CA1230923A - Apparatus for determination of aluminium oxide content of the cryolite melt in aluminium electrolysis cells - Google Patents
Apparatus for determination of aluminium oxide content of the cryolite melt in aluminium electrolysis cellsInfo
- Publication number
- CA1230923A CA1230923A CA000494348A CA494348A CA1230923A CA 1230923 A CA1230923 A CA 1230923A CA 000494348 A CA000494348 A CA 000494348A CA 494348 A CA494348 A CA 494348A CA 1230923 A CA1230923 A CA 1230923A
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- CA
- Canada
- Prior art keywords
- aluminium
- electrode
- melt
- cryolite
- measuring electrode
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
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- Electrolytic Production Of Metals (AREA)
Abstract
ABSTRACT
An apparatus for determination of aluminium oxide content of the cryolite melt in aluminium electrolysis cells includes 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 measuring electrode covered with aluminium. The reference elec-trode is a cryolite melt supersaturated with Al2O3. The aluminium coating of the measuring electrode is the aluminium melt in the cell into which the conductor of the electrode protrudes. The conductor of the measuring electrode has a boron nitride protective coating.
An apparatus for determination of aluminium oxide content of the cryolite melt in aluminium electrolysis cells includes 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 measuring electrode covered with aluminium. The reference elec-trode is a cryolite melt supersaturated with Al2O3. The aluminium coating of the measuring electrode is the aluminium melt in the cell into which the conductor of the electrode protrudes. The conductor of the measuring electrode has a boron nitride protective coating.
Description
~309~3 .
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 ~b~
. ~Z3~923
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 ~b~
. ~Z3~923
- 2 -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 - ~LZ3~ 3 -;~ ,., 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 ~;~30923 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 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/.
~Z;~923 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 .
31 ;2 30923 ~ 6 ~
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 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 ~230~Z3 ' ' ' 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.
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 - ~LZ3~ 3 -;~ ,., 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 ~;~30923 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 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/.
~Z;~923 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 .
31 ;2 30923 ~ 6 ~
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 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 ~230~Z3 ' ' ' 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.
Claims (3)
1. An apparatus for determination of aluminium oxide content of the cryolite melt in aluminium electro-lysis 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 measuring electrode covered with aluminium, said reference electrode being a cryolite melt super-saturated with Al2O3 and the aluminium coating of the measuring electrode being the aluminium melt in the cell into which the conductor of the electrode protrudes.
2. The apparatus as claimed in claim 1 wherin the conductor of the measuring electrode has a boron nitride protective coating.
3. The apparatus as claimed in claim 1 or 2 wherein the reference electrode is placed in a solid electrolyte which inturn is placed in a zirconium oxide tube closed at one end.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000494348A CA1230923A (en) | 1985-10-31 | 1985-10-31 | Apparatus for determination of aluminium oxide content of the cryolite melt in aluminium electrolysis cells |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000494348A CA1230923A (en) | 1985-10-31 | 1985-10-31 | Apparatus for determination of aluminium oxide content of the cryolite melt in aluminium electrolysis cells |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1230923A true CA1230923A (en) | 1987-12-29 |
Family
ID=4131772
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000494348A Expired CA1230923A (en) | 1985-10-31 | 1985-10-31 | Apparatus for determination of aluminium oxide content of the cryolite melt in aluminium electrolysis cells |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1230923A (en) |
-
1985
- 1985-10-31 CA CA000494348A patent/CA1230923A/en not_active Expired
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| MKEX | Expiry |