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New approach controls dust at the collector dump point

Article · June 2004

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William Randolph Reed

Centers for Disease Control, Pittsburgh Mining Research Division

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New Approach Controls Dustat the Collector Dump Point

NIOSH Finds a Simple, Cost-Effective Solution for Reducing Dust for Blasthole DillsBy Wm. Randolph "Randy" Reed, John A. Organiscak, and Sleven 1. Page

Schematic diagram of dust collector Large arrows represent airilow.

Collceltxl [}uH

~~p~])ust-LadenIntake AIr

Shroud

Drill Deck

Stem

Drill Stem

Bail Air

dust, a trend that continues today based on MSHA'srespirable dust sampling database.

A review of the MSHA database for the years 1985through 1992 shows that 81 % of the respirable si 1­ica dust samples taken exceeded the permissibleexposure limit for the highwall drill operator at sur­face coal mines. A more recent review of the MSHAdatabase, for the years 1996 through 2000, showsthat only 31% of the respirable silica dust samplestaken exceeded the respirable silica dust limit forthe highwall driller at surface coal mines. This sug­gests a substantial improvement in preventing theexposure of silica dust to the highwall drill operator.The dri II operator, however, sti II tops the Iist of joboccupations that have the highest exposure to res­pirable silica dust.

Respirable dust has been shown to be harmful tohuman health, especially when silica is present inthe dust. It causes a disease known as silicosis,which can occur in three levels: chronic silicosis,which occurs after 10 years of exposure; acceleratedsilicosis, which occurs between 5 and 10 years ofexposure; and acute silicosis, which can occur with­in a few weeks to five years of very high exposure tosilica dust. Respirable dust consists of the dust par­ticle size fraction whose median diameter is 4micrometers (~m).

The dri II ing mach ine generates high concentrations ofrespirable dust from several sources: drill table shroudleakage, dust leakage through the table bushing, dustdischarge from the dust collector exhaust due toimpaired filters, and dust entrained from the dust

Dri II operators have one of the highest exposure lev­els to respirable dust. This has been proven throughmany reviews of the MSHA dust sampling database.Prior to 1987, the highwall driller at surface coalmines had the most severe exposure to respirable silica

Dust (including the respirable size fraction) becomes entrained in theatmosphere tram the dumping of fine material from the drill dust collector

Researchers studyi ng dust generation fromblasthole drills developed a simple, quick fixthat reduced respirable dust concentrations

by more than 63% at the dump point. This device,which requires almost no maintenance, is small andinexpensive, and it will help operators maintain com­pliance to the dust standard. The thought processwas that a reduction of respirable dust at any of themultiple sources on the drill, in this case the dustcollector dump point, should reduce the total res­pirable dust generated by the drill.

The U.S. Mine Safety & Health Administration(MSHA) has a coal respirable dust standard of 2 mil­ligrams per cubic meter (mglm 3 ) for an 8-hour shift.MSHA and mine operators periodically collect per­sonal dust samples to determine compliance withthis dust standard.

~\N.e.rnj.com .July 2004 • E&MJ 29

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collector fines dumped onto the bench. The NationalInstitute for Occupational Safety and Health's(N 10SH) Pittsburgh Research Lab (PRU has beenstudying different methods to reduce respirable dustfrom these sources. During its testing, the instituteidentified a simple solution that reduces the amount ofrespirable dust generated at the collector dump point.

The Problem:Dust Collector Fines DumpThe drill's dust collector dumps fine material onto thebench and generates respirable dust. The device col­lects dry dust from the space below the drill table thatis enclosed by the shroud. This area contains respirabledust from the fine material removed from the drill holeby the bailing air. This respirable dust frequently con­tains high silica content.

A high-speed fan is used to pull dust-laden air fromunderneath the drill table into the collector. This airis then routed through filters to remove the dust par­ticles and clean air is discharged through the fan.The removed dust particles are captured on the fil­ters. The collector then uses a high-pressure pulsejet of air to backflush the filters, removing the mate­rial off the filters at regular intervals. The back­flushing occurs several times per minute. The back­flushed material drops from the bottom of the col­lector onto the ground beside the drill machine.

The dust collector dump point rests anywhere from24 to 36 inches above the bench. Dumping the finematerial from this height causes entrainment of therespirable size fraction of this material into the air.Recently, the average respirable dust concentrationmeasured at the dump location at a surface coalmine in Kentucky varied from approximately 0.53 to1.34 mgim3 , in time periods ranging from 2.9 to 5

hours. These concentrations are an average of threedust measurements taken with one personal-data­ram gravimetric and two gravimetric filters all using10-mm Dorr-Ol iver cyclones.

