Investigation of calcium carbonate synthesized by steamed ammonia liquid waste without use of additives

Abstract

The aim of this work is to study the effect of reaction conditions using steamed ammonia liquid waste without the use of additives on the crystallization of calcium carbonate. CaCO₃ was prepared by steamed ammonia liquid waste (CaCl₂) and (NH₄)₂CO₃ solution. The produced crystals were characterized by scanning electron microscopy (SEM), Fourier transform infrared spectrometry (FTIR) and X-ray diffraction (XRD). We have investigated the effect of the concentration of reactants, stirring speed, Ca²⁺ : CO₃²⁻ ratio, aging time and adding mode on the particle size and size distribution, final morphology and polymorph of calcium carbonate crystals during precipitation. The influence of concentration of reactants, stirring speed, Ca²⁺ : CO₃²⁻ ratio, aging time and adding mode on the morphology, size and polymorph of CaCO₃ particles and possible formation mechanism were discussed. The exploration provides the possibility for large-scale synthesis of CaCO₃ materials with controllable morphology and crystallographic structure by steamed ammonia liquid waste without use of additives at room temperature.

---

1. Introduction

Western Mining Group Co., Ltd. uses salt lake bischofite and NH₃ as raw materials to prepare magnesium hydroxide. The formation process of magnesium hydroxide uses the following three steps, which are based on the reaction below: (1–3).

At present, the annual production of magnesium hydroxide by Western Mining Group Co., Ltd. has reached 100 000 tons, and the annual output of steamed ammonia waste liquid is nearly 200 000 tons. The large amount of discharged steamed ammonia waste liquid causes serious resource waste and environmental pollution, which restricts the further development of the enterprise. It is of great value to realize the recovery and utilization of steamed ammonia liquid waste. Steamed ammonia liquid waste reacts with ammonium carbonation products CaCO₃, and is a useful way of treating steamed ammonia liquid waste.

Calcium carbonate (CaCO₃) is one of the most studied inorganic chemical materials, which has the advantages of cheap availability, good heat resistance, nontoxicity and good biocompatibility with the human body, so it is widely used as a filling and reinforcing agent for coating, paper, printing ink, textile, rubber and plastics,^1–4 as well as nutritive agent for toothpaste, food, medicine and feed.⁵ The preparation of CaCO₃ from steamed ammonia liquid waste has great economic and environmental effects. However, the reaction of steamed ammonia liquid waste with (NH₄)₂CO₃ is complex, and the product CaCO₃ is quite sensitive to process parameters. At the same time application field of CaCO₃ particles with different properties and functions are different, the morphology of CaCO₃, dispersion and particle size on its physical and chemical properties have large influence. CaCO₃ properties largely depends on its morphology characteristics, particle size and dispersion, such as the preparation of a special morphology, crystal structure, controllable size and good dispersion. CaCO₃ material is an important index in the inorganic chemical materials field. To control the crystallization process of CaCO₃, reduce the production cost, improve the product performance and broaden the application range of the product is the focus of the field of CaCO₃ preparation.

CaCO₃ has calcite (rhombohedral), aragonite (orthorhombic) and vaterite (hexagonal) three anhydrous polymorphs.⁶ It has been well established that the different polymorphs of CaCO₃ synthesized in an aqueous solution possess different morphologies: rhombohedral phased calcite is usually found as cubic particles, orthorhombic aragonite usually found as needle-like particles, while vaterite has a hexagonal structure that normally leads to spherical particles,⁷ the CaCO₃ morphology, size, crystal structure, density, color, brightness and other physicochemical properties of a material are strongly dependent on its preparation technology. Several techniques have been used to manufacture special morphologies, different polymorphs, controllable size, well dispersed CaCO₃, such as carbonation,^8–12 solvo or hydrothermal synthesis^12,13, microwave-assisted synthesis,^14–17 sonochemical synthesis,¹⁸ double water-in-oil-in-water emulsion,¹⁹ wet precipitation^20,21 and other special synthesis processes and methods.²²

