I've been doing some research for a safety lecture I'm giving next week on the topic of bench grinders. I already knew that grinding aluminum without water or some other coolant is a fire hazard, but the story I found is a real eye-opener.
Project Hanford Lessons LearnedTitle: Fireball from Aluminum Grinding Dust
Date: September 21, 2001
Identifier: 2001-RL-HNF-0036
Lessons Learned Statement:
Aluminum grinding dust can create a serious explosive fire hazard when it is mixed with steel or iron grinding dust.
Discussion of Activities:
A safety coordinator at the Esso Oil Company plant in Longford, Australia, was using a belt grinder in his home workshop to smooth the edge of a hacksaw cut on a 2" length of 1.5" angle iron. He had been grinding for about 1.5 to 2 minutes when there was a loud "THUMP" accompanied by an approximately 2-foot diameter brilliant yellow orange fireball. The fireball lasted no more than 1 second and then completely extinguished itself. It completely enveloped the machine, his hands to half way up his forearms, and the front of his torso.
Injuries included deep second-degree burns to about 60% of the victim's left hand and 50% of his right hand and first degree burns to his neck, chin, cheeks, lips, and the end of his nose. The right cuff of his shirt was smoldering, his face felt a burning sensation, and he could hear the front of his hair sizzling. Nothing on the bench was burning. A few streaks of white powder were deposited on the bench top and on a few items lying on the bench. The workshop was filled with dense white smoke with very little odor. His fingers and the ends of his thumbs escaped relatively unscathed as they were protected from the heat flash. He was wearing glasses, which protected his eyes. He also lost half his moustache, one of his eyebrows, and about 1 inch off the front of his hair. His eyelashes were curled by the heat but not singed. The burns to his face were caused solely by radiant heat, as the fireball did not come that high.
Analysis:
A few days before the event, the man's son had ground the heads off about twelve aluminum pop rivets. Finely divided aluminum mixed with finely divided ferrous oxide (the black powder residue from grinding steel) produced a compound called thermite. Thermite is used to fill incendiary bombs and is used commercially to weld large steel items. It burns at approximately 3500C (6300F), hence the extensive burns from such a short exposure time.
more...
http://www.hanford.gov/lessons/sitell/ll01/2001-36.htm:wow:
The article goes on to explain that the grinding wheel and the surrounding area should be cleaned of all aluminum dust before grinding steel, but I would take that a step further and advise against grinding aluminum...period. Aluminum dust with a high magnesium content is especially prone to spontaneously combust, but it's not really worth taking any chances.
Consider this post a public service, since I know many of you DUers are pretty handy and may have bench grinders in your garage or workshop.
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