Mesoporous monoliths for REE recovery from atypical sources

Rare earth elements (REEs) are a group of 17 elements consisting of the lanthanide metals as well as scandium and yttrium. REEs are vital components of various developing technologies, particularly in the area of clean energy, and they are therefore critical to the international economy. However, because more than 90% of global REE production comes from a single source (China), the supply of REEs is potentially at risk due to political and economic concerns. It is therefore crucial to explore new REE feedstocks and to develop new and improved methods for REE purification. Solid-phase extraction (SPE) is one prominent class of REE recovery techniques currently under development. SPE involves the application of a solid material for selective REE adsorption from a liquid feedstock. The solid adsorbents have a high affinity for REEs compared to other metals, and thus will concentrate REEs in the solid phase. Mesoporous silica, carbon, and carbon-silica composite materials are SPE adsorbents that exhibit particular promise for REE extraction. Although significant progress has been made towards designing these materials for industrial-scale REE extraction applications, several major obstacles must be overcome before they are ready for real-world use. First, for a SPE scheme to be feasible, the adsorbent must be rapidly exposed to high volumes of REE feedstock, which is typically achieved by pumping the feedstock through a volume packed with the adsorbent. Unfortunately, most mesoporous materials are powders or other structures that can cause clogging under continuous flow. To address this issue, the adsorbent materials will be synthesized as hierarchically porous monoliths, which have been shown to eliminate clogging while still presenting a high surface area for adsorption. The low REE adsorption capacity of many mesoporous materials compared to other adsorbents is also a potential problem; however, novel functionalization techniques, including ion templating, will improve both the adsorption capacity and selectivity for REEs. Finally, little work has been done to test mesoporous materials with real REE feedstocks. Investigations have so far been largely limited to synthetic solutions, which may not be representative of the extremely complex real-world feedstocks. The functionalized mesoporous monoliths will be used to recover and purify REEs directly from real feedstocks to take the next step towards industrial-scale application. The use of mesoporous monoliths will ultimately permit the recovery and purification of individual rare earth elements from non-traditional feedstocks, which would help to alleviate the economic and environmental issues associated with modern REE mining practices both by making recycled REE feedstocks practical and by providing a green alternative to current REE purification methods.

The rare earth elements (REEs) are a group of 17 elements consisting of the lanthanide series metals as well as scandium and yttrium. REEs are vital components of many developing technologies, particularly in the area of clean energy, and they are therefore critical to the international economy. However, REEs are produced in only a few locations worldwide, which puts the supply of REEs potentially at risk due to political and economic issues. It is therefore crucial to explore new REE feedstocks and to develop new and improved methods for REE purification. In this study, two related experimental materials, hierarchically-porous silica monoliths and silica monolith powders, were assessed for REE recovery. These materials are intended to selectively recover REEs from aqueous feedstocks, such as acid mine drainage or geothermal fluids, via a technique called solid phase extraction (SPE), in which a solid material adsorbs specific metals from a liquid feedstock, concentrating and purifying those metals on the solid. These two sorbents offer numerous advantages for the SPE of REEs, most notably their high surface area for adsorption (with or without grafted ligands on the silica surfaces) and the possibility of rapid, high-volume feedstock transport through the sorbent. Unfunctionalized silica is especially well-suited for scandium (Sc) extraction, given its naturally high affinity for that metal. For this project, monoliths of several different types were synthesized according to established methods, with an emphasis on larger monoliths in an effort to progress the technique closer to the industrial scale. While a novel technique for applying larger monoliths for REE extraction was developed over the course of the project, there are significant present and future barriers to synthesizing large enough monoliths for practical industrial use. Therefore, powdered silica monoliths were tested as an alternative, and these powders may offer even additional advantages for selective metal recovery. The application of the powders circumvents the challenges of producing very large monoliths and offers a more flexible package that can be adapted to many different extraction systems. These monolith powders have been systematically tested for Sc extraction in both batch and continuous-flow column systems, and they demonstrate excellent potential for Sc recovery at a range of conditions, even without the surface modification that is commonly applied to most advanced, synthetic sorbents. Leachates from red mud, which is a type of waste from aluminum mining, will soon be tested as a potential Sc feedstock that could be exploited using this powdered monolith sorbent. Future work will build upon the highly adaptable foundation of hierarchically-porous silica monolith powders, chemically altering them to target other specific metals, as well as developing the overall SPE process for their implementation.