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Contact Angle Studies of Ethyl Xanthate Coated Galena Particles
JOURNAL OF COLLOID AND INTERFACE SCIENCE ARTICLE NO.
184, 512–518 (1996)
0646
Contact Angle Studies of Ethyl Xanthate Coated Galena Particles CLIVE A. PRESTIDGE 1
AND
JOHN RALSTON
Ian Wark Research Institute, University of South Australia, The Levels, S.A. 5095, Australia Received February 2, 1996; accepted July 11, 1996
The wettability of ethyl xanthate coated galena particles has been determined from measurements of the equilibrium capillary pressure across a packed bed. Both the advancing liquid powder contact angle, up , and the specific wettable surface area, Awet , of galena particles have been obtained. The influence of particle size and ethyl xanthate surface coverage on the powder contact angle have been investigated and compared with floatability. Ethyl xanthate at a surface coverage of less than ten percent of an equivalent monolayer significantly increases both up and particle floatability. q 1996 Academic Press, Inc.
Key Words: galena particles; ethyl xanthate; contact angles; wettability and galena; flotation.
INTRODUCTION
The flotation separation of sulfide minerals is controlled to a large degree by the relative wettabilities of mineral particles in a pulp and is therefore related to their respective surface hydrophobicities. Sulfide minerals can be rendered selectively hydrophobic in the absence of collector molecules with, in general, only a rather modest impact on their floatability. Selective collector adsorption is normally required to confer the necessary hydrophobicity for strong, specific bubble–particle interaction and thus rapid flotation. For lead sulfide (galena) much effort has been directed at correlating surface chemistry, wettability, and flotation behavior (1–6). Generally the solid–liquid–vapor contact angle is used as a measure of wettability and is determined with the sessile drop or bubble technique on either freshly cleaved or highly polished galena surfaces. Surface roughness, contamination with grinding/polishing media, and associated oxidation effects, among others, have together conspired to prevent an unequivocal link between collector coverage, contact angle, and flotation being established. Sarkar and Gaudin (3) showed that upon increasing xanthate surface coverage on galena from 1.3 to 1.7 monolayers, the contact angle increased from a minimum to a maximum value. Mok and Salman (2), however, suggested that 5 to 1
To whom correspondence should be addressed.
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• precise control of the surface chemistry of galena; • accurate and reproducible determination of contact angles, which are representative of galena particles; and • accurate determination of xanthate adsorption with the subsequent conversion into a surface coverage expressed as ‘‘equivalent monolayers.’’
With respect to the surface chemistry of galena, there is now direct evidence for considerable surface heterogeneity, with lead-oxide, -hydroxide, -carbonate, -hydroxycarbonate, -sulfate, -thiosulfate, and metal-deficient/sulfur-rich layers having been reported (9–14). The relative predominance of these layers is controlled by the pH, electrochemical potential (Eh ), and hydrodynamic conditions (13) used during mineral conditioning. In addition, impurities present in natural galena samples dramatically effect the rate of surface oxidation (14), with the source of the mineral becoming an important variable. In many of the earlier contact angle studies of galena these variables were not rigorously controlled, making the precise surface chemistry impossible to define. It is essential to obtain precise control of galena surface chemistry, enabling reproducible and accurate contact angles
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20 monolayers were required for the same contact angle changes. Gaudin and Preller (1) reported that hydrophobic surfaces, with a water advancing contact angle greater than 907, are generated at less than a monolayer of xanthate coverage, an observation which agreed with the voltammetric studies of Gardner and Woods (7), who suggested that less than a monolayer of xanthate is required to float galena. Granville et al. (5) reviewed the earlier studies on the hydrophobicity of galena surfaces and concluded that ‘‘although many attempts had been made at relating contact angle measurements with collector adsorption and floatability, they were unlikely to yield definite information on the detailed chemical mechanisms of the interactions at the galena surface.’’ These earlier studies and their modern versions [e.g., (6)] have not identified the exact relationship which should exist between collector coverage, contact angle, and flotation recovery for galena, in contrast to quartz (8). For galena the major requirements for determining this relationship between collector coverage, contact angle, and flotation recovery are:
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to be obtained, if a sensible correlation with floatability is to be made. The majority of contact angle studies on sulfide minerals have been performed using the sessile drop or sessile bubble techniques (4–6). As noted above, their reliability is suspect. Furthermore, these techniques take measurements from single surfaces, which may not be representative of mineral particles in a pulp. Contact angle measurements on powders reduce the problems associated with single-surface studies. In this study, advancing liquid powder contact angles ( up ) are determined from equilibrium measurements of the capillary pressure increment required to prevent the movement of liquid through a packed bed of mineral particles. The Laplace–White equation (15) gives a strict expression for the equilibrium capillary pressure, Dp, in porous media, Dp Å glv cos up
rAwetf , (1 0 f )
[1]
where glv is the liquid–vapor surface tension, f is the volume fraction of the solid in the packed bed, r is the density of solid, and Awet is the specific wettable surface area of the solid (effectively the area of the solid–liquid interface). It should be noted that for sulfide mineral particles, with inherent surface chemical heterogeneity and anisotropic surface tensions, the contact angle measured by this approach is an average value. Using this approach, Prestidge and Ralston (13) have recently used the capillary method of Diggins et al. (16, 17) to determine the powder contact angles of galena particles in the absence of collector. The aim of the present work was to extent this study to investigate the influence of ethyl xanthate surface coverage on the wettability of galena particles. Powder contact angle measurements are compared with microflotation data. EXPERIMENTAL
Reagents High purity water was produced by reverse osmosis, passage through two stages of mixed bed ion exchange resin followed by two stages of activated carbon and a final filtering step through a 0.22-mm filter. The conductivity was less than 0.5 mS m01 with a surface tension of 72.8 mN m01 at 207C. High purity nitrogen and oxygen were used throughout the study. Potassium ethyl xanthate (KEX) was purchased from Kodak as an industrial grade reagent and was subsequently purified by recrystallization from petroleum ether, then vacuum dried and stored under vacuum (18). The purity of the KEX was confirmed from the UV extinction coefficient at 301 nm. A value of 17,600 dm3 mol 01 cm01 was obtained, which compared favorably with previously reported values (19, 20).
