ABSTRACT
The Bayer process holds an exclusive status for alumina extraction, but
a massive amount of caustic ”red mud” waste is generated. In this work,
three oxalate reagents: potassium hydrogen oxalate
(KHC2O4), potassium tetraoxalate
(KHC2O4·H2C2O4),
and oxalic acid (H2C2O4)
were investigated for the Al and Fe extraction process from NIST SRM 600
– Australian Darling range bauxite ore. More than 90% of Al and Fe was
extracted into the aqueous phase in less than 2 h with 0.50 M\(\mathrm{C}_{\mathrm{2}}\mathrm{O}_{\mathrm{4}}^{2\mathrm{-}}\) for all
three reagents. The Fe and Al can be selectively precipitated by
hydrolyzing the aqueous phase. By acidifying the Al and Fe free
filtrate, 80% of the\(\mathrm{C}_{\mathrm{2}}\mathrm{O}_{\mathrm{4}}^{2\mathrm{-}}\) can be
precipitated as
KHC2O4·H2C2O4.
Greater than 90% of the aqueous acid can also be recycled using a
cation exchange resin. The proposed closed-loop process is an
energy-efficient, cost-effective, environmentally-friendly route for
extracting Al and Fe from bauxite ore.
KEYWORDS. bauxite, metal extraction, ore refining, oxalic acid,
red mud, leaching
INTRODUCTION
Aluminum is a lightweight, durable, recyclable metal with its primary
use in the production of high strength alloys in combination with other
metals like Ni, Zn, Cu, and Mn. These alloys are used in a broad range
of industries that vary from automobile manufacturing to aeronautical
applications.1 The primary source of Al is aluminum
oxide (alumina) present in bauxite ore. More than 90% of globally mined
bauxite is used for Al production. On average, a bauxite ore contains
30-60 wt% of alumina, the rest being a mixture of iron oxides and
quartz.2 Commercially, bauxite is refined using the
Bayer process to produce smelter-grade alumina that is then converted to
Al metal using the Hall–Héroult process.1, 3
In bauxite, Al is present in the form of aluminum oxide trihydrate like
gibbsite (Al2O3·3H2O or
Al(OH)3) and monohydrate minerals such as boehmite
(Al2O3·H2O or γ-AlO(OH))
and diaspore (Al2O3·H2O
or α-AlO(OH)). Other minerals which can be found in bauxite include
hematite (Fe2O3) and quartz
(SiO2). The Bayer process involves the digestion of
crushed bauxite in concentrated NaOH solution at high temperatures. The
Al present in the ore reacts with NaOH to form water soluble sodium
aluminate (NaAlO2) leaving behind an insoluble solid
residue (red mud). However, the dissolution of SiO2 and
Al in concentrated NaOH solution makes the Bayer process inefficient for
low-grade bauxite ores (greater than 10 wt% SiO2content).
The bauxite refining industry faces a global environmental issue because
of the disposal problems associated with the caustic bauxite tailings
commonly referred to as red mud.4-5 The red mud is
discharged from the process as an alkaline slurry with a pH
> 12. It primarily consists of bauxite tailings like iron
oxide (Fe2O3) and quartz
(SiO2).6 Typically, about 1.0-1.5 tons
of red mud are produced per ton of alumina in the Bayer
process.7 For the disposal of red mud, methods like
landfills, deep-sea dumping, or storage in open ponds or reservoirs are
utilized. The high alkalinity of the red mud pollutes the land and
threatens plant growth and wildlife.8-9 With the
growing demand for Al, the disposal methods of red mud are an issue that
needs global attention. In the past two decades, there have been
numerous red mud incidences because of its disposal. The most disastrous
incident occurred in Hungary in 2010, when the Ajka refinery dam
collapsed, resulting in red mud flooding the nearby area. The release of
approximately 1 million cubic meters of red mud contaminated more than
40 square kilometers of land and led to 9 deaths and 122 severely
injured.10-11
To solve the problems associated with red mud, either
environmentally-friendly techniques have to be utilized to dispose of
red mud responsibly, or it can be eliminated at the source by developing
a closed-loop bauxite refining process. Numerous researchers have worked
on the recovery of valuable metals from red mud using both
pyrometallurgy and hydrometallurgy.12-16Pyrometallurgy is energy intensive, whereas hydrometallurgical processes
using inorganic acids (e.g., sulfuric acid and nitric acid) poses
significant environmental risk from the emission of SOXand NOX. The large amount of acid initially involved for
neutralizing the caustic red mud and the handling of effluents create an
additional burden on the red mud processing.10 In this
work, we are investigating a closed-loop hydrometallurgical approach for
bauxite refining with minimal waste to eliminate the concerns associated
with red mud. In our alternative approach for bauxite refining, oxalic
acid (H2C2O4) and two of
its derivatives with potassium oxalate
(K2C2O4·H2O):
potassium hydrogen oxalate (KHC2O4) and
potassium tetraoxalate
(KHC2O4·H2C2O4)
are investigated as reagents to extract Al and Fe from bauxite.
