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)
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)