2 Trace Impurities in Coal by Wet Chemical Methods EUGENE N. POLLOCK
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Ledgemont Laboratory, Kennecott Copper Corp., Lexington, Mass. 02173
In determining trace elements in coal by wet chemical methods, conventional atomic absorption spectroscopy (AAS) was used to determine Li, Be, V, Cr, Mn, Co, Ni, Cu, Zn, Ag, Cd, and Pb after dry ashing and acid dissolutions. A graphite furnace accessory was used for the flameless AAS determination of Bi, Se, Sn, Te, Be, Pb, As, Cd, Cr, Sb, and Ge. Mercury can be determined by flameless AAS after oxygen bomb combustion. Arsenic and antimony can be determined as their hydrides by AAS after low temperature ashing. Germanium, tin, bismuth, and tellurium can be determined as their hydrides by AAS after high temperature ashing. Selenium can be determined as its hydride by AAS after a special combustion procedure or after oxygen bomb combustion. Fluorine can be determined by specific ion analysis after oxygen bomb combustion. Boron can be determined colorimetrically.
A s trace i m p u r i t i e s enter t h e e n v i r o n m e n t i n i n c r e a s i n g q u a n t i t i e s , the ^ ^ materials that c a n h a v e a n i m p o r t a n t i m p a c t o n t h e e n v i r o n m e n t are c o m i n g u n d e r c a r e f u l s c r u t i n y . C o a l a n d other e n e r g y sources are of m a j o r interest because t h e y c o n t a i n elements that c a n h a v e u n d e s i r a b l e p h y s i o l o g i c a l effects o n p l a n t a n d a n i m a l life, s u c h as H g , B e , Se, A s , C d , Pb, and F . Increased environmental concern
has a c c e l e r a t e d research o n t h e
analysis of trace elements i n fuels i n m a n y u n i v e r s i t y a n d g o v e r n m e n t a l facilities.
B e c a u s e i n s t r u m e n t s s u c h as mass spectrometers a n d n u c l e a r
reactors f o r n e u t r o n a c t i v a t i o n analysis a r e a v a i l a b l e , m u c h of this r e search uses sophisticated i n s t r u m e n t a t i o n a n d t e c h n i q u e s .
However, the
w e t c h e m i s t r y l a b o r a t o r y is s t i l l t h e o n l y a v a i l a b l e source of c h e m i c a l 23 Babu; Trace Elements in Fuel Advances in Chemistry; American Chemical Society: Washington, DC, 1975.
24
TRACE
ELEMENTS
IN FUEL
analysis c a p a b i l i t y for the service l a b o r a t o r y of a c o a l m i n i n g f a c i l i t y , even a v e r y l a r g e c o a l m i n i n g c o m p a n y . T h e c o a l service l a b o r a t o r y , w h i c h i n the past was c o n c e r n e d
with
d e t e r m i n i n g the A S T M P r o c e d u r e s for U l t i m a t e a n d P r o x i m a t e A n a l y s i s , is n o w responsible for a n a l y z i n g trace elements i n c o a l . W i t h this sort of f a c i l i t y a n d t e c h n i c a l s k i l l i n m i n d , o u r o w n w e t c h e m i c a l l a b o r a t o r y d e v i s e d r e l a t i v e l y r o u t i n e p r o c e d u r e s for d e t e r m i n i n g trace elements i n c o a l that c o u l d h a v e a n u n d e s i r a b l e e n v i r o n m e n t a l i m p a c t i n c l u d i n g H g , B e , Se, As, C d , P b , F , C u , N i , Z n , C r , Te, G e , M n , Sn, B , B i , Sb, V , L i , C o , and
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Ag. Instrumentation
for a Wet
Chemical
Laboratory
A l l of t h e i n s t r u m e n t a t i o n a n d the a n a l y t i c a l c h e m i s t r y skills that are n e e d e d for t r a c e element analysis are essentially present i n the n o r mal
wet
chemistry
laboratory.
Necessary
equipment
includes
the
following: 1.
Spectrophotometer
2.
p H M e t e r , e x p a n d e d scale w i t h fluoride electrode
3.
Parr oxygen b o m b
4.
A t o m i c a b s o r p t i o n s p e c t r o p h o t o m e t e r ( A A S ) w i t h accessories
5.
F u r n a c e for h i g h t e m p e r a t u r e a s h i n g ( Η Τ Α )
6.
S p e c i a l i z e d glassware ( m i n o r i n cost )
7. G e n e r a l miscellaneous glassware a n d a p p u r t e n a n c e s of the c l a s s i cal wet laboratory. W h i l e our research w a s c o n c e r n e d
with developing wet
chemical
m e t h o d s , w e c o n f i r m e d o u r d a t a w i t h analyses f r o m a n a v a i l a b l e spark source mass spectrometer ( S S M S ) . T h e S S M S o p e r a t i n g parameters are g i v e n i n T a b l e I. T h e i n s t r u m e n t u s e d was a n Α Ε Ι M S - 7 (1,2) w i t h electrical detection.
