Depression mechanisms of sodium bisulphite in the xanthate-induced flotation of copper activated sphalerite

Int. J. Miner. Process. 79 (2006) 61 – 75 www.elsevier.com/locate/ijminpro

Depression mechanisms of sodium bisulphite in the xanthate-induced flotation of copper activated sphalerite T.N. Khmeleva, J.K. Chapelet, W.M. Skinner, D.A. Beattie * Ian Wark Research Institute, The ARC special Research Centre for Particle and Material Interfaces, University of South Australia, Mawson Lakes, SA 5095, Australia Received 12 October 2005; received in revised form 8 December 2005; accepted 9 December 2005 Available online 8 February 2006

Abstract The effect of sodium bisulphite on the xanthate-induced flotation of copper-activated sphalerite has been studied using batch flotation testing, surface analysis techniques (XPS and ToF-SIMS), and FTIR. The various techniques have been used to identify the mechanisms of interaction of sulphite ions with both collector and the sphalerite surface. The results indicate that sodium bisulphite depressed the flotation of sphalerite particles pre-treated with copper and xanthate at pH 9 with nitrogen and air purging. It was found that sodium bisulphite interacts with the sphalerite surface, as well as with xanthate in its adsorbed state. Based on the evidence obtained in the present study, and in conjunction with previous work, the mechanisms involved in the depression of the xanthate-induced flotation of copper-activated sphalerite by sulphite are proposed. It is suggested that copper xanthate decomposition on the surface of the activated sphalerite and the decomposition of the hydrophobic copper-sulphide-like species on the sphalerite surface are the active mechanisms for sphalerite depression by sodium bisulphite. D 2005 Elsevier B.V. All rights reserved. Keywords: flotation; copper-activated sphalerite; sodium bisulphite; isobutyl xanthate; in situ FTIR spectroscopy; x-ray photoelectron spectroscopy; time of flight secondary ion mass spectroscopy

1. Introduction It is well known that copper-activated sphalerite interacts readily with xanthate species, forming a hydrophobic surface layer (Fuerstenau, 1982a,b; Finkelstein and Allison, 1976; Laskowski et al., 1997). Both copper–xanthate complexes and copper-sulphide-like phases, formed on the sphalerite surface due to activation, are responsible for sphalerite flotation. Sulphoxy reagents such as sodium bisulphite and sulphur dioxide have been used as depressants in industrial processes for selective depression of cop* Corresponding author. Fax: +61 8 83023683. E-mail address: [email protected] (D.A. Beattie). 0301-7516/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.minpro.2005.12.001

per-activated sphalerite and pyrite from valuable sulphide minerals (e.g. chalcopyrite and galena) during their flotation. A number of mechanisms have been proposed to explain the manner in which these reagents function as depressants (Miller, 1970; Luthy and Bruce, 1979; Peres, 1979; Pattison, 1981; Yamamoto, 1980; Moses et al., 1984; Misra et al., 1985; Li et al., 1995; Grano, 1997; Shen et al., 2001). It has been generally considered that sulphoxy depressants interact with the mineral surface and with collector species. In spite of this, not all investigators have agreed on the effectiveness of sulphoxy reagents as depressants for sphalerite in complex sulphide ores, and there is little clarity in the mechanisms proposed for sphalerite depression by sulphite.

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In our previous work (Khmeleva et al., 2005a) the effect of sodium bisulphite on the collectorless flotation of copper-activated sphalerite and mechanisms of interactions of sulphite ions with the sphalerite surface have been studied. It was found that sodium bisulphite acts as an effective depressant for the collectorless flotation of copper-activated sphalerite and has a specific effect on the surface chemistry of sphalerite particles under the experimental conditions studied. It was suggested that sulphite ions adsorb on the surface of copperactivated sphalerite and subsequently decompose the hydrophobic sulphur-like species formed as a result of the activation process. We concluded that sulphite ions specifically interact with the reduced coordination sulphur associated with copper activation (which results in a trigonally co-ordinated Cu–S species, as opposed to the quadruple coordinated Zn–S species) and report into solution as a thiosulphate, which is then oxidised to sulphate. At the same time, zinc hydroxide is formed at the sphalerite surface and in solution. The resultant reduction in surface hydrophobicity explains the sphalerite depression in the presence of sulphite. The present study has focused on the effect of sodium bisulphite on the xanthate-induced flotation of copper-activated sphalerite at pH 9 with nitrogen and air purging. Additional analytical techniques were employed in the current study to pin-point the mechanisms which are involved in the interaction of sulphite ions both with the sphalerite surface and collector species in its adsorbed state. The order of presentation of the data is as follows. First, we present an investigation into the effect of sodium bisulphite on the flotation of copper activated sphalerite; the investigation was designed to allow us to ascertain the degree of effectiveness of sodium bisulphite as a depressant and to optimise the conditioning parameters for the subsequent surface analysis of the treated sphalerite. In addition, size-by-size analysis of the flotation results is presented. Second, we present detailed surface analysis of the flotation feed using X-ray photoelectron spectroscopy (XPS) and of the con and tail samples from the flotation experiments using time-of-flight secondary ion mass spectrometry (ToF SIMS). The surface analysis allows us to determine the effect of sodium bisulphite on the surface species present on the copper-activated sphalerite and to identify the species responsible for flotation/depression of the mineral. In addition, the surface analysis in the presence of collector allows us to determine whether the same mechanisms of depression are observed as in the case of collectorless flotation (Khmeleva et al., 2005a). Third, we present a detailed study into the interaction between sodium bisulphite

and the collector, sodium isobutylxanthate (IBX), in its adsorbed state on the sphalerite surface using ex situ DRIFT spectroscopy and in situ particle film ATR FTIR (McQuillan, 2001; Chiem et al., in press). This last technique enables us to monitor the effect of sodium bisulphite on adsorbed collector species in situ on an activated mineral surface, and allows us to determine whether sodium bisulphite actually removes IBX from the surface of the activated sphalerite or whether its effect is due to the reduction of the concentration of collector in solution. 2. Experimental 2.1. Materials and reagents The natural sphalerite sample was obtained from the Elmwood Mine, USA. The elemental composition of sphalerite analysed by ICP-MS (Inductively Coupled Plasma- Mass Spectroscopy) is shown in Table 1. Synthetic sphalerite particles (used in the ATR FTIR experiments) were produced by bubbling H2S through a solution of 0.2 M Zn(NO3)2. The particles were centrifuged and washed in deionised water, re-centrifuged and dried in a N2 atmosphere. The particles were stored in a desiccator once dry. The particles were confirmed to be sphalerite by XRD and Raman spectroscopy. The average size of the particles (from comparing multiple SEM images) is around 2 Am. The morphology of the particles indicates that each particle is made up of many smaller rod-like crystals of sphalerite. BET surface area analysis confirmed the expected high surface area, yielding a value of 115 m2/g. All chemicals were of analytical grade. 2.2. Flotation experiments 50 g of sphalerite was wet ground at 30% solids in a Fritsch ball centrifuge zirconium mill (200 ml volume) for 7 min. The d 80 of the ground mineral was around 38 Am. After grinding, the slurry was washed from the mill into a 500 ml Gliwice flotation cell. The pulp was conditioned in 5 stages (gas purging — 3 min, activation — 2 min, Table 1 Chemical assay of the sphalerite sample Mineral

