Iranian Journal of Materials Science and Engineering

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Two commercial methods are used for the production of strontium carbonate:1) Direct conversion of Celsetite to strontium carbonate by hot sodium carbonate,2) Carbothermic reduction of celestite with coal followed by water leaching of strontium sulfide(SrS) and its conversion to strontium carbonate.The present study has been made on the carbothermic reduction of celestite ores of Varamin (Iran) mines. Effects of temperature, time, pellet size, particle size of celestite ore, pellet compactness and type of reducing agent have been studied. In the range of 800-1100°C, reduction rate increases notably with temperature, which may mean that the reduction is predominantly chemical controlled. Activation energy of around 22.5 kcal/mol supports the idea of chemical control mechanism. Further support for this postulation is provided by the following facts:1) Increasing rate with carbon reactivity (graphite, coal, and charcoal)2) Small dependency of rate on pellet compactness.3) Small dependency of rate on pellet size

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This study was launched to investigate the effects of heating rate and aging parameters on the kinetic of precipitation reactions in a high alloy high strength steel having Ni, Co, Mo and Ti. For this purpose, as quenched specimens were subjected to three types of aging methods with different heating rates. These methods consisted of aging in Pb bath, salt bath, and furnace at different aging cycles. The kinetic of precipitation in each method was studied by hardness measurements and was described adequately by the Johnson-mehl-Avrami equation. Remarkable increase in hardness and its rate is observed when the rate of heating increases. The substantial increase in hardness of the specimens aged rapidly in salt & Pb baths, compared with those aged normally in furnace, seemed to be due to the formation of thermo elastic stresses during sudden expansion of the substance subjected to rapid heating. According to the results obtained in this research, increase in the Avrami constants, n & k, and decrease in the start time of transformation, ts, are associated with heating rate increasing. Analysis of the observed and calculated data for hardness using Arrhenius equation, shows that for the same amount of volume fraction of precipitates, the activation energy of precipitates decreased for f=25 and 50%, while at f=90 % it increased by increasing heating rate.

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Abstract: In this paper solid state reduction of high carbon ferrochromium-chromite composite pellets in the temperature range of 900-1350°C was investigated. A two stage reduction mechanism is proposed. The first stage is likely to be controlled by the chemical reaction with activation energy of 127.2kJ/mol. In the second stage, solid state diffusion of carbon through the reaction product layer is suggested to be rate controlling. The activation energy of this stage was calculated to be 93.1kJ/mol. The reduction process was found to be favored by high temperatures as well as high vacuum. The results also show that pre-milling of initial mixture has a negative effect on the reduction degree.

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Abstract: The possibility of vanadium carbide coating formation on AISI L2 steel was studied in molten salt bath containing 33 wt% NaCl- 67 wt% CaCl2. In this research, the effects of time, temperature and bath composition on growing layer thickness were studied. The vanadium carbide coating treatment was performed in the NaCl-CaCl2 bath at 1173, 1273 and 1373 K temperatures for 3, 6, 9 hours and in bath containing 5, 10, 15, 25 wt% ferrovanadium. The presence of VC formed on the surface of the steel substrate was confirmed by optical microscopy, scanning electron microscopy (SEM) and X-ray diffraction analysis. The layer thickness of vanadium carbide and surface hardness ranged between 4.8 to 25.7 µm and 2645 to 3600 HV, respectively. The kinetics of layer growth was analyzed by measuring the depth of vanadium carbide layer as a function of time and temperature. The mean activation energy for the process is estimated to be 133 kJ/ mol.

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  Abstract

  The effect of mechanical activation using an attrition mill on the particle size of an ilmenite concentrate and its effect on the ability of the concentrate for Iron separation during hydrochloric acid leaching and the kinetics of the dissolution process have been investigated. It was observed that mechanical activation in an attritor significantly enhances the dissolution of iron in hydrochloric acid while have a slight effect on dissolution of titanium. With the mechanically activated ilmenite using an attrition mill, leaching conversion at 90 ^oC reached to 80%. The kinetic data of leaching of mechanically activated ilmenite was found to follow shrinking core model. Mechanically activating ilmenite using the attrition mill was found to cause the activation energy of leaching to be decrease from 43.69 , found for samples leached without mechanically activated, to 18.23 .

