Binders for Carbon Electrodes in the Aluminum Industrv J
L. P. CHARETTE AND G . T. BISCHOFBERGER Aluminium Laboratories, Ltd., Arvida, Que.
c
OAL-tar pitch, derived from by-product coke ovens, is the preferred material for use as a binder in the manufacture of carbon electrodes in the aluminum industry. Variations in the type of coke oven and coke oven practice and the diversity of sources of supply give rise to variations in pitch quality which affect electrode performance and result, in the case of some pitch binders, in poor performance of reduction cells. T o date no simple laboratory or small scale test exists which could be used to predict, with any degree of certainty, the suitability of a pitch for electrode manufacture. A good binder can be defined as one which, when mixed with adequate coke aggregate and baked, forms a carbon electrode that is characterized b y good properties contributing to low electrode consumption, good electrical efficiencies, and freedom from disintegration in the reduction cells. Experience has shown that in some cases pitch quality might be related to some of its properties such as coking value, carbon 1 (insoluble in nitrobenzene), and free carbon, but in general this does not hold, especially when pitches of different origins are compared. The only test which has been found significant in evaluating pitches is that b y which the compressive strength of the final electrode product or especially prepared test specimens-prepared from pitch and coke aggregate of well-defined granulometry-is determined. This test, however, because of the complexity of the compounding of the mixtures, the baking operation, and subsequent compression testing, is far from simple to perform for the user or supplier. As part of a general program to find a pitch characteristic, or combination of several characteristics, which would be useful in evaluating binders of coal-tar or petroleum origin, an investigation was undertaken to study the effect of aromaticity, percentage of benzene-insoluble, and coking value of coal-tar and petroleum pitches on their suitability for carbon electrode manufacture, the compressive strength of test electrodes being taken as criterion of pitch quality. The aromaticity is expressed as the atomic ratio of carbon to hydrogen content (C/H). The coking value, expressed in percentage, is the residue after baking pitch in a nonoxidizing atmosphere.
hour at the boiling point and the mixture is centrifuged. The solid residue is washed with benzene, dried, and weighed. The combined benzene solutions are distilled and the residue is dried and weighed. Drying is done in a vacuum oven a t 90' C. for 36 hours. Compressive Strength of Test Electrodes. For data on compressive strength, cylindrical test electrodes (3 inches in diameter by 6 inches high) were used. They were cast in a steel mold from a n intimate mixture of pitch and standardized coke aggregate, baked in a specially built furnace, and allowed to cool before being withdrawn from the furnace. The method is essentially the same as that described in the literature ( 4 ) . T h e baking cycle was, however, extended to 46 hours, the maximum temperature being 1000" C. Determination of Coking Value. A high-form porcelain crucible of 30-ml. capacity, provided with a cover, is ignited and weighed. About 3 grams of the material to be examined are weighed into the crucible, and the volatile matter is carefully evaporated over a low flame, the uncovered crucible being placed in a hole in the center of a shield made from asbestos sheeting. The pitch must not be boiled too violently, as it has a tendency to creep up the wall of the crucible. This operation must be carried out until a dry porous mass remains in the bottom of the crucible; in most cases this requires half an hour. The crucible is now covered and packed in a nickel crucible of
Table I. Analytical Data on Coal-Tar and Petroleum, Pitches, and Their Benzene-Insoluble and -Soluble Fractions Melting Fnt,
Coking Value, Wt. %
145 106 67 67 68
c-1
8.5 90 70 64 70 68 65 107
69.5 57.8 53.9 51.1 45.0 41.5 49.9 53.0 46.1 50.5 43.3 43.2 32.9 57.8
D-1 D-2 D-3
111 111 111
Petroleum Pitches 64.9 39 1.42 53.3 9 0.98 43.1 3 0.93
E-1 E-2
94 118
47.2 45.5
3 3
0.93 0.92
F-1 F-2
112 69
57.6 46.0
39 21
1.57 1.54
Sample Id entifi cationa
-
A- 1 A-2 A-3 A-4 A-5 A-6 B-1 B-2 B-3
MATERIALS
B-4
A total of 24 pitches-14 coal-tar and 10 petroleum pitchesfrom 9 different sources were used.
