Challenges and opportunities in the recovery of gold from electronic waste

Rapid global technological development has led to the rising production of electronic waste that presents both challenges and opportunities in its recycling. In this review, we highlight the value of metal resources in the printed circuit boards (PCBs) commonly found in end-of-life electronics, the differences between primary (ore) mining applications and secondary (‘urban’) mining, and the variety of metallurgical separations, in particular those that have the potential to selectively and sustainably recover gold from waste PCBs.

Rapid global technological development has led to the rising production of electronic waste that presents both challenges and opportunities in its recycling.

The rapid global rise in technology, tied in with consumer pressures for upgrades in functionality and design, has generated advanced electrical and electronic equipment with short lifespans. A consequence of this is the production of electronic waste (e-waste) which, in 2018 amounted to 50 million tonnes,^1,2 with a projected annual growth of 3–5%, three times more than for other waste streams.³ Reports on recycling rates vary, with estimates of around 20–30%.^1,4 It is estimated that more than 70% of globally produced waste electronics and electrical equipment (WEEE) enter China, Africa and India for reprocessing, much of it illegally, and often using crude, hazardous and inefficient processes.^5,6 Dumping and incinerating large amounts of WEEE has severe impact on human life and the environment,⁷ as it leads to the release of toxic heavy elements such as lead, mercury, chromium, nickel, beryllium, arsenic and antimony into the air, soil and water cycles.⁸An end-of-life printed circuit board (PCB) may contain up to 60 different chemical elements,⁹ and have a metal content as high as 40% by weight,¹⁰ so should be viewed as a valuable secondary source of precious and base metals. The metal content of a PCB is typically ten to a hundred times higher than that of conventionally mined ores.¹¹ It is estimated that recycling one ton of mobile phones could produce on average 130 kg of copper, 3.5 kg of silver, 0.34 kg of gold and 0.14 kg of Pd.¹² On this basis, the global e-waste management market is projected to produce an annual revenue of USD 62.5 billion by the end of 2020.^2,13 With an estimated 97% of the world population owning a mobile phone,¹⁴ it can be viewed as a plentiful feedstock for a recycling process. As such, the treatment of e-waste not only helps minimise the environmental impact of our technology-driven society by reducing pollution and energy demands compared to conventional mining practice,¹⁵ it also presents economic drivers for wealth creation and circular economies.^16–21In this review, we outline some of the latest chemical approaches that have been reported for the recovery of gold from discarded mobile phones and other WEEE.^22,23 Gold is the most valuable component of e-waste, with estimates for its consumption to fuel our technology-driven society at 263.3 MT per year.^7,24 We provide an overview of metal concentrations that are present in waste PCBs from end-of-life mobile phones, analyse the different pre-treatment steps that can be used to separate the metallic and non-metallic components of PCBs, and highlight various metallurgical methods for the extraction of gold from waste PCBs. For this latter aspect, we focus on methods in the primary research literature for which an understanding of the chemical mode of action has been developed; as such, a detailed analysis of the patent literature is not in the scope of this review.