Kinetics of dissolution of aluminum in bismuth and zinc in mercury

The kinetics of dissolution of rotating aluminum cylinders in liquid bismuth and zinc single crystal disks in mercury were investigated under controlled hydrodynamic conditions, and solution rate constants were evaluated with the aid of the Nernst-Brunner relation. Dissolution of aluminum in bismuth was studied in the temperature range from 300° to 600°C over the range of Reynolds numbers from 13,500 to 54,000; the experimental results obeyed the Eisenberg correlation and indicated a diffusion-controlled corrosion process. On the other hand, zinc single crystal disks dissolved in the geometry of a rotating disk in nearly-saturated zinc amalgams at 50°C showed that the rate of crystal attack was independent of hydrodynamic conditions in the range of Reynolds numbers from 20,000 to 30,000 and that the overall dissolution process was surface-controlled. T. L. SRI KRISHNA, formerly Graduate Student, School of Materials Science and Metallurgical Engineering, Purdue University, Lafayette, Ind.     

The kinetics of dephosphorization of carbon-saturated iron using an oxidizing slag

The kinetics of dephosphorization of carbon-saturated iron by oxidizing slags were studied at 1330 °C. Nine slag compositions were investigated in the systems CaO-Fe₂O₃-SiO₂-CaF₂ and CaO-Fe₂O₃-SiO₂-CaCl₂. Increasing Fe₂O₃ up to 50 pct was found to increase the rate and extent of dephosphorization, whereas further increases were found to decrease the rate and extent of dephosphorization. This was explained in terms of two competing effects on the driving force, where increased levels of iron oxide increase the oxygen potential for dephosphorization, hence the driving force, but simultaneously dilute the basic components in the slag, lowering the driving force for dephosphorization. CaF₂ and CaCl₂ were found to decrease the rate and extent of dephosphorization at levels higher than 12 pct. The rate of dephosphorization was found to be first order with respect to phosphorous in the metal and was controlled by mass transport in the slag. The oxygen potential at the slag/metal interface was controlled by the FeO activity in the slag. When the kinetic results were analyzed to take account of different driving forces, Fe₂O₃, CaF₂ and CaCl₂ were all found to increase the mass transfer coefficient of phosphorous in the slag, and a quantitative relationship has been demonstrated between these mass transfer coefficients and the slag viscosity for each system studied.     

Kinetics and mechanisms of the non-oxidative dissolution of sphalerite (zinc sulphide)

The kinetics of the non-oxidative dissolution of four samples of sphalerite (ZnS) of different origin were studied. It was concluded that the dissolution is independent of the stirrer speed and is first order in [H⁺], and that the activation energies for the removal and deposition reactions are not sensitive to the impurity content of the solid. The rate of reaction is described by an ionic charge transfer mechanism. A large addition of Zn²⁺ retards the initial rate because equilibrium conditions are established, whereas the addition of H₂S to the reaction system lowers the final extent of reaction. The addition of Fe (III) retards the initial rate of reaction due to an anodic shift in the potential difference at the surface-solution interface, but increases the final extent of reaction as a result of the consumption of H₂S by Fe (III) to form elemental sulphur and Fe (II). The observed inhibition of the initial rate for the impure samples is explained in terms of an electron-transfer theory similar to that proposed for non-stoichiometric metal sulphides.     

Kinetics of zinc oxide formation from zinc sulfide by reaction with lime in the presence of water vapor

A novel reaction scheme for transforming certain metal sulfides to the corresponding oxides has been developed. In this process, steam oxidizes the sulfide into the oxide, and the hydrogen sulfide produced reacts with lime to form calcium sulfide and regenerate steam. There is no net consumption or generation of gaseous species. Thus, the overall reaction can be carried out in a closed system as far as the gas phase is concerned. This eliminates the possibility of emitting hydrogen sulfide out of the reactor. Only certain metal sulfides are thermodynamically amenable to this treatment. In this paper, the reaction of ZnS to ZnO by this scheme is described, together with a detailed formulation of the rate equation for the overall reaction based on the kinetics of the component gas-solid reactions. Although the present work was done with CaO, other suitable oxides may be used in its place. A further potential application of this process is to the selective oxidation of certain sulfide(s) from complex sulfide ores as a treatment prior to the separation of minerals.     

