Synthesis Of ZIT Composite Material And Immobilization Of Lanthanides(Nd,Ce)

ZIT(Zinc Titanate)composite material is composed of four components:Ca HPO₄,Zn₂Ti O₄,B₂O_3,and Si O₂.Numerous studies have found that ZIT composite materials have excellent solidification properties for high-level radioactive wastes,just like a high tolerance to radioactive lanthanide waste,lower sintering temperature,and higher stability.In this paper,Zn₂Ti O₄,the precursor of the ZIT composite material,was prepared by the molten salt method,and the properties of the ZIT composite material to immobilize the lanthanide elements including Nd and Ce were studied.To prepare high-purity zinc titanate Zn₂Ti O₄,the effects of the calcination process,calcination temperature,the molar ratio of Zn O to Ti O₂ in raw materials,and calcination time on the types of Zn₂Ti O₄ were studied.Through comparative experiments of different processes,it is confirmed that the crystal grains of Zn₂Ti O₄ synthesized by the molten salt method had the best growth,complete crystal form,and uniform size.Through comparative experiments with different temperature gradients set at 550?to 1000?,the synthesis process of Zn₂Ti O₄ was explored and the best synthesis temperature was determined to be 1000?.By changing the molar ratio of Zn O to Ti O₂ in the raw materials,the study determined that the optimal ratio of Zn₂Ti O₄ synthesized was 1.5:1.In this paper,neodymium oxide(Nd₂O₃)and cerium oxide(Ce O₂)were both used to simulate radioactive+3 and+4 valence lanthanides,and the monazite structure(Ln PO₄)as the main curing structure was synthesized by a high temperature calcination in ZIT ceramic waste form.Some test methods including XRD,SEM-EDS,and ICP were used to characterize and analyze the phase structure,micromorphology,and stability of ZIT ceramic waste form.The effects of different heat treatment processes on the ZIT ceramic waste form were studied,and it was determined that the pre-sintering and thermal treatment can improve the performance of the ZIT ceramic waste form.The effects of different mass fractions of lanthanide oxides on the properties of the ZIT ceramic waste form were studied,and it was finally determined that the maximum encapsulation capacities of the ZIT composite material for Nd₂O₃ and Ce O₂ were 45 wt.%and20 wt.%,respectively.Through the PCT-B static immersion method and ICP test method,the chemical stabilities of the ZIT ceramic waste form with Nd₂O₃ or Ce O₂ were explored.The study found that the leaching rate of the ZIT ceramic waste form was higher at the initial stage of immersion and then gradually decreased.After soaked for 7 days,it gradually became stable.Finally,the 28-day leaching rate stabilized at the order of 10^-6 g·m^-2·d^-1.