Electrochemical preparation of precursor phases for obtaining alpha-alumina from aluminium scrap

This work presents a study to obtain α-Al2O3 from thermal treatment of the precursor α-Al(OH)3 (bayerite). The precursor was prepared from a cathodic electrosynthesis employing direct current (DC) and alternating current (AC) electrochemical techniques and using an aluminium solution prepared with scrap aluminium cans. Characterization techniques including X-ray diffraction (XDR), scanning electron microscopy (SEM) and thermogravimetric analysis (TG) were used to analyse the thermal behaviour, phase transformation, morphology and particle size of the products obtained. The range of potentials for electrodeposition was between −2.0 and 2.4 V with both DC and AC techniques. The presence of crystalline bayerite in the deposits obtained during all DC and AC experiments was observed, although a species of aluminium oxide (Al2.427O3.64) with AC was also identified. The surface morphology of the deposit with DC presented a compact uniform film, whereas with AC a granular form morphology with compact grains in the order of 1-3 µm was evident. In addition, the formation of an amorphous and low crystallinity bayerite precipitate was obtained from the solution, which presented a surface morphology of non-uniform fine grains and agglomerates. The thermal behaviour (TG) indicates three regions of change in the phase, which were verified via the thermal treatment; a transition of bayerite to η-Al2O3 in a temperature range between 290 and 300 °C and subsequently to α-Al2O3 at a temperature of 1100 °C was determined. The α-Al2O3 presents high purity, a surface morphology with fine grains and agglomerates in the order of 1-10 µm and an appreciable porosity.