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The role of surfactant salts on the spherical agglomeration of hematite suspension
Colloids and Surfaces A: Physicochemical and Engineering Aspects 173 (2000) 211 – 217 www.elsevier.nl/locate/colsurfa
The role of surfactant salts on the spherical agglomeration of hematite suspension Zygmunt Sadowski Department of Chemical Engineering and Heating Equipment, Wroclaw Uni6ersity of Technology, 52 370 Wroclaw, Poland Received 13 December 1999; accepted 3 March 2000
Abstract With the recognition of the importance of beneficiation of fine mineral particles, there has been a tremendous increase in research to aid our understanding of the role of precipitated surfactant salts in the spherical agglomeration process. In turn this has led to the development of special techniques for instance: z-potential and contact angle measurements, to explain this process. This work provides information on the condition of spherical agglomeration of fine hematite suspensions and it better characterizes a more detailed look at some problems of adhesion of surfactant salts. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Surfactant salts; Spherical agglomeration; Hematite suspension
1. Introduction Among the modern colloidal methods of enrichment of fine mineral particles, a process of spherical agglomeration deserves particular attention. That is because oil agglomeration, as initial agglomeration of very fine minerals, is important for facilitating further separation in a subsequent step [1]. In any case the use of agglomeration processes requires the knowledge of the interaction mechanism between the surfactant salts and the hydrophobic surface of mineral particles. According to early works [2 – 4], the adhesion of precipitated surfactant salts is a crucial part of the mechanism of spherical agglomeration. An attachment of a particle to a collector surface is controlled by
various surface interaction forces acting over relatively short distances. In the DLVO theory, the variation in net surface interaction energy with separation distance was determined from the interaction of EDL and van der Waals forces [5]. In practice, the interaction between the surfactant salts and the mineral surface is determined by a strain hydrophobic interaction [6]. Better knowledge of this interaction enhances a development of spherical agglomeration method. The purpose of this paper, therefore, is therefore to study the agglomeration of hematite suspension. The agglomeration condition of hydrophobic particles of hematite in the presence of precipitated surfactant salts will be considered in this work.
0927-7757/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 7 7 5 7 ( 0 0 ) 0 0 5 4 4 - 6
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Z. Sadowski / Colloids and Surfaces A: Physicochem. Eng. Aspects 173 (2000) 211–217
2. Experimental section
3.2. Spherical agglomeration experiment
2.1. Materials
The agglomeration experiments took place in a Teflon vessel, which contained 100 cc of the mineral suspension. The mixing unit was based on the homogenizer type 302 Unipan (Poland). In each of the experiments, 2.0 g of mineral powder was used. The suspensions were agitated with the surfactant at a low speed for 10 min. Then, a small amount of n-heptane was added to the suspension. The mixing of suspension was continued for 50 s at relatively high speed (10 000 rpm). The agglomerates formed in this procedure were collected on a 100-mesh (150 mm) sieve. The agglomeration recovery was calculated as a weight percentage of material remaining on the sieve.
Hematite was obtained in the form of single mineral crystals (Ward Scientific Establishment Inc., New York) and prepared by crushing (cone crusher), grinding and sieving to recover the size fraction of − 38 mm. Sodium oleate was obtained from Riedel-de Haen (Germany) and used with further purification. The surfactant salts were prepared by mixing 2 g of sodium oleate with calcium, ferric and barium soluble salts. These salts (CaCl2, FeCl3 and Ba(N03)2) were purchased from POCH in Gliwice (Poland). Cationic surfactant cetyltrimethylammonium bromide (Aldrich) was used in the n-heptane (Loba-chemie), emulsion preparation. Diagnostic liquids used for the contact angle measurements were diiodomethan (Sigma), 1bromonaphthalene (Riedel-de Haen), formamide (Sigma), hexadecan (Riedel-de Haen) and water. Water was doubly distilled and deionized in a Milipore purification system. The pH was modified using NaOH and HCl (0.1 M solution). All tests were carried out using double destilled water. Sodium dodecyl sulphate (SDS) anionic surfactant, was obtained from Sigma Chemicals.
