铁饼状碳酸钙的制备及性能研究

本文以氯化钙和氨水的混合溶液为水相,聚氧乙烯辛基苯酚醚-10(OP-10)为表面活性剂,正辛醇为助表面活性剂,环己烷为油相配制了反相微乳液.并通过向其中通入CO2气体制备微/纳米碳酸钙,得到了铁饼状的微米碳酸钙.采用扫描电子显微镜(SEM),X射线粉末衍射(XRD)和傅里叶变换红外光谱(FTIR)表征了微米碳酸钙的形貌...     

DTA studies on the effect of impurities on the phase transformations of barium carbonate

Orthorhombic () barium carbonate, undergoes a reversible transformation to hexagonal () form around 1093 K which in turn undergoes a reversible transformation to cubic () form around 1253 K. Cationic impurities such as Ca²⁺, Sr²⁺, Cd²⁺, Zn²⁺, have been doped in pure barium carbonate and their effect on the kinetics and energetics of its transformations were studied. By using DTA technique, the enthalpy and energy of activation for the transformation of pure and doped samples of barium carbonate have been calculated and compared.One of the authors (C. R. M. Rao) thanks the University Grants Commission, New Delhi (India), for the award of a research scholarship. The authors are grateful to Dr. I. P. Saraswat, Head of the Chemistry department, for providing the research facilities.     

Buffering dissociation/formation reaction of biogenic calcium carbonate

The oscillating stability of coral reef seawater pH has been maintained at around physiological pH values over the past 300 years (Pelejero et al., 2005). The stability mechanism of its pH has been interpreted in terms of the buffering dissolution/formation reaction of CaCO(3) as well as the proton consumption/generation reaction in CaCO(3)-saturated water. Here the pH-dependent solubility product [HCO(3)(-)][Ca(2+)] has been derived on the basis of the actual pH-dependent reactions for the atmospheric CO(2)/CO(2 (aq.))/HCO(3)(-)/CO(3)(2-)/Ca(2+)/CaCO(3) system. Overbasic pH peaks appeared between pH approximately 8 and approximately 9.5 during sodium hydroxide titration, as a result of simultaneous CaCO(3) formation and proton generation. The spontaneous and prompt water pH recovery from the acidic to the physiological range has been confirmed by the observation of acid/base time evolution, because of simultaneous CaCO(3) dissolution and proton consumption. The dissolution/formation of CaCO(3) in water at pH 7.5-9 does not take place without a proton consumption/generation reaction, or a buffering chemical reaction of HCO(3)(-)+Ca(2+)right arrow over left arrowCaCO(3)+H(+). SEM images of the CaCO(3) fragments showed that the acid water ate away at the CaCO(3) formed at physiological pH values. Natural coral reefs can thus recover the physiological pH levels of seawater from the acidic range through partial dissolution of their own skeletons.     

Thermal behavior and phase transformations of nanosized carbonate apatite (Syria)

The phase transformations of Syrian phosphorite upon mechanochemical activation are examined in the present work. The latter is carried out in planetary mill equipped with 20 mm steel milling bodies and duration from 30 to 300 min. The established by means of DTA, DTG, TG analyses transformation of non-activated carbonate fluorine apatite type B into the carbonate hydroxyl fluorine apatite (COHFAp) mixed type A2-B leads to substantial changes in the properties of the activated samples expressed in lowering the degree of crystallinity, strong defectiveness of the structure, and increase of the citric solubility. The thermal analysis gives evidence for the decomposition of the carbonate-containing component within the phosphorite, as from the positions placed in the vicinity of the hexagonal 6₃ axis (type A2), as well as from the positions of the phosphate ion (type B), and from the free carbonates. The data from the thermal analysis, the powder X-ray analysis and the infrared spectroscopy give also evidence for phase transformations of the activated apatite (with admixtures of quartz and calcite) into Ca₁₀FOH(PO₄)₆, β-Ca₃(PO₄)₂, Ca₄P₂O₉, Ca₃(PO₄)₂ · Ca₂SiO₄ and for that one of the quartz—into larnite and wollastonite. The influence of the α-quartz as a concomitant mineral is considered to be positive. The α-quartz forms Si–O–Si–OH bonds retaining humidity in the solid phase thus facilitating the isomorphous substitution OH^? → F^? with the subsequent formation of partially substituted COHFAp. Calcium silicophosphate and Ca₄P₂O₉ are obtained upon its further heating. The presented here results settle a perspective route for processing of low-grade phosphate raw materials by means of tribothermal treatment aiming at preparation of condensed phosphates suitable for application as slowly acting fertilizer components.     

