Mineralizing CO2 as MgCO3Â·3H2O Using Abandoned MgCl2 Based

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Mineralizing CO as MgCO•3HO Using Abandoned MgCl Based on a Coupled Reaction-Extraction-Alcohol Precipitation Process Guilan Chen, Xingfu Song, Chunhua Dong, Shuying Sun, Ze Sun, and Jianguo Yu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01297 • Publication Date (Web): 01 Aug 2016 Downloaded from http://pubs.acs.org on August 10, 2016

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Mineralizing CO2 as MgCO3·3H2O Using Abandoned MgCl2 Based on a Coupled Reaction−Extraction−Alcohol Precipitation Process Guilan Chen, Xingfu Song,* Chunhua Dong, Shuying Sun, Ze Sun, Jianguo Yu* National Engineering Research Center for Integrated Utilization of Salt Lake Resources, East China University of Science and Technology, Shanghai 200237, China Abstract: A novel coupled reaction−extraction−alcohol precipitation process was proposed to mineralize CO2 as MgCO3·3H2O directly by abandoned MgCl2. Rod-like crystal MgCO3·3H2O was obtained and the conversion rate of MgCl2 increased sharply by using this novel coupled reaction−extraction−alcohol precipitation process. The effect of added C1−C3 alcohol precipitation agent on the conversion rate of MgCl2 was in order of: ethanol > isopropanol > n-propanol > methanol. Moreover, the optimal conditions for the highest conversion rate of MgCl2 by single factor experiments were obtained as follows: initial concentration of MgCl2 solution is 2 mol·L−1, volume ratio of ethanol and aqueous phase is 2, mole ratio of N235 and aqueous phase is 2, volume ratio of diluent and N235 is 0.5, with a stirring rate of 300 r·min−1 at 298.15 K and an atmospheric pressure. 1. INTRODUCTION The increasing emissions of CO2 have so serious impact on global climate, that it is necessary to develop effective methods to sequestering this greenhouse gas1. Numerous approaches such as ocean storage2 and geological storage3 have been studied extensively. However, potential issues associated with sequestration in

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geologic formations include: permanence, long-term monitoring, and verification, with many unknown effects and potential risks still to be determined4. An approach of CO2 mineralization via reaction of CO2 with metal cations such as magnesium, calcium, and iron4-5 offers an attractive option for the permanent and safe storage of CO2 in solid form because of the “highly verifiable and unquestionably permanent”4 nature of the neoformations. Numerous efforts have been made to mineralize CO2 with ores such as forsterite6, wollastonite7-9, olivine10, and serpentine11. Unfortunately, the industrial extraction of Ca and Mg from ores is rate-limiting and requires expensive pre-processing, which limits the application of these methods12. In fact, many Mg2+/Ca2+-rich aqueous resources, such as seawater, brine, and industrial effluents exist on the planet, all of which have potential to fix CO2. In comparison to solid materials, the precipitation of Ca2+ and Mg2+ from aqueous is much more rapidly than the cations locked in a silicate structure13. Thus this method may be widely applicable in countries like China rich in brine resources containing considerable amounts of MgCl2. It is a challenging process to sequestering gaseous CO2 directly by Mg2+/Ca2+ chloride solution, for the reaction between Mg2+/Ca2+-rich chloride and gaseous CO2 is not spontaneous at room condition. The barrier is pH control, which has an important effect on the formation of CO2– and carbonate 3 precipitation. Ammonia13-16 is usually used as pH regulator to make the reaction occurring continuously. But the difficult in dealing with the byproduct of NH4Cl and regeneration of ammonia limit the development of this process. Coupled reaction–extraction–crystallization process has been a hotspot in the field


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of chemical engineering. Chemical reaction, combined with liquid–liquid extraction and crystallization, can improve the overall yields of the product and overcome limitations of reaction equilibrium and product inhibition17. An important application of reactive extraction is for the recovery of products from fermentations, such as lactic acid17-18

and

carboxylic

acid19-20.

In

recent

years,

using

the

coupled

reaction–extraction–crystallization process to mineralize CO2 is also receiving attentions. In the coupled process, insoluble amine is applied as an extractant to remove the produced HCl out of the aqueous phase by forming an amine hydrochloride complex and enhances the pH of the aqueous phase, thus making the reaction occurs continuously. Zhang et al21 proposed a new CO2 sequestration approach through indirect wollastonite carbonation. Wollastonite was dissolved by HCl to form CaCl2 solution firstly, and then CaCl2 solution reacted with CO2 to form CaCO3 with the addition of triisooctylamine. Vinoba et al22 found that CO2 could be mineralized as various polymorphs of CaCO3 by using sterically hindered amine 2-amino-2-methyl-1-propanol. Wang et al5 and Liu et al23 used concentrated seawater to CO2 mineralization by adding insoluble amine extractant tributylamine. They found that 90% of Ca2+ ions could be converted to carbonate, while Mg2+ could not be directly precipitated. Therefore, in their further work24, the extract system was optimized and diisobutylamine−n-octanol system was selected as extractant. As a result, 80% of Mg2+ could be converted to magnesium carbonate. Zhou et al25 have studied the crystalline mechanism of the coupled reaction–extraction–crystallization process. They found that the solid products obtained from the coupled process were


