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Determination of the critical surface tension of wetting of minerals treated with surfactants by shear flocculation approach
Journal of Colloid and Interface Science 277 (2004) 437–442 www.elsevier.com/locate/jcis
Determination of the critical surface tension of wetting of minerals treated with surfactants by shear flocculation approach A. Ozkan ∗ Department of Mining Engineering, Selcuk University, 42031 Konya, Turkey Received 20 February 2004; accepted 23 April 2004 Available online 2 June 2004
Abstract This paper contributes the shear flocculation method as a new approach to determine the critical surface tension of wetting of minerals treated with surfactants. This newly developed approach is based on the decrease of the shear flocculation of the mineral suspension, with decreasing of the surface tension of the liquids used. The solution surface tension value at which shear flocculation does not occur can be defined as the critical surface tension of wetting (γ c ) of the mineral. By using the shear flocculation method, the critical surface tensions of wetting (γ c ) for calcite and barite minerals, treated with surfactants, were obtained as 30.9 and 35.0 mN/m, respectively. These values are in good agreement with data reported previously on the same minerals obtained by the contact angle measurement and flotation methods. The chemical agents used for the treatment of calcite and barite particles were sodium oleate and sodium dodecyl sulfate, respectively. 2004 Elsevier Inc. All rights reserved. Keywords: Critical surface tension of wetting; Shear flocculation; Contact angle; Flotation; Calcite; Barite
1. Introduction Shear flocculation is the aggregation of fine particles in a convenient stirring regime after hydrophobization by the adsorption of surfactants [1]. The shear flocculation effect is normally observed for fine particles suspended in an aqueous solution in the presence of a surfactant by applying a shear field of sufficient magnitude. To provide hydrophobization of particle surfaces, surfactants known as flotation collectors are often used. When two particles, made hydrophobic by adsorption of a surfactant, collide and adhere, part of the interface between the hydrocarbon chains and the aqueous solution will disappear to be replaced by an area of contact between hydrocarbon chains, thereby reducing the surface energy of the system. This is achieved by throwing particles toward each other with a force exceeding the repulsive energy barrier [2]. Fine particles produced during the grinding process in mineral processing operations decrease the efficiency of the concentration processes. Slimes or ultrafines are difficult to * Fax: +90-332-2410635.
E-mail address: [email protected]. 0021-9797/$ – see front matter 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2004.04.038
separate [3]. The slimes fraction often contains significant quantities of valuable minerals. Therefore, slimes represent a potential resource if a process can be developed to economically recover the valuable minerals [4]. The recovery of the mineral particles by froth flotation decreases dramatically with decrease in particle size [5]. One way to recover valuable minerals from slimes is to increase their size by selective flocculation and then to float the flocs. Also selective flocculation or selective shear flocculation is used to separate valuable minerals from fine particle mixtures, with the aggregation of the desired mineral [1]. Similarly to the flotation process, the shear flocculation technique also utilizes differences in wettability of minerals. Wettability characteristics of mineral surfaces can be defined in terms of their values of critical surface tension of wetting (γc ), which is an essential property to achieve selectivity in surface-chemistry-based processes. This paper introduces a new approach, which is shear flocculation, for determining the critical surface tensions of wetting (γc ) of minerals treated with surfactants. The aim of this work is to support the development of selective shear flocculation by determining the critical surface tension of the flocculation medium in connection with the minerals considered.
