Rheological investigations of galena particle interactions

COLLOIDS Colloidsand Surfaces A: Physicochemicaland EngineeringAspects 126 (1997) 75-83

ELSEVIER

AND SURFACES

A

Rheological investigations of galena particle interactions C.A. Prestidge Ian Wark Research Institute, University of South Australia, The Levels, SA 5095, Australia

Received 8 May 1996; accepted 20 June 1996

Abstract

The rheological behaviour of ultrafine (< 5 ~tm) galena particle slurries has been studied using a concentric cylinder rheometer. The influence of solids content and pH on viscosities and extrapolated yield values are reported and discussed in the context of the level of particle interaction. These studies, coupled with microflotation, microelectrophoresis and X-ray photoelectron spectroscopy (XPS) surface chemical analysis, have furthered our understanding of the role of interparticle forces in controlling the collectorless floatability of galena. In the pH range 6-10, the rheological behaviour of oxidized and effectively hydrophilic galena particles is controlled by electrostatic repulsive forces and the yield values scale with the zeta potentials squared. At pH values less than 6 this relationship breaks down and yield values cannot be predicted by electrostatic and van der Waals considerations alone. A non-DLVO contribution to the extrapolated yield value z~ is required to describe the observed rheological behaviour. The magnitude of z~ is influenced by pH, hydrodynamic conditions, the particle volume fraction and time scale of any conditioning periods. z~ is controlled to a minor degree by hydrodynamic, gravitational and inertial forces, but is mainly due to an attractive hydrophobic force between galena particle surfaces. This attractive hydrophobic particle interaction has been probed by microflotation. XPS has confirmed a pH-controlled surface cleaning mechanism which is responsible for the formation of hydrophobic galena surfaces. © 1997 Published by Elsevier Science B.V. Keywords: Floatability; Galena particles; Interparticle forces; Rheology

1. Introduction

The flotation separation of sulphide minerals is strongly influenced by particle interactions in mineral pulps; the level of which are controlled by processing conditions such as pH, electrolyte concentration, shear environment and reagent concentrations. Attractive hydrophobic interactions between collector-coated sulphide particles have been reported to be advantageous, as in the case of shear flocculation and carrier flotation [1]. Conversely, the interaction between hydrophilic gangue particles and sulphide mineral surfaces detrimentally effect flotation performance through slime coating [2]. Rheological studies offer a direct

approach for investigating such phenomena, furthering our understanding of interparticle interactions and, potentially, bubble-particle interactions in sulphide mineral slurries at pulp densities akin to those experienced during mineral processing. Rheology has been extensively used to investigate particle interactions in slurries of oxide minerals [3,4], clay minerals [5] and coal slurries [6], providing information on the state of aggregation over a range of process related conditions. However, until recently, rheological studies on sulphide mineral slurries have been limited to viscosity studies during grinding [7]. A possible reason for the lack of such studies is the fact that

0927-7757/97/$17.00 © 1997Publishedby ElsevierScienceB.V. All rights reserved. PH S0927-7757 (96) 03872-1

76

C.A. Prestidge/ ColloidsSurfaces A: Physicochem. Eng. Aspects 126 (1997) 75-83

sulphide minerals are inherently difficult to work with experimentally; their surface chemistry is extensively heterogeneous and is controlled by electrochemical oxidation processes and collector interactions. With these complexities in mind, rheological investigations have recently been undertaken on slurries of natural sphalerite particles [8] and synthetically prepared zinc sulphide spheres [9]. In the latter study [9], rheological measurements have been correlated with direct force measurements determined by colloid probe atomic force microscopy (AFM), and attractive hydrophobic forces have been measured. In the present study, we are concerned with galena (lead sulphide) particles. In recent times a considerable amount of work has been directed at deconvoluting the mechanisms of surface oxidation [10-14] and collector adsorption for galena [14,15]. In situ topographical surface imaging of galena with atomic resolution using scanning tunneling microscopy (STM) and AFM [10], plus direct measurements of the particle contact angles [ 11 ], are furthering our understanding of the complex surface chemistry. Moreover, models [12,13] are now available that predict the level of galena surface heterogeneity. Findings from these models correlate well with surface analysis [12] and electrophoretic mobility data [13]. We are now in a position to correlate galena surface chemistry with particle interactions brought about by changes in processing conditions. Rheological investigations offer further insight in this area. This study aims to determine the rheological behaviour of galena slurries, determining the influence of solids content and pH. Microelectrophoresis measurements were used to determine the zeta potentials and X-ray photoelectron spectroscopy (XPS) to characterize the surfaces.

