SE531540C2 - Alkaline pulp preparation process and system for hardwood - Google Patents

Description

531 Boat] .2_ Kappa number of about 30 would result in a pulp rejection level of about 30% or more.

For hardwood, conventional cooking conditions are thus such that a Kappa number below about 20 is achieved, and typically below about 18, to result in an acceptably low amount of pulp reject.

It has now been found that rejects from hardwood pulp at a given Kappa number can be reduced by subjecting the pulp to specific cooking conditions at the end of cooking. According to the present invention, the hardwood pulp is provided with alkaline cooking processes and a device which are particularly beneficial for hardwoods and which aim to increase the Kappa number after cooking above normal, while reject levels are significantly lower than normal. The present invention can be combined with an effective oxygen delignification step to lower the Kappa number below normal due to the low amount of hexenuronic acid (HexA) present in the pulp. The present invention thus contributes to the overall pulp yield and effluent benefits.

It has been found that s-lignin reacts faster than g-lignin and that there is a greater proportion of g-lignin at the end of hardwood cooking compared to the proportion of g-lignin at the beginning of cooking. It has therefore been found that relatively more severe cooking conditions (e.g. higher cooking temperatures) at the end of cooking reduce the g-lignin content in hardwood cooking, which in turn reduces the percentage of pulp rejects at a given Kappa number.

According to certain aspects of the invention, methods and systems are provided for the continuous production of chemical cellulose pulp from a slurry of finely divided hardwood material by subjecting the slurry of finely divided hardwood material to a first cooking step under conditions sufficient to reduce the syringyl lignin content (s-lignin) in the hardwood material relative to the guaiacyl lignin content (g-lignin) therein, and then by subjecting the slurry of finely divided hardwood material to a second cooking step under conditions sufficient to reduce the g-lignin content remaining therein after the first cooking step. In some embodiments, the first cooking stage is performed such that the slurry of hardwood material is exposed to a temperature of about 130 °C - about 170 °C at an alkali charge of about 2% - about 10% of wood calculated as effective alkali (EA) NaOH and/or such that the Kappa number of the cooked hardwood material leaving the first cooking stage is about 30 - about 100, preferably about 40 - about 60.

According to other preferred embodiments, the first coking stage is carried out at a low sulfidity ratio below about 20% sulfidity. Such a low sulfidity ratio in the first coking stage is most preferably achieved by adding a sufficient amount of anthraquinone and/or polysulfide.

The second cooking stage according to some embodiments of the invention is carried out so that the slurry of hardwood material is exposed to a temperature of about 100 °C - about 180 °C. preferably about 130 °C - about 180 °C at an alkali charge of about 2% - about 10% of wood calculated as effective alkali (EA) NaOH and/or so that the Kappa number of the cooked hardwood material leaving the second cooking stage is 15 - 30, preferably about 20 - 30. The Kappa number at the beginning of the second cooking stage can be about 30 - about 100 (preferably about 40 - about 60). At this stage, the temperature can be lowered so that it is below the temperature in the first cooking stage.

If not enough alkali was initially added at the beginning of the second cooking stage, it may be necessary to add white liquor in the second cooking stage. If white liquor is added, it is preferred that the hardwood material is allowed to continue cooking at a temperature of at least about 130°C.

It is preferred in some embodiments of the invention that the second coking stage is carried out at a high sulfidity ratio exceeding about 20% sulfidity, preferably about 25% - about 40% sulfidity. Such a high 10 15 20 25 53% Eli-Ü -4- sulfidity ratio in the second coking stage can be achieved by using anthraquinone, polysulfide and/or sulfite.

The finely divided hardwood material can optionally be subjected to a pre-treatment. If pre-treatment is used, it is carried out before the first cooking stage at an alkali charge of about 40% - about 60% of the wood calculated as effective alkali (EA) NaOH at a temperature of about 80 °C - 120 °C, preferably about 100 °C - about 110 °C.

It may be necessary to wash the hardwood material after the first cooking step before the second cooking step is performed. In one embodiment, the cooking steps are performed in separate digesters. It is also possible that the cooking steps, and the optional washing step, are performed in a single digester vessel.

According to one embodiment of the present invention, the hardwood material is subjected to two separate cooking stages. The H-factor achieved at the end of the second cooking stage is at least 50% of the total H-factor achieved for the entire cook. In other words, the H-factor achieved by the first cooking stage is less than 50% of the H-factor achieved for the entire cook (i.e., at the end of the second cooking stage).