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A. Ozkan / Journal of Colloid and Interface Science 277 (2004) 437–442
2. Theoretical background
3. Experimental method 3.1. Materials
The wettability of solid or mineral particles is known to be an important parameter which affects many technological processes such as froth flotation, oil agglomeration, solid– liquid separation, and dust abatement [6]. The wettability properties of solids or minerals are assessed quantitatively by a number of experimental and empirical techniques. One of these quantifying parameters mentioned above is the critical surface tension of wetting (γc ) to achieve selectivity in surface chemistry processes [7]. The γc parameter is determined by contact angle measurement, flotation, immersion time, bubble pickup, film flotation, automatic wetting balance, and modified Hildebrand–Scott equation methods [8–14]. The Zisman contact angle measurement technique and the flotation method are the two major techniques used to determine the (γc ) values of minerals among these experimental and empirical techniques [15–18]. In the Zisman contact angle method [8], cosines of measured contact angles are plotted against solution surface tension (γLV) and the intercept of this line at the x-axis (for cos θ = 1 or θ = 0) is γc . As can be seen from Fig. 1, the liquid spreads on the solid (mineral) at γc γLV , the liquid forms a contact angle (θ > 0) at γc < γLV . The flotation method [9] estimates the γc value of any mineral by plotting % flotation recovery (FR%) versus solution surface tension (γLV ) with the extrapolation of the linear part of the FR–γLV curve to the surface tension axis in order to obtain an intercept at %FR = 0 as given in Fig. 2.
Pure calcite and barite minerals were used in this study. The chemical analysis results of these mineral samples are given in Table 1. The samples were dry ground to below a 38-µm (i.e., −38 µm) size fraction by a ceramic ball mill. Sodium oleate for calcite and sodium dodecyl sulfate for barite were used as surfactants in the shear flocculation tests. Sodium oleate (C17 H33 COONa) was prepared from oleic acid (C17 H33 COOH) (Carlo Erba) and sodium hydroxide (Merck). Sodium dodecyl sulfate (C12 H25 SO4 Na) was purchased from Merck. All of these chemicals were analytical grade. 3.2. Wettability tests 3.2.1. Solutions Methanol–water solutions of different concentrations (w/w%) were prepared and employed in order to obtain wettability data for the construction of the shear flocculation recovery (SFR%) versus γLV plot. The surface tensions of methanol solutions were determined by the drop-weight measurement method [19]. The measured values of the surface tensions of the methanol solutions are shown in Fig. 3,
Fig. 2. Schematic representation of flotation method for γc determination [9]. Table 1 The chemical composition of the material samples used in the experiments (values in %)
Fig. 1. Schematic representation of contact angle measurement method for γc determination [8].
Calcite
CaCO3 99.14
SiO2 0.30
MgO 0.31
R2 O3 0.25
Barite
BaSO4 97.63
SiO2 0.78
CaCO3 0.12
SrSO4 1.29
Fe2 O3 0.14
Al2 O3 0.04
A. Ozkan / Journal of Colloid and Interface Science 277 (2004) 437–442
439
Fig. 3. Variation of the surface tensions of methanol solutions with their concentrations.
with a comparison of the experimental data to the literature [20]. The solutions used in all experiments were prepared with distilled water. The methanol (>99.9% in purity) was purchased from Merck and the solutions were maintained at 18 ± 2 ◦ C.
Fig. 4. The γc values of calcite treated with sodium oleate by the contact angle measurement and flotation methods [21].
3.2.2. Shear flocculation tests The shear flocculation tests were carried out in a 300 cm3 cylindrical cell with four baffles using 1.5 g solid and 300 cm3 methanol solution. The mineral suspension was conditioned for 3 min at an impeller speed of 500 rpm. After a settling time of 1 min, the supernatant was siphoned off, at a fixed distance of 5 cm below the air–liquid interface, by a special system. The settled fraction was filtered, dried, and weighed in order to determine the % shear flocculation recovery (SFR) [(% shear flocculation recovery = settled weight/feed) × 100)]. The concentration values of sodium oleate and sodium dodecyl sulfate used for calcite and barite minerals in the shear flocculation tests was 100 mg/dm3.
