The phase transition in Bayer red mud from China in high caustic sodium aluminate solutions

• Cancrinite converts to zeolites in high NaOH and low Al2O3 liquors with no lime.
• Transition from cancrinite to CaNaHSiO4 is a fast process.
• Isomorphous substitution is important in recovering Al2O3 from red mud.
• Addition of CaNaHSiO4 accelerates these two reactions for Al2O3 recovery.

The phase transitions of the main substances in Bayer red mud in high caustic sodium aluminate solutions were studied. Without addition of lime, cancrinite was not found to transform to CaNaHSiO4 even with 16 wt.% CaO present in the original residue up to 270 °C. However, this transition was verified to be a fast reaction which was completed in just 10 min with the temperature higher than 240 °C after adding lime, thus enabling the extraction of Al2O3 from the red mud. Whether additional CaO was supplemented or not, the isomorphous substitution of Fe to Al atoms occurred in hydrated andradite as long as the temperature was more than 240 °C, providing another way to extract Al2O3 from Bayer red mud. Accordingly the A/S (weight ratio of solid Al2O3 to SiO2) of red mud would reduce to 0.134 after the complete transition from Ca2.93Al1.97Si0.64O2.56(OH)9.44 to Ca3(Fe0.87Al0.13)2(SiO4)1.65(OH)5.4 in our study, assuming no other Al or Si containing phases. Furthermore, the addition of CaNaHSiO4 seed was confirmed to accelerate both the transition of cancrinite to CaNaHSiO4 and the reaction of isomorphous substitution in the andradite.