The Influence of Impurities on Crystallization Kinetics of Sodium Chloride

The influence of impurities on the crystallization kinetics of NaCl was investigated in a fluidized bed crystallizer. The growth and dissolution rates were related to the supersaturation and impurity concentrations. The effect of different impurities on the growth rate of NaCl crystals can be divided into thermodynamic effects where the impurities influence the solubility and kinetic effects where the impurities will suppress the growth rate compared to the pure NaCl. A mathematical model describing crystal growth rates from aqueous solution as a function of impurity concentration is presented. The model explains impurity concentration effects on the crystal growth rate in terms of an impurity effectiveness factor and a Langmuir adsorption isotherm for the impurity.     

Inhibition of Calcium Oxalate Monohydrate Crystal Growth in High and Low Ionic Strength Solutions

The effect of additives on the kinetics of growth of calcium oxalate monohydrate crystals has been studied. Conductivity and potentiometry measurements have been compared. Growth rates were calculated from precipitate curves by a cubic spline method. An approach consisting on the calculation of rate constants and orders of reaction from logarithmic plots of growth rate versus supersaturation has been followed to study crystal growth kinetics. This method revealed that the presence of additives is causing not only a decrease on the rate constant but an increase on the order of reaction as well. The effect of additives (EDTA, citrate and phytate) was considerably weaker in high ionic strength media. Phytate produced a complete blockage of crystal growth in concentrations as low as 2 × 10^—6 mole/L in both methods.     

The influence of supersaturation on crystal morphology – experimental and theoretical study

The surface docking approach of molecular modeling for prediction of crystal morphology in the presence of additives is further developed in this work. It is modified in order to take into account the influence of supersaturation on the crystal habit. With the introduction of the BCF theory and the 2‐D growth mechanism in the above mentioned approach predictions have been carried out for paracetamol and caprolactam crystals, respectively. The verifications by crystals grown from solutions as simulated by the computer model give good agreements. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The influences of supersaturation on LPE growth of GaN single crystals using the Na flux method

The dependency of LPE growth rate and dislocation density on supersaturation in the growth of GaN single crystals in the Na flux was investigated. When the growth rate was low during the growth of GaN at a small value of supersaturation, the dislocation density was much lower compared with that of a substrate grown by the Metal Organic Chemical Vapor Deposition method (MOCVD). In contrast, when the growth rate of GaN was high at a large value of supersaturation, the crystal was hopper including a large number of dislocations. The relationship between the growth conditions and the crystal color in GaN single crystals grown in Na flux was also investigated. When at 800 °C the nitrogen concentration in Na–Ga melt was low, the grown crystals were always tinted black. When the nitrogen concentration at 850 °C was high, transparent crystals could be grown.     

Effect of organic impurities on the surface morphology and growth mechanism of KBP crystals (C8H5O4K)

The surface morphology of the (010) face of potassium biphthalate (KBP) crystals grown from aqueous solutions under the supersaturation ranging within 0.029–0.04 has been studied by the methods of optical and electron microscopies. It was revealed that the (010) surface has polygonal growth macrohills of the dislocation nature, small hillocks developing by the mechanism of successive two-dimensional nucleation, and numerous two-dimensional nuclei. The density of small hillocks (10⁴–10⁵ cm^?2) exceeds the dislocation density in KBP crystals by one to two orders of magnitude. It is shown that at low supersaturations, the (010) face grows simultaneously by the dislocation mechanism and the mechanism of successive two-dimensional nucleation. It is also established that the tangential velocity of growth-step motion on the (010) face increases in the presence of organic impurities. This effect can be used as one of the factors increasing the growth rates of crystal faces at low impurity concentrations (the so-called catalytic effect of impurities).     

