USGS Circular 735: Methods

GUIDELINES FOR SAMPLE COLLECTING AND ANALYTICAL METHODS USED IN THE U.S. GEOLOGICAL SURVEY FOR DETERMINING CHEMICAL COMPOSITION OF COAL

ANALYTICAL METHODS FOR DETERMINATION OF MAJOR, MINOR, AND TRACE ELEMENTS IN COAL

The analytical work performed on coal samples received in the U.S. Geological Survey laboratories is outlined in figure 1. An undried 600 g (about 1 qt) split of samples representative of the set collected is sent to the U.S. Bureau of Mines at Pittsburgh, Pa., for the routine coal analysis. This analysis sequence includes (1) proximate analysis (percent ash, moisture, fixed carbon, and volatile matter), (2) ultimate analysis (percent carbon, hydrogen, oxygen, nitrogen, and sulfur), (3) Btu determination, and (4) sulfur analysis (percent organic sulfur, pyrite sulfur, and sulfate sulfur). The analytical methods used by the Bureau of Mines have been described in U.S. Bureau of Mines Bulletin 638 (Staff Office of the Director of Coal Research 1967) and will not be described in this report. The remainder of all analytical work and sample preparation shown in the analysis sequence flow diagram (fig. 1) is performed by the U.S. Geological Survey laboratories.

The Geological Survey laboratories routinely provide the following analytical determinations on coal samples:

Except for the specialized analyses run by the U.S. Bureau of Mines, all the analyses indicated for coal (fig. 1) are those routinely run on rock and soil samples. The forms of sulfur in rocks and soils are determined in our laboratory, and the organic carbon, carbonate, and total carbon contents are also determined.

Each coal sample is poured, as received, into a cone-shaped pile on kraft paper, flattened, and portions separated and collected with a scoop or spatula from random locations in the sample. A 600-g (about 1 qt) sample of each coal sample is thus collected in a plastic bag, placed in a 1-quart ice cream container, and transmitted to the U.S. Bureau of Mines, Pittsburgh, Pa., for ultimate and proximate analysis, and for Btu and forms of sulfur determinations. The remainder of the sample is placed in disposable aluminum pie pans and dried at 25° to 32° C and at about 30 percent relative humidity in an air-circulating oven. Some coal samples take as long as 82 hours to dry thoroughly.

The dried coal sample is crushed, and a 300-g (about 1-pint) reserve is split out for storage to be used for future petrographic, mineralogic, or chemical studies. The balance of crushed coal is ground in a vertical Braun¹ pulverizer equipped with ceramic plates set to pass about 80 mesh, and mixed. The ground coal sample (analytical split) is distributed to the analytical laboratories.

Atomic absorption spectrometry methods are used for the determination of Mg, Na, Cd, Cu, Li, Mn, Pb, and Zn in the ash of coal, and X-ray fluorescence spectroscopy methods are employed for Al, Ca, total Fe, K, P, Si, total S, Ti, and Cl. In addition, 63 elements are looked for by a semiquantitive six-step emission spectrographic method. Of these, 17 are generally found to be of interest in the coal ash: B, Ba, Be, Co, Cr, Ga, Ge, La, Mo, Nb, Ni, Sc, Sr, V, Yb, Y, and Zr. The other 46 elements are also reported when detected by the spectrographic method (table 1), even though 18 of these elements are quantitatively determined by other methods.

A portion of the ground raw coal (25 to 75 g) is weighed and transferred to a 100-ml fused silica dish. The dish is placed in a cold muffle furnace and, with the furnace door partly open, the temperature is gradually elevated over a 4-hour period to 450° C. The temperature is then increased to 525° C and maintained until the sample is completely ashed. An occasional stirring or mixing of the sample during ashing is desirable. The ash is weighed, and the percentage of ash calculated. The ash is mixed thoroughly with a spatula and transferred to a suitable container. About 3 g of coal ash is required for the analyses by six-step spectrographic, X-ray fluorescence, and atomic absorption methods.

