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.     

Methods for determination of effective diffusion coefficients in ternary alloys (II). Indirect methods of calculating ternary diffusion matrix

We summarised, in part one of this paper, procedures of measuring diffusion matrix in three-component single phase alloys direct from the concentration-distribution profiles in infinite diffusion couples. In this paper (Part II) some procedures have been outlined which enable calculate ternary diffusion matrix from easily measurable tracer coefficients in ideal and non-ideal single phase solid solutions. These procedures have been illustrated with examples, so that it becomes apparant that in many cases the experimental difficulties faced by the experimentalists in direct measurement of ternary diffusion coefficients can be reduced to a large extent.     

Application of the thermal history of copper sulfate solution to industrial crystallization (I). Crystallization with pilot plant

The relationship between the thermal history and the waiting time in nucleation decided for the aqueous solution of sodium chlorate was verified for the aqueous solution of copper sulfate using pilot crystallizers. The following items were clarified as the result of the experiments.

• (1) The cooling method is more stable to nucleation in comparison with the vacuum cooling method even at the same operational conditions.
• (2) The desirable effect of the thermal history is recognized only in the case in which the solution is heated up to a high degree of undersaturation.
• (3) The residence time at an undersaturated state has an important role to dissociate embryos in solution.

     

A method for determination of size-dependent crystal growth kinetics from batch experiments (I). One-experiment estimation

The method developed for determining size-dependent crystal growth kinetics makes use of the assumption that the linear growth rate is a product of a size-dependent factor and a supersaturation-dependent one. The transformed form of the population balance equation, obtained by means of this assumption, is the basis of the evaluation utilizing measurement data from one seeded batch crystallization experiment, namely the size distributions of the seed and product crystals as well as the supersaturation course. The application of the method is illustrated by evaluating an experiment with potash alum crystals.     

Crystallization of calcium sulfate dihydrate at different supersaturation ratios and different free sulfate concentrations

Free Sulfate is a major parameter affecting gypsum crystallization during phosphoric acid production. Gypsum crystal size, shape and filtration rate are significantly affected by the concentration of free sulfate. It is, therefore, important to evaluate the effectiveness of different sulfate levels from 1.5% to 3.5% on induction time and gypsum morphology. The crystallization of gypsum was carried out under simulated conditions of phosphoric acid production by the dihydrate process. Calcium hydrogen phosphate and sulfuric acid were mixed with dilute phosphoric acid at 80 °C, and the turbidity of the reaction mixture was measured at different time periods to calculate the induction time of gypsum crystals formation. With increasing free sulfate concentration, the induction time was significantly decreased. Chemical processing of Central and South Florida phosphate concentrates under different concentrations of free sulfate from 1.5% to 5.5% was carried out. The change on crystal size distribution and filtration rate were traced with free sulfate concentrations. The results show that, filtration rate of phosphogypsum was correlated to the mean diameter of crystals. In addition, induction time and co‐crystallized (lattice) P₂O₅ % in gypsum are decreased with increasing free sulfate content from 1.5% to 3.5%. Morphology of formed gypsum crystals at different sulfate contents and different supersaturation ratios are investigated. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis and structural characterization of dihexamethylenetetraminetetraaquocobalt(II) hexaaquocobalt(II) sulfate hexahydrate

The synthesis and crystal structure of a new three dimensional cobalt(II) complex [Co(C₆H₁₂N₄)₂(H₂O)₄Co(H₂O)₆][SO₄]₂.6H₂O are reported. It crystallizes in the triclinic space group P-1 with a = 13.394(3), b = 16.077(3), c = 9.282(2) Å, = 89.71(2), = 90.42(2), = 114.02(2)°, V = 1825.6(7)ų and Z = 2. The structure is comprised of [Co(C₆H₁₂N₄)₂ (H₂O)₄Co(H₂O)₆][SO₄]₂ units and H₂O molecules. Two molecules of ligand and four molecules of water chelate one type of Co^II in an octahedral fashion through two N and four O atoms: the four O atoms form the equatorial plane and the two N atoms occupy the apical positions. The other type of Co^II cation is chelated by six water molecules.     

