The influence of convection on zone refining

Zone refining is a process commonly used to purify metals or semiconductors to ultra low (ppb) impurity levels. The purpose of this project is to improve the zone refining process by applying knowledge of the fundamental transport mechanisms. A dimensional analysis and experimental study was undertaken to determine the appropriate operating conditions for improving the yield and/or separation efficiency. Using lead as a model material, instrumented zone refining experiments were carried out on an industrial unit. An analysis of thermocouple data revealed that convection complexity was dependent on ingot geometry and external temperature gradients. Segregation efficiency was calculated by fitting chemical analysis data gathered via glow discharge mass spectrometry to a mass balance model of the molten zone. The best separation efficiency was measured during low convection complexity at the solid/liquid interface. Recommendations for enhancing process efficiency include decreasing ingot geometry and reducing temperature gradients using insulation.