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.     

Crystallization behavior of solid solutions from aqueous solutions: An environmental perspective

Aqueous–solid solution (AQ-SS) processes have garnered increasing attention from geochemists and environmental engineers because they play major roles in the fate and transport of elements in Earth surface environments. The reasons for this interest include: (i) the primary crystallization of minerals from multicomponent aqueous solutions leads to the formation of solid solutions in which different ions are substituted for one another in equivalent structural positions; (ii) the interaction between pre-existing minerals and water frequently yields surface precipitation and dissolution–recrystallization processes in which such substituting ions redistribute to adapt to new physicochemical conditions; (iii) the concentrations of specific minor elements in biogenic and abiogenic minerals have been shown to correlate with various parameters characterizing the growth environment (temperature, pH, nutrient levels, salinity, etc.) and the corresponding compositional signatures can be powerful tools in reconstructing the past from the sedimentary record; (iv) the aqueous concentration of heavy metals and other harmful ions can be significantly reduced by their incorporation into the structure of suitable host minerals and as such a ‘reduction of solubility’ can be exploited as a remediation strategy or used to design engineered barriers for the retention of metals, radionuclides, and other industrially generated inorganic wastes. In this review, the thermodynamics driving of AQ-SS processes is presented using examples of environmentally-relevant systems. The reaction pathways in AQ-SS processes depend not only on thermodynamic factors but also on kinetic and mechanistic effects, which operate at different scales in space and time. Examples of such effects include non-equilibrium ion partitioning, surface passivation, and compositional (sectorial, concentric, oscillatory) zoning. Finally, we discuss the contribution of both state-of-the-art characterization techniques and molecular simulation methods for the development of predictive models.     

Crystallization of inorganic salts from aqueous solutions in a microwave field

The crystallization of some inorganic salts (KH₂PO₄, NaCl, Sr(NO₃)₂, KNO₂, Ca(OH)₂) by the thermal-gradient (with decreasing temperature) and solvent-evaporation methods using microwave heating of solutions is investigated. It is established that the growth rates of single crystals in a microwave field are an order of magnitude higher than obtained in other known techniques at comparable crystallization temperatures and supersaturations. For example, the growth rate of prismatic faces {100} of KH₂PO₄ crystals is as high as 11 mm/day at supersaturations of ～1.2%. The results obtained are discussed in the context of the effect of microwave radiation on the adsorption surface layers of crystals. Fine-grained phases of the salts under study are obtained by evaporation of the solvent.     

Growth mechanism of crystals from aqueous solutions

Microscopic processes occurring on the surface of a growing crystal or a dissolving one were observed by microcinematography. The crystals under observation were grown either in a drop of solution by evaporation or in a constant-temperature microscope stage at a chosen supersaturation. Small (approx. 0.1 mm) and large (approx. 10 mm) crystals of NaCl, Pb(NO₃)₂, NaNO₃, CdI₂, KDP and ADP were studied. It is concluded qualitatively that the layers, in general polygonal, originating in one or several active centres, are formed on the crystal face, never at the corners or edges. – The average velocity of layer motion was studied quantitatively in dependence on their thickness and supersaturation. The layer motion at constant supersaturation considerably fluctuated. – Surface patterns created by moving layers agree in most cases with predictions of the dislocation theory. Two categories of steps were found on the surface: ”︁real”︁ macrosteps and shock waves. – The velocity of layer motion for most compounds lies within (1–10) · 10⁻⁴ cm · s⁻¹.     

Growth morphology of potassium bichromate crystals from aqueous solutions

The results of a systematic study of growth morphology of potassium bichromate single crystals on seeds and by self-nucleation at different temperatures and supersaturations are presented. The observed growth morphology is analysed in the light of theoretical predictions. An increase in both growth temperature and supersaturation leads to a symmetrical morphology, and the transition from asymmetrical to symmetrical morphology is monotonous. The predicted sequence of morphological importance agrees only in the case of crystals grown at low temperatures and supersaturations.     

Crystal growth from concentrated aqueous solutions of calcium nitrate

The crystal growth kinetics was studied in the system of calcium nitrate – water at concentrations 14.3, 11.1, and 9.1 mole% of Ca(NO₃)₂ within the supercooling region ΔT = 0–65 K. The mechanism of the growth was suggested and the concentration dependence of the crystal growth rate was discussed.     

Crystallization of potassium sulfate from pure aqueous solution and in presence of surfactants

The use of surfactants offers an interesting possibility to control the crystallization from solutions. There are already some applications in technical scale. Yet, little knowledge exists on the mechanism of the effect and his dependence on surfactant concentration. Therefore, the displacement rates of selected single crystal faces of K₂SO₄ in pure aqueous solutions and in presence of different amounts of tensides have been measured. Furthermore, in order to get insight into the state of surfactant solutions existing in the interesting concentration interval, the critical micelle concentrations have been determined. In comparison with growth from pure solution, the admixtures caused at concentrations below 10⁻⁵ mol/l no drastic alterations. On the contrary, above a transition region characterized by a very irregular growth, a practically totally inhibition of growth as well as of dissolution was observed. The changes of surface morphology induced by surfactants have been investigated using the phase contrast microscopy.     

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)     

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.     

Morphology and characterization of ammonium bichromate crystals grown from aqueous solutions

The morphology of ammonium bichromate single crystals grown from aqueous solutions by solvent evaporation method at temperatures between 22 and 56 °C is investigated. It is found that the crystal morphology changes monotonically with an increase in growth temperature. Chemical etching is used to characterize the as-grown crystals.     

