Advanced Materials Research Online: 2013-05-14 ISSN: 1662-8985, Vols. 690-693, pp 1455-1460 doi:10.4028/www.scientific.net/amr.690-693.1455 2013 Trans Tech Publications, Switzerland Using of magnesium hydroxide and sodium hydroxide in DP beaching of NaOH-AQ wheat straw pulp Wenbo Wang 1,a, Fangong Kong *1,b, Shoujuan Wang 1,c, Yajie Zhong 1,d, Guihua Yang 1,e 1 Key Laboratory of Pulp and Paper Science and Technology of Education Ministry of China, Shandong Polytechnic University, Ji nan, Shandong, 250353, P. R. China. a wenbochn@163.com, * b kfgwsj1566@163.com, c nancy5921@163.com, d shandongzhongyajie@163.com, e ygh2626@126.com Keywords: hydrogen peroxide, chlorine dioxide, bleaching, wheat straw pulp, Magnesium hydroxide Abstract. Sodium hydroxide (NaOH) substituted by magnesium hydroxide (Mg(OH) 2 ) as control agent and alkali source were investigated in chlorine dioxide bleaching and hydrogen peroxide bleaching of NaOH-AQ wheat straw pulp. The effects of alkali sources on pulp properties, including brightness and tensile strength, were investigated. The four experimental groups, including D NaOH P NaOH, D NaOH P Mg(OH)2, D Mg(OH)2P NaOH and D Mg(OH)2 P Mg(OH)2, were studied in this paper. The results showed that the brightness and tensile strength of D Mg(OH)2 P Mg(OH)2 pulp was slightly lower than that of D NaOH P NaOH pulp. But there were more residual ClO 2 and residual H 2 O 2 in the effluents of D Mg(OH)2 P Mg(OH)2 bleaching process than that in the effluents of D NaOH P NaOH bleaching process, which illustrated that D Mg(OH)2 P Mg(OH)2 bleaching process had higher bleaching efficiency than D NaOH P NaOH bleaching process. The influence of Mg(OH) 2 replacing NaOH was more obvious when it was used in the bleaching process of P stage than that in the D stage. The replacement of NaOH with Mg(OH) 2 in D Stage can lower the dosage of alkali used in the following P stage. Introduction A number of studies have shown that magnesium hydroxide (Mg(OH) 2 ) is a promising weak alkali to substitute for sodium hydroxide (NaOH). Some scholars pointed out that Mg(OH) 2 -based ClO 2 brightening stages can cut down the formation of chlorate and chlorite, which improved the final brightness[1]. Some literatures also supported the conclusion that using Mg(OH) 2 instead of NaOH as the alkali source in peroxide bleaching of spruce SGW gave a lower tensile strength and higher brightness[2]. Fangong Kong et al[3]. found that, for a aspen chemi-thermomechanical pulp (CTMP), the Mg(OH) 2 -based peroxide bleaching gave lower brightness and tensile strength but higher pulp yield and peroxide bleaching efficiency, compared with the NaOH-based peroxide bleaching. The objective of this study was to investigate the use of Mg(OH) 2 as a replacement of NaOH as alkali source in chlorine dioxide bleaching and peroxide bleaching of NaOH-AQ wheat straw pulp. Experiment Material A NaOH-AQ wheat straw pulp which was made in the laboratory was used for the bleaching experiments. And its Kappa number was 13.4. The pulp sample was stored in a refrigerator until use; Chlorine dioxide was provided by Shandong Shanda Huate Technology Co., Ltd.; other chemicals were all reagent grades. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, www.ttp.net. (ID: 130.203.136.75, Pennsylvania State University, University Park, USA-05/03/16,01:26:44)