Kinetics of the dissolution of zinc-magnesium ferrites in sulphuric acid solutions related to zinc leach processes

The presence of sparingly soluble zinc and magnesium ferrites in roasted zinc concentrates can cause problems during leaching. This study aimed at the determination of the dissolution kinetics of zinc ferrite-magnesium ferrite solid solutions. The investigations have been performed for the (Zn_1?xMg_x)Fe₂O₄ powder specimens (x = 0, 0.25, 0.50, 0.75 and 1) in solutions of 100–200 g/l H₂SO₄ at 323–363 K. Dissolution of all specimens has been found to be chemically controlled and homothetic, being described by the “unreacted-core shrinking” model. The dissolution rate constant of zinc ferrite was about seven times higher than that of magnesium ferrite (e.g. 3.83·10^?6 against 0.53·10^?6 mol/m²s in 100 g/l H₂SO₄ at 363 K). The relation between rate constant and the x value was not linear. The activation energy was not dependent on the x value and amounted to ca. 75 kJ/mol. The results have important relevance to the “magnesium problem” often experienced in roach-leach processes in hydrometallurgy.     

Kinetics and mechanism of the oxidative dissolution of a zinc sulphide concentrate in ferric sulphate solutions

The kinetics of the oxidative dissolution of a zinc sulphide (sphalerite) concentrate was studied. It was observed that the dissolution of the concentrate continued beyond 90% conversion in two hours at 80°C. The kinetics of dissolution are successfully described by an electrochemical mechanism in which the charge transfer from the solid to the oxidant is rate-limiting. The rate of reaction is proportional to the sum of the concentrations of the Fe³⁺ (aq) and FeHSO₄²⁺ complexes with a reaction order of one-half. The addition of Fe (II) to the solution had an indirect effect on the reaction rate, by decreasing the concentrations of the electro-active ions. Addition of ZnSO₄ did not affect the reaction rate.     

Limestone dissolution and decomposition in steelmaking slag

Based 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.     

Kinetics of dissolution of sulfur in tetrachloroethylene

The kinetic regularities of the dissolution of sulfur in tetrachloroethylene (TCE), which depend on the temperature, hydrodynamic conditions, and TCE concentration, are presented. Using the rotating disc method, it is revealed that this process occurs in a mixed mode; its diffusion and kinetic components are revealed. The reaction order of the kinetic component of the dissolution rate of sulfur by TCE is 1.5, while the experimental activation energy of the process is 39.2 ± 2.0 kJ/mol. The activation energy of the diffusion component of the convective mass transfer during the dissolution of sulfur in TCE is 28.0 ± 2 kJ/mol.     

Kinetics of oxygen dissolution in molten lead

An electromotive force cell employing the ZrO₂-CaO solid electrolyte has been set up to investigate the kinetics of dissolution of oxygen in molten lead under various oxidizing conditions. The cell may be written as The cell has been so designed that the electrode chambers are completely isolated from each other. The oxygen in lead is first reduced to a very low value by bubbling hydrogen. subsequently, oxygen dissolution is achieved through gas diffusion across the free surface or through bubble metal interaction. During all these reactions the electromotive force of the cell changes continuously and is automatically recorded. The diffusion coefficient of oxygen in lead, as calculated from the experimental data, is 1.29×10⁻⁵ sq cm per sec and the liquid phase mass transfer coefficient is estimated to be 0.035 cm per sec at 750°C.     

