Environ. Sci. Technol. 2010, 44, 406–411
Aqueous Carbonation of Natural Brucite: Relevance to CO2 Sequestration LIANG ZHAO,† LIQIN SANG,† JUN CHEN,† J U N F E N G J I , † A N D H . H E N R Y T E N G * ,†,‡ Department of Earth Sciences, Nanjing University, Nanjing, Jiangsu, 210093, People’s Republic of China and Department of Chemistry, the George Washington University, Washington, DC 20052, United States of America
Received June 18, 2009. Revised manuscript received October 31, 2009. Accepted November 2, 2009.
Carbonation of natural brucite in H2O and diluted HCl is investigated at room temperature and moderate pCO2 to explore the products’ mineralogy and reaction kinetics. Results show nesquehonite is by far the dominant carbonate species formed, despite its poorer thermodynamic stability relative to magnesite and possibly hydromagnesite. Time-dependent measurements reveal carbonate formation within 30 min, regardless of the original acidity of the slurry. However, while the fraction of reacted brucite in H2O increases gradually over time and approaches unity (∼98%) at 2.5 h, it rises rapidly in HCl within the first hour and levels off thereafter, leaving a significant amount of brucite unreacted. Such behavior suggests that the initial quantity of Mg2+ affects the reaction kinetics. Fitting a pseudo firstorder rate law to the data yields a higher rate constant for the HCl experiments. These observations may imply that the carbonation does not proceed through heterogeneous reaction between gaseous CO2 and solid brucite. Solution chemistry analysis indicates that most CO2 stays in aqueous phase in both media; however, the concentration of HCO3- becomes high in H2O after about 2 h, agreeing with the observed inferior carbonation extent in HCl.
Introduction It is very likely that increased concentrations of anthropogenic greenhouse gases, CO2 being the leading culprit, have caused most of the increases in global average temperatures since the mid-20th century (1). As the reality that alternative energy sources are not likely to replace fossil fuels any time soon sets in, concerns over global climate change begin to lead to investigations into methods to sequester CO2. At present, carbon dioxide capture and storage (CCS) is considered one of the promising options for this cause (2). While underground CO2 storage is a more widely advocated sequestration strategy, mineral carbonation through chemical processing provides a potential alternative (3, 4), particularly in regions where proper underground geological formations are unavailable, where the risk of CO2 leakage from underground site is considered unacceptable, or where materials suitable for carbonation abound in the vicinity of large carbon sources (such as coal-fired power plants). To * Corresponding author phone: 202-994-0112; e-mail: hteng@ gwu.edu. † Nanjing University. ‡ George Washington University. 406
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date, effort in mineral sequestration is focused on two broadly defined fronts, one is a direct carbonation involving heterogeneous reaction of powdered silicate minerals and high pressure CO2 in gaseous or aqueous media, and the other is an indirect route normally consisting of a cation extraction step followed by an acid-base reaction between metal (hydro)oxides and CO2. Up to ∼90% (depending upon the feed mineral) conversion (relative to cations) can be achieved in a few hours by direct carbonation (5-7). This process, however, requires mechanical or thermal (600-800 °C, primarily for hydrated silicates) (8, 9), pretreatment of the feedstock in addition to high pCO2 (up to 150 atm) and hydrothermal (up to 185 °C) conditions. However, utilization of alkaline wastes (10, 11) can largely eliminate the need for such preprocessing steps. Currently, estimated total cost for this method is around ∼$54/ton (CO2) (5), economically unviable for industrial scale carbon sequestration. Indirect carbonation has not been studied as comprehensively as the direct method has but seems to be gaining traction lately (12-14). The multistepped method can be applied at moderate temperature ( 10-2.0 atm (24); increasing temperature (>∼ 40 °C) will bring up various basic species (19, 25-27). Magnesite has been reported to form only at temperature greater than 60-100 °C and elevated pCO2 (28-30). Materials and Experimental Method. Feedstock material used in this study was ore brucite extracted directly from the production pit of Jiguanshan mine (Dandong, China). The original mineral chunks with an average dimension of ∼10 × 10 cm were ground to sub-mm sized particles in a ball mill and further sorted using appropriate sieves. The size fraction of
