阅读说明：本技术 生产溶解浆料的方法 (Method for producing dissolving pulp ) 是由 奥沃·凯图纳 萨普赛·拉克索 蒂纳·哈泰宁 马库斯·帕纳宁 于 2019-03-06 设计创作，主要内容包括：本发明涉及一种用于由粉碎的木基纤维材料生产溶解浆料的工艺。该工艺包括以下接连阶段：在硫酸盐蒸煮工艺中用碱性蒸煮液蒸煮粉碎的纤维材料,以生产浆料；在处于70-110℃的温度下且有效碱浓度为60-120g/l的碱提取中处理蒸煮后的浆料至少5分钟；以及洗涤碱提取后的浆料并对其进行氧脱木素处理。(The present invention relates to a process for producing dissolving pulp from comminuted wood-based fibre material. The process comprises the following successive stages: cooking the comminuted fibrous material with an alkaline cooking liquor in a kraft cooking process to produce a slurry; treating the cooked slurry in an alkaline extraction at a temperature of 70-110 ℃ and an effective alkali concentration of 60-120g/l for at least 5 minutes; and washing the slurry after the alkali extraction and performing oxygen delignification treatment on the slurry.)

1. A method for producing dissolving pulp from comminuted hardwood-based fibre material, the method comprising the following successive steps:

treating the comminuted fibrous material under acidic conditions such that a P-factor of 5-250 is achieved;

digesting the comminuted fibrous material which has been treated under acidic conditions with an alkaline cooking liquor in a kraft cooking process at a temperature of about 120-;

treating the cooked pulp in an alkaline extraction at a temperature of 70-110 ℃ for at least 5 minutes, wherein the effective alkali concentration of the liquid phase of the pulp suspension is 60-120g/l,

washing the slurry after alkali extraction, and

after the washing, the alkali extracted slurry is subjected to an oxygen delignification treatment.

2. A method according to claim 1, characterized in that white liquor with an effective alkali concentration of more than 90g/l is introduced into the pulp discharged from the cooking before alkali extraction.

3. A method according to claim 1 or 2, characterized in that the filtrate is separated from the slurry after alkali extraction.

4. A method according to claim 3, characterized in that after alkali extraction, a first filtrate is extracted from the slurry, which first filtrate is conveyed counter-currently to the slurry stream as a slurry washing liquid.

5. A method according to claim 3 or 4, characterized in that a second filtrate is separated from the pulp, which second filtrate is conveyed to the cooking to comprise at least a part of the cooking liquor used for the cooking.

6. A method according to any of the preceding claims, characterized in that the comminuted fibre material is treated in acid hydrolysis before the cooking stage.

7. The method according to any of the preceding claims, characterized in that the temperature of the alkali extraction is 80-100 ℃.

8. Method according to any of the preceding claims, characterized in that in alkali extraction the effective alkali concentration of the liquid phase of the slurry suspension is 60-120g/l, preferably 70-110 g/l.

9. A method according to any of the preceding claims 3-8, characterized in that the slurry is treated in a fractional washing to form a filtrate.

10. A method according to claim 4, characterized in that the first filtrate is conducted to a digester wash.

11. Method according to any of the preceding claims, characterized in that the further treatment of the pulp after the oxygen delignification treatment comprises a treatment of the pulp in an acid stage.

12. The process according to any of the preceding claims, characterized in that the cooking is carried out in a continuous single or two vessel hydraulic or vapor phase digester.

13. The method according to any of the preceding claims 1 to 12, characterized in that the cooking is performed as a batch digester process.

14. Method according to any of the preceding claims, characterized in that it comprises some of the following steps:

a) treating the comminuted fibrous material under acidic conditions such that a P-factor of 5-250 is achieved;

b) digesting the fibrous material with an alkaline cooking liquor at a cooking temperature of about 120-; d) feeding white liquor to the cooked slurry and mixing therewith, e) treating the slurry at 70-110 ℃ for 5-120 minutes; f) removing the first filtrate from the slurry after step e), the filtrate thus produced being conveyed counter-currently to the slurry stream as a slurry washing liquid; and g) separating a second filtrate from the slurry after step e), the second filtrate being passed to step b) to constitute at least a part of the cooking liquor; and h) after step g) conveying the slurry to further processing.

