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Contact angle measurements at the colemanite and realgar surfaces
Applied Surface Science 225 (2004) 347–355
Contact angle measurements at the colemanite and realgar surfaces Sabiha Koca*, Mehmet Savas Mining Engineering Department, Osmangazi University, 26480 Eskisehir, Turkey Received 2 September 2003; received in revised form 21 October 2003; accepted 21 October 2003
Abstract Colemanite is one of the most important boron minerals and covers an important part of Turkey’s boron mineral deposits. The friable nature of the colemanite tends to produce a large amount of fines. Flotation appears to be a promising technique to recover colemanite from such fines. During flotation process, selectivity problem arises between colemanite and associated gangue minerals such as realgar. There is a close relationship between floatability of minerals and contact angle. Therefore, surface hydrophobicity of colemanite and realgar minerals were investigated by receding contact angle measurements in the absence and presence of flotation reagents. The water contact angle values at the colemanite surface remained almost unchanged at 32–358 in the solutions of potassium amyl xanthate (KAX), potassium ethyl xanthate (KEX) and petroleum sulphanate (R825) while another petroleum sulphanate (R840), sodium oleate and tallow amine (Armac-T) affected hydrophobicity of colemanite, and the contact angle values increased up to 478. The contact angle values of 62, 63, 45, 46, 39, and 438 at the realgar surface were obtained in the solutions of KAX, KEX, sodium oleate, R825, R840 and Armac-T, respectively. # 2003 Elsevier B.V. All rights reserved. PACS: 68.10.Cr Keywords: Colemanite; Realgar; Contact angle; Floatability; Flotation reactives
1. Introduction Colemanite is a hydrated semi-soluble salt mineral and is used in the manufacture of a variety of industrial products including advanced technological compounds such as penta borane, sodium borane and boron tri-chloride. Similar to other boron minerals, the major accompanying gangue minerals associated with colemanite are clays, carbonate minerals and to a *
Corresponding author. Tel.: þ90-222-2393750; fax: þ90-222-2393613. E-mail address: [email protected] (S. Koca).
lesser extend arsenious compounds. Beneneficiation of colemanite at coarser sizes is accomplished by scrubbing for clay removal followed by classification. The friable nature of the colemanite tends to produce a large amount of fines mostly below 0.1 mm. Flotation appears to be a promising technique to recover colemanite from such fines. However, arsenious compounds mainly realgar cause selectivity problems in colemanite flotation since both colemanite and realgar can easily be floated by means of anionic and cationic collectors [1,2]. Traditionally, froth flotation involves the aggregation of air bubbles and hydrophobic particles in
0169-4332/$ – see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2003.10.024
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S. Koca, M. Savas / Applied Surface Science 225 (2004) 347–355
aqueous media with subsequent levitation of the bubble particle aggregates to the surface and transfer to the froth phase. Whether or not bubble attachment occurs is determined by the degree to which the particle surface is wetted by water. When the surface shows little affinity for water, the surface is said to be hydrophobic, and air bubble attachment will occur. If the surface has an affinity for water, the surface is hydrophilic, and attachment of the air bubble will be impossible. The hydrophobic/hydrophilic balance at the surface of the mineral can be evaluated in terms of the contact angle, developed between the three phases: liquid, solid and gas [3–8]. If the air bubble does not displace the aqueous phase, the contact angle is zero, and complete displacement of the water represents a contact angle of 1808. Values of contact angle between these two extreme provide an indication of the degree of surface hydration or conversely the hydrophobic character of the surface [9]. The contact angle is a very common measure of the hydrophobicity of a solid surface. There is also a relationship between contact angle and floatability of minerals [10]. Therefore, the contact angle is an important parameter in processing of minerals. Contact angles probe the physics and chemistry of the region near the contact line [9,11,12]. For an ideal homogenous solid surface, the three phase equilibrium between the air bubble, mineral surface and water can be described by the respective interfacial tensions (the solid–gas, the solid–liquid and the liquid–gas). Because of the difficulties involved in measuring directly the surface tension of a solid phase, indirect approaches are used including direct force measurements, contact angle measurements, examination of sedimentation of particles, film flotation tests, etc. [13–17]. Among the used methods, contact angle measurements are believed to be the simplest [15,18]. It is well established in the literature that meaningful contact angle measurements can be used in the calculation of solid surface tensions [6–8, 18–20]. Flotation chemistry has been fairly well established for the concentration of minerals [21–26]. Unfortunately, fundamental understanding of the interfacial phenomena involved in boron mineral systems is very much limited. Although previous flotation and adsorption studies on boron minerals have shed some light on the basics of collector adsorption in these systems
[4,27–33], there is only a few data in the literature on colemanite/arsenious minerals flotation systems [1,2,32]. Therefore, further research is warranted in order to develop a complete understanding of the colemanite/realgar flotation systems. In the present work, the contact angle values on the colemanite and realgar surfaces have been measured to further understand the interfacial phenomena involved in colemanite/realgar flotation systems. In this regard, pure colemanite and realgar minerals were subjected to a series of contact angle measurement studies by means of dynamic captive bubble method in the presence of anionic and cationic reagents at different pH values.
