Formation of nepheline in the heating process of mould fluxes and its effect on sintering behaviour

Abstract

This study explores the formation mechanism of nepheline (NaAlSiO4) in the 500–900 °C heating process of mould flux and the influence of Al-containing materials on the nepheline formation using thermogravimetric-differential scanning calorimetry, scanning electron microscope and X-ray diffraction. The results show that the formation behaviour of nepheline depends on the type of Al2O3-containing materials. At 500 °C, flint clay (Al2SiO5) forms nepheline by solid-state reaction with sodium carbonate (Na2CO3). During the solid-state reaction process, F− improves the kinetic conditions by promoting the diffusion of Na+ into the flint clay particle. F- is derived from the tiny NaF formed by the reaction of cryolite (Na3AlF6) with sodium carbonate. Unlike flint clay, bauxite (Al2O3) reacts with liquid phase at 700 °C to form nepheline. Therefore, the low-temperature sintering of mold flux bauxite-containing is weaker than the flux containing cryolite and flint clay. At 500 °C, the sintered rate of bauxite-containing flux is 22.55%, and the sintered rate of flux containing cryolite and flint clay is 39.50%. With the increase in temperature, the sintered rate of bauxite-containing flux increases, which is gradually close to that of flux containing cryolite and flint clay. Until 900 °C, the sintered rate of bauxite-containing flux (63.45%) is the same as that of flux containing cryolite and flint clay (62.15%).