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Highly purified pulps from x giganteus. A comparative study of a new TCF bleaching sequence applied to organosolv pulps

Juan José Villaverde, Pablo Ligero, Alberto de Vega Department of Physical Chemistry and Chemical Engineering, University of A Coruña, A Coruña 15071, Spain.

Pulpas de altamente purificadas. Un estudio comparativo de una secuencia de blanqueo TCF aplicadas a nueva organosolv

Polpes de Miscanthus x giganteus altament purificades.Estudi comparatiu d’una seqüència de blanqueig TCF aplicada a nova pastes organosolv

Recibido: 3 de noviembre de 2009; revisado: 15 de marzo de 2010

lulose source in applications where high purity is necessary. Resumen The effectiveness of this sequence was analysed in terms of oxidation equivalents (OXE) for each . Se ha utilizado una nueva secuencia alcalina de blanqueo (EPabO(PO)P) sobre tres pastas organosolv (Acetosolv, For- Keywords: Miscanthus x giganteus, Acetosolv, Formosolv, mosolv y Milox). Los resultados demuestran que esta se- Milox, TCF bleaching, carbohydrate protectors. cuencia puede ser una buena alternativa para el tratamiento del Miscanthus x giganteus como fuente de celulosa de alta pureza. La eficacia de esta secuencia en cada una de las Resum pastas ha sido analizada en función de los equivalentes de oxidación (OXE). S’ha utilitzat una nova seqüència alcalina de blanqueig (EPabO (PO) P) sobre tres pastes organosolv (Acetosolv, Palabras clave: Miscanthus x giganteus, Acetosolv, Formo- Formosolv i Milox). Els resultats demostren que aquesta solv, Milox, TCF blanqueo, protectores de carbohidratos. seqüència pot ser una bona alternativa per al tractament del Miscanthus x giganteus com a font de cel.lulosa d’alta puresa. L’eficàcia d’aquesta seqüència en cada una de les Summary pastes ha estat analitzada en funció dels equivalents d’oxi- dació (OXE). A new sequence (EPabO(PO)P) in alkaline medium was used on three acid organosolv pulps (Acetosolv, Formosolv Paraules clau: Miscanthus x giganteus, Acetosolv, Formo- and Milox). The results showed this sequence is a good al- solv, Milox, TCF blanqueig, protectors de carbohidrats. ternative to exploit Miscanthus x giganteus stems as cel-

