Method for recovering chemicals and by-products from high-sulphidity pulping liquors

Abstract

A method used in connection with the recovery of pulping chemicals from spent pulping liquor produced by kraft-type pulping at very high sulphidity. In the method, spent pulping liquor is acidified to a relatively low pH which converts a most or all of the sulphide and hydrosulphide in the liquor to hydrogen sulfide. Sulphur containing gases released from the acidification of the spent pulping liquor, together with other sulphur gases collected at the pulp mill, are converted into an acid compound. This acid compound is employed as an acidification agent in the acidification of the spent pulping liquor. The amount of acid compound generated by the conversion of sulphur containing gases may be sufficient to provide most, if not all, of the acid needed for the acidification of the spent pulping liquor.

Claims

1. A method to be used in connection with recovery of pulping chemicals from spent pulping liquor produced by kraft-type alkaline pulping at a sulphidity in a range of 50 percent to 100 percent, the method comprising: a) acidifying at least part of spent pulping liquor in one or more stages to a pH low enough to convert both hydrosulphide ions and the sulphide ions in the spent pulping liquor into hydrogen sulphide, b) converting to an acid compound sulphur containing gases released in the acidification process of step a), wherein the sulphur containing gases comprise hydrogen sulphide; c) the acid compound generated in step b) is employed in step a) and the amount of the acid compound generated in step b) is sufficient in quantity to provide at least most of the acid required in step a); d) after the release of the sulphur containing gases in step b), combusting the spent pulping liquor containing sodium sulphate in a recovery boiler; e) generating smelt containing sodium sulfide from the combustion in step d); f) dissolving the smelt in a liquid, and g) using the dissolved smelt to produce a pulping liquor.

2. The method as in claim 1, wherein the pH reached in the acidification step is below 7.

3. The method as in claim 1, wherein, in conjunction with the acidification of the spent pulping liquor in step a), one or more by-products are partially or totally recovered from the liquor and/or one or more non-process elements are partially or totally removed from the liquor.

4. The method as in claim 1, wherein the acidification of the spent pulping liquor is carried out in a stepwise manner and one or more by-products are partially or totally recovered and/or one or more non-process elements are partially or totally removed.

5. The method as in claim 1 the spent pulping liquor is combusted in a chemical-recovery boiler after step a).

6. The method as in claim 5, further comprising applying an evaporation process to the spent pulping liquor before step a).

7. The method as in claim 1, wherein the spent pulping liquor is from a pulping stage and is split into two or more streams and at least one by-produce or non-process element is removed from one of the streams of the spent pulping liquor.

8. The method as in claim 1, wherein the spent pulping liquor is from a pulping stage, and the spent pulping liquor is split into streams, and steps a), b) and c) are applied to at least one of the streams but not to all of these streams.

9. A method to be used in connection with recovery of pulping chemicals from spent pulping liquor produced by kraft-type alkaline pulping at a sulphidity of 50% or greater and complements a kraft pulping process, the method comprising: a) acidifying at least part of spent pulping liquor in one or more stages to a pH low enough to convert both hydrosulphide ions and the sulphide ions in the spent pulping liquor into hydrogen sulphide, b) converting to an acid compound sulphur containing gases released in the acidification process of step a), wherein the sulphur containing gases comprise hydrogen sulphide, and c) the acid compound generated in step b) is employed in step a) and the amount of the acid compound generated in step b) is sufficient in quantity to provide at least most of the acid required in step a), at least part of the chemical-recovery process being common to both the very high sulphidity process and the kraft pulping process.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present new method is described in more detail with reference to the drawings, FIGS. 1-3, each depicting one embodiment of the invention. The numbers and letters in the figures refer to the following streams and processing stages:

(2) 1. Raw material for pulping, such as wood chips or straw 2. Washed pulp 3. Spent pulping liquor 4. Spent pulping liquor 5. Spent pulping liquor 6. Spent pulping liquor 7. Processed spent pulping liquor/slurry 8. Processed spent pulping liquor/slurry 9. Concentrated H.sub.2SO.sub.4 10. Processed spent pulping liquor/slurry 11. Concentrated H.sub.2SO.sub.4 12. Concentrated H.sub.2SO.sub.4 13. Sulphur containing gases 14. Sulphur containing gases 15. Sulphur containing gases 16. Sulphur containing gases 17. Sulphur containing gases 18. Sulphur containing gases 19. Sulphur containing gases 20. Auxiliary fuel 21. Smelt 22. Water and/or aqueous solution 23. Liquor stream from smelt dissolving stage 24. Regenerated pulping liquor 25. Lignin slurry 26. Filtrate 27. Processed filtrate/slurry 28. Filtrate from lignin washing 29. Washed lignin 30. Make-up H.sub.2SO.sub.4 31. Spent pulping liquor A. Stage encompassing pulping and pulp washing B1. Evaporation stage B2. Evaporation stage C1. Acidification stage C2. Acidification stage D. H.sub.2SO.sub.4 production plant E. Flashing and/or stripping stage F. Recovery boiler G. Smelt dissolving stage H. Causticization process, including lime slaking, filtration and lime-stone mud washing I. Lime kiln J. Filtration stage K. Washing stage

