Interstitial atom configurations in stable and metastable Fe-N and Fe-C solid solutions

Mössbauer Fe⁵⁷ spectroscopy allows comparison of Fe?N and Fe?C interstitial solid solutions. The spectra of Fe?N retained austenite indicate that nitrogen atoms are randomly distributed on octahedral sites in the austenite and in the virgin martensite. On heating, austenite decomposes directly to the equilibrium phases α iron and Fe₄N at temperatures above 160°C. Virgin martensite ages at room temperature by local ordering of nitrogen atoms. In that process, three new iron atom environments develop, characteristic of the Fe₁₆N₂ (α″) structure. However, the excessive width of the peaks indicate the perfect order of the Fe₁₆N₂ precipitate is not achieved, except after very long times. Further aging at 100°C leads to the complete decomposition of the virgin martensite to the discrete phases α iron and Fe₁₆N₂. This two phase structure is stable up to 160°C, above which the precipitation of Fe₄N occurs. These results are in contrast to Fe?C data. Carbon atoms in retained austenite tend to be far apart in their octahedral sites, and this nonrandom distribution is inherited by the virgin martensite. Fe?C austenite decomposes by the formation of ∈ carbide below 160°C and precipitation of Fe₃C above 180°C. The carbon atoms in virgin martensite agglomerate at room temperature and regions of ordered Fe₄C are believed to result. Subsequently ∈ carbon is formed at 80°C and Fe₃C precipitates above 160°C.¹     

Study on removing Mo from tungstate solution using coprecipitation adsorption method based on novel Mo sulphidation process

Abstract

A coprecipitation–adsorption method based on a novel Mo sulphidation process is proposed for deep removal of molybdenum from tungstate solutions. First, a new sulphur source, P₂S₅, is used to provide HS^? for the Mo sulphidation process. The MoO₄^2? can be thoroughly converted into MoS₄^2? while tungsten still exists as WO₄^2?. Next, in the sodium tungstate solution, 99·4% of MoS₄^2? is removed with precipitated ‘nascent’ CuS under 6 h reaction at 30°C when the molar ratio Cu/Mo is 3. Similarly, in a commercial (NH₄)₂WO₄ solution, by controlling the molar ratio Cu/Mo at 6, 98·4% of MoS₄^2? is removed after 1 h at 30°C. Subsequently, phosphorus introduced during Mo sulphidation is removed using the magnesium ammonium phosphate method allowing the ammonium paratungstate to reach the GB 10116-88 APT-0 standard.

On propose une méthode de co-précipitation et adsorption basée sur un nouveau procédé de sulfuration du Mo pour l’enlèvement en profondeur du molybdène de solutions de tungstate. Premièrement, on utilise une nouvelle source de soufre, P₂S₅, pour obtenir le HS^? pour le procédé de sulfuration du Mo. Le MoO₄^2? est entièrement converti en MoS₄^2? alors que le tungstène existe encore sous la forme de WO₄^2?. Ensuite, 99·4% du MoS₄^2? est enlevé de la solution de tungstate de sodium avec le CuS ‘naissant’ précipité, en moins de 6 heures de réaction à 30°C lorsque le rapport molaire de Cu/Mo est de 3. De façon similaire, 98·4% du MoS₄^2? est enlevé d’une solution commerciale de (NH₄)₂WO₄, après une heure à 30°C, en contrôlant à une valeur de 6 le rapport molaire de Cu/Mo. Subséquemment, le phosphore introduit lors de la sulfuration du Mo est enlevé en utilisant la méthode du phosphate ammoniaco-magnésien, permettant ainsi au paratungstate d’ammonium d’atteindre la norme GB de 10116-88 APT-0.     