The measurements were taken within a distance of 2to 3 ft from the dust collector dump point. The pre­vailing wind directions were used to determine theplacement of the sampling equipment with the sam­plers being placed downwind of the dump point. Oncethe location was set with respect to the drill, it waskept constant throughout the day of testing. However,the sampling location did move with the drill to main­tain the 2 to 3 ft distance from the dump point.

The Solution: Brattice InstallationTo reduce the respirable dust concentrations at the col­lector dump point, a piece of brattice cloth was attachedto the dust collector dump point using a large hoseclamp. This dust shroud is installed over the existingrubber boot attached to the dust collector dump point.

The length of brattice cloth (or similar material)should be sufficient to allow it to extend from the dustcollector dump point to the ground. It should be cutso that it is only long enough to Just touch the groundwhen the drill is lowered. When wrapping the clotharound the dust collector dump, the overlap should beplaced so that it is on the outside of the dust collec­tor dump (it should be visible as the installer looksdirectly at the dust collector dump). This overlapallows the cloth to expand as fine material is dumpedto the ground, while containing the entrained res­pirable fraction within its confines Placement of theoverlap on the outside also keeps the fine material offthe drill tracks, which otherwise could cause re­entrainment of the respirable size fraction of thematerial when the drill starts in motion.

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Sept. 25. 2003

Uncontrolled Dust Collector

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Comparison of dust concentrations of uncontrolled dust collector dump to shrouded dust collector dump.

30 E&M.J· ..July 2004 www.e-rnj.com

Dust collector dump point prior to installation.

This simple procedure of creating a dust collectordump shroud is very effective in reducing the res­pirable dust. Respirable dust concentrations mea­sured after installation of the dust collector shroudranged from 0.16 to 0.24 mg/m3 It can be seenthat the reduction of respirable dust generated bythe dust collector dump point can be reduced bybetween 63% and 88%. This reduction is highlydependent upon wind direction and wind speed.

Dustin~Off: AdditionalSafetylips for Drill Operators

Maximize Dry Dust Collection• Provide inside corner flaps for deck shrouds (skirts).• Install an air-ring seal where the drill pipe passes through

the drill deck.• Install or upgrade dust collector fans so the collector air­

flow/bailing airflow ratio is greater than 31.• When safe to do so, raise the level of the drill in steps as

the job progresses to minimize dust leakage between thedeck shrouds (skirt) and the ground.

• Enclose the area around the collector discharge with brat­tice cloth or a flexible hose to prevent recirculation of dustthat has already been collected.

• Lower the deck shroud (skirt) after it has been lifted toshovel cuttings.

• Use proper dust control equipment.

Properly Maintain Dry Dust Collectors• Change collection filters according to the manufacturers'

directions or when they no longer clean properly.• Maintain deck shrouds (skirts). Replace torn shroud mate­

rial and make sure the shroud is as long as possible, buttry not to drag it on the ground when the drill is lowered.

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Dust collector dump shroud after installation

Advantages to th is method of respirable dust reduc­tion are that the material is inexpensive and requiresalmost no maintenance. If the shroud becomesdamaged, it can easily be replaced in 10-15 min­utes requiring little, if any, downtime for the drill.

Author Information: Reed is a mining engineer for NIOSH-PRL alld call bereached at +1.4123865205 (or [-mail· Rreed@cdc.gov). Orgal7lScak also isa mining engilleer for NlOSH and Page is aresearch physicist for NIOSH-PRL.

Maximize Wet Suppression• Add small amounts of water into bailing air This is known

as "wet drilling"• Use equipment with a needle valve control for proper regu­

lation of the water flow rate.• Add water slowly until the visible dust cloud has been sig­

nificantly reduced.

Properly Maintain Wet Suppression Devices• Use awater filter in front of tile needle valve control so that

it does not get blocked by small debris in the water system.• To increase bit life, do not use more water than is needed to control dust.

Properly Use and Maintain Enclosed Cabs• Make sure enclosed cabs provide a sufficient flow of heat­

ing and cooling air.• Wash out the cab frequently, but do not sweep or use com-

pressed air to blowout dust.• Replace torn or missing cab seals.• Keep the cab door closed when drilling.• Change air filters frequently.

Source: This information was produced by an interagency team fromthe Mine Safety and Health Administration (MSHA), OccupationalSafety and Health Administration (OSHA), and National Institute forOccupational Safety and Health (NIOSH).

..July 2004 • E&M.J 31

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