Ind. Eng. Chem. Res. 2005, 44, 9105-9112
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Effect of Shearing Forces and Flocculant Overdose on Filler Flocculation Mechanisms and Floc Properties Angeles Blanco, Carlos Negro,* Elena Fuente, and Julio Tijero Department of Chemical Engineering, Faculty of Chemistry, Complutense University of Madrid, Avda Complutense S/N, 28040 Madrid, Spain
The increasing use of fillers makes filler retention a more critical issue. The effects of both shearing and flocculant overdose on filler flocculation kinetics and mechanisms and floc properties, in the absence of fibers, are presented. The flocculation by cationic polyacrylamide (CPAM) or poly(aluminum chloride) (PAC) was determined using a focused beam reflectance probe. The effect of shear forces on aggregation kinetics depends on particle size. PAC produces soft flocs. At 250 rpm, the number of counts decreases due to flocculation, while, at 750 rpm, flocculation of particles smaller than 0.5 µm increased the number of counts. Filler flocs induced by the optimal CPAM dosage, which was 3 times lower than the one that neutralized the ζ potential, were partially reversible, although they are generally believed to be irreversible; an excess of CPAM inhibited the reflocculation of the particles and decreased the flocculation rate, but a moderate excess improved floc strength and floc stability. These observations are consistent with the hypothesis of polymer flattening. Introduction Historically, the addition of fillers to paper has been driven by the desire to substitute them for more expensive fibers and thus to reduce production costs. Minerals also give specific properties to the paper, such as improved printability, brightness, opacity, smoothness, and dimensional stability. However, the addition of fillers results in sheet strength reduction. This means that papermakers must manage the filler level to optimize the benefits, but in general, they have always been keen to increase the proportion by weight of filler, or the loading level, used in the paper furnish. However, to reach their aim, the most important aspect is to have good filler retention when paper is formed. Fillers are used nowadays to reduce the cost of raw materials and to increase the quality of the paper, mainly in relation to optical properties and printability. The net cost savings per ton of fibers replaced by fillers can reach 100 U.S. dollars. Furthermore, as fillers do not have any lumen to retain water, they increase the press solids, saving energy. Filler usage has increased by around 1% per annum in some paper grades during the last 15 years, and this trend is expected to continue. The amount of fillers in some specific paper grades can reach 50% of fibers, improving the sheet properties while retaining a high enough sheet strength.1,2 Among the different available minerals, calcium carbonate has become widely used as filler in alkaline papermaking. Its typical properties and ready availability around the globe are seen as the key elements for its success. The main technical reasons for its application are its high brightness and opacity potential.3 Calcium carbonates are supplied as a natural ground product (GCC) made by grinding chalk, limestone, or marble and as a precipitated product (PCC) made * To whom correspondence should be addressed. E-mail:
[email protected]. Tel.: +34 91 394 42 42. Fax: +34 91 394 42 43.
