| With the continuous development of science and technology and the fast upgrading of electronic products,waste electrical and electronic equipment(WEEE)has become a typical type of municipal solid waste which has rapid growth rate and is difficult to handle.As the essential component of electronic products,printed circuit boards complex physical structure,diverse chemical composition and abundant metal resources especially precious metals.As a result,waste printed circuit boards(WPCBs)have attracted great attention.Unfortunately,there is no proper method to recycle WPCBs now.How to recycle the mixed metals in WPCBs environmentally friendly and comprehensively is an urgent problem.During solidification,liquid-liquid phase separation alloy may form two immiscible liquids,which can be used in metal purification and recycling.In this work,in view of reducing environment pollution and recycling valuable materials,green and efficient methods combining pyrolysis(including pyrolysis in furnace,molten salt pyrolysis and supercritical water pyrolysis)and low-temperature alkaline smelting with liquid-liquid phase separation are proposed to separate and recycle mixed metals in WPCBs.The main research work and results are shown as follows:The pyrolysis characteristics of WPCBs of mobile phones,pyrolysis products and the effects of parameters were studied experimentally.It is found that the decomposition of organics mainly occurs at 460-790 K.The pyrolysis of WPCBs is divided into three stages approximately.The first stage is the breaking of weak bonds and the elimination of active side groups,the second stage is the breaking of the main chain of epoxy resin,and the third stage is the secondary reaction of the previous decomposition products.The main components of the pyrolysis oils are phenol and its substituents,while the solid products are mainly glass fiber,coke and mixed metal.Molten salt pyrolysis can remove epoxy resin and glass fiber in WPCBs at the same time by eutectic NaOH-KOH mixture.The mass loss rate of WPCBs can reaches more than 40%.Most of the pyrolysis products are absorbed by the molten alkali.In the supercritical water pyrolysis process,the pyrolysis effect increases with the increase of temperature,reacting time,and water/feedstock ratio.The addition of iron-based amorphous alloy(Fe7sSi8B14)promotes the generation of hydroxyl radicals and plays as catalyst in the degradation of organics.Pyrolysis in fumaceis relatively simple and efficient,and is used to pretreat WPCBs in this study.Based on the composition of mixed metals obtained from pyrolyzed WPCBs,the liquid-liquid phase separation behavior of Fe-Cu-Pb ternary alloys was investigated.(Fe0.4Cu0.6)72Pb28 ternary alloy is characterized by double liquid-liquid phase separations.When the homogeneous melt is cooled,the first liquid-liquid phase separation(L?L(Fe)+L(Cu,Pb))occurs,producing immiscible Fe-rich and Cu,Pb-rich liquids.Then the second liquid-liquid phase separation(L'(Cu,Pb)?L(Cu)+L(Pb))occurs in the remaining(Cu,Pb)-rich liquid phase.Finally,the Fe-rich,Cu-rich and Pb-rich phase separated structure is formed.The immiscible gaps of Fe-Cu and Cu-Pb-based alloys and the distribution ratio of minor elements were calculated by sub-regular solution model.The results show that C can be used as a separating agent for Fe-Cu based alloys,while Al can be used as a separating agent and trapping agent for Cu-Pb based alloys.The minor elements are selectively distributed into the primary Fe,Cu,and Pb metal liquids.The elements Cr,Co,Ni,and Si are mainly enriched in the Fe-rich liquid phase;Zn and precious metals Au,Ag are trapped in the Cu-rich phase;the Pb-rich phase collects low-melting point metals Bi,In,Cd and Sn.Based on the liquid-liquid phase separation behavior and theoretical calculation,a method of self-assembly liquid-liquid hierarchical separation system is designed to separate the mixed metals in WPCBs.In the super-gravity field,three-layer separation and double two-layer separation are used to separate the mixed metal into Fe-rich,Cu-rich and Pb-rich substances.The recycling rates of Cr,Co,Si,Bi,and In exceed 90%;the recycling rates of Ni and Cd are beyond 80%;the recycling rates of the precious metals Au and Ag reach 95%and 92%,respectively.The separation of amphoteric metals from mixed metals by low-temperature alkaline smelting was studied.The separation rates of Al,Pb,Si,Sn and Zn are 99.5%,81.6%,97.8%,88.4%and 95.7%,respectively.Then the Fe-Cu self-assemble liquid-liquid separation system is used to recycling the mixed metals after reduction smelting of LTAS residue.The results show that the metals Ni(75.4%)and Cr(95.6%)are mainly concentrated in the Fe-rich substance,while almost all Au(98.1%)and Ag(99.8%)are enriched in Cu-rich substance. |
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