Precipitation kinetics of sodium bicarbonate in an industrial bubble column crystallizer

Sodium bicarbonate is a substance which is produced in the middle stages of the soda ash production process. In this precipitation process, carbon dioxide gas is continuously injected into the bubble column reactor which contains carbonate and bicarbonate solutions. To elucidate the key parameters affecting precipitation kinetics, an experimental study was conducted to understand nucleation and growth in conditions of industrial reactor. The composition of the solution is followed during the crystallization process by titration. Magma density is also monitored and crystal size distribution (CSD) is obtained by sieving. The method of moments was used to determine nucleation and growth rates of crystals. The nucleation and growth rate correlations for sodium bicarbonate precipitation in industrial scale were correlated by empirical power laws as B = 26.685M_T^0.42Δw^1.31 and G = 1.381×10^–4Δw^1.53. The nucleation and growth rate correlations obtained in this study can be used to simulate the crystallization of sodium bicarbonate plants. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Kinetics of precipitation of nickel ammonium sulphate hexahydrate from aqueous solutions

The precipitation of nickel ammonium sulphate [Ni(NH₄)₂(SO₄)₂·6H₂O] prepared by mixing of equimolar aqueous solutions of (NH₄)₂SO₄ and NiSO₄ has been studied at 25 °C. Precipitation induction periods and the number of crystals formed per unit volume have been determined experimentally as functions of supersaturation for values of S_{a, H} < 2.6. The mechanisms of nucleation and crystal growth are assessed by comparing the experimental results with theoretical predictions. In this system, heterogeneous nucleation and crystal growth are assessed by comparing the experimental results with theoretical predictions. In this system, heterogeneous nucleation appears to operate at S_{a, H} < 1.3, whereas at S_{a, H} > 1.6 crystals are generated predominantly by homogeneous nucleation and grow by a polynuclear mechanism. The interfacial tension of nickel ammonium sulphate crystals in contact with their supersaturated solution is estimated to be about 20 mJ/ m².     

Precipitation of Strontium Sulphate

Precipitation of unseeded supersaturated aqueous solutions of SrSO₄ and growth of SrSO₄ crystals at the size of approximately 20 μm were studied. Within the examined range of supersaturations, i.e. S_a < 16, the nucleation in aqueous solutions is controlled by the heterogeneous nucleation mechanism. The induction period of precipitation depends on the rates of both the nucleation and growth of the formed nuclei. The growth of SrSO₄ crystals in supersaturated solutions at S ⪅ 5.6 is of the 2nd order with respect to the concentration and at temperatures 25 ÷ 45°C the process is controlled by the surface reaction mechanism. The number of crystals in the precipitated slurry is decreasing rapidly as a result of the Ostwald maturing.     

Effect of Ca on HoPO4·nH2O (n = 1 and 2) crystallization from phosphoric acid solution

Ca‐substituted holmium phosphate, isomorphic with tetragonal form of HoPO₄·H₂O was synthesized by crystallization from boiling phosphoric acid (2 M H₃PO₄) solution containing 0.02M of Ho and 0–0.2 M of Ca. Calcium concentration, higher than 0.2 M Ca in solution resulted in biphasic solid crystallization: tetragonal HoPO₄·H₂O and orthorhombic HoPO₄·2H₂O. Ca incorporation in the solid according to substitution mechanism: Ca²⁺ + H⁺ ↔ Ho³⁺ was limited to ∼3 wt.% and was coupled with simultaneous incorporation of HPO₄²⁻. Ca for Ho substitution caused an expansion of the tetragonal unit cell of HoPO₄·H₂O, resulted from differences in the ionic radii (r_Ca > r_Ho). Effects of thermal treatment at 900 °C were as follows: (i) the orthorhombic admixture of HoPO₄·2H₂O re‐crystallized into tetragonal anhydrous HoPO₄, (ii) Ca–at first dissolved in crystal structure of HoPO₄·H₂O was expelled from it during re‐crystallization to form Ca(PO₃)₂, and that was associated with a contraction of the unit cell; a ‐ and c‐ axes went down to the level of Ca‐free anhydrous tetragonal form of HoPO₄. (iii) HPO₄²⁻ present in the solids as prepared underwent condensation according to reaction 2HPO₄²⁻ → P₂O₇⁴⁻ + H₂O. Scanning electron micrographs revealed significant changes in size and morphology of the crystals ranging from spherical globules of HoPO₄·H₂O formed in Ca‐free H₃PO₄ with increasing diameter in the presence of lower Ca concentration to rod‐like crystals organized in bundles resembling the “scheaf of wheat”, while crystallized from phosphoric acid solution with higher than 0.2 M Ca.     

