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Ming, X., Li, Q., and Jiang, W. (2021). "Application of aluminum sulfate in the treatment of papermaking white water," BioResources 16(1), 1382-1393.

Abstract

Physical chemical methods were used to treat papermaking white water used to produce plant fiber mulch sheet that contained fine fibers and inorganic fillers as suspended solids. The ordinary chemical oxygen demand (CODCr) was obviously reduced after the papermaking white water was treated by the flocculant. By comparing three different coagulants (aluminum sulfate (Al2(SO4)3), poly-aluminum chloride (PAC), and poly(diallyldimethylammonium chloride) (PDADMAC)) and flocculant (poly-acrylamide copolymer (PAM)) to process papermaking white water, it was found that Al2(SO4)3 had the best coagulation effect and the lowest cost. The best flocculation conditions were 2,733 mg/L of Al2(SO4)3 and 4.52 mg/L of PAM to treat the papermaking white water. Under the best flocculation conditions, the CODCr was less than 300 mg/L. The goal of closed recycling and zero discharge of white water in the production process of plant fiber mulch sheet was realized.


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Application of Aluminum Sulfate in the Treatment of Papermaking White Water

Xianglan Ming,a Qichao Li,a and Wei Jiang b,*

Physical chemical methods were used to treat papermaking white water used to produce plant fiber mulch sheet that contained fine fibers and inorganic fillers as suspended solids. The ordinary chemical oxygen demand (CODCr) was obviously reduced after the papermaking white water was treated by the flocculant. By comparing three different coagulants (aluminum sulfate (Al2(SO4)3), poly-aluminum chloride (PAC), and poly(diallyldimethylammonium chloride) (PDADMAC)) and flocculant (poly-acrylamide copolymer (PAM)) to process papermaking white water, it was found that Al2(SO4)3 had the best coagulation effect and the lowest cost. The best flocculation conditions were 2,733 mg/L of Al2(SO4)3 and 4.52 mg/L of PAM to treat the papermaking white water. Under the best flocculation conditions, the CODCr was less than 300 mg/L. The goal of closed recycling and zero discharge of white water in the production process of plant fiber mulch sheet was realized.

Keywords: Papermaking white water; Physical chemical method; Flocculation; Aluminum sulfate

Contact information: a: College of Mechanical and Electrical Engineering, Lingnan Normal University, Zhanjiang 524048, China; b: College of Information Engineering, Lingnan Normal University, Zhanjiang 524048, China; *Corresponding author: jwlingnan@163.com

INTRODUCTION

In order to strengthen the control of pollutants and protect the human environment, the Ministry of Environmental Protection of the People’s Republic of China implemented the standard GB 3544-2008 (2008), “Discharge standard of water pollutants for pulp and paper industry”. The standard requires existing pulp and paper companies to implement the upper limit of water pollutant emissions. According to the standard GB 3544-2008 (2008), the common chemical oxygen demand (CODCr) has a limit of 80 to 100 mg/L, and the five-day biological oxygen demand (BOD5) has a limit of 20 mg/L. The standard GB 3544-2008 (2008) also adds indicators for water pollutants such as nitrogen, phosphorus, adsorbable organic halides (AOX), and chroma (Wang et al. 2007; Mänttäri and Nyström 2010; Huang et al. 2012; Mänttäri et al. 2015; Zhao et al. 2016; Toczyłowska-Mamińska 2017; Wang et al. 2018).

Domestically made chemical pulp, especially wet pulp (under 10% consistency), has an anionic trash content that is typically 1 to 5 times higher than that of imported chemical mechanical pulp, such as Canada-Crystal brand and other high-quality brands of bleached chemical thermomechanical pulp (BCTMP) (Agarwal et al. 2001; Avci et al. 2002; Sui et al. 2007; Wang et al. 2014; Sun et al. 2018). The higher anionic trash containing pulp will cause low fiber retention of paper machines and difficulties in sizing with alkylketene dimer (AKD) or alkenylsuccinic anhydride (ASA). The higher anionic trash containing pulp can cause excessive foam in the white water system and the buildup of sediment in the stock and white water systems. The higher anionic trash containing pulp will result in unstable production, more frequent paper breaks, and reduced paper strength and uniformity (Gao et al. 2005; Li et al. 2010; Zhan et al. 2010). As the white water system in a paper machine becomes more closed, the problem tends to be aggravated, so it is urgent to deal with the anionic waste of chemical pulp.

