Melamine Powder is a trimer of cyanamide, and its chemical shape is a white powder. According to the content, the commonly used specifications are divided into melamine 99% and 99.8%. The resin made of it will release a large amount of nitrogen when heated and decomposed to be used as flame retardant, water reducer, formaldehyde cleaner and so on. It is also a metabolite of the insecticide cyromazine in animals and plants.
The primary raw materials for the production of it are dicyandiamide and urea. In 2003, there were about 90 producers globally, with a total capacity of 1 million 334 thousand and 600 tons per year, with the Chinese mainland's annual power reaching 300 thousand tons, accounting for 22% of global capacity. Europe is still the leading producer of it globally, accounting for about 34% of the total global production capacity; The United States accounts for 12%; Japan accounts for 12%; Other countries account for 20%. Changchun enterprise group among Taiwanese manufacturers also produces. Its products manufacture heat hardening resin, moulding materials, cosmetic boards, coatings, adhesives, textiles, paper, etc.
This was first synthesized by Li Bishi in 1834. In the early stage, the dicyandiamide method was used: calcium cyanamide (CaCN2) was prepared from calcium carbide (CaC2), dimerized into dicyandiamide after hydrolysis of calcium cyanamide, and then heated and decomposed to prepare it. Due to the high cost of calcium carbide, the dicyandiamide method has been eliminated. Industrial synthesis mainly uses urea as raw material. Under the conditions of heating and absolute pressure: that reacts with formaldehyde (HCHO) to synthesize resin, which will remove water. This Melamine resin is thermosetting.
According to different reaction conditions, synthesis processes can be divided into the high-pressure method, low-pressure method and atmospheric pressure method.
1、. Production of melamine by the high-pressure method
Pressurize to 9 8Mpa molten urea is sent to the compression quencher and then enters the synthesis reactor after quench: in addition, the liquid ammonia is pressurized to 9.5Mpa 8Mpa, heated to 400C in the preheater, gasified and sent to the reactor, which is heated with molten salt. The residual ammonia is cooled in the ammonia stripper and then sent to the ammonia stripper to generate ammonia. Melamine is separated from the slurry in the centrifuge, and the mother liquor is used as the absorbent of the ammonia absorption tower. After absorption, the ammonia separated in the ammonia distillation tower and stripping tower is rectified and returned under atmospheric pressure as liquid ammonia for recycling. The separated melamine is dried and made into powder in a pulverizer to obtain the refined melamine product.
2. Production of melamine by a low-pressure process
After the fertilizer grade, a urinary cord is melted in the storage tank. It is sprayed into the reactor with several nozzles. With the fluidized alumina as the catalyst, the preheated circulating ammonia is introduced into the reactor to maintain fluidization. The reaction pressure is atmospheric pressure or slightly higher than atmospheric pressure. The reactor is equipped with a heating coil to maintain the reaction temperature with molten salt as the heating medium. The injected urea evaporates and reacts to produce melamine, carbon dioxide and ammonia, with a conversion rate of 95%. The reaction gas comes out from the top of the reactor and enters the gas cooler first. The temperature after cooling is above the dew point of melamine. At this temperature, high boiling point by-products such as Miller amine and Mibai amine crystallize and precipitate, and the catalyst powder is removed through the filter. The filtered gas enters the sublimator, and the sublimation temperature is maintained at 170C ~ 200 ° C with the cooled circulating gas. 98% of melamine is precipitated by particle crystallization, while the unconverted urea remains in the gas. The melamine crystal and gas are separated by a cyclone separator, and the purity of the product is up to 99 9%, and the separation efficiency is 99%. The circulating gas from the cyclone separator enters the urea scrubber for cooling. The unrecovered solid and gas melamine and unconverted urea in the circulating gas are washed and recovered in the urea scrubber.
3. Production of melamine by atmospheric pressure method
The ammonia gas from the synthetic ammonia section, with a pressure of 0.10 ~ 0.15Mpa, is directly sent to the melamine section. After being preheated to 350 ~ 400 ℃ by steam and molten salt, it is used as the carrier gas ammonia of the fluidized bed; The gas from the liquid urea scrubber is divided into two parts, one is returned to the crystallizer as cold air, and the other is directly sent to the absorption post of ammonium bicarbonate/soda ash and ammonium chloride to produce ammonium bicarbonate/soda ash and ammonium chloride It integrates the atmospheric pressure two-step melamine co-production method and one-step melamine method. Therefore, melamine powder products have good quality, low raw material consumption, small labour intensity, large scale and low investment; The investment is saved by 40%, and the power consumption is reduced by 60% compared with the standard pressure one-step melamine method; It uses pure ammonia as a carrier gas, which can improve the efficiency of equipment and catalyst by 30%; There is no need to build a new tail gas treatment device, and the tail gas can be used economically and reasonably.