HIGH YIELD CHEMICAL PULPING AND BLEACHING PROCESS

Abstract

A method of producing high yield chemical cellulosic pulp includes: (a) chemically pulping wood chips to separate lignin and liberate cellulosic fibers from the wood chips to generate a cellulosic pulp; (b) washing and screening the pulp of step (a); (c) pre-treating the washed pulp with oxygen; (d) optionally washing the treated pulp of step (c); (e) bleaching the pre-treated pulp in an extended duration oxidative bleaching stage; (f) optionally washing the bleached pulp of step (e); and (g) optionally further oxidatively or reductively bleaching the bleached pulp in a shorter duration bleaching stage, wherein the bleached pulp is produced at a yield of greater than 60% based on the weight of the pulped wood chips (dry basis).

Claims

1. A method of producing high yield chemical cellulosic pulp comprising: (a) chemically pulping wood chips to separate lignin and liberate cellulosic fibers from the wood chips to generate a cellulosic pulp; (b) washing and screening the pulp of step (a); (c) pre-treating the washed pulp with oxygen; (d) optionally washing the treated pulp of step (c); (e) bleaching the pre-treated pulp in an extended duration oxidative bleaching stage; (f) optionally washing the bleached pulp of step (e); and (g) optionally further oxidatively or reductively bleaching the bleached pulp in a shorter duration bleaching stage, wherein the bleached pulp is produced at a yield of greater than 60% based on the weight of the pulped wood chips (dry basis).

2. The method of producing high yield chemical cellulosic pulp according to claim 1, wherein the bleached pulp is produced at a yield of greater than 65% based on the weight of the pulped wood chips (dry basis).

3. The method of producing high yield chemical cellulosic pulp according to claim 1, wherein the bleached pulp is produced at a yield of 80% or more based on the weight of the pulped wood chips (dry basis).

4. The method of producing high yield chemical cellulosic pulp according to claim 1, wherein bleached pulp produced by the process has an ISO Brightness of 80 or more.

5. The method of producing high yield chemical cellulosic pulp according to claim 1, wherein the process removes less than 75% of the lignin present in the wood chips.

6. The method of producing high yield chemical cellulosic pulp according to claim 1, wherein the process removes less than 50% of the lignin present in the wood chips.

7. The method of producing high yield chemical cellulosic pulp according to claim 1, wherein the process removes less than 20% of the lignin present in the wood chips.

8. The method of producing high yield chemical cellulosic pulp according to claim 1, wherein the pulp produced has a Kappa number higher than 15.

9. The method of producing high yield chemical cellulosic pulp according to claim 1, wherein the pulp produced has a Kappa number higher than 30.

10. The method of producing high yield chemical cellulosic pulp according to claim 1, wherein the pulp produced has a Kappa number from 15 to 65.

11. The method of producing high yield chemical cellulosic pulp according to claim 1, wherein the wood chips are chemically pulped in a Kraft Process or a Sulphite Process.

12. The method of producing high yield chemical cellulosic pulp according to claim 11, wherein the wood chips are chemically pulped in a Kraft Process.

13. The method of producing high yield chemical cellulosic pulp according to claim 1, wherein the step of pre-treating the washed pulp with oxygen is carried out at an oxygen pressure of from 0.25-1 MPa.

14. The method of producing high yield chemical cellulosic pulp according to claim 1, wherein the step of pre-treating the washed pulp with oxygen is carried out in the presence of an alkaline agent.

15. The method of producing high yield chemical cellulosic pulp according to claim 14, wherein the step of pre-treating the washed pulp with oxygen is carried out for a residence time in a bleaching vessel of from 30 to 150 minutes.

16. The method of producing high yield chemical cellulosic pulp according to claim 1, wherein the step of bleaching the pre-treated pulp in an extended duration oxidative bleaching stage comprises: (i) providing the pre-treated pulp in aqueous form to the extended duration bleaching stage at a consistency of from 10% to 30% along with a peroxy bleaching agent and an alkaline agent effective to adjust pH of the charge to 9.5 to 12.5; (ii) bleaching the pulp in a bleaching vessel while maintaining an extended duration bleaching temperature of from 110° F. (43° C.) to 135° F. (57° C.) and a pH of the charge from 9.5 to 12.5 for a bleaching retention time in the extended duration bleaching stage for at least 6 hours.

