Nitrogen solubility and nitride formation in austenitic Fe-Ti alloys

The equilibrium nitrogen solubility and nitride formation in austenitic Fe and Fe-Ti alloys were measured in the temperature range from 1273 to 1563 K. Specimens 0.5 mm thick were equilibrated with four different nitrogen-argon gas mixtures containing 1 pct hydrogen. The nitrogen solubility in austenitic iron obeys Sieverts' law. The equilibrium nitrogen content was determined to be log (wt pct N)_γ-Fe, P_{N2=1 atm} = (539 ± 17)/T − (2.00 ± 0.01). The precipitated titanium nitride was identified as cubic TiN, and the solubility product was determined to be log(wt pct Ti) (wt pct N) = −14,400/T + 4.94.     

The apparent solubility of aluminum in cryolite melts

The apparent solubility of aluminum in cryolite melts saturated with A1₂O₃ has been determined by titration with electrolytically generated O₂. The results may be expressed by wt pct Al = − 0.2877 + 0.0268 (NaF/AlF₃ wt ratio) + 2.992 × 10⁻⁴ (temp °C) − 0.00192 (% CaF₂) −0.00174 (% Li₃AlF₆) −0.00288 (% NaCl) with a standard deviation of ±0.017. Ranges covered were ratio 0.8 to 2.3, temperatures 969° to 1054°C, CaF₂ ≤ 14 pct, Li₃AlF₆ ≤ 20 pct, and NaCl ≤ 10 pct. There was no significant effect of adding 0 to 38. pct K₃A1F₆ or 0 to 10 pct MgF₂. It was found that solubility was approximately proportional to activity of aluminum when Al-Cu alloys were used. Possible mechanisms of solution are discussed. Monovalent aluminum is ruled out on the basis of the variation of solubility with NaF/AlF₃ ratio and a_Al. The favored, but not proven, mechanism involves formation of both sodium atoms and a colloidal dispersion of aluminum.     

Influence of Al2O3 on the reoxidation of aluminum in molten cryolite

The influence of Al₂O₃ content on the reoxidation reaction at 1293 K between CO₂ and Al dissolved in molten cryolite was examined. The rate and extent of the reaction were evaluated as the concentration of Al₂O₃ was varied from 0 to 12 wt pct at 4 pct intervals. The cryolite, under an argon atmosphere, was saturated with Al before CO₂ was introduced. High initial reaction rates were observed with a quasisteady state condition established 30 to 60 min after beginning an experiment. Increasing the Al₂O₃ content of the melt significantly reduced the initial rate, but it was not found to alter the steady rate for a given flow system. This phenomenon was observed for systems where CO₂ was passed over or bubbled through the molten cryolite. Bubbling the CO₂ through the molten cryolite led to increases in both the initial and steady state rates. M. K. Han, formerly a Graduate Fellow with the University of Washington.     

Loss of current efficiency in aluminum electrolysis cells

Consideration of the mechanism of loss of current efficiency (CE) leads to a form of equation which is simple, likely to give reasonable extrapolation beyond the range where experimental data are available, and convenient for responding to practical questions. With coefficients generated from plant experiments (performed by others), the equation is log (pct loss of efficiency) = 0.0095 (superheat) -−0.019 (pct A1F₂) − 0.060 (pct LiF) + const where superheat is the difference (in °C) between cell temperature and the pseudo-binary eutectic temperature with A1₂O₃, and pct A1F₃ is excess A1F₃. The coefficient for CaF₂ is zero. The constant is characteristic of the cell design. The question of reconciling the values of the coefficients with literature data on the solubility of Al in cryolite melts and current theories of loss of efficiency is discussed. Formerly Principal Scientist, Kingston Laboratory, Alcan International Ltd., Kingston, ON K7L 5L9, Canada, is retired.     

Solubility of some transition metal oxides in cryolite-alumina melts: Part II. Solubility of TiO2

The solubility of TiO₂ in cryolite-alumina melts at 1020 °C was measured; it decreased with increasing alumina concentration up to ∼3.5 wt pct total oxide and then increased at higher alumina concentrations. The solubility was found to be 3.1 wt pct Ti in cryolite, and 2.7 wt pct Ti in an alumina-saturated melt. Modeling indicated that the most probable titanium species are TiOF₂ and Na₂TiO₃, which coexist in the solution; the former dominates at low alumina concentrations and the latter at high alumina concentrations. Additional unknown amounts of fluoride may also be associated with these species. Determination of the solubility of TiO₂ in alumina-saturated melts as a function of temperature showed that the solubility increased from 1.9 wt pct Ti at 975 °C to 2.8 wt pct Ti at 1035 °C, the apparent partial molar enthalpy of dissolution of TiO₂ being 88±4 kJ mol⁻¹.     

