Study On The Formation Process And Properties Of Cerium Containing Iron Phosphate Glass-ceramics

Imobilization by using glass-ceramics is a potential method to improve the waste loading and stability of glass waste forms,which is an expected technology to solve the problems of low waste loading and unsatisfactory stability of glass waste forms.Therefore,Imobilization by using glass-ceramics is an important research direction for the treatment of high-level muclear waste.In this paper,in order to immobilize the components（such as actinides）contained in the high-level wastes which have low solubility in the widely used borosilicate baseline glasses,the iron-borophosphate-based（Fe₂O₃-B₂O₃-P₂O₅）glass-ceramics ae used as the baseline material,Cerium（Ce）element is used as a surrogate for actinide nuclides,the cerium containing iron phosphate-based glass-ceramic waste forms were synthesized by a simple melt-quenching process.The structure and properties of the formed cerium containing iron phosphate-based glass-ceramic waste forms were characterized,and the effect of ball milling on the formation process and properties of the glass-ceramic waste forms was discussed in detail.The following conclusions were drawn:（1）Phosphate based glass-ceramic waste forms containing monazite CePO₄ as crystalline phase were prepared by a melt-quenching process using NH₄H₂PO₄,H₃BO₃,Fe₂O₃ and CeO₂ as the feed materials and the formation process was analyzed.The results show that,the decomposition of H₃BO₃ at the heating temperature below 150 ℃ and the reaction between Fe₂O₃ and NH₄H₂PO₄ to form NH₄FeP₂O₇ happen firstly.Then phosphate phases such as Fe₄（PO₄）₂O,Fe（PO₃）₃,FePO₄ and CePO₄ form below 750 ℃,and these phosphate phases are continually dissolved into phosphate glass melts with further increasing temperature.Finally,CePO₄ phase is the only crystalline phase at higher temperatures such as 1100 ℃ due to its good stability and glass-ceramic waste forms with CePO₄ as crystalline phase were obtained by quenching and annealing.Moreover,the content of CeO₂ between 8 mol% and 14 mol% has insignificantly effect on the formation process.（2）The monazite CePO₄ containing glass-ceramic waste forms can be prepared by a simple glass melt-quenching method in the Fe₂O₃-B₂O₃-P₂O₅ composition containing 8-16 mol%CeO₂.Fe（PO₃）₃ and trace Fe₄（PO₄）₂O crystal phase can be detected in the glass ceramics heated at low temperature.The results show that the Hardness and density of the phosphate based glass-ceramic waste forms increases with the increase of the CeO₂ content.The network structure of the studied glass-ceramics mainly consists of Q⁰ and Q¹ units.The LR of samples heated at different temperature decreased with the immersion time and ramian almost unchanged after 35 days,LRFe remaining at a low value of about 3.5×10^-7 g·m^-2·d^-1,LRCe remaining at a low value of about 2.4×10^-7 g·m^-2·d^-1 at 35 days.With the increasing heating temperature and the decreasing content of CePO₄ crystal phase,the thermal stability（T_r-T_g）of the glass-ceramic is improved and their glass transition temperatures decrease.The Q¹ units and Me PO₄（Me= Ce,Fe）groups decrease.Accordingly,before 35 days,the LR_Fe and LR_Ce of Ce12 samples heated at 1180 ℃ are lower than Ce12 samples heated at 1150 ℃ and 1200 ℃.These conclusions indicate that the glass-ceramic preparation simple traditional glass melt-quenching method are chemically durable and suitable waste forms for the immobilization of actinide-rich radioactive wastes with processing chemicals.Appropriate of sintering temperature can improving the structure and performance of glass-ceramic.（3）Ball milling can reduce the particle size of raw materials,especially Fe₂O₃ and CeO₂,which accelerates the speed of all raw materials participating in the reaction among various feed materials,reduces the intermediate side reaction and the formation of each intermediate iron phosphate crystal phase,makes the formed intermediate crystal phases melt into the glass melt more faster.Thus phosphate glass-ceramics can be obtained at a lower heating temperatures such as 1100 ℃.Although the formation process is similar,more NH₄FeP₂O₇crystalline phase is formed at 300 ℃ by ball milling the batches,which makes NH₃ and H₂ O discharge at a lower temperature,and the volume of the feed samples decreases faster,dense glass-ceramic waste forms were obtained.Moreover,ball milling does not reduce the thermal stability and the chemical durability of the final glass-ceramics.