Evaluation of an Alternative Flocculation System for Manufacture of

Figure 8 shows the evolution of the mean chord size during flocculation and the evolution of the ... Figure 8 Effect of flocculant dosages and PFR/PEO...
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Evaluation of an Alternative Flocculation System for Manufacture of Fiber-Cement Composites Carlos Negro,* Elena Fuente, Luis M. Sa´ nchez, A Ä ngeles Blanco, and Julio Tijero

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Department of Chemical Engineering, Complutense UniVersity of Madrid, AVenida Complutense s/n, 28040 Madrid, Spain

Flocculation has been a key issue for fiber cement manufacturing since cellulose or poly(vinyl alcohol) (PVA) fibers are used to replace asbestos. Due to the complexity of flocculation, many fiber cement companies face difficulties in optimizing it, which leads to unpredicted production problems and lower process efficiency. This paper studies the behavior of a dual system (PFR/PEO) and compares it with two anionic polyacrylamides commonly used in this industry. The work was carried out using a focused beam reflectance measurement as a sensor to monitor the flocculation process in real time. Results show that the proposed alternative dual system induces the formation of larger but less stable flocs than the ones obtained with the anionic polyacrylamides. A flocculation mechanism was proposed for the dual system consisting of the initial adsorption of the phenol-formaldehyde resin (PFR) onto the particles, which provides junction points for bridge formation by poly(ethylene oxide) (PEO). The flocculation kinetics induced by the dual system is determined by the PFR dosage. Introduction The prohibition of asbestos in fiber cement manufacturing has pushed the industry to replace asbestos fibers by others, e.g., cellulose and poly(vinyl alcohol) (PVA). Asbestos, cellulose, and PVA fibers have surfaces covered with hydroxide ions and are negatively charged under industrial conditions (pH ∼12). Therefore, they interact with cement particles due to the presence of calcium ions; this interaction is necessary to form the composite. Asbestos fibers are thinner and denser than cellulose or PVA fibers, which enhances these interactions since, on one hand, the specific surface of asbestos is higher and, on the other hand, asbestos fibers tend to settle at rates similar to those of cement and filler minerals in the mix. Therefore, the manufacture of asbestos cement did not require flocculants. However, flocculation aids are necessary to improve the process efficiency when cellulose or PVA fibers are used, due to the lower natural propensity of these fibers to flocculate with cement. In this case, flocculants have to be used to improve the retention of minerals and fine fiber fragments during the formation of films and to reduce the recirculating load of fine particles in the water system. Furthermore, flocculation also limits the separation of the nonfibrous and fibrous materials due to their different densities. Therefore, polyacrylamides (PAMs) are commonly used nowadays in fiber cement plants. However, it has been observed that they reduce the bending strength of the product, reducing its quality.1 Therefore, flocculant selection and flocculant dosage optimization are important to improve the production process and to minimize its effect on product quality. Recently, a methodology based on monitoring flocculation and measuring retention and drainage2 has been developed for flocculant selection.2 This method is complemented by taking into account the flocculation kinetics, the behavior and evolution of the formed flocs, and the effect of the flocculant on product strength.1,3 Thus, different studies have been carried out considering the flocculation efficiency and the product quality: (1) Fibers have been treated with different sizing aids that increased the strength of the product.1,4,5 * To whom correspondence should be addressed. Tel.: + 34 91 394 42 42. Fax: + 34 91 394 42 43. E-mail: [email protected].

