XXXI International Mineral Processing Congress 2024 Proceedings Page 1670 (1694 of 3836)

XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3 1759
For the concentrate evaluated in this study, a thiosulfate
concentration of 0.05 M proved to be optimal for gold dis-
solution under the applied conditions.
Effect of Ammonia on Gold Extraction
The use of ammonia in a copper-thiosulfate leaching system
is to minimize the decomposition of thiosulfate into poly-
thionates (Yazıcı et al., 2018), stabilize copper (Yener Yazıcı
et al., 2011), stabilize pH, and improve leaching kinetics
(Lampinen et al., 2015).The effect of ammonia on gold
dissolution was examined using various concentrations of
ammonia, and the results are shown in Figure 4b. A lower
concentration of NH3 did not yield much gold dissolu-
tion but increased with NH3 up to 0.3 M only. However,
the decline in Au extraction at a much higher concentra-
tion (more than 0.3 M) of NH3 could be because the pH
reduces the area of the regions of thermodynamic stabil-
ity of Cu(NH3)42+ and Cu(S2O3)35–, while widening the
regions of thermodynamic stability of solid copper species
such as CuO and Cu2O (Abbruzzese et al., 1995 Zipperian
et al., 1988).
12.78% 10.98% 8.52%
0%
20%
40%
60%
80%
100%
0.05 M 0.10 M 0.15 M
Thiosulfate concentration (M)
(a)
9.08% 12.26% 11.76%
0%
20%
40%
60%
80%
100%
0.1 M 0.5 M 0.9 M
Ammonia concentration (M)
(b)
18.90% 17.80% 15.69%
0%
20%
40%
60%
80%
100%
1.0 mM 1.5 mM 3.0 mM
Copper ions concentration (mM)
(c)
30.78% 30.39%
25.08%
0%
20%
40%
60%
80%
100%
pH 9.0 pH 10.0 pH 11.0
pH level
(d)
(a) Conditions: pH 10.0–10.5, shaking rate 180 rpm, pulp density 10%, at 1 hour. Reagents concentrations: 0.1 M Cu2+, 0.5 M NH3
(b) Conditions: pH 10.0–10.5, shaking rate 180 rpm, pulp density 10%, at 1 hour. Reagents concentrations: 0.1 M Cu2+, 0.05 M S
2 O
3
2–
(c) Conditions: pH 10.0–10.5, shaking rate 180 rpm, pulp density 10%, at 1 hour. Reagents concentrations: 0.3 M NH3, 0.05 M S2O32–
(d) Conditions: shaking rate 180 rpm, pulp density 10%, at 4 hours. Reagents concentrations: 0.05 M S2O32–, 0.3 M NH3, 1.0 mM Cu2+
Figure 4. Effects of (a) thiosulfate, (b) ammonia, (c) copper ions concentration, and (d) pH level
Leaching
efficiency
(%)
Leaching
efficiency
(%)
Leaching
efficiency
(%)
Leaching
efficiency
(%)

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Extracted Text (may have errors)

XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3 1759
For the concentrate evaluated in this study, a thiosulfate
concentration of 0.05 M proved to be optimal for gold dis-
solution under the applied conditions.
Effect of Ammonia on Gold Extraction
The use of ammonia in a copper-thiosulfate leaching system
is to minimize the decomposition of thiosulfate into poly-
thionates (Yazıcı et al., 2018), stabilize copper (Yener Yazıcı
et al., 2011), stabilize pH, and improve leaching kinetics
(Lampinen et al., 2015).The effect of ammonia on gold
dissolution was examined using various concentrations of
ammonia, and the results are shown in Figure 4b. A lower
concentration of NH3 did not yield much gold dissolu-
tion but increased with NH3 up to 0.3 M only. However,
the decline in Au extraction at a much higher concentra-
tion (more than 0.3 M) of NH3 could be because the pH
reduces the area of the regions of thermodynamic stabil-
ity of Cu(NH3)42+ and Cu(S2O3)35–, while widening the
regions of thermodynamic stability of solid copper species
such as CuO and Cu2O (Abbruzzese et al., 1995 Zipperian
et al., 1988).
12.78% 10.98% 8.52%
0%
20%
40%
60%
80%
100%
0.05 M 0.10 M 0.15 M
Thiosulfate concentration (M)
(a)
9.08% 12.26% 11.76%
0%
20%
40%
60%
80%
100%
0.1 M 0.5 M 0.9 M
Ammonia concentration (M)
(b)
18.90% 17.80% 15.69%
0%
20%
40%
60%
80%
100%
1.0 mM 1.5 mM 3.0 mM
Copper ions concentration (mM)
(c)
30.78% 30.39%
25.08%
0%
20%
40%
60%
80%
100%
pH 9.0 pH 10.0 pH 11.0
pH level
(d)
(a) Conditions: pH 10.0–10.5, shaking rate 180 rpm, pulp density 10%, at 1 hour. Reagents concentrations: 0.1 M Cu2+, 0.5 M NH3
(b) Conditions: pH 10.0–10.5, shaking rate 180 rpm, pulp density 10%, at 1 hour. Reagents concentrations: 0.1 M Cu2+, 0.05 M S
2 O
3
2–
(c) Conditions: pH 10.0–10.5, shaking rate 180 rpm, pulp density 10%, at 1 hour. Reagents concentrations: 0.3 M NH3, 0.05 M S2O32–
(d) Conditions: shaking rate 180 rpm, pulp density 10%, at 4 hours. Reagents concentrations: 0.05 M S2O32–, 0.3 M NH3, 1.0 mM Cu2+
Figure 4. Effects of (a) thiosulfate, (b) ammonia, (c) copper ions concentration, and (d) pH level
Leaching
efficiency
(%)
Leaching
efficiency
(%)
Leaching
efficiency
(%)
Leaching
efficiency
(%)

