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Contents lists available at ScienceDirect
Hydrometallurgy
journal homepage: www.elsevier.com/locate/hydromet
Electrochemical behaviour and surface analysis of chalcopyrite
in alkalineglycine solutions
G.M. O'Connor, K. Lepkova, J.J. Eksteen⁎, E.A. OrabyWestern
Australian School of Mines, Minerals, Energy and Chemical
Engineering, Curtin University, GPO Box U1987, Perth, WA 6845,
Australia
A R T I C L E I N F O
Keywords:ChalcopyriteGlycineElectrochemical
dissolutionLeachingPassivationRamanXPS
A B S T R A C T
Electrochemical experiments with a chalcopyrite rotating disk
electrode were carried out in alkaline glycinesolutions. This
showed no apparent passivation behaviour during anodic dissolution
that is observed in acidsolutions. The current increased with
applied potential from the open circuit potential with no
resemblance tothe passivation region seen in acid solutions. A
loosely held porous layer developed on the surface
consistinglargely of iron oxyhydroxides that had a limited effect
on the anodic current. Elemental sulfur and a disulfidespecies were
detected using XPS and Raman spectroscopy but did not passivate the
surface as has been proposedfor acid solutions. The disulfide
species is sometimes used to infer a metal deficient sulfide or
polysulfide that isresponsible for passivation but in this study it
had no passivating influence. Current-potential curves
showedfeatures of a non-ideal semiconductor that were explained by
charge transfer via surface states.
1. Introduction
Recent research at the Western Australian School of Mines has
fo-cussed on alkaline leaching of gold, silver and base metals
using glycineas a complexing agent (Oraby and Eksteen 2014, Tanda
et al., 2019).The complexing action of glycine with copper is
enhanced above pH 9.8where the glycinate anion is dominant
(O'Connor et al., 2018). Theanodic dissolution of chalcopyrite in
glycine solutions may form ele-mental sulfur or sulfate by the
following half reactions:
+ + ⇌ + + +
+
CuFeS 2Gly 19OH Cu(Gly) Fe(OH) 2SO 8H O
17e2
‐ ‐2 3 4
2‐2
‐ (1)
+ + ⇌ + + +CuFeS 2Gly 3OH Cu(Gly) Fe(OH) 2S 5e2 ‐ ‐ 2 3 ‐
(2)
Other iron species such as FeOOH, Fe2O3 or Fe3O4 may also form
inalkaline solutions and are collectively referred to as iron
oxyhydroxides(Grano et al., 1997). Other sulfur species such as
thiosulfate may alsoform and will be the topic of further
research.
Chalcopyrite dissolves at a slower rate than supergene and
copperoxide minerals in glycine solutions (Eksteen et al., 2017;
Tanda et al.,2017, 2018a,b). This is also true for a variety of
leaching systems in theacidic pH range and in alkaline solutions
with ammonia or cyanide ascomplexing agents (Marsden, 2006; Razzell
and Trussell, 1963;Stanczyk and Rampacek, 1966; Watling, 2013).
Such slow rates ofdissolution have prevented the development of a
financially viable
hydrometallurgical process for leaching copper from
chalcopyrite.The slow rate of dissolution has been known since the
early 20th
century, particularly for acid solutions (Greenawalt, 1912; Pike
et al.,1930; Sullivan, 1933). Strategies developed to enhance the
leach rateincluded ultra-fine grinding and using near boiling
temperatures. Fromthe 1970s, the slow rate has sometimes been
attributed to the formationof a layer of sulfur, jarosite or a
metal deficient sulfide/polysulfide. Thisinhibits leaching and is
termed “passivation” due to some similaritieswith the well-known
phenomenon in corrosion science. The proposedpassivating species
have been described and critiqued at length in manyreviews (Córdoba
et al., 2008; Klauber, 2008; Li et al., 2013; Watling,2013). Of the
proposed species, the metal-deficient sulfide or poly-sulfide
Cu1-xFe1-yS2 has gained popularity, but the literature is
incon-sistent with regard to its electronic and physical
properties(Ghahremaninezhad et al., 2013; Hackl et al., 1995;
Nicol, 2017c;Parker et al., 1981).
In alkaline systems such as ammoniacal solutions, iron
oxyhydr-oxides are readily observed on the chalcopyrite surface. It
is sometimessuggested that these species act as an inhibiting
species (Beckstead andMiller, 1977; Guan and Han, 1997; Nabizadeh
and Aghazadeh, 2015).Some authors have however noted its porous
nature, suggesting that itis unlikely to be completely protective
(Burkin, 1969; Cattarin et al.,1990; Warren and Wadsworth, 1984).
An alternative proposal is that apassivating metal-deficient
sulfide layer, such as that observed in acidsystems, underlies the
iron oxyhydroxides (Buckley and Woods, 1984;
https://doi.org/10.1016/j.hydromet.2018.10.009Received 17 May
2018; Received in revised form 3 October 2018; Accepted 13 October
2018
⁎ Corresponding author.E-mail address:
[email protected] (J.J. Eksteen).
Hydrometallurgy 182 (2018) 32–43
Available online 15 October 20180304-386X/ © 2018 Elsevier B.V.
All rights reserved.
T
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Cattarin et al., 1990; Chander, 1991; Smart, 1995; Warren
andWadsworth, 1984; Wills and Finch, 2016; Yin et al., 2000). This
layermight inhibit leaching in the same way as proposed for acid
solutions.
The need for a passivation model in alkaline systems seems
super-fluous when electrochemical experiments are examined. In
alkalinesolutions with a complexing agent, the apparent passivating
effect thatis observed in the acid range is notably absent. This
can be seen in theanodic sweeps in alkaline ammonia solutions in
Fig. 1 (Warren andWadsworth, 1984; Warren et al., 1982). Here, the
current increaseswith applied potential. By contrast, in sulfuric
acid solution little cur-rent flows up to a potential of about 0.8
V (vs Ag/AgCl). This lack of apassivating region has also been
noted by other researchers in alkalineammonia solutions or at pH 13
where copper and iron are soluble (Nicoland Zhang, 2017; Yin et
al., 1995).
