XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3 1785
Thus, excessive Cu could oxidize thiosulfate, which is con-
sidered an undesirable reaction. This could be the reason
for the high amount of thiosulfate that decomposed at 2
mM Cu in this study.
Column Scale Test Results
Effect of Initial pH
This section investigates the impact of the initial pH of the
feed solution on column leaching. According to Figure 7
(a), pH levels of 9.5 and 10 experience a temporary decrease
before gradually returning to their original levels. The pH
value of the heap solution may be impacted by various fac-
tors such as the ore’s characteristics and the acidity/alkalin-
ity level before thiosulfate leaching [14]. This could explain
why the initial leach solutions drained from the column
may have a lower pH than the feed solution. However, a
pH level of 9.0 will increase to 9.5 within a day, suggesting
that Mg(OH)2 releases hydroxide ions gradually at lower
pH levels, leading to an increase in pH.
Irrespective of the pH level, Au recovery shows almost
identical behavior, as depicted in Figure 7 (b). Figure 7 (c)
shows that Cu concentration from all conditions drops to
0 mM on day one and then slowly increases to the initial
Cu concentration. As previously observed, higher pH lev-
els lead to lower Cu concentrations. In future research, the
reason of the low Cu concentration and formation of Cu
precipitation should be investigated further by analyzing
the Cu concentration at different depths of the column and
performing surface analysis.
As demonstrated in Figure 7 (d), thiosulfate decom-
poses over time. It is believed that tetrathionate and other
sulfur byproducts decomposed from thiosulfate and can
facilitate thiosulfate decomposition even if the fresh feed
solution is injected into the column.
Effect of Mg(OH)2 Concentration
The experiment aimed to examine the impact of Mg(OH)2
on gold leaching with ammonium thiosulfate over a more
extended period. As illustrated in Figure 8 (a), the recov-
ery of Au followed a similar pattern for all cases, except
for 0.05 gMg(OH)2/gore, which yielded only a 14% recov-
ery rate. Interestingly, a lower concentration of Mg(OH)2
resulted in slightly higher Au recovery rates compared to
no Mg(OH)2, with 48% and 53% recovery rates observed
with 0.01 and 0.02 gMg(OH)2/gore, respectively, and 46%
observed without Mg(OH)2. Figure 8 (b) displays the pH
levels during the leaching process. The tests with Mg(OH)2
maintained an initial pH of 9.5, with 0.05 gMg(OH)2/gore
showing a slightly higher pH than 9.5. The test without
Figure 6. Kinetic plots of leaching with several Cu concentrations, a) Au recovery b) Cu concentration, c) pH, and
d) thiosulfate concentration (Test condition: 0.1 M S
2 O
3 2–, 0.5-2 mM Cu, 0.01 g
Mg(OH)2 /g
Ore Mg(OH)
2 ,pH 10.0, 20±3°C)
Thus, excessive Cu could oxidize thiosulfate, which is con-
sidered an undesirable reaction. This could be the reason
for the high amount of thiosulfate that decomposed at 2
mM Cu in this study.
Column Scale Test Results
Effect of Initial pH
This section investigates the impact of the initial pH of the
feed solution on column leaching. According to Figure 7
(a), pH levels of 9.5 and 10 experience a temporary decrease
before gradually returning to their original levels. The pH
value of the heap solution may be impacted by various fac-
tors such as the ore’s characteristics and the acidity/alkalin-
ity level before thiosulfate leaching [14]. This could explain
why the initial leach solutions drained from the column
may have a lower pH than the feed solution. However, a
pH level of 9.0 will increase to 9.5 within a day, suggesting
that Mg(OH)2 releases hydroxide ions gradually at lower
pH levels, leading to an increase in pH.
Irrespective of the pH level, Au recovery shows almost
identical behavior, as depicted in Figure 7 (b). Figure 7 (c)
shows that Cu concentration from all conditions drops to
0 mM on day one and then slowly increases to the initial
Cu concentration. As previously observed, higher pH lev-
els lead to lower Cu concentrations. In future research, the
reason of the low Cu concentration and formation of Cu
precipitation should be investigated further by analyzing
the Cu concentration at different depths of the column and
performing surface analysis.
As demonstrated in Figure 7 (d), thiosulfate decom-
poses over time. It is believed that tetrathionate and other
sulfur byproducts decomposed from thiosulfate and can
facilitate thiosulfate decomposition even if the fresh feed
solution is injected into the column.
Effect of Mg(OH)2 Concentration
The experiment aimed to examine the impact of Mg(OH)2
on gold leaching with ammonium thiosulfate over a more
extended period. As illustrated in Figure 8 (a), the recov-
ery of Au followed a similar pattern for all cases, except
for 0.05 gMg(OH)2/gore, which yielded only a 14% recov-
ery rate. Interestingly, a lower concentration of Mg(OH)2
resulted in slightly higher Au recovery rates compared to
no Mg(OH)2, with 48% and 53% recovery rates observed
with 0.01 and 0.02 gMg(OH)2/gore, respectively, and 46%
observed without Mg(OH)2. Figure 8 (b) displays the pH
levels during the leaching process. The tests with Mg(OH)2
maintained an initial pH of 9.5, with 0.05 gMg(OH)2/gore
showing a slightly higher pH than 9.5. The test without
Figure 6. Kinetic plots of leaching with several Cu concentrations, a) Au recovery b) Cu concentration, c) pH, and
d) thiosulfate concentration (Test condition: 0.1 M S
2 O
3 2–, 0.5-2 mM Cu, 0.01 g
Mg(OH)2 /g
Ore Mg(OH)
2 ,pH 10.0, 20±3°C)