2. Experimental 2.1. Sample Colemanite and realgar crystals of high purity were used in this investigation. Samples were carefully picked by hand from Emet Colemanite Boron deposits of Turkey. The chemical analysis of samples is given in Tables 1 and 2. Samples for contact angle measurements were prepared from large pieces of crystals. They were placed into plastic moulds and covered with resin. This resin blocks were left overnight in open air for curing. After curing, the specimens were removed from the mould, and a fresh surface was exposed by wet grinding and polishing. The top surface of the specimens were wet ground under a stream of deionised distilled water with a series of silicon carbide abrasive papers (from 400 to 1200 grits). Finally, the specimens were wet polished with 0.05 mm alumina powder. After careful Table 1 The chemical composition of colemanite sample Component
Weight (%)
B2O3 CaO H2O SiO2 Na2O K2O MgO FeO þ TiO2 þ Al2O3
50.26 27.74 21.21 0.09 0.27 0.15 0.25 0.03
S. Koca, M. Savas / Applied Surface Science 225 (2004) 347–355 Table 2 The chemical composition of realgar sample Component
Weight (%)
As S Others
69.17 29.56 1.27
polishing, polished surfaces of the specimens were cleaned with a jet of deionised distilled water to remove any slimes and alumina powder. Plastic surgical gloves were used throughout the sample preparation procedure to avoid any contamination. The specimens were kept under deionised distilled water prior to contact angle measurements. In the experiments, both anionic and cationic collectors were used. Anionic collectors used included two different xanthates (potassium amyl and ethyl xanthates, KAX and KEX), two different sulphanates (R825 and R840) and a carboxylate (sodium oleate). Armac-T (amine) is cationic collector. Other chemical used in this study included analytical grade sodium silicate (SiO2:Na2O ratio 2:1). KAX, KEX, R825 and R840 were produced and supplied by Cytec, and Armac-T by Akzo Chemie. Sodium oleate and sodium silicate were purchased from Riedel-de Haen Chemical Company. The pH of the suspension was adjusted with analytical grade sodium hydroxide and hydrochloric acid, purchased from Aldrich Chemical Company Inc. Deionised distilled water were used throughout the experiments. 2.2. Method Contact angle measurements were performed by dynamic captive bubble method by using NRL Contact Angle Goniometer Model 100-00. The prepared specimen was placed on two stable supports with the polished surface facing downward in the rectangular glass chamber of the apparatus. The chamber was filled with the desired solution. The sample was then treated in the solution of reagents with different concentrations by waiting for 10 min before measurements. A small air bubble was produced at the tip of a specially designed U shaped needle using a microsyringe. The bubble was then released from the needle tip from a fixed distance (1 cm) below the submerged specimen surface. Released bubble
349
was captured at the solid surface as a result of buoyant transport and attachment. After the air bubble attached to the solid surface, the angles at each side of the bubble were measured. The measurements were repeated four times for each condition and mean value is presented in this paper. The temperature remained at 20–22 8C in all measurements. Colemanite form buffer solutions at about pH 9.3 at which they acquire minimum solubility [27]. Since the solubility of colemanite increases at acidic media, the contact angle measurements at the colemanite surface were carried out above pH 7. The contact angle measurements at the realgar surface were performed between pH 6 and 11. Contact angles were first measured in deionised distilled water for both minerals, and the results were then compared with contact angles that were obtained after treating minerals with reagents of different concentrations.
3. Results and discussion The Young equation is the thermodynamic equilibrium of condition for an ideal solid-liquid-gas capillary system. The contact angle, y, is determined by three interfacial tensions, gsg, gsl and glg as an equilibrium property of the system (gsg, gsl and glg are the solid–gas, the solid–liquid and the liquid–gas interfacial tensions, respectively). However, the validity of Youngs equation requires that the solid surface is smooth, homogenous, inert, non-porous and nondeformable quality which is usually not met by real surfaces [7,14]. As a consequence, the observed contact angle is not unique but falls into a more or less wide interval between advancing (largest) and receding contact angle (smallest). The difference between them is called contact angle hysteresis. The contact angle hysteresis has been studied extensively in the past several decades and will not be discussed in this paper [5,7,12,19,20,34–36]. Previous studies showed that the receding contact angles are usually preferred for flotation systems because these contact angles are supposed to correlate much better with flotation response [37–39]. Therefore, receding contact angles were measured in this study. The measured contact angle values at the colemanite and realgar surfaces in deionised distilled
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40
38
Colemanite
39
None
36
Realgar
38
1E-4 M
Contact Angle, Degrees
Contact Angle, Degrees
40
34 32 30 28 26 24 22
1E-3 M
37
1E-2 M
36 35 34 33 32 31
20
30
5
6
7
8
9
10
11
12
5
6
7
8
pH
9
10
11
12
pH
Fig. 1. The effect of pH on contact angles at the colemanite and realgar surfaces in deionised distilled water.
Fig. 3. The effect of pH on contact angles at the colemanite surface as a function of KEX concentration.
water are shown in Fig. 1. It can be observed from Fig. 1 that an increase in pH caused a slight decrease in the contact angle at the colemanite surface and a small increase in the contact angle at the realgar surface. For example the contact angle value decreased from 34 to 328 at the colemanite surface when pH increased from 7 to 11. An increase in pH from 6 to 10.50 had also almost no effect on the contact angle values, 26–278, at the realgar surface.