Sphalerite (ZnS) (Elmwood Mine, USA)

Chemical elements present (wt.%) Zn

S

Cu

Fe

Si

Mg

Ca

66.6

32.7

0.075

0.5

0.3

0.055

0.09

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collector addition — 1 min, depressant addition — 1 min, frother addition — 30 s) and four concentrates were collected for 0.5, 2, 4 and 8 min. The overall time of flotation was 8 min. Copper nitrate (Cu(NO3)2d 3H2O) was used to introduce the activating copper ions during the conditioning time. Sodium isobutyl xanthate (C4H9OCSSNa, abbreviated as IBX) and polypropylene oxide methanol (Dowfroth 250) were added in the experiments as a flotation collector and frother, respectively. Sodium bisulphite (NaHSO3) was used in the present studies as a depressant. The slurry pH values of 9 were controlled using a carbonate/bicarbonate buffer. Deionised water was used throughout the experimental work. Wet and dry weights were measured for all flotation products (four concentrates and the final tail). Both nitrogen and air purging were used during this study. The pulp Eh was measured using an Hg/Hg2Cl2 electrode. Size-by-size analysis of the flotation concentrate was performed using a Malvern Master Sizer laser diffraction instrument. The mineral samples were prepared in a similar manner to the flotation experiments. From each flotation experiment, four concentrates were combined together to provide sufficient mass for analysis. After flotation in the absence and presence of depressant, the concentrates and tails were analysed. The results are presented as cumulative mass recovery versus particle size. The percent recovery versus top particle size was calculated based on the mass of mineral recovered for that size fraction divided by the total mass of particles of that size fraction in the original feed. Entrainment has not been subtracted from these results, as it was assumed to be relatively low. 2.3. Surface analysis 2.3.1. X-ray photoelectron spectroscopy (XPS) XPS analyses were performed with a Physical Electronics (PHI) 5600 spectrometer using a Mg Ka irradiation source operated at 300 W. A spectrometer pass energy of 17.9 eV was used for all elemental spectral regions. The pressure in the analyser chamber was 10 7 Pa. All measurements were performed at a take-off angle of 458. The escape depth of photoelectrons was approximately between 2–5 nm. The results are presented as atomic surface concentrations and XPS spectra. 2.3.2. Time of flight secondary ion mass spectroscopy (ToF-SIMS) ToF-SIMS measurements were performed using a PHI model TRIFT 2100 spectrometer equipped with Ga liquid metal ion gun (LMIG). The system uses a pulsed primary ion beam to desorb and ionise species

63

from a sample surface. Damage to the uppermost monolayer is minimised by applying extremely low primary ion fluxes. With the analysis time ranged from 4 to 5 min, the primary ion fluence was ~3  1012 ions/cm2, which ensured static conditions and minimal surface damage, i.e. less than 1 in 1000 atoms struck in the time of the measurement. The quantitative comparison of the surface components for the samples selected has been derived from the ToF-SIMS data with the aid of statistical analysis (analysis of means). Each result (exposure of surface component like copper, iron, sulphur, etc) is expressed by an average value with the associated evaluation of its variability (confidence interval) (Piantadosi et al., 2000; Piantadosi and Smart, 2002). Prior to statistical analysis the SIMS data were normalised by dividing the intensity of the ion of interest by that of the ion yield for the sum of all peaks of interest selected from the spectrum. Each sample was dispersed in water at pH 9, deslimed (i.e. removal of dispersible, non-adsorbed fine particles) and deposited onto a piece of indium and whilst still wet, introduced into the spectrometer preparation chamber with out exposure of the mineral surfaces to air. Prior to analysis the samples were outgassed under vacuum for around 12 h. 2.4. Infrared (FTIR) spectroscopy 2.4.1. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) DRIFT spectra were obtained using a Nicolet Magna 750 spectrometer. The sphalerite samples for infrared analysis were prepared in a similar manner to the flotation experiments. The pulp pH was kept constant at around 9, while the slurry was nitrogen purged. After conditioning and filtration, the sphalerite sample was gently mixed together with ground KBr (by mortar and pestle). Spectroscopic KBr was used as a background and to dilute the sphalerite samples to the optimal 3% by weight. Then, the sphalerite samples were placed in the cup of a Spectra Tech diffuse reflectance accessory apparatus. The samples were allowed to dry out in the spectrometer. The IR spectra of KBr were pre-recorded and subtracted from the IR spectrum, thus obtaining the peaks associated with the adsorbed collector on the sphalerite surface and with the sphalerite itself; the respective subtraction factors were close to 1. 2.4.2. Attenuated total reflection Fourier transform infrared spectroscopy (ATR FTIR) The experimental arrangement used in this work has been described previously (Chiem et al., in press).

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In the present experiments, a germanium prism (rather than a ZnSe prism) with incident angle 458 was used. The germanium crystal was polished with an aqueous alumina (Al2O3) powder (BDH) and then washed with detergent and rinsed with milli-Q water to remove any remaining Al2O3 particles. The crystal was then rinsed with ethanol, milli-Q water, and dried with N2. A spectrum of the clean prism was used as a background for all subsequent recorded spectra. A suspension of synthetic sphalerite was prepared using 0.0209 g of particles and milli-Q water to give a suspension concentration of 0.1 wt.%. The suspension was sonicated for 80 min prior to deposition on the clean prism, to ensure full dispersion of the particles. A 0.300 ml ZnS suspension volume was placed on the crystal, and dried by vacuum evaporation for 30 min. The ZnS layer was copper activated for 30 min using 0.300 ml of a 3  10 3 M solution of CuSO4. After activation the CuSO4 solution was removed and the particle film rinsed with KNO3 solution. A flow through ATR cell and ATR accessory (Harrick FastIR) was used to allow the collector and depressant solutions to flow over the activated particle film. A Masterflex peristaltic pump (model 77120–70) with Tygon R LFL tubing was used to pump the solution from the reservoir to the ATR cell. The flow rate from the solution reservoir was fixed at 5 mL min 1. Prior to use, the KNO3, SIBX, and NaHSO3 solutions were purged with N2 and their pH adjusted to 9 using KOH and HNO3. The sphalerite layer was firstly exposed to the 10 2 M KNO3 solution and a spectrum recorded to allow subtraction of the electrolyte background from the adsorbed collector spectra. The reservoir of electrolyte solution was then exchanged for a sodium isobutyl xanthate (SIBX) solution. The concentration of SIBX in the solution was 3  10 3 M in 10 2 M KNO3. Spectra were recorded (64 scans at 4 cm 1 resolution) as a function of time until the adsorption had reached equilibrium. After this point a longer scan (1024 scans at 4 cm 1 resolution) was recorded to obtain a spectrum with better signal to noise. Once complete, a further set of spectra were required after the substitution of the collector solution for either a 3  10 3 M solution of NaHSO3 in 10 2 M KNO3, or a solution of 10 2 M KNO3. Spectra were recorded over 15 min (64 scans) to show the desorption of the collector species. Finally a longer scan (1024 scans) of the adsorbed layer in equilibrium with the new solution was recorded to compare with the spectrum of the adsorbed layer in equilibrium with the collector solution.