synthetically. PCC is widely used in the U.S.A., where the natural form is not particularly available. It is manufactured in satellite plants near the paper mills, allowing its cost to be reduced. Furthermore, it is possible to tailor this filler to give it specific properties. However, in Europe, limestone is very common and GCC is extensively used.4 The efficient retention of fillers with the fibers requires the addition of retention aids, either flocculants, coagulants, or combined systems. The flocculation process and the floc properties depend on many factors, e.g., nature of the retention aid, dosage and conformation of the polymer, filler nature, pulp properties, concentration of anionic trash, etc.5-8 In the last few years, filler preflocculation has become a strategy to improve filler retention. This has increased the need for a fundamental understanding of filler flocculation and floc properties in the paper industry.8-10 During papermaking, chemical flocculation affects retention, drainage, and formation. In general, retention programs not only enhance retention but also improve the removal of water from papermaking stock. However, on the other hand, flocculation can affect the formation negatively if big flocs are present in the suspension during the formation of the sheet.10 Although the floc size and floc properties controlled by the addition of chemicals are important, the ability of flocs to resist degradation by shear and turbulence in the paper machine headbox and forming elements is the main factor influencing retention, drainage, and formation. Ideally, a retention system needs to flocculate effectively fines and fillers on the fibers, without producing high fiber flocculation, at a minimum dosage level, and to be somewhat shear resistant without adversely affecting sheet structure. Many paper mills have difficulties controlling the optimal chemical doses in real time, sometimes causing flocculant overdosage. This excess of flocculant may not only increase the cost of the paper but also affects the flocculation process and the floc properties. Further-
10.1021/ie050870v CCC: $30.25 © 2005 American Chemical Society Published on Web 10/26/2005
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more, nonretained products accumulate in the process and affect the water chemistry and the wet-end process. In summary, flocculation is a complex phenomenon because of the wide variety of additives used in the paper machine, the different properties of the flocs, according to the predominant flocculation mechanism, and the numerous interactions that may occur between the chemicals and the different pulp fractions, depending on process conditions. Therefore, a deeper knowledge of filler flocculation mechanisms is required to improve their retention. The objective of this work was to study some aspects of the flocculation process and of the floc properties when poly(aluminum chloride) (PAC) or polyacrylamide (CPAM) is added to a ground calcium carbonate (GCC) suspension using a methodology based on monitoring, in real time, the particle chord size distribution with a focused beam reflectance measurement device. Both polymers are commonly used in papermaking and produce very different floc properties.11-16 Due to the complexity of the flocculation processes, the study was carried out in the absence of fibers, which facilitates the interpretation of the results. Materials and Methods Experiments were carried out with a suspension of 1% ground calcium carbonate (GCC) (Hidrocarb CL from OMYA, L’Arbo´s, Spain), whose surface area is 2.2 m2/g. The suspension was continuously stirred at 250 or 750 rpm during trials. Two different flocculants were used: a high molecular weight (>106 g/mol) CPAM with a low charge density (Percol 292 supplied by Allied Colloids, Yorkshire, NY) and PAC (PAC 18 supplied by Atofina, France) with a low molecular weight (175 g/mol) and a high charge density. Method. The flocculation process and the floc properties were studied by using a M500L focused beam reflectance measurement (FBRM) probe manufactured by Lasentec, Mettler Toledo, Seattle, WA. The application of this laser technology to optimize flocculation during papermaking was developed in the early 1990s.17-19 It has also been used to study the resistance of flocs to shear forces by studying the flocculation, deflocculation, and reflocculation processes from a kinetic point of view.20,21 The device generates a laser beam that is focused on a focal point that describes circles at a very high speed (2000 rpm). The detector measures the time duration of the light pulses produced by the light backscattered from the particles. The number of particles that cross the focal point path each second, which is denominated as the number of counts per second, increases with the number of particles per volume unit.22,23 The principle of the measurement and the details of the applied methodology have been described in previous references.18,21 The evolution of the particle chord size distribution was studied by monitoring the number of counts per second, which reflects the particle concentration, and the mean of the chord size distribution (mean chord size). The kinetics of the flocculation, deflocculation and reflocculation processes were studied by following a model based on the existence of two simultaneous processes: the aggregation of particles, with secondorder kinetics, and the aggregate breakage, with firstorder kinetics. The evolution of the particle number concentration is represented by eq 1.24,25
