METHODS FOR SAMPLING AND INORGANIC ANALYSIS OF COAL
U.S. Geological Survey Bulletin 1823

Edited by D.W. Golightly and F.O. Simon

Preparation of Coal for Analysis

By F.G. Walthall and S.L. Fleming, II

Abstract

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 instrumental analyses.

INTRODUCTION

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, 1985).

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 the submitter.

EQUIPMENT AND PROCEDURES

Sample Preparation (Grinding) Laboratory-- Equipment

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 chemical analysis.

1. Jaw crusher, partially corrugated manganese steel jaw plates, model 2X6, Sturtevant Mill Company, Boston, MA.
2. Vertical grinder, alumina ceramic plates with aluminum ring, aluminum ore pan, model 6R (catalog no. 242-72A), Bico Braun, Inc., Burbank, CA.
3. Rolls crusher, 8X5, Sturtevant Mill Company, Boston, MA.
4. Mixer-mill, model 8000, Spex Industries, Edison, NJ.
5. Kiln, 6 kW.
6. Laboratory balance, 0.005 to 500 g.
7. Riffle splitter, with pans, Wards Natural Science Establishment, Inc., Rochester, NY.
8. Plastic bag sealmaster, Packaging Aids Corporation, San Francisco, CA.
9. Sieves, nylon screen in methacrylate rings, 100 mesh, Spex Industries, Edison, NJ.
10. Exhaust hoods, size and location shown in figure 8.
11. Aluminum pans (23 cm circular "pie" pans).
12. Plastic drying pans, sides less than 38 mm high, sample spread less than 25 mm deep.
13. Porcelain crucibles, 65 to 70 g.
14. Wire brush, stainless steel.
15. Spatula, stainless steel.
16. Polystyrene jars, 120 mL, 5.8-cm diameter, with lids.
17. 17. Polystyrene vials (26 mL) with caps.
18. 18. Polyethylene vials (polyvials, 3.7 mL) with snap caps.
19. 19. Paper cartons, 0.55 L, 8.5-cm diameter.
20. 20. Plastic bags, 15 ´ 30 cm, sealed by plastic bag sealmaster (item 8).
21. 21. Wax paper, 30 ´ 46 cm.
22. 22. Lint-free paper towels.
23. Cleaning sand (clean quartz sand).
24. 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 (30 psi).)

Safety Equipment and Provisions

1. Safety goggles for protection of eyes from small projectiles and dust from grinding machines, Macalaster Bicknell Company, Millville, NJ.
2. Ear covers (muff type) or inserts for protection from loud noises emitted by grinding machinery.
3. Laboratory coats or coveralls.
4. Plastic gloves, disposable.
5. Safety shoes with protective steel toes.
6. Fire extinguishers (ABC tri-class dry chemical, 5.25 kg).
7. Explosion-proof switches and light fixtures in grinding laboratory.
8. Respirator masks with interchangeable paper filters.
   

Safety Procedures

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 for coal.

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 laboratory.

1. Identify all grease fittings and keep the fittings capped.
2. Lubricate each piece of equipment in accordance with an established schedule that is posted near the device.
3. Replace worn parts on jaw crusher.
4. Resurface worn ceramic plates that are used on the vertical grinder.
5. Replace worn drive belts.
6. Keep drawings and brochures related to equipment on file for lists of proper replacement parts and for instructions on proper lubrication.
   

Sample Preparation (Grinding) Laboratory--Facilities

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 halon^TM are suggested.

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

1. 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.
2. 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.
3. Containers needed for sample splits from the crushing process are cleaned with a stream of air and labeled prior to the crushing procedure.
4. 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.
5. Just prior to crushing the first sample, the gap between the jaws of the jaw crusher is adjusted to approximately 4 mm.
6. 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 submitter.
7. 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).
8. 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.

Ashing Procedure

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.

1. 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 cycle.
2. 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.
3. 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.
4. 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.
   
5. Forty 118 mL polystyrene jars are labeled, and three 6-mm-diameter glass beads are placed in each jar.
6. 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 Equipment

1. Make certain that the vents on the hoods are open; switch on electrical power to the exhaust system.
2. For the jaw crusher,
   1. Remove loose dust on the plates and surrounding area with a fast stream of air from the compressed-air line.
   2. 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.
   3. Wipe off the plates with a water-dampened sponge, and dry the plates with a stream of air from the compressed-air line.
   4. Finally, wipe off the plates with a Kimwipe^TM tissue soaked with acetone.
   5. Blow away loose dust from the pan with a stream of air; then, clean the pan with a water-dampened sponge.
3. For the vertical grinder,
   1. Blow away loose dust from the plates and pan with a stream of air.
   2. Pass clean sand through the grinder, as you would in grinding a coal sample, and repeat step 3.a.
4. In the work area and hood,
   1. Wipe the inside of the hood and the counter space with a water-dampened sponge.
   2. Dry the cleaned surfaces with a stream of air.
   3. 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.

REFERENCES

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. 298-312.

_______ 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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