Experimental determination of the solid–liquid equilibrium,metastable zone,and nucleation parameters of the flunixin meglumine–ethanol system

Measurements of the metastable zone and solubility for flunixin meglumine–ethanol system were obtained. The solubility was measured within the temperature range from 288.15 to 328.15 K. The mole fraction solubility was correlated satisfactorily with the temperature by the equation: x_eq=2.35×10^?12e^0.07121T. The value of enthalpy of dissolution, enthalpy of fusion and enthalpy of mixing were determined to be 49.04, 64.03 and ?14.99 kJ mol^?1 respectively. The metastable zone width of flunixin meglumine was measured by an electric conductivity method. A comparison of the nucleation temperatures from electric conductivity measurement and from focused beam reflectance measurement (FBRM) shows that both detection techniques give almost the same results for flunixin meglumine. The nucleation parameters of flunixin meglumine in ethanol were determined from the metastable zone data. Over the equilibrium temperature range from 312.28 to 325.55 K, the nucleation rate constant was varied from 0.00001 to 0.00120 #/m² min, whereas the nucleation order was varied from 2.23022 to 3.39299. The obtained high values of nucleation order indicated a high rate of nucleation.     

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.     

Kinetics and thermodynamics of ferrous-ferric equilibrium in sodium aluminoborate glasses

The oxidation-reduction equilibrium between ferrous and ferric iron in sodium aluminoborate glass melts having molar composition 15Na₂O·10Al₂O₃·75B₂O₃ has been studied as a function of temperature. The reaction was observed to follow first order kinetics. The energy of activation for the above reaction was calculated. Equations previously suggested to represent the Fe²⁺–Fe³⁺ equilibrium were discussed and a new equation is proposed. The enthalpy of the reaction was ?8.80 kcal mol^?1.     

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.     

Kinetics of sodium fluosilicate precipitation

The formation of Na₂SiF₆ by discontinuous precipitation of dilute H₂SiF₆ with a 40% excess of an aqueous solution of NaCl under various conditions was studied. The values of induction time, number of crystals formed, their final size and their habit were determined during precipitation from solutions, whose initial supersaturation was 2 < S < 11. At S < 7–8 the crystals were formed by heterogeneous nucleation, whereas at S ≳ 7–8 homogeneous nucleation mechanism began to prevail. Once formed, the Na₂SiF₆ crystals were growing according to the screw-dislocation mechanism till they reached visible size; the corresponding values of kinetic order of nucleation and of the growth rate constant were g = 1.35 and k_g = 4.32 × 10⁻⁸ cm^2.05 sec⁻¹ g^−0.35, resp. The value of interfacial tension on the phase boundary Na₂SiF₆ crystal — saturated solution was determined (σ ∼ 52 erg/cm²). The resulting Na₂SiF₆ crystals conformed to log-normal distribution irrespective of conditions of precipitation. The dependence of the final size of crystals on supersaturation exhibited a maximum at S ∼ 6. Crystals of Na₂SiF₆ had a hexagonal habit, which was near to a spherical form at lower supersaturations, while dendritic crystals were formed at higher supersaturations.     

Thermodynamische Berechung des Systems Ge/H2O/H2 und experimentelle Untersuchung mit der Nahabstandsanordnung

In the temperature range from 950 to 1200 K K_p-values result from 5 · 10⁻⁵ to 6,9 · 10⁻³ for the heterogeneous reversible reaction Ge_(s) + H₂O_(g) ⇋ GeO_(g) + H₂O_(g), the average reaction enthalpy being 46,5 ± 05 kcal/mol. Etching rates calculated with these equilibrium constants for closed systems are 20-25% larger than the experimental etching rates for (110)-Ge in the sandwich device. The ratio of the etching rates is for (111)-, (100)- and (110)-Ge 1:1,4:1,8; the average reaction enthalpy calculated experimentally is 46 ± 1 kcal/mol for the temperature range 1000-1200 K. The dependence of transport rate on distance between source and substrate characterizes the sandwich device as a quasi-closed system with the diffusion as the rate controlling step of the material transport.     