Technical Field

The present invention relates to a method for producing dissolving pulp.

Background

In recent years, there has been a strong demand for development of new fiber raw materials for use in the textile industry and the demand of other polymer industries. One solution for producing fibers is to increase the production of dissolving pulp, so that viscose fibers partly replace cotton in the textile industry, but they have some other applications.

Dissolving pulp differs in nature and chemical composition from pulp intended for papermaking. The production of dissolving pulp aims at forming pulp with the highest possible cellulose concentration and the lowest possible hemicellulose (such as xylan) concentration, and at the same time at removing lignin from the bleached pulp during cooking and bleaching, so that cellulose and hemicellulose remain in the pulp as much as possible. In addition to the main component, i.e. cellulose (known as alpha-cellulose), the pulp may contain up to 25% hemicellulose, while the dissolving pulp always contains more than 90% alpha-cellulose, and the amount of hemicellulose must usually be below about 5%.

Low hemicellulose concentration of dissolving pulp is generally sought by treating the chips and/or pulp under strongly alkaline and acidic conditions. Dissolving slurries are conventionally prepared using the sulfite process or the sulfate process equipped with acid prehydrolysis. If the sulphate process is used in the production of dissolving pulp, the wood chips are subjected to prehydrolysis prior to alkaline cooking, wherein a large amount of hemicellulose is removed under acidic conditions prior to alkaline cooking. The strength of the pretreatment is represented by the P factor, which in the kraft process equipped with prehydrolysis typically varies between 500 and 1000 depending on the type of wood. The concept of P-factor is explained, for example, in the pulp handbook compiled by Sixta, H. (Vol.2006, page 343-.

Wherein k is_reiIs the relative rate of acid-catalyzed hydrolysis and is dependent on temperature, an

t equals time.

At the end of the pulping line, the pulp is treated in a bleaching stage, similar to pulp, where the most important difference is the alkaline bleaching stage, which is carried out at a higher temperature than in bleaching where the maximum yield is maintained. Furthermore, to produce viscose pulp, both kraft cooking and sulfite cooking are typically cooked to lower kappa numbers than in pulp production.

As mentioned above, typically in dissolving pulp production, either an alkaline extraction is performed after the acidic cooking process, or the crumb is subjected to an acid prehydrolysis stage at elevated temperature and pressure before alkaline cooking. Cooking the crumb under acidic conditions is more demanding than under alkaline conditions. Acidic conditions require better materials and, in the absence of alkaline lubrication, the equipment wears more. For this reason, it would be advantageous to be able to produce dissolving slurries under acidic conditions or when using as mild an acid treatment as possible without having to cook the crumb. Another problem with acid treatment may be that, in addition to removing hemicellulose, acid treatment also results in a decrease in cellulose yield, and thus, the stronger the acid treatment, the lower the pulp yield generally.

In cork, hemicellulose is mainly composed of glucomannan and xylan. The hemicellulose of hardwoods is almost entirely composed of xylan. Xylans are typically dissolved under strongly alkaline conditions.

The term "effective alkali" is used in pulp production to indicate the amount of cooking chemicals involved in cellulose cooking. The value of the effective alkali concentration describes the hydroxide ion (OH) concentration of the cooking liquor. In the present application, the effective base (g/l) is expressed as NaOH.

