Volume 59, Nº 1 (2018)

A structural formula and quantum-chemical characteristics of the most energetically probable stable conformation of a bioreagent molecule, which is formed upon oxidizing iron(II) ions by Acidithiobacillus ferrooxidans autotrophic mesophilic iron-oxidizing bacteria in a sulfuric acid solution consisting of iron(III) ions and three acidic residues of glucuronic acid, are determined. The bioreagent oxidant is widely applied in industry for leaching metals from sulfide ores of nonferrous metals and concentrates of concentration. Quantum- chemical characteristics of the bioreagent molecule are analyzed in comparison with anhydrous iron(III) sulfate, which is also used in hydrometallurgy as an oxidant. To investigate the structure and quantum- chemical characteristics, the molecular computer simulation method, the theory of boundary molecular orbitals, and the Pearson principle are used. It is established that the most energetically probable stable conformation of the bioreagent molecule contains acidic residue of glucuronic acid with a noncyclic structure. According to the results of investigations, the bioreagent is referred to more rigid Lewis acid (the electron acceptor) than Fe₂(SO₄)₃. The bioreagent molecule is less polarized and has lower absolute electronegativity and a twofold larger volume. The theoretical substantiation of the larger persistence of primary sulfides (pyrite, pentlandite, and chalcopyrite) relative to secondary minerals (pyrrhotine, chalcosine, and covellite) is proposed based on calculated values of boundary molecular orbitals; absolute rigidity; and the electronegativity of iron, copper, and nickel sulfides. Characteristics determining the interaction efficiency (volume, heat of formation, steric energy and its components, total energy, etc.) of the bioreagent are multiply larger than for Fe₂(SO₄)₃. The larger oxidative activity of the bioreagent relative to Fe₂(SO₄)₃ can be substantiated by a higher partial charge of the iron atom and a longer bond length between the atoms, the lower energy of the lowest free molecular orbital, and increased degree of the charge transfer during the bioreagent interaction with sulfide minerals.

Polymetallic ores always contain precious rare metals which have high economic value. There is a large-scale copper (Cu)–zinc (Zn)–stannum (Sn) polymetallic ore deposit in Dulong, Yunnan Province, China. The polymetallic ore deposit contains a lot of silver (Ag) and indium (In). In this paper, the polymetallic ore is investigated to explore the characteristics of its process mineralogy by X-ray diffraction (XRD), X-ray fluorescence (XRF), optical microscopy (OM), and electron probe micro-analyser (EMPA). The contents of the main valuable elements in the ore are Cu 0.20%, Zn 3.93%, Sn 0.47%, Fe 22.70% and S 9.90%. Cu, Zn, Sn, and Fe mainly occur in chalcopyrite, marmatite, cassiterite and magnetite, respectively. In addition, as rare metal elements, there are also Ag 4.9 g/t and In 90.50 g/t in the ore. Ag mainly occurs in matildite and an unknown mineral (Ag_0.75(Zn, Fe)_0.25S) and these two minerals are all enclosed and disseminated in marmatite. In does not form an independent mineral, but an isomorphism element which mainly occurs in marmatite. Based on the results of the process mineralogy and actual conditions, a feasible flowsheet for this polymetallic ore is designed and optimized. The industrial experimental results show that processing capacity of the plant and the indexes of the concentrates are improved.

Samples of ML19 magnesium alloy with composition, wt %, (0.1–0.6)Zn–(0.4–1.0)Zr–(1.6–2.3)Nd–(1.4–2.2)Y have been investigated. The influence of Nd, Y, Zn, and Zr on equilibrium phase-transition temperatures and phase composition using Thermo-Calc software is established. The Scheil–Gulliver solidification model is also used. We show the significant liquidus temperature increase if the zirconium content in alloy is higher than (0.8–0.9) wt %. Thus, a higher melting temperature is required (more than 800°C). This is undesirable when melting in a steel crucible. The change in equilibrium fractions of phases at different temperatures in ML19 magnesium alloy with a minimum and maximum amount of alloying elements are calculated. Microstructures of alloys with different amounts of alloying elements in as-cast and heat-treated condition has been studied using scanning electron microscopy (SEM). We investigate the concentration profile of Nd, Y, Zn, and Zr in the dendritic cell of an as-cast alloy. The amount of neodymium and zinc on dendritic cell boundaries increased. A high concentration of yttrium is observed both in the center and on the boundaries of the dendritic cell. A high zirconium concentration is mainly observed in the center of the dendritic cells. A small amount of yttrium is also present in zirconium particles. These particles act as nucleation sites for the magnesium solid solution (Mg) during solidification. The effect of aging temperature (200 and 250°C) on the hardness of the samples after quenching was studied. Aging at 200°C provides a higher hardness. The change in the hardness of quenched samples during aging at 200°C is investigated. Maximum hardness is observed in samples aged for 16–20 h. The two-stage solution heat treatment for 2 h at 400°C and 8 h at 500°C with water quenching and aging at 200°C for 16 h is performed. This heat treatment enables us to get tensile strength 306 ± 8 MPa and yield strength 161 ± 1 MPa with elongation 8.7 ± 1.6%.

