When observing yellow smoke emanating from the chimney of a drying unit located downstream of the fertiliser plant granulator, an employee notified a control room technician; the workshop extraction fan was turned off at 10:30 am in order to limit discharges of both nitrous and chlorine gases, which were beginning to fill the workshop. A technician was overcome by the gas and required hospitalisation for a few hours as a precautionary measure. The internal emergency plan was activated at 10:58 am, and fire-fighters arrived on the scene at 11:10. The dryer was started and then flooded; the incident was brought under control at 12:34 pm. The device was drained, with all fire extinction water collected in a retention basin; recovered sludge was recycled over the following week. A few hundred kg of aggregates were decomposed; the 20 tonnes of system load were recycled that very night.Thermal decomposition had occurred inside a dryer, of the rotating tube variety, fed with hot air by a 7 MW/hr natural gas generator containing 20 tonnes of aggregates, over which an ammonium phosphate slurry had been sprayed. This ammonium phosphate supply was obtained by means of a chemical reaction between phosphoric acid (H3PO4) and ammonia (NH3). The site operator considered the possibility of accidental overheating, since the NPK 11-11-32 fertiliser had not been prone to self-sustaining decomposition. The unit had been shut down for the maintenance of a chain conveyor. The pertinent operating instructions would have been followed for this mission, given that the granulation loop contained dry matter, with the burner operating at the minimum setting (35%) and the drying drum no longer rotating.Subsequent to an excessive production temperature (> 300°C), the “dryer input” temperature, which was also abnormally high, surpassed the temperature at the onset of dry fertiliser decomposition (i.e. > 170°C). This thermal disequilibrium was caused by use of an H3PO4 at 38% concentration, which was more diluted than the normal level (53%). With a slurry containing a large quantity of water to be evaporated, gas temperature at the reactor output (set at 110°C) decreased, while the drying air temperature was automatically increased as a compensation to 300°C, by exceeding the typical threshold of 240°C. No alarm was triggered, with the 300°C value remaining below the 370°C threshold recorded on the “hot air intake” temperature probes. Afterwards, the dryer required more time to cool.The acidic dilution stemmed from an incident that had occurred 10 days prior, involving the unit’s three H3PO4 tanks, two containing a 53% acid and the other a diluted corroded acid (< 30%). With its shell leaking at a height of 1.5 m above the bottom, tank contents had been transferred into the other two tanks, thus diluting the acid used for manufacturing purposes.The operator modified production standards, by introducing a "hot air intake temperature" alarm threshold adapted to each production run (260°C for ammonium nitrate fertilisers), along with the relevant maintenance shutdown procedure by indicating temperature controls and thresholds correlated with the steps required for installation shutdown, plus an internal emergency plan reminder to avoid stopping the fan in the event of toxic gas emissions.