Activation Energy For Dissolution Research Articles

Dissolution of Manganese Oxides of Various Compositions in Sulfuric Acid Solutions Studied by Kinetic Methods

The dissolution of manganese oxides in sulfuric acid solutions of various concentrations has been studied by kinetic methods. We have investigated the dissolution of manganese oxides of various compositions in sulfuric acid solutions of various concentrations. The composition of the manganese oxides and the sulfuric acid concentration have been shown to influence the dissolution rate. We have calculated kinetic parameters of the dissolution of manganese oxides in sulfuric acid solutions (dissolution rate, activation energy for dissolution, and reaction orders) and proposed a model for the dissolution of manganese oxides in sulfuric acid solutions.

Modeling of dissolution kinetics of rare earth crystals in a borosilicate glass melt

The novelty of this work is the comparison of two methods to determine the dissolution kinetics in order to evaluate the applicability of the Johnson-Mehl-Avrami-Kolmogorov (JMAK) model for the crystals dissolution in silicate melts. For this, this work focuses on the dissolution of rare earth (RE) silicates (Ca2RE8(SiO4)6O2 where RE=Nd) with an apatite structure in a sodium-borosilicate glass melt. The first approach consists in the characterization of the dissolution kinetics by following the crystalline area fraction by image analysis as a function of time and temperature. Then, a model based on the generalized JMAK equation is proposed to fit the experimental data. This model enables to determine both the mechanism limiting the dissolution (i.e. the diffusion) and the activation energy of crystals dissolution in the studied glass system (496kJ/mol). To support these results, a second and most common approach is the measure of the chemical profiles at the crystal/melt interfaces by microprobe. The conclusions obtained by this last method are in agreement with the conclusions based on JMAK model. All these results allowed to confirm that the JMAK model is well suited to model crystals dissolution in silicate melts.

Limestone dissolution and decomposition in steelmaking slag

ABSTRACTBased on thermal simulation experiment, SEM analysis and mathematical simulation, limestone dissolution and decomposition mechanism in steelmaking slag were studied. The results showed that limestone decomposition and dissolution happen simultaneously in molten slag, and influence each other. Owing to high-activity lime product and CO2 from limestone decomposition, the dissolution rate of limestone is greater than that of lime under the same conditions in slag, and the calculated activation energy of limestone dissolution is 226.8 kJ mol−1. On the other hand, as decomposition product lime dissolves into slag, a sloughing-type unreacted shrinking core model was proposed to describe limestone decomposition behaviour in slag. In addition, mathematical simulation results showed that heat transfer is the rate-controlling step for limestone decomposition in slag.

Kinetic study of the magnesium oxychloride cement cure reaction

Magnesium oxychloride (MOC) is a ceramic material with significant fire-resistant properties and growing potential as an alternative building material for passive fire protection systems. The present study examined the magnesium oxychloride 5-phase cure reaction at temperatures from 35 to 55 °C using time-resolved quantitative X-ray diffraction and differential scanning calorimetry to monitor kinetics. The reaction was characterized as a two-step process: dissolution of magnesium oxide followed by crystallization of magnesium oxychloride from the solvated state. With stoichiometric proportions, 37% of the MgO dissolves before the onset of crystallization at a critical amorphous concentration. A maximum crystallinity of 82–84% was achieved for each temperature. Assuming first-order kinetics for both MgO dissolution and MOC crystallization, a kinetic model predicts 42.4 and 26.1 kJ/mol for dissolution and crystallization activation energies, respectively. This model was applied to pilot-scale production and accurately predicts the cure reaction time as a function of cure temperature. In an alternative approach to modeling the cure reaction, the Avrami nucleation and growth model was fit to calorimetric measurements. This model predicts diffusion-controlled, one-dimensional growth with an activation energy of 72.4 kJ/mol, which accounts for both dissolution and crystallization.

