Improving Oxygen Delignification with Peroxymonosulphate

Chapter 8

Improving Oxygen Delignification with Peroxymonosulphate: Fundamentals 1

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Downloaded by OHIO STATE UNIV LIBRARIES on September 17, 2012 | http://pubs.acs.org Publication Date: March 26, 2001 | doi: 10.1021/bk-2001-0785.ch008

Jean Bouchard , Richard Berry , and Vince Magnotta 1

Pulp and Paper Research Institute of Canada, 570 St. John's Boulevard, Pointe Claire, Quebec H9R 3J9, Canada Air Products and Chemicals Inc., 7201 Hamilton Boulevard, Allentown, P A 18195-1501

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Peroxymonosulphate (PMS) has been evaluated as a delignifying agent under neutral or acidic conditions. Our findings show that under alkaline conditions, PMS, although unstable, is a very selective and efficient delignifying agent. PMS can be added directly to an oxygen delignification stage. Using a 1% active oxygen charge, a softwood pulp delignification greater than 70% is obtained with good selectivity. This provides higher delignification in the Ο stage, resulting in increased system closure and reduced chlorine dioxide requirement.

There has been a steady increase in the use of oxygen delignification world wide since the first commercial installation in South Africa in 1970 (1). The pace of acceptance has increased with the need to decrease the discharge of organic compounds, and particularly chlorinated organic compounds, into the environment (2) . These discharges are decreased because of the decrease in lignin content of the pulp entering the bleach plant after oxygen delignification. The degree of delignification achieved in the oxygen stage is ultimately limited by pulp strength considerations but can also be limited by system configurations (3)> Recent surveys (4-5) have shown that the degree of delignification in mills can vary from 20-60% with an average around 40%. Most mills using oxygen delignification would like to be able to obtain further selective delignification in this stage and continue the process of progressive system closure. One approach is to add recovery-compatible oxygen-containing chemicals to the stage. © 2001 American Chemical Society

In Oxidative Delignification Chemistry; Argyropoulos, D.; ACS Symposium Series; American Chemical Society: Washington, DC, 2001.

149


150 Peroxymonosulphuric Acid Bleaching Under Acidic and Neutral Conditions Peroxymonosulphuric acid (PMS) has been used to increase the delignification prior to the bleaching stages. Since thefirstpatentfromCael (6) on the use of PMS in a single stage, many reports have been published on PMS delignification. PMS treatment has been proposed as a single stage (7-10), as a pretreatment including a wash prior to oxygen delignification or ozone (11-13), as an acidic post-treatment after oxygen delignification (with a wash in between) (14), as an interstage (with wash) in the middle of two-stage oxygen delignification (15-17), in combination with ozone (18), or as afinalbrightening stage (19). Table I shows the various experimental conditions used for the different types of pulp treatments with PMS (the unofficial symbol Px will be used to identify a PMS stage). In almost all these studies, the Px stage is shown to be quite selective toward lignin. However, a patent by Lam et al (9) indicates that the selectivity is about three times greater under neutral conditions than slightly alkaline conditions. In all cases except one (18), PMS has been used in a separate stage with a full wash and in no case has it been used as a single stage combined with oxygen delignification. In most cases the Px stage has been run under acidic conditions which requires long reaction times even at elevated temperatures. Peroxymonosulphate Decomposition Peroxymonosulphuric acid (HOSO2OOH) has two ionization constants. The hydroxy proton is strongly acidic (pKai < 0) while the perhydroxyl proton is weakly acidic (pKa =9.4) (20). Initially studied by Ball and Edwards (21), the kinetics of spontaneous PMS decomposition (Equation 1) has been revisited recently (22-23). 2

(1)

2

2 HSOs —>2 H* + 2 S07 + Oz

In the pH range from 5 to 9, the spontaneous decomposition of PMS follows second order kinetics and the dependence on the concentration of hydrogen ion is of -0.7 order as represented by Equation 2. The maximum rate of decomposition occurs at a pH corresponding to the pKa (9.4) where the halflife of PMS is less than two minutes (23). This rapid decomposition is the main reason why researchers have been reluctant to study PMS delignification under alkaline conditions. 2

d[HSOS 1 , IHSQ}]

2

=

(2)


