Submicron γ-, γ’-, θ-, and κ-Al 2 O 3 Powders from Alkaline Waste

Hydrated γ-, γ’-, θ-, and κ-Al 2 O 3 powders were precipitated with nitric acid, HNO 3 , from Na[Al(OH) 4 ] alkaline solutions at pH = 5.0–5.5. Waste materials from α-B 12 Al and B 12 (C–Al–C) laboratory production were used. The precipitates were rinsed and dried at 420 → 570 K (S specific BET = 213 m 2 ⋅ g –1 , d CSD crystallites ≈ 10 nm, and d SEM agglomerates ≈ 200 nm). The samples were characterized by X-ray diffraction, X-ray diffractometry in the coherent scattering domain (CSD), fluorescence analysis, scanning electron microscopy (SEM), chemical elemental and phase analyses, and thermal desorption of nitrogen calculated by the BET method (S specific BET ) for interpreting surface measurements of nonporous bodies. The concentrations of gas-forming elements (hydrogen, nitrogen, oxygen, and carbon) were determined by reductive and oxidative extraction in helium and oxygen flow, gas chromatography, and coulometry. The multiphase γ-, γ’-, θ-, and κ-Al 2 O 3 samples treated at 1370–1470 K in air exhibited an α-Al 2 O 3 structure. According to X-ray diffraction in hkl 012 CSD, the α-Al 2 O 3 crystallites had d CSD ≈ 48 nm. Based on SEM analysis, the sizes of the α-Al 2 O 3 powder agglomerates did not exceed d SEM = 200–300 nm. The specific surface area of the powder, S specific BET , determined by thermal desorption of nitrogen calculated with the BET method, was equal to 8.6 m 2 ⋅ g –1 . The weight content of α-Al 2 O 3 was 99.69%, while SiO 2 impurities accounted to 0.31%, according to X-ray fluorescence analysis. The crystallites, as components of the alkaline γ-, γ'-, θ-, κ-Al 2 O 3 , and α-Al 2 O 3 powder agglomerates, showed a lamellar shape. The thickness of the lamellas was close to the calculated d CSD values for crystallites. The submicron γ-, γ’-, θ-, and κ-Al 2 O 3 particles had a ‘sandglass’ shape, determined by the dynamics of precipitating flat crystallites of the Al(OH) 3 solid phase (nanosized thickness) as pH decreased. The α-Al 2 O 3 agglomerates consisted of fused local particles connected by bridges.