A process is described for separating a component of a solution or of a liquid mixture by evaporation which comprises feeding said solution or mixture to a heated rotating ..plate so as to convert a part of said solution or mixture to vapour form and condensing at least a part of said evolved vapour on an adjacent relatively cool rotating plate. Devices for use in the process are described. Weak aqueous caustic soda solutions may be concentrated and saline water may be purified by the process of the present invention.