Thermal Activation of Antigorite for Mineralization of CO2

Article pubs.acs.org/est

Thermal Activation of Antigorite for Mineralization of CO2 Reydick D. Balucan and Bogdan Z. Dlugogorski* Priority Research Centre for Energy, The University of Newcastle, Callaghan, NSW 2308, Australia S Supporting Information *

ABSTRACT: This contribution demonstrates the sensitivity of antigorite dehydroxylation to treatment conditions and discusses the implications of the observations for scientific (i.e., dehydroxylation kinetics) and technological (i.e., energy efficient conditions and design of practical activation reactors) applications. At present, the energy cost of dehydroxylation of serpentinite ores represent the most important impediment for a large scale implementation of sequestering CO2 by mineralization. We have analyzed changes in antigorite’s derivative thermogravimetric curves (DTG) and deduced factors affecting the mass loss profiles. The imposed heating rate, type of purge gas, type of comminution and sample mass all influence the dehydroxylation curve. However, the results show no influence of material of construction of the heating vessel and flow rate of the purge gas. We report an important effect of oxidation of Fe2+ under air purge gas that occurs prior to dehydroxylation and leads to formation of hematite skins on serpentinite particles, slowing down subsequent mass transfer and increasing the treatment temperature. From the process perspective, 75 μm particles afford optimal conditions of temperature and rate of dehydroxylation. Overall, the practical considerations, in thermally activating serpentinite ores for storing CO2 by carbonation, comprise rapid heating, proper size reduction, prior demagnetisation, and fluidization of the powder bed. currently used (−38 to −75 μm) 18−21 in serpentine carbonation at a laboratory scale. The influence of the material of construction of a unit operation and type of a purge gas on efficiency of the processing operation remain poorly understood. Both variables require attention due to variable mineralogical and chemical composition of serpentinite rocks.22−24 Not only do these rocks host the rock-forming serpentine minerals (lizardite, antigorite, and chrysotile), they may retain their relict peridotitic minerals (forsterite and enstatite) as well as contain various amounts of metal oxides associated with the serpentinisation process (i.e., the exothermic hydration of the peridotitic minerals). Cations such as Fe2+, Fe3+, and Al3+ are incorporated into the octahedral and tetrahedral sheets of the serpentine minerals.23,25−28 Normally, Fe2+ replaces Mg2+ in octahedral sheet, whereas Fe3+ and Al3+ may appear both in octahedral and tetrahedral sheets, replacing Si4+ in the latter.29 The oxidation of Fe2+ was reported to influence the dehydroxylation process.30,31 It is therefore of practical significance, to the design of a dehydroxylation reactor, to understand the suitability of a refractory vessel (e.g., alumina) and purge gases (e.g., CO2, water vapor or air).

1. INTRODUCTION Accurate measurements of the dehydroxylation of serpentine minerals during thermal treatment allow deriving the thermokinetic parameters and calculating the necessary heat requirements. Such measurements may also serve to develop new technologies for activating serpentines, and to design equipment items, for implementing CO2 sequestration by mineralization at a realistic scale. However, despite the numerous thermal studies on serpentine minerals1−15 only a small number of investigations examined the effect of treatment conditions.9,14,16,17 To the best of our knowledge, no study evaluated the influence of these conditions for preparing activated serpentine minerals for their carbonation. Investigations are needed to identify and quantify the effect of the treatment parameters on the thermal activation of serpentines. Such investigations must supply information of kinetics of serpentine dehydroxylation, that, in combination with heat transfer parameters, could serve to design unit operations (equipment items) for testing the viability of mineral carbonation at a pilot plant scale. Outstanding questions include the determination of the suitable feedstock for heat activation (crushed versus ground), the appropriate operational sequence and cost efficiency (kWhe) for each option. Operational viability and cost efficiency of thermal activation of serpentines for mineral carbonation must dictate the material’s particle size,14 and hence comminution technology. Small scale experiments could assist in identifying the practical particle size among those © 2012 American Chemical Society