3. Methods
3.1. Adsorption of surfactant The adsorption of sodium oleate was measured according to the method described by Fowler and Steel [7]. Pure hematite (1 g) was added to 100 cc of sodium oleate solution. The initial concentration of sodium oleate was changed from 5 ×10 − 5 to 2 ×10 − 3 M. The suspension was stirred vigorously using a mechanical shaker for 8 h to obtain equilibrium of adsorption, then centrifuged to separate the solid. Clear supernatants were used for the sodium oleate determination. The residual concentration of sodium oleate in the supernatant was determined by application of a colorimetric method [7].
3.3. Electrophoretic mobility The electrophoretic mobility of surfactant salt particles was determined for water suspension using Zeta Plus Apparatus (Brookhaven Instr. Corp., USA). Mobility was measured at room temperature and averages were taken over three independent sets of measurements. The value of pH for the surfactant salt suspension was measured with a Precision Digital pH-meter OP-208/1 (Hungary). Suspension pH was adjusted by addition of 0.1 M HCl or NaOH solution. Surfactant salt suspension was prepared by mixing of 2.0 g of sodium oleate with subsequent quantity of 0.5 M solution of CaCl2, FeCl3, Ba(NO3)2, respectively. The oil–water (n-heptane–water) emulsions were prepared by dissolving the surfactant (SDS or CTAB) in water and stirring this in an ultrasonic mixer while slowly adding the oil (n-heptane). Mixing was continued for a short time (10–15 s) to produce an emulsion.
3.4. Contact angle measurements The contact angle measurements were conducted using a homebuilt goniometer. The droplet of probing liquid was placed on the surface of special prepared surfactant salt pellets. The pellets were obtained by compressing KBr powder (first step) so that the suitable surfactant salt was compressed to the KBr disc. Pellets were kept in a
Z. Sadowski / Colloids and Surfaces A: Physicochem. Eng. Aspects 173 (2000) 211–217
desiccatior before making the contact angle measurements. The results reported here are the average of five independent measurements.
Fig. 1. Adsorption isotherms of sodium oleate onto hematite.
Fig. 2. Spherical agglomeration of hematite as a function of the sodium oleate concentration.
213
4. Results and discussion Spherical oil agglomeration can be realized when the surface of interacting particles is hydrophobic [8]. The adsorption density in micromoles of sodium oleate adsorbed per gram of hematite at pH 2.5, 7.5 and 10.5 are presented in Fig. 1. As can be seen the highest adsorption coincided with an acidic pH range. And the shape of this isotherm shows that the precipitation of surfactant salt (ferric oleate) occurs at the high concentration levels. Similar results of the adsorption of sodium oleate for calcite, magnesite and barite have been reported earlier [9–11]. Adsorption results confirmed the finding of Pascoe and Dohery [12] that shear-flocculation of hematite using sodium oleate as a selective flocculant reagent was favored under both neutral and acid conditions. Fig. 2 represents the recovery of spherical agglomeration of hematite a function of the initial sodium oleate concentration at a constant pH range (pH 7.5). Also, the efrect of an addition of surfactant salts on the spherical agglomeration of hematite is presented. The recovery curves have a characteristic shape; a dramatic increase of recovery corresponds to greater spherical agglomeration and maximum recovery is realized at the critical surfactant concentration (csc). The value of a critical surface concentration depends on a quantity of precipitated surfactant salt which is added to the hematite suspension. It can be hypothesized that an attractive interaction between surfactant salt particles and the hematite particles contributes to the spherical agglomeration process. This contribution will be discussed later on the basis of zeta potential results. In the case of oil agglomeration of both coal and pyrite particles [13], the oil droplets adhere to the solid particles and collect these particles together into an aggregate. It is interesting to note that electrokinetic–pH characteristics of coal and oil droplets are very similar. Such similarity occurs for materials with carboxylic groups at the interface [14]. The results of electrophoretic mobility of n-heptane–water emulsion, as a function of pH is presented in Fig. 3. The effect of an
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Fig. 3. The effect of pH and surfactants on the zeta potential of n-heptane.