The slower leach rate of chalcopyrite compared to other
coppersulfides in alkaline solutions is likely not due to
passivation, but isdependent on the presence and concentration of a
suitable oxidant.Studies of oxygen reduction on chalcopyrite in
alkaline solutions haveshown that it is a poor oxidant, with copper
(II) being more effective inammonia solutions (Moyo et al., 2015;
Nicol, 2017b). Further researchinto viable oxidants is needed for
alkaline systems.
An alternative explanation for the apparent passive region in
Fig. 1is that the electronic structure of chalcopyrite dictates its
dissolutionbehaviour. Chalcopyrite, like many sulfide minerals, is
well known forits semiconducting properties and was one of the
first semiconductorsdiscovered (Braun, 1875; Shuey, 1975). The
application of semi-conductor theory to mineral leaching has been
described in detail bymany researchers, based on the work of
Gerischer in the 1960s (Brysonet al., 2016; Crundwell, 1988;
Crundwell, 2015; Gerischer, 1961;Gerischer, 1969; Hiskey, 1993;
Osseo-Asare, 1992; Zevgolis and Cooke,1975).
According to this theory, electron exchange between a
semi-conductor and a redox couple in solution can only occur
between equalenergy levels in both species (Gerischer, 1961;
Gerischer, 1969). Thatis, the potential of a redox couple in
solution must correspond withpopulated energy levels of the
semiconductor. There is however anunpopulated, forbidden region of
energy levels in a semiconductorknown as the band gap. For
chalcopyrite this corresponds to the stan-dard redox potential of
couples such as the ferric-ferrous couple(Crundwell, 1988).
Consequently, the exchange of electrons andtherefore the rate of
dissolution is slow. A suitable redox couple wouldhave a standard
potential outside the energy levels of the band gap,which might
explain the higher rates of leaching with strong oxidants athigh
redox potentials or when the potential is controlled at
relativelylow values (Watling, 2013).
The precise energy levels of the band gap have not been
directlymeasured due to the antiferromagnetic nature of
chalcopyrite (Shuey,
1975). In a theoretical study the energy at the centre of the
band gap,the Fermi level, was calculated from electronegativity
values of theconstituent atoms to be 5.15 eV on the absolute scale,
or 0.45 V (vs Ag/AgCl) (Xu and Schoonen, 2000). With a band gap of
0.6 eV spanning theFermi level, the band edges would be expected to
be at 0.75 V (vs Ag/AgCl) for the valence band and 0.15 V (vs
Ag/AgCl) for the conductionband. A suitable oxidant should
therefore have a potential outside thisrange. This is consistent
with the relative rates of reduction observedfor various oxidants,
and the rapid increase in the slope of current-potential curves at
about 0.75 V (vs Ag/AgCl) (Crundwell, 2015; Parkeret al.,
1981).
Many electrochemical studies have shown that chalcopyrite has
amuch lower open circuit potential (OCP) in alkaline compared to
acidsolutions (Azizkarimi et al., 2014; Moyo et al., 2015; Nicol,
2017b; Yinet al., 2000). For the study in Fig. 1 it is −0.1 V (vs
Ag/AgCl), which isless than the anticipated conduction band edge at
0.15 V (vs Ag/AgCl)and outside the band gap. Electron transfer
therefore initially occursreadily with increasing potential from
the OCP. However, in Fig. 1 thecurrent continues to increase beyond
0.15 V (vs Ag/AgCl) and into theband gap where electron exchange
should not occur for an idealsemiconductor. If the semiconductor
model is to apply, the inclusion ofa more complex model is
required, such as that involving surface states.These states are
energy levels that occur in the band gap at the surface,allowing
electron exchange. This is well known in semiconductor sci-ence and
has been applied to other mineral systems to explain
non-idealbehaviour (Bryson and Crundwell, 2014; Mishra and
Osseo-Asare,1992; Morrison, 1980; Springer, 1970; Tributsch and
Bennett, 1981a).Other complexities to the chalcopyrite band gap may
also cause de-viations from ideal behaviour, particularly with
photo-processes andreflectivity (Bryson et al., 2016; Oguchi et
al., 1980).
The purpose of this study was to fit the dissolution behaviour
ofchalcopyrite in alkaline glycine solutions to either passivation
orsemiconductor theory using electrochemical and surface
analysismethods. The methods used follow closely those of other
researchersand include staircase potential step voltammetry,
chronoamperometryand capacitance measurements as well as the
surface techniques XPSand Raman spectroscopy (Crundwell et al.,
2015; Ghahremaninezhadet al., 2013; Nicol, 2017c; Parker et al.,
1981).
2. Experimental
2.1. Sample details
A high purity chalcopyrite sample was obtained
fromGeodiscoveries Australia. Optical microscopy at various
timesthroughout the study confirmed purity at 95% with small
inclusions ofquartz and feldspars identified by SEM-EDS. The sample
was analysedfor stoichiometry by electron microprobe and showed a
slight excess ofmetal over sulfur over 140 spot locations. This is
consistent with naturalchalcopyrite showing n-type conductivity
(Shuey, 1975). Metal im-purities were highly variable from 0 to
0.02% depending on the spotlocation, consistent with small mineral
inclusions. The thermoelectriccurrent was measured by heating the
positive electrode with a solderingiron and measuring the current
with a digital multimeter. The resultingcurrent was highly variable
but positive, indicating an n-type semi-conductor.
2.2. Electrochemical experiments
All electrochemical tests were carried out using a Bio-logic
VMP3potentiostat. The working electrode was a chalcopyrite core
embeddedin epoxy resin with an exposed surface area of 0.78 cm2.
