Study On Valuable Metals Recycling From Waste Electrical Contacts By The Combined Process Of Dilute Acid/Oxidant Leaching And Evaporation Crystallization

Electrical contacts are the core contact elements of electrical switches and instrumentation.In recent years,with the production scale of the electronic device manufacturing industry gradually developing,the number of waste electrical contacts has also increased.There was a large amount of secondary metal resources in waste electrical contacts?WECs?.If they could be properly disposed,it would not only benefit the environment,but also bring great economic benefits.A research hotspot for recovery of metals from Waste Electrical and Electronic Equipment?WEEE?focused on how to make it high efficiency,low risk and precise to recover secondary metal resources from WEEE.However,there was few researches about recovery metals in WECs.So in this study,Ag/Cu WECs were investigated by the combined process of dilute acid/oxidant leaching and evaporation crystallization to recycle valuable metals of Cu and Ag.This study has exploreded the effect of recovering silver under different pretreatment and leaching conditions,and probed the feasibility of evaporating crystallization to recover copper from the leachate,and analyzed its mechanism by different characterization.The main conclusions are as follows.The effects of different pretreatment conditions on the recovery of Ag in WECs were verified by mechanical crushing and roasting.The results of polarizing microscope,SEM,EDS and XRD showed that the dissociation degree of Cu and Ag reached 100%when the particle size was<0.15 mm.Roasting changed the phase composition of Cu.Different roasting conditions had an important influence on the structure of Cu.The oxidation products of Cu were mainly Cu₂O and CuO below 500? and SnO₂ was formed above 500? as well.However,Ag remained at zero valence throughout the roasting process.The results of ICP-OES,AAS and XRD showed that increasing roasting temperature or prolonging roasting time was beneficial to the conversion of Cu to copper oxide,which could reduce the reaction activation energy with dilute sulfuric acid and be beneficial to leaching.Increasing leaching time and acid concentration could help to improve the contact of Cu with H⁺,making the reaction to proceed more fully.The optimal experimental conditions for recovering Ag were confirmed that 96.94%of Cu was leached and 96.95 wt.%of Ag was recovered by roasting at500? for 4 h,leaching for 4 h with 1 mol/L H₂SO₄.Therefore,it was indispensable for recovring Ag from WECs to crush and roast.H₂O₂ was used as an oxidant to promote the leaching of Cu from WECs by dilute H₂SO₄,and the effects of recovered products were studied by controlling different conditions.The results of XRD,SEM and EDS showed that when the roasting temperature was lower than or equal to 450?,it was benefit for Cu to transform copper oxide by increasing roasting temperature or prolonging roasting time and without generating SnO₂.The results of ICP-OES and AAS showed that H₂O₂ could improve the oxidation of the system under acidic conditions and increase the reaction potential so that the dissolution of Cu into Cu²⁺and the recovery grade of Ag were promoted and improved.As the leaching time and the concentration of sulfuric acid increasing,the extraction of Cu gradually increased as well.Raising the reaction temperature would increase the molecular thermal motion and diffusion rate so that the solid-liquid phase could fully have contact to accelerate the reaction rate.The optimal experimental conditions for recovering Ag were confirmed that 99.67%of copper was leached and 99.99%Ag recovery rate with 99.53 wt.%purity was obtained by roasting at400? for 3 h,leaching for 4 h with 1 mol/L H₂SO₄ and 10 mL of H₂O₂ as an oxidant at a reaction temperature of 80?.Therefore,it was more positive and precise to recover Ag from WECs by Suitable roasting conditions and oxidant.Cu was recovered by evaporation crystallization from the leachate.The XRD showed that the recovered product was CuSO₄·5H₂O crystal.The results showed that evaporation temperature was higher,the solution was saturated faster.After evaporating 82.5 g of water per 100 g,crystals began to appear.Microscope showed that cooling and stirring rate were the main factors affecting the products size.Faster cooling rate or stirring speed would make crystals finer and more sporadic.But slower cooling rate or stirring speed would make crystals stick together.The optimal recovered condition was as follows:after the solution was evaporated at 90? for 2.5 h,and then temperature was cool down to room temperature for 2h under 200 r/min magnetic stirring.The CuSO₄·5H₂O with uniform particle size and high purity of 99.47 wt.%was directly obtained.The recovered crystal purity reaches GB437-2009?98.0 wt.%?,which can be directly used in agricultural production.