- Email: [email protected]
The effect of polyacrylamide charge density and molecular weight on the flocculation and sedimentation behaviour of kaolinite suspensions
Separation and Purification Technology 52 (2006) 241–252
The effect of polyacrylamide charge density and molecular weight on the flocculation and sedimentation behaviour of kaolinite suspensions M.S. Nasser, A.E. James ∗ School of Chemical Engineering and Analytical Science, The University of Manchester, PO Box 88, Manchester M60 1QD, UK Received 8 March 2006; received in revised form 19 April 2006; accepted 19 April 2006
Abstract The effects of the surface charge and molecular weight of anionic and cationic polyacrylamide (PAM) on the surface chemistry, settling rates, floc sizes and sediment bed compactness of kaolinite suspensions has been investigated at pH 7. At optimum polymer concentrations, the kaolinite floc sizes were larger and the settling rates greater in the presence of anionic PAM than cationic PAM. Optimum flocculation for these anionic flocculants was linked to a small reduction in the magnitude of the zeta potential. In the case of cationic polymer, the optimum flocculation was linked to the reduction of the magnitude of the zeta potential to zero by charge neutralization. The results show that the magnitude of the compressive yield stress Py (φ) is strongly dependent upon the floc structure; with greater compressive yield stress being observed for the cationic PAM than for the anionic PAM. The difference in the compression sensitivity of the flocculated slurries may be attributed to floc structure-related adsorption. Cationic polymer chains adsorb via hydrogen bonding interactions between the silanol and aluminol OH groups at the particle surface and polymer’s primary amide functional groups, these electrostatic attractions between the positively charged polymer and the negatively charged kaolinite produce strong and less compressible floc structures. For anionic PAM, however, although the adsorption still occurs through hydrogen bonding between the silanol and aluminol OH groups at the particle surface and polymer’s primary amide functional groups, the interactions appear to be weakened as a consequence of electrostatic repulsion between the negatively charged polymer and negatively charged kaolinite surface. These repulsive forces allow the polymer molecules to be extended and produce loops and tails, which lead to the formation of large open-structure flocs having less resistance to compression loads and, subsequently, produce compact sediments having greater gel points φg following the application of some compression load. It is found that the compressive yield stress and gel point are important factors that should be considered when selecting the type of polyelectrolyte for use in a particular solid liquid separation. A strong correlation between the polymer type and flocculation, compression sensitivity of flocs and sedimentation behaviour of kaolinite is established. © 2006 Elsevier B.V. All rights reserved. Keywords: Kaolinite flocculation; Polyacrylamide; Charge density; Compressive yield stress; Gel point
1. Introduction In the mineral industry, waste residues known as tailings are produced and these must be disposed of in an environmentally and economically acceptable manner. Many of these wastes contain problem minerals such as kaolinite, which due to small particle size and charge properties are difficult to settle and consolidate. The conventional method used to flocculate colloidal particles (e.g. kaolinite) involves high molecular weight polymer-aided flocculation and subsequent sedimentation in thickeners or centrifuges. Several types of polymeric flocculants are currently used with kaolinite systems. Of these, polyacry-
∗
Corresponding author. Tel.: +44 161 306 4368; fax: +44 161 306 4399. E-mail address: [email protected] (A.E. James).
1383-5866/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2006.04.005
lamides form the most common type of polymeric flocculant and are employed because of their effectiveness as flocculants and ability to produce good settling performance for relatively low cost. Kaolinite, known chemically as a 1:1 tetra octahedral aluminium silicate having the general formula Al2 O3 ·2SiO2 ·2H2 O, has two different basal cleavage faces [1]. One basal face consists of tetrahedral siloxane (–Si–O–Si–) species [1], while the other consists of an octahedral alumina (Al2 O3 ) sheet. The basal face, displaying an inert siloxane structure carries a permanent negative charge as a result of isomorphous substitution of Si4+ by Al3+ groups [1]. At the edge of the crystal, the octahedral alumina and tetrahedral silica sheets are disrupted and broken bonds exposing aluminol (Al–OH) and silanol (Si–OH) groups occur. The edge face may be charged as a result of protonation and deprotonation of the surface hydroxyl groups, depending on pH [1,2].
242
M.S. Nasser, A.E. James / Separation and Purification Technology 52 (2006) 241–252
Nomenclature CAK dA g h0 h∞ Py (φ) Q Q0 VSA
ratio of volume concentration of the aggregates to kaolinite average (equivalent) aggregate diameter (m) local gravitational acceleration (980 cm/s2 ) initial height of the suspension (m) equilibrium height of the sediment bed (m) compressive yield stress (Pa) settling rate of slurry-supernatant interface (cm/h) initial settling rate (cm/h) Stokes’ velocity for single aggregates (cm/h)
Greek symbols ε void fraction εd dielectric constant μ viscosity of the suspending liquid (cp) μw viscosity of water, 0.893 cp (at 25 ◦ C) ρw density of water (g/cc) density of kaolinite (2.6 g/cc) ρK φK kaolinite volume fraction φA aggregate volume fraction φ0 initial volume fraction φg volume fraction at gel point ωc volume solids per unit area
Understanding the interaction between kaolinite and polyacrylamide flocculant is important in enabling the mining industry to optimize flocculant performance. Several studies have been performed, examining the adsorption of polyacrylamide on
to kaolinite [3–7]. All authors have concluded that the adsorption of polyacrylamide occurs primarily onto the edge surface of kaolinite (i.e., on the broken bonds of aluminol (Al–OH) and silanol (Si–OH) groups) via hydrogen bonding, with Lee et al. [7] stating that 94% of adsorption takes place at the edge surface. Flocculation of fine particles may occur by polymer bridging, charge neutralization, polymer–particle surface complex formation and depletion flocculation, or by a combination of these mechanisms [8]. The bridging mechanism requires that the polymer chains be adsorbed on the particle surfaces, with only a few points of attachment, with the bulk of the chains projecting into the surrounding solution for contact and adherence to other particles [9]. Strong adsorption does not favour the flocculation process, because strong adsorption can cause surface saturation, preventing effective bridging and restabilising fine particles (see Fig. 1). The flocculation of negatively charged fine particles by high molecular weight cationic polyacrylamide occurs by the adsorption of the polyacrylamide chains on to the particle surface. Thus charge neutralization becomes a major mechanism, where the cationic polyacrylamides will locally reverse the particle surface charge. Collision with negative patches on another particle allows bridging then aggregation [9]. On the other hand, with low molecular weight cationic flocculants adsorbing on to the negative particles, bridging would not be favoured due to the short macro-ion length and strong adsorption (see Fig. 2). Several studies have shown that high molecular weight anionic polymers (e.g. polyacrylamide) are commonly used in the settling of negatively charged kaolinite [10–13]. In this case, the polymer bridging mechanism is of primary importance, whereas charge neutralization will be of secondary or little importance [10]. The use of high molecular weight anionic
Fig. 1. Polymer conformation on adsorption.
Fig. 2. Differences in bridging mechanism for high and low molecular weight cationic polymers.
M.S. Nasser, A.E. James / Separation and Purification Technology 52 (2006) 241–252
polyacrylamide in flocculating negatively charged particles has the advantage of being more effective than cationic polymers by increasing settling rate and producing a distinct sediment structure, and in addition the restabilisation of kaolinite particles by excessive polymer adsorption driven by strong electrostatic attraction may be avoided. The electrostatic repulsion between kaolinite particles and the anionic polyacrylamide allows only limited polymer adsorption. On the other hand, the polymer molecule expansion arising from charge repulsion produces loops and tails, which lead to the formation of large openstructure flocs. This effect has been seen to be effective in flocculating negatively charged kaolinite dispersions [14]. Extensive research is required to optimize the use of different molecular weight and surface charge polymers as flocculants to improve the settling of fine particle suspensions. Toward this aim, the settling rate, supernatant turbidity, polyelectrolyte adsorption, zeta potential, floc size, and sediment bed compactness have been measured for kaolinite suspension at natural pH. Kaolinite tailings are commonly generated in the mineral industry at natural pH, where the kaolinite particles are invariably negatively charged and consequently tend to form stable dispersions with poor flocculation characteristics. 2. Theory
From a material balance on the kaolinite, it follows that ρs − ρw = φA (ρA − ρw ) = φk (ρk − ρw )
(3)
Eq. (2) is rewritten as Q0 =
g(ρk − ρw )dA2 (1 − CAK φK )4.65 18μw CAK
(4)
where CAK =
φA volume of aggregate = φk volume kaolinite in aggregates
(5)
If dA is expressed in microns, density of kaolinite, ρK , is 2.6 g cm−3 and viscosity of water, μw , is 0.893 cp at 25.0 ◦ C, then VSA , the Stokes’ settling velocity for single aggregate is VSA = 0.349
dA2 CAK
(6)
Finally, Eq. (4) can be rewritten [2]: 1/4.65
Q0
1/4.65
= VSA
(1 − CAK φK )
(7)
1/4.65
Therefore, if one plots Q0 against the corresponding value of φK , a straight line should result. From the values of the slope and intercept one, can estimate the aggregate size. 2.2. Sediment bed behaviour
2.1. Floc size measurements The Richardson and Zaki [15] equation for the group-settling rate for uniform spherical particles can be written: Q = VSA ε4.65
(1)
Michaels and Bolger [2] suggest that the aggregate diameter, dA , is relatively independent of kaolinite concentration over the “dilute” range, and that dA dos not change once settling has begun. The dilute settling rate should then be given by Eq. (1) in the form: Q0 =
243
g(ρA − ρw )dA2 (1 − φA )4.65 18μw
(2)
The sediment bed of flocculated suspension has a concentration gradient from a maximum at the base up to a critical volume fraction, the gel point, φg at the top of the sediment (see Fig. 3). Above the top of the sediment, the volume fraction will be unchanged from φ0 and above this is a clear liquid region. This critical volume fraction, φg , may be considered to be the lowest volume fraction at which all primary flocs are interconnected throughout the container, and a network is just formed [16,17]. The volume fraction of solids will increase from the gel point to the maximum packing fraction by the application of the consolidation pressure, P. As this pressure is applied, the network structure will resist further compression until the compressive forces become so strong that the structure begins to deform irreversibly. Bonds between particles will rupture, the
Fig. 3. Schematic of a sedimentation process, showing flocculated kaolinite suspension.
