A freeze-thawing tank assembly includes an upright vessel having a hollow defined therein for accommodating therein a quantity of fluid medium to be treated. The vessel is closed at an upper end by a lid and at a lower end by a normally closed discharge valve. A plurality of double-walled structures are accommodated within the hollow of the vessel in equally spaced and parallel relation with each other. Each of the double-walled structures has a fluid passage defined therein for the flow of a heat-exchanging fluid medium and also has a fluid inlet and outlet communicated with the fluid passage. The fluid medium to be treated may be alternately frozen and thawed by passing the heat-exchanging medium through the fluid passages in the double-walled structures.