Effect of ferrite content on dissolution kinetics of gibbsitic bauxite under atmospheric pressure in NaOH solution

The dissolution property of high-ferrite gibbsitic bauxite and the effect of ferrite content on the dissolution kinetics of gibbsitic bauxites in sodium hydroxide solution under atmospheric pressure from 50 to 90 °C were systematically investigated. The dissolution property of high-ferrite gibbsitic bauxite is increased by increasing the dissolution temperature and the NaOH concentration or decreasing the particle size of bauxite, which is easy to dissolve under atmospheric pressure. The kinetic equations of gibbsitic bauxites with different ferrite contents during the dissolution process at different temperatures for different times were established, and the corresponding activation energies were calculated. The ferrite in the gibbsitic bauxite reduces the dissolution speed and increases the activation energy of dissolution, the diffusion process of which is the rate-controlling step.

Consideration of enthalpic and entropic energy contributions to the relative rates of chalcopyrite dissolution in the presence of aqueous cationic impurities

To understand the effect of aqueous impurities on chalcopyrite dissolution during acid metalliferous drainage and hydrometallurgical processes, batch dissolution experiments were carried out at 650 and 750mV (SHE), pH1 and 35–75°C in the presence or absence of aqueous cationic additives. Activation energies (Ea) for chalcopyrite dissolution in the presence of additives at 750mV, derived using a modified ‘time to a given fraction’ method, demonstrate that Ea varies with reaction extent. The overall trend of evolution from interface- to transport-controlled mechanism was independent of additive type or addition. This suggests that it is not primarily variation in the enthalpy of dissolution that controls the significant changes in relative dissolution rates on addition of additives.Rather, the relative dissolution rates on addition of aqueous cations are found to relate to the M–O framework volume (M and O being the cationic additive and oxygen within H2O in the first sphere of hydration, respectively). There is good inverse linear correlation between this volume and the relative dissolution rate per unit ionic strength which varies as K+>Al3+>no additive>Mg2+>Na+>Ca2+. It is proposed that this effect is related to the relative strength of hydration of the additives with the commensurate entropic effect on solute hydration being advantageous for chalcopyrite dissolution on K+ and Al3+ addition and detrimental for the other additives. H4SiO4, the dominant aqueous species upon Si addition under the conditions examined, has low affinity for water, resulting in a detrimental entropic contribution to solute hydration and dissolution. The effect of Fe addition on relative dissolution rate is convoluted by the role of Fe3+ as the primary oxidant for chalcopyrite dissolution. On addition of aqueous Fe to dissolution at 750mV, the effect is detrimental due to the detrimental effect on the entropy of hydration of the solute and the relative abundance of Fe3+. At 650mV the effect of the greater presence of Fe3+ due to Fe addition dominates and the dissolution rate is enhanced.

The Dissolution Kinetics of MgO into CaO-MgO-Fe2O3 Slag

Calcium ferrite is the main binding phase for high-basicity sinter. The production and structure of calcium ferrite greatly influence the quality of the sinter. With the change in gangue composition, MgO becomes an important factor in the generation of calcium ferrite. In this study, the rotating cylinder method was used to study the dissolution kinetics of MgO into CaO-MgO-Fe2O3 melt. The experimental variables included the temperature, the initial composition of the melt, the Fe2O3/CaO mass ratio, the rotation time, and the rotation speed. The results indicate that the dissolution rate increases with increasing dissolution time, temperature, and rotation speed but decreases with increasing MgO content and Fe2O3/CaO mass ratio in the initial slag. The dissolution rate was observed to increase and then decrease with the addition of SiO2 in the initial slag. The activation energy and diffusion coefficient for MgO dissolution were found to range from 117.31 to 234.24 kJ mol−1 and from 1.03 × 10−6 to 1.18 × 10−5 cm2 s−1, respectively. The concentration difference between the solid and liquid phases is the main driving force for dissolution, but the viscosity and magnesium ion diffusivity of the melt also affect the process.