The present work aims to report and discuss the development of a novel grain refiner (Al–Y–B master alloys) focusing on the characterization of the phenomena that exist during their production. Al–Y–B master alloy is produced by the combined employment of yttrium and boron, instead of yttrium or boron individually. It is discovered as a highly effective grain refiner for inoculating the grain size of Al–Si alloys. The crystallized microstructure can be refined though the effect of Y-based intermetallic on heterogeneity nucleus. The Y-based intermetallic is formed in the melts (Al–Y–B master alloy) by the addition of yttrium and KBF₄ powers. A approach to produce Al–Y–B master alloys as well as its characterization by means of optical micrographs and SEM is presented. The study is assessed by testing the grain refining potency of the produced Al–Y–B master alloys in binary Al–20Si alloy. It is revealed that the approach employed to produce the Al–Y–B master alloys is suitable because the size of the primary phases is significantly reduced in each of the case investigated.

Results of investigations into the formation of the crystallographic orientation of the structure and anisotropy of properties during rolling sheets of the aluminum–lithium 1420 alloy of the Al–Mg–Li system are given. Hot-rolled billets of the 1420 alloy were cold-rolled with intermediate quenching according to the following schedule: 7.3 mm → 4.8 mm → 3.0 mm → 1.8 mm. The samples were selected after each passage to perform mechanical testing and analyze the structure using optical microscopy and diffractometry. A deformed fibrous structure and considerable anisotropy of mechanical properties is characteristic of sheets of all considered states. Herewith, the maximal plasticity is observed at an angle of 45° to the rolling direction. The character of anisotropy of properties formed at the hot-rolling stage is not varied during cold rolling. Sheets of the 1420 alloy have a sharp deformation texture at all rolling stages due to the conservation of the unrecrystallized structure. For example, when analyzing pole figures and preferential orientations, an increase in volume fractions of rolling texture is revealed (the slow one of the brass type and more rapid of the S type) with the rise of summary deformations of cold rolling. The recrystallization texture (of the R type) is present in small amounts only after hot rolling. The volume fraction of the texture-free component decreases with an increase in summary deformations. It is concluded based on these results that, in order to decrease the fraction of the deformation texture and lower anisotropy of properties in sheets of the 1420 alloy, it is first and foremost necessary to provide the running of recrystallization at the hot-rolling stage in order to fabricate the recrystallized hot-rolled billet for subsequent cold rolling.

Phase transformations in the Al–Ca–Mg–Si system in the region of aluminum–magnesium alloys are investigated using the Thermo-Calc program. The liquidus projection of the quaternary system is constructed with a Mg content of 10% and it is shown that phases Al4Ca, Mg₂Si, and Al₂CaSi₂ can crystallize (in addition to the aluminum solid solution (Al)) depending on the calcium and silicon concentrations. The crystallization character of quaternary alloys is investigated with the help of a polythermal cross section calculated at concentrations of 10% Mg and 84% Al. Based on the analysis of phase transformations occurring in alloys of this section, the presence of the Al–Al₂CaSi₂–Mg₂Si quasi-ternary section in the Al–Ca–Mg–Si system was assumed. Three experimental alloys were considered from a quantitative analysis of the phase composition, notably, Al–10% Ca–10% Mg–2% Si, Al–4% Ca–10% Mg–2% Si, and Al–3% Ca–10% Mg–1% Si. Metallographic investigations and electron-probe microanalysis were performed using a TESCAN Vega 3 scanning electron microscope. Critical temperatures are determined using a DSC Setaram Setsys Evolution differential calorimeter. The experimental results agree well with the calculated data; in particular, a peak at t ~ 450°C is revealed for all alloys in curves of the nonequilibrium solidus and invariant eutectic reaction L → (Al) + Al₄Ca + Mg₂Si + Al₃Mg₂. It is established that the structure of the Al–3% Ca–10% Mg–1% Si alloy is closest to the eutectic alloy. It is no worse that the AMg10 alloy in regards to density and corrosion resistance and even surpasses it in hardness, which allows us to consider this alloy as the basis for the development of a new cast material: “natural composites.”