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Abstract:

titanium sheets in pure molten aluminum at 750

and X-Ray Diffraction Analysis results, TiAl

intermetallic layer thickness increases slowly at primary stages. After that an enhanced growth rate occurs due to layer

cracking and disruption. Presumably, reaction starts with solving titanium into the molten aluminum causing in

titanium super saturation and TiAl

intermetallic layer which consequently leads to TiAl

energy of intermetallic layer formation and growth was developed by measuring titanium thickness decreases.

In this work, kinetics of intermetallic compounds formation in Al-Ti system was studied by immersingoC, 850 oC and 950 oC. According to Scanning Electron Microscopy3 is the only phase can form at the interface. Observations revealed that3 formation. At this stage, growth may be controlled by aluminum diffusion through3 formation at the interface of Ti-TiAl3. Furthermore, activation

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Abstract:

nanocrystalline nickel samples with the grain size of ~25 nm were prepared via direct current electrodeposition and

aluminized for different durations by pack cementation method at 500

means of SEM, EDS and XRD techniques. According to results, short time aluminizing resulted in the formation of a

single aluminide layer whereas at long duration two distinct aluminide layers were formed. The growth kinetics of the

coating was non-parabolic at short times while it obeyed the parabolic law at long duration. The parabolic growth

rate constant of single phase coating formed on electrodeposited samples was about 30 ìm / h1/2 approximately 3 times

greater than the data reported for coarse grained nickel (8.4 ìm / h1/2). Meanwhile, the overall growth rate constant

was decreased to 11.7 ìm / h1/2, when double aluminide layers formed on nanocrystalline nickel.

In this research, aluminizing behavior of ultra fine-grained nickel was investigated. For this purpose,oC. The aluminide layers were examined by

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Abstract:

has been investigated. It has been shown that the mechanism of leaching reaction is diffusion through the product layer

and does not undergo any change as a result of mechanical activation in a wide range of experimental conditions.

Leaching rate is strongly influenced by milling intensity and the effect of ball to powder mass ratio is stronger than

milling time. Curve fitting of experimental data shows that leaching rate constant is approximately a linear function

of ball to powder mass ratio, while it obeys a power function with regard to the milling time.

The kinetics of chalcopyrite leaching in a ferric sulfate media for raw and mechanically activated samples

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Abstract:

the competition between the precipitation of cobalt ions and evolution of hydrogen gas on the cathode surface during

the reduction process in a sulfate bath, investigation on the mechanism of metal precipitation is of great importance.

In the present work, study on the kinetics of cobalt electrowinning and the mechanism of the involved reactions have

been carried out. The obtained results, confirm the mechanism of cobalt precipitation by depletion of hydroxides. The

effects of temperature and scan rate parameters were studied on electrowinning of cobalt by cyclic voltammetry

technique. The diffusion coefficient and rate constant of the reactions were measured and calculated by performed

experiments.

Nowadays cobalt is mostly produced through the electrowinning process of sulfate solutions. Regarding to

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Abstract: In this research work, crystallization kinetics of Fe55Cr18Mo7B16C4 alloy was evaluated by X-ray diffraction, TEM observations and differential scanning calorimetric tests. In practice, crystallization and growth mechanisms were investigated using DSC tests in four different heating rates. Results showed that a two -step crystallization process occurred in the alloy in which - Fe phase was crystallized in the first step after annealing treatments. Activation energy for the first step of crystallization i.e. - Fe was measured to be 276 (kj/mol) according to Kissinger model. Further, avrami exponent calculated from DSC curves was 2 and a three -dimensional diffusion controlled mechanism with decreasing nucleation rate was observed in the alloy. It is also known from the TEM observations that crystalline á – Fe phase nucleated in the structure of the alloy in an average size of 10 nm and completely mottled morphology.