These and other aspects and advantages will become more apparent upon careful consideration of the following detailed description of the preferred exemplary embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS In the following, reference will be made to the accompanying drawings, in which like reference numerals designate corresponding structural elements throughout the various figures, of which 10 15 20 25 30 531 Sëlü -5.

FIG. 1 is a schematic illustration of an exemplary apparatus for practicing the method of the present invention, and including an exemplary system of the present invention; FIG. 2 is a graph showing the percentage of lignin remaining from the original cellulose midlamella at each of the sample points during a hardwood cooking cycle; and FIG. 3 is a graph showing the percentage of s-lignin relative to g-lignin at various stages of a hardwood cooking cycle.

DETAILED DESCRIPTION OF THE INVENTION The accompanying FIG. 1 shows a preferred embodiment of the present invention.

More specifically, Fig. 1 shows a system 10 for treating finely divided cellulosic fibrous material, which system essentially consists of or includes a continuous digester 11 having a feed system 12 for the continuous digester. The feed system 12 may be a LO-LEVEL® feed system or a TURBOFEED® system, both of which are sold by Andritz, Inc., but any conventional feed system for feeding, basing and forming a slurry of finely divided cellulosic fibrous material may be used, and/or one or more separate impregnation vessels may be used; and/or unconventional systems, e.g., those comprising devices and/or a chip silo in the woodyard with pumping to the digester. In some circumstances, a plurality of impregnation vessels may also be used so that slurry is pumped from a particular vessel as soon as the impregnation with or without additives for cooking has been completed (or will be completed during pumping).

Finely divided cellulosic fibrous material, for example in the form of hardwood chips 13, is fed to the feed system 12, as described in U.S. Patent Nos. 5,763,075; 6,106,668; 6,325,890; 6,551,462; 6,336,993; 6,841,042 (the entire contents of which are expressly incorporated herein by reference), and marketed by Andritz Inc. under the trademark TURBOFEED®; or a feed system as described in U.S. Patent Nos. 5,476,572; 5,700,355; 5,968,314 (the entire contents of which patents are expressly incorporated herein by reference), and marketed by Andritz Inc. under the trademark LO-LEVEL®, which system may include passing the chips to be treated with steam in a vessel 14, which is preferably a vessel as described in U.S. Patent Nos. 5,500,083; 5,617,975; 5,628,873; 4,958,741; and 5,700,355 (the entire contents of which patents are expressly incorporated herein by reference), and marketed by Andritz Inc. under the trademark DIAMONDBACK®, although other types of basting vessels may also be used. From vessel 14, the chips pass through a device, e.g. a metering device connected to a conduit, which may preferably be a Chip Tube of Andritz Inc. The slurry of chips and liquid is fed to the impregnation vessel (if used) or to the digester 11 through a pressurizing means.

The slurry is pressurized typically to a pressure of about 5-15 bar and is passed through a line 21 to the upper part of the continuous digester 11. The excess liquid contained in the slurry is removed from the slurry at the inlet of the digester 11 by a separator 22, typically a conventional overhead separator, and the excess liquid is removed and returned to the feed system 12 via a line 23. During the treatment within the vessel 14, the slurry of hardwood material may be pretreated at an alkali charge of about 40% - about 60% of wood calculated as effective alkali (EA) NaOH at a temperature of about 80°C - 120°C, preferably about 100°C - 110°C.

The feed system 12 typically also includes conventional devices, not illustrated, such as an in-line drain, a level tank, and a make-up liquor pump. Cooking liquor, for example, kraft white liquor (WL), is typically supplied to the level tank (not shown), as is conventional. 10 15 20 25 30 Uni CJJ ...is Ül »Û Ü -7- The pretreated hardwood material (which may, but need not, be pretreated in an impregnation vessel) fed through line 21 is subjected to a first cooking step in the digester 11. According to the present invention, however, the above-described pretreatment of the hardwood material is not absolutely necessary, since the hardwood material can be fed directly to the digester 11. Whether the hardwood material is pretreated or not, the first cooking within the digester 11 is carried out at low sulfidity conditions (for example 0-20% sulfidity) by using a beneficial additive, for example anthraquinone (AQ) and/or polysulfide. The first cooking stage in the digester 11 is carried out at temperatures of about 130°C - about 170°C using an alkali charge of about 2% - about 10% of the wood calculated as effective alkali (EA) NaOH. The cooked hardwood material exits the digester 11 through a line 30 at a Kappa number of about 30 - about 100 (preferably about 40 - about 60).

