Cu adsorción en esfalerita

7/30/2019 Cu adsorcin en esfalerita

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.Int. J. Miner. Process. 55 1999 247265

Copper activation of sphalerite and its reaction withxanthate in relation to flotation: an X-ray absorption

spectroscopy reflection extended X-ray absorption/fine structure investigation

R.A.D. Pattrick a,), K.E.R. England a, J.M. Charnock a,b,J.F.W. Mosselmans b

aDepartment of Earth Sciences, Uniersity of Manchester, Manchester, M13 9PL, UK

bCCLRC, Daresbury Laboratory, Daresbury, Warrington, WA4 4AD, UK

Received 5 January 1998; revised 19 August 1998; accepted 4 November 1998

Abstract

The technique of fluorescence REFLEXAFS was used to examine the activation by copper of

sphalerite, ZnS, and the subsequent interaction with a xanthate collector at pH 10 and 12. ZnSe

was used as an isostructural analogue of ZnS to avoid the contribution from the xanthate S on the

mineral surface being swamped by that from the substrate S. On activation, the copper interaction

with the Se or S atoms is clearly seen and CuO bonds are formed. On the addition of xanthate,

the oxygen is displaced by sulfur and a covellite-like species develops at the surface. The iron at

the surface in natural sphalerites is bonded to oxygen, inhibiting initial copper activation but

appears to catalyse covellite formation on xanthate addition. The technique succeeds in producingdirect structural information, providing information complementary to other investigative tech-

niques. q1999 Elsevier Science B.V. All rights reserved.

Keywords: flotation; sphalerite; copper activation; X-ray absorption spectroscopy

1. Introduction

Reactions during flotation between mineral surfaces and dissolved ions involve both

sorption onto surfaces and displacement of surface cations. The nature of the surface

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Corresponding author. Fax: q44-161-275-3947; E-mail: [email protected]

0301-7516r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. .P I I : S 0 3 0 1 - 7 5 1 6 9 8 0 0 0 3 6 - 2


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( )R.A.D. Pattrick et al.rInt. J. Miner. Process. 55 1999 247 265248

species formed in such processes can be examined by a series of techniques XPS, UV,.IR but it is difficult to obtain direct structural information on the sorbatemineral

interface. In this study, the activation of the ZnS surface by Cu and the subsequent .Cu xanthate interaction were examined using one X-ray absorption spectroscopy XAS

technique, namely fluorescence reflection extended X-ray absorption fine structure

.fluorescence REFLEXAFS , with the aim of providing information on the mechanismsinvolved. Sphalerite, cubic ZnS, has a poor flotation response to xanthate collectors due

to the solubility of Znxanthate species, but activation of the ZnS surface with Cu by

reaction with CuSO results in an excellent flotation response. Studies of the Cu4activation of ZnS have been extensive. During activation, it is generally accepted that

2q .Cu from solution, in a thermodynamically driven process Jain and Fuerstenau, 1985 ,

replaces the Zn2q at the mineral surface and the xanthate collector reacts with the Cu

species formed. Experiments show that the displacement of Zn increases with activation

time and is a two-stage process involving an initial fast reaction followed by a slower

second stage involving Cu-diffusion Finkelstein and Allison, 1976; Jain and Fuerstenau,.1985; Buckley et al., 1989; Prestidge et al., 1994 . The number of Cu monolayers

formed is dependent on the initial Cu concentration, and XPS depth profiling suggests

Cu penetration to depths of several nanometers if ZnS is reacted for a considerable time .Prestidge et al., 1994 . Studies of the oxidation state of the Cu on the ZnS surface

. .indicate Cu I only to be present Kartio et al., 1996 and references therein .

The ZnS surface is resistant to oxidation but oxidation of the ZnS surface after Cu .activation is apparent Buckley et al., 1989; Kartio et al., 1996 and Popov and Vucinic

.1990 suggested Cu-hydroxides may be present. The latter authors also noted that the

.main surface species in alkaline media after xanthate addition was Cu I ethylxanthate . .while Persson and Persson 1991 , using FTIR, noted that Cu I alkylxanthate and

dialkyl dixanthogen formed on the surface. A high Fe content is known to be an .inhibitor to ZnS flotation in the presence of oxygen and Zhang et al. 1992 suggested

.2qthat Fe OH forms on the surface of ZnS if Fe ions are present. The same authors also .noted that Fe OH RX species subsequently form with the addition of alkylxanthate2

. .RX , resulting in the mineral adopting the flotation characteristics of Fe RX .2 .Persson and Persson 1991 observed that on unactivated ZnS, alkylxanthate is

bonded to the Zn atoms at the surface, but a study of the kinetics of the reactions

.involved in the CuZnS xanthate system led Prestidge et al. 1994 to conclude that noxanthate species formed on the ZnS surface, and that previous sightings were due to

the inadvertent activation of the ZnS surface. They also noted that no diethyldixanthogen

was formed, except by precipitation from Cuqxanthate reaction in solution.