17. The method of producing high yield chemical cellulosic pulp according to claim 16, wherein the peroxy bleaching agent utilized in the extended duration bleaching stage is hydrogen peroxide applied to the fiber in an amount of from 0.35% on dried pulp to 4% on dried pulp.

18. The method of producing high yield chemical cellulosic pulp according to claim 1, wherein the method comprises one or more Q stages.

19. The method of producing high yield chemical cellulosic pulp according to claim 1, comprising bleaching the pulp in a shorter duration bleaching stage following bleaching in the extended duration oxidative bleaching stage.

20. A high brightness, high yield cellulosic pulp produced by the method according to claim 1.

Description

DETAILED DESCRIPTION

[0046] The invention is described in detail below for purposes of illustration only. The invention is defined in the appended claims. Unless otherwise indicated, terminology used herein is given its ordinary meaning consistent with the exemplary definitions set forth immediately below; g, or G refers to grams, MT means metric ton, percents, ppm and like terminology relates to weight percent, parts per million by weight unless otherwise indicated and so forth. Unless otherwise indicated, or readily apparent, chemical cooking and bleaching stages are carried out in aqueous media as will be appreciated by one skilled in the art.

[0047] An alkaline agent means a compound used to adjust the pH of the bleaching liquor to relatively high values. Hydroxides such as caustic, sodium hydroxide are preferred.

[0048] Bleaching temperature and like terminology refers to the temperature maintained in a bleaching stage or a vessel over the retention time of bleaching, and may be referred to as retention temperature.

[0049] “ISO Brightness” or simply brightness as used herein refers to the measured brightness of the pulp made into handsheets in accordance with TAPPI Test Method T 525 om-17 or equivalent, with C-illumination. Diffuse reflectance is measured in the wavelength range of 400-520 nm with an effective wavelength of 457 nm by using a suitable filter set or an equivalent device for modifying the spectral response and an instrument having diffuse illumination and perpendicular observation geometry. The measurements are made in terms of absolute reflectance factors. Brightness testing is done on handsheets using a MacBeth Ci5 instrument. Brightness gain is expressed as a percentage relative to the brightness of the pulp prior to bleaching.

[0050] Chemical pulping, chemical cooking and like terminology refers to separating cellulosic fiber pulp from wood or other plant-based materials from which non-cellulose components are removed by chemical pulping without substantial mechanical post-treatment; preferably with no mechanical grinding at all. In case of chemical pulping processes such as the Sulfite or Sulfate (Kraft) process, primarily the lignin components and the hemi-cellulose components are dissolved from the wood to varying degrees depending on the desired end product. The Kraft Process involves treatment of wood chips with a hot mixture of water, sodium hydroxide (NaOH), and sodium sulfide (Na.sub.2S), known as white liquor, that breaks the bonds that link lignin, hemicellulose, and cellulose. The Sulfite process produces wood pulp by treating wood chips with solutions of sulfite and bisulfite ions. These chemicals cleave the bonds between the cellulose and lignin components of the lignocellulose. A variety of sulfite/bisulfite salts are used, including sodium (Nat), calcium (Ca.sub.2.sup.+), potassium (K.sup.+), magnesium (Mg.sub.2.sup.+), and ammonium (NH.sub.4.sup.+). The lignin is converted to lignosulfonates, which are soluble and can be separated from the cellulose fibers.

[0051] “Chemical wash”, “wash”, “displacement wash” or like terminology refers to a washing step with water to remove chemicals and bleaching residues from the pulp. A washing step is usually advisable following a peroxy bleaching step, before a subsequent stage, and is usually required between peroxy bleaching steps to optimize results. When implementing the present invention, a wash may be provided between pre-treatment and bleaching stages and between bleaching stages unless otherwise indicated.