less than 0.5 pSm -1 with a surface tension of 72.8 mN m -1 at 20°C. The pH of aqueous solutions was controlled by small additions of sodium hydroxide and nitric acid solutions and purged with high purity nitrogen. Any other reagents used were analytical grade, unless otherwise stated. Natural galena samples originated from Brushy Creek, MO and were supplied by Ward's Natural Science Establishment Laboratories, New York. Microelemental analysis showed >99.8% purity, with 2 ppm copper, 11 ppm zinc and 72 ppm iron as the major impurities. Natural galena samples were ground in a colloidal mill (Department of Chemical Engineering, Nanjing University of Science, People's Republic of China) and stored in a vacuum desiccator. Particle size distributions were monitored using a Malvern Mastersizer X laser diffractometer. Surface area measurements were performed by nitrogen and krypton adsorption using a Coulter Omnisorp Model 100. Average particle diameters, particle size spans, surface areas Aesd based on equivalent spherical diameters determined by laser diffraction, and Brunauer-Emmett-Teller surface areas determined by gas adsorption are reported in Table 1. Concentrated slurries of particle volume fraction ~bswere prepared by adding the required mass of dry galena particles to a measured volume of 10 -2 M KNO 3 electrolyte solution in a conditioning vessel. The resultant slurries were continuously stirred by a magnetic follower with nitrogen purging and the pH adjusted to and maintained at pH 10. After a 2 h conditioning period, 10 ml of slurry was transferred to the rheometer for analysis. Between rheological measurements the pH was controlled and the slurries stirred and conditioned for 20 min. 2.2. Experimental techniques

2. Experimental 2.2.1. Electrophoretic mobilities 2.1. Materials and slurry preparation

High purity water was produced by reverse osmosis, two stages of mixed-bed ion exchange, two stages of activated carbon, and a final stage involving 0.22 ~tm filtration. The conductivity was

Electrophoretic mobilities were determined using a Rank Brothers Particle Microelectrophoresis Apparatus (Mark II) employing a flat cell. Galena slurries ( 1.00 g 1- 1) were prepared in 10-2M KNO 3 supporting electrolyte solutions, conditioned for 2 h at 25°C and at pH 10. The

C.A. Prestidge / Colloids Surfaces A: Physicochem. Eng. Aspects 126 (1997) 75-83

77

Table 1 Particle size and surface area data for ultrafine galena Particle size, D~5oj" (Bin)

Size distribution span b (pm)

Specific surface area based on equivalent spherical diameter, Aesd (m 2 g - l )

Specific surface area by gas adsorption, A~t (m 2 g - l )

1.96

1.23

0.76

1.79

aDtsoj = 50% point of the cumulative undersize distribution. bThe span of the distribution measures the spread between the 10 and 90% points of the cumulative undersize distribution (Dilo)- Ooo~).

slurries were nitrogen purged and electrophoretic mobilities determined on application of a 40 V potential. Further measurements were then made under different p H conditions with a 20 min equilibration time between measurements. The electrophoretic mobility was converted to the zeta potential ( using the Smoluchowski equation (since ~ca>> 1) [16].

> 90% of the signal from < 3 nm (based on attenuation lengths for sulphur 2p photoelectrons [17]). Atomic concentrations for each element were determined from XPS peak areas and the relevant sensitivity factor [18]. To preserve the surface chemistry of the galena mineral-aqueous solution interface, wet slurries were transferred directly to the introduction chamber of the XPS instrument as discussed elsewhere by Smart [19].

