Kinetics Of Dissolution Reaction Research Articles

DSC analysis of dissolution reaction in an as-cast and HPT-processed Mg-Gd alloy

The dissolution reaction kinetics were determined by differential scanning calorimetry (DSC) analysis for Mg-0.6Gd (wt.%) in an as-cast condition and after processing by high-pressure torsion (HPT) at room temperature for 20 turns. For the as-cast condition, the dissolution temperature was in the range of 634–658 K with an activation energy of 135 ± 9 kJ/mol. After HPT, the range of dissolution temperature decreased to 601–633 K, with a lower activation energy of 118 ± 10 kJ/mol. In both conditions, the Avrami and growth exponents are around 1.5, which suggests that the dissolved precipitates exhibit polyhedron-like morphology and that the dissolution process is dominated by bulk dissolution with a constant rate controlled by diffusion.

Synthesis of KAlSiO4 by Hydrothermal Processing on Biotite Syenite and Dissolution Reaction Kinetics

To make potassium from K-bearing rocks accessible to agriculture, processing on biotite syenite powder under mild alkaline hydrothermal conditions was carried out, in which two types of KAlSiO4 were obtained successfully. The dissolution-precipitation process of silicate rocks is a significant process in lithospheric evolution. Its effective utilization will be of importance for realizing the comprehensiveness of aluminosilicate minerals in nature. Two kinds of KAlSiO4 were precipitated in sequence during the dissolution process of biotite syenite. The crystal structures of two kinds of KAlSiO4 were compared by Rietveld structure refinements. The kinetics model derived from geochemical research was adopted to describe the dissolution behavior. The reaction order and apparent activation energy at the temperature range of 240–300 °C were 2.992 and 97.41 kJ/mol, respectively. The higher dissolution reaction rate of K-feldspar mainly relies on the alkaline solution, which gives rise to higher reaction order. During the dissolution-precipitation process of K-feldspar, two types of KAlSiO4 with different crystal structure were precipitated. This study provides novel green chemical routes for the comprehensive utilization of potassium-rich silicates.

Dissolution reaction kinetics and mass transfer during aqueous choline chloride pre-treatment of oak wood

Lignocellulosic biomass processing employing ionic liquids is of recent research interest for the biorefinery industry. The data on biomass dissolution kinetics in ionic liquids is important for designing scale-up pre-treatment reactor design. In this study, the reaction mechanism and kinetics of oak wood dissolution in aqueous choline chloride was investigated. In an extended effort, a correlation of dimensionless numbers was developed for the estimation the mass transfer coefficient. The analyses suggested that oak wood dissolution in choline chloride occurred in two stages. The diffusion of ionic liquid through the product layer was the dominating rate-controlling step in the first stage of dissolution followed by the surface chemical reaction in the second stage. The diffusivity of choline chloride into the oak wood matrix was ranging between 2.96E−14 and 2.84E−13 m2/s. The activation energy of the diffusion controlled stage and surface chemical reaction controlled stage was approximately 24.2 and 40.3 kJ mol−1, respectively. The proposed mathematical correlation for mass transfer coefficient fitted well with the experimental mass transfer coefficient values.

Sintering behavior of Y‐doped BaZrO3 refractory with TiO2 additive and effects of its dissolution on titanium melts

AbstractIn this study, the sintering behavior of Y‐doped BaZrO3 with TiO2 additive and effects of its dissolution on titanium melts were investigated. In order to overcome the difficulty of Y‐doped BaZrO3 sintering performance, the 0‐5 wt% TiO2 was added into Y‐doped BaZrO3 and its densification was investigated by density analyzer, scanning electron microscope (SEM) and X‐ray diffraction (XRD). Thereafter, the interface reaction between two crucibles (without and with 2 wt% TiO2) and titanium alloys, the thermodynamics and kinetics of dissolution reaction were also investigated. The results showed that Y‐doped BaZrO3 was rarely dense without TiO2 additive and its relative density was just 88%, while after doping 2 wt% TiO2, the relative density was more than 97%. However, with the excessive TiO2(>2 wt%) doping, the secondary phase was observed by SEM and XRD. After melting titanium‐rich alloys (Ti2Ni, 66 mol% Ti) by Y‐doped BaZrO3 crucibles without and with 2% TiO2 additive, the erosion layer of the two crucibles was approximately 4000 and 1700 μm, respectively. It was also found that the dissolved reaction rate was related to the density and grain size of Y‐doped BaZrO3 ceramic; the higher density and larger grain size ceramics can effectively prevent the crucible from being eroded by Ti2Ni melt.