Anodic dissolution behavior of zirconium in Bun4NBr-containing isopropanol solution

Anodic dissolution behavior of zirconium in Bun4NBr-contaning isopropanol solution was investigated using cyclic voltammetry, linear sweep voltammetry and chronoamperometry, complemented with a scanning electron microscope (SEM). The voltammograms did not exhibit active dissolution until the breakdown of passive layer induced by aggressive bromide anions. SEM images confirmed the existence of pits on zirconium surface. The depth and breadth of pits were intensified with increasing potential. The pitting potential shifted negatively as either temperature or Bun4NBr concentration was increased, while it increased with increasing scan rate. The corrosion current density increased with increasing temperature. The apparent activation energy of anodic dissolution of zirconium was 21.88 kJ/mol. The chronoamperometry revealed that increasing Bun4NBr concentration shortened the incubation time for passivity breakdown and accelerated the pit nucleation and growth. The experimental results were helpful to obtain the optimum conditions for electrosynthesis of zirconium isopropoxide.

(Invited) Effect of Mechanical Stress on the Passivity of Steel

The influence of the elastic and plastic tensile stress on the electrochemical properties of mild carbon steel and austenitic stainless steel was studied using Scanning Kelvin Probe (SKP) and Localized Electrochemical Impedance Spectroscopy (LEIS) techniques. These two methods give complementary information on the surface of the stressed electrode. SKP characterizes the thermodynamic (surface potential) and LEIS is informative on the conditions of the passive oxide (capacitance of the film). The Volta potential and the admittance were mapped across an area with stress concentrator (notch) “in situ” under load. It was found that the elastic deformation slightly influences both the potential and the capacitance. The plastic strain decreases the potential measured in the lab air for both steels by 150 to 300 mV. LEIS shows an increase of the capacitance measured in borax electrolyte above the yielded surface in the notch relatively to the surrounding surface. Thus, the change inf the potential is reciprocal to the change of the capacitance. After straining, in load free conditions, stainless steel saved low potentials in the area containing residual stresses. It corresponds to LEIS data that shows an increased capacitance. In opposite, unloading of strained mild steel redistributes the surface potential. Mainly the surface increased the potential showing the passivation. However, low potential locations related to residual stress were also found by SKP. Mapping of the capacitance by LEIS also shows partial passivation of the steel. Thus, locations with concentrated stress, which are prone to localized corrosion can be recognized using local electrochemical techniques.After experiments, the strained surfaces were characterized by XPS and AFM. The effect of the plastic tensile strain was explained by the formation and emerging of dislocations and dislocations pile-ups. The dislocations rupture the oxide film making it more defective that weaken the steel passivity. On the other hand, the formation of the stepped surfaces decreased the potential that decreased the activation energy of anodic dissolution of the steel.

Experimental Evaluation of Kinetic and Thermodynamic Reaction Parameters of Colloidal Nanocrystals

The unique properties of colloidal semiconductor nanocrystals, or quantum dots, have attracted enormous interest in a wide range of applications, including energy, lighting, and biomedical fields. However, widespread implementation is hampered by the difficulty of developing large-scale and inexpensive synthesis routes, mainly due to our limited knowledge of formation reaction parameters. We report here a simple yet powerful method to experimentally determine critically important reaction parameters such as rate constants, activation barriers, equilibrium constants and reaction enthalpies. This method was applied to wurtzite cadmium selenide nanocrystals, yielding activation energies for growth and dissolution of 14 ± 6 kJ mol–1 and 27 ± 8 kJ mol–1, respectively, and a reaction enthalpy for nanocrystal growth of −15 ± 7 kJ mol–1. Moreover, the Gibbs free energy for growth was found to be negative at low temperatures, whereas dissolution becomes the spontaneous process above 150 °C.

Atomic steps on an ultraflat Si(111) surface upon sublimation

The kinetics of atomic steps on an ultraflat Si(111) surface is studied by in situ ultrahigh-vacuum reflection electron microscopy at temperatures of 1050–1350°C. For the first time it is experimentally shown that the rate of displacement of an atomic step during sublimation nonlinearly depends on the width of the adjacent terrace. It is established that the atomic mechanism of mass-transport processes at the surface at temperatures higher than 1200°C is controlled by nucleation and the diffusion of surface vacancies rather than of adsorbed Si atoms. The studies make it possible to estimate the activation energy of the dissolution of vacancies from the surface into the bulk of Si. The estimated activation energy is (4.3 ± 0.05) eV.