The effect of crystal surface roughness on impurity adsorption

The effect of crystal surface roughness on impurity adsorption was investigated in a fluidized bed crystallizer and in a batch crystallizer. The crystallisation of sucrose in pure and impure systems was the study subject. Calcium chloride was utilized as impurity in this work. The results show that the impurity adsorption is growth rate dependent and is strongly influenced by the crystal surface properties. Crystals with high surface roughness have lower impurity adsorption. Based on experimental evidences, a new theoretical model is proposed to quantify the surface roughness influence on the impurity adsorption, allowing, by operating at the more adequate supersaturation, to control the impurity transfer into crystals. The used impurity does not have a significant influence on the growth rates at the studied temperatures. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Measurement and analysis of the dextran partition coefficient in sucrose crystallization

The effect of crystallization conditions on the dextran partition coefficient between impure syrup and sugar crystal has been investigated in a batch crystallizer. The crystallizer is operated isothermally at temperatures of 30, 40, and 50 °C, at constant relative supersaturations of 0.05, 0.07, and 0.09, and with mother liquor dextran concentrations of 1000 and 2000 ppm/Brix. The dextran content has been determined by the CSR method. A 1:1 mass ratio of high-fraction dextran (approximately 250,000 Da) and low-fraction dextran (60,000-90,000 Da) is used to represent a wide range of dextran contamination. It is seen that the dextran partition coefficient in sucrose crystallization increases with both increasing supersaturation and increasing crystallization temperature. However it appears that these are secondary effects, with the partition coefficient strongly correlating with crystal growth rate alone, despite the regressed data having large variations in temperature, mother liquor dextran content, and supersaturation. Dextran incorporation into the sugar crystal results from both dextran adsorption onto the crystal surface and mother liquor inclusions. The explanation for the variation in the dextran content in sugar crystal with respect to the growth rate is due to increased adsorption due to the higher surface roughness of crystals grown at high growth rates. Although the dextran concentration in the solution affects the dextran content in the crystal, it does not strongly affect the dextran partition coefficient.     

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)     

Crystallization of zinc lactate in presence of malic acid

The influence of malic acid, which acts as an impurity on the cooling crystallization of zinc lactate is investigated in this paper by monitoring the relative supersaturation and the number of crystals during crystallization. The presence of malic acid increases the solution solubility and makes the metastable zone wider; it also changes the habit of the crystal. The purity of the final products is shown to be influenced by the amount and size of seed crystals, cooling rate, seeding temperature and final temperature, but appears to depend mainly on the particle size and level of supersaturation. Residual supersaturation thresholds are observed that depend on the final temperature. A model is proposed to predict the steady-state supersaturation value from the final temperature at a given impurity concentration. This model is based on Kubota and Gibbs equations.     

Growth of Crystals in Presence of Impurities. A Hypothesis based on a Kinetic Approach

A hypothesis is proposed concerning the growth of crystals in the presence of impurities soluble in the parent phase but insoluble in the crystal. The hypothesis is based on: (i) a kinetic model developed by Bliznakov, where it is assumed that the impurity layers exert a passive resistance towards the growth of crystals, (ii) results obtained by Kaischew concerning the nucleation of crystals on a foreign substrate, according to which the heterogeneous nucleation is more favourable than the homogeneous one, in the presence of the same supersaturation, and (iii) a statement of Distler and co-workers that a foreign intermediate layer contains “single crystal” structural information about the crystal face where the layer has been formed.     

Growth Mechanism and Twinning of Sucrose Effected by Surface Adsorption Processes

Growth experiments with sucrose dependent on temperature and supersaturation of solutions were carried out to find the conditions for appearance of the three different growth types. Measurements of growth rates of {110} and {1 1 0} faces at 50°C and supersaturation c_s = 10% resulted in the known feature that in pure solutions a crystal grows faster in positive b-direction than in the negative one. Addition of raffinose stopped growth of {110}. In an electric field generated by 20 kV direct voltage (crystal orientation: negative pole of b-axis pointed towards the negative electric pole) the crystal grew in negative b-direction three times faster than in the positive one. In the presence of raffinose the growth rates in both directions become equal. If the crystal is turned by 180° in the solution (pure solution and in the presence of raffinose) the growth behaviour is the same as without an electric field. These results can be interpreted in such way that in pure solutions sucrose molecules adsorb to {1 1 0} (bonds exist between the fructose rings of molecules in liquid and solid phases) and that in presence of raffinose these molecules adsorb to {110} (bonds exist between glucose (solid) and galactose rings (liquid phases)). By the electric field the adsorption is hindered. The theory is in accordance with the fact that crystals situated in a suspension of the solution can intergrow with a host crystal in twin positions (in pure solutions on {1 1 0} and in the presence of raffinose on {110}).     

