An Enviromental-Friendly Process for the Recovery of Tantalum from Waste Capacitors

Tantalum (Ta) is a metallic element naturally occurring within the mineral columbite, which also encompasses niobium. The extraction of pure tantalum from this mineral necessitates highly costly processes such as pyrometallurgy and the utilization of hazardous chemical agents. Tantalum has been identified as one of the Critical Raw Materials for the European Union in 2023. It possesses a Supply Risk (SR) of 1.3 and a moderate Economic Importance (EI) of 4.8. Its criticality is attributed due to the extraction sites primarily situated in conflict-affected regions. Moreover, the tantalum end-of-life recycling input rate (EOL-RIR) indicates a restricted contribution to the production system from recycling end-of-life scrap, with only 1%. The most common methods for its recovery from e-waste include the use of strong acids, hazardous chemicals, or high-temperature treatment. Tantalum capacitors are typically a casing of epoxy resin loaded with glass particles, containing three main components: an anode, typically composed of tantalum wire; a cathode, made of manganese oxide or conductive polymer; and a dielectric layer of tantalum pentoxide. Additional conductive layers, such as graphite and silver, are incorporated to minimize resistance to the terminals, often made of Fe-Ni or Cu. In this work, a process toward a low-cost and sustainable method for the recovery of tantalum from capacitors was performed. The first step was focused on the physical-mechanical treatment. In particular, a batch of capacitors was treated by means of milling, size classification, and magnetic separation, where the terminals, made of Fe-Ni or Cu, were recovered. The electrostatic separation was performed on the non-magnetic fraction, removing the non-conductive product (polymers). The conductive product, containing mainly Ta, MnO2, Ag, was next subjected to the chemical treatments, first with ascorbic acid and then with a solution of ammonium thiosulfate, ammonia and copper sulphate, removing MnO2 and Ag, respectively. Leaching products have been analysed by means of scanning electron microscopy (SEM) and X-ray diffraction (XRD), revealing good results for the application of a green process for tantalum recovery from spent capacitors. The final product, in which 99.9% of Mn and 50% Ag have been removed, consists of a powder with high concentrations of tantalum (up to 94%).