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Carbon Sequestration Kinetic and Storage Capacity of Ultramafic Mining Waste Julie Pronost,† Georges Beaudoin,*,† Joniel Tremblay,† Faïc-al Larachi,‡ Josee Duchesne,† Rejean Hebert,† and Marc Constantin† †
Departement de geologie et genie geologique & GEOTOP, Universite Laval, 1065 avenue de la Medecine, Quebec (QC), Canada G1V 0A6 ‡ Departement de genie chimique, Universite Laval, 1065 avenue de la Medecine, Quebec (QC), Canada G1V 0A6
bS Supporting Information ABSTRACT: Mineral carbonation of ultramafic rocks provides an environmentally safe and permanent solution for CO2 sequestration. In order to assess the carbonation potential of ultramafic waste material produced by industrial processing, we designed a laboratory-scale method, using a modified eudiometer, to measure continuous CO2 consumption in samples at atmospheric pressure and near ambient temperature. The eudiometer allows monitoring the CO2 partial pressure during mineral carbonation reactions. The maximum amount of carbonation and the reaction rate of different samples were measured in a range of experimental conditions: humidity from dry to submerged, temperatures of 21 and 33 °C, and the proportion of CO2 in the air from 4.4 to 33.6 mol %. The most reactive samples contained ca. 8 wt % CO2 after carbonation. The modal proportion of brucite in the mining residue is the main parameter determining maximum storage capacity of CO2. The reaction rate depends primarily on the proportion of CO2 in the gas mixture and secondarily on parameters controlling the diffusion of CO2 in the sample, such as relative saturation of water in pore space. Nesquehonite was the dominant carbonate for reactions at 21 °C, whereas dypingite was most common at 33 °C.
’ INTRODUCTION The increasing concentration of greenhouse gases in the atmosphere, such as methane and carbon dioxide, has led to the development of several mitigation strategies to reduce anthropogenic impact on climate.1 Sequestration of CO2 by reaction with Ca or Mg-rich natural minerals has been suggested as an environmentally safe and permanent method for storage of CO2.26 Due to their high content in Ca and Mg, mafic and ultramafic rocks are the most reactive rocks for CO2 capture and storage. They are found in the Earth’s crust as greenstone belts, ophiolites, volcanic and intrusive rocks, and they are abundant enough to potentially store the carbon that would be produced by combustion of the world’s known coal reserves.7 Carbonation of these rocks is a thermodynamically favored exothermic reaction that occurs naturally during weathering and which exerts a first order control of atmospheric CO2 concentration over geological time scales.8 However, the kinetics of the reaction at Earth’s surface conditions is too slow to be suitable for industrial processes. Several studies showed that the reactivity of olivine and serpentine is enhanced by thermal and mechanical activation,4,9,10 but this treatment is energy intensive and would lead to storage cost of ∼54 USD/tonne of CO2, which is not economically viable.11 Chemical activation using acids and bases was also studied.12 It should however be noted that, even r 2011 American Chemical Society
if mineral carbonation is a costly process, it has benefits compared to storage in geological formations because the carbon is stored in stable, environmentally benign minerals that do not require long-term monitoring. As the price of open-pit mining and crushing for ultramafic rocks is low (45 USD/ton13), partial carbonation at ambient conditions could prove to be a cost-effective option compared to complete carbonation at high temperature and pressure, to offset the required energy intensive or chemical pretreatment with associated environmental and capital costs. Olivine, though less naturally available, displays high reaction rates,14,15 but the preferential mobilization of Mg in aqueous solution leads to the formation of a silica-rich passivating layer coating the olivine grains thus hindering the carbonation reaction.1618 Experiments of gassolid carbonation of chrysotile at atmospheric pressure show that the atoms of Mg that react are those from the outside, brucitic, layer of the chrysotile structure that are not bound to Si atoms, which remain unreacted in the chrysotile lattice.19
Received: May 17, 2011 Accepted: September 15, 2011 Published: September 15, 2011 9413
