Showing papers in "International Journal of Minerals Metallurgy and Materials in 2019"

Anisotropy in mechanical properties and corrosion resistance of 316L stainless steel fabricated by selective laser melting

Abstract: The corrosion behavior and mechanical properties of 316L stainless steel (SS) fabricated via selective laser melting (SLM) were clarified by potentiodynamic polarization measurements, immersion tests, and tensile experiments. The microstructural anisotropy of SLMed 316L SS was also investigated by electron back-scattered diffraction and transmission electron microscopy. The grain sizes of the SLMed 316L SS in the XOZ plane were smaller than those of the SLMed 316L SS in the XOY plane, and a greater number of low-angle boundaries were present in the XOY plane, resulting in lower elongation for the XOY plane than for the XOZ plane. The SLMed 316L was expected to exhibit higher strength but lower ductility than the wrought 316L, which was attributed to the high density of dislocations. The pitting potentials of the SLMed 316L samples were universally higher than those of the wrought sample in chloride solutions because of the annihilation of MnS or (Ca,Al)-oxides during the rapid solidification. However, the molten pool boundaries preferentially dissolved in aggressive solutions and the damage of the SLMed 316L in FeCl3 solution was more serious after long-term service, indicating poor durability.

An analytical solution for evaluating the safety of an exposed face in a paste backfill stope incorporating the arching phenomenon

Abstract: In current underground mining, the stability of the exposed backfill face is a basic issue associated with mining design and has been the subject of considerable research in mining safety and efficiency. In this study, an improved analytical solution for evaluating the safety of vertically exposed faces in backfilling was proposed. Based on a differential slice method, the proposed solution emphasizes the arching effect as having the advantages of more rigor and wider scalability. Feasibility of the proposed solution was validated with classic centrifuge results. Good agreement between compared results indicated that the proposed solution skillfully predicts the behavior of the paste centrifuge model. Additionally, calculation of exposed face safety in sequential filling was presented. The proposed solution has practical significance in mine backfill design.

Experimental study on dynamic mechanical property of cemented tailings backfill under SHPB impact loading

Abstract: Cemented tailings backfill (CTB) have increasingly been used in recent years to improve the stability of mining stopes in deep underground mines. Deep mining processes are often associated with rock bursting and high-speed dynamic loading conditions. Therefore, it is important to investigate the characteristics and dynamic mechanical behavior of CTB. This paper presents the results of dynamic tests on CTB specimens with different cement and solid contents using a split Hopkinson pressure bar (SHPB). The results showed that some CTB specimens exhibited one to two lower stress peaks after reaching dynamic peak stress before they completely failed. The greater the cement-to-tailings ratio is, the more obvious the strain reaction. This property mainly manifested as follows. First, the dynamic peak stress increased with the increase of the cement-to-tailings ratio when the impact velocity was fixed. Second, the dynamic peak stress had a quadratic relationship with the average stress rate. Third, the cement-to-tailings ratio could enhance the increase rate of dynamic peak stress with strain rate. In addition, the dynamic strength enhancement factor K increased with the increase of strain rate, and its value was larger than that of the rock samples. The failure modes of CTB specimens under low-speed impact were tensile failure and X conjugate shear failure, where were nearly the same as those under static uniaxial and triaxial compression. The CTB specimens were crushed and broken under critical strain, a failure mode similar to that of low-strength concrete. The results of the experimental research can improve the understanding of the dynamic mechanical properties of CTB and guide the strength design of deep mining backfills.

