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Polymer depressants at the talc–water interface: adsorption isotherm, microflotation and electrokinetic studies
Int. J. Miner. Process. 67 (2002) 211 – 227 www.elsevier.com/locate/ijminpro
Polymer depressants at the talc–water interface: adsorption isotherm, microflotation and electrokinetic studies Gayle E. Morris *, Daniel Fornasiero, John Ralston Ian Wark Research Institute, University of South Australia, Mawson Lakes, SA 5095, Australia Received 25 October 2001; received in revised form 30 April 2002; accepted 30 April 2002
Abstract The behaviour of polymer depressants at the talc – water interface was investigated as a function of ionic strength and pH. Adsorption isotherms, microflotation and electrokinetic studies were used to examine the surface interactions involved. The polymers examined were carboxymethyl cellulose (CMC) and two synthetic polyacrylamides (PAM-A and PAM-N). The adsorption of the two anionic polymers, CMC and PAM-A, on talc, and hence, talc depression, is greatest when electrostatic repulsion is minimized. At high pH values and low ionic strength, the adsorption density of the anionic polymers on talc is low whilst at either high ionic strength or low pH, the adsorption density increases; talc depression is therefore largely influenced by variations in solution conditions. The adsorption of the nonionic polymers, PAM-N, on talc is not influenced by ionic strength or pH. The polymers exhibit Langmuir adsorption behaviour with adsorption occurring on the talc face surface and, possibly, the edge surface. The adsorbed polymer layer thicknesses, calculated from electrophoretic mobility measurements, corresponded to a monolayer indicating that adsorption of the polymers onto the talc surface occurs in a flat conformation. D 2002 Elsevier Science B.V. All rights reserved. Keywords: polymer adsorption; talc; depressants; polyacrylamide; polysaccharide
1. Introduction Polymers have been used in the minerals industry to depress hydrophobic gangue minerals such as talc for several decades (Bakinov et al., 1964; Pugh, 1989a; Rath et al., *
Corresponding author. Tel.: +61-8-8302-3750; fax: +61-8-8302-3683. E-mail address: [email protected] (G.E. Morris).
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 4 8 - 0
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1997). Both natural (e.g., carboxymethyl cellulose, CMC) (Bakinov et al., 1964) and synthetic (e.g., polyacrylamide) (Nagaraj et al., 1987) polymers have been used as depressants. Although the effect of these depressants on mineral flotation is known, the specific interaction of the polymers with the talc surface as a function of solution conditions is generally not well understood. The adsorption of polymers at the solid – liquid interface is largely dependent on both the polymer solution chemistry and the solid surface properties of the system. These studies form the backbone to the adsorption work presented here. Polymer adsorption on talc and the subsequent depression of talc is influenced by many variables, including polymer type and concentration, molecular weight, degree of substitution, pH, ionic strength and the presence of other chemical species, such as cations (Gomes and Oliveira, 1989). Talc is a 2:1 layer silicate, Mg3(Si2O5)2(OH)2, comprised of sheets linked by weak van der Waals’ forces (Gomes and Oliveira, 1991). The layered structure consists of two tetrahedral silica sheets with an octahedral brucite sheet sandwiched between the two silica sheets. The talc surface is comprised of two different surfaces, the basal cleavage face and the edge. The face surface, which occupies approximately 90% of the talc surface, consists of a tetrahedral siloxane surface with inert – Si– O –Si – links and is nonpolar, and therefore, hydrophobic. The edge surface is hydrophilic due to the presence of pHdependent SiOH and MgOH groups (Ma¨lhammar, 1990). Possible interaction mechanisms between polymers and the talc surface involve one or more of chemical, electrostatic, hydrogen and hydrophobic bonding contributions (Pugh, 1989b; Rath et al., 1997). These different types of interaction mechanisms reflect the heterogeneous nature of the talc surface. Bakinov et al. (1964) state that infrared studies have shown the carboxyl groups of CMC to interact chemically with cations on the gangue mineral surface. This chemical interaction may involve the carboxylic groups of CMC and the magnesium ions on the talc edge surface (Pugh, 1989b; Rath et al., 1997). Adsorption of anionic polyelectrolytes on negatively charged mineral surfaces generally occurs in the presence of metal cations due to electrostatic charge screening and involves either cation binding or complexation of the anionic groups of the polymer (Liu and Laskowski, 1989). Other workers suggest that polymer adsorption involves physical interaction via hydrophobic (Haung et al., 1978; Miller et al., 1983; Pugh, 1989a) and/or hydrogen bonding (Bakinov et al., 1964; Rhodes, 1981; Steenberg and Harris, 1984; Gomes and Oliveira, 1991). Hydrogen bonding may occur between OH groups of the polymer and the talc edges (Rhodes, 1981). Hydrophobic interaction with the talc face surface may occur via bonding with the hydrocarbon backbone of the polymer (Pugh, 1989a). Adsorption studies involving dextrin and amylose polysaccharide depressants on a hydrophobic surface, such as coal, suggest that physical adsorption was occurring, possibly associated with hydrophobic bonding (Haung et al., 1978; Grano et al., 1991). Miller et al. (1983) clearly showed that dextrin selectively adsorbs on hydrophobic solids such as talc and coal and has little affinity for hydrophilic quartz. Steenberg and Harris (1984) have proposed a combination of hydrophobic and hydrophilic interaction, with the polysaccharides adsorbing firstly on the faces, via hydrophobic bonding, and then onto the hydrophilic talc edges, via hydrogen bonding.
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This paper examines the influence of solution conditions, such as pH and ionic strength, upon adsorption of three different polymer depressants onto the talc surface. In addition to the well-known CMC depressant, two modified polyacrylamides (PAM) were used in this study. As the main objective of this study is to understand the mechanism of interaction of polymers with the talc surface, two classes of polymers were selected; nonionic (PAM-N) and anionic polymers (CMC and PAM-A). Their structures are shown in Fig. 1. The depressant –talc interaction has been investigated using microflotation, polymer adsorption and electrochemical investigations.
Fig. 1. Structure of the carboxymethyl cellulose fragment and polyacrylamide monomers.
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2. Experimental procedure 2.1. Reagents All solutions were prepared using high purity water produced by reverse osmosis, two stages of mixed bed ion exchange, two stages of activated carbon treatment and a final filtering step through a 0.22-Am filter. The conductivity was less than 0.5 10 6 S cm 1 with a surface tension of 72.8 10 3 N m 1 at 20 jC. The high purity talc sample used throughout this study was obtained from BDH Chemicals, Poole, England. The sample purity was estimated to be 96 wt.% with Al2O3 and Fe2O3 as the major impurities. X-ray diffraction, performed using a Philips PW 1050/ 25 X-ray diffractometer, confirmed the major phase to be talc with no minor phases detectable. The particle size distribution was monitored with a Malvern Model 2600C laser diffractometer. Nominally, 90% of the particles were less than 45.3 Am in diameter with a mean diameter of 21.5 Am (assuming an equivalent spherical diameter). The particles have an apparent BET krypton surface area of 1.97 m2 g 1, determined using a Coulter Omnisorb 100. The surface area of the talc sample for the CMC adsorption studies was increased by grinding the talc sample for 60 min using a tungsten carbide grinding mill. Surface contamination and crystallographic changes were not detected by XPS and XRD, respectively, after grinding. The particle diameter and BET surface area were altered to 90 wt.% below 19 Am and 5.19 m2 g 1, respectively. The CMC sample, Finnfix 300 CMC, was obtained courtesy of Tall-Bennett, Sydney, with a quoted purity of 98 wt.%, a degree of substitution of 0.73 and molecular weight of 200 000 Daltons. Synthetic, modified polyacrylamides (PAM), supplied by Cytec Research Laboratories, Stamford, USA, were received as 10% by volume (v/v) aqueous solutions. The polyacrylamides, PAM-N and PAM-A, are nonionic and anionic, respectively, with molecular weights of 8300 and 7000 Daltons, respectively. Other reagents used were analytical grade (unless otherwise stated). 2.2. Microflotation The apparatus used was a modified version of the Partridge and Smith (1971) cell. The gas flow rate during flotation was maintained at 50 cm3 min 1. A 1.0-g sample of talc, with particle diameters between + 38 and 75 Am, was conditioned in N2-purged KCl solution for 5 min inside the reaction vessel. After addition of a known volume of concentrated polymer depressant stock solution, the talc was conditioned for a further 5 min prior to flotation. Within 5 min, the majority ( > 90%) of the polymer is adsorbed and is therefore approaching equilibrium conditions. Microflotation was performed and the concentrate fractions were collected at 0.5, 2, 4 and 8 min. 2.3. Adsorption isotherms Adsorption isotherms were conducted in a glass conditioning vessel at 25 jC. A 10-g talc sample was conditioned for 5 min in KCl solution at the desired pH. A known
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concentration of polymer stock solution was introduced to the vessel and continuously stirred until adsorption equilibrium was obtained at 60 min, verified by equilibration studies (Morris, 1996). A sample was removed and retained to determine the concentration of polymer remaining in solution. Another aliquot of concentrated polymer solution was added to the talc suspension and conditioned for a further 60 min. This procedure was repeated until a total concentration of 55 ppm PAM or 400 ppm CMC was obtained. The extracted samples were centrifuged twice. Colorimetric techniques were then used to determine the concentration of polymer remaining in solution. It was assumed that the amount of polymer depleted from solution had adsorbed onto the talc surface. The detection of CMC and PAM in solution was undertaken using the Dubois method (Dubois et al., 1956) and the colorimetric starch-triiodide method (Scoggins and Miller, 1979), respectively. UV – VIS absorption spectra were measured using a Varian CARY5E spectrophotometer. The experimental error in the colorimetric techniques was F 5 ppm with respect to polymer concentration. 2.4. Electrophoretic mobility measurements Electrophoretic mobilities were measured using a Rank Brothers microelectrophoresis Mark II apparatus. A 1.0-g sample containing particles of talc less than 20 Am in diameter was suspended in KCl electrolyte and conditioned for 15 min at 25 jC in a closed conditioning vessel. The suspensions were stirred continuously, whilst nitrogen was purged during both conditioning and measurement. The electrophoretic mobility of talc in the absence and presence of polymer was measured as a function of pH and polymer. The suspension pH was adjusted using small quantities of KOH and HCl. The zeta potential of talc, as a function of polymer concentration, was determined at pH 5.0. An equilibration time of 15 min was allowed before measurement (Morris, 1996). The zeta potentials were calculated using the Smoluchowski equation (Hunter, 1987) since the particle dimensions considerably exceed the double layer thickness (i.e., jaH1).
