Study On The Component,Structure And Leaching Resistance Performance Of Sulfur-doped Barium Borosilicate Glass-ceramics

Glass-ceramic has become an important development direction in the glass immobilization of high-level radioactive liquid waste?HLLW?.In present,the HLLW stored in China possesses high ammount of both sulfur and soldium.However,due to the low solubility of sulfur in borosilicate glasses,a yellow separate layer?“yellow phase”?is easily formed in vitrification process.The main constituent of the yellow phase is Na₂SO₄.In order to improve the waste loading of sulfate,in the work,the barium borosilicate glass-ceramics containing zirconolite and barite phases were prepared by melting-heat treatment process,and the effects of SO₃ addition on crystalline phases,microstructure,and leaching resistance performance of the barium boro silicate glass-ceramics were systemically studied.The solubility of simulated high-sodium and high-sulfur bearing high level liquid waste in zirconolite-based barium borosilicate glass-ceramics was researched,and the preparation process and the behavior of sulfate phase segregation was also explored.The results showed that the melting temperatures of the barium borosilicate glass-ceramics belonging to SiO₂-B₂O₃-Na₂O-BaO-CaO-TiO₂-ZrO₂-SO₃-Nd₂O₃ are 1200¹250 ?,glass transition temperatures Tg are about 640⁶60 ?.The main crystalline phase is zirconolite along with few spherical-shaped baddeleyite phase and tabular-shaped titanite phase for all of the Nd₂O₃-doped samples with different sulfate contents.When the addition of SO₃ is higher or equal to 4 wt%,rice-shaped barite crystals appear in the bulk of glass-ceramics,and the number increases with the increaseof SO₃ content.Barite phase was formed during the melting stage.When the SO 3 content is 8 wt%,a white separate layer consists of sodium sulfate and barium sulfate was formed on the surface of the glass-ceramics,there are many cracks and holes in the sample,and the structure becomes friable.X-ray fluorescence analysis indicates that about 1.3 wt% SO 3 can be incorporated in the glass-ceramics when the addition of SO₃ is 6 wt%,the sulfate is decomposed and volatilized during the melting process.According to EDX and element mapping analysis,the S element s mainly distribute in the barite crystals,Nd elements homogeneously distribute in zircono lite and residual glass phases.The product consistency test?PCT?shows the leaching resistance performance of barium borosilicate glass-ceramics has no significant change when the content of SO₃ is equal or lesser than 6 wt%.When the content of SO₃ is higher than 8 wt%,the leaching resistance performance is decreases obviously.The homogeneous glass can be obtained under the melting temperature 1150 ? in the process of zirconolite-based barium borosilicate glass-ceramics immobilization of simulated high amount of sodium and sulfur bearing high level liquid waste,glass transition temperature Tg decreases with the increase of simulated HLLW content?580⁶50 ??.The glass-ceramics with 0²0 wt% simulated HLLW possesses mainly zirconolite crystals in the bulk.When the simulated HLLW content is 20 wt%,the baddeleyite and calcium titanate is observed.As the simulated HLLW content increases to 30 wt%,the major crystalline phase is calcium titanate,and the structure becomes friable.The PCT experiment shows that the leaching resistance performance of glass-ceramics has no obvious change when the simulated HLLW content is below or equal to 20 wt%.The viscosity of sample with 16 wt% simulated HLLW is about 30 d Pa×s at 1150 ?.The yellow phase,consists of sodium sulfate and barium sulfate along with chromates and molybdate,formed at early stage of the batch melting,the content reached a maximum value at 1100 ?,begins to decompose at 1150 ? and no yellow phase is found and the melt surface after 2 h at 1150 ?.With the method of rapid cooling,there is only few baddeleyite observed at low temperature,zirconolite crystals are formed when the temperature achieves 850 ?.