Solvothermal synthesis and growth mechanism of asymmetric ZnO hierarchical structures in diethylene glycol with ammonia solution

Several novel asymmetric ZnO hierarchical structures were synthesized in diethylene glycol (DEG) with different amount of ammonia solution via solvothermal process. The submicron‐rods or nanorods as the building units are distributed on the two sides in an asymmetric manner, resulting in formation of the shiitake‐like or bouquet‐like morphology. A possible formation mechanism was proposed on the basis of the experimental result. The consumption of the precursor could lead to a two‐step nucleation and growth process. The relative content of the [Zn(OH)₄]²⁻ and the [Zn(NH₃)₄]²⁺ precursors varies with the amount of ammonia solution, which affect the size and morphology of the asymmetric structures. The [Zn(OH)₄]²⁻ complex and the [Zn(NH₃)₄]²⁺ complex are absorbed on the positive (0001)‐Zn polar surface and the negative (000–1)‐O polar surface respectively, which lead to the alteration of growth rate of these polar surfaces.     

Crystallization of potassium sulphate from glaserite

The crystallization of K₂SO₄ from aqueous solutions of glaserite (a double salt of sodium and potassium sulphate) was investigated in an MSMPR cooling apparatus at 35 °C. The precipitation of K₂SO₄ at an average purity of 93 wt.% was obtained and the crystallization kinetics were derived from the experimental data. The nucleation and growth rates here obtained were, respectively, remarkably higher and slightly lower than those relevant to pure K₂SO₄: this behaviour may be explained by a faster nucleation and a slower growth of crystals containing also Na₂SO₄. The effect of small (0.05%) calcium additions to the feed was also tested, which caused an increase of supersaturation and a depression of the growth kinetics, whereas the nucleation one was not appreciably modified.     

Some Factors Influencing the Rate of Heterogeneous Nucleation of Strontium Sulphate

The effect of the agitation intensity on nucleation, agglomeration, ripening, and recrystallization of SrSO₄ crystals prepared by batch precipitation of equimolar solutions of SrCl₂ and Na₂SO₄ (0.01, 0.015, and 0.02 M) at 25 °C was investigated. The dependence of the SrSO₄ crystals shape and number on the time of the suspension ageing was also studied.     

Crystal and Molecular Structure of 1-Ethyl-3-[tris(trimethylsiloxanyl)silyl]pyrrolinium Hydrochloride

The crystal and molecular structure of 1-Ethyl-3[tris(trimethylsiloxyl)silyl]pyrrolinium hydro-chloride (C₁₅H₃₈N⁺O₃Si₄ · C¹⁻) has been determined by direct methods. The title compound crystallizes in the monoclinic space group C2/c with a = 20.640(3), b = 19.494(2), c = 27.34(3) Å, β = 90.60(4)°, V = 11000(13) ų, Z = 16, D_x = 1.034 Mg m⁻³. There are two molecules with different conformations in the crystal. The pyrroline rings are non-planar.-The Si O Si angles range from 149(1)° to 163(1)°. Two of the SiMe₃ groups are disordered. All molecules are connected by C¹⁻ – N⁺ contacts and C¹⁻ - HN⁺ hydrogen bonds to form double chains.     

A study of the vacancy-defect distribution in a GaAs/AlxGa1−xAs multi-layer structure grown at low temperature

We have grown a multilayer structure of GaAs and Al_xGa_1−xAs (x=0.25) by molecular beam epitaxy at low substrate temperature (250°C) in order to investigate the effects of annealing on arsenic precipitation and vacancy-defect formation. The as-grown heterostructure contained ∼5×10¹⁷ cm⁻³ gallium vacancies, but information about the individual layers was lost because the layer width (∼45 nm) was smaller than the average positron diffusion length (∼70 nm). Annealing at 500 and 600°C showed increases in the S-parameter (above the bulk value) of ∼2.5% and ∼1.5%, respectively, smaller than for a single LT-GaAs layer (∼3–4%). We explain this phenomenon by suggesting that the excess arsenic which migrates from the material with the higher precipitate/matrix energy (LT-Al_xGa_1−xAs) reduces the gallium vacancy concentration in the LT-GaAs and hence the S-parameter. This hypothesis is supported by SIMS data which shows a build-up of As in the LT-GaAs layers, and TEM images which indicate that arsenic precipitation occurs to a greater extent in the LT-GaAs than in the LT-Al_xGa_1−xAs.     

