METHOD FOR PROCESSING BLACK LIQUOR IN PULP PRODUCTION

Abstract

A method for processing black liquor includes operations of: (A1) ultrafiltering the black liquor; (A2) mixing the ultrafiltration concentrate with carbon dioxide and/or sulfuric acid to precipitate lignin in the ultrafiltration concentrate, and obtaining a first inorganic aqueous solution; (A3) mixing the first inorganic aqueous solution with the ultrafiltration permeate to obtain a mixture, and nanofiltering the mixture; (A4) mixing the nanofiltration concentrate with carbon dioxide and/or sulfuric acid to precipitate lignin in the nanofiltration concentrate, and obtaining a second inorganic aqueous solution; (A5) performing a first electrodialysis treatment or reverse osmosis treatment on the second inorganic aqueous solution, and obtaining a remaining inorganic aqueous solution; (A6) mixing the remaining inorganic aqueous solution with the lignin obtained from operation (A2) and/or operation (A4) to obtain a first fuel liquid; and (A7) processing the first fuel liquid to produce sodium sulfide, sodium carbonate, carbon dioxide, and steam.

Claims

1. A method for processing black liquor in pulp production, the method comprising operations of: (A1) ultrafiltering black liquor generated in pulp production to obtain an ultrafiltration permeate and an ultrafiltration concentrate, wherein the black liquor comprises at least lignin, sodium hydroxide, sodium sulfide, sodium sulfate, sodium sulfite, sodium carbonate, and water; (A2) mixing the ultrafiltration concentrate with carbon dioxide and/or sulfuric acid to precipitate lignin in the ultrafiltration concentrate and obtain a first inorganic aqueous solution, wherein the ultrafiltration concentrate reacts with carbon dioxide and/or sulfuric acid to produce sodium carbonate and/or sodium sulfate dissolved in the first inorganic aqueous solution; (A3) mixing the first inorganic aqueous solution with the ultrafiltration permeate to obtain a mixture, and nanofiltering the mixture to obtain a sodium hydroxide aqueous solution as a permeate and a nanofiltration concentrate as a retentate; (A4) mixing the nanofiltration concentrate with carbon dioxide and/or sulfuric acid to precipitate lignin in the nanofiltration concentrate and obtain a second inorganic aqueous solution, wherein the nanofiltration concentrate reacts with carbon dioxide and/or sulfuric acid to produce sodium carbonate and/or sodium sulfate dissolved in the second inorganic aqueous solution; (A5) subjecting the second inorganic aqueous solution to a first electrodialysis treatment or a reverse osmosis treatment, wherein: while the first electrodialysis treatment is performed, sodium carbonate in the second inorganic aqueous solution is electrolyzed to produce sodium hydroxide, and a sodium hydroxide aqueous solution is obtained at a cathode of the first electrodialysis treatment, and a remaining inorganic aqueous solution is obtained at an anode of the first electrodialysis treatment; while the reverse osmosis treatment is performed, water is obtained as a permeate and a remaining inorganic aqueous solution is obtained as a retentate; (A6) mixing the remaining inorganic aqueous solution with a portion of the lignin obtained from operation (A2) and/or operation (A4) to obtain a first fuel liquid; and (A7) processing the first fuel liquid to produce sodium sulfide, sodium carbonate, carbon dioxide, and steam.

2. The method according to claim 1, wherein operation (A7) comprises the acts of: (A7-1) evaporating a portion of water in the first fuel liquid to obtain a concentrated fuel liquid and steam; (A7-2) mixing the concentrated fuel liquid with a portion of the lignin obtained from operation (A2) and/or operation (A4) to obtain a second fuel liquid; and (A7-3) combusting the second fuel liquid to generate sodium sulfide, sodium carbonate, carbon dioxide, and steam.

3. The method according to claim 1, wherein operation (A2) comprises mixing the ultrafiltration concentrate with carbon dioxide produced from operation (A7); and/or operation (A4) comprises mixing the nanofiltration concentrate with carbon dioxide produced from operation (A7).

4. The method according to claim 1, after operation (A7), further comprising operations of: (A8) mixing water with sodium sulfide and sodium carbonate produced from operation (A7) to obtain a green liquor; and (A9) subjecting the green liquor to a second electrodialysis treatment, electrolyzing the sodium carbonate in the green liquor to produce sodium hydroxide, wherein a sodium hydroxide aqueous solution is obtained at a cathode of the second electrodialysis treatment, and a sulfuric acid aqueous solution is obtained at an anode of the second electrodialysis treatment.

5. The method according to claim 4, further comprising operations of: (A10) mixing sodium sulfide with the sodium hydroxide aqueous solution obtained from operation (A9), the sodium hydroxide aqueous solution obtained from operation (A3), and the sodium hydroxide aqueous solution obtained from operation (A5) to obtain a white liquor; and (A11) applying the white liquor to the pulp production; wherein operation (A5) comprises performing the first electrodialysis treatment.

6. The method according to claim 1, after operation (A7), further comprising operations of: (A8) mixing water with sodium sulfide and sodium carbonate produced from operation (A7) to obtain a green liquor; and (A12) subjecting the green liquor to a causticizing reaction with calcium oxide to obtain an aqueous solution comprising sodium hydroxide and sodium sulfide.

7. A method for processing black liquor in pulp production, the method comprising operations of: (B1) ultrafiltering black liquor generated in pulp production to obtain an ultrafiltration permeate and an ultrafiltration concentrate, wherein the black liquor comprises at least lignin, sodium hydroxide, sodium sulfide, sodium sulfate, sodium sulfite, sodium carbonate, and water; (B2) nanofiltering the ultrafiltration permeate to obtain a sodium hydroxide aqueous solution as a permeate and a nanofiltration concentrate as a retentate, and mixing the ultrafiltration concentrate with the nanofiltration concentrate to obtain a concentrated black liquor; (B3) mixing the concentrated black liquor with carbon dioxide and/or sulfuric acid to precipitate lignin in the concentrated black liquor and obtain an inorganic aqueous solution, wherein the concentrated black liquor reacts with carbon dioxide and/or sulfuric acid to produce sodium carbonate and/or sodium sulfate dissolved in the inorganic aqueous solution; (B4) subjecting the inorganic aqueous solution to a first electrodialysis treatment or a reverse osmosis treatment, wherein: while the first electrodialysis treatment is performed, sodium carbonate in the inorganic aqueous solution is electrolyzed to produce sodium hydroxide, a sodium hydroxide aqueous solution is obtained at a cathode of the first electrodialysis treatment, and a remaining inorganic aqueous solution is obtained at an anode of the first electrodialysis treatment; while the reverse osmosis treatment is performed, water is obtained as a permeate and a remaining inorganic aqueous solution is obtained as a retentate; (B5) mixing the remaining inorganic aqueous solution with a portion of the lignin obtained from operation (B3) to obtain a first fuel liquid; and (B6) processing the first fuel liquid to produce sodium sulfide, sodium carbonate, carbon dioxide, and steam.

8. The method according to claim 7, wherein operation (B6) comprises acts of: (B6-1) evaporating a portion of water in the first fuel liquid to obtain a concentrated fuel liquid and steam; (B6-2) mixing the concentrated fuel liquid with a portion of the lignin obtained from operation (B3) to obtain a second fuel liquid; (B6-3) combusting the second fuel liquid to produce sodium sulfide, sodium carbonate, carbon dioxide, and steam.

9. The method according to claim 7, wherein operation (B3) comprises mixing the concentrated black liquor with the carbon dioxide produced from operation (B6).

10. The method according to claim 7, after operation (B6), further comprising operations of: (B7) mixing water with sodium sulfide and sodium carbonate produced from operation (B6) to obtain a green liquor; and (B8) subjecting the green liquor to a second electrodialysis treatment, electrolyzing the sodium carbonate in the green liquor to produce sodium hydroxide, wherein a sodium hydroxide aqueous solution is obtained at a cathode of the second electrodialysis treatment, and a sulfuric acid aqueous solution is obtained at an anode of the second electrodialysis treatment.

