AU5317700A - Bleaching pulp with high-pressure O2 - Google Patents

Description

WO 00/75420 PCT/US00/15263 BLEACHING PULP WITH HIGH-PRESSURE O Background of the Invention This invention relates to bleaching wood pulp, in kraft and other wood pulping 5 processes, to be manufactured into paper products (including paper, tissue, paper board, bleached pulp bales, wet laps, rolls, and slush to be de-inked). For example, in a kraft process for producing paper from wood pulp, chemical (NaOH and Na 2 S) treatment breaks up the wood chips or other particles leaving a product having cellulose and some residual lignin in tight association. To 10 produce pulp suitable for manufacturing paper products (including tissue, paper board, bleached pulp bales, wet lap, rolls, and slush to be de-inked), the majority of the remaining lignin is removed and the cellulose and any residual lignin are bleached, while minimizing degradation of the cellulose. Steps in the standard kraft process include: (a) alkali and sulfur treatment of the wood pulp to produce treated 15 wood pulp suitable for further delignification; and (b) bleaching the treated wood pulp. The by-products of step (a), above, are processed by a chemical recovery cycle which includes an unbleached stock wash and screening, evaporation, the use of a chemical recovery boiler, and recausitcizing. In some cases, an 02 delignification step is used as part of step (a) (before the bleaching step), the filtrate from this 20 delignification step being sent to the chemical recovery system. Insufficient lignin removal during such processes yields a product with undesirable color, brightness and cleanliness. Over-bleaching results in cellulose degradation and weak cellulose fibers, yielding a weak paper, tissue, paper board, bleached pulp bales, wet lap, rolls and slush to be de-inked. Thus, there is a trade-off 25 between lignin removal and bleaching on the one hand, and, on the other hand, protecting cellulose fibers so as to ensure a strong, stable, bright paper, tissue, paper board, bleached pulp bales, wet lap, rolls and slush to be de-inked. Wood pulp to be manufactured into paper products can be treated in a number of ways other than kraft processes, including various sulfite processes, 30 alcohol-based processes, various sulphite processes, MgO processes, and waste paper de-inking processes. Any of these processes typically produces a pulp that has a lignin content of from 0-6%. No matter which way the pulp is treated, it typically must be bleached before it can enter a paper mill and be made into a product suitable WO 00/75420 PCT/US00/15263 for white printing paper, tissue and white packing paper. When waste paper is de-inked to be used in printing paper, tissue or board manufacture, it is difficult to restore adequate brightness and cleanliness to the paper. Typically, an NaOH and sodium hydrosulfide and/or NaOH and H 2 02 mixture is used 5 for this purpose. Often "stickies" (e.g., small pills or balls which form from extraneous matter such as envelope adhesives) pose a problem in de-inking and recycling office waste. Environmental impact is another important consideration for pulp production. It is increasingly important to control discharge of chlorinated and other 10 by-products, as well to minimize the use of energy and raw materials. Summary of the Invention We have discovered a powerful high pressure oxygen (02) bleaching process for wood cellulose that has been pre-treated and has a lignin content below 6%. 15 A. Strong 02 Bleaching of Pretreated Pulp Without Chemical Recovery In general, wood pulp having a kappa number of between 10 and 30 is treated at low (e.g., between 2.0 and 6.0%) consistency and pH between 2.0 and 4.0 20 (preferably pH 2.0-3.0), at 110-185EF for 15 to 60 minutes. Pulp treated by this step is clean, well washed, and suitable for oxygen bleaching to a lower kappa number without excessive degradation of cellulose. A powerful oxygen bleaching step is then performed. The filtrate from this bleaching step is not processed by any chemical recovery system(s) that may be used to recover chemicals used in the pulp pre 25 treatment. The pretreatment can be essentially any process which results in wood pulp with the specified lignin content and is otherwise suitable for bleaching. One important example is, of course, a traditional kraft process as described above, in which the wood is converted to pulp suitable for manufacturing a paper or tissue 30 product with kraft steps (a) and (b) described in the background above. The 2 WO 00/75420 PCT/US00/15263 invention focuses on step (b), which includes an acidification step in which wood pulp having a kappa number