Quantitative Infrared Multicomponent Determination of Minerals Occurring in Coal Patricia A. Estep, J o h n J. Kovach, a n d Clarence Karr, Jr. U.S. Department of the Interior, Bureau of Mines, Morgantown Coal Research Center, Morgantown, W . Va. 26505 The quantitative infrared multicomponent analysis of five commonly occurring mineral constituents of coal-quartz, calcite, gypsum, pyrite, and kaoliniteis described. A successful analysis of these minerals from coal can be developed, using controlled sampling conditions. The infrared region of 650 to 200 cm-I for these minerals contains absorption bands that are valuable for the initial qualitative analysis, and bands that can be used for quantitative analysis. The accuracy of the method was evaluated with tests on synthetic mixtures. The method was applied in the analysis of unaltered mineral matter obtained from bituminous coal by use of the new technique of lowtemperature ashing in an oxygen plasma.
THEDEVELOPMENT of a new low-temperature ashing technique (1) for obtaining unaltered mineral matter from coal, combined with the extension of the midinfrared region to 200 cm-’, has shown clearly that infrared spectrometry is a valuable tool for use in coal mineralogical studies (2). The direct infrared analysis of minerals in coal has been hindered previously by the broad background absorption of the coal itself (3) as shown in Figure 1, curve (b), coupled with the paucity and often nonspecificity of mineral absorption bands in the rock-salt region of 5000 to 650 cm-l. However, when the coal substance, approximately 90% of the sample, is removed at low temperatures in an oxygen plasma, there remains essentially unaltered mineral matter with an infrared spectrum exhibiting many diagnostic and analytically useful absorption bands as shown in Figure 1, curve (c). The disappearance of organic absorption bands indicates a complete removal of the coal substance. The improved quality of the spectrum obtained for this enriched mineral matter has increased the possibilities for quantitative analysis. In this paper we describe the development of an infrared solid-state quantitative analysis for five commonly occurring coal minerals-quartz, calcite, gypsum, pyrite, and kaolinite. The presence of many other minerals in coal in a wide range of concentrations has been shown. However, those included in this analysis were selected on the basis of their frequency of occurrence and relative abundance in the bituminous coal samples examined in this laboratory. We anticipate that in this continuing broad program using infrared spectrometry for coal mineralogical studies, qualitative and quantitative analysis can be developed for other minerals. A large part of the published infrared data for minerals (4-6) and inorganic compounds (7) is limited t o the rock-salt region of 5000 t o 650 cm-l. Recently, spectral data in the
(1) H. J. Gluskoter, Fuel, 44, 285 (1965). (2) C. Karr, Jr., P. A. Estep, and J. J. Kovach, Chem. Znd. (London), 9, 356 (1967). (3) R. A. Friedel and J. A. Queiser, ANAL.CHEM., 28, 22 (1956). (4) H. H. Adler, E. E. Bray, N. P. Stevens, J. M. Hunt, W. D. Keller, E. E. Pickett, and P. F. Kerr, Am. Petrol. Insr., Project 49, Preliminary Rept. No. 8, July 1950, 146 pp. (5) J. M. Hunt and D. S. Turner, ANAL.CHEM., 25,1169 (1953). (6) K. Omori, Sci. Repr. Tohoku Unic., Third Ser., 7, 101 (1961). (7) F. A. Miller and C. H. Wilkins, ANAL.CHEM.,24, 1253 (1952).
