Article pubs.acs.org/IECR
Mechanistic Study of Magnesium Carbonate Semibatch Reactive Crystallization with Magnesium Hydroxide and CO2 Bing Han,*,†,‡ Haiyan Qu,§ Harri Niemi,† Zuoliang Sha,‡ and Marjatta Louhi-Kultanen† †
Department of Chemical Technology, Lappeenranta University of Technology, P.O. Box 20, FI 53851 Lappeenranta, Finland Tianjin Key Laboratory of Marine Science and Chemistry, Tianjin University of Science & Technology, 300457, Tianjin, People’s Republic of China § Department of Chemical Engineering, Biotechnology and Environmental Technology, Faculty of Engineering, University of Southern Denmark, Odense, DK 5230, Denmark ‡
ABSTRACT: This work investigates semibatch precipitation of magnesium carbonate at ambient temperature and pressure using Mg(OH)2 and CO2 as starting materials. A thermal analysis method was developed that reflects the dissolution rate of Mg(OH)2 and the formation of magnesium carbonate. The method was successfully used to determine the composition of solids precipitated from the reaction. Concentration profiles of Mg2+ and total carbon over time were determined from the liquid phase. The influence of CO2 flow rate and stirring rate on precipitation was analyzed based on comprehensive information from the liquid and solid phases. A stirring rate of 650 rpm was found to be the optimum speed as the flow rate of CO2 was 1 L/min. Precipitation rate increased with gas flow rate, which indicates that mass transfer of CO2 plays a critical role in this precipitation case. Magnesium carbonate trihydrate (nesquehonite) was formed as a bunch of needle-like primary crystals.
1. INTRODUCTION Recently, carbonation of magnesium has become a subject of great research interest. Chemical trapping is considered to be one of the most promising methods for capture and storage of CO2, and there are abundant magnesium sources that can be used for preparation of magnesium carbonate, such as byproducts or waste from the fertilizer industry and magnesium-rich rocks.1−3 OH− ions are always needed to support the conversion of Mg2+ to magnesium carbonate salts by CO2 gas. Consequently, pH is crucial for the whole process. Ammonia has been used to generate OH− to synthesize nesquehonite4 and hydromagnesite5 from magnesium chloride solutions. If Mg(OH)2 which has higher reactivity toward CO2 is used as a reactant, there is no need to add other chemicals since Mg(OH)2 provides OH− directly. In-depth understanding of the reaction mechanisms between Mg(OH)2 and CO2 can give guidance on the use of alkaline solutions containing magnesium ions in the production of magnesium carbonates through carbonation, which would be practically valuable and of importance for optimal design of chemical trapping of CO2. To date, a number of studies have considered the Mg(OH)2 and CO2 system. Macroscopic experiments for brucite (Mg(OH)2) carbonation have been studied in dry and wet supercritical CO2 by various novel tools such as in situ infrared spectroscopy,6 in situ high pressure X-ray diffraction,7 static high-pressure and -temperature 13C NMR,8 and atomic force microscopy.9 The magnesium hydroxide dehydroxylation/ carbonation reaction process has also been studied at high CO2 pressure and temperature.10 These studies, however, concentrate on dissolution of Mg(OH)2 or transformation from Mg(OH)2 to magnesium carbonate in a static system. Only a few studies11−13 have investigated the reaction process in mixing reactors, which are widely used in industry operations. The mechanism and kinetics of reactive crystallization in this © 2014 American Chemical Society
special and complicated system have not been clarified exhaustively. In this reactive crystallization system, the reaction starts from a suspension (heterogeneous reaction system), which is quite different from a crystallization that starts from a clear solution (homogeneous reaction system). In a heterogeneous reaction system, the solid phase and liquid phase exist throughout the whole process. Therefore, in order to have comprehensive understanding of the process, the concentrations of the different species in the solution phase and the composition of the solid phase both have to be quantified. The present study focuses on the precipitation