NZ282616A

NZ282616A - Improving pulp brightness by rapid displacement heating and alkali charge pretreatment

Info

Publication number: NZ282616A
Application number: NZ282616A
Authority: NZ
Inventors: Nam Hee Shin
Original assignee: Beloit Technologies Inc
Priority date: 1994-03-04
Filing date: 1995-03-02
Publication date: 1997-05-26
Also published as: FI963448A0; ATE172503T1; WO1995023891A1; RU2127342C1; AU684623B2; NO963520L; DE69505503T2; MX9603876A; FI963448A; AU1978995A; BR9506974A; ZA951777B; DE69505503D1; ES2126263T3; CA2184706C; TW270159B; EP0748412A1; CN1143398A; RO117929B1; JPH09505115A

Abstract

A method for improving pulp brightness utilizes a combination of a distributed total white liquor charge during the warm fill, hot fill and cooking stages of a batch cooking process and low cooking temperatures during the cooking stage. The high total white liquor charge ranges between 15 % AA &tilde& 35 % AA, while the cooking temperatures range between 150 &tilde& 168 DEG C._________________________

Description

<div class="application article clearfix" id="description"> New Zealand No. 282616 International No. PCT/US95/02719 Priority Complete Specification Fll«d: CJftas: (6) 3?a.ica|.^;.."3?.a\c.u./.fie; Publication Date: 2.J&..MAY..139Z. P.O. Journal No: J.Wr.!.W. NEW ZEALAND PATENTS ACT 1953 COMPLETE SPECIFICATION Title of Invention: Impact of temperature and alkali charge on pulp brightness Name, address and nationality of applicant(s) as in international application form: BELOIT TECHNOLOGIES, INC., of Suite 3001, 3513 Concord Pike, Wilmington, Delaware 19803-3001, United States of America WO 95/23891 PCT/US95/02719 28261 6 TITLE; "IMPACT OF TEMPERATURE AND ALKALI CHARGE ON PULP BRIGHTNESS" BACKGROUND OF THE INVENTION The present invention relates to a process for improving the final brightness of pulp. More particularly, the present invention relates to modifications in both the cooking temperature and white liquor charge for a rapid displacement heating cooking system. Rapid Displacement Heating ("RDH") is a low energy batch cooking process for producing kraft pulp. Combining the inherent advantages of batch cooking with the energy efficiencies of a continuous digester, RDH reuses the spent black liquors that are displaced from a cooked digester to pretreat the wood chips in a consequent cook. Thus, both the chemicals and the heat in these spent liquors are recycled to a consequent cook. The pretreatment of fresh wood chips in a consequent cook begins with lower temperature liquors (approximately 80 ~ 130°C), and is followed by high temperature liquors (approximately 130° to 165°C) which heat the digester to the highest possible temperature before raising the temperatures to the final cooking temperature (3170°C) with steam. RDH and other alkaline cooking processes produce pulp that is relatively dark in color. Greater contrast is usually needed for the many uses of pulp and paper, so pulp is usually bleached to a high brightness in order to make white pulp for writing and printing papers and paperboard. Pulp color arises from changes in the iignin component of the raw material which occur in the pulping process. Unfortunately, with the use of high cooking temperatures and low black liquor strength in the RDH process, low bleachabiiity problems have WO 95/23891 PCT/US95/02719 2 occurred following the use of conventional, ECF and TCF bleaching processes. High cooking temperatures and low black liquor strength seem to accelerate condensation reactions, resulting in the condensation of lignin with lignin and other wood extractives. As a result, the bleachability of pulp decreased. An alternative method is, therefore, needed in the RDH cooking process to eliminate such adverse side reactions and improve pulp bleachability. SUMMARY QF THE INVENTION The present invention provides a method for improving pulp brightness. Based on modifications to a batch cooking process utilizing rapid displacement heating, the method of the present invention combines the steps of adding white liquor solution (% active alkalinity (AA) or effective alkalinity (EA» or NaOH to both the warm fill and initial hot fill stages and cooking wood chips at lower temperatures than previously used in a batch type operation to produce pulp that has improved bleachability. In this regard, a total white liquor charge ranging from 15% AA - 35% AA is distributed over the warm, hot and cooking stages in a predetermined amount. If a cool pad is used in practicing the invention, cool white liquor is also added to the black liquor that is released from the cool liquor accumulator. Essentially, white liquor is added to every stage of the batch cooking process prior to the actual cook. During the cooking of the chips, white and black liquors are present in the digester. Cooking temperatures are low, ranging from 150° - 167°C. With the combination of a high AA or EA white liquor charge and low cooking temperatures, final brightness of pulp is improved. As a result, pollutants and bleaching chemical usage are decreased in pulp mill operations. WO 95/23891 PCT/US95/02719 3 BRIEF DESCRIPTION QF THE DRAWINGS FIG. 1 illustrates a schematic view of a digested and its associated equipment used in the current RDH cooking system. FIGS. 2A, 2B and 2C each illustrate white liquor profiling or the addition of white liquor at various stages of the RDH cooking process. In FIG. 2A, plot A represents the addition of a small amount of white liquor at the beginning of the warm fill mode. Plot B represents the cooking stage and illustrates the presence of white liquor in the digester during the actual cooking of the chips. FIG. 2B illustrates the continuous addition of white liquor to the black liquor at each stage of the RDH