Magnesium Hydroxide Extracted from a Magnesium-Rich Mineral for

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Environ. Sci. Technol. 2008, 42, 2748–2752

Magnesium Hydroxide Extracted from a Magnesium-Rich Mineral for CO2 Sequestration in a Gas–Solid System PAO-CHUNG LIN, CHENG-WEI HUANG, CHING-TA HSIAO, AND HSISHENG TENG* Department of Chemical Engineering and Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan 70101, Taiwan

Received August 22, 2007. Revised manuscript received October 26, 2007. Accepted November 13, 2007.

Magnesium hydroxide extracted from magnesium-bearing minerals is considered a promising agent for binding CO2 as a carbonate mineral in a gas–solid reaction. An efficient extraction route consisting of hydrothermal treatment on serpentine in HCl followed by NaOH titration for Mg(OH)2 precipitation was demonstrated. The extracted Mg(OH)2 powder had a mean crystal domain size as small as 12 nm and an apparent surface area of 54 m2/g. Under one atmosphere of 10 vol% CO2/N2, carbonation of the serpentine-derived Mg(OH)2 to 26% of the stoichiometric limit was achieved at 325 °C in 2 h; while carbonation of a commercially available Mg(OH)2, with a mean crystal domain size of 33 nm and an apparent surface area of 3.5 m2/g, reached only 9% of the stoichiometric limit. The amount of CO2 fixation was found to be inversely proportional to the crystal domain size of the Mg(OH)2 specimens. The experimental data strongly suggested that only a monolayer of carbonates was formed on the crystal domain boundary in the gas–solid reaction, with little penetration of the carbonates into the crystal domain.

Introduction Binding CO2 chemically with abundant minerals or solid wastes to form stable carbonates would be an economic way of disposing CO2 emitted from burning fossil fuels (1–9). Magnesium silicates, such as olivine (Mg2SiO4) and serpentine (Mg3Si2O5(OH)4), are available worldwide in huge amounts and have been considered the potential mineral substrates for CO2 fixation (10–12). Because of the massive mineral requirements and the associated energy costs and mining problems, using olivine or serpentine to form stable carbonates may not be a complete solution to sequester CO2 emitted from fossil fuel combustion (12). However, carbonation of these minerals could be part of an integrated CO2 sequestration approach that involves, for example, the use of readily available solid wastes (12). The most economical process would be a direct carbonization of a mineral powder through a heterogeneous gas–solid reaction under atmospheric pressure. However, previous studies (1, 2) and our preliminary work found that carbonation of magnesium silicates under such a mild condition was not feasible. Magnesium hydroxide that can be extracted from magnesium silicates, on the other * Corresponding author e-mail: [email protected]; tel: 8866-2385371; fax: 886-6-2344496. 2748

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hand, exhibited a higher carbonation rate (13–15). The present work intended to introduce a route for extracting Mg(OH)2 from serpentine and to examine the performance of the serpentine-derived Mg(OH)2 in CO2 sequestration. A previous study showed that exaction of Mg(OH)2 from magnesium-bearing minerals could be initiated by decomposing the minerals in hydrochloric acid to yield a MgCl2 solution, which was subsequently transformed into Mg(OH)2 upon further heating (2). However, it was energy-consuming to have a complete decomposition of serpentine by thermal treatment with HCl solution. Here we propose a method that employs hydrothermal treatment to efficiently decompose serpentine. A base solution was subsequently added to the resulting mixture for Mg(OH)2 extraction. The extracted Mg(OH)2 was subjected to carbonation with CO2 in a gas–solid system under atmospheric pressure. Carbonation of a commercially available Mg(OH)2 powder was also conducted for the purpose of comparison. The present work demonstrated that the serpentine conversion route produced a Mg(OH)2 powder exhibiting a high capacity for CO2.

