Trace Elements in Fuel - ACS Publications

6 X-Ray Fluorescence Analysis of Whole Coal

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on February 12, 2016 | http://pubs.acs.org Publication Date: September 1, 1975 | doi: 10.1021/ba-1975-0141.ch006

JOHN K. KUHN, WILLIAM F. HARFST, and NEIL F. SHIMP Illinois State Geological Survey, Urbana, Ill. 61801

X-ray fluorescence analysis proved to be rapid, simple, and reasonably accurate for determining the concentration of 21 minor and trace elements in whole coal. Development of the method was facilitated by the availability of a set of samples that had been analyzed during the investigation of over 100 coals for trace and minor elements by optical emission spectroscopy, atomic absorption spectroscopy, neutron activation, and wet chemical techniques. Although major elements in coal, carbon, hydrogen, oxygen, and nitrogen, cannot be analyzed by x-ray fluorescence, most other elements at levels greater than a few parts per million are readily determined. Tables present data supporting the conclusion that x-ray fluorescence may be the best method for analyzing large numbers of coal samples.

T l e c e n t interest i n the trace element content of c o a l has i n c r e a s e d the A

^

need for r a p i d a n d accurate analytical methods for their determina-

tion. Because x-ray

fluorescence

analysis has d e m o n s t r a t e d its usefulness

i n d e t e r m i n i n g m a j o r , m i n o r , a n d trace elements i n n u m e r o u s other types of m a t e r i a l s , i t w a s felt that this m e t h o d c o u l d b e e x t e n d e d to trace element determinations i n w h o l e coal.

I n t h e past, s u c h analyses w e r e

seriously h a m p e r e d b y t h e l a c k of s t a n d a r d samples. H o w e v e r , r e s e a r c h b e i n g c o n d u c t e d i n o u r laboratories u n d e r t h e s p o n s o r s h i p of t h e U . S. E n v i r o n m e n t a l P r o t e c t i o n A g e n c y p r o d u c e d a large n u m b e r of c o a l s a m ples f o r w h i c h trace elements h a d b e e n d e t e r m i n e d b y t w o or m o r e i n dependent analytical procedures, for example, optical emission, neutron activation, atomic absorption, a n d wet chemical methods. w e r e u s e d as standards to d e v e l o p a n x - r a y

fluorescence

T h e s e coals method that

w o u l d d e t e r m i n e m a n y trace a n d m i n o r elements i n pressed w h o l e c o a l samples. 66 In Trace Elements in Fuel; Babu, Suresh P.; Advances in Chemistry; American Chemical Society: Washington, DC, 1975.

6.

KUHN E T AL.

X-Ray

Fluorescence

67

Analysis

T h e i n s t r u m e n t u s e d i n this project w a s a P h i l l i p s m a n u a l v a c u u m x-ray

fluorescence

spectrometer.

A l l analyses w e r e m a d e i n t h e A n a l y t i c a l

C h e m i s t r y L a b o r a t o r i e s of t h e I l l i n o i s State G e o l o g i c a l S u r v e y . Preliminary

Investigation of Major and Minor Elements

in Whole Coal and Coal Ash


Two

different types of m a t e r i a l s , c o a l ash a n d w h o l e c o a l ,

were

a n a l y z e d , a n d s a m p l e p r e p a r a t i o n was v a r i e d a c c o r d i n g l y . W h o l e c o a l w a s g r o u n d w i t h a b i n d e r ( 10 w t % ) a n d pressed i n t o a d i s k , w h i c h w a s u s e d as the a n a l y t i c a l s a m p l e . T h e b i n d e r w a s a c o m m e r c i a l p r o d u c t , S o m a r M i x , a n d the s a m p l e w a s g r o u n d i n a N o . 6 W i g - L - B u g for 3 m i n . Pellets 1 % i n . i n d i a m e t e r w e r e t h e n f o r m e d at 40,000 p s i i n a d i e d e s i g n e d for that p u r p o s e . S a m p l e p r e p a r a t i o n t e c h n i q u e s are g i v e n i n d e t a i l i n a p r e v i o u s p u b l i c a t i o n ( 1 ). T w o g of c o a l m a d e a d i s k that w a s i n f i n i t e l y t h i c k , i.e., no x-rays p e n e t r a t e d the s a m p l e , for soft x-rays e m i t t e d b y l i g h t elements s u c h as magnesium,

silicon, aluminum, and

calcium.