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Galena (Brushy Creek, Missouri) was purchased from Wards Natural Science Establishment Laboratories, New York, and subsequently analyzed by the Australian Mineral Development Laboratories (AMDEL). The elemental analyses suggested that the galena was ú99.8% pure, with 2 ppm copper, 11 ppm zinc, and 72 ppm iron as the major impurities. Other reagents used were of analytical grade (unless otherwise stated). Mineral Grinding and Conditioning Galena samples were prepared by wet-grinding in a ceramic mortar and pestle at pH 7; size fractions of /75–150mm, /38–75mm, and /20–38mm were obtained by sieving through stainless steel sieves. Samples were deslimed by cycles of 1 min sonication followed by decantation of the fines until a clear supernatant resulted. Particle size distributions and specific surface areas based on the equivalent spherical diameter distribution of particles (Aesd ) were measured using a Malvern Model 2600C laser diffractometer. Surface area measurements were also performed by nitrogen and krypton adsorption using a Coulter Omnisorp model 100. In general the average BET surface areas (Abet ) were three to four times greater than those calculated on a geometric basis (Aesd ), suggesting surface roughness and/or the presence of strongly bound submicron particles, which were not removed during preparation. The latter have been observed by high resolution scanning electron microscopy. Nonsphericity in galena particle shape may also contribute to the observed differences in surface area. For all galena samples used in this study, average particle diameters (D ( 50 ) ), particle size spans (D ( 10 ) to D ( 90 ) ), Aesd values, Abet values, and volume fractions ( f ) of the packed mineral bed are reported in Table 1. The freshly ground and sized galena particles were either dried as described below or presented directly to the reaction vessel for conditioning with ethyl xanthate prior to drying. Galena samples of 20 g were conditioned in a 300 cm3 reaction vessel with air-equilibrated ethyl xanthate solutions, while the pH was maintained at 7 by small additions of 10 02 M sodium hydroxide and nitric acid. Ethyl xanthate adsorption was determined by UV spectrophotometry and the surface coverages were calculated as described below. Prior to contact angle and flotation measurement the galena samples were filtered, then dried in a vacuum desiccator over silica gel. In agreement with previously reported FTIR studies (20), FTIR studies on the galena particles presently under investigation have revealed surface lead(II) ethyl xanthate species and confirmed that the particle treatment, as described above, had negligible influence on the form or surface coverage of adsorbed ethyl xanthate. Techniques Capillary pressure measurements. The capillary pressure measuring device was based on that originally reported
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TABLE 1 Particle Size, Specific Surface Area, and Volume Fraction Data for Powdered Galena Samples
Galena Sample
Da(50) (mm)
Spanb (mm)
Aesd (m2 g01)
Abet (m2 g01)
Volume fraction of particle bed, f
A (150–75 mm) B (75–38 mm) C (38–20 mm)
108 54.3 30.1
97.4 42.9 21.4
0.012 0.020 0.032
0.027 0.080 0.165
0.601 0.602 0.611
a b
Awet (m2 g01) 0.011 0.021 0.0305
D(50) Å 50% point of the cumulative undersize distribution. The span of the distribution measures the spread between the 10 and 90% points of the cumulative undersize distribution, D(90) 0 D(10) .
by Diggins et al. (16) and modified by Prestidge and Ralston (13). Details of the instrument and experimental method have been previously reported (13). UV-Vis spectrophotometry. Sized galena particles were combined with KEX solutions at concentrations in the range from 10 04 to 10 06 M, depending on the particle size and the required collector coverage. The mineral slurries were then stirred and their supernatants circulated through a 10-mm quartz UV cell. UV spectra from 200–400 nm were recorded as a function of time using a Philips Model PU8470, singlebeam, UV-Vis spectrophotometer. Details of the experimental arrangement have been reported previously (19, 20). Ethyl xanthate may be converted to ethyl monothiocarbonate (MTC), ethyl perxanthate (EPX), or ethyl dixanthogen ((EX)2 ) by sulfide mineral surfaces (19). Therefore the amount of ethyl xanthate (EX0 ) adsorbed at the mineral surface may be accurately determined from a mass balance for all of the ethyl xanthate species:
ther ethyl perxanthate or diethyl dixanthogen, with only a small contribution from ethyl monothiocarbonate. These findings are in agreement with previous UV studies on freshly prepared galena particles (20). Ethyl xanthate surface coverages for galena sample B (38 to 75 mm), determined using Eq. [3] with Abet used for As , along with relevant ethyl xanthate concentrations, are given in Table 2. At pH 7 with air equilibrated aqueous solutions, relatively high affinity ethyl xanthate adsorption behavior was exhibited, with ú95% of the initial ethyl xanthate removed from the solution within the 30 min of contact. Galena samples A and C showed effectively identical behavior. Mechanisms for ethyl xanthate adsorption on galena surfaces have been extensively discussed in the literature (e.g., 5–7, 20, 22, 23), although some controversy still remains. It is now generally considered that for an unoxidized galena surface, lead(II) ethyl xanthate forms through an electrochemically driven reaction:
[EX0 ]ads Å [EX0 ]0 0 [MTC] 0 [EPX] 0 2[(EX)2 ] 0 [EX0 ]t ,
PbS / 2EX0 r Pb(EX)2 / ‘‘S 0 ’’ / 2e 0 ,
where [EX0 ]0 is the original concentration of EX0 and [EX0 ]t is the concentration at any time, t. Ethyl xanthate surface coverages, G ( EX ) , were determined from the concentration of adsorbed ethyl xanthate, [EX0 ]ads (mol m03 ), the solution volume, V (m3 ), the mineral surface area, As (m2 ), and by assuming the area occupied by an adsorbed ethyl xanthate molecule, Aa , to be 1.73 1 10 5 m2 ˚ 2 per molecule (1)): mol 01 (28.8 A [EX0 ]adsV Aa . [3] As Microflotation. Microflotation measurements were performed in a modified Partridge and Smith cell (21); details were reported previously (13).
with oxygen reduction generally the other half-cell reaction. However, the precise form of the adsorbed lead(II) ethyl xanthate and whether ‘‘S 0 ’’ is elemental sulfur, polysulfide, or a metal-deficient sulfide are still under debate. It is unequivocal that the rate and extent of lead(II) ethyl xanthate formation is strongly dependent on Eh and the presence of other reagents [e.g., cyanide (20)]. For oxidized galena samples, lead(II) xanthate may form through precipitation with dissolved lead ions,
G ( EX ) Å
RESULTS AND DISCUSSION
Ethyl Xanthate Treatment of Galena Particles UV studies. UV spectra of the supernatants from ethyl xanthate treated galena slurries showed no evidence for ei-
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[4]
[2]
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Pb 2/ / 2EX0 r Pb(EX)2 ,
[5]
and/or an ion-exchange with oxidized surface groups, PbS.Pb(OH)2 / 2EX0 r PbS.Pb(EX)2 / 2OH 0 .
[6]
In the present work, all galena samples were sieved, deslimed, and conditioned for 30 min in air-equilibrated water at pH 7 (Eh / 400 mV) prior to ethyl xanthate addition.
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TABLE 2 Ethyl Xanthate Surface Coverage on Galena Sample B as a Function of Initial Ethyl Xanthate Concentration Galena sample B: maximum theoretical ethyl xanthate surface coverage/equivalent monolayers 0 0.1 0.2 0.5 1.0 2.0 4.0
Initial EX0 concentration, [EX]0 (M)
EX0 concentration after 30 min [EX]tÅ30 min (M)
0 1 1 1 1 1 1
0 õ1007 õ1007 9 1 1007 2.2 1 1006 4.3 1 1006 7 1 1006
3.08 6.17 1.54 3.08 6.17 1.23
1006 1006 1005 1005 1005 1004
Adsorbed EX0 concentration, [EX]ads (M)
Actual EX0 surface coverage, G(EX) (equivalent monolayers)
0 1 1 1 1 1 1
0 0.1 0.2 0.47 0.93 1.86 3.77
3.08 6.17 1.45 2.86 5.74 1.16
1006 1006 1005 1005 1005 1004
Note. Surface coverages determined after 30 min contact between galena particles and air purged solution of ethyl xanthate at pH 7. (In all cases [EPX0] and [EX2] where õ1007 M and [MTC0] õ 1006 M).