Cementitious property of NaAlO 2 -activated Ge slag as cement supplement

Abstract: Germanium (Ge), a waste residue leaching from zinc (Zn) smelting process, has potential cementitious properties and could be recycled as a cement supplement activated by chemical reagents. In this work, a test was conducted to determine the hydration properties of Ge slag-cement-based composites with Ge slag (GS)/ordinary Portland cement (PC) contents of 0wt%, 5wt%, 10wt%, 15wt%, 20wt%, and 25wt% and water-to-binder ratio (w/b) of 0.4. The activators Ca(OH)2, AlCl3, NaAlO2, and Na2CO3 were mixed under 1wt%, 2wt%, 3wt%, and 4wt% dosages of GS weight. The composition and microstructure of the hydration products were investigated by the combined approaches of X-ray diffraction (XRD), thermogravimetry–differential scanning calorimetry (TG-DSC), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS). First, the GS cementitious property is attributed to the high content of CaSO4•2H2O. Second, the activators affected the acceleration performance in the following order: NaAlO2, Na2CO3, AlCl3, and Ca(OH)2. More importantly, the 28-day unconfined compressive strength (UCS) is 45.34 MPa at the optimum formula of 0.6wt% NaAlO2, 15wt% GS, and 85wt% PC, which is 9.16% higher than the control. Thus, NaAlO2 is beneficial for the ettringite (AFt) generation, resulting in the C–S–H structure compaction. However, the Zn2+ residue inhibited the AFt formation, representing an important challenge to the strength growth with curing age. Consequently, the GS could be recycled as a supplement to the cement under the activator NaAlO2.

Review on nanocomposites fabricated by mechanical alloying

Abstract: Composites are composed of multiphase materials, where each phase has specific properties that differ from those of the other phases which can effect on the whole properties of composite. Nanocomposites are class of materials that contain at least one phase in the nanometric size range and can be produced by any suitable technique for preparing nanomaterials. Composites are an interesting class of materials that have recently been used in numerous applications, including structural, biomedical, electronics, and environmental applications. In composites, reinforcements might be fibers, particulates, or whiskers. Mechanical alloying (MA) is a promising technique for producing nanocomposite materials that are difficult or impossible to prepare via conventional techniques. In this review, we provide an overview of nanocomposites prepared by the MA process. The mechanism of milling and other milling parameters are overviewed, and insights into sintering categories and parameters are also presented.

Investigation of the medium calcium based non-burnt brick made by red mud and fly ash: durability and hydration characteristics

Abstract: Red mud is a type of highly alkaline waste residue produced in the process of alumina smelting by the Bayer process. Based on the idea of medium calcium content, solid wastes such as red mud and fly ash were used to prepare non-burnt bricks; and the mass ratio of CaO/SiO2 was selected in the range of 0.88–1.42. Mechanical properties and durability were investigated with a compressive strength test. X-ray diffractometry (XRD), scanning electron microscope (SEM), and Fourier transform infrared spectroscopy (FTIR) techniques were used to characterize the hydration characteristic. The environmental performance was analyzed by Inductively Coupled Plasma Mass Spectrometry (ICP). The results indicated that the mechanical properties and the durability were optimal when the mass ratio of CaO/SiO2 was 1.23. The hydration products were mostly C-S-H gel, ettringite, Na4Ca(Si10Al16)O32·12H2O and Ca3Al2(SiO4)(OH)8. They were responsible for the strength development, and the CaO/SiO2 mass ratio of 1.23 had the best polymerized structure. The results of an environmental performance test showed that the heavy metals in the raw materials were well-solidified in the brick. Therefore, this paper provides an effective solution for use of solid wastes in building material.

Effect of reductant type on the embedding direct reduction of beach titanomagnetite concentrate

Abstract: Iron and titanium were recovered from beach titanomagnetite (TTM) concentrate by embedding direct reduction and magnetic separation. The reduction products and the effects of the reductant type and reduction temperature on the reduction behavior were investigated. The results showed that the reduction of TTM concentrate was strongly related to the gasification reactivity of the reductant. Bitumite presented a better product index than wheat-straw biochar and coke, mainly because the gasification reactivity of bitumite was better than that of the other reductants. In addition, high temperatures were not beneficial to embedding direct reduction because of the emergence of a molten phase and iron-joined crystals, which in turn reduced the diffusion rate of the reducing gas and impeded the reduction reaction in the central area of the roasted briquette. The use of bitumite as the reductant at a C/Fe molar ratio of 1.4 and a reduction temperature of 1200°C for 120 min resulted in direct-reduction iron powder assaying 90.28wt% TFe and 0.91wt% TiO2 with an iron recovery of 91.83% and titanium concentrate assaying 46.01wt% TiO2 with a TiO2 recovery of 91.19%. Titanium existed mainly in the form of anosovite and ilmenite in the titanium concentrate.

