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Aggregate size distributions in flocculation
Colloids and Surfaces A: Physicochemical and Engineering Aspects 177 (2001) 87 – 98 www.elsevier.nl/locate/colsurfa
Aggregate size distributions in flocculation C. Rattanakawin *, R. Hogg Mineral Processing Program, Department of Energy and Geo-En6ironmental Engineering, The Pennsyl6ania State Uni6ersity, Uni6ersity Park, PA 16802, USA
Abstract The evolution of aggregate size distributions resulting from flocculation under a variety of conditions has been investigated using a light scattering technique. By measuring floc size distributions it is possible to distinguish clearly between initial floc formation and growth after the flocs have been formed. Coagulation of dispersed suspensions, under conditions of vigorous agitation, by charge reduction or electrical double layer compression generally produces unimodal size distributions which shift progressively to coarser sizes. Low molecular weight polymers produce size distributions similar to those obtained with salts but with somewhat coarser limiting floc sizes. Flocculation of stable dispersions by high molecular weight polymers gives rise to bimodal size distributions comprising a fine mode consisting of the primary particles and a coarse mode made up of large flocs. Polymer addition leads initially to the formation of some flocs. Flocculation then proceeds by a continuous reduction in the quantity of residual primary particles accompanied by an increase in both the amount and size of the flocs. Complete elimination of the fine mode, corresponding to incorporation of all primary particles into flocs, generally requires high polymer dosage (about 10 mg l − 1 or more for the system studied here). In contrast, the addition of high molecular weight polymers to unstable (pre-coagulated) suspensions causes rapid floc growth at low polymer dosage. (less than 2 mg l − 1), with essentially complete incorporation of the primary particles into the flocs. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Aggregate size; Distributions; Flocculation
1. Introduction Aggregation of colloidal particles suspended in aqueous media can be achieved by flocculation. In most applications the process is used as a pretreatment step prior to solid – liquid separation by sedimentation or filtration. Ideally, all of the fine, dispersed particles should be incorporated into relatively large flocs that settle rapidly, and/or produce permeable filter cakes leaving clear super* Corresponding author.
natants or filtrates. Typically the effectiveness of flocculation processes is measured by indirect performance criteria such as settling rate and supernatant turbidity. These criteria are obviously relevant to solid–liquid separation and provide the information specifically needed for the design and operation of such processes. However, they provide only rather limited information on the flocculation process itself. Since flocculation is a process of aggregation, its performance should be directly measured in term of aggregate (floc) size distribution. The objectives of the study described
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C. Rattanakawin, R. Hogg / Colloids and Surfaces A: Physicochem. Eng. Aspects 177 (2001) 87–98
in this paper were to describe the evolution of floc size distributions under different conditions and to evaluate flocculation mechanisms using the size distributions as direct performance criteria.
2. Experimental Alumina A16SGD obtained from the Aluminum Company of America (Alcoa) was used in this study. This material has a median particle size of 0.43 mm, a density of 3.96 g cm − 3, a BET surface area of 9.51 m2 g − 1, and a point of zero charge (PZC) at pH 8.6 ([1]). The flocculants used in the study were polyacrylamide-based reagents obtained from Betz Mining Chemicals, Trevose, PA and included a high molecular weight nonionic polymer, a high molecular weight and highly anionic polymer and a lower molecular weight cationic polymer. The concentration of the polymer stock solution used was 300 mg l − 1 in all cases. All the experiments were conducted in a 300 ml standard mixing tank of the type described by [2] and applied to flocculation studies by [3]. The tank was equipped with a six flat blade turbine driven by a controllable stirrer. The stirrer speed was monitored by an optical tachometer. The standard test procedure, using continuous flocculant addition at a controlled rate under controlled agitation conditions, described by [3] and [4], was used in all tests. A syringe pump was used to inject the flocculant into the mixing tank at a location adjacent to the turbine. The floc size distributions were measured by a light scattering technique using a Microtrac X-100 (Honeywell, Minneapolis, MN). To avoid breakage of the aggregates the normal Microtrac sample recirculation system was not used. Instead, a modified single pass method was adopted. The modified system employed a 1250 ml sample dilution tank containing a solution with the same pH and ionic strength as the particle suspension medium. A pump, placed after the Microtrac sample cell, was used to withdraw suspension from the dilution tank through the cell where the floc size distribution was measured. After passing through the cell, the suspension was discarded.
