sent directly to the low pressure column while the rest is expanded in a valve sent to the top condenser 29 of the intermediate pressure column where it is vaporized at least partially before being sent to the low pressure column 103 a liquid air flow 12 is withdrawn from the high pressure column cooled in the subcooler 83 expanded and sent to the low pressure column the bottom reboiler 24 of the intermediate pressure column 102 is heated by means of an argon enriched gas stream 233 containing about 5 to 15 mol preferably 8 to 10 mol argon coming from the low pressure column 103 this stream is condensed at least partially in the reboiler 24 before being sent back to the low pressure column 103 a waste nitrogen stream 72 is withdrawn from the top of the low pressure column 103 sent to the subcooler 83 and then to the heat exchanger 100 where it is warmed a stream 31 of 177 nm 3 h oxygen at 99 5 mol is withdrawn in liquid form from the low pressure column pumped in the pump 19 to 40 bara and vaporized in the heat exchanger 100 in order to form a pressurized gas stream it is possible to withdraw liquid products from the apparatus but the apparatus does not produce any argon enriched fluid the advantages of the invention will appear clearly on studying the table below other alternative or additional refrigerating means can be envisaged such as an air blowing turbine a claude turbine or another turbine which is not fed by a liquid stream or.