As a pretreatment process to replace conventional shredding and grinding, microwave pyrolysis of waste PCBs was carried out at a microwave power level of 350 W for a processing time period of 60 min based on the experimental results of TGA and microwave heating. The metals in waste PCBs were leached out by a two-stage leaching process. In the first stage, the leaching efficiency of copper was up to approximately 95% when using a lixiviant composed of sulfuric acid and hydrogen peroxide, so oxidative leaching should be a feasible process for copper recovery from waste PCBs. In the second stage, the leaching efficiencies of gold were approximately 59, 95 and 95%, when using thiourea, thiosulfate and aqua regia, respectively. The high leaching efficiency of gold using thiosulfate should be attributable to the high selectivity offered by thiosulfate leaching. Considering the possible problems caused by aqua regia, the leachate produced by thiosulfate leaching of pyrolyzed PCBs was selected to be applied to the following solvent extraction and oxidative precipitation processes.

The gold ions contained in the leachate previously produced by the leaching processes were not satisfactorily extracted by using D2EHPA, TBP, DBC and TOA organic solvents. The highest extraction efficiency of gold was only approximately 25% by using D2EHPA. Therefore, solvent extraction would not be suitable for gold recovery, and the leachate was decided to bypass solvent extraction and directly apply to the oxidative precipitation process. The precipitation efficiencies of gold were as high as approximately 95 and 99% when using the oxidants of hydrogen peroxide and perchloric acid, respectively. However, it was only approximately 19% when hypochlorous acid was used, possibly due to its weak oxidizing power that was not able to sufficiently decompose the gold thiosulfate complex. The final recovery rates were approximately 90 and 93% when using the oxidants of hydrogen peroxide and perchloric acid, respectively. Therefore, high recovery rate of gold can be obtained by using sulfuric acid leaching, thiosulfate leaching and oxidative precipitation processes. 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. To achieve high recovery and purity of gold, it would be necessary to repeat the acid leaching process to let the copper concentration in the finally obtained leachate and also the copper content in the precipitate both be as low as possible.

Based on the correlation between microwave power level and heating rate, the maximum processing capacity of microwave pyrolysis of waste PCBs would be as high as 1.23 kg. For the processing of 1 t of waste PCBs, the costs would be approximately USD 34 for the energy consumption of microwave pyrolysis and approximately USD 638 for the consumption of chemicals used for hydrometallurgical processes. The gold recovered from 1 t of waste PCBs can be sold for approximately USD 9600, and thus the ROI can be as high as approximately 1400%. Therefore, high economic benefit can be offered by the recycling of waste PCBs using microwave pyrolysis and hydrometallurgical methods. Although this study focused on the gold recovery from waste PCBs, the following treatments for the residues and wastewater generated during microwave pyrolysis and hydrometallurgical processes would be of great importance. Ideally, all of the materials contained in the waste PCBs can be recovered as resources. However, due to the concerns of processing difficulty, health risk and economic efficiency, proper treatments for the residues and wastewater should be necessary and inevitable.