Comparison of sulphur in HNO3-extracted and physically cleaned coals

Comparison of sulphur in HNO,-extracted and physically cleaned coals John T. Riley and Gary M. Ruba Center for Coal Science, Department of Chemistry, Western Kentucky Green, KY 42101, USA (Received 20 October 1988; revised 26 June 1989)

University,

Bowling

The sulphur contents of eight bituminous and subbituminous coals, after extraction with nitric acid, are compared with the sulphur contents of physically cleaned samples of the coals. Samples of -60 mesh (250 pm) coal were extracted with boiling 2 M HNO,, which removes essentially all mineral sulphur. After washing and drying, the extracted samples were analysed for moisture, ash, and total sulphur. The dry, ash-free (daf) sulphur values for the eight coals obtained by this method show excellent agreement with the daf sulphur values for physically cleaned samples of the coals. The physically cleaned samples were prepared by float/sink separation of - 60 mesh coal in 1.30 specific gravity media, followed by milling the float coal to particle sizes less than 10 pm and subsequent float/sink-centrifugation cleaning. The daf sulphur values determined in the HNO,-extracted and physically cleaned samples were less than those obtained using ASTM Method D 2492 and differed by as much as 1.3%. (Keywords: organosulphur compounds; desulphurization; coal)

The types of sulphur

FUEL,

1989,

in coal, as well

Vol 68, December

to account for these mineral sulphur forms results in high values for the organic sulphur. A normal assumption to make about the physical cleaning of coal is that deep-cleaning coal to a very low mineral matter content would remove almost all the mineral sulphur, leaving organic sulphur as the only sulphur species in the cleaned coal. This may be true, but the organic sulphur values determined by ASTM Method D 2492 are usually higher than the sulphur values for physically cleaned coals lo. The direct determination of the organic sulphur in coal has been the goal of researchers for many years. Kuhn et al.” developed a method for the direct determination of organic sulphur in coal by first removing the sulphate sulphur and nonpyritic iron by extraction with dilute HCl, followed by extraction with LiAlH, to remove pyrite, before determining total sulphur in the residue. They assumed that all mineral sulphur was removed in the two extractions, leaving only that sulphur associated with the hydrocarbons in the coal. The determined organic sulphur values for nine coals were generally 0.2-0.3% lower than the calculated ASTM organic sulphur values. Microprobe analysis of sulphur in coal’offers a method of direct determination of organic sulphur concentrations in coal. The microprobe uses a finely focussed electron beam which strikes a carefully selected area in a coal particle to produce X-rays characteristic of the elements present 12,13. Pretreatment of the coal with hydrochloric acid to remove nonpyritic iron and sulphate sulphur is necessary for best results. As the electron beam traverses the sample, X-rays are produced and the intensities from both sulphur and iron are measured. Since the spatial distributions of mineral sulphur are generally clustered while the spatial distribution of organic sulphur is uniform, the relative amounts of organic and mineral sulphur (primarily pyritic) can be measured. These are relative methods, and the use of good calibration

Comparison

of sulphur

in HNO,-extracted

and physically

standards allows reasonably good agreement between organic sulphur values calculated by ASTM Method D 2492 and those obtained by this method13. The use of a scanning electron microscope (SEM) in conjunction with an energy dispersive X-ray spectrometer (EDX) allows better identification of lithotypes in prepared coal samples and thus a better analysis for organic sulphur’““. SEM-EDX is a relatively fast technique for organic sulphur analysis, but has received little attention as a reliable method. Reasons for this lack of attention are the cost of the instrumentation, skills needed for performing the analysis, and unacceptable accuracy. One study reported analysis for nine coals with organic sulphur values ranging from 1.27 to 3.25% (dmmf basis) with an average deviation of 0.35% (17.8% relative) from the ASTM organic sulphur values14. Some of this deviation may be due to the lack of a direct method for organic sulphur analysis that could serve as a reference method. A method for the direct determination of organic sulphur in coal using a transmission electron microscope has been reported’8-20. An electron beam is focussed on a thin section of coal and the X-rays emitted from the elements in the irradiated volume are analysed by X-ray emission spectroscopic methods. Since this is a relative method, the accuracy of the organic sulphur measurement is dependent on the reliability of the standards used for calibration. A procedure for the direct determination of the sulphate, sulphide, pyritic, and organic sulphur in a single sample of coal has been reported by McGowan and Markuszewski2’. The method uses various strengths of perchloric acid as the selective oxidizing agent. The results obtained for the analysis of three coals were comparable with ASTM results and the relative standard deviation for the determinations of the four sulphur forms ranged from 2.4% to 3.4%. This paper contains the results of a study comparing the sulphur contents of eight coals after extraction with dilute nitric acid with the sulphur contents of physically cleaned samples of the eight coals22. The dry, ash-free

