5 Major and Minor Constituents in Siliceous Materials by Atomic Absorption Spectroscopy RICHARD B. MUTER and LARRY L. NICE Coal Research Bureau, West Virginia University, Morgantown, W. Va. 26506
Siliceous materials—Si, Al, Fe, Ti, Ca, Mg, Νa, K, Mn, Ni, Ba, Ag, Au, Ca, Cr, Cu, Ga, In, Mo, Sb and Zn—may be analyzed by a lithium tetraborate fusion-acid dissolution technique using atomic absorption spectroscopy.
Mercury,
tin, and lead volatilize by this technique, and gold and silver in concentrations above 0.5 wt% cannot be held in solution. Coal ash is preconcentrated prior to analysis, and there is possible silica interference. Analytical results, where pos sible, are compared statistically with other reported values.
" i n c r e a s i n g n a t i o n a l c o n c e r n over t h e e c o l o g i c a l a n d e n v i r o n m e n t a l effects A
of coal combustion
coupled
w i t h t h e desire to b e c o m e m o r e
self
sufficient i n m i n e r a l p r o d u c t i o n l e d t h e C o a l R e s e a r c h B u r e a u at W e s t V i r g i n i a U n i v e r s i t y to e x a m i n e t h e major a n d m i n o r constituents i n c o a l ash.
B e c a u s e o f t h e n e e d f o r a c c u r a t e results at t h e l o w trace e l e m e n t
concentrations, i t w a s felt that a t o m i c a b s o r p t i o n s p e c t r o s c o p y
could
provide a r a p i d a n d routine method for analytical determinations. T h e i n t r o d u c t i o n o f a t o m i c a b s o r p t i o n spectroscopy has r e s u l t e d i n m a j o r advances i n t h e r a p i d analysis o f m a n y elements.
Initially, atomic
a b s o r p t i o n w a s a p p l i e d o n l y to aqueous systems o r to m a t e r i a l s t h a t c o u l d b e r e a d i l y s o l u b i l i z e d . T h e r e are m e t h o d s to a n a l y z e major elements i n s u c h c o m p l e x m a t e r i a l s as silicates a n d v i t r e o u s siliceous c o a l ashes
(1-5).
M o r e r e c e n t l y , l i t h i u m m e t a b o r a t e has b e e n r e p o r t e d t o b e a g o o d f l u x i n g agent (6) a n d has also b e e n u s e d i n c o n j u n c t i o n w i t h a t o m i c a b s o r p t i o n analysis i n silicate analysis ( 7 ) . T h i s p a p e r describes a l i t h i u m t e t r a b o r a t e - a t o m i c a b s o r p t i o n a n a l y t i c a l t e c h n i q u e w h i c h is b e i n g u s e d t o a n a l y z e c o a l ash. W h i l e the a m o u n t of a s h i n c o a l varies f r o m r a n k to r a n k , t h e c a r b o n content is r e l a t i v e l y h i g h , a n d at least o n e p r e c o n c e n t r a t i o n step, a s h i n g 57
58
TRACE
ELEMENTS
the s a m p l e , is r e q u i r e d f o r the l i t h i u m tetraborate t e c h n i q u e .
IN
FUEL
Only two
of the a v a i l a b l e a s h i n g t e c h n i q u e s r e q u i r e m i n i m u m s a m p l e h a n d l i n g — high temperature ashing ( Η Τ Α )
a n d low temperature ashing ( L T A ) .
H i g h t e m p e r a t u r e a s h i n g is the s i m p l e s t to p e r f o r m as it n o r m a l l y r e q u i r e s that t h e c o a l b e h e a t e d for 2 hrs at 8 0 0 ° C .
U n f o r t u n a t e l y s o m e of
t h e m o r e v o l a t i l e trace elements s u c h as m e r c u r y , t i n , a n d l e a d are lost b y this t e c h n i q u e .
