Activated Oxygen, A Selective Bleaching Agent for Chemical Pulps. Part I: Laboratory Bleaching with Isolated and In-Situ-Generated Activated Oxygen

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1 . Activated Oxygen, A Selective Bleaching Agent for Chemical Pulps. Part I: Laboratory Bleaching with Isolated and In-Situ-Generated Activated Oxygen C.-L. LEE, K. HUNTand R.W. MURRAY A new class of oxidative bleaching agent, termed activated oxygen (designated as A in a bleaching sequence) has been tested for the first time at Paprican and the University of Missouri, St. Louis. Activated oxygen renders the residual lignin soluble in the following caustic extraction stage in a way similar to elemental chlorinekhlorine dioxide. A Kappa number reduction of greater than 80% was accomplished by AE stages for mill-produced unbleached and oxygen-deligniped softwood kraf pulps. All pulp strengths and viscosity remained virtually the same as conventionally bleached pulps. A totally chlorine-freehlly bleached softwood krafi pulp was produced via an OAE,,QP sequence. This is attributed to the effective and selective activated oxygen which delignifies pulp to a low Kappa (<3 units) before the final QP brightening stages. JP ps C.-L. Lee and K. Hunt Paprican 3800 Wesbrook Mall Vancouver, B.C. V6S 2L9 R.W. Murray Dept. Chemistry University of Missouri 8001 Natural Bridge Road St. Louis, MO, USA INTRODUCTION In response to concerns about the discharge of chlorinated organic materials into the environment, technologies to reduce the use of elemental chlorine have been evaluated and retrofitted, where feasible, into existing mills. The approaches undertaken were to lower the Kappa number of pulps before the chlorination stage by extended delignification [ 1,2], oxygen delignification [3], or by using high chlorine dioxide substitution in the chlorination stage [4,5]. As a result, the chlorinated organic compounds in the corresponding effluent decreased to a very low level. As a potential alternative to elemental chlorine, oxygen delignification has a limitation of less than 50% lignin removal before the degradation of the cellulose becomes unacceptable [6-91. Ozone was tested in an OZPZP sequence for a fully bleached softwood TCF (totally chlorine-free) kraft pulp to a brightness level of 89% IS0 [lo]. However, the strength properties of the pulp were found inferior to those of a conventionally bleached pulp which could result from ozone s unselective attack on both lignin and cellulose [ll]. Liebergott et al. reported that strength properties of softwood kraft pulps bleached in the laboratory to 88-90% IS0 brightness by an OZEPY sequence were the same as those obtained by a conventional bleaching sequence, CDEDED [ 121. Overall, however, the reports of pulp strength properties from ozone bleaching of softwood kraft pulp to a fully bleached brightness without chlorine-containing compounds are inconsistent and conflicting. Hydrogen peroxide under alkaline conditions is not an effective delignifying agent for chemical pulp [13]. Peracids for kraft pulp bleaching resulted in severe viscosity loss compared to chlorine dioxide [14]. This was further realized by significant loss of tensile and tear strengths when peroxyformic acid was tested in a prebleaching stage [15]. Therefore, a bleaching sequence using non-chlorine-containing agents to produce a high-brightness pulp with consistently good strength under industrial conditions has yet to be proven. The nature of the bleaching reaction by the various reagents provides insight into their selectivity and reactivity and, therefore, allows one to seek other chemicals which can react in a preferred fashion. Gierer, for example, classified the bleaching agents in three categories: cationic, radical and anionic, according to their reacting species [ 161. The cationic species acts as an electrophile and the anionic as a nucleophile, while the radical species can be either one depending on the ph. Electrophilic reagents react with electron-rich aromatic and olefinic JOURNAL OF PULP AND PAPER SCIENCE: VOL. 20 NO. 5 MAY 1994 J125

2 . structures in lignin matrices and consequently cause lignin degradation while leaving electron-poor cellulose and hemicellulose almost unaffected. Nucleophilic reagents react with carbonyl and conjugated carbonyl structures to remove chromophores, but not to delignify pulp. Ozone, like elemental chlorine, is a strong electrophile [17] and, therefore, is an effective delignifying agent [18]. Unfortunately, ozone promotes aggressive radical reactions which cause unwanted side reactions with the carbohydrate fractions. With the pressing need to find a selective chlorine-compound-free bleaching agent for chemical pulp bleaching, a parameter which can relate to selectivity is highly desirable. We propose that electrophilicity may be an appropriate parameter