Extraction of tantalum and niobium from tin slags by chlorination and carbochlorination

Chlorination and carbochlorination of tantalum and niobium low-grade concentrate (LGC) and high-grade concentrate (HGC), obtained by leaching of tin slag, were studied using Cl₂ + N₂ and Cl₂ + CO + N₂ gas mixtures. Thermogravimetric analysis and conventional boat experiments were performed between 200 °C and 1000 °C. Chemical analysis, X-ray diffraction (XRD), and scanning electron microscopy (SEM) were used to characterize the samples and reaction products. Chlorination of LGC led to the recovery of about 95 pct of tantalum and niobium compounds at 1000 °C. However, the tantalum and niobium chlorinated compounds were contaminated by chlorides of Fe, Mn, etc. For HGC, chlorination at 1000 °C allowed the extraction of about 84 and 65 pct of the niobium and tantalum compounds, respectively. The recovered condensates were composed of pure tantalum and niobium chlorinated compounds. The apparent activation energies E _a for the chlorination of LGC and HGC, between 850 °C and 1000 °C, were 166 and 293 kJ/mole, respectively. At temperatures lower than 650 °C, the apparent activation energies for the LGC and HGC carbochlorination were 116 and 103 kJ/mole, respectively. Total extraction of the tantalum and niobium compounds was achieved by the carbochlorination of the LGC at 1000 °C. The generated tantalum and niobium chlorinated compounds were contaminated by the chlorides of Fe, Mn, Al, and Ca. The carbochlorination of the HGC at 500 °C allowed complete extraction and recovery of pure tantalum and niobium compounds. These results confirm the importance of obtaining an HGC from tin slag before its subsequent chlorination. The carbochlorination of such a concentrate could be an efficient process for the recovery of relatively pure tantalum and niobium chlorinated compounds at low temperatures.     

Kinetics of carbochlorination of chromium (III) oxide

Kinetics of the carbochlorination of Cr₂O₃ has been studied with Cl₂+CO gas mixtures between 500 °C to 900 °C using thermogravimetric analysis. The apparent activation energy is about 100 kJ/mol. Mathematical fitting of the experimental data suggests that the shrinking sphere model is the most adequate to describe the carbochlorination mechanism of chromium oxide and that is controlled by the chemical reaction. In the temperature range of 550 °C to 800 °C, the reaction order is about 1.34 and is independent of temperature. Changing the Cl₂+CO content from 15 to 100 pct increases the reaction rate and does not affect the reaction mechanism. Similarly, changing the ratio of Cl₂/(Cl₂+CO) from 0.125 to 0.857 does not modify the carbochlorination mechanism of Cr₂O₃. In these conditions, the reaction rate passes through a maximum when using a chlorinating gas mixture having a Cl₂/(Cl₂+CO) ratio of about 0.5.     

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.     

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 of tantalum pentoxide in mixture with sucrose carbon by chlorine gas

The mechanism and kinetics of β-Ta₂O₅ chlorination, mixed with sucrose carbon, have been studied by a thermogravimetric technique. The investigated temperature range was 500 °C to 850 °C. The reactants and reaction residues were analyzed by scanning electronic microscopy (SEM), X-ray diffraction (XRD), and Brunauer-Emmett-Teller method for surface area (BET). The effect of various experimental parameters was studied, such as carbon percentage, temperature, chlorine partial pressure, and flow, use of the multiple sample method, and carbon previous oxidation. The carbon percentage and previous treatment have an effect on the system reactivity. The temperature has a marked effect on the reaction rate. In the 500 °C to 600 °C temperature interval, the apparent activation energy is 144 kJ/mol of oxide, while at higher temperatures, the activation energy decreases. With high chorine partial pressures, the order of reaction is near zero. The kinetic contractile plate model, X=kt, considering carbon oxidation as the controlling stage, is the one with the best fit to the experimental data. A probable mechanism for the carbochlorination of β-Ta₂O₅ is proposed: (1) activation of chlorine on the carbon surface, (2) chlorination of Ta₂O₅, (3) oxidation of carbon, and (4) recrystallization of β-Ta₂O₅.     

