Editors' Choice—Behaviors of Si, B, Al, and Na during Electrochemical Reduction of Borosilicate Glass in Molten CaCl₂

Abstract

The electrochemical reduction of borosilicate glass in molten CaCl 2 at 1123 K was investigated. The behaviors of the constituent elements, i.e., Si, B, Al, Na, and K, were estimated using potential-pO 2− diagrams constructed from the thermodynamic data for the species. The diagrams suggested that the first cathodic wave in the cyclic voltammogram results from the reduction of the B 2 O 3 component. The dissolution of the Na 2 O and K 2 O components, which was predicted from the diagrams, was confirmed by energy dispersive X-ray analysis of a borosilicate glass plate after immersion into molten CaCl 2 without electrolysis. The scanning electron microscopy/wavelength dispersive X-ray mappings and Raman spectrum for borosilicate glass reduced at 0.9 V vs. Ca 2+ /Ca indicated that SiO 2 and B 2 O 3 are reduced to Si and B-Si compound. The formation of Ca-Al-O compounds owing to the increase of O 2− ions is suggested. The pO 2− range during electrolysis at 0.9 V was indicated to be 2. In 2015, more than 11% of global electricity was produced by nuclear power plants.1 Indeed, nuclear power generation is expected to replace conventional fossil-fuel-based thermal power generation in order to accommodate the energy demands of the growing global population. Considering the controls on carbon dioxide emissions, nuclear energy is attractive as a low-carbon-emission power source. However, the disposal of radioactive waste is a serious problem for nuclear power generation, especially in countries frequently hit by volcanic tremors and earthquakes, such as Japan, where the selection of sites suitable for the geological disposal of radioactive wastes is very difficult. Therefore, the development of an alternative to geological disposal is required.In Japan, a new process for the disposal of radioactive wastes has been proposed. 2 In the first step of this process, long-lived fission products (LLFPs) such as 135 Cs, 79 Se, 93 Zr, and 107 Pd are separately recovered from high-level nuclear waste. They are then disposed as stable waste after their conversion by nuclear transmutation into shortlived or stable nuclides. If this process is established as a new disposal process for nuclear wastes, the amount of nuclear waste will be significantly reduced. This process has another potential advantage in that it may be used to recover valuable elements like platinum group metals, which can then be utilized for automobiles and fuel-cell catalysts. Moreover, if this process is realized, it is also applicable to the LLFPs already present in the large amount of existing vitrified waste. However, to accomplish this, each LLFPs must first be recovered from the vitrified waste.Recently, the authors proposed a new method to recover LLFPs from vitrified wastes by electrochemical reduction in molten CaCl 2 .The three-dimensional (3-D) network structure of the glass (Si-O) is electrochemically destroyed. The reduction product is then subjected to the separation of each element. This process is superior to the conventional wet process using HF ac...

Electrolytic Reduction of Solid Al2O3to Liquid Al in Molten CaCl2

“…4, to predict suitable conditions for the production of Al metal and to interpret the electrolysis results. 20 PotentialpO 2− diagrams for the Al-Ca-O-Cl system in Ref. 20 were updated as those in Fig.…”

“…We already reported the potentialpO 2− diagram for the Al-Ca-O-Cl system at 1123 K as a part of the study on the electrochemical reduction of borosilicate glass whose components were SiO 2 , B 2 O 3 , Na 2 O, K 2 O, and Al 2 O 3 . 20 In the present study, we update the diagram in which the Ca content in liquid Al is newly considered. Second, cyclic voltammetry is conducted to investigate the reduction behavior of Al 2 O 3 , whereby the electrode potential is accurately controlled and measured with a three-electrode system by using a Ag + /Ag reference electrode.…”

Electrochemical reduction behavior of vitrified nuclear waste simulant in molten CaCl2

“…The recovery of LLFPs from the vitrified waste requires the decomposition of the Si-O network structure of the glass in the initial step. Recently, we proposed electrochemical reduction in molten CaCl2 for the decomposition of the glass network structure [5][6][7]: SiO2 + 4 e − → Si + 2 O 2− (in molten CaCl2) (1) The details of this molten salt electrolysis process have been presented in our previous paper [7]. Realizing the new disposal process requires clarification of the behavior of the constituent elements, including LLFPs, during the electrolysis of the vitrified waste in molten CaCl2.…”

“…In our previous studies, electrochemical reduction of borosilicate glass and two types of simplified simulants of vitrified radioactive waste was investigated [5][6][7]. Their compositions are summarized in Table 1.…”

“…Their compositions are summarized in Table 1. In the electrochemical reduction of borosilicate glass, the main component, SiO2, was reduced to crystalline Si by potentiostatic electrolysis at 0.9 V vs. Ca 2+ /Ca in molten CaCl2 at 1123 K [5,6]. In addition, the reduction of B2O3 and dissolution of Na2O were indicated [5,6].…”