The austenite formation under isochronal conditions in Nb microalloyed low carbon steel was studied by means of dilatometric analysis and the data was adjusted to the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation, for different heating rates and for three initial microstructures. It was shown that the kinetics of austenitization of a pearlite+ferrite structure is faster than that of martensite (tempered martensite) at a heating rate of 0.1ºC/s. For heating rates higher than 0.1ºC/s, the kinetics of austenitization of a martensite structure is faster than of pearlite+ferrite one. The K parameter of the JMAK equation increases with the heating rate for the three previous microstructures and it is greater for the initial microstructure composed of ferrite+pearlite. At lower heating rates, the formation of austenite in this steel is controlled by carbon diffusion, independently of the initial microstructure. At higher heating rates, the formation of austenite from an initial microstructure composed of pearlite and ferrite is controlled by interface-controlled transformation.
Uma amostra de bauxita contendo 45% de gibbsita e 43% de boehmita foi submetida a uma lixiviacao caustica a 160oC para a remocao da gibbsita. O residuo da lixiviacao, completamente isento de gibbsita, foi tratado com o sistema ditionito-citrato-bicarbonato para a remocao dos oxi-hidroxidos de ferro, resultando em uma amostra contendo cerca de 85%-89% de boehmita e 9% de TiO2. Curvas de calibracao correlacionando as intensidades dos picos de difracao com os teores das fases mineralogicas foram obtidas para padroes preparados pela mistura de quantidades conhecidas de gibbsita, hematita e goethita com a amostra purificada de bauxita. Correlacoes significativas foram obtidas para os planos cristalograficos (020), (051), (002) e (251) da boehmita; (110), (200), (11-2) e (024) da gibbsita; (012), (024) e (110) da hematita; e (110) e (021) da goethita. Estas correlacoes podem ser utilizadas para se estimar o teor destas fases em amostras semelhantes de bauxita.
A representative diamond population from the Borrachudo River was described for the first time as to their physical characteristics, among them weight, morphology, dissolution figures, and abrasion. Most stones have low weight (~51% less than 0.30 ct) and only ~3.5% above 3.0 ct. However, considering the total weight ~47% concentrates in the weight range above 3 ct, occurring even diamonds with dozen of carats. The most common crystallographic forms are originated from the octahedral form by the dissolution, although irregular shapes like chips and flats also occur. The diamonds show differences in form compared to their weight ranges; those up to 0.30 ct have various shapes; in the range of 0.31 to 1 ct the most common are octahedral forms and their descendants; and in stones larger than 1 ct chips and flats predominate. The general habitus of the crystals, identified by their final tetrahexahedroid shapes, chips and flats, corroborated by the dissolution figures, indicate that the diamond were submitted to strong dissolution in the magmatic environment. Residual hillocks and holes represent the final stage of dissolution. The study indicates that the abrasion by the fluvial transport was not expressive to cause mechanical wear, thus ~97% show no sign of wear. This fact suggests a proximal source for most of these diamonds.
A problem for solving mass balances in mineral processing plants is the calculation of circulating load in closed circuits. A family of possible methods to the resolution of this calculation is the iterative methods, consisting of a finite loop where each iteration the initial solution is refined in order to move closer to the exact solution. The present work shows a low-complexity iterative algorithm for circulating load calculation in mineral processing closed circuits, thus enabling the construction of mass, metallurgist and water reliable balances. The proposed equations on the algorithm were obtained through the analysis of many industrial systems, taking into account the process operational parameters. A validation was performed with real industrial data, in order to ensure a greater reliability of the obtained results. Four different types of closed circuits are presented, each one with different levels of complexity, to clarify the proposed algorithm. With the obtained results it is possible to affirm that the proposed iterative algorithm can be successfully applied to any kind of closed circuit in mineral processing. The results obtained were satisfactory with respect to processing speed, convergence of the solution and the number of iterations required for the circulating load calculation.
A new computational tool for advanced static and dynamic analyses of steel framed structures based on the Finite Element Method has been developed and is presented herein. Two sources of nonlinearity can be considered in these analyses, i.e.: the geometric, which considers the nonlinear effects of structure displacements; and the physical, which considers the nonlinear effects of the mechanical characteristics of the material used in civil construction. Loading, geometric and residual stress imperfections can also be considered. To illustrate some of the features and capabilities that differentiate this tool from existing commercial programs, static and dynamic stability analyses of some structural systems with rigid and semi-rigid connections are evaluated. The results obtained by other researchers are used to validate the nonlinear formulations and numerical solution methodologies implemented in CS-ASA, and to attest the efficiency of the structural analysis program presented herein.
Computerized drilling and the electronic timing of detonations are two technological breakthroughs which have had an important role in updating drilling and blasting excavation methods, although the electronic timing of detonators is still a comparatively infrequent technical solution to precision blasting problems. On the basis of an extensive collection of published cases, this paper reviews the successes achieved and the main expected advantages from the electronic ignition devices. After describing the primary characteristics of these detonators, some elements will be considered, in order to better understand their applications in different conditions, both in open pit and underground sites: extension of the time delay number, freedom in the choice of time intervals between detonations, timing accuracy, reduction of vibrations, control of back-break and fragmentation. The results are compared to those obtained by pyrotechnical timing devices, and summarized in the concluding remarks.
