Acta Chim. Slov. 2005, 52, 67–72 67 Scientific Paper Study of the Influence of Dissolved and Colloidal Components on Paper Sizing by Simulation of Process Water Loop Closure Nejc Zakrajšek,a* Janja Zule,a Adolf Može,a and Janvit Golobb a Pulp and Paper Institute Ljubljana, Bogišičeva 8,1000 Ljubljana, Slovenia, E-mail: nejc.zakrajsek@icp-lj.si b Faculty of Chemistry and Chemical Technology, Aškerčeva 5, 1000 Ljubljana Received 10-02-2004 Abstract Simulation of process water loop closure in a papermaking system was performed on laboratory scale by means of the Rapid-Köthen sheet former. The concentrations of sulphate and dissolved inorganic ions, expressed as conductivity, as well as the concentration of dispersed rosin size Sacoccel 309 were systematically measured dur-ing the process. The influence of increasing concentrations on paper sizing efficiency was the main objective of our research. Therefore, we determined the highest degree of water closure at which the examined dissolved and dispersed components do not negatively interfere with the paper sizing process yet. Key words: paper sizing, water loop closure, simulation Introduction In paper industry, water is used for the preparation of fiber suspensions and chemicals, their transportation to the sieves of paper machine, as well as for cleaning purposes and energy transfer.1'2 In addition, it has a major role in forming bonds betvveen pulp fibers.3 During the past years, fresh water consumption has strongly decreased owing to economical and ecologi-cal reasons such as higher costs of fresh water, loss of process substances and strict environmental legislation. Water systems can be closed to various extents. In completely open systems, only fresh water is used, whereas in closed loops ali water is recycled. Thus, there are practically no effluents, and minimal fresh water consumption serves only as compensation for the loss in drying section of the paper machine.4 Process water reuse has economical and eco-logical benefits, yet on the other hand, it can cause technical problems. The most serious disadvantages are increased concentrations of dissolved and col-loidal compounds, the rise of water temperature, intensified microbiological activity and corrosion. Ali the mentioned phenomena affect the system, and they have to be evaluated for each čase separately.5 Paper properties may significantly change due to the water closure process. Any quality reduction should be predicted, which is partly possible by simulating real sys-tems with experiments on a laboratory level. The results obtained help paper mills cut down fresh water consumption without significantly changing the paper properties. The simulation of paper machine wet end process can be efficiently performed by appropriate tests on a lab-oratory sheet former. Although it works discontinuously in contrast to the industrial paper machine, the acquired results are often a good prediction of the real situation. Paper sheets are formed on the laboratory sheet former by applying different pulp suspensions and process chemicals. Either fresh water or process water can be applied for paper sheet formation. In both cases, the whole amount of water may be reused during the experiment, meaning that the system approaches or reaches zero effluent conditions. Paper properties and process water quality can be monitored throughout this test in order to evaluate the influence of decreas-ing fresh water consumption on paper properties. In reality, it is very important to maintain constant paper quality despite the changes in technological conditions. One of the most important characteristics of printing paper is the degree of sizing. The main pur-pose of paper sizing is to protect the surface of paper fibers against the penetration of water and printing inks. This can be achieved by internal sizing during which size is added to paper suspension, as well as by surface sizing, during which size is applied directly to paper surface. The negative charges of paper fibers and rosin size make sizing impossible if no positiveh/ charged ions are added. Fibers and rosin size are bound by means of aluminium ions from aluminium sulphate.6 Laboratory simulation was conducted to deter-mine the influence of water closure on paper sizing. The main objectives of our research were to determine the Zakrajšek et al. Influence of Dissolved and Colloidal Components on Paper Sizing 68 Acta Chim. Slov. 2005, 52, 67–72 Table 1. Plan of the experiments. Experiment number 1 Cone. of suspension (g/L) Water used for dilution Water in the tank at the beginning of the experiment (V=10L) Number of cycles______________ PW...process water. 