Materials Science Forum Vols. 498-499

Abstract: The main objective of this work is to present the preliminary results on the analysis of the signals arising from the electrical arc, during the vacuum arc remelting of Zircaloy electrodes, aiming the automation of the fusion process. Zircaloy electrodes were made from compacted chips resultant of the machining of Zircaloy rods. The melts were performed in a prototype (vacuum arc remelting) VAR furnace under low pressure of argon and the arc was fed by a constant DC power source. Both filtered and unfiltered signals were recorded by means of a data acquisition system. The fast Fourier transforms FFT and autocorrelation integral were used as tools for data analysis. The result showed that the events occurring within the electric arc have a strong influence on the electric signals. The analysis allowed inferring that the VAR electric arc system has mainly a chaotic behaviour and sporadic periods of linear behaviour. The conclusion of this work is that a control system may be developed, based on the modelling of the non-linear behaviour of the arc, mainly chaotic. This may allow the achievement of an automatic control for the process and yield better quality products.

Abstract: Hydrocyclones are centrifugal devices employed on the solid-liquid and liquid-liquid separation. The operation and building of these devices are relatively simple, however the flow inside them is totally complex and its prediction is very difficult. The fluid moves on all possible directions (axial, radial and swirl), the effects of turbulence can not negligible and an air core along the center line of the hydrocyclone can appear when the operational conditions are favorable. For that reason, the most models that are used to predict the hydrocyclone performance are empirical and require the collection of the main operational and geometric variables in order to validate them. This work objectified to apply Computational Fluid Dynamics (CFD) on Bradley Hydrocyclone and compare the results from this technique to empirical models. The numerical simulation was made in a computational code called Fluent® that solves the transport equation by finite volume technique. The turbulence was described by Reynolds Stress Model (RSM) and the liquid-gas interface was treated by Volume of Fluid Model (VOF). In agreement with the results from the simulation, it was possible to predict the internal profiles of velocity, pressure, air core, particle trajectories, efficiencies, pressure drop and underflow-to-throughput ratio.

Abstract: Spouted bed systems have emerged as very efficient fluid-particle contactors and find many applications in the chemical and biochemical industry. Some important applications of spouted beds include coal combustion, biochemical reactions, drying of solids, drying of solutions and suspensions, granulation, blending, grinding, and particle coating. An extensive overview can be found in Mathur and Epstein[1]. The pattern of solid and gas flows in a spouted bed was numerically simulated using a CFD modeling technique. The Eulerian-Eulerian multifluid modeling approach was applied to predict gas-solid flow behavior. A commercially available, control-volume-based code FLUENT 6.1 was chosen to carry out the computer simulations. In order to reduce computational times and required system resources, the 2D axisymmetric segregated solver was chosen. The typical flow pattern of the spouted bed was obtained in the present calculation. The simulated velocity and voidage profiles presented a good agreement qualitative and quantitative with the experimental results obtained by He et al. [4].

Abstract: Performing of the flotation process in columns started in the year 1981. Since then, this new flotation technique became usual in many countries, including Brazil, where nowadays flotation columns are installed in plants for iron and phosphate ore concentration. Sees the need of a rational use of the phosphate of Barreiro, Araxá (MG), whose medium useful life is esteemed in 30 years, Federal University of Uberlândia in partnership with Bunge Fertilizers looks for in this work to develop conditions for use of the rejects of the flotation process. A laboratory flotation column was operated in discontinuous mode in order to examine the mixture of two different types of collectors, ore conditioning time and reagents dosages. Contents between 22 and 29 % P2O5 were obtained and Apatite recovery varied in the range of 18 to 58 wt %. The results shows that the use of mixture of the synthetic reagent KE and the rice oil soap provided an increase considerable in the selectivity of the concentrate.

