Iso-concentration hydrogen peroxide bleaching of poplar chemi-thermomechanical pulp

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1 Journal of Bioresources and Bioproducts. 2018, 3(1): ORIGINAL PAPER DOI: /jbb.v3i1.110 Iso-concentration hydrogen peroxide bleaching of poplar chemi-thermomechanical pulp Xianqi Sun, Qingxi Hou* Tianjin Key Laboratory of Pulp & Paper, Tianjin University of Science & Technology, Tianjin , China *Corresponding author: ABSTRACT Maintaining a constant chemical concentration (Iso-concentration, IC) has been found to be an effective method to improve peroxide bleaching efficiency for chemi-thermomechanical pulp (CTMP). In this study, the mechanism of IC bleaching was investigated. The concentrations of H2O2 and total alkali in IC bleaching were monitored and compared with those in conventional bleaching processes. Control experiments without pulp were carried out to explore the effects of bleaching additives on H2O2 decomposition. The results showed that the concentrations of H2O2 and the total alkali at the early and later stages of IC bleaching were relatively low and high, respectively; thus, undesired decomposition of H2O2 was inhibited, and brightness was improved. Moreover, the stabilizer played an important role in inhibiting the undesired H2O2 decomposition. This may explain why high bleaching efficiency was achieved in IC bleaching. These findings would provide valuable insight for the production of high-brightness bleached chemi-thermomechanical pulp (BCTMP). 1. INTRODUCTION In virtue of high yield, low cost and low environmental impact, chemi-mechanical pulping has become a key pulping method.1 Unlike chemical pulping, lignin is maintained in the bleaching process of chemi-mechanical pulping to increase the pulp yield. Hydrogen peroxide can alter the lignin chromophore structure to improve pulp brightness, instead of elimination of lignin. Hence, it has been widely applied to bleach chemi-mechanical pulp.2 Peroxide bleaching in chemi-mechanical pulping is a sophisticated process. The color of pulp is dependent on several chromophores (e.g., coniferyl aldehyde, α-carbonyl groups, and quinone groups) in lignin. Also, colorless phenolic groups in lignin may be converted into colored structures by condensation reactions at elevated temperatures.2 The reaction rate of chromophore destruction proceeds via a rapid phase initially followed by the second phase, which is much slower and asymptotically approaches a minimum of chromophore concentration.3 This minimum in concentration may be improved by increasing the concentrations of the chemicals in the second phase. Chemical concentrations in the bleaching system are also related to the decomposition of hydrogen peroxide. The high chemical concentrations in the bleaching system, especially that of hydrogen peroxide and total alkali, can accelerate the decomposition of hydrogen peroxide and lead to an undesirable increase in the consumption of hydrogen peroxide. Although low chemical concentrations are beneficial for inhibiting the decomposition of hydrogen peroxide, they often result in lower brightness of bleached pulp.4 In the traditional bleaching processes, all the bleaching chemicals are added simultaneously, resulting in undesirable chemical concentrations in the bleaching system. In addition, the bleaching reaction between lignin and peroxide is accompanied by various side reactions, including the decomposition of peroxide and those between peroxide and carbohydrates.5 Therefore, as peroxide is consumed in side reactions, it is associated to the chemical concentrations in the bleaching system, and this will have an impact on the bleaching efficiency. Apart from the chemical concentrations, other factors, such as temperature, reaction time, pulp consistency, and penetration of bleaching agents also affect the peroxide bleaching reaction to some extent.1,6 In high-consistency bleaching, the pulp consistency is typically above 30%. As a result, the rate of reaction for bleaching increases, resulting in a reduced reaction time and a high brightness of the bleached pulp. In displacement bleaching, the bleaching solution is passed through a bed of relatively stationary fibers. In this process, the water membranes on the surface of the fibers, which may retard the permeation of the bleaching reagent, are broken. The advantages of this process are the reduction of reaction times, and higher bleaching efficiencies.7-10 An iso-concentration (IC) bleaching method has recently been reported by Hou.11 In this study, the bleaching efficiency was significantly improved by continuously replenishing the reagents. To further understand the mechanism of the IC bleaching, some chemical concentrations in the bleaching system were investigated and compared to those in the conventional bleaching processes. 2. EXPERIMENTAL 2.1 Materials Poplar CTMP with an ISO brightness of 46.07% was 35