4. Results and discussion The critical surface tension of wetting (γc ) values of calcite and barite minerals treated with the same surfactants were determined using contact angle measurement and the flotation methods of Ozkan and Yekeler [21]. As shown in Figs. 4 and 5, the γc values of calcite and barite in that study were found by the contact-angle approach to be 30.5 and 31.2 mN/m, respectively. The γc values of these minerals were also determined as 31.4 and 34.5 mN/m by the flotation method. These minerals have a hydrophilic character whose γc > 72 mN/m, therefore the water spreads completely on the mineral surfaces when no chemical treatment is applied to the mineral surfaces. Figs. 6 and 7 show the effect of solution surface tension on the shear flocculation of calcite and barite suspensions with surfactant, respectively. As clearly seen from Figs. 6 and 7, the shear flocculation recoveries for both of the minerals decreased with decreasing solution surface tension. That
Fig. 5. The γc values of barite treated with sodium dodecyl sulfate by the contact angle measurement and flotation methods [21].
is, as the surface tension of the solution used for shear flocculation medium decreases, the wettability of particle surfaces treated with the surfactant increases. As compared in Figs. 4 and 5, it can be concluded from Figs. 6 and 7 that the shear flocculation recovery is directly related to the contact angle (θ ) and the flotation recovery, except for the flotation recovery values obtained in water. This result indicates the importance of surface hydrophobization in the extent of shear flocculation of a mineral suspension. The maximum flotation recovery values obtained at a surface tension value
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Fig. 6. Effect of the solution surface tension on the shear flocculation of calcite suspension with sodium oleate.
Fig. 7. Effect of the solution surface tension on the shear flocculation of barite suspension with sodium dodecyl sulfate.
of approximately 57 mN/m were due to the frother effect of methanol. This surface tension value corresponded to a low concentration (∼10%) of methanol. 4.1. Determination of the γc value of minerals by the shear flocculation method The shear flocculation recovery values obtained without surfactants were due to sedimentation process governed by gravitational, hydrodynamic, and inertial effects [22] and
Fig. 8. The γc value of calcite treated with sodium oleate by the shear flocculation method.
these recovery values increased at high concentrations of methanol (see Figs. 6 and 7). Essentially, the shear flocculation of the suspension did not occur at solution surface tension values below the γc values determined by contact angle measurement and flotation methods. This was also partially observed during the experiments. However, the observation of mineral suspensions to determine the solution surface tension value at which shear flocculation does not occur is very difficult when very small flocs occur in the suspension with decreasing surface tension. As seen from Figs. 6 and 7, at solution surface tension values below the γc values of these minerals, the curves obtained without surfactant follow approximately the same trend as curve obtained with surfactant. This evidence affirms that the shear flocculation of suspension did not take place at surface tensions below the γc value of mineral. Therefore, the effect of sedimentation must be eliminated for correct determination of the γc value of mineral. Thereby, the solution surface tension value at which effective shear flocculation does not occur can be determined accurately, and this value of surface tension at which the liquid wets the surface completely by spreading can also be defined as the critical surface tension of wetting (γc ) value of the mineral. For this reason, effective shear flocculation recovery (%ESFR) values were calculated for each solution surface tension value as follows: % effective shear-flocculation recovery (%ESFR) = shear flocculation recovery (%SFR) − sedimentation recovery (without surfactant). The effective shear flocculation recovery (%ESFR) versus solution surface tension (γLV ) curves obtained for chemically treated calcite and barite minerals are given in Figs. 8 and 9, respectively. The γc values of minerals can be determined as shown in Figs. 8 and 9, with the extrapolation of the linear part of the %ESFR − γLV curve to
A. Ozkan / Journal of Colloid and Interface Science 277 (2004) 437–442
Fig. 9. The γc value of the barite treated with sodium dodecyl sulfate by the shear flocculation method.
