Valorization of Aluminum Chips into γ-Al2O3 and η-Al2O3 with High Surface Areas via the Precipitation Route

Abstract

In this study, pseudo-boehmite and bayerite precursors have been prepared from aluminum chips collected from metal manufacturing industry in the region of Sfax, Tunisia for the production of γ-Al2O3 and η-Al2O3, respectively. The alkaline attack of the scraps with 2 M NaOH solution provides the sodium aluminate solution which was neutralize with 2 M HNO3 acid solution at room temperature to different pH values of 6–10. The neutralization pH is an important parameter that controls the nature of the precipitated phase. At pH 6, the XRD pattern of the as-dried sample showed the characteristic peaks of well crystallized bayerite. The increase of the neutralization pH to 7–9, there is precipitation of pseudo-boehmite. At pH 10, we obtained a mixture of bayerite and pseudo-boehmite. Through the calcination of bayerite at 400 °C, η-Al2O3 was obtained with specific surface area of 455 m2/g, whereas the calcination of pseudo-boehmite leads to globular aggregate made by fibrous crystals of γ-Al2O3. During the structural transformation of pseudo-boehmite into γ-Al2O3 and bayerite into η-Al2O3 a portion of the aluminum atoms migrate from octahedral sites to tetrahedral sites in the alumina lattice. The tetrahedral to octahedral Al ratios are about 1–3 and 1–4 for γ-Al2O3 and η-Al2O3, respectively.