The integration of hemicelluloses pre-extraction step prior to pulping and the subsequent alkaline pulping of the extracted residue is evaluated as the potential method to address pulp mill biorefinery. Hemicellulose was pre-extracted from sugarcane bagasse (SCB) through dilute sulfuric acid or mild alkaline processes. The effects of sulfuric acid or sodium hydroxide concentration, temperature and time on hemicellulose hydrolysis were studied using a statistical experimental design. Xylan content of 62% on dry mass was hydrolyzed from SCB with 0.5% v/v H2SO4, 140 °C and 60 minutes and 88.6% of xylan on dry mass was extracted with 2.3M NaOH, 65 °C and 180 minutes. Acid hydrolysis of SCB showed that, hemicelluloses can be quantitatively extracted from SCB, however subsequent sodaAQ pulping of residue reduced the pulp properties even when small fraction of hemicellulose was extracted on the material. Selective hydrolysis of 11% xylan with autohydrolysis improved the sodaAQ pulp yield by 2.8 % at kappa number 20.9 compared to the control at kappa number 22.8. The pulp brightness, burst and tear index was improved by 5.3%, 5.0% and 18.7% respectively. SodaAQ pulping of 73% xylan extracted residue with alkaline methods improved the pulp yield by 10.8% at kappa number 15.5 compared to the control. The brightness of the pulps was higher by 24.0% and the tear index was superior by 56.4%. Breaking length and the best index of the pulps were reduced by 10.6% and 9.3% respectively; however they are in an acceptable range. These results allow the extraction of a portion of hemicellulose from SCB with autohydrolysis and alkaline methods prior to sodaAQ pulping owing to the improvement in physico-chemical properties of the pulps.
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Keywords: biorefinery, hemicelluloses extraction, sugarcane bagasse, soda pulping, sodaAQ pulping, pulp quality, handsheets strengths
Sugarcane bagasse (SCB) represents an alternative fiber input into the South African pulp industry. Around 70 000 tons of unbleached and 60 000 tons of bleached bagasse pulp grades are produced per annum  . The soda is the preferred method for chemical sulphur free SCB pulping in South Africa. SCB consists approximately of 43% cellulose, 25% hemicellulose which is mainly xylan and 25% lignin [2-4]. In addition to lignin, approximately 70% of the hemicelluloses are dissolved in the black liquor stream during the cooking process. Currently the black liquor stream and their degradation products are typically concentrated and combusted to produce the heat and power requirements for the pulp mill . In the combustion process, hemicellulose is underutilised considering the lower heating value of hemicellulose (13.6 MJ.kg-1) to that of lignin (27 MJ.kg-1) .
Fractionation of lignocellulosic biomass into its three major components , cellulose hemicellulose and lignin has been proposed as the first step of biomass refining to high value-added products [7, 8] . Within the pulp mill biorefinery concept, extraction of hemicelluloses from suitable biomass prior to pulping, followed by the fermentation of the monomeric sugars with genetically engineered yeast to produce bioethanol, could contribute to address the growing biofuel demand . The extracted hemicelluloses oligomers or polymers may then be used as a feedstock for paper additives, polymers and chemicals [9-11]. At the same time, extraction of hemicelluloses enhanced pulp production by improving the overall alkaline pulping processes. Cooking time can be reduced and cooking liquor impregnation enhanced. Such integration of hemicelluloses extraction improved pulp properties and improved production capacity, as already demonstrated in recovery furnace limited Kraft pulp mills .
Treatment with dilute acid, hot water, alkaline/peroxide and other methods has generated technological interest to extract hemicellulosic components from SCB [4, 12-14]. However, it has to be noted that, certain physical properties of pulp require hemicellulose in the fiber matrix [15, 16]. The desired amount of hemicelluloses required in the pulp product depends entirely on the type of biomass, pulping process and the end use of the pulp . Therefore, while developing the hemicellulose pre-extraction process, one should consider not only the yield and the composition of extracted hemicellulose but also the pulp properties produced from hemicellulose pre-extracted solid residue.