However,

for

elements

h e a v i e r t h a n b r o m i n e , the s a m p l e w e i g h t h a d to be i n c r e a s e d to a t t a i n infinite s a m p l e thickness. The

use of x - r a y

fluorescence

was

o r i g i n a l l y i n t e n d e d to

obtain

i n f o r m a t i o n a b o u t the major element m a t r i x of c o a l ashes that w e r e to b e a n a l y z e d for trace elements b y o p t i c a l e m i s s i o n spectroscopy. B o t h l o w t e m p e r a t u r e ( < 1 5 0 ° C ) a n d h i g h - t e m p e r a t u r e ( 4 5 0 ° C ) c o a l ashes, p r e p a r e d as d e s c r i b e d b y R u c h et al. ( 1 ), w e r e a n a l y z e d , a n d the m e t h o d of R o s e et al. (2)

w a s a d a p t e d to d e t e r m i n e the m a j o r a n d m i n o r elements

(Si, T i , A l , Fe, M g , C a , K , and V ) .

T h e i n s t r u m e n t a l parameters u s e d

for these elements are g i v e n i n T a b l e I. T o assess the v a l i d i t y of this p r o c e d u r e , a series of c o a l ashes a n a l y z e d b y the British C o a l U t i l i z a t i o n Research Association

(BCURA)

were

a g a i n a n a l y z e d w i t h the t w o types of ash p r e p a r e d i n o u r laboratories. C a l i b r a t i o n s for these analyses w e r e p r e p a r e d f r o m U . S. G e o l o g i c a l S u r v e y a n d N a t i o n a l B u r e a u of S t a n d a r d s r o c k standards, I B , G l , W l , a n d N o s . 78, 79, 88. T h e values d e t e r m i n e d for the B C U R A c o a l ashes a g r e e d e x c e l l e n t l y w i t h results o b t a i n e d at B C U R A b y D i x o n et al. ( 3 ) . deviations were

Standard

c a l c u l a t e d f o r the d u p l i c a t e c o a l ash d e t e r m i n a t i o n s :

Si_0.07%, Ti—0.01%, Al—0.06%, Fe—0.05%, Mg—0.02%, C a - O . 0 3 % , Κ — 0 . 0 1 % , Ρ — 0 . 0 1 % , a n d V — 1 . 3 p p m . T h e s e d e v i a t i o n s are c o m p a r a b l e w i t h C l a s s A w e t silicate analyses a n d i n d i c a t e a h i g h d e g r e e of p r e c i s i o n . B e c a u s e of these e n c o u r a g i n g results a n d p r e v i o u s w o r k o n b r o w n coals b y S w e a t m a n et al. (4)

a n d K i s s ( 5 ) , w h i c h i n d i c a t e d that m a j o r

a n d m i n o r elements c o u l d be d e t e r m i n e d i n w h o l e c o a l , a series of 25 coals w a s p r e p a r e d for x - r a y

fluorescence

analysis.

F o r each coal, a

l o w - t e m p e r a t u r e ash, a h i g h - t e m p e r a t u r e ash, a n d the w h o l e c o a l itself

In Trace Elements in Fuel; Babu, Suresh P.; Advances in Chemistry; American Chemical Society: Washington, DC, 1975.

68

TRACE

Table I.