3. Results and discussion 3.1. Talc microflotation in the presence of polymer depressants: effect of pH and ionic strength Fig. 2 shows the talc flotation recovery in the absence and presence of CMC and the modified polyacrylamides, PAM-N and PAM-A, as a function of pH. As expected, the talc flotation recovery is very high (>95%) over the entire pH range tested in the absence of polymer depressant. At an ionic strength of 0.001 M, the nonionic PAM-N is an effective talc depressant over the whole pH range. Both anionic polymers, CMC and PAM-A, have a minimal to modest effect on talc flotation at neutral and alkaline pH values, but talc flotation is strongly depressed at pH 3.5. This behaviour is in agreement with the findings of Gomes and Oliveira (1989, 1991). The talc flotation recovery was also investigated in the presence of CMC, PAM-A and PAM-N as a function of ionic strength at pH 9.0 (Fig. 2). An increase in ionic strength
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Fig. 2. Talc recovery after 8 min of flotation in the (––) absence and presence of (–n–) CMC, (– –) PAM-A and (–E–) PAM-N in 0.001 M KCl at varying pH. Talc recovery in 0.1 M KCl at pH 9 in the presence of (5) CMC, (o) PAM-A and (4) PAM-N is also presented. Polymer concentration of 100 ppm. Talc concentration of 2 g dm 3.
increased talc depression by both CMC and PAM-A, whilst the action of PAM-N was not influenced by ionic strength, with effective talc depression occurring at both high and low ionic strengths. 3.2. Adsorption isotherms: influence of pH and ionic strength 3.2.1. CMC Adsorption isotherms of anionic CMC on talc were obtained at both high and low pH values and ionic strengths and are presented in Fig. 3. The solution conditions clearly influence the adsorption density of CMC on the talc surface: the adsorption of CMC increases with KCl concentration and with decreasing pH. The adsorption isotherms are of the high affinity type (Theng, 1979) under all conditions. At pH 9.0 in 0.001 M KCl, CMC adsorption is lowest in agreement with the weak talc depression observed during flotation (Fig. 2). At pH 9.0, the carboxyl groups of CMC are dissociated (pK0 = 3.7; Morris, 1996) and electrostatic repulsion occurs between the negatively charged CMC and talc, hindering polymer adsorption. Plateau coverage was reached at 0.6 mg m 2, a value which is significantly higher than the 0.22 mg m 2 observed by Steenberg (1982) at pH 7 in 5 10 3 M ionic strength solution. The higher adsorption plateau value obtained in this study may be due to inherent differences in the talc sources (surface impurities) or differences in the degree of substitution of the CMC; the latter was not stated by Steenberg (1982). At pH 9.0 and high ionic strength (0.1 M KCl), adsorption of CMC onto talc increases from that observed at low ionic strength. At high KCl concentrations, the negative charge of both the talc and the CMC carboxyl groups are shielded and therefore, electrostatic
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Fig. 3. Adsorption isotherm of CMC on talc under varying solution conditions. (n) 0.001 M KCl, pH 9.0; (5) 0.1 M KCl, pH 9.0; ( ) 0.001 M KCl, pH 3.5; (o) 0.1 M KCl, pH 3.6. Talc concentration of 20 g dm 3. Lines obtained from the Langmuir equation fit.
.
repulsion is reduced between the polymer and the negative talc surface, resulting in an increased CMC adsorption (Fig. 3). In addition, the change in CMC conformation from extended at low ionic strength to a more coiled conformation at high ionic strength (in solution, the radius of gyration of CMC is 176 nm in 0.001 M KCl and 87 nm in 0.1 M KCl; Morris, 1996) may also contribute to the increased CMC adsorption density observed at high ionic strength. At pH 3.5 and at both high and low ionic strength, adsorption of CMC onto talc increases compared with pH 9.0. In previous studies, it has also been observed that adsorption of anionic polysaccharides, such as CMC and substituted guar gum, onto talc increases with a decrease in pH (Steenberg, 1982; Gomes and Oliveira, 1991). At pH 3.5, as the carboxyl groups of CMC are protonated and therefore neutral at both high (pK0 = 3.7) and low ionic strengths (pK0 = 4.3; Morris, 1996) and the talc surface charge is less negative than at pH 9 (Morris, 1996), electrostatic repulsion between the talc surface and CMC is reduced compared with pH 9.0. The decrease in electrostatic repulsion at pH 3.5 and 0.1 M KCl has resulted in an increase in the CMC adsorption density and, consequently, in the enhanced CMC talc depression observed (Fig. 2). In addition, the reduction in CMC radius of gyration at pH 9.0 from 176 to 159 nm at pH 3.5 (Morris, 1996) may also contribute to the increased CMC adsorption density at this pH. The adsorption isotherms obtained in this study under all solution conditions show a single plateau, in contrast to the three-step isotherm obtained by Steenberg (1982). 3.2.2. Polyacrylamides Fig. 4 shows the adsorption isotherms for PAM-A on talc as a function of both ionic strength and pH. The adsorption isotherms are of the high affinity type under all conditions. The adsorption density of PAM-A is largely dependent upon the solution conditions, with adsorption following the same trends as in the case of CMC: PAM-A adsorption increases with an increase in KCl concentration and a decrease in pH. As the
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Fig. 4. Adsorption isotherm of PAM-A on talc under varying solution conditions. (n) 0.001 M KCl, pH 9.0; (5) 0.1 M KCl, pH 9.0; ( ) 0.001 M KCl, pH 3.5; (o) 0.1 M KCl, pH 3.5. Talc concentration of 20 g dm 3. Lines obtained from the Langmuir equation fit.
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pK0 of PAM-A is 4.2 and 4.0 in 0.001 and 0.1 M KCl, respectively (Morris, 1996), PAMA is uncharged at pH 3.5, so that there is no electrostatic repulsion between the polymer and the talc surface. As in the case for CMC, the extent of talc depression with PAM-A correlates with the amount of PAM-A adsorbed onto the talc surface. The same trends with ionic strength and pH were observed in the adsorption studies involving kaolinite and an anionic polyacrylamide (Hollander et al., 1981). In this instance, the changes were attributed to variations in electrostatic repulsion. Broseta and Medjahed (1995) also observed significant increases in anionic polyacrylamide adsorption on quartzitic sands at low pH or high ionic strength, i.e., when electrostatic interactions were minimal. Adsorption isotherms for PAM-N on talc, determined at both high and low pH values and ionic strengths, are presented in Fig. 5. Within experimental error, the adsorption of PAM-N onto the talc surface shows little variation with pH and ionic strength, behaviour which is also reflected in the flotation results (Fig. 2). High affinity adsorption occurs initially until a pseudo-plateau is reached; this is then followed by increased adsorption. The adsorption increase after the plateau may indicate either an increase in the packing density of the adsorbed layer, a ‘‘looped’’ structure (Theng, 1979) or multilayer adsorption. 3.3. Langmuir adsorption model The Langmuir adsorption model (Shaw, 1992) was applied to the polymer systems in order to distinguish the various contributions to the polymer – talc interaction. The adsorption of CMC, PAM-A and PAM-N all conform to an ‘‘apparent’’ Langmuir adsorption isotherm, at low polymer concentrations. The results for PAM-N on talc were obtained using data acquired before the second rise in the adsorption isotherm. The free energy of adsorption, DGads, was calculated from the Langmuir equilibrium constant, K,
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Fig. 5. Adsorption isotherm of PAM-N on talc under varying solution conditions. (n) 0.001 M KCl, pH 9.0; (5) 0.1 M KCl, pH 9.0; ( ) 0.001 M KCl, pH 3.5; (o) 0.1 M KCl, pH 3.5. Talc concentration of 20 g dm 3. Lines obtained from the Langmuir equation fit.
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i.e., DGads = RTlnK, where R is the gas constant and T is the temperature. DGads is represented as the sum of four free energy components (Mackenzie, 1986): DGads ¼ DGchem þ DGelec þ DGHbonding þ DGhydrophobic
ð1Þ
where DGchem, DGelec, DGH-bonding and DGhydrophobic are the chemical, electrical, hydrogen bonding and hydrophobic components of the free energy of adsorption. The values of Cmax in Table 1 clearly show the variation in adsorption density of CMC and PAM-A on talc with solution conditions; the adsorption density of PAM-N is unaltered. Both polyacrylamides have a similar adsorption density under the most favourable conditions for PAM-A. The calculated values of DGads (Table 1) indicate that adsorption of the polymer onto the talc surface is favourable, i.e., DGads < 0. These values are in good agreement with the Table 1 Langmuir adsorption isotherm parameters for the polymer depressants adsorbed on talc as a function of solution conditions Solution conditions
0.001 M KCl/pH 9 0.1 M KCl/pH 9 0.001 M KCl/pH 3.5 0.1 M KCl/pH 3.5
CMC
PAM-A
PAM-N
Cmax (mg m 2)
DGads (kJ mol 1)
Cmax (mg m 2)
DGads (kJ mol 1)
Cmax (mg m 2)
DGads (kJ mol 1)
0.58 F 0.03 1.02 F 0.02 1.02 F 0.03 1.22 F 0.01
12.6 F 0.6 9.9 F 0.3 12.5 F 0.6 15.8 F 0.7
0.11 F 0.01 0.26 F 0.03 0.44 F 0.03 0.50 F 0.01
19.3 F 0.9 17.1 F 0.8 16.7 F 0.4 16.2 F 0.2
0.57 F 0.04 0.56 F 0.07 0.63 F 0.03 0.58 F 0.04
14.7 F 0.3 15.2 F 0.6 15.9 F 0.3 15.3 F 0.4
Cmax is the maximum amount of polymer adsorbed and DGads is the free energy of adsorption.
