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The long-term sequestration of CO2 in solid form: the application of nesquehonite

VINCENZO FERRINI, CATERINA DE VITO, SILVANO MIGNARDI

Dipartimento di Scienze della Terra, Università di Roma “La Sapienza”, P.le A. Moro, 5 – 00185 Roma, Italy vincenzo.ferrini@uniroma1.it, cdevito@uniroma1.it, silvano.mignardi@uniroma1.it

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We describe here encouraging results of our experiments on the synthesis of nesquehonite [MgCO3·3H2O]* by flushing CO2 in a MgCl2 solution with a view to investigate a possible role for nesquehonite in a “CO2 sequestering process”.

Our option could result a cost-effective niche CO2 mineral sequestration process

*Nesquehonite is a rare low-temperature carbonate encountered in alkaline soils and in cave deposits. It generally forms euhedral prismatic crystals, but also is found in fibroradial and botryoidal arrays. Ideal formula contains 29.13% MgO, 39.06% H2O, 31.81% CO2

Our option…

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Sources of magnesium available: Potential magnesium sources involve •seawater

•artificial saltpans •evaporitic saline deposits

They locally can represent point sources for small-scale industrial applications of the proposed method of carbonation.

A massive supply of magnesium could be provided by saline aqueous wastes as a by-product of oil and gas production, the so-called produced water (PW, 70 billion barrels worldwide), as well as reject brines from the desalination process.

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The experiments in the first step: using both doubly distilled and tap water, compressed CO2 from

SAPIO (Italy)

analytical grade reagents (MgCl2·6H2O and NH3, Merck p.a.)

the suitable range of pH for the optimum formation of nesquehonite in our experimental conditions (7.8–8.2) was adjusted by adding about 2% of ammonia solution

in thirty-two experiments, we synthesized nesquehonite by sparging CO2 at a rate of ~100 mL/min through 200 mL of a MgCl2·6H2O solution (~ 7 g/L of Mg) at 20 ± 2ºC

the suspension was filtered using 0.20 µm Nucleopore polycarbonate membrane filters, and washed with doubly distilled water and dried in air

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Results

The reaction rate is rapid, with carbonate deposition almost complete in about 10 minutes (Fig. 2). Nesquehonite exhibits well-formed needles up to 0.5 mm in length and 30 m Ø (Fig. 3a,b).

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Fig. 2

Fig. 3a

Fig. 3b

The kinetics of the formation of the solid products were followed by sampling the solution at appropriate time intervals and measuring the concentration of Mg

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Fig. 4 shows a typical XRD pattern (sample AC12); the precipitate has a very high degree of crystallinity. All patterns are in agreement with those reported in JCPDS card 20–669 for nesquehonite.

The TG-DTG curves (Fig. 5) document the thermal decomposition of nesquehonite during gradual heating, proceeding via dehydration at low temperature (below 350°C) and, above that threshold, complete loss of CO2 (427°C).

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Sample AC 12

card 20.669

Fig. 5

…..results

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The efficiency of the CO2 mineralization process

On the basis of these experimental data, 81.7 0.7% of the sparged CO2 was captured to form nesquehonite

About 5% of the starting concentration of Mg was left in the solution after the carbonate formation

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Thermal behavior of nesquehoniteThe results of our study, by in situ using real-time laboratory parallel-beam X-ray powder diffraction, show that nesquehonite appears to be stable up to 373 K suggesting that its storage as “sequestering medium of CO2” remains stable under the temperature conditions that prevail at the Earth's surface

At temperature above 373 K the process of thermal decomposition of nesquehonite (via intermediate hydrated magnesium carbonate phases) ultimately produces magnesite in the range 423 – 483 K.

This sequence involves the formation of carbonate minerals thermodynamically more stable than nesquehonite, resulting in a CO2 storage stable for millions of years.

Therefore, if the decomposition of nesquehonite would occur into underground storage facilities, this process further on would increase the safety of CO2 disposal.

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Possible uses for the nesquehonite and by-products

The sequestration of CO2 via carbonation produces a solid material that can be utilized as aggregate in bricks, blocks, mortars, and other building materials.

This mineral can be used in the production of eco-cement* concretes because it contributes to strength of the concrete.

The ammonium chloride solution produced in the process could be treated for recoverying the salt or decomposed by heat (~ 350°C) to obtain NH3 and HCl.

* F. Pearce, Green Foundations, New Scientist 175 (2002) 39-40. J. Harrison, Tececo eco-cement masonry product update. www.tececo.com.

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Why nesquehonite option merits further research ? our method as complementary solution could be applied in the countries where

the other solutions are not applicable and in those where suitable MgCl2 sources exist, also as by-product of several industrial processes

the process is rapid, simple and environmentally friendly

nesquehonite is a light, thermodynamically stable solid product allowing for the long-term storage of CO2

the starting reactants are easy to be found nesquehonite can be used for industrial and agricultural purposes, and its near

surface or underground disposal involves limited environmental risks by-products of the process are sought for a large number of industrial

applications

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Aqueous saline wastes having different salinities and pollutants (e.g., Fe, Pb, Cu, Zn, etc.)

CO2 fluxes with different amount of the greenhouse gas

Tests of the process at power plants

Work in progress

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“Nesquehonite solution”

Uses hazardous wastes (CO2, saline wastewaters, PW) Produces solid products stable for millions of years

Recycles by-products of other industrial processes

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…our idea in a “naïve image”

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Ferrini V., De Vito C., Mignardi S. (2009) Synthesis of nesquehonite by reaction of gaseous CO2 with Mg chloride solution: Its potential role in the sequestration of carbon dioxide, J. Hazard. Mater. 168, 832-837.

Ballirano P., De Vito C., Ferrini V., Mignardi S. (2010) The thermal behavior and structural stability of nesquehonite, MgCO3·3H2O, evaluated by in situ laboratory parallel-beam X-ray powder diffraction: New constraints on CO2 sequestration within minerals. J. Hazard. Mater. 178, 522–528.

Main relevant products of the research