الفهرس 
ABSTRACTOnly 14 pages are availabe for public view
 |  | from | 193 |  |  |
| | | from | 193 | | |
Abstract
The electrolysis of aluminium in aluminum reduction cells was carried out at about 960 °C in the presence of different fluoride compounds. The major factors affecting the potlife are carbon quality, preheating, start-up, early operation, normal operation, design and installation. The preheating, start up and early cell operation were represented 25 % from the factors affecting potlife. Thermal behavior of three cells (number 617, 619, and 634) working at Egyptalum smelter was studied during the preheating, start up and early operation stages. Usually, the preheating process and start up stages were taken about 5-6 days. While the early operation duration was varied and based on many cell operational parameters. These parameters include: cell voltage, bath composition and temperature, metal and bath heights, iron and silicon analysis in the produced aluminium and side ledge formation. These parameters were investigated. The preheating time was increased and the heating rate was decreased for the studied cells in order to overcome the problem of cells instability during the start-up period of the cell. Results of preheating and start-up stages of these cells were evaluated. The preheating time for these cells was approximately 71 hours with an average electrical energy consumption of 18276 kWh. Increase the preheating time from two to three days decreases the cell instability during the early operation period from 0.087 to 0.064 µΩ, respectively, and will be reflected on cells performance. The final average cathode surface temperature was 852 °C with a relative standard deviation (RSD) of 2.3 %. The heating rates ranged between 2 and 12 °C/h, and the RSD for the anodic current distribution was about 10 %. Basic bath was added to the preheated cells with an average temperature of 1001 °C. Average time and cell voltage during the initial anode effect were one hour, and 14.6 V, respectively. Molten aluminium was added after 52 hours from bath addition. Measurement the sidewall carbon block temperatures provides a valuable information in both the operation and diagnostics of the aluminum reduction cells. So twenty thermocouples were embedded in the back sidewall carbon blocks to monitor the thermal behavior of these cells during the preheating, start up and early operation stages. The temperatures in these locations at the end of preheating stage were ranged between 71 and 113°C and gradually increased after metal addition to the temperatures in the range between 558 and 737 °C, and then decreased at the end of program measurements to the temperatures in the range between 546 and 691°C.
During the preheating stages, the average heat-up rate in the longitudinal sides were 1, 1.03, and 0.9°C/h, and in the transverse sides were 1.22, 1.02, and 0.95 °C/h for the three studied cells, respectively. While, the average heat-up rate during the period from electrolyte addition up to end the metal addition in the longitudinal sides were 11.55, 11.39, and 10.57 °C/h, and in the transverse sides were 8.8, 8.9 and 8.7 °C/h, respectively. The average temperatures in these locations during the preheating and start-up stages were linearly correlated with the average steel shell temperatures. The results also show the sidewall carbon blocks temperatures were unaffected by small voltage changes during the preheating stage, but it was little affected by small voltage changes during the start-up stage. After two weeks from metal addition, the cell voltage was reduced from 4.9 to 4.4 volts, and the average temperature of twenty thermocouples was decreased from about 690 to 610 °C. In the following period up to end the program of measurements, the cell voltage was stabilized, and the average temperature of twenty thermocouples was decreased from about 610 to 582 °C. The results of cells parameters show that the cell voltage was stabilized after 35days from cell start-up until the end of the program of measurements, and the average cell voltage during that period was 4.4 volts. The average metal height, electrolyte height, electrolyte temperature, cryolite ratio, iron and silicon analysis of these cells were stabilized after 25, 24,
45, 85, 25, and 40 days, respectively from cell start up until end the program of measurements, with an average heights of 27 cm, 22 cm, 960 °C, 2.53, 0.07 %, and 0.04 %, respectively The measured ledge at different locations in the longitudinal sides of the cells showed ledge formation with thickness of about 1, 2 and 3 cm at the electrolyte- aluminium interface at 35, 40, and 45 days, respectively from cell start-up. Ledge thicknesses were increased to about 5.4 cm at 60 and 75 days, and extended to 7.7 and 9.2 cm with increasing the pot life to 90 and 105 days, respectively. The stable ledge was apparently formed in the fourth month of the cell operation with ledge thickness of 10 cm, and this thickness can withstand large cell disturbances. The occurred anode effect during the early cell operation was changed the ledge profile, and the ledge was needed to about 12 hours to return to the normal situation. The results show, that increasing or decreasing the bath temperatures by about 5 to 10 °C was affected on the ledge area and its thickness, and hence cell performance.