cooking process, beginning with the warm fill and continuing through the end of the hot fill. White liquor, as shown, is also present in the digester during the actual cook. FIG. 2C illustrates the continuous addition of white liquor at each RDH stage including the addition of white liquor to the washer filtrate from the displacerr »nt tank. FIG. 3 illustrates a Stage 3 RDH system without white liquor addition during the warm and hot fill modes. FIG. 4 illustrates a Stage 3 RDH system with the addition of white liquor during the warm and hot fill modes. FIG. 5 illustrates a plot of D1-brightness versus total (D100 + D1) available chlorine charge for the best case and baseline case RDH pulps. Plot A represents RDH pulp R3 (0.225 Kappa factor). Plot B represents RDH pulp R4 (0.27 Kappa factor). Plot C represents RDH pulp R7 (0.225 Kappa factor). Plot D represents RDH pulp R8 (0.27 Kappa factor). WO 95/23891 PCT/U S95/02719 4 FIG. 5A illustrates a plot of D1-brightness versus D1-chlorine dioxide charge. Plot A represents RDH pulp R3 (0.225 Kappa factor). Plot B represents RDH pulp R4 (0.27 Kappa factor). Plot C represents RDH pulp R7 (0.225 Kappa factor). Plot D represents RDH pulp R8 (0.27 Kappa factor). FIG. 6 illustrates the D1-brightness versus the total available chlorine charge in the D100- and D1 -stages for all 0.225 Kappa factor bleaches. Plot A represents RDH pulp R3. Plot B represents RDH pulp R12. Plot C represents RDH pulp R7. FIG. 6A illustrates the D1-brightness versus the D1-stage chlorine dioxide charges. Plot A represents RDH pulp R3 (0.225 Kappa factor). Plot B represents RDH pulp R12 (0.225 Kappa factor). Plot C represents RDH pulp R7 (0.225 Kappa factor). FIG. 7 illustrates the D1-brightness versus the total available chlorine charge in the D100- and D1-stages for all 0.27 Kappa factor bleaches. Plot A represents RDH pulp R4. Plot B represents RDH pulp R12. Plot C represents RDH pulp R8. FIG. 7A illustrates the D1-brightness versus the D1-stage chlorine dioxide charges. Plot A represents RDH pulp R4 (0.27 Kappa factor). Plot B represents RDH pulp R12 (0.27 Kappa factor). Plot C represents RDH pulp R8 (0.27 Kappa factor). DETAILED DESCRIPTION QF THE PRESENTLY PREFERRED EMBODIMENTS The present invention provides a method for improving pulp bleachability, which is based on modifications to the existing RDH Cooking System for the WO 95/23891 PCT/US95/02719 5 digestion of wood chips. More specifically, the method involves the addition of a white liquor charge commencing at the start of the RPH cooking cycle and continuing until the time to temperature stage of the process, at which time the actual cook begins. The method of the present invention is also predicated on the use of somewhat lower cooking temperatures for the actual cook as compared to cooking temperatures commonly used in the RDH pulping process. In accordance with the present invention, a total white liquor charge ranging between approximately 15%AA ~ 35%AA is distributed over the warm black liquor, initial hot black liquor and cooking stages. When used, the cool pad or cool liquor accumulator also receives a white liquor charge. In addition to the use of a distributed white liquor charge, the present invention utilizes lower cooking temperatures ranging between approximately 150°C - 167°C. As a result, pulp is produced which, upon bleaching with any combination of bleaching chemicals, is improved in final brightness. The operational stages for a typical RDH Cooking System are as follows: (1) chip fill; (2) cool black liquor fill; (3) warm black liquor fill; (4) hot black liquor fill; (5) time to temperature; (6) time at temperature; (7) displacement; and (8) pump out. The basic principles of RDH operation are described in U.S. Patent No. 4,578,149 (issued March 25,1986), the entire contents of which are hereby incorporated by reference into this disclosure. Accordingly, details of RDH operations will be discussed only to the extent necessary for one of ordinary skill in the art to appreciate the modifications in the RDH cooking system, which produce the bleachable grade pulp described herein. FIG. 1 schematically illustrates the type of apparatus for RDH that is used for the digestion of pulp. It should be understood that this figure illustrates very general features of the cooking apparatus, and modifications and WO 95/23891 PCT/US95/02719 6 variations in this system are indeed made as will be discussed in greater detail below. Many instrumentalities such as gauges, pressure vents, pumps and valves have been eliminated from the figures disclosed herein for reasons of simplicity. FIG. 1 is used to illustrate the existing RDH cooking process and to facilitate an understanding of the improvements to the process in accordance with the principles of the present invention. Referring to FIG. 1, a digester is illustrated at 10 of the type generally used in the chemical digestion of wood chips. The digester 10 has a truncated bottom 12. An inlet valve 14 controls the entry of various reactive liquors into digester 10. Although not shown, the contents of digester 10 can be heated to a final cooking temperature by pumping cooking liquor through a heat exchanger or steam sparger which is connected to digester 10 by a valve-controlled line. After the wood chips are added to digester 10, cool black liquor (temperature around 70°-95°C) from the cool liquor accumulator (A tank) 16 is pumped by means of pump 18 through line 20 which is controlled by a valve 22 into the bottom of the digester 10 through an inlet valve 14. Next, warm black liquor (temperature between approximately 90°-150°C) from the warm liquor accumulator 24 is pumped out by means of a pump 18 through a valve 22 and through valve 14 into the bottom of