Now, precipitation method is the most important method to prepare CaCO₃, the relationship between precipitation conditions and morphology of CaCO₃ is the object of many experimental studies but it still is disputed,²³ it has been reported that synthetic factors including concentration of reactants, stirring speed, Ca²⁺ : CO₃²⁻ ratio, aging time and adding mode may significantly affect the formation of the polymorphs CaCO₃. By adjusting the preparation conditions of CaCO₃, the crystal phase, morphology and particle size of CaCO₃ can be controlled. For example, Yongsheng Wang²⁴ et al. compared the influence CaCO₃ in the poly(sodium 4-styrene sulfonate) presence and absence, the investigated show that when the poly(sodium 4-styrene sulfonate) presence the crystal shape and particle size of prepared CaCO₃ can be changed by changing the concentration of Ca²⁺ and CO₃²⁻ ions in the initial solution. The Yong Sheng Han and coworkers²³ study result of showed that the initial CaCl₂ concentration it can affect the pH of the reaction system and thus ultimately determine the prepared calcium carbonate crystal type. Yohta Mori²⁵ research indicated that the stirring method can effected the particle size of CaCO₃ crystal. Santos Rafael M²⁶ and coworkers by reducing the CO₂ flow rate and forming a higher Ca²⁺ : CO₃²⁻ ratio finally found higher Ca²⁺ : CO₃²⁻ ratio is beneficial to the formation of aragonite CaCO₃. Hongxia Guo²⁷ et al. investigated the reaction concentration and reaction time both could effected the polymorphs of prepared CaCO₃. Ashvin T. Nagaraja²⁸ reported that CaCO₃ particle size, dispersion and surface charge can be controlled by controlling PVSA concentration, reaction temperature and reagent addition sequence, and finally CaCO₃ with an average particle size of 150–500 nm can be obtained under the best preparation conditions. The effects of reaction temperature (T = 30–90 °C) and stirring speed (200–600 rpm min⁻¹) on the crystal structure of CaCO₃ prepared without additives were studied by Radek Ševčík²⁹ et al. Çağatay M. Oral³⁰ synthesised different morphologies and polymorphs CaCO₃ by changing the ratio of Ca²⁺ and CO₃²⁻ of precursor solution under different pH conditions. Haichun Dang³¹ prepared hydrophobic spherical aragonite CaCO₃ by high shear stirring.

Although numerous previous studies have focused on the morphology, structure, particle size, specific surface area, polymorphs, chemical purity and so on of CaCO₃ with of preparation condition. However, the determination of the relationship between precipitation conditions and product morphology is still a major challenge for research scientists. Therefore, developing an effective strategy to fabricate a crystalline material with controlled size, morphology, and crystal structure is of significance in chemical engineering. In this study, influence of concentration of reactants, stirring speed, Ca²⁺ : CO₃²⁻ ratio, aging time and adding mode on the morphology, size and polymorph of CaCO₃ particles were investigated without referring to the use of any additives at initial pH and ambient temperature condition. In this paper explain the effect of preparation conditions on the physicochemical and structural properties such as the pore size distribution, specific surface area, brightness, adsorption capacity and chemical purity of CaCO₃ prepared from steamed ammonia liquid waste, elucidate their exact features and to provide fundamental knowledge and insight into how to control the physicochemical and structural properties of CaCO₃, and finally it provides theoretical support for the large-scale industrial production and application of to prepare CaCO₃ by steamed ammonia liquid waste.

2. Material and methods

2.1 Raw material

The raw materials steamed ammonia liquid waste were from Western Mining Group Co., Ltd. after three times of filtration to remove the solid impurities, the filtrated solution was used as the calcium source(CaCl₂ solution) and the initial pH 11.17. Ammonium carbonate ((NH₄)₂CO₃) was purchased from Tianjin Kemiou Chemical Reagent Company, China, and used without any further purification.

2.2 Synthesis of calcium carbonate

The CaCO₃ was synthesized via a precipitation. Steam ammonia waste liquid (CaCl₂) and ammonium carbonate ((NH₄)₂CO₃) was varying molar concentrations ranging from 0.15 to 0.60 mol L⁻¹ by mixing with magnetic stirring. The steam ammonia waste liquid (CaCl₂) and ammonium carbonate ((NH₄)₂CO₃) were keep initial pH and no effort was made to adjust the pH of the solution. We selected room temperature because it does not need any temperature control which is not only inconvenient but also required specialized devices and thus incur additional cost and not feasible for industrial applications. Take a certain quantity of steam ammonia waste liquid and the (NH₄)₂CO₃ solution was prepared by dissolving a certain amount of (NH₄)₂CO₃ in deionized water. At the beginning of the reaction, the prepared steam ammonia waste liquid and the (NH₄)₂CO₃ solution were immediately poured into a 500 mL conical bottle. After stirring a certain period of time, the product were collected by filtered, washed several times with deionized water and dried at 105 °C for 6 h.

2.3 Characterization

For characterization of the precipitated CaCO₃, a X-ray diffraction (XRD, DX-2700BH), a Fourier-transform infrared (FTIR, Cary 630) spectrometer and a scanning electron microscopy (SEM, phenom pro) was used. The resulting powders crystalline phases of was carried out using a Cu Kα source was used over a 2θ range of 20° to 60° and a step size of 0.02° with dwell time of 0.05 s was applied during the analyses. From intensity peaks of XRD patterns, the compositions of vaterite were estimated by eqn (4) as follows:³²

f_V = 7.691_I110V/(7.691_I110V+I104C) (4)

where I_C and I_V are the intensity of calcite and vaterite respectively, three suffixes are Miller indexes of each phase, and f_V is the content of vaterite in precipitates.

Fourier transmission infrared spectroscopy was performed on uniaxially pressed powder pellets mixed with KBr. The FTIR analyses were carried out in the 4000–400 cm⁻¹ range with a resolution of 4 cm⁻¹ and with 32 spectral scan repeats for each sample. The size and morphological structures of the precipitated CaCO₃ were examined by scanning electron microscopy. Powder samples for SEM were uncoated and observed at a working distance of 3.5 mm and an accelerating voltage of 0.7 kV.