In situ structural characterization of ageing kinetics in aluminum alloy 2024 across angstrom-to-micrometer length scales

The precipitate structure and precipitation kinetics in an AlCuMg alloy (AA2024) aged at 190 °C, 208 °C, and 226 °C have been studied using ex situ Transmission Electron Microscopy (TEM) and in situ synchrotron-based, combined ultra-small angle X-ray scattering, small angle X-ray scattering (SAXS), and wide angle X-ray scattering (WAXS) across a length scale from sub-Angstrom to several micrometers. TEM brings information concerning the nature, morphology, and size of the precipitates while SAXS and WAXS provide qualitative and quantitative information concerning the time-dependent size and volume fraction evolution of the precipitates at different stages of the precipitation sequence. Within the experimental time resolution, precipitation at these ageing temperatures involves dissolution of nanometer-sized small clusters and formation of the planar S phase precipitates. Using a three-parameter scattering model constructed on the basis of TEM results, we established the temperature-dependent kinetics for the cluster-dissolution and S-phase formation processes simultaneously. These two processes are shown to have different kinetic rates, with the cluster-dissolution rate approximately double the S-phase formation rate. We identified a dissolution activation energy at (149.5 ± 14.6) kJ mol−1, which translates to (1.55 ± 0.15) eV/atom, as well as an activation energy for the formation of S precipitates at (129.2 ± 5.4) kJ mol−1, i.e. (1.33 ± 0.06) eV/atom. Importantly, the SAXS/WAXS results show the absence of an intermediate Guinier-Preston Bagaryatsky 2 (GPB2)/S″ phase in the samples under the experimental ageing conditions. These results are further validated by precipitation simulations that are based on Langer-Schwartz theory and a Kampmann-Wagner numerical method.

Effect of liquid structural transition on the dissolution of solid copper in liquid eutectic tin–bismuth

Abstract

Kinetics of electric arc furnace slag leaching in alkaline solutions

The dissolution of electric arc furnace steel slag (EAFSS) in highly alkaline MOH solution (M=alkali metal Na or K) has been investigated with emphases on the influence of the initial alkali concentration, solid to liquid (S/L) ratio and temperature on the kinetics of Si and Al dissolution from EAFSS. The shrinking core model (SCM) was used to analyze experimental data and the results have shown that the Si and Al dissolution from EAFSS was greatly influenced by MOH concentration, alkaline ion type and temperature. The dissolution process was controlled by the bulk diffusion through the product layer crystals. The activation energy for Si and Al dissolution was 55.27kJ/mol and 48.05kJ/mol in NaOH solution and 90.68kJ/mol and 33.62kJ/mol in KOH solution, respectively.

Dissolution behaviour of the γ′ precipitates in two kinds of Ni-based superalloys

The effects of high heating temperature and short dwell time on the evolution of γ′ precipitates were investigated in two kinds of Ni-based superalloys. The microstructural evolutions of the samples were characterised by qualitative and quantitative metallography during various heating cycles, and the dissolution kinetics were then evaluated. The results illustrated that the heating temperature plays a more predominant role in the dissolution of γ′ precipitates than the dwell time. Meanwhile, the γ′ morphology in the dissolution process was strongly influenced by the elastic strain energy associated with the lattice mismatch between γ and γ′ phases. For IN 100 alloy, the γ′ evolution was dominated by the elastic interaction energy between adjacent precipitates, but for DS Rene 125 alloy, the γ′ precipitates kept a regular arrangement along the [001] softest directions in the dissolution process. On increasing the dwell time for the two kinds of superalloys, the dissolution rate of γ′ gradually decreased, and the dissolution activation energy gradually increased. However, the dissolution activation energy for IN 100 alloy was different from that of DS Rene 125 alloy under the same heating conditions.

Coupled population balance and heat transfer model for the description of ice recrystallization during long-term storage of ice cream

A coupled population and energy balance model describing separately growth and dissolution of the ice crystals was developed to simulate recrystallization by Ostwald ripening during long-term storage of ice cream. The model reproduced accurately (R2 > 0.90) measurements of the ice crystal average diameter for two ice creams (ICA and ICB) stored at temperatures between −5 and −18 °C for 104 days. Simulations indicated that ice crystal dissolution controls recrystallization during the early stages of storage, after which both growth and dissolution occur at decreasing rates. Carrageenan, ICB primary stabilizer, seemed to better preserve small ice crystals than locust bean gum, ICA primary stabilizer, by reducing ice crystal growth and dissolution rates. A sensitivity analysis indicated that the activation energy for ice crystal dissolution is the most significant model parameter and that the impact of heat transfer parameters is negligible because of the rapid change of the ice cream temperature.

