Polymorph and morphology of CaCO3 in relation to precipitation conditions in a bubbling system☆

Jian Sun,Lisheng Wang*,Dongfang Zhao

School of Chemistry and Chemical Engineering,Beijing Institute of Technology,Beijing 100081,China

1.Introduction

The increasing consumption offossilfuels to meetglobaldemand for energy had resulted in an increase in CO2emission.Severalstudies have reached the conclusion that the rise in atmospheric CO2concentration is closely connected to global warming and consequent climate change[1,2].Most agree that carbon capture and sequestration(CCS)is necessary for greenhouse gas(GHG)reduction in the immediate future[3].Therefore,the development of appropriate technologies for CCS has attracted considerable attention worldwide.

Among many technologies,mineralCO2sequestration is highlighted by its reliability.This method is based on the reaction of calcium(magnesium)silicates with gaseous CO2leading to solid carbonates and SiO2.Precipitated calcium carbonate(PCC)can be stored permanently and safely.Mineral CO2sequestration can be broadly classified as direct and indirect route.In the view of practical purpose,an indirect carbonation has advantages over a directcarbonation.In an indirectcarbonation,Ca2+is leached from rich calcium solids in solution first and then carbonate with CO2in a separate step.Pure CaCO3can be produced for future usage in this method.However,CaCO3with impurity is obtained in direct carbonation process,wherein solids containing calcium and other metals in an aqueous suspension carbonate with gaseous CO2directly[4,5].

In an indirectcarbonation,many materials have been tested as feedback for extraction of Ca2+.Besides natural minerals,industrial wastes are another important resource of calcium.Vast amount industrial wastes consist of combustion residues,steel slags and fly ashes which can provide calcium resources with low cost.NH4NO3,CH3COONH4,NH4HCO3and NH4Clare favorite asleaching agentin extraction process.Ammonium salts strikingly stand outbecause ofhigh calcium extraction efficiency and selectivity.More importantly,they can be easily recovered by spontaneous reactions for reutilization[6,7].After calcium extraction,gaseous CO2are input into leachate containing calcium and ammonia in carbonation step.A briefprocedure consisting of extraction and carbonation steps using ammonium salts as leaching agents is described as in Eqs.(1)and(2).

A significant amount of researches have been conducted to investigate extraction and carbonation reactions by using NH4X(X=NO3?1,CH3COO?1,HCO3?1,Cl?1,etc.).For instances,Heet al.[8]investigated the carbonation of coal fly ash using NH4Cl,NH4NO3and CH3COONH4.Kodama and coworkers[6]developed a pH swing CO2mineralization process with a recyclable reaction solution.Calcium extraction from steel-making slag by using NH4Cl in the first step,CaCO3precipitation and regeneration of NH4Cl occurred in the second step.Joet al.[9]evaluated the effects of CH3COONH4,NH4Cl,NH4NO3and(NH4)2SO4on the cation extraction and mineral carbonation of waste cement.

Effects of various experimental conditions,covering temperature,pH,CO2pressure and CO2concentration,on carbonation efficiency were fully discussed in these articles,but knowledge regarding polymorph and morphology of PCC in relation to experimental conditions was provided inadequately.The application of calcium carbonate particles is determined by a great number of strictly de fined parameters,such as particle polymorph,morphology,size,specific surface area,brightness,oil adsorption,chemical purity and so on.One of the most important factors is particle polymorph[10].Calcium carbonate particles have three crystal polymorphs:rhombic calcite,needle-like aragonite and spherical vaterite.Calcite is the most stable phase at room temperature under normal atmospheric conditions,while aragonite and vaterite are metastable polymorphs which readily transform into the stable phase,calcite.Few articles discussed dependence of CaCO3polymorph on precipitation conditions in a bubbling system involved with ammonia.The relationship between precipitation conditions and morphology of CaCO3is the object of some experimental studies but it still is disputed.Takahashiet al.presented vaterite and calcite coprecipitated at pH=11.1,and proportion of calcite decreased as pH value dropped.Pure spherical vaterite was observed at pH=7.9[11].However,Olaru and coworkers reported that above 99%purity of vaterite precipitated at pH range 9.8–7.5[12].

