Thermo-Mechanical Separation by Atomic Diffusion for Refinement and Recycling of Alloys

Yale University, New Haven CT

Investigators

Abstract

Research supported by this grant focuses on a separation method for efficient recycling of metallic alloys, which is important for achieving a circular economy. Inefficient separation of alloy scrap leads to loss of valuable alloying elements as impurities. Current alloy separation methods are based on chemical processes, which are energy intensive and hazardous. This research develops a thermo-mechanical separation method based on differences in atomic diffusion of the alloying elements in scrap metal. The project develops theory to predict the separation rates of the alloying elements based on the laws governing diffusion for a given alloy of interest. The thermo-mechanical separation process uses moderate temperatures and pressures and is thus less energy intensive. This diffusion-based separation process lays the foundation for the development of an energy-efficient, economic, and environmentally viable method to separate elements from alloys and scrap metal, which has significant impact on the US economy. The project involves students, especially women and under-represented minorities, who are trained in multi-disciplinary research and who become the next generation of skilled engineers and scientists. The students learn advanced materials fabrication and characterization techniques, data analytics and its adaptation for solving materials science problems. Thermo-mechanical separation is the separation of alloying elements from alloys utilizing their differences in diffusion rates under applied temperature and pressure. This research is based on results from a recently developed thermo-mechanical nanomolding process, in which an alloy is extruded through nanocavities at elevated temperatures and large applied pressure gradients. Preliminary results from the thermo-mechanical nanomolding work revealed separation of elements at temperatures above approximately 40% of the liquidus temperature. This thermo-mechanical separation process is essentially controlled by differences in atomic diffusion rates of the alloy constituents. The established pressure gradient, which originates from the applied pressure and nanocavity geometry, motivates diffusion of the elements. Models based on atomic diffusion are developed that quantify the separation rate for different alloys and model predictions are compared against experimentally determined separation rates for alloys with various compositions. For the experiments, alloys are selected based on their technological importance, the element combinations resulting from mining, extraction, and recycling, the criticality of the elements, and limitations of alternative refinement and recycling processes. The outcome of the project is to reveal for which alloys the use of thermo-mechanical separation to separate desired elements is the most effective, impactful, and superior over existing technologies. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.