Leaching of niobium and tantalum from a low-grade ore using a KOH roast–water leach system

A new process is proposed for the leaching and recovery of niobium and tantalum from a low-grade refractory niobium–tantalum ore after adding pure Nb₂O₅ to adjust the niobium to tantalum ratio. The ore was roasted and decomposed with KOH then leached with water. Experiments were carried out to study the effect of the Nb₂O₅-to-Ta₂O₅ mass ratio, decomposition temperature, alkali-to-ore mass ratio and decomposition time on the leaching of niobium and tantalum, as well as the associated impurity elements, such as titanium, iron, manganese, silicon and tin. The optimal conditions were determined to be: Nb₂O₅-to-Ta₂O₅ mass ratio 2.33:1; KOH-to-ore mass ratio 2:1; reacting for 60 min at 400 °C. Leaching with water extracted ~ 95% Nb and 94% Ta together with about 80% Si and Sn, 50% Ti and < 20% Fe and Mn. The niobium and tantalum was recovered as high purity (Nb,Ta)₂O₅ (99.3%) through evaporation, crystallization and phase transformation processes.     

Factors Affecting Impact Toughness in Stabilized Intermediate Purity 21Cr Ferritic Stainless Steels and Their Simulated Heat-Affected Zones

The correlation between simulated weld heat-affected zone microstructures and toughness parameters has been investigated in four intermediate purity 21Cr ferritic stainless steels stabilized with titanium and niobium either separately or in combination. Extensive Charpy V impact toughness testing was carried out followed by metallography including particle analysis using electron microscopy. The results confirmed that the grain size and the number density of particle clusters rich in titanium nitride and carbide with an equivalent circular diameter of 2 µm or more are statistically the most critical factors influencing the ductile-to-brittle transition temperature. Other inclusions and particle clusters, as well as grain boundary precipitates, are shown to be relatively harmless. Stabilization with niobium avoids large titanium-rich inclusions and also suppresses excessive grain growth in the heat-affected zone when reasonable heat inputs are used. Thus, in order to maximize the limited heat-affected zone impact toughness of 21Cr ferritic stainless steels containing 380 to 450 mass ppm of interstitials, the stabilization should be either titanium free or the levels of titanium and nitrogen should be moderated.     

Extraction kinetics of alunite in sulfuric acid and hydrochloric acid

Extraction kinetics of alunite in sulfuric acid and hydrochloric acid were studied in a batch reactor. The effects of reaction temperature, acid concentration, particle size, calcination temperature, calcination time and KF/Al₂O₃ molar ratio on the extraction process were investigated. Experimental studies were carried out in the ranges of 35–95 °C for reaction temperature, 0.25–3.0 M for sulfuric acid, 0.5–0.6 M for hydrochloric acid, 76–182 μm for average particle size, 100–900 °C for calcination temperature, 15–60 min for calcination time and 0.15–0.90 for KF/Al₂O₃ molar ratio. The calcination temperature was the most important parameter affecting the extraction process followed by reaction temperature, particle size and acid concentration. Others had less effect. It was determined that the extraction process is controlled by diffusion through a product layer. The activation energies of the processes were found to be 19.1 and 18.5 kJ/mol for sulfuric acid and hydrochloric acid, respectively. The apparent rate constants were similar for both acids and found to be a function of acid concentration as C^0.33 and particle size r^−0.8.     

Kinetics of Silicothermic Reduction of Manganese Oxide for Advanced High-Strength Steel Production

The kinetics of silicothermic reduction of manganese oxide from MnO–SiO₂–CaO–Al₂O₃ slags reacting with Fe-Si droplets were studied in the temperature range of 1823 K to 1923 K (1550 °C to 1650 °C). The effects of initial droplet mass, initial droplet silicon content, and initial slag manganese oxide content were studied. Data obtained for 15 pct silicon showed agreement with control by mass transport of MnO in the slag with a mass transfer coefficient (k _s) of 4.0 × 10^?5 m/s at 1873 K (1600 °C). However, when this rate-determining step was tested at different initial silicon contents, the agreement was lost, suggesting mixed control between silicon transport in the metal and manganese oxide transport in the slag. Increasing the temperature resulted in a decrease in the rate of reaction because of an increase in the favorability of SiO as a product. Significant gas generation was found during all experiments, as a result of silicon monoxide production. The ratio of silicon monoxide to silica formation was increased by factors favoring silicon transport over that of manganese, further supporting the conclusion that the reaction is under mixed control by transports of both silicon and manganese oxide.     

