Process for the preparation of melamine

In a process for the preparation of melamine from urea or thermal decomposition products thereof wherein a gaseous reaction mixture containing melamine, ammonia, and carbon dioxide is formed, and wherein

• said ammonia and carbon dioxide, after separation of melamine therefrom, and water vapor are condensed in a condensing zone maintained at a pressure of between about 0.5 and 70 bar to form an aqueous solution of ammonium carbamate, and
• said aqueous solution of ammonium carbamate is increased in pressure to at least about 100 bar, heated and introduced into a urea synthesis zone wherein at least a portion of said ammonium carbamate is converted into urea at a pressure of between about 100 and 200 bar to form a urea synthesis effluent containing urea, unconverted ammonium carbamate and water,
• the improvement essentially comprising
• heating said aqueous solution of ammonium carbamate from said condensing zone, after being increased in pressure to at least about the pressure in said urea synthesis zone, sufficiently to decompose at least a portion of said ammonium carbamate into carbon dioxide and ammonia, to form a gas-liquid mixture, and
• introducing said gas liquid mixture into said urea synthesis zone in which said ammonia and carbon dioxide are recondensed to form ammonium carbamate, and ammonium carbamate is converted into urea,
• wherein the amount of ammonium carbamate decomposed in said aqueous solution and introduced into said urea synthesis zone is such that the heat released by said recondensation of ammonia and carbon dioxide to ammonium carbamate is at least sufficient to provide all of the heat required for said conversion of ammonium carbamate into urea.