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1758 XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3
of the flotation concentrate, confirming the association of
gold with sulfide minerals such as pyrite and galena.
Gold Leaching Experiments
All leaching tests were carried out using a 50 mL coni-
cal flask, placed in a shaking bath (TBK102HA, AXEL,
Japan), and the volume for leaching was fixed at 20 mL.
Predetermined volumes of Cu2+, NH3, and S2O32–
were transferred into the flask, followed closely by relevant
amounts of solids and water. The slurry pH and ORP were
measured, and H2SO4 and NaOH were used to make
relevant pH adjustments. After leaching, the filtrate and
the residues were separated by filtration. Filtration of the
leached slurry was performed using a vacuum filter, and
the residues were dried at 80 °C in an electric drying oven
(ADVANTEC-DRD420DA) for 6 hours. The leach filtrate
was diluted where necessary and analyzed for Au using an
MP-AES machine. The solid leach residues were further
studied using XRF, XRD, and SEM.
To optimize the leaching conditions that increase the
amount of Au dissolved, leach control variables such as the
leaching reagent concentration, pH, and duration of the
leaching process were tested separately. The range tested for
each leach control parameter are: [S2O32–] 2.5 mM–0.4 M,
[NH3] 0.1 M–0.9 M, [Cu2+] 0.3 mM–20 mM, slurry pH
10.00–12.00, leaching time 1 h–10 h.
Pretreatment Experiments
Pretreatment tests were conducted in a shaking bath sys-
tem, utilizing various parameters to optimize the condi-
tions for maximum gold extraction. The primary reagents
for pretreatment included ferric ions and manganese, using
manganese oxide and iron sulfate. These tests involved a
50 mL conical flask set in a shaking bath (TBK102HA,
AXEL, Japan). The pretreatment volume was maintained at
20 mL, with stirring achieved through shaking. After pre-
treatment, filtration of the oxidized slurry was performed
with a vacuum filter, and the residues were subsequently
dried at 80 °C in an electric drying oven for 6 h, and then
further analyzed by XRF, XRD, and SEM-EDS, while the
solution was analyzed by MP-AES.
Recovery Experiments
For the recovery of gold from the pregnant leaching solu-
tion, first, the pyrite (FeS2) model sample, obtained from
Huanzala Mine in Peru with 86% purity, was used as a main
recovery agent. Experiments were conducted using a 50 mL
conical flask, placed in a shaking bath, and the volume for
leaching was fixed at 10 mL. 1 g of pyrite was mixed with
stock ammonium thiosulfate solution containing 100 ppm
of Au ions in 50-ml Erlenmeyer flasks at 25 °C (shaking
amplitude of 40 mm and frequency of 120 min−1). After
recovery, the subsequent separation and analysis procedures
are the same as above.
RESULTS AND DISCUSSION
Effects of Various Parameters on Leaching of Gold
from Flotation Concentrate
The ammonia-copper-thiosulfate leaching system is very
dynamic, and extraction depends on the interaction of
ammonia, copper, and thiosulfate within a set operating
condition. To increase gold extraction, optimum operating
conditions for leach control variables such as time and pH
were individually tested.
Au +5S2O3 2– +Cu(NH3)4 2– =
Au(S2O3)2 3– +Cu(S2O3)3 5– +4NH3 (1)
2Cu(S2O3)3 5– +8NH3 +0.5O2 +H2O =
2Cu(NH3)4 2– +2OH– +6S2O3 2– (2)
The leaching reagents [Cu2+, NH3, and S2O32–] simultane-
ously interact with each other (eqs. 1 and 2) as well as the
solids in the slurry.
Effect of Thiosulfate on Gold Extraction
The effect of thiosulfate on leaching is shown in Figure 4a. At
lower concentrations of thiosulfate, gold leaching tends to
be less effective, potentially due to insufficient reagent avail-
ability, and highest gold leaching efficiency was obtained at
0.05 M of thiosulfate concentration. Conversely, at con-
centrations exceeding 0.05 M, an increase in thiosulfate
concentration leads to a reduction in gold dissolution. The
decline in dissolution could be linked to the creation of
by-products such as elemental sulfur, which can inhibit
the dissolution of gold by causing passivation (Feng &van
Deventer, 2006 Feng &Van Deventer, 2011 Mahmoud
et al., 2015) as well as the formation of polythionates, sul-
fites, and sulfates. These compounds deplete thiosulfate
and further obstruct the dissolution process (Mohammadi
et al., 2017). Additionally, elevated thiosulfate levels tend
to enhance copper stability, expanding the stability region
of [Cu(S2O3)3]5– over [Cu(NH3)4]2+ (eq. 3) and may also
lead to the precipitation of copper (II) (Lampinen et al.,
2015 Navarro et al., 2002). This, in turn, can impede cop-
per’s effectiveness as an oxidizing agent and further restrict
the dissolution of gold.
[Cu(NH3)4]2+ +3S2O2 3– +e– →
[Cu(S2O3)3]5– +4NH3 (3)