Afinidad LXVI, 546, Març - Abril 2010 123 Introduction around 20, were prepared in a 3 L batch reactor using experimental procedures described previously (Villaverde Cellulose consists of polydisperse linear glucose polymer et al., 2009b, 2009c). chains which form hydrogen-bonded supramolecular structures. Cellulose is the most abundant bio-renewable Bleaching material and represents a vast potential feedstock. De- All pulps were bleached under identical conditions with rivatised products have many important applications in an EPabO(PO)P sequence. Epsom salt (magnesium the fibre, , membrane, polymer and paints indus- sulfate) and DTPA (diethylenetriaminepentaacetic acid) tries. Today, researchers are developing methods to iso- were used as chelating chemicals (both 1% by weight late cellulose from wood and non-wood fibres. Cellulose with respect to oven dry pulp) to minimise radical degra- isolation requires the removal of other substances, such dation reactions of the carbohydrates (Villaverde et al., as hemicelluloses and . Organic acids have been 2009b). proposed as agents for fractionation. Among them, ace- tic acid pulping has proven to be an effective method The bleaching tests with alkaline treatments (E-stages), for fractionating lignocellulosic materials (Ligero et al., peroxyacetate treatments (Pab-stages) and peroxide 2007a, 2008a). Acetic acid is advantageous because it bleaching (P-stages) were carried out at the desired can be followed immediately by bleaching. The addition consistency in sealed polyethylene bags. The bags were of hydrogen peroxide to acetic acid generates peroxy- immersed in a thermostatic water bath at the desired acetic acid. Formic acid has also received attention as temperature and the samples were kneaded several a solvent for fractionation, and can be used in aqueous times during the reaction (Villaverde et al., 2009c). solution with hydrochloric acid as a catalyst (for short, Formosolv) (Ligero et al., 2007b, 2008a) or in combina- Bleaching with oxygen (O-stages) and oxygen-pres- tion with hydrogen peroxide (Milox) (Ligero et al., 2008b). surised hydrogen peroxide (PO-stages) were performed These organic acid-based fractionation processes are in a 500 cm3 stainless steel Parr high-pressure reac- good options for dissolving pulps to manufacture feed- tor (Model 4560, Parr Instrument Company, Moline, stock for cellulose derivatives and cellulosic fibres. IL) under oxygen pressure at the chosen consistency. A helicoidal rod was used to adequately stir the reac- For years, Miscanthus species were considered to be in- tion mixtures. The reactor pressure was measured with vasive grasses in Europe. However, in the last 20 years, a pressure gauge and the temperatures were controlled European authorities have regarded them as excellent by a Parr controller (Model 4842). After each bleaching sources (Lewandowski, et al., 2000). Among treatment, the pulps were washed with an aqueous so- them, “Miscanthus x giganteus Greef Deuter ex Hodkin- lution at pH 11 to keep the pH alkaline and prevent lignin son and Renvoize” (Hodkinson and Renvoize, 2001), a precipitation. The specific bleaching conditions of each sterile hybrid recognised for its controlled propagation, stage are listed in Table 1. has been the subject of numerous research efforts in dif- ferent areas (Barba et al., 2002; El Mansouri and Salvadó, Table 1. Bleaching conditions in each stage 2006; Lundquist et al., 2004; Michel et al., 2006; Salvadó et al., 2003; Vega et al., 1997; Villaverde et al., 2009a, 2009b, 2009c; Ye et al., 2005; Yuan et al., 2008). Large Bleaching stage economical benefits can be obtained from dissolving pulps obtained from TCF sequences at Lignocellulosic Feedstock (LCF) organosolv (Fernando et al., 2006). M. x giganteus, with a productivity of 30 tons E Pab O (PO) P d.m. ha-1 year-1 in plantations in southern Europe (Lewan- dowski et al., 2000), could be a remarkable raw material for these industries using organic acids as fractionation Consistency (%) reagents and TCF sequences as purification technolo- Time (min) 10 10 10 10 10 gies. T (ºC) 60 30 40 30 60 Sodium hy- 70 55 80 90 60 This work is a comparative study of the responses of droxide (%) 6.0 - 5.0 2.5 4.0 three organosolv pulps (Acetosolv, Formosolv and Milox) Hydrogen - 7.0 - 5.0 6.0 that have kappa numbers suitable to obtain bleached peroxide (%) - 11.0 - - - pulps by a novel TCF sequence, EPabO(PO)P. This se- pH - - 4.0 4.0 - quence was optimised for Acetosolv pulps and the aim of this work was to explore its ability to efficiently bleach Pressure (bar) - - - 1.0 1.0 MgSO (%) - - - - 1.0 other Miscanthus organosolv pulps. 4 DTPA (%)

MATERIAL AND METHODS

Raw material and pulping Pulp analysis Depithed M. x giganteus stems were used to produce For all experiments, kappa number (KN) and ISO bright- pulps for the bleaching experiments. A detailed descrip- ness (BR) were measured according to TAPPI standards tion of the characteristics of this material has been re- (T 236, and T 525, respectively), and viscosity according ported elsewhere (Hodkinson and Renvoize, 2001). Ace- to UNE-039-92. Pulp yields (PY) were determined gravi- tosolv, Formosolv and Milox pulps, with kappa numbers

124 Afinidad LXVI, 546, Març - Abril 2010 metrically after oven drying until a constant weight was Table 2. Properties of unbleached organo- obtained. solv pulps from M. x giganteus.

Oxidation equivalents (OXE) and effectiveness of bleaching Pulping process The utilisation of oxidation equivalents (OXE) allows for comparison among the different oxidative bleaching Acetosolv Formosolv Milox stages. The chemicals can be converted to oxidation equivalents and be expressed as OXE per metric ton of bleached pulp. An OXE is defined as “the quantity of Pulp yield (%) 55.2 48.7 55.0 substance which receives 1 mol of electrons when the Kappa number 17.9 23.0 19.0 substance is reduced” (Grundelius, 1993). Intrinsic viscosity (cm3/g) 1005 1197 915 Brightness (% ISO) 33.6 30.6 40.8 Since the quantities of oxidizing agents used in each stage are in stoichiometric excess, there are always enough bleaching medium and active chemicals to pro- duce the reaction, and a comparison among the four Assessment of the bleaching effectiveness systems can be done in terms of “OXE effectiveness”; Figure 2 shows the effectiveness of the oxidative stages defined as a function of consecutive values of bright- in the EPabO(PO)P sequence for each organosolv pulp. ness and kappa number (de la Rosa, 2003): Among the effectiveness values for KN and BR, in regards to OXE, stands out the O-stage, for which higher have been obtained, and also high values of . Although to a lesser extent, the Pab- and PO-stages were also effective in increasing brightness (the Pab-stage was most effective for Acetosolv/Milox pulps and the PO-stage for Formosolv pulps). The PO- and P-stages showed their effectiveness as brightness developers rather than as delignifying agents.

