1779
The Role of Magnesium Hydroxide for Optimal Gold Leaching
in the Ammonium Thiosulfate
Sujin Chae, Farzaneh Sadri, Yeonuk Choi, Ahmad Ghahreman
Queen’s University, The Robert M. Buchan Department of Mining, Queen’s University
ABSTRACT: In this study, the practicality of using magnesium hydroxide in ammonium thiosulfate leaching
as a substitute for cyanidation in gold mining is explored. However, ammonia loss which leads to thiosulfate
decomposition and pH reduction will result in excessive consumption of thiosulfate and needs to be addressed
before implementing the method in heap leaching applications. The matter is resolved by utilising the limited
solubility of magnesium hydroxide in alkaline solutions, wherein it gradually dissolves and serves to restore the
pH levels by helping the regeneration of ammonia. The effect of magnesium hydroxide is ascertained through
bench and column scale leaching tests employing different chemical concentrations.
INTRODUCTION
Traditional gold leaching methods involve cyanidation,
which can have a negative impact on the environment
due to its toxicity. As high-grade ores become more scarce,
lower-grade complex ores, such as refractory and carbona-
ceous preg-robbing ores, are being utilized for gold extrac-
tion, where cyanidation is not effective [1–3]. Thiosulfate is a
promising alternative reagent for gold leaching, as it is rela-
tively inexpensive, non-toxic, and forms strong complexes
with gold [4,5]. In fact, thiosulfate is the only alternative
lixiviant that has been successfully used on an industrial
scale (at Barrick Gold Corp.) for highly carbonaceous preg-
robbing ores [6].
The process of gold dissolution in thiosulfate solution
is described by equations 1–3 [7–9]. The cupric tetraamine
complex, acting as an oxidizing agent for gold dissolution,
significantly enhances the gold leaching rate in the presence
of ammonia and cupric ions. Ammonia stabilizes the auro-
thiosulfate and copper complex [6,10].
Au O O
Au^S O
4 8S2 2H2O
4 4OH
3
2-
2
2 3 2
3-
+++=
+-h
(1)
Cu^S O O H O
Cu^NH O
2 8NH 2
1
2 2OH 6S
2 3
5
3 2 2
3 4
2+
2 3
+++=
++--
h3-
h
(2)
Au O Cu^NH3h2+
Au^S O Cu^S O
5S2
4NH
3
2-
4
2 3
3
3 2 3 3
5-
++=
++h2- h
(3)
Thiosulfate is a highly reactive species that is prone to
decompose in leaching solutions. The decomposed prod-
ucts of thiosulfate include sulfite, sulfate, and polythionates
such as tetrathionate and trithionate, depending on various
factors. However, an excessive amount of sulfite can lead to
the reduction of copper concentration, while polythionate
ions can cause passivation, which hinders gold dissolution
[8,11]. The decomposition of thiosulfate can be influenced
by several factors, such as the presence of sulfur-reducing
bacteria, pH, thiosulfate concentration, metal ions, and cat-
alysts, oxygen concentration, and other oxidants [10,12–14].
In the presence of [Cu(NH3)4]2+, thiosulfate is read-
ily oxidized to polythionate ions such as S4O62– as shown
in equation 4 [15]. The [Cu(NH3)4]2+ acts as a catalytic
oxidant that significantly increases gold dissolution while
The Role of Magnesium Hydroxide for Optimal Gold Leaching
in the Ammonium Thiosulfate
Sujin Chae, Farzaneh Sadri, Yeonuk Choi, Ahmad Ghahreman
Queen’s University, The Robert M. Buchan Department of Mining, Queen’s University
ABSTRACT: In this study, the practicality of using magnesium hydroxide in ammonium thiosulfate leaching
as a substitute for cyanidation in gold mining is explored. However, ammonia loss which leads to thiosulfate
decomposition and pH reduction will result in excessive consumption of thiosulfate and needs to be addressed
before implementing the method in heap leaching applications. The matter is resolved by utilising the limited
solubility of magnesium hydroxide in alkaline solutions, wherein it gradually dissolves and serves to restore the
pH levels by helping the regeneration of ammonia. The effect of magnesium hydroxide is ascertained through
bench and column scale leaching tests employing different chemical concentrations.
INTRODUCTION
Traditional gold leaching methods involve cyanidation,
which can have a negative impact on the environment
due to its toxicity. As high-grade ores become more scarce,
lower-grade complex ores, such as refractory and carbona-
ceous preg-robbing ores, are being utilized for gold extrac-
tion, where cyanidation is not effective [1–3]. Thiosulfate is a
promising alternative reagent for gold leaching, as it is rela-
tively inexpensive, non-toxic, and forms strong complexes
with gold [4,5]. In fact, thiosulfate is the only alternative
lixiviant that has been successfully used on an industrial
scale (at Barrick Gold Corp.) for highly carbonaceous preg-
robbing ores [6].
The process of gold dissolution in thiosulfate solution
is described by equations 1–3 [7–9]. The cupric tetraamine
complex, acting as an oxidizing agent for gold dissolution,
significantly enhances the gold leaching rate in the presence
of ammonia and cupric ions. Ammonia stabilizes the auro-
thiosulfate and copper complex [6,10].
Au O O
Au^S O
4 8S2 2H2O
4 4OH
3
2-
2
2 3 2
3-
+++=
+-h
(1)
Cu^S O O H O
Cu^NH O
2 8NH 2
1
2 2OH 6S
2 3
5
3 2 2
3 4
2+
2 3
+++=
++--
h3-
h
(2)
Au O Cu^NH3h2+
Au^S O Cu^S O
5S2
4NH
3
2-
4
2 3
3
3 2 3 3
5-
++=
++h2- h
(3)
Thiosulfate is a highly reactive species that is prone to
decompose in leaching solutions. The decomposed prod-
ucts of thiosulfate include sulfite, sulfate, and polythionates
such as tetrathionate and trithionate, depending on various
factors. However, an excessive amount of sulfite can lead to
the reduction of copper concentration, while polythionate
ions can cause passivation, which hinders gold dissolution
[8,11]. The decomposition of thiosulfate can be influenced
by several factors, such as the presence of sulfur-reducing
bacteria, pH, thiosulfate concentration, metal ions, and cat-
alysts, oxygen concentration, and other oxidants [10,12–14].
In the presence of [Cu(NH3)4]2+, thiosulfate is read-
ily oxidized to polythionate ions such as S4O62– as shown
in equation 4 [15]. The [Cu(NH3)4]2+ acts as a catalytic
oxidant that significantly increases gold dissolution while