Trace Elements in Fuel

2 Trace Impurities in Coal by Wet Chemical Methods EUGENE N. POLLOCK

Downloaded by UNIV LAVAL on July 11, 2016 | http://pubs.acs.org Publication Date: September 1, 1975 | doi: 10.1021/ba-1975-0141.ch002

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


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


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


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

Trace Elements in Fuel

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