The hardwood material after the first cooking stage in the digester 11 has a higher proportion of g-lignin in relation to s-lignin than before, i.e. compared to the proportion of g-lignin in relation to s-lignin at the beginning of the first cooking stage. High concentration of s-lignin in the surrounding liquor can interfere with the dissolution of g-lignin remaining in the hardwood material. To reduce this interference, it may be necessary to wash the hardwood material after the first cooking stage in the digester 11.

The hardwood material can thus optionally be transferred to a washing vessel 32 through line 30 and subjected to washing such that a washing liquor is fed through line 34. Washing liquor is removed from the washing vessel 32 through line 36. Since the washing step within the washing vessel 32 reduces the s-lignin content of the surrounding liquor, any cooking additives that could be utilized later would not be detrimentally consumed by the already dissolved s-lignin in the surrounding liquor.

The washed pulp exiting the optional washing vessel 32 through a line 38 is directed to a digester 40, where a second washing step is performed to further decompose and reduce the g-lignin content of the hardwood material. The decomposition of g-lignin can be achieved by using high-sulfidity cooking liquor (i.e., greater than about 20% sulfidity, and preferably about 25-about 40% sulfidity) through the use of AQ and/or polysulfide. (See, for example, U.S. Patent No. 6,576,084, the entire contents of which document are expressly incorporated herein by reference). Other cooking chemicals that perform well under cooking conditions, such as sulfite, could also be used alone or in conjunction with AQ and/or polysulfide, as mentioned above.

The second cooking stage in the digester 40 is carried out at a temperature of about 100 - about 180 °C, preferably about 130 °C - about 180 °C, with an alkali charge of about 2% - about 10% of wood (original wood charge) calculated as EA and a Kappa number of about 30 - about 100 (preferably about 40 - about 60) at the beginning of the second cooking stage. At this stage, it may be advantageous (but not always necessary) to lower the temperature so that it is lower than the temperature in the first cooking stage.

If not enough alkali was initially added at the beginning of the second wash stage, it may be necessary to add white liquor to the digester 40. If white liquor is added, it is preferred that the hardwood material be allowed to continue to cook at a temperature of at least about 130°C. At the point where the cellulosic hardwood material exits the digester 40 through a line 42, the Kappa number is preferably 15-30, more preferably about 20-30.

In one embodiment of the present invention, the hardwood material is divided into two separate cooking stages. The l-l factor achieved at the end of the second cooking stage is at least 50% of the total H factor achieved for the entire cook. In other words, the H factor achieved by the first cooking stage is less than 50% of the H factor achieved for the entire cook (i.e., at the end of the second cooking stage).

It is also possible that the cooking steps, and the possible washing step, are carried out in a single cooking vessel. If only one cooking vessel is used, the same operating parameters for each described step would be used, and the end of the step would not require an intermediate transfer of material to multiple vessels.

The first and second cooking stages carried out in the respective digesters 11 and 40, or in a single digester vessel, are preferably carried out in accordance with one or more of the processes described in more detail in U.S. Patent Nos. 5,489,363; 5,536,366; 5,547,012; 5,575,890; 5,620,562; and 5,662,775 (the entire contents of which patents are expressly incorporated herein by reference). The processes and apparatus described in these patents are marketed under the trademark LO-SOLIDS® Pulping by Andritz Inc., Glens Falls, New York.

The processes and apparatus of the present invention produce a pulp having a low HexA content as a result of the relatively high temperature, low alkali conditions of the new treatment and/or the cooking in the second cooking stage. This low HexA pulp favors oxygen delignification conditions, which in turn provide a low Kappa number pulp for the subsequent bleaching process. In this regard, see U.S. Patent Nos. 6,776,876 and 6,475,338, the entire contents of which are expressly incorporated herein by reference. The low Kappa number pulp can thus be bleached using fewer chemicals, resulting in lower effluent loads. In other words, since the pulp has fewer HexA compounds, the Kappa number at discharge from the cooking process can be higher and be followed by bleaching to a lower Kappa number through less overall chemical use.

The following non-limiting examples further illustrate the present invention.

EXAMPLE A hardwood cooking cycle was conducted using a total EA charge (effective alkali, such as NaOH) of 17.5% added during impregnation, the first (or upper) cooking sequence and the second (or lower) cooking sequence.