3. Results and discussion 3.1. Flotation study The effect of isobutyl xanthate on the flotation behaviour of copper-activated sphalerite was examined at pH 9 in a nitrogen atmosphere. The results of these experiments are shown in Fig. 1. It is evident from the data that the addition of xanthate to the system without prior copper activation of sphalerite particles results in no change in the sphalerite recovery. These results confirm that sphalerite flotation depends on copper activation, as it is known (Fuerstenau, 1982a,b; Finkelstein and Allison, 1976; Laskowski et al., 1997) that sphalerite floats poorly with xanthate collectors without prior activation. The data in Fig. 1 also shows that addition of xanthate at all concentrations tested (50, 100 and 200 g/t) increased the hydrophobicity of the sphalerite surface, as the sphalerite recovery was increased (50.8% to 83.2%, 91.5% and 95.1%, respectively). As a relatively high recovery of copperactivated sphalerite was obtained at 50 g/t of xanthate addition, this concentration was chosen for further experiments using surface analysis. Fig. 2 shows the effect of sodium bisulphite on the flotation recovery of copper-activated sphalerite pretreated with isobutyl xanthate at pH 9. For comparative purposes these flotation experiments were carried out with both nitrogen or air purging. Based on the flotation data obtained in the previous study (Khmeleva et al., 2005a), the following order of reagent addition and their concentrations were used in the current flotation 100

Sphalerite Recovery, %

64

80 60 40 20 0 0

2

4 Time, min

6

8

Sph only Sph + IBX 50g/t Sph + Cu(NO3)2 Sph + Cu(NO3)2 + IBX 50g/t Sph + Cu(NO3)2 + IBX 100g/t Sph + Cu(NO3)2 + IBX 200g/t

Fig. 1. Effect of isobutyl xanthate concentration on the flotation of copper-activated sphalerite at pH 9 with nitrogen purging.

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100

(a)

Sphalerite Recovery, %

Sphalerite Recovery, %

100

65

80 60 40 20

(b) 80 60 40 20

Time, min 0

Time, min 0

0

2

4

6

8

0

2

4

6

8

Sph + Cu(NO3)2 in N2 Sph + Cu(NO3)2 + IBX in N2

Sph + Cu(NO3)2 + IBX in Air Sph + Cu(NO3)2 + IBX + NaHSO3 in Air

Sph + Cu(NO3)2 + IBX+ NaHSO3 in N2 Fig. 2. Effect of sodium bisulphite on the flotation recovery of copper-activated sphalerite pre-treated with isobutyl xanthate at pH 9 with nitrogen (a) and air (b) purging; [Cu(NO3)2] — 2000 g/t, [SIBX] — 50 g/t, [NaHSO3] — 2000 g/t.

experiments: (1) copper nitrate at 2000 g/t; (2) isobutyl xanthate at 50 g/t; and (3) sodium bisulphite at 2000 g/t. The results demonstrate that sodium bisulphite has a depressing effect on the xanthate-induced flotation of copper-activated sphalerite. It was found that the effect of sodium bisulphite on the sphalerite flotation was similar either with nitrogen or air purged conditions. The addition of sodium bisulphite decreased the sphalerite recovery from 83.2 to 55.4 in the nitrogen atmosphere; and from 71.3% to 43.8% in the air atmosphere. In the case of air purging, the recoveries of copperactivated sphalerite in the absence and presence of sulphite ions were initially 12% lower compared to those in nitrogen. These data indicate that the combination of air purging and sodium bisulphite addition has a slightly greater depressing effect on the sphalerite flotation performance, possibly as more oxidation products would form on the sphalerite surface in air purged conditions. It was also noted that after sodium bisulphite addition to the system, the pulp Eh dropped by 60 mV, suggesting some oxygen consumption due to sulphite oxidation to sulphate. Consequently, sodium bisulphite addition may also reduce the xanthate adsorption on the mineral surface due to the lack of oxygen in solution, which can limit copper–xanthate complex formation (Miller, 1970). Since the copper(I)– xanthate species are partially responsible for sphalerite flotation (Prestige et al., 1994; Shen et al., 2001; Boulton, 2002), a reduction in xanthate adsorption will lead to depression. Further analysis of flotation results on a size-by-size basis was employed to investigate the effect of sodium

bisulphite on the flotation response of copper-activated sphalerite pre-treated with xanthate using an established experimental method. The results are presented as cumulative mass recovery versus particle size. The percent recovery vs. top particle size was calculated based on the mass of sphalerite recovered for that size fraction divided by the total mass of particles of that size fraction in the original feed. Entrainment has not been subtracted from these results, as it was assumed to be relatively low. Table 2 show the particle size distribution of the sphalerite feed sample. The data illustrate that 80% of the ground material was size under 38 Am (d 80 = 38 Am). Fig. 3 shows the effect of sodium bisulphite on the floated particle size distribution. The high recovery of all size fractions was observed in the absence of depressant, suggesting that the concentrations of copper and xanthate species used were very close to optimum/ maximum; even the fine fraction of the sphalerite particles (b 8 Am) floated very well under the experimental conditions studied. The addition of sodium bisulphite to the system decreased the recovery of sphalerite over the entire particle size range. The best depression was achieved for sphalerite particles N 20 Am, while the depression of the 5 Am fraction was only moderate. Evidently, the depression of sphalerite particles by sul-

Table 2 Particle size distribution of the sphalerite feed sample Top size, Am Under %

5 16

10 29

20 53

38 80

53 90

75 96.5

106 99.4

150 100

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Cumulative Sphalerite Mass Recovery, %

