Submicron γ-, γ’-, θ-, and κ-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 materials from α-B₁₂Al and B₁₂(C–Al–C) laboratory production were used. The precipitates were rinsed 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 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-Al₂O₃ samples treated at 1370–1470 K in air exhibited an α-Al₂O₃ structure. According to X-ray diffraction in hkl₀₁₂ CSD, the α-Al₂O₃ crystallites had d_CSD ≈ 48 nm. Based on SEM analysis, the sizes of the α-Al₂O₃ 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 m² ⋅ g^–1. The weight content of α-Al₂O₃ was 99.69%, while SiO₂ impurities accounted to 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, 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-Al₂O₃ 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-Al₂O₃ agglomerates consisted of fused local particles connected by bridges.