CHLORINE DIOXIDE DELIGNIFICATION PRACTICES IN CANADA

Analysis of ECF Bleaching Practices in Canada

Part 1: Softwood

a report prepared for the

Pulp and Paper Technical Association of Canada

(PAPTAC)

Bleaching Committee

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. The balance is produced using high substitution of chlorine dioxide for chlorine in the first stage of bleaching, i.e., ≥ 50%.

Oxygen delignification of softwood pulp is growing. In 1996, 28% of softwood kraft pulp was oxygen-delignified. In 1998, this figure grew to 39%, representing 3.3 million tonnes of production.

Analysis of bleaching conditions shows a number of developing trends since 1996:

An increase in post oxygen kappa no. (17.3 to 18.0)

A doubling of the number of kappa analyzers

A modest increase in the number of medium consistency Do stages (typically associated with introduction of oxygen delignification)

An increase in the temperature of the Eop stage (76°C to 81.4°C)

Fewer mills using NaOH for D1 pH control

A doubling of the mills using hydrogen peroxide in the second extraction stage

There has not been a significant change in the overall bleaching chemical demand for conventionally-delignified softwood pulps. The average sequence kappa factor increased from 0.44 in 1996 to 0.46 in 1998. The average bleaching costs are unchanged at ~ $50-$51/ADMt.

Similarly there has not been a significant change in the overall bleaching chemical demand for oxygen-delignified softwood pulps. The average sequence kappa factor increased from 0.57 in 1996 to 0.59 in 1998. Average bleaching costs are unchanged at ~ $38-40/ADMt.

Oxygen-delignified softwood pulps require more oxidizing equivalents per unit of unbleached pulp kappa number. In spite of this, on the average, oxygen-delignified pulps have a bleaching chemical cost advantage of $10-$13 Cdn/ADMt compared to conventionally-delignified pulps.

An assessment of bleaching practices shows that the difference between mills with relatively "high" chemical consumption and mills with relatively "low" chemical consumption is ~ $5 Cdn/ADMt. In some cases, conventionally-delignified mills had the same bleaching cost as some oxygen-delignified mills.

The data showed that mills with post oxygen kappa no. of 20 had significantly better bleaching efficiency than mills with a post oxygen kappa no. of 16.

The bleaching efficiency for the high post oxygen kappa no. pulps was so much better that the bleaching cost for the 20 kappa no. pulps was $4.63/ADMt less than for the 16 kappa no. pulps. This is a surprising result and raises questions regarding pulp bleachability.

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 softwood mills.

Softwood 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. Softwood accounts for 75% 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 and 2 of this report summarize the bleaching practices for two softwood pulp categories: oxygen-delignified and conventionally-delignified. In Appendix 2, these two categories were further subdivided into "low" and "high" relative chemical consumption to determine "best practices." The best practices are summarized in Table 2 later in the report. Appendix 3 summarizes responses to questions regarding scaling experience in the bleach plant. The data was edited so that only high brightness ECF grades are analyzed.

Softwood Analysis

Kappa Number to Bleaching

For conventionally-delignified pulps, the kappa number from the digester averages 29.8 and declines slightly to 28.7 before bleaching. Enzymes are also used to a limited extent before the bleach plant to enhance bleaching efficiency. In 1998, three softwood bleach plants (with conventional delignification) reported using enzymes representing 0.5 million tonnes of pulp. Typical operating conditions for enzyme applications are: pH 6-7 at 10% consistency for 1 hour at 50-60°C. No oxygen-delignified mills reported using enzymes.

Figure 2a: Conventionally-delignified Softwood Kappa No. to Bleaching

Oxygen delignification of softwood pulp is increasing. Production grew from 28% of softwood kraft pulp in 1996 to 39% in 1998 for a total of 3.3 million tonnes. The digester kappa number for oxygen-delignified mills average 30.8, and declines slightly to 29.4 before entering the oxygen delignification stage. Mills average 39% delignification and the post-oxygen kappa no. averages 18, a 5% increase from the 17.3 kappa no. average in 1996.

The systems are typically single stage operating at medium consistency.

Figure 2b: Oxygen-delignified Softwood Kappa No. to Bleaching

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 ~ 8-10 kg/ADMt.

