METHOD FOR HANDLING SPENT WASH SOLUTION OF A LIGNIN-RECOVERY PROCESS

Abstract

A method for handling spent wash solution produced in the washing of lignin that has been precipitated and then separated from spent alkaline pulping liquor. At least part of the spent wash solution is introduced into the chemical recovery cycle at a location in that part of the cycle which starts at, and includes, the smelt dissolving stage and ends at, and includes, the pulping stage. In preferred embodiments, input of other water and/or aqueous solution into the chemical recovery cycle after the recovery boiler is adjusted so that, as a consequence of the introduction of the spent wash solution, the amount of water in the white liquor going to the pulping line is not significantly increased. Among others, two preferred locations for the introduction of the spent wash solution are (1) the smelt dissolver and (2) the location where unwashed lime mud is diluted.

Claims

1. A method for handling spent wash solution produced during washing of lignin that has been precipitated and then separated from spent alkaline pulping liquor at a pulp mill, in which method the separated lignin is washed in at least one washing step being conducted under acidic conditions, wherein at least part of the spent wash solution from the at least one washing step is introduced into a chemical recovery cycle at a location in that part of the cycle which part includes a smelt dissolving stage, a green liquor treatment, white liquor preparation, and a pulping stage.

2. The method according to claim 1, wherein input of other water and/or aqueous solution into the chemical recovery cycle in that part of the cycle which starts at, and includes, the smelt dissolving stage and ends at, and includes, the pulping stage is adjusted so that, as a consequence of the introduction of the spent wash solution, an amount of water in white liquor going from the white liquor preparation to the pulping stage is not significantly increased.

3. The method according to claim 1, wherein sulphuric acid introduced in at least one washing step.

4. The method according to claim 1, wherein spent wash solution is introduced as part of an aqueous medium in the smelt dissolving stage used to dissolve smelt formed in a chemical recovery boiler.

5. The method according to claim 1, wherein the spent wash solution is introduced as part of an aqueous medium to prepare a diluted lime-mud slurry prior to a final washing and dewatering of lime mud.

6. The method according to claim 1, wherein at least part of an aliphatic carboxylic acid fraction dissolved in the spent wash solution is recovered from the solution before the solution is introduced into the chemical recovery cycle.

7. A method comprising: washing lignin under an acidic condition, where the lignin has been precipitated from spent alkaline pulping liquor and a spent wash solution is discharged from the washing of the lignin; and introducing the spent wash solution into at least one of a smelt dissolving stage, a green liquor treatment stage, a white liquor preparation stage and a pulping stage of a chemical recovery cycle.

8. The method of claim 7, further comprising producing white liquor in the white liquor preparation stage and feeding the white liquor to the pulping stage, and introducing an aqueous solution to the chemical recovery cycle in addition to the introduction of the spent wash solution.

9. The method according to claim 7, wherein sulphuric acid is added to the step of washing lignin.

10. The method according to claim 7, wherein the spent wash solution is introduced in the smelt dissolving stage and contributes to dissolving smelt formed in a chemical recovery boiler.

11. The method according to claim 7, further comprising introducing the spent wash to dilute a lime-mud slurry which flows to a lime-mud slurry washing stage.

12. The method according to claim 7, recovering from the spent wash solution an aliphatic carboxylic acid fraction before the introduction of the spent wash solution at least one of the smelt dissolving stage, the green liquor treatment stage, the white liquor preparation stage and the pulping stage.

Description

[0036] The present new method is discussed further with reference to the drawings FIGS. 1-3. FIG. 1 depicts a typical lignin-recovery process while FIGS. 2 and 3 each highlight a preferred location for introducing spent wash solution to the recovery cycle according to the new method.

