Comprehensive Elemental Analysis of Coal and Fly Ash

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Comprehensive Elemental Analysis of Coal and Fly Ash R. A . NADKARNI Exxon Research and Engineering Company, Analytical Research Laboratory, Baytown, T X 77520

All major ash elements and some trace elements are determined in coal or fly ash by inductively coupled plasma emission spectrometry. Parr oxygen bomb combustion followed by ion selective electrode, X-ray fluorescence or atomic absorption spectrometric measurements are used to determine halogens, sulfur, nitrogen, mercury, arsenic, selenium, and phosphorus. Hydride generation-atomic absorption spectrometry is used to determine traces of As, Se, Sn, Sb, Te, Pb, and Bi. Spectrophotometric determinations are used for gallium and germanium. With the increased emphasis on the development of synt h e t i c f u e l s to supplement the d e p l e t i n g n a t u r a l petroleum resources, c o a l i s coming i n t o prominence as a v i a b l e a l t e r n a t i v e f u e l source. Since c o a l i s a heterogeneous mixture o f many m i n e r a l s , i t i s important to have a n a l y t i c a l methods to measure the inorganic c o n s t i t u e n t s a c c u r a t e l y and thus to be able t o f o l l o w t h e i r path through various stages of c o a l production and utilization. This paper describes the approach o f Exxon's Baytown Research and Development D i v i s i o n to a n a l y z i n g c o a l and f l y ash samples with some e s t a b l i s h e d techniques and some newer improvements that have been incorporated i n them. The f i r s t paper i n t h i s s e r i e s was published by us e a r l i e r (I) emphasizing the a n a l y s i s o f major elements. Experimental Sample Preparation. F i n e l y ground ( u s u a l l y -300 mesh) c o a l samples are ashed at 750°C i n a muffle furnace to a constant weight. Alternatively, an RF plasma low temperature asher can be used; however, the time needed f o r ashing i s o f the order of a few days. The c o a l ash or

American Chemical 0097-651/82/0205-0l47$06.00/0 © li(HWewrMffT3tônûcal Society In Coal and Coal Products: Techniques; Fuller, E.; 1155 Analytical 16th St.Characterization N. W. ACS SymposiumWwfctoette. Series; American Society: Washington, DC, 1982. 0.Chemical C. 2003·

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f l y ash thus prepared i s brought i n t o s o l u t i o n by d i s s o l v i n g i n a mixture o f aqua r e g i a + HF i n P a r r T e f l o n bombs heated a t 110°C f o r two hours. The d e t a i l s are d e s c r i b e d elsewhere (jO.


I n d u c t i v e l y Coupled Plasma Emission Spectrometry (ICPES). The sample prepared by d i s s o l u t i o n i n a P a r r bomb i s analyzed by ICPES. We use a J a r r e l l - A s h Plasma AtomComp 90-975 w i t h 90-55 spectrum s h i f t e r f o r background c o r r e c t i o n . The d e t a i l s o f our i n s t r u m e n t a t i o n and the o p e r a t i o n have been described elsewhere ( 2 ) . Atomic A b s o r p t i o n Spectrophotometer

(AAS).