(CTAB) surfactant on the oil–water interface causes the oil droplets to have a positive zpotential. An increase in cationic surfactant addition to the oil–water emulsion, has an interesting effect on the spherical agglomeration of hematite suspension. Degree of agglomeration is shown in Fig. 4 as a function of the concentration of cationic surfactant added to the oil–water emulsion. As the concentration of CTAB (cationic surfactant) was raised, the recovery of agglomerates increased. However, beyond a certain cationic surfactant concentration, agglomeration decreased. This suggests that an excess of cationic surfactant adsorbed at the hematite particles causes a strong repulsion between oil droplets and mineral surface. However, this hypothesis needs more experimental work. Another factor which may be contributing to agglomeration may be the adhesion of precipitated surfactant salt. Zeta potentials of surfactant salt particles determined by microelectrophoresis are plotted as a function of pH in Fig. 5. An
Fig. 4. The effect of cationic surfactant (CTAB) on the spherical agglomeration.
addition of both cationic (CTAB) and anionic surfactants is also shown in this Figure. It can be seen from Fig. 3 that the adsorption of cationic
Fig. 5. The effect of pH on the zeta potential of surfactant salts.
Z. Sadowski / Colloids and Surfaces A: Physicochem. Eng. Aspects 173 (2000) 211–217
215
Table 1 Contact angles for fatty acid soaps Fatty acid soap
Ca(OL)2 Ba(OL)2 Fe(OL)3
Liquid Water
Formamide
Diiodomethane
a-Bromonaphthalene
26° 48% 14° 30% 6° 26%
30° 18% 27° 41% 11° 15%
45° 38% 48° 22% 39° 40%
41° 18% 40° 20% 24° 18%
Table 2 Characteristics of used liquids at 20°C Liquid
gL (mJ m−2)
g LW (mJ m−2)
g− (mJ m−2)
g+ (mJ m−2)
Water Formamide Diiodomethane a-Bromonaphthalene
72.8 58.0 50.8 44.4
21.8 39.0 50.8 43.5
25.5 2.28 – –
25.5 39.6 – –
isoelectric point (iep) of precipitated surfactant salts is extrapolated near pH 3. According to Laskowski [15], the isoelectric points for fatty acids are around pH 3. Subsequently, it was found that the electromobility of surfactant salt particles changed in the following order NaOL\ Ca(OL)2 \Ba(OL)2 \Fe(OL)3. The negative surface charge on precipitated fatty acid soaps originates from the heterocoagulation of precipitated soap particles with the oleate ions. The experimental data obtained by direct contact angle measurements on a flat surface of fatty acid soaps are shown in Table 1. These contact angles are used to calculate the surface free energy components. The surface tension and its components of the liquid probes adapted to the contact angle measurement are listed in Table 2. The surface tension of the solid surface (gs) can be expressed as a sum of surface tension components which due to a particular type of molecular interaction. These components were divided into two main categories: dispersive (Lifshitz –van der Waals) and non-dispersive (polar). The OCG ( Oss – Chaudhury – Good) equation provides a simple means to characterize a solid surface by using contact angle measurement [16,17]. The equation OCG was derived by combining Young’s equation with Fowkes’ equation:
− 1/2 (l+ cos U)gL = 2(g dS g dL)1/2 + (g+ S gL ) + 1/2 + (g − S gL )
where U and gL are the contact angle and the surface tension of the probing liquid, respectively. The subscript ‘S’ represent the surfactant salt. The superscripts ‘d’, ‘+ ’ and ‘−’ indicate the dispersion, acid and base components of the surface free energy, respectively. The OCG equation was used for the surface free energy components calculation. Table 3 shows the surface free energy components of fatty acid soaps. The results presented in Table 3 demonstrated that the Lifshitz–van der Waals components are almost identical for all fatty acid soaps. By contrast, the Lewis base components are significant great. These great affinity for a base can be reflected the linking of oleate ions to the fatty acid soap particles [18]. Table 3 Value of surface free energy components of fatty acid soaps Fatty acid soap
g LW s (mJ m−2)
g− s (mJ m−2)
g+ s (mJ m−2)
Ca(OL)2 Ba(OL)2 Fe(OL)3
35.56 35.18 35.92
124.19 138.26 135.68
31.57 32.92 40.14
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During the spherical agglomeration process four types of forces are important: van der Waals attraction, electrostatic repulsion, hydrophobic interaction and hydrodynamic force. Aggregation of fine hydrophobic particles, in a shear field was developed as shear-flocculation by Warren [19]. In the presence of electrostatic repulsion between negatively charged particles (hematite and surfactant salt), high shear rates can increase the chance of colliding particles overcoming a potential energy barrier. The hydrophobic attraction forces should be responsible for the surfactant salts adhesion to the hematite surface. Direct experiments have confirmed that precipitated surfactant forms created adsorbed patches or islands on the mineral surface rather than a uniform layer [18,20]. The fact that increasing hydrophobicity makes the negative values of the zeta potential more negative is known from the literature [21]. However, the more hydrophobic and negatively charged the hematite surface, the more extensively it should interact with the positively charged n-heptane droplets. Thus, we can observe a spherical agglomeration. In addition, we can assume that the surfactant salts act much like an additional bridging agent. The spherical shape of aggregates has been preserved after the oil (n-heptane) vaporization.