3. Results and discussion

3.1 Effect of initial CaCl₂ and (NH₄)₂CO₃ concentration on particle polymorph and morphology

A typical X-ray diffraction pattern of the CaCO₃ particles prepared at various initial CaCl₂ and (NH₄)₂CO₃ concentration is shown in Fig. 1. The characteristic the strongest detected (hkl) peaks are at 2θ values of 23.1°, 29.4°, 36.0°, 39.4°, 43.2°, 47.5°, 48.5°correspond to the (012), (104), (110), (113), (202), (018), (116), (122), (214), (300) crystallographic planes of calcite, respectively. The X-ray diffraction pattern with peaks at 2θ values of values of 24.9°, 27.1°, 32.8°, 43.8°, 50.1°, 55.6°(Fig. 1b) correspond to the (100), (101), (102), (110), (104), (202) indicates that the composition of the CaCO₃ microspheres are phase of vaterite. As shown in Fig. 1a, c and d with only calcite phase, however the main crystalline phase is vaterite and contains weak calcite (104) peak in samples b. We know from eqn (4), when the initial CaCl₂ and (NH₄)₂CO₃ concentration is 0.3 mol L⁻¹ the content of the calcite and vaterite in the products are 7.51% and 92.49%, the content of the calcite and vaterite are 97.88%, 97.38%, 97.60% and 2.12%, 2.62%, 2.40% at 0.15 mol L⁻¹, 0.45 mol L⁻¹ and 0.60 mol L⁻¹ initial CaCl₂ and (NH₄)₂CO₃ concentration, respectively.

Fig. 1 XRD image of CaCO₃ crystal obtained with different initial CaCl₂ and (NH₄)₂CO₃ concentration. (Stirring speed = 400 rpm min⁻¹; aging time: 10 min; C: calcite, V: vaterite.)

FT-IR was used as a secondary characterization technique to identify the various initial CaCl₂ and (NH₄)₂CO₃ concentration in the crystals. The FTIR spectra of the CaCO₃ crystals obtained in the 0.15 mol L⁻¹, 0.45 mol L⁻¹ and 0.60 mol L⁻¹ initial CaCl₂ and (NH₄)₂CO₃ concentration are shown in Fig. 2a, c and d displays three characteristic peaks of calcite centered at 711 cm⁻¹, 871 cm⁻¹. As shown in Fig. 2a, c and d a single calcite phase can be confirmed by the appearance of characteristic υ₂ band at 871 cm⁻¹ and υ₄ band at 711 cm⁻¹. When the initial CaCl₂ and (NH₄)₂CO₃ concentration is 0.3 mol L⁻¹ results in the occurrence of a new absorption peak located at 745 cm⁻¹ and 1087 cm⁻¹ (Fig. 2b), which is the fingerprint υ₄ deformation band of CO₃²⁻ in vaterite form, indicating the sample is vaterite phase CaCO₃. According to the XRD and FTIR results, it can be concluded that the obtained product is calcite CaCO₃ at 0.15 mol L⁻¹, 0.45 mol L⁻¹ and 0.60 mol L⁻¹ initial CaCl₂ and (NH₄)₂CO₃ concentration, respectively, but the sample prepared at the condition of 0.30 mol L⁻¹ initial CaCl₂ and (NH₄)₂CO₃ concentration are mainly vaterite and a little calcite CaCO₃, the results are consistent with the XRD peak. When the initial CaCl₂ and (NH₄)₂CO₃ concentration is 0.15 mol L⁻¹, the relative content of CaCO₃ products obtained in the reaction system is relatively low, which is not conducive to the stability of vaterite so that with more stable thermodynamics calcite type CaCO₃ products are obtained. When the initial CaCl₂ and (NH₄)₂CO₃ concentration is 0.45 and 0.60 mol L⁻¹, since the initial solution concentration is too high so that the nucleation rate of CaCO₃ is fast and vaterite can be formed in a short time.³² Since the reaction time does not change, vaterite products formed are converted into calcite through the process of dissolution and recrystallization.³³ Therefore, When the initial CaCl₂ and (NH₄)₂CO₃ concentration is 0.30 mol L⁻¹, a mixture composed mainly of vaterite and a small amount of calcite CaCO₃ were obtained.

Fig. 2 FTIR spectra of CaCO₃ prepared with different initial CaCl₂ and (NH₄)₂CO₃ concentration. (Stirring speed = 400 rpm min⁻¹; aging time: 10 min.)

In order to understand the morphological and size changes at various initial CaCl₂ and (NH₄)₂CO₃ concentration of transformation, SEM images were taken for all the samples. It can be found in Fig. 3a, c and d that the CaCO₃ is well-defined hexahedral crystals prepared with a size ranging from 8 to 10 μm under 0.15 mol L⁻¹, 0.45 mol L⁻¹ and 0.60 mol L⁻¹ initial CaCl₂ and (NH₄)₂CO₃ concentration, respectively. Through Fig. 3b we can know that the sample prepared at the condition of 0.30 mol L⁻¹ initial CaCl₂ and (NH₄)₂CO₃ concentration is 1 to 2 μm size spherical CaCO₃, but the Fig. 3b indicated that the sample contains a certain amount of cubic calcite CaCO₃ the particle size about 8 μm (red circle), it is consistent with the Fig. 1b XRD and Fig. 2b FTIR pattern. From Fig. 3a, c and d it can be seen the particle size of calcite particles has a little increases with concentration of reactants.