2 collected from a pulp mill in Shandong Province, China. The sampling site was located at the discharge outlet of the HC refiner prior to the bleaching step. The pulp sample was first washed with deionized water, then thickened to a consistency of about 30%, and stored in a cold storage room at approximately 4 C. 2.2 Chemicals The chemicals used in this study were: Na 2 SiO 3 (41 ºBé) purchased from the China PQ (Tianjin) Silicates Technology Co. Ltd., Stabilizer P provided by the Chunjiang Chemicals Co. Ltd (Changzhou, China), magnesium hydroxide emulsion (content of magnesium hydroxide: 80 g/l) purchased from the Haililong Magnesium Technology Co. Ltd (Weifang, China), and Diethylenetriaminepentaacetic acid (DTPA) purchased from Akzo Nobel N.V (Amsterdam, Netherlands). Unless otherwise specified, all chemicals used were of analytical grade. Figure 1. Apparatus for IC bleaching (1- thermostatic bath, 2- reactor, 3- stirrer, 4- peristaltic pump for H 2 O 2, 5- plastic container for H 2 O 2, 6- computer, 7- plastic container for NaOH, 8- peristaltic pump for NaOH) 2.3 IC experimental apparatus The designed IC bleaching test device in the present study (Figure 1) was used to control the chemical concentrations at their constant levels. A 2L beaker, used as the bleaching reactor, was placed in a water bath at constant temperature. A mixer was used for mixing the pulp and chemicals, as well as for continuous stirring during the bleaching process. Two peristaltic pumps (Model: BT100S-YZ15; Baoding Leadfluid technology Co., Ltd., Hebei, China) continuously replenished the chemical solutions into the pulp as bleaching proceeded. The bleaching chemicals for replenishment were stored in two separate 500 ml plastic bottles. The flow rates of each peristaltic pump were controlled by the software V1.0 (provided by the pump manufacturer), so that the amount of chemical added matched the amount consumed during the chemical reaction IC experimental procedure The IC bleaching process was divided into two phases: one, constant chemical concentration, and two, residual peroxide consumption. For constant chemical concentration, g of oven-dried unbleached CTMP was placed in the reactor and then diluted with deionized water to a pulp consistency of 5%, followed by mixing at 200 rpm. When the pulp formed a stable suspension and the temperature reached 75 C, the following bleaching additives were added to the pulp: 0.2% Mg(OH) 2, 0.3% stabilizer P, and 0.2% DTPA. Subsequently, the following chemicals were successively added to the pulp according to their initial concentrations in the bleaching system (Na 2 SiO 3 : 2.5 g/l; NaOH: 0.5 g/l; H 2 O 2 :4.5 g/l). Finally, the pulp was diluted to 4.0% consistency, and bleached at 75 C for 50 minutes. During bleaching, consumed chemicals were replenished by pumping the 60 g/l H 2 O 2 and 30 g/l NaOH solutions into the bleaching system according to the set flowrate. After bleaching, the pulp in the reactor was transferred to a cloth bag and thickened to a consistency of about 30%. The resultant filtrate was cooled down to room temperature for analysis. For residual peroxide consumption, g of the thickened wet pulp (about 18.0 g of oven-dried pulp) was bleached in a plastic bag at 90 C for 70 minutes. After bleaching, the residual H 2 O 2 concentration in the pulp and the final pulp brightness were measured. The bleaching efficiency was also calculated according to the following formula: Bleaching efficiency = brightness gains (%ISO)/H 2 O 2 consumption on pulp (%) 2.5 Comparison experiment The high consistency (HC) and low consistency (LC) bleaching experiments were conducted as comparison experiments of the LC bleaching. The fractions of bleaching chemicals, commonly used in pulp mills, were as follows: H 2 O 2, 5.5%; NaOH, 2.75%; Na 2 SiO 3, 3%; and DTPA, 0.2%. The bleaching experiments were conducted with 40 g (od) pulp in plastic bags under conditions of 25% (for HC bleaching) and 4% (for LC bleaching) consistencies