22 A New Method for Analysis of Pyrite in Coal
Downloaded via TUFTS UNIV on August 5, 2018 at 17:56:14 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
and Lignite M. H Y M A N and M . W. ROWE Department of Chemistry, Texas A&M University, College Station, T X 77843
Measurement of the pyrite in coal and lignite by a new method is presented. The method combines the techniques of thermo-gravimetry and magnetometry and utilizes oxidizing and reducing gases. The feasibility of thermo-magnetometry has been established by measuring the magnetite contents of carbonaceous chondrites. We have found that a precision of~2-5%in the magnetite measurements was typical, so that the method appears promising. Coals often contain sizeable amounts of FeSO4 (from air oxidation of FeS2); these would be recorded as FeS by our technique. Our method thus gives an estimate of the FeS2 prior to oxidation. No additional error is introduced due to the air oxidation of FeS2 to FeSO4. Soluble iron species, predominantly FeCO3, could be removed prior to the reduction of the FeS2 to metallic iron by H2 at elevated temperatures (~400°) in the balance, with the measurements being conducted part of the time in a strong magnetic field. The 2
formation of m e t a l l i c i r o n i s thus e a s i l y d e t e c t a b l e through obs e r v a t i o n of the s a t u r a t i o n magnetization which i s p r o p o r t i o n a l to the amount of i r o n formed. Our measurements of the p y r i t e i n samples of c o a l and l i g n i t e from the Coal Research S e c t i o n of The Pennsylvania State U n i v e r s i t y are compared with previous measurements on the same samples. Reasonable agreement i s observed. We present here the p r e l i m i n a r y r e s u l t s of our attempt to develop a new method f o r the a n a l y s i s of p y r i t e i n c o a l and l i g nite. I t i s w e l l known that s u l f u r i n c o a l i s present i n d i f f e r ent forms. In p a r t i c u l a r , although the i r o n s u l f i d e i n c o a l i s g e n e r a l l y p y r i t e (_1), other i r o n s u l f i d e s are f r e q u e n t l y present. For example, i r o n d i s u l f i d e occurs as m a r c a s i t e , a rhombic c r y s t a l l i n e form, as w e l l as p y r i t e , a cubic c r y s t a l l i n e form. Perhaps the term ' d i s u l f i d e s u l f u r ' should be used to r e p l a c e the ' p y r i t i c s u l f u r ' more commonly quoted, as r e c e n t l y suggested by Youh ( 2 ) . Since the chemical r e a c t i v i t y of these two d i s u l f i d e s of i r o n i s s i m i l a r , our method w i l l record them e q u a l l y w e l l . Nonetheless, we w i l l continue to r e f e r to the p y r i t e determinat i o n s here, although we are r e a l l y t a l k i n g about the chemical species FeS2 r a t h e r than a p a r t i c u l a r c r y s t a l l i n e s t r u c t u r e .
0097-6156/81/0169-0389$05.00/0 © 1981 American Chemical Society
Blaustein et al.; New Approaches in Coal Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
NEW
390
APPROACHES IN
COAL CHEMISTRY
The r e s u l t s obtained so f a r i n d i c a t e that the method i s f e a s i b l e , w i t h the l i m i t e d s t u d i e s we have made agreeing i n gen e r a l w i t h p r e v i o u s l y determined r e s u l t s . The primary advantages of our procedure are: (1) The method i s s t r a i g h t f o r w a r d . With about 10 minutes i n s t r u c t i o n , s i x d i f f e r e n t graduate students i n chemistry at Texas A&M U n i v e r s i t y were a b l e to measure the FeS2 i n a c o a l sample, a r r i v i n g at a v a l u e which agreed w i t h i n the estimated p r e c i s i o n (±1σ) w i t h the average v a l u e that we had p r e v i o u s l y determined. Under r o u t i n e o p e r a t i o n a t e c h n i c i a n w i t h minimal q u a l i f i c a t i o n s could be t r a i n e d e a s i l y to conduct the analyses. (2) The apparatus employed i s commonly found i n l a r g e r chemistry departments, where i t i s c a l l e d a Faraday balance, i n geology departments c a l l e d a C u r i e balance, and i n e n g i n e e r i n g departments, as w e l l as i n d u s t r i a l l a b o r a t o r i e s . (3) Automation, i f d e s i r e d , could be implemented r e a d i l y . The p r i n c i p a l d i s a d vantage i s the tdme r e q u i r e d f o r an a n a l y s i s . Each measurement of the p y r i t e content takes about 2 hours, although s i n c e the system records c o n t i n u o u s l y , the operator can attend to other d u t i e s w i t h only i n t e r m i t t e n t a l t e r a t