I n l o w t e m p e r a t u r e a s h i n g , t h e c o a l is h e a t e d i n a n
a t o m i c o x y g e n atmosphere i n w h i c h the r e a c t i v e o x y g e n c o m b i n e s c a r b o n at temperatures a r o u n d 1 0 0 ° - 1 5 0 ° C .
with
T h i s t e m p e r a t u r e r a n g e is
l o w e n o u g h t h a t the m i n e r a l m a t t e r i n c o a l is s u b s t a n t i a l l y u n a l t e r e d b y a s h i n g , b u t a s h i n g t i m e is i n c r e a s e d f r o m 2 hrs to 24 hrs o r longer.
In
a d d i t i o n i t has b e e n r e p o r t e d t h a t some of t h e m o r e v o l a t i l e trace e l e ments m a y also b e lost b y L T A ( 8 ) .
Because a r a p i d routine method
f o r c o a l m i n e r a l m a t t e r analysis w a s n e e d e d , i t w a s felt that h i g h t e m p e r a t u r e a s h i n g f o l l o w e d b y l i t h i u m tetraborate f u s i o n w o u l d p r o d u c e a s a m p l e s u i t a b l e for a t o m i c a b s o r p t i o n analysis.
Element Si Al Fe Ti Ca Mg Na Κ Mn Ni Ba Ag Au Co Cr Cu Ga Hg In Mo Pb Sb Sn Zn
Standard G-2 G-2 G-2 BCR-1 W-l W-l W-l W-l G-2 Si-Al Synthetic Synthetic Noble-G Synthetic Si-Al Synthetic Si-Al Synthetic Noble-G Si-Al Synthetic Synthetic Synthetic Si-Al
3° l a
4" 1" 1"
1" 2" 2"
32.34 8.12 1.93 1.34 7.79 3.98 1.56 0.55 0.023 1.0 2.45 0.63 0.50 8.48 1.0 0.63 1.0 1.57 0.50 1.0 1.26 4.34 8.86 1.0
30.45 7.45 2.14 1.29 7.70 4.09 1.61 0.54 0.024 0.969 2.333 0.609 0.515 8.49 1.031 0.626 1.00 1.599 0.41 0.90 1.26 4.30 8.53 0.966
° Synthetic standards were not fused but were prepared from aqueous standards with
5.
M U T E R AND NICE
Technique
and
Atomic
Absorption
59
Spectroscopy
Instrumentation
Apparatus. A P e r k i n - E l m e r m o d e l 303 a t o m i c a b s o r p t i o n s p e c t r o m eter e q u i p p e d w i t h a D C R - 1 r e a d o u t accessory a n d a strip c h a r t r e c o r d e r was u s e d for a l l d e t e r m i n a t i o n s . A B o l i n g b u r n e r was u s e d for a l l determinations made i n the air-acetylene
flame
single-slot, h i g h - s o l i d s b u r n e r w a s used. u s e d for r e f r a c t o r y elements.
except for c o p p e r w h e r e a
T h e nitrous oxide b u r n e r w a s
B u r n e r a n d i n s t r u m e n t settings u s e d w e r e
those r e c o m m e n d e d b y the m a n u f a c t u r e r ' s h a n d b o o k . Preparation of Standards.
S t a n d a r d s for ash analysis w e r e p r e p a r e d
f r o m c o m m e r c i a l l y a v a i l a b l e p u r e salts i n aqueous s o l u t i o n w i t h a p p r o p r i a t e acids a d d i t i o n w h e r e necessary to m a t c h a c i d concentrations i n the samples as w e l l as to h o l d m a t e r i a l s i n s o l u t i o n .
