METHODS FOR SAMPLING AND
INORGANIC ANALYSIS OF COAL
U.S. Geological Survey Bulletin 1823
Edited by D.W. Golightly and F.O. Simon
Sampling of Coal Beds for Analysis
By Ronald W. Stanton
Channel and core samples are the primary types of samples collected and analyzed to establish the chemical composition, or quality, of coal beds. The choice between channel or core samples may be predicated on cost and accessibility to the coal bed. The purpose of the investigation or program determines the nature of the sample to be taken and the materials to be included in the sample. Impurities, such as mineral-rich layers, may be included in the sample, depending on the intended use of the data.
A channel or core sample should (1) represent an equal volume from the top to the bottom of the coal bed and (2) experience minimal comminution and (3) not be split in the field. Discrimination between impure coal, carbonaceous shale, and coal sometimes is subjective and consequently affects the precision of the sampling method. A coal bed can be described and sampled by facies by delimiting the laterally continuous subunits of the bed and by evaluating their effect on bed quality, particularly in the collection of channel samples. Descriptions of core samples are facilitated by geophysical logging techniques and by X-ray radiography.
The number of samples required to characterize adequately a coal bed depends on the thickness, quality variability, and size of the deposit. In general, the locations for sampling in mines should be separated by distances that are comparable to spacings of other outcrop or core locations in the same bed. Additionally, two to three closely spaced locations (10 to 100 m) may yield useful data on local variability of certain quality parameters.
PURPOSES OF SAMPLING A COAL BED
Samples submitted for chemical and physical analyses are collected for a variety of reasons, but the collection of each sample should always conform to certain guidelines. The application of precise techniques in sample collection helps to ensure that data from each analysis performed on the samples will be useful. For interpretations and comparisons of elemental compositions of coal beds to be valid, the samples must be collected so that they are comparably representative of the coal bed. Such interpretations and comparisons should never be based on data from different types of samples.
The purpose in sample collection and the kinds of analyses to be performed on the sample dictate the type and nature of the sample that should be collected (see table 3). For samples used in determining rank, mineral-rich layers greater than 1 cm must be excluded so that only the coal intervals are sampled (ASTM, 1986a). In contrast, samples intended for chemical analysis commonly include mineral-rich layers that are less than 10 cm thick (Swanson and Huffman, 1976); samples obtained for petrographic analysis generally consist of lithologically distinct parts of the coal bed taken at separate intervals; and samples intended for washability testing may consist of all material, rock and coal, between the roof and the floor of the coal bed.
Other purposes may also be best served by channel or core samples. If the objective is to represent the resources of a coal bed for mine planning and future extraction methods, channel and core samples are necessary. In resource and reserve characterization, coal sample subtypes include those that represent the whole bed, coal facies (laterally extensive parts of the bed), and total coal interval. Stratigraphic descriptions of the coal bed at and between sample localities should enable correlations of subunits, or facies, of the coal bed (fig. 2). This procedure will aid in understanding abrupt quality or thickness changes in the bed. In contrast, if the purpose in sample collection is to represent a mined product, a properly obtained conveyor-belt sample (ASTM, 1986c) may be preferable to channel or core samples.
In some cases, obtaining channel samples may not be possible, thus leaving core samples as the only practical alternative. Some supplementary coal-quality data can be obtained by down-hole geophysical techniques (Lavers and Smits, 1976), regardless of whether a core is recovered.
HISTORICAL PERSPECTIVE OF SAMPLING
The methods of sampling described by Burrows (1907) and Holmes (1911) involve an exclusionary procedure in which the sampler eliminates partings or impurities (mineral-rich layers) greater than 1 cm and lenses or concretions of sulfur or other impurities greater than 5 cm in diameter and 1.3 cm thick "if in the judgement of the sampler, they are being excluded by the miner from the coal as loaded out of the mine or as shipped" (Holmes, 1911, p. 1). The reason for such a procedure is "to obtain samples that represent, as nearly as possible, the coal that is produced commercially from the mine" (Holmes, 1911, p. 8). This was the purpose of sampling coal during the early 1900's by the U.S. Bureau of Mines and the U.S. Geological Survey (Burrows, 1907; Campbell, 1907). For the fuel inspectors of the U.S. Bureau of Mines, the sampling purpose was qualified to represent the particular mining practice at an individual mine and at a specific time. In contrast, the samplers from the U.S. Geological Survey attempted to approximate the average composition of produced coal without regard to site-specific mining practices; therefore, their samplers strictly followed the exclusionary procedure. In both methods, partings and pyrite of certain thicknesses were excluded from the sample. The U.S. Bureau of Mines inspectors excluded only those partings excluded by the miner, and the U.S. Geological Survey samplers excluded layered, mineral-rich material of certain thicknesses.
