Vogrin, J., Vaughan, J., Peng, H., Santini, T.

Increasing levels of reactive silica in bauxites can present significant challenges for the alumina industry. This study details a systematic approach to the synthesis and characterisation of Bayer process desilication products (DSPs). DSPs were synthesised from synthetic Bayer liquors at industry relevant conditions spanning gradients of digest time and temperature, anion concentrations, and Si sources. Characterisation techniques including conventional and synchrotron XRD, ICP and SEM were undertaken to describe crystallographic properties, phase distribution, chemical composition and particle morphology. It was found that at concentrations of 4-5 M NaOH and 0.5-2 M Al(OH)3, the anions shifted XRD peak positions with the order: Al(OH)4- > SO42- > CO32- ≥ Cl-. The presence of anions also promoted the phase transformation from sodalite to cancrinite following a similar order, except for the aluminate anion. Rietveld refinement was calculated the average size of the sodalite unit cell, ranging from 8.89 to 9.02 Å depending on the solution composition. Varying the digest time, digest temperature and silicate source also influenced the phase transformation but otherwise did not significantly influence the DSP crystal structure. Predesilication Al(OH)3 concentrations led to an increase in aluminium content, however no other parameters significantly affected the Na2O or Al2O3 composition of DSPs. Although the best existing structures to describe the DSPs formed in this study were identified, further work is needed to fully describe their structures, especially in NaOH-NaAl(OH)4 systems.