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RE: Recycling of Grinding Swarf



In a 10/26/99 post to P2TECH, Julie Finn (julie.finn@eqonline.com) asked (in
part):

	> I'm looking for information on recycling of grinding swarf and
thought that members of this list could help.

I wanted to share a quick personal anecdote regarding swarf which ought to
be considered as you are coming up with a recycling plan.  This is another
of 
those unanticipated conflicts between environmental and safety goals that
Nicholas Ashford at MIT has done such a fine job studying and documenting.  
In this case the conflict was not unavoidable, but may not have been
intuitive.

Several years ago, I was talking with a colleague who worked for a large
aircraft manufacturer in our region.  They had been recovering aluminum
swarf from a grinding/polishing step 
which used a water curtain (similar to those sometimes used in paint shops)
to collect the fine Al dust that came from the equipment.  The sludge from
the bottom of the 
water curtain was dewatered and put in 55 gal drums for collection and
ultimate disposal.

Unfortunately, finely divided aluminum presents a lot of surface area, and
is fairly reactive with water. The reaction between Al and water liberates
hydrogen and lots of heat, neither of which 
are particularly conducive to safe storage.  This problem came to their
attention only after the contents of the sealed drum exploded, sending the
drum lid some 35 feet into the air, where it 
bounced off a ceiling girder and fell back to the shop floor.  It was not
clear whether the explosion was due to accumulation of steam (from the heat
of reaction) or H2 but my guess is that enough H2 accumulated and was
somehow ignited -- hydrogen gas has a pretty broad explosivity range.  

In any event, the point of this story is just to remember the fact that many
types of metal fines (not just aluminum) pose several special safety issues
during collection, transfer and storage, and that these issues need to be
addressed in the development of a recycling plan.  IN addition to the
reaction with water, Al and Mg dust can also be very explosive in air
suspension -- I recall a second incident at a computer manufacturer I once
worked for, where grinding dust from a hard-disk manufacturing operation
exploded in a collection bin, causing significant damage.  Fortunately, it
occurred on a Sunday, when the facility had far fewer people onsite than
normal.

Scott Butner (rs_butner@pnl.gov)
Senior Research Scientist, Environmental Technology Division
Pacific NW National Laboratory
4500 Sand Point Way NE, Seattle WA  98105 3900
206-528-3290 voice/206-528-3552 fax
http://www.chemalliance.org/