The thermal behavior of mechanically activated galena by thermogravimetry analysis

The thermal decompositions of mechanically activated and nonactivated galenas were studied by thermogravimetry analysis (TGA) at the heating rate of 10 K min⁻¹ in argon. Results indicate that the initial temperature of thermal decomposition (abbreviated as T _di ) in the TGA curves for different galenas decreases gradually with increased grinding time. The specific granulometric surface area (S _G ), the structural disorder, and the content of elemental sulfur of mechanically activated galenas were analyzed by an X-ray diffraction (XRD) laser particle-size analyzer, XRD analysis, and the gravimetric method, respectively, which shows that the specific granulometric surface area of mechanically activated galenas remains almost constant after a certain grinding time, but the lattice distortions (ɛ) rise, the crystallite sizes (D) decrease, and the elemental sulfur contents of mechanically activated galenas increase with increased grinding time. The results imply that the decrease of the initial temperature of thermal decomposition in the TGA curves for mechanically activated galenas is mainly caused by the increase of lattice distortions, and the formation of new dangling bonds resulted from the production of elemental sulfur of mechanically activated galenas with increased grinding time. Finally, the differences in the thermal-decomposition reactivity between nonactivated and mechanically activated galenas were also discussed.     

The oxidation behavior of unactivated and mechanically activated sphalerite

The oxidation behaviors of unactivated and mechanically activated sphalerites were investigated using the thermogravimetry method (TG) in flowing highly pure oxygen at the heating rate of 10 K min⁻¹. It is found that the remaining mass between 400 and 873 K in the TG curves of mechanically activated sphalerites rises with increasing grinding time. The difference in oxidation reactivity of unactivated and mechanically activated sphalerites was also discussed. The specific granulometric surface area (S _G), the structural disorder, and the content of elemental sulfur of unactivated and mechanically activated sphalerites were determined by X-ray diffraction (XRD) laser particle size analysis, X-ray diffraction, and the gravimetric method, respectively. The results show that the specific granulometric surface areas of mechanically activated sphalerites remain almost constant after a certain grinding period. The elemental sulfur contents of unactivated and mechanically activated sphalerites were determined to be 0.5 mg/g, and the lattice distortions (ɛ) increase but the crystallite sizes (D) decrease with increasing grinding time. All the results imply that the mass increase between 400 and 873 K in the TG curves of mechanically activated sphalerites depends mainly on the increase of lattice distortions (ɛ) and the decrease of the crystallite sizes (D) with increasing grinding time. It was concluded that TG is a useful method for characterizing mechanically activated sphalerites.     

Effect of aging conditions on the leaching of mechanically activated pyrite and sphalerite

The leaching of mechanically activated pyrite and sphalerite exposed to nitrogen (99.999 vol pct) or air at ambient temperature or 573 K was investigated. The results indicate that at the same leaching time, the iron-leaching ratio of mechanically activated pyrite or sphalerite aged in nitrogen at both ambient temperature and 573 K decreases slightly with increasing aging time and remains constant after a certain aging period. The iron-leaching ratio of mechanically activated pyrite exposed to ambient air varies with the exposure period. But, at the same leaching time, the zinc-leaching ratio of mechanically activated sphalerite aged at ambient temperature does not change with the aging atmosphere. The structures of mechanically activated pyrite and sphalerite after being aged were determined. The specific granulometric surface area of mechanically activated pyrite and sphalerite decreases with increasing aging time, but keeps constant after a certain aging period. The X-ray diffraction patterns of mechanically activated pyrites aged in nitrogen do not change with aging time; neither do the X-ray diffraction patterns of mechanically activated sphalerites aged either in air or in nitrogen. For mechanically activated pyrite exposed to ambient air for 3 and 6 months, new phases were found. The lattice distortion and the elemental sulfur content of pyrite and sphalerite mechanically activated in nitrogen were also investigated. The results indicate that the elemental sulfur content of mechanically activated pyrite rises noticeably, and its lattice distortion (ε) rises slightly, with increasing grinding time. The elemental sulfur content of mechanically activated sphalerite remains constant at 0.5 mg elemental sulfur per gram of sphalerite, and its lattice-distortion ratio increases apparently with increasing grinding time. These observations provide further evidence for our opinion that the formation of dangling bonds on the surface of mechanically activated pyrites and the lattice distortion on the surface of mechanically activated sphalerites may mainly result in the enhancement of hydrometallurgical process for corresponding sulfide minerals.     