Chlorination and carbochlorination of magnesium oxide

Thermogravimetric technique and boat experiments were used to study the chlorination of MgO and its reactivity with respect to Cl₂ + air, Cl₂ + N₂, and Cl₂ + CO gas mixtures at temperatures lower than 1000°C. Oxychlorination of MgO occurs at temperatures higher than that of its carbochlorination. Effects of experimental parameters such as gas flow rate, temperature, and partial pressure of the carbochlorinating gas mixture on the reaction rate were examined. At 550 °C, the apparent reaction orders with respect to Cl₂ + CO, Cl₂, and CO were 2.37, 1.47, and 0.89, respectively. At this temperature, the maximum reaction rate was obtained using a Cl₂ + CO gas mixture having a Cl₂/CO molar ratio equal to about 0.6. The apparent activation energy of carbochlorination of MgO was calculated as 49 kJ/mol between 425 °C and 600 °C.     

Kinetic study of the chlorination of gallium oxide

The kinetics of the chlorination of gallium oxide in chlorine atmosphere was studied between 650 °C and 800 °C. The calculations of the Gibbs standard free energy variation with temperature for the reaction Ga₂O_3(S)+3Cl_{2 (g)}→2GaCl_3(g)+1.5O_{2 (g)} show that direct chlorination is favorable above 850 °C. Thermogravimetric experiments were performed under isothermal and nonisothermal conditions. The effect of temperature, gas flow rate, and Cl₂ partial pressure were studied. The solids were characterized by X-ray diffraction (XRD) and scanning electronic microscopy (SEM). The nonisothermal results showed that chlorination of Ga₂O₃ starts at approximately 650 °C, with a mass loss of 50 pct at 850 °C. The isothermal results between 650 °C and 800 °C indicated that the reaction rate increased with temperature. The correlation of the experimental data with different solid-gas reaction models showed that the results are adequately represented by the model proposed by Shieh and Lee: X=1−{1−b ₂₂[b ₂₁ t+e ^−b ₂₁ ^t−1]}^1/(1−γ). From this model, it was found that the rate of reaction for the chlorination of Ga₂O₃ is of the order 0.68 with respect to Cl₂ and the activation energy is 113.23 kJ/mol. On the other hand, the order of the activation rate of the interface surface is 0.111 with respect to Cl₂ and its activation energy is 23.81 kJ/mol.     

Rare earth extraction and separation from mixed bastnaesite-monazite concentrate by stepwise carbochlorination-chemical vapor transport

A stepwise carbochlorination-chemical vapor transport (SC-CVT) process is proposed for the rare earth extraction and separation from a mixed bastnaesite-monazite concentrate based on thermodynamic and kinetic analysis using carbon as reductant, chlorine gas as chlorination agent, SiCl₄ as defluorination agent, and AlCl₃ as vapor complex former. Between 500 °C and 800 °C, apparent activation energy of the carbochlorination within 2 hours changed from 22 to 16 kJ/mol roughly for the initial half hour and final 1 hour, respectively, in the absence of SiCl₄; but these values reduced to 15 and 2.1 kJ/mole under 2 kPa of SiCl₄ gas. The rare earth chloride yield for 2 hours was 56 to 88 mol pct in the absence of SiCl₄ and 92 to 99 mol pct in the presence of SiCl₄; but carbochlorination at above 1000 °C yielded a large amount of acid-insoluble residue. This, together with the negligible equilibrium vapor pressure of ThCl₄ at below 600 °C, suggests that carbochlorination of the mixed concentrate at temperatures as low as 500 °C in the (Cl₂ + SiCl₄) atmosphere is suitable for rare earth extraction and thorium-free volatile by-product release, which is different from the conventional Goldschmidt process at 1000 °C to 1200 °C. The CVT reaction of the carbochlorination product was performed at 800 °C for 0.5 hours in the (Cl₂ + SiCl₄ + AlCl₃) atmosphere and then at 1000 °C for 6 hours in the (Cl₂ + AlCl₃) atmosphere along different temperature gradients, leading to complete thorium removal and efficient rare earth separation, respectively. Their combination allows an efficient and environmentally conscious extraction and separation of rare earth elements from the mixed concentrate.     

