Synthesis of high purity calcium carbonate micro‐ and nano‐structures on polyethylene glycol templates using dolomite

This research paper describes the synthesis of nano‐ and micro‐structures of high purity precipitated calcium carbonate (PCC) on poly(ethylene glycol)(PEG) templates for broad‐range industrial applications, using readily available and cheap impure dolomitic marbles. In the method, calcium components of impure dolomitic marbles are extracted as calcium sucrate which is then bubbled with carbon dioxide gas using a carbonation column in the presence of PEG. The effects of concentration of PEG, pH of calcium sucrate solution and temperature on the final yield, morphology and polymorphism of PCC have been studied. Vaterite and calcite are the crystalline forms of calcium carbonate found in final PCC products. The vaterite is observed as hollow spheres with particle diameter of 1.5‐2 μm which is formed by aggregation of vaterite nanoparticles with particle size of 20 nm on PEG templates. Optimum conditions for the highest PCC yield of 79.94% are 0.4 mol dm⁻³ of PEG, pH of 6.5 and temperature of 80 °C. The purity of PCC products is about 99%. Therefore, the synthesized PCC products are of required purity and quality for industrial applications.     

Controlled deposition and transformation of amorphous calcium carbonate thin films

Controlled synthesis of amorphous calcium carbonate (ACC) films was realized by using the multiple templates, which are composed of a self‐assembled film (SAF, insoluble Poly (ε‐caprolactone) film) and a soluble modifier (poly allylamine), as modifiers. The formation of self‐assembled film was analyzed by monitoring the morphologies using atomic force microscopy. Even more noteworthy, using anhydrous ethanol as media, the ACC‐to‐vaterite‐to‐calcite transformation was also investigated, and the obtained products were characterized by scanning electron microscopy, Fourier transform infrared spectroscopy, and X‐ray diffraction. The results demonstrated that organic solvent has profound influence on transformation of amorphous calcium carbonate thin films. In the air of anhydrous ethanol, the controlled synthesis of calcium carbonate films with different morphologies, such as planar films with a few sporadic particles, symmetric rhombohedral crystals, novel crystals with symmetrical terraced convexity formation of calcite, was obtained by the fine‐tuning of induction time. It provides a new and simple method to prepare polymorphic CaCO₃ crystal films from the ACC films by controlling the crystallization process (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Fine-grained amorphous calcium silicate CaSiO3 from vacuum dried sol–gel – Production,characterisation and thermal behaviour

The production of amorphous calcium silicate, CaSiO₃, by vacuum drying a sol–gel is described. The material exhibits a highly porous, irregular extended aggregate morphology. Its amorphous nature is confirmed by X-ray powder diffractometry and its silicate nature by vibrational spectroscopy. The latter is used to characterise the bridging/non-bridging oxygen distribution and hydration state, which comprises SiO₃ bonded hydroxyl and interstitial water. No indicators of silicon hydride defects were observed. From Raman/FTIR spectroscopy, the Si-O tetrahedral species are predominantly peaked around SiO₃^2?, as predicted by the stoichiometry of the starting materials. Variable temperature X-ray diffraction was used to measure the silicate's thermal expansion behaviour up to the point of crystallisation, where the mineral phase wollastonite was observed. Refined lattice parameters were in good agreement with standard values for wollastonite. The procedure as implemented does result in the formation of a small amount of poorly ordered carbonate, which can be removed by post-drying annealing. The carbonation in the presence of CO₂ of the dried amorphous silicate was also investigated, showing the conversion to CaCO₃ calcite and SiO₂.     

Textural properties of synthetic nano-calcite produced by hydrothermal carbonation of calcium hydroxide

The hydrothermal carbonation of calcium hydroxide (Ca(OH)₂) at high pressure of CO₂ (initial P_CO2=55 bar) and moderate to high temperature (30 and 90 °C) was used to synthesize fine particles of calcite. This method allows a high carbonation efficiency (about 95% of Ca(OH)₂–CaCO₃ conversion), a significant production rate (48 kg/m³ h) and high purity of product (about 96%). However, the various initial physicochemical conditions have a strong influence on the crystal size and surface area of the synthesized calcite crystals. The present study is focused on the estimation of the textural properties of synthesized calcite (morphology, specific surface area, average particle size, particle size distribution and particle size evolution with reaction time), using Rietveld refinements of X-ray diffraction (XRD) spectra, Brunauer–Emmett–Teller (BET) measurements, and scanning electron microscope (SEM) and transmission electron microscope (TEM) observations. This study demonstrate that the pressure, the temperature and the dissolved quantity of CO₂ have a significant effect on the average particle size, specific surface area, initial rate of precipitation, and on the morphology of calcium carbonate crystals. In contrast, these PTx conditions used herein have an insignificant effect on the carbonation efficiency of Ca(OH)₂.

