Flocculation and dewatering of kaolin suspensions in the presence of polyacrylamide and surfactants

Int. J. Miner. Process. 66 (2002) 203 – 232 www.elsevier.com/locate/ijminpro

Flocculation and dewatering of kaolin suspensions in the presence of polyacrylamide and surfactants L. Besra a,*, D.K. Sengupta a, S.K. Roy b, P. Ay c a Regional Research Laboratory (CSIR), Bhubaneswar, 751-013, Orissa, India Department of Metallurgical and Materials Engineering, IIT Kharagpur, 721-302, WB, India c Lehrstuhl Aufbereitungstechnik, Brandenburgische Technische Universitaet, 03044 Cottbus, Germany b

Received 20 July 2001; received in revised form 20 May 2002; accepted 21 May 2002

Abstract Flocculation, as a result of the interaction between non-ionic polyacrylamide polymer (PAM-N) and kaolin surface in aqueous suspension, has been discussed both in the absence and in the presence of surfactants namely, cationic cetyl trimethyl ammonium bromide (CTAB), anionic sodium dodecyl sulphate (SDS) and non-ionic TX 100. The results of separation properties have been discussed in the light of kaolin surface charge, PAM-N and surfactant adsorption including conformation of the adsorbed polymer and properties of solution due to mutual interaction of polymer and surfactants. The kaolin settling rate improves by more than twentyfold through flocculation by PAM-N. Flocculation also reduces the specific resistance to filtration (SRF) from 7.8
1011 to 1.1
1011 m/ kg. The high molecular weight polymer, however, entraps excess water in the flocs resulting in very high cake moisture content. Pretreatment with either of the surfactants reduces the adsorption of nonionic PAM due to blocking of some surface sites by surfactant molecules. The polymer under these circumstances assumes different conformation favouring conditions for better flocculation by bridging and increases settling rate. Though flocculation of the surfactant pretreated kaolin does not reduce SRF, the cake moisture is reduced substantially. Addition of PAM-N from a mixture with surfactants leads to increase in PAM-N adsorption on kaolin, but it decreases settling rate as well as moisture content of the filter cake without any change in the SRF value. D 2002 Elsevier Science B.V. All rights reserved. Keywords: flocculation; dewatering; polymer; surfactant

* Corresponding author. Tel.: +91-674-481-635; fax: +91-674-581-637. E-mail address: [email protected] (L. Besra).

0301-7516/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 7 5 1 6 ( 0 2 ) 0 0 0 6 6 - 2

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1. Introduction Salts of polyelectrolytes and high molecular weight long chain synthetic polymers are being increasingly applied for flocculating fine particles in suspension in many mineral processing, hydrometallurgical and in waste water treatment operations (Oliver, 1963; Healy, 1973; Mueller and Beck, 1978; Theng, 1979, 1982; Baltar and Oliveira, 1998). These polymers in solution generally act by adsorbing onto the solid surfaces and aggregating them together to form flocs either by bridging, charge neutralisation, charge patch mechanism, depletion flocculation or by combination of them, facilitating faster sedimentation of the flocs and thus, enhancing separation of solids from liquid. Out of the many polymers available, polyacrylamide (PAM) and its derivatives are among the most important water-soluble polymers used as flocculants in industrial applications. The use of polyacrylamide flocculants in mineral processing has been reviewed in detail (Moody, 1992). In addition to flocculation, polymers have also been used as filtration and dewatering aids with the aim of increasing the filtration rate as well as reducing residual filter cake moisture content. However, the role played by polymeric flocculants has long been disputed. The key point of the issue is whether the flocculants are beneficial to lowering filter cake moisture or not. There are reports showing increase in filter cake moisture content due to flocculation (Xiaomin et al., 1996). Some studies on flocculation of kaolin suspension have also shown similar results, i.e., an increase in cake moisture content although a substantial enhancement in filtration rates has been achieved through flocculation by PAM (Besra and Ay, 1999). Entrapment of water in the flocs as intrafloccular water might be the reason for increase in moisture content of the filter cake. Besides the use of long chain polymers, surfactants are also in use with the purpose of reducing the filter cake moisture. 1.1. Surfactants as dewatering aids Surface-active substances have been employed in many cases as dewatering aids and have been successful in decreasing the filter cake moisture content substantially (Ayub et al., 1987; Cooper et al., 1988; Stroh and Stahl, 1990; Mwaba, 1991; Besra et al., 1998a). The effectiveness of surfactants as dewatering aids have been quantified through the Laplace –Young relationship: DP ¼

2ccosh r

ð1Þ

where, DP is the capillary pressure differential required for dewatering, c is the liquid surface tension, h is the solid –liquid contact angle and r is the capillary radius in a filter cake (Besra et al., 1998b). Reduction in liquid surface tension and increase in hydrophobicity or combinations of both have so far been agreed upon as the basic factors for improved dewatering by surfactants (Nicol, 1976; Puttock and Wainwright, 1984). Although the surfactants help in reducing moisture content of cake, depending on their nature, sometimes they also tend to stabilise the suspension resulting in reduction in filtration rate substantially.