Replacement of alkali silicate solution with silica fume in metakaolin-based geopolymers

Abstract: A metakaolin (Mk)-based geopolymer cement from Tunisian Mk mixed with different amounts of silica fume (SiO2/Al2O3 molar ratio varying between 3.61 and 4.09) and sodium hydroxide (10 M) and without any alkali silicate solution, is developed in this work. After the samples were cured at room temperature under air for 28 d, they were analyzed by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, environmental scanning electron microscopy, mercury intrusion porosimetry, 27Al and 29Si nuclear magnetic resonance (NMR) spectroscopy, and compression testing to establish the relationship between microstructure and compressive strength. The XRD, FTIR, and 27Al and 29Si NMR analyses showed that the use of silica fume instead of alkali silicate solutions was feasible for manufacturing geopolymer cement. The Mk-based geopolymer with a silica fume content of 6wt% (compared with those with 2% and 10%), corresponding to an SiO2/Al2O3 molar ratio of 3.84, resulted in the highest compressive strength, which was explained on the basis of its high compactness with the smallest porosity. Silica fume improved the compressive strength by filling interstitial voids of the microstructure because of its fine particle size. In addition, an increase in the SiO2/Al2O3 molar ratio, which is controlled by the addition of silica fume, to 4.09 led to a geopolymer with low compressive strength, accompanied by microstructures with high porosity. This high porosity, which is responsible for weaknesses in the specimen, is related to the amount of unreacted silica fume.

Investigating the mechanical and acoustic emission characteristics of brittle failure around a circular opening under uniaxial loading

Abstract: The size of underground openings in rock masses in metal mines is critical to the performance of the openings. In this study, the mechanical and acoustic emission (AE) characteristics of brittle rock-like specimens containing a circular opening with different ratios of opening diameter to sample size λ (λ = 0.1, 0.13, 0.17, 0.2, and 0.23) were investigated under uniaxial compression with AE monitoring. The results indicate that the opening size strongly affected the peak strength and the elastic modulus. Crack initiation first started from the upper surface of the specimens, not from the periphery of the openings. Tensile and shear cracks coexisted on the roof and floor of the specimens, whereas tensile cracks were dominant on the two sides. The fracture mode of samples with openings was partially affected by the relative size of the pillars and openings. The AE response curves (in terms of counts, cumulative energy, cumulative counts, and b-value) show that brittle failure was mainly a progressive process. Moreover, the AE information corresponded well with microcrack evolution in the samples and thus can be used to predict sample failure.

Reconstruction of broken Si–O–Si bonds in iron ore tailings (IOTs) in concrete

Abstract: This paper reports a study on the reconstruction of broken Si-O-Si bonds in iron ore tailings (IOTs) in concrete. Limestone and IOTs were used to investigate the influence of different types of coarse aggregates on the compressive strengths of concrete samples. The differences in interfacial transition zones (ITZs) between aggregate and paste were analyzed by scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS). Meanwhile, X-ray diffraction (XRD) and infrared spectroscopy (IR) were used to study microscopic changes in limestone and IOTs powders in a simple alkaline environment that simulated cement. The results show that the compressive strengths of IOTs concrete or paste are higher than those of limestone concrete or paste under identical conditions. The Ca/Si atom ratios in the ITZs of IOTs concrete samples are lower than those of limestone concrete; the diffraction peak of the calcium silicate phase at 2θ = 29.5°, as well as the bands of Si-O bonds shifting to lower wavenumbers, indicates reconstruction of the broken Si-O-Si bonds on the surfaces of IOTs with Ca(OH)2.

Azo dye degradation behavior of AlFeMnTiM (M = Cr, Co, Ni) high-entropy alloys

Abstract: Because of the potential carcinogenic effects and difficult degradation of azo dyes, their degradation has been a longstanding problem. The degradation of azo dye Direct Blue 6 (DB6) using ball-milled (BM) high-entropy alloy (HEA) powders was characterized in this work. Newly designed AlFeMnTiM (M = Cr, Co, Ni) HEAs synthesized by mechanical alloying (MA) showed excellent performance in the degradation of azo dye DB6. The degradation efficiency of AlFeMnTiCr is approximately 19 times greater than that of the widely used commercial Fe-Si-B amorphous alloy ribbons and more than 100 times greater than that of the widely used commercial zero-valent iron (ZVI) powders. The galvanic-cell effect and the unique crystal structure are responsible for the good degradation performance of the BM HEAs. This study indicates that BM HEAs are attractive, valuable, and promising environmental catalysts for wastewater contaminated by azo dyes.