Test solutionswere made from analytical grade glycine (99%, Sigma
Aldrich) andsodium hydroxide (98%, Sigma Aldrich) using Mili-Q
deionised waterwith a resistivity of 18.2MΩ.cm.
The working electrode was progressively polished to a 3 μm
Fig. 1. Comparison of alkaline (A) and acid (B) current
potential curves. Scanrate 30mV/min (Warren and Wadsworth, 1984;
Warren et al., 1982).
G.M. O'Connor et al. Hydrometallurgy 182 (2018) 32–43
33
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diamond finish, rinsed with DI water and immediately placed in
the testsolution. The electrode was rotated at a set rate and the
temperatureadjusted to target within±1 °C accuracy. Tests were
carried out at1000 rpm and 25 °C unless otherwise specified.
The experiments were carried out in a three-electrode
electro-chemical cell with a working volume of 500mL. The reference
elec-trode was single junction Ag/AgCl (3.5M) held in a Luggin
capillaryplaced close to the working electrode to minimize any
error due to iRdrop. The same distance was maintained between the
reference andworking electrodes for all tests. The counter
electrode was Hastelloy Cwith a surface area of 5.5 cm2. The
working electrode was allowed60min to stabilise before beginning
the experiment.
A staircase potential sweep measurement was carried out with
20steps from the open circuit potential to 1.0 V (vs Ag/AgCl). Each
stepwas held for 20min and the current was recorded, followed by
capa-citance measurements at 1 to 100 kHz with a sinus amplitude of
17mV.Capacitance effects at 1 kHz are presented here, consistent
with otherstudies (Ghahremaninezhad et al., 2010; Olvera et al.,
2016; Warrenet al., 1982) Chronoamperometry measurements were
performed at aplateau in the current- potential curve at 0.65 V (vs
Ag/AgCl) for per-iods of 300 s with intermittent rest intervals at
the open circuit poten-tial.
2.3. Surface analysis
The chalcopyrite electrode was removed from the solution after
atest at a specified potential, rinsed, dried under vacuum before
im-mediate analysis by XPS analysis or Raman spectroscopy. The
solutionused was 0.3 M glycine at pH 10.5. For XPS a Kratos Axis
Ultra DLD X-ray photoelectron spectroscope with a monochromatic Al
source at1486.7 eV was used to characterise the surface species. A
survey spec-trum was collected at binding energies between 0 and
1200 eV andhigh-resolution regional spectra were collected for
copper, iron, sulfur,oxygen and carbon. Charge compensation was
used where samplecharging occurred, typically where thick surface
layers were presentwhich resulted in a reduction in spectrum
resolution. For Ramanspectroscopy, a Labram 1B dispersive Raman
spectrometer with a632.817 nm source and 2mW power was used to
determine the sulfurspeciation.
3. Results and discussion
3.1. General features of current-voltage behaviour
Several researchers have noted that high scan rates for
anodicsweeps can hide the effects of a passive layer due to
insufficient time forit to form (Ghahremaninezhad et al., 2010;
Viramontes-Gamboa et al.,2007). In order to allow time for the
formation of a possible passivelayer at a given potential, a
staircase potential step method was usedwith each step held for
20min while recording the current. Unlessotherwise specified, only
the final current after 20min is presentedhere.
3.2. Effect of pH
The effect of pH on the current-potential curve is shown in Fig.
2.The 0.3 M sulfuric acid run was with no glycine. These curves
show theincreased current response above pH 9, due to the higher
mole fractionof glycinate anion described in other studies (Aksu
and Doyle, 2001;O'Connor et al., 2018). Two plateau regions are
present for all alkalinepH values. The apparent passive region for
the acid conditions is ob-vious, from the open circuit potential to
about 0.77 V (vs Ag/AgCl)where currents are about 0.02mA/cm2. A
rapid increase in currentoccurs above this potential and continued
to increase off the scale of thegraph. At alkaline pH in glycine
the increase in current is more mod-erate, likely limited by the
complexing ability of the glycinate anion.
The current eventually drops or forms a plateau at higher
potentials asglycine is depleted, possibly due to some inhibition
by an oxyhydroxidelayer on the surface.
Higher current densities were observed at pH 10.5 than 11.5
atpotentials above 0.5 V (vs Ag/AgCl), this may be due to copper
oxidesbeing more stable than copper glycinate at this pH. Similar
behaviourhas been observed in copper metal electrochemistry
(O'Connor et al.,2018).
3.3. Effect of glycine concentration
The effect of glycine concentration can be seen in Fig. 3. The
generalshape of the current-potential curves is similar at all
concentrationvalues, but with a less pronounced plateau as the
glycine concentrationincreases. For all concentrations, no
significant effect on the currentdensity was observed up to 0.5 V
(vs Ag/AgCl). In the plateau regionfrom 0.5 to 0.8 V (vs Ag/AgCl)
considerably higher current densitieswere recorded as the
concentration of glycine increased. This is con-sistent with
limitation by transport through a porous layer as opposedto
solution diffusion, which would be observed with changes in
rotationspeed as discussed in Section 3.4. This plateau is unlike
the passivationfor chalcopyrite in acid solutions, where the
current is close to zero. Thecurrent density is eventually limited
by glycine diffusion through thebulk solution at potentials greater
than 0.85 V (vs Ag/AgCl) for the0.1 M and 0.3M solutions as also
shown by the effect of rotation speed.
Fig. 2. Current-potential curves at different pH values in 0.3M
glycine com-pared to 0.3M H2SO4 solution.
Fig. 3. Effect of glycine concentration on current potential
curves at pH 10.5.Glycine concentration is 0.3M and temperature is
25 °C.