244
M.S. Nasser, A.E. James / Separation and Purification Technology 52 (2006) 241–252
system will consolidate and, consequently, new and more bonds will form until the network has become strong enough to oppose the compression once more. Hence, the compressive yield stress Py (φ) is the stress which must be exceeded by an applied load, P, before the consolidation will occur and the local volume fraction, φ, will increase. In other words, the compressive yield stress Py (φ) can be thought of as a measure of the resistance of the network to consolidate to a higher volume fraction under an applied compressive pressure [17]. Thereafter, the values of the gel point, φg , and compressive yield stress; Py (φ), can be used to show the compactness of kaolinite sediment beds for different polyacrylamide types. 2.2.1. Compressive yield stress The compressive behaviour of a networked suspension can be characterised by the compressive yield stress Py (φ). It is well known that sediments containing strongly attractive particles produce sediment beds of lower density than weakly attractive particles [18,19]. As mentioned before the volume fraction of solids in a sediment network can be increased from gel point to the maximum packing fraction by the application of a consolidation pressure. Networks produced from particles with differing magnitude of attraction evolve their structure and volume fraction differently in response to compression, i.e., it takes a greater pressure for strongly attractive networked of high compressive yield stress to reach maximum packing fraction than weakly attractive networks [18,19]. Hence, the compressive yield stress is important in both dewatering operations and the down stream processes. For instance, if compact sediment is required in batch sedimentations or a concentrated underflow suspension is needed in a continuous thickening the compressive yield stress should be low. In contrast, in filtration operations where the strength flocs are important, the compressive yield stress should be significantly higher. Buscall and White [17] estimated the compressive yield stress of latex and bentonite suspensions from batch centrifuge experiments. The experiments utilized a centrifuge to consolidate to equilibrium sediments of material at each of a series of consecutively increasing gravitational fields. It was concluded that, for most practical applications, the use of a mean value approximation for the determination of Py (φ) from equilibrium sediment height data was sufficiently accurate. The mean value approximation allows determination of Py (φ) [17]: h∞ Py (φ) = ρgφ0 h0 1 − (8) 2R
2.2.2. Gel point The gel point, φg , of a suspension is a useful parameter for gaining knowledge regarding the nature of the compressibility and structure of sediments. The gel point is related to the strength of attractive bonds between the particles, with stronger attractions creating less compressible (high compressive yield stress) flocs of lower gel points. Direct determination of the gel point is not often undertaken, due in part to the difficulty of measurement [20,21]. As mentioned previously, φg is the lowest concentration at which flocs are able to form a self-supporting network (this is a similar concept to null stress solids, εs0 , defined in other work [21]). If a sediment at the gel point is considered, the weight of each individual particle and floc because of the structure, is transmitted through the network and as a result, flocs within the sediment are subjected to compressive forces arising from the self-weight of the over laying material [16,17]. Hence, material at the top of the sediment bed will experience no compressive force but this increases with depth. The compressive force at the base of any sediment bed will thus be a maximum. In order to measure the gel point, the volume fraction at the top of the sediment bed, to which the flocs settle in the absence of compression, needs to be measured. An indirect method for obtaining gel point used by Tiller–Khatib [21] is based upon determination of the volume of solids per unit area as a function of the ultimate height of the sediment. The volume of the solids/unit area is given by h ωc = φ dx (10) 0
Assuming that φ is a unique function of the distance x below the surface, differentiation of Eq. (10) yields [19]: dωc (11) dh where φ(h) is the volume fraction of solids at distance h. This relationship allows the determination of the solids concentration at given depth below the top of the sediment from a set of equilibrium batch settling tests, each starting with a different amount of solids suspensions height (as given by wc , which is equal to the product φ0 h0 ) [20]. This relation can be used in determining the gel point φg Eq. (11) can be written as [20]: φ(h) =
φg (h∞ ) =
and
+s φ0 h0 1 − h∞ 2R φ= 2 h∞ ∞ (h∞ + s) 1 − R + h2R
[24]. In some of these studies, data obtained using equilibrium sediment techniques has been shown to agree with data obtained from other compressive yield stress determination techniques.
(9)
where g is the centrifugal acceleration (g = ω2 R), R the radius of rotation at the bottom of the sediment, s = (dh∞ /d ln g). This approach has been utilized in several other subsequent investigations for various materials such as cement paste [22], clay-based coal tailings [23], alumina suspension and zirconia
d(φ0 h0 ) dh∞
(12)
where φ0 and h0 are the initial volume fraction of solids and initial height of suspension, respectively, and h∞ is the equilibrium height of the sediment bed. The initial volume fraction should be much lower than the volume fraction at which the gel point is expected to be found. Otherwise, the resultant intercept, which is simply the solids concentration at the top of the bed, will give the initial solids concentration. Eq. (12) shows that different h∞ can be achieved by either holding φ0 and changing h0 , or h0 constant and changing φ0 or
M.S. Nasser, A.E. James / Separation and Purification Technology 52 (2006) 241–252
allowing both to vary. Thus the gel point, which is effectively the solids concentration at the surface of the sediment, can be found from the initial slope (h∞ → 0) of φ0 h0 as a function of h∞ obtained from sequences of settling tests. 3. Experimental 3.1. Materials The kaolinite used was ‘Supreme’ grade china clay provided by courtesy of Imerys, UK. The major dimension of a kaolinite particle is between 2.0 and 0.1 m in diameter and the thickness ratio is between 30:1 and 10:1. The five different polyacrylamides used as flocculants in this study were provided by Cytec industries Inc., Bradford, UK. The characteristics of the polyacrylamides are summarized in Table 1. 3.2. Methods 3.2.1. Kaolinite purification It is well known that some deflocculants are added to the kaolinite for stabilisation purposes, and there is a low level of natural impurity (Imerys literature). In order to insure that the kaolinite suspensions created in the experiments were of a consistent nature, the surface preparation methods of Williams and Williams [25] were used to prepare homoionic sodium kaolinite suspensions. The technique involved repeated washing of the kaolinite in 1 M NaCl at pH 3; the kaolinite–NaCl suspension was then prepared and allowed to settle over a 24 h period, supernatant was removed and fresh 1 M NaCl was added. The electrophoretic mobility of the kaolinite suspension was checked periodically until consistent measurements were obtained. In our case, it was found that the mobility became constant after six washes. Following this technique, further washing with distilled water caused the sodium ions to be expelled from the kaolinite by osmotic diffusion. About 8–12 washes, each lasting 24 h, were required to reduce the supernatant conductivity to 2 S and to produce the kaolinite particle in its natural unsubstituted form. Subsequently, the kaolinite slurries were stored under distilled water. 3.2.2. Polymer adsorption experiments The adsorption measurements were performed by batch technique to obtain equilibrium data. For isotherm studies, adsorpTable 1 Polyacrylamide characteristics Polymer
Molecular weight × 10−6 (g mol−1 )
Charge type
Charge density (%)
C 446 C 496 C 492 A130LMW A130 A100
3–4 5–7 5–7 3–4 10–12 10–12
Cationic Cationic Cationic Anionic Anionic Anionic
35 35 10 35 35 10
245
tion experiments were carried out in 250 ml flasks by adding a known concentration of flocculants to 100 ml of kaolinite dispersion (of 2.0% by volume solids). The suspension was agitated by a rotating shaker (Infros AG CH-4103, Bottnigen, UK) at 120 rpm for 24 h. The suspended colloidal particles were centrifuged at high speed until all the residual colloidal particles had subsided, leaving clear supernatant, which was transferred for titration with a solution of potassium polyvinylsulfate (KPVS), and using tluidine blue dye as the indicator. Such types of titrant were used to estimate the changes in cationic polyacrylamide concentration [26], because only a cationic polyacrylamide will form a polyelectrolyte complex with negatively charged KPVS. When the dye is present in solution, the excess KPVS complexes with the oppositely charged cationic dye will produce a colour change from blue to purple at the end point. As for determination of the changes in anionic polyacrylamide concentration, the technique required some modification. The negatively charged polyacrylamide solution must be first treated with excess solution of polydimethydiallylammonium chloride (PDMDAAC), which is then back titrated with KPVS using the tluidine blue dye as indicator. 3.2.3. Zeta potential measurements The zeta potential of kaolinite suspensions was measured using a Zetasizer 3000 (Malvern Instruments Ltd., UK). The calculation of the zeta potential was assisted with modern software. The equipment measures the electrophoretic mobility of the particles, and converts it to the zeta potential using the von Smoluchowski equation. The von Smoluchowski equation, the most elementary expression for zeta potential, provides a direct relationship between zeta potential and electrophoretic mobility: ξ=
4πμ U εd
(13)
where U is electrophoretic mobility, μ the viscosity of the suspending liquid, εd the dielectric constant, π the constant and ξ is the zeta potential. The zeta potential measurements were carried out as a function of polyacrylamide concentration at pH 7. In this test, 50 ml of 2.0 g l−1 of kaolinite suspension were transferred to a 250 ml flask, to which 50 ml of the required polyacrylamide solution was added, yielding a final kaolinite concentration of 1.0 g l−1 . The kaolinite suspension was agitated by a rotating shaker at 120 rpm for 1 h; the samples were allowed to settle for 20 min to allow the larger particles to settle. An aliquot taken from the supernatant was used to measure the zeta potential. 3.2.4. Settling tests The treated kaolinite slurry was used to prepare 50 ml of kaolinite suspension, at 4% solids by volume, in a series of 250 ml flasks. The 2% by volume suspensions used in the experiments were produced by adding 50 ml of polyacrylamide in varying concentrations (1–300 mg l−1 ), and adjusted to pH 7 using appropriate amounts of 0.1 M NaOH or HCl. Following polyacrylamide addition, the suspension was mixed for 1 min, and thereafter the prepared kaolinite suspensions were transferred to
246
M.S. Nasser, A.E. James / Separation and Purification Technology 52 (2006) 241–252
Table 2 Optimum polymer concentration corresponding to the maximum settling rate and minimum turbidity of kaolinite suspension in 10−3 M NaCl solution and pH 7 Polymer
Maximum settling rate (cm min−1 )
Minimum turbidity (NTU)
Optimum concentration (mg l−1 )
C 446 C 496 C 492 A130 LMW A130 A100
4.1 4.8 5.7 4.6 5.8 7.9
7 7 6 41 26 18
12 12 12 6 3 3
100 ml measuring cylinders (30 mm in diameter). Subsequently, the thickness of the interface was recorded with settling time. 3.2.5. Turbidity of the supernatant Turbidity of kaolinite suspensions was measured in nephelometric turbidity units (NTU) using a turbidity meter (WPA TU100, UK). The flocculation was carried out by 1 min rapid mixing (200 rpm) of the 10 g l−1 kaolinite suspensions of known polymer concentration at pH 7, then 10 min slow mixing (20 rpm) to promote the flocculation. Subsequently, the suspensions were allowed to settle for 20 min. After that, a 5 ml aliquot of the supernatant liquid was pipetted off and transferred to a glass cuvette to determine the turbidity. 3.2.6. Floc size The treated kaolinite slurry was used to prepare 500 ml of kaolinite suspension of different volume fractions in the range 0.00125–0.02. The preparation was made using the optimum polymer concentration (see Table 2) at pH 7. The suspension was mixed for 1 min, and thereafter the prepared kaolinite suspensions were transferred to 500 ml measuring cylinders (5.5 cm in diameter). Subsequently, the settling rate was recorded as a function of time. Therefore, considering Eq. (7), if one plots the 1/4.65 initial settling rates Q0 as a function of the corresponding kaolinite volume fractions φK a straight line should result and from the slope and intercept one can estimate the floc size. 3.2.7. Compressive yield stress measurements A batch centrifuge (Mistral 1000, UK) was used in the compressive yield stress measurements. The centrifuge tubes used in these measurements held 50 ml of suspension and had a diameter of 28 mm. Different rotational speeds in the range of 200–1000 rpm were used giving accelerations 7–180 g. In these measurements the optimum polyacrylamide concentrations were used to study the consolidation rate for kaolinite suspensions. Fig. 4 shows the systematic procedure used to determine the compressive yield stress Py (φ) as function of φ. 3.2.8. Gel point A series of 100 ml measuring cylinders were used for the gel point measurements and both φ0 and h0 were varied. However, in these experiments, four kaolinite suspensions having different φ0 in the range 0.25–1.0% by volume were used, and
Fig. 4. Systematic procedure to measure the compressive yield stress Py (φ).