Table 2 Comparison of the γc values of calcite and barite obtained from the contact angle measurement, flotation, and shear flocculation methods Method Contact angle measurement Flotation Shear flocculation
γc values (mN/m) Calcitea
Bariteb
30.5 31.4 30.9
31.2 34.5 35.0
441
Fig. 10. Schematic representation of the shear flocculation method for γc determination.
conditions are dynamic. Therefore, these differences are in the acceptable range [16,23]. Consequently, the shear flocculation method for determination of the critical surface tension of wetting (γc ) of minerals treated with surfactants can be presented as shown in Fig. 10. As easily seen in Fig. 10, while effective shear flocculation of the mineral suspension occurs at γc < γLV , no effective shear flocculation takes place at γc γLV .
a Treated with sodium oleate. b Treated with sodium dodecyl sulfate.
5. Conclusions the solution surface tension axis in order to obtain intercept at %ESFR = 0, as similar to the flotation method. Consequently, the critical surface tension of wetting (γc ) values for calcite and barite minerals treated with surfactants were determined as 30.9 and 35.0 mN/m, respectively. A comparison of the γc values of calcite and barite treated with surfactants obtained from the shear flocculation tests with values reported previously in the literature [21] for the same minerals treated also with the same surfactants obtained from contact angle measurement and flotation methods is given in Table 2. As can be seen, the γc values obtained by the shear flocculation tests are very close to the values obtained by the contact angle measurement technique and especially by the flotation method, indicating that the new approach performs well. The differences between the γc values determined by flotation and shear flocculation are smaller with respect to those that were found by contact angle measurement method. These differences arise from the nature of the method used; contact angle measurement is based on the static conditions as a sessile drop is measured on a pelleted surface, while flotation and shear flocculation
The shear flocculation of the mineral suspension decreased in tandem with the decrease of the surface tension of the liquids used. By using this property, the critical surface tension of wetting value of a mineral treated with surfactant can be determined by the shear flocculation method. The shear flocculation of a mineral suspension does not occur below that value of the critical surface tension. The values of the critical surface tension of wetting (γc ) for chemically treated surfaces of calcite and barite minerals were obtained as 30.9 and 35.0 mN/m by this new approach, respectively. These γc values are in good agreement with those obtained with other techniques, namely, contact angle measurement and flotation. Accordingly, it was concluded that the shear flocculation can be used for determination of the critical surface tension of wetting (γc ) of minerals coated with surfactants. Since most minerals are hydrophilic and require surfactant adsorption for their flotation and shear flocculation, the shear flocculation method, introduced in this study, can be easily used to determine the γc value of any mineral when a treatment with surfactants is applied to the mineral suspensions.
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References [1] L.J. Warren, J. Colloid Interface Sci. 50 (1975) 307. [2] L.J. Warren, in: J.S. Laskowski, J. Ralston (Eds.), Colloid Chemistry in Mineral Processing, Elsevier, New York, 1992, p. 309, ch. 10. [3] R. Sivamohan, Int. J. Miner. Process. 28 (1990) 247. [4] D.P. Patil, J.R.G. Andrews, P.H.T. Uhlherr, Int. J. Miner. Process. 61 (2001) 171. [5] W.J. Trahar, L.J. Warren, Int. J. Miner. Process. 3 (1976) 103. [6] D.W. Fuerstenau, J. Diago, M.C. Williams, Colloids Surf. A Physicochem. Eng. Asp. 60 (1991) 127. [7] M. Yekeler, B. Yarar, in: SME Annual Meeting, Albuquerque, New Mexico, 1994, Preprint 94-17 . [8] E.G. Shafrin, W.A. Zisman, J. Phys. Chem. 64 (1960) 519. [9] B. Yarar, J. Kaoma, Colloids Surf. A Physicochem. Eng. Asp. 11 (1984) 429. [10] S. Garshva, Contreras, J. Goldfarb, Colloid Polym. Sci. 256 (1978) 241. [11] S.C. Sun, R.C. Troxell, Eng. Min. J. 158 (1957) 79. [12] M.C. Williams, D.W. Fuerstenau, Int. J. Miner. Process. 20 (1987) 153.
