide solutions. The solubility of a paste in alkali thus provides information on cellulose degradation and loss or retention of hemicelluloses during the pulp and bleaching process. In the typical process for measuring the solubility S10, a sample of 10 gram dried oven-dried pasta is placed in a beaker and a solution of 75 ml of NaOH with 10% by weight is added to the paste. The mixture is stirred with a dispersion apparatus for long enough until the paste has completely dissolved. An example of a dispersion apparatus may contain a variable speed motor and a stainless steel stirrer with a housing. The motor speed and the angle of the blades are adjusted so that no air is drawn into the pulp suspension during agitation. After the paste has been completely dispersed, another 25 ml with 10% by weight of NaOH is added to the mixture to ensure that all the fibers in the paste are covered by an alkaline solution. The beaker containing the mixture is kept in a water bath at 25 ± 0.2 ° C for 60 min from the moment of the first addition of the NaOH reagent. After this time, approximately 50 ml of the filtrate is collected in a clean and dry filtration flask. A 10 ml portion of the filtrate is mixed with 10.0 ml of a 0.5 N potassium dichromate solution in a 250 ml flask. To this, 30 ml of sulfuric acid are added

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concentrated by stirring, during the time when the solution is heated from chemical reactions. The solutions are stirred for 15 minutes while keeping warm. 50 ml of water are 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 a 0.1 N ferrous ammonium sulfate solution. The titration was repeated for 10 ml of the 10% NaOH solution. S10 solubility is calculated using the following formula:

S,% = [(V2-V1 (* N * 6.85 * 10] / (A * W)

wherein, V1 is the volume of the ferrous ammonium sulfate solution used to titrate the filtrate and the unit is milliliter. V2, also in milliliters is the volume of the ammonium sulfate solution used to titrate a 10% pure NaOH solution, N is the normality of the ferrous ammonium sulfate solution; A, with one unit in milliliters, is the volume of pasta filtrate used in oxidation; and W is the oven-dried weight of the paste in gram.

The procedure is the same for the determination of S18 solubility except that a solution with 18% NaOH replaces the 10% solution of NaOH used previously.

The viscosity of the paste in the cupriethylenediamine solution (CED) is determined using a procedure based on the SCAN Standard CM 15-99. The procedure determines the index of intrinsic viscosity in the pulp in the diluted CED solution. In a typical procedure, a sample of paste dissolved in the CED solution. The amount of paste is chosen with respect to the intrinsic viscosity index. The heavy paste sample is placed in a polyethylene bottle (approx. 52 ml per volume) in which residual air is expelled by squeezing the bottle. From 5 to 10 pieces of copper wire and 25 ml of deionized water are added to the paste, and the mixture is stirred with an appropriate stirring device until the paste is completely disintegrated. The typical time interval for disintegration ranges from 10 to 30 minutes. Another 25.0 ml of CED solution is added to the mixture. After the residual air is expelled, the bottle is sealed and stirred again for approximately 30 minutes or until the paste sample is completely dissolved. The temperature of the control solution and the viscometer are adjusted to 25 ° C. A portion of the control solution is aspirated into the test viscometer by suction. The sliding time is measured, that is, the time it takes for the meniscus to fall from the upper to the lower mark of the viscometer. The relative viscosity is calculated using the equation:

F

(ƞrel) = x T

Tced

in which, F is a calibration factor of the viscometers; Tced, in seconds, is the slip time for a 50% CED solution; T is the slip time for the control solution, also in seconds. The equivalent value (ƞ * c) can be found in the unit table to the SCAN standard, where ƞ is the intrinsic viscosity of the paste with a unit of ml / g, and c is the concentration of the control solution calculated as the dry weight of the paste divided by the volume of the control solution, which in this example is 50 ML.

