XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3 3399
sulfuric acid compared to none. It was also noted in this
study that decreasing temperature, pH and sulfur dioxide
concentrations were beneficial to mitigate the co-dissolu-
tion of iron.
Dithionate Formation and its Significance
While sulfur dioxide has many benefits as a reductant, the
major draw-back to its use is highlighted in Equation 15.
The formation of manganese dithionate is known to occur
during the reductive leaching of manganese with sulfur
dioxide and results in compromised purity of the final
product and increased reductant consumption (Acharya
et al., 1999). The specification for manganese in its sulfate
product is 31.8%, which cannot be achieved if significant
Mn occurs as dithionate (Ke-jie He et al., 2018).
Ke-jie He et al. showed that the formation of dithi-
onates is promoted at higher sulfur dioxide concentrations
and elevated pHs. Dithionate formation could be inhibited
by leaching at a higher pulp density since this resulted in
more manganese dioxide being present as an oxidant, thus
preferentially oxidising sulfur dioxide to sulfate rather than
dithionate. Dithionate formation could also be inhibited
by leaching at elevated temperatures.
The removal of dithionate from solution using man-
ganese dioxide as oxidant was studied in the presence of
acid (Qu et al., 2018). The proposed mechanism is shown
below:
MnO2 (s) +MnS2O6 (aq) → 2MnSO4 (aq) (20)
It was found that manganese dioxide was an effective oxi-
dant of dithionate. The oxidation was improved by increas-
ing the dosage of manganese dioxide and concentration of
sulfuric acid, as well as running the oxidation at elevated
temperature.
MANGANESE SOLVENT EXTRACTION
Solvent Extraction Background
Solvent extraction (SX) is a liquid-liquid separation pro-
cess in which the desired elements are transferred from an
aqueous feed solution into an organic solvent that is immis-
cible in the solution. The solvent is a combination of an
active extractant (organic) and a carrier (diluent) which is
incorporated to reduce the viscosity of the organic. SX is
employed in a wide range of industries, including nuclear,
pharmaceuticals, food, and the petroleum industry. The
technology can to quantitatively extract metals and com-
pounds of interest, with low energy requirements and a
high degree of selectivity.
The disadvantages of SX are that it has significant
capital and operating cost, and it requires technical staff
MnO2 (s) +2SO2 (aq) → MnS2O6 (aq) (15)
2MnO2
(s) SO2
(aq) +H2O
(l) →
MnOOH
(s) +HSO3–
(aq) (16)
MnOOH
(s) +HSO3–
(aq) → MnSO4
(aq) +H2O
(l) (17)
MnO2 (s) +3SO2 (aq) +H2O (l) →
Mn(SO3)22–
(aq) +HSO3–
(aq) (18)
2MnO2
(s) +3SO2
(aq) → MnSO4
(aq) +MnS2O6
(aq) (19)
The use of aqueous sulfur dioxide for extracting manga-
nese from a low-grade ferromanganese ore was investigated.
It was found that residence time and SO2 flowrate were
strongly and positively correlated with the extraction of
manganese (Kallam et al., 2022). The study also found that
manganese extraction was not strongly influenced by pulp
density up to 20%, but that co-extraction of iron could
be mitigated by increasing the pulp density of the leach. A
similar observation was made by Naik et al., finding that co-
extraction of iron, aluminium, silica and sodium could be
mitigated at higher pulp densities. The use of ammonium
sulfate as buffer to precipitate leached iron as a jarosite has
also been reported (Acharya et al., 1999)
Kallam et al also noted that leaching at lower tempera-
tures were preferable owing to the diminished solubility of
sulfur dioxide at higher temperatures. Deng et al., on the
other hand, found increasing temperature to be beneficial
to the extraction of manganese using sulfur dioxide in the
presence of sulfuric acid. Deng et al. noted only a marginal
improvement in manganese extraction in the presence of
Figure 2. Relationship between SO
2 speciation and
solubility of SO
2 as a function of pH (Kejie He et al., 2018)
sulfuric acid compared to none. It was also noted in this
study that decreasing temperature, pH and sulfur dioxide
concentrations were beneficial to mitigate the co-dissolu-
tion of iron.