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distributed in two regions. The large crystals were obtained by radial growth in the free aqueous phase and small crystals were obtained in water-in-oil structures with growth space constraints. Additionally, the mineralization of CO2 with CaCl2 solution by using commercial tertiary amine N235 has been reported in our previous publications26-28. The effects of extractant, diluent, concentration of CaCl2, phase ratio and feeding pattern were investigated, and an ion-pair extraction mechanism in the coupled process was proposed. In China, 20 million tons of MgCl2 are usually treated as byproduct or waste of potassium fertilizer industry annually29. Valuable magnesium resources are usually discarded back into the saline lakes instead of effective utilization. If the abandoned MgCl2 could be applied for mineralization of CO2 like CaCl2, not only the greenhouse effect can be relieved but also the magnesium resources can be utilized effectively. In this work, the mineralization of CO2 with abandoned MgCl2 based on a novel coupled reaction−extraction−alcohol precipitation process was studied. N235 was used as extractant in this work. Comparing with diisobutylamine used in Wang’s work24, N235 has advantages of lower price and lower solubility in water, which makes the process more economical in industrial production. The thermodynamic calculation of precipitation of MgCO3·3H2O in CO2−MgCl2−H2O system and feasibility analysis of the novel coupled reaction−extraction−alcohol precipitation process were discussed in this paper. Then the effects of factors such as alcohol precipitation agent, extractant, diluent, temperature and pressure on the coupled process were studied. This study provided a new perspective to the understanding of


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CO2 mineralization with tons of waste brine by the coupled process. 2. EXPERIMENTAL SECTION 2.1. Materials MgCl2·6H2O used as Mg source was purchased from Shanghai Lingfeng Chemical Reactant Co. Ltd. Methanol (Sinopharm Chemical Regent Co. Ltd.), ethanol (Sinopharm Chemical Regent Co. Ltd.), n-propanol (Shanghai Lingfeng Chemical Reactant Co. Ltd) and iso−propanol (Shanghai Lingfeng Chemical Reactant Co. Ltd) were used as alcohol precipitation agent. Commercially available extractant N235, which is a mixture of tertiary amines R3N (R: ~C8 − C10) with an effective concentration of amine 2.04 mol·L−1, was purchased from Shanghai Rare-earth Chemical Co. Ltd., China. Isoamylol (Shanghai Lingfeng Chemical Reactant Co., LTD) was used as diluent. CO2 with a mole fraction of 0.995 was supplied by Shanghai Hukang Industrial Gas Co. Ltd. Sodium hydroxide with a purity of 96.0% (Shanghai Ling Feng Chemical Reagent Co. Ltd.) was employed in the titration of organic samples. All reagents were used as received without further purification. 2.2. Experimental Setup The experiments were performed in a jacket reactor shown in Figure 1. A certain amount of MgCl2 solution, alcohol precipitation agent, N235 and isoamylol were added in the reactor. A water bath was used to adjust temperature. Besides, a serpentine condenser was used to avoid the evaporation of alcohol and water when the temperature is higher than 308.15 K. The solution was stirred with an electric stirrer (speed adjustable) for about 20 min to make the organic and aqueous phases mixing


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well. Then CO2 was released from the gas cylinder and bubbled into the reactor through a core distributor at a rate of 30 mL/min for 120 min. After reaction, the suspension was filtered and the solid was washed with anhydrous ethanol and deionized water and then dried at 60 °C in a dryer. The crystal structure, morphology and thermal analysis of the product was characterized by XRD (D/MAX-B, Rigaku Co., Japan), SEM (Quanta 250, FEI Co., US) and TG/DTG (STA449, NETZSCH Co., GER), respectively. The leachate was separated in a separating funnel after standing 12 h. The concentration (mol·kg−1) of H+ in organic phase was determined by dissolving organic phase in ethanol and titrating using a calibrated NaOH solution (0.1 mol·L−1) with phenolphthalein as the indicator. The conversion rate of MgCl2 can be calculated by the ratio of moles of produced HCl in organic phase and the initial moles of Cl− in MgCl2 solution (shown in equation (1)).

ε (MgCl2 ) =

m(o) ⋅ y(H + ) (o) 2V(aq) ⋅ c(MgCl2 ) (init)

(1)

where ε (MgCl2) is the conversion rate of MgCl2, m(o) is the mass (kg) of organic phase after reaction, y(H+)(o) is the equilibrium concentration (mol·kg−1) of HCl in organic phase, V(aq) is the initial volume (L) of aqueous phase, and c(MgCl2)(init) is the initial concentration (mol·L−1) of MgCl2.


