Hydrothermal Valorization of Steel Slags—Part I: Coupled H2 Production and CO2 Mineral Sequestration

Abstract

A new process route for the valorization of BOF steel slags combining H2 production and CO2 mineral sequestration is investigated at 300°C (HT) under hydrothermal conditions. A BOF steel slag stored several weeks outdoor on the production site was used as starting material. To serve as a reference, room temperature (RT) carbonation of the same BOF steel slag has been monitored with in situ Raman spectroscopy and by measuring pH and PCO2 on a time-resolved basis. CO2 uptake under RT and HT are, respectively, 243 and 327 kg CO2/t of fresh steel slag, which add up with the 63 kg of atmospheric CO2 per ton already uptaken by the starting steel slag on the storage site. The CO2 gained by the sample at HT is bounded to the carbonation of brownmillerite. H2 yield decreased by about 30% in comparison to the same experiment performed without added CO2, due to sequestration of ferrous iron in a Mg-rich siderite phase. Ferric iron, initially present in brownmillerite, is partitioned between an Fe-rich clay mineral of saponite type and metastable hematite. Saponite is likely stabilized by the presence of Al, whereas hematite may represent a metastable product of brownmillerite carbonation. Mg-rich wüstite is involved in at least two competing reactions, i.e., oxidation into magnetite and carbonation into siderite. Results of both water-slag and water-CO2-slag experiments after 72 h are consistent with a kinetics enhancement of the former reaction when a CO2 partial pressure imposes a pH between 5 and 6. Three possible valorization routes, (1) RT carbonation prior to hydrothermal oxidation, (2) RT carbonation after hydrothermal treatment, and (3) combined HT carbonation and oxidation are discussed in light of the present results and literature data.

Image-Based Analysis of Weathered Slag for Calculation of Transport Properties and Passive Carbon Capture

“…While the estimated uptake of CO 2 in this study is only a fraction of ex situ carbonation which ranges from 107 to 283 kg CO 2 /1,000 kg slag (Chang et al, 2011), it reflects CO 2 uptake from the atmosphere achievable at no cost, compared to that of ex situ carbonation that requires energy-intensive operations, such as crushing of slag to the microscale and performing the reaction at elevated temperature and pressure that can reach 160°C and 4.8 MPa, respectively (Chang et al, 2011). Crouzet et al (2017) reported a passive CO 2 uptake equal to 63 kg CO 2 /1,000 kg slag (based on TGA analysis) in a slag heap at a French production site. Our calculated value of CO 2 uptake has the same order of magnitude of the value reported by Crouzet et al (2017) despite being lower.…”

“…Crouzet et al (2017) reported a passive CO 2 uptake equal to 63 kg CO 2 /1,000 kg slag (based on TGA analysis) in a slag heap at a French production site. Our calculated value of CO 2 uptake has the same order of magnitude of the value reported by Crouzet et al (2017) despite being lower. In addition to the differences in the measuring methods, several factors justify the observed difference.…”

“…As precipitation occurred, the connected space changed and consequently resulted in a reduction of fluid accessibility to the reactive site within the slag. The study of Crouzet et al (2017) showed that passive carbonation occurred at an early stage after slag production (i.e., slag aged in weeks had CO 2 content of 5.9 wt% while slag aged in years had CO 2 content of 6.3 wt%), possibly indicating reduced access of reactive components after precipitation which slowed the carbonation process.

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

“…In both hydrated slag, silica is bounded to Ca in the form of calcium silicate hydrates and Mg is incorporated into brucite, Mg(OH) 2 (Crouzet et al, 2017b), and no magnesium silicate is produced which could host iron in solid-solution. Crouzet et al (2017c) showed, however, that under high CO 2 partial pressure, iron carbonate can form and sequester iron which is no longer available for magnetite formation.…”