four different h0 in the range 39–185 mm were used for each φ0 . The optimum polyacrylamide concentrations were used in these measurements. The suspension was then left to settle, and h∞ was recorded when it became constant. Subsequently, the gel point was estimated using Eq. (12) by plotting the initial volume fraction per unit area (φ0 h0 ) as function of equilibrium height (h∞ ). 4. Results and discussion 4.1. Adsorption experiments Fig. 5 shows the adsorption of polymers in 10−3 M NaCl solution at pH 7.0 as a function of polymer concentration. The cationic polymer chains adsorb through hydrogen bonding interactions between the silanol and aluminol OH groups at the particle surface and the polymer’s primary amide functional groups, these electrostatic attractions between the positively charged polymer and the negatively charged kaolinite promotes the adsorption mechanism. It was found that the amount adsorbed increased with increasing a polymer concentration. On the other hand, the adsorption behaviour of polyacrylamide
M.S. Nasser, A.E. James / Separation and Purification Technology 52 (2006) 241–252
Fig. 5. Adsorption isotherms for 50 g/l (2% volume fraction) of kaolinite in 10−3 M NaCl solution as a function of polymer dose at pH 7.
on to kaolinite particles is dependent on the charge density of each polyacrylamide. The isotherm for the adsorption of the cationic C 492, which has a charge density of 10%, approached a plateau at 150 mg l−1 of polymer dosing and the maximum polymer adsorbance achieved is 1.5 mg g−1 kaolinite. The adsorption values increased to 3.4 mg g−1 kaolinite for the case of C 496, because of the higher charge density of 35% and no plateau state reached indicating strong adsorption caused by the surface charge. Fig. 5 also shows a marked inflection (i.e., a levelling off before a steep rise) for the cationic polymers C 496 and C 446 at 250 mg l−1 . A possible interpretation is that a first layer of polymer is absorbed, neutralizing the surface charge, and then further layers start to form once the polymer is able to absorb through other bonding mechanisms (e.g. hydrogen or hydrophobic bonding). The molecular weight has only small effect on the adsorption isotherms, where similar behaviour was observed for both C 496 and C 446. In the case of anionic polymers, however, although the adsorption still occurs through hydrogen bonding between the silanol and aluminol OH groups at the particle surface and polymer’s primary amide functional groups, the interactions appear to be weakened as a consequence of electrostatic repulsion between the negatively charged polymer and negatively charged kaolinite surface. These repulsive forces minimises the adsorption rate. It was found that the maximum adsorption value for A100 is 0.6 mg g−1 , and this is considerably smaller compared with cationic polymer. On the other hand, increasing the charge density of anionic polymer to 35% in the case of A130 A130LMW further reduces the adsorption values. 4.2. Zeta potential Fig. 6 shows the zeta potential of kaolinite in 10−3 M NaCl solution at pH 7.0 as a function of polymer concentration.
247
Fig. 6. Electrokinetic zeta potentials for 1.0 g/l of kaolinite in 10−3 M NaCl solution as a function of polymer dose at pH 7.
Prior to flocculant addition, the particles have moderate negative charge and corresponding zeta potential (−33 mV). The reduction in the zeta potential to zero by the cationic polyacrylamide arises from charge neutralization through electrostatic attraction between the positively charged polymer and negative surface sites of the particle, and the adsorption of large amount of positively polymer onto the surface of kaolinite gives rise to charge reversal. The amount of polymer required to neutralize the particle charge is inversely proportional to charge density. This confirms the charge neutralization mechanism; where the high charge density polymers neutralize the surface charge of kaolinite more rapidly than the low charge density polymers, and have require a lesser amount of polymer. In addition, at a concentration of 250 mg l−1 both polymers C 496 and C 446 show inflections (i.e., a levelling off before a steep rise), which are similar to the inflection point shown in the adsorption experiment (see Fig. 5). This seems to confirm the ability of the polymers to neutralize the particles’ surface charge, and to form further layers through other bonding mechanisms (e.g. hydrogen or hydrophobic bonding). In the case of anionic polyacrylamide, the small decrease in the magnitude of the zeta potential with increasing polymer concentration does not arise from charge neutralization but rather from the shift in position of the plane of shear, due to the adsorbed layer, which is caused by the negatively polymer chains. Similar behaviour has been reported by Patience et al. [10]. 4.3. Determination of optimum dosages of polyacrylamide The settling rate and turbidity measurements for kaolinite suspensions were plotted as a function of polyacrylamide concentration at constant pH (see Figs. 7 and 8). The maximum settling rates of flocs produced by cationic and anionic polyacrylamides decrease with charge density and increase with molecular weight. The increase of charge density in polymers
248
M.S. Nasser, A.E. James / Separation and Purification Technology 52 (2006) 241–252
Fig. 7. Settling rates of 50 g/l (2% volume fraction) of kaolinite and turbidity measurements of 10 g/l (0.4% volume fraction) of kaolinite in 10−3 M NaCl solutions as a functions of cationic polymer dose at pH 7.
results in a loss of flocculation power, because the strong adsorption of the cationic polymer chains could rapidly saturate the particle surface sites, preventing effective bridging (see Fig. 1). In addition, the low molecular weight of the cationic flocculants adsorbing on to the negative particles does not favour bridging, because of the short macro-ion length and strong adsorption (see Fig. 2). As a result, the lowest settling rate was observed for the cationic polymer C 446. In the case of high molecular weight anionic polymers, the settling rate observed was faster than with the cationic polymers. Because the kaolinite has a negative zeta potential this prevents the strong adsorption of anionic polymers, leaving long polymer chains to form bridges with other particles and resulting in large flocs (see Section 4.4). On the other hand, increasing the anionic charge density up 35% in the case of A130 and A130LMW flocculants will reduce the settling rate. Therefore, many factors have to be considered in order to select the best charge type and density, and molecular weight of polymer, for any given application. The turbidity results show good agreement with the settling rates; the maximum settling rates were found to correspond to minimum turbidity for all polymers. The optimum concentration
corresponding to the maximum settling rate and minimum turbidity is summarized in Table 2 for each polymer investigated, and the results show that the high negative zeta potentials of the particles treated with anionic polymer are linked to lower supernatant clarities than those treated with cationic polymers. On the other hand, because of the effective bridging, optimum conditions are reached using smaller doses of the anionic polymers. 4.4. Floc size Fig. 9 shows the settling rate as a function of kaolinite volume fraction applying Eq. (7) for the A100 polyacrylamide–kaolinite system at pH 7 using the optimum polymer concentration. Volume fractions of kaolinite in the range of 0.00125–0.015 were found to delineate the limits of the dilute region for all types of polyacrylamide used in this study. The parameters VSA , CAK and dA determined for each of the five different polyacrylamides used in this study are shown in Table 3. The results show that the floc sizes of the kaolinite produced by anionic polyacrylamides are larger than those for cationic polyacrylamides. Anionic polyacrylamides are negatively charged and as mentioned earlier
M.S. Nasser, A.E. James / Separation and Purification Technology 52 (2006) 241–252
249
Fig. 8. Settling rates of 50 g/l (2% volume fraction) of kaolinite and turbidity measurements of 10 g/l (0.4% volume fraction) of kaolinite in 10−3 M NaCl solutions as functions of anionic polymer dose at pH 7.
the adsorption is through hydrogen bonding between the silanol and aluminol OH groups at the particle surface and polymer’s primary amide functional groups, the repulsion between the negatively charged polymer and negatively charged kaolinite surface allow the polymer molecules to be extended and produce loops and tails, which lead to the formation of large and
open-structure flocs. On the other hand, increasing the anionic surface charge from 10% to 35% reduces the floc size. This again in evidence confirming that increasing the anionic charge will reduce the flocculation power. In the case of the cationic flocculants the floc sizes of the kaolinite were found to decrease as the adsorption rates increase. This is because very strong adsorption will saturate the particles surface and that restrict the bridging process. This is clear from the values of the floc size, which decrease as the surface charge increases due to the strong adsorption. In addition, loss of bridging and strong adsorption is found in the results for the low molecular weight cationic polymer and these factors also reduce the floc size (see C 446 in Table 3).
Table 3 Floc structure parameters obtained from dilute settling rate data at pH 7
Fig. 9. Settling rate–volume fraction correlation of dilute settling rates for kaolinite suspensions flocculated by 3 mg/l of A100 polymer in 10−3 M NaCl solutions and pH7.
Polymer
VSA (cm h−1 )
CAK (–)
dA (m)
C 446 C 496 C 492 A130 LMW A130 A100
1334 1542 1620 1486 1652 1718
4.2 5.3 5.7 4.6 5.9 6.9
127 153 163 141 167 184
250
M.S. Nasser, A.E. James / Separation and Purification Technology 52 (2006) 241–252
4.5. Compressive yield stress Fig. 10 shows the compressive yield stress–volume fraction relationships for sediment using different polyacrylamides at optimum concentrations considered (see Table 2) and pH 7. The flocculated kaolinite suspensions exhibit a power law dependence on volume fraction. The compressive yield stress, Py (φ), follows the trend: C 446 > C 496 > C 492 > A130LMW > A130 > A100. The compressive yield stress, Py (φ), for the anionic polymers is smaller than for the cationic polymers. An explanatory summary of sediment bed behaviour of kaolinite flocculated by cationic and anionic polyacrylamides is depicted in Fig. 11. Starting with similar volume fraction of kaolinite, and settling/centrifuging columns, both flocculated suspensions show similar initial suspension heights (h0 ) but different floc structures. The electrostatic attraction between the positively charged polymer and the negatively charged kaolinite
Fig. 10. Compressive yield stresses for different polyacrylamides in 50 g/l (2% volume fraction) of kaolinite in 10−3 M NaCl, using the optimum polymer concentration at pH 7.
