MURDOCH RESEARCH REPOSITORY This is the author’s final version of the work, as accepted for publication following peer review but without the publisher’s layout or pagination. The definitive version is available at : http://dx.doi.org/10.1016/j.hydromet.2011.11.011 Senanayake, G. (2012) Gold leaching by copper(II) in ammoniacal thiosulphate solutions in the presence of additives. Part I: A review of the effect of hard– soft and Lewis acid-base properties and interactions of ions. Hydrometallurgy, 115-116 . pp. 1-20. http://researchrepository.murdoch.edu.au/7498/ Copyright: © 2011 Elsevier BV It is posted here for your personal use. No further distribution is permitted.
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MURDOCH RESEARCH REPOSITORY
This is the author’s final version of the work, as accepted for publication following peer review but without the publisher’s layout or pagination.
The definitive version is available at :
http://dx.doi.org/10.1016/j.hydromet.2011.11.011
Senanayake, G. (2012) Gold leaching by copper(II) in ammoniacal thiosulphate solutions in the presence of additives. Part I: A review of the effect of hard–
soft and Lewis acid-base properties and interactions of ions. Hydrometallurgy, 115-116 . pp. 1-20.
http://researchrepository.murdoch.edu.au/7498/
Copyright: © 2011 Elsevier BV It is posted here for your personal use. No further distribution is permitted.
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Gold leaching by copper(II) in ammoniacal thiosulphate
solutions in the presence of additives I. A review of the effect of
hard-soft and Lewis acid-base properties and interactions of
ions
G.Senanayake* Parker centre, Faculty of Science and Engineering, Murdoch University, Perth, WA 6150 Australia
Abstract
Gold leaching in thiosulphate media has attracted renewed interest of many
researchers over the last three decades due to fast kinetics in the presence of some
oxidants/ligands and adaptability to gold ores which are unsuitable for direct cyanidation.
The acidity (pH) and concentration of copper(II), ammonia, thiosulphate and
polythionates, produced by the reaction between copper(II) and thiosulphate and other
added background reagents including chloride, carbonate, sulphite, sulphide, phosphate
and cations such as silver(I), lead(II) and counter ions of thiosulphate salts affect the rate
per unit area and extent (%) of gold dissolution. The literature data on beneficial and
detrimental effects of background reagents on gold dissolution from rotating discs,
suspended foils and particles of gold ores/concentrates are reviewed. Reaction
mechanism(s) with applications to leach systems are also reviewed and discussed on the
basis of the Lewis acid-base and hard-soft properties and interactions of metal ions and
anions. These interactions determine the ability of background reagents to affect the
residual copper(II) concentration and EH, surface reaction with gold by catalytic action
and/or by avoiding/removing passivating layers on gold.
Keywords: Gold, thiosulphate, leaching, background reagents, ion-properties, redox
potentials
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1. Introduction
Berezowsky and Sefton (1979) demonstrated that precious metals in the residue
from oxidative leaching of copper sulphide concentrates in ammonia/ammonium sulphate
can be subjected to cupric promoted ammoniacal thiosulphate leach. Since then, the
thiosulphate leaching and recovery of gold and silver has attracted the attention of many
researchers. Thiosulphate can be a possible alternative for cyanidation, especially for
copper-gold, pregrobbing, refractory, or high-silver ores/concentrates where cyanidation
is unsatisfactory due to low gold/silver extraction and/or high reagent consumption
(Berezowsky and Sefton, 1979; Zipperian et al., 1988; Abbruzzese et al., 1995; Schmitz
et al., 2001; Ritchie et al., 2001; Aylmore and Muir, 2001; Kononova et al., 2001;
Molleman and Dreisinger, 2002; Navarro et al., 2002; Ficeriova et al., 2002, 2004;
Fleming et al., 2003; Grosse et al., 2003; Wan and LeVier, 2003; Muir and Aylmore,
2004, 2005; West-sells and Hackl, 2005). Copper(II) which can be maintained at a higher
concentration compared to dissolved oxygen (0.25 mM) at ambient conditions, is a faster
oxidant than oxygen in thiosulphate medium. It acts as a redox mediator in ammoniacal
thiosulphate solutions for gold oxidation according to the anodic and cathodic reactions
Au = Au(I) + e- and Cu(II) + e
- = Cu(I), respectively, where Cu(I) produced in the
overall reaction in Eq. 1 is oxidised to copper(II) by oxygen to complete the redox cycle
(Ritchie et al., 2001).
Au + Cu(NH3)42+
+ 2S2O32-
= Au(S2O3)23-
+ Cu(NH3)2+ + 2NH3 (1)
2Cu(NH3)42+
+ 8S2O32-
= 2Cu(S2O3)35-
+ 8NH3 + S4O62-
(2)
The anodic oxidation of gold in ammonia free thiosulphate solutions is also
enhanced by the presence of copper(II). This has been confirmed by electrochemical
studies at pH 7 and 12 and stirred reactor leaching tests of gold in oxygenated
thiosulphate solutions. The presence of copper in the form of ions, metal or oxide
enhanced the rate of gold dissolution (Zhang and Nicol, 2005). The use of thiosulphate-
sulphite mixtures in non-ammoniacal solutions in the presence of Cu(II) and Tl(I) also
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avoid the formation of passivation products and enhance gold and silver leaching which
can be employed for residues of autoclave and bacterial operations (Gudkov et al., 2010).
One of the major issues in thiosulphate leaching is the reaction between Cu(II)
and S2O32-
ions, as it can lead to a lower residual Cu(II) concentration and diminished
rate of gold leaching. Moreover the formation of tetrathionate (Eq. 2), which undergoes
further degradation/hydrolysis, and the production of polythionate/sulphide ions, can be
detrimental to leaching, reagent consumption and downstream processing (Chu et al.,
2003; Breuer and Jeffrey, 2003, Feng and van Deventer, 2006). For example, the
decomposition products of S4O62-
can lead to the formation of S and CuS which may
passivate the gold surface (Feng and van Deventer, 2006). Muir and Aylmore (2005)
reviewed the early work and highlighted the lack of understanding related to the
passivation products on a gold surface during anodic oxidation or leaching in thiosulphate
media, and the beneficial role of ammonia.
The degradation products of S2O32-
can also compete with Au(S2O3)23-
ions for the
adsorption onto anion exchange resins during separation (Nicol and O’Malley, 2001,
2002). Unlike Au(CN)2-, Au(S2O3)2
3- is not adsorbed by carbonaceous matter. This is
advantageous in the leaching of carbonaceous gold ores, but detrimental in a thiosulphate
flowsheet for gold as activated carbon cannot be used to separate gold from the pregnant
thiosulphate liquors. However, Young et al. (2008) demonstrated a potential application
of activated carbon, loaded with copper(I) cyanide, for the adsorption of gold(I) from
thiosulphate leach liquors by displacement (Eq. 3).
Au(S2O3)23-
+ Cu(CN)2- = Au(CN)2
- + Cu(S2O3)2
3- (3)
Drawbacks such as the toxicity of ammonia, pregrobbing by clay minerals,
consumption of reagents due to side reactions of copper(II) with thiosulphate and/or host
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minerals, and the detrimental effects of the degradation products of thiosulphate and/or
host minerals on downstream processing have impeded the development of a thiosulphate
leaching system (Aylmore and Muir, 2001; West-Sells et al., 2003; Muir and Aylmore,
2004, 2005; Feng and van Deventer, 2007a,b). Yet, the non-suitability of cyanidation for
some ores/concentrates and the growing concerns of toxicity and environmental issues
related to the use of cyanide in some parts of the world have led to further interest and
studies on leaching, separation and recovery of gold using solvent extraction, ion-
exchange, cementation, or precipitation in thiosulphate media (Zhao et al., 1997; Kejun et
al., 2004; Nicol and O’Malley, 2001, 2002; Choo and Jeffrey, 2004; Hiskey and Lee,
2003; Lee and Hiskey, 2008; Navarro et al., 2004; West-Sells and Hackl, 2005; Young et
al., 2008, Jeffrey et al., 2010). Research activities related to non-ammoniacal and non-
copper thiosulphate and mixed thiourea-thiosulphate leaching systems using iron(III) as
the oxidant with oxalate or EDTA as the stabilising ligands, and anaerobic in-situ
leaching operations, show continuing interest in developing a thiosulphate based gold
extraction process (Ji et al., 2003; Arima et al., 2004; Chandra and Jeffrey, 2005; Zhang
et al., 2005; Heath et al., 2008; Feng and van Deventer, 2010b).
The pre-oxidation of sulphide ores with oxygen, prior to the introduction of
thiosulphate, can be beneficial in some cases (Feng and van Deventer, 2007b; 2010a;
Alonso-Gomez and Lapidus, 2008). Although the rate and extent of leaching and reagent
consumption are governed by the mineralogy of gold-silver ores/concentrates, various
background reagents can cause beneficial or detrimental effects depending on their ability
to interact with Cu(II) and S2O32-
species involved in Eqs. 1-2, gold and/or silver
surface, and host minerals. The leaching of silver and gold-silver alloys is faster than pure
gold in both cyanide and thiosulphate media (Li et al., 1992; Webster, 1996; Jeffrey,
2001). Addition of low concentrations (< 2-3 mg/L) of silver(I) or lead(II) in the form of
nitrate has a beneficial effect on gold cyanidation, but excess lead(II) precipitated as PbO
shields gold from the solution and retards dissolution (Wadsworh and Zhu, 2003,
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Tshilombo and Sandenbergh, 2001; Sandenbergh and Miller, 2001). The beneficial effect
of low concentrations of lead(II) on gold cyanidation kinetics is due to the underpotential
deposition of lead (Sandenbergh and Miller, 2001). Low addition of lead(II) (< 2-3 mg/L)
is also beneficial for gold leaching in thiosulphate media, but excess lead(II) causes
passivation due to the formation of PbO or Pb(OH)2 on the gold surface (Feng and van
Deventer, 2002b; Xia and Yen, 2008). The beneficial role of silver(I) in thiosulphate
medium remains unclear. In the case of cyanidation, a film of mixed AuCN.AgCN (or
AgAu(CN)2) formed on the gold surface is considered to be more porous than a tight film
of AuCN (Wadsworth and Zhu, 2003). The higher solubility of gold in oxygenated
thiosulphate solutions in the presence of silver has been related to the formation of mixed
metal ion complexes of the type AgAu(S2O3)22-
(Webster, 1986).
Metallic iron and ferric ions decrease gold leaching efficiency due to passivation
of the gold surface by iron hydroxides (Feng and van Deventer, 2010c). The addition of
de-passivating reagents has beneficial effects (Xia and Yen, 2008; Jeffery et al., 2008b;
Feng and van Deventer, 2010c; 2011a,b). For example, Na2CO3 or Na3PO4 remove the
passivation layer of lead(II), and improve gold leaching (Xia and Yen, 2008; Alonso-
Gomez and Lapidus, 2009). As shown in Fig. 1a, some reagents (Na2SO4, EDTA, Na2S,
glycine, Na2SO3) have beneficial effects causing higher gold extraction as well as lower
thiosulphate consumption after 24 h (Xia et al., 2003). The beneficial effect of sulphide
in Fig. 1a during thiosulphate leaching is consistent with the successful operation of some
gold plants with pregnant solutions containing up to 15 mg/L sulfide ions (Weichselbaum
et al., 1989). A low concentration of sulfide up to 1.6 mg/L is beneficial for the
cyanidation of pure gold, but sulphide has a detrimental effect on the cyanidation of gold-
silver alloys due to passivation by Ag2S/Au2S which can be removed by adding Pb(II) to
precipitate PbS (Tshilombo and Sandenbergh, 2001, Jeffrey and Breuer, 2000;
Senanayake, 2008a). The use of CaS2O3 instead of Na2S2O3 also has beneficial effects on
gold extraction using thiosulphate (Feng and van Deventer, 2010d). Fig. 1b shows the
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beneficial effect of other additives such as EDTA, sodium hexametaphosphate (SHMP)
and carboxymethyl cellulose (CMC) on gold/silver extraction where the thiosulphate
consumption follows the order: none < EDTA < SHMP < CMC (Feng and van Deventer,
2010b; 2011a,b). A common feature of the structures of these beneficial reagents in Fig.
2 is the presence of phosphate, carboxylate and amino functional groups which can
coordinate/chelate with transition metals.