to predict the selectivity of a bleaching agent for chemical pulp bleaching. Electrophilicity of a reagent is measured as a p value by the Ha"ett equation via the Linear Free Energy Relationship (LFER). Hydrogen peroxide under alkaline conditions has a p value of [19], is classified as a nucleophile, and acts as a brightening agent. On the other hand, hydrogen peroxide under acidic conditions has a p value of [20], is classified as an electrophile, and is a more effective delignifying agent than it is under alkaline conditions [21,22]. Dimethyldioxirane has a p value of or for the epoxidation of cinnamates or substituted styrenes respectively [23,24]. Therefore, the family of dioxiranes should be ideal for delignification of chemical pulps. A detailed analysis of electrophilicity of various bleaching agents using LFER and the relationship between electrophilicity and selectivity will be reported later. The backbone structure of dioxiranes has a cyclic three-member ring moiety containing two oxygen and one carbon atoms, as shown in Fig. 1 where R1 and R:! can be the same, or different, and may be linked to form cyclic compounds. Although the dioxiranes can be prepared in low yield via gas-phase ozonolysis of alkenes [25,26] or oxidation of carbenes [27-291, the most efficient method is generation by the reaction of monoperoxysulphuric acid with acetone [30]. In 1974, Montgomery observed that certain ketones catalyzed the oxidation of organic dyes using monoperoxysulphate [313. Since then, Edwards and Curci have shown that dioxirane was the intermediate in such oxidation reactions using ketone and monoperoxysul- phate. Evolution of this activated oxygen chemistry finally led to the isolation of dimethyldioxirane in an acetone solution in 1985 by R.W. Murray et al. [32]. Dioxirane is a class of powerful electrophilic oxidants, efficient in oxygen transfer and selective in reactivity. They are also catalytic in that the ketone is not consumed, as illustrated in Eqs. (1) and (2). They have been characterized as environmentally friendly oxidants [331. IS0,- + 9 OGO 2\- 2\ R, R2 R, R2 + HS0,- (1) The unique feature of this new class of oxidants is their readiness to transfer a single activated oxygen atom. We have, therefore, chosen to refer to this class of oxidants as activated oxygen in reference to its use in the bleaching of pulps. The reactions of activated oxygen with residual lignin are characterized by the oxidation of alkenes [34-361, arenes [ and the selective oxidation of the C=C double bond in allylic alcohols [38]. These reactions demonstrate a unique electrophilic oxidation of the electron-rich C=C double bonds in aliphatic and aromatic structures which are abundant in residual lignin, but not in cellulosic or hemicellulosic fragments. Activatedoxygen was first used in the pulp bleaching process by researchers at Paprican and the University of Missouri, St. Louis. This oxidant was found to be extremely selective towards lignin and the results are discussed in this report. EXPERIMENTAL Three kraft pulps were used in,the experiments: a laboratory-prepared aspen pulp (Kappa no. 16.4); a mill-produced Canadian western hemlock kraft pulp of 31.5 Kappa number, 33.5 mf%.s viscosity and 24.4% IS0 brightness; and a mill-produced oxygen-delignified central Canadian mixed softwood kraft pulp of 12.6 Kappa, 22.6 mpa-s viscosity and 39.2% IS0 brightness. Each pulp was screened when necessary, dewatered in a centrifuge to about 30% consistency, shredded and then stored in a freezer at -7 C before use. The bleaching conditions and chemical charges for chlorination (C5o+Dso), caustic extraction (E), oxygen delignification (0), oxygen-peroxide reinforced extraction (Eop), chelating treatment (Q), peroxide brightening (P) and chlorine dioxide (D) stages are listed in Table I. Bleaching with Isolated Activated Oxygen The letter A will be used to denote activated oxygen stage and A 0 for the activated oxygen charge in this report. The chemical charge, AO, is calculated based on one available oxygen atom per molecule of dioxirane. The isolated activated oxygen was prepared in a laboratory-scale reactor according to the established procedure [32]. The activated oxygen concentration was determined by the stoichiometric oxidation of thioanisole to methyl phenyl sulphoxide by GC analysis. Dodecane was used as internal standard. Measurement of the A0 concentration can also be achieved via UVNIS spectroscopic analysis (at 335 nm) using a standardized curve of absorption vs. A0 concentration. Bleaching with isolated activated oxygen was carried out in acetone solution 0-0 \/ I I Fig. 1. Chemical structure of dioxiranes JOURNAL OF PULP AND PAPER SCIENCE: VOL. 20 NO. 5 MAY 1994

