(Galena> ZnS) coek.info_leaching-behaviour-of-sulphides-in-ammoniacal-thio.pdf

Leaching behaviour of sulphides in ammoniacal
thiosulphate systems
D. Feng, J.S.J. Van Deventer*
Department of Chemical Engineering, The University of Melbourne, Parkville, Victoria 3010, Australia
Received 23 July 2001; received in revised form 27 September 2001; accepted 23 November 2001
Abstract
Systematic studies have been conducted to understand the leaching behaviour and dissolution mechanisms when common
gold-host sulphides such as chalcopyrite, pyrite, arsenopyrite and pyrrhotite are treated by oxidative ammoniacal thiosulphate
leaching. Leach solution composition strongly influenced the sulphide leaching, and the presence of sulphides also enhanced
the decomposition of thiosulphate. The XRD patterns of the binary mixtures before and after leaching indicated that the relative
leaching rates of the sulphides in the ammoniacal thiosulphate system were in the order chalcopyrite>pyrrhotite>arsenopyr-
ite>pyrite, which was in accordance with the observations made from the leaching tests. SEM analysis with the aid of EDAX
indicated the formation of iron oxide at the chalcopyrite, pyrrhotite and pyrite surfaces and the formation of iron arsenate at the
arsenopyrite surface after leaching. SEM analysis also demonstrated that the high-energy defect sites and crystal boundaries
favoured the sulphide leaching. Raman spectroscopy indicated that haematite was formed during the leaching of chalcopyrite.
Iron and arsenic concentrations in the leach solutions were very low due to the formation of iron oxide and iron arsenate during
the leaching reactions. Pyrite enhanced chalcopyrite and sphalerite dissolution. Chalcopyrite and sphalerite also enhanced pyrite
dissolution. D 2002 Elsevier Science B.V. All rights reserved.
Keywords: Sulphide; Oxidation; Leaching; Thiosulphate; Ammonia; Galvanic effect
1. Introduction
Thiosulphate, as an alternative lixiviant for gold,
has received much attention in recent years due to the
growing environmental and public concerns over the
use of cyanide. Acceptable gold leaching rates using
thiosulphate are achieved in the presence of ammonia
with cupric ion acting as the oxidant (Abbruzzese et
al., 1995; Cao et al., 1992; Gong et al., 1993; Langhans
et al., 1992; Jiang et al., 1993; Tozawa et al., 1981;
Zipperian et al., 1988). Despite the multitude of papers
in this area, the industrial applications where thiosul-
phate is used instead of cyanide are apparently limited
to a silver ore plant in Mexico (Wan, 1997). Research
has shown that some ores are well suited to thiosul-
phate leaching, while others show hardly any extrac-
tion. An important reason for this lack of acceptance
into the industry is that the solution chemistry and the
mineralogical factors affecting the effectiveness of
ammoniacal thiosulphate systems are not understood
adequately. Since modern gold and silver leaching has
0304-386X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved.
PII: S0304-386X(01)00225-0
*
Corresponding author. Tel.: +61-3-9344-6620; fax: +61-3-
9344-4153.
E-mail address: jannie@unimelb.edu.au (J.S.J. Van Deventer).
www.elsevier.com/locate/hydromet
Hydrometallurgy 63 (2002) 189–200

been directed to very complex ores, the successful
application of thiosulphate leaching depends not only
on the dissolution behaviour of gold and silver, but
also critically on the behaviour of the associated mi-
nerals. Surprisingly, there is hardly any publication
that deals with mineralogical aspects of ores during
thiosulphate leaching.
Pyrite, chalcopyrite, chalcocite, bornite, pyrrhotite
and arsenopyrite are the main sulphide minerals lock-
ing gold in refractory sulphide gold ores. The leaching
behaviour of these minerals could play an important
role in the thiosulphate leaching of gold sulphide ores.
However, the leaching behaviour of the sulphides still
remains unknown in such a leaching system.
Sarveswara Rao and Ray (1998) conducted sys-
tematic studies on the oxidation behaviour and dis-
solution reaction mechanisms operating while pure
copper, zinc and lead sulphide minerals and mixtures,
and single concentrates are treated by oxidative
ammonia leaching. The sequence of sulphide mineral
dissolution from a quaternary mixture of CuS, ZnS,
PbS and FeS2 follows the order PbS, CuS and ZnS.
FeS2 does not react. In ammonia leaching, the oxida-
tion of sulphur in sulphide minerals is complicated.