Figure 3: Carryover to Bleach Plant - Conventional Softwood Pulps

Figure 4: Carryover to Bleach Plant - Oxygen-delignified Softwood

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. Eighty per cent of the bleach plants operate at low consistency and twenty per cent at medium consistency. Furthermore, 75% are up-flow towers and 25% up-flow/down-flow. There is a modest shift toward more medium consistency up-flow down-flow towers as 90% were up-flow in 1996.

Retention time is 37 minutes with a range of 15-80. Temperature averages 54°C ranging from 40 to 70°C. Residual chlorine dioxide is typically zero. Approximately 90% of oxygen delignification bleach plants and 50% of conventional bleach plants control the Do end pH at 2.6-2.7. Most mills control the pH with addition of H₂SO₄. Four mills are using CO₂for brownstock acidification and one mill is using spent sodium sesquisulphate from the chlorine dioxide generator.

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 sensor as well. Approximately 25% control to an extracted kappa number. Forty-five per cent have on-line kappa number analyzers. This is almost double the number in 1996 - indicating the growing use of this technology.

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 oxygen-delignified pulps, averaging 0.22, while for conventionally-delignified pulps the average was 0.20 as shown in Figure 5. Oxygen-delignified pulps are relatively more difficult to further delignify with chlorine dioxide. This is reflected in the higher average D₀ kappa factor.

Figure 5a: Oxygen-delignified Softwood Do Kappa Factor

Figure 5b: Conventionally-delignified Softwood Do Kappa Factor

First Extraction Stage, Eop

Operating Conditions

The Eop stage retention time is 75 minutes with a range of 50-134, operating at 78-80°C with a range of 65-100°C. There has been a shift toward higher temperature in the Eop stage. The mills with conventional delignification averaged over 80°C. Most stages operate under hydraulic pressure, with time at pressure, typically 20-40 minutes.

Approximately 10% of oxygen delignification bleach plants and 35% of conventional lines utilize Papricycle^® to minimize sodium hydroxide consumption. The NaOH:Equivalent Cl₂ ratio averages 0.46 without Papricycle^® and 0.3 with. This represents a saving of approximately 9 kg NaOH/ADMt for conventionally-delignified pulps.

Mixing and Chemical Addition

Hydrogen peroxide is typically added just before oxygen. Oxygen mixing is split between sparger injection into the pulp in a fluidized zone and dedicated oxygen hi-shear mixers as shown in Figure 6.

Figure 6: Eop Oxygen Mixing Distribution

Sensors and Control

Eop stages are controlled to an end pH of 10.7. Approximately 40% of oxygen delignification lines and 20% of conventional lines control to an Eop brightness target. Eop kappa number control is used in about 30% of the lines.

Eop Kappa Number and Kappa Factor

Both hydrogen peroxide and oxygen are used in the extraction stage in 90% of the bleach plants. Oxygen is applied typically at 5-6 kg/ADMt and hydrogen peroxide at 3-4 kg/ADMt. The Eop kappa number averages 4.2-4.4 for both conventional and oxygen-delignified pulps. The kappa factor for the D₀ plus the Eop stage is higher for oxygen-delignified pulps averaging 0.39, while conventionally-delignified pulps average 0.32 as shown in Figure 7.

Figure 7a: Oxygen-delignified DEop Kappa Factor

Figure 7b: Conventionally-delignified DEop Kappa Factor

Chlorine Dioxide Brightening: (D₁) Stage

Operating Conditions

The D₁ stage retention time is 150-160 minutes with a range of 90-265 operating at 70-74°C with a range of 60-82°C. Approximately 40% of oxygen delignification bleach plants and 50% of conventional lines add NaOH to control pH. In 1996, 85% of conventional lines added NaOH. When NaOH is added, the NaOH:ClO₂ ratio is 0.30 kg/kg. The upflow pH averages 3.8-4.0 and end pH 3.2-3.5.

Sensors and Control

Most stages are controlled using pre-tower optical and residual sensors to a compensated brightness target and 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 40 mg/L measured in the D₁ washer vat.

D₁ stage brightness averages 83.5% ISO for both oxygen-delignified pulps and conventional pulps as shown in Figure 9. Chlorine dioxide is applied at 9.8 kg/ADMt for oxygen-delignified pulps compared to 11.6 kg/ADMt for conventional pulps. The application rate for both pulps is approximately 3 kg ClO₂/ADMt per unit Eop kappa.