(2) Cationic, anionic, and nonionic PAMs have been studied, and it has been concluded that anionic PAMs are the most suitable ones to induce the cement flocculation process, because of the interactions between the Ca2+ ions and the carboxylic groups of the PAM.6 (3) The effects of molecular weight and charge density of anionic PAMs have been determined, with the conclusion that the negative effect on product strength is lower when the charge density increases and the molecular weight decreases.3 Although some advances have been achieved, it is of interest for the industry to find alternative flocculation aids that improve the process efficiency without affecting the product quality. Therefore, in this work an alternative product is studied and its behavior is compared with two high molecular weight PAMs, of different anionicities, commonly used in fiber cement mills. The proposed system is formed by a phenol-formaldehyde resin (PFR) and poly(ethylene oxide) (PEO); it is a popular retention aid system found in paper mills, especially in highly contaminated closed systems.7 Flocculation mechanisms have been widely studied in the paper industry, where flocculants are used as retention or drainage aids. However, there is very limited information about flocculation mechanisms and floc behavior in the fiber cement industry. Many authors have studied the mechanism of the flocculation of different suspensions, even cement suspensions, induced by PAM, concluding that it involves bridge formation.3,6,8-12 However, the mechanism of cement flocculation induced by the PFR/PEO system has not been studied. Different mechanisms have been proposed for the flocculation of papermaking suspensions induced by this dual system. For example, the formation of a network, between the two components of the system and the fibers, that occluded the fillers,13,14 was the most accepted theory until 1996, when van de Ven, Alince,15 and Xiao16 demonstrated that it could not explain their observations and proposed complex bridging flocculation, which is the most accepted theory nowadays. In this case, the components of the dual system form a complex that is adsorbed on the particle surface and interacts with other particles, forming bridges between them, in a way similar to the PAMs. This was strongly supported by Modgi et al.,17 who demonstrated the

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existence of aromatic hydrogen bonding interactions between PFR and PEO, which has been also observed by many other authors.15-22 This dual retention system is successfully used in the papermaking industry, and that is the reason this polymer was selected as a potential alternative flocculation system, even though it has not yet been applied to fiber-cement suspensions. Since the fiber-cement mixture is quite complex, it is necessary, as a first step, to study the behavior of the main component (cement) in the presence of the traditional and alternative flocculants. Thus this paper studies the cement flocculation kinetics induced by the three flocculants considering that, after adding the flocculant, two processes take place: the aggregation of particles forming the flocs and the floc breakage due to hydrodynamic forces. Therefore, it is possible to use a model that describes the evolution of the particle concentration with two terms: (1) that flocculation takes place when two particles collide successfully (second-order kinetics) and (2) that floc breakage is first-order kinetic.23 This model is represented by the equation

dn ) -k1n2 + k2n dt

(1)

where n is the concentration of particles, t is the time (in seconds), and k1 and k2 are the kinetic constants of the flocculation and deflocculation processes, respectively. The integrated solution of this differential equation that has been used to fit the experimental evolution of the flocs is given by

n)

k2 k1 k2 n0 k1 -k2t 1e n0

(2)

where n0 is the initial concentration of particles. The experimental study of flocculation kinetics requires a method able to measure, in real time, the properties that characterize the flocculation process. This study has been carried out following the methodology developed by Blanco et al. to monitor flocculation processes based on the evolution of the particle size using a focused beam reflectance measurement (FBRM) probe.24 This method was developed to avoid the limitations of methods based on electrokinetic or optical properties of the particles, and it has been successfully applied in the paper industry for monitoring flocculation and floc breakage.2,23-29 Methods and Materials The trials were carried out with 400 mL of a 5 wt % suspension of ASTM type II cement in water saturated with Ca(OH)2; the pH of the suspension was 12. The behaviors of three flocculants were studied: two commercial anionic polyacrylamides (A-PAM 1 and A-PAM 2) whose molecular weight is 7.5 × 106 and 6.3 × 106 g/mol and anionic charge density is -0.6 and -1.6 mequiv/g, respectively; and a dual retention system formed by the PFR, a commercial phenol-formaldehyde resin, NETBOND FRB, supplied by Kemira Kemi AB, and a high-molecular-weight nonionic PEO, UCARFLOC 309, with a molecular weight of 8 × 106 g/mol, supplied by Dow Chemical Company.

