Location Study of Cold Caustic Extraction (CCE) Stage in Bleaching Sequence of eucalyptus Dissolving Pulp

Transcription

1 Location Study of Cold Caustic Extraction (CCE) Stage in Bleaching Sequence of eucalyptus Dissolving Pulp Janaina de O. Resende: UFV, Viçosa, Brazil, Jorge L. Colodette: Full Professor, UFV, Brazil, Abstract The conversion of conventional kraft pulp to the dissolving pulp requires the hemicelluloses removal and activation of remaining cellulose. The extraction of the hemicelluloses was performed by alkali treatment using cold caustic extraction (CCE) and the activation of cellulose by peracetic acid (Paa) oxidation. The economic viability of the CCE process requires the recovery of alkali used and hemicellulose extracted in the process. In this study, it was evaluated the better position of the CCE stage in the pulp bleaching process, so as to facilitate recovery of the CCE filtrate. The raw material used was an industrial eucalyptus kraft pulp previously delignified with oxygen. The location of the CCE stage as the first stage of the bleaching sequence shown to be the most suitable for the production of viscose-grade pulp with the lower consumption of the bleaching chemicals and the best removal of the hemicelluloses from the pulp. The location of the CCE stage as the last stage of bleaching stage contributed to preserve the pulp viscosity, which would be interesting for acetate-grade pulp, which was less effective to remove the hemicelluloses. Keywords: Eucalyptus, Cold Caustic Extraction (CCE), Hemicelluloses, Dissolving pulp. Introduction Dissolving pulp are special pulps produced from cotton or wood. From wood, they are as usual produced by process of acid sulfite cooking or pre-hydrolisis Kraft. As the main characteristics are included the high α-cellulose content, uniformity of cellulose polymerization s degree as well as the low degraded cellulose content, hemicelluloses, lignin, extractives and inorganic acids [1]. The purpose of the cellulose derivatization consist of turning the soluble cellulosic polymer into solvents such as alkali, acetone, chloroform, and others. These derivatives, which are denominated as soluble pulp, allow the formation of fibers, films, plastics and even water soluble derivatives after being solubilized. However, most of it is allocated for the formation of artificial cellulosic fibers as viscose and cellulose acetate [2]. In spite of being well-established, the technique of dissolving pulp production from wood chips requires a high capital investment. It would be a highly desirable aim to produce dissolving pulp by converting Kraft pulp through a lower capital investment and operating costs. There is a difference between Kraft pulp and dissolving pulp regarding the content of hemicellulose. The first one has a high content of hemicellulose whereas second one has a low content of it. Usually the dissolving pulp quality parameters are presented in tracks, since the levels of these parameters depend on the final use. For example, the alpha cellulose content in the range of 90-92% and serves the production of rayon and cellophane while for the production of cellulose acetate and nitrate, alpha-cellulose levels in the range of 95-98% are desirable [3]. In the case of dissolving pulps intended for xanthation, special attention should be given the reactivity of the pulp, measured by filtration of xanthate, which is related to the accessibility of carbon disulfide (CS2) to cellulose, how this reagent can interact with the hydroxyl groups of cellulose chains [4;5;6]. The most well-known technique regarding the remotion of hemicelluloses from Kraft pulp is by using its treatment with cold sodium hydroxide (CCE), in which is utilized a high-concentration of alkali. The xylans extracted from CCE liquor must be meanwhile recuperated and can be utilized in the same industry as an additive for paper-making process, or sold for multi-purposes [7]. On the other hand, the alkaline solution must be recuperated to be utilized in the same CCE treatment and its remains must be used in different industry operations such as Kraft cooking and oxygen delignification. The CCE stage can be applied to unbleached, semi-bleached or bleached pulp [7]. For instance, in a Kraft pulp industry operating a bleaching process by the sequence D(EP)D (D = delignification with chlorine dioxide; (EP) = alkaline extraction with hydrogen peroxide and D = bleaching with chlorine dioxide) the CCE stage can be implemented by the following order: (1) before the first stage with chlorine dioxide CCED(EP)D, (2) before the stage (EP) DCCE(EP)D, or (3) after the second stage of DIOXIDACAO -