11. The method according to claim 10, further comprising operations of: (B9) mixing sodium sulfide with the sodium hydroxide aqueous solution obtained from operation (B8) and the sodium hydroxide aqueous solution obtained from operation (B2) and the sodium hydroxide aqueous solution obtained from operation (B4) to obtain a white liquor; and (B10) applying the white liquor to the pulp production; wherein operation (B4) comprises performing the first electrodialysis treatment.

12. The method according to claim 7, after operation (B6), further comprising operations of: (B7) mixing water with sodium sulfide and sodium carbonate produced from operation (B6) to obtain a green liquor; and (B11) subjecting the green liquor to a causticizing reaction with calcium oxide to obtain an aqueous solution comprising sodium hydroxide and sodium sulfide.

13. A method for processing black liquor in pulp production, the method comprising operations of: (C1) ultrafiltering black liquor generated in pulp production to obtain an ultrafiltration permeate and an ultrafiltration concentrate, wherein the black liquor comprises at least lignin, sodium hydroxide, sodium sulfide, sodium sulfate, sodium sulfite, sodium carbonate, and water; (C2) distributing the ultrafiltration concentrate into a first ultrafiltration concentrate and a second ultrafiltration concentrate; (C3) mixing the first ultrafiltration concentrate with carbon dioxide and/or sulfuric acid to precipitate lignin in the first ultrafiltration concentrate and obtain a first inorganic aqueous solution, wherein the first ultrafiltration concentrate reacts with carbon dioxide and/or sulfuric acid to produce sodium carbonate and/or sodium sulfate dissolved in the first inorganic aqueous solution; (C4) mixing the first inorganic aqueous solution with the ultrafiltration permeate to obtain a mixture, and nanofiltering the mixture to obtain a sodium hydroxide aqueous solution as a permeate and a nanofiltration concentrate as a retentate; (C5) mixing the nanofiltration concentrate with carbon dioxide and/or sulfuric acid to precipitate lignin in the nanofiltration concentrate and obtain a second inorganic aqueous solution, wherein the nanofiltration concentrate reacts with carbon dioxide and/or sulfuric acid to produce sodium carbonate and/or sodium sulfate dissolved in the second inorganic aqueous solution; (C6) subjecting the second inorganic aqueous solution to a first electrodialysis treatment or a reverse osmosis treatment, wherein: while the first electrodialysis treatment is performed, sodium carbonate in the second inorganic aqueous solution is electrolyzed to produce sodium hydroxide, and a sodium hydroxide aqueous solution is obtained at a cathode of the first electrodialysis treatment, and a remaining inorganic aqueous solution is obtained at an anode of the first electrodialysis treatment; while the reverse osmosis treatment is performed, water is obtained as a permeate and a remaining inorganic aqueous solution is obtained as a retentate; (C7) mixing the remaining inorganic aqueous solution, the second ultrafiltration concentrate obtained from operation (C2), and a portion of the lignin obtained from operation (C3) and/or operation (C5) to obtain a first fuel liquid; and (C8) processing the first fuel liquid to produce sodium sulfide, sodium carbonate, carbon dioxide, and steam.

14. The method according to claim 13, wherein operation (C8) comprises acts of: (C8-1) evaporating a portion of water in the first fuel liquid to obtain a concentrated fuel liquid and steam; (C8-2) mixing the concentrated fuel liquid with a portion of the lignin obtained from operation (C3) and/or operation (C5) to obtain a second fuel liquid; (C8-3) combusting the second fuel liquid to produce sodium sulfide, sodium carbonate, carbon dioxide, and steam.

15. The method according to claim 13, wherein: operation (C3) comprises mixing the ultrafiltration concentrate with the carbon dioxide produced from operation (C8); and/or operation (C5) comprises mixing the nanofiltration concentrate with the carbon dioxide produced from operation (C8).

16. The method according to claim 13, after operation (C8), further comprising operations of: (C9) mixing water with the sodium sulfide and sodium carbonate produced from operation (C8) to obtain a green liquor; and (C10) subjecting the green liquor to a second electrodialysis treatment, electrolyzing the sodium carbonate in the green liquor to form sodium hydroxide, wherein a sodium hydroxide aqueous solution is obtained at a cathode of the second electrodialysis treatment, and a sulfuric acid aqueous solution is obtained at an anode of the second electrodialysis treatment.

17. The method according to claim 16, further comprising operations of: (C11) mixing sodium sulfide with the sodium hydroxide aqueous solution obtained from operation (C10) and the sodium hydroxide aqueous solution obtained from operation (C4) and the sodium hydroxide aqueous solution obtained from operation (C6) to obtain a white liquor; and (C12) applying the white liquor to the pulp production; wherein operation (C6) comprises performing the first electrodialysis treatment.

18. The method according to claim 13, after operation (C8), further comprising operations of: (C9) mixing water with the sodium sulfide and sodium carbonate produced from operation (C8) to obtain a green liquor; and (C13) subjecting the green liquor to a causticizing reaction with calcium oxide to obtain an aqueous solution comprising sodium hydroxide and sodium sulfide.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0003] Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying FIGS. It is noted that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion.

[0004] FIG. 1 is a flowchart illustrating a method for processing black liquor in pulp production, according to some embodiments of the present disclosure.

[0005] FIG. 2 is a schematic diagram illustrating an equipment system for implementing the method, according to some embodiments of the present disclosure.

[0006] FIG. 3 is a flowchart illustrating a method for processing black liquor in pulp production, according to yet some embodiments of the present disclosure.

[0007] FIG. 4 is a schematic diagram illustrating an equipment system for implementing the method, according to yet some embodiments of the present disclosure.

[0008] FIG. 5 is a flowchart illustrating a method for processing black liquor in pulp production, according to yet some embodiments of the present disclosure.

[0009] FIG. 6 is a schematic diagram illustrating an equipment system for implementing the method, according to yet some embodiments of the present disclosure.

DETAILED DESCRIPTION

[0010] The following disclosure provides many different embodiments, or examples, for implementing different features of the provided subject matter. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first operation could be termed a second operation, and, similarly, a second operation could be termed a first operation, without departing from the scope of the embodiments. As used herein, the term and/or includes any and all combinations of one or more of the associated listed items. It will be appreciated that although these methods each illustrate a number of operations, acts and/or features, not all of these operations, acts and/or features are necessarily required, and other un-illustrated operations, acts and/or features may also be present. Also, the ordering of the operations and/or acts in some embodiments can vary from what is illustrated in these FIG. 1n addition, the illustrated acts can be further divided into sub-acts in some implementations, while in other implementations some of the illustrated acts can be carried out concurrently with one another.

[0011] Several conventional terms are used in this disclosure, such as black liquor, green liquor, and white liquor. These are terms of art in the technical field to which the present disclosure pertains. One skilled in the art would understand the meanings without ambiguity.

[0012] During the pulp production process, black liquor is generated. Black liquor includes lignin, cellulose and hemicellulose and derivatives thereof, sodium hydroxide, sodium sulfide, sodium sulfate, sodium carbonate, and water. In some embodiments, the solids content of the black liquor ranges from about 12% to about 20%. Various embodiments of the present disclosure provide more efficient treatment of black liquor to recover and reuse the components therein.

[0013] FIG. 1 is a flowchart illustrating a method 1 for processing black liquor in pulp production, according to some embodiments of the present disclosure. The method 1 includes at least operation A1 to operation A7. FIG. 2 is a schematic diagram illustrating an equipment system 101 for implementing method 1 according to some embodiments of the present disclosure.

[0014] Referring to FIG. 1, method 1 includes operation A1 of ultrafiltering black liquor generated in pulp production to obtain an ultrafiltration permeate and an ultrafiltration concentrate. In some embodiments, referring to FIG. 2, black liquor 12 produced from the pulping process 10 is fed to an ultrafiltration membrane unit UF for filtration, resulting in an ultrafiltration permeate 14 that passes through the membrane and an ultrafiltration concentrate 16 that does not. The ultrafiltration membrane unit UF is capable of filtering out most of the lignin, so the ultrafiltration concentrate 16 contains a relatively large amount of lignin, whereas the ultrafiltration permeate 14 contains a relatively small amount of lignin. The ultrafiltration membrane unit UF does not filter out the inorganic compounds and water present in the black liquor; therefore, both the ultrafiltration permeate 14 and the ultrafiltration concentrate 16 still include sodium hydroxide (NaOH), sodium sulfide (Na.sub.2S), sodium sulfate (Na.sub.2SO.sub.4), sodium sulfite (Na.sub.2SO.sub.3), sodium carbonate (Na.sub.2CO.sub.3), and water.