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extended region of 650 t o 400 cm-l has been presented for some minerals (8-9) and down to 240 cm-1 for some inorganic compounds (10-13). However, these collections offer very few high-resolution spectra for naturally occurring minerals t o 200 cm-’. The published papers on quantitative infrared analysis of minerals have dealt with a single mineral o r the detailed studies of sampling parameters, while very little has appeared on multicomponent mixtures (5,14). EXPERIMENTAL Several specimens of each of the five minerals were obtained from different localities and their infrared spectra checked qualitatively for associated mineral contamination and phase purity by three means: agreement of the spectra for samples from several sources, literature infrared data, and x-ray powder data. On the basis of this qualitative screening, two sources were selected for detailed grinding studies in order to establish the infrared absorptivity calibration data. The mineral was placed with an agate ball into an agate vial, approximately one-third full, and ground mechanically in a Spex Mixer Mill, A sample was removed from the vial for infrared analysis at 5-minute intervals with a 5-minute cooling period. The grinding was continued until measured band absorptivities became constant, at which point a particle size determination was made using a Coulter Counter, Model A (Coulter Electronics). Pellets for the infrared analysis were prepared by weighing 1 mg of the preground mineral and 500 mg of cesium iodide powder (Harshaw, 100 t o 325 mesh, median particle size 62 microns) on a microbalance and blending by hand for 5 minutes in a mullite mortar. This mixture was transferred quantitatively to a die and pressed into a pellet according to a triple-press method (15). For this the pellet was placed under vacuum, pressed for 5 minutes at a pressure of 23,000 pounds total load, the pressure was relaxed for 5 minutes, and the procedure was repeated twice. The additional pressing steps presumably relieve the strain introduced by the original pressing. The resulting pellet of 0.80 by 13 mm was then scanned immediately on a Perkin-Elmer 621 infrared grating spectrophotometer purged with dry air. Preground coal samples were ashed in Tracerlab’s Model LTA-600 low-temperature asher at 145” C. For this lowtemperature oxidation technique, the sample was exposed to a stream of excited oxygen produced by a radio frequency (8) K. Omori, Sci. Repr. Tohoku UHiu., Third Ser., 9, 65 (1964). (9) W. B. White and R. Roy, Am. Mineralogist, 49, 1670 (1964). (10) N. T. McDevitt and W. L. Baun, Tech. Doc. Rep!. RTD-TDR63-4172, AF Materials Lab., Wright-Patterson Air Force Base, Ohio, January 1964, 77 pp. (11) F. A. Miller, G. L. Carlson, F. F. Bentley, and W. H.Jones, Spectrochem. Ac!a, 16, 135 (1960). (12) Sadtler Research Laboratories, Philadelphia, Pa., “High Resolution Spectra of Inorganics and Related Compounds,” Vols. 1 and 2, 1965, Supplement, 1967. (13) S.R. Yoganarasimhan and C. N. R. Rao, Chemist-Analyst,51, 21 (1962). (14) R. J. P. Lyon, W. M. Tuddenham, and C . S.Thompson, Econ. Geol., 54, 1047 (1959). (15) The Harshaw Chemical Co., Cleveland, Ohio, “Pressed Pellet Techniques,” Data Sheet 023, November 1965.
(a%)
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Figure 1. Infrared spectra of ( a ) cesium iodide, ( b ) Pittsburgh-seam coal, and (c) low-temperature ash from Pittsburgh-seam coal electromagnetic field (1 3.56MHz). Commercial grade oxygen was passed at low pressure (1 mm Hg) through five chambers, each containing a borosilicate glass boat filled approximately Under these conditions, organic l / h full with 1 gram of coal. matter was completely removed from the sample in about 70-90 hours. The oxidation temperature, which can be controlled by the power input to the r.f. discharge, was measured with a Huggins Laboratories Mark I Infrascope. Each of the five minerals was exposed to the same ashing conditions as the coal samples. There were no sample alterations, except for the complete conversion of gypsum t o the hemihydrate. RESULTS AND DISCUSSION
Sampling Parameters. Tuddenham and Lyon (16) have shown the application of the potassium bromide pellet method to mineral analysis and demonstrated its quantitative potentialities, The many difficulties encountered in quantitative solid phase spectrometry have been reviewed by Duyckaerts (17) and Baker (18). PARTICLE SIZE. Probably the most important single physical factor to be considered in quantitative solid state spectrometry is that of particle size (16-17). As particle size is reduced there is a reduction of light loss by reflection and scattering and the intensity of an absorption band increases from an apparent absorptivity to its true absorptivity value. The commonly used sedimentation technique for obtaining the required particle size of the sample, while