The anionic charge densities of the PAMs were measured by colloidal titration using the commercial particle charge detector PCD 03, manufactured by Mu¨tek. The flocculation process and the floc properties were studied using a FBRM probe M500L, manufactured by Mettler Toledo, Lasentec, Seattle, WA. This device measures the particle chord size distribution in real time by detecting the light backscattered by tens of thousands of particles per second, obtaining results representative of the particle population.30,31 Each measured particle is denominated “count”. From the chord length distribution it is possible to calculate different statistics such as the mean chord size and the total number of counts or the number of counts in different length intervals. Although the relationship between the particle size distribution and the particle chord distribution is quite complex,30,31 the evolution of this last one allows monitoring and studying of any process that affects the aggregation state, size, or shape of the particles. The principles of the measurement and the details of the applied methodology to monitor flocculation have been described in previous papers.2,23,27 Images of the suspension were taken by a continuous videomicroscopy device, PVM 800, manufactured by Mettler Toledo, Lasentec, Seattle, WA. The procedure for taking images with the PVM and for monitoring flocculation with the FBRM is the same; the only difference is the probe used. In a typical experiment, the probe was introduced in 400 mL of cement suspension stirred at 300 rpm, and the evolution of the system was monitored for 20 min before the flocculant was added. The evolution of the chord size distribution can be monitored by using different characteristics such as the mean chord size and the total number of counts, namely, the number of particles the device has detected per second. To optimize the dual system, trials were carried out with different PFR/PEO addition orders and ratios and different PEO doses (doses are given as the milligrams of polymer per kilogram of dry cement). Two kinds of trials were carried out: (1) flocculation trials at 300 rpm for 15 min and (2) flocculationdeflocculation-reflocculation trials, modifying the stirring intensity. In the second trial, after the flocculant was added, the system was allowed to evolve at 300 rpm (Re ) 6.7 × 104) for 5 min, then the stirring was increased to 800 rpm (Re ) 1.8 × 105) for 5 min to break down the flocs, and, finally, the stirred speed was decreased back to 300 rpm to let the system reflocculate. Trials with different doses of the PAMs were carried out in the same way. Finally, the flocculation and the evolution of the flocs were studied at the determined optimal conditions. Results Addition Order of PEO and PFR. The effect of the order of adding the components of the dual system on both flocculation and floc properties was studied first. Figure 1 shows images of the suspension before the addition of the flocculant and in the moment of maximum flocculation. The formation of flocs with irregular shapes can be observed when PEO is not added before the PFR, and the largest flocs were formed by adding the PEO 60 s after the PFR. Figure 1 also shows that no flocs were formed when PEO was added first. Figure 2 shows that, in this case, the chord length distribution did not change after adding PEO or after the PFR addition, indicating that there was no flocculation process, confirming quantitatively, the observations made from Figure 1. Figure 3 shows that when PEO was added first no change in the mean chord size was observed despite the addition of PFR 60 s after. Therefore, the PEO itself

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Figure 1. Images of the suspension corresponding to the maximum mean chord size.

Figure 2. Effect of PEO addition and PFR addition on chord length distribution of cement suspension.

does not interact with the cement particles, or if it does, it does not have any flocculant properties. When the PFR was added alone, no flocculation was observed. It is well-known that PEO chains tends to interact with the phenolic groups of different cofactors such as phenolic resin to form a complex that induces flocculation of other kind of suspensions such as calcium carbonate, clay, kaolinite, latex, and papermaking suspensions.32 Several authors have observed that the order of adding the PEO and the cofactor was not critical, and some of them have concluded that the two aids form a complex responsible for the flocculation.13,16 However, results shown in Figure 3 indicate that flocculation takes place when the PFR is added first, and that the addition of PEO as the first component inhibits flocculation. Therefore, when PEO is added first, either the PEOPFR complex is not formed or a different PEO-PFR complex without flocculant properties is formed. The formation of the PFR-PEO complex is quite fast; therefore, it can also take place when both components are added simultaneously. In this case, the flocculation obtained was similar to the one produced by adding the PEO 30 s after the PFR addition, but the interaction of the PFR with the suspension during 60 s improved the flocculation process, as shown in Figures 2 and 3. This indicates that PFR could adsorb onto the particle surfaces, providing junction points for PEO chains that adsorb on them forming bridges between the particles. PFR adsorption onto cement particles could be possible due to the