dnc ) -kc1nc2 + kc2nc dt
(1)
where nc is the number of counts measured per second, t is the time (s), and kc1 and kc2 are kinetic constants. This relationship allows us to compare the flocculation and the deflocculation kinetics of different polymers. The relationship between both sets of kinetics allows us to obtain the equilibrium situation toward which the system tends in each particular case. To study the effect of shear forces on GCC flocculation, the FBRM sensor was introduced into 200 mL of filler suspension, which was stirred for 2 min at 250 or 750 rpm, depending on the desired experiment. During this time, the signal of the FBRM was stable. After 2 min of stirring, the flocculant was added. Experiments were performed with the flocculant dosage that neutralized the streaming current obtained using a charge detector MUTEK PCD 3. These doses were 67 mg/L of PAC and 60 mg/L of CPAM. To study floc properties, the following experimental sequence was carried out: flocculation at 250 rpm during 240 s, allowing the flocs to evolve; then increasing the stirring intensity to 750 rpm, during 180 s, to deflocculate the system; and finally, decreasing the stirring speed again to 250 rpm to reflocculate the suspension. During all the experiments the behavior of the suspended particles was monitored. The effect of CPAM dosage on flocculation was studied by adding different CPAM doses to the suspension. The ζ potential of the GCC suspension was measured by electrophoresis with a Zeta-Meter 3.0 from ZetaMeter Inc., Staunton, VA. The suspension was stirred for 10 min after adding the flocculant, and then it was diluted 100 times before measuring the ζ potential. The voltage used for the measurement was 150 V. The ionic strength and the pH of the suspension during the measurements were 5.5 × 10-4 M and 9.6, respectively. To determine the adsorption isotherm, several doses of PAM were added to different GCC suspensions, whose concentration was 10 g/L. After 1 min of mixing, the suspensions were gently stirred for 24 h to reach equilibrium. The suspensions were centrifuged, and the total organic carbon content of the supernatants was measured to determine the polymer concentration using calibration curves. Results and Discussion Figure 1 shows the evolution of the measured number of counts, after adding the polyelectrolyte, at two shear forces (250 and 750 rpm). As was expected, the number of counts after flocculation increased with the stirring intensity. This effect was more pronounced for PAC. The final number of counts is higher for PAC because of the formation of relatively weak flocs that are broken by hydrodynamic shear forces under the conditions of testing. This is due to the low molecular weight and high charge of the PAC that favors the flocculation by charge neutralization. Figure 2 shows that the increment in the mean chord size produced by the flocculant addition at 250 rpm is higher than the one produced at 750 rpm. These results indicate that at 750 rpm flocculation produces fewer and smaller flocs, since shear forces break down and disperse the flocs (see Table 1). When GCC flocculation was induced by CPAM, the evolution of the number of counts per second was
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Figure 1. Effect of the shearing forces on the flocculation of a 1% GCC suspension: CPAM dose, 60 mg/L; PAC dose, 67 mg/L.
Figure 3. Chord size distribution of a 1% GCC suspension at 750 rpm, without flocculant.
or larger than 0.5 µm, and the flocculation of particles whose diameter is smaller than 0.5 µm and that form flocs larger than 0.5 µm. The aggregation of a nonmeasurable particle to one particle that is already being measured does not change the concentration of measurable particles. Thus, the evolution of the measured number of counts is
dnv ) -kv1nv2 + kv2nv + ksns2 dt Figure 2. Effect of the stirring intensity on the mean chord size increment produced by the GCC flocculation: CPAM dose, 60 mg/ L; PAC dose, 67 mg/L. Table 1. Kinetics of Flocculation Induced by CPAM stirring intensity (rpm) kc1 (1/(count s)) kc2 (1/s) nc,equilibrium (counts)
250 7.8 × 10-6 3.3 × 10-2 4192
750 8.4 × 10-6 1.0 × 10-1 11600