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Considering the opposite effects rendered by flocculants and surfactants on filtration and cake moisture, it is of interest to investigate whether a combined use of them could possibly improve both filtration rate and reduction of cake moisture. In such a system where both polymer and surfactant co-exist, there could be mutual interaction between them besides their interaction with the solid surface. Such interactions, regardless of the exact molecular explanation, can significantly alter the macroscopic characteristics of the system and ultimately its application. Therefore, it is necessary to understand interaction between flocculants, surfactants and the surfaces to obtain the best result for improved dewatering characteristics. 1.2. Polymer– surfactant interaction It is generally recognized that surfactant – polymer interactions may occur between individual surfactant molecules and the polymer chain (i.e., simple interaction), or in the form of polymer –aggregate complexes. In the latter case, there may be complex formation between the polymer chain and surfactant micelles or between polymer chains and premicellar aggregates (Tirtaatmadja et al., 1998). Other associations may result in the formation of so-called hemi-micelles along the polymer chain. The nature of the surfactant –polymer complex may significantly alter the overall energetics of the system so that major changes in polymer chain conformation will result. Any and all of those changes may result in major alterations in the macroscopic and microscopic properties of the system. The largest volume of published work in the field of surfactant – polymer interactions has involved non-ionic polymers and ionic surfactants (preferred surfactant has been sodium dodecyl sulfate, SDS) and non-ionic polymers such as polyvinylpyrrolidone (PVP) (Chari and Lenhart, 1990), polyvinyl alcohol (PVA) and polyethylene oxide (PEO) (Brackman and Engbert, 1990; Dubin et al., 1992), poly (ethylene oxide) PEO – cetyl trimethyl ammonium bromide (CTAB) in water (Brackman and Engbert, 1992). In general, the results of the above investigations indicate that the more hydrophobic the polymer is, the greater is the interaction of anionic surfactant with it. The primary driving force for polymer –surfactant interaction in such a system will be the van der Waals forces and the hydrophobic effect. Dipolar and acid – base interactions may be present, depending upon the exact nature of the system. Ionic interactions will be minimal or non-existent. For the polymer, the impact of the hydrophobic effect will be related to the ability of the polymer to undergo hydrogen bonding with the solvent, as well as the relative availability of non-polar binding sites along the polymer chain. Though the mutual interaction and adsorption of polymers and surfactants onto particles in suspension are well-studied, no data is found on their role in determining flocculation and dewatering for the case when they co-exist. Recently, a few literatures (Magdassi and Rodel, 1996; Myagchenkov and Bulidorova, 1997; Somasundaran et al., 1998; Fan et al., 1999) reported flocculation by polymers in presence of surfactants. However, detailed studies on the interfacial phenomena and their consequence on flocculation and dewatering are still lacking. Considering the interaction effect of polymers and surfactants in changing their solution properties, attempts have been made through this study to systematically establish a suitable polymer– surfactant association for

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use as a flocculation and dewatering agent. Attempts have also been made to correlate the surface properties for illustrating mechanisms behind the interfacial processes and the flocculation and dewatering characteristics of kaolin suspension.

2. Experimentals 2.1. Materials 2.1.1. Kaolin The kaolin clay used in this study is of high quality paper coating grade obtained from Jashipur, Orissa, India. The particle size analysis using a Malvern Laser particle size analyser Model 3600E, from Malvern Instruments, UK, showed all the particles to be below 20 Am with only 11% particles are above 10 Am, and the d50 to be 4.8 Am. Its BET specific surface area is 8.62 m2/g and the point of zero charge is at pH value of 2.2. The other characteristic details of the sample are reported elsewhere (Besra et al., 2000). 2.1.2. Flocculant The non-ionic polyacrylamide (PAM-N) flocculant used in the investigation was kindly supplied by Allied Colloids, UK. The molecular weight as determined from the intrinsic viscosity method using the Mark – Houwink constants K = 6.31
10  3 cm3 g  1 and a = 0.80 for PAM in distilled water (Brandup and Immergut, 1989) was estimated to be about 5
106. The viscosity measurements of varying concentrations of polymer in water were carried out using a Fann viscometer. 2.1.3. Surfactants The surfactants used in the investigation were supplied by Merck, Germany. Some of their characteristics are given in Table 1. 2.2. Methods 2.2.1. Studies on polymer – surfactant interaction The mutual interactions of polymers and surfactants in solutions were characterised through measurement of surface tension. For surface tension measurements, incremental addition of surfactant from its solution of a particular concentration was made to a fixed initial volume of a polymer solution of a particular concentration such that after addition, a desired concentration of both polymers and surfactants are achieved. Surface tension measurements were carried out using a Du-Nuoy ring type tensiometer from Fischer Scientific, USA, using a platinum – iridium ring. In the cases of mixed solutions at low surfactant concentrations of surfactants and polymers, the time to attain equilibrium was more than 3 h. Therefore, it was assumed arbitrary that equilibrium has been reached when the surface tension variation was less than 0.1 mN/m over 10 min. The reproducibility, including long equilibrium time was 0.1 mN/m for mixed solution.

Surfactant

Molecular formula

Molecular weight, g/mol

Ionic type

Critical micelle concentration (cmc), mM

HLB

Aggregation no.

Measured value

Literature value

Cetyl Trimethyl Ammonium Bromide (CTAB) Sodium Dodecyl Sulphate (SDS)

C16H33N(CH3)3 + Br 

364.4

Cationic

0.90

0.90 (Pagac et al., 1998; Seng et al., 1999)

–

91

n-C12H25SO 4 Na +

288.3

Anionic

8.20

17.2

62

Triton-X 100

p-t-C8H17C6H4O(C2H4O)10H

628

Non-ionic

0.28

8.20 (Biggs et al., 1992) 8.27 (Minatti and Zannette, 1996) 0.27 (Colic et al., 1998)

13.5

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Table 1 Characteristics of surfactant used

207

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2.2.2. Surface charge measurement Measurement of surface charge of particles was carried out using a particle charge detector (PCD-03-pH) from Muetek, Germany. It consists of a cylindrical test cell with fitted displacement piston which oscillates at a frequency of about 4 Hz forcing a relative motion of liquid and the particles inducing development of streaming potential of either negative or positive sign. The exact charge quantity was then estimated by titrating the sample with an oppositely charged standard polyelectrolyte titrant until neutralisation of the streaming potential to zero value using a compatible standard automatic titrator (702 SM Titrino). The titrant used were 0.001 N sodium polyethylene sulphonate (PES-Na) as anionic, or 0.001 N poly-diallyl-dimethyl-ammonium-chloride (Poly-DADMAC) as cationic standard, respectively. The total charge quantity (in Aeq/gm) is then calculated from the titrant consumption according to the following expression: V
c
1000 ð2Þ w where V = volume of titrant required (ml); c = normality of the titrant ( = 0.001 N); 1000 = calculation factor for the unit of charge density; w = amount of solid in the suspension (g). The total quantity of charge (in C/g) can be obtained by multiplying the specific charge (in eq/g) with the Faradays constant ( = 96485 C/eq). The magnitude of error during the charge measurement is very small and is about F 2.0%. q¼