100 80 60 40 20 0 0

20

40

60

80

100

120

140

Top particle size, µm Con 1-4 (Sph + Cu + IBX) Con 1-4 (Sph + Cu + IBX + NaHSO3) Fig. 3. Effect of sodium bisulphite on the sphalerite size-by-size recovery at pH 9 with air purging (Cumulative recoveries of Cons 1 – 4 combined); [Cu(NO3)2 — 2000 g/t; [SIBX] — 50 g/t; [NaHSO3] — 2000 g/t.

phite was a result of depression of the intermediate and coarse particles (N20 Am). 3.2. Surface analysis 3.2.1. XPS X-ray photoelectron spectroscopy was used to investigate the effect of sodium bisulphite on the surface chemistry of copper-activated sphalerite pre-treated with isobutyl xanthate at pH 9 with nitrogen purged conditions. Two feed samples of copper-activated sphalerite conditioned with xanthate in the absence (a) and presence (b) of sulphite ions were analysed. Table 3 shows the elemental surface concentrations of sphalerite. Carbon atomic concentrations were ratioed to zero for better comparison of O, S, Zn and Cu stoichiometry. It was assumed that the amount of copper measured by XPS was related to the copperactivation of sphalerite surface, as the sphalerite sample on its own contained very small amounts of impurities. The data in Table 3 illustrate that after sodium bisulphite addition, the atomic concentration of oxygen increased from 33.8% to 44%. The amount of sulphur and zinc decreased after depressant addition from 44.9% to 41.6% and from 15.9% to 8.6%, respectively, whilst the atomic concentration of copper on the surface of copper-activated sphalerite did not change dramatically. Fig. 4 show the XPS spectra of the carbon, oxygen, copper, and zinc present on the sphalerite surface. The XPS spectra for copper-activated sphalerite pre-treated

with isobutyl xanthate before and after sulphite addition look similar to those reported previously (Khmeleva et al., 2005a) for copper-activated sphalerite in the absence of collector. The C 1s spectra of the feed samples before (a) and after (b) sulphite addition were similar to each other and are composed of two peaks at 284.6 and 288.7 eV. The first peak (284.6 eV) is attributed to unintentional hydrocarbon contamination of the mineral surface (Buckley et al., 1989). The second peak (288.7 eV) is attributed to carbonate species which can be formed due to the adsorption of CO2 onto the surface oxide and hydroxide groups (Smart, 1991) and/or derived from the carbonate/bicarbonate buffer, which was used for conditioning at pH 9. The spectra for the Zn 2p region are also shown in Fig. 4. The addition of sodium bisulphite did not affect the Zn 2p signal, as the spectra are very similar in shape. In the absence and presence of sulphite, the zinc spectra have only one peak at 1021.5 eV, which is attributed to both zinc sulphide and zinc hydroxide (Fairthorne et al., 1997; Prestidge et al., 1997; Shen et al., 2001). It is not possible to distinguish between ZnS and Zn(OH)2 in the Zn 2p spectra unless there is significant charging of the insulating oxidation product (Shen et al., 2001; Khmeleva et al., 2005b). The Cu 2p spectra (Fig. 4) for samples (a) and (b) show only two signals at 932.2 and 952 eV which are characteristic peaks for copper(I)-substituted zinc sulphide (Prestidge et al., 1997). Only a very small band is seen at 942 eV (due to Cu(II) shake-up satellites), suggesting that in the absence and presence of sodium bisulphite, the copper on the sphalerite surface is mainly present in the cuprous state (Fairthorne et al., 1997). Also, no differences were observed in the Cu 2p spectra between the feed samples before and after sulphite addition. Fig. 4 shows no or little effect of sodium bisulphite on the O 1s spectra. These oxygen spectra have a broad peak at around 531.8 eV due to hydroxide (most likely Zn(OH)2) and sulphoxy species (Prestidge et al., 1997; Grano, 1997). The O 1s spectra of the feed samples are similar in shape in both the absence (a) or presence (b) of sodium bisulphite. Table 3 Elemental surface concentrations of sphalerite by XPS at pH 9 with nitrogen purging (carbon omitted from concentration calculation) Sample

Conditions

Elements (at.%; +/ 0.5) O1s

S2p

Zn2p3

Cu2p

(a) (b)

Sph + Cu + IBX Sph + Cu + IBX + NaHSO3

33.8 44.0

44.9 41.6

15.9 8.6

5.4 5.8

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67

CH

ZnS/Zn(OH) 2

C 1s

Zn 2p

N(E)

N(E)

b) Sph + Cu + IBX + NaHSO3

2-

CO3

b) Sph + Cu + IBX + NaHSO3

a) Sph + Cu + IBX

a) Sph + Cu + IBX

300 298 296 294 292 290 288 286 284 282 280

1037 1035 1033 1031 1029 1027 1025 1023 1021 1019 1017

Binding Energy (eV)

Binding Energy (eV) Cu-S CuS

OH-

Cu 2p O 1s b) Sph + Cu + IBX + NaHSO3

N(E)

N(E)

b) Sph + Cu + IBX + NaHSO3

a) Sph + Cu + IBX a) Sph + Cu + IBX

974 969 964 959 954 949 944 939 934 929 924

545 543 541 539 537 535 533 531 529 527 525

Binding Energy (eV)

Binding Energy (eV)

Fig. 4. C 1s, Zn 2p, Cu 2p, and O 1s XPS spectra of: (a) copper-activated sphalerite conditioned with isobutyl xanthate; and (b) copperactivated sphalerite conditioned with both isobutyl xanthate and sodium bisulphite at pH 9 with nitrogen purging; [Cu(NO3)2] — 2000 g/t, [SIBX] — 50 g/t,[NaHSO3] — 2000 g/t.

