In a plant manufacturing mineral products, a leak of acid effluent occurred on a 20 cu.m. tank at the site’s neutralisation station. When employees discovered this leak, the effluent had already spread over the asphalt ground and was flowing into a sewer outlet located 12 m away. The acid effluent lift pumps were shut down, as were the washing operations. The discharged effluent was slightly acidic (0.022%) and contained inert white corundum grains. To neutralise the effluent, four 25-kg bags of lime stored at the station were poured onto the ground, in the 20 cu.m. tank and into the pumping tank (2 bags). Although the bulk of the corundum (several tonnes) had been decanted (i.e. remained in the tank), a significant portion escaped and was deposited on the ground. No visual impact (i.e. fish mortality, presence of suspended solids or sedimentation) on the Isere River was observed following the accident. A break in the concrete part of the tank, above the foot valve, caused this accident. The localized concrete attack was due to acid infiltration through a section of tiling displaying signs of mechanical wear, the result of abrasion caused by the stirring rod used from time to time to free the agitator blades when too many corundum grains arrived at once. Various measures were implemented in order to reduce the probability that such an accident recurs, including limiting the risk of agitator immobility by periodically draining the tank and assessing the efficiency of wet grain sieving, and designing a new agitator release method (stirring rod less aggressive on the tank sidewalls, specifying place, time, etc.).