Influence of chloride on sodium aluminosilicate solubility in Bayer liquor

The solubility of sodium aluminosilicate and the effects of NaCl in the NaOH-NaAl(OH)(4)-NaCl system were studied under equilibrium conditions in synthetic Bayer liquors. Sodium aluminosilicate was hydrothermally synthesised from kaolin in a rotating bomb heater at 90 degrees C, 150 degrees C and 250 degrees C with NaCl concentration spanning 0-0.40 M. Cl- incorporation into sodium aluminosilicate increased with increasing NaCl concentration and reaction temperature to a maximum occupancy of 85%, primarily substituting for OH-, and to a lesser extent, Al (OH)(4)(-). NaCl enhanced desilication (measured by solution SiO2 removal) by up to 70% compared to control samples, although a diminishing effect was observed as chloride concentration increased. X-ray diffraction analyses revealed that the precipitates contained a mixture of hydroxide and chloride-type sodalites, hydroxide and Bayer cancrinites, along with X-ray amorphous material. The presence of NaCl promoted the formation of spherical nano-sized particles instead of polyhedral micron-sized particles, and increased the thermal stability of the desilication product. Results in this study provide fundamental insights into the relationship between the chloride ion and the composition, structure and solubility of sodium aluminosilicate, required for the development of kinetic and thermodynamic models.