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Correlation of shear flocculation of some salt-type minerals with their wettability parameter
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Chemical Engineering and Processing 46 (2007) 1341–1348
Correlation of shear flocculation of some salt-type minerals with their wettability parameter A. Ozkan ∗ , Z. Uslu, S. Duzyol, H. Ucbeyiay Department of Mining Engineering, Selcuk University, Konya, Turkey Received 16 December 2005; received in revised form 21 September 2006; accepted 27 October 2006 Available online 6 December 2006
Abstract This paper considers the importance of surface hydrophobicity in the shear flocculation process and presents a correlation between the shear flocculation and the wettability parameter for barite, celestite and calcite as salt-type minerals. The critical surface tension of wetting (γ c ) as a wettability parameter describes wetting characteristics of any mineral. The variation of the shear flocculation behaviours of barite, celestite and calcite with sodium oleate concentration in various methanol solutions was investigated. The shear flocculation of these minerals in the methanol solutions increased rapidly towards the optimum surfactant concentration, and thereafter remained relatively constant or increased slightly. On the other hand, the shear flocculation of the minerals decreased with increasing methanol concentration, depends on decreasing surface tension. The γ c values of these minerals as a function of surfactant concentration were determined using a shear flocculation approach. It was found that the γ c values did not change much at surfactant concentrations above the optimum. This result provides a reason for the observed lack of significant increase in the shear flocculation of the mineral suspensions when surfactant concentrations higher than the optimum are used. Furthermore, a strong correlation between the effective shear flocculation and the critical surface tension of wetting (γ c ) value was established. As the effective shear flocculation of these salt-type minerals increased sharply below a particular γ c value, it was not much improved after reaching the γ c value obtained at the optimum concentration of sodium oleate. © 2006 Elsevier B.V. All rights reserved. Keywords: Shear flocculation; Critical surface tension of wetting; Barite; Celestite; Calcite
1. Introduction Shear flocculation is the aggregation of fine particles in a convenient stirring regime after hydrophobization by the adsorption of surfactants. 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 the hydrophobization of particle surfaces, surfactants known as flotation collectors are often used. The aggregation of particles in the suspension is due to the hydrocarbon chain association when the surfactant adsorption layers on particles contact each other [1–4]. 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
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contact between hydrocarbon chains, thereby reducing the surface energy of the system. This is achieved by throwing particles towards each other with a force exceeding the energy barrier [3]. Long-chain surfactants are more effective for obtaining a good flocculation, because the hydrocarbon chain association depends on the carbon number of adsorbed surfactant chain [5]. On the other hand, relationships between shear flocculation, flotation and contact angle for some minerals were investigated by various researchers [4,6,7]. It was reported that high recoveries of flotation and shear flocculation and great contact angles for the studied minerals were obtained in the same pH range and the shear flocculation was closely correlated with the contact angle. Also, Song et al. [7] indicated that the shear flocculation and the contact angle of sphalerite increased sharply after a critical surfactant concentration. As the grade of mineral deposits decreases, the valuable minerals often appear in the form of finely disseminated grains in an ore. Therefore, size reduction by crushing and grinding is required to liberate the valuable constituents. However, fine
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particles produced during the comminution processes decrease the efficiency of the concentration processes in mineral processing operations [8–10]. These fines, technically known as slimes, are difficult to separate by gravity separation and flotation. Therefore, slimes represent a potential resource if a process can be developed to economically recover the valuable minerals [11]. One way to recover fine valuable minerals from slimes is to increase their size by selective flocculation and then to float the flocs. Secondly, one of the selective polymeric flocculation, selective shear flocculation or selective oil agglomeration methods is used to separate valuable minerals from fine particle mixtures, with the aggregation of the desired mineral [1,10]. A successful industrial example of the application of the shear flocculation technique to mineral processing was the Yxsjoberg scheelite plant in Sweden. In this plant, the scheelite ore was conditioned with fatty acid as surfactant and the scheelite was selectively flocculated and then floated by flotation [12]. A pilot plant test of scheelite ore from King Island suggested that pre-treatment of the fine particles by shear flocculation improved subsequent flotation recovery of scheelite mineral. Also, it has been shown that scheelite flocs formed by shear flocculation effect float 10 times faster than the original particles [3,13]. The application of shear flocculation as a pre-treatment before flotation to the Rey de Plata sulfide ore of Mexico showed that shear flocculation not only reduced the losses of the valuable minerals in the tailing through recovering more valuable mineral fines, but also considerably increased the separation efficiency in the flotation through increasing the flotation rate of valuable minerals, in comparison to conventional flotation [12]. Similar results for several sulfide ores whose particle sizes were finer than 10 m were also obtained by Bulatovic and Salter [14]. The advantage of shear flocculation is the production of relatively stable hydrophobic flocs which should be tough enough to withstand the turbulence in subsequent mineral processing operations. The hydrophobic flocs can be separated from remaining hydrophilic dispersed particles by sedimentation or flotation [3]. Therefore, the importance of the shear flocculation in mineral processing increases gradually [3,9,11,12]. Similar 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 (γ c ), which is an essential property to achieve selectivity in surface chemistry-based processes. In this study, the shear flocculation behaviours of barite, celestite and calcite as salt-type minerals were investigated using sodium oleate at various solution surface tensions. The critical surface tension of wetting (γ c ) values of these minerals were determined as a function of surfactant concentration by the shear flocculation approach. We aimed to reveal the correlation of the shear flocculation of some salt-type minerals with their critical surface tension of wetting (γ c ) values. Determination of such a correlation experimentally regarding to minerals will help fine particle processing of ores. This paper also indicates the importance of surface hydrophobization in the extent of shear flocculation of mineral suspensions.
