NEAR-NEUTRAL FINAL CHLORINE DIOXIDE BRIGHTENING: THEORY AND PRACTICE

Transcription

1 NEARNEUTRAL FINAL CHLORINE DIOXIDE BRIGHTENING: THEORY AND PRACTICE ZHIHUA JIANG* AND RICHARD BERRY ABSTRACT It is well known that ph affects the effectiveness of chlorine dioxide treatment, but the standard recommendation has been to maintain a poststage ph range for brightening with chlorine dioxide between 3.5 and 4. In this paper, we confirm the optimum ph for final chlorine dioxide bleaching is dependent on chlorine dioxide charge, but that this stage should be operated with a final ph close to neutral when a typical chlorine dioxide charge is applied. We show that maintaining a ph close to neutral throughout the bleaching stage is the ideal strategy. This result can be achieved under unbuffered conditions in some instances, but is more consistently obtained by generating sodium bicarbonate in situ by adding carbon dioxide to alkaline pulp or sodium carbonate to acid pulp. In addition, we highlight the theory and practice of the nearneutral final chlorine dioxide brightening technology. INTRODUCTION Fine paper producers have recently raised the market standard for brightness of office papers [1]. These new brightness levels are greatly assisted by the application of fluorescent whitening agents (FWAs) but are also dependent on the level of brightness of the original fully bleached pulps used to produce the paper. Obtaining the highest brightness economically from any given furnish requires the application of bleaching chemicals under optimum conditions. For most North American mills, chlorine dioxide is the bleaching chemical of choice. It is well known that ph affects the effectiveness of chlorine dioxide treatment [23]. References 2 and 3 show the optimum poststage ph range for delignification with chlorine dioxide is between 2 and 3 and for brightening with chlorine dioxide is between 3.5 and 4. These recommendations have led to the common practice of operating both the intermediate and final chlorine dioxide bleaching stages to a final ph between 3.5 and 4. [13]. More recently, work by Hart and Connell [4] has shown that identifying the optimum ph for brightening stages is more complex than first thought and is dependent on chlorine dioxide charge. The common practice of using a final ph between 3.5 and 4 is therefore a special case that works well when a high charge of >.75 chlorine dioxide on pulp is used which for many mills is the operating practice in the intermediate chlorine dioxide stage. This charge, however, is greater than what is normally applied in a final chlorine dioxide stage. The discussion is still further complicated by considering not just the final ph from a stage but also the ph during a stage. Early work by Rapson in 1956 showed that, by using soluble buffers such as potassium dihydrogen phosphate, the maximum brightness development was achieved by keeping the ph during the final brightening stage very close to neutral throughout the stage [5]. In 1964, Sepall described a process for neutral chlorine dioxide bleaching in which neutral conditions were maintained by the addition of carbonates and oxides of low solubility alkaline earth metals [6]. However, neither of these processes is practised commercially; the soluble buffers such as potassium dihydrogen phosphate are too expensive for industrial application, while ZHIHUA JIANG FPInnovations 57, Boul. StJean, PointeClaire, Qc, Canada, H9R 3J9 *Contact: hua.jiang24@gmail.com RICHARD BERRY FPInnovations 57, Boul. StJean, PointeClaire, Qc, Canada, H9R 3J9 14 JFOR Journal of Science & Technology for Forest Products and Processes: VOL.1, NO.1, 211

2 PAPTAC I.H. WELDON AWARD WINNER the handling of sparingly soluble buffers such as sodium bicarbonate is difficult. In this paper, we describe the development of nearneutral chlorine dioxide brightening technology [7]. We confirm the optimum ph for final chlorine dioxide bleaching is dependent on chlorine dioxide charge but that this stage should be operated with a final ph close to neutral when a typical chlorine dioxide charge is applied. We also show that maintaining a ph close to neutral is the ideal strategy. This result can be achieved under unbuffered conditions in some instances but is more consistently obtained by generating sodium bicarbonate in situ by adding carbon dioxide to alkaline pulp or sodium