A short description of the atomic absorption methods for the determination of MgO, Na₂O, Cu, Li, Mn, and Zn follows: 0.500 g of coal ash is weighed into a 100-ml platinum dish. Ten millilitres of demineralized water, 10 ml HNO₃, and 10 ml of HF are added to the dish. The dish is covered and allowed to stand overnight. Seven millilitres of HClO₄ is added to the dish. The dish is placed on a steam bath for 1 hour and then placed on a hot plate to fume off the acids to near dryness. The dish is removed from the hot plate and the sides of the dish are washed down with water. Five millilitres of HClO₄ is added and the dish is returned to the hot plate. The dish is heated until all acids are evaporated. Twenty-five millilitres of water and 5 ml of HCl are added to the dish. The dish is covered, placed on a steam bath, and digested for 30 minutes. The solution is transferred to a 100-ml volumetric flask and diluted to volume with water. Aliquots or dilutions of this sample are then aspirated into the air-acetylene flame of an atomic absorption spectrometer to determine the elements listed. The sample aliquot used for the determination of Mg was made to contain 1 percent La. The instrumental parameters used for the listed elements are those recommended by the Perkin-Elmer Corp.

Cd and Pb are also determined by atomic absorption spectroscopy on a separate split. In this method 1.000 g of coal ash is weighed and transferred to a 150-ml beaker. Ten millilitres of water and 10 ml HNO3 are added to the beaker. The beaker is covered with a watch glass, placed on a shaking hot plate, and boiled until the volume of the solution is reduced to about 5 ml. The beaker is removed from the hot plate and the sides of the beaker and lid are washed down with about 20 ml water. The beaker is placed on a steam bath and the contents digested for 1 hour. The contents of the beaker are transferred to a 50-ml volumetric flask, cooled, and diluted to volume with water. The solids are allowed to settle overnight. A portion of the clear sample solution is aspirated into the air-acetylene flame of the atomic absorption instrument using deuterium background correction to determine both Cd and Pb. The instrumental parameters used are those recommended by the Perkin-Elmer Corp.

The sample weights and the lower limits of determination by atomic absorption methods for each of the elements in coal ash are as follows:

Element	Sample wt. (g)	Lower limit
Mg	0.5	0.01 percent
Na	0.5	0.01 percent
Cd	1.0	01 ppm
Cu	0.5	10 ppm
Li	0.5	05 ppm
Mn	0.5	25 ppm
Pb	1.0	25 ppm
Zn	0.5	10 ppm

The concentration of each element determined in the coal ash is normally converted to the whole-coal basis using the percent ash value in this calculation. A coal that contains 10 percent ash would lower the above limits of determination by a factor of 10 for the converted values.

X-ray fluorescence methods developed in our laboratory by James S. Wahlberg are employed for the determination of Al, Ca, total Fe, K, P, Si, total S, Ti, and Cl in the coal ash. In this method 0.800 g of coal ash is fused with 6 g of flux (mixture of 43 percent Li₂B₄O₇, 55 percent Na₂B₄O₇, and 2 percent NaBr) in a 20-ml platinum crucible. The NaBr is added to the fusion mixture to facilitate easy removal of the solidified button from the platinum crucible. This fused button is X-rayed and counted to determine the listed elements.

An Automated General Electric¹ vacuum spectrometer is used to determine the listed elements. The instrument parameters used are given in the following tabular form:

		MA² on	X-ray tube	2q angle
Element	Crystal¹	X-ray tube	target	(degrees)
Al	PET	60	Cr	144.67
Ca	LiF	20	Cr	113.08
Total Fe	LiF	20	W	57.52
K	PET	60	Cr	50.64
P	PET	60	Cr	89.40
Si	PET	60	Cr	109.06
Total S	NaCI	60	Cr	144.53
Ti	LiF	60	Cr	86.13
Cl	NaCI	60	Cr	113.91
¹PET, pentaerythritol		²MA, megaamperes

