Kinetics of chlorination and carbochlorination of pure tantalum and niobium pentoxides

Kinetics of chlorination and carbochlorination of pure Nb₂O₅ and Ta₂O₅ were studied by thermogravimetric analysis between 385 °C and 1000 °C using Cl₂-N₂ and Cl₂-CO-N₂ gas mixtures. Standard free energy changes of the reactions and phase stability diagrams of Nb-O-Cl and Ta-O-Cl systems were calculated. The chlorination reaction order, for both oxides, with respect to Cl₂ in the Cl₂-N₂ gas mixture was 0.82. The apparent activation energies (E _a ) for Nb₂O₅ chlorination were 208 and 86 kJ/mole for temperatures lower and higher than 850 °C, respectively. The experimental data could be described by a shrinking sphere model between 700 °C and 1000 °C. The chlorination mechanism, between 700 °C and 850 °C, was likely controlled by the chemical reaction. For T > 850 °C, the overall Nb₂O₅ chlorination rate was affected by the allotropic transformation of the Nb₂O₅ T form to M form. Between 925 °C and 1000 °C, E _a for Ta₂O₅ chlorination was 246 kJ/mole. In this case, the most appropriate model was also that of shrinking sphere suggesting that the chlorination of Ta₂O₅ was controlled by the chemical reaction. For both oxides, the carbochlorination reaction order with respect to Cl₂+CO partial pressure, in the gas mixture, was about 2. The mathematical analysis of carbochlorination data indicates that Nb₂O₅ and Ta₂O₅ reactions could be described by shrinking sphere or cylinder, respectively. Below 600 °C, the E _a values of Nb₂O₅ and Ta₂O₅ carbochlorination were 74 and 110 kJ/mole, respectively. Chemical reaction was probably the rate controlling step in both cases. An anomaly characterized by a decrease of the reaction rates occurs in the Arrhenius plots between 600 °C and 800 °C. This anomaly could be attributed to the thermal decomposition of COCl₂ formed in situ during the carbochlorination.     

Separation of niobium from ferroniobium by chlorination

Separation of niobium from ferroniobium by chlorine metallurgy has been investigated in order to effect a new process for the mass production of niobium due to the increasing demand for this element. From thermodynamical considerations, the chlorination of ferroniobium by chlorine gas was con-ducted under several conditions, and the most effective conditions were determined. Ferroniobium chlorides obtained were shown to be ferric chloride and niobium pentachloride. Preliminary separation of niobium pentachloride from ferric chloride is possible by selective condensation with temperature gradient techniques. Selective reduction of ferric chloride to ferrous chloride by iron powder was conducted to separate niobium pentachloride from ferrous chloride by their volatility difference. Separation of niobium from ferroniobium by chlorine metallurgy is summarized in a flowsheet, and a new dry methods process, which is simplified compared with the current process, is obtained.     

Kinetics of chlorination and carbochlorination of molybdenum trioxide

Kinetics of chlorination of MoO₃ with Cl₂-air, Cl₂-N₂, and Cl₂-CO-N₂ gas mixtures have been studied by nonisothermal and isothermal thermogravimetric measurements, between ambient temperature and 900 °C. Between 500 °C and 700 °C, the chlorination reaction of MoO₃ with Cl₂-N₂ gas mixture has an apparent activation energy of about 165 kJ/mole, reflecting that a chemical reaction is the rate-controlling step. The reaction order with respect to Cl₂ partial pressure is about 0.75. The apparent activation energy for carbochlorination with Cl₂-CO-N₂ gas mixture is about 83 kJ/mole, between 400 °C and 650 °C. The carbochlorination of MoO₃ was controlled by the chemical reaction, probably affected by the pore diffusion regime. The maximum reaction rate is obtained by using a Cl₂-CO-N₂ gas mixture, having a Cl₂/CO volume ratio equal to about 1. The total apparent reaction order with respect to Cl₂ + CO in Cl₂-CO-N₂ gas mixture is about 1.5 for a Cl₂/CO ratio equal to 1. Laboratoire Environnement et Minéralurgie, associated with the Centre National de la Recherche Scientifique, Mineral Processing and Environmental Engineering team.     

