Kinetics of cellular dissolution in an AlZn alloy

Dissolution of the lamellar precipitate by cell boundary migration has been studied in an Al-18.9at.% Zn alloy in the temperature range 554–637 K. Microstructural observations have revealed that the process of dissolution in this alloy is cellular mode of transformation in the early stages. The boundary diffusivities were calculated by using the theory of Petermann and Hornbogen modified for cellular dissolution. The diffusivities calculated from the experimental data are four orders of magnitude higher than the volume diffusivities. From the Arrhenius plots, activation energies of 51.5 ± 2 and 47.5 ± kJ mol⁻¹ were obtained from the temperature dependence of diffusivity and mobility respectively. These values are about half the activation energy required for volume diffusion of Zn in Al and compare very well with the activation energy of 60 kJ mol⁻¹ for the cellular precipitation in this alloy system. There exists a range of temperature between 502 and 532 K where the two kinetic processes, cellular precipitation and dissolution, are equally probable. The forward migration of the grain boundary during cellular precipitation is acted upon by the back pull of dissolution and the migrating grain boundary remains motionless.     

Kinetics of sulphuric acid leaching of a zinc silicate calcine

Recent developments of acid leaching and solvent extraction of zinc silicate ores have produced renewed commercial interest. However, the leaching kinetics of these concentrates has received little attention. This work, therefore, addresses the leaching of a zinc silicate concentrate in sulphuric acid. The effects of particle size (0.038–0.075mm), temperature (30–50°C) and initial acid concentration (0.2–1.0mol/L) were studied. The results show that decreasing the particle size while increasing the temperature and acid concentration increase the leaching rate. As leaching occurs, there is a progressive dissolution of willemite while the quartz and iron-containing phases remain inert. Among the kinetic models of the porous solids tested, the grain model with porous diffusion control successfully described the zinc leaching kinetics. The model enabled the determination of an activation energy of 51.9 ± 2.8kJ/mol and a reaction order of 0.64 ± 0.12 with respect to sulphuric acid, which are likely to be a consequence of the parallel nature of diffusion and chemical reaction in porous solids.     

The kinetics of dissolution of sphalerite in ferric chloride solution

The dissolution of sphalerite in acidic ferric chloride solution was investigated in the temperature range 320 to 360 K. Both sized particles from three sources and polished flat surfaces were used as samples. The effect of stirring rate, temperature, ferric and ferrous ion concentration, purity, and particle size on the dissolution rate were determined. During the initial stages of the process chemical reaction at the mineral surface is rate controlling while during the later stages diffusion through the product sulfur layer is rate controlling. Overall the process follows the mixed-control model embodying both chemical reaction and diffusion. The activation energy for the dissolution of sphalerite particles was found to be 46.9 kJ/mol.     

Initial-stage Sintering Kinetics of Nanocrystalline Tungsten

Initial-stage sintering kinetics of nanocrystalline tungsten has been studied in the temperature range of 1273–1473 K (1000–1200 °C). Nanocrystalline tungsten sinters initially through a grain boundary diffusion mechanism. The calculated activation energy was 388 ± 11 kJ/mol at low temperatures (1273–1373 K (1000–1100 °C)) and 409 ± 7 kJ/mol at high temperatures (1373–1473 K (1100–1200 °C)), which are close to the experimentally measured activation energy for grain boundary diffusion (385 kJ/mol).     

Kinetics of leaching of lead sulfate in sodium chloride solutions

A powder dissolution method is used to study the kinetics of leaching of lead sulfate from a lead cake in the temperature range 298–353 K and the concentration range 50–270 g/l of a sodium chloride solution. The results obtained are used to calculate the apparent activation energy (E _a = 12.41 kJ/mol); its value indicates that the process of leaching of lead sulfate from a lead cake occurs in a diffusion zone. This process is shown to be described by a first-order kinetic equation.     

