Note: Descriptions are shown in the official language in which they were submitted.
~309~3
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APPARAT~S FOR DETERMINATION OF ALUMINIUM OXIDE CONTENT
OF TH~ CRYOLITE MELT IN ALUMINIUM ELECTROLYSIS CELLS
BACKROUND OF ~HE INVENTION
The invention relates to an apparatus for determination
of aluminium oxide content of the cryolite melt in aluminium
electrolysis cells comprising an oxygen ion conducting solid
electrolite containing oxygen galvanic cell equipped with a
reference electrode of given oxygen potential arranged in
a zirconium oxide tube closed on one end and with a measur-
ing electrode covered with aluminium and being in connection
with the cryolit melt.
It is known that the current efficiency can be inreased
by keeping the appropriate alumina concentration in coarse
of aluminium electrolysis and, thus, it is an im~ortant
economical aspect.
Measuring of the alumina in an aluminium electrolysis
cell is done almost exclusively by classical analytical
methods after sampling. This is very slow and cumbersome and
therefore there is no chance to intervene quickly in the
technology. The instrumental analytical methods - x-ray
diffraction and microscope - works only on the basis of a
previous sampling.
The aluminium oxide content of cryolite melts has been
evaluated by electrochemical methods. Such a procedure is
described in the Hungarian Patent No. 175438, In this case
the electrolytic potential is increased on a graphite
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electrode of known surface and this results in the increase
of the current density. The A1203 concentration of the
cryolite melt can be calculated from the actual anodic
marginal current. This is an intermittent method and the
data obtained are characteristic only in a short time in-
terval.
Further disadvantage of this method i3 that the use
of a polarizing unit is necessary for the operation and
polarization programs should be start and run in each test.
Sp~C;~O~I f~o~7s
The apparatus described in the German patent
~k~ Mo. ~ 798 248, No. 1 798 307 and No. 23 50 485
measure the oxygen content of the melts by means of oxygen
ion conducting solid electrolite containing oxygen galvanic
cell.
These apparatus contain zirconium oxide tubes with
reference electrodes therein. The electrodes are materials
of different oxygen potential for example mixture of metal
and metal oxide, gases for example air etc. The measuring
electrode is a conductor usually fitted onto the outer
surface of the zirconiu~ oxide tube or placed beside it.
The conductor has a covering which makes possible the
tr.ansmission of electrons.
In spite of its simplicity, this apparatus is suitable
for continous measuring. Its disadvantage is, however, that
it is difficult to keep the advantageous oxygen potential
of the reference electrode. In case of gases the constant
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oxygen potential while in case of' other reference ma-
terials the choice of material producing appropriate
results and the assembly of the electrocle are the
problems.
There are difficulties also with the measuring
- electrode and with its covering especially in case of
melts at high temperature. In case of cryolite melts
the conductor has to be covered with aluminium, bu~ at
high temperature it gets damage very quickly and forms
more interfaces which cause inaccuracy in the measuring.
.
SUMMARY OF THE INVENTION
TherePore the object o~ ~he present invention is to
provide an apparatus which measures simply and continous-
ly the aluminium oxide content of the cryolite melt in
the electrolysis cell, the preparation of which is simple
and contains reliable electrodes,
The apparatus according to the invention is an oxygen
ion conducting solid. electrolite containing oxygen gal-
vanic cell equipped with a reference electrode of given
oxygen potential arranged in a zirconium oxide tube
closed on one end and with a measuring electrode covered
with aluminium and being in connection with the cryol.ite
melt. The reference electrode according to the invention
is cryolite meltsupersaturated. with A1203 while the
aluminium coating of the measuring electrode is the
molten aluminium in the furnace into which the electrode
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is immerse,d.
Practically the conductor of the measuring electrode
is covered with boron nitride protective coating.
The apparatus can be used under technological con-
ditions as a probe which provides high accuracy and
comfort even at high temperature and in contaminated en-
vironment.
There are no special requirements to meet with the
probe in order to provide accuracy, that means there is
n,eed to de~ine exactly the sur~ace of the measuring
electrode~ because the aluminium melt itself forms the
surface and choosin~ the potential of this melt the
average concentration of the aluminium oxide can be measur-
ed well~
BRIEF DESCRIPTION OF THE DRAWINGS
Fig 1 shows the schematic illustration of the apparatus
under technological conditions
Fig 2 shows the diagram obtained by means of the
apparatus
DESCRIPTION OF THE PREFERRED EM30DIMENTS
On the Fig 1, on the bottom of an electrolysis cell
/1/ there is the aluminium melt /2/, over it the cryolite-
alumina melt is placed while the slag /4/ is on the top.