Rare earth extraction from bastnaesite concentrate by stepwise carbochlorination-chemical vapor transport-oxidation

A stepwise carbochlorination-chemical vapor transport-oxidation process is developed for the green rare earth extraction from a bastnaesite concentrate using carbon as reductant, chlorine gas as chlorination agent, SiCl₄ gas as defluorination agent, AlCl₃ as vapor complex former, and (O₂+H₂O) mixed gas as oxidant. Between 500 °C and 800 °C, the apparent activation energy of the carbochlorination within 2 hours changed from 17 to 10 kJ/mole roughly for the initial 20 minutes and final 1.5 hours, respectively, in the absence of SiCl₄, but these values reduced to 15 and 5.9 kJ/mole under 10 kPa of SiCl₄ gas, while the rare earth chloride conversion for 2 hours was 43 to 81 mol pct in the absence of SiCl₄ and 55 to 99 mol pct under 10 kPa of SiCl₄ gas. After carbochlorination at 550 °C for 2 hours in the (Cl₂+SiCl₄) atmosphere for efficient rare earth extraction and thorium-free volatile by-product release, throium was removed by chemical vapor transport at 800 °C for 0.5 hours in the (Cl₂+SiCl₄+AlCl₃) atmosphere and alkaline earths were separated from rare earths by oxidation at 700 °C to 1000 °C in the (O₂+H₂O) atmosphere for 0.5 hours, followed by water leaching at room temperature. Their combination allows a clean and efficient rare earth extraction from the concentrate.     

Kinetics of chlorination and oxychlorination of chromium (III) oxide

Thermogravimetric analysis (TGA) is used to study the kinetics of chlorination of Cr₂O₃ with Cl₂+N₂ and Cl₂+O₂ gas mixtures in the temperature range of 550 °C to 1000 °C. The reactivity of Cr₂O₃ toward the chlorine-oxygen gas mixture is higher than that toward the chlorine-nitrogen one. Chlorination of Cr₂O₃ proceeds with an apparent activation energy of about 86 kJ/mol between 550 °C and 1000 °C. The apparent reaction order with respect to chlorine is about 1.23 at 800 °C. At temperatures lower than 650 °C, the shrinking sphere model is the most appropriate for describing the reaction kinetics. Oxychlorination of Cr₂O₃ is characterized by an apparent activation energy of about 87 and 46 kJ/mol for temperatures lower than 650 °C and higher than 700 °C, respectively. At 800 °C and using a Cl₂+O₂ gas mixture, the maximum reaction rate is obtained when the Cl₂/O₂ molar ratio is equal to 4, confirming the formation of chromium oxychloride. At this temperature, the reaction orders with respect to chlorine, oxygen, and Cl₂+O₂ are about 1.08, 0.23, and 1.29, respectively. Mathematical fitting of the experimental data is discussed.     

A study of chromite carbochlorination kinetics

The carbochlorination of a chromite concentrate was studied between 500 °C and 1000 °C using boat experiments. The reaction products were analyzed by scanning electron microscopy (SEM), x-ray diffraction (XRD), and chemical analysis. The carbochlorination of a chromite concentrate at about 600 °C led to the partial selective elimination of iron, thus increasing the Cr/Fe ratio in the treated concentrate. Total carbochlorination of the chromite concentrates and volatilization of the reaction products was achieved at temperatures higher than 800 °C. The kinetics of the chromite carbochlorination was studied between 750 °C and 1050 °C using thermogravimetric analysis (TGA). The results were discussed in terms of the effects of gas flow rate, temperature, partial pressure of Cl₂+CO, and Cl₂/CO ratio on the carbochlorination process. It was observed that the temperature effect changed significantly with the progress of the reaction. The initial stage of the carbochlorination was characterized by an apparent activation energy of about 135 and 74 kJ/mol below and above 925 °C, respectively, while a value of about of 195 kJ/mol was found for the remainder of the carbochlorination process.     

Kinetics and Mechanism of Chlorination of Alumina Grains with He–CO–Cl2 Gas Mixtures—II. Intrinsic Rates and Effectiveness Factors

Abstract

The experimental results reported in Part I[1] for the chlorination of porous samples composed of alumina grains are correlated here using a coupled pore-diffusion/surface-reaction model. The reactant gases CO and Cl₂ diffuse inwards through the interstices between the grains and simultaneously react with grain-surfaces producing AlCl₃ and CO₂, Values for the intrinsic rate constant, k_w, were deduced from the measured rates by correcting for the mass-transfer effects.

ln k _w = ?2.1535(±0.3729) ? 9615(±426)T^?1

where k_w is mole Al₂O₃ reacted per g of solid-grain-sample per atm² per s. The effectiveness factors for the chlorination reaction were small, ranging between 0.136 and 0.374, indicating that the pore-diffusion of reactant species is relatively slow as compared to the second-order surface reaction. The activation energy for chlorination was found to be 79.94 (± 3.54) kJ.mol^?1 in the temperature range 800–950°C.     

