Enhancement of CO2 Mineralization in Ca2+-/Mg2+-Rich AqueousSolutions Using Insoluble AmineWenlong Wang, Xin Liu, Peng Wang,* Yanli Zheng, and Man Wang
National Engineering Laboratory for Coal-fired Pollutants Emission Reduction, Shandong University, Jinan, China, 250061
ABSTRACT: The potential for using concentrated seawater to fix CO2 by adding insoluble amine extractant was tested andverified. The experimental results showed that over 90% of Ca2+ ions could be converted to precipitation. Ammonia was chosenas regenerant to regenerate the extracting agent; the regeneration rate can reach 95%. On the basis of the analysis of MgCO3precipitation properties, a new CO2 mineralization process was proposed in which CaO is employed to react with Mg2+ insolution. Mg(OH)2 precipitation and Ca2+-rich aqueous solutions were produced, and both performed well in CO2mineralization. This new process can produce different kinds of byproducts such as MgCO3, CaCO3, and NH4Cl. Sincethere is no energy consumption from phase separation, nor is there a heat requirement, it is therefore definitely less energyintensive. This approach has great application potential.
1. INTRODUCTIONCarbon capture and storage (CCS) is the primary means toreduce carbon emissions at present. A variety of technologicalapproaches, such as chemical absorption, physical adsorption,membrane absorption, etc.,1−6 have been studied extensively.However, there are still some disadvantages in almost all of theCCS processes, including the high cost, the required energyconsumption that leads to new carbon emissions, the absenceof byproduct output during the process, and the risk anduncertainty in geological7−9 and ocean storage,10,11etc. There-fore, the development of energy-saving, low-cost, and safe CCSprocesses are needed, and carbon capture, utilization, andstorage (CCUS) approaches, which stress the utilization ofCO2, represent an encouraging new technological strategy.Carbon mineralization is a method for CO2 storage, but it
has more promising applications in the utilization of CO2.Initially, CO2 mineralization was developed by reactionsbetween CO2 and silicates, such as serpentine.12 However,the costs and energy consumption are very high in theseprocesses and the low reaction rate is not suitable forsequestration of CO2 from emission sources functioning onengineering time scales.13 A great deal of research has beendone to improve mineralization processes.14−18 Using naturalminerals and solid waste to fix CO2 is one of the mostpromising techniques because it is simultaneously a low-energyprocess and is capable of producing a high value-added product.For instance, Kodama et al.19 studied the mineralization processusing steelmaking slag and ammonium chloride solution toimmobilize CO2 while producing pure CaCO3. Nduagu et al.20
utilized ammonium sulfate and magnesium silicates to producemagnesium hydroxide, which was used to absorb CO2 andcogenerated valuable product such as MgCO3. Xie et al. studiedphosphogypsum waste, magnesium chloride, and potashfeldspar to achieve CO2 fixation and produce other valuablebyproducts.21−24 Almost all of the mineralization reactionswere realized by using magnesium and calcium in differentminerals.Since some aqueous resources, such as seawater, subsurface
brine, and industrial effluents, are also rich in magnesium and
calcium, they have potential applications in CO2 mineralization.Our research group has proved the feasibility of CO2 fixation byenhancing the formation of carbonate precipitation.25,26 In ourwork, theoretical analyses indicated that the carbonationreaction could be enhanced by raising the pH or the CO2
partial pressure, and experiments confirmed that over 90% ofthe Ca2+and Mg2+ ions in seawater could be converted byprecipitation in the forms of MgCO3 and dolomite [MgCa(CO3)2]. NH3/NH4Cl buffer solution was used effectively asthe pH regulator in the experiments. (However, NH3/NH4Clbuffer solution added to seawater is difficult to reclaim andreuse with low energy consumption, and future improvementsin the method are needed.) Different soluble amines, such asMEA, MDEA, TEA, etc., were tested as pH regulators, andmost of them performed well in enhancement of carbonateprecipitation in seawater. Nanofiltration, which has fairly goodseparation effects on organic molecules (molecular mass >150)and multivalent ions, was used to reclaim the amines. However,results showed that the amines could not be completelyseparated from the mixed seawater solution. In addition, energyconsumption was high.Subsequently, insoluble amine was tested by our working
group. Tributylamine was adopted as the pH regulator. It wasfound that the tertiary amine could not only enhance thereactions between CO2 and calcium ions but also perform wellin regeneration or separation from the aqueous solution.Therefore, CO2 mineralization in Ca2+-/Mg2+-rich aqueoussolutions was proved feasible with the help of insoluble amine.This paper will provide a detailed introduction to this process.