G.M. O'Connor et al. Hydrometallurgy 182 (2018) 32–43
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3.4. Effect of rotation speed
The effect of rotation speed is shown in Fig. 4. There is no
significantdifference in the current potential curves at 500 rpm
and above, exceptat high potentials above 0.9 V (vs Ag/AgCl). This
indicates that diffu-sion through the bulk solution is not the
limiting factor for most of thepotential range at 500 rpm and
above. Other researchers have seen nosignificant effect of rotation
rate in the alkaline studies with ammoniasolutions, although not
all of these have investigated the effects at0 rpm. (Guan and Han,
1997; Reilly and Scott, 1977; Warren andWadsworth, 1984). Above 0.9
V (vs Ag/AgCl), there is a dependence ofcurrent density for
rotation rates of 500 rpm and above. This suggestssolution
diffusion plays a role at these potentials, but a final
steadylimiting current density was not observed.
3.5. Effect of temperature
Increasing the temperature from 25° to 60 °C had a positive
effect onthe current density up to about 0.77 V (vs Ag/AgCl), after
which it wasslightly lower (Fig 5).The curve still shows an
increase at 0.77 V (vs Ag/AgCl), but not as great as at room
temperature. There is no drop incurrent at high potentials at 60
°C, which in the previous section wasattributed to the transfer of
glycine to the surface
3.6. Effect of a pre-oxidised surface layer
During the test program, surface species were readily observed
at allpotentials after removing the chalcopyrite electrode from the
solution.Generally, these were a slightly tarnished surface at
potentials less than0.4 V (vs Ag/AgCl), above this potential was a
brown, loosely held layerthat is likely to be iron oxyhydroxides
that are observed in other studies(Grano et al., 1997). The effect
of this layer was investigated by pro-gressively stepping the
potential to 0.65 V (vs Ag/AgCl) before startinga new scan. This
generated a thick oxide layer, with the main effectbeing a lower
current in the plateau regions and above 0.9 V (vs Ag/AgCl) as can
be seen in Fig. 6. The thicker oxide layer had little effect
oncurrent in the active regions around 0.3 and 0.8 V (vs
Ag/AgCl).
3.7. Effect of potential step duration
The time held at each potential step was varied to determine if
therewas an effect from allowing surface layers more time to
thicken andimpede transport of reactants to and from the surface.
No significanteffect was observed up to about 0.5 V (vs Ag/AgCl) as
can be seen inFig. 7. The sample held for 60min at each step
returned a lower currentin the plateau region above 0.5 V (vs
Ag/AgCl).
3.8. Capacitance
Capacitance measurements have been used by several authors
forelectrochemical impedance and Mott Schottky analyses of
chalcopyritein an effort to understand layer formation in situ
(Crundwell et al.,
Fig. 4. Effect of rotation speed on the current potential curves
at pH 10.5 and0.3M glycine concentration at 25 °C.
Fig. 5. Effect of temperature at 25 °C and 60 °C. Glycine
concentration 0.3Mand pH 10.5.
Fig. 6. Effect of a surface layer generated at 0.65 V (vs
Ag/AgCl). Glycineconcentration is 0.3M and pH 10.5.
Fig. 7. Effect of different times at each step at pH 10.5 and
0.3M glycine.
G.M. O'Connor et al. Hydrometallurgy 182 (2018) 32–43
35
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2015; Ghahremaninezhad et al., 2013; Nicol, 2017c). The basis of
thesemeasurements is that a mineral-solution interface is
considered to act asa parallel plate capacitor where the
capacitance, C, is given by Eq. (3)(Bard and Faulkner, 2001):
=C kε A/d0 (3)
where k is the relative permittivity of the dielectric material
betweenthe plates, ε0 is the permittivity of free space, A is the
area of thecharged plate and d is the separation of the plates.
Changes in capaci-tance are therefore a function of these
variables. It has been suggestedthat as a polysulfide passive layer
thickens on a chalcopyrite surface,the separation of charge between
the bulk mineral and solution growswider and so the capacitance
decreases (Nicol, 2017c). This behaviourof chalcopyrite in acid
media was said to be similar to a thickeningoxide layer on a metal.
At high potentials the layer is thought to beoxidised and breaks
down, so the gap narrows, and capacitance in-creases along with an
increase in current. Results in acid solution on thechalcopyrite
used in this study are shown in Fig. 8 and appear to beconsistent
with this theory and compare well with the trends in thestudy by
(Nicol, 2017c).
However, this model based on charge separation does not fit
theobserved behaviour of chalcopyrite in an alkaline glycine
solution. Ascan be seen in Fig. 8, the capacitance decreases with
applied potentialup to 0.77 V (vs Ag/AgCl). This decrease clearly
does not reflect athickening passive layer because the current
increases during this time.At 0.77 V (vs Ag/AgCl) the slope of the
capacitance curve changes signat the same time as an increase in
current, as was observed in acidsolution. While not consistent with
a passive layer formation andbreakdown, this behaviour is as
expected for the inversion region of ann-type semiconductor seen in
Mott Schottky studies (Crundwell, 2015).The curves show little
resemblance to copper with a genuine oxidepassive layer in glycine
solutions, which show a complex behaviour dueto porosity and
semiconducting properties of the duplex Cu2O/CuOlayer (O'Connor et
al., 2018).
3.9. Chronoamperometry
Several authors have proposed that a passivated chalcopyrite
sur-face will reactivate upon removal of potential, based on
interpretationof chronoamperometry (Lu et al., 2000; Nicol, 2017a;
Parker et al.,1981). This has been attributed to the thermal
breakdown of thepolysulfide or solid state diffusion of iron and
copper in the mineral. Asimilar reactivation effect was observed
for this system when an elec-trode was held at 0.65 V (vs Ag/AgCl)
for five-minute periods withvarying rest times at the open circuit
potential. The degree of re-activation increased with increased
rest times as can be seen in Fig. 9,consistent with previous
research in acid solutions (Parker et al., 1981).Copper metal
showed a similar behaviour in alkaline glycine solutions,but with a
strong dependence on stir rate (O'Connor et al., 2018).
