CA2184706C - Impact of temperature and alkali charge on pulp brightness - Google Patents

Abstract

A method for improving pulp brightness utilizes a combination of a distribut ed total white liquor charge during the warm fill, hot f ill and cooking stages of a batch cooking process and low cooking temperatures d uring the cooking stage. The high total white liquor cha rge ranges between 15 % AA ? 35 % AA, while the cooking temperatures range betwe en 150 ? 168 .degree.C.

Description

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 lignin 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 bleachability problems have 2 ~ g q. 7 Q ~ PCTIUS9SI02719 occurred following the use of conventional, ECF and TCF bleaching processes.
High cooking temperatures and low black liquor strer~th 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 OF 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/p2719 BRIEF DESCRIPTION OF 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 displacement 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).

FIG. 5A illustrates a plot of D1-brightness verses 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 far 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).
yFTOU Fn r~ESCRIPTION OF
IHE 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 .~_ 2184706 digestion of wood chips. More specifically, the method involves the addition of a white liquor charge commencing at the start of the RDH 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).
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 PCTIUS95I02719 ..r..

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 of the hot fill, hot white liquor stored in the hot white liquor accumulator 38 is pumped out by means of a pump 30 where it combines with the hot black liquor leaving the hot liquor accumulator 28, the combined liquors thin 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 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 and initial hot fill stages. This is in contrast to the existing RDH cooking process, wherein white liquor is added only during thecmiddle 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.
F1G. 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 . M WO 95123891 PCT/US95/02719 s 2184706 warm and hot fill modes. Only warm black liquor is leaving the warm liquor accumulator (B tank) 24 to flow through line 56 during~the 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 the hot white liquor accumulator 38 is mixed with black liquor leaving hot liquor accumulator 28 by line 72, and is further mixed with the black liquor exiting 'the second hot liquor accumulator 58 by lines 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/US95102719 21 ~4~lOb Without White Liquor Addition at Warm and Hot Fill Operations (FIG. 3) Initial Hot Fill Operation Total Flow. aallcook 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 eration Total Flow. gallcook Dissolved Organic.
C1 Black liquor 19877 10.1 C2 black liquor 7971 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 gll AA) and hot black liquor (temperatures between approximately 100° and 168°C and its strength between 8 and 30 gll 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

W O 95123891 21 ~ 4 7 0 ~ PCTIUS95J02719 controls the total dissolved solids (TDS) concentration and black liquor strength using any combination of black liquor, white liquor anc~ 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.

RDH Pulp i~gi Conditions and Results - "Best Case"
Post Post Post Post Cook NumberR1 R1 R2 R2 R3 R3 R4 R4 H factor 937 532 475 452 (TAPPI) 16.0 16.0 ' 16.0 16 to Hot Fill, a Sulfidity 30.4 30 30.3 30.

(TAPPI),% 2 on AA

aximum 160 160 160 160 temp, C

Time to 16 20 17 19 max, min Time at 130 37 60 57 max, min WO 95123891 PCTItJS95/02719 Kappa, 7.2 8.9 9.2 9.8 unscreened Kappa, 7.1 8.2 8.8 a 9.3 screened Total 46.3 47.3 48.2 1.7 yield,%

Total 0.9 1.2 1.4 1.7 rejects,%

Screened 45.4 46.1 46.8 46.

vield,%

iscosity,0 26 39 40.7 44.

.5% CED, cp End of Cook Residual:

AA(Na20), 28.2 31.6 32.9 31.

g/L 6 EA (Na20), 21.1 24.2 24.2 24.

g/L 2 14.

Na2S 14.3 14.9 17.4 9 (Na20), g/L

TTA(Na20),g /L

Solids,% 14.8 15.9 16.7 16.