A rather efficient method for dissolving hemicellulose from cooked pulp is alkali extraction, wherein the cooked pulp is treated with alkali. The treatment method is cold alkali extraction or hot alkali extraction. In cold caustic extraction, the effective caustic concentration is at a level of 60-110g/l and the temperature is typically at a level of 20-50 ℃. Another method used is hot caustic extraction, in which the effective alkali concentration is generally at a level of 4-20g/l and the temperature is 80-140 ℃. These processes are widely handled in the pulping process of Rydholm, S. (1967, p. 992-1023). The efficiency of hot caustic extraction is significantly lower than that of cold caustic extraction and is generally used only in the case of acid sulfite cooking. In industrial processes, the low temperature of cold caustic extraction is inconvenient because it requires additional cooling and the difficulty of washing the cold slurry is greatly increased due to its poor filterability. It is well known that alkaline extraction can be carried out with concentrated sodium hydroxide solution or white liquor used in cooking. For example, patent application WO2013/178608 proposes a solution with which pulps produced with the kraft cooking (kraftcooking) of normal alkali concentration can be used for producing dissolving pulps using alkali extraction carried out at temperatures of 65 ℃ or lower. In this solution, cold caustic extraction is performed after the cooking and oxygen stages, and residual chemicals of caustic extraction are utilized during the oxygen stage and on the parallel cooking line. In this process, the xylan-rich alkali solution can be used for cooking in a parallel line. One difficulty with this solution is that the residual sulphides of the white liquor need to be oxidised with chemicals prior to acid treatment of the slurry to prevent the formation of dangerous hydrogen sulphide. The acid treatment may for example be a first bleaching stage.

Disclosure of Invention

It is an object of the present invention to eliminate the above problems and to provide a process wherein the residual alkali of alkali extraction can be utilized in cooking without extensive xylan reabsorption in the same pulping line and wherein the acidic conditions of dissolving pulp production can be alleviated compared to the production of dissolving pulp without alkali extraction.

Surprisingly, it has been observed in experiments that xylan is also selectively dissolved from the unbleached pulp after cooking at higher temperatures at levels of 70-110 ℃ when the effective alkali concentration is at levels of 60-120 g/l. The higher the alkali concentration, the more xylan can be dissolved. Thus, alkaline extraction at higher temperatures can also be used to remove large amounts of hemicellulose from hardwood pulp. Conversely, it has been observed that another important hemicellulose component of cork (glucomannan) does not dissolve in large amounts under these conditions.

A new method for producing dissolving pulp from comminuted hardwood based fibre material, which method comprises the following successive steps:

-treating the comminuted fibrous material under acidic conditions such that a P-factor of 5-250 is achieved;

-cooking the comminuted fibrous material with an alkaline cooking liquor in a kraft cooking process to produce a slurry;

-treating the cooked pulp in alkaline extraction at a temperature of 70-110 ℃ and at an effective alkaline concentration of 60-120g/l for at least 5 minutes,

washing the slurry after alkali extraction, and

and (3) carrying out oxygen delignification treatment on the slurry after the alkali extraction.

In the solution according to the invention, which is suitable for continuous cooking, in particular also suitable for batch cooking, alkali extraction is combined with kraft cooking, which helps to achieve a low xylan concentration in the pulp more efficiently than in the known process. The alkali extraction is performed between the digestion stage and the oxygen stage to allow the alkali remaining in the alkali extraction to be utilized in the same digester plant by a simple connection. The filtrate separated from the slurry after alkali extraction has an effective alkali concentration of at least 50g/l, typically 60-110g/l, and may be passed to digestion. The filtrate is separated, for example, with a press or fractional scrubber, in order to obtain the filtrate with the highest possible alkali concentration. During the caustic extraction stage, fractional washing may be used to enhance caustic accumulation and increase caustic concentration. When the washing liquid with the highest possible alkali concentration is supplied in a washing stage preceding the alkali extraction, such as a digester wash (digsterwash), the alkali concentration of the pulp from the washing stage increases. Then, a higher alkali concentration is achieved after addition of white liquor, resulting in an even higher concentration of wash liquor at the washing stage prior to alkali extraction. In fractional washing, after the caustic extraction, more dilute filtrate is sent to the digestion and therefore the caustic extraction cannot be diluted. At the same time, the alkali concentration is high in the last stage of cooking, which minimizes the resorption of xylan during the cooking of the pulp.