Study of inclusions and growth bands in potassium bichromate crystals grown from aqueous solutions

The nature of inclusions and growth bands in potassium bichromate crystals obtained from aqueous solutions and their formation in relation to growth temperature and supersaturation are investigated. It is observed that impurities present in solution, fluctuations in growth temperature and high growth rates are responsible for their formation.     

Understanding the growth mechanism of CuI crystals during gel growth experiments

Cuprous iodide crystals have been grown with decomplexation method in silica gel. Various crystal morphologies, such as polycrystalline aggregate, skeletal, dendrite, hopper crystals and regular tetrahedron crystals, were observed in different growth regions with an optical microscope. Their surfaces were photographed using a binocular metallographic microscope and the results were explained with the crystal growth mechanism which was determined by supersaturation. These observations support the general hypothesis that the concentration of reactant affects the relative growth rate by controlling the nucleation and diffusion. The mutual influence of the crystals grown in different regions was also discussed. Additionally, the suitable condition for getting regular tetrahedron crystals or large hopper crystals was obtained by changing the concentration of CuI·HI complex in the later period of crystal growth. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth rate of single acetaminophen crystals in supersaturated aqueous solution under different operating conditions

Single crystal growth experiments were conducted to investigate the growth rate of acetaminophen (paracetamol) crystals under carefully controlled conditions of temperature, supersaturation and pH value. The relative growth rate of the different faces was studied. Growth Rate Dispersion (GRD) was observed by studying the effect of supersaturation and temperature on the growth rate. The growth rate in all directions was found to be temperature dependant. Elongation of the crystals was measured due to the faster growth in the Y‐ direction and Xa direction compared to in the Xb direction. Dissolution of the crystals was observed at pH values greater than 8.5, and a growth rate retardation was observed when increasing the pH from 5.6, which is the normal value without additions, to higher values in both acidic and basic mediums up to 8.5. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth kinetics of ammonium oxalate monohydrate single crystals from aqueous solutions containing Co(II) and Ni(II) impurities

The growth kinetics of different faces of ammonium oxalate monohydrate (AO) single crystals from aqueous solutions containing different concentrations of Co(II) and Ni(II) ions at a constant temperature are described and discussed. It was found that: (1) at a given supersaturation σ, both Co(II) and Ni(II) ions lead to a decrease in the growth rates R of different faces of AO crystals, (2) the growth of a particular face of the crystals occurs above a critical supersaturation σ_d, but there is also another supersaturation barrier σ* when the rate abruptly increases with σ, and (3) the values of σ_d and σ* increase with increasing concentration c_i of the impurity. The experimental R(σ) data for different concentrations c_i of the impurities were analysed according to the model involving complex source of cooperating screw dislocations and concepts of instantaneous and time‐dependent impurity adsorption. Analysis of the data showed that: (1) adsorption of Co(II) and Ni(II) impurities occurs on the surface terrace of AO crystals, (2) there is a simple relationship between Langmuir constant K and the impurity concentration c_i* corresponding to maximum surface coverage, and (3) the ratio σ_d/σ* of the supersaturation barriers observed in the presence of both impurities increases with an increase in impurity concentration c_i, and may be explained from the standpoint of the mechanism of adsorption of impurity particles at kinks and ledges. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of electrical field on dendritic growth of boric acid

Growth rates of boric acid crystals have been measured both in pure aqueous solution and under the influence of electrical field as a function of supersaturation and electrical field intensity in a laboratory scale fluidized bed crystallizer at the temperature of 30°C. The effect of electrical field was estimated from the growth rate data to evaluate the relative magnitude of two resistances, diffusion and integration. In absence of electrical field, the obtained results indicate that the controlling mechanism is mainly integration. However, in presence of electrical field, growth rate of boric acid were controlled by diffusion and reaction steps. In the absence of electrical field, boric acid crystals grow dendritically at any level of supersaturation from pure boric acid solution. In case of electrical field, the dendritic structure was depressed.     

