The technique which is used to purify the chemicals is said to be recrystallization.
In this process, solute is dissolved in a particular solvent and according to its principle, the quantity of solute which is dissolved by solvent increases with increase in temperature. The solutes are more soluble at high temperatures in comparison to room temperature (insoluble) or low temperature. Therefore, heating is needed to dissolve the solute in the solvent.
Now, in the case of hexane or diethyl ether, flameless heating is required because of their low boiling points implies if they heat directly on fire, then they have tendency to catch fire. Thus, hexane and diethyl ether are highly flammable.