Crystallization kinetics determination of Pb15Ge27Se58 chalcogenide glass by using the various heating rates (VHR) method

The kinetics of crystallization of Pb₁₅Ge₂₇Se₅₈ was studied by differential scanning calorimetry non-isothermally. Various experimental methods are currently employed for determining the kinetic parameters of crystallization in a glassy system. These parameters include the activation energy of crystallization E (kJ/mol), the kinetic exponent n and the frequency factor K_o (s⁻¹). Recently, a new method (VHR method) has been derived from Johnson-Mehl-Avrami (JMA) transformation rate equation to calculate - in sequence - the crystallization kinetic parameters of a glassy system. The VHR technique has been used to estimate the crystallization parameters of Pb₁₅Ge₂₇Se₅₈ chalcogenide glass under non-isothermal conditions. The average value of E, n and K_o are found equal to 181.74 ± 0.58 (kJ/mol), 1.085 ± 0.023 and (9.196 ± 0.716) × 10¹² (s⁻¹), respectively. The kinetic exponent, n ≈ 1 indicates a surface nucleation mechanism.     

Structure determination of polytypes (II)

In this part different methods for structure determination of polytypes with diffraction methods are described. These methods are applied to periodic polytypes consisting of one or more sorts of layers and to disordered polytypes consisting of one sort of layers. Furthermore an example for the determination of the structural principle is given.     

Cooling crystallization of aluminum sulfate in pure water modulated by sodium dodecylbenzenesulfonate

This study investigated the cooling crystallization of aluminum sulfate to explore the basic data for the recovery of aluminum resources from coal spoil. Cooling crystallization process of aluminum sulfate with sodium dodecylbenzenesulfonate (SDBS) was investigated experimentally. The effects of operating conditions, namely rotate speed and cooling rate on the crystal size (Median diameter, D_0.5) were studied. Based on single factor experimental results, the response surface method (RSM) with a Box–Behnken design (BBD) was used to determine the key operating conditions, from which a predictive equation was established to quantitatively describe the relationships of D_0.5 and there relative parameters. The optimum operating conditions for cooling crystallization of aluminum sulfate were as follows: rotate speed of 200–300 rpm, cooling rate of 4–5 °C /min and n (SDBS)/n (Al₂(SO₄)₃) of around 5E‐4. Molecular dynamics (MD) results reveal that SDBS decreases the diffusion coefficient (D) of Al³⁺ molecules, which inhibits nucleation and promotes crystal growth.     

Crystallization of hydrocarbon (n-C28H58): Fatty acid (CH3(CH2)14cooh) mixtures (II). The thermal effect

Studies on the crystallization of mixtures of palmitic acid: n-octacosane at elevated temperatures is presented. The crystallization temperature is seen to have a pronounced effect on the morphology of the banded structures. The fibre twist which is responsible for the band formation, is interpreted as due to a thermal stress. Crystallization of some other compounds in the presence of n-octacosane is described and the results are discussed.     

Nucleation of sodium sulfate heptahydrate on mineral substrates studied by nuclear magnetic resonance

Porous building materials are very susceptible to the aggressive action of salts, sodium sulfate being one of the most damaging. Laboratory and field experiments show that the intensity of damage depends on the structure of the porous material, as well as the type and amount of salt. In this paper we investigate the nucleation process of sodium sulfate on two kinds of substrates: calcite and quartz particles, the main components of calcitic and quartzitic stones, respectively, to assess the influence of the substrate chemistry on the crystalline phase being formed and on the nucleation process. Nuclear Magnetic Resonance (NMR) is used to investigate the cooling-induced crystallization of sodium sulfate in mixtures of solution with different amounts of substrate particles. Temperature and concentration are measured by NMR and used to calculate the crystallization rate. The phase that precipitates is the metastable heptahydrate. Our experiments show that nucleation is energetically more favorable on a calcitic substrate; that is, the required supersaturation is lower than on the quartzitic substrate. Under isothermal conditions, the precipitation rate is faster in the presence of calcite than quartz.     

Synthesis of magnesium borates using sodium borate and magnesium sulfate

Synthesis conditions of magnesium borate compounds in aqueous medium using Na₂B₄O₇ · 10H₂O and MgSO₄ · 7H₂O were determined. The effects of B/Mg mole ratio, pH, reaction time and precipitation temperature on the reaction performed at 90 °C were examined. 0.04Na₂O · MgO · 1.23B₂O₃ · 3.42H₂O and 0.06Na₂O · MgO · 1.65B₂O₃ · 3.34H₂O were synthesized at optimum B/Mg mole ratios of 3.60 and 4.80 by precipitation at 5 °C and the amorphous compounds formed were identified by B, Mg, Na and XRD analyses and some physical tests.     

