Method of producing chemical pulp

Abstract

A method of producing chemical pulp including at least the following steps: a) wood chips or other comminuted ligno-cellulosic fibrous material is treated with a polysulfide-containing cooking liquor in an impregnation stage at a temperature of 90-145° C., and b) slurry of fibrous material from step a) is heated into cooking temperature and cooked for producing pulp having a desired kappa number. After step a) mercaptide ions are added into the slurry of fibrous material and the fibrous material is treated at cooking temperature in step b).

Claims

1. A method of producing chemical pulp, said method comprising at least the following steps: a) treating wood chips and/or other comminuted lignocellulosic fibrous material with a polysulfide-containing cooking liquor in an impregnation stage at a temperature in a range of 90° C. to 145° C., and b) heating and cooking in a digester vessel a slurry of the treated wood chips and/or other comminuted lignocellulosic fibrous material obtained from step a) to produce pulp; c) extracting cooking liquor from screens of the digester vessel; d) cooling the extracted cooking liquor; e) adding mercaptide ions to the extracted cooking liquor by adsorbing a mercaptan-containing gas into the extracted cooking liquor, and f) adding the extracted and cooled cooking liquor with the mercaptide ions to the slurry of the treated wood chips and/or other comminuted lignocellulosic fibrous material before and/or during step b).

2. The method according to claim 1, wherein in step a) at least 65% of an alkali charge required by the cooking of step b) is dosed into the slurry in a polysulfide liquor.

3. The method according to claim 1, wherein a portion in a range of 0.2% to 35% of an alkali charge required by the cooking of step b) is added to the slurry after step a).

4. The method according to claim 1, wherein the slurry contains the mercaptide ions when the slurry is heated to a cooking temperature of at least 140° C.

5. The method according to claim 1, wherein the extracted and cooled cooking liquor with the mercaptide ions is introduced in cooking liquor added to the slurry during step b).

6. The method according to claim 1, wherein the extracted and cooled cooking liquor with the mercaptide ions is added via a cooking circulation or a liquor circulation of the digester vessel in which step b) is performed.

7. The method according to claim 1, wherein the cooking is a continuous cook and the digester vessel is a single vessel or in separate vessels.

8. The method according to claim 1, wherein the digester vessel in the step b) is a vapor-liquid phase digester vessel and additional mercaptan ions are included in a mercaptan-containing gas that is introduced into a vapor space of the vapor-liquid phase digester vessel.

9. The method according to claim 1, wherein the step a) is carried out in an impregnation vessel and the step b) in carried out in a digester vessel, whereby the extracted and cooled cooking liquor with the mercaptide ions is introduced into a transfer circulation line that conveys the slurry from the impregnation vessel to the digester vessel.

10. The method according to claim 1, wherein the step a) is carried out in an impregnation vessel and the digester vessel in step b) is a vapor-liquid phase digester vessel, whereby the extracted and cooled cooking liquor with the mercaptide ions is introduced to a top separator of the vapor-liquid phase digester vessel.

11. The method according to claim 1, wherein the step a) and the step b) are carried out in a single cooking vessel which includes the digester vessel, wherein absorption of polysulfide into the fibrous material takes place in an upper part of the single cooking vessel.

12. The method according to claim 1, wherein the cooking is a modified batch cook.

13. The method according claim 1, wherein the mercaptide ions originate from sulfur compounds-containing gases produced in a chemical pulp mill.

14. The method according to claim 1, wherein the mercaptide ions are added in the amount in a range of one to ten kg S (sulfur)/adt pulp.

15. A method of producing chemical pulp comprising: impregnating comminuted lignocellulosic fibrous material with a polysulfide-containing cooking liquor in an impregnation vessel at a temperature in a range of 90° C. to 145° C. to produce a slurry containing the comminuted lignocellulosic fibrous material and the polysulfide; conveying the slurry from the impregnation vessel to a digester vessel by a liquor circulation conduit extending from the impregnation vessel to the digester vessel; cooking the slurry in the digester vessel at a temperature above 145° C. to produce a chemical pulp; extracting a cooking liquor from a screen in the digester vessel; cooling the extracted cooking liquor; adding mercaptide into the extracted and cooled cooking liquor; and adding the extracted and cooled cooking liquor with the mercaptide into the slurry during the conveyance of the slurry and/or the cooking of the slurry.

Description

SUMMARY OF THE DRAWINGS

(1) The invention is described in more detail with reference to the appended drawing, in which FIG. 1 presents a two-vessel digester system, in connection with which the present invention can be applied.

DETAILED DESCRIPTION

(2) In the exemplary embodiment presented in FIG. 1 an impregnation vessel 10 is connected to a conventional vertical continuous digester 11, which can be a hydraulic digester or a vapor-liquid phase digester.

(3) White liquor is fed via line 2 to polysulfide production 3, such as to the Moxy-process known per se. At least 65%, preferably over 85% of the alkali charge required by the cooking process is added as polysulfide cooking liquor 6 together with fibrous material 5, such as steamed wood chips, into an impregnation stage 10.

