SUBMICRON g-, g’-, q-, and k-Al₂O₃ POWDERS  FROM ALKALINE WASTE

Abstract

Hydrated γ-, γ’-, θ-, and κ-Al₂O₃ powders were precipitated with nitric acid HNO₃ from Na[Al(OH)₄] alkaline solutions at pH = 5.0–5.5. Waste of α-B₁₂Al and B₁₂(C–Al–C)  laboratory production was used. The precipitates were washed and dried at 420 → 570 K (S_{specific BET} = 213 m² × g^–1, d_{CSD crystallites} » 10 nm, and d_{SEM agglomerates} » 200 nm). The samples were certified 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 BET (S_{specific BET}). Gas-forming elements (hydrogen, nitrogen, oxygen, and carbon) were determined by reductive and oxidative extraction in a helium and oxygen flow, gas chromatography, and coulometry. The multiphase γ-, γ’-, θ-, and κ-Al₂O₃ samples treated at 1370–1470 K in air had the α-Al₂O₃ structure. According to X-ray diffraction in CSD hkl₀₁₂, the α-Al₂O₃ crystallites had d_CSD » 48 nm. The sizes of the α-Al₂O₃ powder agglomerates, according to SEM, did not exceed d_SEM = 200–300 nm. According to thermal desorption of nitrogen, calculated with the BET method, the specific surface area of the powder, S_{specific BET}, was equal to 8.6 m² × g^–1. The weight content of α-Al₂O₃ was 99.69% and that of SiO₂ impurities was 0.31%, according to X-ray fluorescence analysis. The crystallites, as components of the alkaline γ-, γ'-, θ-,  κ-Al₂O₃, and α-Al₂O₃ powder agglomerates, were lamellar in shape. The thickness of the lamellas was close to the calculated d_CSD values for crystallites. The submicron γ-, γ’-, θ-, and κ-Al₂O₃ particles had the ‘sandglass’ shape, determined by the dynamics of precipitating the flat crystallites of the Al(OH)₃ solid phase (nanosized thickness) with decreasing pH. The α-Al₂O₃ agglomerates were fused local particles connected by bridges.