Thermally induced transformations of calcium carbonate polymorphs precipitated selectively in ethanol/water solutions

For the precipitation of calcium carbonate polymorphs in ethanol/water solutions of calcium chloride by the diffusion of the gases produced by sublimation–decomposition of solid ammonium carbonate, polymorph selection and morphology control of the precipitates were demonstrated by the effect of ethanol/water ratio in the mother liquor. The precipitated phases change systematically from gel-like aggregates of hydrated amorphous calcium carbonate in the absolute ethanol solution to well-shaped rhombohedral particles of calcite in the absolute aqueous solution via almost pure phase of vaterite with dendrite structure in 75%-ethanol/25%-aqueous and 50%-ethanol/50%-aqueous solutions. On heating the precipitated sample in flowing dry nitrogen, all the samples transformed to calcite before the thermal decomposition, where the thermal decomposition temperature shifts to higher temperatures with increasing the water content in the mother liquor due to the systematic increase in the particle size of calcite. Accordingly, the present method of controlled precipitation of calcium carbonate polymorphs is also useful to control the particle size and reactivity of calcite produced by heating the precipitates. Selecting vaterite with dendrite structure from the present series of precipitated samples, the structural phase transition to calcite was characterized as the three-dimensional growth of rhombohedral particles of calcite with the enthalpy change ΔH = ? 2.8 ± 0.1 kJ mol^?1 and the apparent activation energy E_a = 289.9 ± 5.8 kJ mol^?1.     

The effects of phase transformations in shock-wave reaction products on the rate of reaction front motion

Reversible phase transformations in the products of shock-wave reactions control the rate of solid-state detonation and make it pulsed in character. The calculated reaction rates are in agreement with experiment. Original Russian Text ? S.S. Batsanov, Yu.A. Gordopolov, 2009, published in Zhurnal Fizicheskoi Khimii, 2009, Vol. 83, No. 6, pp. 1199–1200.     

Direct sulfation reaction of SO2 with calcium carbonate

The direct sulfation reaction of SO₂ with CaCO₃ has been investigated by thermogravimetry (TG) under the condition that the CaCO₃ does not decompose to CaO prior to sulfation by controlling CO₂ partial pressure. The direct sulfation process can be described by using a shrinking-core model for constant particle size. The model shows that the reaction rate and the diffusion rate of SO₂ through the product layer are equally important. Temperature effects can be correlated by the activation energy of 35.9 kJ mol⁻¹ for the sulfation reaction and 66.5 kJ mol⁻¹ for the product layer diffusion. The sulfation reaction is found to be first order with respect to SO₂. With larger pore volume and surface area of limestone samples, the sorbents have a stronger reactivity of SO₂ removal. A 70% CaCO₃ conversion can be achieved in 10 min at 800°C and 2000 ppm SO₂.     

The adsorption and self-assembly of mixtures of alkylbenzene sulfonate isomers and the role of divalent electrolyte

In this paper, the role of the different structural isomers of the anionic surfactant sodium para-dodecyl benzene sulfonate, LAS, on surface adsorption and solution self-assembly has been studied. Using a combination of neutron reflectivity, NR, and small angle neutron scattering, SANS, the effect of mixing an isomer with a short symmetric hydrocarbon chain with one which has an asymmetric hydrocarbon chain on both the equilibrium surface adsorption behavior and the solution microstructure of the mixtures, both in the presence and absence of a divalent cation (Ca(2+)), has been investigated. In the absence of electrolyte, the LAS isomer mixtures form small charged globular micelles throughout the composition range studied. The micelle aggregation number increases with the increase in the asymmetric isomer content, reflecting an increase in the packing efficiency within the micelle. The addition of calcium ions promotes the formation of planar aggregates, as multilamellar vesicles, but only when the symmetric LAS isomer is the major component of the mixture. At a surfactant concentration just above the critical micelle concentration, CMC, and in the absence of electrolyte, the variation in the surface composition is close to the solution composition. Regular solution theory, RST, calculations show that this variation is also close to what is expected for ideal mixing. The addition of Ca(2+) ions induces a different surface behavior, resulting in the formation of multilayer structures at the interface throughout the entire composition range.     

The determination of the linear alkylbenzene sulfonate isomers in water samples by gas-chromatography/mass spectrometry

A number of 20 compounds of linear alkylbenzene sulfonates (LASs) family were identified by electron impact mass spectrometry (EI-MS) in water samples collected from wastewater treatment plants (WWTP). This paper presents the mass spectra of 20 compounds, the proposed mechanism of formation of the diagnostic ions obtained by EI-MS and the distribution of individual isomers in water samples collected from compartments of WWTP. The individual isomers from four homolog series C(10)-, C(11)-, C(12)- and C(13)-LAS were analyzed as methyl derivatives.     