carbonate to acid pulp. We will also highlight the theory and practice of the nearneutral final chlorine dioxide brightening technology. EXPERIMENTAL Four different mill pulps, a mixed (6 softwood and 4 hardwood) Kraft pulp, a mill oxygen delignified hardwood Kraft pulp, a hardwood Kraft pulp partially bleached in a mill with a OD Eop sequence, and a softwood kraft pulp partially bleached in a mill with a DEoDEp sequence, were used for this work. In addition, a hardwood Kraft pulp, laboratory bleached with an OD Eop sequence, was also used. Upon receipt the mill pulps were dewatered in a centrifuge to between 3 and 35 consistency, shredded to produce fibre aggregates of approximately 1 cm in diameter and stored in polyethylene bags in a cold room at 5 C before use. The bleaching procedures were those conventionally used in our chemical bleaching laboratory [8]. The chlorine dioxide stages were done in polyethylene containers or polyester bags. Pulp brightness was determined by the PAPTAC standard method and kappa number was measured using an automated titrator [9]. The ph was measured using a Corning ph meter (Model 445) equipped with an Orion ph electrode (Model 9272BN), a temperature probe and automatic temperature compensation. The ph measurements were done at room temperature. The concentration profiles of the different chlorinecontaining species in filtrates, chloride (Cl ), chlorite ( ) and chlorate (ClO 3 ) were measured by anionexchange chromatography (IC) [1] using a DIONEX Model 45i ion chromatograph equipped with an electrochemical detector set up in pulse amperometric mode. The samples were directly injected into the IC, after sparging with helium to remove residual chlorine dioxide. The filtrate species were separated on a Carbopac PA1 4mm analytical column and a Carbopac PA1 4mm guard column (DIONEX) using a water eluent delivered at 1. ml/min for 1 minutes. NaOH (5 mmol/l) was added as a post column reagent at a flow rate of ~.5 ml/ min using a PC1 pneumatic controller. The TAPPI standard titration method was used to determine the total amount of chlorite and residual. The amount of residual was then obtained by subtracting the amount of chlorite as measured by the IC from the total amount of chlorite and residual, as measured by titration. The difference between the sum of all the inorganic chlorinecontaining species present in the filtrate, and the actual charge of chlorine dioxide applied, was assumed to give the amount of organic chlorine in the filtrate and/or pulp. RESULTS AND DISCUSSION Effect of Chlorine Dioxide Charge Figure 1 shows the trends in brightness with ph using different charges in the final Dstage of a D EopD sequence, applied to a mixed (6 softwood and 4 hardwood) Kraft pulp with a kappa number of 21.1; this pulp was from an integrated mill that did not require market pulp brightness. The trend in brightness depended on the chlorine dioxide charge in the stage. At the highest charge (.85) in the final D stage, the final ISO brightness decreased from 83.9 to 82.5 as the final ph in the stage increased from 3.3 to 5.4. However, at low charges (.3.55), which are more close to the typical range of charges applied in mills, the final ISO brightness increased as the final ph increased over a similar ph range. A similar trend of ph dependence was previously observed for the final D stage in OD EDED bleaching of a softwood Kraft pulp over a range of chlorine dioxide charge between.75 and 1.25 [11]. Optimum final ph for final chlorine dioxide brightening To establish the optimum ph for final chlorine dioxide brightening at typical chlorine dioxide charges, a mill oxygen delignified hardwood Kraft pulp with a kappa number of 8. was bleached with a D EpD sequence. The chlorine dioxide charge in the final D stage was.2 or.4. The final ph of the final Dstage was adjusted by using NaOH or H 2 mixed with pulp before the addition of chlorine dioxide. Figure 2a shows that at a chlorine dioxide charge of.2 in the final Dstage, the highest ISO brightness was achieved at the final ph close to neutral (6.1). The results obtained at a higher chlorine dioxide charge (.4) also show that a significantly higher brightness was achieved at a final ph of 5.8, than at a ph between 3.9 and 4.9. These results agree with those obtained using a buffer solution [5] and those obtained by Milanez and Colodette, who showed