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Figure 1. Flow diagram of the experimental setup. 1. CO2; 2. Flowmeter; 3. Buffer; 4. Stirring; 5. Reactor; 6. Core distributor; 7. Water bath 3. RESULTS AND DISCUSSION 3.1. Thermodynamic Calculation and Feasibility Analysis 3.1.1. Thermodynamic Calculation of Precipitation of MgCO3·3H2O in CO2−MgCl2− H2O System Various of Mg−carbonate can form from the CO2−MgCl2−H2O system30-31, such as nesquehonite

(MgCO3·3H2O),

(MgCO3·Mg(OH)2·3H2O),

lansfordite

hydromagnesite

(MgCO3·5H2O),

((MgCO3)4·Mg(OH)2·4H2O)

artinite and

magnesite (MgCO3). But the most common carbonate obtained from the aqueous solution at ambient temperature and moderate partial pressure of CO2 is nesquehonite32-33, which shows good filtration characteristics and can be used as the raw material of high-quality magnesium oxide. Hence, nesquehonite is selected as target precipitate in this study. The

precipitation of nesquehonite from CO2−MgCl2−H2O system is a

gas−liquid−solid heterogeneous reaction, including the dissolution−dissociation


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equilibrium of CO2, and the dissolution−precipitation equilibrium of nesquehonite. The reaction equations can be concluded in equations (2) − (5). CO 2 (g) + H 2 O(aq) ⇔ H 2 CO 3 (aq)

KH =

H 2 CO 3 (aq) ⇔ HCO 3− (aq) + H + (aq)

K a1 =

HCO 3− (aq) ⇔ CO 32− (aq) + H + (aq) 2+

p [H 2 CO 3 ]

(2)

[H + ][HCO 3− ] [H 2 CO 3 ]

(3)

[H + ][CO32− ] [HCO3− ]

(4)

Ka 2 =

2− 3

2− 3

2+

Mg (aq) + CO (aq) + 3H 2 O → MgCO 3 ⋅ 3H 2 O(s) K sp = [CO ][Mg ] (5) where g, aq and s denote gaseous, aqueous and solid phase, respectively. The units of p and activities of all the ions are bar and mol·kg−1, respectively. KH is the Henry's constant of CO2 in water; Ka1 and Ka2 are the first and second dissociation constants of CO2; Ksp is the solubility product constant of nesquehonite. KH, Ka1, Ka2 and Ksp used in this work are shown in table 1.

Table 1. The constants in equations (2) − (5) (ln K = (A1/T + A2lnT + A3T + A4 or lgK = B1 + B2/T + B3T + B4T2) Parameters A1

A2

A3

A4

KH34

−17060.71

−68.31596

0.06598907

430.1920

Ka134

−7726.010

−14.50613

−0.0279842

102.2755

Ka234

−9137.258

−18.11192

−0.02245619

116.7371

B1

B2

B3

B4

−9.31625

470.193

0.02379

−0.00005

Ksp35

According to equations (2)−(5), the precipitation boundary conditions, the relationship between a required concentration of Mg2+ to form MgCO3·3H2O and the given temperature, pH and pressure, can be determined by the following express:


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lg[Mg 2+ ] = lg K sp − (lg K a 1 + lg K a 2 − lg K H + lg pCO2 + 2 pH)

(6)

Figure 2 plots the precipitation boundary conditions at 298.15 K. It shows that a lower magnesium ion concentration is required to form magnesium carbonate at higher pH and higher pressure. The pH of MgCl2 solution is about 6.1 at an ambient temperature and pressure. However, the pH decreases to 3.6 after introducing carbon dioxide. According to Figure 2, the required Mg2+ concentration is more than 100,000 mol·kg−1 when the pH is less than 4. But the maximum Mg2+ concentration in solution is 3.7 mol·kg−1

36

, meaning that nesquehonite solids cannot be obtained

spontaneously in terms of thermodynamics when CO2 was introduced into MgCl2 solution. Thus it is necessary to enhance the pH of the aqueous solution to ensure the continuous reaction between CO2 and MgCl2 to form nesquehonite.

Figure 2. The precipitation boundary conditions at 298.15 K 3.1.2. Feasibility of Coupled Reaction−Extraction−Alcohol Precipitation Process In our previous work27, CaCO3 was prepared directly by CaCl2 and CO2 when N235 was applied to enhance the pH of aqueous phase and remove the product HCl. However, in our preliminary experiment, when MgCl2 (1 mol·L−1) reacted with CO2


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under the same conditions, there was no solid product obtained. And after standing 24 h, a thin layer of white solid was observed at the bottom of the reactor. Therefore, the aqueous phase withdrawn from the reaction was tested by Raman spectroscopy. The Raman spectrum absorption band of the aqueous phase was shown in Figure 3. An absorption band peak position at 1025 cm–1 was observed. This position was the most prominent feature of HCO–3 37. Hence the product of the reaction was Mg(HCO3)2, which could stay stable in water for a long time. The different precipitation phenomenon between Ca2+ and Mg2+ in this coupled process could be assigned to the difference of solubility product constants Ksp of their carbonate and the different behavior of the Ca2+ and Mg2+ coordination shells with the coordination of the HCO–3 38

. The former influences the thermodynamics of the carbonates, while the later

influences the nucleation kinetic of the carbonates.