A novel process on preparation of ammonium metatungstate solution using bipolar membrane electrodialysis

An industrial (NH₄)₂WO₄ solution was used to directly prepare ammonium metatungstate solution using bipolar membrane electrodialysis. The effects of current density, initial concentrations of NH₄HCO₃ and WO₃ in feeds, temperature and pH of product solution on the process were investigated. A continuous running test was carried out under the optimum conditions and the results showed that the direct recovery of WO₃ reached 99.97% with average current efficiency of 72.5% and power consumption of 0.877?kWh?kg^?1 WO₃ using a feed solution containing 172.3?g?L^?1 WO₃ and a base solution containing 1.5?mol?L^?1 NH₄HCO₃ with a current density of 700?A?m^?2 and a solution pH in salt compartment of 2.8 at 31°C for 600?min, indicating easy and stable operation of the bipolar membrane electrodialysis process.     

Potentiostatic studies of the influence of temperature on lead-silver anodes during electrowinning and decay period

The electrochemical behaviour of the Pb-0.7% Ag anode has been investigated in zinc sulphuric acid electrolyte containing 5, 8, 12?g/L Mn²⁺ by electrochemical measurements at different temperatures of 35°C, 40°C, 45°C. It was observed by cyclic voltammetry before electrolysis that temperature increase from 35°C to 45°C resulted in a decrease in the oxidation peak I (Pb → PbSO₄) and an increase in the oxygen evolution peak II (2H₂O → O₂?+?4H⁺?+?4e). At 2.01?V/SHE, the current densities of Pb-0.7% Ag anode in sulphuric acid solution containing 8?g/L Mn²⁺ at 35°C, 40°C and 45°C increased from 26.5 at 35°C to 28.5 and 37.7?mA/cm² at 40 and 45°C, respectively. In the presence of ZnSO₄ addition to the sulphuric acid solution, the anodic current of oxygen evolution at 2.01?V/SHE decreased by ～ 18% at 40°C. After 2?h of potentiostatic polarisation in the presence of zinc sulphate addition at 2.01?V/SHE, electrochemical Noise Measurements showed that the admittance (corrosion rate) of the Pb-0.7% Ag anode in the zinc electrolyte increased with the increase in temperatures during decay. This tendency was also confirmed by linear polarisation and impedance measurements.     

Hot Corrosion Behaviour of Refractory and Rare Earth Oxide Reinforced CoCrAlY APS Coatings at 700 °C

This paper investigates cyclic hot corrosion of plasma sprayed CoCrAlY?+?Al₂O₃?+?YSZ (C1) and CoCrAlY?+?CeO₂ (C2) composite coatings on MDN 321 and Superni 76 substrates in molten salt (Na₂SO₄-60%V₂O₅) environment exposed to 700 °C. Weight change technique is used to evaluate the corrosion performance. Both C1 and C2 coatings showed better corrosion resistance than uncoated alloy. Both the coatings showed linear weight gain during the initial cycles and parabolic weight gain nature with subsequent hot corrosion cycles. The parabolic rate constant (K_p) of C1 and C2 coating was observed to be in the range 0.29–0.32?×?10^?10 g² cm^?4 s^?1 and 1.0–1.13?×?10^?10 g² cm^?4 s^?1 respectively. In C1 coating, the globular and continuously packed structure on the corroded surface having CoO, Cr₂O₃, CoCr₂O₄ and CoAl₂O₄ spinel oxides provided superior hot corrosion resistance. While in case of C2 coating, the outward growth of CeVO₄ irregular crystals as a corrosion product of CeO₂ and V₂O₅ salt deteriorated the oxide scales resulting in higher corrosion rate.     

Growth of a layer of W6Co7 phase in systems with participation of phases containing cobalt

Growth kinetics for a layer of W₆Co₇ at the interfaces tungsten-cobalt and tungsten-melts based on copper and tin containing cobalt at 1200 °C are studied. It is established that rate constant for growth of this layer in the system with participation of tin (k_Sn) is much greater than in the system with copper (k_Cu). Dependences are determined for these constants on cobalt activity in the melt (a_Co): k_Cu(m²/sec) = = 1.72 · 10^?15(a_Co ? 0.221) + 2.99 · 10^?14 (a_Co ? 0.221)²; k_Sn(m²/sec) = 1.8 · 10^?14 (a_Co ? 0.221) + + 9.077 · 10^?12 (a_Co ? 0.221)³.     