A close-up view of the first two current transients in Fig. 10
shows acomplex decay curve. For the initial transient a high
current drops to aminimum within 1–2 s, the current then rises to
reach a maximum
before slowly decreasing again. For the second and each
subsequenttransient there is a small oscillation that gradually
decays, resemblingan under-damped second order system. Similar
patterns have beenobserved in other alkaline studies of
chalcopyrite but are not well de-scribed or understood (Azizkarimi
et al., 2014; Yin et al., 2000).
A current time transient for a stepped anodic sweep over the
wholepotential range is shown in Fig. 11. The most obvious feature
is thechange in mechanism after 0.77 V (vs Ag/AgCl), where the
currentaccelerates for several steps. Before this point the current
shows a spikeand rapid decay as expected in a relaxation process
(Crundwell et al.,2015). The oscillation feature observed in Fig.
10 is beyond the re-solution of this graph, but was present between
0.26 V (vs Ag/AgCl)and 0.82 V (vs Ag/AgCl). The current decay seen
at each step might beargued as evidence of passivation if longer
times are employed, but the
Fig. 8. Comparison of current and capacitance at 1 kHz in 0.3M
H2SO4, left, and 0.3M glycine at pH 10.5, right.
Fig. 9. Chronoamperometry at 0.65 V (vs Ag/AgCl).
Fig. 10. Chronoamperometry at 0.65 V (vs Ag/AgCl). Close-up view
of the in-itial and second current transients of Fig. 9.
G.M. O'Connor et al. Hydrometallurgy 182 (2018) 32–43
36
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results from longer step times of 1 h as shown in Fig. 7 still
yield acurrent greater than that observed for the passive region in
acid solu-tions.
The process of reactivation after a rest at the OCP seen in this
studyand others suggests that if surface species are responsible
for the currentdecay, they will not be present for analysis at a
later time. Analyses ofthe surface are not likely to be of
passivating species, but of other non-passivating reaction
products. Alternatively they could be daughterproducts of a passive
or inhibiting species that has altered upon re-moval from the
system.
3.10. XPS and Raman spectroscopy analysis
Six samples for surface analysis were stepped to potentials
rangingfrom the open circuit potential to 1.0 V (vs Ag/AgCl) and
measured byXPS and Raman spectroscopy in solutions of 0.3M glycine
at pH 10.5. Afreshly polished sample was exposed to the atmosphere
for 12 h andalso measured. At potentials above 0.4 V (vs Ag/AgCl)
the samples hada friable overlayer. The surface underneath was also
measured in areaswhere it had detached during rinsing and handling
for XPS. Completedeconvolution was only attempted for the sulfur
peak. For iron, peaksdistinguishing oxide iron and lattice sulfide
iron were determined.
3.11. Samples oxidised in air, at the OCP and at 0.15 V (vs
Ag/AgCl)
The spectrum for an air oxidised sample showed typical features
of achalcopyrite surface as can be seen by the uppermost trace in
Fig. 12.Copper is present only as Cu (I). Surface iron is in oxide
or hydroxideform and has a low BE shoulder at 708 eV indicating
lattice iron bonded
to sulfur (Buckley and Woods, 1984; Hackl et al., 1995; Luttrell
andYoon, 1984; McCarron et al., 1990). The sulfur spectrum shows
typicalpeaks for a monosulfide and disulfide species. There is no
appreciableindication of polysulfides or an energy loss peak.
The Raman spectrum for the air oxidised sample shown in Fig.
13shows a relatively broad peak typical of chalcopyrite with the
main A1mode at 292 cm−1. A B2/E mode forms a shoulder at 320 cm−1
withanother at 353 cm−1 (Parker et al., 2008). The broad peak width
in-dicates some degree of poor crystallinity compared to those
observed atpotentials of 0.15 V (vs Ag/AgCl) and above. This is
probably an arte-fact of sample preparation.
No significant change was observed in the XPS or Raman spectra
forthe sample held at the open circuit potential for 12 h compared
to theair-oxidised sample. The iron spectrum shows the same
shoulder at708 eV indicating lattice iron bonded to sulfur, and
sulfur shows thesame ratio of monosulfide to disulfide as the air
oxidised sample. Thisratio is similar in other studies that have
attributed these species to ametal-deficient sulfide
(Ghahremaninezhad et al., 2013; Hackl et al.,1995). A minor sulfate
peak was also detected.
The sample stepped to 0.15 V (vs Ag/AgCl) showed a
tarnishedsurface typical of a weathered chalcopyrite. The overlayer
showed noshoulder at 708 eV in the iron spectrum, indicating the
presence of adifferent surface layer. The surface percentage copper
has dropped from1.0% to 0.3% but still gives a strong signal. The
sulfur peak is morecomplex, with the contribution from monosulfur
dropping con-siderably. A new peak at 163.5 eV is possibly an
indication of elementalsulfur. The presence of elemental sulfur in
an ultra-high vacuum ispossible due to the protective nature of
iron oxides (McCarron et al.,1990; Smart et al., 1999). The small
peak at 167.9 eV is assigned tosulfate. The Raman spectrum showed a
distinct narrowing of the mainchalcopyrite peak compared to
previous samples as seen in Fig. 13. Thiswould be expected for a
highly crystalline specimen, which suggests
Fig. 11. Current vs time for a potential step experiment at pH
10.5 with 0.3Mglycine at 25 °C.
Fig. 12. Copper, iron and sulfur XPS peaks of chalcopyrite. From
top to bottom: (a) air oxidised, (b) oxidised at OCP, (c) stepped
to 0.15 V (vs Ag/AgCl). Solution was0.3M glycine at pH 10.5.
Fig. 13. Raman spectrum of oxidised chalcopyrite. From top to
bottom: (a) Airoxidised, (b) Oxidised at OCP, (c) oxidised at 0.15
V (vs Ag/AgCl). Solution was0.3M glycine at pH 10.5.
G.M. O'Connor et al. Hydrometallurgy 182 (2018) 32–43
37
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that disordered or amorphous chalcopyrite is formed during
prepara-tion and dissolved in the initial potential steps. The B2/E
modes arevisible at 320 cm−1 and 353 cm−1.