2.2.3. Flocculation studies A known weight of the powder material was mixed and stirred in an appropriate volume of distilled water to obtain the desired slurry concentration and was allowed to stand for 24 h. Then, the suspension was stirred in the beaker provided with baffles using a 3-cm threebladed stainless steel stirrer for 10 min at 500 rpm. The requisite amount of flocculant (PAM-N) or its mixture with surfactants was then added drop wise followed by stirring for 1 min after addition. The stirring speed was then reduced to 300 rpm and continued for 1 min. In the case of surfactant pretreatment, the above step was preceded by addition of required amount of surfactant and equilibration for 24 h. The entire suspension was then transferred to a graduated cylinder and allowed to settle after inverting the cylinder five times. A clear liquid slurry interface could be seen descending. The pulp level was noted at a regular interval of time. Settling rates were estimated from the slope of the straight line portion of the plot of pulp level vs. settling time. After the sedimentation test, the suspension was allowed to stand without disturbance for 24 h. The volume occupied by the sediment was then read directly from the graduated cylinder. Duplicate measurements made under similar experimental conditions suggest that the settling rates and sediment volumes are reproducible to within the error of 2.5%. Unless and otherwise mentioned, all the measurements were conducted at the natural pH (6.4) of the slurry except in the study on the effect of pH as a variable, where the pH is adjusted with either 1 M HCl or NaOH solution. 2.2.4. Adsorption studies The adsorption of polyacrylamide (PAM) and surfactants from solution onto the surface of kaolin in suspension was estimated by determining their depletion from the solution.

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About 50 ml of the suspension was first stirred in a 100 ml conical flask with either flocculant or a mixture of flocculant and surfactant at 25 jC in a fashion similar to that used for the flocculation test, followed by equilibration for 24 h on a SPINIT orbital shaking platform, India at the lowest speed set at no.1 to prevent breakage of flocs. The supernatants were then separated from the sediment by centrifugation for 90 min at 8500 rpm using a Remi Research Centrifuge from Remi Instruments, Bombay. Care was taken to see that no solid remained in suspension. This was ensured by turbidity measurements, which showed the supernatant turbidity is close to the distilled water used. Equilibrium concentrations of polyacrylamide (PAM-N) and surfactants in the supernatant were determined spectrophotometrically with a Perkin-Elmer Lambda-2 model UV/VIS spectrophotometer using a 1 cm quartz cell. The PAM-N concentration was determined using the Starch-Tri-iodide method (Scoggins and Miller, 1979). Presence of surfactants did not interfere with the determination of PAM-N by this method. The non-ionic surfactant TX 100 was estimated from the calibration curve drawn at its UV active peak at the wavelength of 276.8 nm. The anionic SDS and cationic CTAB did not show any UV active peaks. So their concentration in supernatant was measured by colorimetric method. For CTAB determination, blue colour was developed by formation of ternary complex with Fe (III) and chrome azurol S (Song and Liang, 1996) and the absorbance measured at the wavelength of 660 nm. For SDS estimation, a blue colour complex was developed with ethyl violet and extracted to toluene before measuring the absorbance at the wavelength of 611 nm (Colic et al., 1998). Calibration curves were used for quantitative estimation of each surfactant. 2.2.5. Filterability measurements Measurements on filterability of distilled water containing varying concentrations of polymer solutions was carried out using a standard Capillary Suction Time (CST) apparatus, TW 166, from Triton Electronics UK using a 10-mm diameter reservoir (Sengupta et al., 1994). The Whatman No. 17 filter paper as suggested by Baskerville and Gale (1968) was used in the CST measurements. The apparatus automatically records the time (in s) taken by the interface between the wet and dry portions of the filter paper exposed to a column of liquid in the reservoir.The lower the CST,the better is the filterability. The reported values are an average of five measurements. The reproducibility of the reading was within 0.5 s. 2.2.6. Filtration and dewatering studies All the filtration experiments were carried out with a vacuum filtration unit already described elsewhere (Besra et al., 2000). It consists of a filter holder, 10 cm diameter with 500 ml graduated reservoir. The base of the filter holder is a perforated plate with a provision to fix the filter paper on it. The Whatman 41 filter paper was used throughout the filtration studies. The slurry after desired treatment with polymers or surfactants is poured in to the filter holder for filtration to occur under a desired pressure difference (49 kPa) applied by vacuum. The volume of filtrate collected in the graduated cylinder was recorded at a regular time interval. These data were used to calculate the specific cake resistance using the integrated form of Darcy’s equation (Sengupta et al., 1997; Besra et al., 2000).

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From the moment at which liquid disappeared from the top surface of the cake formed, a certain desired dewatering time was allowed and then the cake was carefully removed, weighed and dried at 105 jC until it attained a constant weight. Moisture content of the cake (in %) was determined from the loss in weight on drying and is calculated as follows: Moisture content ¼

weight of wet cake  weight of dry cake
100 weight of wet cake

ð3Þ

3. Results and discussion 3.1. PAM-N/surfactant interaction Investigations have been made on the interaction of polymer and surfactants namely SDS, CTAB and TX 100 in aqueous solutions of fixed polymer concentration and varying surfactant concentration. Presence of either of the surfactants in distilled water reduces the surface tension of the aqueous solution up to certain concentration called critical micelle concentration (cmc) beyond which it remains constant. The cmc of SDS, CTAB and TX 100 lies at concentration of 8.2, 0.90 and 0.28 mM, respectively, (Fig. 1) and are in good agreement with the literature values (Colic et al., 1998; Pagac et al., 1998; Seng et al., 1999; Biggs et al., 1992; Minatti and Zannette, 1996). But the presence of polyacrylamide (PAMN) in distilled water does not show any change in surface tension and remains close to that of distilled water in the whole range of concentration studied in the present investigation. It indicates the non-surface-active nature of PAM-N solutions. Fig. 1a shows the surface tension on addition of varying concentration of anionic surfactant SDS to distilled water and to a fixed initial concentration of PAM-N solution, respectively. The surface tension curve for PAM-N/SDS mixture exhibits a break point at concentration (1.5 mM SDS) that is lower than its critical micelle concentration (cmc). In the case of CTAB and TX 100 addition to PAM-N solution, the surface tension – concentration curve is almost same as that of aqueous solution of the respective polymer-free surfactants (Fig. 1b and c). It indicates that there is no role of PAM-N in reduction of surface tension. The reduction in surface tension of PAM-N by TX 100 initiates comparatively at much lower concentration than the other two surfactants. The reduction in surface tension in the case of neutral polymers and anionic surfactants such as SDS, as proposed by several authors, may be due to formation of polymer –surfactant complex by binding of surfactants to polymers driven by the following mechanisms: (a) reduction of the hydrocarbon/water contact area of the alkyl chains of the surfactant (Goddard, 1986a,b), (b) an ion-dipole interaction between the surfactant head group and the polymer (Schwuger, 1973) and (c) the hydrophobic interaction between the polymer and the hydrocarbon chain (Perron et al., 1987). 3.2. Charge characteristics on kaolin surface The structure of kaolinite has been described to compose of a single silica tetrahedral sheet (SiO2) and a single alumina octahedral sheet (Al2O3) combined in a unit so that the tips of the silica tetrahedron and one of the layers of the octahedral sheet form a common

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211

Fig. 1. Surface tension of PAM-N solution in presence of surfactants (pH = 6.4).

layered structure (Grim, 1953; Klein and Hurlbut, 1985). Such layers are electrically neutral and are bonded to one another by weak van der Waals bonds. The electrical neutrality can be disturbed by substituting Al for some of the Si in the tetrahedral sites of

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Fig. 2. Effect of pH on specific surface charge of kaolin suspension (5% w/v).