adjusted to the kinetic model described by eq 1 to obtain the values of kc1 and kc2. Table 1 shows that at a higher stirring rate the deflocculation rate increases (higher kc2); therefore, the final concentration of broken flocs increases. When GCC flocculation was produced by PAC, a different behavior was observed and eq 1 did not fit the experimental data. As is shown in Figure 1, an increase in the number of counts after adding PAC at 750 rpm was observed; also, a high instability of flocs formed at 250 rpm. This requires a further analysis. GCC Flocculation Induced by PAC. At 750 rpm, after adding PAC to the suspension, two effects were observed: the mean chord size slightly increased (Figure 2), which indicates the aggregation of particles, and the number of counts increased (Figure 1). This apparent contradiction can be explained because the detection limit of the instrument (0.5 µm) does not allow particles smaller than 0.5 µm to be quantified. However, when these small particles form flocs larger than 0.5 µm, they are counted. Therefore, to study the evolution of the total number of counts (nc), the behavior of the number of counts that corresponds to particles smaller than 0.5 µm (ns) must also be considered. Consequently, the relationship between the total number of counts and the measured number of counts (nv) is the following: nc ) nv + ns. Thus, the evolution of the measured number of counts is the sum of three terms: the flocculation of particles whose diameter is equal to or larger than 0.5 µm, the deflocculation of particles whose diameter is equal to
(2)
where kv1 and kv2 are, respectively, the flocculation and deflocculation rate constants for the aggregation of the measurable particles (g0.5 µm) and ks is a flocculation rate constant for the small particles (0 dt
(4)
Therefore,
()
ks > kv1
nv ns
2
(5)
The number of counts corresponding to particles smaller than 0.5 µm was estimated by adjusting the measured number of counts distribution obtained by the FBRM to a distribution equation, which was fitted perfectly (r2 > 0.999), as shown in Figure 3. The obtained value of ns represents 11.6% of the total number of counts. Therefore, at 750 rpm, ks is much higher than kv1 (ks > 58kv1). Equation 3 represents the Smoluchowski model26 considering only two particle sizes; therefore, ks and kv1 are functions of the collisions frequency and of the capture or collision efficiency. Table 2 shows the relationship between the flocculation kinetics and the efficiency for orthokinetic flocculation (the movement of particles due to hydrodynamic
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Table 2. Efficient Collision Frequencies for Perikinetic and Orthokinetic Flocculation perikinetic
orthokinetic
measurable particles
8KT Rvp 3η
16 Rvo Ga3 3
small particles
8KT Rsp 3η
Rso
16 3 Ga 3
forces) and perikinetic flocculation (the Brownian movement of particles).The variables Rvp and Rsp are the collision efficiencies for the flocculation of the measurable particles and the small particles, respectively. The variables Rvo and Rso are the capture efficiencies of the measurable and small particles, respectively. K is the Boltzmann constant, T is the absolute temperature of the suspension, η is the absolute viscosity of the fluid, G is the root-mean-square velocity gradient, and a is the radius of the particle. Flocculation is perikinetic when the Peclet number (Pe ) πηNa3/10KT, where N is the stirrer intensity in rpm) is lower than 1 and orthokinetic if it is higher. The particle diameter that makes Pe ) 1 at 250 rpm is 0.7 µm. Therefore, the flocculation of most measurable particles is orthokinetic. At 250 rpm, the value of ks is (1.1 × 10-17)Rsp m3/(particle s) and the value of kv1 varies from 1.1 × 10-17Rvp to (1.7 × 10-15)Rvo m3/(particle s) for the particles whose diameters are in the range 0.7-8 µm. Therefore, at 250 rpm, kv1 can be up to 100 times higher than ks, assuming that Rvo and Rsp are similar. Consequently, the condition expressed by eq 5 is not fitted at 250 rpm, and the flocculation of small particles does not affect the dnv/dt value. At 750 rpm, the value of ks is (1.1 × 10-17)Rsp 3 m /(particle s) and the value of kv1 is from 1.3 × 10-17Rvo to (5.1 × 10-15)Rvo m3/(particle s) for particles whose diameters are in the range 0.5-8 µm. If we assume that the efficiencies Rsp and Rvo are similar, ks is not higher than kv1 either. However, the experimental dnv/dt value obtained demonstrates that the flocculation of smaller particles is predominant. Therefore, the capture efficiency (Ro) must be lower than the collision efficiency (Rp) at 750 rpm. The perikinetic flocculation is not affected by the stirring intensity, but the orthokinetic capture efficiency depends on the stirring intensity. Van de Ven, in 1989, exposed the effect of the hydrodynamic forces, as the function of G indicated in eq 6, on the capture efficiency. As shown in Figure 4,27 the capture efficiency can decrease by 100 times when the stirring intensity increases by 3 times if there are repulsive electrostatic forces between the particles.
