Float Coal Dust Explosion Hazards

Objective

To increase awareness of float coal dust explosion 

hazards in the mining industry. 

Background

In underground coal mining, dust is produced at 

the face, at conveyors, at transfer points, and by the 

normal movement of workers and machines. The 

coarse coal dust particles settle rapidly. However, the 

fine coal particles remain airborne much longer, and 

the ventilating air can move this fine dust relatively 

long distances into the returns before settling. This 

fine dust is called float coal dust. It generally consists 

of particles of coal that pass a 200-mesh sieve 

(particles smaller than 75 Fm). 

Generalized rock dusting is currently the primary 

means of defense against coal dust explosions in U.S. 

mines. 30 CFR 75, Subpart E (Combustible Materials 

and Rock Dusting), requires the use of rock dust in 

bituminous coal mines (30 CFR 75.402). The 

regulations state that rock dust shall be distributed 

upon the top, floor, and sides of all underground areas 

of a coal mine in such quantities that the 

incombustible content of the combined coal dust, rock 

dust, and other dust shall be not less than 65%, and 

the incombustible content in the return air courses 

(where the dust is expected to be finer) shall be no 

less than 80% (30 CFR 75.403). These incombustible 

concentrations assume that the coal and rock dust are 

not layered, but are intimately mixed. Float coal dust 

is a serious explosion hazard if it accumulates on top 

of the rock dust and is not mixed thoroughly with the 

rock dust. An example of this is shown in Figure 1. 

Figure 1.—Cross-section of a very thin (0.01-inch) explosible 

float coal dust layer deposited on top of a 3/4-inch (20-mm) thick 

layer of rock dust.

Approach and Results

 The explosion hazards of float coal dust have 

been studied over many decades in the Experimental 

Mine at Bruceton, PA. The position of coal dust 

along the perimeter of an entry is a more important 

factor affecting explosion propagation than is often 

recognized. The dust on the ribs, roof, and other 

elevated surfaces (overhead dust) can be dispersed 

much more readily by an explosion than dust on the 

floor. If the overhead dust is mainly coal dust, the 

explosion hazard is intensified. If the overhead dust 

is primarily rock dust, the explosion hazard is 

reduced. Depending on the quantity, the overhead 

rock dust can compensate somewhat for a deficiency 

of rock dust on the floor. However, thick layers of 

rock dust on the floor cannot compensate for float 

coal dust on overhead surfaces. 

No. 515 April 2006 
 For some of the Experimental Mine explosion 

tests, trays of color-coded dust layers were substituted 

for the floor layer in strategic locations throughout the 

test zone. The results showed that, for a typical float 

coal dust explosion, only the top 3/32 to 5/32 inches 

(2 to 4 mm) of the floor dust layer is stripped off or 

entrained in the air. It was also found that a minimum 

5/l,000-inch (0.12-mm) thick layer (about the 

thickness of a sheet of paper) of pulverized float coal 

dust deposited on top of a 3/8-inch-thick uniform 

concentration of 80% rock dust and 20% float coal 

dust would propagate an explosion. The thicker the 

float coal floor layer, the more violent the explosion. 

Recommendations 

 Research has shown that when the overhead dust 

is primarily rock dust, the explosion hazard is 

reduced. Depending on quantity, the overhead rock 

dust can compensate for a deficiency of rock dust on 

the floor. However, since rock dust on the floor 

cannot compensate for the float coal dust on surfaces 

above the floor, special attention toward increasing 

the rock dust content on these elevated surfaces is 

recommended. 

 Float coal dust deposits can be neutralized by new 

applications of rock dust (such as trickle rock dusting 

or bulk rock dusting), by mixing the float coal dust 

with the underlying rock dust, by general cleanup, 

and/or by washing down the rib and roof surface. 

For More Information

 For more information on the explosion hazards of 

float coal dust, contact Michael J. Sapko (412–386–

6619, MSapko@cdc.gov), Eric S. Weiss (412–386–

5050, EWeiss@cdc.gov), or Kenneth L. Cashdollar 

(412–386–6753, KCashdollar@cdc.gov), NIOSH 

Pittsburgh Research Laboratory, Cochrans Mill Rd., 

P.O. Box 18070, Pittsburgh, PA 15236–0070; fax: 

412–386–6718. 

To receive other information about occupational safety and 

health topics, call 1–800–35–NIOSH (1–800–356–4674), 

or visit the NIOSH Website at www.cdc.gov/niosh 

Mention of any company name or product does not 

constitute endorsement by the National Institute for 

Occupational Safety and Health.

DHHS (NIOSH) Publication No. 2006–125