International audienceHe production of H2 by oxidation of FeO, taken here as model compound for steel slags, has been investigated both in pure water and under acidic aqueous conditions in the 373–573 K temperature range. Whereas after 65 h, H2 yield was negligible in pure water at 423 K, the reaction 3 FeO(s) + H2O(l) → Fe3O4(s) + H2(aq) reached near completion at the same temperature within 10 h in a solution containing 0.05 mol/l acetic acid. Increasing acetic acid concentration by one order of magnitude did not yield significantly more H2. At identical initial pH, acetic acid was found to be more efficient than oxalic acid and hydrochloric acid at enhancing H2 production. Acidic conditions increased FeO dissolution kinetics and, consequently, improved H2 yield. The specific efficiency of acetic acid resides in its thermal stability as well as in the potential of ligand-promoted Fe(II) dissolution. We show that the positive kinetics effect of mild acetic acid solutions over H2 yield evidenced on FeO does not apply directly to steel slags which buffer the pH to high values due to the presence of large amounts of CaO
The effect of acidic conditions (in a pH range of 3 to 6) and temperature on the kinetics of the hydrothermal oxidation of ferrous iron contained in BOF steel slag has been tested in the 150-350 • C range for acid acetic concentrations from 0 to 4 M. Reaction progress was monitored with the amount of produced H 2 . Higher temperature and lower pH are found to enhance the hydrothermal oxidation kinetics of the slag. These two parameters are believed to increase iron dissolution rate which has already been identified as the rate limiting step of the hydrothermal oxidation of pure FeO. An activation energy of 28 ± 4 kJ/mole is found for the hydrothermal oxidation of the steel slag which compares very well with that of pure FeO under similar conditions. In the case of the slag run in water at 300 • C for 70.5 h, magnetite product has been separated magnetically and characterized. Particles were found to fall in three size ranges: 10-30 nm, 100-300 nm, and 1-10 µm. The smallest fraction (10-30 nm) is comparable to the 10-20 nm size range that is achieved when nanomagnetite are synthesized by co-precipitation methods. Obviously, the production of nanomagnetite enhances the economic interest of the hydrothermal processing of steel slags, which has already proven its capacity to produce high-purity H 2 .
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.
Single-phase marićite, NaFePO4, was synthesized from monosodium phosphate and iron oxalate at 750°C, at atmospheric pressure. Thermal treatment of synthetic marićite in air indicated oxidative decomposition into Na3Fe 3+ 2(PO4)3 nasicon and-Fe2O3 at temperatures above 225°C. Intergrowth of the reaction products is found to occur at the nanoscale without identified crystallographic relationship with the marićite precursor. Electrochemical activity of the reaction product is confirmed with the reversible insertion of one Na at 2.55 V vs Na + /Na 0 .
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