Abstract

Constant growth in waste electrical and electronic equipment (WEEE) levels necessitates the development of new, commercially viable recycling processes. Waste printed circuit boards (WPCBs) are a sub-group of WEEE that are of increasing interest due to their relatively high level of valuable metal content including Au, Ag, and platinum group metals (PGMs). Currently, precious metals like gold are mainly recycled from WEEE streams through copper smelting/refining; however, the possibility to peel gold from WPCBs prior to smelting, could offer advantages for recycling. In this study, the suitability of aqua regia for selective or partially selective gold leaching from un-crushed WPCBs was investigated. The redox potential of aqua regia solutions and the dissolution efficiencies of Au, Cu, and Fe from WPCBs were investigated at different temperatures (40–80 °C) and concentrations (2–32%) in batch leaching tests. The redox potential of aqua regia solution was found to depend on concentration and temperature. It is suggested that Au dissolution in aqua regia requires dissolved Cu2+ ions originating from the WPCB material to work. Au extraction (>50%) was shown to require a redox potential >700 mV with [Cu2+] > 2500 ppm, as a potential >850 mV alone was insufficient without cupric ions. Significant amounts of Au and Cu could be dissolved with only minor Fe dissolution at ≥8% aqua regia at 80 °C. Results suggest that leaching of uncrushed WPCBs in 8% aqua regia (T = 80 °C) can provide the opportunity for partial Au recovery prior to further processing.

Highlights

  • The amount of waste electric and electronic equipment (WEEE) has been constantly growing, which influences the development of recycling processes

  • The redox potential characteristics of aqua regia solutions were determined in this study and were found to increase as a function of both increased concentration and temperature

  • Results showed that redox potential ranged from 540 to 900 mV vs. Ag/AgCl in the studied range

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Summary

Introduction

The amount of waste electric and electronic equipment (WEEE) has been constantly growing, which influences the development of recycling processes. WEEE can be divided in many categories, which are defined in the EU/2012/19 [1] directive. The most valuable sub-group within WEEE are waste printed circuit boards (WPCBs) that contain significant amounts of valuable metals such as Au, Ag, and PGMs, increasing the interest towards the recycling of WPCBs and the recovery of valuable metals. The difficulty of recycling WEEE lies in the heterogeneity of the materials. Aqua regia is the traditional medium for dissolving gold and platinum group metals in acid digestion, which is a common method of analyzing valuable metals [2]. Precious metals, such as gold, Recycling 2017, 2, 14; doi:10.3390/recycling2030014 www.mdpi.com/journal/recycling

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