5. Summary The results presented here show that the spherical agglomeration of fine hematite particles in water suspension is highly dependent on the degree of surface hydrophobicity. The interaction between the hydrophobic surface of hematite and precipitated surfactant salts should be hydrophobic in character because both surfactant salts and hematite particles have a negative value of z-potential. The hypothesis of strong hydrophobic interaction between oil (n-heptane) droplets and both surfactant salts and hematite particles was explained. Furthermore, the addition of oil droplets with cationic emulsifier reagent (CTAB) neutralizes a strong electrostatic repulsion.
Acknowledgements The author wishes to thank Prof. Wojcik and Prof. Chibowski from the Department of Physical Chemistry of Maria Curie-Sklodowska University for a discussion. 1 am grateful to Mrs B. Dzikowska for her assistance in determining the z-potential.
References [1] S.J. Laskowski, An introduction: physicochemical methods of separation, in: J.S. Laskowski, J. Ralston (Eds.), Colloid Chemistry in Mineral Processing, Elsevier, Amsterdam, 1992, pp. 25 – 241. [2] Z. Sadowski, Selective spherical agglomeration of fine salt-type mineral particles in aqueous suspension, Colloid. Surf. 96 (1995) 277 – 285. [3] Z. Sadowski, Spherical agglomeration of flotation tailing, in: A. Kemal, V. Arslan, M. Akar (Eds.), Changing Scopes in Mineral Processing, AA Balkema, Rotterdam, 1996, pp. 681 – 685. [4] Z. Sadowski, Spherical Agglomeration of Fine Mineral Particles, Proceedings of the XX IMPC, Aachen, September, 1997, 415 – 424. [5] V. Shilov, H. Lichtenfeld, H. Sonntag, Fast coagulation of nearly spherical ferric oxide (heamatite particles), Colloid. Surf. 104 (1995) 321 – 325. [6] Y.K.J. Leong, D.V. Boger, P.J. Scales, W.T. Healy, Interparticle forces arising from adsorbed surfactants in colloidal suspension, J. Colloid. Inter. Sci. 181 (1996) 605 – 612. [7] E.W. Fowler, T.W. Steel, T.W., National Institute of Metallurgy, Johannesburg, Report, 5, 1968, 3 – 7. [8] S.J. Laskowski, Oil assisted fine particle processing, in: L.S. Laskowski, J. Ralston (Eds.), Colloid Chemistry in Mineral Processing, Elsevier, Amsterdam, 1992, pp. 361– 394. [9] Z. Sadowski, The influence of SLS on the adsorption of SDS on salt-type mineral surface, Miner. Eng. 5 (1995) 421 – 428. [10] S.Ya Shalyt, L. Dekan, M. Sekeresh, Modification of the surface of baryta by surfactants of diffirent nature, J. Colloid. USSR 46 (1984) 91 – 96. [11] S.Ya Shalyt, L. Dekan, M. Sekeresh, Modification of the surface of barium sulfate by sodium oleate, and its peptization effect, J. Colloid. USSR 47 (1985) 630 – 631. [12] R.D. Pascoe, E. Doherty, Shear flocculation and flotation of hematite using sodium oleate, Int. J. Miner. Process. 51 (1997) 269 – 285. [13] J. Drzymala, R. Markuszewski, T.D. Wheelock, Oil agglomeration of sulfurized pyrite, Miner. Eng. 4 (1991) 161 – 172.
