(579c) Supercritical CO2-Induced Alteration of Polymer-Metal Matrix and Selective Extraction of Valuable Metals from Electronic Wastes

Peng Peng, Alan West, and Ah-Hyung (Alissa) Park

Waste electrical and electronic equipment (WEEE), one of the fastest growing waste segments, has become one of the most difficult challenges faced by humanity. Despite increasing efforts in recycling and reuse, a large fraction of WEEEs, such as waste printed circuit boards (PCBs), are being disposed into the environment. Even worse, the amount of WEEEs being transferred/exchanged between states and countries (particularly to developing countries) are causing significant social and economic complications.

The current PCB recycling procedures include physical separation, followed by metal extraction via pyro/hydro metallurgical pathways. The intensive physical separation of PCBs introduces environmental concerns including toxic gas emissions, and energy deficiency. Pyrometallurgy is also environmentally hazardous due to the formation of toxic gas, slag and other industrial wastes. Hydrometallurgy involves a series of acid or caustic leaches of e-waste followed by metal recovery techniques. Although hydrometallurgy is more controllable, solvents that are economical and less corrosive (e.g. sulfuric acid) cannot be used to extract gold, which is the most valuable metal within most PCBs. The use of concentrated, or highly reactive acids to extract gold is costly and generates gaseous and liquid pollutants.

In this research, we aim to develop a greener, and more selective way to extract both precious (e.g., gold) and common metals (e.g., copper) in waste PCBs utilizing supercritical CO₂ (scCO₂). The use of supercritical CO₂ has resulted in the reduction of the use of concentrated acids. Unlike cyanide or aqua regia leaching, gold in this process is recovered as solids rather than dissolved ions to eliminate additional separation steps, thus making the overall PCB treatment technology more intensified and sustainable. Furthermore, the proposed process can inherently separate metals copper and nickel at different stages of the treatment.

The mechanisms of how scCO₂ interacts with various components in the PCBs are studied and it has been found that the CO₂ interaction with the polymeric portion of PCBs has significantly influenced the subsequent leaching behaviors of metals. Once leached, a wide range of separation options including precipitation and electrowinning are considered to selectively recover different metals.