Experimental Section The serpentine specimen used for Mg(OH)2 production was obtained from Hua-Lien, Taiwan. The specimen was ground into a powder with a particle size of ca. 50 µm before use. A schematic summarizing the procedure for the production of Mg(OH)2 from the serpentine is shown in Figure 1. As indicated, the Mg(OH)2 extraction was initiated by treating 2 g of the serpentine powder with 100 mL of 1 N HCl solution at 150 °C in a Teflon-lined autoclave for 24 h. After the hydrothermal treatment, the resulting mixture was neutralized with an appropriate amount of 0.1 N NaOH solution to reach a pH value of ca. 8, at which the dissolved silica precipitated. After removing the silica, the filtrate was further added with an amount of the NaOH solution to reach a pH of 11, at which the Mg species precipitated as Mg(OH)2. The Mg(OH)2 powder from filtration was dried at 100 °C for 3 h to give the final product. This extraction process gave a magnesium yield of ca. 90%. In addition to the serpentinederived Mg(OH)2, we also subjected a reagent-grade Mg(OH)2 powder (Acros) to analysis for the purpose of comparison. The phase identification of the serpentine and Mg(OH)2 specimens was conducted with powder X-ray diffraction (XRD) using a Rigaku RINT2000 diffractometer equipped with Cu KR radiation. The data were collected for scattering angles (2θ) ranging between 10 and 70° with a step size of 0.01°. The structural models of the magnesium-containing compounds, Mg(OH)2, MgCO3, and MgO, were constructed with the Ca.R.Ine version 3.1 crystallography program package (16). Pore structure of the Mg(OH)2 specimens was characterized by N2 adsorption at -196 °C using an adsorption apparatus (Micromeritics, ASAP 2010). The surface area of the specimens was determined from the Brunauer–Emmett– Teller (BET) equation and the pore volume was determined from the total amount adsorbed at relative pressures near unity (p/p0 ) 0.96 here) (17). To understand the temperature dependence of the dehydroxylation and carbonation of Mg(OH)2, we subjected the serpentine-derived specimen to thermogravimetric analysis by using a Perkin-Elmer TGA 7 thermogravimetric analyzer. The thermogravimetric experiments were conducted by heating the specimen (ca. 10 mg) from room temperature to 400 °C at 10 °C/min under a pure N2 or 10% CO2/N2 environment with a total purge flow rate of 50 cm3/ min. 10.1021/es072099g CCC: $40.75

 2008 American Chemical Society

Published on Web 01/16/2008

FIGURE 1. Flowchart for the production of a Mg(OH)2 powder from serpentine by the hydrothermal extraction method.

FIGURE 3. Unit-cell and the refined crystalline structures: (a) the hexagonal phase Mg(OH)2; (b) the rhombohedral phase MgCO3; (c) the cubic phase MgO.

FIGURE 2. X-ray diffraction patterns of the Mg(OH)2 powders: the serpentine-derived specimen and the reagent-grade specimen from Acros. The standard diffraction pattern of Mg(OH)2 from JCPDS 83-0114 is provided at the bottom. Isothermal carbonation of the Mg(OH)2 specimens was conducted over a fixed bed containing 0.1 g of the Mg(OH)2 specimens. Under atmospheric pressure, the gas mixture for reaction, 10 vol% CO2/N2, was fed to the reactor at a total flow rate of 200 cm3/min. The specimens were heated at a ramp of 10 °C/min from room temperature to 325 °C and held at this carbonation temperature for 2 h before cooling. The degree of Mg(OH)2 carbonation was analyzed by measuring the amount of CO2 evolution from temperature programmed desorption of the CO2 treated Mg(OH)2 specimens. The desorption experiment was conducted from room temperature to 850 °C at a heating rate of 10 °C/min, with N2 purged at 400 cm3/min. Nondispersive infrared analyzers were used to continuously monitor CO2 evolution from the carbonates during the temperature programmed desorption.

