The Suppression Of Yellow Phase And Crystallization Of Borosilicate Glass For The Immobilization Of High-level Liquid Waste

Borosilicate glass is used to vitrify high-level liquid waste（HLW）with high content of sulfur and sodium in China.It should be noted that the solubility of sulfate in borosilicate glass is very low（SO₃～1 wt%）.A yellow separated phase usually forms on the surface of the glass melt（referred to as"yellow phase"）during vitrification process.The main component of the yellow phase is alkali metal sulfate（such as Na₂SO₄ and Li Na SO₄,etc.）,which is easily soluble in water,thus affecting the chemical stability of the glass waste form.In addition,pyroxene crystals are prone to crystallize in the waste glass,which may make glass casting difficult and affect the structure and physical and chemical properties of the waste glass.In this thesis,in order to inhibit the formation of yellow phase,it is proposed that the alkali metal sulfates in the simulated HLW and yellow phase are converted into BaSO₄ by adding Ba（NO₃）₂ solution,and then the borosilicate glass waste form are prepared by melting method.The effects of melting temperature,simulated HLW doping content,and Ba（NO₃）₂ content on the structure and leaching resistance of the glass waste form were studied.The composition and microstructure of the yellow phase generated during the melting process were systematically investigated,and the phase segregation behavior of sulfate during the glass melting process was discussed.Moreover,the effects of P₂O₅ content on the structure and leaching resistance of the borosilicate glass were studied,and the crystallization characteristics of the P₂O₅-doped glass waste form after heat treatment was investigated.The main results are as follows:（1）The effect of Ba（NO₃）₂ pretreatment of simulated HLW on the yellow phase composition and structure was studied.The results show that with the increase of melting temperature,the amount of yellow phase on the surface of the glass increases first and then decreases,reaching the maximum at about 1050℃.For the simulated HLW without Ba（NO₃）₂pretreatment,the main crystalline phase of the yellow phase sample obtained at 900～1000℃is Na₂SO₄,and the minor phases are Na₂Mo O₄,Na₂Cr₂O₇,and so on.Additionally,the microscopic morphology of Na₂SO₄ crystals is blocky or flaky shape（PDF NO.24-1132,orthorhombic system）.When the melting temperature is higher than 1000℃,the main phases of the yellow phase sample are Na₂SO₄ and Li Na SO₄,and the microscopic morphology of Na₂SO₄ crystals is gel-like（PDF NO.99-0103,orthorhombic system）and needle-like（PDF NO.86-0800,hexagonal system）.For the simulated HLW after pretreatment with Ba（NO₃）₂ solution(the molar ratio of Ba²⁺to SO₄^2-is 2:1（Ba/S=2:1）),the main crystalline phases of the yellow phase sample are BaSO₄ and Ba Cr O₄ when the melting temperature is900～1000℃.The main crystalline phases of the yellow phase sample obtained at temperatures higher than 1000℃are BaSO₄,Ba Cr O₄ and Na₂SO₄.With the increase of simulated HLW content,the pretreatment of simulated HLLW has no significant change on the phase composition of the yellow phase sample,and the grain size of the crystals in the yellow phase grows obviously.For the simulated HLW without pretreatment,the Na₂SO₄ in the yellow phase mainly exists with massive or flaky shape crystals.While for the simulated HLW after pretreatment,the BaSO₄ in the yellow phase sample grows from regular massive crystals into rod-like crystals.（2）After pretreatment of simulated HLW with Ba（NO₃）₂ solution,the effect of Ba（NO₃）₂ content on the composition and microstructure of yellow phase,and leaching resistance of the glass waste form was investigated.The results show that the amount of yellow phase on the surface of the waste glass gradually decreases with the increase of Ba（NO₃）₂ content,and the main phases of the yellow phase gradually change from Na₂SO₄ and Li Na SO₄ to BaSO₄.When the molar ratio of Ba²⁺to SO₄^2-is 4:1（Ba/S=4:1）,the formation of the yellow phase is significantly inhibited,the Na₂SO₄ and Li Na SO₄in the yellow phase basically disappear,and the yellow phase disappear when Ba/S=6:1.The solid NMR results of the waste glass show that the relative content of the BO₃structure increases from 29.39%to 36.79%and the total relative content of Q¹ and Q²units in the Si network structure increases from 19.54%to 28.13%with the increase of Ba（NO₃）₂ content（Ba/S=0:1 to 7:1）,indicating that the network structure of the waste glass densities decreases.The leaching resistance also decreases with increasing Ba（NO₃）₂ content.After 28 d,LR_Si,LR_B,LR_Na and LR_Csof the waste glass（Ba/S=4:1）are 0.530,0.463,0.262 and 0.098 g·m^-2·d^-1,respectively.（3）After the yellow phase is reacted with different contents of Ba（NO₃）₂,the waste loading of the yellow phase in the glass was studied.The results show that the waste loading of the yellow phase increases significantly after the reaction of Ba（NO₃）₂.When Ba/S ratio is 4:1,the waste loading of the yellow phase can reach up to 10 wt%,which is much higher than that（～5 wt%）of the sample without reaction with Ba（NO₃）₂.With the increase contents of Ba（NO₃）₂ and yellow phase,the FTIR spectra of the waste glass are significantly red-shifted and the network structure of the waste glass is depolymerized to some extent.When the yellow phase doping content is higher than 10wt%,BaSO₄ microcrystals are uniformly distributed inside the waste glass for the sample with Ba/S=4:1.In addition,for the yellow phase without reaction with Ba（NO₃）₂,the main phase compositions of the yellow phase generated during vitrification are Na₂SO₄ and Li Na SO₄,and the minor phases are Ca Mo O₄ and BaSO₄.For the yellow phase after reaction with Ba（NO₃）₂,the main phase of yellow phase sample is basically BaSO₄.（4）The Mg O and Ca O in the borosilicate glass formula are partially replaced by P₂O₅,and the effect of the P₂O₅ content on the structure and leaching resistance of the waste glass was studied.The results show that with the increase of P₂O₅ content,the content of Q³,Q⁴ and BO₃ structural units in the glass network structure gradually increase.The densification of the overall network structure and leaching resistance of the borosilicate glass are gradually improved.After 28 d,the normalized leaching rates LR_Si,LR_B,LR_Na and LR_Cs of the waste glass with 3%P₂O₅ content are 0.508,0.468,0.533 and 0.280 g·m^-2·d^-1,respectively.When the P₂O₅ content is higher than 3 wt%,Na₃Ca₆（PO₄）₅ crystalizes inside the waste glass.In addition,The crystallization characteristics of the waste glass with P₂O₅ content ranging from 0 to 4 wt%after heating at 850℃for 6 h were studied.The results show that for the heat treated waste glass with 3%P₂O₅ content,a small amount of calcium silicate phase appears,the pyroxene phase almost disappears,and the crystallinity is the lowest.The crystallinity of the waste glass increases when the P₂O₅ content is higher than 3 wt%,and oxyapatite phase appears.