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Environmental Science & Technology Unlike previous experimental studies of mineral carbonation, that were mostly performed on pure phases such as olivine,4,14,18,20 serpentine,21,22 orthopyroxene,23 brucite,10,24 the present study assesses the mineral carbonation potential of ultramafic mining residues from two different deposits: 1) samples from various stages mineral processing pilot tests from the Dumont Nickel deposit (Amos, Canada) and 2) samples from chrysotile milling residue from the Black Lake mine (Thetford Mines, Canada). The surface of ultramafic mining waste piles has been shown to undergo passive mineral carbonation by reaction with meteoric water and atmospheric CO2.26,27 The mineral carbonation reaction also occurs within the mining waste piles as shown by warm, CO2-depleted, air venting at the surface of the piles.28 The estimation of CO2 uptake by analysis of the carbonated product after reaction does not yield insight about the evolution of the reaction rate and dynamics throughout the experiment. In previous studies, real time monitoring has been performed using in situ synchrotron X-ray diffraction (XRD) and Raman spectrometry,10,25 running a set of parallel or sequential experiments that are stopped at different stages of the reaction.11,24 Here we present an original method that allows real time monitoring of CO2 consumption by means of a modified eudiometer, in order to determine the critical parameters controlling the carbonation of ultramafic residues and the kinetics of the reaction. Based on manometric principles, the technique enables following the carbonation kinetics of several samples at the same time via the CO2 partial pressure decrease over periods of several weeks. Samples of ultramafic mining waste from the two sites were studied under variable conditions of CO2 partial pressure, sample size, relative humidity, and temperature, to determine optimal conditions for mineral carbonation. The consumption of gaseous CO2, the amount of C captured, and the mineralogy of the carbonate mineral products are used to understand the reaction and derive the rate of the mineral carbonation reaction under various experimental conditions.
’ ANALYTICAL METHODS All the samples have been characterized before and after carbonation. XRD analyses have been performed at Universite Laval, using a Siemens D5000 diffractometer with Cu Kα radiation. Scans were taken for 2θ ranging from 1° to 65° with steps of 0.02°/s. A JEOL-840A scanning electron microscope (SEM) equipped with energy dispersive X-ray spectroscopy (EDS) was used for imaging and semiquantitative major element analysis. Whole-rock major elements were analyzed by X-ray fluorescence (XRF) by Activation Laboratories (Ancaster Canada). For all the oxides the detection limit is 0.01 wt % except for MnO (0.001 wt %). Analysis of a serpentinite reference material UB-N indicates that accuracy is better than 2% for elements with concentrations higher than 1 wt %. Carbon content was measured by Activation Laboratories (Ancaster Canada) using standard infrared (IR) and IR low-level carbon (LLC) instruments. In both cases the analyses indicate the total carbon content, without any discrimination between inorganic or organic carbon. Low-level carbon analysis allows a detection limit of 0.004 wt % CO2 and is suitable for samples with less than 1 wt % CO2. Reproducibility has been tested by analyzing selected samples in triplicate. The SRM UB-N (0.39% ( 0.08 wt % CO2) yielded a LLC average of 0.5 (
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Figure 1. Schematic representation of the experimental device. (A, B) At instant time t of test (reactive sample) and control (corundum sample) eudiometers: instant atmospheric pressure, Pa(t), instant test and control headspace volumes, Vt(t), Vc(t), and corresponding rising heights of glycerol, dt(t), dc(t). (C) (Identical) initial states in test and control eudiometers: atmospheric pressure, Pa(0), test and control headspace volume, V0, and rising height of glycerol, d0.
0.09 wt % CO2, which indicates good accuracy and precision. Standard infrared analysis has a detection limit of 0.04 wt % CO2 and is appropriate for samples containing more than 1 wt % CO2. Reproducibility of randomly selected duplicates is better than 2%. Analysis of SRM SY-4 (3.5 ( 0.1 wt % CO2) yielded an average of 3.9 ( 0.05 wt % CO2, indicating good precision and accuracy. Amos samples were prepared by wet sorting in a pilot plant. For each sample, an aliquot of process water has been filtered in order to remove fine particles in suspension and has been analyzed for major elements by Exova (Quebec) by inductively coupled plasma atomic emission spectrometry (ICP-AES). The detection limit is between 0.5 and 0.001 ppm depending on the element. Two standard solutions were analyzed at the beginning and at the end of each analytical series and the reproducibility is better than 3.3% and the accuracy better than (10%. The acidity of the process water has also been measured with a pHmeter (Model 415, Denver Instrument) calibrated before each use with three buffer solutions with pH of 4, 7, and 12.