Received: January 24, 2013Revised: April 15, 2013Accepted: May 18, 2013
2. PRINCIPLES OF ABSORBING CO2 USINGINSOLUBLE AMINE
The enhancement of carbonation precipitation using insolubleamine employs a strong extraction function of insoluble aminefor inorganic acid in high pH solution. The process proposed inthis paper consists of the following steps:
+ ↔ * ↔ + ↔
+
+ −
+ −
CO H O H CO H HCO 2
H CO2 2 2 3 3
32
(1)
CO2 dissolves into water and generates the componentdenoted as H2CO3*, which includes true carbonic acid anddissolved CO2.Then, carbon acid dissociates to form H+,HCO3
−, and CO32− by the first and second dissociation.
Finally, CO2 reaches solution equilibrium.
+ + →+ −NR H Cl NR HCl3 3 (2)
+ →+ + −Ca /Mg CO CaCO /MgCO2 23
23 3 (3)
When amine extractants are added into the system, CO2solution equilibrium is broken. As shown in eq 2, H+ ionized bycarbonic acid and Cl− in brine reacts with amine at the interfaceto form amine hydrochloride. After reaction, amine hydro-chloride is extracted back into the organic phase. With theincreasing extraction of H+, the concentration of CO3
2−
increases in solution; CaCO3 and MgCO3 are generated as ineq 3.
+ → + +2NR HCl Ca(OH) 2NR CaCl 2H O3 2 3 2 2 (4)
+ · → + +NR HCl NH H O NR NH Cl H O3 3 2 3 4 2 (5)
Amine extractants contain great amounts of HCl in the formof amine hydrochloride after extraction. HCl can be extractedfrom amine extractants by reacting with inorganic alkaline suchas Ca(OH)2 or ammonia in a normal environment.26 Amineextractants can therefore be regenerated, as shown in eq 4 andeq 5. Thus, amine plays the role of reacting medium in thewhole process.Relevant studies have proven that using insoluble amine
extractants can achieve NaHCO3 precipitation in highconcentration sodium chloride solution (eq 6).27,28 However,there is no previous research for the carbonation process usingamine extractant in solutions. Therefore, the possibility ofcompleting the carbonation process using amine extractant isthermodynamically analyzed in this study. The precipitationformation process can be expressed as eq 7 when insolubleamine is added to Ca2+-/Mg2+-rich aqueous solutions, such asseawater.
+ + + → +NaCl CO H O NR NaHCO NR HCl2 2 3 3 3(6)
+ + + →
↓ + ·
CaCl /MgCl CO H O 2NR CaCO /
MgCO 2NR HCl
2 2 2 2 3 3
3 3 (7)
Gibbs free energy change and enthalpy change of reaction arevery important functions for characterization of spontaneouschange, energy change, and limit of reaction. Under standardcondition, they can be calculated by the formula:29
∑ ∑Δ = Δ − Δθ θ θG n G n G(product) (reactant)r m i f m j f mi j
∑ ∑Δ = Δ − Δθ θ θH n H n H(product) (reactant)r m i f m j f mi j
In this formula, n is the stoichiometric number, i and j representdifferent kinds of substances. ΔfG
θm is the Gibbs energy of
formation, and ΔfHθm is the enthalpy of formation. Though
thermodynamic data for NR3·HCl and NR3 are unknown, thesubtraction between eq 7 and eq 6 can offset enthalpy andGibbs free energy of formation of NR3·HCl and NR3.Therefore, the Gibbs free energy change and enthalpy changeof eq 7 can be evaluated with the help of eq 6. It has beenproven that reaction 6 is thermodynamically possible in apositive direction27,28 (ΔrG6
θ < 0) and exothermic; ΔrH6θ is
about −71 kJ/mol.30
The calculation results show that Gibbs free energy changeand enthalpy change of eq 7 is negative (Table 1). Hence, eq 7is thermodynamically possible to use insoluble amine as the pHregulator to fix CO2 in Ca2+-/Mg2+-rich aqueous solutions.
3. EXPERIMENTSThe above analyses indicate that the process is theoreticallyfeasible. Further understanding must be based on experimentalstudy. Thus, experiments were carried out to explore thepractical process of extraction and carbonation.
3.1. Materials. Concentrated seawater was used as theCa2+-/Mg2+-rich aqueous solution. The concentration ofconcentrated seawater was twice that of natural seawater,which represented the current average industrial level.31
Preparation of concentrated seawater was based on the artificialformula created by Lyman and Fleming (1940).32 Table 2
showed the formulation of artificial concentrated seawater.Tributylamine (AR), a typical insoluble amine, was chosen asextracting agent. N-Butyl alcohol (AR) was used as thecosolvent, to ensure that the extracting complex dissolved inextraction and avoided the emulsion phenomenon.
