Sharif Digital Repository

The leaching kinetics of stibnite in basic solution has been investigated. Spherical pellets of antimony sulphide were dissolved in 1 molar sodium hydroxide solutions at different temperatures. It was found that the shrinking core with ash layer model could satisfactorily explain the dissolution process. Using this model, it was found that initially the rate controlling step was a chemical reaction with activation energy of 10.2 kJ/mol. As the ash layer built up, diffusion through the ash layer became the rate controlling step. The activation energy for this step was found to be 33.4 kJ/mol. It was also observed that smaller particle size, larger solid to liquid ratio, and higher NaOH... 

In this study an artificial neural network (ANN) model was developed to predict the oxidation behavior of magnesia graphite composites. After mechanism evaluation in different conditions, the kinetic parameters such as effective diffusion coefficient and diffusion activation energy of oxidation were calculated from ANN predicted results at different graphite content. The obtained mechanism and kinetic parameters were compared with experimental data. First of all, the reliability of the model was checked with different available data. It was found that the model results were in good agreement with experimental data prediction. The results showed that the main mechanism of oxidation was pore... 

Kinetics of air oxidation of MgO-C-Al refractory at 600°-1300°C were investigated using the software based on the modifled shrinking core model (KDA). Commercial bricks containing 88.5% MgO, 10% residual carbon, and 1.5% aluminum anti-oxidant were oxidized isothermally with air. Combination of experimental data with model calculations indicated gas diffusion through solid material and pores as a major controlling step. Previously observed chemisorption process was eliminated from the rate-controlling mechanism with addition of aluminum antioxidant. Comprehensive rate equations were devised for MgO-C-Al and MgO-C oxidation reactions. Overall activation energies of Q id (internal diffusion) =... 

Continuous adsorption in stirred reactors in the form of carbon in pulp (CIP) and resin in pulp (RIP) is an established process for the extraction of gold and uranium. Under the circumstance of intraparticle diffusion resistance, CIP and RIP have been accurately modeled by the Boyd's series (reversible adsorption) and shrinking core model (irreversible adsorption). The present study, in its first part, introduces an analytical formula that most closely approximates both models. Using such formula, the study addresses a basic algorithm for optimization of single-stage continuous adsorption systems through linking of the major process variables. Furthermore, this study is devoted to developing... 

A mathematical model is developed to investigate the reduction process of hematite in the reduction zone of the Midrex shaft furnace as a moving bed reactor. It is described as a counter-current moving bed cylindrical reactor in which hematite pellets are reduced by a gaseous mixture of hydrogen, water vapour, carbon dioxide and carbon monoxide, namely Syngas. It is laterally injected into the bed at the lower part close to the bottom. Governing equations, including continuity, momentum, energy and mass equations, are derived based on the conceptual model for both gas and solid phases in the cylindrical coordinate system. A three interface unreacted shrinking core model (USCM) is applied to... 

In this work, the effect of carbon dioxide partial pressure on the calcination kinetics of high purity zinc carbonate hydroxide has been studied. Non-isothermal analysis has been performed on samples at different CO 2 partial pressures by TGA and DTA. It has been found that the calcination behaviour of this material corresponds to the shrinking core model and the reaction mechanism is phase boundary controlled. The calcination reaction of zinc carbonate hydroxide starts at 240 °C. Increasing the carbon dioxide partial pressure can result in an increase in the reaction start temperature of up to 30°C. The activation energy for the reaction is calculated as 180 ± 5 kJ/mol at... 

The current study was aimed at developing a package of “model + algorithm” for the design of resin in pulp (RIP) and carbon in pulp (CIP) processes of gold, uranium, and base metals. For this purpose, we first formulated a double-resistance model for irreversible adsorption (accompanied by chemical reaction) in CSTRs, and modified the McKay et al. semi-analytical model for reversible uptake in a similar system. We then devised two algorithms for the design and optimization of reversible and irreversible RIP and CIP cascades. The developed algorithms were applied on the extraction of copper, uranium, and gold. The packages are able to specify the optimum number of stages, reactor volume (V),... 

Kinetics of dissolution of silver present in precious metal scraps in HNO3 was studied in temperature range of 26-85°C. Dissolution rate of silver was much faster than that of copper at all temperatures. Effects of particle size, stirring speed, acid concentration and temperature on the rate of dissolving of silver were evaluated. Dissolution rate decreases with particle size and increases with temperature. Dissolving was accelerated with acid concentrations less than 10 mol/L, Concentrations greater than 10 mol/L resulted in slowing down of the dissolution rate. Shrinking core model with internal diffusion equation t/τ = 1 - 3 (1-x)2/3 + 2 (1-x) could be used to explain the mechanism of the... 