The kappa index (KN) is measured using a procedure similar to the standard T 236 om -99 of the TAPPI. The KN corresponds to the volume (in ml) of a 0.1 N potassium permanganate solution used to oxidize one gram of oven-dried pasta. In a typical procedure a paste sample is disintegrated or dissolved in approximately 300 ml of distilled water. The specimen of disintegrated or dissolved pasta is transferred to a beaker by adding enough water to the pasta mixture to place the total volume of the mixture at approximately 795 ml. 100 ml of a 0.1 N potassium permanganate solution and 100 ml of a 4 N sulfuric acid are mixed in a separate beaker, and the mixture is quickly adjusted to reach 25 ° C. The acidified potassium permanganate solution is immediately added to the control paste. After the addition the total volume of the mixture is approximately 1000 ± 5 ml. The mixture is allowed to react for ten minutes, after which time, 20 ml of a solution of potassium iodide of 1 are added to rapidly cool the reaction. The free iodine content of the mixture is determined immediately afterwards by titrating the paste mixture with a 0.2 N solution of sodium thiosulfate. The end point of the titration is indicated by a starch indicator added towards the end of the reaction. The titration is carried out without removing the pulp fibers. Another titration is carried out with a white solution without paste. KN is calculated using the following formula:

KN = (p * f) / w

wherein, p is the amount of 0.1 permanganate of potassium in milliliters, consumed by the test specimen; f is a correction factor for a 50% permanganate volume and dependent on the "p", which can be found in the Tappi standard; w is the oven-dried weight of the pasta sample; and "p" is determined as follows:

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p = [(b -a) * N] / 0.1

wherein, b is the amount of thiosulfate in milliliters consumed in the titration of the white solution; a is the amount of thiosulfate consumed in the titration of the paste sample; and N is the normal thiosulfate.

Example 1

CCE filtrate concentration

According to a first example, a flow of a very diluted caustic solution with an effective alkali concentration of 5.6 grams of NaOH per liter is introduced in the fifth effect 327 as shown in Figure 3 to start the operation of the plant and to observe its behavior with different levels of alkali concentration. Water is removed from the solution at a reduced pressure of 0.73 bar at a temperature between 51.5 ° C and 56.8 ° C. After 4 hours and 30 minutes, a caustic solution with an effective alkali concentration of approximately 50 grams of NaOH per liter, similar to the raw CCE filtrate, is fed the fifth effect by accessing the exit of the sixth effect from a concentration of the inlet filtrate of approximately 50 grams of NaOH per liter. Table I lists the flow rate, temperature, effective alkali concentration and vacuum level as a function of time.

Table I

Effective alkali (g NaOH / l)

Weather

Flow of Temperature Entrance in Exit in effect 6 Pressure (kPa)

(min.)

feed (m3 / h) (ºC) effect 5

0

350 51.5 5.6 -73

65

370 54.7 14.1 -73

105

370 56.8 36.6 -73

210

370 55.9 27.4 58.1 -73

270

400 53.6 49.8 106.9 -73

290

450 54.1 69.6 104.9 -73

Example 2

Conventional Kraft Process

According to a second example, an experimental kfraft process is carried out in an experimental scale digester (approximately 20 liters per volume) to stimulate industrial treatment. An experimental 20 liter digester is preheated with steam at 120 ° C for a period of 30 minutes. An appropriate amount (for example 4.7 kg of oven drying) of eucalyptus wood chips was added to the digester. The digester is heated at 165 ° C for a period of 60 minutes and maintained at 165 ° C for an additional period of 40 minutes to complete the prehydrolysis stage. For a conventional kraft process (without the use of filtrates from the CCE), 4.51 liters of a first white liquor (“WL1”) with an effective alkali concentration of 124.7 g NaOH per liter was added to the digester during fifteen minutes at a temperature of 152 ° C. The typical alkali charge for neutralization is approximately 12% Effective Alkali ("EA") as NaOH on the weight of dried chips. The digester is then filled with 10.8 liters of hot black liquor with an effective alkali concentration of 25.3 g NaOH per liter ("HBL1") added over a period of 15 minutes at a temperature of 140 ° C for Complete the neutralization stage. Ten liters of a second hot black liquor ("HBL2") of the same concentration were added to the digester to displace the neutralized liquor over a period of 23 minutes at a temperature 143 ° C followed by the cooking liquor consisting of a mixture of 1.0 liters of hot black liquor (“HBL2”) and 4.16 liters of a second white liquor (“WL2”) with an effective alkali concentration of 12.7 g NaOH per liter for a period of 12 minutes at a 10 bar pressure and 152ºC. The typical alkali load for the cooking phase is approximately 12% effective alkali (EA) as NaOH over the weight of dried chips. 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 at 160 ° C for a period of 14 minutes, and maintained at 160 ° C for another 23 minutes. The digester is then cooled, and the reaction mixture is washed twice with a diluted caustic solution. Each wash uses 15 liters of an aqueous solution containing approximately 0.2 g NaOH per liter of solution. The resulting brown paste shows a kappa index of 10.3, a viscosity of 988 ml / g, a solubility S10 of 3.6% and a solubility of S18 of a

