Study On The Synthesis Of ZIT Composite By Molten Salt And Immobilization Of Lanthanide Element La

In the recovery and reuse process of spent fuel,the waste salt after electrolytic reduction and electrorefining is rich in lanthanide oxide precipitation,and these waste oxides need to be effectively treated.The ZIT(Zinc Titanate)composite developed by the Korea Institute of Atomic Energy has become a potential option for the treatment of lanthanide oxides due to its large loading factor,high durability and low processing temperature.However,there are few reports about ZIT matrix at home and abroad,and there is no research on curing mechanism and process.The simulated waste La₂O₃ was treated with ZIT matrix here,and the main contents are as follows:The ZIT matrix material Zn₂TiO₄ was prepared by the molten salt synthesis method.The effects of different molten salt systems,reaction temperature,zinc-titanium ratio of raw materials and holding time on the structure and morphology of the product were studied respectively.Compared with the experimental results,it was determined that Zn O and TiO₂were used as raw materials according to the molar ratio of zinc to titanium was 1.2:1.The optimum synthesis conditions of Zn₂TiO₄ were maintained at 1000?in Na Cl-KCl(molar ratio1:1)for 4 h.It was proved that the molten salt promoted the reaction of the Zn O-TiO₂ system by the three zinc titanates,Zn₂Ti₃O₈,Zn TiO₃ and Zn₂TiO₄,coexisting at the lowest temperature of 700?.The growth mechanism of Zn₂TiO₄ in molten salt was discovered following the Ostwald ripening mechanism.Using Zn₂TiO₄,Ca HPO₄,SiO₂,B₂O₃ and La₂O₃ as raw materials,a high-temperature sintering method was applied to prepare ZIT ceramic materials with La PO₄ monazite phase as the main structure.The analysis results of the formation process showed that the dehydration process of the raw materials occurred below 200?,La PO₄ and Zn₂SiO₄ were generated at 800and 1000?respectively.In the range of room temperature to 1200°C,the morphology of the raw material gradually changed from powder to block,and the flatness and density of the sample surface gradually increased.ZIT samples with different La₂O₃ loading factors were prepared,and the results showed that the ZIT materials with 30 wt.%La₂O₃ loading maintained the internal structure,morphology and good stability of physical and chemical properties.The existing crystal phases are mainly Zn₂SiO₄,La PO₄ and TiO₂.The density of ZIT material with20 wt.%La₂O₃ loading was about 3.56 g·cm^-3.After soaking in deionized water for 28 days,the leaching rate of La was 4.75×10^-6 g·m^-2·d^-1.Using V₂O₅ as an additive into ZIT composite,it was found that V₂O₅ can reduce the experimental temperature of Zn₂TiO₄ converting to Zn₂SiO₄ and inhibit the production of Al₆Si₂O₁₃ in the immobilized body.V could enter the Zn₂TiO₄ lattice to replace Zn or Ti.After the crystal lattice,the lattice distortion caused the increase of the activation energy of the crystal lattice,thereby accelerating the conversion reaction of Zn₂TiO₄ to Zn₂SiO₄.At the same time,the low melting point of V₂O₅ promoted the process of dissolution,diffusion and precipitation of surrounding particles.The density and chemical stability tests showed that the density of the sample with 10 wt.%V₂O₅ substituted was 3.72 g·cm^-3,and the leaching rate of La on the 28th day was 8×10^-6 g·m^-2·d^-1.The experimental results showed that the higher density ZIT composite with V₂O₅ substituted were successfully prepared by the high-temperature sintering method.