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A comparison of methods for the determination of the pyritic sulfur content of coal
Mining Science and Technology, 11 (1990) 153-156 Elsevier Science Publishers BN., Amsterdam
153
A comparison of methods for the determination of the pyritic sulfur content of coal A . H . S t i l l e r a, j . j . R e n t o n b a n d P . A . M o n t a n o c a Department of Chemical Engineering, West Virginia University, Morgantown, WV 26506 (U.S.A.) b Department of Geology and Geography, West Virginia University, Morgantown, WV26506 (U.S.A.) c Department of Physics, West Virginia University, Morgantown, WV26506 (U.S.A.) (Received November 30, 1989; revised and accepted February 13, 1990)
ABSTRACT Stiller, A.H., Renton, J.J. and Montano, P.A., 1990. A comparison of methods for the determination of the pyritic sulfur content of coal. Min. Sci. Technol., 11: 153-156. The accurate determination of the concentration of inorganic sulfur in coal is of major environmental importance. With few exceptions, unless the coal is weathered, the inorganic sulfur present within the coal is iron disulfide minerals, chiefly pyrite and to a lesser extent, marcasite. The accuracy of the presently used ASTM procedure for the determination of iron disulfide sulfur can be questioned because various quantities of the mineral grains encapsulated by the coal matrix are not available to the acid-leaching reaction utilized in the procedure. The goal of this study was to test this hypothesis and to devise a procedure which would give analytical results in agreement with a matrix-independent analytical procedure, namely Mossbauer spectroscopy. The concentration of inorganic sulfur was determined for a suite of coal samples varying in sulfur abundance by the ASTM D2492 procedure on two particle sizes of coal, -60 mesh and -220 mesh. The coal samples were subjected to low-temperature ashing and the resultant ashes were leached with 0.5N hydrochloric acid. The total weight loss from the ashing process was made up by adding sulfur-free spectrographic grade carbon and the sulfur content of the mixture was determined using a LECO 1R32 sulfur analyzer. The inorganic sulfur contents as determined by the three methods were then compared to the sulfur concentrations as calculated from the iron disulfide abundance as determined by Mossbauer spectroscopy. The data show that the sulfur analysis using ASTM D2492 and the -60 mesh coal sample is unsatisfactory. Acceptable sulfur data can, however, be determined using the ASTM method and -220 mesh coal. The best comparison of data was obtained with the sulfur content obtained by the LECO analysis of the acid-leached low-temperature ash. These results strongly suggest a significant error in inorganic sulfur analysis due to the encapsulation of iron disulfide grains within the organic matrix of the coal when a procedure is used based upon the leaching of the coal sample. The problem can be minimized by utilizing fine-grained coal (-325 mesh) and essentially eliminated by utilizing an hydrochloric acid-leached low-temperature ash.
Introduction
the c o n t i n u e d e m e r g e n c e of c o a l - b a s e d synf u e l ; a n a c c u r a t e a n a l y t i c a l m e t h o d is n e e d e d
T h e u s e o f c o a l as a c o m b u s t i o n a b l e f u e l is
for the q u a n t i f i c a t i o n of b o t h the i n o r g a n i c
limited depending upon the total sulfur cont e n t o f t h e coal. D u r i n g c o m b u s t i o n , t h e s u l f u r c o n t a i n e d i n t h e c o a l is o x i d i z e d t o s u l f u r
a n d o r g a n i c s u l f u r c o m p o n e n t s i n coal. Sulfur occurs in coal in two basic forms: (1) o r g a n i c , a n d (2) i n o r g a n i c . O r g a n i c s u l f u r
d i o x i d e w h i c h , if it is a l l o w e d t o e n t e r t h e a t m o s p h e r e , has a deleterious effect o n the e n v i r o n m e n t as e x e m p l i f i e d i n a c i d r a i n . W i t h
is t h a t w h i c h is i n c o r p o r a t e d as p a r t o f t h e organic matrix of the coal and cannot be
0167-9031/90/$03.50
© 1990
removed by standard coN-cleaning processes.
Elsevier S¢icn¢¢ Publi~hcr~ B.V.