of between 10 and 30 from kraft step (a) is bleached under the conditions described above. This step removes at least 10% of the hexeneuronic acids in the treated pulp, some metals, some bound sodium, and soluble 5 organics. Pulp treated by this step is clean, well washed, and suitable for oxygen bleaching to a lower kappa number without excessive degradation of cellulose. The powerful oxygen bleaching step is then performed, and filtrate from this bleaching step is not processed by the chemical recovery system used in the kraft treatment. Such pulp has improved bleachability in subsequent stages, due in part to lower 10 metals content and controlled conditions in the reactors to remove a significant portion of wood resins without cellulose fiber damage. The invention also may be used with other wood pulp processes, including various sulfite processes, alcohol-based processes, various MgO processes, and office waste de-inking processes. Any of these processes typically produces a pulp that has 15 a lignin content of from 0-6% and is suitable for the powerful oxygen bleaching step. Preferably, the method handles pulp with higher hexeneuronic acid content, i.e., some or even all of the pulp is from hardwood trees. However, the process is also suitable for pulp that is entirely softwood or combinations of hardwood or 20 softwood. The pretreatment step may include application of an oxidizing agent such as C10 2 applied at a kappa factor below 0.05 or some other oxidizing agent (such as ozone, per acidic acid, etc.) at an equivalent kappa factor. Acid may be added as part of the pretreatment step, with or without any oxidizing agent. The acid may comprise spent acid from a C10 2 generation process or sulfuric acid or both. Even without the 25 addition of a metal ion removal agent, the pre-treated pulp will contain less than 20 ppm manganese; this figure can be significantly lower, depending on the pulp. Addition of a metal ion removal agent reduces the level to less than 5 ppm manganese. Again, this figure can be significantly lower, depending on the pulp. The pretreatment step may include a washing step in which a liquid acidic filtrate is 30 separated from the pulp, downstream from the reaction tower. Preferably, the acidic filtrate of this washing step is not processed by a chemical recovery system, thus 3 WO 00/75420 PCT/US00/15263 avoiding scaling and/or corrosion in the chemical recovery system. B. Bleaching Kraft-Treated Pulp In Dual Stage Oz Oxygen reactors with a single pump 5 A second aspect of the invention features a kraft method having steps (a) and (b) in which the bleaching stage b) includes a pre-treatment step which produces a clean well-washed pulp and a filtrate, low in transition metals, reduced hexeneuronic acid content, consistency between 9-15% and a kappa number of 10-30. The filtrate of this step is not processed in a chemical recovery system. Thereafter, the method 10 includes a strong 02 bleaching process having two separate 02 reactors operated at high pressure. Pulp flow and system pressure are produced by at least one pump, a high-pressure pump producing a pressure of about 140-180 psi at the inlet to the first reactor. The pressure discharging from the second reactor is at least 50 psi. In this method, the first reactor may be supplied by a medium consistency pump standpipe 15 reservoir, the top of which is usually at atmospheric pressure. The standpipe has a minimum operating level at least 20 feet above the pump suction. The standpipe is subjected to controlled gentle heating by mixing the pulp with steam having a pressure less than 5 psi at the point of mixing, so that the temperature of the pulp prior to entering the medium consistency pump is 170-205EF. There need be no 20 additional heating downstream of the medium consistency pump and prior to the first reactor. The temperature of the pulp prior to entering the medium consistency pump is preferably 190-205EF with no additional heating between the first and the second reactor. Alternatively, the temperature of the pulp prior to entering the medium consistency pump is 170-185EF and additional heating is used between the first and 25 the second reactor. The high-pressure may also provided by a second high pressure pump operated in series with the first high pressure pump. For example, the second high pressure pump may be positioned between the first pump and the first reactor, or the second pump may be positioned between the two reactors. High pressure oxygen is injected into stock at an oxygen injection point at or upstream of a high shear mixer 30 which is positioned below the first reactor, so that piping between the oxygen 4 WO 00/75420 