Figure 3. Raman spectrum of the aqueous phase after reaction A simple thermodynamic calculation can be used to understand the difference between Ca2+ and Mg2+. Similar to equation (6), the precipitation boundary condition


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for Ca2+ can be determined by equation (7). The Ksp of CaCO3 is 4.95×10–9 at 298.15 K, while that of MgCO3·3H2O is 8.1×10–6. The former is 3 orders of magnitude lower than the latter. Supposing the concentration of Ca2+ and Mg2+ needed to be precipitated as solid is 1mol·L−1. According to the Eqs (6) and (7), the desired pH for Ca2+ and Mg2+ to be precipitated from water at 298.15 K can be shown in Figure 4. The figure shows that the desired pH for Mg2+ is higher than 6.0, while that of Ca2+ is lower than 5.0. Apparently Ca2+ is easier to be precipitated from solution than Mg2+ when the same amine is employed. This disparity reveals that decreasing the solubility of nesquehonite is conducive to the precipitation Mg2+ from MgCl2 solution.

lg[Ca 2+ ] = lg K sp − (lg K a1 + lg K a2 − lg K H + lg pCO2 + 2pH)

(7)

Figure 4. The desired pH for Ca2+ and Mg2+ with a concentration of 1 mol·L−1 to be precipitated from water at 298.15 K Alcohol precipitation process has been widely used in crystallization process39-42. With the addition of an alcohol to a water solution, the solubility of the salt in alcohol/water mixture is lower than in pure water and the kinetics of precipitation in water/alcohol mixtures is faster than in pure water. Therefore, in order to obtain


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nesquehonite

directly

from

CO2-MgCl2-H2O

Page 12 of 50

system,

a

novel

coupled

reaction−extraction−alcohol precipitation process was proposed in this work.

3.2. Single-Factor Experiments 3.2.1. Effect of Alcohol Precipitation Agent Water soluble alcohols methanol, ethanol, n-propanol and isopropanol were selected as alcohol precipitation agents. It was observed that the mixed phases became turbid within 5 minutes with the bubbling of CO2 and white particles were occurred at the same time. The results indicated that the coupled reaction−extraction−alcohol precipitation process was feasible. Theoretical investigations43,

44

demonstrate that

crystal surface is covered with a layer of tightly bound water molecules. This layer prevents the cations in solution reaching the crystal surface. Desolvation of the surface, and of the cations, can be a rate-determining kinetic step in crystal growth. The Mg2+, due to the high ratio between the electrical charge and the ion surface area, possesses strong bonded layers of water dipoles. Di Tommaso et al38 distinguish 6 water molecules in primary hydration and 12 water molecules in secondary hydration, forming a barrier of the Mg-carbonate precipitation. Alcohol can partially demolish the Mg2+ hydration shell. It is found that the average number of water molecules in the first solvated layer of Mg2+ ion is between 3.87 and 1.83 with different concentration of ethanol45. Therefore the desolvation of Mg2+ caused by alcohol could be one of the factors responsible for the fast precipitate of Mg-carbonate. The overall reaction between MgCl2 and CO2 in coupled process could be described by equation (8) and (9).


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2 MgCl 2 (aq) + 2 CO 2 (aq) + 12 H 2 O(aq) + 4 R 3 N(o) → 2 Mg(HCO 3 ) 2 (aq) + (R 3 N) 4 ⋅ (HCl) 4 (H 2 O) 8 (o)

(8)

Mg(HCO 3 ) 2 + 2 H 2 O alcohol → MgCO 3 ⋅ 3 H 2 O ↓ + CO 2 ↑

(9)

The effect of alcohol precipitation agents on the conversion rate of MgCl2 is shown in Figure 5. The conversion rate of MgCl2 increases sharply with the addition of alcohol. And the effect of alcohol precipitation agent on the conversion rate of MgCl2 is in order of: ethanol > isopropanol > n-propanol > methanol. Thus ethanol is selected as alcohol precipitation agent in following experiments.