Z. Sadowski / Colloids and Surfaces A: Physicochem. Eng. Aspects 173 (2000) 211–217 [14] L. Drzymala, Z. Sadowski, L. Holysz, E. Chibowski, Ice/water interface: zeta potential point of charge, and hydrophobicity, J. Colloid. Inter. Sci. 220 (12) (1999) 229 – 234. [15] S.J. Laskowski, Electrokinetic measurements in aqueous solutions of weak electrolyte type surfactants, J. Colloid. Inter. Sci. 159 (1993) 349–353. [16] P.M. Costanzo, W. Wu, R.F. Giese, C.J. van Oss, Comparison between direct contact angle measurements and thine layer wicking on synthetic monosized cuboid hematite particles, Langmuir 11 (1995) 1827–1830. [17] Z. Xu, Q. Liu, L. Ling, An evaluation of the van Oss – Chaudhury–Good equation and Neumann’s equation of state approach with mercury substrate, Langmuir 11 (1995) 1044–1046.
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[18] M. Rutland, R.J. Pugh, Calcium soap in flotation deinking; fundamental studies using surface force and coagulation techniques, Colloid. Surf. 125 (1997) 33– 46. [19] L.J. Warren, Shear-flocculation, in: L.S. Laskowski, J. Ralston (Eds.), Colloid Chemistry in Mineral Processing, Elsevier, Amsterdam, 1991. [20] Y. Lu, L. Drelich, D.J. Miller, Oleate adsorption at an apatite surface studied by ex-situ FTIR internal reflection spectroscopy, J. Colloid. Inter. Sci. 202 (1998) 462 – 476. [21] L. Rubio, H. Hoberg, The process of separation of fine mineral particles by flotation with hydrophobic polymeric carrier, Int. J. Miner. Process. 37 (1993) 109– 122.
The role of surfactant salts on the spherical agglomeration of hematite suspension Zygmunt Sadowski Department of Chemical Engineering and Heating Equipment, Wroclaw Uni6ersity of Technology, 52 370 Wroclaw, Poland Received 13 December 1999; accepted 3 March 2000
Abstract With the recognition of the importance of beneficiation of fine mineral particles, there has been a tremendous increase in research to aid our understanding of the role of precipitated surfactant salts in the spherical agglomeration process. In turn this has led to the development of special techniques for instance: z-potential and contact angle measurements, to explain this process. This work provides information on the condition of spherical agglomeration of fine hematite suspensions and it better characterizes a more detailed look at some problems of adhesion of surfactant salts. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Surfactant salts; Spherical agglomeration; Hematite suspension
1. Introduction Among the modern colloidal methods of enrichment of fine mineral particles, a process of spherical agglomeration deserves particular attention. That is because oil agglomeration, as initial agglomeration of very fine minerals, is important for facilitating further separation in a subsequent step [1]. In any case the use of agglomeration processes requires the knowledge of the interaction mechanism between the surfactant salts and the hydrophobic surface of mineral particles. According to early works [2 – 4], the adhesion of precipitated surfactant salts is a crucial part of the mechanism of spherical agglomeration. An attachment of a particle to a collector surface is controlled by
various surface interaction forces acting over relatively short distances. In the DLVO theory, the variation in net surface interaction energy with separation distance was determined from the interaction of EDL and van der Waals forces [5]. In practice, the interaction between the surfactant salts and the mineral surface is determined by a strain hydrophobic interaction [6]. Better knowledge of this interaction enhances a development of spherical agglomeration method. The purpose of this paper, therefore, is therefore to study the agglomeration of hematite suspension. The agglomeration condition of hydrophobic particles of hematite in the presence of precipitated surfactant salts will be considered in this work.
