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Oxidation of pyrite surfaces: a photoelectron spectroscopic study
Applied Surface North-Holland
Science
72 (1993) 157-170
applied surface science
Oxidation of pyrite surfaces: a photoelectron
spectroscopic
study
S. Karthe a, R. Szargan a,* and E. Suoninen b a Fachbereich Chemie, Uniuersitiit Leipzig, Linnbtrasse 2, D-04103
Leipzig, Germany b Materials Science, Department of Applied Physics, University of Turku, It&en Pitkiikatu I, SF-20520 Received
9 February
1993; accepted
for publication
2 June
Turku, Finland
1993
Surfaces of pyrite (Fe&) differently prepared in situ and ex situ have been studied before and after contact to air and air-saturated aqueous solutions of 4 < pH I 10 by means of photoelectron spectroscopy. Pyrite surfaces fractured or scraped in situ revealed FeS-like species concentrated in the surface region. Preparation (polishing, grinding, powdering) and prolonged oxidation in air mainly resulted in basic iron sulphate and iron oxide/ hydroxide. A promoting effect of an increased surface roughness due to the preparation was observed for the formation of iron oxide/ hydroxide compared with sulphate in contrast to the natural oxidation process. Oxidation in air also led to sulphur-rich species identified as iron-deficient regions below monolayer coverage. Similar regions were present at ground surfaces exposed to air-saturated solution of pH4 and pH5. In near-neutral to alkaline solution mainly iron hydroxy-oxide is formed the layer thickness of which was estimated in the range of 0.5 nm (pH5) to 1.7 nm (pH10).
1. Introduction Pyrite (FeS,) is one of the most abundant metal sulphides and often associated with other sulphides usually considered to be more valuable. Generally they are separated from each other by froth flotation. Oxidation phenomena prior to and during the flotation procedure in aqueous solution are known to influence the effectiveness and selectivity of the flotation process [ll. The products formed during surface oxidation of sulphides may not only affect the interaction between the mineral and the constituents of sulphide mineral flotation systems but can also govern the hydrophilic or hydrophobic behaviour of the surface itself. For example metal-deficient sulphides and sulphur were shown to render the surface hydrophobic [2] whereas hydrophilic metal oxides and hydroxides can create sulphide depression [3,4]. The kind of oxidation product is
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strongly dependent on the sulphide and can be controlled by the preparation as well as such flotation parameters like pH and potential [3,5,61. Despite of the various analytical studies [7-121 completed by quantum-chemical considerations [13] the oxidation behaviour of pyrite is still unclear. X-ray photoelectron spectroscopy (XPS) has proven to be a valuable technique for monitoring surface oxidation of metal sulphides [2,9,14-161. Several XPS studies on pyrite have identified, sometimes in contradiction, numerous oxidation products such as FeSO,, Fe,(SO,),, FeOOH, Fe,O,, elemental sulphur, a metal-deficient sulphide Fel_,S2 and polysulphides [9,14,16-201. There are indications that this variety is a result of different preparation routes and oxidizing conditions [14,16,21]. A lack of information about pure pyrite surfaces being free of oxygen and carbon contaminations and the often used nonmonochromatized X-ray excitation make it difficult to determine oxidation products in the monolayer region showing only slightly different chemical states compared to the unoxidized pyrite.
B.V. All rights
reserved
S. Karthe et al. / Oxidation of pyrite surfaces
158
Hence, one part of the present XPS study will concentrate on the characterization of pyrite surfaces prepared in situ and ex situ before and after exposure to air under ambient conditions to gain more information on the influence of the preparation on the oxidation behaviour of pyrite. To identify sulphur-rich species more easily a model system S/FeS, has been studied. Besides that, the present work is directed to the surface oxidation of pyrite in air-saturated solution of pH4-10. Previous XPS measurements on the flotation of pyrite by ethyl xanthate solution provide evidence that surface oxidation and collector adsorption are influenced by each other and seem to be strongly dependent on pH [22,23]. Therefore, knowledge of the pyrite oxidation in aqueous solution is the precondition for further examinations of the pyrite/ xanthate system [24,25]. Furthermore, these experiments allow us to elucidate in more detail why pyrite shows only weak collectorless flotation.
2. Experimental
details
2.1. Samples The samples used were compact pyrite pieces of about 10 X 10 X 1 mm. They were cut along the (100) face from a single natural cubic crystal originating from Peru. No significant impurities could be detected by a SEM microprobe bulk analysis. The sulphur-to-iron ratio was determined as 2: 1. The fractured and scraped surfaces were obtained in situ using the equipment of a PHI 5400 ESCA system. The sample produced by fracture showed the typical conchoidal surface. For comparison, a sample polished in air was cleaned by Art etching to remove any oxygen- and carboncontaining contaminations. Polished surfaces were generated in air by wet grinding (Sic 600) and subsequent wet polishing with diamond paste (7 pm). A ground surface was produced by dry grinding (Sic 600) in air. In both cases the samples were then cleaned 5 min ultrasonically in ethanol p.a. The pyrite powder was produced in an agate mortar and pressed into a gold sheet to
ensure a reasonable conductive back contact. Oxidation of dry-ground surfaces in solution was carried out for 30 min in water of ultra-high quality (Elgastat UHQ MkII). The pH, adjusted with H,SO, and NaOH, was kept constant throughout the whole treatment and controlled by conventional techniques. Surfaces were examined after carefully removing the solution residues. The S/FeS, model system was obtained in UHV by deposition of pure elemental sulphur (S,) onto a freshly polished pyrite surface. The substrate temperature was maintained at 130 K in order to prevent loss of elemental sulphur from the mineral surface. 2.2. Measurements and data treatment The XPS measurements were performed in a commercial PHI 5400 ESCA spectrometer. The base pressure was in the lo-’ Pa range. The photoelectron spectra were recorded with monochromatized Al Ka radiation (1486.6 eV). The Au 4f 7,2 line (84.0 eV> was used to calibrate the binding-energy scale the linearity of which was checked by measuring also the Cu2p,,, line (932.6 eV>. Additional SXPS measurements of a surface dry-ground in air were taken at the BESSY synchrotron lab at Berlin. The used photon energies were 500 and 300 eV. The binding energy was referred in these experiments to the internal standard provided by hydrocarbon contamination of pyrite surfaces (284.6 eV>. More details of the spectrometer system and the experimental procedure are given elsewhere [26,27]. S2p spectra were fitted with doublets. The separation and the intensity ratio of their (l/2, 3/2) components were constrained to 1.12-1.20 eV and to 1 : 2, respectively. Generally the spectra were corrected with a Shirley-type background. In order to separate the Fe(II1) contribution from the Fe(B) components in the Fe2p spectra the background was removed using Tougaard’s universal loss function but fitting the parameters B and C [28,291. This was necessary to be sure that the components of the Fe(III12p as well as of the Fe(II)2p doublet satisfy the 1 : 2 intensity ratio and reveal a reasonable energy
159
S. Karthe et al. / Oxidation of pyrite surfaces
separation and peak shape. The spectra displayed are non-smoothed but normalized.
3. Results and discussion 3.1. Pyrite surfaces prepared in UHV Pyrite surfaces fractured and scraped in UHV showed neither oxygen nor carbon contaminations or oxidation products as shown by the missing 0 1s and C 1s photoemissions. Generally, materials containing sulphur in a unique environment, e.g. elemental sulphur or zinc sulphide, exhibit a well resolved single S2p (l/2, 3/2) doublet with an intensity ratio close to 1: 2. However, in the corresponding S 2p spectra of pyrite additional intensity was observed on the low and high binding energy side of the main doublet (see fig. 1). The spectra can reasonably well fitted with four different components A-D (see fig. 2a). The corresponding energy positions are comparable to those obtained previously for similarly prepared specimens and related sulphur-containing compounds (see table 11. According to that the most intense component C is due to the emission from the pyritic sulphur in the bulk whereas the signals A and B are attributed to FeS-like species. The scraped sample can only be distinguished from the fractured one by its slightly enhanced intensity of component A. Obviously, the more the surface is destroyed mechanically the more FeS-like defects are created,
Table 1 Binding energies (eV> of S 2p,,, to reference values; contribution Sample
Fe& Fe& FeS, Fe, -,S FeS, FeS, FeS, a) Excitation
Binding
energy
Fig. 1. S2p photoelectron spectra from pyrite fractured in UHV and for comparison from elemental sulphur. The spectra are shifted so that the maxima of the main peaks coincide.
giving rise to an increased photoemission in the corresponding signal A (see table 1). The feature D is probably caused by energy-loss processes. Since oxygen is absent sulphoxide species can be excluded although their emission belongs to the energy region around 165 eV [30,31]. We suggest that the doublets A and B are caused by a nonideal pyritic lattice. In that case a lattice defect like a vacancy can perturb the charge distribution in the lattice which is reflected in the observed
components of photoelectron spectra from pyrite surfaces to the whole emission (%) in parentheses
differently
prepared
Sample treatment
E, a)
s
(eV)
A
B
C
Fractured in UHV Scraped in UHV Ar+ etching
1486.6 1486.6 1486.6 1486.6 1486.6 200 500
161.1(2) 161.1(4)
161.8(6) 161.8(S) 161.7(71)
162.6 162.7 162.6
161.8 162.0 161.5(26)
162.5 162.7 162.6
Cut in air (100 face) Cleaved in UHV (100) face Dry-ground in air energy.
b, See text.
‘) This work.
2P,,2
components
Ref.
‘)
C) C) C)
1631
161.1 161.2(10)
in UHV compared
[91 [341 C)
S. Karthe et al. / Oxidation of pyrite surfaces
160
62P
I
165.5
I
163.3
I
161.1
Binding energy / eV Fig. 2. Pyrite S2p spectra from (a) a surface scraped in UHV; (b) a surface after Art etching; (cl the same surface after exposure to air for 1 h. For components A-D see text.
chemical shift. Both p- and n-type semiconduction is known for natural pyrite [32,331, so the existence of sulphur- and iron-deficient regions is possible the latter of which might account for the negatively shifted S 2p photoemission consistent with components A and B. Pettenkofer et al. [341 have obtained from SXPS measurements (hv = 200 eV> of the pyrite (100) face cleaved in UHV a S2p line split into three different doublets. They were interpreted in the order of rising binding energy with FeS-like defects (A), FeS, at the surface (Bl and bulk FeS, (Cl (see table 1). In principle this supports our designation given above. The differences can be explained as follows. The enhanced intensity of A is a result of the reduced escape depth due to the lower excitation energy. It was estimated
that instead of 10% only about 1% would contribute to the S 2p line if Mg Ka radiation is used [34] which is close to the value obtained for our fractured surface irradiated by AlKa radiation. According to Pettenkofer et al. [34] the surfaceshifted core-level emission B reflects the changed surface coordination of one sulphur of the dimer in the (100) plane due to the loss of an iron coordination. This may be considered as a special case of an iron-deficient region. While such surface core-level photoemissions undergo a considerable intensity loss compared to the bulk emission caused by the adsorption of oxygen, hydrocarbons or water, the weak component B in the spectrum of the fractured surface exposed to air is still clearly evident. Thus, we conclude that the iron-deficient species are not restricted to the uppermost layer but are also found in underlying regions. In order to confirm this conclusion SXP spectra of a pyrite surface dry-ground in air were measured (see fig. 3). Despite of the contamination layer and the formation of oxidation products the splitting of the S2p emission is maintained. As predicted, the FeS-like emissions in fact are not as sensitive to adsorption as real surface core-level shifted peaks. The limited energy resolution does not allow a reasonable separation of component A from B. The corresponding emission is greatly enhanced by decreasing the excitation energy from 1486.6 to 300 eV.