Figure 5. Effect of alcohol precipitation agent on the conversion of MgCl2 (50 mL 1mol·L−1 MgCl2 solution, V(MgCl2):V(alcohol)=1:1, n(MgCl2):n(N235)=1:2, V(N235):V(isoamylol)=1:1, 298.15 K, atmospheric pressure, 300 r·min−1)

3.2.2. Effect of Volume Ratio of Ethanol and Aqueous Phase (Ve:Vs) The effect of volume ratio of ethanol and aqueous phase (Ve:Vs) on the coupled reaction−extraction−alcohol precipitation process was shown in Figure 6. For a fixed concentration of MgCl2 solution, the conversion rate of MgCl2 initially increased and then decreased with an increase of Ve:Vs, and reached maximum when the value of Ve:Vs is 2. The solubility of MgCO3·3H2O decreased with the increase of the volume


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of ethanol. Hence the conversion rate of MgCl2 will increase with the increase of the volume of ethanol. However, when the volume ratio of ethanol and aqueous phase is higher, the ethanol dissolved in the organic phase will increase consequently, and this will decrease the concentration of N235. As a result, the conversion rate of MgCl2 will be reduced.

Figure 6. Effect of volume ratio of ethanol and aqueous phase (Ve:Vs). A. 1mol·L−1 MgCl2; B. 2mol·L−1 MgCl2; C. 3mol·L−1 MgCl2; D. 4mol·L−1 MgCl2 (50 mL MgCl2 solution, n (MgCl2): n (N235) = 1:2, V (N235): V (isoamylol) = 1:1, 298.15 K, atmospheric pressure, 300 r·min−1) For different concentration of MgCl2 solution, maintaining all other conditions the same, the conversion rate of MgCl2 increased with the increase in initial concentration of MgCl2. The increase in initial concentration of MgCl2 would lead to the


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precipitation/dissolution reaction equilibrium shifting to the right side. Moreover, N235 extraction efficiency also increased with increasing Cl– concentration, which would improve the performance of the reaction accordingly. But an aggregation phenomenon of the obtained solids, due to the high supersaturation in the aqueous solution, was observed as the increasing of initial concentration of MgCl2. Considering the abovementioned factors comprehensively, the initial concentration of MgCl2 as 2 mol·L−1 and Ve:Vs as 2 were selected as the optimal experimental condition. 3.2.3. Effect of Mole Ratio of N235 and Aqueous Phase (nN:ns) The mole ratio of N235 and aqueous phase has significant impacts on both mineralization and regeneration process. Figure 7 demonstrated the pH of the mixed phase at different nN:ns. The pH of the mix phase increased from 6.65 to 6.95 as nN:ns increased from 1.0 to 3.0. According to thermodynamic calculation, increasing pH shows a positive effect on the precipitation of Mg2+. Therefore, the conversion rate of MgCl2 will be increased with an increase in nN:ns (verified by Figure 8). While the concentration of H+ in organic phase first increased, and then decreased with an increase in nN:ns. The concentration of H+ in organic phase reached highest when nN:ns was 2, which was the stoichiometric ratio of reaction. When the mole ratio of

N235 and aqueous phase is larger than 2, N235 is excess with respect to MgCl2. With the increase of nN:ns, the mass of the organic increases significantly. Consequently, the concentration (mol·kg−1) of H+ in organic phase will decrease.


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Figure 7. The pH of the mixed phase at different nN:ns

Figure 8. Effect of mole ratio of N235 and aqueous phase (nN:ns) (50 mL 2 mol·L−1 MgCl2 solution, V(MgCl2):V(alcohol)=1:2, V (N235): V (isoamylol) = 1:1, 298.15 K, atmospheric pressure, 300 r·min−1) The organic phase would be treated by a temperature swing process for its cyclic regeneration utilization. A large amount of free N235 recycling in regeneration would consume extra energy. The complete conversion of MgCl2 solution could be achieved through sufficient recycling of the MgCl2 solution. Thus maintaining a higher concentration of H+ in organic phase by a lower mole ratio of N235 and aqueous phase was a workable approach to reduce energy consumption. As a result, the


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optimal mole ratio of N235 and aqueous phase was selected as 2. 3.2.4. Effect of Volume Ratio of Diluent and N235 (Vd:VN) Diluent is an indispensable component of the organic phase. It changes the extraction ability of amine, the viscosity of the organic phase and the separation rate of the two phases. Without diluent, a third phase appears which exerts a negative influence on the mass transfer and liquid phase separation. Liu et al23 found that the mineralization ratio of Ca2+ was only 6.5% in the absence of diluent n-butyl alcohol. As far as MgCl2−CO2 system, the reaction even cannot occur. Based on our previous work26, polar diluent isoamylol was used in this work. The effect of volume ratio of isoamylol and N235 on the coupled process was investigated. As shown in Figure 9, the conversion rate of MgCl2 reached 65% when a small amount of isoamylol (Vd:VN=0.25) was added. And with the further increase of Vd:VN, the conversion rate of MgCl2 increased from 65% to 72% and then remained unchanged. Nevertheless, the concentration of H+ in organic phase decreased from 0.802 mol·kg−1 to 0.492 mol·kg−1 as Vd:VN increased from 0.25 to 1.5. This is because the mass of the organic increases with the increase of Vd:VN at a fixed volume of N235. Higher conversion rate of MgCl2 and higher concentration of H+ in organic phase are expected in the process. Considering the conversion rate and concentration of H+ in organic phase comprehensively, a volume ratio of 0.5, smaller than the optimal condition 1 in our previous work, was the optimal experimental condition.