According to a preferred embodiment, the method according to the invention comprises the following successive steps:

a) treating the comminuted fibrous material under acidic conditions such that a P-factor of 5-250 is achieved; b) cooking the fibrous material with an alkaline cooking liquor at a cooking temperature of about 120-; d) feeding white liquor to the cooked slurry and mixing therewith, e) treating the slurry at 70-110 ℃ for 5-120 minutes; f) after step e), removing the first filtrate from the slurry, the filtrate thus produced being conveyed counter-currently to the slurry stream as a slurry washing liquid; and g) after step e). Separating a second filtrate from the slurry, the second filtrate being passed to step b) to constitute at least a part of the cooking liquor; and h) after step g), the slurry is conveyed to an oxygen stage and further processed.

In step a), forming an acidic cooking effluent; the acidic cooking waste liquor can be extracted from the fibrous material if desired. In step d), white liquor may be supplied to the slurry at the bottom of the digester or to the slurry removed from the digester.

The purpose of steps f) and g) is to remove at least two filtrates from the slurry, wherein the first filtrate has the highest possible effective alkali concentration. First, a filtrate with a high effective alkali concentration (at least 50g NaOH/l) is separated from the slurry. The filtrate is used in step c) as a slurry washing liquid counter-current to the slurry flow. A second filtrate is also separated from the alkali extracted slurry, having a lower alkali concentration than the first filtrate. This filtrate is used as a source of alkali in the digester and is added to step b). For example, the first filtrate may be a filtrate produced during the thickening stage of the fractional scrubber, and thus comprises a liquid phase separated from the slurry after alkali extraction. The second filtrate is typically the filtrate produced during the washing stage. The filtrate may be formed in the same piece of equipment, such as a fractional distillation scrubber or a press and wash press in succession. Other arrangements are also possible. The base extraction can also be carried out without fractional washing. The advantage of the fractional washing is that it helps to achieve higher alkali concentration and more efficient hemicellulose removal.

Prior to the alkali extraction stage, the slurry was not subjected to oxygen delignification. When the alkaline extraction is performed before this possible oxygen stage, no conversion of residual sulphides to hydrogen sulphide takes place in the alkaline extraction and in the acidic stage after the oxygen stage.

The oxygen delignification treatment stage is an alkaline stage known per se, which usually takes place under pressure and in which oxygen is present around the fibers for at least part of the reaction time. The oxygen stage may have one, two or more steps, in which case the reaction step includes chemical mixing and reaction vessel or reaction delay through tube completion. Typically, oxygen and alkali and possibly also inhibitors to prevent metal damage to the fibers are dosed into the oxygen stage, or by other means to remove or render unreactive metals entrained in the fibers.

In one embodiment, the cooking stage is carried out in a continuous single or two vessel hydraulic or vapor phase digester. The process may be carried out in one or more cooking vessels, for example with a combination of a digester and a prehydrolysis vessel.

In one embodiment, the cooking stage is performed as a batch digester process.

The dissolved xylan enters the cooking together with the alkaline extraction filtrate. When a sufficiently high effective alkali concentration (at least 20g NaOH/l) is maintained in the cooking, the dissolved xylan from the alkali extraction does not settle in harmful amounts in the fibrous material (such as fines) near the end of the cooking. The first part of the cooking may have a lower alkali concentration, in which case some xylan may precipitate, since once the alkali concentration of the cooking rises to a high level, the precipitated xylan dissolves again.

In said solution according to the invention, all or most of the white liquor required for cooking, i.e. at least 60%, usually at least 80%, most preferably more than 90%, is supplied to and mixed with the post-cooking pre-bleaching (brown stock) alkali extraction. Between the cooking stage and the oxygen stage, the alkali extraction is carried out at a temperature in the range of 70-110 ℃, preferably 80-100 ℃. White liquor can be used as a source of alkali in the alkali extraction. The effective alkali concentration of the white liquor is 90-130g/l NaOH, and is usually 100-120 g/l. According to this new solution, no fresh cooking liquor (i.e. white liquor) is introduced at all, or no more than 40%, typically less than 20%, of the fresh cooking liquor (i.e. white liquor) is introduced into the digester or the cooking stage itself.