RESULTS AND DISCUSSION

In this paper, we examined the ability of the TCF se- quence EPabO(PO)P to produce highly purified pulps of dissolving grade from M. x giganteus organosolv pulps. The E- and Pab-stages for Acetosolv pulp were stud- ied previously (Villaverde et al., 2009c). A significant acylation was observed in the unbleached pulps of the three organosolv methods. Therefore, in order to remove saponifiable groups, an initial alkaline treatment was implemented. Later, we have included and optimise a peroxiacetate stage (Pab) which caused simultaneously important delignification and brightness development. A final kappa number of 10 was attained after peroxyac- etate bleaching. To improve the brightness, the intro- duction of one O-, (PO)- and P-stage within the TCF se- quence was studied (Ramos et al., 2008). The sequence with and without carbohydrate protectors in the last two stages of the Acetosolv pulp is presented in order to illustrate the beneficial effects of chelating chemicals when radical mechanisms are present (Villaverde et al., 2009c).

Several pulps were selected, including an acetic acid pulp (Acetosolv) for which the sequence was optimised, a formic acid pulp (Formosolv) and a peroxyformic acid pulp (Milox). Table 2 shows the characteristics of these pulps. Kappa number values were around 20 and the intrinsic viscosities were above 900 mL/g. Their bright- ness (mainly that of Milox pulp) was high compared to typical bleachable kraft pulps (around 26% ISO), and the pulp yields were similar to those obtained in kraft pulping of wood (i.e., eucalyptus and pine). Therefore, Figure 2. Kappa number (a) and brightness (b) ef- they were suitable for bleaching. fectiveness of each stage of the EPabO(PO)P se- quence in terms of oxidation equivalents (OXE).

Afinidad LXVI, 546, Març - Abril 2010 125 Figure 1. Changes in pulp yield (a), kappa number (b), intrinsic viscosity (c) and bright- ness (d) of M. x giganteus stem organosolv pulps during the bleaching sequence.

The three pulps were submitted to the bleaching se- viscosity and pulp brightness in this last system were quence (EPabO(PO)P). Fig 1 summarises the changes seriously damaged in comparison to the other three. during the bleaching sequence. The accumulated pulp The final values were 1.3 for Acetosolv, 0.5 for Formo- yields (compared to the initial dry weight of Miscanthus) solv and 1.1 for Milox. remained relatively high (above 45%) along the whole sequence. The exception was the Formosolv system, VIS (Fig 1c) tended to reduce slowly as the treatments which solubilised more materials during pulping than proceeded (except as mentioned above). Although the the others (Fig 1a). starting viscosity of the Milox pulp was clearly inferior, it reduced in a smooth curve to a very favourable value Delignification, as measured by kappa number, procee- (Acetosolv = 786 mL/g; Formosolv = 745 mL/g; Milox = ded along an almost linear pathway (Fig 1b). During the 669 mL/g). first stages, Formosolv pulp was the most recalcitrant to this process, although later its O- and (PO)-stages Figure 1d shows that the brightness increased to values induced the biggest delignification percentages among near 90% (Formosolv, 89.2%; Acetosolv, 86.9%; Milox, the four systems. In fact, the final kappa number obtai- 90.7%) in a similar way during the three procedures (ex- ned for this pulp was the minimum of all systems. Milox cept for Acetosolv without chelators, 66.4%). The Milox pulp also reached similar final kappa values. It seemed curve was always above the others. to be particularly activated, especially in the E-stage, during which it reduced its kappa number by ten units. The absence of chelating agents affected the Acetosolv pulp in the two last stages. In this system, the final ka- ppa number was around three times that of the systems where agents were used. In addition, Fig 1c and Fig 1d demonstrate the beneficial effects of the agents. The

