Crystallite Structure of Cokes from Pitch Fractions M.
B. DELL
Alcoa Research Laboratories, Aluminum Co. of America, New Kensington, Pa.
M a n y carbon products a r e made by mixing c o k e with a binder s u c h a s pitch, shaping t h e mass, and baking t h e product at a n elevated temperature. To produce carbon e l e c t r o d e s for t h e aluminum industry, a mixture containing 10 t o 35% of pitch and a c o k e aggregate i s carbonized at 950’ t o llOOo C. Several t e s t s have been proposed t o determine t h e suitability of p i t c h e s for u s e a s binders in t h e manufacture of carbon products. Martin and Nelson (12) suggested that a good pitch should have a high s p e c i f i c gravity and carbon-hydrogen ratio. C h a r e t t e and Bischofberger (7) proposed t h e u s e of t h e product of coking v a l u e times carbon-hydrogen ratio. O n e of t h e most important f a c t o r s is t h e quality of t h e “binder c o k e ” which f o r m s f r o m t h e pitch during baking. Milliken (11) and Blayden, Gibson, and Riley (4) showed that t h e t y p e of c o k e depends upon t h e chemical constitution of t h e materials being carbonized and t h e carbonization procedure. With some exceptions t h e more aromatic compounds formed t h e more graphitic cokes. Van Krevelen and Chermin (13) observed t h a t t h e yield of c o k e on carbonization i s greater for aromatic compounds. Ghosh and Riley (9) divided a pitch into three p a r t s by solvent fractionation and showed that on carbonization t h e insoluble fraction formed a c o k e with t h e l e a s t graphitic structure. From t h e r e s u l t s of a modified Kattwinkel t e s t , Briickner a n d Huber (6) concluded that t h e insoluble fraction a c t s chiefly a s a filler and may even b e harmful t o c o k e quality. T h e s e r e s u l t s l e d to t h e present investigation; for they raised t h e q u e s t i o n s of whether a more e x t e n s i v e fractionation of pitch binders would reveal additional variations in graphitizability, whether any differences e x i s t e d in graphitizability of corresponding fractions from different pitches, a n d what t h e c a u s e s of s u c h differences were. P i t c h e s a r e tar distillation r e s i d u e s having cube-in-water melting points a b o v e 80’F. (1). T h e y c o n s i s t of a n insoluble phase, t h e C-I fraction, d i s p e r s e d in a continuous phase. T h e amount determined a s C-I d e p e n d s upon t h e solvent u s e d in t h e analytical procedure-good pitch sol-
Pitch Coke-oven X Coke-oven Y Coke-oven Z Oil-gas Lignite, low temp.
Quinoline Insoluble,
c
C L
110 109 62 116 109
Ash,
YC
%
13.7 16.5
8.9 13.3
PROCEDURE O n e hundred grams of powdered pitch were stirred under reflux for 1 hour with f i v e t i m e s i t s weight of solvent. The mixture w a s filtered through Whatman No. 12 filter paper and t h e procedure w a s repeated on t h e undissolved matter. T h e undissolved matter w a s s u c c e s s i v e l y extracted by t h i s double extraction procedure with e a c h of a s e r i e s of solvents. In order of increasing solvency for pitch, t h e series c o n s i s t e d of n-hexane, acetone, benzene, and carbon disulfide. Following Dickinson’s nomenclature (8) t h e fractions were c a l l e d tar oil, resin A, resin B, C I I , and G I respectively. For two of t h e pitches, pyridine w a s u s e d in p l a c e of carbon disulfide. Extraction w a s e a s i e r when raw pitch w a s t r e a t e d first with acetone rather than, starting a t t h e beginning of t h e series, with n-hexane. In e a c h c a s e , t h e amount of solvent w a s f i v e t i m e s the weight of undissolved matter, Solvent w a s removed from e a c h fraction by distillation a t 0.5-atm pressure. For e a c h fraction y i e l d s carbon-hydrogen ratio and molecular weight d a t a were obtained. Molecular weights were determined by t h e ebulliometric method using nitrobenzene a s a s o l v e n t Each fraction w a s carbonized i n argon a t a temperaiure r i s e of 1’ per minute t o 1500°C., s o a k e d 4 hours a t 1500 , and cooled. P a r a m e t e r s of t h e c r y s t a l l i t e s i n t h e c o k e s were determined by x-ray diffraction using t h e procedure of Blayden, Gibson, and Riley (5).
DISCUSSION
Table I. Properties of Pitches Melting Pgint,
v e n t s s u c h a s carbon disulfide, pyridine, nitrobenzene, or quinoline being generally used. T h e C-I fraction is derived from three sources: c o k e or char carried over into t h e t a r during carbonization, carbonaceous matter formed by thermal cracking of t a r i n t h e carbonization retort, and insoluble matter formed during distillation of t h e tar. In low-temperature t a r s and p i t c h e s (Parry p r o c e s s , Lurgi-SpUlgas), t h e C-I i s predominantly of t h e first type. In high-temperature t a r s (coke-oven, horizontal-retort, oil-gas), t h e s e c o n d t y p e predominates. T h e continuous p h a s e c o n s i s t s of b a s i c , acidic, and neutral materials, e a c h containing a spectrum of organic compounds. F o r t h i s investigation five p i t c h e s were chosen ( T a b l e I) representing three distinctly different types. By means of a s e r i e s of solvents, e a c h pitch w a s divided into five fractions; a n d t h e structure of t h e c o k e from e a c h fraction w a s determined.