equipped
It w a s u s e d i n the p e a k s w i t c h i n g m o d e o n l y
to p r o v i d e m o r e precise analyses. Sample
Preparation
S a m p l e s for S S M S w e r e p r e p a r e d f r o m representative portions
of
t h e same H T A - p r e p a r e d samples u s e d i n the w e t c h e m i c a l analyses. T h e Η Τ Α p r o c e d u r e is the f o l l o w i n g . W e i g h 5 - 6 g < 100 m e s h c o a l i n t o a p o r c e l a i n c r u c i b l e a n d p l a c e it i n c o l d v e n t e d furnace. E l e v a t e the t e m p e r a t u r e to 3 0 0 ° C for 0.5 h r , to 5 5 0 ° C for 0.5 h r , a n d f i n a l l y to 8 5 0 ° C for 1.0 h r . R e m o v e t h e c r u c i b l e f r o m the f u r n a c e a n d stir the ash w i t h a r o d . R e t u r n the c r u c i b l e to the f u r n a c e at 850 ° C for 1.0 h r w i t h no v e n t i n g . T h e S S M S samples are r e g r o u n d w i t h a b o r o n c a r b i d e m o r t a r a n d pestle a n d d i l u t e d w i t h t w o parts of h i g h p u r i t y g r a p h i t e . T h e samples w i t h g r a p h i t e are p l a c e d i n p o l y s t y r e n e vials w i t h t w o or three % - i n .
Babu; Trace Elements in Fuel Advances in Chemistry; American Chemical Society: Washington, DC, 1975.
2.
POLLOCK
Wet
Chemical
Table I.
SSMS Operating Parameters 35% 100 100 0.002 i n . 0.002 i n . 0.3 n a n o c o u l o m b s Variable according to sample elements and concentration
Spark variac P u l s e r e p e t i t i o n rate (pps) P u l s e l e n g t h (μ sec) Source slit M u l t i p l i e r slit M o n i t o r exposure M u l t i p l i e r a n d a m p l i f i e r gains Electrodes, vibrated
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25
Methods
p o l y s t y r e n e beads a n d m i x e d i n a spex m i l l for 20 m i n . E l e c t r o d e s are p r e p a r e d f r o m the p o w d e r s u s i n g the Α Ε Ι b r i q u e t t i n g d i e a n d p o l y e t h y l e n e slugs. T a b l e I I compares
SSMS
(G)
results a n d c o n v e n t i o n a l s o l u t i o n
a t o m i c a b s o r p t i o n d a t a f r o m coals ashed as d e s c r i b e d above. T h e d a t a i n d i c a t e s reasonable i f not o u t s t a n d i n g agreement e s p e c i a l l y since the coals w e r e ashed separately for the S S M S a n d the A A S samples.
Table II.
Comparison of A A S and SSMS (G)
Cu
(ppm)
Zn
SSMS
AAS
SSMS
14 23 22 33 11 32 12 38 13 18
17 16 18 29 13 29 24 61 14 18
13 41 17 22 41 84 8 660 18 50
Ni
Mn AAS 9 45 12 24 39 12 13 852 20 36
SSMS
AAS
33 200 90 101 62 124 11 143 22 73
37 193 65 93 52 96 19 117 21 49
Cr
SSMS
AAS
SSMS
14 23 21 14 14 28 7 41 7 15
23 30 31 21 22 28 15 81 22 18
26 20 21 12 18 19 13 36 15 37
V AAS 24 12 17 16 19 12 13 31 20 16
SSMS
AAS
25 21 35 43 24 24 15 20 61 27
32 21 23 24 25 21 24 40 37 30
Babu; Trace Elements in Fuel Advances in Chemistry; American Chemical Society: Washington, DC, 1975.
26
TRACE
ELEMENTS
IN
FUEL
T h e selection of t h e c o r r e c t d i s s o l u t i o n step i n c o a l d e c o m p o s i t i o n is v i t a l i n d e t e r m i n i n g trace elements.
S u c h elements as c o p p e r a n d n i c k e l
c a n easily b e p i c k e d u p as c o n t a m i n a n t s f r o m the l a b o r a t o r y e n v i r o n m e n t o r reagents.
O t h e r elements s u c h as m e r c u r y a n d s e l e n i u m c a n b e lost
i n the d i s s o l u t i o n step.
T h e dissolution procedure
i n v o l v i n g t h e least
exposure to c o n t a m i n a t i o n w i t h o u t p o t e n t i a l loss of v o l a t i l e c o m p o n e n t s s h o u l d b e u s e d i n f o r e a c h trace element. D r y a s h i n g is s t i l l the s i m p l e s t p r i o r t r e a t m e n t a n d s h o u l d b e u s e d w h e r e h i g h t e m p e r a t u r e a s h i n g is feasible. a n d Η Τ Α w a s m a d e for n i n e elements.
A c o m p a r i s o n of w e t a s h i n g
T h e results i n d i c a t e n o
appre
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c i a b l e loss f r o m v o l a t i l i z a t i o n ( T a b l e I I I ) . Table III. Cu
Comparison of Η Τ Α and Wet A s h i n g Mn
Ni
Zn
ΗΤΑ
Wet
ΗΤΑ
Wet
ΗΤΑ
Wet
ΗΤΑ
17 12 50 13 9
16 15 50 14 9
67 179 122 12 8
64 169 128 15 7
19 4 84 13 7
15 6 85 12 5
121 7 1420 16 17
Pb
α
Be
Cd Wet
ΗΤΑ
Wet
110 9 1450 23 19
2.8 0.6 13.1 0.6
