Abstract
Rare earth elements have become essential materials in advanced clean energy technologies and national security applications due to their unique properties. Despite their importance, the United States remains almost completely dependent upon foreign supply chains, notably imports from China, for both raw and finished commodities containing rare earth elements. This dissertation explores the implementation of a neutral ligand, N, N, N’, N’ tetraoctyl diglycolamide (TODGA), for the separation of rare earth elements. TODGA offers distinct advantages over traditional phosphonic acid extractants, notably the elimination of saponification to achieve high recovery in a solvent extraction circuit and improved adjacent lanthanide separation factors, ultimately requiring fewer solvent extraction stages to achieve high degrees of purity and recovery. This work marks the first use of TODGA’s unique chemistry to separate and purify the rare earths from each other in hydrochloric acid media using counter-current solvent extraction.
Chapter 1 introduces the concept of rare earths as a critical material and highlights historic and future challenges associated with the rare earth supply chain. Separations remains as one of the greatest challenges due to high capital and operating costs to purify individual rare earth elements, thus emphasizing the need for advances in solvent extraction to enable a viable domestic supply chain in the United States.
Chapters 2 and 3 provide background context that motivated the research by describing commercial rare earth separation processes and an overview of TODGA’s known applications and uses for trivalent lanthanide extraction and separation. While TODGA’s lanthanide extraction chemistry has been studied extensively for separations relevant to the nuclear fuel cycle, it has not been successfully applied in the field of rare earth mining and hydrometallurgy to separate individual lanthanides with high degrees of recovery and purity in a continuous counter-current solvent extraction cascade. TODGA exhibits a unique extraction trend among “light” low molecular weight lanthanides, with an observed 50% increase in adjacent light lanthanide separation factors as compared to the industry standard phosphonic acid PC88A. This suggests that a counter-current solvent extraction cascade with a reduced number of stages may be implemented for the purification of light rare earth elements.
Chapter 4 outlines the various experimental methods that were utilized to conduct this research. A variety of techniques were utilized in the approach, including laboratory batch equilibrium solvent extraction experiments, counter-current mixer-settler testing, and process modeling and simulation using MATLAB/Simulink.
Chapter 5 provides the rationale behind counter-current solvent extraction modeling and simulation for process design. Mass balances around a solvent extraction cascade may be written as a system of ordinary differential equations and coupled with empirical laboratory equilibrium data to model the approach to steady state. Alternative techniques for steady state cascade modeling using algebraic equations written in the form of a tridiagonal matrix and solved using the Thomas Algorithm are discussed. This approach may also be coupled with empirical expressions for calculating distribution ratios as a function of free TODGA and aqueous phase chloride concentration at equilibrium.
Chapter 6 describes experimental results that were used to evaluate the feasibility of TODGA’s extraction chemistry in a counter-current solvent extraction circuit. While TODGA demonstrates improved light rare earth separation factors, they are still relatively low, implying that neighboring lanthanides essentially co-extract. Commercially, high degrees of purity and recovery are achieved by implementing a selective scrubbing technique through which the purified REE product stream is refluxed into the scrub section. Batch solvent extraction experiments and simplified counter-current solvent extraction experiments in mixer-settlers revealed that TODGA is indeed capable of selective scrubbing to purify REEs under proper solvent loading conditions.
Chapter 7 describes the applied culmination of TODGA’s extraction chemistry through the design and experimental testing of a solvent extraction process to produce the permanent magnet precursor material didymium (75% neodymium and 25% praseodymium by mass), from a mixed light rare earth chloride feed representative of that produced from the processing of bastnäsite ore. The chapter includes single metal extraction data with empirically determined expressions for calculating distribution ratios, followed by batch counter-current extraction experiments for light REE separations. Batch experimental results were used to design a 24-stage counter-current solvent extraction cascade to purify PrNd from a mixture of La, Ce, Pr, and Nd. While experimental results of the cascade design did not achieve optimum recovery or purity, they indicate that TODGA can successfully be used to for the continuous separation and purification of light rare earths. Single metal distribution ratio correlations did not accurately model cascade behavior; a “pseudo single-metal” approach is presented to calculate distribution ratios under saturated loading conditions in a solvent extraction cascade.
Chapter 8 discusses the implications of utilizing TODGA in an industrial setting. While the use of a neutral ligand has distinct benefits over phosphonic acids, there are several limitations to the solvent system that require additional research efforts to address. Furthermore, implementing TODGA chemistry has economic impacts that may potentially limit its commercial viability. Notable limitations discussed include organic phase loading capacity, ligand synthesis and production costs, and high molarity salt-bearing raffinate streams that must be recycled or disposed of. A structure/property relationship was identified for DGA extractants with varying alkyl chain substituents, indicating that short alkyl chains make stronger, more selective extractants but are prone to gelling and third phase formation. Longer alkyl chains maintain selectivity and slightly reduce overall extraction strength. Branched alkyl chains prevent gelling and third phase formation but comes at the cost of poor selectivity due to steric hindrance caused by the branched alkyl chains in the outer coordination sphere.
Chapter 9 summarizes the general conclusion of this work: TODGA is capable of performing industrially relevant rare earth separations in continuous counter-current solvent extraction equipment, achieving high degrees of REE recovery and purity. However, its practical application is limited at this time due to its low organic phase loading. Ongoing research in collaboration with Oak Ridge National Laboratory is currently underway to synthesize and test novel DGA extractants with tailored alkyl chain substituents that achieve high degrees of organic phase loading capacity, maintain enhanced adjacent lanthanide selectivity among light rare earths, and demonstrate acceptable hydrodynamic behavior suitable for use in solvent extraction equipment.