XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3 1901
7.5, respectively as shown in Figure 1. Hypochlorous
acid (E0HClO/Cl−= 1.495V) is the most effective oxidizer
among other species which are aqueous chlorine (E0Cl
2 /Cl− =
1.359V) and hypochlorite (E0OCl−/Cl− =0.890V) (Hasab et
al., 2013 Pak et al., 2020 Chung et al., 2021).
Hypochlorite is a strong oxidizing agent that dissolves
sulfides even at room temperature (Hasab et al., 2014). The
pH of hypochlorite solution can be adjusted to pH 3.5–7.5
to form hypochlorous acid which can dissolve gold. Thus,
a process involving pretreatment followed by gold disso-
lution using hypochlorite solution at differing pH can be
employed. In this way, gold encapsulated by sulfide matrix
is exposed first through sulfide oxidation ensuring that gold
is already accessible to the lixiviant for gold dissolution.
The use of hypochlorite solution has been widely
explored in processing refractory gold ores and concentrates
involving pyrite matrix (Hasab et al., 2013 Valenzuela et
al., 2013 Regidor et al., 2018 Pak et al., 2020 Ahtiainen
et al., 2021). Oxidation of pyrite has been the major focus
of studies in sulfide oxidation and gold dissolution of refrac-
tory gold since pyrite is the dominant sulfide encapsulating
gold. It is also the most stable sulfide among the common
sulfide phases associated with gold (Chandra et al., 2010
Dos Santos et al., 2016 Tayebi-Khorami, 2016). Thus,
other sulfides are also oxidized when pyrite is oxidized.
2FeS 7H O 15OCl
2Fe(OH) 4SO 8H 15Cl E
0.51V
2 2
3 4
2-
o
)++
+++
=
-
+-(1)
OCl H HOCl )+-+(2)
2Au 3HOCl 3H 5Cl
2AuCl 3H OE 0.496V
4 2 o
)+++
+=
+-
-(3)
Recovery of other valuable metals from sulfides such as
molybdenum from molybdenite using hypochlorite solu-
tion has also been explored (Cao et al., 2010 Chung et al.,
2020 Hesami et al., 2022).
tMoS 9OCl 6OH
MoO 9Cl 2SO 3H2O
2
4
2-
4
2-
)++
+++
--
-(4)
However, extraction of gold from refractory gold-
molybdenum concentrate using hypochlorite has not yet
been investigated. Current processing of gold from refrac-
tory gold-molybdenum concentrate is through biological
oxidation as pretreatment followed by gold cyanide leach.
This process has limitations in molybdenum content of
the concentrate due to sensitivity of bacteria and this still
involves cyanide for gold leaching.
This study aims to investigate the sulfide oxidation and
gold dissolution of a refractory gold-molybdenum concen-
trate using hypochlorite solution. The developed process
can be a potential eco-friendly and simplified process for
treating refractory gold-molybdenum concentrates.
METHODOLOGY
Materials
Flotation concentrate from a gold-molybdenum process-
ing plant in Runruno, Nueva Vizcaya was used in the
experiment. The as-received concentrate was sun-dried for
2 days and subjected to coning and quartering for sam-
pling. Reagents for leaching were prepared from technical-
grade reagents (NaOCl, Ca(OCl)2, NaCl, NaOH, HCl)
using deionized water for mixing and dilution. Since only
7.81% NaOCl is commercially available, Ca(OCl)2 was
used to prepare hypochlorite solution containing higher
OCl– concentration.
Concentrate Characterization
Determination of total gold was done using fire assay,
sulfur using gravimetric analysis, and molybdenum using
ICP-MS. X-ray fluorescence (XRF) was used for concen-
trate elemental analysis and leached solution gold analysis.
Mineralogical composition was determined using X-ray
diffraction (XRD). Particle size analysis was also done to
determine the P80. Gold occurrence test which involves a
series of selective leaches with increasing oxidative strength
was conducted to determine the distribution of gold in
concentrate occurring in four general forms: free gold,
exposed but associated, gold locked in sulfides, gold locked
in gangue.
Two-Stage Hypochlorite Leaching
Leaching was done using a 1-L beaker with magnetic stirrer
for agitation using the parameters shown in Table 1. The
solution Eh and pH were monitored using ORP meter and
pH meter, respectively. Reagents to produce a 500-mL lix-
iviant solution were mixed for homogenization and stabi-
lization then concentrate was added. Solution Eh and pH
were recorded every 10 minutes for 1st hour and 20 min-
utes for the following hours. Ten (10) mL aliquots were
taken at a given time interval for gold analysis and kinet-
ics study. After leaching, solution was filtered. Samples of
the filtered solution and leaching residue were obtained for
gold, sulfur, and molybdenum analysis. Leaching residue
for the oxidation stages was subjected to XRD and SEM-
EDS for characterization.
