Supercritical Fluid Extraction of Rare Earth Elements from Nickel Metal Hydride Battery

Abstract

Today's world relies upon critical green technologies that are made of elements with unique properties irreplaceable by other materials. Such elements are classified under strategic materials; examples include rare earth elements that are in increasingly high demand but facing supply uncertainty and near zero recycling. For tackling the sustainability challenges associated with rare earth elements supply, new strategies have been initiated to mine these elements from secondary sources. Waste electrical and electronic equipment contain considerable amounts of rare earth elements; however, the current level of their recycling is less than 1%. Current recycling practices use either pyrometallurgy, which is energy intensive, or hydrometallurgy that rely on large volumes of acids and organic solvents, generating large volumes of environmentally unsafe residues. This study put emphasis on developing an innovative and sustainable process for the urban mining of rare earth elements from waste electrical and electronic equipment, in particular, a nickel metal hydride battery. The developed process relies on supercritical fluid extraction utilizing CO 2 as the solvent, which is inert, safe, and abundant. This process is very efficient in the sense that it is safe, runs at low temperature, and does not produce hazardous waste while recovering ∼90% of rare earth elements. Furthermore, we propose a mechanism for the supercritical fluid extraction of rare earth elements, where we considered a trivalent rare earth element state bonded with three tri-n-butyl phosphate molecules and three nitrates model for the extracted rare earth tri-n-butyl phosphate complex. The supercritical fluid extraction process has the double advantage of waste valorization without utilizing hazardous reagents, thus minimizing the negative impacts of process tailings.

Selective Dissolution and Separation of Rare Earths Using Guanidine-Based Deep Eutectic Solvents

“…It is worth noting that the solution at extremely high concentration of La (62000 ppm) did not have obvious glass transition temperature, because much La 2 O 3 was dissolved by −COOH and a large amount of water was produced, thus affecting the measurement of T g . 42 In addition, La 3+ has a coordination number of 7−9 and both of the two components of GUC-LAC have two or more coordination groups, 43,44 which might form macromolecules under this situation. 45 The result of viscosity characterization of GUC-LAC before and after dissolution was shown in Figure 5b.…”

Sustainable Recycling of Rare-Earth Elements from NdFeB Magnet Swarf: Techno-Economic and Environmental Perspectives

“…Rare-earth elements (REEs) have distinctive physical and chemical properties that are increasingly being exploited in many high-tech applications, especially in clean energy-related technologies and consumer electronics sectors. − In the last few decades, the global demand for REEs and REE containing materials has increased by 59% due to their versatile applications, accounting for an ∼$42 billion market share . However, many countries are now facing REE supply uncertainties due to the near monopolistic supply from China (e.g., ∼95% of rare-earth oxide (REO) production). − Moreover, the recent COVID-19 pandemic has created a global supply and demand imbalance for REE compounds, highlighting the importance of source diversification .…”

Aeriometallurgical Extraction of Rare Earth Elements from a NdFeB Magnet Utilizing Supercritical Fluids

“…With the increasing focus on reducing greenhouse gas (GHG) emissions, the use of green energy technologies, such as wind turbines, hybrid and electric vehicles, photovoltaic cells, and fluorescent lighting has been widely promoted . The performance of these emerging green technologies relies upon the unique physiochemical properties of their critical building block materials, such as rare earth elements (REEs) . Neodymium–iron–boron (NdFeB) magnets are a class of permanent magnets widely utilized in hybrid and electric vehicles and wind turbines.…”

“…It has moderate critical points ( T c = 31.1 °C, P c = 7.37 MPa, ρ c = 469 kg/m 3 ) and is easy to recycle, and thus it does not generate secondary waste. Moreover, the final extraction products from this process are easily recoverable . Sc-CO 2 can be considered a switchable solvent; i.e., near the critical point, small changes in pressure or temperature result in large changes in the physical properties of CO 2 , particularly density, allowing many process-critical properties of the solvent, such as the solubility, to become time-separated, and, i.e., the properties can be adjusted, fine-tuned, or switched at different points in time and/or space in response to triggers, in this case, depressurization.…”