| dc.description.abstract | In the U.S., critical metals like aluminum (Al from bauxite), lithium (Li), cobalt (Co), and rare earth elements (REEs) are vital in various emerging technologies such as lithium-ion batteries (LIBs), solar cells, and high-tech electronics. Continuous growth in population with an increasing level of technological innovation has resulted in a rapid increase in resource consumption. Hence, sustainable, environmentally-friendly, and efficient use of available resources is required to preserve the natural resources for future generations. This dissertation introduces the utilization of oxalate chemistry to develop sustainable, environmentally-friendly, and closed-loop processes for recovery of critical metals like Li and Co from waste LIBs, Al and Fe from bauxite ore, and Fe and Ti from ilmenite ore. The oxalate anion (C2O42-) can be derived from organic sources, has minimal environmental impact, and forms moderately acidic reagents like oxalic acid (H2C2O4), potassium hydrogen oxalate (KHC2O4), and ammonium hydrogen oxalate (NH4HC2O4). The oxalate reagents are known for the chelation and precipitation properties, but the leaching and reduction properties had not been previously studied. This dissertation establishes oxalate reagents as an efficient route to recover and separate metals from various mixed metal oxide sources.The demand for LIBs has significantly increased over the last 5 years, leading to a shortage in the supply of Li and Co. The LIBs economy can be stabilized by recycling the critical metals from spent cathodes. Currently, approximately 59% of the LIBs contain lithium cobalt oxide (LiCoO2) as the cathode material. In this work, oxalate chemistry has been used to recover and separate Li and Co from LiCoO2. Traditionally, inorganic acids like sulfuric and nitric acid with a reducing agent like hydrogen peroxide (H2O2) are used to recycle LIBs, but the emission of harmful pollutants like SOX and NOX pose a significant risk to the environment. The clean and green oxalate reagents like H2C2O4 and NH4HC2O4 can extract Li into the aqueous phase as lithium oxalate (Li2C2O4) and precipitate cobalt oxalate (CoC2O4·2H2O) from LiCoO2 in a single step. The optimum acidity for Li and Co extraction and separation using oxalate reagents was pH 13. The Co was separated from CoC2O4·2H2O by dissolving and precipitating the metal oxalate (as Co(OH)2) in the basic solution recovered after Li precipitation. Alternatively, micro-rod structure Co3O4 was synthesized by calcining CoC2O4·2H2O at T > 400 °C in the presence of air. In this work, oxalate chemistry was also used for efficient Fe and Al recovery from bauxite ore. Bauxite ore is the world’s primary source for Al metal, and the Bayer process (based on NaOH) holds an exclusive status for its refining. The Bayer process is efficient for Al extraction, but a massive quantity of “red mud” waste is generated. The red mud is an iron-containing caustic waste and is typically disposed in landfills or open ponds and reservoirs. The high alkalinity of the waste pollutes the land and ecosystem around it. With the growing demand for Al, the disposal methods of red mud needs global attention. Using oxalate chemistry, reagents like H2C2O4, KHC2O4, and H2C2O4∙KHC2O4 can efficiently recover Fe and Al from bauxite ore. From NIST SRM 600 bauxite ore, more than 90% of Fe and Al was extracted into the aqueous phase in less than 2 h with 0.50 M C2O42- at 100 °C for all three reagents. Among the three oxalate reagents, H2C2O4 is the most acidic, followed by H2C2O4∙KHC2O4 and KHC2O4. The Fe can be selectively precipitated by hydrolyzing the aqueous phase to a pH = 13.80. After separating the Fe precipitate, the resulting filtrate can be acidified to a pH = 10.50 for efficient Al precipitation. The recycling of acid after the efficient metal extractions is critical to minimize waste generation and improve economics. In this work, two unique acid recycling processes were developed to efficiently recover and reuse oxalate reagents. The first process utilizes strong acid cation-exchange resins to regenerate the oxalate reagent in acidic form. The amount of resins determines the final pH and the type of oxalate reagent regenerated. To recycle the aqueous phase as H2C2O4, a pH 400 °C in the presence of air. In this work, oxalate chemistry was also used for efficient Fe and Al recovery from bauxite ore. Bauxite ore is the world’s primary source for Al metal, and the Bayer process (based on NaOH) holds an exclusive status for its refining. The Bayer process is efficient for Al extraction, but a massive quantity of “red mud” waste is generated. The red mud is an iron-containing caustic waste and is typically disposed in landfills or open ponds and reservoirs. The high alkalinity of the waste pollutes the land and ecosystem around it. With the growing demand for Al, the disposal methods of red mud needs global attention. Using oxalate chemistry, reagents like H2C2O4, KHC2O4, and H2C2O4∙KHC2O4 can efficiently recover Fe and Al from bauxite ore. From NIST SRM 600 bauxite ore, more than 90% of Fe and Al was extracted into the aqueous phase in less than 2 h with 0.50 M C2O42- at 100 °C for all three reagents. Among the three oxalate reagents, H2C2O4 is the most acidic, followed by H2C2O4∙KHC2O4 and KHC2O4. The Fe can be selectively precipitated by hydrolyzing the aqueous phase to a pH = 13.80. After separating the Fe precipitate, the resulting filtrate can be acidified to a pH = 10.50 for efficient Al precipitation. The recycling of acid after the efficient metal extractions is critical to minimize waste generation and improve economics. In this work, two unique acid recycling processes were developed to efficiently recover and reuse oxalate reagents. The first process utilizes strong acid cation-exchange resins to regenerate the oxalate reagent in acidic form. The amount of resins determines the final pH and the type of oxalate reagent regenerated. To recycle the aqueous phase as H2C2O4, a pH < 1.0 was optimal, whereas, for KHC2O4 and NH4HC2O4, a pH around 2.5 was required. Additionally, the low aqueous solubility of KHC2O4 and H2C2O4∙KHC2O4 was utilized to precipitate 60-80% of acid by acidifying the aqueous phase after metal recovery to a pH = 1.5-2.5. Due to acid recycling, the H2C2O4 + H2O2 process for LIBs recycling produced 50% less waste than the traditional H2SO4 process at a similar cost. The oxalate processes demonstrated in this work were closed-loop, environmentally-friendly, and economical and can offer similar advantages for recycling valuable metals from various waste streams. | |