PCT/US00/15263 injection point and the first reactor is free from bends in excess of 30E or other turbulence producing geometries. Oxygen injected at the oxygen injection point is between 25 and 100% of the total oxygen requirement for the two reactors. Alternatively, high pressure oxygen may be injected into stock at an oxygen injection 5 point at or upstream of a high shear mixer which is positioned below the second reactor, piping between the oxygen injection point and the second reactor being free from bends in excess of 30E or other turbulence producing geometries, oxygen injected at the oxygen injection point being between 0 and 50% of the total oxygen requirement for the two reactors. In a third option, high pressure oxygen injected at 10 the point of highest stock pressure between any pump upstream of the first reactor and a control valve for the pump, and oxygen injected at that point is between 0-50% (in some cases, 0-20%) of the total oxygen requirement for the two reactors. Preferably, the high pressure oxygen and the stock pressure are above 150 psi. The bleaching step may further include a peroxide stage after the second 02 15 reactor, in which a fluidizing device is provided at the exit of the second reactor. Alternatively, the fluidizing device may be positioned between the outlet of the second reactor and the inlet of the hydrogen peroxide stage described below. The fluidizing device mixes the stock and hydrogen peroxide and, if necessary, metals control agents and/or a hydrogen peroxide stabilizer, the hydrogen peroxide charge 20 being 1-8 pounds per ton of pulp, the pulp having a consistency of between 8 and 15%. This reaction continues for 20-60 minutes at a temperature between 170 and 212EF (preferably less than 201F. Preferably, there is no wash between the first and second reactors or between the second reactor and the peroxide stage. Stock may be washed downstream of the 25 second reactor. Alternatively, the stock is washed downstream of the peroxide stage. In either case, filtrate from the wash is not processed by the chemical recovery system. This may be the only wash downstream of the second reactor or peroxide stage. 30 5 WO 00/75420 PCT/US00/15263 C. C10, stage followed by alkaline H10 2 treatment without intermediate washing A third aspect of the invention features further processing of an alkali/sulfur-treated pulp that has been previously delignified down to a kappa number of 4-12. This 5 further treatment includes at least one C10 2 stage followed by an alkaline hydrogen peroxide stage without intermediate washing. A wash is then performed, and the resulting pulp is 80-92 ISO brightness. This further processing may include a second C102 stage and its subsequent wash. The second CIO, stage and its wash may be performed before or after the above-discussed combined C10 2 /hydrogen peroxide 10 process. This second C10 2 stage can also be a combined C102/hydrogen peroxide process. Preferably, the reaction time for hydrogen peroxide stage is 30-60 minutes at a temperature of 140-190EF at a final pH of 8.5-11 and a consistency of 3-6% and could be achieved at medium consistency (8-16%) or high consistency (17-35%). A metals control agent or a hydrogen peroxide stabilizer may be added during the 15 hydrogen peroxide stage. D. Powerful bleaching of low lignin pulp with O A fourth aspect of the invention features powerful bleaching of low lignin pulp with 02, in which the filtrate is not processed by the chemical recovery 20 treatment. The wood pulp used in this process B i.e., pulp having less than 6% lignin B may be obtained by any chemical treatment, including those which comprises a chemical recovery system, for example, a system of brown stock washing, evaporation, recovery boiler and recausticizing. The process for bleaching the low lignin pulp produce a product suitable for 25 paper or tissue manufacture, and it includes: i) processing treated wood pulp having a kappa number of between 10 and 30, at low consistency between 2.0 and 6.0%, at 110-185EF for 15 to 60 minutes, pH between 2.0 and 4.0; and ii) powerful oxygen bleaching in which the filtrate is not processed by 30 the chemical recovery system. 6 WO 00/75420 PCT/US00/15263 Each aspect of the invention features pulp processes for making all types of paper products including paper, tissue, paper board, bleached pulp bales, wet laps, and slush to be de-inked. 