Behind the Tianjin Explosions, a Simple But Deadly Chemistry

The Tianjin warehouse blasts on August 12 caused a weak surface quake in the Chinese port city, made houses uninhabitable in a 2-km radius, absolutely decimated a parking lot of 8,000 cars in its vicinity, blew a crater below the warehouse, released a very-toxic chemical into the surrounding air and water, and consumed the lives of 114 people and counting. The structure was known to contain hazardous substances, and claims have surfaced that the company managing them – Ruihai International Logistics – was transporting obscene quantities illegally.

But beyond the illegality itself, the explosions had an underlying chemistry whose strength far outstripped the weaknesses of structures in the vicinity being susceptible to shockwaves, a strength that exacerbated the government’s failure in not clamping down on Ruihai earlier, in allowing residential settlements in the neighbourhood of such chemicals and, in the aftermath, not reacting swiftly enough to either allegations of cover-ups as well as informing the media of what had gone up in flames. According to one video (link now dead) on weibo, the Chinese microblogging website, one of the principal explosions occurred after the firefighters arrived, implying that the destruction was drawn out by the already-burning warehouse being sprayed with water by authorities in the know.

For instance, the structure was known to contain ammonium nitrate, potassium nitrate, calcium carbide, sodium cyanide, and other dangerous chemicals. Of them, ammonium nitrate and calcium carbide are known to be explosive – but for different reasons – while the rest are deadly in their own right.

Ammonium nitrate

Simply put, ammonium nitrate is the Liam Neeson of fertilisers but also the Ra’s al Ghul of explosives. It was used as the explosive material in the 2013 Hyderabad blasts. Each molecule contains two atoms of nitrogen, three of oxygen and four of hydrogen. The way the atoms are bonded, the molecule as a whole is eager to give away an oxygen atom to any other molecule reacting with it because doing so would send it into a stable, less energetic state. This eagerness is exemplified by the molecule’s low sensitivity to physical shock and heat: just a little jerk or heating will blow it up, generating a shockwave at 5,270 m/s – more than five times the speed of sound.

What could make such a shockwave even more powerful has to do with another property of ammonium nitrate. If not stored in tightly sealed containers, the compound gradually absorbs moisture from the atmosphere and coalesces into solid lumps. In the Tianjin warehouse, then, large quantities of ammonium nitrate could’ve become moist and formed proximate clumps, and when one section of those clumps got heated, it blew up and generated a shockwave that shot the rest of it to Hell as well.

Calcium carbide

By itself, calcium carbide is mostly harmless. But should you spray it with water, it reacts to produce calcium hydroxide (slaked lime) and acetylene. When acetylene is burnt in the presence of oxygen, it produces a flame of 3,600 K, hot enough to melt a metal as sturdy as tungsten – so its application in welding. And like ammonium nitrate, acetylene is susceptible to shockwaves, especially if the surrounding pressure goes beyond 103,421 pascals (almost equal to the atmospheric pressure at sea level). In such situations, it explodes into its constituent atoms – carbon and hydrogen.

And here it gets worse: hydrogen burns violently with… well, the atmosphere. Remember the reactor-3 explosion during the Fukushima disaster in 2011? That wasn’t the work of any nuclear substance as much as leaked hydrogen.

Potassium nitrate and sodium cyanide

While potassium nitrate is used in the preparation of gunpowder, it’s explosive when reacting with reducing agents – i.e. electron-donators. The incredibly poisonous sodium cyanide on the other hand isn’t considered explosive. However, an explosive derivative presents itself. The compound is hygroscopic, absorbing moisture from the atmosphere to form sodium formate and ammonia, and in sufficiently high quantities, ammonia reacts explosively with air, especially if heated up to 200° C.

Sodium cyanide is also wildly toxic, and already reports have emerged that it has been found in “nearby drains after the blasts“. It is very soluble in water, and if it enters the body in quantities as small as 3 mg/kg, it rapidly knocks out the lungs’ ability to take in oxygen and results in death. The Associated Press claims “several hundred tons” of it were present in the warehouse (according to one estimate, 700 tons, a decidedly unholy quantity within a kilometre’s radius of residential dwellings, and enough to poison 91% of all Asia to death*).

So, given the repertoire of substances at the warehouse, the crater-blowing explosions weren’t going to be unexpected in the event of an accident.

According to a BBC report, China is the world’s largest consumer of hazardous chemicals, and so can claim expertise in the storage and transport of large quantities of chemicals and no ignorance of how the warehouse came to store over 3,000 tonnes of the chemicals with the nearest houses less than a kilometre away. The chemicals themselves are frequently used in the metals industry, for the manufacture of synthetic substances, and for extracting some precious metals from their ores.

No wonder then that some Ruihai employees have been arrested, as well as a former government officials who’d served in the area called in for questioning. Beyond the difficulty of cleaning up an area made more potent by the addition of water, the bigger challenge facing the Chinese government is the chemistry itself: without a license to handle hazardous substances between October 2014 and June 2015, why did Ruihai have to amass the ammonium nitrate, calcium carbide, potassium nitrate and sodium cyanide?

(*Assuming the average body mass of an Asian adult is 57.7 kg, and with “people” referring only to Asian adults.)

Note: This article was edited for clarity on August 18, 2015, at 18.18 pm.