Study On Extraction Of Rare Earth Elements From Waste Phosphor By Sodium Carbonate Roasting And Molten Salts Chlorination

Rare earth is an important strategic resource,which is widely used in the field of new materials,traditional petroleum and chemical industry.As a non renewable resource,the rare earth ore is drying up.The content of rare earth oxides?REO?mainly including Y,Eu,Ce,Tb,in waste phosphor is more than 20%,which is much higher than that in the original ore.For this matter,waste phosphor is considered as one kind of secondary rare earth resource with high recovery potential.The utilization of waste phosphor can not only effectively slow down the consumption of rare earth ore,but also avoid the pollution to the environment.At present,the wet process is usually used to recycle rare earth elements from waste phosphor.This recycling process is complicated.A large amount of chemical reagents and clean water are consumed andconsequently a great deal of waste liquidis generated in the above process,and resulting in high processing cost and serious environmental pollution as well as some other problems.In this paper,a new recycling method is proposed by chlorination roasting in molten AlCl₃-KCl.The aluminate green phosphors and blue phosphors are very stable and thus difficult to be leached by acid and alkali,which is the main reason for the low recovery of rare earth elements.In order to improve the chlorination rate of Ce,Tb and Eu in green and blue phosphors,the stable aluminate structure is destroyed by sodium carbonate roasting before chlorinating in molten AlCl₃-KCl in present experiments,focusing on the understanding of reaction mechanisms and the establishment of reaction model.The main research contents and results are as follows:Thermodynamic analysis shows that Y2O3,Eu2O3,CeO₂,Tb2O3 can be chlorinated by AlCl₃ to form the corresponding rare earth chloride,while Si,P and other impurity elements can not be chlorinated by AlCl₃.AlCl₃ is an ideal chlorinating agent of the waste phosphor to separate the rare earth elements and impurities.The effects of reaction conditions?including temperature,time andmaterial ratio?on chlorination rate of Y,Eu,Ce,Tb,have been investigated.The results show that the chlorination rate of Eu in the red phosphors is close to 90%,while chlorination rate of Y can reach more than 99%;nevertheless the chlorination rate of rare earth elements in green and blue phosphors is only 20%-30%.The effects of roasting conditions on the decomposition process of green and blue phosphors have been investigated.The results show that green phosphors could completely dissociate and release cerium terbium oxideunder the calcination conditions with the green/sodium carbonate mass ratio of 1:1,reaction temperature of 1000? and reaction time of 2h,while under the calcination conditions of blue phosphors/sodium carbonate mass ratio 1:4,reaction temperature 1000?,reaction time 6h roasting,blue phosphors can be completely dissociated.Combined with XRD and SEM/EDS analysis,the decomposition mechanisms of green and blue phosphors roasting in sodium carbonate have been investigated and subsequently the reaction model has been established.The dissociation reactions of green and blue phosphors roasting in sodium carbonate are concluded as follows:CeMgAl11O19:Tb³⁺ + Na2CO3 ? Ce0.6Tb0.4O₂-x + MgO + NaAlO₂ + CO₂BaMgAl10O17:Eu²⁺ + Na2CO3 ? BaCO3 + MgO + Eu2O3 + NaAlO₂Referring to the chlorination roasting of pink phosphors,the waste phosphor was directly roasted in molten AlCl₃,rare earth element Y,Eu,has a high chlorination rate of 99%,and the conversion rate of Ce and Tb is low.After roasting with sodium carbonate,the product were chlorinated by AlCl₃-KCl,and the conversion rate of rare earth elements Ce and Tb can be improved to 75.73% and 80.62%,respectively.In conclusion,the rare earth elements can be effectively extracted by the method of sodium carbonate roasting-molten salt chlorination.The results of this study provide a theoretical basis for understanding the dissociation mechanisms of green and blue phosphors sodium carbonate roasting process,which is helpful to promote the establishof an effective and environment-friendly technology for waste phosphor.