Contact angle and surface analysis studies of sphalerite particles

Minerals Engineering, Vol. 9, No. 7, pp. 727-741, 1996

Pergamon PII:S0892--6875(96)00064-7

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Copyright © 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0892~875/96 $15.00+0.00

ANGLE AND SURFACE ANALYSIS STUDIES OF SPHALERITE PARTICLES

T.V. SUBRAHMANYAM§,

C.A. PRESTIDGEt*

and J. RALSTONt

§ On leave from Departamento de Geologia-CCE/UFRN, Campus Universitario, \ 59072-970 Natal-RN, Brazil t Ian Wark Research Institute, University of South Australia, The Levels, S.A. 5095, Australia * Author for correspondence (Received 18 March 1996; accepted 22 April 1996)

ABSTRACT

Capillary penetration techniques were used to determine the wettability of sphalerite particles under flotation related conditions. Particle contact angles were determined separately from measurements of wetting rates (Washburn technique) and equilibrium capillary pressures across a sphalerite particle bed. For sphalerite of different size fractions, particle contact angles ranged from 75 to - 9 0 °, whereas synthetic ZnS (99.9%) gave values of 46° and 53 °. The strong hydrophobic nature of the natural sphalerite samples was due to inadvertent copper activation and the ensuing formation of sulfur-rich surfaces, determined by bulk chemical and surface analysis (X-ray photoelectron spectroscopy and scanning Auger microscopy) studies. Scanning Auger microscopy also revealed variations in the levels of surface copper from mineral grain to grain and confirmed the heterogeneous chemical nature of sphalerite surfaces. The latter enables the contact angle variability to be discussed. Copyright ©1996 Elsevier Science Ltd

Keywords Sulfide minerals; froth flotation; surface modification

INTRODUCTION In a three phase system like froth flotation, recoveries are governed by particle size and hydrophobicity. In general, for particles of a particular size there is a critical contact angle above which flotation is strong [1-3]. Contact angles are commonly determined by sessile drop or sessile bubble techniques, where the angle of contact of a water drop or a bubble on a polished single mineral surface is measured through the vapour or liquid phases. Such contact angles may not, however, be representative of mineral particles in a flotation pulp and there is a need for rapid and reliable techniques for contact angle determinations of mineral particles. A knowledge of particle contact angles is important in defining the role of particle size, surface roughness and surface chemical heterogeneities in controlling the flotation of minerals [4]. Methods for contact angle measurements on particles are based on investigations of either sessile drops or bubbles on compressed discs, or liquid penetration into particle beds [5]. In this work we are concerned with the latter, in particular its application to sulfide mineral particles. Liquid penetration studies include equilibrium capillary pressure and wetting rate measurements, both having been extensively applied to model systems [6], oxide minerals [7-9] and coal [10], but only rarely to sulfide minerals [2,3,11,12].

727

728

T.V. Subrahmanyamet al.

Dynamic methods for particle contact angle measurement utilise the Washburn equation [13] which, for steady flow conditions, relates the capillary driving force of a liquid penetrating through a compact bed of particles and the viscous drag: h2

=

t

2y~cos0, _ A p g h

r~t/ 8r 1l

(1)

rtr

where ~ is the viscosity of the penetrating liquid, reff the effective capillary radius, A0 the difference in density between the liquid and the surrounding medium, h the height of liquid penetrating the bed in time t and 0p the particle advancing contact angle, measured through the liquid phase. For a vertical bed with small pore radius, the A0gh term can be neglected, whereby: h 2 _ rey~vcos0,

t

(2)

2q

Szekely e t al. [14] replaced reff by a tortuosity factor K, which accounts for the complex pathway formed by the channels between particles: h2 _ K-f/vCOS0p t

(3)

21"1

For a particular, fixed packing of particles in a column, K will be a constant and a plot of h 2 versus t will be linear. If the wetting rates for two liquids, of which one is a perfectly wetting (with cOS0p = 1), are considered, then the factors reff or K do not need direct calculation and the particle contact angle can be obtained directly: cos0