(4) The fibrous material is treated in the impregnation vessel 10 at a temperature that is below 145° C., typically 100-135° C. The length of the impregnation can be, depending on the application, even two hours, but most typically the length of polysulfide impregnation is 20-60 minutes. Effects of a shorter period of impregnation have not been verified industrially, but it is highly probable that impregnation periods of 10-20 minutes might be possible, if the chips are properly steamed and thus the penetration and impregnation of orange liquor is quick. The conditions of impregnation are chosen so that the cooking reactions do not essentially start but the temperature is maintained below 145° C. The calculating of the H-factor starts at a temperature of approximately 140° C., and it is commonly considered the lowest limit of starting of the cooking reactions.

(5) The impregnated slurry of fibrous material and cooking liquor is taken via line 7 into the cooking stage 11. In the digester the slurry is heated to cooking temperature of over 145° C. with steam or hot cooking liquor. According to the new method, mercaptide is introduced into the slurry of fibrous material before the cooking temperature is reached.

(6) The mercaptan-containing alkali is introduced into the process so that it is in contact with the chips immediately as the cooking starts in the digester 11. FIG. 1 illustrates a two-vessel system.

(7) If the cook is performed in a hydraulic digester, the mercaptide can be introduced absorbed in cooking liquor, which is added for supplementing the required alkali charge. In FIG. 1 white liquor is led via line 4 into an absorption device 8, into which also odorous gases 12 of the mill are introduced. Mercaptide is absorbed from the odorous gases into the white liquor, and unreacted odorous gas is taken via line 13 into further treatment, such as combustion. The mercaptide-containing cooking liquor is led via line 9 to the upper part of the digester, where it is present when the slurry of fibrous material reaches the cooking temperature. In the cooking stage the cooking liquor thus comprises the polysulfide liquor introduced into the impregnation stage and the mercaptide-containing cooking liquor introduced into the cooking stage, which liquor does not essentially contain polysulfide.

(8) The mercaptide-containing cooking liquor can alternatively or additionally be led via line 15 into a transfer circulation line 7 between the impregnation vessel and the digester, which line extends into the digester 11. In this case the digester 11 can be a hydraulic or a vapor-liquid phase digester.

(9) If the digester 11 is a vapor-liquid phase digester, the mercaptide-containing cooking liquor can be led to a top separator (not shown) of the digester so that it is admixed into the slurry of chips.

(10) Cooked pulp exits from the bottom of the digester and is led to further treatment via line 14.

(11) The digester 11 can comprise, in the upper part of the digester and belonging to the beginning of the cook, a first extraction screen array 16 and a circulation line 17, which are known per se. A portion of the circulation line 17 is shown in FIG. 1 and a portion not shown in the figure returns the liquor in the circulation to the feed line 7 and/or the digester vessel, as is conventional for circulation lines. The digester vessel 11 can comprise also other screen arrays 16, or the digester 11 can comprise any number of screens 16. Via a screen array 16 a portion of the cooking liquor is separated into the circulation line 17. Further, a branch flow, via line 18 can be separated from this, which is cooled 19 and directs cooking liquor to the absorption device 8. While in the absorption device, the cooking liquor adsorbs mercaptan-containing gas, such as odorous gas. The branch flow is returned via return line 20 to the circulation line 17 to be combined with the circulation liquor. Thus, the circulation liquor comprises mercaptide that is taken into the slurry of fibrous material before it is heated into cooking temperature.

(12) If the cook is performed in a vapor phase digester of a two-vessel system, the mercaptide can be introduced entrained in heating steam, which is a steam containing mercaptide and other sulfur compounds, e.g. from the evaporation of black liquor extracted from the digester. Also white liquor is introduced into the cook for supplementing the alkali charge.

EXAMPLE

(13) Air dried softwood chips were cooked in a laboratory under the following conditions with different alkali solutions: a kraft cooking liquor, a methyl mercaptane (MM) cooking liquor, a polysulfide (PS) cooking liquor and a PS-MM-cooking liquor (table 1).

(14) TABLE-US-00001 TABLE 1 Cooking conditions Softwood Dry solids content of the chips 55% Cooking liquor sulphidity (kraft/polysulfide) 41%/30% polysulfide concentration, g/L 7.5 MM charge, kg S/ts 6.0 liquor to wood ratio 3.5 Steaming time (100° C.), minutes 30 Impregnation time (115° C.), minutes 60 Cooking time (varying temperature), minutes 150

(15) The results are presented in table 2. The yield of the PS-MM-cook was 1.7% higher compared to the yield of the kraft-cook.

(16) TABLE-US-00002 TABLE 2 Total alkali (effective Yield Alkali Softwood alkali, (% on Rejets consumption (kappa 30) % on wood) H factor wood) (% pulp) (% on wood) Kraft 19.5 960 47.8 0.02 15.8 reference MM 19.7 870 47.8 0.03 15.7 Polysulfide 20.8 975 48.7 0.05 16.9 Polysulfide + 20.4 950 49.5 0.02 16.7 MM