Accepted Manuscript Title: Oxalate formation during hydrogen peroxide-reinforced oxygen delignification Authors: Yingying Liu, Shujuan Ge, Youming Li, Bingyun Li, Hailong Li PII: DOI: Reference:
S1226-086X(17)30478-1 http://dx.doi.org/10.1016/j.jiec.2017.09.005 JIEC 3608
To appear in: Received date: Revised date: Accepted date:
21-12-2016 1-9-2017 1-9-2017
Please cite this article as: Yingying Liu, Shujuan Ge, Youming Li, Bingyun Li, Hailong Li, Oxalate formation during hydrogen peroxidereinforced oxygen delignification, Journal of Industrial and Engineering Chemistryhttp://dx.doi.org/10.1016/j.jiec.2017.09.005 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Oxalate formation during hydrogen peroxide-reinforced oxygen delignification
Yingying Liu, Shujuan Ge, Youming Li, Bingyun Li, and Hailong Li*
State Key Laboratory of Pulp and Paper Engineering, South China University of Technology Guangzhou, 510640, China
Corresponding author. Tel.: +86 2022236028. E-mail address: [email protected]
Abstract: The effects of different process parameters (alkali dosage, hydrogen peroxide dosage, temperature, reaction time, and oxygen pressure) on oxalate formation during hydrogen peroxide-reinforced oxygen delignification (OP) of Eucalyptus kraft pulp were investigated in the present study. The relationships between oxalate formation and the dosages of both alkali and peroxide were found to be almost linear. Oxalate formation could be divided into “fast” and “slow” periods, depending on the reaction time. The best selectivity for the OP process was achieved using an alkali dosage of 2.0%, a hydrogen peroxide dosage of 2.0%, a temperature of 100°C, an oxygen pressure of 0.6 MPa and a reaction time of 60 min. Oxalate formation was also, to some extent, reduced under these conditions. These results will be very helpful in optimizing the OP process, controlling oxalate formation and improving product quality.
Keywords: Oxalate, Hydrogen peroxide-reinforced oxygen delignification, Control, Kappa number, Viscosity
1. Introduction Bleaching, as applied to lignin, refers chiefly to bleaching of compounds that contain aromatic chromophores, which are broken down and removed after oxidation [1, 2]. The bleaching effluents typically contain a variety of organic and inorganic compounds and have low biodegradability and high chemical oxygen demand, which is used as an indirect measure of the amount of organic compounds present, and hence water quality . The organic compounds present in the bleaching
effluents include adsorbable organic halides and phenolic compounds  and the effluents are, therefore, potentially harmful to the environment [5-8]. A major technical breakthrough was made in the 1970s with the development of oxygen delignification (OD), a process that uses oxygen under alkaline conditions to remove lignin left after the pulping process [9, 10]. OD is a more environmentally friendly bleaching process since both the chemicals used in the process and the materials removed from the pulp can be incinerated in a recovery system. A major goal of the pulping and papermaking (P&P) industry is to produce totally chlorine-free and elemental chlorine-free pulps . Hydrogen peroxide bleaching plays an important role in elemental chlorine-free processes because of stringent environmental legislation and rigorous quality demands . Combining hydrogen peroxide and oxygen (hydrogen peroxide-reinforced oxygen delignification, OP) for the bleaching process has attracted much recent interest because of positive features such as improved bleachability and selectivity. Such processes also reduce emission of hazardous chlorinated lignin and ‘anionic trash’ in the effluent and decrease chemical oxygen demand and biochemical oxygen demand .
