PULPING AND BLEACHING OF SAS-AQ BAGASSE PULPS

Author

Ron Braunstein

Company and address

Sappi Technology Centre, PO Box 3252, Springs 1560, South Africa

email

ron.braunstein@za.sappi.com

Keywords

bagasse, shives, SAS-AQ, ECF, TCF, chlorine dioxide, peroxide, peracetic acid

ABSTRACT

The development of a SAS-AQ pulping procedure to produce pulp with a low shive content, in the region of the current soda pulp, while retaining 8% points yield advantage is described. ECF D-E-D bleaching sequence with kappa factor of 0.5 and TCF X-O1.2 A2.0- Z0.6 /(P1.0O2.0) bleaching sequences were performed and brightnesses of 85% and 82% ISO and yields of 56% and 54.5%were obtained respectively.

The yield advantage of 60-63% unbleached and 55-57.5% bleached of SAS-AQ pulps over the 52-55% unbleached and 48-51% bleached of the soda process was somewhat dimmed by the high shive content of 10-12% in comparison to 3-6%of the soda process. A pre-impregnation stage would reduce the shive content but due to the nature of the screw-feeder type digester, used for bagasse pulping, the inclusion of this stage has not been pursued.

The addition of 0.6-1.3% NaOH to the pulping liquor and varying the AQ charge between 0.2 and 0.5% resulted in reduced shive content, in the region of 4-7% unbleached and 3.5% and 0.5% for ECF and TCF bleached pulp respectively.

1. INTRODUCTION

The vastly superior SAS-AQ bagasse pulping yield advantage of 10-12 percentage points for unbleached pulps with 9-13 kappa numbers over the soda / AQ / Infinity pulps [1,2, 3] has a disadvantage in a form of an increase of 4-9 percentage points in the amount of shives or fibre bundles [3,4, 5 6]. 

Reducing the rejects amount in SAS-AQ bagasse pulp was found possible in two ways (without quantifying shive reduction): with an increase of the AQ charge over the favorable 0.05-0.1% [7,8] and by having a pre-impregnation stage (9). Pre-impregnating the bagasse in a separate vessel prior to the introduction to the digester is problematic.

The reduction of shives with the increase of the AQ charge and low NaOH additions, the ECF and TCF bleaching of the improved bagasse pulp is described in several Sappi R&D investigations reports and were combined in a single report [6]

The relevant information from this report [6] is highlighted in this paper.

2. MAIN FINDINGS

  1. Bagasse pulping with 13% Na2SO3, 2.5% Na2CO3, 0.6-1.3% NaOH (all expressed as Na2O) and 0.2-0.5% AQ (Liquor A) obtained kappa No of 6-7, yield of 59-60%, 4-7% shive content and similar strength properties to the soda / infinity / AQ pulp.
  2. Bagasse  pulping with 13% Na2SO3 as Na2O, 2.5% Na2CO3 (all expressed as Na2O) and 0.05-0.5% AQ (Liquor B) obtained kappa No of 9-13, yield of 61-64%, 9-12% shive content and similar strength properties to the soda / infinity / AQ pulp.
  3. D-E-D bleaching sequence with a kappa factor of 0.5 obtained acceptable brightnesses of 85%ISO and 81% ISO for pulps prepared with liquors A and B respectively. O-D-E-D with similar kappa factor did not reach acceptable brightness.
  4. A xylanase stage prior to the first D (X-D-E-D sequence) had 0.1-0.27%ISO  brightness improvement when liquor A was used and  1-3% ISO brightness improvement when liquor B was used.
  5. Bagasse pulp prepared by liquor A enables a relatively simple and cost effective TCF bleaching sequence X-O1.2 A2.0- Z0.6 /(P1.0O2.0)  that obtained 81.44% ISO      brightness and 0.5% shives. Bagasse pulp prepared by using liquor B obtained       81% ISO brightness with an O-Z0.72 -O-Z0.35 P0.5 sequence.
  6. Other TCF bleaching sequences that did not include an ozone stage, failed to reach acceptable brightness.

