Production and Characteristics of Rock Dust Applied in Underground Mines

April 13, 2017

About Greer Lime

• Greer Lime Company is located near Seneca Rocks, West Virginia.

• It was opened in 1960 under the name of Germany Valley Limestone, and was started in order to provide a source of low silica, High Calcium Limestone for dusting Coal mines.

• Production of Lime for steel making, paper mill recovery, and water treatment began a few years later.

• In 1990, the Company changed its name to Greer Lime Company. Today, Greer is in the third generation of family ownership, dating to the early 1900’s.

Manufacture of Rock Dust

• Rock Dust is produced by several companies, some as a primary product, others as a small offering of a company’s production.

• Rock Dust supply tends to be regional in nature, due to the high cost of freight compared to the value of the product.

• Greer Lime sells Rock Dust in Bulk, 2000# and 3000# Supersacks, and 50# paper sacks. All bagged products are hooded with plastic to prevent moisture from being entrained in the product.

Specifications for Rock Dust

• Rock Dust used in Coal mines must meet the requirements of 30 CFR 75.2 –– Pulverized limestone, dolomite, gypsum, anhydrite, shale,

adobe, or other inert material, preferably light colored.– 100% must pass a 20 mesh sieve, and 70% minimum must

pass a 200 mesh sieve (75 micron).– No more than 5% combustible matter– No more than 4% free and combined silica (SiO2).– The particles of which when wetted and dried will not cohere to

form a cake which will not be dispersed into separate particles by a light blast of air.

Manufacture of Rock Dust

• The manufacturing process varies slightly from facility to facility. Greer Lime operates an open pit quarry.– Production benches are shot to produce material minus 48” in

diameter.– The material is loaded into trucks and hauled to a primary and

secondary crushing plant where the stone is reduced to minus 2-1/2”.

– A secondary screening system supplies +3/4” diameter stone to the Kiln Department where it is processed into lime.

– The minus ¾” stone is used for Rock Dust manufacture, and as scrubbing stone for Coal Fired power plants to reduce SO2 emissions.

Manufacture of Rock Dust

• Greer, like many producers uses a Roller Mill to produce Rock Dust.

Manufacture of Rock Dust

Quality Control

• Greer uses both in-process control and Lab Quality Assurance– Every four hours, the mill operator pulls a finished product

sample from an automatic sampler.– This sample is split, and the operator tests it on an air jet sieve

to determine the per cent passing 200 mesh. If the sample is near or below 70%, the dynamic separator speed is adjusted up. An extremely fine sample (+80% passing 200) results in the separator being slowed down. The goal is to average between 70% and 78% passing 200.

– In reality, the changes are very small and infrequent due to the stability of the process.

Quality Control

• The lab will test Daily composite samples also for fineness to verify the operator results.

• Chemistry is checked daily. Due to the CFR requirements as well as quality requirements of Lime production, stone quality is checked continuously. The lab uses a X-ray diffraction to verify Calcium Carbonate content, Silica Content, and other minerals such as Magnesium Carbonate, Alumina, and Iron.– Each day, sized sample of kiln feed and mill feed are sent to

the laboratory.– Reports are sent daily, bi-weekly, and monthly to our customers

as they require.

Respirable Dust Issues

• Greer participates in a partnership with NIOSH, MSHA, and the IMA-NA on the development of improved rock dusts.– The goal has been to provide a rock dust that will adhere to the

rib, yet disperse readily in case of an explosion.– Early testing favored a change from 70% passing 200 mesh to

95% passing 200 mesh, in an effort to improve dispersability.– Due to lack of a testing facility, it is unproven whether the finer

product will actually react better in an explosion.– The following slides show a particle size distribution of Greer’s

current rock dust versus product averaging 95% passing 325.

Fineness of Rock Dust

• Finer rock dust will increase the minus 10 micron percentage by a minimum of 10%.

• We’ve been asked whether we can remove the minus 10 micron from the rock dust to eliminate a respirable dust hazard, and the answer is “Not at this time, and not at a reasonable cost”.

• Producing a minus 325 mesh product cuts our production in half and doubles our energy consumption.

• Removing the minus 10 micron would require installation of secondary classification systems which efficiencies are questionable, and would cut our production another 50%.

Fineness of Rock Dust

• The more appropriate question might be: “Does the current specification for rock dust provide adequate dispersion in case of explosion, and is the -10 micron content of today’s product acceptable? Would increased --10 micron particles be acceptable?

• We believe more research is needed.• Potential developments include a hydrophobic, treated

rock dust which will readily disperse.• There is ongoing work with foamed rock dusts, but they

currently present their own safety issues, and have not been effectively tested.