Kinetics of manganese oxide reduction from basic slags by carbon dissolved in liquid iron

The reduction of manganese oxide from a basic slag by carbon dissolved in liquid iron is a slow reaction, failing to approach equilibrium closely in 20 hr. Furthermore, the rate of stirring has no apparent effect on the reaction rate. This identifies the rate-controlling step as a chemical reaction at the interface. Only the model for the reactionO ²⁻ =O + 2e ⁻ gave a consistent interpretation as the melt geometry, and concentration of manganese oxide and carbon were varied. The rate constant for this reaction was found to be 1.28 × 10⁻⁵ mole per sq cm per min at 155O°C. The effect of temperature is substantial with a calculated energy of activation for the system of 25 kcal per mole. Formerly Graduate Student, The University of Michigan This paper is based on a portion of a thesis submitted by W. L. DAINES in partial fulfillment of the requirements for the degree Doctor of Philosophy at The University of Michigan.     

A kinetic study of the leaching of chalcopyrite at elevated temperatures

A study of the rate of dissolution of chalcopyrite (CuFeS₂) in acidic solutions under oxygen overpressures was carried out by measuring the rate of formation of cupric ions in solution. Effects of temperature, oxygen partial pressure, surface area, and concentration of sulfuric acid were evaluated. A sized batch of chalcopyrite was leached in the temperature range of 125 to 175° and in the pressure range of 75 to 400 psi of oxygen. In 0.5N H₂SO₄ all products of reaction went into solution except for trace amounts of elemental sulfur. The dissolution of chalcopyrite followed linear kinetics and was essentially independent of hydrogen ion concentration for H₂SO₄ concentrations between 0.2 and 0.5 JV. The oxygen dependence indicated adsorption approaching limiting values with increasing oxygen pressure. The linear mechanism was explained in terms of steady-state adsorption of oxygen at the chalcopyrite surface followed by a surface reaction. The enthalpy of activation for adsorption of oxygen was found to be approximately 33 kcal per mole. An activation enthalpy of approximately 9 kcal per mole was observed for the surface reaction. Charge transfer reaction are not rate controlling in the process.     

Influence of deposit morphology on the kinetics of copper cementation on pure iron

The kinetics of copper cementation on pure iron substrates were studied using a rotating disc geometry. The effect of rotational speed of the iron disc, copper ion concentration, hydrogen ion concentration, and temperature on the kinetic response was investigated. The range of each parameter studied was selected with some consideration for commercial operations for copper recovery using iron as the precipitant metal. The optimum values of each parameter for maximum cementation rate were determined. At low temperatures (10–30°C), an experimental activation energy of 22.89 kcal mol^?1 was calculated indicating that the system is strongly controlled by a surface reaction mechanism or pore diffusion process. At high temperatures (30–60°C), an experimental activation energy of 7.94 kcal mol^?1 was obtained which shows that the system may be controlled by both diffusion and surface reaction mechanisms. The deposits obtained were pure copper and not a copper-iron alloy. The total iron consumption, presented as can factor, was around 1.0.The structural characteristics of the resultant deposits were studied with the help of a scanning electron microscope. The structure and morphology of the deposits were analyzed in conjunction with their respective specific rate constants. A change in the surface roughness of the rotating disc and hence in the effective deposition surface area as a result of modification to the deposit morphology obtained for different experimental conditions was found to be the major reason for variations in the reaction rate. For the experimental conditions employed in this study, most of the deposits seen were bulbous or botryoidal crystal masses of different degrees of texture and size.     

Grain-boundary penetration of type 316 stainless steel exposed to cesium or cesium and tellurium

When 20 pct cold-worked Type 316 stainless steel is exposed to Cs at 700°C under controlled oxygen-chemical potential environment, Cs penetration into the stainless steel grain boundaries occurs at oxygen potentials ΔG_o2 ≥ -96 kcal per mole. At lower oxygen potentials (~ΔG_o2 ≤ —110 kcal per mole), no corrosion occurs. Under the same experimental conditions, when the stainless steel is exposed to Cs:Te (2:1, atomic), corrosion occurs and penetration morphology appears to depend strongly on the oxygen-potential environment. The stainless steel suffers intergranular corrosion by Te (in the presence of Cs-Te) under conditions where chromium oxidation is not expected to occur. The kinetics of grain-boundary penetration by Te have been studied at temperatures between 550 and 700°C. The depth of the penetrated zone varies as (time)^1/2, and the process has an activation energy of 34 kcal per mole. The results are discussed, and the effects of stainless steel microstructure and externally applied stress on corrosion reactions are also described.     