1456 Materials Design, Processing and Applications Bleaching Process The Chlorine dioxide bleaching experiments were conducted in plastic bags using the following conditions: 20kg/t active chlorine, 3kg/t EDTA, 10% pulp consistency, 60 and 45min. After adding all bleaching liquor to the pulp, a good mixing was provided by kneading. The plastic bag was sealed and placed into a water bath for the desired retention time. The hydrogen peroxide bleaching experiments were also conducted in plastic bags, the experiment conditions were 22kg/t H 2 O 2, 0.3kg/t MgSO 4, 10% pulp consistency, 90 and 240min. The process were similar to the chlorine dioxide bleaching experiments. Analysis Methods The value of ph was detected by PHS-3C precision detector; Residual ClO 2 and residual H 2 O 2 were obtained by the method of iodometry. The PTI standard sheet forming machine was used to forming paper of 60g/m 2. and tensile strength were detected according to ISO methods. Results and Discussion Using Mg(OH) 2 as alkali in D bleaching stage and P bleaching stage of NaOH-AQ wheat straw pulp were conducted in this experiment. The four experimental groups D NaOH P NaOH, D NaOH P Mg(OH)2, D Mg(OH)2P NaOH and D Mg(OH)2 P Mg(OH)2 were experimented. Influences of NaOH charge used in D stage on pulp bleaching The value of ph is very important in D bleaching process, the active substance derived from ClO 2 will become ineffective when the ph is either too high or too low. However the value of ph is decided by alkali charge. So, a suitable charge of alkali is very important to D bleaching stage[4]. (%ISO) 75 3.7 70 3.6 3.5 65 3.4 60 3.3 3.2 55 3.1 5.1 5.5 5.7 6.1 Charge of NaOH(%) Tensile strength(kn/m) Fig.1 Influence of NaOH charge on D bleaching Table 1 The correspondence of charge of NaOH and end ph in D bleaching Charge of NaOH[%] 5.1 5.5 5.7 6.1 End ph 2.5 4.2 4.5 6.0 Fig.1 shows the result of using NaOH in D bleaching stage. According to Fig.1, the brightness and tensile strength both increased at first and then decreased with the increasing charge of NaOH. The max brightness and tensile strength were gained when the charge of NaOH was 5.5%(based on O.D. pulp). And the most appropriate ph for D bleaching stage of NaOH-AQ wheat straw pulp was 4.2, refer to Table 1. Using of Mg(OH) 2 and NaOH in P stage of D NaON P bleaching process Some scholars pointed out that the HOO- derived form alkaline H 2 O 2 was considered to be the active chemical group in P bleaching stage[4]. The simple mechanism of P bleaching process as follows:
Advanced Materials Research Vols. 690-693 1457 H 2 O 2 +OH - HOO - + Chromophoric Group HOO - +H 2 O Colorless group The equations show that, H 2 O 2 can t generate HOO - sufficiently if the amount of OH - is too little. But if the amount of OH - is too much, there will be a lot of HOO - produced at the beginning of P bleaching process, which may be a big waste of bleaching agents. And this phenomenon can also result in only very small part of HOO - remained in the end of P bleaching stage which will obviously lower the brightness and tensile strength of bleached pulp. Moreover, large amount of alkali can result in evident degradation of cellulose which will decrease the strength of pulp. So, the suitable amount of alkali is very important to the P bleaching stage. In this part, the using of Mg(OH) 2 and NaOH as alkali sources in P stage of D NaOH P bleaching process, D NaOHP NaOH and D NaOH P Mg(OH)2, were included. The following figures are the results of NaOH- and Mg(OH) 2 -based P bleaching for D NaOH pulp. (%ISO) 86 85 84 83 82 81 80 0.8 1.0 1.2 1.5 1.8 Charge of NaOH(%) Fig2. Influence of NaOH used in P stage on pulp properties 