Mineral carbon dioxide sequestration refers to a technology whereby carbon dioxide is reacted with metal cations in silicate minerals to form solid carbonate minerals. This technology provides permanent removal of carbon dioxide from the atmosphere, while eliminating the need for monitoring for CO₂ leakage. It is also appealing in the potential for its storage capacity to exceed what would be required for the sequestration of 100% of U.S. CO₂ emissions at current levels for centuries. Carbonates are overwhelmingly the most abundant natural storage mechanism for carbon on the surface of the Earth. This is a simple result of the fact that at the temperatures and pressures experienced by the Earth's crust, carbonate minerals are the thermodynamic ground state of carbon. Carbon dioxide is not the unavoidable end product of the oxidation of fossil fuels. Instead, at least in principle, it is possible to extract additional energy from a carbon atom by letting the carbon dioxide react with a base to form carbonates or bicarbonates (exemplified in Fig 1).

Figure 1. Free energy of carbon compounds relative to carbon.

The economic and technical challenges in above-ground mineral sequestration do not so much lie in the mining effort, which is well understood, but in the complications of accelerating the carbonation reaction to the point that they are economically attractive. These weathering reactions occur spontaneously in nature, albeit with extremely slow reaction kinetics. For sequestration, these reactions must be accelerated. For common magnesium silicates, the net reactions are:

½ Mg₂SiO₄ + CO₂ → MgCO₃ + ½ SiO₂ + 88 kJ
  (olivine)    (CO₂)  (magnesite) (silica)

1/3 Mg₃Si₂O₅(OH)₄ + CO₂ → MgCO₃ + 2/3 SiO₂ + 2/3 H₂O + 35 kJ
         (serpentine)    (CO₂) (magnesite)   (silica)       (water)

Currently, all processes involve the carbonation of the mineral in an aqueous medium or salt melt. It has been shown that serpentine minerals can be carbonated with reaction rates that are sufficiently fast when activated with pretreatment. Roasting serpentine at temperatures between 600 and 700°C makes it highly reactive. Similarly, grinding olivine to ultra-fine grain sizes (< 35 mm) increases the reactivity of the mineral. However, the cost of activating the serpentine material in order to raise the reaction rates is still too high. We are exploring pathways that avoid such energy-intensive treatment steps. Research is being performed to determine how varying the composition of the medium may enhance the kinetics of the reaction. Specifically, we are studying aqueous processes of mineral dissolution of Mg-bearing and Ca-bearing silicate minerals (see Fig 2). After the dissolution step, which is the rate-limiting step at present, the carbonate and solid byproducts from the dissolved mineral-bearing rock are precipitated by controlling the pH.

Figure 2. Dissolution studies in mineral carbonation.

Considerations of mineral abundance, percentage cation (magnesium or calcium) content, and chemical reactivity have led researchers to focus on deposits of serpentine, olivine (magnesium silicates), and wollastonite (calcium silicate), as a source of magnesium or calcium for the process. Rocks made of minerals with high percentages of magnesium such as olivine and serpentine are known as ultramafic rocks. Ultramafic rocks on the continents most commonly occur in locations where oceanic lithosphere has abducted onto continental crust. These formations are known as ophiolite suites and occur in abundance in the United States and around the world. In the United States there is estimated to be enough rock to sequester the carbon emissions of the United States for more than 100 years.

Figure 3. Ultramafic rock resources in the United States that may be suitable for mineral sequestration.

While the major focus of current research is on developing a chemical process for the carbonation of silicate minerals that would be suitable to implement on an industrial scale, some niche technologies are being explored where the principles of mineral carbon sequestration may be applied. Mine tailings at asbestos waste dumps, which consist almost entirely of ground serpentine, offer an Mg-rich feedstock for carbonation that are also desirable to dispose of. The alkaline waste from steel mills and cement mills make good candidates for inputs to a carbonation process as well. Finally, because many silicates contain up to about 10% iron by weight, there is potential for integration with the iron and steel industry wherein they may sequester their emissions while utilizing the byproduct iron (see Fig 4).

Figure 4. Iron production integrated with mineral carbon dioxide sequestration.

Even though in the near future, injection underground is certain to be cheaper, the cost of assuring long-term integrity of the underground storage may in the end render mineral carbonation competitive. By developing mineral sequestration to the point of demonstrated feasibility, one can guarantee long-term availability of fossil energy, which should be an important policy consideration.

Researchers

Sam Krevor, Ph.D. Candidate, Earth and Environmental Engineering, sck69@columbia.edu
Klaus Lackner, Ewing-Worzel Professor of Geophysics, klaus.lackner@columbia.edu
Tuncel Yegulalp, Professor of Mining: Earth & Environmental Engineering, yegulalp@columbia.edu
Paul Duby, Professor of Earth and Environmental Engineering, pfd1@columbia.edu
Alissa Park, Lenfest Junior Professor in Applied Climate Science, ap2622@columbia.edu

Publications

Hanchen, M., Krevor, S., Mazzotti, M., and Lackner, K.S. Validation of a population balance model for olivine dissolution. Chemical Engineering Science, 2007. 62(22): p. 6412-6422.

Lackner, K. S. A guide to CO2 sequestration, Science, 300(5626), 1677–1678.

Lackner, K. S. Carbonate chemistry for sequestering fossil carbon, Annual Review of Energy and the Environment, 27, 193–232.

Butt, D. P., K. S. Lackner, C. H. Wendt, K. Nomura, and Y. Yanagisawa. The importance of and a method for disposing of carbon dioxide in a thermodynamically stable form, World Resource Review, 11, 196–219.

Goff, F. and K. S. Lackner. Carbon dioxide sequestering using ultramafic rocks, Environmental Geoscience, 5(3), 89–101.

Lackner, K. S., C. H. Wendt, D. P. Butt, E. L. Joyce, and D. H. Sharp. Carbon dioxide disposal in carbonate minerals, Energy, 20, 1153–1170.