S2P
164.8
162.8 Binding
energy
160.8 1 eV
Fig. 3. Pyrite S2p spectrum from a surface ground in air (excitation energy 300 eV). For components A-C see iext.
S. Karthe et al. / Oxidation of pyrite surfaces Table 2 Binding energies (eV) of Fe2p,,, components of photoelectron spectra from pyrite surfaces differently prepared in UHV compared to reference values Sample
Sample
FeS, FeS, FeS, Fe, -XS FeS,
Fractured in UHV Scraped in UHV Arf etching
treatment
707.4 707.4 707.4
Polished
707.4
in air
Fe 2p3,,
components 708.8 708-710 708-710 708
Ref. a) a) a) [631 [161
‘) This work.
Excluding a surface-shifted core-level photoemission this behaviour indicates FeS-like defects accumulated in the near-surface region. They may be partly inherent but mostly created by the grinding procedure (more crystal edges). It should be noted that FeS-like species have also been found on dry-ground pyrite by Pillai et al. [23]. Moreover Mycroft et al. [9] have published XP spectra of a pyrite crystal fractured in UHV where an additional minor S2p,,, peak with a binding energy of 161.8 eV was always detected but had not been interpreted. This peak position is consistent with those of component B and supports our findings. The Fe2p 3,2 spectrum is dominated by a narrow line (FWHM = 0.9 eV) both for the fractured and the scraped surface. The binding energy of 707.4 eV is in good agreement with previously reported values (see table 2). Since the metal ion in pyrite is in a low-spin configuration 2p63dh; S = 0 only one final state 2p53dh; S = l/2 is possible. Hence multiplet splitting together with a line broadening should not occur as it is indeed observed. No satellite structure appears so the rule that diamagnetic low-spin compounds do not exhibit 2p,,, satellites [35] is confirmed by pyrite. There is evidence for a weak feature with a binding energy of 708.8 eV being typical for FeSlike emissions (see figs. 4a and 6a). Due to the mechanical treatment the scraped surface is marked by a slight broadening and increase of this feature, both indicating an enhanced number of FeS-like surface regions. This corresponds well to the observed differences in intensity with respect to the S2p spectra. Furthermore, the background on the high binding energy side of the
161
Fe 2p 3,2 peak coming down only slowly is dominated by energy-loss processes due to inelastic scattering and multi-electron excitations. These Fe 2p spectra provide a useful basis for the detection of small amounts of iron oxidation products the broad Fe2p emissions of which might be superimposed on the background sometimes in a rather unspecific manner. It is quite common to produce oxygen- and carbon-free surfaces by Art etching. However, as to pyrite the surface structure and composition is changed due to the preferential sputtering of sulphur 1361. This is indicated by the superposition of FeS-type Fe2p and S 2p photoemissions originating from the sputtered surface and of the minor signal coming from the undisturbed pyritic layers beneath (see figs. 2b and 4b). Those men-
Fe2p
FeOOH/Fe,O, 1 FeS
>
.z :
-E
I
724
I
718 Binding
I
I
712 energy
706
/ eV
Fig. 4. Pyrite Fe2p spectra from (a) a surface scraped in UHV; (b) a surface after Ar* etching; Cc) the same surface after exposure to air for 1 h.
lh2
S. Karthe et al. / Oxidation of pyrite surfaces
tioned first will determine the behaviour of the pyrite surface. Therefore, Ar+ etching is not appropriate for surface pretreatment in the case of flotation-related studies of pyrite. However, as it will be shown later, such a sputtered pyrite surface may act as a representative for pyrrhotite-like compounds in order to demonstrate the different oxidation behaviour compared to pyrite. 3.2. Oxidation
qf pyrite
S2P
% .z k -Tz
____._._.---Y.7.-:_;
in air
L.-r 171.6
Generation of fresh pyrite surfaces either by polishing or grinding or powdering is associated with the adsorption of oxygen as well as carboncontaining species and the formation of oxidation products. The corresponding broad 0 1s features (FWHM = 2.5-3.0 eV> therefore include emission from adsorbed water (Es = 532.3-533.0 eV [14,37]), from hydroxy groups (E, = 531.5 eV [38,39]), carbonyl groups (Ea = 531.2-531.6 eV [401), oxides (En = 530.0 eV [41]) and from different sulphur oxidation products (EB = 531.8-532.5 eV 116,421). In comparison to the spectra of the surface fractured in UHV a small intensity gain for the component and a slightly higher backS2P,,, ground between 165 and 170 eV indicate that sulphur oxidation occurs even during the preparation in air. The effects are more pronounced in the case of pyrite powder, giving two new components at 163.9 and 168.5 eV (see fig. 5).
Table 3 Binding energies the corresponding
Fe 2~a,~
Binding
166.0
I
160.4
energy
/ eV
Fig. 5. S2p spectra from fresh pyrite powder (. ‘) and after exposure to air for 1 h (. -. -.). For comparison the pyrite S2p spectrum from a surface fractured in UHV is shown.
An overall but only slightly increased background in the energy range 709.5-713 eV is present in the corresponding Fe2~~,~ spectrum due to iron oxidation. Again the pyrite powder seems to be most reactive. This can be understood in terms of an enlarged surface area associated with a higher surface energy. The results are summarized in table 3. Apart from the S2p signal at 163.9 eV the formation of iron sulphate and iron oxide or hydroxide is indicated. The S 2p3,* peak position at 168.5 eV is too low to assume either Fe&SO,), or FeSO, .7H,O being formed. Thus in agreement with previous studies [16] the for-
(eV) of Fe 2p,,,, 0 1s and S 2p,,, p hotoelectron values of spectra from pyrite surfaces differently
iron oxidation
products
in comparison
Fe.?,
Fe0
a-Fe,O,
FeSO,
FeOOH
KFe,(OH),60,)2
Fe260,),
[391
[I61
ft61
tt61
[t61
it61
707.4
709.5 530.0
711.0 529.9
711.3 532.5
711.5 530.3 531.4
712.2 532.5
713.5 532.4
168.8
169.5
162.6
7H,O
spectra from various prepared in air
[I61 01s S 2P,,,
I
163.2
I
168.8
169.4
FeS, (polished, ground, powdered) *’ Fe ZP,,~ 0 1s S 2Ps,* h) ‘) This work.
707.4 530.0 162.6 b, Component
at 163.9 eV is omitted.
709.5-713.0 531.5 168.5
531.8
532.4
to
S. Karthe et al. / Oxidation
mation of basic iron sulphate similar to jarosite KFe,(OH),(SO,), is proposed. By that the 0 Is peak maximum of 532.0 eV obtained independently of the preparation can be interpreted as a superposition of 0 1s emissions from the OHand SO,-groups. From the mineralogical point of view pyrite is known to oxidize to Fe(H) and Fe(II1) sulphates such as Fe,SsO,, . 9H,O (coquimbite), Fe(OH)SO, .3H,O (amerantite), Fe(OH)SO, . 4.5H,O (fibrogerrite) [43]. Since reference binding energies are not available the real composition of the basic sulphate identified in our measurements can not be determined. The presence of an oxide-related contribution to the 0 1s emission below 531 eV coupled with additional Fe2p intensity in the energy range 709.5-711.5 eV gives evidence for iron oxides or iron hydroxy-oxide (see table 3). The results are in accord with those published by Brion [16] but in sharp contrast to the mineralogical findings where iron sulphate was produced initially before the formation of oxides and hydroxides like goethite (FeOOH) starts [43]. On the other hand, observations made by Buckley and Woods [141 agree with this natural oxidation process. In order to elucidate the situation the naturally occurring weathering of pyrite was simulated. For that reason a pyrite surface fractured in UHV was first exposed to air for 10 min. This provides no clear indication for any oxidation product. In contrast to that pyrite powder is significantly oxidized after the same exposure time to air. Thus the reactivity of a fractured surface to oxygen seems to be strongly reduced. This is confirmed comparing the Fe2p spectra from pyrite powder and from a fractured surface after 1 h in air. The Fe2p spectrum of the powder is more intensive in the lower energy region around 710-711 eV, indicating more iron oxide and hydroxide than sulphate (see fig. 6~). As to the fractured surface, no sulphate and only a trace of oxidized iron were discernible in the S2p and Fe2p spectra (see fig. 6a and 6b). On the other hand, the sample is significantly oxidized to iron sulphate after prolonged exposure to air as one might expect from the natural weathering process. In that case, the S2p spectrum clearly exhibits additional intensity near 168.5 eV. Further-
163
of pyrite surfaces
Fe2p
I
I
I
724
718
I
712
I
706
Binding energy / eV Fig. 6. Pyrite Fe2p spectra from (a) a surface fractured in UHV; (b) the same surface after exposure to air for 1 h; Cc) a powdered sample after exposure to air for 1 h; Cd) a fractured surface after exposure to air for 14 days.
more, the formation of some iron oxide and hydroxide besides sulphate can be deduced from the intermediate energy position of the Fe(III)2p feature at 711.8 eV (see fig. 6d and table 3). This is confirmed by the 0 1s spectra showing only slight emission from iron oxide or hydroxy-oxide. It seems to us that the more the pyrite lattice at the surface is distorted the more iron oxide and hydroxide is formed compared to iron sulphate. Obviously this is related to an enhanced surface roughness associated with an increased number of crystal edges providing more FeS-like sites and hence favouring the initial formation of iron oxide and hydroxide over sulphate as does pyrrhotite (FeS,_,). Now from that view the quite different results published with respect to pyrite oxidation can be explained as being due to the significant differences in preparation procedure.
S. Karthe et al. / Oxidation of pyrite surfaces
164
The formation of iron oxide and/or hydroxide besides sulphate would require the existence of a further sulphur oxidation product to maintain the original sulphur-to-iron ratio of 2: 1 in the oxidation layer also. Therefore, it was postulated that sulphur remains in the pyrite lattice forming a metal-deficient sulphide in addition to sulphate [14]. Because of the absence of a separate S 2p signal it was suggested that it displays the same binding energy as the sulphur in the mineral itself [14,16]. By observing the S 2p component at 163.9 eV (see fig. 5) first experimental evidence is now provided for such an iron-deficient sulphide species formed on exposure of pyrite to air. Its chemical environment and in-depth distribution will be discussed later in connection with results from oxidation of pyrite in aqueous media. The following equations may describe the simultaneous formation of iron oxide or hydroxide and iron-deficient sulphide: FeS, + +x0,
+ +xH,O
-+ Fe,_,&
FeS, + +x0,
+ Fe,_,S2
+xFeO,
(2)
FeS, + $x0,
--) FelpxS2
+ $xFe,O,.