SiO2 sheets. Because Al is trivalent, whereas Si is tetravalent, each substitution of this kind causes a free electrical to appear on the surface of the tetrahedral –octahedral layer. If Al substitutes to every fourth Si in the tetrahedral basal plane, a negative charge of significant magnitude is produced on the basal plane. So, the heteropolar model of kaolinite views the particles as having two crystallographic surfaces exposed to aqueous medium (Braggs et al., 1994): (i) a pH-independent basal plane with permanent negative charge on it containing exchangeable cation, and (ii) a pH-dependent edge containing positively charged sites in acidic media (Van Olphen, 1977). The positive charge results from the protonation of the edge aluminol group. Al  OH þ H3 Oþ ! Al  ðOH2 Þþ þ H2 O

ð4Þ

With increasing pH, the cationic sites [Al –(OH2) + ] are neutralised by the following reaction with consequent discharge of edge particles resulting in only negative charge of kaolin Al  ðOH2 Þþ þ OH ! Al  OH þ H2 O

ð5Þ

Fig. 2 shows the surface charge of kaolin measured in aqueous medium as well as in 0.01 M KCl as a function of pH. It shows that the net charge on kaolin surface is positive in the acidic region. Reversal of surface charge from positive to negative occurs at pH 2.2, suggesting it to be the point of zero charge. At higher value of pH above 2.2, the kaolin surface becomes more and more negative up to about a pH value of 4.5 after which, it exhibits flattening in the curve. The magnitude of negative charge of kaolin surface is always smaller in the salt solution compared to that in aqueous medium, which maybe because of compression of the diffuse layer in the electrical double layer. Fig. 3a –c shows the charge characteristics of kaolin through treatment with PAM-N, surfactants and their mixtures. Addition of PAM-N results in its adsorption on the kaolin surface rendering its charge less negative and tending towards zero value. This happens up to a certain PAM-N concentration (0.25 mg/g of kaolin) after which saturation occurs and

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213

Fig. 3. Charge characteristics of kaolin by treatment with PAM-N in the presence and absence of surfactants (pH = 6.4).

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a plateau appears. No reversal of surface charge from negative to positive is possible at any dosage of PAM-N. The reduction of the kaolin surface charge is due to the adsorption of the polymer in thick layers at the surface, which moves the plane of shear in the electrical double layer outward from the particle surface (Carasso et al., 1997). As the surface charge decays exponentially with distance from the solid surface, a decrease in surface charge is predicted in the presence of an adsorbed polymer (Fleer et al., 1972). Similar observations have also been reported earlier for adsorption of anionic polyacrylamide on kaolin (Besra et al., in press). The charge characteristics of kaolin by pretreatment with surfactants depend on the nature of surfactants. Anionic surfactant SDS for example, imparts more negative charge to kaolin and increases the negative surface charge from  0.261 to  0.292 C/g at a dosage of 0.2 mg/g. At similar pretreatment dosages, the cationic surfactant CTAB and non-ionic surfactant TX 100 reduces the negative surface charge to  0.087 and  0.108 C/g, respectively. Addition of PAM-N to the surfactant-pretreated kaolin invariably reduces the surface charge at low concentrations followed by attainment of charge plateau at higher concentrations. Addition of 1:1 mixture (w/w) of PAM-N and SDS (which corresponds to 1:4 molar ratio of SDS to monomeric unit of the polymer) tend to neutralize the negative specific surface charge up to a dosage of about 0.2 mg/g of kaolin, followed by a sharp decrease thereafter (Fig. 3a). However, the negative charge quantity of kaolin surface at the peak is still larger in magnitude than surfactant-free PAM-N addition. Fig. 3b shows charge characteristics of kaolin in presence of cationic surfactant CTAB and PAM-N. The addition of 1:1 mixture (w/w) of PAM-N and CTAB (which corresponds to 1:5.13 molar ratio of CTAB to monomeric unit of PAM-N) causes a charge reversal from negative to positive at a dosage of 2 mg mixture/g of kaolin after which it is positively charged. Fig. 3c shows the charge characteristics of kaolin by addition of 1:1 mixture (w/w) of PAM-N and TX 100 (which corresponds to 1:8.84 molar ratio of TX 100 to monomeric unit of PAM-N) reduces the charge and tends to approach close to zero at about 2 mg/g after which a plateau is obtained. 3.3. Adsorption characteristics 3.3.1. Surfactant adsorption Fig. 4 presents the adsorption isotherm of surfactants onto kaolin in which the amount adsorbed is plotted against the equilibrium surfactant concentration. It clearly indicates low adsorption density of the anionic surfactant SDS that soon attains plateau leading to a typical Langmuir-type curve (Fig. 4a). The low adsorption density of SDS could be a result of the electrostatic hindrance caused by the repulsive forces between the negatively charged kaolin and SDS. It further indicates that it becomes increasingly difficult with increasing concentration for the surfactant molecules to find vacant adsorption sites on kaolin surface as most sites are similarly charged or are already occupied by the previously adsorbed surfactant molecules. So the adsorbed SDS must be of high affinity in nature. Owing to the absence of electrostatic repulsion between TX 100 and the surfaces, its adsorption on kaolin surface is much higher (Fig. 4c) and follows a typical ‘S’-type curve (Giles et al., 1960). The TX 100 adsorption data fits well with the Freundlich equation

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215

Fig. 4. Adsorption characteristics of surfactants on kaolin when added separately and from their mixture with PAM-N (pH = 6.4).