151 Table I. Conditions for PMS delignification stages proposed in the literature

pH

Time

Temperature (°C)

PMS (ΨοΑΟ)

3.0-11.0

30-180 min

40-80

0.05-0.31

12) where PMS (S0 ") is the predominant species, the decomposition rate decreases markedly and the sensitivity to copper disappears (24). 2

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152 PMS Delignification Under Alkaline Conditions Thefirstevidence of the high reactivity of PMS with pulp under alkaline conditions is found in a paper showing that the rate of PMS reaction with lignin increases with pH (25). This effect has also been observed for the epoxidation of olefins and attributed to the superior nucieophilicity of the dianion (SO5* ) (26). The reason this positive pH effect has not been shown before is probably related to the mode of addition of PMS on pulp. Since PMS stability is strongly affected by pH preparing an alkaline PMS solution is not easily accomplished because it can decompose before it is used (the effective charge will therefore be much lower than expected). The present work is based on the development of a procedure to mix PMS, caustic and pulp together almost instantaneously which allows a reconsideration of the bleaching efficiency of PMS at high pH and the possibility of integrating such a treatment within an oxygen delignification process. 2


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Experimental Oxygen Delignification of Unbleached Pulp The pulp sample (250g) was mixed at 10% consistency with MgS0 (0.5% on o.d. pulp) and NaOH (2.5% on o.d. pulp except when specified) in a Hobart mixer. The pulp was then rapidly heated in a microwave and put in a high shear mixer (Quantum, Mark IV). When the temperature reached 90°C, the reactor was pressurized to 690 kPa with oxygen. The pulp was mixed for 4 seconds at 2400 rpm, then mixed for 4 seconds every 30 seconds at 400 rpm for the specified reaction time. The pulp was either immediately treated or washed and stored in a cold room. 4

PMS Delignification of Unbleached Pulp The source of PMS for the experiments was Oxone™ (Dupont) which is an acid salt mixture containing potassium peroxymonosulphate, and mono- and dibasic potassium sulphates. Because Oxone is strongly acidic in solution, NaOH had to be added to the pulp to neutralise the acidity of the PMS mixture and to give the appropriate alkaline pH after Oxone™ addition. The experiments on PMS delignification of unbleached pulp were done in plastic bags. Fifty grams of pulp was mixed at 10% consistency with a 6% caustic charge for 5 minutes and then left to sit for five minutes. At that time, 11.1 g of Oxone™ corresponding to 1% active oxygen (AO) on pulp was added (one active oxygen


153 1

per HSOs" ). The suspension was hand-mixed for 10 minutes, and sporadically mixed thereafter for a total of 30 minutes. The pulp was then washed and the pH recorded. To compare the efficiency of in-situ DMD bleaching (27) and PMS bleaching, a control was treated with a mixture of PMS and acetone at the same AO charge by the method described previously (25).


Oxygen and PMS in a Combined Stage Whether the PMS addition was done at the beginning, during or at the end of the oxygen stage - (PxO), (OPxO), or (OPx) - the same addition procedure was followed. At the appropriate time, the Quantum mixer/reactor was opened and the caustic needed for pH adjustment was added to the pulp on one side of the reactor. The PMS charge was added on the other side as a powder and the reactor was resealed. The reactor was repressurized and heated if required. The pulp was subjected to 4 seconds of high-shear mixing and let to react. Acid Wash (A) and Analytical Procedures The acid stage was performed at 2% consistency for one hour at 50°C and followed by thorough washes with deionized water. The pH was adjusted to 1.5 using H2SO4. Kappa number, pulp viscosity and dry zero-span were measured according to PAPTAC Technical Standards G.18, G.24P, and D.27U respectively.

Results and Discussion PMS Delignification Under Mild Alkaline Conditions Our previous results (25) are summarized in Figure 1. The first two bars represent the delignification and viscosity of a black spruce kraft pulp which has been bleached using in-situ dimethyldioxirane (DMD) at pH 7.2. In-situ DMD bleaching means that acetone and PMS are added to the pulp buffered to pH 7.2 with sodium bicarbonate. DMD is generated during the stage. The three other sets of bars are from experiments where acetone has not been added (PMS bleaching) but the same active oxygen charge on pulp (1.5% A.O.) has been used. Figure 1 indicates that increasing the pH of PMS bleaching from 7.3 to 8.2 increased the delignification from 30% to 38% and the viscosity of the pulp by three units. These results led to the evaluation of using PMS to bleach pulp under alkaline conditions.