2 D(EP)CCE. The convenience of realizing the CCE stage in any order depends on its efficiency, the final destination given to the resulting dissolving pulp and the extracted hemicelluloses as well. It was assessed in this study the best location of CCE stage to the bleaching sequence D(EP)Paa aiming to produce dissolving pulp. Materials and Methods Materials It was utilized one industrial eucalyptus Kraft pulp, pre-delignified by using oxygen with the following characteristics shown in the Table 01 below: Table 01. Characteristics of pre-o2 industrial pulp. Pulp characteristics Methods Studies of location of CCE stage Results Kappa 11,7 Brightness, % ISO 52,4 Viscosity, dm 3 /kg 919 S10, % 17,4 S18,% 13,5 -cellulose,% 81,8 Pentosans,% 16,4 Ash (mg/kg) 2206 The CCE stage was assessed by considering three different positions in the sequence D(EP)Paa: (1) CCED(EP)Paa, (2) DCCE(EP)Paa e (3) D(EP)PaaCCE. Provided in the Table 02 below, are the conditions used in each stage of base sequence as well as in the CCE stage. Table 02. Bleaching conditions used in the various stages of the sequences D-(EP)-Paa and also in the CCE stage. Conditions Bleaching Stages D (EP) Paa CCE Consistency, % reaction time, min Reaction temperature, C ph inicial 3,0 11,0 5,0 14 H2SO4, kg/t Variable - - Variable NaOH, kg/t ,5 - LB, kg AE/t ClO2, kg/t H2O2, kg/t - 6,5 - - CH3COOH, kg/t CCE stage The CCE stage was executed in polyethylene bags, also white liquor was used with sulfidity level 23% as a source of alkali. Samples of 250 g were subjected to conditions given in the Table 02 and white liquor containing a desired alkali charge added to the pulp under a room temperature. After manually mixing, it was remained stable at the temperature of 25 degrees Celsius and also the mixture was maintained for 15 minutes in the consistence of 15 %. Having finished the reaction, the mixture was taken to centrifugation in four distinct steps. In each step of washing, the pulp gained the equivalent of 10 m 3 of water for tonnes of pulp, adding up to 40 m 3 of water for tonnes of pulp. Concluded the third step, the pulp was diluted and

3 homogenised with 10 m 3 h20/tas and its ph corrected to approximately 5,5 by using sulphuric acid (35-40 kg/tas). Furthermore, after finishing this procedure, those four centrifugations were deliberately mixed and stored into a fridge in order to be used in later studies of qualification and separation of hemicellulose of white liquor. D Stage The bleaching by using chlorine dioxide was realised into polyethylene bags with samples of 200 g of a.s. pulp. Later on, the pulp gained a bleach liquor containing ClO2, water and H2SO4 to control the ph, which had been studied in advance with samples of 10 g of pulp. After carefully mixing by hand, the material was heated up by a microwave and transferred to temperature-controlled steam bath. The time and temperature had been previously established. After the end of the reaction, samples of residual liquor were extracted in order to analyse ph and residual CIO2 levels. EP Stage The alkaline extraction by using H202 was realised into polyethylene bags with samples of 150 g of a.s. pulp. The bleach liquor containing H2O2, water and NaOH to ph control, was added to the pulp after previous researches with mini samples. After carefully mixing by hand, the material was heated up by a microwave and transferred to a temperature-controlled steam bath. The time and temperature had been previously established. After the end of the reaction, samples of residual liquor were extracted in order to analyse the ph and residual H2O2 levels. Paa Stage The bleaching by using practice acid (Paa) was realised into polyethylene bags with samples of 100 g of a.s. pulp. The bleach liquor containing practice acid, water and NaOH was added to the pulp under a room temperature. After a manual mix, the material was heated up by a microwave and transferred to a temperature-controlled steam bath. The time and temperature had been previously established. Finished the reaction, samples of residual liquor were extracted in order to analyse the ph levels. Analytics Procedures The standards and procedures followed in this study were: Kappa Number (TAPPI T236 om 99), viscosity (TAPPI T230 om 99), whiteness (TAPPI T525 om 92), solubility in alkali 10 % (S10) and 18 % (S18) (TAPPI T235 os 76), and content of a-cellulose (TAPPI 203 cm 99), level of crystallinity (diffraction of X-ray). Results and Discussion Studies of location of CCE stage The CCE stage was assessed in three different positions in the sequence D(EP)Paa. The positions are (1) CCED(EP)Paa, (2) DCCE(EP)Paa e (3) D(EP)PaaCCE. The results achieved can be seen summarised in the Figures Brightness The positioning of the CCE stage is more effective as the first step of bleaching, which resulted in higher whiteness level (92,2 %ISO). On the other hands, the positioning as a second and last step resulted in 90,7% and 90%ISO to charges of similar reagents. We had already expected these results, due to the fact that the CCE treatment and the xylan (xylan-lignin complex) applied at the beginning of the process causes significant remotion of lignin from the pulp and consequently reduce the request of reagents later in the sequence. The Figure 01, provide the whiteness level afterwards each step of bleaching for each sequence given.