[0015] Method 1 proceeds to Operation A2 by mixing the ultrafiltration concentrate with carbon dioxide (CO.sub.2) and/or sulfuric acid (H.sub.2SO.sub.4) to precipitate lignin in the ultrafiltration concentrate and obtain a first inorganic aqueous solution. In some embodiments, referring to FIG. 2, the ultrafiltration concentrate 16 is fed to a first reactor RX1, where CO.sub.2 and/or H.sub.2SO.sub.4 is introduced and mixed and reacted with the ultrafiltration concentrate 16 to produce precipitated lignin 18 and a first inorganic aqueous solution 20. In Operation A2, CO.sub.2 reacts with the NaOH in the ultrafiltration concentrate 16 to produce Na.sub.2CO.sub.3, and H.sub.2SO.sub.4 reacts with NaOH, Na.sub.2SO.sub.3 and Na.sub.2CO.sub.3 in the ultrafiltration concentrate 16 to produce Na.sub.2SO.sub.4. The resulting Na.sub.2CO.sub.3 and/or Na.sub.2SO.sub.4 is dissolved in the first inorganic aqueous solution 20. The chemical reactions described above are as follows:

##STR00001##

[0016] Due to the reaction between the aforementioned CO.sub.2 and/or H.sub.2SO.sub.4 with the ultrafiltration concentrate 16, the pH value of the ultrafiltration concentrate 16 decreases, thereby causing the lignin 18 in the ultrafiltration concentrate 16 to precipitate. In some embodiments, the precipitated lignin 18 is separated and delivered to a collector CL.

[0017] Method 1 proceeds to operation A3 by mixing the first inorganic aqueous solution with the ultrafiltration permeate to obtain a mixture, and nanofiltering the mixture to obtain a NaOH aqueous solution as a permeate and a nanofiltration concentrate as a retentate. In some embodiments, referring to FIG. 2, the first inorganic aqueous solution 20 and the ultrafiltration permeate 14 are mixed to obtain a mixture 22, which is then delivered to a nanofiltration membrane unit NF for filtration. The mixture 22 contains NaOH, Na.sub.2S, Na.sub.2SO.sub.4, Na.sub.2SO.sub.3, Na.sub.2CO.sub.3, water, and a small amount of lignin. NaOH in the mixture 22 can pass through the nanofiltration membrane, thereby separating a NaOH aqueous solution 24 from the mixture 22. The liquid that does not pass through the nanofiltration membrane becomes the nanofiltration concentrate 26, which contains Na.sub.2S, Na.sub.2SO.sub.4, Na.sub.2SO.sub.3, Na.sub.2CO.sub.3, water, a relatively small amount of NaOH, and a relatively small amount of lignin.

[0018] Method 1 proceeds to operation A4 by mixing the nanofiltration concentrate with CO.sub.2 and/or H.sub.2SO.sub.4 to precipitate lignin in the nanofiltration concentrate and obtain a second inorganic aqueous solution. In some embodiments, referring to FIG. 2, the nanofiltration concentrate 26 is delivered to a second reactor RX2, where CO.sub.2 and/or H.sub.2SO.sub.4 is introduced and mixed and reacted with the nanofiltration concentrate 26 to produce precipitated lignin 28 and a second inorganic aqueous solution 30. In some embodiments, the precipitated lignin 28 is separated and delivered to the collector CL. The chemical reaction in operation A4 is substantially the same as that described hereinbefore in connection with operation A2. The produced Na.sub.2CO.sub.3 and/or Na.sub.2SO.sub.4 are dissolved in the second inorganic aqueous solution 30. Therefore, the second inorganic aqueous solution 30 includes Na.sub.2S, Na.sub.2SO.sub.4, Na.sub.2SO.sub.3, Na.sub.2CO.sub.3, and water. In operation A4, organic substances such as lignin in the nanofiltration concentrate 26 are substantially removed, and the nanofiltration concentrate 26 is converted into Na.sub.2CO.sub.3 and/or Na.sub.2SO.sub.4, which are dissolved in the second inorganic aqueous solution 30.

[0019] Method 1 proceeds to operation A5 by subjecting the second inorganic aqueous solution to a first electrodialysis treatment or a reverse osmosis treatment. In some embodiments, referring to FIG. 2, the second inorganic aqueous solution 30 is delivered to a first electrodialysis device ED1. During the first electrodialysis treatment, Na.sub.2CO.sub.3 in the second inorganic aqueous solution 30 is electrolyzed to generate NaOH, producing a NaOH aqueous solution 32 in the cathode chamber of the first electrodialysis device ED1, and a remaining inorganic aqueous solution 34 in the anode chamber of ED1. The remaining inorganic aqueous solution 34 includes Na.sub.2S, Na.sub.2SO.sub.4, Na.sub.2SO.sub.3, water, and a small amount of un-electrolyzed Na.sub.2CO.sub.3. The NaOH aqueous solution 32 obtained in operation A5 is delivered to a second mixer MX2, which will be described in more detail hereinafter.

[0020] In other embodiments, however, the first electrodialysis device ED1 shown in FIG. 2 is replaced with a reverse osmosis device RO. In the reverse osmosis treatment, permeated water 36 and a remaining inorganic aqueous solution 34 (i.e., a retentate) that does not pass through are obtained. The water 36 obtained from the reverse osmosis process may be used in the pulp washing process or for other purposes, and does not enter the second mixer MX2.

[0021] Method 1 proceeds to operation A6 by mixing the remaining inorganic aqueous solution with a portion of the lignin obtained from operation (A2) and/or operation (A4) to obtain a first fuel liquid. In some embodiments, referring to FIG. 2, the remaining inorganic aqueous solution 34 is mixed with a portion of the lignin 18, 28 in the collector CL (i.e., parts of the product obtained from the first and/or second reactors RX1, RX20, thereby obtaining the first fuel liquid 40.

[0022] Method 1 proceeds to operation A7 by processing the first fuel liquid to produce Na.sub.2S, Na.sub.2CO.sub.3, CO.sub.2, and steam. In the processing, the lignin is combusted, and Na.sub.2SO.sub.4 and Na.sub.2SO.sub.3 are reacted and converted into Na.sub.2S, resulting in solid-phase products including Na.sub.2S and Na.sub.2CO.sub.3, along with CO.sub.2 and steam. In some embodiments, operation A7 includes acts A7-1, A7-2, and A7-3 described below. In act A7-1, part of the water in the first fuel liquid is evaporated to obtain a concentrated fuel liquid and steam. Referring to FIG. 2, the first fuel liquid 40 is delivered to an evaporator VE, where a portion of the water in the first fuel liquid 40 is heated and evaporated, yielding a concentrated fuel liquid 42 and steam ST. In act A7-2, the concentrated fuel liquid is mixed with a portion of the lignin obtained from operation A2 and/or operation A4 to obtain a second fuel liquid. Referring to FIG. 2, the concentrated fuel liquid 42 and the lignin 18, 28 in the collector CL (i.e., parts of the product obtain from the first and/or second reactors RX1, RX2) are delivered to a spraying device SP to obtain a second fuel liquid 44. In act A7-3, the second fuel liquid is combusted to produce Na.sub.2S, Na.sub.2CO.sub.3, CO.sub.2, and steam. Referring to FIG. 2, the second fuel liquid 44 is delivered and sprayed from the spraying device SP into a boiler RB for combustion, resulting in a solid-phase product 46 including Na.sub.2S and Na.sub.2CO.sub.3, as well as CO.sub.2 and steam ST. The temperature of the boiler RB is, for example, from about 800 C. to about 1200 C.

[0023] In some embodiments, act A7-3 includes using a portion of the lignin obtained from operation A2 and/or operation A4 as a fuel to combust the second fuel liquid. Referring to FIG. 2, the lignin 18, 28 in the collector CL (i.e., parts of the product obtain from the first and/or second reactors RX1, RX2) is used as a fuel for the boiler RB to heat the second fuel liquid 44 inside the boiler RB, thereby causing the second fuel liquid 44 to combust within the boiler RB.