The effluents from both the OD and OP processes are suitable for brownstock washing and can be recycled back to the recovery furnace because the effluents are free from chloride ions. It is, therefore, possible to reduce fresh water consumption using recycling processes. Unfortunately, oxalate is formed during the oxidative bleaching process, particularly the hydrogen peroxide bleaching process, and contaminates multiple sections of the mills. The oxalate in pulp slurry can be present in either a soluble form or as a precipitate. The precipitated oxalate is partly associated with fines in the bleaching filtrate or occurs as deposits on the pulp fibers . The soluble oxalate
reacts with dissolved calcium ions in the effluents and the sparingly soluble calcium oxalate salt is formed under certain conditions [15-17]. Calcium oxalate forms three hydrates, a triclinic trihydrate, a tetragonal dihydrate and a monoclinic monohydrate [18, 19]. Whereas the trihydrate and dihydrate are metastable, the monohydrate is thermodynamically stable, particularly under the conditions that exist in a bleach mill. Because of this, the monohydrate is the most problematic salt, in terms of calcium oxalate-related scaling, to the P&P industry [20, 21]. Calcium oxalate is a major contributor to inorganic scale in the P&P industry, resulting in severe production disturbances and losses [22, 23]. To reduce both water consumption and pollution, many pulp mills are now working towards ‘closed water system’ but oxalate may be transferred with the effluent into pipelines and screening and filtration equipment during the recycling process [24, 25]. Since the accumulation of oxalate will lead to the precipitation of calcium oxalate, there is an urgent need to understand oxalate formation in the different P&P processes, especially the bleaching process.
Previous studies  have shown that more oxalate is formed in the peroxide bleaching of hardwood pulp, compared with softwood pulp. It has also been shown that oxalate is generally more easily produced from lignin containing syringyl subunits than from lignin containing guaiacyl subunits. Zhang  reported that more oxalate was formed during hydrogen peroxide bleaching in the bleached chemi-thermomechanical pulp process, compared with that released from refining. Oxalate is produced largely by oxidation of lignin and hemicellulose during oxidative bleaching processes using oxidants such as hydrogen peroxide and ozone . Using lignin model compounds, Bailey and Dence , showed that the Dakin reaction occurred during alkaline hydrogen peroxide treatment, resulting in oxalate formation (Fig. 1). The reaction pathway
was suggested to involve formation of muconic acid intermediates by oxidative cleavage of aromatic rings. The precise mechanism of oxalate formation during the OP process is, however, still not completely understood. To our knowledge, this is the first work addressing oxalate formation during OP process. The pH-dependent equilibrium between oxalate and oxalic acid and the solubility product are known to be key factors affecting the precipitation of calcium oxalate. Reducing oxalate formation during the bleaching process would, to some extent, avoid problems with calcium oxalate scale.
The objective of present work was to clarify how operating parameters (temperature, bleaching time, oxygen pressure and chemical charge) affect oxalate formation and process selectivity. Our main focus was on OP bleaching of Eucalyptus kraft pulp. We have investigated oxalate formation and determined the best conditions for selectivity in the OP process. Calcium oxalate scaling in a paper machine system often leads to disturbance in the process, production losses due to downtime needed for cleaning, as well as impaired paper quality. The most significant benefit of this work is to have a better understanding of oxalate formation and its dependence on process conditions to minimize oxalate formation and scaling. Such understanding would reduce the investment in wastewater treatment and bring more profit to the factory to some extent.
2. Materials and methods 2.1 Materials The sulfate pulping process was carried out in a 10-L stationary stainless steel digester. The Eucalyptus kraft pulp obtained in this way was screened and washed to provide material with a
kappa number of 23.0 and a viscosity of 1237.0 mL/g before the OP process.
Analytical grade chemicals, hydrogen peroxide, sulfuric acid, magnesium sulfate, potassium iodate, and barium chloride, were obtained from the Guangzhou Chemical Reagent Factory (Guangdong, China).
2.2 Hydrogen peroxide-reinforced oxygen delignification OP experiments were conducted in a 2-L inclined rotary stirred Parr reactor. The experimental conditions are summarized in Table 1. During a typical experiment, the Eucalyptus kraft pulp (30.0 g o.d) was mixed with the appropriate chemicals and the pulp consistency was adjusted to 10% with deionized water. After placing the fiber-chemical mixture into the autoclave, the cover was fastened. The air in the autoclave was replaced by oxygen through the gas inlet to achieve the target pressure and the reaction mixture was then rapidly heated to the target temperature. After delignification, the reactor was cooled and the oxygen pressure was released. The pulp was pressed and the filtrate was collected for oxalate determination.
