Synthesis of calcium carbonate in a pure ethanol and aqueous ethanol solution as the solvent

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doi:10.1016/j.jcr

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Journal of Crystal Growth 276 (2005) 680–687

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Synthesis of calcium carbonate in a pure ethanol and aqueousethanol solution as the solvent

Kang-Seok Seoa, Choon Hana, Jung-Ho Weeb,�, Jin-Koo Parkc, Ji-Whan Ahnc

aDepartment of Chemical Engineering, Kwangwoon University, Seoul 139-701, Republic of KoreabDepartment of Chemical and Biological Engineering, Korea University, Seoul 136-701, Republic of Korea

cMinerals and Materials Processing Division, Korea Institute of Geoscience and Mineral Resources, Daejon 305-350, Republic of Korea

Received 7 October 2004; accepted 25 November 2004

Communicated by K. Nakajima

Available online 4 January 2005

Abstract

The possibility of formation of precipitated calcium carbonate (PCC) in pure ethanol, not as small additives, but as

the main solvent, was investigated by precipitating a variety of PCC via a carbonation reaction. During the carbonation

in a slaked lime–pure ethanol suspension, three morphology types of CaCO3 were also precipitated, including calcite,

which was the only type of PCC precipitated in the pure water system, and aragonite and vaterite, which were also

precipitated without leaving Ca(OH)2 as the reactants. Their particle size was half of those from pure water. In a pure

ethanol system, calcite was first precipitated from the carbonation in bulk solution as in the pure water system, while the

aragonite and vaterite might be synthesized via other local carbonation routes occurring in the surface of the Ca(OH)2grain following the bulk carbonation in the solution. In this local carbonation, there was little variation of electrical

conductivity and pH. In the aqueous solution of less than 40mol% ethanol, the PCC is all calcite; therefore, water has

dominant effect as the solvent. On the other hand, in the solution of more than 60mol% ethanol, the solvent acts as the

pure ethanol and calcite, aragonite and vaterite can be precipitated.

r 2004 Elsevier B.V. All rights reserved.

Keywords: A1. Carbonation; A1. Precipitation; B1. Calcium carbonate; B1. Ethanol

1. Introduction

Calcium carbonate is one of the abundantmaterials present in nature, and it exhibits three

e front matter r 2004 Elsevier B.V. All rights reserve

ysgro.2004.11.416

ng author. Tel.: 82 2 923 3105;

2.

ss: [email protected] (J.-H. Wee).

anhydrous crystalline polymorphs including cal-cite, aragonite and vaterite [1–4]. The calciumcarbonate used in industries could be classifiedinto limestone powder, ground calcium carbonate,and precipitated calcium carbonate (PCC) by theirshape, particle size and preparation method. PCCis synthesized by modifying the morphology ofcalcium ore and by using the compound that has a

d.

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K.-S. Seo et al. / Journal of Crystal Growth 276 (2005) 680–687 681

calcium and carbonate radical. Due to its proper-ties such as uniform shape and size, which aremore effective than any other type CaCO3, theapplications of PCC are as diverse as rubber,plastics, paper, paints, food, adhesives, and so on.According to increasingly diverse and advanceddemands on PCC in recent years in the industry,the application of CaCO3 particles is determinedby a great number of strictly defined parameters,such as morphology, structure, size, specific sur-face area, brightness, oil adsorption, chemicalpurity, and so on [5–7]. In addition, recent studiesof CaCO3 precipitation have been directed towardits nanoscale and complex shapes [8–12].From an aqueous slurry of Ca(OH)2 suspension,

CaCO3 can be synthesized via a couple ofprocesses such as coprecipitation or the carbona-tion route [13–15]. The coprecipitation method isoften adapted in a laboratory study because of itssimplicity of operation or its ease in the control ofprocess variables. The most widely used method tosynthesize CaCO3 on an industrial scale is thecarbonation route [16,17] which consists of bub-bling CO2 gas through the aqueous slurry ofCa(OH)2 in a batch process, due to its low cost andthe availability of the raw materials.The physicochemical processes involved in the

