Factors that Affect Dust Generation

DOE-HDBK-3010-94

4.0 Solids; Powders

cement: behavior is between the responses of the previous two materials. The

penetration of the pile by the falling stream of powder is not as deep as with

sand and does not compress the pile as much as limestone. The impact does

not result in bouncing of the powder. Most of the material slides down the

sides of the pile.

flour: the only material tested that formed piles with a peaked top. Material

slid down sides of pile without penetrating.

Dust appears to be generated by two mechanisms: impact of the falling stream creates

separation forces; and, the change in flow direction of the entrained air in the impaction area

due to its inability to enter in the solid powder results in radial flow that transport airborne

particles away. The total dust generation rates are substantially different for the four

materials tested and:

increased as fall distance increased. Greater fall distance appears to increase

the normalized entrained air, V, increasing total dust generation. The slopes

of dust generation rates for sand, cement and flour are almost identical

indicating similar responses to energy input.

decreased with increased F (material flow rate). The material in the center of

a falling stream of powder is less exposed to the surrounding air and the

material in the center of the falling stream increases with F. As a result, less

air is entrained, V, in the falling powder stream and reduces the radial flow

that is a factor in the transport of suspended powder from the impact area. For

sand and cement, the decrease in G may also be due to the reduced impact

forces imposed on these materials by their penetration into the pile. The

decrease in G for flour appears to be due to the reduction in entrained air.

The increase in G with F observed for limestone is attributed to the

proportionately greater increase in separation forces resulting from the

formation of a crater by the material in the impact area with the reduced shock

absorption from the reduced layer of powder.

G decreased as W (moisture content) increased for all materials tested.

Cement formed agglomerates with the addition of moisture altering the particle

size distribution of the source material and was not tested. The rates varied

substantially for the other 3 materials. For the crystalline, nonreactive-to-

water materials (which also appear to be noncohesive), additions of small

amounts of water increased the liquid film of the surface of the individual

particles and appear to increase the capillary interparticle binding forces. For

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