A V a r i a n 475 spectrophotometer and M-65 vapor g e n e r a t i o n accessory i s used f o r the d e t e r m i n a t i o n o f mercury through c o l d vapor g e n e r a t i o n , and f o r the d e t e r m i n a t i o n o f As, Se, Sb, B i , Te, Sn, and Pb through hydride g e n e r a t i o n . The procedure f o r mercury i s d e s c r i b e d elsewhere (1). In practice, after d i s s o l u t i o n and d i l u t i o n o f the sample i n a f i x e d volume, an a l i q u o t of the s o l u t i o n i s taken and i s t r e a t e d w i t h enough HC1 t o proper normality. As d e s c r i b e d l a t e r , the s o l u t i o n s are t r e a t e d w i t h a d d i t i o n a l reagents and allowed t o r e a c t f o r d e s i r e d time p e r i o d s . The s o l u t i o n i s t r a n s f e r r e d t o the M-65 hydride generator accessory. The generator i s c o n t i n u o u s l y f l u s h e d w i t h n i t r o g e n gas to c a r r y the hydrides t o the burner. On dropping a p e l l e t o f NaBH4 i n t o the generator, gaseous metal-hydrides are immediately produced and are swept by the n i t r o g e n gas i n t o a heated openended quartz tube l o c a t e d on the burner-head i n the l i g h t path o f the AA-475 instrument. The hydrides are decomposed i n the heated tube w i t h hydrogen burning q u i e t l y a t both ends o f the tube. Some p a r t i c l e s o f NaBIty are p h y s i c a l l y c a r r i e d over i n t o the flame, which imparts a b r i g h t y e l l o w c o l o r t o the flame. The atomized elements are measured by the AA instrument, p r e f e r a b l y i n the peak area mode. A f t e r one measurement i s completed, the s o l u t i o n i n the generator i s drained out, and i t i s ready f o r the next a n a l y s i s . D e t a i l s o f t h i s method a r e p u b l i s h e d elsewhere (3). P a r r Oxygen Bomb. A P a r r 1901 oxygen bomb apparatus i s used w i t h s t a i n l e s s s t e e l combustion c a p s u l e s . Less than 1 gm o f c o a l sample i s mixed w i t h a few drops o f white o i l i n the cup. F i v e mL o f water i s placed i n the bottom o f the bomb. The assembled and c l o s e d bomb i s p r e s s u r i z e d t o 30 atm o f oxygen. A f t e r a few seconds o f combustion ( and we w i l l not d e s c r i b e the d e t a i l s here. Even though our ICPES system i s equipped with 34 element channels, normally data on 20-25 elements can be obtained with good p r e c i s i o n and accuracy i n a c o a l ash s o l u t i o n matrix. Some a d d i t i o n a l trace elements have been determined with a computer c o n t r o l l e d scanning monochromator ICPES by Floyd et a l (5), however, our ICPES i s equipped with f i x e d array of e x i t s l i t s on a polychromator and some t r a c e elements cannot be determined with r e l i a b i l i t y due to a combination of t h e i r low c o n c e n t r a t i o n i n the c o a l or f l y ash, and the background i n t e r ference from major elemental l i n e s i n the determination of weaker trace element l i n e s . A t y p i c a l example of r e s u l t s obtained by the Parr bomb d i s s o l u t i o n procedure, followed by ICPES measurements, i s given i n Table I. Our r e s u l t s f o r N a t i o n a l Brueau of Standards (NBS) Standard Coal-1632 and F l y Ash-1633 are compared with NBS c e r t i f i e d v a l u e s , or "best" values from the l i t e r a t u r e , where a v a i l a b l e . The best values were obtained from over 70 papers i n the l i t e r a t u r e . The obvious o u t l i e r s were omitted i n c a l c u l a t i n g the average v a l u e s . The agreement between our r e s u l t s and the "known v a l u e s " i s s a t i s f a c t o r y . The average accuracy of our data i s ±10%, though some trace elements have

In Coal and Coal Products: Analytical Characterization Techniques; Fuller, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.


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Table I Elemental A n a l y s i s of Coal and F l y Ash by I n d u c t i v e l y Coupled Plasma Emission Spectrometry Element, ppm Al, % Ba Be Ca, % Co Cr Cu Fe, % K, % Mg, % Mn Na Ni Ρ Si, % Ti V Zn

NBS-1632 Coal Found Present(a) 1.78 342 1.45 0.41 5.78 20.2±5 18+2 0.87±0.03 0.29 0.16 40±3 380 15±1 71; 118 3.38 960 35±3 37±4

1.71 219 1.53 0.49 5.5 12 15 0.72 0.20 0.12 39 368 15.7 118 2.41 702 30 39

NBS-1633 F l y Ash Found Present(a) 12.5 2700 11.9 4.60 39.3 131±2 128±5 6.18 1.68 1.55 493±7 3200 98+3 880 20.5 7200 214±8 210±20

12.7 2100 18.7 5.21 25 104 142 5.53 1.71 1.30 483 3000 —

1004 22.7 6800 214 197

(a) From Nadkarni ( 1). Values with standard d e v i a t i o n s are N.B.S. c e r t i f i e d .