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Β Delignification (%) • Viscosity (mPa.s)

DMD-7.2

PMS-7.3

PMS-7.7

PMS-8.2

Figure 1. Delignification (%) and viscosity (mPa.s) obtained after bleaching of a softwood kraft pulp (kappa number, 28.9; viscosity, 38.5 mPa.s) using either in-situ DMD at pH 7.2 or peroxymonosulphate at pH 7.3, 7.7, or 8.2.

Effect of pH A black spruce unbleached kraft pulp (kappa number, 27.6; viscosity, 39.6 mPa.s) was treated with PMS (1% A.O.) at room temperature. The pH of the pulp slurry was adjusted using different combinations of sodium bicarbonate, sodium carbonate and sodium hydroxide. In one case, acetone was added to generate DMD. Several observations can be made from the results presented in Table Π: • •

•

There is a significant increase of delignification when the pH is increased from 8.4 to 11 and delignification levels-off at pHs higher than 11. PMS bleaching at pH around 11 provides better delignification and similar viscosity than that obtained with a PMS/acetone mixture which generates DMD at pH 8. There is no significant viscosity loss during PMS bleaching up to a pH of 11. Higher pHs cause extensive viscosity loss. A selectivity decrease has also been reported for hydrogen peroxide bleaching at very high alkalinity (28) where pH is above the pKa of 11.6. Alternatively, the rapid mixing of a high charge of a strong acid (Oxone™) and a base (NaOH) with pulp may also have contributed to cellulose hydrolysis.


155 Table II. Effect of pH on PMS delignification of a softwood kraft pulp.


pH

Kappa number Delignification (%) Viscosity (mPcus)

Pulp

27.6

-

39.6

8.0*

18.9

31.5

33.7

8.4

21.3

22.8

33.9

9.6

19.1

30.8

33.3

11.0

16.8

39.1

31.0

12.4

16.0

42.0

21.0

12.7

17.3

37.3

20.1

*In that case, acetone was added to generate DMD. From these results, it appears that efficient PMS delignification can be obtained using alkaline conditions similar to those encountered in an oxygen stage. Consequently, we looked at the possibility of combining oxygen delignification and PMS bleaching in a single stage, the goal being to boost the delignification achieved in an oxygen stage without requiring capital expenditure on a washer or a tower. The experiments were done in a high-shear mixer and wefirstassessed whether the mechanical treatment caused damage to the fibres.

Effect of High-Shear Mixing on Oxygen Delignified Pulp Strength A softwood kraft pulp was exposed to different amounts of mixing at 90°C for 60 minutes with and without the oxygen delignification chemicals added. As described in Table ΙΠ, the pulp was mixed with high shear (1200 rpm) for 4 seconds and then mixed at 400 rpm for 4 seconds every 30 seconds for the entire reaction time. At the end of the experiment, the pulp was washed and kappa number, viscosity, and zero-span tensile strength were determined. A decrease of about 10% in the zero-span measurement was observed after the pulp was mixed with high shear whether the chemicals were added or not. It has been shown by Bennington and Seth (29) that pulp beating during highshear mixing causes this effect.


156 Table III. Properties of a softwood kraft pulp mechanically treated in the absence or presence of oxygen delignification chemicals


Kappa Number

Viscosity Zero-span (km) (mPa.s)

Initial Pulp

24.7

31.3

16.8

No Chemicals

23.5

31.2

14.6

O2 Delignification Chemicals

11.8

21.0

14.5

Table ΠΙ shows that viscosity remained unaffected by mixing in the absence of chemicals while oxygen delignification decreased the pulp viscosity significantly without affecting the zero-span tensile strength. These results are in agreement with those of Bennington and Seth (29) who showed that there is no synergistic effect between mechanical and chemical treatment. Different Modes of PMS Addition: (PxO); (OPxO); (OPx) Three modes of PMS addition (1%A0 change) were evaluated in the combined O2/PMS stage where no washing was used between steps: These sequences are represented in Figure 2. (PxO)

(OPxO)

(OPx)

PMS was added at the beginning of the oxygen delignification stage. Total reaction time was 55 minutes but the reactor was opened after 25 minutes for sampling. After sampling, the reactor was repressurized and the pulp mixed with high shear again for 4 seconds. PMS was added to the oxygen stage after 25 minutes. This mode simulates a two-stage oxygen delignification system where PMS would be added just before the MC mixer of the second stage. PMS was added at the end of the oxygen delignification stage. This mode simulates the process where PMS would be added in the blow line of the oxygen stage and allowed to react without further heating and repressurisation. The reaction was completed within 5 minutes.

