The chlorination kinetics of zirconium dioxide in the presence of carbon and carbon monoxide

Chlorination of zirconium dioxide, an important step in the commercial production of reactor grade zirconium metal, has been studied using carbon and carbon monoxide as reductants. Zirconium tetrachloride was produced when the oxide was reacted with chlorine alone above 1000°C; by introducing carbon the reaction temperatures could be lowered 200°C, and when carbon monoxide was used the reaction temperatures necessary for similar rates were even lower. With carbon monoxide the chlorination of zirconium dioxide is described by two regions differentiated by temperature and the dependence of reaction rate upon chlorine and carbon monoxide pressures.     

Entrained-flow chlorination of titaniferous slag to produce titanium tetrachloride

Preparation of titanium tetrachloride was investigated by chlorination of a low-grade titaniferous slag in an entrained-flow reactor over the 550 to 1100 °C temperature range. Chlorination rates were highly dependent on the reactivity of the carbon used in the chlorination reaction. Use of a lignitic char resulted in the higher chlorination rates, whereas use of petroleum coke resulted in lower reaction rates. Use of bituminous coke gave intermediate chlorination rates. Above 700 °C, the chlorination reaction rate is controlled by mass transfer. Chlorine is completely reacted in 2.5 seconds at 1100 °C using slag/char feedstocks of 7-μm median particle diameter, with 10 to 25 pct excess carbon and 15 to 20 pct excess slag.     

Chlorination Kinetics of Titanium Nitride for Production of Titanium Tetrachloride from Nitrided Ilmenite

The kinetics of chlorination of titanium nitride (TiN) was investigated in the temperature range of 523 K to 673 K (250 °C to 400 °C). The results showed that the extent of chlorination slightly increased with increasing temperature and decreasing particle size of titanium nitride at constant flow rate of N₂-Cl₂ gas mixture. At 523 K (250 °C), the extent of chlorination was 85.6 pct in 60 minutes whereas at 673 K (400 °C), it was 97.7 pct investigated by weight loss measurement and confirmed by ICP analyses. The experimental results indicated that a shrinking unreacted core model with mixed-control mechanism governed the chlorination rate. It was observed that the surface chemical reaction of chlorine gas on the surface of TiN particles was rate controlling in the initial stage and, during later stage, internal (pore) diffusion through the intermediate product layer was rate controlling step. Overall the process follows the mixed-control model incorporating both chemical reaction and internal diffusion control. The activation energy for the chlorination of TiN was found to be about 10.97 kJ mol^?1. In processing TiCl₄ from TiN and TiO_0.02C_0.13N_0.85, the solids involved in the chlorination process were characterized by X-ray diffraction (XRD), Scanning electron microscopy (SEM) and Energy-dispersive X-ray spectrometer (EDX). The SEM/EDX results demonstrated the consumption of TiN particles with extent of chlorination that showed shrinking core behavior.     

Chlorination of commercial molybdenite concentrate in a fluidized bed reactor

Studies on recovery of molybdenum from commercial grade molybdenite using the technique of fluidized bed chlorination in the presence of oxygen are presented. Molybdenum recovery above 99 pct at a chlorine utilization efficiency of 84 pct has been achieved for a fluidizing gas flow-rate of 3 L/min of the gases Cl₂, O₂, and N₂ mixed in the proportion of 2∶5∶23, respectively, at 300 °C. The investigations on kinetics showed that the overall oxychlorination reaction is controlled by chemical reaction and is of first order with respect to particle surface area.     

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.     

Correlation of structure and superconducting properties in Nb(Cb)-Ti quaternary alloys

The aging behavior of a Nb-Ti alloy containing 60 wt pct Ti and small additions of oxygen and erbium or scandium was characterized and related to superconducting properties. The ternary and quaternary alloys were cold reduced and aged for various times at temperatures between 250° and 1000°C. ω and α phase transformations and oxide precipitation processes were followed by lattice parameter, diffraction intensity, resistivity, and metallographic studies, and correlated with superconducting critical magnetic field and critical current density measurements. The optimum 1 hr aging temperatures for producing ω and α phase precipitation were found to be 400° and 500°C, respectively. Aging at 1000°C produced only oxide precipitation. It was found that oxygen, erbium, and scandium stabilize the α phase but have little effect on ω precipitation. The ω phase proved the most effective fluxoid pinning precipitate. The fine scale dispersoid provided an extremely high number density of pinning sites. Formerly Research Assistant, Department of Metallurgy and Materials Science, M.I.T., Cambridge, Mass. Formerly Research Assistant, Department of Metallurgy and Materials Science, M.I.T. Formerly Research Associate, Department of Metallurgy and Materials Science, M.I.T.     

