Hydrothermal Production of H2 and Magnetite From Steel Slags: A Geo-Inspired Approach Based on Olivine Serpentinization

Abstract

The interaction between ultramafic rocks and hot seawater at slow-spreading mid-oceanic ridges triggers hydrothermal redox reactions which are known to produce magnetite and H 2 under appropriate pressure and temperature conditions. Steel slags share some common properties with ultramafic rocks. They are composed of anhydrous and refractory minerals formed at temperatures exceeding 1,200 • C and they contain ferrous iron in comparable amounts. Consequently, when submitted to hydrothermal conditions both types of materials, natural and anthropogenic, are prone to form magnetite and H 2 according to the simplified redox reaction:is the ferrous iron component of the corresponding material that can be present under different mineralogical forms. Since H 2 and magnetite are two valuable products for applications in new technologies, the hydrothermal treatment of steel slags can be seen as a way to valorize a byproduct of the steel industry, a few tenths of a billion tons of which are produced yearly. The hydrothermal behavior of steel slags which arise from basic oxygen furnace (BOF) operations and that of olivine (Mg,Fe) 2 SiO 4 , the main mineral constituent of abyssal peridotites, are described here based on data from the literature. The thermochemical characteristics of Reaction 1 are reviewed for both types of materials in the perspective of optimizing a process that would valorize BOF steel slags for the production of nanomagnetite (and high-purity H 2 ). In particular, the kinetics effect of temperature, pH and solution-to-solid mass ratio on the hydrothermal oxidation of wüstite (FeO), considered here as an analog of the ferrous-iron component of steel slags, are modeled. The possible role of additives (impurity) on the hydrothermal oxidation of wüstite through the catalysis of the water-splitting reaction is discussed. Finally, the lack of kinetics constraints on nanomagnetite growth under hydrothermal conditions in a wide range of pH is identified as a major gap in the understanding of two important issues: (1) the catalysis of abiotic molecules in the course of serpentinization reactions, and (2) the tailoring of the size of the magnetite produced by hydrothermal treatment of BOF steel slags.

Hydrogen Emanations in Intracratonic Areas: New Guide Lines for Early Exploration Basin Screening

“…However, the overall assessment and the kinetic of this redox equation (roughly 3[FeO] + H 2 O => Fe 3 O 4 + H 2 ) remain also poorly known. They appear to be strongly dependent on the temperature but also on the presence of catalyzers [17]. Nevertheless, the Iron oxidation is the more probable source of many of the observed H 2 onshore emanations, it is largely present associated with Silicon and Magnesium within the olivine and orthopyroxene in the oceanic crusts.…”

“…We consider that the O_1 is the more probable, it means that the H 2 may come from water/rock interactions in the crust. Knowing the kinetics of these reactions will be key for the H 2 production since we are talking about a flow and not about fossil resources; research is active in that domain [17]. Nevertheless, we will leave the question of origin and flow rate open, they are not the topic of this article, and discuss the favorable geological settings and the surface evidences which indicate that a H 2 generation could be currently active in a given area.…”

A Review of H2, CH4, and Hydrocarbon Formation in Experimental Serpentinization Using Network Analysis

“…As for "Fe-bearing" phases, oxidation of Fe(II) in magnetite and of Ni or reduced sulfur in Ni-bearing phases such as alloys or sulfides may intrinsically produce more H 2 without a true catalytic effect occurring. Indeed, moderate amounts of Ni metal particles have recently been identified as catalysts of FeO oxidation under hydrothermal conditions (Michiels et al, 2018) and this effect may be extrapolated to Fe-bearing minerals such as olivine (Brunet, 2019). Conversely, H 2 concentrations in spinel-bearing experiments cover the whole range of concentrations despite run duration or P-T conditions (all communities) and always lie below the maximum values observed, which explains the negative correlation observed in Figure 4.…”

Dataset for H2, CH4 and organic compounds formation during experimental serpentinization

“…The dataset can also be extended to other reactions under similar conditions using different solid reactants, such as mine-tailing products, to produce H 2 , CH 4 and other carbon species (e.g. Kularatne et al, 2018;Michiels et al, 2018;Brunet, 2019). We hope that this dataset and the analysis by Barbier et al (2020) stimulate complementary experiments that could fill the identified gaps; this would allow editing future versions of this dataset in a few years and potentially help people better understand the fate of carbon under highly reducing conditions such as the one produced during serpentinization.…”