Analysis of ECF Bleaching Practices in Canada
Part 2: Hardwood
a report prepared for the
Pulp and Paper Technical Association of Canada
(PAPTAC)
Bleaching Committee
by
Douglas C. Pryke, P.Eng Chris J. Kanters.
Consultant PCI Chemicals Canada
Erin, Ontario Montreal, Quebec
Ted Tam
Cariboo Pulp & Paper Ltd.
Quesnel, B.C.
May 1999
Summary
Chemical pulp bleaching in Canada continues to evolve. In 1988, bleach plants began adopting substantial substitution of chlorine dioxide for chlorine. The transition to chlorine dioxide-based bleaching is essentially complete. On a production-weighted basis, chlorine dioxide substitution now averages 96% for all bleached chemical hardwood and softwood pulp.
The number of mills practicing complete replacement of chlorine with chlorine dioxide, so-called Elemental Chlorine-Free (ECF) bleaching, continues to grow. Over 92% of bleach plants in Canada produced ECF in 1998, totaling an estimated 9.6 million tonnes of bleached pulp. ECF now accounts for 86% of Canadian bleached chemical pulp production. Over 80% of hardwood pulp production is ECF. The balance is produced using high substitution of chlorine dioxide for chlorine in the first stage of bleaching, i.e., ≥ 50%.
Oxygen delignification of hardwood pulp is growing slowly. In 1996, 33% of softwood kraft pulp was oxygen-delignified. In 1998, this figure grew slightly to 36%, representing ~ 1.0 million tonnes of production.
Conventionally-delignified hardwood pulps require less oxidizing equivalents per unit of unbleached pulp kappa number compared to oxygen-delignified hardwoods. The sequence kappa factor for conventionally-delignified hardwoods was 0.59 and increased to 0.68 for oxygen-delignified hardwoods.
In spite of this, on the average, oxygen-delignified hardwood pulps have a bleaching chemical cost advantage of $5-$6 Cdn/ADMt compared to conventionally- delignified hardwood pulps.
Introduction
Bleaching practices in Canada, particularly in the first stage, continue to evolve. To keep abreast of these developments, the Pulp and Paper Association of Canada’s (PAPTAC) Bleaching Committee issued questionnaires to mills in 1994, 1995, 1996 and 1998. The questionnaires were designed initially to investigate the extent and impact of increased chlorine dioxide substitution. More recently the questionnaires have been designed to discover optimum bleaching conditions for producing high brightness pulps with minimum use of oxidizing chemicals.
In 1998 the questionnaire was sent to forty-two mill sites and all responded. This report summarizes the data reported in the questionnaires for hardwood mills.
Hardwood Pulp Bleaching in Canada
Table 1 summarizes bleached chemical pulp production in Canada. The dominant end use is market pulp. The dominant pulping process is kraft; sulfite pulp is produced at only 2 mills among the mills surveyed. Hardwood accounts for 25% of Canadian bleached chemical pulp.
Table 1: 1998 Canadian Bleached Chemical Pulp Production
(million tonnes)
|
ECF |
TCF |
Other* |
Total |
|
|
Hardwood |
2.28 |
0.00 |
0.54 |
2.82 |
|
Softwood |
7.36 |
0.01 |
1.03 |
8.41 |
|
Total |
9.64 |
0.01 |
1.57 |
11.22 |
* Pulps bleached with substantial substitution of chlorine dioxide, i.e. ≥ 50% ClO2 substitution.
Figure 1 shows the changing nature of bleached chemical pulp production since 1990. ECF pulp production continues to rise, and in 1998 accounted for eighty-six percent of bleached chemical pulp in 1998. ECF is produced at 38 of 42 mill sites and in 49 of 53 bleach plants. In 1998, Totally Chlorine-Free (TCF) was produced at only two sulfite mills.
Figure 1: Canadian Bleached Chemical Pulp Production
ECF Bleaching Practice
As previously discussed, questionnaires were issued to those bleach plants producing ECF to assess bleaching practices. The questionnaires requested information on unbleached pulp, process conditions, chemical consumption, mixing, control strategy, etc. Responses covered 49 bleach plants: 38 softwood and 11 hardwood. Appendix 1 of this report summarizes the bleaching practices for hardwood pulps. Unfortunately, the data base is insufficient to assess "best practices."
Appendix 2 summarizes responses to questions regarding scaling experience in the bleach plant.
Hardwood Analysis
Kappa Number to Bleaching
For conventionally-delignified pulps, the kappa number from the digester averages 14.8 and declines slightly to 14.4 before bleaching. For oxygen-delignified hardwoods, the kappa number from the digester was higher at 16.9. Oxygen delignification averaged 32.5% resulting in a kappa number to the bleach plant averaging 10.3. No hardwood mills reported using enzymes.
Figure 2: Hardwood Kappa No. to Bleaching
Oxygen delignification of hardwood pulp is increasing slowly. Production grew from 33% of hardwood kraft pulp in 1996 to 36% in 1998 for a total of ~ 1 million tonnes. The systems are typically single stage operating at medium consistency.