Kinetics of chlorination and carbochlorination of vanadium pentoxide

Kinetics of chlorination of V₂O₅ with Cl₂-air, C1₂-N₂, and C1₂-CO-N₂ gas mixtures have been studied by nonisothermal and isothermal thermogravimetric measurements. In the temperature range of 500 °C to 570 °C, the chlorination of V₂O₅ with C1₂-N₂ gas mixture is characterized by an apparent activation energy of about 235 kJ/mole. This could be attributed to chemical reaction. Between 570°C and 650 °C, the apparent activation energy is equal to 77 kJ/mole, indicating that the overall reaction rate is controlled by chemical reaction and pore diffusion. The reaction order with respect to chlorine is 0.78. The apparent activation energy of the carbochlorination of V₂O₅ by C1₂-CO-N₂ gas mixture is about 100 kJ/mole in the temperature range of 400 °C to 620 °C. In this case, the chemical reaction is the limiting step. At temperatures higher than 620 °C, an anomaly is observed in the Arrhenius plot, probably due to thermal decomposition of COC1₂ formedin situ and/or transformation of the vanadium oxide physical state. The maximum reaction rate is obtained by using a C1₂-CO-N₂ gas mixture having a C1₂/CO volume ratio equal to about 1. Formerly Graduate Student, Mineral Processing and Environmental Engineering Team. Formerly Graduate Student, Mineral Processing and Engineering Team, Institut National Polytechnique de Lorraine, Vandoeuvre, France.     

Removal of tin from tin coated steel by chlorination in air

Abstract

A high tin content in steel has detrimental effects during processing, fabrication and application of the steel. Removal of tin from the steel is therefore necessary if the scrap has to be recycled. The thermodynamics and kinetics of the removal of tin from steel scrap have been studied. Removal of tin has been demonstrated in air-chlorine gas mixtures (with a 10 : 1 volume ratio of dry air/chlorine) at relatively low temperatures of 403-473 K. Owing to its high vapour pressure, SnCl₄ is readily removed as a vapour from tin coated steel scrap. While tin is preferentially chlorinated, iron is not, but forms a protective layer of iron oxide on its surface. In a laboratory packed bed reactor operating at 403 K, the tin level of baled steel scrap has been shown to reduce from 0·25 to 0·039 wt-% in the presence of air-chlorine mixtures.     

电感耦合等离子体质谱法测定地球化学样品中铌钽钨锡

采用HCl-HNO3-HF-HClO4溶解地球化学样品,以50 mg/L酒石酸-1％ HCl为测定介质克服了铌、钽、钨、锡易水解的特性,选择93 Nb、181 Ta、182W、118Sn为分析同位素及50 ng/mL185Re为内标,建立了电感耦合等离子体质谱法(ICP-MS)同时测定地球化学样品中微量铌、钽、钨、锡的...     

Kinetics of chlorination of niobium pentoxide by carbon tetrachloride

Kinetics of the chlorination of Nb₂O₅ powder by CCl₄ vapor in mixture with N₂ in a static bed in the temperature range of 698 to 853 K were carried out at different partial pressures of CCl₄ (p _{CCl 4}), varying from 0.10 to 0.75 atm. The fraction of Nb₂O₅ chlorinated R at p _{CCl 4} of 0.6 atm in the temperature range of 698 to 773 K was found to be proportional to time t, and the activation energy E is calculated to be 112 kJ/mole. Results on the effect of p _{CCl 4} (0.4, 0.6, and 0.75 atm) at 723 K suggest that the rate v (R/min) is proportional to p _{CCl 4} ^1.5. However, at p _{CCl 4} of 0.2 atm, R is not linear with t, rather, R ^1/2 is linear with t. Based on these results, two mechanisms, one at low p _{CCl 4} (0.2 atm) and another at higher p _{CCl 4} values, in the temperature range of 698 to 773 K have been suggested. Similar studies in the higher temperature range (793 to 853 K), where p _{CCl 4} used to decompose to elemental chlorine and carbon, were also carried out. At all temperatures and p _{CCl 4} values, R is found to be directly proportional to t. At two higher p _{CCl 4} (0.4 and 0.6 atm), v is proportional to p _{CCl 4} ^1.5, whereas at two lower p _{CCl 4} (0.1 and 0.2 atm) it is proportional to p _{CCl 4} ^1.5. The E values obtained in the temperature range of 793 to 853 K at p _{CCl 4} of 0.6 and 0.2 atm are found to be 57 and 115 kJ/mole, respectively. In this higher temperature range, two different reaction mechanisms have been proposed.     

