Dissolution-crystallization Mechanism Research Articles

Solvothermal Treatment of Rare Earth Chloride Hydrates.

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Solvothermal treatment of rare earth chloride hydrates

Abstract The reaction of rare earth (RE) chloride hydrates in 1,4-butanediol (1,4-BG) at 300 °C in the presence of small amounts of n -hexylamine and 1,6-hexanediamine yielded phase-pure RE(OH) 2 Cl for La–Dy, but RE(OH) 2 Cl obtained from Ho to Yb and Y was contaminated with an unknown compound. The space group of RE(OH) 2 Cl formed by this reaction was P 2 1 /m [No. 11]. Thermal analysis of the products in an air flow indicated that the RE(OH) 2 Cl phases for the RE elements other than Ce were dehydrated into REOCl, while Ce(OH) 2 Cl was decomposed at 350 °C yielding CeO 2 without formation of CeOCl. For the RE elements with smaller ionic size, REOCl was further decomposed into RE 2 O 3 but REOCl with larger ionic size was not decomposed even after calcination at 1000 °C. The morphology of RECl 3 was altered by the solvothermal reaction, indicating that RE(OH) 2 Cl were formed via a dissolution–crystallization mechanism.

Uptake of dissolved Cd by biogenic and abiogenic aragonite: a comparison with sorption onto calcite

The uptake of Cd 2+ by aragonite and calcite is investigated by combining macroscopic measurements with some qualitative sorption experiments performed in a hydrogel medium. Both biogenic and abiogenic aragonites were studied in order to evaluate the process on materials with different textures. Assuming that sorption occurs by surface precipitation of metal-bearing solids, the gel produces a drastic decrease in the nucleation density, which allows for the precipitation of crystallites that are large enough to be analysed by scanning electron microscopy and characterized by glancing-incidence X-ray techniques. The macroscopic study reveals that aragonite is a powerful sorbent for cadmium in aqueous environments. Microscopic observations indicate that cadmium is sorbed onto aragonite by surface precipitation of (Cd, Ca)CO 3 solid solutions with a calcite-type structure. The precipitating individuals grow randomly oriented on the surface to reach sizes in the micrometre range. As a consequence, the concentration of cadmium in the aqueous solution decreases dramatically to values controlled by the low solubility of the cadmium-rich end member. This mechanism involves simultaneous dissolution-crystallization and is the same for both abiogenic and biogenic aragonites, the only difference being a result of the higher specific surface area of the biogenic starting material. Long-term uptake of cadmium by calcite occurs through a similar dissolution-crystallization mechanism, the final outcome being virtually the same, that is, surface precipitation of (Cd,Ca)CO 3 solid solutions. In this case, however, substrate and precipitate are isostructural and the process occurs by oriented overgrowth of thin lamellar crystallites, which spread to quickly cover the surface by a layer a few nanometers thick. This epitaxial layer armors the substrate from further dissolution, so that the process stops when only a small amount of cadmium has been removed from the fluid. As a result, the “sorption capacity” of calcite is considerably lower than that of aragonite. The study illustrates reaction pathways and “partial” equilibrium endpoints in surface-precipitation processes involving solid solutions.

Cation composition during recrystallization of layered double hydroxides from mixed (Mg, Al) oxides

Changes in cation composition and M 2+/M 3+ ratio during hydrotalcite regeneration were studied. Regenerated hydrotalcites were obtained by recrystallization of mixed (Mg, Al) oxides in solutions of divalent (Mg, Zn, Co, Ni, Cu) or trivalent (Al, Fe) cations. Heating Mg–Al–CO 3 hydrotalcites with Mg/Al=2, 3 and 3.7, at 600 °C for 2 h yielded periclase-like mixed (Mg, Al) oxides (HT-P). The hydrotalcite structure was restored by dispersing oxides 48 h in water or aqueous solutions of different cations. The presence of Mg 2+, Zn 2+, Ni 2+, Co 2+, Cu 2+ salts or of low soluble hydromagnesite increased the M 2+/Al ratio, reaching a maximum value of 3.8. An incorporation of Zn 2+, Ni 2+, Co 2+ and Cu 2+ cations in the newly formed hydrotalcite was detected, while Mg 2+ remained in solution. In the presence of soluble Al salts or freshly precipitated Al(OH) 3, the M 2+/Al ratio approximated the minimal possible value of 2. The Mg/Al ratio of a hydrotalcite crystallized from a mixture of two HT-P samples with different Mg/Al ratios is equal to the weighted average value. The results obtained support the conception of the dissolution–crystallization mechanism of hydrotalcite regeneration from mixed (Mg, Al) oxides contrary to the widely accepted concept of topotactic processes.