An anode /5/ protrudes into the electrolysis cell /1/.
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The apparatus according to the invention is shown under
technological condition~, From the left a reference
electrode /7/ and the current supplying wire /8/ can be
seen, both are placed in a solid electrolyte tube /6/.
In the middle the measuring electrode /9/ can be seen
in the form of a conductor /11/ covered with a protective
coating /10/,
From the right a thermocouple /12/ measuring the
temperature is located.
The elctrodes /7 and 9/ and the thermocouple /12/ are
connected to the same instrument /13/.
The solid electrolyte /6/ which contains the re~erence
electrode /7/ is in zirconium oxide tube closed at one
end forming this way an oxygen ion permissive container.
The cryolite and the appropriate amount of alumina is
poured into this cotainer. The appropriate amounts means
that the cryolite melt should be supersaturated with
A1203~ It can be realized very simply with the overcharge
of the alumina. This way adjustment and control are not
necessary. The current supplying wire /8/ protrudes into
the so obtained reference electrode /7J and, at the other
end it is connected to the instrument /13/. The solid
electrolyte /6/ is fitted by its upper part to a boron
nitride tube which is cryolite resistant.
The so obtained electrode can work for more hours
; even in a very corrosive melt which is characteristic
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for the alumina electrolysis. Therefore it is suitable
for continous measuring.
The lead-in wire /11/ covered with the protective
coating /10/ forms the measuring electrode /9/, where
the lead-in wire /11/ is made of molybdenum. The protec-
tiye coating /10/ is made of boron nitride, thus also
the measuring electrode /9/ is resistant to corrosion.
To sum up, the present inventior provides an apparatus
being a galvanic cell, where with alumina saturated
cryolite melt and the aluminium forms the reference
. electrode and the aluminium melt itself forms the measur-
in~ electrode as t~e mea~uring elec.trode /9/ protrudes
into the mol.ten aluminium ~2/. The reference el.ectrode
/7/ and the thermocouple /12/ are placed in the cryolite-
-alumina bath /3/ layer.
In the course of measuring it is possible to register
the change of electromotive force and the A1203 content
is calculated from it with following formula:
E E RT ln /%Al 0 /
where
EMF = the measured electromotive force /V/
E = the electromotive force /V/ in the 1 m % cryolite
melt at T temperature
R = the molar gas constant /8,31433 J mol~1 K-1/
F = Faraday constant /96487 C mol~1/
Fig 2. shows the diagram obtained under technological
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conditions.-This Figure shows how EMF changes versus
time when under intensive stirring we increase the
aluminium oxide content o~ the 1 m.% aluminium oxide
containing cryolite melt by adding additionally aluminium
oxide. Because of the sensitive probe we observe a sudden
change of electromotive force after adding 1 % A1203 to
the cryolite melt. After dissolution of alumina a steady
state develops, the level of which is higher than before
the peak. Adding 2 % aluminium oxide the reaction is
analogous but the difference between the level of the
steady state e,m.f /because of the less concentration
difference between the parts divided by the ZrO2 probe/
will be less.
It can be seen that the change of the e.m.f. is
minimal at 5 % and 6 %.
For the industrial point of view the measuring of
this difference is sufficient, because it makes unambigous-
ly possible to determine whether the alumina concentration
in the electrolysis ce]l is low /2-3%/ or high /5-6 %/.
Generally it is sufficient to know the alumina con-
centration in the electrolysis cell quantitatively. Know-
ing the calibration curve relating to the e.m.f and the
concentration of aluminium oxide the unknown concentration
can be measured.
The above example shoes that the apparatus according
to the inventlon can be used simply, safely,relatively
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long lasting and continously for the determination of
the aluminium oxide content of the cryolite melt.
The main advantage of the invention is the simple
construction of electrodes. The reference electrode - as
it has been mentioned before - can be f`illed with the
cryolite melt from the electrolysis cell and adding of
alumina is necessary. The so obtained electrode can be
prepared on the spot safely, very quickly without any
measuring.
The preparation of the measuring electrode is less
difficult, it is enough to immerse the current supplying
wire covered with protective coating into the aluminiurn
melt and the electrode works.
With this method the mea~uring of the spatial in-
homogenities of the aluminium oxide at different points
in the electrolyte helps to develop the optimum construc-
tion of the electrolyzer cell and to determine the
optimum place of input of aluminium oxide.
The speed of measuring enables to observe the dis-
solution of aluminium oxide in the cryolite after the
crust-breaking and alumina charging. It is po~sible this
way to find the technologicaly most appropriate quality
of alumina.
While several embodiments of the invention have
been shown and described, it will be apparent to those
skilled in the art that modifications rnay be made there-
in without departing from the scope of the inventior.