2.3 Oxalate determination Oxalate analyses were carried out using an automated headspace sampler attached to an HP-6890 capillary gas chromatograph (Agilent Technologies, Palo Alto, CA, USA), using a thermal conductivity detector [30-34].
2.4 Kappa number, viscosity, and brightness determination
The kappa number (K) and viscosity (η) were determined using TAPPI standard testing methods T236 cm-85 and T230 om-94, respectively . Pulps were made into sheets to evaluate their optical properties. Brightness was determined using a colorimeter. 3. Results and discussion The operating parameters under investigation were separated into two categories. The first category, chemical charge, comprised alkali dosage and peroxide dosage, which directly affect oxalate formation and pulp properties. The second category, operating conditions, comprised temperature, reaction time, and oxygen pressure. The effect of individual parameters on oxalate formation and pulp properties is discussed in more detail in the following sections. OD is primarily the oxidative degradation and subsequent removal of lignin components that contain aromatic chromophores. It is, however, inevitable that carbohydrate degradation also occurs during this process. The kappa number (K) encompasses all of the different oxidizable components in the pulp, including lignin, non-lignin oxidizable structures and hexenuronic acid (HexA). The viscosity (η) represents carbohydrate in the pulp. K and η are widely used by the P&P industry as the main indicators of pulp and paper properties. The degree of delignification and carbohydrate degradation can, therefore, be defined by K and η, respectively . A large number of models have already been developed to better understand and optimize the OD process . In the present study, we have used the ratio lignin delignification/cellulose degradation (∆K/∆η) to evaluate the OP process. High values of ∆K/∆η indicate that the lignin is removed very rapidly and the carbohydrate is removed more slowly. Since the goal is selective delignification, ∆K/∆η can be used as an index of the selectivity of the OP process.
3.1 Effect of alkali dosage Oxalate formation was found to be approximately proportional to alkali charge (Fig. 2). A strong linear relationship has been observed between alkali charge and oxalate formation. Yu et al  also found that the oxalate formation is proportional to alkali charge during peroxide bleaching. Häärä  found that lignin was the major source of oxalate during the alkaline peroxide bleaching process, compared with cellulose, hemicellulose, extractives and barks. Oxalate may be formed by reactions between oxalate precursors, such as terminal carboxylic and phenolic groups, and the active bleaching agent, i.e., hydroperoxyl anions. Typically, the concentration of hydroperoxyl anions increases with increasing alkali dosage. Other radicals, including oxide radicals (-O•), superoxide radicals (-OO•) and highly reactive hydroxyl radicals (HO•), could also react with residual lignin. These radicals have been shown to be mainly responsible for side-chain elimination, ring opening and demethoxylation reactions, resulting in higher levels of oxalate and pulp brightness . Additionally, Li et al  found that oxalate was formed during pulping process. They also found that the soda cooks resulted in more oxalate being formed than that of the kraft cooks. Therefore, we could demonstrate that oxalate formation occurred in pulping and bleaching process, respectively. Pulping and bleaching process were mainly related to the removal of lignin. Thus, we could conclude that lignin was the major source of oxalate formation.
A strong positive correlation was also seen between brightness and alkali consumption (Fig. 2). Variations in brightness paralleled changes in oxalate formation and can be explained mainly by the higher concentration of reaction chemicals and their improved contact with fibers in the pulp suspension. ∆K/∆η increased as the alkali concentration increased from 0.5% to 2.0% and then
decreased steadily as the alkali concentration increased from 2.0% to 5.0% (Fig. 2). The best selectivity would, therefore, be obtained with an alkali concentration of 2.0%, which would also help to minimize oxalate formation.