The first objective of this study was to investigate the most favourable reaction conditions under which hemicelluloses could be extracted with dilute sulphuric acid or mild alkaline conditions from hemicellulose-rich sugarcane bagasse grown in South Africa, prior to alkaline pulping. In following the biorefinery approach, the second objective was to pulp the solid residue using soda or sodaAQ pulping methods to determine the effect of hemicelluloses pre-extraction on pulp and paper quality. The best extraction method was described as the one in which maximum hemicelluloses was recovered while simultaneously the quality of the pulp was maintained at desired levels.
Material and experimental
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Sugarcane bagasse (Saccharum officinarum) was provided by a local industry TSB (Mpumalanga, South Africa).The sugarcane bagasse (SCB) was depithed and conditioned at 23 °C and 55% relative humidity before use. Sodium hydroxide used for extraction of hemicelluloses (xylan) and pulping process was purchased from Merck. BUSPERSE 2262 Anthraquinone (AQ) was donated by Buckman Laboratories, Hammarsdale, South Africa.
The depithed SCB was prepared in a Retsch mill to 40 mesh size. Oven-dry mass (DM) was obtained using an oven at 105±2 °C until a constant mass was achieved. The composition of the raw material was determined using two methods: the standard methods of the Technical Association of the Pulp and Paper Industry (TAPPI) (T264 om-88, T 211 om-85, T222 om-88; T 223 cm-84)  and the standard Laboratory Analytical Procedures for biomass analysis provided by the National Renewable Energy Laboratory (NREL); Colorado, USA .
Extraction of xylan from SCB by dilute sulfuric acid or mild NaOH (alkaline) methods was studied. Under acid hydrolysis, the depithed SCB of 40 g and the acid solution were mixed in the desired portions and introduced into the micro reactors (bombs). The filled bombs were placed in a digester of 15 dm3 capacity enclosed by heating jackets. The reaction temperatures selected were monitored with thermocouples. Alternatively, alkaline extractions were carried out in 500 mL schott bottles and kept at desired temperature and time in a shaking water bath. The ratio of solid to liquid was 1 g: 6mL. The pre-extraction conditions were varied systematically according to the central composite design as presented in Table 1 . The designs were created and evaluated in STATISTICA 7.1 (Statsoft Inc., Tulsa, USA). Three assays in the centre point were carried out to estimate the random error required for the analysis of variance (ANOVA).
At the end of the desired extraction time, the reaction vessels (bombs and schott bottles) were cooled in water. The insoluble residue after acid extraction was collected by filtration on a 100 mesh screen, whereas after alkaline extraction the fibres were squeezed by hand to recover the xylan. The solid residues were thoroughly washed with water and air dried. The oven dry mass cellulose, xylan and lignin contents of the solid residue were determined. The xylan-rich hydrolyzate was collected and a sample was filtered through 0.2 μm membranes and analyzed for its content of monomeric sugars, soluble oligomers and by-products. Aminex HPX-87H Ion Exclusion Column equipped with a Cation-H cartridge (Biorad, Johannesburg, RSA) was used. The column was operated at 65 °C with a mobile phase of 5mM sulphuric acid and a flow rate of 0.6 mL/min.
Each of the xylan rich filtrate obtained after alkaline extractions was concentrated to approximately 1/3 by rotary evaporator at 40 °C. The filtrates were then dialyzed for 3 days against de-ionized water using dialysis tubing cellulose membrane having 12kDa molecular weight cut off (MWCO). The samples were then conditioned in liquid nitrogen and were finally freeze dried. The oven dry mass of the samples was determined.
Sequence of experiments according to a central composite design for
dilute H2SO4 and NaOH pre-extraction of xylan from sugarcane bagasse
Micro pulping of sugarcane bagasse residue
Solid residues run 9 and run 15 obtained after acid hydrolysis were subjected to soda or sodaAQ micro pulping to pre-screen the pulping behaviour of pre-extracted residues. Pulping of the non extracted SCB (control) was performed for comparison purpose. Run 15 was preferred since maximum glucan content was observed in the solid residue after acid hydrolysis, as illustrated in Fig 3A. Run 9 was selected since only water (Autohydrolysis) was used during hydrolysis process. Prior to pulping the solid residues were thoroughly washed with water to remove the acid and air dried. Alternatively, alkaline pre-extracted residue run 6, and run 15 were also subjected to sodaAQ pulping. In contrast to acid hydrolysis, no washing was done to alkaline pre-extracted residues prior to pulping. The wet residues were directly subjected to pulping and there was no addition of NaOH during pulping.
