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
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