applicable to single component samples, cannot be employed with multicomponent samples (16) because this procedure leads to a differential separation of the individual components. We investigated the effects of particle size on absorptivity for each band being considered for quantitative analysis by conducting grind studies. If a strict grinding schedule is followed, particle size need only be determined on those samples ground for lengths of time necessary to produce constant absorptivity values. Thus, knowledge of particle size requirements for constant absorptivities is available with a minimum of effort. A typical example of such a grind study is shown in Figure 2 for the mineral pyrite. The data show that the apparent absorptivity becomes constant after a minimum grinding time and remains constant even with extended grinding and further particle size reduction. The minimum grinding time for a particular mineral can vary with the type of vial used and the amount of (16) W. M. Tuddenham and R. J. P. Lyon, ANAL.CHEM., 32,1630 (1 960). (17) G. Duyckaerts, Analyst, 84,201 (1959). (18) A. W. Baker, J . Phys. Chem., 61, 450 (1957)
700
I
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I
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-
600
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500
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* 400 >k 2 c & 300
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340 c r n -1 200
100 I
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I I I 40 30 20 GRINDING TIME, minutes
I
50
3
Figure 2. Absorptivity as a function of grinding time for pyrite, in an agate vial
sample loaded into the vial. Grind studies on all five minerals reveal that at the minimum grind time, the mean particle size of the sample falls in the range of 4 to 13 microns. This fulfills the theoretical requirement for solid phase spectrometry that sample particle size should be less than the wavelength of the incident radiation in order to minimize scattering, reflection, and the Christiansen effect. While the absorptivities as determined for use in this analysis are constant, they are not necessarily maximum values. Particle size reduction of the cesium iodide matrix can cause band absorptivities to increase markedly. This was demonstrated in a grind study in which preground kaolinite was added to samples of cesium iodide, also preground, but for different lengths of time. Each mixture was then handblended to prepare a pellet according to the chosen procedure. Absorptivities for several kaolinite bands were seen to increase 30 to 50z when prepared with cesium iodide that had been ground for 20 minutes. True absorptivities could not be determined because, at a very fine particle size, cesium iodide pellets crumble. Thus it is essential to use the same particle size of matrix throughout the analyses. The relatively gentle grinding that the cesium iodide receives during the blending stage of pellet preparation reduces its particle size very little as shown by particle size determinations. Because quantitative VOL 40, NO. 2, FEBRUARY 1968
359
accuracy is more a function of reproducibility rather than absolute measurement, the relatively simple and highly reproducible method of hand-blending was chosen. SAMPLEALTERATION.Extensive grinding can cause polymorphic transformations or alteration of mineral composition. For example, Dachille and Roy (20) have shown that a few minutes grinding in a Wig-L-Bug with a metal vial and ball can transform calcite to the high-pressure phase of aragonite. Morris (21) found that vibrating for more than 1 minute in a stainless-steel Wig-L-Bug will cause dehydration of gypsum. Liberti and Devitofrancesco (22) have reported destruction of the crystalline structure of quartz under certain grinding conditions. None of these alterations were observed by using our chosen procedure, presumably because of the periodic cooling periods during grinding and the use of an agate-grinding ball mill. PARTICLE DISTRIBUTION. If Beer’s law is to be followed, all absorbances must show a linear dependence on effective concentration. Because the infrared beam does not cover the entire pellet, the use of sample weight as a concentration factor requires a high degree of uniform dispersion of sample in the matrix. The dispersion achieved with the chosen sample preparation technique was tested by systematically rotating and
