Nonlinear Behavior During Resource Recovery From Waste Printed Circuit Boards Of Mobile Phones By Slurry Electrolysis

Slurry electrolysis is an effective method to recover copper from waste printed circuit boards,and fractal growth has been found in the cathode.Slurry electrolysis system has the characteristics of fluidity,openness,and electrode potential far from the equilibrium potential,which contains rich nonlinear mechanisms.The nonlinear behavior is an important factor affecting the unstable production and decreasing current efficiency of the electrolysis process.Therefore,in-depth study on the nonlinear mechanism in the process of slurry electrolysis recycling is a prerequisite for realizing high-efficiency and low-consumption treatment.This thesis focuses on the non-equilibrium nonlinear characteristics such as electrochemical oscillation and fractal growth during the process of recycling from waste printed circuit boards from mobile phones（WPCB-MPs）by slurry electrolysis,and reveals the cathodic fractal growth of Au,and electrochemical oscillation of Cu and Au in the slurry electrolysis system.These results are expected to efficiently realize the step-by-step recovery of valuable and precious metals.The main research conclusions are as follows:（1）Aiming at the fractal growth behavior of gold during the process of recycling from WPCB-MPs by slurry electrolysis,two-dimensional growth experiments of gold in point electrode and wire electrode electrodeposition process were used to study the effect of voltage,chloroauric acid concentration and hydrochloric acid concentration on the gold patterns.Meanwhile,the effects of trisodium citrate and PVP concentration on patterns of gold dendrites at the point and wire electrode were investigated.The experimental results of point and line electrodes show that"tip effect"of the electrode and the"shielding effect"of the external dendrites induce the fractal growth behavior of gold.The increase of voltage,chloroauric acid and hydrochloric acid concentrations weaken the"shielding effect"of the outer dendrites,thereby increasing the deposition probability of particles at the inner growth points.Correspondingly,the morphologies of gold deposition vary from loose and open branches to coarse,complex and dense structures,and their fractal dimensions also increase.These results are consistent with the simulation results of the diffusion-limited agglomeration（DLA）model that the fractal degree increases with the increase of particle moving speed,particle number and binding probability.Appropriate addition of PVP and trisodium citrate affect the nucleation and growth process of electro-crystallization,and exhibit a certain inhibitory effect on the fractal growth of gold.The characterization results of gold deposits show that both additives can effectively vary the surface chemical energy of gold particles and promote the growth of each crystal plane,especially the effect of PVP on the（111）crystal plane.These results are consistent with the simulation results of Materials Studio.The adsorption energy of gold atoms on each crystal plane after adsorption or complexation with sodium citrate and PVP is lower than that of single-atom gold.The adsorption energy of complexation of PVP and gold single crystal on the（111）plane is the smallest at-3.3 e V.Under certain conditions,the minimum fractal dimensions obtained by the fractal growth experiments of point and wire electrode gold electrodeposition are 1.742 and 1.662,respectively.（2）For electrochemical oscillation behavior of copper during its recycling from WPCB-MPs by slurry electrolysis,this research applies Cu Cl₂ as electrolyte to study the electrochemical oscillation behavior of electrodeposition process under constant current condition based on the slurry electrolysis system of high concentration copper salt and chloride salt.These results show that a cuprous chloride passivation film is formed on the cathode during copper electrodeposition.The alternating action of passivation and dissolution on the electrode induces a decaying bottom-up electrochemical oscillation phenomenon.The amplitude and frequency of electrochemical oscillations during copper electrodeposition are affected by factors such as copper chloride concentration,current intensity,electrode spacing,temperature,initial p H,sodium chloride concentration,stirring speed,and anode material.The current intensity,initial p H,sodium chloride concentration and stirring speed are particularly significant,all of which had a certain inhibitory effect on the electrochemical oscillation.Under the conditions of copper chloride 0.2 mol/L,0.03 A,electrode spacing 2 cm,initial p H4,55°C,without Na Cl and stirring,the maximum amplitude and frequency is 0.43 V and 0.42Hz,respectively.One-dimensional