2. Theoretical background The wettability of solid or mineral particles is known to be an important parameter which affects many technological processes such as froth flotation, shear flocculation, oil agglomeration, solid–liquid separation, and dust abatement [15]. 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 (γ c ) to achieve selectivity in surface chemistry processes [16]. Contact angle measurement, flotation, immersion time, bubble pickup, film flotation, automatic wetting balance, modified Hildebrand–Scott equation or shear flocculation methods can be used alternatively for determining the γ c values of solids or minerals [17–24]. The Zisman contact angle measurement technique and the flotation method are the two major techniques used to determine the (γ c ) values of minerals among these experimental and empirical techniques [16,25–27]. In the Zisman contact angle method [17], cosines of measured contact angles (θ) are plotted against solution surface tension (γ LV ) and the intercept of this line at the x-axis (for cos θ = 1 or θ = 0) is γ c . The liquid spreads on the solid (mineral) at γ c ≥ γ LV , the liquid forms a contact angle (θ > 0) at γ c < γ LV . The flotation method [18] estimates the γ c value of any mineral by plotting %flotation recovery versus solution surface tension (γ LV ) with the extrapolation of the linear part of the flotation recovery − γ LV curve to the surface tension axis in order to obtain an intercept at %flotation recovery = 0. The shear flocculation method to determine the critical surface tension of wetting of minerals treated with surfactants was devised recently by Ozkan [24]. The solution surface tension value at which effective shear flocculation does not occur can be defined as the critical surface tension of wetting (γ c ) value of the mineral and the γ c value of any mineral can be determined by the shear flocculation approach as similar to the flotation method. Consequently, while the effective shear flocculation of mineral suspension occurs at γ c < γ LV , no effective shear flocculation takes place at γ c ≥ γ LV [24]. 3. Experimental 3.1. Materials High purity barite, celestite and calcite mineral samples were used in this study. The chemical analysis results of these mineral samples are given in Table 1. The samples were dry ground Table 1 The chemical compositions of the mineral samples used in the experiments (values in %) Barite
BaSO4 97.63
SrSO4 1.29
SiO2 0.78
Celestite
SrSO4 99.06
CaSO4 0.58
Others 0.36
Calcite
CaCO3 99.14
SiO2 0.30
Others 0.56
CaCO3 0.12
Others 0.18
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Fig. 1. Particle size distributions of the samples used in the experiments.
in a ceramic ball mill and sieved to −38 m size fraction. The particle size distributions of the prepared samples were determined using an Andreasen pipette and the obtained results are shown in Fig. 1. The mean sizes of barite, celestite and calcite samples were calculated to be 8.9, 15.4 and 15.3 m, respectively. Also, the densities of barite, celestite and calcite samples were determined as 4.38, 3.94 and 2.69 g/cm3 , respectively, by a pycnometer. Sodium oleate was used as anionic surfactant in the shear flocculation tests. Sodium oleate (C17 H33 COONa) was prepared from oleic acid (C17 H33 COOH) (Carlo Erba) and sodium hydroxide (Merck). All of these chemicals were analytical grade. 3.2. Wettability test solutions Methanol–water solutions of different concentrations (wt%) were prepared and the surface tensions of the methanol solutions were determined by the drop-weight measurement method [28]. The measured values of the surface tensions of the methanol solutions are very close to the data in the literature [29]. The solutions used in all experiments were prepared with distilled water. The methanol (>99.9% in purity) was purchased from Akkimya and the solutions were maintained at 18 ± 2 ◦ C. 3.3. Shear flocculation experiments The shear flocculation experiments were carried out in a 400 cm3 cylindrical cell with four baffles using 1 g solid and 300 cm3 methanol solution. Fig. 2 shows the schematic diagram of the stirred cell system. The cell diameter (T) and impeller diameter (D) are 78 and 42 mm, respectively. H and Hb are the height of the cell and baffle, respectively. The stirring of the suspension was provided by a centrally located turbine impeller with four blades. The shaft of the impeller is driven
Fig. 2. Schematic diagram of stirred cell and impeller.
through a variable speed gearbox and the shaft speed is measured with a digital tachometer. The mixture of mineral–solution was pre-conditioned for 1 min in order to obtain a well-dispersed suspension. Thereafter, the dispersed suspension was conditioned with surfactant at an impeller speed of 500 rpm for 3 min. After a settling time of 1 min, 30 cm3 of supernatant was taken out, at a fixed distance of 4.5 cm below the air–liquid interface, by a special system for turbidity measurements. The turbidity of the supernatant was measured by a WTW Turb 550 turbidimeter. The performance of the shear flocculation process was assessed using a formula (Eq. (1)), used previously [30,31], and modified to allow for the measurement of supernatant turbidity: T0 − Tsf × 100 (1) shear flocculation efficiency = T0 where T0 is the turbidity (nephelometric turbidity unit) of well-dispersed suspension of mineral at 1 g/300 cm3 of solid concentration and Tsf is the turbidity of supernatant when sedimentation is assisted by a surfactant. 4. Results and discussion Barite (BaSO4 ), celestite (SrSO4 ) and calcite (CaCO3 ) are salt-type minerals in regard to their physicochemical properties [32]. These minerals have a hydrophilic character whose γ c > 72 mN/m, therefore the water spreads completely on the mineral surfaces when no chemical treatment is applied to the mineral surfaces.
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Fig. 3. Shear flocculation behaviours of barite as a function of sodium oleate concentration in various methanol solutions.
Fig. 5. Shear flocculation behaviours of calcite as a function of sodium oleate concentration in various methanol solutions.
The shear flocculation behaviours of barite, celestite and calcite as a function of sodium oleate concentration in various methanol solutions are given in Figs. 3–5, respectively. As seen in Figs. 3–5, the shear flocculation of these minerals in each methanol solution increased rapidly towards a particular concentration of sodium oleate, and thereafter remained relatively constant or increased slowly. Therefore, the optimum concentration values of sodium oleate under the studied experimental conditions were determined as 10 mg/dm3 for barite and 20 mg/dm3 for celestite and calcite. Generally, the adsorption states of anionic surfactants, including sodium oleate, onto
salt-type minerals such as calcite, apatite, fluorite, celestite and barite can be divided into two regions [33–38]. In region I, chemisorption of oleate at low concentrations is predominated and takes place by the reaction of carboxylate head group with surface cation sites of the mineral surface. The chemisorption increases with increasing surfactant concentration, and finally, it is considered to be limited to a monolayer surface coverage. At higher surfactant concentrations, surface precipitation of cation–surfactant complexes dominates in region II and multilayer coverage occurs on the mineral surfaces [37–39]. High hydrophobicity of mineral surfaces usually corresponds to the monolayer coverage and multilayer adsorption might occur resulting in a decrease in hydrophobicity [33,38,40–42]. Various researchers also stated that the cation–surfactant salts have a hydrophobic character [43,44] but it was considered to be weak or less efficient as a surfactant [43,45]. On the other hand, it is well known that the shear flocculation is closely correlated with the particle hydrophobicity or wettability [3,4,7,24,46]. That is, the shear flocculation increases with increasing hydrophobicity or decreasing wettability. Hence, the shear flocculation of these minerals improved with increasing surface hydrophobicity depending on increasing surfactant concentration. However, at surfactant concentrations higher than the optimum values, the increasing trends in the shear flocculation efficiency curves slowed or the shear flocculation of the mineral suspensions remained almost constant. The dependence of shear flocculation on surface coverage was investigated by various researchers. Warren [47] reported that shear flocculation of cassiterite and tourmaline with sodium oleate and sodium lauryl sulfate required nearly monolayer surface coverage. Beyond monolayer coverage, the polar head groups of the surfactant are orientated toward the bulk suspension and therefore the surfaces of the particle become less hydrophobic. Shear flocculation is less likely in this situation since the hydrocarbon chain associa-
Fig. 4. Shear flocculation behaviours of celestite as a function of sodium oleate concentration in various methanol solutions.