scanning pellets of each of the five minerals. For all positions, the band intensities were identical, indicating that dispersion was adequate, and also that there were no polarization or orientation effects. CHOICE OF MATRIX. Several salts were tested for use as a matrix. Cesium iodide appeared to be the best choice on the basis of its superior transparency at long wavelengths. The low background at shorter wavelengths due to scattering was not objectionable, as most of the useful absorption bands for minerals are found at longer wavelengths. The triple-press method reduced scattering at short wavelengths, often as much as 40% over single pressing. The overall transmission of a blank cesium iodide pellet was acceptable, as shown in Figure 1, curve (a), and therefore no pellet was used in the reference beam. Although cesium iodide is slightly hygroscopic, there was less difficulty from water adsorption than is generally experienced using the more common potassium bromide matrix. While mechanical grinding of cesium iodide intensified the water absorption bands, hand-blending produced no more than 0.02 absorbance unit at the 3430-cm-1 OH stretching frequency. This behavior of cesium iodide has a decided advantage, as coal minerals often have useful OH absorption bands. The pellet thickness must be controlled to prevent interference or fringe patterns. In the longer wavelength region where the requirements for parallel pellet surfaces are relaxed, these fringe patterns can become quite pronounced on thin pellets. In order to eliminate this as a potential source of quantitative error, pellets at least 0.80 mm thick were used. OTHER PARAMETERS. Another sampling problem sometimes encountered in solid state analysis is the interaction of sample and matrix, but this is believed to be negligible for the minerals analyzed here. A large refractive index difference between the sample and the dispersion medium can cause a pronounced Christiansen effect, resulting in high light scattering and errors in quantitative measurements. However, the small particle size used should minimize any error from this effect. One of the limitations in the quantitative analysis of minerals and inorganics is due to their inherently high-band absorptivities. With intense absorption, only a very small amount of sample can be included in the pellet, so that bands will still be measurable, and thus weighing errors are increased. However, reproducible results could be obtained using a microbalance. Quantitative Analysis. Because natural mineral specimens, which were used as standards in the analysis, often occur in polymorphic aggregations, it was necessary to determine the qualitative infrared spectral differences for polymorphs of each mineral. In the coal samples thus far investigated no polymorphic mixtures were encountered, although they can be expected. Table I lists the spectral data from this laboratory for each of the five minerals and some polymorphic forms. QUARTZ. Although quartz is the most common form of crystalline silicon dioxide, the polymorphic forms of tridymite and crystobalite are found in nature, even though they may be metastable. Each of these crystalline forms has a low-temperature (a) and a high-temperature (6) modification. Table I shows the spectral differences for the a-modifications and literature data (23-24) show differences for P-modifications.
(19) E. R. Lippincott, A. VanValkenburg, C. E. Weir, and E. N. Bunting, J. Res. Natl. Bur. Srd,, 61, 61 (1958). (20) F. Dachille and R. Roy, Nature, 186 (4718), 70 (1960). 35, 1489 (1963). (21) R. J. Morris, Jr., ANAL.CHEM., (22) A. Liberti and G. Devitofrancesco, Staub., 25 (4), 28 (1965).
(23) I. Simon and H. 0. McMahon,J. Chem. Phys., 21,23 (1953). (24) J. Zarzycki and F. Neudin, Ch. in “Advances in Molecular Spectroscopy,” Proc. Intern. Meeting on Molecular Spectry., 4th, Bologna and Rome, 1959, Mangini, A., Ed., Macmillan, New York, Vol. 3, 1071 (1962).
Table 1. Infrared Absorption Bands for Minerals Mineral Absorption bands, cm-1“ a-Quartz 256 (w), 363 (mw), 388 (mw), 452 (m),465 (w, sh), 501 (mw), 688 (w), 772 (mw), 790 (m), 1075 (s), 1135 (w), 1160 (w) a-Tridymite (synthetic) 465 (s), 500 (sh), 782 (rn), 1088 (s), 1155 (sh) a-Cristobaliteb 485 (s), 515 (sh), 620 (m), 798 (m), 1104 (s), 1160 (sh), 1204 (w) Calcite 217 (m), 337 (sh), 307 (s), 707 (m), 842 (vw), 869 (m), 1420 (s), 1600 (w), 1792 (vw) Aragonite 205 (sh), 245 (s), 288 (sh), 692 (m), 705 (m), 836 (w), 850 (m), 1076 (w, sharp), 1450 (sh), 1464 (s), 1600 (w), 1780 (w), Gypsum 215 (m), 298 (mw), 412 (w), 445 (w), 594 (m),660 (rn), 998 (vw), 1106 (s), 1 1 32 (s), 1155 (s, sh), 1615 (m),1680 (mw), 3250 (vw), 3400 (m), 3492 (sh, vw), 3550 (m) Hemihydrate of CaSOl 235 (rn), 250 (sh), 412 (w),455 (w),590 (m), 605 (sh, w), 619 (w), 652 (m), 665 (sh, w), lo00 (mw, sharp), 1008 (vw, sh), 1090 (s), 1110 (s), 1128 (s), 1147 (s), 1613 (mw), 3552 (w), 3610 (mw) Anhydrite 240 (m), 502 (vw), 587 (m), 606 (m), 669 (m), 1008 (vw), 1115 (s), 1148 (s), 1175 (sh) Pyrite 284 (w), 340 (m), 391 (vw), 406 (s) Marcasite 285 (w), 321 (m), 350 (rn), 396 (s), 407 (s), 422 (vw) Kaolinite 268 (m), 338 (m),352 (vw, sh), 405 (vw, sh), 422 (m), 460 (s), 528 (s), 689 (m), 747 (w), 782 (w), 908 (s), 929 (w, sh), 1002 (s), 1026 (s), 1095 (s), 3622 (m), 3652 (w), 2670 (vw), 3698 (m) a Very weak = vw; weak = w ; medium = m; strong = s; shoulder = sh Data from reference (19).