2.2.2. Rheology Shear stress vs. shear rate curves were determined at 25°C using a Haake CV20 Couette type rheometer fitted with a Mooney-Ewart concentric cylinder sensor (ME 45; 45 mm diameter and 1 mm gap size). Slurries were pre-sheared at 300 s-1 for 2 min, then rested for 2 min and finally the shear rate D was increased linearly from 0 to 300 s -1 and back to rest in 4 min, a torque sensor continuously determining the shear stress ~ ( N m-2). This procedure produced reproducible shear stress vs. shear rate curves and minimized any pre-shear or settling effects.

2.2.3. X-ray photoelectron spectroscopy XPS spectra were recorded using a Perkin Elmer PHI Model 5100 spectrometer, with an Mg K~ X-ray source operating at 300 W and with a pass energy of 18 eV. The vacuum pressure in the analyser chamber was near 10-STorr 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, 75.13eV). All measurements were performed at a take-off angle of 45 °, corresponding to analysis depths with

2.2.4. Microflotation tests Microflotation tests were carried out in a modified Partridge and Smith cell [20] with a stainless steel frit of 13 lain pore size at the cell base producing a steady, evenly dispersed bubble swarm. The flotation time was 8 rain and the nitrogen carrier gas flow rate was 50 ml min-1. The flotation cell was tested for entrainment using a clean, nominally 12 gm quartz sample, which gave a value of less than 8%. 250 ml of pH-adjusted Milli-Q water were added to 1 g of galena particles, and the slurry was conditioned with nitrogen purging and pH regulation for 20 rain prior to flotation. After flotation, the concentrate and tail were collected, filtered, dried and weighed, enabling the flotation recovery to be determined to within 3%.

3. Results and discussion

3.1. The influence of pulp density 3.1.1. Rheology Shear stress z vs. shear rate D curves for galena slurries at pH 10 and as a function of particle solids volume fraction ~bS are given in Fig. 1. At ~bs values of less than 0.05, the rheological

C.A. Prestidge / Colloids Surfaces A: Physicochem. Eng. Aspects 126 (1997) 75-83

78 8.

100-

" 0.06

QOs=OA

e~

Z

6

A0 =0,0625 • 0s =0.0385 =

"

_

~

_~

"

80-

--

~

"0.05

Z

- 0.04 604-

•

"

"0.03

40-

.~.

r~

o~

. 100

.

. 200

300

behaviour is effectively Newtonian, with the viscosity r/independent of the shear rate: (1)

At higher ~b, values, non-Newtonian behaviour is exhibited, yield values are observed, and, in some cases, extensive pseudoplasticity (shear thinning) is evident. For 0.05<~bs<0.15, near plastic flow is observed and flow curves are best described by the Bingham model: (2)

where ~B is the Bingham (or extrapolated) yield value and %1 is the plastic viscosity. A Bingham fit is included in the shear stress vs. shear rate data in Fig. 1 for ~ = 0 . 1 . For ~b~>0.15, the Herschel-Bulkley model Z=Zo +kD"

-o.o2

20-

"0.01

~,

0 0.00

..... 0.05

, 0.10

-

, 0.15

•

, 0.20

0.00 0.25

,s

Fig. 1. Shear stress vs. shear stress curves for galena particle slurries at pH 10 ( 1 0 - 2 M KNO3) as a function of particle volume fraction.

= z~ + r/plD,

'

•

.

Shear rate, D / s "l

z=r/D.

,~ Z

(3)

gives a better data fit, where n (< 1 for the studied galena slurries) and k are constants, and z o is the Herschel-Bulkley yield value. Galena slurries at ~b,> 0.15 also showed considerable flow curve hysteresis (thixotropy), indicating time-dependent particle interactions. Extrapolated yield values and plastic viscosities are plotted against galena particle volume fraction (~b~) in Fig. 2. The observed behaviour suggests that significant interparticle interactions occur even

Fig. 2. Extrapolated yield values and plastic viscosities for galena particle slurries as a function of particle volume fraction.

