Mass Transfer in Slag Refining of Silicon with Mechanical Stirring: Transient Kinetics of Ca and B Transfer

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Mechanical stirring was demonstrated to be a potent means to increase the mass transfer rates of B and Ca in slag refining of silicon, and optimal stirring conditions were attained with only a modest kinetic energy input into the melt. Experiments were undertaken to measure mass transfer rates between liquid silicon and CaO-SiO2 slags at 1823 K (1550 °C) with varying stirring speed and slag composition. A remarkable feature of this system is the extreme fluctuation in interfacial area—these changes occur very rapidly and are difficult to capture. As little as 50 rpm stirring speed promotes, significant transient increases in interfacial area. At stirring speeds above 100 rpm, the rates of mass transfer do not increase significantly, so it is evident that above this threshold mass transfer in the bulk phases and interfacial area generation are already maximized. With a sufficiently high content of CaO in the slag, interfacial phenomena induced by the mass flux of calcium facilitate emulsification, and this is explained from a thermodynamic standpoint. It is likely that the calcium transfer enhances boron transfer by increasing the available interfacial area.