A plant manufacturing additives and lubricants discharged 110 tons of sulphur dioxide (SO2) to the atmosphere. A unit transforms hydrogen sulphide (H2S), by-product of the production of calcium phenate (40,000 tonnes/year), into sulphur. 6 tons of H2S are handle per day. The process consists in absorption into the wet H2S phase, an oxidation-reduction reaction followed by the extraction, and the treatment of sulphur for subsequent recycling into manufacturing. This pilot unit, which started up in 1997, had already experienced several incidents: defective seal on the absorption tank during start-up in 1997; loss of oxidation-reduction solution activity; and malfunction of the associated incinerator, resulting in shutdown of the H2S treatment unit (6 days in 2002) with discharge of 36 tons of SO2 to the atmosphere. This secondary release of SO2 was due to shutdown of the sulphur extraction installation, as a result of partial sulphur clogging. Technicians were also required to shut down both the H2S treatment unit and phenate production facility in order to enter the installations and proceed with a thorough cleaning. During this shutdown period, reactor vents containing H2S were routed to a flare. The facility cleaning activity thus led to discharging 110 tons of SO2 while producing 50 tons of polluted water to treat. This accident was caused by an abnormal accumulation, for unknown reasons, of sulphur inside the installations. It took some 20 days to return the unit to normal operations. The plant operator, who evaluated the treatment unit availability rate at between 90% and 95%, stayed in constant contact with the company owning the process patent to help carry out unit improvements. Following the incident, installations were modified by: remodelling the catalytic liquid distribution ducts in order to optimise H2S absorption, and adding a stirring system in the zone where sulphur had deposited to prevent any subsequent clogging of the equipment.