of magnesium carbonate from the reaction between Mg(OH)2 and CO2 at ambient temperature and atmospheric pressure. The whole process includes several kinetic steps such as the dissolution rate of Mg(OH)2, the absorption rate of CO2 gas, the formation rates of carbonates from the dissolved CO2 solution, the reaction rate of Mg2+ and OH−, and the crystallization rate of magnesium carbonate. Identification of the rate-controlling step for the whole process could be the essential task for controlling the entire process and obtaining the required target product. The objective of the present work is to develop a method to analyze the kinetics in the process so that the mechanism of the reaction can be better understood and used further in the study of complex reaction systems. Two main variables, flow rate of CO2 and stirring rate, were investigated in a semibatch crystallization system. Besides the analysis of magnesium and carbon concentrations in the liquid phase, simultaneous thermal analysis was used to determine solid composition in the semibatch process. Thus, the kinetic data Received: Revised: Accepted: Published: 12077
April 25, 2014 June 27, 2014 July 10, 2014 July 10, 2014 dx.doi.org/10.1021/ie501706j | Ind. Eng. Chem. Res. 2014, 53, 12077−12082
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2.3. Thermal Analysis Method Development. The thermal measurements, thermogravimetry (TG) and differential scanning calorimetry (DSC) were carried out using a simultaneous thermal analyzer (449 F3 Jupiter, NETZSCH). Samples (15−20 mg) were heated from 30 to 600 °C with a heating rate of 10 °C/min under nitrogen flow. Figure 2 shows
were obtained by this methodology. The data obtained reflect the rate of dissolution and reaction of Mg(OH)2 and formation of magnesium carbonate in the precipitation process.
2. EXPERIMENTAL SECTION 2.1. Experimental Methods. Magnesium hydroxide of analytic reagent grade (99.6%, VWR) was used without further purification. Deionized water was used to prepare slurries of Mg(OH)2. In each batch operation, 100 g of Mg(OH)2 and 2.5 L of deionized water were first mixed in a jacketed glass crystallizer equipped with a thermostat (Lauda T2200). Four baffles were located symmetrically in the reactor in order to promote efficient mixing and prevent vortex formation. The internal diameter of the crystallizer was 150 mm and its height was 200 mm. The temperature of the Mg(OH)2 slurry was controlled with the thermostat. A 50 mm diameter Rushton turbine with six blades was used as a stirrer. When the temperature of the Mg(OH)2 slurry achieved the target temperature of 25 °C, pure CO2 (purity 99.7%, AGA) was introduced through a metal pipe with a porous sparger from the bottom of the crystallizer. The flow rate of CO2 was controlled by a calibrated CO2 flow meter. A pH meter (Metrohm 744) was used to monitor in situ the pH of the reaction system during the precipitation process. A schematic representation of the experimental setup is shown in Figure 1. Slurry samples were taken at different times during
Figure 2. TG-DSC curves of nesquehonite precipitated from the reaction of Mg(OH)2 and CO2 at a flow rate of 9 L·min−1 and rotation speed of 560 rpm.
the TG−DSC curves of the precipitated nesquehonite from one of the three analyses. It can be seen that there are two main steps of mass losses for TG. The first mass loss of TG was accompanied by three overlapped endothermic peaks in DSC from 90 to 300 °C. The three repeated measurements show that the mass loss from 30 to 300 °C is around 34.6 to 35.1%, which approximately corresponds to the mass loss of dehydration as shown in eq 1.4,14 These values are slightly lower than the theoretical value of 39.1%, which may be due to nonstoichiometry of the nesquehonite. In the following step, the mass loss from 300 to 600 °C is around 33.8−34.7% with respect to the total mass of the initial sample, and 51.7−53.4% with respect to the mass of the dehydrated sample. The mass loss with respect to the dehydrated sample corresponds to the release of CO2 as shown in eq 2. On the basis of stoichiometry, the theoretical value of mass loss in eq 2 is 52.2%. The product is thus considered pure nesquehonite, which is consistent with the XRD result.