(3)
Fe,O,
reactions + H,O
2Fe(OH),
may take place,
+ 2FeOOH,
-+ Fe,O,
+ 3H,O.
e.g.: (4) (5)
A S2p feature indicating iron deficiency also appears when Ar+ etched pyrite is exposed to air. Since now the pyrrhotite-like species FeS,-, on pyrite created by etching suffers an iron loss due to oxide and/or hydroxide formation (see fig. 4c) a metal-deficient sulphide with stoichiometry close to pyrite and therefore with a S2p emission near those of pyrite (see fig. 2c) grows in e.g. according to: Fe!&
+ ;yO,
+ $yH,O
+ Fe I--ySZ--x +yFe(OH),.
Sample
Arf
treatment
etched
and 1 h in air
Rel. 0 1s intensity ‘)
Fe, _&
0.70
+
Powdered
and 1 h in air
0.40
+
Fractured
WHV)
0.06
_
a) Emission
and 1 h in air
at 530.0 eV.
b, Emission
‘)
at 163.9 eV.
around 530 eV (see table 4). Again the relation between the degree of surface roughness (increasing from the fractured to the sputtered as well as powdered sample) and the affinity to form iron oxide/ hydroxide associated with an iron-deficient sulphide should be emphasized. 3.3. Oxidation of pyrite in aqueous solution
+xFe(OH),, (1)
Further
Table 4 Relative intensity of 0 Is oxide signal and presence of a sulphur-rich species dependent upon the pyrite surface preparation
(6)
Thus, an Ar+ etched pyrite surface undergoes in analogy with pyrrhotite strong oxidation in air to form iron oxide/ hydroxide which is obvious from the Fe2p spectra and can be proved quantitatively by following the oxide-related 0 1s feature
In the flotation of pyrite the most interesting pH region is between pH4 to pH12. With respect to further experiments on the adsorption of ethyl xanthate our samples were oxidized for 30 min in aqueous phase of pH4 to pH10. The composition of the surface layer of pyrite after immersion in solution of pH4 differs significantly from that observed for samples treated at higher pH. Obviously pH4 is low enough to solve any sulphate and iron oxide/ hydroxide formed during the preparation and to prevent their redeposition. The Fe2p spectra therefore resemble those displayed by pyrite fractured in situ, and the 0 1s spectrum mainly shows the signal from adsorbed water (En = 532.3 eV) (see fig. 7). After immersion of pyrite in solution of 5 I pH 5 10 the XP spectra mainly indicate the formation of rather insoluble Fe(II1) oxidation products with Fe 2p features around 711.5 eV. This is due to the oxidation of the pyritic iron by oxygen solved in water which is promoted by an increasing pH because of the small solubility constant of the Fe(II1) hydroxide being formed. As to this hydroxide, it is not a well defined trihydroxide but is considered to be a water-rich hydrogel of general formula Fe,O, . H,O and often called iron hy-
S. Karthe et al. / Oxidation of pyrite surfaces
droxy-oxide (FeOOHI [44]. It seems to have a composition intermediate between a-FeOOH (goethite) and a-Fe,O, (haematite) [45,46]. A crystal structure has been proposed for a compound whose composition is close to 2Fe,O,. FeOOH .4H,O [47]. The FeOOH gel is supposed to be an amorphous form of this material [48,49]. In agreement, the emissions from oxide groups in Fe,O, and FeOOH (component A) and from OH groups in H,O and FeOOH (component B) dominate the shape and the intensity of the 0 1s spectra (see fig. 7). The corresponding Fe 2p photoemissions display the typical multiplet structure of Fe(II1) in iron hydroxy-oxide at 711.5 eV. It is remarkable that the ratio of iron in the oxidation layer to those in the pyrite beneath (FeO,/FeSUr,) as well as the relative intensity of
1,2
165
*Fe/S
*- l(Ols,)
2,4
..- Feox/Fesub i
/ /’
I’ 0
3
1
I
1
I
I
I
/
4
5
6
7
8
9
10
PH
Fig. 8. Atomic ratio Fe/S, relative 01s intensity of component A and oxidation ratio Feux /Fesub taken from the corresponding S 2p, Fe 2p and 0 1s spectra from a pyrite surface after immersion in distilled water as a function of pH.
534.0
532.0 Binding
energy
530.0 / eV
Fig. 7. Pyrite 01s spectra from surfaces after distilled water at different pH. For components text.
immersion in A and B, see
the 0 1s signal A and the atomic ratio Fe: S follow the same tendency from pH4 to pHl0 (see fig. 8). This correlation unambiguously results from the enhanced deposition of iron hydroxyoxide with decreasing H+ concentration. It is hence possible to use the oxide-related 0 1s emission A which is easy to separate for monitoring the progress of pyrite oxidation in aqueous solution when the pH is increased. The characteristic shape of the curves reveals that pyrite oxidation depends in a complex manner upon pH. The slope of the curve may be affected by thermodynamic aspects like pH-dependent redox potentials or solubility products related to pH and electrokinetic parameters changing with pH. Moreover, according to Brown and Jurinak [12] different preparation methods and further ions present in the solution may influence the oxidation rate at different pH and therefore the shape of the curve. Assuming the pyrite substrate is covered by an iron hydroxy-oxide overlayer its thickness can be
166
S. Karthe et al. / Oxidation of pyrite surfaces
estimated from the Fe(H) and Fe(III) regions of the Fe2p spectra according to Briggs and Seah [50]. In addition, knowledge of the inelastic mean free path A of 800 eV electrons both in pyrite and iron hydroxy-oxide is necessary. Since experimentally determined A-values are not available the value of 2.1 nm [51] calculated for silicon as a semiconductor with a band gap comparable to that of pyrite may serve as a reasonable approximation. With respect to the electron take-off angle of 60” the escape depth is then 1.8 nm. Assuming equal Fe2p ionization cross sections the intensity ratio of the Fe2p signals from nonoxidized pyrite and from iron hydroxy-oxide can be substituted by the ratio F of the iron atoms per space unit in both the substrate and the overlayer. The latter can be determined through the density values p, the molar weight M and the stoichiometry, respectively. Taking into account the complex composition of iron hydroxy-oxide the overlayer thickness is calculated using either goethite with p = 5.0 g/cm3 [52] or haematite with p = 5.0 g/cm3 [52] as a model compound. The experimental intensity rafrom the subtios FeO.JFesUb of the emission strate and from the oxidation layer are determined by curve fitting of the Fe2p spectra. As expected, the layer thickness undergoes a similar change with pH as the terms displayed in fig. 8 (see fig. 9). The layer thickness differs only slightly assuming either goethite or haematite as the representative for the overlayer. Thus, the mixed composition of the iron hydroxy-oxide precipitate disregarding the water content may result only in insignificant deviations from the values given in fig. 9. The thickness of the iron hydroxy-oxide layer after 30 min oxidation in aqueous solution might be in the range of 0.5 nm (pH5) to 1.7 nm (pH10). Slight alterations in the S2p spectra point to the simultaneous formation of sulphur oxidation products. Treatment of pyrite at pH4 increases the relative S2p intensity compared to that of fresh-fractured pyrite, indicating an enhanced surface concentration of sulphur. This is caused by the superposition of a second doublet shifted by 1 eV from the pyrite position (see fig. 10) accounting for about 8% of the total S2p inten-
J
3
4
5
6
7
6
9
10
11
PH Fig. 9. Estimated thickness of the oxidation layer on pyrite after immersion in distilled water as a function of pH assuming either a layer composition equal to goethite or to haematite.
sity. The observation can be understood in terms of a sulphur-rich surface species being formed simultaneously with dissolving of oxidation prod-
S2P
).
c
E
al
-E
I
I
I
I
I
169
167
165
163
161
Binding
energy
/ eV
Fig. 10. Pyrite S2p spectrum from a polished surface after immersion in distilled water at pH4 (’ . -1.For comparison the pyrite S2p spectrum from a surface fractured in UHV is shown.
S. Karthe et al. / Oxidation of pyrite surfaces
167
ucts Gulphate, hydroxide). This overlayer composition is in contrast to that detected on exposure of pyrite to air. In the latter case the sulphur-rich species is still covered with iron oxide/ hydroxide. A sulphur-rich species has also been detected by Buckley and Woods [141 on pyrite oxidized in acetic acid (pH = 3). Such a sulphur-rich surface species was only slightly discernible after immersion of pyrite in solution of pH5 and was no more evident for higher pH. It is therefore suggested that the sulphur-rich species only represents an intermediate product of pyrite oxidation in aqueous solution to form finally sulphate. The major part of this sulphate is transferred to the solution, but a small amount was still retained at the surface. The corresponding S2p intensity at 168.5 eV increases from pH6 to pHl0 up to about 7% of the total S 2p emission. This finding can be explained considering that the precipitation of sulphate ions together with iron hydroxy-oxide gives a basic salt [53-561. 3.4, Iron-deficient sulphide and the model system sulphur /pyrite Mainly an iron-deficient sulphide or elemental sulphur are discussed to describe the chemical binding in the sulphur-rich species observed at the surface of various sulphides, e.g. chalcopyrite, pyrrhotite, pentlandite, bornite [2,14,21,41,57]. XPS analysis of the model system S/FeS, may provide a useful basis for the characterization of the sulphur-rich species identified at the pyrite surface after its exposure to air and to aqueous media of pH4 and pH5 (see previous section). Elemental sulphur has a vapour pressure of about lo* Pa at room temperature. Hence, substrates need to be cooled down below 200 K to prevent loss of elemental sulphur from the surface to UHV. Elemental sulphur, therefore, was evaporated on the pyrite substrate at 130 K. The increasing sulphur coverage gives rise to a continuously growing additional S 2p feature at 164.2 eV. Prolonged sulphur deposition results in an entire depression of the pyritic S2p signal at 162.6 eV. Fitting the S2p spectra taken from samples with different sulphur coverage indicated neither
I
166 Binding
I
I
164
162
energy
/ eV
Fig. 11. S 2p spectra including curve fits from (a) bulk elemental sulphur, (b) a pyrite surface covered with elemental sulphur (c) a polished pyrite surface.
changes in binding energies nor in full half-widths of the S2p doublets compared to those of uncovered pyrite and of bulk elemental sulphur (see fig. 11). If the sample is allowed to warm up to ambient temperature a fast sulphur desorption takes place leaving a pyrite surface indistinguishable from an uncovered surface. A subsequent deposition and desorption cycle of sulphur reveals reversibility of that procedure. Consequently, the deposited sulphur does not influence the chemical environment of the adjacent pyritic sulphur and is completely lost at ambient temper-
S. Karthe et al. / Oxidation of pyrite surfaces
168
Table 5 Ratio F of sulphur atoms per space unit in the pyrite substrate and in the sulphur-rich overlayer (hypothetical composition) calculated from the corresponding densities p, molar weights M, and stoicbiometry V, F
Hypothetical composition
?g/cm’)
FeS, FeS, Fess _
5.0 d’ 3.0 2.4 _
60.0 46.0 39.0
1.00 1.28 1.34
S
2.0 h,
32.1
1.34
d, Ref. [lo].