4 Brightness (% ISO) 100,0 90,0 80,0 70,0 60,0 50,0 82,7 61,1 Brightness 91,6 92,2 88,4 90,7 88,8 90,0 86,2 79,1 73,8 73,8 Figure 01. The effect of location of CCE stage in whiteness (%ISO) of bleached pulp by sequences CCE D (EP) Paa, D CCE (EP) Paa and D (EP) Paa CCE. The worst result appears when the CCE stage is applied in a pulp which is already bleached even though it still results in a considerable gain of whiteness nearly 2% ISO The advantage of the CCE stage as the first step is the possibility of reusing alkali and the xylan, which was removed in a countercurrent with oxygen delignification, once the pulp is not contaminated with chlorides at the stage with chlorine dioxide. Viscosity Viscosity is a parameter utilised to infer the average degree of polymerization (DP) of the polymers contained in the pulp (cellulose and hemicellulose). A higher viscosity is related to the biggest size of the polymer chains. In practical terms, the viscosity indicates how degraded is the pulp due to the occurred process. For instance: pulping and bleaching. In the CCE stage, it is noticed an increase of viscosity after finishing the treatment. For instance, the level went up from 919 dm 3 /kg to 1020 dm 3 /kg in the original pulp (Table 01) after being applied as the first CCE stage and rose from 931dm3/kg to 959 dm3/kg as the second stage and went up from 826 to 880 dm 3 /kg as the third stage (See the viscosity for second and third stage. The achievement above is explained as a result of removing the xylan, which has a level of polymerisation considerably lower than the cellulose. The reduction of xylan content from the pulp by the CCE stage causes a pulp with high levels of a-celulose content and consequently the average level of depolymerization goes up. That is the reason for rising the viscosity level. The highest final viscosity of bleached pulp (880 dm 3 /kg) was obtained when the CCE stage was applied in the last stage of the process (Figure 02). Using the CCE stage as first (456 dm 3 /kg) and second stage (524 dm 3 /kg) resulted in lower viscosity pulps, being the first stage the worst scenario. The remotion of hemicellulose at the beginning of the sequence makes the cellulose susceptible to be degraded by the oxidants CIO2, H2O2 and Paa during the steps of the bleaching. Consequently, at the end of the sequence the pulp has lower viscosity compared to the CCE stage realised at the end of the sequence. There is a desired range for dissolving pulp because the accessibility of the reagents in the derivatives production is related to the size of the chains. These values change according to the final product desired. For instance, the viscosity reached to the manufacture of dissolving pulp grade viscose when the CCE stage was applied is ideal (About 450 dm 3/ kg). On the other hand, the ideal viscosity to produce pulp grade acetate is greater than 750 dm 3 /kg, which could only have been reached when the CCE stage was applied as the last step of the process.