that a final ph of 5.5 was more effective than either 4.5 or 3.5 for the final D stage in ODHT(PO)D bleaching of a Eucalyptus Kraft pulp [12]. Figure 2a also shows the optimal final Fig. 1 The effect of the final ph in the final Dstage of a D EopD sequence on brightness. Pulp source: a 6 softwood and 4 hardwood Kraft pulp blend with a kappa number of Do conditions:.18 kappa factor, 21 minutes, 6 C and 4. consistency; Eop conditions:.5 H 2 O 2, 4.3 minutes with an O 2 pressure of.14 MPag) and 82.7 minutes under atmospheric pressure, 82 C and 11 consistency; Kappa number and ISO brightness after the Eopstage were 4.5 and 62.3, respectively; Final Dstage conditions:.3 to.85, 148 minutes, 74 C and 12 consistency. JFOR Journal of Science & Technology for Forest Products and Processes: VOL.1, NO.1,

3 ISO brightness obtained at a charge of.2 chlorine dioxide was similar to, or higher than that obtained at a charge of.4 Fig. 2a The effect of the final ph in the final Dstage of a D EpD sequence on brightness. Pulp source: a mill oxygen delignified hardwood Kraft pulp with a kappa number of 8.. D Conditions:.2 kappa factor, 54 minutes, 6 o C and 12 consistency; Ep conditions:.4 H 2 O 2, 6 minutes, 8 o C and 1 consistency; Kappa number and ISO brightness after the Epstage were 3.1/ and 88., respectively. Final Dstage conditions:.2 or.4, 18 minutes, 7 o C and 9 consistency. chlorine dioxide with a final ph of between 3 and 4. This demonstrates that optimizing the final ph is an effective way to reduce the chlorine dioxidebleaching requirement. Alternatively, a mill could decrease the first stage chlorine dioxide charge to minimize the sequence chlorine dioxide charge. The chlorine dioxide consumption in the final Dstage decreased as the final ph increased (Fig. 2b), due to the formation of chlorite formed in situ which becomes less reactive as the final ph is increased [5]. Figure 2c shows the efficiency of chlorine dioxide usage in the final Dstage (expressed as units of ISO brightness) gain over the brightness after the Epstage per kg of chlorine dioxide consumption, increased as the final ph was increased at a given charge of chlorine dioxide. This result again demonstrates the final Dstage is more effective at a final ph close to neutral than between 3 and 4. One drawback to working to a final neutral ph by the approach used to obtain these first results is that pulp viscosity declines as ph is increased from 3.5 and it is more significant at a charge of.4 (Fig. 2d). mixed with pulp before the addition of chlorine dioxide. Figure 3a shows a similar trend to the one shown in Fig. 2a; the final brightness increased as the final ph increased from ~2.6 to near neutral. Figure 3a also shows that the brightness measured after conditioning the bleached handsheets for 1 hour at 15 C increased as the final ph increased from ~2.6 to near neutral. The optimum final ph (between 4.5 and 5.3) obtained in this example was lower than that shown in Fig. 2a, indicating the exact optimum final ph can vary within the near neutral range when the target ph is achieved in an unbuffered system. Again the chlorine dioxide consumption in the final Dstage was found to decrease as the final ph increased (Fig. 3b) and the efficiency of chlorine dioxide usage in the final Dstage, (expressed as Fig. 2b The effect of the final ph in the final Dstage of a D EpD sequence on percentage of uptake. Pulp source and bleaching conditions are the same as in Fig. 2a. Fig. 3a The effect of the final ph in the final Dstage applied to a softwood Kraft pulp partially bleached in a mill with a DE DEp sequence with an ISO brightness of 78. on final brightness before and after reversion. Final Dstage.2, 14 minutes, 9 o C and 1 consistency. Fig. 2d The effect of the final ph in the final Dstage of a D EpD sequence on viscosity. Pulp source and bleaching conditions are the same as in Fig. 2a. Fig. 2c The effect of the final ph in the final Dstage of a D EpD sequence on efficiency of chloride dioxide usage. Pulp source and bleaching conditions are the same as in Fig. 2a. To further support that the optimum final ph for final chlorine dioxide brightening at typical chlorine dioxide charges is near neutral, a final Dstage bleaching with a chlorine dioxide charge of.2 was applied to a softwood Kraft pulp partially bleached in a mill with a DE pdep sequence. Again, the final ph of the final Dstage was adjusted by using NaOH or H 2 Fig. 3b The effect of the final ph in the final Dstage applied to a softwood Kraft pulp partially bleached in a mill with a DE DEp sequence with an ISO brightness of 78. on percentage uptake. Final Dstage.2, 14 minutes, 9 o C and 1 consistency. 16 JFOR Journal of Science & Technology for Forest Products and Processes: VOL.1, NO.1, 211

