The distribution of inorganic elements between coal and mineral matter in Rumanian lignite

The distribution of inorganic elements between coal and mineral matter in Rumanian lignite Horea

I. Nascu,

Dorel

I. Comsuiea+

and Gavril

Niac

Department of Chemistry, Technical University of Cluj-Napoca, 15 C. Daicoviciu, RO-3400 C&j-Napoca, Rumania * S.C. CARB0CHIMS.A. Cluj-Napoca, X-ray Laboratory, RO-3400 Cluj-Napoca, Rumania (Received 2 February 1993; revised 28 September 7993)

Correlations resulting from ash level determinations and X-ray fluorescence data performed on lignite ash samples taken from the Cdmpulung coal field, Rumania, were determined. The existence of linear relationships between the concentrations of each analysed element: Si, Al, Ca, Fe, S, Mg, Na and K (converted to oxides), and the ash content (dry basis) for different ash levels has been ascertained, with

standard deviations, s,,, in the range 4-19%, with respect to the mean concentration of the oxide in the coal. The data can be used, following suitable extrapolation, to estimate the oxide fractions which correspond to the parts of a given element bound to the coal’s organic and inorganic matter. (Keywords:

mineral

matter;

organic

matter;

lignite)

The chemical composition, chemical structure and morphology or physico-chemical behaviour of the mineral matter in coal has been widely studied’-3. However, in the ash, the mineral material results from both the organic and inorganic matter of the coal and only a small fraction volatilizes during ashing. Many elements are found in the mineral matter. However, some ‘macrocomponents’ exist whose concentrations generally exceed 1 wt% in the ash. These are firstly Si, Al, Ca, S and Fe, and secondly, Mg, Na, K, Ti and P. They are easily analysed by common analytical techniques. On the other hand, many correlations between the properties and contents of diverse components in a coal have been published4-22; which, through their ability to predict the behaviour of a coal in some processes or reactions, have an important role in the chemistry of coal. Until now a great number of such correlations took into account variables such as heat, ash and water content; but some of them considered other physicochemical parameters, such as volatile content5,22, fixed carbon content5, degree of metamorphosis7 or even the composition in organogenic elements, H, C, S, N and 0, both for organic and for mineral matter12. Correlations between concentrations of the organic functions, as well as the specific volume of coal and ash contents, have also been found’*. In this paper all the inorganic ‘macrocomponents’ of the ash, except P, have been analysed by X-ray fluorescence (XRF) spectrometry. The coal studied was a Pontian lignite, from the Poenari mine located in the Csmpulung-Muscel, Rumania, coal field. XRF analysis was selected because of its general applicability for the analyses of minerals and geological samples23. This method was recently also applied to the analysis of coal ash giving comparable results with atomic absorption spectrometry24. 0016-2361/95/01/0119~5 c 1994 Butterworth-Heinemann

Ltd

Recently, there has been increased interest in the study of the content of mineral elements in the organic matter of coals19320,25V26. Kortensky19, using statistical processing of chemical analysis data for silicates from Sofia coal field samples, found that the ‘affinity’ of elements for inorganic matter increased in the order: Si>P>Mn>Fe>Ca>Mg>Na>Ti>Al>(S,

K)

The same author, searching for the abundance of macrocomponent elements in a bituminous coal from Bulgaria, found that Ca, P, Fe, Ti and Mg were to a great extent bound to the organic part; while the rest of the elements were found to be bound to both organic and mineral matter2’. Kortensky and Bliznakov” looking for the affinity towards organic matter of some microelements in a brown coal from Bulgaria found a linear relationship (with only four experimental points), between copper concentration and ash content. MartinezTarzona et ~1.~~working with a Spanish brown coal and different density fractions of this coal, used ion exchange for replacing and subsequent extraction of the cations fixed by -COOH organic functions. Analysis by atomic absorption spectroscopy (AAS) of their concentration showed that Na was associated mainly with the mineral part, K exclusively with the mineral part, while Mg and Ca (largely) were found to be essentially bound to mineral matter. Ca and Mg presented negative slopes of their plots of wt% of element versus ash content, while for Na and K the slopes were positive. This paper shows that similar results may be obtained more easily by an appropriate mathematical treatment of the data obtained from moisture and ash determinations, as well as from the chemical analyses of elements, or their oxides, for six samples taken from the same coal mine but having different ash levels.

