Crystal growth of FeCO3 in mixed monoethylene glycol and water solvent

The transportation of natural gas in long subsea pipelines is a challenge when it comes to hydrate prevention, corrosion and mineral scaling. When monoethylene glycol (MEG) is injected into carbon steel pipelines to prevent formation of gas hydrates, the solubility of the corrosion products is altered. Understanding the kinetics of FeCO₃ precipitation may make it possible to avoid deposition in the gas liquid separation process and improve solids removal in MEG recovery units. In this work, the growth kinetics of iron carbonate (siderite) has been studied in seeded batch experiments in MEG‐water solutions with 0 and 40 wt% MEG at 50 and 70 °C. Precautions were taken to keep anaerobic conditions and avoid oxidation of ferrous ions. The growth rate (G) was measured as function of supersaturation (S) and fitted to the equation: G = k_r(S‐1)^g. The growth order (g) was approximately 2 independent of the MEG concentration at the two temperatures. The growth rate constant (k_r) was in the range of 6 × 10⁻¹¹ to 1 × 10⁻¹⁰ m/s. Temperature increase from 50 to 70°C had no measurable effect on the growth rate while in the presence of 40 wt% MEG the growth rate constant decreased.     

X‐Ray Powder Diffraction,DTA and Vibrational Studies of CdNH4PO4.6H2O Crystals

Cadmium Ammonium Phosphate Hexahydrate (CAPH) is analogous to naturally occurring struvite. CAPH crystals are grown by slow evaporation technique. These crystals are characterised by X‐ray, TG‐DTA and Infra‐red studies. Powder X‐ray pattern indicates the orthorhombic crystal structure analogous to struvite. TG‐DTA analysis suggests loss of water of hydration (6H₂O) between 113 and 391°C. Later the substance melts and only Cd remains around 780 °C. Infra Red spectrum is characteristic of H₂O, PO₄^3‐ and NH₄⁺ radicals.     

The effect of manganese additions on the structure changes proceeding in rapidly solidified Al-Cu alloys

The effect of Mn additions (0… 2 wt%) on the decomposition of rapidly solidified Al-4.0 wt% Cu alloys (cooling rate 10³ to 10⁴ K/s; LQ treatment) were studied during ageing between RT and 450 °C by hardness, X-ray methods and electron microscopy. The results were compared with alloys homogenized in the region of the solid solution (SQ treatment). (i) The LQ treatment results in a quite better homogeneous distribution of the alloyed elements than the SQ one, that is less particles of intermetallic phases are present in the ascast state. (ii) At T < 250 °C Mn additions affect the decomposition kinetics by trapping of vacancies (retardation) and the diminution of the solubility of Cu atoms (acceleration). The first effect dominates in the stage of G.P. zone formation, the second one during precipitation of intermediate phases. (iii) At T ≧ 300 °C the intermetallic compound Cu₂Mn₃Al₂₀ forms associated with a significant increase of the hardness.     

Kinetic studies on the influence of temperature and growth rate history on crystal growth

Crystallization experiments of sucrose were performed in a batch crystallizer to study the effect of temperature and growth rate history on the crystal growth kinetics. In one of the growth methods adopted, the isothermal volumetric growth rate (R_V) is determined as a function of supersaturation (S) at 35, 40 and 45 ºC. In the other, crystals are allowed to grow at constant supersaturation by automatically controlling the solution temperature as the solute concentration decreased. Using the latter method R_V is calculated as the solution is cooled. The obtained results are interpreted using empirical, engineering and fundamental perspectives of crystal growth. Firstly, the overall activation energy (E_A) is determined from the empirical growth constants obtained in the isothermal method. The concept of falsified kinetics, widely used in chemical reaction engineering, is then extended to the crystal growth of sucrose in order to estimate the true activation energy (E_T) from the diffusion‐affected constant, E_A. The differences found in the isothermal and constant supersaturation methods are explained from the viewpoint of the spiral nucleation mechanism, taking into account different crystal surface properties caused by the growth rate history in each method. Finally, the crystal growth curve obtained in the batch crystallizer at 40 ºC is compared with the one obtained in a fluidized bed crystallizer at the same temperature. Apparently divergent results are explained by the effects of crystal size, hydrodynamic conditions and growth rate history on the crystallization kinetics of sucrose. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Kinetics of crystal growth of strontium tungstate from solutions in sodium tungstate melts by continuous cooling

The crystallisation kinetics of strontium tungstate from unstirred saturated solutions in sodium tungstate melts was studied by continuous cooling from initial crystallisation temperatures T₀ = 1000° to 800°C to room temperature at cooling rates R_T = 0.67° to 3.3°C min⁻¹. The main crystal growth was diffusion rate-controlled; the final crystal growth was rate-controlled by the development rate of excess solute concentration. The estimated diffusion rate constant (k_D) values increased with cooling rates and initial crystallisation temperatures. They are higher than the rate constants for diffusion-controlled growth of calcium tungstate from sodium tungstate melts, but very much smaller than those for strontium tungstate from lithium chloride melts.     