SAMPLING FOR RANK CLASSIFICATION
To classify coal by rank, the American Society for Testing and Materials (ASTM) refers to the publication by Holmes (1911) as the method of sampling (ASTM, 1986a). In addition, the procedure described by Schopf (1960), which restates the rule for exclusion of material, attempts to discriminate obvious partings from impure coal; the latter is sampled as part of the coal bed. For purposes of rank classification, perhaps either variation of the procedure is adequate. However, objective criteria to distinguish precisely coal from noncoal layers are not available. In the calculation of rank-determining values, the fixed carbon or calorific-value mineral matter is "removed mathematically." This continued practice of excluding mineral layers in samples from a bed, even by a mathematical correction for mineral matter, is probably an outgrowth of concerns by such early workers as Fieldner and Selvig (1930), who proposed that coal rank be calculated on a pure-coal or unit-coal basis.
If an approximation of the characteristics of the mined product is needed, perhaps a better sample for determining rank would be some mechanical separation of a whole-bed sample. The original purpose of the exclusion of partings was to approximate or verify more closely ash and sulfur concentrations for coal delivered by train cars from mined areas (Burrows, 1907; Campbell, 1907). During the days of blasting and hand loading of coal, discrimination among impurities could be made by those handling the coal. However, high-production equipment, such as a continuous miner or long-wall mining equipment, does not differentiate between the coal and the partings during extraction. Parting material generally is removed by jigs or air tables in the first stage of coal preparation.
SAMPLING FOR CHEMICAL ANALYSIS
Another purpose for collecting a channel or core sample is to evaluate the chemical composition of the coal. Known concentrations and distributions of major, minor, and trace elements in a coal bed enable an assessment of the possible environmental impacts and technological problems that could occur from use of the coal. A modification of the exclusionary procedure is commonly used because rock partings may contribute to the variability in concentrations of certain elements or to quality parameters. Shale, siltstone, or nonbanded, impure-coal (bone) layers less than 10 cm thick are included in whole-bed channel samples if it is probable that these materials will be mined along with the coal (Swanson and Huffman, 1976). Swanson and Huffman (1976) also recommended that special samples of non-coal materials be collected to determine their contribution to abnormal elemental concentrations. However, "judgement . . . must be applied toward obtaining samples which will be most representative of the coal bed" (Swanson and Huffman, 1976, p. 2).
SAMPLING FOR OTHER PURPOSES
Other purposes of sampling, such as determining the coal and the noncoal facies and predicting the geometry and quality of a coal bed, may require detailed sampling procedures by bench or by coal facies. In these procedures, the effects of including or excluding parting materials must be assessed. The combined effects of the laterally extensive layers of the bed on the quality of the whole bed also can be evaluated.
In all attempts to sample coal beds, the purpose of collecting a coal sample determines the criteria applied to sampling. Precautions must be taken when comparing certain elemental concentration values so that samples taken for different purposes are not directly compared. For example, in samples collected and analyzed to estimate rank, mineral layers greater than 1 cm thick are excluded, and for samples collected and analyzed to determine the trace-element composition, mineral layers less than 10 cm thick are excluded. Direct comparisons of data from these two suites of samples are suspect. Thus, ash or sulfur isopleth maps generated from this mixed-sample-type set should not be interpreted as being related to coal-forming processes. Furthermore, the application of the exclusionary procedure does not reflect present mining practices. Other questionable comparisons may be made if data from bench or facies samples are mixed with data from whole-bed samples.
PROCEDURE FOR CHANNEL SAMPLING
For both surface and deep mines, the ASTM (1986a) standard for collecting channel samples of coal should be followed. The following procedures are used to collect channel samples of coal.
PROCEDURE FOR SAMPLING DRILL CORE
The drill core should be the most representative type of sample of a coal bed. However, in some cases, coal is lost during coring, and the complete bed is not represented by the recovered drill core. In any case, geophysical techniques, such as gamma and gamma-gamma (density) logs, should be used to compare the thickness of the bed to the thickness of the core recovered (fig. 3).
After the drill core barrel is brought to the surface, the collection of a core should involve the following procedure:
Next, X-ray radiography (Standards Association of Australia, 1982; Stanton and others, 1983) should be performed on the core. If the exclusionary procedure is used, radiography is a reliable technique for discerning coal from noncoal layers. The use of PVC pipe (fig. 4) to encase the coal core provides a way to transport the core without major disorientation and, in many cases, without further breakage.
COAL BED DESCRIPTIONS
Channel-sampled coal faces should be described as completely as possible. Minimally, the thickness of the bed, the thickness and stratigraphic positions of partings, the pyrite layers, and the mineral-cleat fillings should be measured and described. Subunits, or facies, that are lithologically distinct and that have lateral continuity can be identified by the frequency and thickness of vitrain bands and attritus (Schopf, 1960). Individual layers, such as vitrain (bright, homogeneous layers greater than 1 mm thick) and fusain ("charcoal"), generally are not laterally continuous; they are commonly lenticular because they form from compressed plant stems, leaves, and roots. For the most part, facies are composed of assemblages of vitrain and attrital layers or other rock that can be identified and recognized in the coal bed (fig. 5). These facies frequently can be mapped for miles, and commonly have a narrow range of coal quality (fig. 6).