Results and Discussion The XRD patterns of the serpentine-derived and reagentgrade Mg(OH)2 powders are shown in Figure 2. All the diffraction peaks can be indexed according to the hexagonal phase Mg(OH)2 shown at the bottom of Figure 2 (P3j m1 with a ) 3.15 Å, b ) 3.15 Å, and c ) 4.77 Å and R ) β ) 90° and γ ) 120°; JCPDS 83-0114) (18), with no other crystalline phase observed. The XRD data indicate that this extraction route produced the Mg(OH)2 material from serpentine with a high purity as well as a high yield. Figure 3a shows the unit-cell

and the refined crystal structure of Mg(OH)2. For the purpose of comparison, Figure 3b and c show the crystal structures of MgCO3 (rhombohedral phase, R3j c with a ) 4.65 Å, b ) 4.65 Å, and c ) 15.2 Å and R ) β ) 90° and γ ) 120°; JCPDS 86-2348) and MgO (cubic phase, Fm3j m with a ) b ) c ) 4.21 Å and R ) β ) γ ) 90°; JCPDS 87-0652), respectively (19, 20). The structures in Figure 3 clearly reflect that both Mg(OH)2 and MgCO3 have the lamellar feature, showing periodic Mg layers along their [001] direction. This similarity demonstrates the feasibility for Mg(OH)2 to incorporate CO2 into the lamellar framework, probably at the expense of the simultaneous H2O loss. On the other hand, Figure 3 also reflects that the addition of CO2 to the compact MgO framework is not plausible. This spatial restriction would explain the negligible absorption of CO2 by MgO under the atmospheric pressure (13). The principal peaks for the diffraction patterns of the Mg(OH)2 specimens are the (001) and (011) diffraction at scattering angles (2θ) of 19° and 38°, respectively. XRD sizing of the principal faces was performed by using the Debye– Scherrer equation to give a qualitative description on the domain size (21). The (001) and (011) peaks were used to estimate the length of the crystal-stacking domain and the results are shown in Table 1. It can be seen that the crystal size was smaller for the serpentine-derived Mg(OH)2 than for the reagent-grade specimen, with mean domain sizes of ca. 12 and 33 nm, respectively. These data reflected that both the Mg(OH)2 specimens were nanocrystalline powders. Concerning the surface activity in a heterogeneous reaction, the domain boundary represents the disordered region of a powder and would be prone to reaction. Assuming a spherical configuration for crystal domains, the spherical area of the domains would be equal to γπD2, where γ is the surface roughness and D is the mean diameter of the crystal domains. In combination with the true density of the materVOL. 42, NO. 8, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Physical Characteristics of the Serpentine-Derived and Reagent-Grade Mg(OH)2 Powders crystal domain size (nm) sample type

(001)

(011)

serpentine-derived reagent-grade

10 34

14 32

surface area pore volume (m2/g) (cm3/g) 54 3.5

0.22 0.01

ial (F), the specific boundary area of the crystal domains (Sc) can be calculated according to Sc ) 6γ/FD. Thus, the value of Sc would be inversely proportional to the domain size if the crystal domains had similar values in surface roughness. On the basis of the above calculation, we could derive an approximation that the serpentine-derived Mg(OH)2 had a specific crystal domain-boundary area ca. 2.8 (i.e., 33/12) times that of the reagent-grade specimen. The porosity of a nanocrystalline powder is mainly contributed by the exposed interstices between the crystal domains. The smaller domain size of the serpentine-derived Mg(OH)2 would lead to a larger specific surface area and pore volume determined from N2 adsorption. Figure 4 shows the isotherms of N2 adsorption onto the serpentine-derived and reagent-grade Mg(OH)2 powders, both with an apparent particle size of 44–110 µm. In agreement with the preceding interpretation, the serpentine-derived specimen exhibits a larger capacity for N2 adsorption than the reagent-grade one. Auxiliary experiments found that the apparent particle size had little influence on the N2 adsorption. This indicates that the surface available for N2 adsorption was mainly contributed by the pores, i.e., the interstices, contained inside the particles. The isotherms in Figure 4 exhibit an obvious hysteresis behavior, indicating that the specimens were mainly mesoporous (17). The pore structures calculated according to the adsorption data are summarized in Table 1. Both the surface area and pore volume were larger for the serpentinederived specimen than for the reagent-grade, in agreement with the N2 capacity data shown in Figure 4. The analysis on the pore structure demonstrates that the route for Mg(OH)2 extraction from serpentine produced powders with a large surface area. This would be advantageous for CO2 absorption in the aspects related to chemical reaction as well as mass transport. To bind CO2 on Mg(OH)2, it has been generally recognized that there is a simultaneous loss of H2O from dehydroxylation of Mg(OH)2 (13), i.e. Mg(OH)2 (s) + CO2 (g) f MgCO3 (s) + H2O(g)