360
ANALYTICAL CHEMISTRY
The form identified in the coal samples investigated was aquartz; it was, therefore, used as the standard for the quantitative analysis. The distinction of a-quartz from its crystalline polymorphs and other modifications of silica can be most readily made by its highly unique bands a t 256, 363, 388, and 688 cm-l. CALCITE.The natural aggregation of calcite and its dimorph aragonite is common. Infrared literature data (25) for the rock-salt region show that hexagonal calcite can be distinguished from orthorhombic aragonite. The data from our laboratory in Table I show the marked difference in their spectra and list the additional bands obtained for the region 650 t o 200 cm-l. Petrographic distinction between these two polymorphs is often inadequate because three samples labeled aragonite were shown by infrared to be calcite. Adler and Kerr (25) found that several specimens of aragonite and calcite differed from their museum labels. GYPSUM.For gypsum (CaS04.2H20, monoclinic), it was necessary t o show that it can be differentiated in the infrared from anhydrite (CaS04, orthorhombic), also found in coal, and the hemihydrate ( C a S 0 4 .1/2Hz0, hexagonal-R), formed during ashing. Data from Morris (21) and from our laboratory in Table I show that gypsum bands are different from the hemihydrate and these differences were used t o show the conversion of gypsum to the hemihydrate. Gypsum was selected for testing in synthetic mixtures because it has been identified and analyzed in coal product samples not subjected to lowtemperature ashing. Anhydrite is also readily distinguishable from gypsum. The primary difference is a splitting of the broad band of gypsum at 594 cm-I into two sharp bands a t 587 and 606 cm-1 for anhydrite. The 660 crn-I band of gypsum shifts to 669 cm-l in anhydrite, and the water vibrations appearing a t 1615,1680, 3400, and 3555 cm-l are unique for gypsum. Data in Table I also show the differences between the hemihydrate and anhydrite. PYRITE.Both polymorphs of FeS2, pyrite (cubic), and marcasite (orthorhombic), have been identified in coal. White and Roy (9) have presented infrared data t o 400 cm-I, which is insufficient for differentiation between these two polymorphs. However, the infrared spectra t o 200 cm-l obtained o n samples in this laboratory show that these two polymorphs are distinguishable in this region. Table I shows that, although marcasite exhibits two bands common t o pyrite, there are four distinctly different bands which uniquely characterize this polymorph. Further, the band at 340 ern-', which is unique for pyrite, allows detection of pyrite in marcasite samples. These infrared differences, along with x-ray confirmation, established the phase purity of the pyrite standards selected for calibration and thus also the qualitative identification of pyrite as the observed form in the coal samples studied. It was particularly important to establish by x-ray that no pyrrhotite (Fel.,S) was present in the standard samples since all specimens of this mineral examined had no absorption in the entire 5000 to 200 cm-1 region. Optical examination is also often unreliable for this mineral class because out of 15 different mineral specimens labeled marcasite, two were pyrrhotite, six were pyrite, and only seven were actually marcasite as shown by infrared and x-ray analysis. KAOLINITE.Clays other than kaolinite have been identified in coal. Montmorillonite and illite often predominate in many coals and Angino (26) has discussed the infrared spectral differences among these to 90 cm-’. Oinuma and Hayashi (25) H. H. Adler and P. F. Kerr, Am. Mineralogist, 47, 700 (1962). (26) E. E. Angino, Naiure, 204, 569 (1964).
