XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3 3529
Hydrometallurgical Processing
The hydrometallurgical approach for recovery of differ-
ent elements consists of multistage leaching for selective
recovery. Oxalic acid is used as a lixiviant for the selective
dissolution of iron. Due to its chelating capability and
stability, oxalic acid, an organic acid, strongly attaches to
iron. (Taxiarchou et al., 1997). Oxalic acid dissociated into
HC2O4– and C2O42– at pH 4 and pH 4, forming a
coordination bond with metal ions (such as Fe3+), result-
ing in dissolution. However, the presence of a high fraction
of alkali requires excess oxalic acid dosage as a part of it is
used for neutralization. Based on our previous work, it was
found that direct leaching of bauxite residue with oxalic
acid also results in the contamination of the iron prod-
uct with sodium and calcium (Tanvar and Mishra, 2021).
Therefore, bauxite residue must first be neutralized without
affecting the iron and other target elements for effective
separation. Hydrochloric acid (HCl) was used to neu-
tralize bauxite residue and selectively remove alkali. HCl
is a more cost-effective option compared to oxalic acid.
Consequently, neutralizing bauxite residue with HCl not
only lowers overall reagent costs but also results in a 32%
reduction in oxalic acid consumption during iron leach-
ing. Additionally, this neutralization process significantly
improves product purity.
First stage leaching (neutralization) with HCl was
performed with 100 g/L pulp density, 850 rpm stirring
speed, at room temperature for 15 min. Figure 3(a) shows
the chemical analysis of neutralized bauxite residue after
leaching at different acid concentrations. High recovery of
sodium and calcium was obtained using 1 M HCl, resulting
in more than 96% dissolution of sodium and calcium along
with 40% silicon and 42% aluminum. The HCl leaching
resulted in neutralization of the alkali content of bauxite
residue and the dissolution of secondary phases, resulting
in the dissolution of approx. 38% solid mass and pH reduc-
tion from ~10.5 to ~2.5. The neutralized bauxite residue
consists of 7.2% Al, 31.0% Fe, 0.1% Ca, 1.36% Si, 6.52%
Ti and 0.67% Na. The leach solution contains 5400.5 ppm
Na, 2165.1 ppm Ca, 4622.5 ppm Al and 3357.3 ppm Si.
The dissolved aluminum, silicon and calcium values can
be subsequently recovered through pH adjustment of the
leach liquor. As illustrated in Figure 3(b), the XRD analy-
sis of solid samples reveals the absence of peaks associated
with sodalite and calcite phases in the residue obtained after
leaching with HCl. The analysis suggests that HCl leach-
ing primarily dissociated these phases, while hematite and
titanium oxide phases remained unaffected.
The neutralized bauxite residue was leached with oxalic
acid for selective recovery of iron. The possible chemical
reactions for the leaching of hematite with oxalic acid are
shown in Eq. (1–3). The dissolution process results in the
formation of [Fe(C2O4)3]3– and [FeHC2O4]2+ at high
(4) and low (2) pH, respectively, as shown in Eq (1)
and (2). The reductive dissolution in Eq (3) is favored at
low temperatures, favoring the transfer of electrons from
adsorbed complex to ferric ion, resulting in the formation
of [Fe(C2O4)2]2– complex. In the reductive mechanism,
the dissolved iron complex can hydrolyze and precipitate
as hydrated ferrous oxalate (FeC2O4.2H2O), resulting in
low dissolution. The non-reductive dissolution mechanism
generates a stable ferric oxalate solution, providing higher
recovery and purity of the final product. Therefore, the
leaching experiments were performed at high tempera-
tures to enhance non-reductive dissolution kinetics and
limit iron loss through reductive leaching and precipita-
tion. Based on our previous work, the activation energy for
the dissolution of hematite in an aqueous oxalic solution
between 65 to 95 °C was established to be 100 to 150 kJ/
Figure 2. SEM analysis and elemental mapping of raw bauxite residue
Hydrometallurgical Processing
The hydrometallurgical approach for recovery of differ-
ent elements consists of multistage leaching for selective
recovery. Oxalic acid is used as a lixiviant for the selective
dissolution of iron. Due to its chelating capability and
stability, oxalic acid, an organic acid, strongly attaches to
iron. (Taxiarchou et al., 1997). Oxalic acid dissociated into
HC2O4– and C2O42– at pH 4 and pH 4, forming a
coordination bond with metal ions (such as Fe3+), result-
ing in dissolution. However, the presence of a high fraction
of alkali requires excess oxalic acid dosage as a part of it is
used for neutralization. Based on our previous work, it was
found that direct leaching of bauxite residue with oxalic
acid also results in the contamination of the iron prod-
uct with sodium and calcium (Tanvar and Mishra, 2021).