Element³	Sample Weight (g)	Lower limit (percent)
Al₂O₃	0.8	0.2
CaO	0.8	0.02
Total Fe (as Fe₂O₃)	0.8	0.02
K₂O	0.8	0.02
P₂O₅	0.8	0.1
SiO₂	0.8	0.2
Total S (as SO₃)	0.8	0.04
TiO₂	0.8	0.02
Cl	0.8	0.1

A six-step semiquantitative optical emission spectrographic method developed in our laboratory by Myers, Havens, and Dunton (1961) and Myers and Havens (1970) is used to look for 63 elements; the trace elements generally found to be of interest are B, Ba, Be, Co, Cr, Ga, Ge, La, Mo, Nb, Ni, Sc, Sr, V, Y, Yb, and Zr. In this method, 1 part coal ash is mixed with 1.15 parts of a mixture of 9 parts quartz (SiO₂) and 1 part Na₂CO₃. Ten milligrams of the resulting mixture is in turn mixed with 20 mg of pure graphite powder, and this final mixture is burned in a dc arc for 120 seconds, collecting the spectra on photographic plates. The resulting spectra are visually compared with reference standards. The element determinations are identified with geometric brackets whose boundaries are 1.2, 0.83, 0.56, 0.38, 0.26, 0.18, 0.12, and so forth, but are reported as midpoints of these brackets, 1., 0.7, 0.5, 0.3, 0.2, 0.15, 0.1, and so forth; there are thus 6 brackets to the decade. The precision of a reported value is approximately plus-or-minus one bracket at the 68-percent confidence level, or two brackets at the 95-percent confidence level.

The approximate lower limits of determination for the elements analyzed by the six-step spectrographic method in the ash of coal samples are shown in Table 1.

The more volatile elements As, F, Hg, Sb, and Se are determined in the ground raw coal sample. U and Th are also determined on the raw coal sample.

The flameless atomic absorption spectroscopy method (Huffman and others, 1972) is used to determine mercury. In this method, 0.200 g of ground coal is digested under oxidizing conditions using the HNO₃-H₂SO₄-HClO₄ digestion procedure developed by V. E. Shaw (oral commun., 1973). Mercury in the sample solution is reduced to its elemental state with stannous chloride and then aerated from solution onto a silver screen placed in the vapor train. This silver screen is subsequently heated, and the mercury vapor is carried by an airstream to an absorption cell, where its concentration is determined by atomic absorption spectrometry. The lower limit of the determination is 0.01 ppm.

The Rhodamine-B spectrophotometric method of Ward and Lakin (1954) is used to determine antimony. D. R. Norton (oral commun., 1973) of our laboratory has modified this method for coal samples to obtain a lower limit of determination. In this method 1.0 g of raw coal is mixed with a slurry of magnesium oxide and magnesium nitrate. The slurry is dried at 110° C and then ashed in a muffle furnace gradually raising the temperature to 550° C. This ashing technique takes 4 to 5 hours. The ashed sample and magnesium salts are fused with 3.0 g potassium pyrosulfate and leached with 6 N HCI containing glycerol. Sodium sulfate is added to reduce antimony to Sb⁺³. The solution is filtered into a 125-ml extraction flask and the residue washed. After cooling to 15° C, the antimony is oxidized to Sb⁺⁵ with ceric sulfate and the excess oxidant reduced with hydroxylamine hydrochloride. After dilution with water to an acid concentration of 1.5 N, the solution is cooled to 15° C and the antimony chloride complex is extracted with isopropyl ether. The extract is washed and then reacted with an acidic solution of Rhodamine-B to form a red-violet dispersion whose absorption at 560 mm is measured with a spectrophotometer. The limit of determination of this method is 0.1 ppm Sb.

The heteropoly blue spectrophotometric method described by Rader and Grimaldi (1961) is used to determine arsenic. Sample decomposition (1 to 2 g) and sample solution is made with HNO₃, HClO₄, and H₂SO₄ acids. In this method, As is distilled as arsenious chloride after reduction with bromide and hydrazine sulfate and is determined spectrophotometrically. The limit of determination on raw coal is 1.0 ppm.

