Effect of SiO₂ on Immobilization of Metals and Encapsulation of a Glass Network in Slag

Abstract

The final disposal of ash from an incinerator is of special concern because of the possibility of its releasing toxic substances. Melting/vitrification has been regarded as a prospective technology of ash treatment. The object of this investigation was to evaluate the effect of silica (SiO 2 ) addition on the immobilization of hazardous metals and the encapsulation of a glass network during the vitrification process. Four specimens with SiO 2 /fly ash mixing ratios of 0, 0.1, 0.2, and 0.3, respectively, were tested. The mobility of metals in slag was then estimated by a sequential extraction procedure. X-ray diffraction analysis indicates that SiO 2 leads to the polymerization of silicates.The encapsulation of aluminum, calcium, and magnesium would not be observed unless adequate amount of SiO 2 was added. It was also found that SiO 2 addition enhances the formation of a compact and interconnected glass network structure and, thus, contributes to the chemical stability of metals in slag. After vitrification, the mobility of cadmium, copper, iron, chromium, nickel, lead, and zinc was significantly reduced. However, there is no significant correlation between the immobilization of these metals and the addition of SiO 2 . INTRODUCTIONIncineration, an important technology for the treatment of municipal solid waste (MSW), is used in many countries. 1 In addition to flue gas, MSW ashes (including fly ash and bottom ash) pose a big threat to the environment. 2 Direct landfilling of MSW ashes is no longer an appropriate strategy because of the dwindling number of final disposal sites and the risk of releasing organic toxics and heavy metals. Melting/vitrification has been developed as a proper technology in the further stabilization of ashes, radioactive waste, or sewage sludge. [3][4][5] This technology not only reduces the volume of waste and the mobility of metals but also destroys toxic substances such as polycyclic aromatic hydrocarbons (PAHs) or dioxins. 6 -9 The leaching of hazardous metals has been controlled by stringent regulations. 10 However, a leaching test does not preclude potential long-term pollution. On the other hand, the behavior of the main constituents in a glass network might serve as an indicator for prolonged structure deterioration in slag. During vitrification, the chemical composition of the ternary system (silica [SiO 2 ]-alumina [Al 2 O 3 ]-lime [CaO]) is a major factor controlling the formation of the slag structure. 3,11 Among these constitutes, SiO 2 is of the greatest importance for the main constituents in a glass network when fly ash is vitrified. 12 Therefore, this study focused on the effect of SiO 2 addition on vitrification.In this investigation, the mobility of anthropogenic metals was evaluated by a sequential extraction procedure. Similar methods have been applied to estimate the phase distributions in our previous work and other reports. 8,13 When the slag is recycled, its properties deserve more investigation to insure environmental safety. In previous studies, the ...

Heavy Metal Volatilization during Vitrification of Tannery Sewage Sludge with Mineral Waste

“…The high degrees of volatilization observed for Cd (≥ 73.4%), Pb (≥ 79.1%) and Zn (≥ 87.9%) shown in Tables 4-6 are in agreement with studies of other authors reporting high emissions of these metals even at lower temperatures [1,28,29]. Moreover attempts made by Tang et al [29] to reduce the emission levels during incineration by using hydroxyapatite didn't provide the desired results for zinc.…”

“…The lowest emission levels of 9.5 ± 6.5% (SS2-FA), 13.2±8.1 (SS1F-FA3) and 13.4 ± 1.3% (SS1P-FS) seem to be in line with Kuo et al [28] who reported low volatilization of this metal when vitrifying MSW fly ash. …”

“…In order to achieve a vitreous product and incorporate heavy metals in a silica matrix in the vitrification process, sewage sludge needs to be vitrified with an addition of materials with high content of SiO 2 [26][27][28].…”

Metal behavior during vitrification of incinerator ash in a coke bed furnace

“…The Pb and Zn contents were extremely high in comparison to those in the incinerator ash, implying that recovering by acid or alkaline leaching deserves consideration [15,16]. Both metals can also be reconcentrated to higher levels by recycling the fly ash into the furnace for more economical recovery [10].…”

“…In our previous work, the mobility and encapsulation of metals during a lab scale vitrification was studied, but the behavior of metal species was not investigated [10]. In this study, metal mass fluxes of input and output material during vitrification of incinerator ash in such a system were estimated.…”