8-9th August, 2008 School of Materials Science & Nanotechnology,Jadavpur University
Gas-Liquid reactive crystallization for the synthesis of CaCO3 nanocrystals
Suraj Varma1*, Pao-Chi Chen2, G. Unnikrishnan1
1Polymer Science and Technology Laboratory, National Institute of Technology Calicut, Kerala, India
2Department of Chemical and Materials Engineering, Lunghwa University of Science and Technology, Taiwan
*E-Mail: surajvarma@rediffmail.com
Calcium carbonate crystals were synthesized using Ca(OH)2- CO2-H2O reactive crystallization system inside a bubble column apparatus. Effect of dispersion agents on the morphology and particle size of precipitated crystals was studied using X-ray diffraction and BET surface area analysis. Citric acid, sodium metaphosphate and polyacrylic acid were used as the dispersion agents. Effect of dispersion agent concentration was also taken into account. Particle size of the crystals was found to decrease in presence of dispersion agents. Scanning electron micrographs and X-ray diffraction studies revealed that the morphology of the precipitated crystals is influenced by the presence of dispersion agents.
Key Words : Reactive Crystallization, dispersion agents.
1. Introduction
Sparingly soluble compounds are often produced by a precipitation process. The characteristics such as the surface area, the particle size and the morphology are influenced by the presence of dispersion agents [1]. An understanding of the effects of dispersion agents on the properties is crucial in the application of these particles. Small concentrations of these agents have a profound influence on the crystal growth kinetics [2]. Calcium carbonate crystals have found applications ranging from their use in biomaterials to reinforcements for high polymers. CaCO3 exists in three polymorphic forms- calcite, aragonite and vaterite. Calcite has a rhombic structure, Aragonite has a demtritic and Vaterite has a spherical morphology. Westin and Rasmuson [3] found that citric acid promotes the formation of small rhombic calcite crystals at all feed times in the semi batch mode. J. Sellami et al [4] used citrate ions as additives in the pH ranging from 8 to 10 in order to modify the influence of citrate concentration on the formation of calcium carbonate polymorphs. They also analyzed the phase equilibria involved in the precipitation reactions. Adsorption of CO2 gas which is a green house gas and converting them to a useful end product is always important in the present situation regarding environmental pollution. In the present work we analyze the effect of citrate ion. Sodium meta phosphate ion and PAA on the precipitation chemistry and
morphology of CaCO3 crystals in a gas-liquid reactive crystallization system in a bubble column. The prepared nanocrystals were characterized by means of XRD, SEM and BET surface area analysis.
2. Experimental
2.1 Materials
Several chemicals were used in this work such as Ca(OH)2, Citric acid monohydrate, sodium metaphosphate and PAA. All the chemicals were reagent grade and were supplied by Nacalai Tesque, INC, Japan.
2.2Preparation of CaCO3
Calcium Carbonate crystals were synthesized by means of gas-liquid reactive crystallization system using a bubble column. 700 ml of slaked lime slurry is introduced into the bubble column and CO2 gas (30%) was introduced from the bottom of the column. A pH electrode, CO2 analyzer and a thermometer were introduced into the system to follow pH, CO2 pressure and temperature respectively. Completion of the reaction is indicated by the lowering of solution pH to the acidic range. The crystals formed were filtered, washed and dried at 100 degree centigrade in an oven for 12 hours. Then the sample is powdered well and used for the further characterization. In the next set of experiments slaked lime slurries are prepared with the addition of citric acid, sodium meta phosphate and PAA solutions having varying concentrations. In all the experiments the composition of CO2 gas was fixed at 30%.
2.4 BET surface area
The surface areas of the prepared crystals were found out by nitrogen gas adsorption method using BET 201-APCW surface area analyzer.
2.5 X-ray diffraction studies
The approximate morphology of the precipitates were determined using X-ray diffraction pattern obtained from RIGAKU X-RAY DIFFRACTOMETER D/MAX 2200.
2.6 Morphological Studies
Dimensional and morphological inferences were also drawn out using electron microscopic technique. A JEOL scanning electron microscope was used for that purpose.
3. RESULTS AND DISCUSSION
3.1 Particle size and BET surface area
Table 1 shows the effect of dispersion agents on the particle size and surface area of calcium carbonate crystals. It is very much clear that, particle size drastically decrease by the presence of dispersion agents. As the concentration of the dispersion agents increases, its dispersion capacity also increases as evident from the results. Citrate ions proved to be a better dispersion agent as compared to the monophosphate ions. According to Sellami et al, inorder to study the transformation kinetics of calcium carbonate precipitation in presence of citrate ions, the knowledge of CaCO3 solubility in citrate medium is of utmost importance.
A drastic decreese in particle size resulted by the addition of citrate ions from 130 nm to about 40nm. Generally speaking, dispersion agents absorb at the solid-liquid interphase and form a layer of molecular membrane to hinder inter-contacy between particles. When we use a high polymer surfactant like PAA as dispersion agent, it is difficult to get powders with uniform particle size unlike in presence of citric acid or sodium metaphosphate.
3.2 XRD analysis
All the peaks obtained for a standard calcite variety are also can be seen here which indicates the precipitate contains more of calcite polymorphs. According to Vodovic and Kraj, the zeta potential of vaterite is positive in the pH range of 8-10. Then the citrate ions reduce the positive charge of vaterite because of their adsorption on the positively charged sites. At higher concentrations, zeta potential of vaterite is significantly reduced and the precipitation of vaterite is inhibited. The other polymorph arogonite is likely to precipitate only at higher operating temperatures. This explains the precipitation of calcite compared to other polymorphs.
3.3 Morphological Studies
Generally nucleation mechanism depends on the supersaturation of the solution. Low supersaturation nucleation is predominently hetrogeneous while at high supersaturation, homogeneous nucleation becomes prominent. Different types of anionic surfactants at higher concentration led to the various morphology and crystal phases. The surfactants with negatively charged head groups turn out to be very effective in controlling the morphology of inorganic materials. Surfactants can influence one or several crystallization steps and can control the formation of various crystal phases which are not easily formed under neutral environments. Morphology of precipitated CaCO3 are shown in the figures (12-14). As reported by previous researchers, the presence of citrate ions produces only rhomohedral calcite polymorph while the presence of MEA enhances the precipitation of spherical polymorph of calcium carbonate.
Conclusion
Calcium Carbonate crystals were prepared via gas-liquid reactive crystallization using a bubble column . It is found that the presence of dispersion agents can control the morphology and dimensions of precipitated crystals. The concentrations of dispersion agents are also limiting factors. The anionic surfactants inhibit the precipitation of the vaterite calcium carbonate and promote calcite precipitation.
References
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Figures:
0E+04E-48E-41E-3Concentration of citric acid (M)04080120160Particle Dimensionsparticle sizes in nmBET surface areas in meter square per gram0E+04E-48E-41E-3Concentration of sodium metaphosphate04080120Particle dimensionsparticle sizes in nmBET surface areas in meter square per gram
Figure 1: effect of dispersion agents on the particle size and surface area of the precipitated crystals. 203040506070802θIntensity
4
3
2
1
Figure 2: XRD diagrams of some of the precipitated crystals.
Figure 3: Scanning electron micrographs of calcium carbonate crystals produced in presence of different dispersion agents.
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