Production methods of melamine

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Melamine resin, with the chemical name melamine and the English name melamine, is referred to as Meirunai in Chinese. It has the advantages of being non-toxic and odorless, as well as resistant to impacts, corrosion, high temperatures, and low temperatures. It has a compact structure, high hardness, is not prone to breaking, and boasts excellent durability. The production methods of melamine are classified by raw materials into the dicyandiamide method (now obsolete) and the urea method. The chemical reaction for producing melamine through the pyrolysis of urea is as follows: under heating and certain pressure conditions, 6 mol of urea yield 1 mol of melamine, while 3 mol of carbon dioxide and 6 mol of ammonia are produced as by-products. The reaction equation is: 6CO(NH2)2 -> C3N6H6 + 6NH3 + 3CO2. The urea process can be divided into dry, wet, and semi-dry methods based on the manufacturing technique used ； Depending on the pressure during the pyrolysis of molten urea, the process routes for producing melamine via the urea method are divided into three types: high-pressure method (7–10 MPa), medium-pressure method (0.5–1 MPa), and atmospheric-pressure method (below 0.3 MPa). Among them, representative high-pressure process technologies include the ACC process from the United States, the ETCE process from Italy, and the NISSAN process from Japan ； Representative low-pressure process technologies include the DSM process from the Netherlands and the OSW process from Austria ； The atmospheric pressure methods include Germany’s BASF process and a modified low-pressure process developed in China. As demand for melamine products grows in both domestic and international markets, it will inevitably drive the development of melamine production technologies. Currently, the development of various melamine production methods is focused on achieving larger scales, lower energy consumption, and reduced environmental pollution. Significant progress has also been made in melamine technology in China. Amidst fierce competition, melamine production technologies are set to develop rapidly. As a cleaning chemical ingredient at present, melamine is closely related to people’s lives, and it holds great market potential. Various technology development companies are actively working to improve their technologies, expand production scales, reduce product costs, and increase their market share. The following describes the characteristics of each process technology, as well as its current status and development. 　　(1) High-pressure method The production of melamine via the high-pressure method is a liquid-phase reaction, with a temperature range of 370–450°C. Its characteristics are: ① The process is a liquid-phase reaction, and it is not prone to crystallization-induced blockages ； However, under high temperature and pressure, the reaction medium is highly corrosive, requiring higher-grade materials for the equipment, a complex control system, and a large initial investment. ②The reaction does not require a catalyst, eliminating concerns regarding catalyst poisoning and product contamination; it operates stably and yields high-quality products. ③The device has high operational flexibility, and it takes a short time to produce qualified finished products on each startup. ④It features high operating pressure, large scale, low energy consumption, and low operating costs. ⑤The by-product exhaust gas, due to its high pressure, is easy to utilize, facilitating the co-production of urea and reducing product costs. Representative technologies include ETCE from the Italian company OTECH and NEW NISSON from Japan. 　　(a) Italian ETCE process The ETCE process involves concentrating the urea solution supplied from the urea production unit to obtain molten urea at 145°C; this molten urea is then pressurized to 8.5 MPa and mixed with ammonia at 8.5 MPa and 420°C before being fed into a melamine reactor. The reaction takes place at a pressure of 8.0 MPa and a temperature of 380°C, during which urea is directly converted into melamine. The liquid mixture containing carbon dioxide, ammonia, melamine, and a small amount of polycondensates exits the reactor and is depressurized to 2.5 MPa before entering a quenching section. In the quenching tower, most of the ammonia and carbon dioxide are vaporized; they are sent out as methanamide for further processing. The melamine solution coming out of the bottom of the quenching tower is then sent to a stripping tower where any remaining ammonia and carbon dioxide are completely removed. After that, the mixture undergoes polycondensate decomposition, removal of solid impurities, adsorption and decolorization processes, followed by crystallization, centrifugal separation, and drying to yield melamine product, with a yield of 85%–90% ； The mother liquor resulting from centrifugal separation undergoes ammonia recovery and wastewater treatment, so as to recycle the ammonia and process water and achieve the goal of reuse. 　　The maximum production capacity of the ETCE process technology is 30,000 t/year, and the product quality meets the top-grade standards of European regulations. In recent years, this technology has been continuously improved, mainly in the following areas: ① Reaction section: New types of reactors have been developed to enable gas phase separation within the reactor, thereby shortening the process flow ； Secondly, apply pre-converter technology to further expand production scale, thereby reducing product costs. ②Recovery section: Utilizes ultrafiltration membrane technology to remove OAT from the process circulating water. ③Wastewater system: Utilizes thermal decomposition to hydrolyze solid substances in wastewater into ammonia, while reusing carbon dioxide in order to reduce wastewater and solid waste emissions