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tion between the particles decreases. Eventually, the aggregation of these minerals was not observed at bilayer surface coverage, which made the surfaces hydrophilic [47]. Shear flocculation of scheelite and fluorite minerals using sodium oleate was investigated by Sivamohan and Cases [48]. They stated that a monolayer coverage is necessary for complete association between the adsorbed surfactant layers on the mineral surfaces. Increasing surface coverage above monolayer led to a decrease in the degree of the aggregation of scheelite and fluorite due to both the increasing resistance of the two-dimensionally condensed surfactant and the decreasing hydrophobicity. Similar results for quartz, hematite and rutile were also obtained by Lu and Song [40]. Song et al. [49] also stated that destabilization of calcium carbonate suspension with alkyl polyglycoside continued until monolayer coverage and the dispersion stability increased beyond monolayer adsorption. The effect of sodium oleate on the agglomeration of calcium carbonate was investigated by Sayan [50]. It was stated that sodium oleate chemically adsorbed on the calcium carbonate surface and the degree of agglomeration varied depending on the amount of adsorbed ions on the surface. Sayan [50] also reported that sodium oleate reached a maximum adsorption on the surface of calcium carbonate at a critical micelle concentration. In light of these investigations, it may be said that the optimum oleate concentrations determined for barite, celestite and calcite approximately corresponded to a monolayer coverage of that mineral. This assumption is reasonable since the γ c values of the minerals also remained relatively constant above the optimum oleate concentration (see Fig. 7), which means that the hydrophobicity of the mineral surfaces did not much increase at oleate concentrations higher than the optimum. It can also be noted that a decrease in the shear flocculation of these minerals was not observed in the studied concentration range of the surfactant. This shows that higher oleate concentrations were required for decline of both shear flocculation and hydrophobicity. Figs. 3–5 also show that the shear flocculation efficiency values of barite, celestite and calcite decreased with increasing methanol concentration. As the methanol concentration in the solution increases, the surface tension of the solution reduces. Consequently, it can be said that the shear flocculation of the mineral suspensions decreased in tandem with the decrease of the surface tension of the solution used. That is, as the surface tension of the solution used for shear flocculation medium decreases, the wettability of particle surfaces treated with the surfactant increases. Eventually, an effective shear flocculation of the mineral suspensions did not occur in 75% methanol solution for barite and calcite, and 62% methanol solution for celestite. Because, the shear flocculation efficiency values obtained in these methanol solutions correspond to the particular values that occurred due to effect of only sedimentation, as seen from Fig. 6. Since the shear flocculation of the particles was not assisted by the addition of the surfactant in these methanol solutions, the effective shear flocculation of the minerals could not be achieved. Moreover, the shear flocculation of the mineral suspensions was not affected by the increased surfactant concentration and therefore the shear flocculation efficiency curves followed approximately a horizontal trend in these methanol
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Fig. 6. Sedimentation behaviours of barite, celestite and calcite in various methanol concentrations.
solutions. As a result, the particle surfaces treated with surfactant were completely wetted by the solution at these surface tension values and therefore the hydrophobic association between particles could not take place. In other words, while the effective shear flocculation of the mineral suspension occurs at γ c < γ LV , no effective shear flocculation takes place at γ c ≥ γ LV , as stated in the literature [24]. It can be noted that the surface tension (γ LV ) values of the methanol solutions indicated above are lower than the γ c values determined for these minerals with the optimum oleate concentration (see Fig. 7). By using data given in Figs. 3–6, the critical surface tension of wetting (γ c ) values of the studied minerals were determined for each sodium oleate concentration using the shear flocculation approach [24]. The variations of the critical surface tension of wetting (γ c ) values of barite, celestite and calcite minerals with sodium oleate concentration are shown in Fig. 7. As seen in Fig. 7, the γ c values of these minerals decreased sharply with increasing sodium oleate concentration. However, the γ c val-
Fig. 7. Variations of the γ c values of barite, celestite and calcite with sodium oleate concentration.
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ues did not much vary at surfactant concentrations higher than 10 mg/dm3 for barite and 20 mg/dm3 for celestite and calcite. These surfactant concentration values found for barite, celestite and calcite interestingly correspond to the optimum surfactant concentrations determined in the shear flocculation of the related mineral. The γ c values obtained for barite, celestite and calcite minerals at the optimum oleate concentration were 29.0, 35.2 and 29.9 mN/m, respectively. The critical surface tension of wetting (γ c ) value is used as a tool to estimate the behaviour of minerals or solids during wettability-based processes [15]. It is a measure of the degree of wettability or hydrophobicity of mineral surfaces. That is, the lower the γ c value, the more hydrophobic the particle surfaces. Therefore, it can be said that the hydrophobicity of barite, celestite and calcite surfaces did not much improve after reaching the γ c value obtained at the optimum concentration of oleate. This is likely to occur under conditions close to the monolayer coverage of sodium oleate on the mineral surfaces. As mentioned above, high hydrophobicity of mineral surfaces is generally obtained at a monolayer coverage. Surfactant concentrations above the optimum value did not also provide a significant increase in the shear flocculation efficiency. As a result, it can be stated that since the γ c values remained relatively constant (meaning similar hydrophobicity or wettability), a significant increase in the shear flocculation efficiency values was not observed as seen in Figs. 3–5. This result provides an explanation for the observed lack of significant increase in the shear flocculation of the mineral suspensions when surfactant concentrations higher than the optimum were used. The correlation between the effective shear flocculation efficiency values obtained in the water (similar to the conventional shear flocculation medium) and the γ c values determined using different concentrations of sodium oleate for barite, celestite and calcite minerals is shown in Fig. 8. As clearly seen from Fig. 8, the effective shear flocculation efficiency values of barite, celestite and calcite were zero when no treatment with sodium
oleate was applied to these minerals. In this case, the γ c values of these salt-type minerals are higher than 72 mN/m, and therefore the water spreads completely on the particle surfaces. With decreasing the γ c value, the effective shear flocculation of these mineral suspensions started to occur, that is the hydrophobic association between surfactant layers adsorbed on the particle surfaces began to take place. Especially, the effective shear flocculation efficiency values for these mineral suspensions increased sharply below a particular γ c value. This result indicates that the γ c value of the mineral should be lower than a particular value for the successful shear flocculation of that mineral. In other words, a sufficient concentration of surfactant should be used to obtain a good shear flocculation. However, the increasing ratios observed in the effective shear flocculation efficiencies generally started to decrease after reaching the γ c values obtained at the optimum concentration of sodium oleate. In brief, surfactant concentrations higher than the optimum did not provide a significant increase in the shear flocculation of barite, celestite and calcite (see Figs. 3–5). On the other hand, the γ c values of these minerals also remained relatively constant at surfactant concentrations above the optimum value (see Fig. 7). Consequently, the effective shear flocculation of the mineral suspensions was not much enhanced after reaching the γ c value obtained at the optimum concentration of sodium oleate, as seen in Fig. 8. The critical surface tension concept when applied in the flotation researches provides a unique and elegant way of testing the thermodynamic flotation criterion [51], that flotation is possible only when θ > 0. The results obtained from this study clearly confirm the importance of the surface hydrophobicity in shear flocculation applications, as similar to the application of the Zisman’s critical surface tension concept in flotation process. Although various applications of the shear flocculation to mineral processing are present, this technique is still under development. It should be an effective method of selectively aggregating fine particles of one mineral from another in mineral processing operations and therefore shear flocculation is a promising technique for fine particle processing of ores [3,9,11,12]. As mentioned above, one of the most important parameters which affect the shear flocculation is the hydrophobicity of the particle surfaces. The hydrophobicity degree of the particle surfaces exhibits a strong dependence on surfactant concentration. It was concluded that the optimum surfactant concentration for the successful shear flocculation of any mineral can be determined using the critical surface tension of wetting (γ c ) parameter, which is an essential characteristic in achieving selectivity in wettability-based processes. This result can be of significance when the shear flocculation technique is applied to mineral processing. Because, high surfactant concentrations can have an adverse effect on the recovery of the valuable minerals by shear flocculation technique and also cause a decrease in the selectivity of the shear flocculation effect [6,40,47,48]. 5. Conclusions
Fig. 8. Relationship between the effective shear flocculation efficiency values obtained in water and the γ c values of barite, celestite and calcite minerals.