[0024] In some embodiments, operation A2 includes mixing the CO.sub.2 generated from operation A7 with the ultrafiltration concentrate. In examples, referring to FIG. 2, the CO.sub.2 generated from combustion may be delivered to the first reactor RX1 (as a CO.sub.2 source of the first reactor RX1), and CO.sub.2 is mixed and reacted with the ultrafiltration concentrate 16. In yet some embodiments, operation A4 includes mixing the CO.sub.2 generated from operation A7 with the nanofiltration concentrate. In examples, referring to FIG. 2, the CO.sub.2 generated from combustion may be delivered to the second reactor RX2 as a CO.sub.2 source for the second reactor RX2, and CO.sub.2 is mixed and reacted with the nanofiltration concentrate 26.

[0025] In some embodiments, method 1 further includes an operation of delivering the steam ST generated in operation A7 to a power generation device GE, as illustrated in FIG. 2, or using the steam ST as a heat source in other processes.

[0026] In some embodiments, after operation A7, method 1 further includes operation A8. In operation A8, water is mixed with the solid-phase products (i.e., Na.sub.2S and Na.sub.2CO.sub.3) generated from operation A7 to produce green liquor. The green liquor includes Na.sub.2CO.sub.3, Na.sub.2S, water, and a relatively small amount of NaOH. In some embodiments, referring to FIG. 2, the solid-phase product 46 and water W are delivered to a first mixer MX1, where the solid-phase product 46 and water W are mixed to generate the green liquor 48. In some embodiments, after operation A8, method 1 further includes operation A9. In operation A9, the green liquor is subjected to a second electrodialysis treatment, in which the Na.sub.2CO.sub.3 in the green liquor is electrolyzed to generate NaOH. In some embodiments, referring to FIG. 2, the green liquor 48 is delivered to a second electrodialysis device ED2. In the second electrodialysis treatment, a NaOH aqueous solution 50 is obtained in the cathode chamber of the second electrodialysis device ED2, and a H.sub.2SO.sub.4 aqueous solution 52 is obtained in the anode chamber of the second electrodialysis device ED2. In some embodiments, H.sub.2SO.sub.4 aqueous solution 52 are delivered to the first reactor RX1 and/or the second reactor RX2 for the reaction.

[0027] In some embodiments, operation A5 includes performing the first electrodialysis treatment, and method 1 further includes operation A10 and operation A11 described below. In operation A10, Na.sub.2S is mixed with the NaOH aqueous solution obtained in operation A9, the NaOH aqueous solution obtained in operation A3, and the NaOH aqueous solution obtained in operation A5 to obtain white liquor. In some embodiments, referring to FIG. 2, Na.sub.2S, the NaOH aqueous solution 50 obtained from the second electrodialysis device ED2, NaOH aqueous solution 24 obtained from the nanofiltration membrane unit NF, and NaOH aqueous solution 32 obtained from the first electrodialysis device ED1 are delivered to a second mixer MX2 for mixing, thereby producing white liquor 54. In operation A11, the white liquor is used in the pulping process. In examples, referring to FIG. 2, the white liquor 54 is delivered to the pulping process 10 as a raw material for the pulping process 10.

[0028] In some embodiments, after operation A8, method 1 proceeds to operation A12 of subjecting the green liquor to a causticizing reaction (not illustrated in FIG. 2) with calcium oxide (CaO) to obtain an aqueous solution including NaOH and Na.sub.2S. Specifically, in operation A12, the Na.sub.2CO.sub.3 in the green liquor reacts with CaO to generate NaOH and precipitated solid calcium carbonate (CaCO.sub.3). The chemical reaction are as follows:

##STR00002##

[0029] In some embodiments, after operation A8, method 1 proceeds to operation A12, whereas the second electrodialysis treatment described in operation A9 is not performed. However, in other embodiments, method 1 may include both operation A12 and operation A9, for example, by subjecting a portion of the green liquor produced in operation A8 to operation A9 and subjecting another portion thereof to operation A12.

[0030] FIG. 3 is a flowchart illustrating a method 2 for processing black liquor in pulp production, according to yet some embodiments of the present disclosure. Method 2 includes at least operation B1 to operation B6. FIG. 4 illustrates a schematic diagram of an equipment system 201 for implementing method 2 according to some embodiments of the present disclosure.

[0031] Referring to FIG. 3, method 2 includes operation B1 of ultrafiltering black liquor generated in pulp production to obtain an ultrafiltration permeate and an ultrafiltration concentrate. In some embodiments, as shown in FIG. 4, black liquor 12 generated from the pulping process 10 is delivered to an ultrafiltration membrane unit UF for filtration, resulting in ultrafiltration permeate 14 that passes through the membrane and ultrafiltration concentrate 16 that does not. Other embodiments and technical features of operation B1 are, for example, as described hereinbefore in connection with operation A1 of method 1.

[0032] Method 2 proceeds to operation B2 by nanofiltering the ultrafiltration permeate to obtain a sodium hydroxide aqueous solution as a permeate and a nanofiltration concentrate as a retentate, and mixing the ultrafiltration concentrate obtained in operation B1 with the nanofiltration concentrate to obtain a concentrated black liquor. In some embodiments, as shown in FIG. 4, ultrafiltration permeate 14 is delivered to a nanofiltration membrane unit NF for filtration. NaOH in ultrafiltration permeate 14 can pass through the nanofiltration membrane, thereby separating a sodium hydroxide aqueous solution 24 from ultrafiltration permeate 14. The liquid that does not pass through the nanofiltration membrane becomes a nanofiltration concentrate 26, which includes Na.sub.2S, Na.sub.2SO.sub.4, Na.sub.2SO.sub.3, Na.sub.2CO.sub.3, water, a relatively small amount of NaOH, and a large amount of lignin. After mixing nanofiltration concentrate 26 with ultrafiltration concentrate 16 obtained in operation B1, a concentrated black liquor 27 is obtained. The concentrated black liquor 27 includes a large amount of lignin from both the nanofiltration concentrate 26 and the ultrafiltration concentrate 16.

[0033] Method 2 proceeds to operation B3 by mixing the concentrated black liquor with CO.sub.2 and/or H.sub.2SO.sub.4 to precipitate the lignin from the concentrated black liquor and obtain an inorganic aqueous solution. In some embodiments, as shown in FIG. 4, the concentrated black liquor 27 is delivered to a reactor RX, into which CO.sub.2 and/or H.sub.2SO.sub.4 is introduced to mix and react with the concentrated black liquor 27, resulting in precipitated lignin 28 and an inorganic aqueous solution 31. In some embodiments, the precipitated lignin 28 is separated and delivered to a collector CL. In operation B3, CO.sub.2 reacts with NaOH in the concentrated black liquor 27 to generate Na.sub.2CO.sub.3, and H.sub.2SO.sub.4 reacts with NaOH, Na.sub.2CO.sub.3, and Na.sub.2SO.sub.3 in the concentrated black liquor 27 to generate Na.sub.2SO.sub.4. The resulting Na.sub.2CO.sub.3 and/or Na.sub.2SO.sub.4 are dissolved in the inorganic aqueous solution 31. Other embodiments and technical features of operation B3 are, for example, as described hereinbefore in connection with operation A2 and/or operation A4 of method 1.

[0034] Method 2 proceeds to operation B4 by subjecting the inorganic aqueous solution to a first electrodialysis treatment or a reverse osmosis treatment. In some embodiments, as shown in FIG. 4, the inorganic aqueous solution 31 is delivered to a first electrodialysis device ED1. In the first electrodialysis treatment, Na.sub.2CO.sub.3 in the inorganic aqueous solution 31 is electrolyzed to generate NaOH, producing a NaOH aqueous solution 32 in the cathode chamber of the electrodialysis device ED1, and a remaining inorganic aqueous solution 34 in the anode chamber. The remaining inorganic aqueous solution 34 includes Na.sub.2S, Na.sub.2SO.sub.4, Na.sub.2SO.sub.3, water, and a small amount of Na.sub.2CO.sub.3 that has not been electrolyzed. The obtained NaOH aqueous solution 32 is delivered to a second mixer MX2, which will be described in more detail hereinafter.

