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  • Open access
  • 134 Reads
Leaching kinetics of tellurium, selenium and silver from copper anode slime by sulfuric acid leaching in the presence of manganese(IV) oxide and graphite

Leaching of constituent metals from copper anode slime/CAS (22.23% Se, 1.53% Te, 9.66% Ag) by sulfuric acid in the presence of manganese(IV) oxide (MnO2) and graphite has been investigated at a range conditions by varying stirring speed (200–600 rpm), H2SO4 concentration (0.5–3.0 M), MnO2/CAS and graphite/CAS mass ratio (0–1/1) and temperature (25–90 °C). The metal leaching was facilitated by the galvanic interaction with MnO2, while graphite play role as a catalyst. Under the most suitable conditions (500 rpm, 2.0 M H2SO4, 0.8/0.8/1 MnO2/graphite/CAS mass ratio and 90 °C temperature), the extent of leaching of Se, Te and Ag from the CAS, together with Mn from MnO2 after 6 h was 90.8%, 81.9%, 80.7% and 84.1%, respectively, as compared to 76.8% Te, 69.4% Se, 67.2% Ag and 55.9% Mn after leaching of CAS with MnO2 without graphite under the similar conditions, and against still lower leaching in the absence of MnO2 (32.6% Te and negligible Se and Ag). The leaching results (CAS+MnO2+graphite) of tellurium and silver at temperature 25–50 °C well fitted to the mixed and surface chemical reaction models, respectively, and changed to follow the diffusion and mixed control models in the temperature range 60–90 °C with the respective apparent activation energies of 17.8 and 12.2 kJ/mol. That of selenium was controlled by the surface chemical reaction with the estimated activation energy of 27.6 kJ/mol in the temperature range 25–90 °C. And manganese leaching followed the mixed-control model (activation energy of 56.7 kJ/mol) at the all tested temperature.

  • Open access
  • 62 Reads
Niobium oxide and tantalum oxide micro- and nanostructures grown using material recovered from mining tailings

Two main problems faced by the increasingly technological society are the huge amount of waste that humans generate and the scarcity or criticality of many of the materials used. In this sense, one of the EU's priorities is to promote the transition to a circular economy, where the materials and products manufactured with them are kept in the life cycle as long as possible. Among the strategic materials, Niobium, Tantalum and Rare Earth Elements (REE) are included in the 2020 year list of the 30 critical raw materials of European Union due to their importance for high-tech products and emerging innovations and the risk in the security of supply and economic importance.

In this work, the oxides of two of these strategic materials, Niobium and Tantalum, are recovered from the tailings of the Penouta Sn–Ta–Nb deposit (located in Galicia, Spain) via hydrometallurgical route [1,2]. The recovered oxides have been used to obtain micro- and nanostructures by a simple thermal evaporation method. The structures have been characterized by means of X-ray diffraction, scanning electron microscopy, luminescence and Raman spectroscopy. The possibility of using these micro- and nanostructures for optical, sensing and energy storage applications will be discussed and related to the crystal structure of the oxides obtained [3,4].

[1] F. A. López, et al. (2018) Minerals 8, 20.

[2] O. Rodríguez et al. (2020) RSC Adv., 10, 21406-21412

[3] C. Nico, et al. (2016) Progress in Materials Science 80, 1-37.

[4] S. Xia, et al. (2018) Nano Energy 45, 407-412.

  • Open access
  • 62 Reads
Defect reduction through modelling plastic deformation and metallurgical evolution in ferritic stainless steels.

Flat products made of EN 1.4512 ferritic steel may present some defects after the hot rolling process, such as irregular jagged edges. In order to identify the origin of this type of defect to help their reduction, a study has been carried out considering the hot rolling conditions of flat bars made of EN 1.4512 steel. Many semi-empirical models regarding the microstructural evolution during hot deformation of austenitic stainless steels have been developed by many authors. On the contrary, no many studies have been performed about ferritic stainless steels. The main novelty of this work is in the development of a recrystallization and grain growth model applied to EN 1.4512 ferritic steel grade. The microstructural evolution of the ferritic grain size and the damage of the material were calculated through the coupling of metallurgical and damage models. In the thermomechanical simulations of the roughing passes, three granulometry levels (PFGS) and three heating furnace temperatures were considered. The ferritic grain evolution metallurgical model was obtained introducing adequate equations. The results show that the defect can be produced by process conditions that trigger abnormal heating which induces an uncontrolled growth of the grain on the edges. The work-hardened grains undergo elongation during hot deformation but are not able to recrystallize. Consequently, these grains “squeezes” the surrounding recrystallized matrix towards the edges of the bar. Thus, on the edges occurs fractures that macroscopically manifest themselves as jagged edges.