Reactions [4], [5], and [6] may all therefore be postulated as feasible routes for ethyl xanthate adsorption. Reaction [4] is responsible for ethyl xanthate surface coverages up to 1 monolayer, with multilayer surface coverages presumably forming through reactions [5] and [6]. Interestingly, recent scan probe microscopy studies (24) have shown ethyl xanthate treated galena surfaces to be topographically heterogeneous in nature, with islands or colloidal particles of lead ethyl xanthate. Surface heterogeneity of sulfide mineral particles may influence wettability and hence flotation behavior. The precise influence of physical heterogeneities on the powder contact angles of sulfide mineral particles is beyond the scope of the present work, but should be addressed in the future. In this work we consider the influence of chemical heterogeneities on the powder contact angle of galena. Wettability Studies Capillary pressure studies. During the operation of the equilibrium capillary pressure measurement device, or any other apparatus for particle contact angle determination, it is desirable that the wetting liquid does not influence the surfaces under study. With sulfide mineral surfaces, which are inherently reactive toward aqueous solutions, this is obviously a difficult requirement to meet. UV studies of both water and cyclohexane after contact with ethyl xanthate treated galena particles showed no evidence for extracted ethyl xanthate or lead ethyl xanthate species. Interestingly, spectral features characteristic of sulfur or polysulfide species (25) were detected in the cyclohexane extract, which were presumably formed through reaction [4]; this area of investigation clearly warrants further study. We are confident that these findings in no way influence the determination of particle contact angles as a function of ethyl xanthate surface coverage, as described below. Examples of capillary pressure against time plots for galena sample B (at ethyl xanthate surface coverages of zero
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and one equivalent monolayer), using water and cyclohexane as the wetting liquid, are given in Fig. 1. This data clearly demonstrates that the rate of capillary pressure increase and the equilibrium capillary pressure reached is dependent on the wetting solvent and the collector coverage. Equilibrium capillary pressures are apparently reached significantly more quickly with cyclohexane than water. Equilibrium capillary pressure values have been shown to give a reliable measurement of particle wettability and have enabled contact angles of galena particles to be determined at various surface coverages of ethyl xanthate. Adjusted equilibrium capillary pressures ( Dp(1 0 f )/ f ) (16) for galena sample B (at various ethyl xanthate surface coverages), determined using both cyclohexane and water as the wetting liquids, are plotted against the reciprocal of the average particle size in Figs. 2a and 2b, respectively. Adjusted equilibrium capillary pressures determined using both cyclohexane and water as the wetting liquids are plotted
FIG. 1. Capillary pressure versus time plots for powdered galena sample B with ethyl xanthate surface coverages of zero and one equivalent monolayer, using water and cyclohexane as the wetting liquids.
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Awet values are independent of ethyl xanthate coverage and are in better agreement with Aesd than Abet . This finding suggests that the moving wetting front in the capillary pressure experiment only accesses the geometric area of the particles, without probing the surface roughness, porosity, or attached fines which are accessed by gas adsorption. An equivalent finding has been reported for untreated galena particles (13). Water as the wetting liquid. For water-wetted galena particles, plots of adjusted equilibrium capillary pressures against the reciprocal of the average particle size (Fig. 2b) and Aesd (Fig. 3b) are linear, but, unlike the equivalent data with cyclohexane as the wetting liquid, are strongly dependent on ethyl xanthate surface coverage. That is, with water as the wetting liquid the equilibrium capillary pressure is dependent on both the particle size and the surface hydrophobicity, which in this case is controlled by collector coverage. The differences in the gradients of the plots in Fig. 3b and 4b are related to the hydrophobicities of the galena samples and their respective particle contact angles. Powder contact angles and collector coverage. Equilibrium capillary pressures for galena sample B, wetted with both cyclohexane and water, are plotted against ethyl xanthate surface coverage in Fig. 4a. When cyclohexane is the
FIG. 2. Adjusted equilibrium capillary pressures for (a) cyclohexane wetted and (b) water wetted, ethyl xanthate coated galena particles, plotted against the reciprocal of the average particle diameter.
against Aesd in Figs. 3a and 3b, respectively. The linearity of the plots in Figs. 2 and 3 validates White’s theory (15) for ethyl xanthate treated galena particles and was previously observed in examinations of the wetting of angular quartz particles (16) and untreated galena particles (13). Equilibrium capillary pressures determined using cyclohexane and water as the wetting liquids are discussed separately in the following sections. Cyclohexane as the wetting liquid. Previously (13) it was established that cyclohexane ( glv of 25.5 mN m01 ) is a perfect wetting liquid for uncoated galena particles. That is, galena particles have a zero contact angle with cyclohexane and capillary pressure studies using cyclohexane probe the geometry of the particle bed rather than surface chemistry. Further, the data in Figs. 2 and 3 show that equilibrium capillary pressures determined with cyclohexane are dependent on particle size or surface area, but independent of the surface chemistry, as controlled by ethyl xanthate surface coverage in this case. These findings confirm that cyclohexane is a perfectly wetting liquid for ethyl xanthate coated galena particles. In Table 1 the determined Awet values are compared with specific surface areas based on geometric considerations (Aesd ) and from gas adsorption measurements (Abet ). The
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FIG. 3. Adjusted equilibrium capillary pressures for (a) cyclohexane wetted and (b) water wetted, ethyl xanthate coated galena particles, plotted against the specific surface area (Aesd ).
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up Å cos 01
FIG. 4. (a) Equilibrium capillary pressures (cyclohexane and water wetted) and (b) particle contact angles for galena particles (sample B), plotted against ethyl xanthate surface coverage.