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Inorganic

elements in coal: H. I. Nascu et al. Calculation

EXPERIMENTAL

The analytical data in Table I were processed separately for each element and the coefficients of the regression equation: y=a+ bx were calculated, where y stands for the percentage of the oxide content and x for the dry base ash level, Ad. The regression with respect to y was chosen since Ad is less affected by errors. Values for a and b were computed conventionally. The s,, values, the spread of data points around the straight line (parallel to the y axis), were computed by the equation:

Major elemental chemical analyses were carried out using an automatic X-ray fluorescence spectrometer equipped with 12 channels and a microcomputer for data processing. The two computer programs used were ‘MET and ‘DELTA’, based on the Lucas-Tooth and Paine algorithm for calibration and for chemical analyses. The samples used for XRF analyses were prepared by fusion. Spectroscopy grade lithium tetraborate and lithium metaborate were used. The standard glass discs used in this work were prepared from reference materials certified as standards for spectroscopy (Catalogue No. 534, March 1984, Bureau of Analysed Samples Ltd, Newham Hall, TS8 9EA codified BCS-CRM numbers 269,315,375 and 376). In addition, samples of aluminosilicate materials with their composition certified by the ‘Institutul de Cercetari Miniere’, Cluj-Napoca, Rumania, were used. The coal samples taken from the same coal seam and the same mine were stored in tight, thick walled polyethylene bags, preserving the water content, were first broken up by hammering and transferred to sealed glass vials, until the moisture content determination was made. Then, they were ground to a particle size smaller than 0.2mm, air dried and kept in closed vials for ash content determination. The ash content on a dry basis, Ad, was performed by calcination for 1 h in an electrical furnace, keeping the temperature constant at 825°C and using porcelain crucibles. The weighed samples for both ash and water content were about 1 g dried coal powder (drying in an oven at 105”C, if necessary). The ash samples were kept in tightly sealed test tubes with rubber stoppers, and were subsequently subjected to XRF analyses. Measurements and XRF calculations were performed by using relative intensities, the exposure time being 3 x 100 s-l (three readings per sample). With these precautions the reproducibility of readings was within the range f0.183 for Si, and +O.OOOlfor Ti (expressed as standard deviations from the mean in relative intensities). The spread of the method expressed by root mean square error, s, is given by the equation:

s; = (Z:yi- aZy, - bCXiYi)/@- 2)

A,= 0 = Ad(for y = 0)

RESULTS

The data obtained from the ash determinations and by XRF chemical analyses are given in Table 2. The plots of experimental points, as well as of regression lines, are

1

20

30

of ash and principal

40

50

60

70

80

90

100

dry (wt %)

Figure 1 Weight per cent (0) SO2 and (0) Al,O, versus ash content in the Clmpulung

oxides in Cimpulung

lignite, Rumania

lignite coal, on dry basis, found by XRF analyses, in six different

Concentration Ad

10

Ash,

where Ri is the individual measured relative intensity and R the mean value for n = 30 standard samples.

Sample number

AND DISCUSSION

(1)

Table 1 The per cent concentration samples of the same coal

(3)

The correlation coefficient, Y,was computed as normal.

0

s2 = C(R, - R)2/(n - 1)

(2)

where II is the number of data points and the pair Xi, yi is the coordinates of an experimental point. A,=,, is the intercept of the regression line with the ash axis, so that:

(wt%) Na,O

SiO 2

40,

CaO

TiOt

Fe&&

22.0

9.7

3.5

3.1

0.13

1.8

0.35

2.8

0.36

0.90

22.5

10.0

3.7

3.0

0.14

1.9

0.38

2.4

0.38

0.88

21.6

14.1

4.1

2.5

0.19

2.1

0.58

1.9

0.52

0.99

27.0

12.5

5.2

2.4

0.20

1.9

0.51

2.9

0.43

0.92

28.7

14.5

5.5

2.1

0.20

1.8

0.52

2.0

0.43

0.92

28.6

15.0

5.7

2.4

0.20

1.9

0.54

1.3

0.40

0.89

4

16.7

6.2

2.4

2.8

0.08

1.5

0.22

2.1

0.40

0.17

5

23.5

11.7

3.9

2.4

0.14

1.7

0.47

2.2

0.42

0.87

23.1

11.9

4.0

2.4

0.17

1.7

0.48

1.8

0.43

0.88

16.4

5.3

2.1

3.4

0.07

1.7

0.16

2.0

0.44

0.87

2

3

6

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Fuel 1995 Volume 74 Number 1

GO

SO,

MgO

Inorganic

illustrated in Figures 1-5 for all the oxides studied, as well as for the carbon content of the coal. The parameters of the regression line plotted in Figures Z-5 and their s0 values, as well as the associated correlation coefficients, are given in Table 2. The spread of experimental points is only in part due to analytical error, as may be seen by comparing the spread of the standard deviation, s, with the spread of the points around the straight line, measured by s,,, both given in Table 2. Sometimes, the correlation coefficient found was remarkably high (for TiO, and SiO,), although

1.0

1.0

0.5

0.5

0.0

0

20

10

30

40

Ash,

50

60

70

80

90

elements in coal: H. I. Nascu et al.

the data were affected by their natural spread. The measure of spread, the s0 value, was in some cases comparable with the analytical error, s (for SiO,, Al,O,, Na,O) and in other cases, because of the natural inhomogeneity of their content, considerably larger (CaO, Ti02, Fe,O,, KzO, MgO). As can be seen from Table 2 and Figures 2,3 and 5 this natural spread of their content is especially high for CaO, Fe,O, and SO,. It would be interesting to learn about the spread of the contents of these elements when larger samples are analysed. In the case of calcium it is believed that the cause may be the

0.0 100

10

0

20

dry (wt%)

30

40

Ash,

50

60

70

80

90 100

dry (wt8)

Figure 4 Weight per cent (0) Na,O and (0) K,O versus ash content in the Campulung lignite, Rumania

Figure 2 Weight per cent (0) CaO and (0) MgO versus ash content in the Campulung lignite, Rumania

70

5

60

4

50 33

L-z 40 r ;; 30

r 2 $”

20 1 10 0

.O

0

Ash,

Oxide

20

dry (wt%)

30

40

Ash,

50

60

70

80

0 90 100

dry (wt%)

Figure 5 Weight per cent (0) carbon and (0) SO, versus ash content in the Campulung lignite, Rumania

Figure 3 Weight per cent (0) FeO, and (0) TiO, versus ash content in the Clmpulung lignite, Rumania