The shear flocculation of barite, celestite and calcite in various methanol solutions increased sharply towards the optimum
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surfactant concentration, and thereafter remained relatively constant or increased slowly. The shear flocculation of these mineral suspensions also decreased in tandem with the decrease of the surface tension of the solution used. That is, as the surface tension of the solution decreases, the wettability of particle surfaces treated with the surfactant increases. Eventually, the effective shear flocculation of the mineral suspension did not occur at surface tensions lower than the γ c value of that mineral. The critical surface tension of wetting (γ c ) values of these minerals as a function of surfactant concentration was determined using the shear flocculation approach. It was found that the γ c values did not change much at surfactant concentrations higher than the optimum. The surfactant concentration at which the γ c value reaches a relatively stable point interestingly corresponds to the optimum surfactant concentration determined in the shear flocculation of the related mineral. This finding provides an explanation for the observed lack of significant increase in the shear flocculation of the mineral suspensions at surfactant concentrations above the optimum. That is, since the γ c value does not much vary (meaning similar hydrophobicity), a significant improvement in the shear flocculation of these minerals does not occur. A correlation between the effective shear flocculation efficiency and the critical surface tension of wetting (γ c ) value for barite, celestite and calcite minerals was also established. The effective shear flocculation of these mineral suspensions increased sharply below a particular γ c value. Therefore, for the successful shear flocculation of a mineral, the critical surface tension of wetting (γ c ) value of that mineral should be lower than a particular value, indicating a sufficient surface hydrophobicity. It was also concluded that the effective shear flocculation of these mineral suspensions did not much improve after reaching the γ c value obtained at the optimum concentration of the surfactant. Consequently, it is a clear result that there is a strong correlation between the shear flocculation process and the critical surface tension of wetting (γ c ). The shear flocculation technique utilizes differences in wettability of minerals and the γ c value estimates the behaviour of minerals during the wettabilitybased processes. Therefore, this finding can be useful when selective shear flocculation of the desired mineral from ores is practiced. Acknowledgement The financial support given by the Selcuk University Scientific Research Project Fund on this study is acknowledged. References [1] L.J. Warren, Shear flocculation of ultrafine scheelite in sodium oleate solutions, J. Colloid Interface Sci. 50 (1975) 307–318. [2] L.J. Warren, Shear flocculation, Chem. Technol. 11 (1981) 180–185. [3] L.J. Warren, Shear flocculation, in: J.S. Laskowski, J. Ralston (Eds.), Colloid Chemistry in Mineral Processing, Elsevier, New York, 1992, pp. 309–329 (Chapter 10). [4] S. Song, A. Lopez-Valdivieso, J.L. Reyes-Bahena, H.I. Bermejo-Perez, O. Trass, Hydrophobic flocculation of galena fines in aqueous suspensions, J. Colloid Interface Sci. 227 (2000) 272–281.
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[28] J.F. Padday, The measurement of surface tension, in: E. Matijevic (Ed.), Surface Colloid Sci., vol. 1, Wiley-Interscience, New York, 1968, pp. 101–149. [29] R.C. Weast, Handbook of Chemistry and Physics, 68th ed., CRS Press, New York, 1987. [30] D.G. Osborne, Recovery of slimes by a combination of selective flocculation and flotation, Trans. Inst. Min. Metall. Sec. C 87 (1978) C189–C193. [31] A. Ozkan, M. Yekeler, Coagulation and flocculation characteristics of celestite with different inorganic salts and polymers, Chem. Eng. Process. 43 (2004) 873–879. [32] H.S. Hanna, P. Somasundaran, Flotation of salt-type minerals, in: M.C. Fuerstenau (Ed.), A.M. Gaudin Memorial Volume, vol. 1, AIME, New York, 1976, pp. 197–272. [33] L.R. Plitt, M.K. Kim, Adsorption mechanism of fatty acid collectors on barite, Trans. AIME 256 (1974) 188–193. [34] K.P. Ananthapadmanabhan, P. Somasundaran, Surface precipitation of inorganics and surfactants and its role in adsorption and flotation, Colloids Surf. 13 (1985) 151–167. [35] A. Martinez-Luevanos, A. Uribe-Salas, Interfacial properties of celestite and strontianite in aqueous solutions, Miner. Eng. 8 (1995) 1009–1022. [36] A. Martinez-Luevanos, A. Uribe-Salas, A. Lopez-Valdivieso, Mechanism of adsorption of sodium dodecylsulfonate on celestite and calcite, Miner. Eng. 12 (1999) 919–936. [37] C.A. Young, J.D. Miller, Effect of temperature on oleate adsorption at a calcite surface: an FT-NIR/IRS study and review, Int. J. Miner. Process. 58 (2000) 331–350. [38] H. Sis, S. Chander, Adsorption and contact angle of single and binary mixtures of surfactants on apatite, Miner. Eng. 16 (2003) 839–848. [39] P. Somasundaran, S. Krishnakumar, Adsorption of surfactants and polymers at the solid–liquid interface, Colloids Surf. A: Physicochem. Eng. Aspects 123–124 (1997) 491–513.
