XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3 1781
All chemicals used in the experiments were of reagent
grade. Ammonium thiosulfate ((NH4)2S2O3) (Sigma
Aldrich, 99%), magnesium hydroxide (Mg(OH)2) (Fisher
Scientific, 95–100.5%), and copper sulfate pentahydrate
(CuSO4·5H2O) (Fisher Scientific, 98%) were utilized.
Ca(OH)2 (Sigma Aldrich, 95%) was used to adjust the
pH level depending on the test conditions, while sodium
hydroxide (NaOH) (VWR Chemicals BDH ®, 97%) was
used to increase the pH of the cyanide solution.
Experimental Setup
Thiosulfate Leaching Test
Figure 1 presents schematic diagrams of the bench and col-
umn tests. Both tests were conducted using unsaturated
magnesium hydroxide in the copper-ammonium thiosul-
fate system. The tests were conducted at various chemical
concentrations of thiosulfate, copper, magnesium hydrox-
ide that are known to affect gold recovery in the ammo-
nium thiosulfate system.
The present study involved bench-scale experiments
wherein 60 g of the sample was subjected to the test solution,
prepared by adding chemicals to the 240 ml of deionized
water (DI water) to achieve various concentrations within a
500 mL Erlenmeyer flask.The slurry was stirred using a stir
bar maintained at a speed of 600 rpm on a hot plate for 24
hours at 25 °C. The pH of the slurry was then adjusted using
Ca(OH)2 based on the test conditions, following which the
desirable amount of copper sulfate was added as a catalytic
oxidant upon achieving the desired pH level.
Subsequently, at different time intervals (0.5, 1, 3, 5, 8,
and 24 hours), 5 ml of the sample was extracted and filtered
using a 0.45 µm syringe filter after centrifuging the pulp.
The gold and copper concentrations were determined using
atomic absorption spectrometry (AAS), which enabled the
quantification of gold leaching efficiency. In addition, ion
chromatography (IC) was employed to monitor the decom-
position of thiosulfate to other sulfur species. The gold con-
tent of the leaching residue was analyzed through fire assay.
The column was prepared with a height of 12 inches
and an inner diameter of 2 inches of PVC pipe. The sam-
ple, crushed using a jaw crusher, had a top size of 0.5 inches
and weighed approximately 0.7 kg upon being introduced
into the column. The ore was mixed with Mg(OH)2 prior
to being placed in the column, following which a 2 cm bed
and 0.25 cm plastic glasses were positioned on top and bot-
tom of the samples to enhance irrigation. The feed solution
was prepared daily and injected into the top of the column
at an irrigation rate of 18 ml/h through a peristaltic pump
at 25 °C. The chemical composition of the feed solution
varied based on the tests conducted.
The pregnant solution was collected beneath the col-
umn through gravity flow, and the volume of the solution
was measured daily. The Au, Cu, and sulfur contents in
the solution were quantified using AAS and IC after filter-
ing using a syringe filter. The duration of the tests varied
depending on the type of feed solution used, ranging from
7 to 21 days and continued until gold recovery leveled off.
Figure 1. Schematic diagrams of the test (a) bench test and (b) column test

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Extracted Text (may have errors)

XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3 1781
All chemicals used in the experiments were of reagent
grade. Ammonium thiosulfate ((NH4)2S2O3) (Sigma
Aldrich, 99%), magnesium hydroxide (Mg(OH)2) (Fisher
Scientific, 95–100.5%), and copper sulfate pentahydrate
(CuSO4·5H2O) (Fisher Scientific, 98%) were utilized.
Ca(OH)2 (Sigma Aldrich, 95%) was used to adjust the
pH level depending on the test conditions, while sodium
hydroxide (NaOH) (VWR Chemicals BDH ®, 97%) was
used to increase the pH of the cyanide solution.
Experimental Setup
Thiosulfate Leaching Test
Figure 1 presents schematic diagrams of the bench and col-
umn tests. Both tests were conducted using unsaturated
magnesium hydroxide in the copper-ammonium thiosul-
fate system. The tests were conducted at various chemical
concentrations of thiosulfate, copper, magnesium hydrox-
ide that are known to affect gold recovery in the ammo-
nium thiosulfate system.
