4. Conclusions
The authors investigated factors related to the ignitability of dust of discarded
electrical appliances (the sample mainly used was polyurethane), and the following
conclusions are drawn:
(1) The minimum explosive concentrations were 30–40 g/m3 (polyurethane and
plastic dusts) and 70 g/m3 (toner). The minimum explosive concentration was
reduced by about 40% when combustible gas (cyclopentane: 4500 ppm) existed
together with polyurethane dust. The ignition temperature of the dust cloud was
around 500 1C.
(2) The minimum ignition energy for the dust foamed by cyclopentane was about
11 mJ and the dust foamed by chlorofluorocarbon was about 24 mJ. The ignition
energy was the minimum when the spark duration was about 0.2 ms or more. The
explosibility was the maximum when the spark gap was 5–10 mm, and the
ignition energy was the minimum when the electrode tip angle was around 30 1C.
(3) These findings show that the dusts produced in the recycling process can lead to
dust explosion easily. The explosibility data collected should be useful in
preventing dust explosion. Also, this information on ignition should be helpful in
establishing measuring standards for the ignition energy.