3. PULPING

3.1 Shive Reduction Investigation
Pulping was performed on a bagasse sample, obtained from Stanger mill. Two liquor compositions were used as a charge:

Liquor A: 13% Na2SO3, 2.5% Na2CO3 and 1,3% NaOH

Liquor B: 13% Na2SO3 and 2,5% Na2CO3

Both expressed as Na2O on B D bagasse.

Three AQ charges were investigated: 0.05%, 0.2% and 0.5%.

A ramp up time of 45 min and the retention was 20 min at 175C. were investigated.  Kappa No was determined by the Pulping and Bleaching lab which also prepared the handsheets. The pulps were tested for their strength properties by the Technology Centre Paper Section and analyzed for shive content by the Pulmac Shive analyzer at the Ngodwana Mill Lab. Results are presented in Table 1 and illustrated in Figures 1 and 2.

Table 1: Bagasse Pulping Liquor and AQ Charge Comparison

AQ (%)

Kappa Number

Yield (%)

Shives (%)

 

Liquor A

Liquor B

Liquor A

Liquor B

Liquor A

Liquor B

0.05

9.3

14.0

61.41

62.20

10.5

12.4

0.2

5.9

9.4

60.86

62.70

7.2

9.6

0.5

6.3

10.8

58.76

61.50

4.6

10.4

    Note:
    Liquor A composition:  13% Na2SO3, 2,5% Na2CO3 and 1,3% NaOH all as Na2O.
    Liquor B composition: 13% Na2SO3 and 2,5% Na2CO3 all as Na2O

3.2 AQ Charge Variations Effect
The increase in the AQ charge from 0.05% to 0.2% reduced the kappa number by about 35% and the shive content by about 30% for both liquors. Further AQ charge  increase to 0.5% did not impact on the kappa number but reduced the shive content in the pulp prepared with liquor A.  Refer to Table 1.

Figure 1

3.3 Liquor Composition Effect
Pulping with Liquor A (1.3% Caustic addition) resulted in a 35% reduction in the kappa number compared to the pulping with Liquor B at the equivalent AQ level. Shives were reduced by a greater proportion with the AQ increase (about 15% with 0.05% AQ to about 65% with 0.5%AQ) showing greater catalyst activity with the added caustic. Refer to Table 1.

3.4 Yield
Relatively small variation of the yield due to the reduced kappa numbers were observed with 0.8 2.8 percentage points yield advantage to the Liquor B pulps as a result from their higher kappa pulps.

Figure 2

4. BLEACHING

4.1 Bleaching of Bagasse Pulp Prepared with Liquor A

4.1.1 Pulp
Bagasse ex Stanger mill was pulped under the following charges and conditions: Active charge of 13.0% Na2SO3, 2.5% Na2CO3,, 0.6% NaOH and 0.2% AQ. Non process elements: 0.31% NaCl, 1.03% SiO2 and 5.06% K2SO4. Time to and at 175C were 45 min and 20 min respectively, L/B was 6:1. Eight cooks were mixed for a composite, which had the following parameters: Kappa of 6.99 and average pulping yield of 59.34%.

4.1.2 X-D-E-D Bleaching
The pulp was subjected to X-D-E-D Bleaching. X stage with 10u/g of Iogen-P pH=6.2, 1hr at 60C. D stages with kf=0.3. 0.4, 0.5 and 0.75. D1 1 hr and D2 3 hrs all at 70C. Low pH in D1 of ~2 acted as a combination of an A-D stage. E Stage with 2.5% NaOH and 2 hrs at 70C.

Results are presented in Table 2 and Figure 3. From these results a brightness ceiling of ~ 86 % ISO for this sequence can be derived (for practical charges and conditions).