ML/V, (g/m011
‘) Ref. [44].
ature. In contrast, no intensity loss of the S2p feature assigned to the sulphur-rich species at the oxidized pyrite surface was evident, even after the sample had been subjected to ultra-high vacuum for 24 h. This, in agreement with Buckley and Woods [14], clearly supports the conclusion that an iron-deficient sulphide species rather than elemental sulphur in a stable S, configuration is formed on pyrite. Obviously, the sulphur-rich species only then behaves like elemental sulphur including its volatility if pyrite is strongly oxidized electrochemically [9,58] or by treatment in hot concentrated hydrochloric or nitric acid [14]. The thickness of the iron-deficient sulphide species can be estimated from the S2p emission analogous to that of the iron hydroxy-oxide layer (see previous section). In the case of the kinetic energy of the S 2p electrons (1325 eV) experimental data for the inelastic mean free path of inorganic compounds compiled by Seah and Dench
Table 6 Layer thickness of sulphur-rich FeS,, FeS, and S, respectively
species
System
Sample treatment
Fe,-,S,/FeS,
Polished/polished 30 min at pH5
Fe,_,S,/FeS,
Polished and 30 min at pH4
on pyrite
calculated
depending
x (%) a’ and
5
10
‘) Contribution of Fe, _xS, to the total S 2p photoemission. ‘) Lattice constant of pyrite [ 101.
[59] are in the range from 1.5 to 5 nm. Again the value calculated for silicon [51] as 3 nm may serve as a reasonable approximation. Difficulties arise as to the ratio F of the S2p emission from the bulk substrate and bulk overlayer due to the unknown intermediate composition of the irondeficient sulphide between pure pyrite and pure elemental sulphur. Nevertheless, the composition of iron-deficient sulphides can be simulated by increasing stepwise the sulphur content in the formula (see table 5). The density values p for are calculated from the corresponding Fe%+, values of elemental sulphur (ps = 2.0 g/cm3 [44]) and pyrite (pFeS, = 5.0 g/cm3 [60]) according to +x). The proportion of the P = [P&S +xp,]/(l intensity *created by the iron-deficient layer was obtained by curve fitting. The estimation procedure gives a layer thickness which varies only slightly with the sulphur content of the iron-deficient layer (see table 6). Assuming no lattice reconstruction in the iron-deficient region and considering the nearly (100) orientation of the surface the lattice constant of pyrite (0.54 nm [lo]> was used as a reasonable approximation for the interatomic distance in the iron-deficient species. Dividing the layer thickness by this value gives the coverage in monolayers. It can be seen that the thickness of the iron-deficient layer is only in the range below one nanometer. Even if the layer composition is assumed to be equal to elemental sulphur the thickness is far from one monolayer. Therefore, the term “iron-deficient sulphide” generally used for iron-deficient layers on various sulphides for more accuracy should be
on the sample
preparation
and assuming
a composition
F
d (nm)
a (nm)
ML
1.oo 1.28 1.34 1.00 1.28 1.34
0.13 0.17 0.18 0.27 0.35 0.36
0.54 b’
0.24 0.31 0.33 0.51 0.64 0.67
of
S. Karthe et al. / Oxidation of pyrite surfaces
substituted in the case of pyrite by “iron-deficient surface regions” to emphasize the difference to a bulk-like phase. As to that pyrite behaves significantly different from chalcopyrite which is known to form iron-deficient layers up to l-2 nm [El. On the basis of the results presented and discussed above the generally high flotation recovery of pyrite in acidic media of pH4 [61,62] now can be attributed to the existence of iron-deficient surface regions combined with a low amount of sulphate and iron hydroxy-oxide deposition. At pH5 already the hydrophobizing effect of the sulphur-rich areas is counteracted by the hydrophilic oxidized surface regions being twice as large as the former one (compare fig. 9 and table 6). Consequently, both the weak tendency of pyrite to form sulphur-rich surface species and its strong affinity to form quite insoluble iron oxidation products are now suggested to be the main reasons for the known small ability of pyrite to reveal self-induced flotation.
4. Conclusions Fractured and scraped pyrite surfaces prepared in UHV are characterized by minor FeSlike species accumulated in the near-surface region. They are partly inherent due to lattice pertubations but may also be created by mechanical roughening of the surface. The corresponding emissions have to be taken into account when analysing the Fe 2p and S 2p spectra of differently prepared pyrite. Ar+ etching disturbs the pyrite structure to form FeS-type surface species. Hence, this preparation method is not recommended for flotationrelated surface studies. Preparation (polishing, grinding, powdering) and prolonged oxidation of pyrite in air produces mainly a basic iron sulphate similar to jarosite and iron oxide/ hydroxide that is expected to be hydrophilic. The increased surface roughness due to the preparation seems to favour the formation of iron oxide/ hydroxide over sulphate in contrast to the natural weathering process of pyrite. First experimental evidence is given for a sulphur-rich species produced during oxidation of pyrite in air.
169
These species were identified as iron-deficient surface areas below monolayer coverage. Similar iron-deficient regions were observed for ground pyrite surfaces oxidized for 30 min in aqueous solution of pH4-5. Starting at pH5 iron hydroxy-oxide is precipitated on pyrite the more the pH is increased. The corresponding layer thickness was estimated in the range of 0.5 nm (pH5) to 1.7 nm (pH10). The small ability of pyrite to form hydrophobic Fe 1_xS2 species combined with a high affinity to cover itself with hydrophilic iron oxidation products can now be assumed as the main reason for its comparatively low recovery observed in collectorless flotation.
Acknowledgements The project was partly supported by the Bundesministerium fur Forschung und Technik (BMFT No. NT 2082/4). The authors would like to thank I. Kartio and M. Heinonen for their help in the XPS measurements.
References 111 M.C. Fuerstenau, J.D. Miller and M.C. Kuhn, Chemistry of Flotation (AIME, New York, 1985). [2] A.N. Buckley, I.C. Hamilton and R. Woods, in: Flotation of Sulphide Minerals, Ed. K.S.E. Forssberg (Elsevier, Amsterdam, 1985) p. 41. [3] B. Ball and R.S. Rickard, in: Flotation, Vol. 1, Ed. M.C. Fuerstenau (AIME, New York, 19761 p. 458. [4] K.L. Sutherland and I.W. Wark, in: Principles of Flotation (Aust. Inst. Min. Met., Melbourne, 1955) p. 164. [51 P.J. Guy and W.J. Trahar, in: Flotation of Sulphide Minerals, Ed. K.S.E. Forssberg (Elsevier, Amsterdam, 19851 p. 91. I61 R.A. Hayes and J. Ralston, Int. J. Miner. Process. 23 (1988155. [7] S. Chander and A. Briceno, Miner. Met. Process. 4 (1987) 171. [81 J. Pang, A. Briceno and S. Chander, J. Electrochem. Sot. 137 (1990) 3447. [9] J.R. Mycroft, G.M. Bancroft, N.S. McIntyre, J.W. Lorimer and I.R. Hill, J. Electroanal. Chem. 292 (1990) 139. [lo] R.T. Lowson, Chem. Rev. 82 (1982) 461.
170 [ll] [12] [13] 1141 [15] [16] [17] [18]
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S. Karthe et al. / Oxidation of pyrite surfaces C.O. Moses, D.K. Nordstrom, J.S. Herman and A.L. Mills, Geochim. Cosmochim. Acta 51 (1987) 1561. A.D. Brown and J.J. Jurinak, J. Environ. Qual. 18 (19891 545. G.W. Luther, Geochim. Cosmochim. Acta 51 (1987) 3193. A.N. Buckley and R. Woods, Appl. Surf. Sci. 27 (19871 453. A.N. Buckley and R. Woods, Aust. J. Chem. 37 (19841 2403. D. Brion, Appl. Surf. Sci. 5 (19801 133. G.N. Raikar, J. Phys. Condens. Matter 3 (1991) 1931. A.N. Buckley and R. Woods, in: Interfacial Phenomena in Mineral Processing, Eds. B. Yarar and D.J. Spottiswood (Engineering Foundation, New York, 19821 p. 3. G. Panzner and B. Egert, Surf. Sci. 144 (1984) 651. J.P. Baltrus and A. Proctor, Appl. Surf. Sci. 44 (1990) 147. A.N. Buckley, I.C. Hamilton and R. Woods, in: Proc. Int. Symp. on Electrochemistry in Mineral and Metal Processing, Vol. I, Eds. P.E. Richardson and R. Woods (Electrochemical Society, Pennington, NJ, 1988). S.U.M. Khan, J.P. Baltrus, R.W. Lai and A.G. Richardson, Appl. Surf. Sci. 47 (1991) 355. K.C. Pillai, V.Y. Young and J.O’M. Bockris, J. Colloid Interf. Sci. 103 (1985) 145. S. Karthe, Thesis, Universitat Leipzig (1992). R. Szargan, S. Karthe and E. Suoninen, Fresenius J. Anal. Chem. 346 (1993) 303. W. Jaegermann and H. Tributsch, Prog. Surf. Sci. 29 (1988) 1. W. Jaegermann and C. Pettenkofer, Ber. Bunsenges. Phys. Chem. 92 (1988) 1354. S. Tougaard, Surf. Sci. 216 (1989) 343. S. Tougaard, J. Electron Spectrosc. Rel. Phen. 52 (1990) 243. C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder and G.E. Muilenberg, Handbook of X-Ray Photoelectron Spectroscopy (Perkin-Elmer, Physical Electronics Division, Eden Prairie, MN, 1979). B.J. Lindberg, K. Hamrin, G. Johansson, V. Gelius, A. Fahlmann, C. Nordling and K. Siegbahn, Phys. Ser. 1 (1970) 286. D.J. Vaughan and J.R. Craig, in: Mineral Chemistry of Metal Sulfides (University Press, Cambridge, 1978). R.T. Shuey, Semiconducting Ore Minerals (Elsevier, London, 1975). C. Pettenkofer, W. Jaegermann and M. Bronold, Ber. Bunsenges. Phys. Chem. 95 (1991) 560. L. Yin, I. Adler, T. Tsang, L.J. Matienzo and S.O. Grim, Chem. Phys. Lett. 24 (1974) 81.