A fluoride specific-ion electrode is used to determine fluorine. In this procedure 0.250 g of ground coal is mixed in a zirconium crucible with a slurry of MgO and MgNO3. The mixture is dried at 110° C, then ashed in a muffle furnace which is gradually raised to 525° C. The ashed mixture is fused with 1.0 g NaOH over an open burner with the zirconium crucible covered. The crucible and lid are placed in a plastic beaker, water is added to dissolve the fused mass, and then filtered into a 100-ml volumetric flask. The residue is washed with about 5 ml of a 1 percent w/v solution of NaOH, diluted to volume with water, and mixed. A 50-ml aliquot of the sample solution is transferred to a 100-ml volumetric flask, diluted to volume with 1 M ammonium citrate solution, and mixed. Fifty millilitres of this solution is poured into a plastic beaker and the potential is measured by the fluoride-ion electrode. In some cases, about 10 minutes is required for equilibrium to be reached. The lower limit of determination of the method is about 20 ppm.

An X-ray fluorescence method developed by J. S. Wahlberg (written communication, 1972) is followed in the determination of selenium. In this method 2.000 g of raw coal is decomposed with a sodium peroxide fusion. Selenium is then reduced and precipitated with hydrazine sulfate, potassium iodide, and sodium sulfite, with Te added as carrier. The precipitate is collected on a millipore filter for X-ray determination. The lower limit of determination is 0.1 ppm Se.

A delayed neutron activation method described by Amiel (1962) is used to determine these two elements. The raw coal sample of 5.000 g is irradiated in a neutron flux of 2 ´ 10¹² n/cm²/s (neutrons per square centimeter per second) for 1 minute, and within seconds after irradiation is counted for 2 minutes with a ring of 6 boron trifluoride detectors. The lower limit of the determination is 0.1 ppm U and 2.0 ppm Th.

The accuracy of analytical methods as applied to coal samples is rather difficult to evaluate because of the lack of standard samples of coal. Only two National Bureau of Standards standard coal samples are available, NBS Standard Reference Material 1632 and NBS Standard Reference Material 1630. Of these, the NBS-1630 coal has been certified only for its mercury content. The EPA-NBS coal sample (NBS-1632) has been analyzed for selected trace elements by the National Bureau of Standards and also by an interlaboratory roundrobin comparison initiated by the U.S. Environmental Protection Agency. Elements determined in the roundrobin included: As, Be, Cd, Cr, Cu, F, Fe, Hg, Mn, Ni, Pb, S, Se, Tl, Th, U, V, and Zn. The U.S. Geological Survey laboratory was one of the many participating laboratories. Table 2 compares our results with those obtained by the National Bureau of Standards and with the grand mean of all participating laboratories. Our quantitative values for As, Cu, Hg, Mn, Ni, Pb, Se, Th, and U agreed well with the NBS values. Our F and S values agreed with the grand mean of the few laboratories reporting. Our six-step spectrographic values for Be, Cr, Cu, Mn, Ni, Pb, and V are acceptable but appear to be somewhat low.

Five previously analyzed coal samples obtained from Dr. Harold J. Gluskoter of the Illinois Geological Survey were analyzed in our laboratory for selected major and trace elements. All values are reported on a whole-coal basis, even though many determinations were made on ash. The results obtained in our laboratory are compared with those obtained by the Illinois Survey laboratories in table 3. Our results for the major elements Ca, Fe, K, Mg, Na, and Ti agreed well with theirs. The agreement between laboratories is generally good for the trace elements As, B, Be, Cd, Cr, Cu, F, Ga, Ge, Hg, Mn, Pb, Sb, V, and Zn. The agreement for Co, Mo and Ni is poor enough to suggest need for further study. The analytical methods used by the Illinois Geological Survey on these samples have been described by Ruch, Gluskoter, and Shimp (1974).

National Bureau of Standards coal sample 1630 has been certified to contain 0.13 ppm Hg. This sample has been analyzed in this laboratory many times and our values range from 0.12 ppm to 0.15 ppm Hg, with a standard deviation of about 0.01 ppm.

¹Use of a specific trade name does not necessarily constitute endorsement of the product by the U. S. Geological Survey.