126 Afinidad LXVI, 546, Març - Abril 2010 Finally, the sequence without carbohydrate protectors 3. de la Rosa, A. 2003: «Utilización papelera de fibras no showed differences during the P-stage. As expected, the madereras (kenaf y Miscanthus sinensis): Estudio de brightening was around five times lower and ineffective. A secuencias de blanqueo ECF y TCF». Ph.D. Thesis dis- surprising result was obtained for , since its value was sertation, Polytechnical University of Cataluña. very similar to the other processes. That means, delignifica- 4. El Mansouri, N.-E.; Salvadó, J. 2006: «Structural char- tion proceeded (although in a lesser extent) with a restrained acterization of technical for the production of brightness increase. This, together with an important viscos- adhesives: Application to lignosulfonate, kraft, soda-an- ity loss which was also observable in the PO-stage, demon- thraquinone, organosolv and process lignins». strated the necessity of carbohydrate protectors in the last Industrial Crops and Products, 24, 8–16. two stages of the EPabO(PO)P sequence (Bajpai, 2005). 5. Fernando, S.; Adhikari, S.; Chandrapal, C.; Murali, N. 2006: «Biorefineries: current status, challenges, and fu- ture direction». Energy and , 20, 1727–1737. CONCLUSIONS 6. Grundelius, N.R. 1993: «Oxidation equivalents, OXE–an alternative to active chlorine». Tappi Journal, 76, 133– Among the three organosolv pulps from M. x giganteus con- 135. sidered in this work, the highest brightness obtained after a 7. Hodkinson T.R.; Renvoize S. 2001: «Nomenclature of EPabO(PO)P sequence was with the Milox pulp (90.7% ISO). Miscanthus x giganteus ()». Kew Bulletin, 56, Meanwhile, the lowest gain was measured with the Acetosolv 759–760. pulp (86.9% ISO). The high BR values and very low kappa 8. Lewandowski, I.; Clifton-Brown, J.C.; Scurlock, J.M.O.; numbers (between 0.5-1.3) measured from these bleached Huisman, W. 2000: «Miscanthus: European experience pulps demonstrate their appropriateness as feedstock for with a novel ». Biomass , 19, 209– cellulose derivatives. 227. 9. Ligero, P.; Villaverde, J.J.; Vega, A.; Bao, M. 2007a: The use of an oxidation equivalent helped in the comparison «Acetosolv delignification of depithed cardoon (Cynara of the bleaching stages in the four systems. The oxidation cardunculus) stalks». Industrial Crops and Products, 25, equivalents allowed for quantification of the kappa numbers 294–300. and brightness variations. The results revealed that the O- 10. Ligero, P.; Villaverde, J.J.; Vega, A.; Bao, M. 2007b: «Or- stage was the most effective delignifying stage, followed by ganosolv delignification of cardoon (Cynara carduncu- the other three stages. All stages contributed significantly lus) stems with formic acid». Afinidad, 64, 48–53. to brightness gain, except in the sequence without cellulose 11. Ligero, P.; Villaverde, J.J.; Vega, A.; Bao, M. 2008a: protectors, where the P-stage effectiveness was drastically «Delignification of Eucalyptus globulus saplings in two reduced due to the lack of chelating agents. The best effec- organosolv systems (formic and acetic acid). Preliminary tiveness in the EPabO(PO)P sequence in regards to OXE was analysis of dissolved lignins». Industrial Crops and Prod- the Formosolv pulp for KN and to the Acetosolv pulp for BR. ucts, 27, 110–117. 12. Ligero, P.; Villaverde, J.J.; Vega, A.; Bao, M. 2008b: This novel sequence could be used to exploit Miscanthus in «Pulping cardoon (Cynara cardunculus) with peroxy- coordination with an acidic organosolv fractionation process formic acid (MILOX) in one single stage». Bioresource within the concept. Furthermore, the basic me- Technology, 99, 5687–5693. dium is never removed during the sequence, which saves 13. Lundquist, L.; Arpin, G.; Leterrier, Y.; Berthold, F.; Lind- washing water that is typically used between the bleaching ström, M.; Månson, J.A.E. 2004: «Alkali-methanol-an- stages; especially in TCF sequences including acidic ozone thraquinone pulping of Miscanthus x giganteus for ther- stages which require exhaustive washing. moplastic composite reinforcement». Journal of Applied Polymer Science, 92, 2132–2143. 14. Michel, R.; Mischler, N.; Azambre, B.; Finqueneisel, G.; ACKNOWLEDGMENTS Machnikowski, J.; Rutkowski, P.; Zimny, T.; Weber, J.V. 2006: «Miscanthus x giganteus straw and pellets as sus- The authors wish to thank “Xunta de Galicia” for their grant tainable fuels and raw material for activated carbon». for the research project (PGIDIT04RFO2659912PR: Obten- Environmental Chemistry Letters, 4, 185–189. ción de biopolímeros de orixe vexetal a partir da biomasa 15. Ramos, E.; Calatrava, S.F.; Jiménez, L. 2008: «Bleaching do Miscanthus sinensis, en condicións de cultivo enerxético, with hydrogen peroxide. A review». Afinidad, 65, 366- por procedementos solvolíticos), Dr. D. Manuel Bao Iglesias 373. from the University of Santiago de Compostela for the kind 16. Salvadó, J.; Velásquez, J.A.; Ferrando, F. 2003: «Binder- supply of M. x giganteus and the University of A Coruña for less fiberboard from steam exploded Miscanthus sinen- its grant of a predoctoral fellowship to Mr. J.J. 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