Fig. 3 SEM image of CaCO₃ crystal obtained different initial CaCl₂ and (NH₄)₂CO₃ concentration. (a: 0.15 mol L⁻¹, b: 0.30 mol L⁻¹, c: 0.45 mol L⁻¹, d: 0.60 mol L⁻¹, stirring speed = 600 rpm min⁻¹; aging time: 10 min.)

3.2 Effect of stirring speed on particle polymorph and morphology

A typical X-ray diffraction pattern of the CaCO₃ particles prepared at different stirring speed is shown in Fig. 4. We through the analysis of Fig. 4a the XRD pattern confirmed that prepared CaCO₃ is pure calcite at stirring speed 200 rpm min⁻¹. However, as show that in Fig. 1b and 4b, the XRD pattern confirmed that prepared CaCO₃ is vaterite and contains the peak of calcite (104) at stirring speed 400 and 600 rpm min⁻¹. Calcite and vaterite contents of calcium carbonate products prepared at different stirring speeds were calculated using the method of eqn (4), the products prepared were mainly calcite with a content of 97.17% at 200 rpm min⁻¹ but the products prepared were mainly vaterite with a content of 96.31% at 600 rpm min⁻¹.

Fig. 4 XRD image of CaCO₃ crystal obtained with different stirring speed. (Ca²⁺: 0.3 mol L⁻¹, (NH₄)₂CO₃: 0.3 mol L⁻¹; C: calcite, V: vaterite).

FT-IR was used as a secondary characterization technique to identify the various stirring speed experimental conditions in the crystals. The FT-IR spectra of the CaCO₃ crystals obtained in the various stirring speed experimental conditions are shown in Fig. 5 and Fig. 2b. Fig. 5a displays two characteristic peaks of calcite centered at 711 cm⁻¹, 871 cm⁻¹. Fig. 2b and 5b displays five characteristic peaks of centered at 711 cm⁻¹, 745 cm⁻¹, 871 cm⁻¹, 1087 cm⁻¹, the FTIR spectra (Fig. 2b and 5b) clearly showed typical bands at 745 cm⁻¹, 871 cm⁻¹, 1087 cm⁻¹ which are attributed to the υ₄, υ₂, υ₁, and υ₃ modes of the crystalline vaterite phase, respectively. According to the XRD and FTIR results, it can be concluded that the obtained product is calcite CaCO₃ at the 200 rpm min⁻¹ experimental conditions, but the sample prepared at the condition of stirring speed 400 and 600 rpm min⁻¹ are mainly vaterite and a little calcite CaCO₃.

Fig. 5 FTIR spectra of CaCO₃ prepared with different stirring speed. (Ca²⁺: 0.3 mol L⁻¹, (NH₄)₂CO₃: 0.3 mol L⁻¹; aging time: 10 min).

The SEM shows a visible change in the morphology and polymorphism of the calcium carbonate crystals with the increase stirring speed. The morphology of the calcium carbonate particles obtained in the 200 and 600 rpm min⁻¹ are presented in Fig. 6. We can concluded that the different stirring speed in the system (Fig. 6a–d) was an effective method to control the morphology of the CaCO₃. Fig. 6a and c shows the typical rhombohedral calcite crystals with a size ranging from 5 to 8 μm that are formed under the 200 rpm min⁻¹ experimental conditions. It can be observed in Fig. 3b and 6b, d that the CaCO₃ crystals obtained at the 400 and 600 rpm min⁻¹ were a spherical form. The Fig. 3b SEM showed that the precipitate consisted of a mixture of typical rhombohedral particles and spherical vaterite particles crystals at the 400 rpm min⁻¹ experimental conditions. At the stirring speed of 600 rpm min⁻¹, only the spherical vaterite crystal CaCO₃ was obtained it can be seen in Fig. 6b and d. Combine the Fig. 4 XRD pattern and Fig. 5 FTIR spectrum show that with the increase of stirring speed, the prepared CaCO₃ from calcite crystal convert to vaterite crystal. At the higher stirring speed of preparation condition, it is beneficial to the stability of vaterite polymorph and can also prevent vaterite conversion to calcite.

Fig. 6 SEM image of CaCO₃ crystal obtained different stirring speed. (a and c: 200 rpm min⁻¹; b and d: 600 rpm min⁻¹; Ca²⁺: 0.3 mol L⁻¹, (NH₄)₂CO 3: 0.3 mol L⁻¹; aging time: 10 min).

