GB719662A - Process for the manufacture of melamine - Google Patents

Abstract

Melamine (together with ammonium sulphate) is manufactured by reacting at a temperature above about 280 DEG C. (preferably about 280-390 DEG C.), in the presence of added ammonia and under a pressure of ammonia above about 500 lbs./sq. in. (preferably about 500-6000 lbs./sq. in.), at least one compound selected from urea, biuret and cyanuric acid acid with at least one compound selected from sulphamic acid, ammonium sulphamate, diammonium imidodisulphonate and sulphamide, in the molar ratio within the range equivalent to 0.2-3 (preferably 1-1.5) mol(s) of ammonium sulphamate per mol. of urea. The reactants should preferably be substantially anhydrous. Advantageously urea is reacted with an equimolecular proportion of ammonium sulphamate at 330-330 DEG C. under a pressure of ammonia of 500-2000 lbs./sq. in. The apparatus should be protected by corrosion-resistant linings, e.g. tantalum or certain glasses or enamels. The reaction mixture may be treated to separate melamine from ammonium sulphate and minor by-products (mainly guanidine sulphamate and sulphate) and unreacted started materials (which last together with the guanidine compounds are advantageously returned to the reaction vessel). Thus, the reaction mixture may be leached with cold water, the insoluble residue leached with hot water, the hot water leach solution separated and cooled to precipitate melamine, and the mother liquor advantageously reheated and returned to the hot water leaching step. Alternatively, the reaction mixture, after evaporation of free ammonia therefrom, is leached first with strong aqueous ammonia, then with cold water, and finally with hot water to dissolve the melamine. Examples and tables (including comparative experiments) illustrate the effect of varying the molar ratio of ammonium sulphamate to urea, the pressure of ammonia, the temperature and the reaction time, of replacing urea by biuret or cyanuric acid, and of recycling by-product guanidine compounds. Specifications 598,175 and 713,223 are referred to.ALSO:Ammonium sulphate is obtained as a subsidiary product in the manufacture of melamine by reacting at a temperature above about 280 DEG C., in the presence of added ammonia and under a pressure of ammonia above about 500 lbs./sq. in. at least one compound selected from urea, biuret and cyanuric acid with at least one compound selected from sulphamic acid, ammonium sulphamate, diammonium imidodisulphonate and sulphamide in the molar ratio within the range equivalent to 0 . 2 - 3 mol(s) of ammonium sulphamate per mol of urea. The ammonium sulphate may be recovered from the cooled reaction mixture by leaching with cold water and subsequently separating the ammonium sulphate from the other materials dissolved thereby, or preferably by first evaporating free ammonia, leaching with strong aqueous ammonia to remove minor by-products and uncharged starting materials, and then leaching with cold water to dissolve the ammonium sulphate, which, after filtration, is crystallized from the filtrate. Specifications 598,175, [Group IV], and 713,223 are referred to.