at relatively low ~bs values, in agreement with reported findings for sphalerite [8] and coal slurries [6]. Theoretical calculations based on non-interacting hard spheres [21], however, predict significant increases of viscosity and yield value only at particle volume fractions in excess of 0.65. Factors which may contribute to the rheological behaviour of particulate galena slurries, but not non-interacting hard sphere colloids, include: (1) repulsive forces, e.g. double-layer repulsions and steric forces which result in the effective volume fractions greater than actual volume fractions [22,23]; (2) attractive forces, e.g. interactions between heterogeneous patches on the surfaces or hydrophobic interactions [ 8, 9]; and (3) hydrodynamic consideration, gravitational and inertial effects [24,25]. It is acknowledged that yield value determination is dependent on the rheological method and model used [26], and for highly structured samples the Vane technique [23,26] may give a more representative measure of the true yield value than shear stress vs. shear rate measurements. However, in the following sections of this work we report and discuss rheological data for galena slurries at ~s values between 0.05 and 0.1, where the Bingham model adequately describes the flow curves. In these cases, the error in zB is less than 5%, but this error increases significantly at ~b~ values in excess of 0.15. The Vane technique is less sensitive for

CA. Prestidge / Colloids Surfaces A: Physicochem. Eng. Aspects 126 (1997) 75-83

relatively low yield values [26]. Furthermore, in the present study the extrapolated yield values obtained are less than 2 0 N m -z, and therefore the samples would not be considered extensively structured. The ZB is used as a semi-quantitative measure of the extent of galena particle interaction, as has been used in previous studies on nonsulphide mineral particulate systems [22, 27, 28].

3.2. The influence of pH 3.2.1. Electrokinetics and surface chemistry Zeta potentials of galena particles determined by microelectrophoresis are plotted against pH in Fig. 3. An isoelectric point IEP is observed near pH 2, which corresponds to the reported pH~Ep of unoxidized galena particles [12]. A maximum in the zeta potential data in the vicinity of pH 5.5 is indicative of galena surface oxidation [12]. It is noted that this maximium in zeta potential is reproducible and has been observed in electroacoustic measurements [29] on the same galena particles at ~b, values equivalent to those studied here by rheology. Detailed electrokinetic studies of oxidized galena particles and oxidized lead hydrolysis products [e.g. Pb(OH)2 and Pb3(OH)z(CO3)/] have shown that zeta potentials of galena particles are controlled by the oxidation conditions, e.g. the pH, the time scale of condition-

0-

-10-

-20-

•,~

-30.

~1

-40

-50 2

3

4

5

6

7

8

9

10

pit Fig. 3. Zeta potentials for ultrafine galena particles as a function of pH (10 z M KNO3 as an indifferent electrolyte), determined by microelectrophoresis.

79

ing and the purging gas [12]. The galena particles presently under investigation were ground dry in air and stored dry in a vacuum dessicator prior to conditioning in aqueous solution. This preparation resulted in significant surface coverages of lead oxidation products, with XPS confirming the presence of lead hydroxide, carbonate or hydroxycarbonate and sulphate on the galena particle surfaces. Surface atomic concentrations as determined by XPS are given in Table 2. The surface region analysed by XPS is clearly sulphur rich (i.e. sulphur to lead atomic concentration ratio greater than unity) and the galena surface is best described as lead-deficient galena with an overlayer of oxidized lead species, which may be schematically represented as Pb,_xS,.yPbO, where PbO consists of Pb(OH)2, PbCO3, Pb3(OH)z(CO3)2 and PbSO4. XPS data has confirmed that x>y.

3.2.2. Rheology and particle interactions For galena slurries at ~b, values of 0.0625 and 0.1, the extrapolated yield values and plastic viscosities are plotted as functions of pH in Fig. 4. Generally, at these ~bs values, zR increases with decreasing pH and qpl values are independent of pH. These findings are indicative of increased particle attraction at low pH. The maxima in the yield value plots occur in the region of pH 2, corresponding closely to the IEP of the galena sample. At the pHIEP, repulsive electrostatic forces are absent, causing attractive van der Waals forces to dominate and leading to interparticular attraction and aggregation. Studies on particulate dispersions of oxides [3,4,23] have shown a maximum in the yield value at the IEP; this behaviour is readily predicted by DLVO theory. On close inspection of the rheological data for galena, one observes a second pH region of interparticular Table 2 Surface atomic concentrations (%) for ultrafine galena, as determined by XPS (carbon signal from adventitious hydrocarbon ratioed to zero) Atomic concentration (%) Pb

S

C as carbonate

O

27.9

34.8

4.9

32.4

C.A. Prestidge / Colloids Surfaces A. Physicochem. Eng. Aspects 126 (1997) 75-83

80 4.