Special Issue: Carbon Sequestration Received: Revised: Accepted: Published: 182

September 11, 2012 October 28, 2012 November 28, 2012 December 6, 2012 dx.doi.org/10.1021/es303566z | Environ. Sci. Technol. 2013, 47, 182−190

Environmental Science & Technology

Article

Table 1. Chemical, Mineralogical and Physical Properties of the Sample Fractions; all Particle Sizes are in μm A

B

C

demagnetised fractions chemical composition of starting material, % weightc

mineral composition (International Center for Diffraction Data reference code)

SiO2 MgO FeOa Fe2O3b Al2O3 CaO Na2O LOI starting material

quenched materiald particle size of starting material, μm (±3.8 μm)

d3,2 d4,3 d90 d50 d10 d80e

D

E magnetic fractions

43.2 38.2 3.08 1.74 1.04 0.13 0.11 11.9

41.8 38.2 3.08 3.79 0.89 0.05 0.05 12.0

antigorite-8.0 M (00−007− 0417), dimagnesium oxide dihydroxide (01−070−9187)

antigorite-8.0 M (00−007− 0417) periclase (01−077− 2364), triiron tetroxide (01−089− 0691) forsterite (00−034−0189), enstatite (00−019− 0768) 5.31 4.85 4.37 2.87 44.2 31.0 17.9 6.35 125 86.4 47.2 14.2 15.7 11.8 9.03 4.23 ≥2000 1.82 1.73 1.69 1.39 75.0 52.0 31.0 10.0

a

Obtained from titration, represents the Fe2+ content of the primary rock. bRepresents magnetite (Fe3O4) produced from serpentinisation. cTrace components 30 mg.1 The pertinent thermal signatures include the serpentine doublet which comprises a low temperature shoulder, Tsh and the first peak temperature, Tp1, as well as antigorite’s diagnostic high temperature peak, Tp2 (Table S4, Supporting Information). The low temperature shoulder, Tsh, encompasses 635 to 679 °C and is common among serpentine minerals, whereas the Tp’s are shifted to higher temperature with respect to lizardite and chrysotile. From the present results and those of Viti1 for antigorite, our specimen’s Tp1, falls around 710 to 720 °C, whereas the diagnostic peak, Tp2, lies between 730 and 760 °C. At each respective Tp, the mass loss rate maxima, -(dm/dt)/mo_max_Tp1 and -(dm/dt)/mo_ max_Tp2, are roughly identical at 1.7 × 10−4 ± 1.4 × 10−6 s−1 (for both Tp1 and Tp2). The quenched material, shown in Figure 2, indicates that full dehydroxylation (∼1000 °C) of antigorite results in a recrystallized solid. The fully dehydroxylated mineral shows

3. RESULTS AND DISCUSSION Antigorite dehydroxylation, as seen in the DTG curve in Figure 1, covers a wide temperature region varying from ∼500 to 800 °C. This temperature region defines the removal of structurally bound water, constituting 11.43 ± 0.03% w/w antigorite (Δm105−850). The mass loss of 543 kJ mol−1 and pre-exponential factor, A, varies from 1 × 10−8 to 1 × 108 s−1.2−5,9−11,15,33 Figure 3 shows the DTG curves of antigorite under different experimental conditions. Variations in sample mass, purge gas, type of comminution, and heating rates result in a marked departure from the mass loss profile of the base case, while changes in the type of heating vessel, purge gas flow rate and particle size do not influence the DTG curves. The crucible material (alumina vs platinum) does not affect the mass-loss