CG )
A 36πna3G
(6)
where A is the Hamaker constant. The value of log CG at 750 rpm is 0.5 units lower than the value at 250 rpm, for the same particle size. Thus, the capture efficiency can be considerably lower at 750 rpm if there are electrostatic repulsive forces. In this case, the ζ potential of the suspension flocculated with the PAC dosage that made the streaming current null was -17 ( 2 mV (the ζ potential of the GCC suspension without flocculant was -21 ( 2 mV). Therefore, there are electrostatic repulsive forces between the particles. Results obtained with the FBRM
Figure 4. Effect of the hydrodynamic forces on the orthokinetic capture efficiency. The concrete shape of the curve depends on the intensity of the electrostatic repulsive forces (ER) and on the ratio between the particle radii. The curve for ER * 0 corresponds to the conditions summarized by van de Ven.27
probe show that the capture efficiency, in this case, decreases drastically when the stirring intensity increases. On the other hand, the electrostatic double layer was thick because of the low ionic strength of the suspension (5.5 × 10-4 M). Therefore, the electrostatic repulsion forces could be high enough to decrease the capture efficiency by more than 100 times when the stirring intensity increases by 3 times, explaining why the condition expressed by the eq 5 is fitted in this case. On the other hand, the value of the ζ potential with 67 mg/L of PAC, -17 ( 2 mV, is too negative, considering that the cationic charge of this compound is high and the dosage is more than double the dosage of CPAM necessary to neutralize the ζ potential. This indicates that the PAC degrades under the experimental conditions by alkaline hydrolysis. As shown in Figure 1, at 250 rpm, the number of counts reaches a minimum value after adding the flocculant and, then, it starts increasing slowly until reaching a value similar to the initial one. This could be caused by the flocculation of the small particles, which at 250 rpm is slower than the aggregation of the large ones. The flocculation of small particles did not affect the evolution of the measured number of counts when CPAM was added to the suspension because the flocculation mechanism induced by CPAM is by bridges between particles and does not require neutralization.6,28-30 Therefore, the monitoring of the chord length distribution has also allowed us to have an idea about the existence of repulsive electrostatic forces among particles when the flocculation process occurs. GCC Flocculation Induced by CPAM. When a flocculant acts by bridging, the traditional method for optimizing polymer dosage, based on the electrostatic properties of the particles, for example, the measurement of ζ potential or cationic demand, can lead to polymer overdosing.18 Figure 5 shows the ζ potential of the suspension measured at different CPAM doses. The CPAM dosage that neutralized the ζ potential was 1.1 mg/m2 (25 mg/L). The amount of CPAM that neutralized the streaming current of GCC (2.8 mg/m2 or 60 mg/L) produces electrostatic repulsive forces among particles because of the charge reversion, but the steric repulsion with that dosage is not very high because only 2.1 mg/m2 (40 mg/L) was adsorbed onto the surface of the particles when this dosage of PAM was added; the maximum adsorption of PAM is around 2.8 mg/m2 (60 mg/L) (see Figure 6). Figure 2 shows that, at this dose, very large flocs were formed.
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Figure 5. ζ potential of a GCC suspension with different doses of CPAM.
Figure 8. Effect of the excess CPAM on flocculation and floc properties. Table 3. Adsorption of CPAM on GCC Particles adsorption at the following CPAM dosages (mg/L) [mg/m2]
Figure 6. Adsorption isotherm of CPAM on 10 g/L GCC (20 °C). The dosage necessary to neutralize the ζ potential was 1.1 mg/m2 or 25 mg/L.
Figure 7. CPAM dosage optimization for GCC flocculation.