i o n s being necessary d u r i n g the two-hour r e a c t i o n p e r i o d , or the e n t i r e procedure can be e a s i l y programmed with micro p r o c e s s o r s . Experimental Procedure The system which we u t i l i z e d f o r making the thermogravimetry - magnetometry measurements i s i l l u s t r a t e d s c h e m a t i c a l l y i n F i g ure 1. Richardson (3) used a s i m i l a r system to study the magne t i c p r o p e r t i e s of c o a l char. Our system c o n s i s t s of a Cahn e l e c trobalance w i t h a non-inductively-wound heater capable of l o n g term o p e r a t i o n at ^800 C. A temperature c o n t r o l l e r maintains the temperature at ^±5-10°C which i s adequate f o r our technique. The system i n c o r p o r a t e s the p o s s i b i l i t y f o r changing the gas e n v i r o n ment i n order to c o n t r o l the o x i d a t i o n - r e d u c t i o n c o n d i t i o n s w i t h i n . F i n a l l y a permanent magnet (4600 Oe) w i t h poles shaped f o r Faraday a n a l y s i s i s used and can be moved i n t o p l a c e to act upon the sample or be moved away at w i l l . T h i s system was de signed f o r s t u d i e s on meteorites (4) and a system w i t h l a r g e r c a p a c i t y f o r samples would be more s u i t a b l e f o r c o a l s t u d i e s . Our present system c o n s t r a i n s the s i z e of c o a l samples to ^50 mg. Larger samples would c e r t a i n l y be p r e f e r a b l e f o r p y r i t e a n a l y s i s in coal. To understand the technique we are proposing here, i t i s necessary to r e a l i z e the combined e f f e c t of temperature and a strong magnetic f i e l d on the apparent weight of a ferromagnetic m a t e r i a l suspended from a s p r i n g or balance beam. F i g u r e 2 shows that the apparent weight (the s a t u r a t i o n magnetization) of a ferromagnetic m a t e r i a l i s many times i t s a c t u a l weight i n the presence of a strong magnetic f i e l d . Note that t h i s l a r g e i n crease i n apparent weight, the s a t u r a t i o n magnetization, slowly
Blaustein et al.; New Approaches in Coal Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
22.
H Y M A N
Figure 1.
L O
A N D
Pyrite
ROWE
391
Analysis
Schematic of the Cahn electrobalance used for measuring coal and lignite
ι
ι 200
ι
ι 400
ι
ι
ι
600
ι 800
TEMPERATURE °C Figure 2. Variation of the saturation magnetization of 1 mg iron metal with tem perature as recorded by a Faraday balance. Note that the apparent weight (satura tion magnetization) of iron is much greater than its actual weight.
Blaustein et al.; New Approaches in Coal Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
392
NEW
APPROACHES IN
COAL CHEMISTRY
decreases as the temperature i s i n c r e a s e d u n t i l the apparent weight f i n a l l y becomes n e a r l y equal to the a c t u a l weight of the i r o n at ^770°C, the C u r i e p o i n t of the i r o n . The C u r i e p o i n t (also c a l l e d Curie temperature) i s d i s t i n c t i v e f o r the p a r t i c u l a r ferromagnetic m a t e r i a l under q u e s t i o n . For example, i t i s 390 C f o r N i , 595°C f o r Fe3Û4 (magnetite), 770°C f o r m e t a l l i c i r o n , e t c . The method f o r determination of p y r i t e i s perhaps best demons t r a t e d by examination of F i g u r e 3 which i s a schematic represent a t i o n of the d a t a - t r a c i n g of an a n a l y s i s of a h y p o t h e t i c a l samp l e of c o a l or l i g n i t e . A run i s s t a r t e d by weighing out a samp l e (region A of F i g u r e 3) with the gas i n l e t open to a i r . The presence of i n i t i a l Fe2Û3 can be checked by i n s e r t i n g the magnet (B). Normally only a very s l i g h t change i n weight i s seen at B. An exception, noted w i t h PS0C-625, w i l l be d i s c u s s e d l a t e r . The heater i s then set to 90±10°C which d r i v e s o f f the water and a weight r e d u c t i o n i s observed (C). We now use 105°C, the value recommended by ASTM but have not found any s i g n i f i c a n t d i f f e r e n c e i n water content. In f a c t a l l the o p e r a t i n g c o n d i t i o n s have now been changed to conform w i t h those used i n ASTM procedures. After the water has been removed, the weight becomes constant (D) and t h i s weight i s the dry weight of the c o a l . At that p o i n t , the temperature i s r a i s e d slowly to 400°C. T h i s i s s u f f i c i e n t to o x i d i z e the organic m a t e r i a l i n the c o a l which again r e s u l t s i n a r a t h e r extreme l o s s i n weight (E) as the gaseous r e a c t i o