Compared with calcite, vaterite has a higher specific surface energy, with relatively high surface energies were easily eliminated in the final morphology, so vaterite can quickly become more stable calcite and aragonite in an aqueous solution.³⁴ At lower stirring speeds (200 rpm min⁻¹), the reaction condition of CaCl₂ and (NH₄)₂CO₃ approached the equilibrium state, hence forming rhombohedral or pseudo-cubic calcite. With increasing of the stirring speeds, the probability of collision among the nano-particles in the immediate CaCO₃ precipitates rose, such that the time is insufficient to make a choice for faces with specific surface energies in the formation of calcite phase via compact stacking, to say nothing of hexahedral crystals subsequently. As a result, the present case only crystallized to the metastable vaterite phase.³⁵

3.3 Effect of Ca²⁺ : CO₃²⁻ ratio on particle polymorph and morphology

The XRD phase and FTIR spectra compositions of as-synthesized CaCO₃ with different Ca²⁺ : CO₃²⁻ molar ratio is displayed in Fig. 7 and 8. The Fig. 7 XRD result show that the strongest detected (hkl) peaks are at 2θ values of 24.90°, 27.04°, 32.82° and 29.40° corresponding to the following (hkl) indices: (110), (112), (114) and (104) represent the main peaks of (110), (112), and (114) planes of vaterite (PDF #33-0268) respectively, the (104) is among the main peaks of calcite(PDF #05-0586) polymorph of CaCO₃. By the eqn (4) indicated the 52.99% calcite and 47.01% vaterite CaCO₃ was obtained with Ca²⁺ : CO₃²⁻ ratio is 3 : 1, however, the 81.19% and 91.58% vaterite CaCO₃ was obtained at Ca²⁺ : CO₃²⁻ ratio are 1 : 3 and 1 : 5, respectively. Fig. 8 displays four characteristic peaks of at 711 cm⁻¹, 745 cm⁻¹, 871 cm⁻¹, 1087 cm⁻¹, υ₄ at 711 cm⁻¹ is the peak of calcite, the peak of υ₄ at 745 cm⁻¹ and υ₁ at 1087 cm⁻¹ is vaterite, the peak of υ₂ at 871 cm⁻¹ is vaterite and calcite common peak. According to the Fig. 7 XRD pattern, CaCO₃ obtained with different Ca²⁺ : CO₃²⁻ molar ratio was monoclinic crystal with coexistence of calcite phase and vaterite phase were formed which was further confirmed by Fig. 8 FTIR spectrum. However, when the Ca²⁺ : CO₃²⁻ molar ratio are 3 : 1 the sample obtained main are calcite crystal CaCO₃, when the Ca²⁺ : CO₃²⁻ molar ratio are 1 : 3 and 1 : 5 the sample obtained main are vaterite crystal CaCO₃ by Fig. 7 XRD pattern. It can be known that excess Ca²⁺ is beneficial to obtain calcite crystal CaCO₃, on the contrary excess CO₃²⁻ ion is beneficial to obtain vaterite crystal CaCO₃.

Fig. 7 XRD image of CaCO₃ crystal obtained with different Ca²⁺ : CO₃²⁻ ratios. (Stirring speed = 400 rpm min⁻¹; aging time: 10 min; C: calcite, V: vaterite.)

Fig. 8 FTIR spectra of CaCO₃ prepared at different Ca²⁺ : CO₃²⁻ ratio. (Stirring speed = 400 rpm min⁻¹; aging time: 10 min.)

In order to understand the morphological and size changes at various Ca²⁺ : CO₃²⁻ ratio, SEM images were taken for all the samples. We obtained spherical vaterite and the cubic calcite are clustered together can be found in Fig. 9a–c. The Fig. 9a–c showed that the precipitate consisted of a mixture of main typical spherical vaterite particles and a small amount cubic shaped calcite crystals at the Ca²⁺ : CO₃²⁻ratio 3 : 1. By comparing the experimental results in Fig. 9d–f we can know that the prepared product is formed by the aggregation of products with multiple small particle sizes, after maturation, a number of spherical particles composed the aggregate vaterite and polyhedral diamond calcite were obtained, respectively. Fig. 9a–c illustrated that calcite were surrounded by the planar arrays of vaterite and the Fig. 9d–f show that the a part of the is CaCO₃ polyhedral. According to Ostwald's law, it is believed that the metastable vaterite it is easy to transformation to calcite in the precipitation reaction.³⁶ At different Ca²⁺ : CO₃²⁻ ratio, the initial reaction was all vaterite CaCO₃ prepared, as the reaction time goes on, some of the most unstable vaterite was carried out via Ostwald ripening converted into calcite as a whole. Because of the short reaction time, only part of the vaterite is converted into calcite, thus forming the calcium carbonate product mixed with calcite and vaterite as shown in Fig. 9. This result is consistent with the XRD pattern and FTIR spectra. Combined with Fig. 7 XRD pattern, Fig. 8 FTIR spectra and Fig. 9 SEM image, it can be concluded that when Ca²⁺ ions and CO₃²⁻ ions are not in equal proportion, vaterite and calcite mixed phase CaCO₃ can be formed.

Fig. 9 SEM image of CaCO₃ crystal obtained at different Ca²⁺ : CO₃²⁻ ratio. (a–c: Ca²⁺ : CO₃²⁻ = 3 : 1; d–f,: Ca²⁺ : CO₃²⁻ = 1 : 5; stirring speed = 400 rpm min⁻¹; aging time: 10 min).

