Effect of mineral acids on the production of alumina nanopowder from raw bauxite

In this study a novel synthetic method for the large-scale production of spherical, high surface area and ultra-fine alumina (Al2O3) powder has been described. Synthetic Bayer liquor was extracted by alkali fusion of raw bauxite with sodium hydoxide. Alumina nanopowders were synthesised through a ball mill-aided precipitation method using the synthetic Bayer liquor and mineral acid precipitants. The powders produced were characterised by X-ray diffraction (XRD), particle size distribution (PSD), Fourier transform infrared spectroscopy (FTIR), Brunauer–Emmett–Teller surface area and pore size analysis, energy-dispersive spectroscopy (EDS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). In this article, the effects of precipitants such as H2SO4, HCl and HNO3 on crystallite and particle size, surface area, pore volume, and pore size and shape are reported. The experimental results prove that precipitation leads to an aggregated particle that is disaggregated by the ball-milling method. The ball milling process strongly influences the formation of uniform-sized spherical particles with a high surface area. It was revealed that nitric acid is an effective precipitant for controlling particle size and textural properties of Al2O3 powder. A nanopowder of γ-Al2O3 with an average crystallite size of 3 nm and an average particle size of 58 nm with a specific surface area (SSA) of 190 m2 g− 1 is produced. This article elucidates a new method with a simple reaction scheme for the mass production of Al2O3 nanoparticles from raw bauxite for various commercial applications.

Graphical AbstractThis study provides some support to the selection of mineral acid precipitants for the production of alumina nanopowder from synthetic Bayer liquor. It is observed that precipitation yields aggregated particles that are disaggregated by the ball-milling process. It is noteworthy that the ball-milling process develops reduced particle size with spherical morphology.Figure optionsDownload as PowerPoint slideResearch Highlights
► The spherical-shaped, cubic γ-Al2O3 nanopowder with 95.7% chemical purity was produced from raw bauxite.
► The effect of mineral acid precipitants (H2SO4, HCl and HNO3) on synthesis of alumina nanopowder from the synthetic Bayer liquor was established in terms of crystallite and particle size, surface area, pore volume, pore size and shape.
► The precipitants are the key factors for controlling the textural properties of alumina nanopowder. The obtained results reveal the selection of mineral acid precipitants for the production of alumina nanopowder from synthetic Bayer liquor.
► The use of ball-milling leads to disaggregation of aggregated particles and develops reduced particle size with spherical morphology. It is remarkable that stable γ-Al2O3 can be synthesised in a large quantity under neutral conditions with mineral acid precipitants.
► The present synthesis method can be used for mass production of highly shaped spherical alumina nanoparticles with smaller particle size and high surface area, which is more technological important.