Carbon/oxygen synergism during elevated temperature sustained load cracking

Brittle intergranular cracking was observed on a medium carbon, matensitic steel subjected to sustained loading in air at 185–500°C. The slow crack growth rate increased with temperature in the lower temperature regime with an apparent activation energy of 68.5 kJ/mol. It dropped down rapidly after reaching its maximum at ∼400°C. No slow crack growth was observed above 500°C. Control tests at 300°C in argon and nitrogen gases indicated that the presence of oxygen enhanced the slow crack growth rate. Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) revealed enriched carbon and oxygen all the time and sulfur occasionally on the fractured intergranular facets. The latter was shown likely to be due to free surface segregation and thus not responsible for the embrittlement. Based on AES/XPS analysis and kinetic considerations, it is proposed that a synergetic effect of carbon and oxygen led to the observed embrittlement phenomenon. The kinetic behavior of the embrittlement is modelled based on the diffusion and collection of carbon in the process zone.     

The leaching of galena in cupric chloride media

The kinetics of dissolution of galena (PbS) in CuCl₂-HCl-NaCl media have been investigated using the rotating disk technique. Rapid nonlinear kinetics are observed over the temperature range 45 °C to 90 °C and for NaCl concentrations from 0 to 4.0 M. The leaching rate is in-dependent of CuCl₂ concentrations >0.1 M but increases with increasing concentrations of either HC1 or NaCl. The leaching rate decreases with the accumulation of either the PbCl₂ or CuCl reaction product in the leaching medium but is insensitive to the disk rotation speed. The apparent activation energy for the rate controlling process is 33 kJ/mol, and this value falls to about 15 kJ/mol when interpreted as a dissolution rate for PbCl₂, whose solubility increases with temperature. The above observations are shown to be consistent with rate control by the outward diffusion of the PbCl₂ and CuCl reaction products through the solution trapped in the pores of the constantly thickening elemental sulfur layer which forms on the surface of the galena. CANMET, Energy, Mines, and Resources Canada, 555 Booth Street, Ottawa, ON, K1A 0G1, Canada.     

Study of the mechanism of fast diffusion of cobalt in thorium by diffusion and internal friction measurements

The activation energy for diffusion,Q _d, of cobalt in fcc thorium was determined to be 83.7±4 kJ/mol from chemical diffusion measurements between 900 °C and 1332 °C. An internal friction peak associated with cobalt was identified in thorium at 45°C, and the activation energy for relaxation,Q _R, was found to be 76.0±1.5 kJ/mol by a frequency variation technique. The nearly equal values forQ _D andQ _R eliminate the interstitial-vacancy pair mechanism for fast diffusion in this system. The host-solute diplon and substitutional-interstitial (S-I) pair models for fast diffusion are considered, however, and preference is given to the latter mechanism for cobalt in thorium. S.C. AXTELL, formerly Graduate Student, Ames Laboratory, Iowa State University     

Dilatometric technique for evaluation of the kinetics of solid-state transformation of maraging steel

Solid-state transformation kinetics of a 350 grade commercial maraging steel were investigated using a nonisothermal dilatometric technique. Two solid-state reactions—namely, precipitation of intermetallic phases from supersaturated martensite and reversion of martensite to austenite—were identified. Determination was made of the temperatures at which the rates of these reactions reached a maximum at different heating rates. The kinetics of the individual reactions in terms of activation energy were analyzed by simplified procedures based on the Kissinger equation. An estimated activation energy of 145 ± 4 kJ/mol for the precipitation of intermetallic phases was in good agreement with reported results based on the isothermal hardness measurement technique. Martensite to austenite reversion was associated with an activation energy of 224 ± 4 kJ/mol, which is very close to the activation energy for diffusion of substitutional elements in ferrite. Results were supplemented with microstructural analysis.     

Dilatometric technique for evaluation of the kinetics of solid-state transformation of maraging steel

Solid-state transformation kinetics of a 350 grade commercial maraging steel were investigated using a nonisothermal dilatometric technique. Two solid-state reactions—namely, precipitation of intermetallic phases from supersaturated martensite and reversion of martensite to austenite—were identified. Determination was made of the temperatures at which the rates of these reactions reached a maximum at different heating rates. The kinetics of the individual reactions in terms of activation energy were analyzed by simplified procedures based on the Kissinger equation. An estimated activation energy of 145 ± 4 kJ/mol for the precipitation of intermetallic phases was in good agreement with reported results based on the isothermal hardness measurement technique. Martensite to austenite reversion was associated with an activation energy of 224 ± 4 kJ/mol, which is very close to the activation energy for diffusion of substitutional elements in ferrite. Results were supplemented with microstructural analysis.     

