Hyperfine Interactions 57 (1990) 2187-2194 2187

THE QUANTITATIVE DETERMINATION OF FeS 2 PHASES IN COAL BY MEANS OF S7Fe MOSSBAUER SPECTROSCOPY

B.J. EVANS

Department of Chemistry," The University of Michigan; Ann Arbor, Michigan 48109-1055, U.S.A.

Hobart M. KING, John J. RENTON and A. STILLER

West Virginia Geological and Economic Survey," Post Office Box 879; Morgantown, West Virginia 26505, U.S.A.

A knowledge of the concentration of pyrite and marcasite in coals can provide important insight into the genesis of coal deposits. Determinations of the relative amounts of pyrite and marcasite by traditional methods of coal analysis are, however, beset with many difficulties. Using 57Fe M/Sssbauer spectroscopy and a mild chemical treatment with hydrofluoric acid, a technique has been devised for the quantitative determination of the relative concentrations of pyrite and marcasite in samples of whole coals or their low-temperature ashes. The sample preparation procedure is comparable to less accurate methods. Good qualitative agreement has been obtained between ore microscopic and M~Jssbauer spectroscopic techniques for a series of extensively investigated whole coal samples.

1. Introduction

In preparation for a study of the distribution of iron among the various inorganic phases in coals, the systematics of the 57Fe MtSssbauer spectra of pyrite and marcasite were established in an earlier investigation [1] by means of measurements on 16 pyrite and 12 marcasite samples from a number of different geological settings. In the above investigation [1] a straight-forward method was also developed for determining the relative amount of pyrite and marcasite in laboratory-prepared mixtures consisting only of pyrite and marcasite.

Though several coals have been reported to contain both pyrite and marcasite [2] on the basis of ore-microscopic and X-ray diffraction measurements, definitive 57Fe M~Sssbauer spectra due to either marcasite or pyrite/marcasite mixtures have not been reported for coals even though the presence of marcasite has been suggested [3] as a possible explanation for pyrites with MtSssbauer parameters exhibiting lower values of A EQ (quadrupole splitting) and 6 (isomer shift).

It is to be noted, however, that the distinction between pyrite and marcasite or mixtures of the two by means of 5VFe MiSssbauer spectroscopy would be com- plicated by the presence of other iron-containing phases commonly found in coals. Difficulties in determining whether the iron disulfide in coal is pyrite,

�9 J.C. Baltzer A.G. Scientific Publishing Company

2188 B.J. Evans et al. / Determination of FeS 2 phases in coal

marcasite or a mixture of the two would be pronounced in such instances. Therefore, the conclusion, apparently based on MtSssbauer spectra, that marcasite is " v e r y rarely observed" in coals is at variance with the results of light microscopic investigations, and reflect in all likelihood inherent shortcomings in the MtSssbauer spectroscopic technique as applied to this determination.

In this report, the results of a 5VFe MiSssbauer spectroscopic investigation of coals known from light microscopy to contain both pyrite and marcasite are presented. The complications anticipated when iron-containing silicates and carbonates are present have been encountered and successfully overcome. Chem- ical treatment of the coals was necessary (and sufficient) for precise determina- tions of the relative amounts of pyrite and marcasite in the investigated coals and their low-temperature ash (LTA) samples.

The relative amounts of marcasite and pyrite in the coals were determined utilizing the method presented in the earlier report [1]. The results are in good qualitative agreement with the ore-microscopic determinations [4] but the quanti- tative agreement is poor. X-ray diffraction was less satisfactory than either the ore-microscopic or M~Sssbauer spectroscopic method. The 57Fe M/Sssbauer de- terminations of the relative pyrite and marcasite contents of these coals are more accurate over a wider range of py r i t e /marcas i t e contents than either X-ray diffraction or ore-microscopy.

2. Experimental

Four samples of Permo-Pennsylvania Waynesburg Coal from Monongal ia County, West Virginia (Dippel and Dippel Coal Company, Sugar Grove Mine), were selected for this investigation. The textures of these coal samples, especially with respect to the iron disulfide phases, have been described in detail in several previous reports [4] and the results relative to the iron disulfide phases are given in table 1. Low temperature ash samples of these coals were also obtained and investigated using x-ray diffraction and M/Sssbauer spectroscopy.