Fig. 11. Schematic diagram showing ways in which different in kaolinite floc structures produce different equilibrium sediment height. Although the initial volume fractions of kaolinite and initial sediment heights are the same, application of the same pressure to both sediments (see diagrams (c) and (d)) induces greater deformation of the anionic flocs. The flocs produced by cationic polymer are stronger and have a high compressive yield stress (see diagrams (a) and (c)) while in contrast the weaker flocs produced using anionic polymer have low compressive yield stress resulting in a more compact sediment (see diagram (b) and (d)).
M.S. Nasser, A.E. James / Separation and Purification Technology 52 (2006) 241–252
allows the cationic polymer to produce strong flocs. In case of anionic polymers, the electrostatic repulsion between the negatively charged polymer and negatively charged kaolinite surface, allows the polymer molecules to be extended and produces loops and tails, which leads to the formation of large, open and fragile flocs. Under compressive load, the strong flocs produced by cationic polymer are more capable for resisting deformation, thereby producing a less compact sediment that corresponds to a high compressive yield stress. This was apparent from the observed equilibrium sediment heights (h∞ ) found following centrifugation. The flocs produced by anionic polymers tend to be open and more deformable, and thereby produce more compact sediments. For the practical purposes, if compact sediment is required in, say, batch sedimentations or a concentrated underflow suspension is needed in, say, a continuous thickener the use of anionic polymer is favoured. In contrast, in some filtration operations where the strength of flocs is important, the use of cationic polymer is favoured. This type of behaviour is observed by Pearse and Barnett [9] who carried out filtration experiments using kaolin flocculated with different polyacrylamides. These authors report that the mechanically strong flocs produced by cationic polymers are much more effective in filtration than the weakly structured flocs resulting from the use of anionic polymers. In the present study, the loss of bridging and strong adsorption for the low molecular weight and high surface charge cationic polymer (i.e., C 446) allows most of the primary flocs to remain in suspension and additionally has more resistance to consolidate, so that a less compact sediment is produced during the centrifugation process because of the poor flocculation. 4.6. Gel point The gel point of the suspension was determined using the method of Tiller–Khatib [21]. The results of different initial volume fractions per unit area (φ0 h0 ) as a function of the corresponding h∞ , for kaolinite suspensions flocculated by A100 at the optimum concentration and pH 7, are presented in Fig. 12, and the values of φg for the different polyacrylamides are summarized in Table 4. These results are in agreement with the
Fig. 12. The initial volume fraction per unit area (φ0 h0 ) as a function of equilibrium height (h∞ ) for kaolinite suspension flocculated by 3 mg/l of A100 polymer in 10−3 M NaCl solution and pH 7.
251
Table 4 Gel point volume fraction of kaolinite flocculated by different polymers at pH 7 Polymer
φg (–)
C 446 C 496 C 492 A130LMW A130 A100
0.0281 0.0309 0.0363 0.0389 0.0430 0.0491
other compressive yield measurements made in this study. The gel point of kaolinite flocculated by anionic polymers is shown to be higher than that when cationic polymers are used. This reconfirms the notion that the large open flocs produced when anionic polymer is used are weakly structured and susceptible to deformation so that the interfacial porosity can be reduced readily and compact sediments having a significant gel point are formed (see Fig. 11). The effects of the molecular weight and surface charge on the gel point show the same trends as the effects of the molecular weight and surface charge on the compressive yield stress. In a previous paper [27], we determined the gel point of kaolinite suspensions coagulated using different sodium chloride (NaCl) concentrations at three different pH. In that paper, we showed that the value of φg for the kaolinite coagulated by 1 M NaCl at pH 7 is 0.044. Under the same conditions the value of φg of kaolinite flocculated by 3 mg l−1 of anionic A100 polyacrylamide is 0.0491. This again confirms that polyelectrolyte flocculants are, in general, much more efficient than electrolytic coagulants, since a low concentration of polyacrylamide produces sediment having higher solids content than produced using very high electrolyte concentrations. 5. Conclusions The importance of polymer charge type, density and molecular weight in the flocculation–sedimentation process for negatively charged kaolinite suspensions was investigated to determine optimal flocculation conditions. The following conclusions have been drawn: • Polyelectrolyte structure and type has a marked effect on the particle zeta potential, whose decrease in the magnitude was much greater in the presence of cationic than anionic polyacrylamides. • Anionic polymers have a lower adsorption density than cationic polymers. The result is that the zeta potentials of the kaolinite suspensions, flocculated using anionic flocculant, remain negative and this prevents strong adsorption of the polymer. • The floc size and settling rate show a strong dependence on polymer structure type, and concentration. Larger floc sizes and higher settling rates were achieved using anionic polyacrylamides than when using cationic polyacrylamides. • The magnitude of the compressive yield stress Py (φ) is strongly dependent upon the floc structure; with greater com-
252
M.S. Nasser, A.E. James / Separation and Purification Technology 52 (2006) 241–252
pressive yield stress being observed for the strong cationic flocs than for the weak anionic flocs. • The difference in the compression sensitivity of the flocculated slurries may be attributed to floc structure-related adsorption. Cationic polymer chains adsorb via hydrogen bonding interactions between the silanol and aluminol OH groups at the particle surface and polymer’s primary amide functional groups, these electrostatic attractions produces strong and less compressible floc structures. For anionic PAM, however, although the adsorption is still via hydrogen bonding between the silanol and aluminol OH groups at the particle surface and polymer’s primary amide functional groups. The repulsive forces between the polymer and kaolinite surface allows the polymer molecules to be extended and produces loops and tails, which leads to the formation of large openstructure flocs of less resistance to the compression load, subsequently produces compact sediment of high gel point φg by applying some compression load. • The results suggest that if compact sediment is required in batch sedimentation or a concentrated underflow suspension is needed in a continuous thickener then anionic polymers are useful. In contrast, in filtration operations where the strength of flocs is important, cationic polymers are favoured. • Finally differences between the kaolinite sediments coagulated by electrolyte (NaCl) and kaolinite suspensions flocculated by polyelectrolyte are shown. The results confirm that polyelectrolyte flocculants are, in general, much better than electrolyte coagulants in terms the production compact sediment bed. References [1] H. Van Olphen, Introduction to Clay Colloid Chemistry, Interscience, New York, 1963. [2] A.S. Michals, J.C. Bolger, Settling rate and sediementation voulmes of flocculated kaolin suspensions, Ind. Eng. Chem. Fund. 1 (1962) 24–33. [3] L. Nabzar, E. Pefferkon, R. Varoqui, Polyacrylamide–sodium kaolinite interaction: flocculation behaviour of polymer clay suspensions, J. Colloid Interface Sci. 102 (1984) 380–388. [4] L. Nabzar, E. Pefferkon, An experimental study of kaolinite cystal edge–polyacrylamide interaction in dilute suspensions, J. Colloid Interface Sci. 108 (1985) 243–248. [5] L. Nabzar, E. Pefferkon, R. Varoqui, Stabilty of polymer–clay suspensions. The polyacrylamide–sodium kaolinite systems, Colloids Surf. 30 (1988) 345–353.
[6] D.T. Ray, R. Hogg, Agglomerate breakage in polymer-flocculated suspensions, J. Colloid Interface Sci. 116 (1987) 256–268. [7] L.T. Lee, R. Rahbari, J. Lecourtier, G. Chauveteau, Adsorption of polyacrylamides on the different faces of kaolinites, J. Colloid Interface Sci. 147 (1991) 351–357. [8] J. Gregory, The use of polymeric flocculants, in: Proceedings of the Engineering Foundation Conferences on Flocculation, Sedimentation and Consolidation, Georgia, USA, American Institute of Chemical Enginees, New York, USA, 1985, pp. 253–263. [9] M.J. Peares, J. Barnett, Chemical treatments for thickening and filtration, Filtr. Sep. 17 (1980) 460–470. [10] M. Patience, J. Addai-Menash, J. Ralston, Investigation of the effect of polymer type on flocculation, rheology and dewatering behaviour of kaolinite dispersions, Int. J. Miner. Process. 71 (2003) 247–268. [11] P.F. Richardson, L.J. Connely, Industrial coagulant and flocculants. Reagents in mineral technology, Surfact. Sci. Ser. 27 (1988) 519–528. [12] B.J. Scheiner, G.M. Wilemon, Applied flocculation efficiency: a comparison of polyethylene oxide and polyacrylamides, Flocculation Biotechnol. Sep. Syst. 4 (1987) 175–185. [13] R. Hogg, Flocculation and dewatering, Int. J. Miner. Process. 58 (2000) 223–236. [14] P. Somasundaran, B.M. Moudgil, Reagents in Mineral technology, vol. 27, Marcel–Dekker, New York, 1988. [15] J.F. Richardson, W.N. Zaki, Sedimentation and fluidization. Part I, Trans. Inst. Chem. Eng. 32 (1954) 35–53. [16] R. Buscall, L.R. White, The continuous-flow gravity thickeners: steady state behaviour, AIChE J 34 (1998) 239–252. [17] R. Buscall, L.R. White, Solid/liquid separation of flocculated suspensions, Adv. Colloidal Interface Sci. 51 (1994) 175–246. [18] G.V. Franks, F.F. Lange, Plastic-to-brittle transition of saturated, alumina powder compacts, J. Am. Ceram. Soc. 79 (1996) 3161–3168. [19] M. Colic, G.V. Franks, M.L. Fisher, F.F. Lange, Effect of counterion size on short range repulsive forces at high ionic strengths, Langmuir 13 (1997) 3125–3129. [20] R.G. De Kretser, D.V. Boger, P.J. Scales, Compressive rheology: an overview, rheology reviews, Br. Soc. Rheol. (2003) 125–165. [21] F.M. Tiller, Z. Khatib, The theory of sediment volumes of compressible, particulate structures, J. Colloid Interface Sci. 100 (1983) 56–67. [22] K.T. Miller, Compressive yield stress of cement paste, Mater. Res. Soc. 41 (1995) 285–291. [23] R.G. De Kretser, P.J. Scales, D.V. Boger, Improving clay-based tailings disposal: case study on coal tailings, AIChE J 43 (1997) 1894–1903. [24] G.M. Channell, C.F. Zukoski, Shear and compressive rheology of aggregated alumina suspension, AIChE J 43 (1997) 1700–1708. [25] D.J.A. Williams, K.P. Williams, Electrophoresis and zeta potential of kaolinite, J. Colloidal Interface Sci. 65 (1977) 79–87. [26] W. Chen, Effects of surface charge and shear during orthokinetic flocculation on the adsorption and sedimentation of kaolin suspensions in polyelectrolyte solutions, Sep. Sci. Technol. 33 (1998) 569–590. [27] M.S. Nasser, A.E. James, Settling and sediment bed behaviour of kaolinite in aqueous media, Sep. Purif. Technol., (2006), in press.
