An apparatus for thickening liquids with solid contents has a container receiving the liquid and a vapor outlet and a heater, which is positioned in the liquid column in the container. The heater is constructed as a rotor rotating in the container and is formed from a plurality of substantially parallel plates arranged with a limited axial spacing and extending outwards from the rotor axis, every second space between adjacent plates forming a heating chamber sealed with respect to the liquid, while the spaces located between the same close to the rotor axis and on the circumference thereof are open towards the container interior.