The first step in the experimental procedure was to disperse the alumina in distilled water in the mixing tank at 3 wt.% solids. The procedure has been described in detail elsewhere ([5]). pH adjustment using HCl or NaOH and, where appropriate, inorganic salt additions were made immediately following the dispersion step. The suspension was then agitated for 1 min at 1000 rpm. In the case of polymer flocculant addition, the syringe pump was used to control the rate of addition while the addition time was used to regulate the dosage. After a predetermined time had elapsed, the agitation and polymer addition were stopped simultaneously, and the aggregate size distribution was determined as follows: A sample of the flocculated suspension was withdrawn from the mixing tank using a wide mouth eyedropper (to permit large flocs to be collected without breakage) and transferred into the dilution tank of the modified Microtrac for size analysis.
3. Results and discussion Two kinds of aggregation processes were investigated in this study: simple coagulation, defined here as aggregation due to the elimination of electrical double layer interactions, and flocculation, which involves molecular bridging by means of polymers. It will be shown that these two differ not only in the aggregation mechanisms but also in how the floc size distributions evolve in response to reagent addition.
3.1. Simple coagulation Coagulation occurs when the repulsive interaction forces between particles are drastically reduced, and can be accomplished by eliminating the surface charge, e.g. by approaching a point of zero charge (PZC), or by compression of the double layer, i.e. at high ionic strength.
3.1.1. Effect of pH on coagulation of alumina The evolution of floc size distributions as a function of suspension pH is shown in Fig. 1. At pH values below 5 and above 11 there is essen-
C. Rattanakawin, R. Hogg / Colloids and Surfaces A: Physicochem. Eng. Aspects 177 (2001) 87–98
tially no aggregation and the measured size distributions represent the dispersed primary particles. At intermediate values coagulation takes place; the fine, submicron particles are eliminated and a relatively narrow, unimodal size distribution develops. The median aggregate size reaches a maximum at the PZC, pH 8.6 as shown in Fig. 2. At the PZC the net surface charge is zero, the repulsive force between neighboring particles is eliminated and coagulation of the particles due to van der Waals attractive forces occurs readily. Con-
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versely, below pH 5 and above pH 11 the electrostatic repulsive forces from the similarly charged particles dominate and prevent coagulation. It is postulated that the rather small aggregates obtained in these experiments (volume median diameter 56 mm) reflect a dynamic balance between growth and breakage of aggregates in the vigorously agitated suspension. Both the growth rate and the aggregate strength can be expected to be maximum at the PZC, leading to a maximum in the equilibrium aggregate size.
Fig. 1. Effect of pH on floc size distributions for alumina particles in distilled water (3% solids by weight) agitated at a mean shear rate of about 1000 s − 1.
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Fig. 2. Effect of pH on median floc size for alumina (conditions as in Fig. 1).
3.1.2. Effect of ionic strength on coagulation of alumina The effects of NaCl and Na2SO4 addition on the floc size distributions at pH 5 are shown in Figs. 3 and 4. There was no coagulation for NaCl concentrations below 10 − 2 M. When the concentration of NaCl in the suspension was increased to 5×10 − 2 M, a second mode appeared in the distribution at about 1 mm. With further increase in NaCl concentration the height of the coarser mode increased while that of the fine (primary particle) mode decreased. At the same time the coarse mode shifted to larger sizes. Similar behavior was observed for Na2SO4 addition, except that coagulation began at a concentration between 1×10 − 4 and 5× 10 − 4 M and no bimodal distribution was observed at intermediate concentration. Again the maximum median aggregate sizes were quite small — around 3 mm for NaCl and 6 mm for Na2SO4. This coagulation behavior is attributed to electrical double layer compression in the presence of Brownian motion and shear forces. By adding NaCl the double layer is compressed causing a lowering of the repulsive energy barrier. At the same time the Brownian motion and shear forces
promote coagulation by overcoming the barrier between particles and by increasing collision frequencies. The slightly bimodal character of the floc size distributions observed at NaCl concentrations between 10 − 2 and 10 − 1 M may be a consequence of a complex size–stability relationship. Small, submicron particles may be stable while the larger particles and small flocs are subject to coagulation, probably at the so-called secondary minimum ([6]). For Na2SO4 addition there were two important differences compared to coagulation by NaCl. Firstly, coagulation was seen to occur at a substantially lower concentration (B 5×10 − 4 M) for the sulfate than for the chloride (ca. 5 × 10 − 2 M). Secondly, the median aggregate size reached a maximum at about 10 − 2 M and then decreased with further salt addition. Since the alumina particles are positively charged at pH 5, chloride and sulfate are the counter ions so the concentration effect is in general accord with the well-known Schulze–Hardy rule. Almost identical behavior was observed at pH 11 in the presence of NaCl or CaCl2. In this case the particles are negatively charged and sodium and calcium serve as counter ions in the double layer. The effects of electrolyte concentration on median floc size are summarized in Fig. 5. A plausible explanation for the behavior in the presence of divalent counter ions is the charge reversal phenomenon. [7] found that at high concentration of Na2SO4 the SO24 − ions are specifically adsorbed in the Stern layer and can actually reverse the sign of the z potential. Thus by adding these ions to the suspension beyond the point of charge reversal, coagulation decreases due to the increasing negative charge on the surface. At the same time, however, double layer compression prevents the particles from becoming restabilized. At high concentrations aggregation behavior appears to be essentially the same regardless of the charge on the counter ions.
