Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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This invention relates to a new low volatile binder
pitch. More particularly, this invention relates to a new
coal tar binder pitch for Soderberg electrodes for making
aluminum, meeting governmental regulations, restricting
carcinogenic components and to a method of makin~ same.
BACKGROUND O~ TH~: INVENTION
Coal tar binder pitches having a softening point of
100-110C are well known and are generally used as a
paste in conjunction with coke Eiller. That paste moves
down a reservoir for the electrode paste which gradually
forms a liquid and then carbonize to form a coked carbon
electrode which comes in contact with the liquid aluminum
on one side, the other side being carbon tray forming the
second electrode. During coking of the conventional
binder pitch, fumes are elimited and contain substances
which could be considered carcinogenic.
The~problem was such that in order to comply with
government regulations, a highly sophisticated ventilation
and scrubbing system was required, rendering the Soderberg
system obsolete, because of prohibitive costs.
Also known is the Sumitomo process using pitches
havinc3 soEtening point in the order of 120-125C, where
there is less fumes but the problem associated with
carcinoc~enics emission still prevails.
The invention is directed to a new pitch which is
particularly suitable for Soderberg electrodes and which
helps to meet government fume emission control regula-
tions, and which though having a high softening point, is
still not too viscous to move down into the electrode
reservoir.
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~ roadly stated, the invention is directed to a low
volatile coal tar binder pitch having a low carcinogenic
fume emission meeting governmental regulations. This low
volatile binder pitch which is particularly suitable for
makiny Soderberg electrodes, has a softening point of
L30~140C and preferably l32C or: above, as deEined
according to the ASTMD-310~ method. :[n a particular
elnbodiment the binder pitch has a quinoline insoluble
conterlt oE lO-1~ and an ash content less than 0.3%.
A method of making the sarne is also contemplated
which consists in collecting that fraction of regular tar
which boils over 350C. By so doing applicant has
unexpectedly been able to remove by distillation, that
fraction of the tar which contains a substantial portion
of the carcinogenic substances which have been found to be
present in the benzene soluble part of the condensable
portion of the fume, and thereby has been able to reduce
the prohibitively costly pollution control equipment that
would have been required otherwise, and thereby preventing
the costly Soderberg apparatus becoming obsolete.
In a particular embodiment, the method involves
preparing low volatile binder pitch for making Soderberg
electrodes by vacuum distilling a coal tar material having
:less than 5~ ~.I. and not more than 0.1~ ash, to collect
at Eirst a distillate and maintain said distillation until
the vapour temperature of the distillate to be collected
is at least over 360C, stopping said distillation and
then collecting the residue of said distillation to
produce the binder pitch having a softening point of
130 to 1~0C.
In order to obtain the suitable binder pitch, the
distillation of the coal tar is carried over 350 C under
normal pressure, generally up to 360-400C range, at a
temperature within that range which yield the desired
softening point 130-140C of the pitch, preferably
132C or over. Within that pitch, other components are
adjlJsted to meet the characteristics necessary for making
electrocles. For instance, since a larger fraction of the
1~ distillate is collected as the pitch (or residue) a larger
proportion of the pitch is quinoline insoluble, and so for
ashes. In order to achieve the necessary proportions, an
adjustment in composition of the coal tar has to be made
generaly before distallation by reducing of ashes and of a
portion of quinoline insolubles. Blending of various
pitches may also be made. In order to ease distillation
and to reduce side reactions at these high temperatures,
the distillation is conveniently carried out under reduced
pressure.
The following will now serve to illustrate embodi-
mants of our invention:
EXAMPLE l
Over ~L,000 gallons of a tar feed having ~.3%
cluinoline insoluble, 7.8% xylene insoluble and a specific
Z5 gravity at 15.5 C of 1.182 was vacuum distilled in a
distilling column having 27 actual plate numbers into a
distillate up to 390C. The top of the column was
maintairled under a vacuum of 60 mm of Hg. The residue
namely the pitch was 39.7% of the tar feed, 6003% having
been removed as distillate.
The pitch had a quinol;ne insoluble of 10.7%, benzene
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insoluble of 32.1%, ash content of .12~.
Equiviscous temperature was: 5:203.2
10:192.1
1000:141.2
The softening point was 137.1C, and the cooking value
67.5%. All the tests were made accordiny to the ASTM
tnelthods; the softeniny point accordiny to ASTM D-3104.
This pitch, when compared with the reyular conventional
1~ Soderbery binder pitch provided more than 30% reduction in
carcinoyenic fume content.
E XAMPLE_ 2
In Example 2, the feed tar had a quinoline insoluble
(Q.I.) content of 4.8%, was vacuum distilled under 100 mm
of Hy with a final distillation temperature of 358 C
yielding a pitch of 37.1%, having a softening point of
13~.2C, Q.I. of 12.5~, a coking value of 67.4%, an ash
content of 0.08%. The carcinogenic content of the pitch
fumes was reduced by over 30% when compared with
conventional Soderberg binder pitches having a softening
point of 100-110C, thereby satisfying governmental
regulations.
SAMPLE A
As a comparison, a Sumitomo type of pitch was
prepared, the vacuum distillation was carried at 90 mm of
~Ig with a final distillation temperature of 348C
yielding a pitch of 39.3% having a softening point of
126.5C, a Q.I. of 8.7~, an ash content of 0.12~ and a
cokiny value of 62.5%. The carcinogenic content of the
pitch fumes was not reduced sufficiently to meet
governmental regulations.
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EXAMPLE 3
A feed tar having a Q.I. content of 408%, was vacuum
distilled under 100 mm of Hg with a final distillation
temperature of 370C, yielding a pitch of 37%, having a
softening point of 171.4C. This pitch was blended with
a pitch obtained from vacuum distillation of under 100 ~m
of Hg with a final distillation temperature of 355 C,
with a pitch yield of 48.1%, the pitc}l having a softening
point of 95C. The resulting pitch so blended yielded a
softening point of 131C. The Q.I. content was 11.6%
and a coking value of 64.6%. On trial, the carcinogenic
content of the pitch fumes was reduced by over 30% when
compared with conventional Soderberg binder pitches having
a softening point of 100-110C, thereby satisfying
governmental regulations.
SAMPEE B
A conventional Soderberg pitch was produced in a
4,000 gallon distillation pot with direct gas heating, the
ultimate distillation temperature was under 350C at
standard pressure. The tar had a Q.I. content of 7.8%.
The resulting point had a softening point of 101.1C,
with a Q.I. content of 0.26%, with a coking value of
62.1~. The resulting pitch when heated to 400 C
similarly to previous experiments, produced fumes which
contain over 30% higher carcinogenics than described in
the previous Examples 1 to 3.
The above described procedure can be subject to a
great variety of modifications which will be evident to
those skilled in the art and which fall within the scope
of the appended claims.