Solids.g/L 161 ~ 173 183 185 Sulfidity 51 47.1 52.9 47.
%

on A.A. 1 Hot liquor 18 18 18 18 charge, L

Charge 13 14 13 13 time, min Temperature C

Top 130 127 127 128 Bottom 145 141 141 141 ~K,. WO 95123891 PCT/US95102719 Chemical Coaditions in Accumulator:

(Na20), 22.9 20.6 25.7 21.7 29.2 e24.627. 23.6 g/L 3 EA (Na20),16.1 15.1 1B.1 16.1 20.2 18.3 19. 17.9 g/L 8 Na2S 13.7 11 15.2 11.2 18 12.7 14. 11.4 (Na20), 9 g/L

TTA (Na20),- - - - - - - -g/L

Sulfidity,59.4 53.4 59.1 51.6 61.6 51.2 54. 48.3 on A.A. 9 Solids,% 9.5 10.5 11.1 11.9 11.9 13.6 12. 13.6 Solids, 99.8 111 118 127 127 146 137 146 g/L

Total 45 43 41 42 elapsed time, min*

* includes heating time to 145C and time at 145C after initial hot black liquor injection and final hot BIL with WIL mix.

RDH Pulging Conditions and Results - "Best Case"

Cook Post Post Post Post Number R1 R1 R2 R2 R3 R3 R4 R4 White 6.04 5.65 5.53 5.52 liquor charge,L

Hot B/L,L 5 4.13 5.5 5.8 Charge 11 11 9 9 time, min Temperatur e, C

Top 140 139 140 139 Bottom 141 141 144 143 WO 95123891 ~, 8 4 7 0 b PCTIUS95102719 Chemical in (TAPPI) conditions accumulator:

(Na2o) 98 99.2 10~. 101.

, 2 ~ 5 g/L

EA(Na20) 83.1 84.3 85.8 86.2 g/L

Na2S - - - -(Na20) , g/L

TTA

(Na20), g/L

Sulfidity,30.4 30 30.3 30.2 %on AA

Chip 3,700 3,500 3,50 3,50 charge, 0 0 g Chip 37.3 37.3 37.3 37.3 oisture, O.D. 62.7 62.7 62.7 62.7 Solids, warm Liquor charge, 24.7 24 24 24 L

Exit pH, 12.8 13.5 13.3 13.3 initial Charge 15 15 15 15 time, min Temperatur e, C

Top 100 104 102 100 Bottom 113 112 111 112 Chemical in conditions Accumulator:

(Na20), 23.6 16.4 26.4 17.7 25.719.2 28.5 21.1 g/L

EA (Na20),16.4 10.5 17.4 11.5 18 12.4 18.6 13.6 g/L

Na2S 14.3 11.8 18 12.4 15.513.6 19.8 14.9 (Na20), g/L

TTA - - - - -(Na20), g/L

Sulfidity,60.8 71.7 68.3 70.2 60.371 69.5 70.6 on A.A.

Solids,% 9 9.25 13.6 12.4 14.3 13.1 14.1 13.1 Solids, 94.3 96.4 147 131 155 141 152 141 g/L

Total 40 31 34 33 elapsed time, min*

Displaceme32 32 32 32 nt Volume, L

Charge 26 26 26 26 time, min Chemical Coaditioas in accumulator:
(TAPPI) (Na20), 9.5 10.1 9 10.1 g/L

EA (Na20),9.5 10.1 9 10.1 g/L

Na2S - - - -(Na20), g/L

TTA

(Na20), g/L _ _ _ _ Sulfidity,0 0 0 0 % on A.A.

includes fill time, heating time to 120C and time at 120C after warm fill.

RDH Pul~g Conditions and Results - "Baseline Case"
Cook Post Post Post Post Number R5 R5 R6 R6 R7 R7 R8 R8 H factor 1 765 831 832 AA 10. 10.0 10.0 10.0 (TAPPI) 0 to Hot Fill, Sulfidity 30. 30.3 30.3 30.2 (TAPPI),% 2 on AA

Maximum 170 170 170 170 temp, C

Time to 23 27 21 ' 21 max, min Time at 62 35 41 42 max, min Kappa, 7.6 9.7 9.5 8.9 unscreene d Kappa, 7.2 9.1 8.9 8.8 screened Total 47. 48.3 48.6 49.1 yield, 3 %

Total 1.1 1.6 1.7 1.4 rejects, Screened 46. 46.7 46.9 47.7 yield, 2 %

Viscosity,19. 33.2 33.3 32.2 0.5% 5 CED, cp End of Cook Residual:

