Flávio Teixeira da Silva

Alvejamento químico de caulins Brasileiros: efeito do potencial eletroquímico da polpa e do ajuste do pH

Samples of Kaolin from different regions in Brazil were characterized by XRD, SEM and chemical analysis. A chemical bleaching study with pH adjustment was accomplished with the fractions below 37 μm, after classification by screening. The main objective was to evaluate the conditions of chemical bleaching that most increase the brightness of these kaolins samples. Increases between 2.63 and 2.98% in the brightness (ISO) were observed after the chemical bleaching. We could say that the reduction of Fe3+ to Fe2+ during the chemical bleaching promoted an increase in the brightness, based on the Pourbaix Diagrams.

Thermodynamic Aspects of the Roasting Processes in the Pre-treatment of Nickelferrous Garnierites

Abstract The chemistry of pre-calcination and reduction roast processes with H2 + H2O gaseous mixture was analyzed, in order to find the best nickel and cobalt recoveries after selective roasting of nickelferrous laterites A thermodynamic analysis of the pre-reduction of garnieritic ores aiming NiO and CoO reduction showed that the low NiO activity in the ore demands highly aggressive reducing conditions to attain reasonable nickel recoveries. Between 650-850°C, the highest nickel recoveries through amoniacal leaching was obtained at 750°C with 60% H2 + 40% H20. Tests at 900°C revealed an unexpected high nickel recovery probably due to the presence of metallic iron which acted as a solvent for metallic nickel, favoring in consequence its metallization.

Mechanism of the electrochemical reduction of tantalum(V) in molten fluorides

Abstract The mechanism of the reduction of tantalum(V) in molten fluorides (LiFNaFKF eutectic) was investigated in order to resolve the controversy in the literature. The technique used was mainly chronopotentiometry. It was concluded that at a CEE mechanism applies, that is, a chemical reaction followed by two electrochemical steps, the first (reversible) giving a soluble product, with the transfer of two electrons, and the second a product with transfer of three electrons. The conclusion regarding the two-electron transfer was based on the determination of an average number of 1.94 ± 0.40. The diffusion coefficient for the Ta(V) species, in accordance with the previously proposed dissociation of the TaF 2− 7 ion, was 3.14 × 10 −5 cm 2 s −1 , estimated using the Stokes—Einstein equation; the estimated activation energy for diffusion was 10.34 kcal mol −1 (4.33 × 10 4 J mol −1 ).

Influence of different surface treatments on the fracture toughness of a commercial ZTA dental ceramic

The objective of this study was to investigate how mechanical surface treatments performed for removal of excess of molten glass, influence the fracture toughness of a dental zirconia toughened alumina (In-Ceram® Zirconia). Infiltrated ZTA disks were submitted to three different surface treatments (grinding, sandblasting and grinding + sandblasting + annealing). Fracture toughness was accessed through indentation strength test (IS). X ray diffraction was used to investigate the metastability of tetragonal zirconia particles under all treatments proposed. Kruskall-Wallis non-parametrical test and Weibull statistics were used to analyze the results. Grinding (group 1) introduced defects which decreased the fracture toughness and reliability, presenting the lowest KIC. On the other hand, grinding followed by sandblasting and annealing (group 3) presented the highest KIC. Sandblasting (group 2) presented the highest reliability but lower KIC compared to group 3.

Thermal dehydration of lanthanum chloride heptahydrate: thermodynamic approach

Abstract Production of rare-earth metals from molten chloride electrolysis demands the use of anhydrous chloride in order to increase current efficiency. The main difficulty in obtaining these chlorides is due to oxychloride formation. Any thin film of the oxychloride or oxide will tend to inhibit the reduction process. The main purpose of this work was to develop an operational diagram related to the dehydration of lanthanum chloride heptahydrate. This diagram agrees with previous experimental data which have shown that dehydration of light rare-earth chlorides can be effective at relatively low temperatures in a dry HCl gas atmosphere.Production of rare-earth metals from molten chloride electrolysis demands the use of anhydrous chloride in order to increase current efficiency. The main difficulty in obtaining these chlorides is due to oxychloride formation. Any thin film of the oxychloride or oxide will tend to inhibit the reduction process. The main purpose of this work was to develop an operational diagram related to the dehydration of lanthanum chloride heptahydrate. This diagram agrees with previous experimental data which have shown that dehydration of light rare-earth chlorides can be effective at relatively low temperatures in a dry HCl gas atmosphere.