n products (CO, C02, H2O, e t c . ) and other v o l a t i l e s are d r i v e n o f f . We have found 400°C i s s u f f i c i e n t to o x i d i z e a l l the organic m a t e r i a l . Nonetheless we now use 700°C as noted l a t e r i n our mention of proximate a n a l y s i s . When the o x i d a t i o n i s complete, the weight i s once again observed to l e v e l o f f (F) and the p y r i t e has been o x i d i z e d to Fe2Û3. At t h i s p o i n t , the furnace i s turned o f f and 20% H2, d i l u t e d from 100% by N2 c a r r i e r f o r i n c r e a s e d s a f e t y , i s fed i n t o the system at a r a t e of 65 ml/minute. Ten minutes has been found to be adequate to f l u s h the sample area w i t h Η2· The magnet i s r e - i n s e r t e d which r e s u l t s i n an apparent weight i n c r e a s e due to the s a t u r a t i o n magnetization of the Fe2Û3 (G). For convenience i n p r e s e n t a t i o n of F i g u r e 3, we have shown a s c a l e expansion to accomodate the l a r g e i n c r e a s e i n s a t u r a t i o n magnetization which w i l l r e s u l t as m e t a l l i c i r o n i s formed from the r e d u c t i o n of Fe2Û3 (G). The temperature i s then turned to 400°C with the H2 f l o w i n g and the Fe2Û3 begins to be reduced to m e t a l l i c Fe, measured as a l a r g e i n c r e a s e i n apparent weight (H). When a l l the Fe2Û3 i s reduced to Fe, the s a t u r a t i o n magnetization due to the i r o n becomes constant i n d i c a t i n g the r e a c t i o n i s complete (I) and the furnace i s turned o f f . An i n c r e a s e i n apparent weight i s n o t i c e d as the sample c o o l s due to the i n c r e a s e of s a t u r a t i o n magnetization of the i r o n w i t h decreasing temperature (Figure 2) as e x h i b i t e d by r e g i o n J i n F i g u r e 3. Once again the apparent weight w i l l become constant as the temperature approaches room temperature (K). The magnet i s then removed and the f i n a l weight of the r e s i d u e i s recorded ( L ) . For purposes of determining the p y r i t e content, only three regions
Blaustein et al.; New Approaches in Coal Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
Blaustein et al.; New Approaches in Coal Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
Η« TIME
x20
2
s
Figure 3. Schematic of weight change observed continuously as pyrite in coal or lignite is analyzed. The change in the weight-scale factor from XI to X20 is due to the large increase in apparent weight (saturation magnetization) as the Fe O present at G is reduced to ferromagnetic iron.
X1
—
w
NEW
394
APPROACHES IN
COAL CHEMISTRY
of F i g u r e 3 are of c r i t i c a l importance. They a r e : ( l ) e i t h e r the i n i t i a l weight, Wi, of r e g i o n A, or the dry weight, of r e g i o n D, (2) the f i n a l apparent weight w i t h the magnet i n p l a c e , Wf , of r e g i o n K, and (3) the f i n a l weight, Wf, of r e g i o n L. The p y r i t e content i s then c a l c u l a t e d using s t o i c h i o m e t r y and the s a t u r a t i o n magnetization of the i r o n : W. -W. M.Wt. F e S 7 = f χ 2 J00 * 2 218 M.Wt. Fe * W m
0
f
1 Λ Λ
m
F g S
/o
eb
X
i>d
Where W
fm
= f i n a l weight w i t h magnet i n p l a c e ( s a t u r a t i o n magneti z a t i o n of i r o n )
W^ = f i n a l weight without magnet M.Wt.
FeS
M.Wt.
Fe = molecular weight of Fe
W
i
2
= molecular weight of F e S
2
^ = i n i t i a l weight or dry weight, depending upon whether * the per cent i n i t i a l o r , the per cent dry weight were d e s i r e d and the f a c t o r of 218 i s the room-temperature saturation-magnetization of i r o n .
I t i s a l s o p o s s i b l e to o b t a i n the per cent moisture and per cent of ash i n the c o a l or l i g n i t e samples w i t h t h i s technique. The moisture and ash contents a r e , of course, e a s i l y o b t a i n a b l e from F i g u r e 3. The moisture i s simply given by W -W % moisture = X 100 ττ
and the ash i s given by W
% ash =
—— W
X
100
i,d
depending on whether % ash i n the o r i g i n a l sample or f o r dry weight, r e s p e c t i v e l y , i s d e s i r e d . I t would be a simple matter to o b t a i n the proximate a n a l y s i s of the c o a l by means of very s l i g h t a l t e r a t i o n s i n the procedure (5). I f n i t r o g e n were introduced i n i t i a l l y and the p r o c e d u r e followed as i n F i g u r e 3 but i n c r e a s i n g the temperature to 700 C i n s t e a d of 400 C at point (D), then the percentage of v o l a t i l e matter could be obtained d i r e c t l y . The d i f f e r e n c e i n the weight, W