AA(Na20) 27. 25.4 26 25.4 EA 19. 18 17.4 17.4 (Na20), 2 g/L

Na2S 17. 14.9 ~ 17.4 16 (Na20), 4 g/L

TTA(Na20 ), g/L

Solids,% 19 19.1 19.2 19.1 Solids,g/L210 211 213 210 Sulfidity 62. 58.3 66.2 63 on A.A. 4 ~1~4706 Hot liquor18 18 18 18 charge, L

Charge 12 13 12 13 time, min Temperatu re, C

Top 135 137 137 135 Bottom 153 153 154 155 Chemical conditions in Accumulator:

AA(Na20) 30. 23.6 26 21.1 26.7 20.5 26.4 20.5 g/L 4 EA 22. 16.2 18.6 14.3 18 13.6 18.3 13.6 (Na20), 3 g/L

Na2S 16. 14.9 14.9 13.6 17.4 13.6 16.1 13.6 (Na20), 1 g/L

_ _ _ _ _ _ _ _ (Na20), g/L

Sulfidity,53. 63.6 56.9 64.5 65.2 67.3 61.4 67.3 on A.A. 3 Solids, 16. 17 17.2 17,1 17 17.3 17 16.9 %

Solids, 183 185 189 187 187 189 186 184 g/L

Total 46 44 45 44 elapsed time, min ~"

* includes heating time to 155C and time at 155C after initial hot black liquor injection and final hot B/L. with WIL mix.

RDH Purina Conditions and Results - "Baseline Case"
Cook Post Post Post Post Number R5 R5 R6 R6 R7 R7 R8 R8 ite 3.47 3.45 3.46 3.47 liquor charge,L

Hot 5.4 7 5.6 5 B/L,L

Charge 9 9 time, min Tempera-ture, C

Top 151 152 151 151 Bottom 149 147 145 144 Chemicalconditions in (TAPPI) accumulator:

100.8 101.4 101.2 100.

(Na20), 8 g/L

EA(Na20) 85.6 86 85.9 85.6 g/L

Na2S - - - -(Na20), g/L

TTA 121.6 122.8 123.1 120.

(Na20), 6 g/L

Sulfidit 30.2 30.3 30.3 30.2 y, %
on cuTV Rf:E & R RIHS~:
f'HA LIQUO PA~

Chip 3,500 3,500 3,500 3,50 charge, g Chip 37.3 37.3 37.3 37.3 Moisture O.D. 62.7 62.7 62.7 62.7 Solids, 21$x.7 pb arln 24.4 25 25.2 24 Liquor charge, L

Exit 13.1 13.3 13.5 13.3 pH, initial Charge 15 15 15 15 time, min Temper-a ture, C

Top 109 109 106 106 Bottom 121 120 118 116 Chemical PPI) conditions is Accumulator:
(TA

(Na20) 28.1 19.2 27.9 18.6 26.7 19.2 26.7 19.7 g/L

EA 18.3 12.4 18 11.2 18 11.8 18.6 12.1 (Na20), g/L

Na2S 19.6 13.6 19.8 14.9 17.4 14.9 16.1 15.1 (Na20), g/L

Sulfidit69.8 70.8 71 79.6 65.2 77.1 60.7 77.2 y, %
on .A.

Solids, 14.5 15,.4 15 16 14.7 15.5 14.6 15.9 Solids, 157 167 164 173 159 168 158 172 g/L

Total 35 33 31 30 elapsed time, min*

Displace32 32 32 32 -ment olume, L

Charge 26 26 26 26 time, min Chemical Conditions in accumulator:

4.3 4.31 4.3 4.3 (Na20), g/L

EA 3.7 3.7 3.7 3.5 (Na20), g/L

2~
Na2S 1.24 1.24 1.24 1.74 a (Na20), g/L

Sulfidit27.9 27.9 27.9 27.9 y, %
on .A.

Solids, 9.7 10.6 10.4 10.3 Solids, 102 112 109 108 g/L

* includes fill time, heating time to 120C and time at 120C after wane fill.