• 0.020

•

a,

a.

0.017

4

3.

•

0.014 2.

z~ p H > 6 0.011

Z

1. 0.008 r ~

0 ~1

2:

D-

,

18.

•

b.

0.005

Z

O.lO

m

0 18

,.-

..~

•

b.

150.08

15

0.06

12

•

12-

•

9.

630

•

,

3

•

,

•

5

,

7

•

,

9

•

0.04

9 ~

0.02

6-

• o pH>6

3-

0.00 1

pH

0

Fig. 4. Extrapolated yield values and plastic viscosities for galena particle slurries as a function of pH: (a) ~b,=0.0625; (b) ~b,,= 0.1.

attraction, which would not be expected for a simple electrostatically stabilized colloidal system and not predicted by DLVO theory. On the basis on DLVO-type calculations, Hunter and coworkers [16,27,28] have shown that extrapolated yield values of particulate colloidal dispersions (generally oxides) scale with the ( potential squared [23]. Plots of zB against ~2 are linear with negative slope and are described by: rn = rB(m,x)- k( 2,

o

(4)

where ZB(m,x) is the maximum extrapolated yield value, occurring at the IEP. Equivalent plots for galena particle slurries at 4, values of 0.0625 and 0.1 are given in Fig. 5. Over the pH range 6-10 the data (fitted lines in Fig. 5) can be reasonably well described by Eq. (4), confirming that DLVO behaviour is displayed by galena particles in this pH range. At lower pH values Eq. (4) does not hold and the level of aggregation as measured by the zn is not apparently controlled by repulsive

300

600

900

1200

2 / (mV)2 Fig. 5. Extrapolated yield values for galena particle slurries as a function of the zeta potential squared: (a) ~),=0.0625; (b) ¢,=0.1.

electrostatic and attractive van der Waals forces alone. This data is better described by: zB = ZB(max)-- k~2 + ~ ,

(5)

where z~ is an additional, non-DLVO contribution to the overall yield value and may be due to: (a) attractive interactions between heterogeneous patches on the galena surfaces; or (b) hydrophobic attractions between galena surfaces. It should also be noted that in dispersions with particles of dimaeter > 0.1 ~tm, hydrodynamic, gravitational and inertial forces also contribute to the rheology [25]; these may well be active in the presently studied galena particle slurries and make some contribution to the overall yield value and flow behaviour. However, since these effects are likely to be independent of pH, they are not considered significant in controlling the pH dependent non-DLVO component of the yield value, z~

81

CA. Prestidge / Colloids Surfaces A: Physicochem. Eng. Aspects 126 (1997) 75-83

values are plotted against pH in Fig. 6. Clearly, for both ~bs values studied, z~ is negligible above pH 6 and then increases linearly with decreasing pH. It has been hypothesized that attractive interactions between heterogeneous galena surfaces or heteroaggregation between galena particles and colloidal oxidation products may contribute to z~. However, since the greatest ~ values occur at low pH where the galena particle surfaces are most homogeneous (confirmed by XPS), attractive electrostatic forces between heterogeneous surfaces do not apparently play an important role. Furthermore, in addition to pH and particle volume fraction the magnitude of T~ has been shown to depend on the hydrodynamic conditions and time scale of the conditioning period, which gives further evidence that the non-DLVO effects

2.0

•

1.0- ~

-~

0.0

•

•

are controlled by a surface chemical reaction (see below).

3.2.3. Surface chemistry and hydrophobicity (flotability) To further our understanding of the non-ideal rheological behaviour it is useful to discuss the surface chemistry of galena under the conditions used in this study. At pH 2 an unoxidized galena surface will be close to its IEP and therefore will aggregate in the absence of repulsive electrostatic forces. Incongruent galena dissolution at low pH [11] may result in sulphur-rich galena surfaces: nPbS--*Pb,_ xS, + xPb 2+ + 2xe.