3 E - t at ro-om temperature and 14.2% pulp consistency for 60 min. Bleaching with Activated Oxygen Generated in Situ Activated oxygen bleaching was carried out by mixing the pulp slurry with a dilute solution of acetone in water for 10 min and then adding an appropriate amount of sodium bicarbonate (NaHCO3) for ph control ( ). The acetone charge was calculated based on a l.5 mol ratio of acetone to A0 charge. Monoperoxysulphate (oxone) was then added to the resulting pulp slurry at 25 C and 13.6% pulp consistency for 45 min with only initial kneading. The reaction of acetone with monoperoxysulphate at nearly neutral ph generated activated oxygen in situ which subsequently bleached the pulp. When in-situ-generated activated oxygen is used, the chemical charge AO, is calculated based on one available oxygen per molecule of monoperoxysulphate (KHSO5) used. Pulp Evaluation Pulp brightness was measured with a Technibrite Micro TB-1C apparatus at 457 nm as set out in TAPPI T 452 om-87 and Technical Section, CPPA Standard E.l. Physical properties of the pulps were determined using Technical Section, CPPAStandard Methods. Zero-span tensile strength was measured by a Pulmac Troubleshooter zero-span tester according to TAPPI T 23 1 cm-85. Bleached pulp yields (shrinkage) were determined for A-treated aspen pulp by washing the pulp thoroughly after bleaching. The fibres were then quantitatively collected and water removed by filtration. The wet pulp pad was weighed and an aliquot dried at 105 C to constant weight. The moisture content was used to calculate the yield. RESULTS AND DISCUSSION Bleaching with Isolated Activated Oxygen Aspen pulp (Kappa no. 16.4) was treated with activated oxygen (0.55% A 0 on 0.d. pulp) at 14.2% pulp consistency for lh. This one-stage treatment decreased the Kappa number to 3.4, a 79% delignification, as shown in Table 11. The next chlorine dioxide bleaching stage (0.8% C102 charge by weight on 0.d. pulp) produced a pulp of 86.8% IS0 brightness. This A-bleached aspen pulp had a far superior zero-span tensile strength (17.0 km) to that of the same pulp delignified by oxygen to a level of 67% (10.6 km). It should also be noted that the pulp bleached by an AD sequence had a comparable zero-span tensile strength to those of the unbleached pulp and the pulp fully bleached via a conventional bleaching sequence, (C+D)ED. In a further trial, aspen pulp was treated with activated oxygen (0.55% A 0 on 0.d. pulp) at 7.6% pulp consistency for lh. The resulting pulp of 5.3 Kappa (68% delignification) had a 2.0% yield loss basedon the unbleached pulp. When the aspen pulp was oxygen delignified to 5.4 Kappa it suffered a 7.3% yield loss. This result confirmed the lignin selective nature of activated oxygen bleaching over oxygen delignification. As a result, activated oxygen is a promising candidate for selective delignification of chemical pulp [39-41]. However, the chemical yield of activated oxygen in isolated form based on the charge of monoperoxysulphate is extremely low ( 4 %) via the published method [32]. The use of isolated activated oxygen via this method would not be practical for an industrial application. A method of in-situ generation of activated oxygen in the pulp slurry was therefore tested. Bleaching with Activated Oxygen generated in Situ Bleaching of Hemlock Kraft Pulp A western hemlock kraft pulp (31.5 Kappa) was tested with activated oxygen generated in situ using charges ranging from 0.5 to 5.0%, as shown in Table 111. For example, activated oxygen at a 2.5% A0 charge decreased the Kappa number from 31.5 to A caustic extraction (1% NaOH) following activated oxygen bleaching, for example at a 2.5% A0 charge, further reduced the Kappa number to 6.7. As a result, activated oxygen renders the residual lignin soluble in the following extraction stage in a way similar to elemental chlorine/chlorine dioxide. Additional Kappa number reductions from 9 to 19% were observed by the subsequent caustic extraction of A-bleached pulps depending on the A0 charge. It was demonstrated by this bleaching test that activated oxygen (3.0% AO) followed by extraction can delignify a kraft brownstock from Kappa number of 31.5 to as low as about 5.7 units (82% delignification) with good viscosity retention. The viscosity and Kappa number data from the AE-bleached pulps, shown in Fig. 2, illustrate a relationship close to that of pulps bleached by a conventional process (C + D)E. Activated oxygen delignified unbleached hemlock kraft pulp to almost as low a residual lignin content as in chlorination and much further than oxygen delignification. The viscosity loss per Kappa drop for activated oxygen delignification was maintained at an acceptable level until the Kappa number reached 5.7 units. Strength Properties of AEop-Bleached Pulps An oxygen-delignified mixed softwood kraft pulp of 12.6 Kappa was tested with A0 charges of 0.9,1.5 and 2.5% on 0.d. pulp followed by an oxygen-peroxide-reinforced extraction (Eop). The respective Kappa numbers after AEop were 4.0,2.5 and 2.0 which corresponded to delignification of 68,80 and 84%, as illustrated in Table IV. An AEop treatment using 2.5% A0 in the A stage delignified the kraft-oxygen pulp from a Kappa number of 12.6 to as low as 2.0 while decreasing the viscosity from 22.6 to 18.4, which was the same as that observed JOURNAL OF PULP AND PAPER SCIENCE: VOL. 20 NO. 5 MAY 1994 J127

