Na2CO3–Na2SO4 corrosion resistance of mullite castables

Abstract

Mullite castables, employed as the lining materials in high-salinity organic waste liquids incinerators, are subjected to severe alkali attack. The alkali corrosion resistance of four types of mullite castables based on Mullite60 (M60), Mullite45 (M45), Fire clay (FC), and Glass-rich mullite (GRM) materials, named CM60, CM45, CFC, and CGRM respectively, was investigated using a static crucible testing method with Na₂CO₃ and Na₂SO₄ under 1100°C in this study. The results showed that CGRM demonstrated exceptional resistance to Na₂CO₃ and Na₂SO₄. Among these four kinds of mullite materials, GRM stood out as it not only contained 54 wt.% of mullite but also unique glass phase that filled in the voids of the interwoven mullite crystals. The glass phase within the GRM aggregates provided a protective barrier against severe alkali corrosion for the mullite crystals by maintaining an adequate liquid phase. This liquid phase filled the micropores, resulting in an extremely dense corrosion layer. The high content and viscosity of the liquid phase formed within the CGRM matrix during alkali attack further impeded penetration, thereby CGRM exhibited good resistance to alkaline slag corrosion. However, the mullite phase in the aggregates of M60, M45, and FC was vulnerably corroded by the alkali due to lack of enough glass phase. This then led to the formation of corrosion products including nepheline, lazurite, and Na_1.65Al_1.65Si_0.35O₄, which induced internal stress and finally resulted in the surface cracking of the crucibles. Moreover, the presence of cristobalite in CM45 and CFC promoted the cracks propagation due to phase transformation. The large pores and cracks within the FC aggregates exacerbated the alkali corrosion in CFC. Therefore, GRM materials containing glass-rich liquid phase have great application prospects in mullite castables for enhancing resistance to alkaline slags.