A new method of enlarging the cross-section of KDP using ADP crystals as seed

A new method of enlarging the cross-section of KDP crystals using an easily enlarged ADP crystal as seed for a KDP crystal, has been investigated. The growth characteristics of the ADP seeds (Z-plate, cap, and parallelly spliced seeds) at KDP supersaturation have been observed. Chemical etching was used to characterize the grown crystals, and chemical analysis to determine the composition of both the region of the seed and the grown crystal. The mechanism of this kind of growth is discussed. In conclusion this growing method should be qualified for enlarging the cross-section of KDP crystals.     

Crystallization and Characterization of Orthorhombic β‐MgSO4 · 7H2O

In solution, the growth rate and the crystal habit are influenced by a number of factors such as supersaturation, temperature, pH of the solution, cooling rate, agitation, viscosity, initial state of the seed crystal and the presence of impurities. The crystallization of orthorhombic β‐MgSO₄ · 7H₂O, from low temperature aqueous solution by slow cooling process was studied. The metastable zone width, the induction periods (τ) for different supersaturations and the effect of pH on the growth rate of the crystals were investigated. The increase of pH yielded bigger crystals. The structural, optical, thermal and mechanical properties of β‐MgSO₄ · 7H₂O have been studied using FT‐IR, X‐ray diffraction, TGA‐DTG and micro hardness analyses.     

Effects of ammonium citrate additive on crystal morphology of aluminum phosphate ammonium taranakite

In this communication, we demonstrated the growth of aluminum phosphate ammonium taranakite (NH₄-AlPO₄) crystals from regular hexagonal form into the disk-like form could be controlled by ammonium citrate (AMC) as the effective crystal growth modifier at 90 °C. Prepared crystals were characterized by field emission scanning electron microscopy (SEM) and X-ray diffraction (XRD). The effects of AMC's concentration on the crystal form and morphology of NH₄-AlPO₄ were studied. The results showed that the AMC's concentration is an important parameter to control the size and morphology of NH₄-AlPO₄ crystal. The formation mechanism of the special morphology of NH₄-AlPO₄ crystals was also analyzed.     

Rapid crystal growth without inherent supersaturation induced by nanoscale fluid flows?

Crystal growth is a process that only takes place under non‐equilibrium conditions and a necessary prerequisite is that the crystal is exposed to a phase that is supersaturated in the material the crystal is composed of, be it a solution, a vapour or a supercooled melt. In industrial mass crystallization the growth rate for a population of crystals (in suspension growth processes [1]) rarely exceeds mean linear velocities of 10^‐7 ms^‐1. Here we present a mass crystallization process which is accompanied by rapid crystal growth several orders of magnitude faster and into a region of solution that is without inherent supersaturation. The material investigated is a solid hydrate that exhibits a solution mediated phase transition to its anhydrous form in the presence of methanol [2]. The phase transition is initiated simply by placing an amount of hydrate crystals into the solvent and is characterized by the rapid emergence of needle‐shaped crystals. The needles emanate from the crystal faces of the hydrate crystals and grow into the solution, which is nominally free of the substance to be crystallized. The high growth rate of the crystals, which of the order of up to 10^‐4 ms^‐1 is surprising. Although rapid needle growth has been observed before [3‐9], to date a satisfactory explanation for needles growing under the abovementioned conditions is still outstanding. Based upon the topology of the crystals we propose a tentative mechanism for this phenomenon capable of explaining the unusually rapid growth and highlight those questions that need addressing in order to verify this mechanism. X‐ray powder diffraction is used to characterize the crystal phase of the needles; confocal fluorescence microscopy reveals that the needles are hollow. The width of these needles is between 0.5 and 5 μm, their length appears to be limited only by the amount of hydrate available for their formation. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)