precipitation of calcium carbonate are very inter-esting and need to be understood in order to assessquantitatively the factors governing the formationof PCC. Many works on the operating variablessuch as the effect of pH [18,19] temperature [20],foreign ions [21], organic additives [22–25], and thedegree of supersaturation and so on [13,26,27] inaqueous systems have been studied extensively.However, few investigations have been made onthe effects of the solvent. Previous studies pre-sented amorphous calcium carbonate (ACC) thatcould be synthesized in ethanol as a solvent[28,29]. However, these papers could not clearlyelucidate the mechanism of ACC synthesizing, andother papers [23–25] presented the effects ofethanol on the precipitation of calcium phosphatein ethanol. PCC was synthesized in a calciumnitrate and sodium bicarbonate aqueous solutionadded to less than 10% of ethanol, isopropanoland diethylene glycol [30]. The previous works alsostudied the influence of terpineol (less than 1%) on

the size and morphology of CaCO3 particlesformed via the carbonation route [12] and theinfluence of Poly vinyl alcohol on the morphologyof CaCO3 [31].This paper intends to investigate the effect of

pure ethanol and ethanol–water solution as asolvent on the morphology of precipitation ofcalcium carbonate in the carbonation process. Wefocus on the synthesis of PCC in ethanol as asolvent and how they were synthesized at constantexperimental conditions.

2. Experimental procedure

To investigate the effect of pure ethanol andethanol–water solution as a solvent on the mor-phology of PCC at constant experimental condi-tions in the carbonation process, the calciumcarbonate was precipitated in pure ethanol, inpure water and in the four kinds of ethanol(20–80mol%)–water mixing solutions. Each solu-tion, a suspension of 30 g of Ca(OH)2 (Junsei, EP.)in 500ml of each solvent, was prepared in thePyrex reactor (j ¼ 120mm; h ¼ 150mm) as shownin Fig. 1.A radiate-shaped and glass filter-made sparger,

in which all of the holes are kept at the samedistance from the gas entrance, were used toensure uniform gas dispersion. In this reactor,the carbonation reaction was carried out byintroducing the CO2 at a flow rate of 1 l/minfrom the bottom of the reactor via the sparger. Astirrer was operated at a rotation rate of 800 rpm.The reaction temperature was kept at 25 1C bycirculating water through an outer jacket on thereactor. To monitor the variation of the electricalconductivity (EC) and pH continuously duringthe reaction in solution, a digital pH meter (Hannainstruments, pH 211) and an EC meter (Hannainstruments, EC 214), were used, respectively.This method is normally used as a tool to detectthe states of the carbonation reaction. Before thegaseous CO2 was carried into the reactor, theCa(OH)2 suspension in each solution was suffi-ciently mixed for 1 h by stirring to ensure that theCa(OH)2 sufficiently dissolved and dispersed. Thereaction was stopped until there were no variations

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1. CO2 gas cylinder. 2. Gas regulator. 3. Sparger.4. Stirrer. 5. Thermometer. 6. pH meter.7. Electrical Conductivity meter. 8. Reactor. 9. Water bath.10. Recorder.

8

9

10

3

1

25

4

67

Fig. 1. Experimental apparatus for precipitation of calcium

carbonate in pure ethanol and in ethanol–water as the solvent.

K.-S. Seo et al. / Journal of Crystal Growth 276 (2005) 680–687682

of EC and pH in suspension. Especially, in thereaction in pure ethanol suspension, to investigatehow PCC synthesizes in detail, the four inter-mediate products were sampled at four positionsin the course of the reaction time. After thereaction stopped, the products were obtained bywashing, filtering and drying and were used foranalysis. The phase composition of the precipi-tated particles was qualitatively characterized byX-ray diffraction (Rigaku, RINT 2000). Toinvestigate their shape and particle size, a PSA(Otsuka Electronics, ELS-3000) and scanningelectron microscope (Hitachi, S-2700) were used.

Fig. 2. Variation of EC with the introducing time of CO2 gas

into pure ethanol (K) and into pure water (’), as well as pH

variation in pure ethanol (J) and in pure water (&) at these

conditions.

3. Result and discussion

3.1. Relative solubility of CO2 in pure ethanol and

pure water

The dissolution of CO2 gas could be due to theexistence of an aqueous form of CO2 and lead tothe formation of HCO3

� and CO3�2 with a polar

hydroxyl group in pure water and pure ethanol.The measurement of EC and pH value in thesolution is one of the methods of predictingthe solubility of CO2 gas [13,14,30]. Fig. 2 showsthe different variations of EC in water and inethanol with inflow time of CO2 and also revealsthe pH value at these conditions when CO2 gaswas introduced at a flow rate of 1 l/min intoCa(OH)2 free pure water and pure ethanol.In pure water, the EC increased steadily to