On the kinetic of crystallization of zinc oxalate (II). Determination of the specific surface energy at the crystal solution interface by kinetic studies

An apparatus based on turbidimetry was constructed for studying the kinetics of crystallization of sparingly soluble salts. The kinetics of crystallization of zinc oxalate was investigated at various supersaturations and pHs of the medium. The specific surface energy at the crystal-solution interface was determined from the induction periods of the S-shaped curves by means of the classical theory of nucleation. The calculations were made using the slope of the logarithmic dependence of I or τ, respectively, on supersaturation (equations 1 and 2) and the critical supersaturation which was also determined on the basis of kinetic data. Various values of σ were found for the different supersaturations. Different values for this quantity were also obtained when supersaturation was presented by the concentration ratio (S = C/C₀) or by the ratio of the product of the two ions concentrations in the supersaturated solution to that in the saturated one (S = ab/L_p). These σ values were lower than those obtained using the critical supersaturations at the two different solution pHs. For the present it is impossible to give a definite explanation of the results obtained. The experiments for determining the specific surface energy at the crystal-solution interface will be continued.     

Comparing investigation of the smectic C and A phases of the di-n-alkyl 4,4′-azoxycinnamates (I). Calorimetric investigations

We present calorimetric investigations (DSC) of 9 members of the homologous series of the di-n-alkyl 4-4′-azoxycinnamates. The phase transitions smectic C/A and smectic A/isotropic are characterized by thermodynamic changes which are discussed comparingly. With increasing length of the n-alkyl chains in the series the clearing temperatures are depressed, the S_A regions are diminshed and the character of the phase transitions C/A changes from second order to the direction of first order.     

Mode of solidification and strength properties(II). Naphthalene-benzil binary system

The tensile and compressive strengths of directionally and randomly grown samples of naphthalene, benzil, and their eutectic in the faceted-faceted benzil-naphthalene system have been determined. The tensile strength of naphthalene and the eutectic increased significantly on directional solidification. Directional solidification substantially enhanced the compressive strength of the eutetic whes rea decrease in compressiv strength was observed in case of directionally grown naphthalene. In case of benzil, no appreciable change in strength was observed on directional solidification. A comparison between the tensile and compressive strength was made. The compressive strength was 2.5–3.8 times more than the tensile strength of naphthalene, grown randomly, benzil and their eutectic grown randomly and directionally.     

Crystallization kinetics and optical band gap studies of Se96In4 glass before and after slow neutron irradiation

Results of differential scanning calorimetry (DSC) under non-isothermal condition on Se₉₆In₄ semiconducting chalcogenide glass before and after slow neutron irradiation, for different exposure times, have been reported and discussed. Some of Sn atoms have been injected into the glass by nuclear transmutation processes and the binary glass is converted into a ternary. This is accompanied by an increase in the activation energy of crystallization, E_c, and in the glass transition temperature, T_g and a decrease in the glass transition activation energy, E_t, in the onset crystallization temperature, T_c and in the peak temperature of crystallization T_p. Optical band gap measurements have also been carried out, before and after irradiation, on identical thin pellets of Se₉₆In₄ glass. The energy band gap, E_g, is found to decrease upon irradiation. These effects have been attributed to a structural change upon doping and to irradiation induced defects.     

Vibrational Properties of LiGaO2 (II). Theoretical Model Considerations

On the basis of the correlation method and of a simplified version of the extended Keating model with bond-bending forces neglected sphalerite- and chalcopyrite-like modes can be identified in the lattice vibration spectrum of LiGaO₂. In agreement with recently published results for LiInTe₂ it is found that the Li O bond is considerably weaker than the Ga O bond.     

In-situ doping of LPCVD poly silicon (II). Temperature influence

Poly silicon deposition by pyrolysis of silane under low pressure conditions has been investigated with respect to the influence of temperature when simultaneously in-situ doping of the deposited layer takes place. The growth rate of poly silicon is retarded in the presence of phosphine provided that a certain lower PH₃/SiH₄-ratio has been exceeded. It has been shown how that lower ratio depends on temperature. Increasing PH₃/SiH₄-ratio not only slows down layer growth rate but also the apparent activation energy of the layer forming reaction. An empirical equation describing the temperature dependence of that activation energy has been derived. Phosphine adsorption has been discussed as a cause of both layer growth rate and activation energy reduction. Additionally, incorporation of phosphorus during layer growth has been investigated with respect to the total amount and the electrically active concentration, the latter measured after a postdeposition anneal at 1000 °C.