GB814934A - Method for producing melamine - Google Patents

Abstract

Melamine is produced by reacting dicyandiamide, previously heated to between 100 DEG C. and its melting point, with gaseous ammonia under pressure, the pressure of ammonia being maintained throughout the reaction so as to be at least 50 kg./sq. cm. at the end of the reaction, this pressure being maintained during the cooling of the reaction product as long as its temperature is higher than about 250 DEG C. Preferably the pressure is maintained so as to reach about 80 kg./sq. cm. when the temperature of the melamine reaches 350 DEG C. It is advantageous for the dicyandiamide and the reaction vessel to be pre-heated as uniformly as possible, so that no point of the reaction mixture is at a temperature lower than the condensation temperature of ammonia under the operating pressure. It is also desirable that the size of the reaction vessel be such that the heat of reaction is dispersed without raising the temperature of the melamine substantially above 350 DEG C. Auxiliary compression means may be employed for attaining the final pressure of 50 kg./sq. cm. The reaction may be effected intermittently, e.g. inside a digester which may be enclosed in a container, the cooling being carried out in the digester, or continuously, e.g. by passing pre-heated dicyandiamide and gaseous ammonia into a first closed chamber, transferring the product in a liquid state into a second closed chamber, and feeding the product continuously or periodically into a third closed chamber where it is cooled. If desired, the final melamine product may be purified by recrystallization from hot water. The process is described with reference to drawings (not shown) illustrating the batchwise and continuous procedures.