5 Other aspects of the invention will be apparent to those skilled in the art from the following description of the preferred embodiments and figures, and from the claims. Brief Description of the Drawing 10 Fig. 1 is a general flow diagram of the first stage of a kraft pulp bleaching process. Fig. 2 is a general flow diagram of the second stage of the kraft pulp bleaching process. Description of the Preferred Embodiments 15 The initial stage of the pulp bleaching process is shown in Fig. 1. In an initial kraft stage, wood is subjected to some mechanical treatment, and then to a NaOH/NaS chemical digestion. The result of these initial processes is a treated pulp brown stock 10 which includes metals, bound sodium, lignin, and cellulose in intimate association. The process also produces a by-product stream which includes 20 spent chemicals and other by-products. In view of the cost of the kraft chemicals and the nature of the by-product stream, a system for recovering and treating this by product stream is critical to the kraft process. The chemical recovery system includes brown stock washing, evaporation, recovery boiler and recausticizing. In Fig. 1, brown stock 10 is washed in washer 12 and the pulp is acidified 25 with a mixture of spent acid 14 from a C10 2 generator and/or sulfuric acid and recycled acidic filtrate 28 from the pre-treatment (Ad) stage wash. The pulp is low consistency (between 2.0 and 6.0%) with a kappa number of 10-30. C10O 2 is may be added in one or more stages 16. The mixture is reacted in Ad Tower 20 at 110 185EF for 15 to 60 minutes, pH between 2.0 and 4.0. The product is washed in Ad 30 washer 22 using DNP and 02 filtrate 24 described below. Some fresh water may be used on the lowest shower. MgSO 4 is added to washer material and caustic (NaOH) 7 WO 00/75420 PCT/US00/15263 is added to MC pump suction. The solids are conveyed by a screw conveyor 25 to a steam mixer 26 to a standpipe 30 which feeds the 02 bleaching stage described below. The pre-treatment removes at least 10% of hexeneuronic acid in the treated 5 pulp and some metals, including the majority of the manganese. It also removes some bound sodium and soluble organics. The resulting pre-treated pulp is clean, well washed, and suitable for oxygen bleaching to a lower a kappa number without excessive degradation of cellulose. The pre-treated pulp is then subjected to strong oxygen bleaching in at least 10 two stages. This bleaching takes place under high pressure with efficient contact between oxygen and the pulp. Advantageously, this high-pressure two-stage bleaching can be accomplished with a single medium consistency oxygen pump. In Fig. 1, the top of standpipe 30 is at ambient pressure. Medium consistency pump 32 feeds the high-shear mixer 34 which is positioned directly 15 below the first oxygen reactor 36, without turbulence producing geometries, such as bends. In that way, the pressure generated by pump 32 is effective to feed both 02 reactors. Oxygen 38 is fed to the pulp upstream of the first reactor and at mixer 34. Oxygen can also be added at 99, between the MC pump and its level control valve for additional benefit -- e.g., to add additional oxygen to handle additional kappa drop 20 without exceeding the gas handling capability of the additional mixing points and reactors. Steam 40 is for start-up purposes only, if necessary. The pressure in reactor 36 is high as described above to maintain small bubbles and good reaction efficiency. The stock is then fed to the second reactor 42. Again, a high shear mixer 44 is used to introduce oxygen from line 38. The stock is heated by steam 40 and treated with 25 caustic as indicated. The reference to H 2 0 2 is for start-up purposes only. From the second reactor 42 stock is fed to blow tank 46. Diluted hydrogen peroxide and metals control agents and/or peroxide stabilizers can be added to the fluidizing discharger at 51 on the top of the second reactor to use the blow tank as a hydrogen peroxide stage. From there the pulp flows to washer 48. Filtrate 50 is sewered or available for 30 recycling options. The low chloride content of this filtrate enhances recycling/recovery options. 8 WO 00/75420 PCT/US00/15263 Further whitening is accomplished by a C10 2 (Dl) stage as follows. Pulp is heated in a steam mixer 52 and treated with C10 2 and spent acid. The mixture is reacted in tower 54 and then treated with caustic and H 2 0 2 . Standard metals control agents (chelants, etc.) and/or hydrogen peroxide stabilizers (sodium silicate etc.) can 5 also be added. This mixture is then sent to NP tower 56. There is no wash between tower 54 and tower 56. After tower 56 the pulp is washed in washer 58 and subjected to a second C10 2 treatment in tower 60. The pH of the pulp is then treated with caustic and washed in washer 62. The resulting pulp is ready for paper/tissue/paper board manufacture. 10 Other embodiments are within the following claims. What is claimed is: 9