Table 1

Characterization

cleaned coals: J. T. Riley and G. M. Ruba

sulphur values obtained for the extracted and cleaned coal sample are compared with dry, ash-free sulphur values obtained by ASTM Method D 2492. EXPERIMENTAL Eight coal samples were used in this study. Characterization data and information about the rank and origin of the eight coals are given in Table 1. Whole seam channel samples of the Kentucky and Tennessee coals were collected and prepared according to ASTM Method D 2013. The subbituminous coal from the Wyodak seam (No. 86039) was a run-of-mine sample, and two medium volatile bituminous coals were obtained from the Pennsylvania State University Coal Sample Bank (No. 86040= PSOC 1138 and No. 86041= PSOC 1195). All coal samples were subjected to standard analysis by ASTM methods, or methods with better precision and accuracy, as follows: proximate analysis using the LECO MAC-400 moisture, ash, and volatile matter analyser; ultimate analysis using the LECO CHN-600 carbon, hydrogen, and nitrogen analyser and the LECO SC-l 32 sulphur analyser. The forms of sulphur data in Table 1 represent the averages of four analyses (duplicate analysis on two different days) by ASTM Method D 2492. The nitric acid extractions were performed using 6.0 g of -60 mesh (250 pm) coal with 120 ml of 2 M HNO, in a 250 ml beaker or Erlenmeyer flask. The mixture was heated, with constant stirring, to boiling and boiled gently for 30 min. The mixture was then filtered while hot through two pieces of Whatman No. 1 filter paper, and the extracted coal washed with several portions of hot deionized water. The total washings were 400-500 ml per sample. The extracted coal was dried in a recirculating nitrogen atmosphere at 110°C for 3 h (Ref. 23). The coal was then analysed for moisture, ash, and sulphur before the sulphur values were reported on a dry, ash-free basis. Clean coal samples of the eight coals were obtained using a two-step procedure23. In the first step 500 g portions of -60 mesh coal were cleaned using 4 1 of

of coals 85099

86024

86025

I.D.”

86038

86039

86040

86041

85098

Seam

ETNA

Wyodak

U. Kittanning

L. Kittanmng

WKY

County/State

Marion/TN

Campbell/WY

Cleatfield/PA

Cambria/PA

Muhlenberg/KY

Muhlenberg:KY

Muhlenberg/KY

Muhlenberg/KY

Mine

Sand

Jacobs

Penn

Smclair

SlIlClalr

Glbraltai

Gibraltar

Mountam

Ranch

No. 4

Dean

No.

I

No. 11

WKY

No. 12

WKY

No. 10

WKY No. 9

__~ Proximate % Moisture’

1.5

22.5

1.3

1.8

4.3

5.6

48

4.1

% Ash

9.3

9.3

10.2

11.3

19.0

15.9

23.0

15.5

% Volatile % Fixed

matter carbon

24.9

43.5

21.0

24.8

34.7

33.0

31 6

35.3

65.8

47.2

68.8

63.8

46 3

51.3

45 4

49.2

Ultimate 80.2

53.8

81.3

75.9

63.2

66.3

60.3

66.1

% Hydrogen

4.8

4.6

4.7

47

44

4.4

4.2

4.6

% Nitrogen

1.5

1.0

1.6

1.4

1.3

1.5

I.4

14

% Sulphur

12

0.8

0.7

2.3

6.0

4.0

4.5

4.6

3.2

30.6

1.6

4.4

6.2

7.9

67

72

% Carbon

% Oxygen Miscellaneous

(by dillerencc) analysis

Btu/lb

14 170

11 570

13 850

13 250

I1 830

12 050

II 010

112

70

98

89

62

54

84

HGI F.S.I. Apparent Forms

rank’