at 90 C for 120 minutes. After bleaching, the residual H 2 O 2 concentration in the pulp and the pulp brightness were measured. The bleaching efficiency was also determined. To monitor the chemical concentrations in the bleaching process, the experiments for the IC, HC and LC bleaching were routinely sampled to measure the total alkali concentrations and the peroxide concentrations at times of 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90 minutes, respectively. To estimate the amount of hydrogen peroxide consumed when mixed with the bleaching chemicals, the blank experiments were conducted in the absence of pulp. 12 The experimental conditions and results are shown below in Table 1. The chemicals were mixed with deionized water in a beaker and placed in the water bath at 90 C. At the end of the reaction, the concentration of the residual hydrogen peroxide was determined. 36

3 Table 1. Effect of bleaching additives on peroxide self-decomposition Process type Non-stabilizer Stabilizers Temperature, C Time, min H 2 O 2 concentration, g/l 9 9 NaOH concentration, g/l 2 2 Na 2 SiO 3 concentration, g/l Mg(OH) 2 concentration, mg/l 0 5 Stabilizer concentration, mg/l 0 4 Residual rate of H 2 O 2, % Analysis and measurement The pulp handsheets for brightness measurements were prepared according to the ISO Standard 3688: 1999 (E), and the pulp brightness was measured according to the ISO standard test method : 2009 using an L&W Elrepho Autoline 300 module. FT-IR analysis of pulps was carried out with Fourier Transform Infrared Spectroscopy FTIR-650 module. An AT-510 automatic titrator (Kyoto Electronics Mfg. Co. Ltd., Japan) was used for the analysis of the bleaching solution RESULTS AND DISCUSSION 3.1 Bleaching efficiencies In the comparison of the bleaching efficiencies achieved in various bleaching processes (see Table 2), it can be found that the bleaching efficiency in the IC bleaching process was 16.72, being much higher than that in other processes (7.38 for the HC bleaching process, for the LC bleaching process, and for the displacement bleaching process 10 ). The result indicated that the bleaching efficiency can be improved significantly for the IC bleaching process. Table 2. Bleaching conditions and results for different bleaching processes Bleaching process IC HC LC ISO brightness of unbleached pulp, % ISO brightness of bleached pulp, % Charge of H 2 O 2, % Consumption of H 2 O 2, % Initial H 2 O 2 concentration, g/l Initial total alkali concentration, g/l Bleaching temperature, C Bleaching time, minute Bleaching efficiency For comparison of hydrogen peroxide consumption in different bleaching processes (Table 2), the hydrogen peroxide consumption in the IC bleaching process was 1.77%, which was lower than that in the HC bleaching process (4.11%). However, the ISO brightness values of the bleached pulp in these two bleaching processes were very similar (75.68% and 76.42%). Previous studies reported that the majority of hydrogen peroxide is consumed in its decomposition reaction, but only a small fraction is consumed during bleaching reactions. 14,15 The results indicate that the decomposition of hydrogen peroxide in the IC bleaching process can be inhibited to some extent, resulting in reduced hydrogen peroxide consumption and improved bleaching efficiency. 3.2 Influence of IC bleaching process conditions on the bleaching reaction Effect of hydrogen peroxide and alkali concentrations The concentrations of hydrogen peroxide and total alkali in the three bleaching processes (IC, HC, and LC) were compared and are shown in Figure 2 below. The initial concentrations of hydrogen peroxide and total alkali in the HC bleaching were 19.5 g/l and 10.3 g/l, respectively, which reduced sharply in the early bleaching stage and then decreased slowly after approximately 20 min. The initial concentrations of hydrogen peroxide and total alkali in the LC bleaching process were 2.89 g/l and 1.63 g/l, respectively. The initial concentrations of total alkali and hydrogen peroxide in the IC bleaching were 4.5 g/l and 0.79 g/l, respectively, which were held constant for 50 min. It can be seen that the hydrogen peroxide concentration in the IC bleaching was relatively low, while the total alkali concentration was also low initially, but remained higher than that in both the LC bleaching and HC bleaching after 5 and 15 min. Previous studies reported that high chemical concentrations in the initial stage of bleaching causes much of ineffective decomposition, but the excessively low chemical concentrations in the mid-to-late stages commonly led to a low brightness of bleached pulp, resulting in low bleaching efficiency. 