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Abstract: Reduction of the Titanium dioxide, TiO2, by methane was investigated in this work. The thermodynamic of reaction was examined and found favorable. The reaction of titanium dioxide with methane was carried out in the temperature range 1150°C to 1450°C at atmospheric pressure with industrial high porosity pellets prepared from titanium dioxide powder. The evolved gas analyzing method was used for determination of the extent of reduction rate. The gas products of the reaction are mostly CO and trace amount of CO2 and H2O. The synthesized product powder was characterized by X-ray diffraction (XRD) for elucidating solid phase compositions. The effect of varying temperature was studied during the reduction. The conversion-time data have been interpreted by using the grain model. For first order reaction with respect to methane concentration, the activation energy of titanium dioxide reduction by methane is found to be 51.4 kcal/gmole. No detailed investigation of kinetic and mechanism of the reaction was reported in literatures.

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Abstract:

the reduction agent. Pellets of barite ore containing about 95% BaSO

temperature, time, ore grain size and the type and grain size of the carbon materials. Graphite, coke and charcoal have

been used as the reducing agent and the reduction experiments have been performed in the temperature range of 925-

1150 °C. Apart from conducting the experiments using pellets made of ore powder, kinetic analysis of the experimental

data by use of the reduced (dimensionless) time method has been another unique feature of the present study.

Experimental results show that grain size of either carbon material or barite ore has not appreciable effect on the

reaction rate. Kinetic analysis of the experimental data revealed the rate is strongly controlled by the chemical reaction

of carbon gasification (Boudouard reaction). The reaction rate is very considerably related to the type of carbon

material so that the activation energy varies from 15.6 kcal.mol

kcal.mol

gasification.

The present study deals with the reduction of barium sulfate (Barite) to barium sulfide by use of carbon as4 has been reduced under different conditions of-1 for charcoal to 26.3 kcal.mol-1 for graphite and 20.8-1 for coke. This behavior provides further support for the postulated reaction mechanism, i.e., carbon

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The process of mullitization of kyanite concentrate was studied at different conditions of heat treatment (1400
– 1600 °C and 0.5 – 3.5 hours) and particle size of raw materials (38-300 ?m). Kyanite concentrate was obtained from
ore-dressing of kyanite deposits of Mishidowan-Bafgh region at 100 km northeastern part of Yazd. The results of
microstructure (shape, distribution and size of the grains) and phase evolution studies by SEM and XRD showed that
total transformation of kyanite to mullite takes place by heat treatment between 1500 –1550 °C during 2.5 hours.. At
temperatures below 1500 °C need-like mullite grains are always produced. At higher temperatures the mullite grains
reveal rounded and platelet morphology. At 1550 °C, the rate of mullitization and densification were improved by
increasing soaking time from 1h to 3h and decreasing particle size of materials from 300 to 38 m

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The effect of Y2O3, CeO2, P2O5, ZrO2 and TiO2 in single, double and triple form on crystallization mechanism of Li2OAl2O3- SiO2(LAS) glass-ceramic system was investigated .The nucleation and crystallization peak temperatures of optimized samples in each group were determined by Ray & Day method .The crystalline phase was determined by the X-ray diffractometery .The micro-structure of the samples was studied by SEM techniqe .Crystallization activation energy ,E, and kinetic constants(n ,m) were determined by differential thermal analysis (DTA) through Marotta and Augis-Bennet methods .According to the results ,the Avrami constants(m ,n) derived from the Marotta and Augis- Bennett, glasses containing both ZrO2 and TiO2 nuclei were showed bulk crystallization .The crystallization mechanism of specimens containing ZrO2, TiO2 and CeO2 in the triple nuclei series represent two-dimensional bulk crystallization .By comparison of Avrami constants and activation energy of crystallization of optimized samples with other results they gave much lower value of E(255.5 kJ/mol) and higher value of n in 4.38.The lattice constants of the main phase( -eucryptite solid solution)in samples were determined according to the XRD results