(4)

= (h2/t),,(71v)wq,,

P

(h 2/t), (7,v),,rlw

where, (h2/t) n and (h2/t)w are the wetting rates for non-wetting and wetting liquids with surface tensions, ('rlv)n and (~qv)w and viscosities, r/n and ~/w, respectively. 0p of course refers to the non-wetting liquid. Using this approach contact angles of fine coal [10] and kaolinite [9] particles, for example, have been determined with a reported precision of ___2°. In this approach it is assumed that the solid surface is covered by a duplex film in equilibrium with the saturated vapour of the wetting liquid (i.e. the liquid is volitile). Should this not be the case, the "dry" solid will wet faster [15]. This has not been found to be an issue for, say, coal and quartz particles where cyclohexane is used as a wetting liquid and water is the non-wetting liquid [1,10,15]. With respect to determining contact angles of particles by static or equilibrium methods, the equilibrium capillary pressure increment (Apm) required to prevent the movement of liquid through a packed bed of mineral particles may be equated to the sum of the Laplace pressure (Ap) and the hydrostatic head (Phead): Ap,,

= Ap

+ Ph~,,a -

2Y/vCOSOp + Phe,,a

(5)

rqf

The Laplace-White equation [16] gives a strict thermodynamic expression for Ap in porous media: Ap = 7/vcos0/,PA .~,~ (1 - o )

(6)

where • is the volume fraction of the solid in the packed bed, p is the density of solid and Awet is the specific wettable surface area of the solid.

Contact angle and surface analysis studies of sphalerite

729

Dunstan and White [6] used a capillary method to test the White approach [16]. General agreement between theory and practice was observed for smooth glass Ballotini. Diggins et al. [7] and Diggins and Ralston [8] reported a similar capillary method for determining powder contact angles of angular quartz particles and glass spheres in both hydrophilic and hydrophobic states with a number of wetting liquids; they were able to test the Laplace-White equation and to reliably determine the contact angle and the wettable surface area of powders in the 25 to 300/zm range. Prestidge and Ralston have recently used the capillary method of Diggins et al. [7] to determine the powder contact angles of galena particles in the absence [11] and presence [3,12] of processing reagents. For both the wetting rate and equilibrium pressure techniques, it is of course the advancing water contact angle that is obtained. The major objective of the present work was to compare and contrast the applicability of wetting rate measurements using the Washburn method and equilibrium capillary pressure measurements using the Diggins et al. method [7] to determine the contact angles of sphalerite particles. In particular, the ease of measurement, the reliability, particle size limitations and the hydrophobicity range considered are addressed. Correlations with microflotation data were made. Complementary surface analysis studies using X-ray photoelectron spectroscopy and scanning Auger microscopy were also undertaken.

EXPERIMENTAL

Materials A natural sphalerite (ZnS) sample (Elmwood Mine, Carthage, Tennessee, USA) was obtained from Ward's Natural Sciences Establishment, Inc., Rochester, N.Y. The mineral was crushed in a jaw crusher and selected lumps were picked out under a magnifying lens for grinding. A 1 Kg sphalerite sample was wet ground in 1.5 1 of water for 75 min in a ceramic mill with ceramic balls as grinding media. The ground product was separated into - 1 5 0 +75 /~m and - 7 5 + 38 /zm fractions by wet sieving, and the - 3 8 /~m fraction was further separated by a cyclosizer (Warman) to obtain CS1 to CS5 fractions which correspond to 28.6, 21.5, 15.0, 9.8 and 7.2/zm, respectively. Specific surface areas and particle size distributions of the sphalerite particles were determined by BET gas adsorption (Coulter Onmisorp 100) and laser diffraction (Malvern 2600c) respectively. Dry sphalerite particles were stored in a desiccator over silica gel prior to contact angle measurement. Based on microchemical analysis the sample purity was calculated to be 98.53% ZnS (Table 1). The average density was determined to be 4.0 g cm -3. A synthetic ZnS (99.9%; obtained from Aldrich Chemical Co. Inc., USA) was also investigated in this study. This sample was ground wet in a ceramic mortar and pestle, with the - 7 5 +38 ~m fraction obtained by sieving, then dried at 25°C in air for approximately one hour prior to particle contact angle measurement.

