Explosion in a workshop handling sodium residues

This incident took place at a plant specialised in production of sodium, lithium, and chlorine. A hydrogen explosion occurred during the hydrolysis of sodium residues in a dedicated workshop. During the hydrolysis, the remaining metallic sodium reacted violently with water in an exothermic reaction, releasing hydrogen and causing the explosion.

The explosion was audible from outside the site. The smoke and gases released drifted towards the outdoor parking lot, where the sodium deposited and reacted with moisture to form sodium hydroxide.
The plant operator marked off the area, shut down the equipment, and performed pH tests. Since the situation stabilised immediately, the plant did not activate its emergency response plan.
The incident did not damage the building structure and caused a very light injury.

The INITIATING CAUSE was a reaction between the metallic sodium and water, producing hydrogen. Both the exothermic reaction and the ignition of hydrogen may have contributed to the explosion.

The sodium resides, after having been oxidised and before hydrolysis, were usually crushed to guarantee and control a more complete oxidation. In the case of this incident, the crushing had not been performed. The weekend shift staff was reduced, and the procedures allowed for hydrolysis without crushing during weekends.

It is unknown if the simplified procedures during the weekend had been established on the basis of a risk assessment or only dictated by the lack of availability of human resources. Anyhow, the ROOT CAUSE can be attributed to shortcoming I the operative procedures, and possibly also in inadequate management decisions.

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The operations leading to the incident were executed during the weekend.

A similar event occurred in 2024 at the same site (HIAD_1244). As reprotd there, this ocurence is not uncommon.

DESCRIPTION OF THE PROCESS
The process for disposing of sodium residues involves three steps:
1. Cutting and combustion: The drums containing the sodium residues are cut open, and their contents are burned using propane. This oxidation process transforms the reactive metallic sodium into sodium oxide (Na2O), a stable form.
2. Crushing: The oxidized block is crushed to verify that the oxidation process is complete, particularly in the core of the material.
3. Hydrolysis: If the oxidation is complete (verified by visual inspection: unoxidized sodium has a different color and distinct mechanical properties), the crushed material is subjected to hydrolysis (sprayed with water) to stabilize it in the form of sodium hydroxide (NaOH), before being sent to the wastewater treatment plant for final disposal.

A security guard felt a tingling sensation on his face and received medical attention on site.
The incident did not damage the building structure. Property damage was limited to some equipment in the workshop.
Some visible deposits of sodium hydroxide aerosol were found on parked vehicles. The incident was caused by incomplete oxidation of the sodium residues used in the process.

The plant operator marked off the area, shut down the equipment, and performed pH tests. Since the situation stabilised immediately, the plant did not activate its emergency response plan.
Approximately 3 hours later, a firefighter arrived on site to execute an inspection and confirmed that the situation was under control.

The process in which the incident occurred was the treatment of very reactive metallic sodium to be able to dispose of it in a safe way as sodium hydroxide. Sine the metallic sodium-water reaction is extremely exothermic and therefore with ahigh explosive potential, the sodium was first oxidised. The hydrolysis of sodium oxide (N2O) made react with water to form sodium hydroxide (NaOH). It is important to ensure that all sodium is oxidised before bringing it in contact with water. Therefore, in the plant affected by the incident, the sodium oxide was crushed: it is then possible to visually discover if the core of the oxidised solid product still contains presence of metallic sodium, o the basis of difference in colour.

This control step was not required during weekend operations. The weekend shift staff was reduced, and the procedures allowed for hydrolysis without crushing during weekends. It is unknown if the simplified procedure during the weekend had been established based on (1) a risk assessment or (2) only dictated by the lack of availability of human resources.
(2) would be a classic case where financial interests prevailed on safety concerns. (1) is much more probable. It is possible that the decision to avoid the control step was based on a good record of complete oxidations in the past.
The lesson learnt from this incident is that the possibility of direct hydrolysis of metallic sodium was a non-negligible possibility in this plant. Even if the consequences of this incident were relatively small, it cannot be excluded a priori that incidents with more impact will not happen in the future.