Crystal Dissolution Kinetics Studied by a Combination of Monte Carlo and Voronoi Methods

Kinetic Monte Carlo (kMC) methods have been used extensively for the study of crystal dissolution kinetics and surface reactivity. A current restriction of kMC simulation calculations is their limitation in spatial system size. Here, we explore a new and very fast method for the calculation of the reaction kinetics of a dissolving crystal, capable of being used for much larger systems. This method includes a geometrical approach, the Voronoi distance map, to generate the surface morphology, including etch pit evolution, and calculation of reaction rate maps and rate spectra in an efficient way, at a calculation time that was about 1/180 of the time required for a kMC simulation of the same system size at one million removed atoms. We calculate Voronoi distance maps that are based on a distance metric corresponding to the crystal lattice, weighted additively in relation to stochastic etch pit depths. We also show how Voronoi distance maps can be effectively parameterized by kMC simulation results. The resulting temporal sequences of Voronoi maps provide kinetic information. By comparing temporal sequences of kMC simulation and Voronoi distance maps of identical etch pit distributions, we demonstrate the opportunity of making specific predictions about the dissolution reaction kinetics, based on rate maps and rate spectra. The dissolution of an initially flat Kossel crystal surface served as an example to show that a sequence of Voronoi calculations can predict dissolution kinetics based on the information about the distribution of screw defects. The results confirm that a geometrical relationship exists between the material flux from the surface at a certain point and the distance (or, when considering anisotropy, a function of distance) to the nearest defect. In this study, for the sake of comparability, the calculations are made using input parameters directly derived from the kMC models operating at the atomic scale. We show that, using values of v(rpit) and weighting factors obtained by kMC, the resulting surface morphologies and material flux are almost identical. This implies that discrete Voronoi calculations of starting and end points of the dissolution are sufficient to calculate material flux maps, without the time-consuming overhead of computing the interim reactions at the atomic-scale. This opens a promising new venue to efficiently upscale full-atomic kMC models to the continuum macroscopic level where reactive transport and Lattice Boltzmann calculations can be applied.

CFD simulation of dissolution process of alumina in an aluminum reduction cell with two-particle phase population balance model

The dissolution process of alumina in a three-dimensional 300 kA aluminum reduction cell was modeled and simulated employing our custom code. A two-particle phase population balance model (TPPBM) was proposed by dividing solid phase into two groups consisting of small and large size alumina particles separately. A CFD-TPPBM coupled model was developed to describe the liquid–solid flow and dissolution process based on dissolution reaction kinetics mechanism. The effects of alumina effective diffusion coefficient, bath superheat, alumina preheating temperature and alumina feeding positions were studied numerically. The results show that the transport characteristics of alumina are controlled to great extent by the bubble driven forces in some local areas, while the electromagnetic forces can help alumina transported from the feeding positions to the whole cell. An increase of alumina effective diffusion coefficient can effectively benefit the dissolution process of the small alumina particles. It is advantageous to improve the dissolution process of the large alumina particles using higher bath superheat or alumina preheating temperature. The point feeding zone is preferable located at inter-anode gaps. The small alumina particles are dissolved quickly and completely in about 10 s after the feeding while the large alumina particles are dissolved much slowly.

Effect of water quality on the leaching of a low-grade copper sulfide ore

Copper extractions from a low-grade, ground copper sulfide ore (0.7% Cu) leached in three media were freshwater<seawater>double-strength seawater and pH 1.5⩾pH 2; 84% extraction was achieved in pH 1.5 seawater in 28days at 23°C. Cu-oxide and carbonate dissolved completely and chalcocite was altered to secondary covellite, some of which persisted in all media for the duration of the 28-day experiment. Chalcopyrite and bornite were both oxidised more readily in saline water. Iron, sodium, potassium and sulfur (sulphate) concentrations in leach solutions diminished and the amounts of insoluble iron(III) reaction products increased with increased salinity and increased solution pH. While, overall, silicate dissolution was small, the amounts of poorly crystalline phases (both iron(III) and silica-rich phases) increased with increased salinity and were greater in pH 1.5 media. In the context of heap leaching, the increased amounts of secondary precipitates formed if saline water was used could result in lower extraction efficiency and the increased total dissolved solids, density and viscosity could result in increased energy costs for solution management at operations.The software package Geochemist’s Workbench was evaluated by modelling the synthetic seawater – pH 2 test. It was possible to predict the evolution of the solution composition, the main species and phase boundaries at the start and end of leaching, and the formation of three reaction products in accord with experimental data by applying the React sliding function.The tests were conducted using a pulverised ore sample to increase dissolution reaction kinetics, particularly for chalcopyrite. Future tests should be conducted using ore particle sizes appropriate to heap leaching. The copper distribution within particles indicated that the test ore may not be suited to heap leaching because the surface exposure of copper sulfide grains is limited. Therefore reactor designs better suited to smaller sized particles with/without pre-treatment should be considered.

