Effect of CuCl2 on hydrothermal crystallization of calcium sulfate whiskers prepared from FGD gypsum

Using purified flue‐gas desulfurization (FGD) gypsum as raw material, effects of CuCl₂ on crystal morphology, phase structure, aspect ratio and crystallization of hydrothermal products prepared via hydrothermal crystallization in H₂SO₄‐H₂O solutions were investigated. The results show that dosage of CuCl₂ has a significant effect on the morphology, aspect ratio and crystallization of calcium sulfate whiskers (CSWs), but no effect on their phase transformation . At a dosage of 15 g CuCl₂/kg FGD gypsum, the produced calcium sulfate whiskers had diameters ranging from 1 to 3 μm with average aspect ratio greater than 200 . Transmission electron diffraction patterns and highly magnified surface morphology of CSWs were found different from those of self‐assembly crystals. Compared to self‐assembly crystals, the produced CSWs showed a single crystal structure and their surface was very smooth.     

Study on crystallization of calcium carbonate from calcium sulfate and ammonium carbonate in the presence of magnesium ions

Batch reactive crystallization of calcium carbonate (CaCO₃) from ammonium carbonate ((NH₄)₂CO₃) and calcium sulfate (CaSO₄) was investigated in the presence of magnesium (Mg²⁺) ions. It was observed that Mg²⁺ ions partly inhibited the conversion of CaSO₄ into CaCO₃. When the content of Mg²⁺ was less than 2%, the reduction in conversion rate of CaSO₄ was less than 2%, and the effect of Mg²⁺ ions could be ignored. Effect of impurity on crystallization kinetics of CaCO₃, including the growth rate and nucleation rate, was investigated. The results revealed that when Mg²⁺ ions content was less than 1%, Mg²⁺ could promote the growth of CaCO₃ and inhibit the nucleation process, which was favorable for the filtration of CaCO₃.When the content of Mg²⁺ ions was greater than 1%, Mg²⁺ inhibited the growth of CaCO₃, which resulted in explosion nucleation and led to a large number of particles in the solution, which was unfavorable for the filtration of CaCO₃. Based on the Bransom model, the particle size distribution equations of CaCO₃ were established. X‐ray diffraction patterns and scanning electron microscopy images exhibited the existence of spherical vaterite of CaCO₃ due to the reaction of CaSO₄ with (NH₄)₂CO₃ under the effect of Mg²⁺ ions, which was inconsistent with the results reported in the literatures.     

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)     

Preparation and structure of calcium peroxide‐templated porous calcium carbonate crystals

Crystalline calcium carbonate with randomly dispersed porous structure was prepared through co‐ crystallization with calcium peroxide and the following template elimination by a post heating treatment and washing with water. The artificial CaCO₃ possess abundant macro‐mesopores structures and high surface area. This approach may open a new general route for the preparation of crystals with high porosity and structure specialty. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structural and spectral studies on gypsum crystals under simulated conditions of phosphoric acid production with and without organic and inorganic additives

In the dihydrate process to produce phosphoric acid, phosphate ore [Ca₁₀F₂(PO₄)₆] is leached with sulfuric and weak phosphoric acids to produce phosphoric acid and gypsum as a by‐product. Crystallization of gypsum occurs as the leaching is taking place. The effect of organic and inorganic additives on the structure and spectrum of gypsum crystals under simulated conditions of phosphoric acid production is studied using x‐ray diffraction and infrared spectroscopy. Structure and spectrum of formed gypsum crystals in the absence of additives are slightly different from the standard gypsum crystals (card No. 6‐0046), which reflect the effect of preparation medium on the crystal structure of gypsum crystals. Presence of additives such as cetyl trimethyl ammonium bromide and 1,2‐dihydroxybenzene‐3,5‐disulfonic acid, Al³⁺ and Mg²⁺ increase the crystallinity of gypsum, while presence of additives such as citric acid and sodium dodecyl sulfate decrease the crystallinity of gypsum. Presence of Al³⁺ and Mg²⁺ as additives lead to the formation of calcium sulfate hemihydrate beside calcium sulfate dihydrate. Presence of sodium dodecyl sulfate as an additive inhibits the crystallization of gypsum and leads to the formation of anhydrite and calcium sulfate hemihydrate.     