The effect of polyacrylamide charge density and molecular weight on the flocculation and sedimentation behaviour of kaolinite suspensions M.S. Nasser, A.E. James ∗ School of Chemical Engineering and Analytical Science, The University of Manchester, PO Box 88, Manchester M60 1QD, UK Received 8 March 2006; received in revised form 19 April 2006; accepted 19 April 2006
Abstract The effects of the surface charge and molecular weight of anionic and cationic polyacrylamide (PAM) on the surface chemistry, settling rates, floc sizes and sediment bed compactness of kaolinite suspensions has been investigated at pH 7. At optimum polymer concentrations, the kaolinite floc sizes were larger and the settling rates greater in the presence of anionic PAM than cationic PAM. Optimum flocculation for these anionic flocculants was linked to a small reduction in the magnitude of the zeta potential. In the case of cationic polymer, the optimum flocculation was linked to the reduction of the magnitude of the zeta potential to zero by charge neutralization. The results show that the magnitude of the compressive yield stress Py (φ) is strongly dependent upon the floc structure; with greater compressive yield stress being observed for the cationic PAM than for the anionic PAM. The difference in the compression sensitivity of the flocculated slurries may be attributed to floc structure-related adsorption. Cationic polymer chains adsorb via hydrogen bonding interactions between the silanol and aluminol OH groups at the particle surface and polymer’s primary amide functional groups, these electrostatic attractions between the positively charged polymer and the negatively charged kaolinite produce strong and less compressible floc structures. For anionic PAM, however, although the adsorption still occurs through hydrogen bonding between the silanol and aluminol OH groups at the particle surface and polymer’s primary amide functional groups, the interactions appear to be weakened as a consequence of electrostatic repulsion between the negatively charged polymer and negatively charged kaolinite surface. These repulsive forces allow the polymer molecules to be extended and produce loops and tails, which lead to the formation of large open-structure flocs having less resistance to compression loads and, subsequently, produce compact sediments having greater gel points φg following the application of some compression load. It is found that the compressive yield stress and gel point are important factors that should be considered when selecting the type of polyelectrolyte for use in a particular solid liquid separation. A strong correlation between the polymer type and flocculation, compression sensitivity of flocs and sedimentation behaviour of kaolinite is established. © 2006 Elsevier B.V. All rights reserved. Keywords: Kaolinite flocculation; Polyacrylamide; Charge density; Compressive yield stress; Gel point
1. Introduction In the mineral industry, waste residues known as tailings are produced and these must be disposed of in an environmentally and economically acceptable manner. Many of these wastes contain problem minerals such as kaolinite, which due to small particle size and charge properties are difficult to settle and consolidate. The conventional method used to flocculate colloidal particles (e.g. kaolinite) involves high molecular weight polymer-aided flocculation and subsequent sedimentation in thickeners or centrifuges. Several types of polymeric flocculants are currently used with kaolinite systems. Of these, polyacry-
∗
Corresponding author. Tel.: +44 161 306 4368; fax: +44 161 306 4399. E-mail address: [email protected] (A.E. James).
1383-5866/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2006.04.005
lamides form the most common type of polymeric flocculant and are employed because of their effectiveness as flocculants and ability to produce good settling performance for relatively low cost. Kaolinite, known chemically as a 1:1 tetra octahedral aluminium silicate having the general formula Al2 O3 ·2SiO2 ·2H2 O, has two different basal cleavage faces [1]. One basal face consists of tetrahedral siloxane (–Si–O–Si–) species [1], while the other consists of an octahedral alumina (Al2 O3 ) sheet. The basal face, displaying an inert siloxane structure carries a permanent negative charge as a result of isomorphous substitution of Si4+ by Al3+ groups [1]. At the edge of the crystal, the octahedral alumina and tetrahedral silica sheets are disrupted and broken bonds exposing aluminol (Al–OH) and silanol (Si–OH) groups occur. The edge face may be charged as a result of protonation and deprotonation of the surface hydroxyl groups, depending on pH [1,2].
242
M.S. Nasser, A.E. James / Separation and Purification Technology 52 (2006) 241–252
Nomenclature CAK dA g h0 h∞ Py (φ) Q Q0 VSA
ratio of volume concentration of the aggregates to kaolinite average (equivalent) aggregate diameter (m) local gravitational acceleration (980 cm/s2 ) initial height of the suspension (m) equilibrium height of the sediment bed (m) compressive yield stress (Pa) settling rate of slurry-supernatant interface (cm/h) initial settling rate (cm/h) Stokes’ velocity for single aggregates (cm/h)
Greek symbols ε void fraction εd dielectric constant μ viscosity of the suspending liquid (cp) μw viscosity of water, 0.893 cp (at 25 ◦ C) ρw density of water (g/cc) density of kaolinite (2.6 g/cc) ρK φK kaolinite volume fraction φA aggregate volume fraction φ0 initial volume fraction φg volume fraction at gel point ωc volume solids per unit area
Understanding the interaction between kaolinite and polyacrylamide flocculant is important in enabling the mining industry to optimize flocculant performance. Several studies have been performed, examining the adsorption of polyacrylamide on
to kaolinite [3–7]. All authors have concluded that the adsorption of polyacrylamide occurs primarily onto the edge surface of kaolinite (i.e., on the broken bonds of aluminol (Al–OH) and silanol (Si–OH) groups) via hydrogen bonding, with Lee et al. [7] stating that 94% of adsorption takes place at the edge surface. Flocculation of fine particles may occur by polymer bridging, charge neutralization, polymer–particle surface complex formation and depletion flocculation, or by a combination of these mechanisms [8]. The bridging mechanism requires that the polymer chains be adsorbed on the particle surfaces, with only a few points of attachment, with the bulk of the chains projecting into the surrounding solution for contact and adherence to other particles [9]. Strong adsorption does not favour the flocculation process, because strong adsorption can cause surface saturation, preventing effective bridging and restabilising fine particles (see Fig. 1). The flocculation of negatively charged fine particles by high molecular weight cationic polyacrylamide occurs by the adsorption of the polyacrylamide chains on to the particle surface. Thus charge neutralization becomes a major mechanism, where the cationic polyacrylamides will locally reverse the particle surface charge. Collision with negative patches on another particle allows bridging then aggregation [9]. On the other hand, with low molecular weight cationic flocculants adsorbing on to the negative particles, bridging would not be favoured due to the short macro-ion length and strong adsorption (see Fig. 2). Several studies have shown that high molecular weight anionic polymers (e.g. polyacrylamide) are commonly used in the settling of negatively charged kaolinite [10–13]. In this case, the polymer bridging mechanism is of primary importance, whereas charge neutralization will be of secondary or little importance [10]. The use of high molecular weight anionic
Fig. 1. Polymer conformation on adsorption.
Fig. 2. Differences in bridging mechanism for high and low molecular weight cationic polymers.
M.S. Nasser, A.E. James / Separation and Purification Technology 52 (2006) 241–252
polyacrylamide in flocculating negatively charged particles has the advantage of being more effective than cationic polymers by increasing settling rate and producing a distinct sediment structure, and in addition the restabilisation of kaolinite particles by excessive polymer adsorption driven by strong electrostatic attraction may be avoided. The electrostatic repulsion between kaolinite particles and the anionic polyacrylamide allows only limited polymer adsorption. On the other hand, the polymer molecule expansion arising from charge repulsion produces loops and tails, which lead to the formation of large openstructure flocs. This effect has been seen to be effective in flocculating negatively charged kaolinite dispersions [14]. Extensive research is required to optimize the use of different molecular weight and surface charge polymers as flocculants to improve the settling of fine particle suspensions. Toward this aim, the settling rate, supernatant turbidity, polyelectrolyte adsorption, zeta potential, floc size, and sediment bed compactness have been measured for kaolinite suspension at natural pH. Kaolinite tailings are commonly generated in the mineral industry at natural pH, where the kaolinite particles are invariably negatively charged and consequently tend to form stable dispersions with poor flocculation characteristics. 2. Theory
From a material balance on the kaolinite, it follows that ρs − ρw = φA (ρA − ρw ) = φk (ρk − ρw )
(3)
Eq. (2) is rewritten as Q0 =
g(ρk − ρw )dA2 (1 − CAK φK )4.65 18μw CAK
(4)
where CAK =
φA volume of aggregate = φk volume kaolinite in aggregates
(5)
If dA is expressed in microns, density of kaolinite, ρK , is 2.6 g cm−3 and viscosity of water, μw , is 0.893 cp at 25.0 ◦ C, then VSA , the Stokes’ settling velocity for single aggregate is VSA = 0.349
dA2 CAK
(6)
Finally, Eq. (4) can be rewritten [2]: 1/4.65
Q0
1/4.65
= VSA
(1 − CAK φK )
(7)
1/4.65
Therefore, if one plots Q0 against the corresponding value of φK , a straight line should result. From the values of the slope and intercept one, can estimate the aggregate size. 2.2. Sediment bed behaviour
2.1. Floc size measurements The Richardson and Zaki [15] equation for the group-settling rate for uniform spherical particles can be written: Q = VSA ε4.65
(1)
Michaels and Bolger [2] suggest that the aggregate diameter, dA , is relatively independent of kaolinite concentration over the “dilute” range, and that dA dos not change once settling has begun. The dilute settling rate should then be given by Eq. (1) in the form: Q0 =
243
g(ρA − ρw )dA2 (1 − φA )4.65 18μw
(2)
The sediment bed of flocculated suspension has a concentration gradient from a maximum at the base up to a critical volume fraction, the gel point, φg at the top of the sediment (see Fig. 3). Above the top of the sediment, the volume fraction will be unchanged from φ0 and above this is a clear liquid region. This critical volume fraction, φg , may be considered to be the lowest volume fraction at which all primary flocs are interconnected throughout the container, and a network is just formed [16,17]. The volume fraction of solids will increase from the gel point to the maximum packing fraction by the application of the consolidation pressure, P. As this pressure is applied, the network structure will resist further compression until the compressive forces become so strong that the structure begins to deform irreversibly. Bonds between particles will rupture, the
Fig. 3. Schematic of a sedimentation process, showing flocculated kaolinite suspension.
