Behind the Lebanon explosion, the Tianjin Port explosion in 2015, and the Xiangshui explosion on March 21, 2019, there are nitro substances. This article will uncover the veil of ammonium nitrate from the aspects of chemical properties, explosion mechanism, and thermal stability discovery.
1. The chemical structure of Ammonium Nitrate
The molecular weight of ammonium nitrate at 20 °C is 80.052 g/mol and the density is 1.725 g/cm3
2. Classification of ammonium nitrate
According to ADR2019 or JT617-2018, ammonium nitrate can be classified into Class 1 explosives, Class 5.1 oxidants or Class 9 other dangerous goods according to its concentration. In the classification test of ammonium nitrate, scientific classification is carried out according to the UN test classification method. The following table explains the classification results of different concentrations of ammonium nitrate. The level of concentration determines in turn whether it is category 1, category 5.1 or category 9 dangerous goods.
3. Factors affecting the thermal stability of ammonium nitrate
Ammonium nitrate is one of the fertilizers widely used in industry and the most concentrated form of nitrogen fertilizer. However, ammonium nitrate is often associated with continuous fire and explosion hazards in the past. Ammonium nitrate is not a flammable or combustible material under ambient temperature and pressure, but it is a strong oxidant, which will explode under certain conditions; the thermal stability of ammonium nitrate is related to explosion, and additives mainly affect its thermal stability , Confined space, heating rate, temperature, heating history, sample size, reaction thermodynamics, reaction kinetics, and the role of water as a chemical substance; thermal stability studies have found that ammonium nitrate is stable at a temperature of about 200°C. Sodium sulfate is a good inhibitor for the decomposition of ammonium nitrate, because its presence can reduce the reductive decomposition of nitric acid, while potassium chloride is a promoter because it can enhance the escape reaction (runaway reaction). Even if the inhibitor is mixed with ammonium nitrate, it must be stored separately from the accelerator.
Table 1. Matrix table of different additives of ammonium nitrate (Inhibitor-inhibitor; Promoter-promoter)
The physical and chemical interference of water may lead to related fire scenarios; caution should be exercised when choosing water treatment related fires. There is evidence that insufficient water volume may exacerbate the consequences of fires, because a small amount of water is vaporized by high temperature, which intensifies combustion. Therefore, it is necessary to calculate the amount of water used for fire fighting in advance to ensure sufficient water to extinguish the fire. In addition, avoid storing ammonium nitrate in hot or confined spaces , And limit the size of ammonium nitrate stacks to prevent heat storage and heat accumulation from causing spontaneous combustion and explosion.
4. Statistics of ammonium nitrate explosion accidents at home and abroad
5. The combustion and explosion mechanism of ammonium nitrate
The academic circles have different views on the combustion and explosion mechanism of ammonium nitrate. The first reaction mechanism generally accepted is as follows:
A At 170°C, the melted ammonium nitrate begins to undergo an endothermic and reversible reaction:
(2) NH4NO3 ⇌ HNO3 + NH3 ∆H=176 kJ·mol-1
B Between 170°C and 280°C, the following irreversible exothermic reaction occurs
(3) NH4NO3 → N2O ＋ 2H2O ∆H = -59 kJ·mol-1
(4) NH4NO3 → 1/2N2 + NO + 2H2O ∆H = -257 kJ·mol-1
(5) NH4NO3 → 3/4N2 + 1/2NO2 + 2H2O ∆H = -944 kJ·mol-1
C ammonium nitrate material is suddenly heated, such as at 400°C, a violent explosive decomposition reaction occurs:
(6) 2NH4NO3 → 2N2 ＋ O2 ＋ 4H2O ∆H= -1057 kJ·mol-1
(7) 8NH4NO3 →5N2 + 4NO + 2NO2 + 16H2O ∆H = -600 kJ·mol-1
The second recognized reaction mechanism is as follows. The generally recognized decomposition mechanism is dissociation. HNO3 leads to the subsequent oxidation reaction of NH3; equation (8) is the dissociation reaction of nitric acid to form NO2+, and the reaction equation (9) lists the oxidation of NH3 And produce N2O and water
(8) 2HNO3 ⇌ NO2+ + NO3– + H2O
(9) NH3 + NO2+ = products (N2O, H2O)
(10) 2HNO3 ⇌ 2NO2 + H2O + ½O2