Claims (45)

1. In a method of converting wood to pulp suitable for manufacturing a paper product, said method including a) alkali and sulfur treatment of the wood to produce treated pulp and a chemical recovery system, said alkali and sulfur treatment comprising brown stock washing, evaporation, recovery boiler and recausticizing; 5 and b) bleaching the treated pulp to produce a product suitable for product manufacture; the improvement in which the bleaching stage b) includes: i) a pre-treatment acidification step comprising processing treated wood pulp having a kappa number of between 10 and 30, at low consistency between 2.0 10 and 6.0%, at 110-185EF for 15 to 60 minutes, pH between 2.0 and 4.0, said step removing at least 10% of hexeneuronic acid in the treated pulp, removing some metals, some bound sodium, and soluble organics, the pre-treated pulp being clean and well washed, and suitable for oxygen bleaching to a lower a kappa number without excessive degradation of cellulose; and 15 ii) powerful oxygen bleaching step in which the filtrate is not processed by the chemical recovery system.

2. The method of claim 1 in which said paper product is selected from the group consisting of paper, tissue, paper board, bleached pulp, bales, wet lap, rolls, 20 and slush to be de-inked.

3. The method of claim 1 in which at least some of said wood pulp is from hardwood or softwood trees. 25

4. The method of claim 3 in which said wood pulp is from a combination of hardwood and softwood trees.

5. The method of claim 1 in which the pretreatment step is conducted at pH 2.0-3.0. 30 10 WO 00/75420 PCT/US00/15263

6. The method of claim 1 in which the pretreatment step is conducted on treated pulp in which an oxidizing agent is applied at an equivalent kappa factor of .05 or less. 5

7. The method of claim 6 in which the oxidizing agent is C10 2 applied at a kappa factor below 0.05.

8. The method of claim 1 in which the pretreatment step comprises addition of acid. 10

9. The method of claim 1 in which the pretreatment step comprises addition of acid without any oxidizing agent.

10. The method of claim 7 in which said acid comprises spent acid from 15 a C1O 2 generation process or sulfuric acid or both.

11. The method of claim 1 in which pre-treatment agent step does not include the addition of any metal ion removal agent and the pre-treated pulp comprises less than 20 ppm manganese. 20

12. The method of claim 1 in which the pre-treated step comprises addition of a metal ion removal agent and the pre-treated pulp comprises less than 5 ppm manganese. 25

13. The method of claim 1 in which the pretreatment step includes a washing step in which a liquid acidic filtrate is separated from the pulp, downstream from the reaction tower. 30 11 WO 00/75420 PCT/US00/15263

14. The method of claim 13 in which the acidic filtrate of the washing step is not processed by a chemical recovery system, thus avoiding scaling, corrosion, or both, in the chemical recovery system. 5

15. In a method of converting wood bales or board to pulp suitable for manufacturing a paper product, including a) alkali and sulfur treatment of the wood to produce treated pulp and a chemical recovery system, which includes brown stock washing, evaporation, recovery boiler and recausticizing; and 10 b) bleaching the treated pulp to produce a product suitable for paper or tissue manufacture; the improvement in which the bleaching stage b) includes: i) a pre-treatment step which produces a clean well-washed pulp and a filtrate, said pulp being low in transition metals, reduced hexeneuronic acid content, 15 consistency between 9-15%, a kappa number of 10-30; and thereafter, said filtrate not being processed in a chemical recovery system; ii) a strong 02 bleaching process comprising two separate 02 reactors operated at high pressure, pulp flow and system pressure being produced by at least 20 one pump, which is a high-pressure pump and said pressure is about 140-180 psi at the inlet to said first reactor and the pressure discharging from said second reactor being at least 50 psi.

16. The method of claim 15 in which said first reactor is supplied by a 25 medium consistency pump standpipe reservoir, the top of the standpipe being at atmospheric pressure, with a minimum operating level at least 20 feet above the pump suction.

17. The method of claim 16, in which the pulp in said standpipe is 30 subjected to controlled gentle heating by mixing the pulp with steam having a pressure less than 5 psig at the point of mixing. 12 WO 00/75420 PCT/US00/15263

18. The method of claim 17 in which the temperature of the pulp prior to entering the medium consistency pump is 170-205EF. 5

19. The method of claim 15 in which no additional heating is used downstream of the medium consistency pump and prior to the first reactor.

20. The method of claim 19 in which the temperature of the pulp prior to entering the medium consistency pump is 190-205EF and no additional heating is 10 used between the first and the second reactor.

21. The method of claim 15 in which the temperature of the pulp prior to entering the medium consistency pump is 170-185EF and additional heating is used between the first and the second reactor. 15

22. The method of claim 15 in which said high-pressure is provided by a second pump together with said high pressure pump.

23. The method of claim 22 in which said second pump is positioned 20 between the high-pressure pump and the first reactor, or the second pump is positioned between said two reactors.

24. The method of claim 15 in which high pressure oxygen is injected into stock at an oxygen injection point at or upstream of a high shear mixer which is 25 positioned below the first reactor, piping between said oxygen injection point and said first reactor being free from bends in excess of 30E or other turbulence producing geometries, oxygen injected at said oxygen injection point being between 25 and 100% of the total oxygen requirement for the two reactors. 30 13 WO 00/75420 PCT/US00/15263

25. The method of claim 15 in which high pressure oxygen is injected into stock at an oxygen injection point at or upstream of a high shear mixer which is positioned below the second reactor, piping between said oxygen injection point and said second reactor being free from bends in excess of 30E or other turbulence 5 producing geometries, oxygen injected at said oxygen injection point being between 0 and 50% of the total oxygen requirement for the two reactors.