interaction with the divalent cations in solution, calcium for example, as the anionic polyacrylamides can adsorb. This would explain the obtained results. The flocculation was improved when PEO was added 60 s after the PFR addition because it had enough time to adsorb onto the particles and provide the maximum number of junction points for PEO. The addition of PEO 90 s after the PFR addition produced a poorer flocculation, probably because the longer time allowed the calcium cations to saturate the phenolic groups of the PFR, reducing the possibility of interaction with PEO. When PEO was added first, it could have interacted with other compounds in the system, which would reduce its efficiency before PFR provided connection points on the particles and, therefore, flocculation did not occur. Shear could have also negatively affected the PEO microstate or PFR could have interacted first with the PEO chains (in solution), and therefore not enough resin would have remained to adsorb onto the particles. Optimization of Polymer Dosage. To optimize the PFR/ PEO dose ratio, flocculation was induced by adding different dosages of PFR and PEO. PEO was always added 60 s after the PFR addition, as this was found to be the optimum, as shown in Figures 1 and 3. Different doses of PAM 1 and PAM 2 were also tried for comparison. Figure 4 shows the increase in the mean chord size obtained after adding the PEO or the PAMs calculated as the difference between the maximum mean chord size reached during flocculation and the initial mean chord size before flocculant addition. It increased with the PFR/PEO ratio and with flocculant dosage. Two different behaviors were observed for the dual system: (1) When the dosage of PEO was not lower than the PFR concentration (PFR/PEO e 1), flocculation required PEO doses higher than 50 mg/kg, and the mean chord size increase was very low. (2) However, flocculation took place with 5 mg/kg PEO when the PFR/PEO ratio was 5 or higher. Figure 4 also shows that PFR/PEO ratios over 50 did not improve flocculation significantly. The behavior of the PAMs was between these two behaviors, and PAM 1 produced slightly better results than PAM 2. The maximum mean chord size increment was obtained with a PEO dosage of 100 mg/kg and 50 times more of PFR. However, the addition of a PEO dosage of 5 or 10 mg/kg can

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Figure 3. Effect of order of adding PEO and PFR on flocculation of cement.

Figure 4. Effect of PFR/PEO dosage ratio and dosages of flocculant on the maximum mean chord size increment during flocculation.

Figure 5. Effect of PFR dosage on the mean chord size increment.

increase more than three or four times the mean chord size if the PFR/PEO ratio is not below 5. This option reduces the consumption of the components of this dual system, keeping a high flocculation efficiency. The mean chord size increment produced by the dual system was higher than the one induced by the polyacrylamides even with low PEO doses. However, it is necessary to add a high amount of PFR (at least 250 mg/kg as a part of the optimum dosage), whose phenolic groups could affect the product strength, as do the carboxylic groups of the PAMs. Figure 5 shows the increase in the mean chord size obtained versus the PFR dosage. These results show that the phenolic resin determines the flocculation grade, which increases strongly with PFR doses lower than 250 mg/kg. It would be not worthwhile to use doses higher than this one because the same increment of PFR dosage produces a much lower enhancement of flocculation. It is noticeable that the PFR/PEO ratio necessary to produce significant flocculation is quite high compared to the ones used to induce flocculation in other kinds of suspensions.13,14,23,33 It is possible that the amount of PFR to cover the particle surface is larger than the amount of PEO necessary to form bridges between particles coated by the PFR. The interaction between PFR and PEO is due to hydrogen bonding between the ether

Figure 6. Evolution of mean chord size during flocculation-deflocculation- reflocculation process with PAM and with different PFR/PEO ratios.