High-pressure coal injection in zinc slag fuming

Previous work has shown zinc slag fuming to be kinetically controlled by two basic processes: the rate of coal entrainment in the slag and the rate of oxidation of ferrous to ferric iron in the slag. In the present study, industrial trials and mathematical modeling analysis were undertaken to assess the efficacy of increased coal entrainment on zinc fuming rates. Various systems for increasing coal entrainment were assessed before a high-pressure, high-loading pneumatic system was selected for the tests. Three trials were conducted at the lead smelter of Cominco Ltd. in Trail, BC, under three different charge conditions. In each case, the regular low-pressure coal was cut back during high-pressure injection so that the total coal and air flow rates to the furnace were virtually unchanged. Analysis with the mathematical model showed that highpressure injection increased coal entrainment from about 25 pct, characteristic of low-pressure injection, to 65 to 90 pct. As a result, fuming rates were increased substantially, to between 70 and 90 pct, depending on the charge mix. Since the overall coal rate was unchanged, this also meant a corresponding increase in the amount of zinc fumed per unit of coal injected. These results are an important verification of the kinetics model and represent a potentially significant advance in the technology of slag reduction. This paper is based on a presentation made in the T.B. King Memorial Symposium on “Physical Chemistry in Metals Processing” presented at the Annual Meeting of The Metallurgical Society, Denver, CO, February, 1987, under the auspices of the Physical Chemistry Committee and the PTD/ISS.     

Kinetics of dissolution of synthetic scheelite by an alkaline EDTA leach solution

The dissolution rate of synthetic scheelite in an alkaline EDTA leach solution under atmospheric pressure has been determined by use of a rotating disc method. The effects of disc rotation speed, temperature, reagent concentration and pH of leach solution have been investigated. The dissolution rate was proportional to the square root of the disc rotation speed. The apparent activation energy of dissolution of synthetic scheelite was 22 kJ mol⁻¹ within the experimental temperature region of 24 to 99°C. A first order dependence of dissolution rate on EDTA concentration was observed. It has been shown from the experimental results that the dissolution reaction was controlled by the mass transfer in the aqueous solution.     

Effect of zinc sulfide on the formation of an antiseizing layer in the sintered antifriction materials

Conclusions As a result of deformation and dispersion occuring under frictional conditions in the Fe-ZnS material, a colloid-type lubricating layer consisting of single crystals of ZnO (presumably) and finely dispersed Fe₃O₄ phase in its quasi-crystalline and crystalline states forms up to a depth of 100 nm.The high effectiveness of the lubricating layer may be due to the decreased degree of oxidation of the iron matrix, prevention of seizure of the contacting surfaces, and the easy dispersibility and the low shear strength of the iron oxides (Fe₃O₄).As a rule, wearing out of the Fe-ZnS material occurs through the embrittlement of the antiseizing layer and due to delamination of its individual sublayers.Translated from Poroshkovaya Metallurgiaya, No. 4, pp. 87–94, April, 1992.     

Kinetics of decarburization of iron-chromium melts in highly oxidizing atmosphere

The kinetics of decarburization in Fe-Cr-C melts were studied to determine the rate con-trolling step for the process. The experiments were carried out under nitrogen-oxygen at-mosphere in a resistance-heated vertical-tube furnace. The liquid melt was held in a freshly prepared magnesite crucible. Sampling and chemical analysis of the metal phase led to time-carbon concentration curves for the system. An iron oxide layer just below the impinging are a and a general boil were observed. Results obtained by varying param-eters such as temperature, partial pressure of oxygen, flowrate of the oxidizing gas and amount of melt determined the limiting reaction mechanism. The rate has been found to be almost independent of flow rate and partial pressure of oxygen (between 1.0 to 2.0 l/min. and 0.5 to 1.0 atm of oxygen). The amount of melt and temperature have a marked effect on the reaction rate. The apparent activation energy has been found to be 48.0 ± 5.4 K cal/mol. The carbon oxidation reaction has been proposed to occur predominantly at CO bubble/metal interface. On the basis of the experimental results and discussions reaction involving reduction of oxides by carbon has been proposed to be the rate controlling step. Formerly a graduate student of IIT, Kharagpur     

Studies on dissolution kinetics of dolime in electrothermal magnesium slag

The electrothermal process of magnesium metal production is a promising route, where large sized internally heated reactor is used for magnesium production resulting in less energy and labour intensive and high space-time yield process. However, the dissolution behavior of dolime in the electrothermal slag has been found critical for the process optimization. In this paper, the dissolution kinetics of the dolime in the slag was discussed. Quaternary slag (CaO-Al₂O₃-SiO₂-MgO) was prepared having basicity CaO/SiO₂ ≥ 1.8 and Al₂O₃/SiO₂ ≥ 0.26 for dolime dissolution studies by static hot dip method. Prior to the experiments, FactSage calculations were carried out varying temperatures and slag compositions. In the kinetic studies, dolime particles 10–15 mm size was added in slag melted at 1450, 1500 and 1550°C and samples were taken at various time intervals. The chemical analysis of slag sample was carried out to investigate the dissolution kinetics to establish the rate expression. The activation energy for the process was calculated for different models used in study and was found to be in the range of 130–270 kJ/mol. SEM analysis was done for surface analysis of reacted particles. This study would be helpful in optimizing the dolime charging rate during pilot scale trials for electrothermal magnesium production at CSIR-NML, Jamshedpur.     

