GB735356A - Production of melamine - Google Patents

Abstract

Melamine is prepared by a continuous process by continuously comingling molten melamine and urea or other melamine-forming substance in the liquid state in a reaction zone including ammonia at a temperature of at least 370 DEG C. and an absolute pressure of at least 700 lb. per square inch, whereby the resulting reaction mixture contains reaction products of the melamine forming substance together with molten melamine and is in the liquid state, and the said reaction products are converted to molten melamine, periodically or continuously removing molten melamine from the reaction zone in amounts sufficient to maintain the volume of molten melamine within a desired range and continuously or intermittently removing gases from the reaction zone so as to maintain said pressure, the rate of comingling said melamine forming substance with molten melamine being such that said substance or the intermediate products obtained therefrom at the temperature employed are dispersed in the molten melamine and remain substantially dispersed therein during the time required to convert said substance to molten melamine. The temperature of the reaction zone may be maintained by continuously withdrawing molten melamine from the reaction zone, continuously removing a small proportion of the melamine thus withdrawn, heating the remaining molten melamine to a temperature above 370 DEG C. and recycling this heated molten melamine to the reaction zone. When melamine is formed from urea, it is preferably to introduce urea at such a rate that the amount thereof, together with intermediate products derived therefrom, does not exceed 20 per cent by weight of the reaction mixture-preferably not more than 10 per cent. The process may be carried out at a temperature of 550 DEG or 600 DEG C. using urea as the reactant, a preferred temperature range being between 400 DEG and 550 DEG C. Melamine forming catalysts mentioned are strong mineral acids such as sulphuric, phosphoric (including meta- and pyro-phosphoric) and hydrochloric acids and inorganic and organic salts and anhydrides of such acids particularly ammonium and guanidine salts. The preferred catalysts are the ammonium salt of phosphoric acid and phosphoric acid anhydrides. A suitable quantity of catalyst is from 0.001 to 5 per cent, preferably 0.001 to 2 per cent, by weight based on the total weight of the reaction mass. Suitable melamine forming substances are urea, dicyandiamide, cyanamide, guanylurea, cyanourea, biuret, guanidine, cyanuric acid, ammelide and ammeline. The process is desirably carried out in a vertical or vertically inclined reaction tube or autoclave which is closed at the top and bottom. In the examples melamine is formed from urea with and without diammonium hydrogen phosphate as the melamine forming catalyst. Specifications 598,175 and 601,967 are referred to.