Evolution of crystals in SiO2-Fe2O3-Al2O3 system ceramics derived from copper slag

ABSTRACT Copper slag is a kind of huge metallurgical residue with fayalite as the main phase. Due to the oxidation of fayalite to quartz and hematite during sintering process, quartz became the main phase of ceramics when it contained large proportion of copper slag. The internal stress in the ceramic formed due to the change of α to β increases the potential risk of cracks in the quartz cooling process. In this paper, in order to reduce the precipitation of quartz phase, natural diopside rich in CaO and MgO was introduced into the copper slag-based ceramics. A batch of ceramics were prepared with 50 wt.% copper slag and 5 ~ 25 wt.% diopside as raw materials at different sintering temperatures. Evolution of crystals and densification process for the ceramics were investigated through physical property tests, including XRF, XRD, SEM, EDS and FACTSAGE calculation. The results indicated that fayalite particles derived from the slag formed a core-shell structure during the oxidation process, with the inner core dominated by silica and the outer shell enriched by iron oxide. It was observed that MgO and Fe2O3 reacted outside the shell to form mafic spinel (Fe, Mg) Fe2O4, which greatly promoted the reaction of Ca2+ with SiO2 in the core areas to form anorthite. As the amount of diopside increased, more mafic spinel and anorthite formed, while less quartz appeared. The addition of diopside to the ceramic body did not change its initial sintering temperature at 1170°C but significantly improved its bending strength from 35.8 MPa to 50.3 MPa with 5 ~ 20 wt.% addition of diopside, and more diopside would result in excessive flux and over firing.