[0037] The numbers and letters in FIG. 1 refer to the following streams and processing stages: [0038] 1. Feed black liquor [0039] 2. Carbon dioxide [0040] 3. Primary lignin slurry [0041] 4. Lignin-lean black liquor (filtrate from filtration of primary slurry) [0042] 5. Primary filter cake [0043] 6. Sulphuric acid [0044] 7. Wash filtrate from second wash step (displacement wash on filter) [0045] 8. Lignin slurry from first wash step (re-slurry acidic wash) [0046] 9. Spent wash solution (filtrate from filtration of lignin slurry from first wash step) [0047] 10. Water and/or aqueous solution (fresh wash medium) [0048] 11. Filter cake, comprised essentially of moist lignin. [0049] A. Reactor for acidification of black liquor [0050] B. Filter press [0051] C. Reactor for acidic re-slurry washing step [0052] D. Filter press.

[0053] A typical lignin-recovery process for application at kraft pulp mills is depicted in FIG. 1. A part (1) of the black-liquor stream of the mill is acidified with CO.sub.2 (2) to pH 10 (value 25 C.) in a stirred acidification reactor (A). Lignin is precipitated, thereby forming primary lignin slurry (3) which is then filtered using a filter press (B). The filtrate (4) from this step, the so called lignin-lean black liquor, is recycled to the evaporation plant. The primary filter cake (5) from the filter press (B) is led to a stirred-tank reactor (C). Sulphuric acid (6) and filtrate (7) from the second washing step are also introduced to this reactor (C) and the first washing stepan acidic re-slurry washis thereby accomplished at a pH around 2 (value 25 C.). The lignin slurry (8) formed in this step is filtered using a filter press (D). The filtrate (9) formed in the filtration of the lignin slurry (8) is the spent wash solution of the lignin-recovery process. The second washing step is a displacement wash on the filter (D). Fresh washing medium (10), typically in the form of mill water or condensate, is employed in this step and the filtrate (7) thus formed is recycled to the acid-wash reactor (C). The dry matter of the filter cake (11) exiting the filter press (D) is comprised of lignin and some residual impurities.

[0054] The numbers and letters in FIGS. 2 and 3 refer to the following streams and processing stages: [0055] 9. Spent wash solution from the lignin-recovery plant [0056] 12. Smelt from the recovery boiler [0057] 13. Weak wash (also known as weak white liquor) [0058] 14. Water or aqueous solution [0059] 15. Raw (unfiltered) green liquor [0060] 16. Dregs slurry [0061] 17. Filtered green liquor [0062] 18. Washed and dewatered dregs [0063] 19. Wash water [0064] 20. Filtrate from washing and dewatering dregs [0065] 21. Filtrate from washing and dewatering dregs [0066] 22. Burnt lime [0067] 23. Slurry of limestone and white liquor [0068] 24. Lime-mud filter cake [0069] 25. Wash water [0070] 26. White liquor [0071] 27. Dilution (wash) water and/or aqueous solution [0072] 28. Diluted lime-mud slurry [0073] 29. Washed and dewatered lime mud [0074] 30. Wash water [0075] 31. Weak wash (also known as weak white liquor) [0076] E. Smelt dissolving tank [0077] F. Filter for green liquor [0078] G. Filter for dregs slurry [0079] H. Slaking and causticizing line [0080] I. Filter for slurry of limestone and white liquor [0081] J. Filter for slurry of limestone and weak white liquor