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Figure 9. Effect of volume ratio of diluent and N235 (Vd:VN) (50 mL 2 mol·L−1MgCl2 solution, V(MgCl2):V(alcohol)=1:2, n (MgCl2):n (N235) = 1:2, 298.15 K, atmospheric pressure, 300 r·min−1) 3.2.5. Effect of Temperature Temperature exhibits a complicated effect on the coupled process. For example, for tributylamine − n-butyl alcohol23 and N235 − isoamylol27 system, the mineralization ratio of Ca2+ decreases linearly with the rising temperature. For diisobutylamine − n-octanol system24, the mineralization ratio of Ca2+ keeps stable, while mineralization ratio of Mg2+ increases from 72% to 84% when the reaction temperature increases from 281.15 K to 311.15 K. However, a different phenomenon was observed in our system. As shown in Figure 10, the conversion rate of MgCl2 remained at 72% at a temperature range of 308.15−328.15 K and decreased to 41% when temperature increased from 308.15 K to 328.15 K. A possible explanation about the conversion rate of MgCl2 variation with temperature is as follows. The pH of the mixed phase and the desired pH for Mg2+ to be precipitated from water are shown in Figure 10. Both decrease with the increase of


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temperature. However, the pH differential, driving force for Mg2+ to be precipitated, is almost the same at the temperature range of 308.15−328.15 K, which may result in the conversion rate of MgCl2 remaining unchanged. And when temperature increased from 308.15 K to 328.15 K, the pH differential decreased from 0.38 to 0.27, which results in a decrease in conversion rate of MgCl2.

Figure 10. Effect of temperature on coupled process (50 mL 2 mol·L−1 MgCl2 solution, V (MgCl2): V (alcohol) =1:2, n (MgCl2): n (N235) =1:2, V (N235): V (isoamylol) = 1:2, atmospheric pressure, 300 r·min−1) The component of the obtained crystal at different temperatures was identified using XRD (Figure 11). The XRD peak positions of samples did not vary significantly with the increase of the reaction temperature. However, peak intensity of the product decreased significantly when temperature above 318.15 K. The phase composition of magnesium carbonate hydrates is highly temperature dependent. Generally, rod-like nesquehonite is formed in the interval of 283.15 K−325.15 K35,

46

; sheetlike

hydromagnesite is prepared above 338.15 K30, 33, 47 and nesquehonite transforms to hydromagnesite through amorphous metastable intermediates in the interval of 325.15


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K−338.15 K30. Thus the decrease of the peaks may be attributed to the formation of metastable intermediates. The SEM images of the precipitate product at 318.15 K and 328.15 K were obtained to verify the assumption. As shown in Figure 12, the crystal obtained at 318.15 K is still rod-like nesquehonite, but some amorphous metastable phases are observed at 328.15 K. The results indicate that rod-like nesquehonite can only be obtained below 318.15 K. Overall, the mineralization process can be operated stably with a wide temperature range from 278.15 K to 308.15 K. As higher or lower temperatures need extra energy consumption, room temperature 298.15 K was selected as the optimal temperature.

o

Figure 11. The XRD spectra of the obtained solids with different temperatures


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Figure 12. SEM morphology of the solids product obtained at 318.15 K and 328.15 K 3.2.6. Effect of CO2 Pressure The effect of CO2 pressure on the coupled process was also tested. The MgCl2 solution reacted with CO2 in an autoclave at 298.15 K with the existence of alcohol precipitation agent and extractant. The result was displayed in Figure 13. The conversion rate of MgCl2 was down slightly from 73% to 68% as the increase of pressure from 1 atm to 8 atm. The solubility of CO2 in aqueous phase will increase with increasing of CO2 pressure. This is beneficial to the formation of HCO−3 and CO 2− 3

. However, because Ka2 is small compared to Ka1, simply raising the pressure of CO2

only generates a small amount of CO2− 3 . In that case, CO2 mainly exists in the form of


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Mg(HCO3)2. The stability of Mg(HCO3)2 enhances with the increasing of CO2 pressure. That will weaken the extent of reaction in equation (9), which leads to a decline in the conversion rate of MgCl2. In this work, atmospheric pressure was selected as the optimal pressure.