’ EXPERIMENTAL PROCEDURE Protocol for Carbonation Monitoring. The CO2 instantaneous uptake by Mg-rich milling waste material is monitored using a controlled temperature volumetric technique. The experiments are set in a controlled-temperature room at 20.7 ( 0.7 °C or 33.3 ( 2.7 °C. The eudiometer is a device designed to measure gas volumes,29,30 which has been modified to measure CO2 uptake during periods of time up to three weeks. The setup is sketched in Figure 1, which highlights some of the variables influencing the carbonation reaction monitoring. All the glassware is PYREX. The test (reactive sample) and control (corundum sample) eudiometers comprise a small beaker, and a 500 mL graduated cylinder lowered on it until its open-end, immersed in a glycerol container, delineates a reaction headspace volume (Figure 1A,B). The air trapped in both eudiometers is partially pumped out, and a determined amount of CO2 is injected until the air mixture reaches a desired composition. The test and control beakers contain weighted samples (typically 5 g) of, respectively, milling waste rock and inert corundum that were humidified with distilled water. 9414
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Figure 2. Volume change in eudiometers during carbonation experiments. Dashed line marks a week. A) Measured atmospheric pressure. B) Gas volume change in control eudiometer and in test eudiometers for four samples entirely submerged in distilled water and one sample partially saturated with distilled water. C) Gas volume of reacted CO2 after correction for atmospheric pressure. D) Quantity (mmole, filled symbol) of reacted CO2, for several CO2 injections inside the eudiometer. The molar fraction of CO2 (open symbol) records CO2 consumption and vertical steps show time of injection.
Instantaneous gas volumes, Vt(t) and Vc(t), and atmospheric pressure, Pa(t), are registered during the course of carbonation experiments. However, processes other than CO2 consumption can affect the gas volumes in the device. CO2 leakage via physical absorption and then desorption through glycerol is marginal.30 Also, dynamic changes in the eudiometers are very slow so that a hydrostatic correction of the headspace pressures is reasonably accurate. Hence, spurious deviations in volumes are likely to result only from fluctuations of atmospheric pressure. They are captured in the volumes measured as Vc(t) (Figure 1B), for a fixed surface A equal to the graduated tube cross-section, which deviate from the initial state (V0) according to the following expression (symbols are explained in the caption of Figure 1), based on hydrostatic and volume conservation principles A V0 þ δ0 Vc ðtÞ ¼ Pa ðtÞ Fg A 2Fg ffi# sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 V0 V0 þ δ0 þ 4Fg ðPa ð0ÞFgδ0Þ þ Pa ðtÞ Fg A A
The control eudiometer allows systematic compensation of the external pressure disturbances over the extended observational periods characterizing the measurements in the test eudiometer (Figure 2). Consequently, the running gas volumes, Vt(t), in the test eudiometers can be synchronously corrected to reflect solely the actual conversion of carbon dioxide which is given by X ¼
Vc ðtÞ Vt ðtÞ V0 Fg ðVc ðtÞ þ Vt ðtÞÞ þ δ0 þ Pa ðtÞ Fg RTnc0 A A
The method’s sensitivity is such that volume changes as lows as 1 mL can be measured, which correspond to 1.8 mg (or 41 μmol) of CO2 captured during carbonation. After consumption of the initial CO2 load (nc0), known CO2 amounts can be reinjected using a glass tube connected to a gas cylinder. To assess the materials maximum carbonation capacity, replenishment of CO2 can be repeated until carbonation of the solid sample ceased (Figure 2D). The maximum carbonation capacity accounts only for the added carbon after the materials are carbonated in the eudiometer regardless of their content in 9415
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Environmental Science & Technology native carbonates, a clear advantage over other methods, such as TGA that measure the total carbonate content regardless of their origin. Samples Characterization. The milling residues from Thetford Mines have been sampled on the mining heap and have been crushed to 1 mm (80% passing). XRD analyses indicated the presence of serpentine (dominant chrysotile + lizardite) as well as accessory brucite. Magnetite, chromite, chlorite, and phlogopite have been detected in small amounts. Some fine grained deposits observed on the heap display a small concentration of hydromagnesite, but our samples, sampled at the head of the conveyor from the processing plant, were unweathered. The BET specific surface area of Thetford Mines samples ranged from 5.17 to 10.17 m2/g (n = 10). The composition of Thetford Mines sample is very close to the theoretical composition of pure serpentine with Mg# (Mg/(Fe+Mg)) of 95 (Supporting Information Figure S1). The slight deviation toward higher MgO and loss of ignition (LOI) and lower SiO2 can be ascribed to the presence of brucite. The proportion of brucite over serpentine can be estimated from the relative amount of SiO2 and MgO. Using this method, the estimated amount of brucite is low (1.8 wt %), and it contains less than 3 wt % of the total MgO of the samples. The Amos samples were taken at three steps of the pilot plant concentration process. The fraction produced by the defibering process (fluff) has a grain size of 20 mesh (80% under 840 μm), the fraction produced after desliming (slimes) has a grain size of 100 mesh (80% under 150 μm), and the fraction left after flotation (final tail) has a grain size of 150 mesh (80% under 40 μm). The BET specific surface area for samples from the processing steps were similar, ranging from 9.03 to 11.49 m2/g. XRD patterns of the 12 samples reveal that they are mostly composed of serpentine (chrysotile and lizardite) and brucite. Chlorite and magnetite occur as minor components. The Amos samples plot on a mixing line between serpentine Mg#86 and brucite (Supporting Information Figure S1). The relative proportions of these phases are calculated from the relative amounts of SiO2 and MgO, which yields proportion of brucite ranging from 10 to 15 wt %, such that brucite contains between 18 and 26 wt % of the total MgO of the samples. Spontaneous formation of coalingite (Mg10Fe3+2(CO3)(OH)24 3 2H2O) has been observed on exploration drill core in Amos. The amount of CO2 already present in the samples prior to experimental carbonation was measured by LLC. The fluff fraction of each sample has the lowest carbon content (av. 0.30 wt % CO2), the slimes carbon contents are slightly higher (av. 0.49 wt % CO2), and the final tail fractions contain markedly more carbon (av. 0.88 wt % CO2). In addition to spontaneous carbonation during sorting, it is possible that a small amount of carbonates were present in the rocks before processing and became relatively enriched in the final tail fractions as a consequence of the sorting process. The final tail and slime fraction samples are separated by a wet sorting process and were delivered submerged in water. Acidity measurements for the four final tail fractions fall in a narrow range of basic pH, from 9.61 to 9.66. Process water from three of the four slime fractions have pH in the range 8.38 to 9.17, whereas sample S197B is more acidic (6.18), presumably because of the dissolution of sulfides. Chemical analyses reveal that, among the 29 analyzed elements, only Al, Ca, Mg, K, Si, and Na have significantly higher contents than detection level in all samples. The process water of slime samples tends to have higher content of dissolved ions. The relationship is particularly notable for Si and Fe. However, it should be noted that the concentration
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Figure 3. Comparison of CO2 content in carbonated samples according to eudiometer observations and infrared analysis. Here, only the samples that have reached their ultimate carbonation capacity are shown. Legend displays brucite content of Thetford Mines and Amos in wt%. Submerged samples contain 5992 wt % H2O, whereas the wet samples contain 1652 wt % H2O.
in dissolved Mg, comprised between 16 and 31 ppm, is unaffected by the pH.
’ RESULTS AND DISCUSSION Eudiometer-Monitoring of Carbonation Kinetics. The simple construction of the eudiometers from Pyrex glass, without joints or valves, ensures that no leakage occurs in the instrument over periods of weeks, as tested with empty eudiometers. Tests were carried out by loading the test eudiometer beaker with water only (distilled water or process water) and with dry samples. These experiments yielded undetectable, to low CO2 uptake (0 to 0.34 wt % CO2 in the final product). These tests demonstrate that CO2 is not diffusing through glycerol during the duration of the experiments. Dried solid samples were humidified with distilled water (31 to 92 wt % H2O). Samples with more than 6070 wt % H2O formed slurries where the sample was totally submerged. Atmospheric pressure variation (Figure 2A) causes volume change in the control eudiometer and test eudiometers (Figure 2B), which allows computing the volume change related to reaction with a sample as in Figure 2C. Figure 2D shows an example where a sample reacts with a series of batch of CO2, injected into the eudiometer after either the CO2 was entirely consumed or after the sample became unreactive with respect to CO2. The maximum carbon capture capacity is reached when injection of new CO2 produces no measurable reaction. Characterization of Reaction Products. The samples that have reached maximum capacity after repeated carbonation experiments were dried, and their CO2 content was measured by LLC analysis. In Figure 3, the results of these analyses are compared with the amount of consumed carbon calculated from measurement with the eudiometers. The LLC data were corrected after subtracting the contribution from carbon pre-existing in the samples (0.30 to 0.88% CO2). The two methods yield consistent results, albeit somewhat higher CO2 values are obtained using the eudiometer, likely a result of subtraction of initial carbon from the LLC measurement. XRD analyses identified nesquehonite (Mg(HCO3)(OH) 3 2H2O) as the dominant product of carbonation for the experimental series carried out at 21 °C (Supporting Information 9416
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Figure 4. Secondary electron photomicrographs of carbonate minerals formed by mineral carbonation: (a) numerous flakes, one of them being pointed by an arrow, have grown within a network of chrysotile fibers, (b) flat agglomerate that has probably developed between two grains, (c) elongated crystal in the typical habit of nesquehonite,31 and (d) massive crystal with cracks probably from dehydration after sample removal or in the SEM.