154
Inorganic sulfur occurs primarily in two mineral groups, the iron disulfide minerals, which include pyrite and to a lesser extent marcasite, and a variety of iron and calcium sulfate minerals. Unless the coal is weathered, the concentration of sulfate minerals is usually negligible. As a result, the sulfur content of coal can therefore be considered as being composed of two components, (1) organic sulfur, and (2) sulfur contained in the iron disulfide minerals commonly referred to as pyritic sulfur. At the present time, the method most commonly used to determined the relative abundance of organic and pyritic sulfur in coal is ASTM D2492. Several authors strongly criticize the ASTM method while others suggest that the technique is basically sound [1]. The procedure specifies that the coal be subjected to two acid-leaching steps, one with hydrochloric acid and the other with nitric acid. The hydrochloric acid leach removes sulfate minerals while the nitric acid leach removes the iron disulfide minerals. The respective sulfur association is then calculated based upon the amount of sulfur lost from the leached coal. The organic sulfur is calculated as the difference between the original total sulfur content of the coal and the combined sulfate and disulfide removed by the acid leaches. The controversy centers about the effectiveness of the nitric acid leaching step of the procedure to remove all of the pyrite from the coal. The ASTM procedure assumes that the acid removal of pyrite (and subsequently pyrite sulfur) is complete thereby allowing the relative abundance of organic and pyrite sulfur to be accurately determined based upon the results of the original total sulfur analysis and the sulfur content after the nitric acid leach. The arguments against the ASTM technique are based ~pon the fact that the leach° ing of coal particles which are significantly larger than the contained pyrite grains cannot be expected to be totally effective at
A . H . S T I L L E R E T AL.
removing pyrite. Pyrite is heterogeneously distributed throughout the coal matrix in a variety of down to submicron-sized grains. Unfortunately, the ASTM procedure leaves the dimensions of the coal particles to be leached to the discretion of the analyst. The particle size most commonly used for coal analysis is - 6 0 mesh which results in coal particles with maximum dimensions of 230 ~m. As a result, pyrite grains in coal with dimensions less than 230 /~m, may be completely enveloped by the coal matrix. Such encapsulated pyrite grains cannot be wetted by the acid leachate and subsequently are not removed. Any sulfur contained within these grains will ultimately be reported as organic sulfur thereby inflating the organic sulfur content at the expense of the pyritic sulfur content of the coal. Because the different laboratories may use different coal particle sizes for ASTM procedure, there is little external consistency between measurements.
Experimental design This study was designed to compare four different procedures for the determination of pyritic sulfur: (1) Mossbauer spectroscopy, (2) ASTM D2492 on - 6 0 mesh coal, (3) ASTM D2492 on - 2 2 0 mesh coal, and (4) LECO analysis of an hydrochloric acidleached low-temperature ash. Mossbauer spectroscopy allows a direct determination of the pyrite concentration on the whole coal sample and is used in this study as a comparison standard. A discussion of the use of Mossbauer spectroscopy for the determination of pyrite in coal can be found in several publications [2-6]. The use of the - 2 2 0 mesh coal samples reduces the maximum particle ~ize of the coal to 63 btrn thereby allowing a more complete removal of the iron disulfide mineral by the nitric acid leach. The low-temperature ashing process [71 oxidizes
DETERMINATION
OF THE
PYRITIC
SULFUR
CONTENT
OF COAL
the coal in a oxygen plasma and completely removes the organic sulfur as sulfur dioxide as the organic matrix of the coal is oxidized. The hydrochloric acid leach of the resultant low-temperature ash removes any acid soluble sulfates that may have formed during the oxidation process leaving behind only sulfur contained in the iron disulfide minerals (pyritic sulfur).