Crystalline texture evolution, control of the tribocorrosion behavior, and significant enhancement of the abrasion properties of a Ni-P nanocomposite coating enhanced by zirconia nanoparticles

Abstract: This paper describes an investigation of the effect of ZrO2 nanoparticles on the abrasive properties, crystalline texture developments, and tribocorrosion behavior of Ni-P nanostructured coatings. In the investigation, Ni-P and Ni-P-ZrO2 nanostructured coatings are deposited on St52 steel via the electroless method. Transmission electron microscopy (TEM), field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), cyclic-static polarization lests in 3.5wt% NaCl solution, the tribocorrosion test (by back-and-forth wear in electrochemical cell), and the microhardness test using the Vickers method were performed to characterize and analyze the deposited coatings. The results of this study showed that the addition of ZrO2 nanoparticles to the Ni-P electro-less bath produced the following: a sharp increase in wear and hardness resistance, the change of the wear mechanism from sheet to adhesive mode, the reduction of pitting corrosion resistance, significant reduction in the tribocorrosion protective properties, change in the preferred orientation of the crystalline texture coating from (111) to (200), increase in the sedimentation rate during the deposit process, and a sharp increase in the thickness of the Ni-P nanostructured coatings.

Comparison between the surface defects caused by Al 2 O 3 and TiN inclusions in interstitial-free steel auto sheets

Abstract: Al2O3 and TiN inclusions in interstitial-free (IF) steel deteriorate the properties of the steel. To decrease the defects of cold-rolled sheet, it is important to clearly distinguish between the degrees of damage caused by these two inclusions on the surface quality of the steel. In this study, a nanoindenter was used to test the mechanical properties of the inclusions, and the distribution and size of the inclusions were obtained by scanning electron microscopy (SEM). It was found that when only mechanical properties are considered, TiN inclusions are more likely to cause defects than Al2O3 inclusions of the same size during the rolling process. However, Al2O3 inclusions are generally more inclined to cause defects in the rolling process than TiN inclusions because of their distribution characteristic in the thickness direction. The precipitation of Al2O3 and TiN was obtained through thermodynamical calculations. The growth laws of inclusions at different cooling rates were calculated by solidification and segregation models. The results show that the precipitation regularity is closely related to the distribution law of the inclusions in IF slabs along the thickness direction.

Reaction kinetics modeling for lithium and cobalt recovery from spent lithium-ion batteries using acetic acid

Abstract: Lithium and cobalt recovery from spent lithium-ion batteries (LIBs) is a major focus because of their increased production and usage. The conventional method for recycling spent LIBs using inorganic acids produces harmful byproducts. In this work, the leaching agent was substituted with a less expensive and more environmentally friendly alternative—acetic acid—and a mathematical model was developed to describe the kinetics of the recovery process. The variables used were the pH value, temperature, H2O2 concentration, and the solid-to-liquid (S/L) ratio. The mathematical model used was the shrinking core model, which was modified to accommodate an equilibrium reaction. The experimental results show that the rate of recovery of Li and Co over time was only affected by temperature. The leaching behaviors of Li and Co were found to oppose each other. An increase in temperature resulted in increased recovery of Li but decreased recovery of Co because of the product-favoring endothermic reaction of Li and the reactant-favoring exothermic reaction of Co. The product of Li has a lower entropy value than the reactant as a free-moving ion, whereas the product of Co leaching has a higher entropy value as a stiff crystal complex. Thus, temperature conditioning is a pivotal factor in the leaching of spent LIBs.