The kinetics of disperse order development in α-cual alloys

The kinetic parameters of the Mehl-Johnson-Avrami rate equation for disperse order (DO) formation in a-CuAl alloy were determined from nonisothermal experiments. Activation energy values of 145 ± 7/155 ± 10/178 ± 14 kJ/mol were obtained from rate dependent curves for alloys containing, respectively, 19, 13, and 6.5 pct Al. These values are consistent with those for self-diffusion. Further analysis allowed for the determination of the preexponential factorko and the exponentn (= 1.39 ± 0.2), the latter being found to be insensitive to alloy composition. This indicates a diffusion limited growth of particles from a nonnegligible size, suggesting that the preexisting short range order acts as nuclei for DO development. By employing the absolute reaction rate theory, a virtually constant activation free energy, ΔG* = 163 ± 15 kJ/mol, was measured for the three alloys investigated. These results indicate that the transformation proceeds more rapidly with increasing aluminum content of the alloy. Values for domain size and concentration, computed by the classical theory for nucleation and growth processes, are compared with those obtained experimentally. Also, an (α + DO) field is proposed in the Cu-Al equilibrium diagram.     

Diffusion of cobalt,chromium, and titanium in Ni3Al

Diffusion studies of cobalt, chromium, and titanium in Ni₃Al (γ′) at temperatures between 1298 and 1573 K have been performed using diffusion couples of (Ni-24.2 at. pct Al/Ni-24.4 at. pct Al-2.91 at. pct Co), (Ni-24.2 at. pct Al/Ni-23.1 at. pct Al-2.84 at. pct Cr), and (Ni-24.2 at. pct Al/Ni-20.9 at. pct Al-3.17 at. pct Ti). The diffusion profiles were measured by an electron probe microanalyzer, and the diffusion coefficients of cobalt, chromium, and tita-nium in γ′ containing 24.2 at. pct Al were determined from those diffusion profiles by Hall’s method. The temperature dependencies of their diffusion coefficients (m[su2]/s) are as follows: ~D_(Co) = (4.2 ± 1.2) × 1O^-3exp {-325 ± 4 (kJ/mol)/RT} ~D_(Cr) = (1.1 ± 0.3) × 10^-1 exp {-366 ± 3 (kJ/mol)/RT} and D_(Ti) = (5.6 ± 3.1) × 10¹ exp {-468 ± 6 (kJ/mol)/RT} The values of activation energy increase in this order: cobalt, chromium, and titanium. These activation energies are closely related to the substitution behavior of cobalt, chromium, and titanium atoms in the Ll₂ lattice sites of γ′; the cobalt atoms occupying the face-centered sites in the Ll₂ structure diffuse with the normal activation energy, whereas the titanium atoms oc-cupying the cubic corner sites diffuse with a larger activation energy that includes the energy due to local disordering caused by the atomic jumps. The chromium atoms which can occupy both sites diffuse with an activation energy similar to that of cobalt atoms.     

Kinetics of leaching of copper metal in copper(II)-ammonium sulphate solutions as determined by the rotating disc method

The influence of the concentration of copper(II)-ammine-sulphate complexes, temperature and the concentration of ammonia on the velocity of the leaching of copper metal in sulphate solutions was investigated by application of the rotating disc method.The results show that the following factors determine the velocity of the process: transport of copper(II) complexes to the reaction surface and the rate of the surface reaction (mixed control).An attempt was made to derive a semi-empirical equation describing the kinetics of the process. The value of the diffusion coefficient, calculated on the basis of the derived formula and the experimental data, is equal to (1.2 ± 0.1) × 10^?9 m² s^?1 (for solutions containing from 0.0012 to about 1 mol/l copper, 6.5 mol/l ammonia and 1.2 mol/l sulphates; temperature 25°C).The diffusion activation energy, determined in the range of temperatures from 15 to 50°C equals 30.1 ± 1.7 kJ/mole. This is surprisingly high especially in comparison with the activation energy of viscous flow determined at the same conditions, which is 15.9 ± 0.4 kJ/mole.Discussion of the results leads to the conclusion that free ammonia (the stoichiometric excess in relation to the amount necessary for a complete bonding of copper in a complex) does not participate in the electrode reaction. However, it diminishes the velocity of transport of copper complexes to the reaction surface.     