15 PW* from paper mili PW from paper mill 45 10 PW from paper mill 7/8PW from experiment 1,1/8PW from paper mill 90 15 PW from paper mill 3/4 PW from experiment 2, 1/4PW from paper mill 90 dependence of paper sizing on inereasing concentrations of chosen parameters in process water, and to evaluate the degree of process water loop closure at which dis-solved components do not interfere with paper sizing yet. Experimental STOCK PREPARATION Paper sheets were formed on a laboratory level ac-cording to the ISO 5269-1 standard procedure. The fol-lowing raw materials were applied: fresh chemical pulp fibers of 3.5% consistency, kaolin, Sacoccel 309 rosin size, aluminium sulphate, cationic starch and retention aid. Process water and aH chemicals were obtained from a paper mili producing high quality printing paper. Fibers were diluted with industrial process wa-ter in a mixing vessel to a desired consistency of 1 or 1.5%. Chemicals were added to fiber suspension in the same sequence as in the paper mili, first kaolin and then Sacoccel 309 rosin size. Retention aid, aluminium sulphate and cationic starch were added dur-ing paper formation directly into the sheet former. SIMULATION OF PROCESS WATER LOOP CLOSURE Process water loop closure simulation was accom-plished on the Rapid-Köthen laboratory sheet former (Figure 1). The Rapid-Köthen sheet former is designed to allow produetion of laboratory made hand-sheets. The sheet-forming cycle can be operated completely automatically using one of the pre-set programmes. Either fresh or process water can be selected for the experiments. In our čase, process water from the paper mili was chosen because it enables better simulation of water closure. Since the sheet former works in cycles, one cycle is needed to form one sheet. The sheet-forming cycle starts by process water be-ing poured from the water tank into the former, during which fibers and chemicals are added. Compressed air is used for the agitation of diluted stock. The next stage of the sheet-forming cycle is dewatering. The vacuum pump is designed to suck process water through the wire. Process water is returned to the water tank and re-used for paper formation in the next cycle. The formed hand-sheet is removed from the sieve by means of a car-rier board and is subsequently being put into the dryer. Water loop closures result in a decrease of specific fresh water consumption which is defined as fresh water consumption in m3/t of formed paper. The consumption of fresh water for the preparation of chemicals needs to be taken into consideration as well. The volume of fresh water in the tank at the beginning of the experiment is based on an estimation from the paper mili and includes aH fresh water consumed in the process with the excep-tion of fresh water used for the preparation of chemicals.7 The equation for specific fresh water consumption on the former can be deseribed as: speč. comsumption of FW(L) V + n-V n-m Figure 1. Rapid-Köthen sheet former. In the equation, V is the volume of fresh water in the former at the beginning of the experiment, n is the number of cycles, Vs is the volume of fresh water in fibers and rn, is the average weight of paper sheets. According to the equation, the specific consumption of fresh water depends only on fresh water in the tank at the beginning of the experiment, and on the number of cveles. Three experiments were conducted (Table 1). Process water from the paper mili was used for fiber 2 3 Zakrajšek et al. Influence of Dissolved and Colloidal Components on Paper Sizing Acta Chim. Slov. 2005, 52, 67–72 69 dilution. Chemicals were prepared with fresh water. Paper basis weights, which are defined as mass of pa-per on square meter of paper, and concentrations of chemicals were the same in aH tests. In the first experiment, 10 L of process water from the paper mili was poured into the water tank. In the following two experiments, the water from the previous experiment was reused for paper forma-tion. Thus, the consumption of fresh water decreased while the level of water loop closure increased. 