profiles (Figure 3a). This indicates that a wide range of materials could be appropriate for constructing the dehydroxylation reactor. In a practical sense, serpentine heat treatment (i.e., 25 to >800 °C) could involve a vessel made either of a refractory material (i.e., alumina) or a relatively inert material (i.e., stainless steel), with the material selection based on surface erosion rates during treatment. The identical thermal responses for both purge gas flow rates (Figure 3b) imply that, at 20 mL min−1, the dehydroxylation reaction is neither hindered by entrapment of liberated water vapor within the sample matrix nor limited by any build up of product gas above the sample bed. Furthermore, this suggests that, for the present set of experimental conditions (i.e., 5.5 mg of 75 μm antigorite heated in alumina crucible from 25 to 1000 °C at 10 °C min−1 under argon purge flowing at 20 mL min−1), the dehydroxylation reaction proceeds far from equilibrium; that is, irreversibly, with a negligible rate of reverse reaction. In practical situations, with water vapor accumulating in the purge gas, this deceleration in the reaction may need to be included in the design calculations.8,9 Figure 3c shows similar thermal curves for the particle sizes typically employed in mineral carbonation. Unless required by the carbonation reaction, the result indicates that, stage 2 grinding, to produce feedstocks −38 μm in size at cost of about 186

dx.doi.org/10.1021/es303566z | Environ. Sci. Technol. 2013, 47, 182−190

Environmental Science & Technology

Article

63.5 kWhe (tonne mineral)−1,18 is unnecessary due to similar thermal curves generated by particles of 31 to 75 μm in size. It is indeed more practical and cost efficient to subject larger particles (75 μm) to thermal treatment. Under this option, the total energy requirement, for prior size reduction to produce feedstocks 75 μm in size, amounts to 11.8 kWhe (tonne mineral)−1. This estimate accounts for electrical power required for both crushing (1.8 kWhe (tonne mineral)−1) and stage 1 grinding (10 kWhe (tonne mineral)−1, to −75 μm).18 This translates to energy savings of about 53.5 kWhe by avoiding stage II grinding. The treatment conditions reported in Figures 3d−h influence antigorite’s mass loss profile. Employing larger sample mass and different oxidizing purge gases results in minor variations, while the use of a crushed sample with no demagnetisation and employing high heating rates significantly alter the mass-loss rate. The process parameters that resulted in significant deviations in thermal profiles of antigorite were analyzed in detail to assess the extent of their influence. To simplify the evaluation, we represented the thermal profile (Table S5, Supporting Information) by an average value of the two peak temperatures, Tp_mean, and the average value of the maximum mass-loss rate for the temperature doublet, −(dm/dt)/mo_max_mean. These values, redesignated as T‑H2O and r‑H2O, signify the practical activation temperature (i.e., temperature at which dehydroxylation rate is fastest) and its corresponding mass-loss rate, respectively. Figure 4 summarizes the extent of deviations from the characteristic T‑H2O and r‑H2O of the sample and are expressed in terms of ΔT (°C) and % change, in that order, with respect to the base case, for which T‑H2O = 725 ± 2 °C and r‑H2O = 1.7 × 10−4 ± 1.4 × 10−6 s−1. While all deviations favor increase in T‑H2O (11−39 °C), r−H2O may either decrease by as much as 10% or increase by as high as 192%. This implies that changes in the processing parameters result in delays in the removal of evolved gases, and could either enhance or deteriorate the rate at which escaping product gas leaves the sample matrix. The probable factors explaining this type of behavior and the possible implications of the present observations to design of dehydroxylation reactor are discussed in detail in the subsequent paragraphs. 3.1. Effect of Sample Mass. Mass increase by ∼600% elevated the T‑H2O by 11 °C (Figure 4a) as well as increased the r‑H2O by 10% (Figure 4b). The new Tp mean of 736 °C for 38 mg sample of antigorite falls between the previously reported Tp for ∼30 and 44 mg of antigorite at 731 and 749 °C, respectively.1,13 A slight increase in r‑H2O by 4% with 100% mass increase could be explained in terms of competing effects of the increase in number of hydroxyl sites and reduction in gas permeability within the sample matrix that elevates H2O concentration in void spaces. This means that the evolved gas becomes entrapped in the stationary matrix, delaying its escape into the bulk carrier gas. Evolved water vapor entrained within the powder bed induces the reverse reaction, slowing down the dehydroxylation process. This phenomenon, known as the depth effect, operates similarly in the thermal decomposition of carbonates (50−300 mg),35 owing to increased levels of CO2 that force the reverse reactions. We estimated the sample loading and bed height, based on a tap density of 1.53 g mL−1 sample and crucible capacity of 0.1 mL. While sample mass below 11 mg takes no more than 15% of the crucible’s loading capacity, the ∼38 mg sample occupies