In order to clarify in fluences of precipitation conditions on polymorph and morphology of PCC,in the present research,a series of carbonation reactions were designed and conducted,simulating the carbonation step(Eq.(2)).CaCl2was dissolved in solution to provide Ca2+,pure CO2and NH3·H2O was utilized as feed gas and pH adjust agent,respectively.pH value, flow rate of CO2and temperature were experimental variables.The polymorph and morphology of PCC were examined by Fourier transform infrared spectroscopy(FTIR),X-ray diffraction(XRD)and scanning electron microscopy(SEM),and the correlation to experimental conditions was illuminated.The conclusion obtained from this article can provide theoretical direction for production of vaterite through mineral CO2sequestration under the in fluence of ammonia.Appropriate experimental condition is requisite for generation of pure vaterite with less cost and high efficiency.

2.Experimental

2.1.Materials and methods

In the presentresearch,all carbonation reactions were carried out in a 0.5 L glass reactor at atmosphere.This glass reactor was heated by a water-bath thermostat(Shanghai Lp-laboratory Instrument Works Co.,Ltd).Uniform agitation of500 rpmforslurry was supplied by a magnetic stirrer(78HW-1,Bilon).

Calcium chloride(CaCl299.9 wt%,Beijing Chemical Works)used in experiments for preparation of Ca-rich solution,and the prepared Ca-rich solution were filtered through a 0.22 μm membrane before use.The feed gas used in experiments was pure gaseous CO2.The flow rate(Q)of CO2was measured with a Cole-Parmer flow meter.Because aqueous carbonate precipitation is more favored in higher pH conditions[13],a peristaltic pump(RSC01)was utilized to convey NH3·H2O(25.0 wt%,Beijing Chemical Works)into Ca-rich solution to maintain alkalinity during carbonation.

In a glass reactor,pure CO2was bubbled into Ca-rich solution at designed precipitation conditions,as depicted in Fig.1.During the precipitation process,the Ca2+concentration and pH value were monitored by a calcium ion selective electrode(ISE Ca800,WTW)and a pH meter(PHS-3C).CO2supply was turned off at 20 min,and then PCC sample was extracted immediately to avoid crystal transformation.After that,sample was separated on a filter equipped with polytetra fluoroethylene(PTFE)membrane discs(0.2 μm),washed with deionized water and dried over 24 h at 105°C.

2.2.Characteristic analysis of PCC

First,polymorphs of PCC were examined by Fourier transform infrared spectroscopy(FTIR,IS10,NICOLET).Band assignment for FTIR of CaCO3polymorphs was well established in literature[14–16].These absorption bands correspond to symmetric C--O stretching mode(v1),CO3out-of-plane bending mode(v2),doubly degenerated asymmetric C--O stretching mode(v3)and doubly degenerated in-plane O--C--O deformation bending mode(v4).The spectral region between 950 cm?1and 650 cm?1was selected for qualitative analysis:vaterite:v2 at 849 cm?1and 877 cm?1,v4 at 744 cm?1;calcite:v2 at 877 cm?1,v4 at 712 cm?1and aragonite:v2 at 854 cm?1,v4 at 700 cm?1and 712 cm?1.Apparently,there is overlapping of absorption bands among three polymorphs of CaCO3.An overlapping of vaterite and calcite absorption bands appears at 877 cm?1,and the same occurs at 712 cm?1for calcite and aragonite.

Fig.1.Schematic diagram ofexperimentalsetup for carbonation reaction.1,CO2 cylinder;2,NH3·H2O;3,peristaltic pump;4,magnetic stirring;5,waterbath;6,glass reactor;7,pHmeter;8,calcium ion selective electrode.

Further,PCC samples were examined by an X-ray diffractometer(Rigaku Ultima IV)at 40 kV and 40 mA with CuKαradiation(λ =0.15406nm).The diffraction data were recorded for 2θ angles from 20°to 60°with a scanning step 0.02°.The relative amounts ofthe polymorphic composition can be calculated from intensities ofthe vaterite(1 1 0),calcite(1 0 4),and aragonite(2 2 1)by using the Kontoyannis equation[17].The details of the Kontoyannis equation are presented:

For mixture composed of calcite,vaterite and aragonite:

whereXiis the molar fraction ofcalcite,vaterite or aragonite,Iifis the intensity of diffraction peak of one face.For example,IA221represents intensity of face(2 2 1)of aragonite.

Crystal morphology of PCC samples was observed on a FEI Quanta 200 environmentalscanning electron microscope operating athigh vacuum and 15 kV.The samples were coated with gold prior to imaging.