Formation of zirconia polymorphs under hydrothermal conditions

Using zirconium oxychloride solution as precursor, monoclinic zirconia crystallites with narrow distribution of nanosize were obtained in the hydrothermal reaction. However, when the reaction was in weak acidic medium or base medium, whether directly using the colloidal precipitate prepared from zirconium salt solutions with base solution as precursor added, or using the precipitate after filtrating, washing and drying treatments as precursor, the product of the hydrothermal reaction was the mixture of both monoclinic and tetragonal polymorphs. As the pH of the medium rises, the content of tetragonal phase in the product, the morphologies and size of the crystallites all change. There are three types of formation mechanisms under hydrothermal condition, which can be called as saturation-precipitation mechanism in homogeneous solution, dissolution-crystallization mechanism and in-situ crystallization mechanism, respectively. The formation mechanism of crystallites varies with different hydrothermal conditions, such as the states of the precursor and the pH of the medium, which lead to changes in the phases, morphologies and sizes of the resulting crystallites.

An experimental alteration of montmorillonite to a di + trioctahedral smectite assemblage at 100 and 200°C

AbstractHydrothermal experiments were performed at 100 and 200°C and at different clay:water ratios in order to investigate the transformation of smectitic layers during the alteration of a montmorillonitic starting material. This study focused on three phenomena: (1) the amount and localization of charge within the layer of the newly-formed dioctahedral smectite; (2) the stacking of low- and high-charge layers in the dioctahedral smectitic material; and (3) the neoformation of trioctahedral smectites.In all of the runs, the formation of beidellite from montmorillonite induced morphological changes in clay particles which suggests a reaction proceeding by a dissolution-crystallization mechanism. Illite layers were detected in K-saturated montmorillonite runs after the transformation of ∼50% of the starting montmorillonite into beidellite (i.e. after 5 months of reaction with distilled water at 200°C). These illite layers were interstratified with both high-charge and low-charge dioctahedral smectites in a hypothetical three-component mixed-layer mineral.

The formation of pyrochlore potassium tantalate thin films by soft solution processing

In this paper, a novel procedure to fabricate potassium tantalate thin films (KT), named soft solution processing, has been proposed. KT films have been successfully prepared in concentrated KOH solutions at temperatures below 150°C by electrochemical or hydrothermal–electrochemical method. X-Ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and atomic force microscope (AFM) techniques have been used to characterize the structure and morphology of films. Films show pyrochlore structures without contamination. Crystalline KT grain could be still observed for the films prepared at a temperature as low as 50°C in 5.0 M KOH solution with the galvanostatic treatment at the current density of 10.0 mA cm −2. A dissolution–crystallization mechanism has been used to explain the observed experimental phenomena.

Single-step fabrication of potassium tantalate films by hydrothermal-electrochemical method at lower temperatures

Potassium tantalate thin films (KT) were hydrothermal-electrochemically and electrochemically synthesized on tantalum substrates under galvanostatic conditions in KOH solutions at temperatures from 50 to 150 °C. The pyrochlore structures were characterized by x-ray diffraction and scanning electron microscopy for the films formed under a variety of conditions. It was found that the reaction temperature, potassium hydroxide concentration, and current density significantly affect the formation and the morphology of the KT films. When the reaction temperatures were higher than 80 °C and the KOH concentrations were greater than 2.0 M, very crystalline films with excellent film flatness and good adherence on the substrate were obtained. Based on the experimental results, it was confirmed that the formation and growth of KT films by the hydrothermal-electrochemical and electrochemical method follow a dissolution-crystallization mechanism.

Macrokinetics of thermal explosion in a niobium-aluminum system. II. Phase-formation dynamics

The thermal explosion in a gas-free mixture of metal powders has been studied for the first time using the method of dynamic x-ray structural analysis. The dynamics of formation of new crystalline phases at all stages of the process is determined. It is shown that melting of aluminum is followed by dissolution of niobium in the melt and crystallization of the intermediate product NbAl3, and the melt does not wet niobium particles. A dramatic improvement in wetting is observed at a temperature of ≈ 1040 K (apparently, due to the destruction of oxide films). This results in a thermal explosion, and the Nb2Al phase is formed. The reaction during the thermal explosion also follows the dissolution-crystallization mechanism.

Formation of BaTiO3 and PbTiO3 thin films under mild hydrothermal conditions

Well-crystallized polycrystalline thin films of cubic barium titanate (BaTiO3) have been synthesized on Ti-covered Si substrates by exposing the substrates to a Ba(OH)2 aqueous solution at 160 °C and a low pressure less than 1 MPa. It was presumed that the film formation of BaTiO3 involved a dissolution-crystallization mechanism, which provides an attractive approach to produce other titanate films and powders. Also, we have used for the first time this hydrothermal technique to deposit epitaxial (001)-textured PbTiO3 thin films on SrTiO3(100) substrates.

