Xianping Luo*abcd,
Xuewen Song*a,
Yuwei Caoc,
Lei Songc and
Xianzhong Bub
aCollege of Material Science and Engineering, Xian University of Architecture and Technology, Xian, 710055, China. E-mail: luoxianping9491@163.com; Fax: +86 0797 8312227; Tel: +86 0797 8312553
bSchool of Resources Engineering, Xian University of Architecture and Technology, Xian, 710055, China
cWestern Mining Group Co., Ltd., Xining, 810001, China
dSchool of Resources and Environmental Engineering, Jiangxi University of Science and Technology, Ganzhou, 341000, China
First published on 2nd March 2020
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.
At present, the annual production of magnesium hydroxide by Western Mining Group Co., Ltd. has reached 100000 tons, and the annual output of steamed ammonia waste liquid is nearly 200000 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 synthesis12,13, microwave-assisted synthesis,14–17 sonochemical synthesis,18 double water-in-oil-in-water emulsion,19 wet precipitation20,21 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.
fV = 7.691I110V/(7.691I110V+I104C) | (4) |
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.
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. 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
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). |
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.
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.
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.
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.
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