In all cases, the pulp is treated for a total time of 55 minutes and submitted to two high-shear mechanical treatments. Only the points of addition of the chemicals (PMS, NaOH, and O2) are different as indicated in Figure 2. The results are presented in Table IV where data from an oxygen delignification



157 stage (Ο) and a PMS stage without oxygen (Px) have been included. Also included is an example where the pulp has been acid washed to remove transition metals prior to the stage: A(OPxO). Oxygen or PMS alone provided about 50% delignification with similar viscosity losses. Combining both chemicals in a single stage increased the delignification as compared to the oxygen stage alone. However, the results vary according to the mode of PMS addition selected. Adding PMS in front of die Ο stage (PxO) increased the delignification only from 49% to 63%. It seems that the oxidation of the lignin by PMS renders the residual lignin less susceptible to the slower attack by oxygen. Adding PMS in between (OPxO) or at the end of the stage (OPx) was more efficient and provided in both cases 73% delignification. However, in the case of (OPxO), the cellulose was degraded and the pulp viscosity was low (13.3 mPa.s). Using an acid stage before (OPxO) increased the viscosity by reducing the transition metal ion content of the pulp but decreased the degree of delignification.

(PxO)

PMS: 1% NaOH: 7.5% H-S Mix.

H-S Mix.

(OPxO)

NaOH: 2.5% H-S Mix.

PMS: 1% NaOH: 5% H-S Mix.

(OPx)

PMS: 1% NaOH: 5% H-S Mix. No heating

NaOH: 2.5% H-S Mix.

25

55

Time (min) Figure 2. Sequence of chemical addition and high-shear mixing (H-S Mix) the three modes of PMS addition in an oxygen stage.


158 Table IV. Properties of a softwood kraft pulp treated in combined 0 / PMS stages. 2


Sequence

Kappa number

Delignification Viscosity (mPa.s)

Final pH

Initial Pulp

24.7

-

31.3

-

0

12.7

48.6

20.8

12.2

Px

12.3

50.2

16.3

11.7

(PxO)

9.0

63.6

14.6

10.6

(OPxO)

6.6

73.3

13.3

9.8

A(OPxO)

7.5

69.6

15.2

9.3

(OPx)

6.6

73.3

17.3

10.0

NOTE: All the experimental conditions were identical except the addition point for PMS. Lower delignification appears to be more likely the result of a decrease in the pH of the process than an effect of metal removal. In the case of (OPx), the high delignification level was obtained without inducing much carbohydrate damage. This mode gave a higher viscosity than with PMS alone (Px) probably because the pH of the PMS was lowered in the (OPx) stage. Based on these results, the (OPx) combined stage appears to be the most efficient and selective process and future effort will concentrate on this mode of addition. Effect of pH on (OPx) Treatment of a Softwood Kraft Pulp Data from Table 2 show that the optimal pH for a PMS delignification stage is around 11. It was necessary to confirm that this remained true in an (OPx) combined stage. An unbleached softwood kraft pulp (kappa number, 28.2; viscosity, 41.8 mPa.s) was oxygen delignified (2.5% NaOH, 0.5% MgS0 ) in the Quantum mixer for 60 minutes at 90°C. PMS (1% A.O.) was added after oxygen delignification as described previously and the NaOH charge was varied to give different pHs. The results are presented in Table V and in Figure 3. As shown in the first row of Table V, the intermediate results at the end of the O-stages were very reproducible over 7 runs. The following rows indicate results for the combined (OPx) stages. The highest selectivity in the (OPx) process was obtained at pH 10.1; the delignification of this sample was almost 70%. The optimum pH for PMS reaction in the (OPx) stage appears to be a little bit lower that for a simple PMS stage. Figure 3 shows that there was a large increase in delignification when the pH was increased from 8.3 to 10.1 which corresponds to the change from HSO5" to SO5" (pKa 9.4). These results confirm 4

1

2


159 the greater reactivity of the dianion toward lignin. This behaviour parallels results obtained with hydrogen peroxide which is also more reactive in the anionic form. Higher pHs do not provide any benefit in terms of delignification and damage the carbohydrates.