3.2. Polymer-induced flocculation It has been established ([8]) that the performance of polymer flocculants is strongly influenced by the initial state of dispersion of the
C. Rattanakawin, R. Hogg / Colloids and Surfaces A: Physicochem. Eng. Aspects 177 (2001) 87–98
suspended particles. Flocculation of initially stable, well dispersed suspensions generally requires high polymer dosage while initially unstable dispersions can be easily flocculated at much lower polymer dosage — often less than 1/10th of that needed for stable systems.
3.2.1. Initially stable dispersions The floc size distributions resulting from the addition of the nonionic and anionic flocculants at pH 5 are shown in Figs. 6 and 7. As the polymer dosage was increased bimodal floc-size
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distributions developed in both cases. The distributions consisted of a stationary fine mode and a moving coarse mode. The fine mode (ca. 0.35 mm) remains close to that for the primary particles while the coarse mode shifts progressively to larger sizes with increasing polymer dosage. In general flocculation proceeds by a continuous reduction in the height of the fine mode accompanied by an increase in height as well as a gradual coarsening of the coarser mode. As the fine mode disappears the coarse mode eventually becomes a unimodal size distribution. Complete elimination
Fig. 3. Floc size distributions resulting from addition of a monovalent counter ion to alumina suspensions at pH 5. (other conditions as in Fig. 1).
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Fig. 4. Floc size distributions resulting from addition of a divalent counter ion to alumina suspensions at pH 5. (other conditions as in Fig. 1).
of the fine mode appears to require higher dosage (\ 12 mg l − 1) for the anionic polymer than for the nonionic polymer (ca. 7 mg l − 1). On the other hand, the coarse mode seems to progress to larger sizes for the anionic (ca. 200 mm) than for the nonionic polymer (ca. 55 mm). These differences may be a consequence of the charges on the polymer molecules and their conformation in solution. The negatively charged polyanions adsorb strongly on the positively charged particles, while the extended conformation favors the growth of large flocs.
It is interesting to note that addition of the nonionic and anionic polymers to the negatively charged particles at pH 11 gave almost identical results to those obtained at pH 5 ([1]). While this might have been expected for the uncharged, nonionic polymer, the lack of any significant effect of particle charge for the anionic flocculant was somewhat unexpected. [9] also observed this phenomenon. It appears that if the molecular weight of the polymer is high enough and uncharged groups (e.g. amides) are present, adsorption can occur through hydrogen bonding [10] and the
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large molecules are able to bridge between particles, despite the electrical double layer repulsion between them. Generally, the addition of a high molecular weight polymeric flocculant to a stable dispersion gives a bimodal floc size distribution. Ref. [11] has suggested that particle destabilization by polymer adsorption occurs preferentially on coarser material. Consequently, the larger particles present are effectively destabilized by the added polymer and tend to associate into larger flocs while the finer material remains in the dispersed state. Continued polymer addition leads to further growth of the coarse mode as well as adsorption on the high-surface-area fines. The fine mode disappears through collisions between fines and large flocs and eventually by aggregation of the fine particles themselves when ad-
Fig. 5. Effect of electrolyte concentration on median floc size for alumina suspensions: (a) pH 5, (b) pH 11.
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sorption becomes sufficient for effective destabilization. Floc size distributions obtained by adding a cationic polymer to the alumina suspension at pH 11 are shown in Fig. 8. As for the higher molecular weight nonionic and anionic polymers, the distributions became bimodal initially but, in this case, the fine mode was quickly eliminated at quite low polymer dosage (B 0.5 mg l − 1). Floc growth was much more limited for the cationic polymer; the coarse mode appeared to reach a maximum at about 10 mm. Overall, the flocculation behavior of the cationic polymer at pH values greater than the PZC more closely resembles that obtained with inorganic salts than with the other polymers. Essentially no flocculation was observed when the cationic polymer was added to positively charged particles at pH 5. These observations suggest that the primary mechanism for the flocculation with this polymer is charge patch neutralization [12]. The positive charge on the polymer causes adsorption on the negatively charged alumina particles, not only neutralizing the charge but also producing positive patches on the particle. These patches attach electrostatically to bare negative surfaces of other particles upon collision. Like the aggregates obtained from coagulation processes the flocs obtained are generally small, probably because bridging by this low molecular weight flocculant is limited. However, the effective elimination of the primary particles represents a clear advantage of such polymers over the higher molecular weight flocculants for initially stable suspensions. This result agrees with that reported earlier by Mabire et al. [13] who reported that increasing the molecular weight of a cationic flocculant increases the sedimentation rate but also the supernatant turbidity for silica suspensions.