Kinetics of sphalerite bioleaching by Acidithiobacillus ferrooxidans

In the present study, the effect of some important parameters including particle size, pulp density and temperature on the rate of Zn dissolution from sphalerite concentrate by Acidithiobacillus ferrooxidans was investigated. The highest rate of sphalerite bioleaching was obtained at particle size, pulp density and temperature of 38–150 μm, 4% wt/vol and 33 °C respectively. The formation of a product layer over sphalerite concentrate particles was confirmed by SEM images whereas XRD, EDS and BET analysis showed that this layer is composed of elemental sulfur and is non-porous. Based on these results, it was decided that a kinetic model in which the rate of Zn dissolution is limited by diffusion of ions through a non-porous product layer is appropriate to describe the sphalerite bioleaching process. Determinations of ferrous iron ion concentration during bioleaching showed that the concentration of this ion varies significantly during bioleaching and hence the kinetic model was revised to take account of this fact. The predictions of this model had a good compatibility with the experimental data and the value of activation energy was determined as 39 kJ/mol.     

高磷鲕状赤铁矿酸浸脱磷动力学

在HSC6.0计算软件热力学分析的基础上,采用正交实验确定了高磷鲕状赤铁矿酸浸脱磷保铁的最佳工艺,并以最佳工艺为基础进行了酸浸过程中脱磷和铁损反应的动力学研究。热力学分析表明H_2SO_4为最佳酸浸用酸。正交实验得出最佳酸浸条件为:H+浓度为0.5mol/L的H_2SO_4溶液、酸浸时间40min、温度298K、液固比200mL∶14g、搅拌速度100r/min。在该条件下,脱磷率可达98.89%,铁损率仅为0.51%。通过SEM-EDS对酸浸前后高磷鮞状赤铁块矿试样分析表征得出:经H_2SO_4浸出后,磷灰石基本完全溶解,含铁矿相未发生明显反应。动力学分析显示:优化条件下,酸浸脱磷反应在298~328K内符合收缩未反应核模型,浸出过程主要受内扩散控制,表观活化能为11.24kJ/mol;铁损反应在298~328K内遵循收缩未反应核模型,浸出过程主要受化学反应控制,表观活化能为42.24kJ/mol。     

Kinetics of manganous oxide sulfidization in carbonyl sulfide (COS) contaminated atmospheres

To contribute to the mitigation of man-made emission of sulfurous compounds, the susceptibility of manganous oxide for carbonyl sulfide under reducing atmospheres (C-O-S system) has been investigated over the temperature range 700 to 1010 °C (973 to 1283 K). The kinetic investigation employed thermogravimetry and anin situ solid electrolyte oxygen probe to follow the topochemical reaction of spherical MnO pellets under various experimental conditions. The apparent activation energy of sulfidization of manganous oxide, under measured oxygen potentials in the C-O-S system, was determined to be 12.06 (±1.5) kcal/mol (52.33 (±6.28) kJ/mol). Overall sulfidization appeared to proceed by mixed control involving convective mass transfer of COS across the boundary layer and diffusion through the product layer. Formerly with the Department of Theoretical Metallurgy, Royal Institute of Technology, Stockholm     

Kinetics of manganous oxide sulfidization in carbonyl sulfide (COS) contaminated atmospheres

To contribute to the mitigation of man-made emission of sulfurous compounds, the susceptibility of manganous oxide for carbonyl sulfide under reducing atmospheres (C-O-S system) has been investigated over the temperature range 700 to 1010 °C (973 to 1283 K). The kinetic investigation employed thermogravimetry and anin situ solid electrolyte oxygen probe to follow the topochemical reaction of spherical MnO pellets under various experimental conditions. The apparent activation energy of sulfidization of manganous oxide, under measured oxygen potentials in the C-O-S system, was determined to be 12.06 (±1.5) kcal/mol (52.33 (±6.28) kJ/mol). Overall sulfidization appeared to proceed by mixed control involving convective mass transfer of COS across the boundary layer and diffusion through the product layer. Formerly with the Department of Theoretical Metallurgy, Royal Institute of Technology, Stockholm     