It can be observed from Figs. 2 and 3 that the presence and different concentrations of KAX and KEX did not affect the contact angles at the colemanite surface significantly. It can be concluded from the results that the hydrophobicity of the colemanite surface was not improved after treatment with KAX and KEX solutions. In two different studies, the adsorption mechanisms of KAX and KEX on colemanite surface were studied by means of both electrokinetic potential measurements and infrared spectroscopy [2,32]. 40
39
None
39
None
38
1E-4 M
38
0.01 %
37
1E-3 M
37
0.10 %
36
1E-2 M
36
0.50 %
Contact Angle, Degrees
Contact Angle, Degrees
40
35 34 33 32
35 34 33 32 31
31
30
30
5
6
7
8
9
10
11
12
pH Fig. 2. The effect of pH on contact angles at the colemanite surface as a function of KAX concentration.
5
6
7
8
9
10
11
12
pH Fig. 4. The effect of pH on contact angles at the colemanite surface as a function of R825 concentration.
S. Koca, M. Savas / Applied Surface Science 225 (2004) 347–355
50
48
0.01 %
46
0.05 %
44 42 40 38 36 34 32
Contact Angle, Degrees
Contact Angle, Degrees
50
None
48
351
46 44 42
None
40
1E-4 M
38
1E-3 M
36
1E-2 M
34 32
30
30
5
6
7
8
9
10
11
12
pH
5
6
7
8
9
10
11
12
pH
Fig. 5. The effect of pH on contact angles at the colemanite surface as a function of R840 concentration.
Fig. 6. The effect of pH on contact angles at the colemanite surface as a function of sodium oleate concentration.
These studies revealed that these particular xanthates did not adsorb chemically to the surface of colemanite, providing support for a conclusion that can be drawn from our contact angle results. Results from the contact angle measurements are plotted in Figs. 4 and 5 as a function of system pH for colemanite treated with various R825 and R840 concentrations, respectively. As expected, contact angles, measured on surface as treated with R825 remained constant at 33–378 for all solutions from pH 7 to 11 since R825 does not adsorb on colemanite surface [2,32]. On the other hand, the treatment with R840 enhanced the hydrophobicity of colemanite surface. As the concentration of R840 solution was increased from 0 to 0.01 to 0.05%, measured contact angle increased from 34 to 37 to 458 at pH 7, respectively. Similar effects of R840 concentration on contact angles were observed at other pH values. Previous studies documented that R840 adsorb chemically to colemanite surface [2,32]. Fig. 6 presents the contact angle values obtained for colemanite surface treated with sodium oleate solutions. A change in surfactant concentration from 0 to 104 to 103 M did not affect the contact angle measured on mineral surface. A further increase in the surfactant concentration in treating solutions, up to 102 M, caused an increase in the contact angle at mineral surface. At pH 9, contact angle of 458 was measured at the colemanite surface, indicating a
significant improvement in the degree of hydrophobicity. On the other hand, the contact angle at the colemanite surface slightly decreased with an increase in pH. The variation in contact angle correlate well with results of the colemanite flotation, previously reported in the literature [1,28]. Celik et al. [28] and Savas [1] found that the flotation recovery of colemanite increases with an increase in sodim oleate collector dosage, and decreases with increasing pH. There are other reports showing similar correlation between contact angles and floatability for the same systems [1,4,28]. The cationic collector, Armac-T, increased the contact angle at the colemanite surface up to 478 at 0.10% collector dosage. Fig. 7 shows that at pH 9, the change in collector concentrations in the solution from 0 to 0.01 to 0.10% significantly increased the contact angle at colemanite surface from 32 to 44 to 478, respectively. This was already expected since zeta potential measurements and infrared spectroscopy studies revealed that the adsorption of Armac-T on negatively charged colemanite surface takes place over the pH range studied [2,4,32]. Contact angle measurements were also carried out for realgar treated with the same collector solutions. The adsorption behaviour of these collectors on realgar surface was studied by electrokinetic mobility measurements [32]. These results showed that KAX, KEX, R825, R840, sodium oleate and Armac-T were
S. Koca, M. Savas / Applied Surface Science 225 (2004) 347–355
50
70
48
65
46
60
Contact Angle, Degrees
Contact Angle, Degrees
352
44 42 None
40
0.01 %
38
0.10 %
36 34 32
55
None
50
1E-4 M
45
1E-3 M
40
1E-2 M
35 30 25
30
3
4
5
6
7
8
9
10
11
20
12
5
pH Fig. 7. The effect of pH on contact angles at the colemanite surface as a function of Armac-T concentration.
adsorbed on realgar surface, although the mechanism of adsorption remains unknown [2]. The effect of the KAX, KEX and sodium oleate concentrations on the contact angle values at realgar surface was studied at three levels, i.e. 104, 103, 102 M. As showed in Figs. 8–10 contact angle values increased significantly with an increse in collector concentration. Furthermore, it was observed that the effect of KEX on contact angle values was more
6
7
8
9
10
11
12
pH Fig. 9. The effect of pH on contact angles at the realgar surface as a function of KEX concentration.
predominant than KAX and sodium oleate at low collector dosages. Xanthate type collectors produced contact angle values as high as 628 at 102 M collector concentration at pH 9, indicating a significant improvement in the degree of hydrophobicity of the realgar surface. For pH > 9 contact angle values at realgar surface decreased. This may be due to the instability of the adsorbed collector species with respect to surface hydrolysis at higher pH values.
