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
(%)
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
(%)