At approximately 8 a.m., accidental mixing of sodium hypochlorite and sulphuric acid (H2SO4) resulted in the release of a toxic cloud into a distillery. At 7:35 a.m., a lorry had arrived at the factory to deliver a shipment of H2SO4 (30%). The driver went to the control room to have the night shift technician sign the delivery documents as the day-shift technician had not yet arrived for his shift. At 7:44 a.m., the technician accompanied the driver to the unloading area where he donned his protective clothing. The technician unlocked the barrier in front of the transfer equipment and then removed the lock from the dust cap on the H2SO4 filling line. The driver retrieved the transfer hose at the back of the tanker in order to connect it to the filling line, while the technician returned to his workstation. He connected the hose, checked for leaks, and then opened the valves. A yellowish cloud, coming from the sodium hypochlorite tank, formed shortly before 8 a.m. It covered the entire zone, then migrated off the site. The three technicians in the control room (the night-shift and day-shift technicians and a trainee) quickly became aware of the release owing to the smell which had invaded the building. They tried to retrieve their breathing masks but they were inaccessible. The technicians exited the control room, without protection, passed through the cloud, took up shelter outside and then raised the alarm. The driver, from inside the cab of his lorry, noticed the cloud in his rear-view mirror. He attempted to reach the connection area but was engulfed by the cloud. The manager of the treatment plant then saw the cloud and called out to the driver who managed to join him.
The sodium hypochlorite filling line was unlocked and accessible to the driver. He had inadvertently connected the H2SO4 hose to the hypochlorite tank’s fill line, leading to the accidental mixture of approximately 15,000 litres of H2SO4 with 22,000 litres of sodium hypochlorite. The reaction of these two products resulted in the release of chlorine and other compounds.
The cloud reached heights of over 100 m and migrated slowly toward a zone of heavy traffic. Traffic was halted near the plant. Upon their arrival at the scene at 8:05 a.m., the emergency services requested that the city’s 11,000 inhabitants remain confined for at least 2 hours. Schools were evacuated, 67 persons including 5 rescue personnel, 4 plant employees and the driver were intoxicated by the cloud and treated in hospital. Certain individuals remained in the hospital for several days with respiratory problems associated with the exposure to chlorine.
Analysis of the accident showed that the technicians had not correctly followed the chemical unloading procedures defined by the operator due to the driver arriving at the time of the shift change. In particular, no technician was there when the connection was made. The driver found himself in front of two unlocked lines that were poorly identified. The pipes and fittings were not clearly marked, the fittings were the same size and two lines were located close to one another (45 cm apart).
The chlorine gas entered through the control room’s air intakes, highlighting a flaw in the design and location of the ventilation system. The technicians were unable to access the breathing apparatus as they were kept in locked lockers. No automated or remote-controlled valves were provided. The driver was in the cab of his lorry instead of monitoring the operation. He did not stop the transfer operation or actuate the lorry’s emergency stop, nor did he have breathing apparatus.
The facility operator took the following immediate measures:
- implementation of padlocks with distinct wrenches on the fill line plugs, themselves replaced by plugs with different locking systems;
- placement of colour-coded labels on the fill lines;
- updating of chemical product unloading procedures.
In addition, the operator worked with the transporter to install new fittings. The fittings have the same size and shape for each hose/line pair, identified by a single colour, such that only the appropriate hose can be connected to the H2SO4 line.
Other measures taken:
- a minimum distance is set between each filing line and installation of a secure cage around each line;
- access to the secure loading area (card reader) ;
- addition of monitoring, emergency stop and alarm equipment;
- design change of the control room and the ventilation system;
- unloading prohibited in a 20 min. period around the shift change time;
- an observer must be present during operations;
- regular inspection of the lines (caps blocked, locked, etc.) and verification of a checklist before unloading operations begin;
- breathing masks are to be stored in a cabinet accessible in the control room.