Figure S2), whereas dypingite (Mg5(CO3)4(OH)2 3 5H2O) prevails in samples carbonated at 33 °C. Nesquehonite has previously been identified as the most common product of brucite carbonation under high CO2 pressure.24 SEM observation in secondary electron mode and EDS analysis reveal different carbonates characterized by flakes, agglomerates, massive crystals, and well crystallized prisms (Figure 4). It should be noted that carbonates cover only a small fraction of the total grain surface and that there is no obvious control by sample minerals or habitus. Hence product coating of the reactive surface is not a limiting feature for the reaction. Stability of the carbonated phases has been tested by leaving some samples, partially saturated, in their eudiometers during 18 days after they reached their maximum carbonation capacity. These samples did not release or consume any CO2. Six other postcarbonated samples were kept at atmospheric conditions during 6 to 9 months. XRD analyses indicated that all the samples contained the same carbonates that they did at the end of the experiment. As hydrated carbonates are metastable,32 the carbonation reaction in eudiometers appears to be kinetically controlled.24
’ PARAMETERS DETERMINING THE TOTAL AMOUNT OF CARBONATION AND THE REACTION RATE The content of brucite entwined with serpentine appears to be the major parameter controlling the maximum amount of carbon a sample can store (Figure 3). The brucite-rich Amos milling residues has maximum carbonate uptake up to 9 wt % CO2 (8.6% CO2 by IR), whereas it barely reaches 2 wt % in the brucite-poor samples from Thetford Mines. The preferential consumption of brucite is confirmed by XRD analyses (Supporting Information Figure S2). Out of the 42 carbonated samples from Amos, brucite is not detected in 35 of the samples after reaction though it was a major component of all the samples before the experiments. The percentage of Mg that has been mobilized in the formation of
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carbonates is less than 2 wt % for Thetford Mines samples and is typically in the range of 1520 wt % for Amos samples. In both cases, it is less or equal to the amount of Mg stored in brucite. It is thus possible that serpentine has not been involved in the reaction, or at least, this might have been to a very limited extent. There is no influence of the grain size (0.04 to 0.84 mm) among the Amos samples, which have similar specific surface area (911 m2/g), but the coarser grain size of the Thetford Mines samples (1 mm) and their lower specific surface area (510 m2/g) is perhaps one cause for their lower maximum carbon uptake, in addition to their lower content in brucite. The reaction of CO2 with pure brucite and pure chrysotile is shown in Figure 5A. In that experiment, both brucite and chrysotile initially react at a similar rate, but reaction with chrysotile slows after ∼2 h whereas brucite captures 2.8 times more CO2 until it stops reacting after ∼15 h. Brucite has depleted CO2 from the gas mixture, whereas chrysotile is in contact with a gas mixture with 25% CO2. Experimental data of reacted CO2 have been fitted to appropriate polynomial or exponential equations to study reaction rates for the first injection stage. The derivative yields the reaction rate in the course of carbonation and is used to estimate the influence of different parameters controlling the reaction. The initial reaction rate of chrysotile in the presence of high concentration of CO2 (3334 mol %) is near 2.1 mmol CO2/h, and the rate of reaction decreases linearly with the concentration of CO2 in the gas mixture, except at low reaction rates (Figure 5A). The reaction rate of chrysotile decreases rapidly as it becomes unreactive after 5 h (Figure 5A). Brucite has a lower initial reaction rate (∼1.5 mmol CO2/h), but the rate of reaction displays a shallow slope with concentration of CO2 (Figure 5B). The control of the CO2 concentration in the gas mixture is further shown by reaction of the fluff fraction of sample 184F (Figure 6A), which shows that the initial reaction rate is proportional to the initial concentration of CO2 in the gas mixture. Figure 6A also shows that the sample had a slightly lower rate of reaction at a lower temperature (21 vs 33 °C), but in our test conditions, temperature (21 and 33 °C) has no significant effect on the reaction rate. The maximum rate of reaction of the samples is about 0.14 mmol CO2/h, and the rate displays a linear decrease with CO2 content in the gas mixture (Figure 6B). The initial reaction rates define a near unity partial order for CO2 (Figure 6A). In general, samples that were submerged displayed lower reaction rates in comparison to unsaturated samples, likely