Finally, the results presented here demonstrate that nano-calcite crystals with high specific surface area (S_BET=6–10 m²/g) can be produced, with a high potential for industrial applications such as adsorbents and/or filler in papermaking industry.     

Influence of polyvinylpyrrolidone on the precipitation of calcium carbonate and on the transformation of vaterite to calcite

The precipitation process of calcium carbonate (CaCO₃) in the absence and presence of poly (N-vinyl-2-pyrrolidone) (PVP) was investigated using scanning electron microscopy, powder X-ray diffraction, and Fourier transform infrared spectroscopy methods at room temperature. The results indicate that PVP does not affect the polymorphy, but has influence on their morphology and size of CaCO₃ crystals. With the addition of PVP, the amorphous CaCO₃ could aggregate into bigger amorphous spherulites before transforming to crystalline state. Also, the transformation from the thermodynamically unstable vaterite to the stable calcite was investigated in the presence of PVP. As a particle-stabilizing agent, PVP molecules inhibit the formation of vaterite, however, promote the formation of calcite as well as the rate of the solvent-mediated transformation from vaterite to calcite.     

The precipitation of calcium carbonate via a bubbling method in the presence of Mg2+ and glucose

Abstract

The effect of Mg²⁺ on the crystallization of precipitated calcium carbonate (PCC) via a bubbling carbonation method and the mechanism of eliminating its influence by glucose were investigated. The polymorph and morphology of crystals were characterized by field emission-scanning electron microscopy, X-ray diffraction, and Fourier transform infrared spectroscopy. These results demonstrated that Mg²⁺ and Mg/Ca molar ratio played significant roles on the morphology of PCC. When the Mg/Ca molar ratio was below 0.5, only spindle-like calcite formed. The rod-like aragonite started to form when the ratio was 0.6. As the ratio increased, the amount of aragonite increased and the length of rod-like aragonite became longer. Notably, the effect of Mg²⁺ could be eliminated efficiently when the 1.5?wt% glucose was added into the carbonation system, in which system, the PCC crystals were all spindle-like calcite. Furthermore, the mechanism of the glucose to eliminate the influence of Mg²⁺ on PCC crystallization was proposed.     

Phase‐controlled crystallization of amorphous calcium carbonate in ethanol‐water binary solvents

The evolution of amorphous calcium carbonate (ACC) into crystals in ethanol/water binary solvents under ambient temperature was investigated, and it was found to depend on the volume ratio of ethanol to water (R). Calcite remained dominant when the amount of water was high (R = 1/3). A slight change in the amount of ethanol (R = 3/1) could lead to a dramatic change in the polymorph from calcite to aragonite. However, when poly (allylamine hydrochloride) (PAH) was added at R = 3/1, almost pure vaterite could be obtained, which has a specific morphological variation (from hollow microspheres to cloud‐like). This study provides an alternative polymorphic route for the CaCO₃ mineral by using the evolution of ACC in different solvent environments, which provides some useful clues for understanding the importance of kinetic control of the morphologies and polymorphs of a wide range of inorganic materials. In addition, this simple mild phase‐controlled synthetic method could be scaled up as a green chemistry route for the industrial production of different polymorphs of CaCO₃.     

白云石制备似立方体状方解石型碳酸钙晶体及其机理研究

以轻烧白云石粉、氯化铵和二氧化碳为原料,在未使用晶型控制剂的情况下,通过蒸氨-沉钙过程制备出了似立方体状碳酸钙。研究了反应温度、溶液中钙离子浓度、通气速率、搅拌速度以及陈化时间对碳酸钙中方解石相含量以及晶体形貌的影响,并探索了沉钙反应的晶型控制机理。结果表明,在反应温度40 ℃、钙离子浓度0.05 mol/L、通碳速率100 mL/min、搅拌速度400 r/min和陈化时间2 h的条件下,制备出形貌规整、粒径分布均匀的似立方体状碳酸钙,平均粒径为5~10 μm。该研究为提升白云石的使用价值、生产高附加价值的碳酸钙产品,以及提高白云石资源的利用率提供理论基础。     

Growth of nanofibrous barium carbonate on calcium carbonate seeds

Fibrous barium carbonate (BaCO₃/witherite) crystals 50–100 nm in diameter and several microns in length were grown on calcium carbonate (CaCO₃) seeds at temperatures as low as 4 °C. The BaCO₃ fibers were deposited onto calcite rhombs or CaCO₃ films using the polymer-induced liquid-precursor (PILP) process, which was induced with the sodium salt of polyacrylic acid (PAA). The structure and morphology of the resultant fibers were investigated using scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected-area electron diffraction (SAED), and polarized light microscopy (PLM). Fibers were successfully grown on calcite seeds of various morphologies, with a range of barium concentrations, and PAA molecular weight and concentration. Two categories of fibers were grown: straight and twisted. Both types of fibers displayed single-crystalline SAED diffraction patterns, but after examining high-resolution TEM lattice images, it was revealed that the fibers were in fact made up of nanocrystalline domains. We postulate that these nanocrystalline domains are well aligned due to a singular nucleation event (i.e., each fiber propagates from a single nucleation event on the seed crystal) with the nanocrystalline domains resulting from stresses caused by dehydration during crystallization of the highly hydrated precursor phase. These BaCO₃ fibers grown on calcite substrates further illustrate the robustness and non-specificity of the PILP process.     