TABLE 1 Elemental analysis of sphalerite sample.

Element

Weight, %

Zn

63.00 33.00 0.08 0.32 2.90

S Cu Fe

Si

730

T.V. Subrahmanyam et al.

Reagents Water was purified by reverse osmosis and subsequently passed through a Millipore Super-Q system. At 25 °C the surface tension of the water was 72.8 mN m-1 with a conductivity of < 0.5/zS m-1 and a pH of 5.7+0.2. The pH of solutions was regulated with dilute solutions of HNO 3 and KOH. These and any other reagents used were of analytical grade.

Techniques Liquid penetration rate measurements The technique involves the measurement of the height of a penetrating liquid through a packed bed of particles contained in a capillary tube as a function of time, see Figure 1. The bottom of the capillary was packed with a thin compact layer (1-2 mm) of glass wool to retain the particles. Since the height reached by the wetting liquid is influenced by the uniformity of the packed bed, the capillary tube was gently tapped for a given time after filling with particles to ensure uniform packing [10,15]. The height of the packed bed was approximately 50 nun. The capillary tube was attached to a vertically graduated scale to enable accurate height determination. The wetting liquid, either pH adjusted water or toluene, contained in a petri dish, was introduced to the bottom of the capillary and then the height of the wetting front was recorded against time [10,15].

Capillary packed with particles W e t t i n g front

Height, h

Wetting liquid

Glass wool

"l

/I

Fig. 1 Experimental apparatus for determining the particle contact angle of mineral particles, based on the determination of wetting rate as originally described by Washburn [13]

Equilibrium Capillary Pressure Measurements The experimental apparatus developed by Diggins, Fokkink and Ralston [7] was used to determine equilibrium capillary pressures across a bed of powdered mineral. Prior to analysis a sample column was filled with powdered sphalerite, whilst gently tapping to ensure uniformity of packing. Particle volume

Contact angle and surface analysis studies of sphalerite

731

fractions (Table 4) were determined from the mass difference between packed and empty columns. The liquid under study was allowed to enter the powder bed from a reservoir and is automatically forced into the bed by the combined effects of a steadily increasing curvature of the liquid meniscus and the hydrostatic head. The liquid rises through the bed until the capillary pressure, together with the hydrostatic head is exactly compensated by the overpressure; this is measured by the pressure transducer. Data analysis was undertaken using a software package developed at the University of South Australia [7,8]. The actual time taken for the pressure to reach equilibrium depends on the particle size, the wetting liquid, the column geometry and the contact angle of the particles. It has been shown that APm increases as an exponential function of time and therefore samples requiring long equilibration times, in excess of say 1 hour, can be prematurely terminated and the equilibrium pressure value determined precisely from a fitting routine [7,8].

XPS XPS spectra were recorded using a Perkin Elmer PHI Model 5100 spectrometer with a Mg K,~ X-ray source operating at 300 W and with a pass energy of 89.450 eV. The vacuum pressure in the analyser chamber was 10 -8 Torr during analysis. The energy scale was calibrated using the Fermi edge and the 3d5/2 line (binding energy =367.9 eV) for silver, whilst the retardation voltage was calibrated noting the positions of the Cu 2P3/2 peak (binding energy = 932.67 eV) and the Cu 3P3/2 peak (binding energy = 75.13 eV). The protocol for the introduction of mineral particles into the spectrometer have been reported previously by Smart [17].

Scanning Auger Microscopy (SAM) Auger spectra were recorded using a Perkin Elmer PHI 600 spectrometer with an electron beam source operating at 10 kV. The vacuum pressure in the analyser chamber was 10 -9 Torr during analysis. The energy scale was calibrated using the Cu (LMM) Auger peak at 914 eV. Further details of the experimental technique and sample handling have been reported elsewhere [17].