244
M.S. Nasser, A.E. James / Separation and Purification Technology 52 (2006) 241–252
system will consolidate and, consequently, new and more bonds will form until the network has become strong enough to oppose the compression once more. Hence, the compressive yield stress Py (φ) is the stress which must be exceeded by an applied load, P, before the consolidation will occur and the local volume fraction, φ, will increase. In other words, the compressive yield stress Py (φ) can be thought of as a measure of the resistance of the network to consolidate to a higher volume fraction under an applied compressive pressure [17]. Thereafter, the values of the gel point, φg , and compressive yield stress; Py (φ), can be used to show the compactness of kaolinite sediment beds for different polyacrylamide types. 2.2.1. Compressive yield stress The compressive behaviour of a networked suspension can be characterised by the compressive yield stress Py (φ). It is well known that sediments containing strongly attractive particles produce sediment beds of lower density than weakly attractive particles [18,19]. As mentioned before the volume fraction of solids in a sediment network can be increased from gel point to the maximum packing fraction by the application of a consolidation pressure. Networks produced from particles with differing magnitude of attraction evolve their structure and volume fraction differently in response to compression, i.e., it takes a greater pressure for strongly attractive networked of high compressive yield stress to reach maximum packing fraction than weakly attractive networks [18,19]. Hence, the compressive yield stress is important in both dewatering operations and the down stream processes. For instance, if compact sediment is required in batch sedimentations or a concentrated underflow suspension is needed in a continuous thickening the compressive yield stress should be low. In contrast, in filtration operations where the strength flocs are important, the compressive yield stress should be significantly higher. Buscall and White [17] estimated the compressive yield stress of latex and bentonite suspensions from batch centrifuge experiments. The experiments utilized a centrifuge to consolidate to equilibrium sediments of material at each of a series of consecutively increasing gravitational fields. It was concluded that, for most practical applications, the use of a mean value approximation for the determination of Py (φ) from equilibrium sediment height data was sufficiently accurate. The mean value approximation allows determination of Py (φ) [17]: h∞ Py (φ) = ρgφ0 h0 1 − (8) 2R
2.2.2. Gel point The gel point, φg , of a suspension is a useful parameter for gaining knowledge regarding the nature of the compressibility and structure of sediments. The gel point is related to the strength of attractive bonds between the particles, with stronger attractions creating less compressible (high compressive yield stress) flocs of lower gel points. Direct determination of the gel point is not often undertaken, due in part to the difficulty of measurement [20,21]. As mentioned previously, φg is the lowest concentration at which flocs are able to form a self-supporting network (this is a similar concept to null stress solids, εs0 , defined in other work [21]). If a sediment at the gel point is considered, the weight of each individual particle and floc because of the structure, is transmitted through the network and as a result, flocs within the sediment are subjected to compressive forces arising from the self-weight of the over laying material [16,17]. Hence, material at the top of the sediment bed will experience no compressive force but this increases with depth. The compressive force at the base of any sediment bed will thus be a maximum. In order to measure the gel point, the volume fraction at the top of the sediment bed, to which the flocs settle in the absence of compression, needs to be measured. An indirect method for obtaining gel point used by Tiller–Khatib [21] is based upon determination of the volume of solids per unit area as a function of the ultimate height of the sediment. The volume of the solids/unit area is given by h ωc = φ dx (10) 0
Assuming that φ is a unique function of the distance x below the surface, differentiation of Eq. (10) yields [19]: dωc (11) dh where φ(h) is the volume fraction of solids at distance h. This relationship allows the determination of the solids concentration at given depth below the top of the sediment from a set of equilibrium batch settling tests, each starting with a different amount of solids suspensions height (as given by wc , which is equal to the product φ0 h0 ) [20]. This relation can be used in determining the gel point φg Eq. (11) can be written as [20]: φ(h) =
φg (h∞ ) =
and
+s φ0 h0 1 − h∞ 2R φ= 2 h∞ ∞ (h∞ + s) 1 − R + h2R
[24]. In some of these studies, data obtained using equilibrium sediment techniques has been shown to agree with data obtained from other compressive yield stress determination techniques.
(9)
where g is the centrifugal acceleration (g = ω2 R), R the radius of rotation at the bottom of the sediment, s = (dh∞ /d ln g). This approach has been utilized in several other subsequent investigations for various materials such as cement paste [22], clay-based coal tailings [23], alumina suspension and zirconia
d(φ0 h0 ) dh∞
(12)
where φ0 and h0 are the initial volume fraction of solids and initial height of suspension, respectively, and h∞ is the equilibrium height of the sediment bed. The initial volume fraction should be much lower than the volume fraction at which the gel point is expected to be found. Otherwise, the resultant intercept, which is simply the solids concentration at the top of the bed, will give the initial solids concentration. Eq. (12) shows that different h∞ can be achieved by either holding φ0 and changing h0 , or h0 constant and changing φ0 or
M.S. Nasser, A.E. James / Separation and Purification Technology 52 (2006) 241–252
allowing both to vary. Thus the gel point, which is effectively the solids concentration at the surface of the sediment, can be found from the initial slope (h∞ → 0) of φ0 h0 as a function of h∞ obtained from sequences of settling tests. 3. Experimental 3.1. Materials The kaolinite used was ‘Supreme’ grade china clay provided by courtesy of Imerys, UK. The major dimension of a kaolinite particle is between 2.0 and 0.1 m in diameter and the thickness ratio is between 30:1 and 10:1. The five different polyacrylamides used as flocculants in this study were provided by Cytec industries Inc., Bradford, UK. The characteristics of the polyacrylamides are summarized in Table 1. 3.2. Methods 3.2.1. Kaolinite purification It is well known that some deflocculants are added to the kaolinite for stabilisation purposes, and there is a low level of natural impurity (Imerys literature). In order to insure that the kaolinite suspensions created in the experiments were of a consistent nature, the surface preparation methods of Williams and Williams [25] were used to prepare homoionic sodium kaolinite suspensions. The technique involved repeated washing of the kaolinite in 1 M NaCl at pH 3; the kaolinite–NaCl suspension was then prepared and allowed to settle over a 24 h period, supernatant was removed and fresh 1 M NaCl was added. The electrophoretic mobility of the kaolinite suspension was checked periodically until consistent measurements were obtained. In our case, it was found that the mobility became constant after six washes. Following this technique, further washing with distilled water caused the sodium ions to be expelled from the kaolinite by osmotic diffusion. About 8–12 washes, each lasting 24 h, were required to reduce the supernatant conductivity to 2 S and to produce the kaolinite particle in its natural unsubstituted form. Subsequently, the kaolinite slurries were stored under distilled water. 3.2.2. Polymer adsorption experiments The adsorption measurements were performed by batch technique to obtain equilibrium data. For isotherm studies, adsorpTable 1 Polyacrylamide characteristics Polymer
Molecular weight × 10−6 (g mol−1 )
Charge type
Charge density (%)
C 446 C 496 C 492 A130LMW A130 A100
3–4 5–7 5–7 3–4 10–12 10–12
Cationic Cationic Cationic Anionic Anionic Anionic
35 35 10 35 35 10
245
tion experiments were carried out in 250 ml flasks by adding a known concentration of flocculants to 100 ml of kaolinite dispersion (of 2.0% by volume solids). The suspension was agitated by a rotating shaker (Infros AG CH-4103, Bottnigen, UK) at 120 rpm for 24 h. The suspended colloidal particles were centrifuged at high speed until all the residual colloidal particles had subsided, leaving clear supernatant, which was transferred for titration with a solution of potassium polyvinylsulfate (KPVS), and using tluidine blue dye as the indicator. Such types of titrant were used to estimate the changes in cationic polyacrylamide concentration [26], because only a cationic polyacrylamide will form a polyelectrolyte complex with negatively charged KPVS. When the dye is present in solution, the excess KPVS complexes with the oppositely charged cationic dye will produce a colour change from blue to purple at the end point. As for determination of the changes in anionic polyacrylamide concentration, the technique required some modification. The negatively charged polyacrylamide solution must be first treated with excess solution of polydimethydiallylammonium chloride (PDMDAAC), which is then back titrated with KPVS using the tluidine blue dye as indicator. 3.2.3. Zeta potential measurements The zeta potential of kaolinite suspensions was measured using a Zetasizer 3000 (Malvern Instruments Ltd., UK). The calculation of the zeta potential was assisted with modern software. The equipment measures the electrophoretic mobility of the particles, and converts it to the zeta potential using the von Smoluchowski equation. The von Smoluchowski equation, the most elementary expression for zeta potential, provides a direct relationship between zeta potential and electrophoretic mobility: ξ=
4πμ U εd
(13)
where U is electrophoretic mobility, μ the viscosity of the suspending liquid, εd the dielectric constant, π the constant and ξ is the zeta potential. The zeta potential measurements were carried out as a function of polyacrylamide concentration at pH 7. In this test, 50 ml of 2.0 g l−1 of kaolinite suspension were transferred to a 250 ml flask, to which 50 ml of the required polyacrylamide solution was added, yielding a final kaolinite concentration of 1.0 g l−1 . The kaolinite suspension was agitated by a rotating shaker at 120 rpm for 1 h; the samples were allowed to settle for 20 min to allow the larger particles to settle. An aliquot taken from the supernatant was used to measure the zeta potential. 3.2.4. Settling tests The treated kaolinite slurry was used to prepare 50 ml of kaolinite suspension, at 4% solids by volume, in a series of 250 ml flasks. The 2% by volume suspensions used in the experiments were produced by adding 50 ml of polyacrylamide in varying concentrations (1–300 mg l−1 ), and adjusted to pH 7 using appropriate amounts of 0.1 M NaOH or HCl. Following polyacrylamide addition, the suspension was mixed for 1 min, and thereafter the prepared kaolinite suspensions were transferred to
246
M.S. Nasser, A.E. James / Separation and Purification Technology 52 (2006) 241–252
Table 2 Optimum polymer concentration corresponding to the maximum settling rate and minimum turbidity of kaolinite suspension in 10−3 M NaCl solution and pH 7 Polymer
Maximum settling rate (cm min−1 )
Minimum turbidity (NTU)
Optimum concentration (mg l−1 )
C 446 C 496 C 492 A130 LMW A130 A100
4.1 4.8 5.7 4.6 5.8 7.9
7 7 6 41 26 18
12 12 12 6 3 3
100 ml measuring cylinders (30 mm in diameter). Subsequently, the thickness of the interface was recorded with settling time. 3.2.5. Turbidity of the supernatant Turbidity of kaolinite suspensions was measured in nephelometric turbidity units (NTU) using a turbidity meter (WPA TU100, UK). The flocculation was carried out by 1 min rapid mixing (200 rpm) of the 10 g l−1 kaolinite suspensions of known polymer concentration at pH 7, then 10 min slow mixing (20 rpm) to promote the flocculation. Subsequently, the suspensions were allowed to settle for 20 min. After that, a 5 ml aliquot of the supernatant liquid was pipetted off and transferred to a glass cuvette to determine the turbidity. 3.2.6. Floc size The treated kaolinite slurry was used to prepare 500 ml of kaolinite suspension of different volume fractions in the range 0.00125–0.02. The preparation was made using the optimum polymer concentration (see Table 2) at pH 7. The suspension was mixed for 1 min, and thereafter the prepared kaolinite suspensions were transferred to 500 ml measuring cylinders (5.5 cm in diameter). Subsequently, the settling rate was recorded as a function of time. Therefore, considering Eq. (7), if one plots the 1/4.65 initial settling rates Q0 as a function of the corresponding kaolinite volume fractions φK a straight line should result and from the slope and intercept one can estimate the floc size. 3.2.7. Compressive yield stress measurements A batch centrifuge (Mistral 1000, UK) was used in the compressive yield stress measurements. The centrifuge tubes used in these measurements held 50 ml of suspension and had a diameter of 28 mm. Different rotational speeds in the range of 200–1000 rpm were used giving accelerations 7–180 g. In these measurements the optimum polyacrylamide concentrations were used to study the consolidation rate for kaolinite suspensions. Fig. 4 shows the systematic procedure used to determine the compressive yield stress Py (φ) as function of φ. 3.2.8. Gel point A series of 100 ml measuring cylinders were used for the gel point measurements and both φ0 and h0 were varied. However, in these experiments, four kaolinite suspensions having different φ0 in the range 0.25–1.0% by volume were used, and
Fig. 4. Systematic procedure to measure the compressive yield stress Py (φ).
