NaAlH4 powder mixed with hexane was placed in two metal trays and dried by placement in a glove box antechamber under vacuum. After several days, the trays were moved into the glove box main chamber. As the powder in one of the trays was being transferred to a container involving scraping of a metal sieve and metal milling balls with a metal spatula, a portion of the powder in the tray spontaneously reacted rapidly, creating a pressure pulse which cracked the window at the back of the glove box. No injuries occurred, and the glove box window was resealed using tape within one to two minutes.
In addition to resealing the glove box window, a positive pressure of argon gas was maintained inside the glove box while the course of action was planned. Subsequently, the glove box was cleaned up by specialized hazardous materials personnel using natural bristle brushes and plastic utensils. Also, Teflon-coated magnetic stirring bars were used to separate the milling balls from the powder while avoiding metal-to-metal contact.
While no direct evidence has been obtained, it is possible that a small leak in the antechamber seals or back diffusion from the vacuum pump occurred to expose the NaAlH4 material to oxygen and/or water vapor. Similar sudden reactions within a glove box have been noted by other researchers working with NaAlH4 where contamination by oxygen / water vapor was suspected. A possible material mechanism is detailed in “Ashby's warning” published in Chemical and Engineering News, V47 (1), 1969. In general, researchers working with NaAlH4 or other reactive hydrogen storage materials should take extra precautions with regards to sealing and vacuum pump type/performance when holding such materials under vacuum for extended periods of time.
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.