RDH Pulping ~'~~''~'+~nnc anti RPCnItS - "Best Do-able Case"
Cook Number Post Post Post Post H factor 484 558 483 494 (TAPPI) 18.9 16.0 16.0 16 to Hot Fill, Sulfidity 30.6 30.2 29.8 30.4 (TAPPI),%

on AA

i 160 160 160 160 mum ax temp, C

Time to 20 20 18 16 ax, min Time at 60 72 62 63 max, min Kappa, 9.1 8.9 9.6 10 unscreened Kappa, 8.1 8.5 9.2 9.3 screened Total 48.1 48.1 49.1 49.1 yield,%

Total 1.1 1.2 1.5 1.3 rejects,%

Screened 47 46.9 47.6 47'8 yield,%

Viscosity,033.5 33 39.3 32.1 .5% CED, cp End of Cook Residual:

(Na20), 37.6 32.8 34.1 ~ 33.5 g/L

EA (Na20),28.6 25.3 25.9 25.3 g/L

Na2S 17.9 15.2 16.4 16.4 (Na20), g/L

Solids,% 18.8 19.1 19.2 19.3 Solids, 210 212 215 214 g/L

Sulfidity 47.9 45.7 , 48.1 49 %
on A.A.

Hot liquor18 18 18 18.7 charge, L

Charge 13 13 13 13 time, min Temperature C

Top 129 132 132 132 Bottom 145 147 147 145 Chemical conditions in Accumulator:

(Na20), 27.8 25.9 27.3 25.3 27.1 24.6 26.5 23.4 g/L

EA (Na20),19.9 18.9 19.9 18.3 20.2 18.3 19.5 17 g/L 7 Na2S 15.8 13.4 14.9 13.9 13.9 12.6 13.9 12.6 (Na20), g/L

TTA (Na20),- - - - - - - -g/L

Sulfidity,56.8 54.1 54.2 55.3 50.9 51.2 52.3 54.7 on A.A.

Solids,% 16.2 16.1 16.7 16.7 1? 17.2 17.3 17 Solids, 177 176 182 183 187 189 190 186 g/L

Total 42 42 42 39 elapsed time, min*

* includes heating time to 145C and time at 145C after initial hot black liquor injection and final hot BIL with WIL mix.

WO 95123891 2 ! 8 4 7 0 b PCT/US95102719 F3DH P~~i_na Conditions and Results - "Best Do-able Case"
Cook Post Post Post Post Number R9 R9 R10 R10 R11 R11 R12 R12 White 6.17 5.81 5.79 5.6 liquor charge,L

Hot 4.8 5.2 5.0 6.4 B/L, L

Charge l0 10 10 time, in Tempera-ture, C

Top 139 140 139 141 Bottom 139 145 140 144 Chemical conditions in accumulator:
(TAPPI) 107.3 96.4 96.7 100 (Na20), g/L

EA(Na20) 90.8 81.8 82.3 84.8 g/L

TTA 125.8 115.6 117.8 120.

(Na20), g/L

Sulfidit 30.6 30.2 29.8 30.4 y, % on Chip 3,500 3,500 3,500 3,50 charge, 0 g Chip 37.3 37.3 37.3 37.3 oisture %

O.D. 62.7 62.7 62.7 62.7 Solids, arm 24.5 24.3 23.9 24.2 Liquor charge, L

Exit 13.3 13.2 13.2 13 pH, initial Charge 15 15 15 15 time, min Tempera-ture , C

Top 108 106 104 108 Bottom 121 117 116 118 Chemical conditions in Accumulator:
(TAPPI) (Na20) 27.1 21.1 26.5 21.1 26.5 20.8 25.9 20.8 g/L

EA 20.5 14.5 18.9 14.1 18.3 13.3 17.7 13.3 (Na20), g/L

Na2S 13.3 13.3 15.2 14.1 16.4 15.2 16.4 15.2 (Na20~, g/L

Sulfidit48.7 62.6 57.4 66.4 61.9 72.1 63.3 72.1 y, %
on .A.

Solids, 14.3 14.9 14.8 15.8 15.6 16.1 15 15.6 Solids, 155 161 162 171 170 176 163 170 g/L

Total 30 30 31 30 elapsed time, min*

Displace32 32 32 32 -ment Volume, L

Charge 26 26 26 26 time, min Chemical Conditions in accumulator:
(TAPPI) -.

9.5 9.8 8.B 11.4 (Na20), g/L

EA 8.2 9.2 8.2 10.7 (Na20), g/L

Na2S 2.5 1.3 1.3 1.3 (Na20), g/L

Sulfidit 31.7 13 14.6 13.1 y, % on .A.

