Gold Fractal Growth during Its Recycling from Waste Printed Circuit Boards by Slurry Electrolysis

Waste printed circuit boards, especially from mobile phones, possess an extremely higher resource value. The key to current electronic waste disposal is to efficiently recycle valuable and precious metals under the condition of ensuring its environmental friendliness. Slurry electrolysis has been identified to be an effective method to recover valuable and precious metals from waste printed circuit boards. Meanwhile, copper fractal growth causes higher energy consumption during this process and has been extensively investigated. However, the mechanism of precious metal fractal growth, especially gold, is still unclear. In this work, the factors that impact two-dimensional gold dendritic patterns are discussed in detail. These results indicate that increasing the applied voltage and chloroauric acid and hydrochloric acid concentrations could increase the probability of particle movement, which would enter the inner growing point due to the reduction of the tip shielding effect, further resulting in an increase in the fractal dimension. Additives, trisodium citrate and poly(vinylpyrrolidone) (PVP), can inhibit fractal growth to a certain extent by varying surface chemical properties. The minimum fractal dimension is 1.742 under the conditions of 0.12 mol/L chloroauric acid, 0.25 mol/L hydrochloric acid, 80 mg/L PVP, and 25 V. The diffusion-limited aggregation simulation result is consistent with the gold fractal growth experiment under the condition that the electrocrystallization time is significantly less than that of diffusion. The Materials Studio simulation result indicates that additives, PVP in particular, which provide particles with unique hydrogen bonding and steric hindrance, can promote the growth of gold atoms on crystal planes, especially the (111) crystal planes. Based on the results of fractal growth, the addition of 160 mg/L trisodium citrate and PVP to a slurry electrolysis recovery system of WPCB-MPs significantly increases the gold recovery rate and the current efficiency from 31.5, 0.2% to 77.9, 1.7% and 66.0, 2.2%, respectively. These results are conducive to further enriching the theory and enhancing the recovery of precious metals from e-waste by slurry electrolysis.