Thermal Decomposition of the Froth Flotation Mineral Sulfide Concentrates Under Argon Atmosphere in the Presence and Absence of Carbon

Thermal decomposition of the mineral sulfide concentrates derived from froth flotation was studied in the temperature range between 1073 K and 1473 K. The experiments were carried out by heating the mineral sulfide concentrates under inert (argon) atmosphere and, in the presence and absence of carbon. The thermal decomposition of the mineral sulfide concentrates were analyzed by plotting the weight loss of the sample against the reaction time which were obtained from the thermogravimeric analysis (TGA) and characterizing the reacted samples by X-ray diffraction (XRD) and microscopic techniques. The extent and rate of thermal decomposition of the samples were temperature dependent. The high sulfur minerals (CuFeS₂ and FeS₂) were decomposed to low sulfur and stable phases (Cu_1.1Fe_1.1S₂, Cu₅FeS₄, and FeS). However, the thermal decomposition of the mineral sulfides were complex because of liquid phase formation at high temperature such that the low sulfur phases appear to have precipitated from the liquid phase during cooling of the sample. Metallic copper was produced in the mineral concentrates containing hydrated and carbonated compounds. It was concluded that the formation of copper cannot occur in the absence of internal oxidation in the sample, when the mineral concentrates are heated under inert atmosphere. The phases were compared with the thermodynamic predictions.     

Synthesis of Invar 36 type alloys from elemental and prealloyed powders by mechanical alloying

Invar 36 (Fe₆₄Ni₃₆) nanocrystalline powders were successfully obtained by the mechanical alloying process. The mechanically alloyed Invar 36 powders were obtained from both, Fe–Ni elemental and Fe–Ni₃Fe prealloyed powders. XRD, DSC and magnetic measurements were used to characterise the Invar 36 powders. The lattice parameter evolution versus temperature of Invar 36 powders was investigated by in-situ high-temperature X-ray diffraction (HT-XRD). For both, Invar 36 (Fe, Ni) and Invar 36 (Fe, Ni₃Fe) powders, the lattice parameter values are constant up to about 350°C. The magnetic measurement also indicated that the Invar 36-type alloys are formed after 16?h of milling.     

Carbothermic reduction of llmenite (FeTiO3) and rutile (TiO2)

The mechanically activated carbothermic reduction mechanism of ilmenite has been examined by a combination of steady-state and dynamic thermal techniques coupled with X-ray diffraction. The reaction was found to proceed via an initial, rapid reduction to elemental iron and rutile, which was followed by a slow reduction of rutile to a series of oxides of the general formula Ti _n O_2n−1 until Ti₃O₅ was formed, which was found to be relatively stable. Iron was probably incorporated into the Ti _n O_2n−1 lattice only for n>3, forming mixed oxides of uncertain composition. The formation of TiC was evident at temperatures as low as 1100 °C, but the rate of reaction was extremely slow, presumably due to a solid-state diffusion limitation. Increasing the temperature gave increasing conversion of TiO₂ to TiC until it was the only confirmed product. The effect of iron on the later stages of reduction was removed by examining the reduction of pure rutile. It was found that the reduction of Ti₃O₅ was enhanced by the presence of iron. The separation of iron from the titanium product proved to be high, with > 90 pct of iron removed after the initial reduction. The iron removal increased slowly to almost 100 pct when elemental iron and titanium carbide were the products.     