A deconvolution of the S 2p signal (Fig. 5) into its individual components for copper-activated sphalerite pre-treated with isobutyl xanthate before and after sulphite addition, produced identical spectra to those obtained in the absence of collector (Khmeleva et al., 2005a). Likewise, Fig. 5 illustrates that the addition of sodium bisulphite resulted in the most significant changes to the S 2p region. In the absence of sulphite ions (Fig. 5 (a)), the S 2p spectra show two doublets. The main sulphur doublet at 161.2 eV is due to sulphide (S2 ). An additional doublet at around 162.5 eV is due to formation of hydrophobic copper-sulphide-like species (Cu–S) such as metal deficient sphalerite and polysulphide on the sphalerite surface via activation process (Buckley and Woods, 1984; Prestidge et al., 1997; Gerson et al., 1999; Smart et al., 2000). Adsorbed collector species also may contribute to this region, as the peak for xanthate usually presents at 163.7 eV (Szargan et al., 1992). The addition of sodium bisulphite (Fig. 5 (b)) resulted in a reduction in the high binding energy

component (doublet at 162.5 and 163.7 eV), suggesting that sulphite ions decomposed/removed coppersulphide-like phases from the sphalerite surface and/ or reduced the exposure of xanthate species on the sphalerite surface. This decomposition/removal of the surface sulphur is also supported by a decrease in the atomic concentration of sulphur shown in Table 3. A very small peak related to sulphate (168 eV) was found after sodium bisulphite addition, while no sulphite was detected at 166.2 eV (Buckley and Woods, 1984). 3.2.2. ToF-SIMS The first concentrate and tail samples of copperactivated sphalerite pre-treated with isobutyl xanthate before and after sodium bisulphite addition were analysed by time of flight secondary ion mass spectroscopy (ToF-SIMS). These experiments were carried out at pH 9, in an air atmosphere. Air was chosen as a purging gas, since the depressing effect of sulphite was similar either with air or nitrogen purging, however the com-

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(a) S2-

Poly-S and/or metal-deficient ZnS

172

170

168

166

164

162

160

158

162

160

158

Binding Energy, eV

(b) S2-

Poly-S and/or metal-deficient ZnS

SO42-

172

170

168

166

164

Binding Energy, eV Fig. 5. S 2p XPS spectra of: (a) copper-activated sphalerite conditioned with isobutyl xanthate; and (b) copper-activated sphalerite conditioned with both isobutyl xanthate and sodium bisulphite at pH 9 with nitrogen purging; [Cu(NO3)2] — 2000 g/t, [SIBX] — 50 g/t,[NaHSO3] — 2000 g/t.

bination of sulphite addition and air purging resulted in slightly more efficient depression of sphalerite. As a result, any changes in surface chemistry caused by the sulphite are likely to be amplified and therefore more visible in the ToF-SIMS data. After flotation, twentyfour particles were investigated from each of the concentrate and tail samples to obtain statistically significant results, as a single particle analysis can be significantly different from the average value for the whole population of particles (Piantadosi, 2001, Piantadosi et al., 2000; Piantadosi and Smart, 2002). The

data presented in Figs. 6 and 7 show the average intensities of positive (+) and negative ( ) ion signals with the statistically determined confidence intervals ( P = 95%) for the copper-activated sphalerite particles in the absence and presence of sodium bisulphite, respectively. For each sample the spectra show the intensities of the Mg, Si, K, Ca, Fe, Cu, Zn (+ ions) and C, CH, O, OH, F, S, SO3, SO4 and IBX ( ions). Fig. 6 shows the data obtained for the sphalerite particles in the absence of depressant (NaHSO3). It appears that the tail particles were more oxidised than the concentrate particles; the exposure of oxygen products (O and OH) was higher on the surface of the tail particles compared to those on the concentrate. As expected, the amount of copper (Cu) and sulphur (S) species on the surface of tail particles was lower. The intensities of SO3 and SO4 species on the surface of the tail particles were lower compared to the concentrate particles, indicating that these species were not retained on the surface and did not affect the flotation response significantly. Fig. 7 shows the ToF-SIMS statistics after sodium bisulphite addition. In the presence of sulphite ions the spectra looked similar to those described above and the trends were similar. However, after sodium bisulphite addition, slightly more O and OH were found on the surfaces of both the concentrate and tail particles. Also, in the presence of sulphite ions, the exposure of sulphur on the concentrate and tail particles was lower compared to that in the absence of sulphite ions. In terms of collector, less IBX was found on the tail particles compared to that on the concentrate (Fig. 8 (a)). After depressant addition, the concentration of IBX was statistically less for tail particles compared to concentrate particles (Fig. 8 (b)). The results show that the collector coverage of sphalerite particles was very low (e.g. IBX exposure was around 2.7  10 4 for the first concentrate) either in the absence or presence of sodium bisulphite. Results are also presented as the overall mineral hydrophobicity index: K H = (CH + S) / (O + OH) (Khmeleva et al., 2005b). The values of this index were 1.60 and 1.21 for sphalerite in the first concentrate and tail, respectively (Fig. 8 (c)). This confirms that the high flotation rate of concentrate particles of copper-activated sphalerite pre-treated with xanthate was due to its high hydrophobicity. It is most likely that copper and IBX species induced the sphalerite flotation. The overall hydrophobicity index (K H) for the concentrate and tail particles conditioned with depressant was lower compared to that in the absence of sulphite ions (Fig. 8 (c) and (d)). The data suggest that sphalerite particles were to some extent

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69

0.9 Con1 (Sph + Cu + IBX)

0.8 Tail (Sph + Cu + IBX)

0.7

ntensity I

0.6 0.5 0.4 0.3 0.2 0.1 Mg

Si

K

Ca

Fe

Cu

Zn

(+) Fragment 0.5 Con1 (Sph + Cu + IBX) Tail (Sph + Cu + IBX)

Intensity

0.4

0.3

0.2

0.1

0

C

CH

O

OH

F

S

SO3

SO4

IBX

(-) Fragment Fig. 6. (+) and ( ) ion signals for copper-activated sphalerite conditioned with isobutyl xanthate in the first concentrate and tail at pH 9, with air purging; [Cu(NO3)2] — 2000 g/t; [SIBX] — 50 g/t.

more oxidised when sulphite was present in the system and therefore less floatable. XPS and ToF-SIMS analyses further explain the decrease in sphalerite flotation with the addition of sodium bisulphite. The results obtained in the current study are similar to those reported by Khmeleva et al. (2005a) for collectorless flotation of copper-activated sphalerite. These results show that sodium bisulphite interacts with the surface of copper-activated sphalerite in a similar manner whether isobutyl xanthate is present or absent; sodium bisulphite decomposed the hydrophobic copper-sulphide-like and polysulphide species on the sphalerite surface, rendering it more hydrophilic and less floatable. In addition, XPS and ToF-SIMS analyses show that sodium bisulphite promoted the surface oxidation of sphalerite particles. After sodium bisulphite addition, more oxidation products (O and OH) were found on the sphalerite surface due to zinc

hydroxide formation, as no Cu(II) hydroxide/oxide was observed. 3.3. Sulphite–collector interactions The interaction between xanthates and sulphoxy species in solution is reasonably well understood. Misra et al. (1985) investigated the effect of SO2 on the solution concentration of amyl xanthate, as a function of pH, oxygen concentration in solution, and as a function of the molar ratio of SO2 to xanthate. This work illustrated the importance of oxygen in the decomposition of xanthate by SO2. The mechanism of decomposition proposed by these authors involves the reaction of peroxide (formed in solution by the presence of SO2) with the xanthate species to form an intermediate decomposition product (perxanthate), the presence of which was confirmed by

70

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0.8

Tail (Sph + Cu +IBX +NaHSO3)

0.7

Intensity

0.6 0.5 0.4 0.3 0.2 0.1 0 Mg

Si

K

Ca

Fe

Cu

Zn

(+) Fragment 0.5 Con1 (Sph +Cu +IBX +NaHSO3) Tail (Sph + Cu +IBX +NaHSO3)

Intensity

0.4

0.3

0.2

0.1

0 C

CH

O

OH

F

S

SO3

SO4

IBX

(-) Fragment Fig. 7. (+) and ( ) ion signals for copper-activated sphalerite conditioned with both isobutyl xanthate and sodium bisulphite in the first concentrate and tail at pH 9, with air purging; [Cu(NO3)2] — 2000 g/t; [SIBX] — 50 g/t; [NaHSO3] — 2000 g/t.