155
3
, / " • 60 Mesh
~-" 03 < ~_
•
/
® 220 Mesh
~ /
2
/
5 03
o/t
J
"
Y - 0913X- 236 r = 0.991 N = 15
Experimentation ~''
Fifteen samples of five different coal beds representing a range of total sulfur from 0.3 to 3.5 wt.% were ground to both - 6 0 mesh and - 2 2 0 mesh and were mixed to insure homogeneity. Portions of the homogenized, sized samples were analyzed by Mossbauer spectroscopy to determine the pyrite concentrations. The ASTM D2492 method was then used to determine the pyritic sulfur content of the two sizes of each sample. A comparison of the pyritic sulfur concentration as determined from the pyrite content is determined by Mossbauer spectroscopy and the pyrite sulfur content as determined by the ASTM procedure for the two sample sizes can be seen in Fig. 1. A weighed portion of both sizes of each coal sample was then subjected to low-temperature ashing. If the power applied during ashing is maintained at 15 W per chamber, the mineral components in the ash are not affected by the oxidation process except perhaps for a slight oxidation of some iron disulphide grains. The low-temperature ash was leached with an hydrochloric acid solution to remove the sulfate minerals, dried and weighed. The weight loss due to the ashing and leaching process was made up with analytically pure graphite. The low-temperature ash/graphite mixture was then homogenized and the total sulfur content was determined by LECO. With the organic sulfur having been removed by the low-temperature ashing
j, •
0
/
/
~ /
A
•
."
•
•
r = 0.925
•N-15
•
1 2 % Pyritic Sulfur (Mossbauer)
3
Fig. 1. Relationship between pyritic sulfur content of coal as determined from Mossbauer data and the pyritic sulfur content as determined by the ASTM D2492 procedure on - 6 0 and - 2 2 0 mesh coal.
o
o
<
I
~6 2 --
Q
G9 0
Y=X
£
/
.=1~
•
o~ 1
o
~
~
% Pyritic Sulfur (Mossbauer) Fig. 2. Relationship between the pyritic sulfur content of coal as determined from Mossbauer data and the sulfur determined by LECO 1R32 analysis of an HC1 leached low-temperature ash of the coal.
156
and any sulfate sulfur removed by the hydrochloric acid leaching, the resultant sulfur values represents the pyrite sulfur content of the original coal. The pyrite sulfur content as determined by the low-temperature ashing LECO 11232 analysis was compared to the pyritic sulfur concentration as calculated from the pyrite content as determined by Mossbauer spectroscopy (see Fig. 2).
A.H. S T I L L E R ET AL.
the A S T M D2492 m e t h o d and a - 2 2 0 mesh sample seems quite adequate for normal analysis as only small discrepancies were observed when compared to the pyritic sulfur values generated by Mossbauer spectroscopy. The best comparison of data was, however, that between the pyritic concentration as determined by Mossbauer spectroscopy and the sulfur value obtained using the LECO analysis of the hydrochloric acid-leached low-temperature ash.
Results and conclusions The results of the sizing studies substantiated the concerns expressed relative to the A S T M procedure. Substantial quantities of pyrite remained in the - 6 0 mesh coal following the nitric acid leaching as prescribed by A S T M D2492. However, practically all the pyrite was removed by the acid leaching of a - 2 2 0 mesh sample. The a m o u n t of pyrite remaining in the - 2 2 0 mesh samples after leaching was generally less than 5% of all the pyrite originally contained in the sample. The pyrite remaining after acid leaching of the - 2 2 0 mesh sample is due to the presence of extremely small pyrite grains which are still totally encapsulated by the coal matrix. The data show that the A S T M D2492 m e t h o d utilizing - 6 0 mesh coal is definitely inadequate as a method to accurately determine the concentration of the sulfur species contained in coal. On the other hand the use of
References 1 Given, P.H. and Miller, R.N., Determination of forms of sulfur in coals, Fuel, 57, (1978) 380-381. 2 Levison, L.M. and Jacobs, I.S., Mossbauer spectroscopic measurements of pyrite in coal, Fuel, 56, (1977): 453-454. 3 Greer, R.T., Proceedings of the International Symposium of Analytical Chemistry in the Exploration, Mining and Processing of Materials, Johannesburg, (1976). 4 Montano, P.A., Mossbauer spectroscopy of iron compounds fround in West Virginia coals, Fuel, 56, (1977): 397-400. 5 Huffman, G.P. and Huggins, F.E., Mossbauer studies of coal and coke: quantitative phase identification and direct determination of pyrite and iron sulfide sulfur content, Fuel, 57, (1978): 592-604. 6 Goldanski, V.I. and Herber, R.H., Chemical Applications of Mossbauer Spectroscopy, Academic Press, New York, N.Y. (1968). 7 Gluskoter, H.J., Electronic low temperature ashing of bituminous coal, Fuel, 44, (1965): 285-291.