The oxalate ion is a bidentate ligand with excellent chelation
properties. The H2C2O4and its derivatives (KHC2O4 and
KHC2O4·H2C2O4)
utilize the chelation property along with the acidity to extract metals
from the metal oxides. Previously,
H2C2O4 has been used to
extract metals from various sources ranging from spent lithium-ion
battery cathodes17-18 to ores such as
laterite19 and scheelite.20-21Corbin et al. developed two environmentally-friendly closed loop
processes for extraction of Fe and Ti from ilmenite using ammonium
hydrogen oxalate
(NH4HC2O4)22and trimethylammonium hydrogen oxalate
((NH(CH3)3)HC2O4).23The H2C2O4 and its
derivatives can be advantageous for metal separations in aqueous medium.
Most of the divalent (M2+) metal ions are known to
form insoluble metal oxalate compounds, whereas monovalent
(M+) and trivalent (M3+) metal ions
form soluble metal oxalates.24-25 The difference in
the aqueous solubility can be utilized to separate metal oxalate
compounds.
In this study, a standard bauxite material from the Australian Darling
range (NIST SRM600) has been used to investigate the feasibility of a
closed-loop Al and Fe recovery process using
H2C2O4,
KHC2O4, and
KHC2O4·H2C2O4.
The separation of SiO2 from
Al2O3 and
Fe2O3 is a major advantage of using an
acidic process. The Al and Fe from their respective metal oxides are
leached into the aqueous phase, whereas silica remains in the solid
phase. The Al and Fe extracted in the aqueous phase can be separated
using selective hydrolysis, and pH conditions have been optimized for
efficient separation. However, the
H2C2O4 and
K2C2O4 are more
expensive than inorganic acids such as
H2SO4 and
HNO3.17 To offset the cost of
oxalate-based acids and make this process economical, an ion-exchange
resin and a pH-based separation have been developed to recover the
oxalate-based acids in their original form. To the best of our
knowledge, this is the first study on extraction of metals from bauxite
using oxalic acid and its derivatives. This novel closed-loop process is
an environmentally-friendly and economical route for recovering Al and
Fe from bauxite ore.
EXPERIMENTAL SECTION
Materials. In this study, NIST SRM 600 – Bauxite,
Australian-Darling Range,
H2C2O4·2H2O
(ACROS Organics, 99.5%),
K2C2O4·H2O
(Alfa AesarTM, 98.8%), and deionized water were used
for the metal extraction experiments. Potassium hydroxide, (KOH Pellets,
Fisher Chemical) and Fe metal powder (20 mesh, Alfa
AesarTM, 99%) were used for metal precipitation and
hydrolysis, respectively. Sulfuric acid
(H2SO4, Fisher Scientific, 98%) was
used for acidification and regeneration of oxalate.
Reactor Setup and Sampling. The metal extraction experiments
were carried out in a 1-L glass reactor attached to a 5-neck
Duran® head with two thermocouples, an electric
agitator, and a reflux condenser. The reactor was enclosed in a heating
jacket controlled by a set of PID temperature controllers. The reflux
condenser was connected to a chiller operating at 4 °C to avoid water
loss during the experiment. A detailed reactor schematic can be found in
our previous work.17 The reactor temperature and
agitation speed (N s) were set at 98 °C and 600
rpm, respectively, for all the experiments in this work. The temperature
and agitation speed values were optimized to maximize the kinetics and
avoid any diffusion limitation. Samples were withdrawn from the reactor
at specific intervals using a 20 cm long needle connected to a 5 mL
syringe. The withdrawn samples were centrifuged in a
Falcon® tube for 5 min at N s =
4000 rpm to separate out the solids. The aqueous phase was diluted with
5 wt% nitric acid solution at a ratio of 1:10 for the measurement of Al
and Fe concentrations.
Metal Extraction, Hydrolysis, and Acid Regeneration. Metal
extraction experiments were carried out by mixing the oxalate reagents
and heating them to a set temperature and then adding the required
amount of bauxite. Aqueous
H2C2O4 with or without
K2C2O4 was used in each
experiment. The H2C2O4and K2C2O4 molar ratio
is critical for the synthesis of KHC2O4and
KHC2O4·H2C2O4.
The reaction parameters for an efficient hydrometallurgical extraction
are the acid concentration, temperature, solid-to-liquid ratio (S/L),
and agitation speed. As mentioned in the previous section, the
temperature and agitation speed were kept constant, while the effect of
acid concentration was studied for all three oxalate reagents
(H2C2O4,
KHC2O4, and
KHC2O4·H2C2O4).