Microflotation tests Microflotation tests were carried out in a modified Partridge and Smith cell [18] with a 13/~m pore size stainless steel frit at the cell base producing a steady, evenly dispersed bubble swarm. The flotation time was 8 minutes and the air flow rate was 50 ml/min. The flotation cell was tested for entrainment using a clean, nominally 12/zm quartz sample which gave a value less than 8%. 100 ml of pH adjusted milli-Q water were added to 1 g of dry sphalerite particles and the slurry was conditioned for 10 minutes in an ultrasonic bath to aid dispersion. After conditioning, the pulp was transferred to the flotation cell and the volume of the cell was made up with pH adjusted water. After flotation, the concentrate and tail were collected, filtered, dried and weighed, enabling the flotation recovery to be determined. Flotation recoveries were reproducible to within 3 %.

RESULTS AND DISCUSSION

Particle contact angles from capillary wetting rate studies

The influence of particle size Examples of wetting rate measurements obtained for packed sphalerite particle beds are given in Figure 2. For both water and toluene wetting, plots of h 2 (h is the height of the wetting front) against time are linear and pass through the origin, both of which are critical tests for establishing the applicability of the Washburn equation [19]. Similar behaviour was found for the other sphalerite and synthetic zinc sulfide particle fractions; the corresponding gradients are given in Tables 2 and 3. Toluene is a perfectly wetting liquid for sphalerite [20] and therefore equation (4) may be applied to the wetting data. Using "YIvand ~/

732

T.V. Subrahmanyam et al.

values for water as 72.80 mN m-1 and 0.890 mN m - 2 s respectively and for toluene as 27.95 mN m -1 and 0.558 mN m - 2 s respectively, equation (4) becomes: 0 = cos -I (h 2/t)" 0.6126

l,

(7)

(h 2/t)~

where (h2/t)n and (h2/t)w are obtained directly from the wetting data (e.g. Figure 2).

2500-

2000

~"

O

1500

1000

500

U

erl wm'i7

0 0

50

100

150

200

T i m e / sec Fig.2 Wettability data, using both water and toluene, for sphalerite particles (cyclosized, CS1) determined using Washburn approach

TABLE 2 Wetting rate data, contact angles (from dynamic wetting studies) and microflotation recoveries for various particle size fractions of sphalerite. Particle Size (/zm)

hZ/t (water),

hZ/t (toluene), mm 2

min- 1

Contact angle, degrees

Flotation recovery, %

82.3 82.7 73.6 76.5 78.2 85.3 89.8

19.9 51.7 62.0 83.2 75.6 78.0 84.2

mm 2

min- 1 7.2 9.8 15.0 21.5 28.6 - 7 5 +38 -150+75

4.13 4.27 11.86 12.86 9.18 3.87 0.117

20.5 22.4 27.9 36.6 29.9 31.4 39.9

Particle contact angles and flotation recoveries for different size fractions of sphalerite are given in Table 2. All size fractions are relatively hydrophobic with Op values between 75 and 90 o. For clean quartz, the particles are hydrophilic and their measured contact angles, when hydrophobized, are independent of particle size [6,7,15]. Sulfide minerals are reactive and undergo oxidation, with the surface hydrophobicity

Contact angle and surface analysis studies of sphalerite

733

depending on the degree of oxidation. For example, we [6] have shown that aging of lead sulfide particles increases their apparent hydrophobicity. With respect to sphalerite, details of the surface chemical processes that control hydrophobicity and the influence of particle size on these are discussed below. Particles that are greater than about 20#m in diameter float strongly whereas smaller particles float weakly, even though they are quite hydrophobic. In part this behaviour reflects a low collision efficiency with the collecting bubbles, but may also have other causes [1,21].