UV–visible spectroscopy. The end decomposition product was postulated to be the amyl alcohol. Yamamoto (1980) studied the effect of sulphite ions on ethyl xanthate in solution. In this work, similar changes were seen in the UV–visible spectra of the collector solution, indicating the same decomposition pathway. The mechanism proposed by Yamamoto also requires oxygen to be present. However, the proposed reacting species were different (SO32 and HSO3 ) as no peroxide is likely to form in solution through the addition of a sulphite salt. Solution decomposition of xanthate by sulphite ions will contribute to the depression of copper-activated sphalerite shown in Fig. 2(b) under air purging conditions. This is confirmed by the ToF-SIMS data for con/tail samples obtained under air purging conditions (Figs. 6, 7, and 8) which indicate that collector exposure may be reduced by sulphite ions, even at the

low collector concentrations used in the flotation experiments. However, it is unlikely to be an active mechanism for depression in the flotation experiments conducted with nitrogen purging (Figs. 1 and 2(a)), as both Yamamoto (1980) and Misra et al. (1985) have shown that oxygen is necessary for the solution decomposition of xanthate by sulphoxy species. Although solution decomposition of xanthate is unlikely under nitrogen purged conditions, it is possible that the sulphite ions interact directly with the adsorbed xanthate species. FTIR spectroscopy, both ex situ (DRIFT) and in situ (ATR), has been used to probe the interactions between sulphite ions and the adsorbed collector. 3.3.1. DRIFT spectroscopy study The effect of sodium bisulphite on the adsorbed amount of collector species was investigated using

T.N. Khmeleva et al. / Int. J. Miner. Process. 79 (2006) 61–75 5.0E-04

5.0E-04

(a)

IBX

3.0E-04 2.0E-04 1.0E-04

3.0E-04 2.0E-04

0.0E+00 Con 1

Tail

Con 1

Sph + Cu + IBX

2.0

(c)

Tail

Sph + Cu + IBX + NaHSO3

KH

(d)

KH

1.5

Intensity

1.5

Intensity

IBX

1.0E-04

0.0E+00

2.0

(b)

4.0E-04

Intensity

Intensity

4.0E-04

71

1.0

1.0 0.5

0.5

0.0

0.0 Con 1

Tail

Sph + Cu + IBX

Con 1

Tail

Sph + Cu + IBX + NaHSO3

Fig. 8. Exposures of the isobutyl xanthate (top) and the determined hydrophobicity indices (K H — for details see text) (bottom) on copper-activated sphalerite in the absence (a) and presence (b) of sodium bisulphite (at pH 9, air purging; [Cu(NO3)2] — 2000 g/t; [SIBX] — 50 g/t; [NaHSO3] — 2000 g/t).

DRIFT spectroscopy. Spectra were recorded for a wet ground sphalerite sample and for copper-activated sphalerite pre-treated with sodium isobutyl xanthate in the absence and presence of sulphite ions at pH 9 with nitrogen purging (Fig. 9). The infrared spectra in Fig. 9 show only the 900–1300 cm 1 region, since this area includes most of the absorption bands attributable to the xanthate (Poling, 1976). The spectra for wet ground sphalerite (Fig. 9 (a)) shows characteristic bands for sphalerite between 1000 and 1200 cm 1, these peaks are attributed to oxidation products, such as hydroxy species, on the sphalerite surface (Persson et al., 1991). Persson et al. (1991) have reported that these oxidation products can be effectively removed from the sphalerite surface by using different treatments. Fig. 9 (b) shows the infrared spectra for copper-activated sphalerite conditioned with isobutyl xanthate. Characteristic copper–xanthate bands were observed at 1040, 1111 and 1210 cm 1 (Little et al., 1961; Poling and Leja, 1963; Leja et al., 1963; Termes and Richardson, 1986). Based on the results of Poling and Leja (1963) the strong band at 1040 cm 1 can be attributed to the CS vibration, while the weaker two bands at 1210 and 1111 cm 1 are likely to be associated with C–O–C stretching. Tiny traces of

dixanthogen formation were also found. A small peak at around 1278 cm 1 was identified as a dixanthogenrelated band (Termes and Richardson, 1986). It is known that under some experimental conditions, dissolved Cu(II) salts can oxidise xanthate anions to dixanthogen (Sheikh and Leja, 1974). Overall, the main product of interaction of copper-activated sphalerite with isobutyl xanthate was identified as copper(I)–xanthate species (Termes and Richardson, 1986; Prestige et al., 1994). The addition of sodium bisulphite to the system resulted in the disappearance of the bands attributed to copper–xanthate complexes (1040, 1111 and 1210 cm 1). In the presence of sulphite ions, the spectrum looked similar to that which was recorded for the wet ground sphalerite sample. This indicates that in the presence of sulphite ions, less xanthate species were adsorbed onto the surface of copper-activated sphalerite. As no oxygen should have been present during the conditioning of the samples, the reduced signal must be due to removal of xanthate from the surface. It should be noted that the concentrations of isobutyl xanthate and sodium bisulphite for this analysis were much higher than in the normal flotation experiments (due to the limited sensitivity of the

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1040

963

1190

1060

1008

1118

72

963

1102

964

1190

1160

(b) 1278

Absorption

1210

1152

1111

(a)

1278

(c)

1300

1200

1100

1000

Wavenumbers (cm-1) Fig. 9. DRIFT FTIR spectra of (a) wet ground sphalerite; (b) sodium isobutyl xanthate adsorbed on the copper-activated sphalerite in the absence of sulphite; (c) sodium isobutyl xanthate adsorbed on the copper-activated sphalerite in the presence of sulphite at pH 9 with nitrogen purging; [Cu(NO3)2] — 2000g/t, [SIBX] — 2000 g/t, [NaHSO3] — 10000 g/t.

