FIELD: chemistry.
SUBSTANCE: present invention relates to a method of producing melamine from urea with a cycle for separating and extracting high-purity CO2 and highly pure NH3, including the following production steps: a) synthesis of melamine in the main reactor by pyrolysis of molten urea stream (301) in the presence of NH3 (319) to obtain stream (303) of molten crude melamine and off-gas stream (302) containing NH3 and CO2; b) treating stream (303) of molten crude melamine leaving the main reactor in a post-reactor in the presence of gaseous ammonia (304), to complete the urea pyrolysis reaction to obtain stream (306) of molten crude melamine substantially free of unreacted urea and CO2, and offgas stream (305), mainly consisting of NH3 and containing entire CO2, formed in reaction; c) distillation of offgas stream (302) from step a) and offgas stream (305) from step b) in the presence of aqueous solvent (329, 318) for extraction to obtain gaseous high-purity CO2 stream (314) and aqueous ammonia stream (315) containing CO2; d) removing aqueous ammonia stream (315) from step c) to remove still available CO2; e) cooling and dissolving melted crude melamine stream (303) obtained at step a), and stream (306) from step b) by contacting aqueous ammonium stream (315) from step c) to obtain aqueous ammonia solution (330) containing dissolved melamine, impurities of polycondensates and oxyamino-triazines; f) purifying aqueous ammonium solution (330) containing melamine from step e) by settling in presence of NH3 for converting polycondensates to melamine; g) crystallisation of melamine present in purified aqueous solution (307) coming from step f), and separating melamine crystals from said solution to obtain wet residue (309) of melamine and stream (310) of a crystallisation mother solution containing only oxyamino-triazines, dissolved residues of melamine and NH3; h) distilling the mother solution from step g) to obtain high purity NH3 (317) stream and aqueous stream (322), which does not contain ammonia and contains oxyamino-triazines and melamine residues, which are subsequently separated or removed in specially intended sections of the method downstream; i) drying melamine residue (309) from step (g) with hot air to obtain dried high purity melamine crystals and a waste air stream containing ammonia; wherein in said method it is necessary to stage c) extraction distillation of offgas stream (302) from step a) and offgas flow (305) from step b) in presence of aqueous solvent for extraction at pressure in range from 5.5 to 25 MPa (from 55 to 250 bar).
EFFECT: disclosed is a process for producing melamine by separation and reduction of CO2 and NH3.
18 cl, 3 dwg, 2 tbl, 2 ex
