PNNL

Abstract

This study explores mineral carbonation of industrial stainless steelmaking slags and relevant synthetic constituent minerals via in situ pressurized X-ray diffraction, to clarify carbonation reaction pathways and efficiency for carbon storage and waste valorization. The primary mineral phases of Argon Oxygen Decarburization (AOD) and Continuous Casting (CC) slags, namely, akermanite (Ca2MgSi2O7), bredigite (Ca7Mg(SiO4)4), cuspidine (Ca4Si2O7(F,OH)2), merwinite (Ca3Mg(SiO4)2), and ß- and ?-C2S (Ca2SiO4), were reacted in a custom-built beryllium-capped XRD reactor filled with either water-saturated (wet) gaseous (70°C, 6 bar) CO2 (CO2(g)), or wet supercritical (50 °C, 82 bar) CO2 (scCO2), in a series of carbonation experiments. Overall, formation of calcite and aragonite was observed for most Ca-bearing minerals, while cuspidine carbonation resulted in the initial transient precipitation of metastable vaterite. Bredigite and merwinite showed similar rates of conversion, with aragonite and hydromagnesite formation. Unexpectedly, negligible carbonate formation was detected during akermanite carbonation. Carbonation of AOD and CC slag in scCO2 resulted in hydrated crystalline calcium carbonates, with indirect evidence of amorphous carbonate phases. No consistent differences in carbonation efficiency between wet CO2(g) and wet scCO2 was observed, suggesting that low-pressure gaseous carbonation routes are more attractive for stainless steelmaking slag carbonation process applications.