14,15 Obviously, the aforementioned issue was resolved to an extent in the IC bleaching process. In addition, a relatively low total alkali concentration in the IC bleaching process (0.79 g/l, the ph value of 10.46) was also beneficial to the reinforcement of chelation of chelating agents, weakening the negative influence of metallic ions on hydrogen peroxide decomposition. Therefore, the bleaching efficiency was improved significantly in the IC bleaching Effect of pulp consistency By comparing the pulp consistency for different bleaching processes (Table 2), it can be found that the pulp consistency in the IC bleaching process was only 4%, which is lower than 25% for the HC bleaching process. However, the ISO brightness levels of bleached pulp in both bleaching processes were very close (75.68% and 76.42%). This indicates that the lower bleaching consistency for the IC 37

4 bleaching process did not lead to a significant reduction in pulp brightness. This result is different from that of which literature reports. 1,2 The continuous chemical upkeep in the IC bleaching process may play an important role in achieving a high brightness of the bleached pulp at a low pulp consistency. a H 2 O 2 (g/l) b Total alkali (g NaOH/L) Time (min) Isoconcentration bleaching High-consistency bleaching Low-consistency bleaching Isoconcentration bleaching High-consistency bleaching Low-consistency bleaching Time (min) 3.3. FT-IR analysis of the bleached pulps To investigate the variation of the chemical composition of pulp in the different bleaching processes, the infrared spectroscopic analysis was conducted on the unbleached and bleached pulps (Figure 3). The absorption peaks of the bleached pulps at 1740 cm -1 disappeared for these two bleaching processes. This result implies that the deacetylation and breakage of other lipid bonds may take place. 10,16 In addition, other peak positions of the bleached pulps are nearly the same in these two bleaching processes. This implies that the chemical composition of these two bleached pulps may change almost identically for both bleaching processes. Figure 2. Chemical concentrations in different bleaching systems Effect of reaction temperature By comparing the bleaching temperature for different bleaching processes (Table 2), it can be found that the bleaching temperature in the IC bleaching process is the lowest (75 C). A low temperature is helpful for the inhibition of hydrogen peroxide decomposition, 2 resulting in lower consumption. Therefore, the bleaching efficiency was improved Effect of peroxide stabilizers The effects of the stabilizers on peroxide decomposition were investigated in the bleaching system without pulp to estimate the decomposition behavior of hydrogen peroxide in the bleaching system (Table 1). It can be found that after adding magnesium hydrate and stabilizer P, the residual amount of hydrogen peroxide reached 46.7%, being 13.4% higher than that of the control sample. This result indicates that these two stabilizers have favorable inhibition effects on the decomposition of hydrogen peroxide. The addition of these two stabilizers in the IC bleaching process may also improve the bleaching efficiency. 4. CONCLUSIONS Figure 3. FT-IR analysis of the pulps A high bleaching efficiency achieved in the IC bleaching process may be attributed to constant chemical concentrations, which were relatively lower in the initial bleaching stage to inhibit the decomposition of hydrogen peroxide, and relatively higher in the late bleaching stage to increase the brightness of the bleached pulp. In addition, lower reaction temperatures and the presence of a peroxide stabilizing agent also contributed to the improved bleaching efficiency. REFERENCES 1. Sundholm J. Mechanical pulping. In Gullichsen J, Paulapuro H (ed) Papermaking science and technology, book 5. Finnish Paper Engineers' Association and TAPPI. Helsinki, Ek M., Gellerstedt G., Henriksson G. Pulping chemistry and technology. In: Pulp and paper chemistry and technology, volume 2. KTH-Royal Institute of Technology, Stockholm, Huan Wang, Zhibin He, and Yonghao Ni. Model of the Magnesium Hydroxide Based Peroxide Bleaching Process 38

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