wetting liquid, the adjusted equilibrium capillary pressure and hence Awet values are independent of ethyl xanthate coverage; this again confirms that cyclohexane is a perfectly wetting liquid for galena particles, i.e., up Å 0 at collector coverages from 0 to 4 equivalent monolayers. Small variations in the adjusted equilibrium capillary pressure values with ethyl xanthate surface coverage may reflect changes in wettable surface area brought about by the heterogeneous nature of the ethyl xanthate covered galena surface (24). With water as the wetting liquid, the adjusted equilibrium capillary pressure decreases with increased collector coverage; this is indicative of decreased wettability and increased hydrophobicity. Wettability data for galena samples A and C showed identical trends. Powder contact angles can be calculated directly from equilibrium capillary pressures through the Laplace – White equation, where Awet is approximated by Aesd . An alternative and often more convenient approach for determining up is to compare the wetting behavior of the liquid of interest with that for a perfectly wetting liquid. For galena ( 13 ) and ethyl xanthate treated galena, powder contact angles can be calculated from a ratio of the equilibrium capillary pressures determined using water ( Dp ( water ) ) and cyclohexane ( Dp ( cyclohexane ) ) , accounting for differences in surface tension:
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Dp ( water ) g ( cyclohexane ) . Dp ( cyclohexane ) g ( water )
[7]
This approach removes any errors associated with defining the geometry of the mineral particle bed, i.e., errors in reff , particle surface areas, and particle volume fractions. Advancing water powder contact angles for galena particles determined using both Eq. (1) and Eq. (7) are given in Table 3. For galena sample B, up values, determined using Eq. (7), are plotted against ethyl xanthate surface coverage in Fig. 4b. The data in Table 3 shows there is good agreement between up values calculated by the two approaches, which gives justification for approximating Awet by Aesd and highlights the consistency of the experimental and subsequent analysis methods. Within the experimental errors of the capillary pressure measurements and the subsequent analysis procedure, up values for galena particles are independent of particle size but are dependent on collector coverage. Ethyl xanthate coverages as low as ten percent of an equivalent monolayer significantly increases the powder contact angle, which in turn increases the flotation recovery (Fig. 5). Powder contact angles and flotation. The relationships among ethyl xanthate surface coverage/galena particle contact angle, particle size, and flotation response is given in Fig. 5. We should note that in this work the flotation cell was not optimized for ultimate recovery but did enable the relative floatability to be ascertained. Furthermore, since no stirring was implemented during flotation, hydrophobic aggregation between high contact angle particles may occur and aggregate flotation is possible. With these points in mind, the data in Fig. 5 shows the correlation between flotation recovery and particle contact angle. For any particular particle size-range of galena there is apparently a critical contact angle above which the flotation response significantly increases; similar behavior has been observed for hydrophobised quartz particles (8). In the present work, the TABLE 3 Powder Contact Angles (up , in Degrees) for Ethyl Xanthate Treated Galena Samples, Calculated Using: Eq. [1] with Awet Å Aesd and (b) Eq. [7] Ethyl xanthate surface coverage, G(EX)
0
0.1
0.2
0.5
1.0
2.0
4.0
Sample A (150–75 mm) a b
52.6 49.7
60.6 55.8
68.5 69.2
83 83.5
86.1 85.9
90.0 90.0
94.6 94.5
Sample B (75–38 mm) a b
45.0 47.9
63.6 66.4
73.6 75.6
77.9 78.7
87.7 87.8
94.7 94.1
96.1 95.0
Sample C (38–20 mm) a b
54.0 52.1
69.1 67.5
81.8 81.9
84.6 84.8
86.9 86.6
88.8 88.6
92.1 92.2
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ing water powder contact angles have been reliably determined for galena particles in the size range 20 to 150 mm with surface coverages of ethyl xanthate from zero to four equivalent monolayers; these correlate directly with flotation recoveries and enable the determination of critical contact angles for strong flotation. Adsorbed ethyl xanthate, at as low as ten percent of an equivalent monolayer, significantly increased up and the flotation recovery. ACKNOWLEDGMENTS The Australian Research Council is acknowledged for their financial support of the project. Discussions with and assistance from Robert Hayes, Andrew Robinson, and Roger Smart are warmly acknowledged.
REFERENCES FIG. 5. Flotation recovery against the powder contact angle of ethyl xanthate treated galena particles.
38 to 75 mm range galena particle exhibited the lowest critical contact angle (near 707 ) for strong flotation, this therefore may be thought to be the optimum size range for galena flotation in the flotation cell used. Interestingly, this optimum size range for strong flotation is in good agreement with the reported laboratory and plant scale flotation behavior of galena (26). For particles of identical contact angles, the relatively high flotation recoveries (and rate constants) of intermediate sized particle has been explained in terms of the low efficiency of bubble–particle collision for fine particles and the low bubble–particle attachment efficiency for large particles (8, 26). The challenge for the future is to be able to predict flotation recoveries, more specifically flotation rate constants, from knowledge of particle size, particle contact angle, bubble size, and hydrodynamic conditions. Clearly, the particle contact angle device will be a useful tool in the future work in this area. CONCLUSIONS
The powder contact angle instrument of Diggins et al. (16, 17) has been successfully applied to ethyl xanthate coated galena particles. For water, a partially wetting liquid, the determined equilibrium capillary pressure values are controlled by the size of the galena particles as well as by their surface chemistry, as controlled by collector coverage. For cyclohexane, a perfectly wetting liquid, the equilibrium capillary pressure values are controlled by the size of the galena particles but are independent of collector coverage. Advanc-
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1. Gaudin, A. M., and Preller, G. S., Trans. Am. Inst. Min. Engrs. 169, 248 (1946). 2. Mok, J., and Salman, T., Can. Min. J. 85, 70 (1964). 3. Sarkar, N., and Gaudin, A. M., J. Phys. Chem. 70, 2512 (1966). 4. Woods, R., J. Phys. Chem. 75, 354 (1971). 5. Granville, A., Finkelstein, N. P., and Allison, S. A., Trans. Inst. Min. Met. 81, C1 (1972). 6. Kocabag, D., Kelsall, G. H., and Shergold, H. L., Int. J. Miner. Process. 29, 195 (1990). 7. Gardener, J. R., and Woods, R., Aust. J. Chem. 30, 981 (1977). 8. Crawford, R., and Ralston, J., Int. J. Miner. Process. 23, 1 (1988). 9. Buckley, A. N., and Woods, R., Appl. Surf. Sci. 17, 401 (1984). 10. Fornasiero, D., Li, F., Ralston, J., and Smart, R. St. C., J. Colloid Interface Sci. 164, 333 (1994). 11. Fornasiero, D., Li, F., and Ralston, J., J. Colloid Interface Sci. 164, 345 (1994). 12. Bandini, P., Prestidge, C. A., Ralston, J., and Smart, R. St. C., in preparation. 13. Prestidge, C. A., and Ralston, J., J. Colloid Interface Sci. 172, 302 (1995). 14. Kim, B. S., Hayes, R. A., Prestidge, C. A., Ralston, J., and Smart, R. St. C., Appl. Surf. Sci. 78, 385 (1994). 15. White, L. R., J. Colloid Interface Sci. 90, 536 (1982). 16. Diggins, D., Fokkink, L. K. J., and Ralston, J., J. Colloids Surf. 44, 299 (1990). 17. Diggins, D., and Ralston, J., Coal Preparation 13, 1 (1993). 18. Rao, S. R., in ‘‘Xanthates and Related Compounds,’’ p. 7. Dekker, New York, 1971. 19. Montalti, M., Fornasiero, D., and Ralston, J., J. Colloid Interface Sci. 143, 440 (1991). 20. Prestidge, C. A., Ralston, J., and Smart, R. St. C., Colloids Surf. 81, 103 (1993). 21. Partridge, A. C., and Smith, G. W., Trans. Inst. Min. Metall. 80, 199 (1971). 22. Laajehlato, K., Nowak, P., Pomianowski, A., Suoninen, E., Colloids Surf. 57, 319 (1991). 23. Buckley, A. N., and Woods, R., Colloids Surf. 53, 33 (1991). 24. Kim, B. S., Hayes, R. A., Prestidge, C. A., Ralston, J., and Smart, R. St. C., Colloids Surf. A 117, 117 (1996). 25. Buckley, A. N., and Riley, K. W., Surf. Interface Anal. 17, 655 (1991). 26. Trahar, W. J., and Warren, L. J., Int. J. Miner. Process. 3, 103 (1976).