Master standard
solutions w e r e p r e p a r e d so that s e r i a l d i l u t i o n s for the c o n s t r u c t i o n of w o r k i n g curves w e r e possible. num
( e q u i v a l e n t to 2 0 %
A constant a m o u n t of s i l i c o n a n d a l u m i -
Si—5%
A l ) interference s o l u t i o n w a s
added
to e a c h set of standards a l o n g w i t h l i t h i u m tetraborate to c a r e f u l l y m a t c h Standards Used to Check the Working Curves Absolute Error (%)
Relative Error (%)
Standard Deviation (%)
Coefficient of Variation (%)
1.89 0.67 0.21 0.05 0.09 0.11 0.05 0.01 0.001 0.031 0.117 0.021 0.015 0.01 0.031 0.004 0 0.029 0.09 0.10 0 0.04 0.33 0.034
5.84 8.25 10.88 3.73 1.16 2.76 3.20 1.82 4.35 3.10 4.78 3.33 3.00 0.12 3.10 0.63 0 1.85 18.00 10.00 0 0.92 3.80 3.40
0.77 0.19 0.05 0.085 0.13 0.12 0.14 0.03 0.004 0.020 0.034 0.027 0.012 0.07 0.058 0.012 0 0.145 0.018 0.03 0 0.10 0.19 0.075
2.52 2.55 2.33 6.58 1.69 2.93 8.70 5.56 16.67 2.06 1.46 4.43 2.33 0.82 5.62 1.92 0 9.07 4.39 3.33 0 2.33 2.23 7.76
a p p r o p r i a t e a c i d , l i t h i u m tetraborate, a n d interference solution a d d i t i o n s .
60
TRACE ELEMENTS
Table II.
FUEL
Comparison of U.S.G.S. Trace Element Concentrations
G-2
Cr Mn Ni
IN
GSP-1
AGV-1
Fusion
Reported Value
Fusion
Reported Value
Fusion
Reported Value
100 240 400
7 260 5.1
80 290 350
12.5 331 12.5
130 650 480
12.2 763 18.5
s a m p l e a n d s t a n d a r d s o l u t i o n characteristics.
Standards w i t h
s i l i c o n - a l u m i n u m a n d l i t h i u m tetraborate concentrations w e r e
adjusted prepared
i n the event that s a m p l e d i l u t i o n was n e e d e d (i.e., c a l c i u m d e t e r m i n a t i o n ) for analysis. Sample Preparation.
The following method
was u s e d to
prepare
s o l i d samples for analysis. O n e t e n t h g of s a m p l e is a d d e d to a p l a s t i c v i a l c o n t a i n i n g 1 g of p r e w e i g h e d l i t h i u m tetraborate. T h e v i a l is h a n d shaken to m i x the m a t e r i a l , a n d the contents are p o u r e d into a g r a p h i t e c r u c i b l e . T h e m a t e r i a l is f u s e d at 9 5 0 ° C for 15 m i n i n a muffle f u r n a c e . T h e r e s u l t i n g b e a d is r e m o v e d f r o m the f u r n a c e a n d c a n either be stored i n the o r i g i n a l v i a l or i m m e d i a t e l y s o l u b i l i z e d . T h e b e a d is t r a n s f e r r e d to a T e f l o n b e a k e r c o n t a i n i n g 5 m l of 3N H C 1 , 2 m l of 2N H N 0 a n d 10 m l of w a t e r . T e f l o n is u s e d to e l i m i n a t e s o d i u m c o n t a m i n a t i o n . T h e m a t e r i a l is t h e n b o i l e d u n t i l c o m p l e t e l y d i s s o l v e d a n d i m m e d i a t e l y filtered into a 50 m l v o l u m e t r i c flask. H o t filtration is r e q u i r e d to p r e v e n t s o l i d m a t e r i a l s f r o m c r y s t a l l i z i n g out of s o l u t i o n b e f o r e d i l u t i o n a n d to r e m o v e c a r b o n p a r t i c l e s that result f r o m f u s i o n i n g r a p h i t e c r u c i b l e s . T h e s a m p l e is t h e n d i l u t e d to v o l u m e , s h a k e n , a n d t h e n f u r t h e r d i l u t e d as r e q u i r e d to b r i n g the elem e n t c o n c e n t r a t i o n to w i t h i n r a n g e of t h e w o r k i n g c u r v e of interest. F o r the d e t e r m i n a t i o n of c a l c i u m , m a g n e s i u m , a n d b a r i u m , l a n t h a n u m c h l o r i d e is a d d e d as a r e l e a s i n g agent to a c h i e v e a final l a n t h a n u m c o n c e n t r a t i o n of 1 % as r e c o m m e n d e d b y the spectrometer m a n u f a c t u r e r . 3
Results and
Discussion
A l t h o u g h detectable
concentrations
for several elements
could
be
f o u n d after f u s i o n , i t is felt that the v o l a t i l i t y of m e r c u r y a n d p o s s i b l y l e a d a n d t i n w o u l d m a k e t h e i r d e t e r m i n a t i o n b y l i t h i u m tetraborate f u s i o n questionable.