as much 40%. The latter corresponds to a bed height of 3.2 mm, which is almost half of the entire crucible height of 8 mm. This means that the evolved gases need to negotiate more than twice the distance through the sample matrix. The entrapment of water vapor within the sample bed increases the local partial pressure of water vapor, PH2O. Since serpentine dehydroxylation kinetics is highly dependent on PH2O,9 one would expect the delays in Tp, exactly as observed in this study. These results suggest deployment of fluidized bed reactors for activating serpentine minerals, in preference to moving bed reactors. 3.2. Effect of Purge Gas. The use of an oxidizing gas increases heating requirements and slows down the mass loss rate. Despite the perceived simplicity of air activation, the delay in T‑H2O by 17 °C (Figure 4a) and the decrease in r‑H2O by 10% (Figure 4b) makes this purge gas less desirable for operating a dehydroxylation reactor. Based on the obvious decoloration of the sample to reddish hue, we suggest that the formation of hematite layer on the surface of serpentine grains limits the removal of the liberated water. Hematite formation in serpentine minerals had been described by other workers as “fully ferric chrysotile”,31 hematite formation in carlosturanite,36 and oxidized form of magnetite during lizardite heat treatment.37 Under oxidizing atmosphere, the transformation31,36,38 of Fe2+ to Fe3+ most often concludes prior to the onset of dehydroxylation. Besides the oxidation of magnetite, Fe2+, present in the octahedral sites in serpentinite, oxidizes to Fe3+. Also, Fe3+ present in tetrahedral sites migrates out to octahedral sites.31,39 By conservation of charge, a third of the converted Fe3+ from Fe2+ must migrate toward the surface of serpentinite grains to form Fe2O3 layers, as observed by the reddish hue on surfaces of serpentine activated under air. Based on the chrysotile studies by MacKenzie and MacGavin,31 iron present in the hematite layers corresponds to about 10% of the initial iron content of the mineral. With antigorite having relatively higher proportions of Fe2+ than lizardite or chrysotile,40 the effect of an oxidizing gas is probably more pronounced for antigorite than for the other two polymorphs of serpentine. The removal of Fe3O4 via demagnetisation prior to heat activation reduces the severity of hematite formation. This is exemplified in the slightly faster mass loss rate of the partially demagnetised sample (Sample A, despite its relatively larger size) as compared to the nondemagnetised Sample D. As can be seen in Figures 3g and 4b, the mass loss rate for the magnetic sample (Sample D) slightly decreases by about 3%. However, the ARC (Albany Research Center, now U.S. Department of Energy’s National Energy Technology Laboratory) investigations reported that oxidizing gas used during heat treatment of a partially demagnetised antigorite appeared not to influence the serpentinite conversion during carbonation.37 This means that a sufficiently high amount of Fe2+, was removed by demagnetisation prior to thermal treatment, resulting in formation of minor layers of Fe2O3 during activation that had no influence on the conversion of activated serpentine during its carbonation. But the findings of Connor et al. might be specific to their serpentine mineral (Section 4, Supporting Information). In a practical situation, the use of CO2 as purge gas can provide similar efficiency as that of an inert gas. A possible scenario may involve bleeding some CO2 delivered from a capture plant to satisfy the purge requirements of the activation unit. After exiting the activation unit (calciner), mixture of CO2 and steam could be routed through a heat exchanger for 187