45-90 cycles were conducted in each experi-ment. Hand sheets and process water were sampled for analyses after 5 or 10 cycles. Ali hand sheets were preconditioned at 23 °C and 50% relative humidity. WATER AND PAPER ANALYSES Conductivity was measured with the Tetracom 325 conductivity celi (WTW pH 537pH-meter). The concentrations of sulphate ions were deter-mined on the METROHM 761 compact ion chromato-graph, using the Metrosep Anion Dual 2 column. The mobile phase was composed of a mixture of 2.0 mmol NaHC03, 1.8 mmol Na2C03 x 10 H20 and 15% acetone. The suppressor solution was 50 mmol H2S04. The flow of the mobile phase was 0.8 mL/min and the volume of the injected sample was 20 /jlL. A conductivity detec-tor was applied. Concentrations were calculated from the calibration curve of standard sulphate solutions. The concentration of rosin size in process water was measured on the HP 5890 gas chromatograph. Rosin size was isolated from water by solid phase extrac-tion on the BAKER SPE 12GV extraction apparatus. The BOND ELUT-AL-N (Varian) 3 mL extraction cartridges were used. The aqueous sample was acidified and 50 mL eluted through the column. For extraction, 2 mL of CH2Cl2 was used. The methylenchloride extract containing rosin size was subsequently methylated by gaseous diazomethane which was synthesized in a reac-tion betvveen N-methyl-N-nitroso-4-toluenesulfonamide and 50% potassium hydroxide. 2 g of N-methyl-N-ni-troso-4-toluenesulfonamide (diazald) was dissolved in a mixture of diethyl ether (6 mL) and diethylenglycol-monoethylether (4 mL). 1 mL of 50% KOH solution in water was poured into a 4 mL reaction vial equipped with a small magnet and capped with septum through which a capillary was inserted. 1 mL of diazald solution was added to the same reaction vial. Diazomethane produced during stirring on a phase border betvveen the two solutions was introduced through the capillary into another reaction vessel containing a sample solution. The end of methylation was expressed by yellow colour indicating excessive diazomethane. The methylated sample was concentrated to exactly 0.2 mL and analysed by gas chromatography at the following experimental conditions: capillary column SPB 1 (15 m), injector temp. 250 °C, split ratio 1:15, init. oven temp. 200 °C (2 min), heating rate 3 7min, final oven temp. 300 °C (10 min), det. FID temp.300 °C, N2 flow 1.5 mL/min. Typically, the GC chromatogram was composed of two peaks representing methyl esters of dehydroabietic (DA) and abietic acids (A) being the active compounds of the sizing agent and as such responsible for sizing reaction. The two peaks were integrated and their concentration calculated from the calibration curve. Calibration was performed by applying the following concentrations of a standard acid mixture (DA : A - 75 : 25) in CH2C12: 0.05; 0.1; 0.25; 0.5; 0.75 and 1.0 mg/mL. The weight ratio of dehydroabietic and abietic acids was 75 : 25 in the sizing agent as well as in ali water samples. The calibration graph was prepared by plotting the sum of the two standard peak areas against concentration. It is presented by the equation and correlation coefficient (y = 7E+06x +123920; R2 = 0.9985). AH chromato-graphic determinations were performed in 3 parallels. The Cobb60 test was used for paper sizing evaluation. The result of the test is Cobb60 value defined as the amount of water absorbed into 1 m2 of paper surface in 60 seconds. Obviously, higher Cobb60 values indicate worse paper sizing. The Cobb60 test is particularly suitable for measuring medium sized paper (Cobb60 values 28-65 g/m2).8 Results and discussion The specific consumption of fresh