Low solids content, coal tar based impregnating pitch

A novel coal tar based, low solids content, pitch is produced by oxidizing a selected coal tar distillation fraction with air to oxygen at elevated temperatures. This pitch is applicable to end use in which low solids content is desirable. Specifically it may be advantageously applied to the impregnation of carbon electrodes. The oxidized selected coal tar based low solids material described is characterized by high carbon yield and higher product density and has a higher in situ coking value and lower sulfur content which make it particularly beneficial when it is used as an impregnating pitch as compared to the currently commercially used petroleum based pitch.

The invention relates to an improved coal tar based, low solids content 
pitch which is produced by the oxidation at elevated temperature of a 
selected coal tar distillation cut using air or oxygen to yield a product 
whose end use benefits from the use of a low solids content. 
BACKGROUND OF THE ART 
The current industrial carbon electrodes are typically manufactured by 
blending petroleum coke particles (the filler) with molten coal tar pitch 
(the binder) and extruding the resultant mix to form the "green 
electrode". The green electrode is then baked at approximately 
1300.degree. C. These heat treating processes transform the green body 
from approximately 95% carbon content to greater than 99% carbon. During 
the heat treating process, some of the organics are destructively 
distilled or vaporized and others decomposed, resulting in carbon 
deposition in the electrode. As the vaporized materials exit the body of 
the electrode they channel through its walls producing a porous structure. 
The result of this inherent porosity is reduced density, and reduced 
current carrying capacity. 
In the production of carbon electrodes, the carbon industry produces 
electrodes as large as 28 inches in diameter by 10 feet long for use in 
electric arc furnaces. These electrodes are used for example to carry 
large quantities of current in steel melting processes. The 
characteristics of a desirable carbon electrode are: 
1. high density 
2. high modulus of elasticity 
3. high electrical conductivity 
4. high flexural strength 
To reverse the undesirable effect of channeling, inherent porosity and 
reduced current carrying capacity the electrode is impregnated with an 
impregnating pitch which must have properties particularly suitable for 
this purpose. 
Coal tar pitch has historically been used as the impregnant because of its 
relative high density and carbon content as compared to petroleum pitch. 
However, technological improvements in manufacturing carbon electrodes 
have led to reduced porosity and pore size of the green bocy. As a result, 
impregnating pitch of lower solid content must be used. Ordinary coal tar 
based pitch cannot meet this requirement. While the market is currently 
dominated by petroleum based pitch, this material also has certain 
definite drawbacks. Moreover, it is to be understood that solid content of 
a pitch is only one indicator of pitch quality; the ultimate measure of 
quality pertains to penetration rate (high rates are desired) and ultimate 
yield of coke after rebaking. 
The solids content of a pitch is normally measured in weight percentage of 
the pitch and is determined by ASTM D2318-75 in terms of "quinoline 
insoluble" (QI). 
At this point it is significant to note that the term "pitch" is applied to 
a wide range of compositions and there is a distinct difference between 
pitches used for various purposes. With particular reference to electrode 
production "pitch" may be used in at least three different ways. 
1. Pitch can be coked to form "pitch coke" which is pulverized, sized and 
used as filler. Currently, most coke filler is produced from petroleum (as 
noted above). The manufacture of "pitch coke" from pitch produced by 
oxidizing coal tar at high temperatures is also known. However, it is to 
be noted, that pitch used as precursor of "pitch coke" has no "low solids" 
content requirement as does an impregnating pitch which is the material 
with which the present invention is concerned. 
2. Pitch can be used as a binder or cement to hold the carbon electrode 
during forming and baking. This application requires a coal tar pitch with 
its inherently high quinoline insolubles (QI) content. The significance of 
quinoline insolubles in binder pitches is described, for example, in D. R. 
Ball, "The influence of the type of Quinoline Insolubles on the quality of 
coal tar binder pitch" (Carbon 16, page 205 [1978]). It is generally 
agreed, that the solids content of binder pitches is determined by the 
"QI" test. It should also be noted that previous use of high-temperature 
oxidation of carbonacious materials (petroleum, coal tar, and oils) to 
form pitches suitable for electrode production were directed towared the 
production of binder pitches, and pitches for pitch coke, not for 
impregnant pitches. These prior art pitches usually had a QI content of 
the order of 14 percent. 
3. While reference to "impregnating pitches" for use in electrode 
production have been made, this application requires a pitch with 
distinctly "low solids" content. A discussion of the use of impregnating 
pitch and the physical properties of pitches used as both binders and 
impregnants may be found in Kirk-Othmer, Encyclopedia of Chemical 
Technology, Vol. 4, pg. 168, 181-183. The major difference between binder 
pitches and impregnating pitch can be seen from inspection of the 
"quinoline insoluble" line of Table 3, at page 168 of that reference. 