T a b l e I shows the elements selected for analysis a n d the
a c c u r a c y a n d p r e c i s i o n d a t a for the standards u s e d to c h e c k the f u s i o n method.
E a c h s t a n d a r d i n T a b l e I was of k n o w n c o m p o s i t i o n a n d s i l i -
ceous i n n a t u r e .
T h e standards w e r e separately p r e p a r e d 10 times
that a s t a t i s t i c a l e v a l u a t i o n of the results c o u l d be m a d e .
u s e d w e r e U S G S Standards G - 2 , W - l , B C R - 1 , c o m m e r c i a l l y s i l i c a - a l u m i n a based
standards, a n d u n f u s e d
p a r e d b y the C o a l R e s e a r c h
B u r e a u (9,
10,
prepared
s y n t h e t i c standards 11,
12).
so
T h e standards
The
pre-
synthetic
standards w e r e u s e d because no c o m m e r c i a l l y p r e p a r e d s t a n d a r d h a v i n g
5.
M U T E R AND NICE
Atomic
Absorption
61
Spectroscopy
and Concentration Found by L12B4O7 Fusion (ppm) PCC-1 Fusion 2730 900 2150
BCR-1
DTS-1
Reported Value
Fusion
Reported Value
2730 959 2339
3750 960 2000
4000 969 2269
Fusion 130 1360 280
Reported Value 17.6 1406 15.8
d e t e c t a b l e concentrations of B a , A g , C o , C u , H g , P b , S b , a n d S n w e r e available.
Salts of these eight elements w e r e separately f u s e d to deter-
m i n e i f a n y d i f f i c u l t y m i g h t b e e x p e c t e d i n a n a l y z i n g t h e m b y this t e c h n i q u e . A s expected, elements s u c h as m e r c u r y , t i n , a n d to a lesser extent, l e a d v o l a t i l i z e d d u r i n g f u s i o n . S i l v e r i n h i g h concentrations ( a b o v e 0 . 5 % ) c o u l d not be s o l u b i l i z e d . H o w e v e r , i n l o w concentrations, b o t h s i l v e r and gold could be fused and h e l d i n solution. R e l a t i v e error values for the elements r a n g e d f r o m zero to a h i g h of 1 8 % w i t h most values b e i n g 5 % or less. T h e 1 8 % r e l a t i v e error w a s o b t a i n e d for i n d i u m a n d is a t t r i b u t a b l e to the l o w c o n c e n t r a t i o n of this e l e m e n t i n the s o l u t i o n a n a l y z e d .
M o r e o v e r , the i n d i u m values
were
o b t a i n e d o n the l o w e r e n d of the w o r k i n g c u r v e w h e r e the sensitivity is g r e a t l y r e d u c e d . S t a n d a r d deviations a n d coefficients of v a r i a t i o n for the elements of interest are at acceptable deviation and around 5 %
levels
(less t h a n 1 %
standard
coefficient of v a r i a t i o n ) for this t e c h n i q u e .
A g a i n it s h o u l d be p o i n t e d out that the o r i g i n a l p u r p o s e of the subject m e t h o d was to d e v e l o p a r a p i d r o u t i n e analysis for the major a n d m i n o r constituents i n c o a l ash a n d r e l a t e d m a t e r i a l s w i t h o u t the necessity of several preconcentration
steps,
solvent
extraction t e c h n i q u e s ,
or
pH
adjustments. T h e a p p l i c a t i o n of the l i t h i u m tetraborate f u s i o n t e c h n i q u e to the analysis of siliceous ashes has r e s u l t e d i n over 10,000 e l e m e n t a l determinations.