Reactions of dense MgO with calcium ferrite-based slags at 1573 K

The drop-quench technique was used to investigate the solubility of dense MgO in calcium ferrite-based slags (CaO 20 wt pct) under oxygen potentials from 10⁻⁸ to 10⁻⁴ atm at 1573 K. The effect of copper oxide in the slag on the solubility of MgO was also examined in a CO₂ atmosphere. The results showed that MgO solubility in copper-free calcium ferrite slags was generally less than 2 wt pct and it increased with the addition of Cu₂O (up to 28.5 wt pct). It was found that magnesiowustite or magnesioferrite may form at the slag-refractory interface depending on the prevailing oxygen potential. The activity of MgO was estimated through equilibrium between the slag and the solid solution phases. The activity coefficient of MgO was found to be essentially independent of the oxygen potential within the range studied and to decrease from approximately 15 for the copper-free slag to 7 for slags with 28.5 wt pct Cu₂O.     

On the kinetics of oxidation of liquid copper and copper-sulfur alloys by carbon dioxide

The rates of transfer of oxygen between CO₂-CO gas mixtures and liquid copper and copper-sulfur alloys have been studied by a steady-state electrochemical technique. For sulfur-free stagnant copper, and under the conditions of the experiments, the rates are shown to be controlled by the diffusion of oxygen in the metal. The resulting diffusivities are in close accord with the bulk of the previous determinations. At high sulfur concentrations, the rate is found to be controlled by an interfacial reaction which is first order with respect to the pressure of CO₂ and inversely proportional to the sulfur concentration. The rate constant, in mole (at. pct)cm⁻² s⁻¹ atm⁻¹, is approximately 8 × 10⁻⁹ at 1146°C. Formerly a Graduate Student.     

Chlorination kinetics of a niobium pyrochlore in the Gas-Solid phase

The chlorination kinetics of a niobium (columbium) pyrochlore has been studied in the gas-solid phase, for temperatures between 1373 and 1573 K, using a high temperature differential tungsten reactor. Chlorine-helium mixtures were used which contained between 0 and 60 pet helium. It is shown that the kinetic study reduces to one of CaNb₂O₆ chlorination. In order to obtain information on the true reaction mechanisms involved and avoid side effects like difficulty of access of the reactant gas throughout the sample mass subjected to reaction, the reaction rate has been determined from decreasing amounts of initial solid sample. The reaction rate obtained by extrapolation to nil sample values was considered to be the true reaction rate that would be observed if a single particle were subjected to chlorination with the prevailing conditions. Using a reactant gas flow rate which provided a purely chemical reaction process (no film diffusion effects), it has been found that the reaction is of the continuous-reaction type model, while the reaction rate is nearly first order with respect to the chlorine concentration at the solid-gas interface. The rate constants are 0.21 (at 1373 K), 0.46 (at 1473 K) and 0.92 (at 1573 K) min⁻¹.atm⁻¹. The energy of activation was found equal to 129 KJ/mol. The theoretical maximum error, calculated from a knowledge of the error made on temperature, time, sample weight and Nb₂O₅ analysis, does affect the reaction order by ± 20 pct, the reaction rate by ± 20 pct and the energy of activation by ± 25 pct.     

The diffusivity and solubility of oxygen in liquid tin and solid silver and the diffusivity

The diffusivity and solubility of oxygen in liquid tin and solid silver in the temperature range of about 750° to 950°C (1023 to 1223 K) and the diffusivity of oxygen in solid nickel at 1393°C (1666 K) were determined using the electrochemical cell arrangement of cylindrical geometry: Liquid or Solid Metal + O (dissolved) | ZrO₂ + (3 to 4%)CaO | Pt, air The diffusivity and solubility of oxygen in liquid tin are given by:D _O(Sn) = 9.9 × 10⁻⁴ exp(−6300/RT) cm²/s (9.9 × 10⁻⁸ exp − 6300/RT m²/s) andN _O ^S (Sn) = 1.3 × 10⁵ exp(−30,000/RT) at. pct The diffusivity and solubility of oxygen in solid silver follow the relations:D _O(Ag) = 4.9 × 10⁻³ exp (−11,600/RT) cm²/s ( 4.9 × 10⁻⁷ exp − 11,600/RT m²/s) andN _O ^S (Ag) = 7.2 exp (−11,500/RT) at. pct The experimental value for the preexponential in the expression forD _O(Ag) is lower than the value calculated according to Zener’s theory of interstitial diffusion by a factor of 11. The diffusivity of oxygen in solid nickel at 1393°C (1666 K) was found to be 1.3 × 10⁻⁶ cm²/s (1.3 × 10⁻¹⁰ m²/s). Formerly Graduate Student, Department Formerly Graduate Student, Department Formerly Graduate Student, Department This paper is based upon a This paper is based upon a This paper is based upon a This paper is based upon a     