______________________________________ 
TYPICAL COAL TAR BINDERS IN 
CARBON AND GRAPHITE MANUFACTURE 
SOFT MEDIUM HARD IMPREGNATING 
PITCH PITCH PITCH PITCH 
______________________________________ 
QI % 12 12 15 5 
______________________________________ 
The QI of binders is significantly higher than the QI of impregnants. As 
shown, the QI content of a regular coal tar based impregnant is 5 wt%. 
In recent years, the quality of electrode has improved and the criteria for 
specifying the impregnating pitch has become more stringent. Impregnating 
pitch containing 5 percent QI is no longer satisfactory. This is the 
reason petroleum based pitch displaced coal tar pitch in this application. 
The current industrial standard is a petroleum based pitch which contains 
&lt;0.5% QI. The coal tar pitch of the present invention also contains QI 
&lt;0.5%. Previously no one has demonstrated the feasibility of producing 
high quality impregnating pitch based on coal tax oxidation. 
An important characteristic of petroleum based impregnating pitch resides 
in the fact that it possesses a low solids content over regular coal tar 
pitch. This equates to greater productivity in that it takes less 
processing time to perform an impregnation. However, petroleum pitch 
suffers from the disadvantages of low density, high sulfur and low in-situ 
coking value. In-situ coking value rrefers to the actual yield of carbon 
in the electrode after baking as compared to the quantity of pitch 
originally "picked-up" during the impregnation process. For example, 
suppose an electrode is impregnated, and using "before" and "after" 
weights, it is determined that the electrode "picked-up" 100 pounds of 
impregnating pitch. This pitch is transformed to carbon by baking. During 
baking, low boilers are distilled from the pitch which results in a yield 
loss. The "before" and "after" weights for the baking process are used to 
determine the quantity of pitch remaining in the electrode as carbon. 
Thus, if the electrode after baking weighs 30 pounds more than "before" 
impregnation, then the in-situ coking value is 30/100=30%. 
Typically, the specific gravity at 25.degree. C. of a petroleum 
impregnating pitch is 1.24 and the specific gravity of a coal tar pitch is 
1.30. This difference would equate to a 5% increase in "pick-up" for any 
impregnation step. It should also be noted that sulphur is an undesirable 
constituent of pitch because its presence results in an air pollution risk 
during baking and also produces "puffing" or an undesirable decrease in 
density phenomenon which can occur during graphitization. It is thus seen 
that a need exists for the provision of an improved pitch particularly 
characterized by low solids content, increased in-situ coking value and 
improved penetration and penetration rate. 
SUMMARY OF THE INVENTION 
An objective of the invention is to provide an improved coal tar product as 
a premium impregnating pitch in the manufacture of industrial carbon 
electrode. This improved impregnating pitch provides the following 
advantages over the petroleum based impregnating pitch: 
(a) increased yields 
(b) reduced sulfur content 
(c) increased density 
In addition, as shown by the comparison presented hereinafter, it offers 
high penetration rate (i.e., low solids content) as compared to other coal 
tar based pitches currently available. 
The improved, coal tar based, impregnating pitch is produced by oxidizing a 
selected coal tar distillation fraction with air or oxygen at elevated 
temperatures. This pitch is applicable to end use in which low solids 
content is desirable. Specifically it may be advantageously applied to the 
impregnation of carbon electrodes. High carbon yield, higher product 
density and lower sulfur content are the primary benefits when it is used 
as an impregnating pitch as compared to the currently commercially used 
petroleum based pitch. The pitch is characterized by low solids content 
enhanced impregnation property, and high coke yields. 
Heretofore, it has not been known that it was possible to produce an 
impregnating pitch from coal tar of a suitable practical quality. More 
specifically until the present discovery, the significance of selecting a 
"low solids" content feedstock and processing it at a specified 
temperature range was not recognized. In the invention, a processing 
temperature not greater than 750.degree. F. (400.degree. C.) is employed 
to produce a vastly superior coal tar based impregnant. Particularly 
advantageous properties of the pitch obtained, in accordance with the 
invention, include: 
(a) sulphur content less than 0.5 wt.% 
(b) a density at 77.degree. F. greater than 1.28 grams per cc 
(c) a Cleveland Open Cup flash point greater than 200.degree. C. 
(d) an in-situ coking value of 32 wt.% 
(e) Rate of pick-up of impregnant by the electrode comparable to that of a 
petroleum pitch and exceeding that of other coal tar based pitches. 
Additional advantages and attributes of the present invention will become 
apparent from the detailed description which follows. 
