Abstract
Recycling of waste printed circuit boards (PCBs) has attracted increasing attention because of its high annually produced amount and high content of gold. In this study, gold recovery from waste PCBs was carried out by using the processes including microwave pyrolysis, acid leaching, solvent extraction and oxidative precipitation. The leaching efficiency of copper was approximately 95% when using a lixiviant composed of sulfuric acid and hydrogen peroxide, and the leaching efficiencies of gold were approximately 59, 95 and 95% by using thiourea, thiosulfate and aqua regia, respectively. The gold ions contained in the leachate previously produced by the leaching processes were not satisfactorily extracted by using organic solvents including di-(2-ethylhexyl)phosphoric acid, tributyl phosphate, dibutyl carbitol and trioctylamine, so the leachate was decided to bypass solvent extraction and directly apply to the oxidative precipitation process. By using the oxidants of hydrogen peroxide and perchloric acid, the precipitation efficiencies of gold were approximately 95 and 99%, and the final recovery rates were approximately 90 and 93%, respectively. The high recovery rates of gold can be attributable to the use of microwave pyrolysis that prevents the loss of gold caused by shredding and grinding processes. In addition, perchloric acid can provide higher selectivity for gold recovery than hydrogen peroxide. The maximum processing capacity of microwave pyrolysis of waste PCBs would be approximately 1.23 kg. The gold recovered from 1 t of waste PCBs can be sold for approximately USD 10,000, and thus the return on investment can be as high as approximately 1400%.
Highlights
Electronic waste (e-waste), which refers to electrical and electronic equipment and its assemblies that have been discarded as waste without the intent to reuse [1], is one of the most rapidly growing waste segments in the world [2]
3.1 Microwave pyrolysis To prevent the loss of precious metals and the formation of dangerous metal fines, dust containing brominated flame retardants and dioxins during the shredding or grinding pretreatment [2, 22, 30], the waste printed circuit boards (PCBs) were pretreated by microwave pyrolysis prior to hydrometallurgical metal recovery processes
Before proceeding to microwave pyrolysis experiments, the basic thermochemical property of waste PCBs was investigated by thermogravimetric analysis (TGA)
Summary
Electronic waste (e-waste), which refers to electrical and electronic equipment and its assemblies that have been discarded as waste without the intent to reuse [1], is one of the most rapidly growing waste segments in the world [2]. E-waste contains valuable materials and hazardous or toxic substances [4,5,6]. In most cases, recycling factories just focus on the recovery of valuable materials (in particular precious metals) while release hazardous or toxic substances into the surroundings, resulting in threats to the environment and human health [7]. The precious metals, such as gold, silver and palladium, are widely used as contact materials and plating layers due to high electric conductivity and chemical stability [8]. Compared with other kinds of e-waste, there are the highest contents of precious metals in waste printed circuit boards (PCBs) and
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