[36] Y.C. Lee and P.A. Montano, Surf. Sci. 143 (1984) 442. [37] V. Young, Carbon 20 (19821 35. [38] J. Haber, J. Stoch and L. Ungier, J. Electron Spectrosc. Rel. Phen. 9 (1976) 459. [39] N.S. McIntyre and D.G. Zetaruk, Anal. Chem. 49 (1977) 1521. [40] CD. Wagner, D.A. Zatko and R.H. Raymond, Anal. Chem. 52 (19801 1445. [41] A.N. Buckley and R. Woods, Appl. Surf. Sci. 22/23 (1985) 280. [42] Y. Limouzin-Maire, Bull. Sot. Chim. Fr. 1 (1981) 340. [43] G. Linck and H. Jung, Grundriss der Mineralogie und Petrographie (Gustav-Fischer, Jena, 1954). [44] A.F. Hollemann, E. Wiberg and N. Wiberg, Lehrbuch der anorganischen Chemie (de Gruyter, Berlin, 1985). [45] S. Rajendran, V.S. Rao and H.S. Maiti, J. Mater. Sci. 17 (1982) 2709. [46] S.V.S. Prasad and V.S. Rao, J. Mater. Sci. 19 (1984) 3266. [47] K.M. Towe and W.F. Bradley, J. Colloid Interf. Sci. 24 (1967) 384. [48] I.P. Saraswat, A.C. Vajpei, V.K. Garg, V.K. Sharma and N. Prakash, J. Colloid Interf. Sci. 73 (1980) 373. [49] A.C. Vajpei, I.P. Saraswat and K. Chandra, J. Mater. Sci. 19 (1984) 3341. [50] M.P. Seah, in: Practical Surface Analysis, 2nd ed., Vol. 1, Eds. D. Briggs and M.P. Seah (Wiley, Chichester, 1990) p. 244. [51] S. Tanuma, C.J. Powell and D.R. Penn, Surf. Interf. Anal. 11 (1988) 577. [52] H.J. Rosier, Lehrbuch der Mineralogie (VEB Deutscher Verlag fir Grundstoffindustrie, Leipzig, 1981). [53] H.B. Weiser and W.O. Milligan, J. Am. Chem. Sot. 57 (1935) 238. [54] J. Dousma, T.J. van den Hoeven and P.L. de Bruyn, J. Inorg. Nucl. Chem. 40 (1978) 1089. [55] E. Matijevic, R.S. Sapieszko and J.B. Melville, J. Colloid Interf. Sci. 50 (1975) 567. [56] J. Dousma, D. Ottenlander and P.L. de Bruyn, J. Inorg. Nucl. Chem. 41 (19791 1565. [57] A.N. Buckley and R. Woods, Appl. Surf. Sci. 20 (1985) 472. [58] T. Biegler and D.A. Swift, Electrochim. Acta 24 (1979) 415. [59] M.P. Seah and W. Dench, Surf. Interf. Anal. 1 (1979) 2. [60] R.C. Weast, Handbook of Chemistry and Physics, 53rd ed. (CRC Press, Cleveland, OH, 1972) p. B192. [61] M.C. Fuerstenau, M.C. Kuhn and D.A. Elgillani, Trans. AIME 241 (19681 148. [62] J.O. Leppinen, Int. J. Miner. Process. 30 (1990) 245.
Science
72 (1993) 157-170
applied surface science
Oxidation of pyrite surfaces: a photoelectron
spectroscopic
study
S. Karthe a, R. Szargan a,* and E. Suoninen b a Fachbereich Chemie, Uniuersitiit Leipzig, Linnbtrasse 2, D-04103
Leipzig, Germany b Materials Science, Department of Applied Physics, University of Turku, It&en Pitkiikatu I, SF-20520 Received
9 February
1993; accepted
for publication
2 June
Turku, Finland
1993
Surfaces of pyrite (Fe&) differently prepared in situ and ex situ have been studied before and after contact to air and air-saturated aqueous solutions of 4 < pH I 10 by means of photoelectron spectroscopy. Pyrite surfaces fractured or scraped in situ revealed FeS-like species concentrated in the surface region. Preparation (polishing, grinding, powdering) and prolonged oxidation in air mainly resulted in basic iron sulphate and iron oxide/ hydroxide. A promoting effect of an increased surface roughness due to the preparation was observed for the formation of iron oxide/ hydroxide compared with sulphate in contrast to the natural oxidation process. Oxidation in air also led to sulphur-rich species identified as iron-deficient regions below monolayer coverage. Similar regions were present at ground surfaces exposed to air-saturated solution of pH4 and pH5. In near-neutral to alkaline solution mainly iron hydroxy-oxide is formed the layer thickness of which was estimated in the range of 0.5 nm (pH5) to 1.7 nm (pH10).
1. Introduction Pyrite (FeS,) is one of the most abundant metal sulphides and often associated with other sulphides usually considered to be more valuable. Generally they are separated from each other by froth flotation. Oxidation phenomena prior to and during the flotation procedure in aqueous solution are known to influence the effectiveness and selectivity of the flotation process [ll. The products formed during surface oxidation of sulphides may not only affect the interaction between the mineral and the constituents of sulphide mineral flotation systems but can also govern the hydrophilic or hydrophobic behaviour of the surface itself. For example metal-deficient sulphides and sulphur were shown to render the surface hydrophobic [2] whereas hydrophilic metal oxides and hydroxides can create sulphide depression [3,4]. The kind of oxidation product is
* To whom correspondence should be addressed. (341) 6858 225; Tel.: + 49 (341) 6858 497. 0169-4332/93/$06.00
0 1993 - Elsevier
Science
Fax:
+49
Publishers
strongly dependent on the sulphide and can be controlled by the preparation as well as such flotation parameters like pH and potential [3,5,61. Despite of the various analytical studies [7-121 completed by quantum-chemical considerations [13] the oxidation behaviour of pyrite is still unclear. X-ray photoelectron spectroscopy (XPS) has proven to be a valuable technique for monitoring surface oxidation of metal sulphides [2,9,14-161. Several XPS studies on pyrite have identified, sometimes in contradiction, numerous oxidation products such as FeSO,, Fe,(SO,),, FeOOH, Fe,O,, elemental sulphur, a metal-deficient sulphide Fel_,S2 and polysulphides [9,14,16-201. There are indications that this variety is a result of different preparation routes and oxidizing conditions [14,16,21]. A lack of information about pure pyrite surfaces being free of oxygen and carbon contaminations and the often used nonmonochromatized X-ray excitation make it difficult to determine oxidation products in the monolayer region showing only slightly different chemical states compared to the unoxidized pyrite.
B.V. All rights
reserved
S. Karthe et al. / Oxidation of pyrite surfaces
158
Hence, one part of the present XPS study will concentrate on the characterization of pyrite surfaces prepared in situ and ex situ before and after exposure to air under ambient conditions to gain more information on the influence of the preparation on the oxidation behaviour of pyrite. To identify sulphur-rich species more easily a model system S/FeS, has been studied. Besides that, the present work is directed to the surface oxidation of pyrite in air-saturated solution of pH4-10. Previous XPS measurements on the flotation of pyrite by ethyl xanthate solution provide evidence that surface oxidation and collector adsorption are influenced by each other and seem to be strongly dependent on pH [22,23]. Therefore, knowledge of the pyrite oxidation in aqueous solution is the precondition for further examinations of the pyrite/ xanthate system [24,25]. Furthermore, these experiments allow us to elucidate in more detail why pyrite shows only weak collectorless flotation.
2. Experimental
details
2.1. Samples The samples used were compact pyrite pieces of about 10 X 10 X 1 mm. They were cut along the (100) face from a single natural cubic crystal originating from Peru. No significant impurities could be detected by a SEM microprobe bulk analysis. The sulphur-to-iron ratio was determined as 2: 1. The fractured and scraped surfaces were obtained in situ using the equipment of a PHI 5400 ESCA system. The sample produced by fracture showed the typical conchoidal surface. For comparison, a sample polished in air was cleaned by Art etching to remove any oxygen- and carboncontaining contaminations. Polished surfaces were generated in air by wet grinding (Sic 600) and subsequent wet polishing with diamond paste (7 pm). A ground surface was produced by dry grinding (Sic 600) in air. In both cases the samples were then cleaned 5 min ultrasonically in ethanol p.a. The pyrite powder was produced in an agate mortar and pressed into a gold sheet to
ensure a reasonable conductive back contact. Oxidation of dry-ground surfaces in solution was carried out for 30 min in water of ultra-high quality (Elgastat UHQ MkII). The pH, adjusted with H,SO, and NaOH, was kept constant throughout the whole treatment and controlled by conventional techniques. Surfaces were examined after carefully removing the solution residues. The S/FeS, model system was obtained in UHV by deposition of pure elemental sulphur (S,) onto a freshly polished pyrite surface. The substrate temperature was maintained at 130 K in order to prevent loss of elemental sulphur from the mineral surface. 2.2. Measurements and data treatment The XPS measurements were performed in a commercial PHI 5400 ESCA spectrometer. The base pressure was in the lo-’ Pa range. The photoelectron spectra were recorded with monochromatized Al Ka radiation (1486.6 eV). The Au 4f 7,2 line (84.0 eV> was used to calibrate the binding-energy scale the linearity of which was checked by measuring also the Cu2p,,, line (932.6 eV>. Additional SXPS measurements of a surface dry-ground in air were taken at the BESSY synchrotron lab at Berlin. The used photon energies were 500 and 300 eV. The binding energy was referred in these experiments to the internal standard provided by hydrocarbon contamination of pyrite surfaces (284.6 eV>. More details of the spectrometer system and the experimental procedure are given elsewhere [26,27]. S2p spectra were fitted with doublets. The separation and the intensity ratio of their (l/2, 3/2) components were constrained to 1.12-1.20 eV and to 1 : 2, respectively. Generally the spectra were corrected with a Shirley-type background. In order to separate the Fe(II1) contribution from the Fe(B) components in the Fe2p spectra the background was removed using Tougaard’s universal loss function but fitting the parameters B and C [28,291. This was necessary to be sure that the components of the Fe(III12p as well as of the Fe(II)2p doublet satisfy the 1 : 2 intensity ratio and reveal a reasonable energy
159
S. Karthe et al. / Oxidation of pyrite surfaces
separation and peak shape. The spectra displayed are non-smoothed but normalized.