four different h0 in the range 39–185 mm were used for each φ0 . The optimum polyacrylamide concentrations were used in these measurements. The suspension was then left to settle, and h∞ was recorded when it became constant. Subsequently, the gel point was estimated using Eq. (12) by plotting the initial volume fraction per unit area (φ0 h0 ) as function of equilibrium height (h∞ ). 4. Results and discussion 4.1. Adsorption experiments Fig. 5 shows the adsorption of polymers in 10−3 M NaCl solution at pH 7.0 as a function of polymer concentration. The cationic polymer chains adsorb through hydrogen bonding interactions between the silanol and aluminol OH groups at the particle surface and the polymer’s primary amide functional groups, these electrostatic attractions between the positively charged polymer and the negatively charged kaolinite promotes the adsorption mechanism. It was found that the amount adsorbed increased with increasing a polymer concentration. On the other hand, the adsorption behaviour of polyacrylamide
M.S. Nasser, A.E. James / Separation and Purification Technology 52 (2006) 241–252
Fig. 5. Adsorption isotherms for 50 g/l (2% volume fraction) of kaolinite in 10−3 M NaCl solution as a function of polymer dose at pH 7.
on to kaolinite particles is dependent on the charge density of each polyacrylamide. The isotherm for the adsorption of the cationic C 492, which has a charge density of 10%, approached a plateau at 150 mg l−1 of polymer dosing and the maximum polymer adsorbance achieved is 1.5 mg g−1 kaolinite. The adsorption values increased to 3.4 mg g−1 kaolinite for the case of C 496, because of the higher charge density of 35% and no plateau state reached indicating strong adsorption caused by the surface charge. Fig. 5 also shows a marked inflection (i.e., a levelling off before a steep rise) for the cationic polymers C 496 and C 446 at 250 mg l−1 . A possible interpretation is that a first layer of polymer is absorbed, neutralizing the surface charge, and then further layers start to form once the polymer is able to absorb through other bonding mechanisms (e.g. hydrogen or hydrophobic bonding). The molecular weight has only small effect on the adsorption isotherms, where similar behaviour was observed for both C 496 and C 446. In the case of anionic polymers, however, although the adsorption still occurs through hydrogen bonding between the silanol and aluminol OH groups at the particle surface and polymer’s primary amide functional groups, the interactions appear to be weakened as a consequence of electrostatic repulsion between the negatively charged polymer and negatively charged kaolinite surface. These repulsive forces minimises the adsorption rate. It was found that the maximum adsorption value for A100 is 0.6 mg g−1 , and this is considerably smaller compared with cationic polymer. On the other hand, increasing the charge density of anionic polymer to 35% in the case of A130 A130LMW further reduces the adsorption values. 4.2. Zeta potential Fig. 6 shows the zeta potential of kaolinite in 10−3 M NaCl solution at pH 7.0 as a function of polymer concentration.
247
Fig. 6. Electrokinetic zeta potentials for 1.0 g/l of kaolinite in 10−3 M NaCl solution as a function of polymer dose at pH 7.
Prior to flocculant addition, the particles have moderate negative charge and corresponding zeta potential (−33 mV). The reduction in the zeta potential to zero by the cationic polyacrylamide arises from charge neutralization through electrostatic attraction between the positively charged polymer and negative surface sites of the particle, and the adsorption of large amount of positively polymer onto the surface of kaolinite gives rise to charge reversal. The amount of polymer required to neutralize the particle charge is inversely proportional to charge density. This confirms the charge neutralization mechanism; where the high charge density polymers neutralize the surface charge of kaolinite more rapidly than the low charge density polymers, and have require a lesser amount of polymer. In addition, at a concentration of 250 mg l−1 both polymers C 496 and C 446 show inflections (i.e., a levelling off before a steep rise), which are similar to the inflection point shown in the adsorption experiment (see Fig. 5). This seems to confirm the ability of the polymers to neutralize the particles’ surface charge, and to form further layers through other bonding mechanisms (e.g. hydrogen or hydrophobic bonding). In the case of anionic polyacrylamide, the small decrease in the magnitude of the zeta potential with increasing polymer concentration does not arise from charge neutralization but rather from the shift in position of the plane of shear, due to the adsorbed layer, which is caused by the negatively polymer chains. Similar behaviour has been reported by Patience et al. [10]. 4.3. Determination of optimum dosages of polyacrylamide The settling rate and turbidity measurements for kaolinite suspensions were plotted as a function of polyacrylamide concentration at constant pH (see Figs. 7 and 8). The maximum settling rates of flocs produced by cationic and anionic polyacrylamides decrease with charge density and increase with molecular weight. The increase of charge density in polymers
248
M.S. Nasser, A.E. James / Separation and Purification Technology 52 (2006) 241–252
Fig. 7. Settling rates of 50 g/l (2% volume fraction) of kaolinite and turbidity measurements of 10 g/l (0.4% volume fraction) of kaolinite in 10−3 M NaCl solutions as a functions of cationic polymer dose at pH 7.
results in a loss of flocculation power, because the strong adsorption of the cationic polymer chains could rapidly saturate the particle surface sites, preventing effective bridging (see Fig. 1). In addition, the low molecular weight of the cationic flocculants adsorbing on to the negative particles does not favour bridging, because of the short macro-ion length and strong adsorption (see Fig. 2). As a result, the lowest settling rate was observed for the cationic polymer C 446. In the case of high molecular weight anionic polymers, the settling rate observed was faster than with the cationic polymers. Because the kaolinite has a negative zeta potential this prevents the strong adsorption of anionic polymers, leaving long polymer chains to form bridges with other particles and resulting in large flocs (see Section 4.4). On the other hand, increasing the anionic charge density up 35% in the case of A130 and A130LMW flocculants will reduce the settling rate. Therefore, many factors have to be considered in order to select the best charge type and density, and molecular weight of polymer, for any given application. The turbidity results show good agreement with the settling rates; the maximum settling rates were found to correspond to minimum turbidity for all polymers. The optimum concentration
corresponding to the maximum settling rate and minimum turbidity is summarized in Table 2 for each polymer investigated, and the results show that the high negative zeta potentials of the particles treated with anionic polymer are linked to lower supernatant clarities than those treated with cationic polymers. On the other hand, because of the effective bridging, optimum conditions are reached using smaller doses of the anionic polymers. 4.4. Floc size Fig. 9 shows the settling rate as a function of kaolinite volume fraction applying Eq. (7) for the A100 polyacrylamide–kaolinite system at pH 7 using the optimum polymer concentration. Volume fractions of kaolinite in the range of 0.00125–0.015 were found to delineate the limits of the dilute region for all types of polyacrylamide used in this study. The parameters VSA , CAK and dA determined for each of the five different polyacrylamides used in this study are shown in Table 3. The results show that the floc sizes of the kaolinite produced by anionic polyacrylamides are larger than those for cationic polyacrylamides. Anionic polyacrylamides are negatively charged and as mentioned earlier
M.S. Nasser, A.E. James / Separation and Purification Technology 52 (2006) 241–252
249
Fig. 8. Settling rates of 50 g/l (2% volume fraction) of kaolinite and turbidity measurements of 10 g/l (0.4% volume fraction) of kaolinite in 10−3 M NaCl solutions as functions of anionic polymer dose at pH 7.
the adsorption is through hydrogen bonding between the silanol and aluminol OH groups at the particle surface and polymer’s primary amide functional groups, the repulsion between the negatively charged polymer and negatively charged kaolinite surface allow the polymer molecules to be extended and produce loops and tails, which lead to the formation of large and
open-structure flocs. On the other hand, increasing the anionic surface charge from 10% to 35% reduces the floc size. This again in evidence confirming that increasing the anionic charge will reduce the flocculation power. In the case of the cationic flocculants the floc sizes of the kaolinite were found to decrease as the adsorption rates increase. This is because very strong adsorption will saturate the particles surface and that restrict the bridging process. This is clear from the values of the floc size, which decrease as the surface charge increases due to the strong adsorption. In addition, loss of bridging and strong adsorption is found in the results for the low molecular weight cationic polymer and these factors also reduce the floc size (see C 446 in Table 3).
Table 3 Floc structure parameters obtained from dilute settling rate data at pH 7
Fig. 9. Settling rate–volume fraction correlation of dilute settling rates for kaolinite suspensions flocculated by 3 mg/l of A100 polymer in 10−3 M NaCl solutions and pH7.
Polymer
VSA (cm h−1 )
CAK (–)
dA (m)
C 446 C 496 C 492 A130 LMW A130 A100
1334 1542 1620 1486 1652 1718
4.2 5.3 5.7 4.6 5.9 6.9
127 153 163 141 167 184
250
M.S. Nasser, A.E. James / Separation and Purification Technology 52 (2006) 241–252
4.5. Compressive yield stress Fig. 10 shows the compressive yield stress–volume fraction relationships for sediment using different polyacrylamides at optimum concentrations considered (see Table 2) and pH 7. The flocculated kaolinite suspensions exhibit a power law dependence on volume fraction. The compressive yield stress, Py (φ), follows the trend: C 446 > C 496 > C 492 > A130LMW > A130 > A100. The compressive yield stress, Py (φ), for the anionic polymers is smaller than for the cationic polymers. An explanatory summary of sediment bed behaviour of kaolinite flocculated by cationic and anionic polyacrylamides is depicted in Fig. 11. Starting with similar volume fraction of kaolinite, and settling/centrifuging columns, both flocculated suspensions show similar initial suspension heights (h0 ) but different floc structures. The electrostatic attraction between the positively charged polymer and the negatively charged kaolinite
Fig. 10. Compressive yield stresses for different polyacrylamides in 50 g/l (2% volume fraction) of kaolinite in 10−3 M NaCl, using the optimum polymer concentration at pH 7.