Solids, 10.3 10.6 11.2 -%

Solids, 109 112 118 -g/L

' includes fill time, heating time to 120C and time at 120C after warm fill.

PULPING STUDY SUMMARY
COOK NUMBER

BEST COOK: NEW RDH COOKING PROCEDURE R3 R4 BASELINE COOK: OLD RDH COOKING PROCEDURER7 R8 BEST DO-ABLE COOK: MODIFIED NEW RDH R12 COOKING
PROCEDURE

COOKING CONDITIONS

WARM FILL

EA (g/1) as 18 18.6 18 18.6 Q17.7 Na20 Solids, % 14.3 14.1 14.7 14.6 15 HOT FILL

EA (g/1) Na2020 19.8 18 18.3 19.6 Solids, % 11.9 12.7 17 17 17.3 COOKING STAGE

Charge, 'k 16 16 10 10 16 MAXIMUM TEMP.160 160 170 170 160 DEGREE C

DISPLACEMENT

EA (g/1) as 9 10.1 3.7 3.5 10.7 a20 [Solids, % 0 [ 0 10.4 10.3 10 I

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"R 10 8.5 41.5 R1 1 9.2 40.8 R12 9.3 41.5 Five RDH pulps (R3, R4, R7, R8 and R12) were bleached using an (O)(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.

OXYGEN DELIGNIFICATION CONDITIONS
"Do-able ;gPSt Basel,'-ne "
Case Case Best Case Sample R3 R4 R7 RB R12 identification Species Aspen Aspen Aspen Aspen Aspen Cook type RDH RDH RDH RDH RDH

Kappa g.8 9.2 8.9 8.8 9.3 Viscosity, mPa.s40.7 44.9 33.3 32.2 32.1 Until. 45.3 45 40.6 41.3 41.5 brightness, O-Staae~ 95 ~sig, 99C.
12% cons.

NaOH, % 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.s14.4 13.8 12.6 13.6 12.5 Kappa reduction,46.6 43.5 47.2 48.9 50 Yield on raw 95.2 95.8 94.3 98.8 94.4 stock, %

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.

2~ $q-706 Kappa factor = 0.225 Sample identification R3 Species Aspen Cook type RDH

02 Pulp Kappa 4.7 iscosity, 14.4 mPa.s Chlorination 0.23 factor C102, % as 1.06 available C12*

ctual C102, 1.15 % as avail.
C1, Substitution, 100 %

H2S04. % 1.5 Final pH 2 Residual, 0.14 g/L avail.
C1, NaOH, % 0.8 02 pressure, 25 psig 02 time, 15 minutes Final pH 12.4 K No. (25 2.3 M1) iscosity, 13.7 mPa.s Yield on 93.5 raw stock, %

Sample number#1 #2 #3 #4 #5 #6 #7 #B

C102 as C102,0.1 0.3 0.5 0.7 0.9 1.1 0.9 1.1 %*

ctual C102, 0.11 0.33 0.54 0.76 0.98 1.2 0.98 1.2 % C102 NaOH, % 0 0 0.09 0.16 0.25 0.3 0.33 0.42 H2S04, % 0.1 0.05 0 0 0 0 0 0 Final pH 4 3 3 . 3 2 . 2 ~ 3 ~ 3 . . 3 9 . . .
1 4 6 6 '7 l Residual 0.02 0.01 0.01 0.02 0.02 0.02 0 0 as C102, Brightness, 89.7 90.4 91.2 91.5 91.6 ~ 91.1 91.3 %ISO

Viscosity, 13.1 - - 11.~ - - - 9.2 mPa.s ' Actual C102 concentration x 0.92 Em~~~~~o ~~,~+h (Y;;D100y(Eo)(~) on Optimal RDH Pulp Ka~oa factor = 0.27 Sample identification R4 Species Aspen Cook type ~H

02 Pulp Kappa 5.2 iscosity, mPa.s 13.8 Chlorination factor 0.27 C102, % as available 1.4 Clz*

ctual C102, % as avail.1.53 Clz Substitution, % 100 H2S04, % 2 Final pH 1.9 Residual, g/L avail. 0.09 NaOH, % 0.8 02 pressure, psig 25 02 time, minutes 15 Final pH 12.5 K No. (25 M1) 2 iscosity, mPa.s 13.3 Yield on raw stock, 92.8 %