Mechanical behavior of non veneered three unit fixed partial dentures of alumina-zirconia Under cyclic load in wet environment

The aim of this study was to evaluate the fatigue behavior in wet environment of a ZTA three unit FPDs processed via CAD-CAM. Thirty four three unit FPDs (In-Ceram®Zirconia BZ blanks) were grinded through a CEREC In-Lab equipment followed by glass infiltration and sandblasting. Those frameworks were then submitted to different number of load cycles (400 N, 1 Hz). The FPDs that did not fractured by fatigue were submitted to a bending test in order to determine the influence of load cycling on its residual load. The non-parametric Kruskal-Wallis of those tests have shown with a 95% confidence, that there is a clear relation between the number of load cycles and its residual load. The fracture mode most frequently encountered on those FPDs was through the distal connector.

Method to Separate Nanometric Particles of Clays

There are several types of polymeric nanocomposites, but the most commercially advanced are those that involve dispersion of small amounts of nanoparticles in a polymeric matrix. Polymer/clay nanocomposites have exhibited enhanced mechanical properties at low weight fractions of clay. Addition of these nanoparticles not only improves the mechanical properties, but also has been shown to improve thermal stability. The clay can be considered as being essentially a rock consisting of clay minerals, which also contain organic substance and other impurities. Clay minerals possess nanometric particle sizes, being classified as crystalline silicates with structure in layers or lamellar and crystalline silicates with fibrous structure. In the present work, three clays from Rio de Janeiro, Brazil, were studied in order to identify and characterize the clay minerals presents. X-ray diffraction (XRD), X-ray fluorescence (XRF), thermogravimetric (TG) and differential thermal analysis (DTA), Fourier transform infrared (FTIR) and transmission electron microscopy (TEM) were used to characterize the clay fraction previously separated using NH4OH aqueous solution and submitted to physical-chemistry treatment in an ethylene glycol atmosphere.

Tantalum electrowinning from molten fluorides: characterization of structure and defects

high melting-point and corrosion resistance—in which it is matched only by glass, graphite and PTFE1,2,3—other applications, such as in superalloys for the aircraft industry and in the chemical processing industry, are also important. Other well-established applications include tantalum carbide for metal cutting tools, along with prosthetic devices, optical glass, laboratory ware, electroplating devices, etc.4,5 (Fig. 1). Tantalum metal is obtained from the processing of tantalite–columbite concentrates or tin slags; Fig. 2 summarizes the process. Tantalum-bearing materials are concentrated and leached with hydrofluoric acid, in some cases mixed with sulphuric acid to decrease the vapour pressure of the solution.6–8 Tantalum is then removed from the aqueous solution and separated from niobium by liquid–liquid extraction with methyl isobutyl ketone (MIBK). The alternatives for the next step depend on the final product intended: (1) precipitation with ammonia to produce tantalum hydroxide; or (2) crystallization with a potassium-bearing salt to produce potassium fluorotantalate (K2TaF7), known as K-salt. To produce pure tantalum metal as the final product the K-salt can either be electrolysed or be reduced by liquid sodium, the latter being the usual route. Several authors9–11 have advocated fused salt electrolysis for the production of refractory metals, such as tantalum, because purer metals are produced than by thermal processes.10 Nevertheless, since the 1970s the vast majority of the tantalum industry has adopted sodium reduction because the very fine, porous, flaked powder obtained by this route is more suitable for high-performance capacitors than the dendritic powder produced by molten salt electrolysis.6,12 However, the electrolytic process consumes much less energy than sodium reduction13 and is the method of choice for end-uses in the chemistry industry, direct electroplating on steel, working in very aggressive environments or even electroforming of small parts. For these purposes the electrolytic deposit must be smooth, coherent and free of defects—characteristics that can only be obtained under a narrow range of electrolysis conditions owing to the nature of the diffusion-controlled process