digester 10. During this warm liquor fill, some black liquor is displaced from the digester 10 and then returned by a line 26 to the cool liquor accumulator 16. Hot black liquor (temperature between 150°-168°C) is then pumped from the hot liquor accumulator (C tank) 28 by means of a pump 30 which is controlled by a valve 32 into the bottom of the digester 10 utilizing valve 14. During the hot fill, black liquor is displaced from the digester 10 and returned to the warm liquor accumulator 24 and hot liquor accumulator 28 through lines 34 and 36, respectively. During the middle bf the hot fill, hot white liquor stored in the hot white liquor accumulator 38 is pumped WO 95/23891 PCT/US95/02719 7 out by means of a pump 30 where it combines with the hot black liquor leaving the hot liquor accumulator 28, the combined liquors thgn passing through a valve 32 and into the base of the digester 10. After the hot fill is completed, the inlet and outlet valves to the digester 10 are closed as the time to temperature stage commences. Steam is injected into the digester 10 and the temperature is increased to the cooking temperature, which averages approximately 170°C. The temperature of the digester is maintained at about this temperature until the wood chips are digested, depending on white liquor charge and H-factor. Upon completion of the cooking stage, washer filtrate (temperature approximately 70 ~ 85°C) stored in a displacement tank (D tank) 40 is pumped into the digester 10, utilizing pump 42 and valve 44. The contents are washed and the digester 10 is cooled. As the washer filtrate is added to the digester 10, the spent liquors are displaced and returned to the warm liquor accumulator 24 and the hot liquor accumulator 28 by lines 46 and 48, respectively. The displacement mode is concluded when all washer filtrate is used, which is based on the dilution factor of the washer. After displacement is completed, the digested pulp is then pumped out of the digester 10 to a discharge tank using pump 50. With the current RDH cooking system, cooking temperatures of greater than 170°C are used for rapid cooking, resulting in the acceleration of condensation reactions. As a result, bleachability problems occurred when the pulp was subjected to conventional, ECF and TCF bleaching processes. The present invention overcomes these problems and improves pulp bleachability by modifying the cooking process for wood chips. This improved RDH process utilizes a combination of higher alkalinity (or white liquor charge) and lower cooking temperatures. More specifically, white liquor is added during the warm WO 95/23891 PCT/US95/02719 8 and initial hot fill stages. This is in contrast to the existing RDH cooking process, wherein white liquor is added only during thedniddle of the hot fill mode. Further, when a cool pad is used in the present invention, white liquor is added to the cool black liquor leaving the cool liquor accumulator (or A tank). Thus, from the beginning of the RDH cooking process until the time to temperature stage, white liquor is added during each stage to the black liquor. The addition of white liquor at every stage, also called white liquor profiling, is illustrated in greater detail below in FIGS. 2A, 2B and 2C. In FIG. 2A, plot A illustrates the addition of a small amount of white liquor at the beginning of the warm fill mode when warm black liquor leaves the B tank or warm liquor accumulator and flows to the digester. White liquor can also be added to the A tank or cool pad when used. At the end of the hot fill mode, which utilizes two hot liquor accumulators C1 and C2, the mixture of white and black liquors remains in the digester. Plot B represents the cooking stage and illustrates the presence of white liquor in the digester during the actual cooking of the chips. Black liquor is also present during the cook. FIG. 2B illustrates the continuous addition of white liquor to black liquor at each stage of the cooking process, beginning with the warm fill through the end of the hot fill mode. FIG. 2C illustrates the continuous addition of white liquor throughout the various stages, including the addition of white liquor to the washer filtrate from the displacement tank. The concentration of dissolved organic material in the initial hot fill operation (C1 and C2 tanks containing black liquor) was compared with and without white liquor addition during the warm and hot fill operations. FIG. 3 illustrates a Stage 3 RDH system where no white liquor is added during the WO 95/23891 PCT/US95/02719 9 warm and hot fill modes. Only warm black liquor is leaving the warm liquor accumulator (B tank) 24 to flow through line 56 during<£he warm fill mode and into line 20, which then empties into the digester 10. Although this RDH system contains two hot liquor accumulators 28 (C1 tank) and 58 (C2 tank), respectively, there are RDH pulping processes which utilize only one hot liquor accumulator. In practicing the present invention, it is contemplated that the process of white liquor profiling can be applied to systems having any number of black liquor accumulators. As shown in FIG. 3, during the initial hot fill mode, hot black liquor leaves the hot liquor accumulators 28 and 58 by lines 60 and 62, respectively, and flows to the digester 10 through lines 64 and 20. During the middle of the hot fill, hot white liquor from the hot white liquor accumulator 38 mixes with the hot black liquor leaving hot liquor accumulator 58 by line 66. The mixture then flows through lines 64 and 20 and into the digester 10. FIG. 4 illustrates a Stage 3 RDH System with the addition of white liquor during the warm and hot fill modes. First, during the warm fill, white liquor is added to the warm black liquor leaving the