Feasibility of aluminum recovery and MgAl 2 O 4 spinel synthesis from secondary aluminum dross

Abstract: The feasibility of aluminum recovery from secondary aluminum dross by extraction with NaOH solution and the subsequent synthesis of MgAl2O4 spinel by sintering the extracted slag were studied. The extraction percentage of soluble aluminum from the dross reached 80% at a temperature of 353 K, liquid-to-solid ratio of 12 mL·g−1, stirring speed of 300 r·min−1, and an extraction time of 15 min; the hydrolysis percentage of AlN reached 40% with an extraction time of 30 min. The activation energies of the soluble aluminum and AlN extracted from the dross were 7.15 and 8.98 kJ·mol−1, respectively, indicating that their kinetics were controlled by outer diffusion without a product layer. The extracted slag was sintered in the temperature range 1373–1773 K; MgAl2O4 spinel with a compressive strength as high as 69.4 MPa was produced in the sample sintered at 1673 K for 3 h. This value exceeds the threshold (40 MPa) prescribed by the National Standard for the Magnesia and Magnesia−alumina Refractory Bricks of China (GB/T 2275–2007). These results establish the effectiveness of aluminum recovery from secondary aluminum dross and subsequent MgAl2O4 spinel synthesis.

Recovery of vanadium and molybdenum from spent petrochemical catalyst by microwave-assisted leaching

Abstract: The study of the leaching of vanadium (V) and molybdenum (Mo) from spent petrochemical catalysts in sodium hydroxide (NaOH) medium was performed using two approaches, namely, conventional leaching and microwave-assisted leaching methods. The influence of microwave power, leaching time, leaching temperature, and NaOH concentration on the leaching efficiency of spent petrochemical catalyst was investigated. Under microwave-assisted conditions (600 W, 10 min, 90°C, 2.0 mol·L−1 NaOH, and 0.20 g·mL−1 solid-liquid ratio), the leaching efficiencies of V and Mo reached 94.35% and 96.23%, respectively. It has been confirmed that microwave energy has considerable potential to enhance the efficiency of the leaching process and reduce the leaching time. It is suggested that the enhancement of the leaching efficiencies of V and Mo can be attributed to the existence of a thermal gradient between solid and liquid and the generation of cracks on the mineral surface.

Research progress in enhanced bioleaching of copper sulfides under the intervention of microbial communities

Abstract: Compared with the traditional pyrometallurgical process, copper bioleaching has distinctive advantages of high efficiency and lower cost, enabling efficiently extracts of valuable metal resources from copper sulfides. Moreover, during long-term industrial applications of bioleaching, many regulatory enhancements and technological methods are used to accelerate the interfacial reactions. With advances in microbial genetic and sequencing technologies, bacterial communities and their mechanisms in bioleaching systems have been revealed gradually. The bacterial proliferation and dissolution of sulfide ores by a bacterial community depends on the pH, temperature, oxygen, reaction product regulation, additives, and passivation substances, among other factors. The internal relationship among the influencing factors and the succession of microorganism diversity are discussed and reviewed in this paper. This paper is intended to provide a good reference for studies related to enhanced bioleaching.

Effect of Ce on the cleanliness, microstructure and mechanical properties of high strength low alloy steel Q690E in industrial production process

Abstract: In order to improve the strength and toughness of Q690E steel sheets, the effect of rare earth element Ce on the strength and toughness of Q690E steel was studied by means of transmission electron microscopy, scanning electron microscopy, and metallographic microscope. The results showed that the addition of Ce in steel limited the combination of S with Mn and Ca, transformed Al2O3 inclusion into spherical CeAlO3 inclusion, and modified the precipitate form of some composite inclusions of TiN and sulfide oxides into TiN precipitation alone. The inclusions were spheroidizing. The size of inclusions was decreased from 3–5 μm to 1–2 μm, and the distribution was dispersed. Ce played a role in purifying molten steel through desulphurization and deoxidization. Meanwhile, the addition of Ce in steel effectively increased the nucleation particles in the liquid phase, improved the nucleation rate, enlarged the equiaxed grain refinement area, and limited the development of columnar crystals. The average grain size of slab decreased from 45.76 to 35.25 μm, and the proportion of large grain size (> 50 μm) decreased from 40.41% to 23.74%. The macrostructural examination of slab was improved from B0.5 to C2.0, which realized the refinement of the solidified structure and reduced the banded structure of hot rolled plate. In addition, due to the inheritance of refined structure in the upstream, the recrystallization of deformed austenite and the growth of grain after recrystallization were restrained, and a refined tempered sorbite structure was obtained. When rare earth element Ce was added, the width of the martensite lath bundle was narrowed from about 500 nm to about 200 nm, which realized a remarkable grain refinement strengthening and toughening effect. Mechanical properties such as tensile, yield, and low-temperature impact toughness were significantly improved.

