Direct Observation of Key Aluminum Hydroxide Oligomers for Gibbsite Crystallization in Bayer Liquor by ToF-SIMS

The Bayer process, discovered over a century ago, is still not fully understood with regards to the mechanism of crystallization of γ-aluminum hydroxide (γ-Al(OH)3, gibbsite) from the Bayer liquor. In order to gain a better understanding of this process, we utilized liquid ToF-SIMS to directly observe the evolution of Al oligomers in a sodium aluminate solution simulating Bayer liquor. Our results were then validated through in-situ Raman, SAM-NMR, and DFT simulation. By comparing the change in the relative concentration of Al oligomers with +1 and -1 charge in environments favoring the crystallization of gibbsite and solid sodium aluminate, we were able to identify three major Al oligomer candidates: iso-tetramers, iso-pentamers, and cyclic hexamers. Our in-situ Raman results demonstrated that the relative concentration of iso-tetramers and iso-pentamers saturated when crystallized gibbsite appeared, while cyclic hexamers saturated before the appearance of gibbsite. Liquid SAM-NMR and DFT simulation provided evidence that the Al oligomers were inherent in the solution, rather than fragments from bigger molecules or crystals. We believe that the iso-pentamers and iso-tetramers exhibit a higher contribution than cyclic hexamers in terms of the crystallization of gibbsite. Our study expands knowledge of gibbsite crystallization in Bayer liquor, which is crucial for optimizing Al metal production and cleaning up liquid radioactive waste at the Hanford site.