3.12. Sample stepped to 0.4 V (vs Ag/AgCl)
In this region the current increased with each potential step
and aloose overlayer formed that partially detached from the
surface. Thisallowed both the overlayer and the underlying surface
to be measuredas shown in Fig. 14. Copper is still only present as
Cu (I). For theoverlayer, no shoulder was detected at 708 eV for
lattice iron on thesurface. The sulfur peak was noisy in this
region due to sample char-ging, but a substantial sulfate peak and
decreased monosulfide peaksare evident. Duplicate analyses showed
this was a repeatable peak withabout 5% variation in calculated
sulfur species from the deconvolutionprocess. The underlayer was
similar to the fresh surface, with ashoulder for sulfide iron at
708 eV and a sulfur peak consisting of monoand disulfide.
Raman spectra for the overlayer showed mixed spectra of
poorlycrystalline iron oxyhydroxides, elemental sulfur and
chalcopyrite. Abroad oxyhydroxide peak is at 1313 cm−1 and
overlapping peaks arebetween 650 and 720 cm−1. Elemental sulfur was
detected in cracks inthis oxide overlayer with distinct peaks due
to S-S-S bending at152 cm−1 and 219 cm−1, and SeS stretching at 473
cm−1 (Fig. 15).The surface where the overlayer had detached showed
a highly crys-talline chalcopyrite peak as with the previous sample
and is not re-peated here.
3.13. Sample stepped to 0.65 V (vs Ag/AgCl)
In this plateau region both an underlayer and overlayer were
again
measured (Fig. 16). In the overlayer, again no shoulder denoting
latticeiron was detected. Copper is almost completely obscured from
thesurface in this region with peaks barely above noise. As for the
previoussample, sulfur has a noisy peak due to sample charging
which is diffi-cult to model. It can be stated that the monosulfide
contribution hasdisappeared, leaving a disulfide, elemental sulfur
and a sulfate. Anotherspecies appears to be present at 165.6 eV
that is yet to be positivelyidentified, but is possibly sulfite.
The underlayer has features of achalcopyrite surface in the XPS
spectrum as described in previous sec-tions.
Raman spectroscopy showed the overlayer to be a mix of
poorlycrystalline iron oxyhydroxides and elemental sulfur as shown
in Fig. 17.Thick elemental sulfur was detected in cracks in the
oxide overlayer asevidenced by a mixed spectrum of sulfur and
chalcopyrite. Chalcopyritein the underlayer was in a highly
crystalline form similar to that shownin Fig. 13.
3.14. Sample stepped to 0.85 V (vs Ag/AgCl)
In this active region faint copper peaks were visible in XPS for
theoverlayer but no positive assignments can be made (Fig. 18).
Sulfur ispresent as disulfide, elemental sulfur and sulfate. Raman
spectra mostlyshow a mix of sulfur and chalcopyrite spectra in the
cracks of theoverlayer (Fig. 19).
The underlayer in this region shows a significantly different
sulfurXPS peak compared to other regions, with a contribution from
ele-mental sulfur. Previously it was only detected as a component
in theoverlayer. Iron again showed the shoulder at 708 eV
attributed to ironbonded to sulfur in the lattice.
Fig. 14. Copper, iron and sulfur XPS peaks for chalcopyrite
stepped to 0.4 V (vs Ag/AgCl) in 0.3M glycine at pH 10.5. From top
to bottom: (a) overlayer, (b)underlayer.
Fig. 15. Left: Raman spectrum of overlayer showing mixed spectra
of chalcopyrite, sulfur and oxyhydroxides. Right: elemental sulfur
with minor chalcopyrite inoxide layer cracks. Sample stepped to 0.4
V (vs Ag/AgCl) in 0.3M glycine at pH 10.5.
G.M. O'Connor et al. Hydrometallurgy 182 (2018) 32–43
38
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3.15. Sample stepped to 1 V (vs Ag/AgCl)
The overlayer in this region is similar to that observed at 0.85
V (vsAg/AgCl) as shown in Fig. 20. Sulfur is present as disulfide,
elementalsulfur and minor sulfate with no monosulfide detected. The
underlayershowed a higher than usual disulfide level, but no
elemental sulfur inthis case. Raman spectroscopy again showed
elemental sulfur in theoverlayer cracks and crystalline
chalcopyrite elsewhere in the under-layer (Fig. 21).
3.16. Summary of XPS results
The elemental distribution calculated from survey spectra is
pre-sented as elemental ratios with respect to copper in Table 1.
Thedominant species in most cases are adventitious carbon, oxygen
andnitrogen from the atmosphere. The overlayer shows high iron
valuesdue to the presence of iron oxyhydroxides. The surface is
rich in sulfur(or copper deficient) except for the underlayer at 1
V (vs Ag/AgCl). Theunderlayer at 0.88 V has a higher sulfur to
copper ratio than the otherunderlayers, possibly reflecting its
status as a reaction product in thisregion of high oxidation
rates.
Sulfur speciation is shown in Table 2. The monosulfide species
isattributed to the bulk chalcopyrite lattice (Klauber et al.,
2001). Itscontribution to the total sulfur peak diminishes as the
overlayerthickens and is not present in this layer at 0.65 V (vs
Ag/AgCl) andabove. The disulfide peak by contrast is a major sulfur
species for bothlayers in all samples analysed. The disulfide has
sometimes been at-tributed to a passivating metal-deficient sulfide
at acidic pH, but in thiscase there is no correlation with any
features resembling passivation inthe current potential curves.
4. Discussion
No evidence of the passivation effect that is observed in acid
solu-tions was apparent during anodic dissolution of chalcopyrite
in alkalineglycine solutions. Unlike acidic solutions, the
current-potential curvesshowed no apparent passive region above the
OCP, which is consistentwith other studies in alkaline solutions
with glycine or ammonia (Moyoet al., 2015; Nicol, 2017b; Warren and
Wadsworth, 1984). Trends withcapacitance versus potential show no
resemblance to a metal with athickening passive oxide layer as has
been suggested for chalcopyrite inacid solutions (Nicol, 2017c).