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Contact Angle Studies of Ethyl Xanthate Coated Galena Particles CLIVE A. PRESTIDGE 1
AND
JOHN RALSTON
Ian Wark Research Institute, University of South Australia, The Levels, S.A. 5095, Australia Received February 2, 1996; accepted July 11, 1996
The wettability of ethyl xanthate coated galena particles has been determined from measurements of the equilibrium capillary pressure across a packed bed. Both the advancing liquid powder contact angle, up , and the specific wettable surface area, Awet , of galena particles have been obtained. The influence of particle size and ethyl xanthate surface coverage on the powder contact angle have been investigated and compared with floatability. Ethyl xanthate at a surface coverage of less than ten percent of an equivalent monolayer significantly increases both up and particle floatability. q 1996 Academic Press, Inc.
Key Words: galena particles; ethyl xanthate; contact angles; wettability and galena; flotation.
INTRODUCTION
The flotation separation of sulfide minerals is controlled to a large degree by the relative wettabilities of mineral particles in a pulp and is therefore related to their respective surface hydrophobicities. Sulfide minerals can be rendered selectively hydrophobic in the absence of collector molecules with, in general, only a rather modest impact on their floatability. Selective collector adsorption is normally required to confer the necessary hydrophobicity for strong, specific bubble–particle interaction and thus rapid flotation. For lead sulfide (galena) much effort has been directed at correlating surface chemistry, wettability, and flotation behavior (1–6). Generally the solid–liquid–vapor contact angle is used as a measure of wettability and is determined with the sessile drop or bubble technique on either freshly cleaved or highly polished galena surfaces. Surface roughness, contamination with grinding/polishing media, and associated oxidation effects, among others, have together conspired to prevent an unequivocal link between collector coverage, contact angle, and flotation being established. Sarkar and Gaudin (3) showed that upon increasing xanthate surface coverage on galena from 1.3 to 1.7 monolayers, the contact angle increased from a minimum to a maximum value. Mok and Salman (2), however, suggested that 5 to 1
To whom correspondence should be addressed.
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• precise control of the surface chemistry of galena; • accurate and reproducible determination of contact angles, which are representative of galena particles; and • accurate determination of xanthate adsorption with the subsequent conversion into a surface coverage expressed as ‘‘equivalent monolayers.’’
With respect to the surface chemistry of galena, there is now direct evidence for considerable surface heterogeneity, with lead-oxide, -hydroxide, -carbonate, -hydroxycarbonate, -sulfate, -thiosulfate, and metal-deficient/sulfur-rich layers having been reported (9–14). The relative predominance of these layers is controlled by the pH, electrochemical potential (Eh ), and hydrodynamic conditions (13) used during mineral conditioning. In addition, impurities present in natural galena samples dramatically effect the rate of surface oxidation (14), with the source of the mineral becoming an important variable. In many of the earlier contact angle studies of galena these variables were not rigorously controlled, making the precise surface chemistry impossible to define. It is essential to obtain precise control of galena surface chemistry, enabling reproducible and accurate contact angles
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20 monolayers were required for the same contact angle changes. Gaudin and Preller (1) reported that hydrophobic surfaces, with a water advancing contact angle greater than 907, are generated at less than a monolayer of xanthate coverage, an observation which agreed with the voltammetric studies of Gardner and Woods (7), who suggested that less than a monolayer of xanthate is required to float galena. Granville et al. (5) reviewed the earlier studies on the hydrophobicity of galena surfaces and concluded that ‘‘although many attempts had been made at relating contact angle measurements with collector adsorption and floatability, they were unlikely to yield definite information on the detailed chemical mechanisms of the interactions at the galena surface.’’ These earlier studies and their modern versions [e.g., (6)] have not identified the exact relationship which should exist between collector coverage, contact angle, and flotation recovery for galena, in contrast to quartz (8). For galena the major requirements for determining this relationship between collector coverage, contact angle, and flotation recovery are:
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to be obtained, if a sensible correlation with floatability is to be made. The majority of contact angle studies on sulfide minerals have been performed using the sessile drop or sessile bubble techniques (4–6). As noted above, their reliability is suspect. Furthermore, these techniques take measurements from single surfaces, which may not be representative of mineral particles in a pulp. Contact angle measurements on powders reduce the problems associated with single-surface studies. In this study, advancing liquid powder contact angles ( up ) are determined from equilibrium measurements of the capillary pressure increment required to prevent the movement of liquid through a packed bed of mineral particles. The Laplace–White equation (15) gives a strict expression for the equilibrium capillary pressure, Dp, in porous media, Dp Å glv cos up
rAwetf , (1 0 f )
[1]
where glv is the liquid–vapor surface tension, f is the volume fraction of the solid in the packed bed, r is the density of solid, and Awet is the specific wettable surface area of the solid (effectively the area of the solid–liquid interface). It should be noted that for sulfide mineral particles, with inherent surface chemical heterogeneity and anisotropic surface tensions, the contact angle measured by this approach is an average value. Using this approach, Prestidge and Ralston (13) have recently used the capillary method of Diggins et al. (16, 17) to determine the powder contact angles of galena particles in the absence of collector. The aim of the present work was to extent this study to investigate the influence of ethyl xanthate surface coverage on the wettability of galena particles. Powder contact angle measurements are compared with microflotation data. EXPERIMENTAL
Reagents High purity water was produced by reverse osmosis, passage through two stages of mixed bed ion exchange resin followed by two stages of activated carbon and a final filtering step through a 0.22-mm filter. The conductivity was less than 0.5 mS m01 with a surface tension of 72.8 mN m01 at 207C. High purity nitrogen and oxygen were used throughout the study. Potassium ethyl xanthate (KEX) was purchased from Kodak as an industrial grade reagent and was subsequently purified by recrystallization from petroleum ether, then vacuum dried and stored under vacuum (18). The purity of the KEX was confirmed from the UV extinction coefficient at 301 nm. A value of 17,600 dm3 mol 01 cm01 was obtained, which compared favorably with previously reported values (19, 20).