warm liquor accumulator 24 by line 70. The warm fill flows through lines 56 and 20 into the digester 10. Either cool or hot white liquor may be used during the warm fill mode. During the initial hot fill mode, hot white liquor from +he hot white liquor accumulator 38 is mixed with black liquor leaving hot liquor accumulator 28 by line 72, and is further mixed with the biack liquor exiting 'the second hot liquor accumulator 58 by lines 62 and 66. The mixture of hot white and black liquors flows from the two hot liquor accumulators 28 and 58 through lines 64 and 20 into the digester 10. The results of the comparison are as follows: WO 95/23891 PCT/US95/02719 10 Without White Liquor Addition at Warm and Hot Fill Operations (FIG. 3) Initial Hot Fill Operation Total Flow, aal/cook Dissolved Organic. % C1 black liquor 20799 13.1 C2 black liquor 8709 14.9 With White Liquor Addition at Warm and Hot Fill Operations (FIG. 4) White Liquor charges: 1.5% AA at C1 black liquor 1.5% AA at C2 black liquor Initial Hot Fill Operation Total Flow, gal/cook Dissolved Organic. % C1 Black liquor 19877 10.1 C2 black liquor 797'I 9.8 This case study clearly demonstrates that the concentration of dissolved organic compounds at initial hot fill operation can be adjusted by adding white liquor to the hot fill line. The concentration of dissolved organic compounds in the C1 black liquor and in the C2 black liquor decreases from 13.1% to 10.1% and 14.9% to 9.8%, respectively. In order to maximize bleachability benefits and extend delignification for the RDH process, warm black liquor (temperatures between approximately 70° and 150°C and its strength between 3 and 20 g/l AA) and hot black liquor (temperatures between approximately 100° and 168°C and its strength between 8 and 30 g/l AA) should be reinforced with any combination of white liquor or NaOH solution. As shown in the figures presented above, warm and hot black liquor can be modified using white liquor profiling. These liquors can also be modified by sodium hydroxide (NaOH) profiling. The addition of white liquor or NaOH WO 95/23891 PCT/US95/02719 11 controls the total dissolved solids (TDS) concentration and black liquor strength using any combination of black liquor, white liquor an& NaOH. The washer filtrate displacement stage, in which the black liquor temperature is held between approximately 50° and 105°C and black liquor strength between 1 and 18 g/l AA, can be reinforced with any combination of white liquor or NaOH solution. By way of example, and not limitation, the following examples serve to further illustrate the present invention in its preferred embodiments. As shown below, Tables 1,1A, 2, 2A, 3 and 3A provide the pulping results and conditions for a number of cooks used in preparing the RDH pulps for subsequent bleaching studies. A summary of the pulping results is provided in Table 3B. TABLE 1 RDH Pulping Conditions and Results - "Best Case" Cook Number R1 Post R1 R2 Post R2 R3 Post R3 R4 Post R4 H factor 937 532 475 452 AA(TAPPI) to Hot Fill, % 16.0 16.0 16.0 16 Sulfidity (TAPPI),% on AA 30.4 30 30.3 30. 2 Maximum temp, C 160 160 160 160 Time to max, min 16 20 17 19 Time at max, min 130 37 60. 57 WO 95/23891 PCT/US95/02719 12 Kappa, unscreened 7.2 , 8.9 1 9.2 9.8 Kappa, screened 7.1 8.2 8.8 o 9.3 Total yield,% 46.3 47.3 48.2 1.7 Total rejects,% 0.9 1.2 1.4 1.7 Screened yield,% 45. i 46.1 46.8 46. 9 Viscosity,0 .5% CED, cp 26 39 40.7 44. 9 End of Cook R«aidual: AA(Na20), g/L 28.2 31.6 32. 9 31. 6 EA (Na20), g/L 21.1 24.2 24.2 24. 2 N&2S (Na20), g/L 14.3 14.9 17.4 14. 9 TTA(Na20),g /L • Solids,% 14.8 15.9 16.7 16. 8 Solids.g/L 161 * 173 183 185 Sulfidity % on A.A. 51 47.1 52.9 47. 1 HOT FILL: Hot liquor charge, L 18 18 18 18 Charge time, min 13 14 13 13 Temperature , C Top 130 127 127 128 Bottom 145 141 141 141 WO 95/23891 PCT/US95/02719 13 Chamicvil Conditions in Accumulator: AA(Na20), g/L 22.9 20.6 25.7 21.7 29.2 «24.6 27. 3 23.6 EA (Na20), g/L 16.1 15.1 18.1 16.1 20.2 IS.3 19. 8 17.9 Na2S (Nfi20) , g/L 13.7 11 15.2 11.2 18 12.7 14. 9 11.4 TTA (Na20), g/L • ** - " "* - Sulfidity, k on A.A. 59.4 53.4 59.1 51.6 61.6 51.2 54. 9 48.3 Solids,V 9.5 10.5 11.1 11.9 11.9 13.6 12. 7 13.6 Solids, g/L 99.8 111 118 127 127 146 137 146 Total elapsed time, min* 45 43 41 42 * includes heating time to 145C and time at 145C after initial hot black liquor injection and final hot B/L with W/L mix. TABLE1A RDH Pulping Conditions and Results - "Best Case" Cook Number R1 Post R1 R2 Post R2 R3 Post R3 R4 Post R4 WHITE I.TOTTOR ? White liquor charge,L 6.04 5.65 5.53 5.52 Hot B/L,L 5 4.13 5.5 5.8 Charge time, min 11 11 9 9 Temperatur e, C Top 140 139 140 139 Bottom 141 141 144 143 WO 95/23891 PCT/US95/02719 14 ' Chemical condltlona In accumulator: (TAPPI) AA (Na2o), g/L 98 99.2 10* . 2 • 101. 5 EA(Na20) g/L 83.1 84.3 85.8 86.2 Na2S (Na20), g/L TTA (Na20) , g/L Sulfidity, Von AA 30.4 30 30.3 30.2 CHIP CHARGE & LIQUOR PACKIUGi Chip charge, g 3,700 3,500 3,50 0 3,SO 0 Chip Moisture, * 37-. 3 37.3 37.3 37.3 O.D. Solids, % S2.7 62.7 62.7 62.7 WXRM FIT,!,! Warm Liquor charge, L 24.7 24 24 24 Exit pH, initial 12.8 13.5 13.3 13.3 Charge time, min 15 15 15 15 Temperatur e, C Top 100 104 102 100 Bottom 113 112 111 112 Chemical condltlona In Accumulatori AA(Na20), g/L 23.6 16.4 26.4 17.7 25.7 19.2 28.5 21.1 EA (Na20), g/L 16.4 10.5 17.4 11.5 18 12.4 18.6 13.6 Na2S (Na20), g/L 14.3 11.8 18 12.4 15.5 13.6 19.8 14.9 TTA (Na20), g/L Sulfidity, % on A.A. 60.8 71.7 68.3 70.2 60.3 71 69.5 70.6 WO 95/23891 PCT/US95/02719 15 Solids,% 9 9.25 13.6 12.4 14.3 13.1 14.1 13.1 Solids, g/L 94.3 96.4 147 131 155 141 e 152 141 Total elapsed time, min* 40 31 34 33 DISPLACEMENT: Displaceme nt Volume, L 32 32 32 32 Charge time, min 26 26 26 26 Chemical Con ditlona in accumulator» (TAPPI) AA (Na20), g/L 9.S 10.1 9 10.1 EA (Na20), g/L 9.5 10.1 9 10.1 Na2S (Na20), g/L TTA (Na20), g/L . Sulfidity, % on A.A. 0 0 0 0 * includes fill time, heating time to 120C and time at 120C after warm fill. TABLE 2 RDH Pulping Conditions and Results - "Baseline Casft" Cook Number R5 Post R5 R6 Post R6 R7 Post R7 R8 Post R8 H factor 116 1 765 831 832 AA (TAPPI) to Hot Fill, % 10. 