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l a r g e r e r r o r s . Many other standards have been analyzed by t h i s procedure with e q u a l l y good r e s u l t s .


Parr Oxygen Bomb Procedure. The main advantage of t h i s method i s i n determining what we c a l l the " v o l a t i l e " elements such as halogens, s u l f u r , n i t r o g e n , phosphorus, a r s e n i c , and selenium. Normally these elements have to be determined i n d i v i d u a l l y with separate sample p r e p a r a t i o n i n each case. However, once the c o a l sample i s combusted i n a Parr oxygen bomb, and the gases produced are trapped i n an e x t r a c t a n t such as water or d i l u t e d Na2C03, a l l the above elements can be determined i n the r e s u l t a n t s o l u t i o n . A l l the halogens are determined by i o n s e l e c t i v e e l e c t r o d e s , mercury by Cold VaporAAS, n i t r o g e n by Antek chemiluminescent d e t e c t o r , s u l f u r by XRF, a r s e n i c , phosphorus and some other elements by ICPES. Our r e s u l t s f o r NBS-1632 and 1632a c o a l s and 1571 Orchard Leaves are given i n Table I I . We f i n d good agreement between our r e s u l t s and the l i t e r a t u r e v a l u e s . We have seen no iodine or selenium i n any c o a l samples we have analyzed, because of t h e i r low l e v e l s of c o a l . The only other trace elements we found i n the combustionabsorption s o l u t i o n s were boron and lead; however, n e i t h e r of these elements seem to be q u a n t i t a t i v e l y recovered by t h i s procedure. Vapor Generation-Atomic

Absorption

Spectrometry.

Even though the ICPES system i s extremely s e n s i t i v e f o r most o f the trace elements, i t i s s t i l l not p o s s i b l e to determine s e v e r a l c r i t i c a l l y important elements such as As, Se, Sb, Pb, e t c . i n a c o a l or f l y ash matrix. Though t h e o r e t i c a l s e n s i t i v i t y may be impressive, the presence of large concentrations of the matrix elements, which produce i n t e r f e r e n c e s and high background, make the high ICPES s e n s i t i v i t y i n e f f e c t i v e . Atomic a b s o r p t i o n spectrophotometry with a hydride generation accessory i s one p o s s i b l e way of determining t h i s group of elements. In t h i s mode, the elements of i n t e r e s t are converted i n t o gaseous hydrides, which are then fed i n t o the flame, decomposed t h e r e i n , and the elements are determined by AAS. Mercury, which does not form a hydride, can a l s o be determined i n t h i s same f a s h i o n by forming gaseous elemental mercury. The d e t e c t i o n l i m i t s of ICPES, AAS with flame, and AAS with hydride generation are compared i n Table I I I (6) where i t can be c l e a r l y seen that the l a s t mode of determination i s the best. In a d d i t i o n , the matrix i s e l i m i n a t e d i n the hydride mode but not i n flame AAS or ICPES.


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Table I I


Trace Elements Determination

Element, ppm

Method of Determination (a)

i n Coal Using Parr Oxygen Bomb

NBS-1632a Coal Found (c) Pres. (b) (N=5)

Fluorine

I SE

92

84±8

Chlorine

I SE

784 (12)

770 ± 48

Bromine

ISE

45 (13)

43

Mercury

CV-AAS

0.13 ± 0.03

0.17 ± 0.02

Nitrogen,

Antek

1.27 (12)

1.19 ± 0.08

Sulfur, %

XRF

1.62 ± 0.03 9.3 ± 1

Arsenic

ICPES

Phosphorus ICPES

-

NBS-1632 Coal Près. (d) 90

Found 71

NBS-1571 Orchard Leaves Found (c) Pres. (n=3) (b)

