Steam Explosions of Single Drops of Molten Silicon-Rich Alloys
Commercial quantities of ferrosilicon alloys and silicon metal in the form of granules are available. They are produced in water granulation, a process involving the risk of large-scale steam explosions. To learn about these steam explosions, we have investigated single drops of molten alloys when released into water. When single drops of molten ferrosilicon alloy with 75 wt.% Si (FeSi75) or silicon metal at its liquidus or melting temperature were released into cold, room temperature or hot water, spontaneous explosions never occurred. We could, however, initiate steam explosions by exposing the drops to upwarddirected pressure transients generated in the water with an encapsulated impactor. For water at room temperature, we determined (a) the strength of the pressure transients needed to initiate the steam explosion of a single drop, and (b) the range of fall depths over which the drop can be triggered to explode with this transient. When varying the water temperature we learned that the triggered explosions became less vigorous as the temperature increased. The explosions became more vigorous also when the Al and/or Ca content in the molten drop decreased. Photographic images of the bubbles generated during the steam explosions indicate that the maximum energies released correspond to about 1 % to 2% (dependent on drop size) of the total enthalpy of the molten ferrosilicon at its liquidus temperature. During the explosions, tiny bubbles of a permanent gas, presumably hydrogen, were generated. Video images show that ferrosilicon drops with diameters between 3 mm and 11 mm fall through water at about 0.5 m/s. Shutter wheel photographs confirm these fall rates and also indicate the onset of solidification of the drops and show that steam explosions can occur in partially solidified drops.
Extended Stefan problem for the solidification of binary alloys in a sphere
We study the extended Stefan problem which includes constitutional supercooling for the solidification of a binary alloy in a finite spherical domain. We perform an asymptotic analysis in the limits of large Lewis number and small Stefan number which allows us to identify a number of spatio-temporal regimes signifying distinct behaviours in the solidification process, resulting in an intricate boundary layer structure. Our results generalise those present in the literature by considering all time regimes for the Stefan problem while also accounting for impurities and constitutional supercooling. These results also generalise recent work on the extended Stefan problem for finite planar domains to spherical domains, and we shall highlight key differences in the asymptotic solutions and the underlying boundary layer structure which result from this change in geometry. We compare our asymptotic solutions with both numerical simulations and real experimental data arising from the casting of molten metallurgical grade silicon through the water granulation process, with our analysis highlighting the role played by supercooling in the solidification of binary alloys appearing in such applications.
Denitrification of PVA-immobilized denitrifying photosynthetic bacterium, Rhodobacter sphaeroides
Abstract A newly isolated denitrifying strain, Rhodobacter sphaeroides NII2 was immobilized in polyvinyl alcohol (PVA) gel, and the properties of the cells in the gel were examined. The immobilized cells had low or almost no denitrification activity, but the cells were activated by incubation in light with culture medium for denitrification containing 0.5% nitrate and no other nitrogen source. Cells grown in the dark were activated by incubation at an earlier stage and to a higher rate than the light-grown cells. The activation was markedly enhanced in the PVA gel with a low cell concentration. The immobilized cells consumed nitrate with a temporary accumulation of NO 2 and evolved nitrogen gas. The immobilized cells could use various organic compounds as electron donors for denitrification. Thus, the immobilized cells were applied to a continuous treatment of synthetic wastewater using an aparatus devised by this laboratory. The results showed an efficient removal of NO 3 -N from the test water.