0 10.0 10.0 10.0 Sulfidity (TAPPI), % on AA 30. 2 30.3 30.3 30.2 WO 95/23891 PCT/US95/02719 16 Maximum temp, C 170 170 170 170 Time to max, min 23 27 21 O 21 Time at rr x, min 62 35 41 42 K...h. \ unscreene d 7.6 9.7 9.5 8.9 Kappa, screened 7.2 9.1 8.9 8.8 Total yield,% 47. 3 48.3 48.6 49.1 Total rejects, % 1.1 1.6 1.7 1.4 Screened yield, % 46. 2 46.7 46.9 47.7 Viscosity, 0.5% CED, cp 19. 5 33.2 33.3 32.2 End of Cook Residual: AA(Na20) , g/L CM 25.4 26 25.4 EA (Na20), g/L 19. 2 18 17.4 17.4 Na2S (Na20), g/L 17. 4 14.9 17.4 16 TTA(Na20 ), g/L Solids, % 19 19.1 19.2 19.1 Solids,g/L 210 211 213 210 Sulfidity % on A.A. 62. 4 58.3 66.2 63 WO 95/23891 PCT/US95/02719 17 HOT FILL: Hot liquor charge, L 18 18 18 • 18 Charge time, min 12 13 12 13 Temperatu re, C Top 135 137 137 135 Bottom 153 153 154 155 Chemical conditions in Accumulator: AA(Na20) , g/L 30. 4 23.6 26 21.1 26.7 20.5 26.4 20.5 EA (Na20), g/L 22. 3 16.2 18.6 14.3 18 13.6 18.3 13.6 Na2S (Na20), g/L 16. 1 14.9 14.9 13.6 17.4 13.6 16.1 13.6 TTA (Na20), g/L Sulfidity, % on A.A. 53. 3 63.6 56.9 64.5 65.2 67.3 61.4 67.3 Solids,% 16. 7 17 17.2 17.1 17 17.3 17 16.9 Solids, g/L 183 185 189 187 187 189 186 184 Total elapsed time, min* 46 44 45 44 * includes heating time to 1S5C and time at 1S5C after Initial hot black liquor injection and final hot B/L with W/L mix. WO 95/23891 PCT/US95/02719 18 TABLE 2A RDH Pulping Conditions and Results - "Baseline Case" Cook Number R5 Post R5 R6 Post R6 R7 Post R7 R8 Post R8 WHITE LIQUOR* White liquor charge,L 3.47 3.45 . 3.46 3.47 Hot B/L, L 5.4 7 5.6 5 Charge time, min 9 9 9 9 Temperature , C Top 151 152 151 151 Bottom 149 147 145 144 Chemical oonditlons in acouaulator: (TAPPI) AA (Na20), g/L 100.8 101.4 101.2 100. 8 EA(Na20) g/L 85.6 86 85. 9 85.6 Na2S (Na20), g/L TTA (Na20), g/L 121.6 122.8 123.1 120. 6 Sulfidit y, * on AA 30.2 30.3 30.3 30.2 chtp chahqb £ liotor packtwai Chip charge, g 3,500 3,500 3,500 3,50 0 Chip Moisture , * 37.3 37.3 37.3 37.3 O.D. Solids, V 62.7 62.7 62.7 62.7 WO 95/23891 PCT/US95/02719 19 WARM FILL: Warm Liquor charge, L 24.4 25 25.2 » 24 Exit pH, initial 13.1 13.3 13.5 13.3 Charge time, min 15 15 15 15 Temper-a ture, C Top 109 109 106 106 Bottom 121 120 118 116 Chemical condltlona in Accumulator: (TAPPI) AA(Na20) , g/L 28.1 19.2 27.9 18.6 . 26.7 19.2 26.7 19.7 EA (Na20), g/L 18.3 12.4 18 11.2 18 11.8 18.6 12.1 Na2S (Na20), g/L 19.6 13.6 19.8 14.9 17.4 14.9 16.1 15.1 Sulfidit y, V on A.A. 69.8 70.8 71 79.6 65.2 77.1 60.7 77.2 Solids, % 14.5 15,4 15 16 14.7 15.5 14.6 15.9 Solids, g/L 157 167 164 173 159 168 158 172 Total elapsed time, min* 35 33 31 30 Displace -ment Volume, L 32 32 32 32 Charge time, min 26 26 26 26 Chemical Condltlona in accumulator: AA (Na20), g/L 4.3 4.31 4.3 4.3 EA (Na20), g/L 3.7 3.7 3.7 3.5 WO 95/23891 PCT/US95/02719 20 Na2S (Na20), g/L 1.24 1.24 1.24 9 1.74 Sulfidit y, * on A.A. 27.9 27.9 27.9 27.9 Solids, * 9.7 10.6 10.4 10.3 Solids, g/L 102 112 109 108 4 includes fill time, heating time to 120C and time at 120C after warm fill. TABLE 3 RDH Pulping Conditions and Results - "Best Do-able Case" Cook Number R9 Post R9 R10 Post R10 Rll Post Rll R12 Post R12 H factor 484 558 483 494 AA(TAPPI) to Hot Fill, % 18.9 16.0 16.0 16 Sulfidity (TAPPI),% on AA 30.6 30.2 29.8 30.4 Maximum temp, C 160 160 160 160 Time to max, min 20 20 18 16 Time at max, min 60 72 62 63 Kappa, unscreened 9.1 8.9 9.6 10 Kappa, screened 8.1 8.5 9.2 9.3 Total yield,% 48.1 48.1 49.1 49.1 Total rejects,V 1.1 1.2 1.5 1.3 Screened yield,% 47 46.9 47.6 47.8 Viscosity,0 .5V CED, cp 33.5 33 39.3 32.1 WO 95/23891 PCT/US95/02719 21 End of Cook Residual : AA(Na20), g/L 37.6 32.8 34.1 • 33.5 EA (Na20), g/L 28.6 25.3 25.9 25.3 Na2S (Na20), g/L 17.9 15.2 16.4 16.4 Solids,% 18.8 19.1 19.2 19.3 Solids, g/L 210 212 215 214 Sulfidity % on A.A. 47. 9 45.7 • 48.1 49 HOT FILL: Hot liquor charge, L 18 18 18 18.7 Charge time, min 13 13 13 13 Temperature . c Top 129 132 132 132 Bottom 145 147 147 145 Chamical conditions in Accumulator: AA(Na20), g/L 27.8 25.9 27.3 25.3 27.1 24.6 26.5 23.4 EA (Na20), g/L 19.9 18.9 19.9 18.3 20.2 18.3 19.5 7 17 Na2S (Na20), g/L 15.8 13.4 14.9 13.9 13.9 12.6 13.9 12.6 TTA (Na20), g/L - - - ** - • - Sulfidity, V on A.A. 56.8 54.1 54.2 55.3 50.9 51.2 52.3 54.7 Solids,V 16.2 16.1 16.7 16.7 17 17.2 17.3 17 Solids, g/L 177 176 182 183 187 189 190 186 Total elapsed time, min* 42 42 42 39 * includes heating time to 145C and time at 145C after initial hot black liquor injection and final hot B/L with W/L mix. WO 95/23891 PCT/US95/02719 22 TABLE 3A RDH Pulping Conditions and Results - "Best Do-able Case" © Cook Number R9 Post R9 R10 Post R10 Rll Post Rll R12 Post R12 WHITE LIQUOR< White liquor charge,L 6.17 5.81 5.79 5.6 Hot B/L,L 4.8 5.2 5.0 6.4 Charge time, min 10 10 10 9 Temperature , C Top 139 140 139 141 Bottom 139 145 140 144 Chemical conditions in accumulatort (TAPPI) AA (Na20), g/L 107.3 96.4 96.7 100 EA(Na20) g/L 90.8 81.8 82.3 84.8 TTA (Na20), g/L 125.8 115.6 117.8 120. 