To study the reversibility of the flocs, the GCC suspension was flocculated, deflocculated, and reflocculated. Furthermore, the effect of the CPAM dosage on the floc properties was also studied by carrying out trials with the optimal CPAM dosage, determined with the FBRM probe, with the doses that make the ζ potential and streaming current null, and with twice this last dosage. The optimal dosage of CPAM was determined with the FBRM by adding the polymer gradually to the suspension and monitoring the mean chord size. Figure 7 shows that the mean chord size increases with the CPAM dosage but CPAM doses higher than 7 mg/L did not produce significant changes in the mean chord size. Therefore, the optimal CPAM dosage corresponds to 7 mg/L, which is 9 times lower than the one that makes the streaming current null. Some authors have observed a dispersion of their suspensions induced by the excess of CPAM, due to the electrostatic repulsion and steric forces,12,21,32 but results
CPAM dosage
7 [0.3]
60 [2.8]
120 [5.5]
flocculation deflocculation reflocculation
0.2
1.9 2.0 2.1
2.5 2.6 2.6
shown in Figure 7 contrast with those observations. The electrostatic and steric repulsion forces did not significantly affect the floc size in this case. Figure 8 shows the evolution of the number of counts during flocculation at 250 rpm, during deflocculation (produced by increasing the stirring intensity to 750 rpm), and during reflocculation of the GCC suspension (when the stirring intensity decreases to 250 rpm) for CPAM doses of 7, 25, 60, and 120 mg/L. CPAM addition produced a fast flocculation process that decreased the number of counts to a minimum value, as shown in Figure 8. With 7 mg/L of CPAM, the formed flocs were slowly destroyed, as shown by the slow increase in the number of counts starting 30 s after the flocculant addition. The increase in stirring intensity produced a deflocculation process. When the stirring rate decreased again to 250 rpm, partial reflocculation was observed, except for high doses. High doses produce stable flocs; although the covering grade was high and the charge of the particle surface was reversed, these flocs are broken when stirring increases and they do not reflocculate when the dosage of CPAM was 60 or 120 mg/L. Therefore, although it is generally believed that CPAM induces the formation of irreversible flocs,31-34 partial filler reflocculation is possible with low doses of CPAM, while an overdose of CPAM induces the formation of irreversible flocs. The flocs induced by 25 mg/L of CPAM, which corresponds to a null ζ potential, were more stable than the ones induced by the optimal FBRM dose, and they were the strongest ones because of the higher number of junction points, the higher coverage grade that limits the polymer flattening, and the minimization of electrostatic repulsive forces among particles. Flocs are partially reversible upon reaching, after reflocculation, the lowest value of the number of counts. The adsorption of polymer was measured in order to explain these results. Table 3 shows that almost all the adsorption takes place during the flocculation stage and that the stirring rate changes do not increase the adsorption significantly. Figure 7 shows that very low doses of CPAM induced aggregation despite the low coverage grade of the
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Table 4. Effect of CPAM Dosage on Flocculation Kinetics CPAM (mg/L)
7
60
120
kc1 (1/(count s))
8.0 × 10-6
7.8 × 10-6
2.9 × 10-6
particle surface. This demonstrates that the flocculation mechanism induced by CPAM is the fast formation of bridges. Figure 8 shows that, in most cases, the adsorption of the polymer and the particle aggregation took place during the first 20 s after the CPAM addition, producing large flocs whose stability could be hard enough to maintain at 250 rpm. When the dosage of polymer was 7 mg/L, the coverage grade of the particles was low because the adsorbed amount of CPAM was 10 times lower than the maximum one. Under these conditions, the configuration of the adsorbed chains may evolve toward a flat conformation and the flocculation grade may decrease. This potential flattening would be slower than the aggregation process, as observed by Yu and Somasundaran for polymers with high molecular weight.7 Other authors have also observed this effect by monitoring the streaming potential of a suspension after adding different doses of CPAM.35 When the dosage is 60 or 120 mg/L, the coverage of the surface of the particles would be high enough to limit the polymer