The Kinetics of the Nitriding of Ternary Fe-2 at pct Cr-2 at pct Ti Alloy

The mechanism and the kinetics of growth of the nitrided zone of ternary Fe-2 at pct Cr-2 at pct Ti alloy was investigated by performing gaseous nitriding experiments at temperatures of 833 K and 853 K (560 °C and 580 °C) and at nitriding potentials r _N = 0.004 atm^−1/2 and 0.054 atm^−1/2. The microstructure of the nitrided zone was investigated by transmission electron microscopy and the elemental compositional variation with depth was determined by employing electron probe microanalysis. Fine platelet-type mixed Cr_{1 – x} Ti _x N nitride precipitates developed in the nitrided zone. To describe the evolution of the nitrogen concentration depth profile, a numerical model was developed with the following parameters: the surface nitrogen content, the solubility product(s) of the alloying elements and dissolved nitrogen in the ferrite matrix, and a parameter defining the composition of the inner nitride precipitates. These parameters were determined by fitting model-calculated nitrogen depth profiles to the corresponding experimental data. The results obtained demonstrate that the type of nitride formation (i.e., whether Cr and Ti precipitate separately, as CrN and TiN, or jointly, as mixed Cr_{1 – x} Ti _x N) as well as the amounts of mobile and immobile excess nitrogen taken up by the specimen considerably influence the shape and extent of the nitrogen concentration profiles.     

The interfacial kinetics of the reaction of CO2 with liquid nickel

Measurements of the rate of dissociation of CO2 on liquid nickel have been made by the¹⁴CO2-CO isotope exchange technique between 1490 and 1670 °C at CO2/CO ratios between 0.01 and 7. Apparent first order rate constants are given by the expression:ka = (1 + 2pCO₂/pCO)⁻¹exp(−12700/T - 0.65) mol cm⁻² s⁻¹ atm⁻¹. It is shown that the results are consistent with blockage of the surface by oxygen which exhibits ideal Langmuirian adsorption over the conditions of the experiments. The adsorption coefficient of oxygen with respect to the infinitely dilute solution with 1 wt pct as the standard state is deduced to be given by the equation: logK′_o = 11880/T - 4.6. It is deduced that the interfacial rate of oxidation of nickel by CO₂ is given by the rate of dissociative chemisorption of CO₂. Measurements of the rate of decarburization of liquid nickel are reexamined in the light of the present results.     

Beneficiation of west sibaiya phosphate ores by flotation in alkaline media

Four big head phosphate samples (about 200 kg) were collected from West Sibaiya mines [medium grade (sample 1, southern mine), high grade (sample 2, the mine in the cultivated area), low grade (sample 3, oversize), and tailings (sample 4) phosphate ores]. The representative samples were analyzed chemically for P₂O₅, Fe₂O₃, Al₂O₃, InR, CaO, MgO, TiO, MnO, SO ₄ ²⁻ , F⁻, Cl⁻ loss on ignition (LOI), and U before and after screen analysis (−10.0 +9.50, −9.50 +6.70, −6.70 +2.0, −2.0 +1.0, −1.0 +0.85, 0.85 +0.710, −0.710 +0.500, −0.500 +0.350, −0.350 +0.250, −0.250 + 0.180, −0.180 +0.125, −0.125 +0.075, and −0.075-mm-size fractions). The different grades of phosphate ores (organogenic-granular phosphorite ores with carbonates, calcite, calcedony, etc.) were beneficiated using local crude rice bran oil and imported oleic acid (individuals) as collectors, NaOH or KOH as the pH adjustor, and Na₂SiO₃ as the silica depressant. The results indicate that −0.355, +0.180 mm grain size, pH=9.6, 20 minutes conditioning time, 0.7 kg Na₂SiO₃ (depressant)dose/ton feed ore, 4.25 kg rice bran oil dose/ton feed of phosphate ore or 3.4 kg oleic acid (collector) dose/ton feed of phosphate ore, and cleaning (reflotation) of phosphate concentrates are the optimum conditions for beneficiation of Sibaiya phosphate ores. On cleaning (reflotation) the phosphate ore feeds of samples 1 (medium, P₂O₅=28.67 pct), 2 (high, P₂O₅=31.70 pct), 3 (oversize, P₂O₅=25.30 pct), and 4 (tailings, P₂O₅=22.61 pct)using rice bran oil as a collector, the feeds beneficiate into concentrates of P₂O₅ pct equal to 31.87, 34.70, 26.16, and 31.90 pct (recoveries equal 49.41, 61.41, 52.01, and 29.03 pct P₂O₅) for samples 1, 2, 3, and 4, respectively. On using oleic acid as a collector, P₂O₅ pct in phosphate feeds of samples 1, 2, 3, and 4 increases (upgrades) into 31.28, 33.95, 26.82, and 30.70 pct in concentrates (recoveries equal 61.74, 53.60, 53.31, and 36.30 P₂O₅). Comparing the results obtained on using rice bran oil and oleic acid as collectors from the economic point of view, the costs of producing 1 ton phosphate concentrate by rice bran oil are lower (∼LE 70) than those on using oleic acid (∼LE 100) as a flotation collector.     