3.2 Effect of hydrogen peroxide dosage Hydrogen peroxide is a commonly used oxidant in the P&P industry. At different reaction conditions in the alkaline bleaching process, it can affect oxalate formation. Under the same pH conditions, variations in the hydrogen peroxide charge may be regarded as changes in the concentration of hydroperoxide anion. Oxalate levels in the effluent were found to increase proportionally with increasing hydrogen peroxide dosage (Fig. 3). The brightness, however, increased only slightly with increasing hydrogen peroxide dosage. Yu et al  also found an almost linear relationship between oxalate formation and peroxide consumption during peroxide bleaching of kraft pulp. The radicals (HO•, HOO•) presented in the solution attacked the aromatic ring. The muconic acid-like structures, which could act as oxalate precursors, was formed. The increased oxalate levels at high hydrogen peroxide dosages could be explained by further reaction of lignin degradation products with hydrogen peroxide.
∆K/∆η increased significantly as the hydrogen peroxide dosage increased from 0% to 2.0% and then declined slowly as the hydrogen peroxide dosage increased further (Fig. 3). This could be caused by the instability of hydrogen peroxide in the presence of transition metals. Decomposition to form hydroxyl radicals could result in more extensive degradation of carbohydrates and reduce ∆K/∆η . At a hydrogen peroxide dosage of 2.0%, the OP process had best selectivity and
calcium oxalate scaling was inhibited to some extent.
3.3 Effect of time Differences in oxalate levels, ∆K/∆η values and brightness over time are shown in Fig. 4. There were two distinct phases of oxalate formation in the effluent. In the first 30 min of the reaction, oxalate was formed rapidly (“fast” stage) and then formation slowly reached a constant value (“slow” stage). The rate of formation of oxalate was clearly higher in the initial phase and somewhat slower as the reaction continued. A possible explanation for this phenomenon is the formation of peracetic acid during the initial phase of the OP process, resulting in oxalate formation. The peracetic acid, which could be produced by reaction of hydroperoxy anions with acetyl groups in hemicellulose, is believed to have big impact on brightness . Li et al  demonstrated that the degradation of carbohydrates could contribute to oxalate formation in alkaline conditions. However, they also illustrated that the importance of sugars to oxalate formation was not clear based on their data.
In the present study, brightness followed the same pattern as oxalate formation, showing “fast” and “slow” stages (Fig. 4). It is generally accepted that lignin removal can be divided into “rapid” and “slow” periods, which are attributed to variations in the lignin and reflect the “fast” and “slow” stages of oxalate formation. Krasowski and Marton  showed that the amount of oxalate correlated well with the decrease in lignin content during the bleaching process and that ∆K/∆η and brightness followed a similar trend. The value of ∆K/∆η reached a maximum with a reaction time of 60 min, (Fig. 4), indicating that this is the best reaction time.
3.4 Effect of temperature Temperature is one of the most important factors affecting reaction rate, with increased temperatures generally leading to increased reaction rates. This is especially true in the gas phase. Oxalate levels in the effluent increased as the temperature increased from 80°C to 100°C (Fig. 5) and then leveled off as the temperature increased beyond 100°C. Pulp brightness showed a similar trend but two distinct phases were identified for values of ∆K/∆η. ∆K/∆η increased as the temperature rose from 80°C to 100°C and decreased as the temperature increased from 100°C to 120°C. This phenomenon could be explained by two reactions that occur during the OP process, the bleaching reaction and the decomposition of hydrogen peroxide. At higher temperatures, oxygen-based reagents, including oxygen and hydrogen peroxide, generate radicals that contribute to the bleaching reaction and oxalate formation. As the temperature increases further, however, the solubility of oxygen decreases, together with the rate of the bleaching reaction . Decomposition of hydrogen peroxide also occurs when the temperature exceeds 90°C . Pulp brightness and oxalate levels thus remain at a constant value beyond 100°C. The fall in ∆K/∆η could be caused by degradation of carbohydrates at higher temperatures. The best temperature for the OP process was found to be100°C.
3.5 Effect of oxygen pressure The amount of oxalate produced and the pulp brightness increased slightly when the oxygen pressure rose from 0.2 MPa to 1.0 MPa (Fig. 6). The oxygen pressure had a slight effect on oxalate formation. This is likely because the radicals have a short lifetime and cannot be used with
maximum efficiency. ∆K/∆η reached a maximum value at 0.6 MPa.
