Novel Concentration Process for Platinum Group Metals in Automotive Exhaust Catalyst Using Electroless Copper Plating, Sulfurization, and Flotation

Abstract

Platinum group metals (PGMs) are primarily used in automotive exhaust catalysts (autocatalysits). Spent autocatalysts are the most important secondary resource for PGMs. However, transporting autocatalyst scraps and recovering PGMs from the scraps are costly and time-consuming, owing to the low PGM content in spent autocatalysts. Thus, an effective PGM-concentration technology for the pretreatment of scrap prior to transport is required. This study develops a new pretreatment technique that is applied prior to the flotation concentration of PGMs in autocatalysts. This method utilizes electroless Cu plating followed by sulfurization. In the electroless Cu-plating process, which uses glyoxylic acid as a reducing agent, Cu is deposited on the PGM particles in the washcoat of the autocatalyst. During the sulfurization process, S vapor sulfurizes the deposited Cu into copper sulfide, which is hydrophobic. Prior to the experiments, thermodynamic considerations were made to predict the reactivity of Cu and the representative elements constituting the autocatalyst with S vapor, and the sulfurization conditions were designed. Sulfurization experiments were performed at 850 K (577 °C) in the presence of carbon (C), and the results show the successful conversion of only Cu to copper sulfide without sulfurizing the representative oxides (MgO, Al2O3, SiO2, CeO2, and ZrO2) present in the autocatalyst. Finally, in the flotation process, the copper sulfide-coated PGMs are separated from the ceramic components of the autocatalyst, which is hydrophilic; thus, it is concentrated in the froth. Flotation experiments utilizing a microbubble flotation method were successfully performed to recover the PGM concentrates. This innovative pretreatment technique is expected to reduce the cost and time required for the entire PGM recycling process.