The surface species formed during Cu activation is a matter of debate, with CuS . . .covellite , Cu S chalcocite , Cupolysulfide Cu S and a Zn Cu S species all cited2 2 6 x x

as being present Finkelstein and Allison, 1976; Prestidge et al., 1994; Kartio et al.,. .1996 . In a XPS based study, Buckley et al. 1989 observed that if sulfur was

precipitated on a copper activated ZnS surface, this led to the appearance of a high

.binding energy BE component in the S XPS spectrum. They concluded that aCu-deficient sulfide formed at the surface after xanthate addition, a process which

involved Cu migration from the lattice to the surface. The high BE component in S XPS .data was recognised by Prestidge et al. 1994 as well, who assumed that the Cu in the


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( )R.A.D. Pattrick et al.rInt. J. Miner. Process. 55 1999 247 265 249

ZnS lattice contributes to a rearrangement of the surface after xanthate addition. These

authors also noticed that the activated copper sulfide surface was oxidised, whereas the

unactivated ZnS was not. There is debate concerning the nature of the interactionbetween mineral and xanthate, although the consensus see Finkelstein and Poling, 1977;

.Crozier, 1992 is that metal xanthate is the active species.

i .Sodium isopropyl xanthate Na PrX was used in this study and its structure is shown . . in a . Bonding of xanthate to a surface metal might involve one as in a or both as in

.b the xanthate S atoms.

.The issue of the formation of di-thiols in flotation systems e.g., dixanthogen has

been addressed by many and was extensively reviewed by Finkelstein and Poling . .1977 . It is clear that dixanthogen c can form in flotation systems although it plays, at

most, a limited role in the flotation. Even when it is present, the evidence suggests that a

metal xanthate or a chemisorbed xanthate layer is formed at the mineral surface.

2. XAS and REFLEXAFS

.XAS, comprising extended X-ray absorption fine structure EXAFS and X-ray .absorption near edge structure XANES , is an excellent technique for determining the

local environment of elements in complex structures. EXAFS provides information onthe structural environment of specific atoms by supplying bond distances and the

identity of the atoms surrounding the target atom. It can provide bond lengthinformation accurate to "0.02 A and coordination numbers to "10%, the accuracy of

data depending on the data quality and the use of suitable theoretical and measuredphase shift and amplitude functions for details see Koningsberger and Prins, 1988;

.Brown et al., 1993 . XANES is more sensitive to the geometry of the coordination site

and, although more difficult to model, it can used as a fingerprinting technique. For

REFLEXAFS measurements, the X-ray beam is directed to hit the specimen at a very

low angle where the beam is reflected, rather than absorbed by the surface. Thereflectivity of the sample is measured to determine the critical angle, below which

.there is total external reflection of the beam. The angle of incidence f selected is half

the critical angle which is in the order of 100 mdeg, and gives a penetration depth of


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Fig. 1. Schematic experimental set-up for fluorescence REFLEXAFS measurements. The angle of incidence,

f, is set to half the critical angle.

.-50 A; thus, the technique is surface sensitive Fig. 1 . The signal can either be . .measured using the reflected beam It or by collecting the fluorescence signal If .

Using this technique, atoms at the surface can be examined, even at sub-monolayercoverage, and surface phenomena, such as sorption onto mineral surfaces can be .investigated Greaves, 1991 . As there are contributions to the spectra from both the

substrate and the surface atoms, the best results are produced when the sorbed species

and substrates are chemically distinct. In this study, both fluorescence REFLEXAFS and

Bulk EXAFS were used, the latter involving fine powdered mineral. In the case of the

powdered samples, the increased sample surface area is offset by the fact that all the

matrix contributes to the spectral background so that the signal-to-noise ratio is worse.

In this investigation, to avoid the problem of differentiating between sulfur in the

xanthate and the ZnS substrate, the isostructural analogue ZnSe was used and comple-mentary information provided by ZnS.

3. Experimental

3.1. Samples

The samples used for the fluorescence REFLEXAFS were ca. 2.5=8 cm2

slabs ofpolycrystalline synthetic ZnSe and ZnS and natural Fe-rich sphalerite, all with the cubic

.ZnS structure. For powdered samples, high purity Aldrich99.99% powdered ZnSe .and ZnS with a very fine -10 mm grain size were used. The xanthate reagent used


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i .was sodium isopropyl xanthate, C H OCS Na Na PrX , provided by American3 7 2 .Cyanamid and Laporte Industries UK .