A better understanding of the effect of various background reagents on leaching
performance is useful for the development of thiosulphate gold-silver processing options
using economically viable and effective reagents. The aims of this paper are to:
(i) briefly review the ion properties/interactions which are responsible for the
stability of ions participating in gold leaching and main side reactions,
(ii) briefly review the current understanding of the main reaction
schemes/mechanisms for thiosulphate/gold oxidation by copper(II),
(iii) briefly review the effect of background reagents and thiosulphate
degradation products on residual copper(II) concentration and the rate of
gold dissolution per unit surface area, and
(iv) rationalise the literature data on the effect of background reagents on gold
leaching on the basis of various interactions of background reagents with
key components in solution and/or at the gold surface.
2. Hard/soft and Lewis acid/base properties of ions
Simple cations and anions interact with weakly polarized water molecules
(hydration) by ion-dipole and hydrogen-bond interactions, respectively. According to the
concept of hard and soft acids and bases (HSAB) proposed by Pearson (1973), metal ions
prefer ligands of the same kind (soft-soft or hard-hard) over those of different kinds (soft-
hard) when forming coordinated bonds. Such metal-ligand interactions play an important
role in rationalising the salt solubilities and reactions relevant to hydrometallurgical
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processes. The early data compiled by Hancock and Martell (1989) show a decrease in
hardness of anions/ligands in the order:
OH- (0.0) > SCN
- (-0.082) > NH3 (-0.088) > Cl
- (-0.100) > SO3
2- (-0.107) > Br
- (-0.108)
> S2O32-
(-0.119) > I- (-0.122) > NCS
- (-0.128) > (NH2)2CS (-0.135) > CN
-(-0.148),
and cations in the order:
Ca2+
(12.16) > Mg2+
(10.46) > Pb2+
(6.69) > Zn2+
(4.26) > H+ (3.04) > Cu
2+ (2.68)
> Hg2+
(1.63) > Cu+ (-1.30) > Ag
+ (-10.6) > Au
+ (-16).
Although there is no direct linearity between the softness and hardness of ions,
these two terms are related to the polarizability and electrostatic field strength (ze/r2) of
ions, respectively, where z = valency, e = electronic charge and r = ion radius (Marcus,
1997). The numerical values assigned for softness or hardness for cations (Mz+
) and
anions (Az-
) are based on the overall energy change involved in the two steps for
cations: (i) M(g) → Mz+
(aq) + ze-, (ii) M
z+(g) → M
z+(aq), and
anions: (i) A(g) + ze- → A
z-(g), (ii) A
z-(g) → A
z-(aq).
Step (ii) involves the hydration of the gaseous ion by ion-water interactions due to the
donation of electron pairs (H2O:) to form coordinate bonds with cations or the acceptance
of weakly charged protons of the water molecule (–Hδ+
) to form hydrogen bonds with
anions, respectively.
i.e. H2O:→Mz+
(donation of an electron pair to coordinate H2O with Mz+
),
or Az-…..
δ+H–O–H
δ+ (hydrogen-bond formation of A
z- with H2O).
Aqueous cations (soft or hard) act as Lewis acids by accepting non-bonding electrons
from ligands (Lewis bases) to form coordinative bonds. The Lewis acidity of an ion
(Kamlet-Taft parameter, α) describes the ability to donate a hydrogen bond or accept an
electron pair, and the Lewis basicity (β) describes the ability to donate an electron pair
(Marcus, 1997).
Fig.3a plots the softness of a range of cations and anions of different valency,
with respect to H+ and OH
- ions of zero softness, by convention. Fig.3b plots the Lewis
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acidity and basicity of cations and anions respectively, without changing the order of ions
in the abscissa. The general trend in Figs. 3a and 3b is that the anions of higher softness
have lower Lewis basicity, while divalent cations have higher Lewis acidity compared to
monovalent cations (Cu2+
> Mg2+
> Zn2+
> Pb2+
> Ca2+
>> Ag+ > Cu
+ > NH4
+ > K
+ >
Na+). Fig. 4 plots the Lewis basicity of 14 anions as a function of softness to show the
correlation: Lewis basicity = 1.21 {softness}2 - 2.66{softness} + 1.77, ignoring the two
outliers S2-
and F-. The Lewis basicity of SO3
2- has not been quoted by Marcus (1997).
The relationship in Fig. 4 can be used to predict the Lewis basicity for SO32-
(0.55) based
on the value of softness (0.66) reported by Marcus (1997).
Anions of higher softness in Fig. 4 correspond to poor donators of electron pairs
to metal ions. Thus, softness of ions can be used to rationalise the relative stability of the
cation-anion interaction in water which can influence the stability of ion-pairs and metal-
ligand complexes in water relevant to hydrometallurgical processes (Senanayake, 2004a,
2008b). The measured solubility of metal ions in sea water (pH~8) can be rationalised on
the basis of the solubility products (KSP) of carbonate sediments of divalent or
monovalent metal ions. The values of pKSP (= -log KSP) of carbonates increase with the
increasing softness of cations, causing a lower solubility of metal ions in the ocean
(Senanayake, 2011). In this review, the softness and the Lewis acidity/basicity of ions are
used to rationalise the relative stability of various Au(I) complexes and the effect of
background reagents on the reactions between copper(II), thiosulphate and host minerals.
3. Ion properties and stability constants
Based on the relative values of softness, S2-
is considered as a soft ion (1.09)
compared to PO43-
which is a hard ion (-0.78). As for the gold ligands, the softness
plotted in Fig. 3a (Marcus, 1997) increases in the order SO42-
(-0.38) < Cl- (- 0.1) < OH
-
(0) < Br- (0.17) < CN
- (0.41) < I
- (0.5) < HS
- (0.65) < SO3
2- (0.66) < SCN
- (0.85) < S
2-
(1.09). Thus, as expected from the high softness of Au+
(Fig. 3a) and the HSAB rule
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(Pearson, 1973) of favourable interactions between soft metal ions and soft ligands, the
reported values of stability constants (log βn, n = 1,2) of gold(I) complexes show a
general increase with increasing softness of the ligands (Senanayake 2004a). Likewise,
the values of log βn of complexes formed between S2O32-
and M2+
(M = Pb, Cu, Zn) listed
in Table 1 increase with increasing softness of the cations: Zn2+
< Cu2+
< Pb2+
. In
contrast, the values of log βn (n = 1, 2, 3) of complexes formed between S2O32-
and Mz+
(Mz+
= Na+, NH4
+, Ca
2+, Pb
2+) listed in Table 1 increase in the ascending order of Lewis
acidity: Na+ < NH4
+ < Ca
2+ < Pb
2+, showing the increasing ability of these cations to
accept electron pairs from S2O32-
.
The close values of 0.44 and 0.41 for the monovalent ions Au+ and CN
- (Fig. 3a),
respectively, makes the formation of Au(CN)2- very favourable with a very high stability
constant of β2 = 1038.3
reported in the literature (Hogfeldt, 1982). Although the softness of
S2O32-
ion is not quoted by Marcus (1997), the high softness of sulphur containing
ligands explains the higher stability constants of gold(I) complexes with such ligands
reported in the literature: Au2S22-
(1041.1
) > Au(HS)2- (10
30.1, 10
32.8) > Au(SO3)2
3- (10
26.8)
> Au(S2O3)23-
(1026.0
) > Au(HS)0 (10
24.5) > Au(SO3)
- (10
12.3) > Au(S2O3)
- (10
10.4)
compiled by Senanayake (2004c). Successful stripping of gold(I) thiosulphate loaded
onto anion exchange resins using NaCl/Na2SO3 (Jeffery et al., 2010) and the observed
shift in the reduction potentials of gold thiosulphate solutions with the addition of
sulphite to the gold plating baths (Green and Roy, 2006) can be related to the favourable
soft-soft interactions between Au(I) and SO32-.
This causes the formation of mixed Au(I)-
S2O3-SO3 complexes evident from potentiometric and spectroscopic studies (Perera et al.,
2005).
Whilst Cu(II) reacts with host sulphide minerals lead(II), Ca(II), Na2S have
beneficial effects on gold dissolution, but excessive amounts of Pb(II) and Na2S
passivates the gold surface (Feng and van Deventer, 2002a,b, 2007a; Anderson, 2003;
Senanayake, 2007). These effects as well as the observed beneficial effects of
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background reagents on high gold extraction and low thiosulphate consumption described
in Figs. 1a-b, and the enhanced gold leaching by ammoniacal copper(II) thiosulphate in
the presence of NaCl (Li and Kuang, 1998) highlight the relevance of ionic interactions
of both gold(I) and copper(II) with thiosulphate and other anions in this study. An
enhanced understanding of these aspects would enable rationalisation of the
beneficial/detrimental effects in terms of the interaction of background reagents with (i)
gold surface, (ii) coordination sphere of gold(I) or copper(II), or (iii) host minerals.
4. Anion softness and residual coper(II)
The softness of complex species such as Cu(NH3)42+
is not available for the
purpose of comparison. Nevertheless, Figs. 5 and 6 show that the residual Cu(II)
concentration three hours after mixing solutions of Cu(II)+NH3+Na2S2O3 with NazX, is
larger with anions (Xz-
) having lower softness. As the softness increases the Cu(II)
stabilising power of the anions, by donating electrons to coordinate with Cu(II), seems to
decrease in the order: Na3PO4 > Na2SO4 > NaCl > NaNO3 > Na2SO3 >Na2S, leading to a
decrease in residual Cu(II) (Fig. 5). Very low residual copper(II) with Na2SO3 and Na2S
(Fig. 5) is a result of the direct reaction of Cu(II) with SO32-
and HS- (Eqs. 4-5).
2Cu(NH3)42+
+ 2SO32-
+ H2O = 2Cu(NH3)2+ + 2NH4
+ + SO4
2- (K = 10
21.6) (4)
2Cu(NH3)42+
+ HS- = 2Cu(NH3)2
+ + S + NH4
+ + 3NH3 (K = 10
10.9) (5)
Cu(NH4)42+
+ HS- = CuS + NH4
+ + 3NH3 (K = 10
19.6) (6)
The precipitation of CuS according to Eq. 6 is consistent with the high value of
pKSP representing the higher stability of CuS in Fig. 5. All three reactions are
thermodynamically favourable as revealed by the large equilibrium constants at 25oC
shown in brackets, based on thermodynamic information from Hogfeldt (1982) and the
HSC 6.1 database (Roine, 2002). Moreover, the increase in pKSP of Cu(II) salts in order
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of increasing softness shown in Fig. 5 is also consistent with the decreasing coordinating
ability PO43-
> CO32-
> OH- > S
2-. Thus, the anions (X
z-) with lower softness are better
coordinators leading to stabilisation of Cu(II) and retarding the reaction with S2O32-
.
5. Effect of sulphur species and cations on residual copper(II)
Fig. 6 shows the effect of low concentrations (10 mM) of K2S4O6, Na2S3O6,
Na2SO3, and Na2S2O5 in the absence or presence of a concentrated background salt (0.4
M Na2SO4) on residual copper(II), 3 h after mixing with a standard solution of initial
concentrations: 10 mM Cu(II), 0.1 M Na2S2O3 and 0.4 M NH3, at 30oC (Breuer and
Jeffrey, 2003). The residual Cu(II) concentration in Fig. 6 decreases from 10 mM to
about 5.5 mM after 3 h due to the reaction with S2O32-
(Eq. 2) indicating 45% reaction, in
the absence of other additives. Thus, Fig. 7 shows the effect of low concentrations (5-10
mM) of Na2S4O6, Na2S3O6 or Na2S on the percentage of Cu(II) as well as S2O32-
reacted
as a function of time at 25oC. Here, the initial concentrations were slightly different: 6
mM Cu(II), 0.1 M Na2S2O3 and 0.5 M NH3 (Feng and Van Deventer , 2007a). The figure
shows about 30% reaction of 6 mM Cu(II) after 3 h in the absence of other additives (line
(i)). However, S4O62-
and S3O62-
can also decompose in alkali according to the reactions
in Eqs. 7-11 producing S2O32-
, SO32-
, SO42-
and S2-
(Naito et al., 1970; Zhang and
Dreisnger, 2002), again with large equilibrium constants at 25oC based on the HSC 6.1
database. Thus, negative values of % S2O32-
reacted (≤ -10%) in Fig. 7 indicate the
generation of S2O32-
according to reactions 7-11:
4S4O62-
+ 6OH- = 5S2O3
2- + 2S3O6
2- + 3H2O (K = 10
10.0) (7)
2S3O62-
+ 6OH- = S2O3
2- + 4SO3
2- + 3H2O (K = 10
55.7) (8)
S3O62-
+ 2OH- = S2O3
2- + SO4
2- + H2O (K = 10
40.0) (9)
S4O62-
+ 3OH- = 1.5S2O3
2- + SO3
2- + 1.5H2O (K = 10
18.9) (10)
10S4O62-
+ 34OH- = 13S2O3
2- +38/3SO3
2- + 4/3S
2- + 3H2O (11)
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According to Fig. 6, polythionates, sulphite and metabisulphite cause a decrease
in residual Cu(II) in the order: S3O62-
> SO32-
> S4O62-
> S2O52-
. The residual Cu(II)
concentration after 3 h can be as low as 1 mM in some cases, indicating about 90%
reaction with some of these anions. The rapid reaction of Cu(II) with S2-
and S4O62-
is
evident in Fig. 7, where 68% and 44% of Cu(II) reacted with 5 mM S2-
(line vi) or S4O62-
(line ii) after 1 h, compared to 99% and 64% Cu(II) reacted with 10 mM S
2- (line vii) or
S4O62-
(line iii). In comparison, S3O62-
shows no significant effect on residual Cu(II)
(lines (i), (iv) and (v)). Kinetic studies show that the production of S2O32-
by the alkaline
decomposition of S4O62-
closely agrees with the stoichiometry in Eq. 10 (Zhang and
Dreisinger, 2002). Thus, further discussion on the effect of polythionates on residual
copper(II) would require a proper analysis of sulphur species in such solutions which is
beyond the scope of this review.