water decreased with increasing cycle numbers (Figure 2). Depend-ing on the experiment, curves have approached the limiting values which are defined by fresh water in the tank at the beginning of a particular experiment. The amount of fresh water in process water at the beginning of the tests was estimated in the paper mili. 40 35 30 25 20 - 15 10 5 - 0 • No.1 ¦ No.2 ANo.3 ^ ¦ A A A A A A A 0 20 80 40 60 cycles Figure 2. Specific fresh water consumption in dependency on cycle numbers in the three experiments. Zakrajšek et al. Influence of Dissolved and Colloidal Components on Paper Sizing 70 Acta Chim. Slov. 2005, 52, 67–72 Table 2. Results of water and paper analyses. Experiment number Spec.cons.of FW (m3/t) Sulphate (mg/L) Conductivity (uS/cm) Rosin size (mg/L) Cobb60 (g/m2) 60.4 26.9 778 - 28.3 1 32.7 28.3 787 - 30.2 23.4 29.4 790 - 29.7 18.8 31.1 806 - 32.8 10.8 30.8 848 0.75 31.2 7.6 32.4 918 0.79 33.2 2 6.9 33.2 972 - 36.8 6.6 36.1 1011 1.68 37.8 6.5 37.1 1043 2.34 38.5 13.7 35.6 969 - 27.3 7.6 36.3 976 - 33.9 5.6 36.9 980 1.49 32.3 4.6 37.7 989 - 42.8 3 4.0 37.9 997 1.78 37.7 3.6 38.3 1008 - 44.0 3.3 38.7 1020 2.48 45.9 3.1 39.2 1035 - 44.2 2.9 39.3 1044 3.07 49.6 41 39 -37 35 ] 33 31 29 27 K • No.1 ¦ No.2 ANo.3 0 10 20 30 40 spec.consumption of FW (m3/t) Figure 3. Dependence of sulphate cone. to specific fresh water consumption. As a result of water loop closure, the concentra-tions of sulphate ions and rosin size (Sacoccel 309), which entered the process as papermaking chemicals inereased. Due to the aceumulation of inorganic ions in process water (Table 2), conductivity inereased as well. The concentrations of measured components stronglv inereased when the limiting values of specific fresh water consumption were reached. The limiting values were specific for each component, which is most likelv connected to the equilibrium betvveen the concentrations of individual components in process water and in paper. A strong gradual inerease in sulphate concentrations was noticed when the level of fresh 1100 - • No.1 1050 -1000 950 ¦ No.2 ANo.3 ¦V 900 - 850 800 -750 - • •• • 700 25 30 35 40 cone. of sulphate (mg/L) Figure 4. Dependence of conductivity to the concentration of sulphate. water consumption was lower than 15 m3/t. Presumablv the constant bonding of components in paper strueture which resulted in aceumulation of sulphate ions in process water (Figure 3) occurred at the attained equilibrium when the above-mentioned values were reached. The aceumulation of sulphate ions as the main inorganic species, caused an inerease of conductivitv (Figure 4). Cobb60 values that characterize paper sizing were relativelv constant at above 15 m3/t of specific fresh water consumption (Figure 5). Below this value, paper sizing was less efficient. A substantial inerease of Cobb60 values was noticed under 7 m3/t of specific fresh water consumption. Zakrajšek et al. Influence of Dissolved and Colloidal Components on Paper Sizing Acta Chim. Slov. 2005, 52, 67–72 71 50 1 ^ 45 40 35 30 A • No.1 ¦ No.2 A No.3 • 0 10 20 30 40 speč. consumption of FW (m3/t) Figure 5. Dependence of Cobb60 values to specific fresh water consumption. 55 n 50 - • No.1 ¦ No.2 ANo.3 R2=0,8883 45 - r R2=0,8633 40 ^ 35 R2=0,8601 "^7 30 - 25 - 25 30 35 40 45 conc.of sulphate (mg/L) 55 50 - R2 = 0.9548 45 - 40 - 35 30 25 20 R2 = 0.9689 A R2 = 0.7975 A • No.1 ¦ No.2 ANo.3 750 850 950 1050 conductivity (uS/cm) Figure 6. Cobb60 values in dependence to the conductivitv of water. DA Figure 7. Cobb60 values in dependence to the sulphate ions concentration in water. t (min) Figure 8. Gas chromatogram of extracted water sample (DA - dehvdroabietic acid, A - abietic acid). Paper sizing is not influenced only by temperature, pulp quality and refining procedures but also by the presence of dissolved ions. Above ali, higher concentrations of inorganic ions deteriorate paper sizing. Both fibers and rosin size are negativeh/ charged, while aluminium ions from aluminium sulphate are positively charged and serve as a connection betvveen both negative components. Anions such as sulphate easily react with aluminium to form aluminium salts