Magnetic Separation of Iron Ion from Leaching Solution …
849
Magnetic Separation of the Goethite and Gypsum
Precipitates
Maghemite fine particles were used in the laboratory test under the industrial production conditions of iron removal by the goethite process. In brief, 1 L of sulfuric
acid leachate of zinc oxide dust was introduced into a 2-L jacketed pilot plant reactor
and heated to keep at 85 °C by water bath. High temperature is known to benefits the formation of goethite precipitates. Added into the leachate were 1–7 g/L of
maghemite fine particles, with the mixture being kept under stirring by a mechanical mixer at 200 rpm. Substituting for oxygen-enriched air environment applied in
the industry, 10 mL H 2 O 2 solution (6 wt%) as the oxidant (representing a 22.9%
excess) was pumped into the vessel and reacted for 60 min to oxidize all the ferrous ions to ferric ions in solution as shown in Eq. (1). The fraction of hydrogen
peroxide that contributed to oxidation was 67.1%. The pH during the reaction was
maintained between 4.0 and 4.5 by pumping 50 g/L Ca(OH) 2 slurry into the system.
In the goethite process, a large amount of calcium sulfate dihydrate (CaSO 4 ·2H 2 O,
gypsum) and goethite (α-FeOOH) precipitates were generated as shown by Eq. (2)
and Eq. (3), respectively.
2Fe
2+
+ H 2 O 2 → 2Fe
3+
+ 2H 2 O
( 1 )
Ca(OH) 2 + SO 4
2−
+ 2H 2 O → CaSO 4 · 2H 2 O + 2OH
−
(2)
Fe
3+
+ 3OH
−
→ α-FeOOH + H 2 O
( 3 )
After crystallization by the goethite process, the suspension was pumped into
a 30 × 30 cm magnetic drum separator to separate the magnetic particles from the
suspension. The flow of the suspension was 100 ml/min. The magnetic drum separator
uses electromagnets to generate magnetic field and its magnetic field strength can be
adjusted from 0 to 3000 Gs. After the magnetic separation, the magnetic concentrate
and non-magnetic tailings were obtained after solid–liquid separation by filtration.
Both products were soaked in pH 3.0 sulfuric acid solution for 30 min under vigorous
stirring at the solid-liquid mass ratio of 1:5 to remove the adsorbed zinc ions from
particles surfaces. It should be noted that the zinc co-precipitated inside goethite and
gypsum cannot be removed by this acid washing. After filtration and drying at 80 °C
for 12 h, goethite (the concentrate) and gypsum (the tailings) particles were obtained
that could be potentially used as raw materials for value-added products.
Finally, reduction roasting was used to remove the impurity elements such as S
and As from goethite. Figure 1 is a schematic illustration of the entire process.
The recoveries of Fe and Ca in the concentrates and tailings with different dosages
of α-Fe 3 O 4 or γ-Fe 2 O 3 NPs are shown in Fig. 2a. The Fe recovery in the concentrates increased from 15.01 to 52.43%/29.29 to 96.29% with increasing the dosage
of α-Fe 3 O 4 /γ-Fe 2 O 3 from 1 to 7 g/L, which indicated that the amount of goethite
849
Magnetic Separation of the Goethite and Gypsum
Precipitates
Maghemite fine particles were used in the laboratory test under the industrial production conditions of iron removal by the goethite process. In brief, 1 L of sulfuric
acid leachate of zinc oxide dust was introduced into a 2-L jacketed pilot plant reactor
and heated to keep at 85 °C by water bath. High temperature is known to benefits the formation of goethite precipitates. Added into the leachate were 1–7 g/L of
maghemite fine particles, with the mixture being kept under stirring by a mechanical mixer at 200 rpm. Substituting for oxygen-enriched air environment applied in
the industry, 10 mL H 2 O 2 solution (6 wt%) as the oxidant (representing a 22.9%
excess) was pumped into the vessel and reacted for 60 min to oxidize all the ferrous ions to ferric ions in solution as shown in Eq. (1). The fraction of hydrogen
peroxide that contributed to oxidation was 67.1%. The pH during the reaction was
maintained between 4.0 and 4.5 by pumping 50 g/L Ca(OH) 2 slurry into the system.
In the goethite process, a large amount of calcium sulfate dihydrate (CaSO 4 ·2H 2 O,
gypsum) and goethite (α-FeOOH) precipitates were generated as shown by Eq. (2)
and Eq. (3), respectively.
2Fe
2+
+ H 2 O 2 → 2Fe
3+
+ 2H 2 O
( 1 )
Ca(OH) 2 + SO 4
2−
+ 2H 2 O → CaSO 4 · 2H 2 O + 2OH
−
(2)
Fe
3+
+ 3OH
−
→ α-FeOOH + H 2 O
( 3 )
After crystallization by the goethite process, the suspension was pumped into
a 30 × 30 cm magnetic drum separator to separate the magnetic particles from the
suspension. The flow of the suspension was 100 ml/min. The magnetic drum separator
uses electromagnets to generate magnetic field and its magnetic field strength can be
adjusted from 0 to 3000 Gs. After the magnetic separation, the magnetic concentrate
and non-magnetic tailings were obtained after solid–liquid separation by filtration.
Both products were soaked in pH 3.0 sulfuric acid solution for 30 min under vigorous
stirring at the solid-liquid mass ratio of 1:5 to remove the adsorbed zinc ions from
particles surfaces. It should be noted that the zinc co-precipitated inside goethite and
gypsum cannot be removed by this acid washing. After filtration and drying at 80 °C
for 12 h, goethite (the concentrate) and gypsum (the tailings) particles were obtained
that could be potentially used as raw materials for value-added products.
Finally, reduction roasting was used to remove the impurity elements such as S
and As from goethite. Figure 1 is a schematic illustration of the entire process.
The recoveries of Fe and Ca in the concentrates and tailings with different dosages
of α-Fe 3 O 4 or γ-Fe 2 O 3 NPs are shown in Fig. 2a. The Fe recovery in the concentrates increased from 15.01 to 52.43%/29.29 to 96.29% with increasing the dosage
of α-Fe 3 O 4 /γ-Fe 2 O 3 from 1 to 7 g/L, which indicated that the amount of goethite