Unlike the anion effect summarised in Fig. 7, the change in cation of a 0.1 M
thiosulphate salt solution from Na+ to Ca
2+ has little or no effect on residual Cu(II) at
different time intervals as shown in Fig. 8. The % Cu(II) reacted from 4 mM initial Cu(II)
with 0.1 M Na2S2O3 and CaS2O3 follows the same curve with 82% and 77% Cu(II)
reacted after 24 h, respectively. Moreover, the reaction % of S2O32-
is negligibly small
(<1%) in these two cases (Fig. 8) due to the low initial molar ratio of Cu(II)/S2O32-
of
1/25. However, in the case of (NH4)2S2O3, the % Cu(II) reacted after 24 h is lower (55%)
and % S2O32-
reacted is higher (6%). This has been related to the lower pH (10.3) of
(NH4)2S2O3 due to the buffering effect of the NH4+/NH3 system, compared to the pH
11.5-11.7 of Na2S2O3 and CaS2O3 (Feng and van Deventer, 2010d). As the system was
open to air the reoxidation of Cu(I) at pH 10 in 0.1 M (NH4)2S2O3 produced Cu(II) and
facilitated the oxidation of thiosulphate up to 6%, as shown in Fig. 8. The oxidation of
Cu(I) in the form of the ammine complex Cu(NH3)x+ is faster than that in the form of the
thiosulphate complex Cu(S2O3)x-2x+1
, while a higher ratio of ammonia/thiosulphate
facilitates the oxidation of Cu(I) by oxygen (van Wensveen and Nicol, 2005). In
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contrast, the high pH of 11 in other salt solutions facilitates the hydrolysis of S4O62-
generating S2O32-
(Eqs. 7-10) to keep the S2O32-
reaction to less than 1% in Fig. 8 (Feng
and van Deventer, 2010d).
Fig. 9a shows that higher concentrations of Cu(II) and S4O62-
enhance the rate
(RCu, mol m-3
s-1
) of reduction of Cu(II) to Cu(I) (Breuer and Jeffrey, 2003). This is
consistent with the rapid initial reaction and increase in % Cu(II) reacted in Fig. 7 at
higher concentrations of Na2S4O6 (Feng and van Deventer, 2007b). An interesting feature
to note in Fig. 6 is that the detrimental effect of 0.01 M S4O62-
or S3O62-
on residual
Cu(II) is somewhat diminished in the presence of 0.4 M Na2SO4 indicating (i) the
stabilisation of Cu(II) by excess SO42-
ions and/or, (ii) the stabilisation of S4O62-
/S3O62-
by excess Na+
ions, both due to ion-association (ion-pairing). Fig. 9b also shows the
beneficial effect of a high background concentration of Na2SO4. Here, the rate of reaction
between Cu(II) and S2O32-
decreases at higher concentrations of Na2SO4 which leads to
enhanced levels of residual Cu(II). The addition of 0.1 M Na3PO4 also enhances the
concentration of residual Cu(II) as shown in Fig. 9c. The enhanced residual Cu(II) has
beneficial effects on gold dissolution as described later.
7. Rates of gold and silver dissolution
Rates of dissolution of gold per unit surface area (RAu, mol m-2
s-1
) have been
determined using: anodic dissolution of rotating gold discs in the form of
electrochemical quartz crystal microbalance (REQCM) or ring-disc electrodes (RRDE)
both at different applied potentials, or chemical dissolution of foil (Jeffrey, 2001; Okido
et al., 2002; Feng and van Deventer, 2002b; Baron et al., 2011). The rate of gold or silver
dissolution per unit surface area of a flat surface such as a rotating disc is controlled by
the concentration of ligand and oxidant (c, mol cm-3
) at the surface, or potential in the
case of anodic oxidation. The rate per unit surface area can be described by the Levich
equation (Levich, 1962): RAu,Ag = i/nF = J/b = (0.62/b)ω1/2
υ-1/6
D2/3
c, where i = cuirrent
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density due to metal oxidation M = Mn+
+ ne- (A cm
-2), F = Faraday constant (96487 C
mol-1
), J = flux (mol cm-2
s-1
), b = stoichiometric factor, ω = rotation speed of the disc (s-
1), υ = kinematic viscosity of the fluid (cm
2 s
-1) and D = diffusivity of the species (cm
2 s
-
1) through the fluid film adjacent to the surface. Table 2 compiles the values of D for
dissolved O2, CN-, Cu(II) and S2O3
2- which act as oxidants or ligands for gold/silver
during leaching and shows that (i) 2OD is higher than CND , and (ii) Cu(II)D is slightly
lower than 32OSD and both are 3-5 times lower than
2OD and CND . In both cyanide and
thiosulphate systems the ligands can be maintained at a higher concentration compared to
oxidants. Thus, the rate of gold/silver(5%) alloy leaching by Cu(II) in ammoniacal 0.1 M
thiosulphate medium is controlled by the diffusion of Cu(NH3)42+
to the gold surface at
low concentrations of Cu(II) (<5 mM) (Jeffrey, 2001).
The measured rate of thiosulphate leaching of pure gold by dissolved oxygen in
aerated solutions is an order of magnitude lower than cyanidation based on the results
reported by previous researchers (Li and Wadsworth, 1992; Wadsworth and Zhu, 2003;
Jeffrey, 2001; Jeffrey et al., 2001). The rate of gold leaching by Cu(II) in ammoniacal
thiosulphate depends on the concentration of the three reagents (Jeffrey, 2001). In general
the concentration of Cu(II) is maintained relatively low (<10 mM) compared to the two
ligands S2O32-
(0.1 M) and NH3 (0.4-1 M). The concentration of free ammonia in solution
can be enhanced by increasing the pH. However, it is necessary to maintain the pH
between 9 and 11 to avoid the precipitation of CuO/Cu(OH)2 which is detrimental to gold
leaching (Aylmore and Muir , 2001).
The reported rate data summarised in Tables 3 and 4 compare the effect of the
following variables on RAu based on different techniques.
(i) ligand (cyanide, thiosulphate, ammonia, thiourea, sulphide or chloride),
(ii) oxidant (oxygen or Cu(II)),
(iii) the presence or absence of silver(I), alloyed silver, or polythionates, and
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(iv) the change in cations of the thiosulphate salts and chloride salts from Na+
to NH4+, K
+, Ca
2+, CH3NH3
+, (CH3)2NH2
+ and (CH3)3NH
+.
Some results are plotted in Figs. 10 and 11 for comparison. Table 3 shows the
comparison between cyanide and thiosulphate leaching and the comparability of the rates
based on different techniques as well as the faster dissolution of silver and gold-silver
alloys compared with gold in both cyanide and thiosulphate media (sets A and B).
Ammonia alone as a ligand does not offer a fast dissolution rate for gold. However, the
change in ammonia to thiosulphate or mixed ammonia/thiosulphate enhances gold
dissolution (Set C). Copper(II) at 5 mM has a beneficial effect on gold dissolution in
thiosulphate even in the absence of ammonia (Set D), while polythionates cause a
detrimental effect (Set E).
Table 4 extends the summary of rates to highlight the effect of the change in
cations of the thiosulphate/background salts as well as the co-ligands such as NH3, Cl-,
TU and HS- . In the absence of ammonia the increase in concentration of Na2S2O3 from
0.1 to 1 M or pH from 10.6 to 13 at 1 M Na2S2O3 increases RAu (Set G). The addition of
methyl substituted ammonium chloride also increases RAu, but large cations such as
(CH3)3NH+ lowers RAu due to surface coverage (Set H, Chandra and Jeffrey, 2004). The
increase in concentration of (NH4)2S2O3 from 0.1 to 0.6 M increases RAu (Set I), while
the change in cation from NH4+ to K
+ or Na
+ at 0.2 M thiosulphate lowers RAu (Set J).
These results are consistent with the values of RAu based on the dissolution of gold foils
by copper(II) in ammoniacal thiosulphate solutions which shows the beneficial effect of
cations: Na+ < NH4
+ < Ca
2+ (Set F in Table 3).
In non-ammoniacal solutions of near neutral pH (6-7), the increase in Cu(I) (Set,
K), chloride (Set, L), or thiourea (Set, M) enhances the rate of gold dissolution. However,
AuAg(2%) alloy has a lower rate than pure gold in the presence of 5 mM TU (Set M).
Moreover, the rates in the presence of 1, 5 or 10 mM TU are lower in Na2S2O3 than in
(NH4)2S2O3 (Set M). The use of hydrosulphide ligand (produced from the reaction Na2S
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+ H2O = NaHS + NaOH) instead of thiosulphate offers a very high rate of gold
dissolution (Set N). The increase of ammonia (Set O) and the addition of Cu(II) to
solutions of Na2S2O3 (Set P) or (NH4)2S2O3 (Set Q) also has beneficial effects during the
anodic oxidation of gold at a potential of 0.25-0.30 V. This effect is more pronounced in
the case of gold-silver alloy (Set R). The beneficial effect of adding Cu(I) on RAu is not
affected by tetrathionate (Set. S). The next section is a brief description of reaction
mechanism(s) and the effect of various reagents under different categories.
8. Mechanistic details
A detailed analysis of anodic oxidation of gold in ammonia free thiosulphate in
the absence of copper(II) has shown that the anodic oxidation takes place via the
adsorption of the MS2O3- ion pair (M
+ = Na
+, K
+), as described in Eqs. 12-14
(Senanayake, 2005b). These adsorption, oxidation, desorption and stabilisation
mechanisms explain the faster rates of anodic oxidation in ammonia free 0.25 M K2S2O3
compared to that in 0.25 M Na2S2O3. Due to the higher association constant (Table 1), the
concentration of KS2O3- is higher than that of NaS2O3
- at the same initial concentrations
of the two salts, this leads to the beneficial effect.
Effect of M+ in the absence of NH3
Au + MS2O3- = Au(S2O3)M
-(ads) (adsorption) (12)
Au(S2O3)M-(ads) = Au(S2O3)M
0(ads/aq) + e
- (anodic oxidation) (13)
Au(S2O3)M0
(ads/aq) + MS2O3- = M2Au(S2O3)2
-(aq) (desorption/stabilisation) (14)
In the case of thiosulphate oxidation by copper(II) in ammoniacal solutions the
reaction orders with respect to the key reagents are: [Cu(II)]1, [S2O3
2-]1.2
, [NH3]-1.1
and
[OH-]
0 (Senanayake, 2004b). The first order reaction between Cu(II) and S2O3
2- is
consistent with Fig. 12 which shows a 1:1 association between S2O32-
and Cu(II) prior to
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the redox reaction producing Cu(I) and S4O62-
. The reaction orders for gold oxidation by
Cu(II) are: [Cu(II)]1, [S2O3
2-]1.2
, [NH3]0 and [OH
-]
-0.1. These results have been used to
suggest that the oxidation of gold as well as thiosulphate by Cu(II) involves the mixed
ligand complex Cu(II)(NH3)pS2O3, which is noted in the proposed reaction model
depicted in Fig. 13. As shown in Fig. 13 the radical S2O32-
dimerises to S4O62-
which
undergoes forther reactions to other species according to Eqs. 7-11.
A background ligand L (= Xz-
) can change the coordination sphere of Cu(II), as
shown in Fig.12, which may alter the reactivity of Cu(II) towards thiosulphate as well as
gold. Ions of lower softness in Fig. 5 are better ligands as noted in section 3. They are
capable of replacing S2O32-
from the coordination sphere of Cu(II) in Fig. 12 leading to
lower decomposition of S2O32-
, higher residual copper(II) and accelerated gold leaching.
