Major and Minor Constituents in Siliceous Materials by Atomic

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 tetraborat...
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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

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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.