[0035] In other embodiments, a reverse osmosis device RO is used instead of the first electrodialysis device ED1 shown in FIG. 4. In the reverse osmosis treatment, permeated water 36 and a remaining inorganic aqueous solution 34 (i.e., a retentate) are obtained. The permeated water 36 obtained from reverse osmosis can be used in the pulp washing process or for other purposes and does not enter the second mixer MX2.

[0036] Method 2 proceeds to operation B5 by mixing the remaining inorganic aqueous solution with a portion of the lignin obtained from operation B3 to obtain a first fuel liquid. In some embodiments, referring to FIG. 4, the remaining inorganic aqueous solution 34 is mixed with a portion of the lignin 28 obtained from the reactor RX to obtain the first fuel liquid 40.

[0037] Method 2 proceeds to operation B6 by processing the first fuel liquid to produce Na.sub.2S, Na.sub.2CO.sub.3, CO.sub.2, and steam. In the processing, the lignin is combusted, and Na.sub.2SO.sub.4 and Na.sub.2SO.sub.3 are reacted and converted into Na.sub.2S, resulting in solid-phase products including Na.sub.2S and Na.sub.2CO.sub.3, along with CO.sub.2 and steam. In some embodiments, operation B6 include acts B6-1, B6-2, and B6-3 described below. In act B6-1, part of the water in the first fuel liquid is heated and evaporated to obtain a concentrated fuel liquid and steam. Referring to FIG. 4, the first fuel liquid 40 is delivered to an evaporator VE, where a portion of the water in the first fuel liquid 40 is evaporated, resulting in concentrated fuel liquid 42 and steam ST. In act B6-2, the concentrated fuel liquid is mixed with a portion of the lignin obtained from operation B3 to obtain a second fuel liquid. Referring to FIG. 4, the concentrated fuel liquid 42 and a portion of the lignin 28 from the reactor RX are delivered to a spray device SP, where the lignin 28 and the concentrated fuel liquid 42 are mixed to obtain the second fuel liquid 44. In act B6-3, the second fuel liquid is combusted to produce Na.sub.2S, Na.sub.2CO.sub.3, CO.sub.2, and steam. Referring to FIG. 4, the second fuel liquid 44 is delivered and sprayed from the spraying device SP into a boiler RB for combustion, resulting in a solid-phase product 46 including Na.sub.2S and Na.sub.2CO.sub.3, as well as CO.sub.2 and steam ST. In some embodiments, act B6-3 includes using a portion of the lignin obtained from operation B3 as fuel to combust the second fuel liquid. Referring to FIG. 4, a portion of the lignin 28 from the reactor RX is used as fuel for the boiler RB to heat and combust the second fuel liquid 44 inside the boiler RB. Other embodiments and technical features of operation B6 are, for example, as described hereinbefore in connection with operation A7 of method 1.

[0038] In some embodiments, operation B3 described hereinbefore includes mixing the CO.sub.2 produced from operation B6 with the concentrated black liquor. Referring to FIG. 4, the CO.sub.2 generated from combustion may be delivered to the reactor RX for mixing and reaction with the concentrated black liquor 27. In some embodiments, referring to FIG. 4, method 2 further includes an operation of delivering the steam ST generated in operation B6 to a power generation device GE, or using the steam ST as a heat source in other processes.

[0039] In some embodiments, after operation B6, method 2 further includes operation B7. In operation B7, water is mixed with the solid-phase product (i.e., Na.sub.2S and Na.sub.2CO.sub.3) generated from operation B6 to produce green liquor. The green liquor includes Na.sub.2S, Na.sub.2CO.sub.3, water, and a relatively small amount of NaOH. In some embodiments, referring to FIG. 4, the solid-phase product 46 and water W are delivered to a first mixer MX1 for mixing, thereby obtaining green liquor 48.

[0040] In some embodiments, after operation B7, method 2 may further include operation B8. In operation B8, the green liquor is subjected to a second electrodialysis treatment, in which the Na.sub.2CO.sub.3 in the green liquor is electrolyzed to generate NaOH. In some embodiments, referring to FIG. 4, the green liquor 48 is delivered to a second electrodialysis device ED2. In the second electrodialysis treatment, a NaOH aqueous solution 50 is obtained in the cathode chamber of the second electrodialysis device ED2, and an H.sub.2SO.sub.4 aqueous solution 52 is obtained in the anode chamber thereof. In some embodiments, H.sub.2SO.sub.4 aqueous solution 52 are delivered to the reactor RX for the reaction.

[0041] In some embodiments, operation B4 described hereinbefore includes performing the first electrodialysis treatment, and method 2 further includes operations B9 and B10. In operation B9, a white liquor is obtained by mixing Na.sub.2S with the NaOH aqueous solution obtained from operation B8, the NaOH aqueous solution obtained from operation B2, and the NaOH aqueous solution obtained from operation B4. In some embodiments, referring to FIG. 4, Na.sub.2S, the NaOH aqueous solution 50 obtained from the second electrodialysis device ED2, the NaOH aqueous solution 24 obtained from the nanofiltration membrane unit NF, and the NaOH aqueous solution 32 obtained from the first electrodialysis device ED1 are transported to the second mixer MX2 for mixing, thereby producing the white liquor 54. In operation B10, the white liquor is used in the pulping process. As shown in FIG. 4, the white liquor 54 is delivered to the pulping process 10 to serve as a raw material for the pulping process.

[0042] In some embodiments, after operation B7, method 2 proceeds to operation B11 of subjecting the green liquor to a causticizing reaction (not shown in FIG. 4) with calcium oxide (CaO) to obtain an aqueous solution including NaOH and Na.sub.2S. The reaction equation of the causticizing reaction is as described hereinbefore in connection with operation A12 of method 1. In some embodiments, method 2 proceeds to operation B11, whereas the second electrodialysis treatment described in operation B8 is not performed. In other embodiments, method 2 may include both operations B11 and B8. For example, a portion of the green liquor generated from operation B7 undergoes operation B8, while another portion thereof undergoes operation B11.

[0043] FIG. 5 is a flowchart illustrating a method 3 for processing black liquor in pulp production, according to some embodiments of the present disclosure. Method 3 includes at least operation C1 to operation C8. FIG. 6 is a schematic diagram illustrating an equipment 301 for implementing method 3, according to some embodiments of the present disclosure.

[0044] Referring to FIG. 5, method 3 includes operation C1 of ultrafiltering black liquor generated in pulp production to obtain an ultrafiltration permeate and an ultrafiltration concentrate. In some embodiments, referring to FIG. 6, black liquor 12 produced from the pulping process 10 is delivered to an ultrafiltration membrane unit UF for filtration, resulting in an ultrafiltration permeate 14 that passes through the membrane, and an ultrafiltration concentrate 16 that does not. Other embodiments and technical features of operation C1 are, for example, as described hereinbefore in connection with operation A1 of method 1.

[0045] Method 3 proceeds to operation C2 by distributing the ultrafiltration concentrate into a first ultrafiltration concentrate and a second ultrafiltration concentrate. In some embodiments, referring to FIG. 6, the ultrafiltration concentrate 16 is delivered to a distributor DB, where the ultrafiltration concentrate 16 is distributed into a first ultrafiltration concentrate 17 and a second ultrafiltration concentrate 19. In some embodiments, the mass (or flow rate) of each of the first and second ultrafiltration concentrate 17, 19 is about 25% to about 75% of the ultrafiltration concentrate 16. In some embodiments, the mass (or flow rate) of each of the first and second ultrafiltration concentrate 17, 19 is about 50% of the ultrafiltration concentrate 16.