The present study involved bench-scale experiments
wherein 60 g of the sample was subjected to the test solution,
prepared by adding chemicals to the 240 ml of deionized
water (DI water) to achieve various concentrations within a
500 mL Erlenmeyer flask.The slurry was stirred using a stir
bar maintained at a speed of 600 rpm on a hot plate for 24
hours at 25 °C. The pH of the slurry was then adjusted using
Ca(OH)2 based on the test conditions, following which the
desirable amount of copper sulfate was added as a catalytic
oxidant upon achieving the desired pH level.
Subsequently, at different time intervals (0.5, 1, 3, 5, 8,
and 24 hours), 5 ml of the sample was extracted and filtered
using a 0.45 µm syringe filter after centrifuging the pulp.
The gold and copper concentrations were determined using
atomic absorption spectrometry (AAS), which enabled the
quantification of gold leaching efficiency. In addition, ion
chromatography (IC) was employed to monitor the decom-
position of thiosulfate to other sulfur species. The gold con-
tent of the leaching residue was analyzed through fire assay.
The column was prepared with a height of 12 inches
and an inner diameter of 2 inches of PVC pipe. The sam-
ple, crushed using a jaw crusher, had a top size of 0.5 inches
and weighed approximately 0.7 kg upon being introduced
into the column. The ore was mixed with Mg(OH)2 prior
to being placed in the column, following which a 2 cm bed
and 0.25 cm plastic glasses were positioned on top and bot-
tom of the samples to enhance irrigation. The feed solution
was prepared daily and injected into the top of the column
at an irrigation rate of 18 ml/h through a peristaltic pump
at 25 °C. The chemical composition of the feed solution
varied based on the tests conducted.
The pregnant solution was collected beneath the col-
umn through gravity flow, and the volume of the solution
was measured daily. The Au, Cu, and sulfur contents in
the solution were quantified using AAS and IC after filter-
ing using a syringe filter. The duration of the tests varied
depending on the type of feed solution used, ranging from
7 to 21 days and continued until gold recovery leveled off.
Figure 1. Schematic diagrams of the test (a) bench test and (b) column test

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1780 XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3
also accelerating the decomposition of thiosulfate due to its
strong oxidizing ability.
Cu^NH O
Cu^S O S O
2 8S
2 8NH
3 4
2+
2 3
2-
2 3 3 4 6
2-
3
+=
++h5-
h
(4)
Thiosulfate systems have the potential to be environmen-
tally beneficial. However, they often become uneconomi-
cal due to the decomposition of thiosulfate and excessive
reagent usage [16]. Additionally, the evaporation of ammo-
nia from the solution can lower the pH, destabilizing thio-
sulfate and leading to further decomposition of the reagent
[17]. Although higher ammonia concentrations can reduce
thiosulfate consumption or decomposition, it causes oper-
ating costs to rise and more ammonia to evaporate into the
air. This makes it challenging to apply the system to heap
leaching and in situ leaching [18,19].
To address the issue of pH drop from ammonia evapo-
ration in the ammonia thiosulfate system, adding Mg(OH)2
can be a solution. This is because Mg(OH)2 has low solubil-
ity in alkaline solutions, and its solubility decreases as pH
rises [20]. In a typical ammonium thiosulfate solution, when
ammonia evaporation causes a drop in pH, unsaturated
Mg(OH)2 dissolves in the leaching solution. This generates
hydroxide ions, which increases the pH and restores it to
the appropriate level. Therefore, a cycle of pH correction
with Mg(OH)2 may help maintain an appropriate pH for
thiosulfate stability and gold leaching.
The focus of this study was to examine the effects
of magnesium hydroxide on gold leaching rates and pH
changes over time in an ammonium thiosulfate system. The
concentration of the various chemicals and pH levels that
impact gold leaching were adjusted to observe their effects
on the gold recovery rate. All experimental parameters were
monitored simultaneously to identify interactions among
different parameters. This included changes to the concen-
tration of copper and thiosulfate and system’s pH levels, as
well as the concentration of magnesium hydroxide. Both
bench and column scale experiments were conducted to
determine if the pH correction provided by magnesium
hydroxide could be sustained over a long period of leach-
ing, such as in heap scale or in situ scale, and whether it
affected drainage in column leaching.