Decomposition kinetics of titanium slag in sodium hydroxide system

A novel process was proposed for preparing titanium dioxide by the decomposition of titanium slag in sodium hydroxide system under atmospheric pressure. The kinetics on the decomposition of titanium slag was mainly investigated. The results on effect of reaction temperature, particle size and NaOH-to-slag mass ratio on titanium extraction show that the temperature and particle size have significant influence on the titanium extraction. The experimental data of titanium extraction indicate that the shrinking core model with chemical reaction controlled process is most applicable for the decomposition of titanium slag, with the apparent activation energy of 40.8 kJ mol^? 1. Approximately 95–98% of titanium in the titanium slag could be extracted under the optimal reaction conditions. In addition, the content of TiO₂ obtained in the product is up to 99.3%.     

Kinetics of pyrite oxidation in sodium hydroxide solutions

The kinetics of pyrite oxidation in sodium hydroxide solution were investigated in a stirred reactor, under temperatures ranging from 50 °C to 85 °C, oxygen partial pressures of up to 1 atm, particle size fractions from -150 + 106 to -38 + 10μm (-100 + 150 mesh to -400 mesh + 10 μ), and pH values of up to 12.5. The surface reaction is represented by the rate equation:-dN/dt = Sbk″pO^0.5 ₂[oH^{- 0.25}/(1 +k‴ pO₂ ^0.5) where N represents moles of pyrite, S is the surface area of the solid particles,k″ andk″ are constants,b is a stoichiometric factor, pO₂ is the oxygen partial pressure, and [OH^-] is the hydroxyl ion concentration. The corresponding fractional conversion (X) vs time behavior follows the shrinking particle model for chemical reaction control: 1 - (1 -X)^1/3 =k _ct The rate increases with the reciprocal of particle size and has an activation energy of 55.6 kJ/mol (13.6 kcal/mol). The relationship between reaction rate and oxygen partial pressure resembles a Langmuir-type equation and thus suggests that the reaction involves adsorption or desorption of oxygen at the interface. The square-root rate law may be due to the adsorption of a dissociated oxygen molecule. The observed apparent reaction order with respect to the hydroxyl ion concentration is a result of a complex combination of processes involving the oxidation and nydrolysis of iron, oxidation and hydrolysis of sulfur, and the oxygen reduction. Formerly Graduate Student, Department of Mineral Engineering, Pennsylvania State University     

Effect of Lead Addition and Milling on Densification and Mechanical Properties of 6061 Aluminium Alloys

In the present work, one batch of prealloyed 6061Al powder was mixed with different lead compositions (5, 10, 15 vol.%) and another set with same composition was ball-milled for 5 h at 300 rpm. Microstructural features such as lattice constant, crystallite size, particle size and morphology were studied using XRD, particle size analyzer and SEM. Both the as-mixed as well as ball-milled powders were compacted at 300 MPa and sintered under N₂ atmosphere for 1 h in tube furnace at 590 °C. The ball milling of 6061Al alloy powder improved sinter density and densification while lead addition showed negligible influence on these parameters. The microstructure of as-mixed 6061Al–Pb alloys exhibited equiaxial morphology whereas ball-milling resulted in elongated grains with uniform lead distribution. Quasi-static compressive mechanical behavior was investigated for 6061Al–Pb alloys at 1 × 10^?3 s^?1 strain rate. Results indicated that ultimate compressive and yield strength were sensitive to milling and lead volume fraction.     

Kinetics of Reduction of CaO-FeO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>-MgO-PbO-SiO<Subscript>2</Subscript> Slags by CO-CO<Subscript>2</Subscript> Gas Mixtures

Kinetics of the reaction of lead slags (PbO-CaO-SiO₂-FeO _x -MgO) with CO-CO₂ gas mixtures was studied by monitoring the changes in the slag composition when a stream of CO-CO₂ gas mixture was blown on the surface of thin layers of slags (3 to 10 mm) at temperatures in the range of 1453 K to 1593 K (1180 °C to 1320 °C). These measurements were carried out under conditions where mass transfer in the gas phase was not the rate-limiting step and the reduction rates were insensitive to factors affecting mass transfer in the slag phase. The results show simultaneous reduction of PbO and Fe₂O₃ in the slag. The measured specific rate of oxygen removal from the melts varied from about 1 × 10^?6 to 4 × 10^?5 mol O cm^?2 s^?1 and was strongly dependent on the slag chemistry and its oxidation state, partial pressure of CO in the reaction gas mixture, and temperature. The deduced apparent first-order rate constant increased with increasing iron oxide content, oxidation state of the slag, and temperature. The results indicate that under the employed experimental conditions, the rate of formation of CO₂ at the gas-slag interface is likely to be the rate-limiting step.     