[0052] “Consisting essentially of” and like terminology with respect to compositions refers to the recited components and excludes other ingredients which would substantially change the basic and novel characteristics of the composition, article or process. Unless otherwise indicated or readily apparent, a composition or article consists essentially of the recited or listed components when the composition or article includes 90% or more by weight of the recited or listed components, optionally on a dry basis, that is, without water. The terminology thus excludes more than 10% unrecited components. In connection with methods of bleaching or oxygen treating of pulp, the terminology consisting essentially of recited steps excludes additional bleaching or oxidative treatments, but does not exclude washing steps interposed between bleaching or oxygen treatment steps.

[0053] Consistency, % C or like terminology refers to percent solids of a pulp slurry calculated on a dry basis. A slurry having 80 percent water and 20 percent dry pulp has a consistency of 20 percent. Unless otherwise indicated, dry pulp, dried pulp and like terminology means oven dry pulp, which may have up to a few percent water.

[0054] A Kappa number is determined in accordance with TAPPI Method T236-0M-99 or equivalent. The Kappa number is the volume (in millimeters) of 0.1N potassium permanganate solution consumed by one gram of moisture-free pulp. The results are corrected to 50% consumption of the permanganate added. Kappa numbers are commonly used to determine lignin content and used to determine bleachability of pulp.

[0055] “Oxidative” bleaching and like terminology refers to bleaching or oxidative brightening operations with oxidative beaching agents such as oxygen, ozone, peroxy compounds, chlorine, chlorine dioxide, hypochlorite; for example P stages, EOP stages, Paa stages and so forth.

[0056] “Percent on pulp” “OP” and like terminology refers to the weight ratio of a hydrogen peroxide/dried pulp×100% in a charge to a bleaching stage.

[0057] Percent on pulp, OP, “percent applied to pulp” and the like may likewise be expressed for hydrogen peroxide and other peroxy bleaching agents as kg-moles/metric ton of dried pulp, it being noted that 1% OP or 1% Percent consumed on pulp corresponds to 0.295 kg-moles bleaching agent/metric ton of dried pulp.

[0058] A bleaching or delignification “stage” refers to bleaching or otherwise treating pulp in a vessel under a specified set of conditions. Subsequent stages may be undertaken in the same vessel for batch or semi-batch processes and in downstream vessels for continuous processes.

[0059] “Peroxy compound” and like terminology refers to compounds having a peroxo group. Typically one employs hydrogen peroxide in the bleaching method of the invention; however, one may utilize other peroxy compounds as a bleaching agent or for delignification if so desired. Other suitable peroxy bleaching compounds include peroxyacetic acid, peroxyformic acid, potassium peroxymonosulfate, dimethyldioxirane, peroxymonophosphoric acid and so forth.

[0060] A “reductive bleaching agent” refers to a reducing agent used to bleach pulp. Commercial systems may employ a mix of sodium borohydride and sodium bisulfite that form sodium hydrosulfite either in situ with the pulp or in a mixing step prior to addition to a reductive bleaching stage. Alternatively, sodium hydrosulfite as such may be used as available. These bleaching agents and equivalents are referred to as hydrosulfite bleaching agents. Additional reductive bleaching agents which may be used include formamidine sulfinic acid (FAS) and hydroxymethane sulfinic acid (HAS), as well as dithionites. See U.S. Pat. No. 4,871,423 to Grimsley et al. When a reductive bleaching agent is used in a bleaching stage, the stage is referred to as a reductive bleaching stage, a Y stage or with like terminology.

[0061] “Retention time” and like terminology refers to the duration of bleaching under a specified set of conditions in a bleaching stage. Temperatures, retention temperatures and the like refer to temperatures maintained during the retention time in a bleaching vessel.

[0062] Yield is calculated as the dry weight of the recovered pulp divided by the dry weight of the wood chips processed times 100%. Lignin reduction (%) is calculated as: (the percentage of lignin in the dry wood chips less the percentage of lignin in the dry finished pulp) divided by the percentage of lignin in the dry wood chips times 100%. Thus, a process fed with 100 pounds of wood chips yielding 70 pounds of finished pulp has a yield of 70%. A process where the percentage of lignin in the wood fed is 20% and the percentage of lignin in the finished pulp is 8% has a lignin reduction of 60%.