The filtrate of the thickening and/or washing of the pulp after alkali extraction runs counter-currently to the pulp stream towards the digester or digester apparatus. The white liquor thus supplied accumulates in these cycles, which contributes to achieving the alkali concentration required for alkali extraction. In other words, when the filtrate is circulated in countercurrent, alkali accumulates between thickening and/or washing of the pulp after the digester wash and alkali extraction. Thus, even though the slurry concentration is typically 8-12%, the desired alkali concentration level is achieved.

The white liquor and filtrate may be treated as required to achieve the desired temperature level for alkali extraction, which is 70-110 c, preferably 80-100 c. On an industrial scale, the temperature is generally from 70 to 95 ℃. The treatment time in the alkaline extraction is more than 5 minutes, usually 5 to 120 minutes. In alkaline extraction, the effective alkali concentration of the liquid phase of the slurry suspension is 60-120g/l, preferably 65-110g/l, most preferably 70-110 g/l. Some of the alkali-rich filtrate of the pulp washer is conveyed to the cooking stage and some filtrate is supplied to the end of the cooking stage, e.g. at the bottom of the digester. It is essential that all or almost all (at least 80%) of the filtrate is circulated through the digester, since otherwise valuable chemicals will be lost with the filtrate travelling through the digester to the evaporator apparatus. The alkali-rich black liquor obtained from the cooking stage, the effective alkali concentration of which exceeds 20g NaOH/l, is recycled up to the start of the cooking process, where the alkali is consumed in order to achieve the normal residual alkali level in the black liquor brought to the evaporator equipment, i.e. below 10g NaOH/l.

According to a key feature of the new process, the slurry is not subjected to oxygen delignification between cooking and alkali extraction. After the alkali extraction, the slurry is subjected to further processing, which typically includes an initial oxygen stage. When the alkaline extraction is performed before the oxygen stage, the residual sulphides of the slurry are oxidised during the oxygen stage and there is no risk of hydrogen sulphide formation during the acidic treatment performed after the oxygen stage.

The pulp may be further treated in a bleaching stage, which may include, for example, acidic stages A, Z and D and alkaline stages E and P. During this further treatment stage, the xylan concentration in the pulp may be further reduced. Preferably, xylan removal can be enhanced in the acidic stage, i.e. the A-stage (where the temperature can be 100-. The a stage is performed after the alkali extraction stage and preferably after the oxygen stage.

In the solution according to the invention, the removal of hemicellulose may also be enhanced with acid treatment, for example using a normal prehydrolysis stage or various acid pulp treatments. Said solution according to the invention may advantageously be combined with a mild acid treatment prior to cooking, wherein the P-factor in the acid hydrolysis is between 5 and 250 and a part of the hemicellulose contained by the wood is dissolved. This type of acid treatment can be carried out in a prehydrolysis vessel, as is usually done when using the prehydrolysis kraft cooking process, but at a lower temperature or with a shorter delay than is usually the case. The acid treatment may also be carried out in the vapor phase or liquid phase in the top section of the cooking vessel. The chips are steamed in a continuous feed digester apparatus, typically in a chip silo at atmospheric pressure, with a delay of about 10-45 minutes. Mild acid treatment can be generated by pressurizing the crumb silo to a pressure of about 1-10 bar, at which point the steaming temperature can be increased to over 120 ℃ and the hydrolysis reaction started. The target in the crumb bin is a P factor value of 5-50. Preferably the pressure level of the crumb bin may be about 2 bar and the temperature about 135 c, in which case the atmospheric bin requires only minor changes and the crumb can be supplied to the bin using a low pressure feeder. When the hydrolysis process is carried out in the vapour phase in the chip silo, the actual feeding of the chips into the digester may take place under alkaline conditions, to avoid wear on the chip feeding equipment outside the silo due to acidic conditions. Condensate formed during the vapor phase hydrolysis may be recovered and recycled back into the chips entering the bin, which more quickly lowers the pH of the chips and accelerates the hydrolysis reaction.

Drawings

The new method is explained in more detail with reference to the provided figures, wherein an embodiment of the invention is schematically shown in fig. 1.