-

-

962

915

690

638 ± 27

-

-

-

-

0.12 ± 0.02

0.12

—

—

—

—

—

—

1.48 ± 0.07

1.35

1.32

0.19

0.27 ± 0.04

8.9 ± 1.2

5.9 ± 0.6

5.31

10±2

8.9 ± 2.2

71;118

156

-

-

85±17

Boron

ICPES

53 (12)

22±3

40

29

Lead

ICPES

12.4 ± 0.6

7±1

30±9

19

45±3

31±2

(a) E x p l a n a t i o n i n the text (b) From NBS C e r t i f i c a t e of A n a l y s i s and l i t e r a t u r e values where available (c )Number of r e p l i c a t e analyses; ± values represent one standard d e v i a t i o n from mean value (d) From Nadkarni (1) n


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Table I I I (6) D e t e c t i o n L i m i t s of D i f f e r e n t Spectroscopic Procedures


Element,

ppm

ICPES

As Bi Ge Pb Sb Se Sn Te

0.04 0.05 0.15 0.008 0.20 0.03 0.30 0.08

AAS

AAS

Flame

Hydride

0.0008 0.0002 0.004 0.10 0.0005 0.0018 0.0005 0.0015

0.63 0.044 0.02 0.017 0.06 0.23 0.15 0.044

The s p e c t r a l p r o p e r t i e s of the hydride forming group of elements are given i n Table IV. As i s e v i d e n t , the hydride generation-AAS method i s extremely s e n s i t i v e with a f a i r l y large dynamic range. This higher s e n s i t i v i t y i s obtained by using the peak area r a t h e r than the peak height absorbance mode. Use of e l e c t r o d e l e s s discharge lamps instead of the hollow cathode lamps a l s o increases the s e n s i t i v i t y s e v e r a l f o l d i n the case of a r s e n i c and selenium. In the case of other elements, the EDLs are not that advantageous over HCLs. Table IV AAS Determination of Hydride Forming

Element

Hydride

As

AsH

Bi Ge Pb Sb

BiH GeH PbH SbH

Se Sn Te

H Se SnH H Te

3

3

4

-22 -88.5

4

—

-17

4

°C

-55

3

2

2

B.P.

-42 -52 -4

Wavelength nm 193.7; 197.2 233.1 265.1 217.0 217.6; 231.2 196.0 286.3 214.3

Slit nm 1

Elements Sensitivity ng 0.2

Range ng 30 60

0.2 1 1 0.2

0.4 0.05 1.4 0.2

300 30

1 0.5 0.5

0.6 0.8 0.5

100 100 50

—

Most of these hydrides are generated i n s t r o n g l y a c i d i c s o l u t i o n with some reducing agents such as z i n c , magnesium, t i t a n i u m t r i c h l o r i d e or sodium borohydride. The l a s t reagent i s now almost u n i v e r s a l l y p e r f e r r e d . I t can be used as a d i l u t e s o l u t i o n , which i s somewhat unstable, or as s o l i d p e l l e t s . Since ng amounts of the hydride forming elements are determined, i t i s extremely important to keep the working environment c l e a n . Ultra



