How does liquation separate tin from impurities with higher melting points?

In the context of tin processing, liquation is a fire refining method used to separate tin from impurities that possess a higher melting point than the metal itself.12

The process is carried out using a reverberatory furnace equipped with a sloping hearth. The impure tin or dross is placed on this hearth and heated to a temperature maintained just above the melting point of tin. Because the tin has a lower melting temperature than the associated impurities, it melts and flows down the slope, where it is collected in a separate vessel. The unmelted residues, containing the higher-melting-point impurities, remain on the hearth to be removed and treated subsequently.2