Carryover to the Bleach Plant
Carryover to the first stage of the bleach plant is measured as Na2SO4 or as Chemical Oxygen Demand (COD). As shown in Figures 3 and 4, the carryover typically ranges from 5-20 kg/ADMt for either Na2SO4 or COD. The average value of both Na2SO4 and COD carryover, for both conventional and oxygen-delignified pulps, is ~ 8-12 kg/ADMt.
Figure 3: Carryover to Bleach Plant – Conventionally-delignified Hardwood
Figure 4: Carryover to Bleach Plant – Oxygen-delignified Hardwood Pulps
First Chlorine Dioxide (D0) Stage
Operating Conditions
Typically the first chlorine dioxide stage, D0, is operated at low consistency, reflecting the transition from chlorine to chlorine dioxide. Two-thirds of the bleach plants operate at low consistency and on-third at medium consistency. All but one are up-flow towers.
Retention time is 40-45 minutes with a range of 17-90. Temperature averages 55-60°C ranging from 45 to 65°C. Residual chlorine dioxide is typically zero. All the plants control the Do end pH at 2.5-2.8 using addition of H2SO4 or spent acid. Two mills are using CO2 for brownstock acidification.
Sensors and Control
Optical and residual sensors after chlorine dioxide addition and before the tower are the dominant process control strategy. About 20% use a post tower optical and residual sensor as well. About half control to either Do brightness or Eop brightness and the other half to an extracted kappa number. Seventy-three percent have on-line kappa number analyzers.
Mixing and Chemical Addition
Both mechanical and static mixers are used in widely varying combinations. Where there is more than one mixer, the tendency is to add most of the chlorine dioxide to the first mixer. This is likely a reflection of mixing strategies implemented for substantial substitution of chlorine dioxide designs and associated hydraulic limitations. No deliberate distribution pattern for chlorine dioxide addition or mixing strategy is evident.
D0 Kappa Factor
The kappa factor for conventionally-delignified hardwood pulps averages 0.22, while for oxygen-delignified hardwood pulps, the average increased to 0.32 as shown in Figure 5. Oxygen-delignified hardwood pulps are relatively more difficult to further delignify with chlorine dioxide. This is reflected in the higher average D0 kappa factor.
Figure 5: Hardwood Do Kappa Factor
First Extraction Stage, Eop
Operating Conditions
The Eop stage retention time at pressure ranges from 15 to 90 minutes at 70-80°C. Most stages operate under hydraulic pressure.
Only two bleach plants utilize Papricycle® to minimize sodium hydroxide consumption. The NaOH:Equivalent Cl2 ratio averages 0.43 without Papricycle® and 0.28 with. This represents a saving of approximately 5 kg NaOH/ADMt for conventionally-delignified hardwood pulps.
Mixing and Chemical Addition
Hydrogen peroxide is typically added just before oxygen. All mills but one use hi-shear mixers for oxygen mixing
Sensors and Control
Eop stages are controlled to an end pH of 10.7-10.9. Approximately 40% of the plants control to an Eop brightness target. Eop kappa number control is used in about 20% of the plants.
Eop Kappa Number and Kappa Factor
Hydrogen peroxide is used in 6 of 11 and oxygen is used in 8 of 11 extraction stages. Only 4 plants use both oxygen and hydrogen peroxide. When oxygen is applied, it is used 4-5 kg/ADMt and when hydrogen peroxide is used, at 4-5 kg/ADMt. The Eop kappa number averages 4.4 for conventionally-delignified hardwood and declines to 2.6 for oxygen-delignified hardwood pulps. The kappa factor for the D0 plus the Eop stage is higher for oxygen-delignified hardwood pulps averaging 0.50, while conventionally-delignified hardwood pulps average 0.36 as shown in Figure 6.
Figure 6: Hardwood DEop Kappa Factor
Chlorine Dioxide Brightening: (D1) Stage
Operating Conditions
The D1 stage retention time for conventionally-delignified hardwood averages 125 minutes and oxygen-delignified hardwood lines average 172 minutes with a range of 60-250. Operating temperatures are 72-80°C with a range of 60-85°C. Most bleach plants do not add NaOH to control pH. When NaOH is added, the NaOH:ClO2 ratio is 0.15 kg/kg. The upflow pH averages 3.8-4.3 and end pH 3.7-4.0.
Sensors and Control
Most stages are controlled using pre-tower optical and residual sensors to an end-of-tower pulp brightness. A positive chlorine dioxide residual is retained at the end of the stage, typically in the range of trace to 30 mg/L measured in the D1 washer vat.
D1 stage brightness averages 85.9% ISO for conventional pulps and 87.6% ISO for the oxygen-delignified hardwood pulps as shown in Figure 7. Chlorine dioxide is applied at 9 kg/ADMt for conventionally-delignified hardwood pulps compared to 5.5 kg/ADMt for oxygen-delignified hardwood pulps. The application rate is approximately 2.0-2.3 kg ClO2/ADMt per unit Eop kappa.
Mixing is dominated by hi-shear type driven mixers, but medium consistency pumps are used as well.
