GB583504A - Melamine - Google Patents

Abstract

Melamine is manufactured by heating urea (as such or formed in situ, e.g. by heating ammonium cyanate, ammonium carbamate or ammonia-carbon dioxide mixture), cyanuric acid, ammeline, ammelide or melamine-cyanuric acid condensation product, in an atmossphere consisting essentially of ammonia, under superatmospheric pressure (e.g. 200-450 atmospheres but preferably at least 350 atmospheres), at a temperature in the range 275-500 DEG C. (preferably 300-400 DEG C.). For the best results the molar ratio of ammonia to urea or its equivalent should be high, e.g. from about 4 : 1 at high pressures (600-1000 atmospheres) to 20 : 1 at low pressures (below 350 atmospheres). The process may be carried out in any pressure resistant reaction vessel made of or lined with material which is not excessively corroded by the reaction mixture and which preferably does not catalyse ammonia decomposition, e.g. silver, stainless steel or glass. A convenient apparatus is a silver-lined bubble-plate column into which molten urea is pumped at the top and ammonia injected at the bottom, the non-gaseous reaction products being withdrawn through a hot let-down valve whilst the gaseous products pass out of the top of the column. As alternatives to countercurrent operation, co-current and batch processes are also suitable. The product may be purified by crystallization from a solvent, especially from water, in which by-products such as ammeline, ammelide and cyanuric acid salts, e.g. melamine cyanurate, are insoluble and may thus be separated and recycled. In examples: (1) a mixture of urea and ammonia is passed into the top of one arm of a silver-lined electrically heated U-tube converter at 400 DEG C. and 400 atmospheres pressure; (2) the molar ratio of ammonia to urea and the throughput in (1) are reduced; (3) a mixture of urea and ammonia is heated in a silver-lined bomb at 300 DEG C. and 405 atmospheric pressure; (4) the molar ratio of ammonia to urea in (3) is reduced but the temperature and pressure are raised to 325 DEG C. and 500 atmospheres; (5) an insoluble by-product of (4) is added to the reaction mixture in (3).