flattening. Wagberg et al.36 observed that half of the polymer adsorption was reached 10 s after the polymer addition, which is in accordance with the results shown in Figure 8 where flocculation induced by CPAM was almost completed in 15 or 20 s. When the first chains adsorb onto a particle, they can form bridges with other particles and part of the CPAM could be adsorbed on these aggregated particles before the coverage grade reaches such a value that steric repulsion takes place. In that case, flocs are stable, which is consistent with the hypothesis of flattening because the high coverage grade avoids the flattening of the polymer. When the stirring rate increases, the flocs are broken down. However, many of them are formed again after decreasing the stirring intensity when the polymer dosage is 7 mg/L, reaching a situation similar to the one obtained if the stirring was 250 rpm during the whole trial. Considering the polymer flattening, this would increase the coverage grade and reduce the number of tails and loops available for bridging. Consequently, the system would evolve toward a patching situation. After breaking the flocs, the particles can attach to each other by patches. With this low dosage, the ζ potential is around -10 mV, which is only half of the value without CPAM, and the steric repulsion is negligible because of the low coverage grade. These are good conditions for patching reflocculation. When the CPAM dosage is 60 or 120 mg/L, there are steric and electrostatic repulsion forces among particles, because the amount of the polymer adsorbed on particle surfaces is higher than 50% of the maximum adsorption and because, as shown in Figure 5, the particles are positively charged (the ζ potential was around 10 mV). This causes the flocculation rate to decrease, as shown by the kc1 values presented in Table 4. At 250 rpm, the flocs are stable because polymer flattening is limited at a high coverage grade. When the flocs are broken at 750 rpm, more polymer is adsorbed (see Table 3), increasing the repulsive forces, and polymer chains have enough time to change their conformation, increasing the coverage grade to 100%. Therefore, the repulsive forces increase enough to avoid reflocculation. These results demon-
strate that CPAM overdose affects the flocculation kinetics and show the competition between polymer adsorption and bridge formation, which affects floc properties. Figure 8 also shows that the flocs formed with 25 and 60 mg/L of CPAM were harder than the flocs formed with 7 and 120 mg/L of CPAM, as the effect of increasing the stirring rate on the number of counts was lower. This could indicate that there are more junction points among particles and that those junctions are stronger. The steric repulsive forces explain the lower strength of the flocs formed with 120 mg/L of CPAM. At the end of the trial, the number of counts obtained with 25 and 60 mg/L was similar to the one obtained with the optimal dosage. Therefore, a moderate excess of CPAM can improve the floc stability and increase the floc strength, as shown in Figure 8, when the stirring intensity is increased, but a large excess of CPAM affects the flocculation kinetics and the floc properties negatively. Conclusions A fundamental understanding of filler flocculation is necessary to optimize filler retention and wet-end chemistry. The real-time monitoring of the particle chord size distributions allows us to study flocculation mechanisms and floc properties and to distinguish between the flocculation of particles smaller or larger than 0.5 µm. It allows us to study the effect of hydrodynamic forces on the flocculation process and to optimize the flocculant dosage. The effect of the shear forces on the aggregation kinetics for the small and large particles is different, as is observed in the evolution of the number of counts obtained with the FBRM. In the case of PAC, the dosage that neutralizes the streaming current was not enough to neutralize the ζ potential of the particles, which remained almost the same as it was without PAC, indicating the hydrolysis of this flocculant under experimental conditions because of the high pH and the loss of its cationic charge. In the case of the used CPAM, the optimal dosage was 3 times lower than the one required for neutralizing the ζ potential and 9 times lower than the one that neutralizes the streaming current. This proves that the optimization based on the ζ potential or the streaming current leads to overdosing the polymer, which produces charge reversion in the case of using the