.The procedure involved immersion in 0.5 mol CuSO solution pH 3.9 for 1 min4 .followed by rinsing labelled SampleqCu in the tables and immersion in 1 gr100 ml

i i .Na PrX solution for 10 min and rinsing SampleqCuqNa PrX in the tables . The

additions of Nai PrX were conducted at pH 10 and pH 12, within and above the stability .range of dixanthogen, respectively; pH was controlled using Ca OH and HCl, the2

normal pH of the xanthate was 10.2. No evidence of distinct yellow Cu xanthogen

species forming in the solution was observed when Na i PrX was added to the ZnSe and

ZnS.

Determination of the amount of Zn released from the ZnSe and ZnS slabs on copper

activation was measured by analysis of the CuSO solution that had been in contact with4the minerals for 1 min. The analyses were obtained using inductively coupled plasma-

.mass spectrometry ICP-MS . From the concentration of Zn ions released and the area of

sample exposed to the solution, the equivalent number of Zn layers removed wascalculated. These data show that two to three layers of Zn atoms were removed from the

ZnSe surface and one to two layers from the ZnS surface. The amount of Zn released

from the powdered ZnSe and ZnS starting materials was also determined. Although the

surface area of the powders was not measured, using the range of particle size range .quoted by Prestidge et al. 1994 on similar reagents, our results suggested the amount

of Zn lost from the surface to be in the same range as for the slabs.

3.2. XAS data collection and analysis

.All the XAS data were collected using the Synchrotron Radiation Source SRS at the

CCLRC Daresbury Laboratory, UK, operating at 2 GeV with an average current of 200

mA. The Cu-, Se- and Zn-edge fluorescence REFLEXAFS data were recorded on .Station 9.3, using a Si 220 double crystal monochromator and a beam defined by a

vertical slit of 50 mm and a horizontal slit of 1.5 cm. This gives a footprint of 430 mm 2

at an incident angle of 100 mdeg. S K-edge spectra were recorded in fluorescence mode .on Station 3.4 using a Ge 111 double crystal monochromator. These stations have

focusing mirrors which remove higher harmonics. The fluorescence Cu K-edge andFeK, and transmission Zn K-edge bulk EXAFS data were recorded on Station 8.1

.using a Si 220 double crystal monochromator. The Se K-edge bulk EXAFS were .recorded on Station 7.1 in transmission mode using a Si 111 double crystal monochro-

mator. On Stations 8.1 and 7.1 the crystals were detuned by 50% to exclude higher

harmonics. A 13-element Canberra solid state detector was used to collect the fluores-

cence signal on Stations 8.1 and 9.3, while on Station 3.4 a gas proportional counter was

used. The spectra were calibrated, background subtracted and normalised to an edge stepof unity using the Daresbury Laboratory programs EXCALIB and EXBACK Diakun et

.al., 1984 . EXAFS data analysis was performed using the Daresbury program EX- . 3CURV92 Binsted et al., 1991 . The data were simulated using k -weighted EXAFS

employing theoretical phaseshifts calculated with HedinLundqvist exchange potentials .and von Barth ground state Hedin and Lundqvist, 1969 . The theoretical fits were


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obtained by adding shells of backscattering atoms around the central absorber atom and

refining the Fermi energy, E , the absorberscatterer distances, r, and the DebyeWallerf2 .factors, 2s , in order to minimise the R-factors fits , calculated from the formula:

Rs 1rs experiment i y theory i =100% . . . .i i

where:

3 31rssk i r k j experiment j . . . . .i j

where i and j are independent data points. The absolute value of the R-factor will

obviously depend on how noisy the data are and the data range; for the spectra presented

here, values less than 60 represent a good fit. All the shells of backscattering atoms were .checked for their statistical significance using the Joyner test Joyner et al., 1987 .

.In the tables of data the shell radii r are given in A and the coordination numbers .CN were refined to the nearest whole number. The coordination numbers for distant

atoms are effectively the numbers of scatterers and do not imply bonding. In processing

the data, all potential CN values were examined and the best fits are shown. The2 2 .DebyeWaller factors quoted 2s are given in A and are a function of the relative

motion between the central atom and the scatterer, and also of small variations in

distance of the atoms within a shell from the central atom. DebyeWaller factors can be .used to assess the quality of the fits; an anomalously low less than 0.001 or negative

factor may indicate that the CN is too low or the wrong atom has been chosen as a

Table 1 .Summary of Cu K-edge fluorescence REFLEXAFS data of Cu sorbed onto a ZnSe flat surface REFLEXAFS

.and EXAFS of Cu sorbed onto ZnSe powder Bulk

2 2 . . . .Experiment Type angle, mdeg Atom CN r A 2s A Fit R

.ZnSeqCu REFLEXAFS 125 O 1.0 1.95 0.044 42.3

Se 3.0 2.36 0.018i .