4 PAPTAC I.H. WELDON AWARD WINNER Fig. 3c The effect of the final ph in the final Dstage applied to a softwood kraft pulp partially bleached in a mill with a DE DEp sequence with an ISO brightness of 78. on efficiency of chlorine dioxide usage. Final Dstage.2, 14 minutes, 9 o C and 1 consistency. units of ISO brightness gain over the brightness after the Epstage per kg of chlorine dioxide consumption) increased as the final ph was increased (Fig. 3c). Effect of Initial ph In the previously described work, a nearneutral final ph was obtained by mixing NaOH with pulp before the addition of chlorine dioxide. The charge of chlorine dioxide determines the charge of NaOH required, which in turn determines the initial reaction ph. Figure 4 depicts the relationship between the final D stage ph and the required NaOH addition at the beginning of the stage under the bleaching conditions used to produce the data in Fig. 2. With a charge of.2, a charge of NaOH between.2 and.8 was required to give a final ph between 5. and 6.1. With a charge of.4, a charge of.2 NaOH was Fig. 4. The effect of NaOH charge in the final Dstage of a D ED sequence on the final ph. Bleaching conditions are as described in the caption of Fig. 2. required to give a final ph of 5.8. With a charge of.2 NaOH, the initial reaction ph (defined as the ph immediately prior to addition) was 1.8. The initial ph further increased to 11.4 as the charge of NaOH was increased to.8 and went as high as 12. with a charge of.2. There has been little consideration in the literature of the effect of the initial ph immediately prior to addition on the effectiveness of final chlorine dioxide brightening [1314]. In previous work [13], we found the addition of a high charge of NaOH at the beginning of the chlorine dioxide reaction caused some decomposition of to and ClO 3 through the following reaction: 2 + 2OH + ClO 3 + H 2 O This reaction becomes faster as the ph is raised because of the increased concentration of hydroxyl ions [5,15]. The deviation of the optimum final ph from nearneutral observed with high charges of chlorine dioxide (Fig. 1) can be explained by the needed high charge of NaOH which causes substantial decomposition. A high concentration of hydroxyl ions is also expected to promote the formation of sodium hypochlorite from hypochlorous acid which is formed in situ during chlorine dioxide bleaching. Unlike and ClO 3 which are inactive under alkaline conditions, hypochlorite is a strong bleaching agent but one with much lower selectivity than chlorine dioxide. This hypochlorite formation can explain the viscosity drop observed when operating with higher final phs (Fig. 2d) achieved when using a high initial ph. As Rapson demonstrated half a century ago, optimum bleaching with selectivity is achieved by using a buffered system; Rapson showed that chlorine dioxide had little effect on viscosity at phs between 3 and 7 under buffered conditions [5]. The significance of the near neutral ph is that it maximises the performance of the key brightening agent in the chlorine dioxide stage bleaching liquor; this agent is chlorite. Chlorine dioxide is an electrophilic bleaching agent and as such is involved in some delignification reactions. These reactions are greatly enhanced by the in situ generation of chlorine. Brightening reactions are better accomplished by a nucleophilic reagent and it is brightening that is required during the latter stages of bleaching. The key brightening reagents generated in situ from chlorine dioxide are hypochlorite and chlorite and, of these, chlorite is preferred because of significantly better selectivity. This discussion leads to the conclusion that brightening stages should be operated to maximise the periods of time under near neutral conditions and, as Rapson showed, this is best achieved with a buffered system. Novel Methods for Achieving NearNeutral Final Chlorine Dioxide Brightening Three new approaches for constantly maintaining the optimum nearneutral ph