Material Recycling of Municipal Waste Slags by Using Phase Separation of Glass. Takuya IMAOKA*, Shinichi SAKIDA**, Tokuro NANBA* and Yoshinari MIURA*

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1 * ** * * Material Recycling of Municipal Waste Slags by Using Phase Separation of Glass Takuya IMAOKA*, Shinichi SAKIDA**, Tokuro NANBA* and Yoshinari MIURA* A novel recycling process of municipal waste slags obtaining Fe-free colorless materials was developed by using a phase separation of borosilicate glass. B 2 O 3 was added to a simulated waste slag to promote the phase separation. The slag glasses were heat-treated above glass transition temperatures, from which phase separation was successfully induced. The phase-separated slag glasses were still colored in black due to Fe ions, and after soaking in acid, they were successfully bleached, obtaining colorless solids. According to compositional analyses, no Fe ions were confirmed in the remaining insolubles, indicating that Fe ions were preferentially incorporated into the borate-rich phases soluble in acid. The main constituent of the colorless solids was SiO 2, and the end products obtained in the present process were expected as an alternative of pure silica glass. Key words: slag, recycling, colorless glass, phase separation * 2/3 1/3 SiO 2 CaO Al 2 O 3 % Fe 2 O 3 Fe 2 O 3 * ** Fe Fe (B 2 O 3 SiO 2 ) SiO 2 B 2 O 3 (Charles & Wagstaff, 1968; Haller et al., 197) Fe Fe Fe B 2 O 3 B 2 O 3

2 Fe B 2 O 3 Fe Fig.1 Fe SiO 2 : CaO : Al 2 O 3 : Fe 2 O 3 : Na 2 O = 43 : 25 : 17 : 8 : 7 (mass%) Pt C 3 16 C 3 B 2 O 3 16 C 3 B 2 O 3 DTA Tg SEM XRF X Fig.1 slag B 2 O 3 melting at 16 C-3min slag glass heating above Tg phase-separated glass soaking in aq. HCl & filtration insoluble filtrate rinse & drying end product Schematic flow of the slag recycling process developed in the present study. 3.1 SEM B 2 O 3 SEM Figs.2~4 Fig.2 B 2 O 3 3mol% SEM Figs.2a ~ 2c Figs.2d ~ 2f.3 ~.5 μm (Fig.2a) 1 μm Figs.3a ~ 3c B 2 O 3 mol% SEM 6 C Fig.3d SEM (Doremus, 1973; Mazurin & Porai-Koshits, 1984) Figs.4a ~ 4b B 2 O 3 mol% B 2 O 3 3mol% C B 2 O 3 3 mol% B 2 O 3 3mol% B 2 O Fig.5 XRF Fig.5a Fe Fig.5b Fe

3 (a) 6 C 2 hr, surface (b) 6 C 3 hr, surface (c) 6 C 8 hr, surface (d) 6 C 2 hr, surface (e) 68 C 2 hr, surface (f) 68 C 2 hr, cross section (g) 7 C 2 hr, surface Fig.2 SEM photographs of the slag glasses containing 3 mol% B 2 O 3 (T g = 57 C) (a) 6 C 3 hr, surface (b) 6 C 8 hr, surface (c) 6 C hr, surface (d) 58 C 2 hr, surface Fig.3 SEM photographs of the slag glasses containing mol% B 2 O 3 (T g = 585 C)

4 (a) 6 C 8 hr, surface (b) 6 C hr, surface Fig.4 SEM photographs of the slag glasses containing mol% B 2 O 3 (T g = 6 C) Figs.5a, 5b Na Ca Al B 2 O 3 SiO 2 (a) 4 Bslag Bslag B3slag 2 ~ 3 ev B 2 O 3.5 ~ 6. ev 4 ev 3 (a) 12 (b) atomic% atomic% Na Al Si Ca Fe B@6_8h_5N B@6_8h_5N B3@6_8h_5N Na Al Si Ca Fe Fig.5 Analytical composition of (a) the slag glasses containing B 2 O 3 before phase separation and (b) the insoluble end products after the heat treatment at 6 C 8 hr and the subsequent acid treatment. Fig.6a (b) α/s (Kubelka-Munk units) α/s (Kubelka-Munk units) x= Photon energy (ev) C-3hr 58 C-5hr 6 C-8hr + 5N HCl - 5 days Photon energy (ev) Fig.6 Optical absorption spectra of (a) the slag glasses containing x mol% B 2 O 3 before phase separation and (b) the insoluble end products obtained from the slag glass containing mol% B 2 O 3 after the heat treatment and the subsequent acid treatment.

5 Fig.7 Fig.6a 3 ev Figs.7a,b Fe 6 ev Fig.7c SiO 2 SiO 2 (a) simulated waste slag (b) B 2 O 3 -containing slag glass N 5N 2.5N 5N 2.5N N N B 2 O 3 B 2 O 3 Fe Ca SiO 2 SiO 2 (c) insoluble end product Charles, R.J., Wagstaff, F.E.(1968): Metastable Immiscibility in the B 2 O 3 SiO 2 System, J. Am. Ceram. Soc., 51, pp.16-. Doremus, R.H. (1973): Glass Science, Wiley, p.44. Haller, W., Blackburn, D.H., Wagstaff, F. E., Charles, R. J. (197): Metastable Immiscibility Surface in the System Na 2 O-B 2 O 3 -SiO 2, J. Am. Ceram. Soc., 53, pp Mazurin, O.V., Porai-Koshits, E.A.(1984): Phase Separation in Glass, Morth-Holland, p.163. Fig.7 Photographs of the typical specimens prepared in the present study.