W h i l e detectable g o l d a n d silver concentrations h a v e b e e n
f o u n d , the results are n e a r the detection l i m i t s for those t w o elements. U s i n g the l i t h i u m tetraborate f u s i o n t e c h n i q u e , trace e l e m e n t c o n centrations w e r e
examined.
T a b l e I I shows a c o m p a r i s o n of results
o b t a i n e d for a f e w trace elements w i t h other r e p o r t e d values (12).
As
c a n be seen f r o m the t a b l e , results at h i g h e r concentrations are i n f a i r agreement w i t h the r e p o r t e d values for the three elements e x a m i n e d . M a n g a n e s e c o m p a r e s f a v o r a b l y at a l l concentrations w h i l e c h r o m i u m a n d n i c k e l i n the h i g h e r 2000 p p m range s h o w fair agreement.
Elements
s u c h as a n t i m o n y , t a n t a l u m , z i r c o n i u m , g o l d , a n d s i l v e r w e r e too l o w i n c o n c e n t r a t i o n to b e d e t e r m i n e d b y this t e c h n i q u e . I n o r d e r to m o n i t o r the self-consistency of this t e c h n i q u e , a r a w (as r e c e i v e d ) s a m p l e was a n a l y z e d .
A n o t h e r p o r t i o n of the same
sample
TRACE
Table III.
ELEMENTS
IN
Comparison of R a w Sample and Calculated Materials Balance for R a w Sample Analysis Analyzed Raw Sample Composition (wt %)
Element Al Cr Fe Mg Mn Si
Calculated Raw Sample Composition (wt %)
11.72 0.065 3.09 0.98 0.14 24.00
11.03 0.059 3.21 0.92 0.11 22.88
1
A
;/
/
I
JI Ί1 f 1 1
/ /
//
A
/
>
/
1 /
η
I
!
!
I
I
1
1
J
0
2
4
6
8
10
12
14
PPM-Mn
Figure
FUEL
1.
Manganese
determination ference
illustrating
silicon
inter
5.
MUTER
A N D NICE
Atomic
Absorption
63
Spectroscopy
0.40r
PPM-Ft
Figure
2.
Iron determination
illustrating
silicon
interference
was s e p a r a t e d a c c o r d i n g to screen size, a n d these sizes w e r e also a n a l y z e d . M a t e r i a l s b a l a n c e c a l c u l a t i o n s w e r e p e r f o r m e d o n t h e screened f r a c t i o n s . C h e m i c a l analysis a n d screen f r a c t i o n percentages o f e a c h p o r t i o n w e r e u s e d t o b a c k c a l c u l a t e a t h e o r e t i c a l r a w s a m p l e content a n d w e r e t h e n c o m p a r e d w i t h the a c t u a l r a w s a m p l e . T a b l e I I I shows these results a n d i n d i c a t e s that the d a t a are self-consistent. A l t h o u g h p r i o r r e s e a r c h h a s e s t a b l i s h e d t h e i n t e r f e r e n c e effect o f a l u m i n u m , p r e v i o u s i n - h o u s e studies o n t h e a t o m i c a b s o r p t i o n analysis of s i l i c a t e m a t e r i a l s i n d i c a t e d t h a t s i l i c o n m a y also h a v e a n i n t e r f e r e n c e effect.
T h i s interference is p r o b a b l y c a u s e d b y c o m p o u n d f o r m a t i o n i n
the flame. T h e effect o f s i l i c o n o n t h e d e t e r m i n a t i o n o f a l l o f the elements
64
TRACE
ELEMENTS
IN F U E L
a n a l y z e d b y t h e a i r - a c e t y l e n e flame ( interferences s h o u l d n o t o c c u r w h e n u s i n g n i t r o u s oxide—acetylene
flame)
was examined.
I n some cases a
d e p r e s s i o n i n a b s o r b a n c e w a s f o u n d w h e n s i l i c o n w a s present, a n d i n a f e w cases e n h a n c e m e n t w a s o b s e r v e d .
F o r e x a m p l e , depressions
were
f o u n d f o r i r o n , c a l c i u m , m a g n e s i u m a n d manganese a n d e n h a n c e m e n t for p o t a s s i u m a n d g a l l i u m ( a t h i g h e r c o n c e n t r a t i o n s ) .