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values of 22 to 24 kJ mol 1 determined for dextrin adsorption onto coal (Miller et al., 1983). The electrostatic component of adsorption, DGelec, is obtained from the relationship of DGelec = cFwd, where c is the average charge per molecule (i.e., polymer charge portion of charged groups), F is Faraday’s constant and wd is the potential at the outer Helmholtz plane. wd is assumed to be equivalent to f (Shaw, 1992), the zeta potential of talc in the presence of polymer. The values of c for CMC are 0.73 (i.e., 1 0.73), 0.1 (i.e., 1 0.1) for PAM-A and 0 for PAM-N at pH 9. (The fraction of charged groups is taken to be the degree of substitution by the carboxyl groups). The average electrostatic contribution, DGelec, at pH 9.0 was therefore determined to be 3 kJ mol 1 for CMC, 0.4 kJ mol 1 for PAM-A and 0 kJ mol 1 for PAM-N. At pH 3.5, the anionic polymers are uncharged and therefore DGelec reduces to 0 kJ mol 1. These results suggest that electrostatic interactions due to the presence of the charged carboxyl groups inhibit the adsorption process of both CMC and PAM-A onto the talc surface. After consideration of the electrostatic component, the adsorption thermodynamics of PAM-A, PAM-N and CMC at the talc – water interface would appear to be rather similar under all conditions. 3.4. The effective area occupied per molecule, r0, and the bound fraction The fraction of polymer bound to the talc surface or effective surface coverage can be calculated assuming that polymer adsorption occurs in an extended conformation. Through a comparison of the effective area occupied per adsorbed polymer molecule with the extended area occupied by the polymer, the bound fraction (i.e., trains) can be estimated. The effective area occupied per adsorbed polymer molecule, r0, can be calculated from the following relationship (Hiemenz, 1986): r0 ¼ h
A 1 n NA
ð2Þ
where h is defined as the fraction of the talc surface that can be covered by the polymer, A is the talc surface area, n is the number of moles of polymer adsorbed and NA is Avogadro’s number. The effective area of the talc surface occupied per adsorbed molecule of CMC, PAM-A and PAM-N, as a function of surface area and solution conditions, is presented in Table 2. The h values used were 1.0 for the total surface, 0.89 for the face surface and 0.11 for the edge surface (Morris, 1996). The edge surface fraction value of talc is of similar magnitude to the 0.10 edge surface area fraction for kaolinite (Braggs et al., 1994), an aluminosilicate clay mineral. The values for h in this study were determined from a combination of scanning electron microscopy and carbon microanalysis of a probe molecule (chlorodimethyloctadecylsilane) which specifically interacted with the talc edge surface sites. The errors involved in the calculation of r0 are approximately F 10%. The calculated area occupied by a CMC molecule in an extended, linear conformation is 425 nm2. The number of monomer units was estimated to be 910 monomers per molecule based on a Mw of 200 000. A glucose unit of length 0.52 nm (Brown et al., 1963) and a height of 0.92 nm (Gerson, 1995), giving an area of 0.47 nm2 per monomer, were
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Table 2 The effective area occupied per molecule of polymer depressant adsorbed on the talc surface calculated using Eq. (2), assuming adsorption occurred on all of the talc surface (h = 1), face only (h = 0.89) or edge only (h = 0.11), as a function of solution conditions (see text for explanation) Solution conditions
0.001 M KCl/pH 9 0.1 M KCl/pH 9 0.001 M KCl/pH 3.5 0.1 M KCl/pH 3.5
r0, area per CMC molecule (nm2)
r0, area per PAM-A molecule (nm2)
r0, area per PAM-N molecule (nm2)
Total surface
Face only
Edge only
Total surface
Face only
Edge only
Total surface
Face only
Edge only
572 332 332 277
509 295 295 246
63 37 37 30
104 56 26 23
93 50 23 20
11 6.2 2.9 2.5
32 36 27 32
28 32 24 28
3.5 4.0 3.0 3.5
used in the calculation. The calculated area occupied by PAM-A and PAM-N in an extended, linear conformation was determined to be approximately 29 and 32 nm2, respectively. The conformation was assumed to be linear and flat; standard bond length and angles (Aylward and Findlay, 1974) were assumed. The calculated values of the area occupied by the polymers, when compared to the effective area occupied by the adsorbed polymer (Table 2), suggest that in most cases CMC, PAM-A and PAM-N could be accommodated as a monolayer if adsorption occurred either on the total surface or on the face surface only. If polymer adsorption occurred on only the edge surface, then the bound fraction of CMC, PAM-A and PAM-N, would be a value of 0.1, 0.4 to 0.1 and 0.1, respectively, with the unbound fraction of the adsorbed polymer forming loops and tails. This is highly unlikely, normally the unbound fraction of an adsorbed polymer, which is in the form of loops and tails, is only 0.60 to 0.75 and may be as low as 0.30 (Theng, 1979). Polymer adsorption on the talc surface is therefore likely to occur on either the total surface or on the face surface only. 3.5. Electrochemistry Fig. 6 shows the zeta potential of talc as a function of pH, in the absence and presence of the polymers, CMC, PAM-A and PAM-N. All measurements were performed in 0.01 M KCl electrolyte concentration. The isoelectric point (iep) of talc can be found by extrapolation of the zeta potential data to low pH values; an iep of approximately 2 is found independent of ionic strength (Morris, 1996) and the type and concentration of polymer used. For pH values higher than 2, the zeta potential of talc is negative and increases in magnitude with pH. The effect of polymer adsorption on the zeta potential of talc is, however, different for the anionic and nonionic polymers. For the nonionic polymer, PAM-N, the zeta potential of talc decreases with an increase in polymer concentration for all pH values. For the anionic polymers, CMC and PAMA, a similar trend is observed only at alkaline pH values. For neutral and acidic pH values, the trend with polymer concentration is more complex: the zeta potential increases for low polymer concentrations but decreases at higher polymer concentrations, and reaches a similar value to that observed in the absence of polymer. These trends are illustrated more clearly in Fig. 7 where the zeta potential of talc is measured
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Fig. 6. Zeta potential of talc as a function of pH and: (a) CMC concentration at (+) 0, (5) 1 ppm, (n) 100 ppm; (b) PAM-A concentration at (+) 0, (o) 8 ppm, ( ) 90 ppm; (c) PAM-N concentration at (+) 0, (4) 8 ppm, (E) 90 ppm ([KCl] = 0.01 M).
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223
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Fig. 7. Zeta potential of talc as a function of (n) CMC, ( ) PAM-A and (E) PAM-N concentration at pH 5.0 in 0.001 M KCl.
at a fixed pH of 5.0 and as a function of polymer concentration. The zeta potential of talc more or less remains constant with an increase in PAM-N concentration. For the two anionic polymers it reaches a maximum of 57 mV at a low concentration of approximately 2 ppm, then decreases and remains constant for polymer concentrations above 6 ppm. The different trends in zeta potential observed with polymer concentration in Fig. 6a and b result in clear cross-over points in the zeta potential versus pH curves at pH values between 8.0 and 9.5 in the presence of only CMC and PAM-A. These observations are attributed to competition between shear plane shift, charge on adsorbing polymer and conformation. Similar trends have been reported in the literature for the zeta potential of talc in the presence of CMC (Steenberg, 1982; Steenberg and Harris, 1984). Based only on electrical considerations, the adsorption of higher levels of anionic polymers on the talc surface, as in this study, should result in a more negative zeta potential while no change is expected for the adsorption of a neutral polymer. However, the presence of the polymer shifts the plane of shear away from the surface; as the potential decays exponentially with distance from the solid surface, a decrease in zeta potential is predicted in the presence of an adsorbed polymer (Fleer et al., 1972). This is indeed what is observed elsewhere (Fleer, 1971) and in this study (with PAMN) when a neutral polymer adsorbs on a solid surface. Furthermore, the amount of zeta potential decrease should reflect the thickness of the polymer layer at the solid surface (Fleer, 1971). 3.6. Thickness of the polymer layer The zeta potential at the particle plane of shear in the absence of polymer, f1, is measured at a distance d from the surface, whilst the zeta potential in the presence of polymer, f2, is measured at a distance D from the surface.
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The following equation can be derived between the measured zeta potentials and the distance of the plane of shear from the surface, for an uncharged polymer (Fleer, 1971): tanh
zef2 zef ¼ tanh 1 expðjðD dÞÞ 4kT 4kT
ð3Þ
where z is the valency of the ions in the double layer, e is the electron charge, j is the reciprocal Debye length, k is Boltzmann’s constant and T is the temperature. It is assumed that the polymer does not substantially alter the ion distribution in the double layer. Table 3 shows the adsorbed polymer layer thickness, D, calculated using Eq. (3), assuming a value of 0.4 nm for d as used by Fleer (1971) on a AgI surface. The zeta potential values at the onset of the plateau region (i.e., pH values >7) (Fig. 6a, b and c) were used to calculate the polymer layer thickness. Comparison of the PAM-N calculated layer thickness with the calculated height of the polymer monomer of 0.95 nm (Fig. 1) indicates that the nonionic polymer adsorbs in a relatively flat conformation. The calculated layer thicknesses of the anionic polymers, CMC and PAM-A, are likely to be smaller than the actual values, given that the effect of the negative charge of the polymer was not taken into account in the calculations and are therefore an estimate. The small anionic polymer calculated layer thicknesses still suggest, however, that the polymers adsorb in a flat conformation. The calculated layer thicknesses determined in this study are relatively low in comparison to those obtained for poly(vinylalcohol) (PVA) and guar gum using a similar method of calculation (Fleer, 1971; Mao, 1994). The latter thicknesses ranged up to about 10 nm at the higher adsorption densities of the nonionic polymer. The variation in thicknesses is likely to reflect differences in the adsorbed polymer conformation. Fleer et al. (1972, 1993) determined that at the higher adsorption densities, long loops are present in the adsorbed PVA layer on AgI particles. Adsorption of polymer depressants onto the talc face surface has been further investigated by a combination of direct and indirect techniques. Adsorption of polyacrylamides onto the talc face surface is supported by results from time of flight secondary ion mass spectroscopy (ToF SIMS) of adsorbed PAM-N on the talc surface (Fig. 8). The CN fragment, characteristic of the adsorbed polyacrylamide, was used to monitor the presence of PAM-N and was clearly observed on the face surface of talc (Fig. 8b). It was
Table 3 Thickness of the adsorbed polymer layer at the talc surface, calculated with Eq. (3) at 0.01 M KCl (d = 0.4 nm; T = 25 jC; f1 = 36 mV) Polymer
Polymer concentration (ppm)
f2 (mV)
Adsorbed polymer layer thickness (nm)
CMC
1 100 8 90 8 90
54 42 49 47 43 36
0.6 1.1 0.6 0.7 0.8 1.3
PAM-A PAM-N
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Fig. 8. The negative ion ToF SIMS images of the talc sample with adsorbed PAM-N: (a) total negative ion (0 to 800 g mol 1); (b) CN secondary ion (26 g mol 1). The shaded bar scale indicates the relative intensity of the ion(s).