0927-7757/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 7 7 5 7 ( 0 0 ) 0 0 5 4 4 - 6
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Z. Sadowski / Colloids and Surfaces A: Physicochem. Eng. Aspects 173 (2000) 211–217
2. Experimental section
3.2. Spherical agglomeration experiment
2.1. Materials
The agglomeration experiments took place in a Teflon vessel, which contained 100 cc of the mineral suspension. The mixing unit was based on the homogenizer type 302 Unipan (Poland). In each of the experiments, 2.0 g of mineral powder was used. The suspensions were agitated with the surfactant at a low speed for 10 min. Then, a small amount of n-heptane was added to the suspension. The mixing of suspension was continued for 50 s at relatively high speed (10 000 rpm). The agglomerates formed in this procedure were collected on a 100-mesh (150 mm) sieve. The agglomeration recovery was calculated as a weight percentage of material remaining on the sieve.
Hematite was obtained in the form of single mineral crystals (Ward Scientific Establishment Inc., New York) and prepared by crushing (cone crusher), grinding and sieving to recover the size fraction of − 38 mm. Sodium oleate was obtained from Riedel-de Haen (Germany) and used with further purification. The surfactant salts were prepared by mixing 2 g of sodium oleate with calcium, ferric and barium soluble salts. These salts (CaCl2, FeCl3 and Ba(N03)2) were purchased from POCH in Gliwice (Poland). Cationic surfactant cetyltrimethylammonium bromide (Aldrich) was used in the n-heptane (Loba-chemie), emulsion preparation. Diagnostic liquids used for the contact angle measurements were diiodomethan (Sigma), 1bromonaphthalene (Riedel-de Haen), formamide (Sigma), hexadecan (Riedel-de Haen) and water. Water was doubly distilled and deionized in a Milipore purification system. The pH was modified using NaOH and HCl (0.1 M solution). All tests were carried out using double destilled water. Sodium dodecyl sulphate (SDS) anionic surfactant, was obtained from Sigma Chemicals.
3. Methods
3.1. Adsorption of surfactant The adsorption of sodium oleate was measured according to the method described by Fowler and Steel [7]. Pure hematite (1 g) was added to 100 cc of sodium oleate solution. The initial concentration of sodium oleate was changed from 5 ×10 − 5 to 2 ×10 − 3 M. The suspension was stirred vigorously using a mechanical shaker for 8 h to obtain equilibrium of adsorption, then centrifuged to separate the solid. Clear supernatants were used for the sodium oleate determination. The residual concentration of sodium oleate in the supernatant was determined by application of a colorimetric method [7].
3.3. Electrophoretic mobility The electrophoretic mobility of surfactant salt particles was determined for water suspension using Zeta Plus Apparatus (Brookhaven Instr. Corp., USA). Mobility was measured at room temperature and averages were taken over three independent sets of measurements. The value of pH for the surfactant salt suspension was measured with a Precision Digital pH-meter OP-208/1 (Hungary). Suspension pH was adjusted by addition of 0.1 M HCl or NaOH solution. Surfactant salt suspension was prepared by mixing of 2.0 g of sodium oleate with subsequent quantity of 0.5 M solution of CaCl2, FeCl3, Ba(NO3)2, respectively. The oil–water (n-heptane–water) emulsions were prepared by dissolving the surfactant (SDS or CTAB) in water and stirring this in an ultrasonic mixer while slowly adding the oil (n-heptane). Mixing was continued for a short time (10–15 s) to produce an emulsion.
3.4. Contact angle measurements The contact angle measurements were conducted using a homebuilt goniometer. The droplet of probing liquid was placed on the surface of special prepared surfactant salt pellets. The pellets were obtained by compressing KBr powder (first step) so that the suitable surfactant salt was compressed to the KBr disc. Pellets were kept in a
Z. Sadowski / Colloids and Surfaces A: Physicochem. Eng. Aspects 173 (2000) 211–217
desiccatior before making the contact angle measurements. The results reported here are the average of five independent measurements.