For various reasons, the order of oxidation as esti-
mated from the reaction potential does not agree with
those determined experimentally. Majima and Peters
(1966) studied oxidation rates and compared the oxi-
dation order of single sulphide minerals in terms of
decreasing order of oxidisability in ammonia at ele-
vated temperatures as: chalcocite>bornite>chalco-
pyrite>antimonite>galena>pyrrhotite = pyrite = spha-
lerite. Subsequently, Tozawa et al. (1976) made
attempts to describe the order of oxidation reaction
for a complex sulphide bulk concentrate. Without
referring to pyrite and galena minerals, they indicat-
ed the order to be bornite>chalcocite>chalcopyrite>
sphalerite>horbachite>argentite. There appears to be
some ambiguity about the oxidation of pyrite in
ammoniacal medium. The oxidation rate of pyrite is
stated to be lower than other minerals initially but,
subsequently, it perhaps reacts at the same rate as
pyrrhotite and sphalerite (Tozawa et al., 1976). Based
on these results, pyrite is one of the least attacked
minerals during ammonia leaching. However, it is also
reported that pyrite does become leached to some
extent (Tozawa et al., 1976), or dissolves significantly
in the presence of chalcocite (Williams and Light,
1978). Sarveswara Rao et al. (1991, 1992, 1993) exa-
mined the role of galvanic interaction during ammonia
leaching of multi-metal sulphides whereby pyrite
enhances the dissolution of chalcopyrite, sphalerite,
and galena minerals and is itself nearly inert. For am-
monia–ammonium sulphate leaching, the oxidation of
minerals in bulk concentrate follows an order of gale-
na>sphalerite>chalcoprite, in comparison with the
order pyrrhotite>galena>sphalerite>chalcopyrite >
pyrite>chalcocite>bornite>argentite in sulphuric acid
leaching of multi-metal sulphides (Forward and Velt-
man, 1961).
The chemistry of the ammoniacal thiosulphate sys-
tem is very complicated due to the simultaneous pre-
sence of complexing ligands such as ammonia and
thiosulphate, the cupric and cuprous redox couple, and
the possibility of oxidative decomposition reactions of
thiosulphate involving the formation of additional sul-
phur compounds such as tetrathionate (Kerley, 1983).
The presence of sulphides could affect the above series
of reactions. Therefore, the ammoniacal thiosulphate
leaching system is more complicated than the oxidative
ammonia one.
It is the objective of this paper to investigate the
leaching behaviour of pyrite, chalcopyrite, pyrrhotite
and arsenopyrite in ammoniacal thiosulphate leaching
systems. Also examined is the galvanic interaction of
these sulphides during the leaching of binary sulphide
mixtures. An interdisciplinary approach comprising
chemical phase analysis, X-ray diffraction, SEM and
Raman spectroscopy is used to characterise partially
leached residue and the products formed during leach-
ing. Likely reaction pathways are proposed for the
leaching processes.
2. Experimental work
2.1. Minerals and reagents
Pyrite (py), chalcopyrite (cp), pyrrhotite (pr) and
arsenopyrite (ar) samples were obtained from Geolo-
gical Specimen Supplies, Australia. The sulphides we-
re crushed, milled to 100% under 25 mm and stored
in air-tight plastic bags in a refrigerator. Quantitative
XRD was used to determine the mineralogy of the
samples. The analytical results are shown in Table 1.
The pyrite sample was very pure. The pyrrhotite and
D. Feng, J.S.J. Van Deventer / Hydrometallurgy 63 (2002) 189–200
190

arsenopyrite samples contained a small amount of
quartz, while the chalcopyrite sample contained pyrite
and a small amount of sphalerite. Quartz is inert in
ammoniacal thiosulphate leaching solutions (Abbruz-
zese et al., 1995). Therefore, the presence of quartz
does not affect the leaching behaviour of pyrrhotite and
arsenopyrite. However, the presence of other sulphides
does affect the chalcopyrite leaching behaviour due to
the galvanic interactions in the leaching process. Con-
sequently, the leaching behaviour of the chalcopyrite
sample is the bulk effect of chalcopyrite, pyrite and
sphalerite.
Laboratory-grade ammonium thiosulphate, ammo-
nium sulphate and ammonia water (25%) were pro-
vided by Westlab Chem Supply, Australia. Analytically
pure cupric sulphate, hydrogen peroxide (30% w/v),
and hydrochloric acid were obtained from Merck.
Distilled water was used in the experiments.