9 Sulfidit y, V on AA 30.6 30.2 29.8 30.4 CHIP CHARGE & I.TOTTOR PACKING) I Chip charge, g 3,500 3,500 3,500 3,50 0 Chip Moisture , * 37.3 37.3 37.3 37.3 O.D. Solids, % 62.7 62.7 62.7 62.7 WO 95/23891 PCT/US95/02719 23 WARM FILLi Warm Liquor charge, L 24.5 24.3 23.9 9 24.2 Exit pH, initial 13.3 13.2 13.2 13 Charge time, min 15 15 15 15 Temperature , C Top 108 106 104 108 Bottom 121 117 116 118 Chemical conditions in Accumulatort (TAPPI) AA(Na20) , g/L 27.1 21.1 26.5 21.1 26.5 20.8 25.9 20.8 EA (Na20), g/L 20.5 14.5 18.9 14.1 18.3 13.3 17.7 13.3 Na2S (Na20), g/L 13.3 13.3 15.2 14.1 16.4 15.2 16.4 15.2 Sulfidit y, % on A.A. 48.7 62.6 57.4 66.4 61.9 72.1 63.3 72.1 Solids, V 14.3 14.9 14.8 15.8 15.6 16.1 15 15.6 Solids, g/L 155 161 162 171 170 176 163 170 Total elapsed time, min* 30 30 31 30 dispiacemkmtt Displace -ment Volume, L 32 32 32 32 Charge time, min 26 26 26 26 WO 95/23891 PCT/US95/02719 24 Chemical Conditions in accumulator: (TAPPI) AA (Na20), g/L 9 5 9.8 8.8 » 11.4 EA (Na20), g/L 8.2 9.2 8.2 10.7 Na2S (Na20), g/L 2.5 1.3 1.3 1.3 Sulfidit y, V on A.A. 31.7 13 14.6 13.1 Solids, % 10.3 10.6 11.2 Solids, g/L 109 112 118 * includes fill time, heating time to 120C and time at 120C after warm fill. TABLE 3B PULPING STUDY SUMMARY COOK NUMBER BEST COOK: NEW RDH COOKING PROCEDURE R3 R4 BASELINE COOK: OLD RDH COOKING PROCEDURE R7 R8 BEST DO-ABLE COOK: MODIFIED NEW RDH COOKING PROCEDURE R12 COOKING CONDITIONS WO 95/23891 PCT/US95/02719 25 R3 R4 R7 R8 R12 WARM FILL EA (g/l) as Na20 18 10 co h 18 18.6 3L7.7 Solids, % 14.3 14.1 14.7 14.6 15 HOT FILL EA (g/l) Na20 20 19.8 18 18.3 19.6 Solids, % 11.9 12.7 17 17 17.3 COOKING STAGE AA Charge, % 16 16 10 10 16 H-FACTOR 475 452 831 832 494 MAXIMUM TEMP. DEGREE C 160 160 170 170 160 DISPLACEMENT EA (g/l) as Na20 9 h o h 3.7 3.5 10.7 Solids, % 0 0 10.4 10.3 10 EXAMPLE 1 The following definitive pulps were produced for the bleaching study: Case Cook No. Kappa Brightness, TAPPI "Best" R3 8.8 45.3 R4 9.3 45.0 "Baseline" R7 8.9 40.6 R8 8.8 41.3 "Best Do-able" R10 8.5 41.5 R11 9.2 40.8 R12 9.3 41.5 Five RDH pulps (R3, R4, R7, R8 and R12) were bleached using an (C))(D100)(EO)(D) sequence. However, each of the five RDH pulps were first oxygen delignified in stirred reactors using the conditions shown below in Table 4. WO 95/23891 PCT/US95/02719 26 TABLE 4 OXYGEN DEHGNIFICATION CONDITIONS Best Case Baseline Case "Do-able n Beat Case Sample identification R3 R4 R7 R8 R12 Species Aspen Aspen Aspen Aspen Aspen Cook type RDH RDH RDH RDH RDH Kappa 8.8 9.2 8.9 8.8 9.3 Viscosity, mPa.s 40.7 44.9 33.3 32.2 32.1 Unbl. brightness, % 45.3 45 40.6 41.3 41.5 O-Stape: 95 nsiq. 99°C. 12% cons*. NaOH, V 2 2 2 2 2 02 time, min 60 60 60 60 60 Final pH 12.8 12.9 12.5 12.5 12.3 Kappa 4.7 5.2 4.7 4.5 5 Viscosity, mPa.s 14.4 13.8 12.6 13.6 12.5 Kappa reduction, % 46.6 43.5 47.2 48.9 50 Yield on raw stock, % 95.2 95.8 94.3 98.8 94.4 For the bleaching studies, a 0.225 kappa factor was used in calculating the chlorine dioxide charge in the D100-stage for pulps R3, R7 and R12. A 0.27 kappa factor was used for pulps R4, R8 and R12. Tables 5 through 10 below show the (D100) (Eo)(D) bleaching conditions and results on the oxygen delignified pulps from these cooks. The chlorine dioxide solution concentration was adjusted by a 0.92 factor to compensate for losses of chlorine dioxide in charging the reactors and polyethylene bags during bleaching. WO 95/23891 PCT/US95/02719 27 TABLES Bleaching with (OVDIOOVEolD on Optimal RDH Pulp Kappa factor = 0.225 Sample identification R3 Species Aspen Cook type RDH 02 Pulp Kappa 4.7 Viscosity, mPa.s 14.4 D-1 OO-Staga: 30 min. 68°C. 4.^% cons, Chlorination factor 0.23 C102, \ as available CI}* 1.06 Actual C102, V as avail. CI, 1.15 Substitution, % 100 H2S04, % 1.5 Final pH 2 Residual, g/L avail. CI, 0.14 EO-Staae: 60 min. 74°C. 10% com L. NaOH, % 0.8 02 pressure, psig 25 02 time, minutes 15 Final pH 12.4 K No. (25 Ml) 2.3 Viscosity, mPa.s 13.7 Yield on raw stock, % 93.5 D-Stage: 74°C. 210 min. 10% copp. Sample number #1 #2 #3 #4 #5 #6 #7 #8 C102 as C102, V* O.X 0.3 0.5 0.7 0.9 1.1 0.9 1.1 Actual C102, V C102 0.11 0.33 0.54 0.76 0. 98 1.2 0.98 1.2 NaOH, * 0 0 0.09 0.16 0.25 0.3 0.33 0.42 H2S04, V 0.1 0.05 0 0 0 0 0 0 Final pH 4.1 3.4 3.3 3 2.9 2.6 3.6 3.7 WO 95/23891 PCT/US95/02719 28 Residual as C102, % 0.02 0.01 l-l o o 0.02 0. 02 0.02 0 0 Brightness, %ISO 89.7 90.4 91.2 91.5 91.6 • 92 91.1 91.3 Viscosity, mPa.s 13.1 * 11.2 "" 9.2 * Actual CI02 concentration x 0.92 TABLE 6 Bleachin? with (OMDIOOVEoMDl on Optimal RDH Pulp Kappa factor = 0.27 Sample identification R4 Species Aspen Cook type RDH 02 Pulp Kappa 5.2 Viscosity, mPa.s 13.8 D-100-Stapfi: 30 min. 68°C. 4.2% eons?, Chlorination factor 0.27 C102, % as available Cla* 1.4 Actual C102, % as avail. CI, 1.53 Substitution, % 100 H2S04, % 2 Final pH 1.9 Residual, g/L avail. CI, 0.09 EO-Stage: 60 min. 74°C. 10% cons. NaOH, % 0.8 02 pressure, psig 25 02 time, minutes 15 Final pH H to U1 K No. (25 Ml) 2 Viscosity, mPa.s 13.3 Yield on raw stock, % 92.8 WO 95/23891 PCT/US95/02719 29 D-Staae: 74°C. 210 min. 10% cons. Sample number #1 #2 #3 #4 #5 #6 • #7 #8 C102 as C102, V* 0.1 0.3 0.5 0.7 0.9 1.1 0.9 1.1 Actual C102, % C102 0.11 0.33 0.54 0.76 0.98 1.2 0.98 1.2 NaOH, % 0 0 0.09 0.16 0.25 0.3 0.33 0.42 H2S04, t 0.1 0 .05 0 0 0 0 0 0 Final pH 4.3 3.4 3.4 3.2 2.8 2.7 3.7 3.5 Residual as C102, % 0.01 0.01 0.01 0.02 0.01 0.01 0 0 Brightness, VISO 89.8 90.5 91.2 91.5 91.8 91.8 91.4 91.5 Viscosity, mPa.s 12.5 • - 11.6 9.8 * Actual CI02 concentration x 0.92 TABLE 7 Bleaching with rOWDIOMEoVD) on Baseline RDH Pulp Kappa factor = 0.225 Sample identification R7 Species Aspen Cook type RDH 02 Pulp Kappa 4.5 Viscosity, mPa.s 13.6 (D100)-Stage: 30 min. 68°C. 4.2% cons. Chlorination factor 0.23 C102, % as available Cla* 1.01 Actual C102, % aa avail. CI, 1.1 Substitution, % 100 H2S04, % 2 Final pH 2.7 Residual, g/L avail. Cla 0.01 WO 95/23891 PCTVUS95/02719 30 EO-Staqe: 60 min. 74°C. 10% eonf?, NaOH, % 0.8 P 02 pressure, psig 25 02 time, minutes 15 Pinal pH 12.3 K No. (25 Ml) 2.3 Viscosity, mPa.s 13.3 Yield on raw stock, V 97 D-Statpe: 74°C. 210 min. 10% eo DSJL Sample number #1 #2 #3 #4 #5 #6 C102 as C102, %• 0.1 0.3 0.5 0.7 0.9 1.1 Actual