time-voltage sequence phase space reconstruction results show that the investigated factors have a significant regulating effect on the voltage oscillation behavior of copper electrodeposition process.The system evolves between the near-linear equilibrium region,the periodic oscillation region and the chaotic oscillation region.Voltage oscillation power dissipation analysis results indicate that a typical voltage oscillation（Cu Cl₂concentration 0.2 mol/L,0.03 A,electrode spacing 2 cm,25℃,initial p H 4,without sodium chloride and stirring）can cause an additional power consumption increase of 3.4%.Appropriately increasing the concentration of cupric chloride and temperature can effectively reduce the additional power consumption caused by oscillation,and the average power of the system decreases.Although increasing the current intensity and electrode spacing could reduce the additional power consumption to a certain extent,the average power of the system increases significantly.Furthermore,the additional power consumption and the average power of the system can be significantly reduced by reducing initial p H value,increasing stirring speed and Na Cl concentration.Therefore,adding copper salts,chloride salts,and introducing stirring are potentially effective methods to suppress voltage oscillations and achieve process energy conservation during the process of recovering copper from WPCB-MPs by slurry electrolysis.（3）For the electrochemical oscillation behavior of Au during its recycling from WPCB-MPs by slurry electrolysis,the electrochemical oscillation behavior of gold anode dissolution process under constant current condition was studied with hydrochloric acid system.These results show that the periodic formation and dissolution of the Au（OH）₃ passive film on the anode induces electrochemical oscillations.The amplitude and frequency of electrochemical oscillation are affected by factors such as hydrochloric acid concentration,current intensity,electrode spacing,sodium chloride concentration and stirring speed.Among them,the concentration of sodium chloride and the stirring speed have a certain inhibitory effect on the electrochemical oscillation.Under the conditions of hydrochloric acid concentration of 8mol/L,0.03 A,electrode spacing of 1.5 cm,without sodium chloride and stirring,the maximum amplitude and frequency is 1.17 V and 0.56 Hz,respectively.The phase space reconstruction results show that the investigated factors have a significant regulatory effect on the voltage oscillation behavior of the gold anode dissolution process.The system evolves between the near-linear equilibrium region and the periodic oscillation region.Different hydrochloric acid concentration,current intensity,electrode spacing,stirring speed and Na Cl concentration can induce two oscillation modes:top-down and bottom-up.The top-down and bottom-up oscillations induced by current intensity,hydrochloric acid concentration and electrode spacing can reduce the relative power consumption by 11.8,17.2,11.8%and increase by 14.3,21.2,and 30.1%,respectively.When the current intensity,hydrochloric acid concentration and electrode spacing reach a critical value that can vary oscillation form,where may have a region of lower power consumption.The concentration of sodium chloride and stirring speed significantly suppress the oscillation,which make the system in a low potential for a long time,and the average power reduces from 0.34 to 0.2 W.These results provide theoretical support for the realization of energy saving during the process of recovering precious metals by slurry electrolysis by varying the external control conditions to switch the oscillation mode.（4）For the advanced treatment of WPCB-MPs,the copper and gold in WPCB-MPs were recovered step by step by slurry electrolysis,the factors hydrochloric acid concentration,sodium chloride,solid-liquid ratio affecting on gold recovery rate were investigated.These results show that the slurry electrolysis technology can effectively recover gold from WPCB-MPs residue,and the gold recovery rate is greatly affected by hydrochloric acid concentration.Under the optimal conditions of 40 m A/cm²,hydrochloric acid concentration 4 mol/L,Na Cl concentration 60 g/L,solid-liquid ratio（g/L）1/20,30 wt.%hydrogen peroxide 10 m L,5 h,the gold recovery rate reached 86.5%.In addition,the study indicates that the high copper and low gold WPCB-MPs residue is not conducive to the recovery of gold.However,the addition of160 mg/L sodium citrate and PVP to the system significantly increases the Au recovery rate and current efficiency from 31.5,0.17%to 77.9,1.7%,and 66.0,2.2%,respectively.These results provide the possibility for the stepwise enhanced recovery of valuable metal Cu and precious metal Au from WPCB-MPs.