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, 7%. The reaction has a yield of 39.3%. Once the mixture has been screened, it shows rejections of 0.13%, resulting in a sieve yield of 39.1%.

Example 3

Use of the mild concentration CEE filtrate as neutralization solution and cooking solution

According to a third example, the same pulp production process described in Example 2 is repeated, except that the white liquor for the neutralization and cooking steps is replaced by a filtrate from the CCE stage which has an EA of 5 g of NaOH per liter ("CCE54"). The Neutralisate has a pH of 11.0 and a cooking mixture has an EoC of 18.5 g of NaOH per liter. The P factor for prehydrolysis is 297 and the H factor for the cooking reaction is 419. For this example, the total equivalent effective alkali load on the wood is respectively: 12% EA as NaOH for the phase of Neutralization and 11% of EA as NaOH for the Cooking phase.

The resulting brown paste shows a kappa index of 10.8, a viscosity of 1118 ml / g, an S10 solubility of 4.5% and an S18 solubility of 3.10%. The reaction has a yield of 40.4%. Once the mixture is screened, it has a rejection percentage of 0.09%, resulting in a sieve yield of 40.3%.

Example 4

Use of a highly concentrated CCE filtrate as neutralization solution and cooking solution

According to a fourth example, the same pulp production process described in Example 2 is repeated here, except that two thirds of WL1 and WL2 are replaced by a concentrated CCE filtrate with an effective alkali concentration of 110 g NaOH per liter The resulting brown paste shows a kappa index of 9.5, a viscosity of 990 ml / g, an S10 solubility of 4.1% and an S18 solubility of 3.0%. The reaction has a yield of 39.5%. Once the mixture has been screened, it presents rejections of 0.10%, resulting in a sieving yield of 39.43%.

Compared to the conventional kraft process, the process in which two thirds of white liquor was replaced by the concentrated CCE filtrate produces pastes of similar viscosities (approximately 990 mg / l in this example) and kappa indices similar to those of the process traditional kraft. It is expected that this similar technique should work for a longer period, for example, when between 60% and 75% would be replaced by concentrated CCE filtering. The slightly lower kappa index achieved with the concentrated CCE filtrate suggests that the replacement of white liquors with the concentrated CCE filtrate does not adversely affect delignification. The viscosity of the ratio of the kappa index - a measure of selectivity in the cooking stage - is higher for the process with concentrated CCE filtering (104 versus 96 in the traditional process), indicating a better cooking selectivity using the concentrated CCE filtering.

The solubility S18 increases from 2.7% to 3.10% and the solubility S10 increases from 3.6% to approximately 4.1% when the CCE filtrate replaces part of the white liquors, indicating that a redeposition of Some hemicelluloses. The level of solubility S18 can also be controlled by other means if desired.

It should be possible to maximize the process by also lowering the cooking temperature slightly to achieve the same Kappa index (around 10.8) and a higher viscosity. Based on the experiments performed, it is expected that minor variations in the process can be carried out including alkaline levels, relative amounts of white liquor and concentrated CCE filtrate, cooking temperatures and cooking times, as It could be determined from routine calculations or optimizations based on the principles and techniques described herein, while maintaining the qualities of the resulting brown paste at a potentially desirable level. For example, it is expected that the resulting brown paste can produce a Kappa index below 10.0 a viscosity of 1000 ml / g, a solubility S18 of not more than 3.0% and / or a viscosity ratio with the index Kappa of more than 100.