and thus decrease its positive charge. Cations can precipi-tate with negatively charged rosin size, which results in a reduced efficiency of paper sizing9. During the tests, Cobb60 values of paper increased with the correspond-ing concentrations of sulphate ions and conductivity of water samples. Figures 6 and 7 clearly indicate a trend of linear dependency betvveen conductivity, sulphate ions accumulation in water and paper sizing efficiency. A characteristic chromatogram of rosin size from the extracted water sample is presented in Figure 8. The concentration of rosin size in process water increased during the ongoing process of water clo-sure. Typically, the higher the concentration of rosin size in process water, the greater were Cobb60 values. The analysis of process water loop closure is mostly 55 50 45 40 35 30 - 25 20 ¦ No.2 ANo.3 R2 = 0.991 0 1 2 3 conc.of rosin size (mg/L) Figure 9. Cobb60 values in dependence to rosin size concentra-tions in process water. based on mass balances and equilibrium data, and may be used where equilibrium data is noted. In the čase of rosin size, the equilibrium data is difficult to determine. For this reason, these phenomena may also be caused by the decrease of reactivity of rosin size adsorbed on paper after a higher number of cy-cles. Established logarithmic dependency between Cobb60 values (Figure 9) and the concentration of rosin size in process water occurred repeatedly. 4 6 8 0 2 4 2 Zakrajšek et al. Influence of Dissolved and Colloidal Components on Paper Sizing 72 Acta Chim. Slov. 2005, 52, 67–72 Conclusions The laboratory experiment results indicate that the influence of water closure on paper sizing is negligible at values higher than 7 m3/t of specific fresh water con-sumption. Paper sizing deteriorates below the specified limiting values. Cobb60 values increase almost linearh/ with the accumulation of sulphate ions and logarithmicailv with rosin size concentration in process water. One of the possible solutions to the problem is the surface sizing of paper, which increases the efficiencv of paper sizing and prevents further pollution of process water. Excessive concentrations of anionic species are detrimental since they easilv promote formation of insoluble, stickv deposits on paper machine equip-ment. In addition, they initiate corrosion. Long-term solutions can be found in removing components from process water or in changing the process chemicals. Regarding the acquired and expected results, the Rapid-Köthen laboratory sheet former proves to be an appropriate tool for simulation and research of the influence of water loop closure on paper sizing and can therefore be used for similar purposes in the future. Acknowledgement This work was supported by the Ministry of Edu-cation, Science and Sports of Slovenia and Slovenian papermaking industry. References 1. D. Ravnjak, G. Ilić, A. Može, Chem. Biochem. Eng. Q. 2002, 18, 13-19. 2. G. Bourgogne, J. P. Oy, J. E. Lame, Pap. Puu. 2001, 83, 190-203. 3. T. Uesaka, Handbook of physical and mechanical testing of paper and paperboard, Marcel Dekker Inc., N.Y., 1984. 4. F. Zippel, Water management in paper mills, [SN], Heidenheim, 2001. 5. V. Edgardo, P. H. Pfromm, Tappi J. 1998, 81, 206-213. 6. J. Zule, J. Dolenc,^4cto Chim. Slov. 2003, 50, 115-122. 7. D. Ravnjak, Master Thesis, FKKT, Ljubljana, 2003. 8. F. W. Revnolds, The sizing of paper, Lappi press, Atlanta, 1989. 9. W. E. Scott, Principles of wet end chemistry, Lappi press, Atlanta, 1996. Povzetek Na laboratorijskem oblikovalniku je bila izvedena simulacija zapiranja krogotokov procesne vode v papirni industriji. Zaradi zapiranja je pričela koncentracija sulfatnih in drugih ionov (izražena kot prevodnost) ter klejiva v procesni vodi naraščati. Glavni namen raziskave je bil ugotoviti, kako naraščajoče koncentracije komponent vplivajo na učinkovitost klejenja papirja in do katere meje je mogoče krogotok procesne vode v preiskovanem sistemu zapreti, da bo vpliv spremljanih komponent v procesni vodi na učinkovitost klejenja papirja še zanemarljiv. Zakrajšek et al. Influence of Dissolved and Colloidal Components on Paper Sizing