Hydrothermal and mechanochemical synthesis of crystalline CaCO 3

Introduction

Calcium carbonate (CaCO₃) is a substance widely distributed in nature. It is estimated that 7% of the Earth’s crust, such as marble and limestone, is composed of calcium carbonate. It occurs in three crystalline forms, i.e. calcite (rhomboeder), aragonite (needles) and vaterite (polycrystalline spheres) of different morphology and different shapes of crystals, wherein calcite is the most stable (Peric et al., 1996; Tai and Chen, 1998). This substance has many applications in different branches of industry, among others, as a polymer filler (Gorna et al., 2008), as a pigment (Walton et al., 1973; Yoo et al., 2009), as a template for the encapsulation of bioactive compounds (Sukhorukov et al., 2004) or as an adsorbent for the removal of heavy metals from water (Ahmad et al., 2012; Ma et al., 2012a, 2012b). Its use is defined by a large number of specific parameters, such as crystalline form, morphology, particle sizes and their distribution, specific surface area, chemical purity and the like.

There are many methods of production of calcium carbonate. Commonly, it is prepared in the recarbonization of natural calcium carbonate to the calcium oxide with a subsequent reaction with water to form calcium hydroxide, which then creates a pure precipitated CaCO₃ in the reaction with CO₂. Also an interesting example of calcium carbonate preparation is using the eggshell waste as a source (Yoo et al., 2009). The authors used recovered calcium carbonate powder as an ingredient of coating suspension for ink-jet printing paper.

Nowadays, the main aim of chemistry is to design chemical products and processes that will limit or eliminate the use and generation of hazardous substances and consume the lowest possible energy by applying alternative energy sources, e.g. microwaves, ultrasounds and mechanical energy. It is also important to replace the toxic organic substances with water (hydrothermal processes) or carrying out the processes without the use of solvents. The field of chemistry that deals with those aspects is the ‘green chemistry’. Utilizing the water/ethylene glycol system with the addition of surfactants and microwave energy for the production of different morphologies calcium carbonate in the form of vaterite is a good example (Qi and Zhu, 2006). The authors applied microwave heating in the stage of CaCO₃ nuclei formation. By the use of different heating times (from 8 to 30 min), they obtained the samples with various forms of vaterite. The authors of the article (He et al., 2005) successfully used ultrasound energy to synthesize monodispersed nanoparticles of calcium carbonate. They obtained CaCO₃ in the form of calcite and stated that the use of the batch carbonation method with ultrasonication leads to formation of particles of smaller size compared to the product prepared without ultrasonication. In the article (Hosoi et al., 1997), the hydrothermal method of solidified calcium carbonate preparation was presented. The authors studied the transformations of different crystalline forms of CaCO₃ (aragonite-calcite mixture) during hydrothermal hot pressing in the autoclave at a pressure 40 MPa and a temperature 140–160℃. These experimental results presented a low temperature solidification method for the synthesis of calcite compact bodies. In the article (Tsuzuki et al., 2000), the synthesis of calcite using mechanical energy is reported. The particle size of products was controlled by varying the ratio of the reactants. The authors used NaCl as a solid diluent. As a result, it was stated that the resulting particle size was dependent on the amount of salt used as diluent.

Therefore, the aim of this study was to obtain crystalline CaCO₃ applying two alternative energy sources, i.e. mechanochemical treatment in the planetary mill and microwave-assisted hydrothermal synthesis.

Novelty of this study is that in a simple manner, in a short time and with high yield, the target product can be obtained in the crystalline form. The applied procedure did not use the additional factors in the form of organic templates or inorganic salts as thinners. The target product was obtained in one step without further heat treatment. Hydrothermal treatment in a microwave reactor was used as the method which allows modification of the forms created during high-energy milling.