[0082] An embodiment of the present new method is depicted in FIG. 2. More specifically, FIG. 2 is a simplified diagram of a plant for preparing white liquor from recovery boiler smelt (lime kiln not shown), displaying also one preferred location for introduction of the spent wash solution to the recovery cycle according to the new method. The stream (9) of spent wash solution from the lignin-recovery plant is shown as a double line for emphasis. The flows shown as single lines are those of a conventional white-liquor preparation process. The conventional process is described first. Smelt (12) from the recovery boiler (not shown) is led to the dissolving tank (E) to which weak wash (13) and possibly additional water or aqueous solution (14) are also introduced. Smelt dissolves forming raw (unfiltered) green liquor (15) which is then led to the filter (F). Filtered green liquor (17) is led to the slaking and causticizing line (H) while the dregs slurry (16) goes to a filter (G). Washed and dewatered dregs (18) leave the process while filtrate from this step is recycled as green liquor (20) and/or as weak wash (21) depending on its quality. In the slaking and causticizing line (H), burnt lime (22) is first slaked in green liquor, after which green liquor is causticized through reactions with Ca(OH).sub.2. The slurry (23) of limestone and white liquor thus formed is led to a filter (I). The filtrate (26) from this step is the white liquor that is employed in the pulping stage. Water and/or aqueous solution, such as condensate, (27) is added to dilute the lime-mud filter cake (24) leaving the filter (I), and the diluted lime-mud slurry (28) is led to a filter (J). Washed and dewatered lime mud (29) is sent to the lime kiln for re-burning while the filtrate (31) from this step makes up most of the weak wash that is recycled to the smelt dissolver.

[0083] In the embodiment of the present invention shown in FIG. 2, the spent wash solution (9) from the lignin-recovery plant is introduced to the smelt dissolving tank (E). In order to maintain the amount of water in the white liquor at approximately the same level as previously, the amount of water and/or aqueous solution stream entering the dissolving tank as stream 14 must be correspondingly decreased. At many pulp mills, stream 14 is relatively smallor even non-existent. Thus, at many pulp mills, it will not be possible to apply this embodiment without increasing the amount of water in the white liquor.

[0084] Another embodiment of the present new method is depicted in FIG. 3. In fact, the process depicted in FIG. 3 deviates from that of FIG. 2 only in the different location for inputting the spent wash solution. In the embodiment of FIG. 3, the spent wash solution (9) from the lignin-recovery plant is used to substitute part of the water and/or aqueous solution (27) used for diluting the lime mud before its final washing and dewatering. In other words, in conjunction with the addition of the spent wash solution, the amount of water introduced in stream 27 is decreased so that the total amount of dilution water remains approximately the same. This embodiment is likely to be universally applicablethe new input in the form of spent wash solution does not measurably contribute to the contents of problematic impurities in the final lime-mud stream nor does it have a significant detrimental effect upon the filterability of the dilute lime-mud slurry.

EXAMPLE

[0085] Lignin-recovery technology is applied at a pulp mill producing 2000 t/d of air-dried pulp (2000 ADt/d). 20% of the total black-liquor stream of the mill is fed to the lignin-recovery plant. The dry-solids content of the liquor fed to the plant is 30%. The lignin output is 140 t/d of dry matter. Within the lignin-recovery plant, the lignin precipitated and then separated from the black liquor is washed in two steps, first with an acidic aqueous solution and then with condensate (the fresh wash medium). H.sub.2SO.sub.4 is the acidic compound employed in the first washing step. About 620 t/d of spent wash solution exits the lignin-recovery plant. In accordance with one embodiment of the present invention, the spent wash solution is used to substitute some of the water and/or aqueous solution previously employed for diluting lime mud prior to the final washing and dewatering of the lime mud. The amount of water in the final white liquor utilized in the pulping stage is largely unaltered. In this pulp mill, the total amount of water and/or aqueous solution employed for diluting lime mud is 6500 t/d which is an order of magnitude larger than the amount of spent wash solution. The dominating components dissolved in the spent wash solution are Na.sub.2SO.sub.4 and aliphatic carboxylic acids. The contents of these components in the spent wash solution are as follows:

TABLE-US-00001 Na.sub.2SO.sub.4: 6.9% by weight Aliphatic carboxylic acids: 2.4% by weight

[0086] The composition of the spent wash solution is such that the solution may be employed in the dilution of lime mud (1) without measurably affecting the contents of problematic impurities in the final lime mud going to the lime kiln and (2) without measurably worsening the filterability of the dilute lime-mud slurry.

[0087] The embodiments of the present invention are not limited to those mentioned or described herein.