3.4 Effect of aging time on particle polymorph and morphology

The experiments aiming at revealing the effect of aging time on the CaCO₃ particle polymorph and morphology, the time-dependent shape evolution and phase transformation of intermediates were monitored by using XRD and SEM analysis. According to eqn (4), the content of the calcite and vaterite in the products are 24.99%, 9.07%, 4.56%, 15.62%, 12.07%, 22.23%, 18.17% and 75.01%, 90.93%, 95.44%, 84.38%, 87.93%, 77.77%, 81.83% at 5 s, 1 min, 5 min, 20 min, 30 min, 60 min and 120 min, respectively. The XRD patterns of the samples prepared showed in Fig. 10 that the intensity of the calcite (104) decreased with increasing aging time when the aging time is less than 20 min, but when the aging time is more than 20 min the intensity of the calcite (104) increased with increasing aging time. When the reaction time is less than 5 min in this short reaction time only the nucleating part of the crystal is completed at the reaction system so that prepared amorphous CaCO₃ (through experiments it was verified that the product prepared when the reaction time was less than 5 min could penetrate the filter paper and it was speculated the product prepared was amorphous calcium carbonate). Thermodynamically the most unstable amorphous CaCO₃ experienced the filtration and drying process is randomly converted into calcite and vaterite. Because calcite is more stable than vaterite, so that we can observed in Fig. 10a and b a stronger peak calcite (104) is formed. When the reaction time are 5–20 min that due to the reaction time is shorter and the obtained part of vaterite product is not completely stable, therefore in the process of filtering and drying part of vaterite conversion to calcite. With the extension of reaction time, the vaterite crystal has been completed so that the calcite (104) peak strength decreased. When the reaction time exceeds 20 min, we know vaterite with poor thermodynamic stability so that in the reaction mother liquor environment is can slowly converted into stable and finally we found the strength of calcite (104) peak increases with the increase of reaction time.

Fig. 10 XRD image of CaCO₃ crystal obtained at different aging time. (Stirring speed = 400 rpm min⁻¹; Ca²⁺: 0.3 mol L⁻¹, (NH₄)₂CO₃: 0.3 mol L⁻¹; C: calcite, V: vaterite).

In order to investigate the formation mechanism of CaCO₃, we changed the reaction time without changing other experimental conditions. Fig. 11 shows a representative SEM image of CaCO₃ particles were obtained by different aging time. It can be seen in Fig. 11a and e existed is the multilayer hexahedral crystals of CaCO₃ and is the spherical crystals of CaCO₃ in large quantity (Fig. 11a). Further increase in the aging time to 1 min this can be ascribed to the existence of a mount of spherical crystals of CaCO₃ and small amounts of polyhedron hexahedral crystal in the precipitates (Fig. 11b and f). A aging time 10 and 20 min mainly afforded microspheres in the precipitates of size in the range of 1–2 μm (Fig. 1b and 11c, g). As the aging time went up to 60 min, a large quantity spherical crystals of CaCO₃ aggregate and a small number of the precipitates had transformed to pseudo-cubic structures or randomly aggregated rhomboherdal as shown in Fig. 11d and h.

Fig. 11 SEM image of CaCO₃ crystal obtained at different aging time. (a and e: 10 s; b and f: 1 min; c and g: 20 min; d and h: 60 min; stirring speed = 400 rpm min⁻¹; Ca²⁺: 0.3 mol L⁻¹, (NH₄)₂CO₃: 0.3 mol L⁻¹).

Through the analysis of Fig. 10 XRD pattern and Fig. 11 SEM image, it is obvious that sample are mainly composed of calcite particles; meanwhile samples main contain vaterite and small amount of a mixture of calcite particles prepared from 5 min to 10 min aging time. With the increase of aging time from 10 s to 20 min, the reflection of calcite decreases gradually, and vaterite progressively dominates. However, with the extension of aging time to 60 min, the content of vaterite decreased.

3.5 Effect of adding mode on particle polymorph and morphology

X-ray diffraction pattern of the CaCO₃ particles prepared at different adding mode is shown in Fig. 12. The XRD result in Fig. 12a indicated that the peak are at 2θ values of 23.1°, 29.4°, 36.0°, 39.4°, 43.2°, 47.5°, 48.5°correspond to the (012), (104), (110), (113), (202), (018), (116), (122), (214), (300) are pure crystallographic planes of calcite (PDF #05-0586) without impurities. As can be seen in the sample Fig. 12b the strongest detected (hkl) peaks are at 2θ values of 24.9°, 27.1°, 32.8°, 38.6°, 44.0°, 49.1°, 50.2°and 55.9° indicates that the composition of the CaCO₃ are pure phase of vaterite (PDF #33-0268) without impurities. Different of added mode lead to excessive Ca²⁺ and CO₃²⁻ in the reaction system so that two different crystalline calcium carbonate products are obtained.³⁷

Fig. 12 XRD image of CaCO₃ crystal obtained at different adding mode. (Stirring speed = 400 rpm min⁻¹; adding mode: dropwise; adding time: 5 min; aging time: 5 min; Ca²⁺: 0.3 mol L⁻¹, (NH₄)₂CO₃: 0.3 mol L⁻¹; C: calcite, V: vaterite).