Table V. Effect of the pH of the PMS treatment on the overall efficiency of the (OPx) stage for a softwood kraft pulp. Stage

Final pH

Kappa Number

Viscosity (mPa.s)

o (OPx)

12.1 ±0.2

15.2 ±0.3

22.6 ±0.7

•

2.1

13.6

15.0

1.10

3.5

11.9

16.9

1.42

8.3

11.0

18.9

1.72

10.1

8.6

16.0

1.86

11.9

8.2

13.0

1.58

12.5

8.2

11.5

1.40

2

Selectivity (V/K)

1.30 12.9 7.9 10.3 NOTE: Selectivity is expressed as the ratio of viscosity over kappa number (V/K).

pH

Figure 3. Delignification (m) and viscosity (o) as a Junction of thefinalpH of the OPx stage. Intermediate delignification and viscosity after oxygen are indication (by arrows) on both axes


160 We made a comparison between the efficiency of the (OPx) process and of an (OP) process where hydrogen peroxide is used instead of PMS under the same conditions. An unbleached black spruce kraft pulp (kappa number, 28.2; viscosity, 41.8 mPa.s) was oxygen delignified (2.5% NaOH, 95°C, 55 min) and a 1% active oxygen charge of either PMS or H 0 was added to the pulp following the procedure described previously. Pulp samples were taken after 30 and 90 minutes of the PMS or the H 0 treatment. The results are shown in Table VI. 2


2

2

2

Table VI. Comparison of (OPx) and (OP) (1% A.O. charge) delignification of a softwood kraft pulp. Px

o

2

Time (min) Kappa Number Delignification (%) Viscosity (mPa.s) Peroxide Residual pH Temperature (°C)

18.4 35 28.2 11.6 90

30 11.0 61 20.6 tr. 9.7 66

90 10.8 62 20.1 tr. 9.8 48

0

Ρ

2

18.1 36 27.6 11.8 90

30 16.8 40 26.7 tr. 11.5 64

90 16.5 41 26.4 tr. 11.6 47

The (OPx) process increased the delignification from 35 to 62% while hydrogen peroxide had very little effect on kappa number. It appears that the PMS reaction with pulp is complete after 30 minutes. Despite little delignification, hydrogen peroxide was consumed rapidly. Viscosity was maintained which suggests that peroxide was not lost by decomposition into harmful radicals. Rather than decomposition, it is more likely that the H 0 is consumed by organic material in thefiltrate.PMS and hydrogen peroxide residual concentrations were negligible after 30 minutes. 2

2

Potential and Prerequisite for Industrial Application Adding large quantities of Oxone™ (PMS) and caustic in the Ο stage is not economic and such a process has little chance of being applied industrially. However, research conducted at Paprican (24,30) has shown that it is possible to produce PMS in satisfactory concentration by catalytic oxidation of sodium sulphite with molecular oxygen under alkaline conditions. Practical yields appear to be possible. Using such a generator will minimize the cost of PMS and will allow the in jection of an alkaline PMS stream directly into the blow-line of an oxygen reactor. The effect of large amount of sodium sulphate in the oxygen effluent is also presently been examined and will be the basis of a future report.


161

Conclusions •

•


•

•

With the appropriate mode of addition and mixing, peroxymonosulphate (PMS) can be used to selectively delignify softwood kraft pulp under alkaline conditions. PMS is at its most reactive under such conditions. PMS delignification can be combined with oxygen delignification in a single stage without any washing between steps. The most selective process (OPx) was obtained when PMS was added at the end of the Ο stage. This process would be achieved industrially by adding PMS to the blow-line. Delignification higher than 70% has been obtained with softwood kraft pulp in a combined (OPx) stage with a residence time of 55 minutes.