3.Experimental Design and Result

The entire experimentalscheme constituted ofa series of18 independent carbonation reactions.In these reactions,pH value was regulated by addition of aqueous ammonia at the following three ranges,from initial pH to final pH:10.0–9.0,9.0–8.0 and 8.0–7.0 respectively.The declining rate of pH value of solution was controlled at 0.1 pH unit per 2 min,and thus overall change of pH value is 1.0 in each carbonate reaction(20 min).The initialCa2+concentration was 0.5 mol·L?1;the operational temperature wasT=25℃，45℃and 70°C,respectively;CO2input was maintained at relatively low flow rate,Q=50 and 100 ml·min?1respectively.The details of experimental design and corresponded polymorphic composition of PCC samples are summarized in Table 1.

The carbonation process in the presence of ammonia can be described by the following steps:

Dissolution of CO2

From Eqs.(8)and(9),it can be concluded that amounts of CO2and OH?determine kinetically the carbonation process.When Ca2+is sufficient,high flow rate of CO2and pH value are expected to facilitate carbonation.

Fig.2 is a representative example of carbonation reactions of PCC D,PCC J and PCC P.In Fig.2,there are 3 declining curves,where Ca2+concentration gradually decreases,indicating the precipitation of solidCaCO3was occurring.The ratio between overall consumption of Ca2+and time(20 min)in precipitation is considered as average carbonation reaction rate(mol·min?1).The average carbonation reaction rates of PCC A-R are also listed in Table 1.

Table 1Details of experimental design and polymorphic composition of PCC

Fig.2.A representative example of carbonation reactions of PCC D,J and P.

Fig.3.FTIR spectra of PCC A,C and F.

Fig.4.XRD patterns of PCC A,C and F.

Fig.5.SEM images of PCC A,C and F.(a),PCC A;(b),PCC C(c),PCC F.

4.Discussion

4.1.PCC obtained at high pH

PCC A(Q=50 ml·min?1,T=25 °C),B(Q=50 ml·min?1,T=45 °C),D(Q=100 ml·min?1,T=25 °C)and E(Q=100 ml·min?1,T=45 °C)were obtained at high pH(referring to pH=10.0–9.0).Their FTIR spectra are identical and spectrum of PCC A is selected in Fig.3.In PCC A,absorption peaks at 877 cm?1and 849 cm?1corresponding to vaterite present clearly,and no peak can be found at 712 cm?1and 700 cm?1,indicating sample is vaterite with high purity.XRD analysis was in agreement with this result.Diffraction pattern of PCC A is shown in Fig.4,where all diffraction peaks are associated with vaterite except for a very small peak corresponding to calcite(104)at2θ=29.4°.This result suggests PCC A is vaterite with trace calcite.Calculation of polymorphic composition shows purity of vaterite is 98.6%.SEM images prove that massive spherical shape vaterite mingled with single rhombic shape calcite obtained in PCC A,as it can be seen in Fig.5a.

Metastable vaterite can precipitate probably because of high supersaturation of solution contributed by high pH value.From Eqs.(8)and(9),it can be known that the quantity of OH?controlled the formation of CO32?(aq).At high pH value,CO32?(aq)was formed sufficiently from gaseous CO2.As a result,high supersaturation towards Ca2+and CO32?was achieved in solution.According to Ostwald's rule of phases[18],in aqueous media,the less-stable polymorph can nucleate firstly and then converts into the most stable polymorph later.In the present research,the initial phase may be the amorphous calcium carbonate(ACC)and the subsequent transformation and crystallization of ACC follow a downhill pathway in the free energy γ:ACC → vaterite→aragonite → calcite.The reported values of free energy γs(mJ·m?2)for three polymorphs at 25 °C are γcalcite=7–280,γaragonite=150 and γvaterite=6.8–108[19,20].On the one hand,it can be predicted that if the solution supersaturation reaches the solubility constant value of vaterite,this metastable phase can be formed firstly.On the other hand,high reaction rate contributed by high supersaturation ensures crystallization of vaterite.As expected,PCC A,B,D and E obtained at high pH value were nearly pure vaterite(over 98.6%in purity).The above discussion indicates vaterite is the preferential polymorph of PCC at high pH value in this bubbling system.