Study of the kinetics of silver ions cementation onto copper from sulphuric acid solution

The kinetics of silver cementation with copper from sulphuric acid solution has been studied with a rotating cylinder. Two independent methods have been used to monitor the concentration of Ag⁺ ions in the solution: atomic absorption spectrophotometry (AAS) method and a continuous method with the use of an ion-selective electrode. The reaction has been found to follow first-order kinetics with respect to silver ion concentration. The initial rate of the reaction is limited by diffusion through the mass transfer boundary layer. The rate constant for initial period of cementation is independent of the absence or the presence of oxygen in the solutions and is independent of the presence of Cu²⁺ ions up to the concentration of 2×10⁻⁴ M. After the initial period of cementation a rate enhancement has been observed. In solutions containing 20 mg/L of Ag⁺ ions, the rate enhancement is associated with changes in the structure of the deposit which involve an increase in the effective surface area during the process. However, the rate enhancement phenomenon in the absence of oxygen at 100 mg/L of Ag⁺ is attributed not only to an increase in the effective surface area but also to a chemical reaction between Cu⁺ and Ag⁺ ions.     

The kinetics of the dissolution of chalcocite in alkaline cyanide solution

A kinetic study of the dissolution of chalcocite in an alkaline cyanide solution indicates that the reaction is first order with respect to surface area and free cyanide ion concentration, and inversely proportional to the sulfide ion concentration to approximately the 0.1 power. The experimental rate constant is approximately 7.5 × 10⁻⁵ (mole Cu¹⁺/ min) ([S⁻²]^0.1/[CN¹⁻]/cm²/l at 25°C. The activation energy of 2.5 kcal/mole indicates rate control by diffusion through a limiting boundary layer. The concentrations of the important ions in the cyanide solutions are obtained by solving the ionic equilibria and mass balance equations for the system.     

Kinetics of diffusion in the Nb-Al system

Diffusion kinetics were studied by varying the time and temperature (800° to 1300°C) of dipping solid niobium into molten aluminum. It was found by X-ray diffraction analysis that only the Al₃Nb phase forms at the surface of the niobium specimen and that the thickness of this layer for a given dip temperature varies parabolically with time. The activation energy and the preexponential factor of the diffusion parameter, (δD) were found to be 36.5 ∓ 0.45 kcal per mole and 2.0 ∓ 0.34 sq cm per sec, respectively.     

Deformation mechanisms in commercial Ti (0.5 at. pct oineq) at intermediate and high temperatures (0.3 - 0.6 tinm)

The plastic flow of the commercial titanium material Ti-50A (0.5 at. pct O_eq) of 22 μm grain size was investigated over the temperature range of 600 to 1150 structure) and strain rates of 3 x 10^-5 to 3 x 10^-2 per s employing both constant strain rate and strain rate cycling tests. Dynamic strain aging occurred in the temperature range of 600 to 850 (0.31 to 0.44T_m) with an activation energy of 50 kcal per mole derived from the start of serrations in the stress-strain curves, maxima in strain hardening and minima in ductility. This value is in accord with that for the diffusion of oxygen in titanium. At temperatures above 850 (0.46 to 0.59T_m) the data were very well represented by Weertman’s glide and climb high temperature creep mechanism, giving ε_skT/Dμb= 1.1 x 10⁶ (σ/μ)^4.55 withD = 1.0 x exp (- 57,800/RT). The value of 57.8 kcal per mole is in accord with available self-diffusion data for titanium.     

Initial Kinetics of Tungsten Carburization by Methane

The initial kinetics of the gas-phase carburization of tungsten by methane were studied and found to be controlled by the surface adsorption of methane. It was observed that W₂C was formed as the initial carbide phase. Carburization rates were explained by a topochemical model and an equation was developed which predicts the initial reaction rate for a variety of experimental conditions. The activation enthalpy for the initial gas-phase reaction was determined to be 12.4 kcal/mole.     