Mixing is dominated by hi-shear type driven mixers, but medium consistency pumps are used to a great extent as well as shown in Figure 8.

Figure 8: D1 Stage Chlorine Dioxide Mixing

Figure 9: D1 Brightness

Second Extraction Stage, E₂

Operating Conditions

Recently, a number of mills have incorporated so-called short extraction stages for the E₂ stage. In some configurations, NaOH is added directly to the ring dilution at the bottom of the D₁ tower and the short residence time before the pulp is pumped to the D₁ washer is used for extraction. The pulp is then washed and forwarded to the D₂ stage. In other configurations, NaOH is added to the washed D₁ pulp and the residence time in a medium consistency pump feed chute is used for extraction. The pulp is not washed in this case and chlorine dioxide is added directly to the pulp prior to the D₂ tower. Such short extraction stages are found in 30% of the oxygen delignification lines. The retention times for these stages are in the range 5-25 minutes.

Conventional E₂-stage retention time is 70-75 minutes at a temperature of 75°C. Both the short and longer stages are controlled to an end pH of 10.3-10.5. NaOH consumption is typically 5-6 kg/ADMt.

In conventionally-delignified softwood bleach plants, 60% add hydrogen peroxide to the second extraction stage. This is a significant increase since only 30% of the bleach plants used hydrogen peroxide in the second extraction stage in 1996. In oxygen-delignified softwood bleach plants, 30% add hydrogen peroxide to the second extraction stage. When hydrogen peroxide is used in the second extraction stage, the addition rate is at 1-1.5 kg/ADMt.

Chlorine Dioxide Brightening: (D₂) Stage

Operating Conditions

The D₂ stage retention time is 170 minutes with a wide range of 120-300 minutes operating at 76°C with a range of 65-82°C. The upflow pH averages 4.3 -4.5 and end pH 3.8-3.9.

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 40 mg/L , measured in the D₂ washer vat.

D₂ stage brightness averages 89.6% - 89.7% ISO for both oxygen-delignified and conventional pulps as shown in Figure 10. Chlorine dioxide is applied at 2.8 kg/ADMt for oxygen-delignified pulps compared to 3.4 kg/ADMt for conventional lines. Chlorine dioxide is distributed approximately 75:25 between the D₁ and D₂ stages.

Figure 10: 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 0.59 for oxygen-delignified pulps compared to 0.46 for conventional pulps.

Oxygen-delignified pulps averaged require $2.25 Cdn per unit of unbleached kappa number and conventional pulps require $1.77 Cdn per unit of unbleached pulp kappa number to achieve 89.6% ISO brightness. The bleaching cost for oxygen-delignified pulps was $39.60 Cdn/ADMt compared to $50.67 Cdn/ADMt for conventional pulps, a difference of $11.07 Cdn/ADMt. It is interesting to note in Figure 12, that a number of oxygen-delignified mills have the same bleaching cost as conventionally bleached softwoods.

Figure 11: Sequence Kappa Factor

Figure 12: Bleaching Cost, $/ADMt

Optimization Analysis of DEopDED Bleaching

Data Base

The data supplied by the oxygen-delignified and conventional lines was sorted and analyzed to see if optimum practices could be determined. The data was sorted by relative chemical consumption. In other words, which mills used the least oxidizing chemicals to reach the same final brightness.

The database was sorted into two groups of bleach plants, those plants below the median value and those above the median value. They are marked "low" and "high" relative chemical consumption in subsequent tables and graphs. The full data analysis can be found in Appendix 2.

Optimization Summary - Oxygen-delignified Pulps

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 13, there was a difference in emphasis comparing the "low" relative chemical consumption bleach plants to the "high" ones. The "low" plants had more emphasis on minimizing chlorine dioxide consumption and cost. The "high" bleach plants had greater emphasis on maximizing brightness and compensating for short retention time and short sequences.

Figure 13: Optimization Summary: Oxygen-delignified Pulps

Optimization Analysis: Oxygen-delignified Pulps

The key differences between the "low" and "high" relative chemical consumption bleach lines are shown in the following figures.

Kappa Numbers

The "low" consumption bleach plants are characterized by higher kappa numbers in the digester, after oxygen delignification and the extraction stage. Of particular note is that the "low" bleach plants averaged 33% delignification in the oxygen delignification stage compared to 44% for the "high" bleach plants as shown in Figure 14.