because of slower diffusion of CO2 in the water layer above the sample surface and in the pore space water of saturated samples. In unsaturated samples, CO2 diffuses in pore space filled with the gas mixture, and where water is wetting the grains surfaces. Two of the submerged samples display high reaction rates. One of them contained 60 wt % H2O and was barely submerged, whereas the other contained 78 wt % H2O. The potential causes for the higher reaction rates are unknown, but we speculate that undetected trace carbonate minerals acted as seeds for crystallization. A lower rate of reaction is observed for larger amounts of solid material (Supporting Information Table S3). Because the sample holders have the same cross-section area, the weight of the sample determines the height of the sample in the holder. The rate of reaction is not affected by the sample weight because of the fixed surface area of the sample exposed to the gas mixture. The sample grains closer to the upper surface react first, sometimes forming a cemented crust, whereas particles deeper in the sample holder undergo a delay induced by the diffusion of CO2 in the pore space. 9417
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Figure 5. A) Comparison of the quantity of CO2 (mmole, filled symbols) reacted with brucite (red) and chrysotile (green) as a function of time (h). Reaction of CO2 with the samples decreases the molar fraction of CO2 (open symbols) in the gas mixture. B) Rate of reaction of brucite and chrysotile versus the CO2 content of the gas mixture. Experimental data are superposed on the exponential fit curve.
The CO2 concentrations dealt with in the eudiometer kinetic tests by far outweigh those typically encountered under field conditions. In addition, the time scale for conducting the reaction experiments lasted up to 17 days (Figure 2). Under the assumption of stagnant gas conditions (Sherwood number Shgas = 2 and DCO2 ca. 106 m2/s), the gas-side mass transfer coefficients would be of the order of 105 m/s. This is tantamount to the lower-limit gas-side mass transfer coefficient since, in reality, the atmospheric pressure fluctuations act as gentle convective mixers helping to achieve faster homogenization of the CO2 composition in the eudiometer headspace. Therefore, it is highly unlikely that the eudiometer-measured chemical responses will be prone to mass transfer retardation effects from the gas phase, and one can safely assume the measurements reflect true intrinsic gassolid kinetics.
The results of these experiments are in accord with gassolid carbonation experiments which indicate that CO2 reacts preferentially with the Mg(OH)2, brucitic, layer of the chrysotile structure, leaving the internal silica-rich layers largely unreacted.19 Figure 5 is interpreted to show initial reaction of CO2 with the external brucitic layer of chrysotile after which, the chrysotile reacts slowly whereas brucite reaction depletes the gas mixture in CO2, such that the lower concentration of CO2 slows the reaction until new CO2 is injected (Figure 2D). The initial higher rate of reaction for chrysotile is likely related to the its specific surface areas (∼14.4 m2/g, 19) compared to that of brucite (0.2 m2/g, 50100 um size fraction33). The Amos samples are estimated to contain 1015% brucite (Supporting Information Figure S2), and the fluff samples of Figure 6 display a rate of reaction approximately 10% of that of pure brucite 9418
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Figure 6. Experimental results for unsaturated fluff fractions of sample 184F with ∼50 wt % H2O at 21 and 33 °C. A) Rate of reaction versus time for different CO2 contents in the gas mixture and at temperatures of 21 and 33 °C. The CO2 partial order (n) is based on the initial reaction rates. B) Rate of reaction versus the CO2 content of the gas mixture.
(Figure 5), suggesting that the rate of reaction in the mine waste samples studied is largely controlled by the abundance of brucite in the sample. In southern Quebec only, the amount of ultramafic milling and mining residues stored in heaps from chrysotile mining is estimated to 2 Gt, which could store up to ca. 700 Mt C.30 New mining projects, such as Dumont Nickel near Amos, Canada, can estimate the potential for CO2 capture and storage in mine waste to offset their expected greenhouse gas emissions. The worldwide amount of variably serpentinized peridotites exceeds the
requirement to store the excess of atmospheric CO2, including future emissions.4 As the carbonation potential of ultramafic material is variable and depends on the brucite content, tests in eudiometers can help identify the most reactive materials and determine the optimal conditions for carbonation.
