1750 XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3
contact with thiosulfate ions as well as its oxidation can
address the problem of thiosulfate leaching of gold-bearing
sulfidic ores. Thus, this study investigated the applicability
of microencapsulation (ME)—a technique that forms the
surface protective coating on target materials (Park et al.,
2018, 2019, 2021)—to thiosulfate leaching of a simulated
gold-bearing arsenopyritic ore. In this study, arsenopyrite
(FeAsS) was selected because it is one of the most common
gold carriers. Specifically, this study aimed to clarify: (1) the
effect of arsenopyrite on thiosulfate leaching of gold, (2)
passivation of arsenopyrite by ME treatment using ferrous
and phosphate ions, (3) the effect of ME treatment on thio-
sulfate leaching of a simulated gold-bearing arsenopyritic
ore, and (4) the effect of ME treatment on the leachability
of arsenopyrite.
MATERIALS AND METHODS
Materials
Arsenopyrite used in this study was obtained from Toroku
mine, Miyazaki, Japan. It was crushed by a jaw crusher
(BB 51, Retsch Inc., Germany), ground by a vibratory disc
mill (RS 100, Retsch Inc., Germany), and then screened
to obtain a size fraction of –75 μm. The particle size of
the sieved sample was measured by a laser diffraction
(MT3300SX, Microtrac Inc., Japan), and a 80% pass-
ing size (P80) was ~20 µm. The sample was analyzed by
X-ray diffraction (XRD, MultiFlex, Rigaku Corporation,
Japan) and found to be composed of arsenopyrite (67%),
pyrite (13%), and quartz (15%) (Figure 1). A high-purity
gold powder (99.99%), sodium thiosulfate pentahydrate
(Na2S2O3∙5H2O), ammonium chloride (NH4Cl), ammo-
nia water (NH4OH), and copper chloride (CuCl2) used for
leaching experiments were of reagent grade and purchased
from Wako Pure Chemical Industries, Ltd. (Japan).
Microencapsulation Treatment
For the ME treatment, 20 g of arsenopyrite and 200
mL of 10 mM ferrous phosphate solution, prepared
by FeSO4∙7H2O and KH2PO4 (Wako Pure Chemical
Industries, Ltd., Japan), were put into a 400 mL cell and
mixed at 1000 rpm while introducing air at a flow rate
of 1 L/min for 1 h. The air was introduced to accelerate
the rate of ferrous oxidation during ME treatment. After
ME treatment, the suspension was allowed to stand for
10 min for arsenopyrite particles to be settled down, and
around 10 mL supernatant was collected, filtered through
a 0.2 µm syringe-driven membrane filter, and analyzed by
inductively coupled plasma atomic emission spectrometer
(ICP-AES, ICPE-9820, Shimadzu Corporation, Japan) to
measure the changes in dissolved Fe and P concentrations.
Meanwhile, the residue (i.e., ME-treated arsenopyrite) was
thoroughly washed with deionized (DI) water, dried in a
vacuum drying oven at 40 °C for 24 h, and analyzed by
X-ray photoelectron spectroscopy (XPS, JPS-9200, JEOL
Ltd., Japan) to clarify the formation of FePO4 coating.
Leaching Experiment
Leaching experiments were carried out using a heating
mantle and a 500-mL four-neck round-bottom flask,
equipped with a mechanical stirrer, reflux condenser,
thermometer, and sampling outlet. A 200 mL of leaching
solution containing 1 M Na2S2O3∙5H2O, 0.5 M NH4Cl,
0.5 M NH4OH, and 10 mM CuCl2 was poured into the
flask and heated to 25 ± 0.1 °C while agitating at 400 rpm.
NH4Cl and NH4OH were used as a buffer, while formed
by CuCl2 was used as an oxidant for gold. After the solu-
tion temperature was stabilized at 25 °C, 200 mg of gold
powder with or without 20 g of arsenopyrite was added and
leached until 24 h. At pre-determined time intervals, about
2 mL aliquots were taken and analyzed by ICP-AES to
check the concentration of gold dissolved. The stability of
thiosulfate in the absence and presence of arsenopyrite was
investigated by ultraviolet visible spectroscopy (UV-Vis,
UV-970, JASCO Co., Ltd., Japan).