Kinetic investigations of two processes for zinc recovery from zinc plant residue

Abstract

This paper presents a kinetic study of hydrometallurgical and pyrometallurgical processes employed for the recovery of zinc from a zinc plant residue (ZPR). The hydrometallurgical process investigated involved zinc leaching from the ZPR with sulphuric acid solution, while the pyrometallurgical process was based on zinc oxide reduction from the ZPR using activated carbon as the reducting agent. At the optimum leaching conditions, the zinc recovery was 89·5% at 95°C after 60?min. The data obtained for the leaching kinetics indicated that the dissolution of ZPR is a diffusion controlled process and the activation energy is equal to 21±2?kJ?mol^?1. The maximum zinc recovery, obtained during pyrometallurgical treatment was 72% at 1300°C after 60?min. Kinetic investigations revealed that the zinc oxide reduction is carried out in the transient kinetic area, where both diffusion and the chemical reaction control the overall process rate, with an activation energy of 38±5?kJ?mol^?1.

Cet article présente une étude cinétique d'un procédé hydrométallurgique et d'un procédé pyrométallurgique utilisés pour la récupération du zinc du résidu d'une usine de zinc (ZPR). Le procédé hydrométallurgique investigué impliquait la lixiviation du zinc du résidu de l'usine de zinc avec une solution d'acide sulfurique, alors que le procédé pyrométallurgique était basé sur la réduction de l'oxyde de zinc du résidu de l'usine de zinc en utilisant du charbon activé comme agent de réduction. Aux conditions optimales de lixiviation, la récupération du zinc était de 89·5% à 95°C après 60?min. Les données obtenues pour la cinétique de lixiviation ont indiqué que la dissolution du résidu de l'usine de zinc était un procédé contrôlé par la diffusion et l'énergie d'activation est égale à 21±2?kJ?mol^?1. La récupération maximale de zinc, obtenue lors du traitement pyrométallurgique, était de 72% à 1300°C après 60?min. Les investigations de la cinétique ont révélé que la réduction de l'oxyde de zinc était effectuée dans la zone de cinétique transitoire, où tant la diffusion que la réaction chimique contrôlent la vitesse globale du procédé, avec une activation d'énergie de 38±5?kJ?mol^?1.     

Dissolution of Cu2S in aqueous edta solutions containing oxygen

The mechanism of the reactions taking place in the heterogeneous system: synthetic polydispersive Cu₂S-ethylediaminetetraacetic acid (EDTA)—O₂—H₂O has been investigated. The partial pressure of oxygen and pH of the solution were found to exert a significant effect on the process kinetics. The dissolution rate does not depend, in practice, on the agitation rate and the EDTA concentration exerts an influence only at higher partial oxygen pressures.Dissolution of Cu₂S in aqueous EDTA solutions proceeds in two steps with the formation of CuS as an intermediate. In acid and neutral solutions the final products of dissolution are elementary sulphur and Cu(EDTA)^2- complex ion. The activation energy ΔE = 10.4kJ/mol (2.4 kcal/mol) suggests a diffusion controlled process. In alkaline solutions sulphur is oxidized to the sulphate ion and the dissolution process is kinetically controlled, ΔE = 41.4 kJ/mol (9.9 kcal/mol).     

The sintering of 304L stainless steel powder

An examination of the shrinkage kinetics for a 304L stainless steel powder showed that initial densification is controlled by a strain assisted volume diffusion mechanism. At temperatures above 1330 K, grain growth reduces the shrinkage rate; however, at lower sintering temperatures, the shrinkage rate is temporarily increased by the proximity of the moving grain boundaries to the interparticle necks. The activation energies of volume diffusion (240±20 kJ/mol) and grain growth (285±35 kJ/mol) were in good agreement with prior results.