As temperature increased to 70°C,calcite and aragonite began to appear in PCC samples.FTIR shows that PCC C(Q=50 ml·min?1,T=70 °C)and F(Q=100 ml·min?1,T=70 °C)have multiple polymorphs of CaCO3.In Fig.3,the characteristic absorption peaks marked with “C”corresponding to the calcite,“V”to the vaterite and “A”to the aragonite appear in PCC C and PCC F,proving calcite,aragonite and vaterite coexisted in these samples.The result was further con firmed by XRD analysis.The principal diffraction peaks corresponding to aragonite(A221),vaterite(V104)and calcite(C110)polymorph are noted in patterns of PCC C and PCC F(Fig.4).Subsequently,polymorphic composition calculation from XRD patterns shows that for PCC C,dominant phase is vaterite 45.3%,side phase is calcite 33.1%,least phase is aragonite 21.6%,and PCC F is composed of 51.1%vaterite,24.6%calcite and 24.3%aragonite.

Fig.6.FTIR spectra of PCC G,H,I,J,K and L.

Fig.7.XRD patterns of PCC G,H,I,J,K and L.

Three polymorphs of CaCO3coexisted in PCC C and PCC F can be ascribed to the thermal vibrations.At the beginning,vaterite was formed from ACC,and then gradually transformed into aragonite.Vaterite is a polymorph of CaCO3where six oxygen atoms coordinate to the calcium atom within the structure,while in aragonite,nine oxygen atoms coordinate to the calcium atom.This phase transition occurred at 70°C probably because high temperature increased effective radii of calcium atoms[21].In SEM images,rhombic calcite,needlelike aragonite and spherical vaterite can be seen clearly in Fig.5b and c.In addition,some disc-like particles are found to exist in PCC E and PCC F.These irregularsphericalshape particles are attributed to dissolution of spherical shape vaterite that was undergoing polymorphic transformation.

In order to ascertain the role of calcite in transformation of PCC from vaterite to aragonite,PCC F carbonation was repeated in an additional experiment,namely as PCC F40.In this experiment,after supply of CO2stopped at 20 min,PCC F40was remained in solution for another 20 min to provide longer time for transformation.PCC F40sample was examined by means of FTIR and XRD analysis(see supporting information).The result shows a complex of aragonite and calcite obtained in the sample,indicating vaterite was completely converted into calcite and aragonite in 40 min.Polymorphic composition calculation shows PCC F40has 69.2%aragonite and 30.8%calcite.This result implies that aragonite is more stable than calcite at 70°C,and calcite may be an intermediate phase during transformation.It formed from vaterite and transformed into aragonite.

Fig.8.FTIR spectra of PCC M,N,O,P,Q and R.

4.2.PCC obtained at moderate pH

Polymorphs of PCC obtained at moderate pH value(referring to pH=9.0–8.0)are different as a consequence of change of flow rate of CO2.FTIR spectra and XRD analysis show PCC G(Q=50 ml·min?1,T=25 °C),PCC H(Q=50 ml·min?1,T=45 °C),PCC J(Q=100 ml·min?1,T=25 °C)and PCC K(Q=100 ml·min?1,T=45 °C)are composites of vaterite and calcite,as shown in Figs.6 and 7.Polymorphic composition calculation shows 92.2%and 90.5%vaterite existed in PCC G and H respectively,while vaterite accounts for above 98%of PCC J and PCC K.This result indicates that in fluence of flow rate of CO2on polymorph of PCC is observed at the moderate pH range.At the same pH range(pH=9.0–8.0),when quantity of CO2is adequate,vaterite can precipitate without formation of calcite,but when flow rate of CO2is below a critical level,the transformation from vaterite to calcite can occur.This fact can also be interpreted from decline of average carbonation reaction rate.Average carbonation reaction rates for PCC A and PCC B,which can be found in Table 1,are 0.007435 and 0.007232 mol·min?1respectively,and are 0.006349 and 0.006132 mol·min?1for PCC G and H individually.It is manifest that average carbonation reaction rates of PCC G and H were reduced.As a result,nearly pure vaterite was obtained in PCC A and PCC B,while a part of calcite appeared in PCC G and PCC H.Decreased carbonation reaction rate cannot prevent dissolution of vaterite and crystallization of calcite[12].It can be concluded that carbonation reaction rate is the fundamental factor for precipitation of vaterite.At pH=9.0–8.0,50 ml·min?1CO2flow rate could not meet the requirement of carbonation reaction rate,and thus calcite formed partially in PCC G and PCC H.

Fig.9.XRD patterns of PCC M,N,O,P,Q and R.