In order to explain the reaction equations (8) and (9) in more detail, “acid” is used to represent NH4+, H3O+ or HNO3 in the presence of water, and the decomposition mechanism equations (11) to (13) are explained below. Consider the reaction equation (12) due to the slow reaction speed, which is considered as a control step;
(11) HNO3 + acid ⇌ H2ONO2+ → NO2+ + H2O
(12) NO2+ + NH3 → [NH3NO2+] *
(13) [NH3NO2+] → NO2 + H3O+ → NO2 + H2O
The reaction equation (12) can also be described by the basic reaction. In the temperature range of 342-387°C, NO2+ is subsequently oxidized to NH3, as listed in the reaction equations (14) to (19). Reaction equation (20) is the overall stoichiometric equation derived from this theory
(14) NH3 + NO2 → NH2 + HNO2
(15) NH2 + NO2 → NH + HNO2
(16) NH + NO2 → HNO + NO
(17) NH2 + NO → N2 + H2O
(18) 2HNO → N2O + H2O
(19) 2HNO2 → NO2 + H2O + NO
(20) 4NH3 + 5NO2 → N2O + 2N2 + 6H2O + 3NO
The third hypothetical method is slightly different from the mechanism mentioned above. The other method assumes the formation of ammonium nitrate intermediates and the decomposition of ammonium nitrate, as listed in the reaction equations (21)~(25)
(21) NH4+ + NO3– = NH3 + HONO2
(22) HONO2 → HO + NO2
(23) HO + NH3 → HOH + NH2
(24) NH2 + NO2 → NH2NO2
(25) NH2NO2 → N2O + H2O
6. The thermal stability experiment of ammonium nitrate found
Figure 41: The relationship between self-heating rate and temperature. The heating rate increases gently at around 200°C and rises linearly above 200°C.
Figure 41: The relationship between self-heating rate and pressure. The heating rate increases gently at around 200°C, and the pressure rate rises linearly above 200°C. The experimental results show that when the heating rate is faster, the self-heating rate and pressure rise rate increase, and the reaction becomes more intense.
Figure 46 Decomposition of ammonium nitrate under different heating rates (relationship between temperature and time)
Figure 47 Decomposition of ammonium nitrate under different heating rates (relationship between pressure and time)
On the graph, the pre-heated ammonium nitrate needs a shorter decomposition time, and it takes less time to reach the same temperature; the reason is that when the ammonium nitrate is suddenly heated, the heating process is regarded as continuous, and the waiting and searching time under adiabatic conditions Greatly shortened and increased the self-accelerating ammonium nitrate reaction, decomposition occurs faster, so if the temperature rises suddenly, decomposition is more likely to occur.
7. Characteristics of nitrogen oxides
Nitrogen oxide refers to a compound composed of only two elements: nitrogen and oxygen. Common nitrogen oxides are nitric oxide (NO, colorless), nitrogen dioxide (NO2, reddish brown), nitrous oxide (N2O), nitrous pentoxide (N2O5), etc., except for nitrous oxide under normal conditions Except for solid, other nitrogen oxides are gaseous under normal conditions. As air pollutants, nitrogen oxides (NOx) often refer to NO and NO2. Except for nitrogen dioxide, other nitrogen oxides are extremely unstable. When exposed to light, humidity or heat, they become nitrogen dioxide and nitrogen monoxide, and nitrogen monoxide becomes nitrogen dioxide again. Therefore, the occupational environment is exposed to several gas mixtures, often called smoke (gas), mainly nitrogen monoxide and nitrogen dioxide, and nitrogen dioxide is the main component. Nitrogen oxides have varying degrees of toxicity.
There are also nitric oxide dimer (N₂O₂), nitrosyl azide (N₄O), and nitrogen trioxide (NO₃), but mainly NO and NO2, which are common air pollutants. In addition, trinitroamine (N(NO2)3) is also a compound composed only of nitrogen and oxygen, but it is not an oxide in the strict sense.
Except for dinitrogen pentoxide which is solid, the rest are gases. The molecular formula is NOx. Among them, dinitrogen tetroxide is a dimer of nitrogen dioxide, which is often mixed with nitrogen dioxide to form an equilibrium mixture. A mixture of nitric oxide and nitrogen dioxide, also known as nitrate (smoke). Relative density: Nitrogen monoxide is close to air, and nitrous oxide and nitrogen dioxide are slightly heavier than air. Melting point: Dinitrogen pentoxide is 30℃, the rest are below zero. All are slightly soluble in water, and the aqueous solution is acidic in varying degrees. Nitrogen oxides are non-flammable substances, but they can all support combustion. For example, nitrous oxide (N2O), nitrogen dioxide and nitrous oxide can explode when exposed to high temperatures or combustible substances.