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.     

Phase relations,microstructure, and valence transition studies on CaZr_(1-x)Ce_xTi_2O_7(0.0≤x≤1.0) system

In this study, Ce-doped zirconolite was synthesized through high-temperature solid-state reaction at 1250 ℃ in air for 96 h. The crystal phase.microstructure and valence transition were studied by X-ray diffraction(XRD), scanning electron microscopy(SEM), and X-ray photoelectron spectroscopy(XPS).Phase relations of CaZr_(1-x)Ce_xTi_2O_7 systems were determined by XRD analyses and Rietveld refinements.Four different phases are identified, namely zirconolite, perovskite, pyrochlore, and cerianite. The phase transformation(2M-zirconolite → 4M-zirconolite → Ce-pyrochlore) is caused by cations rearrangement as cerium content increases. The solubility limit of cerium ions in CaZr_(1-x)Ce_xTi_2O_7 system is estimated to be approximately 0.80. Under sintering air atmosphere, partial reduction of Ce~(4+) in Ce~(3+) is detected in Ce 3d XPS spectra, and the ratio of Ce~(3+) and Ce~(4+) significantly decreases as cerium content increases.     

In-Situ Time-of-Flight Neutron Diffraction Study of High-Temperature α-to-β Phase Transition in Elemental Scandium

Lattice parameters for both hcp α-Sc and bcc β-Sc were determined between 1200 °C and 1400 °C from time-of-flight (TOF) neutron diffraction data collected from an elemental Sc sample vacuum sealed inside a niobium crucible. On heating, the high-temperature β-Sc phase first appeared between 1340 °C and 1350 °C, close to the reported transition temperature of 1337 °C. The lattice constants of hcp α-Sc were found to vary between a = 3.3522(4) Å, c = 5.3807(7) Å at 1200 °C and a = 3.3579(6) Å, c = 5.398(1) Å at 1340 °C. The lattice constants of bcc β-Sc were found to vary between a = 3.752(2) Å at 1350 °C and a = 3.7572(8) Å at 1400 °C. The average thermal expansion coefficient for the bcc β-Sc phase was 1.61 × 10^?5 °C^?1 over the temperature range 1360 °C to 1400 °C. The average thermal expansion coefficient along the a-axis of hcp α-Sc between 1200 °C and 1340 °C was 1.46 × 10^?5 °C^?1. The average thermal expansion coefficient along the c-axis of hcp α-Sc between 1200 °C and 1340 °C was 2.22 × 10^?5 °C^?1.     

Kinetics of Dissolution of Tenorite in Ammonium Media

In this work, the dissolution kinetics of tenorite (CuO) in a NH₄OH-H₂O system was studied. The studied temperature range was 5–55°C, ammonium hydroxide concentration between 0.1 and 0.75 M, and a particle size range of 5–24 µm. The stirring speed, pH of the ammonia solution, and various agents were also studied. The results indicated that the leaching of tenorite occurred quickly with a particle size of 5 µm in a 0.45 M solution of NH₄OH for a pH value equal to 10.5. Dissolution of CuO also increases as temperature and the concentration of NH₄OH increase. For concentrations less than 0.10 M, there is almost no leaching tenorite. By decreasing the particle size, the dissolution of CuO increase. Results show the stirring speed had no significant effect on the leaching rate of tenorite for values above 250 rpm. Leaching kinetics was analyzed using the model of the surface chemical reaction. The reaction rate was of the order of 2.2 with respect to the concentration of ammonium hydroxide and inversely proportional to the initial particle size. Activation energy of 59 kJ/mol was estimated for the temperature range of 5–55°C.     

Dissolution Kinetics of Iron from Low-Grade Mn Ore and Optimization Using Response Surface Methodology

In this study an attempt has been made to increase Mn/Fe ratio in dump Manganese ore fines so that it can be used for the production of ferromanganese. For this purpose non-coking coal was used as reductant and dilute hydrochloric acid as leaching medium for the roasted ore. The effects of acid strength, leaching time, leaching temperature, stirring speed, ore particle size and pulp density have been studied. The dissolution of iron follows the kinetic model 1 ? 2x/3 ? (1 ? x)^2/3 = k_dt. Thus product layer diffusion is the controlling mechanism and the activation energy has been determined to be 26.23 kJ/mol at 40–95 °C. Another set of experiments have been conducted according to 2³ full factorial design, and regression equation for iron dissolution has been developed.     