groups of PEO and the phenolic hydroxyl groups of the cofactor34 and possibly aromatic hydrogen bonds between the ether groups of PEO and the aromatic groups of PFR, as concluded by Modgi et al.17 The pH of the cement suspension is over 12, because it is saturated on calcium hydroxide. These conditions would not be suitable for interaction between PFR and PEO because of the hydrolysis of the hydroxyl groups of PFR, which would avoid their interaction with PEO. Therefore, it could be possible that the only groups of PFR able to interact with the oxygen atoms of PEO were the aromatic rings, which do not hydrolyze. Even more, some of the phenolic groups would interact with the calcium cations instead, being blocked to the PEO chains. Other suspensions that flocculate easily with PFR/PEO ratios between 1 and 3 have lower pH values, such as papermaking suspensions, whose pH is near 7. Furthermore, flocculation could require more resin than the amount necessary to cover the particle surface and this excess resin would interact with PEO to form a complex that joins to the junction points formed by the adsorbed PFR to form bridges between particles. Figure 6 shows the evolution of the mean chord size after the addition of 10 mg/kg PAM or PEO with different PFR/ PEO ratios. Flocculation was quite fast due to the bridge formation between particles, but flocs were not stable and they were almost completely broken after 300 s at 300 rpm. When the stirring intensity increased to 800 rpm, flocs were completely broken down, and they did not reflocculate after the stirring intensity was decreased. The flocculation induced by 10 mg/kg PEO and 50 mg/kg PFR produced a maximum mean chord size similar to the one induced by the PAMs, but the flocs are less stable as shown by the fast decrease in the mean chord size after flocculation (Figure 6) that could be due to polymer flattening.35 PFR is supposed to increase the stiffness of the polymer improving the flocculation and the stability of the flocs, when flocculation is induced by the complex, but this is not the case, because bridges between cement particles were formed mainly by PEO chains attached to the particle surface by PFR.

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Figure 7. Chord size distributions of the cement suspension in different stages of the flocculation-deflocculation-reflocculation trial.

The increase of the stirring intensity accelerated the floc breakage. When the PEO dosage was 10 mg/kg, the mean chord size after a few seconds at 800 rpm reached a lower value than the initial one (Figure 6). The comparison between the chord length distributions before the flocculant was added and after the floc breakage at 800 rpm (Figure 7 shows one of these cases: 10 mg/kg PEO and 50 mg/kg PFR) indicates that there were a higher amount of small particles and a lower amount of large particles than the ones present before the flocculant was added. Some authors have demonstrated that the cement suspensions are coagulated because of their low ζ potential and the interaction between the compounds of cement with different charges (heterocoagulation).36 Thus, the hydrodynamic forces at 800 rpm would break down not only the flocs formed by the dual system, but also the cement coagula existing before the flocculant addition. When the stirring intensity increased again, the mean chord size evolved toward the initial value because these coagula are reversible and they are formed again. Therefore, the results in Figures 6 and 7 show that the flocs induced by the dual system and by the PAMs are completely irreversible. Effect of PFR/PEO Ratio and PEO Dose on Flocculation and Floc Strength. Figure 8 shows the evolution of the mean chord size during flocculation and the evolution of the flocs at a constant stirring intensity (300 rpm) and during flocculationdeflocculation-reflocculation trials, with different doses of the dual system. The final mean chord size obtained after the evolution of the flocs for 1000 s at 300 rpm increased with the flocculant dosage, in particular when the PFR/PEO ratio was 20 or 50. In these cases it is possible to observe that the increase of the stirring intensity to 800 rpm produced a irreversible floc breakage. The flocculation process reduces the concentration of particles in the suspension as they aggregate, which is reflected by a sharp decrease of the number of counts to reach a minimum value at the time corresponding to the maximum mean chord size (Figure 7). After reaching this minimum, the number of counts increases slowly because of the slow breakage of the formed flocs. Most of these flocs could not be formed again because of polymer flattening.35 Figure 9 shows, with an example, that eq 1 fits the data correctly (r2 ) 0.999). The final situation toward the system tendencies can be analyzed by representing the constant K, that is the ratio k2/k1, obtained by fitting the evolution of the number of counts per second after flocculation at 300 rpm. Therefore, there are two simultaneous processes that tend to an equilibrium situation given by the constant K represented in Figure 10. The lower value of K indicates a higher amount of flocs in the equilibrium. Figure 10 shows that the value of K decreases with the PEO/PFR ratio for constant PEO dosage and with PEO dosage for constant

Figure 8. Effect of flocculant dosages and PFR/PEO ratio on the mean chord size evolution at constant stirring, 300 rpm (open symbols) and when stirring intensity was changed, 300 rpm-800 rpm-300 rpm (filled symbols).

Figure 9. Example of fitting of number of counts during the evolution of the flocs after maximum flocculation by eq 2.