3.60 3.55 3.50 3.45 3.40 3.35 3.30 (kn/m) (%ISO) 85.0 84.5 84.0 83.5 83.0 82.5 0.9 1.1 1.3 1.5 1.7 2.0 Charge of Mg(OH) 2 (%) 3.40 3.38 3.36 3.34 3.32 3.30 3.28 3.26 3.24 Fig3. Influence of Mg(OH)2 used in P stage on pulp properties (kn/m) According to literatures, NaOH can provide efficient OH - when used in P bleaching, but the milder alkalinity of Mg(OH) 2 system can provide average amount of OH - during the whole process of P bleaching. So, it can result in average amount of HOO - produced which can enhance the efficiency of H 2 O 2 when using Mg(OH) 2 as alkali in P bleaching process[2-3]. In addition, the introducing of Mg 2+ by Mg(OH) 2 can play the same role as MgSO 4 in stabilizing H 2 O 2 and protecting carbohydrate. So, the MgSO 4 can be totally substituted when using Mg(OH) 2 as alkali in p bleaching stage. Fig.2 shows the influence of NaOH used in P stage on D NaOH pulp bleaching. The brightness and tensile strength were all increased at first and then decreased with the increasing of alkali. And the maximum brightness 85.0%ISO and maximum tensile strength 3.55kN/m were obtained when the charge of NaOH was 1.2%. Fig.3 shows the influence of Mg(OH) 2 used in P bleaching process of D NaOH pulp. As shown in Fig.3, when the charge of Mg(OH) 2 was 1.2%, the pulp gained the maximum brightness and maximum tensile strength, they were 84.5%ISO and 3.39kN/m respectively. The maximum brightness and maximum tensile strength in P bleaching stage used different alkali sources were similar, comparing Fig.2 with Fig.3. The little higher tensile strength of D NaOH P NaOH pulp was obtained, which may be due to that the strong base of NaOH can swell the cellulose and cause interweave of fibers.
1458 Materials Design, Processing and Applications Influence of Mg(OH) 2 used in D stage on bleaching Mg(OH) 2 is a weak base, which can provide a relatively stable condition in bleaching process. So, the bleaching agent can be kept in the optimal condition for D bleaching when Mg(OH) 2 was used as ph control agent. (%ISO) 66 64 62 60 58 56 3,15 3,99 4,05 4,07 4,11 Charge of Mg(OH) 2 (%) Fig.4 Influence of Mg(OH) 2 charge on D bleaching 3,35 3,30 3,25 3,20 Tensile strength(kn/m) Table 2 The correspondence of charge of Mg(OH) 2 and end ph in D bleaching Charge of Mg(OH) 2 [%] 3.99 4.05 4.07 4.11 End ph 3.3 4.4 5.6 6.5 Fig.4 shows the results of Mg(OH) 2 used in D stage on bleaching. As shown in Fig.4, the brightness and tensile strength increased at first and then decreased with the increasing of Mg(OH) 2. When the charge of Mg(OH) 2 was 4.07%, the pulp had max. brightness and tensile strength. And the suitable ph was 5.6 according to Table 2. Using of Mg(OH) 2 and NaOH in P stage of D Mg(OH)2 P bleaching process In this part, the using of Mg(OH) 2 and NaOH as alkali sources in P stage of D Mg(OH)2 P bleaching process, D Mg(OH)2P NaOH and D Mg(OH)2 P Mg(OH)2, were included, the results are as follows. (%ISO) 85,6 85,4 85,2 85,0 84,8 84,6 84,4 3,5 0,59 0,62 0,63 0,67 charge of NaOH(%) Fig.5 Influence of NaOH used in P stage on D Mg(OH)2 pulp properties 3,9 3,8 3,7 3,6 Tensile strength(kn/m) Fig.5 shows the results of NaOH used in P bleaching stage of D Mg(OH)2 P process. According to Fig.5, the maximum brightness 85.5%ISO and maximum tensile strength 3.84kN/m were obtained when the charge of NaOH was 0.62% in P stage. According to Fig.6, the suitable charge of Mg(OH) 2 was 0.50% in P stage, and the corresponding brightness and tensile strength were 83.5%ISO and 3.31kN/m respectively.