Sample #1 #2 #3 #4 #5 #6 #7 #8 number o C102 as 0.1 0.3 0.5 0.7 0.9 1.1 0.9 1.1 C102, %*

ctual 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, % 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 0.01 0.01 0.01 0.02 0.01 0.01 0 0 as C102, Brightness,89.8 90.5 91.2 91.5 91.8 91.8 91.4 91.5 %ISO

iscosity, 12.5 - - 11.6 - - - 9.8 mPa.s ' Actual C102 concentration x 0.92 TABLE T
Kappa factor = 0.225 Sample identification R7 Species Aspen Cook type RDH

02 Pulp Kappa 4.5 iscosity, mPa.s 13.6 Chlorination factor 0.23 C102, % as available Clz* 1.01 ctual C102, % as avail. C1, 1.1 Substitution, % 100 H2S04, % 2 Final pH 2.7 Residual, g/L avail. C12 0.01 P' fi0 min. 74C.

10% cons.

NaOH, % 0.8 c' 02 pressure, 25 psig 02 time, minutes 15 Final pH 12.3 K No. (25 M1) 2.3 iscosity, mPa.s 13.3 Yield on raw 97 stock, %

Sample number #1 #2 #3 #4 #5 #6 C102 as C102, 0.1 0.3 0.5 0.7 0.9 1.1 %*

ctual 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, %ISO87.6 88.7 89.7 90.3 90.3 90.6 Viscosity, mPa.s12.6 - - 11.4 - 9.6 ' Actual C102 concentration x 0.92 Bleaching ~rs ith (.Ol(D1 Q~~(Eo~(D_)~ on Baseline RDH Pulp Kappa factor = 0.27 Sample identification R8 Species Aspen Cook type ~H

02 Pulp Kappa 4.7 Viscosity, mPa.s 12.6 Chlorination factor 0.27 C102, % as available C12* 1.27 ctual C102, % as avail. 1.38 WO 95!23891 PCTIUS95/02719 21 gq~~70g Substitution, 100 %

H2S04, % 2 Final pH 1.9 Residual, g/L 0.07 avail. C1~

aOH, % 0.8 02 pressure, 25 psig 02 time, minutes 15 Final pH 12.2 K No. (25 M1) 2.1 iscosity, mPa.s 12.6 Yield on raw 94.2 stock, %

Sample number #1 #2 #3 #4 #5 #6 C102 as C102, 0.1 0.3 0.5 0.7 0.9 1.1 %*

ctual 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 0 0 0 0 0 0 C102, Brightness, 87 88.7 89:3 90.1 90.5 90.5 %ISO

iscosity, mPa.s12.2 - - 11.2 - 9.5 ' Actual C102 concentration x 0.92) Blea~hing with~0)(D100)lEo~(D) on Best "Do-able" R~'JH Puln Kappa factor = 0.225 Sample identification R12 ~

Species Aspen Cook type ~H

02 Pulp Kappa 5 iscosity, mPa.s 12.5 Chlorination 0.03 factor C102, % as available 1.13 Clz*

ctual C1O2, % 1.22 as avail. C12 Substitution, 100 %

H2S04, % 1.5 Final pH 2 Residual, g/L 0.04 avail. C1z NaOH, % 0.8 02 pressure, 25 psig 02 time, minutes 15 Final pH 12.7 K No. (25 M1) 2.3 Viscosity, mPa.s 11.9 Yield on raw -stock, %

o i o Sample number #1 #2 #3 #4 #5 #6 C102 as C102, 0.1 0.3 0.5 0.7 0.9 1.1 %*

ctual 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 4 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 0.01 0.01 Brightness, %ISOB8.9 90 90.8 91 91.6 91.8 Viscosity, mPa.s11.9 - - 10.5 - 9~4 " Actual C102 concentration x u.S~

Bleaching with (O)(D100)(Eo)(D) on Best "Do-able" RDH PuIR
Kappa factor = 0.27 Sample identification R12 Species Aspen Cook type RDH