An atomic force microscopy study of the dissolution of calcite in the presence of phosphate ions

Dissolution of calcite in the presence of phosphate solutions was studied in situ by Atomic Force Microscopy. Results of experiments in slightly alkaline (NH4)2HPO4 solutions showed that dissolution, measured from etch pit spreading, is significantly reduced compared to that observed in pure deionized water, confirming an inhibitory effect of (NH4)2HPO4 on calcite dissolution. However, rates measured in the presence of Na-phosphate solutions at the same pH remained close to that in pure water. This would indicate that the inhibitory effect could be caused by the presence of the NH4+ group. Moreover, for phosphate solution concentrations >5mM, the precipitation of a calcium phosphate phase occurred simultaneously while calcite was dissolving, despite the continuous flow of the reaction solution. Such reactions may play an important role in phosphorus recovery from P-bearing solutions. Importantly this study gives insights into the mechanism of interface-coupled dissolution-precipitation reactions occurring during the interaction of phosphate-bearing solutions with calcium carbonate minerals and emphasizes the importance of performing direct observations when determining the kinetics of dissolution reactions, as they can be significantly affected by the precipitation of secondary phases that could alter dissolution rates determined from measurements of bulk solution composition.

Modeling the growth of stylolites in sedimentary rocks

Stylolites are ubiquitous pressure solution seams found in sedimentary rocks. Their morphology is shown to follow two self‐affine regimes. Analyzing the scaling properties of their height over their average direction shows that (1) at small scale, they are self‐affine surfaces with a Hurst exponent around 1, and (2) at large scale, they follow another self‐affine scaling with Hurst exponent around 0.5. In the present paper, we show theoretically the influence of the main principal stress and the local geometry of the stylolitic interface on the dissolution reaction rate. We compute how it is affected by the deviation between the principal stress axis and the local interface between the rock and the soft material in the stylolite. The free energy entering in the dissolution reaction kinetics is expressed from the surface energy term and via integration from the stress perturbations due to these local misalignments. The resulting model shows the interface evolution at different stress conditions. In the stylolitic case, i.e., when the main principal stress is normal to the interface, two different stabilizing terms dominate at small and large scales which are linked respectively to the surface energy and to the elastic interactions. Integrating the presence of small‐scale heterogeneities related to the rock properties of the grains in the model leads to the formulation of a Langevin equation predicting the dynamic evolution of the surface. This equation leads to saturated surfaces obeying the two observed scaling laws. Analytical and numerical analysis of this surface evolution model shows that the crossover length separating both scaling regimes depends directly on the applied far‐field stress magnitude. This method gives the basis for the development of a paleostress magnitude marker. We apply the computation of this marker, i.e., the morphological analysis, on a stylolite found in the Dogger limestone layer located in the neighborhood of the ANDRA Underground Research Laboratory at Bure (eastern France). The results are consistent with the two scaling regimes expected, and the practical determination of the major principal paleostress, from the estimation of a crossover length, is illustrated on this example.

Electrochemical corrosion of X65 pipe steel in oil/water emulsion

The electrochemical corrosion behavior of X65 pipeline steel in the simulated oil/water emulsion was investigated under controlled hydrodynamic and electrochemical conditions by rotating disk electrode technique. Results demonstrated that mass-transfer of oxygen plays a significant role in the cathodic process of steel in both oil-free and oil-containing solutions. Electrode rotation accelerates the oxygen diffusion and thus the cathodic reduction. The higher limiting diffusive current density measured in oil-containing solution is due to the elevated solubility of oxygen in oil/water emulsion. The anodic current density decreases with the increase of electrode rotating speed, which is attributed to the accelerated oxygen diffusion and reduction, enhancing the steel oxidation. Addition of oil decreases the anodic dissolution of steel due to the formation of a layer of oily phase on steel surface, increasing the reaction activation energy. The steel electrode becomes more active at the elevated temperature, indicating that the enhanced formation of oxide scale is not sufficiently enough to offset the effect resulting from the enhanced anodic dissolution reaction kinetics. The corrosion reaction mechanism is changed upon oil addition, and the interfacial reaction is activation-controlled, rather than mass-transfer controlled. When sand particles are added in oil/water emulsion, there is a significant increase of corrosion of the steel. The presence of sands in the flowing slurry would impact and damage the oxide film and oily film formed on the steel surface, exposing the bare steel to the corrosive solution.