Moreover, despite the high residual copper(II) in the presence of background Na2SO4
(Fig. 9b), the presence of 0.25 M Na2SO4 slightly lowers RAu in 0.01 M CuSO4, 0.4 M
NH3, and 0.1 M Na2S2O3 from 3.3 x 10-5
mol m-2
s-1
to 2.9 x 10-5
mol m-2
s-1
(Chu et al.,
2003). The increase in background sulphate concentration favours the reverse reaction of
equilibrium A in Fig. 13, and thus decreases thiosulphate consumption by Cu(II) due to
reactions B, D and E, as well as the gold oxidation via reaction C. Nevertheless, as shown
in Fig. 1a, background sulphate can be beneficial for gold leaching after 24 h, possibly
due to other interactions with host minerals, the higher residual Cu(II), and mixed
solution potential; and thereby warrants further discussion.
Figs. 9c and 14a show that the decrease in residual copper(II) in the absence of
Na3PO4 lowers the solution potential (EH) measured using a platinum electrode. The
measured solution potentials generally represent the couple Cu(II) + e- = Cu(I) according
to the Nernst equation EH = EoCu(II)/Cu(I) + RT ln{aCu(II)/aCu(I)} where E
oCu(II)/Cu(I) =
Eo{Cu
2+/Cu
+} -RT/F{ln(βCu(II)complex) – ln(βCu(I)complex)} and β represent the stability
constant of Cu(II)/(I) complexes in solution (Zhang et al., 2008). Thus, the decrease in EH
with time in Fig. 14a is consistent with the decrease in residual Cu(II) in Fig. 9c,
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corresponding to a lower activity ratio aCu(II)/aCu(I). The stabilisation of Cu(II) due to the
complexation with phosphate enhances residual Cu(II), but also decreases Eo and hence
the measured EH as well as the mixed potential of gold (Jeffrey et al., 2003). This
rationalises the lower initial RAu in the presence of Na3PO4 , compared to that without
phosphate in Fig. 14a. Nevertheless, the value of RAu in the presence of Na3PO4 remains
relatively stable over a longer period of time, compared to the continuous decrease in RAu
in the absence of Na3PO4. A similar effect has also been observed due to the presence of
1 M Na2SO4 (Jeffrey et al., 2003).
Despite the high residual Cu(II) in Fig. 9c, the dissolution of gold powder in
Cu(II)-NH3-S2O3 is not affected by Na3PO4, as the two curves in Fig. 14b over a period
of 8 h follow the same trend. In contrast, Fig. 14c shows that the addition of (NaPO3)x
(SHMP) decreases the % Cu(II) reacted after 5 h, which in turn enhances the dissolution
of gold foil. This difference can be related to the difference between the two phosphate
salts as well as the high Cu(II) (10 mM) under argon in Fig. 14a-b, compared to low
Cu(II) (0.8 mM) with the reactor open to air in Fig. 14c. In general the stabilisation of
Cu(II) by minimising the reaction with S2O32-
leads to low concentrations of
polythionates/sulfides which can also be beneficial for gold dissolution. Thus, the effect
of background ions (Xz-
) can be rationalised on the basis of interactions between Cu(II)-
NH3-S2O3-X-Au as described in the next section.
9. Comparative effects of ions/reagents on rates
9.1 Na+, NH4
+ and Ca
2+
Fig. 15 is based on the dissolution of gold foil (mol m-2
) immersed in solutions of
0.5 M NH3 and 4 mM Cu(II) over 24 h. It shows that the relative effect of the three salts
is in the order Na+ < NH4
+ < Ca
2+ and increases with the increase in concentration of
thiosulphate from 0.1 M (Fig. 15a) to 0.2 M (Fig. 15b). It is also important to consider the
effect of other factors such as the pH and relative concentration of reactive copper(II)/(I)
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species which control EH during leaching (Feng and van Deventer, 2010d). As noted in
Fig. 8, the % Cu(II) and S2O32-
reacted after 8 h were nearly the same with Na2S2O3 and
CaS2O3. The measured solution potentials in 0.1 M thiosulphate solutions indicates the
stabilisation of anionic Cu(I)-S2O32-
complexes by cations in the order Na+ < NH4
+ <
Ca2+
, which increases EH as a result of the increase in activity ratio aCu(II)/aCu(I) (Feng and
van Deventer, 2010d).
Despite the fast dissolution of gold at the higher thiosulphate concentration in
Fig.15b, compared to Fig. 15a, gold dissolution in Na2S2O3 or (NH4)2S2O3 is retarded in
latter stages compared to that in CaS2O3 in Fig. 15b. This indicates passivation in
Na2S2O3 or (NH4)2S2O3 which does not seem to occur in CaS2O3, as evident by the
constant slopes for CaS2O3 in both Figs. 15a-b and the more prominent beneficial effect
of Ca2+
ions in Fig. 15b. Thus, the cations of high Lewis acidity (Fig.3b) appear to
facilitate gold dissolution (Fig. 15) due to two reasons: (i) interaction with anionic Cu(I)-
S2O3 complexes in solution and the maintenance of a higher solution potential (Feng and
van Deventer, 2010d), and (ii) stabilisation of Au(S2O3)M(ads/aq) formed in the anodic
reaction prior to the formation of Au(S2O3)23-
according to the mechanism in Eqs. 12-14.
In the case of CaS2O3, the latter appears to be avoiding thiosulphate degradation products
on the gold surface and assisting the maintenance of a constant and high leach rate during
prolonged leaching.
9.2 Zn(II), Pb(II), Cu(II)/(I) and Ag(I)
The βn values of other divalent metal ions in Table 1 which follow an increasing
order of softness: Zn2+
< Cu2+
< Pb2+
predict beneficial effects of these cations on gold
dissolution. As expected, the beneficial effect of Pb(II) is much larger than that of Zn(II)
in Fig. 16 at low concentrations. Moreover, due to the higher reagent concentrations of
0.5 M (NH4)2S2O3, 2 M NH3, and 12 mM Cu(II), gold dissolution after 24 h even in the
absence of additives is much larger in Fig. 16 (2.5 mol m-2
) than that in Fig. 15 (≤0.8
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mol m-2
). In contrast, the results for gold dissolution at lower reagent concentrations of
0.1 M (NH4)2S2O3, 0.5 M NH3 and 6 or12 mM Cu(II) in Fig. 17 show that both Pb(II)
and Zn(II) have detrimental effects on gold dissolution (even at a low concentration of 5
mg/L). Thus, the important points to note are:
(i) the beneficial effect of Pb(II) at low concentrations in Fig. 16 is due to
the formation of AuPb2, as in cyanide systems, supported by the
PbOH+/AuPb2 couple appearing in the Eh-pH diagram (Feng and van
Deventer, 2002b);
(ii) the detrimental effect of Pb(II) at high concentrations is a result of
passivation of the gold surface by PbO (Feng and van Deventer,
2002b), which can be removed by the addition of other reagents (Xia
and Yen, 2008) described later.
In the absence of silver, both massive and colloidal gold have comparable, but
very low rates of dissolution in oxygenated (non-copper) thiosulphate solutions (Webster,
1986). However, Fig. 10 and Tables 3 and 4 show very high values of RAu for gold-silver
alloys indicating the beneficial effect of silver. Likewise, the presence of silver in alloyed
or colloidal form enhances the rate of dissolution of gold as shown in Fig. 18. A higher
silver content in the alloy is also beneficial for gold dissolution.
The beneficial effect of high copper on RAu in ammonia free solutions (set D in
Table 3) shows the possibility of forming mixed copper-gold-thiosulphate complexes of
the form (Au,Cu)(S2O3)23-
(Zhang and Nicol, 2005) similar to the (Au,Ag)(S2O3)23-
complexes proposed by Webster (1986). This warrants further studies. Copper, silver
and lead form alloys with gold (AuPb2, Au3Cu, AuCu3) (Feng and van Deventer, 2002b,
Marsden and House, 2006; Lee and Hiskey, 2008). The cations Au+, Cu
2+ and Pb
2+ have
comparable and high values of softness (Fig.3a), while Cu+ and Ag
+ ions have
comparable Lewis acidities which are higher than those of Na+ and K
+ (Fig. 3b).
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Moreover, the calculated electrode potentials using actual Ag(I)-NH3-S2O32-
speciation
and activities lower than unity for Ag metal (as in alloys) show the possibility of
reduction of Ag(I) onto gold surface (Zhang et al., 2008); this warrants further studies on
the beneficial role of silver.
9.3 Co-ligands
The results shown in Fig. 11 represent the anodic oxidation of gold at 0.25 V
(SHE). Curve (a) shows the beneficial effect of increasing the ammonia concentration
from 0 to 0.6 M in 0.1 M Na2S2O3 in the absence of Cu(II). The other curves represent
the effect of increasing thiosulphate (curve b), chloride (curve c) and thiourea (curve d),
all in the absence of NH3 and Cu(II). The increase in thiosulphate concentration from 0.1
to 0.4 M causes an increase in RAu from 7x10-7
to 3 x 10-5
mol m-2
s-1
(curve b). At 0.2 M
(NH4)2S2O3, the increase in chloride ion concentration from 0 to 1 M increases RAu from
7 x 10-6
to 2 x 10-5
mol m-2
s-1
(curve c). At the same concentration of (NH4)2S2O3 the
addition of even low concentrations of 0.001 to 0.01 M thiourea causes a large increase in
RAu to 1 x 10-4
mol m-2
s-1
(line d). Although the softness of neutral ligands such as NH3
and TU have not been quoted by Marcus (1997), the early data compiled by Hancock and
Martell (1989) noted in section 2 shows a decrease in the hardness in the following
order:
OH- > NH3 > Cl
- > S2O3
2- > TU > CN
-
(harder) (softer)
Thus, based on the HSAB rule NH3, Cl-, and TU are likely to act as co-ligands
during the anodic oxidation of gold in thiosulphate media. This explains the beneficial
effect of these ligands (NH3 < Cl- < TU) observed in Figs. 10 and 11. Likewise, the rate
of anodic oxidation of gold in 0.1 M (NH4)2S2O3 is 0.6 x 10-6
mol m-2
s-1
, and increases to
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5.0 x 10-6
mol m-2
s-1
with the addition of 0.4 M NH3 without copper(II) (Jeffrey et al.,
2008b), again portraying the role of NH3 as a co-ligand for gold. The addition of both 0.4
M NH3 and 10 mM Cu(II) increased the rate to 17 x 10-6
mol m-2
s-1
(Table 4, Set Q),
indicating the depassivating role of Cu(II) described in next section. The beneficial effect
of the co-ligand TU is more prominent in (NH4)2S2O3 than in Na2S2O3 (Table 4, Set M).
This indicates the beneficial effect of both NH4S2O3- ion and TU at the gold surface via
interim species such as Au(S2O3)TU-, similar to interim species Au(S2O3)NH3
- proposed
previously (Senanayake, 2005c). However, the AuAg(2%) alloy in (NH4)2S2O3
containing 5 mM TU has a lower rate of dissolution compared to pure gold (Table 4, Set
M) and warrants further investigations.
9.4 Sulphur species
Fig. 19 shows the effect of Na2S4O6, K2S3O6 and Na2S on the dissolution of gold
foil with time in solutions of the same reagent concentrations as in Fig. 7. Despite the
large consumption of over 60% of initial Cu(II) by S4O62-
and S2-
after 3 h shown in Fig.
7, gold dissolution remains high in the order S2-
> S4O62-
> S3O62-
in Fig. 19. This
indicates that the HS- ion, added or formed by Eqs. 7-11, could be involved in the anodic
reaction (Feng and van Deventer, 2007b). Table 5 compares the peak potentials, current
densities, and rates of anodic oxidation of gold in 0.6 M (NH4)2S2O3 and 1.6 M Na2S
solutions at 30oC (Chandra and Jeffrey, 2003; Anderson, 2003). The results suggest a 10
fold increase in gold oxidation rate in sulphide due to dissolution as AuS-, compared to
Au(S2O3)23-
in thiosulphate. The Eh-pH diagram in Fig. 20 also shows the stabilisation of
gold(I) in the form of AuS- at higher pH and lower Eh values, compared to Au(S2O3)2
3-
which is stable at relatively lower pH and higher Eh values. Gudkov et al. (2010) also
reported a decrease in peak potential and an increase in peak current during anodic
oxidation of gold in 0.1 M Na2S2O3 with the addition of 1 mM Na2S at pH 12. However,
as shown in Fig. 21, higher concentrations of Na2S (> 5 mM) are detrimental for gold
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leaching due to the formation of S/CuS which passivates the gold surface. Muir and
Aylmore (2005) also noted the precipitation of Ag-Au-Cu sulphides due to the oxidation
of thiosulphate to unstable polythionates. The passivation has been confirmed by detailed
studies using Raman spectroscopy of the gold surfaces which showed the presence of
S/CuS at higher additive concentrations, compared to the gold surface leached under
standard conditions which was generally free of S/CuS (Feng and van Deventer, 2006).