  • Open access
  • 67 Reads
Microstructural and thermomechanical simulation of the additive manufacturing process in 316L austenitic stainless steel

Controlling the microstructural evolution generated under Additive Manufacturing (AM) conditions is a key aspect to achieving the target mechanical properties. In the present study, an integrated thermomechanical and microstructural simulation of ΑΜ, as applied to an AISI 316L austenitic stainless steel, is presented. A one-way coupled analysis is carried out with the heat transfer, microstructural and mechanical problems solved in sequence. A finite element technique is employed to evaluate the temperature evolution as well as residual stresses and distortions in the processed part, due to the successive material deposition. The material deposition is modelled using quiet elements which are activated as the added material solidifies. These elements are present from the start of the analysis but are assigned properties so they do not affect the analysis. The thermal history generated by two-dimensional heat transfer simulations which is essential in determining the resulting microstructure. The effect of processing parameters on critical microstructural features such as freezing range, phase fractions, and elemental segregation were investigated via CALPHAD-based computational thermodynamic and kinetic modelling, implemented in the Thermo-Calc software. Two limit cases of equilibrium and instantaneous solidification were studied through thermodynamic calculations and the Scheil-Gulliver model. Kinetic analysis followed, using the complete thermal cycle calculated by heat transfer simulations. Solidification and solid-phase transformations were investigated upon thermal cycling via multi-phase and multi-component diffusion simulations. A distinction between eutectic and peritectic solidification modes was made, as both have been observed in AM studies. Model comparison carried out, in agreement with experimental observations, indicated that the peritectic diffusion model resulted to the highest freezing range and the smallest ferrite fraction. The ensuing microstructural properties, including phase fractions and constitutions, as well as the temperature field are provided as an input for a mechanical analysis, to calculate the residual stresses and distortions.

  • Open access
  • 40 Reads
Bubble Behavior on Horizontal and Vertical Carbon Anode Surfaces in Cryolite Melt Applying a See-Through Cell

Gas bubble behavior on a carbon anode in a cryolite melt have been studied by direct observation using a see-through cell. The bubble phenomena studied have been growth, coalescence and detachment during electrolysis. The anode geometry and surface orientation affect bubbles behavior. Therefore, two different anodes were tested, an anode with a horizontal facing-downwards surface and an anode with a vertical surface. Galvanostatic and potentiostatic measurements were performed for different current densities and different potentials with simultaneous video recording. At the horizontal anode for a constant current density/potential it was found that one large bubble was formed by growth and coalescence of smaller bubbles and finally the large bubble detached periodically. The frequency of the bubble release events observed from the video recordings was in agreement with the dominant frequency from the Fast Fourier Transform (FFT) analysis. For the vertical anode surface smaller bubbles were formed and detached either due to being pushed by the formation of other bubbles or by coalescence obtaining enough buoyancy. FFT analysis gave no dominant frequency. The diameter of detached bubbles from the horizontal surface and vertical surface was measured. The value was in a range 5.7 mm to 7.2 mm for the horizontal surface and in a range 1.5 mm to 3.7 mm for the vertical surface, strongly depending on the applied current density. The bubble diameter was decreasing with increasing current density for both surfaces. The smaller bubble diameter might be explained by a larger bubble induced convection and increased wetting.

  • Open access
  • 41 Reads
PREDICTIVE TOOLS FOR IN-LINE ISOTHERMAL EXTRUSION OF 6XXX ALUMINUM ALLOYS

During the last fifty years, metal forming of aluminum alloys grew up significantly, leading to a more competitive market on which production rate and overall quality are kept higher as possible. Within the aluminum industries, extrusion plays an important role, since many industrial products with structural and even aesthetic functions are realized with this technology.

Especially in the automotive industry, the use of aluminum alloys is growing very fast, since it permits a considerable weight loss and thus a reduction of the emission. Nevertheless, the stringent quality standards required don’t allow the use of extruded aluminum alloys produced for the common building applications. An important parameter that can be used as an index of the quality of the extruded product is the emergent temperature: if the temperature at the exit of the press is kept constant within a certain limit, products with homogeneous properties and high-quality surface are obtained and the so called “isothermal extrusion” is achieved.

As extrusion industries are spread all over the world with different levels of automation and control, a universal but simple in-line tool for determining the best process condition to achieve isothermal extrusion, is of particular interest.

The aim of this work is to implement this model, that allows to evaluate the thermal gradient which have to be imposed on the billet. Several experiments have been carried out on an industrial extrusion press, and the outer temperature was recorded and compared with the simulated one to verify the model consistency.