Analysis of Mechanically Induced Reactivity of Boehmite Using Kinetics of Boehmite to <Emphasis Type="Italic">γ-</Emphasis>Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Transformation

This article focuses on the mechanically induced reactivity of boehmite prepared by thermal decomposition of gibbsite. Boehmite, which retained the morphology of gibbsite, was characterized by a specific surface area of 264 m²/g. Mechanical activation (MA) was carried out in a planetary mill up to 240 minutes. The samples were characterized in terms of morphology, characteristic particle diameters, Brunauer Emmett Teller (BET) specific surface area (SSA_BET), microcrystallite dimension (MCD), microstrain (ε) and Fourier transform infrared spectroscopy. The reactivity was construed from the kinetics of thermal transformation of boehmite into γ-Al₂O₃. The transformation observed between 600 K and 900 K (327 °C and 627 °C), manifested itself as two overlapping peaks in the differential thermogravimetric plot. These peaks correspond to two stages of dehydroxylation involving Al₂OH and AlOH groups in succession. The peaks were resolved using Gaussian deconvolution. The reactivity was assessed separately for the two stages by comparing the fraction reacted in MA samples (α) with that of nonactivated sample (α _ref). During both stages, enhanced kinetics, as revealed by α-α _ref plots, indicated an increase in reactivity with MA. The transformation mechanism conformed to n ^th order reaction (f[α] = [1 – α] ⁿ with n = 1.3–1.5 in both stages). Values of n remained similar for the activated and reference samples. Activation energies (E _a) for the first and second dehydroxylation stages were respectively 115 and 300 kJ/mol for the nonactivated sample. E _a for the second stage decreased exponentially to a value of 222 kJ/mol after 240 minutes of milling. An anomalous negative correlation between reactivity and SSA_BET was observed. Reactivity parameters were strongly correlated with MCD and ε. A plausible explanation for the observed correlations is presented.     

Hydration of mechanically activated granulated blast furnace slag

Ground granulated blast furnace slag (GGBFS) is known to possess latent hydraulic activity, i.e., it shows cementitious properties when in contact with water over a long period of time. Results are presented in this article to show that, in sharp contrast to published literature on the hydration of neat GGBFS, the complete hydration of slag is possible in a short time (days), even without a chemical activator. This is achieved if the slag used for hydration is mechanically activated, using an attrition mill. The nature of the hydration product of the mechanically activated slag depends not only on the initial specific surface area (SSA) of the slag but also on the surface activation, as manifested by the change in the zeta potential (ξ) of the slag during the milling process. Depending upon the SSA and the ξ, the hydration product changed from nonreacted slag with high porosity (slag SSA < 0.3 m²/g, ξ>−29 mV) to hydrated slag with a compact structure (SSA=0.3 to 0.4 m²/g, ξ=−29 to −31 mV), and, finally, to fully hydrated slag with high porosity (SSA>0.4 m²/g, ξ ∼ 26 mV). Unlike the poorly crystalline hydration product formed by the nonactivated slag, even after prolonged hydration for years, the hydration product of mechanically activated slag was crystalline in nature. The crystallinity of the product improved as the duration of the mechanical activation increased. The calcium-silicate-hydrate (C-S-H) phases present in the slag hydration product, characterized by a Ca/Si ratio of 0.7 to 1.5, were similar to those found for the hydraulic cement binder, except for the presence of Mg and Al as impurities. In addition, the presence of a di-calcium-silicate-hydrate phase (α-C₂SH), which normally forms under hydrothermal conditions, and a Ca-deficient and Si-Al-rich phase (average Ca/Si mole ratio < 0.1 and Si/Al ∼ 3) is indicated, especially in the hydration product of slag that was activated for a longer time.     

