Note: Descriptions are shown in the official language in which they were submitted.
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Electrode Binder
The present invention relates to a process for
preparing an electrode binder, and an electrode binder
obtainable by said process.
Background of the Invention
Electrodes are used in smelting cells for the
production of metals such as aluminium and steel, at
present nearly all primary aluminium being produced by
electrolysis of alumina (A1~03) in electrolysis cells. In
an electrolysis cell for aluminium production, aluminium
is deposited in molten form onto a carbon cathode whilst
simultaneously oxygen is released at, and eventually
consumes, the cell's anode. The electrodes are prepared
by mixing petroleum coke particles with a binder.
Petroleum coke comprises nearly pure carbon and is formed
during the refining of crude oil by high temperature
carbonisation of heavy residues.
Two main categories of anode are employed in
aluminium electrolysis cells, pre-baked anodes and so-
called Soederberg anodes which are used in Soederberg
cells. In Soederberg cells, a continuous mixture of
petroleum coke and binder is fed into the cell, the anode
being baked in situ by the heat generated in the cell.
Pre-baked anodes are prepared by pressing a mixture of
petroleum coke particles and binder into shape and then
subjecting the anodes to baking or carbonisation in order
to transform the binder into carbon.
The binder usually used in the production of such
electrodes is coal-tar pitch. Coal-tar pitch is a
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distillation product of coal tar, coal-tar being a
product of the carbonisation of coal, and consists of
hydrocarbon oils, and derivatives of phenols and bases
such as pyridine and quinoline etc. Coal-tar pitch is
used as a binder as its high carbon and aromatic content
has meant that after carbonisation electrodes containing
a coal-tar pitch binder contain few non-carbon
impurities. This is important to the performance and
life-time of the electrode as impurities (e. g. metals
such as vanadium, nickel, etc.) in the electrode may
similarly contaminate the product and increase the air-
and carbon dioxide-reactivity of the electrode thus
reducing its operational life-time. However, coal-tar
contains an extremely high proportion of Polycyclic
Aromatic Compounds (PAC), a typical coal-tar pitch having
a PAC content of approximately 100,000 ppm. Some of these
molecules are carcinogenic and for environmental and
health and safety reasons it would be beneficial if there
was an alternative material that could be used as an
electrode binder in place of coal-tar pitch, but which
has as low a PAC content as possible.
The need for a coal-tar pitch replacement has been
increased by recent regulatory trends making it
preferable that PAC levels in commercial materials be
kept to a minimum. A large number of PAC molecules exist.
In Europe certain substances (e. g. coal derived
substances such as coal-tar pitch) are classified
according to their content of specific PACs. One such PAC
is benzo[a]pyrene, which is considered~to be a useful
marker of the overall PAC content of a substance.
Accordingly, the classification of such products in
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respect of carcinogenicity is generally based upon their
benzo[a]pyrene content.
The use of heavy petroleum residues as electrode
binders has been investigated. However, to date such
binders have not been considered industry-acceptable
replacements for coal-tar pitch as their performance with
respect to coal-tar pitch in important parameters such as
their carbon content and density, as well as the air-
reactivity and carbon dioxide-reactivity of electrodes
prepared from such binders has been unsatisfactory.
Further, of the few reported means by which satisfactory
electrode binders may be prepared, the petroleum
feedstocks are highly aromatic, resulting in products
having a PAC content which remains high and which is
undesirable for environmental and health and safety
reasons.
For example, EP-A 0378326 describes a binder pitch
suitable for use in the preparation of graphite
electrodes used in electric arc furnaces for the
production of steel, by subjecting a petroleum aromatic
mineral oil to hydrotreating; thermally cracking the
hydrotreated aromatic mineral oil; subjecting residue
from the thermal cracking to distillation and combining
the topped residue with finely subdivided calcined
premium coke particles having an average diameter between
1 and 40 ~,m. In this process, which relates specifically
to the preparation of binder pitch for steel production,
it is necessary to use a hydrotreated aromatic mineral
oil feedstock i.e. a feedstock which has been pre-treated
with hydrogen in the presence of a catalyst.
Canadian Patent publication 200911 describes a
process for the production of a high quality petroleum
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tar pitch from an aromatic feedstock comprising the steps
of, (a) providing a fresh aromatic feedstock; (b) pre-
heating said feedstock in a furnace to a temperature of
about between 380 to 480 °C; (c) feeding said heated
feedstock to a reactor and treating said feedstock in
said reactor under controlled conditions so as to promote
condensation and polymerization reactions; (d) passing
said treated feedstock to a fractionating tower wherein
the feedstock is fractionated into (1) gases, (2) light
distillates and (3) a bottom fraction stream; (e)
dividing said bottom fraction stream into a recycle
stream and a cracked fraction stream;~and (f) feeding
said cracked fraction stream to a reduced pressure
distillation tower wherein said light cracked fraction is
further fractionated into (1) light gas oil, (2) heavy
gas oil and (3) a high quality petroleum tar pitch.
The feedstock of the process of CA 2009121 is a
highly aromatic hydrocarbon stream having an aromatics
content of 65-85o wt (page 4, lines 1-4 and page 6, line
21 to page 7, line 5)~. In the working example provided,
the feedstock employed is a catalytic cracking decanted
(clarified) oil having an aromatic content of 85 o wt.
Further, in order to obtain a binder pitch having the
correct properties, it is essential to recycle a part of
the heavy cracked fraction using a recycle stream, it
being stated that recycling is highly desirable in order
to optimise the resulting pitch properties (page 10,
lines 1 to 5). Therefore, from the teaching of CA 2009121
the person skilled in the art would be led to conclude
that for an electrode binder to be prepared from a
petroleum residue by thermal cracking of that residue, it
is necessary to use a highly aromatic feedstock, even
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requiring recycle of a part of the thermally cracked
residue.
EP-A 1130077 describes a process for producing a so-
called non-polluting petroleum pitch by subjecting a
petroleum fraction or residue to heat treatment at a
temperature between 350 and 470 °C, for a time of less
than 120 minutes at a pressure of under 20 atmospheres.
The feedstocks employed are a decanted oil obtained by
catalytic cracking and a residue from the production of
ethylene from naphtha. However, the pitches obtained
still contain high quantities of polyeyclic aromatic
compounds, in particular benzo(a)pyrene, the levels of
benzo (a)pyrene in the pitch typically being well above
1000 ppm.
It would be advantageous if there was a means by
which an electrode binder having both industry-acceptable
properties and a low PAC content could be prepared from a
petroleum-based feedstock.
Summary of the Invention
It has now surprisingly been found possible to
prepare from a petroleum residue an electrode binder
which has properties approaching that of coal-tar pitch
and a PAC content significantly lower than that of
existing petroleum-based binders.
The present invention provides a process for
preparing an electrode binder, which process comprises:-
(a) thermally cracking a non-hydrotreated thermal
tar feedstock having an aromatic content of less
than 65 o wt,
(b) separating the thermally cracked product of step
(a) in a separator, into at least a top fraction
and a bottom fraction, and
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(c) subjecting the bottom fraction from step (b) to
vacuum distillation, to yield as a vacuum
distillation residue an electrode binder.
Detailed Description of the Invention
The present invention involves thermally cracking a
non-hydrotreated thermal tar feedstock having an aromatic
content of less than 65 o wt [step (a)). Preferably, the
aromatic content of the thermal tar feedstock is less
than 60 o wt, more preferably less than 55 o wt, and most
preferably less than 50 o wt, aromatic content being the
total amount of mono to hexa+ aromatic compounds as
measured according to test method SMS 2783-95 (Research
Disclosure 451104, November 2001, No. 451, pp 1918-1922).
Preferably, the thermal tar feedstock,has an aromatic
content in the range of from 25 to 65 o wt, more
preferably 30 to 60 o wt and most preferably 30 to 50 0
wt.
Thermal tar is a residual product of thermal cracking
of heavy hydrocarbon feeds; thermal cracking being a
process wherein hydrocarbons are heated to high
temperature (e. g. 400 to 550 °C) at which temperatures
longer hydrocarbon molecules become unstable and break
into smaller molecules. The major applications of thermal
cracking of heavy hydrocarbon feeds in refineries are in
visbreaking (i.e. viscosity reduction) and thermal gas
oil production.
The thermal tar.feedstock of the present invention is
preferably obtained by thermal cracking of a heavy
hydrocarbon feed at a temperature in the range of from
400 to 550 °C, more preferably 450 to 520 °C. The
thermal tar feedstock is preferably obtained by thermal
cracking of an atmospheric distillation residue of a
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crude oil (long residue), a vacuum distillation residue
(short residue) or a heavy or waxy distillate fraction.
It is particularly preferred that the thermal tar
feedstock of the present invention is a residue from the
thermal cracking of a heavy distillate feed, preferably a
waxy distillate feed. Waxy distillates are distillate
fractions having a boiling point range of from 220 °C to
650 °C. Waxy distillates may conveniently be obtained by
vacuum distillation of a long residue, or from a thermal
gas oil unit. Such thermal tar feedstocks are preferred
as they contain fewer metal impurities than thermal tars
obtained by cracking, for example, residual feeds.
Preferably, the thermal tar feedstock is a residue
from a thermal gas oil unit. Most preferably the thermal
tar feedstock of the present invention is obtained by
thermally cracking a heavy, preferably a waxy, distillate
feed in a thermal gas oil unit.
In a preferred embodiment of the present invention
the thermal tar feedstock is obtained from a two-stage
thermal gas oil unit in a process comprising:- i) thermal
cracking of a residual feed, ii) separating the cracked
feed into a gas fraction and a liquid fraction, iii)
separating the gas fraction into at least a gas oil
fraction and a waxy distillate fraction in a
fractionator, iv) thermally cracking the waxy distillate
fraction and v) separating the thermally cracked waxy
distillate fraction to yield, as a residue, a thermal
tar. The thermally cracked waxy distillate fraction may
be conveniently separated in the fractionator of step
iii) .
In this preferred embodiment the residual feed is
preferably thermally cracked at a temperature in the
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range of from 400 to 500 °C, more preferably 430 to 480
°C: the residual feed preferably being a short residue.
The residual feed may conveniently be cracked in a
cracking furnace or in a soaker unit, wherein the feed is
heated to the cracking temperature in a furnace and then
subsequently transferred to a soaker (reaction chamber)
where most of the cracking takes place. The cracked feed
may be separated into at least a gas fraction and a
liquid fraction by any convenient means, but the
separation may very conveniently be performed in a
cyclone. As will be understood by those skilled in the
art, the temperature of the cracked residue entering the
separation means is dependent upon the temperature used
in the thermal cracking step, however, it will preferably
be in the range of from 370 to 470 °C, more preferably
400 to 440 °C. The fractionator of step iii) of the
preferred embodiment is preferably an atmospheric
fractionator, and the waxy distillate fraction is
preferably drawn from the fractionator at a temperature
in the range of from 350 to 450 °C, more preferably of
from 370 to 420 °C. The waxy distillate is preferably
cracked at a higher temperature than the residual feed,
preferably in the range of from 470 to 550 °C, more
preferably 500 to 520 °C.
The thermal tar feedstock of the present invention is
a non-hydrotreated feedstock. By non-hydrotreated it is
meant that the feedstock has not been treated with
hydrogen, as for example in a catalytic hydrotreater.
Hydrotreating a feedstock lowers its carbon content and
density. This is considered disadvantageous for the
production of electrode binders as it is preferred that
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electrode binders have a high carbon content, and thus a
high density.
In step (a) of the present invention the thermal tar
feedstock is subjected to thermal cracking. As will be
understood by those skilled in the art, the cracking
conditions used for step (a) may vary depending on the
type of thermal tar feed, and the equipment used to
perform the cracking. However, it is preferred that the
thermal cracking is performed at a cracking temperature
in the range of from 400 to 550 °C.
The thermal tar feedstock may conveniently be cracked
in a cracking furnace. When a cracking furnace is
employed the feedstock is preferably heated to a
temperature in the range of from 400 to 550 °C, more
preferably 450 to 500 °C. The residence time in the
cracking furnace may vary depending on the feedstock and
cracking temperature, however in refinery operations it
may conveniently be in 'the range of from 1 to 10 minutes,
.preferably 1 to 5 minutes.
Preferably the thermal cracking of step (a) is
conducted in a soaker,unit. A soaker unit comprises a
cracking furnace and soaker (reaction chamber). When
cracked in a soaker unit the thermal. tar is heated to the
cracking temperature in the cracking furnace from where
it is transferred to the soaker (reaction chamber) where
most of the cracking takes place. When a soaker unit is
employed the feedstock is preferably heated in the
furnace to a temperature in the range of from 400 to 500
°C, more preferably 450 to 500 °C, and most preferably
475 to 500 °C and then transferred to the soaker reactor.
The pressure in the soaker reactor is preferably
controlled to be in the range of from 100 kPa to 1000
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kPa, more preferably 150 to 500 kPa, and most preferably
200 to 300 kPa; and the residence time is preferably in
the range of from 10 to 120 minutes, more preferably 15
to 60 minutes, and~most preferably 20 to 40 minutes.
In the process of the present invention, thermal
cracking in a soaker unit is particularly effective when
the feedstock is a thermal tar from a thermal gas oil
unit.
The thermally cracked product of step (a) is
separated into at least a top fraction and a bottom
fraction in a separator [step (b)].
The separator of the present invention may be any
apparatus capable of separating the thermally cracked
product into a top fraction and a bottom fraction i.e. a
lighter fraction comprising lighter molecules (top
fraction) and a residual fraction comprising heavy
molecules (bottom fraction). Examples of apparatus that
may be used as the separator include an atmospheric
distillation unit and a cyclone. Regardless of the type
of separator employed, the bottom fraction preferably
comprises at least 80 o wt of components having an
atmospheric boiling point of at least 300 °C. Separation
step (b) is an important feature of the present invention
as only by removing a proportion of lighter materials
prior to vacuum distillation step (c) is it possible to
prepare an electrode binder having industry-acceptable
properties and a low PAC content.
Preferably, the separator of step (b) is a cyclone..
When a cyclone is employed the thermally. cracked product
of step (a) is separated into a gas fraction (top
fraction) and liquid fraction (bottom fraction). It is
preferred to use a cyclone to separate the thermally
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cracked product of step (a) as the high viscosity of this
material (i.e. a thermally cracked thermal tar) is such
that when introduced into distillation apparatus the
thermally cracked product may block or even coke in the
distillation apparatus.
As will be understood by those skilled in the art,
the temperature of the cracked material entering the
separator will be dependent upon the cracking conditions.
However, when a cyclone is employed for separation step
(b), the temperature of the thermally cracked product
entering the cyclone is preferably in the range of from
350 to 450 °C, more preferably 380 to 430 °C; and the
residence time in the cyclone will preferably be in the
range of from 30 seconds to 20 minutes, more preferably
30 seconds to 8 minutes, even more preferably 1 to 6
minutes.
It is an advantageous feature of the present
invention that it is not necessary to recycle any part of
the bottom fraction of separation step (b) back through
thermal cracking step (a). Accordingly, in a preferred
process according to the present invention, no part of
the bottom fraction of separation step (b) is recycled
back through thermal cracking step (a).
The bottom fraction from the separator is subjected
to vacuum distillation, the residue of said vacuum
distillation being the electrode binder of the present
invention [step (c)].
As will be understood by those skilled in the art,
the conditions used for the vacuum distillation may vary
depending upon the properties of the feed and the type
and properties of the electrode binder to be produced,
and a degree of experimentation may be required to
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establish the optimum distillation conditions. However,
the conditions of vacuum distillation are preferably such
that they correspond to an atmospheric boiling point of
from 450 to 550 °C, more preferably of from 480 to 520
°C, wherein conversion of atmospheric boiling point to
sub atmospheric boiling point is made in accordance with
the Maxwell-Bonell relationship as described in Ind. Eng.
Chem., 49 (1957) pp 1187-1196).
In general, the vacuum distillation of the bottom
fraction may conveniently be carried out at a pressure in
the range-of from 0.3 to 16 kPa, more preferably 1 to 10
kPa, even more preferably 1 to 8 kPa, and most preferably
3 to 7 kPa; and a distillation temperature preferably in
the range of from 310 to 400 °C, more preferably 350 to
390 °C.
As is well known to those skilled in the art, it is
preferred that the carbon content of an electrode binder
be as high as practically possible. This is because in
preparing the electrode the binder will be converted into
carbon. Accordingly, the conditions employed in the
thermal cracking, separation and vacuum distillation
steps are preferably optimised such that the electrode
binder has a Micro Carbon Residue Test (MCRT) value of at
least 45 o wt; more preferably at least 50 % wt (as
measured according to DIN EN ISO 10370).
The electrode binder of the present invention may
comprise a vacuum distillation residue from a sole
process stream according to the present invention or it
may conveniently comprise a blend of two or more such
vacuum distillation residues. Blends of two or more
different vacuum distillation residues prepared according
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to the present invention may be conveniently used to
optimise the properties of the electrode binder.
It is an advantageous feature of the present
invention that the electrode binders comprise only low
levels of PAC molecules as compared to coal-tar pitch and
the hereinbefore described petroleum-based binders.
Preferably, an electrode binder according to the present
invention has a benzo[a]pyrene content of less than 200
ppm, more preferably less than 100 ppm, and most
preferably less than 50 ppm, as measured according to IP
BN/93.
Preferably, the electrode binders have a PAC content
of less than 2000 ppm, more preferably less than 1000
ppm, and most preferably less than 750 ppm, as measured
according to IP BN/93 on the basis of the list of PAC
molecules provided in respect of Examples 1-4.
It is a further preferred feature of the present
invention that the electrode binders have a low sulphur
content, the electrode binders preferably having a
sulphur content of less than 2 o wt, more preferably less
than 1 o wt, as measured according to ASTM 2622-94.
The present invention further provides for an
electrode binder obtainable by the process of the present
invention.
The present invention still further provides for an
anode binder for use in aluminium production obtainable
by the process of the present invention; and for the use
of. said anode binder in aluminium production.
The invention will be further understood from the
following illustrative examples.
In the following Examples, unless otherwise stated,
density values were measured at 25 °C by test method DIN
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52004; Micro Carbon Residue Test (MCRT) values were
measured by test method DIN EN ISO 10370; Sulphur content
was measured by test method ASTM D 2622-94; Softening
point was measured by test method DIN 52011; Viscosity
values were measured at the specified temperature using a
Dynamic Shear Rheometer.
Total aromatic content was determined according to
Shell Method Series (SMS) 2783-95 (Research Disclosure
451104, November 200.1, No. 451, pp 1918-1922) and is
based on the total amount of mono to hexa+ aromatic
compounds present. SMS 2783-95 is a means of ultraviolet
quantitative analysis based on ASTM E169-99. The
ultraviolet spectrometer employed was a single beam
instrument(Varian Cary 50), having a bandwith of 1.0 nm
or less at 220 nm; a photometric repeatability of 0.5 0
Transmission and a slit width of 2 nm. The
spectrophotometer was fitted with a matched stoppered
silica cell of certified pathlength. Absorbance maxima
were measured at three wavelength positions: 190 to 205
nm (7~1) : 218 to 238 nm (~,2) : and 245 to 265 nm (~,3)
which positions correspond to the absorption bands of
mono-, di-, and tri-aromatic compounds respectively.
Wavelength positions were derived for,higher aromatic
compounds from the peak maxima for ?~3 as described in SMS
2783-95. Quantities of each aromatic type were calculated
from the absorptivity of each absorbance maxima by
correlating the data with those of a calibration sample
of known concentration using the method described in SMS
2783-95, using calculation procedure number 1.
PAC content was determined according to IP BN/93,
wherein test samples were sequentially filtered on silica
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with toluene; filtered on silica with heptane; PAC
species separated from aliphatics, naphthenics, mono- and
diaromatic hydrocarbons by high performance liquid
chromatography (HPZC); and finally identification and
quantification of individual PAC species performed by
GC-MS. In this analysis, l2Deutorated benzo[a]pyrene was
employed as an internal standard. The PAC species
measured in determining the PAC content were as follows:-
fluoranthene; pyrene; benzo[a]fluorene;
benzo[b+c]fluorene; benzo[b]naphto[2,1-d]thiophene;
benzo[g,h,i]fluoranthene; benzo[a]anthracene; chrysene
and triphenylene; 1+2+3+4+5+6 methylchrysene;
benzo[b,j,k]fluoranthene; benzo[e]pyrene; benzo[a]pyrene;
perylene; dibenz[a,j]anthracene; indeno[1,2,3-c,d]pyrene;
dibenz[a,h+a,c]anthracene; benzo[b]chrysene;
benzo[g,h,i]perylene; anthanthrene, and coronene.