3.2.2. Initially unstable dispersions High molecular weight polymers are extremely effective for the flocculation of suspensions that have already been destabilized by charge reduction (approaching the PZC) or double layer compression (salt addition). An example of the
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Fig. 6. Floc size distributions resulting from addition of a nonionic polymer to an initially stable alumina suspension at pH 5. (other conditions as in Fig. 1).
floc size distributions resulting from the addition of nonionic flocculant to the unstable alumina suspension at pH 8 is given in Fig. 9. Since the original primary-particle mode was already eliminated in the pre-coagulation step there is no tendency for bimodal distributions to develop. Furthermore, floc growth occurs at substantially lower polymer dosage than for the initially stable suspensions. For example, the floc size distribution obtained with 9.6 mg l − 1 of the polymer at
pH 5 (see Fig. 6) requires only 1.6 mg l − 1 at pH 8. It appears [11] that pre-coagulation reduces the particle surface area available for polymer adsorption and favors adsorption on external floc surfaces where it is most effective in promoting floc growth. Experiments with the anionic polymer at pH 8 and both polymers on suspensions pre-coagulated by salt addition gave essentially identical results to those shown in Fig. 9, suggesting that the manner of destabilization is not critical.
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4. Conclusions It is clear from the results presented in this paper that flocculation involves two distinct, and partially independent steps: particle destabilization and floc growth. Charge control and the use of appropriately charged, low-molecular weight polyelectrolytes are very effective for destabilization but not for growth. High-molecular weight polymers, on the other hand, are inefficient desta-
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bilizers but contribute substantially to floc growth. The following specific conclusions can be drawn from the study: (1) Coagulation by charge reduction or simple double-layer compression under conditions of vigorous agitation leads to a narrow size distribution of small flocs (around 5 mm at a shear rate of about 1000 s − 1). (2) Coagulation at a point of charge reversal seems to produce slightly larger flocs than simple doublelayer compression, possibly due to increased ag-
Fig. 7. Floc size distributions resulting from addition of an anionic polymer to an initially stable alumina suspension at pH 5. (other conditions as in Fig. 1).
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Fig. 8. Floc size distributions resulting from addition of a cationic polymer to an initially stable alumina suspension at pH 11. (other conditions as in Fig. 1).
gregation rate coupled with increased aggregate strength. (3) Low-molecular weight polymer coagulants produce larger flocs than inorganic salts or simple charge elimination, probably due primarily to increased floc strength. (4) High molecular weight polymer addition to stable dispersions leads initially to bimodal floc size distributions. Flocculation then proceeds by deposition of primary particles onto larger flocs. Significant floc growth generally occurs after the dispersed primary parti-
cles have been eliminated. (5) Stable dispersions can be flocculated by high molecular weight polymers alone but excessive polymer dosage is required. Acknowledgements C. Rattanakawin wishes to thank the Royal Thai Government for partial support of this study through a graduate scholarship.
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.
Fig. 9. Floc size distributions resulting from addition of a nonionic polymer to an initially unstable alumina suspension at pH 8. (other conditions as in Fig. 1).
References [1] C. Rattanakawin, Aggregate size distributions in flocculation, MS Thesis, The Pennsylvania State University, 1998. [2] F.A. Holland, F.S. Chapman, Liquid Mixing and Processing in Stirred Tanks, Reinhold, New York, 1966. [3] R.O. Keys, R. Hogg, Mixing problems in polymer flocculation, AIChE Symp. Ser. 75 (190) (1979) 63– 72. [4] R. Hogg, R.C. Klimpel, D.T. Ray, Agglomerate structure in flocculated suspensions and its effects on sedimentation and dewatering, Miner. Metall. Process. 4 (1987) 108 – 113. [5] H. Suharyono, Flocculation and consolidation in thickening process, PhD Thesis, The Pennsylvania State University, 1996.