Experimental

Materials

The starting substrates in the synthesis were CaCl₂ and Na₂CO₃ (POCh, Poland). The powder mixture in the molar ratio of 1:1 (10.5 g Na₂CO₃ and 11.1 g CaCl₂) was closed in a vessel made of Si₃N₄ having a volume of 80 cm³ with 25 balls made of the same material having a diameter of 10 mm. Mechanochemical (MChT) processes were carried out in a planetary mill (Pulverisette 7, FRITSCH, Germany) at different speeds of rotation (500 or 850 rpm), different duration times (30 or 60 min) and in dry state(s) or in the aqueous suspensions (liq) (10 cm³ of water per one portion of mixture). The MChT process at 500 rpm in a dry state (MChT_500−30s) was performed for 30 min for the initial activation of the starting mixture of reactants before the next microwave-assisted hydrothermal treatment (MWT). Milling at 850 rpm in water suspension or in the solid state during 30 or 60 min was performed to synthesize directly the target product (MChT_850−30liq; MChT_850−60liq; MChT_850−60s). After the reaction, the mixture was subjected to washing in order to remove formed NaCl. The reaction proceeded in the following manner:

CaCl 2 + Na 2 CO 3 → CaCO 3 + 2 NaCl

The hydrothermal treatment of the sample MChT_500−30s was conducted in a high pressure microwave reactor, power 300 W (Plazmatronika, Poland) in a saturated water vapour (vap) or under the layer of water (liq). The weighed sample (2 g) was put to the quartz thimble, which was placed in the microwave reactor. In the case of the saturated water procedure (vap), 20 cm³ of water was poured on the bottom of microwave reactor under the quartz thimble. But in the case of modification under the liquid water layer (liq), 10 cm³ of water was put into the quartz thimble and 10 cm³ on the bottom of the microwave reactor. The reaction time was 60 min. The resulting parameters of temperature and pressure were 180℃ and 90 atm. In such a way, there were prepared the two samples MChT_500−30sMWT_vap and MChT_500−30sMWT_liq.

Results and discussion

In Figure 1, the nitrogen adsorption-desorption isotherms and differential pore volume distribution functions for the studied samples are presented. The shape of isotherms corresponds to type II with hysteresis loop H3 of the IUPAC classification (Gregg and Sing, 1982; Rouquerol et al., 1994), corresponding to the textural porosity of aggregates of nonporous nanoparticles. The hysteresis loop shape indicates dominant contribution of mesopores. Comparing the relative position of the curves in Figure 1(a), it can be seen that the hydrothermal modification (MWT) causes a decrease in porous structure parameters as the isotherms for these samples are positioned below the adsorption isotherm of the sample MChT_500−30s. Moreover, the hysteresis loop for the sample prepared in the liquid phase (MChT_500−30sMWT_liq) is shifted to higher p/p₀. This results in the shift of the maximum peak in Figure 1(a) (right picture) in the direction of higher values of the pore diameter compared to the other samples. In the case of isotherms for the materials synthesized mechanochemically (Figure 1(b)), it can be seen that elongation of milling process causes drop of the specific surface area and the sorption volume of pores, but the position of the main peak observed in Figure 1(b) (right picture) for all samples is the same. OPEN IN VIEWER

From the analysis of the data presented in Table 1, it follows that the MChT treatment allows to prepare samples with higher values of specific surface area (S_BET) considering this parameter for the hydrothermally synthesized samples in the microwave reactor. As for the materials prepared by MChT, it can be seen that the highest value of S_BET (10.23 m²g⁻¹) is displayed for the sample modified for 30 min with the addition of water (MChT_850−30liq). Prolongation of time to 60 min (MChT_850−60liq) causes lowering of S_BET to 8.64 m²g⁻¹. Running the process without the addition of water gave a sample of correspondingly lower surface area (3.74 m²g⁻¹, MChT_850−60s). The samples obtained in the microwaves, both at saturated water vapour (MChT_500−30sMWT_vap) or under the layer of water (MChT_500−30sMWT_liq) are characterized by the S_BET values being approximately 2 m²g⁻¹. For comparison, the values of specific surface area obtained for the tested here materials are at a similar level as in the case of the microparticles of CaCO₃ described in work (Sukhorukov et al., 2004).

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Table 1.

Parameters of porous structure of studied samples.