[40] S. Lu, S. Song, Hydrophobic interaction in flocculation and flotation 1. Hydrophobic flocculation of fine mineral particles in aqueous solution, Colloids Surf. 57 (1991) 49–60. [41] Y. Lu, J. Drelich, J.D. Miller, Oleate adsorption at an apatite surface studied by ex-situ FTIR internal reflection spectroscopy, J. Colloid Interface Sci. 202 (1998) 462–476. [42] J. Shibata, D.W. Fuerstenau, Flocculation and flotation characteristics of fine hematite with sodium oleate, Int. J. Miner. Process. 72 (2003) 25–32. [43] N.P. Finkelstein, Review of interactions in flotation of sparingly soluble calcium minerals with anionic collectors, Trans. Inst. Min. Metall. Sec. C 98 (1989) C157–C177. [44] Z. Sadowski, The role of surfactant salts on the spherical agglomeration of hematite suspension, Colloids Surf. A: Physicochem. Eng. Aspects 173 (2000) 211–217. [45] R.B. Bjorklund, H. Arwin, Ellipsometric study of anionic surfactant adsorption on apatite and calcite ore surfaces, Langmuir 8 (1992) 1709–1714. [46] S. Song, S. Lu, Hydrophobic flocculation of fine hematite, siderite, and rhodochrosite particles in aqueous solution, J. Colloid Interface Sci. 166 (1994) 35–42. [47] L.J. Warren, Flocculation of stirred suspensions of cassiterite and tourmaline, Colloids Surf. 5 (1982) 301–319. [48] R. Sivamohan, J.M. Cases, Dependence of shear-flocculation on surface coverage and zeta potential, Int. J. Miner. Process. 28 (1990) 161–172. [49] M.G. Song, J.Y. Kim, J.D. Kim, The dispersion properties of precipitated calcium carbonate suspensions adsorbed with alkyl polyglycoside in aqueous medium, J. Colloid Interface Sci. 226 (2000) 83–90. [50] P. Sayan, Effect of sodium oleate on the agglomeration of calcium carbonate, Cryst. Res. Technol. 40 (2005) 226–232. [51] J.S. Laskowski, The relationship between floatability and hydrophobicity, in: P. Somasundaran (Ed.), Advances in Mineral Processing, 1986, pp. 189–208.
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Correlation of shear flocculation of some salt-type minerals with their wettability parameter A. Ozkan ∗ , Z. Uslu, S. Duzyol, H. Ucbeyiay Department of Mining Engineering, Selcuk University, Konya, Turkey Received 16 December 2005; received in revised form 21 September 2006; accepted 27 October 2006 Available online 6 December 2006
Abstract This paper considers the importance of surface hydrophobicity in the shear flocculation process and presents a correlation between the shear flocculation and the wettability parameter for barite, celestite and calcite as salt-type minerals. The critical surface tension of wetting (γ c ) as a wettability parameter describes wetting characteristics of any mineral. The variation of the shear flocculation behaviours of barite, celestite and calcite with sodium oleate concentration in various methanol solutions was investigated. The shear flocculation of these minerals in the methanol solutions increased rapidly towards the optimum surfactant concentration, and thereafter remained relatively constant or increased slightly. On the other hand, the shear flocculation of the minerals decreased with increasing methanol concentration, depends on decreasing surface tension. The γ c values of these minerals as a function of surfactant concentration were determined using a shear flocculation approach. It was found that the γ c values did not change much at surfactant concentrations above the optimum. This result provides a reason for the observed lack of significant increase in the shear flocculation of the mineral suspensions when surfactant concentrations higher than the optimum are used. Furthermore, a strong correlation between the effective shear flocculation and the critical surface tension of wetting (γ c ) value was established. As the effective shear flocculation of these salt-type minerals increased sharply below a particular γ c value, it was not much improved after reaching the γ c value obtained at the optimum concentration of sodium oleate. © 2006 Elsevier B.V. All rights reserved. Keywords: Shear flocculation; Critical surface tension of wetting; Barite; Celestite; Calcite
1. Introduction Shear flocculation is the aggregation of fine particles in a convenient stirring regime after hydrophobization by the adsorption of surfactants. 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 the hydrophobization of particle surfaces, surfactants known as flotation collectors are often used. The aggregation of particles in the suspension is due to the hydrocarbon chain association when the surfactant adsorption layers on particles contact each other [1–4]. 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
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contact between hydrocarbon chains, thereby reducing the surface energy of the system. This is achieved by throwing particles towards each other with a force exceeding the energy barrier [3]. Long-chain surfactants are more effective for obtaining a good flocculation, because the hydrocarbon chain association depends on the carbon number of adsorbed surfactant chain [5]. On the other hand, relationships between shear flocculation, flotation and contact angle for some minerals were investigated by various researchers [4,6,7]. It was reported that high recoveries of flotation and shear flocculation and great contact angles for the studied minerals were obtained in the same pH range and the shear flocculation was closely correlated with the contact angle. Also, Song et al. [7] indicated that the shear flocculation and the contact angle of sphalerite increased sharply after a critical surfactant concentration. As the grade of mineral deposits decreases, the valuable minerals often appear in the form of finely disseminated grains in an ore. Therefore, size reduction by crushing and grinding is required to liberate the valuable constituents. However, fine
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particles produced during the comminution processes decrease the efficiency of the concentration processes in mineral processing operations [8–10]. These fines, technically known as slimes, are difficult to separate by gravity separation and flotation. Therefore, slimes represent a potential resource if a process can be developed to economically recover the valuable minerals [11]. One way to recover fine valuable minerals from slimes is to increase their size by selective flocculation and then to float the flocs. Secondly, one of the selective polymeric flocculation, selective shear flocculation or selective oil agglomeration methods is used to separate valuable minerals from fine particle mixtures, with the aggregation of the desired mineral [1,10]. A successful industrial example of the application of the shear flocculation technique to mineral processing was the Yxsjoberg scheelite plant in Sweden. In this plant, the scheelite ore was conditioned with fatty acid as surfactant and the scheelite was selectively flocculated and then floated by flotation [12]. A pilot plant test of scheelite ore from King Island suggested that pre-treatment of the fine particles by shear flocculation improved subsequent flotation recovery of scheelite mineral. Also, it has been shown that scheelite flocs formed by shear flocculation effect float 10 times faster than the original particles [3,13]. The application of shear flocculation as a pre-treatment before flotation to the Rey de Plata sulfide ore of Mexico showed that shear flocculation not only reduced the losses of the valuable minerals in the tailing through recovering more valuable mineral fines, but also considerably increased the separation efficiency in the flotation through increasing the flotation rate of valuable minerals, in comparison to conventional flotation [12]. Similar results for several sulfide ores whose particle sizes were finer than 10 m were also obtained by Bulatovic and Salter [14]. The advantage of shear flocculation is the production of relatively stable hydrophobic flocs which should be tough enough to withstand the turbulence in subsequent mineral processing operations. The hydrophobic flocs can be separated from remaining hydrophilic dispersed particles by sedimentation or flotation [3]. Therefore, the importance of the shear flocculation in mineral processing increases gradually [3,9,11,12]. Similar 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 (γ c ), which is an essential property to achieve selectivity in surface chemistry-based processes. In this study, the shear flocculation behaviours of barite, celestite and calcite as salt-type minerals were investigated using sodium oleate at various solution surface tensions. The critical surface tension of wetting (γ c ) values of these minerals were determined as a function of surfactant concentration by the shear flocculation approach. We aimed to reveal the correlation of the shear flocculation of some salt-type minerals with their critical surface tension of wetting (γ c ) values. Determination of such a correlation experimentally regarding to minerals will help fine particle processing of ores. This paper also indicates the importance of surface hydrophobization in the extent of shear flocculation of mineral suspensions.
