Removal of Dissolved Impurities from Molten Metals
Impurities are transferred out at the boundary of the liquid. Velocities normal to the boundary are small. Therefore, for efficient removal contact areas and times should be large. Transfer depends on the chemical and physical properties of the liquid and the phase that captures the impurities at the boundary. This phase may be a liquid, gas (vacuum) or solid. Properties can be described in terms of equilibrium and empirical mass transfer coefficients. Vacuum may be applied to remove volatile elements. Refining can be carried out by partial solidification or fractional crystallisation, using the segregation that occurs during freezing of an alloy. Finally, an element can be added to form a reactive compound followed by removal of the compound by sedimentation or filtration.