(Sengupta, 1985) and the slope of the Freundlich plot is about 1.20. The marked increase in the slope of the adsorption isotherm is either through chemical reaction or through formation of hemi-micelles (Fuesrstenau, 1971) or micelles (Sengupta, 1985). The initial concentration of TX 100 at which the steep and linear rise in the adsorption curve (Fig. 4c)

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occurs is between 100 and 200 mg/dm3. The value of cmc of TX 100 is about 0.28 mM (or 175.8 mg/dm3). It can therefore be predicted that the adsorption of TX 100 takes place through micelle formation. Further, as the electrostatic attraction is non-existent in this case, hydrogen bonding may be considered to be the driving force for adsorption (Tahani et al., 1996). The cationic surfactant CTAB exhibits maximum adsorption on kaolin (Fig. 4b) due to attraction between the negative surface and the positively charged surfactant, and its isotherm follows the typical ‘H’-type of curve as per the classification given by Giles et al. (1960). It indicates that the solute has such high affinity that in the dilute solutions it is completely adsorbed or at least there is no measurable amount remaining in solution. The initial part of the curve is therefore vertical. The surfactant adsorption from a mixture of PAM-N and surfactant will be discussed in Section 3.7. 3.3.2. PAM-N adsorption Preliminary experiments on kinetic of PAM-N adsorption on kaolin surface were carried out at an initial PAM-N concentration of 0.5 mg/g. This concentration was adequate because some amount of residual unadsorbed PAM-N was found in the supernatant after equilibration time of 24 h. The results of the kinetic studies revealed that more than 99% of equilibrium adsorption takes place within first 1 –2 min. Similar observations were made by Lindstrom and Soremark (1976). Table 2 presents the adsorption characteristics of PAM-N onto kaolin along with the flocculation characteristics at acidic, near neutral and alkaline pH. It indicates high density of PAM-N adsorption onto kaolin surface in the whole range of pH studied and there is not much significant change in adsorption density of PAM-N with variation in pH. It has now been well recognized that adsorption of non-ionic polymers normally occur via hydrogen bonding between the solid surface and the hydroxyl group (non-ionic polar group) on the polymer (Michaels and Morelos, 1955). Another important mechanism of adsorption is by hydrogen bonding between the amide groups and aluminol groups at the edges of kaolin particles (Nabzar et al., 1984; Lagaly, 1993). The formation of these hydrogen bonds is competitive with hydrogen bond formation between neighbouring surface OH groups (aluminol and silanol groups). Thus, hydrogen bonding of PAM-N to silanol groups as anchoring sites is promoted when in acidic medium, neighbouring aluminol groups are protonated. In alkaline medium, bonding to aluminol groups is favoured, when the neighbouring silanol groups are

Table 2 Effect of pH on PAM-N adsorption and kaolin flocculation pH

Initial concentration of PAM-N, mg/g

Equilibrium concentration, mg/g

Amount adsorbed, mg/g

Settling rate, cm/s

2.3

0.50 2.0 0.50 2.0 0.50 2.0

0.0 0.0 0.05 0.29 0.0 0.02

0.50 2.0 0.45 1.71 0.50 1.98

0.424 0.376 0.635 0.600 1.216 1.356

6.4 10.95

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dissociated. The flocculation characteristics of kaolin with variation of pH will be discussed in Section 3.4. The results of PAM-N adsorption onto kaolin surface at different mode of additions are shown in Fig. 5. The adsorption isotherm shows the amount of PAM-N adsorbed against the equilibrium PAM-N concentration. The isotherm exhibits a rapid and steep but not infinite, initial slope leading to a rounded knee followed by a linear interval with a small positive slope and finally, a more steep rise. This is close to the H3-type curve according to the classification of adsorption isotherms given by Giles et al. (1960). It indicates the occurrence of multilayer adsorption. The H-type isotherms are a special case of the Langmuir curve, in which the solute has very high affinity such that in dilute solution, it is completely adsorbed, or at least there is no measurable amount remaining in solution. The multilayer nature of adsorption has also been confirmed through fitting our data points according to the Langmuir and Freundlich models. The adsorption data points fit more closely to the Freundlich model. 3.4. Flocculation characteristics The objective of this study was to assess the flocculation and dewatering ability of PAM-N flocculants in presence of surfactants. The effectiveness of flocculation was evaluated by measuring the settling rate and sediment volume. The untreated kaolin suspension exhibits very low settling rate of about 0.03 cm/s. Treatment with surfactant alone in absence of any polymer does not improve settling rate. On addition of PAM-N, the settling rate of surfactant-free kaolin increases with concentration to about 0.6 cm/s at 0.5 mg PAM-N/g of kaolin beyond which it remains constant (Fig. 6). Also, there appears no region of restabilization unlike in many ionic flocculants. The sediment volume curve also exhibits a similar trend (Fig. 7). As indicated in Table 2, even though the quantity of PAM-N adsorbed on kaolin at both acidic (pH = 2.3) and alkaline (pH = 10.95) conditions are same, they exhibit different flocculation properties. The settling rate value at the alkaline pH of 10.95 is almost double of that at low pH 2.3. Such a state could be a result of changes brought about to the surface of kaolin and the conformational state of the adsorbed polymer with variation in pH. Though the polyacrylamide flocculant (PAM-N) used is known to be non-ionic in nature, there is some degree of anionicity (about  1.07 C/g) as found while estimating its charge density. It has also been reported that for the purpose of classifying flocculants, a polymer is considered non-ionic if fewer than 1% of the monomer units are charged (Halverson and Panzer, 1980). Besides this, Sastry et al. (1999) found that non-ionic polyacrylamide polymer undergone about 10% hydrolysis in the alkaline region (pH c 10.5) leading to conversion of some amide groups to carboxylic moieties. Similar hydrolysis of the non-ionic polyacrylamide flocculant (PAM-N) in the alkaline pH region could have resulted in an increase in negative charge density along the polymer chain and thus, more inter unit repulsion. The polymer chain can, thus, acquire an extended form of conformation, which enables it to interact simultaneously with more number of sites favourable for flocculation by bridging (Somasundaran et al., 1998). The polymer molecule at low pH has a more coiled state of conformation compared to that at alkaline conditions and is not favourable for bridging.