The solubility of aluminum in cryolite-alumina melts and the mechanism of metal loss

The solubility of aluminum in cryolite-alumina melts has been determined in laboratory experiments by analyzing rapidly-quenched samples of the melt after equilibration with metal at temperatures between 960 and 1060°C. The solubility in pure cryolite increases from 0.085 at 1020°C to 0.12 wt pct Al at 1060°C. The addition of alumina decreases the solubility at 1020°C to 0.081 with 5 pct A1₂O₃ and to 0.073 wt pct Al in melts saturated with alumina. Quenched samples have been taken from operating 130 kA prebake cells at different heights above the metal pad, both in the center channel and beneath the anodes. Within about 10 mm of the cathode the metal content is close to the equilibrium value obtained in the laboratory but above this level it decreases rapidly. It is suggested that oxidation occurs in a central zone of the electrolyte. Mechanisms of metal loss and implications for current efficiency are discussed.     

Calcium deoxidation and nitrogen distribution in liquid nickel equilibrated with CaO-Al2O3 slags

The Wagner’s first-ordere _o ^Ca and second-order (r _o ^Ca ) interaction parameters between Ca and O in liquid nickel were determined as −1220 and 1.35 x 105, respectively, at 1873 K in the equilibrium experiments between liquid nickel and CaO-Al₂O₃ slags in an A1₂O₃ or CaO crucible. The values fore _Ca ^O , (−3060), r _Ca ^o (8.47 × 105), r _Ca ^O,Ca (6.76 x 10⁵), and r _Ca ^O,Ca (6.76 x 105) were also estimated. Nitride capacities, C(_N), defined by (mass pct N)·P ₂ ^3/4 PskN₂1/2 were obtained by using the present results for nitrogen distribution ratio, L_N =(mass pct N)/[mass pct N], and the reported values for the activity of Al₂O₃.     

Solid-state reaction kinetics of the system CaO-FeO

The solid-state reaction mechanisms of the CaO-FeO system have been studied by a diffusion couple method in the temperature range of 1073 to 1273 K, under controlled oxygen-partial pressures. The interdiffusivities and intrinsic diffusivities of Ca²⁺ and Fe²⁺ have been determined in the limited FeO solid-solution range. Based on the obtained results, a solid-state reaction model of the system has been developed, taking account of the formation of the Ca₂Fe₂O₅ phase. According to the model, the diffusivities of Fe³⁻ in the Ca₂Fe₂O₅ phase have been estimated at 1273 K.     

Experimental study of phase equilibria in the “FeO”-ZnO-(CaO + SiO2) system with the CaO/SiO2 weight ratio of 0.71 at metallic iron saturation

The pseudoternary section “FeO”-ZnO-(CaO + SiO₂) with a CaO/SiO₂ weight ratio of 0.71 in equilibrium with metallic iron has been experimentally investigated in the temperature range from 1000 °C to 1300 °C (1273 to 1573 K). The liquidus surface in this pseudoternary section has been determined in the composition range of 0 to 33 wt pct ZnO and 30 to 70 wt pct (CaO + SiO₂). The system contains primary-phase fields of wustite (Fe _x Zn_1−x O_1−y ), zincite (Zn _z Fe_1−z O), fayalite (Fe _w Zn_2−w SiO₄), melilite (Ca₂Zn _u Fe_1−u Si₂O₇), and pseudowollastonite (CaSiO₃). The phase equilibria involving the liquid phase and the solid solutions have also been measured.     

