Activation mechanism of diasporic bauxite calcined with sodium carbonate

A series of sodium aluminosilicates (Na2-xAl2-xSixO4, x = 0.85, 0.55, 0.45, 0.35, 0.25 and 0.05) can be generated in the soda sintering process, some of which can be dissolved in sodium aluminate solution. This paper focuses on the transformation of aluminum-bearing minerals into Na2-xAl2-xSixO4 during the calcination of diasporic bauxite with different proportions of Na2CO3 at 400–1100 °C. Based on thermodynamic analysis, the thermal behaviors, phase transformations and bonding changes of the aluminum-bearing minerals are investigated using thermogravimetric analysis and differential scanning calorimetric (TGA-DSC), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR) techniques. In addition, the dissolution performances of some Na2-xAl2-xSixO4 are verified by water leaching. The results demonstrate that the aluminum-bearing minerals in diasporic bauxite are diaspore and kaolinite, which are converted into Na2-xAl2-xSixO4 after calcination with Na2CO3 at 700–1100 °C. Increasing the calcination temperature in the range of 700–1000 °C and the proportion of Na2CO3 results in Na2-xAl2-xSixO4 having lower x value and decomposing into NaAlO2 and Na2SiO3 if Na2CO3 is present in excess. As the mass ratio of bauxite to Na2CO3 is 1, Na1.95Al1.95Si0.05O4 is primarily formed at 900–1000 °C, which has excellent dissolution performance, close to that of NaAlO2. Upon further increasing the calcination temperature to 1100 °C, Na1.95Al1.95Si0.05O4 transforms into Na2-xAl2-xSixO4 with a higher x value, which has worse dissolution performance.