[6] H. Chung, R. Hogg, Stability criteria for fine-particle dispersions, Colloids Surf. 15 (1985) 119 – 135. [7] H.J. Modi, D.W. Fuerstenau, Streaming potential studies on corundum in aqueous solutions of inorganic electrolytes, J. Phys Chem. 61 (1957) 640 – 643. [8] R. Hogg, P. Bunnaul, H. Suharyono, Chemical and physical variables in polymer-induced flocculation, Miner. Metall. Process. 10 (1993) 81 – 85. [9] X. Yu, P. Somasundaran, Kinetics of polymer conformational changes and its role in flocculation, J. Colloid Interface Sci 178 (1996) 770 – 774. [10] P.F. Richardson, L.J. Connelly, Industrial coagulants and flocculants, in: P. Somasundaran, B.M. Moudgil (Eds.), Reagents in Mineral Technology, Marcel Dekker, New York, 1988, pp. 519 – 558.
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[11] R. Hogg, The role of polymer adsorption kinetics in flocculation, Colloids Surf. A 146 (1998) 253– 263. [12] J. Gregory, Rates of flocculation of latex particles by cationic polymers, J. Colloid Interface Sci. 42 (1973) 448 – 456.
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[13] F. Mabire, R. Audebert, C. Quivoron, Flocculation properties of some water-soluble cationic copolymers toward silica suspensions: a semiquantitative interpretation of the role of molecular weight and cationicity through a ‘patchwork’ model, J. Colloid Interface Sci 97 (1984) 120–135.
Aggregate size distributions in flocculation C. Rattanakawin *, R. Hogg Mineral Processing Program, Department of Energy and Geo-En6ironmental Engineering, The Pennsyl6ania State Uni6ersity, Uni6ersity Park, PA 16802, USA
Abstract The evolution of aggregate size distributions resulting from flocculation under a variety of conditions has been investigated using a light scattering technique. By measuring floc size distributions it is possible to distinguish clearly between initial floc formation and growth after the flocs have been formed. Coagulation of dispersed suspensions, under conditions of vigorous agitation, by charge reduction or electrical double layer compression generally produces unimodal size distributions which shift progressively to coarser sizes. Low molecular weight polymers produce size distributions similar to those obtained with salts but with somewhat coarser limiting floc sizes. Flocculation of stable dispersions by high molecular weight polymers gives rise to bimodal size distributions comprising a fine mode consisting of the primary particles and a coarse mode made up of large flocs. Polymer addition leads initially to the formation of some flocs. Flocculation then proceeds by a continuous reduction in the quantity of residual primary particles accompanied by an increase in both the amount and size of the flocs. Complete elimination of the fine mode, corresponding to incorporation of all primary particles into flocs, generally requires high polymer dosage (about 10 mg l − 1 or more for the system studied here). In contrast, the addition of high molecular weight polymers to unstable (pre-coagulated) suspensions causes rapid floc growth at low polymer dosage. (less than 2 mg l − 1), with essentially complete incorporation of the primary particles into the flocs. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Aggregate size; Distributions; Flocculation
1. Introduction Aggregation of colloidal particles suspended in aqueous media can be achieved by flocculation. In most applications the process is used as a pretreatment step prior to solid – liquid separation by sedimentation or filtration. Ideally, all of the fine, dispersed particles should be incorporated into relatively large flocs that settle rapidly, and/or produce permeable filter cakes leaving clear super* Corresponding author.
natants or filtrates. Typically the effectiveness of flocculation processes is measured by indirect performance criteria such as settling rate and supernatant turbidity. These criteria are obviously relevant to solid–liquid separation and provide the information specifically needed for the design and operation of such processes. However, they provide only rather limited information on the flocculation process itself. Since flocculation is a process of aggregation, its performance should be directly measured in term of aggregate (floc) size distribution. The objectives of the study described
0927-7757/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 7 7 5 7 ( 0 0 ) 0 0 6 6 2 - 2
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in this paper were to describe the evolution of floc size distributions under different conditions and to evaluate flocculation mechanisms using the size distributions as direct performance criteria.
2. Experimental Alumina A16SGD obtained from the Aluminum Company of America (Alcoa) was used in this study. This material has a median particle size of 0.43 mm, a density of 3.96 g cm − 3, a BET surface area of 9.51 m2 g − 1, and a point of zero charge (PZC) at pH 8.6 ([1]). The flocculants used in the study were polyacrylamide-based reagents obtained from Betz Mining Chemicals, Trevose, PA and included a high molecular weight nonionic polymer, a high molecular weight and highly anionic polymer and a lower molecular weight cationic polymer. The concentration of the polymer stock solution used was 300 mg l − 1 in all cases. All the experiments were conducted in a 300 ml standard mixing tank of the type described by [2] and applied to flocculation studies by [3]. The tank was equipped with a six flat blade turbine driven by a controllable stirrer. The stirrer speed was monitored by an optical tachometer. The standard test procedure, using continuous flocculant addition at a controlled rate under controlled agitation conditions, described by [3] and [4], was used in all tests. A syringe pump was used to inject the flocculant into the mixing tank at a location adjacent to the turbine. The floc size distributions were measured by a light scattering technique using a Microtrac X-100 (Honeywell, Minneapolis, MN). To avoid breakage of the aggregates the normal Microtrac sample recirculation system was not used. Instead, a modified single pass method was adopted. The modified system employed a 1250 ml sample dilution tank containing a solution with the same pH and ionic strength as the particle suspension medium. A pump, placed after the Microtrac sample cell, was used to withdraw suspension from the dilution tank through the cell where the floc size distribution was measured. After passing through the cell, the suspension was discarded.