Figure 7: D1 Brightness
Second Extraction Stage, E2
Operating Conditions
Conventional E2-stage retention time is 25-90 minutes at a temperature of 75°C. Stages are typically controlled to an end pH of 10.5-11.0. NaOH consumption is typically 5 kg/ADMt.
Only one mill adds hydrogen peroxide to the second extraction stage. When hydrogen peroxide is used in the second extraction stage, the addition rate is at 5 kg/ADMt.
Chlorine Dioxide Brightening: (D2) Stage
Operating Conditions
The D2 stage retention time is typically 150 minutes with a wide range of 45-250 minutes operating at 78°C with a range of 68-88°C. The upflow pH averages 4.4 and end pH 3.7-4.1.
Sensors and Control
Few stages are controlled using pre-tower optical and residual sensors. Rather, chlorine dioxide is applied as per cent on pulp and to a final brightness target. A positive chlorine dioxide residual is retained at the end of the stage, typically in the range of trace to 15 mg/L, measured in the D2 washer vat.
D2 stage brightness averages 89-90% ISO for both oxygen-delignified and conventional pulps as shown in Figure 8. Chlorine dioxide is applied at 2.7 kg/ADMt for oxygen-delignified hardwood pulps compared to 3.5 kg/ADMt for conventionally-delignified hardwood lines. Chlorine dioxide is distributed approximately 75:25 between the D1 and D2 stages.
Figure 8: D2 Brightness
Overall Chemical Consumption and Cost
The overall sequence kappa factor (sum of chlorine dioxide in all stages plus oxygen and hydrogen peroxide in extraction stages - all expressed as % equivalent chlorine on pulp divided by unbleached kappa number) averages 0.68 for oxygen-delignified pulps compared to 0.59 for conventional pulps.
Oxygen-delignified hardwood pulps averaged require $2.67 Cdn per unit of unbleached kappa number and conventionally-delignified pulps require $2.35 Cdn per unit of unbleached pulp kappa number to achieve 89-90% ISO brightness. The bleaching cost for oxygen-delignified hardwood pulps was $27.71 Cdn/ADMt compared to $33.35 Cdn/ADMt for conventionally-delignified hardwood pulps, a difference of $5.64 Cdn/ADMt. It is interesting to note in Figure 10, that a number of oxygen-delignified mills have the same bleaching cost as conventionally bleached softwoods.
Figure 9: Sequence Kappa Factor
Figure 10: Bleaching Cost, $/ADMt
Optimization of DEopDED Bleaching
Optimization Summary
In the questionnaire, each bleach plant was assessed to determine what each was optimized to achieve. The following scale was used to assess the relative importance of each criteria:
1 - strongly disagree; 2 - disagree; 3 - no opinion/not applicable; 4 - agree; 5 - strongly agree.
As shown in Figure 11, for oxygen-delignified pulps the emphasis was mostly placed on minimizing cost and maximizing brightness, followed by minimizing chlorine dioxide consumption.
Figure 11: Optimization Summary: Oxygen-delignified Hardwood
For conventionally-delignified hardwoods, the emphasis was placed on minimizing chlorine dioxide consumption, minimizing cost and maximizing brightness as shown in Figure 12.
Figure 12: Optimization Summary: Conventionally-delignified Hardwood
Summary
Chemical pulp bleaching in Canada continues to evolve. In 1988, bleach plants began adopting substantial substitution of chlorine dioxide for chlorine. The transition to chlorine dioxide-based bleaching is essentially complete. On a production-weighted basis, chlorine dioxide substitution now averages 96% for all bleached chemical hardwood and softwood pulp.
The number of mills practicing complete replacement of chlorine with chlorine dioxide, so-called Elemental Chlorine-Free (ECF) bleaching, continues to grow. Over 92% of bleach plants in Canada produced ECF in 1998, totaling an estimated 9.6 million tonnes of bleached pulp. ECF now accounts for 86% of Canadian bleached chemical pulp production. Over 80% of hardwood pulp production is ECF. The balance is produced using high substitution of chlorine dioxide for chlorine in the first stage of bleaching, i.e., ≥ 50%.
Oxygen delignification of hardwood pulp is growing slowly. In 1996, 33% of softwood kraft pulp was oxygen-delignified. In 1998, this figure grew slightly to 36%, representing ~ 1.0 million tonnes of production.
Conventionally-delignified hardwood pulps require less oxidizing equivalents per unit of unbleached pulp kappa number compared to oxygen-delignified hardwoods. The sequence kappa factor for conventionally-delignified hardwoods was 0.59 and increased to 0.68 for oxygen-delignified hardwoods.
In spite of this, on the average, oxygen-delignified hardwood pulps have a bleaching chemical cost advantage of $5-$6 Cdn/ADMt compared to conventionally- delignified hardwood pulps.
Acknowledgment
The authors gratefully acknowledge the support of Paptac Technical Section Bleaching Committee, Sterling Pulp Chemicals, PCI Chemicals Canada and Cariboo Pulp & Paper. Most particularly we wish to acknowledge the mill personnel who took the time to complete the questionnaires. It is their effort that made this possible.