26. The method of claim 15 in which high pressure oxygen is injected at the point of highest stock pressure between any pump upstream of said first reactor and a 10 control valve for said pump, oxygen injected at said point being between 0-50% of the total oxygen requirement for the two reactors.

27. The method of claim 26 in which said high pressure oxygen and the stock pressure are above 150 psi. 15

28. The method of claim 15 in which said bleaching step further comprises a peroxide stage after said second 02 reactor, said peroxide stage comprises, providing a fluidizing device at the exit of said second reactor, said fluidizing device 20 mixing said stock and hydrogen peroxide and, if necessary, metals control agents, a hydrogen peroxide stabilizer, or both, the hydrogen peroxide charge being 1-8 pounds per ton of pulp, said pulp having a consistency of between 8 and 15%, and said reaction continuing for 20-60 minutes at a temperature between 180 and 2121F. 25

29. The method of claim 28 in which the reaction takes place at pH between 9 and 11.5.

30. The method of claim 15 in which there is no wash between the first and second reactors. 30 14 WO 00/75420 PCT/US00/15263

31. The method of claim 28 in which there is no wash between the first and second reactors and no wash between the second reactor and the peroxide stage.

32. The method of claim 30 in which the stock is washed downstream of 5 the second reactor, and filtrate from the wash is not processed by the chemical recovery system.

33. The method of claim 31 in which the stock is washed downstream of the peroxide stage and filtrate from the wash is not processed by the chemical 10 recovery system.

34. The method of claim 32 or 33 in which the wash is the only wash downstream of the second reactor or peroxide stage. 15

35. The method of claim 15 in which said paper product is selected from the group consisting of paper, tissue, paper board, bleached pulp, bales, wet lap, rolls, and slush to be de-inked.

36. In a method of converting wood bales or board to pulp suitable for 20 manufacturing a paper product, said method comprising a) alkali and sulfur treatment of the wood to produce treated pulp; and b) processing said treated pulp to a kappa number of 4-12, the improvement in which a) said pulp is further treated with one or more CO 2 stage; b) at least one of said C10 2 stage(s) is followed by an alkaline hydrogen 25 peroxide stage without intermediate wash; and c) following said alkaline hydrogen peroxide, said method comprising performing a wash, the resulting pulp being 80-92 ISO brightness.

37. The method of claim 36 further comprising a second C10 2 stage and 30 its subsequent wash after said hydrogen peroxide stage. 15 WO 00/75420 PCT/US00/15263

38. The method of claim 36 further comprising a C10I 2 stage and its subsequent wash prior to said at least one C10I 2 stage.

39. The method of claim 37 or 38 in which the second C10 2 stage 5 comprises a hydrogen peroxide treatment.

40. The method of claim 36 in which the reaction time for hydrogen peroxide stage is 30-60 minutes at a temperature of 140-190EF at a final pH of 8.5-11 and a consistency of 3-35% 10

41. The method of claim 40 in which the consistency is in a range selected from the group consisting of 3-6%; 8-16%; or 17-35%.

42. The method of claim 36 in which a metals control agent or a 15 hydrogen peroxide stabilizer or both is added during said hydrogen peroxide stage.

43. The method of claim 36 in which said paper product is selected from the group consisting of paper, tissue, paper board, bleached pulp, bales, wet lap, rolls, and slush to be de-inked. 20

44. In a method of converting wood to pulp suitable for manufacturing a paper product, including a) treating wood pulp by a chemical treatment which comprises a chemical recovery system, said chemical recovery system including brown stock washing, 25 evaporation, recovery boiler and recausticizing, said treated wood pulp having less than 6% lignin; and b) bleaching the treated pulp to produce a product suitable for paper manufacture by i) processing treated wood pulp having a kappa number of between 30 10 and 30, at low consistency between 2.0 and 6.0%, at 110-185EF for 15 to 60 minutes, pH between 2.0 and 4.0, and 16 WO 00/75420 PCT/US00/15263 ii) performing a powerful oxygen bleaching step in which the filtrate is not processed by the chemical recovery system.

45. The method of claim 44 in which said paper product is selected from 5 the group consisting of paper, tissue, paper board, bleached pulp, bales, wet lap, rolls, and slush. 17