A new probe has been developed for measuring bubble frequency,size and velocity in molten metals.The probe detects the change in capacitance that occurs when an alumina sheathed wire.immersed in the metal,encounters a bubble.The probe has been tested in molten aluminum and applied in a study of the removal of impurities(notably magnesium) from aluminum by gas fluxing.Fluxing with chlorine is extensively used in abstracting impurities(calcium,magnesium and sodium) from aluminum,for example in processing recycled beverage cans.The kinetics of the reactions involved in chlorine fluxing for magnesium removal have been studied on the laboratory scale and matched to a mathematical model.Above a certain critical magnesium content,the rate determining step is merely the supply of chlorine to the reactor(at temperatures above the melting point of MgCl_2).Below the critcal magnesium content,mass transport of magnesium to the surface of the gas bubbles is rate controlling and significant emissions of chlorides are observed.The rate of removal of magnesium and the level of emissions then depend on the bubble size.The mathematical model has been extended to the scale of large industrial melts and suggests that emissions can be minimized by tailoring the chlorine content,of the usual chlorine argon fluxing gas mixture,so that it is initially high and then lowered later in the processing of the batch.
展开▼