dx.doi.org/10.1021/es303566z | Environ. Sci. Technol. 2013, 47, 182−190

Environmental Science & Technology

Article

Figure 5b illustrates the decrease in processing time to produce thermally treated antigorite with 20% residual OH content, % OHres. The required treatment time appears to scale with the heating rate as t = 630/β0.942, based on the measurements collected between 10 and 30 °C min−1. Figure 5b extrapolates the treatment time to higher heating rates (40−100 °C min−1). We conclude this section by noting that while slow heating rates are essential for investigating intrinsic kinetics, operation of a practical reactor necessitates more rapid thermal processing. 3.5. Effect of Comminution Type. Figures 4a,b show that while the r‑H2O of the crushed sample increases by 35%, the T‑H2O is 36 °C higher than that of the ground sample. This is as expected for a crystalline solid, evolving water en-bulk at relatively higher temperature due to the imparted structural rigidity of the crystal lattice structure. Further comminution (i.e., grinding) could disrupt this structure,41 rendering the material susceptible to thermal and/or chemical decomposition at relatively lower temperatures. The apparent shift in the Tsh location to ∼725 °C also suggests that, this thermal feature is neither contaminant chrysotile nor partially amorphised material but an intermediate phase associated with thermal dehydroxylation. This is because the reported peak temperature of high purity chrysotile at ∼650 °C,1 and even our experimentally determined Tp’s of chrysotile fiber (Figure S11, Supporting Information, also shown as the table of content graphic) at ∼690 °C are significantly lower. While the direct use of crushed samples in thermal activation may be possible, the apparent savings in electrical power is negated by the likelihood of further comminution after thermal activation. This is because dehydroxylation at especially high temperatures (approaching 820 °C) may induce the formation of enstatite. Further comminution is also necessary to increase solid’s surface area. In general, the smaller particles from ground samples represent the preferred feed for thermal dehydroxylation due to the relative ease of dehydroxylation. While there is no significant differences in the thermal profile among the two types of grinding methods (wet grinding ∼21 min; dry grinding ∼1 min), the significantly faster grind time required to reduce the particle size makes dry grinding attractive. It must also be noted that although wet ground sample initially contains magnesium hydroxide species (Table 1), these hydroxides dehydroxylate prior to antigorite. Hence, the presence of Mg(OH)2 does not have any discernible effect on the subsequent serpentine dehydroxylation process. 3.6. Environmental Technology Implications to Serpentine Activation for CO2 Storage by Mineralisation. The evaluation of the effects of sample mass, purge gas, heating rate, and communition type on the behavior of antigorite has provided new (i) knowledge of the dehydroxylation kinetics of this mineral at elevated temperatures, and (ii) scientific underpinnings for designing larger unit operations needed to for scaling up the process of sequestering of CO2 by mineralization. Particles of less than 75 μm in size afford optimal conditions of temperature and rate of dehydroxylation, and allow savings in electrical power for the size reduction stage. The detrimental effect of oxidation of Fe2+ to Fe3+, owing to formation of hematite layers on activated particles, suggests CO2 as preferred purge gas. We highlight the need to fluidize the powder bed to avoid the entrapment of liberated water that engenders the reverse reaction and results in higher processing temperature. Otherwise, the effect of bed height on inducing the reverse

extraction of heat from the gas. Steam could be then condensed, separated and used in the subsequent processes (i.e., water input for carbonation), prior to recycling of CO2 to the activation unit or its use in the carbonation reactor. While air is readily available for actual operations, it is not the preferred purge gas for activating serpentines. Overall, we conclude that a diffusive barrier of Fe2O3 coating the antigorite surface forces the temperature shift in T‑H2O to higher values and the decrease in the r‑H2O. This effect highlights a need for demagnetisation, especially if thermal treatment involves an oxidizing gas such as air. In other words, demagnetisation serves the dual purpose of removing valuable minerals of iron and chromium for cost offsets, and to decrease the energy load required for activation. 3.3. Effect of Heating Rate. Figures 4a,b shows that the increase in heating rate by 100% (20 °C min−1) and 200% (20 °C min−1) results in impediment in T‑H2O by 11 and 22 °C, respectively. Despite these delays, the increase in r‑H2O (Figure 4b) is extremely high, at 89% and 192%, respectively (Section 5, Supporting Information). As can be seen in Figure 5a, the