Table 2: Bleaching Yield, Brightnesses, Viscosities, Strength Properties and Shive Content of X-D-E D Bleaching Sequence

Kappa Factor

Yield (%)

Brightness

(%ISO)

Viscosity (ml/g)

Tear Index

(mN m2/g)*

Burst Index

(kPa m2/g)*

Breaking Length (km)*

Shive Content (%) **

0.3

95.91

83.11

980

4.0

2.8

5.4

3.3

0.4

95.70

84.18

987

4.0

2.7

5.5

4.3

0.5

95.36

85.14

964

3.9

2.6

5.0

3.3

0.75

94.94

85.77

936

4.0

2.6

4.9

3.4

* Strength properties at nil beating

** Shive content as determined by Ngodwana mill lab; the unbleached and bleached pulps were not centricleaned

4.1.3 D-E-D Bleaching and Quantifying Xylanase Advantage
 D-E-D with kappa factors 0.3 and 0.5 were performed as per 2.1 to quantify the Xylanase stage brightness advantage. Bleaching conditions were kept the same. Results are presented in Table 3 and Figure 3. The brightness advantage in this investigation was marginal. For the kappa factors 0.3 and 0.5 brightnesses advantage were 0.27% and 0.1% respectively. In comparison to pulp, prepared with liquor B, with an 11.3 kappa pulp, a xylanase stage led to brightness advantage of 1-3% ISO. See section 4.2.2

Table 3: Bleaching Yield, Brightnesses, Viscosities and Strength Properties of  D-E D Bleaching Sequence

Bleach Number

Kappa Factor

Yield (%)

Brightness
(%ISO)

Viscosity (ml/g)

Tear Index
(mN m2/g)*

Burst Index
(kPa m2/g)*

Breaking Length (km)*

00/59

0.3

95.81

82.74

990

5.3

2.6

4.7

00/60

0.5

95.66

85.04

965

5.3

2.5

4.9

* Strength properties at nil beating

Figure 3

Figure 3: X-D-E-D and D-E-D Bleaching of Bagasse Pulp Prepared with Liquor A

4.1.4  X-O-A-Z/(PO) Bleaching
The unbleached pulp was subjected to the above TCF bleaching sequence. X stage as in section 4.1.2 (same lot of pulp was treated for both sequences). MC O2 followed with 1.2% NaOH, 0.2% Mg++, 400 kPag O2 pressure, 30 min to and 60 min at 95C. kappa was 4.95 and X-O Yield was 98.53%. Hot acid stage, 2% H2SO4, 95C for 1 hr and end pH of 1.92 preceded the HC ozone stage. O3 charge was 0.6% which lowered the kappa to 1.23. Pressurised peroxide stages completed the bleach sequence with 1% or 3% peroxide and 2%or 3% NaOH charges respectively, 0.1% Mg++ and 2.5 hrs at 95C. Results and charges are presented in Table 4 and Figure 4. Note the low levels of shives.

Table 4: Bleaching Charges, Yield, Brightnesses, Viscosities, Strength Properties and Shive Content of X-O-A-Z/(PO) Bleaching Sequence

H2O2 / NaOH (%)

Yield (%)

Brightness

(%ISO)

Viscosity (ml/g)

Tear Index

(mN m2/g)*

Burst Index

(kPa m2/g)*

Breaking Length (km)*

Shive Content (%) **

1.0/ 2.0

92.69

81.44

723

5.7

2.4

3.9

0.5

3.0/3.0

89.14

82.04

544

4.7

2.3

3.9

0.3

* Strength properties at nil beating

** Shive content as determined by Ngodwana mill lab; the unbleached and bleached pulps were not centricleaned

4.2 Bleaching of Bagasse Pulp Prepared With Liquor B

4.2.1 Pulp
A composite pulp was prepared using 13% Na2SO3 as Na2O, 2.5% Na2CO3, 0.2% AQ, 45 min. to and 20 min at 175 oC, liquor to bagasse ratio 6:1.  Results:  kappa 11.3, viscosity 1093 ml/g avg. yield 62.39%, brightness 43,29% ISO, tear index 5,5 mNm2/g*, burst index 2,3 kPam2/g* and breaking length 5590m*

4.2.2 D-E-D and X-D-E-D
The above pulp was subjected to D-E-D and X-D-E-D bleaching sequences. Kappa factor was 0.3 and 0.6 based on the unbleached kappa and 2.5% NaOH in the E stage 70C in D and E stages, D1 1 hr E 2 hrs and D2 3 hrs. X stage with 10u/g of xylanase Iogen P at 60C and pH 5 for 1 hr. All stages at 10% consistency. Results are presented in Table 5 and Figure 5.