Experimental study and modelling of granite-distilled water interactions at 180°C and 14 bars

Distilled water was percolated for 38 days through Soultz granite (France) from the candidate site of the future Hot Dry Rock (HDR) geothermal exchanger at 180°C and 14 bars in a dynamic experimental system (plug-flow system with Ti reaction cell: I.D. 50 mm and L. 120 mm). The amplitude of dissolution and precipitation reactions was estimated after fluid analysis, and scanning electron microscope observations of the minerals were done following the percolation experiment. There were well developed etch pits on the surface of the feldspars. These were, therefore, the most reactive minerals. Dissolution and precipitation are distributed in zones along the flow path of the reacting fluid. Newly precipitated calcite is observed along the fluid flow path. Ferromagnesian saponite precipitates only at the outlet part of the system. The following mobility order was observed: SiO 2 > HCO 3 > F ∼ Na > Ca > Al > Li > Cs > Mg > Ba > Rb > B > Sr. By combining dissolution reaction kinetics, according to the law of the transition state theory, for the granite minerals, and the local chemical equilibrium approach for the precipitation of the secondary minerals and for the reactions of aqueous complexation, we were able to simulate the first reaction pathway of this experiment using the geochemical code EQ3/6 with the ‘fluid-centred flow-through' calculation mode. The experimental results (observations of minerals and chemial composition of theoutlet fluid) provided model constraints. Particular attention was paid to the effectively reactive surface of the minerals. The correction of the reactive surface in relation to the total measured surface was necessary so that calculated results would be in agreement, at least qualitatively, with observations. A good agreement is obtained only if the effective reactive surface of the minerals is taken to be from 0.03 to 3.2% of the total surface measured by the BET-N 2 method.

The mechanism of surface chemical kinetics of dissolution of minerals

This paper deals with the mechanism of dissolution reaction kinetics of minerals in aqueous solution based on the theory of surface chemistry. Surface chemical catalysis would lead to an obvious decrease in active energy of dissolution reaction of minerals. The dissolution rate of minerals is controlled by surface adsorption, surface exchange reaction and desorption, depending on pH of the solution and is directly proportional to\(a_{H^ + }^{n_\theta } \). When controlled by surface adsorption, i.e.,n θ= 1, the dissolution rate will decrease with increasing pH; when controlled by surface exchange reaction, i. e.,n θ= 0, the dissolution rate is independent of pH; when controlled by desorption,n θ is a positive decimal between 0 and 1 in acidic solution and a negative decimal between −1 and 0 in alkaline solution. Dissolution of many minerals is controlled by surface adsorption and/or surface exchange reactions under acid conditions and by desorption under alkaline conditions.

Influence of the electrolyte composition and pH on the anodic dissolution of p-type Si in aqueous HF solutions

The influence of the electrolyte composition and pH on the anodic currents obtained during electrochemical etching of p-type silicon in aqueous HF solutions has been investigated. Original and accurate pH measurements were performed to characterize the exact composition of the HF + H 2O electrolytes commonly used. It is shown that for these very acid solutions (pH < 2) almost all fluoride is in the form of the non-dissociated HF species which appears to play a preponderant role in the silicon dissolution reaction kinetics. The effect of pH can be restricted to its influence on the modification of the different concentrations by shifting the equilibria.

Kinetic Model of Zeolite Paragenesis in Tuffaceous Sediments

AbstractThe sequence of mineral reactions involving zeolites and other authigenic phases in tuffaceous sedimentary rocks can be explained by growth- and dissolution-reaction kinetics. Kinetic factors may determine the specific authigenic phases which form and the temporal and spatial constraints on the solution composition during irreversible dissolution and growth reactions in glass-bearing rocks. The glass phase generates a high level of supersaturation with respect to a variety of aluminosilicates in the pore fluid. The sequence of assemblages formed during a series of metastable reactions resembles an Ostwald step sequence. Metastable reactions occur because formation of less stable phases such as gels, clays, and disordered zeolites may lower the total free energy of the glass-bearing system faster than the growth of the stable assemblage including ordered feldspars, quartz, and micas. Eventually, after a series of steps, the most stable silicate assemblage for the bulk composition, temperature, and pressure may form. However, the formation of intermediate metastable phases can delay the attainment of equilibrium by as much as tens of millions of years.