Detailed Description

FIG. 1 presents a typical system in which the new method can be implemented. The system comprises at least a cooking vessel 2, an alkali extraction vessel 3 and a scrubber 4. The digester 2 is a vapour phase digester, but it may also be a hydraulic digester. The process may be carried out in one or more cooking vessels, for example using a combination of a digester and a prehydrolysis vessel. In particular in arrangements with a plurality of cooking vessels, the method may be carried out in different details than those described herein, but applying the same operating principle. The system further comprises a hydrolysis reactor 5 having an overhead separator 6 which receives a comminuted hardwood-based fibrous material suspension, such as a crumb slurry, from a crumb supply system (not shown) via line 7.

The pre-hydrolysis vessel 5 may be a vapor phase reactor or a hydraulic vessel with a heating cycle for heating the material to the desired hydrolysis temperature.

The feed material is delivered to an inverted top separator 6 located at the top of the vessel 5. The top section of the vessel may be a vapour phase region through which fibrous material falls from the top separator 6 to the surface of the column of liquid and debris. In the top separator, the liquid is separated from the fibrous material and travels to a chip supply system via line 8. Steam and pressurized air may be introduced to generate the appropriate pressure and temperature for hydrolysis. The temperature of the fibrous material is raised above the autohydrolysis temperature (which may exceed 140 c, for example 155 c) and maintained at that temperature to promote hydrolysis. The target is a P factor value of 5-250, which determines the above conditions. Self-hydrolysis occurs when organic acids are released from the fibrous material. If dilute acid is added, the hydrolysis temperature may be below 150 ℃, for example between 150 and 120 ℃. The fibrous material and the liquid flow co-currently downwards in the container 5. The formed hydrolysate can be removed through screen 9 to line 10 and taken on for further processing.

At the bottom of the hydrolysis vessel 5, dilution liquid is added from the digester vessel 2 to the fibrous material via line 11 to assist in transporting the fibrous material to the top separator 13 of the digester 2 via line 12. The dilution liquid in the return line 11 is alkaline, so that it makes the fibre material alkaline when the material flows from the prehydrolysis vessel to the digester 2. Waste from the black liquor filter may be introduced into line 11 via line 15; the waste material comprises fibres and uncooked fibrous material.

The fibrous material is in an alkaline state, such as at or near pH 13, for example at pH 12-14. As an example, the fiber material may be held in the digester within a temperature range of 120 ℃ - > 175 ℃ or 130 ℃ - > 160 ℃, depending on, for example, the residence time and alkali concentration in the digester. In this case, the H factor is 100-500, typically 200-300.

The temperature in the digester 2 is raised and controlled by adding steam and possibly also air or inert gas. The digester may be a vapor phase vessel or a fully hydraulic vessel. The pressure at the bottom of the hydrolysis vessel is a combination of the steam pressure and the hydraulic pressure of the fibrous material and the liquid column. The combined pressure is higher than the pressure at the top of the digester. This pressure difference transports the fibrous material via lines 12, 14 to the top separator of the digester. Furthermore, when the digester is a hydraulic digester vessel, a heating fluid circulation may be used to heat the fibrous material to a desired temperature.

The digester may include multiple parallel-flow and counter-flow cooking zones. The uppermost cooking zone may be a co-current zone of fibrous material and liquid.

The digester includes screens 16, 17 and 18. The fibrous material is treated with cooking liquor in zone I. The temperature in zone I (controlled for example by feeding steam) is for example 144 ℃. The effective alkali concentration of the supplied cooking liquor, which is typically 20-50g NaOH/l, is consumed in zone I such that the effective alkali concentration of the cooking effluent removed via screen 16 is less than 10g NaOH/l. For example 4g NaOH/l, and its temperature is, for example, 151 ℃. The spent cooking liquor of zone I is typically sent to the evaporator unit via line 19.