Element

2 θ Angle

Si Al Ti Fe Ca Κ Mg V S Cl Ρ Ni Cu Zn Pb Br As Co Mn Mo

108.01 142.42 86.12 57.51 44.85 50.32 136.69 76.93 75.24 64.94 110.99 48.66 45.02 41.79 28.24 29.97 34.00 52.79 62.97 20.33

Cr

69.35

6

a

6

ELEMENTS

IN

FUEL

Spectrometer Parameters (Ka x-ray) Back ground 2 θ 111.01 145.95 89.12 60.51 47.95 53.90 139.69 80.93 78.38 67.94 113.99 50.36 49.67 44.25 31.24 35.12 37.00 53.79 63.97 19.83 20.83 68.53

PHA

Volts

Crystal

X-Ray Tube

Base

EDDT EDDT LiF LiF EDDT EDDT ADP LiF EDDT EDDT Ge LiF LiF LiF LiF LiF LiF LiF LiF LiF

Cr Cr Cr" Cr" Cr Cr Cr Cr° Cr Cr Cr Cr" Cr" Cr" Cr" Cr" Cr" W W W

7 5 5 5 14 14 4 5 12 11 9 10 11 10 22 25 24 13 8 36

17 17 18 25 30 21 8 16 18 19 15 27 28 22 28 23 23 16 12 40

LiF

W

7

15

Window

A t u n g s t e n t u b e is used o n these elements w h e n a l r e a d y i n p l a c e . L@i x - r a y .

w e r e p r e p a r e d b y t h e i n d i c a t e d p r o c e d u r e s . W h e n a l l values w e r e c o n v e r t e d to the w h o l e c o a l basis, the a g r e e m e n t a m o n g the three types of c o a l materials w a s excellent ( T a b l e I I ) , i n d i c a t i n g t h a t t h e s i m p l e r a n d m o r e r a p i d w h o l e c o a l t e c h n i q u e is a c c e p t a b l e for d e t e r m i n i n g m a j o r a n d m i n o r elements. Determination

of Trace Elements in Whole Coal

T r a c e e l e m e n t d e t e r m i n a t i o n s o n w h o l e c o a l h a v e b e e n severely h a n d i c a p p e d b y the l a c k of a n a l y z e d s t a n d a r d s . B e c a u s e of this i t w a s necessary to p r e p a r e c a l i b r a t i o n curves f r o m samples a n a l y z e d i n o u r l a b o r a t o r i e s b y i n d e p e n d e n t m e t h o d s . T h e a c c u r a c y of the x - r a y

fluores

c e n c e m e t h o d is, therefore, d e p e n d e n t o n the a c c u r a c y of t h e m e t h o d s u s e d to a n a l y z e the c a l i b r a t i n g standards. I t w a s too difficult to p r e p a r e standards b y u n i f o r m l y a d d i n g k n o w n q u a n t i t i e s of t r a c e elements to ground whole coal. T h e l i g h t c o a l m a t r i x of c a r b o n , h y d r o g e n , a n d o x y g e n a n d the r e l a t i v e l y s l i g h t v a r i a t i o n of h e a v i e r trace elements p e r m i t t h e i r d e t e r m i n a t i o n w i t h m i n i m u m interferences. T h e same w h o l e c o a l p r o c e d u r e s p r e v i o u s l y


6.

KUHN E T AL. Table II.

Fluorescence

69

Analysis

Mean Absolute Variation Between Values Determined From 2 5 R a w Coals and Their Ashes

Element


X-Ray

Mean Difference (%)

Si Al Ti Fe Ca Κ Ρ Mg

0.10 0.08 0.012 0.10 0.04 0.02 0.002 0.010

Maximum Difference (%) 0.24 0.12 0.030 0.17 0.12 0.04 0.005 0.015

d e s c r i b e d w e r e u s e d , a n d the trace a n d m i n o r elements, Ρ, V , C r , M n , C o , N i , C u , Z n , A s , B r , M o , a n d P b , w e r e d e t e r m i n e d d i r e c t l y i n 50 whole-coals. T h e a c c u r a c y of the x - r a y

fluorescence

method was evaluated by

c a l c u l a t i n g , f r o m the 50 w h o l e coals a n a l y z e d , the m e a n v a r i a t i o n of e a c h e l e m e n t f r o m its m e a n c o n c e n t r a t i o n , d e t e r m i n e d b y the other i n d e p e n d ent m e t h o d s p r e v i o u s l y m e n t i o n e d a n d l i s t e d i n T a b l e I I I . Table III.

