3 Trace Elements in Coal
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by Optical Emission Spectroscopy G. B. DREHER and J. A. SCHLEICHER Illinois State Geological Survey, Urbana,Ill.61801 Two quantitative emission spectrochemical methods—direct-reading and photographic—have been developed to analyze 16 trace elements in high-temperature coal ash. Problems such as possible loss of vanadium and molybdenum by volatilization and an apparent increase in boron concentration with increasing ashing temperature were encountered in ashing coal samples. Because natural and/or analyzed standards were unavailable, synthetic standards were prepared for the direct-reading method. U.S.G.S. standard granite G-2 and iron oxide were combined with various amounts of a mixture of silica, alumina, and 1000 ppm of each of the trace elements of interest. For the photographic method, similar mixtures were prepared without the G-2 base. For each element, usable concentration limits, concentration ranges on the coal basis calculated from the coal ash samples, and the resulting standard deviations were determined.
n v i r o n m e n t a l c o n c e r n i n recent years has c a u s e d i n c r e a s i n g awareness of t h e s m a l l q u a n t i t i e s of substances i n o u r a i r , w a t e r , a n d f o o d w h i c h c a n affect l i v i n g t h i n g s . I t has b e c o m e necessary to k n o w t h e sources o f these p o t e n t i a l l y t o x i c substances. C o a l , w h i c h is b u r n e d i n large q u a n t i ties a n n u a l l y i n t h e U . S . to p r o d u c e energy, is one s u c h source
because
it contains trace concentrations of m a n y p o s s i b l y t o x i c elements that m a y be r e l e a s e d to t h e e n v i r o n m e n t i n c o n s i d e r a b l e q u a n t i t i e s . T o assess this p o s s i b i l i t y , concentrations of these trace constituents m u s t b e d e t e r m i n e d i n t h e w h o l e c o a l before
combustion.
R e l i a b l e a n d a c c u r a t e analysis
m e t h o d s are r e q u i r e d to p r o d u c e t h e d a t a that u l t i m a t e l y m a y b e u s e d b y those c h a r g e d w i t h p r o t e c t i n g o u r e n v i r o n m e n t to set standards a n d b y c o a l users to see that those standards are m e t . 35 Babu; Trace Elements in Fuel Advances in Chemistry; American Chemical Society: Washington, DC, 1975.
36
TRACE
ELEMENTS IN FUEL
M e t h o d s h a v e b e e n d e v e l o p e d at the 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 to d e t e r m i n e the concentrations of 23 p o t e n t i a l l y v o l a t i l e trace elements i n c o a l . S o m e of the concentrations w e r e c h e c k e d b y t w o or m o r e a n a l y t i c a l t e c h n i q u e s . A n a l y s e s h a v e b e e n m a d e b y five different t e c h n i q u e s : n e u t r o n a c t i v a t i o n analysis, a t o m i c a b s o r p t i o n s p e c t r o m e t r y , x-ray rescence s p e c t r o m e t r y , o p t i c a l e m i s s i o n spectroscopy p h o t o g r a p h i c ) , a n d i o n specific electrode.
fluo
( direct-reading and
T h e method used determines
w h e t h e r w h o l e c o a l , l o w - t e m p e r a t u r e ash, or h i g h - t e m p e r a t u r e ash is u s e d as the a n a l y t i c a l s a m p l e .
Since c o m p a r a b l e results for c e r t a i n ele
ments w e r e p r o d u c e d , there is a h i g h d e g r e e of confidence i n the m e t h o d s developed.
O f the t w o o p t i c a l e m i s s i o n spectroscopic m e t h o d s d e s c r i b e d ,
a d i r e c t - r e a d i n g m e t h o d w a s u s e d to a n a l y z e S n , B , P b , C u , C o , N i , B e , C r , V , M o , a n d G e , a n d a p h o t o g r a p h i c t e c h n i q u e w a s u s e d for B , M n , C r , P b , B e , V , A g , C u , Z n , Z r , C o , a n d N i . Sixteen of these
elements
w e r e d e t e r m i n e d i n the h i g h - t e m p e r a t u r e ash of the coals, a n d e i g h t of t h e m w e r e c r o s s - c h e c k e d b y b o t h methods.
T h e s i l v e r content i n t h e ash
samples a n a l y z e d w a s a l w a y s v e r y near the d e t e c t i o n l i m i t , a n d a l t h o u g h b i s m u t h was sought, i t w a s n e v e r detected.