70
50 45
60 55
None
50
1E-4 M
45
1E-3 M
40
1E-2 M
Contact Angle, Degrees
Contact Angle, Degrees
65
35 30
40
None 1E-4 M
35
1E-3 M 1E-2 M
30 25
25 20
20
5
6
7
8
9
10
11
12
pH Fig. 8. The effect of pH on contact angles at the realgar surface as a function of KAX concentration.
5
6
7
8
9
10
11
12
pH Fig. 10. The effect of pH on contact angles at the realgar surface as a function of sodium oleate concentration.
50
50
45
45 Contact Angle, Degrees
Contact Angle, Degrees
S. Koca, M. Savas / Applied Surface Science 225 (2004) 347–355
None
40
0.01 % 0.10 %
35
0.50 %
30 25
353
None 0.01 % 0.10 %
40 35 30 25
20
20
5
6
7
8
9
10
11
12
3
4
5
6
7
8
9
10
11
12
pH
pH
Fig. 13. The effect of pH on contact angles at the realgar surface as a function of Armac-T concentration.
The results correlate with the realgar flotation behaviour reported in the literature [1]. Figs. 11 and 12 present the contact angle values measured at realgar surface as a function of system pH for various R825 and R840 concentrations, respectively. The system shows interesting behaviour for R825. As the collector concentration was increased from 0 to 0.01%, the contact angle values increased from 27 to 458, respectively, at pH 9. A further
increase in the collector concentrations of treatment solutions to 0.10 and 0.50% caused reduction in contact angle values down to 33 and 318, respectively. It may be due to heterogeneity of the mineral surface caused by adsorbed surfactant layer. It was claimed in the literature that a significant scatter of both advancing and receding contact angle values is associated with the heterogeneity of the mineral surface [40]. The addition of R840 did not change the contact angle
40
40
38
38
36
36
1E-3 M
34
1E-2 M
Contact Angle, Degrees
Contact Angle, Degrees
Fig. 11. The effect of pH on contact angles at the realgar surface as a function of R825 concentration.
None
34
0.01 %
32
0.05 %
30 28 26 24
None 1E-4 M
32 30 28 26 24 22
22
20
20
5
6
7
8
9
10
11
12
pH Fig. 12. The effect of pH on contact angles at the realgar surface as a function of R840 concentration.
5
6
7
8
9
10
11
12
pH Fig. 14. The effect of pH on contact angles at the colemanite surface as a function of sodium silicate concentration.
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S. Koca, M. Savas / Applied Surface Science 225 (2004) 347–355
50
1E-4 M
45
Contact Angle, Degrees
4. Conclusions
None
1E-3 M 1E-2 M
40 35 30 25 20
5
6
7
8
9
10
11
12
pH Fig. 15. The effect of pH on contact angles at the realgar surface as a function of sodium silicate concentration.
values at realgar surface at 0.01% collector concentration. A further increase in the collector concentration up to 0.05% increased the contact angle values significantly. It can be observed from Fig. 13 that a change in the Armac-T concentration from 0 to 0.01 to 0.10% increased the contact angle at the realgar surface from 27 to 32 to 428, respectively. It is evident that collector adsorption on the mineral surface as described in the literature [32] induces a significant increase in the degree of realgar hydrophobicity. Sodium silicate were used as a dispersant for gangue minerals during both colemanite and realgar flotation [1]. Therefore, it was decided to carry out contact angle measurements at both colemanite and realgar surfaces in the presence of sodium silicate. Results are illustrated in Figs. 14 and 15 as a function of system pH for various sodium silicate concentrations. The contact angle values at both mineral surfaces were not changed at low surfactant concentrations. At surfactant concentration of 102 M, the contact angle at colemanite surface was decreased down to 278. It is evident that the addition of 102 M sodium silicate induces a hydrophilic character to the surface and attachment was not possible. It is interesting to notice that an increase in surfactant concentration increased the contact angle at realgar surface up to 428.
Receding contact angles at the colemanite and realgar surfaces were measured in the absence and presence of flotation reagents at different pH values. Generally a good agreement was seen with the contact angle measurements results obtained in this investigation and flotation results reported in the literature. In deionised distilled water contact angle values at minerals surfaces were 32–348 for colemanite and 26–278 for realgar, and did not change with varying pH. The contact angle values at the colemanite surface in the presence of KAX, KEX and R825 did not change with increasing collector dosage. In the presence of R840, sodium oleate and Armac-T, the contact angle values at the colemanite surface increased up to 478, indicating a significant improvement in the degree of hydrophobicity that explains the colemanite flotation behaviour, previously reported in the literature. The contact angle values at the realgar surface was affected significantly in the presence of all collector range studied. The contact angles were obtained as high as 62, 63, 45, 46, 39, and 438 for KAX, KEX, sodium oleate, R825, R840 and Armac-T, respectively. These results together with the adsorption behaviours of flotation reagents on colemanite and realgar surfaces explain the lack of selectivity between these two minerals when colemanite is floated. Finally, the results suggest that colemanite and realgar separation can be achieved by floating realgar with either xanthates (KAX or KEX) or R825 in the presence of sodium silicate.