Speciation of Fe(II) and Fe(III) effect on CaCO3 crystallization

The present work was carried out to investigate separately the effect of Fe²⁺ and Fe³⁺ on the precipitation kinetics and the microstructure of CaCO₃. For this an experimental procedure was proposed. Precipitation tests were made by using the dissolved‐CO₂ degassing method. Both air and nitrogen were employed to strip the CO₂ from a Ca(HCO₃)₂ solution initially rich in this gas. At anoxic medium, it was shown that iron (II) prolongs the nucleation step and decelerates the crystalline growth rate. X‐ray diffraction analysis shows that its presence inhibits calcite and promotes aragonite variety. By using air, the reaction medium is rich in oxygen and iron (II) is rapidly oxidized. Seeing the higher solution pH (> 6.5), iron hydroxide forms before the onset of CaCO₃ precipitation and plays a role of seed permitting to initiate CaCO₃ nucleation. So, contrary to the observed effect of iron (II), the presence of iron (III) accelerates the precipitation rate of CaCO₃. As for iron (II), iron (III) inhibits calcite formation but favored the vaterite variety instead of the aragonite one.     

Growth behavior of CuPc films by physical vapor deposition

Various Cu‐phthalocyanine (CuPc) films were grown from physical vapor deposition on top of indium‐tin‐oxide glass substrates by controlling substrate temperature (T_sub), source temperature (T_sou), and growth time. From side‐view SEM pictures, the growth rates for these CuPc films are estimated and can be categorized into three regions. From the Arrhenius plot of growth rate versus 1/T_sub, the activation energy E_A can be obtained. As T_sou = 390 °C, for region (A) with T_sub < 140 °C, the growth of CuPc films is dominated by the adhesion process with E_A = 810 meV. For region (B) with 140 °C < T_sub < 320 °C, the growth is then limited by the steric character associated with the organic molecular solids with E_A = 740 meV. For region (C) with T_sub > 320 °C, the re‐evaporation of the CuPc adhered molecules from the interface becomes dominant. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Precipitation of nesquehonite from homogeneous supersaturated solutions

The homogeneous (unseeded) precipitation of nesquehonite (MgCO₃·3H₂O) was studied over the temperature range of 10‐40 °C. Precipitation was triggered by the supersaturation created by mixing MgCl₂ solution (0.5‐1.5 M) with Na₂CO₃ solution in the same concentration range. The Meissner's method was adopted in the calculation of supersaturations during the MgCl₂‐Na₂CO₃ reaction to monitor the precipitation. Solids were identified using X‐ray diffraction (XRD) analysis and scanning electron microscope (SEM) images. In the temperature range of 10‐40 °C, MgCO₃·3H₂O with needle‐like or gel‐like morphology was precipitated. It was seen that the length, width and surface smoothness of the particles changed with reaction temperature and supersaturation. The supersaturation (S) was in the range of 1.09‐58.68 during titration of Na₂CO₃ solution. The dimension of the crystals increased with longer addition time (or lower initial concentration of reactant) at the same temperature. Slower addition via titration of 2 h followed by 2 h of equilibration at 40 °C proved successful in producing well developed needle‐like MgCO₃·3H₂O crystals of 30‐50 μm long and 3‐6 μm wide. MgCO₃·3H₂O obtained were calcined to produce highly pure magnesium oxide (MgO) at 800 °C. The morphology of MgO was similar to that of their corresponding precursors. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Crystallization of glaserite from aqueous solutions

Crystallization of glaserite (double salt of potassium and sodium sulphate) from aqueous solutions was studied by determining solubility, metastable zone width, growth and dissolution kinetics. Solubility measurements confirmed the presence of a large range of mother liquor compositions from which the precipitation of glaserite at about 73 – 76 wt% K₂SO₄ is obtained: inside this region the solubility lines resulted curvilinear. The metastability zone is quite large, ranging from 14 to 23 °C, and mainly increases by increasing the Na₂SO₄ content in the mother liquor. The growth rate of glaserite is second order, slightly size dependent and primarily controlled by diffusion as for K₂SO₄ but both growth and dissolution constants are quite lower than those for K₂SO₄.     

