Applicability of fine industrial metallic iron-rich waste powders for hydrothermal production of hydrogen gas: The influence of non-ferrous contaminants

Abstract

Low to negative cost fine industrial metallic iron-rich waste powders are available in large amounts all over the world and are currently often landfilled. The goal of this paper is to investigate the applicability of such waste powders as raw materials for a recently developed hydrothermal hydrogen gas production method, optimised for pure metallic iron powder and operating at a mild hydrothermal temperature of 160 °C for 16 h. The influence of several metallic, oxide and salt compounds of elements (M) that are commonly present in metallic iron-rich powder wastes was systematically investigated for two concentration levels, (M:Fe)low and (M:Fe)high. The kinetics of hydrogen gas formation was measured during reaction and the obtained solid residues were analysed. This work shows that the oxidation of metallic iron particles and the consequent H2 gas production in the investigated hydrothermal reaction system were inhibited through three main effects by the here studied contaminants, namely passivation of the reaction surface, carbonate precipitation and/or redox effects. On the other side, Ni addition promotes the reaction kinetics and acts as a catalyst. However, an excess of Ni inhibits complete iron oxidation through fast precipitation of Fe3O4 on the metallic iron particle surface, forming a passivation layer. Finally, three industrial iron waste powders were treated according to the same hydrothermal treatment. Hydrogen gas formation for an iron-rich foundry sand (IWP-A) was inhibited by the formation of a silicate deposit on the iron particles, while the hydrogen gas production from iron works’ waste powders (IWP-B and IWP-C) increased with decreasing particle size of the metallic iron particles in the powder.