3. Results and discussion 3.1. Pyrite surfaces prepared in UHV Pyrite surfaces fractured and scraped in UHV showed neither oxygen nor carbon contaminations or oxidation products as shown by the missing 0 1s and C 1s photoemissions. Generally, materials containing sulphur in a unique environment, e.g. elemental sulphur or zinc sulphide, exhibit a well resolved single S2p (l/2, 3/2) doublet with an intensity ratio close to 1: 2. However, in the corresponding S 2p spectra of pyrite additional intensity was observed on the low and high binding energy side of the main doublet (see fig. 1). The spectra can reasonably well fitted with four different components A-D (see fig. 2a). The corresponding energy positions are comparable to those obtained previously for similarly prepared specimens and related sulphur-containing compounds (see table 11. According to that the most intense component C is due to the emission from the pyritic sulphur in the bulk whereas the signals A and B are attributed to FeS-like species. The scraped sample can only be distinguished from the fractured one by its slightly enhanced intensity of component A. Obviously, the more the surface is destroyed mechanically the more FeS-like defects are created,
Table 1 Binding energies (eV> of S 2p,,, to reference values; contribution Sample
Fe& Fe& FeS, Fe, -,S FeS, FeS, FeS, a) Excitation
Binding
energy
Fig. 1. S2p photoelectron spectra from pyrite fractured in UHV and for comparison from elemental sulphur. The spectra are shifted so that the maxima of the main peaks coincide.
giving rise to an increased photoemission in the corresponding signal A (see table 1). The feature D is probably caused by energy-loss processes. Since oxygen is absent sulphoxide species can be excluded although their emission belongs to the energy region around 165 eV [30,31]. We suggest that the doublets A and B are caused by a nonideal pyritic lattice. In that case a lattice defect like a vacancy can perturb the charge distribution in the lattice which is reflected in the observed
components of photoelectron spectra from pyrite surfaces to the whole emission (%) in parentheses
differently
prepared
Sample treatment
E, a)
s
(eV)
A
B
C
Fractured in UHV Scraped in UHV Ar+ etching
1486.6 1486.6 1486.6 1486.6 1486.6 200 500
161.1(2) 161.1(4)
161.8(6) 161.8(S) 161.7(71)
162.6 162.7 162.6
161.8 162.0 161.5(26)
162.5 162.7 162.6
Cut in air (100 face) Cleaved in UHV (100) face Dry-ground in air energy.
b, See text.
‘) This work.
2P,,2
components
Ref.
‘)
C) C) C)
1631
161.1 161.2(10)
in UHV compared
[91 [341 C)
S. Karthe et al. / Oxidation of pyrite surfaces
160
62P
I
165.5
I
163.3
I
161.1
Binding energy / eV Fig. 2. Pyrite S2p spectra from (a) a surface scraped in UHV; (b) a surface after Art etching; (cl the same surface after exposure to air for 1 h. For components A-D see text.
chemical shift. Both p- and n-type semiconduction is known for natural pyrite [32,331, so the existence of sulphur- and iron-deficient regions is possible the latter of which might account for the negatively shifted S 2p photoemission consistent with components A and B. Pettenkofer et al. [341 have obtained from SXPS measurements (hv = 200 eV> of the pyrite (100) face cleaved in UHV a S2p line split into three different doublets. They were interpreted in the order of rising binding energy with FeS-like defects (A), FeS, at the surface (Bl and bulk FeS, (Cl (see table 1). In principle this supports our designation given above. The differences can be explained as follows. The enhanced intensity of A is a result of the reduced escape depth due to the lower excitation energy. It was estimated
that instead of 10% only about 1% would contribute to the S 2p line if Mg Ka radiation is used [34] which is close to the value obtained for our fractured surface irradiated by AlKa radiation. According to Pettenkofer et al. [34] the surfaceshifted core-level emission B reflects the changed surface coordination of one sulphur of the dimer in the (100) plane due to the loss of an iron coordination. This may be considered as a special case of an iron-deficient region. While such surface core-level photoemissions undergo a considerable intensity loss compared to the bulk emission caused by the adsorption of oxygen, hydrocarbons or water, the weak component B in the spectrum of the fractured surface exposed to air is still clearly evident. Thus, we conclude that the iron-deficient species are not restricted to the uppermost layer but are also found in underlying regions. In order to confirm this conclusion SXP spectra of a pyrite surface dry-ground in air were measured (see fig. 3). Despite of the contamination layer and the formation of oxidation products the splitting of the S2p emission is maintained. As predicted, the FeS-like emissions in fact are not as sensitive to adsorption as real surface core-level shifted peaks. The limited energy resolution does not allow a reasonable separation of component A from B. The corresponding emission is greatly enhanced by decreasing the excitation energy from 1486.6 to 300 eV.
S2P
164.8
162.8 Binding
energy
160.8 1 eV
Fig. 3. Pyrite S2p spectrum from a surface ground in air (excitation energy 300 eV). For components A-C see iext.
S. Karthe et al. / Oxidation of pyrite surfaces Table 2 Binding energies (eV) of Fe2p,,, components of photoelectron spectra from pyrite surfaces differently prepared in UHV compared to reference values Sample
Sample
FeS, FeS, FeS, Fe, -XS FeS,
Fractured in UHV Scraped in UHV Arf etching
treatment
707.4 707.4 707.4
Polished
707.4
in air
Fe 2p3,,
components 708.8 708-710 708-710 708
Ref. a) a) a) [631 [161
‘) This work.
Excluding a surface-shifted core-level photoemission this behaviour indicates FeS-like defects accumulated in the near-surface region. They may be partly inherent but mostly created by the grinding procedure (more crystal edges). It should be noted that FeS-like species have also been found on dry-ground pyrite by Pillai et al. [23]. Moreover Mycroft et al. [9] have published XP spectra of a pyrite crystal fractured in UHV where an additional minor S2p,,, peak with a binding energy of 161.8 eV was always detected but had not been interpreted. This peak position is consistent with those of component B and supports our findings. The Fe2p 3,2 spectrum is dominated by a narrow line (FWHM = 0.9 eV) both for the fractured and the scraped surface. The binding energy of 707.4 eV is in good agreement with previously reported values (see table 2). Since the metal ion in pyrite is in a low-spin configuration 2p63dh; S = 0 only one final state 2p53dh; S = l/2 is possible. Hence multiplet splitting together with a line broadening should not occur as it is indeed observed. No satellite structure appears so the rule that diamagnetic low-spin compounds do not exhibit 2p,,, satellites [35] is confirmed by pyrite. There is evidence for a weak feature with a binding energy of 708.8 eV being typical for FeSlike emissions (see figs. 4a and 6a). Due to the mechanical treatment the scraped surface is marked by a slight broadening and increase of this feature, both indicating an enhanced number of FeS-like surface regions. This corresponds well to the observed differences in intensity with respect to the S2p spectra. Furthermore, the background on the high binding energy side of the
161
Fe 2p 3,2 peak coming down only slowly is dominated by energy-loss processes due to inelastic scattering and multi-electron excitations. These Fe 2p spectra provide a useful basis for the detection of small amounts of iron oxidation products the broad Fe2p emissions of which might be superimposed on the background sometimes in a rather unspecific manner. It is quite common to produce oxygen- and carbon-free surfaces by Art etching. However, as to pyrite the surface structure and composition is changed due to the preferential sputtering of sulphur 1361. This is indicated by the superposition of FeS-type Fe2p and S 2p photoemissions originating from the sputtered surface and of the minor signal coming from the undisturbed pyritic layers beneath (see figs. 2b and 4b). Those men-
Fe2p
FeOOH/Fe,O, 1 FeS
>
.z :
-E
I
724
I
718 Binding
I
I
712 energy
706
/ eV
Fig. 4. Pyrite Fe2p spectra from (a) a surface scraped in UHV; (b) a surface after Ar* etching; Cc) the same surface after exposure to air for 1 h.
lh2
S. Karthe et al. / Oxidation of pyrite surfaces
tioned first will determine the behaviour of the pyrite surface. Therefore, Ar+ etching is not appropriate for surface pretreatment in the case of flotation-related studies of pyrite. However, as it will be shown later, such a sputtered pyrite surface may act as a representative for pyrrhotite-like compounds in order to demonstrate the different oxidation behaviour compared to pyrite. 3.2. Oxidation
qf pyrite
S2P
% .z k -Tz
____._._.---Y.7.-:_;
in air
L.-r 171.6
Generation of fresh pyrite surfaces either by polishing or grinding or powdering is associated with the adsorption of oxygen as well as carboncontaining species and the formation of oxidation products. The corresponding broad 0 1s features (FWHM = 2.5-3.0 eV> therefore include emission from adsorbed water (Es = 532.3-533.0 eV [14,37]), from hydroxy groups (E, = 531.5 eV [38,39]), carbonyl groups (Ea = 531.2-531.6 eV [401), oxides (En = 530.0 eV [41]) and from different sulphur oxidation products (EB = 531.8-532.5 eV 116,421). In comparison to the spectra of the surface fractured in UHV a small intensity gain for the component and a slightly higher backS2P,,, ground between 165 and 170 eV indicate that sulphur oxidation occurs even during the preparation in air. The effects are more pronounced in the case of pyrite powder, giving two new components at 163.9 and 168.5 eV (see fig. 5).
Table 3 Binding energies the corresponding
Fe 2~a,~
Binding
166.0
I
160.4
energy
/ eV
Fig. 5. S2p spectra from fresh pyrite powder (. ‘) and after exposure to air for 1 h (. -. -.). For comparison the pyrite S2p spectrum from a surface fractured in UHV is shown.
An overall but only slightly increased background in the energy range 709.5-713 eV is present in the corresponding Fe2~~,~ spectrum due to iron oxidation. Again the pyrite powder seems to be most reactive. This can be understood in terms of an enlarged surface area associated with a higher surface energy. The results are summarized in table 3. Apart from the S2p signal at 163.9 eV the formation of iron sulphate and iron oxide or hydroxide is indicated. The S 2p3,* peak position at 168.5 eV is too low to assume either Fe&SO,), or FeSO, .7H,O being formed. Thus in agreement with previous studies [16] the for-
(eV) of Fe 2p,,,, 0 1s and S 2p,,, p hotoelectron values of spectra from pyrite surfaces differently
iron oxidation
products
in comparison
Fe.?,
Fe0
a-Fe,O,
FeSO,
FeOOH
KFe,(OH),60,)2
Fe260,),
[391
[I61
ft61
tt61
[t61
it61
707.4
709.5 530.0
711.0 529.9
711.3 532.5
711.5 530.3 531.4
712.2 532.5
713.5 532.4
168.8
169.5
162.6
7H,O
spectra from various prepared in air
[I61 01s S 2P,,,
I
163.2
I
168.8
169.4
FeS, (polished, ground, powdered) *’ Fe ZP,,~ 0 1s S 2Ps,* h) ‘) This work.
707.4 530.0 162.6 b, Component
at 163.9 eV is omitted.