[0063] Conventional pulp bleaching stages are commonly referred to as Y stages, EO stages, D stages, EOP stages, Paa stages or P stages.

[0064] A “Y” stage refers to a reductive bleaching stage utilizing a reductive bleaching agent.

[0065] An “EO” stage refers to an alkaline, oxygen based bleaching stage carried out under oxygen pressure of from 0.25 to 1 Mpa in most cases.

[0066] An “EOP” stage refers to an EO stage with a peroxy bleaching agent present.

[0067] A “P” stage refers to a conventional alkaline peroxy bleaching stage, usually with hydrogen peroxide, referred to as peroxide. Representative operating parameters for a P stage appear in Table 1.

TABLE-US-00001 TABLE 1 Operating Parameters for P Stage bleaching Peroxide NaOH Temperature Retention % OP % OP ° F./° C. Minutes 1% 1% 185/85 60 3% 3% 185/85 60

[0068] A Paa stage refers to treatment of the pulp with a peroxyacid or related compound which is operative to remove lignin and optionally, to bleach the pulp. See U.S. Pat. No. 6,007,678 to Linsten et al.

[0069] A D stage refers to a chlorine dioxide bleaching stage.

[0070] The processes of the invention may include multi-stage shorter duration bleaching protocols with sequential steps, for example a P/Y regimen which indicates a P stage followed by a Y stage and so forth.

[0071] Each of the conventional processes noted above are of shorter duration than the extended duration oxidative bleaching stage described herein. Typically, conventional bleaching is carried out for a retention time of from about 5 minutes to about 3.5 hours, typically from 10 minutes to 2.5 hours of retention time and in many cases for a retention time of from 15 minutes to 1 hour. These processes are likewise carried out at relatively elevated temperatures, generally from 50° C. to about 150° C., with from 65° C. to about 125° C. being typical. Bleaching at over 100° C. requires a pressurized reactor, which involves high capital costs.

[0072] The oxygen pre-treatment stage of the present invention resembles an alkaline oxygen (EO) bleaching stage. Suitable conditions are for example, treating the pulp at 10-30% consistency for 10-200 minutes at 80 to 90 or 95° C. temperature and an oxygen pressure of 0.25 to 1 Mpa (36-145 PSIG). The oxygen charged amounts to approximately 1% oxygen on oven dry (OD) fiber on a weight basis; the actual consumption of oxygen is significantly less. Specific conditions may include 0 bleaching for 60 minutes at 80-87.5° C. temperature at 60 PSIG (0.414 MPa) charge of oxygen at the beginning of the retention and the oxygen vented and recharged at 15 minutes. 0 bleaching is carried out, for example, with 3-9% NaOH OP in the process.

[0073] In the oxygen pre-treatment of the pulp in accordance with the present invention, the alkali dose is selected to provide brightening without excessive delignification, that is a lignin or Kappa reduction of less than 50% based on the Kappa number prior to oxygen pretreatment. For example, if the initial Kappa number of the pulp is 120 before pre-treatment, the Kappa number after oxygen pre-treatment is greater than 60. The lignin content in percent is roughly 0.15 times the Kappa number. In various embodiments, the lignin reduction is less than 40%, 35%, 30%, 25% or 20% based on the lignin content prior to oxygen pretreatment. This limited reduction in lignin contrasts the invention with conventional oxygen delignification, where the lignin loss or Kappa Reduction is typically well in excess of 50%.

[0074] Suitable conditions for pre-treating the pulp with oxygen and caustic prior to further bleaching appear in Table 2 below.

TABLE-US-00002 TABLE 2 O Stage Pretreatment Parameter General Typical Preferred pH  8-13  8.5-12.5  9.5-12.5 % NaOH OP  1-15  3-10 3-6 T, ° C.  60-120  75-115  80-115 O.sub.2 P, PSIG  36-145  75-130  80-120 O.sub.2 P, MPa 0.25-1   0.52-0.9  0.55-0.83 Consistency, %  3-30  5-20  6-15 Residence Time (min)  30-150  65-130  70-115

Acid Chelation (Q Stage)

[0075] Acid chelation may be incorporated into the inventive process for the purpose of removing counterions, particularly transition metals which reduce the stability of H.sub.2O.sub.2.

