GB568015A - Improvements in or relating to the manufacture of melamine - Google Patents

Abstract

<PICT:0568015/IV/1> In the manufacture of melamine by heating cyanamide on dicyandiamide in an autoclave in a solvent comprising liquid ammonia, separating the melamine from the mother liquor and stripping and recovering the solvent for re-use, a strong alkali is added to the mother liquor to free it from combined carbon dioxide, with the advantage, when a steel autoclave is used, that the content of iron in the product and in the solvent is substantially reduced. Advantageously, the solvent contains a non-aqueous diluent, e.g. a lower aliphatic alcohol and the contents of the autoclave are discharged into water or an aqueous medium, to which, after separation of the melamine, the strong alkali is added. In an example, dicyandiamide is dissolved in a mixture of methanol and liquid ammonia delivered from a storage tank 1 into a tank 2 and the mixture is passed into the alloy steel autoclave 3 which is heated for 1-4 hours at 160-200 DEG C. The resulting slurry is blown into the quenching tank 4 containing water to remove basic impurities of the type of guanidine, biguamide and guanyl urea. The quenching water contains a small amount of an organic hydroxy compound, e.g. glycerol or mannitol, added to retain the iron impurity in solution. This solution is cooled to reduce excessive loss of melamine while retaining any unconverted dicyandiamide in solution, and is then filtered in the press 5 from which the melamine is withdrawn through a drier 7 and from which the mother liquor, after addition thereto in a tank 8 of an excess of sodium or potassium hydroxide, milk of lime or other strong alkali, is passed through the steam-heated stripping column 9 and the alcohol and ammonia vapours are condensed in the condenser 12, returned to the storage tank 1. Specification 554,262 is referred to.