5 Solubility (%) Viscosity (dm 3 /kg) Viscosity Figure 02. Effect of the location of the CCE stage in the viscosity (dm3/kg) of bleached pulp by the sequences CCE D (EP) Paa, D CCE (EP) Paa and D (EP) Paa CCE. S10 and S18 solubility The difference between S10 and S18 allows to calculate the degraded the cellulose pulp content in the pulp preparation process. As lower the S10 and S18 values best quality is the pulp to manufacture soluble derivatives. The hemicelluloses content of the dissolving pulp must be low, but it depends on the product to be manufactured. For viscose pulp grade, 4-6% hemicelluloses content are acceptable. However, for acetate pulps degree, hemicelluloses levels above 2% are considered undesirable. The hemicelluloses amount are negative because it harm the reactivity of the pulp during the derivatization process. They tend to block the cellulose chains, and thus, hindering the reagents penetration during the derivatization reactions. Moreover, they also increase the consumption of derivatization reagents, since they also consume. Regarding to the CCE stage location in the sequence, smaller amounts of S10 and S18 were aimed in the bleached pulp when this treatment was done at the beginning of the sequence, with no significant differences in these parameters among the treatments applied in the middle or end of the sequence (Figure 03). The values of 5.1 and 3.7% obtained for S10 and S18, respectively, for the implementation of the CCE stage as a first step are perfectly acceptable for use in the production of viscose dissolving pulp. 10 S10 e S ,07 3,74 6,07 5,94 4,29 4,01 S10 S Figure 03. Effect of the location of the CCE stage in the solubility in the NaOH 10%(S10) and 18% (S18) of bleached pulp by the sequences CCE D (EP) Paa, D CCE (EP) Paa and D (EP) Paa CCE. Teor de α-celulose The α-cellulose content is considered very important to the dissolving pulp production. Much more α- cellulose in the pulp, better is the derivative products, referring to the yield and quality when the process is finished. Easily is observed at Figura 5 that the CCE stage is more efficient to rise the α-celulose when it is located at the beginning of the bleaching sequence. The value reached was 95,9%, an acceptable value for viscose degree dissolving pulp[4]. The CCE stage application at the middle and end of the sequence, results

6 pentosans (%) α-cellulose (%) to lower α-celulose values, however also acceptable for viscose degree dissolving pulps. 100 α-cellulose ,9 95,5 95, Figure 04. Effect of the location of the CCE stage in the α-celulose content of bleached pulp by the sequences CCE D (EP) Paa, D CCE (EP) Paa and D (EP) Paa CCE. Pentosans content As the S18 contents, the pentosans amounts, especially in case of hardwood pulp, provide information about the contamination degree with soluble hemicelluloses in the cellulose pulp. The analysis is performed directly in the pulp, where the hemicelluloses, who are consisted by sugars with five carbon atoms, are destroyed to be quantified. Therefore, other carbohydrates present in the pulp containing five carbon atoms are also quantified. Thus, pentosans are observed values greater than the soluble xylan in pulp. But there is a direct relationship between readings of pentosans and xylan (Palmeiras, 2010). This ratio is 1: 0.9 for hardwood pulp, respectively. The location of the CCE stage at the beginning of the sequence was more effective in reducing the pulp pentosans content in relation to other locations (Figure 05). For the specific case of application in the first stage, the pentosans content reached 5.6%, which is equivalent to a xylans content of 5%. This xylans value is within the acceptable range for viscose grade pulp soluble (4-6%). In the case of applications at the middle and end of the sequence, the pentosans values were 9.3 and 10.3%, respectively. The biggest facility to the CCE stage to remove hemicelluloses when it is applied at the beginning of the sequence, can be explained by xylans-lignins complex removed. The removal of lignins by extraction instead oxidation ends to drags the associated hemicelluloses. Pentosans ,3 10,3 5,6 Figure 05. Effect of the location of the CCE stage in the Pentosans content of bleached pulp by the sequences CCE D (EP) Paa, D CCE (EP) Paa and D (EP) Paa CCE. Ash content In general, pulps treated with NaOH high loads have a high content of ash (minerals). This is due to the sodium presence in the white liquor used at the treatment, which effectively even after washing, leaving inorganic residues in the cellulose pulp. To perform the CCE stage in the beginning of the bleaching