Figures 1 a n d 2
i l l u s t r a t e t h e types o f w o r k i n g curves o b t a i n e d a n d t h e effect o f s i l i c o n o n t h e i r slope.
N o a t t e m p t w a s m a d e t o d e t e r m i n e t h e effect o f i n t e r
ferences other t h a n s i l i c o n because trace elements w e r e present i n s u c h r e l a t i v e l y m i n o r q u a n t i t i e s that interferences f r o m t h e m w e r e n o t c o n sidered a problem.
Interferences o f this t y p e c a n easily b e r e m o v e d b y
m a t c h i n g t h e c o n c e n t r a t i o n o f s i l i c o n a n d a l u m i n u m i n t h e standards t o the concentrations g e n e r a l l y e x p e c t e d i n t h e u n k n o w n . Conclusions L i t h i u m t e t r a b o r a t e has b e e n f o u n d to b e a n excellent f u s i o n agent e n a b l i n g c o m p l e t e d i s s o l u t i o n o f s i l i c a t e m a t e r i a l s i n a c i d for t h e analysis of m a j o r a n d m i n o r constituents i n c o a l .
C a r e f u l l y p r e p a r e d standards
m a t c h i n g t h e a p p r o x i m a t e concentrations o f b o t h t h e s i l i c a a n d a l u m i n a present i n u n k n o w n samples p e r m i t d e t e r m i n a t i o n s t o b e m a d e w i t h p r e cision a n d accuracy.
T h i s method is currently being used to analyze
coal ash a n d related materials.
Literature Cited 1. Muter, R. B., Cockrell, C. F., "The Analysis of Sodium, Potassium, Calcium and Magnesium in Siliceous Coal Ash and Related Materials," Appl. Spectros. (1969) 23 (5), 493. 2. Galle, Ο. Karmie, "Routine Determination of Major Constituents in Geo logic Samples by Atomic Absorption," Appl. Spectros. (1968) 22 (5), 404. 3. Langmyhr, F. J., Paus, P. E., "Hydrofluoric Acid Decomposition Atomic Absorption Analysis of Inorganic Siliceous Materials," At. Absorption Newslett. (1968) 7 (6), 103. 4. Van Loon, J. C., "Determination of Aluminum in High Silica Materials," At. Absorption Newslett. (1968) 7 (1), 3. 5. Yule, John W., Swanson, Glenda Α., "A Rapid Method for Decomposition and The Analysis of Silicates and Carbonates by Atomic Absorption Spectroscopy," At. Absorption Newslett. (1969) 8 (2), 30. 6. Ingamells, C. O., "Lithium Metaborate Flux in Silicate Analysis," Anal. Chim. Acta (1970) 52, 323. 7. Medlin, J. H., et al., "Atomic Absorption Analysis of Silicates Employing LiBO Fusion," At. Absorption Newslett. (1969) 8 (2), 25. 8. Ruch, R. R., Gluskoter, H. J., Shimp, N. F., "Occurrence and Distribution of Potentially Volatile Trace Elements in Coal: An Interim Report," Ill. State Geol. Surv. Environ. Geol. Notes (1973) 61. 9. Shapiro, Leonard, Brannoch, W. W., "Rapid Analysis of Silicate Rocks," U.S. Geol. Surv. Bull. (1956) 1036-C. 2
5.
M U T E R AND NICE
Atomic
Absorption
Spectroscopy
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10. Flanagan, F. J., "U. S. Geological Survey Standards—II. First Compilation of Data for the New U.S.G.S. Rocks," Geochim. Cosmochim. Acta (1969) 33, 81.
11. Flanagan, F. J., "1972 Values For International Geo-chemical Reference Samples," Geochim. Cosmochim. Acta (1973) 37, 1189. 12. Abbey, Sydney, "'Standard Samples' of Silicate Rocks and Minerals—A Review and Compilation," Geol. Surv. Can. (1972) paper 72-30. RECEIVED June 6, 1974. Use of trade names does not imply endorsement by the Coal Research Bureau.