The formation of S/CuS from thiosulphate oxidation and adsorption of polythionates
passivates and inhibits gold dissolution, but the presence of both ammonia and copper(II)
avoids passivation. This has been related to the formation of copper(I)-ammonia-
polysulphide which is considered to be less inhibiting than sulphur/polythionate (Jeffrey
et al., 2008b). For example, the formation of disulfide (S22-
) by the reaction in Eq. 18 has
a large equilibrium constant based on thermodynamic information from Hogfeldt (1982)
and the HSC 6.1 database (Roine, 2002), and warrants further studies.
2Cu(NH3)42+
+ 2HS- = 2Cu(NH3)2
+ + S2
2- + 2NH4
+ + 2NH3 (K = 10
8.3) (18)
9.5 Other reagents
As noted in Fig. 14c, the addition of SHMP enhances the residual Cu(II) due to its
high ability to coordinate with Cu(II), as revealed by the structure of SHMP in Fig. 2.
Likewise EDTA and CMC can interact with the coordination sphere and stabilise Cu(II)
and hence increase the residual copper(II) and facilitate gold dissolutions from foil in
experiments conducted over 24 h as shown in Figs. 22a and 22b. However, excessive
amounts of CMC somewhat nullifies the beneficial effect (Fig. 22b) as in the case of
S4O62-
and S3O62-
(Fig. 21), indicating the surface blockage noted above. The detrimental
effect of EDTA in Fig. 22b at higher concentrations is much larger. This can be related to
a stronger interaction between Cu(II)-EDTA, as in the case of Na2S in Fig. 21. Due to the
chelating ability of EDTA, revealed by the structure shown in Fig. 2, the Cu(II)-EDTA
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complexes have higher stability constants compared to Cu(II)-NH3 complexes. This
changes the predominance area from Cu(NH3)42+
to Cu(EDTA)2+
at higher EDTA
concentrations (Alonso-Gomez and Lapidus, 2009), resulting in a large detrimental effect
on gold dissolution as observed in Fig. 22b.
10. Leaching results
10.1 Beneficial effect of Cl- and Ca
2+
Li and Kuang (1998) reported that NaCl could increase gold extraction from an
oxide ore. For example, the addition of 1 M NaCl enhanced gold extraction from 39% to
84% in ammonium thiosulphate solution. They noted that chloride increased gold
dissolution according to the reaction in Eq. 19, where the interim species AuCl2- with a
lower stability constant (2 109) was subsequently converted to Au(S2O3)2
3- with a
higher stability constant (2 1026
) according to Eq. 20.
2Au + 4Cl- + 0.5O2 + H2O = 2AuCl2
- + 2OH
- (19)
AuCl2- + 2S2O3
2- = Au(S2O3)2
3- + 2Cl
- (20)
The formation of AuCl2- reduces the reaction activation energy, which in turn
increases the reaction rate (Li and Kuang, 1998). However, in a mixed ligand system
Cu(NH3)42+
can associate with a range of anions including chloride, sulphate and
thiosulphate ions as noted in Fig. 12 (Byerley et al., 1973; Breuer and Jeffrey, 2003; van
Wensween and Nicol, 2005; Black, 2006). There is evidence for the formation of mixed
Cu(I)-Cl--S2O3
2- complexes (Black et al., 2003). Mixed Ag(I)-Cl
--S2O3
2- complexes have
also been reported (Hogfeldt, 1982). Despite the insignificant effect of NaCl on
increasing residual copper(II), compared to NaNO3 in Fig. 6, it enhances gold leaching as
evident from the results shown in Figs. 10-11 and Table 4 (Set L). This supports an
anodic reaction mechanism via the mixed complex AuCl(S2O3)-. Indeed, the dissolution
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of gold colloids is aided by the addition of NaCl, compared to NaNO3, although the effect
of these two salts on residual Cu(II) remains the same (Senanayake and Zhang, 2011).
This warrants further studies on the effect of NaCl on the speciation of Au(I) and the
anodic oxidation of gold in thiosulphate solutions.
Feng and van Deventer (2010d) reported the effect of change in cations of the
thiosulphate salt from Na+ to NH4
+ or Ca
2+ during the leaching of a pyrite concentrate and
a sulphide ore. In the case of the pyrite concentrate (4.41% Cu, 33.6% Fe, 42.8% S, 10.8
g/t Ag, 33.9 g/t Au), the cation influence on the efficiency of gold extraction (%) after 24
h followed the same trend as the order of the residual copper(II) in different salts (NH4+
> Ca2+
> Na+). The cation influence on gold extraction from the sulphide ore (0.07% Cu,
5.27% Fe, 4.35% S, 2 g/t Ag, 5.62 g/t Au) showed the same trend as the solution EH (
Ca2+
> NH4+ > Na
+). Therefore, it is important to consider factors such as pH and EH
which are governed by the relative and/or residual concentrations of NH3/NH4+/S2O3
2-
/MS2O3/Cu(II)/Cu(I) during prolonged leaching.
10.2 Beneficial effect of silver
Hiskey and Sanchez (1990) noted that despite the faster kinetics of silver
cyanidation compared to gold, the leaching of silver from natural ores lags behind that of
gold. Fig. 23 shows that higher silver grades in oxide ores cause a higher extraction of
both gold and silver compared to the cyanide system, especially during leaching with
ammoniacal copper(II) thiosulphate (Muir and Aylmore, 2004, 2005). However, the
silver extraction remains lower than gold extraction except with ores of high silver
contents, as in the case of gold cyanidation. The beneficial effect of alloyed silver can be
related to the faster rate of leaching of silver compared to gold noted in Table 3. This is
also evident from the beneficial effect of the added or dissolved silver(I) on the
dissolution kinetics of gold colloids (Zhang et al., 2008; Senanayake and Zhang, 2011).
Precipitated Ag2S at lower thiosulphate concentrations can only be partially leached by
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increasing EH (Briones and Lapidus, 1998; Muir and Aylmore, 2005). Thus, further
studies are essential to rationalise the beneficial/detrimental role of silver in the feed as an
alloy or Ag2S on gold leaching.
10.3 Detrimental effect of lead(II)
Results of leaching studies by Xia and Yen (2008) on a natural quartz ore of 74
μm and composition: 16.26 g/t Au, 0.18% Fe-oxides, 0.002% Cu-oxides, 0.19% C and
0% S at a pulp density of 33.3% and 20-24 oC in the absence or presence of a range of
additives: 16% FeS or FeS2, 0-5% PbS or PbO, 0-6 mM PbCO3 and Pb(NO3)2 are
summarised in Figs. 24-25. The effect of adding FeS or FeS2 on gold extraction with or
without PbS after 3 h is shown in Fig. 24a. When no PbS is added, 16% FeS2 has a
detrimental effect, lowering the gold extraction after 3 h from 92% to 80%. The addition
of 0.4% PbS increases gold extraction to 88%, which is consistent with the beneficial
effect of Pb(II) at low amounts described in Fig. 16. However, PbS alone has a
detrimental effect, which is much larger than that of 16% FeS2. This detrimental effect is
somewhat less in the presence of 16% FeS2 or FeS. Nevertheless, gold extraction
decreases to well below 50% with additions of >0.5% PbS alone or >1% PbS in the
presence of 16% FeS2 or FeS.
The detrimental effect of excess Pb(NO3)2 in Figs. 16-17 is consistent with the
leaching results depicted in Fig. 24b. Higher concentrations of Pb(NO3)2 enhance the
detrimental effect due to the precipitation of Pb(OH)2 with a favourable value of K =
105.2
, according to Eq. 21, which passivates the gold surface. However, the addition of
PbCO3 has no significant effect on gold leaching. This can be related to the low value of
K of 10-6.54
for the formation of passivating Pb(OH)2 from insoluble PbCO3 according to
the reaction in Eq. 22.
Pb2+
+ 2NH3 + 2H2O = Pb(OH)2(s) + 2NH4+ (K = 10
5.20) (21)
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PbCO3(s) +2H2O + NH3 = Pb(OH)2(s) + HCO3- + NH4
+ (K = 10
-6.54) (22)
PbO(s) + H2O = Pb(OH)2(s) (K = 10-0.53
) (23)
PbS(s) + H2O + 0.5O2 = Pb(OH)2(s) + S(s) (K = 1017.0
) (24)
PbS(s) + Cu(NH3)42+
+ 2H2O
= CuS(s) + Pb(OH)2(s) + 2NH3 + 2NH4+ (K = 10
1.66) (25)
In contrast, even a small % of PbO has a large detrimental effect indicating the
passivation of the gold surface (Fig. 24b) as the conversion of PbO to Pb(OH)2 has an
equilibrium constant of 0.3 (Eq. 23). The formation of S/CuS along with Pb(OH)2 from
PbS is thermodynamically feasible with K values of the order 101.66
or 1017.0
(Eqs. 24,
25). However, only a trace amount of sulphur species was detected by XPS on the gold
surface compared to the major precipitate Pb(OH)2 after leaching in Cu(II)-NH3-S2O3 in
the presence of 0.8% PbS (Xia and Yen, 2008). The Eh-pH diagram shows PbOH+ as the
stable species at pH 8-10.4 (Feng and van Deventer, 2002b). Thus, as shown in Fig. 24b,
the increase in concentration of (NH4)2S2O3 from 0.2 M to 0.4 M and 0.8 M decreased
the pH from 10.1 to 9.7 and enhanced the gold extraction from 22% to 88% even in the
presence of 2.4% PbS. Moreover, the addition of 0.1 M NaOH and 0.3 M NaOH to a
quartz bearing ore in the presence of 2.4% PbS decreased the gold extraction from 22%
to 15% and 0% respectively, confirming that the gold passivation was by Pb(OH)2 (Xia
and Yen, 2008).
10.4 Beneficial effect of phosphate, carbonate
In the absence of PbS, the addition of Na3PO4 has a detrimental effect as shown
by the decrease in gold extraction by about 10% in Fig. 25. This is consistent with the
lower values of RAu over the first 3-4 h in the presence of Na3PO4 described in Fig.14a.
However, the detrimental effect of PbS due to Pb(OH)2 precipitation noted in Fig. 24 can
be removed by adding Na3PO4 or Na2CO3, as shown in Fig. 25. Large values of K for the
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formation of PbCO3 or Pb3(PO4)2 from Pb(OH)2 (Eqs. 26, 27) suggest that the
depassivation of gold surface in the presence of Na3PO4 or Na2CO3 is responsible for the
beneficial effect of these salts.
Pb(OH)2(s) + HCO3- + NH4
+ = PbCO3(s) +2H2O + NH3 (K = 10
6.54) (26)
3Pb(OH)2(s) + 2HPO42-
+ NH4+
= Pb3(PO4)2(s) + 6H2O + 4NH3 (K =1021.9
) (27)
10.5 Beneficial effect of SHMP, EDTA and CMC
Figs. 26a-c compare the effect of increasing concentration of the three additives
SHMP, EDTA and CMC on gold and silver extraction from a sulphide ore after 24 or 48
h of leaching with 0.5 M NH3, 0.1 M (NH4)2S2O3, 0.8 mM Cu(II) at 25oC (Feng and van
Deventer, 2010b, 2011a,b). The feed contained 23.5% albite, 1.1% calcite, 21.2%
dolomite, 41.3% quartz, 5.4% pyrite and 0.1% chalcopyrite (0.07% Cu, 5.27% Fe, 4.35%
S, 4.3 g/t Au, 2.0 g/t Ag). The gold extraction reached 100% after 48 h with the additive
concentrations 0.80 g/L SHMP (Fig. 26a), 0.15 g/L EDTA (Fig. 26b) or 0.05 g/L CMC
(Fig.26c). The beneficial effect of the three additives at low concentrations is consistent
with the beneficial effects on gold foils (Figs. 22a-b) as a result of the enhanced
concentration of residual copper(II). However, the colloidal iron oxide coatings
emanating from grinding media and/or host minerals can be detrimental for thiosulphate
leaching of gold ores. Thus, one of the beneficial roles of these additives is to
avoid/remove such coatings (Feng and van Deventer, 2011a).
