Crystallization and vitrification behavior of CaO-SiO2-FetO-Al2O3 slag: Fundamentals to use mineral wastes in production of glass ball

Abstract In this study, we investigated the effect of CaO/SiO 2 (C/S) ratio on the crystallization behavior of the CaO-SiO 2 -Fe t O-Al 2 O 3 slag system, which simulates the mixed mineral wastes of red mud, mine tailings, and waste limestone, using a differential scanning calorimetry (DSC) technique. The crystallization behavior of both isothermal and non-isothermal cooling was simultaneously evaluated using X-ray diffraction (XRD) analysis and scanning electron microscopy (SEM) images. The calculated equilibrium phases, such as pseudo-wollastonite (ps-CaSiO 3 ) and melilite (Ca 2 [Al,Fe][Si,Al] 2 O 7 ), crystallized during the cooling process at different C/S ratios (=0.65–1.02). The C/S ratios were varied by simulating the various mixing ratios among red mud, mine tailings, and waste limestone. Even with a relatively low cooling rate of 5 °C/min, none of the exothermic peaks were detected during cooling, irrespective of the C/S ratio. However, pseudo-wollastonite and melilite phases in the slag samples were identified by the XRD and SEM analyses, except for the C/S = 0.65 slag. Ability of crystallization increased with increasing C/S ratio due to increased fluidity of the slags based on the structural consideration. As a result, our findings show that crystallization is increasingly difficult with a decrease in C/S ratio. Although the isothermal holding process remains between 900 °C and 1200 °C for a long period of time at relatively high basicity (C/S = 1.02) in the present system, weak and delayed crystallization was observed in the slag. Consequently, this study provides evidence that CaO-SiO 2 -Fe t O-Al 2 O 3 (C/S = 0.65–1.02) slags exhibit strong vitrification characteristics and control of melts viscosity is important to granulation process to produce commercial grade of glass ball by using mineral wastes such as red mud, mine tailing and waste limestone.