4 - Y E 20- Y 2..- E - $ OAEop Kappa Number Fig. 2. Comparison of the viscosity and Kappa number relationships of a hemlock kraft pulp bleached by (c50 + D50)E and AE sequences and an oxygen delignification stage. Fig. 3. Tear indices of OAEop- and O(C + D)Eop-bleached pulps refined in a PFI mill to various freenesses. d o OAEop,p1 o O(C+D)Eo Canadian Standard Freeness (ml) I Kappa number before QP Fig. 4. Tensile breaking lengths of OAEop- and O(C + D)Eopbleached pulps refined in a PFI mill to various freenesses. Fig. 5. The final brightness achieved vs the Kappa number of pulps entering the QP stages. Data points of QP, EopQP and OQP were abstracted from results presented by van Lierop et al. [42]. after an O(C + D)Eop sequence. The OA(0.9%)Eop- and O(C + D)Eop-bleached pulps were refined in a PFI mill for strength comparison. Both pulps required the same refining energies to achieve equal Canadian standard freeness. The tear and tensile strengths of conventional and OAEop pulps were equal at all freenesses, as illustrated in Figs. 3 and 4. Brightening of OAEop-Bleached Pulps Activated oxygen delignification made it possible to produce fully bleached TCF Canadian softwood kraft pulp by a final peroxide brightening stage (using 2.5% peroxide charge on 0.d. pulp) preceded by a chelating treatment (Q), as shown in Table V. The final brightness achievable by the OAEopQP sequence was a function of the Kappa number of each pulp entering the QP stages, as illustrated in Fig. 5. For example, the OAEop-delignified pulp of 4.0 Kappa using 0.9% A0 charge (Table IV) was brightened by QP stages to 86.1% IS0 brightness while the pulp of 2.0 Kappa after OAEop using 2.5% A0 charge achieved 90.9% IS0 brightness by the same QP stages. A similar relationship was observed previously for conventional delignification at a Kappa number greater than 10 entering the QP stages [42]. However, the limited delignification capability of conventional oxygen or Eop delignification stages prior to the peroxide brightening stage restricted the brightness to about 80% ISO. Activated oxygen can delignify pulps to a Kappa number as low as 3 or less with pulp viscosity and strengths equivalent to those of the (C+D)Ebleached pulp. This low Kappa pulp resulting from AE delignification makes fully bleached TCF kraft pulp possible. However, both OA( 1.5)Eop- and O(C + D)Eop-bleached pulps, 19.0 mpa.s and 18.4 mpa.s viscosity, suffered 6.3 and 8.0 mpa.s viscosity losses, respectively, when they were brightened by identical QP stages to 89.7 and 89.6% IS0 brightness. J128 JOURNAL OF PULP AND PAPER SCIENCE: VOL. 20 NO. 5 MAY 1994

5 I Therefore, a final peroxide brightening of low Kappa pulp can impose a severe loss of viscosity and possibly strength loss. Further study in this area is needed. CONCLUSIONS A new class of electrophilic oxidant, activated oxygen, tested for the first time by Paprican, has demonstrated good selectivity towards lignin removal in chemical pulp bleaching. The conclusions drawn from the investigation are as follows: 1. Activated oxygen treatment renders the residual lignin soluble in the following caustic extraction stage in a way similar to elemental chlorinekhlorine dioxide. 2. Activated oxygen and subsequent caustic extraction can selectively and effectively delignify an unbleached softwood kraft pulp (Kappa = 30), an oxygen-delignified softwood kraft pulp (Kappa = 13) and an unbleached hardwood kraft pulp (Kappa = 16) to Kappa numbers attainable by a chlorination stage. 3. Activated oxygen treatment followed by an extraction stage can effectively and selectively delignify softwood kraft pulps to a Kappa number less than 3. This makes it possible to produce TCF softwood kraft pulps of 90% IS0 brightness in a single peroxide brightening stage. However, a severe viscosity loss was observed at the peroxide stage. ACKNOWLEDGEMENTS The authors are grateful to J.D. Drummond, A. Kwong, S.M. Reath, T.P.G. Stephens and B.B.K. Yuen, for their skillful technical support in this work. M. Singh s assistance on the first test of pulp bleaching using isolated activated oxygen at the University of Missouri, St. Louis is gratefully acknowledged and we also thank J. Chen, a colleague, for useful discussions. We are also indebted to B. Fleming, A. Garner, J.V. Hatton, J.H. Rogers, J.T. Wearing and RE. Wrist for their useful discussions and guidance throughout the development of this project. 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