DESCRIPTION OF PREFERRED EMBODIMENTS

(3) The embodiment depicted in FIG. 1 does not incorporate recovery of by-products or purging of non-process elements in conjunction with the acidification of the spent pulping liquor. The raw material for the pulping process (1), e.g. wood in the form of chips, is subjected to kraft-type pulping at around 80% sulphidity in stage A, which also includes the pulp-washing operation. Washed pulp (2) exits the stage and is further processed as necessary. The spent pulping liquor (3) exiting stage A is concentrated by evaporation in stage B1 before being subjected to acidification to a pH below 6 in stage C1. The acidifying agent (11) is concentrated H.sub.2SO.sub.4, most, or all, of which is produced on site in stage D. As a result of lignin precipitation, the spent pulping liquor is in the form of dense slurry after the acidification stage (C1). This slurry (8) is subjected to a flashing and/or stripping stage (E) in order to maximize release of the molecular H.sub.2S formed in the acidification stage. Sulphur containing gases (13, 17), comprised particularly of H.sub.2S, are collected from the acidification stage (C1) and the flashing/stripping stage (E), and are combined with sulphur containing CNCG gases (15, 18) from the evaporation stage (B1) and the pulping stage (A). The combined sulphur-gas stream (19) is converted into concentrated H.sub.2SO.sub.4 in the H.sub.2SO.sub.4 production plant (D) known per se. Make-up H.sub.2SO.sub.4 (30) is inputted to the H.sub.2SH.sub.2SO.sub.4 cycle as necessary. The spent pulping slurry (10) exiting the flashing/stripping stage (E) is combusted in a recovery boiler (F) of similar type to the boiler employed in the conventional kraft recovery process. As in the conventional process, fly-ash is separated from the flue gas by e.g. an electrostatic precipitator and recycled. The main component in the smelt (21) exiting the boiler is Na.sub.2S, while another significant component is Na.sub.2CO.sub.3. Complete reduction of Na.sub.2SO.sub.4 to Na.sub.2S is not expected in the recovery furnace, so the smelt generally contains some Na.sub.2SO.sub.4, as well, not to mention other minor components. Water and/or an aqueous solution such as weak white liquor (22) is used to dissolve the smelt in stage G. The liquor so formed (23) is subjected to causticization in stage H in order to convert the greater part of its Na.sub.2CO.sub.3 into NaOH. The causticizing capacity, and so the capacity of the lime kiln (I), are much smaller than those of the corresponding conventional kraft recovery process. After causticization, the liquor is ready for reuse as the pulping liquor (24) in stage A.

(4) Another embodiment, exploiting a pulping sulphidity at or near 100% sulphidity, has many features in common with that depicted in FIG. 1. In addition to the higher sulphidity level, significant differences compared to the embodiment of FIG. 1 are: acidification is carried out to a lower pH, e.g. pH 3 more H.sub.2SO.sub.4 make-up is required; at least part of the sulphur consumed in Reactions 5, 6 and 7 needs to be made up the causticization stage and the lime cycle are eliminated.

(5) The embodiment depicted in FIG. 2 differs from that depicted in FIG. 1 in that the concentrated spent pulping liquor (4) is split into two streams (5, 6). Stream 5 is processed in the same way as in the embodiment of FIG. 1. Stream 6 is not subjected to acidification but is led instead directly to the recovery boiler (F), where it is combusted either as a separate stream or as mixed with the concentrated spent pulping slurry (10). The split of the spent pulping liquor into two streams (5, 6) is such that the level of SO.sub.2 in the flue gas of the recovery boiler remains at an acceptable level.