Table 1 Analysis of West Virginia coals, including relative pyrite and marcasite concentrations

1361 1645 1651 1657

LTA ASH % 21.22 22.44 17.99 31.46 TOTAL SULFUR % 1.613 0.623 1.420 1.110 Pyrite sulfur % 0.974 0.210 0.557 0.350 Sulfate % 0.031 0.007 0.259 0.190 Organic % 0.608 0.406 0.613 0.580 Marcasite % * 26.00 3.0 20.00 82.00

* Values listed are the percentage of the total iron disulfide content, obtained by point-count of 100 observations.

B.J. Evans et al. / Determination of FeS 2 phases in coal

Table 2 MiSssbauer parameters of Waynesburg coals following HF leaching

2189

Sample A EQ 3 F (mm s -l) (ram s -1) (mm s -1 )

1645 0.602 0.329 0.330 1651 0.588 0.301 0.318 1361 0.565 0.300 0.309 1657 0.552 0.320 0.349

Separation of the FeS 2 phases from both the whole coal and LTA samples was at tempted by means of a heavy liquid separation using di idomethane and a high-speed centrifuge. The treatment of the coals and their low-temperature ash with nitric acid for the removal of pyrite and dilute hydrochloric acid for the removal of iron sulfates was in accordance with the prescribed ASTM procedure.

The coals and LTAs were also treated at 295 K with 20% hydrofluoric acid which has been found to be effective in removing all "Fe2+-like '' components from the S7Fe M/Sssbauer spectrum except those due to pyrite or marcasite. This treatment involved the addition of 50 ml of 20% hydrofluoric acid to the coal or its low-temperature ash and stirring the mixture for several hours at 295 K with a teflon coated, magnetic bar stirrer, followed by decanting the liquid. This proce- dure was repeated to give a total of four hydrofluoric acid treatments. After the final treatment, the sample was transferred to filter paper, washed several times with distilled water and permitted to dry in air.

The M/3ssbauer spectroscopic technique, including the least-squares fitting of the data was the same as that reported earlier [1]. All spectra were obtained at 295 K and the isomer shifts are reported relative to iron metal.

3. Resu l t s

In order to verify that the hydrofluoric acid treatment does not alter the FeS z phases, a 57Fe M/Sssbauer spectrum was taken of a pure pyrite specimen that had been a part of our earlier investigation. The spectrum of the treated sample is shown in fig. 1; it is typical of a pure pyrite. The MiSssbauer parameters before and after (values in parenthesis) the hydrofluoric acid leaching are the following:

AEQ: 0.615 mm s -1 (0.612 mm s - l ) ;

3 :0 .315 mm s -1 (0.312 mm s - l ) ;

and F ( F W H M ) : 0.252 mm s -1 (0.302 mm s -a ) ,

The 57Fe MtSssbauer spectra of the untreated whole coals are shown in fig. 2. Each of these spectra has been fitted to two quadrupole doublets. For three of the

2190 B . J . E o a n s e t al. / D e t e r m i n a t i o n o f F e S 2 p h a s e s in c o a l

2 .D k.

v

>-

z i,i

z

I I

V E L O C I T Y ( m m s -~)

Fig. 1. STFe Mrssbauer spectrum at 298 K of a bulk pyrite specimen following treatment with hydrofluoric acid (see text). This is the same specimen as sample 18P in an earlier investigation [1].

s a m p l e s , t h e s t r u c t u r e s in t he a b s o r p t i o n l ine n e a r - 2 . 5 m m s - 1 i n d i c a t e s t h e

p r e s e n c e o f a t l e a s t t w o c o m p o n e n t s a n d t h e s p e c t r a s h o u l d b e f i t t e d to t h r e e

q u a d r u p o l e d o u b l e t s . T h r e e q u a d r u p o l e d o u b l e t s f i ts h a v e b e e n a t t e m p t e d b u t

C

o

I - -

Z I , I I - -

A B

~,.,~.~:..:.. ~; . . . . . . : , ;5~ ."?r", .~g ..... ""~; :: ..... . ~ . - ~ r '" ..- . b ' . " ~-" '"

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. . . . . ,~. ~.-:. -:.. :.. r

. ~ '

- 's-~ -i 6 § ,.'2 .'s " ' - ' s -~, -i 6 7, ,'2 4 VELOCITY(turn s -t)

Fig. 2.57Fe MiSssbauer spectra at 298 K of four, West Virginia, whole coal samples. Specimen 1361 (A), 1657 (B), 1651 (C), 1645 (D).

