Bayerite in aluminum hydroxide effecting the thermal decomposition pathways and reducing density of α-Al 2 O 3

Various phase evolutions of alpha-Al2O3 determine efficiency of aluminum electrolysis and the performance of alumina-bearing materials. Bayerite, an impurity of gibbsite, may complicate the structure and properties of alpha-Al2O3. Biphasic aluminum hydroxide (Al(OH)3) containing bayerite and gibbsite was precipitated from a concentrated sodium aluminate solution by adding NaHCO3, H2O2, and Al2(SO4)3. Superfine gibbsite containing bayerite with good crystallization and uniform particle size transformed (or evolved) into chi-, gamma-, delta-, theta-, and alpha-Al2O3, and bayerite followed the thermal decomposition pathway of eta-, theta-, and alpha-Al2O3. Induced by bayerite, gibbsite with poor crystallization, high surface energy, and coarse particles then readily transformed into boehmite, followed by gamma-, delta-, theta-, and alpha-Al2O3. The superfine bayerite converted into boehmite, gamma-, delta-, theta-, and alpha-Al2O3, whereas the thermal decomposition pathways of the superfine gibbsite were gibbsite chi-, kappa-, and alpha-Al2O3. This observation is mainly attributed to boehmite changing the thermal decomposition pathways of gibbsite through boehmite acting as a seed. The transition from chi-Al2O3 to gamma-Al2O3 was found in the mixture of uniformsized gibbsite and bayerite without particle agglomeration below 700 degree celsius. This is possibly due to eta-Al2O3 from bayerite favoring the reorganization from the chi-Al2O3 (hexagonal close-packed structure) to the gamma-Al2O3 (facecentered cubic structure) by eta-Al2O3 acting as a structural template. Furthermore, bayerite changing the thermal decomposition pathways of gibbsite with high AlO4 structural unit content reduced alpha-Al2O3 density, whereas pure gibbsite received high-density alpha-Al2O3 with maximum AlO6 structural units. The mechanism and effect of bayerite on gibbsite were then illustrated. The results provide a novel approach for producing alumina with high activity in aluminum electrolysis and improving the performance of alumina-bearing materials by regulating bayerite content in aluminum hydroxide.