The present article presents a comprehensive evaluation of the potential use of an Electrical Low Pressure Impactor (ELPI) in the ferroalloy industry with respect to indoor air quality and fugitive emission control. The ELPI was used to assess particulate emission properties, particularly of the fine particles (Dp ≤ 1 μm), which in turn may enable more satisfactory risk assessments for the indoor working conditions in the ferroalloy industry. An ELPI has been applied to characterize the fume in two different ferroalloy plants, one producing silicomanganese (SiMn) alloys and one producing ferrosilicon (FeSi) alloys. The impactor classifies the particles according to their aerodynamic diameter and gives real-time particle size distributions (PSD). The PSD based on both number and mass concentrations are shown and compared. Collected particles have also been analyzed by transmission and scanning electron microscopy with energy dispersive spectroscopy. From the ELPI classification, particle size distributions in the range 7 nm – 10 μm have been established for industrial SiMn and FeSi fumes. Due to the extremely low masses of the ultrafine particles, the number and mass concentration PSD are significantly different. The average aerodynamic diameters for the FeSi and the SiMn fume particles were 0.17 and 0.10 μm, respectively. Based on this work, the ELPI is identified as a valuable tool for the evaluation of airborne particulate matter in the indoor air of metallurgical production sites. The method is well suited for real-time assessment of morphology (particle shape), particle size, and particle size distribution of aerosols.
An industrial oxidative ladle refining process of metallurgical grade silicon has been experimentally examined. An extensive industrial sampling campaign has been performed and samples of silicon and slag have been analyzed by inductively coupled plasma mass spectroscopy (ICP-MS). The elemental concentrations of 45 elements have been evaluated with respect to sampling time during the refining process. Major elements, such as Ca and Al, as well as trace elements are studied. The refining kinetics is discussed and groups of elements with different behaviors are distinguished. For 21 elements, which are responsive to the refining process, kinetic parameters are established. The alkaline and alkaline earth elements are identified as having the highest refining rates, whereas the rare earth elements are slower and most transition metals are quite unresponsive to the oxidative refining operation.
The oxidation of liquid silicon resulting in silica fume has been the subject of previous investigation due to its importance to occupational health in the silicon alloy production industry. Small-scale experiments and industrial measurements have been carried out in order to understand the mechanisms and kinetics of liquid silicon oxidation. Key questions as to the main factors and conditions determining the rate of fume formation in the industry, still remain. In this work the rate of active oxidation of liquid silicon was studied by experimental investigations in a 75 kW induction furnace, where oxidizing gas was introduced via a lance above the liquid silicon surface. The silica formed as a result of the reaction was collected and the silica fuming rate determined as a function of gas composition and gas flow velocity. The system was also modeled using computational fluid dynamics (CFD) and kinetic modeling. The flux of silica increases with increased gas velocity above the liquid surface, and was found to correlate well with mass transfer rates calculated from impinging jet theory. The size of the silica particles was also found to be dependent on the gas flow rate; smaller average particle size was obtained at higher flow rates. It was found that the most important factor for the silicon oxidation reaction rate is the velocity of the gas in vicinity of the silicon surface (i.e. the boundary layer thickness). The velocity is more important than the actual amount of oxygen delivered to the system per unit time, indicating that oxygen ''efficiency'' is not a strong function of oxygen concentration in the gas. Thus, the gas velocity is the rate determining parameter in determining the mass transport of oxygen to the silicon surface. Results from computational fluid dynamics simulations show that the gas flow was laminar in all experiments and that oxidation takes place within 123Oxid Met (2014) 82:395-413 DOI 10.1007/s11085-014-9498-z 0.5 mm from the silicon surface. The results from the experiments and the CFD model were used to suggest a molecular mechanism of the active oxidation of liquid silicon.
During oxidative ladle refining (OLR) of silicon, the metal surface is partly oxidized, resulting in the formation of a condensed silica fume (SiO 2 ). This fugitive emission of silica represents a potential health hazard to the workers in the silicon and ferrosilicon industry. In the current work, industrial measurement campaigns aimed at recording the fume generation during OLR were performed at the Elkem Salten plant in Norway. The measured amounts of silica produced were 2.5-5.1 kg/h, depending on the gas flow rate in the refining process. The rate of silica production correlates with the total flow rate and amount of air in the purge gas, and increases as the flow rate increases. The results of this work suggest that fume generation during OLR primarily results from oxidation of the exposed metal surface, with oxygen transport from the surrounding atmosphere to the metal surface being the limiting factor. Other identified mechanisms of SiO 2 formation were splashing of the metal and/or oxidation of SiO gas carried with the refining purge gas.
Small scale laboratory experiments on the oxidation of liquid silicon have reproduced important features of the industrial refining of liquid silicon: active oxidation led to the formation of amorphous silica spheres as a reaction product. The boundary condition for active oxidation in terms of maximum oxygen partial pressure in the bulk gas was found to lie between 2Á10 -3 and 5Á10 -3 atm at T = 1,500°C. The active oxidation of liquid silicon had linear kinetics, and the rate was proportional to bulk oxygen partial pressure and the square root of the linear gas flow rate, consistent with viscous flow mass transfer theory. Classical theory for unconstrained flow over a flat plate led to mass transfer rates for SiO (g) which were 2-3 times slower than observed. However, computational fluid dynamic modeling to take into account the effects of reactor tube walls on flow patterns yielded satisfactory agreement with measured volatilization rates.
To establish an overview of impurity elemental distribution among silicon, slag, and gas/fume in the refining process of metallurgical grade silicon (MGSi), an industrial measurement campaign was performed at the Elkem Salten MG-Si plant in Norway. Samples of in-and outgoing mass streams, i.e., tapped Si, flux and cooling materials, refined Si, slag, and fume, were analyzed by high-resolution inductively coupled plasma mass spectrometry (HR-ICP-MS), with respect to 62 elements. The elemental distributions were calculated and the experimental data compared with equilibrium estimations based on commercial and proprietary, published databases and carried out using the ChemSheet software. The results are discussed in terms of boiling temperatures, vapor pressures, redox potentials, and activities of the elements. These model calculations indicate a need for expanded databases with more and reliable thermodynamic data for trace elements in general and fume constituents in particular.
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