[13] H.L. Rosano, W. Gerbacia, M.E. Feinstein, J.W. Swaine, J. Colloid Interface Sci. 36 (1971) 298. [14] S. Wu, J. Phys. Chem. 72 (1968) 3332. [15] S. Kelebek, J. Colloid Interface Sci. 124 (1987) 504. [16] B. Yarar, in: S.H. Castro, J. Alvarez (Eds.), Developments in Mineral Processing, Elsevier, New York, 1988, p. 41. [17] B.K. Parekh, F.F. Aplan, in: N.N. Li (Ed.), Recent Development in Separation Science, vol. 4, CRC Press, West Palm Beach, FL, 1978, p. 107. [18] M. Yekeler, B. Yarar, in: M. Anil (Ed.), Cukurova University 15th Anniversary Symposium, Cukurova University, Adana, Turkey, 1994, p. 473. [19] J.F. Padday, in: E. Matijevic (Ed.), Surface and Colloidal Science, vol. 1, Wiley–Interscience, New York, 1968, p. 101. [20] R.C. Weast, Handbook of Chemistry and Physics, 68th ed., CRS Press, New York, 1987. [21] A. Ozkan, M. Yekeler, J. Colloid Interface Sci. 261 (2003) 476. [22] L.J. Warren, in: M.H. Jones, J.T. Woodcock (Eds.), Wark Symposium, Australas. Inst. Min. Metall., Parkville, Australia, 1984, p. 185. [23] J.S. Laskowski, in: P. Somasundaran (Ed.), Advances in Mineral Processing, SME, Littleton, CO, 1986, p. 189.
Determination of the critical surface tension of wetting of minerals treated with surfactants by shear flocculation approach A. Ozkan ∗ Department of Mining Engineering, Selcuk University, 42031 Konya, Turkey Received 20 February 2004; accepted 23 April 2004 Available online 2 June 2004
Abstract This paper contributes the shear flocculation method as a new approach to determine the critical surface tension of wetting of minerals treated with surfactants. This newly developed approach is based on the decrease of the shear flocculation of the mineral suspension, with decreasing of the surface tension of the liquids used. The solution surface tension value at which shear flocculation does not occur can be defined as the critical surface tension of wetting (γ c ) of the mineral. By using the shear flocculation method, the critical surface tensions of wetting (γ c ) for calcite and barite minerals, treated with surfactants, were obtained as 30.9 and 35.0 mN/m, respectively. These values are in good agreement with data reported previously on the same minerals obtained by the contact angle measurement and flotation methods. The chemical agents used for the treatment of calcite and barite particles were sodium oleate and sodium dodecyl sulfate, respectively. 2004 Elsevier Inc. All rights reserved. Keywords: Critical surface tension of wetting; Shear flocculation; Contact angle; Flotation; Calcite; Barite
1. Introduction Shear flocculation is the aggregation of fine particles in a convenient stirring regime after hydrophobization by the adsorption of surfactants [1]. The shear flocculation effect is normally observed for fine particles suspended in an aqueous solution in the presence of a surfactant by applying a shear field of sufficient magnitude. To provide hydrophobization of particle surfaces, surfactants known as flotation collectors are often used. When two particles, made hydrophobic by adsorption of a surfactant, collide and adhere, part of the interface between the hydrocarbon chains and the aqueous solution will disappear to be replaced by an area of contact between hydrocarbon chains, thereby reducing the surface energy of the system. This is achieved by throwing particles toward each other with a force exceeding the repulsive energy barrier [2]. Fine particles produced during the grinding process in mineral processing operations decrease the efficiency of the concentration processes. Slimes or ultrafines are difficult to * Fax: +90-332-2410635.