Freezing of aqueous solutions containing gases

The Mpemba effect and possible relation to the gases dissolved in water is investigated. The time of cooling and freezing of water and the enthalpy of its freezing in relation to the start temperature of cooling and the gases dissolved in water are measured.     

Study of growth kinetics of ammonium oxalate monohydrate crystals from aqueous solutions

Experimental results of the dependence of linear growth rates of ammonium oxalate monohydrate [(NH₄)₂C₂O₄ · H₂O; AO] single crystals on solution supersaturation are presented. The AO crystals were grown by constant-temperature, constant-supersaturation method at 30 and 40 °C in the supersaturation range of 1–9%. It was observed that the supersaturation dependence of growth rates follows the parabolic growth law. Analysis of the supersaturation dependence of linear growth rates of AO crystals showed (1) that growth models involving surface diffusion and direct incorporation of growth units give kinetic parameters similar to those reported for other compounds grown from solutions, and (2) that the the BCF model of cooperating screw dislocations is also applicable. An inverse relationship between the estimated values of the length, L, of the line containing the dislocations and growth rate, R, and a direct relationship between L and interplanar distance, d_hkl, of the face {hkl} were found. Both these relationships are associated with the process of generation of screw dislocations in the growing layer.     

Morphologies of calcium carbonate crystallites grown from aqueous solutions containing polyethylene glycol

Calcium Carbonate has been precipitated from aqueous solutions containing different concentrations and different molecular weight of Polyethylene Glycol (PEG). The precipitations were analyzed by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT‐IR) and X‐ray diffraction (XRD). The results demonstrated that PEG has profound influence on the nucleation and crystal growth of CaCO₃, under condition of low PEG6000 (refer to PEG M_W=6000) concentration, it favored the formation of calcite, while high PEG6000 concentration promoted vaterite formation. Additionally, low molecular weight PEG can stabilize vaterite phase. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The precipitation of lead sulphate from aqueous and aqueous alcohol solutions: Nucleation,final sizes and morphology

The precipitation of lead sulphate was studied from 0.0001 to 0.01 M aqueous solutions (supersaturations 3 to 600) and from 20% aqueous ethanediol, methanol and ethanol solutions, in polypropylene beakers, at ambient temperature: the experimental techniques were conductivity measurements and optical microscopy. The precipitations were heterogeneously nucleated at low supersaturations and homogeneously nucleated at intermediate to high supersaturations. New crystal morphologies generally developed at some what higher supersaturations in the aqueous alcohol systems. The final crystal lengths at first increased with increasing initial metal salt concentration and then decreased with this parameter; the largest crystals at any concentration were obtained from solutions in which lead sulphate solubility was highest. The critical supersaturations (for the onset of homogeneous nucleation) increased from 36 (in water) to 50 (in 20% aqueous ethanol): the surface energies for the formation of nuclei correspondingly increased from 90 to 110 mJ m⁻² in good agreement with the Nielsen-Söhnel relation. The nucleation and crystal growth processes are taking in an aqueous environment of similar water activity to that of the bulk solutions.     

Phosphate glass dissolution in aqueous solutions

Rates of aqueous dissolution are reported for a series of phosphate glasses having the composition (50 ? X)M₂O·(X)CaO·50P₂O₅. Dissolution reactions involving the consumption of H⁺ and OH^? were monitored using pH stat titration techniques, solution analyses using inductively coupled plasma emission, and depth profiling of corroded glass surfaces for H and other elements using elastic recoil detection analysis and Rutherford backscattering. These analytical results indicate that the phosphate glasses dissolve uniformly due to acid- or base-catalyzed hydration of the polymeric phosphate network. The rate and nature of this hydration appears to be controlled by the surface pH and/or charge which develops at the glass/ solution interface. The implication of these results on dissolution models for both the phosphate and silicate glasses is discussed.     

The precrystallization state of aqueous salt solutions

The parameters characterizing the fluctuations in the concentration have been calculated for a great number of aqueous solutions of various salts both in the range of the cluster formation and in the critical range. The existence of an inflection point on the isotherms of concentration susceptibility preceding the transition of the solution to the supersaturated state has been established. It is found that (1) the inflection point indicates the complete hydration of the solution and the beginning of the active cluster formation in solution and (2) that, in fact, the cluster formation in the solution is a diffuse second-order phase transition. It is concluded that the clusters have the liquid-like structure and the diffuse boundaries. At a deviation of the solution concentration from its critical value, the critical index equals unity, in full accordance with the Landau theory of the mean field.     

Rapid growth of KDP crystal from aqueous solutions with additives and its optical studies

Potassium Chloride (KCl) and Ethylene diamine tetraacetie acid (EDTA) as new additives were added into the KDP solutions in a small amount (5M% and 0.01M% respectively). The solubility curve and metastable zone width of KDP solution with 5M%KCl was determined and we explained the mechanism of rapid growth of KDP crystal with KCl additve. The clear transparent crystal of KDP with a dimension of 54 × 54 × 42 mm have been grown rapidly by cooling solution method in 2 days. The crystal grown from additives added solution was subjected to optical transmission and laser damage threshold studies as compared with the crystal grown by traditional growing method. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

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.     

Kinetics of crystallization of potassium chloride from aqueous solutions at 30 °C

The aim of this work is to use a diffusion layer model for the determination of individual rate constants of reaction and diffusion step of KCl at 30 °C. Crystal growth rate was measured by travelling microscope technique. From the measured values of the mean linear growth rate the resulting individual rate constants and the effectiveness factors (GARSIDE) are evaluated as a function of the driving force and of the flow velocity of solution. The significance of diffusional and surface reaction resistances against mass transfer is discussed in relation to the experimental conditions used.