153
A comparison of methods for the determination of the pyritic sulfur content of coal A . H . S t i l l e r a, j . j . R e n t o n b a n d P . A . M o n t a n o c a Department of Chemical Engineering, West Virginia University, Morgantown, WV 26506 (U.S.A.) b Department of Geology and Geography, West Virginia University, Morgantown, WV26506 (U.S.A.) c Department of Physics, West Virginia University, Morgantown, WV26506 (U.S.A.) (Received November 30, 1989; revised and accepted February 13, 1990)
ABSTRACT Stiller, A.H., Renton, J.J. and Montano, P.A., 1990. A comparison of methods for the determination of the pyritic sulfur content of coal. Min. Sci. Technol., 11: 153-156. The accurate determination of the concentration of inorganic sulfur in coal is of major environmental importance. With few exceptions, unless the coal is weathered, the inorganic sulfur present within the coal is iron disulfide minerals, chiefly pyrite and to a lesser extent, marcasite. The accuracy of the presently used ASTM procedure for the determination of iron disulfide sulfur can be questioned because various quantities of the mineral grains encapsulated by the coal matrix are not available to the acid-leaching reaction utilized in the procedure. The goal of this study was to test this hypothesis and to devise a procedure which would give analytical results in agreement with a matrix-independent analytical procedure, namely Mossbauer spectroscopy. The concentration of inorganic sulfur was determined for a suite of coal samples varying in sulfur abundance by the ASTM D2492 procedure on two particle sizes of coal, -60 mesh and -220 mesh. The coal samples were subjected to low-temperature ashing and the resultant ashes were leached with 0.5N hydrochloric acid. The total weight loss from the ashing process was made up by adding sulfur-free spectrographic grade carbon and the sulfur content of the mixture was determined using a LECO 1R32 sulfur analyzer. The inorganic sulfur contents as determined by the three methods were then compared to the sulfur concentrations as calculated from the iron disulfide abundance as determined by Mossbauer spectroscopy. The data show that the sulfur analysis using ASTM D2492 and the -60 mesh coal sample is unsatisfactory. Acceptable sulfur data can, however, be determined using the ASTM method and -220 mesh coal. The best comparison of data was obtained with the sulfur content obtained by the LECO analysis of the acid-leached low-temperature ash. These results strongly suggest a significant error in inorganic sulfur analysis due to the encapsulation of iron disulfide grains within the organic matrix of the coal when a procedure is used based upon the leaching of the coal sample. The problem can be minimized by utilizing fine-grained coal (-325 mesh) and essentially eliminated by utilizing an hydrochloric acid-leached low-temperature ash.
Introduction
the c o n t i n u e d e m e r g e n c e of c o a l - b a s e d synf u e l ; a n a c c u r a t e a n a l y t i c a l m e t h o d is n e e d e d
T h e u s e o f c o a l as a c o m b u s t i o n a b l e f u e l is
for the q u a n t i f i c a t i o n of b o t h the i n o r g a n i c
limited depending upon the total sulfur cont e n t o f t h e coal. D u r i n g c o m b u s t i o n , t h e s u l f u r c o n t a i n e d i n t h e c o a l is o x i d i z e d t o s u l f u r
a n d o r g a n i c s u l f u r c o m p o n e n t s i n coal. Sulfur occurs in coal in two basic forms: (1) o r g a n i c , a n d (2) i n o r g a n i c . O r g a n i c s u l f u r
d i o x i d e w h i c h , if it is a l l o w e d t o e n t e r t h e a t m o s p h e r e , has a deleterious effect o n the e n v i r o n m e n t as e x e m p l i f i e d i n a c i d r a i n . W i t h
is t h a t w h i c h is i n c o r p o r a t e d as p a r t o f t h e organic matrix of the coal and cannot be
0167-9031/90/$03.50
© 1990
removed by standard coN-cleaning processes.
Elsevier S¢icn¢¢ Publi~hcr~ B.V.