Effects of Solute and Surfactant Addition on the Crystallization and Morphology of Hydroxyapatite Powders Synthesized by Hydrolysis of Calcium Hydrogen Phosphate Dehydrate (DCPD)

The effects of the addition of alcohol and cetyltrimethylammonium bromide (CTAB) on the crystallization and the morphology of hydroxyapatite (HA) powders synthesized by hydrolysis of calcium hydrogen phosphate dehydrate (DCPD) in the 2.5 M NaOH solutions at 348 K (75 °C) for 1 hour have been studied. The values of zeta potential have large differences between the sums of DCPD with CTAB (Z _DCPD+CTAB) minus the sum of DCPD and CTAB (Z _DCPD + Z _CTAB), and of HA with CTAB (Z _HA+CTAB) minus the sum of HA and CTAB (Z _HA + Z _CTAB), respectively. When the hydrolysis of DCPD occurred in the 2.5 M NaOH solutions at 348 K (75 °C) for 1 hour both with and without alcohol and CTAB, XRD results show the only one phase of HA in the as-dried powders. When the NaOH solution does not contain CTAB, the crystallite size of HA powders decreased from 23 ± 1 to 16 ± 1 nm as the alcohol content was more than 50 pct. The crystallite size of HA powders obtained from DCPD synthesized in the 2.5 M NaOH solution with 1.0 × 10^?3 M CTAB decreased when the alcohol content was increased to 70 pct, whereas the crystallite size increased when the alcohol concentration was greater than that of 70 pct. SEM images show that the HA powders have a rod-like shape when DCPD was synthesized in the 2.5 M NaOH solution without CTAB or alcohol. When the NaOH solution had 1.0 × 10^?3 M CTAB and various alcohol concentrations, the morphology of HA powder still maintained a rod-like or needle-like shape. The HA powder had a maximum specific surface area of 180.25 m²/g when the hydrolysis of DCPD occurred in a 2.5 M NaOH solution containing 1.0 × 10^?3 M CTAB and 70 pct alcohol at 348 K (75 °C) for 1 hour.     

Phase Selection in Undercooled Ni-3.3 Wt Pct B Alloy Melt

The change of eutectic solidification mode in undercooled Ni-3.3 wt pct B melt was studied by fluxing and cyclic superheating. The eutectic structure is mainly controlled by the undercooling for eutectic solidification, ΔT ₂, instead of ΔT ₁, the undercooling for primary solidification. At a small ?T ₂ [e.g., 56 K (56 °C)], the stable eutectic reaction (L → Ni₃B + Ni) occurs and the eutectic morphology consists of lamellar and anomalous eutectic; whereas at a larger ?T ₂ [≥140 K (140 °C)], the metastable eutectic reaction (L → Ni₂₃B₆ + Ni) occurs and the eutectic morphology consists of matrix, network boundary, and two kinds of dot phases. Further analysis declares that the regularly distributed dot phases with larger size come from the metastable eutectic transformation and are identified as α-Ni structure, whereas the irregularly distributed ones with smaller size are a product of the metastable decomposition and tend to have a similar structure to α-Ni as it grows. Calculation of the classical nucleation theory shows that the competitive nucleation between Ni₂₃B₆ and Ni₃B leads to a critical undercooling, ΔT ₂ ^* [125 K < ΔT ₂ ^* < 157 K (125 °C < ?T ₂ ^* < 157 °C)], for the metastable/stable eutectic formation.     

Hydrogen Storage Properties of Graphite-Modified Mg-Ni-Ce Composites Prepared by Mechanical Milling Followed by Microwave Sintering

The Mg₁₇Ni_1.5Ce_0.5 hydrogen storage composites with different contents of graphite were prepared by a new method of mechanical milling and subsequent microwave sintering. The small particle size (~25 ??m) and the low echo ratio of power indicate that graphite plays an important role not only as a lubricant during mechanical milling but also as a supplementary heating material during microwave sintering. As a catalyst in the hydriding/dehydriding (H/D) reaction, graphite also improved the hydrogen storage properties of the composites. The hydrogen absorption and desorption capacities of Mg₁₇Ni_1.5Ce_0.5 with 5 wt pct graphite were 5.34 and 5.30 wt pct H₂ at 573 K (300 °C), its onset temperature of dehydriding reaction was 511 K (238 °C), and its activation energies of H/D reaction were 40.9 and 54.5 kJ/mol H₂, respectively. The kinetic mechanisms of the H/D reaction are also discussed.     