Preparation of calcium sulfate whiskers by carbide slag through hydrothermal method

In this work, a series of calcium sulfate whiskers were prepared successfully using carbide slag as raw material through hydrothermal method. The prepared gypsum observed is hemihydrate calcium sulfate and the formation of the calcium sulfate whiskers are influenced by preparation parameters. The optimal preparation conditions are as follows: seriflux concentration, 4 %; hydrothermal reaction time, 10 h; hydrothermal reaction temperature, 130°C; seriflux pH, 7; calcium‐magnesium ratio, 12 : 1. Some nanowires appear when seriflux pH is very low (e.g. 1). The length‐diameter ratio of the whiskers is about 60‐80.     

Effects of calcium salts on thermal characteristics of hydroxypropyl methylcellulose films

Cast films comprising hydroxypropyl methylcellulose (HPMC) and calcium chloride (CaCl₂) have higher flexibility than those with HPMC and calcium lactate pentahydrate (CLP). The aim of the present study was to investigate the relationship between the molecular behaviour and the film flexibility of HPMC cast films. In differential scanning calorimetry (DSC) measurements, the HPMC-only cast films exhibited a glass transition temperature (Tg) of 142.1-143.9 °C, which is similar that of HPMC/CLP cast films. In contrast, HPMC/CaCl₂ cast films exhibited Tg of 76.1-77.3 °C, which is lower than that of HPMC-only and HPMC/CLP films. Thermal mechanical analysis (TMA) results indicated that the HPMC-only and HPMC/CLP cast films contracted strongly around the Tg calculated using DSC. In contrast, the cast films comprising HPMC/CaCl₂ blends gradually contracted as the temperature increased; this behaviour is significantly different from that observed in the HPMC-only and HPMC/CLP films. The most probable mechanism for the film flexibility of HPMC/calcium salt blends was clarified through attenuated total reflection Fourier transform-infrared (ATR FT-IR) and thermogravimetric (TG) analysis. The analysis results suggest that the difference in the flexibility of the cast films in the presence of CaCl₂ or CLP depends on the difference affinity between calcium salts and water molecules.     

Growth rate of α-calcium sulfate hemihydrate in K–Ca–Mg–Cl–H2O systems at elevated temperature

Kinetics of calcium sulfate hemihydrate (HH) crystal growth plays an important role in mineralization of calcium sulfate phases in nature. HH crystal growth and the conversion of calcium sulfate phases form the basis for the production and application of gypsum based building material. α-HH crystals have been grown in 3.74 M CaCl₂ solutions at a fixed initial ratio of calcium to sulfate under atmospheric pressure. The variations of sulfate ions were determined to obtain the α-HH crystal growth kinetics information. Effects of Mg²⁺ and K⁺ ions on α-HH growth were investigated to find an optimal composition of solution for α-HH preparation. The orders of α-HH growing in the CaCl₂ solution were found, in most cases, to be near 2.0 in presence or in absence of Mg²⁺ and K⁺ ions. Mg²⁺ ions enhance the growth of α-HH in CaCl₂ solution mainly due to initial supersaturation enhancing effects. K⁺ ions also improve the growth rate, which has been attributed to the reduction of interfacial energy. In a Ca (3.74 M)–Mg (0.20 M)–K (0.09 M) chlorides solution, the growth rate of α-HH increases with temperature from 80 to 100 °C, and the activation energy was calculated to be 40 kJ/mol.     

Influence of stirring speed on the crystallization of calcium carbonate

Control over crystal morphology of calcium carbonate (CaCO₃) was investigated by simply changing the stirring speeds in the process of CaCO₃ formation. Scanning electron microscopy (SEM) and powder X‐ray diffraction (XRD) measurements explore the morphology evolution of CaCO₃ at varying stirring speeds. As the stirring speeds increase, rhombohedral calcite, spherical vaterite, and monoclinic crystal with coexistence of calcite phase and vaterite phase were formed, suggesting a facile control over calcium carbonate crystallization in constructing crystals with desired morphology. Moreover, almost pure vaterite spherical particles of narrow particle size distribution were formed at optimum stirring speed. Finally, also elucidated in this work is the mechanism investigation into the construction of various crystal forms via this simple route. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Investigation of the Conditions for the Production of Calcium Magnesium Acetate (CMA) Road Deicer in an Extractive Crystallization Process