142mS/cm and was thereafter saturated. And 7.5 oforiginal pH in pure water decreased to 4.3 within5min with the inflow of CO2. Afterwards, becausethe bicarbonate alkalinity content increases, the pHvalues increased very slightly with respect to theincrease of EC between 65 and 300min in thereaction. Afterwards, they were kept consistently atsaturated condition.However, in pure ethanol, there was only 1 mS/

cm rise in EC and a fluctuated variation of pH, asshown in Fig. 2. These mean that the introducedCO2 gas in water could make more than 100 timesthe HCO3

� and CO3�2 than those in ethanol, and

the formation of HCO3� and CO3

�2 in ethanol is sounstable that this reaction would be more rever-sible. However, the previous work reported that

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Fig. 4. X-ray diffraction patterns of the sequence of the

precipitating of calcium carbonate in pure ethanol as the

solvent. (a) materials from point (1) in Fig. 3; (b) materials from

point (2) in Fig. 3; (c) materials from point (3) in Fig. 3; (d)

materials from point (4) in Fig. 3; and (e) materials from point

(5) in Fig. 3.


the solubility of CO2 gas at 20 1C in pure waterwas 0.9ml/ml(H2O) and 2.8ml/ml(EtOH) forethanol [32]. Therefore, according to the resultsof this paper and those on lower conductivity,higher and unstable pH in pure ethanol, CO2 gasin ethanol dissolved favorably for the existence ofan aqueous state rather than being in the state ofHCO3

� and CO3�2.

3.2. Precipitation of CaCO3 in pure ethanol as the

solvent

To make the precipitation steps clear for PCC inpure ethanol, the variation of EC and pH in thesuspension of Ca(OH)2–pure ethanol were shownin Fig. 3.During the period without CO2 (from the start

to the position (1) in Fig. 3 the initial EC and pHof 30 g (Ca(OH)2)/500ml (EtOH) were 4 mS/cmand 12.6, respectively. The XRD peaks of samplesat position (1) are in accord undoubtedly withthose of Ca(OH)2 as shown in Fig. 4(a).By introducing the CO2, the protruding peak of

EC was shown for 8.5min. The maximum of ECwas 82 mS/cm at the top point of (2) and the pHdecreased rapidly from 12.5 to 6.3 during theperiod. The XRD peaks of samples at position (2)

Fig. 3. Variation of EC (’) and pH (K) with reaction time in

Ca(OH)2–pure ethanol slurry by introducing CO2 gas.

are shown in Fig. 4(b). This sample was composedmainly of Ca(OH)2 and a trace of calcite forma-tion was confirmed. The behavior during theperiod between (1) and (2) may be attributed tothe accumulation of very slightly soluble ions toform the PCC. A previous work reported thesolubility of Ca(OH)2 in pure ethanol wasincreased by introducing CO2 as in water [33]. Atthe same time during the period, a very smallamount of calcite could be formed. The maincarbonation reaction in the bulk suspension ofCa(OH)2–pure ethanol as in traditional carbona-tion in pure water occurred during the periodbetween (2) and (3). PCC might be synthesized bythe reaction with the accumulated Ca2+ and CO3

�2

as much as was possible.The XRD peaks of samples in Fig. 4(c) validate

this hypothesis. The composition of the sampleat position (4), where about 75min passed sincethe protruding peak has disappeared, were asshown in Fig. 4(d). Even though there was littlevariation of EC and pH, three types of CaCO3,

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such as calcite, aragonite and vaterite, wereprecipitated and a trace of Ca(OH)2 still remained.Furthermore, no peaks of Ca(OH)2 were found inthe sample at position (5) where EC and pH werekept constant at the end of the reaction, as shownin Fig. 4(e). Only peaks of three morphologies ofCaCO3 were found. In spite of no variation of ECand pH during the period between (d) and (e), howwere the PCC precipitated under these conditions?This may be explained from the viewpoint of agrain of each solid Ca(OH)2. When the PCCwere precipitated with much variation of EC andpH in a precipitation solution, it was clearly PCCby carbonation reaction. However, PCC in pureethanol, without the variation of EC and pH ofour results, might be synthesized through thefollowing carbonation routes. First, physicaladsorption of the water from the former carbona-tion ((1)–(3)) held on the suspended solid Ca(OH)2grains’ surface in pure ethanol. This water thenallows the Ca(OH)2 to dissolve into Ca2+ andOH� ions on the surface. These two dissolutionsteps contribute to the precipitation of CaCO3

[34]. At this time, because the carbonation occurs

Fig. 5. SEM images of the sequence of the precipitating of calcium ca

(2) in Fig. 3; (b) materials from point (3) in Fig. 3; (c) materials from

not in bulk solution, but locally on the surfaceof each Ca(OH)2 grain, and the reaction is veryfast, the EC and pH meter may not indicatetheir variation. In addition, the types of PCCsynthesized during this period were relativelystructurally unstable aragonite and vaterite.Fig. 5 displays the image of conversion fromCa(OH)2 (small particle) to PCC (large particle)through position (2)–(5).