2. Theoretical background The wettability of solid or mineral particles is known to be an important parameter which affects many technological processes such as froth flotation, shear flocculation, oil agglomeration, solid–liquid separation, and dust abatement [15]. 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 (γ c ) to achieve selectivity in surface chemistry processes [16]. Contact angle measurement, flotation, immersion time, bubble pickup, film flotation, automatic wetting balance, modified Hildebrand–Scott equation or shear flocculation methods can be used alternatively for determining the γ c values of solids or minerals [17–24]. The Zisman contact angle measurement technique and the flotation method are the two major techniques used to determine the (γ c ) values of minerals among these experimental and empirical techniques [16,25–27]. In the Zisman contact angle method [17], cosines of measured contact angles (θ) are plotted against solution surface tension (γ LV ) and the intercept of this line at the x-axis (for cos θ = 1 or θ = 0) is γ c . The liquid spreads on the solid (mineral) at γ c ≥ γ LV , the liquid forms a contact angle (θ > 0) at γ c < γ LV . The flotation method [18] estimates the γ c value of any mineral by plotting %flotation recovery versus solution surface tension (γ LV ) with the extrapolation of the linear part of the flotation recovery − γ LV curve to the surface tension axis in order to obtain an intercept at %flotation recovery = 0. The shear flocculation method to determine the critical surface tension of wetting of minerals treated with surfactants was devised recently by Ozkan [24]. The solution surface tension value at which effective shear flocculation does not occur can be defined as the critical surface tension of wetting (γ c ) value of the mineral and the γ c value of any mineral can be determined by the shear flocculation approach as similar to the flotation method. Consequently, while the effective shear flocculation of mineral suspension occurs at γ c < γ LV , no effective shear flocculation takes place at γ c ≥ γ LV [24]. 3. Experimental 3.1. Materials High purity barite, celestite and calcite mineral samples were used in this study. The chemical analysis results of these mineral samples are given in Table 1. The samples were dry ground Table 1 The chemical compositions of the mineral samples used in the experiments (values in %) Barite
BaSO4 97.63
SrSO4 1.29
SiO2 0.78
Celestite
SrSO4 99.06
CaSO4 0.58
Others 0.36
Calcite
CaCO3 99.14
SiO2 0.30
Others 0.56
CaCO3 0.12
Others 0.18
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Fig. 1. Particle size distributions of the samples used in the experiments.
in a ceramic ball mill and sieved to −38 m size fraction. The particle size distributions of the prepared samples were determined using an Andreasen pipette and the obtained results are shown in Fig. 1. The mean sizes of barite, celestite and calcite samples were calculated to be 8.9, 15.4 and 15.3 m, respectively. Also, the densities of barite, celestite and calcite samples were determined as 4.38, 3.94 and 2.69 g/cm3 , respectively, by a pycnometer. Sodium oleate was used as anionic surfactant in the shear flocculation tests. Sodium oleate (C17 H33 COONa) was prepared from oleic acid (C17 H33 COOH) (Carlo Erba) and sodium hydroxide (Merck). All of these chemicals were analytical grade. 3.2. Wettability test solutions Methanol–water solutions of different concentrations (wt%) were prepared and the surface tensions of the methanol solutions were determined by the drop-weight measurement method [28]. The measured values of the surface tensions of the methanol solutions are very close to the data in the literature [29]. The solutions used in all experiments were prepared with distilled water. The methanol (>99.9% in purity) was purchased from Akkimya and the solutions were maintained at 18 ± 2 ◦ C. 3.3. Shear flocculation experiments The shear flocculation experiments were carried out in a 400 cm3 cylindrical cell with four baffles using 1 g solid and 300 cm3 methanol solution. Fig. 2 shows the schematic diagram of the stirred cell system. The cell diameter (T) and impeller diameter (D) are 78 and 42 mm, respectively. H and Hb are the height of the cell and baffle, respectively. The stirring of the suspension was provided by a centrally located turbine impeller with four blades. The shaft of the impeller is driven
Fig. 2. Schematic diagram of stirred cell and impeller.
through a variable speed gearbox and the shaft speed is measured with a digital tachometer. The mixture of mineral–solution was pre-conditioned for 1 min in order to obtain a well-dispersed suspension. Thereafter, the dispersed suspension was conditioned with surfactant at an impeller speed of 500 rpm for 3 min. After a settling time of 1 min, 30 cm3 of supernatant was taken out, at a fixed distance of 4.5 cm below the air–liquid interface, by a special system for turbidity measurements. The turbidity of the supernatant was measured by a WTW Turb 550 turbidimeter. The performance of the shear flocculation process was assessed using a formula (Eq. (1)), used previously [30,31], and modified to allow for the measurement of supernatant turbidity: T0 − Tsf × 100 (1) shear flocculation efficiency = T0 where T0 is the turbidity (nephelometric turbidity unit) of well-dispersed suspension of mineral at 1 g/300 cm3 of solid concentration and Tsf is the turbidity of supernatant when sedimentation is assisted by a surfactant. 4. Results and discussion Barite (BaSO4 ), celestite (SrSO4 ) and calcite (CaCO3 ) are salt-type minerals in regard to their physicochemical properties [32]. These minerals have a hydrophilic character whose γ c > 72 mN/m, therefore the water spreads completely on the mineral surfaces when no chemical treatment is applied to the mineral surfaces.
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Fig. 3. Shear flocculation behaviours of barite as a function of sodium oleate concentration in various methanol solutions.
Fig. 5. Shear flocculation behaviours of calcite as a function of sodium oleate concentration in various methanol solutions.