EXPERIMENTAL
Material
In preparation for column leaching, 50 kg gold ore sample
underwent initial crushing with a jaw crusher to reduce it
to a size of –½ inch. The sample didn’t grind fine for the
better irrigation to the column leaching, but the ground
sample was used to the bench scale tests which minimized
the effect of the particle size. To facilitate bench tests, the
sample was then divided into 40 roughly 1 kg bags with
the use of a rotary riffler. Each bag was further processed
with a rod mill for 15 minutes, achieving an 80% passing
size of 75 µm. Table 1 displays the sample size distribution
by wet screening and its use in column and bench tests.
The gold concentration was determined through fire assay,
while other metal concentrations were measured by induc-
tively coupled plasma-optical emission spectrometry (ICP-
OES) and X-ray diffraction (XRD), as listed in Table 2
and Table 3. The total carbon and sulfur content of the
sample were also measured by carbon/sulfur analysis (Eltra
CS 2000, Germany). In cyanidation experiments, the free
cyanide of the initial and final solution was determined
by titration using 0.01M silver nitrate (Acros Organics,
Canada).
Table 1. Size distribution of the ore sample
Size fraction
For the Bench
Leaching
For the Column
Leaching
Wt. (%)Wt. (%)
–75 μm 84.69 33.43
–106 μm +75 μm 11.37 33.50
–150 μm +106 μm 3.10 2.67
-212 μm +150 μm 0.35 1.94
-300 μm +212 μm 0.04 2.63
-1.6 mm +300 μm 0.44 2.81
+1.6 mm 0 23.02
Table 2. Elemental analysis of the ore sample
Metals
Ca Al C Fe Mg Ti S Zn Cu Au Ag
Wt (%)mg/L g/t
Assay 11.50 5.62 4.20 2.50 2.05 0.27 1.79 750.00 87.20 4.45 1.70
Table 3. Mineralogy of the sample concentration
Mineral SiO2 CaCO3 CaMg(CO3)2 Mg2Si2O6 Mg3Al2(SiO4)3 TiO2
Wt (%)42.5 30.1 12.6 9.5 5.0 0.3

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Extracted Text (may have errors)

1780 XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3
also accelerating the decomposition of thiosulfate due to its
strong oxidizing ability.
Cu^NH O
Cu^S O S O
2 8S
2 8NH
3 4
2+
2 3
2-
2 3 3 4 6
2-
3
+=
++h5-
h
(4)
Thiosulfate systems have the potential to be environmen-
tally beneficial. However, they often become uneconomi-
cal due to the decomposition of thiosulfate and excessive
reagent usage [16]. Additionally, the evaporation of ammo-
nia from the solution can lower the pH, destabilizing thio-
sulfate and leading to further decomposition of the reagent
[17]. Although higher ammonia concentrations can reduce
thiosulfate consumption or decomposition, it causes oper-
ating costs to rise and more ammonia to evaporate into the
air. This makes it challenging to apply the system to heap
leaching and in situ leaching [18,19].
To address the issue of pH drop from ammonia evapo-
ration in the ammonia thiosulfate system, adding Mg(OH)2
can be a solution. This is because Mg(OH)2 has low solubil-
ity in alkaline solutions, and its solubility decreases as pH
rises [20]. In a typical ammonium thiosulfate solution, when
ammonia evaporation causes a drop in pH, unsaturated
Mg(OH)2 dissolves in the leaching solution. This generates
hydroxide ions, which increases the pH and restores it to
the appropriate level. Therefore, a cycle of pH correction
with Mg(OH)2 may help maintain an appropriate pH for
thiosulfate stability and gold leaching.