DRIFT technique), hence the results in this section simply confirm the possibility of copper isobutyl xanthate formation and its removal from the surface by sulphite. 3.3.2. In situ ATR FTIR spectroscopy study The results of the DRIFT spectroscopy experiments indicate that sodium bisulphite reduces the adsorbed amount of collector on the surface of sphalerite. However, the ex situ nature of the DRIFT sampling method makes it difficult to study in detail the interaction between the sulphite and the adsorbed xanthate species, and the subsequent desorption. There are only a few techniques that can be used to study the desorption of collector from the surface of treated mineral particles in real time. Yamamoto (1980) studied the desorption of ethyl xanthate from pyrite particles using flow calorimetry. He monitored the removal of collector from a packed column of xanthate treated pyrite

particles exposed to a flowing solution of sulphite ions, in effect monitoring the appearance of xanthate in solution after removal from the pyrite surface. Particle film ATR FTIR (McQuillan, 2001; Chiem et al., in press) is another technique that can be used to study the adsorption/desorption of collector onto mineral surfaces directly. The methodology allows for the study of collector adsorption/desorption due to changing solution conditions, such as the introduction of a depressant molecule or the introduction of a blank electrolyte solution (to simulate collector consumption in solution). Particle film ATR FTIR was used to study the adsorption of isobutyl xanthate onto the surface of a copper-activated layer of synthetic sphalerite. Synthetic sphalerite was used in this part of the study to allow greater control over the particle size and surface area of the sample, both of which are critical parameters in particle film ATR FTIR. A germanium internal reflec-

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tion element was used as the substrate for the particle film. Adsorption of xanthate collectors directly onto germanium has been studied by other researchers (Larsson et al., 2004) and it is anticipated that some adsorption of IBX will occur onto the germanium in this system. However, given the expected similar adsorption affinities for activated sphalerite and germanium (xanthates adsorb chemically on both surfaces) and the huge disparity in available surface area (ZnS surface area is more than 300 times the area of the germanium crystal surface), almost all of the observed signal seen in the spectra will be from IBX adsorbing onto the activated sphalerite. Fig. 10 shows in situ ATR FTIR spectra of sodium isobutyl xanthate adsorbed onto a particle film of synthetic sphalerite that had been activated using CuSO4 solution. The spectra of the collector after equilibrium adsorption had been established (black line in Fig. 10 (a) and (b)) show characteristic peaks of copper isobutyl xanthate (1200 and 1035 cm 1) and a smaller

(a) Absorbance

0.0015

0.0010

0.0005

0.0000 1400

1300

1200

1100

1000

900

1000

900

IR wavenumber / cm-1

(b) Absorbance

0.0015 0.0010 0.0005 0.0000

1400

1300

1200

1100

IR wavenumber / cm-1 Fig. 10. In situ ATR particle film spectrum of sodium isobutyl xanthate ([SIBX] — 3  10 3 M) adsorbing onto copper activated synthetic sphalerite particles at pH 9 with nitrogen purging: (a) black line recorded after 20 min of adsorption, grey line recorded after 20 min of exposure to a sodium bisulphite solution ([NaHSO3] 3  10 3 M in 1  10 2 M KNO3); (b) black line recorded after 20 min of adsorption, grey line recorded after 20 min of flushing with blank electrolyte solution (1  10 2 M KNO3).

73

peak due to the presence of a limited amount of dixanthogen (1261 cm 1), formed, presumably, due to oxidation of the isobutyl xanthate. These peak positions are in broad agreement with those obtained using ex situ DRIFT analysis. The spectra shown in the grey line in Fig. 10 (a) and (b) are those recorded after the collector solution flowing over the activated sphalerite particles was substituted for one containing electrolyte and sodium bisulphite (Fig. 10 (a)) and for one containing just electrolyte (Fig. 10 (b)). In both cases, the layer was allowed to reach equilibrium with the new solution prior to recording the spectra. The reduction of the signal due to the collector can be clearly seen in both cases. However, it is equally clear that the sodium bisulphite solution has a far greater effect on the adsorbed collector, removing significantly more of the copper isobutyl xanthate than the plain electrolyte solution (compare the peak intensities for the 1035 cm 1 peak between the two sets of spectra). In the case of the electrolyte solution, the reduction is due to the adsorbed layer re-acquiring equilibrium with the solution. In the case of the sodium bisulphite/electrolyte solution, we can expect a similar decrease in the collector signal due to the lack of collector in solution. The additional decrease of the signal of the adsorbed collector must be due to a reaction between the sulphite ions in solution and the copper isobutyl xanthate complex on the sphalerite surface. Another feature of interest in Fig. 10 is the difference between the two sets of spectra in the region of the dixanthogen peak at 1261 cm 1. Whereas the electrolyte solution shows a decrease in this peak in line with the reduction of the other peaks, the spectrum of the adsorbed layer exposed to sodium bisulphite shows no reduction in this region (in fact there is a slight increase). This observation would point toward some degree of selectivity in the reaction between the sulphite ions and the adsorbed collector, i.e. the reaction between sulphite ions and copper xanthate species (Cu(ROCSS )2 + SO32 Y 2ROCSSO + CuSO3 (Grano, 1997)), is more favourable than the reaction between sulphite ions and dixanthogen ((ROCSS)2 + SO32 Y 2ROCSSO + S2O6 (Jones and Woodcock, 1983)). It should be noted that the decomposition reaction between sulphite and adsorbed copper xanthate does not involve oxygen and should therefore apply to both air and nitrogen purged flotation conditions. The in situ nature of the experiments allows us to not just collect spectra from the adsorbed collector at equilibrium with the solution, but also to record spectra as a

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1035 cm-1 Absorbance / 10-5

74

the decomposition of hydrophobic copper–sulphide by sulphite ions and the resultant surface oxidation and formation of zinc hydroxide will account for a significant degree of the depression observed in the flotation results. In addition to this previously confirmed mechanism, the ex situ DRIFT and in situ ATR FTIR spectra indicate that sulphite ions remove/decompose adsorbed copper xanthate species, and that the removal of copper xanthate species is favoured over the removal of dixanthogen.

