Recovery of urea from off-gases from the synthesis of melamine from urea in the gas phase which have been freed from melamine

摘要

Urea is recovered from the off-gas obtained in a melamine sythesis from urea in the gas phase and in the presence of catalysts and which has been freed from melamine by fractional condensation at from 150 DEG to 200 DEG C., by bringing said off-gas into intimate contact with a melt of urea or a melt of urea and its thermal decomposition products, both of which are kept up to 50 DEG C. above their melting temperature. Thermal decomposition products which may be present in the urea melt are biuret, triuret, cyanuric acid, ammeline, ammelide, melamine or melamine cyanurate. The presence of 20-25% biuret (obtained by blowing with CO2) is preferred, since this depresses the melting point and allows a very cool melt to be used. Portions of the melt are preferably drawn off after use and passed to the melamine synthesis reactor.