All the pulping experiments were carried out (Table 2 and Table 3) in micro bombs. The maximum cooking temperature was kept constant at 170 °C and the solid to liquor ratio was fixed at 1g: 7mL for all pulping experiments. Temperature and reaction time were monitored during the process. Cooking time was measured from the moment the system reached the maximum temperature. At the end of cooking, the fibers were separated from the black liquor and washed through a 10mesh screen, to separate rejects from the fibers, the accepted pulp was collected on a 100 mesh screen. The pulp was then screened in a Packer slotted laboratory screen. Total pulp yield and rejects were determined as a percentage of original dry mass of the raw material. Pulp kappa number was determined by standard TAPPI methods T236.
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Pulping for handsheets making
Based to the best results obtained from the micro pulping experiments, solid residue run 9 (hot water, 120 °C and 40 min) and run 15 (1.5 M NaOH, 65 °C and 40 min) obtained after hot water hydrolysis and alkaline extraction respectively were subjected to sodaAQ pulping for handsheets making. Pulping was carried in a 15L batch type digester. SodaAQ pulps were generated by exposing 1000g (DM) pre-extracted SCB residue to 14% sodium hydroxide and 0.1% Anthraquinone (AQ) for 30 minutes at 170 °C. At the end of the cooking process, the fibers were treated as explained above. Pulp characteristics and strength properties were determined by TAPPI Standard methods . Pulps were beaten in a Valley beater according to Tappi Standard T200 om-89 and the drainage rate in Schopper Riegler (ËšSR) was measured according to Tappi T227 om-99. The hand sheets of papermaking properties were formed according to Tappi T205 om-88 using British Standard hand sheet making equipment. Burst index, breaking length, and tear strength of the handsheets were measured by TAPPI Standard no T403 om-91, T404 om- 87, and T414 om-88, respectively. The brightness was in ISO standards using a reflectance photometer (Zeiss Elrepho 65843, Germany).
Sequence of experiments for soda pulping
of xylan extracted sugarcane bagasse
Sequence of experiments for sodaAQ pulping of xylan extracted
sugarcane bagasse oven dry mass
Raw material composition
The composition of sugarcane bagasse (SCB) is shown in Table 4. The glucan (46.0%), lignin (18.2%), xylan (23.6), arabinans (2.4), ash (2.6%), ethanol/cyclohexane (1.7%) and water soluble extractives (2.4%) were observed. Since xylan was the major pentosan, it was the considered sugar through out this study. This data was used to generate the entire extraction recovery yield described below.
Chemical composition of sugarcane bagasse
Liquid fraction after xylan pre-extraction
The composition of the liquid fraction obtained after acid and alkaline pre-extraction of xylan from SCB is illustrated in Fig. 1A and 1B respectively. The concentrations were expressed in g/100g and extraction percentage to respect the polymer content in dry raw material. The yields obtained after acid hydrolysis varied within the range 0.3-11.1 g/100g (~1.2- 42.9%) as shown in Fig. 1A. The concentration of the sugars increased to 0.3 -15.98 g/100g (~1.2-62%) when the prehydrolyzate obtained after acid hydrolysis was treated with 4% sulfuric acid at 121 ËšC. In all the experiments the concentration of glucan and acid soluble lignin in the liquid fraction was less than 1 g/100g .On the other hand, alkaline conditions resulted to the xylan concentration varied within the range of 9.8 to 22.8 g/100g (~ 43.3-88.6%) as shown in Fig 1B.
Fig. 1. Total glucan and xylan yield expressed as g/100g of the original material in the liquid fraction under different sulfuric acid (A) and NaOH (B) extraction conditions.
