METHODS FOR SAMPLING AND
INORGANIC ANALYSIS OF COAL
U.S. Geological Survey
Edited by D.W. Golightly and
Preparation of Coal for
By F.G. Walthall and S.L.
Bulk quantities of coal weighing 3 to 15 kg are
individually reduced to approximately 150 m m (100 mesh) by
comminution procedures that minimize contamination by grinding
surfaces or by other samples. Seventy grams of each pulverized
coal sample is oxidized at 525° C for 36 h to determine the
percent ash and to provide ash required for chemical and
All procedures described in subsequent sections
of this bulletin on the chemical and instrumental analysis of
coals depend upon the comminution step. The pulverization of bulk
coal samples from the field serves both to homogenize the coal,
which typically is quite heterogeneous, and to reduce the
material to small particles needed for rapid ashing and
dissolution. The comminution of a field sample and the splitting
of the resulting pulverized sample into portions to be
distributed to various laboratories, while minimizing
contamination from grinding surfaces, sieves, and other coals,
are essential to the success of all subsequent chemical
measurements. Thus, the comminution process is of critical
importance, and all aspects of the laboratory arrangement and of
procedures for grinding coals must be carefully planned (Swaine,
Coals submitted for chemical analysis are first
received and prepared by the sample preparation (grinding)
laboratory. The typical sizes of individual field samples vary
from 3 to 15 kg. The normal preparation procedure requires that
each air-dried coal sample pass through a jaw crusher; one
subsample (split) of the crushed material (2 to 4 mm, or 5 to 10
mesh) is then taken for the ultimate and proximate analyses, and
another split is reduced to approximately 150 m m (100 mesh) by a
vertical grinder for chemical analysis. An additional split is
kept for archival storage, and the excess sample is returned to
EQUIPMENT AND PROCEDURES
Sample Preparation (Grinding)
The instrumentation, equipment, and related
items required for the safe operation and maintenance of a
grinding laboratory for coal are listed here. The kiln and
balance are included because this laboratory both determines the
ash from coal and supplies coal ash to other laboratories for
- Jaw crusher, partially corrugated
manganese steel jaw plates, model 2X6, Sturtevant Mill
Company, Boston, MA.
- Vertical grinder, alumina ceramic plates
with aluminum ring, aluminum ore pan, model 6R (catalog
no. 242-72A), Bico Braun, Inc., Burbank, CA.
- Rolls crusher, 8X5, Sturtevant Mill
Company, Boston, MA.
- Mixer-mill, model 8000, Spex Industries,
- Kiln, 6 kW.
- Laboratory balance, 0.005 to 500 g.
- Riffle splitter, with pans, Wards Natural
Science Establishment, Inc., Rochester, NY.
- Plastic bag sealmaster, Packaging Aids
Corporation, San Francisco, CA.
- Sieves, nylon screen in methacrylate
rings, 100 mesh, Spex Industries, Edison, NJ.
- Exhaust hoods, size and location shown in figure 8.
- Aluminum pans (23 cm circular
- Plastic drying pans, sides less than 38 mm
high, sample spread less than 25 mm deep.
- Porcelain crucibles, 65 to 70 g.
- Wire brush, stainless steel.
- Spatula, stainless steel.
- Polystyrene jars, 120 mL, 5.8-cm diameter,
- 17. Polystyrene vials (26 mL) with caps.
- 18. Polyethylene vials (polyvials, 3.7 mL)
with snap caps.
- 19. Paper cartons, 0.55 L, 8.5-cm
- 20. Plastic bags, 15 ´ 30 cm, sealed by
plastic bag sealmaster (item 8).
- 21. Wax paper, 30 ´ 46 cm.
- 22. Lint-free paper towels.
- Cleaning sand (clean quartz sand).
- Compressed-air supply, filtered, with hose
and nozzle. (For safety, the pressure at which the
compressed air is supplied should be kept below 0.2 kPa
Safety Equipment and Provisions
- Safety goggles for protection of eyes from
small projectiles and dust from grinding machines,
Macalaster Bicknell Company, Millville, NJ.
- Ear covers (muff type) or inserts for
protection from loud noises emitted by grinding
- Laboratory coats or coveralls.
- Plastic gloves, disposable.
- Safety shoes with protective steel toes.
- Fire extinguishers (ABC tri-class dry
chemical, 5.25 kg).
- Explosion-proof switches and light
fixtures in grinding laboratory.