Figure 1. Scheme of experimental setup.
the experiments. CO2 flow was stopped when the pH of the suspension approached a constant value. All samples were filtered immediately after sampling, and crystals were dried at room temperature in order to avoid any change of the solid form or the composition of the samples that might be induced by high temperature drying.11 Concentration of Mg2+ and total carbon from the clear filtrate were measured by ion chromatography (ICS-1100 from Thermo Scientific Inc.) and total organic carbon analysis (TOC-5050A from Shimadzu Corp.), respectively. 2.2. Characterization. The morphologies and surface features of the dried crystals were photographed with a scanning electronic microscope (SEM, JEOL JSM-5800). The product from each batch was analyzed by X-ray powder diffraction (XRD, XD-2, Beijing Purkinje General Instrument Co., Ltd.). The particle size distribution of the product was analyzed with a Beckman Coulter LS13320 laser diffraction analyzer.
MgCO3 ·3H 2O → MgCO3 + 3H 2O
(1)
MgCO3 → MgO + CO2
(2)
The raw material, Mg(OH)2, was also verified three times by simultaneous thermal analysis. The obtained standard deviation was 0.1%. As shown in Figure 3, an endothermic peak in the DSC curve occurred around 375 °C, which corresponds to the dehydroxylation of Mg(OH)2, as shown in eq 3.15 The average value of the mass loss from 30 to 600 °C for the three repeated measurements is 28.95%, which is slightly lower than the theoretical value 30.89%. The measured mass loss, 28.95%, is used in further calculations. Mg(OH)2 → MgO + H 2O
(3)
Since the solid samples taken during the reactive crystallization processes are mixtures of nesquehonite and Mg(OH)2, quantification of the composition of these solid mixtures is required to investigate the kinetics of the process. 12078
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was 50.46%, which agreed quite well with experimental data. Thus, this method can be considered to be reliable and accurate in prediction of the composition of the solid samples taken during the precipitation. Each sample was analyzed twice by the simultaneous thermal analyzer, and the averaged results were used in the prediction. The standard deviation was smaller than 3.3%.
3. RESULTS AND DISCUSSION 3.1. Effect of Flow Rate of CO2. Different equilibria need to be considered, such as the gas−liquid equilibrium of CO2, the reaction and dissociation equilibria of the species in the solution phase, and the solid−liquid equilibrium of Mg(OH)2 and magnesium carbonate. The mechanisms are shown below:
Figure 3. TG-DSC curves of Mg(OH)2.
For this purpose, a simple prediction method was constructed based on comprehensive analysis of the TG of the samples. Figure 4 shows the TG and DSC curves of a mixture containing
CO2 (g) ↔ CO2 (l)
(6)
Mg(OH)2 ⇔ Mg 2 + + 2OH−
(7)
CO2 (l) + OH− ⇔ HCO3−
(8)
HCO3− + OH− ⇔ CO32 − + H 2O
(9)
CO2 (l) + H 2O ⇔ HCO3− + H+
(10)
Mg 2 + + CO32 − ⇔ MgCO3(l)
(11)
MgCO3(l) + x H 2O ⇔ MgCO3 ·x H 2O
(12) 16
Although Mg(OH)2 is only sparingly soluble in water, the pH of the suspension increased to 10.2 at the beginning of each semibatch crystallization when Mg(OH)2 was added to the reactor. The increase in pH allows occurrence of the reaction and shifts the equilibrium of eq 10 considerably more to the right. The gas flow rate of the reactant CO2 is one of the important factors that can affect the mass transfer rate between the gas− liquid phases and thus the precipitation rate. Figure 5a,b shows
Figure 4. TG-DSC curves of a mixture containing 49.96 wt % of Mg(OH)2 and 50.04 wt % of nesquehonite.