The Precipitation of Sparingly-Soluble Alkaline-Earth Metal and Lead Salts with Slow Development of Supersaturation (Slow Anion and Hydroxyl Ion Addition): Kinetics of Nucleation and Induction Periods

Barium and lead sulphate and chromate precipitations were studied at 20° to 95°C at precipitation rates varying from 10⁻³ to 10⁻⁷ ion l⁻¹ sec⁻¹: the supersaturation was developed by slow direct addition of anion to metal nitrate solution and by neutralisation of equivalent metal salt in excess acid solution. Slow heterogeneous nucleation occurred onto particles dispersed within the aqueous solution. The nucleation rate at any time was Where k_n is the rate constant for heterogeneous nucleation, N_∞ is the maximum number of potential nuclei, n_t is the number of nuclei after time t, I. P_t is the ionic product (C_mt)(C_mt) and 2π is the number of metal salt ions in the critical nucleus, generally eight. Crystal growth started after induction periods (t̄) at times just after the times for maximum rate of formation of nuclei. The induction periods (t̄) for precipitations from solutions of initial cation concentration ^cM₀ varied with precipitation rate (R) according to the relation, where γ = π/(π + 1) and k₁ (the unit reciprocal induction period) = . Nucleation rate constants for different precipitations were estimated from the k₁ values and are tabulated. For slow precipitations by direct anion addition, the rate constants were lower for precipitation from solution of the salt of greater solubility. The rate constants for slow precipitation of metal chromates from acid solution were far lower than those for slow precipitation by direct chromate addition. Rate constants decreased somewhat with rise in precipitation temperature.     

The Description of the Isothermal Decomposition Processes in Al?Sc Alloys by means of Electric Resistivity Measurements and TEM Investigations

The decomposition behaviour of two AlSc alloys (c_Sc = 0.18; 0.36 at.%) after direct quench to room temperature and subsequent ageing at temperatures in the range 200 °C ≦ T_a ≦ 550 °C was investigated by means of isothermal resistivity measurements and TEM. In the temperature range 325 °C ≦ T_a ≦ 400 °C both alloys show independent of the Sc content an “inversion” of the decomposition kinetics, obviously caused by the transition of the coherent into the incoherent Al₃Sc phase.     

Growth Kinetics of Adenine Sulphate Crystals

The kinetics of growth and dissolution of adenine sulphate in 1.5 mol · 1⁻¹ H₂SO₄ at different super- and undersaturations has been studied. The solubility of adenine sulphate in H₂SO₄ was determined over the temperature range 20–50 °C. The growth rate of the (100) face is by about 25% higher than that of (111) and the crystal habit is changed by a high supersaturation almost to a bipyramide. The surface diffusion was determined as a prevailing processes and the BaS model gives the best fit with experimental data.     

An Isoperibolic Reaction Twin Calorimeter for Investigating Precipitation

An isoperibolic reaction twin calorimeter is described and its application to studying precipitation is shown. This technique is suitable for determining both the total reaction enthalpy and also, in specific ranges of supersaturation, kinetics of the process. The total enthalpy of BaSO₄ and CaC₂O₄ · H₂O precipitation at 25 °C was determined as −16.2 and −18.1 kJ mol⁻¹, respectively. The spontneous precipitation of these compounds affected by mixing reactant solutions together proceeds through initial surge of nucleation and fast crystal growth that steeply diminishes. No major wave of secondary nucleation during precipitation was observed.     

Effect of sulphate ions on iron precipitation from aqueous solutions

In the present work, the effects of sulphate ions on the iron precipitation from aqueous solution were investigated. It was shown that sulphate ions delayed the iron precipitation when this ion was added in form of Na₂SO₄. This effect became less significant in presence of magnesium or calcium. The iron precipitates were characterized by X‐ray diffraction (XRD), scanning electron microscopy (SEM) and differential scanning calorimetry (DSC). In all experiments iron oxide hydroxide (FeOOH) precipitates were obtained. The sulphate ions were adsorbed on the surfaces of the iron precipitates. The effect of temperature on these precipitates was also studied. At 237 °C, the iron oxide hydroxide precipitates obtained from NaCl solution was transformed in crystallized hematite, Fe₂O₃. At 793 °C, the hematite was partially transformed into magnetite (Fe₃O₄). In presence of sulphate ions, this transformation was not detected. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Impurity adsorption mechanism of borax for a suspension growth condition: A comparison of models and experimental data