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Contact angle measurements at the colemanite and realgar surfaces Sabiha Koca*, Mehmet Savas Mining Engineering Department, Osmangazi University, 26480 Eskisehir, Turkey Received 2 September 2003; received in revised form 21 October 2003; accepted 21 October 2003
Abstract Colemanite is one of the most important boron minerals and covers an important part of Turkey’s boron mineral deposits. The friable nature of the colemanite tends to produce a large amount of fines. Flotation appears to be a promising technique to recover colemanite from such fines. During flotation process, selectivity problem arises between colemanite and associated gangue minerals such as realgar. There is a close relationship between floatability of minerals and contact angle. Therefore, surface hydrophobicity of colemanite and realgar minerals were investigated by receding contact angle measurements in the absence and presence of flotation reagents. The water contact angle values at the colemanite surface remained almost unchanged at 32–358 in the solutions of potassium amyl xanthate (KAX), potassium ethyl xanthate (KEX) and petroleum sulphanate (R825) while another petroleum sulphanate (R840), sodium oleate and tallow amine (Armac-T) affected hydrophobicity of colemanite, and the contact angle values increased up to 478. The contact angle values of 62, 63, 45, 46, 39, and 438 at the realgar surface were obtained in the solutions of KAX, KEX, sodium oleate, R825, R840 and Armac-T, respectively. # 2003 Elsevier B.V. All rights reserved. PACS: 68.10.Cr Keywords: Colemanite; Realgar; Contact angle; Floatability; Flotation reactives
1. Introduction Colemanite is a hydrated semi-soluble salt mineral and is used in the manufacture of a variety of industrial products including advanced technological compounds such as penta borane, sodium borane and boron tri-chloride. Similar to other boron minerals, the major accompanying gangue minerals associated with colemanite are clays, carbonate minerals and to a *
Corresponding author. Tel.: þ90-222-2393750; fax: þ90-222-2393613. E-mail address: [email protected] (S. Koca).
lesser extend arsenious compounds. Beneneficiation of colemanite at coarser sizes is accomplished by scrubbing for clay removal followed by classification. The friable nature of the colemanite tends to produce a large amount of fines mostly below 0.1 mm. Flotation appears to be a promising technique to recover colemanite from such fines. However, arsenious compounds mainly realgar cause selectivity problems in colemanite flotation since both colemanite and realgar can easily be floated by means of anionic and cationic collectors [1,2]. Traditionally, froth flotation involves the aggregation of air bubbles and hydrophobic particles in
0169-4332/$ – see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2003.10.024
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aqueous media with subsequent levitation of the bubble particle aggregates to the surface and transfer to the froth phase. Whether or not bubble attachment occurs is determined by the degree to which the particle surface is wetted by water. When the surface shows little affinity for water, the surface is said to be hydrophobic, and air bubble attachment will occur. If the surface has an affinity for water, the surface is hydrophilic, and attachment of the air bubble will be impossible. The hydrophobic/hydrophilic balance at the surface of the mineral can be evaluated in terms of the contact angle, developed between the three phases: liquid, solid and gas [3–8]. If the air bubble does not displace the aqueous phase, the contact angle is zero, and complete displacement of the water represents a contact angle of 1808. Values of contact angle between these two extreme provide an indication of the degree of surface hydration or conversely the hydrophobic character of the surface [9]. The contact angle is a very common measure of the hydrophobicity of a solid surface. There is also a relationship between contact angle and floatability of minerals [10]. Therefore, the contact angle is an important parameter in processing of minerals. Contact angles probe the physics and chemistry of the region near the contact line [9,11,12]. For an ideal homogenous solid surface, the three phase equilibrium between the air bubble, mineral surface and water can be described by the respective interfacial tensions (the solid–gas, the solid–liquid and the liquid–gas). Because of the difficulties involved in measuring directly the surface tension of a solid phase, indirect approaches are used including direct force measurements, contact angle measurements, examination of sedimentation of particles, film flotation tests, etc. [13–17]. Among the used methods, contact angle measurements are believed to be the simplest [15,18]. It is well established in the literature that meaningful contact angle measurements can be used in the calculation of solid surface tensions [6–8, 18–20]. Flotation chemistry has been fairly well established for the concentration of minerals [21–26]. Unfortunately, fundamental understanding of the interfacial phenomena involved in boron mineral systems is very much limited. Although previous flotation and adsorption studies on boron minerals have shed some light on the basics of collector adsorption in these systems
[4,27–33], there is only a few data in the literature on colemanite/arsenious minerals flotation systems [1,2,32]. Therefore, further research is warranted in order to develop a complete understanding of the colemanite/realgar flotation systems. In the present work, the contact angle values on the colemanite and realgar surfaces have been measured to further understand the interfacial phenomena involved in colemanite/realgar flotation systems. In this regard, pure colemanite and realgar minerals were subjected to a series of contact angle measurement studies by means of dynamic captive bubble method in the presence of anionic and cationic reagents at different pH values.