Leaching kinetics of malachite in ammonium carbonate solutions

Leaching of malachite was conducted with ammonium carbonate as lixiviant and with temperature, lixiviant concentration, and particle size as variables. Two stages of reaction were found. In Stage I, the initial dissolution of malachite proceeds rapidly, but after about 10 pct reaction the rate is reduced by surface blockage due to the presence of a needle-structured intermediate, presumably Cu(OH)₂. Subsequently, malachite and the intermediate dissolve concurrently. In Stage II, after 90 pct reaction, essentially all of the malachite has dissolved and only the intermediate remains. It dissolves in Stage II. The activation energy is 64 kJ/mole (15.3 kcal/mole) for Stage I and 75 kJ/mole (18 kcal/mole) for Stage II. The rate of reaction in Stage I is proportional to the reciprocal of particle size and is 0.8 order with respect to the concentration of ammonium carbonate. The structures of leaching residues were studied using a scanning electron microscope. The kinetic data (activation energy and entropy), particle size and concentration dependence, residue morphology, and general leaching behavior evident from microscopic monitoring during leaching were used to develop the geometric equation for leaching in Stage I. The equation, based on a heterogeneous reaction with geometric rate control, is: 1 − (1 − α^1/3 = K₀₁/r₀/[(NH₄)₂C0₃]^0.8 exp(-64,000/RT)t. It was deduced that initial steps in reaction were: (1) release of Cu²⁺ from malachite; (2) initial complexing with ammonia to form Cu(NH₃)²⁺; and (3) subsequent complexing to produce Cu(NH₃) ₄ ²⁺ which is stable in solution at pH 8.8, the buffered pH of reaction. Stage II appears to be a similar reaction except that the reaction obeys cylindrical geometry instead of spherical geometry as in Stage I.     

Kinetics and mechanism of llmenite reduction with graphite

The reduction of synthetic ilmenite with graphite in the solid state has been studied by ther-mogravimetric analysis. The reaction has been observed to initiate near 860°C at the contact points between the reactants. Up to 1020°C solid state reduction appears to be the main reaction mechanism, while above this temperature a rate increase has been observed and has been attributed to a change of mechanism to gaseous reduction of ilmenite by regenerated CO. Microscopic examination and electron probe analysis of the reduced particles have indicated a tendency toward segregation of the products iron and TiO₂. Iron particles as large as 80 μ can be obtained by keeping the reduced sample at 1025° to 1075°C for several hours. Reduction rate data under isothermal conditions were fitted to different rate equations and have been found to be well represented by the equation. 1 − 2x/3 − (1 −x)^2/3 =K’ This equation is based upon diffusion of reactants through a product layer. CO is suggested as the diffusing species. The activation energy for the reaction in the temperature range 1075° to 1140°C has been calculated to be 64 ± 6 kcal per mole. Formerly Research Associate, Department of Metallurgical Engineering, McGill University, Montreal, Quebec, Can.     

Creep behavior of Ni-Cr lamellar eutectic alloy

The structural changes that occur during creep deformation at 625° and 760°C, with creep and creep rupture data of a directionally solidified Ni-Cr lamellar eutectic alloy are presented and discussed. It is shown that the characteristic features of stage I deformation are the formation of dislocation tangles in the nickel-rich phase and shearing of the cellular structure; these features are then carried into stage II without any additional changes. The onset of accelerated creep is associated with the fracture of the chromium-rich lamellae. During this stage well-defined dislocation cells are formed. More than an order of magnitude increase in lifetime over cast specimens is obtained in the lamellar material with intermediate results for partially dendritic specimens. The activation enthalpy for creep is strain dependent, increasing from about 40 kcal per mole at low strains to a constant value of 80 kcal per mole at about 2 pct plastic strain. Stress dependence of steady-state creep for both test temperatures conforms to the expression έ =Aσ ⁿ witha − 7 for the lamellar eutectic anda − 5 for cast specimens.     