Advanced Materials Research Vols. 690-693 1459 By comparing Fig.2 with Fig.3, Fig.5 with Fig.6, it can be found that when the charge of Mg(OH) 2 was more than optimum charge, the brightness and tensile strength decreased obviously. However, when using NaOH as alkali in DP bleaching, this decrease was very small. The reasons for this phenomenon is not clear enough until now. Comparing of DP bleaching using NaOH and Mg(OH) 2 The brightness and tensile strength were different when Mg(OH) 2 or NaOH was used as alkali in DP bleaching process. The bleaching results and conditions of DP bleaching process were listed in Table3. Table 3 Comparing of DP bleaching using NaOH and Mg(OH) 2 No. 1 2 3 4 Alkali charge in D stage NaOH=5.5% Mg(OH) 2 =4.07% NaOH=5.5% Mg(OH) 2 =4.07% Alkali charge in P stage NaOH=1.2% NaOH=0.62% Mg(OH) 2 =1.5% Mg(OH) 2 =0.5% [%ISO] 85.0 85.5 84.5 83.5 [kn/m] 3.42 3.84 3.39 3.31 Residual ClO 2 [mg/l] 23.5 / / 41.2 Residual H 2 O 2 [mg/l] 31.8 / / 60.9 As shown in Table 3, the brightness and tensile strength were a little decreased when using Mg(OH) 2 as alkali in DP bleaching process. But there were more residual ClO 2 when using Mg(OH) 2 as ph control agent of D stage and more residual H 2 O 2 when using Mg(OH) 2 as alkali source in P stage. It means that, it is more efficient when using Mg(OH) 2 as alkali source in DP bleaching process than using NaOH. This result may attribute to the reason that, Mg(OH) 2 can decompose into OH - slowly and keep sufficient active chemical groups existed in the whole process of bleaching. Table 3 also shows that using Mg(OH) 2 as ph control agent in D bleaching process can obviously cut down the charge of alkali used in subsequent P bleaching stage. Such as, according to Table 3, the charge of alkali used in P stage of No. 1 process and No. 3 process were 0.62% and 0.50%, respectively decreased 48.3% and 66.7%, compared to No. 2 process and No. 4 process. Conclusions Mg(OH) 2 substituted NaOH as alkali in DP bleaching of NaOH-AQ wheat straw pulp can result in higher efficiency of bleaching agents. When the charge of Mg(OH) 2 was more than optimum amount in DP bleaching process of NaOH-AQ wheat straw pulp, the brightness and tensile strength decreased obviously. But when using NaOH as the alkali in DP bleaching, this decrease was very small. There was a little difference on brightness and tensile strength when using Mg(OH) 2 substituted NaOH as alkali in DP bleaching of NaOH-AQ wheat straw pulp. Mg(OH) 2 used in D stage of DP bleaching process as ph control agent can obviously cut down the amount of alkali used in subsequent P stage. The changes of final brightness and tensile strength were more obvious when Mg(OH) 2 substituted NaOH in P stage than in D stage. The replacement of NaOH with Mg(OH) 2 in D stage can lower the dosage of alkali used in the following P stage.
1460 Materials Design, Processing and Applications Acknowledgements This work was financially supported by the National Natural Science Foundation of China (31170547) and Natural Science Foundation of Shandong(ZR2010CQ004). Reference [1] M. Sarwar Jahan; Yonghao Ni; Zhibin He: Chlorine Dioxide Bleaching of Soda-Anthraquinone Jute Pulp to a very high (BioResources, 2010,5(2) : 870-880). [2] Nystrom, M.; Pykalainen, j.; Lehto, J: Peroxide bleaching of mechanical pulp suing different types of alkali (Paperi Ja Puu 1993, 75(6), 419-425). [3] Fangong Kong; Yonghao Ni; Zhibin He. A Partial Magnesium Hydroxide Substitution for Sodium Hydroxide in Peroxide Bleaching of an Aspen CTMP (Journal of Wood Chemistry and Technology, 2009,29 : 136-149). [4] Laisu Xie; Huaiyu Zhan. Pulp and Paper Engineering (China Light Industry Press, 2008).
Materials Design, Processing and Applications 10.4028/www.scientific.net/AMR.690-693 Using of Magnesium Hydroxide and Sodium Hydroxide in DP Beaching of NaOH-AQ Wheat Straw Pulp 10.4028/www.scientific.net/AMR.690-693.1455