2.2. Analytical techniques
Elemental concentrations of Cu, Zn, Fe, S, and As
in solutions were determined by ICP-OES, after
oxidising the sulphur species to stable sulphates prior
to the analysis. After oxidation by hydrogen peroxide,
solutions were acidified by HCl and boiled to ensure
complete conversion of the metal species to the chlo-
ride form. The thiosulphate concentration was deter-
mined by iodometric methods. In order to eliminate
the effect of the cupric ammonia complex on iodine
titration, a certain amount of acetic acid (10% sol-
ution) was added prior to the titration with the in-
dicator Vitex. To determine the products of oxidation
of sulphide minerals present in leach residues, the
samples were analysed by SEM coupled with EDAX
(Philips XL30), and the surface morphology data were
also generated at the same time. Raman spectroscopy
was also attempted to identify the surface species.
Chemical phase analyses of the samples were carried
out with a powder diffractometer (Philips, PC1800) in
a 2h range of 10–75 using a copper target.
2.3. Leaching procedure
Experiments were performed in a 1-L baffled open-
top reactor using a magnetic stirrer; 500 mL of solution
was added to 10 g of the sulphide minerals. The stirrer
was maintained at a speed of 800 min 1
. All experi-
ments were performed at a room temperature of about
25 C. Samples were taken continuously at a certain
interval during a total contact time of 24 h. The sam-
ples were centrifuged and filtered for the subsequent
iodine titration and oxidation for ICP analysis. The
leaching of binary mixtures was conducted by using 10
g of mixtures with two different sulphide samples at
the same percentage, and the mixtures were charac-
terised before and after leaching by XRD. The dis-
solution of chalcopyrite was calculated based on the
difference of copper concentrations in solutions; the
dissolution of sphalerite (sp) was calculated based on
the difference of zinc concentrations, and the other
sulphide dissolution was calculated based on the
difference of sulphur concentrations. The increase in
Cu and Zn concentrations in solutions can be con-
verted to the dissolution of chalcopyrite (CuFeS2) and
sphalerite (ZnS) based on the stoichiometric chemical
equations, respectively. Similarly, the increase in S
concentrations in solutions can be converted to the
dissolution of pyrite (FeS2), pyrrhotite (FeS) and ar-
senopyrite (FeAsS) based on the stoichiometric chem-
ical equations, respectively. The pyrite dissolution in
the chalcopyrite sample was calculated on the basis of
mass balance of sulphur.
3. Results and discussion
3.1. Effect of thiosulphate concentration on sulphide
leaching
Fig. 1 shows a plot of percent sulphide dissolution
vs. thiosulphate concentration in a leach solution of
1.0 M ammonia water, 6 mM Cu2+
and 0.25 M sul-
phate. It can be seen that the sulphide dissolution
Table 1
Quantitative XRD analysis of the sulphide samples (mass%)
Mineral Sample
Pyrite Chalcopyrite Arsenopyrite Pyrrhotite
Pyrite 100 31.2 0 0.0
Arsenopyrite 0.0 0.0 93.5 0.0
Sphalerite 0.0 3.5 0.0 0.0
Chalcopyrite 0.0 65.0 0.0 0.0
Quartz 0.0 0.0 6.5 1.6
Pyrrhotite 0.0 0.0 0.0 98.4
D. Feng, J.S.J. Van Deventer / Hydrometallurgy 63 (2002) 189–200 191

decreased with an increase in the thiosulphate con-
centration up to 0.15 M, after which there was no
further decrease in the sulphide dissolution. In the
case of all the sulphides, the dissolution was more in
the absence of thiosulphate than in the presence of it.
This depression effect appeared to be the most prom-
inent in the case of chalcopyrite. In all cases, chalco-
pyrite had the highest dissolution rate among the four
sulphides investigated, followed by pyrrhotite and
arsenopyrite. Pyrite was nearly inert with the lowest
dissolution rate.
Iron and arsenic concentrations remained very low
in the leaching of sulphides as shown in Fig. 2. The
leach solution originally contained 6 mM Cu2+
, 1.0 M
ammonia, 0.25 M thiosulphate and 0.25 M sulphate. It
can be seen that iron concentrations in all the sulphide
leaching systems remained constantly low. However,
the arsenic concentration in the arsenopyrite leaching
system remained the highest at the very beginning of
the leaching process, then gradually levelled off. This
could be attributed to the formation of iron and arsenic
precipitates in the ammoniacal thiosulphate leaching
systems. Similar to the ammonia–ammonium sulphate
leaching systems, iron would precipitate as hydrated
ferric oxide (Sarveswara Rao et al., 1991) while arsenic
would precipitate as iron arsenate (Errington and
Pattinson, 1991). These leaching products were also
verified with SEM coupled with EDAX analysis and
Raman spectroscopy, which are discussed below.