The first step in the experimental procedure was to disperse the alumina in distilled water in the mixing tank at 3 wt.% solids. The procedure has been described in detail elsewhere ([5]). pH adjustment using HCl or NaOH and, where appropriate, inorganic salt additions were made immediately following the dispersion step. The suspension was then agitated for 1 min at 1000 rpm. In the case of polymer flocculant addition, the syringe pump was used to control the rate of addition while the addition time was used to regulate the dosage. After a predetermined time had elapsed, the agitation and polymer addition were stopped simultaneously, and the aggregate size distribution was determined as follows: A sample of the flocculated suspension was withdrawn from the mixing tank using a wide mouth eyedropper (to permit large flocs to be collected without breakage) and transferred into the dilution tank of the modified Microtrac for size analysis.
3. Results and discussion Two kinds of aggregation processes were investigated in this study: simple coagulation, defined here as aggregation due to the elimination of electrical double layer interactions, and flocculation, which involves molecular bridging by means of polymers. It will be shown that these two differ not only in the aggregation mechanisms but also in how the floc size distributions evolve in response to reagent addition.
3.1. Simple coagulation Coagulation occurs when the repulsive interaction forces between particles are drastically reduced, and can be accomplished by eliminating the surface charge, e.g. by approaching a point of zero charge (PZC), or by compression of the double layer, i.e. at high ionic strength.
3.1.1. Effect of pH on coagulation of alumina The evolution of floc size distributions as a function of suspension pH is shown in Fig. 1. At pH values below 5 and above 11 there is essen-
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tially no aggregation and the measured size distributions represent the dispersed primary particles. At intermediate values coagulation takes place; the fine, submicron particles are eliminated and a relatively narrow, unimodal size distribution develops. The median aggregate size reaches a maximum at the PZC, pH 8.6 as shown in Fig. 2. At the PZC the net surface charge is zero, the repulsive force between neighboring particles is eliminated and coagulation of the particles due to van der Waals attractive forces occurs readily. Con-
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versely, below pH 5 and above pH 11 the electrostatic repulsive forces from the similarly charged particles dominate and prevent coagulation. It is postulated that the rather small aggregates obtained in these experiments (volume median diameter 56 mm) reflect a dynamic balance between growth and breakage of aggregates in the vigorously agitated suspension. Both the growth rate and the aggregate strength can be expected to be maximum at the PZC, leading to a maximum in the equilibrium aggregate size.
Fig. 1. Effect of pH on floc size distributions for alumina particles in distilled water (3% solids by weight) agitated at a mean shear rate of about 1000 s − 1.
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Fig. 2. Effect of pH on median floc size for alumina (conditions as in Fig. 1).