ZnSeqCuqNa PrX; pH 10 REFLEXAFS 140 S 1.0 2.21 0.008 39.1Se 3.0 2.39 0.031i .ZnSeqCqNa PrX; pH 12 REFLEXAFS 135 S 1.0 2.23 0.002 48.2

Se 3.0 2.37 0.031

ZnSeqCu Bulk O 1.0 1.98 0.010 51.8

Se 3.0 2.35 0.022

Se 1.0 2.86 0.009iZnSeqCuqNa PrX; pH 10 Bulk S 2.0 2.23 0.023 42.3

Se 2.0 2.38 0.018

Zn 1.0 3.96 0.017iZnSeqCuqNa PrX; pH 12.3 Bulk S 2.0 2.22 0.012 47.6

Se 2.0 2.42 0.043

.Angle is angle of incidence f for REFLEXAFS, atom is the type of scatterer, CN "20% is the number of 2 .scatterers at distance r "0.02 A, inner shells, "0.05 A outer shells , with DebyeWaller factor 2s

."20% .


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scatterer. An anomalously high factor greater than 0.050 for inner shells, or 0.100 for.outer shells indicates that there is little evidence for the presence of that shell.

4. Results

4.1. Cu actiated surface of ZnSe and ZnS

4.1.1. Cu K-edge surface data .Analyses of the Cu K-edge fluorescence REFLEXAFS spectra Table 1 produced

from ZnSe treated with CuSO reveal the Cu is bonded to the host selenium atoms and4oxygen. Figs. 2 and 3 show experimental EXAFS and Fourier transforms, together with

the best theoretical fits, for the activated ZnSe and ZnS surfaces. The Cu is bonded to three Se atoms at a distance of 2.36 A and one O at 1.95 A; these CN values give betterfits than any other integer combination. Fluorescence REFLEXAFS data from activated

.ZnS Table 2 reveals a similar picture with 3 S and 1 O atoms bonded to the Cu,

. .Fig. 2. Cu K-edge fluorescence REFLEXAFS spectra solid lines and theoretical fits broken lines . . . i corresponding to the analyses in Tables 2 and 3 of a ZnSeqCu; b ZnSqCu; c ZnSeqCuqNa PrX, pH

. . i .10 ; d ZnSqCuqNa PrX, pH 10 .


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.Fig. 3. Fourier transforms of the Cu K-edge fluorescence REFLEXAFS spectra solid lines and theoretical fits . . . . i . . i broken lines of a ZnSeqCu; b ZnSqCu; c ZnSeqCuqNa PrX, pH 10 ; d ZnSqCuqNa PrX, pH

.10 .

lending credibility to the use of the ZnSe as an analogue, although the CuO distance is .longer 2.07 A . This difference is real, and may be due to larger distances between

surface oxygens on the selenide, leading to reduced steric crowding and increased . .attraction to the surface copper. The best fit for the data from the Zn,Fe S Table 3 is

also 3 S and 1 O but the fit for 2 S and 2 O was very similar. In all cases, these CuO,

CuS and CuSe distances are typical for compounds of these elements.

( i )4.2. The addition of Na-isopropyl xanthate Na PrX

4.2.1. Cu K-edge surface datai .The addition of Na PrX to the ZnSe samples Table 1 at pH 10 induces changes in

the copper environment as defined by the Cu K-edge data, with the oxygen displaced

to give an average copper coordination of 1 S and 3 Se atoms. Longer interactions can

be fitted with a second S atom at a distance from the copper of 2.54 A, and a metal shell .either Zn or Cu at 3.94 A but the addition of both these shells does not significantly .improve the fit. The data from the ZnS Table 2 sample are comparable, with the Cu

. .bonded to 4 S atoms and a further S atom at 2.70 A. The Zn,Fe S Table 3 sample also


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Table 2 .Summary of Cu K-edge fluorescence REFLEXAFS data of Cu sorbed onto a ZnS flat surface REFLEXAFS

.and EXAFS of Cu sorbed onto ZnS powder Bulk


ZnSqCu REFLEXAFS O 1.0 2.07 0.030 32.8

.110 S 3.0 2.25 0.018i .ZnSqCuqNa PrX; REFLEXAFS 120 S 4.0 2.25 0.021 41.8

.pH 10 120 S 1.0 2.70 0.032iZnSqCuqNa PrX; REFLEXAFS S 4.0 2.24 0.019 42.5

.pH 12 125 S 1.0 2.66 0.043

ZnSqCu Bulk O 1.0 2.10 0.005 41.3

S 3.0 2.22 0.033iZnSqCuqNa PrX; Bulk S 4.0 2.26 0.028 40.8

pH 10 S 1.0 2.65 0.015iZnSqCuqNa PrX; Bulk S 4.0 2.26 0.032 36.6

pH 12 S 1.0 2.65 0.016

See Table 1 for definitions.

