Recovery of Metals from Simulated High-Level Radioactive Waste Glass through Phase Separation

Abstract

The recovery of metals from simulated high-level radioactive waste (HLW) in a glass form using the phase separation of borosilicate glass was studied in order to satisfy possible future demands such as the adoption of some new treatments for the waste in the glass or use of the vitrified elements as resources. The simulated HLW glass was separated into SiO2-rich and B2O3-rich phases at 973 and 873K when the ternary mass ratio of SiO2:B2O3:Na2O was adjusted to 68:27:5 by the addition of SiO2 and B2O3 to the simulated HLW glass. Annealing at the lower temperature of 878K promoted the distribution of the elements in the B2O3-rich phase. Approximately 90% of the Ni, Zn, Fe, Nd, Te, Zr, and Mo were distributed in the B2O3-rich phase, and these elements, except Zr, were almost completely leached into 1 mol dm−3 nitric acid at 363 K. The leaching of Zr was also achieved using 1.5 mol dm−3 sulfuric acid after the nitric acid leaching. The leached fractions of the glass-network components, such as B and Al, were lower than the other elements at approximately 70–80% due to the existence in the SiO2-rich phase. An increase in the concentration of CaO in the glass in the range of 2–5 wt% inhibited the distribution of elements into the B2O3-rich phase. The increase in the concentration of CaO also changed the structure of the B2O3-rich phase from the continuous unit shape to the discontinuous spherical shape. Consequently, the leaching fraction of every element dramatically decreased. The structure of the B2O3-rich phase returned to continuous when the concentration of CaO was more than 10 wt%. From this glass, the leaching of 69% Zr was possible using only nitric acid without any sulfuric acid.