  • Open access
  • 40 Reads
Optimization of process parameters on microstructure and mechanical properties of ADC 12 alloy Aptomat Contact fabricated by thixo-extrusion

The mechanical properties of thixoextrusion components can be improved by controllable processing parameters such as the solid fraction of alloy, holding time, punch velocity, heat treatment and die temperature. In this study, the effects of thixoforming parameters on the microstructures and mechanical properties of thixoextrusion ADC12 alloy Aptomat Contact are studied. ADC 12 has excellent castability with high fluidity and low shrinkage rate, so it is widely used in industry, especially in automotive and motorcycle engine part casting. It is a near eutectic alloy with high strength and low ductility (1%). The optimization parameters mechanical properties were investigated by changing the punch velocity, specimen temperature and holding time. The results also indicated optimal value at punch velocity (13mm/s), specimen temperature (570oC) and holding time (7 minutes) which was changed microstructure from eutectic dendrite to globular grain increasing the ductility (7%) of this alloy during the semi-solid forming process while remaining mechanical properties leads to an increase in the quality of finished parts.

  • Open access
  • 43 Reads
Feasibility study of ferromagnetic cores fabrication by additive manufacturing process

Currently, the commercial production of ferromagnetic cores involves staking thin sheets of soft magnetic material, alternating with dielectric material to reduce the eddy current losses. High silicon FeSi steels show excellent soft magnetic properties. Anyway, their workability decreases Si content increases thus imposing a technological limit in the production of thin sheets up to 3.5-4% Si. The additive manufacturing (AM) process based on laser powder bed fusion (L-PBF) offers the possibility to redesign the magnetic components, compared to conventional design, allowing to act on the chemical composition of magnetic materials and on the geometry of the components. In the case of FeSi alloys, the additive technology allows to overcome the limit of Si content opening new perspectives for the production of ferromagnetic cores with high magnetic performance.

In this work the feasibility study on the production of FeSi magnetic steel components by L-PBF technology is reported. Two variants of FeSi steels, with Si content of 3.0 wt% and 6.5 wt%, were considered. The effect of process parameters on the densification of manufactured parts was investigated. The best operating window has been identified for both steel chemical compositions, in terms of laser scan speed and power.

  • Open access
  • 23 Reads
Work-hardening behavior of cold rolled EUROFER97 steel for nuclear fusion applications

The reduced activation martensitic steel EUROFER97 is recognized in Europe as the reference steel for structural applications in future nuclear fusion reactors.

Usually, EUROFER97 steel plates are manufactured by hot rolling and successive heat treatments: (1) austenitization at 980°C for 30 minutes, (2) air cooling and (3) tempering at 760°C for 90 minutes. Recently, thermo-mechanical treatments have been investigated by us with the scope to improve the mechanical properties, namely to strengthen the steel without reducing its ductility. The experiments involve cold rolling with three reduction rates (30%, 40%, 50%) and, for each of them, heat treatments at different temperatures in the range from 550 °C to 750 °C. The mechanical and microstructural characterization of the samples after successive stages of the process is now underway and present work reports some preliminary results.

The characteristics of the samples after cold rolling have been examined by means of hardness tests, metallography and X-ray diffraction measurements and work-hardening is discussed in terms of grain refinement and dislocation density.

  • Open access
  • 36 Reads
Impact of strain aging kinetics on the failure of thin steel wire ropes

Under quasi-static loading an irregular failure pattern of high strength thin carbon steel cords were observed after low temperature thermal aging. Character and kinetics of damage in such wire ropes highly depend on the plastic elongation of the steel wires, which is significantly modified by the strain aging effect. Therefore, static strain aging effect on heavily drawn high-carbon steel wires and their cords was experimentally studied in the 100 -200°C temperature range.

The investigated cords had been designed with precisely optimized material as well as stranding parameters in order to possess the highest breaking load with the maximum possible elongation. These are equally important properties in their specific field of application as main reinforcement of high pressure oil&gas rubber hoses.

In case of cords of more complex structures serious loss of balance was found under heavy loading revealed by an adverse failure mode in the inelastic deformation regime. Finally, this instability caused significant drop in effective strain capacity as well as in ultimate strength. This unfavourable effect takes place as the thread experiences heat aging under 200°C for time periods typically less than 1 hour.

The phenomenon was investigated and explained in detail based on the different post-drawn strain aging behaviour of the employed thin pearlitic steel wire filaments. In order to gain reliable information about the material performance in aged condition comprehensive kinetic characterisation was given. Constants were determined for both the relevant Avrami and diffusion models and applicability of them was discussed.

These results directly make aging induced failure mode prediction possible and can also give a basis for structural optimization of wire ropes to avoid irregular breaking. The investigated aging temperature range covers the typical vulcanization condition of the mentioned rubber hoses, so the industrial importance of the present study is also highlighted.

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