Patent ID: 7125992

Claim:
A process for the production of high-purity melamine comprising the steps of: (a) pyrolyzing urea in a reactor operating at a temperature comprised between 360 and 420° C. and at a pressure higher than 7 MPa, the reactor producing reaction products that comprise a gaseous phase and a liquid phase; (b) feeding the liquid phase from the reactor to a downstream post-reactor, the liquid phase from the reactor containing melamine, non-reacted urea, intermediate oxidized products of pyrolysis, dissolved carbon dioxide, and products of de-ammoniating condensation of melamine (“polycondensates”), the post-reactor operating at substantially equal conditions of temperature and pressure as the reactor and producing a liquid product, gaseous superheated anhydrous ammonia being fed under pressure to the post-reactor in order to eliminate the dissolved carbon dioxide and under conditions suitable for completing the pyrolysis reaction of the urea, for transforming the majority of the intermediate oxidized products of pyrolysis into melamine, and for decreasing the polycondensates concentration; (c) dissolving the liquid product from the post-reactor in an aqueous solution in a quenching column in the presence of ammonia and under conditions of temperature and residence time such to provide a solution substantially free of polycondensates, the solution substantially free of polycondensates being successively subjected to crystallization to provide purified melamine in solid phase and an aqueous mother liquor that contains melamine and reduced quantities of the intermediate oxidized products of pyrolysis, the mother liquor being recycled for the greater part directly to the quenching column without any further treatment; and (d) subjecting the anhydrous gaseous phase coming from the reactor and post-reactor to washing with molten urea, thereby recovering the melamine contained in the anhydrous gaseous phase as vapor, prior to returning the anhydrous gaseous phase to the urea synthesis plant for the recovery of the ammonia and carbon dioxide contained in it.