By the eqn (4) computed value of calcite approximately (100%) and vaterite approximately (100%) revealed that the products were pure phase of calcite and vaterite was consistent with that of XRD in Fig. 12a and b, respectively. As shown from Fig. 13a, the calcite characteristic bands of υ₄ at 711 cm⁻¹ and υ₂ at 871 cm⁻¹ appeared in the above samples. Also, vaterite characteristic bands of υ₄ at 745 cm⁻¹ and υ₁ at 1090 cm⁻¹ can be observed in Fig. 13b. The Fig. 13 FTIR spectra results are consistent with those of Fig. 12 XRD pattern.

Fig. 13 FTIR spectra of CaCO₃ prepared at different adding mode. (Stirring speed = 400 rpm min⁻¹; adding mode: dropwise; adding time: 5 min; aging time: 5 min; Ca²⁺: 0.3 mol L⁻¹, (NH₄)₂CO₃: 0.3 mol L⁻¹).

The SEM results show that CaCl₂ and (NH₄)₂CO₃ adding mode influence the CaCO₃ morphologies more obviously when other experimental conditions are the same in Fig. 14. In Fig. 14a and c shows some rhombohedral particle with a size ranging from 1 to 3 μm were produced when was CaCl₂ dropwise added into (NH₄)₂CO₃. However, as shown in Fig. 14b and d, multiple micro/nanosized spherical particles by Ostwald ripening tend to grow larger and become nonuniform 5 to 8 μm aggregates of spherical particles when solution (NH₄)₂CO₃ was dropwise added into solution CaCl₂. Combined with Fig. 12 XRD pattern, Fig. 13 FTIR spectra and Fig. 14 SEM image, it can be concluded that when CaCl₂ dropwise added into (NH₄)₂CO₃, only calcite phase CaCO₃ can be formed, when (NH₄)₂CO₃ add into CaCl₂, only vaterite phase CaCO₃ can be formed.

Fig. 14 SEM image of CaCO₃ crystal obtained at different adding mode. (Stirring speed = 400 rpm min⁻¹; adding mode: dropwise; adding time: 5 min; aging time: 5 min; Ca²⁺: 0.3 mol L⁻¹, (NH₄)₂CO₃: 0.3 mol L⁻¹. (a) and (c) CaCl₂ dropwise added into (NH₄)₂CO₃, (b) and (d) (NH₄)₂CO₃ dropwise added into CaCl₂.)

4. Conclusions

Hexahedral calcite crystals CaCO₃ prepared with a size ranging from 8 to 10 μm have been obtained in 0.15 mol L⁻¹, 0.45 mol L⁻¹ and 0.60 mol L⁻¹ initial CaCl₂ and (NH₄)₂CO₃ concentration and 1 to 2 μm size spherical vaterite crystals CaCO₃ the sample prepared at the condition of 0.30 mol L⁻¹ initial CaCl₂ and (NH₄)₂CO₃ concentration. The obtained product is calcite CaCO₃ at the 200 rpm min⁻¹ experimental conditions, but the sample prepared at the condition of stirring speed 400 and 600 rpm min⁻¹ is micro/nano level vaterite CaCO₃. When Ca²⁺ ions and CO₃²⁻ ions are not in equal proportion, vaterite and calcite mixed phase CaCO₃ can be formed. With the extension of aging time, the prepared products first from calcite crystal convert to vaterite crystal CaCO₃, and with the further extension of aging time, the content of CaCO₃ vaterite decreased. When CaCl₂ add into (NH₄)₂CO₃, only calcite phase CaCO₃ can be formed, when (NH₄)₂CO₃ add into CaCl₂, only vaterite phase CaCO₃ can be formed. These results could offer fundamental insight into how to control the size and polymorphism of CaCO₃ prepared by steamed ammonia liquid waste, eventually allowing large-scale industrial preparation the creation of value-added CaCO₃ products and part of the experimental results will be applied to the recovery and utilization of steam ammonia waste liquid in Western Mining Group Co., Ltd.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was partially supported by the National Key Research and Development Program of China (2018YFC1903805) and Major Science and Technology Projects of Qinghai Province (2020-GX-A1).