C o n v e n t i o n a l d.c. a r c e x c i t a
t i o n is u s e d b y b o t h m e t h o d s . Quantitative spectrographic
analyses for
trace elements
in high-
t e m p e r a t u r e ash samples of c o a l h a v e b e e n r e p o r t e d b y A b e r n e t h y et al. ( 1 ), Z u b o v i c et ai. (2,3,4,5),
R a o (6), a n d H u n t e r a n d H e a d l e e (7).
We
felt t h a t i t was d e s i r a b l e to d e v e l o p a d d i t i o n a l analysis m e t h o d s , e s p e c i a l l y for the d i r e c t - r e a d i n g s p e c t r o m e t r i c t e c h n i q u e , i n w h i c h s m a l l
changes
of s o m e m a t r i x constituents m i g h t cause r e l a t i v e l y l a r g e v a r i a t i o n s i n results because of the i n c r e a s e d s e n s i t i v i t y of t h e d e t e c t i o n system. Table I.
Element (ppm)
Β Pb Cu Co Ni Be Cr V Sn Mo Ge
A
Concentrations in Standard Materials of Synthetic Standard for
Weight of synthetic standard (mg) added to 0.5 g G-2 + 879 mg Al 0 2
3
0.00
1.00
4.04
9.19
1.86 26.7 9.95 4.55 5.95 2.23 8.37 34.4 0.93 1.12 0.66
2.84 28.5 11.8 6.40 7.79 4.08 10.2 36.2 2.78 2.97 2.51
5.78 33.9 17.3 12.0 13.4 9.67 15.8 41.6 8.37 8.57 8.11
10.7 43.0 26.6 21.3 22.6 19.0 25.0 50.6 17.7 17.9 17.5
Babu; Trace Elements in Fuel Advances in Chemistry; American Chemical Society: Washington, DC, 1975.
3.
DREHER AND SCHLEICHER
Optical
m e t h o d for silicate analysis ( 8 )
Emission
37
Spectroscopy
was a d a p t e d for t h e c o a l ash analyses
b y the p h o t o g r a p h i c t e c h n i q u e . Experimental Preparation of High-Temperature Ash. T w o grams of finely g r o u n d c o a l (—60 m e s h or s m a l l e r ) are s e p a r a t e d f r o m the l a r g e r stock b y riffling, w e i g h e d i n t o a t a r e d , u s e d s i l i c a c r u c i b l e , a n d d r i e d i n a n o v e n at 1 1 0 ° C for a p p r o x i m a t e l y 3 hrs. ( G o r s u c h ( 9 ) a n d R u c h et al (10) s h o w e d that the w a l l s of n e w s i l i c a c r u c i b l e s r e t a i n some t r a c e elements to a greater extent d u r i n g d r y a s h i n g t h a n c r u c i b l e s t h a t h a v e b e e n u s e d for s e v e r a l ashings.) T h e samples are c o o l e d a n d w e i g h e d for m o i s t u r e d e t e r m i n a t i o n . T h e w e i g h e d , moisture-free samples a r e t r a n s f e r r e d to a c o l d muffle f u r n a c e , h e a t e d to 5 0 0 ° C i n 1 h r , a n d a s h e d at t h a t t e m p e r a t u r e u n t i l no c a r b o n a c e o u s m a t t e r appears. D u r i n g a s h i n g , the s a m ples are m i x e d w i t h a p l a t i n u m w i r e a p p r o x i m a t e l y every h o u r . T h e samples are c o o l e d a n d w e i g h e d , g r o u n d i n a m u l l i t e m o r t a r , t h e n d r i e d at 1 1 0 ° C , a n d s t o r e d i n a desiccator over M g ( C 1 0 ) . Preparation of Direct-Reading Spectrometric Standards. N o c e r t i fied standards are yet a v a i l a b l e for trace elements i n c o a l , a l t h o u g h t h e N a t i o n a l B u r e a u of S t a n d a r d s is c o n d u c t i n g a n i n t e r l a b o r a t o r y e v a l u a t i o n of a c o m p o s i t e c o a l s a m p l e for use as a S t a n d a r d R e f e r e n c e M a t e r i a l (11). It was necessary, therefore, to p r e p a r e s y n t h e t i c standards. 4
2
For the s p e c t r o m e t r i c m e t h o d , U . S . G e o l o g i c a l S u r v e y s t a n d a r d granite G - 2 is u s e d as a base for the s t a n d a r d . T o 0.5 g portions of G - 2 are a d d e d 379 m g of S p e c p u r e A 1 0 ( J o h n s o n M a t t h e y C h e m i c a l s L t d . , t h r o u g h J a r r e l l - A s h D i v i s i o n , F i s h e r Scientific C o . , 590 L i n c o l n St., W a l t h a m , M a s s . 02154) to adjust the S i 0 : A l 0 r a t i o to 3:1 a n d v a r i o u s amounts of a s y n t h e t i c s t a n d a r d c o n t a i n i n g 1000 p p m ( w / w ) of e a c h of 2
3
2
G-2 Base in 3:1 Silica-Alumina Matrix + Spectrometric Method
2
3
1000 ppm Weight of Na B,0 2
Weight of synthetic standard (mg) added to 0.5 g G-2 + 379 mg Al 0 2
19.79 20.5 61.2 45.1 39.9 41.2 37.6 43.6 68.7 36.4 36.6 36.1
5445 50.2 116 101 96.0 97.3 94.0 99.5 123 92.8 92.9 92.5
3
7
10H O 2
133.22
535.89
6.32
106 220 206 202 203 200 205 226 199 199 199
264 512 504 501 502 500 503 516 500 500 499
1320
Babu; Trace Elements in Fuel Advances in Chemistry; American Chemical Society: Washington, DC, 1975.