709.5-713.0 531.5 168.5
531.8
532.4
to
S. Karthe et al. / Oxidation
mation of basic iron sulphate similar to jarosite KFe,(OH),(SO,), is proposed. By that the 0 Is peak maximum of 532.0 eV obtained independently of the preparation can be interpreted as a superposition of 0 1s emissions from the OHand SO,-groups. From the mineralogical point of view pyrite is known to oxidize to Fe(H) and Fe(II1) sulphates such as Fe,SsO,, . 9H,O (coquimbite), Fe(OH)SO, .3H,O (amerantite), Fe(OH)SO, . 4.5H,O (fibrogerrite) [43]. Since reference binding energies are not available the real composition of the basic sulphate identified in our measurements can not be determined. The presence of an oxide-related contribution to the 0 1s emission below 531 eV coupled with additional Fe2p intensity in the energy range 709.5-711.5 eV gives evidence for iron oxides or iron hydroxy-oxide (see table 3). The results are in accord with those published by Brion [16] but in sharp contrast to the mineralogical findings where iron sulphate was produced initially before the formation of oxides and hydroxides like goethite (FeOOH) starts [43]. On the other hand, observations made by Buckley and Woods [141 agree with this natural oxidation process. In order to elucidate the situation the naturally occurring weathering of pyrite was simulated. For that reason a pyrite surface fractured in UHV was first exposed to air for 10 min. This provides no clear indication for any oxidation product. In contrast to that pyrite powder is significantly oxidized after the same exposure time to air. Thus the reactivity of a fractured surface to oxygen seems to be strongly reduced. This is confirmed comparing the Fe2p spectra from pyrite powder and from a fractured surface after 1 h in air. The Fe2p spectrum of the powder is more intensive in the lower energy region around 710-711 eV, indicating more iron oxide and hydroxide than sulphate (see fig. 6~). As to the fractured surface, no sulphate and only a trace of oxidized iron were discernible in the S2p and Fe2p spectra (see fig. 6a and 6b). On the other hand, the sample is significantly oxidized to iron sulphate after prolonged exposure to air as one might expect from the natural weathering process. In that case, the S2p spectrum clearly exhibits additional intensity near 168.5 eV. Further-
163
of pyrite surfaces
Fe2p
I
I
I
724
718
I
712
I
706
Binding energy / eV Fig. 6. Pyrite Fe2p spectra from (a) a surface fractured in UHV; (b) the same surface after exposure to air for 1 h; Cc) a powdered sample after exposure to air for 1 h; Cd) a fractured surface after exposure to air for 14 days.
more, the formation of some iron oxide and hydroxide besides sulphate can be deduced from the intermediate energy position of the Fe(III)2p feature at 711.8 eV (see fig. 6d and table 3). This is confirmed by the 0 1s spectra showing only slight emission from iron oxide or hydroxy-oxide. It seems to us that the more the pyrite lattice at the surface is distorted the more iron oxide and hydroxide is formed compared to iron sulphate. Obviously this is related to an enhanced surface roughness associated with an increased number of crystal edges providing more FeS-like sites and hence favouring the initial formation of iron oxide and hydroxide over sulphate as does pyrrhotite (FeS,_,). Now from that view the quite different results published with respect to pyrite oxidation can be explained as being due to the significant differences in preparation procedure.
S. Karthe et al. / Oxidation of pyrite surfaces
164
The formation of iron oxide and/or hydroxide besides sulphate would require the existence of a further sulphur oxidation product to maintain the original sulphur-to-iron ratio of 2: 1 in the oxidation layer also. Therefore, it was postulated that sulphur remains in the pyrite lattice forming a metal-deficient sulphide in addition to sulphate [14]. Because of the absence of a separate S 2p signal it was suggested that it displays the same binding energy as the sulphur in the mineral itself [14,16]. By observing the S 2p component at 163.9 eV (see fig. 5) first experimental evidence is now provided for such an iron-deficient sulphide species formed on exposure of pyrite to air. Its chemical environment and in-depth distribution will be discussed later in connection with results from oxidation of pyrite in aqueous media. The following equations may describe the simultaneous formation of iron oxide or hydroxide and iron-deficient sulphide: FeS, + +x0,
+ +xH,O
-+ Fe,_,&
FeS, + +x0,
+ Fe,_,S2
+xFeO,
(2)
FeS, + $x0,
--) FelpxS2
+ $xFe,O,.
(3)
Fe,O,
reactions + H,O
2Fe(OH),
may take place,
+ 2FeOOH,
-+ Fe,O,
+ 3H,O.
e.g.: (4) (5)
A S2p feature indicating iron deficiency also appears when Ar+ etched pyrite is exposed to air. Since now the pyrrhotite-like species FeS,-, on pyrite created by etching suffers an iron loss due to oxide and/or hydroxide formation (see fig. 4c) a metal-deficient sulphide with stoichiometry close to pyrite and therefore with a S2p emission near those of pyrite (see fig. 2c) grows in e.g. according to: Fe!&
+ ;yO,
+ $yH,O
+ Fe I--ySZ--x +yFe(OH),.
Sample
Arf
treatment
etched
and 1 h in air
Rel. 0 1s intensity ‘)
Fe, _&
0.70
+
Powdered
and 1 h in air
0.40
+
Fractured
WHV)
0.06
_
a) Emission
and 1 h in air
at 530.0 eV.
b, Emission
‘)
at 163.9 eV.
around 530 eV (see table 4). Again the relation between the degree of surface roughness (increasing from the fractured to the sputtered as well as powdered sample) and the affinity to form iron oxide/ hydroxide associated with an iron-deficient sulphide should be emphasized. 3.3. Oxidation of pyrite in aqueous solution
+xFe(OH),, (1)
Further
Table 4 Relative intensity of 0 Is oxide signal and presence of a sulphur-rich species dependent upon the pyrite surface preparation
(6)
Thus, an Ar+ etched pyrite surface undergoes in analogy with pyrrhotite strong oxidation in air to form iron oxide/ hydroxide which is obvious from the Fe2p spectra and can be proved quantitatively by following the oxide-related 0 1s feature
In the flotation of pyrite the most interesting pH region is between pH4 to pH12. With respect to further experiments on the adsorption of ethyl xanthate our samples were oxidized for 30 min in aqueous phase of pH4 to pH10. The composition of the surface layer of pyrite after immersion in solution of pH4 differs significantly from that observed for samples treated at higher pH. Obviously pH4 is low enough to solve any sulphate and iron oxide/ hydroxide formed during the preparation and to prevent their redeposition. The Fe2p spectra therefore resemble those displayed by pyrite fractured in situ, and the 0 1s spectrum mainly shows the signal from adsorbed water (En = 532.3 eV) (see fig. 7). After immersion of pyrite in solution of 5 I pH 5 10 the XP spectra mainly indicate the formation of rather insoluble Fe(II1) oxidation products with Fe 2p features around 711.5 eV. This is due to the oxidation of the pyritic iron by oxygen solved in water which is promoted by an increasing pH because of the small solubility constant of the Fe(II1) hydroxide being formed. As to this hydroxide, it is not a well defined trihydroxide but is considered to be a water-rich hydrogel of general formula Fe,O, . H,O and often called iron hy-
S. Karthe et al. / Oxidation of pyrite surfaces
droxy-oxide (FeOOHI [44]. It seems to have a composition intermediate between a-FeOOH (goethite) and a-Fe,O, (haematite) [45,46]. A crystal structure has been proposed for a compound whose composition is close to 2Fe,O,. FeOOH .4H,O [47]. The FeOOH gel is supposed to be an amorphous form of this material [48,49]. In agreement, the emissions from oxide groups in Fe,O, and FeOOH (component A) and from OH groups in H,O and FeOOH (component B) dominate the shape and the intensity of the 0 1s spectra (see fig. 7). The corresponding Fe 2p photoemissions display the typical multiplet structure of Fe(II1) in iron hydroxy-oxide at 711.5 eV. It is remarkable that the ratio of iron in the oxidation layer to those in the pyrite beneath (FeO,/FeSUr,) as well as the relative intensity of
1,2
165
*Fe/S
*- l(Ols,)
2,4
..- Feox/Fesub i
/ /’
I’ 0
3
1
I
1
I
I
I
/
4
5
6
7
8
9
10
PH
Fig. 8. Atomic ratio Fe/S, relative 01s intensity of component A and oxidation ratio Feux /Fesub taken from the corresponding S 2p, Fe 2p and 0 1s spectra from a pyrite surface after immersion in distilled water as a function of pH.
534.0
532.0 Binding
energy
530.0 / eV
Fig. 7. Pyrite 01s spectra from surfaces after distilled water at different pH. For components text.
immersion in A and B, see
the 0 1s signal A and the atomic ratio Fe: S follow the same tendency from pH4 to pHl0 (see fig. 8). This correlation unambiguously results from the enhanced deposition of iron hydroxyoxide with decreasing H+ concentration. It is hence possible to use the oxide-related 0 1s emission A which is easy to separate for monitoring the progress of pyrite oxidation in aqueous solution when the pH is increased. The characteristic shape of the curves reveals that pyrite oxidation depends in a complex manner upon pH. The slope of the curve may be affected by thermodynamic aspects like pH-dependent redox potentials or solubility products related to pH and electrokinetic parameters changing with pH. Moreover, according to Brown and Jurinak [12] different preparation methods and further ions present in the solution may influence the oxidation rate at different pH and therefore the shape of the curve. Assuming the pyrite substrate is covered by an iron hydroxy-oxide overlayer its thickness can be
166
S. Karthe et al. / Oxidation of pyrite surfaces
estimated from the Fe(H) and Fe(III) regions of the Fe2p spectra according to Briggs and Seah [50]. In addition, knowledge of the inelastic mean free path A of 800 eV electrons both in pyrite and iron hydroxy-oxide is necessary. Since experimentally determined A-values are not available the value of 2.1 nm [51] calculated for silicon as a semiconductor with a band gap comparable to that of pyrite may serve as a reasonable approximation. With respect to the electron take-off angle of 60” the escape depth is then 1.8 nm. Assuming equal Fe2p ionization cross sections the intensity ratio of the Fe2p signals from nonoxidized pyrite and from iron hydroxy-oxide can be substituted by the ratio F of the iron atoms per space unit in both the substrate and the overlayer. The latter can be determined through the density values p, the molar weight M and the stoichiometry, respectively. Taking into account the complex composition of iron hydroxy-oxide the overlayer thickness is calculated using either goethite with p = 5.0 g/cm3 [52] or haematite with p = 5.0 g/cm3 [52] as a model compound. The experimental intensity rafrom the subtios FeO.JFesUb of the emission strate and from the oxidation layer are determined by curve fitting of the Fe2p spectra. As expected, the layer thickness undergoes a similar change with pH as the terms displayed in fig. 8 (see fig. 9). The layer thickness differs only slightly assuming either goethite or haematite as the representative for the overlayer. Thus, the mixed composition of the iron hydroxy-oxide precipitate disregarding the water content may result only in insignificant deviations from the values given in fig. 9. The thickness of the iron hydroxy-oxide layer after 30 min oxidation in aqueous solution might be in the range of 0.5 nm (pH5) to 1.7 nm (pH10). Slight alterations in the S2p spectra point to the simultaneous formation of sulphur oxidation products. Treatment of pyrite at pH4 increases the relative S2p intensity compared to that of fresh-fractured pyrite, indicating an enhanced surface concentration of sulphur. This is caused by the superposition of a second doublet shifted by 1 eV from the pyrite position (see fig. 10) accounting for about 8% of the total S2p inten-
J
3
4
5
6
7
6
9
10
11
PH Fig. 9. Estimated thickness of the oxidation layer on pyrite after immersion in distilled water as a function of pH assuming either a layer composition equal to goethite or to haematite.
sity. The observation can be understood in terms of a sulphur-rich surface species being formed simultaneously with dissolving of oxidation prod-
S2P
).
c
E
al
-E
I
I
I
I
I
169
167
165
163
161
Binding
energy
/ eV
Fig. 10. Pyrite S2p spectrum from a polished surface after immersion in distilled water at pH4 (’ . -1.For comparison the pyrite S2p spectrum from a surface fractured in UHV is shown.