conditions in brightening Dstages are evaluated here. The first approach is to allow to react with the pulp for a short period of time and then add NaOH to adjust the ph; this approach has been used in the further development of aldehydeenhanced bleaching [13]. The second approach requires forming a sparingly soluble buffer in situ and adjusting the ph to near neutral. Bicarbonate is an ideal buffer because its pka is in the near neutral range and is easily formed by mixing carbon dioxide with an alkaline solution [7]. Carbon dioxide has been widely used for ph control in various processes, including neutral papermaking [16] and in a twostep highph/lowph chlorine dioxide treatment [17]. The third approach is to use sodium carbonate (Na 2 ) to adjust ph. Table I compares final chlorine dioxide brightening when using these different approaches. A hardwood Kraft pulp partially bleached in a mill with an ODEOP sequence was used. The kappa number of the ODEOP pulp was 2. and the bleaching conditions applied in this laboratory study were the conditions used by the mill. For the conventional ph control method, where NaOH or H 2 was added at the beginning of the reaction, the maximum final brightness was obtained with a final ph close to neutral (5.56.) using a NaOH charge of between.25 and.5. The initial reaction ph after the addition of the charges of NaOH was between 1.4 JFOR Journal of Science & Technology for Forest Products and Processes: VOL.1, NO.1,

5 TABLE l Charge on pulp Final ph Final ISO brightness, Viscosity, mpa.s A comparison of the brightening achieved in a final Dstage with different ph control strategies. Conventional method and In approach 1, we introduced NaOH into the system after a reaction time of 2 minutes. The ph before the addition of NaOH was 5.3. Three charges of NaOH (.17,.34 and.5) were used. The ph immediately after the addition of NaOH was between 6.1 and 7.1 and it dropped to between 5.1 and 6.4 at the end of the stage. As shown in Table 1, the final ISO brightness increased using this strategy. The highest ISO brightness was 92.3;.7 units higher than the highest brightness achieved by the conventional method. The disadvantage of this approach is that typical final Dstage configurations in mills do not have a chemical injection point available for this delayed NaOH addition. In approach 2, carbon dioxide was Approach 1 ph adjusted after 2 minutes Approach 2 initial ph adjusted Final chlorine dioxine brightening conditions:.17, 97 minutes, 12.2 consistency and 7 C Approach 3 initial ph adjusted H 2 NaOH NaOH NaOH and CO 2 Na bubbled for 15 minutes through a NaOH solution, which was then mixed with the pulp. The weight ratio of carbon dioxide and NaOH was kept constant at 1:1. The ph immediately after the addition of NaOH and CO 2 was 7.8 at both charges of NaOH and CO 2 (.14 and.25 each). As shown in Table I, the final ph was between 5.3 and 6.. The charge of NaOH and CO 2 and the ratio of NaOH and CO 2 applied was not optimized, nonetheless, a final ISO brightness of 91.9 was obtained; an increase of.3 units over the highest brightness achieved by the conventional method. In approach 3, Na 2 replaced the NaOH used for ph adjustment at the beginning of the reaction. A charge of between.5 and.15 Na 2 gave an TABLE II. A comparison of the brightening achieved in a final Dstage and the mass balance of chlorinecontaining species with different ph control strategies. Charge, on pulp Final ph Mass balance of chlorinecontaining species (mm) Final brightness, ISO Cl ClO 3 Organic chlorine Organic chlorine + ClO 3 Conventional method Approach 2 Initial ph ajusted Approach 3 Initial ph ajusted H 2 NaOH NaOH and CO 2 Na Final chlorine dioxide brightening conditions:.17, 97 minutes, 12.2 consistency and 7 o C initial ph after the addition of Na 2 between 1. and 1.2 and a final ph between 5.8 and 7.. The highest final brightness was 92.3, which was obtained with a final ph of 5.8. This result was the same as that obtained with the addition of NaOH after a reaction time of 2 minutes. However, the approach has the advantage of being compatible with present mill configurations. Table I shows that viscosity did not decrease during any of the new approaches. This result again demonstrates the drop in viscosity was likely caused by a high initial ph (11.9). Mass balance of the chlorinecontaining species As shown in