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.     

Studies on oxychlorination of MoS2 in a fluid bed reactor

Studies on the chlorination kinetics of molybdenum sulfide in the presence of a mixture of oxygen, chlorine, and nitrogen gases have been carried out in a fluidized bed reactor at temperatures of 250 to 350 ° and particle size range of 75 to 200 μm. The reaction rate with respect to surface area of MoS₂ particles as well as the composition of reactant gases has been determined and the specific rate constants evaluated. The oxychlorination of MoS₂ has been determined to be of first order with respect to surface area of particles and the overall reaction is found to be controlled by chemical reaction.     

Studies on oxychlorination of MoS2 in a fluid bed reactor

Studies on the chlorination kinetics of molybdenum sulfide in the presence of a mixture of oxygen, chlorine, and nitrogen gases have been carried out in a fluidized bed reactor at temperatures of 250 to 350 ° and particle size range of 75 to 200 μm. The reaction rate with respect to surface area of MoS₂ particles as well as the composition of reactant gases has been determined and the specific rate constants evaluated. The oxychlorination of MoS₂ has been determined to be of first order with respect to surface area of particles and the overall reaction is found to be controlled by chemical reaction.     

Carbochlorination kinetics of titanium dioxide with carbon and carbon monoxide as reductant

Kinetic study of the chlorination of titanium dioxide (rutile and anatase) was carried out in a fixedbed reactor at temperature ranging from 800 °C to 1000 °C and normal pressure. In our experiment, titanium dioxide powder and gaseous chlorine with carbon or carbon monoxide as reductant were used. The products of the reaction are all in gaseous phase under the temperatures and pressure studied. With CO as reductant, reaction is of noncatalytic gas-solid nature and experimental data fit the shrinking core model. When using C as reductant, solid-solid reaction is involved. Reactivity is highly enhanced by solid carbon and it is concluded that an activated C-TiO₂-Cl complex contributes to the enhanced reactivity. A reaction model based on phase boundary control applies to the experimental data. Thermodynamic analysis supports the experimental observation.     

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.     

Some factors affecting the chlorination of kaolinic clay

Chlorination of small samples of kaolinic clay was studied by the Bureau of Mines by a continuous weighing technique. The effects of NaCl additions, prior contact with SiCl₄ vapor, and the form of the reductant were qualitatively examined. Optimal chlorination of clay samples weighing less than 0.2 g was achieved in a fast-moving stream of CO and Cl₂ such that 90 pct of the aluminum and <5 pct of the silicon were reacted in 55 min at 625°C when one part NaCl was added to ten parts clay. Similar results were obtained in about 15 min at 750°C using chlorine gas with a mixture of ten parts clay, two parts charcoal and one part NaCl.     

钒页岩氯化挥发富集过程的热力学分析

对钒页岩与氯化钠、氯化钙和氯化铁三种固体氯化剂氯化挥发富集过程进行热力学分析。结果表明,以氯化铁作为固体氯化剂效果较好,可以利用氯化铁对V_2O_4、V_2O_5的直接氯化和氯化铁离解产生的氯气对V_2O_3、V_2O_4、V_2O_5的间接氯化反应,有效地将钒氧化物氯化挥发富集,且反应程度随温度的升高而增大。在高温焙烧过程中,Fe_2O_3、Al_2O_3和SiO_2不能被氯化,钒页岩中氧化物氯化反应自发进行的难易程度由易到难分别为K_2O、Na_2O、CaO、V_2O_3、V_2O_4、V_2O_5、MgO。通过焙烧温度的控制可以将V与Fe、Al、K、Na、Ca和Mg等金属杂质有效分离。     