Fig. 11. Schematic diagram showing ways in which different in kaolinite floc structures produce different equilibrium sediment height. Although the initial volume fractions of kaolinite and initial sediment heights are the same, application of the same pressure to both sediments (see diagrams (c) and (d)) induces greater deformation of the anionic flocs. The flocs produced by cationic polymer are stronger and have a high compressive yield stress (see diagrams (a) and (c)) while in contrast the weaker flocs produced using anionic polymer have low compressive yield stress resulting in a more compact sediment (see diagram (b) and (d)).
M.S. Nasser, A.E. James / Separation and Purification Technology 52 (2006) 241–252
allows the cationic polymer to produce strong flocs. In case of anionic polymers, the electrostatic repulsion between the negatively charged polymer and negatively charged kaolinite surface, allows the polymer molecules to be extended and produces loops and tails, which leads to the formation of large, open and fragile flocs. Under compressive load, the strong flocs produced by cationic polymer are more capable for resisting deformation, thereby producing a less compact sediment that corresponds to a high compressive yield stress. This was apparent from the observed equilibrium sediment heights (h∞ ) found following centrifugation. The flocs produced by anionic polymers tend to be open and more deformable, and thereby produce more compact sediments. For the practical purposes, if compact sediment is required in, say, batch sedimentations or a concentrated underflow suspension is needed in, say, a continuous thickener the use of anionic polymer is favoured. In contrast, in some filtration operations where the strength of flocs is important, the use of cationic polymer is favoured. This type of behaviour is observed by Pearse and Barnett [9] who carried out filtration experiments using kaolin flocculated with different polyacrylamides. These authors report that the mechanically strong flocs produced by cationic polymers are much more effective in filtration than the weakly structured flocs resulting from the use of anionic polymers. In the present study, the loss of bridging and strong adsorption for the low molecular weight and high surface charge cationic polymer (i.e., C 446) allows most of the primary flocs to remain in suspension and additionally has more resistance to consolidate, so that a less compact sediment is produced during the centrifugation process because of the poor flocculation. 4.6. Gel point The gel point of the suspension was determined using the method of Tiller–Khatib [21]. The results of different initial volume fractions per unit area (φ0 h0 ) as a function of the corresponding h∞ , for kaolinite suspensions flocculated by A100 at the optimum concentration and pH 7, are presented in Fig. 12, and the values of φg for the different polyacrylamides are summarized in Table 4. These results are in agreement with the
Fig. 12. The initial volume fraction per unit area (φ0 h0 ) as a function of equilibrium height (h∞ ) for kaolinite suspension flocculated by 3 mg/l of A100 polymer in 10−3 M NaCl solution and pH 7.
251
Table 4 Gel point volume fraction of kaolinite flocculated by different polymers at pH 7 Polymer
φg (–)
C 446 C 496 C 492 A130LMW A130 A100
0.0281 0.0309 0.0363 0.0389 0.0430 0.0491
other compressive yield measurements made in this study. The gel point of kaolinite flocculated by anionic polymers is shown to be higher than that when cationic polymers are used. This reconfirms the notion that the large open flocs produced when anionic polymer is used are weakly structured and susceptible to deformation so that the interfacial porosity can be reduced readily and compact sediments having a significant gel point are formed (see Fig. 11). The effects of the molecular weight and surface charge on the gel point show the same trends as the effects of the molecular weight and surface charge on the compressive yield stress. In a previous paper [27], we determined the gel point of kaolinite suspensions coagulated using different sodium chloride (NaCl) concentrations at three different pH. In that paper, we showed that the value of φg for the kaolinite coagulated by 1 M NaCl at pH 7 is 0.044. Under the same conditions the value of φg of kaolinite flocculated by 3 mg l−1 of anionic A100 polyacrylamide is 0.0491. This again confirms that polyelectrolyte flocculants are, in general, much more efficient than electrolytic coagulants, since a low concentration of polyacrylamide produces sediment having higher solids content than produced using very high electrolyte concentrations. 5. Conclusions The importance of polymer charge type, density and molecular weight in the flocculation–sedimentation process for negatively charged kaolinite suspensions was investigated to determine optimal flocculation conditions. The following conclusions have been drawn: • Polyelectrolyte structure and type has a marked effect on the particle zeta potential, whose decrease in the magnitude was much greater in the presence of cationic than anionic polyacrylamides. • Anionic polymers have a lower adsorption density than cationic polymers. The result is that the zeta potentials of the kaolinite suspensions, flocculated using anionic flocculant, remain negative and this prevents strong adsorption of the polymer. • The floc size and settling rate show a strong dependence on polymer structure type, and concentration. Larger floc sizes and higher settling rates were achieved using anionic polyacrylamides than when using cationic polyacrylamides. • The magnitude of the compressive yield stress Py (φ) is strongly dependent upon the floc structure; with greater com-
252
M.S. Nasser, A.E. James / Separation and Purification Technology 52 (2006) 241–252
pressive yield stress being observed for the strong cationic flocs than for the weak anionic flocs. • The difference in the compression sensitivity of the flocculated slurries may be attributed to floc structure-related adsorption. Cationic polymer chains adsorb via hydrogen bonding interactions between the silanol and aluminol OH groups at the particle surface and polymer’s primary amide functional groups, these electrostatic attractions produces strong and less compressible floc structures. For anionic PAM, however, although the adsorption is still via hydrogen bonding between the silanol and aluminol OH groups at the particle surface and polymer’s primary amide functional groups. The repulsive forces between the polymer and kaolinite surface allows the polymer molecules to be extended and produces loops and tails, which leads to the formation of large openstructure flocs of less resistance to the compression load, subsequently produces compact sediment of high gel point φg by applying some compression load. • The results suggest that if compact sediment is required in batch sedimentation or a concentrated underflow suspension is needed in a continuous thickener then anionic polymers are useful. In contrast, in filtration operations where the strength of flocs is important, cationic polymers are favoured. • Finally differences between the kaolinite sediments coagulated by electrolyte (NaCl) and kaolinite suspensions flocculated by polyelectrolyte are shown. The results confirm that polyelectrolyte flocculants are, in general, much better than electrolyte coagulants in terms the production compact sediment bed. References [1] H. Van Olphen, Introduction to Clay Colloid Chemistry, Interscience, New York, 1963. [2] A.S. Michals, J.C. Bolger, Settling rate and sediementation voulmes of flocculated kaolin suspensions, Ind. Eng. Chem. Fund. 1 (1962) 24–33. [3] L. Nabzar, E. Pefferkon, R. Varoqui, Polyacrylamide–sodium kaolinite interaction: flocculation behaviour of polymer clay suspensions, J. Colloid Interface Sci. 102 (1984) 380–388. [4] L. Nabzar, E. Pefferkon, An experimental study of kaolinite cystal edge–polyacrylamide interaction in dilute suspensions, J. Colloid Interface Sci. 108 (1985) 243–248. [5] L. Nabzar, E. Pefferkon, R. Varoqui, Stabilty of polymer–clay suspensions. The polyacrylamide–sodium kaolinite systems, Colloids Surf. 30 (1988) 345–353.
[6] D.T. Ray, R. Hogg, Agglomerate breakage in polymer-flocculated suspensions, J. Colloid Interface Sci. 116 (1987) 256–268. [7] L.T. Lee, R. Rahbari, J. Lecourtier, G. Chauveteau, Adsorption of polyacrylamides on the different faces of kaolinites, J. Colloid Interface Sci. 147 (1991) 351–357. [8] J. Gregory, The use of polymeric flocculants, in: Proceedings of the Engineering Foundation Conferences on Flocculation, Sedimentation and Consolidation, Georgia, USA, American Institute of Chemical Enginees, New York, USA, 1985, pp. 253–263. [9] M.J. Peares, J. Barnett, Chemical treatments for thickening and filtration, Filtr. Sep. 17 (1980) 460–470. [10] M. Patience, J. Addai-Menash, J. Ralston, Investigation of the effect of polymer type on flocculation, rheology and dewatering behaviour of kaolinite dispersions, Int. J. Miner. Process. 71 (2003) 247–268. [11] P.F. Richardson, L.J. Connely, Industrial coagulant and flocculants. Reagents in mineral technology, Surfact. Sci. Ser. 27 (1988) 519–528. [12] B.J. Scheiner, G.M. Wilemon, Applied flocculation efficiency: a comparison of polyethylene oxide and polyacrylamides, Flocculation Biotechnol. Sep. Syst. 4 (1987) 175–185. [13] R. Hogg, Flocculation and dewatering, Int. J. Miner. Process. 58 (2000) 223–236. [14] P. Somasundaran, B.M. Moudgil, Reagents in Mineral technology, vol. 27, Marcel–Dekker, New York, 1988. [15] J.F. Richardson, W.N. Zaki, Sedimentation and fluidization. Part I, Trans. Inst. Chem. Eng. 32 (1954) 35–53. [16] R. Buscall, L.R. White, The continuous-flow gravity thickeners: steady state behaviour, AIChE J 34 (1998) 239–252. [17] R. Buscall, L.R. White, Solid/liquid separation of flocculated suspensions, Adv. Colloidal Interface Sci. 51 (1994) 175–246. [18] G.V. Franks, F.F. Lange, Plastic-to-brittle transition of saturated, alumina powder compacts, J. Am. Ceram. Soc. 79 (1996) 3161–3168. [19] M. Colic, G.V. Franks, M.L. Fisher, F.F. Lange, Effect of counterion size on short range repulsive forces at high ionic strengths, Langmuir 13 (1997) 3125–3129. [20] R.G. De Kretser, D.V. Boger, P.J. Scales, Compressive rheology: an overview, rheology reviews, Br. Soc. Rheol. (2003) 125–165. [21] F.M. Tiller, Z. Khatib, The theory of sediment volumes of compressible, particulate structures, J. Colloid Interface Sci. 100 (1983) 56–67. [22] K.T. Miller, Compressive yield stress of cement paste, Mater. Res. Soc. 41 (1995) 285–291. [23] R.G. De Kretser, P.J. Scales, D.V. Boger, Improving clay-based tailings disposal: case study on coal tailings, AIChE J 43 (1997) 1894–1903. [24] G.M. Channell, C.F. Zukoski, Shear and compressive rheology of aggregated alumina suspension, AIChE J 43 (1997) 1700–1708. [25] D.J.A. Williams, K.P. Williams, Electrophoresis and zeta potential of kaolinite, J. Colloidal Interface Sci. 65 (1977) 79–87. [26] W. Chen, Effects of surface charge and shear during orthokinetic flocculation on the adsorption and sedimentation of kaolin suspensions in polyelectrolyte solutions, Sep. Sci. Technol. 33 (1998) 569–590. [27] M.S. Nasser, A.E. James, Settling and sediment bed behaviour of kaolinite in aqueous media, Sep. Purif. Technol., (2006), in press.