Selection of Minerals
Selection of element
Both alkali and alkaline earth metals can be carbonated. However, alkali carbonates are too soluble to form a stable product that can be stored above ground and would have to be stored in salt caverns. Of the alkaline earth metals calcium and magnesium are by far the most common in nature. Magnesium and calcium comprise ~2.0 and 2.1 mol% of the earth’s crust (Goff et al., 1998). Thus calcium and magnesium are generally selected for mineral CO2 sequestration purposes. Although carbonation of calcium is easier, for mineral carbonation the use of magnesium-based minerals is favoured, because they are available worldwide in large amounts and in relatively high purity. Furthermore, the amount of oxide required to bind carbon dioxide from burning one ton of carbon also favours magnesium oxide at 3.3ton compared to 4.7ton calcium oxide. Therefore, most attention is paid to magnesium-containing minerals.
Of the non-alkali and non-alkaline earth metals, few metals can be carbonated (e.g. Mn, Fe, Co, Ni, Cu and Zn). However, most of these elements are too rare or too valuable. Iron is available in sufficient amounts, but forming iron carbonates implies consuming valuable iron ore.
Selection of mineral
In order to be able to react with acid CO2, the mineral has to provide alkalinity. Not all alkali or alkaline earth metals containing minerals provide alkalinity. For example NaCl is not a source of alkalinity. Alkalinity is derived from oxides or hydroxide.
Another (weaker) source of alkalinity is carbonates. Although it is easier to convert carbonates into bicarbonates than to carbonate a silicate mineral (Lackner, 2002), oxides and hydroxides are preferred. Controlled storage is only possible for carbonates, because carbonates are almost insoluble in water while bicarbonates are fairly soluble. Part of the sequestrated carbon dioxide would be released, if bicarbonates were dissolved in rainwater.
Calcium and magnesium rarely occur as binary oxides in nature. They are typically found in silicate minerals. These minerals are capable of being carbonated. Although calcium and magnesium carbonates are almost insoluble, small amounts can dissolve in acid rainwater. Thus the sequestrated carbon leaches slowly from the storage and comes back into the carbon cycle. This has to be prevented to avoid a possible change of the atmospheric CO2 concentration in the long-term. A potential option is to seal the mineral carbon dioxide sequestration storage. The rate at which this occurs is probably very low, but needs to be studied.
Igneous rocks are particularly suitable for CO2 fixation because they are essentially free of carbonates. The main candidate magnesium-rich ultramafic rocks are dunites, peridotites and serpentinites. The first two can be mined for olivine, a solid solution of forsterite (Mg2SiO4) and fayalite (Fe2SiO4). Ore grade olivine may contain alteration products, such as serpentine (Mg3Si2O5(OH)4) and talc (Mg3Si4O10(OH)2). Serpentine can take the form of antigorite, lizardite and chrysotile. The main calcium-containing candidate is wollastonite (CaSiO3).The composition of various minerals and their specific CO2 sequestration capacity are given.
Composition of various minerals and carbon di oxide sequestration characteristics.Rc=Mass ratio of rock needed for carbon di oxide fixation to carbon burned.Rco2=Corresponding mass ratio of rock to CO2.(Lackner et al .,1995;Wu et al .,2001)
Serpentine is found in large deposits worldwide, large reservoirs being known, for example, on both the East and West Coast of North America and in Scandinavia. Two selected reservoirs in the United States are Twin Sisters, Washington, and Wilbur Springs, California, which are capable of sequestering the globally emitted carbon dioxide for 2 and 5 years respectively. Lackner indicated a deposit in Oman of 30,000km3 magnesium silicates which alone would be able to store most of the CO2 generated by combustion of the world’s coal reserves. Basalt, which is rich in calcium, is ubiquitous, but it is difficult to extract the reactive components from the mineral matrix .
In the DOE research program olivine and serpentine are selected because of their large abundance in nature and the high molar ratio of the alkaline earth oxides within the minerals (Goff et al., 1998). Other researchers, including Wu, concluded that talc and wollastonite would be the most appropriate minerals.
Alkaline solid wastes
Most available literature deals with mineral CO2 sequestration using mineral rock as feedstock. An alternative source of alkalinity could be the use of solid alkaline waste materials, which are available in large amounts and are generally rich in calcium. Possible candidates are, among others, asbestos waste, iron and steel slag and coal fly ash (NETL, 2001). The carbonation of alkaline waste materials has two potential advantages: these materials constitute an inexpensive source of mineral matter for the sequestration of CO2 and the environmental quality of the waste materials (i.e. the leaching of contaminants) can be improved by the resulting pH-neutralization and mineral neoformation