(mg)
38
TRACE
ELEMENTS
IN
FUEL
t h e 11 elements b e i n g d e t e r m i n e d . T h e mixtures are g r o u n d i n a m u l l i t e m o r t a r a n d t h e n m i x e d i n p l a s t i c v i a l s 1 i n . i n d i a m e t e r X 3 i n . d e e p that c o n t a i n t w o p l a s t i c balls VA i n . i n d i a m e t e r for 1 h r i n a m i x e r m i l l . E i g h t standards are p r e p a r e d i n this m a n n e r . A n i n t h s t a n d a r d is p r e p a r e d s i m i l a r l y b y u s i n g 6.32 m g N a B 0 · 1 0 H O i n a d d i t i o n to t h e 0.5 g of G - 2 a n d 379 m g A 1 0 to o b t a i n a b o r o n c o n c e n t r a t i o n of 1320 p p m . A s u m m a r y of t h e amounts o f s y n t h e t i c s t a n d a r d t a k e n a n d t h e r e s u l t i n g trace e l e m e n t concentrations are g i v e n i n T a b l e I. 2
2
4
7
2
3
A n a r r a y of standards is p r e p a r e d so t h a t one series of t h e n i n e p r e v i o u s l y d e s c r i b e d standards contains a t o t a l i r o n o x i d e
concentration
( i n c l u d i n g t h a t a t t r i b u t a b l e to t h e G - 2 ) of 1 0 % , a s e c o n d series
of
standards contains a t o t a l i r o n o x i d e c o n c e n t r a t i o n of 1 5 % , a n d so o n u p to 3 0 % i r o n oxide, w i t h a t o t a l of 45 m i x t u r e s . F i n a l standards are p r e p a r e d b y a d d i n g 40 m g of the c o r r e s p o n d i n g m i x t u r e to 150 m g of S P - 2 X g r a p h i t e p o w d e r ( S p e x I n d u s t r i e s , I n c . , B o x 798, M e t u c h e n , N . J . 08840) a n d 10 m g of S p e c p u r e B a ( N 0 ) a n d m i x i n g t h e m w e l l i n p l a s t i c v i a l s , Vz i n . i n d i a m e t e r X 1 i n . d e e p , c o n t a i n i n g t w o p l a s t i c b a l l s , % i n . i n d i a m e t e r . M i x t u r e s of h i g h t e m p e r a t u r e a s h samples are p r e p a r e d i n a n analogous m a n n e r . F o r t y m g of finely g r o u n d h i g h - t e m p e r a t u r e a s h is m i x e d w i t h 150 m g of S P - 2 X g r a p h i t e p o w d e r a n d 10 S p e c p u r e B a ( N 0 ) . F i f t e e n m g charges of s t a n d a r d or s a m p l e are w e i g h e d a n d l o a d e d i n t o t h i n - w a l l e d crater electrodes Vs i n . i n d i a m e t e r , N a t i o n a l t y p e L - 3 9 7 9 ( S p e x I n d u s t r i e s , I n c . ) . A r c i n g c o n d i t i o n s are s h o w n i n T a b l e I I . 3
3
2
2
Preparation of Spectrographs
Standards.
F o r the
spectrographic
d e t e r m i n a t i o n s , h i g h - t e m p e r a t u r e ash samples w e r e a g a i n u s e d ; h o w e v e r , analytical procedures
were
different o u t of necessity.
