XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3 1751
complex ([Cu(NH3)4]2+), and then the extracted gold
ion (Au+) is stabilized by thiosulfate ions forming gold-
thiosulfate complex ([Au(S2O3)2]3–) (Grosse et al., 2003).
Under oxidizing conditions, cuprous thiosulfate complexes
formed after gold oxidation can be re-oxidized to cupric
ammonia complexes (Eq. (2)), which can be reutilized as
an oxidant for gold.
Au +5S2O32− +[Cu(NH3)4]2+ →
[Au(S2O3)2]3− +[Cu(S2O3)3]5− +4NH3 (1)
4[Cu(S2O3)3]5− +16NH3 +O2 +2H2O →
4[Cu(NH3)4]2+ +12S2O32− +4OH− (2)
Meanwhile, the presence of arsenopyrite showed a detri-
mental evffect on gold extraction that is, only about 10%
of gold extraction was achieved after 24 h. This decreased
gold extraction was attributed to arsenopyrite that accel-
erates the decomposition of thiosulfate ions (Mhandu et
al., 2023 Xu et al., 2017 Xu et al., 1995). Figure 2(b)
shows UV-vis spectra of fresh thiosulfate solution and the
one reacted with and without arsenopyrite for 40 min. As
shown in the UV-vis spectrum of fresh thiosulfate solu-
tion, a strong absorption peak centered at ~212 nm was
observed, but compared to this, peak intensity decreased
substantially when reacted with arsenopyrite. This supports
that arsenopyrite enhanced the decomposition of thiosul-
fate ions as follows (Xu et al., 2017):
4S2O32− +O2 +2H2O → 2S4O62− +4OH− (3)
4S4O62− +6OH− → 5S2O32− +2S3O62− +3H2O (4)
2S3O62− +6OH− → S2O32− +4SO32− +3H2O (5)
2SO32− +O2 → 2SO42− (6)
This strongly suggests that passivation of arsenopyrite is
essential to improve gold extraction from gold-bearing arse-
nopyrite ores by thiosulfate. In the next section, passivation
of arsenopyrite by microencapsulation using ferrous and
phosphate ions will be discussed.
Passivation of arsenopyrite by microencapsulation
treatment
Figure 3 shows the changes in dissolved Fe and P con-
centrations before and after ME treatment of arsenopyrite
for 24 h. Initial concentrations of ferrous and phosphate
ions were both 10 mM but decreased to 0.3 mM after ME
treatment. This can be explained by the following reactions:
4Fe2+ +O2 +4H+ → 4Fe3+ +2H2O (7)
Fe3+ +H2PO4− → FePO4 +2H+ (8)
Arsenopyrite is known to dissolve electrochemically, indi-
cating that it consists of two distinct sites called cathode
and anode. In the presence of oxygen, the cathodic site
loses electrons, and these electron holes are filled by elec-
trons from the anodic site. As a result, anodic dissolution
of arsenopyrite takes place however, this reaction is sacri-
ficed in the presence of ferrous ions that donate electrons to
electron holes of anodic site of arsenopyrite by being oxi-
dized to ferric ions. The ferrous oxidation by oxygen occur-
ring on the surface of arsenopyrite is illustrated in Eq. (7).
Ferric ions present near the arsenopyrite surface react with
Figure 2. Effect of arsenopyrite on thiosulfate leaching of gold: (a) gold extraction by thiosulfate in the absence and presence
of arsenopyrite and (b) UV-vis spectra of fresh thiosulfate solution and the one reacted with and without arsenopyrite for 40
min (Mhandu et al., 2023)
complex ([Cu(NH3)4]2+), and then the extracted gold
ion (Au+) is stabilized by thiosulfate ions forming gold-
thiosulfate complex ([Au(S2O3)2]3–) (Grosse et al., 2003).
Under oxidizing conditions, cuprous thiosulfate complexes
formed after gold oxidation can be re-oxidized to cupric
ammonia complexes (Eq. (2)), which can be reutilized as
an oxidant for gold.
Au +5S2O32− +[Cu(NH3)4]2+ →
[Au(S2O3)2]3− +[Cu(S2O3)3]5− +4NH3 (1)
4[Cu(S2O3)3]5− +16NH3 +O2 +2H2O →
4[Cu(NH3)4]2+ +12S2O32− +4OH− (2)
Meanwhile, the presence of arsenopyrite showed a detri-
mental evffect on gold extraction that is, only about 10%
of gold extraction was achieved after 24 h. This decreased
gold extraction was attributed to arsenopyrite that accel-
erates the decomposition of thiosulfate ions (Mhandu et
al., 2023 Xu et al., 2017 Xu et al., 1995). Figure 2(b)
shows UV-vis spectra of fresh thiosulfate solution and the
one reacted with and without arsenopyrite for 40 min. As
shown in the UV-vis spectrum of fresh thiosulfate solu-
tion, a strong absorption peak centered at ~212 nm was
observed, but compared to this, peak intensity decreased
substantially when reacted with arsenopyrite. This supports
that arsenopyrite enhanced the decomposition of thiosul-
fate ions as follows (Xu et al., 2017):
4S2O32− +O2 +2H2O → 2S4O62− +4OH− (3)
4S4O62− +6OH− → 5S2O32− +2S3O62− +3H2O (4)
2S3O62− +6OH− → S2O32− +4SO32− +3H2O (5)
2SO32− +O2 → 2SO42− (6)
This strongly suggests that passivation of arsenopyrite is
essential to improve gold extraction from gold-bearing arse-
nopyrite ores by thiosulfate. In the next section, passivation
of arsenopyrite by microencapsulation using ferrous and
phosphate ions will be discussed.
Passivation of arsenopyrite by microencapsulation
treatment
Figure 3 shows the changes in dissolved Fe and P con-
centrations before and after ME treatment of arsenopyrite
for 24 h. Initial concentrations of ferrous and phosphate
ions were both 10 mM but decreased to 0.3 mM after ME
treatment. This can be explained by the following reactions:
4Fe2+ +O2 +4H+ → 4Fe3+ +2H2O (7)
Fe3+ +H2PO4− → FePO4 +2H+ (8)
Arsenopyrite is known to dissolve electrochemically, indi-
cating that it consists of two distinct sites called cathode
and anode. In the presence of oxygen, the cathodic site
loses electrons, and these electron holes are filled by elec-
trons from the anodic site. As a result, anodic dissolution
of arsenopyrite takes place however, this reaction is sacri-
ficed in the presence of ferrous ions that donate electrons to
electron holes of anodic site of arsenopyrite by being oxi-
dized to ferric ions. The ferrous oxidation by oxygen occur-
ring on the surface of arsenopyrite is illustrated in Eq. (7).
Ferric ions present near the arsenopyrite surface react with
Figure 2. Effect of arsenopyrite on thiosulfate leaching of gold: (a) gold extraction by thiosulfate in the absence and presence
of arsenopyrite and (b) UV-vis spectra of fresh thiosulfate solution and the one reacted with and without arsenopyrite for 40
min (Mhandu et al., 2023)