Hazardous residues from ferronickel production are vitrified. The optimum thermal regime for the conversion of the resulting glass into glass-ceramics is estimated by rapid alternative methods: the nucleation step is valued by DTA, and the crystallization step by density measurements. The results show that a short and inexpensive thermal treatment can be used. The evolution of phase formation is highlighted by XRD, SEM–EDS, FESEM and TEM. Due to the presence of 1.5 wt % chromium oxides and high amounts of iron and magnesium oxides, the crystallization process is peculiar. It starts during the melt’s cooling with the precipitation of preliminary Fe-Mg-Cr spinel crystals, which then act as centers for growth of pyroxenes. Simultaneously, due to liquid–liquid immiscibility, the main amorphous phase forms a nonhomogeneous binodal structure, which becomes finer after the nucleation treatment. Subsequently, thus formed iron-rich drops are transformed into tiny secondary magnetite spinel, acting as nuclei for the main crystallization process. As a result, new pyroxenes with sizes of about 200 nm are formed and the total crystallinity reaches about 60%. Regardless of the large amounts of problematic wastes in the batch, the resulting glass shows good chemical durability, while the glass-ceramic is characterized by optimal mechanical characteristics.
Read full abstract