3.1.2. Effect of ionic strength on coagulation of alumina The effects of NaCl and Na2SO4 addition on the floc size distributions at pH 5 are shown in Figs. 3 and 4. There was no coagulation for NaCl concentrations below 10 − 2 M. When the concentration of NaCl in the suspension was increased to 5×10 − 2 M, a second mode appeared in the distribution at about 1 mm. With further increase in NaCl concentration the height of the coarser mode increased while that of the fine (primary particle) mode decreased. At the same time the coarse mode shifted to larger sizes. Similar behavior was observed for Na2SO4 addition, except that coagulation began at a concentration between 1×10 − 4 and 5× 10 − 4 M and no bimodal distribution was observed at intermediate concentration. Again the maximum median aggregate sizes were quite small — around 3 mm for NaCl and 6 mm for Na2SO4. This coagulation behavior is attributed to electrical double layer compression in the presence of Brownian motion and shear forces. By adding NaCl the double layer is compressed causing a lowering of the repulsive energy barrier. At the same time the Brownian motion and shear forces
promote coagulation by overcoming the barrier between particles and by increasing collision frequencies. The slightly bimodal character of the floc size distributions observed at NaCl concentrations between 10 − 2 and 10 − 1 M may be a consequence of a complex size–stability relationship. Small, submicron particles may be stable while the larger particles and small flocs are subject to coagulation, probably at the so-called secondary minimum ([6]). For Na2SO4 addition there were two important differences compared to coagulation by NaCl. Firstly, coagulation was seen to occur at a substantially lower concentration (B 5×10 − 4 M) for the sulfate than for the chloride (ca. 5 × 10 − 2 M). Secondly, the median aggregate size reached a maximum at about 10 − 2 M and then decreased with further salt addition. Since the alumina particles are positively charged at pH 5, chloride and sulfate are the counter ions so the concentration effect is in general accord with the well-known Schulze–Hardy rule. Almost identical behavior was observed at pH 11 in the presence of NaCl or CaCl2. In this case the particles are negatively charged and sodium and calcium serve as counter ions in the double layer. The effects of electrolyte concentration on median floc size are summarized in Fig. 5. A plausible explanation for the behavior in the presence of divalent counter ions is the charge reversal phenomenon. [7] found that at high concentration of Na2SO4 the SO24 − ions are specifically adsorbed in the Stern layer and can actually reverse the sign of the z potential. Thus by adding these ions to the suspension beyond the point of charge reversal, coagulation decreases due to the increasing negative charge on the surface. At the same time, however, double layer compression prevents the particles from becoming restabilized. At high concentrations aggregation behavior appears to be essentially the same regardless of the charge on the counter ions.
3.2. Polymer-induced flocculation It has been established ([8]) that the performance of polymer flocculants is strongly influenced by the initial state of dispersion of the
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suspended particles. Flocculation of initially stable, well dispersed suspensions generally requires high polymer dosage while initially unstable dispersions can be easily flocculated at much lower polymer dosage — often less than 1/10th of that needed for stable systems.
3.2.1. Initially stable dispersions The floc size distributions resulting from the addition of the nonionic and anionic flocculants at pH 5 are shown in Figs. 6 and 7. As the polymer dosage was increased bimodal floc-size
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distributions developed in both cases. The distributions consisted of a stationary fine mode and a moving coarse mode. The fine mode (ca. 0.35 mm) remains close to that for the primary particles while the coarse mode shifts progressively to larger sizes with increasing polymer dosage. In general flocculation proceeds by a continuous reduction in the height of the fine mode accompanied by an increase in height as well as a gradual coarsening of the coarser mode. As the fine mode disappears the coarse mode eventually becomes a unimodal size distribution. Complete elimination
Fig. 3. Floc size distributions resulting from addition of a monovalent counter ion to alumina suspensions at pH 5. (other conditions as in Fig. 1).
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Fig. 4. Floc size distributions resulting from addition of a divalent counter ion to alumina suspensions at pH 5. (other conditions as in Fig. 1).
of the fine mode appears to require higher dosage (\ 12 mg l − 1) for the anionic polymer than for the nonionic polymer (ca. 7 mg l − 1). On the other hand, the coarse mode seems to progress to larger sizes for the anionic (ca. 200 mm) than for the nonionic polymer (ca. 55 mm). These differences may be a consequence of the charges on the polymer molecules and their conformation in solution. The negatively charged polyanions adsorb strongly on the positively charged particles, while the extended conformation favors the growth of large flocs.
It is interesting to note that addition of the nonionic and anionic polymers to the negatively charged particles at pH 11 gave almost identical results to those obtained at pH 5 ([1]). While this might have been expected for the uncharged, nonionic polymer, the lack of any significant effect of particle charge for the anionic flocculant was somewhat unexpected. [9] also observed this phenomenon. It appears that if the molecular weight of the polymer is high enough and uncharged groups (e.g. amides) are present, adsorption can occur through hydrogen bonding [10] and the
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large molecules are able to bridge between particles, despite the electrical double layer repulsion between them. Generally, the addition of a high molecular weight polymeric flocculant to a stable dispersion gives a bimodal floc size distribution. Ref. [11] has suggested that particle destabilization by polymer adsorption occurs preferentially on coarser material. Consequently, the larger particles present are effectively destabilized by the added polymer and tend to associate into larger flocs while the finer material remains in the dispersed state. Continued polymer addition leads to further growth of the coarse mode as well as adsorption on the high-surface-area fines. The fine mode disappears through collisions between fines and large flocs and eventually by aggregation of the fine particles themselves when ad-
Fig. 5. Effect of electrolyte concentration on median floc size for alumina suspensions: (a) pH 5, (b) pH 11.
