Abstract: A process for the synthesis of ammonia, where: a front-end produces a make-up syngas (10) having a substantial excess of nitrogen, the H2/N2 ratio being less than 3; hydrogen is separated from a purge stream (15) taken in the high-pressure synthesis loop, with a molecular sieve or a cryogenic device, obtaining a hydrogen-rich gaseous stream (16); said hydrogen-rich gaseous stream (16) is returned to the ammonia synthesis loop, thus obtaining that the H2/N2 ratio of the gas feed (12) actually converted into ammonia is close to 3 and preferably in the range 2.9-3.1.