Therefore, bauxite residue must first be neutralized without
affecting the iron and other target elements for effective
separation. Hydrochloric acid (HCl) was used to neu-
tralize bauxite residue and selectively remove alkali. HCl
is a more cost-effective option compared to oxalic acid.
Consequently, neutralizing bauxite residue with HCl not
only lowers overall reagent costs but also results in a 32%
reduction in oxalic acid consumption during iron leach-
ing. Additionally, this neutralization process significantly
improves product purity.
First stage leaching (neutralization) with HCl was
performed with 100 g/L pulp density, 850 rpm stirring
speed, at room temperature for 15 min. Figure 3(a) shows
the chemical analysis of neutralized bauxite residue after
leaching at different acid concentrations. High recovery of
sodium and calcium was obtained using 1 M HCl, resulting
in more than 96% dissolution of sodium and calcium along
with 40% silicon and 42% aluminum. The HCl leaching
resulted in neutralization of the alkali content of bauxite
residue and the dissolution of secondary phases, resulting
in the dissolution of approx. 38% solid mass and pH reduc-
tion from ~10.5 to ~2.5. The neutralized bauxite residue
consists of 7.2% Al, 31.0% Fe, 0.1% Ca, 1.36% Si, 6.52%
Ti and 0.67% Na. The leach solution contains 5400.5 ppm
Na, 2165.1 ppm Ca, 4622.5 ppm Al and 3357.3 ppm Si.
The dissolved aluminum, silicon and calcium values can
be subsequently recovered through pH adjustment of the
leach liquor. As illustrated in Figure 3(b), the XRD analy-
sis of solid samples reveals the absence of peaks associated
with sodalite and calcite phases in the residue obtained after
leaching with HCl. The analysis suggests that HCl leach-
ing primarily dissociated these phases, while hematite and
titanium oxide phases remained unaffected.
The neutralized bauxite residue was leached with oxalic
acid for selective recovery of iron. The possible chemical
reactions for the leaching of hematite with oxalic acid are
shown in Eq. (1–3). The dissolution process results in the
formation of [Fe(C2O4)3]3– and [FeHC2O4]2+ at high
(4) and low (2) pH, respectively, as shown in Eq (1)
and (2). The reductive dissolution in Eq (3) is favored at
low temperatures, favoring the transfer of electrons from
adsorbed complex to ferric ion, resulting in the formation
of [Fe(C2O4)2]2– complex. In the reductive mechanism,
the dissolved iron complex can hydrolyze and precipitate
as hydrated ferrous oxalate (FeC2O4.2H2O), resulting in
low dissolution. The non-reductive dissolution mechanism
generates a stable ferric oxalate solution, providing higher
recovery and purity of the final product. Therefore, the
leaching experiments were performed at high tempera-
tures to enhance non-reductive dissolution kinetics and
limit iron loss through reductive leaching and precipita-
tion. Based on our previous work, the activation energy for
the dissolution of hematite in an aqueous oxalic solution
between 65 to 95 °C was established to be 100 to 150 kJ/
Figure 2. SEM analysis and elemental mapping of raw bauxite residue