C102, % C102 0.11 0.33 0.54 0.76 0.98 1.2 NaOH, % 0 0 0.08 0.2 0.33 0.42 H2S04, % 0.1 0.05 0 0 0 0 Final pH 4 3.5 3.4 3.4 3.4 3.8 Residual as C102, % 0 0 0 0 0 0 Brightness, %IS0 87.6 88.7 89.7 90.3 90.3 90.6 Viscosity, mPa.s 12.6 - - 11.4 - 9.6 * Actual CI02 concentration x 0.92 TABLE 8 Bleaching with fOVDIOOVEoVm on Baseline RDH Pulp Kappa factor ■ 0.27 Sample identification R8 Species Aspen Cook type RDH 02 Pulp Kappa 4.7 Viscosity, mPa.s 12.6 (D1001-Staae: 30 min. 68°C, 4.2% cons. Chlorination factor 0.27 C102, % as available CI]* 1.27 Actual C102, % as avail. Cla 1.38 WO 95/23891 PCT/US95/02719 31 Substitution, % 100 H2S04, % 2 Final pH 1.9 > Residual, g/L avail. CI, 0.07 EO-Staae: 60 min. 74°C. 10% co NaOH, % 0.8 02 pressure, psig 25 02 time, minutes IS Final pH 12.2 K No. (25 Ml) 2.1 Viscosity, mPa.s 12.6 Yield on raw stock, % 94.2 D-Staae: 74°C. 210 min. 10% co ns. Sample number #1 #2 #3 #4 #5 #6 C102 as C102, %* 0.1 0.3 0.5 0.7 0.9 1.1 Actual C102, % C102 0.11 0.33 0.54 0.76 0.98 1.2 NaOH, % 0 0 0.08 0.2 0.33 0.42 H2S04, * 0.1 0.05 0 0 0 0 Final pH 3.6 3.1 3.1 3.1 3.2 3.6 Residual as C102, % 0 0 0 0 0 0 Brightness, tISO 87 88.7 89/3 90.1 90.5 90.5 Viscosity, mPa.s 12.2 - - 11.2 - 9.5 * Actual CI02 concentration x 0.92) WO 95/23891 PCT/US95/02719 32 TABLES Bleaching with fOttDIOtWEoKDl on Best "Do-able" RDH Pulp Kappa factor = 0.225 Sample identification R12 Species Aspen Cook type RDH 02 Pulp Kappa 5 Viscosity, mPa.s 12.5 D-100-Staae: 30 min. 68°C. 4.2% cons. Chlorination factor 0.03 C102, V as available CI]* 1.13 Actual C102, % as avail. CI, 1.22 Substitution, V 100 H2S04, % 1.5 Final pH 2 Residual, g/L avail. CI, 0.04 EO-Staae: 60 min. 74°C. 10% co as. NaOH, lr 0.8 02 pressure, psig 25 02 time, minutes 15 Final pH 12.7 K No. (25 Ml) 2.3 Viscosity, mPa.s 11.9 Yield on raw stock, % - D-Staae: 74°C. 210 min. 10% co ns. Sample number #1 #2 #3 #4 #5 *6 C102 as C102, ** 0.1 0.3 0.5 0.7 0.9 1.1 Actual C102, % C102 0.11 0.33 0.54 0.76 0.98 1.2 NaOH, % 0 0 0.08 0.2 0.33 0.42 H2S04, % 0.1 0.05 0 0 0 0 Final pH 4.1 3.7 3.4 3.4 3.4 3.3 Residual as C102, * 0.01 0.01 0.01 0.01 O • o 0.01 Brightness, %IS0 68.9 90 90.8 91 91.6 91.8 Viscos ity, mPa.s 11.9 - - 10.5 - 9.4 * Actual CI02 concentration x 0.92 WO 95/23891 PCT/US95/02719 33 TABLE 10 Bleaching with (OMDIOOMEoMm on Best "Do-able" RDH Pulp Kappa factor * 0.27 Sample Identification R12 Species Aspen Cook type RDH 02 Pulp Kappa 5 Viscosity, mPa.s 12.5 D-1 OO-Staae: 30 min. 68°C. 4.2<* Chlorination factor 0.03 C102, % as available Cl3+ 1.35 Actual C102, % as avail. CI, 1.47 Substitution, % 100 H2S04, % 2 Final pH 2.3 Residual, g/L avail. Cla 0.08 EO-Stage: SO min. 74°C. 10% cqr\s. NaOH, % 0.8 02 pressure, psig 25 02 time, minutes 15 Final pH 12.6 K No. (25 Ml) 2.2 Viscosity, mPa.s 12.2 Yield on raw stock, % - D-Staae: 74°C. 21 0 min. 10% CO ns. Sample number #i #2 #3 *4 #5 #6 C102 as C102, V* 0.1 0.3 0.5 0.7 0.9 1.1 Actual C102, % C102 0.11 0.33 0.54 0.76 0.98 1.2 NaOH, % 0 0 0.08 0.2 0.33 0.42 H2S04, % 0.1 0.05 0 0 0 0 Final pH 3.9 3 .4 3.3 3.3 3.3 3.2 Residual as C102, % 0.01 r4 O o 0.01 0.01 0 0.01 Brightness, %XSO 68.9 90 90.8 91.1 91.5 91.8 Viscosity, mPa.s 12.4 - - 10.7 - 9.5 * Actual CI02 concentration x 0.92 WO 95/23891 FCT/US95/02719 34 As shown in FIGS. 5 and 5A, the use of a higher kappa factor did not appear to reduce the D1 -stage chlorine dioxide requirements. The best case RDH pulps (R3 and R4) produced 1.5 to 2 points higher brightness than the baseline case RDH pulps (R7 and R8) at equivalent chlorine dioxide charges. From FIGS. 6 and 6A, it is shown that the best do-able case RDH pulp (R12) produced intermediate brightness between the best case RDH pulp (R3) and the baseline case RDH pulp (R7). FIGS. 7 and 7A show that the best do-able case RDH pulp (R12) gave intermediate brightness between the best case RDH puip (R4) and the baseline case RDH pulp (R8). From the pulp bleaching studies, a summary of the results is shown below in Table 11. The easiest pulps to bleach were the best case pulps. The most difficult to bleach were the baseline case pulps with the bleachability of the best do-able case falling between the first two cases. Results indicated that a combination of high alkalinity (white liquor addition at the warm and hot fill mode plus cooking stage, AA charge between 15% AA and 35% AA) and a low cooking temperature (approximately 150°C - 167°C) improves pulp bleachability and, thus, final brightness of puip. It should be noted that the black liquor strength during the RDH cook should be maintained. WO 95/23891 PCT/US95/02719 35 • TABLE 11 BLEACHING STUDY SUMMARY COOK NUMBER BEST COOK: NEW RDH COOKING PROCEDURE R3 R4 BASELINE COOK: OLD RDH COOKING PROCEDURE R7 R8 BEST DO-ABLE COOK: MODIFIED NEW RDH COOKING PROCEDURE R12 BLEACHING RESULTS KAPPA 0.225 IN THE D (100% C/02 SUBSTITUTION) FACTO R FINAL BRIGHTNESS, % ISO CI02 CHARGES IN THE LAST D STAGE, % 0.1 0.3 0.5 0.7 0.9 1.1 R3 (BEST COOK) 89.7 90.4 91.2 91.5 91.6 92 R12 (BEST DOABLE) 88.9 90 90.8 91 91.6 91.8 R7 (BASELINE) 87.6 88.7 89.7 00.3 90.3 90.6 KAPPA FACTOR Q 27IN THE D (10Q% CJQ2 SUBSTITUTION) FINAL BRIGHTNESS, %ISO WO 95/23891 PCT/US95/02719 36 CI02 CHARGES IN THE LAST DOTAGE, % 0.1 0.3 0.5 0.7 0.9 1.1 R4 (BEST COOK) 89.8 90.5 91.2 91.5 91.8 91.8 R12 (BEST DOABLE) 88.9 90 90.8 91.1 91.5 91.8 R8 (BASELINE) 87 88.7 89.3 90.1 90.5 90.5 It should be understood the various changes and modifications to the presently preferred embodiments described herein will be apparent to those in the art. Such changes and modifications can be made without departing from the spirit and scope of the present invention and without diminishing its attended advantages. It is, therefore, intended that such changes and modifications be covered by the appended claims. </div>