Figure 13. Effect of pressure on coupled process (50 mL 2 mol·L−1 MgCl2 solution, V (MgCl2): V (alcohol) =1:2, n (MgCl2): n (N235) =1:2, V (N235): V (isoamylol) = 1:2, 298.15 K, 300 r·min−1) 3.2.7. Effect of Stirring Speed Stir has important effect on mixing and mass transfer of two phase’s flows. The influence of the stirring speed on the conversion rate of MgCl2 was tested from 100 to 700 r·min−1. As shown in Figure 14, the conversion rate of MgCl2 increased first and then changed mildly with an increase of stirring speed. A higher stirring speed does not improve the conversion rate of MgCl2. Therefore, a stirring speed of 300 r·min−1 is sufficient to maintain good mixing of the two phases.


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Figure 14. Effect of stirring speed on coupled process (50 mL 2 mol·L−1 MgCl2 solution, V (MgCl2): V (alcohol) =1:2, n (MgCl2): n (N235) =1:2, V (N235): V (isoamylol) =1:2, 298.15 K, atmospheric pressure)

3.3. Characteristics of Product In all, the optimal conditions in the coupled reaction−extraction−alcohol precipitation process, obtained by the single-factor experiments, are as follows: initial concentration of MgCl2 solution is 2 mol·L−1, volume ratio of ethanol and aqueous phase is 2, mole ratio of N235 and aqueous phase is 2, volume ratio of diluent and N235 is 0.5, with a stirring rate of 300 r·min−1 at 298.15 K and an atmospheric pressure. The well-filtrated powders obtained under the abovementioned conditions were characterized by XRD, SEM and TG/DTG. And the purity of the solid product analyzed by the titration of Mg2+ is 99.88%. The crystal phase of the precipitated samples produced under the optimal conditions was identified by using XRD (Figure 15). The narrow peaks reflected the high degree of crystallinity of the precipitate. And the XRD spectrum of the sample match


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precisely with the diffraction patterns of nesquehonite in the database (ICDD PDF-2) provided by the Jade software 5.0, indicating the product obtained from the coupled reaction−extraction−alcohol precipitation process was MgCO3·3H2O.

Figure 15. The XRD spectrum for the solid product Figure 16 showed the morphology of the solids by SEM. It could be observed that there were two obvious morphology distribution of the solid product. The large crystals were in rod shape and small crystals existed as needle-like shape, which presented

a

same

size

distribution

as

Zhou’s

work25.

In

the

coupled

reaction−extraction system, the two phases were well-mixed by high stirring speed, which could easily result in a water-in-oil structure. The large crystals were obtained by radial growth in the free aqueous phase, and the small crystals were obtained in water−in−oil structures with growth space constraints. Surfactant can be used to adjust the size distribution in the coupled process, which will be studied in our further work.


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Figure 16. SEM morphology of the solids The thermal analysis of nesquehonite was shown in Figure 17. The thermal decomposition of nesquehonite proceeded via dehydration at a low-temperature (below 573 K) and decarbonation process above 623 K. It was found that the total amount of H2O liberated was 35.73 wt.% and the total amount of carbon dioxide liberated was 33.39 wt.%. A further mass loss of approximately 2 wt.% occurred until a constant mass was obtained. The total mass loss for the analysis was 71.12 wt.%, which was very close to the theoretical mass loss 71wt.% from nesquehonite.

Figure 17. The TG/DTG curves of nesquehonite Coupled reaction−extraction approach is a promising and potentially valuable


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method in fixing CO2 as MgCO3·3H2O. Based on the experiments result reported here, the carbon mineralization capacity of 1 t abandoned MgCl2 is about 333 kg gaseous CO2 at standard conditions. Furthermore, 1046 kg of MgCO3·3H2O and 553 kg of HCl by-product can also be obtained during the process. The productivity of abandoned MgCl2 in potassium fertilizer industry in China is 20 million tons annually. This means 6.7 million tons of CO2 can be mineralized as MgCO3·3H2O theoretically every year. Teir et al48 reported stability of magnesium and calcium carbonate minerals in nitric acid solutions at various conditions of acidity. They demonstrated that magnesium carbonate was a more stable option than calcium carbonate for storing CO2. And CO2 could not release from magnesium carbonate when it exposed to acid rain (pH 5–7), which means magnesium carbonate could theoretically store CO2 permanently. Thus this method may be widely applicable in countries like China rich in lake saline brine resources containing considerable amounts of magnesium.

4．．CONCLUSIONS Thermodynamic calculation indicates that Mg2+ is hard to be precipitated from MgCl2 solution for the higher solubility of its carbonate in water. Therefore, a novel coupled reaction−extraction−alcohol precipitation process was proposed to make the precipitation of Mg2+ continuously. Ethanol was selected as the best alcohol precipitation agent, with the added of ethanol, the conversion rate of MgCl2 increased from 20% to 52% and MgCO3·3H2O crystals were obtained successfully. Moreover, factors such as volume ratio of ethanol and aqueous phase, mole ratio of N235 and aqueous phase, and temperature were also investigated. The optimal condition by


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single factor experiments was obtained as follows: initial concentration of MgCl2 solution is 2 mol·L−1, volume ratio of ethanol and aqueous phase is 2, mole ratio of N235 and aqueous phase is 2, volume ratio of diluent and N235 is 0.5, with a stirring rate of 300 r·min−1 at 298.15 K atmospheric pressure. The conversion rate of MgCl2 achieved

72%

under

the

optimal

condition.