0.52 0.26 0.22 0.96 0.6
T h e yield d a t a ( T a b l e 11) appear normal. T h e low-melting point coke-oven pitch Z and t h e low-temperature l i g n i t e pitch had t h e largest tar oil fractions. T h e lignite pitch contained less C-I1 and resin B compared with t h e other pitches. For coke-oven p i t c h e s X and Y , carbon-hydrogen ratios of t h e fractions d e c r e a s e d regularly with i n c r e a s e d solubility as w a s found by Dickinson (8). Molecular weights decreased from a value of 1000 for t h e C-I1 t o 230 for t h e
Table II. Yield and Properties of Solvent-Fractions of Pitches Fraction CII
CII
Resin B
Resin A
Tar oil
Benzene
Acetone
n-Hexane
15.0 17.6 12.9 15.1 6.0
21.4 21.1 19.0 22.0 23.5
22.8 18.2 37.5 20.0 43.0
Solvent Insoluble Yields Coke-oven X Coke-oven Y Coke-oven 2 Oil g a s Lignite Atomic C/H Coke-oven X Coke-oven Y Molecular weight Coke-oven Y
328
38.1 28.4 25.5 25.2 23.0
Pyridine
5.7 13.1 7.9 16.0 2.2 1.87
2 43 2.81
INDUSTRIAL AND ENGINEERING CHEMISTRY
Carbon disulfide
L92 1000
1.67 1.62
1.56 1.63
1.45 1.50
800
4-00
230
VOL. 3, NO. 2
Table
Ill. Crystallite Structure of Cokes
Fractions h e a t e d at I 0 / m i n u t e to 1500°C and s o a k e d 4 hours before cooling.
L, i s c r y s t a l l i t e height in Angstroms. c is interplanar spacing in Angstroms.
Fraction c-I
Fraction C o k e o v e n X, L, C
Cake-oven Y, L , C
Coke-oven Z, L, C
Oil g a s , L, C
Lignite, L C
,
Insoluble
50.2 3.47 30.6 3.51 49.2 3.48 27.2 3.52 23.4 3.50
C-tI
Pyridine
C-II Carbon disulfide
72.5 3.47 71.0 3.47 710 3.46 59.8 3.46 e e
Resin R Solvent
Resin A
Tar oil
3enzene
Acetone
n-Hexane
71.0 3.47 70.0 3.47 71.5 3.46 69.0 3.47 43.5 3.47
70.0 3.47 74.2 3.46 69.8 3.46 71.0 3.46 40.2 3.48
70.0 3.47
Untreated pitch
62.5 3.47
e e
59.8 3.46 53.0 3 47 47.7 3.47
7 1.0 3.47 65.6 3.47 51.2 3.47
aDid not form enough c o k e on carbonization
t a r o i l a n d a r e somewhat higher than t h o s e reported by Dickinson for a coke-oven tar. X-ray parameters of t h e c o k e s from e a c h fraction a r e given i n T a b l e 111. C o k e s a r e considered t o c o n s i s t of small graphitelike c r y s t a l l i t e s s u s p e n d e d i n a m a s s of disordered carbon. In T a b l e 111, t h e parameters refer only t o t h e c r y s t a l l i t e phase. I n c r e a s e d h e i g h t s of t h e c r y s t a l l i t e s , L,, a n d d e c r e a s e d interplanar s p a c i n g s , c, i n d i c a t e moregraphitic cokes. A temperature of 150OOC. w a s c h o s e n for calcining t h e c o k e s i n order t o a c c e n t u a t e any differences i n t h e c r y s t a l l i t e structures. A s shown i n F i g u r e 1, t h e s p r e a d i n c r y s t a l l i t e h e i g h t s between c o k e s from a graphi t i z a b l e pitch and from a relatively nongraphitizable pitch i s much greater a s t h e temperature i n c r e a s e s . T h e l i g n i t e pitch i n Figure 1 i s not t h e s a m e a s i n T a b l e I. Recently other methods h a v e been u s e d for calculating L, v a l u e s from x-ray diffraction curves. Hirsch (10) u s e d a Fourier transform method. Alexander and Sommer (3) determined t h e distribution of t h e numbers of l a y e r s constituting t h e crystallites. With i n c r e a s i n g c r y s t a l l i t e s i z e , r e s u l t s a r e obtained ( 2 ) by t h e l a t t e r method which approach t h o s e obtained by t h e “width at half-height” method u s e d i n t h i s paper and by Blayden, Gibson, and Riley (5). F o r L, v a l u e s of 25 A. t h e method u s e d by Alexander g i v e s res u l t s about 50% lower, and at 50 A. t h e difference d e c r e a s e s t o less than 10%. T h e width a t half-height method w a s u s e d h e r e b e c a u s e t h e c a l c u l a t i o n s a r e simpler, and t h e relations h i p s among t h e L, v a l u e s upon which t h e c o n c l u s i o n s a r e b a s e d would b e unchanged r e g a r d l e s s of t h e method of calc u l a t i o n ( 2 ) . Moreover, most of t h e L, v a l