Temperature fluctuations in a LiNbO3 melt during crystal growth

The synthesis of calcium carbonate (CaCO₃) crystals from aqueous solutions containing sodium dodecyl sulfate (SDS), poly(N-vinyl-1-pyrrolidone) (PVP), or SDS/PVP complexes has been performed through a slow titration method. It was found that aragonite and calcite co-existed in the prepared crystals. The formation of aragonite in the precipitation systems without magnesium ions indicates that at ambient temperature ca. 26.0°C, initially formed amorphous CaCO₃ could also transfer into aragonite in the sedimentary phase, which indicates the controlling factor of organic additives in the nucleation and growth process of CaCO₃ crystals. The appearance of hexagonal crystals in the suspensible phase confirmed the hexagonal crystallization cell of vaterite, and revealed the colloidal-dispersion function of the SDS/PVP complex in the crystallization process of CaCO₃.     

Crystallization habit of calcium carbonate in presence of sodium dodecyl sulfate and/or polypyrrolidone

The synthesis of calcium carbonate (CaCO₃) crystals from aqueous solutions containing sodium dodecyl sulfate (SDS), poly(N-vinyl-1-pyrrolidone) (PVP) or SDS/PVP complexes has been performed through a slow titration method. It was found that aragonite and calcite coexisted in the prepared crystals. The formation of aragonite in the precipitation systems without magnesium ions indicates that at ambient temperature ca. 26.0°C, initially formed amorphous CaCO₃ could also transfer into aragonite in the sedimentary phase, which indicates the controlling factor of organic additives in the nucleation and growth process of CaCO₃ crystals. The appearance of hexagonal crystals in the suspensible phase confirmed the hexagonal crystallization cell of vaterite, and revealed the colloidal-dispersion function of the SDS/PVP complex in the crystallization process of CaCO₃.     

Deposition of calcium carbonate films by a polymer-induced liquid-precursor (PILP) process

A polypeptide additive has been used to transform the solution crystallization of calcium carbonate to a solidification process of a liquid-phase mineral precursor. In situ observations reveal that polyaspartate induces liquid–liquid phase separation of droplets of a mineral precursor. The droplets deposit on the substrate and coalesce to form a coating, which then solidifies into calcitic tablets and films. Transition bars form during the amorphous to crystalline transition, leading to sectorization of calcite tablets, and the defect textures and crystal morphologies are atypical of solution grown crystals. The formation of nonequilibrium crystal morphologies using an acidic polypeptide may have implications in the field of biomineralization, and the environmentally friendly aspects of this polymer-induced liquid-precursor (PILP) process may offer new techniques for aqueous-based processing of ceramic films, coatings, and particulates.     

Influence of impurity on rhythmic crystallization of calcium carbonate in agar gel

During the gel growth of calcite crystals in test tubes, for a narrow concentration of inner and outer electrolytes we observed rhythmic crystallization of calcium carbonate. In this paper the effect of adding impurities on the pattern of periodic crystallization of calcium carbonate has been presented. The addition of impurities either increases or decreases the solubility of reaction product and hence may affect the rate of nucleation of calcium carbonate crystals. The variation of spacing coefficient and velocity constant due to the addition of impurities has also been studied.     

Conductometric study of calcium carbonate prenucleation stage: underlining the role of CaCO3o ion pairs

Calcium carbonate crystallization process, especially the prenucleation stage, has increasingly been the subject of several works. In the present work, a simple method based on electrical conductivity modeling applied to the FCP (Fast Controlled Precipitation) method data is used to highlight the role of CaCO₃^o ion pairs on calcium carbonate prenucleation stage. A good agreement was obtained between the resistivity vs pH curves estimated by the McCleskey model equation and obtained experimentally in a FCP test. Results showed that the nucleation process begins with the formation of CaCO₃^o ion pairs as pre‐nuclei as soon as the calcite‐equilibrium pH is reached. Additionally CaCO₃^o content increases with pH to form aggregates, which depend on the saturation state of the solution. Basing on our thermodynamic data, these aggregates do not form amorphous calcium carbonate ACC as an intermediate phase. They lead to the formation of stable calcium carbonate nuclei which will further evolve to crystallize. Furthermore we demonstrate that in addition to their inhibitory effect on the Ca²⁺ and CO₃²⁻ association to form ion pairs, the two scale inhibitors sodium triphosphate (STP) and sodium polyacrylate (RPI) reduce ion pairs aggregation rate.     