E-mail address: [email protected]. 0021-9797/$ – see front matter 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2004.04.038
separate [3]. The slimes fraction often contains significant quantities of valuable minerals. Therefore, slimes represent a potential resource if a process can be developed to economically recover the valuable minerals [4]. The recovery of the mineral particles by froth flotation decreases dramatically with decrease in particle size [5]. One way to recover valuable minerals from slimes is to increase their size by selective flocculation and then to float the flocs. Also selective flocculation or selective shear flocculation is used to separate valuable minerals from fine particle mixtures, with the aggregation of the desired mineral [1]. Similarly to the flotation process, the shear flocculation technique also utilizes differences in wettability of minerals. Wettability characteristics of mineral surfaces can be defined in terms of their values of critical surface tension of wetting (γc ), which is an essential property to achieve selectivity in surface-chemistry-based processes. This paper introduces a new approach, which is shear flocculation, for determining the critical surface tensions of wetting (γc ) of minerals treated with surfactants. The aim of this work is to support the development of selective shear flocculation by determining the critical surface tension of the flocculation medium in connection with the minerals considered.
438
A. Ozkan / Journal of Colloid and Interface Science 277 (2004) 437–442
2. Theoretical background
3. Experimental method 3.1. Materials
The wettability of solid or mineral particles is known to be an important parameter which affects many technological processes such as froth flotation, oil agglomeration, solid– liquid separation, and dust abatement [6]. The wettability properties of solids or minerals are assessed quantitatively by a number of experimental and empirical techniques. One of these quantifying parameters mentioned above is the critical surface tension of wetting (γc ) to achieve selectivity in surface chemistry processes [7]. The γc parameter is determined by contact angle measurement, flotation, immersion time, bubble pickup, film flotation, automatic wetting balance, and modified Hildebrand–Scott equation methods [8–14]. The Zisman contact angle measurement technique and the flotation method are the two major techniques used to determine the (γc ) values of minerals among these experimental and empirical techniques [15–18]. In the Zisman contact angle method [8], cosines of measured contact angles are plotted against solution surface tension (γLV) and the intercept of this line at the x-axis (for cos θ = 1 or θ = 0) is γc . As can be seen from Fig. 1, the liquid spreads on the solid (mineral) at γc γLV , the liquid forms a contact angle (θ > 0) at γc < γLV . The flotation method [9] estimates the γc value of any mineral by plotting % flotation recovery (FR%) versus solution surface tension (γLV ) with the extrapolation of the linear part of the FR–γLV curve to the surface tension axis in order to obtain an intercept at %FR = 0 as given in Fig. 2.
Pure calcite and barite minerals were used in this study. The chemical analysis results of these mineral samples are given in Table 1. The samples were dry ground to below a 38-µm (i.e., −38 µm) size fraction by a ceramic ball mill. Sodium oleate for calcite and sodium dodecyl sulfate for barite were used as surfactants in the shear flocculation tests. Sodium oleate (C17 H33 COONa) was prepared from oleic acid (C17 H33 COOH) (Carlo Erba) and sodium hydroxide (Merck). Sodium dodecyl sulfate (C12 H25 SO4 Na) was purchased from Merck. All of these chemicals were analytical grade. 3.2. Wettability tests 3.2.1. Solutions Methanol–water solutions of different concentrations (w/w%) were prepared and employed in order to obtain wettability data for the construction of the shear flocculation recovery (SFR%) versus γLV plot. The surface tensions of methanol solutions were determined by the drop-weight measurement method [19]. The measured values of the surface tensions of the methanol solutions are shown in Fig. 3,
Fig. 2. Schematic representation of flotation method for γc determination [9]. Table 1 The chemical composition of the material samples used in the experiments (values in %)
Fig. 1. Schematic representation of contact angle measurement method for γc determination [8].