Extended Duration Oxidative Bleaching Also Referred to as Peroxide Sous Vide (P.SUB.sv.)

[0076] Extended duration oxidative bleaching, or P.sub.sv is a new type of bleaching stage developed by Applicant. P.sub.sv literally means “under vacuum” but in actual application it represents a low temperature, long cook stage. This technology can be retrofit for little cost to essentially achieve high brightness at low operating cost. Low temperature, long retention produced a brightness that was 2-3 points lower than other options, but has advantages of lower energy costs and minimal additional capital investment. P.sub.sv can whiten colored fibers that conventional high temperature applications do not. What is likely occurring is the longer residence time is allowing the slower reaction pathways to be completed rendering the slow to oxidize moieties to be brightened.

[0077] A preferred set of ranges for operating parameters for extended duration oxidative bleaching include:

[0078] Temperature: From 110-135° F. (43-57° C.), typically about 125° F. (52° C.);

[0079] Retention time: From 6-200 hours, typically about 15 hours in many cases;

[0080] pH: From 9.5-12.5, typically from 10-11;

[0081] Consistency: From 10-30%, typically about 15%.

Exemplary High Yield Chemical Pulping and Bleaching

[0082] 1. Wood Chips are digested in a Kraft type process with a hot mixture of water, sodium hydroxide (NaOH), and sodium sulfide (Na.sub.2S) (white liquor) that breaks the bonds that link lignin, hemicellulose, and cellulose; [0083] 2. The resulting brown stock is washed and screened to yield cellulosic pulp; [0084] 3. A mild oxygen pre-treatment or activation step is carried out under the following conditions:

TABLE-US-00003 Parameter General pH  8-13 % NaOH OP  1-15 T, ° C.  60-120 O.sub.2 P, PSIG  36-145 O.sub.2 P, MPa 0.25-1   Consistency, %  3-30 Residence Time (min)  30-150 Kappa Reduction  5-35 ISO Increase 0 or less-15 [0085] The process conditions are selected to open up the lignin to bleaching reactions without excessive removal of the lignin; [0086] 4. Chemical or displacement washing to remove excess bleaching chemicals and residuals; [0087] 5. The washed and bleached pulp is bleached at a low temperature, long duration with peroxide (a “Psv”) process for 24-48 hours or longer. This step substantially brightens the pulp without significant lignin loss, preferably under the following conditions at a consistency of from 10-30%: [0088] Temperature: From 110-135° F. (43-57° C.), typically about 125° F. (52° C.); [0089] pH: From 9.5-12.5, typically from 10-11; [0090] 6. Chemical or displacement washing is then performed to remove excess bleaching chemicals and residuals; [0091] 7. A third stage of oxidative or reductive bleaching of shorter duration is then performed. This step could be done with oxygen, hydrogen peroxide, an alkali source, a peroxygen, ozone, a reductive compound, an acid source or combinations of these chemicals and conditions. Chlorine dioxide or other traditional chemicals could be used as long as the conditions were set to brighten the pulp with minimal yield loss. The bleaching at this stage in the process is selected from a P stage, an EOP stage, an EO stage, a Paa stage, a D stage or a Y stage. [0092] 8. Chemical or displacement washing to remove excess bleaching chemicals and residuals; [0093] 9. A 4.sup.th stage of bleaching using any of the chemicals in step 7 above; [0094] 10. Chemical or displacement washing to remove excess bleaching chemicals and residuals; and [0095] 11. The pulp can optionally be subjected to a 5.sup.th stage of bleaching depending on the desired final brightness and raw material and yield requirements.
Q stage(s) are incorporated into the process as necessary or desirable to remove metal ions and the like.

