On July 1, 2009, a plasma experiment was conducted to produce a small quantity of sodium borohydride from anhydrous sodium borate, methane, and hydrogen in an enclosed reaction chamber. The reactants were injected into an argon plasma flame to carry out the synthesis reaction.
After the run was completed, as per work control procedure, the experimenter removed the plasma torch from the top lid of the collection chamber and taped a piece of weighing paper over the opening so air would not get into the chamber and contaminate the product. The experimenter then installed a plastic glove bag over the top lid of the collection chamber and attached it just below the top lid using Velcro. Before final installation, the experimenter placed a screwdriver and a natural bristle paint brush into the glove bag that would eventually be used to gather the product sample. After the glove bag was installed, the weighing paper was punctured and the inert gas was allowed to flush the glove bag continuously for 15-20 minutes to displace the air within the bag before removing the lid.
In accordance with the procedure, the experimenter then used the screwdriver to remove the top lid of the collection chamber with purge gas still flowing. The experimenter donned white cotton liner gloves to reduce the potential of static electricity and absorb any sweat from his hands. He used the brush to collect the material from the wall and bottom of the collection chamber and swept the powder into a glass tube located at the bottom center of the collection chamber. As the product was being collected, the experimenter observed a small flash coming from inside the chamber and extending into the glove bag. The flash produced a whooshing sound. The experimenter, whose hand was in the chamber, felt heat through his glove, and the plastic on the right thumb of the glove was slightly hardened from the heat. The flash and the reaction products were contained by the glove bag and the chamber. The glove bag remained intact and no equipment was damaged. The experimenter instantly removed his hand from the glove bag and called the principal investigator (PI) and the department manager to explain what had happened. All activities in the plasma laboratory were stopped, personnel entry was denied, the laboratory was secured, and the incident scene was preserved. An investigation was launched to determine the causes of the flash.
In order for the flash to occur, both oxygen and an ignition source had to be present inside the glove bag. At the time the incident occurred, the normal inert purge gas (plant nitrogen) was being used in another project and was unavailable. A decision was made to use argon instead of plant nitrogen based on the immediate need for an inert purge gas. Argon had been used for powder feeding during the run and was available, so it was used instead. The most probable scenario for this event involves the changes brought on by substituting argon for nitrogen without evaluating the interaction properties of argon in a glove bag at a low flow rate.
During argon purging of the reactor system, the flow rate was set at 8 liters/minute, the same flow rate used for powder injection. After the incident, the experimenter attempted to determine the plant nitrogen purge gas flow rate, but it was significantly above the capacity of the largest flow meter available (10 liters/minute). Apparently, the plant nitrogen purge gas flow rate was much higher than the argon flow rate used during the incident. There may not have been sufficient volume of argon or enough time to purge the glove bag and create enough positive pressure in the bag during the work activities. Certain motions in the glove bag associated with collection activities may have caused purge gas to exit the bag (as designed), but when the glove bag needed additional purge gas to expand, it may have drawn in room air in addition to the purge gas, thus introducing oxygen into the glove bag.
The ignition source for this event may have been spontaneous combustion with oxygen due to the extremely fine particle size of the produced material (nanoparticles). For a flash to occur, the nanoparticles produced must contain some combustible or pyrophoric materials such as borohydride, hydride, carbon, or sodium metal. An electrostatic spark induced from the plastic glove bag could also have been the ignition source for the flash. However, if the inert atmosphere were of the proper quality, an ignition source would not be a problem.
The following recommended actions were identified:
- Reemphasize the current lab management policies and practices on how process changes are evaluated for direct/indirect impacts on the process.
- Reinforce with lab workers the expectations for bringing issues and concerns to management's immediate attention for evaluation of reporting needs.
- Increase the purge gas flow rate to ensure complete purging of the system.
- Extend the inert gas purging time of the reactor system before the run and use oxygen chemical indicator strips to indicate the quality of inert atmosphere in the enclosed system.
- After the run, extend the inert gas purging time of the installed glove bag above the chamber lid and use a high purge gas flow rate.
- Use oxygen chemical indicator strips to indicate the quality of inert atmosphere in the glove bag before opening the collection chamber.
The importance of purging hydrogen piping and equipment is discussed in the Lessons Learned Corner on this website.
Additional discussion about working with reactive metal-hydride materials in the laboratory can be found in the Lessons Learned Corner on this website and in the Hydrogen Safety Best Practices Manual.