Trap-limited diffusion of hydrogen in precursor derived amorphous Si–B–C–N-ceramics

The tracer diffusion of hydrogen is studied in precursor derived amorphous Si–C–N and Si–B–C–N ceramics using deuterium as a tracer and secondary ion mass spectrometry (SIMS). Since the amorphous ceramics are separated in carbon rich phases (amorphous carbon and amorphous C(BN)_x, respectively) and silicon rich phases (amorphous Si₃N₄ and amorphous Si_3+(1/4)xC_xN_4−x, respectively) we additionally measured the diffusivities of hydrogen in amorphous carbon, in amorphous SiC and in amorphous C–B–N films. The silicon rich phases are identified as diffusion paths for hydrogen in the precursor derived ceramics. Diffusion of hydrogen in these materials is explained with a trap limited diffusion mechanism with a single trap level. We found activation enthalpies of about 2 eV for the precursor derived ceramics, where the activation enthalpy is the sum of a migration enthalpy and a binding enthalpy. The low values for the pre-exponential factors of less than 10⁻⁷ m²/s can be explained with an appropriate expression for the entropy factor.     

Precipitation and dissolution of KCaCl3 impurity in NaCl crystals

Impurity precipitation in NaCl crystals containing CaCl₂ and KCl in various proportions is studied by means of flotation density measurements. The paper is concerned with the determination and dissolution, the kinetics and activation energy, and the mechanism of nucleation. Two temperature regions for solid solution decomposition were revealed. The characteristic stages in density variation upon isochronal annealing are shown to be correlated with radical changes in the X-ray diffraction patterns. Evidence is given that the nucleation is heterogeneous for the stable phase and homogeneous for the metastable one. – The activation energy in the low-temperature region has been found to be 0.8 ± 0.1 eV, which suggests precipitation to be diffusion-limited. From the kinetics studies it follows that the composition of the metastable phase corresponds to KCaCl₃ and the rate-controlling stage in stoichiometric crystals is a K⁺ diffusion.     

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)     

Effect of cationic impurities on solubility and crystal growth processes of ammonium oxalate monohydrate: Role of formation of metal‐oxalate complexes

The effect of different bi‐ and trivalent cationic impurities on the solubility of ammonium oxalate and the composition and distribution of chemical complexes formed in saturated ammonium oxalate aqueous solutions as a function of impurity concentration are investigated. The knowledge of the composition and stability of complexes formed in saturated aqueous solutions is then employed to explain the appearance of dead zones of supersaturation for growth and the difference in the effective segregation coefficient of the impurities. Analysis of the experimental results revealed that: (1) at a constant temperature, the dependence of concentration of complex species formed in saturated solutions on the concentration of different impurities can be described by an equation similar to that of the concentration dependence of density of solutions, (2) the dominant metal‐containing species present in saturated solutions are negatively‐charged, most stable oxalato complexes like Cu(C₂O₄)₂²⁻, Mn(C₂O₄)₃⁴⁻, Zn(C₂O₄)₃⁴⁻, Cr(C₂O₄)₃³⁻ and Fe(C₂O₄)₃³⁻, (3) in the investigated range of impurity concentration, the solubility of ammonium oxalates increases linearly with the concentration of all impurities and the increase is associated with the stability of dominant complexes, (4) appearance of dead supersaturation zones in the presence of impurities is associated with instantaneous adsorption of all growth sites by dominant oxalato complexes in relatively short adsorption time, and (5) the segregation coefficient of an impurity cation M of charge z + increases with a decrease in the solubility product constant K_sp for the hydrolysis products of reactions of the type: M^{z +} ↔ M₁^{(z −1)+} + H⁺ (where the cation M has z + charge, and H⁺ is hydrogen ion). (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The effect of cooling rate on size,quality and morphology of KTiOPO4 (KTP) crystals grown by different nucleation techniques

KTP crystals have been grown by two nucleation techniques namely spontaneous nucleation in flux medium and nucleation on Pt rod using K₆P₄O₁₃ flux.10 °C/h, 7 °C/h, 0.8 °C/h, 0.4 °C/h and 0.2 °C/h cooling rates were applied for spontaneous nucleation and crystals up to 15 × 7 × 4 mm³ in size were grown. 1 °C/h and 0.8 °C/h cooling rates were also used for nucleation on Pt rod and crystals up to 8 × 6 × 3 mm³ in size were grown. The effect of cooling rate on size, morphology and optical quality of grown crystals by both techniques were studied. For nucleation on Pt rod upper and lower rotation rates limits and an optimum rotation rate were distinguished for each cooling rate. Quality of the grown crystals by both techniques was characterized by optical transmission analysis.