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Galena (Brushy Creek, Missouri) was purchased from Wards Natural Science Establishment Laboratories, New York, and subsequently analyzed by the Australian Mineral Development Laboratories (AMDEL). The elemental analyses suggested that the galena was ú99.8% pure, with 2 ppm copper, 11 ppm zinc, and 72 ppm iron as the major impurities. Other reagents used were of analytical grade (unless otherwise stated). Mineral Grinding and Conditioning Galena samples were prepared by wet-grinding in a ceramic mortar and pestle at pH 7; size fractions of /75–150mm, /38–75mm, and /20–38mm were obtained by sieving through stainless steel sieves. Samples were deslimed by cycles of 1 min sonication followed by decantation of the fines until a clear supernatant resulted. Particle size distributions and specific surface areas based on the equivalent spherical diameter distribution of particles (Aesd ) were measured using a Malvern Model 2600C laser diffractometer. Surface area measurements were also performed by nitrogen and krypton adsorption using a Coulter Omnisorp model 100. In general the average BET surface areas (Abet ) were three to four times greater than those calculated on a geometric basis (Aesd ), suggesting surface roughness and/or the presence of strongly bound submicron particles, which were not removed during preparation. The latter have been observed by high resolution scanning electron microscopy. Nonsphericity in galena particle shape may also contribute to the observed differences in surface area. For all galena samples used in this study, average particle diameters (D ( 50 ) ), particle size spans (D ( 10 ) to D ( 90 ) ), Aesd values, Abet values, and volume fractions ( f ) of the packed mineral bed are reported in Table 1. The freshly ground and sized galena particles were either dried as described below or presented directly to the reaction vessel for conditioning with ethyl xanthate prior to drying. Galena samples of 20 g were conditioned in a 300 cm3 reaction vessel with air-equilibrated ethyl xanthate solutions, while the pH was maintained at 7 by small additions of 10 02 M sodium hydroxide and nitric acid. Ethyl xanthate adsorption was determined by UV spectrophotometry and the surface coverages were calculated as described below. Prior to contact angle and flotation measurement the galena samples were filtered, then dried in a vacuum desiccator over silica gel. In agreement with previously reported FTIR studies (20), FTIR studies on the galena particles presently under investigation have revealed surface lead(II) ethyl xanthate species and confirmed that the particle treatment, as described above, had negligible influence on the form or surface coverage of adsorbed ethyl xanthate. Techniques Capillary pressure measurements. The capillary pressure measuring device was based on that originally reported
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TABLE 1 Particle Size, Specific Surface Area, and Volume Fraction Data for Powdered Galena Samples
Galena Sample
Da(50) (mm)
Spanb (mm)
Aesd (m2 g01)
Abet (m2 g01)
Volume fraction of particle bed, f
A (150–75 mm) B (75–38 mm) C (38–20 mm)
108 54.3 30.1
97.4 42.9 21.4
0.012 0.020 0.032
0.027 0.080 0.165
0.601 0.602 0.611
a b
Awet (m2 g01) 0.011 0.021 0.0305
D(50) Å 50% point of the cumulative undersize distribution. The span of the distribution measures the spread between the 10 and 90% points of the cumulative undersize distribution, D(90) 0 D(10) .
by Diggins et al. (16) and modified by Prestidge and Ralston (13). Details of the instrument and experimental method have been previously reported (13). UV-Vis spectrophotometry. Sized galena particles were combined with KEX solutions at concentrations in the range from 10 04 to 10 06 M, depending on the particle size and the required collector coverage. The mineral slurries were then stirred and their supernatants circulated through a 10-mm quartz UV cell. UV spectra from 200–400 nm were recorded as a function of time using a Philips Model PU8470, singlebeam, UV-Vis spectrophotometer. Details of the experimental arrangement have been reported previously (19, 20). Ethyl xanthate may be converted to ethyl monothiocarbonate (MTC), ethyl perxanthate (EPX), or ethyl dixanthogen ((EX)2 ) by sulfide mineral surfaces (19). Therefore the amount of ethyl xanthate (EX0 ) adsorbed at the mineral surface may be accurately determined from a mass balance for all of the ethyl xanthate species:
ther ethyl perxanthate or diethyl dixanthogen, with only a small contribution from ethyl monothiocarbonate. These findings are in agreement with previous UV studies on freshly prepared galena particles (20). Ethyl xanthate surface coverages for galena sample B (38 to 75 mm), determined using Eq. [3] with Abet used for As , along with relevant ethyl xanthate concentrations, are given in Table 2. At pH 7 with air equilibrated aqueous solutions, relatively high affinity ethyl xanthate adsorption behavior was exhibited, with ú95% of the initial ethyl xanthate removed from the solution within the 30 min of contact. Galena samples A and C showed effectively identical behavior. Mechanisms for ethyl xanthate adsorption on galena surfaces have been extensively discussed in the literature (e.g., 5–7, 20, 22, 23), although some controversy still remains. It is now generally considered that for an unoxidized galena surface, lead(II) ethyl xanthate forms through an electrochemically driven reaction:
[EX0 ]ads Å [EX0 ]0 0 [MTC] 0 [EPX] 0 2[(EX)2 ] 0 [EX0 ]t ,
PbS / 2EX0 r Pb(EX)2 / ‘‘S 0 ’’ / 2e 0 ,
where [EX0 ]0 is the original concentration of EX0 and [EX0 ]t is the concentration at any time, t. Ethyl xanthate surface coverages, G ( EX ) , were determined from the concentration of adsorbed ethyl xanthate, [EX0 ]ads (mol m03 ), the solution volume, V (m3 ), the mineral surface area, As (m2 ), and by assuming the area occupied by an adsorbed ethyl xanthate molecule, Aa , to be 1.73 1 10 5 m2 ˚ 2 per molecule (1)): mol 01 (28.8 A [EX0 ]adsV Aa . [3] As Microflotation. Microflotation measurements were performed in a modified Partridge and Smith cell (21); details were reported previously (13).
with oxygen reduction generally the other half-cell reaction. However, the precise form of the adsorbed lead(II) ethyl xanthate and whether ‘‘S 0 ’’ is elemental sulfur, polysulfide, or a metal-deficient sulfide are still under debate. It is unequivocal that the rate and extent of lead(II) ethyl xanthate formation is strongly dependent on Eh and the presence of other reagents [e.g., cyanide (20)]. For oxidized galena samples, lead(II) xanthate may form through precipitation with dissolved lead ions,
G ( EX ) Å
RESULTS AND DISCUSSION
Ethyl Xanthate Treatment of Galena Particles UV studies. UV spectra of the supernatants from ethyl xanthate treated galena slurries showed no evidence for ei-
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[4]
[2]
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Pb 2/ / 2EX0 r Pb(EX)2 ,
[5]
and/or an ion-exchange with oxidized surface groups, PbS.Pb(OH)2 / 2EX0 r PbS.Pb(EX)2 / 2OH 0 .
[6]
In the present work, all galena samples were sieved, deslimed, and conditioned for 30 min in air-equilibrated water at pH 7 (Eh / 400 mV) prior to ethyl xanthate addition.
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TABLE 2 Ethyl Xanthate Surface Coverage on Galena Sample B as a Function of Initial Ethyl Xanthate Concentration Galena sample B: maximum theoretical ethyl xanthate surface coverage/equivalent monolayers 0 0.1 0.2 0.5 1.0 2.0 4.0
Initial EX0 concentration, [EX]0 (M)
EX0 concentration after 30 min [EX]tÅ30 min (M)
0 1 1 1 1 1 1
0 õ1007 õ1007 9 1 1007 2.2 1 1006 4.3 1 1006 7 1 1006
3.08 6.17 1.54 3.08 6.17 1.23
1006 1006 1005 1005 1005 1004
Adsorbed EX0 concentration, [EX]ads (M)
Actual EX0 surface coverage, G(EX) (equivalent monolayers)
0 1 1 1 1 1 1
0 0.1 0.2 0.47 0.93 1.86 3.77
3.08 6.17 1.45 2.86 5.74 1.16
1006 1006 1005 1005 1005 1004
Note. Surface coverages determined after 30 min contact between galena particles and air purged solution of ethyl xanthate at pH 7. (In all cases [EPX0] and [EX2] where õ1007 M and [MTC0] õ 1006 M).