The following conditions prevailed during each cooking sequence: Impregnation: v 50% of total EA or 8.75% of EA input ~ Impregnation temperature of 110 °C o Time to reach impregnation temperature = 15 minutes ~ Time at impregnation temperature = 30 minutes 1st (upper) cook: 0 30% of total EA or 5.25% of EA added at the beginning of the cook v Cooking temperature = 155 °C ~ Time to heat to cooking temperature = 15 minutes v Time at cooking temperature = 60 minutes 2nd (lower) cook: ~ 20% of total EA or 3.5% of EA added at the beginning of the cook 0 Cooking temperature = 156 °C o Time at cooking temperature = 120 minutes A sample of the cellulosic material (wood chips) was taken after completion of the impregnation and tested for s-lignin and g-lignin in the laboratory using the procedure described in Lin et al, "Methods in Lignin Chemistry", Springer-Verlag, Berlin (1992) (the entire contents of which are expressly incorporated herein by reference). In addition, samples of the cellulosic material (wood) were taken in the middle of the boil (for example, about 90 minutes from the start of the impregnation) and at the end of the first (upper) boil (for example, about 120 minutes from the start of the impregnation). Each such sample was analyzed in the same way for the s-lignin and g-lignin content. Finally, a sample was also taken in the middle of the second (lower) boil (for example, about 180 minutes from the start of the impregnation). No analysis for s-lignin and g-lignin was made in the final pulp produced, since the 10 15 20 25 30 53? š-ilü _11- lignin content is usually so low that it is difficult to make an accurate analysis between the s-lignin and g-lignin species.

Attached Fig. 2 is a diagram showing the percentage of lignin in the original cellulose midlamella remaining at each sample point. At the beginning of the impregnation, 100% of the lignin is present, but at the end of the second (lower) cooking zone only about 3% of the lignin remains. Typically, wood chips before treatment contain about 24% lignin, which, based on the data according to Fig. 2, means that the lignin amount only constitutes about 0.72% of the wood chips and therefore no analysis between the s-lignin and g-lignin content would be accurate.

FIG. 3 shows data regarding the change in the ratio of s-lignin to g-lignin as the above-described cooking continued. The first ratio of 2.74 is the original ratio before the addition of any liquor or treatment. The second ratio of 2.87 is present during impregnation. The third and fourth ratios of 2.64 and 2.54 prevail in the middle and end of the first cooking stage, respectively. The fifth and final ratio of 2.47 is present in the middle of the second cooking stage.

The data of FIG. 3 thus suggest that the s-lignin is decomposed or dissolved more rapidly than the g-lignin. Consequently, the g-lignin content becomes present at the cellular level, which requires a different set of conditions to cause decomposition and degradation or dissolution thereof. Therefore, in order to degrade the g-lignin, the operating conditions in the digester need to be changed in the later cooking stages. In detail, according to the present invention, the later cooking stages require a higher temperature compared to the earlier stages to ensure that the g-lignin is degraded and to improve the yield (which reduces rejects) and the physical properties of the pulp. Although the invention has been described in connection with what may be considered to be the most practical and preferred embodiments at present, it is EÉUÉ Eflü _12. It is understood that the invention is not limited to the described embodiment, but on the contrary is intended to cover various modifications and equivalent embodiments that fall within the scope of the appended claims.

Claims (24)