Fig. 1. Adsorption isotherms of sodium oleate onto hematite.
Fig. 2. Spherical agglomeration of hematite as a function of the sodium oleate concentration.
213
4. Results and discussion Spherical oil agglomeration can be realized when the surface of interacting particles is hydrophobic [8]. The adsorption density in micromoles of sodium oleate adsorbed per gram of hematite at pH 2.5, 7.5 and 10.5 are presented in Fig. 1. As can be seen the highest adsorption coincided with an acidic pH range. And the shape of this isotherm shows that the precipitation of surfactant salt (ferric oleate) occurs at the high concentration levels. Similar results of the adsorption of sodium oleate for calcite, magnesite and barite have been reported earlier [9–11]. Adsorption results confirmed the finding of Pascoe and Dohery [12] that shear-flocculation of hematite using sodium oleate as a selective flocculant reagent was favored under both neutral and acid conditions. Fig. 2 represents the recovery of spherical agglomeration of hematite a function of the initial sodium oleate concentration at a constant pH range (pH 7.5). Also, the efrect of an addition of surfactant salts on the spherical agglomeration of hematite is presented. The recovery curves have a characteristic shape; a dramatic increase of recovery corresponds to greater spherical agglomeration and maximum recovery is realized at the critical surfactant concentration (csc). The value of a critical surface concentration depends on a quantity of precipitated surfactant salt which is added to the hematite suspension. It can be hypothesized that an attractive interaction between surfactant salt particles and the hematite particles contributes to the spherical agglomeration process. This contribution will be discussed later on the basis of zeta potential results. In the case of oil agglomeration of both coal and pyrite particles [13], the oil droplets adhere to the solid particles and collect these particles together into an aggregate. It is interesting to note that electrokinetic–pH characteristics of coal and oil droplets are very similar. Such similarity occurs for materials with carboxylic groups at the interface [14]. The results of electrophoretic mobility of n-heptane–water emulsion, as a function of pH is presented in Fig. 3. The effect of an
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Z. Sadowski / Colloids and Surfaces A: Physicochem. Eng. Aspects 173 (2000) 211–217
Fig. 3. The effect of pH and surfactants on the zeta potential of n-heptane.
(CTAB) surfactant on the oil–water interface causes the oil droplets to have a positive zpotential. An increase in cationic surfactant addition to the oil–water emulsion, has an interesting effect on the spherical agglomeration of hematite suspension. Degree of agglomeration is shown in Fig. 4 as a function of the concentration of cationic surfactant added to the oil–water emulsion. As the concentration of CTAB (cationic surfactant) was raised, the recovery of agglomerates increased. However, beyond a certain cationic surfactant concentration, agglomeration decreased. This suggests that an excess of cationic surfactant adsorbed at the hematite particles causes a strong repulsion between oil droplets and mineral surface. However, this hypothesis needs more experimental work. Another factor which may be contributing to agglomeration may be the adhesion of precipitated surfactant salt. Zeta potentials of surfactant salt particles determined by microelectrophoresis are plotted as a function of pH in Fig. 5. An
Fig. 4. The effect of cationic surfactant (CTAB) on the spherical agglomeration.
addition of both cationic (CTAB) and anionic surfactants is also shown in this Figure. It can be seen from Fig. 3 that the adsorption of cationic
Fig. 5. The effect of pH on the zeta potential of surfactant salts.