7 Ashs (mg/kg) sequence, it is noted that the ash content is lower compared to most other applications the end of the sequence (Figure 06). During the bleaching sequence, washing the pulp after each stage assists in sodium removal of pulp, which drastically reduces its ash contents. In the case of application in the first step the total ash content of the pulp was 1340 mg/kg of pulp. This value can be considered high since the limit for viscose grade pulp is soluble de1000 mg/kg. However, when applied in the middle or end of the sequence, the CCE stage resulted in ash content in the order of 1900 and 4700 mg / kg pulp, respectively. These values would be unacceptable for any application of soluble pulps Ash Figure 06. Effect of the location of the CCE stage in the ash content of bleached pulp by the sequences CCE D (EP) Paa, D CCE (EP) Paa and D (EP) Paa CCE. Conclusions The location of the CCE stage as the first stage of the bleaching sequence was the most suitable for the production of viscose grade pulp soluble at lower consumption of bleaching chemicals, better removal of hemicellulose in the pulp and lower mineral contamination compared to other applications. The location of the CCE stage as the first stage of the bleaching sequence produces low viscosity pulps, suitable for the production of viscose, but unsuitable for the production of cellulose acetate. The location of the CCE stage as a last stage of bleaching is favorable to preserve the pulp viscosity, but little effective in the removal of hemicellulose. References 1. SIXTA, H.; SÜSS, H.; POTTHAST, A.; SCHWANNINGER, M.; KROTSCHECK, A. W.; ANDREAS, W. Pulp bleaching. In: SIXTA, H. (Ed.). Handbook of pulp. Weinheim: WILEY-VCH Verlag GmbH &Co. KGaA, p COSTA, M.M.; COLODETTE J.L.; FOELKEL C.E.B. Aspectos básicos da produção de polpa solúvel. II: uma revisão. O Papel, v.59, n.9, p.66-73, GEHMAYR, V., SIXTA, H., Dissolving pulps from enzyme treated Kraft pulps for viscose application. Lenzinger Berichte 89 (2011) BAJPAI, P., BAJPAI, P.K., Development of a process for the production of dissolving kraft pulp using xylanase enzyme. AppitaJ. 54, FISCHER, K., Schmidt, I., FISCHER, S., Reactivity of dissolving pulp for processing viscose. Macromol. Symp. 280, CHRISTOFFERSSON, K.E., SJÖSTRÖM, M., EDLUND, U., LINDGREN, A., DOLK, M., Reactivity of dissolving pulp: characterization using chemical properties, NMR spectroscopy and multivariate data analysis. Cellulose 9, ENGSTRÖM, A.-C.,Ek, M., HENRIKSSON, G., Improved accessibility and reactivity of dissolving pulp for the viscose process: pretreatment with mono component endoglucanase. Biomacromolecules 7,

8 7. GOMES, V. J., Aperfeiçoamento de processos de produção de polpas para dissolução e para papel tissue a partir do eucalipto, Tese de mestrado, Universidade Federal e Viçosa, Viçosa-MG, Acknowledgements Funding provided by the Minas Gerais State Research Foundation (FAPEMIG), from the Brazilian National Council for Science and Technology Development (CNPq), and from the Coordination for the Improvement of Higher Education Personnel (CAPES) is greatly appreciated