has a long CuS distance of 2.75 A and, in contrast to the ZnSe, in both these sphaleritesamples this longer CuS distance is significant. The Cu Kedge data for pH 12 for

both ZnSe and ZnS show similar nearest neighbours to the pH 10 data. Figs. 2 and 3

show experimental EXAFS and Fourier transforms, together with the best theoretical

fits, for the ZnSe and ZnS surfaces treated with xanthate at pH 10, which can be

compared with the activated surfaces.

4.2.2. S K-edge powder data . iThe S K-edge spectra Table 4 from ZnSe reacted with Cu and Na PrX were

collected from bulk samples and, as the S K-absorption edge is at a relatively low

Table 3 .

Summary of Cu K-edge fluorescence REFLEXAFS data of Cu sorbed onto a Zn,Fe S flat surface . . .REFLEXAFS and EXAFS of Cu sorbed onto Zn,Fe S powder Bulk


.Zn,Fe SqCu; REFLEXAFS O 1.0 2.05 0.036 44.7 .8 mol% FeS 130 S 3.0 2.25 0.020

i .Zn,Fe SqCuqNa PrX; REFLEXAFS S 4.0 2.22 0.022 60.1 .8 mol% FeS, pH 10.2 140 S 1.0 2.75 0.012

.Zn,Fe SqCu; Bulk S 4.0 2.29 0.018 46.2

4 mol% FeS S 1.0 2.70 0.008i .Zn,Fe SqCuqNa PrX; Bulk S 4.0 2.25 0.021 42.1

4 mol% FeS, pH 10 S 1.0 2.72 0.007i .Zn,Fe SqCuqNa PrX; Bulk S 4.0 2.27 0.018 34.0

4 mol% FeS, pH 12 S 1.0 2.71 0.027



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Table 4

Summary of S K-edge EXAFS of ZnSe reacted with Nai PrX

2 2 . . .Experiment Type Atom CN r A 2s A Fit RiZnSeqCuqNa PrX; Bulk C 1.0 1.53 0.038 73.3

pH 10 S 1.0 2.09 0.058

Cu 1.0 2.17 0.008iZnSeqCuqNa PrX; Bulk C 1.0 1.55 0.020 44.6

pH 12 S 1.0 2.06 0.034

Cu 1.0 2.16 0.003

Se 1.0 2.60 0.009iZn PrX Bulk C 1.0 1.60 0.060 56.0

Zn 1.0 2.31 0.024

S 1.0 2.94 0.010iZnSeqNa PrX; Bulk C 1.0 1.58 0.080 78.5

pH 5.0 S 1.0 1.99 0.025

Zn 1.0 2.22 0.089


energy and the total amount of S present was low, they are of poor quality Figs. 4 and.5 . Nonetheless, data from samples reacted with xanthate at pH 10 and pH 12 clearly

. .Fig. 4. S K-edge EXAFS spectra solid lines and theoretical fits broken lines corresponding to the analyses

in Table 4 of top, ZnSeqCuqNai PrX and bottom, ZnSeqNai PrX.


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. .Fig. 5. Fourier transforms of the S K-edge EXAFS spectra solid lines and theoretical fits broken lines ofi . i .top, ZnSeqCuqNa PrX pH 10 and bottom, ZnSeqNa PrX pH 5 .

showed S bonded to both C and Cu as well as a SS bond of ca. 2 A; in the pH 12 .spectrum a further S Se interaction at 2.60 A was observed Figs. 4 and 5 . In addition,

a sample of ZnSe reacted with Na i PrX at a pH of 5.0 was examined, this pH being

within the range where flotation without activation has been observed. In this sample, . .both SZn 2.22 A and S S 1.99 A interactions are seen. A Zn xanthate model

compound was also analysed which yielded a SZn distance of 2.32 A and a relatively

long SS distance of 2.94 A which is the distance between the two sulfur atoms inxanthates; no short SS bonds were observed.

