| Waste tantalum capacitors and ceramic capacitors,as the important component of e-waste,are characterized by the large number,multi-component complex,high content of rare metal and difficult recycling.At present,the recycling processes of waste capacitors are usually low efficiency and easily cause secondary pollution.Therefore,it is urgent to develop an efficient,environmentally friendly and industrializable recycling processes for waste capacitors.Herein,this paper proposed an idea of “organic thermal decomposition,metal enrichment-chlorination separation and photocatalyst preparation” based on the composition of tantalum capacitors(containing organic,tantalum,nickel-iron electrode,Si O2)and ceramic capacitors(Ba Ti O3,silver,palladium,tin etc.).Through the experiment and theoretical analysis of organic decomposition,metal enrichment-chlorination separation,we developed an integrated technology containing “argon gas pyrolysis-physical separationchlorination-vacuum nitriding” for recycling waste tantalum capacitors and “chlorination-leaching-physical separation” for recycling waste ceramic capacitors.Furthermore,based on the recycling process,we proposed a new method of preparing photocatalytic materials from waste capacitors,which provided a novel idea for e-waste high value-added utilization.Recycling waste tantalum capacitors Firstly,Ar pyrolysis and supercritical water were used to make a comparison for decomposing the organics of resin package,consequently,Ar pyrolysis was chosen.When the pyrolysis temperature was controlled at 550 oC,holding time was 30 min,argon gas flow rate was 100 ml/min,the organics were efficiently decomposed into oil,gas and carbon residue.The pyrolysis oil mainly contained phenol homologs and benzene homologs,and pyrolysis gas was mainly some hydrocarbons.Pyrolysis of organic followed the free radical theory,that is,bond breaking-free radical formation-recombination.Then,Ni-Fe electrodes were separated,and tantalum was enriched by crushing,screening and high voltage electrostatic separation(CES).The tantalum extraction rate was 97.02%,and the tantalum content in the sample was enriched from 36.49 to 71.35%.Subsequently,chlorination was applied to extract tantalum.The thermodynamic and kinetic results suggested that only tantalum could react with Fe Cl2,and the generated Ta Cl5 would be volatilized in gaseous form,separated and condensed in the collection area during the chlorination process.The chlorination process of tantalum was controlled by the mixture of chemical reaction and ash layer diffusion.When the mass ratio of Fe Cl2/tantalum enriched particles was 1.5,chlorination temperature was 500 oC,and holding time was 120 min,the optimal tantalum extraction rate was 93.67%.Finally,Ta Ox Ny and Ta3N5 were prepared through vacuum nitriding using the recycled Ta Cl5.The as-prepared photocatalyst exhibited enhanced photocatalytic H2 evolution.According to the nitrogen potential theory,vacuum condition could dramatically increase the nitrogen potential,which promoted the collision of active nitrogen with the sample surface and achieved more effective nitrogen doping.Recycling waste ceramic capacitors Firstly,crush and magnetic separation were used to recover nickel.Then,chlorination metallurgy combined with leaching was applied to separate Ba,Ag and Sn.The thermodynamic and kinetic results suggested that Ba Ti O3,Ag and Sn can be chlorinated with the HCl produced by NH4 Cl decomposition,and Ba Cl2,Ti O2,Ag Cl and Sn Cl4 could be generated.During the chlorination process,the generated Sn Cl4 would be evaporated in gaseous form and then be separated and condensed in the collection area,while Ba Cl2 and Ag Cl could be leached and separated by water and Na2S2O3 solution in turn.The chlorination of Ba Ti O3,Ag and Sn were controlled by the chemical reaction.When the chlorination temperature was 400 oC,holding time was 30 min,and the mass ratio of NH4Cl/waste powder was 4,the extraction rate of Ag,Ba and Sn could reach 99.21%,98.76% and 99.83%,respectively.The concentration of Pd could be achieved by high voltage electrostatic separation.Finally,the separated Ti O2 was recycled as a photocatalyst.The composition analysis showed that niobium-lead codoped and palladium loaded Ti O2 was formed.The particle size of Ti O2 and Pd were 20-80 and 5 nm,respectively.The Pd uniformly loaded on the Ti O2 surface.Compared with the pure Ti O2,the optical absorption capacity,charge separation efficiency and photocatalytic performance were significantly enhanced.The density functional theory(DFT)showed that niobium and lead were doped in the band gap and formed impurity energy levels,which reduced the photon energy required for electron transition and made the spectral absorption range redshift.Palladium loading mainly introduced energy level into the band gap,which could accept the excited electrons in the conduction band.Consequently,the photogenerated electron and hole were effectively separated.Preparation of self-doped photocatalyst using waste ceramic capacitors Nb-Pb codoped and Ag-Pd-Sn-Ni loaded Ba Ti O3 nano-photocatalyst was prepared by one-step ball milling method.The particle size of Ba Ti O3 and loading were 100-200 and 5 nm,respectively.The Ag-Pd-Sn-Ni uniformly loaded on the Ba Ti O3 surface.When the mass ratio of NH4Cl/waste powder was 0.5-2,Nb-Pb codoped and Pd loaded Ti O2/Ba Ti O3 nanometer heterojunctions were prepared by the chlorination-leaching process.The particle size of heterojunction and loading were 20-80 and 5 nm,respectively.Compared with the pure samples,the optical absorption capacity,charge separation efficiency and photocatalytic performance of the recycled samples were observably enhanced.The prepared heterojunctions displayed the optimal photocatalytic H2 production rate(576.8 ?molg-1h-1)and Rh B degradation rate(0.29911 min-1),which were 11.3 and 19.1 times higher than those of pure Ti O2,and 5.96 and 8.91 times higher than those of pure Ti O2/Ba Ti O3.The DFT results showed that the impurity levels of Nb-Pb were doped in the band gap and formed impurity energy levels in the recycled Ba Ti O3,while mainly located above the valence band in Ti O2/Ba Ti O3.The Nb-Pb codoping reduced the photon energy required for electron transition and made the spectrum absorption range redshift.Palladium,silver and tin loads mainly introduced energy level into the band gap,which could accept the excited electrons in the conduction band.As a result,the photogenerated electron and hole were effectively separated.In conclusion,through theoretical analysis and parameter optimization,this study established the recycling process of waste tantalum capacitor and ceramic capacitor,which can provide the theoretical basis and technical reference for the recycling of other waste electronic components.Furthermore,this study proposed a novel method of preparing self-doped photocatalysts from waste capacitors.Moreover,the prepared photocatalysts have excellent photocatalytic performance,which realizes the high value-added utilization of e-waste. |
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