Detection

Comparative A c c u r a c y for Whole Coal and Limits of Detection Based on 5 0 Samples

lenient

Accuracy

Limit of Detection

Al Si S Cl κ Ca Mg Fe

per cent ±0.08 ±0.10 ±0.04 ±0.01 ±0.02 ±0.04 ±0.010 ±0.10

per cent 0.012 0.016 0.003 0.0015 0,003 0.0005 0.015 0.005

Ti V Ni Cu Zn As Pb Br Ρ Co Mn Cr Mo

ppm ±6.3 ±3.1 ±1.9 ±2.5 ±23.0 ±4.3 ±7.7 ±1.0 ±15.0 ±1.3 ±3.4 ±2.1 ±5.2

ppm 7.5 2.5 3.5 1.0 2.0 3.2 1.8 0.5 15.0 2.5 4.5 1.5 5.0


70

TRACE

ELEMENTS

IN

FUEL

l i m i t s , t h r e e s t a n d a r d d e v i a t i o n s a b o v e b a c k g r o u n d , for e a c h element also are g i v e n i n T a b l e I I I . T h e r e l a t i v e errors for a l l elements d e t e r m i n e d are g i v e n i n T a b l e IV.

F o r completeness, d a t a o n m i n o r elements i n w h o l e c o a l also are

i n c l u d e d i n t h e trace element tables. T h e s e d a t a i n d i c a t e the p r e c i s i o n o b t a i n e d f o r the x - r a y

fluorescence

analysis o n replicates of 15 samples


of w h o l e c o a l g r o u n d to —325 m e s h . Table IV.

Deviation on —325 Mesh on 15 Samples of Whole Coal Standard Deviation

Element

X-Ray

Relative Deviation (%)

Al Si S Cl κ Ca Mg Fe

per cent 0.02 0.05 0.01 0.003 0.004 0.005 0.002 0.02

1.77 1.96 0.532 1.13 2.26 1.65 3.88 1.26

Ti V Ni Cu Zn As Pb Br Ρ Co Mn Cr Mo

ppm 4.16 1.58 1.12 0.75 3.61 0.94 1.53 0.39 3.41 0.43 4.14 1.14 3.11

0.564 3.84 4.29 3.92 1.37 2.49 2.29 2.11 10.92 4.79 7.53 4.35 23.9

Matrix

Corrections

for Analysis

of Whole Coal

B e c a u s e of the l a c k of standards, v a r i a t i o n s i n analyses m a d e

by

other m e t h o d s , a n d errors c a u s e d b y c o a l s a m p l i n g p r o b l e m s , i t w a s diffi c u l t to evaluate the n e e d for x - r a y m a t r i x corrections a n d to select the best m e t h o d for a p p l y i n g t h e m .

H o w e v e r , corrections w e r e

necessary

because some elements i n w h o l e c o a l s u c h as i r o n , s i l i c o n , a n d s u l f u r m a y vary considerably.

F o r these elements, corrections w e r e a p p l i e d i n d i s

c r i m i n a t e l y to a l l samples, because it w a s i m p o s s i b l e to d e t e r m i n e the p o i n t at w h i c h m a t r i x v a r i a t i o n s r e q u i r e d a c o r r e c t i o n greater t h a n the a c c u r a c y l i m i t s of the m e t h o d .

W e e l e c t e d to use t h e m i n i m u m n u m b e r

of corrections c o m p a t i b l e w i t h r e a s o n a b l y a c c u r a t e results.