Nitric oxide is a nitrogen oxide compound, chemical formula NO, relative molecular weight 30.01, and nitrogen valence +2. It is a colorless, odorless, toxic gas that is difficult to dissolve in water. Since nitric oxide contains free radicals, this makes its chemical properties very active. When it reacts with oxygen, it can form a corrosive gas-nitrogen dioxide (NO2), which can react with water to form nitric acid. The equation is: 3NO2+H2O==2HNO3+NO. Dangerous characteristics: strong oxidizing. Contact with flammable and organic matter is easy to catch fire. Encountered explosive combination of hydrogen. Contact with air will emit a brownish-yellow mist with acidic oxidizing properties.
Nitrous oxide (nitrous oxide), chemical formula N2O. Also known as laughing gas, a colorless and sweet gas, it is an oxidant that can support combustion under certain conditions (same as oxygen, because laughing gas can be decomposed into nitrogen and oxygen at high temperatures), but it is stable at room temperature and slightly It has an anesthetic effect and can cause laughter. Its anesthesia was discovered in 1799 by the British chemist Humphrey David. Relevant theories believe that N2O and CO2 molecules have similar structures (including electronic formulas), so their spatial configuration is linear, and N2O is a polar molecule. Now it is mainly used in performances, and can also be used as a combustion aid in racing accelerators.
Nitrogen dioxide is a brown-red toxic gas at high temperatures. At room temperature (0~21.5℃), nitrogen dioxide and dinitrogen tetroxide are mixed and coexist. Toxic and irritating. Dissolved in concentrated nitric acid to generate fuming nitric acid. Can be stacked to synthesize dinitrogen tetroxide. Reacts with water to generate nitric acid and nitric oxide. Reacts with alkali to generate nitrate. It can react violently with many organic compounds. Nitrogen dioxide plays an important role in the formation of ozone. Man-made nitrogen dioxide mainly comes from the release of high-temperature combustion processes, such as motor vehicle exhaust and boiler exhaust. Nitrogen dioxide is also one of the causes of acid rain, and the environmental effects brought by it are diverse, including: impact on wetland and terrestrial plant species competition and composition changes, reduced atmospheric visibility, acidification and eutrophication of surface water (Due to the lack of oxygen due to the proliferation of algae, which are rich in nutrients such as nitrogen and phosphorus), and increase the content of toxins harmful to fish and other aquatic organisms in the water.
8. Storage requirements for ammonium nitrate
1) The height of the storage building should not exceed one storey, and it must have sufficient ventilation. In the event of a fire, it should be able to automatically ventilate or be able to fully ventilate
2) The warehouse can be automatically ventilated or sufficiently ventilated in the event of a fire,
3) Due to corrosiveness and reactive characteristics of ammonium nitrate, to avoid pollution, buildings and structures should be dry, and roofs, walls and floors should not seep
4) Building materials and containers meet the separation distance requirements
5) For oxidants, it is best if a semi-underground storage room that can be designed to avoid light and heat
6) Taking into account the impact of water on ammonium nitrate, the fire extinguishing medium in the daily storage area is equipped with yellow sand
Ammonium nitrate, a chemical fertilizer used in agriculture, is not an explosive itself, it is a Class 5.1 strong oxidant or a Class 9 dangerous goods. The main reason for the explosion is that the nitro NO3 itself is very unstable. It is easy to be heated and split under high temperature collisions, resulting in unstable nitrogen oxidation. Finally, nitrogen oxides split to produce stable nitrogen and oxygen. Oxygen itself helps combustion and accelerates combustion when a large amount of oxygen is released. Therefore, all kinds of nitro explosions we experience are related to unstable nitro. Faced with the storage and control of such chemical substances, the following measures should be taken:
1) Ventilation, moisture-proof and heat-proof
2) Control the number of stacks and strictly implement the principle of taboo mixing
3) Avoid all kinds of ignition sources (open flames such as cutting and polishing, static sparks, hot work, explosion-proof tools and explosion-proof electrical appliances)
4) Control the entry and exit of irrelevant personnel
5) Control the total amount of storage, ensure first-in-first-out, and ensure that the longest storage period is less than the theoretically calculated safety period
6) Load and unload goods in strict accordance with operating procedures, and barbarous stacking and loading and unloading are strictly prohibited
7) Do a good job in the collection and management of damaged packages, and strictly prohibit sampling in the warehouse
8) Equipped with necessary fire-fighting facilities in accordance with the design specifications of Class A warehouses, and test and check that the fire-fighting facilities are always available
9) Train employees in their daily operations and emergency response capabilities, and conduct regular emergency drills
10) Adhere to the daily inspection system to ensure that problems are rectified immediately without leaving hidden dangers
Source from MOLBASE