The shear flocculation behaviours of barite, celestite and calcite as a function of sodium oleate concentration in various methanol solutions are given in Figs. 3–5, respectively. As seen in Figs. 3–5, the shear flocculation of these minerals in each methanol solution increased rapidly towards a particular concentration of sodium oleate, and thereafter remained relatively constant or increased slowly. Therefore, the optimum concentration values of sodium oleate under the studied experimental conditions were determined as 10 mg/dm3 for barite and 20 mg/dm3 for celestite and calcite. Generally, the adsorption states of anionic surfactants, including sodium oleate, onto
salt-type minerals such as calcite, apatite, fluorite, celestite and barite can be divided into two regions [33–38]. In region I, chemisorption of oleate at low concentrations is predominated and takes place by the reaction of carboxylate head group with surface cation sites of the mineral surface. The chemisorption increases with increasing surfactant concentration, and finally, it is considered to be limited to a monolayer surface coverage. At higher surfactant concentrations, surface precipitation of cation–surfactant complexes dominates in region II and multilayer coverage occurs on the mineral surfaces [37–39]. High hydrophobicity of mineral surfaces usually corresponds to the monolayer coverage and multilayer adsorption might occur resulting in a decrease in hydrophobicity [33,38,40–42]. Various researchers also stated that the cation–surfactant salts have a hydrophobic character [43,44] but it was considered to be weak or less efficient as a surfactant [43,45]. On the other hand, it is well known that the shear flocculation is closely correlated with the particle hydrophobicity or wettability [3,4,7,24,46]. That is, the shear flocculation increases with increasing hydrophobicity or decreasing wettability. Hence, the shear flocculation of these minerals improved with increasing surface hydrophobicity depending on increasing surfactant concentration. However, at surfactant concentrations higher than the optimum values, the increasing trends in the shear flocculation efficiency curves slowed or the shear flocculation of the mineral suspensions remained almost constant. The dependence of shear flocculation on surface coverage was investigated by various researchers. Warren [47] reported that shear flocculation of cassiterite and tourmaline with sodium oleate and sodium lauryl sulfate required nearly monolayer surface coverage. Beyond monolayer coverage, the polar head groups of the surfactant are orientated toward the bulk suspension and therefore the surfaces of the particle become less hydrophobic. Shear flocculation is less likely in this situation since the hydrocarbon chain associa-
Fig. 4. Shear flocculation behaviours of celestite as a function of sodium oleate concentration in various methanol solutions.
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tion between the particles decreases. Eventually, the aggregation of these minerals was not observed at bilayer surface coverage, which made the surfaces hydrophilic [47]. Shear flocculation of scheelite and fluorite minerals using sodium oleate was investigated by Sivamohan and Cases [48]. They stated that a monolayer coverage is necessary for complete association between the adsorbed surfactant layers on the mineral surfaces. Increasing surface coverage above monolayer led to a decrease in the degree of the aggregation of scheelite and fluorite due to both the increasing resistance of the two-dimensionally condensed surfactant and the decreasing hydrophobicity. Similar results for quartz, hematite and rutile were also obtained by Lu and Song [40]. Song et al. [49] also stated that destabilization of calcium carbonate suspension with alkyl polyglycoside continued until monolayer coverage and the dispersion stability increased beyond monolayer adsorption. The effect of sodium oleate on the agglomeration of calcium carbonate was investigated by Sayan [50]. It was stated that sodium oleate chemically adsorbed on the calcium carbonate surface and the degree of agglomeration varied depending on the amount of adsorbed ions on the surface. Sayan [50] also reported that sodium oleate reached a maximum adsorption on the surface of calcium carbonate at a critical micelle concentration. In light of these investigations, it may be said that the optimum oleate concentrations determined for barite, celestite and calcite approximately corresponded to a monolayer coverage of that mineral. This assumption is reasonable since the γ c values of the minerals also remained relatively constant above the optimum oleate concentration (see Fig. 7), which means that the hydrophobicity of the mineral surfaces did not much increase at oleate concentrations higher than the optimum. It can also be noted that a decrease in the shear flocculation of these minerals was not observed in the studied concentration range of the surfactant. This shows that higher oleate concentrations were required for decline of both shear flocculation and hydrophobicity. Figs. 3–5 also show that the shear flocculation efficiency values of barite, celestite and calcite decreased with increasing methanol concentration. As the methanol concentration in the solution increases, the surface tension of the solution reduces. Consequently, it can be said that the shear flocculation of the mineral suspensions decreased in tandem with the decrease of the surface tension of the solution used. That is, as the surface tension of the solution used for shear flocculation medium decreases, the wettability of particle surfaces treated with the surfactant increases. Eventually, an effective shear flocculation of the mineral suspensions did not occur in 75% methanol solution for barite and calcite, and 62% methanol solution for celestite. Because, the shear flocculation efficiency values obtained in these methanol solutions correspond to the particular values that occurred due to effect of only sedimentation, as seen from Fig. 6. Since the shear flocculation of the particles was not assisted by the addition of the surfactant in these methanol solutions, the effective shear flocculation of the minerals could not be achieved. Moreover, the shear flocculation of the mineral suspensions was not affected by the increased surfactant concentration and therefore the shear flocculation efficiency curves followed approximately a horizontal trend in these methanol
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Fig. 6. Sedimentation behaviours of barite, celestite and calcite in various methanol concentrations.
solutions. As a result, the particle surfaces treated with surfactant were completely wetted by the solution at these surface tension values and therefore the hydrophobic association between particles could not take place. In other words, while the effective shear flocculation of the mineral suspension occurs at γ c < γ LV , no effective shear flocculation takes place at γ c ≥ γ LV , as stated in the literature [24]. It can be noted that the surface tension (γ LV ) values of the methanol solutions indicated above are lower than the γ c values determined for these minerals with the optimum oleate concentration (see Fig. 7). By using data given in Figs. 3–6, the critical surface tension of wetting (γ c ) values of the studied minerals were determined for each sodium oleate concentration using the shear flocculation approach [24]. The variations of the critical surface tension of wetting (γ c ) values of barite, celestite and calcite minerals with sodium oleate concentration are shown in Fig. 7. As seen in Fig. 7, the γ c values of these minerals decreased sharply with increasing sodium oleate concentration. However, the γ c val-
Fig. 7. Variations of the γ c values of barite, celestite and calcite with sodium oleate concentration.