Exemplary Embodiments of the Invention

[0096] The invention may thus be practiced in various embodiments, wherein Embodiment No. 1 is a method of producing high yield chemical cellulosic pulp comprising: [0097] (a) chemically pulping wood chips to separate lignin and liberate cellulosic fibers from the wood chips to generate a cellulosic pulp; [0098] (b) washing and screening the pulp of step (a); [0099] (c) pre-treating the washed pulp with oxygen; [0100] (d) optionally washing the treated pulp of step (c); [0101] (e) bleaching the pre-treated pulp in an extended duration oxidative bleaching stage; [0102] (f) optionally washing the bleached pulp of step (e); and [0103] (g) optionally further oxidatively or reductively bleaching the bleached pulp in a shorter duration bleaching stage, wherein the bleached pulp is produced at a yield of greater than 60% based on the weight of the pulped wood chips (dry basis).

[0104] Embodiment No. 2 is the method of producing high yield chemical cellulosic pulp according to Embodiment No. 1, wherein the bleached pulp is produced at a yield of greater than 65% based on the weight of the pulped wood chips (dry basis).

[0105] Embodiment No. 3 is the method of producing high yield chemical cellulosic pulp according to Embodiment No. 1, wherein the bleached pulp is produced at a yield of greater than 70% based on the weight of the pulped wood chips (dry basis).

[0106] Embodiment No. 4 is the method of producing high yield chemical cellulosic pulp according to Embodiment No. 1, wherein the bleached pulp is produced at a yield of greater than 75% based on the weight of the pulped wood chips (dry basis).

[0107] Embodiment No. 5 is the method of producing high yield chemical cellulosic pulp according to Embodiment No. 1, wherein the bleached pulp is produced at a yield of 80% or more based on the weight of the pulped wood chips (dry basis).

[0108] Embodiment No. 6 is the method of producing high yield chemical cellulosic pulp according to any of the foregoing Embodiments, wherein bleached pulp produced by the process has an ISO Brightness of 70 or more.

[0109] Embodiment No. 7 is the method of producing high yield chemical cellulosic pulp according to any of the foregoing Embodiments, wherein bleached pulp produced by the process has an ISO Brightness of 75 or more.

[0110] Embodiment No. 8 is the method of producing high yield chemical cellulosic pulp according to any of the foregoing Embodiments, wherein bleached pulp produced by the process has an ISO Brightness of 77.5 or more.

[0111] Embodiment No. 9 is the method of producing high yield chemical cellulosic pulp according to any of the foregoing Embodiments, wherein bleached pulp produced by the process has an ISO Brightness of 80 or more.

[0112] Embodiment No. 10 is the method of producing high yield chemical cellulosic pulp according to any of the foregoing Embodiments, wherein bleached pulp produced by the process has an ISO Brightness of 85 or more.

[0113] Embodiment No. 11 is the method of producing high yield chemical cellulosic pulp according to any of the foregoing Embodiments, wherein the process removes less than 75% of the lignin present in the wood chips.

[0114] Embodiment No. 12 is the method of producing high yield chemical cellulosic pulp according to any of the foregoing Embodiments, wherein the process removes less than 70% of the lignin present in the wood chips.

[0115] Embodiment No. 13 is the method of producing high yield chemical cellulosic pulp according to any of the foregoing Embodiments, wherein the process removes less than 60% of the lignin present in the wood chips.

[0116] Embodiment No. 14 is the method of producing high yield chemical cellulosic pulp according to any of the foregoing Embodiments, wherein the process removes less than 50% of the lignin present in the wood chips.

[0117] Embodiment No. 15 is the method of producing high yield chemical cellulosic pulp according to any of the foregoing Embodiments, wherein the process removes less than 40% of the lignin present in the wood chips.

[0118] Embodiment No. 16 is the method of producing high yield chemical cellulosic pulp according to any of the foregoing Embodiments, wherein the process removes less than 30% of the lignin present in the wood chips.

[0119] Embodiment No. 17 is the method of producing high yield chemical cellulosic pulp according to any of the foregoing Embodiments, wherein the process removes less than 20% of the lignin present in the wood chips.

[0120] Embodiment No. 18 is the method of producing high yield chemical cellulosic pulp according to any of the foregoing Embodiments, wherein the pulp produced has a Kappa number higher than 15.