Table II, similar results were obtained with two of these new approaches (2 and 3) when they were applied to a hardwood Kraft pulp laboratory bleached with an ODEop sequence to a kappa number of 3.1. With this pulp, we also evaluated the mass balance of the chlorinecontaining species to provide insight into how chlorine dioxide is consumed. Rapson and Anderson showed that optimum brightness of chlorine dioxide bleaching coincided with that of the minimum sum of chlorate (ClO 3 ) and chlorite ( ) [5]. Our results, however, indicate no clear correlation between brightness gain and the sum of chlorate (ClO 3 ) and chlorite ( ) (Table II). This disagreement is not surprising because Rapson and Anderson used a high charge of chlorine dioxide (1.) and chlorite was considered an inactive species [5]. By contrast, our results indicate that as brightness increases as the final ph increases toward near neutral, the amount of chlorite ( ) increased, which suggests that chlorite is a key brightening agent under near neutral ph conditions. In addition, the amount of organic chlorine decreased dramatically, which appears to be the main cause of the improved chemical efficiency. Mill Trial The first mill trial of buffered nearneutral final chlorine dioxide brightening using carbon dioxide was conducted in June 27 at a Kraft mill in Canada produced about 26, admt/year of ECF bleached pulp 18 JFOR Journal of Science & Technology for Forest Products and Processes: VOL.1, NO.1, 211

6 PAPTAC I.H. WELDON AWARD WINNER well received at the mill, as the near neutral brightening showed a strong capacity to stabilize the overall bleaching plant operation. Fig. 5 Comparison of final pulp machine brightness between control and trial periods. with a D E O D 1 E p D 2 sequence. The laboratory results on the mill pulp prior to the trial indicated that a mill trial with buffered nearneutral final chlorine dioxide brightening should allow a 1.5 kg/ton chlorine dioxide charge decrease in the D 2 stage or a 2.7 kg/ton decrease in the Do stage while maintaining the same final brightness. An eightday mill trial was started on June 4, 27. The only equipment change made for the trial was to allow CO 2 injection, which was achieved by installing an injection line, injection point and CO 2 flow control panel. Figure 5 shows the comparison of the timetracked final pulp brightness as measured at the pulp machine between the control and trial periods. The average final brightness of the trial was 89.1 with a standard deviation of.26 as compared with the average final brightness of the control Table III Comparison of the final pulp machine brightness from the control and trial runs Period Brightness range Control (May 22, 7:1 to June 24, 19:1) Frequency Percent, Cumulative, period of 88.7 with a standard deviation of.35. It is clear that nearneutral brightening provides pulps with higher and more stable brightness. Table III further compares the brightness distribution pattern. A total of 27.3 of brightness values obtained from the trial run were 89.2 or higher while only 4.2 of brightness values from the control period reached this brightness level. On the other hand, only 1.2 of brightness values obtained from the trial run were 88.4 or lower while 17.5 of brightness values from the control period were at this brightness level. Table IV shows the average results for both control and trial runs. The results show the total charge was lower by 2.2 kg/adton for the trial run, representing a very significant net savings in total chemical cost. These results were achieved with a more stable final brightness. They were Trial (June 4, 19:58 to June 11, 3:59) Frequency Percent, Cumulative, CONCLUSIONS The common belief that final chlorine dioxide brightening should be operated with a final ph in the range between 3.5 and 4. is questionable. Our results show the optimum ph for final chlorine dioxide bleaching is dependant on chlorine dioxide charge and that this stage should be operated with a final ph close to neutral when a typical chlorine dioxide charge is applied. Our results support the premise that an ideal strategy is to maintain a ph as close to nearneutral as possible during the entirety of this stage. These buffered nearneutral conditions can be achieved by using either carbon dioxide or sodium carbonate to control ph depending on the initial ph of the incoming