Activities in liquid Fe−Al−O and Fe−Ti−O alloys

The solubility and activity of oxygen in Fe?Al and Fe?Ti melts at 1600°C were measured. The activity was measured electrochemically using the following galvanic cells: Cr-Cr₂O₃(s) ? ThO₂(Y₂O₃) ? Fe-Al-O(l), Al₂O₃(s) Cr-Cr₂O₃(s) ? ThO₂(Y₂O₃) ? Fe-Ti-O(l, saturated with oxide) Cr-Cr₂O₃(s) ? ZrO₂(CaO) ? Fe-Ti-O(l, saturated with oxide) Aluminum and titanium decrease the solubility of oxygen in liquid iron to a minimum of 6 ppm at 0.09 wt pct Al and 40 ppm at 0.9 wt pct Ti, respectively. The value of the interaction coefficients ε ₀ ^(Al) and ε ₀ ^(Ti) are ?433 and ?222, respectively. the activity coefficient of aluminum at infinite dilution in liquid iron is 0.021, while that of titanium is 0.038. The value of the aluminum equilibrium constant, the solubility product at infinite dilution, is 5.6×10^?14 at 1600°C. The ThO₂(Y₂O₃) electrolyte exhibited insignificant electronic conductivity at 1600°C down to oxygen partial pressures of 10^?15 atm, which corresponds to about 0.3 ppm O in unalloyed iron.     

Evolution of inclusions and change of as-cast microstructure with Mg addition in high carbon and high chromium die steel

The effects of Mg on the inclusions and the as-cast microstructure of high carbon and high chromium die steel, a grade of cold working die steel with high C content of 1.4–1.6% and high Cr content of 11.0–13.0%, were systematically investigated. It is found that inclusions vary with the route as Al₂O₃ (No Mg) to MgO·Al₂O₃ +?Al₂O₃ (5?ppm Mg), and then to MgO+MgO·Al₂O₃ (11 and 15?ppm Mg). The average diameter of the inclusions decreased from 1.91 μm (no Mg) to 1.29 μm (15?ppm Mg), while the number density increased from 2.69?×?10⁴ mm^??3 (no Mg) to ～5.62?×?10⁴ mm^??3 (15?ppm Mg). The changes in the size and the number density were discussed in terms of the effect of inclusions on the nucleation process and the wettability of them with steel melt. The as-cast microstructures were greatly refined with Mg addition that correlated with the evenly dispersed fine Mg containing inclusions.     

Silico-ferrite of Calcium and Aluminum (SFCA) Iron Ore Sinter Bonding Phases: New Insights into Their Formation During Heating and Cooling

The formation of silico-ferrite of calcium and aluminum (SFCA) and SFCA-I iron ore sinter phases during heating and cooling of synthetic iron ore sinter mixtures in the range 298?K to 1623?K (25?°C to 1350?°C) and at oxygen partial pressure of 5?×?10^?3 atm has been characterized using in situ synchrotron X-ray diffraction. SFCA and SFCA-I are the key bonding phases in iron ore sinter, and an improved understanding of their formation mechanisms may lead to improved efficiency of industrial sintering processes.?During heating, SFCA-I formation at 1327?K to 1392?K (1054?°C to 1119?°C) (depending on composition) was associated with the reaction of Fe₂O₃, 2CaO·Fe₂O₃, and SiO₂. SFCA formation (1380?K to 1437?K [1107?°C to 1164?°C]) was associated with?the reaction of CaO·Fe₂O₃, SiO₂, and a phase with average composition 49.60, 9.09, 0.14, 7.93, and 32.15?wt pct Fe, Ca, Si, Al, and O, respectively. Increasing Al₂O₃ concentration in the starting sinter mixture increased the temperature range over which SFCA-I was stable before the formation of SFCA, and it stabilized SFCA to a higher temperature before it melted to form a Fe₃O₄?+?melt phase assemblage (1486?K to 1581?K [1213?°C to 1308?°C]). During cooling, the first phase to crystallize from the melt (1452?K to 1561?K [1179?°C to 1288?°C]) was an Fe-rich phase, similar in composition to SFCA-I, and it had an average composition 58.88, 6.89, 0.82, 3.00, and 31.68?wt pct Fe, Ca, Si, Al, and O, respectively. At lower temperatures (1418?K to 1543?K [1145?°C to 1270?°C]), this phase reacted with melt to form SFCA. Increasing Al₂O₃ increased the temperature at which crystallization of the Fe-rich phase occurred, increased the temperature at which crystallization of SFCA occurred, and suppressed the formation of Fe₂O₃ (1358?K to 1418?K [1085?°C to 1145?°C]) to lower temperatures.     