B-5 B-6 B-7
PROCEDURES
Determination of Carbon and Hydrogen. The method used for the determination of carbon and hydrogen was essentially that usually employed for organic substances. The combustion train, which includes a three-section electrically heated Fisher Scientific Co. furnace, was set u p according to ASTM standards on coal and coke (2)and methods of analyzing coal and coke (3). Determinations were made in duplicate. Fractionation of Pitch into Benzene-Insoluble and -Soluble Fractions. Fifty grams of pitch are submitted t o a first extraction with 1500 ml. of benzene under reflux and stirring. The extraction is carried out at the boiling point for 2 hours. The mixture is then cooled and the residue separated by centrifuging. The residue is again extracted with benzene (750 ml.) for another
C. b
66
BenzeneInsoluble, Wt. yo
Whole pitch
Coal-Tar Pitches 46 1.88 34 1.75 33 1.83 27 1.77 24 1.64 15 1.61 29 1.64 28 1.66 27 1.71 26 1.74 22 1.57 20 1.52 8 1.48 36 1.87
C/H Rr\$ilo Benzeneinsoluble soluble
s
2.38 2.41 3.17 2.86 2.38 2.13 2.20 2.26 2.37 2.60 2.15 1.96 1.78 2.57 3.43 1.47 1 42 1.43
1.57, 1.55 1,57: 1.59, 1.53. 1.55, 1.49, 1.49 1.59 1.58 1.45 1.45 1.47 1.59, 1.04
097, Q;9&
2.16 2.14
1..3% 1,.45, 1,.3S,
G-1
118
48.9
30
1.49
1.81
H-1
103
43.6
9
1.37
1.98,
1.35,
1-1
70
38.2
2
1.12
1.56,
1.1'4;
a A, B , C, D , E , F, G, H, and I indicate different suppliers. b Cube-in-water procedure used for m.p. < 80° C . (1); oqbe-in:ajr, h& m.p. > 80° C. (6).
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INDUSTRIAL AND ENGINEERING CHEMISTRY
July 1955
I
I
I
I
I
1413
I
0
0 0
E
Coal-Tar pitch Petroleum pitch
\ 0 CI,
Y
'"Or
-0 I
IO
I
I
I
20 30 40 "/. benzene insoluble
-
I
50
1.4
Figure 1. Relationship between compressive strength of test electrodes and percentage benzene-insoluble of pitch
t %
o
1.8.
C/H
2.2
2.6
3.0
3.4
r o t i o , benzene-insoluble
Figure 2. Relationship between compressive strength of test electrodes and atomic carbon-hydrogen ratio of benzene-insoluble pitch fraction
in a muffle furnace, maintained a t 600" to 700" C., where it is left for 5 hours. The crucible is removed and allowed to cool; the inner crucible is dug out, brushed and wiped free from coke, and weighed.
