In the high-pressure processes for preparing melamine, urea is reacted in an endothermic liquid-phase reaction to form melamine. The liquid melamine additionally contains, depending on the pressure and temperature conditions in the reactor, variable amounts of dissolved NH3 and CO2 and also condensation by-products and unreacted urea. The resulting melamine, which is under a high NH3 pressure, is subsequently solidified by, for example, quenching with water or with ammonia, by sublimation with subsequent desublimation or by depressurization under particular conditions. The reactor used is customarily a single apparatus of the stirred tank type.
A significant problem in the preparation of melamine from urea is that the urea used is not reacted completely and also that by-products which subsequently have to be converted into melamine by means of costly and complicated work-up steps are formed in the reactors customary in the prior art. For example, it is known from WO97/20826 that pure melamine can be obtained when the melamine is, prior to solidification, cooled to temperatures which are just above the respective melting point of the melamine, which depends on the prevailing NH3 pressure. The cooling of the melamine prior to solidification is carried out by addition of NH3 or by means of heat exchangers.