The Chemical Analysis of Argonne Premium Coal Samples

Edited by Curtis A. Palmer
U.S. Geological Survey Bulletin 2144

Determination of 25 Elements in Coal Ash from 8 Argonne Premium Coal Samples by Inductively Coupled Argon Plasma-Atomic Emission Spectrometry

By Paul H. Briggs

ABSTRACT

Twenty-five major and trace elements were determined in coal ash material by inductively coupled argon plasma-atomic emission spectrometry (ICAP-AES). Two decomposition techniques were used. Coal ashes were analyzed in triplicate to determine the precision of the method. The National Institute of Standards and Technology (NIST), formerly the National Bureau of Standards (NBS), standard reference material 1632a and U.S. Geological Survey (USGS) standard reference material CLB-1 were used to assess the accuracy of the method.

INTRODUCTION

Inductively coupled argon plasma-atomic emission spectrometry (ICAP-AES) is rapidly becoming a common method to determine many major and trace elements in geologic materials. An overview of ICAP-AES analysis was given by Lichte and others (1987). Recently, the ICAP-AES method was expanded to include the analysis of coal by using an acid dissolution procedure (Doughten and Gillison, 1990). This work included new methods combining results from two different decomposition procedures that were used to determine the concentrations of 25 elements in coal ashes from 8 Argonne Premium Coal samples by ICAP-AES. Fourteen elements were determined by an acid decomposition using a mixture of hydrochloric, nitric, perchloric, and hydrofluoric acids at a low temperature in a method described by Crock and others (1983). Eleven additional elements were determined by a sodium peroxide sinter decomposition technique modified from one described by Borsier and Garcia (1983). The digested sample was aspirated into the ICAP discharge where the elemental emission signal was measured simultaneously for the elements of interest.

EXPERIMENTAL

All ICAP-AES measurements described in this paper were performed on a Thermo Jarrell-Ash model 1160 Plasma Atomcomp simultaneous instrument in the U.S. Geological Survey (USGS) laboratory in Denver, Colo. ICAP-AES calibration was performed using USGS reference material BHVO-1, Canadian Certified Reference Materials Project SY-3, and four multielement solutions. The wavelengths, operating ranges, and decomposition methods are given in table 1. The ICAP-AES operating conditions are given in table 2.

Two decomposition procedures were used to determine the 25 elements. The acid decomposition technique was used for the determination of the trace elements Be, Co, Cr, Cu, Li, Mn, Ni, Sc, Sr, Th, V, Y, and Zn and the major element Na. The trace-element suite was chosen in order to give the best reporting limits for the 100-fold dilution and the ease of solubility by the acid decomposition. Sodium is reported with the trace suite because sodium peroxide was the sintering flux used for the sample decomposition for major-element determinations. Coal ash sample solutions from the acid decomposition were prepared in the following manner: a 0.200-g sample, to which a solution containing 100 µg lutetium had been added as an internal standard, was digested and evaporated to dryness in a 30-mL Teflon vessel with 3 mL HCl, 2 mL HNO₃, 1 mL HClO₄ , and 2 mL HF at 110°C. An additional 1 mL HClO₄ was added to the residue and taken to dryness again at 160°C. One milliliter HNO₃ and one drop 30 percent H₂O₂ were added to the residue, and 20 mL of 1 percent HNO₃ was added to the solution. The solution was transferred to a 13 x 100-mm polypropylene test tube and capped until ready for ICAP-AES analysis. All reagents used in the procedures were reagent grade or better. It should be noted that this solution was used for both ICAP-AES and inductively coupled argon plasma-mass spectrometry (ICAP-MS) to minimize duplication of digestion and maximize the efforts of the laboratory staff. (See ICAP-MS analysis in Meier's paper in this volume)

The 11 elements reported from the Na₂O₂ sinter decomposition are the major elements Al, Ca, Fe, K, Mg, P, Si, and Ti, and the trace elements B, Ba, and Zr. The Na₂O₂ sinter technique was used to decompose resistant mineral phases like barite and zircon (for the elements Ba and Zr) and to make soluble boron and silicon, which are volatilized in the acid decomposition. The large dilution factor (1:400) does not degrade the reportability for the major elements because of their high concentrations in the ashed coals.

Coal ash sample solutions from the sinter decomposition were prepared in the following manner: a 0.100-g sample and 0.5 g of finely ground Na₂O₂ were mixed in a 5-mL graphite crucible with a Teflon stirring rod and sintered for 35 minutes at 445 °C. The crucible was allowed to cool to room temperature and was placed in a 50-mL Teflon beaker. Twenty milliliters of deionized H₂O, 20 mL of 20 percent HNO₃ , and 200 µg of lutetium in solution (an internal standard) were added in that order. The solution was transferred to a 13 x 100-mm test tube and analyzed by ICAP-AES. As discussed earlier, this solution was used for both ICAP-AES and ICAP-MS. (See ICAP-MS analysis in Meier's paper in this volume.)

RESULTS AND DISCUSSION

Table 3 gives the results of triplicate analyses of the eight ashed coal samples digested by the acid decomposition method. Table 4 presents the results of triplicate analyses of the eight ashed coal samples digested by the sinter decomposition technique. Generally, the precision for both decomposition techniques is within ±5-10 percent relative standard deviation (RSD).

The copper content of sample ND PC-8-3 is dissimilar to the contents of the other replicates and is attributed to a contaminated acid digestion. A high value occurs for zinc in sample PITT PC-4-1. Again, the probable explanation is contamination from the acid digestion. Boron and barium have erroneous values for samples UF PC-1-3 and show large disagreement for all splits of WV PC-7. Low values of MgO, CaO, and TiO₂ for sample IL PC-3-3 are attributed to the sinter preparation rather than ICAP-AES analysis.

Confirmation of accuracy was evaluated by data for NIST standard reference material 1632a and USGS standard reference material CLB-1 that have undergone the two decomposition techniques. Tables 5 and 6 compare data for this study from two ashed coal standards with values from other studies (Gladney and others, 1984; J.S. Kane, unpub. data, 1990). The data from this study show good agreement with results from other studies.

REFERENCES

Borsier, M., and Garcia, M., 1983, Analyse automatique d'echantillons geologiques par plasma ICP: Spectrochimica Acta, v. 38B, nos. 1/2, p. 123-127.

Crock, J.G., Lichte, F.E., and Briggs, P.H., 1983, Determination of elements in National Bureau of Standards' geological reference materials SRM 278 obsidian and SRM 688 basalt by inductively coupled argon plasma-atomic emission spectrometry: Geostandards Newsletter, v. 7, no. 2, p. 335-340.

Doughten, M.W., and Gillison, J.R., 1990, Determination of selected elements in whole coal and in coal ash from the eight Argonne Premium Coal samples by atomic absorption spectrometry, atomic emission spectrometry and ion selective electrode: Energy and Fuels, v. 4, no. 5, p. 426-430.

Gladney, E.S., Burns, C.E., Perrin, D.R., Roelandts, I., and Gills, T.E., 1984, 1982 compilation of elemental concentration data for NIST biological, geological, and environmental standard reference materials: National Bureau of Standards Special Publication 260-88, p. 21.

Lichte, F.E., Golightly, D.W., and Lamothe, P.J., 1987, Inductively coupled plasma-atomic emission spectrometry, in Baedecker, P.A., ed., Methods for geochemical analysis: U.S. Geological Survey Bulletin 1770, p. B1-B10.

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