not possible using this technique to ascertain if the polymers had adsorbed on the edge surface. Silane modification of talc surfaces has been used indirectly to investigate the mechanism of polymer adsorption onto the talc surface (Morris et al., 1999). The use of monochlorosilane molecules to react with only sites on the talc edge surface has shown that polymer adsorption on talc increased for anionic polymers but remained unaltered for the nonionic polymer after silane modification of the talc surface. It has been proposed that the dominant mechanism for polymer adsorption onto the talc surface occurs by hydrophobic interaction with the face surface together with a weak repulsive electrostatic interaction with the edge surface for anionic polymers. Consequently, with the information presented in this study in combination with related studies, adsorption of the polymer depressants on talc, as a function of pH and ionic
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strength, is therefore likely to occur via hydrophobic interaction with the face surface with a minor contribution, if any, from the edge surface.
4. Conclusions The depression of talc increases with an increase in the adsorption density of the polymers on the talc surface, as is clearly shown by the microflotation and adsorption isotherm data. The solution conditions, such as ionic strength and pH, strongly influence the adsorption of the anionic polymers, CMC and PAM-A, onto the talc surface. This, however, is not the case for the nonionic polymer PAM-N. Talc conditioned at acidic pH values or high ionic strength increases the adsorption density of the anionic polymer depressants and therefore talc depression, due to a reduction in the electrostatic repulsion between the carboxyl groups of the coiled polymer and the negative edge surface sites of the talc. In addition, the change in the anionic polymer depressant conformation from extended to coiled at acidic pH values or high ionic strength increases the adsorption density at the talc surface. The adsorption density of the nonionic polyacrylamide, PAMN, on the talc surface is not influenced by ionic strength or pH. Geometrically calculated values of the area occupied by the polymers suggest that adsorption is not likely on the edge surface but rather on the face surface or a combination of the two surfaces. Adsorption of the polymers CMC, PAM-A and PAM-N is likely to occur in a flat conformation via hydrophobic interaction of the hydrocarbon backbone at the talc face. The hydrophilic hydroxyl and carboxyl groups of the polymer are therefore extended out from the talc surface imparting some hydrophilic character to the talc particle and thereby causing depression during flotation.
References Aylward, G.H., Findlay, T.J.V., 1974. SI Chemical Data, 2nd edn. Jacaranda, Queensland. Bakinov, K.G., Vaneev, I.J., Gorlovsky, S.I., Eropkin, U.I., Zashikhin, N.V., Konev, A.S., 1964. New methods of sulphide concentrate upgrading. In: Arbiter, N. (Ed.), 7th International Mineral Processing Congress, vol. 1. Gordon and Breach, New York, p. 227. Braggs, B.R., Fornasiero, D., Ralston, J., Smart, R.St.C., 1994. The effect of surface modification by an organosilane on the electrochemical properties of kaolinite. Clays Clay Miner. 42 (2), 123. Broseta, D., Medjahed, F., 1995. Effects of substrate hydrophobicity on polyacrylamide adsorption. J. Colloid Interface Sci. 170, 457. ¨ hman, J., 1963. Studies on cellulose derivatives: Part I. The dimensions and configBrown, W., Henley, D., O uration of sodium carboxymethyl cellulose in cadoxen and the influence of the degree of substitution. Makromol. Chem. 62, 164. Dubois, M., Gilles, K.A., Hamilton, J.K., Reders, R.A., Smith, F., 1956. Colorimetric method for determination of sugars and related substances. Anal. Chem. 28, 350. Fleer, G.J., 1971. Polymer adsorption and its effect on colloidal stability. PhD Thesis, Wageningen, The Netherlands, pp. 34 – 59. Fleer, G.J., Koopal, L.K., Lyklema, J., 1972. Polymer adsorption and its effect on the stability of hydrophobic colloids. I. Characterisation of poly(vinyl alcohol) adsorption on silver iodide. Kolloid-Z. Z. Polym. 250, 689.
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227
Fleer, G.J., Cohen Stuart, M.A., Scheutjens, J.M.H.M., Cosgrove, T., Vincent, B., 1993. Polymers at Interfaces. Chapman & Hall, London. Gerson, A., 1995. Molecular modelling of (1) polymer fragments and (2) surface charge of talc. Ian Wark Research Institute, Internal report. Gomes, L.M.B., Oliveira, J.F., 1989. Floatability control of talc by the use of carboxymethyl cellulose. Metalurgia-abm 45 (382), 908. Gomes, L.M.B., Oliveira, J.F., 1991. The control of natural floatability of talc with carboxymethyl cellulose and aluminium chloride. Fine Particles Processing Flotation. 17th Int. Miner. Process. Congress, vol. 2, p. 353. Grano, S.R., Griffin, L.F., Johnson, N.W., Smart, R.St.C., Ralston, J., 1991. Treatment of naturally hydrophobic gangue minerals at the copper concentrator of Mt. Isa Mines Limited. 4th Mill Operators’ Conference. AusIMM, Burnie, Tasmania. Haung, H.H., Calara, J.V., Bauer, D.L., Miller, J.D., 1978. Adsorption reactions in the depression of coal by organic colloids. Recent Developments in Separation Science, vol. 4. CRC, Florida, p. 115. Hiemenz, P.C., 1986. Principles of Colloid and Surface Chemistry, 2nd edn. Marcel Dekker, New York. Hollander, A.F., Somasundaran, P., Gryte, C.C., 1981. Adsorption characteristics of polyacrylamide and sulfonate-containing polyacrylamide copolymers on sodium kaolinite. J. Appl. Polym. Sci. 26, 2123. Hunter, R.J., 1987. Foundations of Colloid Science, vol. 1. Oxford Univ. Press, New York. Liu, Q., Laskowski, J.S., 1989. The interactions between dextrin and metal hydroxides in aqueous solutions. J. Colloid Interface Sci. 130, 101. Mackenzie, M., 1986. Organic polymers as depressants. In: Malhotra, D., Riggs, W.F. (Eds.), Chemical Reagents in the Mineral Processing Industry. Society of Mining Engineers, Colorado, USA, pp. 139 – 145. Ma¨lhammar, G., 1990. Determination of some surface properties of talc. Colloids Surf. 44, 61 – 69. Mao, M., 1994. Surface and interfacial processes in the selective flotation of zircon. PhD Thesis, University of South Australia, Adelaide, Australia. Miller, J.D., Laskowski, J.S., Chang, S.S., 1983. Dextrin adsorption by oxidised coal. Colloids Surf. 8, 137 – 151. Morris, G.E., 1996. The adsorption characteristics of polymeric depressants at the talc – water interface. PhD Thesis, University of South Australia, Adelaide, Australia. Morris, G.E., Fornasiero, D., Ralston, J., 1999. Low molecular weight polyacrylamide adsorption by silane modified talc. 38th Annual Conference of Metallurgists (Canadian Institute of Mining). Quebec City, Canada. Nagaraj, D.R., Rothenberg, A.S., Lipp, D.W., Panzer, H.P., 1987. Low molecular weight polyacrylamide-based polymers as modifiers in phosphate beneficiation. Int. J. Miner. Process. 20, 291. Partridge, A.C., Smith, G.W., 1971. Small-sample flotation testing: a new cell. Trans. Inst. Min. Metall., Sect. C 80, 118. Pugh, R.J., 1989a. Macromolecular organic depressants in sulphide flotation—a review: 1. Principles, types and applications. Int. J. Miner. Process. 25, 101. Pugh, R.J., 1989b. Macromolecular organic depressants in sulphide flotation—a review: 2. Theoretical analysis of the forces involved in the depressant action. Int. J. Miner. Process. 25, 131. Rath, R.K., Subramanian, S., Laskowski, J.S., 1997. Adsorption of dextrin and guar gum onto talc. A comparative study. Langmuir 13, 6260. Rhodes, M.K., 1981. The effects of the physical variables of carboxymethyl cellulose reagents on the depression of magnesia bearing minerals in Western Australian nickel sulphide ores. In: Laskowski, W.J. (Ed.), Mineral Processing, 13th International Congress. Elsevier, Amsterdam, p. 346. Scoggins, M.W., Miller, J.W., 1979. Determination of water-soluble polymers containing primary amide groups using the starch-triiodide method. Soc. Pet. Eng. J. 19, 151. Shaw, D.J., 1992. Introduction to Colloid and Surface Chemistry, 4th edn. Butterworth-Heinemann, Oxford. Steenberg, E., 1982. The depression of the natural floatability of talc: the mechanism involved in the adsorption of organic reagents of high molecular mass. PhD Thesis, Potschefstroom University of Christian Higher Education, Johannesburg. Steenberg, E., Harris, P.J., 1984. Adsorption of carboxymethyl cellulose, guar gum, and starch onto talc, sulphides, oxides and salt-type minerals. S. Afr. J. Chem. 37, 85. Theng, B.K.G., 1979. Clay – polymer interactions: summary and perspectives. Formation and Properties of ClayPolymer Complexes. Elsevier, Amsterdam, pp. 37 – 61.