[0121] Embodiment No. 19 is the method of producing high yield chemical cellulosic pulp according to any of the foregoing Embodiments, wherein the pulp produced has a Kappa number higher than 30.

[0122] Embodiment No. 20 is the method of producing high yield chemical cellulosic pulp according to any of the foregoing Embodiments, wherein the pulp produced has a Kappa number higher than 40.

[0123] Embodiment No. 21 is the method of producing high yield chemical cellulosic pulp according to any of the foregoing Embodiments, wherein the pulp produced has a Kappa number higher than 50.

[0124] Embodiment No. 22 is the method of producing high yield chemical cellulosic pulp according to any of the foregoing Embodiments, wherein the pulp produced has a Kappa number from 15 to 65.

[0125] Embodiment No. 23 is the method of producing high yield chemical cellulosic pulp according to any of the foregoing Embodiments, wherein the pulp produced has a Kappa number from 20 to 60.

[0126] Embodiment No. 24 is the method of producing high yield chemical cellulosic pulp according to any of the foregoing claims, wherein the pulp produced has a Kappa number from 30 to 60.

[0127] Embodiment No. 25 is the method of producing high yield chemical cellulosic pulp according to any of the foregoing Embodiments, wherein the wood chips are chemically pulped in a Kraft Process or a Sulphite Process.

[0128] Embodiment No. 26 is the method of producing high yield chemical cellulosic pulp according to claim 25, wherein the wood chips are chemically pulped in a Kraft Process.

[0129] Embodiment No. 27 is the method of producing high yield chemical cellulosic pulp according to any of the foregoing Embodiments, wherein the step of pre-treating the washed pulp with oxygen is carried out at an oxygen pressure of from 0.25-1 MPa.

[0130] Embodiment No. 28 is the method of producing high yield chemical cellulosic pulp according to any of the foregoing Embodiments, wherein the step of pre-treating the washed pulp with oxygen is carried out in the presence of an alkaline agent.

[0131] Embodiment No. 29 is the method of producing high yield chemical cellulosic pulp according to any of the foregoing Embodiments, wherein the step of pre-treating the washed pulp with oxygen is carried out at a pH of from 8 to 13.

[0132] Embodiment No. 30 is the method of producing high yield chemical cellulosic pulp according to any of the foregoing Embodiments, wherein the step of pre-treating the washed pulp with oxygen is carried out for a residence time in a bleaching vessel of from 30 to 150 minutes.

[0133] Embodiment No. 31 is the method of producing high yield chemical cellulosic pulp according to any of the foregoing Embodiments, wherein the step of pre-treating the washed pulp with oxygen is carried out while applying from 1% to 15% NaOH to the washed pulp based on the dry weight of the washed pulp.

[0134] Embodiment No. 32 is the method of producing high yield chemical cellulosic pulp according to any of the foregoing Embodiments, wherein the step of bleaching the pre-treated pulp in an extended duration oxidative bleaching stage comprises: (i) providing the pre-treated pulp in aqueous form to the extended duration bleaching stage at a consistency of from 10% to 30% along with a peroxy bleaching agent and an alkaline agent effective to adjust pH of the charge to 9.5 to 12.5; (ii) bleaching the pulp in a bleaching vessel while maintaining an extended duration bleaching temperature of from 110° F. (43° C.) to 135° F. (57° C.) and a pH of the charge from 9.5 to 12.5 for a bleaching retention time in the extended duration bleaching stage for at least 6 hours.

[0135] Embodiment No. 33 is the method of producing high yield chemical cellulosic pulp according to Embodiment No. 32, comprising bleaching the pulp in the bleaching vessel while maintaining an extended duration bleaching temperature of from 110° F. (43° C.) to 135° F. (57° C.) and a pH of the charge from 9.5 to 12.5 for a bleaching retention time in the extended duration bleaching stage for at least 12 hours.