Currently, advanced methods used to treat papermaking white water primarily include the biochemical method and the physical chemical method (Culp et al. 1978; Watkinson et al. 2007; Senta et al. 2011; Wu et al. 2011). On the basis of much engineering practice, physical chemical methods have become widely adopted. Physical chemical methods generally are simple and convenient, with a high efficiency and a low investment cost. Such methods can effectively remove the fine fibers, suspended solid, chroma, and organic content in the recycled papermaking white water. The obtained pulp slurry can also be used to produce boxboard after proper treatment. The treated supernatant can be recycled as industrial water. Therefore, its economic and environmental benefits are quite significant (Yeon et al. 2005; Toyoda et al. 2011; Lüeddeke et al. 2015; Rayne and Ikonomou 2015). A large amount of white water gets produced in a papermaking process, which accounts for a large proportion of the total industrial wastewater and it contains a large amount of soft fibers, fillers, and suspended solids. Thus, the pollution it causes has attracted worldwide attention. In general practice, most of the paper mills’ white water produced in forming section is recovered and sent to the stock preparation section to fan pump through dilution line, and only part of the flow is rejected and goes for the white water treatment (Hubbe et al. 2016; Wei 2016). To treat papermaking white water, it is first filtered in a save-all device to recover the cellulosic fines to reduce the content of the suspended solids. Subsequently, flocculants and coagulants are added to coagulate the remaining fine fibers, fillers, colloidal substances, and partially soluble organic matter in the white water. After settling of the solids in a clarifier, the treated clarified water can be completely reused for production (Wang et al. 2006; Gao and Li 2007; Shi 2010; Yang and Wang 2010; Xie et al. 2012; Zhao et al. 2014).

In this work, the sludge settling effect was studied by comparing the three coagulants (aluminum sulfate (Al2(SO4)3), polybasic aluminum chloride (PAC), polydimethyldiallyl ammonium chloride (PDADMAC)), and the anionic polyacrylamide (PAM) flocculant to treat white water used to produce plant fiber mulch sheet. The influence of the different coagulants on the flocculation effect was examined. In order to provide the theoretical basis and technical support for the appropriate physical chemical methods for the advanced treatment of the papermaking white water used to produce plant fiber mulch sheet, the experiments were conducted to screen out the suitable flocculant system, to compare the effects of the different flocculants and coagulant additives on the treatment of the papermaking white water. The optimal conditions for the reaction were analyzed and evaluated. The goal of closed recycling and zero discharge of white water in the production process of plant fiber mulch sheet is realized.

EXPERIMENTAL

Materials

The three coagulants used were Al2(SO4)3 (15.6% solids based on Al2O3 mass fraction) (Mudanjiang Xinxing Chemical Plant, Mudanjiang, China), poly-aluminum chloride (PAC) (30% solids based on Al2O3 mass fraction) (Zibo Taihua Fine Chemical Co., Ltd., Zibo, China), and poly (diallyldimethylammonium chloride) (PDADMAC) (solid) (Nantong Runfeng Petrochemical Co., Ltd., Nantong, China). The flocculant used was a poly-acrylamide copolymer (PAM) (solid, positive charge, relative molecular mass 8 million, formulated into 0.3% aqueous solution for use) (Dezhou Ruixing Water Purification Material Co., Ltd., Dezhou, China). The water sample used for the water quality index of the papermaking white water was taken from a paper mill in the Heilongjiang province of China that produced plant fiber mulch sheet. The pH of white water was between 7.5 and 7.8, the chemical oxygen demand (CODCr) was 640 to 872 mg/L, and the suspended solid was 160 to 193 mg/L. The solids content of white water was 2.13 g/L. The chemicals used in the papermaking process included the sizing agent dispersed gum rosin, aluminum sulfate, and wet-strength agent (water-soluble, cationic, polyamide polyamine epichlorohydrin resin (PPE)). The type of pulp was a mixture of 30% rice straw fiber pulp and 70% kraft wood fiber pulp (KP). The cationic demand of the white water was 56.1 eq/g (PCD-03 Particle Charge Tester, Mütek, Germany). The zeta potential of white water was -22.3 mV (SZP-06 Zeta Potentiometer, Mütek, Germany). The reagents required for the CODCr test were concentrated sulfuric acid (Jiangcheng Chemical Co., Ltd., Wuhan, China), potassium hydrogen phthalate standard solution (Guangdong Wengjiang Chemical Reagent Co., Ltd., Shaoguan, China), special oxidizer, special catalyst, and masking agent.

