CORE Metadata, citation and similar papers at core.ac.uk Provided by Revistes Catalanes amb Accés Obert Highly purified pulps from Miscanthus 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 Miscanthus x giganteus altamente purificadas. Un estudio comparativo de una secuencia de blanqueo TCF aplicadas a nueva pastas 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 pulp. 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, paper, 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 lignin. 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 biomass 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 biorefineries (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).