[0046] Method 3 proceeds to operation C3 by mixing the first ultrafiltration concentrate with CO.sub.2 and/or H.sub.2SO.sub.4 to precipitate lignin in the ultrafiltration concentrate and obtain a first inorganic aqueous solution. In some embodiments, referring to FIG. 6, the first ultrafiltration concentrate 17 is delivered to a first reactor RX1, and CO.sub.2 and/or H.sub.2SO.sub.4 is introduced into the first reactor RX1 to mix and react with the first ultrafiltration concentrate 17, resulting in precipitated lignin 18 and a first inorganic aqueous solution 20. In some embodiments, the precipitated lignin 18 is separated and delivered to a collector CL. In operation C3, CO.sub.2 reacts with NaOH in the ultrafiltration concentrate to generate Na.sub.2CO.sub.3, and H.sub.2SO.sub.4 reacts with NaOH, Na.sub.2CO.sub.3, and Na.sub.2SO.sub.3 in the ultrafiltration concentrate to generate Na.sub.2SO.sub.4. The resulting Na.sub.2CO.sub.3 and/or Na.sub.2SO.sub.4 are dissolved in the first inorganic aqueous solution. The reaction mechanism of operation C3 is as described hereinbefore in connection with operation A2 of method 1.

[0047] Method 3 proceeds to operation C4 by mixing the first inorganic aqueous solution with the ultrafiltration permeate to obtain a mixture, and nanofiltering the mixture to obtain a NaOH aqueous solution as a permeate and a nanofiltration concentrate as a retentate. In some embodiments, referring to FIG. 6, the first inorganic aqueous solution 20 is mixed with the ultrafiltration permeate 14 to form a mixture 22, which is then delivered to a nanofiltration membrane unit NF for filtration. The mixture 22 includes NaOH, Na.sub.2S, Na.sub.2SO.sub.4, Na.sub.2SO.sub.3, Na.sub.2CO.sub.3, water, and a small amount of lignin. The NaOH in the mixture 22 is capable of permeating through the nanofiltration membrane, and thus is separated from the mixture 22 to obtain a NaOH aqueous solution 24. The liquid that does not pass through the nanofiltration membrane becomes a nanofiltration concentrate 26, which includes Na.sub.2S, Na.sub.2SO.sub.4, Na.sub.2SO.sub.3, Na.sub.2CO.sub.3, water, a relatively small amount of NaOH, and a small amount of lignin.

[0048] Method 3 proceeds to Operation C5 by mixing the nanofiltration concentrate with CO.sub.2 and/or H.sub.2SO.sub.4 to precipitate lignin in the nanofiltration concentrate and obtain a second inorganic aqueous solution. In some embodiments, referring to FIG. 6, the nanofiltration concentrate 26 is delivered to the second reactor RX2, into which CO.sub.2 and/or H.sub.2SO.sub.4 is introduced to mix and react with the nanofiltration concentrate 26, resulting in precipitated lignin 28 and a second inorganic aqueous solution 30. CO.sub.2 reacts with NaOH in the nanofiltration concentrate 26 to generate Na.sub.2CO.sub.3, and H.sub.2SO.sub.4 reacts with NaOH, Na.sub.2CO.sub.3, and Na.sub.2SO.sub.3 in the nanofiltration concentrate 26 to generate Na.sub.2SO.sub.4. The generated Na.sub.2SO.sub.4 and/or Na.sub.2CO.sub.3 is dissolved in the second inorganic aqueous solution 30. In some embodiments, the precipitated lignin 28 is separated and delivered to a collector CL. The chemical reactions of Operation C5 are, for example, as described above in connection with Operation A2 of method 1. The second inorganic aqueous solution includes Na.sub.2S, Na.sub.2SO.sub.4, Na.sub.2SO.sub.3, Na.sub.2CO.sub.3, and water. Other technical features of Operation C5 are, for example, as described above in connection with Operation A4 of method 1.

[0049] Method 3 proceeds to Operation C6 by subjecting the second inorganic aqueous solution to a first electrodialysis treatment or a reverse osmosis treatment. In some embodiments, referring to FIG. 6, the second inorganic aqueous solution 30 is delivered to the first electrodialysis device ED1. In the first electrodialysis treatment, producing a NaOH aqueous solution 32 in the cathode chamber of the electrodialysis device ED1, and a remaining inorganic aqueous solution 34 in the anode chamber. The remaining inorganic aqueous solution 34 includes Na.sub.2S, Na.sub.2SO.sub.4, Na.sub.2SO.sub.3, water, and a small amount of Na.sub.2CO.sub.3 that has not been electrolyzed. The NaOH aqueous solution 32 is delivered to the second mixer MX2.

[0050] In other embodiments, a reverse osmosis device RO is used instead of the first electrodialysis device ED1 shown in FIG. 6. In the reverse osmosis treatment, permeated water 36 and a remaining inorganic aqueous solution 34 (i.e., a retentate) are obtained. The permeated water 36 obtained from reverse osmosis can be used in the pulp washing process or for other purposes and does not enter the second mixer MX2.

[0051] Method 3 proceeds to Operation C7 by mixing the remaining inorganic aqueous solution, the second ultrafiltration concentrate obtained from operation C2, and a portion of the lignin obtained from operation C3 and/or operation C5 to obtain a first fuel liquid. In some embodiments, referring to FIG. 6, the remaining inorganic aqueous solution 34 is mixed with the second ultrafiltration concentrate 19 from distributor DB and the lignin 18, 28 from collector CL (e.g., a portion of the products from the first and/or second reactors RX1, RX2 in Operations C3 and/or C5), to obtain the first fuel liquid 40.

[0052] Method 3 proceeds to operation C8 by processing the first fuel liquid to produce Na.sub.2S, Na.sub.2CO.sub.3, CO.sub.2, and steam. In the processing, the lignin is combusted, and Na.sub.2SO.sub.4 and Na.sub.2SO.sub.3 are reacted and converted into Na.sub.2S, resulting in solid-phase products including Na.sub.2S and Na.sub.2CO.sub.3, along with CO.sub.2 and steam. In some embodiments, operation C8 includes the acts C8-1, act C8-2, and act C8-3 described below. In act C8-1, part of the water in the first fuel liquid is heated and evaporated to obtain a concentrated fuel liquid and steam. Referring to FIG. 6, the first fuel liquid 40 is delivered to an evaporator VE, where part of the water is evaporated, resulting in concentrated fuel liquid 42 and steam ST. In act C8-2, the concentrated fuel liquid is mixed with a portion of the lignin obtained in operation C3 and/or C5 to obtain a second fuel liquid. Referring to FIG. 6, the concentrated fuel liquid 42 and the lignin 18, 28 in the collector CL (e.g., a portion of the products from the first and/or second reactors RX1, RX2 in operations C3 and/or C5) are fed to the spray device SP, where the concentrated fuel liquid 42 and the lignin 18, 28 are mixed to obtain the second fuel liquid 44. In act C8-3, the second fuel liquid is combusted to produce Na.sub.2S, Na.sub.2CO.sub.3, CO.sub.2, and steam ST. Referring to FIG. 6, the second fuel liquid 44 is delivered and sprayed from the spraying device SP into a boiler RB for combustion, resulting in a solid-phase product 46 including Na.sub.2S and Na.sub.2CO.sub.3, as well as CO.sub.2 and steam ST. Other embodiments and technical features of operation C8 are, for example, as described hereinbefore in connection with operation A7 of method 1 above.

[0053] In some embodiments, act C8-3 includes using a portion of the lignin obtained from operation C3 and/or C5 as fuel to combust the second fuel liquid. Referring to FIG. 6, lignin 18, 28 in the collector CL (e.g., a portion of the products from the first and/or second reactors RX1, RX2 in operations C3 and/or C5) is used as fuel for the boiler RB to heat and combust the second fuel liquid 44 inside the boiler RB.

[0054] In some embodiments, operation C3 described hereinbefore includes mixing CO.sub.2 generated in operation C8 with the first ultrafiltration concentrate. Referring to FIG. 6, the CO.sub.2 generated from combustion may be fed into the first reactor RX1 to mix and react with the first ultrafiltration concentrate 17. In yet some embodiments, operation C5 described hereinbefore includes mixing CO.sub.2 generated in operation C8 with the nanofiltration concentrate. Referring to FIG. 6, the CO.sub.2 generated from combustion is supplied to the second reactor RX2 as its CO.sub.2 source to mix and react with the nanofiltration concentrate 26. Referring again to FIG. 6, method 3 further includes an operation of delivering the steam ST produced in operation C8 to a power generation device GE, or using the steam ST as a heat source in other processes.