ANOVA analysis providing information on which process variable had significant effect during pre-extraction are presented by multiple regression equations (1 and 2). In the equations, XA, XT, Xt and XS are the coded values of the operational variables sulfuric acid concentration (% v/v), temperature (ËšC), residence time (minutes) and NaOH concentration (M) respectively. All the interactions were significant at 95% confidence level and the lack of fit of the models was insignificant. The square of the correlation coefficient (R2) for percentage xylan obtained under acidic conditions (Y1) and percentage xylan obtained under alkaline conditions (Y2) was 0.98 and 0.82 respectively, explaining 98% and 82% of the variability in the responses.
Y1 (%) = 4.7+ 3.8XA + 3.5XT + 3.3Xt + 2.1XAXt + 1.6XTXA (1)
Y2 (%) = 19.1 + 5.2XS + 3.9XT - 3.7 XTT (2)
All the process variables showed strong influence on xylan hydrolysis under acidic conditions; as indicated in the multiple regression equation 1. The xylan extractions under alkaline conditions, was influenced by both NaOH concentration and temperature as shown in equation 2.
Response surfaces described by the above model equations were fitted to the experimental data points concerning the xylan recovery as shown in Fig. 2. The response surfaces showed that the increase in variable values would increase the yield of recovered xylan. However, Fig 2B indicated a reduction in xylan yield when the temperature exceeded 90 °C under alkaline conditions.
Fig. 2. (A) Estimated response surface for xylan yield obtained after sulfuric acid hydrolysis showing the influence of temperature and acid concentration for a residence time of 40 minutes. (B) Estimated response surface for xylan yield obtained after NaOH extraction showing the influence of temperature and NaOH concentration for a residence time of 180 minutes.
Solid fraction after pre-extraction
The recovery range of sugars on dry weight basis retained in the solid residue after acid and alkaline pre-extraction of SCB is shown in Fig. 3. Glucan and acid insoluble lignin retained in the solid residue after acid pre-extraction ranged between 41 to 54 g/100g (90%-118%) and 16-19 g/100g (76-104%) respectively. For alkaline conditions the range for glucan and acid insoluble lignin was 51 to 60 g/100g (110-130%) and 6 to 17 g/100g (33%-93%) respectively. The xylan retained in the solid residue ranged between 14 to 25 g/100g (56-97%) after acid hydrolysis and the range was 2 to 16 g/100g (8-62%) after alkaline extractions.
Fig. 3. Total glucan and xylan yield expressed as g/100g of the original material in the solid fraction under different sulfuric acid (A) and NaOH (B) extraction conditions.
3.5. Pulping of xylan extracted sugarcane bagasse
Solid residues run 9 (water, 120 °C, 40 min) and run 15 (0.3% v/v, 120 °C,40 min) obtained after acid hydrolysis were subjected to soda or sodaAQ micro pulping. Conditions used for pulping are described under experimental procedure. The pulp yield, kappa number and reject level of the pulps obtained after soda pulping of both extracted and non extracted SCB (control) are shown in Table 5A. The pulp yields recorded from the control were 32.4-34.5%, compared to 35.1-39.7% and 35.4-37.4% of Run 9 and Run 15 respectively. The kappa number of the control pulps ranged between 33.5-34.8 points and the reject levels range was 6.0-9.9%. Run 9 resulted to pulps with kappa number ranged between 32.0-33.2 points and the reject levels were 4.4-8.0%. Kappa number of pulps produced from Run 15 ranged between 36.9-38.4 points and the reject levels were 9.4-A2.7%.
Table 5B list the comparative pulp characteristics obtained after sodaAQ pulping of both extracted and control SCB. The pulp yields recorded from the control were of 31.9-46.2%, accompanied with reject level of 4.9-9.9% and a kappa number of 31.3-31.9 points. Run 9 and Run 15 resulted to pulp yields of 36.5-53.6% and 38.3-39.9% with kappa number of 28.6-29.4 and 36.9-38.4 points respectively. The reject levels of the pulps were 0.5-11.5% and 9.0-13.1% for Run 9 and Run 15 respectively.