Sample	SBET (m2g−1)	Vp (cm3g−1)	VΣ (cm3g−1)	Vmacro (cm3g−1)	Dp (nm)
MChT500-30s	3.94	0.0194	0.4708	0.4506	19.7
MChT500-30sMWTvap	1.93	0.0074	0.6180	0.6026	15.3
MChT500-30sMWTliq	2.03	0.0071	0.6933	0.6829	13.9
MChT850-30liq	10.23	0.0463	0.5179	0.4436	18.1
MChT850-60liq	8.64	0.0358	0.5767	0.5441	16.6
MChT850-60s	3.74	0.0149	0.5122	0.4951	15.9

Comparing the values of the sorption pore volumes (V_p, Table 1), it can be seen that the MChT procedure allows to synthesize samples with higher V_p in comparison to MWT. For the samples prepared by MChT in the liquid phase (MChT_850−30liq; MChT_850−60liq), these values are about 5 times higher compared to the hydrothermally treated ones (MChT_500−30sMWT_vap, MChT_500−30sMWT_liq). In turn, the values of the total pore volumes V_Σ and consequently the volumes of macropores V_macro are higher for the samples treated hydrothermally. Based on this, it can be stated that in the process of hydrothermal treatment, a macroporous structure of the prepared materials is created.

On the basis of the XRD analysis, it was found that all materials contained the desired product (CaCO₃) in the form of calcite (JCPDS card No. 01-078-3262) and the concentration of this phase was 100%. The patterns at 2θ = 22.95°; 29.3°; 39.3°; 43.06°; 48.4°; 57.3° from (012), (104), (113), (202), (116), (122) planes characterized for calcite are observed for all samples. Figure 2 shows the diffractogram of the sample MChT_850−60liq as an example because the XRD patterns for the other samples were the same, they differed only in the intensity of peaks (Table 2). From the comparison of the data in Table 2, it can be observed that the highest peak intensities (37,909 and 36,424 counts) are found for the samples prepared under hydrothermal conditions both in the saturated water vapour (MChT_500−30sMWT_vap) or under the layer of liquid water (MChT_500−30sMWT_liq). These samples are also characterized by the largest crystallite sizes, i.e. 2844 and 3420 Å, respectively. In Table 2 these values were marked in bold. OPEN IN VIEWER

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Table 2.

Parameters of crystallographic structure.

Sample	D(104) (Å)	d (Å)	Imax (104) (counts)
MChT500−30s	2084	3.0461	31,451
MChT500−30sMWTvap	2844	3.0359	37,909
MChT500−30sMWTliq	3420	3.0416	36,427
MChT850−30liq	917	3.0418	20,629
MChT850−60liq	1127	3.0441	25,059
MChT850−60s	2500	3.0456	33,809

D: average crystallite size in the direction of (104) crystallographic plane; d: distance between the planes; I_max (104): intensity of highest peak at 2Θ = 29.3.

The analysis of the data for the preparations synthesized mechanochemically (Table 2) shows that the samples obtained in the aqueous suspension (MChT_850−30liq, MChT_850−60liq) are characterized by smaller crystallite sizes (917 and 1127 Å) as compared to those obtained without the addition of water, i.e. 2084 Å (MChT_500−30s), and 2500 Å (MChT_850−60s). Analyzing the values of d spacings for all materials, it can be generally stated that these values do not differ substantially from each other.

The dependence of crystallite sizes on the specific surface area for all samples is presented in Figure 3. It is interesting that for most of them, with the exception for MChT_500−30s and MChT_500−30sMWT_liq, this relationship is rectilinear. It means that with the increasing specific surface area of the sample, the size of the crystallites decreases. OPEN IN VIEWER

Figure 4 presents the exemplary SEM images. Here it can be observed that MChT processes performed in the solid state (samples MChT_500−30s and MChT_850−60s, Figure 4(a) and (b)) gave materials with crystallites of higher sizes than the samples prepared in the water suspension (MChT_850−30liq and MChT_850−60liq, Figure 4(c) and (d)). This is consistent with the data in Table 2. Meanwhile treatment under the hydrothermal conditions (MChT_500−30sMWT_vap, MChT_500−30sMWT_liq, Figure 4(e) and (f)) makes that the edges of the calcite particles become smooth. Probably, it is a result of dissolving processes in steam under high pressure (MWT). It can also be observed that the particles of studied materials tend to agglomerate (Figure 4). This trend can be also seen in other publications (Pesenti et al., 2008; Qi and Zhu, 2006; Sukhorukov et al., 2004). OPEN IN VIEWER

Conclusions

MChT and microwave-assisted procedures allow to prepare calcium carbonate in the form of calcite. The samples prepared in the aqueous suspension are characterized by smaller crystallite sizes as compared to those obtained without the addition of water. Higher values of the specific surface area are displayed by the samples prepared by means of energetical milling in relation to this parameter for those synthesized hydrothermally. In the process of hydrothermal treatment, a macroporous structure of materials is created.