(6)

These are hydrophobic, with significant advancing water particle contact angles [11] and likely to result in an additional attractive hydrophobic interaction, which may be partially responsible for the increased yield value in the vicinity of pH 2. The collectorless flotation of these ultrafine galena particles, as shown in Fig. 7, is dramatically enhanced at low pH, confirming the formation of a hydrophobic surface phase. For the particulate galena studied in this work the situation is more complex since the surfaces are significantly oxidized, as confirmed by XPS (Fig. 8 and Table 2) and electrophoretic mobility measurements (Fig. 3). Under acidic pH conditions the lead oxidation products on the galena

*

-os

11-

b.

80. 70.

oo.

7-

o I Additional20min

0

O

5-

i

a-

~

| conditioningsteps

/

30 ~

1 0.0 -I

•

4

6

8

20

lo-

•

2

50 40'

10

pH Fig. 6. The non-DLVO component of the extrapolated yield value (z~) for galena particle slurries as a function of pH: (a) ~b,=0.0625; (b) $,,=0.1.

o

~

3

5

7

9

11

pH Fig. 7. Microflotation recoveries for ultrafine galena particles as a function of pH and conditioning time.

C.A. Prestidge / Colloids Surfaces A: Physicoehem. Eng. Aspects 126 (1997) 75-83

82

10 9 B 7 6

///

5 z

4

3

t

/,' Pb-S

a.

i i

2 1

0

L

I

155

....

I

I

I

150

- - ,

~45

140

135

Binding Energy (eV] Fig. 8. Lead 4f X-ray photoelectron signals from galena particles: (a) conditioned at pH 9 prior to analysis; (b) conditioned for 20 min at pH 4 and washed once with pH 4 aqueous solution prior to analysis.

surface will dissolve: Pb,_ xS,.yPbO + 2yH ÷ ~ Pb,_ xS, + yH20 + yPb 2÷

(7) The lead 4f XPS signals in Fig. 8 confirm this mechanism: high binding energy Pb 4f signals characteristic of oxidized lead species [12,17] are removed after conditioning at pH 4 leaving only the 137.3 and 142.2 eV signals which are characteristic of lead sulphide [11,12,17]. This "surface cleaning" step will be kinetically controlled, with the rate strongly dependent on pH, the solids content of the slurry, the hydrodynamic conditions [10] and the nature of the oxidation products in question. With respect to the time scale of this surface cleaning step, the fact that flotation recovery at pH 4 is enhanced by additional conditioning periods of 20 min suggests that under conditions where Eq. (6) and/or Eq. (7) go to completion, the exposed sulphur-rich surface will be hydrophobic, which, in addition to being responsible for strong collectorless flotation at low pH (Fig. 7),

may result in an attractive hydrophobic interaction and be partially responsible for the increased yield value at p H < 6 . It is clear that interparticle interactions in galena slurries are complex, being controlled not only by DLVO-type forces (i.e. electrostatic and dispersive), but also by hydrophobic interactions which may be influenced by surface chemical reactions.

4. Conclusions

Rheological measurements provide both a qualitative and a quantitative indication of the level of aggregation in galena pulps, enabling interparticle interactions to be analysed, and hence furthering our understanding of fine particle flotation. Galena slurries show non-Newtonian rheological behaviour at pulp densities akin to those experience in flotation practice. The observed yield values and shear thinning nature of the slurries are characteristic of particle aggregation, which results from the

C.A. Prestidge / Colloids Surfaces A: Physicochem. Eng. Aspects 126 (1997) 75-83

complex and often strongly processing-conditiondependent surface chemistry. The level o f galena slurry aggregation, as determined by the extrapolated yield value, is strongly influenced by the pH. Over the p H range 6 - 1 0 galena slurries behave in a equivalent m a n n e r to oxide mineral slurries, with the level o f particle interaction, as measured by the extrapolated yield value, scaling with the square o f the zeta potential. At low p H values strong aggregation results, which is n o t explicable by electrostatic and van der Waals considerations alone. Attractive h y d r o p h o b i c interactions are responsible for the rheological behaviour and also result in strong bubble-particle interaction and strong coUectorless flotation.