Claims

<div class="application article clearfix printTableText" id="claims"> 282616 WO 95/2389'! PCT/US95/C2719 I CLAIM:

1. A batch digesting process of the type using rapid displacement heating to produce delignified pulp, wherein spent liquor produced in a digester (10) as a result of cooking a mass of cellulosic material with cooking liquor is displaced and collected in accumulators (24.28.58) as hot black liquor and as warm black liquor so as to conserve and utilize the heat of the spent liquor to preheat another mass of cellulosic material in warm black liquor and hot black liquor displacement type pretreatments with progressively hotter spent liquors prior to cooking, characterized in that: white liquor is added to the warm black liquor during the warm black liquor pretreatmeni and to the hot black liquor during the hot DlacK liquor pretreatment stages and to the cooking liauor during the cooking stage in the digesting process, the total white liquor aaaed naving a oistriouted charge between approximately 15% active alkaiinity and about 35% active alkalinity: and the temoerature of the cookinc liquor is raised tc between 150' - 155°C for the cooking stage.

2. The process of Claim 1. wherein the white iiquor solution is added to the warm black liquor (temperature between 90: and 150°C) and to the hot black liquor (temperature between 150c and 165°C) in a predetermined amount.

3. The process of Claim 1, further characterized in that white liquor is added to a cool black liquor (temperature between 70e and 90°C), and the mass of cellulosic material is pretreated with said cool black liquor with white liquor added thereto, before the warm black liquor pretreatment. 37/A 28 2

4. The process of Claim 1 wherein the preferred total white liquor charge is > 20% AA and < 35% AA.

5. The process of Claim 1, wherein the preferred cooking temperature ranges between 155° - 165°C.

6. A method for producing bleachable grade pulp according to the process of Claim 1, comprising the steps of: 2 3 2 6 2 6 WO 95/23891 PCT/US95/02719 38 (a) introducing wood chips into a digester (10); (b) pretreating the chips with warm black liquor below cooking temperature: (c) displacing the warm black liquor from the digester (10) with at least one volume of a hot black liquor; (d) increasing the temperature of the digester (10) to a cooking temperature: (e) maintaining said temperature until the chips are digested: (f) displacing the contents of the digester (10) with a liquid filtrate derived from puio washing: and (g) emptying the contents of the digester (10) by applying gas pressure to the interior of the digester (10) or pumping out. cnaracterizec in tnat: white iiquo* is added to tne warm black liauor used in the pretreating siec aoditiona! wnite iiquor is added to tne hot black liquor for the disDiacing steo. anc ouring the steos of increasing and maintaining temoeraiure a maximum temperature of aboui 165eC is achieved.

7. The method of Claim 5. including the step of pretreating the chips with a mixture of cool black iiquor and white iiquor (or NaOH solution), before the step of pretreating with warm black liquor.

8. The method of Claim 6, wherein the total white liquor used has a total charge ranging between 15% AA - 35% AA.

9. The method of Claim 8. wherein the preferred total white liquor charge is > 20% AA. AuV.Z..--- N.2. PATE?-,'7 11 JUN 1996 282616 WO 95/23891 PCT/US95/02719 38/A

10. The method of Claim 6. wherein the preferred cooking temperature ranges between approximately 155c - 165°C.

11. The method of Claim 6. including the step of displacing the contents of the digester (10) with any combination of washer filtrate and white liquor (or NaOH solution). EHD OF CLAIMS </div>