Cooking zone I is followed by a counter current cooking zone II, which is between screens 16 and 17. Although the treatment has been shown as counter-current, the treatment may also be co-current. At the end of zone II, the spent cooking liquor is extracted into a cycle 20 comprising one or more screens 17, a pump 21 and an indirect heat exchanger 22. Cooking liquor is added to the material of the cycle 20 via line 23. A large part of the alkali dosage required for cooking, for example 50%, is added to the fibrous material suspension via line 23 leading to the circulation 20. This will result in a high effective alkali concentration in the digester which exceeds 25g NaOH/l, preferably 35 g/l. The heated cycle 20 typically heats the fibrous material suspension and its cooking liquor to a cooking temperature, typically 120-175 c, before the suspension flows to the co-current cooking zone III. The cooking liquor added via line 23 in order to achieve a high alkali concentration and a high pH may have the following characteristics: the total alkali on the wood is about 8-16%, the effective alkali concentration is about 40-80g/l (usually about 50-70g/l) (measured as NaOH), and the flow rate is about 2.0-6.0m of the pulp³/BDMT(m³Per dry metric ton), typically about 3.0 to 5.0m of the slurry³the/BDMT. The cooking liquor of line 23 has an effective alkali concentration of, for example, 58g NaOH/l and a temperature profileSuch as 94 deg.c.

The white liquor can be fed to the recycle 20 via line 20', if desired.

As the cooking reaction continues, the fibrous material travels co-currently downward in the digester zone III at the cooking temperature. At the lower part of the digester, hot spent cooking liquor is now extracted from the cooked fibrous material (such as chips) by means of the screen assembly 18. Wash filtrate from a more remotely located slurry washer is supplied to the bottom of the digester via one or more conduits 27 to end the digestion reaction and reduce the temperature of the digested chip slurry.

The slurry is then removed from the digester via a discharge 25 to a line 26.

Hot spent cooking liquor is extracted from the digester via screen assembly 18 and conduit 24. The hot liquid has a relatively high fresh alkali concentration, i.e. residual alkali concentration. The effective alkali concentration of the liquid in conduit 24 is typically at least 20g/l, preferably at least about 25g/l, for example 41 g/l. This liquid, comprising alkali and sulphide, is conveyed via conduit 24 to the return line 11 for use in the pre-treatment of the supplied chips or in zone I. The temperature of the liquid in the conduit 24 may for example be 143 ℃.

The cooked slurry is conveyed via line 26 to alkali extraction in vessel 3. The vessel 3 may be a conventional digester discharge tank or another type of vessel. The pulp leaving the digester has an effective alkali concentration of 60-110g NaOH/l, e.g. 91g/l, and a temperature of 70-110 deg.C, e.g. 102 deg.C. White liquor from line 34, required for the cooking process and caustic extraction, is supplied and mixed with the slurry flowing in line 26. The effective alkali concentration of the white liquor is 90-130g/l NaOH, usually 100-120g/l, for example 115 g/l. The alkali extraction is carried out at a temperature of 70-110 deg.C (e.g., 90 deg.C). The temperature of the slurry discharged from the digester may be adjusted by adjusting the temperature of the washing filtrate added to the slurry at the bottom of the digester. The duration of the alkali extraction is 5-120 minutes.

The alkali extracted slurry is brought from vessel 3 via line 28 to a slurry thickener or scrubber 4, which may be, for example, a press, wash press or fractional scrubber, and may have one or more of them. Water or filtrate from the oxygen stage or bleaching stage is conveyed via line 33 to the washer for the washing liquid. The aim is to separate at least two filtrates from the slurry, wherein the first filtrate has a high effective alkali concentration. The first filtrate may be a filtrate produced during the thickening stage of the fractional scrubber, which filtrate thereby comprises a liquid phase separated from the slurry after alkali extraction. The second filtrate is typically the filtrate produced during the washing stage. The filtrate may be formed in the same piece of equipment, such as a fractional scrubber or a press and wash press in succession.

First, a filtrate with a high effective alkali concentration (e.g., 94g NaOH/l) is separated from the slurry. This filtrate from the filtrate tank 29 is used as washing liquid at the bottom of the digester, which helps to achieve the highest possible alkali extraction concentration level. The washing section of the digester is counter-current, wherein the alkali-rich washing liquid of line 27 removes the cooking liquor of cooking section III from the digester via screen 18 and continues the alkali extraction of the pulp in vessel 3.