154
Inorganic sulfur occurs primarily in two mineral groups, the iron disulfide minerals, which include pyrite and to a lesser extent marcasite, and a variety of iron and calcium sulfate minerals. Unless the coal is weathered, the concentration of sulfate minerals is usually negligible. As a result, the sulfur content of coal can therefore be considered as being composed of two components, (1) organic sulfur, and (2) sulfur contained in the iron disulfide minerals commonly referred to as pyritic sulfur. At the present time, the method most commonly used to determined the relative abundance of organic and pyritic sulfur in coal is ASTM D2492. Several authors strongly criticize the ASTM method while others suggest that the technique is basically sound [1]. The procedure specifies that the coal be subjected to two acid-leaching steps, one with hydrochloric acid and the other with nitric acid. The hydrochloric acid leach removes sulfate minerals while the nitric acid leach removes the iron disulfide minerals. The respective sulfur association is then calculated based upon the amount of sulfur lost from the leached coal. The organic sulfur is calculated as the difference between the original total sulfur content of the coal and the combined sulfate and disulfide removed by the acid leaches. The controversy centers about the effectiveness of the nitric acid leaching step of the procedure to remove all of the pyrite from the coal. The ASTM procedure assumes that the acid removal of pyrite (and subsequently pyrite sulfur) is complete thereby allowing the relative abundance of organic and pyrite sulfur to be accurately determined based upon the results of the original total sulfur analysis and the sulfur content after the nitric acid leach. The arguments against the ASTM technique are based ~pon the fact that the leach° ing of coal particles which are significantly larger than the contained pyrite grains cannot be expected to be totally effective at
A . H . S T I L L E R E T AL.
removing pyrite. Pyrite is heterogeneously distributed throughout the coal matrix in a variety of down to submicron-sized grains. Unfortunately, the ASTM procedure leaves the dimensions of the coal particles to be leached to the discretion of the analyst. The particle size most commonly used for coal analysis is - 6 0 mesh which results in coal particles with maximum dimensions of 230 ~m. As a result, pyrite grains in coal with dimensions less than 230 /~m, may be completely enveloped by the coal matrix. Such encapsulated pyrite grains cannot be wetted by the acid leachate and subsequently are not removed. Any sulfur contained within these grains will ultimately be reported as organic sulfur thereby inflating the organic sulfur content at the expense of the pyritic sulfur content of the coal. Because the different laboratories may use different coal particle sizes for ASTM procedure, there is little external consistency between measurements.
Experimental design This study was designed to compare four different procedures for the determination of pyritic sulfur: (1) Mossbauer spectroscopy, (2) ASTM D2492 on - 6 0 mesh coal, (3) ASTM D2492 on - 2 2 0 mesh coal, and (4) LECO analysis of an hydrochloric acidleached low-temperature ash. Mossbauer spectroscopy allows a direct determination of the pyrite concentration on the whole coal sample and is used in this study as a comparison standard. A discussion of the use of Mossbauer spectroscopy for the determination of pyrite in coal can be found in several publications [2-6]. The use of the - 2 2 0 mesh coal samples reduces the maximum particle ~ize of the coal to 63 btrn thereby allowing a more complete removal of the iron disulfide mineral by the nitric acid leach. The low-temperature ashing process [71 oxidizes
DETERMINATION
OF THE
PYRITIC
SULFUR
CONTENT
OF COAL
the coal in a oxygen plasma and completely removes the organic sulfur as sulfur dioxide as the organic matrix of the coal is oxidized. The hydrochloric acid leach of the resultant low-temperature ash removes any acid soluble sulfates that may have formed during the oxidation process leaving behind only sulfur contained in the iron disulfide minerals (pyritic sulfur).
