Observation of sodium titanate and sodium aluminate silicate hydrate layers on diaspore particles in high-temperature Bayer digestion

Abstract During the high-temperature Bayer process, the inhibitory effect of titanium-containing minerals on diaspore digestion is believed to be resulted from the formation of a dense sodium titanate layer covering the diaspore particles, however, there is a lack of direct observational evidence to date. This work adopted X-ray photoelectron spectroscopy (XPS), cross-section SEM and in-situ analysis to clarify the inhibitory mechanism by investigating the interactions between Al-/Ti-/Si-bearing minerals in the simulated high-temperature Bayer digestion process. The observation of a sodium titanate layer provided direct evidence for the solid layer theory. XPS result further indicates that the Ti-O-Al bond is a significant factor in promoting the covering of sodium titanate on alumina-bearing mineral particles. Cross-section SEM and in-situ analyses illustrate that the dissolution of diaspore is carried out not only on the surface of diaspore particles but also on the inner surface such as pores and gaps of particles. Meanwhile, as similar to Ti-bearing minerals, the Si-bearing minerals also inhibit the diaspore digestion by forming a sodium aluminate silicate hydrate layer at an elevated temperature.