Table 11. Analytical Absorption Bands and Absorptivities for Materials Absorption bands, cm-1 and absorptivities, l/mma Mineral Source 360 (448), 388 (525), 452 Quartz Hot Springs, Ark. (lolo), 790 (589) Rock crystal Ward’s Natural Science Establishment, Inc. 360 (430), 388 (500), 452 Berkeley Springs, W. Va. (970), 790 (580) Oriskany Sandstone W. Va. University Geology Department 307 (720), 869 (610), 1420 Calcite Germany Valley, W. Va. (2050) Crystals W. Va. University Geology Department 594 (245), 660 (251) Gypsum Niagara, N. Y . Crystals U. S . National Museum, No. 80022 Washington County, Utah 594 (246), 660 (254) Crystals Ward’s Natural Science Establishment, Inc. Park City, Utah Pyrite 340 (190), 406 (630) Ward’s Natural Science Establishment, Inc. 340 (180), 406 (640) Gilman, Eagle County, Colo. Minerals Unlimited Kaolinite Macon. Ga.. Oneal Pit 338 (3531. 422 (5051. 460 API Reference Clay Mineral (986), ‘528 (1 106), 689 Kaolinite No. 4 (338), 747 (131), 908 (617) Ward’s Natural Science Establishment, Inc. Absorptivities in parentheses.
(27) found that kaolinite can be identified by infrared in mixtures of clay minerals and even in mixed-layer clay minerals, The OH region is especially useful in the qualitative identification of kaolinite. Kodama and Oinuma (28) state that the absorption band at 3698 cm-’, due to free hydroxyl stretching, distinguishes kaolinite from other clay minerals down to a few weight per cent of kaolinite in total amounts of clay minerals. Lyon and Tuddenham (29) have shown that the use of ratios for bands appearing in the O H region, along with other spectral differences, enables a unique distinction to be made among the kaolin group minerals of kaolinite, dickite, and halloysite. This literature data, and data obtained in our laboratory from a number of API reference clay minerals, were used to substantiate the identification of kaolinite in the coal low-temperature ash samples and as a guide in the selection of bands for quantitative analysis. Quantitative Analysis. The apparent absorptivity of a band was calculated from the relationship : a =
where a A b
=
= = c =
A bc
--
absorptivity in units of l/mm absorbance of the band (base line technique) pellet thickness, mm concentration, wt
(27) K. Oinuma and H. Hayashi, Am. Mineralogist, 50, 1213 (1965). (28) K. Kodama and K. Oinuma, Proc. Nut/. ConJ on Clays and Clay Minerals, llth, Canada, 236 (1962). (29) R. J. P. Lyon and W. M. Tuddenham, N a m e , 185,835 (1960). VOL. 40, NO. 2, FEBRUARY 1968
361
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examined. No attempt was made to determine the limit of detectability for each mineral since this varies for different sample compositions. Figure 3 shows the infrared spectrum of synthetic mixture No. 7 (Table 111), which approximates the composition of a typical coal low-temperature ash except for the gypsum-hemihydrate conversion. When band overlapping occurred, the required absorbance correction was obtained from the standard spectra of the interfering mineral, using the standard procedures for quantitative analysis. An expedient technique of chart overlay t o obtain background and band overlapping corrections was found to be as accurate as the more time-consuming procedure of matrix solution. The agreement shown in Table 111, within an average error of 6.1 for all minerals, demonstrates the method to be satisfactory for the multicomponent mixtures investigated. It is difficult, without a n independent reliable method of analysis, to determine the accuracy limits of the infrared method when applied t o coal ash samples. Greater accuracy is expected with the synthetic mixtures because they were prepared under controlled conditions of particle size and contained n o unknown interferences. Coal samples were preground in order to speed oxidation in the low-temperature asher, and this pregrinding was sufficient to achieve the required particle size reduction for an accurate quantitative analysis of the ash. The pregrinding conditions were determined by conducting a grind study on the total coal sample, using the absorption bands of kaolinite that appeared fairly well resolved in the coal spectrum (see Figure 1). Grind curves similar t o those in Figure 2 were obtained with a relatively small increase in absorptivity before becoming constant. From these data, it was seen that 30 minutes of grinding in a tungsten carbide vial gave