In this study the thermal calcination of two materials, high purity zinc carbonate hydroxide and the Calsimin zinc carbonate concentrate have been investigated. Isothermal studies have been performed on samples at different temperatures and times. Non-isothermal analysis has been carried out by TGA and DSC. It has been found that the calcination behaviour of both materials corresponds to the shrinking core model. The activation energy for the calcination of zinc carbonate hydroxide is found to be 88.7 kJ/mol, and that for the Calsimin sample is 97.3 kJ/mol. © 2006 Elsevier B.V. All rights reserved  

The present study evaluates the reductive leaching of indium from indium-bearing zinc ferrite using oxalic acid as a reducer in sulfuric acid solution. The effect of main factors affecting the process rate, including the oxalic-acid-to-sulfuric-acid ratio, stirring rate, grain size, temperature, and the initial concentration of synergic acid, was precisely evaluated. The results confirmed the acceptable efficiency of dissolving indium in the presence of oxalic acid. The shrinking-core model with a chemical-reaction-controlled step can correctly describe the kinetics of indium dissolution. On the basis of an apparent activation energy of 44.55 kJ/mol and a reaction order with respect to the... 

Thermal oxidation of commercial ilmenite concentrate from Kahnouj titanium mines, Iran, at 500-950 C was investigated for the first time. Fractional conversion was calculated from mass change of the samples during oxidation. Maximum FeO to Fe2O3 conversion of 98.63 % occurred at 900 C after 120 min. Curve fit trials together with SEM line scan results indicated constant-size shrinking core model as the closest kinetic mechanism of the oxidation process. Below 750 C, chemical reaction with activation energy of 80.65 kJ mol-1 and between 775 and 950 C, ash diffusion with activation energy of 53.50 kJ mol-1 were the prevailing mechanisms. X-ray diffraction patterns approved presence of... 

A novel microcapsule adsorbent for separation of uranium from phosphoric acid solutions was developed by immobilizing the di(2-ethylhexyl) phosphoric acid-trioctyl phosphine oxide extractants in the polymeric matrix of calcium alginate. Physical characterization of the microcapsules was accomplished by scanning electron microscopy and thermogravimetric techniques. Equilibrium experiments revealed that both ion exchange and solvent extraction mechanisms were involved in the adsorption of ions, but the latter prevailed in a wider range of acid concentration. According to the results of kinetics study, at low acidity level, the rate controlling step was slow chemical reaction of ions with the... 

The leaching of rare earth elements (REEs) including cerium, lanthanum and neodymium from apatite concentrate obtained from iron ore wastes by nitric acid was studied. The effects of nitric acid concentration, solid to liquid ratio and leaching time on the recoveries of Ce, La and Nd were investigated using response surface methodology. The results showed that the acid concentration and solid to liquid ratio have significant effect on the leaching recoveries while the time has a little effect. The maximum REE leaching recoveries of 66.1%, 56.8% and 51.7% for Ce, La and Nd, respectively were achieved at the optimum leaching condition with 18% nitric acid concentration, 0.06 solid to liquid... 

Mechanism of removal of lead from silicate glass containing 68.5 wt% PbO by 0.5 N HNO3 was investigated by incorporation of the chemical-analyses/weight-loss data into shrinking-core model (SCM) and minimization of the difference. Scanning electron microscopy (SEM), emission spectrometry with inductively coupled plasma (ICP), X-ray diffraction (XRD) and energy dispersive X-ray analysis (EDS) were used to determine the compositional changes of the lead-silicate glass (LSG) samples. Dual inter-diffusion chemical reaction mechanisms having respective activation energies of 83.49 and 47.80 kJ/mol dominated the deleading process. © 2009 Elsevier B.V. All rights reserved  

The kinetics of bioleaching of Mo, Ni, and Al from spent hydrocracking catalyst, using Aspergillus niger was studied. The four most effective bioleaching variables were selected in accordance with the Plackett-Burman design and were further optimized via central composite design (CCD). The optimal values of the variables for maximum multi-metal bioleaching were as follows: particle size 150-212. μm, sucrose 93.8. g/L, pulp density 3%. w/v, and pH 7. The maximum metal recoveries corresponding to these conditions were 99.5 ± 0.4% Mo, 45.8 ± 1.2% Ni, and 13.9 ± 0.1% Al. The relatively low Ni extraction was attributed to the precipitation of Ni in the presence of oxalic acid. Under the optimal... 

A novel microspherical adsorbent for the removal of uranium from aqueous solutions was developed by immobilizing of natural bentonite in the polymeric matrix of calcium alginate. Different uptake properties of the prepared microspheres were examined using batch, stirred and column methods. The adsorbent showed high affinity toward uranium ions, especially at pHs above 3. Major uptake mechanisms included ion exchange, chelating of the (UO2)2+ ions to the -OH groups of alginate, and surface complexation with bentonite. Surprisingly, the capacity of microspheres was higher than both its constituents, revealing that a synergetic effect occurs. Adsorption kinetics was controlled by slow chemical...