In the presence of dissolved oxygen, the oxidation
reactions of pyrite, sphalerite, chalcopyrite, and pyr-
rhotite during the ammoniacal thiosulphate leaching
can be represented by the following simplified reac-
tions:
2FeS2 þ ð13=2ÞO2 þ 4CuðNH3Þ2þ
4 þ ð6 þ nÞH2O
gFe2O3 nH2O þ 4ðNH4Þ2SO4
þ4CuðNH3Þþ
2 þ 4Hþ
ð1Þ
ZnS þ 2CuðNH3Þ2þ
4 þ ð3=2ÞO2 þ H2O
gZnðNH3Þ4SO4 þ 2CuðNH3Þþ
2 þ 2Hþ
ð2Þ
2CuFeS2 þ ð13=2ÞO2 þ 6CuðNH3Þ2þ
4
þð6 þ nÞH2Og4ðNH4Þ2SO4 þ Fe2O3 nH2O
þ8CuðNH3Þþ
2 þ 4Hþ
ð3Þ
2FeS þ 4O2 þ 2CuðNH3Þ2þ
4 þ ð3 þ nÞH2O
gFe2O3 nH2O þ 2ðNH4Þ2SO4
þ2CuðNH3Þþ
2 þ 2Hþ
ð4Þ
The oxidation reaction of arsenopyrite in such a
leaching system can be expressed as the following
three steps:
4FeAsS þ ð23=2ÞO2 þ 6CuðNH3Þ2þ
4 þ 9H2O
g4ðNH4Þ3AsO4 þ 4FeSO4
þ6CuðNH3Þþ
2 þ 6Hþ
ð5Þ
Fig. 1. Effect of thiosulphate concentration on sulphide dissolution.
Solution: ammonia—1.0 M, Cu2+
—6 mM, sulphate—0.25 M.
Fig. 2. Variation of iron and arsenic concentrations with contact
time.
192

FeSO4 þ CuðNH3Þ2þ
4 þ 3H2O
gFeðOHÞ3 þ CuðNH3Þþ
2
þðNH4Þ2SO4 þ Hþ
ð6Þ
ðNH4Þ3AsO4 þ FeðOHÞ3
gFeAsO4 þ 3NH4 OH ð7Þ
It should be noted that some intermediate sulphur
products such as polysulphur species and elemental
sulphur could also form in the oxidative leaching of the
sulphides. However, these intermediate sulphur spe-
cies could be further oxidised to sulphate. For the
sake of simplicity, all the sulphur species are assumed
to be completely oxidised to sulphate. All the above
equations are just the overall oxidative reactions of
the sulphides. The actual oxidative reactions are far
more complicated, and have not been reported before
in the literature. In the presence of free thiosulphate,
the Cu(NH3)2
+
complex will be converted to be
Cu(S2 O3)3
5
.
Thiosulphate can be oxidised in the presence of the
oxidants cupric tetra-ammonia complex and O2, with
the reactions involved being:
2CuðNH3Þ2þ
4 þ 2S2O2
3 þ4H2O g2CuðNH3Þþ
2
þ 4NH4OH þ S4O2
6 ð8Þ
2S4O2
6 þ 3OH
gð5=2ÞS2O2
3 þ S3O2
6
þ ð3=2ÞH2O ð9Þ
3S2O2
3 þ 6OH
g 4SO2
3 þ 2S2
þ 3H2O ð10Þ
2CuðNH3Þ2þ
4 þ SO2
3 þ 2OH
þ 3H2O
gSO2
4 þ 2CuðNH3Þþ
2 þ 4NH4OH ð11Þ
2CuðNH3Þþ
2 þ 4NH4OH þ ð1=2ÞO2
g2CuðNH3Þ2þ
4 þ 2OH
þ 3H2O ð12Þ
The leaching of arsenopyrite first formed soluble
ammonium arsenate and then precipitated as ferric
arsenate. This could contribute to the higher levels of
arsenic concentrations at the beginning of the leaching
process. It could be expected that ferric oxide formed
simultaneously along with the dissolution of iron-bear-
ing sulphides dissolved in the leach solutions.
