Effect of Al2O3 mole fraction and cooling method on vitrification of an artificial hazardous material. Part 1: Variation of crystalline phases and slag structures

This study investigated how Al ions affect slag structure. During vitrification, pure Al2O3, CaO, and SiO2 served as the encapsulation phases with the use of Al mol% as an operating parameter. All specimens with the same basicity (mass ratio of CaO to SiO2) of 2/3 were vitrified at 1400 °C and cooled by air cooling or water quenching. XRD was used to measure the volume fractions of crystalline and amorphous phases. In a non-Al environment, CaSiO3 was formed in air-cooled and water-quenched slags. With the addition of Al2O3, no crystalline phases were observed in water-quenched slags. With the increase of Al mol% in specimens, the Al ions in air-cooled slags initially acted as an intermediate linking one tetrahedron chain to another and reducing the amount of crystalline phase, then behaved as a network former making the slags amorphous, and finally replaced Si ions in silicate frames to generate a large amount of CaAl2Si2O8. Air cooling improved the formation of crystallize structures with more leachable metal ions. A highly crystallized Al-framed structure is not suitable for encapsulating hazardous metals in vitrified slags.