A fluidized bed crystallizer is employed to investigate the growth and dissolution rates of MgSO₄·7H₂O from aqueous solutions in the presence of borax as impurity at 25°C. By adding 0.5, 1, 2 and 5 wt % of impurity the pH value changes from 6.7 to 7.11, while the saturation temperature shifts to 24.8, 24.4, 24 and 23.1°C, respectively. The data on crystal growth rates from aqueous solutions as a function of impurity concentration are discussed from the standpoint of Cabrera and Vermileya, and Kubota and Mullin. The value of the impurity effect, αθ_eq, determined from analysis of the data on growth kinetics was found to be in good agreement with the value obtained from direct adsorption experiments. The estimated value of the average spacing between the adjacent adsorption active sites and the average distance between the neighbouring impurity‐adsorbed sites are also reported. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Study on crystallization kinetics and the crystal internal defects of HMX

In this paper, the solubility data of HMX (1,3,5,7‐tetranitro‐1,3,5,7‐tetrazocane) in acetone from 323.15 K to 293.15 K were accurately measured by use of the laser‐monitoring observation technique. Intermittent dynamic method was utilized to study crystallization kinetics of HMX in acetone. The data of crystallization kinetics were obtained by moment analysis, and the parameters of the growth rate and nucleation rate equations were derived by using multiple linear least squares method. Subsequently, growth rate and nucleation rate at different conditions were calculated according to these equations. In addition, Optical Microscopy Images (qualitative) and Particle Apparent Density (quantitative) experiments were applied to study the crystal internal defects of HMX under different crystallization conditions. It can be found that the crystal apparent density of HMX is in the range of 1.8993 g·cm⁻³ to 1.9017 g·cm⁻³, very close to the theory density of HMX; the internal defects and the crystal size do not increase after 25 °C, from which we predict that the HMX crystal growth reaches the steady growth segment. These results suggest that the nucleation rate is a significant factor influencing the crystal internal defects, and larger nucleation kinetics can reduce crystal internal defects.     

Evolution of crystal structure of Zn:N films under high temperature

Zinc‐nitrogen (Zn:N) compound thin film was prepared from a pure metallic Zn target by rf magnetron sputtering at ambient temperature under the mixture of nitrogen and argon gases with the ratio of 1:1. High temperature x‐ray diffraction (HTXRD) measurement under vacuum was used to examine the evolution of structural properties of the Zn:N film. At ambient temperature, the (002), (100), and (101) planes corresponding to Zn structure were observed while at higher temperature, the left shifts corresponding to the increase of lattice constants a and c of Zn were observed. At temperatures of 320 °C, 481 °C and 554 °C, the (222), (321) and (400) planes corresponding to Zn₃N₂ structure were observed with the decrease in the intensity amplitudes of the peaks belonging to the Zn structure. The results indicate the gradual transformation of the Zn₃N₂ phase in the Zn:N films at temperature greater than 320 °C. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Determination of the diffusion coefficient of arsenic in Ga solution from growth rate measurements of LPG GaAs

On the basis of the LPE GaAs growth rate measurements in which we used current marks of time, the diffusion coefficient and the critical supercooling of arsenic in Ga solution by a semiempirical method was determined. Some knowledge of these parameters is indispensable for investigating the growth mechanism and kinetics of epitaxial layers from high temperature solutions. Taking into account the fact that the diffusion coefficient as a function of temperature is of the form D(T) = D₀ · exp (–B/T), the results were extrapolated in a range of temperatures 750–950°C typical for liquid epitaxy of GaAs.     

Effects of impurity ions on solubility and metastable zone width of phosphoric acid

Effects of two typical impurity ions as Fe²⁺ and SO₄^2– in phosphoric acid on solubility and metastable zone width (MZW) had been studied. It was shown that polythermal method was suitable for solubility measurement in this study. The solubility experimental data was correlated with a polynomial equation. Being compared with Fe²⁺, SO₄^2– had a greater impact on the phosphoric acid solubility; and the variation in phosphoric acid MZW was just opposite. Phosphoric acid apparent secondary nucleation order was calculated to be 3.50 by means of modified regression method. And also the influence of impurity ions on apparent secondary nucleation order was investigated in this paper. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effects of cadmium on crystallization of calcium phosphates

The precipitation of calcium phosphates in the presence of increasing cadmium amount was studied at 25 °C in dilute ammoniacal solutions ([Ca] = [P] = 0.005 M). An amorphous precipitate, an apatite‐like calcium phosphate and the compound Cd₅H₂(PO₄)₄·4H₂O are found according to pH and Cd concentration. The nucleation of hydroxyapatite is greatly delayed by cadmium, which influences also its crystallinity, but the effect tends to vanish with time. The role of cadmium is discussed. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)