For preparation of the special oxidizer, a balance was used to accurately weigh 20 g aluminum potassium sulfate (Dezhou Ruixing Water Purification Material Co., Ltd., Dezhou, China), 13g potassium dichromate (Dezhou Ruixing Water Purification Material Co., Ltd., Dezhou, China), and 5 g ammonium molybdate (Dezhou Ruixing Water Purification Material Co., Ltd., Dezhou, China). The weighed substances were placed into a 500 mL beaker and first dissolved with about 200 mL of distilled water; then 100 mL of concentrated sulfuric acid was added, and the temperature was allowed to cool to room temperature. The solution was transferred to a 500 mL volumetric flask and shaken. Regarding the special catalyst, 4.2 g silver sulfate was weighed (Dezhou Ruixing Water Purification Material Co., Ltd., Dezhou, China) with a balance, placed into a 500 mL volumetric flask, dissolved with concentrated sulfuric acid, and diluted to the mark. The prepared solution was stored for 2 to 3 days to make it completely dissolved before use.

For preparation of the masking agent, 20 g mercury sulfate was weighed (Jinan Yuanyi Environmental Protection Technology Co., Ltd., Jinan, China) with a balance, placed into a 100 mL volumetric flask, dissolved with the addition of about 80 mL of distilled water. Then 10 mL of concentrated sulfuric acid was slowly added, distilled water to the mark, and shaken with a stopper. All these chemicals were laboratory grade. Triple distilled water was used to prepare all the solutions.

Flocculation Experiment

For the comparison of the sludge settling performance of the different flocculant systems, 500 mL of raw water sample was added to a 1,000 mL graduated cylinder. The sample was agitated with an electromagnetic stirrer (HMS-901; Shenzhen Boda Jingke Biological Technology Co., Ltd., Shenzhen, China). Different amounts of flocculant and an appropriate amount of coagulant were added and mixed with the water at a speed of 200 r/min for 2 min. The stirrer was then slowed to 50 r/min for 10 min before the stirring was finished. The solution was allowed to stand for 22 min, and the sludge volume fraction was observed at 0, 2, 6, 10, 14, 18, and 22 min intervals. Sludge volume fraction = volume of suspended solids in mixed liquid / total volume of mixed liquid ×100%.

According to the single-factor pre-experiment results and economics (the price of Al2(SO4)3 is half the cost of PAC), the Al2(SO4)3 and PAM flocculant system had the lowest cost and the best effect in the treatment of papermaking white water. The central composite design approach was used to study the effects and interaction of the Al2(SO4)3 and the PAM on the CODCr of the papermaking white water. There were five levels of Al2(SO4)3 (669 to 2931 mg/L) and PAM (1.17 to 6.83 mg/L). The ranges and levels of the independent variables are shown in Table 1. During the test, the pH of papermaking white water was 7.5.

Table 1. Central Composite Design for Two Variables at Five Levels

CODCr Determination

The CODCr determination was conducted according to a closed catalytic digestion method. Three mL of distilled water was used as a blank sample. If the concentration of the sample was too high, it was diluted. The diluted water sample was centrifuged at 6,000 r/min for 10 min, and 3 mL of the supernatant was taken for measurement. One mL each of masking agent and special oxidant were added to each reaction tube. Five mL of special catalyst was also added to each reaction tube, and the samples were shaken well. The reaction tubes were inserted into the furnace. The solution in the reaction tubes began to digest at a temperature of 165 °C. After a digestion period of 10 min, the reaction tubes were removed and placed on a test tube rack and cool to room temperature. Two mL of distilled water was added to the reaction tube after the digestion period. The sum of the masking agent and the finishing water was 3 mL. The reaction tube was covered with a stopper and shaken well. If there was precipitation in the sample, the supernatant was measured while at a standstill. The colorimetric method was used to determine the absorbance of the water sample at a wavelength of 610 nm. The CODCr value of the diluted sample was calculated according to the standard curve equation and then multiplied by the dilution factor to obtain the CODCr value of the fermentation broth.