RESULTS AND DISCUSSION
Effect of Arsenopyrite on the Extraction of Gold by
Thiosulfate Leaching
Figure 2(a) shows gold extraction by thiosulfate leaching in
the absence and presence of arsenopyrite. As can be seen,
thiosulfate leaching was highly efficient in extracting gold
when arsenopyrite was absent that is, more than 90% of
gold dissolved within 8 h, and it was further increased to
99% after 24 h. Gold extraction by thiosulfate leaching is
expressed by Eq. (1). Gold is oxidized by cupric ammonia
Figure 1. XRD pattern of arsenopyrite sample (Park et al.,
2018)
contact with thiosulfate ions as well as its oxidation can
address the problem of thiosulfate leaching of gold-bearing
sulfidic ores. Thus, this study investigated the applicability
of microencapsulation (ME)—a technique that forms the
surface protective coating on target materials (Park et al.,
2018, 2019, 2021)—to thiosulfate leaching of a simulated
gold-bearing arsenopyritic ore. In this study, arsenopyrite
(FeAsS) was selected because it is one of the most common
gold carriers. Specifically, this study aimed to clarify: (1) the
effect of arsenopyrite on thiosulfate leaching of gold, (2)
passivation of arsenopyrite by ME treatment using ferrous
and phosphate ions, (3) the effect of ME treatment on thio-
sulfate leaching of a simulated gold-bearing arsenopyritic
ore, and (4) the effect of ME treatment on the leachability
of arsenopyrite.
MATERIALS AND METHODS
Materials
Arsenopyrite used in this study was obtained from Toroku
mine, Miyazaki, Japan. It was crushed by a jaw crusher
(BB 51, Retsch Inc., Germany), ground by a vibratory disc
mill (RS 100, Retsch Inc., Germany), and then screened
to obtain a size fraction of –75 μm. The particle size of
the sieved sample was measured by a laser diffraction
(MT3300SX, Microtrac Inc., Japan), and a 80% pass-
ing size (P80) was ~20 µm. The sample was analyzed by
X-ray diffraction (XRD, MultiFlex, Rigaku Corporation,
Japan) and found to be composed of arsenopyrite (67%),
pyrite (13%), and quartz (15%) (Figure 1). A high-purity
gold powder (99.99%), sodium thiosulfate pentahydrate
(Na2S2O3∙5H2O), ammonium chloride (NH4Cl), ammo-
nia water (NH4OH), and copper chloride (CuCl2) used for
leaching experiments were of reagent grade and purchased
from Wako Pure Chemical Industries, Ltd. (Japan).
Microencapsulation Treatment
For the ME treatment, 20 g of arsenopyrite and 200
mL of 10 mM ferrous phosphate solution, prepared
by FeSO4∙7H2O and KH2PO4 (Wako Pure Chemical
Industries, Ltd., Japan), were put into a 400 mL cell and
mixed at 1000 rpm while introducing air at a flow rate
of 1 L/min for 1 h. The air was introduced to accelerate
the rate of ferrous oxidation during ME treatment. After
ME treatment, the suspension was allowed to stand for
10 min for arsenopyrite particles to be settled down, and
around 10 mL supernatant was collected, filtered through
a 0.2 µm syringe-driven membrane filter, and analyzed by
inductively coupled plasma atomic emission spectrometer
(ICP-AES, ICPE-9820, Shimadzu Corporation, Japan) to
measure the changes in dissolved Fe and P concentrations.
Meanwhile, the residue (i.e., ME-treated arsenopyrite) was
thoroughly washed with deionized (DI) water, dried in a
vacuum drying oven at 40 °C for 24 h, and analyzed by
X-ray photoelectron spectroscopy (XPS, JPS-9200, JEOL
Ltd., Japan) to clarify the formation of FePO4 coating.
Leaching Experiment
Leaching experiments were carried out using a heating
mantle and a 500-mL four-neck round-bottom flask,
equipped with a mechanical stirrer, reflux condenser,
thermometer, and sampling outlet. A 200 mL of leaching
solution containing 1 M Na2S2O3∙5H2O, 0.5 M NH4Cl,
0.5 M NH4OH, and 10 mM CuCl2 was poured into the
flask and heated to 25 ± 0.1 °C while agitating at 400 rpm.
NH4Cl and NH4OH were used as a buffer, while formed
by CuCl2 was used as an oxidant for gold. After the solu-
tion temperature was stabilized at 25 °C, 200 mg of gold
powder with or without 20 g of arsenopyrite was added and
leached until 24 h. At pre-determined time intervals, about
2 mL aliquots were taken and analyzed by ICP-AES to
check the concentration of gold dissolved. The stability of
thiosulfate in the absence and presence of arsenopyrite was
investigated by ultraviolet visible spectroscopy (UV-Vis,
UV-970, JASCO Co., Ltd., Japan).
RESULTS AND DISCUSSION
Effect of Arsenopyrite on the Extraction of Gold by
Thiosulfate Leaching
Figure 2(a) shows gold extraction by thiosulfate leaching in
the absence and presence of arsenopyrite. As can be seen,
thiosulfate leaching was highly efficient in extracting gold
when arsenopyrite was absent that is, more than 90% of
gold dissolved within 8 h, and it was further increased to
99% after 24 h. Gold extraction by thiosulfate leaching is
expressed by Eq. (1). Gold is oxidized by cupric ammonia
Figure 1. XRD pattern of arsenopyrite sample (Park et al.,
2018)