| The borosilicate glass waste forms for the immo bilizat ion o f high level liquid waste(HLLW) have some drawbacks such as the low solubilit y of act inides and poor thermal stabilit y and chemical durabilit y. In addit ion, a separate yellow phase containing sulfur is usually formed during the vitrificat ion process. Different content(0~55 wt%) of Ca O, TiO2 and Zr SiO4(abbreviated as CTZ) was added to the barium borosilicate matrix. Barium borosilicate glass-ceramic wasteforms containing zirconolite crystalline phase. were prepared by melt ing heat treat ment method. The effect of CTZ content on the crystal phase composit ion, microstructure and chemical stabilit y of barium borosilicate glass-ceramic wasteforms was systemat ically st udied. Studies on the effect of different content of calcium(Ca O:TiO2:Zr O2 = x:2:1, x=0.5~10) on the phase structure and microstructure of barium borosilicate glass-ceramics was evaluated. The orthogonal test method was used to optimize the heat treatment process parameters of barium borosilicate glass-ceramics, the effect of one-step method(nucleation and cryst al growth at the same temperatures) heat treatment process and two-step method(nucleat ion and crystal growth at different temperatures) heat treatment process on the structure of glass ceram ics was compared. The solubilit y of sulfur(in the form of Na2SO4) in barium borosilicate glass-ceramics wasteforms was also studied.The results show that the melt ing temperature of barium borosilicate glass-ceramic wasteforms containing different CTZ content is 1150~1250 ℃. Tg is 615~650 ℃ and increases with increasing CTZ content. When the CTZ content is higher than 35 wt%, zirconolite phase appears in the bulk of glass-ceramics. The amount of zirconolite increases with increasing the content of CTZ. When the content of CTZ is 45 wt%, a number of uniform columnar zirconolite grains were formed in the bulk of the base glass. When the content of CTZ is 55 wt%, glass ceramic wasteforms has poor densit y and contains zirconolite, titanite and silica phase. The chemical stabilit y of glass-ceramics containing 45 wt% is desired. After 42 d, the normalized mass loss(NL) of Na and B is 0.2 g/m2, and the NL Ca and Si is 0.02 g/m2. Furthermore, glass-ceramic containing uniform and single zirconolite phase in the bulk of glass can be performed with Ca O:TiO2:Zr O2 = 2:2:1 and 4:2:1. The orthogonal test reveals that nucleat ion and crystallizat ion temperature has greater effects on the wasteforms than nucleat ion time and crystallization t ime. No obvious infulence of the two-step method and one-step methed on the structure of glass-ceramic was observed. While titanite phase was observed when the crystallizat ion temperature is higher or the crystallizat ion time is longer. When the content of S is less than or equal to 3 mo l%, there is only zirconolite phase in the bulk of glass-ceramic without sulfur-containing phase separation. And the varation of S content has no obvious effect on the amount and distribut ion of z irconolite phase. When the content of S is more than 3mo l%, a white seperation layer composite with sodium sulfate and barium sulfate on the surface as well as some pores in the bulk of glass-ceramics wasteforms were observed. In addit ion, z irconolite and titante phase co-exist in the bulk of the glass-ceramics when the content of S is more than 3mo l%,. The PCT experiment show that the NLB and NLNa is 0.01 g/m2, NLBa is 0.001g/m2,NLNa is 0.1 g/m2 for samples with 0-3 mo l% after 7 d. when samples with above 3 mo l% Na2SO4, NLB and NLNa increase with prolonging time after 7 d. after 28 d, NLBa and NLCa is 10-4 g/m2 and 10-2 g/m2 respectively and almost maintain unchanged. |
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