Polymer depressants at the talc–water interface: adsorption isotherm, microflotation and electrokinetic studies Gayle E. Morris *, Daniel Fornasiero, John Ralston Ian Wark Research Institute, University of South Australia, Mawson Lakes, SA 5095, Australia Received 25 October 2001; received in revised form 30 April 2002; accepted 30 April 2002
Abstract The behaviour of polymer depressants at the talc – water interface was investigated as a function of ionic strength and pH. Adsorption isotherms, microflotation and electrokinetic studies were used to examine the surface interactions involved. The polymers examined were carboxymethyl cellulose (CMC) and two synthetic polyacrylamides (PAM-A and PAM-N). The adsorption of the two anionic polymers, CMC and PAM-A, on talc, and hence, talc depression, is greatest when electrostatic repulsion is minimized. At high pH values and low ionic strength, the adsorption density of the anionic polymers on talc is low whilst at either high ionic strength or low pH, the adsorption density increases; talc depression is therefore largely influenced by variations in solution conditions. The adsorption of the nonionic polymers, PAM-N, on talc is not influenced by ionic strength or pH. The polymers exhibit Langmuir adsorption behaviour with adsorption occurring on the talc face surface and, possibly, the edge surface. The adsorbed polymer layer thicknesses, calculated from electrophoretic mobility measurements, corresponded to a monolayer indicating that adsorption of the polymers onto the talc surface occurs in a flat conformation. D 2002 Elsevier Science B.V. All rights reserved. Keywords: polymer adsorption; talc; depressants; polyacrylamide; polysaccharide
1. Introduction Polymers have been used in the minerals industry to depress hydrophobic gangue minerals such as talc for several decades (Bakinov et al., 1964; Pugh, 1989a; Rath et al., *
Corresponding author. Tel.: +61-8-8302-3750; fax: +61-8-8302-3683. E-mail address: [email protected] (G.E. Morris).
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 4 8 - 0
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1997). Both natural (e.g., carboxymethyl cellulose, CMC) (Bakinov et al., 1964) and synthetic (e.g., polyacrylamide) (Nagaraj et al., 1987) polymers have been used as depressants. Although the effect of these depressants on mineral flotation is known, the specific interaction of the polymers with the talc surface as a function of solution conditions is generally not well understood. The adsorption of polymers at the solid – liquid interface is largely dependent on both the polymer solution chemistry and the solid surface properties of the system. These studies form the backbone to the adsorption work presented here. Polymer adsorption on talc and the subsequent depression of talc is influenced by many variables, including polymer type and concentration, molecular weight, degree of substitution, pH, ionic strength and the presence of other chemical species, such as cations (Gomes and Oliveira, 1989). Talc is a 2:1 layer silicate, Mg3(Si2O5)2(OH)2, comprised of sheets linked by weak van der Waals’ forces (Gomes and Oliveira, 1991). The layered structure consists of two tetrahedral silica sheets with an octahedral brucite sheet sandwiched between the two silica sheets. The talc surface is comprised of two different surfaces, the basal cleavage face and the edge. The face surface, which occupies approximately 90% of the talc surface, consists of a tetrahedral siloxane surface with inert – Si– O –Si – links and is nonpolar, and therefore, hydrophobic. The edge surface is hydrophilic due to the presence of pHdependent SiOH and MgOH groups (Ma¨lhammar, 1990). Possible interaction mechanisms between polymers and the talc surface involve one or more of chemical, electrostatic, hydrogen and hydrophobic bonding contributions (Pugh, 1989b; Rath et al., 1997). These different types of interaction mechanisms reflect the heterogeneous nature of the talc surface. Bakinov et al. (1964) state that infrared studies have shown the carboxyl groups of CMC to interact chemically with cations on the gangue mineral surface. This chemical interaction may involve the carboxylic groups of CMC and the magnesium ions on the talc edge surface (Pugh, 1989b; Rath et al., 1997). Adsorption of anionic polyelectrolytes on negatively charged mineral surfaces generally occurs in the presence of metal cations due to electrostatic charge screening and involves either cation binding or complexation of the anionic groups of the polymer (Liu and Laskowski, 1989). Other workers suggest that polymer adsorption involves physical interaction via hydrophobic (Haung et al., 1978; Miller et al., 1983; Pugh, 1989a) and/or hydrogen bonding (Bakinov et al., 1964; Rhodes, 1981; Steenberg and Harris, 1984; Gomes and Oliveira, 1991). Hydrogen bonding may occur between OH groups of the polymer and the talc edges (Rhodes, 1981). Hydrophobic interaction with the talc face surface may occur via bonding with the hydrocarbon backbone of the polymer (Pugh, 1989a). Adsorption studies involving dextrin and amylose polysaccharide depressants on a hydrophobic surface, such as coal, suggest that physical adsorption was occurring, possibly associated with hydrophobic bonding (Haung et al., 1978; Grano et al., 1991). Miller et al. (1983) clearly showed that dextrin selectively adsorbs on hydrophobic solids such as talc and coal and has little affinity for hydrophilic quartz. Steenberg and Harris (1984) have proposed a combination of hydrophobic and hydrophilic interaction, with the polysaccharides adsorbing firstly on the faces, via hydrophobic bonding, and then onto the hydrophilic talc edges, via hydrogen bonding.
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This paper examines the influence of solution conditions, such as pH and ionic strength, upon adsorption of three different polymer depressants onto the talc surface. In addition to the well-known CMC depressant, two modified polyacrylamides (PAM) were used in this study. As the main objective of this study is to understand the mechanism of interaction of polymers with the talc surface, two classes of polymers were selected; nonionic (PAM-N) and anionic polymers (CMC and PAM-A). Their structures are shown in Fig. 1. The depressant –talc interaction has been investigated using microflotation, polymer adsorption and electrochemical investigations.
Fig. 1. Structure of the carboxymethyl cellulose fragment and polyacrylamide monomers.
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2. Experimental procedure 2.1. Reagents All solutions were prepared using high purity water produced by reverse osmosis, two stages of mixed bed ion exchange, two stages of activated carbon treatment and a final filtering step through a 0.22-Am filter. The conductivity was less than 0.5 10 6 S cm 1 with a surface tension of 72.8 10 3 N m 1 at 20 jC. The high purity talc sample used throughout this study was obtained from BDH Chemicals, Poole, England. The sample purity was estimated to be 96 wt.% with Al2O3 and Fe2O3 as the major impurities. X-ray diffraction, performed using a Philips PW 1050/ 25 X-ray diffractometer, confirmed the major phase to be talc with no minor phases detectable. The particle size distribution was monitored with a Malvern Model 2600C laser diffractometer. Nominally, 90% of the particles were less than 45.3 Am in diameter with a mean diameter of 21.5 Am (assuming an equivalent spherical diameter). The particles have an apparent BET krypton surface area of 1.97 m2 g 1, determined using a Coulter Omnisorb 100. The surface area of the talc sample for the CMC adsorption studies was increased by grinding the talc sample for 60 min using a tungsten carbide grinding mill. Surface contamination and crystallographic changes were not detected by XPS and XRD, respectively, after grinding. The particle diameter and BET surface area were altered to 90 wt.% below 19 Am and 5.19 m2 g 1, respectively. The CMC sample, Finnfix 300 CMC, was obtained courtesy of Tall-Bennett, Sydney, with a quoted purity of 98 wt.%, a degree of substitution of 0.73 and molecular weight of 200 000 Daltons. Synthetic, modified polyacrylamides (PAM), supplied by Cytec Research Laboratories, Stamford, USA, were received as 10% by volume (v/v) aqueous solutions. The polyacrylamides, PAM-N and PAM-A, are nonionic and anionic, respectively, with molecular weights of 8300 and 7000 Daltons, respectively. Other reagents used were analytical grade (unless otherwise stated). 2.2. Microflotation The apparatus used was a modified version of the Partridge and Smith (1971) cell. The gas flow rate during flotation was maintained at 50 cm3 min 1. A 1.0-g sample of talc, with particle diameters between + 38 and 75 Am, was conditioned in N2-purged KCl solution for 5 min inside the reaction vessel. After addition of a known volume of concentrated polymer depressant stock solution, the talc was conditioned for a further 5 min prior to flotation. Within 5 min, the majority ( > 90%) of the polymer is adsorbed and is therefore approaching equilibrium conditions. Microflotation was performed and the concentrate fractions were collected at 0.5, 2, 4 and 8 min. 2.3. Adsorption isotherms Adsorption isotherms were conducted in a glass conditioning vessel at 25 jC. A 10-g talc sample was conditioned for 5 min in KCl solution at the desired pH. A known
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concentration of polymer stock solution was introduced to the vessel and continuously stirred until adsorption equilibrium was obtained at 60 min, verified by equilibration studies (Morris, 1996). A sample was removed and retained to determine the concentration of polymer remaining in solution. Another aliquot of concentrated polymer solution was added to the talc suspension and conditioned for a further 60 min. This procedure was repeated until a total concentration of 55 ppm PAM or 400 ppm CMC was obtained. The extracted samples were centrifuged twice. Colorimetric techniques were then used to determine the concentration of polymer remaining in solution. It was assumed that the amount of polymer depleted from solution had adsorbed onto the talc surface. The detection of CMC and PAM in solution was undertaken using the Dubois method (Dubois et al., 1956) and the colorimetric starch-triiodide method (Scoggins and Miller, 1979), respectively. UV – VIS absorption spectra were measured using a Varian CARY5E spectrophotometer. The experimental error in the colorimetric techniques was F 5 ppm with respect to polymer concentration. 2.4. Electrophoretic mobility measurements Electrophoretic mobilities were measured using a Rank Brothers microelectrophoresis Mark II apparatus. A 1.0-g sample containing particles of talc less than 20 Am in diameter was suspended in KCl electrolyte and conditioned for 15 min at 25 jC in a closed conditioning vessel. The suspensions were stirred continuously, whilst nitrogen was purged during both conditioning and measurement. The electrophoretic mobility of talc in the absence and presence of polymer was measured as a function of pH and polymer. The suspension pH was adjusted using small quantities of KOH and HCl. The zeta potential of talc, as a function of polymer concentration, was determined at pH 5.0. An equilibration time of 15 min was allowed before measurement (Morris, 1996). The zeta potentials were calculated using the Smoluchowski equation (Hunter, 1987) since the particle dimensions considerably exceed the double layer thickness (i.e., jaH1).