The results obtained from alkaline extracted residue run 6 (2M NaOH, 40 °C) and run 15 (1.5 M NaOH, 65 °C) are shown in Table 5C. The average pulp yield, kappa number and reject level obtained from Run 6 was 37.7±2.8%, 16.8±1.0 points and 2.8±0.4% respectively. Run 15 resulted to average pulp yield of 40.1±2.0% accompanied with reject level of 2.1±0.3% and a kappa number of 15.8±0.3 points.
Soda pulping results for 11% (Hot water, 120 ËšC, and 40 min) and 23% (0.3% v/v H2SO4, 120 ËšC, and 40 min) xylan extracted sugarcane bagasse
SodaAQ pulping results for 11% (Hot water, 120 ËšC, and 40 min) and 23% (0.3% v/v H2SO4, 120 ËšC, 40 min) xylan extracted sugarcane bagasse
SodaAQ pulping results for 73% (1.5M NaOH, 65 ËšC, 240 min) and 79% (2M NaOH, 40 ËšC, 240 min) xylan extracted sugarcane bagasse
Based on the best pulping pre-screen results, autohydrolyzed run 9 and alkaline extracted run 15 were pulped with 14% NaOH , and 0.1 % AQ for 30 minutes in a pilot scale for handsheets making .The non extracted SCB was pulped for comparison. The properties of the pulps are presented in Fig. 4. Screened pulp yields obtained from the control, alkaline pre-extracted and autohydrolyzed SCB were 40.12%, 44.97% and 41.28% respectively. The pulp viscosities in centipoise were 7.2, 7.1 and 5.5 and the kappa numbers of the pulps were 22.8, 20.9, and 15.5 respectively. Pulping of non extracted SCB resulted to the reject level of 15.7% compared to 3.3% of NaOH pre-extracted residue and 14.7% of autohydrolyzed residue.
Fig. 4. Pulp properties of non extracted and extracted sugarcane bagasse on pilot scale sodaAQ pulping process. The NaOH concentration =14%, AQ = 0.1%, T = 170 °C, t = 30 min
Handsheets physical properties
The burst index, tear index, breaking length and ISO brightness properties of handsheets produced from sodaAQ pulps are shown in Fig 5. It has been observed that strength properties increased with increasing degree of beating expressed in °SR. The sequence of presentation of the results will be for the control, alkaline and autohydrolyzed pulps respectively. The burst index range was (2.4-4.7 kPa.m2/g), (1.4-4.3 kPa.m2/g), and 1.7-5.0 kPa.m2/g (Fig. 5A). The tear index range was (3.3-4.4 mN.m2/g), (8.8-9.8 mN.m2/g) and 4.7-4.8 mN.m2/g (Fig. 5B). The breaking length property of the handsheets ranged between (2.4-5.0 km), (1.3-4.5 km) and 1.8-5.0 km (Fig. 5C). The optical brightness of handsheets ranged between (39.3-43.5% ISO), (51.7-55.8% ISO), and 41.5-44.2% ISO (Fig.5D).
Fig. 5. Handsheets properties as the function of drainage in ËšSR of sugarcane bagasse after sodaAQ pulping.
The composition of the liquid fraction recovered after hydrolysis of SCB with dilute sulfuric acid is presented in Fig 1A. Analysis of the results revealed that xylan hydrolysis was improved when both acid concentration and temperature were increased from 0.1 to 0.5% and from 100 to 140 °C respectively at 60 minutes. The sugars were mostly oligosaccharides with the maximum yield of 42.87% and this value increased to 62.07% when the prehydrolyzate was subsequently treated with 4% sulfuric acid at 121 °C. Pessoa et al., extracted about 70% of hemicelluloses in SCB by sulfuric acid on a laboratory scale at 140 °C . Attempts to increase temperature above 140 °C to recover more xylan as suggested by the statistical analysis in Fig 2A resulted to the degradation of xylan. This was shown by the presence of 0.3 g/L furfural at temperatures of 154 °C. Neureiter et al., reported that temperature showed the highest impact on the formation of degradation products during dilute acid hydrolysis of SCB  . Combining the analysis of the liquid fraction with the results obtained in the analysis of the solid residue in Fig 3B, it can be observed that high recovered xylan fraction corresponded to the high glucan content (111.9%) retained in the solid residue. This result is important for the utilization of the solid residue for pulp and paper making. The lignin content in the solid residue at this point was 86.3%. Similar results of high glucan recovery (89 to 126%) during liquid hot water pre-treatment of SCB was reported . Cellulose and lignin are resistant to attack by dilute acids at low temperatures, although small fraction of lignin (about 10%) can be dissolved in the process [20, 22].