10 15 20 25 531 511121 _13. PATENT REQUIREMENTS A process for continuously producing chemical cellulose pulp from a slurry of finely divided hardwood material, comprising the steps of: (a) subjecting the slurry of finely divided hardwood material to a first cooking step under conditions sufficient to reduce the syringyl-lignin (s-lignin) content; in the hardwood material compared to the guaiacyl-lignin (g-lignin) content therein, wherein the slurry is exposed to a temperature of about 130 ° C - about 170 ° C at an alkali rate of about 2% - about 10% of wood calculated as effective alkali ( EA) NaOH and under a low sulfidity ratio below about 20% sulfidity, and then (b) the slurry of finely divided leaf material is subjected to a second boiling step under conditions sufficient to reduce the g-lignin content remaining therein after the first cooking step, the slurry exposed to a temperature of about 100 ° C - about 180 ° C, at an alkali rate of about 2% - about 10% of wood calculated as effective alkali (EA) NaOH and under a high sulfidity ratio exceeding about 20% sulphidity, A method according to claim 1, wherein step (a) is performed so that the Kappa number of the cooked hardwood material departing from the first cooking step is about 30 - about 100, preferably about 40 - about 60. Process according to Claim 1 or 2, in which the low sulphidity ratio in the first cooking step is achieved by adding a sufficient amount of anthraquinone and / or polysul fi d. A method according to any one of claims 1 to 3, wherein step (b) comprises exposing the slurry of hardwood material to a temperature of about 130 ° C - about 180 ° C. 10 15 20 25 531 EÅLÜ _14- A method according to any one of the preceding claims, wherein step (b) is performed so that the Kappa number of the cooked hardwood material departing from the second cooking step is about 15 - about 30, preferably about 20 - about 30. A process according to any one of the preceding claims, wherein step (b) is carried out under a high sulphidity ratio of about 25% - 40% sulphidity. A method according to any one of the preceding claims, further comprising a step of pretreating the slurry of finely divided hardwood material before step (a) at an alkali batch of about 40% - about 60% of wood calculated as effective alkali (EA) NaOH at a temperature of about 80 ° C - about 120 ° C, preferably about 100 ° C - 110 ° C. A method according to any one of claims 1 to 7, further comprising a washing step for washing the hardwood material after step (a) and before performing step (b). A method according to any one of claims 1 to 7, wherein there is no washing between steps (a) and (b). A method according to any one of claims 1 to 9, wherein the cooking steps (a) and (b) are performed in separate boilers. A method according to any one of claims 1 to 9, wherein the cooking steps (a) and (b) are performed in a single boiler. A method according to any one of the preceding claims, wherein the H-factor obtained at the end of the second cooking step constitutes at least 50 ° /> of the total H-factor obtained throughout the process. 10 15 20 25 30 53? Esflü .15. A method according to any one of the preceding claims, wherein the H-factor obtained in the first cooking step constitutes less than 50% of the total H-factor obtained throughout the process. A system for continuously producing chemical cellulose pulp from a slurry of finely divided hardwood material, comprising: (a) a first digester for subjecting the slurry of finely divided hardwood material to a first cooking step under conditions sufficient to reduce syringyl lignin (s-lignin ) content of the hardwood material compared to the guaiacyl-lignin (g-lignin) content therein, wherein the first digester exposes the slurry of hardwood material to a temperature of about 130 ° C - about 170 ° C at an alkali rate of about 2% - about 10%. % of wood calculated as effective alkali (EA) NaOH and operated at a low sulphidity ratio below about 20% sulphidity, and (b) a second digester downstream of the first digester to subject the slurry of finely divided hardwood material to a second cooking step under conditions which are sufficient to reduce the g-lignin content remaining therein after the first boiling step, the second digester exposing the slurry of leaf material to a temperature of about 100 ° C - about 180 ° C at an alkali rate of about 2% - about 10% of wood calculated as effective alkali (EA) NaOH and operated at a high sulphidity ratio exceeding about 20% sulphidity. A system according to claim 14, wherein the Kappa number of the cooked hardwood material departing from the first cooking step is about 30 - about 100, preferably about 40 - about 60. A system according to claim 14 or 15, wherein the low sulphidity ratio in the first digester is achieved by adding a sufficient amount of anthraquinone and / or polysulphide. 10 15 20 25 30 531 5413 _16- A system according to any one of claims 14 to 16, wherein the second digester exposes the slurry of hardwood material to a temperature of about 130 ° C - about 180 ° C. A system according to any one of claims 14 - 17, wherein the Kappa number of the cooked Iovved material departing from the second cooking step is about 15 - about 30, preferably about 20 - about 30. A system according to any one of claims 14 to 18, wherein the second digester is operated at a high sulphidity ratio of about 25% - about 40% sulphidity. A system according to any one of claims 14 to 19, further comprising a pretreatment vessel for pretreating the slurry of finely divided wood material upstream of the first digester at an alkali rate of about 40% - about 60% of wood calculated as effective alkali (EA) NaOH at a temperature of about 80 ° C - about 120 ° C, preferably about 100 ° C - about 110 ° C. A system according to any one of claims 14 to 20, further comprising a washing vessel for washing the hardwood material downstream of the first digester and upstream of the second digester. A system according to any one of claims 14 to 20, wherein there is no washing between the first digester and the second digester. A system according to any one of claims 14 to 22, wherein the H-factor obtained at the end of the second cooking step constitutes at least 50% of the total 1-1 factor achieved with the whole system. A system according to any one of claims 14 to 23, wherein the H-factor obtained through the first cooking step constitutes less than 50% of the total H-factor achieved with the whole system.