u e s reported h e r e a r e i n t h e range where differences d u e t o method of calculation would b e of l i t t l e significance. F o r e a c h pitch, c o k e f r o m C-I had t h e s m a l l e s t crystall i t e height, L,. In most c a s e s , t h i s w a s accompanied by a larger interplanar spacing, c. F o r t h e other fractions diff e r e n c e s were minor, although t h e l i g n i t e and oil g a s fract i o n s were l e s s graphitizable than t h e coke-oven fractions. B e c a u s e carbon disulfide i s a poorer solvent for cokeoven p i t c h e s than pyridine, t h e C-I obtained with carbon d i s u l f i d e would b e e x p e c t e d t o contain some of t h e other pitch fractions, e s p e c i a l l y t h e C-11. T h a t t h e s e C-I fract i o n s were contaminated i s shown by comparing t h e crystall i t e parameters of t h e C-I from coke-oven pitch Y with t h o s e from coke-oven p i t c h e s X a n d 2. T h e oil-gas pitch had a l e s s graphitizable C-I fraction than t h e coke-oven pitches. T h e C-I from l i g n i t e pitch w a s t h e l e a s t graphitizable of a l l t h o s e tested. T h i s w a s probably b e c a u s e i t c o n s i s t e d of c h a r p a r t i c l e s rather than lampblack or other thermal decomposition products. T h e t a r o i l fraction of coke-oven pitch Y did not f o r m enough c o k e o n carbonization for characterization by x-ray diffraction. T a r oil contributes very l i t t l e to binder coke, a s i t readily v o l a t i l i z e s during t h e baking p r o c e s s . 1958
I
I
I
1 1 - 1
1
1
I
I
I
I
I
I
I
I
I
I
v)
F o r coke-oven pitch X, t h e uniformity i n t h e c r y s t a l l i t e parameters of t h e c o k e s from C-11, resin A, r e s i n B, and tar oil i n d i c a t e s that although they differ i n molecular weight and carbon-hydrogen ratio, e a c h fraction formed t h e same t y p e of c o k e at 15OO0C. T h i s might b e explained on t h e assumption that although differing i n carbon-hydrogen ratios and molecular weights, t h e t y p e s of hydrocarbons or other compounds present i n e a c h of t h e s o l u b l e fractions i s t h e same for a particular pitch. T h e chemical t y p e s initially present influence greatly t h e graphitizability of t h e coke formed on carbonization ( 4 , 11). In a l l cases, c r y s t a l l i t e parameters of c o k e s derived from binder pitch were intermediate between t h e s e of t h e C-I and t h e other fractions. If t h e C-I exerted a n effect on t h e graphitic structure formed by t h e other fractions, i t w a s not indicated by t h e s e data. L I T E R A T U R E CITED (1) Abraham, Herbert, “Asphalts and Allied Substances,” 5th e & , vol. 1, p. 401, Van Nostrand, New York, 1945. (2) Alexander, L. E , private communication (3) Alexander, L. E., Sommer, E. C., J. P h y s . Chem. 60, 1646 (1956). (4) Blayden, H. E., Gibson, J., Riley, H. L., Znst. F u e l (London), Wartime B u l l . pp. 117-29, Feb. 1945. (5) Blayden, H., Gibson, J., Riley, H. L., “Proceedings of a Conference on t h e Ultra-fine Structure of C o a l s and C o k e s , ” 8.C.U. R A. (London), J u n e 1943. (6) B e c k n e r , H., Huber, G., G0.s-u. Rasserfach 91, t04 (1950). (7) Charette, L P . , Bischofberger, G. T., Znd. E n g . Chem. 47, 1412 (1955). (8) Dickinson, E. J., J. SOC. Chem. Znd. 64, 121 (1945). (9) Ghosh, S. P., Riley, H. L., F u e l 27, 56 (1948). (10)Hirsch, P, B., Proc. R o y . SOC. 226A, 146 (1954). (11) Kinney, C. R., “Proceedings of t h e First and Second Confere n c e s on Carbon,” p. 85, University of a u f f a l o , 1956. (12) Martin, S. W., N e l s o n , H. W., annual meeting, AIME, Chicago, Ill., 1955. (13) Van Krevelen, 4 W., C h e m i n , H. A. G., F u e l 33, 338 (1954). R e c e i v e d for r e v i e w April 10, 1957. Accepted December 26, 1957. Division o f G a s and Fuel Chemistry, 131st Meeting, ACS, Miami, F l a , April 1957. CHEMICAL AND ENGINEERING DATA SERIES
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