XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3 3397
Pyrite
The leaching of manganese nodules using pyrite (FeS2) as
a reducing agent in the presence of sulfuric acid at ambi-
ent temperature was investigated by Torres et al., 2019. The
suggested stoichiometry for the reaction with pyrite in acid,
as per Torres et al. is shown in Equation 1. Some dissolu-
tion of pyrite in sulfuric acid is also to be expected.
15MnO2 (s) +2FeS2 (s) +14H2SO4 (aq) →
Fe2(SO4)3
(aq) +15MnSO4
(aq) +14H2O
(l) (1)
It was observed that a higher ratio of pyrite to manganese
dioxide was beneficial to dissolution and that an increase in
acid concentration from 0.1 M to 1 M had a slight benefit
to the extraction. The highest Mn extraction achieved when
using pyrite in this study was 26.32%. It should be noted
that the residence time for the leach was only 30 min, but
other reductants tested achieved significantly higher extrac-
tions in the same period.
Iron Metal
The leaching of manganese nodules using elemental iron
was investigated by Torres et al. in the same study refer-
enced above. Another investigation was conducted using
high grade manganese ore and iron sponge as a reductant
(Bafghi et al., 2008). The proposed reactions by Bafghi et
al. are as follows:
Fe
(s) +2H+
(aq) → Fe2+
(aq) +H2
(g) (2)
Fe
(s) +2Fe3+(aq) → 3Fe2+(aq) (3)
MnO2 (s) +2Fe2+(aq) +4H+(aq) →
Mn2+
(aq) +2 Fe3+
(aq) +2H2O
(l) (4)
MnO2
(s) +2Fe
(s) +8H+(aq) →
Mn2+(aq) +2 Fe3+(aq) +2H2O
(l) +2H2
(g) (5)
MnO2 (s) +Fe (s) +4H+(aq)→
Mn2+(aq) +Fe2+(aq) +2H2O (l) (6)
MnO2
(s) +2/3Fe
(s) +4H+
(aq) →
Mn2+(aq) +2/3Fe3+(aq) +2H2O
(l) (7)
Of all the iron sources tested by Torres et al., elemental iron
was found to be the most suitable reductant. Similarly, it
was observed that increasing the ratio of iron to manganese
dioxide significantly improved on the extraction of man-
ganese, achieving up to 97.34% extraction in 30 min. The
leaching of manganese was found to be independent of the
concentration of sulfuric acid between 0.1 and 1.0 M.
Aligning with the conclusions drawn by Torres et al.,
the investigation by Bafghi et al. also concluded that the
dissolution of manganese is more sensitive to the ratio of
reductant to manganese dioxide than the concentration of
acid applied. The effect of temperature was also investigated
during this study, finding that increasing the temperature
from ambient to 60°C was beneficial to the rate of extrac-
tion. Under the most favourable conditions, essentially
complete dissolution of Mn was achieved in 10 min.
Slags and Tailings
The use of slag from a smelting plant and flotation tail-
ings from said slag was investigated as reductant for the
extraction of manganese from manganese nodule, owing to
the high magnetite (Fe3O4) content in the material (Toro
et al., 2021, 2019, 2018). Toro proposed that the reduc-
tion occurs as the dissolution of the iron oxide to generate
ferrous sulfate, which in turn acts as reductant, as shown
below:
Fe3O4
(s) +4H2SO4
(aq) →
FeSO4 (aq) +Fe2(SO4)3 (aq) +4H2O (l) (8
2H2SO4 (aq) +2FeSO4 (aq) +MnO2 (s) →
Fe2(SO4)3
(aq) +MnSO4
(aq) +2H2O
(l) (9
Ferrous Ion As a Salt
The use of ferrous sulfate salt as reductant was discussed
by Toro et al. (2021) and investigated by Torres et al. .The
chemical reaction is the same as presented in Equation 9,
above. The findings from Torres et al. regarding the
Figure 1. Pourbaix diagram for manganese at 25°C, 10–6 M
(Sinha and Purcell, 2019)
Pyrite
The leaching of manganese nodules using pyrite (FeS2) as
a reducing agent in the presence of sulfuric acid at ambi-
ent temperature was investigated by Torres et al., 2019. The
suggested stoichiometry for the reaction with pyrite in acid,
as per Torres et al. is shown in Equation 1. Some dissolu-
tion of pyrite in sulfuric acid is also to be expected.