’ ASSOCIATED CONTENT
bS
Supporting Information. Tables providing whole-rock chemical composition of samples, pH of process water, and
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Environmental Science & Technology parameters and results of carbonation experiments. Also included are additional figures showing chemical and experimental data. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Phone: 1-418-656-3141. Fax: 1-418-656-7339. E-mail: beaudoin@ ggl.ulaval.ca.
’ ACKNOWLEDGMENT This research has been funded by a Natural Science and Engineering Research Council of Canada Discovery grant to G. Beaudoin and by Royal Nickel Corporation. M. Plante is gratefully acknowledged for his help with experiments and ingenuity solving experimental problems. ’ REFERENCES (1) Intergovernmental Panel on Climate Change 2007 Synthesis Report of the IPCC Fourth Assessment Report: Summary for Policy Makers. 2007. Available at http://www.ipcc.ch/pdf/assessment-report/ ar4/syr/ar4_syr_spm.pdf (accessed February 5, 2009). (2) Seifritz, W. CO2 disposal by means of silicates. Nature (London, U.K.) 1990, 345, 486. (3) IPCC Special Report on Carbon Dioxide Capture and Storage; Metz, B., Davidson, O., de Coninck, H., Loos, M., Meyer, L., Eds.; Cambridge University Press: New York, 2005. (4) Lackner, K.; Wendt, C.; Butt, D.; Joyce, E.; Sharp, D. Carbon dioxide disposal in carbonate minerals. Energy 1995, 20, 4802. (5) Lackner, K. S. A guide to CO2 sequestration. Science (Washington, DC, U.S.) 2003, 300, 1677–1678. (6) Sundquist, E. T. The global carbon dioxide budget. Science (Washington, DC, U.S.) 1993, 259, 934–941. (7) Goldberg, P.; Chen, Z. Y.; O’Connor, W; Walters, R.; Ziock, H. CO2 Mineral Sequestration Studies. Presented at First National Conference on Carbon Sequestration, Washington, DC, May 1417, 2001. (8) Berner, R. A.; Kothavala, Z. GEOCARB III: a revised model of atmospheric CO2 over Phanerozoic time. Am. J. Sci. 2001, 301, 182–204. (9) O’Connor, W. K.; Dahlin, D. C.; Rush, G. E.; Gerdemann, S. J.; Penner, L. R. Energy and economic consideration for ex situ aqueous mineral carbonation. In Proceeding of the 29th International Technical Conference on Coal Utilization & Fuel System 2004, 71. (10) McKelvy, M. J.; Chizmeshya, A. V. G.; Diefenbahcer, J.; Bearat, H.; Wolf, G. Exploration of the role of heat activation in enhancing serpentine carbon sequestration reactions. Environ. Sci. Technol. 2004, 38, 6897–6903. (11) Gerdemann, S. J.; O’Connor, W. K.; Dahlin, D. C.; Penner, L. R.; Rush, G. E. Ex situ aqueous mineral carbonation. Environ. Sci. Technol. 2007, 19, 95–101. (12) Maroto-Valer, M. M.; Fauth, D. J.; Kuchta, M. E.; Zhang, Y.; Andresen, J. M. Activation of magnesium rich minerals as carbonation feedstock material for CO2 sequestration. Fuel Process. Technol. 2005, 86, 1627. (13) O’Connor, W. K.; Walters, R. P.; Dahlin, D. C.; Rush, G. E.; Nilsen, D. N.; Turner, In Proceedings of the 26th International Technical Conference on Coal Utilization & Fuel Systems 2001, 765. (14) H€anchen, M.; Prigiobbe, V.; Storti, G.; Seward, T. M.; Mazzotti, M. Dissolution kinetics of fosteritic olivine at 90150°C including effects of the presence of CO2. Geochim. Cosmochim. Acta 2006, 70, 4403–4416. (15) H€anchen, M.; Prigiobbe, V.; Baciocchi, R.; Mazzotti, M. Precipitation in the Mg-carbonate system - effects of temperature and CO2 pressure. Chem. Eng. Sci. 2008, 63, 1012–1028.
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