Studies on polysulphide pulping of Populus deltoides

Transcription

1 Journal of Scientific & Industrial Research Vol. 63, October 24, pp Studies on polysulphide pulping of Populus deltoides Suhail Akhtar Rao and J S Upadhyaya* Department of Paper Technology, Indian Institute of Technology,Roorkee, Saharanpur Campus, Post Box No. 83 Saharanpur Received 28 March 23; rev recd 25 February 24; accepted 8 July 24 Addition of polysulphide (PS) during the pulping is investigated in the laboratory. Polysulphide was found to be advantageous to extend the cooking. The present study deals with polysulphide pulping of Populus deltoids Marsh The benefits gained were in terms of increased pulp yield without considerably affecting the quality of the pulp. The tear and burst values of the PS pulps were found to be slightly lower, while the tensile index values were found to be comparable with that of kraft pulp. The results demonstrate the better utilization of fibrous raw material for paper-making in terms of increased pulp yield and better physical and other strength properties of paper. Keywords: Polysulphide pulping, Pulping, Populus deltoids, Marsh IPC Code: Int.Cl. 7 : D 21 B 1/ Introduction Over the past few decades, several chemicals were identified by the researchers, which can improve pulp yield, when used as pulp additives 1-3. Most of these chemicals are capable of inhibiting the peeling reactions during kraft pulping, by converting the carbohydrate aldehydic end groups to alkali-stable alcohols or carboxylic acids. The carbohydrate end groups can be reduced to alcohols with sodium borohydrides 4 and to thiols with hydrogen sulphide. This can also be oxidized to carboxylic acids with polysulphide or anthraquinone (AQ) 2,3. Among these four additives only polysulphide and anthraquinone have been applied commercially due to their relatively low cost and easy handling. The first report of increase in kraft pulp yield in the presence of polysulphide, made by dissolving elemental sulfur in kraft cooking liquor, was given by Hagglund 5. Subsequently, several kraft mills applied Hagglund s technique to generate polysulfide in their white liquor, was confirmed in their mills 6,7. However, polysulphide (PS) pulping was introduced in late 197s as a means of increasing the pulp yields along with decreasing the wood and energy costs of kraft pulping 4,8,9. Due to the increasing cost of wood the efforts have been made to increase the pulp yield. The pulp yield can be increased by increasing the carbohydrate retention during the *Author for correspondence course of pulping operation. The use of polysulphide or anthraquinone (AQ) or in combination, proved beneficial in improving the pulp yield at the same kappa no. without sacrificing the strength properties 1,11. Polysulphide does not affect the delignification rate significantly but an effective pulping additive for increasing kraft pulp yield. PS oxidises the reducing end group in polysaccharides to carboxylic groups rendering the polysaccharide, in particular glucomannans, stable against the alkali assisted end wise degradation, resulting in an increase in pulp yield 1,3,7-9. In the early stages of the polysulphide pulping, most of the polysulphide is consumed by reacting with the carbohydrates 5. R-CHO + Na 2 S NaOH RCOONa + 2Na 2S + 2H 2 O This equation depicts the oxidation of the carbohydrate reducing carbonyl end group to carboxylic group, thus preventing the extent of peeling reaction and stabilizing the carbohydrates against alkaline degradation. The stabilization of carbohydrates is one of the reason to increase the pulp yield in PS pulping over kraft pulping. In the above reaction, it has been seen that the reaction of polysulphide and carbonyl end group generates sodium sulfide. The polysulphide at 14 C also generates sodium sulfide. 4Na 2 S 2 + 6NaOH 6 Na 2S + Na 2S 2 O 3 + 3H 2 O

