Abstract: An apparatus for concentrating and solidifying a salt solution includes a vessel for containing the salt solution having an exposed surface, the vessel having a generally open upper portion, a cover for sealing the generally open upper portion of the vessel and having a generally hood shaped portion, the cover including at least one inlet conduit for admitting the salt solution to the vessel and a discharge conduit for vapor, and a magnetron for generating microwaves for heating the solution in the vessel. A microwave waveguide is disposed between the magnetron and the vessel and incorporates the cover, for directing microwaves from the magnetron into the vessel, the waveguide transitioning within the cover from the generally hood shaped portion into a horn having a variable cross-section and/or variable vertical spacing with respect to the vessel. This structure enables microwaves to be radiated directly onto the exposed surface of the salt solution in the vessel.