DESCRIPTION OF THE PREFERRED EMBODIMENT 
The improved impregnating pitch of the present invention comprises a 
product of oxidation of a high residue, low solids content, coal tar oil. 
The oil used as the precursor in making the desired pitch is obtained by 
isolating a middle cut during the distillation of crude coke oven tar. The 
quality of the precursor oil is critical. It is qualified by a filtration 
test and the solids content of the oil must be less than 0.05% as 
determined by ASTM D2318-76. The low solids heavy oil is oxidized by 
sparging with air at 300.degree.-700.degree. F. to yield an intermediate 
product substantially higher in average molecular weight than the 
precursor. The surface temperature of the reaction vessel is crucial. It 
is preferred to be kept below 700.degree. F. and should not exceed 
800.degree. F., otherwise solids formation cannot be controlled. The 
intermediate product is then stripped with an inert gas (steam and 
nitrogen may be used) to remove undesirable low boiling constituents. 
The endpoint of the oxidation period is determined by two criteria: (1) the 
yield of intermediate and (2) the softening point, as determined by ASTM 
D-3104-77. 
As a guideline, the yield of intermediate product usually is 30-70% by 
weight. However, it is a function of the residue content of the feed stock 
determined by ASTM D246-73. The softening point of the intermediate should 
be approximately 30.degree.-120.degree. C. At this stage, stripping is 
commenced and continued until another 10% by weight of the original charge 
is removed. At this point the pitch is characterized according to the 
following criteria: 
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1. Softening Point (.degree. C.) 
(ASTM D3104-77) 
100-150 
2. Coking Value-Conradson 
(ASTM D2416-73) 
45 min. 
(wt. %) 
3. Flash Point C.O.C. (.degree.C.) 
(ASTM D92-72) 200 min. 
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In accordance with the invention, the new coal tar based impregnating pitch 
is prepared by oxidation of a coal tar distillation cut. 
To obtain a feedstock for production of the new improved coal tar based 
impregnating pitch, the crude tar is distilled to obtain a heavy creosote 
cut described as having a distillation residue at 355.degree. C. of 
between 25 and 100 weight percent. 
The two criteria used for choosing the feedstock are: 
(1) the quinoline insoluble (QI) content must be less than 0.05 weight 
percent as determined by ASTM D-2318-76; and 
(2) the distillation residue according to ASTM D246-73 is greater than 
about 25%, with about 60% preferred. 
Other methods may also be used to suitably qualify heavy oils as 
satisfactory feedstocks, for example, as indicated in TABLE A below: 
TABLE A 
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FILTRATION TIME 
500 grams filter #4 
Whatman filter paper on 
QI TI 
Oil steamheated Buchner 
ASTM ASTM 
Sample funnel with 20" Hg. Vac. 
D- D- 
# @ 100.degree. C. 
2318-76 4072-81 
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Satis- A .03 
factory B 21 seconds .004 
Precursor 
C 36 seconds .05 
Unsatis- 
D .45 
factory E 19.3 minutes .47 
Precursor 
F 14.7 minutes .23 
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In preparing the oxidized coal tar component, as illustrated by reference 
to the figure of the drawing, the creosote starting material is heated in 
the vessel 10 at a temperature between about 300.degree. F. (149.degree. 
C.) and 750.degree. F. preferably between about 600.degree. F. 
(315.degree. C.) and 725.degree. F. (385.degree. C.), while sparging 
copious amounts of air, as shown at 12, through the fluid and thereafter 
as it is being heated. The simultaneous heating and sparging effectively 
(a) strips off low boilers which are shown being removed at 14 and (b) 
oxidizes the residual tar shown as being withdrawn at 16 as it is being 
heated. When the desired temperature limit is attained, typically at about 
725.degree. F., (385.degree. C.) although it will be apparent that steady 
state oxidation may be accomplished at lower temperatures probably down to 
300.degree. F. (149.degree. C.) the air sparging is continued at that 
temperature and until the desired oxidized intermediate product is 
obtained. The non-condensable vapors are removed at 18 and light oil 
withdrawn at 20. 
After obtaining the desired intermediate product the oxidation is 
terminated and stripping commenced with an inert gas, such as steam or 
nitrogen. In the stripping operation, steam is preferred because it is 
economical and is easily condensed out of the vapor stream. This reduces 
off-gas scrubbing equipment requirements. The inert gas stripping step, as 
a separate step, can be eliminated by using higher heat input during the 
oxidation step. In the stripping operation, undesirable low boiling 
constituents are removed from the pitch leaving the high molecular 
components. The endpoint of the stripping process is characterized by a 
softening point between 115.degree. and 150.degree. C., a Conradson coking 
value greater than 45% and a flashpoint greater than 392.degree. F. 
(200.degree. C.). 
The invention will be further described by the following specific examples. 