Study of a titaniferous magnetite chlorination at high temperature

The chlorination of a titaniferous magnetite with low content in Ti and Fe has been studied between 1273 and 2273 K. Most of the hercynite and ilmenite initially present are decomposed during the gas-solid phase reaction between 1273 and 1823 K. Considerable ilmenite decomposition and FeCl₃evolution already occur at 1273 K, leaving a residue consisting of TiO₂, Fe₂O₃-TiO₂ (pseudobrookite), and about 50 pct of each of the Cr and Mg initially present. X-ray diffractograms shown the formation of Al₂TiO₅ which contributes to the stabilization of TiO₂ up to 1773 K, above which temperature significant decomposition of Al₂TiO₅ is observed. At the melting point of the titaniferous magnetite sample (around 1823 K), the presence of both solid and liquid phases result in a considerable decrease in the chlorination rate. In this respect, heating the sample under helium up to the melting point, so that liquid and solid phases are obtained at equilibrium, yields two structures replacing the magnetite present just prior to melting. One of these structures is of the spinel Fe₂TiO₄ type, while the other is a combination of the spinel types MgAl₂O₄, FeAl₂O₄, and MgCr₂O₄. When the sample is chlorinated, a high proportion of the initial Cr (90 pct) and Ti (80 pct) are found in the chlorination residue at the early stages of fusion, together with 13 pct of the initial Fe. Chlorination of the liquid phase between 1823 and 2273 K shows a steady decrease of Ti and Cr in the chlorination residue, associated with an increase of Fe content.     

The solubility of hydrogen in liquid binary Al-Li alloys

The solubility of hydrogen in liquid binary aluminum alloys with 1, 2, and 3 wt pct lithium has been determined for the temperature range of 913 to 1073 K and pressure 5.3 × 10⁴ to 10.7 × 10⁴ Pa, using an appropriate version of Sieverts’ method. The results fit the Van’t Hoff isobar and Sieverts’ isotherm and the solubility,S, is given by: Al-1 pct Li: log(S/S°) − 1/2 log(P/P°) = −2113/T/k + 2.568 Al-1 pct Li: log(S/S°) − 1/2 log(P/P°) = −2797/T/k + 3.329 Al-1 pct Li: log(S/S°) − 1/2 log(P/P°) = −2889/T/k + 3.508 whereS° is a standard value of solubility equal to 1 cm³ of diatomic hydrogen measured at 273 K and 101,325 Pa per 100 g of metal, andP° is a standard pressure equal to 101,325 Pa. Added lithium progressively increases the solubility of hydrogen in liquid aluminum, due more to its effect on the entropy of solution of hydrogen, through its influence on the liquid metal structure than to an increase in the solute hydrogen atom binding enthalpy.     

The solubility of hydrogen in liquid binary Al-Li alloys

The solubility of hydrogen in liquid binary aluminum alloys with 1, 2, and 3 wt pct lithium has been determined for the temperature range of 913 to 1073 K and pressure 5.3 × 10⁴ to 10.7 × 10⁴ Pa, using an appropriate version of Sieverts’ method. The results fit the Van’t Hoff isobar and Sieverts’ isotherm and the solubility,S, is given by: Al-1 pct Li: log(S/S°) − 1/2 log(P/P°) = −2113/T/k + 2.568 Al-1 pct Li: log(S/S°) − 1/2 log(P/P°) = −2797/T/k + 3.329 Al-1 pct Li: log(S/S°) − 1/2 log(P/P°) = −2889/T/k + 3.508 whereS° is a standard value of solubility equal to 1 cm³ of diatomic hydrogen measured at 273 K and 101,325 Pa per 100 g of metal, andP° is a standard pressure equal to 101,325 Pa. Added lithium progressively increases the solubility of hydrogen in liquid aluminum, due more to its effect on the entropy of solution of hydrogen, through its influence on the liquid metal structure than to an increase in the solute hydrogen atom binding enthalpy.     

Equilibrium distribution of Fe,Ni, Sb,and Sn between liquid Cu and a CaO-rich slag

Equilibrium measurements of the distribution of Fe, Ni, Sb, and Sn between a liquid Cu-O solution and a CaF₂-CaO-MgO-SiO₂ were carried out at 1500 K in a magnesia crucible. The results show that the studied solutes were in the states Fe(III), Ni(II), Sb(III), and Sn(IV), in the slag, for metal O contents ranging from 100 ppm to saturation at 2.1 pct. The Cu oxide solubility in the slag was also measured in absence of the solute elements. Its maximum solubility is about 4 ± 1 mass pct Cu₂O. The compositions at equilibrium allow determination of the activity coefficients (referred to pure oxide) of the four solute oxides in the slag. These values, expressed in round figures to take into account the experimental uncertainties, are 10 for Fe₂O₃, 20 for NiO, 10 for SnO₂, 1.6 10⁻² for SbO_1.5, and 60 for Cu₂O.