Coal-Tor pitch
pitch --- Petroleum Coal-Tar pitch only
400
Ym
Coking value =
Y
rl
-0 W
g
c 0
-
0
0
c
=
e
c 0
fm
E l a
c
'1 W
.-
0
In
:2 0 0 Q
E
0
0
0
0
I-
i-
i o
I I o
I
C/H ratio, whole pitch
Figure 3. Relationship between compressive strength of test electrodes and atomic carbon-hydrogen ratio of whole pitch
100-ml. capacity with screened calcined coke (-20 to +65 mesh) in such a way that it is completely surrounded and covered by the coke. The lid is then placed on the nickel crucible and it is set
weight of residue X 100 weight of sample
This method is a slightly modified version of that described in the literature ( 4 ) . RESULTS
Table I shows the analytical data obt,ained on coal-tar and petroleum pitches and their benzene-insoluble and -soluble fractions. Figures 1 to 5 show the variation of compressive strength of test electrodes with the following pitch properties: coking value, per cent benzene-insoluble, and atomic carbon-hydrogen ratio of whole pitch, atomic carbon-hydrogen of benzene-insoluble, and of benzene-soluble. DISCUSSION
A s shoxn by Figures 1 to 5 , there is no really satisfactory relationship, common to both coal-tar and petroleum pitches, between compressive strength of test electrodes and any of the individual pitch properties. The curves (Figures 1 and 2 j relating compressive strength to percentages of benzene-insoluble fractions and their atomic carbon-hydrogen ratios show a certain trend, but the relationships, on the basis of the standard deviations about the curves ( u = 34 and 47 kg. per sq. cm., respectively), are not considered satisfactory. [Standard deviation about the curve or standard deviation about regression of compressive strength ( y ) upon per cent benzene-insoluble ( r j is equal t o the square root of the sum
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t
o
C o o l - T a r pitch
Petroleum p i t c h
-
-
20
40
60
80
Chorocterizotion foctor I, Coking value x whole pitch
100
120
C/H rotio of
Figure 6. Relationship between compressive strength of test electrodes and characterization factor 1
of the squares of the vertical deviations from the straight line.] Examination of Figures 3 and 4,where atomic carbon-hydrogen ratios of whole pitch and benzene-soluble fraction are plotted
regation between coal-tar and petroleum pitches. It also indicates clearly why curves were drawn only for the former type of pitch. Quality of coal-tar pitch appears to be associated with atomic carbon-hydrogen ratios, especially ratio of whole pitch, b u t for petroleum pitches such a relationship is far from being well defined. Although it is generally agreed that the coking value is an important property of pitch, its relationship to compressive strength, taking into consideration both types of pitch samples, coal-tar and petroleum, is not good, as illustrated b y Figure 5, curve C, the standard deviation about the curve being 50 kg. per sq. cm. If, however, separate curves are drawn for the two different types (Figure 5, curves A and B ) , a much lower standard deviation is obtained for coal-tar pitches ( u = 28 kg. per sq. cm.), but for petroleum pitches it is still fairly high ( v = 40 kg. per sq. cm.). It is evident that none of these properties taken individually can be used as a common criterion of pitch quality for both coal-tar and petroleum pitches. If, however, certain of these properties are studied collectively, some interesting relationships can be established. For instance, if the coking values are multiplied by the atomic carbon-hydrogen ratios of the whole pitches and the products (characterization factor 1, Table 11) plotted against compressive strengths as in Figure 6, a fairly good relationship common t o both types of pitch is obtained, the standard deviation about the curve being 19 kg. per sq. cm. Taking into consideration the precision of the compressive strength test, mhich is of the order of 15 kg. per sq. cm., this value of 19 kg. per sq. cm. is close to the lowest one that can be obtained to express the precision of the correlation of pitch properties to compressive strength. Another relationship t o compressive strength was derived from the product of ‘(per cent benzene-insoluble, atomic carbonhydrogen ratio of benzene-insoluble fraction, and atomic carbonhydrogen ratio of benzene-soluble fraction” (characterization factor 2, Table I1 and Figure 7). Although the standard devia-
INDUSTRIAL AND ENGINEERING CHEMISTRY
July 1955
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Table 11. Correlation between Compressive Strength of Test Electrodes and Characterization Factors Sample Identification A- 1 A-2 A-3 A-4 A-5 8-6
B-1
B-2 B-3 B-4 33-5 B-6 B-7
c-1
Compressive Strength, Kg./Sq. Cm.“
Characterization Factor l b 2c
Coal-Tar Pitches 440 127 352 100 99 375 340 91 289 74 67 273 263 82 300 88 320 79 330 88 240 68 229 66 48 181 366 108
172 125 164 123 87 50 96 94 99 108 68 57 21 147
Petroleum Pitches D-1 319 92 135 52 12 D-2 210 D-3 169 40 4 44 4 E-1 180 E-2 167 42 4 F- 1 315 90 112 F-2 242 71 65 255 G-1 73 75 23 254 H-1 60 136 43 4 1-1 z, Pitch samples are usually considered good, fair, or bad when compressive strengths a€ test electrodes are >320 kg./sq. om., 240 t o 320, or
