Cu extraction from waste copper clad laminate sorting residue in a two-stage bioleaching process: Process optimization and mechanism

Much waste copper clad laminate sorting residue is generated from the flotation process of recovering copper resources from waste printed circuit boards. The improper treatment and disposal of waste copper clad laminate sorting residue harms the environment and human health. According to the National Hazardous Waste List (2021 edition) of China, this waste belongs to HW13 (900-451-13) hazardous waste. The sorting residue contains approximately 1% copper, which is similar to the average copper grade of 0.8% in China. Therefore, this residue is an important copper renewable resource and has a high potential for copper recycling. To optimize the effective factors, including the Fe²⁺ concentration, initial solution pH value, and pulp density, and clarify the mechanism during the bioleaching process of waste copper clad laminate sorting residue, a Box–Behnken design of response surface methodology was first used, and a scheme consisting of 17 experiments was designed in the present study. Through the multiple regression fitting analysis of experimental results, a quadratic polynomial regression model was established. The regression model showed high reliability and simulation accuracy and was then used to optimize the bioleaching process. Under the optimal conditions (6.13 g·L⁻¹ Fe²⁺, initial leaching solution pH value of 1.65, and pulp density of 30%), 92.2% maximum Cu extraction was obtained. Then, a modified scale-up bioleaching experiment in a 100-L stirred tank was performed. The results indicated that the maximum copper recovery reached 98%, and less than 0.02% of copper was detected in the bioleaching residue after 6 h of bioleaching because of the improved bioleaching operating conditions in the 100-L stirred tank, including slowly adding the sorting residue, additional stirring (200 r·min⁻¹), aerating (20 L·h⁻¹), and controlling the bulk pH value (solution pH value <2.5 adjusted with 50% (v/v) H₂SO₄). Leaching kinetic data described by a modified shrinking core model indicated that interfacial transfer and diffusion across the solid film layer controlled the copper dissolution kinetics. In conclusion, copper in the sorting residue was dissolved primarily by Fe³⁺ oxidation and secondarily by H⁺ attack throughout the bioleaching process. Notably, the continuous regeneration of Fe³⁺ by an iron-oxidation microbial consortium led to more Fe³⁺ distributed across the solid film layer of residual iron/calcium compounds and accumulated on the reacted core, which not only reduced total iron consumption (particularly Fe³⁺) but also substantially improved copper extraction from waste copper clad laminate sorting residue. These findings should have important implications for the green recycling and reuse of waste printed circuit boards and other waste electronic appliances.