Mechanism and kinetics of the reaction: 1 dolomite + 2 quartz = 1 diopside + 2 CO2: a comparison of rock-sample and of powder experiments

Two experiments using cylindrical samples of a dolomite-quartz rock were carried out in a conventional hydrothermal apparatus for the forward reaction: 1 dolomite + 2 quartz = 1 diopside + 2 CO2, in order to compare the mechanism and the kinetics with results from experiments using mineral powders of dolomite and quartz at the same P-T-X conditions. Experimental conditions were as follows: total pressure 500 MPa; temperature 680° C (overstepping 65° C); CO2 content of the fluid phase, consisting of carbon dioxide and water, was nearly 90 mol%; the fluid/rock ratio was 1:37, and the H2O/rock ratio was about 1:740; run duration was 92 days. Scanning electron microscope (SEM) examination of a polished axial section of the rock cylinders after the run, using back-scattered electrons (BSE), shows that the reaction produced corona textures. The diopside crystals nucleate and grow exclusively on dolomite surfaces adjacent to quartz grains, i.e. in regions where there is intimate contact between the reactants. The dolomite matrix, in contrast, is diopside free. A concept of microsystems is used to compare directly the rock cylinder results with those from runs done with mineral powders. The microsystems, which consist of quartz, dolomite and diopside, are connected by the intergranular space which is filled by the fluid phase. The SEM analysis of the rock cylinders indicates a dissolution-crystallization mechanism operating in the microsystems; this is consistent with the results of experiments using dolomite quartz powders (Luttge et al. 1989). It can be demonstrated that reaction kinetics in mineral powder runs are interface controlled as long as the newly formed diopside crystals do not cover the dolomite surfaces completely (Luttge and Metz 1991 c). This result is applicable to each microsystem of the rock cylinder, since the reaction mechanism and the resulting textures are the same in both kinds of experiments. The reaction is much slower outside the microsystems, i.e. in the dolomite matrix but in the close vicinity of the quartz grains. At these places, the reaction is controlled by the transport of Si-species in the CO2-rich fluid phase filling the intergranular space. The reaction is absent in quartz-free regions of the dolomite matrix. Calculations and measurements of the extent of reaction progress in both kinds of experiments give results of the same order of magnitude: the conversion, and therefore the reaction rate, differs by less than a factor of two. The conclusion is that there are no differences, in principle, concerning mechanisms, rate controls, rates, and resulting textures between rock cylinder experiments, and mineral powder experiments.

The mechanism of the reaction 1 tremolite+3 calcite+2 quartz =5 diopside+3 CO2+1 H2O: results of powder experiments

The mechanism of the reaction 1 tremolite +3 calcite+2 quartz=5 diopside+3 CO2+1 H2O was investigated at 2 and 5 kb, \(X_{CO_2 }\), using powder experiments lasting from 14 to 170 days. Because experiments were at high ratios of fluid to solids, the study identified the mechanism under surface-control conditions and thus establishes which reactant surface determines the kinetics. To achieve a diopside nucleation rate high enough to gain detectable reaction in the time of experimentation, the equilibrium boundary had to be overstepped by 30°–60° C at 5 kb. Experiments in which diopside successfully nucleated show that the reaction proceeds by a dissolution-crystallization mechanism. Experimentally-produced textures are presented in a series of SEM images and demonstrate that diopside nucleates and grows topotactically exclusively on tremolite. The mechanism of the forward reaction is modeled by a simplified scheme consisting of three processes, each comprising formation, transport and incorporation of 1) the Ca-, 2) the Mg-, and 3) the Si-bearing species in the fluid in response to dissolution of the reactants and crystallization of diopside. Using the dependence of the overall-reaction rate on the surface area of the reactants, it was experimentally determined that process 2) (dissolution of tremolite, transport of the Mg-bearing species in the fluid and crystallization of diopside) will be rate-limiting in most cases where metamorphism occurs in an internally controlled system. Due to the experimental design chosen, the dissolution of tremolite at the beginning of process 2) is rate-limiting in the experiments. The magnitude of the probable temperature-overstep necessary to achieve a significant nucleation rate during metamorphism is discussed on the basis of the experimental evidence and a simple nucleation rate model.

The Early Stages of Hydration of CaO⋅2Al<sub>2</sub>O<sub>3</sub>

The early stage of hydration of CaO⋅2Al2O3 was investigated. Hydration was carried out mainly in a diluted water suspension. The rate of hydration of CaO⋅2Al2O3 was greatly affectted by the water/solid ratio and the synthetic condition of specimen, while the mechanism of hydration remained unchanged under these conditions. In particular, formation of crystalline hydrates was retarded with increasing water/solid ratio. At the earliest period of hydration, the reaction rate was slow and hydrates were not found. It appears that this period corresponds to a nucleation stage of gibbsites. In the following period precipitation of gibbsite was observed, in which the concentrations of CaO and Al2O3 in the liquid increased rapidly, accompanied by an increase in CaO/Al2O3 molar ratio. In this period the reaction proceed by the dissolution-crystallization mechanism. Finally, different hydrates such as 2CaO⋅Al2O3⋅8H2O began to precipitate and the rate of hydration was accelerated.

Kinetics of hydration of calcium sulphate hemihydrate

The kinetics of hydration of α-CaSO4·0.5H2O has been studied by calorimetry, analysis of the liquid phase, measurement of the combined water, and electron microscopic observations. Hemihydrate particles dissolve immediately after mixing with water and subsequently form hydrated layers on their surfaces by intrusion of water molecules, which grow to a thickness of ca. 0.09 µm according to a parabolic law and then undergo destruction, when the dihydrate nuclei are generated for the first time. At this point in time, the induction period terminates, and subsequent hydration of hemihydrate proceeds in accordance with the previously proposed rate equation based on the dissolution–crystallization mechanism.