49.96 wt % Mg(OH)2 and 50.04 wt % nesquehonite. The mass loss of the first step is attributed to the loss of water molecules from nesquehonite crystals, and the second step mass loss of the TG results to the combination of decomposition of anhydrous MgCO3 and Mg(OH)2, which both occur above 300 °C, as shown in Figures 2 and 3. The composition of the solid can be predicted from the mass loss in the second stage by taking into account the mass loss of decomposition of Mg(OH)2 and anhydrous MgCO3, as expressed by eq 3 and eq 2, respectively. The prediction method is shown in eq 4 and eq 5: x + y = m0(1 − w1)
(4)
28.95%x + 52.3%y = m0w2
(5)
Figure 5. Concentration profiles of Mg2+ and total carbon over time with a CO2 flow rate of 1 L·min−1 (a) and 9 L·min−1 (b) at constant temperature of 25 °C and rotation speed of 560 rpm.
where x and y are the mass of Mg(OH)2 and anhydrous magnesium carbonate, respectively; m0 is the mass of the sample for simultaneous thermal analysis measurement; w1 and w2 are the mass loss percents with respect to the initial sample for the first and second mass loss stage of TG, respectively. The weight percent of Mg(OH)2 in the mixtures, expressed by x/ m0, is then known and reflects the kinetics of the consumption of Mg(OH)2 in the reactive crystallization. The accuracy of this method was checked as follows. The sample with a mass fraction of Mg(OH)2 of 49.96% prepared using the raw material Mg(OH)2 and the precipitated product found to be pure nesquehonite were analyzed three times. Figure 4 shows one of the analyzed results. The predicted result
the concentration profiles of Mg2+ and total carbon over time for two CO2 flow rates, 1 L·min−1 and 9 L·min−1 respectively, where the rotation speed was kept constant. It can be seen that magnesium concentration gently increased over time and then gradually decreased for both cases. Moreover, the concentration profiles of carbon are similar to the concentration profiles of magnesium. The molar ratio of Mg/C in the final liquid sample was around 1/2, which means that the final mother liquid contains Mg2+ and HCO3− due to the high solubility of Mg(HCO3)2. The concentrations of species in the liquid phase indicate that the peak appeared earlier with a flow rate of 9 L/ 12079
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min (Figure 5b) than a flow rate of 1 L/min (Figure 5a). Generally, the decrease in concentration is indicative of the occurrence of crystallization. Therefore, a large amount of precipitate starts to form from 35 min for a flow rate of 1 L/ min, and from 20 min for a flow rate of 9 L/min. Five solid samples taken over time for each semibatch were analyzed by simultaneous thermal analyzer. The consumption of Mg(OH)2 over time predicted from the solid phase by simultaneous thermal analysis is shown in Figure 6 for the two
agrees quite well with the trend of ion concentration in the liquid phase shown in Figure 5. The concentrations of the species in the liquid and solid phases indicate that the main subprocesses happening in the initial period are the dissolution of Mg(OH)2 and the reaction between CO2 and OH−. The crystallization process of nesquehonite starts when the highest level of the concentrations of magnesium and carbon is reached. These observations indicate that precipitation of magnesium carbonate starts from 20−35 min and crystals grow in the following period. This finding is consistent with the results obtained by Zhao et al.,13 who observed that carbonate formed within 30 min. From the results of size distribution measurement, it can be concluded that at constant stirring speed, a larger particle size of final products can be obtained with a higher flow rate of CO2, as shown in Figure 8. The result can be explained by the
Figure 6. Comparison of Mg(OH)2 consumption over time with a CO2 flow rate of 1 L·min−1 and 9 L·min−1 at a constant temperature of 25 °C and rotation speed of 560 rpm. Figure 8. Particle size distribution of magnesium carbonate precipitated with a flow rate 1 and 9 L·min−1 at constant temperature of 25 °C and rotation speed of 560 rpm.