Physicochemical aspects of technology of the Al2O3-Al layered cermet obtained using reaction sintering

Certain features of the technology of the Al₂O₃-Al layered cermet obtained by the reaction sintering (RS) in air of powdered billets that were fabricated by pressing the charge from the PAP-2 plateletshaped aluminum powder. The RS process was activated by the introduction of a dry residue of liquid glass (DRLG) into the charge (C = 3?C28%). According to the data of the X-ray phase analysis, the sintered material contains (vol %): Al (61?C82), ??-Al₂O₃ (8?C25), Na₂Si₂O₅ (2?C15), and Si (1?C14). Oxide phases and silicon are nanodimensional morphological objects (13?C100 ??m) in the layered aluminum host. The cermet density is 2.1?C2.45 g/cm³, tensile strength is 45?C90 MPa, and the ultimate bending strength is 320 MPa. The activation of RS by a small DRGL additive (3%) retains the layered cermet structure. It is leveled at a considerable increase in the RS time (up to 600 min) or C (up to 28%) because of recrystallization and involves plateletshaped aluminum particles in the chemical interaction.     

Numerical Simulation of Inclusion Modification During Calcium Treatment Process in Ladle

In order to investigate the inclusion modification process and predict final inclusions after the injection of calcium, based on actual data of injection of calcium and inclusion thermodynamic calculation, a three-dimensional model for calcium treatment in LF refining furnace for pipeline steel was established. The evolution of concentrations and distribution of the modification inclusions in the liquid steel during the injection of calcium were investigated. The results showed that when the length of feeding calcium was 600 m, after about 1000 s, the average calcium equilibrium concentration was 1.86?×?10^?5 and the recovery of calcium was 23.2%. The average equilibrium concentration of C3A, C12A7, CA(C stands for CaO and A stands for Al₂O₃) and CaS, respectively, was 0, 25.7?×?10^?6, 3.04?×?10^?6 and 13.7?×?10^?6. The final modified inclusion was related to the amount of feeding calcium. The equilibrium value of C12A7 and CaS increased with the increase of feeding amount; however for CA, the regularity was opposite.     

Reduction of lead in the lead sulfide-molten sodium hydroxide system

Based on thermodynamic investigations of the reduction of lead from its sulfide compounds using the electron-donating properties of sulfur of its sulfide, we studied the effect of various factors on the reduction of lead (“metallization”) in molten sodium hydroxide. In the range of temperatures of smelting PbS with the alkali (470–510°C), there occurs a “burst” metallization reaching 95% at T = 550°C; at T = 650°C, the process is completed in 15 min; at 650–500°C, the reduction of the metal occurs in the diffusion region. The NaOH: PbS weight ratio (α) exerts a direct effect on the yield of the compact metal (‘lens’). At α = 1.8?0.8, the yield of the compact metal is 96–98%. It has been confirmed experimentally that the chemism of the utilization of elementary sulfur is related to a disproportionation of the latter with the accumulation of S^2? and SO ₄ ^2? ions and the possible formation of polysulfide sulfur S _n ^2? .     