Elemental sulfur and a disulfide speciesthat might be attributed to
a metal-deficient sulfide were present insignificant amounts in
plateau regions and regions of increasing cur-rent. These species
are clearly not passivating at alkaline pH. This lackof a passive
region in alkaline solutions has also been noted by
otherresearchers (Nicol and Zhang, 2017; Yin et al., 1995).
An alternative to the passivation proposal is to apply
semiconductortheory. Chalcopyrite in glycine solutions does not
behave as an idealsemiconductor, but has characteristics
intermediate between a semi-conductor and metal. Metal-like
behaviour can be seen in semi-conductors in several ways. One is by
impurities substituting into thelattice, which is utilized in the
well-known doping process in thesemiconductor industry. Doping has
been suggested by some authors asa reason for metal-like behaviour
in chalcopyrite, the reasoning beingthat natural chalcopyrite has
high impurity levels (Nicol et al., 2016).Such a heavily doped or
degenerate semiconductor is often char-acterised by the absence of
a thermoelectric effect and has been ob-served for synthetic
chalcopyrite doped with zinc (Xie et al., 2016).However, a
thermoelectric effect was observed for the sample used inthis study
and in many others, so there is some doubt that doping of
thechalcopyrite lattice is a cause of metal-like behaviour. Studies
haveshown that impurities in chalcopyrite have little influence on
the
Fig. 16. Copper, iron and sulfur XPS peaks of chalcopyrite
stepped to 0.65 V (vs Ag/AgCl). “S*” denotes an unidentified
species- possibly sulfite in 0.3M glycine atpH 10.5. From top to
bottom: (a) overlayer, (b) underlayer.
Fig. 17. Left: Raman spectrum of mixed sulfur and oxyhydroxide
in the surface overlayer. Right Elemental sulfur in cracks in the
overlayer in 0.3M glycine atpH 10.5.
G.M. O'Connor et al. Hydrometallurgy 182 (2018) 32–43
39
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charge carrier density (Pridmore and Shuey, 1976).Another way in
which metal-like behaviour can be observed in a
semiconductor is when surface states are present in high
density. Thesestates occur through the termination of the lattice
or by adsorbedspecies that create energy levels within the bandgap
(Morrison, 1980).These allow electron exchange at energy levels
within the gap, but thesurface limitation means a thermoelectric
effect is still observed in thesemiconductor bulk. This concept of
a surface state mechanism hasbeen used to explain the
electrochemical behaviour of chalcopyrite andother minerals for
many years (Bryson and Crundwell, 2014; Mishraand Osseo-Asare,
1992; Olvera et al., 2016; Springer, 1970; Tributschand Bennett,
1981b).
In addition to the presence of surface states in the band gap,
metal-like behaviour can be observed when the semiconductor has an
accu-mulation or inversion layer at the surface (Gomes and Cardon,
1982;Morrison, 1980). An accumulation layer is proposed here to
occur onchalcopyrite in alkaline solutions from the OCP at −0.1 V
(vs Ag/AgCl)
up to the conduction band edge at about 0.15 V (vs Ag/AgCl).
Theseelectrons are readily removed from the conduction band when
thepotential is applied, in a similar manner as for a metal (Gomes
andCardon, 1982). Also, if the potential is increased beyond the
valenceband edge, an inversion layer is created and current will
flow via a holemechanism. For chalcopyrite this would occur at
about 0.95 V (vs Ag/AgCl).
Taking these factors into account, band diagrams can be used
tovisualise the mechanisms via accumulation/inversion layers and
viasurface states (Fig. 22). Between the open circuit potential and
0.15 V(vs Ag/AgCl), electrons are removed from the accumulation
layer in aone-step process. At potentials in the band gap between
0.15 and 0.75 V(vs Ag/AgCl), electrons tunnel from surface states
to the conductionband. In the inversion region electrons tunnel
from the valence bandedge at the surface to the conduction band
with mobile holes generatedat the surface. This tunnelling can
occur directly or via bulk defects inthe mineral.
These three mechanisms are distinguished by different
behavioursshown in the current potential curves in Fig. 2 to Fig.
7. From the opencircuit potential to 0.15 V (vs Ag/AgCl), current
increases with eachpotential step followed by a current decay with
time typical of a re-laxation process. A small plateau is observed
at 0.15 V (vs Ag/AgCl),which coincides with the potential of the
conduction band edge. Fromthis point, electron exchange is via
surface states and the current decaycurves resemble an under damped
second order system shown in Fig. 9.This feature remains up to
about 0.82 V (vs Ag/AgCl), after the appliedpotential crosses the
valence band edge.
The two-step surface state mechanism described by other
re-searchers is proposed for the band gap region (Crundwell et al.,
2015;Gerischer, 1969; Vanmaekelbergh, 1997). First, electrons are
removedfrom the surface to form holes at a rate proportional to the
density ofoccupied surface states and the applied potential across
the spacecharge layer. The second step is the reaction of the hole
at the surface to
Fig. 18. Copper, iron and sulfur XPS peaks of chalcopyrite
stepped to 0.85 V (vs Ag/AgCl) in 0.3M glycine at pH 10.5. From top
to bottom: (a) overlayer, (b)underlayer.
Fig. 19. Raman spectrum of mixed sulfur, oxyhydroxide and
chalcopyrite inoverlayer at 0.85 V (vs Ag/AgCl) in 0.3M glycine at
pH 10.5.
Fig. 20. Copper, iron and sulfur XPS peaks of chalcopyrite
stepped to 1.0 V (vs Ag/AgCl) in 0.3M glycine at pH 10.5. From top
to bottom: (a) overlayer, (b)underlayer.
