Research On Methods And Mechanisms Of Hydrometallurgical Step-by-Step Recovery Of Metals From Waste Printed Circuit Boards

Waste printed circuit boards（WPCBs）contain a wide variety of metals which are rich in contents.And some of the metals are of much higher contents than those in corresponding ores.Therefore,WPCBs are important secondary resources for metal materials.The recovery of metals from WPCBs is very important for resource recycling as well as environmental protection.Hydrometallurgy is a widely used technique for recovery of metals from WPCBs,since it has advantages of high recovery efficiency,low energy consumption,simple operation,and short experimental period.At present,most of the researches on metal recovery from WPCBs focus on metals with higher economic value such as copper（Cu）and gold（Au）,etc.,and researches on recovery of base metals tin（Sn）and lead（Pb）are limited.However,the recoveries of both Sn and Pb are also crucial for environmental protection and economic benefits.At the same time,the influence of various parameters and their interactions on the leaching of base metals cannot be ignored.The influences and the leaching mechanisms are still unclear.Meanwhile,there still exists some problems in the present Cu and Au recycling methods.In the process of Cu recovery by direct-current（DC）electrodeposition,there are concentration polarization and hydrogen evolution reaction at the cathode.The traditional gold leaching system copper-ammonia-thiosulfate system has problems such as large consumption of reagents,instability,and environmental pollution.Therefore,a complete and efficient hydrometallurgical recovery process is urgently needed to recover metals from WPCBs.To this end,in view of the problems existing in the metal recovery processes,this research developed different leaching systems and recovering methods to realize the sequential separation and recoveries of Pb,Sn,Cu and Au from WPCBs according to the differences in the properties of each metal.And a high-efficiency process of hydrometallurgical step-by-step separation and recovery of metals from WPCBs was finally established.The main research contents and results are as follows:（1）Research on methods and mechanisms of leaching Pb,Sn and Cu from WPCBs.In the research,the powder obtained by pretreatment of WPCBs was used as the raw material for metal recovery.HCl solution was used to leach Sn and Pb,and H₂SO₄-H₂O₂ solution was used to leach Cu.The influences of processing parameters and their interactions on leaching efficiencies of metals were studied used response surface method,and the processing parameters were optimized accordingly.In the leaching process using HCl solution,the interaction terms of temperature and rotational speed,temperature and solid-liquid ratio,temperature and HCl concentration have significant effects on leaching efficiency.After parameter optimization,the highest leaching rates of Sn and Pb were 97.56%and 98.26%,respectively.In the leaching process using H₂SO₄-H₂O₂ solution,the interaction terms of temperature and H₂O₂ concentration,temperature,and solid-liquid ratio,H₂SO₄ concentration and solid-liquid ratio also have significant effects on leaching efficiency.Under the optimal parameters,the leaching rate of Cu was 99.97%.Kinetic studies showed that the Sn and Pb leaching processes are controlled by mixed-controlled reaction and diffusion-controlled reaction,respectively.And the Cu leaching process had a high initial leaching rate,which conforms to the Avrami model,and the leaching was demonstrated to be controlled by the diffusion process.Meanwhile,based on the leaching experimental data,the kinetic equations of Sn and Pb and the kinetic equations corresponding to each experimental parameter of Cu were obtained,which reflects the effect of each experimental parameter on the leaching rate.（2）Study on principles and methods of step-by-step recovery of Sn,Pb and Cu from leaching solutions.In the leaching solution,Pb existed in the form of complex with chloride ion.A simple cooling precipitation method was developed to precipitate PbCl₂ from the solution,and 100%of Pb was recovered in the end.According to the fact that the solubility product constant of Sn（OH）₂ is much smaller than those of other metal hydroxides,NaOH solution was used to adjust pH of the leaching solution,and 99.70%of Sn was precipitated and recovered.A pulse electrodeposition method was proposed to recover Cu from the leaching solution.The results showed that copper ion in the cathode region can be replenished in time during the off-time of the pulse current,and hence the concentration polarization can be alleviated.Moreover,the pulse electrodeposition has a large overpotential,which can effectively suppress the hydrogen evolution reaction.The pulse electrodeposition effectively improved the current efficiency to 95.1%,and realized efficient recovery of Cu.（3）Development of the novel Co（Ⅱ）-glycine-thiosulfate system for Au leaching,and recovery of Au from the leaching solution by extraction.The newly developed Co（Ⅱ）-glycine-thiosulfate system has the advantages of high leaching efficiency,low reagent consumption,and environmental friendship.Study on the Au leaching process showed that glycine formed complexes with Co（Ⅱ）,in which Co（Ⅱ）acted as a transporter of oxygen and formed a Co（ⅡI）/Co（Ⅱ）electric pair in the solution to catalyze the leaching of Au.Electrochemical tests and first-principles calculations showed that Co（Ⅱ）-glycine complexes have better catalytic ability than that of Cu（Ⅱ）-ammonia complexes of traditional thiosulfate leaching gold system.And the redox potential of Co（ⅡI）/Co（Ⅱ）pair is lower than that of Cu（Ⅱ）/Cu（I）pair.Therefore,the new Co（Ⅱ）-glycine-thiosulfate system can improve the Au leaching rate and reduce the consumption of thiosulfate.After removing impurities from the base metal leaching residue by high temperature oxidative roasting and H₂SO₄ leaching,the leaching rate of Au was increased to 97.00%,and the consumption of thiosulfate was further decreased to 5.6 kg/t.The Au in the leaching solution was extracted by N1923,and the results showed that Au was extracted in the form of[Au（S₂O₃）₂]^3-through anion exchange mechanism,and the extraction rate was 99.67%.Therefore,efficient extraction of Au was realized.