Hydrometallurgical metal recovery from waste printed circuit boards pretreated by microwave pyrolysis

Waste printed circuit boards (PCBs) were pretreated by either microwave or conventional pyrolysis prior to acid leaching processes. Microwave pyrolysis can provide higher weight loss of waste PCBs and better metal recycling than conventional pyrolysis. After microwave and conventional pyrolysis of waste PCBs, the overall copper recovery rates of two-stage acid leaching can be up to approximately 96 and 75%, respectively. The gold recovery rates of thiourea leaching from microwave- and conventional-pyrolyzed PCBs were approximately 80 and 69%, respectively. Therefore, size-reduction processes for the pretreatment of waste PCBs would not be necessary, as long as microwave pyrolysis is applied prior to hydrometallurgical processes. The gold recovery rates of this study should be credible, since the sum of recovery and non-recovery rates was close to 100%. The initial acid leaching of copper may be governed by the copper content in the PCBs as well as the concentration of oxidizing agent in the leaching solution. Besides, the reaction rate of thiourea leaching of microwave-pyrolyzed PCBs may be largely influenced by the thickness of product layer. Both acid and thiourea leaching experimental results were better fitted by the pseudo-second-order reaction model. However, the maximum recovery rate determined by using the pseudo-first-order reaction model should be more reasonable.