In the ammoniacal thiosulphate leaching systems,
both the cupric tetra-ammonia complex and O2 acted as
oxidants. Thiosulphate was not stable and could easily
be oxidised especially in the presence of the cupric
tetra-ammonia complex, following the reaction path-
ways shown in Eqs. (8)–(12). The oxidation of thio-
sulphate would consume O2, which served as the
oxidant itself for sulphide oxidation as well as the oxi-
dant for the conversion of the Cu+
to Cu2+
ammonia
complexes. Thiosulphate could be considered as the O2
scavenger, resulting in an insufficient O2 content in the
leach solutions for sulphide oxidation. Therefore, thio-
sulphate depressed the oxidative dissolution of sul-
phides. The leaching of chalcopyrite generated a large
amount of copper ions, which combined with ammonia
to form more of the cupric ammonia complex for chal-
copyrite oxidation. The formation of a large amount of
the cupric ammonia complex would enhance chalco-
pyrite dissolution, contributing to the highest dissolu-
tion rate among the four sulphides. On the other hand,
the high dissolution rate of chalcopyrite also demanded
a high oxygen supply. Consequently, the presence of
thiosulphate had the most significant depressing effect
on chalcopyrite dissolution (Fig. 1).
3.2. Effect of ammonia concentration on sulphide
leaching
Ammonia water was used as the slurry pH regu-
lator as well as the ligand for the formation of the
cupric tetra-ammonia complex. In addition, ammonia
itself could also act as the ligand for the leached heavy
metal ions such as iron in the leaching process. The
iron ammonia complex so formed was quickly con-
verted to FeOOH and then to Fe2O3nH2O in such an
oxidative basic environment. Therefore, the ammonia
concentration would play an important part in the
sulphide leaching. Fig. 3 shows the effect of ammonia
concentration on the sulphide leaching. The leach
solutions originally contained 6 mM Cu2+
, 0.25 M
thiosulphate and 0.25 M sulphate. It can be seen that
the sulphide dissolution rates increased with an
increase in the ammonia concentration as expected.
The ammonia concentration had the most significant

effect on chalcopyrite dissolution. The leach rates of
pyrrhotite, arsenopyrite and pyrite showed only a
marginal increase with an increase in the ammonia
concentration. The ammonia will complex copper in
the leaching of chalcopyrite as shown in Eq. (3). The
high ammonia concentration would shift the equili-
brium to the right, enhancing the chalcopyrite dis-
solution.
3.3. Effect of cupric concentration on sulphide lea-
ching
Fig. 4 shows the effect of cupric concentration on
sulphide leaching. The leach solutions originally con-
tained 0.25 M thiosulphate, 1.0 M ammonia and 0.25
M sulphate.
Fig. 4 shows that the sulphide leaching rates were
much lower in the absence of the cupric ion. The
sulphide leaching rates increased with an increase in
the cupric concentration. However, a further increase in
the cupric concentration beyond about 6.3 mM would
no longer increase the sulphide dissolution rates. With-
out the cupric ion, the leaching system was very similar
to ammonia–ammonium sulphate leaching and only
oxygen acted as the oxidant. The presence of the cupric
ion would shift the oxidative dissolution reactions of
the sulphides Eqs. (1)–(5) to the right. In the leaching
processes, the stirring speed remained constant at all
conditions without an extra addition of oxygen. It could
be expected that the oxygen supply in the leach
solutions remained constant in all cases. The leaching
processes were dependent not only on the cupric
concentration but also on the dissolved oxygen content,
which converted Cu+
to Cu2+
. At a fixed level of oxy-
gen content, the sulphide dissolution rates would no
longer increase with increase in the cupric concentra-
tion beyond a certain value, as shown in Fig. 4.
3.4. Effect of sulphate on sulphide leaching
In the ammoniacal thiosulphate leaching system,
sulphate is used to stabilise the thiosulphate in the
leach solutions. Based on the oxidative reactions of
thiosulphate shown in Eqs. (8)–(11), the final oxida-
tion product of thiosulphate is sulphate. The presence
of sulphate would shift the equations back to the left
hand side, hence stabilising thiosulphate. However,
the oxidative dissolution of the sulphides would gen-
erate sulphate as shown in Eqs. (1)–(5). Therefore, it
is expected that the presence of sulphate in the leach
solutions would depress the sulphide dissolution. Fig.
5 shows the effect of sulphate on sulphide leaching.
The leach solutions originally contained 6 mM Cu2+
,
0.25 M thiosulphate and 1.0 M ammonia. As indica-
ted in Fig. 5, sulphide dissolution decreased with the
addition of sulphate. Fig. 5 shows that the leaching
kinetics of chalcopyrite were almost linear, while the
leaching of the other three sulphides was fast at the
beginning and became linear with time. The formation
of elemental sulphur, iron oxide and iron arsenate pre-
cipitates would cover the reaction sites at the sulphide
particle surfaces as the sulphide leaching proceeds.
Fig. 3. Effect of ammonia concentration on sulphide leaching.
Solution: Cu2+
—6 mM, thiosulphate—0.25 M, sulphate—0.25 M.