Therefore,


6.

KUHN

X-Ray

ET AL.

Fluorescence

71

Analysis

the elements M g , A l , S i , P , S, C l , K , a n d M o w e r e left u n c o r r e c t e d .

While

these d e t e r m i n a t i o n s p r o b a b l y c o u l d be i m p r o v e d ( 6 ) , t h e y w e r e s h o w n to b e a d e q u a t e for o u r purposes ( T a b l e I I ). T h e t i t a n i u m a n d v a n a d i u m values w e r e c o r r e c t e d b y u s i n g the v a r i a t i o n s i n the i r o n content of the whole

coal.

T h e m e t h o d u s e d for c o r r e c t i n g the other elements for m a t r i x v a r i a -


tions w a s p r o p o s e d b y S w e a t m a n et al. (4).

T o t a l mass a b s o r p t i o n w a s

d e t e r m i n e d b y m e a s u r i n g the a t t e n u a t i o n of the r a d i a t i o n i n q u e s t i o n b y a t h i n l a y e r of the s a m p l e to b e a n a l y z e d . T h e mass a b s o r p t i o n coefficient M was calculated b y M = (cm ), W

(In Cs/Cx);

A/W

w e i g h t of s a m p l e

=

2

( c o u n t s / s e c ) , a n d Cx =

(g),

Cs

where A = =

area of s a m p l e

i n t e n s i t y of t h e s t a n d a r d

i n t e n s i t y of the s t a n d a r d ( c o u n t s / s e c ) a t t e n u -

a t e d b y the t h i n l a y e r c o a l elements d e t e r m i n e d . U s i n g these coefficients, a c o r r e c t e d v a l u e w a s o b t a i n e d for the elements d e t e r m i n e d , e v e n w h e n matrix variations were considerable.

G r e a t c a r e was t a k e n to press t h e

coals to a u n i f o r m thickness so that the mass a b s o r p t i o n coefficient

was

affected o n l y b y d e n s i t y ( for w h i c h c o m p e n s a t i o n was m a d e ) a n d m a t r i x considerations. Effect of Coal Particle Size on Analytical

Precision of Trace Elements

O u r results i n d i c a t e d that coals g r o u n d to —60 m e s h f a i l e d to y i e l d a consistently a c c e p t a b l e p r e c i s i o n for most trace element d e t e r m i n a t i o n s . T h e r e f o r e , i t w a s necessary to e v a l u a t e the errors associated w i t h trace element d e t e r m i n a t i o n s i n coals g r o u n d to v a r i o u s p a r t i c l e sizes. N i n e coals, r e p r e s e n t i n g a range of trace element concentrations, w e r e c a r e f u l y g r o u n d to pass screens of v a r i o u s m e s h sizes ( T a b l e V ) . D u p l i c a t e 2-g c o a l samples for e a c h m e s h size w e r e w e i g h e d a n d t h e n g r o u n d 3 m i n i n a N o . 6 W i g - L - B u g to f u r t h e r r e d u c e p a r t i c l e size. T h e final g r i n d i n g e l i m i n a t e d , as n e a r l y as possible, a n y v a r i a t i o n i n the pressed c o a l d i s k s , w h i c h w e r e s u b s e q u e n t l y p r e p a r e d for analysis ( I ) . M o r e t h a n 1000 i n d i v i d u a l d e t e r m i n a t i o n s w e r e m a d e i n this s t u d y . T a b l e V gives the c o m b i n e d means of the differences b e t w e e n d u p l i cate trace element d e t e r m i n a t i o n s for e a c h c o a l p a r t i c l e size a n a l y z e d . B o t h the means of the absolute differences Table V .

(in ppm)

a n d the means of

Mean E r r o r for A l l Elements at Various Coal Particle Sizes

Mesh Size -60 -100 -200 -325 -400

Trace Elements in Fuel - ACS Publications

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