Effects of mechanical activation on the HCl leaching behavior of plagioclase, ilmenite and their mixtures

Plagioclase (NaAlSi₃O₈–CaAlSi₂O₈) is one of the dominant impurity minerals in the ilmenite concentrate from the Panxi area of China. The effects of mechanical activation on the structural changes of plagioclase and ilmenite single minerals from Panxi were investigated using X-ray diffraction (XRD), particle size analysis, and scanning electron microscope (SEM). Hydrochloric acid leaching behaviors of mechanically activated plagioclase and ilmenite single minerals and their mixtures were also studied. The results show that with increasing milling time, the crystallite size, lattice strain and the particle size changed continuously but differed between plagioclase and ilmenite. Mechanical activation enhanced the leaching reactivity of plagioclase and ilmenite with 20% HCl at 105 °C, consistent with the lattice disorder and decreased particle size. The leaching of plagioclase was hardly affected by the existence of ilmenite, but the leaching of ilmenite was inhibited when plagioclase co-existed in the sample which is attributed to silica gel produced from leached plagioclase blocking the surface of unleached ilmenite.     

Preparation and performance of Pr-doped Ba_(0.5)Sr_(0.5)Co_(0.8)Fe_(0.2)O_(3-δ) cathode for IT-SOFCs

(Ba0.5Sr0.5)1-xPrxCo0.8Fe0.2O3-δ(BSPCFx;x=0.00-0.30) oxides were synthesized by a sol-gel thermolysis process using combination of PVA and urea,and were also investigated as cathode material for intermediate temperature solid oxide fuel cells(IT-SOFCs).X-ray diffraction(XRD) results showed that all the samples formed a single phase cubic pervoskite-type structure after being calcined at 950 oC for 5 h and the lattice constant decreased with the Pr content increasing.The electrical conductivity of Ba0.5Sr0.5Co0.8Fe0.2O3-δ(BSCF) was greatly enhanced by Pr-doping.The thermal expansion coefficient(TEC) of BSPCFx was increased with the content of Pr increasing,and all the thermal expansion curves had an inflection at about 250-400 oC due to the thermal-induced lattice oxygen loss and the reaction of Co and Fe ion.Ac impedance analysis indicated that BSPCFx possessed better electrochemical performance.The polarization resistance of the sample with x=0.2 was only ～0.948 Ω cm2 at 500 oC,significantly lower than that of BSCF(～2.488 Ω cm2).     

A study of the thermal decomposition of BaCO3

In the present work, the decomposition reaction, BaCO₃ (solid) = BaO (solid) + CO₂ (gas), was investigated by thermogravimetric analysis (TGA) and differential thermal analysis (DTA) methods. Both shallow powder beds and densely compacted spheres of the carbonate were employed. In the case of the shallow powder beds, TGA and DTA were carried out simultaneously. The DTA curves showed that BaCO₃ exhibited two phase transformations, the transformation of orthorhombic to hexagonal occurring at 1079 K and that of hexagonal to cubic at 1237 K. The activation energy and the forward reaction rate constant of the decomposition of BaCO₃ were evaluated from the thermogravimetric results of the powder beds. The activation energy of the decomposition was found to be 305(± 14) kJ • mole⁻¹. The experimental results obtained with the compacted spheres were compared with those corresponding to the powder beds. After the initial stages, the formation of liquid due to the eutectic reaction between BaCO₃ and BaO appears to play an important role in the reaction kinetics.     

机械活化氧化锌在稀硫酸溶液中的量热研究

用Calvet型微量热计,通过测定机械活化前后氧化锌在稀硫酸中的溶解热,从而确定机械活化氧化锌的机械活化储能。对行星式球磨和搅拌式球磨氧化锌在不同活化时间及不同转速下储能结果进行了比较。结果表明:随着活化时间的增加,氧化锌的储能依次增加;增加的趋势随活化时间增加而减慢,最后达到极限值。搅拌式球磨效率较行星式球磨高,达极限储能时间较短。在不同气氛下,机械活化氧化锌储能基本一致。     

The intrinsic thermal decomposition kinetics of SrCO3 by a nonisothermal technique

The kinetics of the decomposition of SrCO₃ in argon to SrO and CO₂ were studied in the temperature range 1000 to 1350 K. The thermal decomposition was followed simultaneously by thermogravimetric analysis (TGA) and differential thermal analysis (DTA) during linear heating. By using a nonisothermal method, the complete rate expression was determined from a relatively small number of experimental runs. Shallow beds of fine synthetic powder as well as thin flakes of pressed powder were employed to obtain the kinetics of decomposition in the absence of heat- and mass-transfer effects. The thermal decomposition started at about 1000 K. The recommended rate expression for the SrCO₃ decomposition is