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Fig. 5. Adsorption characteristics of polyacrylamide (PAM-N) on kaolin in presence and absence of surfactants (pH = 6.4).

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Fig. 6. Effect of PAM-N on settling rate of 5% (w/v) kaolin suspension in presence and absence of surfactants (pH = 6.4).

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Fig. 7. Effect of PAM-N on sediment volume of 5% (w/v) kaolin suspension in presence and absence of surfactants (pH = 6.4).

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It is however imperative to consider the variation in kaolin surface charge with pH, which can influence the floc microstructure. At higher pH it is found that the kaolin surface is more negatively charged (Fig. 2) which is expected to cause more mutual repulsion between the particles within the floc. This could lead to formation of more open but bigger flocs compared to those formed at lower pH. 3.5. Filtration and dewatering characteristics The filtration characteristics have been assessed through the measurement of specific resistance of the cake to filtration (SRF).The lower the SRF, the better is the filtration characteristics. The filtrate volume collected at regular time intervals has been used to calculate the specific cake resistance using the integrated form of Darcy’s equation (Besra et al., 2000). Treatment of kaolin slurry with surfactants alone reduces SRF to some extent (Fig. 8). The decrease in SRF could be a result of increased hydrophobicity of the surface (Pearse and Allen, 1983; Wainwright et al., 1995), reduction of surface tension (Nicol, 1976; Puttock and Wainwright, 1984; Stroh and Stahl, 1990) or a combination of both (Mwaba, 1991). The filtration and dewatering characteristics of kaolin suspensions flocculated by PAM-N are shown through Figs. 8 and 9. In the absence of any surfactant, the SRF of kaolin suspension is very high (7.8
1011 m/kg) with about 36.15% cake moisture content. The SRF reduces rapidly with increasing concentration of PAM-N up to about 0.25 mg/g, but it increases cake moisture as well. This may be due to entrapment of excess water by the strongly adsorbing PAM-N. The cake resistance profile clearly shows a distinct minimum in the region of 0.25 –0.4 mg/g PAM-N concentration. Addition of more PAM-N results in poor filtration as indicated by a rise in the SRF value (Fig. 8). Considering the flocculation results (Fig. 6), which show a constant settling rate beyond 0.5 mg/g PAM-N addition, one would expect a similar trend in cake resistance profile also. The poor filtration characteristics indicated by increase in SRF may be a result of blocking of pores in the filter cake and the medium by the long chain high molecular weight PAMN flocculant or due to compression of the large voluminous flocs restricting the passage of liquid through the cake bed. This explanation can be supported by Fig. 10, which indicates that when water contains more amount of PAM-N, the capillary suction time (CST) is higher, i.e., poor filterability. The residual flocculant in the liquid component of the suspension may block the pores restricting the passage of liquid. 3.6. Effect of surfactant pretreatment Fig. 5 also includes the adsorption isotherms of PAM-N onto kaolin pretreated with 0.2 mg surfactant/g of kaolin. Pretreatment of kaolin with all the surfactants reduces the adsorption of PAM-N substantially, and also limits its adsorption only to monolayer coverage as evident by the appearance of a distinct plateau in the isotherm typical of Langmuir adsorption model. The results on flocculation of kaolin pretreated with 0.2 mg surfactant/g of kaolin are presented in Fig. 6. Pretreatment with all the three surfactants leads to substantial improvement in settling rates. The settling rate increases to 1.2 cm/s on addition of 0.5

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Fig. 8. Effect of PAM-N on SRF of 5% (w/v) kaolin suspension in presence and absence of surfactants (pH = 6.4).

mg PAM-N/g of surfactant-pretreated kaolin. The sediment volumes (Fig. 7) at the conditions of maximum settling in the cases of surfactant-pretreated kaolin are marginally smaller in comparison to the one not treated with surfactant. It suggests that pretreatment with surfactant in general results in more compact and higher density flocs. This type of

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Fig. 9. Effect of PAM-N on filter cake moisture content in presence and absence of surfactants (pH = 6.4).

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Fig. 10. Filterability of distilled water containing varying concentrations of PAM-N.

flocculation behaviour can be explained on the basis of conformation of adsorbed polymer. As most of the active sites are blocked by the surfactants due to pretreatment, only a fraction of the PAM-N adsorbs on the surface and a large segments of its chain extends into solution as tails making them available to interact with unoccupied sites of other particles. It may be supported by Fig. 11, which shows variation of kaolin surface charge with increasing concentration of surfactants. It indicates that there is some extent of change in the surface condition. CTAB being cationic in nature changes the surface charge towards positive side and anionic SDS towards more negative side. But the non-ionic TX 100 changes the charge marginally towards positive side. It can therefore be concluded that some of the active sites are blocked by the surfactants even at a low concentration. A schematic illustration for flocculation of surfactant-pretreated kaolin is proposed in Fig. 12. As more number of flocculant segments is available to interact with the neighboring particles, flocculation is better resulting in higher setting rate even though the total flocculant adsorption is less. The sediment volume is slightly less compared to the one with only PAM-N. This may be due to formation of denser flocs by the surfactant-pretreated kaolin with less number of polymer molecules taking part in flocculation process. Fig. 8a –c also showS the SRF values of surfactant pretreated kaolin. When the kaolin is pretreated with 0.2 mg/g CTAB, and then flocculated by PAM-N, a distinct minimum value of SRF is attained at 0.125 mg PAM-N/g of kaolin. This concentration is sufficiently lower than that required (0.25 – 0.4 mg/g) when PAM-N is added to surfactant-free kaolin. Secondly, the value of minimum SRF is same (1.10
1011 m/ kg). At concentrations higher than 0.125 mg/g, the SRF values are always higher than those in the case of surfactant-free kaolin. At the CTAB concentration of 0.2 mg/g (corresponding to 1.33 mg/dm3) there is no reduction in surface tension of the liquid (Fig. 1). Hence, one of the possibilities of improved filtration can be due to hydrophobisation of kaolin surface by CTAB adsorption. Pretreatment of kaolin with 0.2 mg/g of SDS or

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Fig. 11. Effect of surfactants concentration on specific surface charge of kaolin suspension (5% w/v in distilled water; pH = 6.4).

TX 100 does not improve the filtration characteristics of kaolin suspension. The SRF curves of SDS and TX 100 pretreated kaolin lie along and above, respectively, to that with PAM-N flocculated cake of kaolin not pretreated with any surfactant (Fig. 8a and c).