Visual descriptions of core are difficult to make unless the core is broken. Visual descriptions can be complemented with data from common down-hole, geophysical measurement techniques, such as gamma, gamma-gamma (density), and resistivity (Lavers and Smits, 1976). A high-resolution geophysical density log can provide data that aid in the identification of coal facies (fig. 3). High-resolution density logging also can provide measurements of thickness to the nearest 3 cm and a profile of the quality of the coal bed (fig. 3). X-ray radiography (fig. 7), combined with visual observation of the core and density log, provides another tool for recognizing the dominant facies in a core. Comparison of an X-ray radiograph to a density log can be used to (1) discriminate between layers of pyrite and clay, impure coal, and coal, (2) determine which parts of the coal bed were not recovered in the core, and (3) identify the different facies of the coal bed.
IDENTIFICATION OF CARBONACEOUS SHALE LAYERS
High-ash coal or mineral-rich partings generally are visually distinct in a coal bed. However, in most cases, criteria or tests cannot be applied objectively in the field to differentiate impure coal (25 to 50 weight percent ash) from carbonaceous shale (>50 weight percent ash). Any classification that involves such terms as "bone," "billy coal," or "rash" generally has only local significance and is not useful for coals of different rank or type (Schopf, 1960). Some field methods may provide aid in discriminating carbonaceous shale from coal. In surface mines, a gamma-ray scintillometer (a hand-held instrument) is used to locate impure layers in low-ash coal. For the drill core, an X-ray radiograph and a gamma-gamma (density) down-hole geophysical log can be compared with the coal core.
For sample collection from deep mines, no comparable instrument that is "mine safe" is available. This deficiency in instrumentation creates difficulty when applying the exclusionary procedure. However, even if this procedure is not followed, another difficulty exists in precisely sampling a coal bed at the points of contact with adjacent rock strata. Commonly, coal bed contacts between the floor and roof rocks are sharp. However, in places, contacts are interbedded, transitional, or located in a nonbanded layer, particularly at the top of the bed.
SAMPLING THICK COAL BEDS
A common practice of sampling thick coal beds (greater than 10 m thick), either by core or by channel, is to divide the bed into intervals no thicker than 1.5 m. In many cases, thick coals are sampled in 0.7 m sections; such sampling provides a detailed stratigraphic section of the bed. Core samples are best for thick-bed sampling. Beds greater than approximately 3 m thick are difficult to sample reliably by the channel method, and in many places the thick beds are mined in layers in surface mines, thus decreasing the probability of obtaining a fresh sample of the entire bed. Where a high resolution density log is obtained, the core can be subdivided into lithologically distinct units.
Proper collection of coal samples requires representative material from a coal bed. Uniform volumes by core samples, by channel samples, or by properly collected stream (run-of-mine) samples (ASTM, 1986b) are the common types. The selection of sample types is determined by the purposes of the sampling project or program. Certain sample types may not be appropriate for particular analyses or comparisons.
Valid comparisons of analytical data are the user's responsibility and depend largely on (1) the types of samples, (2) the nature of the samples, such as whole bed or bench, and (3) whether mineral layers or coal partings are excluded from the sample or are sampled separately.
American Society for Testing and Materials (ASTM), 1986a, Standard practice for collection of channel samples of coal in the mine (D4596), in 1986 annual book of ASTM standards, pt. 5.05, p. 519-521.
________ 1986b, Classification of coals by rank (D388), in 1986 annual book of ASTM standards, pt. 5.05, p. 247-252.
________ 1986c, Standard methods for collection of a gross sample of coal (D2234), in 1986 annual book of ASTM standards, pt. 5.05. p. 336-352.
Burrows, J.S., 1907, The importance of uniform and systematic coal-mine sampling: U.S. Geological Survey Bulletin 316, p. 486-517.
Campbell, M.R., 1907, The value of coal-mine sampling: Economic Geology, v. 2, no. 1, p. 48-57.
Fieldner, A.C., and Selvig, W.A., 1930, Present status of ash corrections in coal analysis: Classification of coal, American Institute of Mining and Metallurgical Engineers (A.I.M.M.E.), p. 51-65.
Holmes, J.A., 1911, The sampling of coal in the mine: U.S. Bureau of Mines Technical Paper 1, 22 p.
Lavers, B.A., and Smits, L.J.M., 1976, Recent developments in coal petrophysics: Transactions of Annual Logging Symposium of the Society of Professional Well Log Analysis, 17th, Denver, June 9-12, 1976.
Schopf, J.M., 1960, Field description and sampling of coal beds: U.S. Geological Survey Bulletin 1111-B, 67 p.
Standards Association of Australia, 1982, Guide to the evaluation of hard coal deposits using borehole techniques: Australian Standard 2519-1982, North Sydney, Standards Association of Australia, 58 p.
Stanton, R.W., Moore, T.A., Ruppert, L.F., and Cecil, C.B., 1983, X-ray radiographic description of coal core as an aid for estimating coal quality: Geological Society of America Annual Meeting Abstracts, v. 15, no. 6, p. 695.
Swanson, V.E., and Huffman, Claude, Jr., 1976, Guidelines for sample collecting and analytical methods used in the U.S. Geological Survey for determining chemical composition of coal: U.S. Geological Survey Circular 735, 11 p.
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