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324 arise from the formation of silica or carbonate layers on the mineral surface during carbonation that tend to hinder further reaction and to limit conversion (Butt et al., 1996) and from the rate of CO2 uptake from the gas phase in the case of aqueous reactions. The challenge for mineral carbonation is to find ways to accelerate carbonation and to exploit the heat of reaction within the environmental constraints, for example with minimal energy and material losses. 7.2.3 Sources of metal oxides IPCC Special Report on Carbon dioxide Capture and Storage Most processes under consideration for mineral carbonation focus on metal oxide bearing material that contains alkaline-earth metals (such as calcium and magnesium) as opposed to alkali metals (such as sodium and potassium) whose corresponding carbonates are very soluble in water. Oxides and hydroxides of calcium and magnesium would be the ideal source materials, but because of their reactivity they are also extremely rare in nature. Therefore, suitable metal oxide bearing minerals may be silicate rocks or alkaline industrial residues, the former being abundant but generally difficult to access and the latter scarcer but easily available. On a smaller-scale, industrial wastes and mining tailings provide sources of alkalinity that are readily available and reactive. Even though their total amounts are too small to substantially reduce CO2 emissions, they could help introduce the technology. Waste streams of calcium silicate materials that have been considered for mineral carbonation include pulverized fuel ash from coal fired power plants (with a calcium oxide content up to 65% by weight), bottom ash (about 20% by weight CaO) and fly ash (about 35% by weight CaO) from municipal solid waste incinerators, de-inking ash from paper recycling (about 35% by weight CaO), stainless steel slag (about 65% by weight CaO and MgO) and waste cement (Johnson, 2000; Fernández Bertos et al., 2004; Iizuka et al., 2004). Among silicate rocks, mafic and ultramafic rocks are rocks that contain high amounts of magnesium, calcium and iron and have a low content of sodium and potassium. Some of their main mineral constituents are olivines, serpentine, enstatite (MgSiO3), talc (Mg3Si4O10(OH)2) and wollastonite. Although molar abundances of magnesium and calcium silicates in the Earth’s crust are similar, rocks containing magnesium silicate exhibit a higher MgO concentration (up to 50% by weight, corresponding to a theoretical CO2 storage capacity of 0.55 kg CO2/kg rock), than rocks containing calcium silicates, for example basalts, that have CaO content of about 10% by weight only (with a theoretical CO2 storage capacity of 0.08 kg CO2/kg rock) (Goff and Lackner, 1998). Deposits of wollastonite, the most calcium-rich silicate, are much rarer than those of magnesium-rich silicates. 7.2.4 Processing 7.2.4.1 Mining and mine reclamation Serpentine and olivine are mainly found in ophiolite belts – geological zones where colliding continental plates lead to an uplifting of the earth’s crust (Coleman 1977). For example, considering ultramafic deposits containing serpentine and olivine in the Eastern United States and in Puerto Rico, it was found that they have RCO2 values between 1.97 and 2.51, depending on purity and type (the RCO2 is the ratio of the mass of mineral needed to the mass of CO2 fixed when assuming complete conversion of the mineral upon carbonation, that is the reciprocal of the theoretical CO2 storage capacity introduced above). Peridotites and serpentinites exceed the total Mg requirement to neutralize the CO2 from all worldwide coal resources estimated at 10,000 Gt (Lackner et al., 1995). Specific ore deposits identified in two studies in the USA and Puerto Rico add to approximately 300GtCO2 (Goff and Lackner, 1998; Goff et al., 2000). This should be compared to CO2 emissions of about 5.5 GtCO2 in the United States and about 24 GtCO2/yr-1 worldwide. No comprehensive mapping of the worldwide storage potential in ophiolite belts has been reported. However, their total surface exposure is estimated to be of Mining serpentine would not differ substantially from conventional mining of other minerals with similar properties, for example copper ores. Serpentine and olivine are both mined already, although rarely on the scale envisioned here (Goff and Lackner, 1998; Goff et al., 2000). Like in other mining operations, disposal of tailings and mine reclamation are important issues to consider. Tailing disposal depends on the material characteristics – particle size and cohesion, moisture content and chemical stability against natural leaching processes – and these depend in turn on the specific process. It is likely that carbonation plants will be located near the metal oxide bearing material, either the factory producing the residues to be treated or the silicate mine, to avoid transport of solid materials (see Figure 7.2). the order of 1000 km by 100 km (Goff et al., 2000). It is well known however that magnesium silicate reserves are present in all continents, but since they tend to follow present or ancient continental boundaries, they are not present in all countries. The feasibility of their use for ex-situ or in-situ mineral carbonation is yet to be established (Brownlow, 1979; Newall et al., 2000). Economies of scale applying to today’s mining technology suggest a minimum mining operation of 50,000 to 100,000 tonnes day–1 (Hartman, 1992), which translates into a minimum mineable volume of about 0.3 km3 for a mine with a 30 year life. This is a rather small size for ophiolite ore bodies, which are often kilometres wide and hundreds of meters thick (Goff and Lackner, 1998; Goff et al., 2000; Newall et al., 2000). Since coal, in contrast to ophiolite bodies, occurs in thin seams and is buried under substantial overburden, it has been argued that a typical above ground coal mine must move more material (Lackner et al., 1995) and disturb a far larger area (Ziock and Lackner, 2000) for the same amount of carbon atoms treated than the equivalent ophiolite mine, assuming maximum conversion of the mineral to carbonate (one carbon atom yields one CO2 molecule upon combustion, which has to be fixed in one molecule of carbonate). Serpentine can take many different forms, from decorative stones to chrysotile asbestos (O’Hanley, 1996). The possibility of encountering asbestos requires adequate precautions. With current best practice it would reportedly not be an obstaclePDF Image | CARBON DIOXIDE CAPTURE AND STORAGE
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