Fig. 4. Effect of cupric concentration on sulphide dissolution.
Solution: thiosulphate—0.25 M, ammonia—1.0 M, sulphate—
0.25 M.
194

This could be the reason why the sulphide leaching
kinetics slowed down with time.
3.5. Effect of sulphides on thiosulphate decomposition
Fig. 6 shows the thiosulphate decomposition in the
presence of sulphides. The leach solutions originally
contained 0.25 M thiosulphate, 6 mM Cu2+
, 1.0 M
ammonia and 0.25 M sulphate.
Fig. 6 shows that the presence of sulphides in the
leaching systems enhanced the decomposition of
thiosulphate. In the absence of sulphides, the thiosul-
phate decomposition was only about 3% during 24 h
of contact. However, the degree of thiosulphate de-
composition was different for different sulphides. Py-
rite induced the highest thiosulphate decomposition
(28%), followed by arsenopyrite (18%), chalcopyrite
(12%) and pyrrhotite (10%).
3.6. Leaching of binary artificial mixtures
The chalcopyrite sample was a mixture of chalco-
pyrite, pyrite and sphalerite. Therefore, the mechanical
mixture of chalcopyrite and pyrite contained a higher
percentage of pyrite and lower percentages of chalco-
pyrite and sphalerite than the natural chalcopyrite
sample. Fig. 7 shows the leaching of the chalcopyrite
and pyrite mixture at a ratio of 1:1.
Fig. 7 shows that the chalcopyrite and sphalerite
leaching was enhanced by the addition of the extra
pyrite. The pyrite leaching was also enhanced in the
presence of chalcopyrite and sphalerite in comparison
with the pure pyrite leaching. Sphalerite had the highest
leaching rate in the bulk leaching of sphalerite, chalco-
pyrite and pyrite, followed by chalcopyrite. Pyrite had
the lowest leaching rate among the three minerals.
Sphalerite and chalcopyrite leaching kinetics were app-
roximately linear, while the leaching of pyrite gradually
levelled off, similar to the observation in the pure pyrite
leaching. The leaching behaviour of pure chalcopyrite
and sphalerite was unknown in the ammoniacal thio-
sulphate leaching system, so that the galvanic effects of
pyrite–chalcopyrite, pyrite–sphalerite and sphalerite–
Fig. 5. Effect of sulphate on sulphide leaching. 0 M denotes no
sulphate and 0.25 M denotes 0.25 M sulphate. Solution: Cu2+
—6
mM, thiosulphate—0.25 M, ammonia—1.0 M.
Fig. 6. Effect of sulphides on thiosulphate decomposition. Blank
denotes no sulphides in the leach solutions. Solution: thiosulphate—
0.25 M, Cu2+
—6 mM, ammonia—1.0 M, sulphate—0.25 M.
Fig. 7. Leaching of the artificial mixture of pyrite and chalcopyrite.
(cp) means the chalcopyrite sample, (py) the pyrite sample and
(cp + py) the mixture of pyrite and chalcopyrite samples. Solution:
Cu2+
—6 mM, thiosulphate—0.25 M, sulphate—0.25 M, ammo-
nia—1.0 M.

chalcopyrite coupling could not be quantified in the
experiments. However, it can be inferred that the
sphalerite and chalcopyrite leaching was enhanced in
the presence of pyrite, based on their higher leaching
rates at higher pyrite ratios in the mix-ture. The pyrite
leaching was also much improved in the presence of
chalcopyrite and sphalerite, although the overall leach-
ing rate of pyrite was still very low. This is in accord-
ance with the findings of Sarveswara Rao et al. (1992),
where pyrite enhanced the dissolution of chalcopyrite
and sphalerite, and chalcopyrite and sphalerite im-
proved the dissolution of pyrite as well, due to the
galvanic coupling between sulphide minerals.
Fig. 8 shows the leaching behaviour of chalcopyrite
and sphalerite in the binary mixtures of chalcopyrite
and the other three sulphide samples. It can be seen
that both chalcopyrite and sphalerite leaching rates
increased in the presence of pyrite, pyrrhotite and ar-
senopyrite. The effect of pyrite, arsenopyrite and py-
rrhotite on chalcopyrite leaching enhancement was in
the order pyritearsenopyritepyhrrotite. Similarly,
the effect of pyrite, arsenopyrite and pyrrhotite on
sphalerite leaching enhancement was in the order
prpyar. Because of the presence of pyrite in the
chalcopyrite sample, it was impossible to quantify the
leaching behaviour of arsenopyrite and pyrrhotite in
the mixtures, which will be discussed qualitatively in
the following section by use of XRD analysis.
