ARTICLE pubs.acs.org/IECR
Influence of Sodium Borohydride on Kraft Pulping of European Black Pine as a Digester Additive Sezgin Koray Gulsoy* and Hudaverdi Eroglu Faculty of Forestry, Forest Products Engineering Department, Bartin University, 74100 Bartin, Turkey ABSTRACT: In this study, the effects of sodium borohydride (NaBH4) on properties of kraft pulp and paper obtained from European black pine (Pinus nigra Arn.) were investigated. Kraft-NaBH4 cookings were done under fixed cooking conditions by adding NaBH4 in the amounts of 0.5%, 1%, 1.5%, and 2% (oven-dried wood) to the cooking liquor. NaBH4-free kraft cooking was carried out as control. The results indicated that addition of NaBH4 to cooking liquor increased the pulp yield. However, the kappa number of pulp and beating time decreased. The brightness of kraft-NaBH4 papers was found to be higher than that of control papers. Addition of NaBH4 significantly decreased pulp viscosity and slightly decreased strength properties of papers.
’ INTRODUCTION Alkaline pulping was developed in 1851 by Watt and Burgess. Since then, most researchers have aimed to increase the pulp yield by reducing carbohydrate losses. In chemical pulping, pulp yield depends on holocellulose content of the wood. To understand the reasons of pulp yield variations during cooking, the mechanism of degradation and losses in polysaccharides caused by alkaline solutions should be known well.1 Cooking chemicals break carbohydrate chains by means of degradation reactions such as peeling and alkaline hydrolysis. By changing the structure of carbohydrate reducing end, pulp yield drops can be prevented. The reducing ends are transformed into the hydroxyl or carboxyl group by adding reducing (e.g., sodium borohydride) or oxidizing (e.g., anthraquinone and polysulfide) additives to cooking liquor. Thus, the reducing ends become stable and polysaccharides are not affected by the degradation reactions mentioned above. Pulp yield can be increased by up to 3-5% (on oven-dried wood) depending on the charge of the additives. A preliminary report on the use of NaBH4 in kraft pulping was published by Hartler in 1959.2 The results of this study revealed that NaBH4 increases the pulp yield. Subsequently, investigations in the 1960s and 1970s showed similarity with the results of Hartler.3-9 In the last decades, some investigations focused on feasibility of using boron compounds with different species and additive ratios in pulp and paper industry.10-20 In this scope, European black pine, a widespread species, is used with different ratios of NaBH4 to determine the effects on kraft pulp and paper properties in this study. ’ EXPERIMENTAL METHODS European black pine (Pinus nigra Arn.) spreads over more than 3.5 million hectares from western North Africa through southern Europe to Asia Minor.21 A single log of 102-year-old P. nigra Arn., obtained from the Bartin province of Turkey, was used in this study. It was chipped to 3.5-1.5-0.5 cm size for pulping. The wood chips were air-dried to a final humidity of 10% and stored in dry conditions. Specimens were sampled and prepared according to the TAPPI T 257 standard for chemical tests. The holocellulose,22 R-cellulose,23 and lignin (TAPPI T 222) contents of samples were r 2011 American Chemical Society
determined according to the relevant methods. The solubility properties were also determined based on alcohol-benzene (TAPPI T 204), cold-hot water (TAPPI T 207), and 1% NaOH (TAPPI T 212) methods. The conditions for the kraft cookings were as follows: 18% active alkali as Na2O, 35% sulfidity, 4:1 liquor/wood ratio, 170 °C cooking temperature, 90 min to cooking temperature, and 60 min at cooking temperature. NaBH4-added cookings were carried out by adding 0.5% (KB1), 1% (KB2), 1.5% (KB3), and 2% (KB4) NaBH4 (oven-dried wood) into the cooking liquor. NaBH4-free kraft cooking (K) was also done as control. In each experiment, 600 g of oven-dried wood chips were cooked in a 15-L electrically heated laboratory cylindrical-type rotary digester. At the end of pulping, the pressure was reduced to atmospheric pressure. After digestion, pulps were washed to remove black liquor and were disintegrated in a laboratory-type pulp mixer with 2-L capacity. Disintegrated pulps were screened using a Somerville-type pulp screen with 0.15-mm slotted plate (TAPPI T 275). Pulps were then beaten to 35 and 50°SR in a Valley Beater according to TAPPI T 200. Kappa number (TAPPI T 236), screened yield (TAPPI T 210), viscosity (SCAN-CM 15-62), and freeness of pulps (ISO 5267-1) were determined according to relevant methods. Handsheets (75 g/m2) made by a RapidKothen Sheet Former (ISO 5269-2) were conditioned (TAPPI T 402). Determination of tensile index (TAPPI T 494), burst index (TAPPI T 403), tear index (TAPPI T 414), opacity (TAPPI T 519), brightness (TAPPI T 525), roughness (ISO 8791-2), and air permeability (ISO 5636-3) of the handsheets were performed in accordance with relevant standard methods.