Abstract: Mathematical modelling has been developed to improve processes control and has helped theories about complex systems to be written. Nitriding is a multivariable system that needs the convergence of efforts to allow its improvement. Computer simulation is an important tool to give a simultaneous evaluation of all variables and respective effects in the result of the process. Surface conditions have an important effect in the interaction between gases and metals. Its influence can only be studied under two-dimensional and 3-D simulations. The presence of oxides and other barrier have a critical influence on the kinetic and on the result of the process. Secondary reactions such as precipitation and nitride layer formation also affect the process. Particularly, porosities can have an important effect by enhancing the gas permeation inside the sintered material. Depending on the variables that might be deleterious to the process, the simulations give some indications how to proceed and how to control these process variables. This work show, through simulation tools, how variables such as gas atmosphere, temperature, powder surface conditions, porosity and alloy composition will affect the final result of the nitriding process of sintered and non-sintered powders. A methodology was developed using numerical methods and systemic algorithms.

Abstract: The humidity sensing behavior of a ceramic oxide based on the ZnO-TiO2-Ta2O5 system was analyzed. Samples were uniaxially pressed and sintered at temperature range from 1000 to 1200oC. Electrical impedance and capacitance measurements were realized in different values of relative humidity (from 11 to 100%). The microstructures of the sintered samples were characterized by X-ray diffractions and SEM observations. Mercury porosimetry analyses were carried out to determine the open porosity and the surface area was measured by using the BET nitrogen method. The obtained results were correlated and discussed.

Abstract: The zinc oxide varistors are ceramic semiconductor devices doped with some metallic oxides such as: bismuth, nickel, manganese, cobalt, and others. The aim of this work is to characterize the varistors, obtained by the Pechini’s method, regarding their microstructure and electrical features. By using chemical synthesis, it has been produced ceramic powders. The powders have been compacted in the form of pellets and sintered at 1200°C/1h. The samples have been characterized by measurements of J x E, XRD and by SEM. The results show that the powders obtained through Pechini’s synthesis method and calcined at 700°C/1h have a particle size of less than 100 nm. The resulting microstructures show heterogeneous morphologies in a shape of pins and presence of inter and intragrain porosity. The electrically characterized results show that the produced varistors present a nonlinearity coefficient of 21 and breakdown voltage of 1590 V/cm.

Abstract: The use of yttria concentrates was investigated in this study for synthesis and processing of zirconia based ceramics applied as solid electrolyte materials. Terbium, dysprosium, holmium, erbium and ytterbium are the chemical elements, classified as heavy rare earths, that can be found in those concentrates due to their association with yttrium ores. The ceramic characteristics were compared to zirconia-yttria and zirconia-yttria-heavy rare earth oxide systems, containing 3 and 9 mol% of dopant. Powders were prepared by the coprecipitation route and ceramic processing conditions were established to attain relative densities up to 95%. The characterization of assintered pellets was performed by apparent density measurement by Archimedes method, X-ray diffraction, scanning electron microscopy and electrical resistivity measurement by impedance spectroscopy. It was observed that the presence of heavy rare earths in a concentrate containing 85 wt% of yttria has no significant influence on the total ionic resistivity of zirconia based ceramics.

Abstract: Strontium ferrites powders were obtained by high energy milling process after calcinations of iron oxide and barium carbonate. Phase formations and crystallite size was determined using X-ray diffraction. Morphology, particle size and agglomeration stages were analyzed using scanning and transmission electron microscopy. Results show particles in the range of 14 to 40 nanometers, large agglomerates and crystalline phases formation.

Abstract: The Bi-Sr-Ca-Cu-O system has been one of the most studied superconducting ceramic materials for industry applications. The most of the studies with this aim are on silver/ceramic composites, due to the benefits and great compatibility of this metal with the oxide. Tapes made by the powder in tube (PIT) method have been successfully tested in pilot power plants in many countries but in Brazil. In this paper, 5, 10, and 20-wt% silver powders are introduced to compose the core of the tape along with the Bi:2212 ceramic powder. The results of electrical experiments are compared with those made with no silver addition Ag tapes. The best current density, at 60 K and no applied magnetic field, was found for the 10-wt% silver proportion, doubling the value obtained for the tape with no silver in the core.