S. Karthe et al. / Oxidation of pyrite surfaces
167
ucts Gulphate, hydroxide). This overlayer composition is in contrast to that detected on exposure of pyrite to air. In the latter case the sulphur-rich species is still covered with iron oxide/ hydroxide. A sulphur-rich species has also been detected by Buckley and Woods [141 on pyrite oxidized in acetic acid (pH = 3). Such a sulphur-rich surface species was only slightly discernible after immersion of pyrite in solution of pH5 and was no more evident for higher pH. It is therefore suggested that the sulphur-rich species only represents an intermediate product of pyrite oxidation in aqueous solution to form finally sulphate. The major part of this sulphate is transferred to the solution, but a small amount was still retained at the surface. The corresponding S2p intensity at 168.5 eV increases from pH6 to pHl0 up to about 7% of the total S 2p emission. This finding can be explained considering that the precipitation of sulphate ions together with iron hydroxy-oxide gives a basic salt [53-561. 3.4, Iron-deficient sulphide and the model system sulphur /pyrite Mainly an iron-deficient sulphide or elemental sulphur are discussed to describe the chemical binding in the sulphur-rich species observed at the surface of various sulphides, e.g. chalcopyrite, pyrrhotite, pentlandite, bornite [2,14,21,41,57]. XPS analysis of the model system S/FeS, may provide a useful basis for the characterization of the sulphur-rich species identified at the pyrite surface after its exposure to air and to aqueous media of pH4 and pH5 (see previous section). Elemental sulphur has a vapour pressure of about lo* Pa at room temperature. Hence, substrates need to be cooled down below 200 K to prevent loss of elemental sulphur from the surface to UHV. Elemental sulphur, therefore, was evaporated on the pyrite substrate at 130 K. The increasing sulphur coverage gives rise to a continuously growing additional S 2p feature at 164.2 eV. Prolonged sulphur deposition results in an entire depression of the pyritic S2p signal at 162.6 eV. Fitting the S2p spectra taken from samples with different sulphur coverage indicated neither
I
166 Binding
I
I
164
162
energy
/ eV
Fig. 11. S 2p spectra including curve fits from (a) bulk elemental sulphur, (b) a pyrite surface covered with elemental sulphur (c) a polished pyrite surface.
changes in binding energies nor in full half-widths of the S2p doublets compared to those of uncovered pyrite and of bulk elemental sulphur (see fig. 11). If the sample is allowed to warm up to ambient temperature a fast sulphur desorption takes place leaving a pyrite surface indistinguishable from an uncovered surface. A subsequent deposition and desorption cycle of sulphur reveals reversibility of that procedure. Consequently, the deposited sulphur does not influence the chemical environment of the adjacent pyritic sulphur and is completely lost at ambient temper-
S. Karthe et al. / Oxidation of pyrite surfaces
168
Table 5 Ratio F of sulphur atoms per space unit in the pyrite substrate and in the sulphur-rich overlayer (hypothetical composition) calculated from the corresponding densities p, molar weights M, and stoicbiometry V, F
Hypothetical composition
?g/cm’)
FeS, FeS, Fess _
5.0 d’ 3.0 2.4 _
60.0 46.0 39.0
1.00 1.28 1.34
S
2.0 h,
32.1
1.34
d, Ref. [lo].
ML/V, (g/m011
‘) Ref. [44].
ature. In contrast, no intensity loss of the S2p feature assigned to the sulphur-rich species at the oxidized pyrite surface was evident, even after the sample had been subjected to ultra-high vacuum for 24 h. This, in agreement with Buckley and Woods [14], clearly supports the conclusion that an iron-deficient sulphide species rather than elemental sulphur in a stable S, configuration is formed on pyrite. Obviously, the sulphur-rich species only then behaves like elemental sulphur including its volatility if pyrite is strongly oxidized electrochemically [9,58] or by treatment in hot concentrated hydrochloric or nitric acid [14]. The thickness of the iron-deficient sulphide species can be estimated from the S2p emission analogous to that of the iron hydroxy-oxide layer (see previous section). In the case of the kinetic energy of the S 2p electrons (1325 eV) experimental data for the inelastic mean free path of inorganic compounds compiled by Seah and Dench
Table 6 Layer thickness of sulphur-rich FeS,, FeS, and S, respectively
species
System
Sample treatment
Fe,-,S,/FeS,
Polished/polished 30 min at pH5
Fe,_,S,/FeS,
Polished and 30 min at pH4
on pyrite
calculated
depending
x (%) a’ and
5
10
‘) Contribution of Fe, _xS, to the total S 2p photoemission. ‘) Lattice constant of pyrite [ 101.
[59] are in the range from 1.5 to 5 nm. Again the value calculated for silicon [51] as 3 nm may serve as a reasonable approximation. Difficulties arise as to the ratio F of the S2p emission from the bulk substrate and bulk overlayer due to the unknown intermediate composition of the irondeficient sulphide between pure pyrite and pure elemental sulphur. Nevertheless, the composition of iron-deficient sulphides can be simulated by increasing stepwise the sulphur content in the formula (see table 5). The density values p for are calculated from the corresponding Fe%+, values of elemental sulphur (ps = 2.0 g/cm3 [44]) and pyrite (pFeS, = 5.0 g/cm3 [60]) according to +x). The proportion of the P = [P&S +xp,]/(l intensity *created by the iron-deficient layer was obtained by curve fitting. The estimation procedure gives a layer thickness which varies only slightly with the sulphur content of the iron-deficient layer (see table 6). Assuming no lattice reconstruction in the iron-deficient region and considering the nearly (100) orientation of the surface the lattice constant of pyrite (0.54 nm [lo]> was used as a reasonable approximation for the interatomic distance in the iron-deficient species. Dividing the layer thickness by this value gives the coverage in monolayers. It can be seen that the thickness of the iron-deficient layer is only in the range below one nanometer. Even if the layer composition is assumed to be equal to elemental sulphur the thickness is far from one monolayer. Therefore, the term “iron-deficient sulphide” generally used for iron-deficient layers on various sulphides for more accuracy should be
on the sample
preparation
and assuming
a composition
F
d (nm)
a (nm)
ML
1.oo 1.28 1.34 1.00 1.28 1.34
0.13 0.17 0.18 0.27 0.35 0.36
0.54 b’
0.24 0.31 0.33 0.51 0.64 0.67
of
S. Karthe et al. / Oxidation of pyrite surfaces
substituted in the case of pyrite by “iron-deficient surface regions” to emphasize the difference to a bulk-like phase. As to that pyrite behaves significantly different from chalcopyrite which is known to form iron-deficient layers up to l-2 nm [El. On the basis of the results presented and discussed above the generally high flotation recovery of pyrite in acidic media of pH4 [61,62] now can be attributed to the existence of iron-deficient surface regions combined with a low amount of sulphate and iron hydroxy-oxide deposition. At pH5 already the hydrophobizing effect of the sulphur-rich areas is counteracted by the hydrophilic oxidized surface regions being twice as large as the former one (compare fig. 9 and table 6). Consequently, both the weak tendency of pyrite to form sulphur-rich surface species and its strong affinity to form quite insoluble iron oxidation products are now suggested to be the main reasons for the known small ability of pyrite to reveal self-induced flotation.
4. Conclusions Fractured and scraped pyrite surfaces prepared in UHV are characterized by minor FeSlike species accumulated in the near-surface region. They are partly inherent due to lattice pertubations but may also be created by mechanical roughening of the surface. The corresponding emissions have to be taken into account when analysing the Fe 2p and S 2p spectra of differently prepared pyrite. Ar+ etching disturbs the pyrite structure to form FeS-type surface species. Hence, this preparation method is not recommended for flotationrelated surface studies. Preparation (polishing, grinding, powdering) and prolonged oxidation of pyrite in air produces mainly a basic iron sulphate similar to jarosite and iron oxide/ hydroxide that is expected to be hydrophilic. The increased surface roughness due to the preparation seems to favour the formation of iron oxide/ hydroxide over sulphate in contrast to the natural weathering process of pyrite. First experimental evidence is given for a sulphur-rich species produced during oxidation of pyrite in air.
169
These species were identified as iron-deficient surface areas below monolayer coverage. Similar iron-deficient regions were observed for ground pyrite surfaces oxidized for 30 min in aqueous solution of pH4-5. Starting at pH5 iron hydroxy-oxide is precipitated on pyrite the more the pH is increased. The corresponding layer thickness was estimated in the range of 0.5 nm (pH5) to 1.7 nm (pH10). The small ability of pyrite to form hydrophobic Fe 1_xS2 species combined with a high affinity to cover itself with hydrophilic iron oxidation products can now be assumed as the main reason for its comparatively low recovery observed in collectorless flotation.
Acknowledgements The project was partly supported by the Bundesministerium fur Forschung und Technik (BMFT No. NT 2082/4). The authors would like to thank I. Kartio and M. Heinonen for their help in the XPS measurements.
References 111 M.C. Fuerstenau, J.D. Miller and M.C. Kuhn, Chemistry of Flotation (AIME, New York, 1985). [2] A.N. Buckley, I.C. Hamilton and R. Woods, in: Flotation of Sulphide Minerals, Ed. K.S.E. Forssberg (Elsevier, Amsterdam, 1985) p. 41. [3] B. Ball and R.S. Rickard, in: Flotation, Vol. 1, Ed. M.C. Fuerstenau (AIME, New York, 19761 p. 458. [4] K.L. Sutherland and I.W. Wark, in: Principles of Flotation (Aust. Inst. Min. Met., Melbourne, 1955) p. 164. [51 P.J. Guy and W.J. Trahar, in: Flotation of Sulphide Minerals, Ed. K.S.E. Forssberg (Elsevier, Amsterdam, 19851 p. 91. I61 R.A. Hayes and J. Ralston, Int. J. Miner. Process. 23 (1988155. [7] S. Chander and A. Briceno, Miner. Met. Process. 4 (1987) 171. [81 J. Pang, A. Briceno and S. Chander, J. Electrochem. Sot. 137 (1990) 3447. [9] J.R. Mycroft, G.M. Bancroft, N.S. McIntyre, J.W. Lorimer and I.R. Hill, J. Electroanal. Chem. 292 (1990) 139. [lo] R.T. Lowson, Chem. Rev. 82 (1982) 461.
170 [ll] [12] [13] 1141 [15] [16] [17] [18]
[19] [20] [21]
[22] [23] [24] [25] [26] [27] [28] [29] [30]
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