2. Experimental 2.1. Sample Colemanite and realgar crystals of high purity were used in this investigation. Samples were carefully picked by hand from Emet Colemanite Boron deposits of Turkey. The chemical analysis of samples is given in Tables 1 and 2. Samples for contact angle measurements were prepared from large pieces of crystals. They were placed into plastic moulds and covered with resin. This resin blocks were left overnight in open air for curing. After curing, the specimens were removed from the mould, and a fresh surface was exposed by wet grinding and polishing. The top surface of the specimens were wet ground under a stream of deionised distilled water with a series of silicon carbide abrasive papers (from 400 to 1200 grits). Finally, the specimens were wet polished with 0.05 mm alumina powder. After careful Table 1 The chemical composition of colemanite sample Component
Weight (%)
B2O3 CaO H2O SiO2 Na2O K2O MgO FeO þ TiO2 þ Al2O3
50.26 27.74 21.21 0.09 0.27 0.15 0.25 0.03
S. Koca, M. Savas / Applied Surface Science 225 (2004) 347–355 Table 2 The chemical composition of realgar sample Component
Weight (%)
As S Others
69.17 29.56 1.27
polishing, polished surfaces of the specimens were cleaned with a jet of deionised distilled water to remove any slimes and alumina powder. Plastic surgical gloves were used throughout the sample preparation procedure to avoid any contamination. The specimens were kept under deionised distilled water prior to contact angle measurements. In the experiments, both anionic and cationic collectors were used. Anionic collectors used included two different xanthates (potassium amyl and ethyl xanthates, KAX and KEX), two different sulphanates (R825 and R840) and a carboxylate (sodium oleate). Armac-T (amine) is cationic collector. Other chemical used in this study included analytical grade sodium silicate (SiO2:Na2O ratio 2:1). KAX, KEX, R825 and R840 were produced and supplied by Cytec, and Armac-T by Akzo Chemie. Sodium oleate and sodium silicate were purchased from Riedel-de Haen Chemical Company. The pH of the suspension was adjusted with analytical grade sodium hydroxide and hydrochloric acid, purchased from Aldrich Chemical Company Inc. Deionised distilled water were used throughout the experiments. 2.2. Method Contact angle measurements were performed by dynamic captive bubble method by using NRL Contact Angle Goniometer Model 100-00. The prepared specimen was placed on two stable supports with the polished surface facing downward in the rectangular glass chamber of the apparatus. The chamber was filled with the desired solution. The sample was then treated in the solution of reagents with different concentrations by waiting for 10 min before measurements. A small air bubble was produced at the tip of a specially designed U shaped needle using a microsyringe. The bubble was then released from the needle tip from a fixed distance (1 cm) below the submerged specimen surface. Released bubble
349
was captured at the solid surface as a result of buoyant transport and attachment. After the air bubble attached to the solid surface, the angles at each side of the bubble were measured. The measurements were repeated four times for each condition and mean value is presented in this paper. The temperature remained at 20–22 8C in all measurements. Colemanite form buffer solutions at about pH 9.3 at which they acquire minimum solubility [27]. Since the solubility of colemanite increases at acidic media, the contact angle measurements at the colemanite surface were carried out above pH 7. The contact angle measurements at the realgar surface were performed between pH 6 and 11. Contact angles were first measured in deionised distilled water for both minerals, and the results were then compared with contact angles that were obtained after treating minerals with reagents of different concentrations.
3. Results and discussion The Young equation is the thermodynamic equilibrium of condition for an ideal solid-liquid-gas capillary system. The contact angle, y, is determined by three interfacial tensions, gsg, gsl and glg as an equilibrium property of the system (gsg, gsl and glg are the solid–gas, the solid–liquid and the liquid–gas interfacial tensions, respectively). However, the validity of Youngs equation requires that the solid surface is smooth, homogenous, inert, non-porous and nondeformable quality which is usually not met by real surfaces [7,14]. As a consequence, the observed contact angle is not unique but falls into a more or less wide interval between advancing (largest) and receding contact angle (smallest). The difference between them is called contact angle hysteresis. The contact angle hysteresis has been studied extensively in the past several decades and will not be discussed in this paper [5,7,12,19,20,34–36]. Previous studies showed that the receding contact angles are usually preferred for flotation systems because these contact angles are supposed to correlate much better with flotation response [37–39]. Therefore, receding contact angles were measured in this study. The measured contact angle values at the colemanite and realgar surfaces in deionised distilled
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40
38
Colemanite
39
None
36
Realgar
38
1E-4 M
Contact Angle, Degrees
Contact Angle, Degrees
40
34 32 30 28 26 24 22
1E-3 M
37
1E-2 M
36 35 34 33 32 31
20
30
5
6
7
8
9
10
11
12
5
6
7
8
pH
9
10
11
12
pH
Fig. 1. The effect of pH on contact angles at the colemanite and realgar surfaces in deionised distilled water.
Fig. 3. The effect of pH on contact angles at the colemanite surface as a function of KEX concentration.
water are shown in Fig. 1. It can be observed from Fig. 1 that an increase in pH caused a slight decrease in the contact angle at the colemanite surface and a small increase in the contact angle at the realgar surface. For example the contact angle value decreased from 34 to 328 at the colemanite surface when pH increased from 7 to 11. An increase in pH from 6 to 10.50 had also almost no effect on the contact angle values, 26–278, at the realgar surface.
