Summary of incident
In May 2017, during a maintenance shutdown at a processing plant, a tank that normally held sulphuric acid slurry was temporarily used for storing limestone slurry.
When the tank was returned to service, it was drained of limestone slurry and filled to launder level with acidic slurry. However, although the tank had been emptied, limestone residue had accumulated at the bottom. When the tank was filled with acidic slurry and the agitator turned on, there was a rapid acid-base reaction between the free sulphuric acid and limestone, liberating carbon dioxide gas which could not readily escape. The tank’s fibreglass roof burst and about 10,000 litres of hot slurry (87°C) escaped from the tank.
A process technician working on the platform of the tank fled the bulk of the hot slurry by jumping over a nearby handrail onto an adjacent tank platform. He was treated for minor injuries, including burns.
Direct causes
- The reaction between the free sulphuric acid and limestone in solution resulted in the uncontrolled liberation of carbon dioxide gas.
Note: The rate of reaction was increased by the high temperature of the slurry (87°C) and agitation of the solution, which made the residual limestone more readily available for reaction with the free acid.
Contributory causes
- The risk assessment was not adequate and failed to identify the hazard associated with mixing of the acid slurry with limestone that might accumulate on the bottom of the tank.
- Control measures, such as cleaning the tank to remove residual limestone before refilling it with acidic slurry, were not implemented.
- There was only one means of access or egress from the tank platform.