The value of absorptivity is specific for a 13-mm diameter pellet. Beer's law was obeyed for the concentration range of 0.02 to 0.2 wt %. After completion of grind studies for each mineral, bands were selected for quantitative analysis. These selections, shown in Table 11, were based on both their specificity and the constancy of their absorptivities with prolonged grinding. When it is necessary, for the sake of specificity, t o use a band which does not remain constant with particle size reduction beyond the minimum grind time the error introduced must be considered. The data in Table I1 are the results of reproducible duplicate grind studies. Only one source for kaolinite was used for the calibration data because of its high purity and there is extensive mineralogical data for it in the literature. Also, only one source of calcite was used because of the wide variation of absorptivity values from different sources, even though their spectra were qualitatively identical and their particle sizes were the same. Therefore, calcite from the same source was used for both calibration data and for synthetic mixtures. The source selected was of high optical quality and had the highest absorptivities. Examination of these samples by other analytical techniques may determine the cause for such a variation and resolve this limitation. Several synthetic mixtures, listed in Table 111, were prepared to test the accuracy of the multicomponent analysis. Preground standard mineral samples of the required particle size were used and the same technique for pellet preparation used as described for the mineral standards. The concentration ranges of the pure minerals were selected to represent the concentration ranges commonly encountered in the coal samples
Table 111. Analysis of Synthetic Mixtures of Minerals Kaolinite, Calcd. Found 71.9 69.6 53.1 41.5 54.6 29.9 35.7
72.7 71.2 43.7 41.0 55.2 29.4 31.1
Av. Error 2.6
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ANALYTICAL CHEMISTRY
Quartz, Z Calcd. Found 28.1 30.4 23.2 19.3 18.5 12.5 24.1 12.2 22.9 15.2
27.1 29.1 19.7 19.3 19.7 11.5 20.8 14.2 19.0 12.0 1.9
Gypsum, Z Calcd. Found 23.7 39.2 26.9 47.3 31.7 33.1 31.3 30.5
20.1 33.5 27.0 48.1 30.8 31.1 31.2 30.8 1.7
z
Calcite, Calcd. Found
10.3 8.5 12.2 8.4 10.6
10.3 10.2 12.4 7.0 11.3
0.8
Pyrite, Calcd. Found
42.5 37.4 43.7
44.9 30.0 35.1
6.1
kaolinite bands with constant absorptivities and a sample particle size of 20 microns. I t is essential to conduct a grind study on any mineral mixture prior to analysis in order t o establish when the required particle size reduction for each component in the mixture has been achieved. The infrared analysis should be made only after absorptivity values have become constant and particle size has been checked. We have found that particle size reduction and the high degree of blending required for such small samples used in a pellet can be achieved using a Fisher automatic grinder equipped with a n agate mortar and pestle. While the particular ball mill used in this work was excellent for grinding pure minerals, it produced poorly blended mixtures, as shown from grind studies conducted on synthetic mixtures and replicate sampling. A limitation of the application of infrared spectrometry t o mineral mixtures in coal is in the analysis of pyrite in the presence of high amounts of kaolinite (2). Both of the analytical bands of pyrite are overlapped considerably by kaolinite bands and amounts of pyrite as high as 2 0 z might be undetected when kaolinite is present t o the extent of 3 0 z or greater. F o r this reason, no attempt was made to analyze synthetic mixtures containing both. Work is continuing to develop a satisfactory analysis for kaolinite-pyrite mixtures when kaolinite content is high.