TABLE 3 Wetting rate data and particle contact angles of sphalerite and synthetic zinc sulfide particles obtained from dynamic wetting studies. Sample/ Particle Size (/~m)

h~/t (water), mm 2 min- 1

h~/t (toluene), mm 2 min- 1

Contact Angle, degrees

Flotation Recovery, %

Synthetic ZnS ( > 9 9 . 9 % pure); - 7 5 + 3 8 #m Sphalerite (Elmwood Mine); - 7 5 + 3 8 / z m

21.39

19.05

44.0

28.4

3.87

31.4

85.3

78.0

The influence of mineral source Particle contact angles and flotation recoveries for - 7 5 +38 #m sphalerite and zinc sulfide particles are given in Table 3. The natural sphalerite sample was significantly hydrophobic, with 0p values greater than 75 °, whereas the synthetic zinc sulfide sample was less so with a 0p value in the vicinity of 46 °. Flotation recoveries were in agreement with the particle contact angle values, with the natural sphalerite sample having a significantly greater flotation recovery than the synthetic zinc sulfide sample. The synthetic zinc sulfide is clearly more water-wettable than sphalerite, the reasons for which are discusssed below.

Reproducibility and applicability of the wetting rate technique The reproducibility of the measured h2/t values depends at least on the hydrophobicity of the mineral particles, the wetting liquid used and the efficiency of the column packing. The natural sphalerite particles studied here gave more reproducible wetting rates with toluene (error __+3%) rather than water (error +7 %). With water, variations in the h2/t values are believed to be due to localized channelling during the wetting process, i.e. non-uniform penetration of water through a partially hydrophobic mineral particle bed. For the synthetic zinc sulfide particles both water and toluene wetting occurred in a more uniform fashion and h2/t values were very reproducible (error +2%). With respect to particle size effects, it is desirable to use narrow size fractions of mineral particles so as to minimise the variation in the effective capillary radii in the particle bed. For capillary beds with particles of a broad size range the largest capillary radii, i.e. those between the larger particles, will dominate. Wetting rates may be artifactually high, leading to non-representative particle contact angle values. Capillary beds with small particles, i.e. with small effective capillary radii, give relatively low wetting rates which can be measured more accurately. For relatively large particles (i.e. > 75/~m) it can be difficult to precisely determine the relatively fast wetting process (i.e. the time is short), leading to larger errors in the measured h2/t values. The wetting rate technique also has the advantage that its operation is simple and quick. A measurement can be made in less than 10 minutes, enabling multiple measurements to be taken with a corresponding improvement in reliability.

Particle contact angles from equilibrium capillary pressures

The influence of particle size and sample Sphalerite particle contact angles calculated by the Laplace-White equation (equation (6)), with Abet used for Awet, are shown in Table 4. For intermediate sized particles (15-75/zm), 0p values from equilibrium

T.V. Subrahmanyamet aL

734

capillary pressure measurements are in good agreement with those from wetting rate measurements (c.f. Table 2 and 4). However, a significant variation in particle contact angle values were observed for the fine ( < 15 /~m) and coarse particle size ranges (-150+75 #m). The sphalerite sample had 0p >75 °, whereas for synthetic zinc sulfide 0p was 53.2 °.

TABLE 4 Volume fraction (40, surface area and contact angles (obtained through equation 6) for various size fractions of sphalerite using the equilibrium capillary pressure measurement method. Sample/ Particle Size (/~m)

volume fraction

(,i,)