Acknowledgements The authors acknowledge Dr. Argyropoulos and Dr. Chen for useful suggestions and discussion. Special thanks to C. Maine and A. Audet for their technical skills. Thefinancialcontributions of Industry Canada and Air Products and Chemicals Inc. are also acknowledged.

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Rowlandson, G. Tappi J. 1971, 54 (6), 962-967. Johnson, A.P. Pulp Pap Can 1993, 67 (3), 103-112. Bennington, C.P.J.; Pineault, I. . PAPTAC Annual Meeting Proceedings, PAPTAC, Montreal, Canada., 1999; pp B223-B232. Reid, D.W.; Ayton, J.; Mullen, T. CPPA Annual Meeting Proceedings, PAPTAC, Montreal, Canada, 1998; pp A25-A30. Carter, D.; Johnson, T.; McKenzie, D.; Idner, K. Tappi Pulping Conf. Proceedings, Tappi Press, Atlanta, GA, 1996; pp 469-479. Gael, J.J. US Patent 4,404,061, 1983. Springer, E.L. TappiJ.1990, 73 (1), 175-78. Seccombe, R.; Martens, H.; Haakana, Α. Ο Papel, August, 1996, 37-42 (1996). Lam, J.N.; Yasnovsky, V.M.; Bhattacharjee, S.S. PCT Patent Application, WO 97/19222, 1997. Arnold, G.W.; Klykken, P.G.; Kronis, J.D.; Walters, C.R. Tappi Pulping Conf. Proceedings, Tappi Press, Atlanta, GA, 1995; pp 897-902. Springer, E.L.; McSweeny, J.D. Tappi J. 1993, 76 (8), 194-199. Meier, J.; Arnold, G. US Patent 5,091,054, 1992. Meier, J.; Arnold, G.; Helmling, O. US Patent 5,246,543, 1993.


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Troughton, Ν.; Hoyos, M.; Robberechts, M.; Vrambout, G. US Patent 5,698,075, 1997. Allison, R.; Lachenal, D.; McGrouther, K.; De Choudens, C.; Angelier, R. Cell. Chem. Technol., 1995, 29, 451-462. McGrouther, K.; Allison, R. 49 ΑΡΡITΑ Annual Conf. Proceedings, 1995; pp 41-47. Allison, R.; McGrouther, K.; Ellis, M.J. J. Pulp Pap. Sci., 1997, 23 (9), 433-438. Francis, R.C.; Zhang, X.-Z.; Devenyns, J.; Troughton, N.A.; Tappi J., 1997, 80 (7), 171-176. Sundara, R.P.; Kronis, J.D. Tappi Pulping Conf. Proceedings, Tappi Press, Atlanta, GA, 1998; pp 863-880. Curci, R. Edwards, J.O. in Organic Peroxides; Swern D., Ed.; Wiley -Interscience: New-York, 1970; Vol. 1, pp 199. Ball, D.L.; Edwards, J.O. J. Am. Chem. Soc., 1956, 78, 1125-1129. Francis, R.C.; Zhang, X.-Z.; Froass, P.M.; Tamer, Ο. Tappi J., 1994, 77 (6), 133-140. Bouchard, J.; Maine, C.; Berry, R.M.; Argyropoulos, D.S. Can. J. Chem., 1996, 74, 232-237. Chen, J.; Robertson, R.; Hogikyan, R.; Wearing, J. PPR 1306, (1997), unpublished. Bouchard, J.; Maine, C.; Argyropoulos, D.S.; Berry, R.M. Holzforschung 1998, 52, 499-505. Wang, Z.-X.; Tu, Y.; Frohn, M.; Shi, Y. J. Org. Chem., 1997, 62, 23282329. Lee C.-L.; Murray R.W.; Hunt K. PCT patent application, Intern. Publication Number WO92/13993, 1992. Troughton, N. Sarot, P. Tappi Pulping Conf. Proceedings, Tappi Press, Atlanta, GA, 1992; pp 519. Bennington, C.P.J. Seth, R.S. Intern. Pulp Bleaching Conf. Proceedings, Helsinki, Finland, 1998; pp 167-173. Shaharuzzaman, M. Bennington, C.P.J. International Pulp Bleaching Conf. Proceedings, PAPTAC, Montreal, Canada, 2000; pp 267-273. th


Improving Oxygen Delignification with Peroxymonosulphate

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