’ RESULTS AND DISCUSSION The lignin, holocellulose, and R-cellulose content of European black pine used in this study were determined as 27.84%, 69.01%, and 50.99%, respectively. Also, solubility values of European Received: September 30, 2010 Accepted: December 17, 2010 Revised: December 6, 2010 Published: January 19, 2011 2441
dx.doi.org/10.1021/ie101999p | Ind. Eng. Chem. Res. 2011, 50, 2441–2444
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Figure 1. Viscosity versus kappa number with NaBH4 ratio as parameter.
black pine were found as 12.35%, 1.53%, 0.98%, and 4.52% in respect to 1% NaOH, hot water, cold water, and alcohol-benzene. These results revealed that the chemical composition of European black pine used in this study was similar to results determined in a previous study of this species.24 The kappa number of pulp gives information regarding the efficiency of cooking, residual lignin content in pulp, and the delignification degree of pulp. The kappa number of K, KB1, KB2, KB3, and KB4 pulps was determined as 46.39, 44.70, 43.74, 44.40, and 37.47, respectively. The lowest kappa number was found in 2% NaBH4-added KB4 cooking. The kappa number decreased with addition of NaBH4. Earlier studies have shown similar results.10-16 Strong correlation (r = 0.95) was seen between pulp viscosity and the kappa number of kraft and kraft-NaBH4 pulps (Figure 1). This finding indicates that the kappa number can be used to estimate the pulp viscosity for kraft and kraftNaBH4 methods. The pulp viscosity is related to the DP of polysaccharides (especially cellulose). Therefore, the degradation of polysaccharides during cooking leads to a decrease in pulp viscosity. The pulp viscosities of K, KB1, KB2, KB3, and KB4 pulps were determined to be 1281.12, 1170.65, 1225.41, 1153.24, and 1003.74 cm3/g, respectively. The lowest viscosity was found in 2% NaBH4-added KB4 cooking. Pulp viscosity of kraft-NaBH4 pulp was lower than that of kraft pulp. The cause of pulp viscosity decrease can be attributed to higher hemicellulose retention in kraft-NaBH4 pulps. The pulp viscosity results in earlier studies related to kraft and kraft-NaBH4 pulping were not found to be consistent with others. Some authors reported that pulp viscosity is decreased by adding NaBH4.12,14 Conversely, the others stated that pulp viscosity is increased by adding NaBH4.10,11,13,15 The kraft method has a relatively low pulp yield. Therefore, researchers and producers aim to increase pulp yield using modified kraft methods such as kraft-AQ, kraft-PS, and kraftNaBH4. Earlier studies have indicated that pulp yield is increased by adding NaBH4.2-17 The total pulp yield of K, KB1, KB2, KB3, and KB4 pulps was determined as 48.88%, 51.65%, 52.20%, 53.15%, and 52.55%, respectively. This result corresponds well with earlier investigations related to kraft-NaBH4 pulping. The cause of the pulp yield increase can be ascribed to the prevention of degradation reactions by NaBH4 during cooking. As can be seen in Figure 2, there is a significant correlation (r = 0.52) between total pulp yield and the kappa number of kraft and kraft-NaBH4 pulps. On the other hand, the screened pulp yield values were found to be 46.42%, 49.02%, 49.47%, 50.13%, and 50.80% in respect to K, KB1, KB2, KB3, and KB4. In the KB3 cooking with the highest total yield, the yield increased by 4.27% (from 48.88% to 53.15%). If this increase ratio is considered in a 500 tons/day pulp mill, it means 43.68 tons/day of additional
ARTICLE
Figure 2. Total yield versus kappa number with NaBH4 ratio as parameter.