Fig. 12. Schematic illustrations showing adsorption of PAM-N, and flocculation (a) of kaolin, (b) kaolin some of whose surface sites are blocked by surfactant pretreatment.

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At a surfactant concentration of 0.2 mg/g (1.33 mg/dm3), there is no reduction in surface tension of either PAM-N/SDS or PAM-N/TX 100 system (Fig. 1). Pretreatment of kaolin with only surfactants CTAB substantially reduces the moisture content of the filter cake. Addition of PAM-N to the CTAB pretreated sample helps in aggregating the particles into large flocs, but entraps some amount of water in the floc structure resulting in higher cake moisture. However, the moisture content obtained is much lower than that obtained by PAM-N flocculation of surfactant-free kaolin (Fig. 9b). It indicates that presence of CTAB has a role to play in decreasing the cake moisture content. Similarly, the moisture content of TX 100 pretreated kaolin flocculated by PAMN is reduced sufficiently in comparison to that for surfactant-free kaolin (Fig. 9c) although there is no reduction in specific cake resistance. Pretreatment with SDS also reduces the cake moisture content, but the reduction is small in comparison to that by CTAB and TX 100 pretreatment (Fig. 9a). 3.7. Effect of simultaneous addition of PAM-N and surfactants from 1:1 mixture (w/w) In order to identify if there is a competitive adsorption between surfactants and polymers, adsorption studies from a 1:1 mixture (w/w) of flocculant and surfactant on kaolin have been carried out. Additions from mixtures with all the three surfactants lead to increase in adsorption of PAM-N onto kaolin (Fig. 5). The increase in adsorption of PAMN may be a result of interaction of surfactant with polymer and polymer – surfactant complex which behaves as polyelectrolyte (Otsuka and Esumi, 1997). In the case of addition of mixture of CTAB and PAM-N, the free cationic surfactant can adsorb much faster on kaolin than PAM-N due to its smaller size and the electrostatic attractive force. The positive ions of the surfactant can neutralise some of the negative surface sites AlO  . The polymer PAM-N can adsorb on it due to hydrophobic interaction between the alkyl groups of surfactant ions and hydrophobic part of PAM-N chains (Lange, 1971; Moudgil and Somasundaran, 1985; Goddard, 1986a,b; Zhang et al., 1998). Simultaneous addition of PAM-N and TX 100 from its 1:1 (w/w) mixture enhances PAM-N adsorption simultaneously but it reduces TX 100 adsorption as indicated in Figs. 4 and 5, respectively. Similarly, adsorption of SDS or CTAB also reduces at low equilibrium concentration when added from their mixture with PAM-N (Fig. 4). Such a reduction in surfactant adsorption is a result of competition between polymer and surfactant (Ghodbane and Denoyel, 1997). Fig. 6 shows that simultaneous addition of 1:1 mixture (w/w) of PAM-N and surfactants are generally detrimental to flocculation as it does not improve but decreases settling rate. It also results in a lower sediment volume in comparison to that due to PAM-N addition to untreated kaolin (Fig. 7). Simultaneous addition of PAM-N and surfactants in general have not improved the filtration characteristics as no further reduction in SRF is noticed in any case (Fig. 8). The minimum value of SRF attained in each case is same (1.10
1011 m/kg) as that due to PAM-N addition alone. But the simultaneous addition of PAM-N and surfactants could reduce cake moisture content significantly in comparison to the suspension treated with PAM-N only (Fig. 9). For example, the simultaneous addition of PAM-N and CTAB brings down the moisture content to the level of CTAB pretreated kaolin. The advantageous effect on addition of surfactants may be due to improved hydrophobicity effected

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by this surfactant for which the aggregates entraps less amount of water compared to PAM-N flocculated aggregates. The moisture content of filter cake due to simultaneous addition of PAM-N with TX 100 and SDS is sufficiently lower than PAM-N addition to surfactant-free as well as surfactant-pretreated kaolin. The best performance of PAM-N/ TX 100 (1:1) may be due to combined effect of hydrophobicity imparted by TX 100 adsorption and reduction in surface tension as it can be seen from Fig. 1 that reduction in surface tension by TX 100 happens at a much lower concentration compared to the other two surfactants. 3.8. Effect of washing To establish the nature of PAM-N adsorption on kaolin, we have carried out desorption experiments by washing. For desorption experiments, we have treated the kaolin in a manner similar to adsorption studies except that at the end of equilibration time, the contents were filtered and washed repeatedly with distilled water till the washed water does not contain any measurable quantity of polymer. Table 3 presents the results of desorption studies. It indicates that there is very little ( < 11%) desorption of PAM-N on washing even in presence of surfactants. Since major quantity of PAM-N adsorbed on kaolin is irreversible and of high affinity, the adsorption could be due to chemical interaction. The interaction between PAM-N and kaolin surface may be considered to occur via hydrogen bonds. Maslenkova (1961) has observed using infrared spectroscopy that hydroxyl groups of kaolinite are the surface active sites for PAM adsorption. Nabzar et al. (1984) and Nabzar and Pefferkorn (1985) reported the formation of Na-kaolinite –PAM complex by hydrogen bonding between the amide groups of polymer and the free hydroxyls of the edge surface. They further corroborate that the two basal faces of the kaolinite platelets are non-adsorbing surfaces. The silicate cleavage face has no adsorption active sites, which can form hydrogen bonding with PAM, whereas alumina cleavage face Table 3 Summary of the results of desorption studies Reagent

Substrate

Initial concentration of PAM-N, mg/dm3

Total amount of PAM-N adsorbed, mg/g

PAM-N desorbed by washing, mg/g

PAM-N PAM-N

Kaolin 0.2 mg/g CTAB pretreated kaolin 0.2 mg/g SDS pretreated kaolin 0.2 mg/g TX 100 pretreated kaolin Kaolin

90.90 90.90

1.212 0.692

0.0988 0.0233

8.24 3.37

90.90

0.71

0.0254

3.58

90.90

0.82

0.00139

0.17

45.45

0.7484

0.0398

5.32

Kaolin

45.45

0.778

0.0832

10.70

Kaolin

45.45

0.724

0.0078

1.08

PAM-N PAM-N PAM-N/CTAB (1:1) PAM-N/SDS (1:1) PAM-N/TX 100 (1:1)