Calcite
CaCO3 99.14
SiO2 0.30
MgO 0.31
R2 O3 0.25
Barite
BaSO4 97.63
SiO2 0.78
CaCO3 0.12
SrSO4 1.29
Fe2 O3 0.14
Al2 O3 0.04
A. Ozkan / Journal of Colloid and Interface Science 277 (2004) 437–442
439
Fig. 3. Variation of the surface tensions of methanol solutions with their concentrations.
with a comparison of the experimental data to the literature [20]. The solutions used in all experiments were prepared with distilled water. The methanol (>99.9% in purity) was purchased from Merck and the solutions were maintained at 18 ± 2 ◦ C.
Fig. 4. The γc values of calcite treated with sodium oleate by the contact angle measurement and flotation methods [21].
3.2.2. Shear flocculation tests The shear flocculation tests were carried out in a 300 cm3 cylindrical cell with four baffles using 1.5 g solid and 300 cm3 methanol solution. The mineral suspension was conditioned for 3 min at an impeller speed of 500 rpm. After a settling time of 1 min, the supernatant was siphoned off, at a fixed distance of 5 cm below the air–liquid interface, by a special system. The settled fraction was filtered, dried, and weighed in order to determine the % shear flocculation recovery (SFR) [(% shear flocculation recovery = settled weight/feed) × 100)]. The concentration values of sodium oleate and sodium dodecyl sulfate used for calcite and barite minerals in the shear flocculation tests was 100 mg/dm3.
4. Results and discussion The critical surface tension of wetting (γc ) values of calcite and barite minerals treated with the same surfactants were determined using contact angle measurement and the flotation methods of Ozkan and Yekeler [21]. As shown in Figs. 4 and 5, the γc values of calcite and barite in that study were found by the contact-angle approach to be 30.5 and 31.2 mN/m, respectively. The γc values of these minerals were also determined as 31.4 and 34.5 mN/m by the flotation method. These minerals have a hydrophilic character whose γc > 72 mN/m, therefore the water spreads completely on the mineral surfaces when no chemical treatment is applied to the mineral surfaces. Figs. 6 and 7 show the effect of solution surface tension on the shear flocculation of calcite and barite suspensions with surfactant, respectively. As clearly seen from Figs. 6 and 7, the shear flocculation recoveries for both of the minerals decreased with decreasing solution surface tension. That
Fig. 5. The γc values of barite treated with sodium dodecyl sulfate by the contact angle measurement and flotation methods [21].
is, as the surface tension of the solution used for shear flocculation medium decreases, the wettability of particle surfaces treated with the surfactant increases. As compared in Figs. 4 and 5, it can be concluded from Figs. 6 and 7 that the shear flocculation recovery is directly related to the contact angle (θ ) and the flotation recovery, except for the flotation recovery values obtained in water. This result indicates the importance of surface hydrophobization in the extent of shear flocculation of a mineral suspension. The maximum flotation recovery values obtained at a surface tension value
440
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Fig. 6. Effect of the solution surface tension on the shear flocculation of calcite suspension with sodium oleate.
Fig. 7. Effect of the solution surface tension on the shear flocculation of barite suspension with sodium dodecyl sulfate.
of approximately 57 mN/m were due to the frother effect of methanol. This surface tension value corresponded to a low concentration (∼10%) of methanol. 4.1. Determination of the γc value of minerals by the shear flocculation method The shear flocculation recovery values obtained without surfactants were due to sedimentation process governed by gravitational, hydrodynamic, and inertial effects [22] and
Fig. 8. The γc value of calcite treated with sodium oleate by the shear flocculation method.