Dithionate Formation and its Significance
While sulfur dioxide has many benefits as a reductant, the
major draw-back to its use is highlighted in Equation 15.
The formation of manganese dithionate is known to occur
during the reductive leaching of manganese with sulfur
dioxide and results in compromised purity of the final
product and increased reductant consumption (Acharya
et al., 1999). The specification for manganese in its sulfate
product is 31.8%, which cannot be achieved if significant
Mn occurs as dithionate (Ke-jie He et al., 2018).
Ke-jie He et al. showed that the formation of dithi-
onates is promoted at higher sulfur dioxide concentrations
and elevated pHs. Dithionate formation could be inhibited
by leaching at a higher pulp density since this resulted in
more manganese dioxide being present as an oxidant, thus
preferentially oxidising sulfur dioxide to sulfate rather than
dithionate. Dithionate formation could also be inhibited
by leaching at elevated temperatures.
The removal of dithionate from solution using man-
ganese dioxide as oxidant was studied in the presence of
acid (Qu et al., 2018). The proposed mechanism is shown
below:
MnO2 (s) +MnS2O6 (aq) → 2MnSO4 (aq) (20)
It was found that manganese dioxide was an effective oxi-
dant of dithionate. The oxidation was improved by increas-
ing the dosage of manganese dioxide and concentration of
sulfuric acid, as well as running the oxidation at elevated
temperature.
MANGANESE SOLVENT EXTRACTION
Solvent Extraction Background
Solvent extraction (SX) is a liquid-liquid separation pro-
cess in which the desired elements are transferred from an
aqueous feed solution into an organic solvent that is immis-
cible in the solution. The solvent is a combination of an
active extractant (organic) and a carrier (diluent) which is
incorporated to reduce the viscosity of the organic. SX is
employed in a wide range of industries, including nuclear,
pharmaceuticals, food, and the petroleum industry. The
technology can to quantitatively extract metals and com-
pounds of interest, with low energy requirements and a
high degree of selectivity.
The disadvantages of SX are that it has significant
capital and operating cost, and it requires technical staff
MnO2 (s) +2SO2 (aq) → MnS2O6 (aq) (15)
2MnO2
(s) SO2
(aq) +H2O
(l) →
MnOOH
(s) +HSO3–
(aq) (16)
MnOOH
(s) +HSO3–
(aq) → MnSO4
(aq) +H2O
(l) (17)
MnO2 (s) +3SO2 (aq) +H2O (l) →
Mn(SO3)22–
(aq) +HSO3–
(aq) (18)
2MnO2
(s) +3SO2
(aq) → MnSO4
(aq) +MnS2O6
(aq) (19)
The use of aqueous sulfur dioxide for extracting manga-
nese from a low-grade ferromanganese ore was investigated.
It was found that residence time and SO2 flowrate were
strongly and positively correlated with the extraction of
manganese (Kallam et al., 2022). The study also found that
manganese extraction was not strongly influenced by pulp
density up to 20%, but that co-extraction of iron could
be mitigated by increasing the pulp density of the leach. A
similar observation was made by Naik et al., finding that co-
extraction of iron, aluminium, silica and sodium could be
mitigated at higher pulp densities. The use of ammonium
sulfate as buffer to precipitate leached iron as a jarosite has
also been reported (Acharya et al., 1999)
Kallam et al also noted that leaching at lower tempera-
tures were preferable owing to the diminished solubility of
sulfur dioxide at higher temperatures. Deng et al., on the
other hand, found increasing temperature to be beneficial
to the extraction of manganese using sulfur dioxide in the
presence of sulfuric acid. Deng et al. noted only a marginal
improvement in manganese extraction in the presence of
Figure 2. Relationship between SO
2 speciation and
solubility of SO
2 as a function of pH (Kejie He et al., 2018)