The focus of this study was to examine the effects
of magnesium hydroxide on gold leaching rates and pH
changes over time in an ammonium thiosulfate system. The
concentration of the various chemicals and pH levels that
impact gold leaching were adjusted to observe their effects
on the gold recovery rate. All experimental parameters were
monitored simultaneously to identify interactions among
different parameters. This included changes to the concen-
tration of copper and thiosulfate and system’s pH levels, as
well as the concentration of magnesium hydroxide. Both
bench and column scale experiments were conducted to
determine if the pH correction provided by magnesium
hydroxide could be sustained over a long period of leach-
ing, such as in heap scale or in situ scale, and whether it
affected drainage in column leaching.
EXPERIMENTAL
Material
In preparation for column leaching, 50 kg gold ore sample
underwent initial crushing with a jaw crusher to reduce it
to a size of –½ inch. The sample didn’t grind fine for the
better irrigation to the column leaching, but the ground
sample was used to the bench scale tests which minimized
the effect of the particle size. To facilitate bench tests, the
sample was then divided into 40 roughly 1 kg bags with
the use of a rotary riffler. Each bag was further processed
with a rod mill for 15 minutes, achieving an 80% passing
size of 75 µm. Table 1 displays the sample size distribution
by wet screening and its use in column and bench tests.
The gold concentration was determined through fire assay,
while other metal concentrations were measured by induc-
tively coupled plasma-optical emission spectrometry (ICP-
OES) and X-ray diffraction (XRD), as listed in Table 2
and Table 3. The total carbon and sulfur content of the
sample were also measured by carbon/sulfur analysis (Eltra
CS 2000, Germany). In cyanidation experiments, the free
cyanide of the initial and final solution was determined
by titration using 0.01M silver nitrate (Acros Organics,
Canada).
Table 1. Size distribution of the ore sample
Size fraction
For the Bench
Leaching
For the Column
Leaching
Wt. (%)Wt. (%)
–75 μm 84.69 33.43
–106 μm +75 μm 11.37 33.50
–150 μm +106 μm 3.10 2.67
-212 μm +150 μm 0.35 1.94
-300 μm +212 μm 0.04 2.63
-1.6 mm +300 μm 0.44 2.81
+1.6 mm 0 23.02
Table 2. Elemental analysis of the ore sample
Metals
Ca Al C Fe Mg Ti S Zn Cu Au Ag
Wt (%)mg/L g/t
Assay 11.50 5.62 4.20 2.50 2.05 0.27 1.79 750.00 87.20 4.45 1.70
Table 3. Mineralogy of the sample concentration
Mineral SiO2 CaCO3 CaMg(CO3)2 Mg2Si2O6 Mg3Al2(SiO4)3 TiO2
Wt (%)42.5 30.1 12.6 9.5 5.0 0.3

1782 XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3
Cyanidation Test
The purpose of the cyanidation test was to evaluate the
leachability and refractoriness of the sample. To achieve
this, sodium hydroxide was added to adjust the pH of the
leaching reactor to 11.0, followed by the introduction of 2
g/t of sodium cyanide. Throughout the test, the pH level
was kept constant. The bench test was conducted at 25°C
and 100 rpm, using 150 g of the sample and 150 ml of
solution for 24 hours. The column test, which lasted for
7 days, utilized the same setup as the thiosulfate tests. The
feed solution was comprised of pH 11.0 and 2 g/t of sodium
cyanide. The results of both tests revealed that 67.3% and
64.9%, respectively, of the gold could be leached from the
ore under these conditions.
RESULTS AND DISCUSSION
Bench Scale Test Results
Effect of Initial pH
The behavior of various parameters in thiosulfate leaching
tests conducted at different pH values adjusted by Ca(OH)2
are depicted in Figure 2 (a) to (d). It was observed that the
pH of the solution decreased during the first 30 minutes
of leaching when the initial pH was above 9.0, as shown in
Figure 2 (a). Conversely, an increasing trend was observed
in the initial pH of 9.0.