Figure 5. (a) Practical activation temperatures (T‑H2O) and estimated mass loss rates (r‑H2O) with respect to the increase in heating rate, β. (b) Required time at various heating rates (β) for the production of thermally treated antigorite containing 20% OHres.

response in both T‑H2O and r‑H2O with increased heating rates follows an increasingly linear trend. The shifts in T‑H2O and r‑H2O toward higher values, as function of the increasing heating rate, arise as a result of the dependence of the mass-loss rate on the Arrhenius expression. Further analysis indicates that while higher heating rates elevate T‑H2O, a significant increase in r‑H2O drastically reduces the processing time, to reach target degree of dehydroxylation. 188

dx.doi.org/10.1021/es303566z | Environ. Sci. Technol. 2013, 47, 182−190

Environmental Science & Technology

Article

(10) Weber, J. N.; Greer, R. T. Dehydration of serpentine: Heat of reaction and reaction kinetics at PH2O = 1 atm. Am. Mineral. 1965, 50, 450−464. (11) Brindley, G. W.; Hayami, R. Kinetics and mechanisms of dehydration and recrystallization of serpentine: I. In Proceedings of the 12th National Conference on Clays and Clay Minerals, Atlanta, Georgia, September 30 - October 2, 1963; Atlanta, Georgia, 1964; pp 35-47, 49−54. (12) Ball, M. C.; Taylor, H. F. W. The dehydration of chrysotile in air and under hydrothermal conditions. Mineral. Mag. 1963, 33, 467−482. (13) Franco, F.; Perez-Maqueda, L.; Ramirez-Valle, V.; PerezRodriguez, J. Spectroscopic study of the dehydroxylation process of sonicated antigorite. Eur. J. Mineral. 2006, 18, 257−264. (14) Martinez, E. The effect of particle size on the thermal properties of serpentine minerals. Am. Mineral. 1961, 46, 901−912. (15) Cattaneo, A.; Gualtieri, A. F.; Artioli, G. Kinetic study of the dehydroxylation of chrysotile asbestos with temperature by in-situ XRPD. Phys. Chem. Minerals. 2003, 30, 177−183. (16) Maroto-Valer, M. M.; Fauth, D. J.; Kuchta, M. E.; Zhang, Y.; Andresen, J. M. Activation of magnesium rich minerals as carbonation feedstock materials for CO2 sequestration. Fuel Process. Technol. 2005, 86, 1627−1645. (17) Li, W.; Li, W.; Li, B.; Bai, Z. Electrolysis and heat pretreatment methods to promote CO2 sequestration by mineral carbonation. Chem. Eng. Res. Des. 2009, 87, 210−215. (18) Gerdemann, S. J.; O’Connor, W. K.; Dahlin, D. C.; Penner, L. R.; Rush, H. Ex- situ aqueous mineral carbonation. Environ. Sci. Technol. 