G.M. O'Connor et al. Hydrometallurgy 182 (2018) 32–43
40
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form oxidation products such as copper glycinate, iron oxides
and sulfuror sulfate. The first step is rate controlling up to
about 0.45 V (vs Ag/AgCl) and is characterised by an increasing
current with each potentialstep. Surface layers have no effect on
the rate in this region, as evi-denced by Fig. 6 and Fig. 7. As
glycine is depleted at the surface by
transport through the porous surface layer, the second step is
ratelimiting resulting in the plateau region.
The plateau region ends at 0.76 V (vs Ag/AgCl) which is the
an-ticipated potential for the valence band edge. At this point
electronexchange is no longer via surface states and so the
reaction is notlimited by this mechanism. A change in the slope of
the capacitance-potential curve indicates an inversion region and a
p-type conductionmechanism as seen in other studies (Crundwell et
al., 2015). Currentincreases with potential in this region until it
is finally limited by so-lution diffusion at about 0.9 V (vs
Ag/AgCl). The current potentialcurves obtained in this study
closely resemble the calculated curves fora surface state mechanism
published by Vanmaekelbergh (1997).
The question remains as to why the behaviour is different to
that foracid systems. The surface of chalcopyrite is different
under each system,so it is possible that different surface states
are involved. The currentdensities observed are a function of the
density and occupancy of sur-face states. At around 0.75 V (vs
Ag/AgCl) both systems undergo anincrease in current and a change in
slope of the capacitance curves. Inthis region the surface states
would not be expected to play a role.
5. Conclusion
Anodic dissolution in alkaline glycine solutions did not show
apassive region that is often seen in acidic solutions. This is
despitesurface species being formed that are often attributed to
passivation,such as elemental sulfur and the metal-deficient
sulfide. The anodicdissolution behaviour of chalcopyrite can be
attributed to it being anon-ideal n-type semiconductor, with a high
density of surface states.This research is consistent with recent
studies that show the semi-conducting properties should be
considered in the anodic dissolution ofchalcopyrite (Bryson et al.,
2016; Crundwell et al., 2015; Olvera et al.,2016; Zhao et al.,
2017; Zhou et al., 2015).
Future research on leaching chemistry should focus on applying
amore effective oxidant than dissolved oxygen from air. A
primaryconcern is that it should not break down glycine. Some
progress has
Fig. 21. Left: Raman spectrum of mixed oxide/sulfur in
overlayer; right: elemental sulfur in overlayer crack in 0.3M
glycine at pH 10.5.
Fig. 22. Band diagram for chalcopyrite in alkaline glycine
solutions at differentapplied potentials (vs Ag/AgCl).
Table 1Surface elemental distribution from survey scans.
Holding potential Cu Fe S C O N
Air 1.0 0.6 5.2 78 13 1.6OCP 1.0 0.6 5.6 67 13 3.50.15 V 1.0
12.0 6.0 199 101 140.4 V over layer 1.0 3.5 2.6 95 35 6.40.4 V
under layer 1.0 1.0 2.9 8.1 5.0 0.40.65 V over layer 1.0 14.6 6.9
152 97 150.65 under layer 1.0 1.0 2.6 4.5 1.9 0.30.88 V over layer
1.0 11.7 9.3 190 100 210.88 V under layer 1.0 1.2 5.7 153 16 51 V
over layer 1.0 24.3 9.0 176 114 101 V under layer 1.0 0.7 1.8 5.8
3.5 0.4
Table 2Sulfur species on the friable overlayer and the exposed
surface beneath.
Monosulfide Disulfide Elemental sulfur Sulfate Sulfite
Sample (eV) % (eV) % (eV) % (eV) % (eV) %
Air 160.7 60 162.0 40OCP 160.5 56 162.0 41 167.9 30.15 V 160.5
36 161.7 37 163.5 18 167.8 90.4 V over layer 160.6 9 161.7 40 163.4
33 167.8 170.4 V under layer 160.8 59 162.1 410.65 V over layer
161.8 29 163.9 40 167.9 19 165.7 120.65 V under layer 160.8 58
162.2 420.88 V over layer 162.1 24 163.7 59 167.2 170.88 V under
layer 160.7 52 161.7 28 163.5 211 V over layer 162.0 37 163.9 50
167.7 131 V underlayer 160.9 36 162.1 64
G.M. O'Connor et al. Hydrometallurgy 182 (2018) 32–43
41
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already been made in this area, where Cu2+ was shown to be a
moreeffective oxidant than oxygen in glycine solutions (Nicol,
2017b). Op-timisation of the process may yield more satisfactory
results.
Acknowledgements
The authors acknowledge Dr. JP Veder and Mr. Peter Chapman
ofCurtin University for XPS and Raman spectroscopy and Dr.
MacolmRoberts of the University of Western Australia for microprobe
analysis.Professor Brian Kinsella and staff of the Curtin Corrosion
EngineeringIndustry Centre are thanked for the allocation of
laboratory resources,technical assistance and discussions. Mr
O'Connor's PhD research wasfunded by the Research Training Scheme
through Curtin CurtinUniversity.
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Electrochemical behaviour and surface analysis of chalcopyrite
in alkaline glycine solutionsIntroductionExperimentalSample
detailsElectrochemical experimentsSurface analysis
Results and discussionGeneral features of current-voltage
behaviourEffect of pHEffect of glycine concentrationEffect of
rotation speedEffect of temperatureEffect of a pre-oxidised surface
layerEffect of potential step
durationCapacitanceChronoamperometryXPS and Raman spectroscopy
analysisSamples oxidised in air, at the OCP and at 0.15 V (vs
Ag/AgCl)Sample stepped to 0.4 V (vs Ag/AgCl)Sample stepped to
0.65 V (vs Ag/AgCl)Sample stepped to 0.85 V (vs Ag/AgCl)Sample
stepped to 1 V (vs Ag/AgCl)Summary of XPS results
DiscussionConclusionAcknowledgementsReferences