Abstract/Summary

The Rare Earth Elements (REEs) are becoming increasingly vital in today’s modern society, with uses in mobile phones, televisions, wind turbines, light bulbs and medical equipment. Despite the fact they are used so regularly, only 1% of all REEs are recycled from end-of-life products. REEs are currently some of the most critical elements on the critical elements list and with their demand so high, strategies must be put in place to keep up with this increasing demand. Chapter 1 This work summarizes the occurrence of these elements in the Earth’s crust and their uses and demands in modern life. The mining and processing of REEs involves multiple stages including leaching, benefaction, pyrometallurgy and hydrometallurgy; either ion exchange or solvent exchange to afford pure REEs. The reprocessing of REEs from end-of-life products is also discussed. Chapter 2 Neocuproine immobilized silica gel (NC-Si) contains an N-donor phenanthroline substituent able to bind to REEs and provide a way to separate REEs from each other providing a potential method for the reprocessing of REEs. This study explores the adsorption and desorption of REEs from a fixed-bed column of NC-Si. Thermogravimetric analysis (TGA) confirmed that approximately 5% w/w of neocuproine content was present on the NC-Si, resulting in a molarity of 0.1466 mmol per gram of adsorbent. The adsorption capacities of NCSi for all REEs (including Sc and Y) ranged from 0.0005 to 0.0012 mg g-1, with a preference for the late REEs (Ho-Lu), with an overall adsorption capacity of 0.0153 mg g-1. Breakthrough times (tb) were used to classify the REEs series into three groups; early (Y, La-Pr) (<3 mins), mid (Sc, Nd-Dy) (3.5 - 15 mins), and late (Ho-Lu) (>44 mins), indicating the potential of NC-Si for REE separations. Different models; Adams-Bohart, Thomas, Yoon Nelson, Modified Dose Response (MDR), and Lagergren's pseudo first and second order rate kinetics were applied to describe the adsorption of the multicomponent solution. The Yoon-Nelson model had the highest correlation coefficient (R2 ≈ 0.95) and effectively fit the mid and later REEs (Nd-Lu) among all the models tested. The MDR and Thomas models showed good fitting for early (R2 ≈ 0.95) and early/mid (R2 ≈ 0.93) REEs, respectively. The Adams-Bohart model provided the lowest correlation coefficient (R2 ≈ 0.81) but was able to describe all ions. Chapter 3 Similarly to NC-Si, CyMe4-BTPhen also contains a phenanthroline functional group and is able to bind to REEs, however separation of REE is not as efficient. CyMe4-BTPhen was immobilised onto silica (BTPhen-Si) and fixed-bed column techniques were carried out. BTPhen-Si had an overall adsorption capacity of 0.048 mg g-1 with adsorption capacities ranging from 0.0016 to 0.0038 mg g-1 for Y and Sc, respectively. Based on the breakthrough times (tb) the series could be separated into two groups, mid REE ions (Pr-Eu) (>3 mins) and late REE ions (Gd-Lu) (<1.5 mins), however Sc and Ce would elute with the mid REE ions, whereas Y and La elute with the late REEs. Adams-Bohart and Thomas models were able to describe all ions (Sc, Y and all REEs), however they gave the lowest correlation coefficients at R2 = ~0.86 and 0.9, respectively. Yoon-Nelson and MDR were not able to describe Sc, Pr, Nd, Sm and Eu. BTPhen-Si shows potential as a material for extracting REE ions, but further improvements are needed to enhance separation and adsorption capacities. Chapter 4 Four ligand systems were developed to investigate extraction and partition capabilities of REEs in a fixed-bed column system which allowed comparison with NC-Si and BTPhen-Si. All ligands contained the phenanthroline functional group but all had different linkages to the silica gel. ANC-Si, BNC-Si, HANC-Si and TBTPhen-Si were successfully synthesized, characterised and measured for REE extraction. The models; Adams-Bohart, Thomas, Yoon Nelson and Modified Dose Response (MDR) were applied to the breakthrough curves of each of the ligand systems. HANC-Si could not be described by any model, however BNC-Si and TBTPhen-Si were best described by MDR and ANC-Si described by AdamsBohart and Thomas. Adsorption capacities of the four ligand systems were as followed for ANC-Si, BNC-Si, HANC-Si and TBTPhen-Si; 0.0106, 0.0157, 0.0101 and 0.00484 mg g-1. Cost, REE separation, w/w% and adsorption capacities were compared to for all four ligands as well as with NC-Si and BTPhen-Si. Overall, NC-Si was the most efficient at separating the REEs from each other into small sub-groups of REEs. BTPhen-Si had the highest adsorption capacity out of the six ligands (0.0484 mg g-1).