3. Results and discussion 3.1. Talc microflotation in the presence of polymer depressants: effect of pH and ionic strength Fig. 2 shows the talc flotation recovery in the absence and presence of CMC and the modified polyacrylamides, PAM-N and PAM-A, as a function of pH. As expected, the talc flotation recovery is very high (>95%) over the entire pH range tested in the absence of polymer depressant. At an ionic strength of 0.001 M, the nonionic PAM-N is an effective talc depressant over the whole pH range. Both anionic polymers, CMC and PAM-A, have a minimal to modest effect on talc flotation at neutral and alkaline pH values, but talc flotation is strongly depressed at pH 3.5. This behaviour is in agreement with the findings of Gomes and Oliveira (1989, 1991). The talc flotation recovery was also investigated in the presence of CMC, PAM-A and PAM-N as a function of ionic strength at pH 9.0 (Fig. 2). An increase in ionic strength
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.
Fig. 2. Talc recovery after 8 min of flotation in the (––) absence and presence of (–n–) CMC, (– –) PAM-A and (–E–) PAM-N in 0.001 M KCl at varying pH. Talc recovery in 0.1 M KCl at pH 9 in the presence of (5) CMC, (o) PAM-A and (4) PAM-N is also presented. Polymer concentration of 100 ppm. Talc concentration of 2 g dm 3.
increased talc depression by both CMC and PAM-A, whilst the action of PAM-N was not influenced by ionic strength, with effective talc depression occurring at both high and low ionic strengths. 3.2. Adsorption isotherms: influence of pH and ionic strength 3.2.1. CMC Adsorption isotherms of anionic CMC on talc were obtained at both high and low pH values and ionic strengths and are presented in Fig. 3. The solution conditions clearly influence the adsorption density of CMC on the talc surface: the adsorption of CMC increases with KCl concentration and with decreasing pH. The adsorption isotherms are of the high affinity type (Theng, 1979) under all conditions. At pH 9.0 in 0.001 M KCl, CMC adsorption is lowest in agreement with the weak talc depression observed during flotation (Fig. 2). At pH 9.0, the carboxyl groups of CMC are dissociated (pK0 = 3.7; Morris, 1996) and electrostatic repulsion occurs between the negatively charged CMC and talc, hindering polymer adsorption. Plateau coverage was reached at 0.6 mg m 2, a value which is significantly higher than the 0.22 mg m 2 observed by Steenberg (1982) at pH 7 in 5 10 3 M ionic strength solution. The higher adsorption plateau value obtained in this study may be due to inherent differences in the talc sources (surface impurities) or differences in the degree of substitution of the CMC; the latter was not stated by Steenberg (1982). At pH 9.0 and high ionic strength (0.1 M KCl), adsorption of CMC onto talc increases from that observed at low ionic strength. At high KCl concentrations, the negative charge of both the talc and the CMC carboxyl groups are shielded and therefore, electrostatic
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Fig. 3. Adsorption isotherm of CMC on talc under varying solution conditions. (n) 0.001 M KCl, pH 9.0; (5) 0.1 M KCl, pH 9.0; ( ) 0.001 M KCl, pH 3.5; (o) 0.1 M KCl, pH 3.6. Talc concentration of 20 g dm 3. Lines obtained from the Langmuir equation fit.
.
repulsion is reduced between the polymer and the negative talc surface, resulting in an increased CMC adsorption (Fig. 3). In addition, the change in CMC conformation from extended at low ionic strength to a more coiled conformation at high ionic strength (in solution, the radius of gyration of CMC is 176 nm in 0.001 M KCl and 87 nm in 0.1 M KCl; Morris, 1996) may also contribute to the increased CMC adsorption density observed at high ionic strength. At pH 3.5 and at both high and low ionic strength, adsorption of CMC onto talc increases compared with pH 9.0. In previous studies, it has also been observed that adsorption of anionic polysaccharides, such as CMC and substituted guar gum, onto talc increases with a decrease in pH (Steenberg, 1982; Gomes and Oliveira, 1991). At pH 3.5, as the carboxyl groups of CMC are protonated and therefore neutral at both high (pK0 = 3.7) and low ionic strengths (pK0 = 4.3; Morris, 1996) and the talc surface charge is less negative than at pH 9 (Morris, 1996), electrostatic repulsion between the talc surface and CMC is reduced compared with pH 9.0. The decrease in electrostatic repulsion at pH 3.5 and 0.1 M KCl has resulted in an increase in the CMC adsorption density and, consequently, in the enhanced CMC talc depression observed (Fig. 2). In addition, the reduction in CMC radius of gyration at pH 9.0 from 176 to 159 nm at pH 3.5 (Morris, 1996) may also contribute to the increased CMC adsorption density at this pH. The adsorption isotherms obtained in this study under all solution conditions show a single plateau, in contrast to the three-step isotherm obtained by Steenberg (1982). 3.2.2. Polyacrylamides Fig. 4 shows the adsorption isotherms for PAM-A on talc as a function of both ionic strength and pH. The adsorption isotherms are of the high affinity type under all conditions. The adsorption density of PAM-A is largely dependent upon the solution conditions, with adsorption following the same trends as in the case of CMC: PAM-A adsorption increases with an increase in KCl concentration and a decrease in pH. As the
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Fig. 4. Adsorption isotherm of PAM-A on talc under varying solution conditions. (n) 0.001 M KCl, pH 9.0; (5) 0.1 M KCl, pH 9.0; ( ) 0.001 M KCl, pH 3.5; (o) 0.1 M KCl, pH 3.5. Talc concentration of 20 g dm 3. Lines obtained from the Langmuir equation fit.
.
pK0 of PAM-A is 4.2 and 4.0 in 0.001 and 0.1 M KCl, respectively (Morris, 1996), PAMA is uncharged at pH 3.5, so that there is no electrostatic repulsion between the polymer and the talc surface. As in the case for CMC, the extent of talc depression with PAM-A correlates with the amount of PAM-A adsorbed onto the talc surface. The same trends with ionic strength and pH were observed in the adsorption studies involving kaolinite and an anionic polyacrylamide (Hollander et al., 1981). In this instance, the changes were attributed to variations in electrostatic repulsion. Broseta and Medjahed (1995) also observed significant increases in anionic polyacrylamide adsorption on quartzitic sands at low pH or high ionic strength, i.e., when electrostatic interactions were minimal. Adsorption isotherms for PAM-N on talc, determined at both high and low pH values and ionic strengths, are presented in Fig. 5. Within experimental error, the adsorption of PAM-N onto the talc surface shows little variation with pH and ionic strength, behaviour which is also reflected in the flotation results (Fig. 2). High affinity adsorption occurs initially until a pseudo-plateau is reached; this is then followed by increased adsorption. The adsorption increase after the plateau may indicate either an increase in the packing density of the adsorbed layer, a ‘‘looped’’ structure (Theng, 1979) or multilayer adsorption. 3.3. Langmuir adsorption model The Langmuir adsorption model (Shaw, 1992) was applied to the polymer systems in order to distinguish the various contributions to the polymer – talc interaction. The adsorption of CMC, PAM-A and PAM-N all conform to an ‘‘apparent’’ Langmuir adsorption isotherm, at low polymer concentrations. The results for PAM-N on talc were obtained using data acquired before the second rise in the adsorption isotherm. The free energy of adsorption, DGads, was calculated from the Langmuir equilibrium constant, K,
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Fig. 5. Adsorption isotherm of PAM-N on talc under varying solution conditions. (n) 0.001 M KCl, pH 9.0; (5) 0.1 M KCl, pH 9.0; ( ) 0.001 M KCl, pH 3.5; (o) 0.1 M KCl, pH 3.5. Talc concentration of 20 g dm 3. Lines obtained from the Langmuir equation fit.
.
i.e., DGads = RTlnK, where R is the gas constant and T is the temperature. DGads is represented as the sum of four free energy components (Mackenzie, 1986): DGads ¼ DGchem þ DGelec þ DGHbonding þ DGhydrophobic
ð1Þ
where DGchem, DGelec, DGH-bonding and DGhydrophobic are the chemical, electrical, hydrogen bonding and hydrophobic components of the free energy of adsorption. The values of Cmax in Table 1 clearly show the variation in adsorption density of CMC and PAM-A on talc with solution conditions; the adsorption density of PAM-N is unaltered. Both polyacrylamides have a similar adsorption density under the most favourable conditions for PAM-A. The calculated values of DGads (Table 1) indicate that adsorption of the polymer onto the talc surface is favourable, i.e., DGads < 0. These values are in good agreement with the Table 1 Langmuir adsorption isotherm parameters for the polymer depressants adsorbed on talc as a function of solution conditions Solution conditions
0.001 M KCl/pH 9 0.1 M KCl/pH 9 0.001 M KCl/pH 3.5 0.1 M KCl/pH 3.5
CMC
PAM-A
PAM-N
Cmax (mg m 2)
DGads (kJ mol 1)
Cmax (mg m 2)
DGads (kJ mol 1)
Cmax (mg m 2)
DGads (kJ mol 1)
0.58 F 0.03 1.02 F 0.02 1.02 F 0.03 1.22 F 0.01
12.6 F 0.6 9.9 F 0.3 12.5 F 0.6 15.8 F 0.7
0.11 F 0.01 0.26 F 0.03 0.44 F 0.03 0.50 F 0.01
19.3 F 0.9 17.1 F 0.8 16.7 F 0.4 16.2 F 0.2
0.57 F 0.04 0.56 F 0.07 0.63 F 0.03 0.58 F 0.04
14.7 F 0.3 15.2 F 0.6 15.9 F 0.3 15.3 F 0.4
Cmax is the maximum amount of polymer adsorbed and DGads is the free energy of adsorption.