Kinetics of the thermal dissociation of Co4S3 under reduced pressure

Kinetics of the thermal dissociation of Co₄S₃ were investigated under reduced pressures of 5 × 10^-2 and 1.5 × 10^-4 mm Hg in the temperature range 1120° to 1405°C. The initial 10 pct dissociation was found to take place in accordance with a linear rate law, with activation energies of 20 and 17 kcal per mole under low and high vacuums, respectively. Subsequent dissociation up to about 35 pct was observed to be parabolic in nature, with activation energies of 38 and 40 kcal per mole under the respective vacuums. Further dissociation up to about 90 pct was found to fit into another linear rate law, with activation energies of 47 and 49 kcal per mole under the reduced pressures of 5 × 10^-2 and 1.5 × 10^-4 mm Hg, respectively. Beyond 90 pct, the dissociation was found to be very sluggish. These results have been interpreted with the help of the Co-S phase diagram. It has also been possible to achieve more than 99 pct dissociation of Co₄S₃ in 2 hr at 1405°C under low vacuum or 1355°C under high vacuum.     

Leaching of chrysocolla with Ammonia-Ammonium carbonate solutions

Chrysocolla was leached in solutions of ammonium hydroxide and ammonium carbonate as a function of the variables: temperature (25 to 55 ‡C), ammonia-ammonium ratio (0.0:1.0 to 1.0:0.0), total ammonia concentration (0.25 to 6.0 M), and particle size (100 to 400 mesh). A model of the leaching behavior was deduced based on: (1) the activation energy of 60.75 kJ/mole (14.51 kcal/mole) for 3 M total NH₃ which was dependent on both total ammonia concentration and temperature; (2) first-order dependence of rate on [(NH₄)₂CO₃]; (3) dependence of initial reaction rate on reciprocal of particle diameter; and (4) morphological evidence from SEM and ED AX measurements of diffusion and leaching occurring primarily in surface microcracks and not in the submicroscopic pores. In addition to the importance of diffusion through microcracks in rate control chemical reaction at active surface sites to produce the species, CuNH ₃ ^{2 +}, is also important. Only a fraction of the Cu atoms react that are exposed to lixiviant. Higher ammonia-ammonium ion concentrations, higher temperatures, or much longer times are required for more refractory Cu atoms to dissolve. Formerly Graduate Student, Department of Metallurgy and Metallurgical Engineering, University of Utah     

The thermodynamic properties of carbon in body-centered cubic iron

The temperature dependence of the equilibrium between m ethane-hydrogen gas mixtures of varying composition and carbon dissolved in bcc iron has been investigated. The data obtained have been analyzed to give values of the partial enthalpy and excess entropy of carbon in ferrite. The results have been discussed and compared to other investigations of the thermodynamic properties of the C-bcc-Fe system. From this investigation the partial excess entropy of the carbon is 6.56k, the partial enthalpy with respect to a solute atom at rest in a vacuum is −144.06 kcal per mole and the corresponding relative partial enthalpy is 23.34 kcal per mole at 1000°K. Formerly Graduate Student, William Marsh Rice University, Houston, Tex. This paper is based on a thesis submitted by WILLIAM W. DUNN in partial fulfillment of the requirements for the degree of Doctor of Philosophy at William Marsh Rice University.     

Embrittlement of titanium alloys by long time,high temperature exposure

α stabilized titanium alloys are known to exhibit embrittlement after long-time exposures above ∼800°F. The time-temperature dependency of this embrittlement phenomenon in the Ti-6Al-2Sn-4Zr-2Mo and Ti-5Al-6Sn-2Zr-lMo-0.25Si alloys was observed using a substandard fracture mechanics test. Room temperature slow-bend tests of fatigue precracked Charpy specimens were used to monitor toughness degradation after unstressed thermal exposures in the temperature range of 800° to 1100°F for times to 5000 hr. The activation energy for the embrittlement process was found to be ∼25 to 28 kcal per g mole, which approximates that for diffusion of aluminum or tin in α-Ti. The embrittlement is attributed to the Ti₃X (X = Al, Sn) phase with the rate controlling step that of diffusion controlled growth of the Ti₃X phase domains. The embrittlement process is reversible by heat treatment at temperatures above the α + Ti₃X two phase region.     

