XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3 1903
Solution Eh was stable at 600 mV (higher than pyrite oxi-
dation potential, E° =510 mV) as shown in Figure 3, indi-
cating that the first stage involves oxidation of pyrite by
hypochlorite ions. Since pyrite is more stable than molyb-
denite, molybdenite oxidation by hypochlorite occurs first.
It can be inferred then that molybdenite oxidation also
occurs in this stage indicating that hypochlorite solution
can also be used to dissolve molybdenum from the con-
centrate. Products of the sulfide oxidation reactions such
as Fe(OH)3, SO42–, and MoO42– shown in Eq 1 and 4
are stable in this condition. This confirms that the sul-
fides encapsulating gold are oxidized in the first stage, thus
exposing the locked gold and already amenable to react
with the lixiviant for gold dissolution. In the 2nd stage,
pH was adjusted to 6 to ensure gold-chloride complex sta-
bility. Gold dissolution rapidly increased together with Eh
upon pH adjustment. The increase in Eh indicates forma-
tion of hypochlorous acid, which has the highest oxidation
power among chlorine species. The increase in Au dissolu-
tion indicates start of dissolution of the exposed gold by
hypochlorous acid. During 2nd stage, decrease in Eh is
observed indicating consumption of hypochlorous acid and
continuous dissolution of gold. Solution Eh continues to
decrease until it becomes stable at 700 mV and reaching Au
dissolution of 93.51%. Gold dissolved during this stage is
refractory (gold that was exposed during sulfide oxidation).
Remaining 7% of Au that is not dissolved could have been
gold locked in silicates (6.69% of Au based on gold occur-
rence) that are not dissolved by hypochlorite.
During leaching of the sulfide concentrate in hypo-
chlorite, pyrite being the major phase is oxidized to iron
hydroxide following Eq 1. The product layer is an amor-
phous oxide layer based on the SEM-EDX analysis as shown
in Figure 4 and Table 2. The XRD pattern of the residue
(Figure 5) also shows presence of hydroxides and sulfates
as products of sulfide oxidation. This is in agreement with
the observed iron hydroxide product layer surrounding the
unreacted pyrite core indicating that the sulfide oxidation
stage is product-diffusion controlled (Regidor et al., 2018).
Valenzuela et al. (2013) also observed deposits of sulfates
and hydroxide on the particles of the refractory gold oxi-
dized by hypochlorite.
Figure 3. Solution Eh and pH of two-stage hypochlorite leaching using optimum sulfide oxidation parameters
Figure 4. SEM micrograph of sulfide oxidation residue
Table 2. Elemental analysis of concentrate and oxidation
residue
Sample
Oxygen,
wt%
Sulfur,
wt%
Iron,
wt%
Concentrate 16.7 19.7 41.3
Sulfide Oxidation Residue 36.4 2.1 6.1
Solution Eh was stable at 600 mV (higher than pyrite oxi-
dation potential, E° =510 mV) as shown in Figure 3, indi-
cating that the first stage involves oxidation of pyrite by
hypochlorite ions. Since pyrite is more stable than molyb-
denite, molybdenite oxidation by hypochlorite occurs first.
It can be inferred then that molybdenite oxidation also
occurs in this stage indicating that hypochlorite solution
can also be used to dissolve molybdenum from the con-
centrate. Products of the sulfide oxidation reactions such
as Fe(OH)3, SO42–, and MoO42– shown in Eq 1 and 4
are stable in this condition. This confirms that the sul-
fides encapsulating gold are oxidized in the first stage, thus
exposing the locked gold and already amenable to react
with the lixiviant for gold dissolution. In the 2nd stage,
pH was adjusted to 6 to ensure gold-chloride complex sta-
bility. Gold dissolution rapidly increased together with Eh
upon pH adjustment. The increase in Eh indicates forma-
tion of hypochlorous acid, which has the highest oxidation
power among chlorine species. The increase in Au dissolu-
tion indicates start of dissolution of the exposed gold by
hypochlorous acid. During 2nd stage, decrease in Eh is
observed indicating consumption of hypochlorous acid and
continuous dissolution of gold. Solution Eh continues to
decrease until it becomes stable at 700 mV and reaching Au
dissolution of 93.51%. Gold dissolved during this stage is
refractory (gold that was exposed during sulfide oxidation).
Remaining 7% of Au that is not dissolved could have been
gold locked in silicates (6.69% of Au based on gold occur-
rence) that are not dissolved by hypochlorite.
During leaching of the sulfide concentrate in hypo-
chlorite, pyrite being the major phase is oxidized to iron
hydroxide following Eq 1. The product layer is an amor-
phous oxide layer based on the SEM-EDX analysis as shown
in Figure 4 and Table 2. The XRD pattern of the residue
(Figure 5) also shows presence of hydroxides and sulfates
as products of sulfide oxidation. This is in agreement with
the observed iron hydroxide product layer surrounding the
unreacted pyrite core indicating that the sulfide oxidation
stage is product-diffusion controlled (Regidor et al., 2018).
Valenzuela et al. (2013) also observed deposits of sulfates
and hydroxide on the particles of the refractory gold oxi-
dized by hypochlorite.
Figure 3. Solution Eh and pH of two-stage hypochlorite leaching using optimum sulfide oxidation parameters
Figure 4. SEM micrograph of sulfide oxidation residue
Table 2. Elemental analysis of concentrate and oxidation
residue
Sample
Oxygen,
wt%
Sulfur,
wt%
Iron,
wt%
Concentrate 16.7 19.7 41.3
Sulfide Oxidation Residue 36.4 2.1 6.1