3.2. Experimental Processes. The CO2 mineralizationprocess was performed as follows: Concentrated seawater (100mL), tributylamine, and n-butyl alcohol were added into thereactor at the volume ratio of 1:1:2. The solution was stirred
with an electric stirrer at a rate of 500 rpm throughout theprocess. After it was well-distributed, pure CO2 was releasedfrom a gas cylinder and entered the reactor through a gasdistributor. A Mettler-Toledo pH meter was employed tomonitor the chemical reaction by monitoring the pH change.The pH of the mixed solution was 10.0 before the experiments.When the pH decreased to 8.30, the experiments wereterminated. The reaction temperature was set to 25 °C, andthe CO2 flow rate was controlled at 0.3L/min. Figure 1presents a schematic diagram of the experimental setup.
After a standing and layering process, organic liquid wasseparated into a collector from the reaction solution. Theprecipitation (in brine phase) was filtered through 0.20 μmnucleopore polycarbonate membrane filters.The concentrations of Ca2+ and Mg2+ in the separated brine
phase were measured by ion chromatography. Analysismethods of X-ray powder diffraction (XRD) and X-rayfluorescence (XRF) were jointly applied to determine thechemical compositions of the precipitation products. XRDpatterns were obtained on a diffractometer operated at 40 kVand 40 mA and were recorded from 10° to 80° (2θ) at a rate of0.06° per step. The elemental contents were tested by an ARLQuant’x X-ray fluorescence (XRF) spectrometer.Retrievability and recyclability are important foundations for
selecting a pH regulator. Insoluble amine meets thoseconditions. It is easily regenerated by alkaline substances suchas ammonia, Mg(OH)2, MgO, CaO, etc. In this study,ammonia was chosen as regenerant to regenerate the extractingagent after reaction. The regeneration process was performed asfollows: the extraction solvent (after the experiment) was mixedwith a regenerant (ammonia) at the volume ratio of 20:1; themixed phases were stirred with an electric stirrer at a rate of 500rpm. The reaction lasted for 15 min at 25 °C. Theconcentration of tributylamine hydrochloride in organic phasewas analyzed by NaOH titration.3.3. Results and Discussion. During the experiments,
transparent amine and seawater were changed into a slightlyepinephelos solution by stirring. With CO2 bubbling into thereactor, the mixture solution gradually became more turbid andthe pH of the system declined without any sudden changes.When the stirring stopped after reaction, the solution wasquickly separated into two phases with a clear-cut interface;large amounts of white fine precipitation were dispersed in the
water phase. The whole process was finished within a fewminutes.Through the analysis of solution and precipitation, the
changing rules of Ca2+ and Mg2+ concentration during theextraction and carbonation process were revealed. As shown inFigure 2, the concentration of Ca2+ declined sharply during the
first minute from 800 to 200 mg/L and then dropped slightlyfrom 200 to 65.5 mg/L in the following 3 min. This indicatedthat about 90% of Ca2+ was precipitated in 4 min. Figure 3
showed the XRD diffractogram pattern of the final precip-itation; it determined that the precipitation was only CaCO3 bycomparison with standard CaCO3, so Ca2+ had goodperformance in CO2 fixation by using insoluble amine. Inaddition, the reaction can be completed in a short period, whichhas potential to meet industrial applications of CO2 fixation.Results also confirmed that the concentration of Mg2+
remained the same during the process, the reason being thatMgCO3 is unsaturated in the solution. According to Pitzerelectrolyte solution theory,33 ionic strength will decrease ionicactivity and increase solubility when the background concen-tration of electrolyte solution is high in solutions such asseawater, garbage percolate, and activated sludge. The solubilityproduct constant of MgCO3 is 1.15 × 10−5 in dilute electrolyticsolution at 25 °C; it becomes higher in concentrated seawater.Therefore, the solubility of MgCO3 in strong electrolytesolutions and CO3
2− concentrations are the main influencingfactors for the formation of MgCO3 precipitation. These couldeasily be improved by changing the flow mass of CO2 in orderto increase contact time, using higher concentrated solution,
Figure 1. Experimental setup.
Figure 2. Changing rule of Ca2+ and Mg2+.
Figure 3. XRD diffractogram of the final precipitation.