This study is devoted to the fabrication of the ZrB₂–SiC–(MoSi₂) compact ceramics according to hybrid technology (self-propagating high-temperature synthesis (SHS) + hot pressing), as well as to investigating its phase composition, structure, and high-temperature oxidation kinetics. Reaction mixtures are prepared according to the following scheme: mechanical activation (MA) of Si + C powders; wet admixing of Zr, B, and Si + C MA-mixture powders; and drying mixtures in a drying oven. The ZrB₂–SiC SHS composite powder is formed in a reactor in a combustion mode by elemental synthesis. Compact samples with a homogeneous structure and low residual porosity not exceeding 1.3% are formed by hot pressing the SHS powder. Two compositions are selected for testing, notably, the first one calculated for the formation of ZrB₂ + 25% SiC; the second composition is similar to the first one, but with the addition of 5% of the MoSi₂ commercial powder. The microstructure of the samples is presented by dispersed dark gray rounded SiC grains distributed among light faceted ZrB₂ grains. The sample with the MoSi₂ additive has a more finely dispersed structure. The high-temperature oxidation of the samples at 1200°C results in the formation of SiO₂‒ZrB₂–(B₂O₃) complex oxide films on their surface with a thickness on the order of 20–30 μm, which serve as an efficient diffusion barrier and lower the oxidation rate. Their structure also contains ZrSiO₄ complex oxide after prolonged holding (longer than 10 h). In addition, an insignificant weight loss of the samples is observed after 10 h testing, which is caused by the volatilization of gaseous oxidation products (B₂O₂, CO/CO₂, MoO₃). The sample with the MoSi₂ additive shows better resistance to oxidation.

In this article, Welding of AA2219 aluminium alloy using Gas tungsten arc welding process (GTAW) and evaluation of metallurgical, mechanical and corrosion properties of the joints are discussed. The weld samples were subjected to ageing process at the temperature range of 195°C for a period of 5 h to improve the properties. AA2219 aluminium plates of thickness of 25 mm were welded using gas tungsten arc welding (GTAW) process in double V butt joint configuration. The input parameters considered in this work are welding current, voltage and welding speed. Tensile strength and hardness were measured as performance characteristics. The variation in the properties were justified with the help of microstructures. The same procedures were repeated for post weld heat treated samples and a comparison was made between as weld condition and age treated conditions. The post weld heat samples had better tensile strength and hardness values on comparing with the as weld samples. Fracture surface obtained from the tensile tested specimen revealed ductile mode of failure.

The diverse abilities such as the antioxidant effect of cerium oxide nanoparticles (CeO₂-NPs) have encouraged researchers to pursue CeO₂-NPs as a therapeutic agent to treat a number of diseases, including cancer and diabetes. The synthesis method of CeO₂-NPs affected on its abilities. In this study, nanosize ceria powders were synthesized by combustion of aqueous containing corresponding cerium nitrate, ammonium nitrate, and glycine redox mixtures. Solution combustion synthesis is a fast and cost-efficient process with high purity product. The crystallite structures were characterized by various methods, including X-ray diffraction technique, high-resolution scanning electron microscopy, transmission electron microscopy, and UV–vis spectroscopy technique. The combustion was flaming and yields voluminous oxides with nano size (20–30 nm). In addition, no diffraction patterns that are characteristic of impurities were observed, indicating the purity of the CeO₂-NPs. In vitro cytotoxicity studies on L929 cells, a non-toxic effect in all concentration (up to 1000 μg/mL) was indicated and it can be believed that this nanoparticle will have viable applications in different medical fields.

In-situ Al matrix composite was synthesized from Al–TiO₂–C powder mixtures using mechanical alloying and heat treatment, subsequently. The effect of ball milling on reaction processes of the resulting nanocomposite was investigated. The evaluation of powder mixture without mechanical activation showed that at 900°C aluminum reduced TiO₂, forming Al₃Ti and Al₂O₃. After 20 h mechanical activation of powder mixture, Al₃Ti and Al₂O₃ were fabricated. After that, by increasing milling time up to 30 h, no new phases formed. The DTA analysis of 30 h milled powder indicated two peaks after aluminum melting at 730 and 900°C. The XRD results confirmed that at 730°C, molten Al reacted with TiO₂ and C, forming Al₃Ti, Al₂O₃ and Al₄C₃. After that, at 900°C, Al₃Ti reacted with Al₄C₃, causing TiC formation. This results proposed that the TiC formation is associated by a series of reactions between intermediate products, Al₃Ti and Al₄C₃ and the resultant nanocomposite was successfully synthesized after 30 h milling and heated by DTA analysis up to 1200°C.