Pyrite oxidation with ozone: stoichiometry and kinetics

One of the most frequent causes of refractoriness in precious metals leaching is their occlusion or fine dissemination into a pyritic matrix. This study experimentally explores the acid leaching of pyrite with ozone, suggests the stoichiometry of the reaction, estimates its activation energy and defines the effect of the main variables on the leaching kinetics. The results of stoichiometry tests allow establishing that one mole of pyrite requires 7.7 moles of ozone to produce one mole of ferric ion and 2 moles of HSO₄? ions. A decrease in the particle size, solution pH and solids’ concentration of the leaching system increases pyrite dissolution. The type of acid (nitric, sulphuric and hydrochloric) does not affect pyrite dissolution rate. Up to 60% of pyrite is dissolved when the optimal experimental conditions are employed (1?g pyrite (?25?µm), 800?mL of 0.18?M of H₂SO₄, 800?rev?min^?1, 1.2?L?min^?1 gas stream O₂/O₃ with 0.079?g O₃?L^?1 and 25°C). The apparent activation energy of the pyrite-ozone reaction is 14.92?kJ?mol^?1, and the absence of a passive layer on the pyrite surface and the linearity of the dissolution profiles suggest that the dissolution kinetics is controlled by the chemical reaction.     

Synthesis and characterisation of Al2O3/Ni-type composites obtained by spark plasma sintering

This paper presents the influence of sintering on the structure, morphology and compressing strength of alumina/nickel composite compacts obtained by spark plasma sintering (SPS). Al₂O₃/Ni composites were prepared by SPS in argon atmosphere at temperatures in the range of 1000–1200 –C with a holding time of 2, 5 and 10?minutes. The heating rate was 200 C?min^?1. These composites have been characterised by X-ray diffraction, SEM and EDX. The relative density and compressive strength of the as-obtained compacts were determined. The results showed that the alumina particles are uniformly dispersed in a quasi-continuous Ni network, and there was no sign of phase changes during sintering. The maximum strength of the alumina/nickel composite with a content of 75 vol. ? Al₂O₃ and 25 vol. ? Ni was about 240?MPa for the samples sintered at 1200?C for 10?minutes.

Special block from the conference RoPM2017 guest edited by Ionel Chicinas, Technical University, Cluj-Napoca.     

Efficient removal of K2O and Fe2O3 impurities from bauxite tailings through active calcination combined with acid leaching

In the present study, direct acid and active calcination–acid leaching processes were employed to remove K₂O and Fe₂O₃ impurities from bauxite tailings. For direct acid leaching, the results indicated that Fe₂O₃ can be removed with the addition of 3?mol?L^?1 HCL, whereas K₂O cannot be removed even when the concentration of HCl is higher than 7?mol?L^?1. As the bauxite tailings were calcined at 400–700°C, both K₂O and Fe₂O₃ were removed at low HCl concentration (3?mol?L^?1), which was ascribed to the removal of hydroxyls and increased K activity, derived from X-ray diffraction, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy.     

Influence of silicide coatings on the mechanical properties of the NT-50 alloy

We have investigated the effect of silicide coatings on the ultimate strength of NT-50 alloy over the temperature range ?196...+1100 °C. From 196 to 800 °C the ultimate strength of silicidated specimens was lower than that of the initial and annealed alloy whereas from 900 °C it was slightly higher. We have also examined the effect of silicide coatings on the relative elongation of NT-50 alloy samples. Over the temperature range ?196...+1100 °C, the relative elongation of specimens annealed in a vacuum of 6.7 · 10^?3 Pa at 1250 °C for 4 h was found to be lower than that of the silicidated specimens. This is attributed to the dissolution of the residual gases (O₂, N₂, H₂, CO, and CO₂) under vacuum annealing.     