(6) The embodiment depicted in FIG. 3 incorporates recovery of by-product lignin. In many other respects it is similar to the embodiment depicted in FIG. 2. After the first evaporation stage (B1), the spent pulping liquor (4) is split into two streams (5, 6) in the same way as in the embodiment of FIG. 2. Stream 5 is first acidified to a pH of around 9 in stage C1 using concentrated H.sub.2SO.sub.4 (11) from the H.sub.2SH.sub.2SO.sub.4 cycle. The lignin slurry (25) exiting stage C1 is subjected to filtration in stage J. The filtrate (26) from stage J is acidified further to a pH below 6 in stage C2 using concentrated H.sub.2SO.sub.4 (12) from the H.sub.2SH.sub.2SO.sub.4 cycle. The sulphur containing gases (13, 14) exiting stages C1 and C2 are collected to be part of the sulphur-gas stream that is fed to the H.sub.2SO.sub.4 production plant (D). From stage C2, the processed filtrate (27), in the form of slurry, is mixed with the non-acidified stream (6) of spent pulping liquor. The pH of the mixed spent pulping liquor stream (7) is only a little lower than that of the non-acidified spent pulping liquor (6). Solids in stream (27) re-dissolve when the stream is mixed with the non-acidified liquor (6). Lignin filter cake from stage J is washed in at least two steps in stage K, thus yielding the desired by-productwashed lignin (29). At least one washing step is conducted under acidic conditions using H.sub.2SO.sub.4. Filtrate (28) from the lignin-washing stage K is led to the evaporation stage B2. Other features of the embodiment depicted in FIG. 3 are similar to the corresponding features of the embodiment depicted in FIG. 2. The higher the extent of withdrawal of by-product lignin, the more likely is the need for auxiliary fuel (20) in the recovery boiler (F). Obviously the extent of withdrawal of lignin can be decreased by bypassing the first acidification stage (C1), i.e. by leading part (31) of stream 5 directly to the second acidification step (C2).

EXAMPLE

(7) Mass flows of the main components in various streams of an example recovery process incorporating the new method are given in the following Tables 1-5. The example recovery process does not incorporate withdrawal of by-products or non-process elements in conjunction with the acidification of the spent pulping liquor. The acidification process is applied to the whole stream of spent pulping liquor. Where applicable, the flows are compared to those of a reference conventional kraft recovery process. In the case of the new method, pulping of softwood is carried out at 80% sulphidity and 17.5% EA (effective alkali as NaOH on wood), while, in the reference process, softwood pulping is carried out at 35% sulphidity and 19.5% EA. Other key assumptions are: (1) Na.sub.2S is completely hydrolyzed in the pulping liquor, i.e. sulphide is completely converted to hydrosulphide according to Reaction 2, (2) the reduction efficiency in the recovery furnace is 95% and (3) the causticization degree is 85%. The unit of mass flow is kg per air-dried metric ton of pulp (kg/ADt).

(8) TABLE-US-00001 TABLE 1 Mass flows of liquor components after pulping, kg/ADt of pulp Sulphidity 35% Sulphidity 80% EA 19.5% EA 17.5% Conventional High-sulphidity pulping kraft process and new recovery process NaOH 65 38 NaHS 92 277 Na.sub.2CO.sub.3 71 27 Na.sub.2SO.sub.4 15 44 Na.sub.2S.sub.2O.sub.3 1 9 Na in lignin 53 58 Na in acids 139 139 S in lignin 10 18 Organics 1140 1140 Total solids 1585 1750

(9) TABLE-US-00002 TABLE 2 Mass flows of liquor components after acidification, kg/ADt of pulp Sulphidity 80% EA 17.5% High-sulphidity pulping and new recovery process Na.sub.2SO.sub.4 819 Na.sub.2S.sub.2O.sub.3 9 Na in lignin 0 Na in acids 92 S in lignin 18 Organics 1140 Total solids 2080

(10) TABLE-US-00003 TABLE 3 Mass flows of components in the H.sub.2SH.sub.2SO.sub.4 cycle, kg/ADt of pulp Sulphidity 80% EA 17.5% High-sulphidity pulping and new recovery process S in H.sub.2S from acidification 158 S in other collected CNCG gases 7 H.sub.2SO.sub.4 produced from S gases 505 H.sub.2SO.sub.4 make-up 30 H.sub.2SO.sub.4 consumed in acidification 535

(11) TABLE-US-00004 TABLE 4 Mass flows of compounds in recovery-boiler smelts, kg/ADt of pulp Sulphidity 35% Sulphidity 80% EA 19.5% EA 17.5% Conventional High-sulphidity pulping kraft process and new recovery process Na.sub.2S 158 456 Na.sub.2SO.sub.4 15 44 Na.sub.2CO.sub.3 470 181 Total 645 680

(12) TABLE-US-00005 TABLE 5 Mass flows of compounds in the regenerated pulping liquors, kg/ADt of pulp Sulphidity 35% Sulphidity 80% EA 19.5% EA 17.5% Conventional High-sulphidity pulping kraft process and new recovery process Na.sub.2S 158 456 NaOH 302 116 Na.sub.2SO.sub.4 15 44 Na.sub.2CO.sub.3 71 27 Total solids 545 645

(13) The embodiments of the present invention are not limited to those mentioned or described herein.