Statistical Analysis

The experimental results were processed with Microsoft Excel 2010 (Redmond, WA), Design Expert 6.0.10.0 (Stat-Ease, Minneapolis, MN), and Origin Pro 9 software (OriginLab, Northampton, MA) (Shi and He 2003; Ay et al. 2009; Montgomery 2017). The multivariate regression analysis, the optimization process, and the analysis of variance (ANOVA) test were performed using the Design Expert software. The optimum values of the independent variables were determined by conducting three-dimensional response surface analysis of the independent and dependent variables (Sen and Swaminathan 2004; Ravikumar et al. 2005; Tavares et al. 2009; Chen et al. 2015; Ming et al. 2019; Ming and Chen 2020). The detailed experimental design is shown in Table 2.

RESULTS AND DISCUSSION

Comparison of the Sludge Settling Performance of Different Flocculant Systems

Physical chemical methods are used to treat wastewater. The sedimentation performance of sludge is one of the most important indicators for the selection of flocculants. The sedimentation performance affects the wastewater treatment effect and the sludge disposal. When the Al2(SO4)3, PAC, and PDADMAC (1,800 mg/L) coagulants were used in conjunction separately with the PAM (4 mg/L) flocculant, the sedimentation performance of the sludge was as shown in Fig. 1.

Fig. 1. Comparison of the sludge settling performance. The dosage of the Al2(SO4)3, PAC, and the PDADMAC was 1,800 mg/L, and the dosage of the PAM was 4 mg/L.

According to the flocculation theory, the longer the molecular chain, the higher the degree of connection between particles in the suspension. The larger the expansion space of the flocculant in the water, the more distant particles can be bridged. The more chances of collision with particles, the formation of relatively long and stable bridges and networks. The higher the cationic degree, the stronger the adsorption. Therefore, it is easier to adsorb on the surface of two or more negatively charged colloids or particles, and then form a network, with the help of the adsorption of colloidal particles and mechanical retention to form large floccules. As can be seen in Fig. 1, when the three coagulants were used separately with the PAM to treat the papermaking white water, the Al2(SO4)3 + PAM and the PAC + PAM had an equivalent effect on the sludge settling speed. The PDADMAC + PAM had a detrimental effect on the sludge settling speed. However, the price of Al2(SO4)3 is 750 a ton, which is twice cheaper than PAC (1450 a ton). Therefore, this study investigated the combined flocculation effect of Al2(SO4)3 and PAM. As a coagulant, aluminum sulfate mainly destabilizes the colloids in the white water. The colloidal particles agglomerate with each other and form larger-particle flocs through adsorption and bridging.

Effect of Different Additions on the Flocculation Effect

Regression models

Sixteen experiments were carried out to study the effects and interactions of the Al2(SO4)3 and PAM on the CODCr of the papermaking white water (Ming 2016). These are represented in Table 2. Table 2 also shows the values of the papermaking white water CODCr at each of the experimental designed conditions.

Table 2. Experimental Design and Results

The regression model results indicated that the quadratic model was the best fit for the CODCr of the papermaking white water. Table 3 shows the significance of the regression model parameters.

Table 3. ANOVA Table of the CODCr

The ANOVA of the reduced model showed that the fitted model was highly significant, with an F-value of 186.27. The lack-of-fit F-value of 0.98 implied that the lack of fit was not significant relative to the pure error. From all these statistical parameter values obtained from ANOVA, it can be concluded that the second-order quadratic model obtained is reliable, stable, and predictable. Following the experimental design presented in Table 2, the full quadratic model was simplified and reduced to include only significant terms, as shown in Eq. 1,

 (1)

where y is the CODCr of the papermaking white water (mg/L), x1 is the Al2(SO4)3 (mg/L), and x2 is the PAM (mg/L).