Transformation of mechanically alloyed Nb-Sn powder to Nb<Subscript>3</Subscript>Sn

Nb₃Sn was processed via mechanical alloying (MA). The powder mixture comprised of stoichiometric proportions of elemental niobium and tin powder was mechanically alloyed for 3 hours and the mechanically alloyed powder mixture was heat treated. While MA resulted in Nb-Sn solid solution, the reaction leading to the formation of Nb₃Sn occurs during the subsequent heat treatment of the powder mixture.     

Thermally assisted and mechanically driven solid-state reactions for formation of amorphous AI33Ta67 alloy powders

The rod milling technique using the mechanical alloying (MA) process has been employed for preparing amorphous Al₃₃Ta₆₇ alloy starting from elemental Al and Ta powders. X-ray diffraction (XRD), differential thermal analysis (DTA), differential scanning calorimetry (DSC), optical microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) are utilized to follow the progress of amorphization. The results show that during the first few kiloseconds of MA time, layered composite particles of Al and Ta are intermixed and form an amorphous phase upon heating to 685 K by DTA. This process is called thermally assisted solid-state amorphization (TASSA). During the early stage of milling, the number of layers of the composite particles increases. This leads to an increase in the heat formation of amorphous Al₃₃Ta₆₇ alloyvia the TASSA process, ΔH _TASSA ^a . After 360 ks (100 h) of the MA time, all Al atoms emigrate to Ta lattices to form a solid solution phase and the powder particles have no more layered structure. At this stage of milling, the value of ΔH _TASSA ^a becomes zero. This solid solution phase is not stable against the shear forces that are generated by the rods and transforms completely to an amorphous phase upon milling for 720 ks (200 hours). This phase transformation is attributed to the accumulation of several lattice imperfections, such as point and lattice defects, which raise the free energy from the more stable phase (solid solution) to a less stable phase (amorphous). After 1440 ks (400 hours) of MA time, a homogeneous amorphous phase is formed. The amorphization process in this case is attributed to a mechanical driven solid-state amorphization (MDSSA). The heat of formation of the amorphous phase formedvia the MDSSA process, ΔH _MDSSA ^a , has been calculated. Moreover, the crystallization characteristics indexed by the crystallization temperature, and the enthalphy of crystallization, of the amorphous phases formed by TASSA and MDSSA processes are investigated as a function of MA time. The role of amorphizationvia each process has been discussed. Formerly lecturer of Materials Science, Department of Mining and Petroleum Engineering, Faculty of Engineering, AI-Azhar University, Nasr City 11884, Cairo, Egypt     

The kinetics and mechanism of the pyrite-to-pyrrhotite transformation

The kinetics of the transformation of pyrite to pyrrhotite have been investigated. The study was performed using thermogravimetric analysis over the temperature range of 620 to 973 K in atmospheres of H₂, He, Ar, and in vacuo over a wide range of pressures: 0.20 Pa to 4.24 MPa. Based on the kinetic results, a mechanistic picture of the various steps exerting control over the transformation is proposed. The thermal decomposition proceeds via a two-step, consecutive process. The rate-controlling step is the desorption of sulfur vapor from the surface. The presence of H₂ introduces different rate-controlling steps into the sequence, providing the H₂ exists at a pressure sufficiently high to suppress the rate of thermal decomposition. Rates at which the H₂ reduction occurs with pyrite samples from different sources depends upon the samples’ impurity level and the extent to which various crystallographic faces are exposed.     

