The U.S. Department of Energy's National Energy Technology Laboratory (NETL) located in Albany, OR (formerly the Albany Research Center) has studied ex situ mineral carbonation as a potential option for carbon dioxide sequestration. Studies focused on the reaction of Ca-, Fe-, and Mg-silicate minerals with gaseous CO2 to form geologically stable, naturally occurring solid carbonate minerals. The research included resource evaluation, kinetic studies, process development, and economic evaluation. An initial cost estimate of approximately $69/ton of CO2 sequestered was improved with process improvements to $54/ton. The scale of ex situ mineral carbonation operations, requiring 55 000 tons of mineral to carbonate, the daily CO2 emissions from a 1-GW, coal-fired power plant, may make such operations impractical.

Direct mineral carbonation has been investigated as a process to convert gaseous CO 2 into a geologically stable final form. The process utilizes a slurry of water, with bicarbonate and salt additions, mixed with a mineral reactant, such as olivine (Mg 2 SiO 4 ) or serpentine [Mg 3 Si 2 O 5 (OH) 4 ]. Carbon dioxide is dissolved into this slurry, resulting in dissolution of the mineral and precipitation of magnesium carbonate (MgCO 3 ). Optimum results have been achieved using heat pretreated serpentine feed material and high partial pressure of CO 2 (P CO2 ). Specific conditions include: 155°C; P CO2 =185 atm; 15% solids. Under these conditions, 78% conversion of the silicate to the carbonate was achieved in 30 minutes. Process mineralogy has been utilized to characterize the feed and process products, and interpret the mineral dissolution and carbonate precipitation reaction paths.

Research on carbon capture and storage has been focused on CO 2 storage in geologic formations, with many potential risks. An alternative to conventional geologic storage is carbon mineralization, where CO 2 is reacted with metal cations to form carbonate minerals. Mineralization methods can be broadly divided into two categories: in situ and ex situ. In situ mineralization, or mineral trapping, is a component of underground geologic sequestration, in which a portion of the injected CO 2 reacts with alkaline rock present in the target formation to form solid carbonate species. In ex situ mineralization, the carbonation reaction occurs above ground, within a separate reactor or industrial process. This literature review is meant to provide an update on the current status of research on CO 2 mineralization.

In aqueous mineral carbonation for carbon sequestration, high-pressure

The dramatic increase in atmospheric carbon dioxide since the Industrial Revolution has caused concerns about global warming. Fossil-fuel-fired power plants contribute approximately one third of the total human-caused emissions of carbon dioxide. Increased efficiency of these power plants will have a large impact on carbon dioxide emissions, but additional measures will be needed to slow or stop the projected increase in the concentration of atmospheric carbon dioxide. By accelerating the naturally occurring carbonation of magnesium silicate minerals it is possible to sequester carbon dioxide in the geologically stable mineral magnesite (MgCO 3 ). The carbonation of two classes of magnesium silicate minerals, olivine (Mg 2 SiO 4 ) and serpentine (Mg 3 Si 2 O 5 (OH) 4 ), was investigated in an aqueous process. The slow natural geologic process that converts both of these minerals to magnesite can be accelerated by increasing the surface area, increasing the activity of carbon dioxide in the solution, introducing imperfections into the crystal lattice by high-energy attrition grinding, and in the case of serpentine, by thermally activating the mineral by removing the chemically bound water. The effect of temperature is complex because it affects both the solubility of carbon dioxide and the rate of mineral dissolution in opposing fashions. Thus an optimum temperature for carbonation of olivine is approximately 185 o C and 155 o C for serpentine. This paper will elucidate the interaction of these variables and use kinetic studies to propose a process for the sequestration of the carbon dioxide.