The effect of increasing the S/L ratio was studied only with
KHC2O4·H2C2O4.
Once the Al and Fe metals were extracted into the aqueous phase, the Fe
was precipitated as Fe(OH)3 via hydrolysis by increasing
the pH using KOH. After removal of the Fe, the Al was precipitated from
the solution by lowering the pH to an appropriate range by adding either
H2SO4 or
H2C2O4. The specific
range of pH for efficient precipitation of Al and Fe is discussed in the
results and discussion section. The precipitation experiments were
performed at 20 °C to maximize the precipitation efficiency using
minimum energy.
The oxalate reagents were regenerated using two methods. The first
approach involved using a strong acid cation exchange resin such as
Amberlyst-15 H-form to decrease the pH. In this work, a batch process
was used for the acid regeneration by mixing the activated resins with
the filtrate in a benchtop shaker while monitoring the pH. After
achieving the desired pH, the resins were regenerated by soaking them in
a 1 M sulfuric acid solution for 24 h. In the presence of a strong acid,
the resins regain their initial H-form. The regenerated resins were
washed with DI water until the effluent was pH neutral before performing
another ion exchange. The washing step removed any excess acid present
on the resin beads. The washed and regenerated resins can be repeatedly
used for additional metal precipitation. The second method for
regenerating the oxalate reagents involved acidification of the filtrate
post metal precipitation to the initial pH using
H2SO4. The acidification will
precipitate either KHC2O4 or
KHC2O4·H2C2O4,depending on the pH range. To minimize the amount of water added, 98
wt% H2SO4 was used in the acidification
process. This pH-based process utilizes solubility differences for
separation and is discussed in detail in the results and discussion
section.
Characterization. The metal concentrations in the solid and
aqueous phase were measured using an inductively coupled plasma –
optical emission spectrometer (ICP-OES). A Varian/Agilent 725 ES ICP-OES
with simultaneous CCD detector was used for the measurements, and a
Varian/Agilent SP3 autosampler was used to sequence multiple samples.
The aqueous phase samples were diluted 100 times with 5 wt%
HNO3 before analyzing them by ICP. Elemental
compositions of the solid phases were identified using X-Ray
fluorescence (XRF) using a Malvern Panalytical Zetium (1 kW) instrument
with a Rh anode and a 75 μm Be window with a duplex detector. Phase
identification and crystallinity measurements were performed on a Bruker
D2 phaser powder X-ray diffraction (PXRD) with a Co Kα radiation source
(λ = 1.78897 Å). The source voltage and current were set at 30 kV and 10
mA, respectively. The data were collected in the 2θ range of 10−70° with
a step size of 0.02° and dwell time of 0.40 s per step. XRD patterns
were analyzed using MDI Jade 6 software.
RESULTS AND DISCUSSION
Synthesis of Potassium Hydrogen Oxalate and Potassium
Tetraoxalate. H2C2O4 is
a diprotic acid with pKa1 = 1.23 and
pKa2 = 4.19 at 20 °C. Both
KHC2O4 and
KHC2O4·H2C2O4contain the binoxalate anion
(\(\mathrm{H}\mathrm{C}_{\mathrm{2}}\mathrm{O}_{\mathrm{4}}^{\mathrm{-}}\)).
Based on the speciation of oxalic acid,
H2C2O4 is the
predominant species below a pH = 1.23,\(\mathrm{H}\mathrm{C}_{\mathrm{2}}\mathrm{O}_{\mathrm{4}}^{\mathrm{-}}\)is the predominant species between pH = 1.23 and 4.19, and\(\mathrm{C}_{\mathrm{2}}\mathrm{O}_{\mathrm{4}}^{2\mathrm{-}}\) is the
predominant species above pH = 4.19.24 The
KHC2O4 can be synthesized using a 1:1
molar ratio of H2C2O4and K2C2O4, as shown in
eq 1. The KHC2O4 is sparingly soluble in
the resulting solution shown in eq 1 at 20 °C and a white precipitate
was observed. The precipitate was filtered and identified as
KHC2O4 using PXRD.
KHC2O4·H2C2O4is another derivative of
H2C2O4 that can be
synthesized by mixing
H2C2O4 and
K2C2O4 in a 3:1 molar
ratio, as shown in eq 2. The
KHC2O4·H2C2O4was also found to be sparingly soluble in the resulting solution shown
in eq 2 at 20 °C. A white precipitate was observed, filtered, and
identified using PXRD as
KHC2O4·H2C2O4·2H2O.
These two derivatives provide an alternative to
H2C2O4 with moderate
acidity and similar chelation properties. The low solubility of these
acids in comparison to the
H2C2O4 provides a
convenient means to recover the acids after metal extraction. The
details for the acid recovery will be discussed in the next section.