Neutron diffraction and phase evolution of the mechanically alloyed intermetallic compound ζ-FeZn13

High-energy ball milling with subsequent annealing is used to synthesize the intermetallic compound ζ-FeZn₁₃. The mechanically alloyed phase in the as-milled state is determined to be nonequilibrium, or metastable. Transmission electron microscopy (TEM) studies show a highly defective microstructure with undefined grain areas, and the alloy can be described as a mechanical mixture of elemental Fe and Zn, based on neutron diffraction measurements. Characteristic stages associated with its transformation to the equilibrium state are identified based on differential scanning calorimetry (DSC) measurements. The activation energies corresponding to these stages are 128, 202, and 737 kJ/mole, respectively, with increasing transformation temperatures. The first stage is related to limited atomic diffusion or rearrangements, such as recovery, during thermal treatment, while the second stage depicts continued recrystallization and long-range atomic diffusion leading to a stable phase formation. The third and final stage marks structural decomposition of the equilibrium structure due to phase transition. Neutron diffraction of the equilibrium alloy confirmed that the structure is C2/m, with lattice parameters of a=13.40995 Å, b=7.60586 Å, c=5.07629 Å, and β=127 deg 18 minutes. The atomic positions of Fe and Zn compared well to reported values.     

Fabrication, characterization, and electrical conductivity properties of Pr6O11 nanoparticles

Pr6O11 nanoparticles were obtained by subsequent thermal decomposition of the as-prepared precipitate formed under ambient temperature and pressure using NaOH as precipitant.The calcination process was affected,for 1 h in static air atmosphere,at 400-700 °C temperature range.The different samples were characterized using X-ray diffraction(XRD),transmission electron microscopy(TEM),field emission scanning electron microscopy(FE-SEM),thermogravimetric analysis(TGA),in situ electrical conductivity,and N 2 adsorption/desorption.The obtained results demonstrated that nano-crystalline Pr6O11,with crystallites size of 6-12 nm,started to form at 500 °C.Such value increased to 20-33 nm for the sample calcined at 700 °C.The as-synthesized Pr6O11 nanoparticles presented high electrical conductivity due to electron hopping between Pr(III)-Pr(IV) pairs.     

Synthesis and structure of a praseodymium (III) complex with carboxylate ligand: A thermal and spectroscopic study

One Pr(III) lanthanide ion complex was initially synthesized and characterized by TGA-DSC in air atmosphere, as well as characterized by CHN elemental analysis, defining the stoichiometric ratio as Pr(DMBz)₃. The gaseous products evolved during the thermal decomposition were also monitored in N₂ atmosphere employing TGA/FT-IR system. A crystal structure is obtained by state-of-the-art powder X-rays diffraction methods measured in conventional laboratory equipment and refined by the Rietveld method, which defined it as a monoclinic system of the space group P2₁/c with a polymeric crystal structure, [Pr(DMBz)₃]_n. FT-IR theoretical spectrum and time-dependent density functional theory (TD-DFT) were calculated from TGA-DSC and crystalline system data. The experimental and theoretical FT-IR spectra present a high correlation degree when the main stretching bands are compared, while the energy transfer (HOMO – LUMO) in their neighborhoods suggests the main contributions of the light-emitting states.     

Intrinsic Properties and Structure of AB2 Laves Phase ZrW2

Using the first-principle calculations along with the quasi-harmonic Debye model, we explore the structural, thermodynamic, mechanical, and electronic properties of ZrW₂ intermetallic considering temperature or pressure effect. The computed equilibrium lattice parameter here is highly consistent with previous available results. The obtained formation enthalpy reveals that the ZrW₂ is structurally stable in the pressure range of 0 to 100 GPa. The pressure and temperature dependences of V/V ₀ ratio, constant volume specific heat capacity, thermal expansion coefficient, and Debye temperature of ZrW₂ have been obtained. The calculated minimum thermal conductivity k _min of ZrW₂ is fairly small and shows anisotropy, which implies that ZrW₂ has promising thermal-insulating application in engineering and may be competent for the thermal barrier materials. Moreover, from the results of elastic properties, we found the ZrW₂ is mechanically stable and exhibits elastic anisotropy and the extent of elastic anisotropy increases with pressure. Additionally, ZrW₂ shows ductile nature and its mechanical moduli all enhance as pressure increases, which is further confirmed by the findings from the electronic properties.