T h e micropho-
tometer u s e d is c a p a b l e of a greater r e s o l u t i o n t h a n t h e d i r e c t - r e a d i n g spectrometer b e c a u s e of its i n h e r e n t m a g n i f i c a t i o n ( 1 7 χ ) , r e c i p r o c a l l i n e a r dispersions of t h e s p e c t r o g r a p h
although the
a n d spectrometer
are
v e r y n e a r l y t h e same ( 5 A / m m ). I t w a s f o u n d , therefore, that t h e b a c k g r o u n d effects w e r e less significant i n t h e p h o t o g r a p h i c m e t h o d , a l t h o u g h i t is m o r e t i m e c o n s u m i n g . A c c o r d i n g l y , a set of s y n t h e t i c standards w a s p r e p a r e d b y m i x i n g c o m m e r c i a l S i 0 base standards c o n t a i n i n g 1000 p p m of 49 elements ( S p e x I n d u s t r i e s , I n c . #1006) w i t h c o r r e s p o n d i n g 1000 p p m A 1 0 base standards ( S p e x I n d u s t r i e s , I n c . #1007) i n t h e ratio of 3:1 ( S i 0 : A l 0 ) . T h i s m i x t u r e w a s t h e n d i l u t e d w i t h a 3:1 m i x t u r e of S p e c p u r e S i 0 a n d A 1 0 to concentrations of 1000, 500, 250, 100, 50, a n d 10 p p m . T h e 10 p p m s t a n d a r d w a s l a t e r f o u n d to b e b e l o w the l i m i t of d e t e c t a b i l i t y u n d e r t h e exposure c o n d i t i o n s u s e d . T o n i n e parts of each of these mixtures w a s a d d e d one p a r t F e 0 , p r o d u c i n g final trace element c o n centrations of 900, 450, 225, 90, 45, a n d 9 p p m . T h e s e standards w e r e , i n t u r n , m i x e d i n a 1:1 r a t i o w i t h S P - 2 X g r a p h i t e p o w d e r c o n t a i n i n g 0 . 1 1 1 % i n d i u m as i n t e r n a l s t a n d a r d . H o w e v e r , the i n t e r n a l s t a n d a r d w a s discontinued because it decreased rather than increased precision, prob a b l y o w i n g to t h e r e f r a c t o r y n a t u r e of t h e m a t r i x . 2
2
3
2
2
2
2
3
2
3
Babu; Trace Elements in Fuel Advances in Chemistry; American Chemical Society: Washington, DC, 1975.
3
3.
DREHER AND SCHLEICHER Table II.
Optical
Emission
Standard Operating Conditions Spectrometric
Instrument
J a r r e l l - A s h M o d e l 750 Atomcounter
Arc current Arc voltage E l e c t r o d e gap Exposure time
15 a m p (short c i r c u i t ) 6 mm 65 sec
Electrodes
sample electrode, N a t i o n a l L-3979 counter electrode, N a t i o n a l L-4036 (ASTM C-l) 8 0 % argon, 2 0 % oxygen at 10 S C F H 15 m g
Electrode atmosphere S a m p l e charge Attenuation" S t e p sector E n t r a n c e slit width E x i t slit w i d t h Development
39
Spectroscopy
10 μτη
Spectrographic J a r r e l l - A s h 3.4 m E b e r t w i t h 3-lens c o l l i m a t i n g system 11 a m p (true) 250 V D C 4 mm c o m p l e t i o n + 10 sec ( u s u a l l y a b o u t 150 sec) s a m p l e electrode, N a t i o n a l L - 3 9 0 3 ( A S T M C-13) counter electrode, N a t i o n a l L - 4 0 3 7 ( A S T M C - l with radial tip) 8 0 % argon, 2 0 % oxygen at 14 S C F H 20 m g n e u t r a l d e n s i t y filter, 6% Τ 1:1.585, 8 steps 25 Mm
50 μϊϊϊ 3.0 m i n i n E a s t m a n D - 1 9 , 30 sec i n 2 . 5 % acetic a c i d , 4.0 m i n i n E a s t man fixer
B e c a u s e the concentrations of m o s t trace elements i n the ash were too h i g h to be recorded o n a t o t a l energy i g n i t i o n , it was necessary to insert a n e u t r a l d e n s i t y filter i n the light p a t h after the second lens of the three lens c o l l i m a t i n g s y s t e m . α
T h e samples ( h i g h - t e m p e r a t u r e ash) a n d s t a n d a r d s , b o t h d i l u t e d w i t h a n e q u a l w e i g h t of S P - 2 X g r a p h i t e p o w d e r , w e r e i g n i t e d spectrographically under conditions shown i n T a b l e II. Results and
Discussion
T w o types of c o a l ash samples h a v e b e e n p r e p a r e d r o u t i n e l y for analysis at the I l l i n o i s G e o l o g i c a l S u r v e y . L o w - t e m p e r a t u r e ash samples (12),
i n w h i c h the b u l k of the m i n e r a l m a t t e r r e m a i n s u n c h a n g e d , are
p r e p a r e d b y r e a c t i o n of frequency
field.