In accordance with certain embodiments disclosed herein, it is possible to bake for the same or similar viscosity and Kappa index levels using concentrated CCE filtering as a traditional kraft process using only fresh white liquor, thereby causing a increased efficiency

Although preferred embodiments of the invention have been described herein, many variants that are included in the concept and scope of the invention are possible. Such variants should be apparent to the person skilled in the art after the study of the specification and the drawings. The invention, therefore, is not restricted except by the scope of any of the appended claims.

Claims (11)

5 10 fifteen twenty 25 30 35 40 E10771810 12-01-2014 1.-A paper pulp manufacturing process using a cold caustic extraction for production of dissolving paste, comprising: the delignification (221) of organic materials (218) in one or more digesters and the treatment (222) of a resulting brown paste (212) to achieve a semi-purified paste for use in the production of paste for dissolution; the extraction of the semi-purified paste with a caustic solution during a cold caustic extraction process (223) to obtain a purified paste and a solution containing hemicellulose; the separation of the solution containing hemicellulose from the purified paste; washing (224) of the purified paste and collecting the spent washing liquid therefrom; the combination of the spent washing liquid and the solution containing hemicellulose to form an alkaline filtrate (216); the concentration (225) of the alkaline filtrate (216) to form a concentrated alkaline filtrate (217); and the use of at least a portion of the alkaline filtrate (217) concentrated in at least one of said digesters in connection with the production of dissolving paste. 2. The method of claim 1, wherein the concentration of the alkaline filtrate is carried out with a evaporation process (225). 3. The method of claim 2, wherein the evaporation process (225) is carried out with a plurality of effects connected in series. 4. The method of claim 2, wherein said evaporation process (225) is carried out at a temperature range between about 50 and 60 ° C. 5. The method of claim 2, wherein the evaporation process (225) is carried out at a pressure of -60 kPa and -84 kPa. 6. The method of claim 2, wherein the evaporation process (225) is carried out until said concentrated alkaline filtrate (217) has an effective alkali concentration of between about 95 and 125 grams of NaOH per liter . 7. The method of claim 2, wherein the evaporation process (225) is carried out until said concentrated alkaline filtrate (217) has an effective alkali concentration of between about 100 and 110 grams of NaOH per liter . 8. The method of claim 1, wherein the alkaline filtrate (216) is obtained by: separating the solution containing hemicellulose from the purified paste; and washing the purified paste and collecting the crude alkaline filtrate obtained therefrom. 9. The method of claim 1, further comprising adding white liquor to the alkaline filtrate (217) concentrate used in said one or more digesters. 10. The method of claim 8, wherein the ratio of the white liquor to the concentrated alkaline filtrate (217) is between about 1: 1.5 and 1: 2.5. 11. The method of claim 1, wherein said caustic solution comprises NaOH and Na2S. 12. The method of claim 11, further comprising the use of a second portion of the alkaline filtrate (217) concentrated on a different pulp production line. 13. The method of claim 1, further comprising the steps of: the production of a second brown paste from the cooking of a second batch of organic materials in the one or more digesters; washing and sieving the second brown paste to obtain semi-purified paste; Y 13 E10771810 12-01-2014 the use of a cold caustic extraction, extracting the semi-purified paste derived from the second brown paste to obtain a second purified paste and a second solution containing hemicellulose. 14. The method of claim 13, wherein: 5 the second brown paste has an S18 solubility of no more than 3.0% for a cold caustic extraction; or the second brown paste has a kappa index of less than 10.0 before cold caustic extraction; the second brown paste has a viscosity of approximately 1000 milliliters per gram before cold caustic extraction; or the second brown paste has a viscosity with a ratio of the kappa index of 100 or higher before cold caustic extraction. 15. The method of claim 1, wherein the delignification of the organic materials comprises cooking (221) of organic materials in a plurality of discontinuous digesters using at least a portion 15 of a concentrated caustic solution derived from the cold caustic extraction stage downstream and in which the process further comprises the return of at least a portion of the concentrated alkaline filtrate (217) to the discontinuous digesters as at least one cooking fluid . twenty 14