Figure 3. Beating time versus kappa number with NaBH4 ratio as parameter.
pulp production which provides an important economical benefit to the pulp mill. The addition of digester additives also allows for more efficient utilization of wood resources. The reject ratio, which provides information about pulping efficiency and penetration of cooking liquor into chips, of K, KB1, KB2, KB3, and KB4 pulps were found to be 2.46%, 2.63%, 2.73%, 3.02%, and 1.75%, respectively. The reject ratio increased with the addition of NaBH4 except the case of 2% NaBH4 added KB4 cooking. In this case, the reject ratio results of KB4 cooking showed similarity with literature.10-16 Beatability of pulps is very important for the energy consumption of mills and generally depends on the chemical composition of pulps. The beating time decreased with addition of NaBH4 (Figure 3). Increasing the ratio of NaBH4 led to easier beating. The beating time reduction means energy saving during the beating in refiner. Better response to beating of kraft-NaBH4 pulps can be ascribed to the high hemicellulose content and to the low lignin content. The correlation between the beating time and the kappa number of beaten kraft and kraft-NaBH4 pulps was positively linear, as seen in Figure 3. The tear, burst, and tensile indices of handsheets obtained from beaten and unbeaten kraft-NaBH4 pulps were determined to be lower than those of kraft pulp (Table 1). Furthermore, the burst index of KB2 pulp and the tensile index of KB1 pulp were found to be higher than those of kraft pulp. The highest strength losses in NaBH4-added beaten pulps were determined in 2% NaBH4-added pulp (KB4). Similar strength losses were reported in literature.11-13 Strength losses of pulp can be ascribed to increases in pulp yield. Increasing pulp yield leads to decrease in the fiber amount per unit weight of oven-dried pulp and cellulose/ hemicellulose ratio of pulp. The correlation between the tear index and tensile index of unbeaten and beaten to 35°SR and 50°SR kraft and kraft-NaBH4 pulps was positively linear (r = 0.42, r=0.56, and r=0.85, respectively). This result shows that the tear index and tensile index of beaten and unbeaten pulps decrease in parallel with the addition of NaBH4 to the pulps. 2442
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Table 1. Some Mechanical, Surface, and Optical Properties of Kraft and Kraft-NaBH4 Handsheets cooking
NaBH4 ratio
freeness level
tear index
burst index
tensile index
air permeability
roughness
opacity
brightness
(%)
(°SR)
(mN 3 m2/g)
(kPa 3 m2/g)
(N 3 m/g)
(mL/min)
(mL/min)
(%)
(%)
unbeaten
30.5
2.7
45.3
1191.1
99.57
19.62
35
14.4
6.9
103.7
36.9
669.2
98.87
17.29
50
13.4
6.9
99.8
5.3
605.9
98.92
17.20
unbeaten
29.5
2.7
47.6
1143.3
99.50
20.00
35
13.8
6.3
95.3
53.5
637.0
98.65
17.81
50
13.0
6.6
94.6
8.3
653.3
98.28
17.28
unbeaten 35
25.3 12.6
2.9 6.1
45.5 95.3
55.4
1095.7 584.7
99.14 98.46
20.46 18.27
8.6
kraft
KB1
KB2
KB3
KB4
0.5
1
1.5
2
50
12.5
6.5
95.3
unbeaten
24.0
2.6
45.2
35
12.2
6.2
97.2
50
12.3
6.3
87.9
unbeaten
27.3
2.7
44.9
35
13.2
5.7
93.5
50
12.3
6.1
91.9
599.9
98.36
17.24
1188.8
99.51
20.39
70.3
586.5
98.94
18.43
20.3
622.3
98.67
17.84
1143.0
99.36
21.29
90.0
651.1
98.83
19.35
14.4
796.0
98.09
18.66
additive. Moreover, it decreases paper strength. If paper strength is not a necessity, NaBH4 can be used in both pulping and bleaching. Also, more studies on cheaper production technology of NaBH4 have to be carried out.