As noted in section 9 the formation of polythionate and S/CuS on the gold surface
leads to inhibition of gold leaching (Jeffrey et al., 2008b). The stabilisation of Cu(II) and
hence thiosulphate due to the presence of EDTA is responsible for the decrease in
formation of these passivating layers on the gold surface (Feng and van Deventer,
2010b). In addition, the presence of EDTA decreases the interference of heavy metal ions
such as lead(II) due to chelating effect and thus causes high gold and silver extraction
from sulphide ores by ammoniacal copper(II) thiosulphate systems (Alonso-Gomez and
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Lapidus, 2009; Feng and van Deventer, 2010b). Some of the important features in Fig. 26
are summarised below.
(i) In all cases silver extraction remains lower than gold but generally follows the
gold extraction curve.
(ii) The formation of Fe(OH)3 and FePO4.H2O based on the Eh-pH diagrams has been
used to rationalise the beneficial role of SHMP. This additive converts the
passivating or surface blocking Fe(OH)3 precipitated on the gold surface to
colloidal FePO4.H2O, a reaction which appears to be thermodynamically
favourable (Eq. 28).
Fe(OH)3(s)+ PO43-
+ 3NH4+ = FePO4(s) + 3NH3 + 3H2O (K = 10
0.77) (28)
(iii) Higher additions of SHMP over 0.8 g/L decreases both gold and silver extraction
(Fig.26a). However, this detrimental effect is not observed with the leaching of
gold foil in the presence of SHMP, in the absence of host minerals in Fig. 22a.
Thus, the detrimental effect of excess SHMP on gold leaching from the sulphide
ore is a result of the high dissolution of iron, as evident from the increase in iron
concentration in leach liquor. The precipitation of excess iron as phosphate slimes
retards the gold and silver leaching in Fig. 26a (Feng and van Deventer, 2011b).
(iv) In other cases higher additive concentrations increase both gold and silver, where
the addition of 0.13 g/L CMC causes 100% gold extraction after 24 h (Fig. 26c).
Likewise, the higher additions of EDTA increase the gold extraction after 24 h,
though this does not reach 100%. The detrimental effect of excess EDTA or CMC
on gold foil in Fig.22b is not observed with the sulphide ore in Fig. 26, showing
the interference with host minerals.
10.6 Thiosulphate consumption (TSC)
Fig. 27 shows TSC during 3 h leach tests for an oxide ore (Xia and Yen, 2008),
compared with the 24 h leach test results for a sulphide ore shown in Fig. 2. A lower TSC
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was observed in the presence of Na2CO3 in Fig. 27. This is due to the stabilisation of
Cu(NH3)42+
through interaction with CO32-
predicted from Fig. 5. This stabilisation
lowers thiosulphate degradation via reaction C in Fig. 13. A high TSC in the presence of
Na3PO4 is contrary to the beneficial effect of Na3PO4 on residual copper(II) in Fig. 9c.
Thus, the higher TSC in Fig. 27 in the presence of Na3PO4 can be related to the enhanced
iron dissolution, which may facilitate thiosulphate degradation. This is also consistent
with the higher TSC with SHMP compared to the other two additives described in Fig. 2.
The higher TSC with Pb(NO3)2 may be related to the reaction with NO3- according to Eq.
29, with an equilibrium constant of K = 2 at 25oC (HSC 6.1). The oxidation of NO2
- to
NO3- by dissolved O2 according to Eq. 30 also has a favorable equilibrium constant of K
= 1030
(HSC 6.1). Likewise, the presence of FeS and FeS2 is responsible for higher values
of TSC, as the thiosulphate oxidation by dissolved oxygen is catalysed by these solids
(Xia and Yen, 2005; Feng and van Deventer, 2005; Zhang and Jeffrey, 2008).
NO3- + S2O3
2- + H2O = NO2
- + S4O6
2- + 2OH
- (29)
NO2- + 0.5O2 = NO3
- (30)
A summary of the TSC after 24 or 48 h depicted in Fig. 28 shows the relative
effects of SHMP, EDTA and CMC. The important points to note are:
(i) all additives lower the TSC compared to leaching without additives,
(ii) the TSC decreases with increasing concentration of additives and the leaching
time from 24 to 48 h,
(iii) the general order of TSC is: none > SHMP > EDTA > CMC.
Low TSC in the presence of EDTA leads to higher rate of gold dissolution in Fig. 22b
and gold extraction in Fig. 26b (Feng and van Deventer, 2010b). These results show that
TSC in leaching gold/silver ores is governed by the reaction between Cu(II), thiosulphate,
host minerals, and additives. For example, despite the low TSC with SHMP in Fig. 28 at
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higher additions, the gold/silver leaching decreases in Fig. 26a. This is caused by the
interaction between the additive and iron minerals as noted in section 10.4. This appears
to be a common feature in both Figs. 25 and 26a where the excess phosphate retards gold
leaching.
Summary and conclusions
Gold dissolution by ammoniacal thiosulphate is a result of the reaction between
gold, thiosulphate and ammoniacal copper(II) ions. The reaction between thiosulphate
and ammoniacal copper(II) produces polythionate/sulphide ions which block or passivate
the gold surface with S/CuS and cause detrimental effects on gold dissolution. A
comparison of the rates of anodic/chemical dissolution of pure gold and gold-silver
alloys/ores/concentrates in the absence or presence of the main reagents and additives
rationalises the beneficial/detrimental roles of different additives:
(i) Thiosulphate degradation via Cu(NH3)xS2O3 can be reduced by anions of lower
softness which represent higher coordinating ability (higher Lewis basicity)
decreasing in the order: PO43-
> SO42-
> CO32-
> Cl- > SO3
2- > S
2-. Thus, enhanced
residual Cu(II) in the presence of phosphate, carbonate, EDTA, CMC and SHMP
increases gold dissolution, but excess EDTA can be detrimental by changing
Cu(NH3)xS2O3 to the more stable Cu(EDTA)2-
which retards gold dissolution.
(ii) Direct involvement of co-ligands in the anodic surface reaction accelerates gold
dissolution in the order NH3 < Cl- < HS
- < TU as expected by the increasing order
of softness of these ligands which facilitates complexation with Au+ via soft-soft
interactions.
(iii) Depending upon their Lewis acidity or softness, some cations (Mz+
) are involved in
the anodic reaction of gold due to the adsorption of the ion-pair AuS2O3M-2+z
(ads),
which avoids/minimises passivation by thiosulphate oxidation products by
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dissolving gold as AuS2O3M-1+z
(ads/aq) species prior to conversion to Au(S2O3)23-
in
solution.
(iv) Other cations may improve gold dissolution via the formation of mixed-metal ion
complexes M(I)Au(I)(S2O3)22-
(M = Cu, Ag) or assist depassivation of gold by
producing a surface product Cu(I)(NH3)x.Sy which is more porous than S/CuS and
warrants further studies.
(v) Pyrite and pyrrhotie in large quantities (16%) assist the catalysis of thiosulphate
oxidation and cause high thiosulphate consumption and low gold extraction from
ores. In comparison, even low quantities of PbS, PbO, Pb(NO3)2 largely retard gold
leaching due to passivation by PbO/Pb(OH)2 which can be removed by adding
phosphate or carbonate to precipitate Pb3(PO4)2 or PbCO3. Likewise, phosphate and
CMC can also improve gold leaching by removing Fe(OH)3 slimes on the gold
surface, but excess phosphate can produce FePO4 slimes which retard gold
leaching.
(vi) Low Cu(II) addition is advantageous as it lowers thiosulphate consumption, avoids
passivating products, and results in 100% gold extraction despite the slow rates
which require prolonged leaching.
(vii) The dissolution of silver and gold-silver alloys is faster than pure gold. Although
the leaching curves for silver from ores/concentrates generally follow the same
trend as gold, silver leaching is lower than gold, indicating the possible involvement
of Ag2S, which warrants further studies.
Acknowledgement
Financial assistance and support from the Parker CRC for Integrated Hydrometallurgy
Solutions and Murdoch University are gratefully acknowledged.
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Highlights
Stabilisation of Cu(II) by coordination enhance residual Cu(II) and gold
leaching
Co-ligands and thiosulphate ion-pairs are beneficial for anodic reaction of
gold
Removal of degradation/passivation products on the gold surface enhances
leaching
Ionic hard-soft and Lewis acid-base interactions/properties explain leach
results
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Tables
Table 1. Equilibrium constants
Equilibrium log βn at 25 oC Softness
of cation
Lewis acidity
of cation
Na+ + S2O3
2- = NaS2O3
- 0.68 -0.60 0.89
NH4+ + S2O3
2- = NH4S2O3
- 0.93 -0.60 0.98
K+ + S2O3
2- = KS2O3
- 1.0 -0.58 0.85
Ca2+
+ S2O32-
= CaS2O30 1.90 -0.66 3.54
Ca2+
+ 2S2O32-
= Ca(S2O3)22-
4.00
Pb2+
+ S2O32-
= PbS2O30 3.35 0.41 3.91
Pb2+
+ 2S2O32-
= Pb(S2O3)22-
5.64
Pb2+
+ 3S2O32-
= Pb(S2O3)34-
6.86
Cu2+
+ S2O32-
= CuS2O30 2.40 0.38 4.77
Cu2+
+ 2S2O32-
= Cu(S2O3)22-
5.20
Zn2+
+ S2O32-
= ZnS2O30 0.96 0.35 4.65
Zn2+
+ 2S2O32-
= Zn(S2O3)22-
1.94
Zn2+
+ 3S2O32-
= Zn(S2O3)34-
3.30
log βn from Sillen and Martell (1964), Hogfeldt (1982), Senanayake (2005c) and From
HSC 6.1 data base (Roine, 2002); Softness and Lewis acidity from Marcus (1997).
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Table 2. Diffusivity of oxidant and ligands in cyanidation and thiosulphate leaching of gold/silver
Process Species
(X)
105
Dx
(cm2 s
-1)
Reference
Gold leaching by oxygenated cyanide O2 2.20 Kudryk and Kellogg, 1954
Gold leaching by oxygenated cyanide CN- 1.75 Kudryk and Kellogg, 1954
Diffusion limiting current of Cu(NH3)42+
Cu(II) 0.46 Nicol, 1975
Thiosulphate leaching of Au-Ag alloy Cu(II) 0.46 Jeffrey, 2001
Thiosulphate leaching of Ag Cu(II) 0.50 Zhang et al., 2008
Anodic oxidation of thiosulphate S2O32-
0.66 Sabzi, 2005
Ammoniacal Cu(II) leaching of Ni S2O32-
0.72 Senaputra et al., 2008
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Table 3. Chemical or electrochemical dissolution rates of gold in cyanide or thiosulphate based on rotating discs
Set Oxidant/ligand/pH/other conditions O2 or Cu(II) (mM) CN- or S2O32-
(mM)
106 RAu
(mol m-2 s-1)
Cyanide systema
A Au/air/CN-
Air or O2 saturated
10 4.5(4.4)
Au/air/CN- 5 1.0*
Au/air/CN- (with 3 ppm Ag(I)) 10 17.5(11.5)
Au/O2/CN- (with 3 ppm Ag(I)) 10 18.0
Au-Ag(1%)/CN- Electrochemical 10 (14.0)
Au-Ag(5%)/air/CN- Air saturated 5 41*
Ag/air/CN- Air saturated 10 48
Thiosulphate systemb
B Au/O2/Na2S2O3 O2 saturated 200 0.1
Au/O2/Na2S2O3 100 (~0.4**)
Au/O2/(NH4)2S2O3 200 4.0
Au-Ag(5%)/
Cu(II)/S2O32-/NH3 (0.4 M)
10 100 38
Ag/Cu(II)/S2O32-/NH3 (0.84 M) 25 400 270
Thiosulphate systemc
C Au/NH3 +NH4+ (0.2 M) / 0.1 M NaOH 0 <0.01
Au/S2O32-
Electrochemical
1000 0.66
Au/S2O32-/NH3+NH4
+(0.2 M) 1000 1.42
Thiosulphate systemd
D Au/S2O32-, pH 12
Electrochemical
200 0.021
Au/S2O32-/Cu(II) (0.5 mM), pH 12 200 0.021
Au/S2O32-/Cu(II) (5 mM), pH 12 200 0.065
Thiosulphate systeme
E Au/S2O32-/ NH3+NH4
+ (0.4 M) 10 200 33 (31)
+ 25 mM Na2S3O6 10 200 29 (25)
+ 25 mM Na2SO4 10 200 29 (19)
+ 25 mM Na2S4O6 10 200 27 (23)
+ 25 mM Na2S2O5 10 200 1.8 (1.8)
Thiosulphate system j
F Au/S2O32-/Cu(II), NH3+NH4
+ (2 M) 12 100 28 (NH4+)
Au/S2O32-/Cu(II), NH3+NH4
+ (0.5 M)k 4 200 11 (Ca2+)
4 200 8 (NH4+)
4 200 5 (Na+)
Rates based on REQCM (Jeffrey, 2001; Jeffrey et al., 2001) or chemical analysis of solutions (Zhang and Nicol, 2003,
2005); rates marked by * are also from Jeffrey et al. (2001); rates marked by ** are in O2/Na2S2O3 from Baron et al.