I2 350 66

8.5

Cl

5.0

75

3

3

3

4

mvb

sub B

mvb

mvb

hvAb

hvBb

hvBb

hvAb

of sulphur

Pyritic

0.67

0.16

0.11

0.81

2.85

1.62

2.43

1.73

Sulphate

0.05

0.07

0.02

0.36

0.55

0.51

004

051

Orgamc

0.44

0.53

0.52

1.12

2.56

I 89

I 98

2 31

~~~_ ’ Accession

number,

’ Moisture

is as-determined;

’ LJslng as-determined

Center

for Coal Science all other

analyses

are reported

on a dry basrs

moisture

FUEL, 1989, Vol 68, December

1595

Comparison of sulphur in HNO,-extracted

and physically cleaned coals: J. T. Riley and G. M. Ruba

1.30 specific gravity ZnCl, solutions in a float/sink apparatusz4. The coal was slowly mixed with the ZnCl, solution to ensure thorough wetting of the coal surface. The mixture was allowed to set overnight to allow the float and sink portions to separate and the float and sink portions were then filtered using two sheets of Whatman No. 1 filter paper in Buchner funnels. The cleaned coal was then washed repeatedly on the filter with deionized water until the filtrate did not give a positive test for the chloride ion. The coal was then dried in a recirculating nitrogen atmosphere at 110°C for 3 h. The particle size of the precleaned coals was reduced to mean diameters near 10 pm by milling in a stirred-ball slurry attritor mill. Slurries containing 20% by weight coal dispersed in deionized water were milled for 2 min using 6.3 mm diameter stainless steel media (140 cc slurry per kg media) and an agitator shaft speed of 290 rpmZ3. The slurries were then filtered and dried in a recirculating nitrogen atmosphere at 110°C for 3 h. Float/sink separations were carried out on the milled products using aqueous solutions of zinc chloride (1.30 specific gravity). Samples (20 g) of the dried coals were thoroughly mixed with 150 ml of aqueous ZnCl, before centrifuging at 1700 rpm for 45 min with a model K, size 2, centrifuge from International Centrifuge Co. The float and sink fractions were separated, filtered, washed repeatedly with deionized water and dried for 3 h at 110°C in a nitrogen atmosphere.

Table 3

Sulphur values” for eight coals after physical cleaning Cleaned coal

Coal No.

Moisture (%)

Ash (% dry)

Sulphur (% )

85098 85099 86024 86025 86038 86039 86040 86041

2.08 1.23 5.38 3.06 0.46 0.15 0.94 1.16

1.50 1.80 2.45 1.20 2.35 6.79 1.62 2.20

2.17 1.40 1.14 1.89 0.58 0.56 0.59 0.81

’ All sulphur values are reported on a dry, ash-free basis

Table 4

Summary of organic sulphur data’

Coal No.

ASTM D 2492 organic sulphur Extracted coal by difference (%) sulphur (%)

Clean coal sulphur (%)

85098 85099 86024 86025 86038 86039 86040 86041

3.16 2.25 2.57 2.73 0.48 0.58 0.58 1.26

2.17 1.40 1.14 1.89 0.58 0.56 0.59 0.81

2.22 1.33 1.24 1.84 0.50 0.44 0.54 0.87

’ All sulphur values are reported on a moisture and ash-free basis

RESULTS

AND DISCUSSION

The sulphur contents for the eight coals after treatment with 2 M HNO, are given in Table 2. The reported values are the averages of at least four determinations (duplicate determinations on different days). The nitric acid extractions of the eight coals reduced the mineral matter content of the coals by an average of about 30%. Carbonates, sulphates, and other minerals also dissolve in the nitric acid solution used to extract pyrite. Table 3 lists the dry, ash-free sulphur values for the in the nitric acid solution used to extract pyrite. Table 3 lists the dry, ash-free sulphur values for the eight coals after they are physically cleaned to reduce the mineral matter. The moisture and dry ash values for the cleaned coals are also listed. One can see from the results of the cleaning that the sulphur contents of the eight coals were substantially reduced. Three sets of sulphur data are given in Table 4. The organic sulphur values determined by ASTM Method D 2492, which are the averages of duplicate runs on two different days, are reported for the eight coals. Sulphur Table 2 Analytical values” for eight coals before and after extraction with 2 M HNO, Coal No.

Sulphur in raw coal (%)

Sulphur in coal after extraction (%)

85098 85099 86024 86025 86038 86039 86040 86041

7.35 4.78 5.78 5.78 1.28 0.84 0.72 2.58

2.22 1.33 1.24 1.84 0.50 0.44 0.54 0.87

’ All sulphur values are reported on a dry, ash-free basis

1596

FUEL, 1989, Vol 68, December

eight coals after they are physically cleaned to reduce the mineral matter. The moisture and dry ash values for the cleaned coals are also listed. One can see from the results of the cleaning that the sulphur contents of the eight coals were substantially reduced. Three sets of sulphur data are given in Table 4. The organic sulphur values determined by ASTM Method D 2492, which are the averages of duplicate runs on two different days, are reported for the eight coals. Sulphur values determined by the extraction method and clean coal sulphur values are reported for comparison. One can see from the data that there is very good agreement between the dry, ash-free sulphur values obtained for the physically cleaned coals and those obtained by the HNO, extraction method. The mean difference between the two sets of sulphur values is f0.07% (absolute) with the clean coal sulphur values being slightly higher by an average of 0.02%. Considering