Reactions [4], [5], and [6] may all therefore be postulated as feasible routes for ethyl xanthate adsorption. Reaction [4] is responsible for ethyl xanthate surface coverages up to 1 monolayer, with multilayer surface coverages presumably forming through reactions [5] and [6]. Interestingly, recent scan probe microscopy studies (24) have shown ethyl xanthate treated galena surfaces to be topographically heterogeneous in nature, with islands or colloidal particles of lead ethyl xanthate. Surface heterogeneity of sulfide mineral particles may influence wettability and hence flotation behavior. The precise influence of physical heterogeneities on the powder contact angles of sulfide mineral particles is beyond the scope of the present work, but should be addressed in the future. In this work we consider the influence of chemical heterogeneities on the powder contact angle of galena. Wettability Studies Capillary pressure studies. During the operation of the equilibrium capillary pressure measurement device, or any other apparatus for particle contact angle determination, it is desirable that the wetting liquid does not influence the surfaces under study. With sulfide mineral surfaces, which are inherently reactive toward aqueous solutions, this is obviously a difficult requirement to meet. UV studies of both water and cyclohexane after contact with ethyl xanthate treated galena particles showed no evidence for extracted ethyl xanthate or lead ethyl xanthate species. Interestingly, spectral features characteristic of sulfur or polysulfide species (25) were detected in the cyclohexane extract, which were presumably formed through reaction [4]; this area of investigation clearly warrants further study. We are confident that these findings in no way influence the determination of particle contact angles as a function of ethyl xanthate surface coverage, as described below. Examples of capillary pressure against time plots for galena sample B (at ethyl xanthate surface coverages of zero
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and one equivalent monolayer), using water and cyclohexane as the wetting liquid, are given in Fig. 1. This data clearly demonstrates that the rate of capillary pressure increase and the equilibrium capillary pressure reached is dependent on the wetting solvent and the collector coverage. Equilibrium capillary pressures are apparently reached significantly more quickly with cyclohexane than water. Equilibrium capillary pressure values have been shown to give a reliable measurement of particle wettability and have enabled contact angles of galena particles to be determined at various surface coverages of ethyl xanthate. Adjusted equilibrium capillary pressures ( Dp(1 0 f )/ f ) (16) for galena sample B (at various ethyl xanthate surface coverages), determined using both cyclohexane and water as the wetting liquids, are plotted against the reciprocal of the average particle size in Figs. 2a and 2b, respectively. Adjusted equilibrium capillary pressures determined using both cyclohexane and water as the wetting liquids are plotted
FIG. 1. Capillary pressure versus time plots for powdered galena sample B with ethyl xanthate surface coverages of zero and one equivalent monolayer, using water and cyclohexane as the wetting liquids.
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Awet values are independent of ethyl xanthate coverage and are in better agreement with Aesd than Abet . This finding suggests that the moving wetting front in the capillary pressure experiment only accesses the geometric area of the particles, without probing the surface roughness, porosity, or attached fines which are accessed by gas adsorption. An equivalent finding has been reported for untreated galena particles (13). Water as the wetting liquid. For water-wetted galena particles, plots of adjusted equilibrium capillary pressures against the reciprocal of the average particle size (Fig. 2b) and Aesd (Fig. 3b) are linear, but, unlike the equivalent data with cyclohexane as the wetting liquid, are strongly dependent on ethyl xanthate surface coverage. That is, with water as the wetting liquid the equilibrium capillary pressure is dependent on both the particle size and the surface hydrophobicity, which in this case is controlled by collector coverage. The differences in the gradients of the plots in Fig. 3b and 4b are related to the hydrophobicities of the galena samples and their respective particle contact angles. Powder contact angles and collector coverage. Equilibrium capillary pressures for galena sample B, wetted with both cyclohexane and water, are plotted against ethyl xanthate surface coverage in Fig. 4a. When cyclohexane is the
FIG. 2. Adjusted equilibrium capillary pressures for (a) cyclohexane wetted and (b) water wetted, ethyl xanthate coated galena particles, plotted against the reciprocal of the average particle diameter.
against Aesd in Figs. 3a and 3b, respectively. The linearity of the plots in Figs. 2 and 3 validates White’s theory (15) for ethyl xanthate treated galena particles and was previously observed in examinations of the wetting of angular quartz particles (16) and untreated galena particles (13). Equilibrium capillary pressures determined using cyclohexane and water as the wetting liquids are discussed separately in the following sections. Cyclohexane as the wetting liquid. Previously (13) it was established that cyclohexane ( glv of 25.5 mN m01 ) is a perfect wetting liquid for uncoated galena particles. That is, galena particles have a zero contact angle with cyclohexane and capillary pressure studies using cyclohexane probe the geometry of the particle bed rather than surface chemistry. Further, the data in Figs. 2 and 3 show that equilibrium capillary pressures determined with cyclohexane are dependent on particle size or surface area, but independent of the surface chemistry, as controlled by ethyl xanthate surface coverage in this case. These findings confirm that cyclohexane is a perfectly wetting liquid for ethyl xanthate coated galena particles. In Table 1 the determined Awet values are compared with specific surface areas based on geometric considerations (Aesd ) and from gas adsorption measurements (Abet ). The
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FIG. 3. Adjusted equilibrium capillary pressures for (a) cyclohexane wetted and (b) water wetted, ethyl xanthate coated galena particles, plotted against the specific surface area (Aesd ).
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up Å cos 01
FIG. 4. (a) Equilibrium capillary pressures (cyclohexane and water wetted) and (b) particle contact angles for galena particles (sample B), plotted against ethyl xanthate surface coverage.