This

novel

coupled

reaction−extraction−alcohol precipitation process proposed in this work also has potential in preparing Na2CO3 or K2CO3 directly from CO2-NaCl/KCl-H2O system without the thermal decomposition of NaHCO3 and KHCO3. Overall, this novel method not only can apply in mineralization of carbon dioxide, but also can take advantage of waste MgCl2.

AUTHOR INFORMATION

Corresponding Author

* Phone/fax: +86-021-64252170. E-mail: [email protected]; [email protected]

Notes The authors declare no competing financial interest.

REFERENCES

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Figure 1. Flow diagram of the experimental setup. 1. CO2; 2. Flowmeter; 3. Buffer; 4. Stirring; 5. Reactor; 6. Core distributor; 7. Water bath 66x43mm (300 x 300 DPI)


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Figure 2. The precipitation boundary conditions at 298.15 K 65x47mm (300 x 300 DPI)


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Figure 3. Raman spectrum of the aqueous phase after reaction 71x57mm (300 x 300 DPI)


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Figure 4. The desired pH for Ca2+ and Mg2+ with a concentration of 1 mol•L−1 to be precipitated from water at 298.15 K 74x56mm (300 x 300 DPI)


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Figure 5. Effect of alcohol precipitation agent on the conversion of MgCl2 (50 mL 1mol•L−1 MgCl2 solution, V(MgCl2):V(alcohol)=1:1, n(MgCl2):n(N235)=1:2, V(N235):V(isoamylol)=1:1, 298.15 K, atmospheric pressure, 300 r•min−1) 61x42mm (300 x 300 DPI)


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Figure 6. Effect of volume ratio of ethanol and aqueous phase (Ve:Vs). A. 1mol•L−1 MgCl2; B. 2mol•L−1 MgCl2; C. 3mol•L−1 MgCl2; D. 4mol•L−1 MgCl2 (50 mL MgCl2 solution, n (MgCl2): n (N235) = 1:2, V (N235): V (isoamylol) = 1:1, 298.15 K, atmospheric pressure, 300 r•min−1) 119x93mm (300 x 300 DPI)


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Figure 7. The pH of the mixed phase at different nN:ns 67x60mm (300 x 300 DPI)


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Figure 8. Effect of mole ratio of N235 and aqueous phase (nN:ns) (50 mL 2 mol•L−1 MgCl2 solution, V(MgCl2):V(alcohol)=1:2, V (N235): V (isoamylol) = 1:1, 298.15 K, atmospheric pressure, 300 r•min−1) 64x46mm (300 x 300 DPI)


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Figure 9. Effect of volume ratio of diluent and N235 (Vd:VN) (50 mL 2 mol•L−1 MgCl2 solution, V(MgCl2):V(alcohol)=1:2, n (MgCl2):n (N235) = 1:2, 298.15 K, atmospheric pressure, 300 r•min−1) 64x46mm (300 x 300 DPI)


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Figure 10. Effect of temperature on coupled process (50 mL 2 mol•L−1 MgCl2 solution, V(MgCl2):V(alcohol)=1:2, n (MgCl2):n (N235) = 1:2, V (N235): V (isoamylol) = 1:2, atmospheric pressure, 300 r•min−1) 68x52mm (300 x 300 DPI)


Energy & Fuels

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Figure 11. The XRD spectra of the obtained solids with different temperatures 73x60mm (300 x 300 DPI)


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Figure 12. SEM morphology of the solids product obtained at 318.15 K and 328.15 K


Energy & Fuels

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Figure 13. Effect of pressure on coupled process (50 mL 2 mol•L−1 MgCl2 solution, V (MgCl2): V (alcohol) =1:2, n (MgCl2): n (N235) =1:2, V (N235): V (isoamylol) = 1:2, 298.15 K, 300 r•min−1) 66x50mm (300 x 300 DPI)


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Energy & Fuels

Figure 14. Effect of stirring speed on coupled process (50 mL 2 mol•L−1 MgCl2 solution, V (MgCl2): V (alcohol) =1:2, n (MgCl2): n (N235) =1:2, V (N235): V (isoamylol) =1:2, 298.15 K, atmospheric pressure) 71x57mm (300 x 300 DPI)


Energy & Fuels

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Figure 15. The XRD spectrum for the solid product 72x58mm (300 x 300 DPI)


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Figure 16. SEM morphology of the solids


Energy & Fuels

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Figure 17. The TG/DTG curves of nesquehonite 69x54mm (300 x 300 DPI)


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Mineralizing CO2 as MgCO3Â·3H2O Using Abandoned MgCl2 Based

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