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In the present study, reduction of zinc oxide from the pellets made of steelmaking electric arc furnace dust has been investigated. Effects of such parameters as the type of carbon material (graphite, coke and charcoal) as well as time and temperature on the reduction reaction have been examined. The reduced (dimensionless) time method was applied to perform a kinetic analysis of the system. Experimental results showed that increasing the temperature in the range of 925-1150°C results in a remarkable increase in the reduction rate. It was also shown that the reduction process is controlled by chemical reaction. Meaningful difference in the activation energy values calculated for reduction with graphite (24.75 kcal/mol), coke (18.13 kcal/mol) and charcoal (11.52 kcal/mol) indicate the predominant role of chemical reaction (carbon gasification) in the overall reaction rate and its rate-controlling mechanism. Carbothermal reduction of pelletized EAF dust proved to be an efficient reduction method, so that above 90% reduction was achieved in about one hour at temperatures around 1100°C.

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In this paper, gold leaching of a refractory sulfide concentrate by chloride–hypochlorite solution was investigated and effects of stirring speed, temperature and particle size on the leaching rate were reported. Experimental data for leaching rate of gold were analyzed with the shrinking–core model. Results were consistent with chemical reaction control mechanism in the first 1 h of leaching and diffusion control mechanism in the second 1 h. Apparent activation energy also was found to be 22.68 kJ/mol in the first step and 3.93 kJ/mol in the second step of leaching.

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In this study, the crystallization behavior of a 65GeO₂-15PbO-10MgF₂-10MgO glass (prepared by the conventional melt quenching technique) has been investigated. The microstructure and crystallization behaviors of this glass were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), non-isothermal differential thermal analysis (DTA) and Fourier transform infrared spectroscopy (FTIR). The results demonstrated that a fully glassy phase can successfully be prepared by the conventional melt quenching technique exhibiting one-stage crystallization on heating, i.e., the glassy phase transforms into crystalline MgGeO₃ and Pb₅GeO₇ phases. The activation energy for the crystallization, evaluated from the Kissinger equation, was approximately 202±5 kJ/mole using the peak temperature of the exothermic reaction. The Avrami exponent or reaction order, n, indicates the nucleation rate in this glass to increase with time and the crystallization to be governed by a three-dimensional interface-controlled growth.

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An accurate prediction of reaction kinetics of silicon nitridation is of great importance in designing procedure of material production and controlling of reaction. The main purpose of the present study is to investigate the effect of temperature on the kinetics of reaction bonded silicon nitride (RBSN) formation. To achieve this, nitrogen diffusion in the silicon nitride layer is considered as a reaction controlling factor and sharp interface method based on this theory is used to develop the analytical model. In the developed model, the variations in the size of silicon particles are calculated for the whole reaction. In the experimental phase, the extent of nitridation is measured for different reaction temperatures and 4 different reaction times and then, the occurrence of full nitridation is shown by EDS analysis. Furthermore, an analytical approach was established for describing the kinetics of compound formation and the performance of the developed model is evaluated through statistical analysis. There was good agreement between experimental data and predictions of the developed model which demonstrates the accuracy of considered presumptions and reaction mechanisms. An accurate prediction of reaction kinetics of silicon nitridation is of great importance in designing procedure of material production and controlling of reaction. The main purpose of the present study is to investigate the effect of temperature on the kinetics of reaction bonded silicon nitride (RBSN) formation. To achieve this, nitrogen diffusion in the silicon nitride layer is considered as a reaction controlling factor and sharp interface method based on this theory is used to develop the analytical model. In the developed model, the variations in the size of silicon particles are calculated for the whole reaction. In the experimental phase, the extent of nitridation is measured for different reaction temperatures and 4 different reaction times and then, the occurrence of full nitridation is shown by EDS analysis. Furthermore, an analytical approach was established for describing the kinetics of compound formation and the performance of the developed model is evaluated through statistical analysis. There was good agreement between experimental data and predictions of the developed model which demonstrates the accuracy of considered presumptions and reaction mechanisms.