Table 2

IO

The parameters of the linear regression equation illustrated in Figuresf-5 (I

i-

b

A,=0

?‘a

Y95

s

so

SiO,

- 6.590

0.752

0.992

8.70

0.000

64.90

1.400

0.9000

Al,% CaO

-2.196 3.970

0.264 - 0.052

0.985 -0.668

8.33 76.00

0.100

23.10

0.670

0.1000

3.520

0.97

0.860

0.0130

TiO,

- 0.095 1.190

0.010

0.986

9.50

0.87

0.024

0.0001

0.026

0.690

- 0.008 1.420

3.66

0.390

0.0120

-0.295 2.910

0.030 -0.037

0.958 - 0.408

- 0.034 2.590

2.56 -0.61

0.135 1.200

0.0460 0.1700

Na,O

0.353

0.003

0.290

0.380

0.64

0.140

0.1700

Mg0

0.678

0.009

0.716

0.760

1.53

0.130

0.0400

FeA K,O SO,

-41.00 9.83 79.00 -118.00 - 75.00

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crystallization of CaSO, in the glass disc, during sample preparation. Although this fact has not been proved, it is believed that another flux composition would improve, at least in the case of calcium, the results. In addition, it should be noted that sodium and magnesium have a very low correlation coefficient, because of the mixed origin in the ash from both the mineral and organic matter. The same thing may be observed in the case of iron. By examining all nine curves it may be ascertained that there are three types of graphs. These are illustrated in Figure 6. The curves identified with (2), as in the case of SiO,, Al,O,, TiOz and KzO, intersect the ash axis near the same value. This is consistent with the assumption that these oxides originate exclusively from the inorganic matter of the coal. This means, that the ash level corresponding to the intercept is due exclusively to the ions fixed to the organic part (by ionic or complex bonds). Lines of type (l), have, at the minimum ash level previously mentioned (corresponding to their intercept with x=8.7 wt%), a positive value (the case of Fe, Na, Mg). Graphs of type (3), in which the weight per cent in oxide becomes lower as the ash level increases, imply that those oxides originate predominantly from the organic matter (the case of Ca and S). The oxide total (yO in Figure 6) equals the ash at the intercept of the ash axis for the plots of K, Si, Ti, Al, i.e. oxides which are not of organic origin. Thus, the oxide calculated from the respective regression lines for the ash level of about 8.7 wt% [see Figure 6, graphs (1) and (3)] corresponds to that part of the oxide originating from organic matter. Since the lignite studied does not contain carbonates, the intercept of the regression line of carbon content (wt%) versus ash level on dry basis, Ad (see Figure 5) corresponds to the ash which results exclusively from mineral matter: 95 wt%27. Since the extrapolation is made to a greater distance from the zone in which the experimental points are grouped, the experimental error will be greater. Nevertheless, valuable information about mineral matter composition can be obtained. From the materials balance, the contribution of those elements which, during ashing, react and volatilize is missing. Particularly, in the present case, it was presumably the entire pyritic sulfur content. As can be seen from Table 3 the total is close to 95 wt%, which is the ash level for zero carbon content, i.e. of pure mineral matter (without the ions bound to organic molecules).

a.7

95

Ad (wt%l

CONCLUSIONS The extrapolation of the regression lines for any oxide to the ash level for zero organic carbon content, in the case under consideration, to 95 wt% (y& ash level, gives the oxides of the mineral matter. For the elements originating exclusively from inorganic matter (especially Si or K) the intercept with the ash axis, Ad, gave an ash percentage which comes exclusively from the organic matter; this being a very good estimate for the total ash content originating from ions bound to organic matter. Inorganic elements originating almost exclusively from organic matter had negative slopes for plots of per cent oxide versus per cent ash, and their extrapolation intercept, the x = Ad axis at a point AYZo (for SiO,), reflects the effective content of the inorganic element in the pure coal mass (converted to oxide in the burning process) bound to -COOH or -SH groups of the organic matter by ionic or complex bonds. Extrapolation to x = 8.7 wt% ash for the nil organic part (about 95 wt% in the present case) gives the oxide coming exclusively from inorganic matter. Extrapolations to 0 or 100 wt% ash, used by some authors, are without a physical meaning. The correlations found in this paper and their possible refinement, present a new way of determining the proportion of a given element bound to organic or mineral matter.

Table 3 The content of different oxides in organic and mineral matter, as well as of elements bound to pure coal

Oxide

Organic matter

Inorganic matter

Element in pure coal (wt%)

SO, GO, CaO Fe,O, SO, TiO, MgO Na,O K,O

None None 3.52 1.42 2.59 None 0.76 0.38 None

64.90 22.88 None 3.66 None 0.86 1.53 0.64 2.56

None None 2.51 0.99 1.04 None 0.46 0.28 None

Total

8.66

97.26

5.28

Wt% oxide content corresponding to

a.7

95

Ad (wt%l

Figure 6 Types of linear regression lines found between weight per cent oxide and ash content on dry basis. A, wt% oxide from an element bound to the organic matter of coal. B, wt% oxide originating in the inorganic part of the coal

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inorganic elements in coal: H. I. Nascu et al.

