Crystallization process of thermally treated vitrified EAFD waste

Electric arc furnace dust (EAFD) powder was mixed and vitrified with 55% SiO2, 10% Na2CO3 and 35% CaCO3 raw materials. The resulting glass was thermally treated at the same temperature at different annealing times. The process towards crystallization of the glass–ceramics produced, as a function of annealing time, was examined by means of optical microscopy, electron microscopy techniques and X-ray diffraction. While at small crystallization times wollastonite was the only crystalline phase separated from the amorphous matrix, as the time of crystallization increased crystals of diopside emanating from wollastonite were also observed. The emergence of diopside crystals was attributed to the tendency of the metallic elements of EAFD, incorporated in the wollastonite structure, to move towards the neighbouring glass matrix causing local maxima of concentration in the vicinity of wollastonite crystals. This diffusive motion of the metallic elements of EAFD, such as Fe, Zn, was found to follow the 2nd Fick law and plays a crucial role in the stability region of the crystallized structures of the system.