BET specific surface area / m2g-1

7.2 9.8 15.0 21.5 - 7 5 +38 - 7 5 +38 (synthetic ZnS) -150+75

0.348 0.355 0.489 0.544 0.556 0.575

0.363 0.305 0.180 0.118 0.095 --

76.1 73.9 75.9 76.7 85.2 53.2

0.574

0.0587

76.3 -t-2

Contact Angle, degrees -t-10 +10 +3 +2 +3 +5

Reproducibility and applicability of the equilibrium capillary pressure technique With respect to particle size, as with the wetting rate technique, the equilibrium capillary pressure technique requires a narrow size fraction of mineral particles so as to ensure uniform packing and as narrow as possible a distribution of effective capillary radii in the particle bed. With the equilibrium capillary pressure apparatus used in this work, it proved difficult to determine reproducible data for particles < 15 t~m (the values given in Table 4 for the smallest two size fractions were not reproducible). The issue of minimum size has been discussed in previous studies [7,11] of contact angles using equilibrium capillary pressure measurements. This minimum value relates to the fact that for capillary beds with small particles, i.e. with small effective capillary radii, the overpressure developed by capillary rise in the finest capillaries is directed out of the lower end of the column through the wider channels, making equilibrium measurements irreproducible. We also suspect that for fine capillaries liquid, intrusion is kinetically slow [11]. The equilibrium capillary pressure technique can certainly be reliably used for sphalerite particles of contact angles from 0 to 90 ° in the range from 15 to 200/zm. Larger particles are accessible and their reliable contact angle determination largely reflects the sensitivity of the pressure transducer used.

Surface chemistry and hydrophobicity

Surface analysis by X-ray photoelectron spectroscopy (XPS) and scanning Auger microscopy (SAM) The particle contact angle and microflotation data reported in Tables 2 to 4 have confirmed that the natural sphalerite particles are considerably more hydrophobic than are the synthetic zinc sulfide particles. Surface analysis using XPS and SAM has been used to compare the surface chemistry of the samples. S 2p, combined Cu 2p/Zn 2p and individual Cu 2p XPS spectral regions from the surfaces of synthetic zinc sulfide and Elmwood Mine sphalerite, both sieved and cyclosized, are given in Fixures 3, 4 and 5, respectively. The corresponding XPS surface atomic concentrations for these samples are given in Table 5. In some cases argon ion etching has been used between XPS analyses to "depth profile", i.e. to distinguish between bulk and surface species; in these cases the relevant atomic concentrations are also given in Table 5.

Contact angle and surface analysis studies of sphalerite

735

m

ta

n-

174

172

170

168

166

164

162

160

158

Binding Energy, eV

Fig.3 Sulfur 2p XPS photoelectron spectra for: a. synthetic zinc sulfide particles (38-75/zm), b. sieved sphalerite particles (38-75 #m) and c.cyclosized sphalerite particles (CS 2 fraction)

Zn 2p

Cu 2p


a

=

b

1060

1040

1020

1000

980

960

940

920

Binding Energy, eV

Fig.4 Copper and zinc 2p XPS photoelectron regions for: a. synthetic zinc sulfide particles (38-75 #m), b. sieved sphalerite particles (38-75/zm) and c.cyclosized sphalerite particles (CS 2 fraction)

736

T. V, Subrahmanyam et al.

Cu 2p

I ~\

b

.~=

_,==

_=

!

>=

i°

965

960

955

950

945

940

935

930

925

Binding Energy, eV

Fig.5 Copper 2p XPS photoelectron spectra for: a. synthetic zinc sulfide particles (38-75/~m), b. sieved sphalerite particles (38-75 /zm) and c.cyclosized sphalerite particles (CS 2 fraction)

TABLE 5 XPS atomic concentration data.

Elemental atomic concentration, % Sample

Zinc

Synthetic ZnS, 38-75#m As above Ar + ion etched for 5 min

35.5 45.8

43.6 41.9

20.9 12.3

--

Sphalerite, 3875#m As above Ar ÷ ion etched for 5 min Sphalerite, CS5 fraction As above Ar ÷ ion etched for 5 min As above Ar* ion etched for 15 rain

18.9

33.0

41.2

4.8

2.1

35.6

30.2

27.1

3.5

3.6

9.5

27.5

48.2

10.9

3.9

12.0

32.3

40.2

12.6

2.9

28.0

34.2

22.5

10.2

5.1

Iron

The XPS spectra of the synthetic zinc sulfide sample contain signals due to only zinc, sulfur-species and oxygen, with no evidence for activating metal ions. The XPS atomic concentration data (Table 5) indicates a partially sulfur-rich surface (a S:Zn ratio > 1), which is removed by a 5 min argon ion etch. This low level of sulfur-richness is presumably responsible for the non-zero contact angle of the synthetic zinc sulfide sample.