In the case of the CBmpulung lignite studied, SiO,, Al,O,, K20 and TiO, had an almost exclusively mineral origin, but Na,O, MgO and Fe,O, were bound partly to organic matter, as in other low rank coals. This is in agreement with other published data. Elements such as Ca and S proved to be bound only to pure coal matter. By comparing the data with other published data it seems that excluding Si, which is almost entirely of mineral origin, the other inorganic elements have a mixed abundance in mineral and organic matter in a proportion which depends on the place and time of their genesis. It is believed that, without taking into account the perspectives related to the chemistry of coal itself, a better understanding of its geological origin is possible, by using the derived correlations. Correlations established between ash and heat content*-“, and that found in this paper between SiO, content and ash indicate a possible method for the determination of heating values of coals by means of XRF analysis.

4 6

8 9 10 II

12

13 14 15 16 17

18

ACKNOWLEDGEMENTS

19

We would like to express our appreciation to Mr Csapo Ernest for his help in computing and preparing this manuscript and to thank Dr Michael Cloke (University of Nottingham) for reading it and for his critical appreciation of our work.

20 21 22 23 24

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Jenkins, R. G. and Walker, P. L. Jr in ‘Analytical Methods for Coal and Coal Products’ (Ed. C. Carr Jr), Vol. 2, Academic Press, New York, 1978, pp. 265-280 Shpirt, M. I. Khim. Tverdovo. Topliva 1982, 16, 35 Valkovic, V. ‘Trace Elements in Coal’, Vol. 1, CRC Press Inc., Boca Raton, 1983, pp. 91-l 13

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Abraham, L. Bull. Sot. Politehnice Romane 1938,52, 3 Carlos. A. L. Inst. Tecknol. Rioarande delSul Bol. 1971.55.35 Anghei, V. Rev. Minelor (Buciarest) 1971, 22, I4 Buk, S. I., Fedushshak, M. Y. and Radchenko, L. M. Geol. Geochim. Gornii Ishop. 1973,36, 75 Niac, G. Mine Petrol si Gaze 1977, 28, 267

Niac, G., Enache. C., Kraus, H. and Kraus, S. Mine Petrol si Guze 1977, 28. 127 Niac. G., Enache, C.. Kraus, H. and Kraus, S. MQje Perrol si GNZP1977, 18, 175 Niac, G.. Enache, C., Kraus, H. and Kraus, S. Mine Perrol si Go:e 1977, 28, 303 Weddell. C.. Davis. A.. Soackman. W. and Griffifths. J. C. ‘Study of Interrelationship Among Chemical and Petrographic Variables of United States Coals’, Pennsylvania State University, 1978 Niac, G. and Enache, C. Mine Petrol si Gu:e 1978. 29,48 Niac, G. and Gonteanu, A. Mine Petrol si Gaze 1978, 29, 257 Niac, G.. Horovitz, 0. and Enache, C. Mine Petrol si Gu:e 1979. 30, 505 Gonteanu, A. and Niac, G. Mine Petrol si Gaze 1980, 31, 123 Neavel, R. C., Smith, E. S., Hippo, E. J. and Milles, R. N. Fur/ 1986, 65, 312 Niac, G., Damian, L. and Nascu, H. Bull. Inst. Politehnir Cluj-Nupoca, Ser. Chimie-Metalurgie 1985, 27-28, 49 Kortensky, 1. Annual, Higher Inst. Mining & Geol.. Sofa 1986, 32, 179 Kortensky, 1. Annual, Higher Inst. Mining & Geol., Sofia 1988, 34,297 Kortensky, I. and Bliznakov, A. Annual, Higher Inst. Mining & Geol., Sofia 1988, 34, 311

Martinez-Tarzona, M. R., Martinez-Alonzo, A. and Tascon, J. M. D. Fuel 1990,69, 362 Oliver, G. J. Inter. Ceram. 1981, 30, 110 Martinez-Tarzona, M. R., Spears, D. A., Palacios, J. M., Martinez-Alonzo, A. and Tascon, J. M. D. Fuel 1992, 71, 367 Martinez-Tarazona, M. R., Palacios, J. M. and Tascon, J. M. D. in ‘Institute of Physics Conference Series’ No. 98, Chapter 7 (paper presented at EMAG-MICRO 89, London, 13-15 September 1989) Martinez-Tarzona, M. R., Palacios, J. M., Martinez-Alonzo, A. and Tascon, J. M. D. Fuel Process. Technol. 1990, 25, 81 Niac, G. Unpublished data

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