(1)

However, the hydrated form of magnesium carbonate, hydromagnesite (Mg5(CO3)4(OH)2 · 4H2O), is a stable compound and would be formed as an intermediate product for Mg(OH)2 carbonation (22, 23), i.e. 5Mg(OH)2 (s) + 4CO2 (g) f Mg5(CO3)4(OH)2 · 4H2O(s) (2) At appropriate temperatures the hydromagnesite formed would become a transient intermediate and proceed with prompt dehydration to form MgCO3 (22, 23). It has to be noted that the dehydroxylation of Mg(OH)2 resulting from the thermal effect can as well occur irrespective of the CO2 presence (24), i.e. Mg(OH)2 (s) f MgO(s) + H2O(g)

(3)

The solid-phase product of R3, MgO, is known to be inactive in carbonation under a low CO2 pressure. Thus, temperature 2750

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FIGURE 4. N2 adsorption (empty symbol)-desorption (full symbol) isotherms for the serpentine-derived and reagent-grade Mg(OH)2 specimens.

FIGURE 5. Variation of mass (a) and mass loss rate (b) with temperature during the thermogravimetric analysis of the serpentine-derived Mg(OH)2 under a pure N2 or 10% CO2/N2 environment with a heating rate of 10 °C/min. optimization to render an effective CO2 capture on the Mg(OH)2 surface through R1 or R2 while minimizing the intrusion of R3 becomes an important issue here. By employing the thermogravimetric analysis we monitored the mass variation of the serpentine-derived specimen with temperature under one atmosphere of N2 or 10 vol% CO2/N2. The thermogravimetric analysis data are shown in Figure 5, in which the temperature was linearly increased from room temperature to 500 °C. The mass loss of the Mg(OH)2 in N2 can be entirely attributed to dehydroxylation through R3. The mass loss rate increased with the temperature and reached a maximum at a temperature of ca. 300 °C. In the presence of CO2, there was mass gain observed at temperatures near 300 °C, indicating the uptake of CO2 to form the carbonate even if the dehydroxylation through R3 was still in progress. At temperatures above 325 °C, the dehydroxylation became dominant and the mass loss rate reached a maximum at ca. 340 °C. The mass loss was delayed because of the presence of CO2. The carbonate formed on the Mg(OH)2 specimen must have inhibited the dehydroxylation (13). Thus, we did not use the difference between the two curves to calculate the amount of CO2 fixation on the Mg(OH)2 specimen.

FIGURE 6. Evolution of CO2 during temperature programmed desorption of the serpentine-derived and reagent-grade Mg(OH)2 specimens that had been treated in a 10% CO2/N2 flow at 325 °C for 2 h.

TABLE 2. Amounts of CO2 Absorbed by the Serpentine-Derived and Reagent-Grade Mg(OH)2 Powders during Carbonation in a 10% CO2/N2 Flow at 325 °C for 2 h and the Corresponding Fractional Occupancies of the CO2 Stoichiometric Limit (17 mmol/g for Mg(OH)2) sample type

CO2 absorbed (mmol/g)

