The Definition of Rock Dusting

Definition of Rock Dusting

Rock dusting is the application of finely pulverized inert material, usually limestone and sometimes dolomite, to the roof, ribs and floor of an underground coal mine so coal dust cannot propagate an explosion.

Coal dust is a combustible dust. Mining, cutting and hauling coal create fine particles that settle on every surface. A small methane ignition at the face can create a pressure wave that lifts this float coal dust into the air. The dust cloud then ignites and can carry the explosion through miles of entries, causing far more damage than the methane alone. Rock dust mixes with the coal dust, absorbs heat and keeps the flame from spreading.

Rock dusting applies to underground bituminous coal mines regulated by MSHA. It involves rock dusting crews, section foremen, examiners and mine management. It is one of the most important explosion prevention controls in coal mining, along with methane control, ventilation, cleanup of loose coal and sound ground control in dusted entries.

How Rock Dusting Works

Why Inert Dust Stops Explosions

• Dilution: rock dust raises the share of incombustible material in the mixed dust so there is not enough fuel to sustain a flame.
• Heat absorption: inert particles absorb heat from the flame front and cool it below the point where coal dust ignites.
• Total incombustible content: the measure that matters is the combined incombustible content of coal dust, rock dust and other dust, including moisture and ash.

Application Methods

• Hand dusting: workers spread rock dust by hand or shovel in small areas and near the face.
• Bulk dusters: machines blow large volumes of dry rock dust through hoses to coat entries quickly.
• Trickle dusters: machines that release a steady stream of rock dust into the air current, often in return airways, so it settles over time.
• Wet dusting: rock dust mixed with water into a slurry and sprayed on surfaces. It reduces airborne dust during application.

Sampling and Testing

• MSHA sampling: MSHA inspectors collect mine dust samples and analyze them for incombustible content.
• Coal Dust Explosibility Meter (CDEM): a handheld device that gives on-the-spot readings so operators can find and fix low areas right away.
• Cleanup: loose coal, coal dust and float coal dust must be cleaned up and not allowed to accumulate. Rock dust does not replace cleanup.

Regulations and Standards

MSHA Underground Coal: 30 CFR Part 75

• 30 CFR 75.400: coal dust, including float coal dust deposited on rock-dusted surfaces, loose coal and other combustible materials must be cleaned up and not permitted to accumulate.
• 30 CFR 75.402: all underground areas of a coal mine must be rock dusted to within 40 feet of all working faces, unless those areas are too wet or too high in incombustible content to propagate an explosion.
• 30 CFR 75.403: the total incombustible content of the combined coal dust, rock dust and other dust must be maintained at no less than 80 percent in all underground areas of bituminous coal mines. Where methane is present in any ventilating current, the incombustible content must be increased by 0.4 percent for each 0.1 percent of methane.

The 2010 Change

Before 2010, the minimum in non-return areas was 65 percent. After the April 2010 Upper Big Branch explosion killed 29 miners, MSHA issued an emergency temporary standard in 2010 and a final rule in 2011 that raised the requirement to 80 percent in all underground areas. Coal dust played a major part in spreading that explosion.

Good Practices for Rock Dusting

• Dust close to the face: keep rock dusting current to within 40 feet of each working face as mining advances.
• Clean first: remove loose coal and float coal dust before applying rock dust, so the inert layer is not buried.
• Test often: use a CDEM during workplace examinations to spot low areas between MSHA samples.
• Adjust for methane: use gas detection readings to raise incombustible content where methane is present in the air current.
• Cover every surface: apply rock dust to the roof, ribs and floor, including behind equipment and in crosscuts.
• Watch returns and belts: pay extra attention to return airways and belt entries, where float dust builds up quickly.
• Record the work: log where and when rock dust was applied and the results of each test.

Example of Rock Dusting

An underground coal operator ran CDEM checks during weekly examinations. An examiner found readings below 80 percent along a belt entry near a transfer point. Black float coal dust had settled on top of older rock dust. The belt crew had been dusting on a fixed schedule, but higher production had increased dust buildup and the schedule had not kept up.

The mine cleaned the belt entry, applied fresh rock dust and installed a trickle duster at the transfer point. It changed from a fixed schedule to one based on CDEM results, with more frequent checks on belts and returns. Examiners now record each reading on a map, and any area below the required level is dusted before the next shift.

Frequently Asked Questions: Rock Dusting

What is rock dust made of?

Rock dust is usually finely ground limestone, and sometimes dolomite. It must be inert and fine enough to mix with coal dust in the air during an explosion.

What is the required incombustible content for rock dusting?

Under 30 CFR 75.403, the combined coal dust, rock dust and other dust must be at least 80 percent incombustible in all underground areas of bituminous coal mines. The requirement goes up by 0.4 percent for each 0.1 percent of methane in the ventilating current.

How close to the face must rock dust be applied?

30 CFR 75.402 requires rock dusting of all underground areas to within 40 feet of all working faces. Areas that are too wet or already too high in incombustible content are exempt.

Why did the rock dust standard change in 2010?

The April 2010 Upper Big Branch explosion killed 29 miners, and coal dust helped spread it. MSHA then raised the minimum incombustible content to 80 percent in all underground areas through an emergency temporary standard and a 2011 final rule.