Effect of intermetallic compounds on the fracture behavior of dissimilar friction stir welding joints of Mg and Al alloys

Abstract: Joining Mg to Al is challenging because of the deterioration of mechanical properties caused by the formation of intermetallic compounds (IMCs) at the Mg/Al interface. This study aims to improve the mechanical properties of welded samples by preventing the fracture location at the Mg/Al interface. Friction stir welding was performed to join Mg to Al at different rotational and travel speeds. The micro structure of the welded samples showed the IMCs layers containing Al12Mg17 (γ) and Al3Mg2 (β) at the welding zone with a thickness (> 3.5

Research progress in lead-less or lead-free three-dimensional perovskite absorber materials for solar cells

Abstract: The trend toward lead-free or lead-less perovskite solar cells (PSCs) has attracted increasing attention over the past few years because the toxicity of lead (Pb) is one of the substantial restrictions for large-scale applications. Researchers have investigated the viability of substituting Pb with other elements (group 14 elements, group 2 elements, transition-metal elements, and group 13 and 15 elements) in the three-dimensional (3D) perovskites by theoretical calculations and experimental explorations. In this paper, recent research progress in Pb-less and Pb-free PSCs on the perovskite compositions, deposition methods, and device structures are summarized and the main problems that hinder the enhancement of device efficiency and stability are discussed in detail. To date, the fully Sn-based PSCs have shown a power conversion efficiency (PCE) of 8.12% and poor device stability. However, lead-less PSCs have shown higher PCE and a better stability. In addition, the introduction of double-perovskite materials also draws researchers’ attention. We believe that the engineering of elemental composition, perovskite deposition methods, and interfacial modification are critical for the future development of Pb-less and Pb-free PSCs.

Microstructure, mechanical, and corrosion properties of extruded low-alloyed Mg- xZn-0.2Ca alloys

Abstract: The microstructure, mechanical, and corrosion properties of extruded low-alloyed Mg-xZn-0.2Ca (x = 0,1.0,2.0, 3.0) alloys were investigated in this study. Findings from scanning electron microscope, X-ray diffraction and transmission electron microscopy results indicate that the amount of ternary Ca2Mg6Zn3 phase, as the only secondary phase in 1.0Zn, 2.0Zn, and 3.0Zn alloys, gradually increases with the addition of Zn, while the Mg2Ca phase was observed in the Mg-0.2Ca alloy only. Zn has a strong effect on the orientation and intensity of textures, which also influence mechanical behaviors, as revealed by electron back-scatter diffraction. Among all the alloys, the Mg-2.0Zn-0.2Ca alloy obtains the maximum tensile strength (278 MPa) and yield strength (230 MPa). Moreover, Zn addition has an evident influence on the corrosion properties of Mg-xZn-0.2Ca alloy, and Mg-l.0Zn-0.2Ca alloy exhibits the minimum corrosion rate. This paper provides a novel low-alloyed magnesium alloy as a potential biodegradable material.

Effects of small additions of Zn on the microstructure, mechanical properties and corrosion resistance of WE43B Mg alloys

Abstract: Zn is a commonly used alloying element for Mg alloys owing to its beneficial effects on mechanical properties. To improve the mechanical and corrosion properties of WE43B Mg alloys, the effects of 0–0.7wt% Zn addition on the microstructure and properties of sample alloys were investigated. Addition of Zn to as-cast WE43B alloy promoted the formation of the Mg12Nd phase; by contrast, after T6 heat treatment, the phase composition of WE43B alloys with and without Zn addition remained mostly the same. A long-period stacking ordered phase was predicted by CALPHAD calculation, but this phase was not observed in either the as-cast or heat-treated Zn-containing WE43B alloys. The optimum temperature and duration of T6 heat treatment were obtained using CALPHAD calculations and hardness measurements. Addition of Zn resulted in a slight reduction in the average grain size of the as-cast and T6 heat-treated WE43B alloys and endowed them with increased corrosion resistance with little effect on their mechanical properties.