different CO2 flow rates. The standard deviation for each data point is also shown in the figure. Concurrently, the reaction rate can be well expressed by the mass fraction of Mg(OH)2 over time using the simple predictive method given above. Precipitation experiments with a gas flow rate of 1 L·min−1 and 5 L·min−1 at rotation speed 650 rpm were additionally performed. The trends of the profiles for concentration and Mg(OH)2 consumed over time are similar to the profiles shown in Figure 5 and Figure 6, respectively. It can be seen from Figure 6 that in the initial period only a few MgCO3 crystals are precipitated from the reaction. Analyzed by SEM, the morphology of crystals in the initial stage is flaky, which is almost the same as the starting material Mg(OH)2, shown in Figure 7a. Some nuclei may form in this period. Subsequently, more crystals were gradually formed. Figure 6 shows that the consumption rate of Mg(OH)2 is faster with a higher flow rate. The consumption rate corresponds to the formation rate of magnesium carbonate, and this result
nucleation having occurred earlier with a higher gas flow rate, which promotes crystal growth. Therefore, the precipitation is faster with a higher gas flow rate due to the enhancement of the mass transfer and chemical reaction rate. 3.2. Effect of Stirring Rate. At a constant CO2 flow rate of 1 L·min−1, three impeller speeds (560, 650, and 750 rpm) were used to study the effect of stirring rate on the reactive crystallization. Figure 9 shows the concentration profiles of Mg2+ and total carbon in the liquid phase as a function of time with impeller speed of 650 rpm. The concentration profile with impeller speed of 750 rpm is similar to that of 560 rpm shown in Figure 5a. As mentioned in section 3.1, for the three different stirring rates the concentration of magnesium and total carbon increases with time, approaches a maximum level, and then
Figure 7. Characteristic SEM images of (a) Mg(OH)2 (scale bar 10 μm), (b) crystal mixtures of Mg(OH)2 and magnesium carbonate formed at 30 min (scale bar 5 μm), and (c) final product nesquehonite (scale bar 25 μm). 12080
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Figure 9. Concentration profiles of Mg2+ and total carbon over time with a rotation speed of 650 rpm at a constant temperature of 25 °C and a CO2 flow rate of 1 L·min−1.
Figure 11. Particle size distribution of magnesium carbonate precipitated with various mixing rates at a constant temperature of 25 °C and a CO2 flow rate of 1 L·min−1.
starts to decrease. These concentration profiles indicate that Mg(OH)2 dissolves and reacts with CO2 in the initial period. Subsequently, magnesium carbonates are gradually precipitated. The concentration peak appeared at 25 min for stirring speed of 650 rpm, and at 30−35 min for the other two stirring speeds. From the solid phase composition profiles shown in Figure 10,
obtained at the highest rotation speed. It can be concluded that the mass transfer of CO2 is the rate controlling step in this precipitation process. This result is consistent with our previous research work.18 3.3. Characterization of Magnesium Carbonate. All the final precipitate was identified by XRD. Figure 12 shows the
Figure 10. Comparison of Mg(OH)2 consumption over time with various rotation speeds at a constant temperature of 25 °C and a CO2 flow rate of 1 L·min−1.
Figure 12. Typical XRD patterns of nesquehonite.
typical XRD patterns together with the literature data. It can be seen that XRD data obtained from the present research agree well with reference data JCPDS 20-0699, which indicates that only nesquehonite was precipitated from the reaction. This result is consistent with reports19 that nesquehonite MgCO3· 3H2O normally forms at room temperature. The shape of the XRD peak demonstrates that the nesquehonite is well crystallized. The morphology of the crystals precipitated over time was examined by SEM. In the preliminary period, there are flaky crystals, which is similar to the raw material Mg(OH)2, because most Mg(OH)2 has not reacted. Elongated crystals are observed after the concentration achieved the highest point, a characteristic image of which is shown in Figure 7b. According to the simultaneous thermal analysis results, the crystals are mixtures of Mg(OH)2 and magnesium carbonate. There are lots of small platy crystals on the surface of the elongated crystals, which might be unreacted Mg(OH)2 particles or an intermediary. Further research is needed to clarify this phenomenon. Figure 7c shows the morphology of the nesquehonite precipitated from the reaction of Mg(OH)2 and CO2 at ambient atmosphere. The crystals have a needle-like