PFR/PEO ratio. This could be due to the excess of PFR that does not adsorb on the particles but interacts with the PEO chains, improving flocculation and increasing the strength of the bridges with more flocs remaining in the equilibrium. Figure 11 shows that for PFR/PEO ratios less than 50, the value of K depends strongly on the dosage of PFR, and it is not dependent on the PEO dosage. Figure 11 also shows that it is not worthwhile using a PFR/PEO ratio of 50 and that using PFR doses lower than 250 mg/kg allows us to obtain good results with low PFR consumption.

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Figure 10. Effect of PFR/PEO ratio and PEO dosage on the value of K during the evolution of the flocs after maximum flocculation.

Figure 11. Effect of PFR dosage on the value of K during the evolution of the flocs after maximum flocculation.

Figure 14. Effect of PFR dosage on the value of k1 during the flocculation stage.

Figure 14 shows the value of k1 in the flocculation stage, just after the PEO is added. During this stage, the aggregation of particles is the predominant process; this allows us to consider that the second term of eq 1 is negligible with respect to the first one. A higher value of k1 indicates a faster flocculation process. It can be observed that the flocculation kinetics increases with the PFR dosage and when the PFR dose is low enough it depends also on the PEO dosage. At low PFR doses, the increase in PFR dosage increases the number of junction points to form bridges, but it is also necessary to increase the PEO dosage to form those bridges. A PFR/PEO ratio about 10, with a PFR dosage of 100 or 200 mg/kg, gives the best flocculation kinetics with the lowest PFR consumption. Conclusions

Figure 12. Effect of PFR/PEO ratio and PEO dosage on the value of k2 during the evolution of the flocs after maximum flocculation.

Figure 13. Effect of PFR dosage on the value of k2 during the evolution of the flocs after maximum flocculation.

Figures 12 and 13 show that the value of k2 decreases with the PFR dose. This represents the deflocculation kinetics; therefore, a lower value of k2 indicates a higher floc strength. Thus the floc strength increases with the PFR dose and the PEO dose has a lower effect on it when the PFR/PEO ratio is less than 50. This could be due to the large size of the flocs induced by this PFR/PEO ratio. Large flocs are easily broken by hydrodynamic forces. Results in Figures 11 and 13 corroborate the proposed mechanism. Higher PFR dosage allows the formation of more bridges and the existence of a higher concentration of dissolved PFR that can interact with PEO chains forming strong bridges between particles.

The evaluation of an alternative flocculation system (PFR/ PEO) for fiber-cement composite manufacture has been carried out by comparing its behavior during cement flocculation with the behavior of two traditional anionic PAMs of different charge densities. The three flocculants induced the formation of flocs that could not reflocculate after decreasing stirring intensity, when they had been broken down by shear forces. The differences between the results obtained with the two PAMs were negligible compared to the results obtained with the dual system. This induced the formation of larger and less stable flocs, even with low PEO doses. The required PFR/PEO ratio was very high compared to the optimal PFR/PEO ratio commonly used in the paper industry A flocculation mechanism has been proposed for the aggregation process induced by the PFR/PEO system; that is, the PFR adsorbs first on the particle surfaces and provides junction points for anchoring the PEO that forms bridges between the particles. It is hazardous to extrapolate these findings to fiber cement mill conditions, where the scale is higher, the shearing schemes are very different, and the suspension also contains fibers and additives. However, these findings provide useful information about the behavior of the polymers and about the flocculation mechanism induced by the proposed dual system. It could be an alternative way to increase the retention of cement because it induces the formation of larger flocs without negatively affecting the formation of the sheet, since the flocs would break down more easily than the ones formed by the PAMs. Therefore, this dual system could be an alternative flocculant, providing that the negative effect on the final product strength is not higher than the effect produced by the traditional flocculants. Literature Cited (1) Negro, C.; Sa´nchez, L. M.; Fuente, E.; Blanco, A. Effects of Flocculants and Sizing Agents on Bending Strength of Fiber Cement Composites. Cem. Concr. Res. 2005, 35, 2104-2109.

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ReceiVed for reView May 10, 2006 ReVised manuscript receiVed July 18, 2006 Accepted August 10, 2006 IE060580U