B.J. Eoans et al. / Determination of FeS 2 phases in coal 2191

k-

z W k-- Z

-g . - 2

A

! t

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B

" ~ ,'"-~.~ ~ "m~.~ �9 .~. Z.. e~

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i :t I

6

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~ i,"

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. . . . 6 ' +2 * 4 - - * 2 + 4

VELOCITY(ram s -I}

Fig. 3. 5VFe MOssbauer spectra at 298 K of four, West Virginia whole coal samples following treatment with hydrofluoric acid. 1361 (A), 1657 (B), 1651 (C), and 1645 (D).

due to the strong overlap of lines between - 0 and - 0.5 mm s -a no consistent set of parameters was obtained.

However, it is quite clear that an accurate analysis of the spectral components due to the iron disulfide phases can be made only if the other spectra components are accounted for properly. In only one case (sample 1361) does the two quadrupole doublet fit lead to parameters for the component with the smaller splitting that are consistent with a pyrite/marcasite mixture. This is not the sample, however, with the highest relative marcasite content, but rather is the one with the weakest absorption due to iron silicates or other iron-containing phases.

Computer-assisted, spectrum-stripping techniques proved unsatisfactory due to difficulties in defining the various spectral components in the whole-coal M/Sssbauer spectrum. Heavy-liquid separation was also unsuccessful due to an appreciable decrease in overall intensity of the spectrum and a failure to remove the asymmetry in the spectral component that would be attributable to pyri te / marcasite.

Treating the whole coals with hydrofluoric acid in the manner described above resulted in the spectra shown in fig. 3. Fits of a single-quadrupole doublet to these spectra, indicated by the line drawn through the points, showed all of them to have MOssbauer parameters intermediate to those of pyrite and marcasite. The

2192 B.J. Evans et al. / Determination of FeS 2 phases in coal

Table 3 Comparison of MiSssbauer and ore microscopic determinations of pyrite and marcasite in Waynes- burg coals

Method of determination Percent marcasite

Sample

1645 1651 1361 1657

Ore microscopy 3 20 36 82 MSssbauer spectroscopy 11 23 42 53

spectra in fig. 3 can, therefore, be employed to determine the relative amounts of pyrite and marcasite in these coals by means of the model previously developed [1]. The equation used to determine the relative amounts of pyrite and marcasite in the coals is given below:

A E g = 0.114Xpy (mm s -1) + 0.500 (mm s-1), (1)

where Xpy is the mole fraction of pyrite in the mixture and A E ~ is the quadrupole splitting of the spectrum for the pyri te/marcasite mixture. The fraction of the iron disulfides that is marcasite as determined from the above equation is presented in table 3 for each of the coal samples.

4. Discussion

The determination of the presence of marcasite and its fraction of the total iron disulfide phases in coals by means of 5VFe MiSssbauer spectroscopy has been successfully accomplished. Chemical treatment of the coal will, in general, be necessary if quantitative estimates of the marcasite/pyri te ratio are desired. A satisfactory chemical treatment, involving cold hydrofluoric acid, that leaves the iron disulfide phases unaffected while removing other iron-containing phases has been devised. If the non-iron disulfide, Fe-containing phases are present in coals in amounts small compared to the iron disulfides, a qualitative determination of the relative marcasite/pyrite contents is possible for untreated coals (as in the case of sample 1361).

The differences between the determinations of the marcasite/pyri te contents by MSssbauer spectroscopy and optical microscopy (cf. tables 1 and 3) occasion no lasting concerns. The sampling technique operational in MSssbauer spec- troscopy is clearly superior to that of optical microscopy as submicroscopic particles and particles not visible on the surface of a sample still contribute to the total intensity of the MSssbauer patterns.

Thus, the technique presented here for the quantitative determination of marcasite and pyrite in coals is superior in analytical precision and accuracy to

B.J. Evans et al. / Determination of FeSe phases in coal 2193

bo t h X- ray d i f f rac t ion and opt ical microscopy. In addi t ion , the t echn ique is appl icable to whole coals and does not requi re ashing the coal samples which can lead to compl ica t ions occas ioned by t r ans fo rma t ions of the inorganic phases.

References

[1] B.J. Evans, R.G. Johnson, F.E. Senftle, C. Blaine Cecil and F. DuLong, Geochim. et Cosmo- chim Acta 46 (1982) 761.

[2] W.C. Grady, Amer. Inst. of Mining, Metall., and Pet. Engineers, Trans. 262 (1977) 268. [3] G.P. Huffman and F.E. Huggins, Fuel 57 (1978) 592. [4] H.M. King, Master thesis, Dept. of Geology, West Virginia University, Morgantown, WV, 1978.