it can be seen that the precipitation rate of the magnesium carbonate does not accelerate with increasing impeller speed. The trend obtained from the solid phase is consistent with the trend from the liquid phase. The results also show that the precipitation rate is fastest with a stirring speed of 650 rpm. According to the pH results recorded from the process, pH did not drop faster with the highest speed, which supports the results mentioned above. Thus, for the currently studied system, a stirring rate of 650 rpm is the optimum speed. Figure 11 shows the particle size distribution of final precipitate for three different stirring rates at constant CO2 flow rate. The smallest particles were obtained at rotation speed of 750 rpm. Smaller bubbles can be generated by a higher stirring rate, which increases the interfacial area between gas and liquid. Thus, higher supersaturation was generated and results in the formation of small particles. Moreover, small bubbles have strong energy which has a complicated impact on the surrounding particles.17 This influence depends on the relationship of bubble size and particle size. In addition, aggregation of crystals is weakened with high rotation speed. These might be the reasons why the smallest particles were 12081
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(2) Teir, S.; Eloneva, S.; Fogelholm, C.; Zevenhoven, R. Fixation of carbon dioxide by producing hydromagnesite from serpentinite. Appl. Energy 2009, 86, 214−218. (3) Kelemen, P. B.; Matter, J. M.; Streit, L.; Rudge, J. F.; Curry, W. B.; Blusztajn, J. S.; Streit, E. E. Rates and mechanisms of mineral carbonation in peridotite: Natural processes and recipes for enhanced, in situ CO2 capture and storage. Annu. Rev. Earth Planet. Sci. 2010, 39, 545−576. (4) Ferrini, V.; Vito, C. D.; Mignardi, S. Synthesis of nesquehonite by reaction of gaseous CO2 with Mg chloride solution: Its potential role in the sequestration of carbon dioxide. J. Hazard. Mater. 2009, 168, 832−837. (5) Wang, D.; Li, Z. Gas−liquid reactive crystallization kinetics of hydromagnesite in the MgCl2−CO2−NH3−H2O system: Its potential in CO2 sequestration. Ind. Eng. Chem. Res. 2012, 51, 16299−16310. (6) Loring, J. S.; Thompson, C. J.; Zhang, C.; Wang, Z.; Schaef, H. T.; Rosso, K. M. In situ infrared spectroscopic study of brucite carbonation in dry to water−saturated supercritical carbon dioxide. J. Phys. Chem. A 2012, 116, 4768−4777. (7) Schaef, H. T.; Windisch, C. F., Jr.; McGrail, B. P.; Martin, P. F.; Rosso, K. M. Brucite [Mg(OH)2] carbonation in wet supercritical CO2: An in situ high pressure X-ray diffraction study. Geochim. Cosmochim. Ac. 2011, 75, 7458−7471. (8) Andrew Surface, J.; Skemer, P.; Hayes, S. E.; Conradi, M. S. In situ measurement of magnesium carbonate formation from CO2 using static high−pressure and −temperature 13C NMR. Environ. Sci. Technol. 2013, 47, 119−125. (9) Hövelmann, J.; Putnis, C. V.; Ruiz-Agudo, E.; Austrheim, H. Direct nanoscale observations of CO2 sequestration during brucite dissolution. Environ. Sci. Technol. 2012, 46, 5253−5260. (10) Béarat, H.; McKelvy, M. J.; Chizmeshya, A. V. G.; Sharma, R.; Carpenter, R. W. Magnesium hydroxide dehydroxylation/carbonation reaction process: Implications for carbon dioxide mineral sequestration. J. Am. Ceram. Soc. 2002, 85, 742−748. (11) Botha, A.; Strydom, C. A. Preparation of a magnesium hydroxyl carbonate from magnesium hydroxide. Hydrometallurgy 2001, 62, 175−183. (12) Mitsuhashi, K.; Tagami, N.; Tanabe, K.; Ohkubo, T.; Sakai, H.; Koishi, M.; Abe, M. Synthesis of microtubes with a surface of “House of cards” structure via needlelike particles and control of their pore size. Langmuir 2005, 21, 3659−3663. (13) Zhao, L.; Sang, L.; Chen, J.; Ji, J.; Henry Teng, H. Aqueous carbonation of natural brucite: relevance to CO2 sequestration. Environ. Sci. Technol. 