4.3. Cu K-edge powder data

.The Cu K-edge EXAFS data Tables 13 on powdered samples activated with

copper confirm the Cu to be bonded to SerS and O, although when Nai PrX is added the

best fits for the ZnSe sample are with 2 S and 2 Se atoms around the Cu, rather than 1 S

and 3 Se atoms. These spectra are noisier than those derived by fluorescenceREFLEXAFS and therefore, less reliable and as a result the subsequent discussion

focuses on the REFLEXAFS data. Furthermore, although the bulk samples were rinsed

after reaction with xanthate, it is feasible that some interstitial Cu i PrX remained and


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Table 5 .Summary of Fe K-edge EXAFS data from Zn,Fe S

2 2 . . .Experiment Type Atom N r A 2s A Fit R

.Zn,Fe SqCu; Bulk O 1.0 2.02 0.002 47.5

4.0 mol% FeS S 3.0 2.35 0.013

Zn 12.0 3.82 0.036i .Zn,Fe SqCuqNa PrX; Bulk S 4.0 2.32 0.016 38.7

4.0 mol% FeS, pH 10 Zn 12.0 3.81 0.040

S 12.0 4.39 0.049i .Zn,Fe SqCuqNa PrX; Bulk S 4.0 2.32 0.015 50.2

4.0 mol% FeS, pH 12 Zn 12.0 3.82 0.034

S 12.0 4.38 0.043


therefore, these bulk data should be used with caution. Nonetheless, the observeddifferences in the analyses may reflect genuine differences in Cu coordination and the

.development of relatively more CuS bonds Cu-sulfide on the surface.

4.4. Other data

Fluorescence REFLEXAFS and powder EXAFS data for Se K-, Zn K- and Fe .K-edges were also collected from the ZnSe, ZnS and Zn,Fe S samples. As expected, the

high concentration of these elements in the substrates swamped any distinct contributionto the EXAFS from the surface atoms in almost all cases. The Zn spectra showed normal

sphalerite structure coordination and bond lengths with no evidence of O even in the

fluorescence REFLEXAFS spectra. 1 This contrasts with the FeK edge EXAFS spectra

of the Fe-bearing sphalerite reacted with Cu which showed significant O to be bonded to .Fe Table 5 which is lost on the addition of xanthate, suggesting a FeS interaction.

5. Discussion

The CuK edge REFLEXAFS and the S K-edge EXAFS data have clearly demon-

strated the interaction of the Cu with the mineral surface and xanthate. The simplest

interpretation of an average Se or S:O ratio of 3:1 for Cu coordination on the activated

ZnSe and ZnS surface is that the Cu is bonded to three surface SerS atoms and an

oxygen atom, forming surface tetrahedral sites. However, the X-ray spectra havecontributions from all Cu in the top 50 A and, as previous work and our ICP-MS results

demonstrate, Cu displaces Zn in the ZnSe or ZnS lattice which will contribute four

.CuSe S bonds per Cu to the average Cu coordination observed. In the experiments

1The details of the XAS analyses of the Se and Zn in these samples are available on request.


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conducted here the evidence is for ca. two layers of Zn exchanged with Cu and,

therefore, a significant contribution to the Cu REFLEXAFS will have come from Cu

within the host lattice. The Cu that has displaced the Zn within the lattice will be fully

bonded to Se, suggesting that the outermost Cu at the surface must be bonded to more

than one oxygen. This surface Cu is likely to be bonded to the mineral via the outer

.originally surface SerS atoms which prior to Cu addition, were in three-fold coordina- .tion with Zn Vaughan et al., 1997 . This surface copper is available for bonding to

oxygen that forms a new outer layer. The data are not good enough to make a more

detailed assessment of the nature of this oxide species; a schematic representation of the .type of interaction at the ZnS Se surface is shown in Fig. 6.

The addition of Nai PrX results in the development of CuS bonds, this is seen in the

Cu K-edge data on all samples and in the S K-edge from ZnSe; it confirms the reaction

of the xanthate sulfur with the surface Cu-species. Previous work indicates Cu-xanthates

are formed at the surface but the appearance of longer CuS distances on addition of the

Nai PrX is also evidence of the development of a more ordered Cu-sulfide phase at thesurface at this time. The development of SCu bonds on the ZnSe surface due to the

xanthatesurface copper interaction shows that the CuS species can form without the

addition of sulfur from the mineral substrate. The S K-edge data from the ZnSe also .reveal the presence of disulfide groups S S . These could be due either to the presence

of dixanthogen or a Cu-polysulfide or a Cu-sulfide. As the samples prepared at pH 12,

which is outside the stability field of dixanthogen also show SS bonds, a Cu-sulfide or

polysulfide species is favored. The most likely form of Cu-sulfide to be present is

.Fig. 6. Schematic diagram showing a two-dimensional section through ZnS or Se activated with Cu.


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.covellite, CuS, in which 2r3 of the S is in S S groups Fig. 7 ; the longer range order

indicated by the REFLEXAFS data lends support to CuS formation.

To assist in the identification of the surface Cu species the XANES of the Cu spectra

can be used to fingerprint possible species present. Fig. 8a shows the Cu XANES

derived from the REFLEXAFS spectra and Fig. 8b some CuS species for comparison.