7.5, respectively as shown in Figure 1. Hypochlorous
acid (E0HClO/Cl−= 1.495V) is the most effective oxidizer
among other species which are aqueous chlorine (E0Cl
2 /Cl− =
1.359V) and hypochlorite (E0OCl−/Cl− =0.890V) (Hasab et
al., 2013 Pak et al., 2020 Chung et al., 2021).
Hypochlorite is a strong oxidizing agent that dissolves
sulfides even at room temperature (Hasab et al., 2014). The
pH of hypochlorite solution can be adjusted to pH 3.5–7.5
to form hypochlorous acid which can dissolve gold. Thus,
a process involving pretreatment followed by gold disso-
lution using hypochlorite solution at differing pH can be
employed. In this way, gold encapsulated by sulfide matrix
is exposed first through sulfide oxidation ensuring that gold
is already accessible to the lixiviant for gold dissolution.
The use of hypochlorite solution has been widely
explored in processing refractory gold ores and concentrates
involving pyrite matrix (Hasab et al., 2013 Valenzuela et
al., 2013 Regidor et al., 2018 Pak et al., 2020 Ahtiainen
et al., 2021). Oxidation of pyrite has been the major focus
of studies in sulfide oxidation and gold dissolution of refrac-
tory gold since pyrite is the dominant sulfide encapsulating
gold. It is also the most stable sulfide among the common
sulfide phases associated with gold (Chandra et al., 2010
Dos Santos et al., 2016 Tayebi-Khorami, 2016). Thus,
other sulfides are also oxidized when pyrite is oxidized.
2FeS 7H O 15OCl
2Fe(OH) 4SO 8H 15Cl E
0.51V
2 2
3 4
2-
o
)++
+++
=
-
+-(1)
OCl H HOCl )+-+(2)
2Au 3HOCl 3H 5Cl
2AuCl 3H OE 0.496V
4 2 o
)+++
+=
+-
-(3)
Recovery of other valuable metals from sulfides such as
molybdenum from molybdenite using hypochlorite solu-
tion has also been explored (Cao et al., 2010 Chung et al.,
2020 Hesami et al., 2022).
tMoS 9OCl 6OH
MoO 9Cl 2SO 3H2O
2
4
2-
4
2-
)++
+++
--
-(4)
However, extraction of gold from refractory gold-
molybdenum concentrate using hypochlorite has not yet
been investigated. Current processing of gold from refrac-
tory gold-molybdenum concentrate is through biological
oxidation as pretreatment followed by gold cyanide leach.
This process has limitations in molybdenum content of
the concentrate due to sensitivity of bacteria and this still
involves cyanide for gold leaching.
This study aims to investigate the sulfide oxidation and
gold dissolution of a refractory gold-molybdenum concen-
trate using hypochlorite solution. The developed process
can be a potential eco-friendly and simplified process for
treating refractory gold-molybdenum concentrates.
METHODOLOGY
Materials
Flotation concentrate from a gold-molybdenum process-
ing plant in Runruno, Nueva Vizcaya was used in the
experiment. The as-received concentrate was sun-dried for
2 days and subjected to coning and quartering for sam-
pling. Reagents for leaching were prepared from technical-
grade reagents (NaOCl, Ca(OCl)2, NaCl, NaOH, HCl)
using deionized water for mixing and dilution. Since only
7.81% NaOCl is commercially available, Ca(OCl)2 was
used to prepare hypochlorite solution containing higher
OCl– concentration.
Concentrate Characterization
Determination of total gold was done using fire assay,
sulfur using gravimetric analysis, and molybdenum using
ICP-MS. X-ray fluorescence (XRF) was used for concen-
trate elemental analysis and leached solution gold analysis.
Mineralogical composition was determined using X-ray
diffraction (XRD). Particle size analysis was also done to
determine the P80. Gold occurrence test which involves a
series of selective leaches with increasing oxidative strength
was conducted to determine the distribution of gold in
concentrate occurring in four general forms: free gold,
exposed but associated, gold locked in sulfides, gold locked
in gangue.
Two-Stage Hypochlorite Leaching
Leaching was done using a 1-L beaker with magnetic stirrer
for agitation using the parameters shown in Table 1. The
solution Eh and pH were monitored using ORP meter and
pH meter, respectively. Reagents to produce a 500-mL lix-
iviant solution were mixed for homogenization and stabi-
lization then concentrate was added. Solution Eh and pH
were recorded every 10 minutes for 1st hour and 20 min-
utes for the following hours. Ten (10) mL aliquots were
taken at a given time interval for gold analysis and kinet-
ics study. After leaching, solution was filtered. Samples of
the filtered solution and leaching residue were obtained for
gold, sulfur, and molybdenum analysis. Leaching residue
for the oxidation stages was subjected to XRD and SEM-
EDS for characterization.