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 SiO 2 -rich and B 2 O 3 -rich phases at 973 and 873 K when the ternary mass ratio of SiO 2 :B 2 O 3 :Na 2 O was adjusted to 68:27:5 by the addition of SiO 2 and B 2 O 3 to the simulated HLW glass. Annealing at the lower temperature of 878 K promoted the distribution of the elements in the B 2 O 3 -rich phase. Approximately 90% of the Ni, Zn, Fe, Nd, Te, Zr, and Mo were distributed in the B 2 O 3 -rich phase, and these elements, except Zr, were almost completely leached into 1 mol dm À3 nitric acid at 363 K. The leaching of Zr was also achieved using 1.5 mol dm À3 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 SiO 2 -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 B 2 O 3 -rich phase. The increase in the concentration of CaO also changed the structure of the B 2 O 3 -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 B 2 O 3 -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.

Phase separation of borosilicate glass containing sulfur

“…As above mentioned, Nanba et al 18) reported the prediction of BO 4 fraction, in which it was revealed that N 4 value increased with increasing SiO 2 /B 2 O 3 molar ratio. Before phase separation, Table 2).…”

“…N 4 value of the sulfur-containing as-prepared glass is 23.3%, and the amount of Na 2 O used to produce BO 4 units is therefore expected as 6.5 mol %. In addition, 2 mol % of Na 2 O is required to form AlO 4 units, and total amount of Na 2 O consumed for the BO 4 and AlO 4 units is 8.5 mol %. However, the analytical amount of Na 2 S + Na 2 O is only 7.1 mol %, which is insufficient to compensate the negatively-charged species.…”

“…Based on their prediction, 1.9 mol % of Na 2 O is enough to obtain N 4 = 23.3% for the glass at B 2 O 3 :SiO 2 = 27.7:65.2. Then, 1.9 and 2.0 mol % of Na 2 O is consumed to form BO 4 and AlO 4 units, respectively. Consequently, 3.2 © 2 mol % of Na + ions is remaining to compensate 6.8 mol % of negatively-charged sulfur atoms, and the average charge of sulfur anions is thereby estimated at ¹0.94.…”

“…1), 2) It has been widely used in production of porous glass and SiO 2 substitution. Recently, novel utilization methods of phase separation of borosilicate glass has been developed for the recycling of wastes containing SiO 2 , such as waste glass, 3) radioactive waste, 4) blast furnace slag 5) and municipal waste slag. 6) In the recycling process, B 2 O 3 is added to the wastes, preparing borosilicate glasses, from which high SiO 2 glass can be recovered by using phase separation.…”

“…These negatively-charged sulfide anions are associated with Na + ions as a charge compensator. Na + ions also compensate negatively-charged BO 4 and AlO 4 units. At phase separation, Na + ions are preferentially distributed to B 2 O 3 -rich phase, and the distribution shift of Na + ions to B 2 O 3 -rich phase is accompanied by the segregation of negatively-charged species, resulting in the condensation of sulfurs and Al 2 O 3 into B 2 O 3 -rich phase.…”