[0055] In some embodiments, after operation C8, method 3 further include operation C9. In operation C9, water is mixed with the solid-phase product 46 (i.e., Na.sub.2S and Na.sub.2CO.sub.3) produced from operation C8 to generate green liquor 48. The green liquor 48 include Na.sub.2S, Na.sub.2CO.sub.3, water, and a relatively small amount of NaOH. In some embodiments, as shown in FIG. 6, the solid-phase product 46 and water W are delivered to a first mixer MX1 for mixing, thereby obtaining the green liquor 48.

[0056] In some embodiments, after operation C8, method 3 further includes operation C9. In operation C9, water is mixed with the solid-phase product (i.e., Na.sub.2S and Na.sub.2CO.sub.3) generated in operation C8 to produce green liquor. The green liquor includes Na.sub.2S, Na.sub.2CO.sub.3, water, and a relatively small amount of NaOH. In some embodiments, referring to FIG. 6, the solid-phase product 46 and water W are delivered to a first mixer MX1 for mixing, thereby producing green liquor 48.

[0057] In some embodiments, after operation C9, method 3 further includes operation C10. In operation C10, the green liquor is subjected to a second electrodialysis treatment, in which the Na.sub.2CO.sub.3 in the green liquor is electrolyzed to generate NaOH. In some embodiments, referring to FIG. 6, green liquor 48 is delivered to a second electrodialysis device ED2. In the second electrodialysis treatment, a NaOH aqueous solution 50 is obtained in the cathode chamber of the second electrodialysis device ED2, and an H.sub.2SO.sub.4 aqueous solution 52 is obtained in the anode chamber thereof. In some embodiments, the H.sub.2SO.sub.4 aqueous solution 52 may be delivered to the first reactor RX1 and/or the second reactor RX2 for reaction.

[0058] In some embodiments, the operation C6 described hereinbefore includes performing the first electrodialysis treatment, and method 3 further includes operations C11 and C12. In operation C11, Na.sub.2S is mixed with the NaOH aqueous solution obtained from operation C10, the NaOH aqueous solution obtained from operation C4, and the NaOH aqueous solution obtained from operation C6 to produce white liquor. In some embodiments, referring to FIG. 6, Na.sub.2S, the NaOH aqueous solution 50 obtained from the second electrodialysis device ED2, the NaOH aqueous solution 24 obtained from the nanofiltration unit NF, and the NaOH aqueous solution 32 obtained from the first electrodialysis device ED1 are delivered to a second mixer MX2 for mixing, thereby producing white liquor 54. In operation C12, the white liquor is used in the pulping process. Referring to FIG. 6, white liquor 54 is delivered to the pulping process 10 as a raw material.

[0059] In some embodiments, after operation C9, method 3 proceeds to operation C13 by subjecting the green liquor to a causticizing reaction (not shown in FIG. 6) with calcium oxide (CaO) to obtain an aqueous solution including NaOH and Na.sub.2S. Other embodiments and technical features of operation C13 are, for example, as described hereinbefore in connection with operation A12 in method 1.

[0060] In some embodiments that include operation C13, method 3 does not perform the second electrodialysis treatment described in operation C10. However, in other embodiments, method 3 may include both operation C13 and operation C10. For example, a portion of the green liquor produced in operation C9 undergoes operation C10, while another portion thereof undergoes operation C13.

[0061] According to various embodiments of methods 1, 2, and 3, referring again to FIGS. 2, 4, and 6, the NaOH aqueous solution 24 separated by the nanofiltration unit NF and the NaOH aqueous solution 32 separated by the first electrodialysis device ED1 both contain a large amount of water. As a result, the total amount of water entering the evaporator VE is reduced, thereby effectively decreasing the energy required by the evaporator VE and the boiler RB.

[0062] In addition, according to some embodiments, both the NaOH aqueous solution and the H.sub.2SO.sub.4 aqueous solution obtained from the second electrodialysis treatment can be reused. Therefore, various embodiments of methods 1, 2, and 3 can substantially achieve zero discharge to meet environmental protection requirements.

[0063] According to various embodiments, a method for processing black liquor in pulp production is disclosed. The method includes operations of: (A1) ultrafiltering black liquor generated in pulp production to obtain an ultrafiltration permeate and an ultrafiltration concentrate, wherein the black liquor comprises at least lignin, sodium hydroxide, sodium sulfide, sodium sulfate, sodium sulfite, sodium carbonate, and water; (A2) mixing the ultrafiltration concentrate with carbon dioxide and/or sulfuric acid to precipitate lignin in the ultrafiltration concentrate and obtain a first inorganic aqueous solution, wherein the ultrafiltration concentrate reacts with carbon dioxide and/or sulfuric acid to produce sodium carbonate and/or sodium sulfate dissolved in the first inorganic aqueous solution; (A3) mixing the first inorganic aqueous solution with the ultrafiltration permeate to obtain a mixture, and nanofiltering the mixture to obtain a sodium hydroxide aqueous solution as a permeate and a nanofiltration concentrate as a retentate; (A4) mixing the nanofiltration concentrate with carbon dioxide and/or sulfuric acid to precipitate lignin in the nanofiltration concentrate and obtain a second inorganic aqueous solution, wherein the nanofiltration concentrate reacts with carbon dioxide and/or sulfuric acid to produce sodium carbonate and/or sodium sulfate dissolved in the second inorganic aqueous solution; (A5) subjecting the second inorganic aqueous solution to a first electrodialysis treatment or a reverse osmosis treatment, wherein: while the first electrodialysis treatment is performed, sodium carbonate in the second inorganic aqueous solution is electrolyzed to produce sodium hydroxide, and a sodium hydroxide aqueous solution is obtained at a cathode of the first electrodialysis treatment, and a remaining inorganic aqueous solution is obtained at an anode of the first electrodialysis treatment; while the reverse osmosis treatment is performed, water is obtained as a permeate and a remaining inorganic aqueous solution is obtained as a retentate; (A6) mixing the remaining inorganic aqueous solution with a portion of the lignin obtained from operation (A2) and/or operation (A4) to obtain a first fuel liquid; and (A7) processing the first fuel liquid to produce sodium sulfide, sodium carbonate, carbon dioxide, and steam.

[0064] In some embodiments, operation (A7) includes the acts of: (A7-1) evaporating a portion of water in the first fuel liquid to obtain a concentrated fuel liquid and steam; (A7-2) mixing the concentrated fuel liquid with a portion of the lignin obtained from operation (A2) and/or operation (A4) to obtain a second fuel liquid; and (A7-3) combusting the second fuel liquid to generate sodium sulfide, sodium carbonate, carbon dioxide, and steam.

[0065] In some embodiments, operation (A2) includes mixing the ultrafiltration concentrate with carbon dioxide produced from operation (A7); and/or operation (A4) includes mixing the nanofiltration concentrate with carbon dioxide produced from operation (A7).

[0066] In some embodiments, after operation (A7), the method further includes operations of: (A8) mixing water with sodium sulfide and sodium carbonate produced from operation (A7) to obtain a green liquor; and (A9) subjecting the green liquor to a second electrodialysis treatment, electrolyzing the sodium carbonate in the green liquor to produce sodium hydroxide, wherein a sodium hydroxide aqueous solution is obtained at a cathode of the second electrodialysis treatment, and a sulfuric acid aqueous solution is obtained at an anode of the second electrodialysis treatment.

[0067] In some embodiments, the method further includes operations of: (A10) mixing sodium sulfide with the sodium hydroxide aqueous solution obtained from operation (A9), the sodium hydroxide aqueous solution obtained from operation (A3), and the sodium hydroxide aqueous solution obtained from operation (A5) to obtain a white liquor; and (A11) applying the white liquor to the pulp production; wherein operation (A5) includes performing the first electrodialysis treatment.

[0068] In some embodiments, the method further includes transporting the steam produced in operation (A7) to a power generation device.

[0069] In some embodiments, after operation (A7), the method further includes operations of: (A8) mixing water with sodium sulfide and sodium carbonate produced from operation (A7) to obtain a green liquor; and (A12) subjecting the green liquor to a causticizing reaction with calcium oxide to obtain an aqueous solution including sodium hydroxide and sodium sulfide.