wetting liquid, the adjusted equilibrium capillary pressure and hence Awet values are independent of ethyl xanthate coverage; this again confirms that cyclohexane is a perfectly wetting liquid for galena particles, i.e., up Å 0 at collector coverages from 0 to 4 equivalent monolayers. Small variations in the adjusted equilibrium capillary pressure values with ethyl xanthate surface coverage may reflect changes in wettable surface area brought about by the heterogeneous nature of the ethyl xanthate covered galena surface (24). With water as the wetting liquid, the adjusted equilibrium capillary pressure decreases with increased collector coverage; this is indicative of decreased wettability and increased hydrophobicity. Wettability data for galena samples A and C showed identical trends. Powder contact angles can be calculated directly from equilibrium capillary pressures through the Laplace – White equation, where Awet is approximated by Aesd . An alternative and often more convenient approach for determining up is to compare the wetting behavior of the liquid of interest with that for a perfectly wetting liquid. For galena ( 13 ) and ethyl xanthate treated galena, powder contact angles can be calculated from a ratio of the equilibrium capillary pressures determined using water ( Dp ( water ) ) and cyclohexane ( Dp ( cyclohexane ) ) , accounting for differences in surface tension:
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Dp ( water ) g ( cyclohexane ) . Dp ( cyclohexane ) g ( water )
[7]
This approach removes any errors associated with defining the geometry of the mineral particle bed, i.e., errors in reff , particle surface areas, and particle volume fractions. Advancing water powder contact angles for galena particles determined using both Eq. (1) and Eq. (7) are given in Table 3. For galena sample B, up values, determined using Eq. (7), are plotted against ethyl xanthate surface coverage in Fig. 4b. The data in Table 3 shows there is good agreement between up values calculated by the two approaches, which gives justification for approximating Awet by Aesd and highlights the consistency of the experimental and subsequent analysis methods. Within the experimental errors of the capillary pressure measurements and the subsequent analysis procedure, up values for galena particles are independent of particle size but are dependent on collector coverage. Ethyl xanthate coverages as low as ten percent of an equivalent monolayer significantly increases the powder contact angle, which in turn increases the flotation recovery (Fig. 5). Powder contact angles and flotation. The relationships among ethyl xanthate surface coverage/galena particle contact angle, particle size, and flotation response is given in Fig. 5. We should note that in this work the flotation cell was not optimized for ultimate recovery but did enable the relative floatability to be ascertained. Furthermore, since no stirring was implemented during flotation, hydrophobic aggregation between high contact angle particles may occur and aggregate flotation is possible. With these points in mind, the data in Fig. 5 shows the correlation between flotation recovery and particle contact angle. For any particular particle size-range of galena there is apparently a critical contact angle above which the flotation response significantly increases; similar behavior has been observed for hydrophobised quartz particles (8). In the present work, the TABLE 3 Powder Contact Angles (up , in Degrees) for Ethyl Xanthate Treated Galena Samples, Calculated Using: Eq. [1] with Awet Å Aesd and (b) Eq. [7] Ethyl xanthate surface coverage, G(EX)
0
0.1
0.2
0.5
1.0
2.0
4.0
Sample A (150–75 mm) a b
52.6 49.7
60.6 55.8
68.5 69.2
83 83.5
86.1 85.9
90.0 90.0
94.6 94.5
Sample B (75–38 mm) a b
45.0 47.9
63.6 66.4
73.6 75.6
77.9 78.7
87.7 87.8
94.7 94.1
96.1 95.0
Sample C (38–20 mm) a b
54.0 52.1
69.1 67.5
81.8 81.9
84.6 84.8
86.9 86.6
88.8 88.6
92.1 92.2
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ing water powder contact angles have been reliably determined for galena particles in the size range 20 to 150 mm with surface coverages of ethyl xanthate from zero to four equivalent monolayers; these correlate directly with flotation recoveries and enable the determination of critical contact angles for strong flotation. Adsorbed ethyl xanthate, at as low as ten percent of an equivalent monolayer, significantly increased up and the flotation recovery. ACKNOWLEDGMENTS The Australian Research Council is acknowledged for their financial support of the project. Discussions with and assistance from Robert Hayes, Andrew Robinson, and Roger Smart are warmly acknowledged.
REFERENCES FIG. 5. Flotation recovery against the powder contact angle of ethyl xanthate treated galena particles.
38 to 75 mm range galena particle exhibited the lowest critical contact angle (near 707 ) for strong flotation, this therefore may be thought to be the optimum size range for galena flotation in the flotation cell used. Interestingly, this optimum size range for strong flotation is in good agreement with the reported laboratory and plant scale flotation behavior of galena (26). For particles of identical contact angles, the relatively high flotation recoveries (and rate constants) of intermediate sized particle has been explained in terms of the low efficiency of bubble–particle collision for fine particles and the low bubble–particle attachment efficiency for large particles (8, 26). The challenge for the future is to be able to predict flotation recoveries, more specifically flotation rate constants, from knowledge of particle size, particle contact angle, bubble size, and hydrodynamic conditions. Clearly, the particle contact angle device will be a useful tool in the future work in this area. CONCLUSIONS
The powder contact angle instrument of Diggins et al. (16, 17) has been successfully applied to ethyl xanthate coated galena particles. For water, a partially wetting liquid, the determined equilibrium capillary pressure values are controlled by the size of the galena particles as well as by their surface chemistry, as controlled by collector coverage. For cyclohexane, a perfectly wetting liquid, the equilibrium capillary pressure values are controlled by the size of the galena particles but are independent of collector coverage. Advanc-
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1. Gaudin, A. M., and Preller, G. S., Trans. Am. Inst. Min. Engrs. 169, 248 (1946). 2. Mok, J., and Salman, T., Can. Min. J. 85, 70 (1964). 3. Sarkar, N., and Gaudin, A. M., J. Phys. Chem. 70, 2512 (1966). 4. Woods, R., J. Phys. Chem. 75, 354 (1971). 5. Granville, A., Finkelstein, N. P., and Allison, S. A., Trans. Inst. Min. Met. 81, C1 (1972). 6. Kocabag, D., Kelsall, G. H., and Shergold, H. L., Int. J. Miner. Process. 29, 195 (1990). 7. Gardener, J. R., and Woods, R., Aust. J. Chem. 30, 981 (1977). 8. Crawford, R., and Ralston, J., Int. J. Miner. Process. 23, 1 (1988). 9. Buckley, A. N., and Woods, R., Appl. Surf. Sci. 17, 401 (1984). 10. Fornasiero, D., Li, F., Ralston, J., and Smart, R. St. C., J. Colloid Interface Sci. 164, 333 (1994). 11. Fornasiero, D., Li, F., and Ralston, J., J. Colloid Interface Sci. 164, 345 (1994). 12. Bandini, P., Prestidge, C. A., Ralston, J., and Smart, R. St. C., in preparation. 13. Prestidge, C. A., and Ralston, J., J. Colloid Interface Sci. 172, 302 (1995). 14. Kim, B. S., Hayes, R. A., Prestidge, C. A., Ralston, J., and Smart, R. St. C., Appl. Surf. Sci. 78, 385 (1994). 15. White, L. R., J. Colloid Interface Sci. 90, 536 (1982). 16. Diggins, D., Fokkink, L. K. J., and Ralston, J., J. Colloids Surf. 44, 299 (1990). 17. Diggins, D., and Ralston, J., Coal Preparation 13, 1 (1993). 18. Rao, S. R., in ‘‘Xanthates and Related Compounds,’’ p. 7. Dekker, New York, 1971. 19. Montalti, M., Fornasiero, D., and Ralston, J., J. Colloid Interface Sci. 143, 440 (1991). 20. Prestidge, C. A., Ralston, J., and Smart, R. St. C., Colloids Surf. 81, 103 (1993). 21. Partridge, A. C., and Smith, G. W., Trans. Inst. Min. Metall. 80, 199 (1971). 22. Laajehlato, K., Nowak, P., Pomianowski, A., Suoninen, E., Colloids Surf. 57, 319 (1991). 23. Buckley, A. N., and Woods, R., Colloids Surf. 53, 33 (1991). 24. Kim, B. S., Hayes, R. A., Prestidge, C. A., Ralston, J., and Smart, R. St. C., Colloids Surf. A 117, 117 (1996). 25. Buckley, A. N., and Riley, K. W., Surf. Interface Anal. 17, 655 (1991). 26. Trahar, W. J., and Warren, L. J., Int. J. Miner. Process. 3, 103 (1976).
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