Kinetics of oxychlorination of magnesium oxide

The kinetics of oxychlorination of MgO by Cl₂ + O₂ were studied in the temperature range from 850 °C to 1025 °C, using thermogravimetric analysis (TGA). The effects of Cl₂/O₂ ratio, gas velocity, temperature, and partial pressure of reactive gases on the reaction rate were investigated. The oxychlorination process was characterized by an apparent activation energy of about 214 kJ/mol. The reaction orders with respect to O₂, Cl₂, and Cl₂ + O₂ at 950 °C were about −0.37, 0.98, and 0.65, respectively. Data concerning oxychlorination of MgO, Cr₂O₃, and MgCr₂O₄ contained in chromite were compared. The effectiveness of using oxychlorination to extract iron oxides contained in magnesia was demonstrated.     

The use of nitride intermediates in the preparation of metals. A study of the reduction of Nb2O5 with NH3

A kinetics study of the reduction of Nb₂O₅ with NH₃ was conducted at 600° to 1300°C, using vertical fixed-bed, flow-through reactors, with the goal of using the nitride as an interme-diate in the preparation of niobium (columbium) metal via a thermal decomposition step. The effects of reactor materials (stainless steel, nickel, molybdenum, graphite, alumina, and Vycor) upon ammonia reactivity toward Nb₂O₅ were investigated. At low temperatures, the metal reactor systems were more catalytically reactive, yielding faster rates of reac-tion and a greater degree of nitride conversion, whereas at high temperatures, the non-metal reactor systems performed better. In general, the initial reaction rate-temperature data exhibited a maximum, associated with oxynitride formation, near 700°C for the metal reactor systems and 800° to 900°C for the nonmetal reactor systems, followed by a mini-mum, associated with NbO₂ formation, at 800° to 850°C for the metal reactor systems and 950° to 1000°C for the nonmetal reactor systems where NbN formation commences. A sec-ond maximum, associated with the hexagonal NbN phase, occurred at 1200°C. The ranges of activation energies for these regions were from 15 to 30 kcal/mole for region I, 8 to 22 kcal/mole for region II, and 10 to 22 kcal/mole for region III.     

Photolytic effects in alumina chlorination

The temperature dependence of the rate of chlorination of α-alumina with CO/Cl₂ gas mixtures exhibits an anomaly, a departure from the normal Arrhenius behavior, in the range 650 to 850°C; it is manifested as a local maximum in the Arrhenius plot at 670°C followed by a local minimum in the range 770 to 850°C. By carefully studying the effect of irradiation of the CO/Cl₂ gas mixtures on the rate of chlorination of α-alumina, it is shown that such an anomaly, which has been observed in the chlorination of various metallic oxides, is most likely due to the photochemical formation of phosgene (COCl₂) by ambient light incident on the reactant gas mixture during its transport to the main reactor. Phosgene is a better chlorinating agent than a CO/Cl₂ mixture. The mechanism of chlorination of α-Al₂O₃ by CO/Cl₂ mixtures subjected to the light emitted by a high-pressure Hg-vapor lamp is elucidated.     

Oxidation processes of alloys of the Ni—Ta system. II. Oxidation of the intermetallic compound NiTa

The oxidation kinetics of the intermetallic compound NiTa was studied by the continuous thermogravimetry in air at temperatures ranging from 600 to 1000°C. The scale formed was subjected to X-ray and metallographic sectioning phase analysis. Oxidation of NiTa was shown to occur because of the preferential diffusion of oxygen toward the scale-alloy interface. The kinetics is described as a parabolic function of time. The isotherms indicate that the parabolic oxidation rate constant K _p periodically decreases for t≤800°C but increases and periodically decreases for t>800°C. The temperature dependence of K_p is exponential. At t∼850°C the oxidation rate decreases, indicating a change in the oxidation mechanism. The scale formed on NiTa was found to contain the oxides NiO, NiO·Ta₂O₅(NiTa₂O₆), and Ta₂O₅, as well as Ni. The solid solutions Ni(Ta) and Ni₃Ta were detected in the sublayer of scale adjacent to the alloy. At high temperatures those phases are distributed among the layers: NiO+NiTa₂O₆+Ta₂O₅ in the first, NiTa₂O₆+Ta₂O₅ in the second, Ni+Ta₂O₅ in the third, and Ta₂O₅+Ni(Ta)+Ni₃Ta in the fourth. By analogy with the oxidation of unalloyed tantalum the explanation for the experimental results is that at a p→n phase transition. accompanied by the formation of oxygen vacancies and tantalum interstitials, occurs in the lattice at t∼850°C. Institute for Problems of Materials Science, Ukraine National Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 5–6(407), pp. 75–82, May–June, 1999.     

