Study On Crystallization Behavior And Leaching Resistance Of Borosilicate Glass

The safe immobilization and disposal of high-level liquid waste is a common problem in the world,and it is also an important factor for the sustainable development of nuclear industry.Vitrification is the only kind of technology to industrially immobilize the high-level liquid waste,and borosilicate glass is the preferred matrix for immobilization.At present,the first vitrification factory has been constructed in China and it is in cold operation.It is very important to study the crystallization behavior and leaching property of glass for safe production,temporary storage,and disposal of the waste glass.In this thesis,borosilicate glass with simulated high-level liquid waste was prepared by melting method.The waste loading was 16 wt%.The effects of heat treatment temperature and time on the crystal phases and microstructure of the waste glass were studied.Static Leaching of Monolithic Waste Forms（MCC-1）and the Product Consistency Test（PCT）were used to study the leaching properties.The effects of temperature（40～150℃）,soaking time（1～364 d）,soaking agents（deionized water and simulated groundwater）,and pH value（5.5～10）on the alteration layer structure and leaching resistance of the waste glass were studied,and the Monte carlo method was used to simulate the glass dissolving process.The main research results are as follows:（1）The Augite crystalline phase appears after heat-treatment of the waste glass powders at 600～900℃for 12h.The X-xay diffraction peak intensity is weak at 600℃and 900℃,and there is a lot of crystals in the glass powder at 700～860℃.After the glass powder was heat-treated at 700℃for different time,the crystal size of the Augite increases with time increasing,reaches the maximum for 24h,and no obvious change with time further increasing.A small number of crystals precipitate on the surface of glass only at 700℃,and the crystallization rate reaches 14.71%at 28 days.After heat-treatment for 28 d,the leaching resistance ability of the glass is lower than that of the non-treated glass.The glass before and after crystallization for 28 d,the normalized leaching rate of Na,B,Si and Cs elements are on the order of 10^-1 g·m^-2·d^-1 in deionized water.（2）The effects of temperature on the altereation layer structure and leaching resistance ability of the glass in simulated groundwater and deionized water for 42 d were studied.The results show that there is no obvious change at 40℃and a little sediment appears at 70℃on the surface of glass.The Mg-rich phyllosilicate and aluminosilicate alteration layer form on the surface of the glass when the temperature increases to 90～150℃.In addition,after the glass was immersed in simulated groundwater at 150℃for 42 d,the secondary sedimentary phase barium sulfate appears on the surface.The results of MCC-1 leaching experiments show that the normalized leaching rates of B,Si,Cs elements increase gradually with inceasing temperature.The dissolution of glass in simulated groundwater at 40～150℃is dominated by ion exchange,and the apparent activation energy（E_a）was calculated to be 27.8 k J/mol based on the leaching rate of element B.The dissolution of the glass in deionized water at 40～90℃is dominated by ion exchange（E_a≈45.63 k J/mol）,and in deionized water at 90～150℃it is dominated by network dissolution（E_a≈28.12 k J/mol）.（3）The effects of leaching agents（simulated groundwater and deionized water）with different pH values on the altereation layer structure and leaching resistance ability of the glass were studied.The results show that at 90℃in deionized water and simulated groundwater for42 d,honeycombed phyllosilicate forms on the surface of the glass.Honeycomb structure dissolves in deionized water when pH=10.A few hyllosilicates forms on the surface of the glass in the deionized water when pH=7,and the content of Mg and Al is relatively low.The results of MCC-1 leaching experiment show that there is no significant effect of the pH value on the leaching resistance ability of the glass,and normalized leaching rates of B,Si,Cs elements are 10^-1 g·m^-2·d^-1 order of magnitude.（4）The altereation layer structure and leaching resistance ability of the glass at 90℃for182 d and 364 d were studied.The results show that after 182 d,honeycombed phyllosilicate and secondary sedimentary phase form on the surface of the glass,and the main elements are Mg,Si and Al.After 364 d,the alteration layer is composed of Ba SO₄ phase and gel layer,and the main elements are Mg and Si on the surface.The normalized leaching rates of B,Si,Cs elements tend to be stable after 182 d.According to the equilibrium equation of precipitation and dissolution,the dissolution process of B and Si of borosilicate glass was simulated by Monte Carlo method.It is found that the leaching behavior of B and Si in borosilicate glass was consistent with the experimental results,verifing the reliability of the simulation method.