Among these comparisons is primitive covellite which requires explanation here. Theprimitive covellite structure is shown in Fig. 9 and is a metastable CuS phase formed by

.precipitation from aqueous solution Pattrick et al., 1997 . Spectroscopic evidencesuggests that this phase contains the two types of layer found in covellite the

.Cu SCuS and the CuS Cu layers of Fig. 7 but they are linked via a CuS bond3 3 2 .rather than the trigonal bipyramidal S Figs. 7 and 9 . EXAFS of the primitive phase

.showed a CuS distance Fig. 9 of ca. 2.8 A which is not found in covellite. In a matterof hours at room temperature this metastable phase changes to an amorphous covellite

structure which results in the appearance of longer distance scatterers, at 3.59 A and

3.74 A in the EXAFS spectra. XPS analysis of sulfur in the primitive covellite structure .Pattrick et al., 1997 revealed it to have the characteristics of covellite but with a high

BE component, indicating the presence of a copper deficient sulfide. This is of

significance because XPS investigations have revealed a high BE component, attributed

to a metal-deficient sulfide species, at the ZnS surface during activation by Buckley et . .al. 1989 and Prestidge et al. 1994 , although the identity of the species is not

confirmed.

In the case of the ZnSe sample, the activated surface has a relatively featureless Cu

XANES spectrum with some similarity to the chalcocite XANES which contains

disordered Cu in a large number of sites. The addition of Na i PrX results in an increasein the peak at 9000 eV and the Cu XANES spectrum resembles primitive covellite. The

Cu XANES of the ZnS samples show a similarity to primitive covellite in both the

Fig. 7. The structure of covellite, CuS, showing the S -layers and Cu S CuS layers after Evans and2 3 3.Konnert, 1976 and from Pattrick et al., 1997 . Thick bonds are SS bonds, the shaded atoms are Cu and the

unfilled circles are S.


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.Fig. 8. The Cu XANES derived from a the fluorescence REFLEXAFS spectra of copper activated surfaces . .ZnSe, ZnS; and Zn,Fe S, and b the EXAFS of other coppersulfur phases for comparison. The CuSqCuO

is a spectrum from the heavily oxidised surface of CuFeS which is shown to contain these two components.2

samples activated with Cu and the ones with Nai PrX added, although the latter also .shows some features of true covellite. The Cu XANES of the Zn,Fe S samples are

strikingly different from the others. From our experience, the best comparison that can

.Fig. 9. The primitive covellite structure after Pattrick et al., 1997 showing the two covellite layers and the ca. .2.8 A interaction dotted line .


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be made is with a copper spectrum derived from an oxidised chalcopyrite surface which .comprises a mixture of CuS covellite and CuO. The Cu K-edge REFLEXAFS data

also indicated that the activated surface of these Fe-bearing samples were more oxidised

than the ZnS and ZnSe samples, and the Fe K-edge EXAFS data revealed that the

surface Fe is bonded to O. The implication is that the O on the surface of the Fe-rich

samples results in the formation of a more distinct CuO species than in the othersamples and covellite is also more easily formed. The XANES data do not support the

presence of copper xanthate on the surface as the prominent feature in the model copper .xanthate at 9000 eV Fig. 8b is not evident. This does not necessarily mean it is totally

absent, just that in samples prepared under the conditions reported here, it is not presentas a major Cu species in the outer 50 A of the mineral.

If primitive covellite is present on the surface, the REFLEXAFS data should show an interaction at ca. 2.8 A. A CuS distance of 2.652.75 A is seen in all the spectra

.derived from the ZnS and Zn,Fe S samples after xanthate addition. As this interaction is

not found in either ZnS or true covellite structures, it provides evidence for a primitivecovellite-type structure to be present. Unfortunately, the data are not good enough to

confirm the presence of the longer distance interactions seen in true covellite.

Thus, the REFLEXAFS and XANES suggest that on the addition of xanthate, a CuS .species forms on the activated ZnSe S mineral surfaces. The XANES also indicate that

.on the ZnS and Zn,Fe S surfaces this species may already be present to some extent on

the activated surface prior to treatment with xanthate, while the EXAFS demonstrate that

on addition of Nai PrX this structure becomes more ordered. On the activated ZnSe

surface the XANES indicate that a more disordered CuSe species is present but it is

clear that in this case, the addition of Nai PrX provides the S source to form a CuS .phase. In the case of the Zn,Fe S the presence of the Fe appears to aid in the

development of a true covellitic structure.