Combustion characteristics of unburned pulverized coal and its reaction kinetics with CO 2

Abstract: The combustion characteristics of two kinds of unburned pulverized coal (UPC) made from bituminous coal and anthracite were investigated by thermogravimetric analysis under air. The reaction kinetics mechanisms between UPC and CO2 in an isofhermal experiment in the temperature range 1000–1100°C were investigated. The combustion performance of unburned pulverized coal made from bituminous coal (BUPC) was better than that of unburned pulverized coal made from anthracite (AUPC). The combustion characteristic indexes (S) of BUPC and AUPC are 0.47 × 10−6 and 0.34 × 10−6%2·min−2·°C−3, respectively, and the combustion reaction apparent activation energies are 91.94 and 102.63 kJ·mol−1, respectively. The reaction mechanism of BUPC with CO2 is random nucleation and growth, and the apparent activation energy is 96.24 kJ·mol−1. By contrast, the reaction mechanism of AUPC with CO2 follows the shrinkage spherical function model and the apparent activation energy is 133.55 kJ·mol−1.

Gasification kinetics of bulk coke in the CO 2 /CO/H 2 /H 2 O/N 2 system simulating the atmosphere in the industrial blast furnace

Abstract: The gasification characteristics and gasification kinetics of coke in complex CO2/CO/H2/H2O/N2 systems similar to the gas system of industrial blast furnace (BF) were studied by the method of isothermal thermogravimetric analysis. The experimental gas compositions and the corresponding temperature were chosen according to data reported for industrial BFs. The gasification behavior of coke was described by the Random Pore Model (RPM), Volumetric Model (VM), and Grain Model (GM). Results showed that the gas composition of the coke gasification zone in BF changes slightly and that the temperature is the most important factor affecting coke gasification. The lower activation energy of coke samples (Coke Reaction Index (CRI) > 50) is due to the high Fe2O3 in the ash, lower degree of graphitization, and larger pore structure. In addition, the choice of kinetic model does not differ substantially in describing the gasification mechanism of coke in a BF.

Influence of rolling temperature on the interfaces and mechanical performance of graphene-reinforced aluminum-matrix composites

Abstract: To study the influence of rolling on the interfaces and mechanical performance of graphene-reinforced Al-matrix composites, a rolling method was used to process them. Using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy, and tensile testing, this study analyzed the micromorphology, interfaces, and mechanical performance of the composites before and after rolling. The experimental results demonstrates that the composites after hot rolling has uniform structures with strong interfacial bonding. With an increase in rolling temperature, the tensile strength and elastic modulus of the composites gradually increase. However, when the rolling temperature is higher than 500°C, granular and rod-like Al4C3 phases are observed at the interfaces and the mechanical performance of the composites is degraded. When the rolling temperature is 480°C, the composites show the optimal comprehensive mechanical performance, with a tensile strength and elastic modulus of 403.3 MPa and 77.6 GPa, respectively, which represent increases of 31.6% and 36.9%, respectively, compared with the corresponding values prior to rolling.

Hot-corrosion behavior of Cr 2 O 3 -CNT-coated ASTM-SA213-T22 steel in a molten salt environment at 700°C

Abstract: The present work investigates the hot-corrosion behavior of carbon nanotube (CNT)-reinforced chromium oxide coatings on boiler steel in a molten salt (Na2SO4-60wt%V2O5) environment at 700°C under cyclic conditions. The coatings were deposited via the high-velocity oxygen fuel process. The uncoated and coated steel samples were subjected to hot corrosion in a silicon tube furnace at 700°C for 50 cycles. The kinetics of the corrosion behavior was analyzed through mass-gain measurements after each cycle. The corrosion products were analyzed by X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray analysis techniques. The results revealed that uncoated steel suffered spallation of scale because of the formation of nonprotective Fe2O3 scale. The coated steel samples exhibited lower mass gains with better adhesiveness of oxide scale with the steel alloy until the end of exposure. The CNT-reinforced coatings were concluded to provide better corrosion resistance in the hot-corrosion environment because of the uniform dispersion of CNTs in the coating matrix and the formation of protective chromium oxides in the scale.