Thermochemical and kinetic investigations of amorphous silica dissolution in NaOH solutions

Dissolution of amorphous silica in 4, 8 and 11 mass% of sodium hydroxide solutions was followed by microcalorimetry at temperature between 353.15 and 403.15 K. Decreasing the Na2O/SiO2 molar ratio leads to the successive formation of the following dissolved entities Si4O11Na6, Si2O5Na2, Si3O7Na2 and Si5O11Na2. Their formation enthalpies were determined at 393.15 K as −5640.8, −2492.2, −3401.8 and −5214.7 kJ mol−1, respectively. A kinetic investigation showed that dissolution cannot be described by one-step controlled process. Isoconversional model allows determining a complex variation in the activation energy of dissolution in the range 6–21 kJ mol−1.

Kinetics of nickel extraction from Indonesian saprolitic ore by citric acid leaching under atmospheric pressure

The kinetics of leaching of a saprolitic ore from Indonesia by citric acid solution under atmospheric pressure was investigated. An examination of the effects of leaching temperature, citric acid concentration, pulp density and ore particle size on the dissolution rate ofnickel found that they all had significant influence on the rate. The highest nickel recovery (95.6 percent) was achieved under the leaching conditions of ore particle size of 212–355 microns, citric acid concentration of 1 M, leaching time of 15 days, pulp density of 20 weight/volume percent, leaching temperature of 40°C and shaker speed of 200 rpm. The shrinking core model was found to be appropriate for describing the leaching kinetics of this ore in citric acid solutions at atmospheric pressure. The experimental data were well interpreted by this model with rate ofreaction controlled by diffusion through the solid product layer. Using the Arrhenius expression, the apparent activation energy for nickel dissolution was evaluated as 12.38 kJ/mol. Finally, on the basis of the shrinking core model, a proposed empirical kinetic model for the leaching of nickel from this Indonesian saprolitic ore was expressed as a mathematical model, which was verified as consistent with the obsenved experimental results.

Dissolution of interdendritic σ phase in cast Ni–Fe–Cr alloy

Various sizes, fractions and morphologies of interdendritic σ phase in the as cast N08028 alloy due to different solidification conditions are characterised. Dissolution behaviours of the secondary phase in both the as cast and the forged alloys are investigated during heat treatment at 1100, 1150 and 1200°C. A recently developed analytical model, subjected to necessary modification, is applied to describe the dissolution process of σ phase in the alloys with and without prior hot deformation. A recipe derived from transformed fraction is applied to evaluate the effective activation energy for dissolution. It has been found that, the prior hot deformation accelerates the dissolution process, which can be attributed to the decreased particle size and the reduced effective activation energy.

In situ quantification of electrochemical dissolution of hafnium-tantalum alloys in acidic media

Abstract A thin film hafnium-tantalum combinatorial library with a compositional spread of over 70 at.% was used for electrochemical dissolution experiments in nitric acid. Surface microstructure analysis and crystallographic characterization of individual Hf–Ta alloys confirmed a hexagonal to tetragonal transition from pure Hf to pure Ta accompanied by a change in the surface grain structure. A flow-type scanning droplet cell microscope coupled to downstream analytics was used for the quantification of Hf and Ta dissolution rates along the entire compositional spread. Potentiostatically applying 3 V vs. SHE for 120 s for an electrolyte flow of 0.46 ml min − 1 resulted in dissolution rates of pure Hf and pure Ta in the ng s − 1 cm − 2 and pg s − 1 cm − 2 range, respectively. For both species, the average dissolution rate was independent of the compositional gradient, indicating a dissolution enhancement of minor species. A decrease in their activation energy for dissolution triggered by a surface energy modification was the reason for the observed behavior.