Calcium magnesium acetate (CMA) is considered as the best road deicer to replace the environmentally unacceptable NaCl and CaCl₂. However, the high cost of CMA prohibits its widespread use. The present study is dealing with the investigation of a crystallization method for the production of deicing CMA crystals of desired physical properties and the elucidation of the conditions under which such a product can be formed. Extractive crystallization is promising for the low cost production of CMA crystals considering that acetic acid is produced by a biochemical method and removed from the fermentation broth in situ by organic extractant systems. In this method, this organic phase, which contains the acetate ions is contacted with an aqueous phase which is the source of calcium and magnesium ions. The extractive crystallization process resulted in the production of well‐formed, large, and non‐spherical crystals of calcium acetate (CA), magnesium acetate (MA), and calcium magnesium acetate double salt (CMADS). The crystal size was affected by the concentration of acetic acid in both the organic and aqueous phases, whereas the crystal type and hydration level were determined primarily by the acetic acid concentration in the aqueous phase. The molar ratio of the precursor salts (CaCO₃/MgCO₃) in the reaction mixture was found to be the major factor for determining the habit and Ca/Mg content of crystals. Crystallization of CMADS was favored at high concentrations of acetic acid in the aqueous phase and at higher temperatures as shown from supplementary evaporation‐to‐dryness experiments.     

Formation of various polymorphs of calcium carbonate on porous membrane by electrochemical approach

Calcium carbonate (CaCO₃) formation was observed without surface modification of the organic template and in the absence of chemical additives such as macromolecules and divalent cations. Our innovative electrochemical approach that involves the use of an alternating current facilitated the crystallization of CaCO₃ polymorphs on a porous polymer membrane. A solution of calcium chloride (CaCl₂) and sodium carbonate (Na₂CO₃) was filled in a glass cell, and the porous membrane was interposed in the cell. A sine waveform of 10 Hz was applied to the platinum electrodes using a high-speed bipolar power supply. An alternating current was generated for 60 min. The crystal morphology and crystal structure of the resulting hybrid membrane were studied. In this electrochemical approach, versatile polymorphs of vaterite, aragonite, and calcite were formed on the membrane, thereby showing that the alternating current induced the formation of various polymorphs of CaCO₃ on the porous membrane even in the absence of any additives.     

Conductometric study of calcium carbonate prenucleation stage: underlining the role of CaCO3o ion pairs

Calcium carbonate crystallization process, especially the prenucleation stage, has increasingly been the subject of several works. In the present work, a simple method based on electrical conductivity modeling applied to the FCP (Fast Controlled Precipitation) method data is used to highlight the role of CaCO₃^o ion pairs on calcium carbonate prenucleation stage. A good agreement was obtained between the resistivity vs pH curves estimated by the McCleskey model equation and obtained experimentally in a FCP test. Results showed that the nucleation process begins with the formation of CaCO₃^o ion pairs as pre‐nuclei as soon as the calcite‐equilibrium pH is reached. Additionally CaCO₃^o content increases with pH to form aggregates, which depend on the saturation state of the solution. Basing on our thermodynamic data, these aggregates do not form amorphous calcium carbonate ACC as an intermediate phase. They lead to the formation of stable calcium carbonate nuclei which will further evolve to crystallize. Furthermore we demonstrate that in addition to their inhibitory effect on the Ca²⁺ and CO₃²⁻ association to form ion pairs, the two scale inhibitors sodium triphosphate (STP) and sodium polyacrylate (RPI) reduce ion pairs aggregation rate.     

Influence of a polyacrylate antiscalant on gypsum nucleation and growth

Calcium sulphate dihydrate (gypsum) crystallization was studied under conditions, of supersaturation and temperature, simulating a brackish water desalination unit using solar energy. The effect of an commercial sodium salt of poly(acrylic acid), based compound known as RPI, on homogeneous nucleation and growth of gypsum was also examined. Gypsum was precipitated by mixing aqueous CaCl₂ and Na₂SO₄ solutions. It was found that, with increasing temperature or supersaturation, the induction time decreases and the growth rate increases. By using classical nucleation theory, the interfacial tension and the nucleation rate values were estimated. It was shown that the interfacial tension is temperature dependent. The addition of increasing quantities of RPI, in the same conditions of temperature and supersaturation, prolongs the induction time, decreases the nucleation rate and increases the interfacial tension. The addition mode of RPI (in calcium or in sulphate solution) was found as an important parameter in controlling the inhibition process of gypsum crystallization. XRD and SEM analysis showed that RPI antiscalant strongly affected the texture and the morphology of the deposit gypsum. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth of calcium phosphate crystals in Silica gel

The results of a study of the crystal growth of calcium phosphates in silica gel employing double diffusion system with CaCl₂ and KH₂PO₄ aqueous solutions are reported. Liesegang rings, spherical crystalline agglomerates and brushite dendrites are formed near the CaCl₂ solution. It was found that the agglomerates consist of octacalcium phosphate and hydroxyapatite crystals with the predominance the former.     

