GB628255A - Process for the production of melamine - Google Patents

Abstract

Melamine is produced by heating to 275-550 DEG C., preferably 385-450 DEG C. in the absence of added ammonia, and at a pressure of 200-1000 atmos., urea, ammeline, ammelide, cyanuric acid or the melamine-cyanuric reaction product present in the by-product of heating urea under pressure, or urea-forming substances, e.g. ammonium cyanate or carbamate. Diluent gases such as carbon dioxide, nitrogen or nitrogen-hydrogen may be present. Molten urea may be conducted through a tube preferably at about 500 atmos. and 400 DEG C. Contact times may range from 3 1/2 minutes or less to 40 minutes or more. The results of experiments at various temperatures, pressures, and contact times in the absence of added ammonia, and with addition of ammonia or carbon dioxide are given. Melamine may be isolated by injecting into the hot reaction product, water or mother liquid under high pressure, as described in Specification 628,250, and crystallizing. Reactors of the internally braced cartridge type comprising, for example, a silver cartridge where pressure is balanced by injecting a liquid between it and an exterior steel shell, may be used. The preheated urea may be passed through the reactor, the product treated with recycled mother liquor, filtered, the filtrate passed to a flash evaporator to separate ammonia, carbon dioxide, and some water which is treated first with ammonium nitrate solution, then with water to remove ammonia which is recovered, as is carbon dioxide. The liquid from the evaporator is crystallized. Stabilizers such as caffeine may be used in heating the urea as described in U.S.A. Specification 2,346,425. Pressure and temperature may be varied along the reactor. Specifications 583,504 and 598,175 also are referred to. Reference has been directed by the Comptroller to Specification 598,175.