Table 5: D-E-D and X-D-E-D Bleaching Results

Sequence

Kappa Factor

 Yield (%)

 Brightness (%ISO)

Viscosity

(ml/g)

Tear Index (mNm2/g)*

Burst Index (kPam2/g)*

Breaking length (m)*

D-E-D

0.3

93.69

78.82

1066

5.7

2.6

5244

D-E-D

0.6

93.13

79.24

1006

5.2

2.7

5923

X-D-E-D

0.3

93.86

79.89

1091

4.9

2.9

5649

X-D-E-D

0.6

93.63

82.19

996

5.0

3.0

5066

* All strengths at nil beating

4.2.3 O-D-E-D and O-(qPa)-P

4.2.3.1 O2 Stage
The unbleached pulp, refer to 4.2.1 was subjected to a MC O2 stage. Charges and conditions: 1.4% NaOH, 0.2%Mg++, 10% consistency 400kPag O2 pressure, 30 min to and 60 min at 95C. Results: kappa 7.01, yield 99.21% viscosity 1066 ml/g, brightness 44.61% ISO, tear index 4.7 mNm2/g*, burst index 2.9 kPam2/g*, and breaking length 5243 m*.

4.2.3.2 D-E-D
D-E-D stages were performed as per section 4.2.2 with kappa factors 0.3 and 0.5 based on the O2 pulp kappa. See results in Table 6 and Figure 5.

4.2.3.3 (qPa)-P
The O2 pulps were treated with 0.5% Warwick PQ 100 (a chelating agent) at pH 8 and 10% consistency. A charge of either 0.3% or 0.5% TAED (the peracetic acid precursor) was added to the stage for 2 hrs at 75 C. P stage followed with 3% H2O2, 2% NaOH, 10% consistency for 3 hrs at 75 C. Results are presented in Table 7 and Figure 6.

Table 6: O-D-E-D Bleaching Results

Sequence

Kappa Factor

 Yield (%)

Brightness (%ISO)

Viscosity

(ml/g)

Tear Index (mNm2/g)*

Burst Index (kPam2/g)*

Breaking length (m)*

O-D-E-D

0.3

93.81

72.93

1087

5.8

3.4

5975

O-D-E-D

0.5

93.41

75.69

1045

5.1

3.3

5925

* All strengths at nil beating

Table 7: O-(qPa)-P Bleaching Results

Sequence

TAED

(%)

 Yield (%)

Brightness (%ISO)

Viscosity

(ml/g)

Tear Index (mNm2/g)*

Burst Index (kPam2/g)*

Breaking length (m)*

O-(qPa)-P

0.3

96.37

62.35

1076

5.3

3.7

5343

O-(qPa)-P

0.5

96.14

63.11

1082

4.7

3.5

5754

* All strengths at nil beating    

4.2.4 X-O-(qPa)-P
The unbleached pulp was subjected to an X stage as per section 4.2.2 followed by an MC O2 stage. Charges and conditions as per section 4.2.3.1. Results: kappa 7.27and viscosity 1108 ml/g. The pulp was further (qPa)-P bleached as per section 4.2.3.3. Results are presented in Table 8 and Figure 6.

4.2.5 A-O-(qPa)-P
The unbleached pulp was subjected to an hot acid stage: 2% H2SO4, 95C, 10% consistency and end pH was 2.10. A MC O2 stage followed. Charges and conditions as per section 4.2.3.1. Results: kappa 6.52 and viscosity 1089 ml/g. The pulp was further (qPa)-P bleached as per section 4.2.3.3. Results are brought in Table 8 and Figure 6.

