NOVELTY - Cryogenic separation of a low boiling CO2-rich gas mixture is carried out by rectification using a highly thermally conductive a gas permeable layer, within which a solid CO2 layer is formed.    USE - Cryogenic separation of gases produced by bacterial decomposition of organic matter, especially for treating natural gas to separate carbon dioxide from the methane.    ADVANTAGE - Cryogenic separation of CO2-rich gas mixtures from biological processes is effected at acceptable costs, without the need for the usual purification by freezing of the undesired CO2 in order to recover pure methane, and formation of solid CO2 is reliably avoided so that the CO2 can be efficiently recovered as liquefied gas.    DETAILED DESCRIPTION - Cryogenic separation of a low boiling CO2-rich gas mixture is carried out using a highly thermally conductive a gas permeable layer by (a) introducing an expanded gas/liquefied gas mixture at the lower end of a rectification column; (b) simultaneously passing an additional coolant around and/or through the layer to cause solid CO2 formation in the layer; and (c) simultaneously producing different rectification zones with different temperature and pressure ranges.    An INDEPENDENT CLAIM is also included for equipment for carrying out the above process, the equipment including a rectification column (1) which (i) is equipped with a high pressure valve (3), a transport device (4) for the expanded gas/liquid mixture (5), an inlet (6), a differential pressure gauge (7), a regulator (8) and a medium pressure valve (9); (ii) contains a thermal separation layer (10) located above the inlet (6) and having an internally formed solid CO2 layer (11), an adjacent cooling region (12) and an overlying liquefied gas collection region (13); and (iii) includes upper and lower rectification zones (15, 17).    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of the rectification column.    Rectification column (1)    Distillation plates (2)    High pressure valve (3)    Transport device (4)    Expanded gas/liquefied gas mixture (5)    Inlet (6)    Differential pressure gauge (7)    Regulator (8)    Medium pressure valve (9)    Thermal separation layer (10)    Solid CO2 layer (11)    Cooling region (12)    Collection region (13)    Upper rectification zone (15)    Lower rectification zone (18)