Figure 5. Flowsheet for the proposed bauxite refining process
using oxalate precipitation.
CONCLUSIONS
Commercially valuable metals like Al and Fe can be recovered efficiently
from NIST SRM 600 bauxite ore using an environmentally-friendly process
based on oxalate chemistry. In this work, an efficient Al and Fe
extraction process using three oxalate reagents:
H2C2O4,
KHC2O4, and
KHC2O4·H2C2O4has been demonstrated. The three oxalate reagents were synthesized using
various molar ratios of
H2C2O4 and
K2C2O4. The amount of
K2C2O4 determines the
reaction pH and the rate of metal dissolution. The kinetics of Al
extraction had a negligible dependence on the pH; however, the presence
of H+ ions are necessary for the metal dissolution.
The kinetics of Fe extraction was dependent on pH and the fastest rate
was measuring for KHO at a pH = 2.50. KHO is an effective and safe acid
for Al and Fe extraction from bauxite ore compared with the strong acids
and bases currently used in the Bayer process. The amphoteric behavior
of Al(OH)3 allows selective precipitation of Fe at a pH
= 13.80 and Al can be subsequently precipitated by acidifying the
filtrate to a pH = 10.50. The efficient extraction of Al and Fe from
bauxite ore produces only quartz (SiO2) as the final
residue.
In the described processes, 98-99% pure alumina can be produced. A
critical step in the proposed process is the recovery and recycling of
oxalate. After precipitating and separating the metals, an ion-exchange
resin process was developed to regenerate the
H2C2O4,
KHC2O4·H2C2O4,or KHC2O4 in solution. An alternative
approach that acidifies the precipitation filtrate takes advantage of
the lower solubility of
KHC2O4·H2C2O4and KHC2O4. Both reagents can be
precipitated from the aqueous solution and recovered as solids. The
closed-loop process utilizing oxalate chemistry does not produce any
hazardous waste, unlike the alkaline red mud produced in the Bayer
process. The simple and novel process using the combination of
H2C2O4 and
K2C2O4 provides a safe,
efficient, sustainable, and environmentally-friendly route for
recovering commercially valuable metals such as Al and Fe from bauxite
ore.
SUPPORTING INFORMATION
The supporting information is available free of charge on the journal
website.
ACKNOWLEDGEMENTS
The authors would like to acknowledge the contributions of Alexander
Henne for his assistance with the metal precipitations and ion-exchange
resin work and Dr. Laurence Weatherley for his expertise on ion exchange
resins.
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