DEVELOPMENT OF A CO2 SEQUESTRATION MODULE BY INTEGRATING

MINERAL ACTIVATION AND AQUEOUS CARBONATION

The Pennsylvania State University

Grant number: DE-FG-03NT41809

OBJECTIVES
Carbon dioxide is a greenhouse gas that contributes to global climate warming. Mean annual concentrations
of gaseous CO2 have increased by 19.4% from 315.98 parts per million by volume (ppmv) of dry air in
1959 to 377.38 ppmv in 20041. To limit the risk of a climate change to acceptable levels, the emission of
CO2 and other greenhouse gases should be reduced. Mineral carbonation can provide benign and long-term
storage capabilities. The formation of stable mineral carbonates can occur by the reaction of carbon dioxide
with metals such as calcium and magnesium. Research has focused on calcium and magnesium silicates
due to favorable thermodynamics for carbonation and their relative abundance and availability (Goff and
Lackner, 1998). The natural weathering process is unfortunately limited in extent by slow reaction
kinetics; in an effort to expedite the reaction, the use of sulfuric acid as an accelerating medium has been
investigated. According to current CO2 emission levels, the serpentine requirements for a mineral
carbonation plant are not trivial. Therefore a better understanding of the dissolution of serpentine is
warranted. In addition to dissolution studies, the reaction of the magnesium-rich leachate with carbon
dioxide provides its own challenges.

The overall objective of the proposed research program is to optimize the active carbonation process in
order to design an integrated CO2 sequestration module for Vision 21 plants. This research program can be
divided into the following four tasks, as described in the research proposal. Task 1 ‘Mineral activation’
will conduct a parametric study to optimize the operation conditions for the mineral activation, where
serpentine and sulfuric acid are reacted. The optimization of these processing parameters will yield a
maximum dissolution of magnesium cations and also produce a high surface SiO2 solid. Under Task 2
“Aqueous carbonation” the extracted Mg2+ will then be reacted with CO2 at various temperatures and
pressures in a CSTR to optimize the variables (temperature, pressure, pH, and stirring speed) that
predominantly affect the carbonation reactions. Task 3 “Integration of the activation and carbonation units
into a CO2 sequestration module” will involve the integration of the two CSTR reactors and their optimum
operating conditions into a module. Finally, Task 4 “Economic assessment” will be conducted to evaluate
the economic viability of the proposed CO2 sequestration module2.

ACCOMPLISHMENTS TO DATE
The mineral dissolution study samples consisted of serpentine from the Cedar Hills quarry in SE
Pennsylvania. The sample was provided by the Department of Energy - Albany Research Center (ARC)
and consisted of an 80% passing 400 mesh (-37 µm) ground sample that underwent magnetic separation
from a separation pilot-plant study (O’Connor, 2003). Analyses of chemical composition, solutions and
solids, were carried out on a Leeman Labs PS3000UV Inductively Coupled Plasma – Atomic Emission
Spectroscopy (ICP-AES). Carbonation experiments were carried out in a 300 ml Parr – 4651 stirred high
pressure/high temperature reactor. X-ray diffraction analyses were conducted with a Philips Analytical
1050 XRD with a Hilton Brooks x-ray generator and scanning electron microscopy images were obtained
on an FEI Quanta 600.

The treatment of serpentine with sulfuric acid has shown to be effective in the leaching of magnesium for
its subsequent carbonation. The baseline experiment was performed at 6 hours and room temperature with a
particle size of <37 µm and a 2 M sulfuric acid concentration. This experiment yielded 7125 ppm of Mg2+.
Although the effects of time are too slight to appear within shorter time periods, a 24 hour reaction was
shown to provide a 46 % improvement in the extraction of Mg2+. The effects of increased reaction
temperature from 25oC to 50oC provided a 70% improvement on dissolution. Additionally, multi-stage
leaching provided little gain in Mg2+ extraction whether it was a 2 stage acid leaching or 2 stages of acid
leaching with 1 stage of base leaching in between. Although research efforts into the dissolution of
serpentine will continue to optimize the reaction variables for the dissolution stage, the ability to carbonate
these magnesium ions is central to mineral carbonation.

In contrast to the dissolution process where extreme conditions are favored, the carbonation reaction
requires a more careful balance of reaction variables. Magnesite, although stable under ambient conditions,
is vulnerable to dissolution in an acidic environment. Initial investigations revealed that a partial pressure
of 650 psi resulted in too acidic of conditions for the formation of magnesium carbonates, regardless of the
addition of a base. Further experimentation showed that the bicarbonate ion concentration remains two
orders of magnitude too low without the addition of a buffer such as sodium bicarbonate, even at lower
partial pressures. In addition, experimentation was also able to show that it is sodium bicarbonate
responsible for the formation of carbonates.

FUTURE WORK

The magnesium ion presents a unique challenge with respect to reaction products and reaction kinetics.
Magnesite, although the thermodynamically favored magnesium carbonate, does not predominate under
wide-ranging carbonation conditions due to kinetically favored metastable products such as nesquehonite,
hydromagnesite, and lansfordite. Accordingly, future research will assess the minimum carbonation
conditions required for the formation of the magnesium carbonates.

PAPERS PUBLISHED
M.M. Maroto-Valer, D.J. Fauth, M.E. Kuchta, Y. Zhang, and M. Andresen (Submitted). Activation of
Magnesium Rich Minerals as Carbonation Feedstock Materials for CO2 Sequestration. (Phase I)

PATENT APPLICATIONS
M.M. Maroto-Valer, D.J. Fauth, M.E. Kuchta, Y. Zhang, and M. Andresen (Submitted). Activation of
Magnesium Rich Minerals as Carbonation Feedstock Materials for CO2 Sequestration. (Phase I)

AWARDS RECEIVED AS A RESULT OF SUPPORTED RESEARCH

STUDENTS SUPPORTED UNDER THIS GRANT

REFERENCES
1. Keeling & Whorf, 2005. Atmospheric CO2 records from sites in the SIO air sampling network. In
Trends: A Compendium of Data on Global Change. CDIAC
2. Alexander, G., Maroto-Valer, M., Gafarova-Aksoy, P., Schobert, H.H., 2005. Development of a
CO2 Sequestration Module by Integrating Mineral Activation and Aqueous Carbonation.
University Coal Research/Historically Black Colleges and Universities & Other Minority
Institutions Contractors Review Meeting