Examples 1-4: Preparation of Electrode Binders
An electrode binder was prepared using a feedstock
and process according to the invention (Example 1). In
addition, comparative binders were prepared using
alternative types of feedstock (Examples 2-4). The
feedstocks employed in Examples 1-4 were:-
Example 1: (According to the Invention) Feedstock (A). A
~thermal,tar from a two-stage thermal gas oil unit
(aromatic content 49.5 o wt).
Example 2: (Comparative) Feedstock (B). Clarified oil; a
highly aromatic oil (aromatic content 76.0 o wt) which is
the residue from an atmospheric fractionator on a
catalytic cracking unit.
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Example 3; (Comparative) Feedstock (C). A short residue
from a North Sea crude, which is the bottom product of a
vacuum distillation unit (aromatic content 30.40 wt).
Example 4: (Comparative) Feedstock (D): An ethylene
cracker residue (ECR), which is the residue from an
atmospheric fractionator of an ethylene cracker unit
(aromatic content 70.70 wt).
The properties of feedstocks A to D are shown in
Table 1.
TABLE 1
Feedstock Thermal Clarified Short ECR
Tar Oil Residue (D)
(A) (B) (C)
Density (g/cm3) 1.029 1.119 1.018 1.137
MCRT (o wt) 12.5 13.5 21.6 25.0
Sulphur (o wt) 0.72 2.50 2.20 0.23
Softening Liquid liquid 51.0 44.5
Point (C)
Viscosity 248 56 32718 5191
80 C (mm2/s)
PAC (ppm) 6800 20132 34 32472
Vanadium (ppm) <5 3 95 <1
Nickel (ppm) <5 3 25 <1
Total Aromatic 49.5 76.0 30.4 70.7
(o wt)
Electrode binders were prepared from feedstocks A-D
as follows.
Feedstock was passed through a filter manifold and
stored in a heavy oil weight tank. From the weight tank,
the feedstock was pumped to a cracking furnace. The
cracking furnace comprised eight separate tube coils each
immersed in a lead pot with each lead pot separately
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heated by an electrical heater. The total volume of the
coils was approximately 3000 cm3. The residence time of
the feedstock in the furnace was dependant upon the
density of the feedstock and the feed rate, however a
feed rate, of 5Kg/h gave a residence time of approximately
40 minutes, and a feed rate of 2.5 Kg/h gave a residence
.time of approximately 80 minutes.
After passing through the cracking furnace the total
thermally cracked product was fed into a flash column
separator wherein overhead vapours, butane and lighter
fractions were removed. The bottom fraction from the
separator (fractionator bottoms) was then fed into a
vacuum tower wherein it was further separated into two
fractions by vacuum flash distillation, an over-head
fraction and a residue fraction, which residue fraction
was collected for use as an electrode binder.
The yields and properties of the electrode binders
obtained, are shown in Table 2, together with the
conditions employed in the cracking furnace and vacuum
flasher,. For each Feedstock the conditions were optimised
to obtain an electrode binder having a MCRT value of at
least 45o wt. In Example 1 (Feedstock (A)), two runs
were performed yielding two binders, (A1) and (A2).
Similarly, in Example 2 (Feedstock (B)) two runs were
performed yielding binders (B1) and (B2).
CA 02468061 2004-05-21
WO 03/046111 PCT/EP02/13313
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o ~ r~ ov ~, -~ -~ ~I ~a -~I o 0 0
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CA 02468061 2004-05-21
WO 03/046111 PCT/EP02/13313
- 19 -
From Table 2 it can be seen that the electrode binders
of Example 1, (A1) and (A2), prepared from a thermal tar
feedstock, have a low sulphur content and low PAC /
benzo[a]pyrene content. In this regard they compare
favourably to the binders of Example 2, (B1} and (B2),
prepared from a clarified oil feedstock, which have a high
sulphur and PAC content, and to electrode binder (D1)
prepared in Example 4 from an ethylene cracker residue
feedstock which has an even higher PAC content. Whilst
electrode binder (C1), prepared in Example.3 from a short
residue feedstock had low PAC content, the density of the
binder obtained was low.
Examples 5-9: Preparation and testing of laboratory
electrodes
Test electrodes were prepared using the electrode
binders of Examples 1-4.
In Example 5 (according to the invention) an electrode
was prepared from a blend of binders (A1) and (A2).
Similarly, in Example 6 a comparative electrode was
prepared from a blend of binders (B1) and (B2). In Example
8, binder (D1) was blended with a small amount of over-head
fraction (OH) obtained from the vacuum tower as the
viscosity of D1 alone was too high to prepare a laboratory
scale electrode. A further comparative electrode was
prepared using a coal-tar pitch binder (Example 9). The
composition and properties of the binders used to prepare
the test electrodes are shown in Table 3.
Test electrodes were prepared as follows. Graded
petroleum coke was preheated to a temperature of at least
110 °C greater than the softening point of the binder to be
used. The petroleum coke was then placed in a similarly
CA 02468061 2004-05-21
WO 03/046111 PCT/EP02/13313
- 20 -
preheated mixer and cold crushed binder blended into the
preheated petroleum coke. Mixing was continued until a
homogenous blend was obtained, after which the mixture was
transferred to a preheated steel mould and the electrode
shaped by means of a hydraulic press with an applied
pressure of approximately 10 tonnes. The electrodes
comprised 14 o wt binder, 19.7 o wt course petrol coke,
26.5 o wt medium petrol coke, 15.8 a wt fine petrol coke
and 24 o wt dust petrol coke.
The electrodes were calcified by oven baking, applying
a final temperature of 1190 °C, for 24 hours. During the
baking process, the electrodes were protected against
oxidation by a covering of petrol coke and by flushing the
oven with nitrogen. The electrodes were then tested for
air-reactivity, and carbon dioxide-reactivity according to
the methods described in Fischer W.K. et al, Journal of
Metals 39 (11), 43-45, 1987. Electrical resistance was
tested according to test method DIN 51919. The results are
shown in Table 3.
CA 02468061 2004-05-21
WO 03/046111 PCT/EP02/13313
of
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CA 02468061 2004-05-21
WO 03/046111 PCT/EP02/13313
- 22 -
From Table 3 it can be seen that the electrode of
Example 5, pxepared according to the present invention,
was prepared from a binder having a very low PAC/
benzo[a]pyrene content, and displayed a performance in
terms of air-reactivity, carbon dioxide-reactivity and
electrical resistance similar to that of a conventional
electrode prepared from coal-tar pitch (Example 9).
The electrodes of Examples 6 and 8, were prepared
from binders having a high PAC'/benzo[a]pyrene content
and their use would therefore be undesirable from an
environmental and health and safety standpoint. Whilst
the electrode of Example 7 was prepared from a binder
with a low PAC content, it displayed very low air
reactivity and thus the binder from which this electrode
was prepared would not be considered acceptable as a
replacement for coal-tar pitch by the aluminium and steel
industry.
Example 10
An electrode binder was prepared from a thermal tar
taken from the bottom of an atmospheric fractionator of a
two-stage thermal gas oil unit (TGU). The electrode
binder was prepared by thermally cracking the thermal tar
through a cracking furnace (temperature 480 °C), passing
the thermally cracked product through a soaker reactor
(pressure 2.5 bar (250 kPa), residence time 25 to 35
minutes, temperature 450 °C), separating the thermally
cracked product in a cyclone (inlet temperature 390 °C)
into a top and bottom fraction, (residence time 4 to 6
minutes) and subjecting the bottom fraction to vacuum
distillation in a vacuum flasher (temperature 370 °C,
pressure 55 mbar (5.5 kPa). The properties of the thermal
CA 02468061 2004-05-21
WO 03/046111 PCT/EP02/13313
- 23 -
tar feedstock and the electrode binder obtained are shown
in Table 4.
An electrode was prepared from the electrode binder
of Example 10 in the same manner as the electrodes of
~ Examples 5-9 were prepared; and tested for air
reactivity, carbon dioxide reactivity, and electrical
resistance. The results are shown in Table 4.
From Table 4 it can be seen that the binder of
Example 10 had a low PAC / benzo[a]pyrene content and the
electrode prepared from the binder had air-reactivity,
carbon dioxide-reactivity and electrical resistance
similar to that of an electrode prepared from coal-tar
pitch.
CA 02468061 2004-05-21
WO 03/046111 PCT/EP02/13313
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