In a mineral processing plant, a convoy was in the process of delivering a storage silo containing a powdery substance (sodium carbonate). During the delivery, one tonne of product was accidentally discharged through the silo’s vent, dispersing product into the surrounding area.

No environmental consequence was identified due to the non-toxic nature of the substance involved. However, production operations at a neighbouring company were disrupted. The ground, vehicles, pallets and a building are also contaminated by white dust. The operator contracted 2 specialised companies to carry out the clean-up operation.

Causes

During the unloading process, the pressure of the compressed air used by the delivery man to expel the material from the tank was excessive, causing the opening of the safety valve at the top of the silo, and then did not close. The valve was not equipped with counterweights allowing it to close automatically. Owing to the high pressure and the vent remaining open, sodium carbonate was able to escape through the vent. As the top of the silo was not visible, the technician didn’t notice anything. The neighbouring company raised the alert.

Circumstances

During delivery, the driver was accompanied by a site operator who was called upon to perform other tasks.

Measures taken

Following the event, the operator undertook the following corrective actions:

• pressure control in the silo during unloading operations;
• slaving of the discharge valve to the silo’s pressure;
• installation of an automatic valve closing device;
• an unloading procedure was drafted to take into account the reception of the delivery personnel;
• installation of an audible alarm on the silo’s high level.