Figure 14: Kappa Number Profile – Oxygen-delignified Pulps

Kappa Factor

The kappa factors are higher for the "high" bleach plants throughout the sequence. The biggest difference is in the Do stage where the first stage kappa factor averages 0.18 for the "low" mills compared to 0.27 for the "high" mills. The oxygen and hydrogen peroxide in the Eop stages are comparable as is the chlorine dioxide application in the D₁ . However in the case of the "low" mills, 10 kg/ADMt chlorine dioxide raises a 5 extracted kappa no. to 83.5% ISO. For the "high" mills, it takes the same 10 kg/ADMt to raise a 3.0 extracted kappa no. to 83.5% ISO. This indicates a change in bleachability.

Furthermore, chlorine dioxide requirement in the final D₂ stages is greater for the "high" bleach plants as shown in Figure 15. The greater chlorine dioxide application is consistent with the emphasis on maximizing brightness, and compensation for short retention time as noted in the optimization summary.

Figure 15: Kappa Factor Profile – Oxygen-delignified Pulps

Figure 16: Chlorine Dioxide Profile – Oxygen-delignified Pulps

Optimization Summary - Conventionally-delignified Pulps

As discussed previously, bleach plants were assessed to determine what each was optimized to achieve.

As shown in Figure 17, there was a difference in emphasis comparing the "low" relative chemical consumption bleach plants to the "high" ones. The "low" relative consumption bleach plants had slightly greater emphasis on minimizing bleaching cost and minimizing chlorine dioxide consumption.

Figure 17: Optimization Summary – Conventionally-delignified Pulps

Optimization Analysis: Conventionally-delignified Pulps

The key differences between the "low" and "high" relative chemical consumption bleach lines are shown in the following figures.

Kappa Numbers

The "low" consumption bleach plants are characterized by higher kappa numbers to the bleach plant and after the extraction stage as shown in Figure 18.

Figure 18: Kappa Number Profile – Conventionally-delignified Pulps

Kappa Factor

The kappa factor after each stage is consistently higher for the "high" bleach plants as show in Figure 19. Chlorine dioxide application in the D₁ and D₂ stages is greater for the "high" bleach plants as shown in Figure 20.

Figure 19: Kappa Factor Profile – Conventionally-delignified Pulps

Figure 20: Chlorine Dioxide Profile – Conventionally-delignified Pulps

Bleaching Cost

As shown in Figure 21, for oxygen-delignified pulps, the bleaching cost for the "high" lines was higher than the "low" bleaching lines. This is in spite of the fact that the post-oxygen kappa number was lower i.e., 16.1 vs. 19.9. The bleaching chemical cost advantage for the "low" grades, was $4.63 Cdn/ADMt. This is surprising as one would expect lower kappa numbers to require less bleaching chemicals and therefore less cost. Does this raise a question regarding pulp bleachability due to conditions in the digester, an optimum level of delignification for bleaching chemical efficiency and bleaching cost? Or is driving the kappa number to lower and lower levels cost-effective?

For conventionally-delignified pulps, the "low" bleaching lines achieved a bleaching cost advantage of approximately $5.31 Cdn/ADMt compared to the "high" bleaching lines.

Figure 21: Bleaching Cost

Summary

Oxygen delignification of softwood pulp is growing. In 1996, 28% of softwood kraft pulp was oxygen-delignified. In 1998, this figure grew to 39%, representing 3.3 million tonnes of production.

Analysis of bleaching conditions shows a number of developing trends since 1996:

An increase in post oxygen kappa no. (17.3 to 18.0)

A doubling of the number of kappa analyzers

A modest increase in the number of medium consistency Do stages (typically associated with introduction of oxygen delignification)

An increase in the temperature of the Eop stage (76°C to 81.4°C)

Fewer mills using NaOH for D1 pH control

A doubling of the mills using hydrogen peroxide in the second extraction stage

Similarly there has not been a significant change in the overall bleaching chemical demand for oxygen-delignified softwood pulps. The average sequence kappa factor increased from 0.57 in 1996 to 0.59 in 1998. In addition bleaching costs are unchanged at ~ $38-40/ADMt.

The data showed that mills with post oxygen kappa no. of 20 kappa no. had significantly better bleaching efficiency than mills with a post oxygen kappa no. of 16.

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.