The more dilute filtrate obtained from the slurry is used as a source of alkali in the digester and is taken from the filtrate tank 30 via line 23 to the cycle 20, where it is added to the cooking zone via the cycle 20. The major part of the alkali dosage (at least 50%) required for cooking is added to the fibrous material suspension via line 23 and cycle 20.

The filtrate contains xylan separated from the fibrous material during alkali extraction. Because a sufficiently high effective alkali concentration (at least 20g NaOH/l) is maintained near the end of the cook, dissolved xylan from alkali extraction does not precipitate in harmful amounts in the fibrous material (such as fines) during the cook.

By arranging an indirect heat exchanger (not shown) for the line, the heat of the spent digester liquor 24 and/or 19 extracted from the digester can be used to heat the filtrate of the line 23.

First, the slurry is removed from the scrubber 4 via a dropper 31 and line 32 to further processing, which typically includes an oxygen stage. The pulp may be further treated in a bleaching stage, which may for example comprise: acid stages A, Z (ozone) and D (chlorine dioxide) and alkaline stages E (extract) and P (peroxide). During the further treatment stage, the xylan concentration in the pulp may be further reduced.

Preferably, the removal of xylan can be further enhanced in the acid stage, i.e. the A stage (where the temperature can be 100-130 ℃ and the pH 2-3). The a stage follows the alkali extraction stage, preferably the oxygen stage.

Example 1:

the method according to the invention was analyzed in the laboratory. The starting material was hardwood chips with a xylan concentration of 12.1%. When the crumb was cooked at normal alkalinity, the cooking yield was 53.3% with a kappa number of 17.1 and the xylan concentration in the pulp was 14.5%, meaning that 62% of the original xylan in the crumb was retained.

When cooking crumb according to this method at higher alkali concentrations, the cooking yield was 50.4% with a kappa number of 14.5 and the xylan concentration in the pulp was 12.3%, which means that 50% of the original xylan in the crumb was retained. When the pulp was subjected to alkaline extraction at a temperature of 50 ℃, it produced a pulp with a kappa number of 8.7 and a xylan concentration of 5.0%. Thus, only 16% of the original xylan in the chips was retained. When the temperature of the corresponding alkali extraction was 90 ℃, the kappa number of the pulp was 8.8 and its xylan concentration was 5.9% and 20% of the original xylan in the chips was retained. Laboratory tests have shown that both pulps can be used as dissolving pulps, especially after suitable further treatment and/or pretreatment, and also within the normal pre-rinse washing temperature range of 70-100 ℃, alkali extraction can be performed very successfully, and for successful alkali extraction high alkalinity cooking yields a better starting point than normal.

Example 2:

the method according to the invention was analyzed in the laboratory. The starting material was hardwood chips with a xylan concentration of 15.5%. When the crumb was first subjected to a prehydrolysis stage with a factor of 200P and a cooking stage at high alkali concentration, the cooking yield was 44.2% with a kappa number of 10.2 and the xylan concentration in the pulp was 5.5%. Thus, 16% of the original xylan in the crumb was retained. When this pulp was subjected to alkaline extraction at a temperature of 90 ℃ and an alkaline concentration of about 80g/l, it produced a pulp with a kappa number of 6.9 and a xylan concentration of 2.6%. The total yield after prehydrolysis, cooking and alkaline extraction was 42.3%. Thus, only 7% of the original xylan was retained. When the same raw materials were used in the laboratory to produce dissolving pulp with a conventional prehydrolysis digestion with a factor of 500P, the yield was 39.4%, the kappa number was 6.6, and the xylan concentration in the pulp was 2.5%. These laboratory tests show that with alkaline extraction, a good quality dissolving pulp can be produced with a significantly higher yield than when using a conventional prehydrolysis process.

The advantages of the new solution are:

this method is a simpler and more economical cooking process than before, connecting the alkali extraction to the same production line, since the alkalinity of the cooking avoids precipitation of excessive xylan in the chips. When the alkaline extraction is carried out before the oxygen stage, the conversion of residual sulphides to hydrogen sulphide does not take place in the subsequent acidic stage. With the alkaline extraction according to the method, the prehydrolysis stage can be greatly reduced, which significantly improves the slurry yield.