At 11:30 a.m., nitrogen dioxide was released from the stack of a shop of a Seveso plant located at a chemical park. The operator immediately halted the chemical reaction, placed the plant in a safe state and gave the alert. The park’s firefighters secured and cordoned off the area. At 11:35 a.m., the park’s security office reported a vividly coloured gas exiting from the stack and activated the gas alert. The operator purged the reactor. This influx of water halted the reaction and stopped the release of any more gas. The release was declared ended at 11:45 a.m. The gas alert was lifted at 12:30 p.m.

The operator estimated that less than 2 kg of nitrogen dioxide was released from the 20-m-high stack.

The release was caused by a pilot test of recycling abrasives by means of nitric acid oxidation. The reaction produces gaseous nitrogen dioxide (NO2), which is washed with sodium hydroxide in a scrubber. During the test, the temperature and pressure inside the reactor rose (+4 °C and +23 mbar, respectively) at around 11:15 a.m. This led to a runaway reaction and, consequently, overproduction of nitrogen dioxide. As the scrubber was not designed to handle such an influx of gas, the nitrogen dioxide was released to atmosphere. The reactor temperature rose due to a build-up of gas inside it. The discharge rate of this gas was reduced by a liquid buffer (condensation) at a cold point on the gas discharge line leading to the scrubber.

The pilot test will have to be improved before it may be resumed. The operator implemented the following actions:

• the temperature measuring point was moved to obtain faster response times;
• a NOx analyser was fitted on the stack’s outlet with the parameters being monitored in the control room;
• a condenser was fitted on the reactor outlet to dry the vapours and the gas pipe leading to the scrubber was lagged to eliminate the risk of formation of liquid buffers;
• the gas scrubber was resized.