Disclosed is a process for the integrated production of urea and melamine. A urea production zone produces a urea synthesis stream comprising urea, water and ammonium carbamate. This stream is subjected to processing, preferably involving stripping, so as to separate an aqueous urea stream from residual dissociated carbamate vapor comprising ammonia, carbon dioxide, and water. The urea is fed to a melamine synthesis zone and subjected to melamine forming conditions so as to form melamine and off-gas comprising carbon dioxide and ammonia. The dissociated carbamate vapor and the melamine off-gas are subjected to combined condensation so as to form a dilute melamine off-gas condensate.

Disclosed is a process for the integrated production of urea and melamine. A urea production zone produces a urea synthesis stream comprising urea, water and ammonium carbamate. This stream is subjected to processing, preferably involving stripping, so as to separate an aqueous urea stream from residual dissociated carbamate vapor comprising ammonia, carbon dioxide, and water. The urea is fed to a melamine synthesis zone and subjected to melamine forming conditions so as to form melamine and off-gas comprising carbon dioxide and ammonia. The dissociated carbamate vapor and the melamine off-gas are subjected to combined condensation so as to form a dilute melamine off-gas condensate.

In an integrated process for urea and melamine production, urea is produced in a urea plant (10) comprising a high pressure urea synthesis section (11) from which an aqueous solution comprising urea, ammonium carbamate and ammonia is obtained and a urea recovery section (21) operating at low pressure, and melamine is produced in a melamine plant (40) wherein off-gases resulting as by-products of the melamine synthesis are discharged from said plant at a medium pressure and recycled to the high-pressure urea synthesis section (11).

A process for synthesis of urea from ammonia and carbon dioxide comprising the synthesis of urea in parallel in a first urea reactor (1) at a first urea synthesis pressure and in a second urea reactor (2) at a second and lower urea synthesis pressure; a stripping step of the reaction effluent of the first reactor, which is performed in a stripper (4) operating at a stripping pressure lower than the first urea synthesis pressure; the reaction effluent (21) of the second reactor (2) and the stripper liquid effluent (11) are sent to a recovery section (13) where a carbamate-containing recycle solution (17) is produced, and said recycle solution (17) is sent partly to said first reactor and partly to said second reactor.

A process for synthesis of urea from ammonia and carbon dioxide comprising the synthesis of urea in parallel in a first urea reactor (1) at a first urea synthesis pressure and in a second urea reactor (2) at a second and lower urea synthesis pressure; a stripping step of the reaction effluent of the first reactor, which is performed in a stripper (4) operating at a stripping pressure lower than the first urea synthesis pressure; the reaction effluent (21) of the second reactor (2) and the stripper liquid effluent (11) are sent to a recovery section (13) where a carbamate-containing recycle solution (17) is produced, and said recycle solution (17) is sent partly to said first reactor and partly to said second reactor.

A process for the purification of a melamine melt (5) containing melamine and by-products, comprising the steps of: (a1) In quenching of said melamine melt; (a2) decomposition of by-products with alkali, providing an alkaline aqueous solution of melamine (26); (b) stripping of said alkaline aqueous solution of melamine (26); (c) crystallization of melamine with a first alkali-containing solution (6b) and separation of solid melamine (7) from a mother liquor (8); (d) treatment of said mother liquor, providing a waste water stream (11) containing carbonates; (e) decomposition of at least part of the carbonates contained in said waste water stream (11) into carbon dioxide and alkali, providing a second alkali-containing aqueous solution (30); (f) recycle of at least part of said alkali-containing aqueous solution (30) to at least one of said steps (a1), (a2) and (c).

A process for the purification of a melamine melt (5) containing melamine and by-products, comprising the steps of: (a1) In quenching of said melamine melt; (a2) decomposition of by-products with alkali, providing an alkaline aqueous solution of melamine (26); (b) stripping of said alkaline aqueous solution of melamine (26); (c) crystallization of melamine with a first alkali-containing solution (6b) and separation of solid melamine (7) from a mother liquor (8); (d) treatment of said mother liquor, providing a waste water stream (11) containing carbonates; (e) decomposition of at least part of the carbonates contained in said waste water stream (11) into carbon dioxide and alkali, providing a second alkali-containing aqueous solution (30); (f) recycle of at least part of said alkali-containing aqueous solution (30) to at least one of said steps (a1), (a2) and (c).

A process for the purification of a melamine melt (5) containing melamine and by-products, comprising the steps of: (a1) In quenching of said melamine melt; (a2) decomposition of by-products with alkali, providing an alkaline aqueous solution of melamine (26); (b) stripping of said alkaline aqueous solution of melamine (26); (c) crystallization of melamine with a first alkali-containing solution (6b) and separation of solid melamine (7) from a mother liquor (8); (d) treatment of said mother liquor, providing a waste water stream (11) containing carbonates; (e) decomposition of at least part of the carbonates contained in said waste water stream (11) into carbon dioxide and alkali, providing a second alkali-containing aqueous solution (30); (f) recycle of at least part of said alkali-containing aqueous solution (30) to at least one of said steps (a1), (a2) and (c).

A process for the purification of a melamine melt (5) containing melamine and by-products, comprising the steps of: (a1) In quenching of said melamine melt; (a2) decomposition of by-products with alkali, providing an alkaline aqueous solution of melamine (26); (b) stripping of said alkaline aqueous solution of melamine (26); (c) crystallization of melamine with a first alkali-containing solution (6b) and separation of solid melamine (7) from a mother liquor (8); (d) treatment of said mother liquor, providing a waste water stream (11) containing carbonates; (e) decomposition of at least part of the carbonates contained in said waste water stream (11) into carbon dioxide and alkali, providing a second alkali-containing aqueous solution (30); (f) recycle of at least part of said alkali-containing aqueous solution (30) to at least one of said steps (a1), (a2) and (c).

A process for the purification of a melamine melt (5) containing melamine and by-products, comprising the steps of: (a1) In quenching of said melamine melt; (a2) decomposition of by-products with alkali, providing an alkaline aqueous solution of melamine (26); (b) stripping of said alkaline aqueous solution of melamine (26); (c) crystallization of melamine with a first alkali-containing solution (6b) and separation of solid melamine (7) from a mother liquor (8); (d) treatment of said mother liquor, providing a waste water stream (11) containing carbonates; (e) decomposition of at least part of the carbonates contained in said waste water stream (11) into carbon dioxide and alkali, providing a second alkali-containing aqueous solution (30); (f) recycle of at least part of said alkali-containing aqueous solution (30) to at least one of said steps (a1), (a2) and (c).