- Respirator masks with interchangeable
As in all procedures for the comminution of
geologic materials, precautions must be taken to protect the
operator of grinding equipment from dust inhalation, the noise of
the machinery, small projectiles emanating from the grinding
process for a sample, and injury (especially to the hands) that
can be inflicted by the powerful machinery required for grinding
of samples. Moreover, high concentrations of coal dust in air can
constitute an explosion hazard. Thus, an adequate ventilation
system and the complete absence of high-temperature sources
(cigarettes, sparks from electrical switches, electrostatic
sparks, flames, etc.) are essential for a safe grinding facility
Safety goggles, ear protection, a laboratory
coat, and an air-filter mask should be used at all times while
operating the crushing and grinding equipment. A rapid stream of
compressed air, used in cleaning grinder surfaces, should always
be directed away from the operator. Electrical power to grinding
equipment should be switched off before hands or tools are
inserted into the machinery. Periodic medical examinations, which
may include chest X-ray examinations to reveal developing
respiratory disorders such as silicosis, are generally considered
to be a good preventive measure.
Maintenance of Equipment
Adequate maintenance of equipment basically
consists of regular lubrication and replacement of worn
components. The following procedure is suggested for the grinding
- Identify all grease fittings and keep the
- Lubricate each piece of equipment in
accordance with an established schedule that is posted
near the device.
- Replace worn parts on jaw crusher.
- Resurface worn ceramic plates that are
used on the vertical grinder.
- Replace worn drive belts.
- Keep drawings and brochures related to
equipment on file for lists of proper replacement parts
and for instructions on proper lubrication.
Sample Preparation (Grinding)
The grinding laboratory, which contains heavy
machinery that is capable of producing significant floor
vibrations, should be located on the ground floor of a building.
The laboratory should have no overhead water plumbing, including
fire-sprinkler systems, because the failure, or breakup, of such
plumbing presents a serious hazard to both the equipment and the
operator. Fire extinguishers that use carbon dioxide or halonTM
The arrangement of the laboratory currently
used by the U.S. Geological Survey for all grinding procedures on
coal is shown in figure
8. This laboratory occupies an area of
51 m2 and has four specially constructed exhaust hoods
that are ducted to a "rotocone" dust collector.
Electrical power for equipment is made available through eight
duplex outlets (single phase, 110-V alternating current, 20 A)
and four outlets providing three-phase, 220-V alternating
current, 30 A. Compressed air is provided at five outlets, and
hot and cold tapwater are available at the sink. A water drain is
located in the center of the floor.
Adjacent to the laboratory is an office area
(31 m2), in which field samples are received and
information concerning each sample is recorded. A sample-drying
room (20 m2) is used both for short-term storage and
for drying of samples. Archives of samples are stored in a
separate area of the facility.
Crushing and Grinding
The coal sample, as received from the field,
consists of 3 to 15 kg of material that first is reduced to a
particle size of 2 to 4 mm (5 to 10 mesh) in a jaw crusher. This
crushed material is then divided into three splits. The first
split is forwarded to a laboratory for standard ultimate and
proximate analyses. The second split, which is intended for
chemical analysis and archiving within the U.S. Geological
Survey, is pulverized to 150 m m (100 mesh) in a vertical
grinder. The remaining split is returned to the submitter. These
and subsequent steps are outlined in figure 9, which is a flowchart
that shows the treatment and routing of each sample.
Description of Procedure
- Generally, samples are received in plastic
bags that are tightly packed into a cubic box that has a
30-cm edge. The individual plastic bags, which have been
labeled in the field, are removed from the box and
necessary recordkeeping is first completed. Special
grinding and routing instructions that accompany the
sample are noted on a form that also has descriptive
information pertaining to the coal.
- Samples that need air drying are
identified. Typically, moisture is readily apparent on
these samples, or the coal powder commonly present in the
samples does not move about freely as the container is
agitated. Each of these samples is poured into a plastic
or aluminum pan, and each of the sample numbers is
written on a strip of masking tape attached to the pan.
The empty plastic bag is placed under the pan for later
use. Wet samples typically are air dried for one week;
however, longer drying times may be required for samples
that tend to stick to surfaces during grinding.
- Containers needed for sample splits from
the crushing process are cleaned with a stream of air and
labeled prior to the crushing procedure.
- Samples are taken into the grinding
laboratory to be crushed one at a time, while the
remainder of the samples are stored on a cart outside the
door of the grinding laboratory. This practice reduces
the possibility of cross contamination.
- Just prior to crushing the first sample,
the gap between the jaws of the jaw crusher is adjusted
to approximately 4 mm.
- After crushing the entire sample, the
sample is homogenized by rolling it on a sheet of waxed
paper. Then, the homogeneous, crushed sample is fed into
a riffle splitter (fig. 10) to produce
splits A and B. A minimum of 100 g of sample from the
first split is poured into the container for samples that
are intended for ultimate and proximate analyses (fig. 10, split A). The second split (fig. 10, split B) is fed again into the riffle splitter (fig. 10, splits C and
D), and a 15-cm-long plastic
bag is filled from split D for return to the submitter.
Split C (fig.