a p p r o x i m a t e l y 150 ° C . spectroscopic
the coal w i t h activated oxygen
T h e effective
analysis.
in a radio-
t e m p e r a t u r e p r o d u c e d b y this d e v i c e
S u c h samples w e r e u n s a t i s f a c t o r y f o r It is p o s t u l a t e d that the presence
is
emission
of
largely
u n a l t e r e d m i n e r a l matter, s u c h as carbonates, sulfides, a n d h e m i h y d r a t e d sulfates (12),
c a u s e d the o b s e r v e d n o n r e p r o d u c i b i l i t y of results.
High-
t e m p e r a t u r e ash samples, p r e p a r e d i n a muffle f u r n a c e , consisted m a i n l y
Babu; Trace Elements in Fuel Advances in Chemistry; American Chemical Society: Washington, DC, 1975.
40
TRACE
ELEMENTS
IN
FUEL
of oxides of t h e elements present i n the o r i g i n a l m i n e r a l m a t t e r a n d w e r e u s e d t h r o u g h o u t this i n v e s t i g a t i o n . R e s u l t s o b t a i n e d f r o m h i g h - t e m p e r a ture ash, c a l c u l a t e d to t h e w h o l e c o a l basis, c o m p a r e w e l l w i t h c o r r e sponding
results
from
low-temperature
cross-checks are a v a i l a b l e . (See
Ref.
ash
and
whole
coal
where
10.)
Variation of Trace Element Concentration with Ashing Conditions. T o d e t e r m i n e w h e t h e r or not the concentrations of any of t h e elements of interest w e r e
altered w i t h increasing ashing temperature,
samples
w e r e a s h e d i n u n c o v e r e d , u s e d p o r c e l a i n c r u c i b l e s at 300, 400, 500, 600, a n d 700 ° C u n t i l the carbonaceous matter w a s no l o n g e r a p p a r e n t .
Two
coals w e r e s t u d i e d , a n d trace e l e m e n t analyses w e r e c a r r i e d out o n e a c h of the 10 r e s u l t i n g ash samples.
T h e results are s h o w n i n T a b l e I I I .
N o n e of t h e d a t a e x h i b i t losses or gains of trace e l e m e n t c o n c e n t r a t i o n i n samples a s h e d b e t w e e n 300 a n d 700 ° C , other t h a n a g a i n i n b o r o n c o n c e n t r a t i o n a n d a possible loss of l e a d . A loss of l e a d w i t h i n c r e a s i n g t e m p e r a t u r e is expected. creased
with
F o r b o t h coals, the b o r o n concentrations i n
a s h i n g t e m p e r a t u r e f r o m 300 to 6 0 0 ° C ,
s l i g h t l y b e t w e e n 600 a n d 7 0 0 ° C .
Williams and Vlamis
then
dropped
(13)
noted
a
s i m i l a r effect w h e n t h e y a s h e d p l a n t m a t e r i a l w i t h c a l c i u m h y d r o x i d e i n muffle furnaces a n d also w h e n t h e y h e a t e d c a l c i u m h y d r o x i d e i n the same furnaces i n the absence of p l a n t m a t e r i a l . T h e increase i n b o r o n c o n c e n t r a t i o n w a s greater i n u n c o v e r e d c r u c i b l e s or dishes t h a n i n c o v e r e d .
It
was t h o u g h t t h a t b o r o n v a p o r f r o m the f u r n a c e w a l l s c o n d e n s e d i n i n c r e a s i n g a m o u n t s i n the a l k a l i n e m a t e r i a l as temperatures i n c r e a s e d for a g i v e n h e a t i n g t i m e or as h e a t i n g times i n c r e a s e d at a g i v e n t e m p e r a t u r e . A n o t h e r p a i r of coals w a s chosen to d e t e r m i n e w h e t h e r or not a s i m i l a r effect took p l a c e w h e n c o a l w a s a s h e d w i t h o u t a l k a l i n e m a t e r i a l . Table III.
Concentrations (ppm) of Trace Elements Samples Prepared at
Sample Element
300°C
400°c
Β Pb Cu Co Ni Be Cr V Sn Mo Ge
62 77 21 7.9 39 2.5 21 42 4.4 12 16
90 61 20
— 39 2.5 19 45