A study of niobium and tantalum sulfides

Summary A study was made of the reaction of metallic niobium and tantalum powders with hydrogen sulfide; niobium sulfide, NbS_1.6, was obtained at 1000–1300°C and tantalum disulfide at 1400°C.The lattice parameters and pycnometric density of the sulfides were determined, and their stability in different media and oxidation resistance were studied.     

A New process for the separation of tantalum from niobium

A new process for the separation of tantalum from niobium (Cb) has been developed. This process consists of repeatedly reacting the mixed pentachloride vapors of niobium and tantalum with an alkali-metal chloride powder to form complex compounds of the type, AMCI₆, where A is an alkali metal, and M is Nb of Ta. Tantalum tends to concentrate in the condensed AMCl₆ phase leaving a vapor enriched in niobium. The results of tests conducted with NaCl, KC1, RbCl, and CsCl are presented along with a discussion of the thermodynamics of the process. The best separation was achieved with KC1, where the molar ratio of pentachloride to KC1 was 2:1, the pentachlorides were heated to 229°°C, and the KC1 was heated to 342°°C. Under these conditions the concentration of tantalum was reduced by a factor of almost three in one separation step.     

Protecting niobium and tantalum from high-temperature gas corrosion (review)

The survey deals with methods of raising the heat resistance of niobium, tantalum, and alloys, based on them, and the main lines of development are distinguished for protective coatings that increase the working life of refractory-metal specimens. Ternopol Pedagogic Institute. Translated from Poroshkovaya Metallurgiya, Nos. 1–2, pp. 85–93, January–February, 1997.     

Kinetics of the vapor phase chlorination of niobium oxychloride by phosgene

The kinetics of the homogeneous gas phase reaction between niobium oxychloride and phosgene was studied between 380° and 450°C in a constant volume reactor. The reaction was found to be essentially irreversible with the only detected reaction products being niobium pentachloride and carbon dioxide. The data obtained were fit by an elementary second-order rate equation. Values of the Arrhenius frequency factor and activation energy were found to be 1.33 × 10⁶ liter per g mol sec and 21.9 kcal per g mol with standard deviations of 0.05 × 10⁶ and 1.4, respectively.     

Solid-gas reactions driven by mechanical alloying of niobium and tantalum in nitrogen

Solid-gas reactions of niobium and tantalum with molecular nitrogen driven by mechanical alloying (MA) have been investigated by X-ray diffraction, transmission electron microscopy, and differential thermal analysis. It was found that the phase transition followed a sequence of Nb₂N → Nb₃N₄ → NbN when Nb was milled with N₂. However, Ta₂N and an amorphous phase were formed when Ta was milled with N₂. The chemosorption of nitrogen onto the clean metal surfaces created by ball milling is believed to be the fundamental process governing solid-gas reactions, and the defects generated during MA can promote the diffusion of adsorbed nitrogen, and consequently the formation of metal nitrides. The difference in phase transition between the two systems is discussed.     

Oxygen effects on the corrosion of niobium and tantalum by liquid lithium

The effect of oxygen on the corrosion of niobium and tantalum by liquid lithium at 600°C was studied with static capsules. An increase in the oxygen concentration of the lithium from 100 to 2000 ppm had no measurable effect on the dissolution of either refractory metal. Exposure to lithium reduced the oxygen content of niobium and tantalum to less than or equal to 20 ppm regardless of the oxygen concentration of the lithium, results that agree with thermodynamic calculations. When the initial oxygen concentration of niobium and tantalum exceeded a threshold level, lithium (with no oxygen added) penetrated the refractory metal. Penetration resulted from the formation of a ternary oxide on grain boundaries or preferred crystallographic planes and proceeded by a wedging mechanism, which is caused by stresses generated by the corrosion product. Penetration was along grain boundaries at low oxygen concentrations, the attack depth and number of affected boundaries increasing with increasing oxygen concentration. At higher oxygen concentrations, transgranular attack also occurred. The threshold oxygen concentrations for attack were 400 and 100 ppm for niobium and tantalum, respectively.     