Process mineralogy as a key factor affecting the flotation kinetics of copper sulfide minerals

Abstract: The aim of this study is to apply process mineralogy as a practical tool for further understanding and predicting the flotation kinetics of the copper sulfide minerals. The minerals’ composition and association, grain distribution, and liberation within the ore samples were analyzed in the feed, concentrate, and the tailings of the flotation processes with two pulp densities of 25wt% and 30wt%. The major copper-bearing minerals identified by microscopic analysis of the concentrate samples included chalcopyrite (56.2wt%), chalcocite (29.1wt%), covellite (6.4wt%), and bornite (4.7wt%). Pyrite was the main sulfide gangue mineral (3.6wt%) in the concentrates. A 95% degree of liberation with d80 > 80 µm was obtained for chalcopyrite as the main copper mineral in the ore sample. The recovery rate and the grade in the concentrates were enhanced with increasing chalcopyrite particle size. Chalcopyrite particles with a d80 of approximately 100 µm were recovered at the early stages of the flotation process. The kinetic studies showed that the kinetic second-order rectangular distribution model perfectly fit the flotation test data. Characterization of the kinetic parameters indicated that the optimum granulation distribution range for achieving a maximum flotation rate for chalcopyrite particles was between the sizes 50 and 55 µm.

Austenite grain growth of medium-carbon alloy steel with aluminum additions during heating process

Abstract: In this study, the effects of heating temperature (850–1100°C) and holding time (30–150 min) on the grain growth behavior of austenite in medium-carbon alloy steel were investigated by conducting experiments. The abnormal grain growth and mixed grain structure phenomenon are explained using an equilibrium precipitation phase diagram calculated by Thermo-Calc software package. The AlN particles were observed by field-emission scanning electron microscopy (FESEM), and the amount of AlN precipitations was detected by electron probe microanalysis (EPMA). Based on the research results, it was found that the average grain size of austenite in the test steel increased continuously with the increase of temperature and holding time. Furthermore, the abnormal growth of austenite occurred in the test steel at 950°C, and the heating temperature affected the austenite grain size more significantly. In addition, the decline in the amount of AlN second-phase particle in the test steel, which weakened the “pinning” effect on austenite grain boundaries, resulted in abnormal growth and the development of mixed austenite grain structures. The prediction model for describing the austenite grain growth of medium-carbon alloy steel during heating was established by regression analysis of the experimental data, and the model was verified to be highly accurate.

Effect of ash on coal structure and combustibility

Abstract: Four bituminous coals and one anthracite were used in this study. On the basis of the similar volatile matter contents of the four bituminous coals, the effects of ash in coal on the microstructure, carbonaceous structure, and chemical composition of pulverized coal were studied. Thermogravimetric analysis was used to study the effect of the addition of anthracite on the combustibility of four different bituminous coals. The results showed that with the increase of ash content in pulverized coal, the microstructure of carbon particles in coal was not much different. However, the analysis results of Raman spectroscopy and X-ray diffraction pattern showed that as the ash content increased, the degree of graphitization of coal carbonaceous structure gradually decreased. The combustibility of the four bituminous coals were better than that of the anthracite. When bituminous coal and anthracite were mixed and burned, the combustibility of the mixed sample decreased as the ash content increased.

Experimental investigation of the cyclic degradation of the one-way shape memory effect of NiTi alloys

Abstract: Based on stress- and strain-controlled cyclic tension-unloading-heat-cooling tests, cyclic degradation of the one-way shape memory effect (OWSME) of NiTi shape memory alloys (SMAs) was investigated. It was seen, in thermo-mechanical coupled cyclic tests, that residual strain after each cycle accumulated, but the martensite reorientation stress and dissipation energy-per-cycle decreased as the number of cycles increased. Meanwhile, the cyclic degradation of OWSME was aggravated by increasing the stress/strain amplitude. In addition, the stress-strain response of NiTi SMAs was further investigated by performing simultaneous thermo-mechanical coupled cyclic tests with various phase-angle differences between the mechanical and thermal cyclic loadings. It can be concluded that such cyclic response depends significantly on prescribed phase-angle differences. Obtained experimental results are helpful for both the development of constitutive models and engineering applications of NiTi SMAs.