Kinetics of vaporization of lead sulfide

The kinetics of vaporization of lead sulfide were investigated in the temperature range 785 to 993 °C using a thermogravimetric balance. The effect of nonreactive ambient atmosphere on the vaporization kinetics was examined in some detail. Under He-rich He-N₂ atmosphere the rate of vaporization was found to be about four times as large, as that under N₂-atmosphere. A mathematical model was developed taking into consideration the chemical kinetic and mass-transfer factors. It was found that faster vaporization rates obtained with He-rich atmosphere are due to the higher diffusivity of PbS(g) in He(g). The chemical kinetic aspects of vaporization appear to be unaffected by the ambient atmosphere. The activation energy for vaporization has a value of 198.95 kJ·mole^-1 in the temperature range investigated. The evaporation coefficient was found to have values ranging from 0.05 to 0.14.     

Kinetics of oxidation of copper sulfide

The oxidation rate of a cuprous sulfide pellet suspended in a stream of air was followed by measuring the evolution of SO₂ titrimetrically. Thin thermocouples embedded in the center of the sample recorded the variation of temperature during oxidation. The reaction was found to be topochemical and the sample temperature was found to be higher than its surroundings initially for about half an hour. After this initial period, the sample temperature decreased to that of the surroundings and remained constant during the rest of the period of over 5 hr. The apparent activation energy from the experimental data was found to be different for the initial (nonisothermal) and subsequent (isothermal) periods. Rate controlling mechanisms for these two intervals have been proposed based on interface chemical reaction, mass transfer resistance, and heat transfer concepts. Fair agreement is found between the theoretical rates based on transport mechanisms and those obtained experimentally.     

BaO-CaO渣系不锈钢氧化脱磷研究

针对不锈钢氧化脱磷容易导致合金元素铬损耗的问题,在中频感应炉中采用20%CaO-45%BaO-15%Fe2O3-10%Cr2O3-10%CaF2渣系进行氧化脱磷试验。在热力学计算的基础上,通过调配BaO含量、钢液中初始碳含量和钢水处理温度,获得脱磷率不低于50%,脱磷过程中铬氧化量不超过8%的试验结果。试验研究表明:该试验渣系的脱磷效果比CaO渣系好,与BaO渣系相当;随着钢液中碳含量的提高,脱磷率增加、铬损减小;随着熔炼温度的升高,脱磷率降低、铬损减小,在1 750 K时脱磷率和铬损耗处于最优化水平。     

Kinetics of galena dissolution in ferric chloride solutions

A leaching investigation of galena with ferric chloride has been carried out as a function of concentration of ferric chloride and sodium chloride, temperature, and particle size. Three size fractions were considered in this investigation, namely, 48 × 65, 35 × 48, and 28 × 35 mesh. The concentration ranges of ferric chloride and sodium chloride used in this investigation were 0 to 0.25 M and 0 to 3 M, respectively. The reaction rate mechanism has been discussed in terms of a shrinking core model developed for cubic systems. Mass transport of ferric chlorocomplex through the product sulfur layer appears to be responsible for establishing the overall leaching rate under most of the conditions used in this investigation. The apparent activation energy for the leaching of 28 × 35 mesh galena with 0.1 M FeCl₃, 1 M HC1, and 3.0 M NaCl was found to be about 8.05 kcal/mol (33.7 kJ/mol), which was partially contributed by diffusion and partially by the heat of reaction of the formation of ferric chlorocomplexes. Rate of dissolution at both 50° and 90 °C is greatly affected by ferric chloride concentration up to 0.2 M and is essentially constant with ferric chloride concentration above this value.