Oxidation of alloys of the system Ni−Ta. I. Oxidation of the intermetallic Ni3Ta

Oxidation kinetics for the intermetallic Ni₃Ta in air at 600–1000°C are studied by a thermogravimetric method. The alloy has an ordered crystal structure (D¹³ _2h-Pmmn) with rhombic lattice parameters a=0.512 nm, b=0.423 nm, and c=0.452 nm. The kinetic isotherms of Ni₃Ta oxidation are described by a parabolic equation. With t≤800°C there is a periodic increase in the rate constant of parabolic oxidation, but with t>800°C there is a periodic decrease of it. In the range 850–875°C the alloy oxidation rate decreases as a result of scale sintering. Oxygen diffusion slows down in the compact scale. X-ray and metallographic analysis of the scale that forms on Ni₃Ta indicates that it contains NiO, NiO·Ta₂O₅, Ta₂O₅ and also Ni and the solid solution Ni(Ta). These phase components are distributed in layers of the scale: NiO (first), NiO+NiO·Ta₂O₅ (second), NiO·Ta₂O₅+Ni+Ta₂O₅ (third), Ta₂O₅+Ni(Ta) (fourth). With a low temperature and short periods of heating there is no Nio·Ta₂O₅ or Ni(Ta) in the scale. Oxidation of Ni₃Ta is controlled by oxygen diffusion in the scale over the direction towards the alloy. With t>850°C this mechanism changes. By analogy with oxidation of tantalum it is assumed that structural changes in the Ta₂O₅ lattice may be responsible for this. Institute for Problems of Materials Science, Ukraine National Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 3–4(406), pp. 80–87, March–April, 1999.     

The thermodynamics of volatilization of chromic oxide: Part II. the species CrO2 CI{8in2}

A study has been made, using the transpiration technique, of the volatility of chromic oxide in oxygen-chlorine-argon mixtures in the temperature range 627 to 977°C. Under the conditions of the experiments, the volatilization is shown to occur by the reaction Cr₂O₃(s) + 2Cl₂(g) + 1/2O₂(g) = 2CrO₂Cl₂(g). The standard Gibbs free energy change is given by AG° = 90,900(±1200) + 16.46(±1.2)TJ, where the reference state for the gases is one standard atmosphere. Combination of this work with previous structural studies leads to a value for the heat of formation of CrO₂Cl₂ (g) at 298 K of + 518.2 (±2) kJ/mole.     

High-temperature thermodynamic stability of ditantalum carbide (Ta2C)

The CO(g) pressure in equilibrium with a Ta₂C-Ta₂O₅-Ta mixture has been measured at temperatures between 1740 and 1900 K using the torsion-effusion technique. From the equilibrium data, the following equation for ΔG°₂ of Ta₂C has been obtained: ΔG°₂ (±300) = −47,000 (±2200) +.IT From the enthalpy term in the ΔG°f equation, a value of —47.9 (±2.3) kcal/mole has been calculated for ΔH°₂₉₈ of Ta₂C which is in good agreement with several calorimetric results. This paper is based upon a thesis submitted by A. D. KULKARNI in partial fulfillment of the requirements of the degree of Doctor of Philosophy at the University of Pennsylvania.     

High temperature chlorination reaction of a pyrochlore mineral

The reaction of chlorine with a pyrochlore at 1000, 1400 and 1800°C was investigated. Compounds resulting from the reaction at 1000°C could be detected by means of X-ray diffraction analysis. They essentially contained CaNb₂O₆, Ti₂Nb₁₀O₂₉, NaCl and a small amount of calcium chloride. The condensed vapor obtained from reaction carried out at 1000°C was subjected to thermal treatment. Crystallized forms Nb₂O₅, NbO₂F, NaNbO₃, NaCl and Fe₂O₃ were obtained. Samples from this condensed vapor were also hydrolyzed, the hydrolysis product being further subjected to calcination, 5-Nb₂O₅ was obtained. The results suggest the possible formation of NbOCl₃ and NbOF₃ at this temperature. At 1800°C, the reaction of chlorine with pyrochlore appeared to be very fast, 84 pct of the initial Nb₂O₅ pyrochlore content being collected from the hydrolyzed condensed vapor after 10 min reaction.