The nature, harm and prevention of nitrogen trichloride (NCl3)

In the chlor-alkali production liquid chlorine system, there is a danger of potential nitrogen trichloride explosion, which causes great accidents in chlor-alkali production. There have been cases of nitrogen trichloride explosions in several domestic chlor-alkali production plants, causing casualties. In particular, the explosion of the Chongqing Tianyuan Chemical Plant on April 16, 2004 caused 9 deaths, 3 injuries, and 150,000 people were evacuated, causing heavy losses to the country and the masses.

There was no sign of the explosion of nitrogen trichloride beforehand, and it all happened suddenly. A loud bang, sometimes accompanied by a flash, is highly destructive. Nitrogen trichloride is very powerful, and its destructive power is determined by the amount of nitrogen trichloride. The location of the explosion can occur in any part of the accumulation of nitrogen trichloride, such as pipes, sewage tanks, gasifiers, cylinders, etc.

The explosion of nitrogen trichloride is determined by its physical and chemical properties. The molecular weight of NCl ₃ is 120.5, which is a yellow viscous oily liquid at room temperature, the density is 1.653 g/cm ³ , solidified below -27 ° C, the boiling point is 7 l ° C, and the spontaneous combustion explosion point is 95 ° C. When the volume ratio reaches 5 to 6%, it can spontaneously ignite at 90 °C. In the industry, nitrogen trichloride can be extracted into carbon tetrachloride for storage as a detonating agent and a chemical reagent. Pure nitrogen trichloride can be exploded when heated above the boiling point or in contact with or attacked with organic substances such as rubber or oil, and decomposes into Cl ₂ and N ₂ , and decomposes into ammonia and hypochlorous acid in the presence of water. If it is more likely to cause explosion under the influence of light or collision, the temperature can reach 2128 °C and the pressure is up to 531.6Mpa when the volume is constant. The explosion temperature in the air can reach 1698 °C. The explosion equation is:

2NCl ₃ →N ₂ +3Cl ₂ +459.8kJ

Since it has a high boiling point and is easily separated from liquid chlorine, it is also easy to accumulate and concentrate. Therefore, when chlorine trichloride is present in the chlorine gas, the safety of liquid chlorine production is threatened. Nitrogen trichloride is mainly formed when ammonium ions enter the electrolysis unit and react with chlorine. In the electrolysis process, when the pH of the solution is less than 5, nitrogen trichloride is produced, and the reaction formula is:

NH ₃ +3HClO→NCl ₃ +3H ₂ O

In addition, chlorine gas may also form nitrogen trichloride in contact with water containing ammonium ions during cooling.

NH ₄ Cl+3Cl ₂ →NCl ₃ +4HCl

The danger of nitrogen trichloride in the chlor-alkali industry is so great that we should attach great importance to it and take measures to prevent the harm of nitrogen trichloride. The following aspects should be done:

First, preventing ammonium ions from entering the electrolytic cell is the cure for the harm of nitrogen trichloride. The sources and pathways of ammonium ions are mainly:

(1) The original salt contains ammonium.

(2) The chlor-alkali plant salt contains ammonia and ammonium salts.

(3) It is produced by directly cooling chlorine gas with water containing ammonia and ammonium salts.

From the above situation, it is crucial to turn off the good salt water. The total ammonium content in the controlled brine should be less than 1 mg/L (factory standard), and if it is higher than it, the total ammonium content of the raw salt and the salt should be checked immediately.

Second, strict monitoring indicators of nitrogen trichloride, the content of nitrogen trichloride in gas chlorine and liquid chlorine should be less than 50ppm. If you find that the standard is exceeded, you should do the following:

1. Switch to other water sources and replace the original salt. Increase monitoring efforts to verify and ensure the effects of changing water sources and raw salt.

2. The presence of nitrogen trichloride in liquid chlorine is not easy to explode. It can be discharged with liquid chlorine during the sewage discharge, and then treated with alkali neutralization, and the number of sewage discharges is increased to reduce the accumulation of nitrogen trichloride. When the sewage tank is drained to recover residual chlorine, it is a process of concentrating nitrogen trichloride, which is a very dangerous process. Because the boiling point of liquid chlorine is -34.05 ° C (standard pressure), the boiling point of nitrogen trichloride is 71 ° C (standard pressure), when the spray is cooled, nitrogen trichloride will first accumulate to reach the explosion limit. Therefore, when the nitrogen trichloride exceeds the standard, the water spray recovery should be stopped, and the residual liquid is directly discharged into the lime milk or the alkali liquid for neutralization.

3. In the actual situation of domestic chlor-alkali production enterprises, there are many other methods for treating ammonium in the brine process, such as ammonium removal by liquid phase oxidation and ammonium removal by analytical method.

In addition, the inclusion of ammonium ions in liquid chlorine chilled brine is also a major accident. In the process of liquid chlorine production, due to corrosion and perforation of the chlorine cooler, a large amount of ammonium-containing brine directly enters the liquid chlorine system, generating extremely harmful nitrogen trichloride explosives, which will explode when accumulated to the explosive concentration. This is the direct cause of the "4·16" explosion accident in Chongqing Tianyuan. Some domestic chlor-alkali production facilities have replaced liquid chlorine refrigerants with lithium bromide or Freon, eliminating the formation of such causes.

At present, there is no relevant technical data on the mechanism of explosion and explosion of nitrogen trichloride in China. The relevant safety technical standards for how to avoid the explosion of nitrogen trichloride are not perfect. Therefore, it is necessary to explore technically and form a set of safety as soon as possible. Mature and reliable emergency plan for the prevention and treatment of nitrogen trichloride. As the company's production and dispatching department, it should actively cooperate with the chlor-alkali production unit to control the production of nitrogen trichloride from the source. It is necessary to pay attention to the total ammonium content of the brine and the presence of ammonium pollution in the company's water to eliminate trichlorine. The occurrence of a nitrogen explosion accident.

(From: Haichuan Chemical Forum)