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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