Free-Fall Spill of Powder with Air Velocity Normal to the Direction of Fall

DOE-HDBK-3010-94

4.0 Solids; Powders

Four readily available materials in powder form were allowed to free-fall spill known

distances and the material airborne was collected as a function of aerodynamic particle

diameter in a cascade impactor. Two powders were nonporous and nonreactive to water,

inorganic, crystalline materials (sand and limestone). One porous, reactive, inorganic

material (cement) and one porous, reactive, organic material (flour) were also tested. Sand

and limestone are similar to some process generated wastes (e.g., slag and crucible) and

coarse fractions of process materials (e.g., heavy metal oxides). The finer cohesive heavy

metal oxides are similar in characteristics to cement but a nonreactive with water. The solid

salts of other heavy metal compounds (e.g., fluoride, chloride, oxalate, hydride) were not

represented here.

The powder fell in the test apparatus from a funnel with an interchangeable tube through a

hole in the center of the lid of the receiving hopper. The interchangeable diameter tubes

(24-, 37- and 49-mm) adjusted the material flow rates. The total mass dropped ranged from

2 to 10 kg. Flow rates (ranging from 0.1 to 10 kg/s) were determined by the time required

to release a known mass of material. The fall distance was measured from the bottom of the

tube to the top of the pile of material. The stream of falling powder impacted a natural pile

of powder formed of the same material under the same conditions. Air was entrained in the

falling powder stream.

The dusty air generated was drawn into the second section (the aerosol collection section) of

the apparatus by a fan that was turned off after all the powder had been released. A second

fan circulates the air through the elutriation column (flow velocity designed to prevent

particles >25 m AED from being carried to the cascade impactor) and air return channel.

The mass collected per impactor stage or filter was used to determine the size specific dust

generation rate, Gi (fraction of dust particles generated with an aerodynamic diameter i).

The summation of all fraction collected was the total dust generation rate, G. (The total dust

generation rate, G, is expressed as mg airborne/mg source and is equivalent to the ARF.)

The size distribution and moisture content for the source powders were determined.

At low Fs (material flow rates), the falling stream of material stops abruptly on top the pile

and slides down the sides. At higher Fs, the individual materials exhibit different behavior:

sand: the falling stream penetrates the pile and displaces material radially.

limestone: the pile is compressed by the falling stream and forms a crater, 70-

to 150-mm in diameter. The diameter of the crater increases with fall

distance. The falling powder stream strikes the center of the crater and

bounces back in all directions.

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