Fabrication of new glass–ceramic materials from float glass and slag waste by modulation of the cooling rate

This study explores an alternative route to recycle waste materials from float glass (FG, 30 wt%) and copper slag (CS, 70 wt%). The FG is a silica-rich glass, while the CS is rich in Fe and Zn. They were melted at 1550 °C to obtain a homogeneous glass that was then re-melted and cooled at 10 (low) and 500 (high) °C/h to produce a glass-ceramic. X-Ray Powder Diffraction, Scanning Electron Microscope and Electron microprobe characterisations show that both products contain spinel crystals within an abundant glassy matrix. At 500 °C/h, unexpectedly, the glass-ceramic contains a higher content (30.0 ± 5.5 area%) of tiny and long dendrites (spinifex) of spinels than at 10 °C/h (13.7 ± 2.2 area%); at the low rate, spinels are skeletal (large crystals) to dendritic (tiny and short) and larger than at high rate. This unveils that the estimated crystal growth rate (10^-7 cm/s) is higher at 500 °C/h. The crystal-chemistry of spinels results in more enriched Fe and Zn at 10 °C/h than at the high rate. This approach is promising for various applications or for concentrating valuable transition metals (Fe, Zn) as a function of cooling rate and type and quantity of starting waste materials; also, it avoids treatments with additives or fluxing agents and it provides, thanks to the dielectric properties shown, a strong potential for industrial use as a microwave absorber.