3430 XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3
The much higher concentration of sodium in the fil-
trate than in the feed was a result of the use sodium hydrox-
ide (NaOH) as pH modifier.
SELECTIVE PRECIPITATION
MANGANESE
This section describes the oxidative precipitation of
manganese(II) as manganese dioxide (MnO2) using potas-
sium permanganate (KMnO4) as oxidant. The aim was to
selectively precipitate the manganese while leaving as much
of the impurity ions in solution. The target reaction is as
shown in equation 2 (EQ 2).
2MnO4–(aq) +3Mn2+(aq) +2H2O(l) ↔
5MnO2(s) +4H+
(aq) (2)
Analytical grade potassium permanganate (KMnO4)
was dissolved in deionised water (4% w/v). The feed, which
was the PLS post iron removal, was transferred into a
multi-neck glass vessel. The solution was stirred (200 rpm)
until a steady temperature and pH were achieved. Then, the
permanganate solution was added slowly using a pump to
allow a steady stream. Both the temperature and pH were
monitored but not controlled. Stirring of the solution was
continued for two more minutes after completion of the
permanganate addition. The slurry was then filtered under
pressure using glass fibre filter.
The precipitate was washed three times (3×) with dilute
sulfuric acid (50 mL of 0.1 M) and then twice (2×) with
deionised water. Both washes were done by repulping.
The residue was then filtered under vacuum, oven dried at
110 °C overnight, and weighed. Both the filtrate and resi-
due were assayed.
Consistent with the equation of the reaction (EQ 2),
the pH of the solution decreased as the added permanga-
nate increased (Figure 3).
The temperature of the solution slowly increased as
KMnO4 was added, i.e., from ~17 °C of the PLS feed
before any KMnO4 was added, peaking at 22 °C when the
stoichiometric amount of KMnO4 had been added, where
it stayed even with further of addition of the oxidant. This
indicated that the oxidation reaction had reached comple-
tion at approximately that point. This consistent with the
known exothermic nature of this reaction.
Table 4 shows the composition of the MnO2 precipi-
tate. The significant amount of co-precipitated potassium,
despite being a soluble metal, was a standout. This was owing
to the high concentration potassium in the solution owing
to the use of potassium permanganate as oxidant. Similarly,
the significant amount of sodium in the precipitate, which
is also a soluble metal, was owing to the high concentration
of sodium in the feed as sodium hydroxide was used as pH
modifier in the preceding unit step. Obviously, they were
entrained in the MnO2 crystals, which can be addressed in
the optimisation stage of the work.
The nominal purity of the MnO2 precipitate was
98.56 but there is significant uncertainty in this value. As
shown in Table 5, the accountability ranges from 97% for
calcium to 4,893% for copper with majority substantially
Figure 3. Cumulative KMnO
4 added and slurry pH vs time
The much higher concentration of sodium in the fil-
trate than in the feed was a result of the use sodium hydrox-
ide (NaOH) as pH modifier.
SELECTIVE PRECIPITATION
MANGANESE
This section describes the oxidative precipitation of
manganese(II) as manganese dioxide (MnO2) using potas-
sium permanganate (KMnO4) as oxidant. The aim was to
selectively precipitate the manganese while leaving as much
of the impurity ions in solution. The target reaction is as
shown in equation 2 (EQ 2).
2MnO4–(aq) +3Mn2+(aq) +2H2O(l) ↔
5MnO2(s) +4H+
(aq) (2)
Analytical grade potassium permanganate (KMnO4)
was dissolved in deionised water (4% w/v). The feed, which
was the PLS post iron removal, was transferred into a
multi-neck glass vessel. The solution was stirred (200 rpm)
until a steady temperature and pH were achieved. Then, the
permanganate solution was added slowly using a pump to
allow a steady stream. Both the temperature and pH were
monitored but not controlled. Stirring of the solution was
continued for two more minutes after completion of the
permanganate addition. The slurry was then filtered under
pressure using glass fibre filter.
The precipitate was washed three times (3×) with dilute
sulfuric acid (50 mL of 0.1 M) and then twice (2×) with
deionised water. Both washes were done by repulping.
The residue was then filtered under vacuum, oven dried at
110 °C overnight, and weighed. Both the filtrate and resi-
due were assayed.
Consistent with the equation of the reaction (EQ 2),
the pH of the solution decreased as the added permanga-
nate increased (Figure 3).
The temperature of the solution slowly increased as
KMnO4 was added, i.e., from ~17 °C of the PLS feed
before any KMnO4 was added, peaking at 22 °C when the
stoichiometric amount of KMnO4 had been added, where
it stayed even with further of addition of the oxidant. This
indicated that the oxidation reaction had reached comple-
tion at approximately that point. This consistent with the
known exothermic nature of this reaction.
Table 4 shows the composition of the MnO2 precipi-
tate. The significant amount of co-precipitated potassium,
despite being a soluble metal, was a standout. This was owing
to the high concentration potassium in the solution owing
to the use of potassium permanganate as oxidant. Similarly,
the significant amount of sodium in the precipitate, which
is also a soluble metal, was owing to the high concentration
of sodium in the feed as sodium hydroxide was used as pH
modifier in the preceding unit step. Obviously, they were
entrained in the MnO2 crystals, which can be addressed in
the optimisation stage of the work.
The nominal purity of the MnO2 precipitate was
98.56 but there is significant uncertainty in this value. As
shown in Table 5, the accountability ranges from 97% for
calcium to 4,893% for copper with majority substantially
Figure 3. Cumulative KMnO
4 added and slurry pH vs time