References

1. T. D. Lam, T. V. Hoang, D. T. Quang and J. S. Kim, Mater. Sci. Eng., A, 2009, 501, 87–93.
2. F. Karakaş, B. V. Hassas and M. S. Çelik, Prog. Org. Coat., 2015, 83, 64–70.
3. J. Shen, Z.-Q. Song, X.-R. Qian and F. Yang, Carbohydr. Polym., 2010, 81, 545–553.
4. A. Said, H.-P. Mattila, M. Järvinen and R. Zevenhoven, Appl. Energy, 2013, 112, 765–771.
5. D. B. Trushina, T. V. Bukreeva, M. V. Kovalchuk and M. N. Antipina, Mater. Sci. Eng., C, 2014, 45, 644–658.
6. A. Sarkar and S. Mahapatra, Cryst. Growth Des., 2010, 10, 2129–2135.
7. M. Ni and B. D. Ratner, Surf. Interface Anal., 2008, 40, 1356–1361.
8. T. Beuvier, B. Calvignac, G. J.-R. Delcroix, M. K. Tran, S. Kodjikian, N. Delorme, J.-F. Bardeau, A. Gibauda and F. Boury, J. Mater. Chem., 2011, 21, 9757–9761.
9. I. Udrea, C. Capat, E. A. Olaru, R. Isopescu, M. Mihai, C. D. Mateescu and C. Bradu, Ind. Eng. Chem. Res., 2012, 51, 8185–8193.
10. Y.-H. Lai, L.-S. Chen, W.-C. Bao, Y.-H. Ren, Y.-X. Gao, Y.-W. Yin and Y.-F. Zhao, Cryst. Growth Des., 2015, 15, 1194–1200.
11. G. Yuan, X.-F. Chen, X. Li, Q.-M. Liang, G.-H. Miao and B. Yuan, Powder Technol., 2015, 284, 253–256.
12. P.-Y. Chen, H.-H. Ma, Y. Xu and Z.-W. Shen, Int. J. Mater. Res., 2017, 108, 600–606.
13. Z.-X. Zhao, L. Zhang, H.-X. Dai, Y.-C. Du, X. Meng, R.-Z. Zhang, Y.-X. Liu and J.-G. Deng, Microporous Mesoporous Mater., 2011, 138, 191–199.
14. C. Qi, Y.-J. Zhu and F. Chen, ACS Appl. Mater. Interfaces, 2014, 6, 4310–4320.
15. R.-J. Qi and Y.-J. Zhu, J. Phys. Chem. B, 2006, 110, 8302–8306.
16. Y.-X. Chen, X.-B. Ji and X. B. Wang, J. Cryst. Growth, 2010, 312, 3191–3197.
17. J. A. Juhasz-Bortuzzo, B. Myszka, R. Silva and A. R. Boccaccini, Cryst. Growth Des., 2017, 5, 2351–2356.
18. L.-P. Liu, D.-W. Fan, H.-Z. Mao, X. Fang and J.-C. Hao, J. Colloid Interface Sci., 2007, 306, 154–160.
19. R. K. Pai and S. Pillai, CrystEngComm, 2008, 10, 865–872.
20. Z.-G. Wu, Y. Guo, J. Wang and Y.-R. Jia, Int. J. Mater. Res., 2017, 108, 245–248.
21. L. Amer, S. Ouheniaa, I. Belabbas and D. Chateigner, J. Cryst. Growth, 2018, 501, 49–59.
22. H. Casanova and L. P. Higuita, Chem. Eng. J., 2011, 175, 569–578.
23. Y.-S. Han, G.-W. Hadiko, M. Fuji and M. Takahashi, J. Eur. Ceram. Soc., 2006, 26, 843–847.
24. Y.-S. Wang, Y.-X. Moo, Ch-P. Chen, P. Gunawan and R. Xu, J. Colloid Interface Sci., 2010, 352, 393–400.
25. Y. Mori, T. Enomae and A. Isogai, Mater. Sci. Eng., C, 2009, 29, 1409–1414.
26. M. Santos Rafael, P. Ceulemans and T. V. Gerven, Chem. Eng. Res. Des., 2012, 90, 715–725.
27. H.-X. Guo, P.-Z. Sun, Z.-P. Qin, L.-L. Shan, G.-J. Zhang, S.-P. Cui and Y.-C. Liang, Eur. J. Inorg. Chem., 2014, 1001–1009.
28. A. T. Nagaraja, S. Pradhan and M. J. McShane, J. Colloid Interface Sci., 2014, 418, 366–372.
29. R. Ševčík, M. Pérez-Estébanez, A. Viani, P. Šašek and P. Mácová, Powder Technol., 2015, 284, 265–271.
30. C. M. Oral and B. Ercan, Powder Technol., 2018, 339, 781–788.
31. H.-C. Dang, Z.-Z. Xu, Z.-S. Chen, W.-L. Wu, J. Feng, Y.-Y. Sun, F.-C. Jin, J. Li and F. Ge, Cryst. Res. Technol., 2019, 243, 1–7.
32. C. G. Kontoyannis and N. V. Vagenas, Analyst, 2000, 125, 251–255.
33. S. Karthika, T. K. Radhakrishnan and P. Kalaichelvi, Cryst. Growth Des., 2016, 16, 6663–6681.
34. L. A. Estroff and D. R. Hamilton, Chem. Mater., 2001, 13, 3227–3235.
35. F.-W. Yan, S.-F. Zhang, C.-Y. Guo, X.-H. Zhang, G.-C. Chen, F. Yan and G.-Q. Yuan, Cryst. Res. Technol., 2009, 44, 725–728.
36. X.-d. Yang, G.-y. Xu, Y.-j. Chen, F. Wang, H.-z. Mao, W.-P. Sui, Y. Bai and H.-j. Gong, J. Cryst. Growth, 2009, 311, 4558–4569.
37. S. Kirboga and O. Mualla, Powder Technol., 2013, 24, 95–104.

---