Contact angle and surfaceanalysisstudies of sphalerite

737

In addition to zinc, sulfur-species and oxygen signals, XPS spectra of the natural sphalerite samples contain signals due to both copper (oxidation state (I), see Figure 5) and iron species. The copper, in particular, is known to activate sphalerite flotation [22-23] and is likely to be responsible for the high level of hydrophobicity in these samples. These natural sphalerite samples also showed XPS signals characteristic of metal-deficient sulfide or polysulfide species [23], i.e. high binding energy components in the S 2p signal in Figure 3. In agreement with the finding that samples of the cyclosized sphalerite are more hydrophobic than the sieved samples, a greater surface copper atomic concentration was determined for the cyclosized sample. The level of charge shifting in the XPS spectra (see Figures 3-5) also relates directly to the extent of metal ion activation [23]. In this case sphalerite samples with the higher copper levels showed least charge shifting, with correspondingly highest particle contact angles and floatabilities. Argon ion etching of the sphalerite samples reduced their Cu:Zn ratio, which is indicative that copper is concentrated at the surface and is in agreement with previously reported copper activation studies of zinc sulfide [23]. The XPS atomic concentration data (Table 5) confirms the presence of both surface copper species and metal-deficient surfaces (S:Zn ratio > 1) for the natural sphalerite samples; these species clearly contribute to the particle hydrophobicity, mechanistic details are discussed below. SAM micrographs from the surfaces of the cyclosized natural sphalerite samples (without the application of a conducting surface coating, e.g. carbon or gold) are given in Figure 6. Examples of Auger spectra from two points on the sphalerite particle surfaces are given in Figure 7; these are used to ascertain the relative level of surface copper, see below. The non-conducting nature of the synthetic zinc sulfide particle surfaces resulted in very poor quality electron images. In fact, attempts to image their surfaces without employing a surface coating technique proved unsuccessful. The conducting nature of the natural sphalerite sample, also confirmed from the charge shifting in the XPS spectra, immediately suggests that the band gap has been reduced by metal ion activation [22,23]. Both copper and iron species have been identified as potential activating species, being present at 800 and 3200 ppm, respectively, as shown from bulk analysis. It is also clear that copper, in particular, has been concentrated at the sphalerite surface during particle preparation.

Fig.6 Scanning Auger microscopy images from cyclosized sphalerite particles: a. sample CS2 and b. sample CS5

ME 9:7-B

738

T.V. Subrahmanyamet al. 10" &.

8-

4-

2-

0 700

9~o lO;)O Electron Energy/eV

11oo

Fig.7 Examples of Auger electron spectra from individual sphalerite particles Due to the overlapping nature of the Auger spectra from zinc (major Zn Auger peak at a kinetic energy of 994 eV, with minor secondary peaks at 916 and 830 eV) and copper (major Cu Auger peak at a kinetic energy of 920 eV) [24], it is not trivial to quantitatively determine the surface coverage of copper on sphalerite. A semi-quantitative indication of the level of surface copper on a sphalerite particle can be gained from the ratio of the Auger spectral intensities at 920 eV and 994 eV. The 920 eV to 994 eV intensity ratio for zinc is reported to be 0.3 [24], but will increase with increasing surface coverages of copper. The 920 eV to 994 eV intensity ratios from the spectra in Figure 7 are 0.3 and 0.49, which suggests a greater level of copper in spectra b. Numerous Auger spectrum from the surfaces of natural sphalerite particles have shown 920 eV to 994 eV intensity ratios in the range 0.3 to 0.6, confirming the presence of surface copper and showing that its coverage varies from mineral grain to grain. This finding suggests that in any particular sample of sphalerite particles there will be a range of particle contact angles, which will be influenced by the extent of activation and oxidation. The challenge for the future is to fully deconvolute the Auger signals from copper and zinc and therefore quantify the level of copper on sphalerite samples with spatial resolution. Surface chemistry

Surface chemical processes which influence the surface chemistry of sphalerite include metal ion activation, which is invariably used in a controlled manner to introduce hydrophobicity to sphalerite and facilitate selective flotation in plant practice. In the present study it is clear that copper activation is partially responsible for the fact that the natural sphalerite particles are considerably more hydrophobic than the synthetic zinc sulfide particles. Trace quantities of copper may be leached out from the bulk of the natural sphalerite sample during wet grinding and then exchange with zinc ions at the surface, which under mildly acidic condition can be represented as:

