Application of electrolyte temperature and molecular ratio control modes in the automatic control system for aluminum electrolysis

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________________________________________ The process parameters in modern aluminum electrolysis industry are evolving from the \"four lows and one high\" principle (low alumina concentration, low temperature, low molecular ratio, low efficiency factor, and high cell voltage) toward even higher standards. That is, efforts are being made to achieve low cell voltage and low aluminum levels on the basis of these \"four lows\", and considerable progress has been made in this regard. In this process, daily control of the temperature and molecular ratio in aluminum electrolysis cells is of great importance. I. Control concept: As is well known, as the molecular ratio of industrial electrolytes decreases, both the melting point and the electrolysis temperature of these electrolytes drop accordingly. Production experience has shown a high degree of correlation between the electrolysis temperature and the molecular ratio. With other conditions remaining essentially unchanged, we analyzed a large amount of data and found that the correlation coefficient between the cell temperature and the molecular ratio is 0.89. Due to this high degree of correlation, it becomes possible to control the electrolysis temperature and the molecular ratio by adding aluminum fluoride. Based on this, a simple regression analysis was conducted, and the relationship between the tank temperature and the molecular ratio for our company’s 114.5KA series was determined to be: T=65CR+812. It was verified that the temperatures of over 90% of the tanks in this series conform well to this equation in terms of the molecular ratio. Of course, the cell temperature is also primarily related to the cell voltage VS (set voltage), and there is a corresponding relationship between the cell voltage VS and the molecular ratio CR. The control mechanism for cell temperature and molecular ratio in our company’s aluminum electrolysis automatic control system is based on keeping the cell voltage VS relatively constant, while ensuring good control over the effect coefficient and fluctuation coefficient. In other words, the molecular ratio is controlled by regulating the amount of aluminum fluoride added, thereby achieving control over the electrolysis temperature. II. Temperature and Mole Ratio Control Mode 2.1 Establishment of the control table: Based on the process control objectives, the relationship between tank temperature and the mole ratio, as well as the capacities of the aluminum fluoride storage tank and the constant-volume feeder, the following control table for the amount of aluminum fluoride to be added is formulated: Temperature (°C) Mole Ratio Basic addition frequency Additional addition frequency 980 Hot tank – Either the mole ratio or tank pressure is too high, or the aluminum level is too low. Using this table, we can determine the basic amount of aluminum fluoride to be added as well as the additional amount to be added, thereby achieving control over the mole ratio and tank temperature by regulating the amount of aluminum fluoride added. This requires measuring the tank temperature once a day and analyzing the molecular ratio 1-2 times per week. As can be seen from the table above, our process control objectives are: temperature of 950°C–960°C, and molecular ratio of 2.1–2.3. If the molecular ratio analysis frequency does not meet this requirement, we can also calculate the molecular ratio directly based on temperature, as the two are highly correlated; determining the addition of ALF3 based on the molecular ratio is equivalent to determining its amount of addition using temperature measurement results. 2.2 Implementation steps: 1) The electrolysis workshop and the central laboratory provide the computer center with the temperature of each cell and the molecular ratio for each cell on a daily basis; the staff on duty at the computer center enter this information into the mainframe (it is also possible for the workers in the workshops to enter it via the X terminals in the workshops). 2) The workshop technicians (at the X terminal) or process engineers (on the host computer) determine the basic feeding amount and additional amount of aluminum fluoride for the day based on the aluminum fluoride addition schedule. As shown in the table below: Slot number N-add N-bas R-0 R-1 R-2 R-3 T-0 T-1 T-2 T-3 ALF3-1 ALF3-2 ALF3-3 101 0 0 2.18 2.42 956 962 970 971 20 50 85 102 10 20 2.31 2.15 957 962 956 953 20 0 0 In the table, N-add and N-bas represent the additional and basic addition frequencies of aluminum fluoride today, respectively. R-0, R-1, R-2, and R-3 represent the molecular ratios for today, yesterday, the day before yesterday, and the day before that, respectively. T-0, T-1, T-2, and T-3 represent the tank temperature for today, yesterday, the day before yesterday, and the day before that, respectively. ALF3-1, ALF3-2, and ALF3-3 represent the number of times aluminum fluoride was added yesterday, the day before yesterday, and the day before that, respectively. The table above is a display table for the addition of aluminum fluoride in the control system; it lists the tank temperature, molecular ratio, and the number of times aluminum fluoride was added over the past four days. This is done so that those responsible for making adjustments can clearly see the trends in tank temperature, molecular ratio, and aluminum fluoride usage over these days, as well as the effectiveness of the control measures. As can be seen from the table above, after 155 additions over 3 days in trough No. 101, the molecular ratio decreased from 2.42 to 2.18, and the temperature dropped from 917°C to 956°C, thus meeting our control targets; therefore, there was no need to add aluminum fluoride any further, resulting in 0 additions of aluminum fluoride both as part of the regular dosage and as additional additions on that day. The temperature and molecular ratio of slot No. 102 have increased from 953°C and 2.15 the day before yesterday to 967°C and 2.31 today, thus deviating from the target range for control; corrections should be made in accordance with the control table. Therefore, the basic feeding frequency and additional feeding frequency of aluminum fluoride for that day are 20 and 10 times respectively. It can also be seen from the table above that samples were not taken for molecular ratio analysis in the areas where grooves 101 and 102 are located yesterday and the day before; in such cases, the molecular ratio can be calculated back using the formula T=65CR+812. If the tank temperature differs significantly from that of the previous day, a case-by-case approach is required. If an event occurred shortly before temperature measurement, the base amount and additional amount of aluminum fluoride to be added on that day should remain the same as those from the previous day; adjustments can be made after temperature measurement is taken on the following day ； If the cause is an imbalance between the two levels, then these two levels should be adjusted to normal promptly. 3) Feed fluoride salt into the tank above the groove at regular and irregular intervals. Timed feeding: A dedicated cart adds fluoride salt to the feed tanks at the top of all troughs every 4 days ； Ad-hoc feeding: For special tanks where fluoride salts are consumed rapidly, they are refilled to 10% remaining level based on their capacity and consumption rate. That is, to ensure there is always material in the fluoride salt tank. III. Control Effectiveness: After operating for a period of time, it was evident that this control model was highly effective. Among the 100 electrolyzers in the entire plant, 90% had temperatures and molecular ratios within the desired control ranges. This enabled a shift from rough control to precise control of the electrolyzers’ operation, resulting in excellent technical performance. The specific details are shown in the table below:
Temperature range: 935–985°C
Molecular ratio range: 1.9–2.6
Current efficiency: 90%
Effect coefficient: 0.6–0.7 units per electrolyzer per day
Aluminum consumption: 75 kilograms
Electricity consumption: 14,730 kWh

After operation:
Temperature range: 950–960°C
Molecular ratio range: 2.1–2.3
Current efficiency: 92.50%
Effect coefficient: 0.2–0.3 units per electrolyzer per day
Aluminum consumption: 30 kilograms
Electricity consumption: 14,100 kWh

IV. Conclusions:
1) Stabilizing the temperature and molecular ratio of aluminum electrolyzers is an essential prerequisite for their stable and efficient operation. It is feasible and highly effective to control the tank temperature and molecular ratio by specifically adding aluminum fluoride. 2) In the absence of this mode in the automatic control system, as well as a fluoride salt tank and a feeding mechanism, manual addition according to the amounts indicated in the control sheet can also be used, with good results. 3) The application of this mode can significantly improve the stability and technical performance of aluminum electrolysis. 4) This molecular ratio approach is simpler than the control based on tank conditions, yet it is very practical. It relies on stable tank conditions; to achieve truly automated optimal control of both the molecular ratio and tank temperature, it is necessary to combine this approach with an expert system for tank condition diagnosis, and this is exactly what we are working towards.