Magnesia- and aluminabased refractory ceramics within the MgO-Al2O3-ZrO2-SiO2 system

Abstract

The main target of this work is to prepare refractory magnesia- and aluminabased ceramics within the MgO-Al₂O₃-ZrO₂-SiO₂ system. These include periclase (MgO) and its composites with MgAl₂O₄-spinel and forsterite (MgSiO₄)-zirconia (ZrO₂) as well as corundum (α-Al₂O₃) and its composite with mullite (Al₆Si₂O1₃)- zirconia (ZrO₂). These were processed from recycled pure and fine magnesium and aluminium hydroxide precipitates as well as raw zircon, ilmenite and rutile raw materials. Up to 5.0% lmenite and rutile as well as Mg(OH)₂ , Al(OH)₃ or MgO were used as doping materials to accelerate sintering rate of the periclase and corundum products. Also 5-20% of calcined alumina and raw zircon were mixed with the magnesium as well as aluminium hydroxides to prepare varieties of periclase-spinel, periclase-forsterite-zirconia as well as corundum-mullitezirconia composites, respectively. The corresponding batches were prepared, semi-dry pressed and fired at two-stages up to 1600 °C. The densification rate was followed by determining linear shrinkage, bulk density and apparent porosity. Phase composition as well as microstructure and microchemistry were investigated by XRD as well as EPMA techniques, respectively. The refractory quality was assessed by determining load-bearing capacity and volume stability up to1600 °C. The results indicate that doping magnesia with 1% rutile accelerates its rate of solid-state sintering by lattice diffusion with a maximum relative density (RD) of 97%. The dense bodies have developed periclase network with a high degree of direct bonding and encloses some fine euhedral spinel [MgO(Al,Fe,Ti)₂O₄] solid-solutions. Similarly, RD of the alumina is improved up to a maximum of only 93%, after doping with 1.0 % MgO. The dense bodies are mainly composed of rounded corundum (α-Al₂O₃) network enclosing some pores. This indicates that such doping material and/or firing temperature (1600 °C) are insufficient for sintering of the corundum bodies. Meanwhile, dense periclasespinel and periclase-forsterite-zirconia as well as corundum-mullite-zirconia composite bodies could be prepared. All of the processed periclase- and alumina- based bodies show high refractory quality, except those of the corundum as indicated from the high temperature corresponding to maximum expansion (To = ≥ 1500 °C) and beginning of subsidence under load (Ta ≥ 1500 °C) and rate of creep at 1500 °C (<0.02 mm/hr) as well as limited permanent linear change (PLC) on re-firing at 1600 °C (<1.0%).