Sulfation of a nickeliferous laterite

The sulfur trioxide roasting of a ferruginous, nickeliferous laterite was investigated at temperatures from 500° to 800°C. The kinetics of the fixed bed sulfation appeared to be logarithmic and this was interpreted to indicate a transport controlled process. Reaction rate constants were evaluated and it was found that the time for equivalent sulfation of nickel was more than 5 orders of magnitude greater at 800°C than at 600°C. The composition of the products appeared to dominate the reaction kinetics through changes in their physical nature. Nickel and cobalt sulfated quite rapidly at temperatures from 600° to 700°C but favorable selectivity over iron could be obtained only at temperatures >750°C where the nickel sulfation rate falls off rapidly. A two-step sulfation process was investigated that takes advantage of the rate characteristics of the low temperature process and attains selectivity by reversing the iron reaction at a higher temperature. Approximately 87 pct of the nickel, 97 pct of the cobalt, and 2 pct of the iron were extracted in a two-step process without additives.     

Ductilization of a powder metallurgy Al-17 wt pct Cu by means of channel-die compression and extrusion

Metal powders always contain a surface oxide layer, which is particularly tenacious in aluminum alloys. After hot pressing, this oxide coats the particle boundaries and reduces the ductility. In this article, a study of the Al-17 wt pct Cu alloy densified from rapidly solidified powder is presented. Different thermomechanical treatments were investigated to improve the ductility of this material. Channel-die (CD) forging was performed at two temperatures (430 °C and 500 °C). Eight compression runs were applied to the samples in each CD treatment. At 430 °C, three strain values per run were investigated (35, 50 and 70 pct). A bar was also extruded with a 40:1 ratio. Because of the small size of the samples, the ductility was assessed by means of the ring expansion test and analyzed by post mortem (fracture surface and cross section) observations. No ductility was measured after CD compression at 430 °C, although it appears from the fracture surface observations that increasing the strain per run has a beneficial effect. The CD compression at 500 °C and extrusion were both successful at promoting ductility, extrusion being more effective.     

Kinetics of reduction of ilmenite with graphite at 1000 to 1100 °C

Pellets prepared from mixtures of synthetic ilmenite and graphite were heated in argon at 1000 to 1100 °C and the rate of reduction was followed gravimetrically. No measurable reaction was obtained at 1000 °C but the rate was high at 1100 °C. An inflection in the reduction curve at about 2 pct weight loss at 1050 and 1100 °C was due to the difficulty in nucleating iron. The rate is increased significantly by the addition of ferric chloride which promotes the nucleation of iron. The addition of rutile decreases the rate of reduction and the addition of manganese stabilizes a pseudobrookite phase.     

Intrinsic Kinetics of Chlorination of WO<Subscript>3</Subscript> Particles With Cl<Subscript>2</Subscript> Gas Between 973 K and 1223 K (700 °C and 950 °C)

Chlorination is one of the methods applied in extractive metallurgy for the treatment of minerals to obtain valuable metals, such as titanium and zirconium. The possibility of applying chlorination metallurgy to other metals such as tungsten was the major aim of this study. The kinetics of the chlorination of tungsten oxide (WO₃) particles has been investigated by thermogravimetry between 973 K and 1223 K (700 °C and 950 °C) and for partial pressures of chlorine ranging from 15 to 70 kPa. The starting temperature for the reaction of WO₃ with chlorine is determined to be about 920 K (647 °C). The influence of chlorine diffusion through the bulk gas phase and through the particle interstices in the overall rate was analyzed. In the absence of these two mass-transfer steps, a reaction order of 0.5 with respect to chlorine partial pressure, and an activation energy of 183 kJ/mol were determined. For tungsten oxide particles of less than 50-μm size, a complete rate expression has been obtained.     

Physicochemical Fundamentals for the Synthesis of Zinc Oxide in Molten Chlorides

The oxidation of chloride ions in molten mixtures of zinc chloride and alkali metals (Na, K) is studied during oxygen bubbling at a temperature of 600°C. The dependence of the quantity of formed chlorine on the molar ratio of the chlorides in a mixture is found. The quantity of formed chlorine is shown to be directly proportional to the effective polarizing power of a cation. The following kinetic characteristics of the oxidation of chloride ions with respect to chlorine are calculated: the order of reaction, the reaction rate constants, and the apparent activation energy. Zn oxide is extracted and the dispersity, the chemical composition, and the specific surface area of some samples are determined.