2007, 41 (7), 2587−2593. (19) Huijen, W. J. J.; Comans, R. N. J. Carbon Dioxide Sequestration by Mineral Carbonation: Literature Review, ECN-C-05-022; Energy Research Centre of the Netherlands: The Netherlands, 2005. (20) Huijen, W. J. J.; Comans, R. N. J. Carbon Dioxide Sequestration by Mineral Carbonation: Literature Review, ECN-C-03-016; Energy Research Centre of the Netherlands, The Netherlands. 2003. (21) Sipila, J.; Teir, S.; Zevenhoven, R. Carbon dioxide sequestration by mineral carbonation-Literature review update. Report VT 2008−1; Abo Akademi University, Heat Engineering Laboratory: Turku, Finland, 2008; http://web.abo.fi/∼rzevenho/ MineralCarbonationLiteratureReview05-07.pdf. (22) Davis, M. The CO2 Sequestration Potential of the Ultramafic Rocks of the Great Serpentinite Belt, New South Wales. Honours Thesis, The University of Newcastle, Newcastle, 2008. (23) Moody, J. B. Serpentinization: A review. Lithos 1976, 9, 125− 138. (24) O’Hanley, D. S.; Wicks, F. Conditions of formation of lizardite, chrysotile and antigorite, Cassiar, British Columbia. Can. Mineral. 1995, 33, 753−773. (25) Auzende, A. L.; Guillot, S.; Devouard, B.; Baronnet, A. Serpentinites in an Alpine convergent setting: Effects of metamorphic grade and deformation on microstructures. Eur. J. Mineral 2006, 18, 21−33. (26) O’Hanley, D. S. Serpentinites: Records of Tectonic and Petrological History: Oxford University Press: Oxford, United Kingdom, 1996. (27) O’Hanley, D. S.; Dyar, M. D. The composition of lizardite 1T and the formation of magnetite in serpentinites. Am. Mineral. 1993, 78, 391−404. (28) O’Hanley, D. S.; Dyar, M. D. The composition of chrysotile and its relationship with lizardite. Can. Mineral. 1998, 36, 727−739. (29) Burzo, E. Serpentines and related silicates. In Phyllosilicates.; Springer-Verlag: Berlin Heidelberg. 2009; col. 27I5b. (30) O’Connor, W. K.; Dahlin, D. C.; Nilsen, R. P.; Rush, G. E.; Walters, R. P.; Turner, P. C. In Carbon Dioxide Sequestration by Direct Mineral Carbonation: Results from Recent Studies and Current Status, 1st Annual DOE Carbon Sequestration Conference, DOE/ARC-2001-029; National Energy Technology Laboratory, United States Department of Energy: Washington, DC, May 14−17, 2001. (31) MacKenzie, K. J. D.; McGavin, D. G. Thermal and mossbauer studies of iron-containing hydrous silicates. Part 8. Chrysotile. Thermochem. Acta 1994, 244, 205−221.