3.3. Precipitation of CaCO3 in pure water and in

ethanol– water mixing solution as the solvent

Under the same conditions of precipitationwith pure ethanol, the traditional carbonationexperiment in pure water was carried out. Its ECand pH variation are shown in Fig. 6 and thetraditional carbonation trend clearly appears.After 36min of carbonation reaction, the dissol-ving rate of CO2 is larger than its consumptionrate, owing to the limited amount of Ca2+,resulting in the accumulation of HCO3

–, CO32–,

and H+ ions and the increase of the EC.

rbonate in pure ethanol as the solvent: (a) materials from point

point (4) in Fig. 3; and (d) materials from point (5) in Fig. 3.

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Fig. 6. Variation of EC (’) and pH (K) with reaction time in

Ca(OH)2–pure water slurry by introducing CO2 gas.

Fig. 7. Variation of EC with reaction time in Ca(OH)2–etha-

nol–water aqueous slurry by introducing CO2 gas: (a) 20mol%

EtOH (K); (b) 40mol% EtOH (’); (c) 60mol% EtOH (m); (d)

80mol% EtOH; (.); and (e) pure EtOH (E).

Fig. 8. X-ray diffraction patterns of the calcium carbonate

precipitated: from (a) pure water; (b) 20mol% EtOH; (c)

40mol% EtOH; (d) 60mol% EtOH; (d) 80mol% EtOH; and

(e) pure EtOH.


However, very different EC variations of morethan 20mol% ethanol added to the pure water asthe solvent were as shown in Fig. 7.

The five kinds of solvent including pure ethanolhave protruding peaks. The height and width ofthe peaks become even bigger with the increase inwater composition. The XRD peaks of CaCO3

synthesized under these conditions including purewater are shown in Fig. 8.In the solution of less than 40mol% ethanol, the

PCC is all calcite; therefore, water has a dominanteffect as the solvent. However, their particle sizeswere even smaller than those from pure water, aslisted in Table 1.Furthermore, with the increase of ethanol

composition, the crystallinity of calcite becameweaker and aragonite and vaterite began toappear. That is, the effect of ethanol as the solventbecame dominant. The particle size of PCC fromethanol was half that from pure water. Table 1shows a variety of data on carbonations of sixkinds of solutions.

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Table 1

Various data of six kinds of solution on carbonations (30 g of Ca(OH)2/500ml of solvent)

Pure water 20% EtOH

solution

40% EtOH

solution

60% EtOH

solution

80% EtOH

solution

Pure EtOH

Initial EC without CO2

(mS/cm)8100 420 75 19 4.6 1.8

Maximum EC with CO2

(mS/cm)8100 2370 1340 710 330 82

Carbonation time (min) 36 30 25 15 11 8.5

Type of synthesized

CaCO3

Calcite Calcite Calcite Calcite,

aragonite,

vaterite

Calcite,

aragonite,

vaterite

Calcite,

aragonite,

vaterite

Average particle size 208.4 101.5 123.9 108.4 116.5 92.4


4. Conclusion

The formation of PCC in pure ethanol as thesolvent was possible without leaving Ca(OH)2 asreactants. Among them, calcite was first precipi-tated from the carbonation in bulk solution as inthe pure water while the aragonite and vateritemight be synthesized via local carbonation occur-ring in the surface of the Ca(OH)2 grain followingthe bulk carbonation in the solution. The particlesize of the PCC was half of those from purewater. In more than 60mol% of ethanol aqueoussolution, the solvent acts as pure ethanol andcalcite, aragonite and vaterite can be precipitated.Our results present the possibility of formation

of PCC in pure ethanol not as the small additives,but as the main solvent to precipitate a variety ofCaCO3 via carbonation reaction.

Acknowledgement

This work was supported by grants from KoreaInstitute of Geoscience and Mineral Resources(KIGAM).

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Synthesis of calcium carbonate in a pure ethanol and aqueous ethanol solution as the solvent