Hydrothermal Oxidation of Coarse Aluminum Granules with Hydrogen and Aluminum Hydroxide Production: The Influence of Aluminum Purity

This study is devoted to the hydrothermal oxidation of aluminum—the exothermic process in which hydrogen and aluminum oxide (or hydroxide) are produced. In this work, the influence of the chemical purity of aluminum on the conversion degree of coarse aluminum at hydrothermal oxidation was studied. Distilled water and coarse granules of aluminum with an average size of ~7–10 mm and three different aluminum purities—99.7, 99.9, and 99.99%—were used in the experiments. The oxidation experiments were carried out inside a 5 liter autoclave in an isothermal mode at temperatures from 200 to 280 °C, with a step of 20 °C. The holding time at the set temperature varied from 4 to 10 h. It was shown that the chemical purity of aluminum considerably influences the oxidation kinetics. More chemically pure aluminum oxidizes much faster, e.g., at a temperature of 280 °C and a holding time of 10 h, the conversion degree of granules with a chemical purity of 99.9% and 99.7% was less than 2%, while 99.99% aluminum was almost fully oxidized. The conversion degree of 99.99% aluminum decreased with the reduction in temperature and holding time, to 66–68% at 280 °C, 4 h, and 2–3% at 200 °C, 10 h.