16. The process according to claim 14, wherein hot condensate produced in the heat-drying system is subjected to air stripping at high pH with added caustic to remove ammonium-nitrogen, and the ammonia-laden air is subjected to wet acid scrubbing at low pH with added sulphuric acid or phosphoric acid to recuperate ammonia in the form of a concentrated solution of ammonium sulphate or ammo10 nium phosphate.