Fe(III)–As(V) precipitates obtained from a sulphate system and their leach stability

Fe(III)–As(V) precipitates were synthesised from the Fe(II)-As(V)-SO₄^2--H₂O system at a temperature of 90°C and a constant pH maintained at 1.5?±?0.05. The precipitates obtained were characterised by X-ray diffraction (XRD), chemical composition analysis, scanning electron microscope (SEM) and Raman spectrum, thermogravimetric analyzer (TGA) and electron probe micro-analyzer (EPMA). The precipitates were in irregular aggregation of about 1–4?μm in size. The precipitates consisted of scorodite, ferric arsenate and an amorphous ferric hydroxide sulphate formulated as Fe(OH)_x(SO₄)_y. The precipitates were stable in modified Toxicity Characteristic Leaching Procedure (TCLP) tests at pH 4.93 for 60?h. Arsenic concentrations in the leaching solutions of 0.27?mg?L^?1 and 0.59?mg?L^?1 were obtained for the precipitates prepared initial Fe(II)/As(V) molar ratios of 4.0 and 5.0, respectively. Significantly more iron than arsenic was dissolved with up to 280?mg?L^?1 of iron reporting to solution. Long-term stability tests of the precipitates were carried out by leaching them for 40 days at 25°C under various media of pH between 9.50 and 10.57. The results showed that the precipitates tested in this study were more stable than those by previous researchers owing to a preferential dissolution of the amorphous ferric hydroxide sulphate.     

Effect of Sn Grain Orientation on the Cu6Sn5 Formation in a Sn-Based Solder Under Current Stressing

A SnAgCu-based solder stripe between two Cu electrodes is current stressed with a density of 5?×?10⁴?A/cm² at 393?K (120?°C). After current stressing for 24?hours, electromigration induces the Cu dissolution from the cathode-side Cu electrode, leading to the Cu₆Sn₅ formation in the solder stripe. Very interestingly, the Cu₆Sn₅ phase is selectively formed within a specific Sn grain. Electron backscattering diffraction analysis indicates the crystallographic orientations of Sn grains play an important role in the selective Cu₆Sn₅ formation.     

Dissolution behaviour of a beryl ore for optimal industrial beryllium compound production

This study involves the leaching of the beryl ore with sulphuric acid (H₂SO₄) solution for predicting optimal beryllium extraction conditions with the aim of assessing the importance of leachant concentration, reaction temperature and particle size on the extent of dissolution. A kinetic model to represent the effects of these variables on the leaching rate was developed. It was observed that the dissolution of beryl ore increases with increasing H₂SO₄ concentration, temperature, decreasing particle size and solid to liquid ratio. At optimal leaching conditions, 89.3% of the ore was reacted by 1.25?mol/L at 75°C temperature and 120 minutes with moderate stirring, where 1612.0?mg/L Be²⁺, 786.7?mg/L Al³⁺, 98.1?mg/L Fe³⁺ and 63.4?mg/L Ag⁺ were found as major species in the leach liquor. The unleached products constituting about 10.7% were examined by X-ray diffraction (XRD) and found to contain primarily, siliceous compounds such as Xonotlite, Antigorite, Chrysolite and Kaolinite.     

Solubility of carbon in CaO-Al2O3 melts

Carbon distribution ratios between CaO-Al₂O₃ slags and Fe-0.0003 to 0.8 mass pct Al-0.2 to 5.6 mass pct C alloys were measured at 1873 K in an Al₂O₃, CaO, or graphite crucible. The carbon distribution ratios were dependent on the oxygen potential, determined by theAl/(Al₂O₃) equilibrium, not by theC/CO (P _co = 1 atm) equilibrium. The (mass pct C)/a _c ratios were proportional to the activity of Al in logarithmic form with a slope of 2/3, indicating that carbon in slag is dissolved as C^2? ion. Solubilities of carbon in CaO-Al₂O₃ slags were also measured at 1873 K under the CO-CO₂-Ar gas mixtures in an Al₂O₃ or graphite crucible. It was found that C^2? ion is present in the range of log $P_{O_2 } $ (atm) < ?15 and CO ₃ ^2? ion in the range of log $P_{O_2 } $ (atm) > ?7.