2 RAO & UPADHYAYA: POLYSULPHIDE PULPING OF POPULUS DELTOIDES 821 Experimental Methodology Polysulphide liquor was prepared in laboratory by reacting sulfur powder with high sulphidity white liquor in the presence of oxygen between 7-8 C for 3h under N 2 atmosphere. The active alkali and polysulphide sulfur contents of the liquor were determined by Tappi method 694 om-9. The polysulphide per cent applied on wood is not independent variable and is determined by the polysulphide concentration in the liquor. The polysulphide concentration depends upon the sulfidity of the white liquor, higher the sulphidity the more will be the amount of polysulphide formed. During the course of polysulphide pulping, the sulphidity of the polysulphide liquor continuously decreases and attains a low value in spite of white liquor. To keep the kappa no. of polysulphide pulp comparable to conventional kraft pulp the active alkali charge for the polysulphide cooking is kept slightly higher than the conventional kraft pulping -14. The chips of poplar deltoides were screened in order to separate the oversize chips and undersize chips from the chips of acceptable quality. The chips of poplar deltoides retained on 32 mm screen, were utilized for pulping studies. All the cooks were done in CCL digester, using a charge of 3 g od chips at wood to liquor ratio 1:3, maintaining the following cooking conditions: Time from room temperature to 15 C=45 min Time from 15 C to maximum temperature 165 C=45 min Time at maximum temperature= min Optimum Pulping Conditions The pulping studies were made, using different concenteration of polysulphide to study the optimum ratio of these chemicals for the purpose of delignification. The cooking was also carried out for different time, at 165 o C to study the effect of cooking time during the course of delignification reaction. At the end of the cook the pulp was defibrated, washed, and screened with a plate clearance of.15 mm. These pulps were evaluated for the screened pulp yield, kappa number, and rejects. Results and Discussion Effect of Polysulphide Concentration on Pulp Yield The results of the different doses of polysulphide pulping are reported in Table 1 and 2. Addition of polysulphide increases the pulp yield. From the results of polysulphide pulping of P. deltoides it has been observed that with the addition of 1 per cent polysulphide, there is an increase of 1.5 per cent in pulp yield in the kappa range of Similarly on addition of the polysulphide dose of 2 and 3 per cent, the increase in yield was found to be and 2.8, per cent respectively. The results of the effect of PS concentration at 16 and 17 per cent active alkali on pulp yield and kappa number are plotted in Figs 1-4. The data fitted statistically smooth curves having concavity upwards with R 2 values ranging from The R 2 values indicate the closeness of the fit of the data. From Table 1, it has been observed that the increase in pulp yield with polysulphide is more for higher kappa pulps. However, the use of PS and AQ are less effective for yield improvement in the low kappa range. These results indicate that the yield improvement by PS addition decreases in low kappa number range may be due to instability of hemicellulose at higher degree of delignification. The increase in pulp yield by the addition of polysulphide or other additives is due to the higher retention of carbohydrate materials in the pulp. The hemicelluloses are less stable than cellulose at higher cooking temperature; longer time and higher active alkali charge degrades the carbohydrate materials and decreases the retention of hemicelluloses. In the case of PS pulping the pulp yield increases, while the black liquor solids decreases by about 5 per cent compared to conventional kraft pulping at a same kappa number. From the results it can be concluded that a 2 per cent dose of polysulphide may be considered as optimum dose in order to get better pulp yield without affecting the physical strength properties. Effect of Alkali Charge and Sulphidity Polysulphide liquor is generated from the oxidation of sodium sulfide in kraft white liquor. Since the generation of PS consumes sodium sulfide the nominal sulphidity of the PS liquor produced is decreased to about half to that of the original white liquor. When the PS liquor is heated during pulping, a portion of polysulphide in the liquor is converted back to sodium sulfide. Despite this reversion the sulphidity of PS cook remains lower than that of conventional kraft cooks. The lower sulphidity of PS liquor needs to increase the active alkali in PS cook for obtaining pulp with same kappa no. as in conventional 16 per cent kraft cook. Therefore, in PS pulping, a little higher active alkali charge (17 per cent) was used in order to obtain the pulp with same