(2011)
(a) Li et al. (1992) and Wadsworth and Zhu (2003), 25oC; values in parentheses based on corrosion currents
(b) Jeffrey (2001) (pH 10); Jeffrey et al.( 2001), Chandra and Jeffrey (2003) (pH <7) ; 300 rpm, 30oC
(c) Zhang and Nicol (2003), at 0.25 V and pH 10.6 adjusted with NaOH, 25oC
(d) Zhang and Nicol (2005), at 0.3 V and pH 12, 25oC
(e) Chu et al. (2003), 0.25 V, REQCM, anaerobic in fresh solutions (results in parentheses correspond to 1 h after
mixing)
(j) Feng and van Deventer (2002b), gold foil in (NH4)2S2O3 open to air, ambient temperature, cations in parentheses
show the different thiosulphate salts.
(k) Feng and van Deventer (2010d), 20oC, initial 5-6 h in different thiosulphate salt solutions.
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Table 4. Effect of cations and co-ligands on dissolution rates of gold in thiosulphate with or without ammonia
Set Thiosulphate Salt Ammonia
addition
/M
Other additives
(or conditions)
106
RAu
mol m-2
s-1
Salt M
G
Na2S2O3
0.1
nil
pH 13
0.06
0.5 0.74
1.0 1.66
1.0 pH 12 0.88
1.0 pH 10.6 0.66
H Na2S2O3
0.2
nil
none 0.96
0.05 mM CH3NH3Cl 2.22
0.05 mM (CH3)2NH2Cl 1.33
0.05 mM (CH3)3NHCl 0.68
I (NH4)2S2O3
0.1
nil
none 0.74
0.2 none 8.14
0.4 none 22.2
0.6 none 32.6
J Na2S2O3
0.2
nil
pH 6-7
none 0.95
K2S2O3 none 1.89
(NH4)2S2O3 none 5.68
K
Na2S2O3
0.2
nil
pH 7
none 0.21
0.5 mM Cu(I) 0.70
5 mM Cu(I) 2.04
L
(NH4)2S2O3
0.2
nil
pH 6-7
0.05 M CsCl 12.4
0.2 M CaCl2 14.9
1 M NH4Cl 20.9
1 M (NH4)2SO4 24.2
M (NH4)2S2O3
0.2
nil
0 mM TU 5.68
1 mM TU 37.0
5 mM TU 89.2
5 mM TU, 2%Ag alloy 16.8
10 mM TU 127
Na2S2O3
0.2
nil
0 mM TU 0.95
1 mM TU 2.49
5 mM TU 7.46
10 mM TU 9.45
N Na2S 1.6 nil none 280
O Na2S2O3
0.1
0 none 1.10
0.2 none 4.07
0.4 none 6.39
0.6 none 9.01
P Na2S2O3
0.1
no none 1.28
0.4 none 6.39
0.4 10 mM Cu(II) 20.0
Q
(NH4)2S2O3
0.1
0 none 0.59
0.4 none 5.00
0.4 10 mM Cu(II) 17.6
R (NH4)2S2O3 0.4 0.44 10 mM Cu(II) , 2%Ag alloy 78.8
S Na2S2O3 0.1 0.4 none 8.74
0.4 0.5 mM Cu(I) 18.5
0.4 0.5 mM Cu(I) + 0.25 mM S4O62-
18.5
0.4 10 mM Cu(II) 28.6 Based on linear sweep voltammograms, potential 0.25 V, 300 rpm, 30oC,
G. Zhang and Nicol (2003)
H. Chandra and Jeffrey (2004)
I. Chandra and Jeffrey (2003)
J. Chandra and Jeffrey (2004)
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K. Zhang and Nicol (2005)
L. Chandra and Jeffrey (2004)
M. Chandra and Jeffrey (2004); Chandra and Jeffrey (2003)
N. Anderson (2003)
O. Breuer and Jeffrey (2002)
P. Breuer and Jeffrey (2003)
Q. Jeffrey et al.(2008b)
R. Jeffrey (2001)
S. Breuer and Jeffrey (2002)
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Table 5.
Comparison of anodic oxidation rate of gold in thiosulphate and sulphide media at 30oC
[Ligand]
Potential
/ V
Current (I)
A m-2
105 Rate
mol m-2 s-1
Equilibrium potential Ref
Couple Eo / V
0.6 M (NH4)2S2O3 0.25 3.1 3.2 Au(S2O3)2-/Au 0.16 a
0.2 M (NH4)2S2O3 0.25 -
1.6 M Na2S
-0.42
27
28
AuS-/Au -0.46 b
Au(HS)2-/Au
-0.09
a. Chandra and Jeffrey (2003), 300 rpm , 1 mV s-1 scan rate
b. Anderson (2003)
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Figures
(a)
78
80
82
84
86
88
90
22 24 26 28 30
Thiosulphate consumption / (kg/t)
Go
ld e
xtr
acti
on
% (
afte
r 2
4 h
)
EDTA
Na2S
Glycine
Na2SO3
None
Na2SO4
(b)
0
20
40
60
80
100
None EDTA SHMP CMC
Additive
0
2
4
6
Au (%) Ag (%) TSC (kg/t)
Th
iosu
lph
ate
con
sum
ptio
n (
kg
/t)
Go
ld,
silv
er e
xtr
actio
n %
(af
ter
24
h)
Fig. 1. Effect of additives (0.1 M) on gold/silver extraction and thiosulphate consumption
(TSC) after 24 h leaching at 25oC: (a) a mild-refractory copper bearing ore with 0.3 M
Na2S2O3, 0.03 M CuSO4, 3 M NH3, pH 10.2, 0.1 M additive (Xia et al., 2003); (b) a low
copper sulphidc ore with 0.1 M (NH4)2S2O3, 0.78 mM CuSO4. 0.5 M NH3, 0.15 g/L
EDTA, 0.80 g/L SHMP and 0.13 g/L CMC (see Fig. 2 for structures) (Feng and van
Deventer, 2010b, 2011a,b).
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Fig. 2
(a) Carboxymethyl cellulose (CMC)
(b) Ethylenediaminetetraacetic acid
(EDTA)
(c) Sodium hexametaphosphate (NaPO3)x
(SHMP)
(d) Glycene
(a) http://en.wikipedia.org/wiki/Carboxymethyl_cellulose
(b) http://en.wikipedia.org/wiki/Ethylenediaminetetraacetic_acid
(c) http://en.wikipedia.org/wiki/Sodium_hexametaphosphate
(d) http://en.wikipedia.org/wiki/Glycine
Fig. 2. Structures of beneficial additives for thiosulphate leaching of gold
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Fig. 3
(a)
-0.8
-0.4
0.0
0.4
0.8
1.2
S2
-
SC
N-
SO
32
-
HS
- I-
CN
-
Br-
NO
3-
OH
-
Cl-
SO
42
-
CO
32
-
F-
PO
43
-
Ca2
+
NH
4+
Na+ K+
Mg
2+
Cu
+
H+
Ag
+
Fe3
+
Zn
2+
Cu
2+
Pb
2+
Au
+
Ion
So
ftn
ess
of
anio
ns
-0.8
-0.4
0.0
0.4
0.8
1.2
Anions
Cations
So
ftn
ess
of
cati
on
s
(b)
0.0
1.0
2.0
3.0
4.0
5.0
S2
-
SC
N-
SO
32
-
HS
- I-
CN
-
Br-
NO
3-
OH
-
Cl-
SO
42
-
CO
32
-
F-
PO
43
-
Ca2
+
NH
4+
Na+ K+
Mg
2+
Cu
+
Ag
+
Zn
2+
Cu
2+
Pb
2+
Ion
0.0
1.0
2.0
3.0
4.0
5.0Lewis basicity of anions
Lewis acidity of cationsL
ewis
aci
dit
y o
f ca
tio
ns
Lew
is b
asic
ity
of
anio
ns
Fig. 3. Ion properties selected anions and cations: (a) softness, (b) Lewis acidity or
basicity (data from Marcus, 1997, except SO32-
in (b) see text).
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Fig. 4
y = 1.21x2 - 2.66x + 1.78
R2 = 0.98
0
1
2
3
4
5
-1.0 -0.5 0.0 0.5 1.0 1.5
Softness of anions
Lew
is b
asi
cit
y o
f an
ion
sS
2-
F-
Fig. 4. Correlation between Lewis basicity and softness of anions (data from Figs. 3a,b).
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Fig. 5
0
2
4
6
8
10
-0.8 -0.4 0 0.4 0.8 1.2Softness of anion X
z-
Res
idual
Cu(I
I) /
mM
0
10
20
30
40
pK
SP o
f C
uzX
2
PO43-
PO43-CO3
2-
SO42-
SO32-
S2-
S2-
OH-
Cl- NO3
-
Fig. 5. Correlation between anion softness, residual Cu(II) and pKSP of Cu(II) salts CuzX2
(softness from Fig. 3, residual Cu(II) after 3 h at 0.1 M anion, 0.1 M Na2S2O3, 0.4 M
NH3, 10 mM Cu(II), 30oC from Breuer and Jeffrey, 2003; pKSP from HSC 6.1).
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Fig. 6
Fig. 6. Effect of background reagents (anions) on residual Cu(II) after 3 h at 0.1 M anion,
0.1 M Na2S2O3, 0.4 M NH3, 10 mM Cu(II), 30oC from Breuer and Jeffrey, 2003.
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Fig. 7
-20
0
20
40
60
80
100
0 2 4 6 8
Time / h
Cu
(II)
o
r S
2O
32- re
acte
d (
%)
(i)
(ii)
(iii)
(iv)
(v)
(vi)
(vii)
None
5 mM S4O62-
10 mM S4O62-
5 mM S3O62-
10 mM S3O62-
5 mM HS-
10 mM HS-
Fig.7. Effect of Na2S4O6, K2S3O6 and Na2S on reaction % of Cu(II) (solid lines) or S2O32-
(dashed lines) reacted during gold foil dissolution (0.1 M Na2S2O3, 0.5 M NH3, 6 mM
CuSO4, 25 oC; Feng and van Deventer, 2007b).
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Fig. 8
0
20
40
60
80
100
0 8 16 24
Time / h
Cu(I
I) o
r S
2O
32- r
eacte
d (
%)
(i)
(ii)
(iii)
(i)
(ii)
(iii)
Na+
NH4+
Ca2+
Na+
NH4+
Ca2+
Fig.8. Effect of thiosulphate salts Na2S2O3, (NH4)2S2O3 and CaS2O3 on reaction % of
Cu(II) (solid lines) or S2O32-
(dashed lines) reacted during gold foil dissolution (0.1 M
thiosulphate, 0.5 M NH3, 4 mM CuSO4, 20 oC (open to air); Feng and van Deventer,
2010d).
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Fig. 9
(a)
0
10
20
30
40
50
0 5 10
[Cu(II)] / mM
R
Cu /
10-4
mo
l m
-3 s
-1
(i) No tetrathionate
(ii) 2 mM K2S4O6
(iii) 10 mM K2S4O6
(i)
(ii)
(iii)
(b)
0
2
4
6
8
10
0 0.5 1
Na2SO4 / M
Res
idu
al [
Cu
(II)
] /
mM
or
RC
u/
10-4
mo
l m
-3 s
-1
Residual [Cu(II)], after 3 h
Initial [Cu(II)] = 10 mM
RCu
(c)
0
2
4
6
8
10
12
0 4 8 12
Res
idu
al [
Cu
(II)
] /
mM
(i) No Na3PO4
(ii) 0.1 M Na3PO4
Time / h
(i)
(ii)
Fig.9. Effect of (a) Cu(II) and K2S4O6 , (b) Na2SO4 , and (c) Na3PO4 on residual Cu(II)
and initial rate of Cu(II) reaction with thiosulphate or tetrathionate at 0.4 M NH3, 10 mM
CuSO4, 30 oC; other conditions: (a,b) 0.1 M Na2S2O3 (Breuer and Jeffrey, 2003); (c) 0.2
M Na2S2O3 (Jeffrey et al., 2003).
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Fig. 10
0.E+00
2.E-05
4.E-05
6.E-05
8.E-05
1.E-04
Na2
S2O3
K2S2O
3
(NH
4)2S2O
3
(NH
4)2S2O
3 + 0
.05
M C
sCl
(NH
4)2S2O
3 + 0
.001
M T
U
(NH
4)2S2O
3 (2
%A
g/A
u)
RA
u /
mol
m-2
s-1
Fig. 10. Effect of different thiosulphate salts (0.2 M), background reagents, thiourea and
alloyed silver on initial gold dissolution rate based on mass loss from gold electrode with
REQCM at 0.25 V, 0.4 M NH3, 10 mM Cu(II), 30oC from Breuer and Jeffrey, 2003.