15MnO2 (s) +2FeS2 (s) +14H2SO4 (aq) →
Fe2(SO4)3
(aq) +15MnSO4
(aq) +14H2O
(l) (1)
It was observed that a higher ratio of pyrite to manganese
dioxide was beneficial to dissolution and that an increase in
acid concentration from 0.1 M to 1 M had a slight benefit
to the extraction. The highest Mn extraction achieved when
using pyrite in this study was 26.32%. It should be noted
that the residence time for the leach was only 30 min, but
other reductants tested achieved significantly higher extrac-
tions in the same period.
Iron Metal
The leaching of manganese nodules using elemental iron
was investigated by Torres et al. in the same study refer-
enced above. Another investigation was conducted using
high grade manganese ore and iron sponge as a reductant
(Bafghi et al., 2008). The proposed reactions by Bafghi et
al. are as follows:
Fe
(s) +2H+
(aq) → Fe2+
(aq) +H2
(g) (2)
Fe
(s) +2Fe3+(aq) → 3Fe2+(aq) (3)
MnO2 (s) +2Fe2+(aq) +4H+(aq) →
Mn2+
(aq) +2 Fe3+
(aq) +2H2O
(l) (4)
MnO2
(s) +2Fe
(s) +8H+(aq) →
Mn2+(aq) +2 Fe3+(aq) +2H2O
(l) +2H2
(g) (5)
MnO2 (s) +Fe (s) +4H+(aq)→
Mn2+(aq) +Fe2+(aq) +2H2O (l) (6)
MnO2
(s) +2/3Fe
(s) +4H+
(aq) →
Mn2+(aq) +2/3Fe3+(aq) +2H2O
(l) (7)
Of all the iron sources tested by Torres et al., elemental iron
was found to be the most suitable reductant. Similarly, it
was observed that increasing the ratio of iron to manganese
dioxide significantly improved on the extraction of man-
ganese, achieving up to 97.34% extraction in 30 min. The
leaching of manganese was found to be independent of the
concentration of sulfuric acid between 0.1 and 1.0 M.
Aligning with the conclusions drawn by Torres et al.,
the investigation by Bafghi et al. also concluded that the
dissolution of manganese is more sensitive to the ratio of
reductant to manganese dioxide than the concentration of
acid applied. The effect of temperature was also investigated
during this study, finding that increasing the temperature
from ambient to 60°C was beneficial to the rate of extrac-
tion. Under the most favourable conditions, essentially
complete dissolution of Mn was achieved in 10 min.
Slags and Tailings
The use of slag from a smelting plant and flotation tail-
ings from said slag was investigated as reductant for the
extraction of manganese from manganese nodule, owing to
the high magnetite (Fe3O4) content in the material (Toro
et al., 2021, 2019, 2018). Toro proposed that the reduc-
tion occurs as the dissolution of the iron oxide to generate
ferrous sulfate, which in turn acts as reductant, as shown
below:
Fe3O4
(s) +4H2SO4
(aq) →
FeSO4 (aq) +Fe2(SO4)3 (aq) +4H2O (l) (8
2H2SO4 (aq) +2FeSO4 (aq) +MnO2 (s) →
Fe2(SO4)3
(aq) +MnSO4
(aq) +2H2O
(l) (9
Ferrous Ion As a Salt
The use of ferrous sulfate salt as reductant was discussed
by Toro et al. (2021) and investigated by Torres et al. .The
chemical reaction is the same as presented in Equation 9,
above. The findings from Torres et al. regarding the
Figure 1. Pourbaix diagram for manganese at 25°C, 10–6 M
(Sinha and Purcell, 2019)