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215
Table 1 Contact angles for fatty acid soaps Fatty acid soap
Ca(OL)2 Ba(OL)2 Fe(OL)3
Liquid Water
Formamide
Diiodomethane
a-Bromonaphthalene
26° 48% 14° 30% 6° 26%
30° 18% 27° 41% 11° 15%
45° 38% 48° 22% 39° 40%
41° 18% 40° 20% 24° 18%
Table 2 Characteristics of used liquids at 20°C Liquid
gL (mJ m−2)
g LW (mJ m−2)
g− (mJ m−2)
g+ (mJ m−2)
Water Formamide Diiodomethane a-Bromonaphthalene
72.8 58.0 50.8 44.4
21.8 39.0 50.8 43.5
25.5 2.28 – –
25.5 39.6 – –
isoelectric point (iep) of precipitated surfactant salts is extrapolated near pH 3. According to Laskowski [15], the isoelectric points for fatty acids are around pH 3. Subsequently, it was found that the electromobility of surfactant salt particles changed in the following order NaOL\ Ca(OL)2 \Ba(OL)2 \Fe(OL)3. The negative surface charge on precipitated fatty acid soaps originates from the heterocoagulation of precipitated soap particles with the oleate ions. The experimental data obtained by direct contact angle measurements on a flat surface of fatty acid soaps are shown in Table 1. These contact angles are used to calculate the surface free energy components. The surface tension and its components of the liquid probes adapted to the contact angle measurement are listed in Table 2. The surface tension of the solid surface (gs) can be expressed as a sum of surface tension components which due to a particular type of molecular interaction. These components were divided into two main categories: dispersive (Lifshitz –van der Waals) and non-dispersive (polar). The OCG ( Oss – Chaudhury – Good) equation provides a simple means to characterize a solid surface by using contact angle measurement [16,17]. The equation OCG was derived by combining Young’s equation with Fowkes’ equation:
− 1/2 (l+ cos U)gL = 2(g dS g dL)1/2 + (g+ S gL ) + 1/2 + (g − S gL )
where U and gL are the contact angle and the surface tension of the probing liquid, respectively. The subscript ‘S’ represent the surfactant salt. The superscripts ‘d’, ‘+ ’ and ‘−’ indicate the dispersion, acid and base components of the surface free energy, respectively. The OCG equation was used for the surface free energy components calculation. Table 3 shows the surface free energy components of fatty acid soaps. The results presented in Table 3 demonstrated that the Lifshitz–van der Waals components are almost identical for all fatty acid soaps. By contrast, the Lewis base components are significant great. These great affinity for a base can be reflected the linking of oleate ions to the fatty acid soap particles [18]. Table 3 Value of surface free energy components of fatty acid soaps Fatty acid soap
g LW s (mJ m−2)
g− s (mJ m−2)
g+ s (mJ m−2)
Ca(OL)2 Ba(OL)2 Fe(OL)3
35.56 35.18 35.92
124.19 138.26 135.68
31.57 32.92 40.14
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During the spherical agglomeration process four types of forces are important: van der Waals attraction, electrostatic repulsion, hydrophobic interaction and hydrodynamic force. Aggregation of fine hydrophobic particles, in a shear field was developed as shear-flocculation by Warren [19]. In the presence of electrostatic repulsion between negatively charged particles (hematite and surfactant salt), high shear rates can increase the chance of colliding particles overcoming a potential energy barrier. The hydrophobic attraction forces should be responsible for the surfactant salts adhesion to the hematite surface. Direct experiments have confirmed that precipitated surfactant forms created adsorbed patches or islands on the mineral surface rather than a uniform layer [18,20]. The fact that increasing hydrophobicity makes the negative values of the zeta potential more negative is known from the literature [21]. However, the more hydrophobic and negatively charged the hematite surface, the more extensively it should interact with the positively charged n-heptane droplets. Thus, we can observe a spherical agglomeration. In addition, we can assume that the surfactant salts act much like an additional bridging agent. The spherical shape of aggregates has been preserved after the oil (n-heptane) vaporization.
5. Summary The results presented here show that the spherical agglomeration of fine hematite particles in water suspension is highly dependent on the degree of surface hydrophobicity. The interaction between the hydrophobic surface of hematite and precipitated surfactant salts should be hydrophobic in character because both surfactant salts and hematite particles have a negative value of z-potential. The hypothesis of strong hydrophobic interaction between oil (n-heptane) droplets and both surfactant salts and hematite particles was explained. Furthermore, the addition of oil droplets with cationic emulsifier reagent (CTAB) neutralizes a strong electrostatic repulsion.
Acknowledgements The author wishes to thank Prof. Wojcik and Prof. Chibowski from the Department of Physical Chemistry of Maria Curie-Sklodowska University for a discussion. 1 am grateful to Mrs B. Dzikowska for her assistance in determining the z-potential.
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