An explosion in chlor-alkali electrolyser

An explosion occurred in a electrolyser cell, blowing off the cell's vent cover and severing a low-pressure chlorine gas pipe. This caused a release of chlorine in the building, The event occurred when two operators were trying to improve the tightness of the diaphragm. 

They were only very lightly injured. A reaction between hydrogen and chlorine in the cell's headspace was responsible for the explosion. The migration of hydrogen towards the chlorine, producing an explosive mixture, was due to excessive pressure in the cathodic compartment.

The INITIATING CAUSE was the diffusion of hydrogen into the chlorine half-cell, creating an explosive mixture. 

The post-incident analysis revealed a poor quality of the cell's diaphragm: it was part of a batch of diaphragms whose manufacture did not adhere to standard procedures (crosslinking temperature of 275°C instead of 355°C due to a furnace regulation failure) and which subsequently exhibited premature aging.

chlor-alkali electrolyser

During routine controls a malfunction was observed at the level of the diaphragm. 

The accident occurred when proceeding to a cell ‘vaccination’ procedure, consisting in improving the seal of the orifice plate (diaphragm) separating chlorine and hydrogen. This is achieved by injecting suspended silica and magnesium chloride or glass fibres.

Two workers were very lightly injured, both having experienced a shock from the explosion and having skin irritations in the legs caused by the acidic electrolyte with a pH=3. 

The chlorine discharged inside the workshop amounted to between 2 and 3 kg. The series of 132 electrolytic cells in the shop were shut down, followed by the entire production chain.

The production unit was stopped

On top of the defective diaphragm, the increase hydrogen pressure and its migration could have been the result of three situations: an incorrect positioning of the caustic soda tube, leading to an excessively high caustic soda level in the cell; a blockage of the caustic soda outlet pipe; or blockage of the H2 outlet pipe. 

Therefore, the corrective measures put in place by the operator included 

1. Replacing cells whose diaphragms were produced under the same temperature conditions. 
2. Improving the instructions related to the 'vaccination' of the cells, 
3. Modifying the hydrogen outlet pipes to facilitate checks and cleaning and thus to limit the risk of blockage.