A spill of 5 to 800 kg of zirconium tetrachloride (ZrCl4) occurred around 1 am in the carbo-chlorination unit of a chemical site as the result of rupture of the vent pipe on a storage tank downstream of the compactors. The product mainly spread in the workshop, but some reached the outside through small vents in the roof and cladding corrugation. Between 3:45 am and 5 am, during clean-up of the ZrCl4 spilled outside, a cloud of hydrogen chloride (HCl) formed by hydrolysis. The exact same event then recurred at 9 am while collecting ZrCl4 in the workshop, leading the operator to trigger the IOP. Three employees of a neighbouring company, overcome by the fumes, were placed under observation in the infirmary. The operator issued a press release.
The process in question used two compactors supplying 3 ZrCl4 storage tanks of 150 t each via a pneumatic transport pipe under nitrogen. During the accident, both compactors were supplying the 1st storage tank loaded at 132 t, according to the weighing scale, while the other two remained empty. At 10:14 pm, the high pressure threshold (90 mbar) was exceeded repeatedly, though only briefly, at the 1st storage tank’s pressure sensor, which shut down the first compactor. At 12:50 am, the high pressure threshold for the tank was being continuously exceeded, thus indicating that the rupture disk had been broken. The HCl sensors for the building exceeded their alarm threshold (5 ppm) 1 minute later. Lastly, the 2nd compactor was only shut down at 1:50 am by staff; the ZrCl4 escaped through the rupture disk and the broken PVC vent pipe.
An analysis of the causes of this accident revealed that the instrumentation system associated with the pressure sensor had only been attached to the 1st compactor and had no effect on the 2nd. The transfer of ZrCl4 therefore continued for an hour after both the disk and pipe broke. This situation resulted from poor management of installation modifications: the compactors, initially set up to supply just one tank each, had been changed to allow supplying several tanks through a switch system, without the safety instrumentation being upgraded.
In addition, control room technicians did not intervene during repeated activation of the pressure alarms, preferring to trust the weighing scale data, which indicated a fill level of 132 t for a capacity of 150 t; the most recent metrological intervention on these scales dated from more than 6 years prior.