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Fig. 11
0.0E+00
2.0E-05
4.0E-05
6.0E-05
8.0E-05
1.0E-04
1.2E-04
0 0.2 0.4 0.6 0.8 1Concentration of
NH3, (NH4)2S2O3, Cl- or TU / (mol L
-1)
RA
u / (m
ol
m-2
s-1
)
(c)
(a)
(b)
(a) 0.1 M Na2S2O3 + 0-0.6 M NH3
(b) 0.1-0.4 M (NH4)2S2O3
(c) 0.2 M (NH4)2S2O3
+ CsCl, CaCl2 or NH4Cl
(d ) 0.2 M (NH4)2S2O3 + TU
(d)
Fig. 11. Effect of ammonia, thiosulphate, chloride or thiourea concentration on initial
gold dissolution rate based on mass loss from gold electrode with REQCM at 0.25 V, 0
mM Cu(II), 30oC and 0 M NH3 except in (a) from Chandra and Jeffrey, 2003, 2004.
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Fig. 12
H2O H2O
NH3
NH3 H3N
H3N
Cu
H2O
H2O
NH3
NH3 H3N
H3N
Cu
S2O32-
NH3
NH3 H3N
H3N
Cu
L
L S2O3
2-
Cu(I) + tetrathionate S2O32-
Fig. 12. Reaction equilibria involved in the oxidation of thiosulfate by opper(II).
( Ligand L = NH3, SO42-
, Cl- ; van Wensveen and Nicol, 2005).
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Fig. 13
(stable) Cu(S2O3)35-
NH3 + Cu(S2O3)23-
Cu(NH3)p(S2O3)- + S2O3
- Cu(NH3)p(S2O3)2
2-
+ S2O32-
2Cu(NH3)p+
+ S4O62-
+( Au + S2O32-
)
Cu(NH3)p+ + S2O3
-
Cu(S2O3)35-
Cu(NH3)p+ + Au(S2O3)2
3-
(stable) (stable)
Cu(NH3)m2+
+ S2O32-
Cu(NH3)p(S2O3)0 + (m-p)NH3 (A)
(B2)
(D)
(C)
(E)
(B1)
Fig. 13. Proposed reaction model for the oxidation of thiosulfate and gold by copper(II)
in ammoniacal solutions (Senanayake, 2004b, 2005a,b).
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Fig. 14
(a)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
0 2 4 6 8Time / h
RA
u /
(10
-5 m
ol
m-2
s-1)
150
200
250Rate (without phosphate)
Rate (with phosphate)
Potential (without phospahte)
EH (
mV
)
(b)
0
20
40
60
80
100
0 2 4 6 8
Time / h
Au f
rom
pow
der
dis
solu
tion (
%)
Without phosphate
With phosphate
(c)
30
36
42
0.0 1.0 2.0
SHMP / (g L-1
)
Cu
(II)
reacte
d (
%)
0.02
0.04
0.06
0.08
Au
fo
il r
eacte
d /
mo
l m
-2
Fig.14. Effect of Na3PO4 (a,b) or (NaPO3)x (SHMP) (c) on RAu and Cu(II) and gold
reacted; conditions (a,b) 0.2 M Na2S2O3, 0.4 M NH3, 10 mM Cu(II) at 30oC in the
absence of oxygen in argon atmosphere (Jeffrey et al., 2003); (c) 0.1 M (NH4)2S2O3, 0.4
M NH3, 0.8 mM CuSO4, 20 oC after 5 h (Feng and van Deventer, 2011b).
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Fig. 15
0
0.1
0.2
0.3
0.4
0 8 16 24
Time / h
Au
dis
solu
tio
n /
mo
l m
-2Ca
2+
NH4+
Na+
(a) 0.1 M thiosulphate
0
0.2
0.4
0.6
0.8
0 8 16 24Time / h
Au
dis
solu
tio
n /
mo
l m
-2
Ca2+
NH4+
Na+
(b) 0.2 M thiosulphate
Fig. 15. Effect of 0.1 M or 0.2 M thiosulphate salts of Na+, NH4
+ and Ca
2+ on dissolution
of gold foil after 7 h or 24 h at 0.5 M NH3, 4 mM Cu(II), 25oC (from Feng and van
Deventer, 2010d).
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Fig. 16
1.5
2.0
2.5
3.0
0 0.4 0.8 1.2 1.6
[Pb(II) or [Zn(II)] / mM
Au
dis
solu
tio
n /
mo
l m
-2
Pb(II)
Zn(II)
Fig. 16. Effect of Pb(II) and Zn(II) concentration on gold dissolution from foil immersed
in 0.5 M (NH4)2S2O3, 2 M NH3, 12 mM Cu(II) after 24 h at 25oC from Feng and van
Deventer (2002b).
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Fig. 17
0
0.2
0.4
0.6
None Zn(II) Pb(II)
Additive
Au
dis
solu
tio
n /
mo
l m
-2
6 mM Cu(II)12 mM Cu(II)
Fig. 17. Effect of 5 mg/L Zn(II) and Pb(II) on gold dissolution from foil immersed in 0.1
M (NH4)2S2O3, 0.5 M NH3, 6-12 mM Cu(II) after 24 h at 25oC from Feng and van
Deventer (2002b).
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Fig. 18
0
0.1
0.2
0.3
0.4
0.5
0.6
0 10 20 30 40 50
Time/ Weeks
Au
or
Ag
dis
solu
tio
n
mM
Au(64 mol% Ag) Ag(64 mol% Ag)Au(Au-Ag(col) 8:1) Ag(Au-Ag(col) 8:1)Au(massive) Au(colloid)
Fig. 18. Gold and silver dissolution from massive or colloidal material in 0.1 M sodium
thiosulphate in the presence of oxygen (Webster, 1986).
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Fig. 19
0.00
0.05
0.10
0.15
0 2 4 6 8
Time / hA
u d
isso
luti
on
/ m
ol
m-2
Na2S4O6
K2S3O6
Na2S
None
Fig. 19. Effect of sulphur species (anions), on gold dissolution (mol/m2) from foil with
time at 0.5 M NH3, 0.1 M Na2S2O3, 6 mM Cu(II), 5 mM Na2S4O6, K2S3O6, Na2S, 25oC
from Feng and van Deventer, 2007b.
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Fig. 20
Au3+ Au(OH)3
Au
AuS- Au
Au+ Au(OH)4
-
Au(S2O3)3- 2
pH
Eh /V
2.0
1.5
1.0
0.5
0
- 0.5
- 1.0 -2 0 2 4 6 8 10 12 14 16
Fig. 20. Eh-pH diagram for the Au-S-H2O system (Chen et al. 1996)
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Fig.21
0
0.1
0.2
0.3
0 5 10 15 20
Concentration / mMA
u d
isso
luti
on
/ m
ol
m-2
Na2S4O6
Na2S3O6
Na2S
Fig. 21. Effect of concentration of sulphur species on gold dissolution (mol/m2) from foil
after 24 h at 25oC. Conditions: (a) 0.5 M NH3, 0.1 M Na2S2O3, 6 mM Cu(II), 2.5-20 mM
Na2S4O6, K2S3O6, Na2S (from Feng and van Deventer, 2006)
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Fig.22
(a)
0.00
0.05
0.10
0.15
0.20
0.25
0 1 2Additive / g L
-1A
u d
isso
luti
on
afte
r 2
4 h
/ m
ol
m-2
SHMP
(b)
0.00
0.05
0.10
0.15
0.20
0.25
0 0.05 0.1 0.15
Additive / g L-1
Au
dis
solu
tio
n
afte
r 2
4 h
/ m
ol
m-2
EDTA
CMS
Fig. 22. Effect of concentration of (a) SHMP & (b) EDTA and CMS on gold dissolution
(mol/m2) from foil after 24 h at 25
oC. Conditions: 0.5 M NH3, 0.1 M (NH4)2S2O3, 0.8
mM Cu(II) (from Feng and van Deventer 2010b, 2011a,b)
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Fig. 23
0
20
40
60
80
100
0 5 10 15Silver grade of oxide ore (g/t)
Au
or
Ag
ex
trac
tio
n (
%)
Au (CN) Ag (CN)Au (TS)(max) Ag (TS)(max)Au (TS)(min) Ag (TS)(min)
(CN =cyanide,TS = thiosulfate)
Fig. 23. Effect of silver content in ore on metal extraction in cyanide and dilute
ammoniacal copper(II) thiosulphate (Muir and Aylmore, 2004, 2005).
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Fig. 24
(a)
0
50
100
0 0.5 1 1.5 2 2.5
PbS (%)
Go
ld e
xtr
acti
on
fro
m o
re
(%)
(aft
er 3
h)
Ore + 16% FeS2 + PbS
Ore + 16% FeS + PbS
Ore + PbS
(b)
0
50
100
0 1 2 3 4 5 6Concentration of added PbCO3 (mM), Pb(NO3)2 (mM),
PbO (%) or PbS(%)
Au
ex
trac
tio
n %
(af
ter
3 h
) PbCO3
Pb(NO3)2
PbO
PbS
0.8 M (NH4)2S2O3, pH 9.7
0.4 M (NH4)2S2O3, pH 9.9
0.2 M (NH4)2S2O3 , pH 10.1
Fig. 24. Effect of Pb(II) compounds on gold extraction after 3 h from a natural quartz
gold ore of -74 μm, 33% solid, 20-24 oC, 1.2 mM CuSO4, 0.2 M (NH4)2S2O3, 0.9 M NH3,
air; data from Xia and Yen, 2008.
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Fig. 25
0
50
100
0 10 20 30 40
Na3PO4 or Na2CO3 (mM)
Go
ld e
xtr
acti
on
% (
afte
r 3
h)
0% PbS, Sodium phosphate
2.4% PbS, Sodium phosphate
2.4 % PbS, Sodium carbonate
Fig. 25. Effect of Na3PO4 or Na2CO3 in the absence or presence of 2.4% PbS on gold
extraction after 3 h from a natural quartz gold ore, conditions same as in Fig. 16; data
from Xia and Yen, 2008.
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Fig. 26
(a)
40
60
80
100
0 1 2[SHMP] / g L
-1
Au
or
Ag
extr
acti
on
(%
)
Ag (24 h)
Ag (48 h)
Au (24 h)
Au (48 h)
(b)
40
60
80
100
0.0 0.2 0.4 0.6
[EDTA] / g L-1
Au o
r A
g e
xtr
acti
on (
%)
Ag (24 h)
Ag (48 h)
Au (24 h)
Au (48 h)
(c)
40
60
80
100
0.00 0.05 0.10 0.15
[CMC] / g L-1
Au
or
Ag
ex
tracti
on
(%
)
Ag (24 h)
Ag (48 h)
Au (24 h)
Au (48 h)
Fig. 26. Effect of SHMP (a), EDTA (b) and CMC (c) on gold and silver extraction from a
sulphide ore after 24 or 48 h at 0.5 M NH3, 0.1 M (NH4)2S2O3, 0.8 mM Cu(II) 25oC from
Feng and van Deventer, 2010b, 2011a,b.
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0
5
10
15
20
25
0 10 20 30 40 50 60 70 80
PbS (%) or Pb(NO3)2, Na3PO4, Na2CO3 (mM)
Th
iosu
lph
ate
Co
nsu
mp
tio
n (k
g/t
)
(aft
er 3
h)
Na2CO3
PbS (+16% FeS2)
PbS (+16% FeS)
Na3PO4
Pb(NO3)2
Fig. 27. Effect of background reagents in the absence or presence of 16% FeS or FeS2
on thiosulfate consumption after 3 h of leaching, conditions same as in Fig. 17; data from
Xia and Yen, 2008.
ACC
EPTE
D M
ANU
SCR
IPT
ACCEPTED MANUSCRIPT
79
Fig. 28
0
4
8
12
None SHMP
(0.8 g/L)
SHMP
(1.8 g/L)
EDTA
(0.15 g/L)
EDTA
(0.6 g/L)
CMC
(1.3 g/L)
Additive
TS
C /
kg t-1
24 h
48 h
Fig. 28. Effect of SHMP, EDTA and CMC on thiosulphate consumption during gold and
silver extraction from a sulphide ore after 24 or 48 h at 0.5 M NH3, 0.1 M (NH4)2S2O3,
0.8 mM Cu(II) 25oC from Feng and van Deventer, 2010b, 2011a,b.
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