streaming current for optimization. The optimal dosage of CPAM corresponded to a low coverage grade of the particles; it was around 10% of the maximum CPAM adsorption. This and the fact that it is not necessary to neutralize the ζ potential to induce particle aggregation indicate that the flocculation mechanism is bridging. These results are consistent with the hypothesis of polymer fattening. At the optimal dosage determined by FBRM, the flocs are slightly instable because of the potential flattening of the polymer, but they are reversible. An excess of CPAM, enough to reverse the charge of the particles and to almost completely cover the particle surface, does not affect the floc size significantly but affects the GCC flocculation rate negatively and prevents the reflocculation process because of the steric and electrostatic repulsive forces that are generated among the particles. A moderate excess of CPAM increases the floc strength without greatly affecting the flocculation because it limits the
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potential flattening of the polymer; however, the formed flocs are not reversible when the ζ potential becomes positive. Results indicate that the electrostatic repulsion forces inhibit mainly the reflocculation process and that the steric forces slow the flocculation process. It is challenging to extrapolate these results to the papermaking process, where there are fibers, fines, and additives with the GCC and where the hydrodynamic forces are more intense. However, the relevance of these findings is quite high for filler preflocculation processes, showing the importance of optimizing the polymer dosage to obtain the required floc properties for maximized retention, formation, and drainage rate and minimized polymer consumption. In presence of fibers, three flocculation processes compete: fiber-fiber flocculation, filler attachment on a fiber, and filler-filler flocculation. It is expected that the filler-fiber attachment and the fiber-fiber flocculation will be faster than the filler-filler aggregation because of the influence of size on collision frequencies and that, on the other hand, fiber-fiber flocs would be more susceptible to being affected by the hydrodynamic forces because of their larger size. The mean chord size of the suspension without flocculant would be higher due to the higher diameter of the fibers with respect to filler diameter, and the flocculation mechanism would be similar due to the high molecular weight of the PAM. An increase in the PAM dosage would first increase the floc size, but when the maximum floc size is achieved, a further increase of the dosage would mainly affect floc strength due to an increase of bonding. Acknowledgment The authors wish to express their kind appreciation to Professor Theo van de Ven from the Pulp and Paper Research Centre of McGill, Montreal, for his comments and close collaboration. We also thank the European Commission for financing the project QLK5-CT-200100749, the “Comisio´n Interministerial de Ciencia y Tecnologı´a” for supporting the project CICYT PPQ 30000711.C03-02, and the product suppliers Atofina, Ciba, and Omya for their support. Finally, we thank the “Instituto de Cera´mica y Vidrio, CSIC”, where we carried out the electrophoresis measurements. Literature Cited (1) Thorp, B. Engineered Fillers: An Agenda 2020 Goal. Solutions! 2005, 5 (5), 45-48. (2) Baker, C. F. Assesing the Global Fillers and Pigments Market. Presented at Scientific Advances in Wet End Chemistry: Fillers and Pigments for Papermakers, Lisbon, Portugal, June 2-3, 2005; Paper 1. (3) Huggenberger, L.; Arnold, M.; Ko¨ster, H. H. Ground Calcium Carbonate. Pigment Coating and Surface Sizing of Paper; Papermaking Science and Technology Series; Tappi Press: Finland, 2000; Vol. 11, pp 95-105. (4) Baker, C. F. Emerging Technologies for Fillers and Pigments; Pira International: Leatherhead, U.K., 2005. (5) Eklund, D.; Lindstro¨m, T. Paper Chemistry-An Introduction; DT Paper Science: Grankulla, Finland, 1991. (6) Litchfield, E. Dewatering Aids for Paper Applications. Appita J. 1994, 47 (1), 62-65. (7) Yu, X.; Somasundaran, P. Kinetics of Polymer Conformational Changes and Its Role in Flocculation. J. Colloid Interface Sci. 1996, 178, 770-774. (8) Holm, M.; Manner, H. Increasing Filler Content of Fine Paper by using Preflocculation. In Technical, Technological and
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Received for review July 26, 2005 Revised manuscript received September 26, 2005 Accepted September 26, 2005 IE050870V
