Reacting
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| source: goldberg 9 |
The entire premise of mineral carbonation rests on reacting carbon dioxide with magnesium or calcium to form carbonates like magnesite or calcite in a thermodynamically favored, exothermic reaction. The simplest form of the reaction is that of carbon dioxide with pure magnesium or calcium oxide.
CaO + CO2 → CaCO3 + 179 kJ/mole
MgO + CO2 → MgCO3 + 118 kJ/mole
It is interesting that even when compared to the heat of combustion of carbon, 394 kJ/mole, these reactions release a significant amount of heat. [1] In fact, because the reaction generates heat, it is hoped that the process will be self-sufficient and no external heat will be required. [2] Calcium and magnesium, however, are not found in nature as oxides, but rather as silicates. The general reaction for silicates is:
(Mg, Ca)xSiyOx+2y+zH2z + xCO2 → x(Mg, CA)CO3 + ySiO2 + zH2O [3]
The heat release for the carbonation reaction with silicates is less than with pure oxides, but it is still exothermic and favored at low temperatures. [4] Two examples of the general reaction with common minerals are given below:
Forsterite (olivine): Mg2SiO4 + 2CO2 → 2MgCO3 + SiO2 + 95 kJ/mole
Serpentine: Mg3Si2O5(OH)4 + 3CO2 → 3MgCO3 + 2SiO2 + 2H2O + 64 kJ/mole [5]
These reactions occur in nature as chemical weathering and also in the biological activity in corals and coccoliths. [6] Chemical weathering happens naturally on a geologic time scale and eventually all the carbon dioxide in the atmosphere would be disposed of this way [7] (we are just speeding up the process). The reaction occurs in nature when naturally rich CO2 fluids percolate through mineral deposits forming silica and stable magnesite in serpentized rocks. [8]
Reaction Rate
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| source: goldberg 10 |
The biggest issue with the carbonation reaction is the reaction rate. The reactions occur in nature in geological time, but we don't have that long to wait. For the process to be economically feasible the resonance time for the solids must be on the order of one hour. Goldberg offers the following suggestions to increase the reaction rate: pre-treat the serpentine by heating at 600-650 °C to release chemically bonded water (recall serpentine has 13% by weight water) and create an open structure and add sodium bicarbonate (increases the HCO3- concentration) and NaCl solution (aids in the release of magnesium ions from the silicate) to the reaction. With these modifications to the reaction, 78% conversion can be achieved in 30 minutes at 185 bar and 155°C. [9] O'Connor gives the following results for direct carbonation of olivine and serpentine:
§ Up to ~80% efficiency achieved in 1 hour @ PCO2 = 150 atm, T=155ēC
§ Up to ~50% efficiency achieved in 1 hour @ PCO2 = 20 atm, T=155ēC
§ Up to ~40% efficiency achieved in 1 hour @ PCO2 = 20 atm, T=50ēC [10]
Thus, it appears that researchers are very close to finding a "best" method for reacting the carbon dioxide in an appropriate amount of time. Still, "direct carbonation of the mineral rock is feasible but at this point still too slow to be economic. An aqueous process using hydrochloric acid that is completely recovered within the process has been demonstrated to be fast. However, the number of steps is too large and the large amount of steam generated makes the process energy inefficient even though the overall reaction is exothermic." [11]
Of course, when the reaction takes place the products are not pure as the idealized reaction indicates. There are always impurities. O'Connor's study showed the following make-up of products by weight percent when material which had undergone ~80% conversion was tested: for olivine, the final product had 30% CO2, 25-30% free SiO2 (no silica pattern - amorphous), 35% MgO, 65% MgCO3 (calculated), and residual olivine and enstatite and for serpentine, the final product contained 30% CO2, 25-30% free SiO2 (no silica pattern - amorphous), 35% MgO, 65% MgCO3 (calculated), residual olivine, and no chrysotile (asbestos). [12] Although, the make-up was not pure MgCO3, as would be ideal, all components are environmentally benign and are not a problem to dispose of.
Cost
The cost of mineral sequestration is predicted to be quite reasonable. "The study noted that the difficulty lies in the design of an efficient chemical process. Simple processes that bring together the CO2 and the serpentine rock in a direct reaction are potentially very low in cost. If the size of the plant is determined by a residence time of the solids that is on the order of an hour, the containment vessels, even if they are pressure vessels, will add little to the cost of the plant." [13] "However, reasonable assumptions about thermodynamically efficient processes suggest cost for entire disposal process, including rock mining, crushing and milling, of about $20/ton of CO2. For a 66% efficient power plant this would add less than 1 cent to the cost of a kilowatt hour." [14] Thus, once the methodology for chemically processing the magnesium minerals is refined, mineral sequestration will become a very economically competitive option with the added advantage of permanent disposal of carbon dioxide with no possibility for leakage.