fractional occupancy of CO2 stoichiometric limit

serpentine-derived reagent-grade

4.4 1.6

26% 9%

On the basis of the thermogravimetric analysis, we explored the CO2-binding capability of the Mg(OH)2 specimens over the fixed bed system at 325 °C. This temperature was expected to give an efficient CO2 fixation on Mg(OH)2 while refraining from a significant progress of dehydroxylation. As a matter of fact, we have confirmed that 325 °C was the optimal temperature for CO2 fixation with auxiliary experiments (see Supporting Information). The carbonation of the Mg(OH)2 specimens over the fixed bed was conducted in a 10 vol% CO2/N2 flow at 325 °C for 2 h. A longer period of carbonation (>2 h) or higher CO2 concentration (>10 vol%) did not give a higher degree of CO2 fixation. Following the carbonation, temperature programmed desorption was used to determine the amount of CO2 uptake on the specimens. It was known that magnesium carbonate would completely decompose into magnesium oxide and CO2 at temperatures lower than 800 °C (22, 23). Figure 6 shows the CO2 evolution profile during the temperature programmed desorption. The evolution was stronger for the serpentine-derived Mg(OH)2 than for the reagent-grade specimen, reflecting a larger CO2binding capacity of the serpentine-derived Mg(OH)2 surface. The carbonate decomposition mainly occurred in a temperature range of 300–700 °C. The CO2 evolution patterns were similar for both specimens, indicating that similar carbonate compounds were created on the surfaces of the two Mg(OH)2 specimens during the carbonation. The amounts of CO2 bound to the Mg(OH)2 specimens are summarized in Table 2. The stoichiometric limit for CO2 uptake on Mg(OH)2, based on R1, is calculated to be 17 mmol CO2 per gram of Mg(OH)2. Table 2 also shows the corresponding fractional occupancies of the stoichiometric limit achieved by the specimens in the carbonation. About 26% of the Mg atoms in the serpentine-derived Mg(OH)2 were in the carbonate form after the carbonation while only 9% was achieved for the reagent-grade Mg(OH)2. The CO2 capacity of the serpentine-derived specimen was larger and ca. 2.8 times that of the reagent-grade one. The preceding result

also showed that the serpentine-derived specimen had a specific domain-boundary area ca. 2.8 times that of the reagent-grade one. This coincidence reflects that the crystal domain size (or the boundary area of the crystal domains) was the principal factor governing the CO2 capacity of Mg(OH)2. The specific surface area determined by N2 adsorption, on the other hand, was not a critical factor in determining the CO2 capacity. Because the amount of CO2 fixation was proportional to the domain-boundary area, it is likely that in the carbonation only a monolayer of CO2 molecules was absorbed on the crystal domain boundary of the Mg(OH)2 specimens. Assuming a surface roughness factor of unity, the specific domain-boundary area of the Mg(OH)2 specimens was calculated, from 6γ/FD, to be 210 and 76 m2/g, respectively, for the serpentine-derived and reagent-grade specimens. Thus, by dividing the domain-boundary area with the CO2 capacity shown in Table 2, the area occupied per CO2 molecule was calculated to be ca. 0.08 nm2 for both the Mg(OH)2 specimens. The actual area occupied should be larger than 0.08 nm2, however, owing to the fact that a crystal domain would never be a perfect sphere and a surface roughness factor of greater than unity is expected. As to the Mg atom on the domain surface of the Mg(OH)2 crystal (as shown in Figure 3a), each Mg(OH)2 unit on the (001), (010), and (100) faces of the crystal occupies an area of 0.086, 0.15, and 0.15 nm2, respectively. It is of interest to observe that the area occupied by a CO2 molecule on Mg(OH)2 had a range close to that for a Mg(OH)2 unit on the crystal surface. This coincidence demonstrates a low degree of carbonate penetration for CO2 fixation on Mg(OH)2 crystals. A monolayer of MgCO3 formation on the domain boundary of the Mg(OH)2 specimens was, therefore, very likely. This explicitly explained why the CO2 capacity of Mg(OH)2 was determined by the crystal domain size. The present work demonstrated that the carbonation of Mg(OH)2 in a gas–solid system was restricted under the atmospheric pressure. The migration of the carbonate species formed on the Mg(OH)2 surface must have been obstructed at the carbonation temperature. This has led to only a monolayer of carbonate coverage on the domain boundary of the Mg(OH)2 crystals in the carbonation. To activate carbonate migration by elevating the reaction temperature would as well promote dehydroxylation to form MgO, which is inactive in binding CO2. The restricted carbonate migration into the crystal interior might have prohibited a higher degree of carbonation on Mg(OH)2. Knowledge obtained from this carbonation analysis will be useful in the preparation or processing of Mg-rich lamellar-hydroxide minerals for CO2 sequestration (24).

Acknowledgments This research was supported by the National Science Council of Taiwan (NSC 95-2218-E-006-019).

Supporting Information Available The amounts of CO2 fixation on the Mg(OH)2 specimens during carbonation at different temperatures. This material is available free of charge via the Internet at http:/pubs. acs.org.

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