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values of 22 to 24 kJ mol 1 determined for dextrin adsorption onto coal (Miller et al., 1983). The electrostatic component of adsorption, DGelec, is obtained from the relationship of DGelec = cFwd, where c is the average charge per molecule (i.e., polymer charge portion of charged groups), F is Faraday’s constant and wd is the potential at the outer Helmholtz plane. wd is assumed to be equivalent to f (Shaw, 1992), the zeta potential of talc in the presence of polymer. The values of c for CMC are 0.73 (i.e., 1 0.73), 0.1 (i.e., 1 0.1) for PAM-A and 0 for PAM-N at pH 9. (The fraction of charged groups is taken to be the degree of substitution by the carboxyl groups). The average electrostatic contribution, DGelec, at pH 9.0 was therefore determined to be 3 kJ mol 1 for CMC, 0.4 kJ mol 1 for PAM-A and 0 kJ mol 1 for PAM-N. At pH 3.5, the anionic polymers are uncharged and therefore DGelec reduces to 0 kJ mol 1. These results suggest that electrostatic interactions due to the presence of the charged carboxyl groups inhibit the adsorption process of both CMC and PAM-A onto the talc surface. After consideration of the electrostatic component, the adsorption thermodynamics of PAM-A, PAM-N and CMC at the talc – water interface would appear to be rather similar under all conditions. 3.4. The effective area occupied per molecule, r0, and the bound fraction The fraction of polymer bound to the talc surface or effective surface coverage can be calculated assuming that polymer adsorption occurs in an extended conformation. Through a comparison of the effective area occupied per adsorbed polymer molecule with the extended area occupied by the polymer, the bound fraction (i.e., trains) can be estimated. The effective area occupied per adsorbed polymer molecule, r0, can be calculated from the following relationship (Hiemenz, 1986): r0 ¼ h
A 1 n NA
ð2Þ
where h is defined as the fraction of the talc surface that can be covered by the polymer, A is the talc surface area, n is the number of moles of polymer adsorbed and NA is Avogadro’s number. The effective area of the talc surface occupied per adsorbed molecule of CMC, PAM-A and PAM-N, as a function of surface area and solution conditions, is presented in Table 2. The h values used were 1.0 for the total surface, 0.89 for the face surface and 0.11 for the edge surface (Morris, 1996). The edge surface fraction value of talc is of similar magnitude to the 0.10 edge surface area fraction for kaolinite (Braggs et al., 1994), an aluminosilicate clay mineral. The values for h in this study were determined from a combination of scanning electron microscopy and carbon microanalysis of a probe molecule (chlorodimethyloctadecylsilane) which specifically interacted with the talc edge surface sites. The errors involved in the calculation of r0 are approximately F 10%. The calculated area occupied by a CMC molecule in an extended, linear conformation is 425 nm2. The number of monomer units was estimated to be 910 monomers per molecule based on a Mw of 200 000. A glucose unit of length 0.52 nm (Brown et al., 1963) and a height of 0.92 nm (Gerson, 1995), giving an area of 0.47 nm2 per monomer, were
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Table 2 The effective area occupied per molecule of polymer depressant adsorbed on the talc surface calculated using Eq. (2), assuming adsorption occurred on all of the talc surface (h = 1), face only (h = 0.89) or edge only (h = 0.11), as a function of solution conditions (see text for explanation) Solution conditions
0.001 M KCl/pH 9 0.1 M KCl/pH 9 0.001 M KCl/pH 3.5 0.1 M KCl/pH 3.5
r0, area per CMC molecule (nm2)
r0, area per PAM-A molecule (nm2)
r0, area per PAM-N molecule (nm2)
Total surface
Face only
Edge only
Total surface
Face only
Edge only
Total surface
Face only
Edge only
572 332 332 277
509 295 295 246
63 37 37 30
104 56 26 23
93 50 23 20
11 6.2 2.9 2.5
32 36 27 32
28 32 24 28
3.5 4.0 3.0 3.5
used in the calculation. The calculated area occupied by PAM-A and PAM-N in an extended, linear conformation was determined to be approximately 29 and 32 nm2, respectively. The conformation was assumed to be linear and flat; standard bond length and angles (Aylward and Findlay, 1974) were assumed. The calculated values of the area occupied by the polymers, when compared to the effective area occupied by the adsorbed polymer (Table 2), suggest that in most cases CMC, PAM-A and PAM-N could be accommodated as a monolayer if adsorption occurred either on the total surface or on the face surface only. If polymer adsorption occurred on only the edge surface, then the bound fraction of CMC, PAM-A and PAM-N, would be a value of 0.1, 0.4 to 0.1 and 0.1, respectively, with the unbound fraction of the adsorbed polymer forming loops and tails. This is highly unlikely, normally the unbound fraction of an adsorbed polymer, which is in the form of loops and tails, is only 0.60 to 0.75 and may be as low as 0.30 (Theng, 1979). Polymer adsorption on the talc surface is therefore likely to occur on either the total surface or on the face surface only. 3.5. Electrochemistry Fig. 6 shows the zeta potential of talc as a function of pH, in the absence and presence of the polymers, CMC, PAM-A and PAM-N. All measurements were performed in 0.01 M KCl electrolyte concentration. The isoelectric point (iep) of talc can be found by extrapolation of the zeta potential data to low pH values; an iep of approximately 2 is found independent of ionic strength (Morris, 1996) and the type and concentration of polymer used. For pH values higher than 2, the zeta potential of talc is negative and increases in magnitude with pH. The effect of polymer adsorption on the zeta potential of talc is, however, different for the anionic and nonionic polymers. For the nonionic polymer, PAM-N, the zeta potential of talc decreases with an increase in polymer concentration for all pH values. For the anionic polymers, CMC and PAMA, a similar trend is observed only at alkaline pH values. For neutral and acidic pH values, the trend with polymer concentration is more complex: the zeta potential increases for low polymer concentrations but decreases at higher polymer concentrations, and reaches a similar value to that observed in the absence of polymer. These trends are illustrated more clearly in Fig. 7 where the zeta potential of talc is measured
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Fig. 6. Zeta potential of talc as a function of pH and: (a) CMC concentration at (+) 0, (5) 1 ppm, (n) 100 ppm; (b) PAM-A concentration at (+) 0, (o) 8 ppm, ( ) 90 ppm; (c) PAM-N concentration at (+) 0, (4) 8 ppm, (E) 90 ppm ([KCl] = 0.01 M).
.
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223
.
Fig. 7. Zeta potential of talc as a function of (n) CMC, ( ) PAM-A and (E) PAM-N concentration at pH 5.0 in 0.001 M KCl.
at a fixed pH of 5.0 and as a function of polymer concentration. The zeta potential of talc more or less remains constant with an increase in PAM-N concentration. For the two anionic polymers it reaches a maximum of 57 mV at a low concentration of approximately 2 ppm, then decreases and remains constant for polymer concentrations above 6 ppm. The different trends in zeta potential observed with polymer concentration in Fig. 6a and b result in clear cross-over points in the zeta potential versus pH curves at pH values between 8.0 and 9.5 in the presence of only CMC and PAM-A. These observations are attributed to competition between shear plane shift, charge on adsorbing polymer and conformation. Similar trends have been reported in the literature for the zeta potential of talc in the presence of CMC (Steenberg, 1982; Steenberg and Harris, 1984). Based only on electrical considerations, the adsorption of higher levels of anionic polymers on the talc surface, as in this study, should result in a more negative zeta potential while no change is expected for the adsorption of a neutral polymer. However, the presence of the polymer shifts the plane of shear away from the surface; as the potential decays exponentially with distance from the solid surface, a decrease in zeta potential is predicted in the presence of an adsorbed polymer (Fleer et al., 1972). This is indeed what is observed elsewhere (Fleer, 1971) and in this study (with PAMN) when a neutral polymer adsorbs on a solid surface. Furthermore, the amount of zeta potential decrease should reflect the thickness of the polymer layer at the solid surface (Fleer, 1971). 3.6. Thickness of the polymer layer The zeta potential at the particle plane of shear in the absence of polymer, f1, is measured at a distance d from the surface, whilst the zeta potential in the presence of polymer, f2, is measured at a distance D from the surface.
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The following equation can be derived between the measured zeta potentials and the distance of the plane of shear from the surface, for an uncharged polymer (Fleer, 1971): tanh
zef2 zef ¼ tanh 1 expðjðD dÞÞ 4kT 4kT
ð3Þ
where z is the valency of the ions in the double layer, e is the electron charge, j is the reciprocal Debye length, k is Boltzmann’s constant and T is the temperature. It is assumed that the polymer does not substantially alter the ion distribution in the double layer. Table 3 shows the adsorbed polymer layer thickness, D, calculated using Eq. (3), assuming a value of 0.4 nm for d as used by Fleer (1971) on a AgI surface. The zeta potential values at the onset of the plateau region (i.e., pH values >7) (Fig. 6a, b and c) were used to calculate the polymer layer thickness. Comparison of the PAM-N calculated layer thickness with the calculated height of the polymer monomer of 0.95 nm (Fig. 1) indicates that the nonionic polymer adsorbs in a relatively flat conformation. The calculated layer thicknesses of the anionic polymers, CMC and PAM-A, are likely to be smaller than the actual values, given that the effect of the negative charge of the polymer was not taken into account in the calculations and are therefore an estimate. The small anionic polymer calculated layer thicknesses still suggest, however, that the polymers adsorb in a flat conformation. The calculated layer thicknesses determined in this study are relatively low in comparison to those obtained for poly(vinylalcohol) (PVA) and guar gum using a similar method of calculation (Fleer, 1971; Mao, 1994). The latter thicknesses ranged up to about 10 nm at the higher adsorption densities of the nonionic polymer. The variation in thicknesses is likely to reflect differences in the adsorbed polymer conformation. Fleer et al. (1972, 1993) determined that at the higher adsorption densities, long loops are present in the adsorbed PVA layer on AgI particles. Adsorption of polymer depressants onto the talc face surface has been further investigated by a combination of direct and indirect techniques. Adsorption of polyacrylamides onto the talc face surface is supported by results from time of flight secondary ion mass spectroscopy (ToF SIMS) of adsorbed PAM-N on the talc surface (Fig. 8). The CN fragment, characteristic of the adsorbed polyacrylamide, was used to monitor the presence of PAM-N and was clearly observed on the face surface of talc (Fig. 8b). It was
Table 3 Thickness of the adsorbed polymer layer at the talc surface, calculated with Eq. (3) at 0.01 M KCl (d = 0.4 nm; T = 25 jC; f1 = 36 mV) Polymer
Polymer concentration (ppm)
f2 (mV)
Adsorbed polymer layer thickness (nm)
CMC
1 100 8 90 8 90
54 42 49 47 43 36
0.6 1.1 0.6 0.7 0.8 1.3
PAM-A PAM-N
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Fig. 8. The negative ion ToF SIMS images of the talc sample with adsorbed PAM-N: (a) total negative ion (0 to 800 g mol 1); (b) CN secondary ion (26 g mol 1). The shaded bar scale indicates the relative intensity of the ion(s).