10) is distributed into a paper
carton (550 mL), a polystyrene vial (26 mL), and a
snap-cap polyvial (3.7 mL); the paper carton and the
polystyrene vial are filled approximately two-thirds
full, and the polyvial is filled to within 3 mm of the
top. These individual portions are to be used for ashing
and for subsequent chemical and instrumental analyses.
All remaining sample is placed in the original plastic
bag, which is then resealed and returned to the
- The simultaneous use of two vertical
grinders is recommended. This arrangement enables a
machine operator to pulverize one sample while the
previously pulverized sample is being split and the other
grinder is being cleaned. Each sample is split by a
riffle splitter inside a hood (46 ´ 61 ´ 61 cm).
- Splits must be prepared for five
laboratories: a 20-g split for analysis by neutron
activation is packaged in a paper carton; a separate
3.5-g sample for delayed-neutron determinations of
uranium and thorium is placed in a polyvial; a 140-g
split for the ultimate and proximate analyses is placed
in a metal can; 70 g of coal is split for
high-temperature (525° C) ashing; and the submitter
receives a 140-g portion in plastic bags.
A similar method for preparing coal samples is
described in the ASTM (1984a) book of standards.
Pulverized coal samples are ashed in an
electrically heated kiln that can ash 40 samples simultaneously.
The steps followed in the ashing process are listed here.
- Forty sequentially numbered porcelain
crucibles (or evaporating dishes) are cleaned and dried
prior to ashing the coal samples. The weight of each
crucible, approximately 70 g, is sufficiently constant to
make repeated weighings unnecessary for each ashing
- Approximately 70 g of sample is weighed
into each crucible. Appropriate records are maintained
for the weights of the sample and the crucible and for
the sample number or name used by the laboratory.
- The crucibles are placed in the kiln. The
electrical power to the kiln is switched on, and the kiln
is slowly heated to 200° C. After the kiln is operated
at 200° C for 1.5 h, the temperature is increased to
350° C and is maintained at that temperature for 2 h.
Finally, the temperature is increased to 525° C and the
ashing is completed at that temperature; generally, a
period of 36 h is required.
- After 36 h, the electrical power is
switched off and the kiln and samples are allowed to cool
(1 to 2 h). After the crucibles have cooled to room
temperature, the "crucible-plus-ash" weight is
measured for each sample. These data are recorded, and
the percent ash is calculated.
- Forty 118 mL polystyrene jars are labeled,
and three 6-mm-diameter glass beads are placed in each
- Ash from each crucible is then transferred
into an individual polystyrene jar, and the jar is
closed. Each jar subsequently is placed into a mixer-mill
and agitated for 30 s. These homogenized samples are
provided to the chemical laboratories for analysis.
Another method for determining ash in coal and
coke is described by the ASTM (1984a) book of standards.
Cleaning of Work Area and
- Make certain that the vents on the hoods
are open; switch on electrical power to the exhaust
- For the jaw crusher,
- Remove loose dust on the plates
and surrounding area with a fast stream of air
from the compressed-air line.
- Remove buildup of sample on the
crusher plates with a wire brush, and again blow
away loose material with a fast stream of air
from the compressed-air line.
- Wipe off the plates with a
water-dampened sponge, and dry the plates with a
stream of air from the compressed-air line.
- Finally, wipe off the plates with
a KimwipeTM tissue soaked with
- Blow away loose dust from the pan
with a stream of air; then, clean the pan with a
- For the vertical grinder,
- Blow away loose dust from the
plates and pan with a stream of air.
- Pass clean sand through the
grinder, as you would in grinding a coal sample,
and repeat step 3.a.
- In the work area and hood,
- Wipe the inside of the hood and
the counter space with a water-dampened sponge.
- Dry the cleaned surfaces with a
stream of air.
- Once each week, thoroughly clean
the entire floor with a broom and dust pan.
Vacuum cleaning with a cleaner that does not
generate sparks is quite appropriate.
American Society for Testing and Materials
(ASTM), 1984a, D2013-72(1978) Standard method of preparing coal
samples for analysis: 1984 annual book of ASTM standards,
Petroleum products, lubricants, and fossil fuels, sect. 5, v.
05.05: Gaseous fuels, coal, and coke: Philadelphia, ASTM, p.
_______ 1984b, D3174-82 Standard test method
for ash in the analysis sample of coal and coke from coal: 1984
Annual Book of ASTM Standards, Petroleum products, lubricants,
and fossil fuels, sect. 5, v. 05.05: Gaseous fuels, coal, and
coke: Philadelphia, ASTM, p. 401-404.
Swaine, D.J., 1985, Modern methods in
bituminous coal analysis: trace elements: CRC Critical Reviews in
Analytical Chemistry, v. 15, no. 4, p. 323.
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