Kinetic studies of the reaction between Cr23 C6 particles and Cr2O3 particles

The kinetics of reaction between Cr₂₃C₆ particles and Cr₂O₃ particles which yields metallic chromium was studied. This reaction is one of the basic reactions in the Simplex process and is also related to the carbon reduction of metal oxide. The rate of reaction between these particles in a pellet of the mixture was measured by thermogravimetry in vacuum at 1050, 1075, and 1100°C. The molar ratio of Cr₂₃C₆ to Cr₂O₃ in the pellet was maintained at 1:8. At the earlier stages of the reaction, a rate equation for interfacial reaction control was applied and the rate constant α was observed to be inversely proportional to the reciprocal effective radius of Cr₂₃C₆ particles. The indirect gaseous reactions are predominant: 1/6 Cr₂₃C₆ + CO₂ = 23/6 Cr + 2CO 1/3 Cr₂O₃+ CO = 2/3 Cr + CO₂ The reduction of Cr₂O₃ with CO gas is presumed to proceed so rapidly and the equilibrium is attained instantaneously. The reaction of CO₂ with Cr₂₃C₆ particles occurs in the sequence co₂ (g) +c< ⇋ c(o) + co(g) c(o) → CO(g) +c< The latter reaction was thought to be the rate determining step, and the activation energy of this reaction was estimated to be approximately 60 kcal per mole.     

The rate of chlorination of metals and oxides: Part I. Fe,Ni, and Sn in chlorine

The rates of chlorination of Fe (528 to 921 K), Ni (890 to 1249 K), and Sn (340 to 396 K) in Cl₂-He andCl₂-Ar mixtures were measured. In the temperature range of the respective investigations the overall reaction products are gaseous: (FeCl₃)₂, NiCl₂, and SnCl₄. Transport through the gas film boundary layer at the surface of the sample plays a major role in controlling the rate of the reactions over large temperature ranges for all three metals. In the high-temperature range for iron (> 680 K) and nickel (> 993 K) as well as for the entire temperature range studied for tin, the transport of Cl2₂(g) through the gas film boundary layer to the surface of the sample controlled the rates for the experimental conditions in the present work. The transport of NiCl₂(g) controlled the rate of the Ni-Cl₂ reaction at lower temperatures. The rate of the Fe-Cl₂ reaction at lower temperatures is controlled by a slow surface reaction between Cl₂(g) and FeCl₂(s) which covers the surface of the iron. The overall activation energy for the formation of the activated complex (FeCl₃)₂‡ is 23 kcal/g-atom Fe.     

Electrotransport and thermotransport of oxygen and nitrogen in vanadium metal

The effective valence of oxygen in vanadium was measured by a steady state electrotransport technique at six different temperatures in the range of 1500 to 2060 K. A temperature dependence inZ∗ was observed decreasing from 1.51 at 1500 to 1.15 at 2060 K. In thermotransport experiments the heats of transport were measured for oxygen and nitrogen in vanadium as a function of solute concentration. Positive values ofQ* were obtained for both solutes at all concentrations which, in view of their positive sign ofZ* in vanadium, is somewhat anomalous. The heat of transport of oxygen increases from 4.6 kcal/mole at 0.25 at. pct oxygen to 7.6 kcal/mole at 2.0 at. pct oxygen and above. Nitrogen shows a similar trend. This work was done while O. N. Carlson was a visiting scientist with the Max-Planck Institut.     

Electrotransport of carbon in molybdenum and uranium

The electrotransport velocity of carbon in molybdenum and uranium was measured over a temperature range slightly below their melting points. Carbon was found to have a positive effective valence of 2.26 to 1.74 in molybdenum over the temperature range of 1890 to 2320°C and a negative value of - 5.0 in γ-uranium between 850 and 1000°C. The effective valences of nitrogen and oxygen were also observed to be positive in molybdenum and negative in uranium but their magnitudes were not determined. The diffusion coefficients for carbon in both metals were determined over the same temperature ranges.¹⁴Carbon was used as a tracer in the molybdenum work. The diffusion coefficient for carbon in molybdenum is described by the equationD = D ₀ exp (-†H/RT) whereD ₀ and †H are 0.033 cm²/s and 153 kJ/mole (36.60 kcal/mole), respectively. The values forD ₀ and †H for carbon in γ-uranium were determined as 0.218 cm₂/s and 123 kJ/mole (29.40 kcal/mole), respectively. Electrotransport was shown to be an effective method of purifying a small amount of each metal with regard to carbon as indicated by resistance ratio measurements and chemical analysis. A correlation is also presented showing the relationship between the atomic size of the solvent metal and the sign of the effective charge of the migrating solute.