Investigating the recovery of rare earth elements by ionic liquids and bacteria

Abstract

The work presented in this thesis focuses on developing and understanding practices and reagents that can be more efficient at rare earth element (REE) recovery and separation than current industrial reagents and practices. With demand for REEs rapidly increasing due to their use in modern technologies and clean energy production, the sustainable supply of these elements has become vitally important for society. The concepts and difficulties that underpin current REE recovery are introduced in Chapter 2 followed by an appraisal of how an ammonium ionic liquid (IL) transports these elements from an aqueous phase into an immiscible organic phase during solvent extraction. Prior to this work the extraction of REEs using ILs was known but an understanding of the chemical transport mechanisms was limited. A large variety of analytical, spectroscopic and computational techniques have been used: ICP-OES measurements confirmed that REE extraction from nitrate solutions is maximised under low acid, high salt conditions with preferential extraction of the lighter REEs occurring; Karl-Fischer water content measurements confirmed only modest water transport, allowing for (reverse)-micelle formation in the organic phase to be rejected. The combined experimental, analytical and computational data alludes to the transport of REEs through a microhydration mechanism pathway. Organic phase REEs comprising both inner-sphere bound water molecules and nitrate anions are encapsulated and stabilised in the organic phase by multiple lipophilic IL cations through a hydrogen-bonding network. Chapter 3 builds upon the understanding of REE extraction by ILs gained in Chapter 2. The extraction of REEs using neutral reagents (malonoamides and diglycolamides) is introduced, highlighting their effectiveness but also their tendency to form undesirable precipitates (3rd phase formation). Incorporating chemical characteristics of these reagents such as the amide functional group into an IL results in a stronger extractant with improved selectively for lighter REEs and no solubility issues. The effective separation of lighter REEs (e.g. Ce or Nd) from heavier (e.g. Tb or Dy) REEs using an amide functionalised IL is presented with potential industrial applications discussed. Experimental, analytical and computational work suggests microhydrated REEs are extracted and a more extensive hydrogen-bonding network comprising amides, nitrate anions and water molecules enhances the stability of the formed organic phase assemblies. Chapter 4 compares the leaching of REEs from eudialyte, a zirconate REE mineral by highly acidic solutions against mildly acidic solutions containing the bacterialstrain Acidithiobacillus thiooxidans. The percentage of REEs leached using mild conditions is noticeably reduced over highly acidic conditions. Even so, and with no pH adjustment, the leached lighter REEs could be effectively separated from other elements within the leach solutions using an amide functionalised IL, potentially providing a more environmentally friendly REE purification process. Chapter 5 explores the development of multiple variations of the amide functionalised ILs introduced in Chapter 3, highlighting their similarities and differences. The variants include the incorporation of more amide functionalities and increasing the lipophilicity of the ILs. While initial screening of these reagents did not indicate any improvements over the ILs introduced in Chapter 3, the experimental data collected for these reagents helps to validate the importance of the amido-ammonium function