It should be understood, however, that although these examples may 
describe in detail certain preferred operating conditions of the 
invention, they are given primarily for purposes of the illustration, and 
the invention in its broader aspects is not limited thereto. Parts 
expressed are parts by weight unless otherwise stated. 
EXAMPLE 1 
In this run, a total of 117,600 pounds (53390 Kg) of heavy coal tar oil 
were charged to a nominal 10,000 gallon (37800 liters) still in two 
increments. Using direct fire the contents were heated to 690.degree. F. 
(365.degree. C.) while sparging with an average 200 SCFM (5663 liter/min.) 
of air. 61% of the precursor oil was stripped off, either during oxidation 
or during the stripping cycle. 
Seventy-four percent of the oxygen which was fed reacted with the coal tar 
oil. 
Twenty thousand pounds (5952 Kg) of material were stripped off during the 
stripping period and steam was used as the stripping medium. 
The finished impregnating pitch properties were: 
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(A) Softening Point (ASTM 
123.8.degree. C. 
D3104-77) 
(B) Q.I. (wt. %) .29 
(C) T.I. (wt. %) 31.1 
(D) Ash (wt. %) .009 
(E) Coke Conradson (wt. %) 
50.3 
(F) Sp. Gr. @ 77.degree. F. 
1.298 
(G) Flash C.O.C. .degree.F. 
450 
(H) C-9 Dist. % to 
270.degree. C. 0.0 
300.degree. C. 0.0 
360.degree. C. 0.5 
______________________________________ 
The actual properties of the heavy coal tar oil precursor although not 
recorded for this run were estimated as: 
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Sp. g. @ 100.degree. F. 
1.150 
Distillation, (wt. %) To 
235.degree. C. 0.0 
270.degree. C. 0.0 
315.degree. C. 2.2 
355.degree. C. 31.0 
% Residue at 355.degree. C. 
68.9 
Xylene Insoluble Content (wt. %) 
0.02 
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EXAMPLE 2 
2067 grams of heavy coal tar oil were charged to a 1/2 gallon reactor. The 
reactor was heated to 200.degree. C. at which time the air flow rate was 
adjusted to 130 cc/min. (standard cubic centimeters per minute). The 
contents were continually oxidized as they were heated to 375.degree. C. 
51.7% of original charge was stripped off during oxidation period. The 
average air flow rate was approximately 450 scc/min (standard cubic 
centimeters per minute) and the softening point at the end of the 
oxidation period was 75.4.degree. C. The pitch was then stripped with 
nitrogen until another 10% was stripped off based on the original charge. 
The final yield was 38%, the softening point was 126.degree. C., the 
coking value was 55%. 
A comparison of the coal tar pitch of the present invention with other 
pitch standards as shown in TABLE B below. The correlation between QI 
content, low solids content and penetration rate is demonstrated by this 
data. The rate of impregnant penetration of the carbon artifact is 
critical in juding an impregnant's quality. Assuming filtration of the 
pitch simulates the impregnating process, the data indicates a significant 
advantage for the coal tar pitch of the present invention. It is thus seen 
that the low solids composition of the invention which is a measure of the 
quantity of solids, as exhibited by filtration rates is significantly 
superior. 
TABLE B 
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"LOW SOLIDS" CONTENT PITCH VS. PRIOR ART 
Penetration 
Quinoline Insolubles 
Simulation.sup.(1) 
By ASTM By 
WT. % Filtration Rate 
______________________________________ 
Instant Invention 
&lt;.5 90 g/10 seconds 
Prepared During 
Example #1 
Typical Binder 
13.5 2 g/15 minutes 
Pitch.sup.(2) 
Petroleum Pitch.sup.(3) 
&lt;.5 50 g/15 minutes 
Low QI Coal Tar.sup.(4) 
7. 5 g/15 minutes 
Based pitch 
Prior Art 
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.sup.(1) Filtration of a designated pitch quantity through a 40 micron 
porous metal plate (3/4" Diameter .times. 1/4" thick) @ 225.degree. C. @ 
75 PSIG Differential Pressure. 
.sup.(2) 110.degree. C. Binder Pitch available from Allied Corp., Detroit 
MI. 
.sup.(3) Ashland Oil A240 Pitch, Available from Ashland Oil Co., Ashland, 
KY. Current industrial standard for impregnating pitch. 
.sup.(4) 15-V Pitch available from Allied Corp., Detroit, MI., previous 
industrial standard for impregnating pitch. 
It will be apparent that various modifications may be made without 
departing from the scope and spirit of the invention. Accordingly, the 
several details disclosed herein as illustrative are not be be construed 
as placing limitations on the invention, except as such limitations may be 
recited in the appended claims.