It can be observed from Figs. 2 and 3 that the presence and different concentrations of KAX and KEX did not affect the contact angles at the colemanite surface significantly. It can be concluded from the results that the hydrophobicity of the colemanite surface was not improved after treatment with KAX and KEX solutions. In two different studies, the adsorption mechanisms of KAX and KEX on colemanite surface were studied by means of both electrokinetic potential measurements and infrared spectroscopy [2,32]. 40
39
None
39
None
38
1E-4 M
38
0.01 %
37
1E-3 M
37
0.10 %
36
1E-2 M
36
0.50 %
Contact Angle, Degrees
Contact Angle, Degrees
40
35 34 33 32
35 34 33 32 31
31
30
30
5
6
7
8
9
10
11
12
pH Fig. 2. The effect of pH on contact angles at the colemanite surface as a function of KAX concentration.
5
6
7
8
9
10
11
12
pH Fig. 4. The effect of pH on contact angles at the colemanite surface as a function of R825 concentration.
S. Koca, M. Savas / Applied Surface Science 225 (2004) 347–355
50
48
0.01 %
46
0.05 %
44 42 40 38 36 34 32
Contact Angle, Degrees
Contact Angle, Degrees
50
None
48
351
46 44 42
None
40
1E-4 M
38
1E-3 M
36
1E-2 M
34 32
30
30
5
6
7
8
9
10
11
12
pH
5
6
7
8
9
10
11
12
pH
Fig. 5. The effect of pH on contact angles at the colemanite surface as a function of R840 concentration.
Fig. 6. The effect of pH on contact angles at the colemanite surface as a function of sodium oleate concentration.
These studies revealed that these particular xanthates did not adsorb chemically to the surface of colemanite, providing support for a conclusion that can be drawn from our contact angle results. Results from the contact angle measurements are plotted in Figs. 4 and 5 as a function of system pH for colemanite treated with various R825 and R840 concentrations, respectively. As expected, contact angles, measured on surface as treated with R825 remained constant at 33–378 for all solutions from pH 7 to 11 since R825 does not adsorb on colemanite surface [2,32]. On the other hand, the treatment with R840 enhanced the hydrophobicity of colemanite surface. As the concentration of R840 solution was increased from 0 to 0.01 to 0.05%, measured contact angle increased from 34 to 37 to 458 at pH 7, respectively. Similar effects of R840 concentration on contact angles were observed at other pH values. Previous studies documented that R840 adsorb chemically to colemanite surface [2,32]. Fig. 6 presents the contact angle values obtained for colemanite surface treated with sodium oleate solutions. A change in surfactant concentration from 0 to 104 to 103 M did not affect the contact angle measured on mineral surface. A further increase in the surfactant concentration in treating solutions, up to 102 M, caused an increase in the contact angle at mineral surface. At pH 9, contact angle of 458 was measured at the colemanite surface, indicating a
significant improvement in the degree of hydrophobicity. On the other hand, the contact angle at the colemanite surface slightly decreased with an increase in pH. The variation in contact angle correlate well with results of the colemanite flotation, previously reported in the literature [1,28]. Celik et al. [28] and Savas [1] found that the flotation recovery of colemanite increases with an increase in sodim oleate collector dosage, and decreases with increasing pH. There are other reports showing similar correlation between contact angles and floatability for the same systems [1,4,28]. The cationic collector, Armac-T, increased the contact angle at the colemanite surface up to 478 at 0.10% collector dosage. Fig. 7 shows that at pH 9, the change in collector concentrations in the solution from 0 to 0.01 to 0.10% significantly increased the contact angle at colemanite surface from 32 to 44 to 478, respectively. This was already expected since zeta potential measurements and infrared spectroscopy studies revealed that the adsorption of Armac-T on negatively charged colemanite surface takes place over the pH range studied [2,4,32]. Contact angle measurements were also carried out for realgar treated with the same collector solutions. The adsorption behaviour of these collectors on realgar surface was studied by electrokinetic mobility measurements [32]. These results showed that KAX, KEX, R825, R840, sodium oleate and Armac-T were
S. Koca, M. Savas / Applied Surface Science 225 (2004) 347–355
50
70
48
65
46
60
Contact Angle, Degrees
Contact Angle, Degrees
352
44 42 None
40
0.01 %
38
0.10 %
36 34 32
55
None
50
1E-4 M
45
1E-3 M
40
1E-2 M
35 30 25
30
3
4
5
6
7
8
9
10
11
20
12
5
pH Fig. 7. The effect of pH on contact angles at the colemanite surface as a function of Armac-T concentration.
adsorbed on realgar surface, although the mechanism of adsorption remains unknown [2]. The effect of the KAX, KEX and sodium oleate concentrations on the contact angle values at realgar surface was studied at three levels, i.e. 104, 103, 102 M. As showed in Figs. 8–10 contact angle values increased significantly with an increse in collector concentration. Furthermore, it was observed that the effect of KEX on contact angle values was more
6
7
8
9
10
11
12
pH Fig. 9. The effect of pH on contact angles at the realgar surface as a function of KEX concentration.
predominant than KAX and sodium oleate at low collector dosages. Xanthate type collectors produced contact angle values as high as 628 at 102 M collector concentration at pH 9, indicating a significant improvement in the degree of hydrophobicity of the realgar surface. For pH > 9 contact angle values at realgar surface decreased. This may be due to the instability of the adsorbed collector species with respect to surface hydrolysis at higher pH values.