3.7. XRD diffraction patterns for binary sulphide mix-
tures before and after leaching
XRD diffraction patterns were recorded for the
binary sulphide mixtures of cp–py, cp–pr, cp–ar,
py–pr, py–ar and pr–ar before and after 24 h leach-
ing, respectively. The leach solution originally con-
tained 6 mM Cu2+
, 0.25 M thiosulphate, 0.25 M sul-
phate and 1.0 M ammonia water.
In the binary system of pyrite and chalcopyrite, the
pyrite intensity increased markedly after leaching,
while the chalcopyrite intensity decreased correspond-
ingly. The change for the sphalerite intensity was very
hard to identify due to its minor amount. Although a
small amount of pyrite was dissolved in the leaching
process, the pyrite intensity still increased after leach-
ing because the relative percentage of pyrite in the
leach residue increased due to the sharp decrease in
the chalcopyrite amount.
The chalcopyrite intensity shown in the binary
system of chalcopyrite and pyrrhotite decreased notice-
ably after leaching, while the pyrrhotite intensity re-
mained almost the same. Therefore, chalcopyrite dis-
solved to a larger extent and the pyrrhotite dissolved to
a lesser degree. In the binary system of chalcopyrite and
arsenopyrite, the chalcopyrite intensity decreased to
some extent after leaching, while the arsenopyrite
intensity decreased to a lesser extent. It could be ex-
pected that chalcopyrite dissolved more than arseno-
pyrite.
The pyrite intensity in the binary system of pyrite
and pyrrhotite increased markedly after leaching,
while the pyrrhotite intensity decreased slightly. It
could be expected that pyrrhotite dissolved more than
pyrite. In the binary system of pyrite and arsenopyrite,
the pyrite intensity increased slightly and the arsen-
opyrite intensity decreased slightly. Furthermore, the
quartz intensity also increased slightly. It can be
inferred that arsenopyrite dissolved more than pyrite.
The quartz intensity in the binary system of pyr-
rhotite and arsenopyrite greatly increased after leaching
and the arsenopyrite intensity also increased noticeably,
while the pyrrhotite intensity decreased correspond-
ingly. It could be inferred that pyrrhotite dissolved more
than the arsenopyrite.
In summary, the XRD patterns of the binary mix-
tures before and after leaching indicated that the
leaching of the sulphides in the ammoniacal thiosul-
Fig. 8. Leaching behaviour of chalcopyrite and sphalerite in the
binary mixtures of chalcopyrite and the other three sulphide samples.
(cp) means the chalcopyrite sample, (cp + py) the mixture of pyrite
and chalcopyrite samples, (cp + ar) the mixture of chalcopyrite and
arsenopyrite samples, and (cp + pr) the mixture of chalcopyrite and
pyrrhotite samples. Solution: Cu2+
—6 mM, thiosulphate—0.25 M,
sulphate—0.25 M, ammonia—1.0 M.
196

phate system was in the order cpprarpy, which
was in accordance with the observations on the leach-
ing tests.
3.8. Topological studies
In order to study the reaction products at the
leached particle surfaces as well as the surface mor-
phology, the leached sulphide lumps were subjected to
SEM analysis coupled with EDAX. The sulphide
lumps (about 10 mm in diameter) were polished with
a 1200 grade sand paper for the removal of the
oxidation layer prior to the 24-h leach. The leached
sulphide lumps were rinsed with distilled water and
dried under vacuum overnight. Figs. 9–12 show the
SEM images of chalcopyrite, pyrite, pyrrhotite and
arsenopyrite, respectively.
There was a large area of erosion at the chalcopyr-
ite surface (Fig. 9) and the surface was very rough and
loose, approaching a porous structure. The EDAX
analysis indicated the presence of iron oxide at the
erosion surfaces, especially at the surface defect sites
and the crystal boundaries. This may be because the
leaching of chalcopyrite started from the high-energy
defect sites and the crystal boundaries. Also illustrated
in Fig. 9, there was some localised erosion at the
surface of the small pyrite particle surrounded by the
chalcopyrite matrix. Erosion was also observed at the
boundaries between the pyrite and chalcopyrite crys-
tals. This phenomenon could explain the galvanic
effect between chalcopyrite and pyrite in the ammo-
niacal thiosulphate leaching system.
The pure pyrite surface was still very smooth after
leaching except for some minor erosion at the surface
defects (Fig. 10). The EDAX analysis demonstrated
that only a small amount of oxygen associated with the
sulphur and iron was present. It can be inferred that
only a thin layer of iron oxide formed on the pyrite sur-
face, which would hinder the further oxidative leach-
ing of pyrite. This observation was in accordance with
the above experimental results, i.e. the pyrite leaching
rate was higher at the beginning and gradually levelled
off.