3 822 J SCI IND RES VOL 63 OCTOBER 24 Pulp Table 1 Effect of time on different polysulphide concentration during polysulphide pulping of P. deltiodes at 165 C 16 per cent AA 2 per cent, (Kraft pulp) per cent,. per cent, sulf. per cent,. per cent,. per cent, per cent,. Time at Screened pulp yield, temperature (min) per cent (±.8) Screen rejects, per cent (±.2) Total pulp yield, per cent (±1.) Kappa number Lignin, per cent (±.1) Table 2 Effect of active alkali, sulphidity and polysulphide concentration on pulp and black liquor properties, at following conditions Max. temperature : 165 C Time at temperature : min Bath ratio : 1:3 Active alkali, (per cent) as Na 2 O Sulphidity, per cent Polysulphide, per cent Screened pulp yield, per cent (+.8) Screen rejects, per cent (+.2) Total pulp yield, per cent (+1.) Kappa number(+1.) Black liquor properties ph (+.1) Residual alkali, gpl (+.2) B/L solids(+.3) kappa no. It has been observed that the kappa no. of conventional kraft pulp is comparable to that of polysulphide pulping with 17 per cent active alkali along with 2 per cent PS concentration. The effect of polysulphide pulping at 16 and 17 per cent active alkali charge along with 1-3 per cent polysulphide concentration on pulp yield and kappa no. are plotted in Fig The R 2 values are between , which indicates the closeness of the fit of the datas. These results indicates that for obtaining lower kappa number pulp, a little higher active alkali is required during polysulphide as compared to kraft pulping. Effect on Pulp Properties The pulps were beaten in PFI mill at different revolutions as per Tappi method (28cm-85). The hand sheets of 6 gsm were made on British sheet forming machine, according to Tappi method T25. The hand sheets were conditioned at 27±1 o C and 65±2 per cent relative humidity and evaluated for various physical strength properties.. The results of the pulp evaluation are reported in Table 3.

4 RAO & UPADHYAYA: POLYSULPHIDE PULPING OF POPULUS DELTOIDES 823 Fig. 1 Plot of pulp yield vs time at temperature at different polysulphide at 16 per cent active alkali Fig. 3 Plot of kappa number vs time at temperature at different polysulphide at 16 per cent active alkali Fig. 2 Plot of pulp yield vs temperature at different polysulphides at 17 per cent active alkali The results of the tear strength showes that PS pulp has slightly lower tear than the conventional kraft pulp. The tear index of kraft, 2 per cent PS pulp at the freeness of around 31 (ml) CSF are 7.2 and 6.8, respectively. The tensile index of polysulphide pulp is found to be comparable to that of kraft pulp, at equal freeness level. The tensile index values at around 31 ml CSF of kraft pulps at 16 per cent active alkali (2 per cent sulphidity) and polysulphide pulps at 16 per cent active alkali and 2 per cent PS( per cent sulphidity) were found to be 71. and 7, respectively, polysulphide improves the tensile properties. In the case of polysulphide pulping there is no noticeable effect on burst and tensile properties, in addition to higher hemicellulose retention. Tear tensile relationship for the handsheets of kraft, polysulphide pulps is shown in Fig. 5 and 6. The polysulphide pulp shows slight decrease in tear strength when refined to lower CSF, while on the other hand the tensile index values show a slight improvement at the same freeness level. These results indicate that pulping to higher kappa number Fig. 4 Plot of kappa number vs time at temperature at different poloysulphide at 17 per cent active alkali produces the pulp with higher tear, while the low kappa pulping has lower tear due to higher carbohydrate degradation. The burst index values of polysulphide pulp have been found to be slightly lower than that of kraft pulp. The effect of freeness on the tear, tensile and burst for kraft, and PS pulps indicates that the burst and tensile values improve up to 3mL CSF, while the tear declines. The tensile and burst index depend on the extent of pulping and refining conditions. The more delignified pulp shows poor tensile and burst index due to higher carbohydrate degradation, although the differences in burst strength of kraft and polysulphide pulps (PS) have been found to be negligible. The freeness value data indicate that the polysulphide pulp can be refined with lesser energy than the conventional kraft pulps, due to comparatively higher retention of hemicellulose contents. Conclusions The use of polysulphide PS shows a significant increase in the pulp yield. The use of 1-2 per cent

