Hydrogen production from hydrolysis of magnesium wastes reprocessed by mechanical milling under air

Abstract

Magnesium-based wastes were reprocessed by mechanical milling under air atmosphere and used to produce hydrogen by hydrolysis on a laboratory scale. The evolution of the material during reprocessing and the generation of hydrogen in a 0.6 M MgCl2 aqueous solution at 24 °C are reported. The morphology, microstructure and phase abundance change with milling time. During mechanical processing, (i) particle size and crystallite size reduce, (ii) microstrain accumulates in the material, (iii) Al dissolves in Mg, (iv) the amount of Mg17Al12 (β-phase) increases and (v) small quantities of Fe from the milling tools are incorporated in the material. By hydrolysis, hydrogen yields in the 70–90% range after 30 min of reaction have been obtained, depending on milling time. Reactants are not exhausted during the hydrolysis reaction in the saline solution, due to the formation of a Mg(OH)2 layer that produces a passivating effect. Higher generation has been observed for larger particles and for materials reprocessed for longer milling times. Reaction kinetics also improves with milling time, with faster rates observed for the smaller particles. The shape of the hydrolysis curves can be fitted with a model that corresponds to a reaction limited by a three dimensional geometric contraction process. Mg17Al12 and Fe favor hydrogen production by acting as micro-galvanic cathodes during the reaction.