Abstract: Process for the preparation of urea in whicha urea synthesis solution containing carbamate and free ammonia is formed in a high-pressure part in a synthesis zone at an NH.sub.3 /CO.sub.2 molar ratio of up to 4:1, a temperature of at least 175.degree. C.

Abstract: An improved process for the preparation of melamine from urea or thermal decomposition products thereof wherein ammonia, carbon dioxide, and water vapor are condensed at a pressure of between about 0.5 and 70 bar to form an aqueous solution of ammonium carbamate, and such solution is increased in pressure to at least 100 bar and heated prior to being introduced into a urea synthesis zone wherein at least a portion of the ammonium carbamate is converted into urea. The aqueous solution of ammonium carbamate is heated sufficiently to decompose a portion of the ammonium carbamate into ammonia and carbon dioxide to form a gas-liquid mixture for introduction into the urea synthesis zone such that the heat released by recondensation of the ammonia and carbon dioxide is at least sufficient to provide all of the heat required for the conversion of ammonium carbamate into urea.