[0136] Embodiment No. 34 is the method of producing high yield chemical cellulosic pulp according to Embodiment No. 32, comprising bleaching the pulp in the bleaching vessel while maintaining an extended duration bleaching temperature of from 110° F. (43° C.) to 135° F. (57° C.) and a pH of the charge from 9.5 to 12.5 for a bleaching retention time in the extended duration bleaching stage for at least 24 hours.

[0137] Embodiment No. 35 is the method of producing high yield chemical cellulosic pulp according to Embodiment No. 32, comprising bleaching the pulp in the bleaching vessel while maintaining an extended duration bleaching temperature of from 110° F. (43° C.) to 135° F. (57° C.) and a pH of the charge from 9.5 to 12.5 for a bleaching retention time in the extended duration bleaching stage for at least 48 hours.

[0138] Embodiment No. 36 is the method of producing high yield chemical cellulosic pulp according to Embodiment No. 32, comprising bleaching the pulp in the bleaching vessel while maintaining an extended duration bleaching temperature of from 110° F. (43° C.) to 135° F. (57° C.) and a pH of the charge from 9.5 to 12.5 for a bleaching retention time in the extended duration bleaching stage for at least 96 hours.

[0139] Embodiment No. 37 is the method of producing high yield chemical cellulosic pulp according to any of Embodiment Nos. 32 to 36, wherein the extended duration bleaching temperature is from 118.4° F. (48° C.) to 131° F. (55° C.).

[0140] Embodiment No. 38 is the method of producing high yield chemical cellulosic pulp according to any of Embodiment Nos. 32 to 37, wherein the extended duration bleaching stage is carried out at a pH of from 10 to 11.

[0141] Embodiment No. 39 is the method of producing high yield chemical cellulosic pulp according to any of Embodiment Nos. 32 to 38, wherein the extended duration bleaching stage is carried out with a peroxy bleaching agent applied to the cellulosic pulp in an amount of from 0.1 kg-moles bleaching agent/metric ton of dried pulp to 2.4 kg-moles bleaching agent/metric ton of dried pulp.

[0142] Embodiment No. 40 is the method of producing high yield chemical cellulosic pulp according to any of Embodiment Nos. 32 to 39, wherein the peroxy bleaching agent utilized in the extended duration bleaching stage is hydrogen peroxide applied to the fiber in an amount of from 0.35% on dried pulp to 4% on dried pulp.

[0143] Embodiment No. 41 is the method of producing high yield chemical cellulosic pulp according to any of the foregoing Embodiments, wherein the method comprises one or more Q stages.

[0144] Embodiment No. 42 is the method of producing high yield chemical cellulosic pulp according to any of the foregoing Embodiments, comprising bleaching the pulp in a shorter duration bleaching stage following bleaching in the extended duration oxidative bleaching stage.

[0145] Embodiment No. 43 is the method of producing high yield chemical cellulosic pulp according to Embodiment No. 42, wherein the shorter duration bleaching stage comprises a bleaching stage selected from a P stage, an EOP stage, an EO stage, a Paa stage, a D stage or a Y stage.

[0146] Embodiment No. 44 is the method of producing high yield chemical cellulosic pulp according to Embodiment No. 42, comprising bleaching the pulp in at least two shorter duration bleaching stages following bleaching in the extended duration oxidative bleaching stage, wherein the shorter duration bleaching stages are selected from P stages, EOP stages, EO stages, Paa stages, D stages or Y stages.

[0147] Embodiment No. 45 is a high brightness, high yield cellulosic pulp produced by the method according to any of the foregoing Embodiments.

[0148] While the invention has been described in detail, modifications within the spirit and scope of the invention will be readily apparent to those of skill in the art. Such modifications are also to be considered as part of the present invention. In view of the foregoing discussion, relevant knowledge in the art, the related copending cases and references discussed above in connection with the foregoing description including the Cross Reference to Related Applications, the Detailed Description and Background of the Invention, the disclosures of which are all incorporated herein by reference, further description is deemed unnecessary. In addition, it should be understood from the foregoing discussion that aspects of the invention and portions of various embodiments may be combined or interchanged either in whole or in part. Furthermore, those of ordinary skill in the art will appreciate that the foregoing description is by way of example only, and is not intended to limit the invention.