155
3
, / " • 60 Mesh
~-" 03 < ~_
•
/
® 220 Mesh
~ /
2
/
5 03
o/t
J
"
Y - 0913X- 236 r = 0.991 N = 15
Experimentation ~''
Fifteen samples of five different coal beds representing a range of total sulfur from 0.3 to 3.5 wt.% were ground to both - 6 0 mesh and - 2 2 0 mesh and were mixed to insure homogeneity. Portions of the homogenized, sized samples were analyzed by Mossbauer spectroscopy to determine the pyrite concentrations. The ASTM D2492 method was then used to determine the pyritic sulfur content of the two sizes of each sample. A comparison of the pyritic sulfur concentration as determined from the pyrite content is determined by Mossbauer spectroscopy and the pyrite sulfur content as determined by the ASTM procedure for the two sample sizes can be seen in Fig. 1. A weighed portion of both sizes of each coal sample was then subjected to low-temperature ashing. If the power applied during ashing is maintained at 15 W per chamber, the mineral components in the ash are not affected by the oxidation process except perhaps for a slight oxidation of some iron disulphide grains. The low-temperature ash was leached with an hydrochloric acid solution to remove the sulfate minerals, dried and weighed. The weight loss due to the ashing and leaching process was made up with analytically pure graphite. The low-temperature ash/graphite mixture was then homogenized and the total sulfur content was determined by LECO. With the organic sulfur having been removed by the low-temperature ashing
j, •
0
/
/
~ /
A
•
."
•
•
r = 0.925
•N-15
•
1 2 % Pyritic Sulfur (Mossbauer)
3
Fig. 1. Relationship between pyritic sulfur content of coal as determined from Mossbauer data and the pyritic sulfur content as determined by the ASTM D2492 procedure on - 6 0 and - 2 2 0 mesh coal.
o
o
<
I
~6 2 --
Q
G9 0
Y=X
£
/
.=1~
•
o~ 1
o
~
~
% Pyritic Sulfur (Mossbauer) Fig. 2. Relationship between the pyritic sulfur content of coal as determined from Mossbauer data and the sulfur determined by LECO 1R32 analysis of an HC1 leached low-temperature ash of the coal.
156
and any sulfate sulfur removed by the hydrochloric acid leaching, the resultant sulfur values represents the pyrite sulfur content of the original coal. The pyrite sulfur content as determined by the low-temperature ashing LECO 11232 analysis was compared to the pyritic sulfur concentration as calculated from the pyrite content as determined by Mossbauer spectroscopy (see Fig. 2).
A.H. S T I L L E R ET AL.
the A S T M D2492 m e t h o d and a - 2 2 0 mesh sample seems quite adequate for normal analysis as only small discrepancies were observed when compared to the pyritic sulfur values generated by Mossbauer spectroscopy. The best comparison of data was, however, that between the pyritic concentration as determined by Mossbauer spectroscopy and the sulfur value obtained using the LECO analysis of the hydrochloric acid-leached low-temperature ash.
Results and conclusions The results of the sizing studies substantiated the concerns expressed relative to the A S T M procedure. Substantial quantities of pyrite remained in the - 6 0 mesh coal following the nitric acid leaching as prescribed by A S T M D2492. However, practically all the pyrite was removed by the acid leaching of a - 2 2 0 mesh sample. The a m o u n t of pyrite remaining in the - 2 2 0 mesh samples after leaching was generally less than 5% of all the pyrite originally contained in the sample. The pyrite remaining after acid leaching of the - 2 2 0 mesh sample is due to the presence of extremely small pyrite grains which are still totally encapsulated by the coal matrix. The data show that the A S T M D2492 m e t h o d utilizing - 6 0 mesh coal is definitely inadequate as a method to accurately determine the concentration of the sulfur species contained in coal. On the other hand the use of
References 1 Given, P.H. and Miller, R.N., Determination of forms of sulfur in coals, Fuel, 57, (1978) 380-381. 2 Levison, L.M. and Jacobs, I.S., Mossbauer spectroscopic measurements of pyrite in coal, Fuel, 56, (1977): 453-454. 3 Greer, R.T., Proceedings of the International Symposium of Analytical Chemistry in the Exploration, Mining and Processing of Materials, Johannesburg, (1976). 4 Montano, P.A., Mossbauer spectroscopy of iron compounds fround in West Virginia coals, Fuel, 56, (1977): 397-400. 5 Huffman, G.P. and Huggins, F.E., Mossbauer studies of coal and coke: quantitative phase identification and direct determination of pyrite and iron sulfide sulfur content, Fuel, 57, (1978): 592-604. 6 Goldanski, V.I. and Herber, R.H., Chemical Applications of Mossbauer Spectroscopy, Academic Press, New York, N.Y. (1968). 7 Gluskoter, H.J., Electronic low temperature ashing of bituminous coal, Fuel, 44, (1965): 285-291.