Thermodynamics of hydrothermal solutions: Calcium carbonate and calcium sulphate solubility in a four-phase system

A thermodynamic model for the system CaCO₃-CaSO₄·2H₂O-NaCl-CO₂-H₂O has been developed for calculating calcium carbonate solubilities as a function of the carbon dioxide pressure under vapour-liquid-solid equilibrium conditions.The model has been tested against available data and its predictive capability compares favourably with that of other proposed models. Numerical values for model parameters are also given.The main feature of the model used in the present work is that the excess Gibbs energy is the sum of three terms: a Debye-Huckel contribution, a Born term as a correction for the change in dielectric constant and a short range interactions contribution calculated according to the non-random two-liquid (NRTL) equation.Our results may be useful in describing and interpreting the general characteristics of hydrothermal solutions with a view to their utilization for energy production.     

Pentafluorophenyl sulfonate ester as a protecting group for the preparation of biaryl- and heterobiaryl sulfonate esters

The use of the pentafluorophenyl (PFP) group as a sulfonic acid protecting group has allowed the synthesis of new biaryl- and heterobiaryl-PFP-sulfonate esters by use of the Suzuki-Miyaura reaction. The successful employment of a novel inorganic base, anhydrous sodium tetraborate, was crucial to give the products in excellent yields. The PFP-sulfonate ester has been previously shown to be an excellent alternative to sulfonyl chlorides in the synthesis of sulfonamides. [structure: see text]     

Polycarbonate synthesis by the melt phase carbonate-ester interchange reaction of bisphenol-A diacetate with dimethyl carbonate

The molecular weight of polycarbonate formed by the carbonate-ester interchange reaction of bisphenol-A diacetate (BPAC₂) and dimethyl carbonate (DMC) was found to depend on the composition of the oligomer formed in its first stage. At a [DMC] : [BPAC₂] ratio of 2, an oligomer with relatively higher acetate end group concentration was formed. This upon further polycondensation yielded a polycarbonate of inherent viscosity, 0.2-0.25 dL/g. Upon addition of DMC at a later stage in the oligomer-forming reaction, an oligomer containing relatively higher carbonate end groups could be obtained. These oligomers gave polycarbonates higher inherent viscosity, 0.4-0.5 dL/g. It is therefore proposed that oligomers having carbonate end groups are more reactive in self-polycondensation than those containing acetate end groups. © 1995 John Wiley & Sons, Inc.     

Chemical reaction and phase transformations of the components of low temperature shift catalyst in various atmospheres

The systems CuO–Al₂O₃ and ZnO–Al₂O₃, as model systems of low temperature shift catalysts, have been studied in the temperature interval 100–700°C. The changes of their phase composition and surface area, influenced by gas atmosphere have been studied by X-ray, IR analysis and low temperature nitrogen adsorption.

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CuO–Al₂O₃ ZnO–Al₂O₃ CO 100–700°C. , - .

     

Synthesis and catalytic applications of sulfonate β-ketoimine and β-diimine in the Suzuki reaction in aqueous phase

A series of sulfonated β-ketoimine and β-diimine compounds were synthesized and characterized. As supporting ligands they were used in the Suzuki cross-coupling reaction in aqueous phase with PdCl₂ as catalyst. The new catalytic system can tolerate a wide range of substrates.     

Rearrangement of aromatic sulfonate anions in the gas phase

Collisionally activated dissociation of deprotonated aromatic sulfonic acids in the gas phase causes rearrangement and fragmentation to produce the corresponding phenoxide ions. The mechanism for this reaction has been investigated and the results of this study favor initial intramolecular nucleophilic addition of a sulfonate oxygen atom to the aromatic ring, a process which is followed by heterolytic cleavage of the carbon–sulfur bond to rearomatize the ring. The product from this addition–elimination sequence is the anion of a sulfurous acid half-ester, which loses SO₂ to generate the corresponding phenoxide ion.     

Effect of calcium and carbonate concentrations on anionic membrane fouling during electrodialysis

A previous study on electrodialysis of calcium and carbonate high concentration solutions demonstrated that calcium migrated through the cation-exchange membrane (CEM) was blocked by the anion-exchange membrane (AEM) where it formed another fouling. The aim of the present work was to complete the identification of the deposit formed on AEM during electrodialysis and to characterize its physical structure at the interface of the membrane. No fouling was found on the anionic membranes treated without calcium chloride in presence of sodium carbonate, while membranes used during ED process of solutions containing calcium chloride and sodium carbonate were slightly fouled. A thin layer of precipitates was observed on the anionic membrane surface. The appearance of precipitates was typical of a crystalline substance. The size and form of crystal increased in proportion to the concentration of calcium chloride in solution. Large and cubic crystals were the best defined on the membrane treated at 1600 mg/L of CaCl2. The precipitate was identified as calcium hydroxide. However, this fouling was not found to affect significantly the electrical conductivity and the thickness of the membranes. Furthermore, the fouling formed was reversible.     