Contact angle and surface analysis studies of sphalerite

nZnS

+

Cu 2+

--->

Zn(n.1)CuS n +

Zn 2+

739

(8)

reflecting a combination of exchange and redox processes [22,23] which increases the collectorless flotation of sphalerite. In addition to metal ion activation of sphalerite, surface oxidation reactions may also influence surface hydrophobicity. Surface oxidation mechanisms will be dependent on whether the sphalerite is in air or aqueous solution. In aqueous solution surface oxidation will be influenced by pH and E h, but will be modified by dissolved gases, other reagents or contaminants and galvanic interactions with other minerals. Schematically the oxidation process can be represented by: ZnS

+

1/2 02

q-- H20

--- >

Zn(n_l)S

+

Zn(OH) 2

(9)

producing a surface which is deficient in zinc and enriched in sulfur. Such surfaces display collectorless flotation and finite contact angles. The resultant hydrophobicity of the sphalerite surface will be controlled by: the extent to which the zinc hydroxide (or oxides and carbonates, depending on conditions adheres to the underlying sulfur-rich surface, as dictated by dissolution and dispersion processes. the oxidation rate of the sulfur-rich sphalerite surface species, e.g.: Zn(n_l)S

+

202

+

2e-

--->

n-IZnS

+

5042-

(10)

In some cases, meta-stable, metal-deficient sulfide or polysulfide surface phases dominate, resulting in hydrophobicity, whereas, in others, the hydrophilic oxidation products dominate the surface chemistry with ensuing hydrophilicity [11]. The control of hydrophobic and hydrophilic surface layers on sulfide mineral surfaces is ultimately critical in achieving flotation selectivity in practice [ 17] and is dependent on numerous variables, including the mineral type and origin, pH, E h and shear conditions [11]. During particle contact angle measurements from either wetting rates or equilibrium capillary pressures, dry sphalerite particles are packed in a capillary and will therefore be exposed to air. Sphalerite oxidation in air may by schematically represented by: ZnS

+

1/202

--->

Zn(n_l)S. ZnO

(11)

which may be further modified by oxidation of the sulfur-rich component as described by reaction (10). Obviously in the dry state the hydrophilic oxidised zinc species (e.g. zinc oxide/hydroxide/sulfate or carbonate) will be fixed at the surface and will control the surface chemistry. However, during a particle bed wetting experiment the advancing wetting front may dissolve or dislodge any oxidised zinc products and, in so doing, probe the underlying, potentially hydrophobic layer [12]. It is clear that some of these potential mechanisms for introducing surface hydrophobicity need further investigation. However, it is encouraging that the present wettability measurements were able to effectively differentiate between synthetic zinc sulfide and natural sphalerite particles.

CONCLUSIONS Contact angles of sphalerite particles from different sources and of different size fractions were determined from wetting rate and equilibrium capillary pressure measurement. Both capillary wetting techniques used in this work are simple and rapid and can routinely determine the advancing water contact angle of

740

T.V. Subrahmanyamet al.

sphalerite particles over a wide range of sizes and hydrophobicities. Natural sphalerite particles were shown to be significantly more hydrophobic than synthetic zinc sulfide particles. Copper activation of the natural sphalerite during particle preparation was confirmed by XPS and SAM surface analysis, with both surface copper sulfide and sulfur rich surfaces detected.

ACKNOWLEDGEMENTS Financial support from CNPq- Conselho Nacional de Desenvolvimento Cient'fico e Tecnol--gico (Brazilian National Research Council) to one of the authors (TVS) is gratefully acknowledged. Financial support from the Australian Research Council and Australian Minerals Research Association is gratefully acknowledged. Roger Smart, William Skinner, Angus Netting and Darren Simpson of South Australian Surface Technology Centre are thanked for the surface analysis of samples.

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