Figure 2 (b) shows that similar gold dissolution patterns
and leaching efficiencies of around 65% were measured at
all initial pH values, except for the experiment conducted
at an initial pH of 11.5, in which a sharp decrease of gold
recovery to 28% was obtained. Most of the copper precipi-
tates within 30 minutes when the initial pH increases from
9.5 to 10.3, as shown in Figure 2 (c). According to the
previous research, the precipitation of Cu as CuS and CuO
could hinder gold dissolution at this pH level [21]. Despite
the interaction between copper and thiosulfate, several
investigations have found that higher copper concentra-
tions benefit gold leaching [22–25]. This could support the
higher gold recoveries obtained at pH 9.0 and pH 9.5 with
higher Cu contents. Molleman and Dreisinger explained
that the favorable pH range for thiosulfate leaching with
copper is 9.0 to 10 [16].
Thiosulfate decomposition displayed similar trends at
all pH levels (Figure 2 (d)), and 0.03 M of ammonium
thiosulfate was consumed during leaching. This demon-
strated that similar mechanisms of thiosulfate decomposi-
tion occurred from pH 9.0 to pH 10.3. A specific range of
ammonia to thiosulfate ratio should be maintained to allow
regeneration of the cupric ion. However, over-concentrated
Figure 2. Kinetic plots of leaching at various pH levels adjusted by Ca(OH)
2 ,a) pH, b) Au recovery, c) Cu concentration,
d) thiosulfate concentration (Test condition: 0.1 M S
2 O
3 2–, 1 mM Cu, 0.01 g
Mg(OH)2 /g
Ore Mg(OH)
2 ,20±3°C)

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1782 XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3
Cyanidation Test
The purpose of the cyanidation test was to evaluate the
leachability and refractoriness of the sample. To achieve
this, sodium hydroxide was added to adjust the pH of the
leaching reactor to 11.0, followed by the introduction of 2
g/t of sodium cyanide. Throughout the test, the pH level
was kept constant. The bench test was conducted at 25°C
and 100 rpm, using 150 g of the sample and 150 ml of
solution for 24 hours. The column test, which lasted for
7 days, utilized the same setup as the thiosulfate tests. The
feed solution was comprised of pH 11.0 and 2 g/t of sodium
cyanide. The results of both tests revealed that 67.3% and
64.9%, respectively, of the gold could be leached from the
ore under these conditions.
RESULTS AND DISCUSSION
Bench Scale Test Results
Effect of Initial pH
The behavior of various parameters in thiosulfate leaching
tests conducted at different pH values adjusted by Ca(OH)2
are depicted in Figure 2 (a) to (d). It was observed that the
pH of the solution decreased during the first 30 minutes
of leaching when the initial pH was above 9.0, as shown in
Figure 2 (a). Conversely, an increasing trend was observed
in the initial pH of 9.0.
Figure 2 (b) shows that similar gold dissolution patterns
and leaching efficiencies of around 65% were measured at
all initial pH values, except for the experiment conducted
at an initial pH of 11.5, in which a sharp decrease of gold
recovery to 28% was obtained. Most of the copper precipi-
tates within 30 minutes when the initial pH increases from
9.5 to 10.3, as shown in Figure 2 (c). According to the
previous research, the precipitation of Cu as CuS and CuO
could hinder gold dissolution at this pH level [21]. Despite
the interaction between copper and thiosulfate, several
investigations have found that higher copper concentra-
tions benefit gold leaching [22–25]. This could support the
higher gold recoveries obtained at pH 9.0 and pH 9.5 with
higher Cu contents. Molleman and Dreisinger explained
that the favorable pH range for thiosulfate leaching with
copper is 9.0 to 10 [16].
Thiosulfate decomposition displayed similar trends at
all pH levels (Figure 2 (d)), and 0.03 M of ammonium
thiosulfate was consumed during leaching. This demon-
strated that similar mechanisms of thiosulfate decomposi-
tion occurred from pH 9.0 to pH 10.3. A specific range of
ammonia to thiosulfate ratio should be maintained to allow
regeneration of the cupric ion. However, over-concentrated
Figure 2. Kinetic plots of leaching at various pH levels adjusted by Ca(OH)
2 ,a) pH, b) Au recovery, c) Cu concentration,
d) thiosulfate concentration (Test condition: 0.1 M S
2 O
3 2–, 1 mM Cu, 0.01 g
Mg(OH)2 /g
Ore Mg(OH)
2 ,20±3°C)