70
50 45
60 55
None
50
1E-4 M
45
1E-3 M
40
1E-2 M
Contact Angle, Degrees
Contact Angle, Degrees
65
35 30
40
None 1E-4 M
35
1E-3 M 1E-2 M
30 25
25 20
20
5
6
7
8
9
10
11
12
pH Fig. 8. The effect of pH on contact angles at the realgar surface as a function of KAX concentration.
5
6
7
8
9
10
11
12
pH Fig. 10. The effect of pH on contact angles at the realgar surface as a function of sodium oleate concentration.
50
50
45
45 Contact Angle, Degrees
Contact Angle, Degrees
S. Koca, M. Savas / Applied Surface Science 225 (2004) 347–355
None
40
0.01 % 0.10 %
35
0.50 %
30 25
353
None 0.01 % 0.10 %
40 35 30 25
20
20
5
6
7
8
9
10
11
12
3
4
5
6
7
8
9
10
11
12
pH
pH
Fig. 13. The effect of pH on contact angles at the realgar surface as a function of Armac-T concentration.
The results correlate with the realgar flotation behaviour reported in the literature [1]. Figs. 11 and 12 present the contact angle values measured at realgar surface as a function of system pH for various R825 and R840 concentrations, respectively. The system shows interesting behaviour for R825. As the collector concentration was increased from 0 to 0.01%, the contact angle values increased from 27 to 458, respectively, at pH 9. A further
increase in the collector concentrations of treatment solutions to 0.10 and 0.50% caused reduction in contact angle values down to 33 and 318, respectively. It may be due to heterogeneity of the mineral surface caused by adsorbed surfactant layer. It was claimed in the literature that a significant scatter of both advancing and receding contact angle values is associated with the heterogeneity of the mineral surface [40]. The addition of R840 did not change the contact angle
40
40
38
38
36
36
1E-3 M
34
1E-2 M
Contact Angle, Degrees
Contact Angle, Degrees
Fig. 11. The effect of pH on contact angles at the realgar surface as a function of R825 concentration.
None
34
0.01 %
32
0.05 %
30 28 26 24
None 1E-4 M
32 30 28 26 24 22
22
20
20
5
6
7
8
9
10
11
12
pH Fig. 12. The effect of pH on contact angles at the realgar surface as a function of R840 concentration.
5
6
7
8
9
10
11
12
pH Fig. 14. The effect of pH on contact angles at the colemanite surface as a function of sodium silicate concentration.
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50
1E-4 M
45
Contact Angle, Degrees
4. Conclusions
None
1E-3 M 1E-2 M
40 35 30 25 20
5
6
7
8
9
10
11
12
pH Fig. 15. The effect of pH on contact angles at the realgar surface as a function of sodium silicate concentration.
values at realgar surface at 0.01% collector concentration. A further increase in the collector concentration up to 0.05% increased the contact angle values significantly. It can be observed from Fig. 13 that a change in the Armac-T concentration from 0 to 0.01 to 0.10% increased the contact angle at the realgar surface from 27 to 32 to 428, respectively. It is evident that collector adsorption on the mineral surface as described in the literature [32] induces a significant increase in the degree of realgar hydrophobicity. Sodium silicate were used as a dispersant for gangue minerals during both colemanite and realgar flotation [1]. Therefore, it was decided to carry out contact angle measurements at both colemanite and realgar surfaces in the presence of sodium silicate. Results are illustrated in Figs. 14 and 15 as a function of system pH for various sodium silicate concentrations. The contact angle values at both mineral surfaces were not changed at low surfactant concentrations. At surfactant concentration of 102 M, the contact angle at colemanite surface was decreased down to 278. It is evident that the addition of 102 M sodium silicate induces a hydrophilic character to the surface and attachment was not possible. It is interesting to notice that an increase in surfactant concentration increased the contact angle at realgar surface up to 428.
Receding contact angles at the colemanite and realgar surfaces were measured in the absence and presence of flotation reagents at different pH values. Generally a good agreement was seen with the contact angle measurements results obtained in this investigation and flotation results reported in the literature. In deionised distilled water contact angle values at minerals surfaces were 32–348 for colemanite and 26–278 for realgar, and did not change with varying pH. The contact angle values at the colemanite surface in the presence of KAX, KEX and R825 did not change with increasing collector dosage. In the presence of R840, sodium oleate and Armac-T, the contact angle values at the colemanite surface increased up to 478, indicating a significant improvement in the degree of hydrophobicity that explains the colemanite flotation behaviour, previously reported in the literature. The contact angle values at the realgar surface was affected significantly in the presence of all collector range studied. The contact angles were obtained as high as 62, 63, 45, 46, 39, and 438 for KAX, KEX, sodium oleate, R825, R840 and Armac-T, respectively. These results together with the adsorption behaviours of flotation reagents on colemanite and realgar surfaces explain the lack of selectivity between these two minerals when colemanite is floated. Finally, the results suggest that colemanite and realgar separation can be achieved by floating realgar with either xanthates (KAX or KEX) or R825 in the presence of sodium silicate.
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