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pure reagents o n l y , where a v a i l a b l e , should be employed, although sometimes we have found even these to be contaminated. Thus, an i d e n t i c a l blank must be c a r r i e d through the e n t i r e procedure each time. V a r i a t i o n i n the a r s e n i c content of the NaBIty p e l l e t s from 3 ng to 90 ng per p e l l e t has been r e p o r t e d ( 7 ) . The con c e n t r a t i o n of a c i d i n s o l u t i o n has pronounced e f f e c t on the a b s o r p t i o n s i g n a l . No e f f e c t on absorbance of As, B i , and Sb was observed on changing the HC1 c o n c e n t r a t i o n from 1 to 9 N. The selenium response i s optimum i n the 2.5 to 5 Ν range of HC1 s t r e n g t h . The optimum HC1 c o n c e n t r a t i o n f o r the I^Te p r o d u c t i o n i s between 2.5 to 3.6 N. The production of SnH4 i s markedly dependent on the HC1 c o n c e n t r a t i o n , the optimum being 0.6 to 1 N. Outside these l i m i t s , the absorbance dropped s h a r p l y . For the r e s t of t h i s work 4 N HC1 s o l u t i o n s t r e n g t h was used i n the a n a l y s i s of As, B i , Sb, and Se. O x i d a t i o n s t a t e of some elements i s a l s o c r i t i c a l i n a n a l y s i s . Thus, a r s e n i c and antimony have to be i n +3 form, and selenium i n +4 s t a t e . This i s achieved by a p p r e c i a b l e amounts of c o a l m a t r i x — N a , A l , K, Ca, Mg, Fe, T i , P, L i , Sr, Ba, N i , Cr, S i , e t c . — w e r e found not to i n t e r f e r e w i t h the hydride d e t e r m i n a t i o n . Lead i s determined by hydride-AAS procedure f o l l o w i n g the method of Fleming and Ide ( 8 ) . T a r t a r i c a c i d and l ^ C ^ O y at pH 1.5 to 2 are used f o r optimum Pblty p r o d u c t i o n . R e s u l t s of u s i n g the hydride-AAS method f o r two NBS standard c o a l s and a f l y ash are summarized i n Table V. Each sample was run on f i v e r e p l i c a t e a l i q u o t s . No bismuth was detected i n any sample. No t e l l u r i u m was detected i n c o a l 1635. There are no l i t e r a t u r e values a v a i l a b l e f o r e i t h e r of these c o a l s ; two l i t e r a t u r e values a v a i l a b l e f o r t e l l u r i u m i n the f l y ash are w i d e l y d i f f e r e n t from each other and from our v a l u e , p o i n t i n g to the d i f f i c u l t y i n determining t h i s element by most a n a l y t i c a l techniques. Agreement of other elements between our r e s u l t s and the NBS values i s s a t i s f a c t o r y . The o v e r a l l p r e c i s i o n of the r e s u l t s i s of the order of ±10%. Spectrophotometric Determination of Ga and

Ge.

The s i g n i f i c a n c e of these two elements i n c o a l or f l y ash i s from the p o i n t of view of t h e i r recovery from c o a l ashes or f l y ashes. The c o n c e n t r a t i o n l e v e l s of these two t e c h n o l o g i c a l l y v a l u a b l e elements i n other m i n e r a l sources ( p r i n c i p a l l y z i n c and aluminum ores) i s about the same l e v e l as i n c o a l or f l y ashes (Table V I ) . Both of these elements are d i f f i c u l t to determine by most a n a l y t i c a l techniques. We have chosen to use two s e n s i t i v e and r e l a t i v e l y s e l e c t i v e reagents to e x t r a c t these elements i n t o organic s o l v e n t s and then spectrophotom e t r i c a l l y measure them. The flow scheme of a n a l y s i s i s given i n Table V I I .


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Table V Determination o f Trace Elements i n Coal by Hydride Generation-Atomic Absorption Spectrometry

Element Arsenic

Coal-1632a Found (b) Present (a) (n=5) 9.3±1

9.5 ±

0.6 Selenium

2.6±0.7

3.1

Coal-1635 Found (b) Present (a) (n-5) 0.42 0.15 0.9±0.3

0.41

0.14

Lead

8.1±1

12.4

Tellurium

12.4

0.79

0.13

8.8±0.5

6.9

6.0*0.6

0.01

-

-

1.9

1.5

±

±

±

±

0.6

0.4

0.2

0.2

0.50 + 0.05

-

-

9.4±0.5

±

0.15

-

0.02

0.07

±

Tin

64±4

±

0.2 0.58

61±6

±

±

±

Antimony

0.28

F l y Ash-1633 Found Present (b) (a) (n-5.)

Comprehensive Elemental Analysis of Coal and Fly Ash

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