02 Pulp Kappa 5 Viscosity, mPa.s 12.5 Chlorination 0.03 factor C102, % as available 1.35 C12*

ctual C102, 1.47 % as avail.
ClZ

Substitution, 100 %

H2S04, % 2 Final pH 2.3 Residual, g/L 0.08 avail. C1, NaOH, % 0.8 02 pressure, 25 psig 02 time, minutes 15 Final pH 12.6 K No. (25 M1) 2.2 iscosity, mPa.s 12.2 Yield on raw -stock, %

Sample number #1 #2 #3 #4 #5 #6 C102 as C102, 0.1 0.3 0.5 0.7 0.9 1.1 %*

ctual 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 0.01 0.01 0.01 0.01 0 0.01 C102, %

Brightness, 88.9 90 90.8 91.1 91.5 91.8 %ISO

Viscosity, mPa.s12.4 - - 10.7 - 9.5 Actual C102 concentration x 0.92 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 pulp (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 pulp. It should be noted that the black liquor strength during the RDH cook should be maintained.

BLEACHING STUDY SUMMARY
COOK NUMBER
BEST COOK: NEW RDH COOKING PROCEDURE R3 R4 BASELINE COOK: OLD RDH COOKING R7 R8 PROCEDURE

BEST DO ABLE COOK: MODIFIED NEW RDH R12 COOKING PROCEDURE

BLEACHING RESULTS
KAPPA 0.225 IN THE D (900% CI02 SUBSTITUTION) FACTO
R
FINAL BRIGHTNESS, % lS0 CHARGES
IN THE
LAST
D STAGE, 0.1 0.3 0.5 0.7 0.9 1.1 R3 89.7 90.4 91.2 91.5 91.6 92 (BEST

COOK) R12 88.9 90 90.8 91 91.6 91.8 (BEST

DO-BLE) R7 87.6 88.7 89.7 90.3 90.3 90.6 (BASE-LINE) KAPPA
FACTOR 0,27 IN THE D (700% CI02 SUBSTITUTION) FINAL BRIGHTNESS, % ISO

CHARGES
IN THE
LAST
D~,STAGE, 0.1 0.3 0.5 0.7 0.9 1.1 R4 89.8 90.5 91.2 91.5 91.8 91.8 (BEST

COOK) R12 88.9 90 90.8 91.1 91.5 91.8 (BEST

DO-BLE) Rg 87 88.7 89.3 90.1 90.5 90.5 (BASE-LINE) 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.

Claims (9)

CLAIMS:

1. A batch digesting process of the type using rapid displacement heating to produce delignified pulp, wherein spent liquor produced in a digester as a result of cooking a mass of cellulosic material with cooking liquor is displaced and collected in accumulators 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, wherein:
white liquor is added to the warm black liquor during the warm black liquor pretreatment and to the hot black liquor during the hot black liquor pretreatment stages and to the cooking liquor during the cooking stage in the digesting process, the total white liquor added having a distributed charge between approximately 15%
active alkalinity and about 35% active alkalinity; and the temperature of the cooking liquor is raised to between 150° ~ 167°C for the cooking stage.

2. The process of claim 1, wherein the white liquor solution is added to the warm black liquor (temperature between 90° and 150°C) and to the hot black liquor (temperature between 150° and 167°C) in a predetermined amount.

3. The process of claim 1, wherein white liquor is added to a cool black liquor (temperature between 70° 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.

4. The process of claim 1, wherein the preferred total white liquor charge is > 20% AA.

5. The process of claim 1, wherein the preferred cooking temperature ranges between 155° ~ 167°C.

6. A method for producing bleachable grade pulp according to the process of claim 1, comprising the steps of:
(a) introducing wood chips into the digester;
(b) pretreating the chips with a mixture of warm black liquor and white liquor below cooking temperature;
(c) displacing the mixture from the digester with at least one mixture of a hot black liquor and hot white liquor;
(d) increasing the temperature of the digester to the cooking temperature;
(e) maintaining said temperature until the chips are digested;
(f) displacing the contents of the digester with a liquid filtrate derived from pulp washing; and (g) emptying the contents of the digester by applying gas pressure to the interior of the digester or pumping out.

7. The method of claim 6, including the step of pretreating the chips with a mixture of cool black liquor and white liquor (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 6, including the step of displacing the contents of the digester with any combination of washer filtrate and white liquor (or NaOH
solution).