Crystallization of potassium sulfate by cooling and salting‐out using 1‐propanol in a calorimetric reactor

A study was made on a isothermal process for the crystallization of potassium sulfate as an alternative to the cooling process. The process employs addition of 1‐propanol to aqueous salt solutions to achieve the “saltingout” of the K₂SO₄. This work was carried out using an automated Mettler Toledo model RC1 reactorcrystallizer with 800 ml capacity, and controlled isothermally at 25 °C to test the crystallization of K₂SO₄ by addition of the alcohol, and from 50 to 10 °C for the cooling crystallization. In both systems, the line of nucleation points was shown to be approximately parallel to the saturation curve, with an average width of 13°C or 3 % mass for crystallization by cooling, compared with 0.2 to 1 % by salting‐out. In experiments on crystallization by cooling, the K₂SO₄ crystals were 0.27 mm in mean size, showed 7 % agglomeration, and contained 8.5 % moisture. Crystals obtained by salting‐out had a mean size of 0.79 mm, 28 % agglomeration, and 9‐10 % moisture content. A crystal shape factor of approximately of 0.7 was obtained in both systems, apart from the agglomeration.     

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.     

机械力活化对脱硫石膏制备硫酸钙晶须的影响

对脱硫石膏原料进行机械力活化,然后采用降温重结晶法制备出硫酸钙晶须.借助扫描电镜、图像粒度分析仪和相关软件进行表征,并初步讨论了机械力活化对晶须成核和生长过程的影响.结果表明:经过机械力活化的脱硫石膏可以制备有较大长径比的硫酸钙晶须.在水浴反应温度为75℃,硫酸浓度为2.5 mol/L,机械力活化时间为3.5 h,浆料浓度为2;的条件下,制备的硫酸钙晶须平均长径比为150,平均直径为2.1μm.     

Study of calcium phosphate precipitation at 37 °C

The nature of the calcium phosphate formed when equal volumes of 20 mM CaCl₂ and 15 mM or 12 mM K₂HPO₄ with pH 7.4 are rapidly mixed at 37 °C has been investigated by following the pH of the suspension as a function of time, by calorimetric and light scattering methods, chemical analysis and TEM. It is found that in the early stages of this process, there appears a spherular amorphous form of calcium phosphate, called ACP1, which transforms into a floccular amorphous form, called ACP2. We suggest that this transformation is solution-mediated. ACP2 has not previously been clearly described as a separate phase and has not before been shown to be amorphous.     

Physico-chemical processes in single crystal growing of calcium aluminates by zone melting

The growing of single crystals of calcium aluminates of compositions 12 CaO · 7 Al₂O₂, CaO · Al₂O₃, and CaO · 2 Al₂O₃ by zone melting under vacuum of 10⁻⁵ mm Hg permitted to establish that some of the Al³⁺ ions in octahedral coordination have been driven back by the moving crystallization front. Energetically, this process can be represented on the basis of the viscous flow model with the activation energy of −45 kcal/mole. The possible mathematical models have been considered for the processes of preparation of single phase crystals of the aluminates mentioned above which take account of incongruent vaporization of the component oxides and refining of the melt from structural impurities by the moving crystallization front.     

Etch pit morphology and calcium sulphate growth on {111} faces of calcium fluoride crystals etched in sulphuric acid

An investigation of the etching behaviour of cleaved octahedral faces of calcium fluoride crystals in sulphuric acid of different concentrations and at various temperatures is carried out. It is observed that the morphology of dislocation etch pits depends on the temperature and concentration of the acid. With an increase in the acid concentration, growth of whiskers, elongated platelets, hexagonal and dice-shaped elevations, spherulites and sheaves is observed. Temperature of the etchant also has an effect on their formation. By X-ray diffraction techniques, the compounds forming the whiskers and elongated platelets, and hexagonal and dice-shaped features are identified as CaSO₄ · 2 H₂O and CaSO₄, respectively. The mechanism of the growth of CaSO₄ · 2 H₂O and CaSO₄ and the change in the morphology of selective etch pit are discussed.