these recovery values increased at high concentrations of methanol (see Figs. 6 and 7). Essentially, the shear flocculation of the suspension did not occur at solution surface tension values below the γc values determined by contact angle measurement and flotation methods. This was also partially observed during the experiments. However, the observation of mineral suspensions to determine the solution surface tension value at which shear flocculation does not occur is very difficult when very small flocs occur in the suspension with decreasing surface tension. As seen from Figs. 6 and 7, at solution surface tension values below the γc values of these minerals, the curves obtained without surfactant follow approximately the same trend as curve obtained with surfactant. This evidence affirms that the shear flocculation of suspension did not take place at surface tensions below the γc value of mineral. Therefore, the effect of sedimentation must be eliminated for correct determination of the γc value of mineral. Thereby, the solution surface tension value at which effective shear flocculation does not occur can be determined accurately, and this value of surface tension at which the liquid wets the surface completely by spreading can also be defined as the critical surface tension of wetting (γc ) value of the mineral. For this reason, effective shear flocculation recovery (%ESFR) values were calculated for each solution surface tension value as follows: % effective shear-flocculation recovery (%ESFR) = shear flocculation recovery (%SFR) − sedimentation recovery (without surfactant). The effective shear flocculation recovery (%ESFR) versus solution surface tension (γLV ) curves obtained for chemically treated calcite and barite minerals are given in Figs. 8 and 9, respectively. The γc values of minerals can be determined as shown in Figs. 8 and 9, with the extrapolation of the linear part of the %ESFR − γLV curve to
A. Ozkan / Journal of Colloid and Interface Science 277 (2004) 437–442
Fig. 9. The γc value of the barite treated with sodium dodecyl sulfate by the shear flocculation method.
Table 2 Comparison of the γc values of calcite and barite obtained from the contact angle measurement, flotation, and shear flocculation methods Method Contact angle measurement Flotation Shear flocculation
γc values (mN/m) Calcitea
Bariteb
30.5 31.4 30.9
31.2 34.5 35.0
441
Fig. 10. Schematic representation of the shear flocculation method for γc determination.
conditions are dynamic. Therefore, these differences are in the acceptable range [16,23]. Consequently, the shear flocculation method for determination of the critical surface tension of wetting (γc ) of minerals treated with surfactants can be presented as shown in Fig. 10. As easily seen in Fig. 10, while effective shear flocculation of the mineral suspension occurs at γc < γLV , no effective shear flocculation takes place at γc γLV .
a Treated with sodium oleate. b Treated with sodium dodecyl sulfate.
5. Conclusions the solution surface tension axis in order to obtain intercept at %ESFR = 0, as similar to the flotation method. Consequently, the critical surface tension of wetting (γc ) values for calcite and barite minerals treated with surfactants were determined as 30.9 and 35.0 mN/m, respectively. A comparison of the γc values of calcite and barite treated with surfactants obtained from the shear flocculation tests with values reported previously in the literature [21] for the same minerals treated also with the same surfactants obtained from contact angle measurement and flotation methods is given in Table 2. As can be seen, the γc values obtained by the shear flocculation tests are very close to the values obtained by the contact angle measurement technique and especially by the flotation method, indicating that the new approach performs well. The differences between the γc values determined by flotation and shear flocculation are smaller with respect to those that were found by contact angle measurement method. These differences arise from the nature of the method used; contact angle measurement is based on the static conditions as a sessile drop is measured on a pelleted surface, while flotation and shear flocculation
The shear flocculation of the mineral suspension decreased in tandem with the decrease of the surface tension of the liquids used. By using this property, the critical surface tension of wetting value of a mineral treated with surfactant can be determined by the shear flocculation method. The shear flocculation of a mineral suspension does not occur below that value of the critical surface tension. The values of the critical surface tension of wetting (γc ) for chemically treated surfaces of calcite and barite minerals were obtained as 30.9 and 35.0 mN/m by this new approach, respectively. These γc values are in good agreement with those obtained with other techniques, namely, contact angle measurement and flotation. Accordingly, it was concluded that the shear flocculation can be used for determination of the critical surface tension of wetting (γc ) of minerals coated with surfactants. Since most minerals are hydrophilic and require surfactant adsorption for their flotation and shear flocculation, the shear flocculation method, introduced in this study, can be easily used to determine the γc value of any mineral when a treatment with surfactants is applied to the mineral suspensions.
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