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ues did not much vary at surfactant concentrations higher than 10 mg/dm3 for barite and 20 mg/dm3 for celestite and calcite. These surfactant concentration values found for barite, celestite and calcite interestingly correspond to the optimum surfactant concentrations determined in the shear flocculation of the related mineral. The γ c values obtained for barite, celestite and calcite minerals at the optimum oleate concentration were 29.0, 35.2 and 29.9 mN/m, respectively. The critical surface tension of wetting (γ c ) value is used as a tool to estimate the behaviour of minerals or solids during wettability-based processes [15]. It is a measure of the degree of wettability or hydrophobicity of mineral surfaces. That is, the lower the γ c value, the more hydrophobic the particle surfaces. Therefore, it can be said that the hydrophobicity of barite, celestite and calcite surfaces did not much improve after reaching the γ c value obtained at the optimum concentration of oleate. This is likely to occur under conditions close to the monolayer coverage of sodium oleate on the mineral surfaces. As mentioned above, high hydrophobicity of mineral surfaces is generally obtained at a monolayer coverage. Surfactant concentrations above the optimum value did not also provide a significant increase in the shear flocculation efficiency. As a result, it can be stated that since the γ c values remained relatively constant (meaning similar hydrophobicity or wettability), a significant increase in the shear flocculation efficiency values was not observed as seen in Figs. 3–5. This result provides an explanation for the observed lack of significant increase in the shear flocculation of the mineral suspensions when surfactant concentrations higher than the optimum were used. The correlation between the effective shear flocculation efficiency values obtained in the water (similar to the conventional shear flocculation medium) and the γ c values determined using different concentrations of sodium oleate for barite, celestite and calcite minerals is shown in Fig. 8. As clearly seen from Fig. 8, the effective shear flocculation efficiency values of barite, celestite and calcite were zero when no treatment with sodium
oleate was applied to these minerals. In this case, the γ c values of these salt-type minerals are higher than 72 mN/m, and therefore the water spreads completely on the particle surfaces. With decreasing the γ c value, the effective shear flocculation of these mineral suspensions started to occur, that is the hydrophobic association between surfactant layers adsorbed on the particle surfaces began to take place. Especially, the effective shear flocculation efficiency values for these mineral suspensions increased sharply below a particular γ c value. This result indicates that the γ c value of the mineral should be lower than a particular value for the successful shear flocculation of that mineral. In other words, a sufficient concentration of surfactant should be used to obtain a good shear flocculation. However, the increasing ratios observed in the effective shear flocculation efficiencies generally started to decrease after reaching the γ c values obtained at the optimum concentration of sodium oleate. In brief, surfactant concentrations higher than the optimum did not provide a significant increase in the shear flocculation of barite, celestite and calcite (see Figs. 3–5). On the other hand, the γ c values of these minerals also remained relatively constant at surfactant concentrations above the optimum value (see Fig. 7). Consequently, the effective shear flocculation of the mineral suspensions was not much enhanced after reaching the γ c value obtained at the optimum concentration of sodium oleate, as seen in Fig. 8. The critical surface tension concept when applied in the flotation researches provides a unique and elegant way of testing the thermodynamic flotation criterion [51], that flotation is possible only when θ > 0. The results obtained from this study clearly confirm the importance of the surface hydrophobicity in shear flocculation applications, as similar to the application of the Zisman’s critical surface tension concept in flotation process. Although various applications of the shear flocculation to mineral processing are present, this technique is still under development. It should be an effective method of selectively aggregating fine particles of one mineral from another in mineral processing operations and therefore shear flocculation is a promising technique for fine particle processing of ores [3,9,11,12]. As mentioned above, one of the most important parameters which affect the shear flocculation is the hydrophobicity of the particle surfaces. The hydrophobicity degree of the particle surfaces exhibits a strong dependence on surfactant concentration. It was concluded that the optimum surfactant concentration for the successful shear flocculation of any mineral can be determined using the critical surface tension of wetting (γ c ) parameter, which is an essential characteristic in achieving selectivity in wettability-based processes. This result can be of significance when the shear flocculation technique is applied to mineral processing. Because, high surfactant concentrations can have an adverse effect on the recovery of the valuable minerals by shear flocculation technique and also cause a decrease in the selectivity of the shear flocculation effect [6,40,47,48]. 5. Conclusions
Fig. 8. Relationship between the effective shear flocculation efficiency values obtained in water and the γ c values of barite, celestite and calcite minerals.
The shear flocculation of barite, celestite and calcite in various methanol solutions increased sharply towards the optimum
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surfactant concentration, and thereafter remained relatively constant or increased slowly. The shear flocculation of these mineral suspensions also decreased in tandem with the decrease of the surface tension of the solution used. That is, as the surface tension of the solution decreases, the wettability of particle surfaces treated with the surfactant increases. Eventually, the effective shear flocculation of the mineral suspension did not occur at surface tensions lower than the γ c value of that mineral. The critical surface tension of wetting (γ c ) values of these minerals as a function of surfactant concentration was determined using the shear flocculation approach. It was found that the γ c values did not change much at surfactant concentrations higher than the optimum. The surfactant concentration at which the γ c value reaches a relatively stable point interestingly corresponds to the optimum surfactant concentration determined in the shear flocculation of the related mineral. This finding provides an explanation for the observed lack of significant increase in the shear flocculation of the mineral suspensions at surfactant concentrations above the optimum. That is, since the γ c value does not much vary (meaning similar hydrophobicity), a significant improvement in the shear flocculation of these minerals does not occur. A correlation between the effective shear flocculation efficiency and the critical surface tension of wetting (γ c ) value for barite, celestite and calcite minerals was also established. The effective shear flocculation of these mineral suspensions increased sharply below a particular γ c value. Therefore, for the successful shear flocculation of a mineral, the critical surface tension of wetting (γ c ) value of that mineral should be lower than a particular value, indicating a sufficient surface hydrophobicity. It was also concluded that the effective shear flocculation of these mineral suspensions did not much improve after reaching the γ c value obtained at the optimum concentration of the surfactant. Consequently, it is a clear result that there is a strong correlation between the shear flocculation process and the critical surface tension of wetting (γ c ). The shear flocculation technique utilizes differences in wettability of minerals and the γ c value estimates the behaviour of minerals during the wettabilitybased processes. Therefore, this finding can be useful when selective shear flocculation of the desired mineral from ores is practiced. Acknowledgement The financial support given by the Selcuk University Scientific Research Project Fund on this study is acknowledged. References [1] L.J. Warren, Shear flocculation of ultrafine scheelite in sodium oleate solutions, J. Colloid Interface Sci. 50 (1975) 307–318. [2] L.J. Warren, Shear flocculation, Chem. Technol. 11 (1981) 180–185. [3] L.J. Warren, Shear flocculation, in: J.S. Laskowski, J. Ralston (Eds.), Colloid Chemistry in Mineral Processing, Elsevier, New York, 1992, pp. 309–329 (Chapter 10). [4] S. Song, A. Lopez-Valdivieso, J.L. Reyes-Bahena, H.I. Bermejo-Perez, O. Trass, Hydrophobic flocculation of galena fines in aqueous suspensions, J. Colloid Interface Sci. 227 (2000) 272–281.
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