pulp. Nearneutral final chlorine dioxide brightening is an effective way to reduce bleaching costs by maximizing chlorine dioxide bleaching efficiency with no detrimental effect on pulp viscosity and brightness stability. The improved bleaching efficiency observed with a nearneutral ph is caused by a decrease in the amount of organic chlorine formed. The results obtained in a mill trial show a net estimated savings in total chemical costs was C$2.68/ADton at an equivalent feed kappa number and final brightness. These results were achieved with a more stable final brightness, which was well received at the mill as the near neutral brightening showed a strong capacity of stabilizing the overall bleaching plant operation. The nearneutral final chlorine dioxide brightening technology (NNB) has now been widely implemented in Kraft mills across Canada. Table IV. The average timetracked results from the control and trial runs Period Production rate, ADton/day Final machine brightness Total charge, kg/adton Control (May 22, 7:1 to June 24, 19:1) Trial (June 4, 19:58 to June 11, 3:59) JFOR Journal of Science & Technology for Forest Products and Processes: VOL.1, NO.1,

7 ACKNOWLEDGEMENTS The authors thank all FPInnovations personnel and groups involved in this work. Special thanks are due to Andre Audet, Michel Faubert and Aline Nolin for performing the various laboratory trials. The authors would also like to thank the many mill personnel who participated in the mill trials and implementations of the nearneutral final chlorine dioxide brightening technology (NNB). REFERENCES 1. Earl, P., and Pryke, D., Softwood Bleaching Practices in Canada: Analysis of the 23 Paptac Bleaching Committee Best Practices Survey 91st Annual Meeting, Preprints, Paptac, Montreal, B281 (25). 2. Reeve, D.W., Chlorine Dioxide in Bleaching Stages in Pulp Bleaching Principle and Practice, Section IV, Chapter 8, Dence, C.W. and Reeve, D.W., (eds.), TAPPI Press, Atlanta (1996). 3. Rapson, W.H., and Strumila, G.B., Chlorine Dioxide Bleaching in The Bleaching of Pulp, Chapter 6, Singh, R.P., (ed.), TAPPI Press, Atlanta (1976). 4. Hart, P., and Connell, D., Improving Chlorine Dioxide Bleaching Efficiency by Selecting the Optimal ph Targets TAPPI Fall Conference Proceedings, TAPPI press, Section 5 (26). 5. Rapson. W.H., The Role of ph in Bleaching Pulp Tappi Journal, 39(5): (1956). 6. Sepall, O., Neutral Chlorine Dioxide Pulp Bleaching Process Canadian Patent No , issued in April Jiang, Z.H., and Richard, R.M., Near Neutral Chlorine Dioxide Bleaching of Pulp International Patent Publication: WO/27/14128 (27). 8. Jiang, Z.H., van Lierop, B., Nolin, A., and Berry, R.M., How to Improve Bleached Pulp Yield Measurement Journal of Pulp and Paper Science, 27(1): (21). 9. Jiang, Z.H., Audet, A., van Lierop, B., Berry, R.M., and Menegotto, R., Kappa Number Testing with Better Repeatability and at Lower Cost Preprints, PAPTAC Annual Meeting, C (24). 1. Sullivan, J., and Douek, M., Determination of inorganic chlorine species in kraft mill bleach effluents by ion chromatography J. Chromatography A, 84 (1): (1998). 11. Olli, D., Jouko, N., Tapio, T., and Hannu, K., Bleaching Softwood Kraft Pulp: the Role of Chlorine dioxide Dosage and Final ph in the D Stages Paper Timber, 79(8):56564 (1997). 12. Milanez, A., and Colodette, J.L., Optimal Conditions for Bleaching Eucalyptus Kraft Pulp with Three Stage Sequence 25 International Pulp Bleaching Conference Proceedings, TAPPI press, Atlanta, 1724 (25). 13. Wafa AlDajani, Z.H., Berry, R., and van Lierop, B., The Optimum ph for AldehydeEnhanced Bleaching 28 International Pulp Bleaching Conference Proceedings, PAPTAC, Montreal, 2128 (28). 14. He, A., and Ni, Y., Improving Chlorine Dioxide Bleaching of a Softwood Kraft Pulp by using Magnesium Hydroxide for ph control Preprints PAPTAC Annual Meeting, (29). 15. Wartiovarra, I., Reaction Mechanism of Effective Chlorine Dioxide Bleaching Tappi Journal, 69(2):8285 (1986). 16. Hua, X., Owston, T., and Dorris, G., Carbon Dioxide for ph Control in Neutral Papermaking Wet End and Water Systems Management for Papermakers, Pira Conference, Charleston (26). 17. Seger, G.E., Jameel, H., and Change, H.M., A 2Step HighpH LowpH Method for Improved Efficiency of DStage Bleaching Tappi Journal, 75 (7):174 (1992). 2 JFOR Journal of Science & Technology for Forest Products and Processes: VOL.1, NO.1, 211