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and altering amine, etc. All of which can be studied in futureexperiments. In addition, Ca2+ efficiently fixed CO2, which wasproven in the experiments, so Mg2+ in solution can be replacedby Ca2+. Mg2+ can react with CaO to form a Ca2+-rich aqueoussolution and Mg(OH)2, and then a Ca2+-rich aqueous solutionand Mg(OH)2 can be used to fix CO2. Therefore, usinginsoluble amine as the pH regulator is feasible.In the regeneration experiment, the mixed solution was
quickly separated into two phases with a clear-cut interfacewhen the stirrer stopped, which was similar to the CO2 fixationprocess. The regeneration rate could be calculated as follows:
=− ′
×RC C
C100%HCl HCl
HCl
CHCl is the concentration of tributylamine hydrochloride in theextracting agent after reaction, and CHCl′ is the concentration oftributylamine hydrochloride in the extracting agent afterregeneration. As shown in Figure 4, the concentration of
tributylamine hydrochloride in the extracting agent increasedwith the increase in reaction time. Figure 5 indicates that theregeneration ratio remained around 95% even though theconcentration of tributylamine hydrochloride was different invarious reaction times. Therefore, ammonia had high efficiencyin amine regeneration, and the concentration of tributylaminehydrochloride in the extracting agent did not influence theregeneration process under all experimental conditions.
Furthermore, NH4Cl was enriched in the ammonia regenerantduring the regeneration process, so the ammonia regenerantalso had great application potential in NH4Cl production.
4. APPLICATION DESIGNThe above experiments primarily proved that using amineextractant to fix CO2 in Ca2+-/Mg2+-rich aqueous solutions isfeasible. Because of the mutual insolubility between amineextractants and seawater and easy regeneration process ofinsoluble amine, the process will be attractive in application.Although Mg2+could not be directly precipitated, we can firstreplace them with calcium ions and then realize theircarbonation indirectly. On the basis of this idea, an applicationdesign is proposed and illustrated in Figure 6. The whole
process can be divided into 3 sections: CO2 fixation bymagnesium hydroxide, CO2 fixation by amine extractants, andamine extractants regeneration. Initially, CaO is added intoconcentrated seawater to react with Mg2+ to form Mg(OH)2,and then, Mg(OH)2 is used to fix CO2. Compared to othermethods of enhancing MgCO3 precipitation, using CaO as asubstitute for Mg2+ is more promising. It not only immobilizesCO2 but also realizes the separation of different byproducts.Mg(OH)2 precipitation and Ca2+-rich aqueous solutions canreact with CO2 respectively. CaO is produced by calciumcarbonate decomposition, converting back to calcium carbonateafter CO2 fixation, so it has no net CO2 fixation effect.Consequently, CaO acts as a reacting medium in this processsimilar to that in chemical-looping combustion.After Mg(OH)2 precipitation separation, the Ca2+-rich
aqueous solution is mixed with an extracting agent to absorbCO2 in the reactor. After a standing and layering process, thewater phase and precipitation are expelled out of the reactor,and the extracting agent after reaction is regenerated byammonia. Regenerated amine extractant can be reused directly.Although each step was carried out separately in our previousexperiments, the whole process could also be proven to befeasible.Compared to conventional methods, this new CO2
mineralization process has unique advantages. From theperspective of CO2 fixation, this new process can achieve
Figure 4. Concentration of HCl in the extracting agent after reaction.
Figure 5. Regeneration ration of extracting agent containing differentconcentrations of HCl.
Figure 6. Process schematic drawings.