[0070] According to yet various embodiments, a method for processing black liquor in pulp production is disclosed. The method includes operations of: (B1) ultrafiltering black liquor generated in pulp production to obtain an ultrafiltration permeate and an ultrafiltration concentrate, wherein the black liquor includes at least lignin, sodium hydroxide, sodium sulfide, sodium sulfate, sodium sulfite, sodium carbonate, and water; (B2) nanofiltering the ultrafiltration permeate to obtain a sodium hydroxide aqueous solution as a permeate and a nanofiltration concentrate as a retentate, and mixing the ultrafiltration concentrate with the nanofiltration concentrate to obtain a concentrated black liquor; (B3) mixing the concentrated black liquor with carbon dioxide and/or sulfuric acid to precipitate lignin in the concentrated black liquor and obtain an inorganic aqueous solution, wherein the concentrated black liquor reacts with carbon dioxide and/or sulfuric acid to produce sodium carbonate and/or sodium sulfate dissolved in the inorganic aqueous solution; (B4) subjecting the inorganic aqueous solution to a first electrodialysis treatment or a reverse osmosis treatment, wherein: while the first electrodialysis treatment is performed, sodium carbonate in the inorganic aqueous solution is electrolyzed to produce sodium hydroxide, a sodium hydroxide aqueous solution is obtained at a cathode of the first electrodialysis treatment, and a remaining inorganic aqueous solution is obtained at an anode of the first electrodialysis treatment; while the reverse osmosis treatment is performed, water is obtained as a permeate and a remaining inorganic aqueous solution is obtained as a retentate; (B5) mixing the remaining inorganic aqueous solution with a portion of the lignin obtained from operation (B3) to obtain a first fuel liquid; and (B6) processing the first fuel liquid to produce sodium sulfide, sodium carbonate, carbon dioxide, and steam.

[0071] In some embodiments, operation (B6) includes acts of: (B6-1) evaporating a portion of water in the first fuel liquid to obtain a concentrated fuel liquid and steam; (B6-2) mixing the concentrated fuel liquid with a portion of the lignin obtained from operation (B3) to obtain a second fuel liquid; (B6-3) combusting the second fuel liquid to produce sodium sulfide, sodium carbonate, carbon dioxide, and steam.

[0072] In some embodiments, operation (B3) includes mixing the concentrated black liquor with the carbon dioxide produced from operation (B6).

[0073] In some embodiments, after operation (B6), the method further includes operations of: (B7) mixing water with sodium sulfide and sodium carbonate produced from operation (B6) to obtain a green liquor; and (B8) subjecting the green liquor to a second electrodialysis treatment, electrolyzing the sodium carbonate in the green liquor to produce sodium hydroxide, wherein a sodium hydroxide aqueous solution is obtained at a cathode of the second electrodialysis treatment, and a sulfuric acid aqueous solution is obtained at an anode of the second electrodialysis treatment.

[0074] In some embodiments, the method further includes operations of: (B9) mixing sodium sulfide with the sodium hydroxide aqueous solution obtained from operation (B8) and the sodium hydroxide aqueous solution obtained from operation (B2) and the sodium hydroxide aqueous solution obtained from operation (B4) to obtain a white liquor; and (B10) applying the white liquor to the pulp production; wherein operation (B4) includes performing the first electrodialysis treatment.

[0075] In some embodiments, the method further includes transporting the steam produced from operation (B6) to a power generation device.

[0076] In some embodiments, after operation (B6), the method further includes operations of: (B7) mixing water with sodium sulfide and sodium carbonate produced from operation (B6) to obtain a green liquor; and (B11) subjecting the green liquor to a causticizing reaction with calcium oxide to obtain an aqueous solution including sodium hydroxide and sodium sulfide.

[0077] According to yet various embodiments, a method for processing black liquor in pulp production is disclosed. The method includes operations of: (C1) ultrafiltering black liquor generated in pulp production to obtain an ultrafiltration permeate and an ultrafiltration concentrate, wherein the black liquor includes at least lignin, sodium hydroxide, sodium sulfide, sodium sulfate, sodium sulfite, sodium carbonate, and water; (C2) distributing the ultrafiltration concentrate into a first ultrafiltration concentrate and a second ultrafiltration concentrate; (C3) mixing the first ultrafiltration concentrate with carbon dioxide and/or sulfuric acid to precipitate lignin in the first ultrafiltration concentrate and obtain a first inorganic aqueous solution, wherein the first ultrafiltration concentrate reacts with carbon dioxide and/or sulfuric acid to produce sodium carbonate and/or sodium sulfate dissolved in the first inorganic aqueous solution; (C4) mixing the first inorganic aqueous solution with the ultrafiltration permeate to obtain a mixture, and nanofiltering the mixture to obtain a sodium hydroxide aqueous solution as a permeate and a nanofiltration concentrate as a retentate; (C5) mixing the nanofiltration concentrate with carbon dioxide and/or sulfuric acid to precipitate lignin in the nanofiltration concentrate and obtain a second inorganic aqueous solution, wherein the nanofiltration concentrate reacts with carbon dioxide and/or sulfuric acid to produce sodium carbonate and/or sodium sulfate dissolved in the second inorganic aqueous solution; (C6) subjecting the second inorganic aqueous solution to a first electrodialysis treatment or a reverse osmosis treatment, wherein: while the first electrodialysis treatment is performed, sodium carbonate in the second inorganic aqueous solution is electrolyzed to produce sodium hydroxide, and a sodium hydroxide aqueous solution is obtained at a cathode of the first electrodialysis treatment, and a remaining inorganic aqueous solution is obtained at an anode of the first electrodialysis treatment; while the reverse osmosis treatment is performed, water is obtained as a permeate and a remaining inorganic aqueous solution is obtained as a retentate; (C7) mixing the remaining inorganic aqueous solution, the second ultrafiltration concentrate obtained from operation (C2), and a portion of the lignin obtained from operation (C3) and/or operation (C5) to obtain a first fuel liquid; and (C8) processing the first fuel liquid to produce sodium sulfide, sodium carbonate, carbon dioxide, and steam.

[0078] In some embodiments, operation (C8) includes acts of: (C8-1) evaporating a portion of water in the first fuel liquid to obtain a concentrated fuel liquid and steam; (C8-2) mixing the concentrated fuel liquid with a portion of the lignin obtained from operation (C3) and/or operation (C5) to obtain a second fuel liquid; (C8-3) combusting the second fuel liquid to produce sodium sulfide, sodium carbonate, carbon dioxide, and steam.

[0079] In some embodiments, operation (C3) includes mixing the ultrafiltration concentrate with the carbon dioxide produced from operation (C8); and/or operation (C5) includes mixing the nanofiltration concentrate with the carbon dioxide produced from operation (C8).

[0080] In some embodiments, after operation (C8), the method further includes operations of: (C9) mixing water with the sodium sulfide and sodium carbonate produced from operation (C8) to obtain a green liquor; and (C10) subjecting the green liquor to a second electrodialysis treatment, electrolyzing the sodium carbonate in the green liquor to form sodium hydroxide, wherein a sodium hydroxide aqueous solution is obtained at a cathode of the second electrodialysis treatment, and a sulfuric acid aqueous solution is obtained at an anode of the second electrodialysis treatment.

[0081] In some embodiments, the method further includes operations of: (C11) mixing sodium sulfide with the sodium hydroxide aqueous solution obtained from operation (C10) and the sodium hydroxide aqueous solution obtained from operation (C4) and the sodium hydroxide aqueous solution obtained from operation (C6) to obtain a white liquor; and (C12) applying the white liquor to the pulp production; wherein operation (C6) includes performing the first electrodialysis treatment.

[0082] In some embodiments, after operation (C8), the method further includes operations of: (C9) mixing water with the sodium sulfide and sodium carbonate produced from operation (C8) to obtain a green liquor; and (C13) subjecting the green liquor to a causticizing reaction with calcium oxide to obtain an aqueous solution including sodium hydroxide and sodium sulfide.

[0083] The foregoing outlines features of several embodiments so that those skilled in the art may better understand the aspects of the present disclosure. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure.