the effect of binder pitch quinoline insolubles content - Coke Oven ...

COKE OVEN MANAGERS’ YEAR BOOK
THE EFFECT OF BINDER PITCH QUINOLINE
INSOLUBLES CONTENT ON ALUMINUM ANODE
PHYSICAL PROPERTIES
Robert H. Wombles
Koppers Inc. – Pittsburgh, PA, USA
womblesRH@koppers.com
Barry Sadler
Net Carbon Consulting Pty Ltd – Melbourne, Australia
barry.sadler@bigpond.com.au
This paper was presented to the 8 th Australasian Aluminium
Smelting Technology Conference and the centre for Electrochemical
and Minerals Processing, University of New South Wales.
The Conference was held on 3 – 8 October 2004 at the Rydges
Capricorn Resort in Yeppoon, Queensland. We thank the authors
and the Conference for being able to republish it here.
R H Wombles is Vice President Technology, Koppers, Harmaville
Center, Pitt Way, Pittsburg, USA.
Barry Sadler is Consultant with Net Carbon Consulting Pty Ltd.,
Melbourne, Australia.
Introduction
The first coal chemical recovery ovens were installed in the United
States in 1893. By 1915, by-product ovens accounted for 97 % of
the metallurgical coke produced in the United States. These byproduct
ovens produced coal tar as one of the major by-products. An
industry developed around distillation of coal tar to produce various
products. One of the major products produced is coal tar pitch.
Since that time, coal tar pitch has become the binder of choice for the
aluminum, commercial carbon, and graphite industries.
119

BINDER PITCH QI CONTENT – ALUMINIUM ANODE
Coal tar pitch contains small particles that are insoluble in the strong
solvent quinoline (hence the name Quinoline Insolubles or QI).
These high carbon content particles are formed during the coking
process by the polymerization of coal volatiles during the high
temperature coking of coal in metallurgical coke ovens. QI
formation will be discussed in a later section of this paper. During
the distillation of coal tar almost 100 % of the QI in the tar is
incorporated into the coal tar pitch product. Over the years the
importance or lack of importance of QI on the quality of coal tar
pitch for use as a binder has been reported in the literature. No clear
conclusion regarding the importance of QI in determining the quality
of coal tar pitch as a binder can be drawn from these reports. The
remainder of this paper will examine the importance of QI for coal
tar pitch as a binder.
Production of Coal Tar
Coal tar is produced in a coke oven. Coke ovens are built in a series
called a battery. Figure 1 shows a coke oven battery
The coking process involves filling each coke oven with coal,
closing the oven after it has been filled, and heating the oven to a
temperature of approximately 1100°C. A schematic of the coke
oven process is given in Figure 2.
120

COKE OVEN MANAGERS’ YEAR BOOK
Figure 1. Coke Oven Battery
Figure 2 – Schematic of Coke Oven Process
121

BINDER PITCH QI CONTENT – ALUMINIUM ANODE
122

COKE OVEN MANAGERS’ YEAR BOOK
As the coal is heated, it begins to devolatilize. The volatile products
consist of liquid and gaseous products. The gaseous products are
usually utilized as fuel for the coke ovens or an associated blast
furnace. The liquid products are condensed and sent to a vessel
called the decanter. In the decanter the large solid particles entrained
in the volatiles in the coke oven settle to the bottom of the vessel.
The tar is decanted from the solids and collected for further
processing. As the name implies, a coke oven is operated to produce
metallurgical coke for use in a blast furnace. The solid and liquid
products produced are by-products and are not the desired products.
Figure 3 gives a material balance for a typical coke oven operation.
As Figure 3 indicates the coke oven yields approximately 70 weight
% solid products and 30 weight % gas and liquid products. The
yield of coal tar is approximately 5 weight % of the coal feed. One
component of the coal tar is the QI. The next section will discuss the
types of QI present in coal tar and coal tar pitch.
Quinoline Insolubles In Coal Tar
One of the most discussed components of coal tar and coal tar pitch
is the component called QI. The QI content of coal tar or coal tar
pitch is determined by a test method designated as ASTM D 2318.
The test is performed by digesting coal in hot quinoline for twenty
minutes followed by filtration of the hot quinoline. The insolubles
are dried and weighed and the QI content reported as weight %.
There are two major types of QI present in coal tar or coal tar pitch
as listed in Table 1.
123

BINDER PITCH QI CONTENT – ALUMINIUM ANODE
Table 1 – Types of QI
Coke Oven Derived Thermal Treatment of Tar/Pitch
Normal: Mesophase:
~1 µm 4+ µm (ASTM)
Cracking of coal volatiles C/H ~2
C/H 3.5 – 5.5 Mesogens:
Carry-Over:

COKE OVEN MANAGERS’ YEAR BOOK
Figure 4 – Scanning Electron Microscope Photo Of Normal QI.
QI present in coal tar or coal tar pitch as a result of thermal treatment
is called mesophase. The mesophase content of coal tar or coal tar
pitch is determined by a test method designated as ASTM D 4616.
The method involves optical microscopic examination of a sample of
polished pitch under cross-polarized light. The mesophase appears
as multicolored spheres. The ASTM method defines mesophase as
any sphere with a size of greater than 4 microns. Mesophase has a
much lower carbon to hydrogen ratio than normal QI. Figure 5 gives
a picture of mesophase present in coal tar pitch.
125

BINDER PITCH QI CONTENT – ALUMINIUM ANODE
Figure 5 – Photograph of Mesophase
Modern distillation practices limit the amount of mesophase formed
in coal tar pitch. The remainder of this paper will concentrate on the
effect of coke oven derived QI on the quality of a binder pitch. The
effect of the presence of mesophase on the binder characteristics of a
coal tar pitch has been the subject of many publications.
126

COKE OVEN MANAGERS’ YEAR BOOK
As has been discussed, coke oven derived QI is either the result of
the cracking of coal volatiles in the coke oven or entrainment of
larger solids in the coal derived volatiles as they are formed. The
large majority of the coke oven derived QI is normal QI formed by
cracking coal volatiles in the coke oven. With the large variation of
the QI content in coal tar previously discussed, one can assume that
the operation of the coke oven has an effect on the amount of normal
QI in coal tar. The following paragraphs discuss the factors which
control the amount of normal QI formed in the coke oven.
Formation of Normal QI
A coke oven is operated by heating coal to a temperature of
approximately 1100°C. As the coal is heated, it begins to emit
volatiles. A result of the loss of the volatiles is the shrinking of the
coal in the coke oven. This shrinkage forms a headspace in the coke
oven called the “tunnel head”. The “tunnel head” acts as a reactor
for the formation of normal QI. Figure 6 gives a picture of a coke
oven “tunnel head”.
Figure 6 gives a picture of a coke oven
127

BINDER PITCH QI CONTENT – ALUMINIUM ANODE
The effect of conditions in the “tunnel head” on the amount of
normal QI formed is shown in Figure 7.
16
14
12
10
Effect of Tunnel-Head Temperature
on Tar QI Content
8
6
8.0
7.5
7.0
6.5
6.0
5.5
870 900 925 955 980 1010 1035
Tunnel-Head Temp., °C
Effect of Coal Bulk Density
on Tar QI
640 655 670 690 705 720 735 750
Bulk Density (Dry, Cone) Kg/m 3
Figure 7 – Tunnel Head Effect on QI Formation
As Figure 7 indicates, there are two major factors which affect the
amount of normal QI which is formed in the coke oven. These
factors are the temperature in the “tunnel head” and the bulk density
of the coal packed in the coke oven. The affect of temperature is
easy to explain, but the affect of the bulk density of the coal is not as
128

COKE OVEN MANAGERS’ YEAR BOOK
obvious. As mentioned previously, the formation of QI is a result of
thermal cracking and polymerization of the coal volatiles in the
“tunnel head”. Therefore, it follows that an increase in temperature
in the “tunnel head” would increase cracking and polymerization
resulting in an increase in the amount of QI formed. The bulk
density of the coal in the coke oven is inversely related to the size of
the “tunnel head”. Therefore, as the bulk density of the coal in the
coke oven decreases the size of the “tunnel head” increases. As the
size of the “tunnel head” increases, the residence time of the coal
volatiles in the “tunnel head” increases.
Increased residence time results in increased coal volatiles cracking
and polymerization giving an increase in QI formation.
Production of Coal Tar Pitch
Coal tar pitch is produced by distilling coal tar. The distillation can
be performed in either a batch or continuous mode. Modern coal tar
distillation is performed using continuous vacuum distillation, which
reduces the thermal exposure of pitch reducing the chances of
mesophase formation. A schematic of the coal tar distillation
process is given in Figure 8.
Coal tar distillation produces three major distillation products
1) chemical oil,
2) distillate,
3) pitch
. Figure 9 summarizes the yields of these distillation products from
distillation of a typical coal tar.
129

BINDER PITCH QI CONTENT – ALUMINIUM ANODE
Figure 8 – Coal Tar Distillation Process
20%
%
50%
Figure 9 – Coal Tar Distillation Fractions
130
Chemical Oil
Distillates
Pitch

COKE OVEN MANAGERS’ YEAR BOOK
As Figure 9 indicates, the yield of coal tar pitch from a typical coal
tar is approximately 50 wt. %. Since the QI in the tar is not volatile
and will not distill, almost 100 % of the QI in the tar remains in the
pitch product. With the yield of pitch being 50 wt. % of the tar, a
good rule of thumb is that the pitch QI content is double that of the
coal tar QI content.
Effect Of Pitch QI Content On Binder Quality
The question that has been asked for many years is “What is the
effect of pitch QI content on the binder quality of the pitch?” This
question has been discussed many times in publications with no
definitive answer given. This study addressed the question by
conducting a literature survey and performing a laboratory scale
anode study. The results of these are discussed in the remainder of
this paper.
Literature Survey
Green anodes consist of calcined coke and recycled anode butt filler
particles coated with binder pitch, with the binder in this coating
penetrating porosity in the filler down to a size of about 5µm and
largely filling the interstices between the filler particles. During the
anode baking process the pitch is carbonised to binder coke. This
transformation has several stages [1]:
Anode temperature ( o C) Pitch transformation
Thermal expansion with the
0 - 200
release of stresses induced
during forming and cooling
Redistribution of pitch into voids
by pitch expansion leading to
150 - 350
further pitch impregnation of the
filler aggregate
350 - 450 Release of light pitch volatiles
131

BINDER PITCH QI CONTENT – ALUMINIUM ANODE
450 – 600
600 - 900
900 - 1200
132
Coking – Formation and growth
of mesophase leading to the
transition from a plastic to a
solid binder matrix. The binder
coke forms “bridges” between
the filler particles. Release of the
major amount of non-coking
volatiles.
Post coking – Release of heavy
cracked volatiles, methane, and
hydrogen. Annealing of tensions
in the coked structure
Crystalline re-orientation and
growth of the binder carbon and
possibly the lowest calcined
filler coke.
The formation and growth of mesophase in the binder pitch has a
strong influence on anode properties [2]. If mesophase can grow and
coalesce unhindered, the resulting binder coke will be well ordered
with an anisotropic “flow” optical texture when examined under a
microscope using cross-polarised light. Anisotropic binder cokes
tend to reduce the electrical resistivity and reactivity of the anode;
however the anode strength can also be reduced due to cleavage
between the layers in the structure [2]. Disruption of mesophase
development results in a more disordered or isotropic coke structure
that imparts greater strength to the anode. The influence of normal
QI on this carbonization process, and hence anode properties,
has been studied extensively. A feature of these studies, however is
wide disagreement on what the effect is of varying amounts of
normal QI on binder pitch performance.
A review of the literature shows that the following have been
attributed to the presence of normal QI in pitch:
Wetting of the filler particles: There is disagreement as to whether
normal QI improves the coating of petroleum coke particles by pitch
[3,4], makes it worse [5,6] or has no influence [7].

COKE OVEN MANAGERS’ YEAR BOOK
Controls the penetration of pitch into coke porosity by forming a
filter cake at the pore entrance. [2,3,8,9]. This filter cake of normal
QI carbonizes to a tough, strong carbon imparting strength to the
coke/pitch interface.
Provides a pathway for the release of volatiles from the binder
during baking [3,4].
Increases the coking value of pitch due to its own high coking value
[3]
There is general agreement that the presence of normal QI increases
the amount of disorder (isotropy) in the binder coke
[2,3,5,8,10,11,12,13]; however how this isotropy is achieved is not
agreed:
One view is that normal QI influences the chemistry of the
carbonization process by acting as a site for mesophase nucleation
[3,14], thereby increasing the coking value of pitch beyond the
contribution of its own coking value; or by acting as a cross linking
agent to reduce the size of the binder coke optical texture [11,13]. It
has also been reported that normal QI slows the carbonization
process up to temperatures of 850 – 900 o C [15].
The alternative viewpoint suggests that the influence of normal QI
on the carbonization process is only physical, i.e. the normal QI acts
as additional aggregate particles [2,5,12,16,17,18] that are physically
absorbed onto the surface of the growing mesophase, inhibiting their
ability to grow by coalescence [2,3,10,12]. This results in pockets of
high normal QI and isotropic carbon in a more anisotropic binder
matrix.
Greater levels of normal QI, increase the anode optimum pitch
requirement [5,14,19,20,21]. This is due to the normal QI particles
being wet by, and hence effectively consuming, liquid pitch and by
these particles taking up volume in the binder that could otherwise
be occupied by liquid pitch. The reported increase in optimum pitch
requirements is about 0.1% for each additional 1% of normal QI in
the binder [5].
133

BINDER PITCH QI CONTENT – ALUMINIUM ANODE
Increasing the amount of normal QI in pitch has been found to have
the following effects on anode properties:
Baked density – increase [6,16,17,23], decrease [14], or not strongly
influenced [20,24]
Strength - increase [2,6,8,13,16, 17,21,23,25], or little change
[14,19]
Electrical resistivity – increase [2,8,17], decrease [6], or little change
[19,20,21]
Air Permeability – decrease [6] or little change [19]
Coefficient of thermal expansion – increase [22]
Reactivity – increase [2,8,15,17,25,26], little change [19,21,23,27]
Sodium sensitivity – decreases [25]
Anode consumption – little change [20,21,23,27]
While there is a lot of variation in these reported anode property
results, it is worth noting that in virtually all of the studies reviewed
(covering normal QI levels from 1 – 20%) the measured anode
properties were in the typical ranges for each test. Several studies
showed clear evidence or included the comment that low (e.g.

COKE OVEN MANAGERS’ YEAR BOOK
Table 2 – Physical Properties of Pitches in Anode Study
Property
Softening
Point, °C
Quinoline
Insolubles,
wt.%
Toluene
Insolubles,
wt.%
Beta
Resins,
wt.%
Coking
Value,
wt.%
Density,
g/cc
3.8% 5.6% 10.1% 16.1% 20.0% 24.3%
QI QI QI QI QI QI
112 110 112 111 107 107
3.8 5.6 10.1 16.1 20.0 24.3
20.3 22.1 25.6 29.5 33.5 37.4
16.4 16.5 15.5 13.1 13.5 13.1
53.8 51.9 56.2 57.5 59.7 60.5
1.30 1.25 1.33 1.34 1.35 1.36
Sized coke received from a United States smelter was mixed using
the following standard anode coke formation:
Coarse 31.5 %
Intermediate 14.6 %
Fines 37.0 %
Butts 16.9 %
A series of cylindrical laboratory size anodes (10.2 cm in diameter
and 15.2 cm in height) was prepared using each of the six pitches.
The details of the anode mixing, molding, and baking protocol are
given in Table 3.
135

BINDER PITCH QI CONTENT – ALUMINIUM ANODE
Table 3 – Anode Production Protocol
Mixing and Molding Conditions
Materials Not Preheated
Mixing Time 19-25 Minutes
Mixing Temperature 159-163°C
Molding Temperature 149-153°C
Vibrating Time 65 Seconds
Baking Protocol:
0-600°C 10°C/hr
600-1170°C 25°C/hr
1170°C 14 hr hold
Center Retort Temperature 1100-1120°C
After baking, the anodes were cored and the cores were tested for ten
typical anode properties. Plots of the results of the testing are given
in Figures 10-19. As the plots indicate, the anode properties are
quite consistent considering the wide range of QI content pitches.
136

g/cc
Vol. %
COKE OVEN MANAGERS’ YEAR BOOK
Green Apparent Density
1.680
1.660
1.640
1.620
1.600
1.580
1.560
1.540
1.520
13 14 15 16 17 18 19
% Pitch
5.6% QI Pitch 10.1% QI Pitch 24.3% QI Pitch 20.0% QI Pitch
16.1% QI Pitch 3.9% QI Pitch
Figure 10. Green Apparent Density
Shrinkage
2.50
2.00
1.50
1.00
0.50
0.00
-0.50
-1.00
-1.50
-2.00
13 14 15 16 17 18 19
% Pitch
5.6% QI Pitch 10.1% QI Pitch 16.1% QI Pitch
20.0% QI Pitch 24.3% QI Pitch 3.9% QI Pitch
Figure 11. Shrinkage
137

g/cc
nPm
1.600
1.580
1.560
1.540
1.520
1.500
BINDER PITCH QI CONTENT – ALUMINIUM ANODE
Core Baked Apparent Density
1.480
13 14 15 16
% Pitch
17 18 19
5.6% QI Pitch 10.1% QI Pitch 16.1% QI Pitch 20.0% QI Pitch
24.3% QI Pitch 3.9% QI Pitch
2.50
2.00
1.50
1.00
0.50
Figure 12. Core Baked Apparent Density
Air Permeability
0.00
13 14 15 16
% Pitch
17 18 19
5.6% QI Pitch 10.1% QI Pitch 16.1% QI Pitch 20.0% QI Pitch
24.3% QI Pitch 3.9% QI Pitch
Figure 13. Air Permeability
138

% Residue
% Residue
COKE OVEN MANAGERS’ YEAR BOOK
Air Reactivity
85.00
80.00
75.00
70.00
65.00
60.00
55.00
50.00
45.00
40.00
13 14 15 16 17 18 19
% Pitch
5.6% QI Pitch 10.1% QI Pitch 16.1% QI Pitch 20.0% QI Pitch
24.3% QI Pitch 3.9% QI Pitch
96
94
92
90
88
86
Figure 14. Air Reactivity
CO2 Reactivity
84
13 14 15 16
% Pitch
17 18 19
5.6% QI Pitch 10.1% QI Pitch 24.3% QI Pitch 20.0% QI Pitch
16.1% QI Pitch 3.9% QI Pitch
Figure 15. CO2 Reactivity
139

µ - Ohms / m
W / m-K
BINDER PITCH QI CONTENT – ALUMINIUM ANODE
Electrical Resistivity
120.0
110.0
100.0
90.0
80.0
70.0
60.0
50.0
40.0
13 14 15 16 17 18 19
% Pitch
5.6% QI Pitch 10.1% QI Pitch 16.1% QI Pitch 20.0% QI Pitch
24.3% QI Pitch 3.9% QI Pitch
Figure 16. Electrical Resistivity
Thermal Conductivity
3.10
2.90
2.70
2.50
2.30
2.10
1.90
1.70
1.50
13 14 15 16 17 18 19
% Pitch
3.9% QI Pitch 5.6% QI Pitch 10.1% QI Pitch 16.1% QI Pitch
20.0% QI Pitch 24.3% QI Pitch 3.9% QI Pitch
Figure 17. Thermal Conductivity
140

Avg. Alpha * 10 -6 (@300° C)
Mpa
4.700
4.500
4.300
4.100
3.900
3.700
COKE OVEN MANAGERS’ YEAR BOOK
Coefficient of Thermal Expansion
3.500
13 14 15 16
% Pitch
17 18 19
5.6% QI Pitch 10.1% QI Pitch 16.1% QI Pitch 20.0% QI Pitch
24.3% QI Pitch 3.9% QI Pitch
Figure 18. Coefficient of Thermal Expansion
Flexural Strength
14.00
12.00
10.00
8.00
6.00
4.00
2.00
0.00
13 14 15 16 17 18 19
% Pitch
5.6% QI Pitch 10.1% QI Pitch 16.1% QI Pitch 20.0% QI Pitch
24.3% QI Pitch 3.9% QI Pitch
Figure 19. Flexural Strength
141

BINDER PITCH QI CONTENT – ALUMINIUM ANODE
Table 4 gives the same ten anodes properties at optimum baked
anode density. Again a comparison of the ten anode properties
indicates quite consistent properties.
Green
Apparent
Density
Table 4. Optimized Anode Properties
Pitch
Level
QI
3.9
wt.%
142
QI
5.6
wt.%
QI
10.1
wt.%
QI
16.1
wt.%
QI
20.0
wt.%
QI
24.3
wt.%
15 16 16 17 18 18
g/cc 1.614 1.630 1.621 1.641 1.646 1.633
Shrinkage Vol. % 0.65 -0.24 1.27 0.68 0.84 1.11
Baked
Apparent
Density
Air
Permeability
Air
Reactivity
CO2
Reactivity
Electrical
Resistivity
Thermal
Conductivity
Coefficient
of Thermal
Expansion
Flexural
Strength
g/cc 1.544 1.551 1.569 1.569 1.580 1.581
nPm 0.45 0.54 0.32 0.73 0.30 0.25
%
Residue
%
Residue
µ
Ohms/m
68.09 64.50 74.25 76.80 74.00 81.50
88.96 84.80 92.22 94.10 94.20 92.80
61.9 64.7 58.2 60.1 58.1 60.9
W/mK 2.64 2.52 2.85 2.72 2.77 2.75
*10^-6
@300°C
3.759 4.297 4.469 4.443 4.564 4.404
MPa 6.08 5.31 6.87 6.49 7.70 5.92

Conclusions
COKE OVEN MANAGERS’ YEAR BOOK
Despite all of the contradictions in the literature studied, we can
draw some conclusions:
Good quality anode carbon requires a balanced optical texture in the
binder coke bridges that bond filler particles together – it should be
sufficiently isotropic (i.e. disordered) to give the anode the required
mechanical properties, most notably sufficient strength; but with
enough anisotropy (order) to give acceptable levels of reactivity and
electrical conductivity.
Quite fortuitously, the presence of normal QI in pitch delivers such a
heterogeneous structure. Normal QI physically inhibits the
coalescence of mesophase by absorbing on to their surface during
anode baking. As the mesophase grows, they push the normal QI
into pockets – these pockets of isotropy in the baked anode impart
the necessary strength to the binder coke bridges. The remaining
matrix of binder coke, which is still relatively anisotropic, gives the
binder coke bridges the desired reactivity and electrical resistivity
properties.
Since anodes made from 0% normal QI pitch have been found to
have unacceptable anode properties [28], it is reasonable to assume
that there will be a minimum normal QI level, below which the
concentration of normal QI will be too low to deliver this desirable
heterogeneous anode structure.
The results of the studies reviewed suggest that this minimum
normal QI level is less than 2% and may be as low as 1%. This needs
to be confirmed experimentally.
In several of the studies, reducing normal QI levels from >10% to

BINDER PITCH QI CONTENT – ALUMINIUM ANODE
Although a number of different “ideal” ranges are given in the
literature for normal QI levels [4,13,14,21,28], however most of
these are based on historical results. As normal QI levels have come
down, so has the bottom limit of the ideal ranges – when put to the
test, the lower normal QI pitches have performed acceptably.
The situation is well described in [4]: “The importance of normal QI
in binder pitch is well established. However, there is some
disagreement as to the optimal content” and “……..the amount of
normal QI must be consistent”
The anode study conducted did not reveal anything that would
contradict the concept that acceptable anodes can be produced with
pitches with as little as 3.9 wt. % normal QI. The lower Baked
Apparent Density at lower normal QI levels warrants further
attention as there is potential to improve this through modification of
the anode formulation process [5].
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144

COKE OVEN MANAGERS’ YEAR BOOK
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146