In comparison, alkaline conditions have been shown to improve its potential for xylan extraction (Fig 1B). The maximum xylan recovered up to 88.6% under 2.34 M NaOH and 65 °C was observed. The statistical correlation presented in Fig 2B against temperature and NaOH concentration showed that increased in temperature beyond 65 °C would decrease the xylan recovery yield. Optimum xylan recovery was accompanied by high dissolution of lignin, where only 49% lignin was obtained in the solid residue. For pulp mill biorefinery to be effective, the lignin content of the extracted hemicellulose should be relatively low, and the DP of cellulose and lignin reactivity in the extracted chips remains high during subsequent pulping . Treatment of non wood material under alkaline conditions at room temperature has been reported to solubilised up to 50% of lignin present in the material [23, 24]. Brienzo et al., extracted 94.5% hemicellulose from SCB associated with more than 88% lignin using alkaline peroxide .
The effect of xylan pre-extraction on pulp yield, kappa number and level of rejects after soda and sodaAQ pulping was evaluated. As can be seen in Table 5A the entire tested residues resulted to pulp yields less than 40% with similar kappa number under soda pulping. Soda pulping of alfafa stems resulted to low screened pulp yield (16-21%) and high percentage rejects (18-35%) . Addition of anthraquinone (AQ), during the soda pulping process improved the yields, kappa number and reject levels of the pulps as shown in Table 5B. AQ has maximum effect on degradation of lignin and stabilization of celluloses in pulping process . Table 5B showed that autohydrolysis of SCB prior to sodaAQ pulping improved pulp properties when compared to both the non extracted and acid extracted pulps. Comparing the best pulping results (Run 2 in Table 5B), autohydrolyzed SCB produced 14% higher pulp yields than those of non extracted pulps and 26% higher than those of acid extracted pulps. The delignification efficiency was also improved due to autohydrolysis. The kappa number for autohydrolyzed pulps was 5.6 points lower than for the non extracted pulp and 9.4 lower than for acid extracted pulps. At this point the reject level for autohydrolyzed pulps was only 0.5%. It is reported that lignin in SCB is covalently bonded to arabinoxylans through ferulic acid bridges . Solubilization of hemicelluloses under the hot water extraction at mild temperatures could have resulted to the cleavage of the ester linkages between arabinose and the ferulic acid. This resulted to the open structure of the cell wall which promoted the facilitation of cooking chemicals during pulping, hence high pulp yields and low rejects levels . Previously a hot water pre-extracted SCB with kappa number 12.4 and screened pulp yield 52.95 versus kappa 16.6 and screened pulp yield 53.27 from non extracted SCB was obtained . The data presented in Table 5B showed that, the combined process of hemicellulose pre-extraction with sulfuric acid and sodaAQ pulping is not the best option following the poor pulping response on separation of only small fraction of hemicelluloses in prehydrolysis. In comparison, sodaAQ pulping of 73% xylan extracted SCB resulted to pulp yield of 40.1% accompanied with the kappa number of 15.8 points. Lopez et al., reported pulp yield range of 40.5-45.9% accompanied with kappa number range 12.9-17.7 during sodaAQ pulping of 31-72% hemicellulose extracted barley straw .
Based on the best results obtained from laboratory scale pulping (Table 5B and 5C), sodaAQ of autohydrolyzed and alkaline extracted SCB was performed in a batch type pilot scale digester to evaluate the effect of xylan extraction on strength properties of the pulps. The results were compared with those of the non extracted SCB pulps treated under similar pulping conditions. SodaAQ pulping in a pilot digester reduced the pulp yield of non extracted and autohydrolyzed SCB by 13.2% and 23% respectively. This difference could be related with the high pith content in the raw SCB, since depithing was done by hand.