4. Conclusions Oxalate was inevitably generated during the OP process. Alkali and hydrogen peroxide dosage were the major factors affecting oxalate levels. Variations in reaction time, oxygen pressure, and temperature had smaller effects. Oxalate formation increased linearly with increasing dosage of alkali and hydrogen peroxide. The formation of oxalate could be divided into “fast” and “slow” stages, depending on the reaction time. Oxalate levels in the effluent increased as the temperature increased from 80°C to 100°C and then stabilized but oxygen pressure had only a slight effect on oxalate formation. The value of ∆K/∆η reached a maximum as the time increased and then leveled off as the time increased beyond 60 min. However, ∆K/∆η increased as alkali charge increased from 0.5% to 2.0% and then decreased as the alkali charge beyond 2.0%, which was similar to that of hydrogen peroxide dosage, temperature and oxygen pressure, respectively. Thus, the best selectivity for the OP process was achieved using an alkali dosage of 2.0%, a hydrogen peroxide dosage of 2.0%, a temperature of 100°C, an oxygen pressure of 0.6 MPa and a reaction time of 60 min. These conditions provided good quality pulp and, to some extent, reduced oxalate formation. It is important to have a better understanding of oxalate formation and its dependence on process conditions to minimize oxalate formation and scaling. Revealing the oxalate formation during the process is useful in that it could be used to adjust the bleaching process, and scaling problems could be solved fundamentally to some extent.
Acknowledgements The authors acknowledge the Natural Science Foundation of China (No. 31370585, 31670586), Science and Technology Planning Project of Guangdong Province (2016A020221010), State Key Laboratory of Pulp and Paper Engineering 2106C05) and the Fundamental Research Funds for the Central Universities (2015ZZ048) for sponsoring the research.
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Fig. 1 The proposed oxidative reaction between creosol and alkaline hydrogen peroxide（adapted from Bailey and Dence 1969）
Fig. 2 Effect of alkali dosage on oxalate formation, ∆K/∆η value, and brightness (Hydrogen peroxide: 2.0%; Time: 60min; Oxygen pressure: 0.6MPa; Temperature: 100°C; Pulp consistency: 10%; Magnesium sulfate: 0.2%)
Fig.3 Effect of hydrogen peroxide dosage on oxalate formation, ∆K/∆η value, and brightness (Alkali dosage: 2.0%; Time: 60min; Oxygen pressure: 0.6MPa; Temperature: 100°C; Pulp consistency: 10%; Magnesium sulfate: 0.2%)
Fig. 4 Effect of time on oxalate formation, ∆K/∆η value, and brightness (Alkali dosage: 2.0%; Hydrogen peroxide: 2.0%; Oxygen pressure: 0.6MPa; Temperature: 100°C; Pulp consistency: 10%; Magnesium sulfate: 0.2%)
Fig. 5 Effect of temperature on oxalate formation, ∆K/∆η value, and brightness (Alkali dosage: 2.0%; Hydrogen peroxide: 2.0%; Oxygen pressure: 0.6MPa; Time: 60min; Pulp consistency: 10%; Magnesium sulfate: 0.2%)
Fig. 6 Effect of oxygen pressure on oxalate formation, ∆K/∆η value and brightness (Alkali dosage: 2.0%; Hydrogen peroxide: 2.0%; Time: 60min; Temperature: 100°C; Pulp consistency: 10%; Magnesium sulfate: 0.2%)
Table 1 Conditions of hydrogen peroxide reinforced oxygen delignification Parameter
NaOH (% odp)
0.5, 1.0, 2.0, 3.0, 4.0, 5.0
H2O2 (% odp)
0, 1.0, 2.0, 3.0, 4.0
10, 20, 30, 40, 50, 60, 90
0.2, 0.4, 0.6, 0.8, 1.0
80, 90, 100, 110, 120
Pulp consistency: 10%; Magnesium sulfate: 0.2%; % odp: percent of oven dried pulp