’ AUTHOR INFORMATION Corresponding Author
*Tel.: þ90 378 2235073. Fax: þ90 378 2235066. E-mail:
[email protected]. Figure 4. Relationship between brightness and kappa number of unbeaten and beaten kraft and kraft-NaBH4 pulps.
The brightness of handsheets obtained from beaten and unbeaten pulps was found to increase with the addition of NaBH4. The brightness of pulps increased in direct proportion to the increase of the NaBH4 ratio. The highest brightness was determined in KB4 pulp. These results were in quite good agreement with literature.11-13,16 Figure 4 shows a strong correlation between brightness and the kappa number of unbeaten and beaten to 35°SR and 50°SR kraft and kraft-NaBH4 pulps. This result indicates that the kappa number can be used to estimate the brightness of kraft and kraft-NaBH4 pulps. The opacity of handsheets obtained from beaten and unbeaten pulps was decreased by adding NaBH4. This result was similar to earlier studies reported in the literature.12,16 The air permeability results of handsheets obtained from beaten pulps showed that the air permeability increased with the addition of NaBH4 (Table 1). Conversely, another study showed that air permeability decreased with the addition of NaBH4.13 The roughness of handsheets obtained from unbeaten and beaten to 35°SR pulps was increased by adding NaBH4 (Table 1). However, the roughness was decreased in handsheets obtained from beaten pulps to 50°SR freeness level.
’ CONCLUSIONS Using NaBH4 in kraft pulping offers various benefits. The most important advantage of using NaBH4 is the significant yield increase. Other advantages are energy savings during refining and increased pulp brightness. However, NaBH4 is an expensive
’ ACKNOWLEDGMENT This research was supported by The Scientific and Technological Research Council of Turkey (TUBITAK, Project 107M208). We especially thank TUBITAK for their support. Also, thanks to the OYKA Pulp and Paper Mill technical staff for their technical assistance. ’ REFERENCES (1) Courchene, C. The tried, the true, and the new: getting more pulp from chips modifications to the kraft process for increased yield. In Breaking the Pulp Yield Barrier Symposium; TAPPI: Atlanta, GA, 1998; pp 11-20. (2) Hartler, N. Sulphate cooking with the addition of reducing agents. Part 1. Preliminary report on the addition of sodium borohydride. Sven. Papper. 1959, 62 (13), 467–470. (3) Pettersson, S. E.; Rydholm, S. A. Hemicelluloses and paper properties of birch pulps, Part 3. Sven. Papper. 1961, 64 (1), 4–17. (4) Meller, A. Retention of polysaccharides in kraft pulping. Part 1. The effect of borohydride treatment of Pinus radiata wood on its alkali stability. Tappi 1963, 46 (5), 317–319. (5) Aurell, R; Hartler, N. Sulphate cooking with the addition of reducing agents. Part III. The effect of sodium borohydride. Tappi 1963, 46 (4), 209–215. (6) Annergren, G.; Rydholm, S. A; Vardheim, S. Influence of raw material and pulping process on the chemical composition and physical properties of paper pulps. Sven. Papper. 1963, 66 (6), 196–210. (7) Meller, A.; Ritman, B. L. Retention of polysaccharides in kraft pulping. Part II. The effect of borohydride addition to kraft liquor on pulp yield, chemical characteristics and papermaking properties of Pinus radiata pulps. Tappi 1964, 47 (1), 55–64. 2443
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