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Hypochlorite plus Urea Explosion

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Michael Fox, PhD.



Hypochlorite plus Urea Explosion

A municipal water treatment company decided to switch from chlorine gas (Cl2) disinfectant to liquid 12.5% sodium hypochlorite (NaOCl). Part of the motivation for the switch was to avoid the OSHA Process Safety Management (29 CFR 1910.119) requirement for a Process Hazard Analysis. They also believed the liquid would be safer than the chlorine gas.

Hypochlorite plus Urea Explosion
Exploded tank
At one of their remote facilities, they had a 2500-gallon polyethylene tank that previously held a liquid fertilizer consisting of 78% urea and sulfuric acid (1:1 ratio). Someone in management decided that they could reuse the liquid fertilizer tank for the sodium hypochlorite, most likely to save a few dollars on a new tank. 

On the day that the liquid sodium hypochlorite tanker arrived, the 2500-gallon polyethylene tank still had a few inches of the leftover liquid fertilizer. The site safety manager was there along with another employee of the water treatment company as well as the truck driver. The site safety manager decided to try to offload a little of the sodium hypochlorite and see “what would happen.” There was an immediate and strong reaction that generated either a white cloud of gas and/or fizzing. 

The initial reaction “scared” the site safety manager who then decided to get a new tank from another location. While the new tank was being delivered, the site supervisor showed up at the remote site. He cut the drainpipe to the 2500-gallon tank and ordered the contents to be poured out onto the ground and the tank flushed with water. Even though the new tank had already arrived at the site, the site supervisor decided to try to offload the sodium hypochlorite into the used tank one more time. At first, it seemed to go okay, and so he gave the order to increase the flow of sodium hypochlorite. Just about then, the tank exploded, as seen in the accompanying photo. 

The site supervisor and other employees present were seriously injured. The site supervisor died within about 48 hours. The injured employees sued the trucking company as well as their employer. 

Chemaxx was hired to determine the underlying cause of the explosion. Other issues in the case included the roles and responsibilities of the tanker transport company, the employer (the water treatment company), and the individual employees. The employer’s state of knowledge and OSHA Hazard Communication Standard training were important factors in the analysis. 

A number of experimental methods were used to investigate this occurrence, including mixing experiments, the analysis of the solutions after mixing, and the analysis of the gases given off via GC/MS. 

The mixing experiments revealed that the order of mixing was important. When the sodium hypochlorite was added to an excess of liquid fertilizer, there was only a modest reaction or heat generated. On the other hand, when the liquid fertilizer was added to an excess of sodium hypochlorite, an extremely vigorous reaction plus significant heat was generated. One possible explanation seemed that when there is excess of alkaline sodium hypochlorite, the sulfuric acid is neutralized and what remains is sodium hypochlorite and urea, a mixture said to produce explosive nitrogen trichloride: 

It’s suspected that there were also some bulk layering and subsequent mixing involved in the incident such that the sodium hypochlorite layered on top of the more dense liquid fertilizer. This layering may have allowed the accumulation of excess NaOCl and NaOH needed to neutralize the H2SO4 and produce the urea plus NaOCl solution. 

It is worth noting that the MSDS for the sodium hypochlorite listed urea and acids as incompatible materials and that the MSDS for the liquid fertilizer listed chlorites and chlorine bleach as incompatibles. The site supervisor knew the two materials were incompatible and the site safety coordinator (who left to get a new tank) did the site OSHA training on sodium hypochlorite. Furthermore, a sales representative from the liquid sodium hypochlorite company had told the site supervisor (and others) that brand new tanks needed to be used for the sodium hypochlorite. 

This seemed to be a classic case of management being in a hurry, taking risks to meet a schedule, and trying to save a few dollars whenever possible.

Dr. Fox is an explosion expert, fire expert, and chemical expert with extensive experience in OSHA chemical regulations and chemical safety.