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1760 XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3
Effect of Copper Ions on Gold Extraction
Gold dissolution in thiosulfate solution in the presence of
oxygen is very slow, but copper (II) has been reported to be
a better oxidant to dissolve gold at a faster rate (Aylmore,
2016). Figure 4c shows the amount of gold extracted with
various dosages of copper (II). A lower concentration of
copper did not promote gold dissolution. This could be due
to the limited catalyst in the thiosulfate leaching system.
Gold dissolution increased at low copper concentrations of
up to 1 mM, beyond which gold dissolution declined. A
similar pattern has been reported by Income et al. (2021)
(Income et al., 2021) among others. Low gold extraction
at high copper (II) dosage could be a consequence of the
precipitation of copper into CuO, Cu2O, CuS, and Cu2S
which coats the surface of gold. Precipitation of cop-
per minerals usually occurs when the stability region for
Cu(NH3)42+ complex is narrowed as a result of rapid oxida-
tion of thiosulfate by copper (II) (Abbruzzese et al., 1995
Income et al., 2021 Sitando et al., 2020).
Effect of pH on Gold Extraction
The role of pH in the ammonia-copper-thiosulfate leaching
system is crucial, as it directly influences the solution’s Eh
(Redox potential). A pH above 9 is commonly maintained
in this system because thiosulfate tends to decompose at
a faster rate at pH levels below 9. In this study, pH levels
between 8.5 and 12.0 were tested to determine the opti-
mum condition for leaching the concentrate. Contrary to
previous reports suggesting a stable pH of 10 is necessary
(Lampinen et al., 2015), our current results indicate that a
pH of 9.5 is more favorable, resulting in higher gold dis-
solution rates as shown in Figure 4d.
The optimized leaching conditions established for the
flotation concentrate were 0.05 M S2O32–, 0.3 M NH3,
1.0 mM Cu2+, pH 9.5, and 8 h. Under the optimized con-
ditions, the extraction of gold was 44% and this low extrac-
tion can be attributed to the double refractoriness feature
of the sample. To further refine and enhance the leaching
efficiency of gold, the pretreatment process was included in
the technological scheme.
Effects of Pretreatment on the Leaching of Gold from
Flotation Concentrate
The gold particles are locked in sulfide minerals, mostly in
pyrite, making direct leaching non-efficient. To increase the
percentage of gold leached, it is necessary to oxidize pyrite,
which leads to the destruction of the mineral layer. The
oxidation of pyrite was conducted using ferric ions with
atmospheric oxygen:
FeS2 +8H2O +14Fe3+ → 15Fe2+ +2SO4 2– +16H+ (4)
2FeS2 +2H2O +7O2 → 2Fe2+ +4SO4– 2 +4H+ (5)
Initially, the effect of pH under various times was carried
out for the pretreatment of the flotation concentrate. From
the pH range between 1.0–1.8, the results showed little
effect on the oxidation rate (Garrels R. M. &Thompson
M. E., 1960), with a minor advantage at pH 1.2, which was
chosen as the optimum (Figure 5).
Initially, the oxidation of pyrite occurs rapidly as fer-
ric iron leaches the pyrite and is subsequently reduced to
2.3 2.4 2.1 1.9 1.5
6.1 6.2 6.4
5.2
4.5
0
2
4
6
8
10
pH 1.0 pH 1.2 pH 1.4 pH 1.6 pH 1.8
(a)
Ferric ions Ferrous ions
7.4%
8.2% 7.9%
5.0% 4.7%
0%
2%
4%
6%
8%
10%
1.0 1.2 1.4 1.6 1.8
pH level
(b)
Experiment condition: 10% pulp density, 180 rpm shaking rate, 80 °C temperature, 4 hours. Reagent concentration: 6 g/L Fe3+
solution
Figure 5. (a) Effect of pH on ferric/ferrous ion concentration (b) and pyrite oxidation
Concentration
(g/L) Pyritoxidati
(%)

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