GKAIN-DUST EXPLOSIONS. 11 



At first the hood of the attrition mill Was removed, so that the 

 sparks emitted could be observed. When a single nail (eightpenny, 

 but clipped to a length of about one-half inch) or small piece of stone 

 was passed thi-ough the mill, sparks were, in nearly every case, 

 emitted from diiferent points of the periphery of the disks. Feeding 

 1 to 2 ounces of tliese clipped nails, small pieces of flmt, or other hard 

 materials, either singly or in combination, resulted in a large number 

 of sparks, as in the case of an emerj^ wheel. These sparks were tlu-own 

 a distance of from 10 to 16 inches from the peripheiy of the disks. 

 The sparks were emitted radially when both discs were running, and 

 tangentially when one disk was stationary. When matches were fed 

 into the mill, usually no sparks were emitted from the periphery of 

 the disc, as the rapidly revolving arms of the disk generally ignited 

 the matches before they had fairly entered the eye of the runner. 



After making these observations of the sparks produced by foreign 

 materials passing through the attrition mill, the hood was replaced, 

 so that various kinds of gi-ain products coidd be fed into the mill, 

 with pieces of flint, nails, matches, etc. Three to Ave quarts of oat 

 hulls, corn, elevator dust, flour, wheat scourings, wheat shorts, and 

 floor sweepings were fed into the mill, at flrst singly, then in various 

 combinations, with about 2 ounces of foreign materials, a quantity 

 which probably would gi'eatly exceed the amount of such materials 

 finding their way into an attrition mill with the grain during a com- 

 paratively long period, under ordinary milling conditions. The 

 disks were set veiy close m order to grind as fine as or even finer than 

 is usual. Repeated attempts with such materials as flour, dried ele- 

 vator dust, and dust settlings failed to produce a single explosion. 

 Further efforts were made to obtain explosions by duplicating, as 

 far as possible, the actual conditions obtaining in an attrition mill, by 

 feeding the grain at normal rate until several bagfuls had been passed 

 through. At frequent intervals pieces of nails, flint, etc., were added 

 with the grain, but never was there the sUghtest evidence of an 

 explosion. 



The first explosion in the experimental attrition mill was obtained 

 shortly after the preceding series of runs at the time when J. K. 

 Clement, of the Bureau of Mines, was present to witness the tests. 

 The bottom of the hopper was closed by the slide so as to retain the 

 ground material and not permit any dust to escape except through 

 the hand hole, which was open. When a quantity of elevator dust 

 and ground oat hulls was fed into the mill, a rather dense cloud of 

 dust issued from the hand hole, due to the fan-like action of the grind- 

 ing disks. Upon placing a gasoline blow torch near -this hand hole, 

 the dust ignited and a flame, 6 feet long, shot out toward the wall. 

 The hopper was immediately withdrawn, and about a quart of the 

 same materials thrown into the mill, while the torch was introduced 