that daf sulphur values are calculated using three determined parameters (moisture, ash, and sulphur) the agreement between the two sets of sulphur values is excellent. Upon examining the daf organic sulphur results obtained by ASTM D 2492 and the sulphur contents of the HNO,-extracted and physically cleaned coals given in Table 4, one can see there is poor agreement between the two sets of data. The large differences between the results for the sulphur values can possibly be explained by the inability of the D 2492 method to account for sulphur forms other than sulphate and pyritic. The sulphur in any mineral sulphide such as FeS, ZnS, or PbS, and any elemental sulphur, is not analysed separately in Method D 2492, but is included in the total sulphur. Consequently, the presence of sulphur in these forms results in the calculation of high organic sulphur values. Any nonstoichiometric pyrite, which would have

Comparison

of sulphur

Table 5 Sulphur values for HNO,-extracted float/sink separations

in HNO,-extracted sink portions

from

Coal No.

Sulphur (% dry, ash-free basis)

85098 85099 86024 86025

2.23 1.32 1.22 1.80

something other than a 2:l sulphur:iron ratio, would also lead to an incorrect value for the pyritic sulphur. The discrepancy between the ASTM organic sulphur values for coals and the sulphur that remains in ‘deep-cleaned’ coals has been noted elsewhere”. The ASTM organic sulphur values obtained with -60 mesh raw coal samples are usually higher than the total sulphur values in deep-cleaned ultratine coal used in slurries for fuels. It is proposed that the HNO,-extraction method for sulphur can be used to estimate an accurate value for the residual sulphur content in coals that are candidates for deep-cleaning. To answer the question of whether or not the physical cleaning of coal would preferentially remove coal components enriched in organic sulphur, in addition to the mineral sulphur forms, an experiment was run on the sink portions of four coals. These portions were the sink fractions of coal with specific gravities greater than 1.3 that were separated in float-sink experiments. These mineral matter enriched samples were extracted with 2 M HNO, (20 ml per g of coal), filtered, washed with deionized water, dried in a nitrogen atmosphere, and the dried residue analysed for moisture, ash, and total sulphur. The results of quadruplicate analyses of the sink portions are given in Table 5. The dry, ash-free sulphur values for the four sink portions are almost identical to those obtained for the analysis of the -60 mesh raw coals (Tables 2 and 4). This indicates that the organic sulphur present in the coals is not extracted by 2 M HNO,, and is essentially the same as the sulphur remaining in the coal after an efficient cleaning process to remove the mineral matter. The mean per cent relative standard deviation for the determination of sulphur (on a dry, ash-free basis) in the residues of the HNO,-extracted coals was 3.6%. This measure of precision was based on 4-6 analyses of each of the eight coals. This level of precision is comparable to that of other reported methods for the determination of forms of sulphur in coa12’p25. The good agreement between the dry, ash-free sulphur values for the physically cleaned coals and the HNO,-extracted samples of these coals is a strong indication that the HNO,-extraction method may be useful in predicting the sulphur content of deep-cleaned coals. CONCLUSIONS The dry, ash-free sulphur values of eight bituminous and subbituminous coals extracted with dilute nitric acid show very close agreement with the daf sulphur values of

and physically

cleaned coals: J. T. Riley and G. M. Ruba

deep-cleaned samples of the coals. The daf sulphur values for the nitric acid extracted and deep-cleaned coal samples are generally lower than the daf organic sulphur values determined (calculated) by ASTM Method D 2492. It is proposed that the nitric acid extraction method can be used as a better method for estimating the clean coal sulphur content in coals that are candidates for deep-cleaning. ACKNOWLEDGEMENTS The authors gratefully acknowledge support of this work through funding from the United States Department of Energy under contract DE-FG22-85PC80514. REFERENCES 1

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14 15 16 17

Microsc.

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Hsieh, K. C. and Wert, C. A. Fuel 1985, 64,255 Tseng, B. H., Buckentin, M., Hsieh, K. C., Wert, C. A. and Dyrkacz, G. F. Fuel 1986,65, 385 Wert, C. A., Ge, Y., Tseng, B. H. and Hsieh, K. C. J. Coal Quality 1988, 7 (4) 118 McGowan, C. W. and Markuszewski, R. Fuel 1988, 67, 1091 Ruba, G. M. M.Sc. Thesis, Western Kentucky University, Bowling Green, KY, USA, 1987 Lloyd, W. G., Riley, J. T., Kuehn, K. W. and Kuehn, D. W. ‘Chemistry and Reactivity of Micronized Coal’, Final Report, USDOE Contract No. DE-FG22-85-PC 80514, February 1988 ‘Test Method for Determining the Washability Characteristics of Coal’, Method D 4371, Annual Book of ASTM Standards, Vol. 5.05, Amer. Sot. for Testing and Materials, Philadelphia, PA, USA, 1988 Janke, L. ‘Report of Sulfur Forms Task Group to ASTM D05.21.03 Subcommittee’, May 1986

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