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Advanced Separation Technologies for Extraction of Rare Earth Elements (REE)


OBJECTIVE: The objective of this research is to conduct fundamental surface chemistry measurements and demonstrate the use of these data to laboratory and small scale froth flotation systems so that more effective recovery can be achieved than with existing methods. DESCRIPTION: A critical step in the extraction of elements from ore, especially rare earth elements that are found in complex minerals, is separation. Froth flotation is a highly versatile method for physically separating particles based on differences in the ability of air bubbles to selectively adhere to specific mineral surfaces in a mineral/water slurry. The particles with attached air bubbles are then carried to the surface and removed, while the particles that remain are completely wetted stay in the liquid phase. Froth flotation is an attractive approach, but its effectiveness is limited for the rare earth minerals as they occur as phosphates, carbonates, fluorides, silicates and oxides with gangue minerals, which often share physical properties. By providing another tool for separation, increased understanding of localized surface chemistries in complex rare earth minerals could enable affordable processes that improve grades, recoveries, capital costs and operating costs for separation of rare earth elements from their ores. The techniques used to characterize surface chemistry in flotation relate to methods to make selective minerals hydrophobic by adjusting the surface charge so that ionic collectors may be adsorbed. In the case of non-sulfide minerals this is complicated by the fact that the waste materials are also non-sulfide, so very small differences in surface chemistry properties are observed. Finding chemical methods to selectively adsorb collectors onto the desired minerals requires additional fundamental understanding of the surface ions (potential determining ions) and charges (electrochemical potentials) encountered. The work, coupled with the development of a fundamental understanding can lead to greatly improved processes for concentration by froth flotation. PHASE I: In the phase I effort, the investigators need to explore the fundamental surface chemistry measurements (zeta potential, contact angle, micro-flotation tests) on pure rare earth mineral samples to evaluate various alternatives chemistries for selective froth flotation. Pure mineral samples need to be acquired, crushed, ground, screened, and analyzed using chemical and X-ray diffraction techniques. The surface chemistry measurements will be made as a function of collector type, pH, feed rate, particle size, mineral composition (phosphate, carbonate, fluoride, oxide, silicate), surface modification chemicals, etc. Attention will also be paid to the principal gangue minerals that occur in these ore bodies. Models are to be developed to describe and understand the surface chemistry and relate this to separation efficiency. Process environmental impact will also be a factor of evaluation. PHASE II: In the phase II effort, the investigators shall evaluate and validate the process models, modify the process models and analyze and characterize the efficiency and the environmental impact of the separation methodology using real crushed ores using standard large scale laboratory flotation equipment. Modification of the models, as necessary, based on the test results, will be conducted and retested to determine the range of their applicability. This will demonstrate the effect of this increased understanding on the grades and recoveries obtained by determining (1) the Ratio of Concentration and (2) the Percent REE Recovered. This then could be compared to conventional methods in order to demonstrate increased value and/or reduced operating costs as a function of ore type and original concentration. Separation variability as a function of REE ore composition should be assessed. If viable, scalability will be evaluated and preliminary drawings of pilot plant floth floatation system will be planned. . PHASE III: Working with industry, a pilot plant sized floth floatation system is constructed and various crushed commercial ores will feed to determine separation efficiency based on (1) the Ratio of Concentration and (2) the Percent REE Recovered. PRIVATE SECTOR COMMERCIAL POTENTIAL/DUAL-USE APPLICATIONS: The modeling of surface chemistries for froth floatation will lead to greater separation efficiencies and benefit the domestic mineral extractive companies specializing in rare earth recovery and production. More secure, domestic REE sources would be of great strategic importance to the Department of Defense for many applications where REE are utilized.
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