Johan Fagerlund

Mineral carbonation is a carbon dioxide capture and storage (CCS) route that warrants further investigation. Although most of the CCS research to date has been concerned with underground storage in liquefied form, mineral carbonation is the only method that disposes CO₂ in a permanent and inherently safe manner. Here, we consider the gas–solid conversion of both MgO and Mg(OH)₂ with CO₂ in the presence and absence of steam in an attempt to model and predict the optimum conditions for rapid and complete carbonation. Results from pressurised thermogravimetric analysers (PTGA) and a laboratory scale pressurised fluidised bed (PFB) are presented. The results show that the carbonation of Mg(OH)₂ is much faster (∼50% in 4 min) in a PFB than the carbonation of comparatively fine MgO (2 carbonation. In the case of MgO, steam (>10%) accelerates the carbonation considerably. However, in the case of Mg(OH)₂, the addition of steam to the CO₂ is less important as it is provided intrinsically, as a result of the dehydroxylation of Mg(OH)₂ at elevated temperatures. Still, humidifying the gas stream can help control dehydroxylation, thereby sustaining carbonation, which typically levels out short of completion. A careful control of the carbonation conditions (temperature, pressure, fluidising velocity, gas composition) and particle properties should allow for close to complete carbonation (>90%) without compromising the carbonation kinetics. Because the PFB carbonation step considered here is part of a larger CCS process (Mg extraction from a natural and abundant mineral followed by production of MgCO₃), the precipitation stage [Mg(OH)₂ formation] may be tailored to obtain the necessary particle properties (surface area, porosity).

This paper concerns the growing role of cheap and potentially recyclable ammonium salts in CO₂ mineralization. The powerful hyphenated technique TG-FTIR, along with XRD and ICP-AES, were used to shed light on the underlying chemistry and measure the efficiency of magnesium ion extraction from a Finnish serpentinite in contact with molten ammonium sulfate above 300 °C. From micro- and gram-scale tests, flux extraction as epsomite [MgSO₄·7H₂O] proceeds via the intermediacy of Tutton salts, NH₄/Mg double sulfates increasingly rich in Mg. Extraction is effected through the agency of acidic derivatives, principally ammonium bisulfate and sulfamic acid, which are created sequentially from ammonium sulfate in situ. However, sulfamic acid volatilizes and/or decomposes at a significant rate by 400 °C. This loss mechanism is primarily responsible for the modest recovery of Mg (50–60%). An improved process would operate below 400 °C where Mg extraction as efremovite [(NH₄)₂Mg₂(SO₄)₃] is effective. Future experiments evaluating the use of humid air to stabilize the bisulfate and impede the loss of flux are recommended.