PAM-N desorbed by washing, %

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Table 4 Summary of comparative studies on flocculation and dewatering experiments on washed and unwashed kaolin suspension treated with 2.0 mg/g PAM-N Substrate

Kaolin 0.2 mg/g SDS pretreated kaolin 0.2 mg/g CTAB pretreated kaolin 0.2 mg/g TX 100 pretreated kaolin

Unwashed

Washed

Settling rate

Sediment volume cm3/gm

Specific cake resistance, m/kg

Cake moisture, %

Settling rate, cm/s

Sediment volume, cm3/gm

Specific cake resistance, m/kg

Cake moisture, %

0.60 1.20

3.01 2.89

3.31
1011 5.10
1011

42.01 41.25

0.6244 0.6666

2.78 2.80

1.11
1011 1.11
1011

39.59 40.96

1.1166

2.82

4.44
1011

40.64

0.5311

2.82

1.11
1011

40.11

1.22

2.72

3.83
1011

40.24

0.7244

2.82

1.11
1011

39.42

is highly hydrated and energetically stabilised by hydrogen bonding of basal aluminol groups with water molecules impeding any polymer adsorption on this face. Table 4 summarises the flocculation and dewatering characteristics of washed and unwashed samples. It indicates that washing of the PAM-N treated kaolin practically does not show any change in settling rate and is almost same as that for unwashed sample. Washing, however, leads to a substantial decrease in sediment volume. In the cases of surfactant-pretreated kaolin, washing reduces the settling rate but no change in sediment volume. The weaker flocs formed by PAM-N in the beginning breaks into smaller units while washing leading to close packing and consequently to reduction in sediment volume. But these smaller units are strong enough to withstand the pressure during filtration and are less compressible than their parent flocs. There is also a substantial reduction in SRF value, and they all attain a constant value of 1.11
1011 m/kg. This constant value of SRF on washing is same as that of kaolin flocculated with PAM-N only.

4. Conclusions PAM-N on kaolin forms a strong bond with the surface and their interaction reduces the overall negative charge on the surface. Improvement in flocculation and settling rate is due partly to this reduction in negative charge and mainly due to bridging. The filtration characteristics of the flocculated kaolin is improved manifold as suggested by a decrease in SRF from 7.8
1011 m/kg for the untreated kaolin to 1.11
1011 m/kg for the flocculated suspension. Flocculation of kaolin by PAM-N however has a negative influence on the moisture content of the filter cake. The high molecular weight polymer entraps excess water with them in the flocs resulting in very high cake moisture content. Treatment with only surfactants reduces moisture content of the filter cake mainly due to improved hydrophobicity caused by the adsorbed surfactants. The reduction in moisture content occurs in the following order: TX 100>CTAB>SDS. Pretreatment with either TX 100

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(non-ionic), CTAB (Cationic) or SDS (anionic) reduces the adsorption of non-ionic PAM due to blocking of some surface sites by the surfactants. The polymer conformation under these conditions is suitable for enhanced flocculation by bridging and form flocs with improved settling rates. PAM-N adsorbs more on kaolin surface when added from a mixture with surfactants, but it decreases settling rate. It does not reduce specific resistance of the cake to filtration (SRF) any further, but yields cake with lower moisture content than those for surfactant-free kaolin. Repeated washing of the flocculated kaolin with distilled water removes only a fraction of the adsorbed PAM-N from the surface indicating strong interaction through hydrogen bonding and hydrophobic association. The resuspended washed kaolin exhibits almost same settling rate with that of the unwashed sample, however washing leads to a substantial decrease in sediment volume which may be due to breakage of the weaker flocs formed by PAM-N into smaller units. But the smaller units are strong enough to withstand the pressure during filtration and are less compressible than their parent flocs. It also releases some amount of entrapped water while washing resulting in reduction of cake moisture. Acknowledgements One of the authors (LB) is thankful to the German Academic Exchange Service (DAAD) for the financial grant to carry out a part of this work. We are thankful to Ms. Valeria Thalheim for assistance in quantitative analysis of PAM-N and surfactants using UV/VIS spectrophotometer. We also thank Mr. S.K. Bhaumick for useful discussion during preparation of the manuscript. Permission of the Director, Regional Research Laboratory, Bhubaneswar for publishing this paper is duly acknowledged. References Ayub, A.L., Sheppard, J.D., Kwack, J.C.T., 1987. The effect of surfactant and polymer addition to a fuel grade peat: adsorption electrokinetics and dewatering. Colloids Surf. 26, 305 – 315. Baltar, C.A.M., Oliveira, J.F., 1998. Flocculation of colloidal silica with polyacrylamide and the effect of dodecylamine and aluminium chloride pre-conditioning. Miner. Eng. 11 (5), 463 – 467. Baskerville, R.C., Gale, R.S., 1968. A simple automated instrument for determining the filterability of sewage sludge. Water Pollut. Control 67, 233 – 241. Besra, L., Ay, P., 1999. Flocculation and dewatering of kaolin suspensions with the help of combinations of polymeric flocculants and surface-active substances. Forum Forsch. 9, 68 – 74. Besra, L., Singh, B.P., Reddy, P.S.R., Sengupta, D.K., 1998a. Influence of surfactants on filter cake parameters during vacuum filtration of iron ore sludge. Powder Technol. 96, 240 – 247. Besra, L., Sengupta, D.K., Roy, S.K., 1998b. Flocculant and surfactant aided dewatering of fine particles: a review. Miner. Process. Extr. Metall. Rev. 18, 67 – 103. Besra, L., Sengupta, D.K., Roy, S.K., 2000. Particle characteristics and their influence on dewatering of kaolin, calcite and quartz suspension. Int. J. Miner. Process. 58 (2), 89 – 112. Besra, L., Sengupta, D.K., Roy, S.K., Ay, P., 2002. Studies on flocculation and dewatering of kaolin suspensions by anionic polyacrylamide flocculant in the presence of some surfactants. Int. J. Miner. Process. (in press). Biggs, S., Selb, J., Candau, F., 1992. Effect of surfactant on the solution properties of hydrophobically modified polyacrylamide. Langmuir 8, 838 – 847. Brackman, J.C., Engbert, J.B.F.N., 1990. Polymer induced breakdown of rod like micelles. A striking transition of a non-Newtonian to a Newtonial fluid. J. Am. Chem. Soc. 112 (2), 872 – 873.

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