Novel Membrane and Electrodeposition-Based Separation and Recovery of Rare Earth Elements from Coal Combustion Residues

Scope and Objectives. The goal of this project is to develop and demonstrate bench-scale technology to separate and concentrate rare earth elements (REEs) from coal fly ash and other combustion residues. Coal combustion residues (CCRs) are the solid wastes generated at coal-fired power plants and represent one of the largest industrial waste streams in the United States. While some CCRs are used for beneficial purposes, more than half of this waste is disposed in landfills and retention ponds every year. In our previous work, we collected dozens of fly ash and other CCR samples from power plants across the U.S. and have determined that many have REE concentrations that could be high enough for resource recovery. Moreover, we have found that the REE content in the CCRs and REE recovery potential depend on properties such as feed coal origin and ash geochemical characteristics. As part of this work we will use these parameters to identify promising feedstocks for REEs recovery and develop a bench scale system that can extract and concentrate REEs from CCR feedstock. Technical Approach. This project will use a representative selection of CCRs to develop hydrometallurgical-based technology to extract and concentrate REEs. The novelty of our approach is the application of nanofiltration membranes in combination with electrodeposition to separate rare earth ions from acid leachates of CCRs. We have developed carbon nanotube electrodes that can efficiently extract REEs from other metal ions in complex aqueous mixtures. The advantage of this separation approach is a reduction in hazardous chemicals and waste products that would be necessary for conventional methods that use solvent-based separations. Specific aims for Phase 1 of this project are to: 1) Identify and characterize a representative selection of CCR samples as candidates for REE recovery; 2) Evaluate the efficiency of hydrometallurgical acid extraction techniques as a function of major CCR characteristics and extraction conditions; 3) Optimize nanofiltration membranes and carbon nanotube-based electrochemical deposition techniques for concentration of REEs from CCR extracts; 4) Perform a technical and economic feasibility study of the proposed separation methods; 5) Develop an implementation plan for a bench-scale system. A variety of nanofiltration and electrodeposition configurations will be tested with the CCR samples, and from this work we will identify the most promising system for further scale-up for Phase 2. The work will involve partners in the electric power and CCR industries, who will supply feedstock materials for this project, and partners experienced in the waste recovery industry, who will provide assistant with technical feasibility and scale-up. Benefits and Expected Outcomes. The expected benefits of this project will be the development of technologies to recover REEs from a highly abundant waste material (coal combustion residues) and the development of methods to determine the most promising CCR materials for REE recovery. The outcomes will include a new approach for REE extraction that utilized advanced separation technologies that enable more environmentally benign processes than more conventional approaches that use hazardous chemical for separations.