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CO2 capture and storage simultaneously. Gaseous CO2 isconverted to solid carbonate directly in the process. Comparedwith conventional technologies, our method is comparativelysimple because the procedure of CO2 transportation andstorage can be eliminated. Carbonate is the most stable form ofCO2 and perhaps the safest way for humans to store carbondioxide.From the perspective of byproduct production, the major
advantage of this process is that all kinds of byproducts can beobtained in pure forms. MgCO3 is obtained during CO2fixation by magnesium hydroxide; CaCO3 is separated fromthe reactor, and NH4Cl is produced after the regenerationprocedure. In the material flow of the entire process, only cheapraw materials such as NH3·H2O, CaO, and CO2 are added intothe system while a variety of pure and valuable byproducts areproduced, e.g., MgCO3, CaCO3, and NH4Cl. As byproducts,CaCO3 and MgCO3 are widely used. For instance, the CaleraCorporation is using magnesium and calcium carbonateproduced in the CO2 mineralization process to manufacturecarbon-negative building materials. CaCO3 and MgCO3 canalso sequestrate CO2 through the bicarbonate process.34
5. PROSPECTSThe approach of using amine extractants as the pH regulator tofix CO2 is a promising and potentially valuable method. On thebasis of the theoretical analyses and experiments reportedherein, the potential of this method to fix CO2 is evaluated. Thecalculation is based on consumption as follows: all of the Mg2+
is replaced by Ca2+, and the mole ratio of Mg/Ca is 1:1; theutilization ratio of Ca2+ was 90% in the experiments. CO2fixation mainly comes from 3 resources in this process: Mg2+
and Ca2+ in concentrated seawater and Ca2+ from CaO addedinto solution, so it can be calculated as follows:
= − +C C C C0.9fixation Ca,system CaO Mg
CCa,system is the total CO2 fixed by calcium in the process. CCaOis the CO2 produced by calcium carbonate decomposition.Because CaO is produced by calcium carbonate decomposition,the fixed CO2 by Ca2+ from CaO needs to be excluded fromthis process. CMg is the CO2 that can be fixed by the Mg(OH)2generated in the replacement reaction between calcium andmagnesium. Actually, CCaO is equal to CMg. The value of Cfixationcan be calculated as 0.114 mol of CO2 per liter of concentratedseawater. That is, the carbon fixation capacity of 1m3concentrated seawater is about 2.55 m3 or 5.02 kg gaseousCO2 at standard conditions. Furthermore, 8.98 kg of MgCO3,6.10 kg of NH4Cl, and 12.50 kg of CaCO3 are produced duringthe process. In addition, the whole process is less energyintensive. There is no energy consumption from phaseseparation, nor is there a heat requirement. Therefore, theenergy used to pump and transport seawater is the main energyconsumption. According to the above calculation, 199.2 m3
concentrated seawater is needed for 1 ton of CO2 fixation.Typical pump parameters (2000 m3/h, 400 kW, lift of 50 m,and pump efficiency of 0.85) are used to drive the concentratedseawater. That is, the pump needs 470.59 kWh to drive 2000m3 of water, so driving 199.2 m3 of concentrated seawaterrequires 46.87 kWh. Considering other energy consumption,even if they triple, it only results in 140.6 kWh, so this processis favorable from the viewpoint of power consumption.According to relevant statistics, the desalination capacity
worldwide is about 40 million m3 per day.35 The productivity ofdesalination of seawater and bitter in China was 60 million tons
per day in 2011. “The Suggestions of Accelerate Developmentof Seawater Desalinization Industry” points out that theproductivity of seawater and bitter after desalination willreach 220−260 million tons per day in 2015.36 With thedevelopment of the desalination industry, an increasing amountof wastewater will be produced. At present, wastewater isdirectly discharged into the sea. It is not only environmentallyharmful but also resource-wasting. In addition to highconcentrated seawater, solutions containing Ca2+ and Mg2+
are widely found in sources such as waste brine produced bysalt and potassium carbonate industries. In addition, bitter andsalt lakes are widely distributed in China. Therefore, the processof using Ca2+-/Mg2+-rich aqueous solutions to fix CO2permanently by amine extractants has broad potential as wellas a wide array of sources of raw materials.This paper presents a preliminary concept that is still far
from commercial application. As research continues, theprocess will be improved and optimized. Almost all of theenergy consumption is caused by the large volume ofconcentrated seawater and its large space requirements.Therefore, a high concentration of Ca2+-/Mg2+-rich aqueoussolutions or preconcentrated seawater by waste heat37 is one ofthe future research directions.
6. CONCLUSIONSBoth theoretical analyses and experimental results confirmedthat concentrated seawater could fix CO2 by using insolubleamine extractant. The use of tributylamine and artificialconcentrated seawater to fix CO2 was studied at roomtemperature. It was found that over 90% of Ca2+ ion couldbe converted to precipitation. The regeneration ratio was up to95% when stronger ammonia−water was employed as thereclaiming agent. The solubility of MgCO3 in strong electrolytesolutions and CO3
2− concentrations are the main influencingfactors for the formation of MgCO3 precipitation. Compared toother approaches for improving MgCO3 precipitation, usingCaO to substitute for Mg2+ is more promising. It not only fixesCO2 permanently but also realizes valuable byproductcoproduction. This new process can realize CO2 capture andstorage simultaneously.Although this method appears sound, some technical points
must be well researched, and the design should be enhanced toachieve practical application. Further work in these areas isunder way in our laboratory.
■ ACKNOWLEDGMENTSThe authors acknowledge the support from National NaturalScience Foundation of China (No. 51106088).
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