Perovskite type lanthanum manganite: Morpho-structural analysis and electrical investigations

Perovskite-type oxides of LaMnO_3 were synthesized by means of the sol-gel method, in the presence of citric acid as gelling agent. The precursors used were Mn(NO_3)_2·H_2O, La_2O_3, and NaOH,mixed in the stoichiometric ratio to obtain perovskite materials. The obtained gel was heat-treated at 400, 600 and 800 ℃ respectively, for 6 h. X-ray diffraction and FT-IR spectroscopy were used to analyze the phase transformation as a function of temperature, and the Rietveld refinement was used in order to characterize the materials obtained structurally. The average crystallite size of the products was calculated from XRD data and the average particle size was measured from the TEM micrographs. At 600 ℃, the synthesized compound is well-crystallized, showcases a perovskite structure(Pm-3m space group), and exhibits uniform and homogeneous hexagonally-shaped particles, with sizes in the 20-50 nm range.Complex impedance measurements in the 20-2 × 10~6 Hz frequency range were carried out at different temperatures(26-115 ℃), and the electrical conduction mechanism is discussed.     

The Effect of Cooling Rate from Solution on the Lattice Misfit During Isothermal Aging of a Ni-Base Superalloy

The evolution of lattice misfit in the polycrystalline nickel-base superalloy, RR1000, has been investigated using high resolution neutron diffraction at interrupted time intervals during an aging heat treatment. Samples were subjected to a super-solvus heat treatment followed by either a 100 or a 1 K min^?1 cooling rate prior to aging. Irrespective of cooling rate, the lattice misfit remained unchanged at approximately 0.1 pct throughout the aging cycle, indicating the microstructure remained stable. Microstructural observations validated this result for samples cooled at 1 K min^?1. However, for the faster, 100 K min^?1, cooling rate, whilst the secondary γ′ remained unchanged, the tertiary γ′ showed significant coarsening. Simulated diffraction patterns were used to investigate the influence of volume fraction, particle size, and lattice parameter of individual γ′ distributions on the measured lattice misfit. The results obtained indicate that conventional methods of measuring lattice misfit will be dominated by the γ′ distribution with the highest volume fraction, and may therefore obscure subtle changes in the γ′ distributions with lower a volume fraction.     

Kinetics of Ammonia Leaching of Multimetal Sulphides

This paper briefly describes the studies carried out on oxidative ammonia leaching of Cu-Zn-Pb multimetal sulphides. Kinetics of zinc and copper dissolution were studied with ? 200 + 300 mesh BSS fraction and 1% solids in the slurry. It is observed that the dissolution of sphalerite proceeds by a phase boundary reaction model and that of copper via diffusion through product layer in the temperature range of 70-100°C. The rate equations for zinc and copper dissolution are given by:

1 ? (1 ? α)^1/3 = k _Zn[NH₃][pO₂]^1/2

1 ? 2/3α ? (1 2/3α )^2/3 = k^Cu[NH₃]²[pO₂]^1/2

where the symbols have the usual meanings.

Activation energies for zinc and copper dissolution reactions are estimated to be 66.5 and 55.4 kJ/mole, respectively. Activation energy values thus obtained are also comparable to those obtained using a differential approach.

The leaching results obtained with 10% solids using a wide range of particle size (? 140 + 500 mesh) indicate that copper dissolution is chemically controlled in ammonia as well as ammonia-ammonium sulphate medium in the temperature range of 115-135°C. However, at lower temperature (?55°C). the leaching reaction follows a diffusion model. Zinc dissolution data show deviations from the shrinking core model due to high extractions in the initial stages.     

Structural states and electrical conductivity of oxidized niobium nanopowders

The structure of niobium nanopowders (particle size 0.03–0.07 μm) oxidized in air is studied by X-ray diffraction. The nanopowder particles have a significant fraction of an amorphous phase. The amorphous component is likely to block the well-known mechanism of niobium oxidation Nb → Nb_s.s → Nb₆O → NbO → NbO_x, which was proposed on the basis of the results of studying the oxidation of niobium powders at high temperatures. Here, Nb_s.s is the solid solution of oxygen in niobium and NbO_x are the higher niobium oxides NbO₂ and Nb₂O₅. The amorphization of the surface of niobium nanopowders oxidized at 20°C can be one of the main causes of a rather high electrical resistivity (ρ ≈ 10⁸ Θ cm) of the samples compacted from these powders.