A possible method for the characterization of amorphous slags: Recovery of refractory metal oxides from tin slags

As X-ray, neutron scattering, and vibrational spectroscopy are not useful for amorphous solids characterization, microprobe analysis is used in determining the composition of these materials. The correlation coefficient matrix between the slag’s elements is obtained by a simple computer program which is commercially available. This matrix is employed for the constitution of the neighborhood of an element, which may be called the “pseudo-structure” (PS). The proposed method is a statistical view of the probable associations between the elements. It gives an insight into the amorphous solids' structure. The lixiviation of tin slags in order to recover the refractory metals they contain is used to illustrate the importance of the PS. A multistage acid-basic (AB) leaching leads to the dissolution of the matrix composed of Ca, Al, Fe, Mn, Si, ... oxides and the concentration of refractory metal oxides in the residues. The optimum tantalum and niobium recovery rates are 93 and 78 pct, respectively. The results of this research indicate that the leaching of the amorphous tin slag is a structure-sensitive operation. However, one may emphasize that the PS of amorphous solids is a simplification of the real neighborhood of ele-ments. It may be considered as a complement to other methods of investigation of the amorphous solids and may facilitate the hydrometallurgical process planning. K. MALAU, formerly Researcher Mineral Processing and Environmental Engineering, INPL. M.-CH. MEYER-JOLY, formerly Researcher, Mineral Processing and Environmental Engineering.     

High temperature chlorination kinetics of a niobium pyrochlore

The chlorination kinetics of a niobium (Cb) pyrochlore has been studied between 1873 and 2223 K, the chlorine concentration in helium varying between 0 and 20 pct. The pyrochlore was subjected to a preliminary thermal treatment at 1473 K in order to remove fluorine which escaped under the form of niobium oxyfluorides. This left NaNbO₃, CaNb₂O₆ and residual refractory oxides. The large chlorination reaction rate difference between NaNbO₃ and CaNb₂O₆ made possible the definition of distinct chlorination reaction rates for these constituents. It was found that the decomposition of CaNb₂O₆ is the controlling step in the chlorination of this constituent, while Nb₂O₅ (NbO₂ + NbO₂ at the prevailing temperatures) chlorination is very fast. The reaction is second order with respect to CaNb₂O₆ concentration and first order with respect to chlorine partial pressure between 1873 and 2023 K, a distinct reaction rate equation being obtained at 2223 K. Reaction rate constants have been calculated and vary between 3 and 10 moleJ.kg ·min for the temperature range considered. The NaNbO₃ reaction rate is first order with respect to total Nb₂O₅ concentration and 2.5 order with respect to chlorine partial pressure for the temperature range covered (1973 to 2223 K). Reaction rate constants are much higher than in the former case, being respectively 148 (1873 K), 214 (2023 K) and 518 (2223 K) mole/kg-min. Reaction orders may be affected by an error varying between 16 and 40 pct. The reaction rate constants are found accurate within 40 pct for CaNb₂O₆ and 25 pct for NaNbO₃.     

Activity and diffusivity of oxygen in tantalum and niobium base-alloys

An emf technique with a ThO₂-Y₂O₃ electrolyte has been used to measure oxygen activity and diffusivity in Nb-V, Nb-Mo, Ta-W, Ta-Re, and Ta-N alloys between 850 and 1130 °C. Oxygen activity is decreased by V additions to Nb and increased in the other alloys. Oxygen diffusivity is mostly lower in comparison with the base metal; however, for Re and W at low contents in Ta it was shown unambiguously by a difference technique that diffusivity is higher at higher temperatures and lower at lower temperatures. The results are discussed within the framework of a two-level system assuming different potential energies for oxygen atoms being either in octahedral sites far away from substitutionally dissolved foreign atoms or in the neighborhood of these atoms. Formerly Ph. D. Student at the Max-Planck-Institut     

Deformability of niobium and tin bronze during compression

The deformability of Cu–14.5 wt % Sn–0.25 wt % Ti bronze and pure niobium (99.84%) is studied during compression tests. It is found that an increase in the strain rate or a decrease in the test temperature can improve the strength characteristics of both materials. The strength properties of bronze are more sensitive to a change in the temperature–rate deformation conditions.