Similarly,as temperature increased to 70°C,vaterite,calcite and aragonite were formed simultaneously in PCC samples.FTIR reveals that PCC I(Q=50 ml·min?1,T=70 °C)and PCC L(Q=100 ml·min?1,T=70°C)are mixtures of aragonite,vaterite and calcite.As shown in Fig.6,absorption peak at 854 cm?1,712 cm?1and 700 cm?1of aragonite,peaks at 877 cm?1,849 cm?1and 744 cm?1of vaterite,and peaks at 877 cm?1and 712 cm?1of calcite are apparently presented in PCC I and PCC L.The XRD analysis of PCC I and PCC L was also performed.Bragg re flections of principal peaks of PCC I and PCC L are noted in Fig.7.The calculated relative amounts of the three polymorphs of PCC I are 39.2%vaterite,22.6%calcite and 24.0%aragonite respectively,and for PCC L are 44.3%vaterite,24.8%calcite and 21.9%aragonite respectively.By comparison,it was found that polymorphic compositions of PCC I and PCC L have no significantdifference from PCC C and PCC F.This result indicates polymorphic transformation from vaterite to aragoniteviacalcite is mainly subject to elevation of temperature,and decrease of pH range from 10.0–9.0 to 9.0–8.0 has no significant impacts here.

4.3.PCC obtained at low pH

At low pH value(referring to pH=8.0–7.0),massive calcite precipitated in PCC samples.FTIR spectra and XRD analysis found PCC M(Q=50 ml·min?1,T=25 °C),N(Q=50 ml·min?1,T=45 °C),P(Q=100 ml·min?1,T=25 °C),and Q(Q=100 ml·min?1,T=45°C)are mixtures of calcite and vaterite(Figs.8 and 9).Polymorphic composition calculation shows calcite is main polymorph for PCC M(calcite 52.3%)and N(calcite 59.7%).From SEM images,it can be seen substantial rhombic calcite appeared in PCC M and PCC N(Fig.10a and b).This result demonstrates calcite is the dominant polymorph at low supersaturation attributed to low pH value and low flow rate of CO2.Dissolution of vaterite and crystallization of calcite substantially occurred at this experimental condition.In addition,it can be found that PCC Nhas higher contentofcalcite than PCC M.Temperature elevation from 25 °C to 45 °C can account for this fact.As temperature increases,solubility of CO2was dropped in aqueous solution[22],which led to less formation of CO32?and lower supersaturation.Therefore,more calcite converted from vaterite in PCC N than in PCC M.PCC P and PCC Q have higher supply of CO2,so vaterite remains the dominant polymorph in these two samples.PCC P and PCC Qhave 73.4%and 70.9%vaterite,respectively.This result suggests in fluence of flow rate of CO2on polymorph of PCC became more important at low pH range.

At 70°C,FTIR spectra and XRD analysis just found calcite and aragonite coexisted in PCC samples.PCC O(Q=50 ml·min?1,T=70 °C)has 41%calcite and 59%aragonite,and PCC P(Q=100 ml·min?1,T=70°C)has 44.7%calcite and 55.3%aragonite.This result demonstrates preservation of vaterite at low pH value and high temperature is very difficult.SEM images are consistent with this result that only rhombic calcite and needle-like aragonite can be seen in PCC O and PCC R(Fig.10c and d).

Fig.10.SEM images of PCC M,N,O and R.(a),PCC M;(b),PCC N;(c),PCC O,(d)PCC R.

5.Conclusions

A series of PCC samples were obtained by carbonation reaction in a bubbling system at regulated pH ranges.Polymorph and morphology ofPCC were found to be closely connected with precipitation conditions.Vaterite was the preferable polymorph at a high pH value.pH condition(10.0–9.0)can be applied for a relatively low CO2flow rate leading to pure vaterite,while pH condition(9.0–8.0)requires 100 ml·min?1for pure vaterite formation.This finding could guide synthesis of vaterite in emission reduction of greenhouse gases by mineral CO2sequestration.Calcite is the preferential polymorph of PCC at low pH value range(8.0–7.0)because low supersaturation would not prevent dissolution of vaterite and recrystallization of calcite.In addition,high temperature was found to be a critical factor for the formation of aragonite.At 70°C,transformation from vaterite to aragoniteviacalcite was detected in the presentresearch.In an additional carbonation reaction that provides 40 min for polymorphic transformation,vaterite vanished and only major aragonite and minor calcite were found in sample.This finding suggests aragonite is more stable than calcite at high temperature,and calcite is an intermediate phase during this transformation.

Supplementary Material

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.cjche.2016.12.004.

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