Table 8: X-O-(qPa)-P and A-O-(qPa)-P Bleaching Results

Sequence

TAED

(%)

 Yield (%)

 Brightness (%ISO)

Viscosity

(ml/g)

Tear Index (mNm2/g)*

Burst Index (kPam2/g)*

Breaking length (m)*

X-O-(qPa)-P

0.3

95.77

64.03

1091

4.9

3.3

5797

X-O-(qPa)-P

0.5

94.43

66.86

1092

4.9

3.1

5847

A-O-(qPa)-P

0.3

94.44

68.60

1066

5.1

3.6

5836

A-O-(qPa)-P

0.5

93.48

70.81

1078

5.1

3.3

5808

* All strengths at nil beating

Figure 5

Figure 5: X-D-E-D, D-E-D and O-D-E-D Bleaching of Bagasse Pulp Prepared with Liquor B

Figure 6

Figure 6: O-(qPa)-P, X-O-(qPa)-P and A-O-(qPa)-P Bleaching of Bagasse Pulp Prepared with Liquor B

4.2.6 O-Z-O-Z-P
TCF bleaching sequence that can obtain acceptable brightness of about 80 %ISO was found in the form of O-Z-O-Z-P. Other sequences like A-Z-(EP)-P even with a 1.2% ozone charge and over 3% peroxide yielded brightnesses which were lower than 76% ISO [9]. A composite pulp was prepared as in section 4.2.1. The pulp had an 11.3 kappa. The pulp was subjected to an MC O2 stage with 1.2% caustic, 400 kPag O2, 0.2% Mg++, and 30 min at 95C. MC ozone stage followed with 0.72% O3 charge. A second MC O2 stage with 1.0% caustic, 400 kPag O2, 0.2% Mg++, and 30 min at 95C followed by a second ozone stage with 0.35% O3 charge. One half of the pulp was subjected to 0.5% peroxide and the other half to 2%. In both cases 1% caustic and 0.2% Mg++ was charged with 4 hrs duration at 95C. Results are presented in Table 9 and Figure 7.

Table 9: O-Z-O-Z-P Bleaching Results

Sequence*

Brightness (%ISO)

Viscosity

(ml/g)

Bleach Yield (%)

Total Yield (%)

O-Z0.72 -O-Z0.35 P0.5

80.7

579

88.3

55.3

O-Z0.72 -O-Z0.35 P2.0

82.8

518

87.2

54.6

Note: * Ozone and peroxide charges as per subscripts

Figure 7

Figure 7: O-Z0.72-O-Z0.35-P Bleaching of Bagasse Pulp Prepared with Liquor B

5. REFERENCES

1. S. J. Raubenheimer, SAS-AQ Pulping and Bleaching of Stanger Bagasse Fibre, Sappi Research and Development Report No. R&D 83/014, 23 February, 1983.

2. 2. S. J. Raubenheimer, An Investigation on the Pulping and Bleaching of Bagasse Fibre (Report 1), Sappi Research and Development Report No. R&D 83/005, 26 January, 1983.

3. R. Braunstein, Stanger SAS-AQ Mill Trial 03/2002 Results Investigation , Sappi Technology Centre report, TC 2002/003,PB 24 May, 2002-06-21.

4. Laboratory Analysis Results from Ngodwana Mill lab, LAB.2000/08/04/01, Analyst: A. Siguka, dated 4 August, 2000.

5. R. Braunstein and A. du Plooy, Extending Pulping Time for Stanger Digester, Sappi Technology Centre memo M2000/330, 17 November, 2000.

6. R. Braunstein, SAS-AQ pulping and Bleaching Studies , Sappi Technology Centre report, R&D2001/002, 22 January, 2001.

7. D. L. K. Wang, F. Ramirez and R. Patt, Alkaline Sulfite-Anthraquinone Pulping of Bagasse, Holzforschung 43, 261-264 (1989).

8. E. J. Smith, High Shives / Rejects of Laboratory Alkaline Sulphite / Anthraquinone  Unbleached Stanger Mill Bagasse Pulp, June 1999, Research and Development memo M99/234, 20 October, 1999

9. R. Braunstein, Further SAS-AQ pulping and Bleaching Studies on Bagasse for Stanger Mill, Sappi Research and Development report, R&D97/007, 27 August, 1997

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