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sorption becomes sufficient for effective destabilization. Floc size distributions obtained by adding a cationic polymer to the alumina suspension at pH 11 are shown in Fig. 8. As for the higher molecular weight nonionic and anionic polymers, the distributions became bimodal initially but, in this case, the fine mode was quickly eliminated at quite low polymer dosage (B 0.5 mg l − 1). Floc growth was much more limited for the cationic polymer; the coarse mode appeared to reach a maximum at about 10 mm. Overall, the flocculation behavior of the cationic polymer at pH values greater than the PZC more closely resembles that obtained with inorganic salts than with the other polymers. Essentially no flocculation was observed when the cationic polymer was added to positively charged particles at pH 5. These observations suggest that the primary mechanism for the flocculation with this polymer is charge patch neutralization [12]. The positive charge on the polymer causes adsorption on the negatively charged alumina particles, not only neutralizing the charge but also producing positive patches on the particle. These patches attach electrostatically to bare negative surfaces of other particles upon collision. Like the aggregates obtained from coagulation processes the flocs obtained are generally small, probably because bridging by this low molecular weight flocculant is limited. However, the effective elimination of the primary particles represents a clear advantage of such polymers over the higher molecular weight flocculants for initially stable suspensions. This result agrees with that reported earlier by Mabire et al. [13] who reported that increasing the molecular weight of a cationic flocculant increases the sedimentation rate but also the supernatant turbidity for silica suspensions.
3.2.2. Initially unstable dispersions High molecular weight polymers are extremely effective for the flocculation of suspensions that have already been destabilized by charge reduction (approaching the PZC) or double layer compression (salt addition). An example of the
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Fig. 6. Floc size distributions resulting from addition of a nonionic polymer to an initially stable alumina suspension at pH 5. (other conditions as in Fig. 1).
floc size distributions resulting from the addition of nonionic flocculant to the unstable alumina suspension at pH 8 is given in Fig. 9. Since the original primary-particle mode was already eliminated in the pre-coagulation step there is no tendency for bimodal distributions to develop. Furthermore, floc growth occurs at substantially lower polymer dosage than for the initially stable suspensions. For example, the floc size distribution obtained with 9.6 mg l − 1 of the polymer at
pH 5 (see Fig. 6) requires only 1.6 mg l − 1 at pH 8. It appears [11] that pre-coagulation reduces the particle surface area available for polymer adsorption and favors adsorption on external floc surfaces where it is most effective in promoting floc growth. Experiments with the anionic polymer at pH 8 and both polymers on suspensions pre-coagulated by salt addition gave essentially identical results to those shown in Fig. 9, suggesting that the manner of destabilization is not critical.
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4. Conclusions It is clear from the results presented in this paper that flocculation involves two distinct, and partially independent steps: particle destabilization and floc growth. Charge control and the use of appropriately charged, low-molecular weight polyelectrolytes are very effective for destabilization but not for growth. High-molecular weight polymers, on the other hand, are inefficient desta-
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bilizers but contribute substantially to floc growth. The following specific conclusions can be drawn from the study: (1) Coagulation by charge reduction or simple double-layer compression under conditions of vigorous agitation leads to a narrow size distribution of small flocs (around 5 mm at a shear rate of about 1000 s − 1). (2) Coagulation at a point of charge reversal seems to produce slightly larger flocs than simple doublelayer compression, possibly due to increased ag-
Fig. 7. Floc size distributions resulting from addition of an anionic polymer to an initially stable alumina suspension at pH 5. (other conditions as in Fig. 1).
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Fig. 8. Floc size distributions resulting from addition of a cationic polymer to an initially stable alumina suspension at pH 11. (other conditions as in Fig. 1).
gregation rate coupled with increased aggregate strength. (3) Low-molecular weight polymer coagulants produce larger flocs than inorganic salts or simple charge elimination, probably due primarily to increased floc strength. (4) High molecular weight polymer addition to stable dispersions leads initially to bimodal floc size distributions. Flocculation then proceeds by deposition of primary particles onto larger flocs. Significant floc growth generally occurs after the dispersed primary parti-
cles have been eliminated. (5) Stable dispersions can be flocculated by high molecular weight polymers alone but excessive polymer dosage is required. Acknowledgements C. Rattanakawin wishes to thank the Royal Thai Government for partial support of this study through a graduate scholarship.
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Fig. 9. Floc size distributions resulting from addition of a nonionic polymer to an initially unstable alumina suspension at pH 8. (other conditions as in Fig. 1).
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[13] F. Mabire, R. Audebert, C. Quivoron, Flocculation properties of some water-soluble cationic copolymers toward silica suspensions: a semiquantitative interpretation of the role of molecular weight and cationicity through a ‘patchwork’ model, J. Colloid Interface Sci 97 (1984) 120–135.