5 824 J SCI IND RES VOL 63 OCTOBER 24 Table 3 Strength properties of PS pulps at various freeness levels Pulp PFI rev. (Nos) Freeness CSF (ml) (+1) Drainage time (s) (+.1) Apparent density (g/cm 3 ) (+..2) Tensile index (N.m/g) (+1.) Burst index (kpa.m 2 /g) (+.2) Tear index (mn.m 2 /g) (+.3) 16 per cent AA 2 per cent, (Kraft pulp) per cent,. per cent, per cent, per cent, per cent,. per cent, Fig. 5 Plots of tear tensile of the polysulphide pulp at 16 per cent active alkali polysulphide helps in increasing the pulp yield to about 1.5- per cent. PS processes can be used to decrease the kappa number at the same yield or increase the yield at the same kappa number. However, in polysulphide pulping, slightly higher active alkali is used then kraft pulp because the sulphidity of the polysulphide liquor falls during PS formation. Fig. 6 Plots of tear tensile of the polysulphide pulp at 17 per cent active alkali The polysulphide pulp was found to be easily beatable than kraft pulps. Compared to kraft process the addition of 2 and 3 per cent PS decreases about 1-15 per cent refining energy at a given freeness. These results are in good agreement with the literature and confer the higher retention of hemicellulose by PS pulp. The tear and burst values of the PS pulps were found to be slightly lower, while the tensile index values were found to be comparable with that of kraft pulp.

6 RAO & UPADHYAYA: POLYSULPHIDE PULPING OF POPULUS DELTOIDES 825 The present study demonstrates the better utilization of fibrous raw material for paper-making in terms of increased pulp yield and better physical and strength properties of paper. Acknowledgement Authors are grateful to CSIR for the financial assistant to Suhail Akhtar Rao for carrying out research for his Ph. D degree at Department of Paper Technology, Indian Institute Technology Roorkee, Saharanpur Campus References 1 Aurell R & Hartler N, Part III, Sulphate cooking with the addition of reducing agents: The effect of sodium boro hydride, Tappi J, 46(4) (1963) Simson H A, Towards kraft mill, AF IPK Study Book II, (1992) p Teder A, Some aspects of the chemistry of polysulphide pulping, Svensk Papperstid, 72(9) (1969) Green R D & Prusas Z C, Pulp Paper Canada, 76(9) (1975) T272 5 Hagglund E, Sulfate pulp produced by cooks with addition of sulfur, Svensk Papperstid, 49(9) (1946) Hilmo P & Johnsen K, Worlds only commercial polysulphide mill reveals advantages and problems, Pulp Paper, 43(5) (1969) Kleppe P J, Polysulphide pulping in a dual-vessel Kamys digester, Tappi J, 58(8) (1975) Jian Er Jiang, Tappi J, 78(2) (1995) 6. 9 Jian Er Jiang, Extended modified cooking with polysulphide: Effect on pulp yield and physical properties, Tappi J, 77(2) (1994). 1 Kibble R P & White Dell Bewden, Sructural organization and paper making qualities of kraft, soda-aq, neutral sulfite-aq, Polysulphide and polysulphide-aq pulps, Appita J, 42(4) (1989) Pitt Y, Sturgeoff L G & Kanters C J, Extended modified cooking with polysulphide: Effect on pulp yield and physical properties, Pulp Paper Canada J, 97(1) (1996) T23. Rao Suhail Akhtar Studies on pulping and bleaching of poplar deltoides, PhD Thesis, Indian Institute of Techno-logy, Roorkee, July Rao Suhail Akhtar & Upadhyaya J S, Studies on Kraft and Kraft - AQ Pulping and bleaching of P.deltoides J Sci. Ind Res, 61(3) (22) Dutt Dhram, Upadhyaya J S, Rao Suhail Akhtar & Rao G N, Studies on use of oxidised white liquor in kraft pulping for prevention of digester corrosion and improvement in pulp quality, J Sci Ind Res, 56(2) (1997) 99.