Interactive effects of the Al2(SO4)3 and PAM on the CODCr

The combined effect of the Al2(SO4)3 and PAM on the CODCr of the papermaking white water is shown in Fig. 2. It was observed that the CODCr of the papermaking white water decreased as the dose of the Al2(SO4)3 was increased. This is likely because the Al2(SO4)3 contains a large amount of aluminum ions and has a high positive charge density, which can neutralize a large amount of colloidal substances and macromolecular organic substances in the papermaking white water to form flocs. In addition, the paper machine pulp and white water system treated with Al2(SO4)3 greatly reduce the colloidal substances, especially in the flow system. Meanwhile, the system will not become dirtier and turbid, and the charge in the system will not become more negative with the closed cycle of white water. The CODCr of the papermaking white water first decreased and then increased as the PAM dosage increased. However, the variation was not significant. This is mainly because after adding the coagulants to the wastewater, an electrical neutralization reaction occurred and the electric double layer on the surface of the particles was compressed, so the colloid lost its stable state (Li 2012).

Fig. 2. Effects of the interactive factors on the CODCr

The pollutant particles in the water formed into larger flocs through the effects of adsorption, bridging, net capture, etc. The particles then separated from the water by means of precipitation or air flotation to achieve the purpose of purifying the water (Yazdanbakhsh et al. 2012; Wang et al. 2014; Lindholm-Lehto et al. 2015; Ming 2016). According to Fig. 2, the effect of the Al2(SO4)3 on the CODCr of the papermaking white water was stronger than that of the PAM based on the ANOVA analysis. The interaction of the Al2(SO4)3 and the PAM had a significant effect on the CODCr of the papermaking white water. The lowest CODCr of the papermaking white water was achieved at 2,931 mg/L of Al2(SO4)3 and 4 mg/L of PAM.

Optimization Analysis

The optimization experiment was based on the Chinese Standard GB 3544-2008 (2008), the “Discharge standard of water pollutants for pulp and paper industry.” This standard refers to the production stability, paper strength up to the standard, and the principles of saving energy, reducing costs, and reducing secondary pollution. Based on the design model, the constraints as previously described, and the graphic optimization using the Design Expert software, the optimum condition were 669 to 2931 mg/L of Al2(SO4)3 and 1.17 to 6.83 mg/L of PAM (Fig. 3). At the optimum condition, the CODCr of the papermaking white water was lower than 300 mg/L, which can meet the requirements of repulping and papermaking of plant fiber mulch sheet.

Fig. 3. Optimum analysis of the technology parameters

Model Validation

The optimum model was validated by testing the CODCr of the papermaking white water at 2733 mg/L of Al2(SO4)3 and 4.52 mg/L of PAM. The CODCr of the papermaking white water obtained from the validation experiment was 289.7 mg/L. This agreed with the predicted result, that the CODCr of the papermaking white water was 300 mg/L. Therefore, the optimum condition obtained from response surface methodology design was reliable and applicable, and it indicates that response surface methodology is a powerful tool for system optimization.

CONCLUSIONS

  1. The papermaking white water that was coagulated with Al2(SO4)3 exhibited fast settling of flocs, an obvious turbidity removal effect, and a good flocculation effect, which can greatly reduce the cost of water production, and the treated white water can be reused.
  2. The Al2(SO4)3 coagulant was used in conjunction with the PAM flocculant. The best flocculation conditions were 2,733 mg/L of Al2(SO4)3 and 4.52 mg/L of PAM when combined to treat the papermaking white water used to produce plant fiber mulch sheet.
  3. Under the best flocculation conditions, the CODCr was less than 300 mg/L, which can meet the requirements of repulping and papermaking of plant fiber mulch sheet.

ACKNOWLEDGMENTS

The authors would like to thank a paper mill in Heilongjiang, China for providing the papermaking white water and the valuable support for the treatment of the papermaking white water.

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Article submitted: September 7, 2020; Peer review completed: November 21, 2020; Revised version received: November 30, 2020; Published: January 6, 2021.

DOI: 10.15376/biores.16.1.1382-1393