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[Chemical Knowledge]：NDI production and manufacturing methods and processes.

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NDI (diisocyanate) is an important chemical raw material, widely used in polyurethane, coatings, adhesives and other fields. A manufacturing method of the NDI will be described below.

1. phosgenation

phosgenation is a commonly used NDI manufacturing method. The method uses aniline as raw material, generates diphenyl methane diisocyanate (MDI) through phosgenation reaction, and then obtains NDI through separation, purification and other steps. The specific steps are as follows:

1. reacting aniline with phosgene to generate aniline hydrochloride.
2. Aniline hydrochloride is reacted with excess phosgene at low temperature to form diphenylmethane diisocyanate (MDI) and hydrogen chloride gas.
3. MDI and unreacted raw materials are separated by distillation and other methods to obtain MDI with high purity.
4. MDI is cracked at high temperature to obtain NDI and methane gas.
5. High purity NDI is obtained by condensation, separation and other methods.

phosgenation production process is more mature, can obtain high purity NDI products. However, this method uses toxic and harmful raw materials such as phosgene, which causes certain harm to the environment and human health.

2. nitrification

nitrification is a new type of NDI manufacturing method. The method takes aniline as raw material, generates dinitroaniline through nitration reaction, and then obtains NDI through reduction, cyclization and other steps. The specific steps are as follows:

1. reacting aniline with mixed acid (nitric acid and sulfuric acid) to generate dinitroaniline.
2. Dinitroaniline undergoes a reduction reaction to generate diamino aniline.
3. Cyclization of diaminoaniline at elevated temperature affords NDI.
4. Purify NDI by crystallization, filtration and other methods to obtain a higher purity product.

nitrification process is relatively simple, safe operation, and does not use toxic and harmful raw materials, environmentally friendly. However, the purity of the product obtained by this method is low and further purification treatment is required.

3. other methods

In addition to the phosgenation method and the nitrification method, there are some other NDI manufacturing methods, such as electrochemical method, gas phase plasma method, etc. These methods are still in the laboratory research stage and have not yet been industrialized.

To sum up, the manufacturing methods of NDI mainly include phosgenation method and nitrification method. Among them, the phosgenation production process is more mature, can obtain high purity products, but the use of toxic and harmful raw materials, causing certain harm to the environment and human health. Nitrification production process is relatively simple, safe operation, and do not use toxic and harmful raw materials, environmentally friendly, but the product purity is low, need further purification treatment. In the future, with the continuous advancement and application of technology, there may be more environmentally friendly, efficient and economical NDI manufacturing methods.

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