not possible using this technique to ascertain if the polymers had adsorbed on the edge surface. Silane modification of talc surfaces has been used indirectly to investigate the mechanism of polymer adsorption onto the talc surface (Morris et al., 1999). The use of monochlorosilane molecules to react with only sites on the talc edge surface has shown that polymer adsorption on talc increased for anionic polymers but remained unaltered for the nonionic polymer after silane modification of the talc surface. It has been proposed that the dominant mechanism for polymer adsorption onto the talc surface occurs by hydrophobic interaction with the face surface together with a weak repulsive electrostatic interaction with the edge surface for anionic polymers. Consequently, with the information presented in this study in combination with related studies, adsorption of the polymer depressants on talc, as a function of pH and ionic
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strength, is therefore likely to occur via hydrophobic interaction with the face surface with a minor contribution, if any, from the edge surface.
4. Conclusions The depression of talc increases with an increase in the adsorption density of the polymers on the talc surface, as is clearly shown by the microflotation and adsorption isotherm data. The solution conditions, such as ionic strength and pH, strongly influence the adsorption of the anionic polymers, CMC and PAM-A, onto the talc surface. This, however, is not the case for the nonionic polymer PAM-N. Talc conditioned at acidic pH values or high ionic strength increases the adsorption density of the anionic polymer depressants and therefore talc depression, due to a reduction in the electrostatic repulsion between the carboxyl groups of the coiled polymer and the negative edge surface sites of the talc. In addition, the change in the anionic polymer depressant conformation from extended to coiled at acidic pH values or high ionic strength increases the adsorption density at the talc surface. The adsorption density of the nonionic polyacrylamide, PAMN, on the talc surface is not influenced by ionic strength or pH. Geometrically calculated values of the area occupied by the polymers suggest that adsorption is not likely on the edge surface but rather on the face surface or a combination of the two surfaces. Adsorption of the polymers CMC, PAM-A and PAM-N is likely to occur in a flat conformation via hydrophobic interaction of the hydrocarbon backbone at the talc face. The hydrophilic hydroxyl and carboxyl groups of the polymer are therefore extended out from the talc surface imparting some hydrophilic character to the talc particle and thereby causing depression during flotation.
References Aylward, G.H., Findlay, T.J.V., 1974. SI Chemical Data, 2nd edn. Jacaranda, Queensland. Bakinov, K.G., Vaneev, I.J., Gorlovsky, S.I., Eropkin, U.I., Zashikhin, N.V., Konev, A.S., 1964. New methods of sulphide concentrate upgrading. In: Arbiter, N. (Ed.), 7th International Mineral Processing Congress, vol. 1. Gordon and Breach, New York, p. 227. Braggs, B.R., Fornasiero, D., Ralston, J., Smart, R.St.C., 1994. The effect of surface modification by an organosilane on the electrochemical properties of kaolinite. Clays Clay Miner. 42 (2), 123. Broseta, D., Medjahed, F., 1995. Effects of substrate hydrophobicity on polyacrylamide adsorption. J. Colloid Interface Sci. 170, 457. ¨ hman, J., 1963. Studies on cellulose derivatives: Part I. The dimensions and configBrown, W., Henley, D., O uration of sodium carboxymethyl cellulose in cadoxen and the influence of the degree of substitution. Makromol. Chem. 62, 164. Dubois, M., Gilles, K.A., Hamilton, J.K., Reders, R.A., Smith, F., 1956. Colorimetric method for determination of sugars and related substances. Anal. Chem. 28, 350. Fleer, G.J., 1971. Polymer adsorption and its effect on colloidal stability. PhD Thesis, Wageningen, The Netherlands, pp. 34 – 59. Fleer, G.J., Koopal, L.K., Lyklema, J., 1972. Polymer adsorption and its effect on the stability of hydrophobic colloids. I. Characterisation of poly(vinyl alcohol) adsorption on silver iodide. Kolloid-Z. Z. Polym. 250, 689.
G.E. Morris et al. / Int. J. Miner. Process. 67 (2002) 211–227
227
Fleer, G.J., Cohen Stuart, M.A., Scheutjens, J.M.H.M., Cosgrove, T., Vincent, B., 1993. Polymers at Interfaces. Chapman & Hall, London. Gerson, A., 1995. Molecular modelling of (1) polymer fragments and (2) surface charge of talc. Ian Wark Research Institute, Internal report. Gomes, L.M.B., Oliveira, J.F., 1989. Floatability control of talc by the use of carboxymethyl cellulose. Metalurgia-abm 45 (382), 908. Gomes, L.M.B., Oliveira, J.F., 1991. The control of natural floatability of talc with carboxymethyl cellulose and aluminium chloride. Fine Particles Processing Flotation. 17th Int. Miner. Process. Congress, vol. 2, p. 353. Grano, S.R., Griffin, L.F., Johnson, N.W., Smart, R.St.C., Ralston, J., 1991. Treatment of naturally hydrophobic gangue minerals at the copper concentrator of Mt. Isa Mines Limited. 4th Mill Operators’ Conference. AusIMM, Burnie, Tasmania. Haung, H.H., Calara, J.V., Bauer, D.L., Miller, J.D., 1978. Adsorption reactions in the depression of coal by organic colloids. Recent Developments in Separation Science, vol. 4. CRC, Florida, p. 115. Hiemenz, P.C., 1986. Principles of Colloid and Surface Chemistry, 2nd edn. Marcel Dekker, New York. Hollander, A.F., Somasundaran, P., Gryte, C.C., 1981. Adsorption characteristics of polyacrylamide and sulfonate-containing polyacrylamide copolymers on sodium kaolinite. J. Appl. Polym. Sci. 26, 2123. Hunter, R.J., 1987. Foundations of Colloid Science, vol. 1. Oxford Univ. Press, New York. Liu, Q., Laskowski, J.S., 1989. The interactions between dextrin and metal hydroxides in aqueous solutions. J. Colloid Interface Sci. 130, 101. Mackenzie, M., 1986. Organic polymers as depressants. In: Malhotra, D., Riggs, W.F. (Eds.), Chemical Reagents in the Mineral Processing Industry. Society of Mining Engineers, Colorado, USA, pp. 139 – 145. Ma¨lhammar, G., 1990. Determination of some surface properties of talc. Colloids Surf. 44, 61 – 69. Mao, M., 1994. Surface and interfacial processes in the selective flotation of zircon. PhD Thesis, University of South Australia, Adelaide, Australia. Miller, J.D., Laskowski, J.S., Chang, S.S., 1983. Dextrin adsorption by oxidised coal. Colloids Surf. 8, 137 – 151. Morris, G.E., 1996. The adsorption characteristics of polymeric depressants at the talc – water interface. PhD Thesis, University of South Australia, Adelaide, Australia. Morris, G.E., Fornasiero, D., Ralston, J., 1999. Low molecular weight polyacrylamide adsorption by silane modified talc. 38th Annual Conference of Metallurgists (Canadian Institute of Mining). Quebec City, Canada. Nagaraj, D.R., Rothenberg, A.S., Lipp, D.W., Panzer, H.P., 1987. Low molecular weight polyacrylamide-based polymers as modifiers in phosphate beneficiation. Int. J. Miner. Process. 20, 291. Partridge, A.C., Smith, G.W., 1971. Small-sample flotation testing: a new cell. Trans. Inst. Min. Metall., Sect. C 80, 118. Pugh, R.J., 1989a. Macromolecular organic depressants in sulphide flotation—a review: 1. Principles, types and applications. Int. J. Miner. Process. 25, 101. Pugh, R.J., 1989b. Macromolecular organic depressants in sulphide flotation—a review: 2. Theoretical analysis of the forces involved in the depressant action. Int. J. Miner. Process. 25, 131. Rath, R.K., Subramanian, S., Laskowski, J.S., 1997. Adsorption of dextrin and guar gum onto talc. A comparative study. Langmuir 13, 6260. Rhodes, M.K., 1981. The effects of the physical variables of carboxymethyl cellulose reagents on the depression of magnesia bearing minerals in Western Australian nickel sulphide ores. In: Laskowski, W.J. (Ed.), Mineral Processing, 13th International Congress. Elsevier, Amsterdam, p. 346. Scoggins, M.W., Miller, J.W., 1979. Determination of water-soluble polymers containing primary amide groups using the starch-triiodide method. Soc. Pet. Eng. J. 19, 151. Shaw, D.J., 1992. Introduction to Colloid and Surface Chemistry, 4th edn. Butterworth-Heinemann, Oxford. Steenberg, E., 1982. The depression of the natural floatability of talc: the mechanism involved in the adsorption of organic reagents of high molecular mass. PhD Thesis, Potschefstroom University of Christian Higher Education, Johannesburg. Steenberg, E., Harris, P.J., 1984. Adsorption of carboxymethyl cellulose, guar gum, and starch onto talc, sulphides, oxides and salt-type minerals. S. Afr. J. Chem. 37, 85. Theng, B.K.G., 1979. Clay – polymer interactions: summary and perspectives. Formation and Properties of ClayPolymer Complexes. Elsevier, Amsterdam, pp. 37 – 61.