(a)

120

90

60

30

0 0

1035 cm-1 Absorbance / 10-5

150

500

1000 1500 2000 2500 3000 3500

(b)

Acknowledgements

120

90

60

30 0

1000

2000

3000

4000

5000

6000

7000

Adsorption time / sec Fig. 11. Kinetic data for the adsorption and desorption of sodium isobutyl xanthate: (a) adsorption and then desorption due to sodium bisulphite. (b) adsorption and then desorption due to flushing with electrolyte solution. In both (a) and (b) the dashed line indicates the start of desorption/flushing.

function of time. The intensity of the 1035 cm 1 copper isobutyl xanthate peak can then be used to monitor the kinetics of the collector adsorption and desorption. This data is shown in Fig. 11 for both the sulphite/ electrolyte flush and the plain electrolyte flush. The differences in the effect on the adsorbed collector between the two experimental conditions are equally clear in the kinetic data, with the sulphite solution having a larger effect in reducing the collector concentration on the surface of the sphalerite. 4. Conclusions The results obtained in the current study show that sodium bisulphite has a complex effect on copperactivated sphalerite flotation in the presence of isobutyl xanthate. The depression of sphalerite by the sulphite ions is clearly seen in the batch flotation data (in either a nitrogen or air atmosphere). The surface analysis of feed and concentrate/tail samples indicates that similar sulphite depression mechanisms to those observed in the absence of collector (Khmeleva et al., 2005a) are also at work in the presence of collector:

TNK would like to acknowledge the Australian Research Council for their support in awarding her an APA scholarship. JKC would like to thank the University of South Australia for awarding her an exchange scholarship. DAB would like to thank Margaretta Lidstro¨m Larsson (Lulea˚ University of Technology, Sweden) for supplying the synthetic sphalerite sample. The authors would also like to acknowledge the Australian Research Council and the University of South Australia for their financial support of this work through the Special Research Centre Scheme and the UniSA Development Grant Scheme, respectively. References Boulton, A.B., 2002. Improving sulphide mineral flotation selectivity against iron sulphide gangue, PhD Thesis, University of South Australia. Buckley, A.N., Woods, R., 1984. An X-ray photoelectron spectroscopic study of the oxidation of chalcopyrite. Australian Journal of Chemistry 37, 2403 – 2413. Buckley, A.N., Woods, R., Wouterlood, H.J., 1989. An XPS investigation of the surface of natural sphalerites under flotationrelated conditions. International Journal of Mineral Processing 26, 29 – 49. Chiem, L.T., Huynh, L., Ralston, J. Beattie, D.A., in press. An in situ ATR-FTIR study of polyacrylamide adsorption at the Talc Surface, Journal of Colloid and Interface Science. Fairthorne, G.A., Fornasiero, D., Ralston, J., 1997. Effect of oxidation on the collectorless flotation of chalcopyrite. International Journal of Mineral Processing 49, 31 – 48. Finkelstein, N.P., Allison, S.A., 1976. The chemistry of activation, deactivation and depression in the flotation of zinc sulfide: a review. In: Fuerstenau, M.C. (Ed.), Flotation: A.M. Gaudin Memorial Volume, vol. 1. American institute of Mining, Metallurgical and Petroleum Engineering, New York, pp. 414 – 451. Fuerstenau, D.W., 1982. Activation in the flotation of sulphide minerals. In: King, R.P. (Ed.), Principles of Flotation. South African Institute of Mining and Metallurgy, Johannesburg, pp. 183 – 199. Fuerstenau, M.C., 1982. Adsorption of sulphydryl collectors. In: King, R.P. (Ed.), Principles of Flotation. South African Institute of Mining and Metallurgy, Johannesburg, pp. 91 – 108.

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Persson, P., Malmensten, B., Persson, I., 1991. Interactions between sulphide minerals and alkylxanthate ions: Part 1. A vibrational spectroscopic study of the interactions between sphalerite and synthetic zinc(II) and cadmium(II) sulphides, and ethyl xanthate ions in aqueous and acetone solutions. Journal of Chemical Society of Faraday Transactions 87 (17), 2769 – 2777. Prestidge, C.A., Skinner, W.M., Ralston, J., Smart, R.St.C., 1997. Copper (II) activation and cyanide deactivation of zinc sulphide under mildly alkaline conditions. Applied Surface Science 27, 437 – 452. Prestige, C.A., Thiel, A.G., Ralston, J., Smart, R.St.C., 1994. The interaction of ethyl xanthate with copper(II)-activated zinc sulphide: kinetic effects. Colloids and Surfaces A: Physicochemical and Engineering Aspects 85, 51 – 68. Piantadosi, C., 2001. Competitive collector adsorption in the selective flotation of galena and chalcopyrite from iron sulphide minerals, PhD Thesis, University of South Australia. Piantadosi, C., Smart, R.St.C., 2002. Statistical comparison of hydrophobic and hydrophilic species on galena and pyrite particles in flotation concentrates and tails from TOF-SIMS evidence. International Journal of Mineral Processing 64 (1), 43 – 54. Piantadosi, C., Jasieniak, M., Skinner, W.M., Smart, R.St.C., 2000. Statistical comparison of surface species in flotation concentrates and tails from TOF-SIMS evidence. Minerals Engineering 13 (13), 1377 – 1394. Poling, G.W., 1976. Reactions between thiol reagents and sulphide minerals. In: Fuerstenau, M.C. (Ed.), Flotation: A.M. Gaudin Memorial Volume, vol. 1. American institute of Mining, Metallurgical and Petroleum Engineering, New York, pp. 334 – 363. Poling, G.W., Leja, J., 1963. Infrared study of xanthate adsorption on vacuum-deposited films of lead sulphide and metallic copper under conditions of controlled oxidation. Journal of Physical Chemistry 67, 2121 – 2126. Sheikh, N., Leja, N., 1974. Precipitation and stability of copper ethyl xanthate in hot acid and alkaline solutions. Journal of Colloid and Interface Science 47, 300 – 308. Shen, W.Z., Fornasiero, D., Ralston, J., 2001. Flotation of sphalerite and pyrite in the presence of sodium sulfite. International Journal of Mineral Processing 63 (1), 17 – 28. Smart, R.St.C., 1991. Surface layers in base metal sulphide flotation. Minerals Engineering 4 (7–11), 891 – 909. Smart, R.St.C., Jasieniak, M., Prince, K.E., Skinner, W.M., 2000. SIMS studies of oxidation mechanisms and polysulphide formation in reacted sulphide surfaces. Minerals Engineering 13 (8–9), 857 – 870. Szargan, R., Karthe, S., Suoninen, E., 1992. XPS studies of xanthate adsorption on pyrite. Applied Surface Science 55, 227 – 232. Termes, S.C., Richardson, P.E., 1986. Application of FT-IR spectroscopy for in situ studies of sphalerite with aqueous solutions of potassium ethylxanthate and with diethyldixanthogen. International Journal of Mineral Processing 18, 167 – 178. Yamamoto, T., 1980. Mechanism of depression of pyrite and sphalerite by sulphite. In: Jones, M.J. (Ed.), Complex Sulphide Ores. Institute of Mining and Metallurgy, London, pp. 71 – 78.