Results in Fig 4 showed that pulps prepared from alkaline extracted SCB on pilot scale improved the yield by 10.8% at kappa number 15.5 than the control at kappa number 22. 8 and improved by 8.2% than autohydrolyzed pulps at kappa number 20.9. The viscosity of alkaline extracted pulps was 1.4% less than the control pulps, showing minimal cellulose degradation. This result is very important since the alkaline pre-extraction process was able to maintain the quality of the pulp at desired levels while simultaneously achieving acceptable levels of hemicellulose pre-extraction. The increased cellulose retention with AQ addition is caused by stabilization of cellulose reducing ends by oxidation with AQ  . The viscosity of autohydrolyzed pulps was lower by 23.6% than the control, probably due to the cellulose loss through peeling reaction during cooking.
All the pulps were beaten to improve handsheets strength properties and the optimum strengths were reached at the beating degree of 20 ËšSR as shown in Fig 5. Alkaline extraction show a 56.4% improvement in tear index and a 24% improvement in optical brightness compared with the control. However, both breaking length and burst index were reduced by 10.6% and 9.3% respectively. These properties depend on fiber collapse and interfiber bonding . The reduction in the strength properties can be traced back to the high percentage of xylan (73%) extracted prior to pulping Hemicelluloses enhanced fiber bonding during beating of pulps [10, 16]. Reduction in tear index by 2% and burst index by 8% due to sodaAQ pulping of 57% xylan extracted barley straw has been reported . In comparison, autohydrolyzed pulps showed an improvement in optical brightness, tear and burst index by 5.3%, 18.72% and 5.0% respectively than the control. The breaking length of the pulp was 0.4% less compared to control pulps. These results are slightly different to those obtained in the literature during sodaAQ of autohydrolyzed SCB although the pulping conditions in the compared study were slightly higher . The results obtained in the referenced study showed that autohydrolyzed pulps improved the optical brightness and tear index by 6% and 10.6% compared to the soda pulps obtained from non extracted SCB. The breaking length and burst index of autohydrolyzed pulp were reduced by 21.8% and 26.3% respectively compared to the control . This finding could be related to the use of higher temperatures in the referenced study (160 °C versus 120 °C) combined with higher NaOH concentration (15.5% versus 14%) during pulping. The process modification prior to pulping and the pulping cooking conditions may affect the physico-chemical properties of pulp [8, 30].
Of the two pre-extraction methods, autohydrolysis had the high breaking length and burst index, while alkaline extraction has the highest pulp brightness and superior tear index at maximum beating degree of 20 °C. In literature, it has been mentioned that, the reduction in burst and tensile index is not the limiting factor for the selection of high paper grades, as long the pulps still have more adequate tensile strength .
Dilute sulfuric acid hydrolysis of sugarcane bagasse under low temperature and mild acid concentration is the efficient method to extract xylan from sugarcane bagasse. About 62% of xylose can be recovered with 0.3 g/L furfural present in the hydrolysate. However, the removal of small fraction of xylan (up to 23% dry mass) through dilute sulfuric acid hydrolysis step affected the subsequent sodaAQ pulping. Pulp yields less than 40% with kappa number around 35 points were obtained. When 11% of the orgininal SCB xylose was extracted through autohydrolysis at pilot scale, and the residue subjected to sodaAQ pulping, the pulp yields were slightly improved and the kappa number of the pulps were lower than those of the control . The physical strength properties of the pulps were improved, except for slight reduction in breaking length by 0.4%. The combination of 73% pre-extracted xylan under mild alkaline conditions with subsequent sodaAQ pulping improved the delignification rate. Pulp yields were high and the kappa number of the pulps was less than 20 points. The viscosity of the pulps were comparable to those of control. These conditions provided pulps with higher brightness and superior tear index. On contrary the breaking length and burst index was reduced. According to these results both alkaline and hot water pre-extraction of sugarcane bagasse had a great potential for the integration process of xylan extraction prior to pulping and subsequent sodaAQ pulping depending on the desired final product of the of paper.