A large area of erosion was observed at the leached
pyrrhotite surface (Fig. 11). The surface also appeared
like a porous structure with very fine pores evenly
distributed across it. There was some erosion at the
large defect sites at the surface. The EDAX analysis
indicated that a large amount of iron oxide phase was
present at the eroded pyrrhotite surface. Similarly, the
layer of iron oxide formed at the pyrrhotite surface
would hinder the further oxidative leaching of pyrrho-
Fig. 9. SEM image of leached chalcopyrite.

tite, which was in accordance with the above exper-
imental observations.
Like pyrrhotite, the leached arsenopyrite surface
also showed a large area of erosion (Fig. 12). The
leached surface also had fine pores on it. Severe ero-
sion was observed at the defect sites and the crystal
boundaries. This was because the high-energy defect
sites and crystal boundaries favoured the leaching
process. The EDAX analysis demonstrated that a type
of iron arsenate existed at the leached sites.
Fig. 11. SEM image of leached pyrrhotite.
Fig. 10. SEM image of leached pyrite.
198

3.9. Characterisation of the leach product by Raman
spectroscopy
Attempts were made to use Raman spectroscopy to
identify the reaction species on the leached sulphide
lump surfaces. However, the only reaction product
identified was haematite, which was formed at the
leached chalcopyrite surface (Fig. 13). The failure of
this method to identify other products is due to the
small amounts of products in comparison with the
unreacted matrices. Chalcopyrite dissolved substan-
tially in the leaching process, and hence a high ratio of
haematite was formed in the leach residue.
4. Conclusions
The leaching of sulphides in an ammoniacal thio-
sulphate system is dependent on the solution compo-
sition and the mineral types. On the other hand, the
leaching of sulphides influences the oxidation of thio-
sulphate. The following conclusions can be drawn
from the results of this study:
. The relative leaching rates of sulphides were in
the order chalcopyritepyrrhotitearsenopyritepy-
rite. The chalcopyrite leaching kinetics were nearly
linear. However, the leaching of the other three sul-
phides was fast at the start and gradually levelled off.
This was attributed to the formation of an iron oxide or
arsenate layer at the particle surface, hindering further
exposure of the sulphides to the leach solution. The
iron and arsenic concentrations remained at low levels
in the leaching process due to the formation of pre-
cipitates.
. Thiosulphate depressed the leaching of sulphides
due to its preferential oxidation over sulphides. Thio-
sulphate consumed the dissolved oxygen in the leach
Fig. 12. SEM image of leached arsenopyrite.
Fig. 13. Raman spectra of chalcopyrite surface before and after
leaching.

solution, which was essential for the direct oxidation
of sulphides as well as the conversion of the Cu+
to
Cu2+
ammonia complexes.
. Ammonia acted as the pH regulator and the
ligand for Cu2+
to form the cupric ammonia complex.
An increase in ammonia concentration up to a certain
level resulted in an enhanced sulphide leaching.
. An increase in Cu2+
concentration up to a certain
level enhanced the sulphide leaching.
. Because sulphate was one of the final leaching
products for all the sulphide minerals, the presence of
sulphate would shift the equilibrium back to the sul-
phide side. Therefore, the presence of sulphate de-
pressed the sulphide leaching.
. The presence of sulphides in the leach solution
resulted in an increased oxidation rate of thiosulphate.
Pyrite induced the highest thiosulphate decomposition,
followed by arsenopyrite, chalcopyrite and pyrrhotite.
. Pyrite enhanced chalcopyrite and sphalerite dis-
solution. In return, chalcopyrite and sphalerite also
enhanced pyrite dissolution. This could be clearly ob-
served in the SEM image, where pyrite revealed ero-
sion at the pyrite–chalcopyrite boundaries.
. SEM analysis with the aid of EDAX indicated the
formation of iron oxide at the chalcopyrite, pyrrhotite
and pyrite surfaces and the formation of iron arsenate at
the arsenopyrite surface after leaching. In addition,
SEM analysis demonstrated that the high-energy defect
sites and crystal boundaries favoured the sulphide
leaching. Raman spectroscopy indicated that haematite
was formed during the leaching of chalcopyrite.
Acknowledgements
The financial support from Newcrest Mining Li-
mited, Placer Dome Technical Services Limited and
the Australian Research Council is gratefully ack-
nowledged. Appreciation is also expressed to Hui Tan
and Fay Lim for assistance in the experimental work.
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