Acknowledgment The Australian Research Council is acknowledged for their financial support o f the project. Discussions with J o h n Ralston, R o b e r t Hayes and other members o f the Particle and Surface Technology G r o u p are warmly acknowledged. Fengsheng Li at Nanjing University o f Science, People's Republic o f China is acknowledged for grinding the galena samples. R o g e r Smart and Bill Skinner o f the South Australian Surface Technology Centre are t h a n k e d for XPS analysis.

References [1] T.V. Subrahmanyam, K.S.E. Forssberg, Int. J. Miner. Process 30 (1990) 265. [2] A. Gaudin, D. Fuerstenau, H. Maiw, Can. Min. Metall. Bull. 56 (1960) 960. [3] A.S. Rao, J. Dispersion Sci. Technol. 8 (1987) 457. [4] Y.K. Leong, D.V. Boger, J. Rheology 35 (1990) 149.

83

[5] D. Heath, Th.F. Tadros, J. Colloid Interface Sci. 93 (1983) 307. [6] R.W. Lai, L.G. McMahan, A.G. Richardson, S.H. Chiang, Advances in Coal and Mineral Processing Using Flotation, Society for Mining, Metallurgy, and Exploration, Inc., Littleton, CO, 1989. [7] R.R. Klimpel, R.D. Hansen, Miner. Metall. Processing 6 (1989) 35. [8] T.H. Muster, C.A. Prestidge, Miner. Engng 8 (1995) 1541. [9] T.H. Muster, G. Toikka, R.A. Hayes, C.A. Prestidge, J. Ralston, Colloids Surf. A. Physiochem. Engng Aspects 106 (1996) 203. [10] B.S. Kim, R.A. Hayes, C.A. Prestidge, J. Ralston, R.St.C. Smart, Langmuir 11 (1995)2554. [1l] C.A. Prestidge, J. Ralston, J. Colloid Interface Sci. 172 (1995) 302. [12] D. Fornasiero, F. Li, J. Ralston, R.St.C. Smart, J. Colloid Interface Sci. 164 (1994) 333. [13] D. Fornasiero, F. Li, J. Ralston, J. Colloid Interface Sci. 164 (1994) 345. [14] J. Ralston, Miner. Engng 7 (1994) 715. [15] C.A. Prestidge, J. Ralston, R.St.C. Smart, Colloids Surf. 81 (1993) 103. [16] R.J. Hunter, Zeta Potential in Colloid Science, Principles and Applications, Academic Press, London, 1981. [17] A.N. Buckley, R. Woods, Appl. Surf. Sci. 17 (1984) 401. [ 18] G.E. Muilenberg, The Handbook of X-ray photoelectron spectroscopy, Perkin Elmer Corp., Eden Prairie, MN, 1979, p. 188. [19] R.St.C. Smart, Miner. Engng 4 (1991) 891. [20] A.C. Partridge, G.W. Smith, Trans. Inst. Min. Metall. 80 (1971) 199. [2l] I.M. Krieger, Adv. J. Colloid Interface Sci. 3 (1972) 111. [22] C.A. Prestidge, Th.F. Tadros, J. Colloid Interface Sci. 124 (1988) 660. [23] Y.K. Leong, P.J. Scales, T.W. Healy, D.V. Boger, R. Buscall, J. Chem. Soc. Faraday Trans. 89 (1993) 2473. [24] L.S. Laskowski, R.J. Pugh, in: D.W. Fuerstenau, J.S. Laskowski, J. Ralston (Eds.), Colloid Chemistry in Mineral Processing: Developments in Mineral Processing, vol 12. Elsevier, Amsterdam, 1992, pp. 115-171. [25] L. Warren, in: M.H. Jones, J.T. Woodcock (Eds.), Principles of Mineral Flotation, Australasian Inst. Min. Metall., 1984, p. 185. [26] Q.D. Nguyen, D.V. Boger, J. Rheology 29 (1985) 335. [27] B.A. Firth, R.J. Hunter, J. Colloid Interface Sci. 57 (1976) 266. [28] R.J. Hunter, Adv. Colloid Interface Sci. 17 (1982) 197. [29] C.A. Prestidge, W.N. Rowlands, Miner. Eng., in press.