The bulk EXAFS data are similar to those from REFLEXAFS although there is anindication in the ZnSe data of a CuSe interaction at 2.86 A suggesting that primitive

CuSe forms on activation, and true covellite on addition of the xanthate. This may be a

function of the reactivity of the very fine grain sized ZnSe powder which has grains -5 .mm but, as Prestidge et al. 1994 found, probably contains 90% of the surface area on

grains -0.5 mm.

The suggestion that covellite-like structures form on the surface of activated spha- .lerite was first discussed in detail by Finkelstein and Allison 1976 and this work

provides direct evidence to support their conclusions. Copper chalcogenides are invari- . .ably Cu I see van der Laan et al., 1992 , and therefore, the Cu incorporated into the

.ZnS is likely to be reduced to Cu I . The solubility of Cu in ZnS at room temperature is

very low, of the order of 0.2 wt.%, and therefore cannot exist in the ZnS structure in any

significant concentration. It is also likely that the Cu reduction will involve a corre-

sponding oxidation of the sulfur, forming SySy bonds. This provides one of the basic

building blocks of CuS, covellite, and the surface layers of the ZnS must progressively

resemble covellite as the substitution of Cu for Zn proceeds. This work shows that thisdevelopment is via a primitive covellite structure, aided by the addition of Na i PrX. The

apparent diffusion of copper back to the surface may, in reality, be a reconstruction of

the surface to covellite.


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This work also provides some evidence for the role of Fe in ZnS flotation, and the

unactivated flotation of ZnS. As seen by the FeK edge EXAFS, surface Fe forms FeO

species that are not displaced during Cu activation but do form FeS bonds with

xanthate sulfur, thus affecting the flotation characteristics of the ZnS. There is also

evidence that the presence of Fe in the ZnS accelerates the process of covellite

formation and also the formation of a CuO species. It has long been known that Cudiffusion into Fe-bearing ZnS leads to chalcopyrite formation but there is no evidence in

. .the XANES spectra Fig. 8a and b of the Zn,Fe S samples that this has happened in

these experiments.

The analysis of the S K-edge data from unactivated ZnSe reacted with Nai PrX at pH

5 indicates that there is a direct interaction between the xanthate S and Zn, and theobserved SS bonds at 1.99 A are evidence for dixanthogen formation during this

process. This is evidence to corroborate the observation that flotation of ZnS is possible

without activation as these powders were 99.99% pure.

6. Summary

XAS and fluorescence REFLEXAFS in particular have provided direct structural

information on the binding of atoms on surfaces relevant to flotation processes. The

following information has been derived: . The Cu interacts with the Se or S at the ZnSe S surface during activation.

The bonding of the Cu at the surface involves O as well as Se or S.

On addition of Nai PrX, O is displaced by S from the xanthate.

The xanthate S bonds to the Cu and a primitive covellitic species forms. . Fe at the Zn,Fe S surface is predominantly bonded to O.

In the presence of Fe, a covellitic species develops faster than in Fe-free ZnS.

Evidence was observed of direct interaction of xanthate with unactivated ZnSe at pH

5.

This study reveals both the potential usefulness and the limitations of fluorescence .REFLEXAFS and XAS in general in mineral surface studies. Direct structural

information can be obtained for particular atoms in complex systems. However, where

the element, such as in this case Cu, may be in more than one site, interpretation is

limited in the absence of complementary evidence from other analytical techniques asXAS only provides an average coordination environment for that element. Situations

where fluorescence REFLEXAFS will be a particularly suitable analytical method to

employ include problems relating to sorption of metals on mineral surfaces not

involving exchange reactions. In these cases, the metal concentration on the surface will

be high and the surface sensitivity of REFLEXAFS provides an ideal way to obtain

direct structural information, for example, investigating the poisoning of mineral sur-

faces by metals precipitated from aqueous slurries.

Future work must focus on activation of sphalerite using lower copper concentrations

and varying both conditioning times and solution pH. In the testing of flotation ofactivated sphalerite, the mineral may be considered as having the flotation characteristics

of either amorphous CuS or a CuO species. Once xanthate is added the mineral

particles will behave like CuS.


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Acknowledgements

The authors wish to acknowledge the support of the EPSRC for funding this work,

the Director of the Daresbury Laboratory for access to the SRS and the assistance of the

staff at CCLRC Daresbury, especially Drs. A.J. Dent and A.D. Smith. Mr. D. Johnson

and the late Mr. K. Brown of the Department of Earth Sciences, University of

Manchester, are thanked for their help in preparing the polished samples, and Mr. P.Lythgoe for help with the ICP-MS analyses. Laporte Industries Cavendish Mill,

.Derbyshire are thanked for providing the organic reagents. We are grateful to Profes-

sors C.M.B. Henderson, D.J. Vaughan and Dr. K. Wright for useful discussions.

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