2010, 44, 406−411. (14) Sawada, Y.; Yamaguchi, J.; Sakurai, O.; Uematsu, K.; Mizutani, N.; Kato, M. Thermal decomposition of basic magnesium carbonates under high−pressure gas atmosphere. Thermochim. Acta 1979, 32, 277−291. (15) Liu, P.; Guo, J. S. Organo-modified magnesium hydroxide nanoneedle and its polystyrene nanocomposite. J. Nanopart. Res. 2007, 9, 669−673. (16) Haynes W., Ed. CRC Handbook of Chemistry and Physics, 93rd ed.; CRC Press: Boca Raton, FL, Internet version 2012−2013. (17) Belien, I. B.; Cashman, K. V.; Rempel, A. W. Gas accumulation in particle-rich suspensions and implications for bubble populations in crystal-rich magma. Earth Planet. Sci. Lett. 2010, 297, 133−140. (18) Han B.; Qu H.; Niemi H.; Sha Z.; Louhi-Kultanen M. Mass transfer and kinetics study of heterogeneous semi-batch precipitation of magnesium carbonate. Chem. Eng. Technol. 2014, 37, DOI: 10.1002/ceat.201300855. (19) Xiong, Y.; Lord, A. S. Experimental investigations of the reaction path in the MgO−CO2−H2O system in solutions with various ionic strengths, and their applications to nuclear waste isolation. Appl. Geochem. 2008, 23, 1634−1659. (20) Guo, M.; Li, Q.; Ye, X.; Wu, Z. Magnesium carbonate precipitation under the influence of polyacrylamide. Powder Technol. 2010, 200, 46−51.
shape with a smooth surface and bind together, which is a morphology similar to the nesquehontie synthesized by Guo et al.20 from the reaction of MgCl2 with Na2CO3 by adding polyacrylamide.
4. CONCLUSIONS By appropriate adjustment of the experimental conditions, nesquehonite (MgCO3·3H2O) can be obtained by a reaction of Mg(OH)2 and CO2 at ambient temperature and pressure. The formation of MgCO3·3H2O occurred in the pH range of 7.2− 8.0, which is consistent with the results obtained by Ferrini et al.4 On the basis of TG and DSC results from simultaneous thermal analysis, a simple method was developed and successfully used to determine the composition of the solid phase. This method provides kinetic information about the dissolution of Mg(OH)2 and crystallization of nesquehonite during the semibatch precipitation process. Kinetic phenomena and mechanisms were examined based on data collected from the liquid and solid phases. The initial period exhibits mainly dissolution of Mg(OH)2, after which the precipitation of magnesium carbonate starts. The precipitation rate of the magnesium carbonate is enhanced with increasing CO2 flow rate. Mixing intensity affects the precipitation rate in several ways and correlates with factors such as gas flow rate, suspension density, etc. In the present study, a rotation speed of 650 rpm is found to be the optimum speed based on the obtained crystal size at a gas flow rate of 1 L/min. Mass transfer of CO2 as the main controlling factor plays a key role in the precipitation. The particle size of nesquehonite increased with an increase of flow rate. The reaction of Mg(OH)2 is completed in 1 h, which is a simple and fast operation. Furthermore, the final product has trihydrate concentration that is higher than 96%. The product presents a bind formed by needle-like primary crystals. The synthetic nesquehonite has good potential for use in capture and storage of CO2 due to its stability up to 300 °C. The thermal analysis method developed in the present work provides a simple and effective way to determine the kinetics of the formation of magnesium carbonate crystals. The approach could be applied to other carbonation processes as well. Many magnesium sources exist in saline deposits, seawater, and saline wastewater, so comprehensive understanding of the mechanism of this heterogeneous reaction process could serve as a background for further investigation of reactive crystallization of magnesium salts and for finding optimal operational conditions for industrial crystallization processes.
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +358-451185268. Fax: +358-5-411-7201. E-mail: bing. han@lut.fi. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank the Academy of Finland (Project 260141) for financial support.
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REFERENCES
(1) Hsu, K. J. Process and apparatus for recovery of lithium in a helminthoid evaporator. U.S. Patent 6197152B1, 2001. 12082
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