凝胶体系中生物矿物生长的研究进展

近年来,生物矿化的模拟和仿生材料的研究十分引人注目.凝胶体系是模拟生物矿化的有效介质.本文综述了凝胶中羟基磷灰石、透钙磷石、磷酸钙等磷酸盐,文石、方解石、球霰石等碳酸钙,碳酸钡和草酸钙等生物矿物生长的研究进展,讨论了生长参数如凝胶种类、凝胶化学性质、吸附到凝胶表面的物质种类、介质pH、生长时间等对这些矿物晶体生长的影响,最后指出了该领域所面临的问题和将来的发展方向.     

Influence of stirring speed on the crystallization of calcium carbonate

Control over crystal morphology of calcium carbonate (CaCO₃) was investigated by simply changing the stirring speeds in the process of CaCO₃ formation. Scanning electron microscopy (SEM) and powder X‐ray diffraction (XRD) measurements explore the morphology evolution of CaCO₃ at varying stirring speeds. As the stirring speeds increase, rhombohedral calcite, spherical vaterite, and monoclinic crystal with coexistence of calcite phase and vaterite phase were formed, suggesting a facile control over calcium carbonate crystallization in constructing crystals with desired morphology. Moreover, almost pure vaterite spherical particles of narrow particle size distribution were formed at optimum stirring speed. Finally, also elucidated in this work is the mechanism investigation into the construction of various crystal forms via this simple route. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Application of seeded batch crystallization methods for reduction of the scaling tendency of seawater – A study of growth kinetics of calcium carbonate in seawater

The objective of this study is to evaluate the applicability of a seeded batch crystallization method as a pre‐treatment stage to the reduction of the the scaling potential of seawater. The crystallization process of calcium carbonate in supersaturated seawater is studied experimentally in a seeded batch crystallizer. Results show that the initial pH value of seawater must be adjusted to be in the range of 8‐9. Calcite seeds will not have the potential to start the growth process in seawater at he the normal pH (7.36) compared to the above mentioned range. The growth kinetic parameters are determined from the measured desupersaturation curves. It is found that the growth process of calcite is controlled by surface integration step. The growth rate of calcite increases with increasing temperature and seeding ratio (up to 1 g/L), while it decreases with increasing the salinity of seawater. Crystals of calcite morphology are found to be growing faster than seeds of aragonite morphology. Stirrer speed has no distinct effect on the growth rate of calcite in seawater. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Compositional analysis of a polymer-induced liquid-precursor (PILP) amorphous CaCO3 phase

Amorphous calcium carbonate (ACC) has been of keen interest in the biomimetics field because of recent evidence which suggests it plays an important role in biomineralization. In this report, an in vitro model system is used to examine the composition of an amorphous phase generated by polyanionic process-directing agents, such as the sodium salt of polyaspartic acid (Pasp), which is considered a simple mimic to the proteins associated with calcific biominerals. This additive leads to the formation of a highly hydrated, amorphous mineral precursor to calcium carbonate (CaCO₃), referred to as a polymer-induced liquid-precursor (PILP) phase. The precursor phase was collected by centrifugation, and the quantity of precursor phase and the water content were determined. It was found that Pasp promotes and stabilizes the amorphous precursor, which has a composition that steadily changes with time as the polymer and water are excluded. Elemental analysis was used to investigate the role of the polymer in influencing the calcium/carbonate ratio, the water content, and the amount of precursor phase. Raman and ATR-FTIR spectroscopy were used to compare the compositions of the precursor phases generated with different polymeric concentrations. The role of Pasp in generating and stabilizing the ACC precursor phase is discussed.     

在卵磷脂溶液中仿生合成无定形碳酸钙

在含有卵磷脂的水溶液中仿生合成了无定形碳酸钙.所得样品用扫描电镜(SEM)、傅立叶红外光谱(FT-IR)、X射线衍射(XRD)和热重分析(TG)等分析方法进行了表征.研究表明:具有一定浓度的卵磷脂可以利用其端基磷酸根基团稳定无定形碳酸钙,这为理解在生物体内的环境中生物矿物的形成机理提供了一种新的途径.     

The onset of spherulitic growth in crystallization of calcium carbonate

Batch and semi-batch crystallization experiments were performed in aqueous solutions for the three anhydrous polymorphs of calcium carbonate vaterite, aragonite and calcite to investigate the effect of crystallization parameters on the onset of spherulitic growth and particle morphology.