Role of sodium ions in the vitrification process: Glass matrix modification, slag structure depolymerization, and influence of metal immobilization

Abstract

This study investigates the role of Na ions, a common flux, in the vitrification process. Artificial glass systems composed of Al₂O₃, CaO, and SiO₂ with various Na concentrations were melted at 1450 °C. The specimens were cooled by air cooling and water quenching and the metal mobility was evaluated using a sequential extraction procedure. The X-ray diffraction analysis and scanning electron microscopy observations showed that Na ions governed the air-cooled slag’s structure. Na ions initially depolymerized CaSiO₃-linked chains into CaSiO₃ chains, and further cut them into shorter and nonuniform ones, making the slag structure amorphous. With even more Na ions, CaSiO₃ chains were divided into single SiO₄ tetrahedrons and formed Na-related crystals (Na₂Ca₃Si₂O₈ and NaAlSiO₄). The phase distributions of Al, Cr, Cu Mn, and Ni showed that Na has a positive effect on the immobilization of heavy metals at suitable concentrations, but a negative effect when in excess amounts.

Implications:

Vitrification has been widely used to treat hazardous materials. The Na-bearing additives were often used as a flux to improve the melting process. This study described the role of Na played in the vitrification process. The Na ions acted as glass modifier and depolymerize the chain structure of slag. With adequate addition amount of Na ions, the immobilization of heavy metals was improved. The results provided much information about the crystalline phase variation, metal mobility, and surface characteristics while Na serves as a flux.

Funding

The author acknowledges the financial support by the National Science Council of Taiwan under grant NSC 101-2628-E-273-001-MY3.

Additional information

Notes on contributors

Yi-Ming Kuo

Yi-Ming Kuo is an associate professor in the Department of Safety, Health, and Environmental Engineering, Chung Hwa University of Medical Technology.