Structural transformations in a crystal during the photochemical reaction of 2-benzyl-5-benzylidenecyclopentanone

The studies presented are the first example in which the substrate and product molecules were monitored separately over many stages during the chemical reaction progress in a crystal to understand the reaction path of the molecules in detail during the whole transformation. The BBCP, 2-benzyl-5-benzylidenecyclopentanone, single crystal was irradiated without any destruction over several steps. After each of step the X-ray structure with a different substrate/product ratio and with a disorder was determined. The study revealed that the photochemical [2+2]-cycloaddition reaction of BBCP in a crystal does not exhibit cooperative character and can be regarded as of the first-order. During the transformation the product molecules do not adopt the position of the molecules in the final pure dimer crystal, but move gradually and smoothly in this direction. The movement of the molecules was also observed for the substrate. This remarkable behaviour of the product and reactant molecules bears a rotational component. The distance between reaction centres of adjacent BBCP substrate molecules decreases fluently with the progress of the reaction, but it does not get smaller than a sum of van der Waals radii.     

Effect of calcium and carbonate concentrations on cationic membrane fouling during electrodialysis

Fouling, which is the accumulation of undesired solid materials at the phase interfaces of permselective membranes, is one of the major problems in electrodialysis. The aim of the present work was to investigate the effect on the fouling of cation-exchange membranes of the composition in calcium and carbonate of a model solution to be treated by electrodialysis. No fouling was observed at 400 and 800 mg/L of CaCl(2) in the absence of carbonate, while at only 400 mg/L CaCl(2) with carbonate, a deposit was observed. This difference could be explained by the buffering capacity of the carbonate, which affects the treatment duration with and without sodium carbonate. Since the duration was longer with carbonate, more calcium ions were able to migrate across the CMX-S membrane, which explained the higher deposit on its surface. Furthermore, whether there was carbonate in the solution treated by electrodialysis or not, the deposit on the surface of the cationic membrane was calcium hydroxide. However, this fouling formed during conventional ED was easily cleaned by an acid procedure.     

Effects of microscale calcium carbonate and nanoscale calcium carbonate on the fusion,thermal, and mechanical characterizations of rigid poly(vinyl chloride)/calcium carbonate composites

A Haake torque rheometer equipped with an internal mixer has been used to study the influence of microscale calcium carbonate (micro‐CaCO₃) and nanoscale calcium carbonate (nano‐CaCO₃) on the fusion, thermal, and mechanical characteristics of rigid poly(vinyl chloride) (PVC)/micro‐CaCO₃ and PVC/nano‐CaCO₃ composites, respectively. The fusion characteristics discussed in this article include the fusion time, fusion temperature, fusion torque, and fusion percolation threshold (FPT). The fusion time, fusion temperature, and FPT of rigid PVC/calcium carbonate (CaCO₃) composites increase with an increase in the addition of micro‐CaCO₃ or nano‐CaCO₃. In contrast, the fusion torque of rigid PVC/CaCO₃ composites decreases with an increase in the addition of micro‐CaCO₃ or nano‐CaCO₃. The results of thermal analysis show that the first thermal degradation onset temperature (T_onset) of rigid PVC/micro‐CaCO₃ is 7.5 °C lower than that of PVC. Meanwhile, the glass‐transition temperature (T_g) of rigid PVC/micro‐CaCO₃ is similar to that of PVC. However, T_onset and T_g of PVC/nano‐CaCO₃ composites can be increased by up to 30 and 4.4%, respectively, via blending with 10 phr nano‐CaCO₃. Mechanical testing results for PVC/micro‐CaCO₃ composites with the addition of 5–15 phr micro‐CaCO₃ and PVC/nano‐CaCO₃ composites with the addition of 5–20 phr nano‐CaCO₃ are better than those of PVC. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 451–460, 2006     

Free energy and structure of calcium carbonate nanoparticles during early stages of crystallization

We introduce a metadynamics based scheme for computing the free energy of nanoparticles as a function of their crystalline order. The method is applied to small nanoparticles of the biomineral calcium carbonate to determine the preferred structure during early stages of crystal growth. For particles 2 nm in diameter, we establish a large energetic preference for amorphous particle morphologies. Particles with partial crystalline order consistent with vaterite are also observed with substantially lower probability. The absence of the stable calcite phase and stability of the amorphous state support recent conjectures that calcite formation starts via the deposition of amorphous calcium carbonate.