GB628250A - Production of melamine - Google Patents

Abstract

Melamine is separated from the product obtained by the pyrolysis of urea by subjecting the product to the action of at least two parts of water per part of melamine by weight under conditions of elevated temperature and pressure and subsequently removing the aqueous solution from the region of elevated pressure for separation of the melamine. Preferred features of the invention include the addition of the water to the pyrolysate while it is in a molten condition, the removal of insoluble impurities by filtration before reducing the pressure, and the use instead of pure water as the aqueous additive liquor, of a mother liquor obtained in a melamine crystallization process. The filtration may be carried out at either the full original pressure or at a somewhat reduced but still superatmospheric pressure. The initial temperature and pressure range may be from 275-550 DEG C. and 100-1000 atmospheres. In examples: (1) molten urea is passed through a silver tube at 400 atmospheres at 400 DEG C. and the resultant fluid subjected to a stream of water in the proportion of 4 parts by weight per part of urea charged; (2) a mixture of urea and ammonia is passed through a silver lined tube at 400 DEG C. under a pressure of 400 atmospheres with a constant time of 10 minutes. The product is quenched by water in amount of 20 parts of water per part of urea charged. The resulting aqueous solution is evaporated and the melamine crystallized; (3) the aqueous mother liquor obtained in the crystallization of malamine in example (2) is preheated to 150 DEG C. under 400 atmospheres and used as the aqueous reactant. Specifications 583,504 and 628,255 are referred to. The Specification as open to inspection under Sect. 91 refers to the separation of melamine from any mixture containing melamine by subjecting the mixture to the action of water as indicated above. This subject-matter does not appear in the Specification as accepted.