CONCLUSIONS
Although this method for mineral analysis was developed for bituminous coal, it would be equally applicable t o coals of different rank. The calibration data have been used in this laboratory for analysis of other materials connected with the mining and utilization of coal. For example, we have determined these minerals directly in coal mine refuse samples, which often have low organic matter content. The development of the analysis demonstrates that infrared spectrometry is a valuable tool for differentiating among various polymorphic forms of naturally occurring minerals and this makes it useful for a wide variety of mineralogical problems. ACKNOWLEDGMENT
Special thanks are due to Edward E. Childers for assistance in obtaining infrared spectra and to John J. Renton, Geology Department, West Virginia University, for providing x-ray data o n the mineral samples. RECEIVEDfor review September 27, 1967. Accepted November 14, 1967. Presented before the Division of Fuel Chemistry, 154th Meeting, ACS, Chicago, Ill., September 1967.
Thermal Decomposition Kinetics of Silver Carbonate as Determilned by the Infrared Disk Technique Theodore Wydeven and Mark Leban Ames Research Center, NASA, Moffett Field, Calq. 94035 The infrared disk technique has been used to study the thermal decomposition kinetics of silver carbonate. The technique has been improved by the design and construction of a heated cell which permits continuous in situ quantitative analysis of infrared active reactants and products. Beer’s law was found to be obeyed up to a concentration of at least 10 weight per cent silver oxide in silver carbonate. The activation energy for the thermal decomposition of inactive silver carbonate was determined to be 18,000 cal/mole for the induction period and 20,000 cal/mole for the decay period. The first order rate equation: In C A ~ , C= O ~-k(t
- tinduetion)
satisfactorily described the decay period of the thermal decomposition. The first-order rate constants can be represented by the equation: k(SeC-’) = 1.46
x
106 exp ( - 2 0 , 0 0 O / ~ T )
The value of the activation energy for the decay period agrees well with that previously found for active silver carbonate. Agreement between the experimental rate of decomposition and the rate predicted by the PolanyiWigner equation provides circumstantial evidence in favor of the interface mechanism for the decomposition of inactive silver carbonate.
THEINFRARED DISK TECHNIQUE has been employed infrequently in the past for studying the kinetics and mechanism of the thermal decomposition of solids. This technique is not only an alternate to thermogravimetric analysis and gas pressure methods for studying decomposition kinetics but can provide in certain instances additional information about a reaction
mechanism which is not obtainable by the other methods (1-6).
One goal of this work was to determine the feasibility of the infrared disk technique for studying the decomposition of metal carbonates, specifically, silver carbonate. Silver carbonate was chosen because an understanding of its thermal decomposition kinetics is essential in the development of a silver oxide-silver carbonate system as a regenerable chemisorbent for carbon dioxide (7). Furthermore, different preparations of silver carbonate exhibit significantly different decomposition kinetics (8, 9). Preparations classified as active or normal are characterized in their decomposition kinetics by a short induction period followed by an acceleration and decay (1) H. A. Bent and B. Crawford, Jr., J . Am. Chem. SOC.,79, 1793 (1957). (2) K. 0. Hartman and I. C. Hisatsune, J . Phys. Chem., 69, 583
(1966). (3) Ibid., 70, 1281 (1966). (4) Ibid., 71, 392 (1967). (5) F. E. Freeberg, K. 0. Hartman, I. C. Hisatsune, and J. 1% Schempf, Ibid., p. 397. (6) K. 0. Hartman, Ph.D. Thesis, Department of Chemistry, the Pennsylvania State University, September 1965. (7) W. J. Culbertson, Jr., “Investigation and Design of a Regenerable Silver Oxide System for Carbon Dioxide Control,” Rept. No. AMRL-TR-64-119, Denver Research Institute, 6570th
Aerospace Medical Research Laboratories, Wright-Patterson AFB, Ohio, December 1964. (8) W. R. Spencer and B. Topley,J. Ckem. SOC.,1929,2633. (9) 0. Kadlets and 11. AI. Dubinin, Zza. Akad. Nauk SSSR Old. Khim. Nauk, 1961, No. 3, 390 (English trans. p. 363). VOL. 40, NO. 2, FEBRUARY 1968
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