A Z ELÉRT EREDMÉNYEK GYAKORLATI HASZNOSÍTÁSA

1. A laboratóriumi mérések hozzájárultak az alumíniumelektrolizáló kádakban végbemenő, az áramhatásfokot csökkentő folyamatok részarányának meghatározásához.

2. Az extrapolált feszültség meghatározásával gyors áramhatásfok-meghatározásra nyílt lehetőségem. Nemzetközi megbízásaink révén gyors és hathatós segítséget kellett adni az üzemek technológiai problémáira, intézkedésekre kellett javaslatot tenni. Ebben számomra nélkülözhetetlen volt az extrapolált feszültség meghatározása és az abból számolt áramhatásfok.

3. A kapott eredmények modellekbe, szakértői rendszerbe épültek be. A mérési módszer, mint a folyamat elektrokémiai ismerete, szerves része a 16 csatornás mérésadatgyűjtő és feldolgozó rendszernek.

4. A laboratóriumi és üzemi vizsgálatok során kapott ismeretek oktatási anyagokban, nemzetközi workshopokon és egyetemi előadásokban is hasznosultak.

5. THESIS POINTS

5.1 . Summary of results and findings

1.)The dissolved metal in cryolite-alumina melts consists of two components: sodium and aluminum. For their separation and determination an analytical method was developed. This maintain the determination of their solubility in cryolite-alumina meltt and it was the base of the determination of the reoxidation process causing decrease in current efficiency.

2.)On the base of the study of reoxidation process parameters, interaction between dissolved metals and CO2 gas was investigated at plant temperature in different electrolyte compositions. It was found that the diffusion process through the boundary layer was the rate-determining step when the convection effect was small and the inlet of CO2 gas was far from the aluminum surface. In the case, when CO2 gas disturbs the aluminum/electrolyte surface due to the extended convection, both sodium and aluminum content increased in the electrolyte and the rate of the reoxidation process increased as well. The relationship was determined among the electrolyte composition, the electrolyte temperature and the anode-cathode distance in industrial applied parameters. The rate of the reoxidation process was also determined in the decrease of the current efficiency by equation. This process is considered as an atomic solution and solubility at given temperature and electrolyte composition is determined by convection transport of gases.

3.)The depolarisation effect of dissolved aluminium existing in ionic form was measured by current-interruption-techniques. The anode current density of the secondary electrochemical process taking place on the anode was determined using literature activity data of aluminum-monofluoridedata.Accordingly to knowledge of the anode current density the rate of dissolved aluminium-taking part in in the anode secondary reaction-was determined onto the decrease of current efficiency.

4.)It was stated that the accumulation of the sodium-fluoride in the aluminum/electrolyte boundary layer causes the decrease of the current efficiency in the cathode process. The primary process, - the reduction of the aluminum containing components - takes place through this boundary layer. Here takes place the secondary process, as well-the decomposition of the sodium-causing decrease in current efficiency. Due to research work of Solli the equation of the secondary process taking place on the cathode is known, so my own results onto the sodium concentration data were used to describe the process causing decrease of current efficiency in the cathode process.

5.2. Determination of the Electrochemical Components of Existing Aluminum Electrolysis Cells

1.A measurement method was created to determine the extrapolated voltage at I= 0 kA using the characteristics of the current line-cell voltage in electolysis cells.

2. It was verified, instead of assumption of constant electrochemical components tha calculated electromotive force( EMF) is practical to use in the process control of aluminium electrolysis cells.The calculation of electromotive force (EMF) is based on the parameters of reduction cells.

The components of the electromotive force (EMF) are the Nernst potential and overvoltages ( anode reaction overvoltage, anode diffusion overvoltage and cathode overvoltage), nevertheless electromotive force (EMF) doesnot have thermodynamic effect to theprocess.

The extrapolated voltage varied in wide range between tha value of 1,5-2,5V. To these changes of the operation parameters the reduction cell were assigned. The diffusion overvoltage from the components of the extrapolated voltage increases in cases when the alumina concentration is below 3% in the electrolyte. In such cases, the extrapolated voltage is higher than the calculated electromotive force, caused by the increase of the anode diffusion overvoltage. This makes extrapolation to I=0kA a difficult task.

4. The measured extrapolated voltages were in all cases less than the electromotive force calculated for the given reduction cells in the range of 3-8% alumina concentration. To the explanation of this difference serves the depolarisation effect of the dissolved aluminium among laboratory condition, because this anode urrent density decreasing component influences the current efficiency loss.

5. It was concluded that the measurement of the extrapolated voltage is suitable to characterise the operation. The measurements carried out in reduction cells of different constructions and operations proved the relationship between current efficiency and extrapolated voltage.

6. A method was created and applied to determine the stable operation range of the existing reduction cells by using the measurements of the extrapolated voltage.

7. It was concluded that with a small change in the line current the change of cell resistance and time function can be used to correctly adjust the proper electromotive force (EMF) and to improve alumina feeding. In addition, it can also lead to the decrease of the number of anodeffects and the early detection of abnormal operational activity.

8. The relationship between current-efficiency-decreasing processes and extrapolated voltage measured in industrial electrolysis cells was stated. In the value of the extrapolated voltage appear processes occuring in the boundary layer. These processes can be influenced by the change of the boundary layer caused by the convection. The effect of the secondary process taking place on the anode is the effect of depolarization of dissolved ionic aluminum. It occures in the value of extrapolated voltage and decreases it. This depolarization effect is the highest rate in the current efficiency loss. This loss completing with the losses of reoxidation process and cathode process a current efficiency indicator was determined which can be easily applied to characterize the electrolysis process.

5.3. Practical implications

1. The laboratory measurements contribute to the determination of the rate of the current-efficiency-decreasing processes appearing in the aluminum electrolysis cells.

2. Using the determination of the extrapolated voltages an opportunity opened to determine quickly the current efficiency. The achieved results were identified and used in the frame of international contracts to improve the smelter performance parameters.

3. The results achieved from the measurements and calculations were incorporated into models and expert systems. The measurement method is an organic part of the 16-Channel-Data-Collection and Processing system as the electrochemical knowledge of the process.

4. The knowledge received during laboratory and industrial measurements was utilized in education, in international workshops and in university lectures.