reactions must be included in the design of a moving bed reactor, as the results suggest that the build-up of localized PH2O is likely to occur in moving bed reactors with increase in bed height. We also recommend the rapid thermal treatment as a practical way to increase the throughput, and minimize the reactor’s size.

■

ASSOCIATED CONTENT

S Supporting Information *

Further information on the material standard pretreatment process, mineralogy, thermal properties and details on the deviations in T‑H2O and r‑H2O are found in Sections 1−3 of the Supporting Information. Sections 4−7 provide additional notes on the effects of purge gas, heating rate and comminution type. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*Phone: +61 2 4985 4433; fax: +61 2 4921 6893; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This study was funded by an internal grant (ref. No. G0189103) from the University of Newcastle. We gratefully acknowledge valuable discussions with Prof. Eric Kennedy during the course of this research. The first author thanks the University of Newcastle for a Postgraduate Research Scholarship. Material and analytical assistance from Prof. Erich Kisi, Dr. Judy Bailey, Ms. Monica Davis, and Ms. Jennifer Zobec (EM-Xray Unit) are greatly appreciated.

■

REFERENCES

(1) Viti, C. Serpentine minerals discrimination by thermal analysis. Am. Mineral. 2010, 95, 631−638. (2) Inque, T.; Yoshimi, I.; Yamada, A.; Kikegawa, T. J. Time-resolved X-ray diffraction analysis of the experimental dehydration of serpentine at high pressure. J. Mineral. Petrol. Sci. 2009, 104, 105−109. (3) Llana-Funez, S.; Brodie, K. H.; Rutter, E. H.; Arkwright, J. C. Experimental dehydration kinetics of serpentine using pore volumometry. J. Metamorph. Geol. 2007, 25, 423−438. (4) Candela, P. A.; Crummett, C. D.; Earnest, D. J.; Frank, M. R.; Wylie, A. G. Low pressure decomposition of chrysotile as a function of time and temperature. Am. Mineral. 2007, 92, 1704−1713. (5) Perrillat, J. P.; Daniel, I.; Koga, K. T.; Reynard, B.; Cardon, H.; Crichton, W. A. Kinetics of antigorite dehydration: A real-time x-ray diffraction study. Earth Planet. Sci. Lett. 2005, 236, 899−913. (6) McKelvy, M. J.; Chizmeshya, A. V. G.; Diefenbacher, 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. (7) MacKenzie, K. J. D.; Meinhold, R. H. Thermal reactions of chrysotile revisted: A 29Si and 25Mg MAS NMR study. Am. Mineral. 1994, 79, 43−50. (8) Tyburczy, J. A.; Ahrens, T. J. Dehydration kinetics of shocked serpentine. In Proceedings of the 18th Lunar and Planetary Science Conference, Houston, Texas, March 16−20, 1987; Houston, Texas, 1988. (9) Brindley, G. W.; Narahari, A.; Sharp, J. H. Kinetics and mechanism of dehydroxylation processes: II. Temperature and vapor pressure dependence of dehydroxylation of serpentinite. Am. Mineral. 1967, 52, 1697−1705. 189

dx.doi.org/10.1021/es303566z | Environ. Sci. Technol. 2013, 47, 182−190

Environmental Science & Technology

Article

(32) Khoo, H. H.; Tan, R. B. H. Life cycle evaluation of CO2 recovery and mineral sequestration alternatives. Environ. Prog. 2006, 25 (3), 208−217. (33) Balucan, R. D.; Kennedy, E. M.; Mackie, J. F.; Dlugogorski, B. Z. Optimization of antigorite heat pre-treatment via kinetic modeling of the dehydroxylation reaction for CO2 mineralization. Greenhouse Gas Sci. Technol. 2011, 1, 294−304. (34) Wicks, F. J. Status of the reference x-ray powder-diffraction patterns for the serpentine minerals in the PDF database-1997. Powder Diffr. 2000, 15 (1), 42−50. (35) Sharp, J. H.; Wilburn, F. W.; McIntosh, R. M. The effect of procedural variables on TG, DTG and DTA curves of magnesite and dolomite. J. Therm. Anal. 1991, 37, 2021−2029. (36) Compagnoni, R.; Ferraris, G.; Mellini, M. Carlosturanite, a new asbestiform rock-forming silicate from Val Varaita, Italy. Am. Mineral. 1985, 70, 767−772. (37) O’Connor, W.; Dahlin, D. C.; Nilsen, R. P.; Rush, G. E.; Walters, R. P.; Turner, P. C. In CO2 Storage in Solid Form: A Study of Direct Mineral Carbonation. 5th International Conference on Greenhouse Gas Technologies, DOE/ARC-2000-01; Cairns, Australia, August 14− 18, 2000. (38) Gallagher, K. J.; Feitknecht, W.; Mannweiler, U. Mechanism of oxidation of magnetite to γ-Fe2O3. Nature 1968, 217, 1118−1121. (39) Malysheva, T. V.; Satarova, L. M.; Polyakova, N. P. Thermal transformations of layer silicates and the nature of iron-bearing phase in CII-type Murray carbonaceous chondrite. Geochem Int. 1977, 14, 117−128. (40) Page, N. J. Chemical differences among the serpentine “polymorphs”. Am. Mineral. 1968, 53, 201−215. (41) Drief, A.; Nieto, F. The effect of dry grinding on antigorite from Mulhacen, Spain. Clay Clay Mineral. 1999, 47 (4), 417−424.

190

dx.doi.org/10.1021/es303566z | Environ. Sci. Technol. 2013, 47, 182−190